Zirconium-based MOFs for light-driven reactions: a critical assessment of recent progress

Abstract

Light-driven reactions are crucial processes in sustainable chemistry and environmental remediation. These reactions harness the energy of light, typically solar or artificial, to drive chemical transformations that rely on the fundamental principle of photocatalysis, where a semiconductor material absorbs light and generates electron–hole pairs. This review provides a critical and comprehensive survey of recent progress in utilizing zirconium-based Metal–Organic Frameworks (Zr-MOFs), including the UiO series, PCN, BUT, and MOF-801, for light-driven reactions, focusing on CO₂ reduction, hydrogen evolution, and contaminant degradation-crucial processes for addressing global challenges in energy and environmental sustainability. It examines the structural attributes of Zr-MOFs that underpin their photocatalytic activity, such as high stability, tunable porosity, and versatile chemical functionality, and analyzes strategies to enhance performance, including co-catalyst integration and ligand modification, which are vital for optimizing light absorption and charge transfer. The review consolidates mechanistic insights derived from both experimental and computational studies, aiming to clarify the complex reaction pathways involved. By critically assessing current research, it identifies key challenges and opportunities, particularly in designing efficient co-catalysts and elucidating reaction mechanisms. This analysis underscores the significant potential of Zr-MOFs in addressing climate change, energy scarcity, and environmental pollution through efficient light-driven transformations. The review aims to stimulate further research towards developing highly efficient, stable, and sustainable photocatalytic systems, ultimately contributing to the advancement of green technologies and the realization of a sustainable future.

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1. Introduction

The relentless surge in atmospheric carbon dioxide (CO₂) levels, coupled with the escalating demand for clean energy and the pervasive issue of environmental contamination, has spurred intense research into sustainable and efficient technologies.^1,2 Photocatalysis, harnessing the abundant and renewable energy of sunlight, presents a compelling approach to address these interconnected challenges.^3,4 Among the burgeoning class of materials explored for photocatalytic applications, Metal–Organic Frameworks (MOFs) have emerged as highly promising candidates, owing to their exceptional structural versatility, high porosity, and the ability to tailor their chemical functionalities at the molecular level.^5–7 Within the MOF family, zirconium-based MOFs (Zr-MOFs) have garnered particular attention due to their remarkable chemical robustness, thermal stability, and the inherent redox activity of zirconium nodes, which are crucial for facilitating electron transfer processes.^8–10 Characterized by robust structures, exceptional stability, and versatile application properties, Zr-MOFs constitute a significant class of materials. These frameworks are composed of zirconium-containing inorganic nodes, often Zr₆ oxo/hydroxo clusters, combined with diverse organic linkers. This composition leads to a wide array of topologies and pore sizes, exemplified by structures such as UiO-66 and NU-1000.^11,12 The inherent chemical, thermal, and hydrolytic stability of Zr-MOFs, derived from their strong Zr–O bonds, permits their deployment in demanding environments.¹³ This stability, coupled with high porosity and tunable pore functionalities, makes them ideal for applications in catalysis, including CO₂ reduction and photocatalysis, gas adsorption and separation, drug delivery, environmental remediation, and sensing.^9,13,14 The ability to precisely control their structural features through linker selection and post-synthetic modifications further enhances their adaptability for various technological applications, solidifying their position as a promising material platform.^15,16 The structural diversity of Zr-MOFs, arising from the versatile coordination chemistry of zirconium and the vast library of organic linkers, allows for the precise control of pore size, shape, and functionality. This tunability enables the optimization of light harvesting, charge separation, and reactant accessibility, all of which are critical factors in determining photocatalytic performance.⁹ Moreover, the incorporation of various co-catalysts, such as noble metals, metal oxides, and semiconductors, into Zr-MOF frameworks can further enhance their photocatalytic activity by facilitating charge transfer and providing additional active sites.^17–20

The photocatalytic reduction of CO₂ to value-added chemicals, such as fuels and feedstocks, is a pivotal strategy for mitigating greenhouse gas emissions and establishing a sustainable carbon cycle.^21–23 Zr-MOFs, with their ability to precisely engineer active sites and enhance reactant adsorption, offer a unique platform for optimizing CO₂ photoreduction efficiency.^24–26 Beyond CO₂ conversion, the sustainable production of hydrogen (H₂) through photocatalytic water splitting is a cornerstone of the transition to a clean energy economy.^27,28 Zr-MOFs, by judiciously selecting linkers and incorporating co-catalysts, can significantly enhance the efficiency of hydrogen evolution reactions, overcoming the kinetic barriers associated with water splitting.^19,29 Furthermore, the burgeoning problem of organic pollutants and other contaminants in water and air necessitates the development of effective remediation technologies.^30,31 Photocatalytic degradation, leveraging the oxidative and reductive power of photogenerated charge carriers, offers a promising solution.³² Zr-MOFs, with their ability to concentrate pollutants within their pores and facilitate charge transfer processes, demonstrate excellent potential for contaminant degradation under light irradiation.^10,33

This review aims to provide a comprehensive and critical assessment of the recent advancements in the application of Zr-MOFs for light-driven reactions, with a specific focus on CO₂ reduction, hydrogen evolution, and contaminant degradation. We will delve into the structural features of Zr-MOFs that contribute to their photocatalytic activity, discuss the various strategies employed to enhance their performance, and highlight the mechanistic insights gained from experimental and theoretical studies. By critically analyzing the current state of research, this review seeks to identify the key challenges and opportunities in the field, including the development of the design of efficient co-catalysts, and the elucidation of reaction mechanisms. Ultimately, this review aims to contribute to the development of highly efficient, stable, and sustainable photocatalytic systems based on Zr-MOFs for addressing the pressing challenges of climate change, energy scarcity, and environmental pollution.

2. Photocatalytic reduction mechanisms

The foundational concept of photocatalysis relies on the direct excitation of a single semiconductor material.³⁴ Upon irradiation with photons possessing energy exceeding the semiconductor's band gap, electrons are promoted from the valence band to the conduction band, leaving behind positively charged holes. This process generates electron–hole pairs, which act as the fundamental redox species.³⁵ The conduction band electrons, possessing a more negative potential, can reduce adsorbed molecules, while the valence band holes, with a more positive potential, can oxidize them. However, the efficiency of this traditional approach is often hampered by several factors. Firstly, the rapid recombination of electron–hole pairs within the bulk or on the surface of the semiconductor diminishes the availability of charge carriers for redox reactions. Secondly, the limited spectral response of a single semiconductor restricts its ability to utilize a broad range of solar irradiation. Finally, the redox potentials of the photogenerated charge carriers may not be sufficiently strong to drive thermodynamically challenging reactions.³⁴

To address the limitations of single-component photocatalysts, heterojunctions, specifically Type II heterojunctions, were introduced.³⁶ These systems strategically combine two semiconductors with staggered band gaps, enabling efficient charge separation (Scheme 1a). Upon illumination, electron–hole pairs are generated in both constituent semiconductors. The staggered band alignment facilitates the transfer of electrons from the conduction band of one semiconductor to the conduction band of the other, while holes migrate in the opposite direction along the valence bands. This spatial separation of charge carriers effectively reduces the probability of electron–hole recombination, thereby enhancing photocatalytic activity. However, it is crucial to recognize that the charge transfer process in a Type II heterojunction can lead to a reduction in the redox potentials of the separated charge carriers. Consequently, while charge separation is improved, the overall driving force for certain redox reactions may be compromised. Careful selection of the constituent semiconductors and optimization of the heterojunction interface are therefore essential to balance charge separation and redox potential.³⁷ Another mechanism known as Type I heterojunctions, characterized by a “straddling gap” band alignment, confine both photogenerated electrons and holes within the semiconductor possessing the smaller band gap.³⁸ This confinement leads to an increased probability of electron–hole recombination, significantly diminishing the availability of charge carriers for surface redox reactions. Consequently, Type I heterojunctions are generally unsuitable for enhancing photocatalytic activity, as the heightened recombination rate impedes efficient charge separation and utilization (Scheme 1b). Instead, their charge carrier confinement properties find applications in optoelectronic devices such as LEDs and lasers, where the localization of charge carriers is beneficial for light emission.³⁹

Scheme 1 Schematic illustration of electron–hole pairs transfer in (a) type-II, (b) type-I, (c) Z-scheme, (d) S-scheme and (e) p-n (RP: reduction photocatalyst; OP: oxidation photocatalyst). Reproduced with permission from ref. 44. Copyright 2023 Elsevier.

The Z-scheme mechanism, inspired by the natural photosynthetic process, offers a more sophisticated approach to photocatalytic charge transfer.⁴⁰ It involves the integration of two semiconductors with appropriate band positions and a redox mediator or direct interfacial contact. Upon irradiation, photogenerated electrons from the conduction band of one semiconductor recombine with the holes in the valence band of the other, effectively eliminating the need for a backward electron transfer. This recombination process leaves behind strong reducing electrons in the conduction band of one semiconductor and strong oxidizing holes in the valence band of the other (Scheme 1c). The Z-scheme mechanism effectively mimics the natural Z-scheme of photosynthesis, enabling the preservation of strong redox abilities while simultaneously enhancing charge separation. This strategic charge transfer process leads to improved photocatalytic performance, making Z-scheme heterojunctions highly promising for a wide range of applications. Direct Z-schemes, which achieve charge transfer through direct interfacial contact, further simplify the system by eliminating the need for redox mediators.⁴¹

The S-scheme mechanism, a more recent advancement in heterojunction photocatalysis, offers a refined understanding of charge transfer dynamics.⁴² This mechanism emphasizes the critical role of Fermi level alignment and the formation of built-in electric fields at the heterojunction interface (Scheme 1d). In an S-scheme system, the internal electric field acts as a driving force for charge separation. Upon illumination, photogenerated electrons and holes with weak redox abilities recombine at the interface, while those with strong redox abilities are retained. This selective recombination process ensures that the system maintains strong redox potentials while achieving effective charge separation. The S-scheme mechanism is highly dependent on the Fermi level alignment of the constituent semiconductors and can be realized in various heterojunction configurations, including n-n, p-n, and p-p types. This mechanism offers a more accurate representation of charge transfer in certain heterojunctions, particularly those with closely matched Fermi levels, leading to enhanced photocatalytic performance (Scheme 1e). The ability to finely tune the built-in electric field and Fermi level alignment provides a powerful tool for optimizing S-scheme photocatalysts for specific redox reactions.⁴³

A p-n type photocatalytic mechanism utilizes a heterojunction formed between a p-type semiconductor and an n-type semiconductor.⁴⁵ When these two materials are in contact, a built-in electric field is established at the interface due to the difference in their Fermi levels. Upon illumination, electron–hole pairs are generated in both semiconductors. The built-in electric field then facilitates the separation of these charge carriers, driving electrons towards the n-type semiconductor and holes towards the p-type semiconductor. This separation reduces the recombination rate, enhancing the availability of charge carriers for surface redox reactions. The effectiveness of this mechanism depends on the band alignment, the strength of the built-in electric field, and the ability of the separated charges to migrate to the surface for interaction with reactants. The p-n heterojunctions are widely explored for photocatalytic applications due to their potential for improved charge separation and enhanced photocatalytic activity.⁴⁶

3. Photocatalytic CO₂ reduction

Zr-MOFs have garnered significant interest as promising candidates for photocatalytic CO₂ reduction due to their inherent structural stability, tunable porosity, and exceptional versatility.^47–49 Their robust framework allows for the incorporation of photocatalytically active components, such as metal complexes or semiconductor nanoparticles, enabling efficient light absorption and subsequent redox reactions necessary for CO₂ conversion. The high surface area and customizable pore structures of Zr-MOFs facilitate enhanced CO₂ adsorption, bringing reactant molecules into close proximity with catalytic sites, which is crucial for improving reaction efficiency. Furthermore, the modular design of MOFs permits precise control over their physicochemical properties through the selection of organic linkers and metal nodes, allowing for the optimization of pore size, surface area, and active site distribution. This design flexibility offers a powerful approach to engineer advanced photocatalytic systems capable of effectively capturing and converting CO₂ into valuable chemical feedstocks, driving ongoing research towards further enhancing their performance through meticulous structural and compositional optimization.⁵⁰

In 2017, Lin and co-workers successfully synthesized a series of zirconium(IV)-porphyrin-based metal–organic frameworks (ZrPP-n-M) (n = 1,2), (M = Zn, Cu, Fe, Co), utilizing a top-down fabrication strategy, resulting in materials with extended porous structures comprised of eclipsed porphyrin arrays connected by zirconium oxide rods (Fig. 1a).⁵¹ Notably, the prototypic ZrPP-1 exhibited exceptional chemical stability, remaining intact in both highly acidic and basic solutions, a critical feature for potential applications in harsh environments. Furthermore, the cobalt-metallated analog, ZrPP-1-Co, demonstrated remarkable photocatalytic activity for the selective reduction of CO₂ to CO (≈90 cm³ g⁻¹ at 1 atm, 273 K), achieving a high CO/CH₄ yield ratio without the need for co-catalysts and maintaining stability over multiple cycles (Fig. 1b and c). Through Density functional theory (DFT) calculations, a plausible mechanism for this photoreduction was proposed. This work not only expands the structural diversity of Zr-MOFs by demonstrating the rational design of non-interpenetrated isoreticular frameworks with large pore apertures but also provides a robust platform for CO₂-to-CO photoreduction by uniformly isolating active cobalt centers within the metal–organic lattice. The high stability and catalytic performance of these materials highlight their potential for advanced photocatalytic applications.

Fig. 1 (a) Crystal packing of ZrPP-1 and ZrPP-2. (b) CO₂ adsorption behaviors of ZrPP-1-M at different temperatures. (c) Time courses of CO obtained from CO₂ photoreduction with catalysts of ZrPP-1-M under visible-light irradiation. Reproduced with permission from ref. 51. Copyright 2018 WILEY.

More recently, Liu and co-workers introduced a novel approach to designing MOF photocatalysts for CO₂ reduction, addressing the challenge of achieving efficient light-harvesting and photoexcitation.⁵² The core innovation lies in the utilization of a newly synthesized aggregation-induced emission (AIE)-active ligand, tetraphenylpyrazine (PTTBPC), which, through coordination with stable zirconium-oxo clusters, forms a Zr-MOF with a unique two-fold interpenetrated scu topology. This structural design positions the PTTBPC ligands to effectively absorb and convert light energy, while the Zr-oxo clusters facilitate CO₂ adsorption and activation, and provide potential sites for further metal modification. The resulting Zr-PTTBPC MOF exhibits exceptional stability across various solvents and pH conditions, coupled with a high CO₂ adsorption capacity of 73 cm³ g⁻¹ at 273 K and 1 atm. To further enhance photocatalytic performance, cobalt and nickel metal sites were introduced by anchoring them to the hydroxyl and water groups of the Zr-oxo clusters, creating unsaturated coordination sites. These dopants significantly improved charge carrier separation and transfer, minimizing recombination losses. DFT calculations revealed that the cobalt-modified Zr-PTTBPC demonstrated a lower energy barrier for the rate-determining step in CO₂ reduction compared to the nickel-modified variant, indicating stronger bonding and enhanced charge transfer from cobalt atoms to reaction intermediates. Consequently, Zr-PTTBPC, particularly when modified with cobalt, achieved a remarkable CO production rate of 293.2 μmol g⁻¹ h⁻¹ under 420 nm LED light, surpassing the performance of many porphyrin-based MOFs. This achievement highlights the synergistic effect of the single-phase multicomponent system, where the AIE-active ligand, Zr-oxo clusters, and metal dopants work in concert to optimize CO₂ adsorption, light absorption, charge separation, and catalytic activity.

Further advancing MOF photocatalysis, in 2023, Wei et al. addressed the challenge of enhancing MOF photocatalytic performance, which is often hindered by complex and costly modification methods (Fig. 2a).⁵³ The authors introduce a simplified strategy focused on regulating the atomic interface structure of metal active sites within the MOF. Specifically, they developed a hybrid material, cellulose acetate@PCN-222 (CA@PCN-222), by growing cellulose acetate (CA) in situ on the surface of PCN-222, a Zr-based MOF, achieving a strong electronic coupling at the interface (Fig. 2b). This hybridization resulted in a lowered average valence state of the Zr ions, indicating a modification of the electronic environment. Comprehensive synchrotron radiation-based X-ray spectroscopy and transmission electron microscopy were employed to meticulously characterize the interface effects, revealing the formation of highly correlated nanocable-like heterogeneous structures that facilitated efficient charge-transfer channels and active site accessibility. The study demonstrated a significant improvement in the photocatalytic CO₂ reduction reaction (CO₂RR) to formate, with CA@PCN-222 achieving a formate production rate of 2816.0 μmol g⁻¹, a substantial increase compared to the 778.2 μmol g⁻¹ observed for pristine PCN-222. This enhancement is attributed to the improved electron migration efficiency at the MOF-cellulose interface, highlighting the synergistic role of the strongly coupled metal–ligand moiety in heterogeneous catalysis. The findings suggest that strategically modulating the atomic interface structure of MOFs through simple hybridization techniques can lead to significant improvements in photocatalytic activity, offering a practical and efficient approach for designing advanced photocatalysts.⁵³

Fig. 2 (a) Cheap and stable in situ strategy using cellulose for enhancing the photocatalytic activity of MOF-based photocatalysts. (b) Schematic of the synthetic procedures of CA@PCN-222. Reproduced with permission from ref. 53. Copyright 2023 WILEY.

Continuing the advancements in CO₂ utilization, in a 2025 study, Liu and colleagues addressed the challenge of directly synthesizing boron dipyrromethene (BODIPY)-based Zr-MOFs, materials recognized for their strong light-harvesting capabilities and potential in artificial photosynthesis, but previously hindered by ligand synthesis difficulties. This research achieved the first direct synthesis of such Zr-MOFs, ingeniously utilizing CO₂ as a feedstock. By precisely controlling synthetic parameters, they generated two distinct Zr-MOFs: CO₂-Zr₆-DEPB, featuring a face-centered cubic (fcu) topology with a Zr₆(μ₃-O)₄(μ₃-OH)₄ node, and CO₂-Zr₁₂-DEPB, possessing a hexagonal closed packed (hcp) topology built around a Zr₁₂(μ₃-O)₈(μ₃-OH)₈(μ₂-OH)₆ node (Fig. 3). Both MOFs exhibited high crystallinity, confirmed through powder X-ray diffraction and high-resolution transmission electron microscopy, validating the successful formation of well-structured materials. These MOFs demonstrated suitable photocatalytic redox potentials for CO₂ reduction to CO, utilizing water as an electron donor, eliminating the need for co-catalysts or toxic sacrificial reagents. Under light irradiation, CO₂-Zr₁₂-DEPB and CO₂-Zr₆-DEPB yielded 16.72 and 13.91 μmol g⁻¹ h⁻¹ of CO, respectively, with nearly 100% selectivity, showcasing their efficiency and specificity in CO₂ conversion. CO₂ uptake and photoelectrochemical experiments were conducted to elucidate the mechanisms driving the differing catalytic activities between the two MOFs, revealing key insights into their structure–activity relationships. This work not only introduces a novel dual CO₂ utilization strategy, integrating CO₂ as a reactant for MOF synthesis and as a substrate for photocatalytic reduction, but also establishes BODIPY-based Zr-MOFs as a promising platform for developing efficient, light-responsive photocatalysts, potentially extending to applications such as photodynamic therapy.

Fig. 3 Synthetic routes of CO₂-Zr₆-DEPB with fcu topology and CO₂-Zr₁₂-DEPB with hcp topology. Reproduced with permission from ref. 54. Copyright 2025 WILEY.

Parallel to the direct synthesis advancements, Serre and coworkers introduced a novel, scalable room-temperature synthesis for creating core–shell composites of ultrasmall copper nanoclusters (Cu NCs) encapsulated within benchmark zirconium-based metal–organic frameworks (Zr-MOFs) like MOF-801 and UiO-66-NH₂, addressing the challenge of preventing Cu NC aggregation during encapsulation (Fig. 4).⁵⁵ This method, employing a seed-mediated growth mechanism, enabled gram-scale production and provided insights into the formation of these core–shell structures. Notably, the resulting Cu NCs@MOF composites exhibited significantly enhanced CO₂ photoreduction activity compared to Cu NCs simply confined within MOF pores, despite their similar intrinsic atomic-level properties, highlighting the crucial role of the core–shell architecture. Furthermore, the introduction of polar functional groups, specifically amino groups in UiO-66-NH₂, further boosted both catalytic reactivity and selectivity towards formic acid production, demonstrating the linker's influence on product distribution. Mechanistic studies, utilizing XANES/EXAFS and in situ IR spectroscopy, revealed that CuI sites at the interface between the Cu NCs and the MOF support served as the active sites, effectively catalyzing CO₂ photoreduction. A comparative analysis demonstrated a threefold increase in catalytic activity for the core–shell composites compared to Cu NCs on the MOF surface or within pores, emphasizing the synergistic effect of the close host-guest packing. The Cu NCs@MOF-801 composite achieved a CO₂ photoreduction rate of 94 μmol h⁻¹ g⁻¹, while Cu NCs@UiO-66-NH₂ showed a 36% higher rate and improved formic acid selectivity. This work not only provides a sustainable and efficient route for synthesizing these complex core–shell materials but also underscores the importance of interface engineering and polar group modification in designing high-performance MOF-based heterogeneous catalysts for CO₂ conversion, paving the way for low-cost, high-value chemical feedstock production.⁵⁵

Fig. 4 Illustration of the “bottle-around-ship” strategy for synthesis of UiO-66-NH₂ and MOF-801. Reproduced with permission from ref. 55. Copyright 2022 WILEY.

In another report, Xu and coworkers (2024) developed a novel Zr-MOF photocatalyst, Zr-Co MOF@TBAPy, designed for enhanced CO₂ reduction coupled with the selective oxidation of benzyl alcohol.⁴⁸ Recognizing the potential of Zr-MOFs for photocatalysis due to their stability and porosity, the researchers strategically incorporated three functional components: cobalt-tetrakis(4-carboxyphenyl)porphyrinate (Co-TCPP) as a visible light sensitizer, 1,3,6,8-tetra(4-carboxyphenyl)pyrene (TBAPy) as an electron transport ligand, and Zr clusters as active sites. Synthesized via a solvothermal method, this composite MOF leverages the photo-responsiveness of Co-TCPP, the catalytic activity of Zr clusters, and the electron transfer capability of pyrene. The result was a catalyst exhibiting superior photocatalytic performance, achieving a CO production rate of 127.42 μmol g⁻¹ h⁻¹, significantly exceeding that of Zr-TBAPy and Zr-Co MOF alone. This enhanced performance was attributed to the synergistic effect of the components, where Co-TCPP effectively absorbs visible light, the pyrene moiety facilitates efficient electron transfer, and the Zr clusters provide stable active sites for CO₂ reduction. Furthermore, the catalyst demonstrated effective coupling of CO₂ reduction with benzyl alcohol oxidation to benzaldehyde, showcasing its ability to simultaneously utilize photogenerated electrons and holes for valuable product synthesis. Density functional theory (DFT) calculations corroborated the proposed electron transfer pathway, revealing how the pyrene ligand mediates electron transfer from the photosensitive unit to the active site. The study highlights the importance of integrating functional moieties into MOFs to optimize photocatalytic performance, demonstrating that strategic design can lead to catalysts with improved charge carrier mobility, enhanced charge separation, and efficient electron transfer. This work not only establishes Zr-Co MOF@TBAPy as a high-performance photocatalyst for CO₂ reduction and coupled oxidation reactions but also provides a valuable framework for the development of advanced MOF-based photocatalysts.⁴⁸

Further refining Zr-MOF photocatalysts, Wang et al. (2022) explored the impact of defect engineering in Zr-MOFs, specifically UiO-66-NH₂, on photocatalytic CO₂ reduction, addressing the nascent understanding of the interplay between structural defects and photocatalytic properties.⁴⁹ They synthesized a series of UiO-66-NH₂ variants, each exhibiting distinct defect types: ligand-vacant (LV), missing-cluster (MC), monocarboxylate compensated, and non-defective, to systematically investigate their influence on CO₂ photoreduction via a solvent-free reaction route. Experimental results revealed that the ligand-vacant UiO-66-NH₂ (UiO-66-NH₂-LV) demonstrated superior photocatalytic activity, achieving a CO yield of 30.5 μmol g⁻¹ h⁻¹, which was significantly higher than the missing-cluster variant (UiO-66-NH₂-MC), and exhibited the highest quantum yield (QY) of 0.90%. DFT calculations corroborated these findings, elucidating that the variations in photocatalytic activity were attributed to the tunable electronic properties of the defect structures, specifically the absorption energy (E_abs) and charge transfer energy (E_LMCT). The ligand-vacant defect, characterized by the lowest sum of E_abs and E_LMCT, effectively lowered the photocatalytic reaction energy barrier in the rate-limiting step of CO₂ photoreduction. Furthermore, DFT modeling of the CO₂-to-CO reaction pathways confirmed that ligand vacancies reduced this energy barrier, thus enhancing photocatalytic performance.⁴⁹

Complementing this, Xia et al. (2023) focused on optimizing Zr-MOF photocatalysts for efficient CO₂ reduction by strategically introducing electron-rich ferrocene (Fc) ligands into UiO-66 derivatives.⁵⁶ Recognizing the inherent trade-off between enhancing ligand-to-metal charge transfer (LMCT), which facilitates light-driven electron excitation, and maintaining sufficient Lewis acidity, crucial for CO₂ adsorption and activation, the researchers employed a simple solvent-assisted ligand incorporation (SALI) method to synthesize a series of Fc-modified NH₂-UiO-66-Fc MOFs (Fig. 5). Through detailed theoretical calculations and experimental analyses, including photoelectrochemical measurements, electron paramagnetic resonance, fluorescence probe analysis, and X-ray photoelectron spectroscopy, they demonstrated that increasing Fc content progressively lowers the LMCT energy, enhancing electron transfer, while simultaneously reducing the Lewis acidity of zirconium (Zr) sites, thereby weakening CO₂ adsorption. Notably, an optimal Fc content in NH₂-UiO-66-Fc(2.0) achieved a critical balance, maximizing LMCT enhancement while preserving adequate Lewis acidity, resulting in a significantly improved CO production rate of 90.65 μmol g⁻¹ h⁻¹, a 13-fold increase compared to the unmodified MOF. DFT calculations revealed that this enhanced performance is attributed to a lowered energy barrier for the rate-limiting *COOH formation step. Importantly, the study highlights that excessive Fc incorporation disrupts CO₂ adsorption and shifts the rate-limiting step to *CO desorption, diminishing catalytic efficiency. In essence, this work provides a valuable strategy for fine-tuning MOF photocatalysts by carefully balancing LMCT and Lewis acidity through controlled ligand modification, offering a new avenue for designing high-performance CO₂ reduction systems.⁵⁶

Fig. 5 Schematic illustration of the synthesis of the high-performance photocatalysts based on a trade-off between enhancing LMCT and frustrating Lewis acid. Reproduced with permission from ref. 56. Copyright 2023 Elsevier.

Further advancing CO₂ photoreduction, Qin and co-workers in 2024 explored the enhancement of CO₂ photoreduction by meticulously modulating the local microenvironment within Zr-MOFs.⁵⁷ Recognizing the potential of solar-driven photocatalysis for sustainable fuel production, they addressed the challenge of creating efficient and stable photocatalysts by synthesizing two iron-porphyrin-based Zr-MOFs: MOF-526-H and MOF-526-NH₂ (Fig. 6). These materials were designed to integrate light-harvesting sites and catalytic centers within a single phase, utilizing iron-containing porphyrin ligands (FeTCBPP and FeTCBPP-NH₂, respectively). The key distinction between these MOFs lies in the presence of amino groups in MOF-526-NH₂, which significantly impacted its photocatalytic performance. Under visible light irradiation and without the need for a photosensitizer, MOF-526-NH₂ achieved a CO yield of 21.2 mmol g⁻¹, a fourfold increase compared to MOF-526-H. This enhanced performance was attributed to the amino groups' ability to tailor the local microenvironment and electronic structure, facilitating CO₂ enrichment and improved light absorption. Furthermore, MOF-526-NH₂ demonstrated excellent stability over four cycles, maintaining approximately 95% selectivity for CO production. DFT calculations provided insights into the photocatalytic mechanism, supporting the role of the amino groups in optimizing the reaction pathway. This study not only offers a comparative analysis of MOF structures to elucidate the impact of amino functionalization on CO₂ reduction but also establishes a framework for designing molecular-level photocatalysts. The successful implementation of amino group modification highlights its potential as a promising strategy for developing advanced MOF-based photocatalysts for various applications.⁵⁷

Fig. 6 The preparation route of MOF-526-H and MOF-526-NH₂. Reproduced with permission from ref. 57. Copyright 2024 Elsevier.

Further exploring modifications for CO₂ reduction, Ce-doped NH₂-UiO-66(Zr) photocatalysts have also emerged as strong candidates for CO₂ reduction in aqueous systems. In 2022, Yu et al. optimized bimetallic NH₂-UiO-66(Zr/Ce) catalysts for photocatalytic CO₂ reduction under simulated sunlight (Fig. 7).⁵⁸ Incorporating cerium (Ce) into the framework significantly enhanced performance, with the NH₂-UiO-66(Zr/Ce 1 : 1) catalyst showing superior activity due to an optimized electronic structure, increased CO₂ adsorption, and improved charge separation. It achieved a CO production rate of 30.04 μmol g⁻¹ h⁻¹, a 1.66-fold increase over the Zr-only catalyst, and 97.2% CO selectivity. The improved performance was linked to expanded porous volumes and the presence of Ce⁴⁺ ions. This work underscores the importance of bimetallic doping in enhancing photocatalytic efficiency without needing additional electron donors or co-catalysts.⁵⁸

Fig. 7 Schematic illustration of design of Ce/Zr bimetallic NH₂-UiO-66. Reproduced with permission from ref. 58. Copyright 2024 Elsevier.

Moreover, a recent study in 2024 addressed the limitations of MOFs' large band gaps and insufficient ligand-to-metal charge transfer (LMCT) in CO₂ photoreduction.⁵⁹ Researchers implemented a dual-modification strategy using a simple one-pot solvothermal synthesis to create an amino-functionalized MOF, aU(Zr/In). This novel MOF incorporates an amino-functionalizing ligand linker and indium-doped zirconium-oxo clusters, strategically designed to enable efficient CO₂ reduction under visible light irradiation. Unlike previous approaches that typically focused on single modifications like metal doping or ligand exchange, this research employs a dual-modification strategy, simultaneously engineering both the Zr-oxo clusters and the organic ligand. The introduction of amino groups significantly reduces the MOF's band gap, facilitating visible light absorption, and induces charge redistribution, enhancing the separation of photogenerated charge carriers. Furthermore, the non-equivalent substitution of Zr⁴⁺ ions with In³⁺ ions creates oxygen vacancies near the indium sites, which not only promotes the LMCT process but also lowers the energy barrier for CO₂-to-CO conversion by stabilizing *HCOOH intermediates. This synergistic effect between amino functionalization and indium doping results in a significantly enhanced CO production rate of 37.58 ± 1.06 μmol g⁻¹ h⁻¹ with 100% selectivity, surpassing the performance of isostructural UiO-66- and MIL-125-based photocatalysts. The reduction of the band gap caused by the amino groups is achieved by the raising of the valence band maximum (V_BM). The In-doping also increases the amount of oxygen vacancies (VOs).⁵⁹

In a parallel approach, Yaseen and co-workers in 2024 study explored a strategic approach to enhance the photocatalytic CO₂ reduction performance of Zr-MOFs by meticulously engineering structural defects and forming heterojunctions.⁶⁰ Recognizing the importance of controlled atomic-level manipulation for industrial scalability, the research focuses on developing a robust methodology for designing efficient MOF-based photocatalysts. Initially, a series of defective NUiO-66 (D-NUiO-66) materials were synthesized via an acidic modulated hydrothermal route, effectively introducing structural defects. Subsequently, these D-NUiO66 materials were combined with cerium dioxide (CeO₂) to create D-CeNUiO-66 heterojunctions, aiming to improve CO₂ reduction capabilities. Morphological analysis revealed that the resulting D-CeNUiO66 composite exhibited a mesoporous structure with favorable adsorption properties, crucial for efficient CO₂ uptake. The optimized D-CeNUiO66 photocatalyst demonstrated a significant CO₂ reduction activity, achieving a CO production rate of 38.6 μmol g⁻¹ h⁻¹, while also showcasing excellent repeatability. The introduction of defects in the NUiO-66 structure led to an enhanced light response in the visible region, resulting in a reduced band gap of 2.9 eV. This research highlights the synergistic effect of defect engineering and heterojunction formation in optimizing the photocatalytic performance of Zr-MOFs for CO₂ reduction, offering a promising strategy for developing advanced photocatalytic materials. Notably, the direct coupling of CeO₂ with defective MOFs, as demonstrated in this study, represents a relatively unexplored avenue, providing a novel perspective for the design of high-performance MOF-based photocatalysts.⁶⁰

Expanding beyond defect engineering, other researchers have explored bimetallic compositions and heterojunctions to boost CO₂ reduction efficiency. In 2023, Ezugwu et al. synthesized a series of robust, octahedral bimetallic NH₂-UiO-66(Zr/M) MOFs (Zr/M-ATA, where M = Fe, Co, or Cu) via a solvothermal de novo reaction, bypassing the time-consuming post-synthetic metal exchange method.⁶¹ These bimetallic MOFs, characterized by spectroscopic and microscopic analyses, retained the crystal structure, morphology, and stability of the monometallic Zr-ATA framework, demonstrating successful incorporation of the secondary metal ions. Photocatalytic evaluations revealed that all bimetallic MOFs exhibited superior performance compared to Zr-ATA in visible-light-driven CO₂ reduction and H₂ generation. Notably, Zr/Cu-ATA demonstrated exceptional efficiency, achieving a formate formation rate of 122 μmol h⁻¹, placing it among the top-performing NH₂-UiO-66 based MOFs, and generating 12.8 mmol of H₂ in 2 hours. This enhanced performance was attributed to a significantly narrowed band gap (1.93 eV compared to 2.95 eV for Zr-ATA) due to the presence of Cu-oxo clusters, a red-shifted light absorption band extending into the near-IR region, and improved charge transfer efficiency, as confirmed by photoelectrochemical studies. DFT calculations and spectroscopic studies also indicated that this material had a large amount of active sites. This study highlights the effectiveness of rationally designed bimetallic MOFs as a promising strategy for enhancing artificial photosynthesis, particularly in visible-light-driven CO₂ reduction and H₂ generation.⁶¹ In a related study, Li and co-workers in 2023 developed a novel photocatalytic system for CO₂ reduction by creating an S-scheme heterojunction between mixed-valence bimetallic Ce/Zr-NH₂-UiO-66 (Ce-NU66) and CdIn₂S₄ (CIS) (Fig. 8).⁶² This heterojunction, denoted as Ce-NU66/CIS, was synthesized via a one-pot hydrothermal co-synthesis, ensuring close interfacial contact between the MOF and semiconductor. A key feature of this system is its ability to perform CO₂ reduction under visible light without the need for sacrificial agents. The introduction of cerium (Ce) ions into the Zr-based MOF structure induced oxygen vacancies and increased electron density around Zr⁴⁺ ions, which significantly boosted photocatalytic activity. The resulting Ce-NU66/CIS heterojunction exhibited enhanced charge separation, a high surface area, and a well-dispersed heterojunction structure compared to the individual components. Notably, the optimized 8 wt% Ce0.2NU66/CIS composite demonstrated a CO production rate of 6.01 μmol g⁻¹ h⁻¹ and a CO selectivity of 54.98%, representing significant improvements over pristine CIS and NU66. The researchers proposed a charge transfer pathway indicating that the heterojunction interface facilitated efficient charge separation and transfer, leading to improved photocatalytic performance. This study highlights the potential of mixed-valence bimetallic MOF-semiconductor heterojunctions for solar-driven CO₂ conversion, offering a rational design strategy for advanced photocatalytic systems.⁶²

Fig. 8 Synthetic route of Ce-NU66/CIS composites. Reproduced with permission from ref. 62. Copyright 2024 Elsevier.

Similarly focusing on the impact of structural defects, Xing et al. in 2024 study delves into the impact of structural defects on the photocatalytic CO₂ reduction capabilities of Zr-MOFs, specifically focusing on enhancing visible light-mediated conversion.⁶³ The researchers synthesized a series of defective Zr-MOF-X materials, varying defect densities through acid-regulated defect engineering using acetic acid as a modulator, and compared their performance to pristine, defect-free NNU-28. The introduction of defects resulted in increased nitrogen uptake, larger pore spaces, and higher Brunauer–Emmett–Teller surface areas, creating mesoporous structures within the MOF. Electrochemical tests revealed that these defective Zr-MOF-X materials exhibited more negative reduction potentials and enhanced photocurrent responses, indicating improved charge transfer properties. Significantly, the defective samples demonstrated a substantially higher efficiency in the photoreduction of CO₂ to formate. Transient absorption spectroscopy revealed that the structural defects altered the excited-state behavior of Zr-MOF-X, promoting efficient charge separation and prolonging the lifetime of excited states compared to the pristine MOF. Electron paramagnetic resonance and in situ X-ray photoelectron spectroscopy further validated the enhanced photocatalytic performance, confirming the presence of abundant Zr(III) catalytically active sites with extended lifetimes. The study highlighted that the defects facilitated improved charge transfer, leading to the formation of long-lived Zr(III) species that were fully exposed to CO₂, thus enhancing the photocatalytic efficiency. The authors concluded that the controlled introduction of structural defects in Zr-MOFs, through defect engineering, significantly improves their physicochemical and photoelectrochemical properties, leading to superior CO₂ photoreduction performance. The prolonged lifetime of the active Zr(III) species, attributed to efficient charge separation and transfer, was identified as a key factor in this enhancement.⁶³

Furthermore, Navalón and co-workers, in their 2024 study, addressed the critical area of solar-driven CO₂ conversion by developing multifunctional UiO-66-based photocatalysts for gas-phase CO₂ hydrogenation.⁶⁴ They synthesized a series of RuOx nanoparticle-supported UiO-66 frameworks, incorporating either nitro or amino functional groups on the ligands and utilizing zirconium or mixed zirconium/titanium metal nodes, to create materials with varying energy band level diagrams. The focus was on achieving efficient CO₂ methanation under simulated concentrated sunlight irradiation. Notably, RuOx nanoparticles (1 wt%, 2.2 ± 0.9 nm) supported on nitro-functionalized UiO-66(Zr/Ti) demonstrated exceptional performance, achieving a CO₂ methanation yield of 5.03 mmol g⁻¹ after 22 hours, with corresponding apparent quantum yields (AQY) of 1.67%, 0.25%, and 0.01% at 350, 400, and 600 nm, respectively, surpassing previous reports by 3–6 times. The photocatalytic activity followed a clear trend: UiO-66(Zr/Ti)–NO₂ > UiO-66(Zr/Ti)–NH₂ ∼ UiO-66(Zr)–NO₂ > UiO-66(Zr)–NH₂, highlighting the superiority of nitro-functionalized ligands over amino-functionalized ones for this application. This study diverged from typical UiO-66 photocatalysts based on 2-aminoterephthalate ligands, emphasizing the critical role of 2-nitroterephthalate ligands in achieving high activity with Zr(IV) or Zr(IV)/Ti(IV) nodes, attributed to the unique energy band level diagram of these solids. Comprehensive characterization using advanced spectroscopic techniques, including femto- and nanosecond transient absorption, spin electron resonance, photoluminescence, and operando FTIR, alongside electrochemical measurements, revealed that the most active photocatalyst operates through a dual photochemical and photothermal mechanism. This research underscores the potential of strategically designed multifunctional MOFs, particularly RuOx@UiO-66(Zr/Ti)–NO₂, as highly efficient and reusable photocatalysts for solar-driven CO₂ recycling, offering a significant advancement in the development of sustainable fuel production technologies.⁶⁴ Further contributing to CO₂ conversion, NNU-28, a novel microporous and robust Zr-MOF functionalized with an anthracene-based organic ligand, was designed for highly efficient visible-light-driven CO₂ reduction to formate.⁶⁵ NNU-28 exhibits desirable properties including exceptional stability, high CO₂ uptake, broad visible light absorption, and efficient photoinduced charge generation. Notably, it demonstrates a remarkable formate formation rate of 183.3 mmol h⁻¹, significantly surpassing the performance of previously reported Zr-MOFs in CO₂ photoreduction. This high efficiency is attributed to the unique dual catalytic routes within NNU-28, where both the anthracene-based organic ligand and the Zr₆ oxo cluster act as photocatalytic centers, contributing synergistically to the CO₂ reduction process. Through photocatalytic experiments and electron paramagnetic resonance (EPR) studies, the authors confirmed that this dual catalytic mechanism is more effective than the conventional ligand-to-metal charge transfer process, providing a new strategy for designing and synthesizing highly efficient visible light responsive MOF photocatalysts.⁶⁵ Complementing these advancements, other research has focused on optimizing Zr-MOFs through structural modifications. In 2019, Fan and co-workers demonstrated a temperature-controlled defect engineering strategy to enhance the photocatalytic CO₂ reduction performance of NH₂-UiO-66(Zr) MOFs.⁶⁶ By precisely adjusting the synthesis temperature and utilizing concentrated hydrochloric acid (HCl) as a modulator, they successfully synthesized a series of NH₂-UiO-66(Zr) materials with varying degrees of structural defects. These defective MOFs exhibited significantly improved photocatalytic activity for CO₂ reduction compared to their defect-free counterparts. The introduction of defects, specifically linker deficiencies, resulted in the creation of active binding sites and a more open framework structure, which facilitated efficient photo-induced charge transfer and suppressed the recombination of photogenerated charges. This work highlights the critical role of defect engineering in optimizing MOF photocatalysts, revealing that controlled introduction of defects can substantially enhance catalytic performance. The authors successfully tailored linker deficiencies by manipulating synthesis temperature, leading to a direct correlation between defect concentration and photocatalytic activity.⁶⁶ Similarly, in 2018, Xing and coworkers reported the synthesis and characterization of Zr-SDCA-NH₂, a porous and visible-light-responsive zirconium MOF (Fig. 9a).⁶⁷ This was achieved by utilizing an amine-functionalized dicarboxylic ligand (H₂SDCA-NH₂). Zr-SDCA-NH₂ exhibited commendable chemical stability and a broad visible light absorption range, extending to approximately 600 nm. Notably, it demonstrated enhanced photocatalytic activity for CO₂ reduction to formate, achieving a formation rate of 96.2 μmol h⁻¹ mmol_MOF⁻¹, surpassing other reported amine-functionalized Zr-MOFs (Fig. 9b). Through Mott–Schottky measurements, photoluminescence studies, and photocatalytic experiments, the authors established that both the Zr₆ oxo cluster, via a ligand-to-metal charge transfer (LMCT) process, and the H₂SDCA-NH₂ ligand contributed synergistically to the CO₂ photoreduction (Fig. 9c). This dual catalytic pathway, involving both the inorganic and organic components, was identified as the key to the enhanced performance. The researchers emphasized that incorporating amino groups into highly conjugated molecules effectively extends the light absorption range of the organic ligand, offering a straightforward strategy for designing visible-light-responsive MOF photocatalysts.⁶⁷

Fig. 9 (a) View of the 3D network of Zr-SDCA-NH₂. (b) The amount of HCOO⁻ produced as a function of irradiation time under different conditions. (c) Schematic illustration of photocatalytic CO₂ reduction to formate over Zr-SDCA-NH₂ under visible light irradiation. Reproduced with permission from ref. 67. Copyright 2018 RSC.

A recent investigation in 2025 by Chen and co-workers describes the creation of three hydrophobic porphyrin Zr-MOFs with distinct structural arrangements: OPA@PCN-222, OPA@PCN-223, and OPA@PCN-224.⁶⁸ These materials were synthesized using zirconium tetrachloride and tetra(4-carboxyphenyl)porphyrin, with the in situ introduction of octadecylphosphonic acid (OPA). Compared to their unmodified counterparts, these OPA-modified Zr-MOFs demonstrated superior hydrophobic properties and maintained excellent stability in humid conditions. Leveraging the inherent characteristics of the porphyrin ligands, the modified MOFs also exhibited broad visible light absorption and a robust, rapid photocurrent response. Under visible light irradiation, these hydrophobic Zr-MOFs efficiently converted CO₂ to formate (HCOO⁻), achieving average reaction rates of 330, 260, and 258 μmol h⁻¹ g⁻¹ for OPA@PCN-222, OPA@PCN-223, and OPA@PCN-224, respectively. These rates represent a 1.13 to 1.41-fold increase over the original porphyrin Zr-MOFs. Mechanistic studies indicated that both the porphyrin ligands and the Zr–O clusters functioned as catalytically active sites for this CO₂ to formate conversion. However, a subsequent study by Saouma et al. (2020) offered a crucial challenge to the established understanding of photocatalytic CO₂ reduction to formate using amino-substituted UiO-66 Zr-MOFs.⁶⁹ Their work revealed that the observed catalytic activity might not be intrinsic to the MOF structure itself but rather arises from leached zirconium species. Through comprehensive experiments, including recycling studies and SEM imaging, they demonstrated a direct correlation between the amount of leached zirconium and the formate production rate. They found no relationship between crystal size and catalytic activity. Furthermore, all tested UiO-66 frameworks that exhibited CO₂ reduction also showed zirconium leaching, suggesting a universal instability under typical photochemical conditions. The study noted a correlation between the MOF's band gap energy and formate production, indicating its impact on the leached zirconium species' activity, likely via linker coordination. This research critically highlighted the role of leached metals in catalysis, emphasizing the need for meticulous analysis of leached components when employing UiO-66 Zr-MOFs and their derivatives in photochemical reactions using basic sacrificial reductants, as traditional interpretations focusing solely on the MOF structure may overlook the actual catalytic mechanisms.⁶⁹

Despite these mechanistic complexities, research continues to push the boundaries of Zr-MOF performance in CO₂ reduction. In 2022, Xing and co-workers investigated the synthesis and application of PCN-223, a zirconium-porphyrin MOF with shp-a topology, as an efficient visible-light-driven photocatalyst for CO₂ reduction to formate.⁷⁰ Utilizing a solvothermal reaction, the authors constructed PCN-223 from a highly conjugated 4-connected tetrakis(4-carboxyphenyl)porphyrin (TCPP) ligand and 12-connected zirconium-based metal units, resulting in an open-framework structure with a large specific surface area and excellent chemical stability (Fig. 10a). The material exhibited broad light absorption due to the TCPP ligand and a rapid photocurrent response, indicating efficient charge separation. Notably, PCN-223 achieved a high average formate formation rate of 65.2 mmol h⁻¹ g_MOF⁻¹ under visible light irradiation, surpassing the performance of many reported Zr/Ti-MOFs under similar conditions (Fig. 10b). Through a combination of photocatalytic experiments, Mott–Schottky measurements, EPR, and photoluminescence studies, the authors demonstrated that PCN-223 facilitates CO₂ reduction through dual catalytic routes, where both the TCPP ligand and the Zr–O clusters act as active catalytic centers (Fig. 10c). This synergistic effect, where the porphyrin ligand and zirconium clusters both contribute to the catalytic process, is proposed as the key to PCN-223 enhanced performance.⁷⁰

Fig. 10 (a) Synthetic route of PCN-223. (b) Amounts of HCOO⁻ produced as a function of visible-light irradiation time. (c) Photocatalytic pathway of CO₂ reduction over PCN-223. Reproduced with permission from ref. 70. Copyright 2022 RSC.

In a complementary study in 2022, Draznieks and co-workers provided a comprehensive mechanistic understanding of the photocatalytic reduction of CO₂ to formate using porphyrinic Zr-based MOF-545 (PCN-222) in acetonitrile/triethanolamine solutions.⁷¹ Employing a combined experimental and computational approach, they first optimized the MOF catalyst by metalating the porphyrin linkers and synthesizing nanosized materials (150–200 nm) under microwave conditions, achieving a remarkable formate production rate of 6000 mmol g⁻¹ after 4 hours for Nano MOF-545(Fe), with 100% selectivity. Contrary to the prevailing belief of ligand-to-Zr₆ cluster charge transfer, photophysical measurements and DFT calculations revealed a novel mechanism: visible light irradiation activates the porphyrinic linkers to photo-oxidize TEOA, generating TEOAc radicals. These radicals then act as hydride donors, transferring a formal hydride to CO₂ molecules activated by the Zr^IV centers, which function as Lewis acids. Radical quenching between reduced linkers and dehydrogenated donors terminates the reaction. This mechanism, validated through additional photocatalytic experiments and explaining the observed activity trends with different metalations, highlights the crucial role of porphyrinic linkers in TEOA photo-oxidation. Furthermore, the nanosized MOF-545(Fe) exhibited significantly enhanced activity compared to micron-sized crystals due to increased solvent-accessible catalytic sites.⁷¹ Following this mechanistic elucidation, Su et al. reported the successful synthesis and application of MOF-OH, a phenolic hydroxy-modified iron-porphyrinic Zr-MOF, for highly efficient photocatalytic CO₂ reduction.⁷² They employed a post-synthetic modification strategy, utilizing demethylation to convert MOF-OCH₃, a precursor MOF, into MOF-OH, overcoming the synthetic challenges posed by the direct assembly of FeTCBPP-OH ligands with Zr⁴⁺ centers. Notably, MOF-OH exhibited a remarkable CO production yield of 26.8 mmol g⁻¹ after 72 hours under visible light irradiation, without the need for an additional photosensitizer, significantly outperforming its precursor MOF-OCH₃. This enhanced performance was attributed to the presence of phenolic hydroxyl groups, which increased local proton concentrations, facilitated ultrafast electron migration, and improved charge separation. Comprehensive characterization and DFT calculations elucidated the reaction mechanism, revealing that MOF-OH's structural features allowed for easier CO release by overcoming a lower potential barrier in the *CO intermediate compared to MOF-OCH₃. The stability of MOF-OH was demonstrated through four consecutive CO₂ reduction cycles, underscoring its potential for practical applications. This work not only provides a molecular-level design strategy for MOF photocatalysts but also establishes a model system for investigating structure-performance relationships, highlighting the importance of multifunctional coordination spheres and pore microenvironments in optimizing CO₂ reduction efficiency.⁷² Further contributing to the field, Goyal et al. in 2021 demonstrated the synthesis and evaluation of novel copper-zirconia imidazolate (CuZrIm) framework photocatalysts for the visible-light-driven reduction of CO₂ to methanol in a continuous-flow stirred photoreactor at ambient conditions.⁷² They synthesized a series of CuZrIm catalysts via a hydrothermal method with varying zirconia molar ratios. Photoluminescence (PL) spectroscopy confirmed the catalysts' visible light absorption capabilities. Notably, the CuZrIm1 catalyst, synthesized with a Cu/Zr/Im/NH₄OH molar ratio of 2 : 1 : 4 : 2, showed the highest methanol activity, reaching 818.59 μmol g⁻¹ L⁻¹, attributed to the presence of Cu²⁺ oxidation states and uniformly dispersed metal ions. Response surface methodology (RSM) optimized reaction parameters, achieving an enhanced methanol yield of 1054 μmol g⁻¹ L⁻¹ under specific light intensity, stirring rate, and catalyst loading. The favorable redox potential of CuZrIm1 further supported its efficacy. This study underscores the potential of imidazolate frameworks as effective catalyst supports for CO₂ photoreduction to methanol, offering a promising pathway for converting greenhouse gases into valuable liquid fuels under mild conditions. Way for converting greenhouse gases into valuable liquid fuels under mild reaction conditions. The catalytic performance of some reported Zr-MOFs for CO₂ photoreduction is further detailed in Table 1.

Table 1 Catalytic performance of some Zr-MOFs for CO₂ photoreduction

Entry	Photocatalyst	Major product	Light source	Product yield	Ref.
1	ZrPP-1-Co	CO/CH4	Visible light (λ > 400 nm)	90 cm^3 g^−1	51
2	CO2-Zr12-DEPB	CO	Simulated solar irradiation	16.72 g^− 1 h^−1	54
3	CO2-Zr6-DEPB	CO	Simulated solar irradiation	13.91 μmol g^−1 h^−1	54
4	Cu NCs@MOF-801	CO	UV irradiation (385 nm)	94 μmol g^−1 h^−1	55
5	CA@PCN-222	CO	UV irradiation	2816.0 μmol g^−1	53
6	Zr-PTTBPC	CO	420 nm LED light	293.2 μmol g^−1 h^−1	52
7	Zr-Co MOF@TBAPy	CO	Visible light (λ > 400 nm)	127.42 μmol g^−1 h^−1	48
8	Zr-MOF-160	Format	UV irradiation	733 μmol h^−1	63
9	Ce/Zr-UiO-66-NH2/CdIn2S4	CO	Simulated solar irradiation	6.01 mmol g^−1	62
10	Zr/Cu-ATA	Format	Xe lamp	122 μmol h^−1	61
11	aU(Zr/In)-3	CO	Visible-light (λ > 420 nm)	37.58 μmol g^−1 h^−1	59
12	NH2-UiO-66(Zr/Ce1 : 1)	CO	300 W Xe lamp	30.04 μmol g^−1 h^−1	58
13	MOF-526-NH2	CO	Visible light(λ > 400 nm)	21.2 mmol g^−1	57
14	NH2-UiO-66-Fc(x)	CO	Xe lamp with 400 nm	90.65 μmol g^−1 h^−1	56
15	UiO-66-NH2	CO	300 W Xe arc lamp	30.5 μmol g^−1 h^−1	49
16	CuZrIm1	CH4	500 W Xe lamp	1054 μmol g^−1 h^−1	73
17	MOF-OH	CO	420 nm LED light	26.8 mmol g^−1	72
18	PCN-222	Format	280 W Xe lamp	6000 mmol g^−1	71
19	PCN-223	Format	300 W Xe lamp	65.2 mmol h^−1 gMOF^−1	70
20	ZrSDCA-NH2	Format	Visible light	96.2 μmol h^−1 mmolMOF^−1	67
21	UiO-66(Zr/Ti)–NO2	CH4	Simulated sunlight irradiation	5.03 mmol g^−1	64
22	Bi2S3/UiO-66	CO/CH4	UV-visible light	20.54, 0.31 μmol g^−1 h^−1	74
23	Defective Zr-MOF	Format	MOF 300 W Xe lamp	129.8 μmol g^−1 h^−1	66
24	Cu-UiO-66	CH3OH/C2H6O	UV-vis light	5.33, 4.22 μmol g^−1 h^−1	75
25	D-CeNUiO66	CO	300 W Xe lamp	38.6 μmol g^−1 h^−1	60
26	ZnO/Gr-UiO-66-NH2	CH3OH/HCOOH	300 W Xe lamp	34.83, 6.41 μmol g^−1 h^−1	76
27	Gr-UiO-66-NH2	Format	300 W Xe lamp	35.5 μmol g^−1 h^−1	77

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4. Photocatalytic hydrogen evolution

Photocatalytic hydrogen evolution using Zr-MOFs is a promising area of research for sustainable energy production.^20,78 The concept revolves around harnessing light energy to drive the splitting of water molecules into hydrogen and oxygen, with MOFs acting as the photocatalyst. Zr-MOFs, with their unique structural properties like high surface area, tunable porosity, and customizable composition, offer several advantages in this process.⁹ When a MOF-based photocatalyst is exposed to light, the photoactive components within the MOF (either the organic ligands or the metal nodes, or intentionally incorporated sensitizers) absorb photons and generate electron–hole pairs. These photogenerated charge carriers then migrate to the surface of the MOF, where they participate in the redox reactions necessary for water splitting. Specifically, the photogenerated electrons reduce protons (H⁺) to hydrogen gas (H₂), while the photogenerated holes oxidize water molecules (H₂O) to oxygen gas (O₂). The efficiency of this process is highly dependent on the MOF's design, including its ability to absorb light effectively, facilitate efficient charge separation and transfer to minimize electron–hole recombination, and provide active sites for the catalytic reactions. Researchers are exploring various strategies to enhance the photocatalytic activity of MOFs for hydrogen evolution, such as incorporating co-catalysts (like noble metals or metal oxides), modifying the organic ligands to improve light absorption and charge transport, and engineering the MOF structure to optimize porosity and accessibility of active sites.⁷⁹

One of the first research of Zr-based MOF for hydrogen evolution reported by García in 2010.⁸⁰ This study investigates the photocatalytic hydrogen generation capabilities of two water-resistant Zr-MOFs: UiO-66 and UiO-66(NH₂), which utilize terephthalate and 2-aminoterephthalate ligands, respectively. Both MOFs demonstrated photocatalytic activity for hydrogen production when irradiated with light longer than 300 nm in methanol or a water/methanol mixture, with UiO-66(NH₂) achieving an apparent quantum yield of 3.5% at 370 nm in a 3 : 1 water/methanol solution. Using laser-flash photolysis, the researchers detected a long-lived charge-separated state in both MOFs, persisting for up to 300 ms after excitation, indicating efficient initial charge separation upon light absorption. Notably, the introduction of the amino group in the UiO-66(NH₂) ligand resulted in a bathochromic shift in the optical spectrum, extending light absorption into the visible region, without altering the fundamental photochemistry of the material. This finding suggested a strategy for enhancing the light-harvesting properties of MOFs for water splitting. The authors conclude that the inherent water stability and semiconducting characteristics of these Zr-MOFs make them promising photocatalysts for hydrogen generation, and while the current quantum yields are modest, the vast structural diversity of MOFs offers significant potential for future development of more efficient water splitting catalysts.⁸⁰ Recent research highlights Zr-MOFs as promising materials for next-generation energy storage. For example, in 2025, Ghanem and co-workers developed a novel supercapattery electrode material by synthesizing a hybrid composite of chromium oxide and a Zr-MOF embedded with graphene quantum dots (GQDs@Zr-MOF/Cr₂O₃).⁸¹ This innovative design leverages the benefits of MOFs, such as their high surface area and tunable pore volume, to create an energy storage device that combines the high energy density of batteries with the superior power density of supercapacitors. When configured as an asymmetric supercapacitor (GQDs@Zr-MOF/Cr₂O₃/A.C), the device demonstrated excellent performance, achieving a maximum specific capacity of 247 C g⁻¹ at 1.0 A g⁻¹. It also delivered a notable power density of 850 W kg⁻¹ and a maximum energy density of 61.44 Wh kg⁻¹. Furthermore, the device exhibited remarkable stability, retaining 87% of its cycle stability after 10 000 cycles. These impressive characteristics position the GQDs@Zr-MOF/Cr₂O₃ hybrid material as a suitable and promising candidate for industrial supercapattery applications.

Further studies highlight the versatile application potential of Zr-MOFs, spanning both photocatalysis and electrocatalysis. In photocatalysis, a 2019 study by Su and co-workers explored the use of ZrT-1-NH₂, a water-stable, amine-functionalized zirconium metal–organic polyhedron (Zr-MOP), for visible-light-driven hydrogen production.⁸² While MOPs have shown promise in various fields, their use in photocatalysis, particularly for hydrogen evolution, was previously unexplored. The study demonstrates that ZrT-1-NH₂ exhibits significantly enhanced photocatalytic activity (510.42 μmol g⁻¹ h⁻¹) compared to other homogeneous crystalline materials under visible light irradiation. Furthermore, the hydrogen production efficiency of ZrT-1-NH₂ was further improved upon the introduction of platinum nanoparticles as a co-catalyst. The enhanced performance of ZrT-1-NH₂ is attributed to its efficient electron–hole separation, targeted electron transfer pathways, and effective suppression of electron–hole recombination. Notably, ZrT-1-NH₂ also demonstrated excellent stability over five continuous cycles of photocatalytic hydrogen evolution. The authors conclude that MOP-based photocatalysts, as exemplified by ZrT-1-NH₂, hold significant potential for applications in clean energy production, offering a promising avenue for efficient hydrogen generation from water splitting under visible light.⁸² In a separate but related advancement in catalysis, Kung and co-workers in another report demonstrated a novel approach to developing efficient electrocatalysts for water oxidation by synthesizing nanocomposites of zirconium-based UiO-66 MOF nanocrystals grown on carboxylic acid-functionalized multi-walled carbon nanotubes (CNTs).⁸³ The authors successfully controlled the MOF-to-CNT ratio, thereby tuning the nanocomposite's porosity and electrical conductivity. Subsequently, they introduced spatially dispersed iridium sites onto defect sites within the UiO-66 crystals using a self-limiting solution-phase method, creating Ir-functionalized UiO-66, CNT, and UiO-66-CNT nanocomposites. These materials were then evaluated for electrocatalytic water oxidation in acidic solutions. The study found that while Ir-functionalized UiO-66 exhibited the fastest reaction kinetics due to a high density of accessible iridium sites, its overall activity was limited by sluggish charge transport. Incorporating a small amount of CNT in the nanocomposites facilitated electron and ion transport between MOF crystals, leading to enhanced electrocatalytic performance. The optimal nanocomposite, Ir-UiO-66-10CNT, achieved an overpotential of 430 mV at a current density of 10 mA cm⁻², outperforming both Ir-functionalized UiO-66 and Ir-CNT. Electrochemical impedance spectroscopy (EIS) was crucial in decoupling the reaction kinetics at the iridium sites from the transport limitations within the catalytic thin films. Post-electrocatalysis characterizations confirmed the structural integrity of the MOF (Fig. 11).⁸³

Fig. 11 Schematic representation for the synthesis of iridium-functionalized UiO-66-CNT nanocomposites and the redox-hopping process during electrochemical operations. Reproduced with permission from ref. 83. Copyright 2022 Wiley.

Zr-MOFs' versatile properties make them promising candidates for various catalytic applications, including both CO₂ reduction and hydrogen evolution. For instance, Biswas and colleagues (2021) reported the development of a Zr-MOF incorporating anthracene for the selective photochemical conversion of carbon dioxide to formic acid, simultaneously oxidizing water.⁸⁴ This study details the development and application of an anthracene-based zirconium metal–organic framework (Zr-MOF) as a catalyst for the selective photochemical reduction of carbon dioxide to formic acid, coupled with the oxidation of water. The synthesized Zr-MOF exhibits significant CO₂ reduction activity alongside simultaneous water oxidation, facilitated by its ability to absorb both broadband visible and UV light, a property confirmed through electronic absorption spectroscopy. Notably, formic acid was the sole reduced product, achieved with a turnover frequency (TOF) of 0.69 h⁻¹, while oxygen was produced from water oxidation at a TOF of 0.54 h⁻¹. The MOF functions without an external photosensitizer, as the anthracene-based ligand itself acts as the light-harvesting component. The researchers reported a formic acid yield of 82.5 μmol per gram of catalyst loading. The selective formation of formic acid over other potential products is attributed to changes in the frontier orbital energy of the Zr-MOF compared to the ligand. The MOF's high CO₂ adsorption capacity is believed to enhance its interaction with CO₂ under ambient reaction conditions, eliminating the need for a sacrificial electron donor. The synergistic effect of the Zr-oxo cluster and the anthracene ligand enables the coupling of CO₂ reduction with water oxidation.⁸⁴

Similarly, in another facet of energy conversion, Toyao et al. (2016) demonstrated the potential of Pt-incorporation into Zr-MOFs for hydrogen evolution.⁸⁵ This study reports the successful construction of a platinum (Pt) complex within a Zr-MOF containing bipyridine units (Zr-MOF-bpy-PtCl₂) and investigates its photocatalytic activity for hydrogen production under visible-light irradiation. The Zr-MOF-bpy-PtCl₂ was synthesized by first creating a Zr-MOF with 2,2′-bipyridine-5,5′-dicarboxylic acid ligand (Zr-MOF-bpy) and then reacting it with K₂PtCl₄ to form the Pt complex within the MOF framework. Characterization using XRD and N₂ adsorption–desorption confirmed that both Zr-MOF-bpy and Zr-MOF-bpy-PtCl₂ retained a UiO-type structure. Further analysis using UV-vis and XAFS spectroscopy revealed that the incorporated Pt species adopted a square planar geometry, coordinated with two nitrogen atoms from the bipyridine units and two chlorine atoms. When tested for hydrogen production from water containing a sacrificial electron donor under visible light, Zr-MOF-bpy-PtCl₂ demonstrated steady hydrogen evolution, reaching 8.3 μmol after 9 hours, while the unmodified Zr-MOF-bpy showed no activity. Notably, the MOF-supported catalyst exhibited superior activity compared to its homogeneous molecular analogue, (bpy)PtCl₂. The researchers attribute this enhanced performance to the close proximity of the Pt complexes within the MOF framework.⁸⁵ Recent research highlights diverse strategies for enhancing photocatalytic hydrogen evolution using MOFs. In one study, nitrogen and sulfur co-doped carbon quantum dots (NS-CQDs), derived from high-sulfur lignite, were employed to boost the performance of a nickel-based MOF (Ni-MOF) modified with cobalt and zirconium (Co/Zr-MOF) (Fig. 12).¹⁸ The researchers successfully prepared NS-CQDs and used them to modify the MOFs, creating composite photocatalysts. The optimal composite, 5%NS-CQDs/NCZ2, exhibited a significantly improved hydrogen precipitation rate of 3662.68 μmol g⁻¹ h⁻¹, which is 5.35 times higher than that of Ni-MOF alone and 3.61 times higher than Co/Zr-MOF alone. Through analysis of the conduction and valence band positions of the three components, the authors propose the formation of a double S-type heterojunction within the composite material. The presence of thiophene and pyrrole nitrogen in the NS-CQDs is believed to facilitate efficient electron transport and reduce electron–hole recombination, thereby boosting the hydrogen evolution activity.

Fig. 12 Preparation chart of 5%NS-CQDs/NCZ2 catalyst. Reproduced with permission from ref. 18. Copyright 2024 Elsevier.

Simultaneously, other innovative MOF designs are emerging for related applications. For instance, a separate significant work introduced MFM-808, a novel MOF with a unique topology and a layered structure of Zr₆-clusters.⁸⁶ By substituting the bridging formate ligands within the Zr₆-layers with sulfate ligands, the modified material, MFM-808-SO₄, exhibits an exceptionally high proton conductivity of 0.21 S cm⁻¹ at 85 °C and 99% relative humidity, attributed to an efficient two-dimensional proton-conducting network facilitated by the sulfate-Zr₆-layers, as confirmed by molecular dynamics simulations. Furthermore, MFM-808-SO₄ also displays excellent photocatalytic activity for water splitting to produce hydrogen, suggesting its potential to bridge solar energy conversion with hydrogen fuel cell technology. The authors successfully synthesized and characterized MFM-808, highlighting its unique structural features, and subsequently transformed it into MFM-808-SO₄via a single-crystal-to-single-crystal sulfation process. The resulting material's dual functionality, showcasing both remarkable proton conductivity and photocatalytic hydrogen generation, positions it as a promising candidate for advancing hydrogen-based proton-exchange membrane fuel cells and contributing to the development of a renewable hydrogen energy cycle.⁸⁶

UiO-66-based MOFs continue to show remarkable versatility in water splitting, extending beyond traditional catalytic roles. Recently, Ma and co-workers reported the successful application of UiO-66-based MOFs in radiolytic water splitting. Their study demonstrated that nanoscale zirconium/hafnium-based UiO-66 MOFs serve as highly efficient and stable radiation sensitizers, significantly enhancing hydrogen production from both purified and natural water under gamma-ray irradiation (Fig. 13).⁸⁷ Through a combination of scavenging and pulse radiolysis experiments, supported by Monte Carlo simulations, the researchers discovered that the unique structure of UiO-66, featuring 3D arrays of ultrasmall metal-oxo clusters and high porosity, promotes significant scattering between secondary electrons and confined water molecules. This scattering leads to an increased generation of precursors for solvated electrons and excited states of water, which are the key species responsible for the enhanced hydrogen production. Notably, the addition of a small quantity (<80 mmol L⁻¹) of UiO-66-Hf-OH resulted in a gamma-ray-to-hydrogen conversion efficiency exceeding 10%, surpassing the performance of both zirconium/hafnium oxide nanoparticles and existing radiolytic hydrogen promoters. The robustness of the UiO-66 framework against continuous gamma radiation was confirmed through various characterization techniques. This work demonstrates a promising and competitive method for green hydrogen production and ionizing radiation storage through MOF-assisted radiolytic water splitting.

Fig. 13 Schematic representation of nanoscale UiO-66 MOF-assisted green H₂ production via renewable high-energy radiation. Reproduced with permission from ref. 87. Copyright 2023 ACS.

Complementing this advancement in radiolytic water splitting, UiO-66 MOFs have also been strategically integrated into electrocatalytic systems. In 2021, a study focused on developing an efficient and stable non-precious metal-based catalyst for electrocatalytic water splitting. This was achieved by creating a hybrid material composed of molybdenum carbide (Mo₂C) nanoparticles and a Zr-MOF (UiO-66) using a solvothermal method.⁸⁸ The synergistic interaction between Mo₂C and UiO-66 was found to lower the hydrogen adsorption energy, resulting in excellent catalytic activity for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in an alkaline environment. The optimized hybrid, MCU-2, containing a 50 : 50 weight ratio of Mo₂C and UiO-66, demonstrated the best performance, achieving a low overpotential of 174.1 mV for HER and 180 mV for OER at specific current densities, along with good stability over extended periods and cycles. Electrochemical analysis indicated that the hybrid's enhanced activity is due to the complementary properties of Mo₂C, which provides stability and catalytic activity, and UiO-66, which offers a large surface area, improved electrical conductivity, and efficient charge transport. The findings suggest that Mo₂C/UiO-66 hybrids are promising candidates for cost-effective and highly active catalysts for overall water splitting, highlighting the potential of designing bifunctional MOF-based catalysts with tailored properties for enhanced electrochemical performance.⁸⁸

Douhal and co-workers investigated the impact of cerium doping on UiO-66 using naphthalene-2,6-dicarboxylate (NDC) as a linker, aiming to identify the optimal Ce concentration for improved activity (Fig. 14a and b).⁸⁹ They synthesized a series of mixed-metal MOFs (MMOFs) with varying Ce content (0% to 100%). Their detailed characterization, including structural, chemical, and photodynamic analyses, revealed that Ce concentrations of 12% or higher led to the formation of a secondary crystalline phase (NDC-Ce) alongside the primary UiO-type structure. Spectroscopic analyses, including steady-state and femtosecond to millisecond time-resolved techniques, revealed two competing photoprocesses within these MMOFs: linker excimer formation and ultrafast ligand-to-cluster charge transfer (LCCT) from the organic linker to the Zr/Ce metal clusters. The LCCT process, occurring on a femtosecond timescale, results in long-lived charge-separated states, and the study demonstrated that a Ce content of 9% maximized the efficiency of photoproducing these states, suggesting this composition would exhibit the highest photoactivity. Photoaction spectroscopic measurements corroborated this prediction, showing that the MOF with 9% Ce displayed the maximum photocatalytic efficiency, leading to a 20% improvement in overall water splitting efficiency compared to the purely Zr-based MOF (Fig. 14c). By correlating structural and morphological characterization with detailed photodynamic studies and photoactivity measurements, this research provides clear insights into the relationship between the photophysical properties of Zr/Ce MMOFs and their photocatalytic performance, offering valuable guidance for the design of future MOFs with enhanced photocatalytic capabilities.⁸⁹

Fig. 14 (a) Molecular structure of the NDC Linker. (b) Crystalline Structure of Zr/Ce (NDC-Ce(x)) mixed-metal MOFs. (c) Photocatalytic efficiency carried out employing NDC-Ce(0) (black) and NDC-Ce(9) (blue) with the time. Reproduced with permission from ref. 89. Copyright 2023 ACS.

Expanding on the potential of Ce-doped Zr-MOFs, Parida and co-workers in 2022 reported on a novel mixed-valency bimetallic Zr/Ce MOF, denoted as UNH (Ce/Zr), synthesized via a one-step solvothermal method.⁹⁰ This study focused on its efficiency for visible-light-driven hydrogen (H₂) generation and ciprofloxacin (CIP) degradation. The incorporation of Ce³⁺/Ce⁴⁺ ions into the Zr-MOF structure resulted in a narrower band gap, enhanced exciton separation, and improved ligand-to-metal charge transfer (LMCT) compared to the pristine MOF. The presence of cerium ions increased the electron density around the zirconium ions and generated oxygen vacancies, collectively boosting the photocatalytic activity. The optimized UNH (Ce/Zr 1 : 1) photocatalyst exhibited a significantly enhanced H₂ production rate of 468.30 μmol h⁻¹ (apparent conversion efficiency of 3.51%), which is four times greater than that of the unmodified MOF. Additionally, UNH (Ce/Zr 1 : 1) demonstrated superior CIP photodegradation efficiency of 90.8%, following pseudo-first-order kinetics with a rate constant of 0.0363. Mechanistic studies revealed that hydroxyl radicals (OH˙) and superoxide radicals (O₂˙^–) were the primary active species in the CIP photodegradation process. The synergistic effects of the mixed-valency bimetallic node and the amine-functionalized linker in UNH (Ce/Zr) led to a blue shift in the band gap, higher exciton stability, effective band-edge positions, and improved electrochemical properties, contributing to the enhanced photocatalytic performance despite a slight decrease in surface area. This research showcases the potential of mixed-valency bimetallic MOFs as robust and efficient photocatalysts for both energy conversion and environmental remediation applications.⁹⁰

Building on this success, the same research group in 2025 unveiled a mixed-valence bimetallic Eu–Zr MOF for H₂ evolution.⁹¹ This single-component MOF, also synthesized via a one-step solvothermal method, proved effective as a photocatalyst for converting visible light into green energy sources like H₂ and H₂O₂ (Fig. 15). The incorporation of Eu³⁺ ions into the Zr-MOF structure results in enhanced visible light absorption, improved charge carrier separation, a narrowed band gap, and superior LMCT properties compared to the pristine MOF, primarily due to the presence of the interconvertible Eu³⁺/Eu²⁺ ion pair. X-ray photoelectron spectroscopy (XPS) confirmed an increase in electron density around the Zr⁴⁺ ions upon Eu addition, while PL and EIS analyses indicated enhanced exciton segregation, contributing to the improved catalytic performance. The synthesized EZUNH-2 MOF exhibited a significantly increased photocatalytic H₂ generation efficacy of 331.26 μmol h⁻¹ (ACE = 2.42%), approximately three times higher than that of the original MOF. Furthermore, it showed a four-fold increase in photocatalytic H₂O₂ production, reaching 35.2 μmol h⁻¹. Scavenger tests identified electrons as the main active species for H₂O₂ production. The study concludes that this one-pot synthesized bimetallic MOF provides a promising pathway for achieving high performance in photocatalytic H₂O₂ and H₂ production by leveraging the synergistic effects of mixed valency and bimetallic composition within a single framework.⁹¹

Fig. 15 One-step synthetic method of (Eu/Zr) bimetallic MOF. Reproduced with permission from ref. 91. Copyright 2025 ACS.

Beyond mixed-valency approaches, researchers have also explored incorporating co-catalysts and quantum dots to enhance the photocatalytic performance of Zr-MOFs. In 2019, Tian et al. investigated boosting photocatalytic hydrogen evolution performance by using Ti₃C₂ nanosheet-modified Zr-MOFs.⁹² This study explores the use of MXenes, specifically Ti₃C₂ nanosheets, as non-noble metallic co-catalysts to enhance the photocatalytic hydrogen production activity of porous UiO-66-NH₂ MOFs. The researchers successfully synthesized Ti₃C₂ nanosheets via an intercalation method and then integrated them with UiO-66-NH₂ using a one-pot hydrothermal process, ensuring face-to-face intimate contact between the two materials (Fig. 16). The resulting composite, denoted as TU10, exhibited the highest hydrogen evolution rate within the TU series (204 μmol g⁻¹ h⁻¹), achieving a rate approximately eight times greater than that of pure UiO-66-NH₂. This significant enhancement is attributed to the presence of exposed active sites at the edges of the Ti₃C₂ nanosheets and the formation of a Schottky junction at the interface between Ti₃C₂ and UiO-66-NH₂. This junction facilitates the spatial separation and transfer of charge carriers, leading to the accumulation of photo-induced electrons on the Ti₃C₂ nanosheet surface, thereby improving its electron-donating ability. DFT calculations further revealed that oxygen-terminated Ti₃C₂ possesses the most favorable Fermi level and a low Gibbs free energy for hydrogen adsorption, indicating its superior catalytic activity.⁹²

Fig. 16 Preparation method of Ti₃C₂ nanosheets and TU series. Reproduced with permission from ref. 92. Copyright 2019 Elsevier.

Further demonstrating the versatility of Zr-MOFs, He and co-workers in 2023 reported the creation of a highly efficient bifunctional photocatalyst, g-CNQDs@Zr-MOFs(Pt)(Zr/Ti), for both hydrogen evolution and nitric oxide (NO) removal under visible light irradiation.⁹³ The material was synthesized by incorporating uniformly distributed graphitic carbon nitride quantum dots (g-CNQDs) within a porphyrin-based MOF framework. This MOF utilized PtTCPP as the organic ligand and mixed zirconium/titanium-oxo clusters as the metal nodes. Compared to other zirconium-based porphyrin MOFs, this novel composite demonstrated superior performance in both hydrogen evolution (14511.57 μmol g⁻¹) and NO removal (31.9%). The enhanced photocatalytic activity is attributed to the synergistic effects of the mixed Zr/Ti-oxo clusters and the in situ generated g-CNQDs. The presence of mixed metal clusters facilitates efficient charge separation and transfer through the rapid Ti³⁺/Ti⁴⁺ redox cycle. Simultaneously, the uniformly dispersed g-CNQDs within the MOF cavities improve visible light utilization, particularly through up-conversion, and further promote charge carrier separation. The synthetic strategy involved metal cluster exchange and in situ quantum dot loading, resulting in a homogeneous distribution of both bimetallic clusters and g-CNQDs within the composite. The resulting g-CNQDs@Zr-MOFs(Pt)(Zr/Ti) also exhibited good photostability over repeated reaction cycles. This research offers insights into designing advanced MOF composites by functionalizing the host MOF matrix and incorporating guest active species for improved performance in solar energy applications.⁹³

A recent study focused on photocatalysis, detailing the design and synthesis of a robust, water-stable mixed-linker Zr-MOF for efficient visible-light-driven photocatalytic H₂ production and tetracycline hydrochloride (TCH) degradation.⁹⁴ Employing a single-step solvothermal method, the researchers utilized terephthalic acid-based ditopic linkers, including amino-terephthalic acid, to create a single-component photocatalyst. This mixed-linker approach successfully tuned the MOF's band gap, enhancing visible light absorption, promoting exciton segregation, and introducing oxygen vacancies, which collectively resulted in superior photocatalytic performance compared to pristine Zr-MOFs. Spectroscopic and electrochemical analyses corroborated these findings. The synthesized mixed-linker MOF, designated UML-2, exhibited a remarkable H₂ evolution rate of 247.88 μmol h⁻¹ (apparent conversion efficiency of 1.9%), doubling the performance of its parent UiO-66-NH₂ MOF. Furthermore, UML-2 demonstrated enhanced TCH degradation efficacy of 91.8%, following pseudo-first-order kinetics with a rate constant of 0.032. Mechanistic studies revealed that hydroxyl radicals and superoxide radicals were the primary active species in TCH degradation. The mixed-linker MOF maintained its stability under aqueous and photocatalytic conditions, highlighting its potential as a highly effective MOF-based photocatalyst for both pollutant abatement and hydrogen energy production, outperforming several reported Zr-MOF-based nanomaterials in both applications.⁹⁴

Shifting focus to electrocatalysis, researchers in 2024 explored the development of cost-effective and highly efficient non-noble metal electrocatalysts for the HER using Zr-MOFs.⁹⁵ While MOFs are promising due to their customizable structure and high surface area, their low conductivity and instability often hinder their electrocatalytic performance. To overcome these limitations, the researchers introduced an innovative strategy involving the creation of interfaces with precisely controlled proportions of phosphide nanostructures. Specifically, they successfully synthesized Zr-MOF/Ni₂P@nickel foam (NF) electrodes, where the interface was regulated by Ni₂P nanostructures (Fig. 17). The presence of these Ni₂P nanostructures, with abundant active sites at the Zr-MOF@NF interface, significantly boosted the electronic conductivity and local charge density of the hybrid electrocatalysts, leading to improved reaction kinetics and enhanced electrocatalytic activity across a wide pH range. By optimizing the amount of Ni₂P, the Zr-MOF/Ni₂P@NF electrode demonstrated impressive stability and superior HER performance, achieving low overpotentials of 149 mV in acidic and 143 mV in alkaline electrolytes at a current density of 10 mA cm⁻².

Fig. 17 Synthesis process of the Zr-MOF/Ni₂P@NF. Reproduced with permission from ref. 95. Copyright 2024 Elsevier.

The versatility of Zr-MOFs in catalysis has been further expanded by strategies involving post-synthetic modification and the creation of hybrid structures. In 2020, Costantino and colleagues explored modifying the UiO-66 MOF after its initial construction to introduce defects, which then allowed for the attachment of an iridium(III)-HEDTA complex.⁹⁶ This study investigates the viability of employing a defective Zr-MOF with the UiO-66 topology as a support for the heterogenization of a high-performance iridium complex, [Ir(HEDTA)Cl]Na, intended for catalytic water oxidation, a critical process in the generation of renewable fuels. The introduction of defects within the MOF structure was achieved through the partial substitution of terephthalic acid linkers with formate groups. The iridium complex was successfully immobilized onto the MOF support via a post-synthetic modification strategy involving the exchange of formate anions coordinated to the zirconium clusters with the carboxylate moiety of the [Ir(HEDTA)Cl]^– complex. The resultant hybrid material was evaluated as a heterogeneous catalyst for the water oxidation reaction, utilizing cerium ammonium nitrate (CAN) as a sacrificial oxidant. In a separate but related effort to develop efficient water splitting catalysts, researchers reported the development of a highly efficient and stable non-noble metal bifunctional electrocatalyst for both the HER and OER.⁹⁷ The researchers synthesized a two-dimensional (2D) selenium-blended zirconium dioxide (Se–ZrO₂) nanostructure supported on a nitrogen-doped carbon heterostructure (NC), denoted as Se–ZrO₂@NC, which was derived from a Zr-MOF. This composite material was then loaded onto a stainless-steel mesh electrode. The resulting electrode demonstrated exceptional electrocatalytic performance, achieving a remarkably low overpotential of 48 mV for HER in acidic media and 251 mV for OER in alkaline media, both at a current density of 10 mA cm⁻². Furthermore, a complete water electrolysis cell utilizing this catalyst for both HER and OER required a cell potential of only 1.58 V to reach 10 mA cm⁻², while maintaining long-term stability over 48 hours. The study highlights the potential of this Se-blended 2D transition metal dioxide on a 2D nitrogen-doped carbon heterostructure platform for creating a range of highly active and stable electrocatalysts for alkaline water splitting. The optimized Se–ZrO₂@NC composite, obtained at 500 °C, exhibited a high electrochemical surface area, excellent electroconductivity, favorable stability, and fast ion transfer, making it a promising candidate for efficient and cost-effective water electrolysis.⁹⁷

Further demonstrating the breadth of Zr-MOF applications, researchers in 2024 considered Ag/Pd bimetallic nanoparticle-loaded Zr-MOF as a novel catalyst for H₂O₂ and H₂ production.⁹⁸ This report addresses the limitations of pristine MOF-based photocatalysts, namely low visible light absorption and rapid exciton recombination, by modifying the UiO-66-NH₂ MOF with noble bimetallic nanoparticles (Ag/Pd) using a simple adsorption-reduction method. The resulting composite photocatalyst, particularly with a 1 : 2 Ag/Pd ratio, demonstrated significantly enhanced performance in both H₂O₂ and H₂ production under visible light irradiation. The H₂O₂ production rate reached 39.4 mmol h⁻¹, a four-fold increase compared to the unmodified MOF and twice that of monometallic counterparts. Similarly, the H₂ evolution capacity was 448.2 mmol h⁻¹, also a substantial improvement. The enhanced photocatalytic activity is attributed to the bimetallic nanoparticles' ability to suppress exciton recombination, improve photon capture, and facilitate fast charge transfer. Mechanistic studies revealed that the Ag component promotes visible light absorption through the localized surface plasmon resonance (LSPR) effect, further enhanced by the Pd support, which also aids in electron transfer from the MOF to the bimetallic nanoparticles. The formation of a Schottky barrier at the MOF-nanoparticle interface further reduces electron–hole recombination. The bimetallic nanoparticles act as efficient electron traps, providing active sites for the production of both H₂O₂ and H₂, establishing this composite material as a promising approach for sustainable green energy production.⁹⁸

Another strategy for boosting photocatalytic efficiency involves constructing heterojunctions. A highly efficient Z-scheme catalyst for H₂ production was reported as Zr-MOF/g-C₃N₄ by Xing et al. in 2023.⁹⁹ The researchers explored an effective strategy to enhance photocatalytic activity by constructing heterojunction photocatalysts, specifically focusing on new composites of a robust Zr-MOF and low-cost graphitic carbon nitride (g-C₃N₄). These composites were successfully fabricated using a simple mechanical grinding method followed by low-temperature heating, resulting in materials that exhibit broad visible-light absorption, good long-term stability, and significantly improved photocatalytic hydrogen production (Fig. 18). The optimized Zr-MOF/g-C₃N₄ heterojunction (with 5.0 wt% g-C₃N₄) achieved a remarkable hydrogen production rate of 1.252 mmol g⁻¹ h⁻¹, which is 41.7 times higher than that of the pure Zr-MOF and 15.7 times higher than that of pure g-C₃N₄. The enhanced performance is attributed to the formation of a Z-scheme charge transfer mechanism at the heterojunction interface, which effectively promotes charge separation and maintains the high redox potential of the photogenerated electrons and holes, thereby greatly boosting the photocatalytic hydrogen production activity. This study also demonstrated a rational and facile approach to fabricate heterojunction photocatalysts and introduces new, high-performance, and cost-effective MOF/g-C₃N₄ composites for hydrogen production.⁹⁹

Fig. 18 Synthesis process of Zr-MOF/g-C₃N₄ heterojunctions. Reproduced with permission from ref. 99. Copyright 2023 RSC.

One notable approach, investigated by Pal and co-workers, involves a synergistic combination of defect-rich UiO-66 (UiO-66-D), graphitic carbon nitride (g-C₃N₄), and nickel nanoparticles (Ni NPs) to boost photocatalytic hydrogen evolution.¹⁰⁰ Their work, published in 2023, deliberately engineered defects in the UiO-66 framework to tune its photonic and electrical properties. The resulting UiO-66-D@g-C₃N₄/Ni composite exhibited an unprecedented hydrogen production rate of 2.6 mmol g⁻¹ h⁻¹ with an apparent quantum yield (AQY) of 6.41% at 420 nm, a substantial improvement compared to the pristine UiO-66-based photocatalyst. Theoretical calculations and photocatalysis trends support the strong interaction between UiO-66-D and g-C₃N₄, with the enhanced activity attributed to the formation of an abundant type-II heterojunction. This heterojunction, along with the defect-induced local strain and modified electronic structure at the interface, plays a crucial role in promoting catalytic activity. The well-coordinated Ni NPs on g-C₃N₄ further contribute to the structural stabilization of the composite and facilitate efficient charge transfer. Comprehensive photoelectrochemical and spectroscopic analyses confirmed accelerated kinetics and enhanced electron–hole pair dissociation in the UiO-66-D@g-C₃N₄/Ni system.¹⁰⁰

Complementing this, another study explored the use of different ligand-functionalized UiO-66 MOFs (UiO-66-X: –OH, –(OH)₂, –NH₂) as supports for ZnIn₂S₄ nanosheets.¹⁰¹ This research, also from 2023, focused on creating heterostructures for visible-light-driven hydrogen peroxide (H₂O₂) production in pure water. Composites featuring UiO-66-NH₂ and UiO-66-(OH)₂ showed significantly higher H₂O₂ yields (799 and 733 μmol L⁻¹, respectively) compared to other UiO-66 variations. This superior performance was linked to favorable visible-light response and the formation of Z-scheme heterostructures, promoting efficient charge separation and transfer. This enhanced performance is attributed to the favorable visible-light response and the formation of Z-scheme heterostructures in ZnIn₂S₄/UiO-66-NH₂ and ZnIn₂S₄/UiO-66-(OH)₂, which facilitate efficient charge separation and transfer. The study also determined that the H₂O₂ evolution proceeds through an indirect oxygen reduction pathway, with superoxide radicals (˙O₂⁻) acting as intermediate species. Furthermore, the ZnIn₂S₄/UiO-66-NH₂ composite demonstrated comparable H₂O₂ yields in tap water and lake water, suggesting its potential for practical applications.¹⁰¹

Further demonstrating the versatility of Zr-MOFs, researchers in 2023 introduced a novel room temperature synthesis (RTS) approach for highly redox-active cerium(IV)-MOFs, including Ce-UiO-66-NH₂ and other derivatives.¹⁰² This mild synthesis condition allowed for the creation of highly crystalline Ce-UiO-66-NH₂ and various other Ce-MOF derivatives and topologies with excellent space-time yield. The photocatalytic HER and OER activities of these Ce-MOFs under simulated sunlight correlated well with their energy level band diagrams, with Ce-UiO-66-NH₂ and Ce-UiO-66-NO₂ showing the highest activity for HER and OER, respectively, outperforming other metal-based UiO-type MOFs. Furthermore, combining Ce-UiO-66-NH₂ with supported platinum nanoparticles resulted in a highly active, stable, and reusable photocatalyst for overall water splitting, attributed to its efficient photoinduced charge separation. The study highlights the potential of this scalable green RTS approach for creating versatile redox-active Ce-based MOFs and demonstrates their promising applications in photocatalytic water splitting, paving the way for future work focused on fine-tuning the energy level diagrams of Ce-UiO-66-X MOFs to optimize the thermodynamic requirements of the process.¹⁰²

Lastly, researchers also reported the successful synthesis of a novel litchi-like nanocomposite, MoS_x–Fe@UiO-66-(OH)₂, designed for efficient and durable HER catalysis in acidic environments.¹⁰³ Fabricated through a sequential room-temperature redox and coordination reaction, this nanocomposite features amorphous molybdenum sulfide-iron (MoS_x–Fe) nanoparticles uniformly anchored on a hydroxylated UiO-66 MOF. The resulting material demonstrated exceptional catalytic performance, achieving a current density of 1000 mA cm⁻² at an overpotential of −297 mV, outperforming commercial Pt/C catalysts under high current densities. The remarkable activity and stability are attributed to the porous UiO-66-(OH)₂ support, which effectively disperses and firmly anchors the MoS_x–Fe nanoparticles, leading to a low Tafel slope of 41 mV dec⁻¹. This work introduces a facile phenol-Fe(III) coordination approach for synthesizing highly active and robust HER catalysts, positioning the developed nanocomposite among the top-performing molybdenum-based catalysts reported to date.¹⁰³

Photocatalytic production of hydrogen peroxide and hydrogen stands as a promising strategy to address the escalating energy crisis. In a 2025 study, Parida and co-workers explored enhancing the performance of the inherently stable but photon- and exciton-deficient UiO-66-NH₂ MOF for these reactions.¹⁰⁴ They achieved this by integrating inexpensive carbon nanoparticles via a simple solvothermal procedure. The resulting composite, specifically UC-2, showed significantly improved photocatalytic activity. This enhancement was attributed to the composite's ability to reduce exciton recombination, boost photon absorption, and facilitate quicker charge transfer, as evidenced by various spectroscopic and electrochemical analyses. Under visible light irradiation, UC-2 demonstrated a hydrogen peroxide generation rate of 33.2 μmol h⁻¹ in oxygen-saturated conditions with isopropyl alcohol and water, nearly tripling the rate of the pristine UiO-66-NH₂. Similarly, UC-2 exhibited a maximum hydrogen evolution capacity of 298.1 μmol h⁻¹. The authors concluded that the superior light-trapping, electron transfer, and electron capture capabilities of the carbon nanoparticle co-catalyst were crucial in boosting the overall photoreaction efficiency, thus offering a sustainable energy alternative in both H₂O₂ and H₂ production.

In the following, the catalytic performance of some reported Zr-MOFs for H₂ evolution has been added to Table 2.

Table 2 Catalytic performance of some Zr-MOFs for H₂ evolution

Entry	Photocatalyst	Time/h	Light source	Product yield	Ref.
1	UML-2	1	300 W Xe-lamp	247.88 μmol h^−1	94
2	5%NS-CQDs/NCZ2	5	300 W Xe-lamp	3662.68 μmol g^−1 h^−1	18
3	Zr-MOF-bpy-PtCl2	9	Visible light (λ > 420 nm)	8.3 μmol	85
4	ZrT-1-NH2	30 min	300 W Xe-lamp	510.42 μmol g^−1 h^−1	82
5	Pt/ZrT-1-NH2	30 min	300 W Xe-lamp	1057.5 μmol g^−1 h^−1	82
6	Pt/UiO-66-NH2	30 min	300 W Xe-lamp	47.5 μmol g^−1 h^−1	82
7	UiO-66-NH2	3	200 W Xe-lamp	2.8 mL	80
8	UiO-66	3	200 W Xe-lamp	2.4 mL	80
9	Pt@Ce-UiO-66-NH2	3	Visible light (λ > 420 nm)	1200 μmol g^−1	102
10	Ce-UiO-66-NH2	3	Visible light (λ > 420 nm)	600 μmol g^−1	102
11	UNH (Ce/Zr 1 : 1)	—	Visible light (λ > 420 nm)	468.30 μmol h^−1	90
12	EZUNH-2	2	Visible light (λ > 420 nm)	331.26 μmol h^−1	91
13	g-CNQDs@Zr-MOFs(Pt)(Zr/Ti)	30 min	Visible light (λ > 420 nm)	14 511.57 μmol g^−1	93
14	Ag/Pd@UiO-66-NH2	2	Visible light (λ > 420 nm)	448.2 μmol h^−1	98
15	Zr-MOF/g-C3N4	4	Visible light (λ > 420 nm)	1.252 μmol g^−1 h^−1	99
16	g-C3N4/UiO-66-D/Ni	—	Visible light (λ > 420 nm)	2621 μmol g^−1 h^−1	100
17	g-C3N4/UiO-66/Ni	—	Visible light (λ > 420 nm)	2057 μmol g^−1 h^−1	100
18	UiO-67	3	Visible light (λ > 400 nm)	26.9 μmol g^−1 h^−1	105
19	UiO-67-Ce	3	Visible light (λ > 400 nm)	269 μmol h^−1 g^−1	105
20	UiO-67-NS-Ti	12	300 W Xe-lamp	31 μmol g^−1 h^−1	106
21	WP/UiO-66/CdS	5	300 W Xe-lamp	79 μmol g^−1 h^−1	107
22	UiO-66-NH2/CdS	6	Xe lamp sunlight	64 μmol g^−1 h^−1	108
23	UiO-66-NH2/Au/CdS	—	Xe lamp sunlight	39.5 μmol g^−1 h^−1	109
24	UN-CdS	4	Xe lamp sunlight λ > 400 nm	487.5 μmol g^−1 h^−1	110

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5. Cr(VI) photocatalytic reduction

Zr-based MOFs have emerged as promising photocatalysts for the degradation of highly toxic hexavalent chromium (Cr(VI)) into the less harmful trivalent chromium (Cr(III)).¹¹¹ This application leverages the unique properties of Zr-MOFs, including their high surface area, tunable pore structure, and the ability to incorporate active sites for photocatalysis. Upon irradiation with light, the MOF's organic ligands act as light harvesters, exciting the Zr-oxo clusters or incorporated catalytic centers to generate electron–hole pairs. These photogenerated electrons then participate in the reduction of Cr(VI) to Cr(III), while the holes can oxidize water or other electron donors present in the solution. The efficiency of this process is often enhanced by modifying the MOFs, such as through doping, ligand functionalization, or creating heterojunctions with other semiconductors, to improve light absorption, charge separation, and Cr(VI) adsorption. Studies have shown that Zr-MOFs, particularly UiO-66 and its derivatives, exhibit significant potential for Cr(VI) removal from wastewater under both UV and visible light irradiation, offering a stable and reusable platform for environmental remediation.¹¹²

A critical challenge in developing highly accurate water-stable Zr-MOF sensors for Cr(VI) detection stems from the vulnerability of single emission signals to external interferences. To address this, Zhu et al. in 2025 devised a novel strategy involving the intercalation of luminescent guest species into a Zr-MOF host to achieve dual-emission signals.¹¹³ They synthesized a highly water-stable, blue-fluorescent Zr-MOF (Zr-bcu-22bipy44dc) and incorporated Rhodamine B (RhB) into its framework, creating a unique dual-emissive platform (RhB@Zr-MOF) with emission peaks at 410 nm (host) and 580 nm (guest). This innovative design enabled precise detection of trace Cr(VI) ions by evaluating the relative fluorescence intensity difference between the host and guest emissions. Importantly, this interstitial incorporation effectively suppressed the aggregation-caused quenching (ACQ) typically observed with RhB molecules. This approach led to a significant enhancement in detection limits for CrO₄²⁻ (6.13 ppb) and Cr₂O₇²⁻ (10.04 ppb) ions, demonstrating an approximately 4.92-fold and 3.66-fold improvement, respectively, compared to the pristine Zr-MOF. Contrary to common assumptions of FRET or previously identified PET, DFT and TD-DFT calculations confirmed that the orange-yellow photoluminescence of RhB@Zr-MOF primarily arises from a photoinduced electron transfer (PET) process inherent to the intercalated RhB molecules. The synergistic effect of RhB's antenna function and the Zr-MOF's optimized band structure also endowed RhB@Zr-MOF with a remarkable ability to photochemically detoxify Cr(VI) and bleach reactive dyes.

Rapid reduction of Cr(VI) via Zr-Porphyrin based MOF investigated by Xing and co-workers in 2021.¹¹⁴ They explored the application of Zr-porphyrin MOFs, specifically the isostructural series PCN-222(M) [M = H₂, Zn(II), Fe(III), Co(II)], in a novel sonophotocatalytic system for the reduction of highly toxic Cr(VI) to the less harmful Cr(III) in water under visible light irradiation. The researchers investigated the synergistic effect of combining sonochemistry and photocatalysis, demonstrating that PCN-222(M) exhibited significantly enhanced Cr(VI) reduction activities compared to photocatalysis alone, with reaction rate constants being 1.5 to 3.3 times higher in the combined system. Through fluorescence and UV-vis absorption spectroscopy, they elucidated that the sonophotocatalytic process promotes the transfer of photoinduced electrons from the PCN-222(M) catalysts to Cr(VI), thereby boosting the catalytic performance. The authors concluded that the innovative integration of porous MOFs with sonophotocatalytic technology presents a promising strategy for improving the efficiency of existing MOF-based photocatalytic systems for environmental remediation.¹¹⁴ Further expanding the scope of Cr(VI) photoreduction, Peng et al. in 2021 investigated CdIn₂S₄/MOF-808 heterostructures as highly efficient and multifunctional photocatalysts for removing Cr(VI) from wastewater using solar light.¹¹⁵ The researchers prepared composites of cadmium indium sulfide (CdIn₂S₄) combined with the metal–organic framework MOF-808 (CIS/MOF-808) to enhance the photoreduction of Cr(VI). The close interaction between MOF-808 and CdIn₂S₄ facilitated efficient separation and transfer of photogenerated electron–hole pairs, while MOF-808 provided ample adsorption sites for Cr(VI) and improved photon capture. The resulting CIS/MOF-808 hybrids exhibited superior Cr(VI) removal activity compared to the individual components. The study investigated the impact of CdIn₂S₄ content, pH, and the presence of inorganic salts on the Cr(VI) removal efficiency and the concentration of the less toxic Cr(III) in the solution. The optimal composite, 4-CIS/MOF-808 with 16 wt% CdIn₂S₄, achieved a Cr(VI) removal efficiency of 95.45% within 150 minutes. While acidic conditions accelerated the reduction of Cr(VI) to Cr(III), they also contributed to metal leaching and photocorrosion, leading to a decrease in efficiency to 88.17% after five cycles. The authors concluded that the rapid charge transfer at the interface between MOF-808 and CdIn₂S₄ in acidic conditions promotes Cr(VI) reduction, but future research should prioritize improving the photostability of CIS/MOF composites for long-term wastewater remediation applications.¹¹⁵

Also in 2021, another study focused on highly stable, bcu topological Zr-MOFs for the detection, adsorption, and catalytic reduction of Cr(VI) ions.¹¹⁶ These isoreticular MOFs, designated as 1 (with H₂L₁ ligand) and 2 (with H₂L₂ ligand), feature stretched square channels enriched with bipyridine/biquinoline nitrogen atoms and eight-connected Zr₆ clusters with Zr–O defect sites, giving them high porosity and photosensitivity. MOF 1 demonstrated excellent performance in fluorescence sensing and adsorption trapping of Cr(VI), achieving an ultralow detection limit of 0.0176 ppm and a significant saturated adsorption capacity of 145.77 mg g⁻¹. Conversely, MOF 2 exhibited superior photochemical removal of Cr(VI), achieving a remarkable reduction efficiency of 98.05% within 70 minutes, and maintaining 92.21% efficiency over five consecutive photocatalytic cycles. The researchers proposed potential photoluminescence quenching and reduction mechanisms, suggesting that the formation of coordination bonds between the –C=O or Zr–O sites of the MOFs and Cr(VI) ions is crucial for their excellent performance (Fig. 19).¹¹⁶

Fig. 19 The schematic illustration of MOF 1 and MOF 2 for removal of Cr₂O₇²⁻ and photocatalytic reduction of Cr(VI). Reproduced with permission from ref. 116. Copyright 2021 ACS.

In 2022, a significant advancement was reported by researchers who successfully synthesized NH₂-UiO-66 (Zr)/PDI (NUPDI) n-n heterojunction photocatalysts via a simple self-assembly method.¹¹⁷ This work focused on enhancing the photocatalytic reduction of highly toxic Cr(VI). The resulting NUPDI heterojunctions demonstrated high efficiency and stability in Cr(VI) removal, achieving a remarkable 99.5% removal rate with the optimized NUPDI (1 : 8) composition under visible light irradiation within 120 minutes. The improved performance is attributed to the synergistic effects of incorporating SA-PDI, which broadens the visible light absorption range of NH₂-UiO-66 (Zr) and generates more photogenerated charge carriers. Additionally, the formation of strong π–π interactions between the NH₂-UiO-66 (Zr) and SA-PDI components significantly facilitates charge separation, effectively inhibiting the recombination of photogenerated electrons and holes. The researchers propose and analyze a Z-scheme mechanism operating within the heterojunction, which further explains the enhanced photocatalytic reduction capability.¹¹⁷

Expanding on the design of efficient Zr-MOF-based systems for similar environmental challenges, other recent studies have introduced novel material architectures and compositions. Researchers introduced a novel Zr-MOF, termed NU-1400-py, which features a pyrimidine ring in place of the benzene ring found in the topologically identical NU-1400.¹¹⁸ Leveraging its inherent porosity and semiconducting properties, NU-1400-py demonstrated a significant adsorption capacity for Cr₂O₇²⁻ ions (136.43 mg g⁻¹) and exhibits photocatalytic purification of Cr(VI) and various reactive dyes (K-, X-, M-types) from water. Furthermore, the researchers developed a composite material, EY@Zr-MOF, by intercalating the luminescent molecule Eosin Y (EY) into the Zr-MOF. This modification significantly enhanced the material's interface resistance, transient photocurrent intensity, and charge carrier migration efficiency compared to the pristine Zr-MOF. Consequently, EY@Zr-MOF displayed dramatically improved photochemical purification efficiencies towards Cr₂O₇²⁻, Reactive Blue 13 (RB13), Reactive Red 2 (RR2), and Reactive Red 11 (RR11), with reaction kinetics accelerated by factors of 1.24, 4.82, 2.48, and 6.23, respectively, compared to the unmodified Zr-MOF. The study also proposes tentative photochemical decomposition and deoxidation mechanisms for EY@Zr-MOF in the removal of these contaminants, highlighting a promising strategy for designing multifunctional materials to address environmental issues related to Cr(VI) and reactive dyes.¹¹⁸ In a systematic approach, researchers enhanced the photocatalytic efficiency of a Zr-MOF for Cr(VI) detoxification and reactive dye degradation by constructing heterojunctions with carefully selected metal sulfides.¹¹⁹ The researchers deposited six different metal sulfides onto the Zr-MOF and found that the In₂S₃/Zr-MOF heterojunction exhibited superior optical-electronic properties and photocatalytic performance due to a highly matched band structure, leading to improved UV-visible light utilization, charge carrier migration, electron–hole separation, and reduced recombination. This optimized heterojunction, particularly with a 50% mass of Zr-MOF, demonstrated remarkable dual-functionality in purifying water contaminated with ultra-stubborn reactive dyes (RR11, RB21) and highly toxic Cr(VI) ions under a 500 W xenon lamp, achieving degradation/reduction efficiencies of 91.8%, 93.3%, and 97.9%, respectively. Compared to the individual components, the In₂S₃/Zr-MOF heterojunction significantly increased the photocatalytic degradation kinetics of RR11 by 27.18 times and of RB21 by 38.50 times, while also showing good recyclability. Mechanism studies revealed that superoxide radicals (˙O₂⁻) play a dominant role in dye removal, facilitated by efficient electron transfer from In₂S₃ to Zr-MOF, which enhances the conversion of dissolved oxygen to ˙O₂⁻. For Cr(VI) reduction, the cooperative effect of electrons in the Zr-MOF's conduction band with oxygen species is proposed as the key mechanism.¹¹⁹

Researchers also explored a novel approach for simultaneous dichromate and dye removal by constructing a heterojunction photocatalyst based on a zirconium(IV)-biquinoline framework (Zr-MOF) and in situ generated In₂S₃ species.¹²⁰ The resulting In₂S₃/Zr-MOF heterojunction, with a bcu topology, exhibits optimized visible light utilization, inhibited photogenerated electron–hole recombination, and enhanced carrier migration kinetics due to the well-matched band structures of its components. Notably, the 10%-Zr-MOF composite demonstrated excellent photocatalytic redox abilities against both highly toxic Cr(VI) ions and the dye RhB. Specifically, it achieved a Cr(VI) deoxidation efficiency of 96.7%, which is 3.14 and 1.42 times higher than that of pristine Zr-MOF and sole In₂S₃, respectively. Furthermore, the heterojunction displayed a significantly faster photocatalytic reduction kinetic rate (0.031 min⁻¹) for Cr(VI) compared to its individual components. The study also proposes detailed photocatalytic reduction and degradation mechanisms based on comprehensive experimental and instrumental characterization. This work presents a viable post-modification strategy for developing novel Zr-MOF-based photocatalytic platforms for the effective decontamination of water from Cr(VI) and dyes.¹²⁰

In another study, researchers reported the fabrication of novel Z-scheme PDINH/NH₂-UiO-66(Zr) (PNU) heterojunctions using a simple ball-milling method for efficient chromium(VI) reduction and antibacterial applications (Fig. 20).¹²¹ The optimized composite, PNU-1, demonstrated exceptional photocatalytic activity, achieving 97% reduction of Cr(VI) within 60 minutes under LED light irradiation, significantly outperforming the individual components. Furthermore, PNU-1 exhibited superior antibacterial performance, completely eradicating E. coli and S. aureus within 4 hours. The enhanced performance is attributed to a broader light absorption range, promoted charge separation facilitated by the Z-scheme mechanism, and the efficient generation of reactive oxygen species (¹O₂, ˙O₂⁻, and ˙OH). The stability and reusability of the composite were confirmed through five cycles, showing only a slight decrease in performance. This research provides a promising strategy for designing high-efficiency Z-scheme heterostructures combining MOFs and organic PDINH for effective wastewater remediation.

Fig. 20 Schematic illustration of construction of PDINH/NH₂-UiO-66(Zr) composites. Reproduced with permission from ref. 121. Copyright 2023 Elsevier.

For the first time, in 2022, researchers used a novel microwave-assisted synthesis method for the rapid preparation of an amino-functionalized zirconium-based metal–organic polyhedron (MW-ZrT-1-NH₂), significantly reducing the reaction time from 12 hours in traditional solvothermal methods to just 2 minutes, while maintaining good yield and product purity.¹²² The synthesized MW-ZrT-1-NH₂ was then evaluated as a photocatalyst for the reduction of highly toxic Cr(VI) under visible light irradiation, demonstrating superior photocatalytic performance compared to its conventionally synthesized counterpart. Through ESR measurements and radical trapping experiments, the researchers proposed a plausible photocatalytic mechanism, identifying photogenerated electrons (e⁻) and superoxide radicals (˙O₂⁻) as the primary active species responsible for the conversion of Cr(VI) to the less harmful Cr(III). Complementing this focus on rapid synthesis, earlier work by Wang et al. in 2019 investigated the photocatalytic reduction of Cr(VI) using UiO-66-NH₂(Zr/Hf) MOF membranes.¹²³ This study addressed the challenge of separating powder photocatalysts from water after Cr(VI) reduction by utilizing MOF membranes. Specifically, UiO-66-NH₂(Zr/Hf) MOF membranes were successfully fabricated on alumina porous supports using a reactive seeding method. These membranes exhibited excellent photocatalytic performance in reducing toxic Cr(VI) ions under both simulated and real sunlight irradiation, achieving high efficiency and enabling easy separation from the treated wastewater. Notably, the UiO-66-NH₂(Zr) membrane maintained over 94% Cr(VI) reduction efficiency after 20 cycles, demonstrating exceptional chemical and water stability. Furthermore, the presence of common foreign ions found in surface water did not significantly inhibit the Cr(VI) reduction activity of the membrane. The authors conclude that these MOF membranes provide a promising new approach for the efficient photocatalytic removal of pollutants from wastewater, offering facile reuse and avoiding the need for organic hole scavenger reagents.

Expanding on the work in remediation, researchers in 2022, demonstrated the development of a dual-emissive composite material, RhB@Zr-MOF, by embedding the luminous dye RhB into a zirconium-biquinoline-based MOF using an in situ encapsulation method (Fig. 21).¹²⁴ This approach effectively avoided the aggregation-caused quenching (ACQ) effect of RhB. The resulting RhB@Zr-MOF exhibits a rapid and selective fluorescence quenching response towards Cr(VI) ions, demonstrating high sensitivity and resistance to interference. Unlike typical MOF-guest fluorescence resonance energy transfer (FRET), the mechanism behind the enhanced sensing is attributed to photoinduced electron transfer from RhB to the Zr-MOF, which was confirmed through DFT and time-dependent DFT (TD-DFT) calculations. This electron transfer significantly improves the detection sensitivity for both Cr₂O₇²⁻ and CrO₄²⁻ ions, achieving ultralow detection limits of 6.27 ppb and 5.26 ppb, respectively, representing a 6 to 9-fold increase compared to the pristine Zr-MOF. Furthermore, the favorable conduction band (CB) and valence band (VB) potentials of RhB@Zr-MOF endow it with excellent photochemical scavenging ability for Cr(VI) and methyl orange (MO) contaminants. The coordination reactions between Cr(VI) metal centers and carboxyl oxygen atoms on RhB@Zr-MOF are also identified as crucial for both the efficient fluorescence sensing and photochemical reduction capabilities.

Fig. 21 The schematic illustration of RhB@Zr-MOF for removal of Cr₂O₇²⁻ and CrO₄²⁻ ions - and photocatalytic reduction of Cr(VI). Reproduced with permission from ref. 124. Copyright 2022 ACS.

Continuing the exploration of Zr-MOFs in environmental applications, several studies highlight their versatile capabilities in both photocatalytic chromium reduction and sensitive Cr(VI) detection. In a significant contribution, researchers designed a series of novel molybdenum/zirconium bimetallic MOFs (MZMs), synthesized via a simple hydrothermal method.¹²⁵ This study investigated their efficiency in the photocatalytic degradation of methylene blue (MB) and the reduction of chromium(VI) ions under visible light. Among the synthesized materials, 2MZMs exhibited the highest photocatalytic performance, achieving degradation rates of 94.36% for MB within 120 minutes and 91.20% for Cr(VI) reduction to Cr(III) within 100 minutes. ESR experiments revealed that superoxide radicals (˙O₂⁻) and hydroxyl radicals (˙OH) play a crucial role in the degradation process. The excellent catalytic activity of 2MZMs is attributed to their large surface area, high pore volume and size, negative zeta potential, and suitable band structure. Furthermore, this research demonstrates the potential of these cost-effective MZMs, synthesized from readily available materials, for practical applications in treating water pollution caused by organic dyes and heavy metal ions, offering a promising avenue for environmental remediation.

More recently, a study demonstrated the design and fabrication of a novel Zr-MOF by combining In₂S₃ with the Zr-MOF (In₂S₃/Zr-MOF) specifically tailored for both detecting and remediating water pollutants.¹²⁶ This Zr-MOF exhibits exceptional hydrolytic stability and unique structural features, enabling it to function as a highly selective fluorescent sensor for Cr₂O₇²⁻ and CrO₄²⁻ ions, even in complex water samples. The MOF demonstrated impressive detection limits of 3.98 ppb and 5.82 ppb for these ions, with high fluorescence quenching constants, indicating its sensitivity and quantitative detection capability. The mechanism of fluorescence quenching was attributed to competitive absorption of excitation light and coordination of Cr(VI) with the Zr-MOF. Furthermore, the inherent semiconductor properties of the Zr-MOF allowed for effective photochemical decolorization of the reactive dye RB13 and reduction of Cr(VI) under ultraviolet light. To enhance its performance under lower energy light, the researchers fabricated novel type-II heterojunction materials by combining In₂S₃ with the Zr-MOF. The resulting In₂S₃/Zr-MOF composites showed significantly improved photochemical purification efficiency for both Cr(VI) reduction (98.4% reduction) and RB13 dye decontamination, with reaction rate constants substantially higher than those of the individual components.¹²⁶

In 2020, a research reported the development of a series of multifunctional Zr(IV)-porphyrin MOFs, denoted as PCN-222(M) (where M represents various metal centers), for the highly efficient visible-light-assisted photocatalytic reduction of toxic Cr(VI) to less harmful Cr(III) in water.¹²⁷ These Zr-MOFs exhibit remarkable chemical stability in aqueous environments and possess visible-light harvesting capabilities due to the porphyrin linkers. Notably, the metal-free PCN-222 demonstrated unprecedentedly high photocatalytic activity, achieving complete reduction of Cr(VI) within 25 minutes with a rate constant of 0.1289 min⁻¹, surpassing previously reported MOF photocatalysts for this reaction. The superior performance of PCN-222 compared to its metallated counterparts (PCN-222(M)) is attributed to its lower band gap and efficient transfer of photo-excited electrons to the catalytic site, a conclusion supported by both experimental and theoretical investigations. Furthermore, PCN-222 exhibited excellent recyclability, maintaining its catalytic activity over 10 cycles. The study also provides a detailed mechanistic understanding of the photoreduction process through combined experimental and theoretical approaches, highlighting the significant impact of band gap engineering in porphyrin-based MOFs for efficient aqueous-phase Cr(VI) detoxification under visible light irradiation. Further advancing Cr(VI) photoreduction, Nguyen and co-workers in 2024 introduced a novel hydroxyl-functionalized Zr-MOF, HCMUE-1 (Fig. 22).¹²⁸ Addressing limitations of existing MOF photocatalysts such as low Cr(VI) uptake and poor visible light response, the authors successfully prepared and thoroughly characterized HCMUE-1, demonstrating its excellent Cr(VI) photoreduction efficiency, reaching almost complete removal (98% in 90 minutes and nearly 100% in 120 minutes) in a low acidic environment under visible light. Notably, HCMUE-1 maintained its Cr(VI) removal rate over at least seven cycles, indicating high stability and reusability. Further investigations confirmed the structural integrity and surface morphology of HCMUE-1 post-photoreduction, and a plausible photocatalytic mechanism was elucidated through systematic experimental measurements and kinetic modeling.¹²⁸

Fig. 22 Structures of the DUT-52 and HCMUE-1 backbones generated from the Zr₆O₄(OH)₄ SBUs with H₂NDC and H₂NDC(OH), respectively. Reproduced with permission from ref. 128. Copyright 2024 RSC.

Expanding on the versatility of Zr-MOFs in addressing chromium contamination, recent research has yielded exciting advancements in both its detection and removal. In 2023, a research presented the design and synthesis of a novel dual-functional platform, Tb@Zr-MOF, by anchoring terbium(III) (Tb³⁺) ions onto a highly water-stable Zr(IV)-bipyridine MOF.¹²⁹ This modification aimed to simultaneously enhance both the photoluminescence ability and the photocatalytic reduction efficiency of the MOF towards hexavalent chromium (Cr(VI)). The resulting Tb@Zr-MOF exhibits a unique double-enhancement feature due to a significantly promoted photo-induced electron transfer (PET) effect. The anchored Tb³⁺ ions emit a characteristic bright green fluorescence signal via the “antenna effect,” enabling the Tb@Zr-MOF to function as a self-calibrating photochemical sensor for the high-precision fluorescence quenching detection of Cr(VI) ions, achieving a high Stern–Volmer constant (K_sv) and an ultra-low detection limit. Furthermore, the efficient PET process, combined with a suitable energy band structure featuring a sufficiently negative conduction band potential, grants Tb@Zr-MOF excellent capability for the photochemical reduction of Cr(VI) to Cr(III) under UV light, demonstrating a significantly enhanced reduction efficiency compared to the pristine Zr-MOF. DFT and time-dependent DFT (TDDFT) calculations confirmed the highly efficient PET from the organic modules of the Zr-MOF to the anchored Tb³⁺ ions. This work innovatively combines the advantages of porous MOFs with lanthanide metal ion anchoring to provide a promising strategy for improving existing dual-functional MOF platforms for fluorescence sensing and photocatalytic reduction of Cr(VI).¹²⁹ In a separate but complementary approach, 2023 also saw the first report of a novel MOF@polymer system used for photocatalytic Cr(VI) reduction.¹³⁰ This report investigated the development of an integrated system for the removal and detoxification of Cr(VI) from contaminated water sources. The core of the system is a novel composite material, Zr-BDC-(NH₂)₂@PB, created by incorporating a double amino-functionalized UiO-66 MOF into chemically modified, hydrophilic polymeric beads. This composite uniquely functions as both an adsorbent, efficiently capturing Cr(VI) from water, and a photocatalyst, converting the captured Cr(VI) into the less toxic Cr(III) upon light irradiation (Fig. 23). The study demonstrates that the composite exhibits exceptionally high Cr(VI) uptake capacity and selectivity, even in real river water samples containing various competing ions. Furthermore, the researchers designed a simple two-column system where the composite beads are housed and exposed to an external light source to drive the photoreduction process during adsorbent regeneration. This integrated approach successfully reduces Cr(VI) concentrations to below EPA drinking water standards, eliminates the need for post-capture treatment by directly converting Cr(VI) to Cr(III), and allows for the release of Cr(III) for potential recovery or safe disposal.¹³⁰

Fig. 23 Schematic representation of capture and photoreduction of Cr(VI) by Zr-BDC-(NH₂)₂@PB composite. Reproduced with permission from ref. 130. Copyright 2020 RSC.

Moreover, in 2019, Liu and co-workers reported the successful synthesis of a highly stable and luminescent Zr-MOF, JLU-MOF60, constructed from 8-c Zr₆ clusters and tetrakis(4-carboxyphenyl)pyrazine (H₄TCPP) ligand, exhibiting a sqc topology and aggregation-induced emission (AIE) effect.¹³¹ JLU-MOF60 demonstrated remarkable chemical stability across a wide pH range (0–11) in water, which, combined with the presence of Zr-bond hydroxides in its framework, enables high capture capability (149 mg g⁻¹) and a rapid adsorption rate (38 mg g⁻¹ min⁻¹) for Cr₂O₇²⁻ ions in aqueous solutions. Furthermore, leveraging the AIE properties of the H₄TCPP ligand, the MOF can sensitively and selectively detect Cr₂O₇²⁻with a high K_SV of 5.91 × 10⁴ M⁻¹ and a low detection limit of 0.38 μM, and also efficiently photocatalytically reduce Cr(VI) to Cr(III) with a rate constant (κ) of 0.058 min⁻¹. These combined functionalities position JLU-MOF60 as a promising multifunctional material for the effective removal, detection, and photoreduction of Cr₂O₇²⁻in aqueous environments, highlighting the potential of stable and functional MOFs in addressing water contamination challenges. Furthering the understanding of enhanced photocatalytic performance, Feng et al. investigated the impact of defect engineering and surface chemistry on the photocatalytic performance of UiO-66-NH₂ for the reduction of Cr(VI).¹³² The researchers synthesized UiO-66-NH₂ using different zirconium precursors and then treated the samples with titanium substitution. They found that using ZrOCl₂·8H₂O as the zirconium precursor, followed by titanium substitution, resulted in a material (U-OCl-Ti) with significantly enhanced Cr(VI) adsorption (20.9 mg g⁻¹) and photocatalytic reduction under visible light. This improvement is attributed to the creation of a highly defective framework with a higher concentration of surface-exposed –NH₂ groups, leading to increased light absorption and a reduced rate of electron–hole recombination. The findings of this study provide valuable insights into the relationship between defects, surface chemistry, and photocatalytic activity in UiO-66-NH₂, offering guidance for the design of more efficient MOF-based photocatalysts for environmental remediation. In a related development, a 2021 study described the successful synthesis and characterization of Zr-BBI, a novel luminescent zirconium-based MOF (Fig. 24).¹³³ Constructed from eight-connected Zr₆ clusters and a rationally designed four-connected bisimidazole tetracarboxylic acid ligand (H₄BBI), Zr-BBI's high connectivity contributes to its exceptional stability in both acidic and basic aqueous environments. Notably, the inherent bisimidazole units within Zr-BBI impart multifunctionality, enabling both intense fluorescent emission and pH-dependent protonation/deprotonation behavior. Consequently, Zr-BBI exhibits a highly sensitive fluorescence response to varying pH values in aqueous solutions, making it a promising candidate for pH sensing, particularly within the range of 4.6 to 7.12. Furthermore, Zr-BBI demonstrates effective fluorescence-based detection of Cr₂O₇²⁻ at low concentrations, achieving a high K_SV of 6.49 × 104 M⁻¹. Significantly, Zr-BBI also functions as an efficient photocatalyst for the reduction of toxic Cr(VI) to less harmful Cr(III) in aqueous solution under visible light irradiation, with the addition of benzyl alcohol as a hole scavenger further enhancing its photocatalytic efficiency, achieving a reaction rate constant (k) of 0.073 min⁻¹, which surpasses that of many recently reported MOF photocatalysts. The authors conclude that Zr-BBI's combination of high stability, pH sensing capability, Cr(VI) detection, and photocatalytic reduction of Cr(VI) highlights the potential of incorporating specific functional groups within MOF frameworks to create multifunctional materials for environmental applications. Demonstrating the versatility of Zr-MOFs in environmental remediation, Yuan et al. recently reported in 2025 on a novel S-scheme heterojunction, IS-Zr-MOF, synthesized using Zr-bcu-22bipy44dc.¹³⁴ This design, which positions the Zr-MOF's LUMO favorably closer to the valence band of In₂S₃, significantly optimized the material's optoelectronic properties. As a result, the IS-Zr-MOF-2 photocatalyst achieved remarkable efficiency, reducing Cr₂O₇²⁻ by 98.6% in just 30 minutes under a 500 W xenon lamp. Furthermore, it effectively degraded reactive dyes like RB13, RB21, and RR15 when exposed to a 500 W mercury lamp. This work highlights a practical method for developing new bi-functional photocatalysts for effective water environment remediation.

Fig. 24 (a) Structure of H₄BBI ligand. (b) Eight-connected Zr₆ cluster. (c) 3D framework of Zr-BBI. (d) Topological structure of Zr-BBI, where the purple and turquoise polyhedra represent four- and eight-connected nodes, respectively. Reproduced with permission from ref. 133. Copyright 2021 ACS.

Table 3 summarizes the catalytic performance of some reported Zr-MOFs for Cr(VI) photoreduction.

Table 3 Catalytic performance of some Zr-MOFs for Cr(VI) photoreduction

Entry	Photocatalyst	Time/min	Light source	Product yield/efficiency%	Ref.
1	Zr-MOF-2	70	UV light	145.77 mg g^−1/98.05%	116
2	CdIn2S4/MOF-808	30	Xe lamp	16%w/95.45%	115
3	NH2-UiO-66(Zr)/PDI	60	300 W Xe lamp	99.5%	117
4	NU-1400-py	30	UV light	136.43 mg g^−1/97.88%	118
5	EY@Zr-MOF	30	UV light	99.66%	118
6	In2S3/Zr-MOF	70	500 W Xe lamp	97.9%	119
7	In2S3/Zr-MOF	100	Visible light (λ > 420 nm)	96.7%	120
8	PDINH/NH2-UiO-66(Zr)	60	LED light	97%	121
9	MW-ZrT-1-NH2	60	300 W Xe lamp	100%	122
10	UiO-66-NH2(Zr)	120	300 W Xe lamp	98%	123
11	2MZMs	100	Visible light (λ > 420 nm)	91.20%	125
12	In2S3/Zr-MOF	60	Xe lamp	98.4%	126
13	HCMUE-1	120	Visible light (λ > 420 nm)	100%	128
14	Tb@Zr-MOF	60	UV light	97.5%	129
15	PCN-222	25	Visible light (λ > 420 nm)	100%	127
16	JLU-MOF60	10	300 W Xe lamp	149 mg g^−1/98%	131
17	U-OCl-Ti	120	Visible light (λ > 420 nm)	20.9 mg g^−1	132

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6. Photocatalytic degradation of antibiotics

Photocatalytic degradation of antibiotics is a promising technology for removing these persistent pollutants from water.^135,136 It utilizes a photocatalyst material, often a semiconductor like TiO₂ or a MOF, which, upon absorbing light (UV or visible), generates electron–hole pairs.^137–139 These highly reactive charge carriers initiate redox reactions that can break down complex antibiotic molecules into simpler, less harmful substances, and ideally, completely mineralize them into CO₂ and water. The general mechanism involves the photocatalyst absorbing photons with energy greater than its band gap, leading to the excitation of electrons from the valence band to the conduction band, creating electron–hole pairs. These pairs can then migrate to the surface of the photocatalyst where they can react with adsorbed molecules. The photogenerated electrons typically reduce oxygen to superoxide radicals (O₂˙^–), while the holes can oxidize water or hydroxide ions to produce highly reactive hydroxyl radicals (˙OH). Both superoxide and hydroxyl radicals, along with the direct oxidation or reduction by the holes and electrons, respectively, contribute to the degradation of the antibiotics.¹⁴⁰ While photocatalytic degradation offers advantages like the potential use of sunlight, operation under mild conditions, and the ability to completely mineralize pollutants, it also faces limitations. These include the efficiency of the photocatalyst itself, which can be affected by factors like its band gap, surface area, and charge separation capabilities.¹⁴¹ Furthermore, the presence of other substances in the water matrix can interfere with the degradation process. Research is ongoing to develop more efficient photocatalysts, including MOFs, and to optimize reaction conditions to overcome these limitations and make photocatalytic degradation a more widely applicable and sustainable solution for antibiotic removal.^142–144

In 2020, Lou and coworkers investigated the development of a novel 3D flower-on-sheet structured Zr-MOF adsorbent, UiO-67/NSC, for the selective and effective removal of tetracycline (TC) from contaminated water.¹⁴⁵ The synthesis involved using nitrogen and sulfur co-doped carbon dots (NSC) as heterogeneous nucleation sites to grow UiO-67 into a hierarchical structure. This design leverages the functional groups of UiO-67/NSC to specifically recognize TC molecules, while the NSC component facilitates easy separation of adsorbed TC within the UiO-67 framework (Fig. 25). The resulting UiO-67/NSC exhibited excellent TC recognition performance, with a wide sensitive range (0.08–20.0 mg L⁻¹) and a low detection limit (0.063 mg L⁻¹). Notably, the vertically oriented 2D nanosheets in UiO-67/NSC minimized diffusion barriers and enhanced mass transfer, significantly boosting the TC adsorption capacity compared to the parent UiO-67, and reaching 427.35 mg g⁻¹ under optimized conditions. Cytotoxicity tests confirmed the non-toxic nature of UiO-67/NSC towards human cells, indicating its safety for practical applications. The enhanced adsorption capacity is attributed to hydrogen bonding, Lewis acid–base interactions, and π–π interactions between TC and the active sites of UiO-67/NSC. This research demonstrates the potential of UiO-67/NSC as a safe and efficient adsorbent for detecting and removing TC residues, offering a new strategy for designing multifunctional and environmentally friendly MOF-based adsorbents for water purification.¹⁴⁵

Fig. 25 The synthesis of UiO-67/NSC and the possible mechanism of UiO-67/NSC in TC detection and removal. Reproduced with permission from ref. 145. Copyright 2020 Elsevier.

Further expanding the utility of Zr-MOFs in water treatment, Gao et al. investigated the successful synthesis of PCN-134, a mixed-linker Zr-MOF, utilizing a thermodynamic approach.¹⁰ This MOF, designed with a 3D layer-pillar structure, incorporates benzene tribenzoate (BTB) as the primary linker forming 2D layers, which are then pillared by tetrakis(4-carboxyphenyl)porphyrin (TCPP) linkers (Fig. 26a). The researchers systematically controlled the defect density and properties of PCN-134 by varying the TCPP ratios, capitalizing on the ability of Zr-MOFs to tolerate missing linkers while maintaining structural integrity. Among the synthesized PCN-134 variants, PCN30 demonstrated the highest uptake of diclofenac (DF), a representative emerging contaminant, due to an optimized balance between porosity and structural defects. PCN-134 exhibited excellent adsorption performance for DF removal from water, with the adsorption process being influenced by pH and ionic strength, and fitting well with the pseudo-second-order and Langmuir isotherm models, yielding a maximum adsorption capacity of approximately 2.04 mmol g⁻¹. Furthermore, PCN-134 displayed high photocatalytic activity for DF degradation under visible light irradiation, primarily through a type II photosensitization reaction involving the generation of singlet oxygen, with productivity surpassing that of the single-linker PCN-224 (Fig. 26b). The electron transfer between the TCPP linker and Zr₆ clusters also contributed to the photodegradation process. The enhanced performance in both adsorption and photocatalysis is attributed to the MOF's open porous structure, increased defect sites, efficient energy transfer, and the synergistic effect between the two organic linkers, positioning PCN-134 as a promising recyclable photocatalytic adsorbent for wastewater treatment.¹⁰

Fig. 26 (a) Synthetic procedure of PCN-134. (b) Degradation mechanism of DF by PCN-134. Reproduced with permission from ref. 10. Copyright 2019 Elsevier.

In 2021, a porphyrinic Zr-MOF, PCN-224, investigated for the removal of antibiotics, specifically TC and ciprofloxacin (CIP), from water through adsorption and photocatalysis.¹⁴⁶ The researchers synthesized PCN-224 with varying particle sizes and found that the 300 nm particles exhibited the best adsorption performance due to their large surface area (1616 m² g⁻¹), achieving high adsorption capacities of 354.81 mg g⁻¹ for TC and 207.16 mg g⁻¹ for CIP with fast removal rates. The adsorption process followed a pseudo-second-order kinetic model and Langmuir isotherm, indicating homogeneous monolayer chemisorption, and was determined to be exothermic. Furthermore, under visible-light irradiation, the 300 nm-PCN-224 demonstrated significant photocatalytic activity for the degradation of both TC and CIP, with the adsorption occurring via hydrogen bonding, π–π interactions, and electrostatic attraction. Notably, the adsorbent could be easily regenerated through photocatalysis under visible light and maintained over 85% efficiency for adsorption and desorption over five cycles.

Expanding on the capabilities of Zr-MOFs, Hong-Cai Zhou's research group reported the design, synthesis, and application of two novel, chemically stable, and highly porous Zr-MOFs, BUT-12 and BUT-13, for the simultaneous selective detection and removal of antibiotics (nitrofurazone -NZF, and nitrofurantoin -NFT) and organic explosives (2,4,6-trinitrophenol -TNP, and 4-nitrophenol -4-NP) from water.¹⁴⁷ Guided by a topological design approach, the researchers rationally designed two new ligands to construct MOFs with the predicted the-a topology, representing the first examples of this topology among Zr-MOFs. The introduction of methyl groups into the ligands enhanced the fluorescence properties of the resulting MOFs by increasing steric hindrance and reducing non-radiative relaxation pathways. Both MOFs exhibited excellent selective detection capabilities towards the target analytes based on significant fluorescence quenching, achieving detection limits in the parts per billion range, with BUT-13 showing particularly low detection limits for TNP. The high quenching efficiencies are attributed to a combination of electron and energy transfer mechanisms between the MOFs and the guest molecules. Furthermore, both MOFs demonstrated good adsorption abilities towards the selected contaminants, with uptakes for 4-NP and TNP comparable to other reported porous materials. In parallel to these developments, further research in 2023 focused on the synthesis and application of Zr-MOFs for the removal of doxycycline hydrochloride (DOC), a common antibiotic, from wastewater.¹⁴⁸ The Zr-MOFs were synthesized using a solvothermal method and characterized using techniques such as XRD, SEM, FTIR, and TGA. The adsorption performance of the Zr-MOFs for DOC was evaluated, revealing a maximum adsorption capacity of 148.7 mg g⁻¹ within 5 hours. Kinetic studies indicated that the adsorption process followed a pseudo-second-order model, suggesting chemisorption as the rate-limiting step. Isotherm analysis using the Freundlich model suggested that the adsorption of DOC onto the Zr-MOFs occurred via a multilayer mechanism. The study also explored the effect of pH on DOC removal, identifying pH 6 and pH 10 as optimal conditions. In another work, researchers focused on developing an efficient photocatalyst for the removal of the antibiotic TC from the environment, driven by solar energy.¹⁴⁹ The researchers successfully synthesized a novel organic-inorganic composite, BiOBr/UiO-66 nanoplates, using a facile in situ assembly method. This composite material exhibited significantly enhanced photocatalytic degradation and mineralization of TC under visible light irradiation, achieving a degradation activity 2.15 times higher than that of pristine BiOBr, and an 83.84% mineralization rate within 150 minutes when UiO-66 constituted 8% of the composite's mass. Through characterization of the material's morphology, phase structure, optical properties, and electrochemical behavior, the improved performance was attributed to enhanced transfer and separation efficiency of photogenerated electron–hole pairs. Radical scavenging experiments identified superoxide radicals and holes as the primary active species responsible for TC degradation. Furthermore, the study proposed a probable degradation pathway for TC based on high-performance liquid chromatography and LC/MS-MS analysis, and explored the photo-oxidative mechanism using energy band structure measurements. The authors conclude that the synergistic interaction between BiOBr and UiO-66, facilitated by the hydrothermal treatment, leads to improved light absorption, enhanced charge mobility, and reduced electron–hole recombination, ultimately boosting the photocatalytic efficiency of the composite for antibiotic removal and mineralization.

Beyond direct degradation, Zr-MOFs are also being engineered for precise extraction and sensing. Recently, Gao et al. studied the development of a series of multivariate MOFs (MTV-MOFs), denoted as PCN-224-DCDPSx, synthesized via a one-pot solvothermal method using a multiple ligand strategy for the extraction and removal of sulfonamide antibiotics (SAs).¹⁵⁰ By adjusting the doping ratios of medium-tetra(4-carboxylphenyl) porphyrin and 4,4′-dicarboxydiphenyl sulfones, the researchers were able to fine-tune the pore structure and adsorption performance of the MTV-MOFs. The optimized material, PCN-224-DCDPS1.0, exhibited a high specific surface area of 1625 m² g⁻¹. This MOF was then utilized as a sorbent in a dispersive solid-phase extraction method for the extraction and preconcentration of SAs from complex matrices such as water, eggs, and milk, prior to high-performance liquid chromatography analysis. The method achieved low detection limits, ranging from 0.17 to 0.27 ng mL⁻¹, with enrichment factors between 214 and 327. Notably, the adsorption process was rapid, reaching completion within 30 seconds, and the sorbent maintained a recovery rate above 80% over 10 repeated uses. Adsorption capacity studies revealed high capacities for various sulfonamides, ranging from 300 to 621 mg g⁻¹. The adsorption mechanism was attributed to a combination of π–π interactions, hydrogen bonding, and electrostatic interactions between the SAs and the MOF material. Besides, researchers studied the fabrication of a novel, cost-effective Zr-MOF with a dual-ligand composition (MA-MOF) for the efficient removal of TC from the environment.¹⁵¹ The MA-MOF was synthesized using a facile solvothermal method with fumaric acid and 2-aminoterephthalic acid as ligand precursors. A key aspect of the synthesis was the intentional absence of monocarboxylic acid modulators, which typically lead to the formation of single-ligand MOF-801. Instead, this inverted modulator strategy successfully yielded the crystalline double-ligand MA-MOF. The researchers demonstrated that by altering the ratio of the two ligands, they could tune the properties of the MA-MOFs, including surface area, porosity, charge transfer resistance, and energy level positions. The optimized material, MA-MOF2 (with a fumarate to 2-aminoterephthalate molar ratio of 2 : 1), exhibited a total TC removal of 94.6% through a combination of adsorption and photodegradation, significantly outperforming its single-ligand counterparts. Under visible light irradiation, MA-MOF2 generated holes, hydroxyl radicals (˙OH), and superoxide radicals (˙O₂⁻), which synergistically contributed to the effective photodegradation of TC. The authors propose that this innovative inverted modulator strategy can be extended to synthesize other double-ligand MOFs for various applications, including energy storage/conversion, catalysis, and sensing, in addition to environmental remediation.¹⁵¹ In a distinct but equally impactful study, researchers in 2025 developed a sensitive voltammetric sensor for determining trace Lomefloxacin (LOM) residues in milk (Fig. 27).¹⁵² The fabrication of the UiO-66(Zr/Ce)/rGO composite aimed to enhance the electrochemical performance by increasing the electroactive surface area and reducing charge transfer resistance. The synergistic interaction between the electrocatalytically active UiO-66(Zr/Ce) MOFs and the highly conductive rGO nanosheets resulted in a significantly enhanced peak current response for LOM, which was 4.2 times higher than that of a bare glassy carbon electrode (GCE). Consequently, the UiO-66(Zr/Ce)/rGO/GCE sensor exhibited remarkable electrocatalytic activity towards LOM oxidation, demonstrating two broad linear detection ranges (0.03–2.0 μM and 2.0–30 μM), high sensitivities (5.1254 and 0.2198 μM μA⁻¹), and a low detection limit of 5.0 nM. Furthermore, the sensor showed strong anti-interfering capabilities against various potential interfering species and maintained robust voltammetric responses over 25 days. Importantly, the UiO-66(Zr/Ce)/rGO/GCE accurately determined trace LOM residues in liquid milk samples with good recovery rates (96.11–103.2%), indicating its potential for practical food safety applications.

Fig. 27 Schematic preparation route of bimetallic UiO-66(Zr/Ce) MOFs encapsulated rGO nanosheets for voltametric detection of trace LOM residues. Reproduced with permission from ref. 152. Copyright 2025 Elsevier.

Expanding on their utility in environmental cleanup, Zr-MOFs have also been investigated for removing various micropollutants. In 2023, researchers explored the synthesis and application of a zirconium-porphyrin MOF, PCN-134, and its iron (PCN-134(Fe)) and copper (PCN-134(Cu)) derivatives for the removal of six structurally diverse micropollutants present at very low concentrations (100 μg L⁻¹).¹⁵³ The initial adsorption rates of five pharmaceuticals (antipyrine, sulfadimethoxypyrimidine, clofibric acid, diclofenac, and ibuprofen) were significantly higher over PCN-134(Cu) compared to the pristine PCN-134. Specifically, PCN-134(Fe) exhibited extended visible light absorption up to around 750 nm and a transient photocurrent intensity 3.1 times higher than PCN-134(Cu). Both PCN-134 and PCN-134(Fe) were found to be favorable for generating superoxide radicals. Notably, PCN-134(Fe) demonstrated superior degradation efficiency for naproxen, antipyrine, and clofibric acid compared to PCN-134. Quenching experiments and electron paramagnetic resonance confirmed that superoxide radicals and singlet oxygen were the primary reactive oxygen species responsible for the degradation. Addressing other contaminants, Yang and co-workers in 2023 also investigated novel Bi₂MoO₆/NH₂-UiO-66(Zr/Ce) photocatalytic materials with a heterojunction structure for the degradation of oxytetracycline (OTC).¹⁵⁴ The synthesized composites demonstrated superior photocatalytic degradation efficiency compared to their individual components. Notably, the BMNU-2 composite exhibited the highest photodegradation efficiency, removing 93.7% of OTC after 150 minutes of simulated solar irradiation, with a kinetic constant significantly higher than that of NH₂-UiO-66(Zr/Ce) and Bi₂MoO₆ alone. Through intermediate product analysis and theoretical calculations, the researchers identified superoxide radicals (˙O₂⁻), photogenerated holes (h⁺), and hydroxyl radicals (˙OH) as the key active species in the degradation process. The degradation mechanism of OTC was thoroughly elucidated based on energy band theory. Furthermore, toxicity tests revealed a decreasing trend in toxicity as OTC degraded into intermediate products. The enhanced photocatalytic performance of BMNU-2 is attributed to a combination of factors: the incorporation of cerium into the mixed-metal MOF, which narrowed the band gap; the Ce⁴⁺/Ce³⁺ redox cycle, which improved charge separation; and the formation of a heterojunction, which accelerated charge transfer and broadened the light absorption range. This research highlights the effectiveness of this novel heterojunction photocatalyst for OTC degradation and provides insights into the underlying mechanisms and toxicity reduction during the process.¹⁵⁴ Furthermore, an interesting study by Wang and co-workers introduced a novel iron-based MOF, Fe-UiO-68-terNap, for the capture and subsequent solar-light-driven photodegradation of micro-organic pollutants like dyes and personal care products.¹⁵⁵ The material was constructed using a ternaphthalene-based ligand, chosen for its π–π* stacking capabilities to enhance the adsorption of these pollutants from wastewater. The incorporation of iron ions into the UiO-68 framework resulted in the formation of Fe–O–Zr linkages, which significantly improved the charge-separation efficiency within the material, a crucial factor for photocatalytic activity. The synthesized Fe-UiO-68-terNap was thoroughly characterized using single-crystal X-ray diffraction and Fe K-edge XANES, providing detailed insights into its structure at the molecular level. The material demonstrated excellent adsorption properties for the target pollutants and successfully degraded them under simulated solar light irradiation within 240 minutes. The authors attribute this performance to the combined effect of enhanced pollutant adsorption due to the ternaphthalene ligand and improved charge separation facilitated by the Fe–O–Zr units, highlighting the potential of functionalized Zr-based MOFs with tailored pore sizes and active metal centers for effective water pollutant abatement.¹⁵⁵ In parallel with their environmental remediation capabilities, Zr-MOFs are also being explored for antibacterial applications. Researchers developed a highly effective antibacterial photodynamic therapy (aPDT) composite, UiO-66-(SH)₂@TCPP@AgNPs (Fig. 28a).¹⁵⁶ This nanocomposite, approximately 90 nm in size, was created by encapsulating silver nanoparticles (AgNPs) and a porphyrin-based photosensitizer (TCPP) into a UiO-66 framework modified with –SH groups. The combination of AgNPs and TCPP significantly enhanced visible light absorption and reactive oxygen species (ROS) generation, leading to a substantial increase in aPDT efficacy compared to composites with only AgNPs or TCPP. The resulting UiO-66-(SH)₂@TCPP@AgNPs exhibited remarkable antibacterial activity against Gram-negative (E. coli), Gram-positive (S. aureus), and drug-resistant (MRSA) bacteria, with minimal bactericidal concentrations (MBCs) of 19.5, 39, and 19.5 ppm, respectively (Fig. 28b). The strong interaction between the –SH groups and AgNPs effectively prevented AgNP leakage, ensuring biocompatibility and mitigating cytotoxicity. Furthermore, in vitro experiments demonstrated that the composite promoted cell proliferation, highlighting its potential for safe and effective antibacterial applications.

Fig. 28 (a) Synthetic process of UiO-66-(SH)₂@TCPP@AgNPs and (b) mechanism of antibacterial photodynamic therapy. Reproduced with permission from ref. 156. Copyright 2023 ACS.

Addressing the critical need for advanced antibacterial strategies, Ma et al. in 2025 reported the development of a highly effective and stable Zr-MOF (Zr-TSS-1) designed for photocatalytic antibacterial applications.¹⁵⁷ This novel MOF incorporates a photoactive tetrathienylethene-based organic linker, which significantly enhances its ability to harvest visible light and generate free charge carriers when compared to conventional all-carbocyclic Zr-MOFs. To validate its potential as an antibacterial protective material, the researchers created a composite by growing Zr-TSS-1 onto bacterial cellulose (BC) using an in situ method. This composite exhibited near-complete eradication of both Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus within one hour under mild light conditions, maintaining its strong antibacterial efficacy after five cycles of reuse. Furthermore, the material demonstrated high biocompatibility, showing low cytotoxicity towards human skin fibroblasts, suggesting its suitability for various biomedical and healthcare uses. This study successfully showcases the synergistic integration of a deliberately engineered photosensitive porous framework with a sustainable substrate, providing a practical pathway for developing next-generation, high-efficiency antimicrobial technologies. In 2023, a separate study reported the successful development of a novel dual Z-scheme Zr-MOF/Ti-MOF/g-C₃N₄ heterojunction photocatalyst.¹⁵⁸ This material was specifically designed for efficient C–C bond formation via the Gomberg–Buchmann–Hey reaction under visible light. The researchers employed a strategic approach, first synthesizing UiO-66(Zr) microporous MOFs as host structures, and then growing MIL-125(Ti)–NH₂ crystals and g-C₃N₄ nanosheets within these hosts to create the heterojunction. This architecture aimed to enhance charge separation and improve photocatalytic performance. Spectroscopic analysis confirmed the epitaxial growth of UiO-66(Zr) with g-C₃N₄ onto MIL-125(Ti)–NH₂, validating the formation of the composite. Under visible light irradiation (λ > 420 nm), the resulting dual Z-scheme heterojunction exhibited remarkable photocatalytic activity, competing with the best performing photocatalytic systems for this reaction. The enhanced performance is attributed to the synergistic effect of Zr and Ti metals within the MOF-on-MOF structure, along with the g-C₃N₄ nanosheets, which significantly improved the separation of photogenerated electron–hole pairs.¹⁵⁸ Continuing the theme of enhanced photocatalytic activity, Zhang et al. focused on improving the photocatalytic degradation of tetracycline (TC) by developing a novel composite, ZIS/MOF-525.¹⁵⁹ This material combined a zirconium-based porphyrin MOF (MOF-525) with ZnIn₂S₄ (ZIS). Recognizing that the recombination of photogenerated carriers limits the performance of pure MOF-525, the authors introduced ZIS, a metal sulfide semiconductor with a suitable visible band gap, to create a type-II heterojunction. This heterojunction formation, achieved through a solvothermal method, aimed to improve electron transport and suppress electron–hole recombination. The synthesized ZIS/MOF-525 composites, with varying ZIS loadings (ZIS/MOF-525-1, ZIS/MOF-525-2, ZIS/MOF-525-3 and ZIS/MOF-525-4), were characterized using various techniques, confirming the formation of a strong interfacial contact and the desired heterojunction. Notably, the ZIS/MOF-525-3 composite, with 30 mg of MOF-525, exhibited the highest TC degradation efficiency, reaching 93.8% within 60 minutes under visible light irradiation, significantly outperforming pure MOF-525 (37.2%) and ZIS (70.0%). The catalyst also demonstrated excellent stability and recyclability over five cycles. The superior photocatalytic performance of ZIS/MOF-525–3 is attributed to its enhanced structural, optical, and electrochemical properties, which facilitate efficient electron transport and minimize charge carrier recombination. This study highlights the effectiveness of constructing type-II heterojunctions with Zr-PMOFs and metal sulfides for improved photocatalytic degradation of organic pollutants. Further expanding the application scope, researchers explored the development and performance of electrocatalytic membranes composed of UiO-66-X (X = H, NH₂, Br, NO₂) blended with conductive graphite and polyvinylidene fluoride (PVDF) for TC removal from water.¹⁶⁰ This study systematically investigated the impact of ligand substitution in UiO-66 on the membrane's physicochemical and electrochemical properties. They found that ligand substitution, particularly with electron-withdrawing groups like Br and NO₂, led to a reduction in the membrane's molecular weight cutoff (MWCO) and pure water flux due to size exclusion effects. Conversely, the introduction of an electron-donating group (NH₂) enhanced the membrane's anti-fouling ability, using bovine serum albumin (BSA) as a model foulant, but diminished TC removal and self-cleaning properties. In contrast, electron-withdrawing groups (Br and NO₂) promoted electrocatalytic activity and suppressed the oxygen evolution side reaction, resulting in improved TC removal and self-cleaning, despite a decrease in anti-fouling performance. Notably, the UiO-66-NO₂/Graphite/PVDF membrane achieved 99.5% TC removal and demonstrated excellent electrically-driven self-cleaning capabilities over six cycles. The study elucidated that hydroxyl (˙OH) and superoxide (˙O₂⁻) radicals played dominant roles in TC degradation, with three distinct degradation pathways identified through liquid chromatography-mass spectrometry. The electrically-driven self-cleaning mechanism was attributed to the reduced fouling potential of BSA after oxidation degradation. The study concludes that ligand substitution in UiO-66 significantly influences membrane performance, with electron-withdrawing groups enhancing TC removal and self-cleaning, while electron-donating groups improve anti-fouling characteristics. The UiO-66-NO₂/Graphite/PVDF membrane emerged as the most effective, showcasing high TC removal, excellent self-cleaning, and electrochemical/thermal stability under low current density.¹⁶⁰

Complementing the efforts in membrane-based removal, a 2023 study introduced a novel series of hierarchically porous Zr-MOFs, denoted as HP-NU-902-X, synthesized via a modulator-induced defect-guided formation strategy.¹⁶¹ This series was designed for the efficient removal and preconcentration of sulfonamide antibiotics (SAs) from contaminated water and food matrices. By systematically adjusting the concentration of monocarboxylic acid modulator (propionic acid), the researchers precisely tuned the pore size and specific surface area of the HP-NU-902-X series, demonstrating that defect formation significantly enhanced adsorption performance. Notably, HP-NU-902-80 exhibited the highest adsorption efficiency for SAs, achieving low detection limits (0.08–0.25 ng mL⁻¹) and high spiked recoveries (70.34–103.4% for water, 73.83–100.5% for milk). The material demonstrated rapid adsorption kinetics, completely removing SAs within 10 minutes, and followed a pseudo-second-order kinetic model. Langmuir isotherm modeling revealed high maximum adsorption capacities for various SAs, ranging from 279.9 to 467.7 mg g⁻¹, attributed to the high surface area and defect sites within the MOF structure. The adsorption process was primarily driven by van der Waals forces, π–π interactions, and hydrogen bonding between the sorbent and target analytes (Fig. 29).

Fig. 29 Illustration of adsorption mechanisms of HP-NU-902-80. Reproduced with permission from ref. 161. Copyright 2023 Elsevier.

Complementing this adsorption-focused research, Lu and co-workers reported the development of a novel (Zr/Ce)U(NH₂)@CN composite for efficient TC removal from water.¹⁶² This was achieved by integrating co-modified zirconium/cerium-substituted UiO-66 (U(NH₂)) with thin-layered graphitic carbon nitride (g-C₃N₄) to form a Z-scheme heterojunction. Characterization confirmed successful modification, partial Zr–Ce substitution, and Z-scheme formation, which collectively resulted in enhanced light harvesting, improved photogenerated carrier migration, strong redox capability, and a high specific surface area. The g-C₃N₄ acted as a support, facilitating uniform dispersion and reduced particle size of the (Zr/Ce)U(NH₂) component. Consequently, the (Zr/Ce)U(NH₂)@CN composite exhibited a significantly higher TC degradation rate, with a kinetic constant 23.73 times greater than pure UiO-66 and 5.89 times greater than g-C₃N₄. This enhanced photocatalytic performance was attributed to effective electron transfer via the MOF unit (improved by Zr/Ce substitution) and strengthened interfacial photogenerated carrier transfer due to the Z-scheme heterojunction. Hydroxyl radicals (˙OH) were identified as the primary species responsible for TC degradation.¹⁶² These studies underscore the versatility of Zr-MOFs in tackling antibiotic contamination, whether through highly efficient adsorption in hierarchically porous structures or enhanced photocatalytic degradation via tailored composites and heterojunctions. The catalytic performance of some reported Zr-MOFs for antibiotic photoreduction has been further detailed in Table 4.

Table 4 Catalytic performance of some Zr-MOFs for antibiotic photoreduction

Entry	Photocatalyst	Antibiotic	Time/min	Light source	Efficiency	Ref.
1	MOF-801	TCH	90	UV light	94.6%	151
2	UML-2	TCH	60	Xe lamp	91.8%	94
3	(Zr/Ce)U(NH2)@CN	TC	120	Visible light (λ > 420 nm)	97.89%	162
4	UiO-66-NO2/Graphite/PVDF	TCH	180	—	99.5%	160
5	UiO-67/NSC	TC	—	—	427.35 mg g^−1	145
6	PCN-134	DF	5h	500 W Xe lamp	2.04 mmol g^−1/99%	10
7	ZnIn2S4/MOF-525	TC	60	Visible light (λ > 420 nm)	93.8%	159
8	BiOBr/UiO-66	TC	150	Visible light (λ > 420 nm)	83.84%	149
9	PCN-224	TC		500 W Xe lamp	354.81 mg g^−1	146
10	PCN-224	CIP	90	500 W Xe lamp	207.16 mg g^−1	146
11	Bi2MoO6/NH2-UiO-66(Zr/Ce)	OTC	150	350 W Xe lamp	93.7%	154
12	UiO-67/CdS/rGO-1	OFL	30	Simulated sunlight	93.4%	163
13	NH2-UiO-66	Ketorolac Tromethamine (KTC)	60	Solar light	68.3%	164
14	NH2-UiO-66	TC	280	Solar light	71.8%	164
15	Niln2S4/UiO-66	TC	60	Visible light (λ > 420 nm)	90%	165
16	UiO-66-NH2@WO3	TC	60	Visible light (λ > 420 nm)	100%	166
17	CFC/UiO-66-NH2/BiOBr	CIP	120	Visible light (λ > 420 nm)	92%	167
18	CFC/UiO-66-NH2/BiOBr	Levofloxacin (LVFX)	120	Visible light (λ > 420 nm)	86%	166
19	UiO-66-NH2	Metrindazole (MZN)	6h	Xe lamp	—	168
20	Fe-UiO-66	Sulfameter	300	Visible light (λ > 420 nm)	89.9%	169
21	UiO-66@MoS2	Lomefloxacin (LOM)	90	Visible light (λ > 420 nm)	87%	170

---

7. Conclusion

In conclusion, this review has illuminated the significant strides made in leveraging Zr-MOFs for crucial light-driven reactions, specifically CO₂ reduction, hydrogen evolution, and contaminant degradation. These processes are pivotal in addressing the pressing global challenges of climate change, energy scarcity, and environmental pollution, and Zr-MOFs, with their inherent structural advantages, offer a promising platform for advancing these technologies. The critical analysis of recent research has underscored the importance of Zr-MOF structural attributes, such as their exceptional stability, tunable porosity, and versatile chemical functionality, in achieving efficient photocatalytic performance. Strategies such as co-catalyst integration and ligand modification have been shown to be instrumental in optimizing light absorption and charge transfer, thereby enhancing the overall efficacy of these materials. Moreover, the consolidation of mechanistic insights from experimental and computational studies has provided a deeper understanding of the complex reaction pathways involved, paving the way for more rational material design and process optimization. Despite the significant progress, this review has also highlighted key challenges that remain, particularly in the development of highly efficient co-catalysts and the precise elucidation of reaction mechanisms. Addressing these challenges is paramount to unlocking the full potential of Zr-MOFs in light-driven reactions. By stimulating further research in these areas, this review aims to contribute to the development of robust, sustainable, and scalable photocatalytic systems. Ultimately, the successful implementation of Zr-MOF-based technologies will play a crucial role in transitioning towards a green energy economy and mitigating the adverse impacts of environmental pollution, contributing to a more sustainable and prosperous future. The insights presented herein serve as a foundation for future innovations, fostering the continued evolution of Zr-MOFs as a cornerstone of advanced photocatalytic materials.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Author contributions

Anita Abedi, Fataneh Norouzi and Vahid Amani conceived the idea and wrote the outline and final draft. Vahid Amani supervised the project and revised the draft through its final format. All authors revised the draft.

Conflicts of interest

The writers state that they are unaware of any opposing monetary interests or individual relations that may affect the work mentioned in this study.

Acknowledgements

The authors are grateful to the Bowie State University and University of Farhangian for financial support.

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