Mechanochemical ZIF-9 formation: in situ analysis and photocatalytic enhancement evaluation

Abstract

Efficient treatment of persistent pollutants in wastewater is crucial for sustainable water management and environmental protection. This study addresses this challenge by investigating the mechanochemical synthesis and photocatalytic performance of ZIF-9, a cobalt-based zeolitic imidazolate framework. Using synchrotron-based powder X-ray diffraction, we provide real-time insights into the formation dynamics of ZIF-9 during mechanosynthesis. Our results show that mechanochemically synthesised ZIF-9 exhibits superior photocatalytic activity compared to its solvothermally prepared counterpart, achieving a 2-fold increase in methylene blue degradation rate. This research not only advances our understanding of the synthesis and properties of ZIF-9, but also demonstrates the potential of mechanochemical approaches in the development of high-performance, sustainably produced materials for water treatment and other environmental applications.

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Introduction

Access to clean water and its sustainable management are critical challenges facing contemporary society, particularly amid increasing scarcity and contamination of water resources. Reuse of treated wastewater in agriculture,¹ industry,² and urban supply³ offers a viable strategy to mitigate the global water crisis. However, this requires the effective removal of recalcitrant contaminants, such as pharmaceuticals,⁴ personal care products,⁵ and hazardous industrial chemicals⁶ among others.⁷

Advanced Oxidation Processes (AOPs) have attracted attention in recent years for their effectiveness in degrading pollutants. Among them, photocatalytic reactions stand out due to their potential for complete mineralization of contaminants, through the generation of reactive species, under light irradiation that break down complex organic molecules.⁸ The photo-Fenton process, based on the addition of H₂O₂, has received much attention as one of the most studied photocatalytic AOPs in water treatment, and has traditionally been conducted using homogeneous catalysts like iron salts under light exposure.^9,10 However, it has limitations related to the homogeneous nature of the catalyst, such as difficulties in catalyst recovery and the generation of iron sludge.¹¹ In contrast, heterogeneous photo-Fenton processes, using metal ions immobilized on solid catalysts, offer advantages in recovery and reuse, reducing leaching and thus, secondary pollution.¹² In this context, metal–organic frameworks (MOFs) have emerged as promising materials in heterogeneous photo-Fenton processes, due to their high surface area, tunable porosity, and structural versatility.^13,14 These materials can encapsulate active sites within their frameworks, enhancing pollutant degradation and improving catalyst stability and reusability.^15,16 However, their widespread industrial application has been limited by the high costs and environmental drawbacks associated with conventional synthesis methods.^17,18 They typically involve dissolving precursors in a solvent and heating them under specific pressure and temperature conditions and often require lengthy reaction times, high energy consumption, and the use of toxic solvents, which pose significant economic challenges and environmental and safety concerns.¹⁹ As a result, there is an urgent need for more sustainable and efficient synthesis methods, in line with the principles of green chemistry.^20–22

Mechanochemical synthesis has demonstrated significant advantages, including rapid reaction times, the ability to use insoluble metal sources and the use of solvent-free or solvent-minimal approaches,²² which significantly reduces the environmental footprint and safety hazards associated with traditional methods.^23,24 Additionally, mechanochemical synthesis is typically faster and can be conducted at ambient temperature and pressure, making it more energy-efficient.^25,26 Moreover, mechanochemical reactions can be understood by observing and analyzing them through time resolved in situ monitoring (TRIS).^27,28 By providing continuous, real-time data, this technique allows detailed insights into reaction mechanisms, kinetics and intermediates formed during the process.^27,29 This real-time monitoring uses analytics such as in situ synchrotron-based powder X-ray diffraction (PXRD), Raman spectroscopy, and infrared (IR) spectroscopy or combinations thereof, integrated directly into the mechanochemical setup.^23,30 The advantage of in situ characterisation is its ability to provide integral information on the evolution of the material, allowing real-time optimisation of reaction conditions and improved reproducibility.^27,31

In the last decade, different functional materials, including MOFs, have been synthesized using mechanochemical methods, offering a more sustainable and efficient alternative to conventional approaches.^24,32–34 However, while it is well known that crystallisation can occur rapidly under mechanical treatment,^35,36 the potential impact of this treatment on the physico-chemical properties of the material remains uncertain. This gap in understanding has prompted us to explore how different synthesis methods influence the photocatalytic properties of ZIF-9, which is a member of the zeolitic imidazolate framework (ZIF) family. ZIF-9 has been widely studied for catalytic processes, such as the oxygenation of aromatic compounds and hydrogen production.³⁷ Specifically, we aim to optimize the synthesis conditions and achieve a reproducible mechanochemical ZIF-9 with well-defined structural properties, including the first-ever in situ monitoring investigation of ZIF-9. Finally, to elucidate how these methodologies affect its functional properties, we have evaluated and compared the effects of solvothermal and mechanochemical synthesis on the photocatalytic performance of this MOF by the degradation of methylene blue (MB) through heterogeneous photo-Fenton process under visible light.

Materials and methods

Materials

Cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O, ≥99%) and ammonium sulfate ((NH₄)₂SO₄, ≥99.5%) were purchased from ACS reagents. Benzimidazole (bIm, ≥99%), Cobalt(II) hydroxide (Co(OH)₂, ≥95%) and methylene blue were obtained from Sigma Aldrich. Ammonium chloride (NH₄Cl, ≥99%) was purchased from Fisher Scientific. Ethanol absolute, methanol, hydrogen peroxide, N,N-dimethylformamide, ammonium nitrate (NH₄NO₃, ≥99%) and ammonium hydroxide 25% (NH₄OH) were acquired from ChemSolute.

Characterization of ZIF-9

ZIF-9 was characterized by X-ray diffraction (XRD) using a Bruker D8 Advance X-ray diffractometer. The Fourier transform infrared (FTIR) spectroscopy measurements were performed using a Thermo Nicolet NEXUS 470 FTIR spectrophotometer and TGA thermogravimetry was conducted to study the thermal stability of the samples using a Thermal Analysis System TGA/DSC 3+ equipment under N₂ atmosphere. Scanning Electron Microscopy (SEM) was carried out under a XL30 ESEM. Transmission Electron Microscopy (TEM) images were obtained using a Talos F200S microscope (Thermo Fisher Scientific). A Ceta 16M camera (TEM mode) and a HAADF and BF detector (STEM mode; Scanning Transmission Electron Microscopy) were used to capture the images. A Super-X G2 detector equipped with two silicon drift detectors (SDD) was used during the TEM analysis for Energy Dispersive X-ray Spectroscopy (EDS) to determine the elemental composition of the samples. BET analyses were performed under ASAP from the Micromeritics Instrument Corporation company and the isotherms were measured with nitrogen at 77 K. The specific surface areas ABET were hereby calculated in a relative pressure range between 0.04 ≤ p/p₀ < 0.22 of the isotherms using the multipoint method of Brunauer, Emmett and Teller (BET) with a minimum of five supporting points. In situ X-ray diffraction monitoring was carried out on the μSpot beamline (BESSY II, Helmholtz Centre Berlin for Materials and Energy), using a beam diameter of 100 μm at a photon flux of 1 × 109 s⁻¹ at a ring current of 100 mA. Scattered intensities were collected with a two-dimensional X-ray detector (MarMosaic, CCD 3072 × 3072) and a time-resolution of 15 s. A Shimazdu UC-1900i UV-vis spectrophotometer was used to determine absorbance during the decontamination tests.

Synthesis of ZIF-9

Solvothermal synthesis of ZIF-9. Solvothermal synthesis of ZIF-9 was carried out according to Park et al., 2006 with a slight modification. For ZIF-9, 1 mM of bIm (0.120 g) and 1.4 mM of cobalt nitrate hexahydrate Co(NO₃)₂·6H₂O (0.420 g) were dissolved in 36 mL of dimethylformamide (DMF) stirring at room temperature. The mixture was then poured into a hydrothermal autoclave and placed in an oven at 5 °C min⁻¹ to 130 °C for 48 h, and cooling at 0.4 °C min⁻¹. The samples were centrifuged and washed three times with DMF (3 mL × 3) and let dry at room temperature. Finally, it was placed in a vacuum dryer at 100 °C for 24 h. The final molar ratio used for the synthesis was 1 : 1.4 : 465 for the reagents bIm : Co(NO₃)₂·6H₂O : DMF.

Mechanochemical synthesis of ZIF-9. Several additives (NH₄Cl, NH₄NO₃, NH₄OH, and (NH₄)₂SO₄) and solvents (water, methanol, ethanol and DMF) were evaluated as coadditives to optimize the synthesis of mechanochemical ZIF-9. The amount of additive was 20 mg, except for NH₄NO₃ which was 13 mg, while 200 μL of solvent was used. The highly optimized synthesis of mechanochemical ZIF-9 was achieved by mixing 1 mM of bIm (0.120 g), 0.5 mM of cobalt(II) hydroxide (0.046 g) and 0.09 mM of NH₄Cl (5 mg) with 40 μL of DMF in a 10 mL stainless steel jar with a 9 mm stainless steel ball for horizontal ball milling and an 8 mm stainless steel ball for vertical ball milling. The mixture was milled for 50 minutes at a rate of 30 Hz using a Retsch MM400 shaker-type mixer mill and at 50 Hz using a Fritsch Pulverisette 23 for vertical milling, as schematically shown in Fig. 1. The powder was collected scratching the wall of the jar. Finally, the MOF was washed three times with DMF (3 mL) and dried at room temperature. The final molar ratio used for the synthesis was 1 : 0.5 : 0.09 : 0.5 for the reagents bIm : Co(OH)₂ : NH₄Cl : DMF.

Fig. 1 Schematic representation of the mechanochemical synthesis using horizontal ball milling (a), PXRD patterns for mechanochemical ZIF-9 according to the additive used. Mechanochemical reactions symbols were represented according to Michalchuk et al.³⁸ (b) and according to the solvents using NH₄Cl as additive (c), being Co(OH)₂ peaks represented by stars (★) benzimidazole represented by rhombuses (◆).

For in situ characterization, the mechanochemical synthesis of ZIF-9 was carried out using the same procedure and optimized conditions applied under vertical ball milling but using a custom-designed 2.3 mL PMMA jar consisting in a transparent Persplex center piece of 0.75 mm thickness and two stainless steel pieces as caps, thus allowing the incidence of X-rays on the sample.

Photo-Fenton and stability experiments

A 50 mL reactor was placed under a 24 W visible lamp and on a magnetic stirrer at 800 rpm. The reactor was covered to ensure maximum light incidence. The reactor was connected by a cooling tube to a water bath, which was also connected to a temperature controller to keep it at 25 °C.

To evaluate the photocatalytic properties of the solvothermal and mechanochemical ZIF-9, a MB solution (50 mL, 5 mg L⁻¹) and 0.5 g L⁻¹ of the catalyst were added to the photoreactor and left in the dark for 60 minutes, stirring at 800 rpm and a temperature of 25 °C. This allows the adsorption of MB on the MOF to be measured until equilibrium is reached. H₂O₂ was then added to reach an initial concentration of 10 mM (35 μL). The light was activated, and irradiation was maintained in the reactor for 165 min. Samples were taken every 15 minutes, centrifuged and their absorbance was measured using a UV/Vis spectrophotometer at 664 nm. To avoid loss of catalyst or solution volume and ensure accuracy of results, all samples were returned to the reactor. All experiments were repeated 3 times to ensure reproducibility.

After photo-Fenton catalysis test, the ZIFs were recovered by centrifuging the final solution and let them dry in an oven for 2 h at 50 °C. After that, an XRD spectroscopy analysis was carried out to check the structural stability of the ZIF. The recovered mechanochemical synthetized ZIF-9 was used over three consecutive cycles to ensure the recyclability of the ZIF.

Kinetic study of photo-Fenton process

Photo-Fenton test kinetics were primarily analyzed using a first-order kinetic model of Langmuir–Hinshelwood (L–H), as this model is effectively accounts for both the adsorption of MB and the catalytic degradation under photo-Fenton conditions. This model can be described by the following equation (eqn (1)):³⁹where r is the reaction rate, k_cat is the catalytic rate constant, K is the adsorption equilibrium constant, and C is the concentration of the reactant. On the assumption that due to the low concentration of MB in the tests (5 mg L⁻¹), the term K·C is sufficiently small, such that 1 + K·C ≈ 1, the reaction rate can be approximated as (eqn (2)):

r ≈ k_cat·K·C = kapp·C (2)

where k_app is the apparent first-order rate constant. Integration of this equation reduces the L–H expression and yields a linear relationship between the natural logarithm of concentration and time (eqn (3)). This approximation, provide a reliable description of the reaction kinetics while allowing for a simpler mathematical treatment.where [c] is the concentration of each point, [c₀] is the initial concentration, k is the value of the velocity constant and t is the time.

Photo-Fenton test data were also fitted to zero-order and pseudo-second-order kinetic models [eqn (4) and (5)], included in the ESI† for comparison.

C₀ − C_t = K_o × t (4)

Langmuir, Freundlich, and Temkin desorption kinetics models^40,41 were also evaluated for mechanochemical synthetized ZIF-9 during dark period. The details can be found on the ESI.†

Results and discussion

ZIF-9 was synthesized by solvothermal and mechanochemical routes and characterized using several techniques to compare both materials. The mechanochemical synthesis was optimized through the addition of different additives and solvents (Fig. 1). Powder X-ray diffraction (PXRD) investigations reveal a material with high crystallinity (Fig. S1†). The intensity and position of the reflections match with those of the solvothermal synthesized ZIF-9 corroborating a high crystallinity of the sample in agreement with the simulated PXRD.⁴² In the absence of additives or in presence of NH₄OH or (NH₄)₂SO₄ (Fig. 1b) as additive, the transformation to ZIF-9 was incomplete. Reflections observed at 18.3°, 19.4° and 22.5° correspond to benzimidazole, while those at 19° and 37.9° are associated with Co(OH)_2, indicating that the staring materials have not been fully reacted. In contrast, the use of NH₄NO₃ or NH₄Cl as additives leads to the successful synthesis of ZIF-9. However, the explosive nature of NH₄NO₃ prevents it from being used on a large scale and therefore this additive cannot be used for the synthesis of ZIF-9 on a large scale. Additionally, it was observed that when using NH₄NO₃ as an additive, the amount required for the synthesis cannot be less than 80 μL of DMF solvent and 13 mg (0.16 mmol) of the additive. However, the optimal synthesis conditions for high crystalline ZIF-9 were attained using only 5 mg (0.09 mmol) of NH₄Cl as the additive and 40 μL of DMF as solvent (Fig. S2 and S3†). No reflection from NH₄Cl could be identified in the structure after synthesis due to its coincident re-reflection at 32° and the low amount of additive used.³⁵ Also, under optimal conditions, complete incorporation of the starting materials was not achieved with any organic solvents other than DMF (Fig. 1c), underscoring the reaction's dependence on both solvent and additive. Under these optimized conditions, mechanochemical synthesis of ZIF-9 was successfully reproducible using both vertical and horizontal movement shaker mill (Fig. S4†), and was scaled up to 2.3 g, maintaining high crystallinity (Fig. S5†).

The FTIR spectra of mechanochemical ZIF-9 display the characteristic absorption bands of the ZIF-9 framework, albeit with slightly lower intensity compared to its solvothermal counterpart (Fig. 2a). This suggests that while the mechanochemical method may introduce some structural defects, the overall integrity of the framework is largely preserved. The highest intensity bands are identified with the C–C single bonds at 1236 cm⁻¹, C=C at 1459 cm⁻¹ characteristic of the aromatic rings of the benzimidazole ligand, followed by the C–N bond at 1297 cm⁻¹ and C–H at 736 cm⁻¹. It is also possible to identify the C–C–C bond at 652 cm⁻¹, characteristic of ZIF-9.⁴³ The presence of all key functional groups in the FTIR spectrum confirms that the mechanochemical approach effectively produces the desired ZIF-9 structure. TGA analysis of both mechanochemical and solvothermal ZIF-9 confirm high thermal stability (Fig. 2b), demonstrating stability up to approximately 500 °C, consistent with previous reports.^42,43 The significant weight loss observed in the thermogram of mechanochemical ZIF-9 is attributed to the presence of low amount of DMF molecules in the framework. The nitrogen adsorption isotherms ZIF-9 from both synthesis methods, measured at 77 K, are presented in Fig. S6†. Similar to previous reports, solvothermal synthesized ZIF-9 exhibit a typical Type I isotherm, characteristic of microporous materials, with a steep rise at low relative pressures and an adsorption capacity nearing 70 cm³ g⁻¹ (STP).^44–46 Whereas, mechanochemically synthesized ZIF-9 shows a typical Type IV isotherm characteristic of a mesoporous system and a adsorption capacity around 50 cm³ g⁻¹ (STP), possibly due to structural defects, how it occurs typically during the use of mechanical forces.⁴⁷ BET surface area of solvothermal ZIF-9 was similar to other previous studies,^48,49 being 274.06 m² g⁻¹ for N₂ adsorption at 77 K, and 10.93 m² g⁻¹ for mechanochemical ZIF-9 (Table S1†).

Fig. 2 FTIR spectra for solvothermal synthetized ZIF-9 (black) and mechanochemical synthetized ZIF-9 (blue) (a) and TGA spectra for solvothermal synthesized ZIF-9 (black) and mechanochemical synthetized ZIF-9 (blue) (b).

SEM and TEM images provide a clear depiction of differences in particle size and morphology of mechanochemically synthesized ZIF-9 and its solvothermal counterpart (Fig. 3). The SEM image of solvothermal ZIF-9 illustrates the formation of large, well-defined, and faceted crystals (Fig. 3c), suggesting that the solvothermal process promotes the growth of highly ordered crystalline structures due to the slow and controlled crystallization conditions.⁵⁰ This is further supported by TEM images (Fig. 3f and g), which display a high degree of internal order with uniform electron transparency, indicating fewer defects and particle size around 600 nm. In contrast, SEM image of the mechanochemically synthesized ZIF-9 (Fig. 3b) reveals significantly smaller particles with irregular plate-like shapes. TEM analysis (Fig. 3d and e) confirms reduced particle sizes, around 150 nm, and increased surface area, characteristic of the high-energy milling process in mechanochemistry.^51,52 (Fig. 3a). The TEM images of mechanochemical ZIF-9 also show varying degrees of opacity, indicating a more complex internal structure with potential areas of disorder and defects (Fig. 3d and e).⁵³ In addition, a similar qualitative distribution between solvothermal and mechanochemical ZIF-9 particles was determined by EDS (Fig. S7†).

Fig. 3 Schematic d of the synthesis and differences in the surface area of ZIF-9 by mechanochemical and solvothermal synthesis (a), SEM images for mechanochemical ZIF-9 (b) and solvothermal ZIF-9 (c), and TEM images for mechanochemical ZIF-9 (d and e) and solvothermal ZIF-9 (f and g).

High-resolution synchrotron-based powder X-ray diffraction data were collected as a function of milling time for ZIF-9 to study the real-time reactions pathway by analyzing the phase composition and crystallinity of intermediates and products (Fig. 4). During the initial phase of grinding, series of peaks corresponding to Co(OH)₂ and the benzimidazole ligand are observed. After 15 minutes of grinding, the two characteristic peaks of ZIF-9 appeared at 5.03q/nm⁻¹ and 5.39q/nm⁻¹. Around the 35 minutes, new diffraction peaks begin to appear, coinciding with a decrease in intensity of the initial benzimidazole and Co(OH)₂ related peaks. This shift is indicative of the formation of ZIF-9 as the reaction progresses towards the assembly of the final MOF structure, finishing after 50 minutes. No peaks belonging to the starting materials are observed, demonstrating pure phase formation. Hence, the above in situ investigation reveals that mechanochemical synthesis of ZIF-9 occurs via direct transformation from starting materials to ZIF-9, without the formation of any intermediates.

Fig. 4 Time-resolved in situ X-ray diffraction data obtained during the mechanochemical synthesis of ZIF-9.

Considering the established linear relationship between decreasing particle size and increasing photocatalytic activity in solid catalysts, as reported in the literature,⁵⁴ we observed a significant difference in the particle size of ZIF-9 synthesized via mechanochemical and hydrothermal methods (Fig. 3). This prompted us to investigate the photocatalytic properties of both materials in heterogeneous photo-Fenton test. To ensure that the observed effects in the photocatalysis assays are due to photocatalytic activity, a series of control experiments were conducted to measure the contributions of photolysis and the effect of visible light combined with hydrogen peroxide in the absence of the catalysts (Fig. S8†). Visible light contributed to 25% degradation of MB after 165 minutes, attributed to the oxidized form of MB absorbing light in the visible region and initiating photochemical reactions from singlet or triplet excited states (¹D*, ³D*).^55–57 On the other hand, the addition of H₂O₂ led to a modest increase in MB degradation, attributed to hydroxyl radicals generated from H₂O₂ under irradiation. Adsorption was then evaluated over 60 minutes in dark conditions to ensure stabilization between the MOF and the contaminant (Fig. 5a). Mechanochemical ZIF-9 adsorbed approximately 20% of MB, consistent with its increased surface area, resulting in a significantly higher concentration of the contaminant on the surface compared to solvothermal ZIF-9. Adsorption isotherms models were evaluated based on Langmuir, Freundlich and Temkin models with a high correlation (Fig. S9, Table S2†). These finding align with the results of Taheri et al., 2020, where the mechanochemical ZIF-8 adsorbed 29.1% of MB and 93.9% of rhodamine B (RhB), while its solvothermal counterpart adsorbed only 3.3% of MB and 41.6% of RhB after 2 hours in dark.⁵⁸ We need to consider that since the size of the MB molecules is notoriously higher than the pore delimiting windows of ZIF-9, albeit its flexibility, it is then expected that the observed molecular capture occurs in the external surface. The low particle size of mechanochemically synthesized ZIF-9, provides more active sites for the adsorption of reactants and subsequent photocatalytic reactions.^54,59 We should note that particle size can be also a factor in favoring external surface area, once smaller particles have higher surface volume ratios.⁵² As a result, the degradation of the pollutant by mechanochemical ZIF-9 was significantly higher than that by the solvothermal method (Fig. 5a).

Fig. 5 Degradation of MB by heterogeneous photo-Fenton process using solvothermal synthesized ZIF-9 (■) and mechanochemical synthesized ZIF-9 (◆) as catalyst (a), velocity of MB degradation for solvothermal ZIF-9 (black) and mechanochemical ZIF-9 (blue) (b), UV-vis spectra for solvothermal ZIF-9 (black) and mechanochemical ZIF-9 (blue) (c) and X-ray diffraction patterns for solvothermal ZIF-9 (black) and mechanochemical ZIF-9 (blue) before and after reaction.

Solvothermal synthetized ZIF-9 exhibited low photocatalytic activity, with only a slight decrease in MB concentration over time, achieving 50% of MB degradation. This suggests that solvothermal ZIF-9 lacks significant photocatalytic properties under the experimental conditions. In contrast, mechanochemical ZIF-9 demonstrated a first-order kinetics constant twice as high, with values of 0.0061 min⁻¹ for solvothermal ZIF-9 and 0.0158 min⁻¹ for mechanochemical ZIF-9, respectively (Fig. 5b). Zero-order and pseudo-second order kinetics model were also analyzed, being mechanochemical ZIF-9 two times faster in all the adjustments (Fig. S10†). The absence of significant differences in the UV-vis absorption spectra (Fig. 5c), with a maximum absorption peak at 600 nm, suggests that the enhanced photocatalytic performance of mechanochemical ZIF-9 is not due to increased light absorption or band gap reduction. This means that there are rather other factors, such as particle size reduction and plates-shaped particle morphology, that significantly influence the photocatalytic process. The availability of more active sites is crucial to improve the efficiency of charge carrier separation and enhance the interaction between the MOF and the target molecules, which directly contributes to the increased photocatalytic activity.⁶⁰ Amano et al., reported similar findings, showing that the increased thickness of platelet-shaped Bi₂WO₆ particles was directly correlated with greater surface area, which in turn was proportional to photocatalytic activity.⁶¹ As previously discussed, TEM images of mechanochemically synthesized ZIF-9 (Fig. 3d and e) suggest the presence of defects. These defects, combined with the high surface area and smaller particle size, can serve as trapping sites for photogenerated electrons and holes, reducing the rate of recombination and thereby prolonging the lifetime of charge carriers and generating more reactive oxygen species (ROS), thus increasing the photocatalytic activity of MOFs.⁶² To ensure the accuracy and reproducibility of the results, photo-Fenton assays were repeated three times, resulting in a maximum error of 8% for the tests performed with the solvothermal ZIF-9 and only 2% for the tests performed with the mechanochemically synthesized ZIF-9 (Fig. 5a). XRD analysis of the mechanochemical and solvothermal ZIF-9 after the photo-Fenton tests (Fig. 5d) shows that the overall structure of the ZIFs remains intact, with characteristic diffraction peaks preserved. While a slight reduction in peak intensity is observed, indicating a minor decrease in crystallinity, the structural integrity is not compromised. Finally, mechanochemical synthetized ZIF-9 was used over three consecutive cycles without significant loss of photocatalytic activity, following a first order kinetic of 0.0158 min⁻¹, 0.0149 min⁻¹, and 0.0142 min⁻¹ for cycles 1, 2, and 3, respectively (Fig. S11†). This demonstrates that mechanochemically synthesized ZIFs are as stable as their solvothermal counterparts, making them equally viable for photocatalytic applications, and highlighting their potential for sustainable, efficient catalyst production.

Conclusions

The mechanochemical synthesis of ZIF-9 offers significant advantages over traditional solvothermal methods, particularly in enhancing photocatalytic performance. In situ XRD monitoring provided valuable insights into the rapid, one-step reaction mechanism, allowing for optimised synthesis conditions. The mechanochemically prepared ZIF-9 exhibited twice the photocatalytic activity of its solvothermal counterpart, due to higher defect density and smaller particle size. These results highlight mechanochemistry as a promising, sustainable approach for the synthesis of high-performance MOFs. In addition to improving existing properties, this method opens up new avenues for tailoring MOF properties, which is particularly beneficial for applications such as water treatment. Our research highlights the potential of mechanochemical synthesis to advance MOF design and expand their applications in catalysis and environmental remediation, paving the way for further innovation in the field.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

Experimental investigations were carried out by NRS, CP and RB. BB and FE conceived, designed, and supervised the project. NRS prepared the manuscript, which was discussed and agreed by MB, ARR, CP, RB, BB, and FE.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank BESSY II (Helmholtz Zentrum Berlin) for beam time and to Dominik Al-Sabbagh for BESSY data manipulation and computational resources. Also thank to Ines Feldmann for SEM images.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4mr00114a

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