Enhanced CO₂ affinity in a metal–organic framework through green incorporation of a dual-functional amino acid

Abstract

Post-synthetic incorporation of amino acids into the Zr-based metal–organic framework DUT-67 via relatively green methods yields materials with enhanced CO₂ affinity. Structural and sorption studies reveal high affinity for DUT-67-Trp, exploiting its dual functionality to achieve an adsorption enthalpy level comparable to those of high-performing frameworks. The findings provide a promising pathway towards developing sustainable amino-functionalized CO₂ sorbents.

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The increasing environmental concerns about climate change and air pollution have resulted in a constant search for innovative solutions that can reduce greenhouse gas emissions, with carbon dioxide being a top priority.¹ Currently, the most common CO₂ removal processes utilize amine-based materials, including liquid solutions^2,3 as well as porous solids like zeolites,⁴ silica,⁵ and covalent-^6,7 or metal–organic frameworks (MOFs).^8,9 The success of this approach relies on the favourable interactions between CO₂ and the sorbent through formation of carbamate/bicarbonate intermediates.¹⁰ A particularly promising method involves incorporation of alkylamines into MOFs,^11–14 producing solid adsorbents with the potential to capture CO₂ directly from air.¹⁵ This strategy has led to considerable advancements of the field, yet improved amine-based methods still need to be optimized to overcome existing operational limitations in capture performance (capacity and selectivity), recyclability (high regeneration energies) and material stability (amine loss over multiple cycles).^16,17 Another crucial, often overlooked factor in these processes is their detrimental effect on the environment potentially caused by their regular use of hazardous amino species like alkylamines. Integrating sustainable chemicals and practices is essential at all stages of functional material development,¹⁸ particularly in materials intended for environmental applications.

In this context, developing MOF sorbents incorporating amino acids offers a sustainable and efficient approach for material design. Amino acids are inexpensive and abundant chemicals that do not pose the hazards of alkylamines. By coupling the various options in length and basicity of amino acids with the high surface areas, stability and tuneable chemistry of MOFs, the pores and surfaces of the resulting functionalized materials can be precisely tuned and hence provide an excellent opportunity to improve CO₂ affinity.^11,16 Furthermore, the carboxylate group in amino acids enables the amino acids to be directly bound to MOFs through M–O coordination bonding, orienting the amino group towards the pores. This arrangement offers new options compared to alkylamine-modified frameworks, as it provides access to inorganic nodes displaying higher stability, reducing the amine loss during CO₂ capture cycles.

Building on this approach, we report the post-synthetic incorporation of two different amino acids, namely L-proline (Pro) or L-tryptophan (Trp), into the much-studied Zr-based MOF DUT-67. The pristine framework is based on 8-connected Zr₆O₈ nodes (Fig. 1A), utilizing 2,5-thiophenedicarboxylate (TDC) as the organic ligand.¹⁹ Its well-known inorganic cluster combines high stability with tuneable composition, containing unsaturated Zr centres for amino-acid functionalization (Fig. 1B). The resulting frameworks, DUT-67-Pro and DUT-67-Trp, are prepared in ethanol and display improved CO₂ affinity compared to the initial DUT-67, as determined from adsorption studies. Notably, DUT-67-Trp exhibits an isosteric enthalpy of CO₂ adsorption of 50 kJ mol⁻¹, a marked improvement over the value observed for the pristine framework (31.8 kJ mol⁻¹), and comparable to those of high-performance amine-modified MOFs without hazardous reagents and with no chemical degradation after capture. Short- and long-range structural studies indicate an increased CO₂ selectivity through multiple potential interaction sites, uniquely facilitated by the presence of Trp. The results represent an important step towards the design of sustainable amine-based sorbents with enhanced potential for CO₂ capture.

Fig. 1 (A) View of the 8-connected Zr₆ cluster in DUT-67 (left) and the framework of DUT-67, highlighting the different types of pores (right). Color coding: Zr = blue, C = grey, O = red. Dark purple spheres in the Zr₆O₈ nodes indicate the potentially available functionalization positions. H atoms have been omitted for clarity. (B) Schematic of the amino-acid functionalization of the inorganic node in DUT-67 as demonstrated in this work.

Pristine DUT-67 was synthesized in high yields following a previously reported procedure (SI, Section S1).²⁰ As corroborated by powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM) data (Fig. 2A, S1 and S2), the resulting solid displayed high crystallinity and phase purity. In particular, the corresponding PXRD pattern closely matched the simulated one from the crystal structure of DUT-67, while SEM images showed crystallites with dimensions in the range of 0.5–1 μm, in line with previous reports.²¹ Further confirming the identity of this material, ¹H nuclear magnetic resonance (NMR) spectroscopy and thermogravimetric analysis (TGA) revealed the incorporation of 2 formate modulator molecules per Zr₆O₈ node (Fig. S3 and S4). These results translate to a chemical formula of [Zr₆O₆(OH)₂(TDC)₄(HCOO)₂(solvent)₄] for the main framework, also in accordance with previous studies using this modulator.^21,22

Fig. 2 (A) PXRD data for all MOFs of this study. (B) Selected peaks of the ¹H-NMR spectra for all materials after digestion, indicating presence of amino acids in the functionalized analogues. The full spectra can be found in the SI (Fig. S5 and S6). (C) PDF data of DUT-67 and DUT-67-Trp (bottom) and the corresponding dPDF signal (top), confirming tryptophan incorporation. (D) Low-pressure region of the CO₂ isotherms for all MOFs recorded at 273 K (full region shown in Fig. S13). (E) CO₂ isotherms for all MOFs recorded at 298 K. (F) Plots of the corresponding isosteric enthalpies of adsorption, calculated using the Clausius–Clapeyron equation from the 273 and 298 K adsorption data.

In the next step of our investigation, we aimed to functionalize DUT-67 with either Pro or Trp by performing a post-synthetic amino-acid insertion within the unsaturated positions of the Zr₆O₈ nodes (Fig. 1B). This functionalization was performed by carrying out solvothermal incorporation experiments, in which the pristine solid was immersed in a solution of the targeted amino acid under predetermined conditions (SI, Section S2). To optimize the amount of amino acid loaded, a thorough screening of various synthetic parameters was carried out, including of different temperatures, solvents, reaction times and pH levels, as summarized in Table S1. The efficacy of each test was initially assessed by taking ¹H-NMR measurements of the resulting solids, allowing us to quantify the amount of amino acid incorporated (Fig. 2B, S5 and S6). Notably, the highest degree of functionalization for both ligands was afforded by performing reactions in ethanol, a green solvent,²³ thus avoiding the need for hazardous solvents such as dimethylformamide that is typically used to modify DUT-67.^20,22,24 As outlined in Section S2, complete cluster saturation was achieved in DUT-67-Pro, with four Pro molecules inserted per Zr₆O₈ node. In contrast, the greater bulkiness and higher pK_aCOO− of Trp resulted in partial replacement of formates, leading to the incorporation of two Trp molecules per inorganic cluster.

Additional characterization was performed to conclusively determine the identities of the functionalized solids. In both materials, crystallinity and structural integrity of the main framework were maintained during the modification process according to the PXRD data (Fig. 2A, S7 and S8). As anticipated, N₂ adsorption studies at 77 K indicated that the introduction of bulkier ligands reduced the porosity of these MOFs, evidenced by decreased values in uptake, specific surface area, pore size and volume (Fig. S9, S10 and Table S2). Complementary Fourier-transform infrared spectroscopy (FT-IR) studies, detailed in Fig. S11, also revealed characteristic signals associated with the presence of the Zr₆O₈ cluster and amino groups, among others. Both spectra confirmed the absence of uncoordinated –COOH groups, indicating that the amino acids coordinated to the inorganic nodes rather than occupying the pores of the framework.

To further investigate the local structural features within these MOFs, pair distribution function (PDF) analysis was applied to total X-ray scattering data collected for representative materials DUT-67 and DUT-67-Trp. As seen in Fig. 2C and S12, in both cases the PDF showed three main peaks, at approximately 2.2, 3.5 and 5.0 Å, corresponding to Zr–O, Zr⋯Zr and Zr⋯Zr_axial pair correlations within the Zr₆O₈ clusters.²⁵ The impact of the post-synthetic modification was also evaluated by performing differential PDF (dPDF) analysis comparing the signals of the two materials. Key features were identified at 1.4 Å (C–C/C–N distances), 2.4 Å (intramolecular N–H⋯O bonding in Trp²⁶), 3.3 and 3.7 Å (local distortion of the Zr₆O₈ nodes upon Trp replacing the initial non-structural ligands^27,28), confirming amino-acid incorporation within the inorganic clusters.

Having assessed structural composition, CO₂ adsorption measurements were taken for all materials at 273 and 298 K. These measurements allowed us to evaluate the CO₂-capture abilities of these MOFs and to characterize the physicochemical nature of each adsorption process. All materials were activated in vacuo at 100 °C for 15 hours. Experiments with a batch of pristine DUT-67 at 273 and 298 K revealed respective uptakes of 2.99 and 1.60 mmol g⁻¹ at a pressure of 1 bar (Fig. 2E and S13). These values were also found to be in very good agreement with those of other literature studies for this material.²⁹ Unsurprisingly, the incorporation of larger amino acids resulted in a significant decrease in uptake for both functionalized materials due to reduced available pore space. Specifically, uptakes of 1.66 (273 K) and 0.84 (298 K) mmol g⁻¹ were observed for DUT-67-Pro at 1 bar, while the corresponding values for DUT-67-Trp were measured to be 1.73 (273 K) and 0.92 (298 K) mmol g⁻¹ under the same conditions. A more thorough analysis of the isotherm data led to additional observations and conclusions. The adsorption branch for all frameworks showed steeper slopes at low pressure levels (up to 0.2 bar), with the steepness decreasing to various degrees for each MOF as the pressure was increased. These features indicated a distinct response for each material as CO₂ was introduced, pointing to differences between the gas-framework interactions of the different frameworks. Notably, at 273 K the Trp-functionalized material showed a higher uptake and steeper slope at low pressures, namely in the 0–2.5 kPa range, than displayed by the Pro-functionalized material, despite Trp being the bulkiest amino acid, suggesting a strong CO₂ preference. Furthermore, the desorption branch for all materials revealed practically reversible sorption processes, although slight hysteresis was observed, particularly at 298 K and more pronounced for DUT-67-Trp, indicating greater CO₂ affinity due to chemisorption.³⁰

To corroborate these observations, the isosteric enthalpy of adsorption (ΔH_ads) was calculated for each of the materials from the Clausius–Clapeyron equation using the above data. For pristine DUT-67, a moderate −ΔH_ads of 31.8 kJ mol⁻¹ was calculated at zero surface coverage. Both modified frameworks showed better adsorption enthalpies, as per the initial hypothesis—with the respective −ΔH_ads for the proline analogue increased to 38.8 kJ mol⁻¹, and that for the tryptophan-functionalized material considerably increased to 50 kJ mol⁻¹. This result provided evidence for the strong preference of DUT-67-Trp for CO₂, comparable to other high-performing MOFs in the field of CO₂ capture, including amino-functionality–containing materials (Table 1). Importantly, the strength of these gas-framework interactions is not excessively high, hence helping to minimize the energy required to desorb CO₂, and facilitating regeneration of the material. Furthermore, the strong affinity in DUT-67-Trp was achieved without the use of hazardous amines, as environmentally-friendly protocols were employed for both the synthesis of the pristine material and the post-synthetic modifications. PXRD and ¹H-NMR measurements following CO₂ sorption and vacuum treatment (Fig. S14 and S16) also showed that both functionalized materials retained their composition and crystallinity, indicating potential reusability over multiple cycles.

Table 1 Synthetic aspects and CO₂ affinity characteristics in representative high-performing metal–organic frameworks, including DUT-67-Trp

MOF material	−ΔHads (kJ mol^−1)	Features	Ref.
HKUST-1	35	Open metal sites	31
Mg-MOF-74	47	Open metal sites	32
SIFSIX-3-Cu	54	Physisorption due to ultramicroporosity	33
		Use of hazardous azine
CAU-1	48	Amino-containing linker	34
NH2-MIL-101(Cr)	∼50	Open metal sites, amino-containing linker	35
Cu-BTTri-en	90	Open metal sites, amino functionalization	30
		Use of hazardous diamine
Hydrazine-Mg-MOF-74	90	Open metal sites, amino functionalization	36
		Use of hazardous diamine
en-MIL-100(Cr)	80	Open metal sites, amino functionalization	37
mmen-MIL-100(Cr)		Use of hazardous amine
MOF-808-Gly	46	Open metal sites, amino functionalization	16
		Non-hazardous amine, green synthesis
MOF-808-DL-Lys	80	Open metal sites, amino functionalization	16
		Non-hazardous amine, DMSO synthesis
DUT-67-Trp	50	Open metal sites, amino functionalization	This work
		Non-hazardous amine, green synthesis

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To evaluate the CO₂-capture performance of the materials under relatively ambient conditions, dynamic breakthrough experiments were conducted (Fig. 3 and Section S5) for DUT-67-Pro and DUT-67-Trp. Samples of a simulated flue gas mixture of 15% CO₂ and 85% N₂ (20 mL min⁻¹ total flow) with different levels of relative humidity (RH), ranging from 0 to 50%, were tested to assess the effect of water vapor on the adsorption process. The results revealed a clear difference between the breakthrough profiles for DUT-67-Pro and DUT-67-Trp. Initially, both materials exhibited a decrease in uptake with increasing humidity (10% RH). This initial decline may be attributed to competitive adsorption of water, likely via hydrogen-bonding interactions with the amino-acid functional groups. Interestingly, at higher relative humidity (25–50%) the tryptophan-functionalised analogue tended to recover its initial adsorption efficiently, eventually approaching the behaviour observed under dry conditions. This result suggested that once a hydration equilibrium was reached, further water uptake no longer compromised the accessibility of CO₂ adsorption sites in the case of DUT-67-Trp. These findings underscored the effects of amino-acid selection and moisture on the adsorption mechanism, revealing a humidity-tolerant behaviour at elevated RH.

Fig. 3 CO₂ uptake values obtained from dynamic breakthrough experiments for DUT-67-Pro and DUT-67-Trp under various relative humidity (RH) conditions (0%, 10%, 25%, and 50%).

In order to assess the cyclability of the materials, we then tested the DUT-67-Pro and DUT-67-Trp systems over three consecutive breakthrough cycles at 50% RH (Fig. S23). The results indicated a slight drop in adsorption capacity (10% for the Trp derivative and 19% for the Pro system) from cycle 1 to cycle 2, followed by subsequent maintenance of adsorption capacity. The larger drop in adsorption capacity for DUT-67-Pro was consistent with its higher sensitivity to moisture. The collective data from CO₂ affinity, breakthrough and recyclability experiments confirmed the superior stability and performance of the Trp-functionalized material in comparison to the Pro-analogue.

Upon considering the chemical composition of our materials, it became evident that multiple factors could be used to optimize CO₂ binding. The presence of amino groups and unsaturated Zr₆O₈ nodes were indicated to be key driving forces: up to four active metal sites per cluster can be made available in pristine DUT-67 after removal of coordinating solvent molecules. With all these positions occupied by –COO_proline groups in DUT-67-Pro, CO₂ affinity was further enhanced by the incorporated amino groups, as the CO₂ molecules and amino groups reacted under dry conditions to form the well-reported carbamate intermediate (Fig. S17).^10,12,16 DUT-67-Trp combined both elements, with each Zr₆O₈ node featuring two amino acid molecules and at least one open metal site after activation, consistent with its superior performance. Moreover, note formation of dipole-π interactions between the adsorbate and the indole ring of the amino acid appearing to be an important factor for boosting CO₂ adsorption preference in the Trp analogue.³⁸ This formation could explain the marked differences in affinity between the two functionalized MOFs, with DUT-67-Trp outperforming DUT-67-Pro despite having fewer amino groups, lower basicity (pK_aTrp = 9.39 and pK_aPro = 10.36) and a bulkier amino acid. While theoretical calculations are currently underway to validate these observations, the results already highlighted tryptophan as a promising option for the greener design of amino-acid–modified MOF systems with improved CO₂ adsorption potential. For all materials, the adsorption enthalpy values slightly decreased with increasing gas loading, indicating that after initial binding to these preferred locations, CO₂ was subsequently adsorbed onto less reactive sites.

Conclusions

In this study, we successfully demonstrated the post-synthetic incorporation of amino acids into the Zr₆O₈-based DUT-67 framework, resulting in materials with enhanced CO₂ affinity. This methodology underscored a sustainable approach to material design and modification by introducing amino-containing molecules of minimal environmental impact via cleaner methods, addressing relevant concerns associated with typically used amines. Our findings indicated both amino groups and unsaturated Zr centres playing crucial roles in CO₂ binding, leading to increased affinity. Notably, DUT-67-Trp exhibited superior performance, attributed to the formation of dipole-π interactions and the availability of multiple active sites. This material achieved an isosteric enthalpy of adsorption of 50 kJ mol⁻¹, demonstrating a performance comparable to those of other leading CO₂ sorbents while eliminating the need for hazardous molecules. Furthermore, DUT-67-Trp retained a similar adsorption behaviour at high relative humidity levels. On the other hand, the large size of the used amino acids caused a trade-off in performance, leading to reduced CO₂ uptakes in comparison to the pristine MOF. Although this aspect limits the potential of DUT-67 as a suitable platform for practical use, this study has overall increased our understanding of the development of amino-acid–modified MOFs as promising CO₂ sorbent candidates, highlighting several beneficial directions. Future work is expected to aim to improve and expand this approach by utilizing other Zr₆O₈-based frameworks with larger pores to maximize CO₂ capacity. Additionally, investigation of a broader range of amino acids is planned to fully elucidate their chemical effects on adsorption enthalpy.

Author contributions

Conceptualization: E. L. and A. E. P. P. Methodology: E. L., J. A. R. N. and A. E. P. P. Formal analysis: E. L., S. B. S., S. M. B and E. B. M. Data curation: E. L., S. B. S., S. M. B and E. B. M. Project administration: E. L. and A. E. P. P. Funding acquisition: E. L. and A. E. P. P. Writing – original draft: E. L., S. B. S. and A. E. P. P. Writing – review and editing: all co-authors.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5nr03069b.

Acknowledgements

This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 101034324. This work was supported by the grants PID2021-123839OB-I00, PID2023-147972OB-I00, RYC2018-024328-I and CNS2022-135261 funded by MICIU/AEI/10.13039/501100011033 and the NextGenerationEU/PRTR. The authors acknowledge the financial support from the Spanish Ministry of Science and Innovation, through the “María de Maeztu” Programme for Units of Excellence in R&D (CEX2018-000805-M and CEX2023-001316-M). The authors acknowledge the European Synchrotron Radiation Facility (ESRF) for provision of synchrotron radiation facilities (PDF experiments, proposal MA-5852), and the authors would like to thank Dr Stefano Checcia for assistance and support in using beamline ID15A.

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Footnote

† Present address: Institute of Catalysis and Petrochemistry (ICP-CSIC), Universidad Autónoma de Madrid, Campus de Cantoblanco, 28049 Madrid, Spain.

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