Control of the pore chemistry in metal-organic frameworks for efficient adsorption of benzene and separation of benzene/cyclohexane

Abstract

•A high uptake of benzene up to 3.92 mmol/g at 1.2 mbar and 298 K•A high selectivity of 166 for an equal volume benzene/cyclohexane (v/v = 1/1) mixture•Analysis of binding mechanism of benzene and cyclohexane within the porous materials•Direct visualization of reversible binding of benzene at an open Cu(II) site Benzene is an important volatile organic compound (VOC) with high prevalence and toxicity, which poses a serious threat to human health. The separation of benzene and cyclohexane is vital for the production of high-purity cyclohexane in petrochemical industries. Conventional adsorbents such as activated carbons and zeolites often suffer from structural disorder, restricting the visualization of binding sites and host-guest interactions. Herein, we report a comprehensive study of adsorption of benzene and cyclohexane in a series of ultra-stable metal-organic framework materials. We demonstrate that the precise control of pore size and pore chemistry provides a powerful strategy to enhance adsorption affinity and selectivity of benzene vs cyclohexane, even in the presence of water. We found that single-atom sites anchored within the pores promote high adsorption of benzene at low pressure. Benzene is an important air pollutant and a key chemical feedstock for the synthesis of cyclohexane. Because of the small difference of 0.6°C in their boiling points, the separation of benzene and cyclohexane is extremely challenging. Here, we report the high adsorption of benzene at low pressure and efficient separation of benzene/cyclohexane, achieved by the control of pore chemistry of two families of robust metal-organic frameworks, UiO-66 and MFM-300. At 298 K, UiO-66-CuII shows an exceptional adsorption of benzene of 3.92 mmol g−1 at 1.2 mbar and MFM-300(Sc) exhibits a high selectivity of 166 for the separation of benzene/cyclohexane (v/v = 1/1) mixture. In situ synchrotron X-ray diffraction and neutron powder diffraction, and multiple spectroscopic techniques reveal the binding mechanisms of benzene and cyclohexane in these materials. We also report the first example of direct visualization of reversible binding of benzene at an open Cu(II) site within metal-organic frameworks. Benzene is an important air pollutant and a key chemical feedstock for the synthesis of cyclohexane. Because of the small difference of 0.6°C in their boiling points, the separation of benzene and cyclohexane is extremely challenging. Here, we report the high adsorption of benzene at low pressure and efficient separation of benzene/cyclohexane, achieved by the control of pore chemistry of two families of robust metal-organic frameworks, UiO-66 and MFM-300. At 298 K, UiO-66-CuII shows an exceptional adsorption of benzene of 3.92 mmol g−1 at 1.2 mbar and MFM-300(Sc) exhibits a high selectivity of 166 for the separation of benzene/cyclohexane (v/v = 1/1) mixture. In situ synchrotron X-ray diffraction and neutron powder diffraction, and multiple spectroscopic techniques reveal the binding mechanisms of benzene and cyclohexane in these materials. We also report the first example of direct visualization of reversible binding of benzene at an open Cu(II) site within metal-organic frameworks. Volatile organic compounds (VOCs) are indoor air pollutants showing increasing emissions from anthropogenic activities. They cause many environmental problems and are linked with millions of premature deaths every year.1McDonald B.C. De Gouw J.A. Gilman J.B. Jathar S.H. Akherati A. Cappa C.D. Jimenez J.L. Lee-Taylor J. Hayes P.L. McKeen S.A. et al.Volatile chemical products emerging as largest petrochemical source of urban organic emissions.Science. 2018; 359: 760-764https://doi.org/10.1126/science.aaq0524Crossref PubMed Scopus (517) Google Scholar,2Chen W.Y. Jiang X. Lai S.N. Peroulis D. Stanciu L. 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Chen W. Xiao Y. Zhang H.-F. Dai B.-L. Chen S.-H. Wu X.-D. Li M. Huang X.-C. Harnessing shape complementarity for upgraded cyclohexane purification through adaptive bottlenecked pores in an imidazole-containing MOF.Angew. Chem. Int. Ed. Engl. 2021; 60: 23590-23595https://doi.org/10.1002/anie.202109964Crossref PubMed Scopus (7) Google Scholar However, the optimization of pore chemistry of MOFs to achieve strong yet reversible adsorption of benzene at low pressure (p <1.2 mbar) or in the presence of cyclohexane remains a significant challenge to date, and the direct observation of host-guest interactions has only been achieved in exceptional cases, hindering the design of new efficient sorbent materials. Herein, we report the designed fine-tuning of pore chemistry in two families of robust MOFs, namely MFM-300 and UiO-66, to deliver high adsorption of benzene at low pressure and efficient separation of benzene/cyclohexane in liquid phase, even in the presence of water. Decoration of the structural defects (missing linkers) in UiO-66-defect with atomically dispersed Cu(II) sites results in an exceptional and reversible adsorption of benzene of 3.92 mmol g−1 at 1.2 mbar and 298 K in UiO-66-CuII, demonstrating its potential for benzene capture. Refinements of the pore size in MFM-300(M) (M = Sc, VⅢ, Cr, Fe, Al, Ga, In) by variation of the size of metal centers but with identical pores decorated with phenyl rings and bridging M–OH–M groups results in a high selectivity [up to 166 for MFM-300(Sc)] for the separation of benzene/cyclohexane (v/v = 1/1) as well as high adsorption of benzene of 3.02 mmol g−1 at 1.2 mbar and 298 K in MFM-300(Sc). A combination of in situ synchrotron X-ray powder diffraction (SXPD) and neutron powder diffraction (NPD), Fourier transformed infrared microspectroscopy (FTIR), solid-state nuclear magnetic resonance (ssNMR) and electron paramagnetic resonance (EPR) spectroscopies has elucidated the mechanism of binding and the basis for the observed preferential adsorption of benzene versus cyclohexane. We report the direct visualization of reversible binding of benzene to the open metal Cu(II) site in UiO-66-CuII. The variation of pore size in MFM-300 has a notable impact on the binding and packing of benzene and cyclohexane molecules, resulting in their separation. This study provides key insights into the design of new MOF materials for the capture of trace benzene and low-carbon purification of cyclohexane in chemical industry. Seven iso-structural MOFs, MFM-300(M) (M = Sc, VⅢ, Cr, Fe, Al, Ga, In)36Guo L. Han X. Ma Y. Li J. Lu W. Li W. Lee D. Da Silva I. Cheng Y. Rudić S. Manuel P. High capacity ammonia adsorption in a robust metal-organic framework mediated by reversible host–guest interactions.Chem. Commun. (Camb). 2022; 58: 5753-5756https://doi.org/10.1039/D2CC01197BCrossref PubMed Google Scholar,37Han X. Godfrey H.G.W. Briggs L. Davies A.J. Cheng Y. Daemen L.L. Sheveleva A.M. Tuna F. McInnes E.J.L. Sun J. et al.Reversible adsorption of nitrogen dioxide within a robust porous metal-organic framework.Nat. Mater. 2018; 17: 691-696https://doi.org/10.1038/s41563-018-0104-7Crossref PubMed Scopus (117) Google Scholar,38Luo T. Li L. Chen Y. An J. Liu C. Yan Z. Carter J.H. Han X. Sheveleva A.M. Tuna F. et al.Construction of C-C bonds via photoreductive coupling of ketones and aldehydes in the metal-organic-framework MFM-300(Cr).Nat. Commun. 2021; 12: 3583https://doi.org/10.1038/s41467-021-23302-wCrossref PubMed Scopus (13) Google Scholar and four UiO-66 materials (UiO-66, UiO-66-defect, UiO-66-CuII, and UiO-66-CuI)39Abdel-Mageed A.M. Rungtaweevoranit B. Parlinska-Wojtan M. Pei X. Yaghi O.M. Behm R.J. Highly active and stable single-atom Cu catalysts supported by a metal–organic framework.J. Am. Chem. Soc. 2019; 141: 5201-5210https://doi.org/10.1021/jacs.8b11386Crossref PubMed Scopus (258) Google Scholar,40Ma Y. Han X. Xu S. Wang Z. Li W. Da Silva I. Chansai S. Lee D. Zou Y. Nikiel M. et al.Atomically dispersed copper sites in a metal–organic framework for reduction of nitrogen dioxide.J. Am. Chem. Soc. 2021; 143: 10977-10985https://doi.org/10.1021/jacs.1c03036Crossref PubMed Scopus (30) Google Scholar,41Ma Y. Lu W. Han X. Chen Y. Da Silva I. Lee D. Sheveleva A.M. Wang Z. Li J. Li W. Fan M. Direct observation of ammonia storage in Uio-66 incorporating Cu(II) binding sites.J. Am. Chem. 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Interfaces. 2021; 13: 29137-29149https://doi.org/10.1021/acsami.1c06003Crossref PubMed Scopus (34) Google Scholar Additionally, UiO-66 and MFM-300 materials provide an excellent platform to tailor the pore interior and pore size, respectively, to identify the optimal pore environments for adsorption of benzene at low concentration or in the presence of cyclohexane. MFM-300(M) (M = Sc, VⅢ, Cr, Fe, Al, Ga, In) are composed of one-dimensional (1D) square-shaped channels of well-defined sizes (8.1, 6.7, 7.7, 6.8, 6.5, 6.7, 7.4 Å, respectively) with bridging hydroxyl groups pointing directly into the pores. Compared to defect-free UiO-66, UiO-66-defect contains –OH/–OH2 defect sites on the {Zr6} clusters resulting from missing carboxylate linkers, with approximately one missing ligand per {Zr6} cluster. UiO-66-CuII and UiO-66-CuI can be obtained via post-synthetic modification by covalently attaching Cu(Ⅱ) and Cu(Ⅰ) ions to the defect sites, respectively. The phase purity and stability toward air and water of these MOFs are confirmed by powder X-ray diffraction (PXRD) and thermogravimetric analysis (TGA) (Figures S1 and S2). The presence of Cu(I) and Cu (II) sites has been confirmed39Abdel-Mageed A.M. Rungtaweevoranit B. Parlinska-Wojtan M. Pei X. Yaghi O.M. Behm R.J. Highly active and stable single-atom Cu catalysts supported by a metal–organic framework.J. Am. Chem. Soc. 2019; 141: 5201-5210https://doi.org/10.1021/jacs.8b11386Crossref PubMed Scopus (258) Google Scholar (Figure S3). Porosity data for these materials are comparable to those reported previously,36Guo L. Han X. Ma Y. Li J. Lu W. Li W. Lee D. Da Silva I. Cheng Y. Rudić S. Manuel P. High capacity ammonia adsorption in a robust metal-organic framework mediated by reversible host–guest interactions.Chem. Commun. (Camb). 2022; 58: 5753-5756https://doi.org/10.1039/D2CC01197BCrossref PubMed Google Scholar,37Han X. Godfrey H.G.W. Briggs L. Davies A.J. Cheng Y. Daemen L.L. Sheveleva A.M. Tuna F. McInnes E.J.L. Sun J. et al.Reversible adsorption of nitrogen dioxide within a robust porous metal-organic framework.Nat. Mater. 2018; 17: 691-696https://doi.org/10.1038/s41563-018-0104-7Crossref PubMed Scopus (117) Google Scholar,38Luo T. Li L. Chen Y. An J. Liu C. Yan Z. Carter J.H. Han X. Sheveleva A.M. Tuna F. et al.Construction of C-C bonds via photoreductive coupling of ketones and aldehydes in the metal-organic-framework MFM-300(Cr).Nat. Commun. 2021; 12: 3583https://doi.org/10.1038/s41467-021-23302-wCrossref PubMed Scopus (13) Google Scholar,39Abdel-Mageed A.M. Rungtaweevoranit B. Parlinska-Wojtan M. Pei X. Yaghi O.M. Behm R.J. Highly active and stable single-atom Cu catalysts supported by a metal–organic framework.J. Am. Chem. Soc. 2019; 141: 5201-5210https://doi.org/10.1021/jacs.8b11386Crossref PubMed Scopus (258) Google Scholar,40Ma Y. Han X. Xu S. Wang Z. Li W. Da Silva I. Chansai S. Lee D. Zou Y. Nikiel M. et al.Atomically dispersed copper sites in a metal–organic framework for reduction of nitrogen dioxide.J. Am. Chem. Soc. 2021; 143: 10977-10985https://doi.org/10.1021/jacs.1c03036Crossref PubMed Scopus (30) Google Scholar,41Ma Y. Lu W. Han X. Chen Y. Da Silva I. Lee D. Sheveleva A.M. Wang Z. Li J. Li W. Fan M. Direct observation of ammonia storage in Uio-66 incorporating Cu(II) binding sites.J. Am. Chem. Soc. 2022; 144: 8624-8632https://doi.org/10.1021/jacs.2c00952Crossref PubMed Scopus (5) Google Scholar confirming the complete activation and phase purity of these materials (Figure S4). Single-component adsorption isotherms of benzene and cyclohexane have been recorded for these porous materials for P/P0 up to 0.9 at 298–323 K (Figures 1A–1F, S5, and S6). All isotherms show characteristic Type-Ⅰ profiles with higher uptakes for benzene than cyclohexane. However, the isotherm for cyclohexane adsorption in MFM-300(In) corresponds to a Type-Ⅳ isotherm likely due to a small degree of structural flexibility of the host material. This has been observed previously in the structural analysis of p-xylene loaded MFM-300(In).19Li X. Wang J. Bai N. Zhang X. Han X. Da Silva I. Morris C.G. Xu S. Wilary D.M. Sun Y. et al.Refinement of pore size at sub-angstrom precision in robust metal–organic frameworks for separation of xylenes.Nat. Commun. 2020; 11: 4280https://doi.org/10.1038/s41467-020-17640-4Crossref PubMed Scopus (38) Google Scholar Adsorption of benzene in these MOFs displays rapid kinetics and equilibrium is reached within several minutes (Figure S7). Especially noteworthy is the sharp increase in the uptake of benzene at low pressure with saturation reached at P/P0 of ∼0.02, demonstrating the potential of these MOFs for capturing trace benzene (Figures 1B and 1E). These MOFs exhibit high benzene uptakes at 298 K and 1.2 mbar and compare favorably with other reported sorbents for benzene (Figure 1G). All benzene uptakes discussed below refer to data collected at 298 K and 1.2 mbar. MFM-300(Sc) with the largest pore size (8.1 Å) displays the highest uptake (3.02 mmol g−1) among the seven MFM-300 materials (2.28–3.02 mmol g−1) (Figure 1H), with the number of adsorbed benzene molecules per metal showing a partial correlation with the pore size (Figure S8). UiO-66-defect exhibits a higher uptake (2.55 mmol g−1) than UiO-66 (1.78 mmol g−1), which can be attributed to the presence of defect sites in the former as well as its slightly higher porosity. Importantly, UiO-66-CuⅡ shows an exceptional uptake of benzene of 3.92 mmol g−1, an enhancement of 220% and 154% compared with UiO-66 and UiO-66-defect, respectively. This uptake is only slightly lower than that of the benchmark BUT-54 (4.31 mmol g−1) under the same conditions.15He T. Kong X.-J. Bian Z.-X. Zhang Y.-Z. Si G.-R. Xie L.-H. Wu X.-Q. Huang H. Chang Z. Bu X.-H. et al.Trace removal of benzene vapour using double-walled metal–dipyrazolate frameworks.Nat. Mater. 2022; 21: 689-695https://doi.org/10.1038/s41563-022-01237-xCrossref PubMed Scopus (29) Google Scholar Single-atom systems incorporating atomically dispersed metal sites have become a major focus in catalysis due to their unique electronic properties, active metal sites, and high atom efficiency.43Mitchell S. Pérez-Ramírez J. Single atom catalysis: a decade of stunning progress and the promise for a bright future.Nat. Commun. 2020; 11: 4302https://doi.org/10.1038/s41467-020-18182-5Crossref PubMed Scopus (100) Google Scholar However, they remain understudied for applications in gas adsorption. The introduction of atomically dispersed Cu(II) sites significantly enhances the adsorption of benzene in UiO-66-CuⅡ, creating a new avenue to improve the adsorption of benzene at low pressure in MOFs. In contrast, UiO-66-CuI, obtained by reduction of UiO-66-CuⅡ, presents a much lower uptake of benzene (2.74 mmol g−1) owing partly to the weaker Lewis acidity of Cu(Ⅰ) sites.25Mukherjee S. Manna B. Desai A.V. Yin Y. Krishna R. Babarao R. Ghosh S.K. Harnessing Lewis acidic open metal sites of metal–organic frameworks: the foremost route to achieve highly selective benzene sorption over cyclohexane.Chem. Commun. (Camb). 2016; 52: 8215-8218https://doi.org/10.1039/C6CC03015GCrossref PubMed Google Scholar All eleven MOFs show lower uptakes for cyclohexane (1.27–2.46 mmol g−1 at 298 K and 1.3 mbar) than benzene, suggesting weaker binding interactions of cyclohexane in these MOFs. All cyclohexane uptakes discussed below refer to data collected at 298 K and 1.3 mbar. The relationship between cyclohexane uptake and pore size of MFM-300 is similar to that observed for benzene, with MFM-300(Sc) exhibiting the highest uptake (2.32 mmol g−1). Also, UiO-66-defect shows a higher uptake (2.10 mmol g−1) than UiO-66 (1.38 mmol g−1). Unlike benzene, the cyclohexane uptakes of UiO-66-CuⅡ (2.46 mmol g−1) and UiO-66-CuI (1.47 mmol g−1) do not show obvious superiority to pristine UiO-66 or UiO-66-defect, suggesting that Cu(II) or Cu(Ⅰ) sites display, perhaps not unexpectedly, poor affinity to cyclohexane. The isosteric heats of adsorption (Qst) were calculated from the isotherms measured at different temperatures (Figures S9 and S10). The values for Qst for these materials show a similar trend of increase at high loading owing to strong guest-guest interactions, whereas at low loadings, the host-guest interactions are dom