Flexible metal-organic frameworks (MOF) can show exceptional selectivity and capacity for adsorption of CO2. The incorporation of CO2 into flexible MOFs that have Cu2+ coordination centers and organic pillar ligands is accompanied by a distortion of the framework lattice arising from chemical interactions between these components and CO2 molecules. CO2 adsorption yields a reproducible lattice expansion that is enabled by the rotation of the pillar ligands. The structures of Cu2(pzdc)2(bpy) and Cu2(pzdc)2(bpe), CPL-2 and CPL-5, were evaluated using in situ synchrotron x-ray powder diffraction at room temperature at CO2 gas pressures up to 50 atm. The structural parameters exhibit hysteresis between pressurization and depressurization. The pore volume within CPL-2 and CPL-5 increases at elevated CO2 pressure due to a combination of the pillar ligand rotation and the overall expansion of the lattice. Volumetric CO2 adsorption measurements up to 50 atm reveal adsorption behavior consistent with the structural results, including a rapid uptake of CO2 at low pressure, saturation above 20 atm, and hysteresis evident as a retention of CO2 during depressurization. A significantly greater CO2 uptake is observed in CPL-5 in comparison with predictions based on CO2 pressure-induced expansion of the pore volume available for adsorption, indicating that the flexibility of the CPL structures is a key factor in enhancing adsorption capacity.
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Insights into dual-functional modification for water stability enhancement of mesoporous zirconium metal–organic frameworks
The stability of metal–organic frameworks (MOFs) in water affects their ability to function as chemical catalysts, their capacity as adsorbents for separations in water vapor presence, and their usefulness as recyclable water harvesters. Here, we have examined water stability of four node-modified variants of the mesoporous MOF, NU-1000, namely formate-, Acac-, TFacac-, and Facac-NU-1000, comparing these with node-accessible NU-1000. These NU-1000 variants present ligands grafted to NU-1000's hexa-Zr( iv )-oxy nodes by displacing terminal aqua and hydroxo ligands. Facac-NU-1000, containing the most hydrophobic ligands, showed the greatest water stability, being able to undergo at least 20 water adsorption/desorption cycles without loss of water uptake capacity. Computational studies revealed dual salutary functions of installed Facac ligands: (1) enhancement of framework mechanical stability due to electrostatic interactions; and (2) transformation and shielding of the otherwise highly hydrophilic nodes from H-bonding interactions with free water, presumably leading to weaker channel-stressing capillary forces during water evacuation – consistent with trends in free energies of dehydration across the NU-1000 variants. Water harvesting and hydrolysis of chemical warfare agent simulants were examined to gauge the functional consequences of modification and mechanical stabilization of NU-1000 by Facac ligands. The studies revealed a harvesting capacity of ∼1.1 L of water vapor per gram of Facac-NU-1000 per sorption cycle. They also revealed retention of catalytic MOF activity following 20 water uptake and release cycles. This study provides insights into the basis for node-ligand-engendered stabilization of wide-channel MOFs against collapse during water removal.
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- NSF-PAR ID:
- 10399140
- Date Published:
- Journal Name:
- Journal of Materials Chemistry A
- Volume:
- 10
- Issue:
- 33
- ISSN:
- 2050-7488
- Page Range / eLocation ID:
- 17307 to 17316
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D2TA03851J
