Abstract :
1) The study delves into how defects, pore dimensions, and functionalization influence the heat of CO2 adsorption in zirconium-based MOFs, specifically UiO-66 and UiO-67.
2) Defect-free UiO-66 exhibited stronger exothermic CO2 adsorption compared to its defective counterpart, a trend reversed in UiO-67.
3) Dehydration of UiO-66 and UiO-67 led to a decrease in exothermicity of CO2 adsorption, except for defective UiO-66, which remained unchanged.
4) The study highlights the complex interplay of adsorption interactions, including pore size, hydrogen bonding, and dispersion forces, on CO2 adsorption.
5) Postsynthetic modification with methanol/methoxy groups significantly impacted defective UiO-66, enhancing its CO2 adsorption enthalpy to the highest value observed.

Background:
1) Industry Challenges: The paper addresses the critical issue of CO2 capture for climate change mitigation, focusing on the development of materials that can selectively adsorb CO2 over other gases.
2) Previous Research: Previous studies have explored MOFs for CO2 sequestration, but there is an ongoing search for optimal materials with high exothermic adsorption enthalpies for stable CO2 capture and low energy desorption.
3) Innovations by Authors: The authors propose examining the impact of defects and pore functionalization on CO2 adsorption in UiO-66 and UiO-67, offering insights into the role of pore size and post-synthetic modifications.

Experimental Details:
1) Synthesis of UiO-66 and UiO-67: The authors synthesized MOFs with and without defects using acetic acid or hydrochloric acid as modulators.
2) Activation and Dehydration: Samples were activated at varying temperatures and subjected to dehydration processes to modify the zirconium clusters.
3) CO2 Adsorption Isotherms: Measurements were conducted at different temperatures (278, 283, and 288 K) to determine the adsorption capacity and enthalpy.

Tests and Anlysisi:
1) Surface Area and Pore Size: The specific surface area and pore size of different UiOs were measured by nitrogen adsorption isotherms. For instance, UiO-66-AA has a BET specific surface area of 1725 m^2/g, while UiO-66-HCl has 1600 m^2/g.
2) Enthalpy of CO2 Adsorption: The enthalpy of CO2 adsorption for different UiOs was obtained by fitting the CO2 adsorption isotherms. For example, the enthalpy of adsorption for defect-free UiO-66-AA is -24.4 kJ/mol, while for defective UiO-66-HCl it is -20.9 kJ/mol.
3) Enthalpy of Adsorption for Thermally Dehydrated Samples: After thermal dehydration treatment, the enthalpy of CO2 adsorption for both UiO-66-AA and UiO-66-HCl is -21 kJ/mol, indicating that hydrogen bonding plays a significant role in the enthalpy of adsorption.
4) Enthalpy of Adsorption for Methanol-Modified Samples: The enthalpy of adsorption for MeOH-UiO-66-HCl significantly increased to -28 kJ/mol, which is the highest adsorption enthalpy observed in this study, indicating that methanol modification greatly enhances the adsorption capacity for CO2.
5) Adsorption Entropy: The adsorption entropy (ΔSads) of MeOH-UiO-66-HCl is 18(4) J/molK lower than that of other UiO-66 samples, indicating a more ordered adsorption process.

Conclusion:
1) The study identifies key structural features, such as pore size and hydrogen bonding, that enhance CO2 adsorption in UiO-66 and UiO-67.
2) Defective UiO-66 showed no change in adsorption enthalpy post-dehydration, indicating the importance of hydrogen bonding sites.
3) Postsynthetic modification, particularly in defective UiO-66, resulted in the highest CO2 adsorption enthalpy, suggesting a new avenue for MOF optimization.
4) It would be valuable to investigate the scalability of the post-synthetic modifications and their impact on other gases.

Pore Perfection vs Defect Design: Examining the Complex Relationship between Pore Structure and Carbon Dioxide Adsorption in Zr-Based MOFs
Mason C. Lawrence, Aidan M. Spoel, and Michael J. Katz*
DOI：10.1021/acs.jpcc.4c02334
Link：https://pubs.acs.org/doi/10.1021/acs.jpcc.4c02334