Enhancing CO2 Separation Ability of a Metal–Organic Framework by Post‐Synthetic Ligand Exchange with Flexible Aliphatic Carboxylates

Abstract A series of porous metal–organic frameworks having flexible carboxylic acid pendants in their pores (UiO‐66‐AD n : n =4, 6, 8, and 10, where n denotes the number of carbons in a pendant) has been synthesized by post‐synthetic ligand exchange of terephthalate in UiO‐66 with a series of alkanedioic acids (HO 2 C(CH 2 ) n −2 CO 2 H). NMR, IR, PXRD, TEM, and mass spectral data have suggested that a terephthalate linker in UiO‐66 was substituted by two alkanedioate moieties, resulting in free carboxyl pendants in the pores. When post‐synthetically modified UiO‐66 was partially digested by adjusting the amount of added HF/sample, NMR spectra indicated that the ratio of alkanedioic acid/terephthalic acid was increased with smaller amounts of acid, implying that the ligand substitution proceeded from the outer layer of the particles. Gas sorption studies indicated that the surface areas and the pore volumes of all UiO‐66‐AD n s were decreased compared to those of UiO‐66, and that the CO 2 adsorption capacities of UiO‐66‐AD n ( n =4, 8) were similar to that of UiO‐66. In the case of UiO‐66‐AD6, the CO 2 uptake capacity was 34 % higher at 298 K and 58 % higher at 323 K compared to those of UiO‐66. It was elucidated by thermodynamic calculations that the introduction of flexible carboxyl pendants of appropriate length has two effects: 1) it increases the interaction enthalpy between the host framework and CO 2 molecules, and 2) it mitigates the entropy loss upon CO 2 adsorption due to the formation of multiple configurations for the interactions between carboxyl groups and CO 2 molecules. The ideal adsorption solution theory (IAST) selectivity for CO 2 adsorption over that of CH 4 was enhanced for all of the UiO‐66‐AD n s compared to that of UiO‐66 at 298 K. In particular, UiO‐66‐AD6 showed the most strongly enhanced CO 2 uptake capacity and significantly increased selectivity for CO 2 adsorption over that of CH 4 at ambient temperature, suggesting that it is a promising material for sequestering CO 2 from landfill gas.