Tuning CO₂ selective adsorption over N₂ and CH₄ in UiO-67 analogues through ligand functionalization

Introducing functional groups into pores of metal-organic frameworks (MOFs) through ligand modification provides an efficacious approach for tuning gas adsorption and separation performances of this type of novel porous material. In this work, two UiO-67 analogues, [Zr6O4(OH)4(FDCA)6] (BUT-10) and [Zr6O4(OH)4(DTDAO)6] (BUT-11), with functionalized pore surfaces and high stability were synthesized from two functional ligands, 9-fluorenone-2,7-dicarboxylic acid (H2FDCA) and dibenzo[b,d]thiophene-3,7-dicarboxylic acid 5,5-dioxide (H2DTDAO), respectively, and structurally determined by single-crystal X-ray diffraction. Notwithstanding skeleton bend of the two ligands relative to the linear 4,4'-biphenyldicarboxylic acid in UiO-67, the two MOFs have structures similar to that of UiO-67, with only lowered symmetry in their frameworks. Attributed to these additional functional groups (carbonyl and sulfone, respectively) in the ligands, BUT-10 and -11 show enhanced CO2 adsorption and separation selectivities over N2 and CH4, in spite of decreased pore sizes and surface areas compared with UiO-67. At 298 K and 1 atm, the CO2 uptake is 22.9, 50.6, and 53.5 cm(3)/g, and the infinite dilution selectivities of CO2/CH4 are 2.7, 5.1, and 9.0 and those of CO2/N2 are 9.4, 18.6, and 31.5 for UiO-67, BUT-10, and BUT-11, respectively. The selectivities of CO2/CH4 and CO2/N2 are thus enhanced 1.9 and 2.0 times in BUT-10 and 3.3 and 3.4 times in BUT-11, respectively, on the basis of UiO-67. The adsorption mechanism of CO2 in BUT-11 has also been explored through computational simulations. The results show that CO2 molecules locate around the sulfone groups in pore surfaces of BUT-11, verifying at the molecular level that sulfone groups significantly increase the affinity toward CO2 molecules of the framework. This provides thus an efficient strategy for the design of CO2 capture materials.