Fine-Tuning Pore Size by Shifting Coordination Sites of Ligands and Surface Polarization of Metal–Organic Frameworks To Sharply Enhance the Selectivity for CO2

Based upon the (3,6)-connected metal–organic framework {Cu­(L1)·2H2O·1.5DMF}∞ (L1 = 5-(pyridin-4-yl)­isophthalic acid) (SYSU, for Sun Yat-Sen University), iso-reticular {Cu­(L2)·DMF}∞ (L2 = 5-(pyridin-3-yl)­isophthalic acid) (NJU-Bai7; NJU-Bai for Nanjing University Bai group) and {Cu­(L3)·DMF·H2O}∞ (L3 = 5-(pyrimidin-5-yl)­isophthalic acid) (NJU-Bai8) were designed by shifting the coordination sites of ligands to fine-tune pore size and polarizing the inner surface with uncoordinated nitrogen atoms, respectively, with almost no changes in surface area or porosity. Compared with those of the prototype SYSU, both the adsorption enthalpy and selectivity of CO2 for NJU-Bai7 and NJU-Bai8 have been greatly enhanced, which makes NJU-Bai7 and NJU-Bai8 good candidates for postcombustion CO2 capture. Notably, the CO2 adsorption enthalpy of NJU-Bai7 is the highest reported so far among the MOFs without any polarizing functional groups or open metal sites. Meanwhile, NJU-Bai8 exhibits high uptake of CO2 and good CO2/CH4 selectivity at high pressure, which are quite valuable characteristics in the purification of natural gases.