Thermodynamic Study of CO2 Sorption by Polymorphic Microporous MOFs with Open Zn(II) Coordination Sites

Two Zn-based metal organic frameworks have been prepared solvothermally, and their selectivity for CO2 adsorption was investigated. In both frameworks, the inorganic structural building unit is composed of Zn­(II) bridged by the 2-carboxylate or 5-carboxylate pendants of 2,5-pyridine dicarboxylate (pydc) to form a 1D zigzag chain. The zigzag chains are linked by the bridging 2,5-carboxylates across the Zn ions to form 3D networks with formulas of Zn4(pydc)4­(DMF)2·3DMF (1) and Zn2(pydc)2­(DEF) (2). The framework (1) contains coordinated DMF as well as DMF solvates (DMF = N,N-dimethylformamide), while (2) contains coordinated DEF (DEF = N,N-diethylformamide). (1) displays a reversible type-I sorption isotherm for CO2 and N2 with BET surface areas of 196 and 319 m2/g, respectively. At low pressures, CO2 and N2 isotherms for (2) were not able to reach saturation, indicative of pore sizes too small for the gas molecules to penetrate. A solvent exchange to give (2)-MeOH allowed for increased CO2 and N2 adsorption onto the MOF surface with BET surface areas of 41 and 39 m2/g, respectively. The binding of CO2 into the framework of (1) was found to be exothermic with a zero coverage heat of adsorption, Qst0, of −27.7 kJ/mol. The Qst0 of (2) and (2)-MeOH were found to be −3 and −41 kJ/mol, respectively. The CO2/N2 selectivity for (1), calculated from the estimated KH at 296 K, was found to be 42. At pressures relevant to postcombustion capture, the selectivity was 14. The thermodynamic data are consistent with a mechanism of adsorption that involves CO2 binding to the unsaturated Zn­(II) metal centers present in the crystal structures.