Thermodynamic Study of CO sub(2) 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 CO sub(2) 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 Zn sub(4)(pydc) sub(4)(DMF) sub(2)-3DMF (1) and Zn sub(2)(pydc) sub(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 CO sub(2) and N sub(2) with BET surface areas of 196 and 319 m super(2)/g, respectively. At low pressures, CO sub(2) and N sub(2) 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 CO sub(2) and N sub(2) adsorption onto the MOF surface with BET surface areas of 41 and 39 m super(2)/g, respectively. The binding of CO sub(2) into the framework of (1) was found to be exothermic with a zero coverage heat of adsorption, Q sub(s)t super(0) of -27.7 kJ/mol. The Q sub(s)t super(0)of (2) and (2)-MeOH were found to be -3 and -41 kJ/mol, respectively. The CO sub(2)/N sub(2) selectivity for (1), calculated from the estimated K sub(H)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 CO sub(2) binding to the unsaturated Zn(II) metal centers present in the crystal structures.