Synthesis and Characterization of a Cu-2(pzdc)(2)(bix) [pzdc: 2,3-pyrazinedicarboxylate; bix: 1,3-bis(imidazol-1-yl)benzene] Porous Coordination Pillared-Layer Network

A porous coordination polymer (PCP) was prepared using pyrazine-2,3-dicarboxylate (pzdc) and the pillar ligand 1,3-bis(imidazol-1-yl)benzene) (bix) in an attempt to improve on the thermal stability and adsorption properties of Cu-2(pzdc)(2)(L) (L = pillar ligand; typically dipyridyl-based). As-synthesized Cu-2(pzdc)(2)(bix) showcases a highly crystalline structure (Monoclinic, P2(1)/c, a = 11.836 angstrom, b = 13.005 angstrom, c = 11.389 angstrom, beta = 107.64) that exhibits characteristic diffraction peaks also found in the dipyridyl-based PCP counterparts. Thermal analysis based on results from thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC) and in situ high temperature X-ray diffraction (XRD) suggests both framework contraction and expansion during the elimination of physisorbed and coordinated water, phase transition, and prior to the eventual destruction of the structure, which takes place at ca. 556 K. The thermal stability limit is an improvement over those displayed by other Cu-2(pzdc)(2)(L) materials by an average margin of ca. 55 K. Comparison of the adsorption of CO2 vs N-2 both at cryogenic temperatures suggests a concomitant structural expansion in the presence of the former. This process appears to take place even at ambient temperatures and CO2 pressures up to 50 atm; the isotherm profile showcases a hysteretic pathway and full adsorption saturation is not apparent. Furthermore, evaluation of CO2 adsorption at different equilibration time intervals indicate that the structure relaxation time scale is larger than that of the adsorption kinetics; similar observations have been made before for Cu-2(pzdc)(2)(bpy) (bpy: 4,4'-bipyridine). The presence of the imidazole groups, however, results in greater interactions with CO2 (maximum Delta H-iso similar to 32 kJ mort). Selectivity for CO2 over CH4 (10/90) based on the Ideal Adsorbed Solution Theory (LAST) reaches 8.4, which is a 10% and 75% increase when compared against Cu-2(pzdc)(2)(bpy) and Cu-2(pzdc)(2)(bpe) (bpe: 1,2-di(4-pyridyl)-ethylene).