Cu-2(pzdc)(2)L [L = dipyridyl-based ligands] porous coordination polymers: Hysteretic adsorption and diffusion kinetics of CO2 and CH4

CPL-n porous coordination polymers (PCPs) in general exhibit concomitant structural expansion during adsorption without a sudden gate-opening feature, but implications on the practical aspects of gas separation or storage are yet to be understood. In an attempt to gain further knowledge, we have gathered CO2 and CH4 adsorption/desorption and fractional uptake data using Cu2(pzdc)2L (pzdc: pyrazine-2,3-dicarboxylate) variants CPL-2 (L: 4,4′-bipyridine) and CPL-5 (L: 1,2-Di (4-pyridyl) ethylene). Both exhibited good affinity toward CO2 probably because of interactions with the adsorbate permanent quadrupole moment. Adsorption and desorption of CO2 also features hysteresis that increases in size upon an increase in the maximum pressure registered during a complete adsorption/desorption cycle. Furthermore, the size of the hysteresis increases with an increase in equilibration time interval (te), suggesting that the structural expansion proceeds at a slower rate compared to the adsorption one. CO2 heats of adsorption (max. −27 kJ mol−1) and desorption (max. 36 kJ mol−1) also reflect the aforementioned framework expansion, particularly in the case of CPL-2. Diffusion time constants calculated using a phenomenological model show hysteresis up to gas pressures of ca. 0.4 atm. The structural changes seem beneficial for the equilibrium separation of bulk quantities of CO2 from CH4 as evidenced by selectivity values based on the Ideal Adsorbed Solution Theory (SIAST ∼ 6.7 avg.), which are larger than those of MOFs with similar pore dimensions. Although kinetic selectivity values are similar to that of other metal organic frameworks (MOFs), these are smaller than those of small pore zeolites.