Mechanism for the Dynamic Adsorption of CO2 and CH4 in a Flexible Linear Chain Coordination Polymer as Determined from In Situ Infrared Spectroscopy

Abstract

Adsorption−desorption cycles for CO2 and CH4 on the one-dimensional coordination polymer, catena-bis(dibenzoylmethanato)-(4,4′-bipyridyl)nickel(II), “Ni-DBM-BPY”, showed pronounced step-shape isotherms, where minimal gas adsorption was detected below a threshold pressure and rapid gas uptake was observed above this threshold. Desorption isotherms from the saturated state displayed significant hysteresis from the adsorption isotherm path. Such behavior is rare in one-dimensional coordination polymers that lack a robust framework with permanent porosity. This step-shape adsorption behavior for CO2 was shown by in situ FT-IR measurements to be the result of a structural phase transition in the Ni-DBM-BPY host which arises from a change in conformation of the DBM ligands. After the structural transition, the adsorption spectrum of the adsorbed CO2 changed significantly due to an enhanced CO2 interaction with the host. A similar mechanism can be inferred for CH4 from the isotherm shape, but the host structural phase transitions could not be observed directly with CH4 uptake, since the threshold conditions were outside the temperature and pressure limits of the instrument. These reported results highlight the importance of in situ FT-IR measurements for determining gas adsorption mechanisms in flexible porous coordination polymers.