Theoretical investigation of the effect of subtle structural dynamics on CO2 sorption in TIFSIX-3-Ni, a hybrid ultramicroporous material (HUM)

The release of carbon dioxide (CO2) emissions into the atmosphere is a leading contributor to global warming. Carbon capture and sequestration (CCS) strives to mitigate the effects of CO2 on the atmosphere, traditionally by expensive and energy-intensive chemisorptive approaches. CO2 capture by physisorbents such as hybrid ultramicroporous materials (HUMs) is a step toward cheaper and more efficient CCS. In this study, the effect of pyrazine ring orientation upon CO2 adsorption is investigated for TIFSIX-3-Ni, a leading HUM for CO2 selectivity. Rigid systems are constructed by eliminating disorder from the unit cell as determined by in situ characterization. Density Functional Theory (DFT) and Grand Canonical Monte Carlo (GCMC) methods are used to investigate the effect of tilting and ordering pyrazine rings upon CO2 loading and isosteric heat of adsorption (Qst). Results show that more edge to face interactions between pyrazine CH moieties and the CO2 molecule induce a preferred binding site. Systems with chemically distinct binding sites exhibit a Qst trend comparable to that which is experimentally observed, showing first preference for binding in smaller pores using models treated with both UFF and OPLS-AA Lennard-Jones parameters. It is also noted that the degree of pyrazine ring tilting affects the energetics of the sorbent-sorbate interactions, meriting further study. This work highlights the importance of subtle structural dynamics in adsorption performance of leading porous materials, and can be used to guide further fine-tuning of physisorbent materials for gas sorption applications.