Theoretical optimisation of pore size and pore chemistry of SIFSIX-3-M hybrid ultramicroporous materials (HUMs)

Three analogues of the SIFSIX-3-M family, namely SIFSIX-3-Zn, SIFSIX-3-Ni, and SIFSIX-3-Cu were investigated using theoretical methods to determine the effect of pore-size and pore-chemistry on the adsorption of CO2 in Hybrid Ultramicroporous Materials (HUMs). Classical Grand Canonical Monte Carlo (GCMC), Molecular Dynamics (MD), and Density Functional Theory (DFT) were used to explore the sorption properties of these HUMs, which show high affinity toward CO2. Simulated adsorption isotherms are in good agreement with experimental results. Also, calculated isosteric enthalpies of adsorption (Qst) follow the trend: SIFSIX-3-Cu > SIFSIX-3-Ni > SIFSIX-3-Zn, which agrees with experimental measurements. The interaction energy between a CO2 molecule and the HUM pore was determined by DFT-D2 level of theory calculations. As might be expected, the strongest interaction energy (56.89 kJ mol-1) was observed in the HUM with smallest pore size (SIFSIX-3-Cu) compared to SIFSIX-3-Ni (52.21 kJ mol-1) and SIFSIX-3-Zn (48.46 kJ mol-1) each with increasing pore dimensions, respectively. This increase in interaction strength is attributed to the shorter distance between the electronegative equatorial fluorine atoms of the SiF62- pillar and electropositive carbon atom of carbon dioxide. Finally, uncertainty about the exact crystallographic position of equatorial fluorine atoms was clarified with DFT-D2 calculations. It was observed that simulated interaction energies are in best agreement with experimental Qst values when equatorial fluorine atoms are at a 45° angle with respect to a-axis. Our observations will be valuable in aiding the rational design of improved variants of SIFSIX-3-M and related materials for the selective adsorption of CO2.