Theoretical study of the effect of halogen substitution in molecular porous materials for CO2 and C2H2 sorption

Grand canonical Monte Carlo (GCMC) simulations of carbon dioxide (CO2) and acetylene (C2H2) sorption were performed in MPM-1-Cl and MPM-1-Br, two robust molecular porous materials (MPMs) that were synthesized by the addition of adenine to CuX2 (X = Cl or Br) by solvent diffusion. Previous experimental studies revealed that both MPMs are selective for C2H2 over CO2 [Xie DY, et al. (2017) CIESC J 68: 154-162]. Simulations in MPM-1-Cl and MPM-1-Br were carried out using polarizable and nonpolarizable potentials of the respective sorbates; this was done to investigate the role of explicit induction on the gas sorption mechanism in these materials. The calculated sorption isotherms and isosteric heat of adsorption (Qst) values for both sorbates are in reasonable agreement with the corresponding experimental measurements, with simulations using the polarizable models producing the closest overall agreement. The modeled CO2 binding site in both MPMs was discovered as sorption between the halide ions of two adjacent [Cu-2(adenine) 4X(2)](2+) (X = Cl, Br) units. In the case of C2H2, it was found that the sorbate molecule prefers to align along the X-Cu-Cu-X axis of the copper paddlewheels such that each H atom of the C2H2 molecule can interact favorably with the coordinated X ions. The simulations revealed that both MPMs exhibit stronger interactions with C2H2 than CO2, which is consistent with experimental findings. The effect of halogen substitution toward CO2 and C2H2 sorption in two isostructural MPMs was also elucidated in our theoretical studies.