Theoretical study of the effect of halogen substitution in molecular porous materials for CO<sub>2</sub> and C<sub>2</sub>H<sub>2</sub> sorption

Abstract

Grand canonical Monte Carlo (GCMC) simulations of carbondioxide (CO$_2$) and acetylene (C$_2$H$_2$) 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 CuX$_2$ (X = Cl or Br) by solvent diffusion. Previous experimental studies revealed that both MPMs are selective for C$_2$H$_2$ over CO$_2$ [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 ($Q_{st}$) valuesfor both sorbates are in reasonable agreement with the corresponding experimental measurements, with simulations using the polarizable models producing the closest overall agreement. The modeled CO$_2$ binding sitein both MPMs was discovered as sorption between the halide ions of two adjacent [Cu$_2$(adenine)$_4$X$_2$]$^{2+}$ (X = Cl, Br) units.In the case of C$_2$H$_2$, it was found that the sorbate molecule prefers to align along the X--Cu--Cu--X axis of the copper paddlewheels suchthat each H atom of the C$_2$H$_2$ molecule can interact favorably with the coordinated X$^-$ ions. The simulations revealed that both MPMs exhibit stronger interactions with C$_2$H$_2$ than CO$_2$, which is consistent with experimental findings. The effect of halogen substitution toward CO$_2$ and C$_2$H$_2$ sorption in two isostructural MPMs was also elucidated in our theoretical studies.