Investigating C₂H₂ Sorption in α-[M₃(O₂CH)₆] (M = Mg, Mn) Through Theoretical Studies

Simulations of C2H2 sorption were performed in α-[Mg3(O2CH)6] and α-[Mn3(O2CH)6], two isostructural metal–organic frameworks (MOFs) that consist of a network of M2+ ions coordinated to formate linkers. Previous experimental studies revealed that both MOFs display high low-pressure uptake and isosteric heat of adsorption (Qst) for C2H2 [Samsonenko, D. G. et al. Chem. Asian J. 2007, 2, 484−488]. Simulations using two recently developed potential energy functions for the sorbate in both MOFs yielded sorption isotherms and Qst values that are in reasonable agreement with the corresponding experimental measurements. Electronic structure calculations revealed that the metal ions are more positively charged in α-[Mg3(O2CH)6] than in α-[Mn3(O2CH)6], which in turn led to greater partial negative charges on the linker O atoms in the Mg variant. This resulted in the former displaying a higher calculated C2H2 uptake than the latter for all state points considered, which is consistent with what was observed experimentally. While repulsion/dispersion interactions dominated the C2H2 sorption mechanism in α-[Mn3(O2CH)6], stationary electrostatic interactions were the main contributor to the total energy for C2H2 sorption in α-[Mg3(O2CH)6]; this is ostensibly a consequence of the different charge environments within the two variants. The simulated C2H2 molecule positions in the pores of both MOFs were found to be in decent agreement with those obtained through X-ray crystallography. Overall, this study demonstrates the accuracy and transferability of two recently established C2H2 potentials and how differences in the C2H2 sorption profiles between two isostructural MOFs can be rationalized by electronic structure calculations and Monte Carlo simulations.