DFT-based evaluation of porous metal formates for the storage and separation of small molecules

Abstract

Microporous metal formates with the α-Mg(fa)2 structure (fa = formate, −OOCH) have attracted considerable interest as materials for the storage and separation of small guest molecules. In this study, dispersion-corrected density-functional theory (DFT-D) calculations are employed to calculate the interaction energies for five guest molecules (acetylene, carbon dioxide, methane, nitrogen, hydrogen) adsorbed in the pores of metal formates with different metal centres (Mg, Mn, Fe, Co, Zn). For each guest-adsorbent combination, the average interaction strength is calculated as Boltzmann-weighted average of the DFT-D interaction energies obtained for individual adsorption sites. For systems for which experimentally measured heats of adsorption have been reported, the averaged energies agree very well with experimental values. Therefore, the DFT-D results can be used to predict the potential of metal formates as adsorbents for different applications. While metal formates may be of limited use for many gas storage applications due to their small free volume, the high affinity of Co(fa)2 and Zn(fa)2 towards methane and acetylene could render them interesting for storage applications in which the affinity is more important than the uptake capacity. With regard to gas separation and purification, Co(fa)2 emerges as the most attractive system for the selective adsorption of acetylene and carbon dioxide over other species, while Zn(fa)2 and Mg(fa)2 are more promising for the separation of methane from nitrogen or hydrogen. Correlations between the averaged interaction energies and simple descriptors of pore size and pore volume are analysed.