DFT calculations for Adsorption of H2S and other natural gas compounds on M-BTC MOF clusters

Abstract

Abstract Desulfurization is a necessary process to reduce the corrosiveness of natural gas. In this regard, H2S adsorption on porous materials is the focus of study for development of new eco-friendly technologies. Although there are many experimental and theoretical studies about gas adsorption on MOFs, so far, there has been no theoretical work about desulfurization of natural gas or biogas through H2S adsorption on MOF BTC. Therefore, the objective of this study is to preselect which metal center, M2+ = Co2+, Ni2+, Cu2+, or Zn2+, has the highest potential for selective desulfurization of natural gas. DFT calculations were performed at B3LYP‑D3/6‑311++G(2d,p)+LanL2DZ level for H2O, H2S, COS, CO2, and CH4 adsorption on M-BTC MOF clusters in order to obtain equilibrium geometries of adsorption complex, adsorption energies and thermodynamic properties. It was found that Zn-BTC MOF cluster has the highest potential for selective H2S removal from dry natural gas streams, as it has an adsorption energy of ‑79.4 kJ mol‑1, which is 2.4 times higher than CH4. Furthermore, it is an exothermic and thermodynamically favorable process. Through NBO and EDA analyses, it was found that d electrons transfer from adsorbate to unoccupied orbitals of metal center contributes mainly to H2S chemisorption on Zn-BTC and Co-BTC, while for CO2 and CH4 adsorption, non-bonded interactions predominate. Most of the gases coordinate to coordinatively unsaturated site of BTC MOF cluster at axial position, indicating a stronger interaction with metal center compared to linkers.