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

Abstract

Desulfurization is a necessary process to reduce the corrosiveness of natural gas. In this regard, H₂S adsorption on porous materials has gained attention in the 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 was to preselect by ab initio calculations which metal center M²⁺, such as 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 adsorption complex equilibrium geometries, 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, since its adsorption energy is −79.4 kJ mol⁻¹, which is 2.4 times higher than CH₄. Furthermore, H₂S adsorption on Zn-BTC MOF is an exothermic process and thermodynamically favorable. Through NBO and EDA analyses, it was found that d electrons transfer from adsorbate to metal center unoccupied orbitals contributes mainly to a possible 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.