DFT calculations for adsorption of H2S and other natural gas compounds on (Fe, Co, Ni, Cu and Zn)–Y zeolite clusters

The acid compounds removal from natural gas by adsorption on zeolites is an environmentally friendly alternative technology for its purification and commercialization. However, despite numerous theoretical studies determining the adsorption energies for gases on zeolites are found, for important natural gas compounds adsorption on Y zeolite they are poorly reported, especially with respect to long range van der Waals interactions, the description of their nature and related thermodynamic properties. Thus, this work aims to pre-select transition metal-exchanged Y zeolite for selective removal of H2S and other acid compounds from natural gas streams by DFT calculations for H2S, COS, CO2 and CH4 adsorption on Fe–Y, Co–Y, Ni–Y, Cu–Y and Zn–Y zeolite clusters. Adsorption energies, thermodynamic properties, chemical hardness, electronegativity, Natural Bond Orbital (NBO) and Energy Decomposition Analysis (EDA) were determined using B3LYP hybrid functional and Grimme's D3 empirical dispersion correction. Comparison between H2S and other compounds adsorption energies indicates that Ni–Y zeolite has potential for selective removal of H2S, despite Co–Y zeolite presents stronger adsorption energy for all compounds. Moreover, the NBO and EDA analyzes reveal that there is a transfer of d electrons from H2S molecule to unoccupied orbitals of the cation. Charge transfer and polarization phenomena contribute mainly to the adsorption energy of H2S, COS and CO2 in transition metal-exchanged Y zeolite clusters, showing the presence of surface chemisorption.