Density functional theory study on the adsorption of H 2S and other claus process tail gas components on copper- and silver-exchanged Y zeolites

The potential use of Cu- and Ag-exchanged Y zeolites as selective adsorbents for hydrogen sulfide (H2S) from Claus process tail gas was investigated with density functional theory (DFT). The adsorption energies of H2S and other Claus tail gas components (CO, H2O, N-2, and CO2) were computed for these zeolites as well as for Li-Y, Na-Y, and K-Y on a cluster model. Comparison of adsorption energies for H2S versus the other components indicated that Ag-Y has potential for selective adsorption of H2S, whereas Cu-Y is subject to strong adsorption of CO, and alkali metal-exchanged Y zeolites are subject to H2O adsorption. Comparison with alkali metal-exchanged Y zeolites was performed to clarify the role of d electrons, while the influence of the zeolite framework was assessed by comparing adsorption energies on the cluster model with those on bare cations. Absolutely localized molecular orbital energy decomposition analysis (ALMO EDA) revealed that for Cu- and Ag-containing systems, transfer of electrons between the cation and the adsorbate, i.e., the donation of d electrons and the acceptance of electrons in the unoccupied orbitals of the cation, plays an important role in determining the adsorption energy. On the other hand, for alkali metals-containing systems, charge transfer is negligible and adsorption energies are dominated by interactions due to electrostatics, polarization, and structural distortions.