Olefin adsorption on silica-supported silver salts - A DFT study

Recent experiments have shown that silver salts supported on mesoporous silicas display excellent adsorption selectivities of ethylene over ethane and propylene over propane. Employing the techniques of density functional theory, we have investigated the fundamental bases of this separation process by examining silver salts dispersed on model silica surfaces. Our model system includes Ag ⁺ cations, their counteranions, silica supports, and surface silanols. Both adsorption geometries and energetics of ethylene and propylene were explored. Our results indicate that the nature of the Ag-olefin interaction is predominantly hybridization between Ag d and olefin n states, which is supported by analyses of electron density difference plots and density of states. The counteranions, such as NO ₃ ^-, were found to interact strongly with surface silanols through multiple hydrogen bonds but have limited effect on the adsorption energy of olefins on the Ag ⁺ cations. The current work supports recent experiments, which indicate that Ag-salt/silica may be a very promising adsorbent for olefin/paraffin separation.