Why Ag(I) grafted porous carbon matrix prefers alkene over alkane? An inside view from ab-initio study

Adsorptive separation of ethylene or propylene from ethane or propane is an attractive strategy for its sustainable and inexpensive nature. Ag (I) grafted porous carbon adsorbents showed an excellent selective adsorption towards ethylene and propylene due to the π complexation. Starting from experimental evidences, Density Functional Theory (DFT) was used to understand the selective adsorption. The DFT computed adsorption energies of ethylene and propylene were 6.1 to 3.6 times higher than those of the saturated hydrocarbons depending on the pore size, which corroborated the higher experimental adsorbed amounts of alkenes. Thus, the DFT calculations highlighted the effect of the pore widths on the adsorption energy by providing an explanation based on both optimized geometries and molecular orbitals analysis. An overlap between d orbitals of Ag and π orbitals, centred on the carbons of unsaturated hydrocarbons, was found. This overlap was not found for the saturated hydrocarbons where, for pores with width around 5.23 Å, the d orbitals of silver overlap with the π orbital of the carbon atoms within the graphene plane placed in front of the Ag functionality, thereby rejecting the alkane. These theoretical findings confirm the empirical hypothesis of selective adsorption of unsaturated hydrocarbons by π−π complexation.