Adsorption of linear hydrocarbons in zeolites: A density-functional investigation

An extensive first-principles periodical study of adsorption properties of linear hydrocarbons in zeolites is presented. The applicability of density-functional theory to weak interactions is inspected within both local-density (LDA) and generalized-gradient (GGA) approaches for C1 to C6 linear hydrocarbons. The LDA adsorption energies are due to the overbinding ∼2.5 times larger than the GGA values. A compact diagram is constructed showing the increase of the adsorption energy with the length of the adsorbed molecule and with the concentration of acid sites in the zeolite support. The flow of the electron density induced by the adsorption indicates that the adsorption on the acid site is realized through the hydrogen bonding between the OH group and the CH3 group. The pattern of the reconstructed bonding, however, is more complex than that of the simple hydrogen bond. The regions of redistributed electron density within the adsorbed molecule are spread over the whole CH3 group and the adjacent C–C bond. The off-centering of the reconstructed regions from atomic positions is in good agreement with recent C13 measurements, showing only slight variation of chemical shifts with the hydrocarbon length for both proton-free and the protonated forms of zeolites.