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-densi ty (LDA) and generalized-gradient (GGA) approaches for C-1 to C-6 linear hy drocarbons. The LDA adsorption energies are due to the overbinding similar to2.5 times larger than the GGA values. A compact diagram is constructed sh owing 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 t he adsorption on the acid site is realized through the hydrogen bonding bet ween the OH group and the CH3 group. The pattern of the reconstructed bondi ng, however, is more complex than that of the simple hydrogen bond. The reg ions of redistributed electron density within the adsorbed molecule are spr ead over the whole CH3 group and the adjacent C-C bond. The off-centering o f the reconstructed regions from atomic positions is in good agreement with recent C-13 measurements, showing only slight variation of chemical shifts with the hydrocarbon length for both proton-free and the protonated forms of zeolites. (C) 2001 American Institute of Physics.