A theoretical study on the adsorption of C_2~6 olefins on the H-ZSM-5 zeolite of periodic model

The adsorption behaviors of linear C_2~6 olefins and butene isomers on the H-ZSM-5 zeolite of periodic model were studied by the PBE-D method. The adsorption energies (EPBE-D) and the dispersive correction energies (E_D) of C_2~6 linear olefins on the zeolite exhibit a linear increase with the number of carbons by -12 kJ/mol and -13 kJ/mol, respectively. The adsorption energies without dispersive correction (E^*) is not changed obviously, but it is decreased in the cases of increased space resistance for pentene and hexane. The value of E_D is much larger than that of E^*, implying that van der Waals' force plays a key role in the adsorption of olefins on the zeolite and its influence on the adsorption is dependent on the carbon number. The adsorption energies of butene isomers decrease in the order of trans-2-butane > cis-2-butene > n-butene > isobutene. Three kinds of n-butene isomers are similar in their E_D value, which are larger than that of n-butene. The difference in adsorption energy among the three isomers is caused by E^*. The differential charge density analysis shows that the electrons between the alkene double bond and the acidic center are gathered, in consistent with the strength of π-coordination; H atom in the acid site turns to be protonated, with electrons transferred to the nearby O and other atoms.