Abstract: The ONIOM method was used to perform the computational study on the acid strength of double acid sites in the 12 member-ring of MCM-22 zeolite. The calculation was carried out by the B3LYP/6-31G**/MNDO method based on the 52T cluster model. The dependency of the acid strength on the interval distance of the framework Al atoms was examined, and the adsorption of ethylene and benzene on the adjacent acid sites was investigated. The calculation results indicated that when two acid sites existed near each other and spaced out by one Si atom, the acidity descends obviously with respect to the isolated acid site. If the Si atom number between two framework Al atoms increases, the acidity rises up. When the Si atom number is more than three, the acid strength reaches a steady value, being equivalent to the isolated system. For C2H4 adsorption, its adsorption energy in all cases is 31-35 kJ/mol, independent on the interval distance between the framework Al atoms. For C6H6 benzene adsorption, its adsorption energy on next nearest neighbour acid sites is higher than that on single acid site, and when the Si atom number is more than three, its adsorption energy is 21-29 kJ/mol. The adsorption energy of either two C2H4 molecules or two C6H6 molecules at the adjacent acid sites is equivalent to that of single molecule on the isolated acid site. The natural bond orbital analysis for the adsorption complexes ascertained their electronic structure, and the nature of C2H4 and C6H6 adsorption on the acid sites was explored.

Key words: density functional theory, MCM-22 zeolite, ethylene, benzene, adsorption energy