Application of ONIOM calculations in the study of the effect of the zeolite framework on the adsorption of alkenes to ZSM-5

The structures and energetics associated with the adsorption of ethene and four butene isomers on H-ZSM-5 zeolite have been studied using a 46T cluster and calculated at the ONIOM2(B3LYP/6-311++G(d,p):UFF) level. The adsorption energy for ethene-zeolite complex is predicted to be -8.17 kcal/mol, which is in good agreement with the experimental data of -9.0 kcal/mol. The trend of the calculated adsorption energies (kcal/mol) for the butene isomers is as follows: 1-butene (-16.06) > cis-2-butene (-13.62) congruent to trans-2-butene (-13.25) > isobutene (-6.96). The isobutene-zeolite complex is the least stable due to the greatest steric repulsion between the methyl substituents around the C=C bond and zeolite framework; the more substituted the lower the adsorption energy. Although our own N-layered integrated molecular orbital and molecular mechanics (ONIOM) calculation results indicate that isobutene hardly approaches the acid site and has a weak interaction with the zeolite framework, NBO analysis shows that it has the maximum charge transfer from the active site and the largest stabilization energy. These findings explain the reason why ZSM-5 is selective towards isobutene produced from n-butene and indicate that the acidic proton from the zeolite is easy to transfer to isobutene. Thus, further catalytic conversion of isobutene would be facile. (c) 2006 Elsevier B.V All rights reserved.