Reaction Mechanisms of the Methylation of Ethene with Methanol and Dimethyl Ether over H-ZSM-5: An ONIOM Study

The mechanisms of ethene methylation with methanol and dimethyl ether have been investigated using a 128T cluster of ZSM-5 zeolite modeled by ONIOM(B3LYP/6-31G(d,p):UFF) and ONIOM(M06-2X/6-311+G(2df,2p):UFF) calculations. The effects of the infinite zeolitic framework on the model of the zeolite nanopocket, which consisted of a quantum cluster of 34 tetrahedral units and of 128 tetrahedral units modeled on the UFF level, were also included. The zeolitic Madelung potential was reproduced by a set of point charges generated by the SCREEP method. The energies for the adsorption of methanol and dimethyl ether on H-ZSM-5 from an ONIOM2(M06-2X/6-311+G(2df,2p):UFF)+SCREEP calculation are −26.3 and −29.4 kcal/mol, respectively, which are in good agreement with the experimental data. Dissociative and associative mechanisms of the ethene methylation by methanol and dimethyl ether have been considered. For the dissociative mechanism, the methylation reaction of ethene starts with the protonation of methanol or dimethyl ether by the acidic zeolite proton to form a surface methoxide intermediate, which subsequently reacts with an ethene molecule forming a propoxide intermediate. The propoxide intermediate is then deprotonated to form the propene product. The activation energies for the first step are computed to be 41.2 and 42.9 kcal/mol for methanol and dimethyl ether, respectively. The activation energies for the subsequent second and third reaction steps are 21.4 and 26.5 kcal/mol, respectively. For the associative mechanism, protonation and methylation take place simultaneously without formation of a surface methoxide. The calculated activation barriers are 29.0 and 33.0 kcal/mol for methanol and dimethyl ether, respectively, suggesting that methanol should be slightly more reactive than dimethyl ether for the methylation of ethene. The final step in the associative mechanism, the deprotonation of the propoxide intermediate to give the adsorbed propene product, has an activation energy of 25.4 kcal/mol. The results indicate that the associative pathway is favored over the dissociative one and that the rate-determining step of this reaction is the ethene methylation step.