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

Abstract

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.

Mechanistic Aspects of the Zeolite Catalyzed Methylation of Alkenes and Aromatics with Methanol: A Review

“…Also, the absolute reaction rates could be calculated with near chemical accuracy, and the qualitative experimental trends were well reproduced. Finally, the intrinsic barrier for direct methylation of ethene has been reported to be 121 kJ/mol in an ONIOM study on H-ZSM-5 [28]. The barriers reported in these three studies agree well with experimental data: 109, 69, and 45 kJ/mol has been measured for the methylation of ethene, propene, and t-2-butene, respectively (see below).…”

“…When employing cluster calculations, this reaction is invariably found to be quite endothermic, the values of the reaction energy range from ?43 to ?57 kJ/mol [13, 21-23, 26, 30]. Employing more sophisticated ONIOM methods, Maihom et al [28] also found a similar value; ?66 kJ/mol. Thus, these computational studies indicate that the formation of methoxy groups from methanol or DME is an endothermic reaction.…”

“…The mechanism of DME formation is well studied, and this reaction is also suggested to proceed via a direct or stepwise mechanism [21][22][23][24][25][26]. However, the mechanism of DME formation will not be discussed further, as its formation is quick compared to methylation [27] and several studies, both experimental and theoretical have shown that DME may undergo quite analogous reactions to those described above for methanol [12,27,28]. That is, both direct methylation and formation of surface methoxy groups from DME is possible, the only difference when compared to the methanol situation is that a methanol molecule is formed as the byproduct rather than water.…”

Mechanistic investigation of methanol to propene conversion catalyzed by H-beta zeolite: a two-layer ONIOM study

“…The products, propene and ethene, are eliminated from the side chain of the aromatic benzene derivative by an internal H-shift step. Two different pathways have been reported for the methylation step in the literature [36][37][38]: a stepwise mechanism that involves a surface-bound methoxide intermediate, and a concerted one in which adsorbed methanol directly attacks the substrate to be methylated. The concerted pathway is widely used in theoretical calculations because the experimental kinetic measurements are readily explained by this pathway [19,37,39], so we adopted this mechanism in the present work.…”

Methylation of benzene by methanol: Single-site kinetics over H-ZSM-5 and H-beta zeolite catalysts

“…As far as the mechanism of the zeolite-catalyzed methylation reaction is concerned, two different proposals have been advocated in literature: a stepwise route that involves a surfacebound methoxy group intermediate, and a concerted mechanism in which physisorbed methanol directly interacts with the species to be methylated [22,26,27]. While spectroscopic evidence for the existence of surface-bound methoxy groups has been presented, and the fundamental viability of a stepwise mechanism has been verified, experimental kinetic measurements are readily explained by the concerted pathway [26,28].…”