Reaction Dynamics of Zeolite-Catalyzed Alkene Methylation by Methanol

A hybrid quantum mechanics/molecular mechanics (QM/MM) model and the quasiclassical trajectory (QCT) method have been combined to study the reaction of alkene methylation by methanol catalyzed by the zeolite H-MFI. The rate-limiting step of this reaction is the methylation of the alkene, and the apparent activation energy calculated at the omega B97X-D/6-31G(d,p)//omega B97X-D/6-311++G(3df,3pd) level of theory for this step agrees well with experiment and previous full QM studies. Following the ethene methylation transition state toward the products along the intrinsic reaction coordinate reveals the existence of a protonated cyclopropane (PCP+) carbocation intermediate. A similar protonated methylcyclopropane (mPCP(+)) carbocation intermediate is found for propene methylation. The intermediates produced during the alkene methylation reaction are metastable with a lifetime of O(1 ps) obtained from QCTs. Because of the short lifetime of these intermediates, the available energy in the carbocation is not in thermal equilibrium distribution with the zeolite lattice before subsequent reaction occurs. The qualitative difference between product distributions obtained by static and dynamic reaction pathways suggests the pathways of zeolite-catalyzed reactions proceed through high-temperature pathways that differ from the 0 K potential energy surface. The transformation of the m-PCP+ intermediate to the longer-lived secondary 2-butyl carbocation observed during QCTs suggests that more stable carbocations can properly thermalize and exist as reaction intermediates for longer than 1 ps.