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The conversion of methanol-to-hydrocarbons (MTH) over an acidic zeolite catalyst represents the final step in upgrading any gasifiable carbon feedstock to gasoline-range hydrocarbons (methanol-to-gasoline, MTG) and light olefins (methanol-to-olefins, MTO). CH₃ groups derived from CH₃OH or CH₃OCH₃ (DME) cannot be coupled directly into hydrocarbons at rates relevant for steady state MTH catalysis. Instead organic species, specifically olefins and arenes, contained inside the zeolite pore act as scaffolds for C-C bond formation. This hypothesis, known as the "hydrocarbon pool" mechanism, is now widely accepted as the mechanism for MTH catalysis. Speciation preferences in this dual olefin and arene methylation cycle determine catalytic rates and selectivity in MTH.

The complex role that the dual olefin-arene methylation cycle plays in the observed catalytic rate and selectivity for MTH implies that it is difficult to isolate one hydrocarbon pool cycle over another. Reduced methylation barriers with increasing olefin chain length, the subsequent isomerization of hydrocarbon products, and formation of olefinic species from both methylation and cracking reactions at conversions relevant for practice of MTH make quantitative evaluation of kinetic parameters of olefin methylation on zeolites experimentally challenging. In this work, methylation kinetics of ethylene and propylene were measured on proton form FER, MFI, MOR, and BEA zeolites at low olefin conversions (<0.2%) and high DME:olefin ratios (15-60:1). We report that the kinetics of olefin methylation are consistent with a mechanism involving a surface predominantly covered by DME derived species (zero-order kinetics) that react with olefinic species in kinetically relevant steps (first-order kinetics). A systematic decrease in activation barriers was noted with increasing substitution of the olefin. These data show that FER, MFI, MOR, and BEA zeolites propagate the olefin methylation cycle to varying extents and thereby explain the marked diversity in selectivity and yield for C₁ homologation using different zeolites (Tables 1 and 2).

Table 1. A comparison of kinetic parameters for ethylene methylation over proton form zeolites

Table 2. A comparison of kinetic parameters for propylene methylation over proton form zeolites

An outstanding question pertaining to the methylation of olefins is the identity of the active DME-derived species on the zeolite surface that is responsible for the methylation of olefins. Surface species that have been investigated to exist at zeolite Brznsted acid sites under olefin methylation conditions include:

(i)             a co-adsorbed DME/methanol and olefin complex

(ii)           methanol dimers, and

(iii)          surface-bound methoxide groups

Co-feed DME and d₆ DME experiments at steady-state olefin methylation reaction conditions were done to provide mechanistic insight regarding the nature of the active surface species and the role of C-H bonds in the rate-determining step. Unreacted DME and d₆ DME scrambling about the C-O bond was observed by the presence of a 1.2:1.5:1 distribution of d₀:d₃:d₆ isotopic distribution in the product stream. Significant scrambling about the C-O bond at the reaction conditions reported herein proves that fast and reversible formation of surface-bound methoxides occurs under olefin methylation reaction conditions. A co-adsorbed mechanism would only break the DME C-O bond in direct association with an adsorbed olefin. The olefin methylation rate decreased by a factor of 1.3 in the presence of a 1.2:1 d₀:d₆ DME feed compared to purely unlabeled reagents. This observed rate difference is consistent with a positive secondary kinetic isotope effect, indicating the transition from a sp³ to sp² hybridized methyl species in the generation of the activated complex without the cleavage of a C-H bond. On the basis of these isotopic experiments, we propose surface methoxide groups are formed under methylation reaction conditions and that the transition state of the rate-limiting step proceeds via an sp² hybridized configuration generated from an sp³ hybridized precursor state without breaking C-H bonds consistent with these surface-bound methoxide species being responsible for the methylation of olefins

In summary, our work has elucidated that olefin methylation reactions proceed via similar mechanistic routes on proton form FER, MFI, MOR, and BEA zeolites; however, these reactions are propagated to varying extents by different zeolites thereby providing an initial hypothesis for the marked diversity in C₁ homologation selectivity and yield observed with varying structure. Our experimental studies done using un-labeled and d₆-labeled dimethyl ether reactants have shown that surface methoxide species are formed under olefin methylation conditions and that the secondary kinetic isotope effect observed is consistent with a planar transition state for these surface methyl groups reacting in kinetically relevant steps with olefin precursors.