Use of isotopic labeling for mechanistic studies of the methanol-to-hydrocarbons reaction. Methylation of toluene with methanol over H-ZSM-5, H-mordenite and H-beta

Methylation of toluene (in one case benzene) with methanol has been investigated over four different zeolites: Small and large crystal ZSM-5, dealuminated mordenite and zeolite beta. The feed ratio methanol/arene was varied over a wide range, mostly >1, using nitrogen or helium as carrier gas. A rather high space velocity was employed (WHSV (methanol + arene): 5–35 h^?1). The reaction temperature was 350°C or 375°C. A parallel set of experiments was performed using ¹³C methanol (99% ¹³C). The products were analyzed by on-line gas chromatography using a GC–MS system allowing determination of isotopic composition of the more important products, i.e., ethene, propene and the arenes when ¹³C methanol was employed, otherwise a FID was used. The goal was to obtain mechanistic information, and no attempt has been made to optimize for any particular reaction product.

The experiments showed that ethene and propene were isotopically mixed, containing 50–75% ¹³C. The 25–50% ¹²C atoms coming from the reactant arene. Over all catalysts the arene was, when fed alone, essentially inert with 0.5% or less conversion, giving neither ethene nor propene in measurable quantities. The isotopic distribution in ethene was indistinguishable from a random distribution. Propene, although being close to, displayed some deviation from randomness. The ¹²C/¹³C isotopic ratio in propene was equal to that in ethene, and they both varied with the methanol/arene ratio in the feed, but much less so than the variation in feed composition.

The results support a pool mechanism where the catalytic activity for converting methanol to hydrocarbons is connected with the presence of adsorbates in the zeolite cavities which add methanol and split off product molecules, notably ethene and propene. Formation of ethene by reaction between C₁ species is at best a minor reaction. While formation of propene may take place by a homologation/cracking mechanism, this route was of minor importance here.

Polymethylbenzenes which were formed in many cases displayed a pronounced isotopic scrambling, containing up to six ¹³C atoms in the benzene ring. Molecules with fewer ¹³C atoms than the number of added methyl groups were also identified.