M. Vandichel

Methylation reactions of naphthalenic species over the acidic microporous zeolite with chabazite topology have been investigated by means of two-layered ab initio computations. Large cluster results combined with van der Waals contributions provide thermodynamic and kinetic results of successive methylation steps. The growth of fused bicyclic species is important as these can act as hydrocarbon pool species within the methanol-to-olefin (MTO) process, but ultimately leads to the deactivation of the catalyst. The influence of the confined space of the zeolite pore on the resulting transition state or product shape selectivity is investigated in detail.

The formation of cyclic hydrocarbons from smaller building blocks such as ethene and propene is investigated in protonated ZSM-5, using a 2-layered ONIOM(B3LYP/6-31+g(d):HF/6-31+g(d)) approach and an additional Grimme-type van der Waals dispersion correction term to account for the long-range dispersion interactions. These cyclic species form precursors for active hydrocarbon pool species and play a key role in activating the acidic zeolite host for successful methanol-to-olefin (MTO) conversion. Starting from trace amounts of ethene and propene that are formed during an initial induction period or during the active phase, dimerization reactions allow for rapid chain growth. The products of these reactions can be neutral alkenes, framework-bound alkoxide species or intermediate carbenium ions, depending on the zeolite environment taken into account. On the basis of rate constants for successive reaction steps, a viable route toward cyclization is proposed, which starts from the formation of a framework-bound propoxide from propene, followed by dimerization with an additional propene molecule to form the 2-hexyl carbenium ion which finally undergoes ring closure to yield methylcyclopentane. This cyclic species in turn forms a precursor for either an active hydrocarbon pool compound or for deactivating coke deposit.

The methanol-to-olefin (MTO) process, catalyzed by acidic zeolites such as H-ZSM-5, provides an increasingly important alternative to the production of light olefins from crude oil. However, the various mechanistic proposals for methanol-to-olefin conversion have been strongly disputed for the past several decades. This work provides theoretical evidence that the experimentally suggested ‘alkene cycle’, part of a co-catalytic hydrocarbon pool, offers a viable path to the production of both propene and ethene, in stark contrast to the often- proposed direct mechanisms. This specific proposal hinges on repeated methylation reactions of alkenes, starting from propene, which occur easily within the zeolite environment. Subsequent cracking steps regenerate the original propene molecule, while also forming new propene and ethene molecules as primary products. Because the host framework stabilizes intermediate carbenium ions, isomerization and deprotonation reactions are extremely fast. Combined with earlier joint experimental and theoretical work on polymethylbenzenes as active hydrocarbon pool species, it is clear that, in zeolite H-ZSM-5, multiple parallel and interlinked routes operate on a competitive basis.