How Many Molecules Can Fit in a Zeolite Pore? Implications for the Hydrocarbon Pool Mechanism of the Methanol-to-Hydrocarbons Process

Abstract

The methanol-to-hydrocarbons (MTH) process is a very advantageous way to upgrade methanol to more valuable commodity chemicals such as light alkenes and gasoline. There is general agreement that, at steady state, the process operates via a dual cycle “hydrocarbon pool” mechanism. This mechanism defines a minimum number of reactants, intermediates, and products that must be present for the reaction to occur. In this paper, we calculate (by three independent methods) the volume required for a range of compounds that must be present in a working catalyst. These are compared to the available volume in a range of zeolites that have been used, or tested, for MTH. We show that this straightforward comparison provides a means to rationalize the product slate and the deactivation pathways in zeotype materials used for the MTH reaction.

Neutron scattering studies of the methanol-to-hydrocarbons reaction

“…The finite size of the channels in the zeolite limit the possible species to methylated benzenes, naphthalenes and anthracene. [86][87][88][89] The INS spectra of benzene and all its methylated derivatives, C 6 H 6−x (CH 3 ) x (x = 0-6), as well as cyclic and polycyclic alkenes and aromatic molecules were recorded as candidate species. 90 Synthetic spectra were generated by adding spectra of the model compounds in an equimolar ratio; similar hydrocarbons were added together in order to highlight common spectral features.…”

Methods for Studies of Reactions on Zeolite Catalysts Occurring by the Hydrocarbon Pool Mechanism

“…Alkenes are formed as the primary products of C-C coupling, then they are methylated with methanol and subsequently cracked in one cycle (alkene cycle), while the aromatic hydrocarbons are methylated and subsequently dealkylated in another (arene) cycle, undergoing isomerization and transalkylation [26,27]. In the olefin cycle, chain propagation is observed, forming higher alkenes, which can transfer hydrogen, forming alkanes and aromatic hydrocarbons (Scheme 1) [28]: Scheme 1. HCP mechanism with a double cycle proposed for zeolite-catalyzed transformation of methanol into hydrocarbons.…”