(445d) Kinetic and Mechanistic Study of the Chemistry Involved in the Deactivation of Zeolite Catalysts during Methanol-to-Hydrocarbons Conversion

The conversion of methanol-to-hydrocarbons (MTH) on microporous solid acid zeolite/zeotype catalysts offers a feedstock agnostic route for the transformation of gasifiable carbon feedstock to polymer precursors and fuels via synthesis gas (CO/H₂ mixtures) and methanol (CH₃OH) as chemical platforms. Currently this technology is industrially deployed in China to produce ~6 MMT per annum of light olefins. Zeolite catalysts require fluidized-bed operation because of deactivation during MTH catalysis, motivating research in understanding the mechanism and kinetics of catalyst deactivation to provide insights on increasing total turnover capacity – the cumulative amount of hydrocarbon products formed from methanol.

Catalyst deactivation during MTH catalysis is mediated by formaldehyde (HCHO), a product formed by the hydride abstraction from methanol. Formaldehyde undergoes condensation reactions with olefins and aromatics during MTH conversion, which purportedly lead to the formation of bulky polycyclic aromatic species that cause deactivation.

We combine steady-state kinetic, chemical transient, and probe molecule reaction studies to measure rates and elucidate the mechanism of formaldehyde condensation with benzene on self-pillared pentasil ZSM-5 samples with ~3 nm diffusion lengths in an effort to circumvent kinetic artifacts introduced by physical rate processes. Diphenylmethane (DPM), the product of condensation reactions of benzene with formaldehyde followed by benzylation, is formed in >99% selectivity in these reactions. Titration of intermediates subsequent to DPM formation in combination with in situ infrared spectroscopic measurements show that stable benzyloxy species are formed as surface intermediates and that reaction kinetics can be described by a Langmuir-Hinshelwood-type rate expression. These measurements in conjunction with density functional theory calculations provide insights into the mechanism and reactivity of aromatic species with formaldehyde during methanol-to-hydrocarbons conversion.