Conversion of Methanol to Hydrocarbons: Why Dienes and Monoenes Contribute Differently to Catalyst Deactivation

Methanol-to-hydrocarbons (MTH) conversion over solid acid catalysts offers a feedstock agnostic platform for fuels and chemicals production. Formaldehyde (HCHO) formed via transfer dehydrogenation of methanol facilitates transformation of unsaturated hydrocarbons—olefins, dienes, and aromatics—to inactive polycylics during MTH with propensity of dienes being considerably higher than that of olefins and aromatics to cause catalyst deactivation. Here, we rationalize the basis for the observed differences in catalyst deactivation rates induced by HCHO reactions with monoenes and dienes by directly probing the mechanism of formaldehyde alkylation of propylene and 1,3-butadiene on self-pillared pentasil ZSM-5 using a combination of steady-state kinetic and chemical transient studies in conjunction with kinetic modeling geared to describing rates during transient and steady-state operation.

Transients were observed during start-up and after step-change in reactant pressures during HCHO alkylation with propylene and 1,3-butadiene to 3,6-dihydro-2H-pyran (C₅H₈O) formation, suggesting the presence of a persistent surface intermediate (I^*) that is formed and consumed by irreversible reaction steps. Continuity in C₅H₈O formation rates with a step-change in hydrocarbon(s) pressure and discontinuities in C₅H₈O formation rates with a step-change in water or HCHO pressure, along with the dependences of steady-state C₅H₈O formation rates on reactant pressures, indicate that hydrocarbons only impact I^* formation rates while water and aldehydes directly impact I^* consumption rates. Stoichiometric reactions where water and/or aldehydes were fed to facilitate I^* desorption enable identification of protonated C₅H₉O⁺ species as the persistent intermediate that differs during HCHO alkylation with propylene and 1,3-butadiene as different rate constants for I^* desorption were quantified. A kinetic model, derived from a proposed reaction mechanism, quantitatively captures the transient and steady-state C₅H₈O formation rates, and suggests that distinct reaction intermediates were formed with distinct rates during HCHO alkylation with propylene and 1,3-butadiene, providing rationale for the varying propensity of monoenes and dienes to foment deactivation during MTH.