(289b) Mechanisms and Rates of Catalyst Deactivation in Methanol-to-Hydrocarbons Conversion

The implementation of methanol-to-hydrocarbons (MTH) technology on a commercial scale has brought renewed interest in mechanistic chemistry and reactor scale processes involved in methanol conversion on solid acid catalysts. The dual aromatic- and olefins-based methylation/cracking cycle has provided over the past two decades a context for describing propagation sequences prevalent in the ‘hydrocarbon pool’ mechanism however, this scheme is devoid of mechanistic guidance on catalyst deactivation. We illustrate, deactivation in MTH catalysis is initiated by unproductive dehydrogenation reactions of methanol to form formaldehyde via methanol disproportionation and olefin transfer hydrogenation. Subsequent alkylation reactions between formaldehyde and active olefinic/aromatic co-catalysts instigate cascades for dehydrocyclization, resulting in the formation of inactive polycyclic aromatic hydrocarbons.¹

Next we inquired of a metric to describe the propensity of olefins, dienes, and aromatics to cause deactivation in these HCHO-mediated alkylation events. We propose metrics for assessing deactivation that consider active sites consumable, akin to reacting species. This in turn allows us to define the rate, yield, and selectivity for the loss of sites during reaction.² We illustrate utility of these metrics by reporting that butadiene is ~100x more pernicious in causing H⁺ site loss upon reactions with formaldehyde than propylene or toluene on a molar basis at typical MTH conditions.³ Therefore, dienes along with formaldehyde should be considered as critical intermediates in fomenting deactivation in MTH catalysis and strategies to eliminate polyunsaturated species and/or intercept reaction sequences of these intermediates with HCHO will likely enhance lifetime.

1. "Deactivation of zeolites and zeotypes in methanol-to-hydrocarbons catalysis: Mechanisms and circumvention" Acc. Chem. Res. 52 (2019) 2647

2. "A method for assessing catalyst deactivation: A case study on methanol-to-hydrocarbons conversion” ACS Catal. 9 (2019) 7065

3. “A kinetic evaluation of deactivation pathways in methanol-to-hydrocarbons catalysis on HZSM-5 with formaldehyde, olefinic, dieneic, and aromatic co-feeds” ACS Catal. 11 (2021) 3628