Mechanistic Basis for Effects of High-Pressure H₂ Cofeeds on Methanol-to-Hydrocarbons Catalysis over Zeolites

Cofeeding high-pressure (16 bar) H₂ with methanol (0.005 bar) during methanol-to-hydrocarbons conversion over acidic zeolites with varying topologies (CHA, AEI, FER, and BEA) results in a ∼2× to >15× enhancement in catalyst lifetime compared to He cofeeds, as determined by the cumulative turnovers attained per proton before the final methanol conversion level drops below 15%C. These beneficial effects of prolonged catalyst lifetime are observed without any impact on the carbon backbone of effluent hydrocarbon products characteristic of the particular zeolite topology. The olefins-to-paraffins ratio of C₂₊ hydrocarbons, however, decreases due to enhanced paraffins production, and the magnitude of this decrement depends on the specific zeolite topology. The observations of marked lifetime improvements and topology-dictated variations in the paraffin make of MTH effluent with H₂ cofeeds can be interpreted based on the different proclivities of zeolitic protons confined in varying topological environments for catalyzing hydrogenation of hydrocarbons that are predominantly formed via formaldehyde-based alkylation routes (e.g., 1,3-butadiene) or methanol-based alkylation routes (e.g., ethene and propene). Independent kinetic studies reveal that measured hydrogenation rates per H⁺ of 1,3-butadiene are at least 1 order of magnitude (∼7× to ∼320×) higher than that of ethene or propene, which provides an explanation for the observed lifetime improvements in MTH with H₂ cofeeds. Further, trends in the reactivities of ethene and propene with H₂ over the different zeolites help explicate the topology-dependent variations in the paraffin content of the effluent hydrocarbons during MTH with H₂ cofeeds.