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.
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© 2019 American Chemical Society.
- alkene hydrogenation