Strategies to modulate selectivity and improve stability in methanol to hydrocarbons catalysis over zeolites or zeotypes and their mechanistic origins

Abstract

The catalytic conversion of methanol-to-hydrocarbons (MTH) over Brønsted acid zeolites/zeotypes is now used to derive an increasing fraction of the annual 250 million metric tons production of light olefins, ethene and propene—feedstocks in plastics, pharmaceuticals, and chemicals manufacturing. Controlling the relative formation rates of ethene and propene, and mitigating deactivation rates of zeolites that necessitates energy-intensive regeneration cycles to restore catalytic activity are the two critical outstanding challenges. This work demonstrates the hitherto unknown role of trace quantities of aldehydes in distinct chemical events that dictate the selectivity ratio of ethene and propene, and the extent of catalyst deactivation during MTH. Specifically, it is shown that formaldehyde, formed in trace quantities by the loss of hydrogen from methanol, facilitates the propagation of reactions that promote ethene production, while simultaneously accelerating rates of chemical transformations that induce catalyst deactivation. This mechanistic understanding is exploited to demonstrate that in MTH, i) the selectivity ratio of propene-to-ethene can be notably varied (1-25 range) by controlling the inlet methanol pressure which directly affects formaldehyde production and consequently its involvement in reactions that produce ethene, and ii) catalyst lifetime can be considerably enhanced (>70x) by co-processing high-pressure H2 which results in the interception of formaldehyde-mediated condensation pathways deleterious to catalyst lifetime by selective hydrogenation of the unsaturated intermediates formed in these reactions.