Confinement-Driven Selectivity for Alkanol Dehydration in Ti-BEA Zeolites

Abstract Zeolite catalysts offer unique opportunities for tuning catalytic behaviors through modifications to both the active site and the surrounding pore environment. Although confinement effects in zeolites have been investigated, their specific contributions—particularly in Lewis acid catalysts—remain difficult to isolate from active-site chemistry and diffusive limitations. Here, we use density functional theory combined with microkinetic modeling to demonstrate confinement effects on the dehydration of ethanol, n-propanol, and n-butanol to alkene and dialkyl ether products catalyzed by Ti-BEA zeolites. Dehydration mechanisms are proposed over “closed” heteroatom-substituted sites, partially hydrolyzed “hydroxy-open” sites, and “alkoxy-open” sites formed through the dissociative adsorption of an alkanol. Closed-site dehydration dominates both alkene and ether formation at low alkanol partial pressures in the absence of co-fed water. Dispersion stabilization increases with alkanol size, decreasing the ratio of alkene-forming reactant-state coverages relative to ether-forming states. For alkanols that are smaller than the surrounding zeolite framework, dispersion stabilizes the closed-site alkene transition state, resulting in similar alkene selectivities for EtOH and n-PrOH. Repulsive interactions between the n-BuOH backbone and the framework raise the closed-site alkene formation energy barrier, yielding an order-of-magnitude lower alkene selectivity compared to EtOH and n-PrOH. With increasing alkanol pressure, site saturation suppresses closed-site alkene formation, causing alkene selectivity to decrease and eventually plateau.