Direct acylation of furanics with carboxylic acids over Bronsted acid zeolites an important reaction for producing both fuels and high-value chemicals. However, furanics, which can serve as acyl acceptors, rapidly undergo coupling reactions in the presence of Bronsted acid surface sites leading to rapid catalyst deactivation. Carboxylic acids themselves can also form reactive ketenes in equilibrium with surface acyl species. In this work, the role of furan self-coupling, side reactions with ketenes, reactions with formed products are decoupled by feeding each species independently and comparing activity loss rates with those of varying surface coverage. The specific reactants employed in this study are 2-methylfuran (2-MF) and acetic acid (AcA) to yield the product was 2-acetyl-5-methylfuran (Ac-MF). We reveal that the deactivation constant associated with direct 2-MF adsorption on a clean catalyst surface is approximately twice that of product self-coupling under identical conditions. Co-feeding carboxylic acids, which after dehydration produces trace levels of deactivating ketene species, improves catalyst stability by diminishing the deactivation rate by several orders of magnitude via inhibiting adsorption of 2-MF and the formed products. Furthermore, evaluating the prominent route toward catalyst deactivation at high acid coverage was determined by systematic modification of surface coverage and the diffusion path of produced products. These results uncover the critical role of acetic acid to limit direct interaction between highly reactive acyl acceptors with surface Bronsted sites. Moreover, the addition of varying partial pressures of inert molecules with significant adsorption constants allows manipulation of surface coverage while maintaining the gas phase chemical potential. These experiments showing that the product (Ac-MF) reacting with gas phase 2-MF is the primary mechanism of deactivation at high acid coverage, not sequential reactions along the diffusion path or direct interaction of 2MF with the zeolite surface.

Critical role of surface coverage on direct acylation reactions with highly reactive ACYL acceptors

ACS Spring Meeting 2021

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