Selectivity Control in Tandem Catalytic Furfural Upgrading on Zeolite-Encapsulated Pt Nanoparticles through Site and Solvent Engineering

Selectivity control is a pressing challenge in developing selective tandem catalytic processes. In this work, we demonstrate that tailoring types of acid sites of zeolite-encapsulated Pt nanoparticles (NPs) and choosing solvents provide a compelling strategy to manipulate product distribution in tandem catalysis. The model tandem furfural conversion with acetone and ethanol (EtOH) is investigated in cyclohexane and EtOH solvents. Pt NPs encapsulated in ZSM-5 with Bronsted acid sites (BAS) only located in intracrystalline mesopores selectively mediate the tandem aldol condensation and subsequent hydrogenation of furfural and acetone by limiting the access of furfural to Pt sites and promoting the aldol condensation. A combined yield of 69% toward hydrogenated aldol adducts is achieved. In contrast, furfural hydrogenation to valeric acid (VA) and ethyl valerate (EV) dominates with a combined yield of 80% on Pt NPs encapsulated in ZSM-5 with Lewis acid sites (LAS) and BAS in both zeolitic micropores and intracrystalline mesopores. The concentration of EtOH in the reaction system is identified as another key parameter in selectivity control: (1) low EtOH concentrations (EtOH to furfural ratio of <5) suppress the catalytic transfer hydrogenation pathway and favor the formation of 2-methylfuran (MF) from furfural, as most hydrogenation and hydrogenolysis steps occur on Pt NPs, (2) modest EtOH concentrations (EtOH to furfural ratio of 5-10) favor VA/EV by enabling the catalytic transfer hydrogenation pathway in zeolitic micropores and exposing intermediates to BAS prior to Pt NPs, and (3) high EtOH concentrations (EtOH to furfural ratio of >10) suppress the production of VA/EV by excessive water formation via acid-catalyzed etherification of EtOH. Therefore, tailoring the nature of active sites and changing the solvent composition are both effective strategies in manipulating the product distribution in tandem catalysis.