Boosting Reductive Etherification of Cinnamaldehyde via Synergistic Lewis/Brønsted Acid Sites in Zr-Doped Molecular Sieves

Abstract

Selective hydrogenation of the conjugated carbon–carbon double bond (C═C) and carbon–oxygen bond (C═O) is an important reaction in organic synthesis, which not only provides access to key intermediates in natural product synthesis but also serves as a probe reaction for the evaluation of catalyst reactivity. However, the higher activation energy of the C═O bond over the C═C bond renders its reduction more challenging. In this work, we report a tailored zirconium (Zr)-doped zeolite catalyst featuring integrated Lewis and Brønsted acid sites for the selective transfer hydrogenation of the C═O bond of cinnamaldehyde (CAL) followed by etherification to yield 1-cinnamyl 2-propyl ether (CPE). A series of Zr catalysts supported on different zeolites were prepared and characterized. Among them, Zr-doped mesoporous SBA-15 (Zr/SBA-15) exhibited the highest reactivity, achieving complete conversion of CAL and 99% yield of CPE at 140 °C in 7 h. Mechanistic studies revealed that balanced Lewis/Brønsted acid sites were key to the reaction, where Lewis acid sites primarily catalyzed the transfer hydrogenation of CAL to cinnamyl alcohol (COL), while Brønsted acid sites promoted the subsequent esterification of COL to CPE. The current work not only establishes an efficient one-pot approach for the synthesis of allyl ethers from α,β-unsaturated carbonyl compounds but also provides insights into tuning catalyst reactivity through support modification, which would benefit both fundamental research and industrial production.