Alkali cations enhance Zr-beta zeolite selectivity in citronellal Meerwein-Ponndorf-Verley reduction

Abstract

Zr-silicate zeolites efficiently catalyze Meerwein-Ponndorf-Verley (MPV) reduction, a key reaction in fine chemical production. However, for substrates such as citronellal, MPV reaction selectivity decreases due to competing Lewis acid-catalyzed reactions, notably carbonyl-ene cyclization, which yields isopulegol as the main product.  Reaction selectivity may be determined by site strength as weaker “closed” sites in Zr-beta catalyze MPV reduction, while stronger “open” Zr sites catalyze carbonyl-ene cyclization. But the precise control over acidity strength in Zr-beta catalysts remains an unsolved challenge. This study aims at modulating Al-free Zr-beta zeolite acidity through ion exchange with Li⁺, Na⁺ and Cs⁺ ions. In citronellal MPV reduction, we increased citronellol selectivity by tuning the Lewis acidity of Zr-beta. As shown by FT-IR spectroscopy of acetone and d₃-acetonitrile, ion exchange weakened interactions between carbonyl compounds and “open” Zr Lewis acid sites, converting strong Zr “open” sites into weaker Lewis acid sites and, thus, switching the reaction selectivity from the isopulegol to the MPV reduction. Case in point, ion-exchanged Zr-beta-1-Na⁺4x provided 77% citronellol selectivity and 23% isopulegol selectivity. In contrast, non-exchanged Zr-beta-1 afforded 26% citronellol selectivity and 73% isopulegol selectivity, at a similar conversion level, without significant changes in the overall rate of citronellal conversion. These findings highlight ion exchange as an effective tool for post-synthesis control of Zr-beta Lewis acidity.