Lewis acid zeolites, primarily Al-free Zr and Sn silicates, catalyze the chemoselective reduction of ketones and aldehydes to the corresponding alcohols through hydrogen transfer (Meerwein-Ponndorf-Verley (MPV) reduction). Sn silicates are more active in the MPV reduction of ketones, whereas Zr silicates are more active in the MPV reduction of aldehydes. However, the catalytic activity of these zeolites has not been accurately ascribed to "open" vs. "closed" Zr sites even though this correlation is crucial for systems whose substrate structure allows competing reaction pathways. For example, MPV reduction of citronellal competes with carbonyl-ene cyclization to isopulegol and acetalization in the citronellal reaction with 2-propanol. Therefore, we aimed to correlate thoroughly characterized Lewis acid sites in Zr-substituted beta and MFI zeolites with their selectivity. For this purpose, we analyzed Zr-zeolite acidity by fourier transform infrared spectroscopy (FTIR) spectroscopy of adsorbed deuterated acetonitrile and acetone because deuterated acetonitrile probes "open" Zr sites without interacting with "closed" sites, but acetone identifies both "open" and "closed" sites. Our results showed that Zr-beta rich in Zr "closed" sites favored MPV reduction. Conversely, Zr-beta rich in "open" sites and reference catalysts yielded isopulegol as the main product. Ion exchange of the Zr-beta "open" sites with Na+ cations deactivated these sites, thereby switching the selectivity to citronellol. In turn, the silanol groups of the catalyst promoted acetalization, regardless of substituting the heteroelement (Zr or Sn). These findings demonstrate that Zr-site Lewis acidity determines terpenoid reduction selectivity, as the relatively weaker Zr-beta "closed" sites catalyze citronellal MPV reduction to citronellol, while the relatively stronger Zr-beta "open" sites catalyze intramolecular carbonyl-ene cyclization to isopulegol. Moreover, this correlation between selectivity and Zr-site Lewis acidity may enable us to design specific catalysts, even for systems with competing reactions, based on quantitative data acquired using our experimental paradigm.
Gołą̨bek et al. (Thu,) studied this question.