gem-Diol-Type Intermediate in the Activation of a Ketone on Sn-beta Zeolite as Studied by Solid-State NMR Spectroscopy

摘要

Lewis acid zeolites have found increasing application in the field of biomass conversion, in which the selective transformation of carbonyl-containing molecules is of particular importance due to their relevance in organic synthesis. Mechanistic insight into the activation of carbonyl groups on Lewis acid sites is challenging and critical for the understanding of the catalytic process, which requires the identification of reaction intermediates. Here we report the observation of a stable surfacegem-diol-type species in the activation of acetone on Sn-beta zeolite.C-13,Sn-119, and(13)C-Sn-119 double-resonance NMR spectroscopic studies demonstrate that only the open Sn site ((SiO)(3)Sn-OH) on Sn-beta is responsible for the formation of the surface species.C-13 MAS NMR experiments together with density functional theory calculations suggest that thegem-diol-type species exhibits high reactivity and can serve as an active intermediate in the Meerwein-Ponndorf-Verley-Oppenauer (MPVO) reaction of acetone with cyclohexanol. Thegem-diol-type species offers an energy-preferable pathway for the direct carbon-to-carbon hydrogen transfer between ketone and alcohol. The results provide new insights into the transformation of carbonyl-containing molecules catalyzed by Lewis acid zeolites.