The Meerwein-Ponndorf-Verley reduction of aldehydes and ketones and Oppenauer’s oxidation of alcohols (MPVO reactions) are important reactions in selective reduction of carbonyl groups under mild conditions. Recently, MPVO reactions have been considered as the bridge that could connect biomass upgrading and chemical industry. As the most promising catalyst, Sn substituted Beta (Sn-Beta) zeolite has attracted wide attention in many processes of biomass upgrading due to its unique activity, including MPVO process. The open site((SiO)3Sn-OH), closed Sn sites ((SiO)4Sn), and extra-framework SnO2 particles are supposed can be formed on Sn-Beta zeolite, depending on the preparation method. The open Sn site has been considered as the active site for many reactions on Sn-Beta zeolite. However, the direct characterization of open Sn site and understanding of their role in biomass-related reactions are challengeable works. Here, two types of open tin sites are unambiguously identified via correlating the hydroxyl groups to Sn atoms by using of one and two-dimensional proton-detected 1H/119Sn correlation solid-state NMR spectroscopy, which only amounts to ca. 17% of the total tin content. A reversible transformation between the open and closed tin site is observed. Additionally, we find the experimental evidence for the activation of carbonyl group of a ketone in the MPVO reaction catalysed by Sn-Beta zeolite. 13C, 119Sn, and 13C–119Sn double resonance NMR spectroscopic studies demonstrate that only the open Sn site acetone is responsible for the activation of acetone to form a stable surface gem-diol-type species. In-situ NMR experiments together with density functional theory (DFT) calculations suggest that the gem-diol-type species exhibits high reactivity and can serve as an active intermediate in the MPVO reaction of acetone with cyclohexanol.
What will audience learn from your presentation?
- Methodology of solid-state NMR and its application heterogenous catalysis.
- This will help the audience in understanding the structure and role of metal Lewis acid sites on zeolite.
- This provides a practical solution to characterize the active sites on zeolites
- This will be helpful for the design of highly active zeolites catalysts in biomass conversion.