1,2-H- versus 1,2-C-Shift on Sn-Silsesquioxanes

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• 作者：Tyler R. Josephson ; Stephen K. Brand ; Stavros Caratzoulas ; Dionisios G. Vlachos
• 刊名：ACS Catalysis
• 出版年：2017
• 出版时间：January 6, 2017
• 年：2017
• 卷：7
• 期：1
• 页码：25-33
• 全文大小：611K
• ISSN：2155-5435

文摘

Lewis acidic zeolites such as Sn-Beta catalyze glucose isomerization to fructose via an intramolecular 1,2-H-shift reaction, a key step for converting lignocellulosic biomass into renewable chemicals. Na-exchange of Sn-Beta titrates the neighboring SiOH group in the open Sn site and shifts catalyst selectivity to mannose formed by a 1,2-C-shift reaction. To probe structure/activity relationships in the zeolite, tin-containing silsesquioxanes with (1a) and without (1b) a neighboring SiOH group were recently synthesized and tested. These molecular catalysts are active for glucose conversion, and the presence (absence) of the SiOH favors fructose (mannose) selectivity by intramolecular H(C)-shift reactions. Using density functional theory, we investigated numerous H/C-shift pathways on these tin-silsesquioxane catalysts. On both 1a and 1b, the H-shift reaction occurs through a bidentate binding mode without participation of the SiOH, while the bidentate binding mode is not favored for the C-shift due to steric hindrance. Instead, the C-shift reaction occurs through different concerted reaction pathways, in which an acetylacetonate (acac) ligand interacts with the substrate in the transition state complexes. Favorable H-shift pathways without SiOH participation and acac ligand promotion of the C-shift pathway explain why 1a produces mannose from C-shift reactions instead of exclusively catalyzing H-shift reactions, as the Sn-Beta open site does.