(79c) Single Site Sn Catalysts for Glucose Conversions

Sn-Beta catalyst was shown to isomerize glucose to fructose with 60-70% selectivity in water[1].  However, the structure of Sn-Beta lacks tunability and Sn loading is low to avoid the formation of extra framework Sn species.  To obtain a more versatile Sn platform, and probe whether zeolitic framework is critical to the high selectivity of fructose, we studied the synthesis, characterization of molecular analogs of active sites of Sn-Beta, and their catalytic performance in glucose conversions.  T₇ polyhedral oligosilsesquioxane (POSS) ligands bearing two or three silanol groups (POSS-R₇-(OH)_x, x =2 or 3) were utilized to anchor Sn(IV) to form discrete Sn-POSS complexes.  The reaction of CH₃SnCl₃ withPOSS-ⁱBu₇-(OH)₃ generated (≡SiO)₃SnCH₃(I) in DMSO solvent.  In contrast, the reaction of Sn(OⁱPr)₄  with POSS-ⁱBu₇-vinyl-(OH)₂ generated (≡SiO)₃Sn(OSi≡) (II), similar to the Sn center in Sn-Beta.  The homogeneous catalytic reactions of sugar conversions in DMSO of these two catalysts were compared.  In addition, II can be heterogenized via reaction of the vinyl groups with silane-terminated silica.  The heterogenized catalyst can also be calcined to remove the ⁱBu groups such that the effect of steric bulk around the catalytic center can be probed.  Glucose reactions were conducted in both water and water-ethanol solvents.  The reaction data were used to generate understandings of the catalytic properties of Sn in different structural configurations as well as different reaction medium.

[1] Moliner, M., Roamn-Leshkov, Y. and Davis, M. E. PNAS 107, 6164 (2010).