I. Tin Silsesquioxanes as Analogs for the Open and Closed Sites in Tin-Containing Zeotype Beta and II. Enantiomerically Enriched, Polycrystalline Molecular Sieves

Citation

Brand, Stephen Kramer (2017) I. Tin Silsesquioxanes as Analogs for the Open and Closed Sites in Tin-Containing Zeotype Beta and II. Enantiomerically Enriched, Polycrystalline Molecular Sieves. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z96T0JPX. https://resolver.caltech.edu/CaltechTHESIS:04182017-134255198

Abstract

The use of biomass as a resource to produce value-added products has garnered significant interest as a means of reducing reliance on fossil fuels. This task is complicated by the complex, highly functionalized nature of abundant biomass derivatives, such as glucose. Tin-containing zeolite Beta (Sn-Beta) has been investigated as a catalyst for isomerizing aldohexoses into ketohexoses through a Lewis acid mediated hydride shift (1,2-intramolecular hydride shift, 1,2-HS). Recent studies on the reactivities of Lewis base-doped and alkali-exchanged Sn-Beta samples have conclusively demonstrated that the open tin site performs the glucose isomerization reaction. With Lewis base doped Sn-Beta, glucose conversion is almost completely eliminated and product selectivity is shifted predominantly to mannose, formed through a 1,2-intramolecular carbon shift (1,2-CS). To understand the structure-activity relationships between the conditions of the active sites in the zeolite, three molecular models (tin silsesquioxanes) of the tin sites in the zeolite are synthesized. Two tin silsesquioxanes that contain an octahedral tin site with and without an adjacent silanol group are prepared and used as catalysts for the reaction of glucose. The catalyst that contains the adjacent silanol group selectively forms fructose through a 1,2-HS while the catalyst without the silanol group yields mannose through a 1,2-CS. These results provide further evidence for the nature of the active sites in Sn-Beta. A methyl-ligated tin silsesquioxane is experimentally and theoretically examined to examine possible reactivities at the closed site. This compound is an active glucose conversion catalyst that selectively produces mannose, although the rates of reaction are far below those obtained from Sn-Beta. Additionally, a hybrid quantum mechanical/molecular mechanics model is constructed, and the complete catalytic cycle is computationally examined via considering ring-opening, three distinct pathways for each hydride- and carbon-shift reaction, and ring-closing. The combined experimental and computational results suggest that there could be reaction pathways that involve Si-O-Sn cleavage that give much slower reaction rates than the open tin site in Sn-Beta.

Zeolite and zeolite-like molecular sieves are being used in a large number of applications such as adsorption and catalysis. Achievement of the long-standing goal of creating a chiral, polycrystalline molecular sieve with bulk enantioenrichment would enable these materials to perform enantioselective functions. In part II of this thesis, the synthesis of enantiomerically enriched samples of a molecular sieve is reported. Enantiopure organic structure directing agents (OSDAs) are designed with the assistance of computational methods, and used to synthesize enantioenriched, polycrystalline molecular sieve samples of either enantiomer. Computational results correctly predicted which enantiomer is obtained, and enantiomeric enrichment is proven by high-resolution transmission electron microscopy. The enantioenriched and racemic samples of the molecular sieves are tested as adsorbents and heterogeneous catalysts. The enantioenriched molecular sieves show enantioselectivity for the ring opening reaction of epoxides and enantioselective adsorption of 2-butanol (R enantiomer of the molecular sieve shows opposite and approximately equal enantioselectivity from the S enantiomer of the molecular sieve, while the racemic sample of the molecular sieve shows no enantioselectivity).

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