The reactivity obtained using weakly acidic surface hydroxyls on silica and alumina for polyglucan hydrolysis, typically catalyzed by strong acid sites, highlights the sensitivity for precise positioning in facilitating catalytic reactivity by weak acid sites. Precise positioning of weak acid sites has an even greater role in inducing bifunctional catalytic reactivity on oxide surfaces. This has been achieved by chemical grafting of amine function adjacent to a hydroxyl function. However, the rigid nature of the metal oxide surface hinders the ability to align the active sites in a manner that juxtaposes the acid and base in an optimal form. Conversely, the use of an amine containing polyhydroxylated polymer such as chitosan offers both the high density of hydroxyls juxtaposed next to an amine function and the processing flexibility of a polymeric system. In this case it is imperative to prevent collapse of the pore system, driven by the tendency to form multiple inter- and intra-molecular hydrogen bonds, limiting accessibility to the active site.Herein we use surface grafting techniques to form inter- and intra-molecular metal oxide crosslinks as mediators between the chitosan chains. The crosslinking in this inorganic-chitosan hybrid material is designed to resist the chain collapse and maintaining accessibility to the bifunctional active sites. The thermal stability of these of metal-crosslinked chitosan materials is discussed. Polymer stretching is used as means to macroscopically orient the position and direction of the molecular functions (amine and hydroxyl) on the polymer backbone. Finally the performance of these hybrid materials is assessed for catalytic aldol condensation reactions with respect to the degree of crosslinking and the type of metal crosslinker.

Crosslinked inorganic-polymer hybrids as tunable bi-functional catalysts

248th National Meeting (2014)

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