The promotion of cooperative catalytic interactions between metal and organic active sites in a self-assembled system is a challenging task. Achieving such precise microstructure organization requires flexibility one the one hand and stiffness on the other. Enzymes leverage on their ability to form highly specific hydrogen bonds to facilitate site cooperativity. To circumvent this rigid constraint we use grafting of metal sites on the backbone chitosan polymer to obtain active site cooperativity in a heterogeneous synthetic system. Chitosan is a naturally abundant polysaccharide. It is largely insoluble in most organic and aqueous solutions and is inherently functionalized with hydroxyl and amine groups. These properties make it a prime candidate as a functional material for a range of adsorption and catalytic applications. A prerequisite for such applications is to overcome the strong molecular hydrogen bonds within chitosan to obtain a high and accessible surface area. We developed a simple yet novel method for the preparation of metal-chitosan hybrid materials with up to 200 m2/g, retaining the primary amine sites free for catalysis. The synthesis of the hybrid catalysts entails aqueous phase dissolution of the chitosan followed by solvent exchange to an aprotic organic solvent. The chitosan gel is then grafted with e.g. SnCl4 or TiCl4 to form alkoxy bonds with the chitosan hydroxyls. The covalent grafting localizes the Sn or Ti in close proximity to the amine site, allowing cooperative catalysis in condensation reactions of biomass derived molecules. Catalytic materials are characterized using XRD, HR-TEM-EDS, TGA-MS, FTIR, and XPS. We find that when the amine is adjacent to the metal-hydroxyls kinetics is significantly faster than when the amine is on its own. Moreover, reaction selectivity can be controlled by manipulating the amount of grafted metal sites. The effect of the metal type and the state of amine is correlated to catalytic results.

Promoting aldol addition by cooperative interactions in metal functionalized chitosan

254th National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.