Low Temperature Biomass Deconstruction by Zeolite-encapsulated Enzyme Mimics
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Low Temperature Biomass Deconstruction by Zeolite-encapsulated Enzyme Mimics L. Shannon Davis*, Thomas T. Eisenhart, Brianna C. Hughes, and Amy L. Pressley Department of Chemistry, 250 Forest Drive, Georgia Southern University, Statesboro, GA, USA 30460 * Corresponding Author, email: [email protected], phone (912) 478-5055, fax (912) 478-0699 ABSTRACT Oxidation catalysis is a reaction necessary for the production of plastics and other materials that seem now essential to our everyday lives. Unfortunately, most oxidation processes suffer from poor selectivity or yields, creating unwanted byproducts and waste. In nature, oxidative enzymes like methane monooxygenase and the family of cytochromes provide a more selective method for oxidation of organic compounds. Of particular interest is the low temperature, selective oxidation of cellulosic biomass for the production of biofuels or other useful replacements for materials currently derived from petroleum feedstocks. An enzymatic approach could replace the high temperature pyrolysis technology in use today. A series of inorganic mimics of some oxidative enzymes, using transition metal – amino acid complexes encapsulated in large pore zeolites have been synthesized and examined as oxidation catalysts under benign conditions. Several of these demonstrate turnovers comparable to native enzymes in the reaction of model compounds for the oxidation of lignin and cellulose. INTRODUCTION The catalytic function of oxidative enzymes such as manganese peroxidase, lignin peroxidase, and laccases are intriguing for their function as natural biomass deconstruction agents [1]. The mechanism of action and the importance of the structure-activity relationships to the catalytic activity of these enzymes have been widely studied; the active site of these enzymes is a transition metal bound by a small coordination sphere of amino acids. However, simple transition metal complexes of amino acids, like metalloporphyrin compounds, suffer from oxidative instability [2], and enzymes themselves remain unable to perform well outside a restrictive range of temperatures and pH values, even with modern technical modifications [3, 4]. A solution to this problem is the use of zeolites to provide the missing structural stability, oxidative inertness, and a source of electrons similar to that found in the native protein. Zeozymes, zeolites acting as inorganic mimics for enzymes [5-7]), are intriguing materials for complex catalytic processes. Novel catalysts such as these fill a gaping hole in the exploration of low temperature deconstruction of biomass. To date, analogous studies focused only on copper amino acid complexes in zeolites have been performed [8-10]. In order to implement a shift in renewable sourcing of fuel feedstocks, the discovery of effective catalysts for the deconstruction of biomass materials is necessary. Catalysts that can carry out the function of hydrolytic and oxidative enzymes without the constraints of pH and temperature will be highly prized. The reduction of biomass
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