Electrochemical Study of Cobalt Salen Compounds for Catalytic Biomass Degradation

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ABSTRACT Heightened interest in sustainable energy sources have led to a multitude of forays into technologies dealing with biomass refining and energy conversion. While certain streams of biomass offer attractive possibilities as a virtually limitless feedstock, the inherent complexities associated with the constituent biomass polymers, especially the heterogeneous lignin polymer make efficient processing a difficult problem. To make efficient use of lignin as a potential feedstock for energy production or as a chemical precursor, high catalytic specificity and yield is desirable to deal with a broad range of chemical linkages in a manner that results in a single (or few) product(s). In this study, we investigate the use of organometallic Cobalt (salen) complexes and their application toward catalytic breakdown or modification of the lignin polymer. Various lignin model compounds have been used to study possible modes of converting lignin into a usable product in batch reactions with mixed results. In nearly all studies, regardless of yield, the catalyst undergoes deactivation, likely due to an irreversible conformational change or a change in oxidation state of the metal center.By employing non-aqueous cyclic voltammetry, we show that unfixed Co(salen) complexes form a highly reversible redox system. The largely conserved shape of the electrochemical response over successive scans suggests that these catalysts may be recoverable within an electrochemical redox system. Using lignin monomer analogs, we studied electro-catalytic reactions utilizing half-cell cyclic voltammetry and various other characterization techniques to study the electrochemical reversibility of the system and examine the reaction mechanism and oxidation products. Depending on the relative rates of the chemical reaction step and the characteristic response to CV scan rate, the catalytic process defined by the electrochemical oxidation of the catalyst and subsequent chemical reaction step between the ‘activated’ catalyst and the substrate should lead to a “steady state” response similar to that observed for a mass transport limited system. Chemical characterization techniques were also utilized to probe the catalyst oxidation state changes throughout the reaction in an attempt to resolve some of the issues of system deactivation. Keywords : lignin, cobalt salen, electrochemical oxidation, non-aqueous cyclic voltammetry ACKNOWLEDGEMENT We gratefully acknowledge the support of the University of Tennessee Office of the Vice President for Research for support of this work.