Center for Direct Catalytic Conversion of Biomass to Biofuels

1. Lignin comprises as much as 25% of terrestrial biomass, but remains one of the most underused carbon sources in the biosphere.1 The main obstacle in utilizing lignin is its irregular, heterogeneous structure,resulting from biosynthetic, free radical polymerization of a small group of monolignols. Thus, developing processes that convert it into a single material in high yieldwould greatly increase its value and utility within the biorefinery. Models of G and S subunits: Proposed mechanism Model of lignin in poplar UV/Vis reveals that the sterically hindered base does not coordinate to the catalyst. We envision deprotonation of the phenol substrate: Primary lignin monomers We seek to convert lignin into a high-value material, p-quinones, which are: • Essential structural blocks for dyes and antibiotics and many have shown biological, pharmacological and antitumor activity. • Molecules that can be functionalized in order to synthesize complex natural products. • Due to their unique proton and electron transfer abilities, they form part of the respiration process of almost all living organisms. Sterics: -Addition of Im or N-MeIm – 10% DMBQ -No added ligand, 30% DMBQ -Addition of 2-MeIm and 1,2-diMeIm - >75% DMBQ promote high yields of DMBQ formation. -Bulky ligands distort salen, prevent addition of a second ligand, and may raise the dz2 energy Electronics: -Co(salen)/2-MeIm is high spin, placing the unpaired electron in the dx2-y2 orbital -Co is displaced from the salen plane to reduce the ligand field strength -The presence of an electron in the highest energy orbital facilitates its redox transfer from CoII to the oxygen ligand, forming the CoIII-superoxo complex Our approach it to focus on the single unifying structural feature of lignin - its network of aromatic rings. We examine a processes that can cause selective oxidation of this aromatic network in models of S and G subunits. Preliminary DFT calculations Gaussian03 B3LYP functional/6-31g basis set Alpha spin density – primarily on O2 Beta spin density – primarily on Co Currently working to locate single electron Models of G subunits More difficult to oxidize2 but yields are improved in the presence of a non-coordinating base. Models of S subunits Oxidized in the presence of a pyridine or imidazole coordinating base. Terminal oxygen spin density When using these oxidizing conditions on lignin itself we observe formation ofquinones among other byproducts.2 In order to identify those byproducts we synthesized 3 dimeric model compounds representative of lignin and carried out the oxidation.3 Preliminary calculations reproduce structure of Co(salen)/pyr complex Impact: We have successfully oxidized models of the S and G subunits in lignin in the presence a variety of axial ligands and hindered bases. In addition, we used these oxidizing conditions in lignin itself. This is a starting point for the conversion of lignin into a single material. Current work is expanding the range of metal catalysts and investigating the mechanism through EPR studies. When using these oxidizing conditions on dimeric models of lignin we observe cleavage only when the β-O-4 bond is present. • References and Notes • Bozell, J. J.; Hames, B. R. J. Org. Chem.1995, 60, 2398. • 2. Superimposed 2D HMQC spectra before (red) and after (gray) oxidation. Isolated lignin was reacted in the presence of methanol, 10% cobalt catalyst, O2 (60psi), 24 hrs. • 3. Crestini, C.; D’Auria, M.; Tetrahedron, 1997, 53, 7877; Sipila, J.; Syrjanen, K.; Holzforschung, 1995, 49, 325; Brunow, G.; Sipila, J.; S. von Unge. Cellulose Chem. Technol. 1998, 22, 191. Although we were not able to identify the byproducts, we learned that the β-O-4 bondis important for the conversion of lignin into p-quinones. Metal Catalyzed Oxidation of Biorefinery Lignin Joseph J. Bozell,† Diana Cedeno† and Thomas Elder‡ †Center for Renewable Carbon, University of Tennessee, 37996. ‡USDA-ForestService Southern Research Station, Pineville, LA 71360. Center for Direct Catalytic Conversion of Biomass to Biofuels Spin density on oxygen increases as the size of the ligand increases. The Center for Direct Catalytic Conversion of Biomass to Biofuels (C3Bio) is an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0000997.