Senior Investigators: Prof. Peter Ford (UCSB), Prof. Susannah Scott (UCSB), Prof. Wes Borden (U. North Texas), Dr. David Thorn (LANL), Dr. Tom Baker (U. of Ottawa).  

Global energy consumption is projected to double over the next three decades, placing severe demands both on the environment and on existing fossil fuel reserves. The development of alternatives to fossilized carbon as feedstocks for fuels and for chemical precursors for plastics, fabrics, and other valuable commodities is thus increasingly urgent. Non-food biomass represents one such alternative, since it is both renewable and, in principle, carbon-neutral. The Department of Energy has estimated that the amount of non-food biomass available annually is equivalent in energy to 3 billion barrels of liquid transportation fuel. This material is largely lignocellulose; thus, developing fundamental catalytic strategies to unlock this resource would have major economic, strategic and environmental ramifications.

The chemical approaches necessary to convert biomass to fuels and chemicals are fundamentally different than those needed for petroleum. In petrochemical catalysis, key tasks are to introduce functionality into unreactive hydrocarbons at elevated temperatures and pressures, and to assemble smaller molecules into larger ones (e.g., polymers). In contrast, lignocellulosic feedstocks require new catalysis strategies to disassemble these highly functionalized biopolymers and to prepare higher value compounds via selective removal and transformations of functional groups.

Cellulose is a linear polymer of D-glucose linked by β(1,4)-glycosidic bonds and the most plentiful biopolymer in the world. The closely associated lignins, which constitute up to 30% of woody biomass, are heterogeneous biopolymers with irregular structures. However, the most characteristic units are methoxylated and hydroxylated 4-propylphenols, which are coupled primarily as ethers but occasionally cross-linked with carbon-carbon bonds. We are pursuing strategies for the disassembly of these complex molecules.

Vanadium-based oxidative lignocellulose disassembly: It has been recognized for some time that vanadium(V) complexes are capable of oxidative C-C bond cleavage reactions. Developing new vanadium-catalyzed aerobic oxidative C-C bond cleavage reactions would complement related oxidation processes catalyzed by other metals and could enable reactions that provide new chemical intermediates from cellulose and lignin.

Our recent studies have focused on the reactivity of vanadium complexes of the ligand dipicolinic acid (H₂dipic). Reaction of the isopropoxide V(V) complex (dipic)V(O)OiPr with pinacol yields the chelating V(V) diolate complex 2. When complex 2 is dissolved in pyridine-d₅ complete disappearance of 2 is observed in 4-6 days at room temperature, along with formation of acetone (1 equiv) and pinacol (0.5 equiv).

Under more forcing conditions complex 3 can facilitate a further C-C bond cleavage reaction. When the reaction mixture containing 3 and 0.5 equiv of pinacol is heheated in pyr-d₅ at 100 °C for 48 h, the pinacol is entirely consumed and acetone (1 equiv) is formed.

The reaction of complex 3 with pinacol to form V(III) complex 4 is remarkable. Reactions of V(V) complexes with alcohols generally afford V(IV) products, and although V(III) complexes have been proposed as intermediates in the vanadium-catalyzed aerobic oxidation of activated alcohols, V(III) products have never been isolated from such reactions. Both complexes 3 and 4 could be oxidized by air, suggesting potential catalytic reactivity of (dipic)V toward polyalcohols. Experiments exploring the scope and mechanism of this reaction in more detail, including reactions with lignin and cellulose, are currently underway.

 
Published Papers:

Hanson, S. K.; Baker, R. T.; Gordon, J. C.; Scott, B. L.; Sutton, A. D.; Thorn, D. L. "Aerobic Oxidation of Pinacol by Vanadium(V) Dipicolinate Complexes: Evidence for Reduction to Vanadium(III)" J. Am. Chem. Soc. 2009, 131 (2), 428–429 (DOI: 10.1021/ja807522n)

Macala, Gerald S.; Robertson, Andrew W.; Johnson, Charles L.; Day, Zachary B.; Lewis, Robert S.; White, Mark G.; Iretskii, Alexei V.; Ford, Peter C. "Transesterification Catalysts from Iron Doped Hydrotalcite-like Precursors: Solid Bases for Biodiesel Production" Catalysis Letters, 2008, 122(3-4), 205-209 (DOI: 10.1007/s10562-008-9480-y)

Macala, G. S.; Matson, T. D.; Johnson, C. L.; Lewis, R.; Iretskii, A. V.; Ford, P. C. “Hydrogen Transfer from Super-critical Methanol over a Solid Base Catalysts. A Model for Lignin Depolymerization” ChemSusChem, 2009, 2, 215-217.
(DOI: 10.1002/cssc.200900033)