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46828-AC3
Dehydrogenation Catalysis Relevant to Biomass Conversion to Chemicals and Fuels

Peter C. Ford, University of California (Santa Barbara)

This project is concerned with the development of new catalysts for the dehydrogenation of alcohols and with elucidating mechanistic principles defining such processes for polyfunctional alcohols that serve as biomass models. The longer-term goal is to utilize such catalytic systems for the degradation of biomass components to dihydrogen (or dihydrogen equivalents) plus carbon dioxide or for the disproportionation of such renewable feedstocks to higher value materials relevant to the petrochemical industry. Of particular interest is the development of new methodologies for the transfer of hydrogen equivalents from carbohydrates (including cellulose) to other substrates such as lignin (Scheme 1).  With future supply challenges anticipated for fossil hydrocarbons, delineating catalytic strategies for the utilization of alternative feedstocks is clearly essential to sustained economic health. Furthermore, materials formed by photosynthesis are CO₂ neutral and represent net solar energy storage, excluding the fossil fuels used in their production, harvesting and transportation. Thus, effective and efficient catalytic conversion of non-food biomass to transportation fuels or chemical precursors would be an important addition to the world's energy and supply portfolio. 

Scheme 1:

The focus of our ongoing studies proposed is the investigation of chemical transformations that are models for key steps in the multiple catalytic cycles necessary for the degradation of a polyfunctional substrate such as glucose or glycerol. Having better mechanistic understanding of operating catalyst will provide better insight into the design of more effective or selective homogeneous heterogeneous catalysts. For example, vicinal diols are common motifs in biomass-derived materials, but their catalytic dehydrogenations have only been briefly studied.¹ In this context, we have initiated an investigation focused on the homogeneous dehydrogenation of the vicinal diols 1,2-propanediol (PDO) and 1-phenyl-1,2-ethanediol (PhEDO), as catalyzed by the Shvo catalyst² (Ru₂{(3,4-Tol₂-2,5-Ph₂C₅O)₂H}(µ-H)(CO)₄, (1) and by Robinson's catalyst³ (Ru(CF₃CO₂)₂(CO)(PPh₃)₂, (2). These dehydrogenations were operated in an open system in the acceptorless mode generating H₂ quantitatively and in a closed system with the H₂ acceptor diphenyl acetylene.⁴ Although, we expected dehydrogenation of the primary alcohol to aldehyde to be kinetically favored, the products appeared to be exclusively the a-hydroxy ketones as illustrated in Scheme 1. Since the latter is thermodynamically favored, it may result from a subsequent isomerization the aldehyde to the ketone. Ongoing studies are attempting to determine the possible intermediates in the reaction. Parallel studies are evaluating the degradation of glycerol and glucose to H₂, CO and CO₂ using more extreme conditions and heterogenized ruthenium catalysts.

In a related study, we are investigating the possible hydrogen transfer from alcohols to lignin models, with the eventual goal of utilizing biomass derived H₂ (or H₂ equivalents) for the disassembly of lignin to more tractable monomeric or oligomeric fragments.  Lignin is a heterogeneous biopolymer composed largely of 4-propylphenol derivatives coupled primarily  as ethers but occasionally cross-linked with carbon-carbon bonds. We have found that porous metal oxides (PMOs) obtained by calcination of transition metal substituted Mg-Al hydrotalcite (HTC) precursors exhibit the properties of strong solid bases.  For example Fe-doped Mg-Al PMOs are catalysts for transesterification reactions relevant to biodiesel production.⁵ It was our premise that it would be possible to enhance the effectiveness of base-catalyzed hydrolysis of the phenyl ethers in lignins by introducing redox active transition metals into the formulation of the hydrotalcite precursors. Somewhat serendipitously, we have found that new PMO catalysts derived from 3:1 Mg:Al HTCs for which some of the Mg²⁺ was replaced by Cu²⁺are effective for transfer of H₂ equivalents from methanol to the lignin model compound dihydrobenzofuran under supercritical conditions.⁶  The reaction appears to occur via sequential hydrogenolysis of the ether linkage and hydrogenation of the aryl group with methanol as the hydrogen source. Ongoing studies are directed toward evaluating such reactions using other alcohols to provide the H₂ equivalents and with lignin models and for lignins themselves. 

References:

(1) D. Morton, D. J. Cole-Hamilton, Polyhedron 1987, 6, 2187-2189.

(2) C. P. Casey, S. W. Singer, D. R. Powell, R. K. Hayashi, M. Kavana. J. Am. Chem. Soc. 2001, 123, 1090-1100.

(3)  A. Dobson, S. D. Robinson. Inorg. Chem. 1977, 16, 137-142.

(4) Catalytic Dehydrogenation of Vicinal Diols, T. D. Matson, P. C. Ford, manuscript in preparation

(5) G. S. Macala, A. W. Robertson, C. L. Johnson, Z. B. Day, R. S. Lewis, M. G. White, A. V. Iretskii, P. C. Ford, Catal. Lets. 2008, 122, 205-209.

(6) Hydrogen Transfer from Methanol over a Solid Base Catalyst.  A Model for Lignin Depolymerization, G. S. Macala, T. D. Matson, C. L. Johnson, R. S. Lewis, A. V. Iretskii and P. C. Ford, manuscript in preparation.

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