(332c) Coupling Chemical and Biological Catalysis to Produce Biobased Chemicals
The prevalence of â??lightâ? (C1-C3) hydrocarbons obtained from shale gas motivates a need to find alternative sources of higher-carbon-number molecules suitable for producing commodity and specialty chemicals. Biomass is attractive in this regard because it is composed of monomers with five or more carbons, and it natively contains the oxygenated functionality that is common to high-value chemicals. However, with a carbon-to-oxygen ratio near unity, selective de-functionalization of biomass is a key challenge for producing biobased chemicals. One attractive strategy for obtaining value-added chemicals from biomass uses a combination of chemical and biological catalysis, where biological catalysts are used to produce selectively-functionalized platform molecules that are subsequently upgraded using heterogeneous chemical catalysts. We will present an analysis that compares the moieties that can be accessed by biological catalysis with the conversions that are available using heterogeneous catalysis. Based on this analysis we suggest that biologically-derived platform molecules possessing three distinct functional groups provide the most flexibility for subsequent catalytic upgrading. However, the successful upgrading of multi-functional molecules will require the careful design of catalytically active sites. For example, using transition state theory applied to thermodynamically non-ideal systems, we have analyzed the reaction kinetics for the hydrogenation of both lactic acid and triacetic acid lactone (TAL) by catalysts intercalated with poly(vinyl alcohol). Based on this analysis, it can be shown that the activity of catalytically active sites can be tuned by solvating transition states and abundant surface intermediates using polymer-derived microenvironments. Similarly, bimetallic nanoparticles contain active sites suitable for the selective hydrogenation of unsaturated carbon-carbon bonds in aromatic-containing platform molecules. For one such reaction, the hydrogenation of 4-hydroxycoumarin, the use of a bimetallic PdAu catalyst leads to a doubling of the turnover frequency coupled with an increase in selectivity from 93% to 97% (at 82-86% conversion).