(766d) Expanding the Biofuel Portfolio By Integrating Chemistry and Biology

Acetone, a product of solventogenic Clostridia’s acetone-butanol-ethanol (ABE) fermentation, harbors nucleophilic α-carbons, which are amenable to C-C bond formation with the electrophilic α-carbon of the alcohols (ethanol and n-butanol).  These inherent functionalities enable coupling chemistry to form higher molecular weight hydrocarbons, similar to those found in current jet and diesel fuels, by a transition-metal-catalyzed alkylation reaction.  Here we describe the integration of biological and chemocatalytic routes to efficiently convert ABE fermentation products into ketones, ranging from 2-pentanone to 6-undecanone, by a palladium or copper-catalyzed alkylation reaction.  Tuning of the reaction conditions permits production of either predominately gasoline or predominately jet and diesel blend stocks.  The integration of the two technologies, ABE fermentation and the alkylation reaction, is made possible by in situ liquid-liquid extraction (LLE) of the ABE with a non-toxic extractant. Glyceryl tributyrate was used for in situ selective extraction of both acetone and n-butanol to enable simple integration of ABE fermentation and chemical catalysis, while reducing the energy demand of the overall process.  A 2L 60-hr extractive fermentation of wild-type C. acetobutylicum in a 1:1 volume ratio of medium:extractant produced 40.8 g of ABE from 105 g of glucose (90% theoretical yield).  Similar titers were observed when sucrose was the major carbon source.  Solvents distilled from the extractant phase were as reactive as the pure chemicals, achieving an overall molar yield of 93% and 97% conversion of acetone.  This process provides a means to selectively produce gasoline, jet, and diesel blendstocks from lignocellulosic and cane sugars at yields near their theoretical maxima.  Replacing acetone with isopropanol in the ABE mixture predictably increases the concentration of diesel-range components in the blendstock.  C. acetobutylicum was metabolically engineered to produce isopropanol-butanol-ethanol (IBE), achieving an alcohol titer of 21.5 g/L from 54 g/L glucose in 35 hours.  Computational modeling of solute/extractant LLE using Cosmo-RS™ identified oleyl alcohol as a superior extractant for isopropanol-butanol-ethanol extractive fermentation on the basis of isopropanol removal.  Extractive fermentation with the metabolically engineered C. acetobutylicum SACE strain produced 39.6 g of IBE from 117 g glucose.  Alcohols distilled from the extractant phase were as reactive in the alkylation reaction as the corresponding synthetic mixtures.  In conclusion, we have leveraged advantages from both theromchemical and biological processes for the efficient conversion of sugars to biodiesel blendstocks.