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Open Access Publications from the University of California

Engineering Microbial Fermentations: Acetone/Butanol/Ethanol to Fungible Fuels

  • Author(s): Baer, Zachary Cardwell
  • Advisor(s): Clark, Douglas S
  • Blanch, Harvey W
  • et al.

The batch fermentation of simple sugars to ethanol has thus far been the most successful process to displace transportation fuel consumption worldwide. However, the physical properities of ethanol limit its application in most of the world beyond 10-15 vol% blends with conventional gasoline. Additionally, ethanol is too volatile to be blended at an appreciable level with jet and diesel fuels. Alternative technologies are necessary to produce compounds that can be blended in all forms of transportation fuel and to higher levels. The biological production of longer chain oxygenates or hydrocarbons has been purposed to address this challenge. However, the increased toxicity of these molecules has limited their usefulness to-date. Alternatively, chemocatalytic routes to produce these fuel molecules do not suffer from toxicity issues but the highly functional saccharide-based feedstocks result in undesirable by-product formation and yield losses.

We propose that by effectively combining these biological and chemocatalytic approaches efficient production of all classes of transportation fuels can be achieved. Clostridium acetobutylicum is a well-studied industrial bacterium capable of fermenting a wide-variety of saccharides into a mixture of acetone, butanol, and ethanol (ABE). These mixed products contain functionalities that are amenable to C-C bond formation through transition-metal catalysis. Thus enabling the production of a suite of long-chain oxygenates from a variety of sugar substrates. This dissertation describes three major topics associated with the integration biological and chemocatalytic processes for fuel production: (1) the initial discovery and evaluation of such a combined process, (2) tailoring the fermentation products and catalyst to favor diesel production, and (3) engineering tunable ethanol and acetone co-production in Escherichia coli. Additionally, a thorough and reproducible protocol describing both biological and chemocatalytic procedures are provided.

Acetone, a product of the acetone-butanol-ethanol (ABE) fermentation, harbors a nucleophilic α-carbon, which is amenable to C-C bond formation with the electrophilic alcohols (ethanol and n-butanol) produced in ABE fermentation. This functionality enables the formation of higher molecular weight hydrocarbons similar to those found in current gasoline, jet, and diesel fuels. Using a palladium-catalyzed alkylation reaction efficient conversion of ABE fermentation products into ketones, ranging from 2-pentanone to 6-undecanone was achieved. Tuning of the reaction conditions permits production of either predominately gasoline or jet and diesel precursors. Glyceryl tributyrate was identified as a suitable extractant for selective in situ removal of both acetone and alcohols. Enabling simple integration of ABE fermentation and water sensitive chemical catalysis, while reducing the energy demand of the overall process. Additionally, extractive fermentation with glyceryl tributyrate removed several lignocellulosic-derived pretreatment inhibitors completely restoring solvent production titers.

Tailoring both the biological fermentation and chemocatalytic reaction conditions predictably increased the production of sustainable diesel blendstocks from lignocellulosic sugar. More specifically, engineering the metabolism of C. acetobutylicum to produce isopropanol-butanol-ethanol (IBE) as opposed to ABE by expression of the secondary alcohol dehydrogenase (sadh) from C. beijerinckii strain B593. Coupled with the optimization of a more water-resistant alkylation catalyst, hydrotalcite-supported copper(II) or palladium (0). Furthermore, in silco predictions of extraction efficiency using COSMO-RS were employed to identify oleyl alcohol as a superior extractant for in situ removal of IBE.

To achieve highly controllable acetone and ethanol production both metabolic and fermentation engineering approaches were employed. Strains capable of producing acetone without carboxylic acid reassimilation through a newly described pathway were developed. Furthermore, a more oxidized sugar-acid (gluconic acid) was used to drive production of acetone through redox balancing of NADH, predicatively reducing the molar ethanol:acetone ratio. Increases in the molar ethanol:acetone ratio were also described by the co-expression of either the aldehyde/alcohol dehydrogenase (adhE) from E. coli MG1655 or pyruvate decarboxylase (pdc) and alcohol dehydrogenase (adhB) from Z. mobilis. Acetone titers reached wild-type C. acetobutylicum levels by controlling the fermentation sparge rate and pH in a bioreactor. Finally, catalytic strategies to upgrade the co-produced ethanol and acetone at both low and high molar ratios are described.

By leveraging the advantages of both biological fermentation and chemocatalytic reactions of fermentation products with well-defined functionality we are able to efficiently convert a variety of saccharides into long-chain oxygenates. These long-chain oxygenates can be blended with all forms of transportation fuels. Additionally, tuning the microorganism's metabolism, fermentation conditions, catalysis reaction, or all of the above can optimize the production for a specific class of fuel. The integration of mixed-product fermentation with chemical catalysis is thus a novel and potentially enabling route for the economical conversion of biomass into liquid transportation fuels.

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