Principal Investigator: Aaron M Scurto Proposal No: 1034433
Intellectual Merit
The production of alcohol-based biofuels from fermentation of renewable biomass feedstocks is one route for securing a sustainable source of transportation fuels. Most fermentations produce alcohols in relatively dilute aqueous solutions that are currently separated by energy-intensive downstream processes such as distillation. However, if the alcohols could be removed in situ from the fermentation broth by solvent extraction, then product inhibition of the microorganisms could be overcome, and downstream energy consumption could be reduced, leading to more efficient alcohol recovery. Optimal solvents must possess three main characteristics. First, the solvent must possess thermodynamic and mass transfer characteristics that facilitate alcohol separation and solvent recovery. For example, the solvent must be immiscible in water, have high solubility for the target biofuel, and possesses a large volatility difference with the biofuel. Second, the solvent must be biocompatible with the fermentation microorganisms. Finally, solvent should be environmentally benign with low impact on humans and the environment. Ionic liquids (ILs) have the potential meet all these requirements.
The overall goal of this research is to provide fundamental understanding of the extraction of alcohol-based biofuels from fermentation broths using molecularly-adaptable ionic liquids. Ionic liquids are liquid organic salts with no volatility. The design of ionic liquids as materials for in situ product recovery of alcohols from fermentation broths will be realized by simultaneous optimization of phase equilibrium thermodynamics, mass transfer, and biocompatibility. Model systems of candidate nonvolatile ionic liquids and volatile alcohol biofuels will be investigated according to their partitioning and mass transfer in aqueous systems, aqueous systems with model fermentation compounds, and actual fermentation broths. S. cerevisiae will serve as the model organism for ethanol fermentation, and C. acetibutylicum will serve as the model organism for butanol fermentation. The molecular and phase toxicity of the ionic liquids to these fermentation microorganisms will be determined. Life-cycle analysis will be used to compare the long-term impact of conventional solvents vs. ionic liquids. Overall, this research plan is designed to elucidate the structure-property relationships that will assess the potential of ionic liquids to accomplish the low-energy separation of alcohol-based biofuels from fermentation broths.
Broader Impacts
Overall, the proposed research seeks to assess alternatives to distillation to lower the energy consumption associated with biofuel production from renewable resources. Educational and outreach activities based on the research are also proposed. Specifically, case studies based on new processes which could result from the research will be developed and used in three current courses at the University of Kansas: Biocatalysis, Environmentally-Benign Reaction Engineering, and Environmental Assessment of Chemical Processing. In collaboration with a local high-school science teacher, incorporation of eco-toxicology experiments with solvents, including ionic liquids, will be incorporated into the science curriculum. These activities will be adapted for curriculum materials at Project Discovery, a summer camp for female high-school students interested in chemical engineering.
The production of chemicals and fuels from fermentation using micro-organisms such as bacteria and yeast is a growing area (~ 5% per year) for future sustainable ("green") processes. However, efficient and low-energy separations will be the key to full implementation and wide-scale societal, environmental, and economic benefit. Most common fermentation methods produce the target compound (such as ethanol) in one stage until the compound accumulates to such levels that it inhibits or kills the microorganism usually resulting in dilute water-based mixtures; e.g. ~15% ethanol in yeast fermentation. Then, the product is separated in another stage ("downstream") usually by distillation which requires large amounts of heat, energy, and fresh water. Alternatively, if the target solute could be removed from the fermentation reaction directly (in situ), then product inhibition/death of the microorganisms could be overcome and lead to more efficient separations. While several in situ methods exist, only solvent extraction (also called extractive fermentation, 2-phase biocatalysis) represents a conceptually simple process that may be highly tuned by the choice of solvent. Here, an immiscible solvent is passed through the water fermentation solution to extract selectively the target product as it is being produced by the microorganism without being toxic to the microorganism itself. For modern sustainable processes, these solvents should have low impact on humans and the environment. Few current organic solvents meet all of these criteria. Ionic liquids (ILs) are liquid organic salts that do not evaporate (and thus NO air pollution potential) and may help optimize the necessary extractive fermentation processes for future sustainable energy needs. Due to the molecular flexibility/designability of ionic liquids, these materials are capable of optimizing nearly any extraction. Biological compatibility affects both the use of ILs with fermentation microorganisms and their ultimate human and environmental impact. Only ionic liquids that have both advantageous extraction properties of the target fuels and bio-compatibility (both to the microorganisms and environment) can produce a feasible separation process for biofuels (primarily ethanol and n-butanol). This novel methodology will uniquely allow the rapid development of benign and sustainable bio-separation processes for both energy production, but also fermentation separations in general. In pursuit of this goal, research was performed to find ionic liquids that have satisfy two main objectives: 1) good extraction of the target solutes; and 2) bio-compatible (nontoxic) IL solvents with the fermentation microorganism. The ability of over a dozen different ionic liquids was investigated as to their ability to extract target solutes from dilute water solutions similar to the fermentation media was investigated. The target solutes include ethanol and butanol as potential biorenewable fuels and (+)-cis-(1R,2S)-dihydroxy-1,2-dihydronaphthalene (NDHD) as a model pharmaceutical and dye compound produced by fermentations to investigate the wider-scale applicability of the technology. Many of the ionic liquids had very good extraction of the target solutes from water. While the ionic liquids are considered immiscible, like all solvents, they still dissolve small amounts of water. The selectivity is measured as the solubility of the target solute compared to the co-extraction of water. Most of the ionic liquids had very good selectivity, meaning that they extracted more of the target than water. The industrial extraction of butanol and ethanol with ILs were modeled and shown to have lower energy requirements than the conventional fermentation, then extraction. These same ionic liquid solvents were also investigated as to their biocompatibility to a model microorganism used universally in industrial fermentations, Escherichia Coli; note that these are not the disease causing strains commonly discussed in the media. They have been used to produce a wide variety of chemicals and fuels. The results here indicate a wide variety of toxicity among the ionic liquid class of solvents. Some of the ionic liquids were perfectly biocompatible with the microorganism. However, some were very effective antimicrobial compounds. This is illustrated in the images attached where some of the ILs left the bacterial cell membranes intact (healthy, image 1) while other ones disturbed the cells allowing some of their inside material to flow out of the cell effectively killing it (image 2). However, a few of the ionic liquids killed the cells without much damage to the cell membranes indicating that there was a difference in mechanism of the toxicity. While the goal here was to find biocompatible ionic liquids, the few ILs that were toxic may be important new molecules for anti-microbial agents used for human health. The biocompatible ILs may now be further exploited for extracting fuels and chemicals from fermentations.
