CO₂ enabled process integration for the production of cellulosic ethanol using bionic liquids

The substantial global supply of sustainable lignocellulosic biomass (e.g., agricultural wastes, forestry wastes, dedicated energy crops, and organic municipal solid waste) makes it a vital feedstock for commercial-scale production of biofuels and renewable chemicals. The efficient and affordable conversion of lignocellulosic biomass into fuels and chemicals is currently limited by, among other factors, its recalcitrance that inhibits efficient saccharification required to produce fermentable sugars. To overcome this recalcitrance, and increase saccharification efficiency and yield, ionic liquid (IL) based pretreatment technologies are showing promise in meeting the desired key characteristics of biomass pretreatment. The IL 1-ethyl-3-methylimidazolium acetate ([C₂C₁Im][OAc]) has been shown to be effective at decreasing the recalcitrance of both single and mixed lignocellulosic feedstocks, including softwoods and hardwoods, with potential for producing renewable aromatics from lignin. IL pretreatment using [C₂C₁Im][OAc] has been demonstrated at high solid loadings, and recently scaled to larger volumes and operated in continuous mode.

Despite the effectiveness of [C₂C₁Im][OAc] and similar ILs in reducing the recalcitrance of lignocellulosic biomass, the inhibition of enzyme activity and microbial toxicity of these top performing ILs often require extensive water washes to remove residual IL from pretreated biomass prior to enzymatic hydrolysis and fermentation. As a result, the associated IL recycling and wastewater treatment costs create significant economic and process engineering challenges for the commercial scale-up of this technology. To reduce water use, an integrated wash-free process using [C₂C₁Im][OAc] was recently developed, where the pretreatment slurry was diluted with water to a final IL concentration of 10-20 wt% and directly hydrolyzed using a thermostable IL tolerant enzyme mixture, liberating 81.2% glucose and 87.4% xylose. This result provides the basis for developing a more economical IL pretreatment process, but requires specialized enzymes and is not compatible with the majority of the commercially available hydrolytic enzyme mixtures. In addition, downstream microbial fermentation is generally inhibited by the presence of residual ILs, and requires further separation and/or cleanup of the hydrolysate prior to fermentation. Even with the recent discovery and expression/activation of efflux pumps in Escherichia coli and the identification of strains of Saccharomyces cerevisiae with improved IL tolerance, establishing an industrially relevant microbial host capable of withstanding the amounts of IL needed to decrease overall operating costs will require extensive research and development.

To address the economic and sustainability challenges associated with conventional ILs used for biomass pretreatment, a new generation of ILs containing ions derived from naturally occurring bases, acids and aldehydes from lignin and hemicellulose have recently emerged. Despite these benefits, these "bionic liquids" (BILs) still operate, in general, at highly basic pH conditions and thus are not compatible with the commercially available cellulase and hemicellulase mixtures, nor are they compatible with microbial fermentation hosts that require neutral or slightly acidic conditions. To overcome this compatibility problem, a neutralization step is required before saccharification and fermentation. This is a common practice for other pretreatment technologies that use acids or bases. BILs have recently shown great potential, but the higher cost of BILs relative to mineral acids necessitates that they are recycled (Fig. 1). A typical neutralization step leads to the formation of complex salts, from which there is no simple solution efficiently recovering and reusing ILs. This is a significant challenge that must be addressed to realize an integrated process and obligates exploration of clever approaches to overcome this present technology gap.

One potential solution to these challenges is to use a reversible and benign chemical input to adjust pH after pretreatment that enables process integration with saccharification and fermentation with no purification. Microbes produce carbon dioxide (CO₂) during anaerobic fermentation, and the production of CO₂ at biorefineries has been considered to be a co-product. It is known that certain ILs can capture high volumes of CO₂ under ambient or low-pressure conditions that decrease pH by forming the corresponding carbonate salts. The process is reversible at elevated temperatures or by sparging nitrogen gas as previously reported for other ILs.

To overcome the problems of IL loss in the current BIL process that would use commercially available enzyme mixtures and wild type fermentation hosts, we further tested the threshold of IL tolerance for cholinium lysinate ([Ch][Lys]) and other ILs. The use of CO₂ as a means of reversibly switching pH after pretreatment in order to develop an integrated process with minimal IL losses addresses several challenges with conventional pH adjustment, such as acid neutralization, and eliminates salt formation. The pretreated biomass generates high ethanol yields using wild type yeast (S. cerevisiae) in the presence of [Ch][Lys]. Recovery and recycle of the [Ch][Lys] was demonstrated and this approach shows significant potential to resolve several of the most significant obstacles towards the realization of an efficient, integrated, affordable and scalable IL conversion technology suitable for deployment at a biorefinery and opens the door to a new approach to biomass conversion into renewable biofuels and chemicals.