Ionic Liquids: A Key Enabler of Biomass Processing?

By Lorenz Bauer, Ph.D., Lee Enterprises Consulting

Special to The Digest

The explosion of interest in using biomass to produce chemicals and fuels has motivated a renewed search for improved methods for pretreatment that enable efficient conversion of the raw materials.   The molecules in cellulosic materials, and particularly lignocellulosic material, are intertwined and held together by intermolecular forces like hydrogen bonding that limits their solubility and makes separation difficult.   Pretreatments that break apart these complex agglomerates and make them more accessible can greatly increase the efficiency of conversion to value added products.   Pretreatments with low temperature liquid salts, known as ionic liquids, have shown great promise.

A patent application from 1930 showed that 1-alkylpyridinium chloride, an ionic liquid, dissolves cellulose.[i]   The patent was largely ignored initially, however, since 2000 there have been over 140 patents extending this work to producing value added materials from cellulose by BASF, the major supplier of ionic liquids, Eastman Chemicals, Bridgestone and Prof Rodgers at the University of Alabama to produce cellulose fibers.

There also has been great interest in using modern ionic liquids for biomass pretreatment.   Over 1000 academic papers and several patents involving the use of ionic liquids and in converting biomass to fuels and chemicals have appeared.[ii] The results of this work show that it is possible to produce separate streams of lignin and cellulose. The treated cellulose is easier to convert to sugars and other products via fermentation approaches   Thermal and chemical conversion like pyrolysis are also facilitated. A recent webinar by Blake Simmons of JBIE, summarized the results of their research into feasibility of a biorefinery that integrates ionic liquid pretreatment in ethanol production.   He concluded with further development it is possible to produce ethanol at costs comparable to the current spot market price of $1.60.[iii]

However, the current prospects for ionic liquid pretreatments is mitigated by economic realities. Two startups, SuGanit and Hyrax, were attempting commercialization, but there have been no recent updates on their progress.   Major advances that lower the costs of the ionic liquids and increases the value of the downstream process are needed. One scenario is that meets this later criteria would be a return to higher priced oil.

Ionic liquids are low melting salts comprised of organic cations and/or anions.   The nature of their components prevents crystallization so that they are liquids at low temperatures.   They can disrupt the intermolecular forces that hold agglomerated biopolymers together. They are often extremely good solvents for oxygen and nitrogen containing molecules.   These include most biomass components like cellulose and lignin. Their low vapor pressure and low flammability makes them more environmentally friendly than low molecular weight polar organic solvents. They are often water soluble and can be separated from products by water washing.   These properties have made them a major topic in “green chemistry” process research.     Some typical examples of ionic liquids are depicted below.

There have been thousands of different structures described in the literature.   This opens up the possibility of tuning the properties of the liquids to match the application.   It also provides a possible route for cost reduction by investigating different raw material sources. Some of the possibilities are renewable biological compounds like amino acid salts.

The vast majority of proposed routes for biomass conversion benefit by pretreatment of the material. Some of the methods that have been evaluated are listed in the Table above.   All them have their own set of advantages and disadvantages. Given that most biomass conversion to fuels and chemicals have questionable economics, cost is the critical factor. Currently there is little information comparing them of the same basis.

Ionic liquids have some clear technical advantages over the other biomass pretreatment approaches. Ionic liquids that are particularly effective at separating lignin from cellulose have been identified.   The cellulose produced is de-agglomerated and can be more easily hydrolyzed to sugars.   Generation of a relatively pure lignin output stream increases the potential of converting this material into a high-value coproduct.   The ionic liquids are stable, do not decompose cellulose, can be used at low temperatures, can be easily separated, and are potentially recyclable.   However, they have a significant limitation in that they are currently much more expensive than other solvents.   They also can be toxic and cannot be released to the environment. They may inhibit microbe and enzymatic activity.   There are significant losses of ionic liquid activity during the recovery and recycling process.

The promise of ionic liquids has motivated great interest in the biotechnology community.   The recently published “Application, Purification, and Recovery of Ionic Liquids”, edited by Olga Kuzmina and Jason Hallett provides an excellent summary of the state of art including a realist economic analysis.

Estimates for the cost of ionic liquid for commercial use range from $2.5-$50 per KG. In contrast methanol, a potential an organic solvent for pretreatment, costs less than $1 per KG on the spot market.   The lower ionic liquid price assumes an economy of scales due to the increased demand that is not currently practiced.   A major investment in ionic liquid production capacity would need to accompany the building of a biorefinery. It also assumes the use of lower cost raw materials to make the liquid.   However, not all ionic liquids perform equally as biomass pretreatment, and it remains to be proven how effective the lower cost ionic liquids would be in applications.

A comprehensive technoeconomic study of ethanol production showed that the key cost factors in using ionic liquids were the ionic liquid price, recyclability, and biomass loading.   High biomass loading (33.3% or higher), low IL price ($2.5/kg or less), and high IL recovery (97% or higher) are needed to ensure an ethanol selling price of $5/gal or less.[iv] Notably the spot price of ethanol is currently about $1.60 per gallon.   To achieve this target price an ionic liquid price of $1.50 per KG is required with over 99% per pass recyclability. Clearly there is a huge gap to overcome by either process improvement or government subsidies.

There is a path forward that can meet these targets. Integrating the ionic liquids into complete ethanol production process could greatly lower costs.   Pretreatment and enzymatic sugar production can be combined in the same reactor.   Also methods for increasing the solids loading can be explored that decrease the relative amount of ionic liquid used.

The most important step would be lowering the cost of the ionic liquid. The Hallet group at Imperial College has determined that triethylammonium sulfate has a raw material cost of $1.50 per KG. They report that it is 80% as effective as more expensive ionic liquids costing more 10 times the amount.[v] It also had the advantage of operating well in the presence of water, which is a perquisite for use in approaches involve single pot processing of the biomass.   New approaches for recycling the ionic liquids are also required to meet the cost targets.  There a several approaches being investigated that would eliminate the need for distilling all of the water used in the ionic liquid recovery step.

An unsubsidized biorefinery would also require lowering the cost of the enzymatic conversion process.   Enzymes represent up to 24% of the raw material costs.   There are continuing efforts to produce more effective enzymes for cellulose to chemicals conversion by companies like Novozymes The ionic liquid residues in the sugars represent a challenge because they can suppress enzyme activity.   .   A part of this effort would be developing ionic liquid tolerant enzymes that can operate function in a one pot process.

One positive factor is that that the above analysis does not include any value added from the lignin stream produced by the pretreatment. It is valued at the combustion fuel level. There are a number of projects underway to valorize lignin that would be greatly improve the economics of a bio-refinery. The lignin stream from the ionic liquid pretreatment would an attractive feedstock for these efforts.  The ultimate value may be equal to or greater than the sugar derived chemicals.   Higher value uses for the cellulose may also be possible. Ionic liquid dissolution has been shown to produce materials suitable for electrospinning into nanofibers.

In conclusion, ionic liquids clearly have great potential but are not ready to be commercialized until further development and demonstrations are completed.   Evaluating proposed integrated process will require extensive review before the risk is acceptable for most investors.   Demonstrating the production of higher value products enabled by the use of ionic liquids could provide the required economic benefit to justify investment.

The cost of the ionic liquids and difficulty in recycling have limited the use of ionic liquids in many of the processes where they have shown promise outside of biomass processing. There are a limited number of commercial processes that use ionic liquids. However, there is clearly a path forward that suggests that ionic liquid pretreatment is viable if the costs could be controlled. Particularly for develop in areas without large hydrocarbon reserves or that are willing to pay a premium for renewables. The use of ionic liquids is like the chicken and egg conundrum, in that the cost of the liquids will not come down until there is a significant demand. Co-developing ionic liquid production along with a bio refinery at an economically viable scale will require some major investments.

About the Author: Lorenz Bauer is a member of Lee Enterprises Consulting, the world’s premier bioeconomy consulting group, with more than 100 consultants and experts worldwide who collaborate on interdisciplinary projects, including those requiring the technologies discussed in this article.  The opinions expressed herein are those of the author, and do not necessarily express the views of Lee Enterprises Consulting.

[i] Charles Graenacher, Manufacture and Application of New Cellulose Solutions and Cellulose Derivatives Produced therefrom, US 1934/1943176

[ii] Amal A. Elgharbawy, Md Zahangir Alam, Muhammad Moniruzzaman, Masahiro Goto, Biochemical Engineering Journal, 109 (2016) 252–267

[iii] Simmons BA. Ionic Liquid Pretreatment. Energy Efficiency and Renewable Energy (EERE) Webinar. 2013. http://energy.gov/eere/bioenergy/downloads/ionic-liquid-pretreatment-technologies.

[iv] Klein‐Marcuschamer, Daniel, Blake A. Simmons, and Harvey W. Blanch. “Techno‐economic analysis of a lignocellulosic ethanol biorefinery with ionic liquid pre‐treatment.” Biofuels, Bioproducts and Biorefining 5.5 (2011): 562-569.

[v] Gschwend, F. J., Brandt, A., Chambon, C. L., Tu, W. C., Weigand, L., Hallett, J. P. Pretreatment of Lignocellulosic Biomass with Low-cost Ionic Liquids. J. Vis. Exp. (114), e54246, doi:10.3791/54246 (2016)