Ionic liquid pretreatments show great potential as a biomass pretreatment for dissolving lignocellulose and helping to hydrolyze the resulting aqueous solution into fuel sugars. But the best of these ionic liquids so far have required the use of expensive enzymes.
Blake Simmons, a chemical engineer who heads the Joint BioEnergy Institute’s (JBEI) Deconstruction Division has taken another step towards meeting this challenge with the development of a new technique for pre-treating cellulosic biomass with ionic liquids – salts that are liquids rather than crystals at room temperature.
The new technique requires none of the expensive enzymes used in previous ionic liquid pretreatments, and makes it easier to recover fuel sugars and recycle the ionic liquid.
Simmons said, “Most of our ionic liquid efforts at JBEI have focused on using enzymes to liberate fermentable sugars from lignocellulosic biomass after pretreatment, but with this new enzyme-free approach we use an acid as the catalyst for hydrolyzing biomass polysaccharides into a solution containing fermentable sugars. We’re then able to separate the pretreatment solution into two phases, a sugar-rich water phase for recovery and a lignin-rich ionic liquid phase for recycling. As an added bonus, our new pretreatment technique uses a lot less water than previous pretreatments.”
Simmons is the corresponding author of a paper describing this research that has been published in the journal Biotechnology for Biofuels. The paper is titled “Production and extraction of sugars from switchgrass hydrolyzed in ionic liquids.” Co-authoring it were Ning Sun, Hanbin Liu Noppadon Sathitsuksanoh, Vitalie Stavila, Manali Sawant, Anaise Bonito, Kim Tran, Anthe George, Kenneth Sale, Seema Singh and Bradley Holmes.
The pitch at JBEI is advanced biofuels – liquid fuels synthesized from the sugars in cellulosic biomass – offer a clean, green and renewable alternative to gasoline, diesel and jet fuels. Bringing the costs of producing these advanced biofuels down to competitive levels with petrofuels, however, is a major challenge.
The press release continues with the burning of fossil fuels adding 9 billion metric tons of excess carbon dioxide to the atmosphere each year, so the need for carbon neutral, cost-competitive renewable alternative fuels has never been greater. Advanced biofuels, produced from the microbial fermentation of sugars in lignocellulosic biomass, could displace gasoline, diesel and jet fuel on a gallon-for-gallon basis and be directly dropped into today’s engines and infrastructures without impacting performance. If done correctly, the use of advanced biofuels would not add excess carbon to the atmosphere.
Environmentally benign ionic liquids are used as green chemistry substitutes for volatile organic solvents. With great potential as a biomass pretreatment for dissolving lignocellulose and helping to hydrolyze the resulting aqueous solution into fuel sugars, the best of these ionic liquids so far have required the use of expensive enzymes.
The state of the art when Simmons group published the study paper showed that acid catalysts, such as hydrochloric or Brønsted, can effectively replace enzyme-based hydrolysis, but the subsequent separation of sugars and ionic liquids becomes a difficult and expensive problem that can require the use of significant amounts of water.
Simmons team was guided by molecular dynamics simulations carried out at DOE’s National Energy Research Scientific Computing Center. Simmon’s team solved this problem by deploying the ionic liquid imidazolium chloride in tandem with an acid catalyst.
Simmons explains, “Imidazolium is the most effective known ionic liquid for breaking down lignocellulose and the chloride anion is amenable with the acid catalyst. The combination makes it easy to extract fermentable sugars that have been liberated from biomass and also easy to recover the ionic liquid for recycling. By eliminating the need for enzymes and decreasing the water consumption requirements of more traditional ionic liquid pretreatments we should be able to reduce the costs of sugar production from lignocellulose.”
Complete separation of the pretreatment solution into sugar-rich water and lignin-rich ionic liquid phases was attained with the addition to the solution of sodium hydroxide. The optimized sodium hydroxide concentration for both phase separation and sugar extraction was 15-percent, resulting in the recovery of maximum yields of 54-percent glucose and 88-percent xylose. The team believes optimizing the process conditions and using more advanced methods of phase separation and sugar recovery can increase these sugar yields.
Looking ahead Simmons said, “After optimizing the process conditions, our next step will be to scale the process up to 100 liters,” Simmons says. “For that work we will use the facilities at the Advanced Biofuels Process Demonstration Unit.”
Ionic liquid ideas have been popping up in papers and press releases over the past months with great hope for a low cost breakthrough for biomass to fuel processes. The Simmons team looks like the leader in two respects; it’s a process development and reduces inputs.
There remains one over riding question, just what does say a hundred thousand gallons cost and what would be involved for a facility to be built? Then the feedstock, transport and all the functional issues come into play.
The Simmons team looks to be getting to a point where those kinds of issues need considered. And that is a breakthrough of it own.