JBEI team develops one-pot, wash-free process for pretreatment and saccharification of switchgrass; avenues for driving down biofuel cost
|Conventional separate pretreatment and saccharification of biofuel feedstock (a) entails water and waste disposal that the new one-pot system (b) eliminates. (Image courtesy of Joint BioEnergy Institute). Click to enlarge.|
Researchers with the US Department of Energy (DOE)’s Joint BioEnergy Institute (JBEI) report the first demonstration of a one-pot, wash-free process that combines ionic liquid (IL) pretreatment and enzymatic saccharification into a single vessel using a thermostable IL-tolerant bacterial consortium comprising several species of thermophiles (microbes that thrive at extremely high temperatures and alkaline conditions).
Using this one-pot system, they liberated 81.2% glucose and 87.4% xylose (monomers and oligomers) at 72 h processing at 70 °C with an enzyme loading of 5.75 mg g−1 of biomass at 10% [C2mim][OAc]. Glucose and xylose were selectively separated by liquid–liquid extraction with over 90% efficiency, thus eliminating extensive water washing as a unit operation.
The results, said Blake Simmons, head of JBEI’s Deconstruction Division and co-author of a paper on the work in the RSC journal Green Chemistry, were very close to the 85-95% sugar yields that are typically achieved with separate pretreatment and saccharification processing steps.
Despite the promising results, further improvements must be realized, he said, primarily in the composition and activity of the enzyme cocktails used, in order to reach target yields of better than 95% for all fermentable sugars.
By combining ionic liquid pretreatment and saccharification into a single vessel we eliminate the excessive use of water and waste disposal currently associated with washing biomass that is pretreated with ionic liquids. We also drastically simplify the downstream sugar/lignin recovery process and enable the ionic liquid to be recycled; all factors that help drive down biofuel production costs.—Blake Simmons, head of JBEI’s Deconstruction Division and co-author
If advanced biofuels are to be a commercial success, they must be cost-competitive with fossil fuels. For certain pathways this means, among other things, means economic technologies must be developed for extracting fermentable sugars from cellulosic biomass and synthesizing them into fuels as well as other valuable chemical products.
In their efforts to develop cost-effective ways to deconstruct cellulosic biomass into fuel sugars, the researchers focused on pretreatments of the biomass with ionic liquids—environmentally benign organic salts often used as green chemistry substitutes for volatile organic solvents. The ionic liquids that have emerged from this JBEI effort as a benchmark for biomass processing are imidazolium-based molten salts.
Imidazolium is the most effective known ionic liquid for breaking down cellulosic biomass for enzymatic hydrolysis into fermentable sugars. However, imidazolium, like other ionic liquids, can inhibit the commercial enzyme cocktails now used for the saccharification of cellulosic biomass and must therefore be removed by substantial amounts of water after the pretreatment. The recycling and waste disposal costs associated with this required washing pose a significant challenge for the commercial scale-up of ionic liquid pretreatment technology.—Seema Singh, director of JBEI’s biomass pretreatment program
To meet this challenge, JBEI researchers led by John Gladden, deputy director for fungal biotechnology, developed the compost-derived consortium of bacterium adapted to grow on switchgrass they named “Jtherm.”
Jtherm is both an ionic liquid and heat tolerant cellulase cocktail that can liberate sugars from biomass in the presence of up to 20-percent ionic liquids.—John Gladden
Simmons says his deconstruction team is planning to have scale-up tests of this one-pot system tested at the Advanced Biofuels Process Demonstration Unit. They are also in the process of further optimizing/augmenting their Jtherm enzyme cocktail to improve sugar yields, and they are seeking to identify other ionic liquids that are even easier on hydrolyzing enzymes than the imidazolium-based molten salts.
This research was supported by the DOE Office of Science.
Jian Shi, John M. Gladden, Noppadon Sathitsuksanoh, Pavan Kambam, Lucas Sandoval, Debjani Mitra, Sonny Zhang, Anthe George, Steven W. Singer, Blake A. Simmons and Seema Singh (2013) One-pot ionic liquid pretreatment and saccharification of switchgrass. Green Chem. doi: 10.1039/C3GC40545A