University of Wisconsin Team Develops High-Yielding Chemical Hydrolysis Process to Release Sugars from Biomass for Cellulosic Fuels and Chemicals
A University of Wisconsin-Madison research team has developed a chemical process for the hydrolysis of biomass into sugars for subsequent processing into fuels and chemicals that delivers sugar yields approaching those of enzymatic hydrolysis. In an open-access paper published in the 9 March issue of the Proceedings of the National Academy of Sciences, they report that the process leads to a nearly 90% yield of glucose from cellulose and 70-80% yield of sugars from untreated corn stover.
Dr. Ronald Raines, a UW-Madison professor of biochemistry and chemistry, and graduate student Joseph Binder say that the new process generates easily recovered sugars that are “superb feedstocks” for microbial growth and biocatalytic ethanol production.
To release the sugars locked into the cellulose polymers in biomass, producers use both physical and chemical methods, with physical chemical pretreatment processes followed by enzymatic hydrolysis being the most common, note Raines and Binder. While the proper combination of pretreatment and enzymes for a given feedstock delivers high yields of sugars from hemicellulose and cellulose components, the hosts of both pretreatment and enzymes are high—up to one-third of the cost of cellulosic ethanol production—and the rate of hydrolysis can be low.
Although concentrated acids can also be used with high conversion rates, the hazards of handling concentrated acids and the complexities of recycling them have limited the adoption of this technology, they say.
Raines’ chemical approach relies on a chloride ionic liquid-containing catalytic acid to dissolve the long chains of sugars in biomass and break them up into individual molecules of glucose and xylose. Over the course of the reaction, they add water to the mixture to prevent unwanted byproducts from forming. After two rounds of such treatment, a sample of corn stover gave up about 70% of its glucose and 79% of its xylose, a 75% sugar yield overall. From there, the researchers used ion-exclusion chromatography to separate the sugars from the reaction mixture, as well as the ionic liquid, for reuse.
By balancing cellulose solubility and reactivity with water, we produce sugars from lignocellulosic biomass in yields that are severalfold greater than those achieved previously in ionic liquids and approach those of enzymatic hydrolysis. Furthermore, the hydrolysate products are readily converted into ethanol by microorganisms. Together, these steps comprise an integrated process for chemical hydrolysis of biomass for biofuel production.
...In comparison to extant enzymatic and chemical processes to biomass hydrolysis, ours has several attractive features. Like concentrated acid processes, it uses inexpensive chemical catalysts rather than enzymes and avoids an independent pretreatment step. Working in concert, [EMIM]Cl and HCl produce high sugar yields in hours at just 105 °C, whereas enzymatic hydrolysis can take days and many pretreatment methods require temperatures of 160–200 °C. Also, lignocellulose solubilization by the ionic liquid allows processing at high concentrations, which can be a problem in enzymatic hydrolysis.
On the other hand, our process improves on typical acid hydrolysis methods by avoiding the use of hazardous concentrated acid. Using catalytic amounts of dilute acid removes the complexity and danger of recycling large volumes of concentrated acid. The ionic liquid used in its place is likely to be far easier to handle. Despite this difference, our process is similar to commercial processes using concentrated acid hydrolysis and consequently can exploit proven engineering and equipment for facile scale-up, particularly for separations and recycling.—Binder and Raines
Raines and Binder subsequently used ethanologenic microbes to ferment the sugars they collected into ethanol. All told, says Raines, using this integrated process, they were able to convert half of the sugars available in plant biomass into liquid fuel.
To make it work at the industrial scale, however, a number of hurdles will need to be overcome, the authors note, including:
Highly viscous biomass-ionic-liquid mixtures might require special handling, and larger scale fermentation of hydrolysate sugars might reveal the presence of inhibitors not detected in our demonstration experiments.
The sugar concentration resulting from stover hydrolysis (about 1%) is too low for practical fermentation and is decreased even further during the chromatographic separation, leading to water-evaporation costs. Methods allowing a higher starting biomass loading and strategies to concentrate rather than dilute the sugars during separation from the ionic liquid would overcome this problem.
Separations and ionic-liquid recycling could pose additional challenges to commercialization.
Raines’ project was supported by the Great Lakes Bioenergy Research Center, a US Department of Energy bioenergy research center located at UW-Madison, as well as a National Science Foundation Graduate Research Fellowship awarded to Binder.
Joseph B. Binder and Ronald T. Raines (2010) Fermentable sugars by chemical hydrolysis of biomass. PNAS vol. 107 no. 10 4516-4521 doi: 10.1073/pnas.0912073107