Producing a concentrated solution of Monosaccharides from biomass is a key bottleneck in the selective production of materials or fuels from biomass. This process is generally constituted of a thermo-chemical or pretreatment stage followed by a biologically catalyzed stage. The goal of the pretreatment is to increase the available surface area of cellulose to the enzymes but also to extract lignin, which can inhibit the enzymatic hydrolysis. Most thermo-chemical pretreatment processes achieve this by de-polymerizing hemicellulose, which can lead to removal of some lignin as well. This increases the porosity in biomass (see figure 1).
Figure 1: Acid pretreated Switchgrass particle viewed with a Scanning Electron Microscope (SEM). Small pores form during pretreatment, presumably as lignin and hemicellulose are extracted.
This process is chemically catalyzed (acid or base) at low temperatures or run using pressurized water (1) at high temperatures. The chemically catalyzed systems are environmentally intensive, while hot water systems suffer from mass-transfer and dilution issues while demanding important amounts of energy. These problems have been addressed by trying to recycle the chemical catalysts or developing packed bed biomass reactors to increase mixing. However, these processes do not completely eliminate environmental or energetic issues and create increased costs.
One approach could be to decrease the solution's viscosity and increase oligomer and biomass solubility. This could be accomplished by using a binary mixture of CO2 and water. Supercritical CO2 can be saturated with up to about 40 mol% water below 260°C and is a low viscosity fluid that has the capacity of solubilizing organic molecules. An additional advantage offered by CO2 is its ability to separate as a gas after pretreatment by simple decompression and obtain a concentrated aqueous product solution. This approach was successfully between 150 and 250°C for various types of biomass (2). Pretreatment at 170°C for 60 min gave glucose yields of 77, 73 and 68% for 20 and 40 wt% solids mixed hardwood and mixed perennial grasses, respectively. Pretreatment at 160°C for 60 min gave glucan to glucose yields of 81% for switchgrass and 85% for corn stover. Yields obtained for switchgrass at 40wt% are shown in Figure 2.
Figure 2: Combined yields from pretreatment of a 40 wt% solids (biomass water mixture) slurry of switchgrass as a function of pretreatment time and temperature at 200 bar. Yields are obtained after 72 hr of enzymatic hydrolysis (15 FPU/g cellulose or 22.8 FPU/g glucan). (A) Glucan to glucose yields. (B) xylan, arabinan and mannan (hemicellulose sugars) to xylose, arabinose and mannose yields. (C) Glucan and mannan (hexose sugars) to 5-HMF yields. (D) Xylan and arabinan (pentose sugars) to furfural yields.
Therefore, pretreatment was seccessfully run in a batch reactor for screening purposes. The pretreated material can then undergo enzymatic hydrolysis and the produced sugars can be fermented into biofuels or other bioproducts. The effect of pretreatment on enzymatic hydrolysis can be evaluated by observing fluorescently labeled enzymes react with the material using confocal microscopy.
Figure 3: Acid pretreated Switchgrass particle with bound fluorescently-labeled Thermobifida Fusca enzymes (Cel5A is shown in green and Cel6B in red and are shown in rows A and B respectively). Row C is the overlay of images A and B. The images in Column 1 were taken after 0 min (Temperature: 40°C; it reached 49°C and stayed constant after 20 min). The images in columns 2 and 3 were taken after 75 and 150 min.
(1) Wyman, C. E.; Dale, B. E.; Elander, R. T.; Holtzapple, M.; Ladisch, M. R.; Lee, Y. Y., Coordinated development of leading biomass pretreatment technologies. Bioresource Technology 2005, 96, (18), 1959-1966.
(2) Luterbacher, J. S., Tester, J. W. and Walker, L. P. 2010. High-solids biphasic CO2-H2O pretreatment of lignocellulosic biomass. Biotechnology Bioengineering, 107: 451-460.