(564e) Effects of Dilute Acid Pretreatment Conditions On Enzymatic Hydrolysis Monomer and Oligomer Sugar Yields for Aspen, Balsam, and Switchgrass

In recent years, growing attention has been devoted to the use of lignocellulosic biomass as a feedstock to produce renewable carbohydrates as a source of energy products, including liquid alternatives to fossil fuels. Currently, research is driven by the need to reduce the cost of biomass-ethanol production. One of the prominent methods is to thermochemically hydrolyze (pretreat) the biomass and subsequently, enzymatically hydrolyze the pretreated material to fermentable sugars that can then be converted to ethanol using specialized microorganisms. A key goal of pretreatment is to produce fermentable sugars from hemicellulose and enhance enzymatic conversion of the cellulose fraction, and, hopefully, obtain a higher ethanol yield. Most research focuses independently on xylose yields from dilute acid hydrolysis or glucose yields after enzymatic hydrolysis. Another important factor to consider is the overall effect of pretreatment and subsequent enzymatic hydrolysis on total xylose yields, total glucose yields, and overall total sugar yields (Lloyd and Wyman, 2005) and the relationship between these processes. It has been shown that these maximums do not occur simultaneously and rather than reporting for glucose or xylose yields independently, total sugar yields should be taken into consideration (Lloyd and Wyman, 2005). Therefore, in order to more closely analyze the differences in the optimum conditions between dilute acid hydrolysis and enzymatic hydrolysis, a series of 27 experiments was conducted using three biomass species: aspen (hardwood), balsam (softwood), and switchgrass (herbaceous energy crop); and three process parameters: acid concentration, temperature, and reaction time. The goal of this project was to study the relationship between dilute acid hydrolysis and enzymatic hydrolysis by analyzing the effects of differing pretreatment conditions on total sugar yields after enzymatic hydrolysis.

A series of 27 experiments was completed for each possible combination of biomass type, reaction temperature (150, 160, 175°C), and acid concentration (0.25, 0.5, and 0.75% (wt.) H2SO4). A system of ten tubular reactors, each containing 0.5g biomass and 4.5ml dilute sulfuric acid, was lowered into an oil bath and each reactor was allowed to remain at the target temperature for a predetermined amount of time. Reaction times were optimized to ensure that the ten reactors were removed from the oil bath beginning with the initial heat-up period through the time required for maximum xylose yields, and then samples up to twice that ?optimum? time. The solids from several of the reactors then underwent enzymatic hydrolysis at an enzyme loading of 60 filter paper units (FPU) SpeezymeCP (Genencor) per dry gram glucan in the feedstock and 120 cellobiohydrolase units (CBU) &beta-glucosidase (Novozyme, Novozym 188) per FPU, pH 4.8, and 50°C for 72 hours.

The results of this study, in the form of theoretical yields and a severity analysis, show that for aspen and balsam maximum dilute acid hydrolysis xylose yields were obtainable at all acid concentrations and temperatures studied as long as reaction time was optimized. Switchgrass shows a dependence on dilute acid hydrolysis acid concentration due to the neutralizing effects of the biomass, with higher acid concentrations needed to obtain maximum yields. Glucose yields post enzymatic hydrolysis for aspen and balsam are most dependent upon dilute acid hydrolysis temperature with the highest yields at the highest temperature, and for switchgrass are dependent upon higher acid concentration and temperature. Minor xylose yields are obtained for aspen and switchgrass from enzymatic hydrolysis due to the cellulase enzymes exhibiting slight xylanase activity. Glucan oligomer yields are negligible and xylan oligomers appear to be most sensitive to the reaction time component of dilute acid hydrolysis severity for aspen and balsam and are already at low levels by the ?optimum? reaction time. For switchgrass, xylan oligomers show an opposite trend to xylose monomers with higher and significant concentrations present at low acid concentration. Overall, high total sugar yields, approaching 78-98% are obtained from dilute acid and enzymatic hydrolysis of aspen and switchgrass at the various conditions, but low yields are achieved for balsam, approximately 20 percent. The severity analysis shows that for the range of dilute acid hydrolysis conditions studied, to obtain maximum total sugar yields post enzymatic hydrolysis, severities within the middle of the range analyzed are the optimum conditions.

References

Lloyd TA, Wyman CE. Combined sugar yields for dilute sulfuric acid pretreatment of corn stover followed by enzymatic hydrolysis of the remaining solids. Bioresour Technol. 2005; 96: 1967-1977.

Jensen JR, Morinelly JE, Gossen KR, Brodeur-Campbell MJ, Shonnard DR. Effects of Dilute Acid Pretreatment Conditions on Enzymatic Hydrolysis Monomer and Oligomer Sugar Yields for Aspen, Balsam, and Switchgrass. Submitted to Bioresour Technol.