Date of Graduation

8-2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Biological and Agricultural Engineering

Advisor

Danielle J. Carrier

Committee Member

Jin-Woo Kim

Second Committee Member

Jackson O. Lay

Third Committee Member

Greg Thoma

Fourth Committee Member

Edgar Clausen

Abstract

Production of fuels and chemicals from biomass is contingent upon economical release of carbohydrates from biomass. Carbohydrates can then be used for production of bio-based products using a biochemical conversion process. Pretreatment, the first step of the biochemical conversion process, has been suggested to be the most costly step of the conversion process. Thus, better understanding the behavior of biomass during pretreatment is imperative for an economically viable production of biofuels and chemicals. Elucidating the physicochemical properties of biomass and developing an understanding the depolymerization patterns of biomass during pretreatment will help progress towards this goal.

In this study, July- and February-harvested switchgrass hemicelluloses were extracted and characterized for monosaccharide constituents, glycosyl linkages, and molecular size using acid hydrolysis, per-O-methylation analysis, and size exclusion chromatography, respectively. The results revealed that the July hemicelluloses contained 13% glucose, 67% xylose, and 19% arabinose, and the February hemicelluloses contained 4.8% glucose, 79% xylose, and 16% arabinose. Glycosyl linkage analysis revealed both hemicelluloses to have similar linkages but in different proportions. Size exclusion chromatography showed that the July hemicelluloses had an average molecular weight of 30,000 g mol-1, and the February hemicelluloses had an average molecular weight of 28,000 g mol-1.

Once characterized, extracted hemicelluloses were used as feedstock for production of xylose oligomers that were then fractionated using centrifugal partition chromatography (CPC) with a butanol:methanol:water (5:1:4, V:V:V) solvent system. Xylose oligomers with a degree of polymerization (DP) from two to six were successfully produced via autohydrolysis and fractionated via CPC. Yields for xylobiose (DP2), xylotriose (DP3), xylotetraose (DP4), xylopentose (DP5), and xylohexose (DP6) were 24, 34, 23, 19, and 38 mg, respectively, per g of hemicelluloses. Purities, as calculated by mass of a given oligomer divided by the total mass of detected oligomers and degradation products and then reported on a percent basis, were 75, 89, 87, 77, and 69% for DP2, DP3, DP4, DP5, and DP6, respectively.

Lastly, depolymerization patterns of CPC-fractionated xylose oligomers were investigated through pretreatment studies and subsequent kinetic modeling. DP6 was pretreated using water at 160 and 180 oC and 1.0 wt % sulfuric acid at 160 oC. Modeling results revealed that degradation rate constants increased with increasing temperature and acid concentrations, and that acid promotes cleavage of end bonds over interior bonds in xylose oligomers.

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