Date of Award

12-2008

Level of Access

Campus-Only Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

Advisor

Adriaan R.P. van Heiningen

Second Committee Member

Barbara J.W. Cole

Third Committee Member

Joseph M. Genco

Abstract

Hemicelluloses derived from biomass are presently underutilized. In order to develop more profitable biorefinery processes, the mechanism responsible for hemicellulose removal by pretreatments must be further explored. The hydrothermal dissolution profile of the main wood components, cellulose, hemicelluloses and lignin, of a hardwood mixture during autohydrolysis were investigated. Xylan initially dissolves as oligosaccharides and then slowly depolymerizes into monomeric xylose. The residual xylan in wood is only slightly deacetylated. A smaller amount of glucomannan is removed as oligosaccharides. Arabinan and galactan are completely removed from wood as monomers at the end of the extraction process. Initially all acetyl groups are removed, while still bound to oligosaccharides. Then acetic acid is released by deacetylation of the dissolved oligosaccharides. Xylan is the predominant component in the extract, especially at temperatures higher than 150°C. At temperatures higher than 150°C a small amount of both furfural and hydroxymethylfurfural is generated. Above 150°C, the component sugars such as arabinose, galactose, glucose and xylose in the extract are further degraded indicating that complete hydrolysis has taken place with additional degradation. In the extract, the ratio of acetyl groups bound to xylooligosaccharides decreases with increasing temperature. The composition of a precipitate formed by addition of 4 parts of ethanol to hemicellulose extract was determined. Almost all lignin in the ethanol precipitate is bound to low molecular weight carbohydrates. However, the majority of carbohydrates present in the precipitates is found to be lignin-free. The amount of cellulose and hemicellulose precipitated with the lignin reaches a maximum around 150 °C and 160°C respectively. The average Mw of hemicellulose and cellulose in the precipitates decreases with increasing temperature. The average degree of polymerization (DP) of the isolated hemicellulose is between 71 and 36. The hydrothermal dissolution profile of the hardwood mixture during autohydrolysis was described as a function of P-factor, a parameter quantifying the effect of time and temperature. The effect of time and temperature on the extraction yield and the dissolution yield of major wood components can be expressed by a single relationship as a function of P-factor.

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