Document Type
Thesis - University Access Only
Award Date
2009
Degree Name
Doctor of Philosophy (PhD)
Department / School
Chemistry and Biochemistry
Abstract
Lignocellulosic materials are renewable biomass that has the potential to serve as sustainable feedstocks in lieu of diminishing crude oil and coal for future biofuels and chemicals. Processes to convert biomass into valuable products through the hydrolysis of cellulosic materials to monosaccharides or the fractionation of the lignocellulosic materials into its three main components and transformation of sugars into biobased products are scrutinized worldwide. New approaches for designing improved energy feedstocks, deconstructing plant cell walls, and transforming their polysaccharides to fermentable sugars are needed. This dissertation investigates the effectiveness of deconstructing lignocellulosic feedstocks to sugars via various pretreatment and enzymatic hydrolysis methods. Distillers dried grains with solubles (DOGS), the residue from current ethanol production, prairie cordgrass and switchgrass were selected as biomass feedstocks for this study. The pretreatment methods were divided into two groups: the hydrolytic methods generating slurries and the fractionation methods which separate the lignocellulosic materials into their three main components (cellulose, hemicellulose, and lignin). Methods involving hydrolysis of lignocellulosic materials heating by heating with convection or microwave irradiation were investigated. The samples were processed using water and chemical additives (0.3% H2O2, 0.2% H2SO4, 0.5% H2SO4, 1.8% glycerol, and 1 % NaOH) at elevated temperatures and pressures (1500 psi). Hydrolysis times (5 - 60 minutes) and temperatures (50 - 200 °C) were investigated and the optimal conditions were obtained at 200 °C for 1 0 minutes. Pretreatments with subcritical fluids or microwave irradiation achieved similar results but pretreatments with subcritical fluids were favored because they resulted in less samples loss during hydrolysis compared to microwave pretreatments Treatments under alkaline conditions resulted in the least amount of lignin (5-6%) remaining in the residues and a percent cellulose digestibility of 58.14 ± 4.56% and 55.48 ± 2.49% (for prairie cordgrass and switchgrass respectively) was achieved. Treatments under acidic conditions yielded the highest amount of monomeric sugars in hydrolyzates, but the lowest percent cellulose digestibility was obtained due to the formation of degradation products or by-products such as acetic acid, lactic acid, furfural, and hydroxymethyl furfural (HMF) that act as inhibitors during the enzymatic hydrolysis. Treatments under neutral conditions with hot compressed water achieved a higher percent cellulose digestibility than acidic pretreatments. The addition of carbon dioxide to the hot compressed water experiments via a two-stage hydrolysis method significantly enhanced the enzymatic hydrolysis yield. Experiments were performed at 175 °C and hydrolysis times and pressure were investigated. The optimal conditions were obtained when samples were treated at 1500 psi for 1 0 to 15 minutes. A cellulose digestibility of up to 75% was achieved. The last part of this dissertation dealt with fractionation methods which were performed with common organic solvents and alternative solvents such as ionic liquids. With the former pretreatment method the biomass samples were hydrolyzed at 140 °C for 56 minutes with five different mixtures of dilute sulfuric acid, ethanol and methyl isobutyl ketone, mimicking the NREL (National Renewable Energy Laboratory) clean fractionation procedure; while with the latter method the samples were dissolved in a mixture of dimethyl sulfoxide and 1-butyl-3 methylimidazolium chloride (BMIMCI) at moderate temperatures. The cellulose was reconstituted using solvents such as water, acetonitrile, methylene chloride, or acetone, and the reconstituted cellulose was characterized with 13C-NMR. The organosolv fractionation process using different solvent mixtures was found to be equivalent to the acid hydrolysis pretreatments. The delignification of samples displayed a high efficiency when solvent mixtures with more organic solvent and less water was used and these results were similar to those obtained when the same samples were treated under alkaline conditions. The enzymatic hydrolysis rate was enhanced because the samples were lignin and hemicellulose free at the end of the process and a cellulose digestibility of up to 88% was achieved. Complete dissolution of samples with 1-butyl-3-methylimidazolium chloride was investigated at 100 - 135 °C and the optimal dissolution temperature was obtained at 100 °C. The 13C-NMR spectra revealed the presence of lipids in biomass samples, while that of DOGS revealed the presence of proteins in addition to lipids. The removal of extractable lipids in samples prior to dissolution with BMIMCI did not result in a total isolation of the former, those results led to the conclusion that the samples still contained a group of persistent lipids (polymeric lipids) that is trapped in the macromolecular structure of cellulose. Hemicellulose and lignin in supernatants could further be fractionated with the organosolv fractionation process. Based on the findings of this research, this dissertation has shown that lignocellulosic materials could be hydrolyzed and fractionated into their main components. Methods that could address and overcome the drawbacks caused by the other pretreatment methods using volatile solvents, and highly corrosive or caustic chemicals were developed. These environmentally friendly methods used hot compressed water and carbonic acid or ionic liquids and they could be used on an industrial scale and ethanol could be produced at quantities and cost competitive with gasoline.
Library of Congress Subject Headings
Lignocellulose
Biomass conversion
Cellulose -- Biodegradation
Biomass energy
Hydrolysis
Cellulosic ethanol
Format
application/pdf
Number of Pages
210
Publisher
South Dakota State University
Recommended Citation
Tenlep, Lisette Ngo, "Pretreatment Strategies for Cellulosic Ethanol" (2009). Electronic Theses and Dissertations. 1620.
https://openprairie.sdstate.edu/etd2/1620
