Date of Award
Doctor of Philosophy
Chemical and Biological Engineering
Robert C. Brown
Solvent liquefaction using polar aprotic solvents is a promising approach for production of solubilized carbohydrates as biofuel precursor from lignocellulosic biomass. However, many technical challenges preclude its application at commercial scale. This research focuses on improving upon these challenges with bench-scale studies on liquefaction of cellulose and hardwood biomass in a variety of polar aprotic solvents.
Cellulose conversion was studied in a variety of polar aprotic solvents at hot, pressurized conditions, including 1,4-Dioxane, ethyl acetate, tetrahydrofuran (THF), methyl iso-butyl ketone (MIBK), acetone, acetonitrile, and γ-valerolactone (GVL). Maximum yield of depolymerized carbohydrate and products of carbohydrate dehydration from cellulose, called solubilized products, was 72 to 98% at 350 oC within 8-16 min of reaction. The most prevalent solubilized carbohydrate product was levoglucosan and it was produced with yields reaching 41% and 34% in acetonitrile and GVL, respectively. Levoglucosan yields increased with increasing polar solubility parameter of the solvent. This behavior of solvents could be attributed to reduction of apparent activation energy of cellulose depolymerization in higher polarity solvents. Recovery of solvents in all cases was high.
The effectiveness of a wide range of polar aprotic solvents, including1,4-Dioxane, ethyl acetate, THF, MIBK, acetone, acetonitrile, and GVL, in depolymerizing cellulose into solubilized carbohydrates in the presence of dilute acid catalyst. While yields of solubilized carbohydrates strongly depended on the polar solubility parameters of solvents, the use of dilute acid catalyst substantially removed differences in the yields for various polar aprotic solvents. The equalized solubilized carbohydrate yields among the polar aprotic solvents were 83-97%. Levoglucosan and solubilized carbohydrates yields in 1,4-Dioxane, THF, and acetone approached that of GVL, along with completely solubilizing cellulose within 1-7 min. The low polarity, low boiling point solvents showed high stability and competitive yields of the anhydrosugar compared to high polarity and high boiling solvent such as GVL due to low initial rates of levoglucosan degradation. The ease of separation of low polarity, low-boiling solvents offers them as attractive media for solubilized carbohydrates production when used in presence of acid catalyst.
Use of 1,4-Dioxane to depolymerize cellulose for production of solubilized carbohydrates was explored. The low boiling point of this low polarity solvent offers inexpensive and simple separation compared to higher boiling point solvents like GVL, which has been previously investigated for acid-catalyzed depolymerization of cellulose. In this work, several key reaction parameters including reaction temperature, acid catalyst concentration, and content of co-solvent water in 1,4-Dioxane were studied for their impact on enhancing sugar production from cellulose. Yield of levoglucosan, the major anhydrosugar product of cellulose depolymerization, was maximized at 71% by operating at high temperature, short reaction time, low acid concentration and low mass loading of cellulose. Use of water as a co-solvent improved cellulose solubilization and promoted solubilized carbohydrates production at low temperature and high mass loading. This behavior of acid-catalyzed co-solvent system could potentially enable processing of cellulose at high solid loadings and milder conditions thus increasing its applicability at large scale.
In this work, a novel two-step liquefaction process was developed for bench-scale production of solubilized, fermentable carbohydrates from hardwood biomass in a mixture of THF, water and dilute sulfuric acid. THF facilitates solubilization of lignin and hemicellulose in the biomass in presence of dilute acid catalyst resulting in 61% lignin extraction and 64% xylose recovery in a mild pretreatment step. The pretreatment loosens up the structure of biomass by delignification and produces a cellulose-rich hardwood, which could be readily solubilized at low temperature in a subsequent solvent liquefaction step using THF/water/acid mixture. The combined pretreatment and solvent liquefaction process produced 60% glucose yield and 89% xylose yields based on initial amounts of glucan and xylan in untreated biomass. Additionally, volumetric productivity of sugars was four orders of magnitude larger than conventional enzymatic hydrolysis. This process, not only achieves comparable sugar yields and significantly enhanced sugar productivity compared to biological processes and state-of-the-art solvent liquefaction techniques, but it also offers prospects for overcoming economic and sustainability barriers of cellulosic ethanol production by using THF which is relatively low-cost and low toxicity, derivable from biomass, and readily separable from sugar solution due to its low boiling point.
Ghosh, Arpa, "Production of solubilized carbohydrates from lignocellulosic biomass using solvent liquefaction" (2018). Graduate Theses and Dissertations. 16357.