Understanding How Tetrahydrofuran and Ethanol Solvent Pretreatments Impact Biological Deconstruction of Lignocellulosic Biomass
The abundance and low cost of cellulosic biomass such as wood, grasses, and agricultural and forestry residues make them very attractive sustainable resources for production of transportation fuels. However, low-cost bioethanol production is hindered largely by the recalcitrant nature of the biomass that results in high enzyme doses for enzymatic hydrolysis to realize high monomeric sugar yields. Biochemical conversion pathways typically apply a pretreatment step to make biomass accessible to enzymes during hydrolysis stage. This thesis focused on application of a new pretreatment technology that applies tetrahydrofuran (THF) in solution with very dilute sulfuric acid and water as a co-solvent pretreatment method to dramatically reduce the recalcitrance of poplar wood and make more sugars accessible to enzymes for hydrolysis along with high lignin separation. It is shown that this Co-solvent Enhanced Lignocellulosic Fractionation or CELF as we have called it is effective for various types of woody biomass for short pretreatment times and moderate temperatures in comparison to various other past pretreatments. Sugar yields from CELF coupled with subsequent 7-day enzymatic hydrolysis were compared with results from dilute sulfuric acid, hydrothermal, and organosolv pretreatments. CELF was able to achieve 100% yields for an enzyme loading of 100mg/g glucan in raw biomass compared to 92% for dilute sulfuric acid pretreatment of the same poplar wood. More importantly, CELF realized much higher combined sugar yields at low enzyme (≤15mg/g glucan in raw biomass) compared to dilute sulfuric acid pretreatment. An important distinction that could account for the substantially better results by CELF from BESC standard poplar was its ability to remove a much greater fraction of lignin. However, CELF achieved higher total sugar yields than ethanol organosolv despite the latter’s ability to also remove lignin. Furthermore, CELF performed better than ethanol organosolv at lower temperatures that resulted in negligible degradation of sugars. Removal of lignin by CELF resulted in an ability to mix higher glucan loadings that resulted in higher sugar titers. Preliminary results showed that a fed-batch strategy reduced hydrolysis times. In other experiments, hydrothermal pretreatment of BESC standard poplar and its variants in combination with consolidated biomass processing (CBP) showed how changes in biomass features could enhance performance. Finally, adding surfactants to enzymatic hydrolysis reactions was shown to reduce enzyme deactivation at the air-liquid interface.