Comparison of the Efficacy of Co-Solvent Enhanced Lignocellulosic Fractionation (CELF) and Dilute Acid Pretreatments at Low to High Solids Loadings for Switchgrass

Over the decades, numerous environmental and political concerns have motivated research into sustainable, domestic sources of fuels to replace those derived from petroleum. Lignocellulosic biomass represents the most abundant renewable resource and provides the only known route to sustainably produce liquid fuels on a large scale and low cost. Due to its recalcitrant nature, raw biomass hinders enzymatic digestion of polysaccharides, thus necessitating pretreatment to deconstruct the plant structure and allowing cellulose and hemicellulose to be more accessible to enzymes for release of fermentable sugars. Several pretreatment methods, including hydrothermal and dilute acid pretreatments, have been developed to improve enzyme accessibility, but each is limited in their ability to remove lignin, a key contributor to biomass recalcitrance.  Recently, we developed a novel pretreatment called Co-solvent Enhanced Lignocellulosic Fractionation (CELF) that applies aqueous tetrahydrofuran (THF) with dilute sulfuric acid to effectively remove lignin, while simultaneously achieving high yields of five carbon sugars from hemicellulose during pretreatment and subsequently of six carbon sugars from glucan using very low enzyme dosages (Nguyen et al., ChemSusChem, 2015). CELF was optimized for corn stover at low solids loading (5 wt %) to counter mass transfer artifacts, however, from energy and capital costs perspectives, pretreatments need to be performed and evaluated at high solids loadings to test their commercial applicability. Therefore, in this study, we first applied CELF on switchgrass to determine the optimum conditions for maximum sugar release at low solids loadings. For comparison, DA pretreatment was also performed at conditions that were previously determined to be optimum (Wyman et al., Bioresource Technology, 2005). We then evaluated pretreatment of varying solids loadings of switchgrass using dilute acid and CELF in terms of changes in solids compositions, sugar recovery, and digestibility of the resulting pretreated solids produced over a range of enzyme loadings for each pretreatment.