Bio-oil via catalytic liquefaction of unhydrolyzed solids in aqueous medium

Bioethanol can be produced from lignocellulosic feedstock using a biochemical route involving enzymatic hydrolysis followed by microbial fermentation. During these processes, about 30 to 40% of the biomass is left behind as wet unhydrolyzed solids (UHS), depending on the type of biomass, enzyme loading, and duration of enzymatic hydrolysis. The UHS is mostly composed of lignin, bound enzymes, undigested recalcitrant carbohydrates and ash. The efficient conversion of UHS into bio-oil will increase the overall conversion efficiency of biomass to liquid fuels (bioethanol and bio-oil) and has the potential to reduce the cost of biofuel production from lignocellulosic biomass. In this paper we report the results of bio-oils production from UHS via hydrothermal liquefaction (HTL) under varying temperatures (280–350°C) and subcritical water conditions. The effects of K₂CO₃ and supported bimetallic CoMo/Al₂O₃ catalysts during HTL process were investigated. The UHS used in this study was produced after enzymatic hydrolysis of Ammonia Fiber Expansion (AFEX) pretreated corn stover (ACS). Bio-oil yields at different HTL temperatures were quantified and characterized using ¹H-NMR, ¹³C-NMR, GC-MS, and elemental analysis. The yield of bio-oil was higher in the presence of 5 wt% of K₂CO₃ during HTL in comparison with CoMo/Al₂O₃ catalyst. The highest degree of liquefaction (DL) and bio-oil yield were respectively 43.4% and 30.1 wt% at 350°C in the presence of K₂CO₃ while the highest ECR was 57.1% at 320°C in the presence of K₂CO₃. The study is one of the first of its kind where unhydrolyzed solids, left behind after bioethanol production, were used for making bio-oils with traditional and reduced bimetallic CoMo/Al₂O₃ catalysts in an aqueous medium. The results of the study can contribute to the development of the lignocellulosic biomass-based biorefinery concept.