Production of upgraded biocrude from hydrothermal liquefaction using clays as in situ catalysts

Abstract Hydrothermal liquefaction (HTL) is a thermochemical process that converts biomass into biocrude. HTL suffers from low yields of water-insoluble biocrude with high oxygen contents and low heating values. Inexpensive clay minerals including montmorillonite, dolomite, kaolinite and sand were used to upgrade HTL biocrude as in situ acid-base catalysts. Batch tests were performed using starch with 5 wt% clay minerals at 300 °C for 1 h. Bio-oil was fractionated into water-soluble and water-insoluble parts to explore potential catalytic mechanisms by analyzing the fractional distribution, elemental composition and chemical composition. Higher carbon recoveries in the bio-oil fraction (approaching 60%) occurred with clay-catalyzed HTL. Energy recovery of both bio-oil fractions increased by approximately 22% for all clays. A base-catalyzed pathway inhibits char formation from catalytic HTL, with dolomite approaching a char yield as low as 3%. Chromatographic analysis of heavy and light oils from both fractions showed that dolomite and montmorillonite play a catalytic effect via base and acid pathways on upgrading biocrude. Clay-catalyzed HTL modified the boiling point distributions by producing more 100–300 °C middle temperature distillates. Overall, catalytic HTL with clay minerals enhanced the heating value and energy recovery of bio-oils.