Systematic evaluation of fractionation and valorization of lignocellulose via two-stage hydrothermal liquefaction

Abstract

Converting lignocelluloses through conventional hydrothermal liquefaction results in a complex mixture of products due to the compositional heterogeneity of the feedstocks. Sequential hydrothermal liquefaction (SeqHTL) was therefore proposed to address this drawback by selectively fractionating biomass in a stepwise manner. In this work, a two-stage SeqHTL process was designed to isolate sugars in the first stage and to enhance the production of sugar-derived biochemicals and lignin monomers in the second stage from poplar biomass. As a unique feature, the SeqHTL process enabled 24.3% conversion yield of hemicellulose to xylose in the first stage at 180 ℃. In addition, the yields of furfural and lignin monomers were improved by 152.9% and 58.7% for the second stage at 280 ℃ in comparison with the one-stage process operating at the same temperature, respectively. The fractionation of the SeqHTL process reduced the coupling reactions between sugar and lignin components in the feedstock at the first stage, reducing the formation of char and high-molecular-weight compounds at the second stage. Besides, the effects of additives (alcohol, acid, and salt) on the hydrothermal conversion of poplar were investigated in terms of product yields, reaction pathways, surface chemistry and morphology of the solid products. Results showed that the presence of ethanol significantly promoted the production of lignin monomers, while phosphoric acid facilitated the formation of levulinic acid. Overall, this study demonstrates that by properly utilizing subcritical water properties, the SeqHTL platform through simultaneous fractionation and conversion, provides a potential for the stepwise production of bio-based chemicals from lignocellulosic biomass.