Qualitative Analysis of Liquid Products Generated from Lignocellulosic Biomass Using Post-Target and Nontarget Analysis Methods and Liquefaction Mechanism Research

摘要

Significant progress has been made in the preparation of levulinic acid and its esters, furfural, 5-HMF, and other small molecular platform compounds by catalytic liquefaction of lignocellulosic biomass. However, the components of the lignocellulosic biomass's liquefaction products are complex, in particular, the liquid phase products. This makes product separation difficult and presents challenges for the full composition of liquefied products. In this study, gas chromatography coupled with quadrupole time-of-flight mass spectrometry (GC/Q-TOF MS), combined with post-target and nontarget analysis methods, was used to analyze the liquid phase products generated from lignocellulosic biomass liquefaction, such as poplar wood, pine wood, cellulose model compounds, including glucose, cellobiose and microcrystalline cellulose, hemicellulose model compounds, including xylose, xylan, glucose and xylose mixture, and microcrystalline cellulose and xylan mixture, liquefied in ethanol catalyzed by sulfuric acid. A total of 72 compounds were identified from nine samples. Fifty-four compounds were identified by post-target analysis, and 18 unknown compounds were identified for the first time by nontarget analysis. Among the 18 unknown compounds, five were intermediates of lignocellulosic biomass liquefaction, and 13 were byproducts. The reaction mechanism of ethyl levulinate by catalytic liquefaction of lignocellulosic biomass acid was further clarified based on the intermediate compound ethyl 2-hydroxy-4-oxopentanoate (EHO), which was first reported. The mechanism was as follows: (i) cellulose is hydrolyzed to generate glucose, from which 1,6-anhydro-beta-d-glucopyranose (AGP), 1,6-anhydro-beta-d-glucofuranose (AGF), 3-hydroxy-4-oxopentanoic acid (HOA), and EHO are gradually generated, and finally ethyl levulinate is produced; and (ii) cellulose was hydrolyzed into glucose, which was gradually generated into 1,4:3,6-dianhydro-alpha-d-glucopyranose (DGP), HOA, and EHO, and finally into ethyl levulinate.