Green degradation of Caragana korshinskii to valuable bio-oils in supercritical CO2-ethanol: Evaluation of CH and O moieties

Abstract

An eco-friendly supercritical CO2 (scCO2)-ethanolysis system was designed to convert Caragana korshinskii (Ck) into valuable bio-oils. Hydrocarbons and O-containing moieties (CH and O classes) in derived soluble portions (SPs) were evaluated in detail. The results show that scCO2-ethanol has synergistic degradation ability and strong permeability to complex structures. Optimal conditions were determined to be 10 mL g−1 ethanol/Ck ratio, 1 MPa initial CO2 pressure, 300 °C, and 60 min holding time based on the SP yield and the maximum yield is 38.2 wt% with high heating value of 24.06 kJ g−1. The difference of SPs between conventional ethanolysis (SPE) and scCO2-ethanolysis (SPSE) was compared. According to FTIR and GC/MS analyses, the distribution of functional groups and detectable group components is similar, and O-containing species are dominant. Tetrahydrofurans & furans, alkoxy-substituted benzenes & alkoxy-substituted phenols, and ketones & esters are main group components. The characteristic differences of >C-O- and >CO classes are related to the following three aspects: (1) the ring-opening, >C-O- bond cleavage, cyclization, and condensation of cellulose/hemicellulose, (2) the destruction of abundant >Car-O- in lignin, especially guaiacyl and syringyl units, and (3) the formation of ethyl esters promoted by the nucleophilic attack of ethoxy group. Orbitrap MS analysis further confirmed that O-containing species of SPE and SPSE are mainly concentrated in O1-O4 classes, and scCO2-ethanolysis promotes the acquisition of sugars and bioactive fragments from Ck. In addition, the characteristic distribution of On classes in SPSE is related to the release of natural oxygenates, cleavage of weak/intermediate-strength covalent bonds, and dissociation of O-containing macromolecules. This study provides a reference perspective and a potential implementation approach for the clean and efficient utilization of Ck to produce valuable bio-oils.