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

An eco-friendly supercritical CO2 (scCO(2))-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 scCO(2)-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 degree celsius, 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 scCO(2)-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 >C=O 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 >C-ar-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 O-1-O-4 classes, and scCO(2)-ethanolysis promotes the acquisition of sugars and bioactive fragments from Ck. In addition, the characteristic distribution of O-n 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.