Impact mechanisms of supercritical CO2-ethanol-water on extraction behavior and chemical structure of eucalyptus lignin.

A compounded medium of supercritical CO2, ethanol, and water (SEW) was used to extract lignin from eucalyptus fiber and the mechanism of the extraction was studied. Compared with the extraction method based on high-temperature ethanol (HTE), the lignin yield of the SEW method was 49.7% higher with higher average molecular weight. Physical and chemical synergies occurred during the extraction process. SEW compound medium penetrated eucalyptus fiber cell walls because of strong permeability, while the fast discharge of the compounded medium facilitated efficient lignin dissociation and removal. Carbonic acid formed from CO2 and water under high temperature and pressure can provide an acidic environment to effectively degrade hemicellulose. Formaldehyde formed from CO2 and ethanol in the process also prevented condensation of the extracted lignin fragments. The obtained lignin had high content of β-O-4 linkages and syringyl units.