Lignin depolymerization in supercritical solvents next to water includes ethanol, methanol, CO2, and CO2/acetone/water. The supercritical properties of these fluids are displayed in Table 17.10.

The choice for using CO2 as solvent is obvious as CO2 is cheap, environmentally friendly and generally recog­nized as safe by the US Food and Drug Administration. scCO2 has other advantages because of its high diffusivity combined with its easily tunable solvent strength. To use CO2 under supercritical conditions, the temperature needed is low (>31 0C) and the pressure needed relatively low (>7.4 MPa) in comparison to other supercritical sol­vents (Table 17.10). Additionally, CO2 is a gas at room temperature and pressure, which leads to a solvent-free product after pressure expansion. A drawback of scCO2 is its low polarity, which is comparable to hexane, but this problem can be overcome by using cosolvents to change the polarity of the SCF (Herrero et al., 2010). Furthermore, SCF processing based on CO2 enables the easy recycling of CO2, which is advantageous for the development of a sustainable process. Research per­formed on supercritical processing of lignin to produce aromatic compounds has been summarized hereafter.

Depolymerization of lignin model compounds and organosolv lignin have been studied in supercritical al­cohols like methanol and ethanol in a temperature range of >239 0C and a pressure of >8.1 MPa. By using bases such as KOH and NaOH a high depolymerization con­version was obtained. The dominant depolymerization route is the solvolysis of ether linkages in the lignin structure while the carbon—carbon linkages are mostly stable (Miller et al., 1999; Minami et al., 2003).

Yuan et al. (2010) used BCD at mild temperatures (220—300 0C) of kraft lignin in water—ethanol into oligo­mers with a negligible char and gas production.

However, under the conditions applied lignin could not be completely degraded into monomers.

Oxidation of lignin and lignin model compounds with peroxide was studied under scCO2 conditions in the absence of alkali. The 5-5 biphenols were shown to be degraded and in this process mostly the formation of carboxylic acids from kraft lignin was observed (Argyropoulos et al., 2006).

Gosselink et al. (2012) found that hardwood and wheat straw organosolv lignins were depolymerized in supercritical carbon dioxide/acetone/water fluid at 300 0C and 100 bar into 10—12% monomeric aromatic compounds. Small amounts of formic acid were intro­duced as in situ hydrogen donor. Furthermore, lignin is converted into a phenolic oil consisting of both mono­meric and oligomeric aromatic compounds. Interest­ingly, maximum individual yields of 3.6% for syringol and 2.0% for syringic acid based on lignin were ob­tained. Depolymerized phenolic products and char were separated during this process by pressure expan­sion. As during this process competition occurs between lignin depolymerization and recondensation of frag­ments a substantial amount of char is formed.