Reactivity of lignin in supercritical methanol studied with various lignin model compounds

Behavior of lignin in supercritical methanol (250-270degreesC, 24-27 MPa) was studied by using lignin model compounds at the tin bath temperature of 270degreesC with a batch-type reaction vessel. Guaiacol and veratrole were selected as a guaiacyl type of aromatic ring in lignin, while 2,6-dimethoxyphenol and 1,2,3-trimethoxybenzene as a syringyl one. In addition, biphenyl and beta-O-4 types of dimeric lignin model compounds were, respectively, studied as condensed and ether linkages between C-6-C-3 phenyl propane units. As a result, both guaiacyl and syringyl types of aromatic rings were very stable, and the biphenyl type was comparatively stable under supercritical conditions of methanol. However, beta-ether linkage in the phenolic beta-O-4 model compound was cleaved rapidly into guaiacol and coniferyl alcohol, which was further converted to its gamma-methyl ether. Non-phenolic beta-O-4 model compound was, on the other hand, converted initially into its alpha-methyl ether and degraded further to produce guaiacol. These lines of evidence imply that in lignin macromolecules, the new phenolic residues are continuously formed and depolymerized repeatedly in supercritical methanol into the lower molecular products, mainly by the cleavage of the dominant beta-ether structure in lignin.