Carbonyl Groups in Lignin. III. Mild Catalytic Hydrogenation of Björkman Lignin.

Abstract

Bjorkman lignin was subjected to catalytic hydrogenation at room temperature in 93% acetic acid in the presence of a Pd-BaSO4 catalyst promoted by HC1 (system A) or in alkaline solution with a nickel boride catalyst promoted by chromium boride (system B). The hydrogen consumption amounted to 0.5-0.6 moles of H2/OCH3 in system A, and to nearly 0.3 moles of H2/OCH3 in system B. Hydrogenation of the aromatic nuclei was negligible in both systems. to system A only a minor fraction of the benzyl alcohol and benzyl ether groups suffered hydrogenolysis. The resistant main part of these groupings may belong to elements of the arylglycerol-[beta]-aryl ether structure, which in model experiments was found to be hydrogenolysed very slowly, to system B the benzyl alcohol and benzyl ether groupings remained entirely intact, to accordance with previous results, Bjorkman lignin was found to contain 0.03-0.04 cinnamaldehyde units per OCH3. to addition, 0.03 ethylene groups were found, presumably present in cinnamyl alcohol units. These conclusions were reached by spectrochemical examination of the hydrogenated lignin products. Hydrogenation in system B indicated a total carbonyl content of about 0.2 CO/OCH3, which is in harmony with the value previously found by oximation.