Electrocatalytic hydrogenation of lignin models at Raney nickel and palladium-based electrodes

Abstract

The electrocatalytic hydrogenation (ECH) of β-O-4 lignin model compounds has been carried out at constant current at Raney nickel and palladium-based cathodes in aqueous sodium hydroxide solution at temperatures ranging from 25 to 75°C. It was found that the hydrogenolysis of phenolic β-arylethyl-aryl ethers with one β-O-4 linkage takes place at both types of electrodes to give a mixture of phenolic compounds. The ECH of 1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-1-ethanol (model 1) at Raney nickel gives guaiacol (9), acetovanillone (11b), alpha-methylvanillyl alcohol (12b), and products derived from their further hydrogenation. The β-O-4 bond is more difficult to cleave upon replacing one hydrogen of the methylene group of 1 by a hydroxymethyl group (see 2). However, model 4 with two β-O-4 linkages obtained by the introduction of an α-methylvanillyl group at the phenolic position of 1 is hydrogenolysed at the same rate as 1 at Raney nickel. Only the C-O aryl bond attached to the hydroxyethylphenol moiety is cleaved. At palladium, the hydrogenolysis of 1 requires only half of the theoretical amount of electricity due to the generation of chemisorbed hydrogen by dehydrogenation of 1 followed by hydrogenolysis of the resulting ketone. A general reaction scheme is proposed for the ECH of model 1.Key words: Lignins, lignin models, electrocatalytic hydrogenation, Raney nickel electrodes, palladium electrodes.