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.

The electrocatalytic hydrogenation (ECH) of phenol has been studied using palladium supported on gamma-alumina (10% Pd-Al2O3) catalysts. The catalyst powders were suspended in aqueous supporting electrolyte solutions containing methanol and short-chain aliphatic acids (acetic acid, propionic acid, or butyric acid) and were dynamically circulated through a reticulated vitreous carbon cathode. The efficiency of the hydrogenation process was measured as a function of the total electrolytic charge and was compared for different types of supporting electrolyte and for various solvent compositions. Our results show that these experimental parameters strongly affect the overall ECH efficiency of phenol. The ECH efficiency and yields vary inversely with the quantity of methanol present in the electrolytic solutions, whereas the presence of aliphatic carboxylic acids increased the ECH efficiency in proportion to the chain length of the specific acids employed. In all cases, ECH efficiency was directly correlated with the adsorption properties of phenol onto the Pd-alumina catalyst in the studied electrolyte solution, as measured independently using dynamic adsorption isotherms. It is shown that the alumina surface binds the aliphatic acids via the carboxylate terminations and transforms the catalyst into an organically functionalized material. Temperature-programmed mass spectrometry analysis and diffuse-reflectance infrared spectroscopy measurements confirm that the organic acids are stably bound to the alumina surface below 200 degrees C, with coverages that are independent of the acid chain length. These reproducibly functionalized alumina surfaces control the adsorption/desorption equilibrium of the target phenol molecules and allow us to prepare new electrocatalytic materials to enhance the efficiency of the ECH process. The in situ grafting of specific aliphatic acids on general purpose Pd-alumina catalysts offers a new and flexible mechanism to control the ECH process to enhance the selectivity, efficiency, and yields according to the properties of the specific target molecule.

The hydrogen evolution reaction on the Raney nickel composite-coated electrode is investigated in 1.04M NaOH aqueous solution at 70~ by ac impedance technique. In case of the chemically activated electrode material, the easiest pathway for the hydrogen evolution reaction (HER) is the Volmer-Tafel mechanism, and the controlling step is the Volmer reaction with a small contribution of the Heyrovsky reaction. The electrode material becomes more electrocatalytic towards the HER after an electrochemical activation. In this case, the easiest pathway for the HER is the Volmer-Heyrovsky mechanism, both steps being rate-controlling.

Electrodes consisting of particles of a transition metal entrapped and dispersed in a reticulated vitreous carbon (RVC) matrix were prepared in situ by stirring the particles in the presence of an RVC cathode. Such electrodes were used for the electrocatalytic hydrogenolysis (ECHsis) of 4-phenoxyphenol, a compound representative of the 4-O-5 type linkage in lignins. The electrolyses were carried out under galvanostatic control in aqueous 1 M NaOH. Raney nickel, nickel boride, and transition metals supported on activated charcoal or alumina were used as catalytic powders. The extent and efficiency of the ECHsis of 4-phenoxyphenol was found to depend on the catalyst and on the temperature.Key words: electrocatalytic hydrogenation, electrocatalytic hydrogenolysis, 4-phenoxyphenol, Raney nickel, palladium on charcoal, palladium on alumina.