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.

Catalytic C−O Bond Cleavage of 2-Aryloxy-1-arylethanols and Its Application to the Depolymerization of Lignin-Related Polymers

“…The selective deuteration result is consistent with a mechanism in which a ruthenium enolate ( 6 ) is formed and trapped by the deuterated silane 13. These experiments indicate that radical and elimination mechanisms are not likely responsible for the observed transformation and support the proposed model outlined in Figure 2 7b, 12…”

“…The β -[O]-4′-glycerolaryl ether linkages ( 1 ) are ubiquitous in lignin found across many species including Miscanthus giganteus , a species of particular interest for industrial lignocellulosic bio-fuels production (Figure 1). 6 2-Aryloxy-1-arylethanols ( 2 ), or “ β -[O]-4′–ethanolaryl ethers”, are often used to model the β -[O]-4′-glycerolaryl ether linkages and were chosen for the purposes of reaction development 7…”

Final Report: Investigation of Catalytic Pathways for Lignin Breakdown into Monomers and Fuels

“…Electrocatalytic hydrogenation has been shown to selectively convert lignin model compounds to reduced phenolic compounds (Mahdavi et al 1997 andCyr et al 2000). For the published reaction, lignin model compounds were dissolved in 1M NaOH and adsorbed hydrogen atoms for reduction and deoxygenation were generated on a carbon cathode loaded with Pd/Al 2 O 3 .…”

“…An electrocatalytic hydrogenation experimental apparatus was built based on the setup described by Cyr and Chiltz (2000), Fig.8. A catalytic cathode (1.5 x 1.0 cm 2 or 0.6 cm 3 ) was cut from reticulated vitreous carbon foam, 100 pores per square inch (ERG aerospace), and rinsed in deionized water and methanol to remove dust and loose carbon particles generated during cutting.…”

Preparation and characterization of electrochemically modified electrodes containing the metal complexing moiety quinolin-8-ol anchored to the polymeric film

“…Over the past two decades, they have been widely applied in electrocatalytic reactions [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] and as electrochemical sensors [2,4,7,14]. Chemical [1], electrochemical [2][3][4][5][6][7][8][9][10][11][12], catalytic [13], photochemical [15] and optical [16] properties can be achieved by modifying selectively the electrodeÕs surface.…”