The electrocatalytic hydrogenation of fused poly cyclic aromatic compounds at Raney nickel electrodes: the influence of catalyst activation and electrolysis conditions

Abstract

. The electrocatalytic hydrogenation (ECH) of phenanthrene, anthracene, and naphthalene has been investigated under constant current at Raney nickel electrodes in a mixed aqueous organic medium. The influence of various parameters on the efficiency of the process determined by the current efficiency (a measure of the competition between hydrogenation and hydrogen evolution, the only two electrochemical processes occumng), the extent of hydrogenation (yield of octahydro-derivatives), and the conversion rate was studied with phenanthrene. The best conditions were ethylene glycol or propylene glycol as cosolvent containing between 1.5 to 5% of water, a neutral or slightly acidic medium containing boric acid (0.1 M) as buffer (initial pH of 2.6, final pH of 6.0-6.2), sodium chloride or tetrabutylamonium chloride as supporting electrolyte, a temperature of 80°C, and a current density of 42 to 84 m~/ c r n~. The most active electrodes (consisting of Raney Ni particles dispersed in a nickel matrix and surrounded by a layer of porous nickel) were obtained by leaching the dispersed alloy particles at 75°C for 7 h in 30% aqueous sodium hydroxide. The electrohydrogenation stopped at derivatives with a single aromatic ring, namely the octahydrophenanthrenes, octahydroanthracenes, and tetralin. In a non-buffered medium, tetrahydrophenanthrene could be obtained with selectivities of 80% or better.

Electrocatalytic hydrogenation of conjugated enones on nickel boride, nickel, and Raney nickel electrodes

“…The selectivity was also greatly dependent on the type of catalytic electrode, increasing from 20% (2 F mol-') and 9% ( 4 F mol-') with the RaNi electrodes (entry 3) to 74 and 46%, respectively, with the Ni2B electrodes (entry 1) and to 86 and 78% with the fractal Ni electrodes (entry 2). This is the order of decreasing activity (apparent activity) of these three electrodes for the ECH of phenanthrene in ethylene glycol-water at 80°C, fractal Ni being much less active than the two other electrodes (5).6 It is reasonable to assume that the order of activity would be the same for the ECH of cyclohexenone in methanol-water at room temperature and the most active RaNi electrodes then gave the lowest selectivity for the hydrogenation of the carbon-carbon double bond, as expected.…”

“…ECH presents several advantages over catalytic hydrogenation. Firstly ECH uses the very mild conditions (room temperature and atmospheric pressure) normally employed in electrochemical synthesis, mainly because there is no mass transport of poorly soluble molecular hydrogen and because the kinetic barrier for its dissociation is bypassed (1). In fact, the chemisorbed hydrogen is produced in situ, at the surface of the catalyst.…”

“…In fact, the chemisorbed hydrogen is produced in situ, at the surface of the catalyst. Furthermore, its generation can be controlled by adjusting the current density, which makes the reaction easier to handle and may also increase the reaction selectivity (1,7,9). Thus, ECH is expected to be more selective than catalytic hydrogenation.…”

“…Thus, ECH is expected to be more selective than catalytic hydrogenation. secondly, in some cases, the adsorption of poisons can be diminished or even prevented by cathodic polarization of the catalyst (1). When compared to electrohydrogenation by the electronation-protonation (EP) process, ECH has the advantage of requiring less energy.…”

Hidrogenação de compostos orgânicos utilizando método eletroquímico para geração de hidrogênio in situ: hidrogenação eletrocatalítica

“…Eletrodos compostos por níquel de Raney foram utilizados na HEC de alguns compostos poliaromáticos (fenantreno, antraceno e naftaleno) 7 , como também do benzeno, anilina e nitrobenzeno 6 . Neste 9 verificaram que a HEC sob corrente controlada, pressão média (3 a 41 atm) de um gás inerte (N 2 ), temperaturas de 81 a 110°C e níquel de Raney como cátodo, levaram a rendimentos da ordem de 99% para a hidrogenação do fenantreno, gerando os dois octahidrofenantrenos (91%) e produtos di-(3%) e tetra-hidrogenados (5%), com uma eficiência eletroquímica de cerca de 94% (Esquema 5).…”