Electrocatalytic Hydrogenation Using Palladium Membrane Reactors

Abstract

Hydrogenation is a crucial chemical process employed in a myriad of industries, often facilitated by metals such as Pd, Pt, and Ni as catalysts. Traditional thermocatalytic hydrogenation usually necessitates high temperature and elevated pressure, making the process energy intensive. Electrocatalytic hydrogenation offers an alternative but suffers from issues such as competing H₂ evolution, electrolyte separation, and limited solvent selection. This Perspective introduces the evolution and advantages of the electrocatalytic Pd membrane reactor (ePMR) as a solution to these challenges. ePMR utilizes a Pd membrane to physically separate the electrochemical chamber from the hydrogenation chamber, permitting the use of water as the hydrogen source and eliminating the need for H₂ gas. This setup allows for greater control over reaction conditions, such as solvent and electrolyte selection, while mitigating issues such as low Faradaic efficiency and complex product separation. Several representative hydrogenation reactions (e.g., hydrogenation of C=C, C≡C, C=O, C≡N, and O=O bonds) achieved via ePMR over the past 30 years were concisely discussed to highlight the unique advantages of ePMR. Promising research directions along with the advancement of ePMR for more challenging hydrogenation reactions are also proposed. Finally, we provide a prospect for future development of this distinctive hydrogenation strategy using hydrogen-permeable membrane electrodes.

Figure 1

(a) Simplified scheme of tPMR for the hydrogenation of an organic substrate (R) using H₂ as the hydrogen source. (b) Gas-phase hydrogen permeation, where H₂ dissociates at the Pd membrane surface and the resulting hydrogen atoms permeate through the membrane. (c) Simplified scheme of ePMR for the hydrogenation of an organic substrate (R) using H⁺ as the hydrogen source. (d) Electrochemical hydrogen permeation, where H⁺ sourced from water is reduced to hydrogen on one side of a palladium membrane electrode and permeates it to the other side.

Figure 2

Hydrogenation of (a) styrene and (b) para-methylstyrene in neat conditions, (c) 1-(but-3-en-2-yl)-4,5-dimethoxy-2-propoxybenzene in benzene, and (d) 1,3-butadiene in the gas phase using ePMR.

Figure 3

Comparison of phenylacetylene hydrogenation in a conventional electrochemical reactor (a) versus an ePMR (b), together with their corresponding ethylbenzene formation (c and d) at applied currents of 20 and 50 mA for 24 h.

Figure 4

Impacts of catalyst (a), solvent (b), and applied current (c) for hydrogenation using ePMR.

Figure 5

(a and b) Time-dependent product conversion for (a) acetophenone and (b) styrene on different MPd/Pd_m membrane electrodes in ePMR. (c) Product selectivity of furfural hydrogenation to furfuryl alcohol (FA), tetrahydrofurfuryl alcohol (THFA), and any other products after 2 h electrolysis at 150 mA using ePMR.

Figure 6

Schematics of hydrogenation of CO₂ (a), CH₃CN (b), N₂ (c), and O₂ (d) using ePMR under different conditions.

Figure 7

(a) Selective hydrogenation of benzaldehyde catalyzed by Pd nanocubes on Pd/Pd_m leads to different products depending on the active sites (e.g., face sites versus edge sites). (b) Integration of PtPd/Pd_m with enzymes enables the asymmetric hydrogenation of biologically relevant substrates.

Figure 8

(a) Paired electrolysis consists of 1-hexyne hydrogenation at the Pd/Pd_m cathode membrane and alcohol oxidation at the anode. (b) Dual hydrogenation of maleic acid taking advantage of the low-potential oxidation of aldehyde to formate at the Pd/Pd_m anode membrane coupled with water reduction at the Pd/Pd_m cathode membrane.