Bioelectrocatalysis with a palladium membrane reactor

Abstract

Enzyme catalysis is used to generate approximately 50,000 tons of value-added chemical products per year. Nearly a quarter of this production requires a stoichiometric cofactor such as NAD+/NADH. Given that NADH is expensive, it would be beneficial to regenerate it in a way that does not interfere with the enzymatic reaction. Water electrolysis could provide the proton and electron equivalent necessary to electrocatalytically convert NAD+ to NADH. However, this form of electrocatalytic NADH regeneration is challenged by the formation of inactive NAD2 dimers, the use of high overpotentials or mediators, and the long-term electrochemical instability of the enzyme during electrolysis. Here, we show a means of overcoming these challenges by using a bioelectrocatalytic palladium membrane reactor for electrochemical NADH regeneration from NAD+. This achievement is possible because the membrane reactor regenerates NADH through reaction of hydride with NAD+ in a compartment separated from the electrolysis compartment by a hydrogen-permselective Pd membrane. This separation of the enzymatic and electrolytic processes bypasses radical-induced NAD+ degradation and enables the operator to optimize conditions for the enzymatic reaction independent of the water electrolysis. This architecture, which mechanistic studies reveal utilizes hydride sourced from water, provides an opportunity for enzyme catalysis to be driven by clean electricity where the major waste product is oxygen gas.

Electrochemical production of methyltetrahydrofuran, a biofuel for diesel engines

“…This –OH cleavage is characteristic of hydride transfer, which has been previously observed in the membrane reactor. 32…”

Electrocatalytic Hydrogenation Using Palladium Membrane Reactors

“…(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 …”

“…Building on the capability of enzymes to facilitate asymmetric transformations via naturally evolved cofactors, there is growing interest in incorporating biocatalytic hydrogenation within ePMR systems to achieve enantioselective outcomes. Recent research has already shown the feasibility of asymmetric hydrogenation of aldehydes and ketones, as well as the reductive amination to produce chiral amino acids . As depicted in Figure b, the interaction between surface hydrogen and water on Pd m facilitates heterolytic cleavage of the Pd–H bond.…”

Developing electrochemical hydrogenation towards industrial application

“…In addition, to using supported electrocatalysts and foilbased electrodes, Berlinguette and coworkers have driven the development of palladium membrane-based reactors for ECH as advanced specialized types of H-type cells. [36][37][38][39][40][41] Specifically, hydrogen species bound to the cathode during electrolysis in aqueous environments can permeate through a Pd-lattice in Pd-membranes towards a separate compartment where the organic substrate is hydrogenated. This approach does not only take advantage of the well-known ability of Pd to perform hydrogenation reactions, but also its ability to transport hydrogen atoms through the Pd lattice, to effectively separate an aqueous environment providing protons from the organic hydrogenation compartment, which enables the use of organic solvents for the hydrogenation of hydrophobic organic substrates.…”

“…This separation also facilitates the investigation of underlying mechanisms and electrocatalytic screenings via metal sputtering methods, 38,42,43 permits the efficient deuteration of organic compounds 44 as well as the regeneration of enzymatic cofactors. 40 Further groups have adopted this design, with Zhang et al expanding the approach for the conversion of acetonitrile to ammonia, 45 Xu et al applying this reactor design for reductive treatment of halogen-containing contaminants in water, 46 and Sun and coworkers even enabled a highly efficient dual hydrogenation by applying Pd-membranes not only as the cathode but also as the anode for formaldehyde oxidation. 47 Notably, Wijaya et al demonstrated the promising perspective of mechanically stirred H-type slurry reactors to reach higher current densities (up to about 250 mA cm À2 ).…”