Ex situ electro-organic synthesis – A method for unrestricted reaction control and new options for paired electrolysis

Classic in situ electro-organic synthesis with substrates in an electrolyzer must compromise process conditions to balance electro- and thermochemical steps at both electrodes. This often restricts efficiency and product selectivity, since requirements may deviate for electrochemical- (catalyst activation) and chemical (organic synthesis) steps, as well as for paired anode- and cathode reactions. Breaking this barrier, we report ex situ electro-organic synthesis as a versatile method that enables unique product selectivity and unusual product pairs. We exemplify the concept for pairing H2 evolution (HER) with anodic alcohol oxidation. The two-step method accomplishes this by separating cathode reactions from organic substrate oxidation, and anodic electrocatalyst activation from chemical conversion of organic substrates in time and space. First, the electro-oxidation of Ni(OH)2 anodes to NiOOH is paired with H2 production by alkaline water electrolysis. Then, “charged” NiOOH electrodes are removed from the electrolyzer and used in external vessels to oxidize model substrate benzyl alcohol under regeneration of Ni(OH)2. Free choice of reaction media outside the electrolyzer allows to selectively obtain benzoic acid (in water) or benzaldehyde (in n-hexane), whereas classic in situ electrosynthesis only produces the acid together with H2. Perspectively, the method enables electrosynthesis of previously inaccessible products paired to H2 generation.