Base-Assisted Nitrate Mediation as the Mechanism of Electrochemical Benzyl Alcohol Oxidation

Abstract

Nitrate anion (NO3 –) oxidation to nitrate radical (NO3 •) is chemically irreversible in acetonitrile (MeCN) solvent due to solvent-based hydrogen-atom transfer (HAT). Introducing benzyl alcohol (PhCH2OH) leads to competition with MeCN for electrochemically generated NO3 • and affords benzaldehyde (PhCHO) product with ∼80% faradaic efficiency (FE) in 250 mM PhCH2OH. Stoichiometric HNO3 forms during HAT reactions (observed by UV–vis spectroscopy) and exists as an electrochemically inert and weak electrolyte; this off-cycle form of nitrate can be reintroduced to the catalytic cycle upon deprotonation by 2,6-lutidine while maintaining the base-free FE. Oxygen reduction complements nitrate oxidation during catalysis and reduced oxygen species (ROS) generated during proton-coupled oxygen reduction are identified through rotating ring-disk electrochemistry; proton-coupled oxygen reduction indicates ROS are capable of rendering NO3 – catalytic when collocal. Directly observing ROS as the stoichiometric base generated during nitrate anion oxidation resolves differences in photocatalytic vs photoelectrochemical reactivity of NO3 – in base-free conditions and points toward HAT as the general mode of reactivity for nitrate radical in acetonitrile solutions.

Oxidizing Ethanol and 2-Propanol by Hypochlorous Acid Generated from Chloride Ions on HxWO3 Photoelectrodes

“…As a result, replacing oxygen evolution with faster anodic reactions that provide valuable products has been investigated . Among them, oxidizing alcohols to aldehydes, ketones, and carboxylic acids is of high importance in producing pharmaceuticals and fine chemicals, especially for the likes of ethanol, which can be derived from biomass feedstock. − While alcohol oxidation is strategically favored, direct alcohol oxidation on photoelectrodes usually requires large applied bias, and the reactant and/or intermediates are prone to adsorb to the electrode . On the other hand, indirect oxidation by introducing redox mediators circumvents these problems.…”

“…5−9 While alcohol oxidation is strategically favored, direct alcohol oxidation on photoelectrodes usually requires large applied bias, and the reactant and/or intermediates are prone to adsorb to the electrode. 10 On the other hand, indirect oxidation by introducing redox mediators circumvents these problems. The mediator is rapidly oxidized at the anode, which subsequently oxidizes the alcohol in solution.…”

Electrochemical Generation of Peroxy Radicals and Subsequent Peroxidation of 1,3-Dicarbonyls in an Undivided Cell

“…The t BuOO radical can also eliminate O 2 to give the t BuO radical . Hydrogen atom abstraction from 1a with the t BuO radical or NO 3 radical leads to the C-centered radical A , which recombines with the t BuOO radical to form product 2a . Based on the observation of benzaldehydes as byproducts, control experiments (Scheme d), and CV study (Figure , f), product 2a can likely undergo oxidative degradation in the presence of peroxy and alkoxy radicals from TBHP.…”

Selective Chloride-Mediated Neat Ethanol Oxidation to 1,1-Diethoxyethane via an Electrochemically Generated Ethyl Hypochlorite Intermediate

“…Among all the compounds obtainable from biomass, bioethanol stands out, and the oxidative conversion of bioethanol has been widely investigated. , In particular, acetaldehyde formation from bioethanol has attracted significant attention due to acetaldehyde’s versality in preparing long-chain organic compounds . Industrially, acetaldehyde is produced at 10 6 tons/year through the Wacker process starting from petroleum based ethylene. , While converting ethanol to acetaldehyde remains a highly sought industry target, preventing overoxidation to acetic acid remains a challenge, and selective ethanol oxidation typically requires either high-temperature gas phase conditions, which are energy- and equipment intensive, or costly noble metal catalysts. − Consequently, electrocatalysis is a promising alternative, where inert reagents can be activated serving as in situ oxidants under ambient conditions. , …”

“…10−12 Consequently, electrocatalysis is a promising alternative, where inert reagents can be activated serving as in situ oxidants under ambient conditions. 13,14 To reduce the need for costly noble metal catalysts while maintaining the selectivity, mechanistic differences must be designed to inhibit overoxidation, particularly under electrochemical (bulk electrolysis) conditions. 1,1-Diethoxyethane (DEE) is a highly desirable oxygenated fuel additive (derived from ethanol and acetaldehyde) for corrosion prevention in combustion engines.…”