Breaking the selectivity barrier in glycerol electrooxidation to glyceraldehyde via redox mediation

doi:10.1016/S1872-2067(25)64816-9

Abstract

Abstract:

Aldehydes are valuable intermediates with widespread industrial applications, and their traditional synthesis relies on chemical oxidation that is often hazardous and environmentally unfriendly. Electrochemical oxidation offers a more sustainable and milder alternative; however, it faces challenges such as aldehyde overoxidation and susceptibility to base-catalyzed Cannizzaro disproportionation. Electrochemical glycerol oxidation to glyceraldehyde is a representative example, which typically requires precious metal-based electrocatalysts but still suffers from low selectivity and activity. Here, we report a metal-free oxidation strategy mediated by 2,2,6,6-tetramethylpiperidine-1-oxyl. By systematically investigating the redox thermodynamics and kinetics of TEMPO across a broad pH range, we construct a Pourbaix diagram and elucidate the relative kinetics of each reaction step. These insights allow us to explain the anomalously high apparent Faradaic efficiency (~200%) observed under acidic conditions, and identify neutral media as the optimal environment for selective glyceraldehyde production. Under optimized conditions, our system achieves a glyceraldehyde Faradaic efficiency exceeding 93% and a partial current density of 23.3 mA cm^-2 at 0.57 V — more than doubling the performance of the best reported precious metal-based systems. Furthermore, the versatility of this strategy extends to the selective oxidation of other primary alcohols to their corresponding aldehydes with near-unity selectivity.

Key words: Electrochemical glycerol oxidation, Glyceraldehyde, Redox mediator, Selectivity, Pourbaix diagram

Zhenghao Mao, Wenjing Xu, Na Han, Yanguang Li. Breaking the selectivity barrier in glycerol electrooxidation to glyceraldehyde via redox mediation[J]. Chinese Journal of Catalysis, 2025, 78: 336-342.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(25)64816-9

https://www.cjcatal.com/EN/Y2025/V78/I11/336

Figures/Tables 4

Fig. 1. Thermodynamic and kinetic properties of the TEMPO redox system. (a) Cyclic voltammograms of 10 mmol L-1 TEMPO in 0.5 mol L-1 buffered aqueous solutions across a pH range of 3.75-11.00, recorded using hydrophilic carbon paper electrodes. (b) Pourbaix diagram constructed from midpoint potentials (E1/2) of the three redox couples. (c) Anodic-to-cathodic peak separations (ΔEp) as a function of scan rate for the TEMPO+/ TEMPO, TEMPO/TEMPOH, and TEMPO/TEMPOH2+ redox couples. (d) Schematic representation of the redox couples involved in the TEMPO-mediated oxidation process: TEMPO+/ TEMPO, TEMPO/TEMPOH, and TEMPO/TEMPOH2+.

Fig. 2. TEMPO-mediated glycerol oxidation reaction at various pH values. (a) Schematic comparison of direct GOR and TEMPO-mediated GOR pathways. (b) Cyclic voltammograms of direct GOR and TEMPO-mediated GOR recorded in 0.5 mol L-1 buffered solutions containing 1 mol L-1 glycerol across a pH range of 4.25-11.00. (c) Chronopotentiometry tests conducted at 0.44 V vs. SCE across various pH values. (d) pH-dependent FEs for TEMPO-mediated GOR at various pH values.

Fig. 3. TEMPO-mediated glycerol oxidation to glyceraldehyde under neutral conditions. (a) Chronopotentiometry curves recorded at various applied potentials (0.38-0.57 V vs. SCE) under neutral conditions (pH = 7.00). (b) FEs for GLAD as a function of potential in TEMPO-mediated GOR. (c) Comparison of GLAD FEs and partial current densities achieved by TEMPO-mediated GOR with previously reported direct and indirect GOR systems. (d) Long-term chronoamperometry performance and corresponding GLAD concentration at 0.44 V.

Fig. 4. Electrocatalytic TEMPO-mediated oxidation of representative alcohols. Potential-dependent FEs for the formation of benzaldehyde (a) and furfural (b) from benzyl alcohol and furfural from furfuryl alcohol via TEMPO-mediated oxidation.

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