Glycerol Electrocatalytic Reduction Using an Activated Carbon Composite Electrode: Understanding the Reaction Mechanisms and an Optimization Study

Abstract

The conversion of biomass-derived glycerol into valuable products is an alternative strategy for alleviating energy scarcity and environmental issues. The authors recently uncovered an activated carbon composite electrode with an Amberlyst-15 mediator able to generate 1,2-propanediol, diethylene glycol, and acetol via a glycerol electrocatalytic reduction. However, less attention to mechanistic insights makes its application to industrial processes challenging. Herein, two proposed intermediates, acetol and ethylene glycol, were employed as the feedstocks to fill the gap in the mechanistic understanding of the reactions. The results discovered the importance of acetol in producing 1,2-propanediol and concluded the glycerol electrocatalytic reduction process has a two-step reduction pathway, where glycerol was initially reduced to acetol and consecutively hydrogenated to 1,2-propanediol. At 353 K and 0.28 A/cm2, 1,2-propanediol selectivity achieved 77% (with 59.8 C mol% yield) after 7 h of acetol (3.0 mol/L) electrolysis. Finally, the influences of the temperature, glycerol initial concentration, and current density on the glycerol electrocatalytic reduction were evaluated. The initial step involved the C-O and C-C bonds cleavage in glycerol plays a crucial role in producing either acetol or ethylene glycol intermediate. This was controlled by the temperature, which low to moderate value is needed to maintain a selective acetol-1,2-propanediol route. Additionally, medium glycerol initial concentration reduced the hydrogen formation and indirectly improved 1,2-propanediol yield. A mild current density raised the conversion rate and minimized the growth of intermediates. At 353 K and 0.21 A/cm2, glycerol (3.0 mol/L) electrocatalytic reduction to 1,2-propanediol reached the maximum yield of 42.3 C mol%.

Electrochemical Valorization of Glycerol via Electrocatalytic Reduction into Biofuels: A Review

“…At the onset of their studies, [21] an interesting observation gathered is that acrolein and 1,3‐PDO were only produced at current densities between 0.14 and 0.24 A/cm 2 , which shows that dehydration of the adsorbed glycerol molecule at the second hydroxyl group is not favored at values below 0.08 A/cm 2 and over 0.27 A/cm 2 . This trend is also in line with the study by Rahim et al [63] . Moreover, a high current can speed up the electrolysis process by enabling the conversion of glycerol to 1,2‐ and 1,3‐PDO.…”

“…For instance, based on works by Lee et al., [62] within the operating temperature range of 25–80 °C, over a Pt catalyst surface, the conversion rate increased gradually from 0.406‐0.774 h −1 , respectively, indicating the glycerol electrochemical conversion is thermally activated. Rahim et al [63] . have evaluated the impact of high process temperature (above 100 °C), which hindered the glycerol conversion due to several factors of vaporization of by‐products with a lower boiling point, and quicker reduction of H + ions into H 2 (see Figure 4).…”

“… Glycerol conversion via ECR and its first‐order kinetic models at different processing temperature – (Δ) 25, (○) 50, 80, and 100 °C. Reproduced from ref [63] . Copyright (2022), with permission from Frontiers.…”

“…In galvanostatic electrolysis, Rahim et al [63] . as well as Hunsom and Saila [21] have discovered that glycerol conversion increased along with the increasing current density applied, which obeyed Faraday's second law of electrolysis [67] .…”

Combined dehydrogenation of glycerol with catalytic transfer hydrogenation of H2 acceptors to chemicals: Opportunities and challenges

“…In general, as a chemical building block, the glycerol can be converted to a series of value-added chemicals, such as lactic acid (LA), propanediol (PDO), ethylene glycol (EG), glyceric acid, dihydroxyacetone, glycolic acid, and tartronic acid. ( Zhang et al, 2019 ; Meng et al, 2020 ; Liu et al, 2021 ; Sun et al, 2021 ; Yan et al, 2021 ; Zhang et al, 2021 ; Md Rahim et al, 2022 ). These products are widely used in food, medicine, organic synthesis, chemical industry, and other fields.…”