An electrooxidation technique effectively changes glycerol, a byproduct from biodiesel creation, into valuable three-carbon compounds.
Researchers at the Tokyo Institute of Technology (Tokyo Tech) have created an electrochemical process that uses sodium borate and a nickel-oxide catalyst, providing a promising method for enhancing the value of glycerol, which is commonly regarded as a low-worth byproduct of biodiesel. By strategically managing the formation of borate-glycerol complexes, this method increases both the selectivity and effectiveness of glycerol’s electrooxidation, transforming it into precious three-carbon compounds like dihydroxyacetone and glyceraldehyde. This improvement contributes to the financial feasibility and ecological advantages of biodiesel production.
Biodiesel serves as an eco-friendly alternative to traditional diesel and is known to decrease carbon dioxide emissions by as much as 74%. It is created through a process called transesterification, which converts triglycerides into biodiesel while yielding glycerol as a low-cost byproduct. Given that glycerol constitutes around 10% of the total output, researchers are keen on enhancing its value. One approach is electrochemical oxidation, a method that converts glycerol into high-value three-carbon compounds, such as dihydroxyacetone (DHA) and glyceraldehyde (GLYD). However, earlier methods have often produced unstable or low-value outputs, particularly under highly alkaline conditions.
A study featured in the Journal of Catalysis on August 15, 2024, led by Associate Professor Tomohiro Hayashi from Tokyo Tech alongside Professor Chia-Ying Chiang from National Taiwan University of Science and Technology, Taiwan, introduces a remarkably selective and efficient glycerol electrooxidation (GEOR) technique that can produce valuable three-carbon (3C) compounds.
“Creating an electrochemical method for a highly selective and effective glycerol electrooxidation process is crucial for biodiesel production,” say Hayashi and Chiang.
However, oxidizing glycerol selectively is a challenge due to its chemical structure. Glycerol features three -OH groups: two on primary carbon atoms and one on a secondary carbon atom. This configuration blocks approaches, making it difficult for reactants to focus on specific -OH groups. In alkaline environments, the -OH groups can also trigger undesired side reactions that break carbon-carbon bonds, leading to the production of two-carbon or one-carbon compounds instead of the intended three-carbon products.
To overcome this issue, the researchers implemented GEOR using sodium borate and a bicarbonate buffer as a gentle alkaline electrolyte alongside a nickel-oxide (NiOx) catalyst. Sodium borate plays a protective role by safeguarding certain -OH groups, thereby improving the reaction’s selectivity, while the NiOx catalyst boosts the efficiency of the electrooxidation process. The sodium borate connects with the primary and secondary alcohol groups in glycerol, aiding in the creation of GLYD and DHA, respectively. However, the final product varies based on the ratio of borate to glycerol. In order to ascertain how these differing concentrations influence the electrooxidation, the researchers explored a constant borate buffer concentration of 0.1 M mixed with varying glycerol concentrations (0.01, 1, 2.0 M), and a fixed 0.1 M glycerol with changing borate buffer concentrations (0.01, 0.05, 0.10, and 0.15 M), while keeping the pH at 9.2.
They found that higher borate levels enhanced the selectivity for 3C products, especially DHA, achieving a peak selectivity of up to 80% at a borate concentration of 0.15 M. This enhancement is linked to the added buffer capacity from the borate solution, which maintains a stable pH during the reaction and stabilizes the borate-glycerol complex for further oxidation into 3C compounds. Conversely, increasing the glycerol concentration lowered both the yield and selectivity of 3C products. At a glycerol concentration of 1 M, the main product was GLYD, with a selectivity of 51%.
The variation in the type of 3C product was associated with the development of different glycerol-borate complexes. Through Raman spectroscopy, the researchers observed that higher borate concentrations favored the formation of six-membered ring complexes, which enhanced secondary -OH oxidation and facilitated DHA production. In contrast, higher glycerol concentrations aligned with five-membered ring complexes that prompted primary -OH oxidation and GLYD formation.
“Five-membered ring complexes were more frequently formed in the electrolyte with a borate-to-glycerol ratio of 0.1, while six-membered ring complexes were more prevalent when the borate-to-glycerol ratio reached 1.5,” state Hayashi and Chiang.
This research presents a promising approach for converting glycerol into valuable products, significantly improving the sustainability and profitability of biodiesel production.
