(636e) Oxidation of Glycerol on Diamond Coated Electrodes

Glycerol oxidation, subject of this ongoing project, has been widely investigated in the past. Several electrode materials, electrolyte compositions, pH-regions and temperature fields have been investigated by different research groups. To briefly sum up the results, the expected optimum set up for highly selective synthesis of a specific product has still not been found.

Method:

The electrochemical oxidation was carried out in batch operation in a thin layer pump cell. Anode and cathode compartment were separated by an anion exchange membrane. A diamond coated electrode was used as anode. This consists of a boron doped polycrystalline diamond layer on a niobium carrier. Diamond coated electrodes have high chemical, mechanical, and thermal stability. The main advantage of diamond coated electrodes for anodic synthesis reactions is their very high oxygen overvoltage. A platinum grid was used as cathode because of its very low overvoltage for hydrogen, the only cathode product. All experiments were carried out with aqueous anolyte mixtures of glycerol. The electrocatalytic activity and the conductivity of the electrolyte were investigated with several additives at selected pH values, such as Na₂SO₄ (moderate acidic), K₂HPO₄ (neutral) and Na₂CO₃ (moderate alkaline). NaOH was used as catholyte. Thus charge transfer through the membrane was based on hydroxide ion transfer. During process optimization the current density, temperature and electrolyte concentration was varied.

Results:

At the beginning of the electrolysis the desired products (glyceraldehyde and dihydroxyacetone) are formed with good selectivity and current yield. With ongoing process the fraction of cleavage products (glycolaldehyde and formic acid) increases. On a limited scale the oxidation of glyceraldehyde to glyceric acid and the oxidation of glycolaldehyde to glycolic acid take place.

The reaction kinetics of glycerol oxidation was determined. At the beginning of the process the degradation of glycerol follows zero-order rate law (electron transfer controlled reaction), with increasing electrolysis time a change to a first order rate of degradation can be observed (mass transport controlled reaction). Formation of gaseous decomposition products (oxygen or carbon dioxide) is negligible.