Non-catalytic selective oxidation of glycerol under mild condition by low-temperature atmospheric pressure plasma irradiation

Abstract: Polyol can be converted to value added chemicals such as pharmaceutical raw material by selective oxidation (e.g. glycerol conversion into glyceric acid (GA)), which is oxygen addition to a specific position using precious metal catalyst. However, selectivity control of this oxidation is very difficult so that many side reactions proceed simultaneously. In our previous study, we clarified that glycerol oxidation selectively proceeds by the reaction with OOH species. When the production of OOH species is promoted, the selective oxidation may proceed even without catalysts. In this study, we focused on the production of reactive oxygen species by an irradiation of low-temperature atmospheric pressure plasma, and oxidation experiments of glycerol under the plasma irradiation were carried out to explore the possibility of non-catalytic selective oxidation of glycerol.

In oxidation experiments, the plasma, which was generated from helium, was irradiated to glycerol aqueous solutions at 20ºC and atmospheric pressure. As an operating factor, pH of reaction solution was varied.

Under alkaline condition, the oxidation proceeded by plasma irradiation at low power of 20 W and atmospheric pressure for 5 min. Glycerol conversion reached around 30%, which is comparable to that for 30 min using Au catalyst at 60ºC, 9.3 atm in our previous study. The selectivity of target GA was around 20%, indicating that plasma irradiation can proceed the selective oxidation of glycerol into GA under mild conditions. On the other hand, under neutral condition where oxidation did not proceed using Au catalyst, the oxidation also proceeded and main productions were not GA but glyceraldehyde and dihydroxyacetone, which are productions using Pt catalyst. These results suggest that the plasma irradiation is a potentially powerful tool for selective oxidation of glycerol instead of the conventional catalytic conversion.