Oxidation of Woody Biomass Dissolved in Ionic Liquids By a Platina-Loaded Titanium Dioxide Electrode
Authors: Yamaki Eri , Ikeda Kazuma , Fujita Kyoko , Ohno Hiroyuki , Nakamura Nobuhumi 
Published: February 2020

Woody biomass is uneatable and plentiful material which is expected to be a new energy resource instead of fossil fuel. Combustion power generation systems which use gasified biomass or intact biomass are generally operated, but their energy conversion efficiency is low because of heat loss. Recently extraction of electricity as electrochemical oxidation is drawing an attention, which has one simple merit that its energy conversion efficiency is high. However, woody biomass has complex structure and that is not soluble in conventional solvents at room temperature. If there are solvents which dissolve woody biomass, we can expect its effective oxidation and electrochemical oxidation.
We focused on the new solvents, ionic liquids. Recently it is reported that some ionic liquids can dissolve cellulose or lignin on comparatively mild conditions [1]. We have also reported some ionic liquids that can dissolve and extract cellulose or lignin at room temperature. Especially tetra-n-butylphosphonium hydroxide (TBPH) containing water can dissolve woody biomass itself [2].
It is known that a platina-loaded titanium dioxide (Pt-TiO2) electrode can catalyse the oxidation of glucose in alkali aqueous solutions [3]. Therefore the effective energy conversion of cellulose is expected by using the Pt-TiO2 electrode in ionic liquids. In this study, we have been investigated the direct electrochemical oxidation of compounds included in the biomass and biomass itself in ionic liquids using the Pt-TiO2 electrode.
First, we focused on the cellulose which is one of the main components in the woody biomass. Four ionic liquids, 1-ethyl-3-methylimidazolium methylphosphonate ([C2mim][(MeO)(H)(PO2)]), 1-butyl-3-methylimidazolium acetate ([C4mim][C3COO]), 1-butyl-1-methylpyrrolidinium hydroxide containing 60 wt% water ([C4mpyrr]OH), and TBPH containing 40 wt% water were chosen to be used as solvents for cellulose. [C2mim][(MeO)(H)(PO2)] and [C4mim][C3COO] were used in the non- hydrated state while the other ionic liquids were used in hydrated state. All of these can dissolve the cellulose at room temperature. 1 wt% cellulose was added to each ionic liquid and the solubility was visually confirmed. The Pt-TiO2 electrode was prepared based on the previous report [3]. Electrochemical measurements were conducted in each solution. Oxidation current was measured by cyclic voltammetry (WE: Pt-TiO2 electrode, CE: Pt wire, RE: Ag/AgCl/NaCl (3M)). The oxidation current was observed only in the presence of the cellulose dissolved in TBPH. It indicates that cellulose was oxidised by the Pt-TiO2 electrode in TBPH. TBPH was selected as a solvent for the following experiments.
High performance ionic liquid chromatography (HPILC), which was developed by our laboratory, is one of the liquid chromatography using ionic liquid as a developing solvent. The influence of cyclic voltammmetry measurement on the molecular weight of the cellulose was analyzed using HPILC. As results, the sharp peak of the cellulose having high molecular weight reduced and the broad peak of the cellulose having low molecular appeared (Fig. 1). This suggests that high molecular weight cellulose was decomposed by the cyclic voltammetry measurement.
Xylan and lignin were also main components of the woody biomass. The similar investigations were conducted to that of cellulose, next. The oxidation current was observed only in the xylan dissolved in TBPH but not in the lignin dissolved in TBPH. The results indicate that xylan can be electrochemically oxidized in TBPH but lignin cannot be electrochemically oxidized.
Finally, we tried to electrochemically oxidize the Japanese cedar powder as a model of real biomass. The oxidation current was detected using the TBPH solution of the Japanese cedar powder. The cellulose and the xylan contained in the Japanese cedar powder could be oxidized.

Figure 1