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要旨:
The oxidation of methane directly to valuable products as an alternative anode reaction to the oxygen evolution reaction is a “dream reaction”. If successful, two major challenges could be addressed by this attempt. On the one hand, a value generating oxidation reaction is introduced as a counter reaction to hydrogen evolution. On the other hand, the never solved challenge of direct, selective oxidation of methane could be achieved. In this thesis, significant contributions to this attempt are presented. A flexible, stable reactor system was developed, wherein electrochemical reactions can be performed under demanding conditions, such as a high pressure, high temperature and a highly corrosive and oxidative environment.
After identifying stable electrode materials, the electro-oxidation of methane to the methanol derivative methyl bisulfate was investigated. The concept to synthesize a derivative is in other words the introduction of a protection group to methanol, which was for the first time successfully done by Periana and coworkers in the past. Product protection is necessary in this case to avoid major further oxidation of the product.
The synthesis of methyl bisulfate in the reactor system was developed without the use of a catalyst using a boron-doped diamond anode. As a second product methanesulfonic acid – also a valuable chemical – was found. The system was somewhat limited in terms of product concentration and selectivity towards methyl bisulfate. After increasing reaction time and/or cell voltage, the formation of carbon dioxide as a result of product oxidation was observed. An alternative methanol derivative would be methyl trifluoroacetate. The electrochemical synthesis of this compound was additionally investigated exploratorily. Since methanesulfonic acid was already obtained by chance in the process that aimed to produce methyl bisulfate, its formation was further studied and optimized. It was found that on a boron-doped diamond anode in fuming sulfuric acid methane could be directly sulfonated without the addition of an initiator molecule. The product concentrations achieved by this method were in the molar range with almost full selectivity. Mechanistic investigations have shown that this was possible due to the formation of a persulfate or persulfate precursor species that is able to initiate the reaction, from which a catalytic cycle is started. As a last reaction connected to the previously studied systems, it was observed that methyl bisulfate can be formed from methanesulfonic acid – in the absence of a catalyst/initiator or a cell voltage – in fuming sulfuric acid with high sulfur trioxide content. An initial study of this reaction was successfully carried out, showing that this proceeds fast under elevated temperature and sulfur trioxide concentration.
