In the early 1980s, Italian EniChem Company realized the industrialization of oxidative carbonylation of methanol to DMC with CuCl as catalyst, which was the first industrialized non-phosgene synthesis process and the most widely used process. The disadvantage of this process is that the catalyst deactivation is serious at high conversion rate, so its one-way conversion rate is only 20%.
In 1990s, the research of DMC synthesis technology developed rapidly: Ube in Japan improved the oxidative carbonylation process of methanol of EniChem Company, and chose NO as the catalyst, thus avoiding the deactivation of the catalyst, making the conversion rate almost reach 100%, and it has been industrialized; Texaco Company of the United States has developed a process in which ethylene oxide reacts with carbon dioxide to produce ethylene carbonate, and then transesterification with methanol to produce DMC. Ethylene glycol was co-produced by this process, and 1992 was industrialized. This method is considered to have low yield and high production cost. The annual output of DMC must be higher than 55 kt, and its investment and cost can compete with other methods. In addition, there is a new process, namely methanolysis of urea, which can reduce the cost if combined with urea production, and this process is expected to be commercialized [7~ 1 1].
Table 3 gives the composition of the operating procedures of the above four processes to compare their complexity. Table 3 DMC process complexity comparison process methanol oxidative carbonylation transesterification
(ethylene carbonate) urea methanol decomposition of copper chloride and carbon monoxide detailed steps feed gas compression feed gas compression methanol evaporation urea dissolution oxidative carbonylation reaction oxidative carbonylation reaction transesterification reaction methanol decomposition reaction gas recovery * * * boiling americium * * removing methyl nitrite from boiling distillate with carbon dioxide to generate methanol, recovering ammonia gas, recovering catalyst, treating methanol, recovering methanol, recovering DMC, recovering ethylene carbonate, recovering DMC, recovering ethylene glycol. The key processes of research and development are still oxidative carbonylation and transesterification. Typical oxidative carbonylation methods include ENI liquid phase method, Dow gas phase method and UBE atmospheric pressure gas phase method, and the usual transesterification process is to obtain DMC by transesterification of ethylene carbonate or propylene carbonate with methanol. It is reported that Shell has developed new processes for producing dimethyl carbonate from propylene oxide and dimethyl carbonate, which can obviously reduce investment and operating costs. Compared with the oxidative carbonylation process, the production cost per ton of carbonate is reduced by 300 dollars. The process uses greenhouse gas carbon dioxide, which is an environment-friendly process and can reduce the carbide emission of 10%. China has also invested a lot of energy in the research of transesterification process, but most of them are concentrated in the laboratory and pilot stage. Only by further simplifying the process and optimizing the catalyst can industrialization be realized [12~ 17].