The preparation by oxidative carbonylation has attracted interest because it requires only C1 precursors: The reaction is catalyzed by Pd2+. The synthesis gas is mostly obtained from coal or biomass. The oxidation proceeds via dinitrogen trioxide, which is formed according to of nitrogen monoxide and oxygen and then reacts according to with methanol forming methyl nitrite: In the next step of dicarbonylation carbon monoxide reacts with methyl nitrite to dimethyl oxalate in the vapor phase at atmospheric pressure and temperatures at 80-120 °C over a palladium catalyst: The sum equation: This method is lossless with respect to methyl nitrite, which acts practically as an carrier of oxidation equivalents. However, the water formed must be removed to prevent hydrolysis of the dimethyl oxalate product. With 1% Pd/α-Al2O3 dimethyl oxalate is produced selectively in a dicarbonylation reaction, under the same conditions with 2% Pd/C dimethyl carbonate is produced by monocarbonylation: Alternatively, the oxidative carbonylation of methanol can be carried out with high yield and selectivity with 1,4-benzoquinone as an oxidant in the system Pd2/PPh3/benzoquinone with mass ratio 1/3/100 at 65 °C and 70 atm CO:
Use
Dimethyl oxalate is used for alkylation and in the cosmetics industry as a chelating agent. For countries with low oil but large coal reserves, the oxidative carbonylation of methanol provides a promising approach to the important C2-basic chemical ethylene glycol. Dimethyl oxalate can be converted into ethylene glycol in high yields by hydrogenation at a copper-containing catalysts: The methanol formed is recycled in the process of oxidative carbonylation; therefore the only raw materials consumed in the overall process are carbon monoxide, hydrogen and oxygen. One plant following that coal-to-MEG process with a production capacity of 200,000 tons ethylene glycol per year is already operational in Inner Mongolia, a second plant with a capacity of 250,000 with tons/year was scheduled for 2012 in Henan. Other plants with a total annual capacity of more than 1 million tons of ethylene glycol per year are planned. Furthermore, dimethyl carbonate is accessible by decarbonylation from dimethyl oxalate at temperatures around 100 °C in the presence of alkali metal alcoholates, which is discussed as a fuel additive from biomass : The formed carbon monoxide can be feed back to the reaction forming dimethyl oxalate. Diphenyl oxalate is obtained by transesterification of dimethyl oxalate with phenol in the presence of titanium catalysts, which is again decarbonylated to diphenyl carbonate in the liquid or gas phase. Diphenyl carbonate can be used as a replacement for the highly toxic phosgene for the production of polycarbonates.
