In a flask, 68.22 g of imidazole was suspended in 505 g of xylene. The mixture was heated to reflux and 5 g of the xylene/water azeotropic mixture was removed by distillation to achieve dehydration. Subsequently, the temperature of the reaction system was lowered to 66°C and 25.2 grams of phosgene was added dropwise at a rate of 50.4 grams/hour over 30 minutes. After about 15 minutes, the viscosity of the reaction mixture increased significantly, exhibiting a chewing gum-like consistency. During the phosgene dropwise addition, the by-product imidazole hydrochloride precipitated in the form of yellow globules. After maintaining this temperature and continuing stirring for 1 hour, the reaction temperature was raised to 130 °C, at which time the imidazole hydrochloride transformed into a brown molten state. The melt was discharged from the reaction system at 130°C and solidified to a dark green solid upon cooling. The supernatant xylene phase was cooled to 0 °C, the precipitated crystals were filtered and dried at 20 mbar and 50 °C. A white crystalline N,N'-carbonyl diimidazole with a small amount of black impurities (Hazen color number: 489) was finally obtained with a purity of 96.8%, corresponding to 70% of the theoretical yield.
