Clean Synthesis of N,N’-diphenylurea from Urea and Aniline
|Keywords||Urea Aniline N,N’-diphenylurea Clean process Process intensification|
N, N’-diphenylurea is an important organic intermediate, which mainly used for the synthesis of sulfa drugs, isocyanate and other chemicals. The traditional method for the preparation of N, N’-diphenylurea utilizes phosgene as a raw material directly or indirectly, which is toxic, corrosive, expensive, and restricted to employ. The industrial synthesis of N, N’-diphenylurea from urea and aniline uses water as solvent and hydrochloric acid as catalyst, giving only 40% yield of N, N’-diphenylurea, and dischargeing aniline waste water and hydrogen chloride waste gas, which are both difficult to be handled. As urea is a cheap material and an important industrial chemical synthesized from CO2, there is high economic and environmental potential in N, N’-diphenylurea synthesis from urea. But how to maximize the utilizing efficiency of raw materials and minimize the quantity of waste need further study.In this dissertation, a process of N, N’-diphenylurea catalytic synthesis from urea and aniline in organic solvents, using neither water nor hydrochloric acid, was investigated. Based on this , a clean process of N, N’-diphenylurea synthesis from urea and aniline without any catalysts and solvents was developed, the reaction mechanism was also mentioned and proved. The main contents studied and results obtained shows as follow:(1) The synthesis of N, N’-diphenylurea from urea and aniline catalyzed by various catalysts in a variety of solvents was investigated. It is found that xylene is the optimal solvent, zinc acetate and p-toluenesulfonic acid - lead acetate catalysts are the optimal catalysts. The yield of N, N’-diphenylurea reached 94.0% and 92.9% under the optimum reaction condition of temperature 144℃, aniline/urea/xylene ratio 3:1:2 (molar ratio), reaction time 1.5 h, catalyzed by zinc acetate and p-toluenesulfonic acid - lead acetate, respectively.(2) The clean process of N, N’-diphenylurea synthesis from urea and aniline without any solvents and catalysts was investigated, the various reaction conditions on the yield of N, N’-diphenylurea were also studied. The results of FT-IR, MS, NMR and UV analysis showed that the product of the developed process was N, N’-diphenylurea with high purity, the yield of N, N’-diphenylurea reached 92.1% under the optimum reaction condition of temperature 180℃, aniline/urea ratio 5:1 (molar ratio), reaction time 1.5 h.(3) Investigation showed that the yield of N, N’-diphenylurea could be further improved by fast reaction and ammonia removal through inputing N2 into reaction liquids or reducing pressure of reaction system. The coupling of reaction and separation can be easily achieved by vacuum filtration, resulting in the promotion of product yield. The yield of N, N’-diphenylurea and material utilizing ratio could be increased by recycle of the reactants (aniline), under the lower temperature. The yield of N, N’-diphenylurea reached 99.1% under the optimum reaction condition of temperature 180℃, aniline/urea ratio 5:1 (molar ratio), reaction time 1.5 h, product separating by vacuum evaporation.(4) The reaction mechanism was discussed by means of HPLC analysis and proving experiment, it has been ascertained that isocyanic acid and phenylurea are the major intermediates in the reaction. Based on the analysis of experimental resultes, the reaction might carry through the following pathway: phenylurea is obtained with aniline reacting with isocyanic acid from urea pyrolysis, and then reacts with aniline to produce N, N’-diphenylurea.(5) A clean production process for synthesizing N, N’-diphenylurea from urea and aniline was advanced at last, which exhibiting the following advantages: free fom catalyst and reaction solvent, high-yield of product, and pollution control from headstream by recycling of the reactants and resource utilizing of ammonia, which is used to manufacture urea.