The reaction between primary amines and phosgene (carbonyl chloride) to synthesize isocyanates is the most widely used method for producing isocyanates. Currently, large-scale isocyanate production facilities predominantly use the phosgenation process.
Phosgene Production
Phosgene is produced by reacting carbon monoxide with chlorine gas after purification and drying. The gases are mixed at a molar ratio of CO:Cl₂ = 1.1:1 and fed into a tubular reactor. Using activated carbon as the catalyst, the reaction occurs at approximately 100°C, achieving a conversion rate of 98–100%. The reaction heat is dissipated through circulating hot water or oil.
Initially, the direct high-temperature phosgenation method was employed to synthesize isocyanates. However, due to numerous side reactions and low yields, this method has been discontinued. It was replaced by a two-step phosgenation process involving cold phosgenation followed by hot phosgenation.
Cold Phosgenation Reaction
The cold phosgenation reaction occurs within a temperature range of 0–70°C, primarily involving the following reactions:
·Primary reactions:
R-NH2+COCl2→R-NHCOCl+HCl
R-NH2+HC→R-NH2·HCl Secondary reactions:
·Secondary reactions:
R-NH2+R–NHCOCl→R-NH2·HCl+R-NCO
R-NH2+R-NCO→R-NHCONH-R
Hot Phosgenation Reaction
In the hot phosgenation step, the reaction mixture from cold phosgenationis further reacted with phosgene within a temperature range of 80-200°Cinvolving these key reactions:
·Primary reactions:
R-NHCOCl→R-NCO-HCl
R-NH2·HCl+COCl2→R-NCO+3HCl
·Secondary reactions:
R-NH2·HCl+R-NCO→R-NHCONH-R+HCl
R-NHCONH-R+COCl2→2R-NCO+2HCl+tarrybyproducts
Challenges and Solutions
During phosgenation, side reactions produce urea derivatives, which consume additional phosgene and form tar-like waste, thereby reducing isocyanate yields. To mitigate this, primary amines are often converted into their hydrochloride or carbonate salts before cold phosgenation, especially for aliphatic amines.
Industrial Production of Aromatic Isocyanates
Although industrial methods for producing aromatic isocyanates vary by company and equipment, the general process includes the following steps:
1.Dissolve the amine compound in inert aromatic solvents, such as monochlorobenzene, dichlorobenzene, or toluene. Phosgene is introduced either as a solution in the same solvent or in gaseous form, reacting with the amine in a cold phosgenation reactor at temperatures below 70°C.
2.The slurry from the cold phosgenation reactor is transferred to one to three hot phosgenation reactors in series. The reaction temperature is gradually increased to 100–200°C, depending on the solvent’s boiling point, with additional phosgene introduced for several hours.
3.The reaction mixture from the final hot phosgenation reactor undergoes inert gas (e.g., nitrogen or methane) purging for several hours to remove byproduct HCl and unreacted phosgene. The mixture is then distilled to separate the solvent, isocyanate product, and residue. The yield of isocyanates typically ranges from 85–95%.
Types of Phosgenation Processes
Five main phosgenation production methods are used:
1.Batch reactor process
2.Continuous stirred-tank reactor process
3.High-temperature one-step process
4.Pressurized process
5.Continuous tower reactor process
