The research group headed by Prof. Deng Youquan of the R&D Center for Green Chemistry and Catalysis of the Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, (LICP, CAS) has achieved the heterogeneous catalyzed non-phosgene synthesis of N-substituted carbamates, an important isocyanate precursor.
Researchers have developed a nickel-promoted magnetic iron oxide catalyst for the effective syntheses of N-substituted carbamates. The catalyst could be easily isolated using external magnetic field and recovered for several runs without deactivation. In general, good to excellent yields were obtained with various amines and alkyl carbamates. Catalyst characterization results suggested the catalytic activity may be derived from the delicate synergy between Ni and Fe species resulted in specific basic sites. Reaction pathway investigations revealed that the N-substituted carbamates were formed via substitute urea intermediate and the catalyst mainly promoted the further alcoholysis of the urea intermediate. The technology provided a cheap, safe and environmental-benign non-phosgene route for the N-substituted carbamates synthesis.
Isocyanates are major raw materials for the manufacture of polyurethane, which have been widely used in producing elastomer, elastic fiber, coatings, and so on. They mainly include hexamethylene diisocyanate (HDI), tolylene diisocyanate (TDI), diphenyl-methane-diisocyanate (MDI) and isophorone diisocyanate (IPDI). Currently, isocyanates are produced on a commercial scale using extremely toxic phosgene as the carbonyl source, which leads to serious equipment corrosion, phosgene leakage, environmental pollution and even personnel death. Thus, there have been increasing interests in developing green alternative methods for isocyanate production. The existing non-phosgene approaches mainly involve reductive carbonylation of nitro-compound, oxidative carbonylation of amine compounds and aminolysis reaction of dimethyl carbonate. These methods avoid using phosgene, however, they have many problems, such as the toxicity of CO, the explosion of CO and O2 under high temperature, high cost of dimethyl carbonate and difficult separation from the methanol. Theoretically, CO2 should be the ideal carbonyl source in carbonylation reactions. Unfortunately, the high chemical inertness of CO2 limits its practical use in carbonylation processes.
In recent years, alkyl carbamates, e.g., methyl carbamate (MC), ethyl carbamate (EC), and butyl carbamate (BC), which can be produced with urea alcoholysis, are promising green carbonyl source with lower cost. Also, the co-produced NH3 could be recycled for urea production. Therefore, the use of CO2 as carbonyl source could be realized. In this way, alkyl carbamates could be developed into a green and economical carbonyl source besides phosgene, carbon monoxide, and DMC. Possibly, it might be a new carbonyl source in the future.
This work has been financially supported by the National Natural Science Foundation of China. The detailed report has been published in Journal of Catalysis (Journal of Catalysis, 2011, 279, 328-336).