Study On The Clean Synthesis Of Methyl N-Phenyl Carbamate And The Coupling With Its Condensation

Methylene diphenyl diisocyanate (MDI) is an important raw material for the production of polyurethane. The synthesis of MDI from aniline and dimethyl carbonate (DMC) belongs to the non-phosgene route, and is composed of synthesis of methyl N-phenyl carbamate (MPC), condensation of MPC with formaldehyde to methylene diphenyl carbamate (MDC) and decomposition of MDC to MDI. However, this method has some drawbacks that to be deal with, such as the usage of homogeneous catalyst and the multistep process to obtain MDI, etc. In this paper, the novel efficient heterogeneous catalyst for MPC synthesis and the coupling of MPC synthesis with its condensation, that is to say, the one-pot synthesis of MDC from aniline and DMC, were studied systematically.The qualitative analysis of the reaction system of aniline and DMC was carried out by a gas chromatography-mass spectrometry (GC-MS), and two unknown by-products were determined as N,N-dimethyl aniline (DMA) and methyl 2-methylphenylcarbamate (or methyl 4-methylphenylcarbamate). The quantitative analysis method for the reaction system was developed using a reversed-phase high performance liquid chromatography and all the components could be separated well.The thermodynamics analysis of the reaction system of DMC and aniline showed that the main reaction and two side reactions, the formation of N-methyl aniline (NMA) and DMA, were exothermal. While,the synthesis of diphenylurea was endothermic reaction. In addition, the synthesis of NMA and DMA were more competitive than the synthesis of MPC in thermodynamics.A novel nano-sized ZrO₂ catalyst for MPC synthesis was prepared through sol-gel process. The influence of preparation conditions on particle size, crystal phase and specific surface area was studied. The preparation conditions, such as sol-gel time, molar ratio of urea to ZrOCl₂, ZrOCl₂ concentration, acetic acid concentration and calcination temperature was correlated quantitatively with the nature of ZrO₂ catalyst, such as particle size, content of tetragonal ZrO₂ and the specific surface area. The influence of ZrO₂ particle size on its acidic nature was explored. It was found that the acid amounts decreased monotonously with the particle size increasing, and there was an instant change around 15 nm. The synthesis of MPC was evaluated over nano-sized ZrO₂ catalyst, and high aniline conversion and MPC selectivity were obtained over ZrO₂ with small size and high content of tetragonal phase.A novel supported ZrO₂/SiO₂ catalyst for MPC synthesis was developed based on the abovementioned study. When ZrO₂ loading was 1 wt% and the catalyst was calcined at 573 K for 2 h, it showed high activity. XRD, XPS, NH3-TPD and FT-IR of the catalyst adsorbed with pyridine were carried out to characterize the supported catalyst. From the results, it can be concluded that the formation of Si-O-Zr bonds was due to the interaction between ZrO₂ and SiO₂. The acid amounts and the content of weak acid sites on the surface of ZrO₂/SiO₂ increased obviously than that of ZrO₂, and Lewis acid sites were dominant. Aniline conversion is 98.6% and MPC yield is 79.8% under the optimized reaction conditions.The reaction scheme of DMC and aniline over ZrO₂/SiO₂ catalyst was proposed and studied by in-situ FT-IR. Firstly, DMC adsorbed on the surface of ZrO₂/SiO₂ catalyst, and C atom in carbonyl group became more electropositive. Then,aniline, the nucleophilic reagent, attacked the C atom and bimolecular nucleophilic substitution (SN2) took place. Based on the reaction scheme, kinetics of MPC synthesis over ZrO₂/SiO₂ catalyst was studied by Reactor IR IC-10. The surface reaction was the control step of reaction rate, and the kinetic equation was as follows.A novel heterogeneous catalyst, ZnO-TiO₂ was designed and prepared for MPC synthesis. ZnO-TiO₂ catalyst, calcined at 673 K, with n(Ti)/n(Zn) equaling to 2 exhibited a better activity. Aniline conversion was 96.9% and MPC yield was 66.7%. The characterization results and catalytic activity was correlated. It was found that the formation of ZnTiO3 and Zn₂TiO₄ provided the Lewis acid sites for MPC synthesis. Furthermore, ZnO-TiO₂ showed better stability, and its catalytic activity could be reactivated almost completely just by calcination.The integrated reaction system of MPC synthesis with its condensation to MDC in microscale was set up. A bifunctional catalyst, H₄SiW₁₂O₄₀-ZrO₂/SiO₂, was designed and prepared for the one-pot synthesis of MDC using aniline, DMC and formaldehyde as raw material. The better reaction conditions were as follows: n(DMC)/n(aniline)/n(formaldehyde) = 20/1/0.1(molar ratio), H₄SiW₁₂O₄₀ load was 10 wt% and the reaction temperature was designed by stages, namely, maintaining 443 K for 7 h and then cooling down to 373 K for 4.5 h. Under above conditions, the MDC yield was 24.9%.