Kinetic Study on the Novel Efficient Clean Decomposition of Methyl N

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Kinetic Study on the Novel Efficient Clean Decomposition of Methyl N‑Phenyl Carbamate to Phenyl Isocyanate Ganyu Zhu,†,‡ Huiquan Li,*,† Yan Cao,† Haitao Liu,† Xintao Li,†,‡ Jiaqiang Chen,† and Qing Tang† †

Key Laboratory of Green Process and Engineering, National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China ‡ University of Chinese Academy of Sciences, Beijing, 100049, China ABSTRACT: This study presents a clean, novel, and highly efficient system for the decomposition of methyl N-phenyl carbamate (MPC) to phenyl isocyanate (PI). MPC conversion of high as 89% and PI selectivity of almost 100% were obtained using chlorobenzene solvent under high pressure without any catalyst. The effects of temperature and time on the conversion of MPC were investigated, and the apparent kinetics model of MPC decomposition reaction was established. Results indicate that the decomposition of MPC is a pseudo-first-order reaction with Arrhenius parameters of Ea = 71.28 kJ/mol and A = 2.74 × 105 min−1, and the model correlated well with experimental data. This study provides not only a new and clean system for the highly efficient production of PI, but also a guide for the design of reactors in the isocyanate industry.

1. INTRODUCTION Phenyl isocyanate (PI) is an important raw material in the production of medicines, pesticides, plant protection products, bleaches, and synthetic resins.1 Traditionally, PI is prepared via the phosgenation of aniline.2 However, the phosgenation of aniline is being abandoned because of its use of the highly toxic raw material phosgene and the mass production of the corrosive byproduct hydrochloric acid that leads to serious environmental pollution. Phosgene-free methods have been extensively investigated to minimize the environmental hazards of phosgene.3−15 In the phosgene-free route to PI, the thermal decomposition of methyl N-phenyl carbamate (MPC) shows good prospects for industrial applications (Scheme 1).

catalyst to increase the conversion and yield. The use of a catalyst makes the side reaction occur more easily, makes the follow-up product separation process difficult, increases the cost of the process, and pollutes the environment. Thus, the search for a clean and highly efficient procedure for the decomposition of MPC is ongoing. Recent research on the decomposition of MPC mainly focused on the screening of catalysts and process optimization, among others, and scarcely investigated the intensive apparent kinetics. However, in this study, the reaction is conducted in a novel high-efficiency system without any catalysts, and the kinetics of MPC to PI is investigated for the first time. Moreover, the impacts of reaction time and temperature are discussed. A kinetic model is proposed and the kinetic parameters are obtained. The results of the kinetics are expected to help in the design of reactors in the industry.

Scheme 1. Decomposition of MPC

2. MATERIAL AND METHODS 2.1. Materials. The chemicals used in this study had the following purities: MPC, 99%, prepared from the reaction between dimethyl carbonate (DMC) and N,N′-diphenylurea (DPU) as our previously reported;19 chlorobenzene, analytical reagent grade; and N2, 99.99%. 2.2. Experimental Procedure. The decomposition of MPC was conducted in a 1 L stainless steel autoclave (Scheme 2). In a typical reaction procedure in this study, MPC (11.50 g) and chlorobenzene (400 mL) were charged into the autoclave. Then, the mixture was heated to a reaction temperature with mechanical stirring. Timing started once the autoclave reached the desired temperature. During the reaction procedure, the methanol was continually removed from the reaction system using a flow of N2 at 800 mL min−1, and a back-pressure valve

Several studies have been devoted to exploring new catalysts for the thermal decomposition of MPC. Valli and Alper16 used chlorocatechloborane and triethylamine to capture the alcohol from the carbamate and obtained a PI yield of 96−100%. However, the need to recycle those reagents limits the industrial application. Uriz et al.17 decomposed MPC using odichlorobenzene as solvent and a relatively large amount of montmorillonite K-10 as catalyst with a conversion of 96%. Using the same solvent Dai et al.18 obtained a conversion of MPC of 86.2% and a 78.5% yield of PI using Bi2O3 as catalyst. Many other attempts for decomposition of MPC were performed. However, most previous studies operated at temperatures equal to or higher than the boiling point of PI under normal or reduced pressure. Thus, not only did the reverse reaction occur relatively easily because PI and methanol evaporated out simultaneously, but also polymerization became prone to occur because of the high concentration of PI in the gas phase. Moreover, almost all of the reported studies used a © 2013 American Chemical Society

Received: Revised: Accepted: Published: 4450

September 29, 2012 February 25, 2013 March 2, 2013 March 3, 2013 dx.doi.org/10.1021/ie302659q | Ind. Eng. Chem. Res. 2013, 52, 4450−4454

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Scheme 2. Apparatus for Decomposition of MPCa

Figure 1. Chromatograms of different samples at reaction times of 0, 75, and 180 min and temperature 503 K. Peaks: 1, MPC; 2, PI (derived by ethanol); 3, DPU; 4, chlorobenzene.

a

1, Autoclave; 2, heating jacket; 3, stirring paddle; 4, inlet air tube; 5, sampling tube; 6, bumper post; 7, outlet air tube; 8, back-pressure valve.

k

was installed at the air outlet to stabilize the system pressure at different temperatures. The reactions were taken at temperatures of 463−513 K and pressures of 0.43−0.98 MPa. Because the product of PI can react with ethanol to ethyl Nphenyl carbamate, which has a different resident time through the column in HPLC compared with MPC, the samples were obtained from a tube inserted in the bottom of the reactor and immediately treated with ethanol for 3 h.20 After dilution to a constant volume with a methanol−water mixture, the samples were analyzed by HPLC (Agilent-1200; column Extend C-18, 4.6 mm × 150 mm, at 245 nm; methanol/water = 50/50 at 1.0 mL min−1).

MPC → PI

(1)

The reaction rate of MPC can be expressed as follows: rMPC = −

d[MPC] = k[MPC]n dt

(2)

where k and n represent the rate constant and reaction order, respectively. Assuming that the decomposition reaction of MPC is a firstorder reaction, the relationship between the concentration of MPC and the reaction time is [MPC] = [MPC]0 e−kt

3. RESULTS AND DISCUSSION 3.1. Model Simplification. The decomposition of MPC is known to be a reversible reaction, and the reverse reaction occurs more easily. Therefore, nitrogen gas was used to remove the methanol generated during the reactions, assuring that the reverse reaction did not occur. Moreover, PI is so active that it can dimerize slowly at room temperature, as shown in Scheme 3A. The dimerization

(3)

k is a function of temperature, and thus, it can be expressed using the Arrhenius equation21 as follows: ⎛ −E ⎞ k = A exp⎜ a ⎟ ⎝ RT ⎠

(4)

where A represents an exponential factor, Ea is the activation energy, and R is a universal gas constant with a value of 8.314 J mol−1 K−1. If the assumption of first-order reaction is correct, then the conversion of MPC should increase exponentially with temperature, as well as reaction time, according to eqs 3 and 4. 3.3. Effects of Reaction Conditions. Several reported highly efficient catalysts18,22,23 were added in our system of chlorobenzene solvent under high pressure. As can be seen in Table 1, the conversion of MPC decreased except when ZrO2 was used but the selectivity of PI significantly decreased from 100 to 56.59%. These results indicated that the addition of a catalyst promotes the side reaction. Therefore, in this study, the reaction was conducted without any catalyst. As shown in Figure 2, thermal decomposition of MPC was conducted to investigate the effect of temperature. The decomposition rate speeded up quickly between the temperatures of 463 and 503 K, while the conversion of MPC increased from 25.42 to 88.18%. Then, the increase of the conversion slowed down, and the conversion of MPC just reached 90.40% at 513 K. Moreover, the selectivity was almost 100%, except for some fluctuations at temperatures higher than

Scheme 3. Side Reactions in Decomposition of MPC

increases with increasing temperature and time. Therefore, the experiments were conducted in low concentration (less than 5 wt %). This dilute solution was useful considering the results of HPLC (Figure 1) and the total mass conservation. Moreover, MPC also decomposed to form DPU and DMC, as shown in Scheme 3B. However, the content of DPU was less than 1% in the system, and this slow reaction can be neglected. 3.2. Kinetics. As mentioned above, the reaction can kinetically be expressed as an irreversible reaction: 4451

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almost 90%. When the temperature was lower than 503 K, the selectivity stabilized at 100% within 300 min, but the reaction rate was low, needing 300 min for both the highest MPC conversion and the PI yield to reach 85%. In addition, the exponential trends can be seen between the conversion of MPC and the temperature, as well as the reaction time, from Figures 2 and 3. This means that the decomposition of MPC fits well with the assumption of a first-order reaction. 3.4. Kinetics Parameter Confirmation. Experiments were conducted at four different temperatures, i.e., 473, 483, 493, and 503 K. Figure 4 shows the relationship between MPC concentration and reaction time.

Table 1. Effect of Catalyst on the Pressure Decomposition Reaction of MPC under Pressurea catalystb none nano Zn + ZnO SnO2 Bu2SnO ZrO2

temp (°C)

press. (MPa)

MPC conv (%)

PI select. (%)

PI yield (%)

210 210

0.59 0.60

62.23 60.87

100 87.27

62.23 53.12

205 205 205

0.54 0.54 0.54

42.51 56.82 75.38

85.05 79.53 56.59

36.15 45.19 42.65

a

Reaction time is 180 min. bMass ratio of the catalyst to the solvent is 0.5%.

Figure 4. Concentration of MPC versus reaction time at different temperatures with N2 flow rate of 800 mL min−1.

Figure 2. Effects of reaction temperature on thermal decomposition of MPC. Reaction time, 180 min; N2 flow, 800 mL min−1; reaction pressure, 0.43, 0.50, 0.59, 0.71, 0.85, and 0.98 MPa.

It is worth noting that the concentration of MPC was not from the same point, as shown in Figure 4, because MPC had a decomposition temperature of 373 K from the thermogravimetric experiment (data not shown). Therefore, the decomposition of MPC had already slowly proceeded before the desired temperature was reached, making the concentrations from different data points. Equation 3 can be converted into a first-order integral as follows:

483 K caused by the side reaction of MPC to DPU and the polymerization of PI. As shown in Figure 3, the effect of reaction time on the decomposition of MPC was also investigated. The MPC

ln C0 − ln C1 = kt

(5)

where C0 represents the initial concentration of MPC. On the basis of eq 5, the value of k can be obtained at different temperatures after linear fittings24 (Figure 5 and Table 2). Moreover, the linear relations between ln C1 and t are very clear and the assumption of a first-order reaction can be further confirmed. The activation energy and exponential factor can be calculated based on the Arrhenius equation. As can be seen in Figure 6, Ea was 71.28 ± 4.32 kJ mol−1 and A was exp(12.53 ± 1.08) min−1. Given the kinetics parameters, k can be expressed as follows: Figure 3. Effects of reaction time on thermal decomposition of MPC. Reaction temperature, 513 K; reaction pressure, 0.98 MPa; N2 flow, 800 mL min−1.

⎛ ( −71.28)(1000) ⎞ k = 2.74 × 105 exp⎜ ⎟ ⎝ ⎠ RT

(6)

4. CONCLUSIONS Using the low boiling point chlorobenzene as a solvent without any catalyst under high pressure, the side reactions for the decomposition of MPC to PI were avoided by effectively controlling the reaction time and temperature, consequently

conversion needed only 50 min to reach 75% and then 250 min to increase from 75 to 96%. However, the selectivity suddenly decreased from nearly 100% to 90% after 180 min in the occurrence polymerization of PI at the temperature of 513 K. Thus, the highest MPC conversion and PI yield were both 4452

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 10 82544825. Fax: +86 10 82544830. E-mail: hqli@ home.ipe.ac.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the Science and Technology Ministry of China (No. 2013BAC11B03) and the Knowledge Innovation Fund of Chinese Academy of Sciences (No. KGCX2-YW-215-2). Figure 5. Plots of ln C1 versus reaction time.

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Table 2. Rate Constants for the Decomposition Reaction of MPC at Different Temperatures temp (K)

103k

R2

−ln k

473 483 493 503

3.78 ± 0.16 5.10 ± 0.21 8.00 ± 0.27 10.81 ± 0.54

0.973 0.980 0.976 0.971

5.578 5.279 4.828 4.527

Figure 6. Arrhenius plots of the first-order rate constant versus 1000/ T.

making the PI selectivity stabilize at 100%. Kinetics study is based on the effective removal of methanol. The results show that the decomposition of MPC is a first-order reaction with an activation energy of 71.28 kJ mol−1 and exponential factor of 2.74 × 105 min−1. The built reaction rate equations were accurate and were able to reflect the concentration and conversion change during the reaction period. Furthermore, although the proposed system poses some problems, i.e., the conversion and yield were still not able to reach the highest point, as indicated in the report,16 and product separation and solvent recovery were not applied, this high pressure chlorobenzene system exhibits a great advantage over other processes at high selectivity, which is an important factor in the reaction. Moreover, the kinetic study provided a theoretical guide for reactor design. 4453

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