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Nov 26, 2012 - The catalytic activities of many transition-metal salts for dimethyl carbonate (DMC) synthesis from methyl carbamate (MC) and methanol ...
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Reaction Mechanism and Kinetics of Dimethyl Carbonate Synthesis from Methyl Carbamate and Methanol Wenbo Zhao,*,†,‡ Xianye Qin,† Yanhong Li,† Zhaoshu Zhang,† and Wei Wei‡ †

Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming, 650500, People’s Republic of China Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, People’s Republic of China

‡

ABSTRACT: The catalytic activities of many transition-metal salts for dimethyl carbonate (DMC) synthesis from methyl carbamate (MC) and methanol were evaluated in a batch reactor. The reaction mechanism and kinetics on the outstanding catalyst ZnCl2 were further investigated in detail. X-ray diffraction (XRD), thermogravimetry (TG), and differential scanning calorimetry (DSC) characterization, quantum chemical calculation, and kinetics experiments all indicated that this reaction could be divided into two processes: (1) two MC molecules coordinated with ZnCl2 via N atom to produce Zn(MC)2Cl2. This intermediate would convert to Zn(MC)(NH3)Cl2 by reacting with one methanol molecule. This process was first order, relative to ZnCl2, and zeroeth order, relative to MC, from a macrokinetics viewpoint. (2) Zn(MC)(NH3)Cl2 further reacted with another methanol molecule to yield DMC and Zn(NH3)2Cl2, one ammonia molecule of Zn(NH3)2Cl2 could be substituted by MC at experimental temperature to form Zn(MC)(NH3)Cl2 again. This process was first order to both ZnCl2 and MC, from a macrokinetics viewpoint. It should be noted that Zn(MC)2Cl2 could not appear again after the first process, and the second process is the real catalysis cycle in DMC synthesis.

1. INTRODUCTION Dimethyl carbonate (DMC), which can be used as organic reaction reagent, green solvent, and transportation fuel additive, is an important green organic compound.1−5 Currently, DMC is mainly produced by the phosgenation of methanol, the oxidation carbonation of methanol, and transesterification in the industry. However, they all have corresponding disadvantages, such as being poisonous, easily explosive, and expensive. Several other synthesis methods have been proposed recently; among them, the methanolysis of urea is considered to be promising for low cost and easy separation of product.6−14 In this synthesis method, urea reacts with methanol to yield methyl carbamate (MC) at first, and then MC reacts with methanol again to produce DMC, with the latter reaction being the rate-limited step. Because the byproduct ammonia from the first reaction is also the byproduct of the second reaction, which will restrict the shift of chemical balance, we proposed a twostep separate technique to produce DMC in our previous work.15−19 The key of this technique is to determine the proper catalyst to accelerate the rate-limited reaction. Although many catalysts have been tested for the urea methanolysis, the exploitation of a catalyst for the isolated reaction of MC and methanol has been rarely reported up to this point. Organic tin compound was considered to be effective to this rate-control reaction, but toxicity limited its application in industry.20 Calcium oxide could activate the reactant methanol to promote the reaction rate.21 The yield of DMC on it is still far from satisfactory. In our previous work, zinc-containing compounds as homogeneous catalysts were found to be highly active for this reaction. 15 Some heterogeneous catalysts were synthesized with zinc as one of active element and their activity were evaluated in batch reactor.17,18 However, the reaction mechanism of elemental zinc was not very clear. A density functional theory (DFT) © 2012 American Chemical Society

study had been carried out recently to illustrate the catalysis reaction mechanism of Zn(NH3)2(NCO)2.19 In the present work, on one side, the seeking of suitable catalysts was further extended; on the other, several characterization methods were explored to uncover the catalysis reaction mechanism of zinc chloride, whose activity is higher than other metal salts. Besides, quantum chemical calculation and reaction kinetics experiments were carried out to elucidate the formation reason of intermediate and to validate the proposed mechanism.

2. EXPERIMENTAL SECTION 2.1. Catalytic Reaction. The catalyst evaluation experiments were conducted in a 350-mL stainless steel autoclave reactor equipped with an electric heater, a reflux column and a magnetic stirrer under the assigned conditions. In a typical process, 5 mmol of catalyst, 7.5 g of MC, and 64 g of methanol were charged into autoclave, then rapidly heated to the desired temperature and kept for a certain time with magnetic stirring. After the reaction, the autoclave was cooled to room temperature, and then product and catalyst in the autoclave were weighed, clarified, and determined. Methanol in this experiment not only was a reactant, but also was a solvent. All the reagents in experiments are commercial products except Zn(NH3)2Cl2, which was prepared by injecting ammonia into a methanol solution of zinc chloride. The kinetics experiments were carried out in a 1000-mL stainless steel autoclave reactor, as shown in Scheme 1. The reactant and catalyst were rapidly heated to the desired temperature, using a stirring rate of 600 ± 50 rpm in 0.5 h. Received: Revised: Accepted: Published: 16580

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agent for amino group, could further react with MC to yield Nmethyl methyl carbamate (NMMC). Thus, the concentration of byproduct NMMC increases correspondingly with the increase of the DMC concentration. At a high temperature, DMC is easily thermal decomposed into dimethyl ether and carbon dioxide with the catalysis effect of solid basic, acidic oxides and residual Brønsted sites of copper zeolite. Furthermore, DMC could also react with water from the reactant to yield carbon dioxide. However, the concentration of water in reactant was so low that this side reaction was not considered in this study.25

Scheme 1. Experimental Setup

3. RESULTS AND DISCUSSION 3.1. Experimental and Characterization. 3.1.1. Catalyst Evaluation. Transition-metal ions are considered to be classical Lewis acids since they have empty d-orbitals to get electrons. They are often used as a catalyst of organic reaction, since they can coordinate with organic substrates with empty d-orbitals. In our previous work, the catalysis activities of a serial of zinccontaining compound were investigated for the reaction of MC and methanol in a batch reactor. The result indicated that the catalytic performances of zinc-containing compounds were related with their solubility in methanol, which is affected by their anions. ZnCl2 showed the highest activity among them; its activity only came from the Zn cation in solution while the Cl− anion did not have any activity. In the present work, the activity of some other transition-metal salts were investigated for the reaction of MC and methanol and their activity were compared with ZnCl2 under the same condition, as shown in Figure 1.

After the reaction temperature was steady, whose error was (NH3)4ZnCl2 > NH3ZnCl2. We also investigated the coordination of MC with zinc chloride in the same method. The calculation result elucidate that the Gibbs energy change for zinc chloride coordinating with MC from 1 to 4 were −35.02, −57.44, −5.34, and −29.43 kJ/mol, respectively (see Figure 6). This meant the stability of the complex of MC and zinc chloride was following this order: (MC)2ZnCl2 > MCZnCl2 > (MC)4ZnCl2 > (MC)3ZnCl2. We further calculated the coordination condition of zinc chloride with MC and ammonia synchronously. Since the above result indicated that the complex with coordination number 2 was the most stable, we only calculated the condition that one ammonia molecule and one MC molecule coordinated with zinc chloride. The result elucidated the Gibbs energy change of (NH3)(MC)ZnCl2 was −101.04 kJ/mol (see Figure 7). By comparing this value with the previous calculation results, we found that the Gibbs energy change for ZnCl2 to coordinate with two ammonias was more than that with one ammonia and one MC and was much more than that with two MC. This result implied that the conversion from (NH3)2ZnCl2 to (NH3)(MC)ZnCl2 was easier than the conversion from (NH3)2ZnCl2 to (MC)2ZnCl2, and further implied the catalysis cycle only occur between (NH3)2ZnCl2 and (NH3)(MC)ZnCl2. 3.3. Reaction Kinetics. In the above work, a possible reaction mechanism of MC and methanol with ZnCl2 as the catalyst was proposed, in which the formation of two DMC molecules occurred one by one. The real catalysis cycle occurred between Zn(NH3)(MC)Cl2 and Zn(NH3)2Cl2, after Zn(MC)2Cl2 had been consumed completely. Therefore, for the purpose of kinetics research, the entire reaction process was

Figure 4. Electron distribution of MC.

In theory, the coordination number of the Zn cation could change from 1 to 6. With regard to ZnCl2, since 2-coordinated spaces were occupied by a Cl− ion, its coordination number with N atom is from 1 to 4. In reality, only diammine zinc chloride, whose coordinated number is 2, was found in our experiments. Quantum chemistry calculation results could explain this phenomenon perfectly. We calculated the electron energy of ammonia, zinc chloride, and four types of ammine zinc chloride, including NH3ZnCl2 with plane structure, (NH3)2ZnCl2 with tetrahedron structure, (NH 3 ) 3 ZnCl 2 with trigonal bipyramid structure and (NH3)4ZnCl2 with octahedron structure. The other stereo-

Figure 5. Structure and relative energy for complex of ZnCl2 and NH3. 16583

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Figure 6. Structure and relative energy for complex of ZnCl2 and MC.

Figure 7. Structure and relative energy for complex of ZnCl2 with NH3 and MC.

Figure 8. DMC concentration profiles ((A) at different temperatures and (B) the corresponding Arrhenius plot).

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Figure 9. (A) DMC concentration change profile with time at different ZnCl2 amounts at 190 °C, and (B) the relationship between reaction rate and ZnCl2 amount.

divided into two separated processes: in first process, ZnCl2 coordinated with MC to form Zn(MC)2Cl2 and then transformed to Zn(NH3)(MC)Cl2; this process occurred only one time. In the second process, Zn(NH3)(MC)Cl2 changed to Zn(NH3)2Cl2 and then changed back to Zn(NH3)(MC)Cl2 through the release of ammonia, this was a cycle process. In the first process, the catalyst was ZnCl2; in the second process, the catalyst was Zn(NH3)2Cl2 actually. In the following work, the kinetics of reaction without catalyst, with ZnCl2 catalyst, and with Zn(NH3)2Cl2 catalyst were investigated. 3.3.1. Reaction Kinetics Experiments without Catalyst. The kinetics research of MC with methanol without catalyst was carried out in a batch reactor in the temperature range of 180− 210 °C. The byproduct NMMC, which was the reaction product of MC and DMC, was not found in these experiments, since the yield of DMC was very low, under the reaction conditions. Figure 8A shows the relationship between DMC concentration and time. The DMC concentration always increased with time linearly, even as the reaction time reached 40 h. This result suggested that the reaction order from MC to DMC should be zero and the model equations should be R0 =

d[DMC] =k dt

was 0.4 mol, the amount of methanol was 0.8 mol, and the ZnCl2 amount was changed from 0.01 mol to 0.04 mol. The overall reaction rate (R) could be obtained by linear fitting of data in the figure. The linear correlation coefficient of four experimental runs were all close to 1; this meant that the first rate method was adequate for determination of the kinetics parameters. From the slope of the lines, we found that the reaction rate increased as the ZnCl2 amount increased. The catalysis reaction rate of ZnCl2 (r) was obtained by subtracting the reaction rate without catalyst (R0) from the overall reaction rate R, as shown in eq 2: r = R − R0

(2)

According to experimental equations, the relationship of the catalytic reaction rate r and substrate concentration could be expressed by the following equation: ln r = ln k + n ln[reactant1] + m ln[reactant2] + ...

(3)

Figure 9B plots the natural logarithm of ZnCl2 concentration versus the natural logarithm of r. The linear fitting of the data indicated that the slope of the line was 1.16. The kinetics equation could be described as

(1)

r = 8.94 × 10−4 × [ZnCl 2]01.16

The reaction rate (R0) could be obtained by linear fitting the data at the same temperature. According to the Arrhenius equation, the apparent activation energy and the pre-exponential factor were found to be 75.3 kJ/ mol and was 812 mol L−1 s−1, respectively (see Figure 8B). 3.3.2. Reaction Kinetics Experiments with ZnCl2 as a Catalyst. The kinetics parameter was determined using the first rate method. In all of the experiments, the reaction time was not longer than 2 h and the conversion of MC was no greater than 5%. The amount of converted MC was lower than the amount of MC coordinated with ZnCl2. By this way, the reaction was limited in the first process, from ZnCl2 to Zn(NH3)(MC)Cl2. The second process, from Zn(NH3)(MC)Cl2 to Zn(NH3)2Cl2, did not happen. Figure 9A shows the DMC concentration profile versus time under the condition that the reaction temperature was 190 °C, the amount of MC

(4)

Thus, we could conclude that this catalysis reaction was first order, with respect to ZnCl2. With the similar method, under the condition that the amount of ZnCl2 was 0.04 mol and the amount of methanol was 0.8 mol, we changed the MC amount from 0.4 mol to 0.7 mol and investigated its effect on the reaction rate (see Figure 10). We found that the reaction rate did not change with the increase of MC concentration. This implied that the reaction was zeroeth order, with respect to MC. According to the reaction mechanism, the reaction of first process can be described as Scheme 2. There are three possible reaction equations based on different reaction essence. If the first reaction is the ratecontrolling step of the entire reaction, according to the mass action law, the reaction model equation should be 16585

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This means that the reaction is second order, with respect to MC, first order with respect to ZnCl2, and first order, with respect to methanol. The concentration of methanol can be considered as a constant, since it is solvent; its concentration is very high and almost does not change at the reaction process. Thus, this condition is the same as the first condition, from a macro viewpoint. Also, this model is not fit for the experimental results. If the second reaction is the rate-controlling step, the first reaction has reached equilibrium and the equilibrium constant is very large. The concentration of Zn(MC)2Cl2 should be the same as the original concentration of ZnCl2, since the ZnCl2 amount is much lower than the MC amount. From this deduction, the reaction model equation should be r = k[ZnCl 2]0 × [Me]0

Because the amount of methanol is excessive and could be considered to be a constant, the reaction is first order, with respect to ZnCl2, and zeroeth order, with respect to MC and methanol, from the macro viewpoint. This is supposed to be consistent with the experimental results. The reaction essence of MC and ZnCl2 is the N atom of MC providing a lone pair of electrons to the Zn cation, which is similar to the acid−base reaction. Thus, the reaction equilibrium constant is large and reaction rate is very fast. The reaction of MC in Zn (MC)2Cl2 with methanol is similar to the organic reaction. The atomic orbital of carbon in the carbonyl species changes from sp2 to sp3, and then back to sp2. This reaction is difficult and, consequently, is the ratecontrolling step of the first process. 3.3.3. Reaction Kinetics Experiments with Zn(NH3)2Cl2 as a Catalyst. The kinetics parameter was also determined using the first rate method. Figure 11A shows the DMC concentration profile versus time under the condition that the reaction temperature was 190 °C, the amount of MC was 0.4 mol, and the amount of methanol was 0.8 mol; the amount of Zn(NH3)2Cl2 was changed from 0.01 mol to 0.04 mol. The overall reaction rate R was obtained by linear fitting. The slope of the line indicated that the reaction rate increased as the

Figure 10. DMC concentration change profile with time at different MC amounts at 190 °C.

Scheme 2. Reaction from MC to DMC with ZnCl2 as a Catalyst

r = k[MC]20 × [ZnCl 2]0

(5)

This means that the reaction is second order, with respect to MC, and first order, with respect to ZnCl2, which is different from the experimental results. If the second reaction is the rate-controlling step, the first reaction has reached equilibrium and the equilibrium constant is very small. According to the rate-control method, the reaction model equation should be r = k[MC]20 × [ZnCl 2]0 × [Me]0

(7)

(6)

Figure 11. (A) DMC concentration change profile with time at different Zn(NH3)2Cl2 amounts at 190 °C, and (B) the relationship between reaction rate and Zn(NH3)2Cl2 amount. 16586

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Figure 12. (A) DMC concentration change profile with time at different MC amounts at 190 °C, and (B) the relationship between the reaction rate and MC amount.

There also are three possible reaction equations, based on different reaction essence. If the first reaction is the ratecontrolling step, the reaction model equation should be

Zn(NH3)2Cl2 amount increased. According to eq 2, the catalysis reaction rate of Zn(NH3)2Cl2 (r) could be obtained by subtracting the rate without catalyst (R0) from the overall reaction rate (R). Compared to the catalysis reaction rate of ZnCl2, the rate of Zn(NH3)2Cl2 was slightly lower. Figure 11B plots the natural logarithm of the Zn(NH3)2Cl2 concentration versus that of r. The slope of the line was found to be 0.932. The kinetics equation could be described as r = 2.54 × 10−4 × [Zn(NH3)2 Cl 2]0 0.932

r = k[MC]0 × [Zn(NH3)2 Cl 2]0

This means that the reaction is first order, with respect to both MC and Zn(NH3)2Cl2. This deduction is consistent with the experimental result. If the second reaction is the rate-controlling step, the first reaction has reached equilibrium and the equilibrium constant is small; the reaction model equation should be

(8)

This results implied that the reaction was first order, with respect to Zn(NH3)2Cl2. With the similar method, under the condition that the amount of Zn(NH3)2Cl2 was 0.04 mol, the amount of methanol was 0.8 mol, and the MC amount was changed from 0.2 mol to 0.5 mol, we investigated the effect of MC concentration on the reaction rate (see Figure 12A). The reaction rate was found to increase as the MC amounts increased. The natural logarithm of MC concentration versus natural logarithm of r is plotted in Figure 12B. The slope of the line was found to be 1.16, The kinetics equation could be described as r = 2.73 × 10−5 × [MC]01.16

(10)

r = k[MC]0 × [Zn(NH3)2 Cl 2]0 × [Me]0

(11)

This means the reaction is first order, with respect to MC, Zn(NH3)2Cl2, and methanol. Since methanol is the solvent, its concentration almost does not change in the reaction. This condition is the same as the above one, from the macro viewpoint. This deduction is also consistent with the experiment result. If the second reaction is the rate-controlling step, the first reaction has reached equilibrium and the equilibrium constant is very large. Because the amount of Zn(NH3)2Cl2 is much lower than amount of MC, the amount of Zn(MC)(NH3)Cl2 should be the same as the amount of Zn(NH3)2Cl2. The reaction model equation should be

(9)

This result indicated that the reaction was first order, with respect to MC. It was different from the result obtained with ZnCl2 as a catalyst, which was zeroeth order, with respect to MC. According to the reaction mechanism, the reaction expressions of the second process are described as Scheme 3.

r = k[Zn(NH3)2 Cl 2]0 × [Me]0

(12)

The concentration of methanol can be considered as a constant. Thus, this reaction is first order, with respect to Zn(NH3)2Cl2, and zeroeth order, with respect to MC, from a macro viewpoint. This is different from the experiment result. Although the first and second suppositions are consistent with the experiment result, the second supposition is closer to the reaction essence. Because the first reaction is unfavorable on reaction thermodynamics, based on the theory analysis and results of quantum chemical calculation, consequently, the equilibrium constant should be very small. Furthermore, the second reaction is also similar to an organic reaction.

Scheme 3. Reaction from MC to DMC with Zn(NH3)2Cl2 as a Catalyst

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4. CONCLUSION The essential reaction mechanism of MC and methanol with ZnCl2 as a catalyst was proposed. At first, MC coordinated with ZnCl2 to form Zn(MC)2Cl2, which would convert to Zn(MC)(NH3)Cl2 via reaction with methanol. Then, Zn(MC)(NH3)Cl2 was attacked by another methanol to yield DMC and Zn(NH3)2Cl2. Finally, one ammonia molecule of Zn(NH3)2Cl2 was substituted by MC at experimental temperature to form Zn(MC)(NH3)Cl2 again. The X-ray diffraction (XRD), thermogravimetry (TG), and differential scanning calorimetry (DSC) characterization results strongly supported the reaction mechanism. Quantum chemical calculation indicated that a complex of ZnCl2 with a coordination number of 2 was most stable and the catalysis cycle most possibly occurred between Zn(MC)(NH3)Cl2 and Zn(NH3)2Cl2. The reaction kinetics experiment, which was divided into two processes, further confirmed the reaction mechanism. At first, the reaction with ZnCl2 catalyst was first order, with respect to ZnCl2, and zeroeth order, with respect to MC. After ZnCl2 converted to Zn(NH3)2Cl2, the reaction was first order, with respect to both Zn(NH3)2Cl2 and MC. The kinetics equation could be deduced from the proposed reaction mechanism and related theory analysis.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 15887851075. Fax: +86 871 5954196. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from Open Foundation of State Key Laboratory of Coal Conversion (1112-609), Yunnan Province Science Foundation (No. 2010ZC034), Research Fund for Doctor Program of Higher Education of China (No. 20105314120005), Scientific Research Fund of Yunnan Provincial Education Department (No. KKJD201051012).

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NOMENCLATURE R0 = noncatalysis reaction rate (mol/(L s)) R = overall reaction rate (mol/(L s)) r = catalysis reaction rate (mol/(L s)) k = reaction rate constant t = time (s) [MC]0 = concentration of methyl carbonate added into the autoclave (mol/L) [Me]0 = concentration of methanol added into the autoclave (mol/L) [ZnCl2]0 = concentration of ZnCl2 added into the autoclave (mol/L) [Zn(NH3)2Cl2]0 = concentration of Zn(NH3)2Cl2 added into the autoclave (mol/L) [reactant] = concentration of reactant (mol/L)

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REFERENCES

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