Investigations of Catalytic Activity, Deactivation, and Regeneration of

The experimental results shows that the Pb(OAc)2 is highly active and .... were obtained with a Rigaku C/max-2500 diffractometer using graphite-filter...
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Ind. Eng. Chem. Res. 2007, 46, 6858-6864

Investigations of Catalytic Activity, Deactivation, and Regeneration of Pb(OAc)2 for Methoxycarbonylation of 2,4-Toluene Diamine with Dimethyl Carbonate Shengping Wang,† Guanglin Zhang,‡ Xinbin Ma,*,† and Jinlong Gong§ Key Laboratory for Green Chemical Technology, School of Chemical Engineering & Technology, Tianjin UniVersity, Tianjin 300072, P.R. China; School of Chemical Engineering & Technology, Hebei UniVersity of Technology, Tianjin 300130, P.R. China; and Department of Chemical Engineering, UniVersity of Texas at Austin, Austin, Texas 78712-0231

Non-phosgene synthesis of dimethyltoluene-2,4-dicarbamate from methoxycarbonylation of 2,4-toluene diamine (TDA) and dimethyl carbonate (DMC) is investigated over a Pb(OAc)2 catalyst. The experimental results shows that the Pb(OAc)2 is highly active and selective in the reactions to produce dimethyltoluene-2,4dicarbamate. The experiment that had been carried out at 443 K with a DMC/TDA molar ratio of 20 and a TDA/Pb(OAc)2 molar ratio of 50 yields a 100% conversion of TDA and a 97.7% selectivity of dimethyltoluene2,4-dicarbamate. Yields of dimethyltoluene-2,4-dicarbamate are strongly dependent on the reaction temperature. X-ray diffraction (XRD) measurements have shown that deactivation of Pb(OAc)2 after reactions can be ascribed to the formation of Pb3(CO3)2(OH)2 from the reaction of Pb(OAc)2, DMC, and methanol. This indicates that methanol, the byproduct from methoxycarbonylation of TDA with DMC, plays a significant role in the catalyst deactivation. However, the deactivated Pb(OAc)2 can be easily regenerated via a treatment by acetic acid. A reaction mechanism of methoxycarbonylation of TDA with DMC over Pb(OAc)2 catalyst to produce dimethyltoluene-2,4-dicarbamate is proposed. Introduction Organic carbamates have been widely used as intermediates for the synthesis of pesticides, medical drugs, and herbicides. This class of compounds has also been utilized in organic synthesis as protective reagents for amine functional groups.1 Specifically, organic carbamates have important applications in the production of isocyanates by self-decomposition. Dimethyltoluene-2,4-dicarbamate 1 is a precursor of 2,4-toluene diisocyanate (TDI), which is a major isocyanate compound used commercially in polyurethanes, surface coatings, adhesives, resins, elastomers (i.e., polyurethane foams), binders, and sealants as well as in various other expanding fields of applications. Such an example is that TDI is a useful “blocked” isocyanate that can provide one-component coatings with hydroxy components during a cure.2 Traditionally, the majority of aromatic diisocyanates are commercially synthesized by the reaction of phosgene (COCl2) or its derivatives with the corresponding aromatic diamines as shown in eq 1. Although phosgene can be utilized as a convenient carbonylation regent, its severe toxicity and thuscaused issues, such as corrosion from byproducts (i.e., HCl) and waste salt, limit the further developments of the phosgene route from environmental and social points of view.

In recent years, there have been increasing demands for safe and environmental friendly processes for chemical synthesis. * To whom correspondence should be addressed. Tel.: +86-2227406498. Fax: +86-22-27890905. E-mail: [email protected]. † Tianjin University. ‡ Hebei University of Technology. § University of Texas at Austin.

Removal of halogen compounds from various chemical processes to avoid environmental pollution is an important subject for the modern chemical production. Using a carbamate compound to replace the phosgene processes has, thus, been considered important from the viewpoint of environmental protection. For the past few years, several synthetic methods of aromatic carbamates have been developed based on environmentally friendly considerations. For example, aromatic carbamates can be synthesized by the oxidative carbonylation of amine with CO in the presence of transition metal compounds, as shown in eq 2.3,4 They can also be produced by the reductive carbonylation of nitro compounds with alcohols and CO in the presence of Ru, Rh, or Pd compounds, as shown in eq 3.5-7Another approach to generate carbamates is based on the transformation of azides in the presence of Me3P and several chloroformates (ClCOOBu, ClCOOMe, ClCOOEt, ClCOOCH2CCl3, and ClCOOCH2CHdCH2).8

Ar-NH2 + 1/2O2 + CO + CH3OH f Ar-NHCOOCH3 + H2O (2) Ar-NO2 + 3CO + CH3OH f Ar-NHCOOCH3 + 2CO2 (3) The methoxycarbonylation of amines with dimethy carbonate (DMC) to produce carbamates has been regarded as an attractive synthetic route because of the absence of phosgene, as shown in eq 4. It has been shown that various catalysts including PbO, Pb(NO3)2, Bi(NO3)2,9,10 and derivatives of Al, Ti, Zr, Zn, and Yb compounds,11-16 as well as strong bases (Group I alkoxides),17,18 are active for carbamate synthesis from methoxycarbonylation of alkyl amine or aromatic amines with DMC. However, to our best knowledge, few studies have been reported so far on the methoxycarbonylation of 2,4-toluene diamine (TDA) with DMC to produce dimethyltoluene-2,4-dicarbamate except with Zn(OAc)2, which has been shown by Baba and coworkers.19,20 In this study, we present results of an investigation

10.1021/ie061537+ CCC: $37.00 © 2007 American Chemical Society Published on Web 09/12/2007

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into the catalytic synthesis of dimethyltoluene-2,4-dicarbamate by methoxycarbonylation of TDA with DMC in the presence of Pb(OAc)2 as well as some other tested catalysts. The toxicity of Pb(OAc)2 is nearly negligible in comparison to that of phosgene, which was once used as a chemical weapon in World War II. Therefore, it is feasible for methoxycarbonylation of TDA with DMC with Pb(OAc)2 catalyst. We have studied the effects of reaction temperature, reaction time, molar ratio of TDA to DMC, and molar ratio of TDA to Pb(OAc)2. We have also explored the origin of deactivation of Pb(OAc)2 and proposed a possible reaction mechanism based on the experimental results.

Table 1. Activities for the Methoxycarbonylation of TDA with DMC over Various Catalystsa catalyst

T (K)

reaction time (h)

conversion of TDA (%)

yield of 1 (%)

yield of 2 (%)

Pb(NO3)2 Pb(OAc)2‚3H2O Mg(OAc)2 Ca(OAc)2 Pb(OAc)2 PbCO3 none

443 443 443 443 443 443 443

4 4 4 4 4 4 4

82.0 100 76.1 100 100 89.3 8.6

11.2 87.0 11.2 37.7 97.7 36.5 0

0 2.7 4.5 10.9 0 4.9 0

a 1 ) dimethyltoluene-2,4-dicarbamate and 2 ) methyl 3-amino-4-methyl phenyl carbamate. Reaction conditions: 0.045 mol of TDA, 0.9 mol of DMC, and 0.9 mmol of catalyst.

reagent, TDA, and is defined as the ratio of the moles of converted TDA to the moles of TDA fed initially to the reactor. The selectivities to product 1 and product 2 were expressed as the moles of 1 and 2 produced per 100 mol of consumed TDA. The yields of product 1 and product 2 were obtained from multiplication of TDA conversion by the selectivities to product 1 and product 2, respectively. Catalyst Characterization. X-ray powder diffraction (XRD) patterns were obtained with a Rigaku C/max-2500 diffractometer using graphite-filtered Cu KR radiation (λ ) 1.5405 Å) at 40 kV and 100 mA with a scanning rate of 8° min-1 from 2θ ) 5° to 2θ ) 80°. The XRD phases present in the samples were identified with the help of JCPDS powder data files. Experimental Section

Results and Discussion

Catalyst Preparation. The reactants and catalysts such as 2,4-toluene diamine, DMC, Pb(OAc)2‚3H2O, Mg(OAc)2, Zn(OAc)2, Ca(OAc)2, PbCO3, and Pb(NO3)2 were commercial analytical reagents. Pb(OAc)2 was obtained by heating Pb(OAc)2‚3H2O at 383 K (dehydration of Pb(OAc)2‚3H2O was observed around 383 K by thermogravimetric/differential thermal analyzer (TG/DTA) measurements) for 5 h under vacuum conditions. Methoxycarbonylation of Dimethyl Carbonate with 2,4Toluene Diamine. Methoxycarbonylation of TDA with DMC was carried out in a stainless steel autoclave (having a volume of 500 mL) that was magnetically stirred. The reaction mixture containing the reactants (TDA and DMC) and the catalyst (if used) were charged into the autoclave at room temperature, followed by being heated to the desirable reaction temperatures (with an experimental error of (2 K). The pressure was atmospheric at room temperature. However, the pressure was about 3-5 Mpa when the temperature rose up to reaction temperatures because of the gasification of DMC and the production of byproduct CO2. After reaction (typically 4 h), the liquid mixture was cooled down to room temperature and taken out of the autoclave. Then, it was evaporated in vacuo until removal of DMC and byproduct methanol. After the residue (unreacted TDA, products, and catalyst) was dissolved in CH2Cl2, the solution was filtrated to remove the catalyst precipitation, which was insoluble in CH2Cl2. Then the resultant solution was evaporated while the white solid was obtained as the final product, which was analyzed by high-performance liquid chromatography (HPLC). The qualitative analyses of the reaction products were carried out on a HP 5890-HP5891 MSD, while a HPLC (Agilent 1100) equipped with a spectrophotometric detector (LC 254UV/V) and a chromatographic column (Zorbax XDB-C18, 4.6 mm × 150 mm, 5 µm) was employed for quantitative analysis of the reaction products. The conversion is on the basis of the limiting

Performances of Catalysts for Methoxycarbonylation of 2,4-Toluene Diamine with DMC. The formation of dimethyltoluene-2,4-dicarbamate 1, from TDA and DMC, requires the methoxycarbonylation of two nonequivalent amino groups of the aromatic diamine, which brings forward the question about which could be functionalized first. Recently, Baba and coworkers reported that methyl-3-amino-4-methyl phenyl carbamate 2, but not methyl-5-amino-2-methyl phenyl carbamate 2′, was the direct intermediate for this reaction.19,20 In other words, the amino group in the para-position can be methoxycarbonylated more easily than that in the ortho-one. Steric hindrance of the methyl group could conduce the better reactivity of the amino group in the ortho-position than that in the paraposition.19 Furthermore, the basicity of the NH2 group at the 4 position in TDA is stronger than that at the 2 position, which is also favorable for the production of 2.20 Thus, only the yields of 1 and 2 are discussed in this study as one of the criteria to evaluate the performance of the catalyst. The methoxycarbonylation of TDA with DMC was carried out in the presence of various Pb compounds and metal acetate compounds as catalysts. As shown in Table 1, Pb compounds and metal acetates catalysts exhibited different activities. In the reaction, DMC served not only as a reactant but also a solvent since metal acetates could be solubilized in DMC solutions at reaction temperature. In this case, metal cation acted as an electron acceptor with strong Lewis acidity. It is notable that, in the absence of catalyst, no product 1 or 2 was obtained, whereas a side reaction to N-methyltoluene diamine proceeded predominantly. Pb(NO3)2, which has been reported to show a high activity for the methoxycarbonylation of hexylamine,21 shows undesirable selectivities to products 1 and 2, accompanied with a large amount of N-methylated compounds formed (not shown in Table 1). The difference in molecular structure between TDA and hexylamine may lead to the significant difference of catalytic performance with the same catalyst, Pb-

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Figure 1. Effect of reaction temperature on the methoxycarbonylation of TDA with DMC: 9, conversion of TDA; b, yield of 2; 4, yield of 1. Reaction conditions: 0.045 mol of TDA, 0.9 mol of DMC, and 0.9 mmol of Pb(OAc)2 catalyst conducted for 4 h.

Figure 2. Effect of reaction time on the methoxycarbonylation of TDA with DMC: 9, conversion of TDA; O, yield of 2; 2, yield of 1. Reaction conditions: 0.045 mol of TDA, 0.9 mol of DMC, and 0.9 mmol of Pb(OAc)2 catalyst conducted at 443 K.

(NO3)2, since hexylamine is an alkyl amine while TDA is an aryl amine. The acetates of Mg and Ca, especially Mg(OAc)2, also show low catalytic activities for the methoxycarbonylation of TDA with DMC. PbCO3 produces a 36.5% yield of 1 and 89.3% conversion of TDA because it is insoluble in DMC solution. Table 1 also shows that Pb(OAc)2‚3H2O can significantly promote the methoxycarbonylation of TDA as well as the yields of 1 (87.0%). A small amount of ureas were also formed in the presence of Pb(OAc)2‚3H2O. Compared to the Pb(OAc)2‚3H2O, the dehydrated Pb(OAc)2 is even more active with respect to TDA conversions (100%) and yield of 1 (97.7%). Notably, no formation of monocarbamate 2 as well as ureas were observed, suggesting that Pb(OAc)2 was the most active and selective catalyst with respect to TDA conversion and yield of 1 among all the tested catalysts. On the basis of the thermodynamic data calculated by Wang et al.,22 the equilibrium constant of the methoxycarbonylation of TDA with DMC to produce product 1 is 9.10 × 109 at 433 K. This result suggests that 100% TDA should be converted to product 1, which is in good agreement with our experimental results. Another point we need to mention here is that, for those reactions catalyzed by Pb(OAc)2‚3H2O, a significant amount of noncondensing gases was produced, which was identified as

Figure 3. Effect of the molar ratio of DMC/TDA on the methoxycarbonylation of TDA with DMC: 9, conversion of TDA; b, yield of 1; 2, yield of 2. Reaction conditions: 0.9 mol of DMC, 50 of TDA/Pb(OAc)2 molar ratio conducted at 443 K for 4 h.

Figure 4. Effect of the molar ratio of TDA/Pb(OAc)2 on the methoxycarbonylation of TDA with DMC: O, conversion of TDA; 9, yield of 2; 2, yield of 1. Reaction conditions: 0.045 mol of TDA, 0.9 mol of DMC conducted at 443 K for 4 h.

CO2 by GC measurements. On the basis of the screened experiments of DMC and Pb(OAc)2‚3H2O or H2O, respectively, we noticed that, in the present of water, DMC could be hydrolyzed to produce CO2 and methanol. Thus, the existence of crystallization water in Pb(OAc)2‚3H2O resulted in the hydrolyzation of DMC and can further accelerate the production of ureas also reported for Zn(OAc)2‚2H2O.19 These arguments can also be verified by the fact that the selectivity to 1 was significantly improved and no ureas were formed when the Pb(OAc)2 was used as a catalyst. Effect of Reaction Temperature. To examine the effect of reaction temperature on the catalytic performances of TDA with DMC, Pb(OAc)2 (0.9 mmol) was tested at various temperatures using a DMC/TDA molar ratio of 20 for 4 h. Shown in Figure 1 are the changes in the conversions of TDA and yields of products as a function of the reaction temperature. The conversion of TDA was only 57% at 393 K but increased sharply in a narrow temperature range between 393 and 423 K, above which TDA is almost completely converted (100%). The yield of monocarbamate 2 reached a maximum of 53% at 393 K, followed by decreasing gradually with the increase in reaction temperature. Comparatively, the yield of 1 was only 3% at 393

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Figure 5. XRD patterns for catalysts after reaction: (a) obtained from reaction of Pb(OAc)2, DMC, and TDA; (b) obtained from reaction of Pb(OAc)2 and DMC; (c) obtained from reaction of Pb(OAc)2 and TDA; (d) obtained from reaction of methanol, Pb(OAc)2, and DMC; (e) obtained from reaction of methanol, Pb(OAc)2, and TDA; (f) obtained from reaction of Pb(OAc)2‚2PbO‚H2O and DMC; and (g) obtained from reaction of Pb3(CO3)2(OH)2 and acetic acid.

K, while a large amount of 1 was produced upon raising the reaction temperature from 403 to 423 K, reaching a maximum of 97.7% at 443 K. This suggests that low temperature is favorable for the production of 2 from TDA and DMC instead of monocarbamate 2 being further converted into dicarbamate 1. By contrast, a higher reaction temperature is advantageous to the formation of 1. However, on the other hand, a higher reaction temperature leads to the polymerization of 1 and more byproducts caused by the N-methylation reaction. As can be seen in Figure 1, therefore, the yield of 1 was lower at 463 K because of the formation of a significant amount of ureas and then further declined to 36% at 483 K. Effect of Reaction Time. Shown in Figure 2 is the timeon-stream behavior of the methoxycarbonylation of TDA (0.045 mol) with DMC (0.9 mol) over a Pb(OAc)2 catalyst (0.9 mmol) at 443 K. As shown in Figure 2, the conversion of TDA increased sharply in a narrow range from 0.5 to 2 h and remained 100% after 2 h. A similar trend observed in the case of the yield of 1 is that the yield of 1 increased abruptly from 2.8% at 0.5 h to 84% by 2 h and reached a maximum of 97.7% after 4 h. Interestingly, the yield of 2 basically remained at a low level all the time. It increased slowly with the increase of reaction time, and a maximum of 7% was obtained at 1 h, followed by gradually decreasing and approaching to zero after 4 h. These observations imply that the formed monocarbamate further reacted with DMC to dicarbamate 1 with the reaction process. It also suggests that the prolonged reaction time was advantageous to the transformation of 2 into 1 based on the fact that the product 2 disappeared while the yield of 1 reached a maximum at 4 h. Effect of the Molar Ratio of DMC/TDA. The effect of the DMC/TDA molar ratio on the methoxycarbonylation of TDA with DMC was examined by varying the amount of TDA while keeping constant the initial amounts of DMC (0.9 mol) and the TDA/Pb(OAc)2 molar ratio (50). From the results shown in Figure 3, the conversions of TDA were all close to 100% at various DMC/TDA molar ratios. The yield of 1 was up to 90% at the DMC/TDA molar ratio of 5 and continued to increase with increasing molar ratio of DMC/TDA, reaching a maximum of 97.7% at a molar ratio of 20. Comparatively, an opposite

Figure 6. XRD patterns for catalysts after reaction: (a) obtained from reaction of Zn(OAc)2, DMC, and TDA; (b) obtained from reaction of Zn(OAc)2 and DMC; (c) obtained from reaction of Zn(OAc)2 and TDA; (d) obtained from reaction of methanol, Zn(OAc)2, and DMC; and (e) obtained from reaction of methanol, Zn(OAc)2, and TDA. Table 2. Catalytic Activities of the Regeneration Catalyst regeneration times

TDA conversion (%)

yield of 2 (%)

yield of 1 (%)

0 1 2 3 4

100 100 100 100 100

0 0.2 0 0 0.6

97.7 95.2 94.3 95.1 95.3

trend in the yield of 2 is observed in Figure 3, which shows that the yield of 2 decreased consistently with the increase in DMC/TDA molar ratio. Moreover, the amount of byproducts of N-methyltoluene diamine and ureas decreased to near zero with the further increase in DMC/TDA molar ratio, indicating that the relative increase of the amount of DMC could avoid the production of byproducts. Effect of the Molar Ratio of TDA/Pb(OAc)2. The effect of the molar ratio of TDA/Pb(OAc)2 on the conversion of TDA, as well as the yields of 1 and 2, is shown in Figure 4. The reaction was carried out at 443 K for 4 h using a DMC/TDA molar ratio of 20, and the initial amount of TDA was 0.045 mol constantly. As mentioned earlier, the conversion of TDA without the catalyst was only 8.6%, and neither 1 nor 2 was produced while the N-methylation of TDA was the exclusive reaction observed. However, the conversion of TDA remained close to 100% regardless of the molar ratio of TDA/Pb(OAc)2 ranging from 2.5 to 75. The yield of 1 was improved with the increase of the molar ratio of TDA/Pb(OAc)2 and reached a maximum of 97.7% at a molar ratio of 50, while the yields of N-methylated compounds of TDA decreased by further increasing the molar ratio of TDA/Pb(OAc)2 until disappearing at a molar ratio of 50. These observations indicate that an optimized amount of the catalyst is necessary to obtain a high selectivity to the target carbamate based on the amine. Another interesting feature we need to mention here is that the molar ratio of TDA/ Pb(OAc)2 has little effect on the yield of 2. On the basis of the results above, it is notable that the selective formation of either monocarbamate 2 or dicarbamate 1 was associated with the reaction temperature, the reaction time, the molar ratio of DMC /TDA, and the molar ratio of TDA/Pb(OAc)2. Specifically, the reaction temperature is the most influential factor. Generally, longer reaction time and higher

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Scheme 1 Proposed Reaction Mechanism for the Synthesis of 1 over Pb(OAc)2

reaction temperature were necessary to obtain a high TDA conversion rate and a desirable selectivity to the dicarbamate 1. More specifically, compared to the effect of the reaction time, the reaction temperature was more crucial for the methoxycarbonylation of TDA and DMC to prepare 1. Deactivation and Regeneration of Pb(OAc)2 Catalyst. The Pb(OAc)2 catalyst after reaction was obtained according to the procedures described in the Experimental Section. The structure and the chemical state of the catalyst had been investigated by employing XRD techniques, and the patterns are shown in Figure 5a. As shown, no diffraction patterns corresponding to Pb(OAc)2 were observed, whereas XRD peaks of Pb3(CO3)2(OH)2 were detected, suggesting that Pb(OAc)2 was converted into Pb3(CO3)2(OH)2 in the reaction process. The mixture after the reaction was also analyzed by GC-MS measurements. The results showed that a new matter, methyl acetate, was formed during the methoxycarbonylation of TDA with DMC over Pb(OAc)2. On the basis of the experimental results, we speculate that the formation of methyl acetate can be ascribed to the reaction of Pb(OAc)2 and DMC or some other species containing a methyl group. In order to get a better understand of how catalyst behaves and deactivates in the reaction, Pb(OAc)2 was treated with DMC or TDA for 4 h at 443 K, respectively. The resultant solids were separated, and then the phase composition of the samples was confirmed by XRD measurements. Surprisingly, as shown in parts b and c of Figure 5, it is apparent that no change occurred on the Pb(OAc)2 after the pretreatments. Because methanol is formed as a byproduct in the methoxycarbonylation of TDA with DMC, methanol was added to react with Pb(OAc)2 and DMC or TDA, respectively. Again, the obtained solids from the reaction of methanol, Pb(OAc)2 and DMC were separated, and their structures were examined by XRD as shown in Figure 5d. Figure 5d shows that no XRD peaks attributed to Pb(OAc)2 were observed and that for the Pb3(CO3)2(OH)2 was observed instead, which is consistent with the observations of Figure 5a. Additionally, the catalyst after reaction with methanol, Pb(OAc)2, and TDA was characterized, and the results as seen in Figure 5e show that Pb(OAc)2 was transformed into a new phase, Pb(OAc)2‚2PbO‚H2O, which was detected by XRD measurements. Moreover, the mixture after reaction was examined by GC-MS, which shows that water and methyl acetate were formed. It indicates that Pb(OAc)2 reacted with methanol and was converted into Pb(OAc)2‚2PbO‚H2O together with the formation of methyl acetate (as shown in eq 5).

3Pb(CH3COO)2 + 4CH3OH f Pb(CH3COO)2‚2PbO‚H2O + 4CH3COOCH3 + H2O (5)

Additionally, the reaction mixture of the above-mentioned experiment, including Pb(OAc)2‚2PbO‚H2O, was further treated with DMC for 4 h at 443 K. The separated catalyst was examined by XRD, as shown in Figure 5f. Surprisingly, the diffraction patterns attributed to Pb3(CO3)2(OH)2 were detected again, implying that Pb(OAc)2‚2PbO‚H2O was transformed into Pb3(CO3)2(OH)2 because of the reaction between Pb(OAc)2‚ 2PbO‚H2O and DMC (as shown in eq 6).

On the basis of the results of these experiments, therefore, it can be deduced that the deactivation of Pb(OAc)2 catalyst was due to the reaction of Pb(OAc)2, DMC, and methanol to produce Pb3(CO3)2(OH)2 accompanied by the formation of methyl acetate (as shown in eq 7). It also suggests that byproduct methanol play a significant role in the process of catalyst deactivation.

The deactivated catalyst, Pb3(CO3)2(OH)2, was treated with acetic acid for 1 h at 298 K, and the obtained solid was characterized by XRD. As shown in Figure 5g, the species of Pb(OAc)2 appears again, strongly suggesting that Pb(OAc)2 could be regenerated easily through the treatment of acetic acid (as shown in eq 8).

Pb3(CO3)2(OH)2 + 6 CH3COOH f 3Pb(CH3COO)2 + 2CO2 + 4H2O (8) The catalytic activities of the refreshed catalyst were shown in Table 2. The results showed that the TDA conversion remained 100% regardless of the regenerated times while the yield of 1 slightly varied around 95%. It indicates that the refreshed catalyst shows robust activities and selectivity (for product 1) for the methoxycarbonylation of TDA with DMC. The deactivation of Zn(OAc)2, which has been reported to show high activity and selectivity (100% TDA conversion and 98% yield of 1) for the methoxycarbonylation of TDA with DMC to produce dimethyltoluene-2,4-dicarbamate,19,20 was also investigated because of the similar properties between Zn(OAc)2 and Pb(OAc)2. The catalyst after reaction was

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investigated by XRD, and the results are shown in Figure 6a. Similarly, in Figure 6a, no Zn(OAc)2 was observed, whereas XRD peaks of ZnO were detected, suggesting that Zn(OAc)2 was converted into ZnO in the reaction process. Again, methyl acetate was detected in the mixture after reaction by GC-MS measurements. We conducted similar experiments to explore the deactivation of Zn(OAc)2. First, Zn(OAc)2 was treated with DMC or TDA for 4 h at 443 K, respectively, and the resultant catalysts were examined by XRD. The results showed that no change occurred on Zn(OAc)2 after the reaction, as shown in parts b and c of Figure 6, which was consistent with the results from the reactions of Pb(OAc)2 with DMC or TDA. Then, methanol was added to react with Zn(OAc)2 and DMC or TDA. The two solid samples obtained from these experiments were characterized by XRD, and the results are shown in parts d and e of Figure 6, respectively, which show that, in both cases, the ZnO, but not the Zn(OAc)2, was the only species observed. From this point, it could be tentatively concluded that causes for deactivation of Pb(OAc)2 and Zn(OAc)2 are different. The following GC-MS measurements for the reaction mixture also showed that water was produced as well as methyl acetate, suggesting that Zn(OAc)2 reacted with methanol and was converted into ZnO accompanied by the formations of methyl acetate and water (as shown in eq 9). Therefore, it can be deduced that byproduct methanol was crucial for the deactivation of both Pb(OAc)2 and Zn(OAc)2 catalysts.

Zn(CH3COO)2 + 2CH3OH f ZnO + 2CH3COOCH3 + H2O (9) As mentioned eariler, the deactivated Pb(OAc)2 could be regenerated easily through the treatment of acetic acid and the refreshed catalyst could be used without losing any activities. However, ZnO, the product of the deactivated Zn(OAc)2, could not be converted into Zn(OAc)2 simply by the reaction between ZnO and acetic acid. Therefore, from the viewpoint of catalyst regeneration, Pb(OAc)2 was more favorable for the methoxycarbonylation of DMC and TDA to produce dimethyltoluene2,4-dicarbamate. Reaction Mechanism. Fu and Ono9 reported about the reaction of dimethyl carbonate with aniline in the presence of PbO and drew a conclusion that the hydroxyl groups or methoxy groups attached to Pb ions may play a key role in the catalysis. As to the reaction system of methoxycarbonylation of TDA with DMC over Pb(OAc)2, a proposed reaction mechanism for the synthesis of 1 is expressed as follows. The Pb2+ as an electrophilic reagent performs an electrophilic attack on the carbonyl group of DMC and enhances the polarization of the carbonyl group in DMC, which is more strongly activated toward the following nucleophilic attack by the amino group of TDA, as shown in Scheme 1. Additionally, the monocarbamate 2 is formed accompanied with the removal of Pb2+ and methanol. Then, the carbonyl group of DMC driven by Pb2+ continues making an attack on another amino group of monocarbamate 2. With the release of the second Pb2+ and methanol, dimethyltoluene-2,4-dicarbamate 1 is produced. Again, the free Pb2+ continues to perform an electrophilic attack. Conclusions We have presented that 2,4-toluene diamine reacted with dimethyl carbonate to efficiently produce the corresponding dimethylcarbamate ester under mild conditions over Pb(OAc)2. A high yield of 97.7% and a selectivity of 97.7% of dimethyl-

toluene-2,4-dicarbamate were obtained for 4 h at 443 K with a DMC/TDA molar ratio of 20 and a TDA/Pb(OAc)2 molar ratio of 50. Compared to other reaction conditions (such as reaction time, molar ratio of DMC/TDA, and so on), the reaction temperature was the most influential factor for the methoxycarbonylation of TDA and DMC to prepare dimethyltoluene2,4-dicarbamate. It is shown that, because of the reaction of Pb(OAc)2 with dimethyl carbonate and methanol, Pb(OAc)2 was deactivated and was converted into Pb3(CO3)2(OH)2 accompanied by the formation of methyl acetate. Specifically, methanol, the byproduct of the methoxycarbonylation of TDA and DMC, plays a significant role in the catalyst deactivation. Fortunately, the deactivated Pb(OAc)2 could be regenerated easily through the reaction between Pb3(CO3)2(OH)2 and acetic acid and effectively kept high activity and selectivity. A possible reaction mechanism for the methoxycarbonylation of TDA and DMC to produce dimethyltoluene-2,4-dicarbamate was proposed. Acknowledgment The financial support by Natural Science Foundation of China (NSFC) (Grant No. 20506018) and Program of Introducing Talents of Discipline to Universities (Grant No. B06006) and Program for New Century Excellent Talents in University (NCET-04-0242) are gratefully acknowledged. Literature Cited (1) Greene, T. W.; Wuts, P. G. M. ProtectiVe Groups in Organic Synthesis; John Wiley: New York, 1991; p 315. (2) Fritsch, K. C.; Klempner, D. ComprehensiVe Polymer Science; Allen, G., Bevington, J. C., Eds.; Pergamon: New York, 1989; p 413. (3) Wan, B. S.; Liao, S. j.; Yu, D. R. Polymer-Supported PalladiumManganese Bimetallic Catalyst for the Oxidative Carbonylation of Amines to Carbamate esters. Appl. Catal., A 1999, 183, 81-84. (4) Kim, H. S.; Kim, Y. J.; Lee, H.; Lee, S. D; Chin, C. S. Oxidative Carbonylation of Aromatic Amine by Selenium Compounds. J. Catal. 1999, 184, 526-534. (5) Cenini, S.; Crotti, C.; Pizzotti, M. Ruthenium Carbonyl Catalyzed Reductive Carbonylation of Aromatic Nitro Compounds: a Selective Route to Carbamates. J. Org. Chem. 1988, 53 (6), 1243-1250. (6) Gasperini, M.; Ragaini, F.; Cenini, S.; Gallo, E. Carbonylation of Nitrobenzene to N-methyl Phenylcarbamate Catalyzed by PalladiumPhenanthroline Complexes Bifunctional Activation by Anthranilic Acid. J. Mol. Catal., A 2003, 204-205, 107-114. (7) Reddy, N. P.; Masdeu, A. M.; Ali, B. E. Palladium ComplexPotassium Carbonate-Catalysed Reductive Carbonylation of Mono- and Dinitroaromatic Compounds. J. Chem. Soc. Chem. Commun. 1994, 7, 863865. (8) Ariza, X.; Urpi, F.; Vilarrasa, J. A Practical Procedure for the Preparation of Carbamates from Azides. Tetrahedron Lett. 1999, 40 (42), 7515-7517. (9) Fu, Z. H.; Ono, Y. Synthesis of Methyl N-phenyl Carbamate by Methoxycarbonylation of Aniline with Dimethyl Carbonate using Pb Compounds as Catalysts. J. Mol. Catal., A 1994, 91, 399-405. (10) Deleon, R. G.; Kobayashi, A.; Yamauchi, T.; Ooishi, J.; Baba, T.; Sasaki, M.; Hiarata, F. Catalytic Methoxycarbonylation of 1,6-Hexanediamine with Dimethyl Carbonate to Dimethylhexane-1,6-dicarbamate using Bi(NO3)3. Appl. Catal., A 2002, 225, 43-49. (11) Hans-Josef, B.; Heinrich, K.; Wolfgang, R. Process for the preparation of N,O-disubstituted urethanes. EP 48371, 1982. (12) Aldo, B.; Pietro, C.; Fausto, C. Process for the production of aromatic carbamates. EP 752413, 1997. (13) Aldo, B.; Pietro, C.; Emanuele, C.; Ignazio, P. Process for the production of aromatic urethanes. EP 752414, 1997. (14) Aldo, B.; Emanuele, C.; Vittorio, C.; Pietro, C. Process for the synthesis of aromatic urethanes. EP 881213, 1998. (15) Gurgiolo, A. E.; Tex, L. J. Preparation of carbamates from aromatic amines and organic carbonates. U.S. Patent 4,268,683, 1981. (16) Gurgiolo, A. E. Preparation of carbamates from aromatic amines and organic carbonates. U.S. Patent 4,268,684, 1981.

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(17) Vauthey, I.; Valot, F; Gozzi, C.; Fache, F.; Lemaire, M. An Environmentally Benign Access to Carbamates and Ureas. Tetrahedron Lett. 2000, 41 (33), 6347-6350. (18) Curini, M.; Epifano, F.; Maltese, F.; Rosati, O. Carbamate Synthesis from Amines and Dimethyl Carbonate under Ytterbium Triflate Catalysis. Tetrahedron Lett. 2002, 43 (28), 4895-4897. (19) Baba, T.; Kobayashi, A.; Yamauchi, T.; Tanaka, H.; Aso, S.; Inomata, M.; Kawanami, Y. Catalytic Methoxycarbonylation of Aromatic Diamines with Dimethyl Carbonate to their Dicarbamates using Zinc Acetate. Catal. Lett. 2002, 82 (3-4), 193-197. (20) Baba, T.; Kobayashi, A.; Kawanami, Y.; Inazu, K.; Ishikawa, A.; Echizenn, T.; Murai, K.; Aso, S.; Inomata, M. Characteristics of Methoxycarbonylation of Aromatic Diamine with Dimethyl Carbonate to Dicarbamate using a Zinc Acetate Catalyst. Green Chem. 2005, 7, 159165.

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ReceiVed for reView November 30, 2006 ReVised manuscript receiVed June 26, 2007 Accepted July 22, 2007 IE061537+