Synthesis of MDI from Dimethyl Carbonate over Solid Catalysts

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Ind. Eng. Chem. Res. 2002, 41, 5139-5144

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APPLIED CHEMISTRY Synthesis of MDI from Dimethyl Carbonate over Solid Catalysts Xinqiang Zhao,†,‡ Yanji Wang,† Shufang Wang,† Hongjian Yang,† and Jiyan Zhang*,‡ School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin, 300130 P.R. China, and School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072 P.R. China

The catalytic synthesis of methylene diphenyl-4,4′-diisocyanate (MDI) consists of three steps. Starting from the catalytic reaction of aniline and dimethyl carbonate (DMC), methyl phenyl carbamate (MPC) is formed. Then MPC condenses with formaldehyde to produce dimethyl methylene diphenyl-4,4′-dicarbamate (MDC). Last, MDC is decomposed to MDI. For the first step, the properties of supported zinc acetate catalysts on different supports have been examined. Supported zinc acetate catalyst on activated carbon (AC) or R-Al2O3 shows good catalytic properties. Over Zn(OAc)2/AC catalyst, MPC yield reaches 78% and the selectivity is 98%. For the second step, when zinc chloride is used as a catalyst and nitrobenzene as a solvent, MDC yield can reach 87.4%. The catalytic activity of AC-supported ZnCl2 catalyst, which is calculated based on 1 mol of ZnCl2, is much higher than that of homogeneous ZnCl2. For the third step, zinc powder and its organic salts show higher catalytic activity; MDI yield is 87.3% over zinc powder catalyst. 1. Introduction Methylene diphenyl-4,4′-diisocyanate (MDI) is a major raw material for synthesizing polyurethane, which is widely used in manufacturing elastomer, elastic fiber, synthetic leather, and so forth. At present, MDI is mainly produced by the phosgene route, which will be eliminated and replaced in the future because of its toxic feedstock, severely corrosive hydrochloric acid as a byproduct, and the contamination of the remaining chlorine anion to final product. Several substitute routes, that is, phosgene-free routes, have been developed, which consist of three basic steps: synthesis of methyl phenyl carbamate (MPC), condensation of MPC with formaldehyde, and decomposition of the condensation product. As for the first step, there are three technological processes in use: reductive carbonylation of nitrobenzene,1 oxidative carbonylation of aniline,2 and reaction between aniline and alkyl carbonate.3 In the first process, only one-third of CO can be utilized effectively, and there exists difficulty in separation of CO with CO2. In the second process, although CO can be used effectively, the formation of water lowers the atom utility of the reactants. In addition, the operational safety resulting from oxygen must be considered. In both of the above-mentioned processes, severe reaction conditions such as high pressure must be adopted, and a noble metal catalyst such as palladium or rhodium must be used for the carbonylation reactions. The reaction between aniline and alkyl carbonate can proceed under mild conditions, and there exist no such problems as in the two above-mentioned processes, so it is a promising process for the manufacture of MDI. * To whom correspondence should be addressed. Phone/ Fax: 86-22-87892212. E-mail: [email protected]. † Hebei University of Technology. ‡ Tianjin University.

Some works have been published about the synthesis of MPC from aniline and DMC. Gurgiolo4 has prepared MPC over zinc acetate catalyst; the selectivity of MPC to aniline is 99.8% under the conditions of 140 °C and 0.88 MPa. Fu and Ono5 have obtained higher aniline conversion and MPC yield over Pb(OAc)2 catalyst. At a temperature of 400 K for 1 h, the conversion of aniline is 96.7% and the yield of MPC is 95.1%. It can be inferred from above that metal acetates possess much higher catalytic activity. However, they bring about troublesome problems such as product separation and recovery of the homogeneous metal acetate catalyst. As for the condensation of MPC with formaldehyde, Matsunaga and Yasuhara6 have prepared MDC in nitrobenzene solvent over ZnCl2 catalyst; MDC yield and selectivity are 81 and 80%, respectively. However, Matsunaga and Yasuhara have not separated solid MDC from the reaction mixture. Clerici et al.7 use zeolites as solid acid catalysts to synthesize MDC; MPC conversion is only 38%. The decomposition of MDC usually proceeds in the liquid phase over costly catalysts.8 In this paper, the emphasis is put on the development of supported catalysts or solid catalysts for the synthesis of MDI using DMC as raw material instead of phosgene, this work is of great importance for realization of the clean production of MDI. 2. Experimental Section 2.1. Synthesis of MPC. 2.1.1. Catalyst Preparation and Characterization. A series of supported zinc acetate catalysts was prepared by the incipient impregnation method. The used supports include alumina, silica, magnesium oxide, ZSM-5, and AC. The characterization of the supported catalysts was made by means of X-ray diffraction (XRD, Rigaku D/max-2500), X-ray

10.1021/ie020084f CCC: $22.00 © 2002 American Chemical Society Published on Web 09/19/2002

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Figure 1. Flow scheme of the apparatus for MDC decomposition: (1) autoclave; (2) distillation column; (3) first stage cooler; (4) phase separator; (5) second stage cooler; (6) phase separator; (7) collector of byproduct; (8) collector of crude product; (9) pressure gauge; (10) thermometer; (11) flowmeter; (12) valve.

photoelectron spectroscopy (XPS, PHI1600), specific surface area measurement (BET, ASAP 2010), and atomic adsorption (Hitachi, 180-80). 2.1.2. Evaluation of Catalyst Activity. A fournecked flask or an autoclave was used for evaluating the properties of catalysts. The separation process of the reaction mixture included removal of the catalyst, vacuum distillation, crystallization, and so forth. 2.1.3. Analysis of Product. A gas chromatograph (type 102G) with a thermal conductive detector (TCD) was used to analyze methanol concentration in the reaction mixture to indicate the degree of reaction. The chromatograph column was filled with the supported PEG 20000 on a 101carrier, and the column temperature was 95 °C. Hydrogen was used as the carrier gas. The analysis for MPC was carried out on HPLC (Waters 2740) equipped with a Nova-Pak C18 column and UV detector. The mobile phase was methanol-water (volume ratio 60/40) and its flow rate was 0.8 mL/min. 2.2. Synthesis of MDC. 2.2.1. Catalyst Preparation and Characterization. Zinc chloride catalyst was used as purchased in the market; AC-supported zinc chloride was prepared by the incipient impregnation method and then calcined at 160 °C in a N2 stream for 2 h. The characterization of ZnCl2/AC was similar to that of the supported zinc acetate catalyst mentioned above. 2.2.2. Evaluation of Catalyst Activity. Evaluation of catalyst activity was conducted in a four-necked flask equipped with a stirrer and a reflux condenser. A novel and efficient separation process was developed comprising five steps: removing nitrobenzene by steam distillation, adding methanol to crystallize MDC, filtering, washing, and drying. 2.2.3. Analysis of Product. The analysis of MDC was carried out on the same HPLC with the same conditions as those for MPC. 2.3. Decomposition of MDC to MDI. Catalysts were used as purchased in the market. The decomposition of MDC was carried out in a vacuum apparatus shown in Figure 1. During the decomposition process, a nitrogen flow of 10 mL/min continuously flowed into the reactor in place of a stirrer. The operational procedure was as follows: introducing heat carrier into the reactor and beginning the temperature rise; vacuuming the system to the desired degree; introducing corresponding coolant into two coolers,

respectively; slowly adding MDC solution in the mixed solvents into the reactor; weighing the crude product; and analyzing the content of MDI. MDI was analyzed by means of determination of the content of the -NCO group. 3. Results and Discussion 3.1. Synthesis Route. The synthesis route starting from DMC instead of phosgene is adopted. The reaction principle is as follows:

The advantages of the route are as follows: 1 DMC replaces noxious phosgene as a raw material; 2 methanol formed in both reactions (1) and (3) can be recycled to synthesize DMC9; 3 if solid catalysts used have high activity, good selectivity, and long service time, the production of MDI with nearly zero emission can be realized in combination of this route with the DMC synthesis process. Therefore, the key is to develop some effective solid catalysts. 3.2. Catalysts for MPC Synthesis. In addition to reaction (1), there are two main side reactions in this reaction system: that is, N-methylation of aniline to N-methyl aniline (NMA) and dimerization of MPC to diphenyl urea (DPU).

3.2.1. Zn(OAc)2/AC Catalyst. As described earlier, zinc acetate catalyst brings about both troubles in recovery of the catalyst and difficulties in purification of the products. To overcome the above-mentioned drawbacks, a series of supported zinc acetate has been prepared by the incipient impregnation method. First, AC-supported zinc acetate catalyst has been investi-

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Figure 2. Effect of molar ratio of DMC to aniline on MPC yield and selectivity of both MPC and DPU over Zn(OAc)2/AC. Reaction temperature: 150 °C. Reaction time: 4 h. (1) Selectivity of MPC. (2) Yield of MPC. (3) Selectivity of DPU.

Figure 3. Dependence of both the yield and selectivity of MPC on reaction temperature over Zn(OAc)2/AC. Reaction time: 4 h. Molar ratio of DMC to aniline: 7.

gated systematically. Figure 2 shows the effect of molar ratio of DMC to aniline on the yield of MPC and the selectivity of both MPC and DPU. As the molar ratio of DMC to aniline increases, both the selectivity and the yield of MPC increase, but the selectivity of DPU decreases. When the molar ratio is above 7, both the yield and the selectivity of MPC become constant. This fact demonstrates that higher DMC concentration can inhibit the dimerization of MPC effectively. From the point of view of kinetics, high DMC concentration lowers the probability of collision between MPC molecules. From the viewpoint of thermodynamics, because the side reaction is reversible, increasing DMC concentration will shift the equilibrium backward. However, too large a molar ratio of DMC to aniline does not result in a significant increase of MPC yield; in contrast, it brings about the problem of recycling DMC. Therefore, the optimal molar ratio of DMC to aniline is 7. Figure 3 shows the effect of reaction temperature on both the yield and the selectivity of MPC over Zn(OAc)2/ AC at the optimal molar ratio of DMC to aniline. There exists an optimal reaction temperature of 150 °C. In addition, Zn(OAc)2/AC shows no catalytic activity below 110 °C. The reason is that, over Zn(OAc)2/AC, the reactants have to diffuse into the inner pores and onto the active surface of the supported catalyst. Lower temperature makes liquid reactants more viscous and the pore diffusion more difficult. The case changes and the reaction rate are promoted at higher temperature.

Figure 4. XP spectra of Zn 2p3/2 in Zn(OAc)2/AC before and after reaction.

Figure 5. Effect of support on the catalytic properties of supported zinc acetate catalyst. Reaction temperature: 150 °C. Reaction time: 4 h. Molar ratio of DMC to aniline: 7. (1) AC. (2) R-Al2O3. (3) MgO. (4) ZSM-5. (5) SiO2.

XRD patterns of Zn(OAc)2/AC before and after reaction show no diffraction peaks of zinc acetate, which attributes to both lower load and good dispersion of zinc acetate on AC. XP spectra of Zn 2p3/2 in Zn(OAc)2/AC in Figure 4 show that electron binding energy of Zn 2p3/2 decreases as much as 2.25 eV after reaction, which demonstrates that the chemical environment of the zinc atom changes. Interaction of zinc acetate with one or more reaction species results in the increase of electron cloud density around the zinc atom after reaction. In addition, atomic adsorption analysis shows that the weight percentage of the zinc atom decreases from 2.1% before reaction to 1.1% after reaction. This result obviously demonstrates serious leakage of the active species, zinc acetate. 3.2.2. Effect of Supports. Effect of supports on the catalytic properties of supported zinc acetate catalysts has been investigated. The supports include alumina, silica, magnesium oxide, ZSM-5, and AC. Figure 5 shows the reaction result over the different supported catalysts under the reaction conditions: temperature, 150 °C; reaction time, 4 h; and molar ratio of DMC to aniline, 7. The highest selectivity of MPC is obtained over Zn(OAc)2/R-Al2O3 and the lowest one is over Zn(OAc)2/ MgO. The sequence of the selectivity is as follows: R-Al2O3 > AC > ZSM-5 > SiO2 > MgO. In addition, the MPC yield increases progressively in the following

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Figure 6. Dependence of MPC yield and aniline conversion on reaction time over Zn(OAc)2/R-Al2O3 or Zn(OAc)2/AC. Molar ratio of DMC to aniline: 7. Reaction temperature: 150 °C. (1) Conversion of aniline over Zn(OAc)2/AC. (2) Conversion of aniline over Zn(OAc)2/R-Al2O3. (3) Yield of MPC over Zn(OAc)2/R-Al2O3. (4) Yield of MPC over Zn(OAc)2/AC.

Figure 7. Effect of molar ratio of MPC to formaldehyde on MDC yield. Reaction temperature: 100 °C. Reaction time: 3 h. Molar ratio of ZnCl2 to MPC: 0.1.

Table 1. Specific Surface Areas of the Supports

MDC n(MPC):n(HCHO) reaction reaction n(ZnCl2):n(MPC) yield no. (mol/mol) temp (h) time (h) (mol/mol) (%)

supports

specific surface area (m2‚g-1)

R-Al2O3 γ-Al2O3 AC

15.0 213.0 569.0

supports ZSM-5 SiO2 MgO

specific surface area (m2‚g-1) 392.0 298.0 10.0

order: MgO < SiO2 < ZSM-5 < R-Al2O3 < AC. Therefore, R-Al2O3 and AC are the two suitable supports for MPC synthesis. The differences in the catalytic properties of supported zinc acetate catalysts can be partly attributed to the difference in physical properties of the supports. Specific surface areas of the supports are listed in Table 1. Among the supports in Table 1, the specific surface area of AC is the largest and that of MgO the smallest. However, there exists no corresponding relationship between the specific surface area and activity of the catalysts. The interaction between the supports and the active species and the acidity or alkalinity of the supports must be considered. Over Zn(OAc)2/MgO catalyst, the selectivity of MPC is rather lower, while that of DPU becomes 92.2%, which means that the alkaline support benefits the formation of DPU. In contrast, it is difficult to obtain higher MPC yield over the Zn(OAc)2/ ZSM-5 catalyst. For further study on the influence of the acidic support, the catalytic properties of Zn(OAc)2/ R-Al2O3 and Zn(OAc)2/γ-Al2O3 have been compared. In the case of the acidic γ-Al2O3 support, a large amount of NMA and CO2 are formed, which means that the acidic support promotes the N-methylation of aniline. In the case of R-Al2O3 support, little NMA is formed. From the above discussion, it is inferred that neutral support is more profitable to MPC synthesis. Figure 6 shows the relationship between MPC yield or aniline conversion and reaction time over the supported zinc acetate catalysts on R-Al2O3 and AC, respectively. It shows that aniline conversion increases with the prolonging of reaction time. However, there exists a maximum of MPC yield because the accumulated MPC promotes the dimerization of MPC to DPU. At the optimal reaction time of 8 h, the MPC yield becomes 78% and selectivity reaches 98% over Zn(OAc)2/ AC catalyst. 3.3. Catalyst for MPC Condensation. 3.3.1. ZnCl2 Catalyst. To get rid of the corrosion of inorganic acid,

Table 2. MDC Synthesis over Homogeneous ZnCl2 Catalyst

1 2 3 4 5 6 7 8 9

2 4 6 2 4 6 2 4 6

80 80 80 100 100 100 120 120 120

5 3 1 3 1 5 1 3 5

1.5 0.5 2.5 0.5 2.5 1.5 2.5 1.5 0.5

71.3 12.7 48.3 11.9 65.6 74.6 42.3 51.0 87.4

ZnCl2 was used as the catalyst to conduct the condensation reaction of MPC with formaldehyde when nitrobenzene was ultilized as the solvent for MPC. Orthogonal experimental design was used in which the molar ratio of MPC to formaldehyde, reaction temperature, reaction time, and molar ratio of ZnCl2 to MPC were chosen as four influencing factors. The result is listed in Table 2. It can be found that the optimal conditions are as follows: molar ratio of MPC to formaldehyde, 6; reaction time, 5 h; reaction temperature, 120 °C; and molar ratio of ZnCl2 to MPC, 0.5. The highest MDC yield is 87.4%, which is higher than that in the literature.7 Data analysis shows that the effect of the four factors on MDC yield is in the following order: reaction time > molar ratio of ZnCl2 to MPC > molar ratio of MPC to formaldehyde > reaction temperature. 3.3.2. Supported ZnCl2 Catalyst. Homogeneous zinc chloride catalyst may bring about a series of troublesome problems, such as separation of product, recovery of the catalyst, and serious corrosion caused by ZnCl2 plus the formed water. Therefore, supported zinc chloride catalyst has been prepared and its catalytic activity has been evaluated. Nitrobenzene is used as the solvent to improve the contact between the reactants. Figure 7 shows the effect of the molar ratio of MPC to formaldehyde on MDC yield. The highest MDC yield is 17.8% when the molar ratio is 4 under the reaction conditions of 100 °C, 3 h, and molar ratio of ZnCl2 to MPC of 0.1. If the molar ratio is 4, low formaldehyde concentration decreases the reaction rate, which also lowers MDC yield. Figure 8 demonstrates that MDC yield increases with the rising of reaction temperature with the other

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Figure 8. Effect of reaction temperature on MDC yield. Molar ratio of MPC to formaldehyde: 4. Reaction time: 3 h. Molar ratio of ZnCl2 to MPC: 0.1.

Figure 9. XP spectra of Zn 2p3/2 in ZnCl2/AC before and after reaction.

conditions remaining the same except for optimal molar ratio of MPC to formaldehyde. When the temperature rises from 80 to 120 °C, MDC yield increases from 2.3% to 37.2% rapidly. However, it increases slowly over 120 °C and is 42.6% at 140 °C. The effect of reaction time has been studied also and the optimal reaction time is 7 h. It can be seen that MDC yield is only 42.6% over ZnCl2/AC catalyst at 140 °C and 3 h. This result is much lower than that over homogeneous ZnCl2 catalyst. But in view of MDC yield per mole of ZnCl2, this value over ZnCl2/AC catalyst is 2.6 times the highest MDC yield over homogeneous ZnCl2. Similar to Zn(OAc)2/AC, XRD patterns of ZnCl2/AC before and after the reaction show no diffraction peaks of zinc chloride, which is also attributable to both lower load and good dispersion of zinc chloride on AC. XP spectra of Zn 2p3/2 in ZnCl2/AC in Figure 9 show that the electron binding energy of Zn 2p3/2 increases as much as 0.25 eV after reaction, which demonstrates that the chemical environment of zinc atom changes. In addition, the atomic adsorption analysis shows that the weight percentage of the zinc atom decreases from 11% before reaction to 5% after reaction. This result obviously demonstrates serious leakage of the active species, zinc chloride. The reason for loss of zinc chloride may be due to both the vigorous stirring and the weak linkage of the active species to the support.

Figure 10. Effect of catalyst on MDC decomposition. Reaction temperature: 280 °C. Reaction time: 2 h. Reaction pressure: 2.7 kPa. (1) Zinc powder. (2) Zinc acetate. (3) Aluminum powder. (4) Uranyl zinc acetate.

3.4. Decomposition of MDC. The decomposition of MDC to MDI is a highly reversible and endothermal reaction. Owing to the high reactivity of the -NCO group in a MDI molecule, several side reactions can take place inevitably along with the decomposition of MDC. Therefore, to inhibit the side reactions effectively, some measures must be taken: operating under a vacuum; adding inert solvent (also as a heat carrier) in the reaction system; and removing the products as fast as possible to shift the reaction equilibrium in the direction of product and to diminish further reactions of MDI. First, the suitable decomposition conditions, that is, temperature of 280 °C and pressure of 2.7 kPa, have been determined after pre-experiments. Then thermal decomposition of MDC is studied, including the investigation of the effect of solvent and heat carrier. Last, catalytic decomposition of MDC is investigated, including the study on effect of catalyst. 3.4.1. Thermal Decomposition. Solid MDC is dissolved in solvent to feed it continuously into the system. Anhydrous ethanol, THF, and cyclobutylsulfone (CBS) can dissolve MDC effectively. However, ethanol and THF are not suitable for MDC decomposition because of their low boiling points. They evaporate under vacuum too quickly, which results in crystallization of the dissolved MDC onto the feeder wall. Although CBS has a higher boiling point (bp) and ideal solubility for MDC, it is expensive. Therefore, the mixed solvents, which consist of two economic solvents with different bp’s, have been selected. THF is chosen as the solvent with a lower bp while nitrobenzene, methyl benzene, or chlorobenzene is chosen as the solvent with higher bp’s, respectively. The effects of the binary mixed solvents on the MDC decomposition reaction have been studied when liquid paraffin is selected as the inert heat carrier. Two grams of MDC is dissolved in 100 mL of mixed solvents with equal volume ratio and then added into the reactor controlled under suitable reaction conditions. The reaction time is 1.5 h. The result shows that nitrobenzene is the ideal solvent with the higher bp and MDI yield is 52.7%. This may attribute to nitrobenzene having the strongest ability to solve MDC. The effect of di-n-octyl sebacate (DOS) as a heat carrier on MDC decomposition has been studied when nitrobenzene-THF is selected as the mixed solvent.

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Under suitable reaction conditions, MDI yield is 65.9%. Therefore, starting from the viewpoint of MDI yield, DOS is the ideal heat carrier for MDC decomposition. 3.4.2. Catalytic Decomposition of MDC. Figure 10 shows the effect of different catalysts on MDC decomposition when the same reaction conditions as above, liquid paraffin as the heat carrier and THF-nitrobenzene as the mixed solvent, have been used. It can be seen that zinc and some of its organic salts have higher catalytic activity for MDC decomposition. When zinc powder is used as the catalyst, MDI yield reaches 87.3%, which increases 65.7% as compared with the thermal decomposition of MDC. 4. Conclusion For the synthesis of MPC from aniline and DMC, ACsupported zinc acetate has excellent catalytic properties: MPC yield is 78% and its selectivity is 98%. An acidic support can promote N-methylation of aniline while an alkaline support benefits the formation of DPU. For the condensation of MPC with formaldehyde, the highest MDC yield can reach 87.4% over ZnCl2 catalyst, while it is 42.6% over AC-supported ZnCl2 under the conditions of 140 °C, molar ratio of MPC to formaldehyde of 4, molar ratio of ZnCl2 to MPC of 0.1, and reaction time of 4 h. However, MDC yield per mole of ZnCl2 over ZnCl2/AC is 2.6 times higher than the highest MDC yield over homogeneous ZnCl2. For MDC decomposition to MDI, the suitable reaction conditions are 280 °C, 2.7 kPa, DOS as the heat carrier, nitrobenzene-THF as the mixed solvent, and zinc powder as the catalyst; the highest MDI yield is 87.3%.

Acknowledgment This work has been supported by the Chinese National Fund for Natural Science (29976010); the authors are grateful for their contribution. Literature Cited (1) Miyata, K. Direct synthesis of isocyanate. Shokubai 1981, 23 (1), 29. (2) Hinotono, T.; Kamina, N.; Watabe, C. Polyisocyanates. JP Patent 59-172451, 1984. (3) Yagii, T.; Itokazu, T.; Oka, K.; Tanaka, Y.; Kojima, H. Manufacture of isocyanate without phosgene. WO Patent 88/ 05430, 1988. (4) Gurgiolo, A. E. Carbamates from aromatic amines and organic carbonates. U.S. Patent 4268683, 1981. (5) 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. 1994, 91, 399. (6) Matsunaga, F.; Yasuhara, M. Method of condensing Nphenyl carbamates. EP Patent 0410684A2, 1992. (7) Clerici, G. M.; Bellussi, G.; Romano, U. Process for preparation of 4, 4′-diaminodiphenyl-methane and its derivatives. U.S. Patent 5241119, 1993. (8) Sundermann, R.; Konig, K.; Engbert, T.; Becher, G.; Hammen, G. Process for the preparation of polyisocyanates. U.S. Patent 4388246, 1983. (9) Wang, Y. J.; Zhao, X. Q.; Yuan, B. G.; Zhang, B. C.; Cong, J. S. Synthesis of dimethyl carbonate by gas-phase oxidative carbonylation of methanol on supported solid catalyst. Appl. Catal. A 1998, L171, 255-260.

Received for review January 29, 2002 Revised manuscript received July 22, 2002 Accepted July 29, 2002 IE020084F