APPLIED CHEMISTRY Transesterification of Dimethyl

http://www.paper.edu.cn Ind. Eng. Chem. Res. 2004, 43, 4027-4030 4027

APPLIED CHEMISTRY

Transesterification of Dimethyl Oxalate with Phenol under SnO2/SiO2 Catalysts

Shengping Wang, Xinbin Ma,* Jinlong Gong, Xia Yang, Hongli Guo, and Genhui Xu Key Laboratory for Green Chemical Technology, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China

The transesterification of dimethyl oxalate (DMO) with phenol to produce methyl phenyl oxalate (MPO) and diphenyl oxalate (DPO) was carried out over a SnO2/SiO2 catalyst. The evaluation results showed that SnO2/SiO2 had better activity and total selectivity to MPO and DPO than many conventional ester exchange catalysts. Especially, SnO2/SiO2 was in favor of the disproportionation of MPO into DPO compared with TS-1, MoO3/SiO2, and MoO3-SnO2/SiO2 catalysts. The conversion of DMO was 46.7% and the selectivity to DPO was 22.5% at 8% Sn loadings, which was the maximum at different Sn loadings. IR of adsorbed pyridine and NH3- TPD characterization indicated that the desirable catalytic activity of SnO2/SiO2 could be ascribed to its weak Lewis acidity. The structure and chemical state of tin species in SnO2/SiO2 had been investigated by X-ray diffraction. SnO2 active centers dispersed on a SiO2 carrier with a high specific surface area were of benefit to the transesterification.

Introduction large-scale commercial production has been established. Polycarbonates (PCs) are important engineering ther- Also, one of the possible applications of this process is moplastics with excellent mechanical and optical prop- to supply DMO for the preparation of DPC.15 erties as well as electrical and heat resistance proper- For the synthesis of DPC from DMO and phenol, the ties. In recent years, there has been an increasing decarbonylation of DPO to DPC could be carried out demand for safer and more environmentally friendly easily over a PPh4Cl catalyst, and the yield of DPO may processes for PC synthesis. One such process is com- be up to 99.5%.16,17 However, the synthesis of DPO from posed of the synthesis of diphenyl carbonate (DPC) the transesterification of phenol with DMO follows a followed by transesterification between DPC and bisphe- two-step reaction module consisting of the transesteri- nol A.1 However, DPC is prepared commercially by the fication of DMO with phenol into methyl phenyl oxalate reaction of phenol and phosgene in the presence of bases (MPO) and the disproportionation of MPO, as shown in such as sodium hydroxide.2 Therefore, to produce PCs the following reactions. without social effects of pollution, the key technology is the preparation of DPC via a green process. + f Several alternative phosgene-free methods for DPC (COOCH3)2 C6H5OH 3-11 + synthesis have been developed or proposed, e.g., C6H5OOCCOOCH3 CH3OH (4) oxidative carbonylation of phenol and a transesterifi- f + cation reaction. Among them, the transesterification of 2C6H5OOCCOOCH3 (COOC6H5)2 (COOCH3)2 dimethyl oxalate (DMO) with phenol to prepare di- (5) phenyl oxalate (DPO), followed by the decarbonylation of DPO to DPC, as shown in reactions (1) and (2), is an Ube Industries reported that conventional Lewis acid 12,13 catalysts such as AlCl3 and Zn(OAc)2‚2H2O and organic available route. Also, methanol and CO produced in 12,13 the transesterification and the decarbonylation reaction Pb, Sn, or Ti compounds are active for this reaction. is reusable in the DMO production via oxidative carbon- However, the selectivity to MPO and DPO was poor, and ylation of methanol as shown in reaction (3).14 the separation of the catalyst from the reaction system was difficult. Therefore, the development of active solid + f + catalysts is highly desirable in view of regeneration and (COOCH3)2 2C6H5OH (COOC6H5)2 2CH3OH separation. Unfortunately, there are few reports on the (1) development of active heterogeneous catalysts for the f + reaction. TS-1 is a solid catalyst that shows excellent (COOC6H5)2 CO(OC6H5)2 CO (2) selectivity for MPO, but the conversion of DMO and the 1 18 2CO + 2CH OH + / O f (COOCH ) + H O (3) selectivity to DPO are not satisfactory. MoO3/SiO2 and 3 2 2 3 2 2 - A pilot-plant test in DMO production has been MoO3 SnO2/SiO2 catalysts show the favorable conver- completed by Ube Industries, and the technology for sion of DMO. However, the selectivity to DPO is still below 20%.19.20 Therefore, it is of interest to find better catalysts for the transesterification of DMO with phenol. * To whom correspondence should be addressed. Tel.: In this work, we proposed the synthesis of MPO and + + 86-22-27406498. Fax: 86-22-27890905. E-mail: xbma@ DPO from the transesterification of DMO with phenol tju.edu.cn.

over a SnO2/SiO2 catalyst. SnO2/SiO2 exhibited the Table 1. Catalytic Activities of SnO2/SiO2 Catalysts with satisfactory activity and excellent total selectivity to Different Sn Loadingsa MPO and DPO in the transesterification of DMO and Sn loadings DMO convn selectivity (%) yield (%) phenol. Especially, SnO2/SiO2 was in favor of the (wt %) (%) AN MPO DPO MPO DPO disproportionation of MPO into DPO. 1 16.3 1.8 78.6 19.6 12.8 3.2 2 31.6 1.0 78.5 21.5 24.8 6.8 Experimental Section 4 45.2 0.7 78.8 20.5 35.6 9.3 8 46.7 0.8 76.7 22.5 35.8 10.5 Catalyst Preparation. To prepare SnO2/SiO2 cata- 12 38.1 0.7 77.9 21.4 29.7 8.1 lysts, SiO2 samples with various Sn loadingss, given in 16 35.2 0.6 87.1 12.2 30.6 4.3 weight percentage based on metal, were impregnated 8*b 45.8 0.8 77.0 22.2 35.2 10.1 with a solution of dibutyltin dilaurate dissolved in SnO2 2.8 7.2 57.1 35.7 1.6 1.0 SiO 1.7 0 100 0 1.7 0 toluene for 24 h. Then, these pretreated samples were 2 dried in an oven for2hat393Kandcalcined in a muffle a Reaction conditions: 0.1 mol of DMO, 0.5 mol of phenol, 1.8 g furnace at 873 K for 4 h. of catalyst, conducted at 180 °C for 2 h. MPO: methyl phenyl oxalate. DPO: diphenyl oxalate. AN: anisole. b 8*: The results Transesterification of DMO with Phenol. The of SnO /SiO with 8% Sn loadings used repeatedly. transesterification of DMO with phenol was carried out 2 2 in a thee-neck flask (250 mL) equipped with a ther- occurred over SnO2 or SiO2, indicating that the catalytic mometer, a reflux condenser, and a stirrer refluxing at activities of SnO2 and SiO2 were low for the reaction atmosphere pressure. The condensing water of 353 K between DMO and phenol. However, the activity was was used to remove methanol from the reaction system increased significantly when SnO2 was supported on so that the reaction equilibrium was shifted toward the SiO2. The results in Table 1 show that the conversion desired product. The reaction mixture contained 0.1 mol of DMO was increased from 16.3% to 46.7% with an of DMO, 0.5 mol of phenol, and 1.8 g of SnO2/SiO2 increase in Sn loadings from 1% to 8%, while the total catalyst. After the raw materials and the catalyst were selectivity to MPO and DPO was kept at about 99%. placed in the batch reactor, nitrogen gas was flowed at Accordingly, the yield of MPO and DPO was improved. 30 sccm to purge the air from the reaction system. After The conversion of DMO and the yield of MPO and DPO 10 min, the nitrogen flow was stopped and the flask was showed the maxima at 8% Sn loadings. Both the heated at a rate of 10 K min-1. The reaction was conversion and the yield decreased when the Sn load- conducted at 453 K and atmospheric pressure. The ings were over 8%. Moreover, after repeatedly using the qualitative and quantitative analyses of the reaction catalyst, we found that the catalytic activity of 8% SnO2/ products and the distillates were carried out on a HP SiO2 had no significant change, indicating that SnO2/ 5890-HP5971MSD and a HP 5890 gas chromatograph SiO2 was stable for the transesterification reaction. (GC) equipped with a flame ionization detector. An OV- We have also reported that TS-1, MoO3/SiO2, and 101 packed column was used to separate products for MoO3-SnO2/SiO2 catalysts were active for the trans- GC analysis. esterification and the Lewis acid sites were the active Characterization of the SnO2/SiO2 Catalyst. The centers. The total selectivity of MPO and DPO was 99% IR spectroscopic measurements of adsorbed pyridine over TS-1. However, the DMO conversion was only were carried out on a Bruker Vector 22 FTIR spectrom- 26.5%, and the selectivity to DPO was only 10%.18,21 For -1 eter. The scanning range was from 500 to 4000 cm , MoO3/SiO2 and MoO3-SnO2/SiO2 catalysts, the selectiv- and the resolution was 4 cm-1. The sample powder was ity to DPO was 14-20% and 15-18%, respectively.19,20 pressed into a self-supporting wafer. Compared with the two series of catalysts mentioned Powder X-ray diffraction (XRD) measurements were above, the SnO2/SiO2 catalyst was in favor of the conducted on a MAC Science D/Max-2500 X-ray diffrac- disproportionation of MPO into DPO and the selectivity tometer using a radiation source of Cu KR (λ ) 1.5405 to DPO was more than 20%. Å) at 40 kV and 100 mA with a scanning rate of 8° IR Characterization of Adsorbed Pyridine. FTIR -1 min . spectra of adsorbed pyridine on SnO2/SiO2 are shown -1 Temperature-programmed desorption of NH3 (NH3- in Figure 1. An IR band at 1455 cm is evidence of TPD) spectra were recorded using a Micromeritics 2910 pyridine adsorbed on the Lewis acid sites and an IR chemical adsorption spectrometer. Samples were purged band at 1545 cm-1 shows pyridine adsorbed on the with argon at 423 K for 1 h and then cooled to ambient Brønsted acid sites. In Figure 1, each absorption curve temperature. The pulses of ammonia were supplied to shows a significant peak at 1455 cm-1 but no peak at the samples to be saturated. Ammonia was replaced 1545 cm-1. These indicate that many Lewis acid sites with argon, and the sample was heated to 873 K at a are present on SnO2/SiO2, while the Brønsted acid sites - rate of 10 K min 1. are absent. Similar to TS-1and MoO3/SiO2, the Lewis The specific surface areas and the pore-size distribu- acid sites are the active centers for the reaction. tion of the catalysts were determined on a constant- Analysis of XRD. The structure and chemical state volume adsorption apparatus (Chembet 3000) by the N2 of the Sn species in SnO2/SiO2 were investigated by Brunauer-Emmett-Teller method at the liquid-nitro- XRD, and the results are shown in Figure 2. It is gen temperature. The pore-size distribution was calcu- observed that the Sn(IV) species are highly dispersed lated using the theory of Barrett-Joyner-Halenda. on the silica below 2% Sn loadings. At 4% and 8% Sn loadings, small XRD peaks of SnO2 are detected. Then Results and Discussion a great deal of crystalline SnO2 is formed at 16% Sn loadings. DMO conversion decreases at high Sn loadings Catalytic Properties of SnO2/SiO2 Catalysts. The because the catalytic activity of crystalline SnO2 is low catalytic properties of SnO2/SiO2, SnO2, and SiO2 cata- for the transesterification. lysts were performed as shown in Table 1. The conver- Analysis of NH3-TPD. NH3-TPD characterization sion of DMO was very low when the transesterification was conducted to survey the acid strength of SnO2/SiO2. 中国科技论文在线 http://www.paper.edu.cn Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004 4029

Figure 3. NH3-TPD profile of SnO2/SiO2 with different Sn loadings: (a) 1%, (b) 2%, (c) 4%, (d) 8%, (e) 16%. Figure 1. IR spectra of pyridine absorbed on 8% SnO2/SiO2 catalysts.

Figure 4. Pore-size distribution of SnO2/SiO2 with different Sn loadings: (a) 1%, (b) 4%, (c) 8%, (d) 16%.

Figure 2. XRD spectra of SnO2/SiO2 with different Sn loadings: Table 2. Amount of NH3 Desorbed at Low Temperature (a) 1%, (b) 2%, (c) 4%, (d) 8%, (e) 16%. 2: SnO2. 4: SiO2. on SnO2/SiO2 with Different Sn Loadings

Sn loadings amount of desorbed NH3 For the NH3-TPD curves, the peaks are generally distributed into two regions, below and above 673 K, (wt %) (mmol of NH3/g of catalyst) referred to as the low-temperature (LT) and high- 1 0.393 temperature (HT) regions, respectively.22,23 The peaks 2 0.481 4 0.544 in the HT and LT regions can be attributed to the 8 0.594 desorption of NH3 from the strong and weak acid sites, 16 0.655 respectively. From the results shown in Figure 3, it can be found that all of the peaks appeared in the LT region the conversion of DMO decreases at 16% Sn loadings of each absorbed curve and belonged to the weak acid even if the amount of acid sites continues to increase. sites though the strength of the acid sites became Therefore, the catalytic performance, especially DMO stronger gradually with an increase in Sn loadings. conversion, is not directly related with the amount of Therefore, there only existed weak acid sites on SnO2/ the surface acid sites. At 16% Sn loadings, the conver- SiO2. Because the weak acid sites were in favor of the sion of DMO decreases because of the form of SnO2 formation of MPO and DPO,21 the high selectivity to supported on silica transferred from the high dispersion MPO and DPO was obtained over SnO2/SiO2. to the crystalline. Table 2 shows the amount of NH3 desorbed at the LT Pore-Size Distribution and Specific Surface region on SnO2/SiO2 with different Sn loadings. The Area. Figure 4 illustrates the pore-size contribution of data in Table 2 show that the amount of acid sites of SnO2/SiO2 with different Sn loadings, which presents SnO2/SiO2 increases with an increase in Sn loadings, that the pore-size range of SnO2/SiO2 is mainly 25-130 indicating that the new acid sites are formed when SnO2 Å. From this figure, we can also see that the pore with is supported on SiO2. Similar to TS-1, the weak Lewis 25-50 Å increases and the pore with 50-75 Å decreases acid sites are the active centers for the transesterifica- when the Sn loadings are increased from 1% to 4%. 19 tion. The activity of SnO2/SiO2 is increased with an However, the pore-size contribution with 75-330 Å has increase in the acid sites until 8% Sn loadings. However, no impact change. With a further increase of Sn load- 中国科技论文在线 http://www.paper.edu.cn

4030 Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004

Table 3. Surface Area of the Catalyst Samples (4) Fu, Z. H.; Ono, Y. Two-step synthesis of diphenyl carbonate from dimethyl carbonate and phenol using MoO /SiO catalysts. Sn loadings (wt %) 3 2 J. Mol. Catal. A: Chem. 1997, 118, 293. SnO2 SiO2 12481216 (5) Kim, W. B.; Lee, J. S. A new process for the synthesis of diphenyl carbonate from dimethyl carbonate and phenol over A (m2/g) 7.5 231.1 252.4 236.1 236.0 175.8 167.3 125.4 heterogeneous catalysts. Catal. Lett. 1999, 59, 83. (6) Gabriello, I.; Ugo, R.; Renato, T. Aromatic carbonates. Ger. ings, the big pore with 50-330 Å decreases greatly, Offen 2528412, 1976. indicating that the big pore is plugged by pore filling. (7) Akinobu, W.; Yoshiyuki, O.; Hideaki, T. Production of Aryl The data of the specific surface area of catalysts are Ester and Catalyst Used Therefore. JP 11279126, 1999. described in Table 3. When SnO2 is supported on SiO2, (8) Oyevaar, M. H.; To, B. W.; Doherty, M. F. Process for the specific surface area is increased largely compared continuous production of carbonate esters. U.S. Patent 6,093,842, with SnO . The specific surface area of SnO /SiO is 2000. 2 2 2 (9) Hideaki, T.; Yoshiyuki, O.; Atusi, M. Process for preparing 2‚ -1 about 230 m g at low Sn loadings, close to the specific carbonate esters. EP 684221, 1995. surface area of SiO2. However, a great deal of crystalline (10) Chalk, A. J. Aromatic carbonates and polycarbonates. Ger. SnO2 is formed, and the pores are plugged by pore filling Offen 2738520, 1978. at high Sn loadings, resulting in a decrease of the (11) Kongen, K.; Arihito, C.; Kashun, S. Preparation of carbon- specific surface area. SnO2/SiO2 shows higher activity ate compounds by esterification of urea derivatives in high-yield for the transesterification of DMO with phenol com- and high-selectivity. JP 08198815, 1996. pared with SnO because of a significant increase of the (12) Nishihira, K.; Tanaka, S.; Nishida, Y.; Hirofumi, I.; Fujistu 2 S.; Harada, K.; Sugise, R. Process for producing diaryl carbonate. specific surface area. SnO2 active centers dispersed on U.S. Patent 5,834,651, 1998. the surface of the carrier are in favor of the transes- (13) Nishihira, K.; Tanaka, S.; Harada, K.; Sugise, R.; Shiatani, terification. A.; Washio, K. Process for producing diaryl carbonate. U.S. Patent 5,922,827, 1999. Conclusions (14) Matsuzaki, T.; Nakamura, A. Dimethyl carbonate synthesis and other oxidative reactionsusing alkyl nitrites. Catal. Surv. SnO2/SiO2 was the active catalyst for the synthesis Jpn. 1997, 1, 77. of MPO and DPO from the transesterification of DMO (15) Uchiumi, S.; Ataka, K.; Matsuzaki, T. Oxidative reactions - with phenol. Especially, it was in favor of the dispro- by a palladium alkyl nitrite system. J. Organomet. Chem. 1999, 576, 279. portionation of MPO into DPO. The conversion of DMO (16) Katsumasa, H.; Ryoji, S.; Kashiwagi, K. Process for was 46.7%, and the selectivity of MPO and DPO was preparing diaryl carbonate. U.S. Patent 5,892,089, 1998. 76.7% and 22.5% at 8% Sn loadings, which were the (17) Wang, S. P.; Ma, X. B.; Li, Z. H.; Xu, G. H. Synthesis of maxima at different Sn loadings. IR of adsorbed pyri- diphenyl carbonate by decarbonylation of diphenyl oxalate over dine and NH3-TPD characterization indicated that the Ph4PCl. Nat. Gas Chem. Eng. (China) 2002, 27,1. (18) Ma, X. B.; Guo, H. L.; Wang, S. P. Transesterification of desirable catalytic activity of SnO2/SiO2 could be ascribed to its weak Lewis acidity. The surface Sn(IV) dimethyl oxalate with phenol over TS-1 catalyst. Fuel Process Technol. 2003, 83, 275. species with high dispersion were dominant at 2% Sn (19) Ma, X. B.; Gong, J. L.; Wang, S. P. Reactivity and Surface loadings, while crystalline SnO2 was detected at high Properties of Silica Supported Molybdenum Oxide Catalysts for Sn loadings. SnO2 active centers dispersed on the SiO2 the Transesterification of Dimethyl Oxalate with Phenol. Catal. carrier with a high specific surface area were also in Commun. 2004, 5, 101. favor of the transesterification. (20) Gong, J. L.; Ma, X. B.; Yang, X. A Bimetallic Molybdenum- (VI) and Stannum(IV) Catalyst for the Transesterification of Dimethyl Oxalate with Phenol. Catal. Commun. 2004, 5, 179. Acknowledgment (21) Wang, S. P.; Ma, X. B.; Guo, H. L.; Wang, B. W.; Li, Z. H.; Support from National Natural Science Foundation Xu, G. H. Synthesis of Diphenyl Oxalate from Transesterification of Dimethyl Oxalate with Phenol over TS-1 Catalyst. Chin. J. Appl. of China (Grant 20276050) and Tianjin Science and Chem. 2002, 19, 832. Technology Committee (TSTC) of China (Grant (22) Lonyl, F.; Valyon, J. On the interpretation of the NH3- 030103511) is very much appreciated. TPD patterns of H-ZSM-5 and H-mordenite. Microporous Meso- porous Mater. 2001, 47, 293. Literature Cited (23) Sawa, M.; Niwa, M.; Murakami, Y. Relationship between Acid Amount and Framework Aluminum Content in Mordenite. (1) Ono, Y. Dimethyl carbonate for environmentally benign Zeolites 1990, 10, 532. reactions. Pure Appl. Chem. 1996, 68, 367. (2) Janatpour, M.; Shafer, S. J. Process for making diaryl Received for review October 31, 2003 carbonates. EP 228672, 1987. Revised manuscript received April 29, 2004 (3) Kim, W. B.; Lee, J. S. Gas-Phase Transesterification of Accepted May 23, 2004 Dimethylcarbonate and Phenol over Supported Titanium Dioxide. J. Catal. 1999, 185, 307. IE034227P