1 Chinese Journal of Catalysis 35 (214) 催化学报 214 年第 35 卷第 7 期 available at journal homepage: Article Transesterification of dimethyl oxalate with phenol over a MoO3/SiO2 catalyst prepared by thermal spreading Fubao Zhang a,c, Xiaopeng Yu b, Fei Ma a,d, Xiangui Yang a,d, Jing Hu a,d, Zhiyong Deng a,d, *, Gongying Wang a,d,# a Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 6141, Sichuan, China b Department of Material and Chemical Engineering, Sichuan University of Science and Engineering, Zigong 643, Sichuan, China c University of Chinese Academy of Sciences, Beijing 149, China d Chengdu Organic Chemicals Co., Ltd., Chengdu 6141, Sichuan, China A R T I C L E I N F O A B S T R A C T Article history: Received 7 December 213 Accepted 2 January 214 Published 2 July 214 Keywords: Thermal spreading method MoO3/SiO2 catalyst Methyl phenyl oxalate Diphenyl oxalate Transesterification MoO3/SiO2 catalysts for the transesterification of dimethyl oxalate (DMO) with phenol were prepared by both the thermal spreading (TS) and incipient wetness impregnation methods. The results showed that the 1%MoO3/SiO2 catalyst prepared by TS (1%MoO3/SiO2 TS) exhibited higher catalytic performance compared with the 1%MoO3/SiO2 catalyst prepared by incipient wetness impregnation (1%MoO3/SiO2 C). The catalysts were characterized by X ray diffraction, Raman spectroscopy, X ray photoelectron spectroscopy, pyridine IR spectroscopy, and NH3 temperatureprogrammed desorption. These analyses indicated that weak Lewis acid sites were formed on the catalyst surfaces and that the Mo species were present as monomeric MoO3 rather than as isolated molybdenum oxide or polymolybdate species on both catalysts, although the 1%MoO3/SiO2 TS exhibited better dispersion of MoO3 and a higher surface Mo content than the 1%MoO3/SiO2 C. Under the optimal transesterification reaction conditions (1.2 g 1%MoO3/SiO2 TS, T = 18 C, n(dmo)/n(phenol) = 2, t = 4 h), the conversion of phenol was 7.9%, and the yields of methyl phenyl oxalate and diphenyl oxalate were 63.1% and 7.7%, respectively. 213, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved. 1. Introduction Diphenyl carbonate (DPC) is an important organic intermediate  primarily used to synthesize polycarbonates, isocyanates, pesticides, and pharmaceuticals. In recent years, the demand for DPC has increased significantly in conjunction with rapid developments in various economies. DPC was originally synthesized from phenol using phosgene or methyl chloroformate. The high toxicity of phosgene and methyl chloroformate, however, has caused serious environmental and safety issues over the years  and, since the 197s, several environmentally benign phosgene free synthesis routes have been explored and developed [3 1]. Among these, an attractive option is the transesterification of phenol with dimethyl oxalate (DMO) or diethyl oxalate (DEO) to diphenyl oxalate (DPO) followed by decarbonylation of DPO to produce DPC, as developed by Ube Industries Ltd. This process has several advantages, including mild reaction conditions, * Corresponding author. Tel: ; Fax: ; E mail: # Corresponding author. Tel: ; Fax: ; E mail: This work was supported by the National Key Technology R&D Program (213BAC11B5) and the Special Project for the Outstanding Youth Innovation Team of Sichuan Province (213TD1). DOI: 1.116/S (14)42 5 Chin. J. Catal., Vol. 35, No. 7, July 214
2 144 Fubao Zhang et al. / Chinese Journal of Catalysis 35 (214) easy separation of products, and the potential to recycle the CO generated during the reaction to synthesize DMO or DEO. The decarbonylation of DPO to produce DPC is easily accomplished, and selectivity for DPC of nearly 1% with an accompanying yield as high as 95% has been reported [11,12]. For these reasons, the synthesis of DPO has been widely studied. The transesterification synthesis of DPO from DMO generally occurs via a two step reaction process consisting of the transesterification of DMO with phenol into methyl phenyl oxalate (MPO), followed by further transesterification or disproportionation of the MPO to DPO, as shown in Scheme 1. Catalyst systems for the phosgene free synthesis of DPO can be divided into homogeneous and heterogeneous systems. The homogeneous systems generally include Ti(OBu)4, AlCl3, H2SO4, and H3PO4 , while the heterogeneous systems involve regular or modified molecular sieves, supported metal oxides, N doped nanoporous carbon materials, and hydrotalcites [14 32]. Among these, the supported MoO3 catalysts have attracted significant attention because of their excellent catalytic performance [13,22 3]. Traditionally, supported MoO3 catalysts are prepared by impregnation of the support with a molybdenum salt solution, usually ammonium heptamolybdate, followed by drying and calcination. However, this preparation method also entails certain challenges, such as uneven drying, ammonia emissions during the calcination process, and deposition of MoO3 crystals on the surface of the support due to agglomeration of Mo species [33,34]. The thermal spreading method (TS), which does not use solvent and does not produce gaseous emissions, can effectively inhibit the agglomeration of active species. The supported MoO3 or V2O5 catalysts prepared by this method are widely used in the metathesis of alkenes, the hydrodesulfurization reaction, oxidation of alcohols, and in other processes [33 4]. To date, however, there have been no reports of the application of MoO3/SiO2 catalysts prepared by the TS method to the transesterification reaction of phenol with DMO. In this work, the catalytic performance of MoO3/SiO2 prepared either by the TS method or the incipient wetness impregnation method was tested in the transesterification of DMO with phenol, and the optimal reaction conditions were investigated. 2. Experimental 2.1. Catalyst preparation The preparation of the MoO3/SiO2 catalyst via the TS method was as follows . SiO2 particles were ground to a 1 mesh size and subsequently mixed with MoO3 in the desired proportion for 3 min. Each mixture was then dried in an oven at 12 C for 2 h and calcined at 55 C for 6 h. The samples are denoted herein as xmoo3/sio2 TS, where x is the percent loading of MoO3. When applying the incipient wetness impregnation method, the MoO3/SiO2 catalyst was prepared by impregnation of SiO2 with an aqueous solution of (NH4)6Mo7O24. 4H2O. The impregnated sample was then dried in an oven at 12 C for 2 h and calcined at 55 C for 6 h. The sample is referred to as ymoo3/sio2 C, where y is the percent loading of MoO Catalyst characterization X ray diffraction (XRD) analysis of the catalysts was performed using a Philips X pert PRO MPD with Cu Kα (4 kv, 4 ma) radiation, with a scan range of 2θ = 1 8 and a scanning velocity of 2 /min. The specific surface areas, total pore volumes, and average pore diameters of the catalysts were determined from N2 adsorption/desorption isotherms at 196 C (Quantachrome). Before each measurement, the sample was degassed under vacuum at 3 C for 3 h. Raman spectra of the catalysts were recorded on a Renishaw Invia Raman microscope using an argon laser (λ = 514 nm) as the excitation source. X ray photoelectron spectroscopy (XPS) was carried out using an XSAM 8 spectrometer (Kratos) with an Al Kα (hν = ev) X ray source, and the binding energies were corrected using the C 1s peak at ev. The infrared (IR) spectroscopic measurements of adsorbed pyridine were performed on a Nicolet 5 Fourier transform IR (FT IR) spectrometer. Prior to each measurement, the catalyst sample was evacuated to remove any physically adsorbed contaminants. After the saturated adsorption of pyridine, the sample was allowed to out gas, and the IR spectrum was recorded at room temperature. NH3 TPD profiles were obtained with a TP58 chemical adsorption spectrometer (Tianjin Xianquan Industry and Trade Development Co., Ltd., China). The catalyst sample was heated to 4 C under a flow of N2 for 1 h and then cooled to room temperature. NH3 adsorption was then carried out at 5 C until the material was saturated. NH3 was replaced with N2, and the sample was heated to C at a rate of 1 C/min while the desorption signal was monitored by TCD Catalytic testing and product analysis The transesterification synthesis of DPO was conducted in a 25 ml glass flask equipped with a thermometer, a distillation H 3 CO C C OCH 3 + OH O C C OCH 3 + CH 3 OH O C C OCH 3 + OH O C C O + CH 3 OH 2 O C C OCH 3 O C C O + H 3 CO C C OCH 3 Scheme 1. Transesterification of DMO with phenol to synthesize DPO.
3 Fubao Zhang et al. / Chinese Journal of Catalysis 35 (214) apparatus, and a stirrer under atmospheric pressure. After the phenol, DMO, and the catalyst were added in the desired proportions, inert gas was introduced to purge the air from the reaction system, and the flask was heated to the specified temperature. The reaction products and distillates were analyzed using a GC 112A gas chromatograph (Shanghai Precision Scientific Instrument Co., Ltd., China) equipped with an SPB TM 5 (Supelco) capillary column and a flame ionization detector. 3. Results and discussion Intensity MoO 3 (3) (2) (1) 3.1. Catalyst characterization Specific surface area measurements The surface areas of the 1%MoO3/SiO2 catalyst and the support material were determined from N2 adsorption desorption isotherms using the BET method. As shown in Fig. 1, the N2 adsorption desorption isotherms of the 1%MoO3/SiO2 TS and 1%MoO3/SiO2 C catalysts both displayed type IV isotherms with an H1 type hysteresis loop, which is typical for a mesoporous structure according to the IUPAC method of classification. The textural properties of the 1%MoO3/SiO2 TS and 1%MoO3/SiO2 C catalysts, as well as the support, are listed in Table 1. Compared with the support material, the 1%MoO3/ SiO2 C catalyst exhibited a decreased specific surface area, likely because the MoO3 species occupied the pores of the support. In the case of the 1%MoO3/SiO2 TS catalyst, however, the extreme decreases in the specific surface area and average pore diameter may be due to a decrease in the number of micropores, because of plugging of pores by migration of additional MoO3 to the surface. Volume adsorbed (cm 3 /g, STP) SiO 2 1% MoO 3/SiO 2-TS 1% MoO 3/SiO 2-C Relative pressure (p/p ) Fig. 1. N2 adsorption desorption isotherms of 1%MoO3/SiO2 catalysts and support. Table 1 Textural characteristics of the 1%MoO3/SiO2 catalysts. Catalyst SBET a (m 2 /g) Vp (cm 3 /g) Dp b (nm) SiO %MoO3/SiO2 TS %MoO3/SiO2 C a BET specific area. b Average pore diameter calculated by the BJH method /( o ) Fig. 2. XRD patterns of catalysts SiO2 (1), 1%MoO3/SiO2 TS (2), and 1%MoO3/SiO2 C (3) XRD patterns of MoO3/SiO2 catalysts The XRD patterns of the 1%MoO3/SiO2 TS and 1%MoO3/ SiO2 C catalysts are shown in Fig. 2. The diffraction peaks observed at 2θ = 12.8, 23.3, 25.7, 27.4, 33.1, 33.7, 39., 39.7, 45.7, 46.3, and 49.2 can be attributed to the presence of an orthorhombic MoO3 phase (JCPDS 5 58) in the 1% MoO3/SiO2 C catalyst. However, no molybdenum phase is evident in the 1%MoO3/SiO2 TS catalyst, indicating that the Mo species were either well dispersed on the SiO2 surface or in a highly amorphous state. The XRD patterns of MoO3/SiO2 TS catalysts with a range of MoO3 loadings between 2% and 18% are presented in Fig. 3. When MoO3 loadings are below 1%, no characteristic peaks associated with an orthorhombic MoO3 phase are observed, indicating that the Mo species were well dispersed or in a highly amorphous state. Diffraction peaks corresponding to the bulk MoO3 phase were however observed in the 14%MoO3/ SiO2 TS catalyst. With increasing levels of MoO3 loading, the diffraction peaks corresponding to the MoO3 phase became both more apparent and sharper as the formulation approaches the 18%MoO3/SiO2 TS catalyst, suggesting an obvious increase in the sizes of the MoO3 crystallites. Intensity MoO /( o ) (6) (5) (4) (3) (2) (1) Fig. 3. XRD patterns of MoO3/SiO2 TS catalysts with different MoO3 loadings (x). (1) ; (2) 2%; (3) 6%; (4) 1%; (5) 14%; (6) 18%.
4 146 Fubao Zhang et al. / Chinese Journal of Catalysis 35 (214) Raman spectra of MoO3/SiO2 catalysts Raman spectra of the MoO3/SiO2 catalysts prepared by the TS and incipient wetness impregnation methods are shown in Fig. 4. All the MoO3/SiO2 catalysts displayed characteristic bands associated with MoO3 at 666, 819, and 995 cm 1 [34,35,4]. Among these, the bands at 666 and 819 cm 1 can be ascribed to Mo O Mo bridge bond vibrations, while the 995 cm 1 band results from the stretching mode of terminal Mo=O groups. There are no bands at 874, 959, or 981 cm 1, which suggests the presence of Mo in the MoO3 monomeric form but not as isolated molybdenum oxide or polymolybdate species  XPS of 1%MoO3/SiO2 catalysts The XPS results obtained for the 1%MoO3/SiO2 catalysts are summarized in Table 2. Both catalysts exhibited the same Mo 3d5/2 binding energies, while the Mo 3d3/2 peak binding energies were only slightly different at and ev. There was therefore no apparent shift in the Mo 3d binding energy [22,25], suggesting that the molybdenum compounds were present only in the (VI) oxidation state as MoO3. This result is in accordance with the information obtained from Raman spectroscopy. In addition, it can be seen that a higher Mo/Si ratio was obtained on the surface of the 1%MoO3/ SiO2 TS catalyst than on the 1%MoO3/SiO2 C material, indicating that the TS method favors the migration of Mo species to the surface. Combined with the XRD results, which showed that no orthorhombic MoO3 phase had formed on the 1%MoO3/SiO2 TS catalyst, it is evident that the MoO3 species exhibited better dispersion in the 1%MoO3/SiO2 TS catalyst than in the 1%MoO3/SiO2 C catalyst Pyridine IR of 1%MoO3/SiO2 catalysts FT IR analysis of adsorbed pyridine allows a clear distinction to be made between types B and L acid sites. In general, the IR band at 145 cm 1 is attributed to pyridine adsorbed on L acid sites, while the band at 154 cm 1 is associated with adsorption on B sites . From Fig. 5, it can be seen that both catalysts exhibit a peak at approximately 145 cm 1, indicating that there were only L acid sites on both NH3 TPD of 1%MoO3/SiO2 catalysts In NH3 TPD curves, desorption peaks are generally found within two regions [13,24]: either below or above 4 C, referred to as the low and high temperature regions, respectively. The peaks in the low temperature region can be attributed to desorption of NH3 from weak acid sites, while the peaks in the high temperature region are due to desorption of NH3 from strong acid sites. As shown in Fig. 6, the desorption peaks of the two catalysts appear at and 19.9 C, suggesting that only weak acid sites were present on both materials [22,3]. Furthermore, the total NH3 desorptions from the 1%MoO3/ SiO2 TS and 1%MoO3/SiO2 C catalysts were approximately.88 and.92 mmol/g (calculated on the basis of the peak integration), respectively. These data suggest that there were no significant differences in the total amounts of acid between the two catalysts. Transmittance (1) (2) Intensity (4) (3) (2) (1) Raman shift (cm 1 ) Fig. 4. Raman spectra of MoO3/SiO2 catalysts. (1) 6%MoO3/SiO2 TS; (2) 1%MoO3/SiO2 TS; (3) 1%MoO3/SiO2 C; (4) 18%MoO3/SiO2 TS Wavenumber (cm 1 ) Fig. 5. IR spectra of pyridine absorbed on the catalysts. (1) 1%MoO3/ SiO2 C; (2) 1%MoO3/SiO2 TS. Intensity (2) Table 2 Mo 3d binding energies and surface element compositions of the 1%MoO3/SiO2 catalysts. Catalyst Binding energy (ev) Mo a Mo/Si Mo 3d3/2 Mo 3d5/2 (mol%) atomic ratio a 1%MoO3/SiO2 C %MoO3/SiO2 TS a Calculated from XPS data. (1) Temperature ( o C) Fig. 6. NH3 TPD profiles of 1%MoO3/SiO2 C (1) and1%moo3/sio2 TS (2).
5 Fubao Zhang et al. / Chinese Journal of Catalysis 35 (214) Catalytic performance Effect of preparation methods on the transesterification reaction The effects of preparation methods on catalytic performance during the transesterification of DMO with phenol are shown in Fig. 7. Compared with the pure MoO3 catalyst, the conversion of phenol increased from 26.7% to 43.8% on the 1%MoO3/SiO2 C catalyst using (NH4)6Mo7O24. 4H2O as the precursor. When the 1%MoO3/SiO2 TS was used as the catalyst, the conversion of phenol reached 56.2% and the selectivities for MPO and DPO were 89.% and 1.9%, respectively. The Raman spectra of the catalysts demonstrated that the element Mo was in the monomeric MoO3 form on the catalysts and that isolated molybdenum oxide species or polymolybdate species were not present. Moreover, the results of pyridine IR and NH3 TPD studies indicated that the 1%MoO3/SiO2 catalyst contained weak Lewis acid sites, suggesting that such sites produced by the interaction of MoO3 and SiO2 were responsible for the transesterification reaction. It was reported by Ma et al.  that the catalytic performance of MoO3/SiO2 catalysts is closely associated with the dispersion state of MoO3. The appearance of agglomerated MoO3 particles in the catalyst is therefore not favorable with regard to the formation of MPO and DPO [23,28]. Based on the results of XRD and XPS analyses, the 1%MoO3/SiO2 TS catalyst exhibited better dispersion of MoO3 and a higher surface Mo content than the 1%MoO3/ SiO2 C catalyst, meaning that it had a greater quantity of weak Lewis acid sites on its surface. Consequently, the 1%MoO3/ SiO2 TS catalyst exhibited enhanced catalytic performance compared with the 1%MoO3/SiO2 C catalyst Yield of MPO Yield of DPO Selectivity for MPO Selectivity for DPO Loading of MoO 3 (%) Fig. 8. Effect of MoO3 loadings on the performance of MoO3/SiO2 TS catalysts. Reaction conditions:.2 mol DMO,.2 mol phenol, 1.2 g catalyst, T = 18 C, t = 3 h Effect of MoO3 loadings on the transesterification reaction The effect of MoO3 loadings on the performance of MoO3/ SiO2 TS catalysts is summarized in Fig. 8. The conversion of phenol and the selectivity for DPO both increased continuously before decreasing as the MoO3 loading went from 2% to 18%. The selectivity for MPO, however, exhibited the opposite trend. Compared with the 1%MoO3/SiO2 TS catalyst, the 14% and 18%MoO3/SiO2 TS catalysts did not show better catalytic performance because the orthorhombic phase MoO3 formed on the surface covered the existing active sites to some extent. Similar phenomena have been reported in TiO2/SiO2, MoO3/γ Al2O3, and MoO3/SiO2 catalysts prepared by the slurry impregnation method [21,23,27,28] Optimization of reaction conditions on the 1%MoO3/SiO2 TS catalyst Effect of catalyst amount The effect of catalyst amount on the transesterification reaction is shown in Fig. 9. The conversion of phenol increased from 33.6% to 56.2%, and the selectivity for DPO also increased from 2.8% to 1.9%, whereas the selectivity for MPO decreased from 97.1% to 89.% when increasing the amount of catalyst from.3g to 1.2 g. The total transesterification selectivity was kept at 99.8%. When the amount of catalyst was 1.5 g, the conversion of phenol decreased slightly, and therefore the optimal amount of catalyst is 1.2 g Effect of reaction temperature The effect of reaction temperature on the transesterification Selectivity for MPO Selectivity for DPO MoO 3 1%MoO 3/SiO 2-C 1%MoO 3/SiO 2-TS Fig. 7. Effect of preparation methods on the performance of catalysts. Reaction conditions:.12 g MoO3,.2 mol DMO,.2 mol phenol, T = 18 C, t = 3 h Amount of catalyst (g) Fig. 9. Effect of catalyst amount on transesterification. Reaction conditions:.2 mol DMO,.2 mol phenol, T = 18 C, t = 3 h.
6 148 Fubao Zhang et al. / Chinese Journal of Catalysis 35 (214) Selectivity for MPO Selectivity for DPO Selectivity for MPO Selectivity for DPO Reaction temperature ( o C) Fig. 1. Effect of reaction temperature on transesterification. Reaction conditions:.2 mol DMO,.2 mol phenol, 1.2 g catalyst, t = 3 h. was studied, and the experimental results are presented in Fig. 1. When the reaction temperature was below 14 C, the transesterification reaction did not proceed to any appreciable extent. The conversion of phenol and the selectivity for DPO, however, both increased sharply with further increases in reaction temperature, suggesting that the transesterification reaction can be improved by operating at higher temperatures, and that the transesterification and disproportionation reactions of MPO are endothermic. The conversion of phenol and the yields of MPO and DPO all reached their maximum values at 18 C, and hence this appears to be the optimal temperature Effect of n(dmo)/n(phenol) The effect of the n(dmo)/n(phenol) ratio on the transesterification was also investigated. As shown in Fig. 11, the selectivities for MPO and DPO changed somewhat as the n(dmo)/ n(phenol) ratio was varied, while the conversion of phenol increased as the ratio was increased from.5 to 2. When the n(dmo)/n(phenol) was further increased to 2.5, the phenol conversion leveled off and so the optimal n(dmo)/n(phenol) ratio is 2. A phenol conversion value of 65.6% along with selectivities of 89.3% and 1.6% for MPO and DPO were obtained when using the 1%MoO3/SiO2 TS catalyst Selectivity for MPO Selectivity for DPO n(dmo)/n(phenol) Fig. 11. Effect of n(dmo)/n(phenol) on transesterification. Reaction conditions:.2 mol phenol, 1.2 g catalyst, T = 18 C, t = 3 h Reaction time (h) Fig. 12. Effect of reaction time on transesterification. Reaction conditions: 1.2 g catalyst,.2 mol phenol, n(dmo)/n(phenol) = 2, T = 18 C Effect of reaction time The transesterification reaction was also monitored while changing the reaction time. As shown in Fig. 12, the conversion of phenol increased significantly while the selectivities for MPO and DPO changed only slightly when the reaction time was increased from 1 to 4 h, suggesting that reaction time was beneficial to transesterification and has little effect on the disproportionation of MPO. After 4 h, the conversion of phenol reached 7.9%, and the yields of MPO and DPO were 63.1% and 7.7%, respectively. Increasing the reaction time to 5 h only slightly improved the conversion of phenol and so the optimal reaction time is 4 h. 4. Conclusions Compared with the 1%MoO3/SiO2 C catalyst, the 1%MoO3/ SiO2 TS catalyst exhibited better dispersion of MoO3, higher surface Mo content, and better catalytic performance during the transesterification of DMO with phenol. Under the optimal reaction conditions, consisting of a catalyst amount of 1.2 g, a reaction temperature of 18 C, a DMO to phenol molar ratio of 2, and a 4 h reaction time, the conversion of phenol was 7.9%, while the yields of MPO and DPO were 63.1% and 7.7%, respectively. The thermal spreading method is both simple and environmentally friendly and therefore will play an important role in future with regard to the preparation of supported MoO3 catalysts. References  Xu K X. Handbook of Fine Organic Chemical Raw Materials and Intermediates. Beijing: Chem Ind Press ( 徐克勋. 精细有机化工原料及中间体手册. 北京 : 化学工业出版社 ), 1998  Mei F M, Li G X, Mo W L. Modern Chem Ind ( 梅付名, 李光兴, 莫婉玲. 现代化工 ), 1999, 19: 13  Andraos J. Pure Appl Chem, 212, 84: 827  Gong J L, Ma X B, Wang S P. Appl Catal A, 27, 316: 1  Wang G Y, Liu S Y, Chen T, Yin X. Fine Chem ( 王公应, 刘绍英, 陈彤, 殷霞. 精细化工 ), 213, 3: 42  Kanega R, Hayashi T, Yamanaka I. ACS Catal, 213, 3: 389
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* - - 100084 Q235B ML15 Ca OH 2 DOI 10. 13204 /j. gyjz201508023 STUDY OF GALVANIC CORROSION SENSITIVITY BETWEEN ANY COUPLE OF STUD WELDMENT OR BEAM Lu Xinying Li Yang Li Yuanjin Department of Civil Engineering
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