Chinese Journal of Catalysis 35 (214) 143 153 催化学报 214 年第 35 卷第 7 期 www.chxb.cn available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/chnjc 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 [1] 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 [2] 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: +86 28 85255; Fax: +86 28 8522713; E mail: zhiyongdeng@cioc.ac.cn # Corresponding author. Tel: +86 28 85255; Fax: +86 28 8522713; E mail: gywang@cioc.ac.cn 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/S1872 267(14)42 5 http://www.sciencedirect.com/science/journal/1872267 Chin. J. Catal., Vol. 35, No. 7, July 214
144 Fubao Zhang et al. / Chinese Journal of Catalysis 35 (214) 143 153 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 [13], 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 [41]. 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 MoO3. 2.2. 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 = 1486.6 ev) X ray source, and the binding energies were corrected using the C 1s peak at 284.6 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. 2.3. 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.
Fubao Zhang et al. / Chinese Journal of Catalysis 35 (214) 143 153 145 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 3.1.1. 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) 18 15 12 9 3 SiO 2 1% MoO 3/SiO 2-TS 1% MoO 3/SiO 2-C..2.4.6.8 1. 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) SiO2 198.96 21.2 1%MoO3/SiO2 TS 89.25 11.2 1%MoO3/SiO2 C 135.9 26.7 a BET specific area. b Average pore diameter calculated by the BJH method. 15 3 45 75 2 /( o ) Fig. 2. XRD patterns of catalysts SiO2 (1), 1%MoO3/SiO2 TS (2), and 1%MoO3/SiO2 C (3). 3.1.2. 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 3 15 3 45 75 2 /( 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%.
146 Fubao Zhang et al. / Chinese Journal of Catalysis 35 (214) 143 153 3.1.3. 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 [4]. 3.1.4. 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 235.7 and 235.6 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. 819 3.1.5. 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 [22]. 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. 3.1.6. 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 192.6 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 666 995 (4) (3) (2) (1) 7 8 9 1 11 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. 17 165 1 155 15 145 14 Wavenumber (cm 1 ) Fig. 5. IR spectra of pyridine absorbed on the catalysts. (1) 1%MoO3/ SiO2 C; (2) 1%MoO3/SiO2 TS. Intensity 192.6 19.9 (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 235.7 232.6.63.24 1%MoO3/SiO2 TS 235.6 232.6.82.33 a Calculated from XPS data. (1) 1 2 3 4 5 Temperature ( o C) Fig. 6. NH3 TPD profiles of 1%MoO3/SiO2 C (1) and1%moo3/sio2 TS (2).
Fubao Zhang et al. / Chinese Journal of Catalysis 35 (214) 143 153 147 3.2. Catalytic performance 3.2.1. 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. [22] 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. 8 4 2 Yield of MPO Yield of DPO 1 8 4 2 Selectivity for MPO Selectivity for DPO 4 8 12 16 2 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. 3.2.2. 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]. 3.3. Optimization of reaction conditions on the 1%MoO3/SiO2 TS catalyst 3.3.1. 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. 3.3.2. Effect of reaction temperature The effect of reaction temperature on the transesterification 1 8 4 2 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...3.6.9 1.2 1.5 1.8 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.
148 Fubao Zhang et al. / Chinese Journal of Catalysis 35 (214) 143 153 1 1 8 4 2 Selectivity for MPO Selectivity for DPO 8 4 2 Selectivity for MPO Selectivity for DPO 14 15 1 17 18 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. 3.3.3. 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. 1 8 4 2 Selectivity for MPO Selectivity for DPO.5 1. 1.5 2. 2.5 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. 1 2 3 4 5 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. 3.3.4. 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 [1] 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 [2] 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 [3] Andraos J. Pure Appl Chem, 212, 84: 827 [4] Gong J L, Ma X B, Wang S P. Appl Catal A, 27, 316: 1 [5] 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 [6] Kanega R, Hayashi T, Yamanaka I. ACS Catal, 213, 3: 389
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15 Fubao Zhang et al. / Chinese Journal of Catalysis 35 (214) 143 153 热扩散法制备 MoO 3 /SiO 2 催化草酸二甲酯和苯酚酯交换反应 张付宝 a,c, 余晓鹏 b, 马飞 a,d, 杨先贵 a,d, 胡静 a,d, 邓志勇 a,d,* a,d,#, 王公应 a 中国科学院成都有机化学研究所, 四川成都 6141 b 四川理工学院材料与化学工程学院, 四川自贡 643 c 中国科学院大学, 北京 149 d 中国科学院成都有机化学有限公司, 四川成都 6141 摘要 : 采用热扩散法 (TS) 和等体积浸渍法制备了 MoO 3 /SiO 2 催化剂用于草酸二甲酯和苯酚酯交换反应. 结果表明, 热扩散法制备的 1%MoO 3 /SiO 2 -TS 催化剂较等体积浸渍法制备的 1%MoO 3 /SiO 2 -C 催化剂具有更好的催化性能. 运用 X 射线衍射 Raman 光谱 X 射线光电子能谱分析 吡啶吸附红外光谱 NH 3 程序升温脱附等手段对催化剂进行了表征, 发现虽然两种方法制备的催化剂都只有弱 Lewis 酸中心, 钼均以氧化钼单体形式存在, 未形成解离和聚合, 但是 1%MoO 3 /SiO 2 -TS 催化剂较 1%MoO 3 /SiO 2 -C 催化剂表面钼含量更高且 MoO 3 分散得更好. 在苯酚用量为.2 mol, 1%MoO 3 /SiO 2 -TS 催化剂用量为 1.2 g, 反应温度为 18 C, 草酸二甲酯与苯酚的摩尔比为 2, 反应时间为 4 h 的优化条件下, 苯酚转化率可达 7.9%, 甲基苯基草酸酯和草酸二苯酯的收率分别达 63.1% 和 7.7%. 关键词 : 热扩散法 ; MoO 3 /SiO 2 催化剂 ; 甲基苯基草酸酯 ; 草酸二苯酯 ; 酯交换 收稿日期 : 213-12-7. 接受日期 : 214-1-2. 出版日期 : 214-7-2. * 通讯联系人. 电话 : (28)85255; 传真 :(28)8522713; 电子信箱 : zhiyongdeng@cioc.ac.cn # 通讯联系人. 电话 : (28)85255; 传真 :(28)8522713; 电子信箱 : gywang@cioc.ac.cn 基金来源 : 国家科技支撑计划 (213BAC11B5); 四川省青年科技创新团队专项计划 (213TD1). 本文的英文电子版由 Elsevier 出版社在 ScienceDirect 上出版 (http://www.sciencedirect.com/science/journal/1872267). 1. 前言碳酸二苯酯 (DPC) 作为一种重要的有机化工中间体 [1], 主要用作合成聚碳酸酯 异氰酸酯及农药 医药等. 近年来, 随着我国经济的高速发展, 国内对 DPC 的需求不断增加. DPC 的合成最初是采用苯酚与光气或氯甲酸甲酯反应, 但光气及氯甲酸甲酯均为剧毒, 存在严重的环境和安全问题 [2]. 上世纪 7 年代以来, 国内外陆续开发了多条非光气清洁生产工艺路线 [3 1], 其中日本 Ube 公司开发的利用草酸二甲酯 (DMO) 或草酸二乙酯和苯酚先进行酯交换合成草酸二苯酯 (DPO), 然后 DPO 再脱羰生成 DPC 这一工艺路线颇具吸引力. 该过程具有反应条件温和 产物易分离及生成的 CO 可回收合成原料草酸二甲酯或草酸二乙酯等优点. DPO 脱羰制备 DPC 较易进行, 催化剂选择性几乎为 1%, DPC 收率可达 95% 以上 [11,12]. 因此, 酯交换合成 DPO 受到更多关注. 以草酸二甲酯为例, 一般认为草酸二甲酯和苯酚酯交换过程分为两步 : 首先, 草酸二甲酯与苯酚生成甲基苯基草酸酯 (MPO); 其次, 甲基苯基草酸酯发生歧化或者进一步与苯酚酯交换生成草酸二苯酯, 有关反应方程式见图式 1. 酯交换合成草酸二苯酯催化剂可分为均相催化体 系和多相催化体系. 均相催化体系主要包括钛酸四丁酯 氯化铝及硫酸 磷酸等 [13], 多相催化体系主要包括分子筛及改性分子筛 负载型金属氧化物 多孔碳材料和类水滑石等 [14 32]. 其中, 负载型三氧化钼催化剂由于具有较好的催化性能引起了研究者广泛关注 [13,22 3]. 传统上制备负载型 MoO 3 催化剂多采用浸渍法. 然而, 浸渍法也存在着一些缺陷, 比如干燥过程中难以保持均匀, 在后期焙烧过程中会产生氨气, 制备的催化剂易出现活性物种团聚 [33,34]. 热扩散法 (TS) 不需要任何溶剂且不产生氨气, 能有效降低催化剂活性物种的团聚. 该法制备的负载型 MoO 3 和 V 2 O 5 等催化剂广泛应用在烯烃复分解 加氢脱硫及醇的氧化等反应中 [33 4]. 截止目前, TS 法制备负载型 MoO 3 催化剂在草酸二甲酯与苯酚酯交换反应体系中尚未见报道. 本文分别采用 TS 法和等体积浸渍法制备了 MoO 3 /SiO 2 催化剂, 并比较了它们的催化性能, 在此基础上优化了草酸二甲酯与苯酚酯交换反应的工艺条件. 2. 实验部分 2.1. 催化剂制备热扩散法 (TS) 制备 MoO 3 /SiO 2 催化剂参照文献 [41]. 将 SiO 2 粉碎成 1 目粉末后, 取一定量与 MoO 3 混合, 研
Fubao Zhang et al. / Chinese Journal of Catalysis 35 (214) 143 153 151 磨 3 min, 置于马弗炉中 12 C 干燥 2 h, 然后加热至 55 C 并保持 6 h. 所得催化剂记为 xmoo 3 /SiO 2 -TS (x 表示 MoO 3 的负载量 ). 对于等体积浸渍法, 配制一定浓度的钼酸铵溶液浸渍 1 目 SiO 2 粉末, 静置过夜, 然后置于马弗炉中 12 C 干燥 2 h, 再加热至 55 C 并保持 6 h. 所得催化剂记为 ymoo 3 /SiO 2 -C (y 表示 MoO 3 的负载量 ). 2.2. 催化剂表征催化剂晶相分析在 Philips X pert PRO MPD 型 X 射线衍射仪 (XRD) 上进行, Cu 靶, K α 辐射源, 管电压 4 kv, 管电流 4 ma, 入射狭缝 (1/6)º, 扫描速度 2 /min, 扫描范围 1 8. 比表面积分析在美国 Quantachrome 公司全自动 NOVA1e 型比表面和孔隙度分析仪上进行, 分析结果采用 BET 法计算. Raman 光谱测试在 InVia 型激光拉曼光谱仪 ( 英国雷尼绍公司 ) 上进行, 激发光波长为 514.5 nm. 样品的 X 射线电子能谱测定在英国 Kratos 公司 XSAM 8 型 X 射线光电子能谱仪 (XPS) 上进行, 以 Al K α (1486.6 ev) 为激发光源, 样品的荷电效应以 C1s (284.6 ev) 为内标加以校正. 催化剂吸附吡啶红外光谱表征在傅里叶红外光谱仪 ( 美国 Nicolet 5) 上进行, 先将脱去表面物理吸附杂质的样品置于含有吡啶蒸气的密闭容器中, 充分吸附后再在真空条件下 18 C 加热 2 h, 冷却后测量. NH 3 -TPD 表征在 TP58 型全自动化学吸附仪 ( 天津先权工贸发展有限公司 ) 上进行, 用高纯氮气 4 C 吹扫 1 h 后降至室温, 在 5 C 化学吸附 NH 3 至饱和, 再切换成氮气以 1 C/min 从 5 C 升温至 C, 然后降至室温, 用热导检测 NH 3 脱附信号. 2.3. 催化剂性能评价草酸二苯酯合成在带有搅拌器和分馏柱的 25mL 三颈烧瓶中进行. 分别将一定量的苯酚 草酸二甲酯和催化剂按比例加入, 通惰性气体保护, 升温到反应温度. 反应结束后的釜液和馏分用带有 FID 检测器的气相色谱 GC112A ( 上海精密科学仪器有限公司 ) 分析, 分离色谱柱型号为 SPB TM -5 (SUPELCO). 3. 结果与讨论 3.1. 催化剂表征结果 3.1.1. 比表面积测试图 1 为载体 SiO 2 和 1%MoO 3 /SiO 2 催化剂的 N 2 吸附 - 脱附曲线. 可以看出, 热扩散法和等体积浸渍法制备的 MoO 3 /SiO 2 催化剂与载体 SiO 2 的吸附 - 脱附等温线形状相似. 根据 IUPAC 吸附等温线的分类标准, 1%MoO 3 / SiO 2 -TS 和 1%MoO 3 /SiO 2 -C 均为 IV 型等温线, 且出现了明显的 H1 型滞后环, 表明两种催化剂仍保持了典型的介孔结构. 表 1 列出了两种不同方法制备的 1%MoO 3 /SiO 2 催化剂比表面积和孔结构参数. 与载体相比, 等体积浸渍法制备的催化剂比表面积和孔体积出现了下降, 表明有 MoO 3 进入并堵塞了载体孔道, 而 TS 法制备的催化剂比表面积和孔体积下降更为明显, 这可能是由于更多的 MoO 3 向载体表面迁移时堵塞了微孔. 3.1.2. XRD 表征图 2 为 TS 法和等体积浸渍法制备的 1%MoO 3 /SiO 2 催化剂 XRD 谱图. 可以看出, 等体积浸渍法制备的 1% MoO 3 /SiO 2 催化剂在 2θ = 12.8º, 23.3º, 25.7º, 27.4º, 33.1º, 33.7º, 39.º, 39.7º, 45.7º, 46.3 º 和 49.2º 等处出现了明显的正交相 MoO 3 (JCPDS 5-58) 特征衍射峰. 而相同负载量经 TS 法制备的催化剂则未出现正交相 MoO 3 的特征衍射峰, 这可能是由于 MoO 3 以高分散形式或者无定形状态分布在载体表面. 图 3 为不同 MoO 3 负载量时 MoO 3 /SiO 2 -TS 催化剂的 XRD 谱图. 可以看出, 当 MoO 3 负载量为 2%, 6% 和 1% 时, 所有催化剂均未观察到正交相 MoO 3 的特征衍射峰, 表明 MoO 3 以高分散形式或无定形形态存在 ; 当 MoO 3 负载量为 14% 时, 有较弱的正交相 MoO 3 特征衍射峰出现 ; 继续增大 MoO 3 负载量至 18% 时, MoO 3 晶粒的衍射峰变得更尖锐, 表明 MoO 3 晶粒变大. 3.1.3. Raman 表征图 4 为 TS 法和等体积浸渍法制备的 MoO 3 /SiO 2 催化剂的 Raman 谱图. 从中可以看出, 两种方法制备的 MoO 3 /SiO 2 催化剂均在 666, 819 和 995 cm 1 处出现了明显的 MoO 3 特征峰 [34,35,4]. 其中 666 和 819 cm 1 为 MoO 3 中 Mo O Mo 桥键振动峰, 995 cm 1 为末端 Mo=O 官能团伸缩峰. 在 874, 959 和 981 cm 1 处并未出现峰, 意味着两种方法制备的催化剂表面活性物种均以 MoO 3 单体形式出现, 未发生解离和聚合 [4]. 3.1.4. XPS 表征表 2 为 TS 法和等体积浸渍法制备的 1%MoO 3 /SiO 2 催化剂表面 XPS 分析结果. 可以看出, TS 法和等体积浸渍法制备的催化剂表面 Mo 的 3d 3/2 结合能分别为 235.7 和 [22,25] 235.6 ev, 而 3d 5/2 结合能均为 232.6 ev, 与文献中报道的 MoO 3 中 3d 3/2 3d 5/2 结合能一致, 无明显位移, 意味着两种方法制备的催化剂中钼都以 MoO 3 形式存在, 与 Raman 表征结果一致. 此外, 表 2 还列出了两种方法制备
152 Fubao Zhang et al. / Chinese Journal of Catalysis 35 (214) 143 153 的 1%MoO 3 /SiO 2 催化剂表面钼含量. 显然, 1%MoO 3 / SiO 2 -TS 催化剂表面 Mo 含量比 1%MoO 3 /SiO 2 -C 催化剂更高, 说明 TS 法有利于 MoO 3 向催化剂载体表面迁移. 结合 XRD 谱图结果, 1%MoO 3 /SiO 2 -TS 催化剂未出现 MoO 3 特征衍射峰, 而 1%MoO 3 /SiO 2 -C 催化剂出现了 MoO 3 晶粒特征衍射峰, 表明 1%MoO 3 /SiO 2 -TS 催化剂中活性组分 MoO 3 较 1%MoO 3 /SiO 2 -C 催化剂分散的更好. 3.1.5. 吡啶吸附 IR 表征吡啶吸附红外光谱可以有效区分催化剂上的酸型. 通常 145 cm 1 附近出现的特征吸收峰为 Lewis 酸中心, 154 cm 1 附近出现的特征吸收峰为 B 酸中心 [22]. 图 5 为 TS 法和等体积浸渍法制备的 1%MoO 3 /SiO 2 催化剂吡啶红外谱图. 可以看出, 两种方法制备的催化剂在 145 cm 1 附近出现吸收峰, 在 154 cm 1 附近没有出现特征吸收峰. 这说明 1%MoO 3 /SiO 2 -TS 和 1%MoO 3 /SiO 2 -C 均产生了 L 酸中心而无 B 酸中心. 3.1.6. NH 3 -TPD 表征在 NH 3 -TPD 曲线中, 低于 4 C 的峰对应着催化剂的弱酸中心, 而大于 4 C 的峰对应着催化剂强酸中心 [13,24]. 图 6 为 1%MoO 3 /SiO 2 催化剂的 NH 3 -TPD 图谱. 可以看出, 1%MoO 3 /SiO 2 -C 催化剂和 1%MoO 3 / SiO 2 -TS 催化剂脱附峰温变化较小, 分别出现在 19.9 和 192.6 C, 意味着两种催化剂都只存在弱 L 酸中心, 无强酸中心出现 [22,3]. 通过对氨脱附的峰面积进行分析, 发现 1%MoO 3 /SiO 2 -TS 和 1%MoO 3 /SiO 2 -C 的氨脱附量约分别为.88 和.92 mmol/g, 即两种催化剂所具有的总酸量相差不大. 3.2. 催化剂性能 3.2.1. 制备方法的影响制备方法对 MoO 3 /SiO 2 催化性能的影响见图 7. 以 MoO 3 作为催化剂时, 苯酚转化率仅为 26.7%. 以钼酸铵为前驱体等体积浸渍法制备的 1%MoO 3 /SiO 2 -C 为催化剂时, 苯酚转化率为 43.8%. 而以 TS 法制备的 1%MoO 3 /SiO 2 -TS 为催化剂时, 苯酚转化率大幅增加到 56.2%, MPO 和 DPO 选择性分别达 89.% 和 1.9%. 催化剂 Raman 表征表明 1%MoO 3 /SiO 2 -TS 催化剂和 1%MoO 3 /SiO 2 -C 催化剂上钼元素均以 MoO 3 单体形式存在, 未发生解离和聚合 ; 吡啶红外光谱和 NH 3 -TPD 表征表明 MoO 3 与 SiO 2 相互作用产生的弱 Lewis 酸中心是草 [22] 酸二甲酯和苯酚酯交换反应的活性中心. 马新宾等 研究发现 MoO 3 /SiO 2 催化剂性能与 MoO 3 的分散状态有 关. 催化剂表面上团聚的 MoO 3 晶粒越多, 越不利于产物 MPO 和 DPO 的生成 [23,28]. 结合催化剂的 XRD 和 XPS 表征结果, 1%MoO 3 /SiO 2 -TS 较 1%MoO 3 /SiO 2 -C 表现出更高的催化性能归因于 1%MoO 3 /SiO 2 -TS 催化剂表面钼含量更高且 MoO 3 分散得更好, 可产生更多的弱酸中心. 3.2.2. MoO 3 负载量对 MoO 3 /SiO 2 -TS 催化剂性能的影响不同 MoO 3 负载量对酯交换反应的影响见图 8. 可以看出, 随着催化剂中 MoO 3 含量增加, 苯酚转化率和 DPO 选择性先逐渐增大然后下降, 而 MPO 选择性则出现完全相反的趋势. 当 MoO 3 负载量为 1% 时, 苯酚的转化率和 DPO 选择性达最大, MPO 选择性为最低, 表明此时有较多的 MPO 转化成 DPO. 与 1%MoO 3 /SiO 2 -TS 催化剂相比, 14%MoO 3 /SiO 2 -TS 和 18%MoO 3 /SiO 2 -TS 催化剂没有表现出更好的催化性能, 可能是由于 MoO 3 负载量过高时, 催化剂表面形成的正交相 MoO 3 晶粒一定程度上覆盖了原有的活性位. 类似的现象在 TiO 2 /SiO 2 催化剂体系 MoO 3 /γ-al 2 O 3 催化体系和泥浆浸渍法制备的 MoO 3 /SiO 2 催化体系中也有报道 [21,23,27,28]. 3.3. 1%MoO 3 /SiO 2 -TS 催化酯交换反应条件优化 3.3.1. 催化剂用量图 9 为 1%MoO 3 /SiO 2 -TS 催化剂用量对草酸二甲酯与苯酚酯交换反应的影响. 可以看出, 随催化剂用量从.3 g 增加到 1.2 g, 苯酚的转化率由 33.6% 增加到 56.2%, DPO 选择性由 2.8% 增加至 1.9%, 而 MPO 选择性则由 97.1% 降至 89.%, 酯交换总选择性在 99.8% 以上. 当催化剂用量为 1.5 g 时, 苯酚转化率反而稍有下降, 但酯交换总选择性基本保持不变. 这表明较适宜的催化剂用量为 1.2 g. 3.3.2. 反应温度图 1 为反应温度对酯交换的影响. 可以看出, 当温度为 14 C, 酯交换几乎不能进行. 随着体系反应温度逐渐升高, 苯酚转化率和 DPO 选择性急剧增大, MPO 选择性不断下降, 说明提高反应温度有利于酯交换反应和 MPO 歧化反应的进行, 这与酯交换和歧化反应均为吸热反应相吻合. 当反应温度为 18 C, 苯酚转化率和产物 MPO 和 DPO 收率达最大值. 因此, 适宜的酯交换反应温度为 18 C. 3.3.3. 草酸二甲酯与苯酚摩尔比图 11 为原料中 DMO 与苯酚摩尔比对酯交换反应的影响. 可以看出, 当草酸二甲酯过量时, 随着 DMO 与苯酚摩尔比由.5 增大到 2, 产物 MPO 和 DPO 的选择性变化
Fubao Zhang et al. / Chinese Journal of Catalysis 35 (214) 143 153 153 不明显, 但苯酚转化率明显提高, 表明 DMO 用量增加有利于苯酚的转化 ; 继续增大摩尔比到 2.5 时, 苯酚转化率增加较小. 因此, 适宜的草酸二甲酯与苯酚摩尔比为 2. 此时, 苯酚转化率可达 65.6%, MPO 和 DPO 的选择性分别为 89.3% 和 1.6%. 3.3.4. 反应时间图 12 为反应时间对酯交换反应的影响. 可以看出, 随着反应时间的延长, 苯酚转化率逐渐增加, 而 MPO 和 DPO 选择性均变化较小, 表明延长反应时间有利于酯交换的进行而对 MPO 歧化反应影响不大. 反应 4 h 时, 苯酚转化率已达 7.9%, MPO 和 DPO 收率分别达 63.1% 和 7.7%. 继续延长反应时间至 5 h 时, 苯酚转化率增加幅度较小. 因此, 较佳的反应时间为 4 h. 4. 结论与等体积浸渍法相比, 热扩散法制备的 1%MoO 3 / SiO 2 -TS 催化剂表面钼含量更高且活性组分 MoO 3 分散得更好, 在草酸二甲酯与苯酚酯交换反应中表现出更好的催化性能. 在优化的工艺条件下, 苯酚转化率可达 7.9%, MPO 和 DPO 收率分别达 63.1% 和 7.7%. 热扩散法处理过程简单, 绿色环保, 为开发负载型 MoO 3 催化剂提供了选择, 具有重要的意义.