Activity and Surface Properties of Titanium Oxide Modified Silica

(DMO) with phenol to produce methyl phenyl oxalate (MPO) and diphenyl oxalate (DPO). The evaluation results showed that MoO3/TiO2-SiO2 catalysts were ...
0 downloads 0 Views 184KB Size
Download PDF
Ind. Eng. Chem. Res. 2007, 46, 1045-1050

1045

Activity and Surface Properties of Titanium Oxide Modified Silica Supported Molybdenum Oxide Catalysts for Transesterification of Dimethyl Oxalate with Phenol Yue Liu, Shengping Wang, and Xinbin Ma* Key Laboratory for Green Chemical Technology, School of Chemical Engineering & Technology, Tianjin UniVersity, Tianjin 300072, People’s Republic of China

In this work, a series of MoO3/TiO2-SiO2 catalysts were tested in the transesterification of dimethyl oxalate (DMO) with phenol to produce methyl phenyl oxalate (MPO) and diphenyl oxalate (DPO). The evaluation results showed that MoO3/TiO2-SiO2 catalysts were more active and selective than both MoO3/SiO2 and TiO2/SiO2 catalysts in the transesterification reaction. Moreover, the catalytic activities of MoO3/TiO2-SiO2 linearly increased with increased TiO2 content up to a dispersion threshold and then decreased. To explore the influence of TiO2 modified silica support on the dispersion and acidic properties of molybdenum oxide species, a series of characterization approaches were performed. The results of XRD, XPS, and BET measurements indicated that the incorporation of dispersed TiO2 could not only enhance the interaction between MoO3 and SiO2 but also promote the dispersion state of MoO3 on the surface of composite support, and further increase the specific surface areas of the catalysts. NH3-TPD measurements demonstrated that the incorporation of amorphous TiO2 generated new weak acid sites on MoO3/TiO2-SiO2 catalysts. In addition, the synergetic effect of MoO3 with TiO2 could be another possible explanation for the increase of DPO selectivity. 2CO + 2CH3OH + (1/2)O2 f (COOCH3)2 + H2O (3)

Introduction Polycarbonates (PCs) are excellent engineering thermoplastics and substitutes for metals and glasses because of their good impact strength and transparency.1,2 Annual market growth for aromatic PCs has been more than 10% from the late 1990s. With increasing demands for safe and clean processes, the hazardous phosgene process has to be improved or essentially replaced by more environmentally friendly or compatible processes.3 One such path is the synthesis of diphenyl carbonate (DPC) followed by the transesterification of DPC with bisphenol A.4 So far, several alternative non-phosgene approaches for DPC synthesis have been explored and developed,5-12 e.g., the oxidative carbonylation of phenol and transesterification reaction. Many patents and papers covering the catalytic oxidative carbonylation of phenol have been issued over the years, but most of them suffer from low phenol conversion and poor diphenyl carbonate selectivity.5-10 On the other hand, dimethyl carbonate (DMC) can be also used as a substitute for phosgene for synthesis of DPC through transesterification with phenol.11,12 Among the developed techniques, the preparation of DPC from dimethyl oxalate (DMO) with phenol via a two-step reaction as shown in reactions 1 and 2 is an attractive and promising alternative to the phosgene route.13,14

(COOCH3)2 + 2C6H5OH f (COOC6H5)2 + 2CH3OH

(1)

(COOC6H5)2 f CO(OC6H5)2 + CO

(2)

In this method, the methanol and CO produced in the transesterification and decarbonylation reactions can be reusable in the DMO production via oxidative carbonylation of methanol as shown in reaction 3.15 * To whom correspondence should be addressed. Tel.: +86-2227406498. Fax: +86-22-27890905. E-mail: [email protected].

A pilot plant for DMO production has been constructed by UBE, and the technology for large-scale commercial production has been established. One of the possible applications of this process is to supply DMO for the preparation of DPC.16 For the synthesis of DPC from the transesterification of DMO with phenol, the decarbonylation of diphenyl oxalate (DPO) to produce DPC can be carried out easily over PPh4Cl catalyst, and the yield of DPC is up to 99.5%.17-19 However, the synthesis of DPO follows a two-step reaction consisting of transesterification of DMO with phenol into methyl phenyl oxalate (MPO) followed by production of DPO via the disproportionation of MPO, as shown in the following reactions.

(COOCH3)2 + C6H5OH f C6H5OOCCOOCH3 + CH3OH (4) 2C6H5OOCCOOCH3 f (COOC6H5)2 + (COOCH3)2

(5)

Supported MoO3 is an important catalyst or catalyst precursor in a number of industrially relevant reactions such as oxidation, dehydrogenation, oxidative dehydrogenation, isomerization, and metathesis.20-24 It is well-known that the type of support plays an important role in the catalytic properties, and for a given reaction the activity and selectivity of the catalyst can be improved by use of an appropriate support oxide.23 Previous studies have focused on alumina, zirconia, active carbon, silica, silica-alumina, etc. as supports for MoO3-based catalysts for the transesterification of DMO with phenol.25-27 Although MoO3/SiO2 showed higher activity and excellent total selectivity to MPO and DPO, low selectivity to DPO was obtained, which motivated part of the work we present here. Over MoO3/SiO2 catalysts, the optimized content of the active component is often close to its monolayer dispersion capacity.25,27 Since the silica surface is quite inert, it is very difficult

10.1021/ie0611812 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/17/2007

1046

Ind. Eng. Chem. Res., Vol. 46, No. 4, 2007

Table 1. Catalytic Activities of MoO3/Ti-Si Catalysts with Different TiO2 Contentsa,b selectivity (%)

yield (%)

catalyst

conversion (%)

AN

MPO

DPO

MPO

DPO

SiO2 8%TiO2/SiO2 MoO3/SiO2c MoO3/2%Ti-Sic MoO3/4%Ti-Sic MoO3/8%Ti-Sic MoO3/10%Ti-Sic MoO3/12%Ti-Sic

1.7 55.2 63.9 66.1 67.7 69.4 65.2 64.3

0.0 0.3 0.4 0.4 0.5 0.7 0.8 0.9

100.0 80.1 84.3 72.9 67.0 60.2 65.7 74.8

0.0 18.6 15.3 26.7 32.5 39.1 33.5 24.3

1.7 44.7 53.9 48.2 45.4 41.8 42.8 48.1

0.0 10.3 9.8 17.6 22.0 26.4 21.8 15.6

a MPO, methyl phenyl oxalate; DPO, diphenyl oxalate; AN, anisole. b Reaction conditions: catalyst 1.8 g; phenol 0.3 mol; PhOH/DMO ) 3.0; reaction time 2 h; reaction temperature 453 K. c MoO3 loading is 16 wt %.

to form highly dispersed metal oxides on the surface. The amount of MoO3 required to form a monolayer on a SiO2 support surface is relatively low, which is probably due to the weak interaction between molybdenum ions and the silica support surface. On the other hand, as a representative of a strong metal-support interaction (SMSI) support, TiO2 shows strong interaction with transition metal oxides.28 Highly dispersed titanium on silica can improve the interaction between MoO3 and SiO2 and increase the dispersion capacity of MoO3 on SiO2 since the interaction between MoO3 and TiO2 is stronger than that between MoO3 and SiO2.29-31 Additionally, supported MoO3 catalysts on TiO2-SiO2 and other composite supports have already been attempted in a number of industrially important catalytic reactions such as hydrodesulfurization of dibenzothiophenes,32 isomerization of butanes,33 dehydration of cyclohexanol,34 and selective catalytic reduction of NOx.35 High activity and selectivity are usually obtained by combining two or more mixed oxides into a composite support. Here, we present our recent investigations on TiO2-SiO2 supported MoO3 catalyst, which has been used for the transesterification of DMO with phenol. TiO2-SiO2 composite supports were prepared by the conventional impregnation method, and the active component MoO3 was deposited by the new slurry impregnation method.36,37 The capability of TiO2SiO2 supported MoO3 catalysts in the synthesis of DPO from DMO and phenol was well exhibited. Moreover, the effect of TiO2 doping on the nature of acid sites was investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), BET, and temperature programmed desorption of ammonia (NH3-TPD) measurements. Experimental Section Catalyst Preparation. Commercial silica (with an average particle size of 4 mm) was used as a support material. Prior to impregnation it was dried at 393 K for 2 h. This treatment results in silica with a surface area of 146 m2/g and average pore size of 100 Å, as measured by nitrogen adsorption. TiO2-SiO2 composite was prepared by a conventional impregnation method using an ethanol solution of Ti(OBu)4 in excess of ethanol. A 15 g sample of the silica support was impregnated with approximate 100 mL of the salt solution until dry. After impregnation, the samples were dried and calcined in a muffle furnace at 393 K for 4 h followed by at 823 K for 5 h. TiO2-SiO2 composites are given in weight percentage based on TiO2 contents and labeled as 2%Ti-Si, 4%Ti-Si, 8%Ti-Si, 10%Ti-Si, and 12%Ti-Si, respectively. MoO3 was ground in an agate mortar before use. Mixtures of the appropriate amounts of MoO3, support, and deionized water were prepared in a set of rotary flasks. Each mixture was heated under a rotary evaporator to 363 K for 24 h. After impregnation, the water was vacuum-removed at 353 K until

Table 2. Surface Areas of Ti-Si and MoO3/Ti-Si Samples support

BET surf. area (m2/g)

catalyst

BET surf. area (m2/g)

SiO2 2%Ti-Si 4%Ti-Si 8%Ti-Si 10%Ti-Si 12%Ti-Si

146 141 145 150 140 135

MoO3/SiO2 MoO3/2%Ti-Si MoO3/4%Ti-Si MoO3/8%Ti-Si MoO3/10%Ti-Si MoO3/12%Ti-Si

72 74 76 81 78 71

the mixture was dry, followed by calcination in a muffle furnace at 433 K for 6 h. MoO3 loading was 16 wt % for each supported MoO3 catalyst. The composition of MoO3 is expressed in weight percent and is based on MoO3 content. The resultant catalysts are named MoO3/Ti-Si and MoO3/SiO2, respectively. For the purpose of convenience, we label MoO3/SiO2 as MoO3/Ti-Si with 0 wt % TiO2 content. Transesterification of DMO with Phenol. The transesterification reaction was conducted in a 250 mL glass flask equipped with a thermometer, distillation apparatus, and a rotor under refluxing condition at atmospheric pressure. The top of the distillation column was kept at 353 K by flowing recycled hot water through it to remove methanol from the reaction system. Thus, the reaction equilibrium limitation in reaction 1 was broken and the reaction was accelerated in the desired direction. The reaction mixture contained 0.1 mol of DMO, 0.3 mol of phenol, and 1.8 g of catalysts. After the raw materials and catalysts were placed into the batch reactor, nitrogen gas was flowed at 30 sccm to purge the air from the reaction system. After 10 min, nitrogen flow was stopped and the flask was heated at a rate of 8 K min-1. The reaction was conducted at 453 K at atmospheric pressure. Quantitative analyses of reaction products and distillates were carried out on a SP3420 gas chromatograph equipped with a flame ionization detector (FID). An HP-5 capillary column (Hewlett-Packard, 15 m × 0.53 mm × 1.5 µm) was used to separate products for gas chromatographic analysis. The products were mainly DPO, MPO, and anisole (AN). An internal standard qualitative analysis method was used where ethyl benzoate was chosen as an internal standard reagent. The conversions were reported on the basis of DMO, the limiting reagent, and were defined as the ratio of the moles of converted DMO to the moles of DMO fed initially to the reactor. The selectivities to MPO and DPO were defined as the moles of MPO and DPO produced per 100 mol of consumed DMO. The yields of MPO and DPO were obtained from multiplication of DMO conversion by the selectivities to MPO and DPO, respectively. Catalyst Characterization. Powder X-ray diffraction crystalline phases were determined at room temperature by use of a powder X-ray diffractometer. A PANalytical X’Pert Highscore (Holland) diffractometer, equipped with a Co KR radiation anode (k ) 1.789 01 Å, 40 kV and 40 mA), was used for these

Ind. Eng. Chem. Res., Vol. 46, No. 4, 2007 1047

Figure 1. XRD spectra of MoO3/Ti-Si catalysts with different TiO2 contents: (A) 0 wt %; (B) 2 wt %; (C) 4 wt %; (D) 8 wt %; (E) 10 wt %; (F) 12 wt %.

Figure 3. Correlation between ITi2p/ISi2p and different TiO2 contents.

(NH3-TPD) was carried out employing a chemical adsorption spectrometer (Model 2910, Micromeritics Co.). The catalysts were heated to 873 K in flowing Ar for 1 h, and then cooled to room temperature. Adsorption of ammonia was carried out at 323 K upon saturation followed by heating of the sample to 873 K at 10 K min-1 to desorb NH3. Results and Discussion

Figure 2. XPS peak intensity ratio of Mo 3d/Si 2p on the MoO3/Ti-Si catalysts with different TiO2 contents.

measurements. Intensity data were obtained by step scanning with a scanning rate of 12° min-1 from 2θ ) 5° to 2θ ) 80°. The specific surface areas of the catalysts were determined on a constant volume adsorption apparatus (CHEMBET-3000) by the N2-BET method at liquid nitrogen temperature. The point surface areas were determined at P/P0 ) 0.1, 0.2, and 0.3. The surface composition and structure of catalyst were studied by X-ray photoelectron spectroscopy (XPS) in a Perkin-Elmer PHI 1600 ESCA system with Mg KR 1253.6 eV radiation as the excitation source. The sample was mounted on the specimen holder by means of double-sided adhesive tape. Spectra were recorded in steps of 0.15 eV. The C 1s peak (284.5 eV) was used as the internal standard for binding energy calibration. An estimated error of (0.1 eV can be assumed for all the measurements. The scanning of the spectra was done at pressures less than 10-8 Torr and the temperature was approximately 293 K. To assess the acid strength and total amount of acid sites of each sample, temperature-programmed desorption of ammonia

Performance of Catalyst. Table 1 presents the comparative results of catalytic activities of pure SiO2, TiO2/SiO2, MoO3/ SiO2, and MoO3/Ti-Si catalysts with different TiO2 contents in the transesterification of DMO with phenol. Therein, Ti-Si gave not only a rather high DMO conversion (55.2%) but also an excellent selectivity to DPO (18.6%) compared to pure SiO2. It is notable that MoO3/Ti-Si catalysts showed higher catalytic efficiencies than both MoO3/SiO2 and TiO2/SiO2 catalyst did. Also, it is obvious that the content of TiO2 had a significant effect on the catalytic efficiencies of MoO3/Ti-Si. At the same experimental conditions, 16%MoO3/8%Ti-Si offered the highest DMO conversion and DPO selectivity compared to 16%MoO3/SiO2 and 8%TiO2/SiO2. In addition, the total selectivities to MPO and DPO were up to 99% for all the MoO3/Ti-Si catalysts. One of the most interesting features we need to address here is that the selectivity to DPO is highly promoted over the MoO3/Ti-Si catalysts, which leads to the high yields of DPO over MoO3/Ti-Si catalysts. In our previous studies, both MoO3/SiO2 and TiO2/SiO2 catalysts exhibited unsatisfactory DPO selectivity for the transesterification of DMO with phenol.25-27,38,39 Here we have shown the DMO conversions for all the results are usually in the range of 64%-69%, while the corresponding selectivities to DPO are 15%-39%. It seems that DMO conversion is not remarkably increasable as the selectivity to DPO is. Therefore, it is deduced that MoO3/ Ti-Si catalysts provide significantly higher selectivity to DPO than both MoO3/SiO2 and TiO2/SiO2 do, which could be ascribed to the synergetic effect between MoO3 and TiO2. The results shown in Table 1 also demonstrate that, in the case of MoO3/Ti-Si catalyst, the DMO conversion increased with the TiO2 content ranging from 2 to 8 wt %, and thereafter decreased to 64.3%. The optimized TiO2 loading is 8 wt %, which provides a DMO conversion of 69.4%. It is notable that the selectivity and yield of DPO increased remarkably with the

1048

Ind. Eng. Chem. Res., Vol. 46, No. 4, 2007

Table 3. Element Surface Contents of MoO3/Ti-Si Catalysts with Different TiO2 Contents element content (atom %) catalyst

O

Si

Mo

Ti

C

MoO3/SiO2 MoO3/2%Ti-Si MoO3/4%Ti-Si MoO3/8%Ti-Si MoO3/10%Ti-Si MoO3/12%Ti-Si

59.4 53.0 42.5 50.8 50.4 45.7

23.9 21.9 17.0 19.3 18.9 17.3

0.7 0.8 0.7 0.9 1.2 1.2

0.0 0.4 0.4 0.7 1.2 1.5

16.0 23.9 39.4 28.3 28.3 34.3

Table 4. Binding Energies of Elements on MoO3/Ti-Si Catalysts with Different TiO2 Contents binding energy (eV) catalyst

Mo 3d5/2

MoO3/SiO2 MoO3/2%Ti-Si MoO3/4%Ti-Si MoO3/8%Ti-Si MoO3/10%Ti-Si MoO3/12%Ti-Si

232.5 232.7 232.7 232.4 232.5 232.7

Ti 2p3/2

Si 2p

459.0 459.0 458.8 459.0 459.0

103.6 103.6 103.6 103.6 103.6 103.6

addition of TiO2 below 8 wt %, and reached maximum values of 39.1% and 26.4%, respectively, with 8 wt % TiO2 content. Interestingly, the selectivity and yield of MPO showed almost an opposite trend compared to those of DPO, indicating that much more MPO was converted into DPO over MoO3/8%TiSi catalyst. Accordingly, most reactivity data show a distinct turning point (at 8 wt % TiO2 content), which may be related to the so-called threshold effect.29 This observation motivated the following work to further explore surface properties of MoO3/Ti-Si catalysts. Powder X-ray Diffractograms. The XRD measurements were undertaken to determine the composition and crystallites of MoO3 on MoO3/SiO2 and MoO3/Ti-Si. In the XRD patterns of all the samples shown in Figure 1, characteristic peaks of crystalline molybdenum oxide were observed for the MoO3/ SiO2 sample with 16 wt % MoO3 loading, which is due to the overloading of MoO3 on SiO2. The appearance of polymolybdate species implies a weak interaction between MoO3 and silica. As for the MoO3/Ti-Si samples, the MoO3 peak intensity was much weaker than those over MoO3/SiO2 samples. It is very likely that highly dispersed Mo species on Ti-Si leads to the decrease of bulk MoO3 species. Furthermore, the molybdenum phase decreased sharply with the increase of TiO2 content ranging from 2 to 8 wt %; thereafter constant MoO3 peak intensity independent of MoO3 loading was observed. Also, Figure 1 shows no characteristic peaks of crystalline TiO2 on MoO3/Ti-Si samples below 8 wt % TiO2 content, indicating that TiO2 was highly dispersed. A small peak of anatase was observed on a MoO3/10%Ti-Si sample, and it got bigger when the TiO2 content was increased to 12 wt %, suggesting that 8 wt % TiO2 is very possibly the uppermost monolayer dispersion capacity. Based on these results, we can speculate that highly dispersed TiO2 on silica can enhance the interaction between MoO3 and SiO2 and thus improve the dispersion state of MoO3 on the surface of silica support. Moreover, increasing the content amount of TiO2 which is below the monolayer dispersion capability can suppress crystallization of MoO3 and thus favor amorphously dispersed MoO3 on the SiO2. However, further deposition of TiO2 may result in the generation of crystalline TiO2, which does not show a significant effect on the interaction between MoO3 and SiO2. Thus, it can be concluded that titanium oxide modified, SiO2-supported MoO3 catalysts favor the transesterification of DMO with phenol and more amorphous MoO3 and TiO2 on the surface of SiO2 makes more catalytic

activity centers. In particular, promotion of disproportionation of MPO into DPO can be ascribed to the synergetic effect of amorphous dispersed MoO3 with amorphous dispersed TiO2. Specific Surface Area Measurement. Table 2 presents the comparative results of surface areas of SiO2, Ti-Si supports, MoO3/SiO2, and MoO3/Ti-Si catalysts. Ti-Si composites exhibited almost the same surface areas as the pure SiO2 does. The surface areas of MoO3/Ti-Si catalysts were slightly larger than that of MoO3/SiO2 catalyst. Based on a previous study by Gong et al.,27 MoO3 is inclined to form crystallites on the surface of SiO2 even at low MoO3 loading because of a weak interaction between the molybdenum ions and the silica support. Therefore the surface areas of MoO3/SiO2 catalysts decrease sharply with the increase of MoO3 loading, and are usually much smaller than that of bare SiO2 at high MoO3 loadings.27 However, as Table 2 shows, it is interesting that the specific surface areas of MoO3/Ti-Si increase with the deposition of TiO2 up to 8 wt %. It is probably due to the addition of dispersed TiO2 leading to the decrease of crystalline MoO3. The surface area of MoO3/ Ti-Si decreased with the further addition of TiO2, which can be ascribed to the growth of crystalline TiO2. Thus, it is deduced that the incorporation of dispersed TiO2 can improve the dispersion state of MoO3 on the surface of SiO2 and further increase the specific area. On the basis of the reactivity tests and surface area measurements in Tables 1 and 2, the activity of catalyst is shown to be strongly dependent on the surface area of the catalyst. The samples with larger surface areas offer more DMO conversion and higher selectivity to DPO. In addition, crystalline TiO2 may fill in small pores of silica and thus reduce the surface area of catalyst, and then suppress the catalytic activity and production of DPO. The dense monolayer dispersion model shows that the interspace occupied by one TiO2 molecule is the same as that occupied by two oxygen anions, if the size of a Ti4+ cation can be negligible compared to that of an O2- anion.29 As the radius of the oxygen anion is 0.14 nm and the average bond length of Ti-O is 0.019 46 nm, it can be calculated that the theoretical monolayer capability of TiO2 on the surface of SiO2 is 0.055 g of TiO2/100 m2 of SiO2. Specifically, for the SiO2 sample with a surface area of 146 m2/g, the monolayer dispersion of TiO2 was approximately 7.5 wt %, which agrees well with our experimental value (8 wt %). X-ray Photoelectron Spectroscopy. To further explore the surface structure of MoO3/Ti-Si catalysts and the dispersion states of MoO3 and TiO2 on SiO2 surface, XPS characterization was carried out. Table 3 summarizes the calculated surface atomic ratios observed on each catalyst sample. As shown in Table 4, the Mo 3d5/2 binding energies of MoO3/SiO2 and MoO3/ Ti-Si catalysts with different TiO2 contents were almost invariant (232.4 ( 0.3 eV), which is the characteristic value of Mo6+ valence and corresponds to the binding energy of MoO3. Shown in Figure 2 is XPS peak intensity ratio of Mo 3d/Si 2p on the MoO3/Ti-Si catalysts with different TiO2 contents. As can be seen, Mo:Si ratios of MoO3/Ti-Si are larger than that

Ind. Eng. Chem. Res., Vol. 46, No. 4, 2007 1049

of MoO3/SiO2 and increase steadily with increasing in TiO2 loadings, indicating that there is more MoO3 on the surfaces of Ti-Si composites than on that of pure silica. The results from XRD, BET, and XPS measurements show that the low activity of the 16%MoO3/SiO2 sample (a multiple phase sitting on a flat support surface is such a case) is probably due to the low ratio of surface Mo atoms to all atoms. Polymerization of surface Mo atoms on the supported MoO3/ SiO2 catalyst may decrease the fraction of Mo-O-Si bonds, and subsequently reduce the activity of the MoO3 activity sites.27 In our case, an additional reason is that a part of the multilayer phase surface is blocked by the micropore walls surrounding it. For the MoO3/Ti-Si catalyst, the ratio of surface Mo atoms to all atoms in it was more than that of the MoO3/SiO2 sample and even gradually increased with the deposition of TiO2. This suggests that the Ti(IV) species contributes to the dispersion of Mo(VI) species on the silica surface. Specifically, Ti(IV) species favors reducing the polymerization of surface Mo atoms on the surface of SiO2, and thus the MoO3/Ti-Si catalysts exhibited much better performances than the MoO3/SiO2 catalyst did. Based on the theory proposed by Kerkhof et al.,40 a linear relation between relative XPS intensity and the bulk ratio of the metal to the support may be expected for monolayer catalysts as well as for catalysts with crystallites of constant sizes. The point of intersection of two lines shows the change from monolayer to crystalline. As shown in Figure 3, two linear lines intersect at 7.9 wt % TiO2 content. Therefore, surface TiO2 was dominant in the form of a submonolayer below 8 wt % TiO2 content, while crystalline TiO2 was formed above 8 wt % TiO2 content. This result agreed well with those from XRD and BET measurements and the theory calculation. Temperature-Programmed Desorption of NH3. NH3-TPD characterization was conducted to survey the surface acid strengths of MoO3/SiO2 catalyst and MoO3/Ti-Si catalysts and the influence of TiO2 content. In the NH3-TPD curves, peaks are generally distributed into two regions: below and above 673 K, referred to as low-temperature (LT) and high-temperature (HT) regions, respectively. The peaks in the HT region can be attributed to the desorption of NH3 from strong Brønsted and Lewis type acid sites, and the assignment of the peaks in the LT region is attributed to the desorption of NH3 from weak acid sites.41,42 From the results shown in Figure 4, it can be seen that the peaks only appear in the low-temperature region, confirming that there only exist weak acid sites on the surfaces of MoO3/SiO2 and MoO3/Ti-Si catalysts. Furthermore, the maximum temperature offset was 15 K between the MoO3/TiSi catalysts and MoO3/SiO2 catalyst studied, implying that the strength of the surface acid on MoO3/Ti-Si is close to that on MoO3/SiO2 and the content amount of TiO2 has a slight impact on the strength of the surface acid of MoO3/Ti-Si. Figure 5 illustrates the amount of NH3 desorbed at low temperature on MoO3/SiO2 and MoO3/Ti-Si catalysts. It shows that MoO3/Ti-Si catalysts had more total acid sites than MoO3/ SiO2 catalyst, and the amount of NH3 desorbed increased with increasing TiO2 content from 2 to 8 wt %, thereafter decreasing slightly with the further deposition of TiO2. Therefore, these data provide reliable evidence that the addition of amorphous TiO2 contributes a number of new weak acid sites. Moreover, although crystalline TiO2 can provide a few acid sites by itself, the number of acid sites it provides is less than it occupies on SiO2, which leads to the decrease of total acid sites on MoO3/ Ti-Si catalysts at higher TiO2 content (above 8 wt %) with the presence of crystalline TiO2 as confirmed by XRD measurements. In addition, we have examined the effect of acid strength

Figure 4. NH3-TPD profile of MoO3/Ti-Si catalysts with different TiO2 contents: (A) 0 wt %; (B) 2 wt %; (C) 4 wt %; (D) 8 wt %; (E) 10 wt %; (F) 12 wt %.

Figure 5. Amount of NH3 desorbed at low temperature on MoO3/Ti-Si catalysts with different TiO2 contents.

on the selectivity to the target products and a part of the results were reported elsewhere.27,38,39 Briefly, the weak acid sites are responsible for the formation of MPO and DPO, while the strong acid sites favor the formation of the byproduct AN. Therefore, the results of NH3-TPD gave the reason for high selectivities to MPO and DPO over MoO3/Ti-Si catalysts. Considering experimental results from reactivity tests and the total acid amount over the MoO3/Ti-Si catalysts, it can be concluded that the total acidity has a direct influence on the activity of the catalysts. Conclusion We have shown that MoO3/Ti-Si was a quite active and selective heterogeneous catalyst for the synthesis of MPO and DPO from the transesterification of DMO with phenol compared to MoO3/SiO2 and TiO2/SiO2 catalysts. The catalytic efficiency of MoO3/Ti-Si linearly increased with TiO2 content up to a value of dispersion threshold observed and then decreased.

1050

Ind. Eng. Chem. Res., Vol. 46, No. 4, 2007

The results from XRD, XPS, and BET measurements indicated that the incorporation of dispersed TiO2 could not only enhance the interaction between MoO3 and SiO2 but also promote the dispersion state of MoO3 on the surfaces of the composite support. The result of NH3-TPD showed that incorporation of TiO2 supplied more weak acid sites on the surface of the catalysts, which resulted in the 99% total selectivity to the target productions over MoO3/Ti-Si catalysts. The desirable catalytic activities of MoO3/Ti-Si may be ascribed to the three following factors: (1) improvement of the MoO3 dispersion state on SiO2; (2) more weak acid sites on the surface of catalysts; (3) a synergetic effect of amorphous MoO3 with amorphous TiO2. Acknowledgment Financial support by the National Natural Science Foundation of China (NSFC) (Grant 20276050), the Program of Introducing Talents of Discipline to Universities (Grant B06006), and the Program for New Century Excellent Talents in University (NCET-04-0242) are gratefully acknowledged. The authors thank J. L. Gong for helpful discussions and for proofreading the manuscript. Literature Cited (1) Gong, J. L.; Ma, X. B.; Wang, S. P. Phosgene-free approaches to catalytic synthesis of diphenyl carbonate and its intermediates. Appl. Catal., A 2006, 316, 1-21. (2) Shaikh, A. G.; Sivaram, S. Organic carbonates. Chem. ReV. 1996, 96 (3), 951-976. (3) Ono, Y. Dimethyl carbonate for environmentally benign reactions. Pure Appl. Chem. 1996, 68 (2), 367-375. (4) Sikdar, S. K. The world of polycarbonates. CHEMTECH 1987, 2, 112-118. (5) Gabriello, I.; Ugo, R.; Renato, T. Aromatic carbonates. Ger. Offen. 2528412, Jan 8, 1976. (6) Akinobu, Y.; Takoshi, Y. Preparation of diaryl carbonates and/or alkyl aryl carbonates. JP 09241218, Sept 16, 1997. (7) Oyevaar, M. H.; To, B. W.; Doherty, M. F. Process for continuous production of carbonate esters. U.S. Patent 6,093,842, July 25, 2000. (8) Hideaki, H.; Yoshiyuki, O.; Atusi, M. Process for preparing carbonate esters. EP 684221, Nov 29, 1995. (9) Chalk, A. J. Aromatic carbonates and polycarbonates. Ger. Offen. 2738520, April 13, 1978. (10) Krimm, H.; Buysch, H. J.; Rudolph, H. Aromatic carbonates. Ger. Offen. 2736062, Feb 22, 1979. (11) Kim, W. B.; Lee, J. S. A new process for the synthesis of diphenyl carbonate from dimethyl carbonate and phenol over heterogeneous catalysts. Catal. Lett. 1999, 59, 83-88. (12) Fu, Z. H.; Ono, Y. Two-step synthesis of diphenyl carbonate from dimethyl carbonate and phenol using MoO3/SiO2 catalysts. J. Mol. Catal. A: Chem. 1997, 118, 293-299. (13) Nishihira, K.; Tanaka, S.; Nishimura, K.; Sugise, R. Process for producing diaryl carbonate. U.S. Patent 5,834,651, Nov 10, 1998. (14) Nishihira, K.; Tanaka, S.; Harada, K.; Sugise, R.; Shiotani, A.; Washio, K. Process for producing diaryl carbonate. U.S. Patent 5,811,573, Sept 22, 1998. (15) Chalk, A. J. Aromatic carbonates and polycarbonates. Ger. Offen. 2738520, April 13, 1978. (16) Uchiumi, S.; Ataka, K.; Matsuzaki, T. Oxidative reactions by a palladium-alkyl nitrite system. J. Org. Chem. 1999, 576 (1), 279-289. (17) Nishihira, K.; Tanaka, S.; Harada, K.; Sugise, R.; Shiotani, A.; Washio, K. Manufacturing method of diphenyl carbonate. U.S. Patent 5,922,827, 1999. (18) Harada, K.; Sugise, R.; Kashiwagi, K.; Dio, T.; Niida, S.; Kurafuji, T. Process for preparing diaryl carbonate. U.S. Patent 5,792,883, 1998. (19) Wang, S. P.; Ma, X. B.; Li, Z. H.; Xu, G. H. Synthesis of diphenyl carbonate by decarbonylation of diphenyl oxalate over Ph4PCl. Nat. Gas Chem. Eng. (China) 2002, 27, 1.

(20) Haber, J. The Role of Molybdenum in Catalysis; Climax Molybdenum Co.: Ann Arbor, MI, 1981. (21) Gates, B. C.; Katzer, J. T.; Schuit, G. C. A. Chemistry of Catalytic Processes; McGraw-Hill: New York 1979; Vol. 5, p 179. (22) Saini, A. R.; Johnson, B. G.; Massoth, F. E. Studies of molybdenumaluminum catalysts: XIV. Effect of Cation-modified Alumina. Appl. Catal. 1988, 40, 157-172. (23) Hu, H.; Wachs, I. E. Catalytic properties of supported molybdenum oxide catalysts: in situ-Roman and methanol oxidation study. J. Phys. Chem. 1995, 99, 10911-10922. (24) Kno¨zinger, H.; Taglauer, E. Toward supported oxide catalysts via solid-solid wetting. Catalysis (R. Soc. Chem., Cambridge) 1993, 10, 1-40. (25) Gong, J. L.; Ma, X. B.; Wang, S. P. Transesterification of dimethyl oxalate with phenol over MoO3/SiO2 catalysts. J. Mol. Catal. A: Chem. 2004, 207, 215-220. (26) Gong, J. L.; Ma, X. B.; Yang, X.; Wang, S. P. A comparative study of supported catalysts prepared by the new “slurry” impregnation method and by the conventional method: their activity in transesterification of dimethyl oxalate and phenol. Appl. Catal., A: Gen. 2005, 280, 215-223. (27) Ma, X. B.; Gong, J. L.; Wang, S. P.; Gao, N. Reactivity and surface properties of silica supported molybdenum oxide catalysts for the transesterification of dimethyl oxalate with phenol. Catal. Commun. 2004, 5, 101106. (28) Goodman, D. W. “Catalytically active Au on Titania”: Yet another example of a strong metal support interaction (SMSI). Catal. Lett. 2005, 99 (1-2), 1-4. (29) Xie, Y. Ch.; Tang, Y. Q. Spontaneous monolayer dispersion of oxides and salts onto surface of supports: Application to heterogeneous catalysis. AdV. Catal. 1990, 37, 1. (30) Xie, Y. Ch.; Yang, N. F.; Liu, Y. J. Spontaneous dispersion of some active components onto the surfaces of carriers. Sci. Sin., Ser. B 1983, 26 (4), 337-350. (31) Deng, C.; Duan, L. Y.; Xu, X. P.; Xie, Y. Ch. Preparation of TiO2/ SiO2 complex support by gas phase adsorption and dispersion state of MoO3 on surface of complex support. J. Catal. 1993, 14 (4), 281-286. (32) Yoshinaka, S.; Segawa, K. Hydrodesulfurization of dibenzothiophenes over molybdenum catalyst supported on TiO2-Al2O3. Catal. Today 1998, 45, 293-298. (33) Hattori, H.; Itoh, M.; Tanabe, K. The nature of active sites on TiO2 and TiO2-SiO2 for the isomerization of butanes. J. Catal. 1975, 38, 172178. (34) Bosman, H. J. M.; Kruissink, E. C.; Vanderspoel, J.; Vandenbrink, F. Characterization of the acid strength of TiO2-ZrO2 mixed oxides. J. Catal. 1994, 148 (2), 660-672. (35) Baiker, A.; Dollenmeier, P.; Glinski, M.; Reller, A. Selective catalytic reduction of nitric oxide with ammonia II. Monolayer of vanadia immobilized on titania-silica mixed gels. Appl. Catal. 1987, 35 (2), 365380. (36) Zdrazˇil, M. Supported MoO3 catalysts: preparation by the new “slurry impregnation” method and activity in hydrodesulphurization. Catal. Today 2001, 65, 301-306. (37) Klicpera, T.; Zdrazˇil, M. High surface area MoO3/MgO: preparation by reaction of MoO3 and MgO in methanol or ethanol slurry and activity in hydrodesulphurization of benzothiophene. Appl. Catal. 2001, 216, 4150. (38) Wang, S. P.; Ma, X. B.; Guo, H. L.; Gong, J. L.; Yang, X.; Xu, G. H. Characterization and catalytic activity of TiO2/SiO2 for Transesterification of Dimethyl Oxalate with Phenol. J. Mol. Catal. 2004, 214, 273. (39) Wang, S. P.; Ma, X. B.; Guo, H. L.; Yang, X.; Xu, G. H. A comparative study of supported TiO2 catalysts and activity in ester exchange between dimethyl oxalate and phenol. J. Mol. Catal. 2004, 222, 183-187. (40) Kerkhof, F. P. T. M.; Moulijn, J. A. Quantitative analysis of XPS intensities for supported catalysts. J. Phys. Chem. 1979, 83, 1612-1619. (41) Lo´nyl, F.; Valyon, J. On the interpretation of the NH3-TPD patterns of H-ZSM-5 and H-mordenite. Microporous Mesoporous Mater. 2001, 47, 293. (42) Sawa, M.; Niwa, M.; Murakami, Y. Relationship between Acid Amount and Framework Aluminum Content in Mordenite. Zeolites 1990, 10, 532.

ReceiVed for reView September 7, 2006 ReVised manuscript receiVed November 9, 2006 Accepted December 2, 2006 IE0611812