Superior Performance of Gold Supported on Titanium-Containing

Apr 22, 2009 - Titanium-containing hexagonal mesoporous silicas (Ti-HMS) with wormhole structure and Si/Ti molar ratios ranging from 10 to 40 have bee...
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J. Phys. Chem. C 2009, 113, 8186–8193

Superior Performance of Gold Supported on Titanium-Containing Hexagonal Mesoporous Molecular Sieves for Gas-Phase Epoxidation of Propylene with Use of H2 and O2 Hongwei Yang, Dingliang Tang, Xinning Lu, and Youzhu Yuan* State Key Laboratory of Physical Chemistry of Solid Surfaces, National Engineering Laboratory for Green Chemical Productions of Alcohols-Ethers-Esters, College of Chemistry and Chemical Engineering, Xiamen UniVersity, Xiamen 361005, China ReceiVed: NoVember 20, 2008; ReVised Manuscript ReceiVed: March 23, 2009

Titanium-containing hexagonal mesoporous silicas (Ti-HMS) with wormhole structure and Si/Ti molar ratios ranging from 10 to 40 have been prepared by using long-chain alkyl primary amines as template agents. The Ti-HMS supported Au catalyst (Au/Ti-HMS) was obtained by a deposition-precipitation method for direct gas-phase epoxidation of propylene with use of O2 and H2. The structures of Ti-HMS and Au/Ti-HMS samples were characterized by X-ray diffraction, N2-physisorption, scanning electron microscopy, transmission electron microscopy, UV-vis diffuse reflectance spectroscopy, UV Raman spectroscopy, ammonia-temperatureprogrammed desorption, and atomic adsorption spectroscopy. The results showed that the Au/Ti-HMS catalyst exhibited superior performance in terms of propylene conversion, propylene oxide (PO) selectivity, and H2 efficiency in comparison with the Au catalysts supported on the conventional Ti-containing mesoporous materials. Besides the Si/Ti molar ratio, the chain length of alkylamine for the Ti-HMS preparation was crucial for the enhancement of catalytic performance. Specifically, 9.0% of propylene conversion, 97.3% of PO selectivity, and 30.4% of H2 efficiency can be obtained at 373 K in the initial 30 min of time-on-stream on the Au/Ti-HMS catalyst, where the Ti-HMS having a Si/Ti molar ratio at 20 was prepared by using tetradecylamine as the template agent. Regeneration of the spent catalyst by calcination in air gave almost no change in the PO selectivity but about 25% loss in the propylene conversion. The enhanced catalytic performance of Au/Ti-HMS catalyst may be essentially attributed to the homogeneous dispersion and uniformity of titanium species in combination with accessible pore structure. Introduction Propylene oxide (PO) is one of the most important synthetic intermediates in chemical industry. Currently, PO is produced mainly through chlorohydrin and hydroperoxidation processes. However, both of these processes produce large amounts of byproducts and are not environmentally benign. In recent years, direct epoxidation of propylene using “green” oxidant has attracted much attention. Among them, considerable interests have been paid on the gas-phase epoxidation of propylene using O2 and H2 with Au nanoparticles deposited on Ti-containing supports since the pioneer work of Haruta and his co-workers.1–3 To date, the gas-phase epoxidation of propylene has been performed over highly dispersed nanosize Au particles supported on a number of Ti-containing materials like TiO2, TiO2/SiO2, TS-1, Ti-MCM-41, Ti-MCM-48, three-dimensional Ti-Si mesoporous materials (3D-Ti-Si), Ti-TUD, and so on.4–15 Attractively, both Au/3D-Ti-Si developed by Haruta et al. and Au/ TS-1 developed by Delgass et al. could afford about 10% of propylene conversion with PO selectivity of 90.3% and 76%, respectively.7,9 Quite recently, Oyama et al. has reported the epoxidation of propylene with H2 and O2 in the explosive regime in a packed-bed catalytic membrane reactor. At 453 K, the C3H6 conversion was 10% and the PO selectivity was 80%, corresponding to a space-time yield (STY) of 150 gpo kgcat-1 h-1, whereas at 485 K, the STY increased to 200 gpo kgcat-1 h-1.15 Nevertheless, based on a rough estimation by Haruta, the catalytic reaction acceptable for commercialization must achieve * To whom correspondence should be addressed. E-mail: yzyuan@ xmu.edu.cn. Phone: +86 592 2181659. Fax: +86 592 2183047.

a propylene conversion of 10% in one pass, with a PO selectivity of 90% and H2 efficiency of 50%.8 Therefore, the development of new catalysts is still demanded to overcome the problems of low propylene conversion, rapid catalyst deactivation, and low H2 efficiency. On the other hand, the hexagonal mesoporous silicas (HMS) with wormhole framework structure prepared by the sol-gel reaction by using cheaper neutral long-chain alkyl primary amines as the template agents at room temperature may offer certain advantages in catalysis.16–18 They usually possess thicker framework walls, small crystallite size of primary particles, and complementary textual porosity to provide better transport channels for reactants and products. Transition metal cations like Ti, Al, Fe, Co, and Cu species can be readily incorporated into the HMS framework uniformly with high contents. The organic templates can be totally removed from the assynthesized HMS samples by simple calcination.16–18 As for the Ti-HMS, it is likely to obtain a high concentration of surfaceexposed titanium sites in homogeneous dispersion and uniformity, which would be beneficial for the dispersion of Au nanoparticles and thus the formation of hydroperoxo species in the process of propylene epoxidation with use of H2 and O2.19–22 However, no work on the gas-phase epoxidation of propylene with H2 and O2 over the Ti-HMS supported Au catalysts (Au/ Ti-HMS) has been reported. In the present work, we have synthesized a series of Ti-HMS samples using alkyl primary amines with different carbon chains as the template agents. After that, we have reported a kind of highly efficient Au/Ti-HMS catalyst prepared by a deposition-

10.1021/jp810187f CCC: $40.75  2009 American Chemical Society Published on Web 04/22/2009

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Figure 1. Nitrogen adsorption-desorption isotherms (I) and the corresponding pore size distributions (II) for Au/Ti-HMS samples: (a) Au/TiHMS-A12-ST10; (b) Au/Ti-HMS-A12-ST20; (c) Au/Ti-HMS-A12-ST30; (d) Au/Ti-HMS-A12-ST40; (e) Au/Ti-HMS-A14-ST20; and (f) Au/Ti-HMS-A16ST20.

precipitation method for the direct vapor-phase epoxidation of propylene using O2 and H2. The effects of titanium content, template agent, reaction conditions, and the regeneration and silylation of the catalysts on the properties of Au/Ti-HMS are systematically investigated in combination with the characteristic results by means of various techniques. Experimental Section Catalyst Preparation. The 3D-Ti-Si and TS-1 were synthesized according to the procedures in the literature, refs 8 and 10, respectively. TiO2 (P-25) was gifted from Degussa AG Co. The Ti-HMS materials were synthesized in a manner similar to that in the literature.23 In the present work, the primary amines with alkyl chains of 12-16 carbon atoms were used as template agents and the Si/Ti molar ratios ranged from 10 to 40. Typically, a solution A was prepared by mixing 0.1 mol of tetraethoxyorthosilicate, 0.01-0.0025 mol of tetrabutylorthotitanate, 0.65 mol of ethanol, and 0.1 mol of isopropyl alcohol. Another solution B was prepared by mixing alkylamine (0.027 mol), water (3.6 mol), and hydrochloric acid (0.002 mol). Solution A was added slowly to solution B under vigorous stirring for about 20 min. The mixture was kept at ambient temperature for 18 h to afford the crystalline templated product. After that, the resulting solids were collected by filtration, washed several times with distilled water, and dried under vacuum at 333 K overnight. Then the as-synthesized solids were calcined at 923 K in air for 5 h to remove the residual organic template materials, yielding the final Ti-HMS samples. They were denoted as Ti-HMS-Am-STn, where A, S, and T stand for alkylamine, Si, and Ti, respectively, and the subscript letters denote the carbon atom numbers of alkylamine and the Si/Ti molar ratio accordingly. Au was deposited onto each of the supports by using a deposition-precipitation method.7,8 An aqueous solution of HAuCl4 · 4H2O (0.02 mol) was heated to 333 K and its pH value was adjusted to 7.0 ( 0.1 by 0.1 M aqueous NaOH solution. The support powder (1.0 g) was added and dispersed into the solution under stirring. The pH value was adjusted to 7.0 ( 0.1 by NaOH solution during 15 min. After that, the mixture was stirred for another 90 min at the same temperature and pH. The solids were collected by filtration, dried under vacuum for 12 h, and calcined at 573 K in air for 4 h, affording the Au/ Ti-HMS samples. Catalyst Characterization. Power X-ray diffraction (XRD) patterns were measured on a Phillips Panalytical X’pert Pro diffractometer equipped with graphite monochromator and Cu

KR radiation (40 kV and 30 mA). In general, the diffraction data were collected by using a continuous scan mode with a scan speed of 1° (2θ) min-1. Nitrogen adsorption-desorption isotherms were measured at 77 K on a Micromeritics TriStar 3000 porosimetry analyzer. The samples were outgassed at 573 K for 3 h before each measurement. The specific surface area was calculated following the Brunauer-Emmett-Teller (BET) method. The average pore diameter and total pore volume were determined by the Barret-Joyner-Halenda (BJH) method according to the desorption isotherm branch. UV-visible diffuse reflectance spectroscopies (UV-vis) were taken on a Varian Cary-5000 spectrometer equipped with a diffuse-reflectance accessory, using dehydrated BaSO4 as a reference in the range of 200-800 nm. UV Raman spectroscopies were collected on a RENISHAW inVia Raman Microscope UV Raman spectrometer. The 244 nm line from a Coherent Innova 300 Fred was used as the excitation source. The power of the UV laser lines was less than 2.0 mW. The spectral resolution was set at 4.0 cm-1. Scanning electron microscopy (SEM) images were obtained on a LEO 1530 scanning microscope. High-resolution transmission electron microscopy (HRTEM) images were performed on a Phillips Analytical FEI Tecnai 30 electron microscope operated at an acceleration voltage of 300 kV. Ammonia-temperature-programmed desorption (NH3-TPD) measurements were performed with a Micromeritics AutoChem II 2920 instrument connected to a Hiden Qic-20 mass spectrometer. Typically, the sample (100 mg) in a quartz tube was first pretreated at 523 K with high-purity He for 1 h. The adsorption of NH3 was performed at 373 K in an NH3-Ar mixture (10 vol % NH3) for 1 h. The remaining or weakly adsorbed NH3 was purged by high-purity He. TPD was performed in He flow by raising the temperature to 873 K at a rate of 10 deg min-1, The desorbed NH3 was detected with the mass spectrometer by monitoring the signal with m/e 16, because the parent peak with m/e 17 possibly could be affected by the desorbed water. The Au and Ti loadings of the samples were determined by atomic absorbance analysis (AAS), using a WFX-1E2 spectrometer after dissolution of the samples in a mixture of 5% HF/HCl-HNO3 (volume ratio 3/1). Catalytic Testing. The catalytic tests were carried out in a vertical fixed-bed quartz reactor (i.d. 8 mm) at 353-413 K by using 150 mg of catalyst at 1 atm. The feed gas was adjusted to 10 vol % each C3H6, H2, and O2 diluted with N2 by mass

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TABLE 1: The Au and Ti Loadings and Structural Properties of Au/Ti-HMS-Am-STn Samples catalyst Ti-HMS-A12-ST10 Ti-HMS-A12-ST20 Ti-HMS-A12-ST30 Ti-HMS-A12-ST40 Ti-HMS-A14-ST20 Ti-HMS-A16-ST20 Au/Ti-HMS-A12-ST10 Au/Ti-HMS-A12-ST20 Au/Ti-HMS-A12-ST30 Au/Ti-HMS-A12-ST40 Au/Ti-HMS-A14-ST20 Au/Ti-HMS-A16-ST20 Au/TiO2 (P-25) Au/TS-1 Au/3D-Ti-Si

finial Si/Ti molar ratio

Au loading/wt %

11.9 20.1 30.5 43.7 20.6 22.7 22.7 20.2 30.5 46.5 21.0 23.1

1.58 1.63 0.91 0.61 1.72 1.58 0.90 0.07 1.30

936.8 50.5

d100a/nm

a0b/nm

wall thicknessc/nm

pore diameterd/nm

SBET/m2 · g-1

pore vole/ cm3 · g-1

4.6 4.6 4.8 4.3 5.1 5.6 4.9 5.0 5.0 5.5 5.4 5.7

5.3 5.3 5.5 5.0 5.9 6.4 5.6 5.7 5.7 6.3 6.2 6.6

1.7 1.6 1.5 1.5 2.0 2.1 1.6 1.6 1.4 1.5 2.0 2.0

6.5

7.5

2.4

3.6 3.7 4.0 4.1 3.9 4.3 4.0 4.1 4.3 4.8 4.2 4.4 17.0 3.7 5.1

685 833 786 774 883 649 569 645 541 610 712 630 49 382 381

0.57 0.72 0.84 0.82 0.96 0.75 0.53 0.58 0.55 0.68 0.68 0.73 0.17 0.32 0.54

Calculated from XRD analysis. a0 ) 2d100/3 . Wall thickness ) a0-pore diameter. d Calculated from the desorption branch of nitrogen isotherm by using the BJH model. e Calculated from the volume adsorbed of P/P0 at 0.99. a

b

1/2 c

Results and Discussion

Figure 2. XRD patterns of several Au/Ti-HMS-Am-STn catalysts: (a) Au/Ti-HMS-A12-ST10; (b) Au/Ti-HMS-A12-ST20; (c) Au/Ti-HMS-A12ST30; (d) Au/Ti-HMS-A12-ST40; (e) Au/Ti-HMS-A14-ST20; and (f) Au/ Ti-HMS-A16-ST20.

flow controllers. The typical reaction was conducted at gashourly space-velocity (GSHV) of 4000 cm3 h-1 gcat-1. The temperature was measured by using a glass tube covered Cr-Al thermocouple located in the center of the catalyst bed. The gas leaving the reactor was heated at about 373 K and analyzed online with use of two gas chromatographs, one equipped with two thermal conductivity detectors (TCD), using a MS 5A packed column (3 mm × 3 m) and a Porapak Q packed column (3 mm × 3 m), respectively, and another equipped with a flame ionization detector (FID) attached to a Porapak T packed column (3 mm × 3 m). The MS 5A and Porapak Q columns were used to detect hydrocarbons (e.g., propylene), H2, O2, COx, N2, and H2O, respectively. The Porapak T column was used to detect oxygenates (e.g., propylene acetaldehyde, PO, acetone, propionaldehyde, acrolein, acetic acid, and the like). Regeneration and Silylation of Catalysts. For the catalyst regeneration, the spent catalyst was regenerated by calcination at 573 K in air for 2 h and then the next catalytic testing was performed. For the silylation of Au/Ti-HMS, the catalyst bed was passed by the stream of methoxytrimethylsilane vapors with N2 as the carrier gas at 298 K for 20 min, followed by flushing with N2 at 473 K for 5 h before the catalytic measurement.

Nitrogen Physisorption Measurements. Nitrogen physisorption measurements were conducted for a series of Ti-HMS derived from different template agents and Au/Ti-HMS samples with different Ti loadings (nominal 2.0-7.0 wt %). The nitrogen isotherms of the Ti-HMS and the corresponding Au/Ti-HMS samples are shown in Figure 1s-I (Supporting Information) and Figure 1-I, respectively. The pore structure parameters, such as the specific area (SBET), cumulative pore volume (Vp), pore diameter, and wall thickness of the Ti-HMS and Au/Ti-HMS samples, as well as those of Au supported on TS-1, TiO2, and 3D-Ti-Si, are summarized in Table 1. All the titanium-containing HMS samples, including Au/Ti-HMS, exhibit Langmuir type IV isotherms with a H1-type hysteresis loop,23,24 corresponding to a typical large pore mesoporous material with 1D cylindrical channels (Figure 1s-I in the Supporting Information for Ti-HMS and Figure 1-I for Au/Ti-HMS). In the case of Ti-HMS samples generated from the primary amines with different chain lengths as templates and various Si/Ti molar ratios, the adsorptiondesorption isotherms are similar to those of Ti-HMS materials reported by other authors.23–27 Capillary condensation of nitrogen with mesopores occurred, causing an increase in nitrogen uptake in the characteristic relative pressure (P/P0) range of 0.2-0.4, suggesting typical mesoporous structure with uniform pore diameters (Figure 1s-I, Supporting Information). In general, the slope at P/P0 of 0.2-0.4 is less steep for the sample with higher Ti contents, implying that the pore size uniformity becomes worse. This point has been proven by the pore size distribution of the samples (Figure 1s-II, Supporting Information). The higher the Ti contents in the Ti-HMS, the broader the pore size distribution. Among the Ti-HMS-A12-STn samples, the results also show that the pore diameter decreases with increasing titanium loading. In addition, as shown by the pore size distributions in Figure 1s (Supporting Information) and Figure 1, the pore size of the samples increases with increasing length of the alkyl chain of the primary amine template. In this case, however, no obvious influences on the pore uniformity are observed. After immobilization of Au nanoparticles onto the Ti-HMS, the amount of adsorbed nitrogen, the BET surface area, and the pore volume decrease, while the pore diameter increases slightly, in comparison with those of the pristine Ti-HMS sample (Table 1). We speculate that the existence of Au nanoparticles may partially occupy or even completely block the mesopores.

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Figure 3. Images of SEM and HRTEM: (a) SEM image of Ti-HMS-A14-ST20; (b) HRTEM image of Ti-HMS-A14-ST20; (c) HRTEM image of Au/Ti-HMS-A14-ST20; and (d) plot of Au particle distribution in Au/Ti-HMS-A14-ST20.

Moreover, some pore structures in the Ti-HMS support may be destroyed or collapsed during the deposition of Au species, which can also bring about a slight increase in the pore size of the Au/Ti-HMS sample. XRD Measurements. The powder XRD patterns for the TiHMS samples are shown in Figure 2s (Supporting Information). All the samples exhibit only (100) reflection and no higher order reflections (beyond 2θ ) 10°) can be observed, indicating the presence of a mesoporous structure without any long-range ordering and the absence of bulk (>1000 Å particle size) anatase in the samples. In addition, increasing titanium amount results in a decrease in the diffraction peak intensity, broadness in the peak width, and lower 2θ value for the d100 reflection. After deposition of Au nanoparticles onto the Ti-HMS, the diffraction peak due to the (100) reflection is still retained, but it becomes broader due to the weaker intensity and wider width (Figure 2). The results indicate that the distortion and collapse of mesoporous structures may occur to some extent during the Au deposition. On the other hand, the diffraction peaks due to the Au nanoparticles are considerably weak, suggesting that there may exist a high dispersion of Au particles in the Au/TiHMS samples. AAS Measurements. We have measured the Au and Ti contents by AAS. The results are incorporated in Table 1. Obviously, the titanium species are incorporated into the HMS sample successfully by the one-pot sol-gel method. Moreover, the Au loading increases with the increment of titanium contents in the case of Si/Ti molar ratio ranging from 20 to 40. Nevertheless, the Au/Ti-HMS samples prepared with different chain length primary amines as templates show varieties in the Au loadings even if the same Si/Ti molar ratio is employed, which may probably be due to their diverse pore properties, surface areas, and available surface-exposed titanium sites.

Measurements of SEM and HRTEM. The morphology and structural ordering of Ti-HMS and Au/Ti-HMS samples were analyzed by SEM and HRTEM. The SEM image of Ti-HMS in Figure 3a shows that the sample is composed of discrete spherical particles with a diameter at about 0.2-2 µm. The HRTEM image in Figure 3b indicates its characteristic wormholelike mesoporous structure. The HRTEM image of Au/Ti-HMS in Figure 3c clearly illustrates that the Au nanoparticles with a mean diameter around 4 nm (Figure 3d) are dispersed well on the Ti-HMS surfaces. Diffuse-Reflectance UV-Visible Spectroscopy Measurements. The UV-vis spectra of Ti-HMS and Au/Ti-HMS samples are shown in Figure 3s (Supporting Information) and Figure 4, respectively. All these samples show a band near 220 nm that is attributed to tetrahedrally coordinated Ti. There is a shoulder at 260-270 nm due to the presence of Ti(IV) species in 6- or 8-fold coordination, which are most likely generated through hydration of the tetrahedrally coordinated sites.11,20,21 With increasing Ti contents in the samples, the UV-vis spectra are found to become broader but no shoulder at ca. 330 nm attributed to bulk titanium oxides can be observed. As for the Au/Ti-HMS samples, there is a plasmon band at 500-600 nm, which is typical for the existence of Au nanoparticles.12,13,28,29 UV Raman Spectroscopy Measurements. Figure 5 displays the UV Raman spectra of Ti-HMS and Au/Ti-HMS samples. As for the Ti-HMS samples, several Raman bands at 491, 532, 813, 939, and 1061 cm-1 are observed. According to literature,30–33 the bands at 813 and 939 cm-1 are assigned to the vibrations of Si-O-Si bonds, while the three Raman bands at 491, 532, and 1061 cm-1 can be attributed to the framework titanium species in tetrahedral coordination. More specifically, the bands at 491 and 532 cm-1 are respectively assigned to the bending and symmetric stretching vibrations of the framework Ti-O-Si

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Figure 4. Diffuse-reflectance UV-vis spectra of several Au/Ti-HMSAm-STn catalysts: (a) Au/Ti-HMS-A12-ST10; (b) Au/Ti-HMS-A12-ST20; (c) Au/Ti-HMS-A12-ST30; (d) Au/Ti-HMS-A12-ST40; (e) Au/Ti-HMSA14-ST20; and (f) Au/Ti-HMS-A16-ST20.

species, while the most enhanced band at 1061 cm-1 is attributable to the asymmetric stretching vibration of the Ti-O-Si species.30–33 The peak intensities tend to correlate with the Si/Ti molar ratios in the samples. With increasing Ti contents (lower Si/Ti molar ratio) in the samples, the peak intensities due to Ti-O-Si bonds become weak. Considering the results of the UV-vis spectra, this trend may be attributed to the amount changes of the extraframework titanium species. It is likely that there are some titanium species existing in the extraframework of Ti-HMS samples in the case of higher titanium contents. These extraframework titanium species absorb the UV Raman scattering, thus diminishing the Raman signal intensity. The stretching vibrations due to the Ti-O-Si species can also be observed in the Au/Ti-HMS samples, but the most enhanced band is slightly shifted to a higher value. The phenomena might be ascribed to the strong interactions between Au nanoparticles and surface-exposed titanium sites. Catalyst Activity Measurements. We examined the catalytic performances of propylene epoxidation over the Au catalysts supported on Ti-HMS, TS-1, 3D-Ti-Si, and TiO2. The results are listed in Table 2. The catalytic reaction, except that over the Au/TiO2, was carried out at 373 K. It can be found that the Au/Ti-HMS catalysts present higher performance for the gasphase epoxidation of propylene, as being reflected by the higher propylene conversion, PO selectivity, and H2 efficiency. Among them, the Au/Ti-HMS-A14-ST20 sample affords a propylene conversion of 9.0% with selectivity to PO up to 97.3% at the initial 30 min of time-on-stream (TOS). Figure 6 shows the effect of Ti content on the catalytic performance of Au/Ti-HMS-A12-STn samples for the gas-phase propylene epoxidation at 373 K. The propylene conversion increases with decreasing Si/Ti molar ratio from 40 to 20 and reaches the highest value at Si/Ti ) 20. After that, the propylene conversion and PO selectivity apparently decline. Considerable influences of the titanium content in the catalysts on the H2 efficiency and PO STY are also observed as depicted in Figure 7. The maximum PO STY is obtained when the Si/Ti ratio is set at 20. The H2 efficiency, however, increases with decreasing titanium content almost linearly. From the UV-vis observations, higher titanium contents in the Ti-HMS and the corresponding Au/Ti-HMS-A12-STn samples may cause transformations of isolated framework titanium to other forms, such as hexahedral and octahedral titanium, which may result in reduction in the active Ti sites effective for the

Yang et al. propylene epoxidation.12,13,34 On the other hand, it has been reported that the hydroperoxide-like intermediate, which is supposed to be produced on the nanosized Au surfaces in the presence of H2 and O2, may be the real oxidizing species for the propylene epoxidation.34–37 In solution reaction, the presence of proper acidic species can inhibit the decomposition of H2O2 effectively.38 The present experimental results indicate that when the titanium content increases, the amount of Lewis acidic sites over the Au/Ti-HMS-Am-STn samples may be increased slightly (see NH3-TPD profiles in Figure 4s in the Supporting Information), which may pay positive effects to restrain the decomposition of hydroperoxo species produced in situ on the nano-Au surfaces. Therefore, proper titanium content might be able to form the most accessible amount of isolated framework titanium sites and also generate adequate acidic sites. In this case, the catalyst thus generated may be able to realize higher efficiencies in propylene conversion and H2 utilization. The reaction temperature is one of the vital parameters which are related to the activation of reactants and the efficiency of hydroperoxide species.39–41 Figure 8 shows the catalytic performances of Au/Ti-HMS-A14-ST20 sample at different temperatures. When the reaction temperature is set at 353 K, the propylene conversion is the lowest at the initial 3 min of TOS. The PO selectivity, on the other hand, is kept at a higher level of 97% during 4 h of run. It is worth noting that the catalyst gives a propylene conversion of 9.0% at the initial 30 min and 3.6% after 4 h of TOS, keeping the selectivity to PO as high as 97% when the reaction is conducted at 373 K. When the reaction is carried out at 393 or 413 K, however, lower propylene conversions and selectivity to PO are obtained. Moreover, the reaction results obtained during 3-30 min indicate that the propylene conversion increases as a function of time at 353 or 373 K, while it decreases at 393 or 413 K. Conceivably, at relatively lower temperatures like 353 or 373 K the initial reaction rate is slow. A prolonged reaction time may be beneficial for the reaction at an early stage. At higher temperatures like 393 or 413 K, however, the reaction can take place quickly, but the decomposition of active hydroperoxo species and the formation of oligomers due to PO deep oxidation may also be accelerated considerably. Therefore, lower propylene conversions and PO selectivity should be the reasonable outcomes in these cases. We have also found that both H2 efficiency and PO STY vary with the reaction temperatures (Figure 9). The H2 efficiency decreases from 48.1% to 10.6% when the reaction temperature is raised from 353 to 413 K, while the PO STY reaches a maximum at 373 K. We believe that the results are closely related to the facile decomposition of the hydroperoxo species and the formation of byproduct at relatively higher temperatures. Catalyst Regeneration. Like other Au catalysts for the propylene epoxidation in H2-O2, the Au/Ti-HMS-Am-STn showed an obvious deactivation in the propylene conversion as a function of TOS. A plausible interpretation for the behavior was essentially ascribed to the formation of oligomers at the active sites of catalyst surfaces.42 We then treated the Au/TiHMS-A14-ST20 sample by calcination in air after the reaction every 2 h and the treatment was repeated consecutively several times. Figure 10 shows the catalytic performances of fresh and regenerated catalysts. As compared with the fresh catalyst, the regenerated one can regain about 75% of the catalytic activity during the first reuse. After that, the consecutive regenerations do not bring about significant changes in the propylene conversion and PO selectivity. The above results reveal that calcination may be one of the feasible approaches to regenerate

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Figure 5. UV Raman spectra of (I) Ti-HMS and (II) Au/Ti-HMS samples: (a) Ti-HMS-A12-ST10; (b) Ti-HMS-A12-ST20; (c) Ti-HMS-A12-ST30; (d) Ti-HMS-A12-ST40; (e) Ti-HMS-A14-ST20; (f) Ti-HMS-A16-ST20; (g) Au/Ti-HMS-A12-ST10; (h) Au/Ti-HMS-A12-ST20; (i) Au/Ti-HMS-A12-ST30; (j) Au/Ti-HMS-A12-ST40; (k) Au/Ti-HMS-A14-ST20; and (l) Au/Ti-HMS-A16-ST20.

TABLE 2: Catalytic Performance of Au Deposited on Several Supports for Gas-Phase Epoxidation of Propylenea selectivity/% support

C3H6 conversion/%

H2 efficiency/%

PO

CH3CHO

C2H3CHO

(CH3)2CO

COx

Ti-HMS-A12-ST20 Ti-HMS-A14-ST20 Ti-HMS-A16-ST20 TS-1b 3D-Ti-Sic TiO2 (P-25)d

6.9 9.0 6.0 2.4 5.3 0.5

30.3 30.4 28.7 3.7 25.7 2.3

96.2 97.3 96.3 47.3 92.1 99.9

1.8 1.4 1.7 33.4 3.9 0.0

0.1 0.0 0.0 6.9 1.7 0.0

0.1 0.0 0.0 12.4 1.3 0.0

1.8 1.3 2.0 0.0 1.0 0.1

a Reaction conditions: feed gas C3H6/O2/H2/N2 ) 1/1/1/7 (vol %); GHSV ) 4000 cm3 h-1 gcat-1; catalyst weight ) 150 mg; reaction temperature ) 373 K; data were taken at 30 min of TOS. b Si/Ti ) 36 (atomic ratio). c Ti content ) 3 mol %. d Reaction temperature ) 343 K.

Figure 7. Variation of H2 efficiency and PO STY as a function of Si/Ti molar ratio over Au/Ti-HMS-A12-STn. The reaction conditions are the same as in Table 2.

Figure 6. Epoxidation of propylene over Au/Ti-HMS-A12-STn with different titanium contents as a function of TOS: 9, ST10; b, ST20; 2, ST30; and 1, ST40. The reaction conditions are the same as in Table 2.

the spent catalyst. We have observed that there are no evident changes in the band at 220 nm in the UV-vis spectra (Figure 5s in the Supporting Information), but there are slight decreases in the BET surface area and the XRD diffraction peak intensity

during the repeat calcinations (Figures 6s and 7s in the Supporting Information). These are believably due to the occurrences of collapse and distortion of the HMS structure. Effect of Silylation. The silylation of supported Au catalysts is known as an effective method to depress the deactivation rate in the case of the vapor-phase epoxidation of propylene in H2-O2.8 We performed the catalyst silylation using methoxytrimethylsilane at room temperature. The catalytic behaviors of Au/Ti-HMS-A14-ST20 before and after the silylation are shown in Figure 11. The results indicate that the silylated catalyst exhibits lower initial propylene conversion and deactivation rate, but higher H2 efficiency and better selectivity to PO. Doubtlessly, the attachment of methyl groups on the catalyst surfaces can bring about the changes in the microenvironments in the

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Figure 8. Epoxidation of propylene over Au/Ti-HMS-A14-ST20 at different temperatures as a function of TOS: 9, 353 K; b, 373 K; 2, 393 K; and 1, 413 K. The reaction conditions are the same as in Table 2.

Figure 9. Variation of H2 efficiency and PO STY as a function of reaction temperature over Au/Ti-HMS-A14-ST20. The data were taken at 30 min of TOS.

catalyst active sites, such as the reduction of active titanium sites and the changes of surface property from hydrophilicity to hydrophobicity. However, such silylation can only make limited contribution to the improvement of the catalyst performance. Conclusions The Ti-HMS materials with different pore distributions and Si/Ti molar ratios were successfully synthesized by the onestep sol-gel method by using primary amines with long alkyl chains (12-16 carbons) as the template agents. The characterizations by XRD, N2 physisorption, and HRTEM confirmed that the samples maintained structural integrity as well as ordered mesoporous nature after the Ti incorporation and Au deposition. The UV-vis and UV Raman spectra indicated that the Ti species mainly existed as tetrahedrally coordinated isolated species in the Ti-HMS and Au/Ti-HMS samples. The Au/Ti-

Yang et al.

Figure 10. Epoxidation of propylene over Au/Ti-HMS-A14-ST20 at 373 K before and after regeneration as a function of TOS: (9) before regeneration; (b) after the first regeneration; (2) after the second regeneration; and (1) after the third regeneration. The reaction conditions are the same as in Table 2.

Figure 11. Epoxidation of propylene over Au/Ti-HMS-A14-ST20 at 373 K before and after silylation as a function of TOS: (9) before silylation and (0) after silylation. The reaction conditions are the same as in Table 2.

HMS sample was found to be a new class of catalyst highly efficient for the direct gas-phase propylene epoxidation in H2-O2. A propylene conversion of 9.0% with 97.3% selectivity to PO and 30.4% H2 efficiency could be achieved at the initial 30 min of TOS for the Au/Ti-HMS-A14-ST20 sample. The spent catalyst can be regenerated by calcination in air, recovering the activity by about 75% and sustaining the PO selectivity almost unchanged. Also, the catalyst deactivation with TOS can be appreciably retarded via the silylation of the catalyst. The superior performance of Au/Ti-HMS-A14-ST20 catalyst was essentially attributed to the homogeneous dispersion and uniformity of titanium species in combination with accessible pore structure and nanosized Au particles. Acknowledgment. The authors gratefully acknowledge the financial support from the NSFC (20433030, 20873108 and

Gas-Phase Epoxidation of Propylene 20423002), the 973 program (2009CB939804), the Key Project of NSF of Fujian Province (2007J0013), the Research Fund for the Doctoral Program of Higher Education (20050384011), and the Key Project of Chinese Ministry of Education (106099). Supporting Information Available: Figures showing nitrogen adsoption-desorption isotherms and the corresponding pore size distributions for Ti-HMS samples (Figure 1s), XRD patterns of Ti-HMS-Am-STn supports (Figure 2s), diffusereflectance UV-vis spectra of Ti-HMS-Am-STn catalysts (Figure 3s), ammonia temperature-programmed desorption for Ti-HMS and Au/Ti-HMS samples (Figure 4s), diffuse-reflectance UV-vis spectra of Au/Ti-HMS-A14-ST20 catalysts (Figure 5s), XRD patterns of Au/Ti-HMS-A14-ST20 catalysts (Figure 6s), and nitrogen adsorptin-desorption isotherm and the corresponding pore suze distribution for Au/Ti-HMS-A14-ST20 catalyst after the 3rd generation. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Hayashi, T.; Tanaka, K.; Haruta, M. J. Catal. 1998, 178, 566. (2) Haruta, M. Catal. SurV. Jpn. 1997, 1, 61, and references cited therein. (3) Haruta, M. Catal. Today 1997, 36, 123, and references cited therein. (4) Qi, C.; Okumura, M.; Akita, T.; Haruta, M. Appl. Catal. A 2004, 263, 19. (5) Uphade, B. S.; Yamada, Y.; Akita, T.; Nakamura, T.; Haruta, M. Appl. Catal. A 2001, 215, 137. (6) Uphade, B. S.; Akita, T.; Nakamura, T.; Haruta, M. J. Catal. 2002, 209, 331. (7) Sinha, A. K.; Seelan, S.; Tsubota, S.; Haruta, M. Angew. Chem., Int. Ed. 2004, 43, 1546. (8) Sinha, A. K.; Seelan, S.; Okumura, M.; Akita, T.; Tsubota, S.; Haruta, M. J. Phys. Chem. B 2005, 109, 3956. (9) Cumaranatunge, L.; Delgass, W. N. J. Catal. 2005, 232, 38. (10) Taylor, B.; Lauterbach, J.; Delgass, W. N. Appl. Catal. A 2005, 291, 188. (11) Chowdhury, B.; Bravo-Sua´rez, J. J.; Date´, M.; Tsubota, S.; Haruta, M. Angew. Chem., Int. Ed. 2006, 45, 412. (12) Dai, M. H.; Tang, D. L.; Lin, Z. J.; Yang, H. W.; Yuan, Y. Z. Chem. Lett. 2006, 35, 878. (13) Dai, M. H.; Tang, D. L.; Yuan, Y. Z. Chin. J. Catal. 2006, 27, 1063. (14) Lu, J. Q.; Zhang, X. M.; Bravo-Sua´rez, J. J.; Bando, K. K.; Fujitani, T.; Oyama, S. T. J. Catal. 2007, 25, 350. (15) Oyama, S. T.; Zhang, X. M.; Lu, J. Q.; Gu, Y. F.; Fujitani, T. J. Catal. 2008, 257, 1.

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