Effect of Synthesis Conditions on the Structure and Catalytic

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Ind. Eng. Chem. Res. 2010, 49, 6790–6802

Effect of Synthesis Conditions on the Structure and Catalytic Performance of Vand Ce-Incorporated SBA-15-like Materials in Propane Selective Oxidation Ozge Aktas,† Sena Yasyerli,† Gulsen Dogu,*,† and Timur Dogu‡ Department of Chemical Engineering, Gazi UniVersity, Maltepe, 06570 Ankara, Turkey, and Department of Chemical Engineering, Middle East Technical UniVersity, Ankara, Turkey

Vanadia- and/or ceria-incorporated SBA-15-like materials were prepared by impregnation and one-pot hydrothermal synthesis procedures at different solution pH values. The pH of the synthesis solution was found to have a very strong effect on the pore structure, chemical composition, and morphology of the synthesized materials. Materials synthesized by impregnation of V and/or Ce and by the one-pot hydrothermal route at pH , 1.0 showed the characteristic ordered pore structure of SBA-15. However, the materials prepared by the one-pot synthesis route at pH 1.5 gave complex pore structures with bottleneck-shaped and/or slit-like interconnected pores. Activity tests in the selective oxidation of propane showed higher activities for the V-incorporated materials than for the materials containing Ce and V-Ce. Catalysts synthesized by impregnation gave higher propane conversions. However, the propylene selectivity values were higher with the catalysts prepared by the one-pot route. An increase in the temperature from 550 to 600 °C led to increases in both the conversion and the propylene selectivity with the V- and V-Ce-incorporated materials prepared by the onepot procedure at pH 1.5. For the V-incorporated catalyst prepared by this route, the propane conversion and propylene selectivity were obtained as 40% and 62%, respectively, at a space time of 0.4 s · g · cm-3 and a temperature of 600 °C. 1. Introduction A new route has been opened in catalysis research by the discovery of silicate structured mesoporous materials with welldefined ordered pore structures. Among the M41S family mesoporous materials, which were discovered by Mobil researchers,1 MCM-41 is the most popular and has been tested in catalytic applications.2 This material has a high surface area with uniform pore sizes in the range of 2-4 nm. SBA-15 is also a highly promising silicate structured mesoporous material, with a narrow pore size distribution, a high surface area (600-1000 m2/g), and larger pore diameters (5-10 nm) than MCM-41.3 Its thicker walls (3.0-6.0 nm) compared to those of MCM-41 were reported to provide more thermal stability.4,5 Improvement of the hydrothermal stability of such silicate structured mesoporous materials has also been achieved by using salt solutions during the hydrothermal synthesis step.6 Although these silicate structured mesoporous materials have high surface area values, they are catalytically inactive for most purposes. The activities of these materials could be enhanced through the incorporation of metals,7 metal oxides,8-10 and/or acid sites.11 In most cases, incorporation of metals or metal oxides were achieved by impregnation procedures.8,9,12,13 However, a onepot hydrothermal procedure was used in some other studies.14 Better catalytic performance of the materials prepared by the one-pot procedure was reported in some applications.2,15 Synthesis conditions used in the one-pot hydrothermal procedure are expected to have a significant effect on the structure of the synthesized materials and also on the extent of incorporation of the metals/metal oxides. The effect of the synthesis temperature on the textural properties of SBA-15 was discussed by Ryoo and co-workers.16,17 The pH of the synthesis solution is also expected to have a significant effect on the pore structure * To whom correspondence should be addressed. Tel.: +90 312 5823559. E-mail: [email protected]. † Gazi University. ‡ Middle East Technical University.

of SBA-15.18 In fact, as discussed by Zhao et al.,3,4 the pH of the solution should be less than the isoelectric point of silica (which is about 2) for the formation of silica gel. Consequently, a pH value of less than 1.0 is generally used in the synthesis of SBA-15. The increasing demand for lower alkenes has greatly increased the research activities directed toward finding new pathways for their production. Synthesis of alkenes through the dehydrogenation of alkanes is thermodynamically limited by equilibrium, and these endothermic reactions require high reaction temperatures. Oxidative dehydrogenation (ODH) of alkanes, at much lower temperatures than conventional dehydrogenation, is a promising and attractive route to alkene production19-24 without any thermodynamic limitations and with much lower coke formation. However, the ODH process has some problems, such as difficulty in controlling the oxidation reactions to carbon oxides. Commercialization of the oxidative dehydrogenation of alkanes to produce alkenes requires further challenging research for the development of active and selective catalysts. Catalysts based on vanadium2,9,10,13,21,25 and cerium14,15,26 oxides have been reported to be quite active in selective oxidation reactions. The redox properties of vanadia-based mixed oxide catalysts and the presence of highly mobile capping oxygen atoms in ceria are considered to be responsible for the favorable catalytic performances of these oxides in selective oxidation. In recent years, the catalytic performance of vanadiasupported mesoporous catalysts that had been prepared by impregnation procedures was tested in the selective oxidation of propane.9,13 Achievement of high alkene selectivity is a major challenge in the development of new catalysts for the selective oxidation of alkanes. To investigate the effect of synthesis conditions of V-, Ce-, and bimetallic V-Ce-incorporated mesoporous SBA-15-like materials on their structure and also on their catalytic performance in selective oxidation, such materials were prepared in the present study both by the one-pot hydrothermal procedure

10.1021/ie901672b  2010 American Chemical Society Published on Web 03/05/2010

Ind. Eng. Chem. Res., Vol. 49, No. 15, 2010

at different solution pH values and also by an impregnation route. The catalytic performances of all of these materials were tested in the oxidative dehydrogenation of propane. The results indicate a major influence of the synthesis procedure on the structures and catalytic performances of these materials. 2. Experimental Details 2.1. Synthesis of SBA-15. Synthesis of mesoporous SBA15 support material was achieved following a hydrothermal route similar to that reported by Zhao et al.3 In this synthesis procedure, the triblock copolymer poly(ethylene glycol)-blockpoly(propylene glycol)-block-poly(ethylene glycol) (Pluronic P123, molecular weight 5800, Sigma-Aldrich) and tetraethylorthosilicate (TEOS, Merck) were used as the surfactant and the silica source, respectively. A solution of surfactant was prepared at 30 °C by continuous mixing. Then, 2 M HCl was added to this solution, and mixing was continued for another hour at 35 °C. TEOS was then added to this solution, to achieve a surfactant/TEOS weight ratio of 1/2, and mixing was continued for another 20 h at 35 °C. The pH of this mixture was much less than 1. This solution was then transferred into a Teflonlined stainless steel autoclave, and hydrothermal synthesis was carried out at 100 °C for 24 h. The solid product formed during hydrothermal synthesis was then washed with deionized water until the pH of the filtrate became constant. This material was dried at 40 °C for 18 h and then calcined at 600 °C for 6 h in a tubular furnace, under a flow of dry air, for removal of the organic template. 2.2. Synthesis of Vanadia- and Ceria-Incorporated SBA-15-like Catalysts. Vanadia- and ceria-incorporated SBA15-like catalytic materials were prepared following the one-pot hydrothermal and impregnation procedures. Materials prepared by the one-pot procedure were synthesized following modified routes at different solution pH values. In all of these synthesis procedures, cerium nitrate hexahydrate (Merck) and ammonium monovanadate (Merck) were used as the cerium and vanadium sources, respectively. In the synthesis of most of the V- and/or Ce-incorporated SBA-15 catalysts, the total amount of metals was adjusted to be 5% (by weight) in the final product. In the case of the bimetallic catalysts containing both vanadia and ceria, 2.5% (by weight) of each metal was incorporated into SBA15, which corresponded to V/Si and Ce/Si molar ratios of about 0.031 and 0.011, respectively, in the synthesis mixtures. In the case of the one-pot procedure, a methanol (Merck) solution of metal salts was added to the surfactant solution, under stirring at 35 °C. As the acid source, either 2 or 0.03 M HCl was used. In the case of 2 M HCl, the pH of the solution became much less than 1. However, for the 0.03 M HCl, the final pH of the solution was 1.5. As discussed in the following sections of this article, the materials synthesized at different pH values showed quite different structural and chemical behaviors. After about 1 h of mixing, TEOS was added dropwise, and the resulting solution was continuously mixed for 20 h. Hydrothermal synthesis was then carried out in a Teflon-lined autoclave at 100 °C for 24 h. The solid product obtained as a result of the hydrothermal synthesis was then filtered and washed with deionized water. This material was then dried at 40 °C for 24 h, after which it was calcined at 600 °C for 6 h in a tubular furnace, in the presence of flowing dry air. In the case of synthesis of catalysts by the impregnation procedure, a methanol solution of metal salts was added to the suspension of pure SBA-15 at 60 °C under stirring. Then, the same steps of drying at 40 °C for 24 h and calcination at 600 °C were followed.

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2.3. Catalyst Characterization. The catalysts synthesized in this work were analyzed by X-ray diffraction (XRD) for the determination of the pore ordering in the mesostructures and for the detection of possible bulk vanadia and/or ceria crystalline phases, by using a Rigaku D/MAX 2200 instrument with a Cu KR radiation source (λ ) 1.5406 Å). Nitrogen adsorption and desorption analyses were carried out for the determination of the Brunauer-Emmett-Teller (BET) surface area, pore size distribution, and pore volume values, using a QuantaChrome Autosorb 1 instrument. Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) analyses of the catalysts were performed with a JEOL JSM-6400 instrument. SEM images gave information about the morphology of the synthesized materials. Transmission electron microscopy (TEM) images of the synthesized materials were obtained with a JEOL model JEM 1010 microscope at the Electron Microscopy Center of Babes-Bolyai University in Romania. Energy-dispersive spectroscopy analysis was used for the determination of bulk compositions of the materials. Fourier transform infrared (FTIR) analysis of the synthesized materials was performed using a Perkin-Elmer instrument. In these analyses, materials were mixed with KBr (KBr/sample mass ratio ) 98/2), and the pellets were placed into the solid cell of the instrument. These spectra were obtained at room temperature in a helium atmosphere. FTIR analyses of the pyridine-adsorbed materials were also performed with the same Perkin-Elmer instrument. For these analyses, materials were prepared in a similar way. Before the pyridine adsorption step, the materials were dried at 100 °C. Differences between the spectra obtained with and without pyridine-adsorbed materials were analyzed for the acidity characterization of the synthesized catalysts. Temperatureprogrammed reduction (TPR) of the synthesized materials was carried out with a 5% hydrogen/95% nitrogen gas mixture, using a QuantoChrome Chembet 3000 instrument. 2.4. Catalytic Tests. Catalytic activity test experiments were performed at atmospheric pressure in a quartz tubular flow reactor packed with 0.2 g of catalyst. The reaction temperature was 550 °C in most experiments. However, some experiments were also carried out at 600 °C. Below 550 °C, no appreciable amounts of reaction products were observed. The feed stream consisted of a propane/oxygen/helium mixture with a composition of 6/3/21 in most experiments and a total flow rate of 30 cm3min-1. Online analysis of the product stream leaving the reactor was performed on a gas chromatograph (SRI 8610C type) equipped with a thermal conductivity detector and two columns (Molecular Sieve 13X and Hayesep D columns) connected in series. The following reactions can take place during the selective oxidation of propane25 1 C3H8 + O2 f C3H6 + H2O 2

(1)

7 C3H8 + O2 f 3CO + 4H2O 2

(2)

C3H8 + 5O2 f 3CO2 + 4H2O

(3)

C3H6 + 3O2 f 3CO + 3H2O

(4)

9 C3H6 + O2 f 3CO2 + 3H2O 2

(5)

Coke formation is expected to be minimal in such an oxidative dehydrogenation process. The objective of the development of

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Figure 1. XRD patterns of pure SBA-15 and V- and/or Ce-incorporated SBA-15 catalysts prepared by the impregnation method: (a) V-Ce@SBA-15, (b) V@SBA-15, (c) Ce@SBA-15, (d) SBA-15.

new catalysts is to maximize propylene selectivity and to minimize total oxidation and formation of carbon oxides. 3. Results and Discussion 3.1. Catalyst Characterization. XRD patterns of Ce- and/ or V-incorporated SBA-15 catalysts prepared by the impregnation method and of pure SBA-15 are shown in Figure 1. The characteristic XRD pattern of SBA-15 was essentially conserved in the catalysts prepared by the impregnation of Ce and/or V (Ce@SBA-15, V@SBA-15, and V-Ce@SBA-15). XRD patterns of the impregnated materials showed the characteristic d100 peak of the SBA-15 structure at a 2θ value of about 1.0°, as well as its two reflections, which correspond to d110 and d200. 2θ values corresponding to d100 for SBA-15, Ce@SBA-15, V@SBA-15, and V-Ce@SBA-15 were found at 0.98°, 1.04°, 1.08°, and 1.10°, respectively. However, the intensity of the main peak observed in the impregnated materials was not as sharp as the intensity of the corresponding peak in pure SBA15. The wide peak observed in the 2θ range of 20-30° corresponds to amorphous silica.15 In the Ce-V-impregnated bimetallic material, XRD peaks corresponding to the CeVO4 phase were also observed. In the cases of V- or Ce-impregnated materials, no diffraction lines of vanadia or ceria were observed, indicating that large crystals of these oxides were not formed and that the impregnated V and Ce were well-dispersed on the pore surfaces of SBA-15. The low-angle XRD patterns of the V- and/or Ce-incorporated materials prepared by the one-pot procedure at pH , 1.0 (VSBA-15, Ce-SBA-15, and V-Ce-SBA-15) were quite similar to the XRD pattern of pure SBA-15 (Figure 2A). For the Vand/or Ce-incorporated materials prepared at this pH, the characteristic peak corresponding to d100 was observed at a 2θ value of 0.88°, and the two reflections were observed at about 1.52° and 1.76°. The wide-angle XRD spectra of these materials showed the presence of amorphous silica, as indicated by the broad XRD peak in the 2θ range of 20-30° (Figure 2B). The absence of peaks corresponding to vanadia or ceria indicates the good dispersion of these oxides within the lattice of these materials. The XRD patterns of the V- and V-Ce-incorporated materials prepared at a solution pH of 1.5 by the one-pot hydrothermal procedure (V-SBA-15m and V-Ce-SBA-15m, respectively) showed a very different behavior (Figure 3) than the XRD patterns of the corresponding materials synthesized at a pH of less than 1 and also than the XRD patterns of the materials prepared by the impregnation method. As shown in Figure 3, the primary peak of the mesoporous structure was observed at

Figure 2. (a) Low-angle and (b) wide-angle XRD patterns of pure SBA-15 and V- and/or Ce-incorporated SBA-15 catalysts prepared by the one-pot procedure at pH , 1.0.

a 2θ value of about 0.7° for V-Ce-SBA-15m. A minor reflection of this peak was observed at about 1.2°. In the case of V-SBA-15m, the primary peak was shifted to a lower angle, and no reflections were observed. These results indicate major distortions in the long-range order of the mesopores of these materials. Nitrogen adsorption-desorption isotherms and SEM images provided further information about the differences in the structures and morphologies of the materials prepared at different solution pH values. Figure 4a,b displays the nitrogen adsorption-desorption isotherms of V- and/or Ce-incorporated SBA15-type catalysts prepared by impregnation and by the one-pot hydrothermal procedure at a solution pH of much less than 1, respectively. All of these isotherms are of type IV according to the IUPAC classification, which are typical for mesoporous materials with ordered pore structures.27,28 Well-defined hysteresis loops with very steep and parallel adsorption and desorption branches were observed for all of the materials synthesized by impregnation or by direct synthesis at pH ,


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Figure 3. XRD patterns of V- and V-Ce-incorporated materials prepared by the one-pot hydrothermal procedure at a solution pH 1.5: V-SBA-15m and V-Ce-SBA-15m.

Figure 4. Nitrogen adsorption-desorption isotherms for V- and/or Ceincorporated SBA-15-like materials prepared by (a) impregnation and (b) the one-pot hydrothermal procedure at pH , 1.0.

1.0. Such hysteresis behavior is denoted as type H1 according to the IUPAC classification (or type A in earlier references), indicating the formation of uniform open-ended unconnected pores with a long-range order. Formation of such an ordered pore structure is the most important characteristics of SBA-15. These results confirm that the ordered pore structure of SBA15 was not significantly distorted by the incorporation of V and/ or Ce through the impregnation method or through the one-pot hydrothermal procedure in a strong acid solution (pH , 1.0). The pore size distribution curves of the materials synthesized by impregnation (Figure 5a) also support the conclusion that these materials had pores within a very narrow size range of 5-7 nm. In the case of the materials prepared by the one-pot procedure (at pH , 1.0), a small shift to higher pore diameters

Figure 5. Pore size distributions of V- and/or Ce-incorporated SBA-15like materials prepared by (a) impregnation and (b) the one-pot hydrothermal procedure at pH , 1.0.

was observed (Figure 5b) as compared to the pore diameters of the materials prepared by impregnation. As a result of these analyses, it was concluded that the pore structure of SBA-15 was not significantly altered by the incorporation of V and/or Ce through the impregnation procedure or the one-pot hydrothermal synthesis at pH , 1.0. The average pore diameter of SBA-15 was found to be 6.3 nm (Table 1). However, in the case of vanadium-incorporated material prepared by the one-pot procedure (at pH , 1.0), the average pore diameter was somewhat higher (6.8 nm). Such an increase in pore diameter upon metal incorporation was also reported by Shah et al.18 for Sn-incorporated SBA-15-like

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Table 1. Physical and Structural Properties of SBA-15 Incorporated Catalysts sample

d100a (nm)

aa,b (nm)

dpc (nm)

δd (nm)

Vpe (cm3/g)

Vmf (%)

SBET (m2/g)

SBA-15 V@SBA-15 Ce@SBA-15 V-Ce@SBA-15 V-SBA-15 (pH , 1.0) Ce-SBA-15 (pH , 1.0) V-Ce-SBA-15 (pH , 1.0) V-SBA-15m (pH ) 1.5) V-Ce-SBA-15m (pH ) 1.5)

9.0 8.2 8.5 8.0 10.0 10.0 10.0 -

10.4 9.4 9.8 9.3 11.6 11.6 11.6 -

6.3 5.8 5.8 5.8 6.8 6.3 6.3 4.0 3.7

3.8 3.6 4.0 3.5 4.8 5.3 5.3 -

1.2 0.85 1.04 1.05 1.51 1.52 1.47 0.82 0.74

35 33 35 35 33 34 35 57 55

855 566 715 742 998 1029 1069 709 830

a From the XRD results of the catalysts. b a ) lattice parameter. c Average pore diameter determined by the Barrett-Joyner-Halenda (BJH) method from nitrogen desorption data. d Pore wall thickness calculated from the difference a - pore size, where a ) 2d100/3. e Pore volume determined by the BJH method from nitrogen adsorption/desorption data. f Volume percent of pores having diameters less than 2.6 nm.

materials prepared by a direct synthesis procedure. This was explained by the presence of Sn in the corona region of the mesopore structure, which was considered to form around the cylindrical aggregates of surfactant during the hydrothermal synthesis step. Upon calcination, this corona region was reported to become microporous.18,29 Similarly to their findings, for the vanadium-incorporated material synthesized in our work, the increase in the pore diameter might be an indication of the presence of V in the corona region of the mesopore structure. The presence of heteroatoms in the corona region might cause less shrinkage of pore structure during calcination. In addition to the variations in pore diameter, the characteristic lattice parameter a and the pore wall thickness values of all of the V- and/or Ce-incorporated materials prepared by the one-pot procedure (at pH , 1.0) were found to be higher than the corresponding values of pure SBA-15 (Table 1). These results can also be considered as an indication of the incorporation of V and Ce into the pore walls of SBA-15. Similar results showing increases in the average pore diameter, characteristic lattice parameter, and pore wall thickness were also reported in our earlier publications related to V- and Pdincorporated MCM-41-like mesoporous materials prepared by the one-pot procedure.2,7 Table 1 presents physical and structural properties of the synthesized catalysts. The average pore diameters of the materials prepared by the impregnation procedure were somewhat lower than the average pore diameters of the materials prepared by the one-pot hydrothermal synthesis procedure (pH , 1.0). The average pore diameter decreased from 6.3 to 5.8 nm upon the impregnation of V and/or Ce into SBA-15. Some reduction of the pore volumes was also observed in the materials prepared by the impregnation of V and/or Ce (Table 1). All of these results indicate some reduction of pore sizes and pore volumes of impregnated materials due to the deposition of vanadia and/or ceria on the pore walls. Similar results were reported in one of our earlier publications for Pd-impregnated MCM-41 materials.7 Closure of some of the pores by Ce or V was also expected. A decrease of the surface area upon impregnation of Ce and/or V also supports these conclusions. The surface area values were on the order of 1000 m2/g for the materials synthesized by the one-pot route (at pH , 1.0), whereas the corresponding values were between 600 and 700 m2/g for the impregnated materials. Analysis of the nitrogen adsorption-desorption isotherms obtained using the QuantaChrome Autosorb 1 instrument showed that about one-third of the total pore volume of V- and/ or Ce-incorporated materials prepared by impregnation and by the one-pot procedures (at pH , 1.0) corresponded to pores having diameters of less than 2.6 nm. (Table 1). Considering that the mesopores of these materials essentially lie in the 5-7nm range, these smaller pores can be ascribed as micropores. The presence of micropores within the SBA-15 structure was

also reported in number of earlier publications.16,17,18,29,30,31 Ravikovich and Neimark30 named these pores intrawall pores and reported that these pores might constitute up to 30% of the total porosity. Our results support their conclusion. Their findings indicated that intrawall pores might occupy 20-40% of the pore wall. As reported by Ryoo and co-workers16,17 and Imperor-Clec et al.,31 the structure of the silica walls of SBA15-like materials is rather complex and contains a microporous corona region. This corona region was shown to be formed around the cylindrical organic aggregates during the hydrothermal synthesis step. Formation of a corona region was believed to be due to the partial occlusion of the poly(ethylene oxide) (PEO) chains of the Pluronic surfactant deep into the silica walls. Upon calcination, removal of occluded PEO was reported to result in the formation of a microporous layer. The temperature of the hydrothermal synthesis step was shown to be a very important factor for the pore structure of the final product.16,17 It was reported in the literature that about one-third of the pore volume of SBA-15 corresponded to micropores when the synthesis temperature was lower than 80 °C. A decrease of microporosity was reported at higher synthesis temperatures. With an increase of temperature, a decrease of ultramicroporosity accompanied by the appearance of secondary porosity due to pore bridging was observed.17 Kruk et al.16 also reported that these complementary pores provided the connectivity between the primary mesopores of SBA-15. As for the metalimpregnated SBA-15 materials, Parathoner et al.’s study29 indicated that the grafting of titania was facilitated by the microporous corona region. In our case, impregnation of V and/or Ce caused reductions in both the total pore volume and the micropore volume. However, the volume fraction of the pores having diameters smaller than 2.6 nm remained about the same after the impregnation of V and/or Ce (Table 1) into SBA-15. Apparently, the pore mouths of a fraction of micropores in the corona region were closed by the V and Ce deposited on the mesopore walls. Some of the V and Ce was also expected to enter into the micropores, causing a reduction in the micropore volume. Nitrogen adsorption-desorption isotherms of V- and V-Ceincorporated materials synthesized at a solution pH value of 1.5 (Figures 6 and 7) showed major differences from the isotherms of the materials prepared at pH , 1.0. This result is consistent with the XRD results discussed above. The importance of pH in the structure of final product was also discussed in the earlier publications of Zhao et al.3,4 and Shah et al.18 Typical S-shaped behavior of the adsorption branch, corresponding to capillary condensation in the mesopores, was not observed in the nitrogen adsorption isotherm of vanadiumincorporated material prepared by the one-pot procedure at a solution pH of 1.5 (Figure 6). However, the desorption branch of the hysteresis loop was found to show a steep inflection at a relative pressure of about 0.5. For this material, the adsorption


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Figure 8. Comparison of pore size distributions of V-SBA-15m (pH ) 1.5) and V-Ce-SBA-15m (pH ) 1.5) with those of V-SBA-15 (pH , 1.0) and V-Ce-SBA-15 (pH , 1.0).

Figure 6. Comparison of nitrogen adsorption-desorption isotherms of (a) V-SBA-15m (pH ) 1.5), (b) V-SBA-15 (pH , 1.0).

Figure 7. Nitrogen adsorption-desorption isotherms of (a) V-Ce-SBA15m (pH ) 1.5), (b) V-Ce-SBA-15 (pH , 1.0).

and desorption branches of the hysteresis loop are closed at a relative pressure very close to saturation. This behavior is more like an H4-type hysteresis loop, according to the IUPAC classification. As discussed by Rouquerol et al.,27 H4-type hysteresis is observed in materials having slit-shaped pores with a significant contribution of micropores. In the case of the nitrogen adsorption-desorption isotherm of V-Ce-SBA-15m (pH ) 1.5), the hysteresis loop was also closed at a quite high relative pressure of nitrogen (Figure 7). In this case, the desorption branch is more peculiar, showing a two-step behavior.

Such a behavior was reported as a characteristic behavior of “bottle-shaped” pores.18 In this case, the shape of the hysteresis loop is more like type H2, indicating the presence of bottleneck pores with large cages interconnected by windows. Closure of the hysteresis loop at about saturation might also be considered as an indication of the presence of some slit-shaped pores within this material. Type H2 hysteresis loops were reported to indicate complex pore structures that tend to be made up of interconnected networks of pores of different sizes and shapes.27 Such an isotherm is an indication of a disordered mesoporous system with substantial pore blocking and constrictions.28 These nitrogen adsorption-desorption isotherms and XRD patterns clearly indicate that the long-range order of the mesopores was significantly distorted for the materials prepared at a solution pH value of 1.5. Pore size distributions evaluated from the desorption branches of the nitrogen adsorption-desorption isotherms of the materials prepared at a solution pH of 1.5 (V-SBA-15m and V-Ce-SBA15m) are shown in Figure 8. As shown in this figure, V-Ce-SBA-15m gave a bidisperse pore size distribution in the mesopore range. Surface area and pore volume values of the V- and V-Ce-incorporated materials prepared at pH 1.5 were found to be about 750 m2/g and 0.75 cm3/g, respectively (Table 1). These values are less than the corresponding values of the materials synthesized at pH , 1.0. Comparison of the pore size distributions of the materials synthesized at pH ) 1.5 and pH , 1.0, also showed major differences (Figure 8). The average pore diameters of the V- and V-Ce-incorporated materials prepared by the direct synthesis route at pH 1.5 were about 4.0 nm. These values were considerably less than the average pore diameters of the materials prepared at pH , 1.0 (Table 1). An increase in the pore diameter with increasing acidity of the synthesis solution was reported to be due to the dehydration of the PEO [poly(ethylene oxide)]18 chains of the P123 surfactant, which was reported to cause a decrease in corona volume and consequently an increase in pore diameter. Another significant difference in the pore structure of these materials is related to the fraction of the pores having diameters less than 2.6 nm. As reported in Table 1, the volume fraction of these smaller pores was more than 50% in the materials prepared at a solution pH of 1.5. This is more than 50% higher than the values obtained for the materials prepared at pH , 1.0. As discussed in the literature,17 such complementary pores having smaller diameters can provide bridging between the primary mesopores and can enhance pore connectivity. These nitrogen adsorption-desorption isotherms and XRD results clearly show that pH is a very important parameter in the

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Table 2. EDS Results for the V- and/or Ce-Incorporated SBA-15 Catalysts mole ratio in solution mole ratio from EDS sample

Ce/Si

V/Si

Ce/Si

V/Si

SBA-15 V@SBA-15 Ce@SBA-15 V-Ce@SBA-15 V-SBA-15 (pH , 1.0) Ce-SBA-15 (pH , 1.0) V-Ce-SBA-15 (pH , 1.0) V-SBA-15m (pH ) 1.5) V-Ce-SBA-15m (pH ) 1.5)

0.023 0.011 0.023 0.011 0.011

0.062 0.031 0.062 0.031 0.062 0.031

-

-

0.017 0.007 0.006

trace 0.010 0.082 0.004

synthesis of V- and/or Ce-incorporated SBA-15-like materials. The materials synthesized at a solution pH of 1.5 had quite different structural properties than SBA-15. In fact, these materials should no longer be called SBA-15; instead, they are denoted as V-SBA-15m and V-Ce-SBA-15m in this article. Energy-dispersive spectroscopic (EDS) analysis was carried out to obtain information about the chemical compositions of the synthesized materials (Table 2). The EDS results indicate that the pH of the synthesis solution in the one-pot hydrothermal procedure is a very important factor affecting the degree of incorporation of vanadia into the synthesized material. The V/Si ratio (0.082) measured in the synthesized material at pH 1.5 was even higher than the corresponding ratio (0.062) in the synthesis solution of this material. This result indicates that some of the silica species remained in the solution without forming the solid material at this pH value. However, in the case of the material prepared at pH , 1.0, incorporation of vanadium into the material was not good. Repeated syntheses of this material showed a maximum V/Si ratio of 0.002 at this very low pH value. It seemed that incorporation of vanadium into the structure of SBA-15 was not highly successful at very low pH

values (less than 1.0). Similar observations have been reported for titanium-incorporated32,33 and iron-incorporated34,35 SBA15-like materials prepared by a one-pot procedure. Studies on Ti-substituted materials showed that Ti incorporation into SBA15 lattice decreased with an increase in hydrochloric acid concentration, and this was explained by the higher solubility of titanium in strongly acidic solutions. Similar observations were made for iron-incorporated materials.34,35 These results were explained by the increase of interaction of silica species and iron hydroxo complexes at higher pH values, which resulted in higher incorporation of Fe into the SBA-15 lattice.35 Hydrolysis of vanadium is considered to have a very important effect on its incorporation into SBA-15 during the one-pot procedure.36 Hydrolysis of vanadium in solution was examined in detail in the early book of Baes and Mesmer,37 as a function of solution pH. At moderate acidities, VO2+ is expected to polymerize to form decavanadates, V10O28-z(OH)z(6-z)-. However, at very low pH values, the solubility of V2O5 becomes very high, yielding only VO2+, and polymerized species due to hydrolysis do not form in the solution. Very low vanadium incorporation into SBA-15 lattice, at pH values less than 1, is then considered to be due to the high solubility of vanadium and the absence of formation of such hydrolyzed species. Apparently, the disappearance of such hydrolysis products of vanadium causes a major decrease in the interaction of silica and vanadium at very low pH values. However, good incorporation of vanadium into the SBA-15 lattice at pH 1.5 indicated the formation of such hydrolyzed vanadium species and good interaction of them with the Si species. The morphology of the mesoporous materials was examined by scanning electron microscopy (SEM). Images of the samples shown in Figure 9 for V- and/or Ce-impregnated materials and images shown in Figure 10 for the materials prepared by a one-

Figure 9. SEM images of V- and/or Ce-incorporated materials prepared by impregnation: (a) V@SBA-15, (b) Ce@SBA-15, (c) V-Ce@SBA-15.

Figure 10. SEM images of V- and/or Ce-incorporated materials prepared by the one-pot procedure at pH , 1.0: (a) V-SBA-15, (b) Ce-SBA-15, (c) V-Ce-SBA15.


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Figure 11. SEM images of V- and/or Ce-incorporated materials prepared by the one-pot procedure at pH 1.5: (a) V-SBA-15m, (b) V-Ce-SBA15m.

pot hydrothermal procedure at pH , 1.0 clearly indicate the formation of rope-like elongated aggregates with relatively uniform sizes (with dimensions of about 0.5-3.0 µm). Such a morphology is quite typical for SBA-15-type mesoporous materials.3,6 However, the morphologies of the materials prepared by the one-pot hydrothermal procedure at pH 1.5 were quite different (Figure 11). In this case, larger sponge-like aggregates were observed. These results also confirm the significance of the synthesis solution pH value in relation to the structure of the synthesized materials. TEM images of the V-incorporated materials prepared by the one-pot procedure at Figure 13. FT-IR spectra of the synthesized materials.

Figure 12. TEM images of (a) V-SBA-15 and (b) V-SBA-15m.

different pH values also support the conclusions reached from XRD, SEM, and nitrogen adsorption analyses. Long-range order of the mesopores of V-SBA-15 prepared at a solution pH of ,1.0 is clearly seen in Figure 12a. However, in the case of V-SBA-15m, which was prepared at pH 1.5, the TEM image indicated major distortions in the long-range order of the mesopores. Some mesopores with short-range order are seen in Figure 12b. FT-IR absorption spectra (in the range of 450-4000 cm-1) of pure SBA-15 and V- and/or Ce-incorporated SBA-15-like materials are shown in Figure 13. In the FT-IR spectra of all of these materials, a broad absorption band of hydrogen-bonded surface silanol groups and adsorbed water situated inside the channels of SBA-15-like materials was observed between 3000 and 3700 cm-1. The presence of adsorbed water can also be identified by the band at 1630 cm-1. A higher absorbance intensity observed at 1630 cm-1 and a broader absorption band observed at 3000-3700 cm-1 for the V- and/or Ce-incorporated materials synthesized at pH , 1.0 indicate higher water adsorption on these materials. The characteristic IR absorption bands of SiO2 in SBA-15 were reported to appear at about 465, 800, and 1090 cm-1 for the rocking, bending (or symmetric stretching) and asymmetric stretching of the intertetrahedral oxygen atoms of SiO2, respectively.14,38 The band corresponding to the asymmetric stretching of O-Si-O was observed at about 1050 cm-1 in the work of Brodie-Linder et al.39 A shoulder of this band was generally observed between 1100 and 1300 cm-1. The presence of the bands at about 1085 and 1200 cm-1 was considered to be due to concerted Si-O-Si ordering of silicate framework.38 As discussed in the literature, the shift of the band of the asymmetric stretching of SiO2 toward lower wavenumbers was considered as an indication of the incorporation of metals into the framework of metal-incorporated silicate structured ordered mesoporous materials.14,40 Such a behavior was also observed for Ce-incorporated MCM-41 prepared by the direct synthesis route.40 In the FT-IR spectrum of pure SBA-15 obtained in our work, a strong band was observed between 1039

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Figure 14. FT-IR spectra of pyridine-adsorbed samples prepared by the impregnation method.

Figure 15. FT-IR spectra of pyridine-adsorbed samples prepared by the one-pot procedure at two different solution pH values.

and 1211 cm-1 (Figure 13). Instead of a shoulder, two maxima were observed at about 1040 and 1210 cm-1 within this wide band, which correspond to the Si-O-Si ordering of silicate framework. A shift of the band corresponding to asymmetric stretching of SiO2 (at about 1040 cm-1) toward lower wavenumbers was observed in the FT-IR spectra of the V- and/or Ce-incorporated materials prepared by the one-pot hydrothermal procedure. For the materials prepared by V and/or Ce incorporation, this band was observed within the 1012-1027 cm-1 range. The shift of this band to lower wavenumbers was accompanied by a shift of the 1210 cm-1 band toward higher wavenumbers. Another important observation related to these twin bands is the decrease of their intensity upon the incorporation of V and/ or Ce into SBA-15. The intensity of the band at about 460 cm-1 (corresponding to rocking of the O atoms of SiO2) also decreased upon Ce and/or V incorporation (Figure 13). All of these observations indicate the incorporation of V and/or Ce into the framework of SBA-15 during the one-pot procedure. FT-IR spectra of pyridine-adsorbed materials observed with pure SBA-15 and with the V- and/or Ce-impregnated materials are shown in Figure 14. In the spectra of SBA-15, Ce@SBA15, and V-Ce@SBA-15, characteristic bands corresponding to the pyridine adsorbed on the Lewis acid sites were observed at 1445-1450 and 1598 cm-1. The strength of the Lewis acid sites due to pure SBA-15 was found to decrease with metal incorporation. In the case of V@SBA-15, bands corresponding to the Lewis acid sites were not apparent. However, for this

catalyst, relatively strong bands were observed at 1541 and 1640 cm-1. These bands correspond to the pyridinium ion adsorbed on the Brønsted acid sites.11,41 These results showed a significant increase in the Brønsted acidity of SBA-15 upon the impregnation of vanadia. In the cases of Ce@SBA-15 and V-Ce@SBA15, the band corresponding to Brønsted acid sites at 1540 cm-1 was quite weak. For the vanadia-incorporated materials prepared by the onepot procedure at two different solution pH values, the FT-IR results for the pyridine-adsorbed samples indicate quite different acidic characteristics. For the material synthesized at a solution pH of 1.5, quite strong bands corresponding to both Brønsted acid sites (at about 1550 and 1640 cm-1) and Lewis acid sites (at about 1450 and 1598 cm-1) were observed (Figure 15). However, for the material synthesized at a solution pH of ,1.0, such bands corresponding to Brønsted and Lewis acid sites were very weak. In the case of vanadia- and ceria-incorporated bimetallic SBA-15-like materials, similar conclusions were reached related to the effect of the synthesis solution pH on the acidic characteristics of the catalysts (Figure 15). All of these results clearly show the significance of the synthesis conditions in terms of the structure, morphology, chemistry, and acidic characteristics of V- and/or Ce-incorporated SBA-15-like mesoporous materials. Consequently, differences were also expected in the catalytic performances of the materials prepared by different procedures. 3.2. Catalytic Activity Tests in the Selective Oxidation of Propane. The catalytic performances of all of the materials prepared in this work were tested in the selective oxidation of propane to produce propylene. Test results obtained with pure SBA-15 showed no activity of this material in this reaction. In the case of V- and/or Ce-incorporated materials, measurable activities were observed at temperatures over 550 °C and at a space time of 0.4 s · g · cm-3. Most of the test reactions were then carried out at 550 °C, for the comparison of the catalytic performances of the synthesized materials. However, some experiments were also conducted with the most promising catalysts at 600 °C. Fractional conversion values obtained in the selective oxidation of propane to propylene over V- and/or Ce-incorporated SBA-15 catalysts synthesized by impregnation and direct synthesis routes (pH , 1.0) are shown in Figure 16a. As shown in this figure, the conversion values were quite stable, especially after the first 20 min. Propylene, carbon monoxide, and carbon dioxide are the main reaction products during the selective oxidation reaction of propane. Catalytic test results for the selective oxidation of propane showed higher propane conversion values over V- and/or Ce-incorporated SBA-15 catalysts prepared by the impregnation method than over the catalysts prepared by the one-pot synthesis route (at pH , 1.0) (Figure 16a). Propylene selectivity was defined as the ratio of the number of moles of propylene formed per mole of propane reacted. The propylene selectivities observed with the V- and bimetallic V-Ce-incorporated materials prepared by the one-pot procedure (at pH , 1.0) were much higher than the corresponding values observed with the catalysts prepared by impregnation (Figure 16b). Among these catalysts, the highest propylene selectivity was observed to be about 70%, for the vanadium-incorporated catalyst prepared by the one-pot procedure. The EDS results indicated only a trace amount of vanadium incorporation into this catalyst. Consequently, its activity was not very high (about 4% propane conversion). However, these quite high selectivity values are highly promising for further investigation with the catalysts synthesized through the one-pot procedure. In the case


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Figure 17. Comparison of (a) fractional conversion of propane and (b) propylene selectivity obtained with catalysts prepared by the one-pot procedure at different solution pH values (T ) 550 °C). Figure 16. (a) Fractional conversion of propane and (b) propylene selectivity obtained with V- and/or Ce-incorporated SBA-15-like catalysts prepared by impregnation and the one-pot synthesis procedure at pH , 1.0 (T ) 550 °C, propane/oxygen/helium ) 6/3/21, total flow rate ) 30 cm3 min-1).

of the catalyst prepared by impregnation of vanadium into SBA15, the propane conversion was higher (about 18%). However, in this case, the selectivity of propylene decreased to values below 50%. This result indicates that the impregnated catalysts enhanced the formation of carbon oxides rather than selective oxidation to propylene. As discussed in an earlier publication of ours2 on the selective oxidation of ethanol over vanadiumincorporated MCM-41-type catalysts, high selectivity for olefins is strongly related to the coordination of the V, Si, and O atoms in the catalyst structure. Functionality of the V-O-Si bridging bonds in the selective oxidation reactions and oxygen liberation of the surface redox sites were also demonstrated in the works of Wachs and co-workers.42,43 The fact that higher propylene selectivity values were observed in the present study for the catalysts prepared by the one-pot procedure than for the impregnated catalysts is considered to be due to the good incorporation of vanadia into the SBA-15 lattice, forming V-O-Si bridges. However, in the case of impregnated catalysts, the presence of extraframework vanadia causes a reduction of the propylene selectivity. In the case of materials prepared by impregnation, the incorporation of Ce into the V@SBA-15 material (V-Ce@SBA15) did not improve the conversion, but caused some decrease in propylene selectivity. These results show that ceria is more effective for the total oxidation reactions, increasing the yields of carbon oxides. Similar results were obtained for the Ceincorporated materials prepared by the one-pot procedure at a solution pH of ,1.0. The variation in the propane conversion and propylene selectivity values demonstrates that they were strongly dependent on the catalyst preparation method. The dispersion of metal oxide on the silica surface is expected to effect the catalytic performance significantly. In the preparation of the Ce-V-incorporated catalysts, the V/Si and Ce/Si molar

Figure 18. Comparison of (a) propane conversion and (b) propylene selectivity at 550 and 600 °C with V-SBA-15m (pH ) 1.5) and V-Ce-SBA-15m (pH ) 1.5).

ratios were adjusted to 0.031 and 0.011 (corresponding to equal weight percentages of V and Ce) in the synthesis solution. To test the effect of the V/Ce ratio of the catalysts on the catalytic performance, another set of catalysts was prepared using a V/Ce molar ratio of 5.6 at both pH ) 1.5 and pH , 1.0. It was interesting to observe that the catalytic performances of these materials were quite similar to the catalytic performances of the materials having a V/Ce molar ratio of 2.8. These results show that the influence of the V/Ce ratio on the propane conversion and propylene selectivity was rather small for the V/Ce ratio values investigated in this work.

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Figure 19. Hydrogen TPR analysis of V- and/or Ce-incorporated materials prepared by (a) impregnation and (b) one-pot hydrothermal procedure at a pH of 1.5.

Considering that V-SBA-15 (pH , 1.0) and V-Ce-SBA15 (pH , 1.0) showed quite high selectivities, these materials were also synthesized at a solution pH of 1.5. The characterization results discussed in the previous sections showed that the pore structures, morphologies, and chemical compositions of the materials prepared at two different pH values were quite different from each other. As shown in Figure 17a, the materials prepared at a solution pH of 1.5 gave much higher propane conversions than the catalysts prepared at a solution pH of ,1.0. The propane fractional conversion value increased from about 4% to about 22% and 15% for the V- and V-Ce-incorporated materials, respectively, when the synthesis solution pH was increased to 1.5. However, this increase in activity caused some reduction in the propylene selectivity for both catalysts (Figure 17b). Better incorporation of vanadium into the structure of the synthesized mesoporous material is considered to be the major reason for the higher activity of the material synthesized at a solution pH of 1.5. The pyridine-adsorbed FT-IR analysis also indicated higher acidities of the materials prepared at a solution pH of 1.5. In the case of V-Ce-incorporated materials, better incorporation of V was not achieved at pH 1.5. However, this material still gave higher propane conversion values. Higher activity of this catalyst could be due to better dispersion of vanadia and/or CeVO4 phase on the surface of this material, than for the material synthesized at pH , 1.0. In fact, the Lewis acidity of this material was also higher than that of the material prepared at pH , 1.0. To see the catalytic performance of a material prepared at an intermediate synthesis solution pH value, vanadium-incorporated SBA-15-like material was also prepared using 0.5 M HCl in the synthesis solution by the one-pot hydrothermal route. The resulting pH of the synthesis solution was about 0.4 in this case. However, EDS analysis indicated that vanadium incorporation was still very low, and no activity enhancement was observed as compared to the catalyst prepared with 2 M HCl (pH , 1.0). To see the effect of temperature on the catalytic performance of the mesoporous materials V-SBA-15m (pH ) 1.5) and V-Ce-SBA-15m (pH ) 1.5), the propane selective oxidation reaction was repeated at 600 °C. The results obtained at 600 °C showed an increase of both propane fractional conversion and propylene selectivity with an increase in temperature for both of these catalysts (Figure 18). This is an interesting result,

indicating the formation of propylene and carbon oxides through parallel reactions. Apparently, the temperature sensitivity of the desired reaction yielding propylene is higher than the temperature sensitivity of undesired total oxidation reactions over these new V- and V-Ce-incorporated materials synthesized by a onepot procedure at a solution pH of 1.5. The best catalytic performance was achieved with V-SBA-15m at 600 °C, yielding about 40% conversion with a propylene selectivity of 62% at a space time of 0.4 s · g · cm-3. In the case of V-Ce-incorporated bimetallic catalyst, 20% propane conversion with a propylene selectivity of over 50% was obtained. The results showed no significant deactivation of the V- and V-Ce-incorporated materials within a reaction period of 100 min. The propylene selectivity values were also highly stable especially after the first 20 min of reaction. In the case of the Ce-impregnated catalyst, some decrease in propane conversion was detected within this reaction period. Another important conclusion reached in this work is related to the activity comparison of vanadia- and ceria-incorporated materials. Because of the presence of highly mobile capping oxygen26 in ceria, activity enhancement of the vanadiaincorporated materials was expected upon ceria incorporation. However, our results clearly show that the vanadia-based catalysts were more active and selective in the selective oxidation of propane than the Ce-incorporated materials. This is thought to be due to the excellent redox ability of vanadiabased catalysts. As shown in Figure 19, hydrogen TPR of vanadia-based catalysts prepared by impregnation and by the one-pot hydrothermal procedure showed reduction peaks in the temperature range of 520-600 °C. However, ceria- and V-Ceimpregnated catalysts (containing CeVO4) did not show a major reduction peak in the temperature range of 200-800 °C. For these catalysts, a broad reduction peak with a very low intensity was observed at about 380 °C. These results confirm the better reducibility of the vanadia-incorporated materials in the temperature range of the reaction (550-600 °C) studied in this work. 4. Conclusions The results of this work demonstrate the importance of the synthesis conditions in terms of the physical and chemical


structures and morphologies of V- and/or Ce-incorporated SBA15-like mesoporous materials, as well as their activities in the selective oxidation of propane. In the case of materials synthesized by a one-pot hydrothermal procedure, an increase of the synthesis solution pH to 1.5 caused major changes in the pore structure of the resulting mesoporous material. The ordered pore structure of SBA-15 changed to complex pore structures that were composed of interconnected bottleneck-shaped and/ or slit-like pores at this pH value. Because of the high reducibility of vanadia in the synthesized materials in the temperature range of 550-600 °C, V-incorporated materials showed better catalytic performance than the Ce- and V-Ceincorporated materials. Vanadium incorporation by the one-pot procedure caused improved propylene selectivity as compared to the catalysts prepared by impregnation. Higher propylene selectivity values observed for the catalysts prepared by the onepot procedure than for the impregnated catalysts is considered to be due to the good incorporation of vanadia into the SBA15 lattice, forming V-O-Si bridges. An increase of the propylene selectivity, as well as the propane conversion, with increasing temperature from 550 to 600 °C indicates the formation of propylene and carbon oxides by parallel reactions over these catalysts. Propylene selectivity values over 60%, obtained at relatively high propane conversions of about 40%, using the V-incorporated catalyst prepared by the one-pot hydrothermal procedure at a synthesis solution pH of 1.5 are considered to be highly promising. Acknowledgment Gazi University Research Found (BAP 06-2007/59) and TUBITAK Scholarship Programme (2210 and 2211) are gratefully acknowledged. We also thank Dr. Meltem Dogan and Saliha Kılıcarslan for the nitrogen adsorption analysis. Literature Cited (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. N. A New Family of Mesoporous Molecular Sieves. Nature 1992, 359, 710. (2) Gucbilmez, Y.; Dogu, T.; Balci, S. Ethylene and Acetaldehyde Production by Selective Oxidation of Ethanol Using Mesoporous V-MCM41 Catalysts. Ind. Eng. Chem. Res. 2006, 45, 3496. (3) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Triblock Copolymer Synthesis of Mesoporous Silica with Periodic 50 to 300 Angstrom Pores. Science 1998, 279, 548. (4) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, F.; Stucky, G. D. Nonionic Triblock and Star Ciblock Copolymer and Oligometric Surfactant Syntheses of Highly Ordered, Hydrothermally Stable Mesoporous Silica Structures. J. Am. Chem. Soc. 1998, 121, 6024. (5) Jin, Z.; Wang, X.; Cui, X. Synthesis and Morphological Investigation of Ordered SBA-15-type Mesoporous Silica with an Amphiphilic Triblock Copolymer Template under Various Conditions. Colloids Surf. A 2008, 316, 27. (6) Kim, J. M.; Jun, S.; Ryoo, R. Improvement of Hydrothermal Stability of Mesoporous Silica Using Salts: Reinvestigation for Time-Dependent Effects. J. Phys. Chem. B. 1999, 103, 6200. (7) Sener, C.; Dogu, T.; Dogu, G. Effects of Synthesis Conditions on the Structure of Pd Incorporated MCM-41 Type Mesoporous Nanocomposite Catalytic Materials with High Pd/Si Ratios. Microporous Mesoporous Mater. 2006, 94, 89. (8) Zhang, X.; Yue, Y.; Gao, Z. Chromium Oxide Supported on Mesoporous SBA-15 as Propane Dehydrogenation and Oxidative Dehydrogenation Catalysts. Catal. Lett. 2002, 83, 19. (9) Liu, Y. M.; Cao, Y.; Yi, N.; Feng, W. N.; Dai, W. L.; Yan, S. R.; He, H. Y.; Fan, K. N. Vanadium Oxide Supported on Mesoporous SBA15 as Highly Selective Catalysts in the Oxidative Dehydrogenation of Propane. J. Catal. 2004, 224, 417. (10) Gucbilmez, Y.; Dogu, T.; Balci, S. Vanadium Incorporated High Surface Area MCM-41 Catalysts. Catal. Today 2005, 100, 473.

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ReceiVed for reView October 27, 2009 ReVised manuscript receiVed February 4, 2010 Accepted February 19, 2010 IE901672B

Effect of Synthesis Conditions on the Structure and Catalytic

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