Vanadium Oxide Supported on Titanosilicates for the Oxidative

Mar 26, 2015 - H2-TPR, O2-TPD, and XPS results show that lattice oxygen exists in the surface of the V-containing titanosilicate catalysts and enhance...
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Vanadium Oxide Supported on Titanosilicates for the Oxidative Dehydrogenation of n‑Butane Cun Wang,† Jian-Gang Chen,† Tian Xing,† Zhao-Tie Liu,*,† Zhong-Wen Liu,† Jinqiang Jiang,† and Jian Lu*,‡ †

Key Laboratory of Applied Surface and Colloid Chemistry (MOE) and School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an 710119, China ‡ Department of Catalytic Technology, Institute of Xi’an Modern Chemistry, Xi’an 710065, China ABSTRACT: Vanadium-containing titanosilicates and V-containing SBA-15 catalysts for oxidative dehydrogenation (ODH) of n-butane at lower temperatures and with lower vanadia contents were contrastively studied. The catalysts were investigated by various techniques, namely, N2 adsorption−desorption, SAXS, TEM, FT-IR, XRD, XRF, H2-TPR, O2-TPD, and XPS, in relation to their performance for the ODH of n-butane. Results reveal that titanosilicate materials synthesized exhibit mesoporous structure, high BET specific surface area, and high total pore volume. H2-TPR, O2-TPD, and XPS results show that lattice oxygen exists in the surface of the V-containing titanosilicate catalysts and enhances the reducibility of vanadia-based catalysts with increasing TiO2 loadings. The V-containing titanosilicate catalysts exhibit much higher catalytic activity for the ODH of n-butane than that of V-containing SBA-15 at a significantly lower temperature of 460 °C, which indicates that lattice oxygen in the catalyst plays an important role in the activation of n-butane.

1. INTRODUCTION Butenes and butadienes, which are valuable intermediates for producing synthetic rubbers, plastics, and various industrial useful chemicals, are obtained traditionally via oxidative dehydrogenation (ODH) of n-butane. Compared with the direct dehydrogenation (DH) of feedstocks containing different hydrocarbons at higher reaction temperatures, the catalytic oxidative dehydrogenation (ODH) for n-butane is an attractive selective way in the synthesis for C4 alkenes that is thermodynamically favored at much lower reaction temperatures and generally will not form coke and deactivate catalytic performances.1,2 Vanadium oxide is an effective oxidation−reduction catalyst in various chemical processes, and it is also taken as a catalyst in the oxidation reaction in ODH.3,4 The catalytic performance over vanadium oxide catalyst for ODH strongly depends on the dispersion degree of vanadium species, and also the types of supports have dramatic effects.4−6 Recently, the ordered mesoporous material SBA-15 has received enormous attention as being used in catalyst supports. Compared with MCM materials and conventional silica, advantages of SBA-15 have been attributed to high surface area, regular framework, large pore sizes with narrow distribution, and lower surface acidity, which will benefit the mass transfer in the reaction conditions and good dispersion of vanadium species, thereby leading to excellent catalytic performance for the ODH of n-butane. In general, V-containing SBA-15 catalysts have excellent catalytic performance at high reaction temperatures (≈560 °C). However, the V-containing SBA-15 catalyst indicates very low catalytic activities for the ODH of n-butane that even can be ignored at a lower temperature of 460 °C. It has been reported that oxygen vacancies reside in the bulk TiO2 (anatase).7−9 Oxygen vacancies play a critical role in the electronic properties of TiO2.10,11 Oxygen (O2) adsorbed over © 2015 American Chemical Society

metal oxide is a critical factor for the catalytic oxidation reaction. The adsorption between O2 and the surface of metal oxide demands transfer of negative charges provided by the oxygen vacancy. Therefore, oxygen vacancies have a critical role for transformation and formation of oxygen species. Results obtained via scanning tunneling microscopy and calculated using theory confirmed the formation of lattice oxygen.9 Based on the Mars−van Krevelen mechanism, lattice oxygen plays a critical role in activation of n-butane and abstraction of the first H atom. Despite important potential uses of TiO2 for the ODH of n-butane, results reported in the literature are rare.12 However, pure TiO2 supports have some defects, such as comparatively low specific surface areas and irregular pore structure.13 Lower surface areas of supports will prevent the dispersion of vanadium oxide species and irregular pore structure does not benefit mass transfer, which lead to the further decrease in selectivity to C4 alkenes. In this work, we reported a one-pot synthetic method for Ti−SBA-15 supports featuring mesopore structure with higher contents of Ti. Synthesized V-containing titanosilicates exhibited excellent catalytic activity for the ODH of n-butane at a significantly lower temperature (at 460 °C). The effect of TiO2 on catalytic activity was discussed based on the physicochemical properties of the catalysts obtained by N2 adsorption−desorption, small-angle X-ray scattering (SAXS), powder X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), hydrogen temperature-programmed reduction (H2Received: Revised: Accepted: Published: 3602

January 1, 2015 March 24, 2015 March 26, 2015 March 26, 2015 DOI: 10.1021/acs.iecr.5b00007 Ind. Eng. Chem. Res. 2015, 54, 3602−3610

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Industrial & Engineering Chemistry Research

were measured with a step size of 0.02° and a scan rate of 0.2 s· step−1. The N2 adsorption−desorption isotherm was taken over a BelSorp-Max (Bel Japan Inc.) instrument at −196 °C. Prior to analysis, each sample (ca. 100 mg) was degassed under vacuum (10−2 kPa) at 300 °C for 10 h to remove any contaminant and physically adsorbed moisture. The surface areas of samples were calculated by the Brunauer−Emmett−Teller (BET) method, and the pore size distributions were resolved on the basis of the Barrett−Joyner−Halenda (BJH) method using the data of desorption branches. Transmission electron microscopic (TEM) images of samples were obtained on a JEM-2100 transmission electron microscope (JEOL, Japan). The sample powder was ultrasonically suspended in ethanol, deposited on a carbon-enhanced copper grid, and dried in air. X-ray fluorescence (XRF) spectra were recorded on a Shimadzu X-ray fluorescence spectrometer. Hydrogen temperature-programmed reduction (H2-TPR) spectra were obtained on a Micromeritics Autochem 2920 (Micromeritics, USA) instrument loaded with 50 mg of catalysts. The samples were pretreated in flowing argon at 300 °C for 1 h and then cooled to 50 °C in argon. Subsequently, the H2/Ar mixture (H2/Ar molar ratio of 10/ 90 and a total flow of 25 mL·min−1) was introduced and the temperature was raised to 900 °C at a heating rate of 10 °C· min−1. H2 consumption was monitored using a thermal conductivity detector. The temperature-programmed desorption of O2 (O2-TPD) was performed in a quartz tube reactor system equipped with a mass spectrometer (QIC-20, Hiden Analytical Ltd.). A 50 mg sample of catalyst (40−60 mesh) was pretreated in Ar at 300 °C for 90 min and then cooled to 50 °C. Thereafter, the catalysts were contacted with pure O2 (30 mL·min−1) for 60 min. Subsequently, He (30 mL·min−1) was then introduced instead of pure O2, and then heated at a rate of 10 °C·min−1 until 900 °C. The mass signals of O2 (m/z = 32) were recorded. The signals were smoothed by the Savitzky−Golay method (points of window = 80, polynomial order = 4). X-ray photoelectron spectra (XPS) were recorded on an AXIS ULTRA X-ray photoelectron spectrometer (Kratos Analytical Ltd.) equipped with an Al monochromatic X-ray source (Al Kα = 1486.6 eV) at a room temperature under vacuum (approximately 5 × 10−9 Torr). The C 1s binding energy of contaminant carbon (284.8 eV) was taken as standard to calibrate any charge-induced peak shifts. 2.3. Activity of Catalyst Testing. The performances of catalysts were carried out in a fixed-bed stainless steel tubular flow reactor (8 mm i.d., 300 mm length). The reactor was equipped with a coaxial thermocouple for temperature measurement in the catalytic bed. The feed was controlled by mass flow controllers and consisted of a mixture of n-butane/ oxygen/nitrogen with a molar ratio of 1/2/7. Typically 100 mg of catalyst (40−60 mesh) was loaded in the catalyst bed and diluted with 500 mg of quartz powder (40−60 mesh) to avoid overheating the catalyst bed. The catalytic activities were investigated at 460 °C under fixed condition, and the time on stream for catalytic results (5 h) is listed in Table 2. In the present study, a space-time yield (STY) of C4 alkenes and gas hourly space velocity (GHSV) are related to the catalyst weight investigated. The starting gases and products were confirmed online by a gas chromatograph (GC-9560, Shanghai Huaai Chromatographic Analysis Corp., Ltd.) equipped with a

TPR), temperature-programmed desorption of oxygen (O2TPD), and X-ray photoelectron spectroscopy (XPS).

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Ti−SBA-15 samples with various Si/Ti molar ratios were synthesized by a sol−gel method reported by Dalai et al.14 The molar gel compositions of the mixture were tetraethyl orthosilicate (TEOS):Ti(OiPr)4:P123:HCl:H2O = 0.988:x:0.016:0.46:127. The typical synthesis procedure of Ti−SBA-15 with molar ratio of Si/Ti = 10 was as follows: 4.69 g of Pluronic P123 (EO20PO70EO20, M = 5800, Aldrich) was dissolved in 115.6 g of deionized water by stirring for 2 h at 40 °C. Thereafter, 2.29 g of HCl (36−38%) was added to the above mixture with stirring for another 2 h. Then, a mixture of 10.61 g of TEOS (Kermel, 98.0%) and 1.49 g of titanium isopropoxide (Heowns, 95%) was added slowly. The synthesized mixture was stirred for 24 h at 40 °C and then for 5 h at 100 °C. Subsequently, the resulting gel was aged under static conditions for 43 h at 100 °C. The precipitate solid was filtered, washed with deionized water, and dried at 100 °C for 12 h. Finally, the solid product was calcined in air at 550 °C for 6 h with a heating rate of 1 °C·min−1 to remove the template. The sample was denoted as Ti−SBA-15(10), where “10” denotes the Si/Ti molar ratio in the material. SBA-15 samples were synthesized in a round-bottomed flask as reported15 by Cejka et al. A triblock copolymer, that is, Pluronic P123 (EO20PO70EO20, M = 5800, Aldrich) was used as a structure directing agent, and TEOS was used as a silica precursor. The synthesis molar ratio was TEOS:HCl:P123:H2O = 1:6.2:0.017:197. Typically SBA-15 was synthesized by mixing Pluronic P123 (EO20PO70EO20, M = 5800, Aldrich) with a mixture of deionized water and HCl. The synthesized mixture was vigorously stirred at 35 °C until a clear solution was obtained. Thereafter, TEOS (Kermel, 98.0%) was added and the solution was stirred for 5 min. Then the synthesized mixture was aged under static conditions at 35 °C for 24 h and then at 97 °C for 48 h. The solid products were obtained by filtration, extensively washed with deionized water and ethanol, and finally dried at 100 °C for 12 h. Calcination was carried out in order to remove the template in a stream of air at 550 °C for 6 h at a heating rate of 1 °C·min−1. The vanadia-based catalysts (1.0 wt % V) were prepared by an excessive impregnation method from a water solution of NH4VO3 (Tianjin Fuchen, 99.0%). After dissolving a fixed amount of NH4VO3 to excessive deionized water at 80 °C, the desired amount of the support was added and the solution was stirred at 80 °C until the solution evaporated completely. The sample prepared via the impregnation method was dried at 100 °C in air for 12 h and then calcined at 550 °C for 4 h under an atmosphere with a heating rate of 2 °C·min−1. The synthesized catalyst was assigned as 1V−Ti−SBA-15(X), where X denotes the Si/Ti molar ratio in the sample. 2.2. Catalyst Characterization. The infrared spectral characterization of samples was performed using a Tensor 27 (Bruker, Germany) FT-IR spectrometer. Samples were pressed into pellets by using dry KBr and then scanned in the range 4000−400 cm−1 with a resolution of 2 cm−1. The blank KBr pellet was also scanned to perform background correction. Small-angle X-ray scattering (SAXS) and powder X-ray diffraction (XRD) data were collected on a D8 Advance X-ray diffractometer (Bruker, Germany) using monochromatized Cu Kα radiation operating at 40 kV and 40 mA. All of the samples 3603

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Industrial & Engineering Chemistry Research thermal conductivity detector (TCD) and a flame ionization detector (FID). The feed (n-butane) conversion and selectivity to product were based on the carbon balance. The carbon balances for all catalysts tested in this work were 98 ± 3%. It is found that there was no coke deposition over the catalyst due to excess oxygen. Blank experiments without catalyst loading revealed that the homogeneous reactions of feed could be ignored under the experimental conditions.

titanium and propyl alcohol generated from hydrolysis of Ti(OiPr)4. Introduction of the titanium causes expansion of the silica framework that resulted from the tension formed from the titanium oxide, resulting in increasing the pore diameter. Moreover, propyl alcohol as micelle expanders also enhances the pore diameter. With incorporation of more titanium in the SBA-15 matrix, titanium oxide is assembled into the pore wall and causes an enlargement in the pore wall size, resulting in decreasing of the pore diameter. An obvious decrease in intensity of the peak of the (100) lattice plane with increasing TiO2 loading for Ti−SBA-15(10) and Ti−SBA-15(5) samples is observed. The decrease in intensity of the peak of the (100) lattice plane may probably be induced by a partial collapse of SBA-15. With incorporation of more titanium in the SBA-15 matrix and assembling of titanium oxide, the partial structural collapse may be caused. TEM images (Figure 2a,b) reveal that an ordered and uniformed mesoporous structure was obtained, which indicated that the well-defined frameworks of SBA-15 were maintained while SBA-15 was modified with titanium isopropoxide at a Si/ Ti molar ratio of 30. However, the mesoporous structure was disordered while SBA-15 was modified with titanium isopropoxide at Si/Ti molar ratios of 10 (Figure 2c) and 5 (Figure 2d). TEM images indicate the partial structural collapse of the SBA-15 ordered mesoporous after incorporation of TiO2, which is consistent with the results obtained from SAXS. However, mesoporous structure was retained as indicated from the results of SAXS and N2 adsorption−desorption data. The N2 adsorption−desorption isotherm is of type IV and shows hysteresis of H1 type. The pore size distribution of the samples shown in the inset of Figure 3 distinctly reveals that mesoporous structure was preserved. The BET specific surface areas, total pore volumes, and pore diameter are listed in Table 1. Both SBA-15 and Ti−SBA-15 samples exhibit reasonable high BET specific surface areas and total pore volumes which are in the ranges 780−990 m2·g−1 and 0.9−1.4 cm3·g−1, respectively. The BET specific surface areas and total pore volumes of vanadium-containing samples are lower than those of the corresponding unsupported samples. The pore diameter of samples dramatically decreases from 6.2 to 3.7 nm after introduction of TiO2 and vanadia except for the Ti−SBA15(30) sample. Compared with the SBA-15, the pore diameter of Ti−SBA-15(30) increased. The IR spectra of samples in KBr pellets are shown in Figure 4 in the range 4000−400 cm−1, in which the models can be assigned as skeletal vibrations. The bands at 1229, 1092, 964, 799, and 465 cm−1 in the spectrum are characteristic for a silica

3. RESULTS 3.1. Characterization of Materials. SAXS patterns are observed if uniformed mesoporous structure is presented in the material. Figure 1 shows the recorded SAXS patterns for

Figure 1. Small-angle X-ray scattering patterns of (a) SBA-15, (b) Ti− SBA-15(30), (c) Ti−SBA-15(10), and (d) Ti−SBA-15(5).

mesoporous titanosilicates with different molar ratios of Si/Ti. The SAXS patterns of SBA-15 display three well-resolved peaks, which can be designated to the (100), (110), and (200) lattice planes. Compared with SBA-15, the peak of the (100) lattice plane of Ti−SBA-15(30) shifts to a lower 2θ value, indicating the increase in the unit-cell size, which is in agreement with the result of the pore diameter measured by the BJH method as shown in Table 1. The SAXS peaks of Ti−SBA15(30) showed that a hexagonal lattice having a d(100) spacing of 96.9 Å was revealed, which demonstrated a unit-cell parameter of a0 = 11.2 nm (a0 = 2[d(100)]/(3)1/2) that was also reported by Luan et al.16 The unit-cell parameter a0 of SBA-15 is 10.9 nm with a d(100) spacing of 94.4 Å. The change in the pore diameter may be caused by introduction of the Table 1. Chemical Composition and Characteristics of Samples samples

TiO2a (wt %)

Va (wt %)

SBET (m2·g−1)

Vpb (cm3·g−1)

Dpc (nm)

Tonsetd (°C)

Tmaxe (°C)

Δef

SBA-15 1V−SBA-15 Ti−SBA-15(30) Ti−SBA-15(10) Ti−SBA-15(5) 1V−Ti−SBA-15(30) 1V−Ti−SBA-15(10) 1V−Ti−SBA-15(5)

− − 4.3 11.6 19.7 4.3 11.4 19.4

− 1.0 − − − 1.0 1.0 1.0

889 731 990 910 786 691 679 632

1.07 0.92 1.31 1.04 0.94 0.90 0.70 0.75

6.2 6.2 7.1 4.8 4.2 3.7 3.7 3.7

− 376 − − − 370 362 349

− 480 − − − 479 474 435

− 1.08 − − − 0.98 0.89 0.82

a Titanium oxide and vanadium content determined by XRF method. bTotal pore volumes calculated by the BET method at p/p0 = 0.990. cDp, pore diameter determined by the BJH method. dOnset temperature of H2-TPR profile. ePosition of maximum temperature of H2-TPR profile. fAverage change of oxidation state per vanadium atom based on H2 consumption during H2-TPR experiment.

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Figure 2. TEM images of (a) SBA-15, (b) Ti−SBA-15(30), (c) Ti−SBA-15(10), and (d) Ti−SBA-15(5).

Figure 3. N2 adsorption−desorption isotherms for (a) SBA-15, (b) Ti−SBA-15(30), (c) Ti−SBA-15(10), and (d) Ti−SBA-15(5). Insets show the pore size distribution (PSD) of the corresponding samples.

Figure 4. FT-IR spectra of (a) SBA-15, (b) Ti−SBA-15(30), (c) 1V− Ti−SBA-15(30), (d) Ti−SBA-15(10), (e) 1V−Ti−SBA-15(10), (f) Ti−SBA-15(5), and (g) 1V−Ti−SBA-15(5).

network.17,18 The bands at 1229 and 1092 cm−1 are assigned to asymmetric stretching vibrations of Si−O−Si. The bands at 799 and 465 cm−1 can be assigned to symmetric stretching vibrations of Si−O−Si and Si−O bending, respectively.17 In general, the band at approximately 960 cm−1 can be assigned to the vibration of Ti−O−Si for titanosilicate materials.19−22 With introduction of the titanium, the intensity of the band at approximately 960 cm−1 increases, indicating that titanium was successfully incorporated into the silica framework. The broad bands assigned to crystalline TiO2 at 600−650 cm−1 were detected in the spectra of all Ti−SBA-15 samples.23−25 Results

indicate that titanium oxide assembles and forms crystalline TiO2. The XRD patterns of supports and catalysts are shown in Figure 5. A wide peak having a lower intensity at 14−30° was detected in the X-ray powder pattern of SBA-15, which can be designated to the existence of amorphous SiO2 wall. Diffraction peaks belong to TiO2 (anatase) crystallites were detected in all Ti−SBA-15 samples, indicating formation of separate crystallites of TiO2 (anatase). However, the absence of diffraction peaks belong to V2O5 crystallites indicates that vanadium 3605

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of V-containing mesoporous titanosilicates is enhanced due to the presence of TiO2. Figure 7 shows the O2-TPD profiles of the catalysts. The desorption peaks from 50 to 900 °C may be assigned to

Figure 5. X-ray diffraction patterns for (a) SBA-15, (b) Ti−SBA15(30), (c) 1V−Ti−SBA-15(30), (d) Ti−SBA-15(10), (e) 1V−Ti− SBA-15(10), (f) Ti−SBA-15(5), and (g) 1V−Ti−SBA-15(5).

species are finely spread and maintain a similar distribution on the surface of supports. H2-TPR profiles of catalysts are depicted in Figure 6. The average change of the oxidation state (Δe) per vanadium atom

Figure 7. O2-TPD results of (a) 1V−SBA-15, (b) 1V−Ti−SBA15(30), (c) 1V−Ti−SBA-15(10), and (d) 1V−Ti−SBA-15(5).

O2(ad), O2−(ad), O22−(ad), O−(ad), and lattice oxygen (O2−), respectively.32,33 Usually, the physically adsorbed oxygen (O2(ad)) is easier to desorb than the lattice oxygen. The peak at the highest temperature (∼800 °C) is assigned to lattice oxygen. The lattice oxygen desorbed in 1V−SBA-15 catalyst is hardly found in the O2-TPD profiles; nonetheless the lattice oxygen desorption peaks are unambiguous after the introduction of TiO2. The results mentioned demonstrated that TiO2 existing in the catalysts increased remarkably the content of the lattice oxygen, which is important for the ODH of n-butane. Figure 8 shows the O 1s XPS spectra of 1V−SBA-15, 1V− Ti−SBA-15(30), and 1V−Ti−SBA-15(5) samples. The O 1s spectrum of 1V−SBA-15 was analyzed in terms of two components assigned to silicon dioxide (∼533.0 eV) and VOx (∼531.6 eV).1 As is seen from Figure 8, the O 1s spectra of 1V−Ti−SBA-15(30) and 1V−Ti−SBA-15(5) samples can be well fitted into seven peaks at ∼529.6, ∼530.1, ∼530.7, ∼531.6, ∼533.0, ∼534.3, and ∼536 eV. Five new peaks are observed. Moreover, the intensity of these peaks increases with increasing TiO2 content. The peak at 529.6 eV is attributed to lattice oxygen.34,35 The O− is presented in the anatase, reflected by EPR in a recent paper.36 Therefore, the peak at 530.1 eV can be expected to be attributed to O−. The peaks at 530.7 and 531.6 eV are attributed to TiO2 and Ti−O−Si groups, respectively.37,38 Compared with the pure TiO2 (anatase), Vcontaining titanosilicate O 1s peaks attributed to TiO2 shifted slightly toward a higher binding energy, which indicated that the electron density of oxygen decreased and more oxygen vacancies formed over V-containing titanosilicates. The peaks at 534.3 and 536 eV can be expected to be attributed to O2− and chemisorbed oxygen (O2), which is consistent with the results of O2-TPD at relatively lower temperature. Relative amounts of O− and O2− are represented in Figure 8 by using the ratios of their intensities, namely, I(O−)/I(O2−). The I(O−)/I(O2−) values range from 0.65 to 0.88 with increasing TiO2 loading. The reason may be that formation of more oxygen vacancies causes decreasing in the electronic density of oxygen, hindering electron transfer among oxygen species.

Figure 6. H2-TPR results of (a) 1V−SBA-15, (b) 1V−Ti−SBA15(30), (c) 1V−Ti−SBA-15(10), and (d) 1V−Ti−SBA-15(5).

based on H2 consumption was calculated and is shown in Table 1. The average Δe varied from 0.82 to 1.08 electrons/V atom, suggesting reduction from V5+ to V4+ or from V5+ to V3+. The H2-TPR profiles of all catalysts exhibit only one sharp reduction peak at 340−600 °C. An advancing shift of the reduction maximum temperature (Tmax) of the reduction peak to a lower temperature is observed for the V-containing mesoporous titanosilicates with an increase in TiO2 loading; a similar trend for the reduction onset temperature (Tonset) is confirmed. By analyzing previous studies on the reducibility of V-containing mesoporous titanosilicates and other V-containing mesoporous materials reported in the literature,13,26−29 the reduction peaks can be assigned to the reduction of both monomeric and oligomeric tetrahedral-like coordinated vanadium species. The presence of TiO2 (anatase) in the support increases the reducibility of vanadia-based catalysts.30,31 In the present work, similar results were obtained. This indicates that the reducibility 3606

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Figure 10. V 2p XPS spectra for (a) 1V−SBA-15, (b) 1V−Ti−SBA15(30), and (c) 1V−Ti−SBA-15(5).

ratio of 1:2:7. The results are presented in Tables 2 and 3. The C4 alkenes identified in the reaction mixture were 1-butene (1C4), cis- and trans-2-butenes (c-C4 and t-C4), and 1,3-butadiene (1,3-C4). The n-butane conversion obtained over SBA-15 under the investigated conditions did not exceed 1.5%. Compared with SBA-15, the catalytic activity of titanosilicate samples is significantly increased. Moreover, the n-butane conversion increases with increasing TiO2 loading and 6% conversion of nbutane is obtained over Ti−SBA-15(5). The alkanes can be activated by the Ti−Si matrix that was also reported in the ODH processes for n-butane and propane.13,40 The catalytic activity of V-containing titanosilicates is higher than that of Vcontaining SBA-15. Moreover, the catalytic activity over Vcontaining titanosilicates enhances with increasing TiO2 loading. The catalytic activity reached a maximum conversion of n-butane (23.6%) over 1V−Ti−SBA-15(5) catalyst under the investigated conditions. The conversion of the feed and the selectivity to the corresponding light alkenes are also important for assessment of the ODH catalyst. The selectivities to ODH products are shown in Table 2. The selectivity to C4 alkenes over 1V−SBA15 and 1V−Ti−SBA-15(30) catalysts reaches about 60%. However, a rapid drop in selectivity to C4 alkenes is presented with increasing TiO2 loading, due to the deep oxidation of the ODH products. The decrease in selectivity to C4 alkenes is more than 27%. Turnover frequencies (TOFs) of C4 alkene products produced over vanadia-based catalysts were calculated and are presented in Table 3. For comparison, the catalytic results obtained over 1V−SBA-15 at 560 °C are also listed in Table 3. The TOF value increases remarkably with increasing TiO2 loading. Moreover, the TOF value over 1V−Ti−SBA-15(5) catalyst presents a maximum value of 3.8 μmol of ∑C4·V−1·s−1 that is higher than over 1V−SBA-15 at 560 °C. Compared with the V/7.11Mg−SBA-15 (Mg/V = 7.11 denotes Mg/V molar ratio, 6.9 wt % V), 24VMgO (24 wt % V2O5, 13.4 wt % V), V− MSM (6.4 wt % V), and V−Ti−SiO2 (2.8 wt % V) catalysts, the 1V−Ti−SBA-15(5) catalyst exhibits a much higher TOF value for the ODH of n-butane.1,12,41 However, compared with the 1V−Ti−SBA-15(5) catalyst, the Mg3V2O8 (at 550 °C and 3.5 wt % V) catalyst reaches a higher TOF value for the ODH of nbutane due to higher reaction temperature.42

Figure 8. O 1s XPS spectra for (a) 1V−SBA-15, (b) 1V−Ti−SBA15(30), and (c) 1V−Ti−SBA-15(5). Insets show the O 1s XPS spectra of the corresponding samples at 528−532 eV.

The Ti 2p XPS spectra are shown in Figure 9. As shown in Figure 9, the binding energies 458.8 and 464.7 eV of Ti 2p on

Figure 9. Ti 2p XPS spectra for (a) 1V−Ti−SBA-15(30) and (b) 1V− Ti−SBA-15(5).

1V−Ti−SBA-15(30) and 1V−Ti−SBA-15(5) are unchanged. Both of them belong to characteristic peaks of TiO2.39 As shown in Figure 10, the binding energy of V 2p2/3 on all samples is broad from 514.8 to 519.0 eV. With increasing TiO2 loading, the binding energies of V 2p2/3 are consistent, which indicates that the existential state of vanadia is similar. 3.2. Catalyst Performance for the ODH of n-Butane. The performance of catalyst was investigated using a fixed-bed stainless steel tubular flow reactor at 460 °C with a constant GHSV of 48 000 L·kgcat−1·h−1 and C4H10:O2:N2 with a molar 3607

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Industrial & Engineering Chemistry Research Table 2. Oxidative Dehydrogenation Results of n-Butane over Samples at 460 °C conva (%)

selectivity (%)

yield (%)

sample

C4H10

1-C4

c-C4

t-C4

1,3-C4

C1−C3b

COxb

∑C4b

∑C4

SBA-15 1V−SBA-15 Ti−SBA-15(30) Ti−SBA-15(10) Ti−SBA-15(5) 1V−Ti−SBA-15(30) 1V−Ti−SBA-15(10) 1V−Ti−SBA-15(5)

1.4 2.1 1.6 3.5 6.0 6.1 13.3 23.6

16.0 20.4 15.9 11.5 10.5 15.1 11.5 7.2

11.4 12.6 11.7 9.7 9.5 14.4 11.8 7.5

12.8 15.4 12.9 11.1 11.3 18.1 15.4 9.9

7.7 11.3 5.0 6.4 6.6 12.4 10.4 7.6

8.5 6.0 9.7 7.4 5.6 2.4 0.9 0.9

43.6 34.3 44.8 53.9 56.5 37.6 50.0 66.9

47.9 59.7 45.5 38.7 37.9 60.0 49.1 32.2

0.7 1.3 0.7 1.4 2.3 3.7 6.5 7.6

Reaction conditions: Wcat = 0.1 g, GHSV = 48 000 L·kgcat−1·h−1, reaction time = 5 h, C4H10:O2:N2 = 1:2:7. bC1−C3, i.e., sum of C1−C3 hydrocarbons and acetaldehyde; COx products, i.e., CO2 and CO; ∑C4, i.e., sum of selectivity and yield to C4 alkenes, respectively. a

Table 3. Oxidative Dehydrogenation Results of n-Butane over Vanadia-Based Catalysts catalyst 1V−SBA-15 1V−SBA-15 1V−Ti−SBA15(30) 1V−Ti−SBA15(10) 1V−Ti−SBA15(5)

T (°C)

TOFa × 1020 (μmol of ∑C4·V−1·s−1)

STY∑C4b

460 560 460

0.6 3.1 1.9

0.15 0.74 0.44

460

3.3

0.78

460

3.8

0.90

high dispersion of vanadia on supported materials. Moreover, the binding energies of V 2p2/3 are consistent, which indicates that the existential state of vanadia is similar. Lattice oxygen was detected by O2-TPD and XPS on Vcontaining titanosilicate catalysts, which was consistent with the results reported by Setvin et al.9 Lattice oxygen over the catalyst surface was highly activated, which was important for the activation of n-butane and removal of the first hydrogen atom from n-butane. H2-TPR profiles of catalysts showed that the reduction onset temperature and maximum temperature shifted to lower temperature with increasing TiO2 loading. The results revealed that lattice oxygen enhanced the reducibility of catalysts. Based on the Mars−van Krevelen mechanism, n-butane is activated by lattice oxygen and abstracts an H atom from the nbutane molecule to form an adsorbed butyl radical and an −OH surface group.43,44 The step required the presence of very reactive oxygen (O2−) over the catalyst surface. In terms of the reaction rate, the step played a key role in the ODH of nbutane. Then, the butyl radical was absorbed and abstracted the second H atom corresponding to the adjacent carbon atom to form C4 alkenes. In terms of catalytic activity, the conversion of n-butane and the number of lattice oxygen species in the catalyst increased with an increase of TiO2 loading. However, the selectivity to C4 alkenes decreased. This may be caused by O−, O2−, and O22−. Surface oxygen species are classified into electrophilic (O−, O2−, and O22−) and nucleophilic (O2−) types.45 Electrophilic oxygen species are electron-deficient and attack the electron-rich CC bonds in alkenes, resulting in the rupture of the carbon skeleton and succeeding inflammation. The I(O−)/I(O2−) values range from 0.65 to 0.88 with increasing TiO2 loading. Figure 8 also presents that the intensities of the O−, O2−, and O22− peaks are enhanced with increasing TiO2 loading. It was clear that increasing electrophilic oxygen species caused decreasing selectivity to C4 alkenes.

a

Rate of conversion of C4 alkenes per V atom of catalyst per time. Rate of formation of C4 alkenes per unit mass of catalyst per time, STY∑C4 (space-time yield) in kg·kgcat−1·h−1.

b

The space-time yield (STY) of C4 alkenes is shown in Table 3. The changed trend of the STY and the TOF of C4 alkenes over vanadia-based catalysts are consistent. Until now, the excellent STY values of C4 alkenes obtained over Mg3V2O8 (at 550 °C and 3.5 wt % V) and V−MSM (at 540 °C and 6.4 wt % V) catalysts were about 4.65 and 1.92 kg·kgcat −1·h −1, respectively.41,42 But the results were obtained at higher reaction temperature and with higher vanadia loading. Table 3 shows that the STY of C4 alkenes obtained over 1V−Ti− SBA-15(5) catalyst was 0.9 kg·kgcat−1·h−1, demonstrating a quite good performance of the 1V−Ti−SBA-15(5) catalyst at lower temperature (at 460 °C) and with lower vanadia loading (1 wt % V).

4. DISCUSSION The FT-IR spectra revealed that titanium was successfully incorporated into the SBA-15 matrix. The increase of the FT-IR spectrum band at 960 cm−1 in intensity, assigned to the vibration of Ti−O−Si for titanosilicate materials, indicated that Ti was incorporated into the SBA-15 matrix. Besides, the features of crystalline TiO2 with the characteristics of a wide band at 600−650 cm−1 were observed in the spectra of all Ti− Si supports. This finding was consistent with the results of XRD measurements. Structure characterization of titanosilicate materials by SAXS, N2 adsorption−desorption, and TEM revealed the presence of mesoporous structure, high BET specific surface areas, and total pore volumes. However, the results indicated the partial structural collapse of the ordered and uniformed mesoporous SBA-15 after incorporation of TiO2. The vanadia-based catalysts obtained by the excessive impregnation method from water solution of NH4VO3 showed

5. CONCLUSIONS A highly active vanadia-based catalyst for the ODH of n-butane at lower temperature and with lower vanadia content was developed by using titanosilicate mesostructure material as a support. The vanadia-containing titanosilicate catalysts are highly active for the activation and selective conversion of nbutane to C4 alkenes at a significantly lower temperature. Compared with the 1V−SBA-15 catalyst, the V-containing titanosilicate catalysts exhibit much higher STY values of C4 alkenes. Moreover, the V-containing titanosilicate catalysts present much higher values than the V/7.11Mg−SBA-15 (Mg/ 3608

DOI: 10.1021/acs.iecr.5b00007 Ind. Eng. Chem. Res. 2015, 54, 3602−3610

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Industrial & Engineering Chemistry Research

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V = 7.11 denotes Mg/V molar ratio, 6.9 wt % V), 24VMgO (24 wt % V2O5, 13.4 wt % V), V−MSM (6.4 wt % V), and V−Ti− SiO2 (2.8 wt % V) catalysts in the TOF value of ∑C4 alkenes reported in the literature.1,12,41 The catalyst characterization results show that mesoporous structure of titanosilicate samples is preserved and show the presence of lattice oxygen over the surface of the V-containing titanosilicate catalysts. Lattice oxygen plays a critical role in the activation of n-butane.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +86 29 81530802. Fax: +86 29 81530727. (Z.-T. Liu). *E-mail: [email protected]. Tel.: +86 29 88291213. Fax: +86 29 88291213. (J. Lu). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (21176151, 21327011, 21306111), the Program for Changjiang Scholars and Innovative Research Team in University (IRT_14R33), the Shaanxi Innovative Team of Key Science and Technology (2012KCT-21, 2013KCT-17), and the Fundamental Research Funds for the Central Universities (GK201401001).

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REFERENCES

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DOI: 10.1021/acs.iecr.5b00007 Ind. Eng. Chem. Res. 2015, 54, 3602−3610