Dehydrogenation of n-butane over vanadia catalysts supported on silica gel

Journal of Natural Gas Chemistry 18(2009)88–93

Dehydrogenation of n-butane over vanadia catalysts supported on silica gel Yuebing Xu, Jiangyin Lu∗ , Mei Zhong, Jide Wang Key Laboratory of Oil & Gas Fine Chemicals, Ministry of Education, Xinjiang University, 14#, Shengli Road, Urumqi 830046, Xinjiang, China [ Manuscript received October 17, 2008; revised November 18, 2008 ]

Abstract VOx /SiO2 catalysts prepared by impregnation method were used for catalytic dehydrogenation of n-butane to butenes and characterized by X-ray diffraction, FT-IR, UV-vis, Raman, and BET measurements. The effects of VOx loading and the reaction temperature on the VOx /SiO2 catalysts and their catalytic performances for the dehydrogenation of n-butane were studied. When the VOx loading was 12% g/gcat and reaction temperature was between 590 ◦ C and 600 ◦ C, n-butane conversion and butenes yields reached the highest value under H2 flux of 10 ml/min and n-butane flux of 10 ml/min. Product distribution, such as the ratio of 2-butene to 1-butene and the ratio of cis-2-butene to trans-2-butene, was mainly influenced by the reaction temperature. Key words: n-butane; catalytic dehydrogenation; butene; VOx supported SiO2

1. Introduction Presently, the output of C4 fraction is approximately 150200 million tons every year around the world. The C4 fraction is mainly produced from catalytic cracking and steam pyrolysis. The C4 fraction is used in chemical industries mainly as olefins, and recently, C4 alkanes are primarily used as fuel. n-Butane is one of the main components of C4 alkanes and is often used for selective oxidation to maleic anhydride [1,2], cracking to light olefins [3], dehydrogenation and isomerization to produce isobutene [4], aromatization [5], and disproportionation [6], etc. Recently, it is estimated that butenes will be another valuable petrochemical material that could be well used after ethylene and propylene. Significant attention has been paid to the study on the n-butane dehydrogenation process because of the growing demand for butene. So, several researchers have studied various dehydrogenation catalysts for n-butane and made effort to develop novel catalysts with high-activity and high-selectivity. VOx -based catalysts are widely used in light alkanes dehydrogenation, which included oxidative dehydrogenation with oxidant, such as oxygen [7-9], carbon dioxide [10], and nitrous oxide [11], and catalytic dehydrogenation without oxidant. Several studies have reported that VOx -supported catalysts can be used for n-butane oxidative dehydrogenation, for

instance, VMgO catalysts [7-14], VOx /SiO2 catalysts [15,16], VOx /USY catalyst [17], and VOx /SBA-15 catalyst [18], and also some researchers [19-24] have used VOx -supported catalysts for n-butane catalytic dehydrogenation, for example, VOx /θ-Al2 O3 catalysts [19-21], VN catalysts [22], and VC catalysts [23]. Murgia et al. [15] synthesized V2 O5 -SiO2 catalysts with tetraethoxysilane, Si(OC2 H5 )4 and vanadium acetylacetonate, and V(CH3 COCH = COCH3 )3 by sol-gel method for the oxidative dehydrogenation of n-butane and obtained less than 15% of butene yield. Santacesria et al. [16] prepared V/TiO2 -SiO2 catalysts by grafting method in oxidative dehydrogenation of n-butane and showed that more COx products can be produced by the oxidative dehydrogenation (ODH) of n-butane. In this study, the VOx /SiO2 catalysts are prepared by impregnation and used for n-butane catalytic dehydrogenation process. The results showed that the catalysts have good catalytic activity and selectivity for n-butane dehydrogenation after activity test and some physical characterization. 2. Experimental 2.1. Catalyst preparation The VOx /SiO2 catalysts were prepared by impregnation method. SiO2 with 40-60 mesh was used and impregnated

∗

Corresponding author. Tel: +86-991-8581012-808; Fax: +86-991-8582807; E-mail: [email protected] The project was supported by Program for New Century Excellent Talents in University (Grant No. NCET-04-0987), and Doctor Fund of Science Research of Xinjiang University (Grant No. BS060101). Copyright ©2009, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi:10.1016/S1003-9953(08)60087-0

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with a given concentration of NH4 VO3 solution. The amounts of VOx loading (in V2 O5 basis) were 0.00, 0.03, 0.06, 0.09, 0.12, 0.15, 0.18, and 0.21 g/gcat , respectively. The impregnation period lasted for 6 h at 70 ◦ C and then dried at 120 ◦ C for 10 h and finally calcined at 550 ◦ C in air for 10 h. After completing the preparation of this series of catalysts, they were marked as 0#, 1#, 2#, 3#, 4#, 5#, 6#, and 7# according to the different amounts of VOx loading. 2.2. Catalyst characterization X-ray diffraction (XRD) patterns were obtained on a DX2000 X-ray powder diffractometer coupled to a copper anode tube. The K α radiation was the light source with applied voltage of 40 kV and current of 30 mA. 2θ ranged from 10 o –50 o with a speed of 5 o per minute was recorded using step scanning and long counting times to determine the composition peaks. Infrared spectra were recorded between 4000 cm−1 and 400 cm−1 with a Bruker Equinox55 on samples dispersed in KBr and pressed in thin wafers. Raman spectroscopy was performed with a Bruker Vertex70 equipment. The spectral resolution was 4 cm−1 , and the spectra acquisition consisted of three accumulations of 20 s for each sample. UV-Vis absorption spectra of the samples were performed on a spectrophotometer (Hitachi U-4100) equipped with the integration sphere diffuse reflectance attachment. BET surface areas were measured by a Micromeritics ASAP 2000 adsorption apparatus. The samples were evacuated under a vacuum of 5×10−3 Torr at 350 ◦ C for 15 h. Specific total surface areas were calculated by the BET equation.

Figure 1. XRD patterns of the samples. (1) V2 O5 , (2) 2# sample, (3) 3# sample, (4) 4# sample, (5) 5# sample, (6) 6# sample

Infrared spectra of the VOx /SiO2 samples in the 4000400 cm−1 region are presented in Figure 2. For sample 0#, namely SiO2 , the following absorption bands were observed: 3445 cm−1 assigned to −OH group of silica surface, 1639 cm−1 attributed to physisorbed water, 1099 cm−1 and the shoulder at 1216 cm−1 due to asymmetric stretching vibrations of the three-dimensional Si−O−Si network; 969 cm−1 corresponding to the presence of surface Si−O− groups in silica gels; 808 cm−1 assigned to symmetric stretching vibrations of the Si−O−Si network, and 469 cm−1 attributed to bending deformation [15].

2.3. Reaction tests The catalytic reaction was carried out in a fixed-bed flow reactor with a 6 mm i. d. quartz tube by passing a gaseous n-butane (10 ml·min−1, 99.9%) in a H2 flow at a total flow rate of 20 ml·min−1 over 300 mg catalyst (total pressure: 1 atm). The reactants and reaction products were analyzed online by a gas chromatograph (SP-3420) equipped with a 50 m KBAl2 O3 /Na2 SO4 capillary column and FID detector. Blank tests on the same reactor showed low activity for dehydrogenation under the described conditions. 3. Results and discussion 3.1. Characterization of catalysts XRD patterns of the samples are presented in Figure 1. The intensity diffraction lines at 20.36o, 26.18o, and 31.06o, corresponding to vanadium species with crystalline structure of V2 O5 , were observed in Figure 1(1). These lines were almost not detected at low VOx loadings because of the characteristic of amorphous silica, and it indicated the vanadium species were well dispersed on silica gel surface. Nevertheless, with increasing the amount of VOx loading, the intensity of these lines intensified, such as the samples 5# and 6#.

Figure 2. FT-IR spectra. (1) 0# sample (SiO2 ), (2) 1# sample, (3) 2# sample, (4) 3# sample, (5) 4# sample, (6) 5# sample, (7) 6# sample

Adsorption bands at 1400 cm−1 and 669 cm−1 around which corresponded to residual ammonia did not appear in the FT-IR spectra for samples 1# to 6#, and it indicated that

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ammonia had decomposed and emerged completely from catalysts at 550 ◦ C calcination temperature. In other studies, bands at 930 cm−1 were attributed to Si−O−V stretching mode [25]. Band at 980 cm−1 was assigned to vibration of Si−O−H groups [26]. It was also connected to asymmetric stretching mode of SiO4 tetrahedrons connected to V-ions, for instance, band at 960 cm−1 for V-silicalites [27] and at 950 cm−1 for V-silica xerogels [28]. IR spectra of samples 1# to 6# were similar to sample 0#. Some bands shifted slightly to lower wavenumbers, and it was attributed to the interaction of vanadia species and silica support. The band at 969 cm−1 corresponding to the presence of surface Si−O− groups disappeared, and the bands at approximate 935 cm−1 assigned to Si−O−V stretch mode were detected in samples 1# to 6#, but the bands were too weak to be detected because of a possible overlapping with the silanols group of SiO2 . The Si−O−V bonds were formed with Si−O−H bonds and vanadia species, but the intensity of the bands at 3445 cm−1 , assigned to −OH bonds, did not weaken. The reason may be that the V−OH bonds were generated. Raman spectra of V2 O5 , SiO2 , and samples 3# to 5# are shown in Figure 3. The intensity of the V2 O5 band had been reduced 40 times in Figure 3. Bands at 723 cm−1 and 801 cm−1 were observed for SiO2 . The spectra of the V2 O5 crystallites showed the bands at 142, 285, 406, 701, and 995 cm−1 . For the samples 3# to 5#, the bands of V2 O5 were detected and intensified with the increase in the loading. A broad band at 1044 cm−1 assigned to the stretching mode of the terminal V = O bond was observed for these catalysts [15]. The band at 775 cm−1 attributed to symmetric stretching modes of V−O−V polyvanadates species was not detected, and it indicated that only monovanadate groups were present for these catalysts.

500 nm. The bands between 280 and 340 nm could be related to the highly dispersed tetrahedral species of V5+ ion. The bands between 350 and 500 nm were assigned to the octahedral species of V5+ ion [15,16]. When the amount of VOx loading was low, such as the samples 1# and 2#, the tetrahedral species were predominant in these catalysts. But the octahedral species would then increase with the higher VOx loading. The two kinds of V5+ species coexisted in high loading catalysts.

Figure 4. UV-vis spectra. (1) 1# sample, (2) 2# sample, (3) 3# sample, (4) 4# sample, (5) 5# sample, (6) 6# sample, (7) 7# sample

Table 1 shows the data of the catalysts of BET surface area, BJH average pore diameters, and volumes. It could be seen from Table 1 that the BET surface area and the BJH average pore volumes were reduced with the increase in the VOx loading, but the BJH average pore diameters changed slightly. When the VOx loading was increased, the BET surface area decreased from 340.0 m2 /g of SiO2 to 302.7 m2 /g of sample 6#. Table 1. BET surface area, BJH average adsorption and desorption pore diameters and volumes of catalysts Sample SiO2 2# 3# 4# 5# 6#

BET surface Pore volumes (cm3 /g)a area (m2 /g) adsorption desorption 340.0 0.940 – 324.6 0.872 0.870 311.3 0.826 0.824 309.7 0.817 0.815 306.3 0.811 0.809 302.7 0.803 0.801

Pore diameters (nm)b adsorption desorption – – 95.01 78.85 95.90 78.93 95.83 79.82 95.72 79.67 95.28 79.46

a

BJH average adsorption and desorption pore volumes, b BJH average adsorption and desorption pore diameters

3.2. Catalytic properties of VOx /SiO2 catalysts Figure 3. Raman spectra. (1) 0# sample (SiO2 ); (2) V2 O5 ; (3) 5# sample; (4) 4# sample; (5) 3# sample

UV-vis spectra of the VOx /SiO2 catalysts are presented in Figure 4. It could be seen that only the characteristic absorption bands of V5+ ion were observed between 280 and

The effect of amounts of VOx loading on the activity and the selectivity of catalysts for the dehydrogenation of n-butane are shown in Table 2, and the relations of products yield, n-butane conversion to the amounts of VOx loading are plotted in Figure 5.

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Table 2. Catalytic activity and the selectivity of VOx /SiO2 for the dehydrogenation of n-butane Catalyst Nonec 0# 1# 2# 3# 4# 5# 6# 7#

Conversion (%) 5.15 5.20 23.39 35.21 38.80 40.95 39.06 34.52 32.19

C1 ∼C3 a 84.77 83.25 28.02 16.54 14.15 13.53 13.38 13.51 13.37

1-Butene 2.71 3.08 21.18 23.30 23.72 24.52 24.41 24.53 24.54

cis-2-butene 2.33 2.85 24.43 26.80 27.31 28.08 28.57 28.27 28.36

Selectivity (%) trans-2-butene 1.79 2.03 18.39 20.38 20.77 21.37 21.25 21.53 21.61

isobutylene 1.83 1.65 3.31 6.53 7.45 5.61 5.93 5.89 6.05

1,3-butadiene 0.11 0.12 3.03 4.58 4.80 5.23 5.07 4.79 4.51

a C ∼C included methane, ethane, propane, ethene, and propene; b Others included mainly isobutane and some carbon; 1 3 substance in tubule; Reaction conditions: 590 ◦ C, H2 /n-C4 H10 = 1 (molar ratio), WHSV of n-butane is 5 h−1 .

There was no catalytic activity on sample 0# without VOx loading from Table 2, and the only n-butane conversion of 5.20% was attributed to n-butane pyrolysis which was similar to the “None” condition in Table 2. When 3 wt% VOx was loaded, namely sample 1#, the catalytic activity increased rapidly to 23.39%, the butenes selectivity gained 64%, and C1 ∼C3 selectivity fell down to 28.02% simultaneously. It showed that the VOx /SiO2 catalysts prepared by impregnation possessed high activity for dehydrogenating n-butane to butenes. It can be seen from Figure 5 that sample 4# achieved the highest n-butane conversion which is 40.95% and highest butene yields. As the VOx loading exceeded 12 wt%, the n-butane conversion and butene yields descended simultaneously shown in Figure 5, whereas the products selectivity changed almost little as shown in Table 2. It can be concluded that when more VOx was added to the catalysts especially over 12 wt%, the catalytic activity was significantly decreased and the products selectivity was slightly influenced.

c

Othersb 6.46 7.02 1.64 1.87 1.80 1.66 1.39 1.48 1.56

None means no

It indicated that the reactions also included dehydroisomerization of n-butane and further dehydrogenation of butenes over VOx /SiO2 catalyst. It can also be seen from Table 2 that the C1 ∼C3 selectivity dropped from 28.02% to around 13.5% in samples 4#∼7# compared with sample 1#, which showed that the catalysts could also restrain the cracking reaction with the increase of VOx loading. To gain more information on the VOx /SiO2 catalysts with the regard to the functions of other factors, we chose sample 4# for further investigation. The effects of reaction temperature on the reactivity and the selectivity of sample 4# are listed in Table 3, and their relations are plotted in Figure 6.

Figure 6. Product yields and n-butane conversion as a function of reaction temperature

Figure 5. Product yields and n-butane conversion as a function of VOx loading

In Figure 5, the vertical distance between the lines of n-butene yield and C4 H8 yield could be designated as isobutylene yield, and the vertical distance between the lines of C4 H8 yield and C= 4 yield could be designated as 1,3-butadiene yield.

It could be seen from Figure 6 that n-butane did not undertake dehydrogenation below about 450 ◦ C, but then the catalytic activity of sample 4# increased notably with the increase in reaction temperature. The products of C1 ∼C3 and 1,3-butadiene were scarcely produced below 540 ◦ C, whereas the yield of C1 ∼C3 increased rapidly over 540 ◦ C. When the reaction temperature was at 600 ◦ C, the yield of butenes reached the highest value of 32.19%. In addition, the yield of butenes was 31.06% at 590 ◦ C and 31.25% at 610 ◦ C, respectively, which were all somewhat less than 32.19% at 600 ◦ C.

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Table 3. Catalytic activity and selectivity of sample 4# for the dehydrogenation of n-butane Temperature (◦ C) 435 455 475 495 540 570 580 590 600 610 620

Conversion (%) 1.14 1.59 2.71 5.00 12.90 23.59 28.52 39.11 42.53 43.76 44.77

C1 ∼C3 a 5.72 5.13 4.72 5.12 3.40 13.23 14.71 14.17 17.45 21.16 27.67

1-Butene 11.28 16.67 21.41 24.55 26.85 24.61 24.26 23.93 22.95 21.39 19.80

cis-2-butene 17.68 25.26 30.83 33.56 35.34 29.07 28.56 28.40 26.06 23.90 21.66

Selectivity (%) trans-2-butene 12.93 18.65 22.77 24.95 26.44 22.09 21.70 21.09 19.97 18.33 16.69

isobutylene 9.45 7.07 4.72 3.34 3.21 5.64 5.35 6.02 6.73 7.81 6.44

1,3-butadiene 0.00 0.00 0.32 0.60 1.25 3.51 3.75 4.45 5.37 6.04 6.72

Othersb 42.94 27.22 15.23 7.88 3.51 1.85 1.67 1.94 1.47 1.37 1.02

a

C1 ∼C3 included methane, ethane, propane, ethene, and propene; b Others included mainly isobutane and some carbon; Reaction conditions: H2 /n-C4 H10 = 1 (molar ratio), WHSV is 5 h−1

Although the conversion of n-butane rose from 590 ◦ C to 610 ◦ C, the yield of C1 ∼C3 also increased. It could be concluded that the higher reaction temperature was helpful to the catalytic activity of VOx /SiO2 catalysts in increasing the cracking products but decreasing the yield of butenes. The suitable reaction temperature was around 590∼600 ◦ C on sample 4# for dehydrogenation of n-butane as seen from Table 3 and Figure 6. Figure 7 plotted the ratio of 2-butene to 1-butene as a function of reaction temperature and VOx loading, and the selectivity of n-butene as a function of reaction temperature.

tion temperature affected the selectivity of n-butene notably and influenced the distribution of 1-butene, cis-2-butene, and trans-2-butene simultaneously. 4. Conclusions The VOx /SiO2 catalysts prepared by impregnation method had good catalytic activity and selectivity for n-butane dehydrogenation to butenes. The V = O bond, V−O−Si bond, and V−O−H bond existed in catalysts according to FT-IR characterization. There were two kinds of V5+ structure including octahedral and tetrahedral species in catalysts. When catalyst was impregnated 12% g/gcat VOx and calcined at 550 ◦ C, n-butane conversion and yield of butenes reached the highest value under the reaction temperature of 590∼600 ◦ C, H2 flux of 10 ml/min, and n-C4 H10 flux of 10 ml/min. When more VOx loading was added, more V2 O5 species were yielded on SiO2 surface, and resulted in the decrease of BET surface area of catalysts. When the VOx loading exceeded 12% g/gcat , many octahedral V5+ species were produced, which would adversely affect the catalyst activity according to the catalytic tests. Acknowledgements This project is supported by the Program for New Century Excellent Talents in University (Grant No. NCET-04-0987) and the Doctor Fund of Science Research of Xinjiang University (Grant No. BS060101).

References Figure 7. Ratio of 2-butene to 1-butene as functions of reaction temperature and VOx loading

It could be seen from Figure 7 that the ratio of 2-butene to 1-butene descended from 2.71 to 1.94 with the increase in temperature from 435 ◦ C to 620 ◦ C but kept around 2.02 all the time with change in VOx loading from 3 wt% to 21 wt%. The ratio of cis-2-butene to trans-2-butene which did not plot in Figure 7 was similar to the previous tendency. It was also found that the selectivity of butenes including 2-butene and 1-butene was highest of 88.63% at 540 ◦ C from Figure 7, but the conversion of n-butane was only 12.9% from the Figure 6, which was very low. These results showed that the reac-

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