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MTS-9
Microporous and Mesoporous Materials 143 (2011) 320–325
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Metathesis of butene to propene and pentene over WO3/MTS-9 Derun Hua, Sheng-Li Chen ⇑, Guimei Yuan, Yulong Wang, Qinfeng Zhao, Xuelian Wang, Bo Fu State Key Laboratory of Heavy Oil Processing, College of Chemical Science and Engineering, China University of Petroleum, Beijing 102249, PR China
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Article history: Received 2 December 2010 Received in revised form 28 January 2011 Accepted 8 March 2011 Available online 13 March 2011 Keywords: 2-butene MTS-9 Isomerization Metathesis
a b s t r a c t A WO3/MTS-9 catalyst for metathesis of butene to propene and pentene was prepared by supporting tungsten oxide on MTS-9. The prepared catalyst was characterized by a number of experimental techniques including XRD, nitrogen adsorption, NH3-TPD, H2-TPR, UV-DRS and UV-Raman, The characterization data showed that three types of tungsten species (tetrahedral, octahedral polytungstate species and WO3 crystallites) are present on the catalyst. In the case of butene metathesis, the performance of WO3/ MTS-9 was tested. On the base of experiment results, we speculated that the tetrahedral and the octahedral polytungstate species may be the active sites rather than crystalline WO3. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction C4 stream from the steam cracker or refinery has been used as fuel in China, and enhancing the value of the C4 stream is an important objective for petrochemical companies. One approach solving the problem is olefin metathesis which is an important reaction for producing a wide variety of more valuable products from available olefin substrates [1]. Now the catalysts used for the olefin metathesis include Ruthenium-Based catalyst [2], Molybdenum-Based catalyst [3] and Tungsten-Based catalyst [4]. Among them, the Tungsten-based catalyst has several unique advantages over the others, listing as follows: (1) considerable resistance to poisons, (2) a long online time, which make it attractive for commercial applications [5,6]. The activity of Tungsten-based catalyst depends mainly on: (1) the oxidation state of tungsten species [5,6], (2) the content of tungsten oxide [5] and (3) the pre-treatment conditions [7]. Besides, supports have a profound effect on the activity of catalyst, so many researchers used mesoporous materials as support, mesoporous materials become candidates because of mesoporous materials with high surface area and tunable channel. According to previous studies [8], mesoporous silica-based catalyst exhibited high activity in olefin metathesis, but operation was at high temperature [9,10]. In order to achieve acceptable metathesis activity at mild temperature, many reports [11] used different materials as supports, and the improved activities of catalyst were partly attributed to the acidity of supports. In addition, the modified support stabilized better the active species [12–15]. In this study, WO3/MTS-9 was prepared and used as the metathesis catalyst. ⇑ Corresponding author. Tel.: +86 010 89733396; fax: +86 010 69724721. E-mail address: [email protected] (S.-L. Chen). 1387-1811/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2011.03.012
The experiment results show that WO3/MTS-9 exhibits excellent catalytic performance for the metathesis of butene to propene and pentene. 2. Experimental 2.1. Synthesis of MTS-9 MTS-9 was hydrothermally synthesized from an assembly of triblock polymers (P123) with preformed titanosilicate precursors in strongly acidic media (pH < 1) by a two-step procedure. First, precursors containing titanosilicate (TS-1) nanoclusters were prepared. Second, the preformed precursors were assembled with triblock copolymers in a strong acid medium (pH < 1) [15,16]. The synthesis procedure for MTS-9 was reported in detail in literature [17]. 2.2. Preparation of WO3/MTS-9 catalyst WO3/MTS-9 catalysts with different tungsten loadings were prepared by wet impregnation method. The WO3 to MTS-9 ratio (in weight) of the catalyst WO3/MTS-9 were 4.0, 8.0, 12.0, and 16.0 wt%, respectively. The impregnated products were dried at 353 K for 12 h, then the catalysts were thermally treated at 823 K for 4 h. 2.3. Characterization of catalyst X-ray powder diffraction patterns of the catalyst WO3/MTS-9 were recorded on a Bruker D8 ADVACE diffractometer, using CuKa (1.5406 Å) radiation in the 2h range of 10°–60° with a scanning rate of 1°/min. N2 adsorption–desorption isotherms at 77 K were
D. Hua et al. / Microporous and Mesoporous Materials 143 (2011) 320–325
recorded with a Micromeritics ASAP 2010 automatic sorption analyzer. The BET surface areas were calculated from the desorption isotherms in the relative pressure range of P/P0 = 0.01–1.0. The acidity of the catalyst samples was obtained via NH3-TPD. Each sample was first treated at 773 K for 30 min at an air flow of 40 mL min1, second cooled to 383 K, then exposed to 20% NH3/ Ar for 30 min, and last purged by Ar at 383 K for 120 min in order to eliminate the physical adsorbed ammonia. Temperature programmed desorption (TPD) was conducted by ramping to 773 K at 10 K min1 and NH3 (m/e = 16) in the effluent was detected and recorded as a function of temperature by a thermal conductivity detector (TCD) (BJGC, GSD301, China). All the catalysts were characterized by H2-TPR using a conventional flow apparatus. The samples (0.15 g) were pretreated at 873 K in air flow for 60 min, and then cooled to 373 K with argon stream. Subsequently, the reduction was performed by using H2/Ar mixture (H2/Ar molar ratio of 5:95 and a total flow rate of 40 mLmin1 at a temperature from 373 to 1373 K with a heating rate of 10 K/min. The consumption of hydrogen was determined with TCD in an online gas chromatograph. The UV–Vis DRS (diffuse reflectance spectroscopy) of the catalyst was recorded in the range of 200–800 nm against the support as reference, on a Hitachi U-4100 spectrophotometer (Japan) equipped with an integration sphere diffuse reflectance attachment. UV-Raman spectra were obtained on a HR800UV-Raman spectrograph (Horiba Jobin Yvon Company, France). The 244.0 nm line from a He–Cd laser is used as the excitation source. The spectrum resolution is estimated to be 4.0 cm1. 2.4. Evaluation of catalyst WO3/MTS-9 The catalytic properties of the WO3/MTS-9 samples were evaluated by the metathesis of butene. The metathesis reactions were carried out in a downflow fixedbed stainless steel microreactor (10 mm i.d.) using 0.375 g WO3/ MTS-9 catalyst. The C4 stream was treated through BASF adsorbents Selexsorb CD and COS for removal of the oxygenated organic compounds and other trace contaminants which can cause catalyst deactivation. Followed by the introduction of the C4 stream into the reactor, the catalyst was pretreated in situ with a N2 and H2 mixture flow for 30 min at 693 K. The weight hour space velocity (WHSV) of the metathesis reaction ranged from 1.6 to 6.4 h1. All the products were analyzed using a gas chromatograph equipped with a flame ionization detector (FID) and a 50 m PONA capillary column. The reaction pathway was illustrated in the Scheme 1 in detail. 3. Results and discussion 3.1. X-ray powder diffraction Fig. 1 shows the XRD spectral analysis of WO3, SiO2 and WO3/ MTS-9 catalysts with different WO3 loadings. Comparison to the
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diffraction pattern of SiO2, MTS-9 has almost the same diffraction pattern as SiO2 because the titanium is dispersed into the silica framework, 4% WO3/MTS-9 shows the same diffraction pattern as them, indicating the absence of very small crystalline WO3. Above 8% WO3 loadings, crystalline WO3 is observed in the diffraction patterns, and the intensity of crystalline WO3 increases with increasing WO3 loadings, indicating that the crystallite size increases with increasing WO3 loadings. 3.2. N2 adsorption–desorption isotherms To further investigate dispersion state of tungsten oxide on the support, the W content (monolayer dispersion) on the surface of support was calculated by the following equation and the results were summarized in Table 1. The W content on the surface was calculated quantitatively as follows [17,18]:
CW ¼
LWO3 =100 No 18 231:8 S 10
Where, CW denotes the surface concentration of tungsten (W6+ cations nm2); LWO3 is the loading amount of tungsten oxide (wt%); the 231.8 is the molecular weight of WO3, N0 is the Avogadro constant (6.023 1023) and S is the specific surface area (m2 g1). The specific surface area, pore size and pore volume of all the samples decreased with increasing WO3 loadings. The dispersion ability of tungsten on support was listed in Table 1, which is different to the theoretical monolayer coverage (5.5 W-atom/nm2) [19,20] and was far lower than monolayer dispersion. The reason for such phenomenon might be that: (1) Hydroxyl was not sufficient to react with ammonium metatungstate. (2) Tungsten oxide should be dispersed randomly on the support. 3.3. NH3-TPD (the amine titration method) To study the effect of WO3 loadings on acidity of WO3/MTS-9 catalyst, the number of acid sites of the catalysts was measured by the amine titration method, and the results were shown in Fig.2. As shown in Fig.2. The neat support exhibited low acidity (Fig. 2), two ammonia desorption peaks were found at 420 and 630 K corresponding to weak acid peak and mild strong acidity peak (Fig. 2). Thus we confirmed that the support did not possess the strong acid sites. Pure WO3 had no acid site. After WO3 supported on MTS-9, the desorption temperature moved to higher temperature, the area of desorption peak became larger, indicating that the density and strength of acid sites increased remarkably with increasing WO3 loadings. Moreover, the desorption peak disappeared at 630 K, and the reason might be that it was covered by the desorption peak of lower temperature. The acid amount of WO3/MTS-9 with loadings 0, 4, 8, 12 and 16% is 0.12, 0.26, 0.31, 0.35 and 0.46 mmol/g, respectively. On the other hand, the desorption temperature shifted from 422 to 470 K, indicating that new acid sites formed and the acid strengths increase.
Scheme 1. Possible reaction pathways of 2-butene.
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WO3
Intensity (a.u)
WO3
16%WO3/MTS-9
Intensity(a.u)
16%WO 3 /MTS-9
12%WO3/MTS-9 8%WO3/MTS-9 4%WO3/MTS-9 MTS-9 SiO2
10
20
30
40
2θ
50
60
Fig. 1. XRD spectra of WO3/MTS-9 with different WO3 loadings.
MTS-9 4W/MTS-9 8W/MTS-9 12W/MTS-9 16W/MTS-9 a
Loading (wt%)
Surface densitya (W atoms/nm2)
BET surface area (m2/g)
Pore volume (cm3/g)
Average pore size (A)
0 4 8 12 16
0 0.2 0.4 0.6 0.8
510.0 506.0 500.4 430.7 408.6
1.2 1.2 1.1 1.1 1.0
97.1 97.0 96.9 96.7 96.6
650 K
Intensity (a.u)
16 %WO3/MTS-9
12 %WO3/MTS-9
8 %WO3/MTS-9
4 %WO3/MTS-9 MTS-9 WO3
400
500
600
700
MTS-9 400
500
600
700
800
900
1000 1100 1200 1300 1400
Temperature(K) Fig. 3. Temperature-programmed reduction profiles of catalysts with different loadings.
Defined as the number of tungsten atoms per square nanometer of the catalyst.
420 K
8%WO 3 /MTS-9
4%WO 3 /MTS-9
Table 1 Structural characteristics and tungsten surface concentration of the support and catalyst with different loadings. Samples
12%WO3 /MTS-9
800
Temperature (K) Fig. 2. Curves of NH3-TPD catalysts with different loadings.
3.4. H2-TPR H2-TPR technique is used to investigate the interaction between W species and the support. The H2-TPR curves of all the catalysts, support and WO3 were shown in Fig. 3. The H2-TPR profile of WO3 exhibited three peaks at 893, 988, and1158 K, which might be assigned to the stepwise reduction of WO3 to W (0) [WO3(VI) ? W20O58(V, VI) ? WO2(IV) ? W(0)] [20,21]. The H2-TPR profile of the plain support only exhibited a very small peak at about
890 K, which was attributed to the reduction of Titanium [22]. For other catalysts, reduction peaks were between 800 and 900 K, meanwhile the reduction temperature shifted toward higher temperature, the intensity increased with increasing WO3 loadings, and reached the maximum for 12% WO3/MTS-9. The peak between 800 and 900 K was attributed to the reduction of W species in octahedral and tetrahedral coordination [20]. According to literature [22], surface Si–O–W species and Ti-O-W could be decomposed completely to W in the process, this decomposition process could be completed prior to the reduction process at 1000–1300 K. According to literature [23], the reduction peak at 1150–1250 K was attributed to the reduction of crystalline WO3 and the intensity of the peak increased with increasing WO3 loadings, indicating the increase of crystalline WO3 on the support. For 4% WO3/MTS-9, the reduction peak at 1150–1250 K disappeared, which indicated no crystalline WO3. These results were in accordance with the results of XRD. However, it should be noted that the reduction recorded for the samples was different from that obtained for crystalline WO3, suggesting that the WO3 supported on the MTS9 presented mainly in the form of Si–O–W or Ti–O–W species reduced more easily than crystalline WO3. 3.5. UV–vis DRS UV–vis DRS is used for distinguishing the structure of tungsten species for catalysts with different loadings [24]. The diffuse reflectance spectra of WO3/MTS-9 catalysts in the UV–vis region were recorded and the results were shown in Fig. 4. Three diffraction peaks appear at 228, 287 and 400 nm respectively. The diffraction peak at 228 nm was assigned to the isolated titanium framework titanium for MTS-9 [25–27]. The bands at 228 and 287 nm suggest that the surface tungstate consisted of distorted isolated WO4 dioxo surface species (Raman band at 1000 cm1) and polytungstate monoxo surface WO5/WO6 (Raman band at 1100 cm1) that may be attributed to a distorted tetrahedral or an octahedral
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228
287
400
16%WO3/MTS-9
Diffraction intensity(a.u)
WO3
12%WO3/MTS-9
16%WO3/MTS-9
12%WO3/MTS-9
8%W0 3 /MTS-9
4%WO3/MTS-9 MTS-9
200 250 300 350 400 450 500 550 600 650 700 750 800
Wavelength(nm)
Intensity(a.u)
8%WO3/MTS-9
4%W0 3 /MTS-9
Fig. 4. UV–vis diffuse reflectance spectra of various samples.
coordination environment [28,29], the diffraction peak at 400 nm was assigned to crystal WO3. The two bands in the UV–vis spectra indicated that two distinct surface tungstate species were present on the catalysts. For loadings above 8%, a peak at 400 nm was observed, and the intensity increases slightly with the increase of WO3 loading, the result was in accordance with that obtained by XRD and H2-TPR. The intensity of peaks at 228 and 287 nm increased slightly with the increase of WO3 loading, and they may be active sites for metathesis reaction. 3.6. Raman spectroscopy The major structural information concerning the surface tungsten oxide species can be derived from Raman spectroscopy because of its ability of distinguishing different tungsten oxide species that may simultaneously be present on the catalysts. The Raman spectra of all catalysts were shown in Fig.5. The SiO2 exhibited Raman bands at 1090, 976, 792, 600 and 489 cm1 [30], bands at 1090 and 792 cm1 were assigned to the transverse-optical (TO) asymmetric stretch and Si–O–Si symmetrical stretching [31], the 976 cm1 Raman band arose from the Si– OH stretching mode of the surface hydroxyls [31,32], bands at 600 and 489 cm1 were attributed to tri- and tetra- cyclosiloxane rings, respectively [32]. MTS-9 also exhibited Raman bands at 488, 797, 978 and 1100 cm1, MTS-9 and SiO2 possessed common bands at 488, 790, 976, and 1090 cm1, but the intensity of the band at 976 and 1090 cm1 for MTS-9 were stronger than SiO2, suggesting that these bands(976 and 1090 cm1) were attributed to the resonance Raman effect [33], others were the characteristics of SiO2. For catalysts with different WO3 loadings, very strong bands at 1000 and 1110 cm1 are observed. In comparison to SiO2 and MTS-9, the bands at 976 and 1090 cm1 for SiO2 and MTS-9 shift to 1000 and 1110 cm1 for catalysts with different WO3 loadings. The intensity of bands at 1000 and 1110 cm1 are stronger for catalysts than for MTS-9. The two bands of the catalysts at 1000 and 1110 cm1 increase in intensity gradually from 4% to 12% loading, and the two bands for 16% loading is almost the same for 12% loading. According to literatures [27,34], the Raman band at 1000 cm1 is assigned to the O@W@O bond of the isolated surface tetrahedral tungsten oxide species. The Raman band at 1100 cm1 corresponds to a surface octahedral structure of polytungstate with monoxo W@O coordinated species. For 4% WO3/MTS-9, except for Raman bands at 1000 and 1100 cm1, no other bands were observed. However, additional Raman bands were observed for the
MTS-9
SiO 2 200
400
600
800
1000
1200
14 00 16 00 1800
2000
Raman Shift(cm-1 ) Fig. 5. The Raman spectra of SiO2 and catalysts with different loadings.
other catalysts. For 8% WO3/MTS-9, additional Raman band was observed at 800 cm1; For 12% WO3/MTS-9 and 16% WO3/MTS9, additional Raman bands were observed at 270, 320, 730 and 800 cm1 due to more crystalline WO3 particles. These bands (270, 320, 730 and 800 cm1) were assigned to the W–O–W deformation mode, the W@O bend mode, the O–W–O bending mode of surface WOx and the W@O stretching mode [35,36], indicating that high loadings of tungsten species may result in the agglomeration of bulk microcrystalline WO3 on the surface. 3.7. Metathesis reaction 3.7.1. Effect of tungsten oxide loading on metathesis activity of catalyst To investigate the influence of the WO3 loading on catalyst activity, WO3/MTS-9 catalysts containing 0, 4, 8, 12, 16 wt% WO3 and tungsten trioxide (WO3) were prepared. Fig. 6 showed the relationship of yield of propene and WO3 loading. It can be seen that the plain MTS-9 and tungsten trioxide possess no activity. For the other catalysts, the initial activity increased with time, up to the point where the steady state was attained. The transition process (the introduction period) was common to a number of catalysts, including silica-supported tungsten oxide used in metathesis of olefins. The introduction period was shortened by reduction pretreatment or reaction at higher temperature, but was not eliminated. For 16% WO3/MTS-9 catalyst, loading was the highest, but the activity was not the highest, indicating that all of WO3 did not act in reaction and WO3 interacting only with support possessed activity, which was verified by 4% WO3/MTS9. The yield of propene increases with increasing WO3 loading, an optimal loading was 12 wt%. Further increasing loading resulted in the decrease of propene yield, the reason might be that the active centers were covered by an excessive of crystalline WO3 and the number of active centers decreased for too much WO3 loading. As shown in profile of H2-TPR, spectra of UV–vis DRS and Raman, peak of surface tungsten oxide species increased with the increase
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40
22
50 4 % WO3/MTS-9
16 % WO3/MTS-9 MTS-9 WO3
20
10
45
Conversion of butene(%)
12 % WO3/MTS-9
20 40 18 35
16
30
25 14
0 0
10
20
30
40
20 660
50
665
670
Fig. 6. Activity of WO3/MTS-9 with different loadings: Reaction conditions P = 0.8 MPa, T = 593 K, WHSV = 6.4 h1.
675
680
685
50
80 70
C2H4
80
C3H6
=
=
1-C4/2-C4
50 40 0.4 30 20
1-C=4/2-C=4
0.6
0.2
ethene propene pentene hexene conversion
40
50 30 40 30
20
Conversion(%)
C6H12
60
Selectivity(%)
60
0.8
C5H10
70
695
Fig. 8. Effect of pretreatment temperature on conversion of butene and yield of propene Reaction conditions: P = 0.8 MPa, T = 593 K, WHSV = 6.4 h1.
1.0
90
690
Temperature(K)
Time on stream (h)
Selectivity(%)
Yield of propene(%)
Yield of propene (%)
8 % WO3/MTS-9
30
20 10
10
10 0
0 660
0.0
670
680
690
0 540 550 560 570 580 590 600 610 620 630 640 650 660 670
Temperature(K)
Temperature(K) Fig. 7. Effect of pretreatment temperature on selectivity of production: Reaction conditions P = 0.8 MPa, T = 593 K, WHSV = 6.4 h1.
Fig. 9. Correlation between temperature and activity Reaction conditions: P = 0.8 MPa, WHSV = 6.4 h1.
of WO3 loading in the range of 4wt%-12wt% and decreased slightly for 16% WO3/MTS-9, the trend was in accordance with the metathesis activity of catalyst, so optimal loading was 12%.
on the temperature of activation. According to the literature [38], the tungsten oxide of intermediate nonstoichiometric oxidation state could be the most active one for olefin metathesis. This point was confirmed by our experiments in this work.
3.7.2. Effect of pretreatment temperature on activity of catalyst According to previous results reported in literatures [35–37], tungsten possessing some intermediate valence state had activity for the tungsten supported catalyst, so they had to be reduced before it was used. So the effect of reduction pretreatment temperature on activity of catalyst was investigated in this study, catalyst used was 12% WO3/MTS-9, and the results were shown in Fig. 7 and Fig. 8. The catalyst was pretreated by using hydrogen and nitrogen mixture flow (with 10% hydrogen). It can be seen that pretreatment temperature had no effect on selectivity of products, which kept constant. But the conversion of butene and yield of propene impacted dramatically by the pretreatment temperature. As shown in Fig. 8, when the catalyst was pretreated at 663 K, the conversion of butene and yield of propene were 41.8% and 35.6%, respectively. When the reduction temperature raised to 673 K, the activity of catalyst reached maximum, however with further increasing temperature, the activity fell off. Hence the activity of the catalyst depends
3.7.3. Effect of reaction temperature on activity of catalyst To investigate the effect of temperature on the activity of catalyst, metathesis of butene was carried out over catalyst 12% WO3/ MTS-9, pretreating temperature was 673 K, and the results were shown in Fig. 9 It can be seen that the activity of catalyst was affected greatly by reaction temperature. The conversion of butene and selectivity of propene increased slightly with increasing reaction temperature, the elevated temperature is favorable to forming 1-butene from isomerization of 2-butene.The increase of 1-butene promoted the reaction to move towards propene and pentene. 4. Conclusions This paper presented a detailed study of the nature of WO3/ MTS-9 catalyst and its catalytic properties in the metathesis of butene to propene and pentene, we made the following conclusions:
D. Hua et al. / Microporous and Mesoporous Materials 143 (2011) 320–325
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Metathesis of butene to propene and pentene over WO3/MTS-9 Derun Hua, Sheng-Li Chen ⇑, Guimei Yuan, Yulong Wang, Qinfeng Zhao, Xuelian Wang, Bo Fu State Key Laboratory of Heavy Oil Processing, College of Chemical Science and Engineering, China University of Petroleum, Beijing 102249, PR China
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Article history: Received 2 December 2010 Received in revised form 28 January 2011 Accepted 8 March 2011 Available online 13 March 2011 Keywords: 2-butene MTS-9 Isomerization Metathesis
a b s t r a c t A WO3/MTS-9 catalyst for metathesis of butene to propene and pentene was prepared by supporting tungsten oxide on MTS-9. The prepared catalyst was characterized by a number of experimental techniques including XRD, nitrogen adsorption, NH3-TPD, H2-TPR, UV-DRS and UV-Raman, The characterization data showed that three types of tungsten species (tetrahedral, octahedral polytungstate species and WO3 crystallites) are present on the catalyst. In the case of butene metathesis, the performance of WO3/ MTS-9 was tested. On the base of experiment results, we speculated that the tetrahedral and the octahedral polytungstate species may be the active sites rather than crystalline WO3. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction C4 stream from the steam cracker or refinery has been used as fuel in China, and enhancing the value of the C4 stream is an important objective for petrochemical companies. One approach solving the problem is olefin metathesis which is an important reaction for producing a wide variety of more valuable products from available olefin substrates [1]. Now the catalysts used for the olefin metathesis include Ruthenium-Based catalyst [2], Molybdenum-Based catalyst [3] and Tungsten-Based catalyst [4]. Among them, the Tungsten-based catalyst has several unique advantages over the others, listing as follows: (1) considerable resistance to poisons, (2) a long online time, which make it attractive for commercial applications [5,6]. The activity of Tungsten-based catalyst depends mainly on: (1) the oxidation state of tungsten species [5,6], (2) the content of tungsten oxide [5] and (3) the pre-treatment conditions [7]. Besides, supports have a profound effect on the activity of catalyst, so many researchers used mesoporous materials as support, mesoporous materials become candidates because of mesoporous materials with high surface area and tunable channel. According to previous studies [8], mesoporous silica-based catalyst exhibited high activity in olefin metathesis, but operation was at high temperature [9,10]. In order to achieve acceptable metathesis activity at mild temperature, many reports [11] used different materials as supports, and the improved activities of catalyst were partly attributed to the acidity of supports. In addition, the modified support stabilized better the active species [12–15]. In this study, WO3/MTS-9 was prepared and used as the metathesis catalyst. ⇑ Corresponding author. Tel.: +86 010 89733396; fax: +86 010 69724721. E-mail address: [email protected] (S.-L. Chen). 1387-1811/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2011.03.012
The experiment results show that WO3/MTS-9 exhibits excellent catalytic performance for the metathesis of butene to propene and pentene. 2. Experimental 2.1. Synthesis of MTS-9 MTS-9 was hydrothermally synthesized from an assembly of triblock polymers (P123) with preformed titanosilicate precursors in strongly acidic media (pH < 1) by a two-step procedure. First, precursors containing titanosilicate (TS-1) nanoclusters were prepared. Second, the preformed precursors were assembled with triblock copolymers in a strong acid medium (pH < 1) [15,16]. The synthesis procedure for MTS-9 was reported in detail in literature [17]. 2.2. Preparation of WO3/MTS-9 catalyst WO3/MTS-9 catalysts with different tungsten loadings were prepared by wet impregnation method. The WO3 to MTS-9 ratio (in weight) of the catalyst WO3/MTS-9 were 4.0, 8.0, 12.0, and 16.0 wt%, respectively. The impregnated products were dried at 353 K for 12 h, then the catalysts were thermally treated at 823 K for 4 h. 2.3. Characterization of catalyst X-ray powder diffraction patterns of the catalyst WO3/MTS-9 were recorded on a Bruker D8 ADVACE diffractometer, using CuKa (1.5406 Å) radiation in the 2h range of 10°–60° with a scanning rate of 1°/min. N2 adsorption–desorption isotherms at 77 K were
D. Hua et al. / Microporous and Mesoporous Materials 143 (2011) 320–325
recorded with a Micromeritics ASAP 2010 automatic sorption analyzer. The BET surface areas were calculated from the desorption isotherms in the relative pressure range of P/P0 = 0.01–1.0. The acidity of the catalyst samples was obtained via NH3-TPD. Each sample was first treated at 773 K for 30 min at an air flow of 40 mL min1, second cooled to 383 K, then exposed to 20% NH3/ Ar for 30 min, and last purged by Ar at 383 K for 120 min in order to eliminate the physical adsorbed ammonia. Temperature programmed desorption (TPD) was conducted by ramping to 773 K at 10 K min1 and NH3 (m/e = 16) in the effluent was detected and recorded as a function of temperature by a thermal conductivity detector (TCD) (BJGC, GSD301, China). All the catalysts were characterized by H2-TPR using a conventional flow apparatus. The samples (0.15 g) were pretreated at 873 K in air flow for 60 min, and then cooled to 373 K with argon stream. Subsequently, the reduction was performed by using H2/Ar mixture (H2/Ar molar ratio of 5:95 and a total flow rate of 40 mLmin1 at a temperature from 373 to 1373 K with a heating rate of 10 K/min. The consumption of hydrogen was determined with TCD in an online gas chromatograph. The UV–Vis DRS (diffuse reflectance spectroscopy) of the catalyst was recorded in the range of 200–800 nm against the support as reference, on a Hitachi U-4100 spectrophotometer (Japan) equipped with an integration sphere diffuse reflectance attachment. UV-Raman spectra were obtained on a HR800UV-Raman spectrograph (Horiba Jobin Yvon Company, France). The 244.0 nm line from a He–Cd laser is used as the excitation source. The spectrum resolution is estimated to be 4.0 cm1. 2.4. Evaluation of catalyst WO3/MTS-9 The catalytic properties of the WO3/MTS-9 samples were evaluated by the metathesis of butene. The metathesis reactions were carried out in a downflow fixedbed stainless steel microreactor (10 mm i.d.) using 0.375 g WO3/ MTS-9 catalyst. The C4 stream was treated through BASF adsorbents Selexsorb CD and COS for removal of the oxygenated organic compounds and other trace contaminants which can cause catalyst deactivation. Followed by the introduction of the C4 stream into the reactor, the catalyst was pretreated in situ with a N2 and H2 mixture flow for 30 min at 693 K. The weight hour space velocity (WHSV) of the metathesis reaction ranged from 1.6 to 6.4 h1. All the products were analyzed using a gas chromatograph equipped with a flame ionization detector (FID) and a 50 m PONA capillary column. The reaction pathway was illustrated in the Scheme 1 in detail. 3. Results and discussion 3.1. X-ray powder diffraction Fig. 1 shows the XRD spectral analysis of WO3, SiO2 and WO3/ MTS-9 catalysts with different WO3 loadings. Comparison to the
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diffraction pattern of SiO2, MTS-9 has almost the same diffraction pattern as SiO2 because the titanium is dispersed into the silica framework, 4% WO3/MTS-9 shows the same diffraction pattern as them, indicating the absence of very small crystalline WO3. Above 8% WO3 loadings, crystalline WO3 is observed in the diffraction patterns, and the intensity of crystalline WO3 increases with increasing WO3 loadings, indicating that the crystallite size increases with increasing WO3 loadings. 3.2. N2 adsorption–desorption isotherms To further investigate dispersion state of tungsten oxide on the support, the W content (monolayer dispersion) on the surface of support was calculated by the following equation and the results were summarized in Table 1. The W content on the surface was calculated quantitatively as follows [17,18]:
CW ¼
LWO3 =100 No 18 231:8 S 10
Where, CW denotes the surface concentration of tungsten (W6+ cations nm2); LWO3 is the loading amount of tungsten oxide (wt%); the 231.8 is the molecular weight of WO3, N0 is the Avogadro constant (6.023 1023) and S is the specific surface area (m2 g1). The specific surface area, pore size and pore volume of all the samples decreased with increasing WO3 loadings. The dispersion ability of tungsten on support was listed in Table 1, which is different to the theoretical monolayer coverage (5.5 W-atom/nm2) [19,20] and was far lower than monolayer dispersion. The reason for such phenomenon might be that: (1) Hydroxyl was not sufficient to react with ammonium metatungstate. (2) Tungsten oxide should be dispersed randomly on the support. 3.3. NH3-TPD (the amine titration method) To study the effect of WO3 loadings on acidity of WO3/MTS-9 catalyst, the number of acid sites of the catalysts was measured by the amine titration method, and the results were shown in Fig.2. As shown in Fig.2. The neat support exhibited low acidity (Fig. 2), two ammonia desorption peaks were found at 420 and 630 K corresponding to weak acid peak and mild strong acidity peak (Fig. 2). Thus we confirmed that the support did not possess the strong acid sites. Pure WO3 had no acid site. After WO3 supported on MTS-9, the desorption temperature moved to higher temperature, the area of desorption peak became larger, indicating that the density and strength of acid sites increased remarkably with increasing WO3 loadings. Moreover, the desorption peak disappeared at 630 K, and the reason might be that it was covered by the desorption peak of lower temperature. The acid amount of WO3/MTS-9 with loadings 0, 4, 8, 12 and 16% is 0.12, 0.26, 0.31, 0.35 and 0.46 mmol/g, respectively. On the other hand, the desorption temperature shifted from 422 to 470 K, indicating that new acid sites formed and the acid strengths increase.
Scheme 1. Possible reaction pathways of 2-butene.
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WO3
Intensity (a.u)
WO3
16%WO3/MTS-9
Intensity(a.u)
16%WO 3 /MTS-9
12%WO3/MTS-9 8%WO3/MTS-9 4%WO3/MTS-9 MTS-9 SiO2
10
20
30
40
2θ
50
60
Fig. 1. XRD spectra of WO3/MTS-9 with different WO3 loadings.
MTS-9 4W/MTS-9 8W/MTS-9 12W/MTS-9 16W/MTS-9 a
Loading (wt%)
Surface densitya (W atoms/nm2)
BET surface area (m2/g)
Pore volume (cm3/g)
Average pore size (A)
0 4 8 12 16
0 0.2 0.4 0.6 0.8
510.0 506.0 500.4 430.7 408.6
1.2 1.2 1.1 1.1 1.0
97.1 97.0 96.9 96.7 96.6
650 K
Intensity (a.u)
16 %WO3/MTS-9
12 %WO3/MTS-9
8 %WO3/MTS-9
4 %WO3/MTS-9 MTS-9 WO3
400
500
600
700
MTS-9 400
500
600
700
800
900
1000 1100 1200 1300 1400
Temperature(K) Fig. 3. Temperature-programmed reduction profiles of catalysts with different loadings.
Defined as the number of tungsten atoms per square nanometer of the catalyst.
420 K
8%WO 3 /MTS-9
4%WO 3 /MTS-9
Table 1 Structural characteristics and tungsten surface concentration of the support and catalyst with different loadings. Samples
12%WO3 /MTS-9
800
Temperature (K) Fig. 2. Curves of NH3-TPD catalysts with different loadings.
3.4. H2-TPR H2-TPR technique is used to investigate the interaction between W species and the support. The H2-TPR curves of all the catalysts, support and WO3 were shown in Fig. 3. The H2-TPR profile of WO3 exhibited three peaks at 893, 988, and1158 K, which might be assigned to the stepwise reduction of WO3 to W (0) [WO3(VI) ? W20O58(V, VI) ? WO2(IV) ? W(0)] [20,21]. The H2-TPR profile of the plain support only exhibited a very small peak at about
890 K, which was attributed to the reduction of Titanium [22]. For other catalysts, reduction peaks were between 800 and 900 K, meanwhile the reduction temperature shifted toward higher temperature, the intensity increased with increasing WO3 loadings, and reached the maximum for 12% WO3/MTS-9. The peak between 800 and 900 K was attributed to the reduction of W species in octahedral and tetrahedral coordination [20]. According to literature [22], surface Si–O–W species and Ti-O-W could be decomposed completely to W in the process, this decomposition process could be completed prior to the reduction process at 1000–1300 K. According to literature [23], the reduction peak at 1150–1250 K was attributed to the reduction of crystalline WO3 and the intensity of the peak increased with increasing WO3 loadings, indicating the increase of crystalline WO3 on the support. For 4% WO3/MTS-9, the reduction peak at 1150–1250 K disappeared, which indicated no crystalline WO3. These results were in accordance with the results of XRD. However, it should be noted that the reduction recorded for the samples was different from that obtained for crystalline WO3, suggesting that the WO3 supported on the MTS9 presented mainly in the form of Si–O–W or Ti–O–W species reduced more easily than crystalline WO3. 3.5. UV–vis DRS UV–vis DRS is used for distinguishing the structure of tungsten species for catalysts with different loadings [24]. The diffuse reflectance spectra of WO3/MTS-9 catalysts in the UV–vis region were recorded and the results were shown in Fig. 4. Three diffraction peaks appear at 228, 287 and 400 nm respectively. The diffraction peak at 228 nm was assigned to the isolated titanium framework titanium for MTS-9 [25–27]. The bands at 228 and 287 nm suggest that the surface tungstate consisted of distorted isolated WO4 dioxo surface species (Raman band at 1000 cm1) and polytungstate monoxo surface WO5/WO6 (Raman band at 1100 cm1) that may be attributed to a distorted tetrahedral or an octahedral
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228
287
400
16%WO3/MTS-9
Diffraction intensity(a.u)
WO3
12%WO3/MTS-9
16%WO3/MTS-9
12%WO3/MTS-9
8%W0 3 /MTS-9
4%WO3/MTS-9 MTS-9
200 250 300 350 400 450 500 550 600 650 700 750 800
Wavelength(nm)
Intensity(a.u)
8%WO3/MTS-9
4%W0 3 /MTS-9
Fig. 4. UV–vis diffuse reflectance spectra of various samples.
coordination environment [28,29], the diffraction peak at 400 nm was assigned to crystal WO3. The two bands in the UV–vis spectra indicated that two distinct surface tungstate species were present on the catalysts. For loadings above 8%, a peak at 400 nm was observed, and the intensity increases slightly with the increase of WO3 loading, the result was in accordance with that obtained by XRD and H2-TPR. The intensity of peaks at 228 and 287 nm increased slightly with the increase of WO3 loading, and they may be active sites for metathesis reaction. 3.6. Raman spectroscopy The major structural information concerning the surface tungsten oxide species can be derived from Raman spectroscopy because of its ability of distinguishing different tungsten oxide species that may simultaneously be present on the catalysts. The Raman spectra of all catalysts were shown in Fig.5. The SiO2 exhibited Raman bands at 1090, 976, 792, 600 and 489 cm1 [30], bands at 1090 and 792 cm1 were assigned to the transverse-optical (TO) asymmetric stretch and Si–O–Si symmetrical stretching [31], the 976 cm1 Raman band arose from the Si– OH stretching mode of the surface hydroxyls [31,32], bands at 600 and 489 cm1 were attributed to tri- and tetra- cyclosiloxane rings, respectively [32]. MTS-9 also exhibited Raman bands at 488, 797, 978 and 1100 cm1, MTS-9 and SiO2 possessed common bands at 488, 790, 976, and 1090 cm1, but the intensity of the band at 976 and 1090 cm1 for MTS-9 were stronger than SiO2, suggesting that these bands(976 and 1090 cm1) were attributed to the resonance Raman effect [33], others were the characteristics of SiO2. For catalysts with different WO3 loadings, very strong bands at 1000 and 1110 cm1 are observed. In comparison to SiO2 and MTS-9, the bands at 976 and 1090 cm1 for SiO2 and MTS-9 shift to 1000 and 1110 cm1 for catalysts with different WO3 loadings. The intensity of bands at 1000 and 1110 cm1 are stronger for catalysts than for MTS-9. The two bands of the catalysts at 1000 and 1110 cm1 increase in intensity gradually from 4% to 12% loading, and the two bands for 16% loading is almost the same for 12% loading. According to literatures [27,34], the Raman band at 1000 cm1 is assigned to the O@W@O bond of the isolated surface tetrahedral tungsten oxide species. The Raman band at 1100 cm1 corresponds to a surface octahedral structure of polytungstate with monoxo W@O coordinated species. For 4% WO3/MTS-9, except for Raman bands at 1000 and 1100 cm1, no other bands were observed. However, additional Raman bands were observed for the
MTS-9
SiO 2 200
400
600
800
1000
1200
14 00 16 00 1800
2000
Raman Shift(cm-1 ) Fig. 5. The Raman spectra of SiO2 and catalysts with different loadings.
other catalysts. For 8% WO3/MTS-9, additional Raman band was observed at 800 cm1; For 12% WO3/MTS-9 and 16% WO3/MTS9, additional Raman bands were observed at 270, 320, 730 and 800 cm1 due to more crystalline WO3 particles. These bands (270, 320, 730 and 800 cm1) were assigned to the W–O–W deformation mode, the W@O bend mode, the O–W–O bending mode of surface WOx and the W@O stretching mode [35,36], indicating that high loadings of tungsten species may result in the agglomeration of bulk microcrystalline WO3 on the surface. 3.7. Metathesis reaction 3.7.1. Effect of tungsten oxide loading on metathesis activity of catalyst To investigate the influence of the WO3 loading on catalyst activity, WO3/MTS-9 catalysts containing 0, 4, 8, 12, 16 wt% WO3 and tungsten trioxide (WO3) were prepared. Fig. 6 showed the relationship of yield of propene and WO3 loading. It can be seen that the plain MTS-9 and tungsten trioxide possess no activity. For the other catalysts, the initial activity increased with time, up to the point where the steady state was attained. The transition process (the introduction period) was common to a number of catalysts, including silica-supported tungsten oxide used in metathesis of olefins. The introduction period was shortened by reduction pretreatment or reaction at higher temperature, but was not eliminated. For 16% WO3/MTS-9 catalyst, loading was the highest, but the activity was not the highest, indicating that all of WO3 did not act in reaction and WO3 interacting only with support possessed activity, which was verified by 4% WO3/MTS9. The yield of propene increases with increasing WO3 loading, an optimal loading was 12 wt%. Further increasing loading resulted in the decrease of propene yield, the reason might be that the active centers were covered by an excessive of crystalline WO3 and the number of active centers decreased for too much WO3 loading. As shown in profile of H2-TPR, spectra of UV–vis DRS and Raman, peak of surface tungsten oxide species increased with the increase
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40
22
50 4 % WO3/MTS-9
16 % WO3/MTS-9 MTS-9 WO3
20
10
45
Conversion of butene(%)
12 % WO3/MTS-9
20 40 18 35
16
30
25 14
0 0
10
20
30
40
20 660
50
665
670
Fig. 6. Activity of WO3/MTS-9 with different loadings: Reaction conditions P = 0.8 MPa, T = 593 K, WHSV = 6.4 h1.
675
680
685
50
80 70
C2H4
80
C3H6
=
=
1-C4/2-C4
50 40 0.4 30 20
1-C=4/2-C=4
0.6
0.2
ethene propene pentene hexene conversion
40
50 30 40 30
20
Conversion(%)
C6H12
60
Selectivity(%)
60
0.8
C5H10
70
695
Fig. 8. Effect of pretreatment temperature on conversion of butene and yield of propene Reaction conditions: P = 0.8 MPa, T = 593 K, WHSV = 6.4 h1.
1.0
90
690
Temperature(K)
Time on stream (h)
Selectivity(%)
Yield of propene(%)
Yield of propene (%)
8 % WO3/MTS-9
30
20 10
10
10 0
0 660
0.0
670
680
690
0 540 550 560 570 580 590 600 610 620 630 640 650 660 670
Temperature(K)
Temperature(K) Fig. 7. Effect of pretreatment temperature on selectivity of production: Reaction conditions P = 0.8 MPa, T = 593 K, WHSV = 6.4 h1.
Fig. 9. Correlation between temperature and activity Reaction conditions: P = 0.8 MPa, WHSV = 6.4 h1.
of WO3 loading in the range of 4wt%-12wt% and decreased slightly for 16% WO3/MTS-9, the trend was in accordance with the metathesis activity of catalyst, so optimal loading was 12%.
on the temperature of activation. According to the literature [38], the tungsten oxide of intermediate nonstoichiometric oxidation state could be the most active one for olefin metathesis. This point was confirmed by our experiments in this work.
3.7.2. Effect of pretreatment temperature on activity of catalyst According to previous results reported in literatures [35–37], tungsten possessing some intermediate valence state had activity for the tungsten supported catalyst, so they had to be reduced before it was used. So the effect of reduction pretreatment temperature on activity of catalyst was investigated in this study, catalyst used was 12% WO3/MTS-9, and the results were shown in Fig. 7 and Fig. 8. The catalyst was pretreated by using hydrogen and nitrogen mixture flow (with 10% hydrogen). It can be seen that pretreatment temperature had no effect on selectivity of products, which kept constant. But the conversion of butene and yield of propene impacted dramatically by the pretreatment temperature. As shown in Fig. 8, when the catalyst was pretreated at 663 K, the conversion of butene and yield of propene were 41.8% and 35.6%, respectively. When the reduction temperature raised to 673 K, the activity of catalyst reached maximum, however with further increasing temperature, the activity fell off. Hence the activity of the catalyst depends
3.7.3. Effect of reaction temperature on activity of catalyst To investigate the effect of temperature on the activity of catalyst, metathesis of butene was carried out over catalyst 12% WO3/ MTS-9, pretreating temperature was 673 K, and the results were shown in Fig. 9 It can be seen that the activity of catalyst was affected greatly by reaction temperature. The conversion of butene and selectivity of propene increased slightly with increasing reaction temperature, the elevated temperature is favorable to forming 1-butene from isomerization of 2-butene.The increase of 1-butene promoted the reaction to move towards propene and pentene. 4. Conclusions This paper presented a detailed study of the nature of WO3/ MTS-9 catalyst and its catalytic properties in the metathesis of butene to propene and pentene, we made the following conclusions:
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