Production of propene from 1-butene metathesis reaction on tungsten based heterogeneous catalysts

Transcription

1 Journal of Natural Gas Chemistry 18(2009) Production of propene from 1-butene metathesis reaction on tungsten based heterogeneous catalysts Huijuan Liu 1,2, Ling Zhang 1,2, Xiujie Li 1, Shengjun Huang 1, Shenglin Liu 1, Wenjie Xin 1, Sujuan Xie 1, Longya Xu 1 1. State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian , Liaoning, China; 2. Graduate University of Chinese Academy Sciences, Beijing , China [ Received February 20, 2009; Revised March 24, 2009; Available online August 14, 2009 ] Abstract A new propene production route from 1-butene metathesis has been developed on heterogeneous 10WO 3 /Al 2 O 3 -HY catalysts with different HY contents. It is found that the catalysts play bi-functionally first for the isomerization of 1-butene to 2-butene and then for the cross-metathesis between 1-butene and 2-butene to propene and 2-pentene. The combination of HY zeolite and Al 2 O 3 is prerequisite for the production of propene. The propene yield keeps increasing with the HY content in the range of wt%, where 10WO 3 /Al 2 O 3-70HY exhibits the highest propene yield. The MS-H 2 -TPR and MS-O 2 -TPO characterizations indicate that the increase of HY content in the catalysts weakens the interaction between W species and supports, in contrast to the probability of coking on the metal species and acid sites. Key words propene; 1-butene; metathesis; 10WO 3 /Al 2 O 3 -xhy; bifunction 1. Introduction Demand for propene due to the increasing demand of propene derivatives is growing rapidly in worldwide chemistry. The technology of propene production from olefin metathesis has become a major focus research in recent years, such as the Lummus ABB process, which converts ethene into propene through cross-metathesis with 2-butenes over a heterogeneous metathesis catalyst [1 4]. According to reaction equations based on widely accepted carbene mechanism, propene can be obtained only between the terminal and internal alkenes, i.e. 1-butene and 2-butene. For the recent published results [5], propene can not be obtained by pure 1-butene feed over homogeneous Grubbs first generation-type ruthenium catalysts. As a result, the commercial application of Phillips Triolefin Process (now licensed as ABB OCT) and Shell Higher Olefin (SHOP) used isomerization catalysts system to get 2-butene from 1-butene or ethene before the metathesis reaction [4]. An alternative way to produce propene is the autometathesis of 1-butene over bifunctional catalysts with both isomerization as well as metathesis activity. In this light, metal oxide (MoO 3 or WO 3 ) supported on acidic aluminazeolite composite can be a candidate for the production of propene from pure 1-butene. The aim of the present work is to report a heterogeneous catalyst, i.e. 10WO 3 /Al 2 O 3 -HY, as well as its catalytic performance in the 1-butene metathesis reaction. A series of catalysts with different HY contents in support are evaluated. The results indicate that the catalytic performances are remarkably influenced by HY content in 10WO 3 /Al 2 O 3 -HY catalysts. The propene molar yield can achieve 21% and 19% on 10WO 3 /30%Al 2 O 3-70%HY and 10WO 3 /50%Al 2 O 3-50%HY respectively. The initial catalytic activity can be correlated with the state of tungsten oxide species as a function of HY content. Furthermore, the coking behavior evidenced by TPO profiles is closely associated with the transformation of tungsten oxides and acidity of the catalysts, which will lead to remarkably different stability in the metathesis reaction. 2. Experimental 2.1. Catalyst preparation and its evaluation Al 2 O 3 -HY support was prepared by extruding a mixture of γ-al 2 O 3 powder and HY zeolite (Wenzhou Zeolite Manu- Corresponding author. Tel: ; Fax: ; lyxu@dicp.ac.cn This work was supported by the National Natural Science Foundation of China (No ) and National 973 Project of China (No. 2005CB221403) Copyright 2009, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi: /s (08)

2 2 Huijuan Liu et al./ Journal of Natural Gas Chemistry Vol. 18 No facture of China, Si/Al 2 = 10, Na 2 O<0.2 wt%) at desirable weight percent into strips with a diameter of about 2 mm. The drawn extrudate was left at room temperature for a few hours and placed in an oven at 393 K for 12 h. Subsequently, the dried extrudate was calcined at 773 K for 2 h and then ground into mesh. Catalysts containing 10wt% tungsten were prepared by impregnation of the Al 2 O 3 -HY support with ammonium metatungstate solution according to the incipient wetness method. The impregnated samples were dried at 393 K for 5 h, and then calcined at 873 K for 2 h. These prepared catalysts are denoted as 10WO 3 /Al 2 O 3 -xhy, where x indicates the weight percent of HY zeolite in the Al 2 O 3 -HY support. The catalysts were evaluated in a fixed-bed flow microreactor of 10 mm inner diameter, and 3 g of catalyst with an average particle size of mm was loaded. An EU-2 type thermocouple was fixed in the middle of the catalyst bed and taken as the reaction temperature. Before evaluation, the catalysts were pretreated at 773 K in high purity N 2 (30 ml/min) for 1 h, then it was cooled down to the desired reaction temperature. The typical reaction conditions are as follows: temperature = 453 K, pressure = 0.1 MPa; WHSV (1-C 4 H 8 ) = 1.5 h 1, and catalyst = 3.0 g. The reaction products are analyzed by a Varian 3800 gas chromatograph equipped with an FID detector and a 50 m Al 2 O 3 -plot column Product analysis Reaction of 1-butene on 10WO 3 /Al 2 O 3 -xhy catalysts may include isomerization, self-metathesis, second metathesis reaction and possible olefin oligomerization as shown below: Scheme 1. Scheme of isomerization, self-metathesis, second metathesis reaction and possible olefins oligomerization It is hard to make a comprehensive list of all the reactions and products due to the complicated side reactions and crossmetathesis reactions. GC results indicate that cis, trans-2- pentene and cis, trans-3-hexene are the major products for C 5 and C 6 component. For a simple treatment, we denote products with the same carbon number as C = n, where n represents carbon number. At the same time, all the heavy products in C 7 C 10 range are denoted as C 7+. The 1-butene conversion is determined by the mass percentage in the exhaust gas, and the molar yield of products is calculated by the following equation: Y (C = n) = W(C = 2 ) 2 + W(C= 3 ) 3 + W(C= 4 ) 4 In the formula, Y (C = n ) is the yield of olefin with carbon number n, X is the 1-butene conversion, and W (C = n ) denotes mass percent of alkene component with different carbon number n. X W(C= n) n + W(C= 5 ) 5 + W(C= 6 ) + W(C= 7 ) + W(C= 8 ) O 2 /Ar flow rate was 50 ml/min. The carbon was monitored by QMS (oministar) instrument, using the fragments with m/e of 28 and 44 as the representative of carbon monoxide and carbon dioxide Catalyst characterization X-ray diffraction (XRD) measurements were made with an X pert PRO/PANalytical Diffractometer using Cu-K α radiation and operated at 40 kv and 40 ma, with a scanning speed of 5 o /min. Patterns were recorded from 5 o to 70 o (2θ). Under 10% H 2 /Ar flow (20 ml/min), H 2 -TPR profiles were obtained in the range of ambient to 1073 K at a programmed temperature rate of 14 K/min after the samples had been pretreated in an Ar flow at 773 K for 30 min. The reduction was monitored by QMS (oministar) instrument, using the fragments with m/e of 18 as the representative of water which were formed from oxidation of H 2. Coke deposition on spent 10WO 3 /Al 2 O 3 -xhy catalysts was determined by temperature programmed oxidation (O 2 - TPO) g sample was loaded into a U-shaped quartz micro-reactor. The catalyst was heated from 323 to 1123 K in a 10% O 2 /Ar stream. The heating rate was 10 C/min, and the 3. Results and discussion 3.1. Catalytic performance of 10WO 3 /Al 2 O 3 -xhy catalysts Table 1 shows the activity and selectivity of 10WO 3 / Al 2 O 3 -xhy catalysts as a function of HY zeolite content. The catalytic performance of catalysts depends on HY content in the support. Although all the catalysts exhibit high 1-butene conversion, the product distributions are quite different. 10WO 3 /Al 2 O 3 proceeds metathesis reaction with 88.9% 1-butene conversion at 453 K, while the total metathesis activity is very poor with 0.5% molar yield. The primary products are cis-2-butene and trans-2-butene, indicating the predominance of isomerization reaction. This is in fair agreement with reference results, which always report poor low metathesis activity for WO 3 /γ-al 2 O 3 under relatively mild conditions [1]. A remarkable change in distributions of products has been ob-

3 Journal of Natural Gas Chemistry Vol. 18 No served with increasing HY contents. As shown in Table 1, 1-butene conversion on 10WO 3 /Al 2 O 3-10HY slightly increases to 93.3%, while propene yield achieves 9.8%. In the range of 10wt% 70wt% HY content, the propene yield keeps increasing with higher HY content. The maximum propene yield of 21.4% belongs to 10WO 3 /Al 2 O 3-70HY. However, further higher HY content will lead to a decreased yield as 18.6% on 10WO 3 /Al 2 O 3-90HY. As another extreme example, the propene yield on 10WO 3 /HY dramatically decreases to 1.4%. Upon the active 10WO 3 /Al 2 O 3 -xhy catalysts, C = 3 and C = 5 yields are around 20% and 15%, whereas C= 2 and C = 6 yields are just around 1% and 5%, respectively. This suggests 1-butene metathesis does not proceed through selfmetathesis reaction but through cross-metathesis route between 1-butene and 2-butene. Such low ethene yield also suggests consumption of this product by successive secondary cross-metathesis reactions. As a comparison, the yield of propene is quite higher than that of associated C = 5 products, which is supportive for the contribution from cross metathesis reactions. Table 1. Reaction performance of 10WO 3 /Al 2 O 3 -xhy with different HY contents Catalyst 1-C = 4 conversion Metathesis yield (%) Polymerization Isomerization yield (%) (%) C = 2 C = 3 C = 5 C = 6 yield (%) cis-2-c = 4 trans-2-c = 4 10WO 3 /Al 2 O WO 3 /Al 2 O 3-10HY WO 3 /Al 2 O 3-30HY WO 3 /Al 2 O 3-50HY WO 3 /Al 2 O 3-70HY WO 3 /Al 2 O 3-90HY WO 3 /HY Reaction conditions: T = 453 K; P = 0.1 MPa; WHSV = 1.5 h 1 ; time on stream = 1 h The propene yield of 10WO 3 /Al 2 O 3 -xhy catalysts as a function of time is compared in Figure 1(a). Low yields (<10%) are still observed on 10WO 3 /HY, 10WO 3 /Al 2 O 3, and 10WO 3 /Al 2 O 3-10HY with extended reaction time of 10 h. Whereas for the active 10WO 3 /Al 2 O 3 -xhy (30 wt% 90 wt% HY) catalysts, propene yield increased in the initial 5 h on each sample, indicating the presence of induction period. The induction time was also observed on WO 3 /SiO 2, indicating a period was needed to form active metal-carbene species [1]. But such phenomena were not observed in metathesis of ethene and 2-butene with the same WO 3 /Al 2 O 3 - xhy catalysts [6]. Although the reason is not clear, this may suggest the difference in the interaction between substrate alkenes and tungsten oxide species for the initial formation of active metal-carbene species. Figure 1(a) displays that the maximum propene yield of 25% belongs to 10WO 3 /Al 2 O 3-70HY after the initial induction period. However, the propene yield on the catalyst sharply decreases after 17 h running time, and only 5% yield can be obtained at 30 h time on stream. As a comparison, catalysts with lower HY content show better stability despite their slightly lower initial metathesis activity. For example, the propene yield on 10WO 3 /Al 2 O 3-50HY can still achieve 17% after 30 h time on stream. While on the same WO 3 /Al 2 O 3 -xhy catalysts for the metathesis between ethene and butene to propene, the stability of catalyst increases with propene selectivity [6], indicating the other different reaction behavior of 1-butene on WO 3 /Al 2 O 3 -xhy catalysts. Since propene is from cross-metathesis between 1-butene and 2-butene, the variation of 2-butene yield and 1-butene conversion were also compared. 2-butene yield on 10WO 3 /Al 2 O 3 and 10WO 3 /Al 2 O 3-10HY are high and stable, indicating the two catalysts are active for 1-butene isomerization reaction. The 2-butene yield on 10WO 3 /Al 2 O 3-30HY and 10WO 3 /Al 2 O 3-50HY shows almost the same tendency, on which 2-butene yield increases Figure 1. Comparison of 10WO 3 /Al 2 O 3 -xhy reaction performance. (a) Propylene yield, (b) 2-butene yield, (c) 1-butene conversion. ( ) 10WO 3 /Al 2 O 3, ( ) 10WO 3 /Al 2 O 3-10HY, ( ) 10WO 3 /Al 2 O 3-30HY, ( ) 10 WO 3 /Al 2 O 3-50HY, ( ) 10WO 3 /Al 2 O 3-70HY, ( )10WO 3 /Al 2 O 3-90HY, ( ) 10WO 3 /HY

4 4 Huijuan Liu et al./ Journal of Natural Gas Chemistry Vol. 18 No with time on stream, indicating more 1-butene converts to 2-butene on these catalysts. For 10WO 3 /Al 2 O 3-70HY and 10WO 3 /Al 2 O 3-90HY 2-butene yield decreases first and then increases. As for 10WO 3 /HY catalysts, 2-butene yield decreases rapidly with time on stream, suggesting a fast deactivation on the catalyst. The initial 1-butene conversion on various catalysts was close to 90% except a lower value of 80% on 10WO 3 /HY. The most distinct difference was observed in the variation of 1-butene conversion as the function of time on stream. 1-butene conversions are quite stable on 10WO 3 /Al 2 O 3 -xhy with HY content lower than 50%. However, higher HY content will lead to a sharper decrease of 1-butene conversion. Especially for 10WO 3 /HY, the 1-butene conversion undergoes a sharp decrease to 43% within 10 h. At the same time, the 2-butene yield on 10WO 3 /HY decreases correspondingly to 40% in 10 h. From the sharp decrease of 1-butene conversion and 2-butene yield, it is inferred that a significant reduction of accessible acid sites on 10WO 3 /HY takes place in the process of 1-butene conversion, these are the active sites for double bond isomerization reaction [7,8]. As shown in Figure 1(b) and (c), the decrease of HY content in support leads to improved performance for the 1-butene conversion and isomerization to 2-butene. The less active metathesis catalysts of 10WO 3 /Al 2 O 3, 10WO 3 /Al 2 O 3-10HY and 10WO 3 /Al 2 O 3-30HY give higher and more stable 2-butene yields. Whereas, such isomerization product yields on 10WO 3 /Al 2 O 3-70HY and 10WO 3 /Al 2 O 3-90HY gradually increase after the decrease in the initial reaction stage. According to the results in Table 1 and Figure 1, combination of Al 2 O 3 and HY in appreciable ratio is a prerequisite for the high active catalysts in 1-butene metathesis, which has also been observed in the metathesis reaction between ethene and 2-butene [6,9]. In preceding study, such support effect has been related with acidity, interaction between metal species and support, and variation of tungsten oxide species [6]. In the case of 1-butene substrate, the distinctive differences in metathesis catalytic performance, isomerization and 1-butene conversion suggest more understandings of support effect apart from other viewpoints as a function of HY content. tic peaks is shown in samples with HY content below 70 wt%. Only HY characteristic peaks, except their difference in intensity, appear as the HY content is higher than 70 wt%, due to the decrease of Al 2 O 3 content as well as the lower sensitivity of Al 2 O 3 to the X-ray compared with HY zeolite. The poor performance of WO 3 /Al 2 O 3 for 1-butene metathesis reaction was partially attributed to the lower Brönsted acidity [6]. However, the high 2-butene yield over 10WO 3 /Al 2 O 3 in Figure 1(b) excludes the potential limitation of acidity on the isomerization for the cross-metathesis reaction between 1-butene and 2-butene. In our case, one of the driving force for the zeolite-alumina composite is to improve the Brönsted acidity of catalysts, which can interact with metal species and results in the active sites precursors [10]. According to previous characterization results by NH 3 - TPD and pyridine-ir adsorption [6], the increase of HY zeolite content enhances the total acidity and Brönsted acidity of catalysts proportionally. As shown in Figure 1 and Table 1, the initial propene yield increases stepwise with higher HY content within 70 wt% range. However, catalysts with further higher HY content, especially 10WO 3 /HY, undergo dramatic decrease in metathesis activity despite the steadily enhanced acidity. This suggests that the metathesis activity of 1-butene is controlled not only by the catalyst Brönsted acidity and total acidity, but also by the other factors such as the oxidization number of W species, the reducibility of catalyst and coordination geometry of W species etc Characterization results and discussion XRD patterns of WO 3 species loaded on Al 2 O 3 -xhy supports with different HY contents are shown in Figure 2. The X-ray diffraction patterns do not show any evidence of the WO 3 phase on 10WO 3 /Al 2 O 3 -xhy with the HY contents in the range of 0 70 wt%, indicating a good dispersion of tungsten species on these supports, or small crystallite agglomerates are outside the XRD crystallite size detection limits. As for 10WO 3 /Al 2 O 3-90HY and 10 WO 3 /HY, there are obvious WO 3 phase formed. The intensities of sharp HY characteristic peaks increase with increase of HY content in the Al 2 O 3 -HY support. The coexistence of broad γ-al 2 O 3 (2θ = o, o, and o ) and sharp HY characteris- Figure 2. XRD patterns of 10WO 3 /Al 2 O 3 -xhy catalysts Interaction between W species and the Al 2 O 3 -HY support was investigated by the H 2 -TPR technique. In this paper, typical comparisons of fresh and spent catalysts such as 10WO 3 /HY and 10WO 3 /Al 2 O 3-70HY were made by MS-H 2 - TPR as shown in Figure 3. A higher-temperature peak denoted as h existed in all of the investigated samples, but for

5 Journal of Natural Gas Chemistry Vol. 18 No the spent 10WO 3 /HY catalyst, it was almost vanished; as for the fresh and spent 10WO 3 /Al 2 O 3-70HY catalysts, it shifted to higher temperature, suggesting an enhancement of interaction between W species and the supports with increase of HY contents. After 1 h time on stream, the curve of spent 10WO 3 /Al 2 O 3-70HY was similar to that of fresh catalyst, indicating there was little variation of tungsten species from the original state after 1 h introduction of alkene. The profile of fresh 10WO 3 /HY catalyst exhibited two reduction peaks, denoted as l and h, suggesting that the 10WO 3 /HY was subject to the deep reduction by alkenes under reaction condition. However, the reduction peak of h at higher temperature almost disappeared after 1 h reaction under the 1-butene atmosphere indicating a complete reduction of high-valence W species in a short time on 10WO 3 /HY catalyst. This might also reflect the serious coking on 10WO 3 /HY. metal species, similar trend can be observed on the catalysts except 10WO 3 /HY. This indicates the differences of tungsten oxide species over HY, γ-al 2 O 3 and γ-al 2 O 3 -HY. As characterized by H 2 -TPR, the increase of HY zeolite weakens the metal-support interaction, and leads to the increased lateral interaction among tungsten oxide species on support. As a result, the structure of tungsten oxide species will change accordingly. Figure 4. O 2 -TPO profiles of 10WO 3 /Al 2 O 3 -xhy catalysts Figure 3. Comparison of reducibility between fresh and spent 10WO 3 /Al 2 O 3 -xhy catalysts O 2 -TPO was used to characterize the coke species on 10WO 3 /Al 2 O 3 -xhy catalysts after reaction. As shown in Figure 4, all catalysts display coke burning peaks after reaction, and the shape and maxima temperatures (T max ) of the profiles strongly depend on the HY contents. The spent 10WO 3 /Al 2 O 3 has two overlapping peaks centered at 673 and 783 K, respectively. With the increase of HY content in support, the intensities of both peaks increase, accompanied by the shift to the higher temperature. When HY content reaches 50%, a small burning peak appears at 550 K. For 10WO 3 /HY, the profile undergoes a remarkable change. The former burning peak shifts oppositely to lower temperature of 660 K with the presence of more intensive peak at 550 K. Sachtler et al. [11] investigated typical TPO profiles of a bifunctional catalyst and assigned the first peak at lower temperature to the coke oxidation catalyzed by metal and the broad one at higher temperature to coke oxidation from the covered acid sites. As shown in Figure 4, coke species related with acid sites become much more severe with increase of HY content, which agrees well with the variation of acidity as the function of HY content in NH 3 -TPD and Py-IR [6]. For coke species related with It is well reported that tungsten oxide species will be present as isolated tetrahedral species on low loading WO 3 /Al 2 O 3 [12,13]. By preceding Raman results [14], characteristic band of W=O bonds ( 970 cm 1 ) in distorted tetrahedral state can be observed on 10WO 3 /Al 2 O 3. With the increase of HY content, tungsten species will turn into surface poly-tungsten oxide species with W-O-W linkages (broad band in cm 1 ). For 10WO 3 /HY, the tungsten species will turn into small crystallites and WO 3 -like polytungstate (overlap bands around 805, 706 and 273 cm 1 band). A positive correlation between tetrahedral tungsten oxide species and metathesis activity has been displayed, suggesting these species to be active sites precursors in reaction between ethene and 2-butene [6,9,11]. Since the quite similar reaction atmosphere, the active sites precursors in 1-butene metathesis should also include these tetrahedral tungsten oxide species. Combining the transformation of metathesis activity and O 2 -TPO profiles, the increase of HY zeolite content will not only improve the amount of the active sites precursors, but also promote the coking related with side reactions. In the case of 10WO 3 /HY, the most severe coking behavior can be observed in the presence of poly-tungsten oxide and WO 3 -like polytungstate species. Instead of being active in metathesis activity, these species can catalyze the coke formation related with side reactions of alkene substrates, leading to

6 6 Huijuan Liu et al./ Journal of Natural Gas Chemistry Vol. 18 No further blockage of tungsten species and acid sites. This can explain the poor metathesis activity and simultaneous quick deactivation of 1-butene isomerization on 10WO 3 /HY. In summary, the incorporation of HY zeolite in 10WO 3 / Al 2 O 3 support affects the acidity of catalyst, metal-support interaction and tungsten active sites, which are responsible for the metathesis activity and catalyst stability. 4. Conclusions The heterogeneous 10WO 3 /Al 2 O 3 -xhy catalysts presents a good performance on the metathesis of 1-butene for the production of propene. The 1-butene metathesis catalyzed by 10WO 3 /Al 2 O 3 -xhy catalysts does not proceed through the self-metathesis reaction but through the cross-metathesis route between 1-butene and 2-butene which is obtained from the isomerization of 1-butene. The metathesis activity is influenced both by the Brönsted acidity of samples and the state of tungsten oxide species. Especially, the increase of HY zeolite content in the 10WO 3 /Al 2 O 3 -xhy samples has the conflicting effects: improves the amount of the active sites precursors and promotes the coking from side reactions. 10WO 3 /Al 2 O 3 -xhy catalysts with 50wt% 70wt% HY content show good 1-butene conversion, as well as propene yield due to their suitable Brönsted acidity and suitable interaction between W species and supports for the formation of active sites precursors. References [1] Ivin K J, Mol J C. Olefin Metathesis and Metathesis Polymerization. San Diego: Academic Press [2] Taoufik M, Roux E L, Cazat J T, Basset J M. Angew Chem Int Ed, 2007, 46(38): 7202 [3] Connon S J, Blechert S. Angew Chem Int Ed, 2003, 42(17): 1900 [4] Mol J C. J Mol Catal A, 2004, 213(1): 39 [5] Meyer W H, Radebe M M D, Serfontein D W, Ramdhani U, Toit M D, Nicolaides C P. Appl Catal A, 2008, 340(2): 236 [6] Huang S, Liu S, Xin W, Bai J, Xie S, Wang Q, Xu L. J Mol Catal A, 2005, 226(1): 61 [7] Yamaguchi T, Tanaka Y, Tanabe K. J Catal, 1980, 65(2): 442 [8] Guo Y, Pu M, Liu L, Li H, Chen B. Comput Mater Sci, 2008, 42(2): 179 [9] Huang S, Liu S, Zhu Q, Zhu X, Xin W, Liu H, Feng Z, Li C, Xie S, Wang Q, Xu L. Appl Catal A, 2007, 323: 94 [10] Xu X, Boelhouwer C, Vonk D, Benecke J, Mol J C. J Mol Catal A, 1986, 36(1-2): 47 [11] Lerner B A, Zhang Z, Sachtler W M H. J Chem Soc, Faraday Trans, 1993, 89(11): 1799 [12] Salvati L, Makovsky J L E, Stence J M, Brown F R, Hercules D M. J Phys Chem, 1981, 85(24): 3700 [13] Horsley J A, Wachs I E, Brown J M, Via G H, Hardcastle F D. J Phys Chem, 1987, 91(15): 4014 [14] Huang S, Chen F, Liu S, Zhu Q, Zhu X, Xin W, Feng Z, Li C, Wang Q, Xu L. J Mol Catal A, 2007, 267(1 2): 224