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Selective hydrogenation of benzene to cyclohexene over Ce-promoted Ru catalysts
Journal of Energy Chemistry 22(2013)710–716
Selective hydrogenation of benzene to cyclohexene over Ce-promoted Ru catalysts Haijie Sunb , Yajie Pana , Shuaihui Lia , Yuanxin Zhanga , Shouchang Liua, Zhongyi Liua∗
Yingying Donga,
a. College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, Henan, China; b. Institute of Environmental and Catalytic Engineering, Department of Chemistry, Zhengzhou Normal University, Zhengzhou 450044, Henan, China [ Manuscript received November 30, 2012; revised January 14, 2013 ]
Abstract Ru-Ce catalysts were prepared by a co-precipitation method. The effects of Ce precursors with different valences and Ce contents on the catalytic performance of Ru-Ce catalysts were investigated in the presence of ZnSO4 . The Ce species in the catalysts prepared with different valences of the Ce precursors all exist as CeO2 on the Ru surface. The promoter CeO2 alone could not improve the selectivity to cyclohexene of Ru catalysts. However, almost all the CeO2 in the catalysts could react with the reaction modifier ZnSO4 to form (Zn(OH)2 )3 (ZnSO4 )(H2 O)3 salt. The amount of the chemisorbed salt increased with the CeO2 loading, resulting in the decrease of the activity and the increase of the selectivity to cyclohexene of Ru catalyst. The Ru-Ce catalyst with the optimum Ce/Ru molar ratio of 0.19 gave a maximum cyclohexene yield of 57.4%. Moreover, this catalyst had good stability and excellent reusability. Key words benzene; selective hydrogenation; cyclohexene; ruthenium; cerium
1. Introduction Cyclohexene is commercially important for the production of adipic acid, nylon 6, nylon 66 and many other fine chemicals [1,2]. Selective hydrogenation of benzene to cyclohexene has drawn much attention due to its inexpensive starting feedstock, atomic economy, low amounts of undesirable products and simplified operation [3−6]. However, it is difficult to obtain a high yield of cyclohexene through this route, because cyclohexane, the complete hydrogenation product, is thermodynamically more favorable [1,2]. It was found that the promoters have great impacts on the yield of cyclohexene for Ru catalysts. Fan et al. [7,8] prepared a Ru-Co-B/γ-Al2O3 catalyst and reached a cyclohexene yield of 34.8% in the presence of ethylenediamine and ZnSO4 . Xue et al. [9] prepared a Ru-Zn/SiO2 catalyst in a water-in-oil microemulsion and obtained a cyclohexene yield of 29.9%. Liu et al. [10] developed a Ru-Cu/ZnO catalyst by a depositionprecipitation method and got a cyclohexene yield of 49.4%. Zhou et al. [11] prepared a Ru catalyst promoted with Mn and Zn by a co-precipitation method and received a cyclohexene
yield of 59.9%. On the other hand, rare-earth elements such as La and Ce have been frequently used as promoter of Ru catalysts for this reaction. Liu et al. [12,13] prepared a Ru-La-B/ZrO2 catalyst and got a cyclohexene yield of 53% in the presence of ZnSO4 . They found that the addition of La could significantly enhance both the activity and the selectivity to cyclohexene. They had confirmed that the La in the catalyst existed as La2 O3 . They suggested that the role of La was mainly ascribed to its dispersion effect on Ru. Liu et al. [14] prepared a Ru-La/SBA-15 catalyst by a “two-solvents” impregnation method and achieved a cyclohexene yield of 57% in the presence of CdSO4 and ZnSO4 as co-modifiers. However, the effect of the La promoter in improving the selectivity to cyclohexene was ignored there. Liu et al. [15] prepared the Ce-promoted Ru/SBA-15 catalysts by a “two-solvents” impregnation method and obtained a cyclohexene yield of 53.8% in the presence of ZnSO4 . Based on characterizations, the beneficial effects of the Ce promoter on the selectivity to cyclohexene were ascribed to the enhacement of the hydrophilicity of the catalyst and to the electron transfer between the Ce(III) species and metallic Ru.
Corresponding author. Tel/Fax: +86-371-67783384; E-mail: [email protected] (Z.Liu) This work was supported by the National Nature Science Foundation of China (21273205), the Innovation Found for Technology Based Firms of China (10C26214104505), the Chinese Post-doctorate Science Fund 51th batch of surface subsidizes (2012M511125) and the Scientific Research Foundation of Graduate School of Zhengzhou University. ∗
Copyright©2013, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.
Journal of Energy Chemistry Vol. 22 No. 5 2013
In our previous work, we prepared Ru-Zn catalysts and achieved a cyclohexene yield of 58.9%. We found that the Zn in the catalysts existed as ZnO. The ZnO on the surface could react with ZnSO4 in the slurry to form (Zn(OH)2)3 (ZnSO4)(H2 O)5 salt. The chemisorbed (Zn(OH)2)3 (ZnSO4 )(H2 O)5 salt play a key role in improving the selectivity to cyclohexene [16,17]. We obtained a cyclohexene yield of 63.6% in the presence of ZnSO4 and diethanolamine over Ru-Zn catalyst. It is found that the (Zn(OH)2)3 (ZnSO4 )(H2 O)3 formed by diethanolamin reacting with ZnSO4 improved the cyclohexene yield [18]. We also got a cyclohexene yield of 61.4% in the presence of ZnSO4 and PEG-20000 over Ru-Zn catalysts [19]. In the present work, we prepared Ru-Ce catalysts by a co-precipitation method. The effects of the Ce precursors with different valences and the Ce contents on the catalytic performance of Ru-Ce catalysts were investigated in the presence of ZnSO4 . 2. Experimental 2.1. Catalyst preparation Ru-Ce catalysts were prepared according to the following procedure. 9.75 g RuCl3 ·H2 O and a desired amount of Ce(NO3 )3 ·6H2 O were dissolved in 400 mL H2 O with agitation. To the stirred solution, 200 mL of a 10% NaOH solution was added instantaneously and the resulting mixture was agitated for an additional 4 h at 353K. This black precipitate was dispersed in 400 mL of a 5% NaOH solution and charged into a 1 L autoclave lined the Teflon. Hydrogen was introduced into the autoclave to raise the total internal pressure to 5 MPa and the reduction was conducted at 423K and at 800 r/min stirring rate for 3 h. The reaction mixture was cooled and the obtained black powder was washed with water until neutrality, subsequently vacuum-dried and the desired catalysts were obtained. The catalyst was divided into two shares, one share was used for activity test and another was used for catalyst characterization. This method ensured that the catalysts with different Ce contents had the same Ru contents (about 1.8 g Ru). The amounts of Ce(NO3 )3 ·6H2 O were adjusted to give the catalysts with different Ce contents which were denoted as Ru-Ce(III, x) catalysts, where x denoted the Ce/Ru molar ratio measured by X-ray fluorescence (XRF). The Ru-Ce(IV, x) catalysts were prepared in accordance with the above procedures except that the Ce(NO3 )3 ·6H2 O was replaced by the equal molar of Ce(SO4 )2 ·4H2 O. 2.2. Catalyst characterization Transmission electron micrographs (TEM) and energy dispersion scanning (EDS) were observed on a JEOL JEM2100 instrument using an accelerating voltage of 200 kV. Auger Electron Spectroscopy (AES) and sputter profiles were taken on a ULVAC PHI-700 Nano-canning Auger system with on-axis scanning argon ion gun and CMA energy analyzer. The energy resolution ratio was 0.1%. The back-
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ground pressure of analysis room was less than 5.2×10−7 Pa. The standard sample was SiO2 /Si. The sputtering rate was 9 nm/min. The Ce/Ru molar ratios and the compositions of Ru-Ce(x) catalysts after hydrogenation were measured by Xray fluorescence (XRF) on a Bruker S4 Pioneer instrument. N2 physisorption was determined on a Quantachrome Nova 100e apparatus at 77K. Temperature-programmed reduction (TPR) measurements were carried out in a U-shaped quartz reactor, using a 5% H2 /He gas flow of 50 cm3 ·min−1 , a thermal conductivity detector (TCD) and about 10 mg of catalyst. 2.3. Activity test The selective hydrogenation of benzene was performed in a 1 L autoclave lined the hastelloy. The autoclave was charged with 280 mL of H2 O containing a share of Ru-Ce(III, x) catalyst, and 49.2 g of ZnSO4 ·7H2 O, then heating commenced with H2 pressure of 5 MPa and stirring rate of 800 r/min. 140 mL of benzene was fed and the stirring rate was elevated to 1400 r/min to exclude the diffusion effect when the temperature reached 423 K. A small amount of reaction mixture was sampled every 5 min and sent for gas chromatographic analysis with a FID detector, and the benzene conversion and cyclohexene selectivity were calculated. At the end of the reaction, the organic phase was removed using a separating funnel. The slurry containing the catalyst was reused in accordance with the above hydrogenation procedures without any addition. After the reaction the organic was removed, the solid sample was washed with distilled water until no Zn2+ was detected and then was vacuum-dried for characterization. The samples after reaction corresponding to Ru-Ce(III, x) and Ru-Ce(IV, x) were denoted as Ru-Ce(III, x) AH and Ru-Ce(IV, x) AH, respectively, where AH stood for After Hydrogenation. 3. Results and discussion 3.1. Catalyst characterization Figure 1 shows TEM image and the crystallite size distribution of Ru-Ce(III, 0.19) catalyst. It is found that the catalyst particles consist of polycrystalline conglomerates of spherical and ellipsoidal crystallites with the crystallite size mainly distributing around 3.8 nm. Figure 2(a) shows the XRD patterns of the catalysts with different Ce/Ru molar ratios, which were prepared using the Ce precursors with different valences. The XRD patterns of all the catalysts display the diffractional peaks of metallic Ru (JCPDS: 01−070−0274), indicating that the Ru in the catalysts were mainly present in metallic Ru. The crystallite sizes were calculated from the Scherrer equation and were listed in Table 1. It is concluded that the Ru crystallite sizes are in the range of 4.4−5.3 nm, which were similar to those derived from the TEM result. This implies that the addition of the Ce species has a negligible effect on the Ru crystallites. Besides, the XRD patterns of the catalysts prepared using the Ce precursors with different valences all show the diffractional
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peaks of CeO2 (JCPDS: 00−004−0593), indicating that the Ce species in all catalysts prepared using the Ce precursors with different valences mainly existed as CeO2 . This suggests that the valences of the Ce precursors have little effect on the valences of the Ce species in the catalysts, since it is easy for the oxide or hydroxide of Ce(III) to be oxidized to the oxide or hydroxide of Ce(IV) in the precipitation process. Figure 2(b) shows the XRD patterns of different catalysts after hydrogenation. It is interesting to find that the diffractional peaks of CeO2 disappeared and the diffractional peaks of the (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt (JCPDS: 01−078−0247) were observed on the XRD patterns of Ru-Ce(III, 0.19), RuCe(IV, 0.21), and Ru-Ce(III, 0.25) catalysts. All these indicated that the promoter CeO2 on the surface of the cat-
alyst had reacted with the reaction modifier ZnSO4 and H2 O to form an insoluble (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt, which was shown in Reaction (1). The intensity of the diffractional peak at 2θ = 11.2o of this salt increases with the CeO2 loading, indicating the increase of the amount of the (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt formed. However, the diffractional peaks of the (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt are not found on the XRD patterns of Ru-Ce(IV, 0.16) catalyst due to its low amount confirmed by the XRF results below. The crystallite sizes of different catalysts after hydrogenation are in the range of 4.6−5.3 nm, similar to those of the corresponding catalysts. This suggests that serious agglomeration of the catalysts did not happen in the hydrogenation processes.
3CeO2 + 8ZnSO4 + 12H2O −→ 2(Zn(OH)2 )3 (ZnSO4 )(H2 O)3 ↓ +3Ce(SO4 )2
Figure 1. TEM image (a) and the crystallite size distribution of Ru-Ce(III, 0.19) catalyst (b)
Figure 2. XRD patterns of different catalysts and their corresponding samples after hydrogenation
(1)
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The XRD results revealed the existence of the (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt. Figure 3 shows the AES Zn LMM spectrum of Ru-Ce(III, 0.19) AH. It is worthy of noting here that all the spectra were recorded after Ar+ sputtering for 1 min to avoid the interruptions of the surface oxidation of catalysts. As can be seen, the kinetic energy (KE) of Zn LMM of Ru-Ce(III, 0.19) AH is 984.5 eV, this contribution being previously assigned to the oxidized Zn [20,21]. This finding suggests that the Zn is present on the surface of Ru-Ce(III, 0.19) AH mainly as oxidized Zn even under the reaction conditions of 150 ◦ C and H2 pressure of 5.0 MPa, which is consistent with the XRD results that the Zn species are mainly present in the (Zn(OH)2)3 (ZnSO4)(H2 O)3 salt. Unfortunately, AES measurements do not allow discerning any additional contribution from metallic Zn (commonly appearing in the 991−995 eV range). This indicates that the chemisorbed Zn2+ on Ru surface could not be reduced to metallic Zn. Moreover, Table 2 shows that the pH values of the aqueous solutions after hydrogenation in the presence of ZnSO4 at room temperature are below 6.32, indicating the acidity of aqueous phase due to the hydrolysis of ZnSO4 . It is well known that increasing temperature favors the hydrolysis. This means that the acidity of liquid phase is much stronger at the reaction temperature of 423 K due to the increase of hydrolysis degree of ZnSO4 . As we know, metallic Zn can hardly exist in the acid solution, which is consistent with no clear evidences of the presence of metallic Zn on the surface of Ru catalysts observed in the AES results.
noted that different catalysts had the same Ru contents. Thus the molar ratios of Ce/Ru, Zn/Ru and S/Ru reflect the compositions of the catalysts after hydrogenation. The Ru catalyst after hydrogenation in the absence of ZnSO4 only contains Ru element. However, the Zn and S elements are found on Ru catalyst after hydrogenation in the presence of ZnSO4 . Moreover, the molar ratio of Zn/Ru is much higher than that of S/Ru, indicating that the chemisorbed Zn species might not be ZnSO4 . AES results have confirmed that the Zn2+could not be reduced on Ru surface. Thus we propose that the Zn2+ are predominately present in the (Zn(OH)2)3 (ZnSO4)(H2 O)3 salt that is formed by the hydrolysis of ZnSO4 . However, the XRD pattern of Ru catalyst after hydrogenation did not show the diffractional peaks of the (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt due to its low amount beyond the detection limit of the XRD instrument. It is interesting to find that only trace amounts of the Ce are detected after hydrogenation, although the CeO2 loadings of the catalysts increase. The molar ratios of Zn/Ru and S/Ru after hydrogenation increase with the CeO2 loadings of the catalysts. All these indicate that the CeO2 mainly exist on Ru surface and most of the CeO2 on the Ru surface had reacted with the ZnSO4 in the slurry to form (Zn(OH)2)3 (ZnSO4 )(H2 O)3 and Ce(SO4 )2 salts. The (Zn(OH)2)3 (ZnSO4 )(H2 O)3 is a insoluble salt and is readily chemisorbed on the surface of the catalyst. The amount of this salt increases with the CeO2 loadings of the catalysts, which is consistent with the XRD results. The Ce(SO4 )2 is dissolved in the aqueous solution. Table 1. Textural properties and the crystallite sizes of different catalysts and their corresponding samples after hydrogenation Catalyst Ru cat. Ru-Ce(IV, 0.16) Ru-Ce(III, 0.19) Ru-Ce(IV, 0.21) Ru-Ce(III, 0.25) Ru cat. AH Ru-Ce(IV, 0.16) AH Ru-Ce(III, 0.19) AH Ru-Ce(IV, 0.21) AH Ru-Ce(III, 0.25) AH a
BET surface area (m2 /g) 69 75 78 74 72 73 65 58 56 51
Pore volume (cm3 /g) 0.18 0.18 0.18 0.19 0.17 0.18 0.18 0.14 0.13 0.10
Pore diameter (nm) 9.63 9.12 8.53 9.16 8.45 8.44 7.85 7.83 7.73 7.45
Ru crystallite size (nm)a 4.7 4.4 4.8 4.9 5.3 4.6 4.8 4.9 4.8 5.3
measured by the XRD instrument
Figure 3. AES Zn LMM spectrum of Ru-Ce(III, 0.19)
Table 1 shows the texture properties of different catalysts and their samples after hydrogenation. It is found that the BET surface areas, pore volumes and pore diameters of the catalysts change little with the increase of the CeO2 loadings. This suggests that the addition of the CeO2 has little effect on the texture properties of the catalysts. However, the BET surface areas, pore volumes and pore diameters after hydrogenation decrease with the CeO2 loadings of the catalysts. This suggests that the (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt covered some of the Ru surface and entered the channels of the pores. Table 2 shows the compositions of different catalysts and their corresponding samples after hydrogenation. It should be
Table 2. Compositions of different catalysts and their corresponding samples after hydrogenation as well as the pH values of aqueous phase after hydrogenationa Catalyst Ru cat. b Ru cat. Ru-Ce(IV, 0.16) Ru-Ce(III, 0.19) Ru-Ce(IV, 0.21) Ru-Ce(III, 0.25) a
Ce/Ru AH 0 0 0.0017 0.0025 0.0030 0.0027
Zn/Ru AH 0 0.0313 0.2561 0.2908 0.3346 0.4206
S/Ru AH 0 0.0026 0.0187 0.0246 0.0275 0.0396
pH value 7.02 5.53 6.27 6.27 6.32 6.30
Reaction conditions: a share of catalyst, 49.2 g of ZnSO4 ·7H2 O, 280 mL of H2 O, 140 mL of benzene, reaction temperature of 150 ◦ C, H2 pressure of 5.0 MPa and stirring rate of 1400 r/min; b without the addition of 49.2 g ZnSO ·7H O 4 2
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Each TPR profile in Figure 4 for the catalysts shows only one peak shoulder between 50−100 ◦ C, ascribable to the reduction of RuO2 . The absence of any additional peaks indicates that the promoter CeO2 could not be reduced within 300 ◦ C. Although bulk CeO2 reduction is thermodynamically feasible, temperature above 630 ◦ C is required [22]. TPR profiles for the catalysts after hydrogenation also show peak shoulders between 50−120 ◦ C, also attributable to the reduction of RuO2 . Specially, the temperatures of complete reduction for all the catalysts after hydrogenation are much lower than the hydrogenation temperature of 150 ◦ C, indicating only the existence of metallic Ru under the hydrogenation conditions of 150 ◦ C and 5 MPa H2 .
In a word, the TPR results have confirmed that the Ru species mainly exist as metallic Ru in the hydrogenation process. Besides, the Ce species mainly exist as CeO2 on the Ru surface. The XRD results, the AES results and the XRF results have confirmed that the promoter CeO2 could react with the reaction modifier ZnSO4 to form a (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt. This salt is readily chemisorbed on the Ru surface. The chemisorbed salt increased with the CeO2 loadings. Based on these, the structures of Ru-Ce and Ru-Ce AH catalysts were sketched as shown in Figure 5. Therefore, the (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt chemisorbed on the Ru surface might directly influence the performance of Ru catalyst.
Figure 4. TPR profiles of (a) the different catalysts and (b) their corresponding samples after hydrogenation
Figure 5. Structure sketches of Ru-Ce and Ru-Ce AH catalysts
3.2. Catalytic performance Before comparing the selectivity over these catalysts, Carberry number (Ca) and Wheeler-Weisz number (ηφ2 ) were calculated according to previous works [23,24]. It was found that Carberry numbers and Wheeler-Weisz numbers were smaller than 0.05 and 0.1, respectively, indicating that external mass transport limitation and pore diffusion limitation could be neglected. This indicates that the reaction was always under kinetic control. Figure 6(a) and 6(b) shows that benzene conversion decreases and cyclohexene selectivity increases with the CeO2 loading of the catalysts in the presence
of ZnSO4 . Figure 6(c) presents the course of the hydrogenation of benzene over Ru-Ce(III, 0.19) catalyst in the presence of ZnSO4 . It is found that Ru-Ce(III, 0.19) catalyst gave a cyclohexene yield of 57.4%, which is among the best results reported so far [14,15,25]. However, the benzene conversion reached 100% at 5 min and cyclohexene was barely detectable in the absence of ZnSO4 , suggesting that the promoter CeO2 alone could not improve the selectivity to cyclohexene. This indicates that the selectivity to cyclohexene of the Ru catalyst might be closely related to the (Zn(OH)2)3 (ZnSO4)(H2 O)3 salt chemisorbed on Ru surface. Figure 7 shows the relationships between the Zn/Ru or S/Ru molar ratios and benzene conversion as well as cyclohexene selectivity at 5 min. It is found that the benzene conversion monotonically decreased and the selectivity to cyclohexene monotonically increased with the molar ratios of Zn/Ru and S/Ru. This indicates that the (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt chemisorbed on the Ru surface directly improved the selectivity to cyclohexene of Ru catalyst. Based on the catalyst characterization and the previous works, the roles of the (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt in improving the selectivity to cyclohexene of Ru catalyst could be attributed to the following reasons. (1) The chemisorbed
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(Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt could selectively cover the most reactive Ru sites, which could reduce the active sites for the chemisorption of cyclohexene and suppress the further hydrogenation of cyclohexene to cyclohexane [16,17,26]. (2) The (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt chemisorbed on Ru surface caused Ru catalyst to be surrounded by a firm stagnant water layer as shown in Figure 5. The existence of the stagnant water layer on the surface of the catalyst could accelerate the desorption and hinder the re-adsorption of cyclohexene for further hydrogenation to cyclohexane due to the very low solubility of cyclohexene [17,27], resulting in the improvement
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of the selectivity to cyclohexene. (3) The Zn2+ of the chemisorbed (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt could form loosely bound adducts with cyclohexene, which could stabilize the formed cyclohexene on the surface of Ru catalyst and improve the selectivity to cyclohexene of Ru catalyst [14,15]. Therefore, increasing the loading of CeO2 increased the formation of the (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt, resulting in the decrease of activity and the increase of the selectivity to cyclohexene of Ru catalyst. Above all, the chemisorbed (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt formed by the CeO2 reacting with ZnSO4 in the slurry directly enhanced the selectivity to cyclohexene of Ru catalyst.
Figure 7. Relationships between the Zn/Ru or S/Ru molar ratios and benzene conversion as well as cyclohexene selectivity at 5 min
Figure 6. Benzene conversion (a) and cyclohexene selectivity over different catalysts (b), as well as reaction course of benzene hydrogenation (c) over Ru-Ce(III, 0.19) catalyst
Figure 8. Resability of the Ru-Ce(III, 0.19) catalyst for selective hydrogenation to cyclohexene
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Figure 8 shows the reusability of the Ru-Ce(III, 0.19) catalyst. As can be seen, although the benzene conversion slightly decreased with cycle times, the selectivity to cyclohexene increased with cycle times and the cyclohexene yields were kept above 57% in the five cycles. This indicates that the Ru-Ce(III, 0.19) catalyst had good stability and potential for industrial applications. 4. Conclusions The Ce species in the catalysts prepared with different valences of the Ce precursors are all present in CeO2 on the Ru surface. The promoter CeO2 alone could not improve the selectivity to cyclohexene of Ru catalysts. However, most of the CeO2 could react with the reaction modifier ZnSO4 to form a (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt. This salt chemisorbed on the Ru surface plays a key role in improving Ru catalysts. That is, the promoters in some catalysts could not directly improve the selectivity to the desired product. However, these promoters could react with the reaction modifier to form a new compound that could significantly enhance the selectivity to the desired product. This finding might open a new avenue for selectivity enhancement for other hydrogenation reactions. References [1] Sun H J, Guo W, Zhou X L, Chen Z H, Liu Z Y, Liu S C. Chin J Catal (Cuihua Xuebao), 2011, 32: 1 [2] Lu F, Liu J, Xu J. Prog Chem (Huaxue Jinzhan), 2003, 15: 338 [3] Zanutelo C, Landers R, Carvalho W A, Cobo A J G. Appl Catal A, 2011, 409-410: 174 [4] Ning J B, Xu J, Liu J, Lu F. Catal Lett, 2006, 109: 175 [5] Sun H J, Zhang C, Yuan P, Li J X, Liu S C. Chin J Catal (Cuihua Xuebao), 2008, 29: 441 [6] Liu S C, Liu Z Y, Zhao S H, Wu Y M, Wang Z, Yuan B. J Nat Gas Chem, 2006, 15: 319 [7] Fan G Y, Li R X, Li X J, Chen H. Catal Commun, 2008, 9: 1394
[8] Fan G Y, Jiang W D, Wang J B, Li R X, Chen H, Li X J. Catal Commun, 2008, 10: 98 [9] Xue W, Song Y, Wang Y J, Wang D D, Li F. Catal Commun, 2009, 11: 29 [10] Liu H Z, Liang S G, Wang W T, Jiang T, Han B X. J Mol Catal A, 2011, 341: 35 [11] Zhou X L, Sun H J, Guo W, Liu Z Y, Liu S C. J Nat Gas Chem, 2011, 20: 53 [12] Liu S C, Liu Z Y, Wang Z, Zhao S H, Wu Y M. Appl Catal A, 2006, 313: 49 [13] Liu S C, Liu Z Y, Wang Z, Wu Y M, Yuan P. Chem Eng J, 2008, 139: 157 [14] Liu J L, Zhu Y, Liu J, Pei Y, Li Z H, Li H, Li H X, Qiao M H, Fan K N. J Catal, 2009, 268: 100 [15] Liu J L, Zhu L J, Pei Y, Zhuang J H, Li H, Li H X, Qiao M H, Fan K N. Appl Catal A, 2009, 353: 282 [16] Sun H J, Zhang X D, Chen Z H, Zhou X L, Guo W, Liu Z Y, Liu S C. Chin J Catal (Cuihua Xuebao), 2011, 32: 224. [17] Sun H J, Wang H X, Jiang H B, Li S H, Liu S C, Liu Z Y, Yuan X M, Yang K J. Appl Catal A, 2013, 450: 160 [18] Sun H J, Pan Y J, Wang H X, Dong Y Y, Liu Z Y, Liu S C. Chin J Catal (Cuihua Xuebao), 2012, 33: 610 [19] Sun H J, Chen Z H, Guo W, Zhou X L, Liu Z Y, Liu S C. Chin J Chem (Zhongguo Huaxue), 2011, 29: 369 [20] Silvestre-Albero J, Serrano-Ruiz J C, Sep´ulveda-Escribano A, Rodr´ıguez-Reinoso F. Appl Catal A, 2005, 292: 244 [21] Ramos-Fern´andze E V, Ferreira A F P, Sep´ulveda-Escribano A, Kapteijn F, Rodr´ıguez-Reinoso F. J Catal, 2008, 258: 52 [22] Zonetti P D S, Landers R, Cobo A J G. Appl Surf Sci, 2008, 254: 6849 [23] Milone C, Neri G, Donato A, Musolino M G, Mercadante L. J Catal, 1996, 159: 253 [24] Hu S C, Chen Y W. Ind Eng Chem Res, 1997, 36: 5153 [25] Nagahara H, Ono M, Konishi M, Fukuoka Y. Appl Surf Sci, 1997, 121/122: 448 [26] Struijk J, Moene R, Vanderkamp T, Scholten J J F. Appl Catal A, 1992, 89: 77 [27] Struijk J, Dangremond M, Lucasderegt W J M, Scholter J J F. Appl Catal A, 1992, 83: 263
Selective hydrogenation of benzene to cyclohexene over Ce-promoted Ru catalysts Haijie Sunb , Yajie Pana , Shuaihui Lia , Yuanxin Zhanga , Shouchang Liua, Zhongyi Liua∗
Yingying Donga,
a. College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, Henan, China; b. Institute of Environmental and Catalytic Engineering, Department of Chemistry, Zhengzhou Normal University, Zhengzhou 450044, Henan, China [ Manuscript received November 30, 2012; revised January 14, 2013 ]
Abstract Ru-Ce catalysts were prepared by a co-precipitation method. The effects of Ce precursors with different valences and Ce contents on the catalytic performance of Ru-Ce catalysts were investigated in the presence of ZnSO4 . The Ce species in the catalysts prepared with different valences of the Ce precursors all exist as CeO2 on the Ru surface. The promoter CeO2 alone could not improve the selectivity to cyclohexene of Ru catalysts. However, almost all the CeO2 in the catalysts could react with the reaction modifier ZnSO4 to form (Zn(OH)2 )3 (ZnSO4 )(H2 O)3 salt. The amount of the chemisorbed salt increased with the CeO2 loading, resulting in the decrease of the activity and the increase of the selectivity to cyclohexene of Ru catalyst. The Ru-Ce catalyst with the optimum Ce/Ru molar ratio of 0.19 gave a maximum cyclohexene yield of 57.4%. Moreover, this catalyst had good stability and excellent reusability. Key words benzene; selective hydrogenation; cyclohexene; ruthenium; cerium
1. Introduction Cyclohexene is commercially important for the production of adipic acid, nylon 6, nylon 66 and many other fine chemicals [1,2]. Selective hydrogenation of benzene to cyclohexene has drawn much attention due to its inexpensive starting feedstock, atomic economy, low amounts of undesirable products and simplified operation [3−6]. However, it is difficult to obtain a high yield of cyclohexene through this route, because cyclohexane, the complete hydrogenation product, is thermodynamically more favorable [1,2]. It was found that the promoters have great impacts on the yield of cyclohexene for Ru catalysts. Fan et al. [7,8] prepared a Ru-Co-B/γ-Al2O3 catalyst and reached a cyclohexene yield of 34.8% in the presence of ethylenediamine and ZnSO4 . Xue et al. [9] prepared a Ru-Zn/SiO2 catalyst in a water-in-oil microemulsion and obtained a cyclohexene yield of 29.9%. Liu et al. [10] developed a Ru-Cu/ZnO catalyst by a depositionprecipitation method and got a cyclohexene yield of 49.4%. Zhou et al. [11] prepared a Ru catalyst promoted with Mn and Zn by a co-precipitation method and received a cyclohexene
yield of 59.9%. On the other hand, rare-earth elements such as La and Ce have been frequently used as promoter of Ru catalysts for this reaction. Liu et al. [12,13] prepared a Ru-La-B/ZrO2 catalyst and got a cyclohexene yield of 53% in the presence of ZnSO4 . They found that the addition of La could significantly enhance both the activity and the selectivity to cyclohexene. They had confirmed that the La in the catalyst existed as La2 O3 . They suggested that the role of La was mainly ascribed to its dispersion effect on Ru. Liu et al. [14] prepared a Ru-La/SBA-15 catalyst by a “two-solvents” impregnation method and achieved a cyclohexene yield of 57% in the presence of CdSO4 and ZnSO4 as co-modifiers. However, the effect of the La promoter in improving the selectivity to cyclohexene was ignored there. Liu et al. [15] prepared the Ce-promoted Ru/SBA-15 catalysts by a “two-solvents” impregnation method and obtained a cyclohexene yield of 53.8% in the presence of ZnSO4 . Based on characterizations, the beneficial effects of the Ce promoter on the selectivity to cyclohexene were ascribed to the enhacement of the hydrophilicity of the catalyst and to the electron transfer between the Ce(III) species and metallic Ru.
Corresponding author. Tel/Fax: +86-371-67783384; E-mail: [email protected] (Z.Liu) This work was supported by the National Nature Science Foundation of China (21273205), the Innovation Found for Technology Based Firms of China (10C26214104505), the Chinese Post-doctorate Science Fund 51th batch of surface subsidizes (2012M511125) and the Scientific Research Foundation of Graduate School of Zhengzhou University. ∗
Copyright©2013, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.
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In our previous work, we prepared Ru-Zn catalysts and achieved a cyclohexene yield of 58.9%. We found that the Zn in the catalysts existed as ZnO. The ZnO on the surface could react with ZnSO4 in the slurry to form (Zn(OH)2)3 (ZnSO4)(H2 O)5 salt. The chemisorbed (Zn(OH)2)3 (ZnSO4 )(H2 O)5 salt play a key role in improving the selectivity to cyclohexene [16,17]. We obtained a cyclohexene yield of 63.6% in the presence of ZnSO4 and diethanolamine over Ru-Zn catalyst. It is found that the (Zn(OH)2)3 (ZnSO4 )(H2 O)3 formed by diethanolamin reacting with ZnSO4 improved the cyclohexene yield [18]. We also got a cyclohexene yield of 61.4% in the presence of ZnSO4 and PEG-20000 over Ru-Zn catalysts [19]. In the present work, we prepared Ru-Ce catalysts by a co-precipitation method. The effects of the Ce precursors with different valences and the Ce contents on the catalytic performance of Ru-Ce catalysts were investigated in the presence of ZnSO4 . 2. Experimental 2.1. Catalyst preparation Ru-Ce catalysts were prepared according to the following procedure. 9.75 g RuCl3 ·H2 O and a desired amount of Ce(NO3 )3 ·6H2 O were dissolved in 400 mL H2 O with agitation. To the stirred solution, 200 mL of a 10% NaOH solution was added instantaneously and the resulting mixture was agitated for an additional 4 h at 353K. This black precipitate was dispersed in 400 mL of a 5% NaOH solution and charged into a 1 L autoclave lined the Teflon. Hydrogen was introduced into the autoclave to raise the total internal pressure to 5 MPa and the reduction was conducted at 423K and at 800 r/min stirring rate for 3 h. The reaction mixture was cooled and the obtained black powder was washed with water until neutrality, subsequently vacuum-dried and the desired catalysts were obtained. The catalyst was divided into two shares, one share was used for activity test and another was used for catalyst characterization. This method ensured that the catalysts with different Ce contents had the same Ru contents (about 1.8 g Ru). The amounts of Ce(NO3 )3 ·6H2 O were adjusted to give the catalysts with different Ce contents which were denoted as Ru-Ce(III, x) catalysts, where x denoted the Ce/Ru molar ratio measured by X-ray fluorescence (XRF). The Ru-Ce(IV, x) catalysts were prepared in accordance with the above procedures except that the Ce(NO3 )3 ·6H2 O was replaced by the equal molar of Ce(SO4 )2 ·4H2 O. 2.2. Catalyst characterization Transmission electron micrographs (TEM) and energy dispersion scanning (EDS) were observed on a JEOL JEM2100 instrument using an accelerating voltage of 200 kV. Auger Electron Spectroscopy (AES) and sputter profiles were taken on a ULVAC PHI-700 Nano-canning Auger system with on-axis scanning argon ion gun and CMA energy analyzer. The energy resolution ratio was 0.1%. The back-
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ground pressure of analysis room was less than 5.2×10−7 Pa. The standard sample was SiO2 /Si. The sputtering rate was 9 nm/min. The Ce/Ru molar ratios and the compositions of Ru-Ce(x) catalysts after hydrogenation were measured by Xray fluorescence (XRF) on a Bruker S4 Pioneer instrument. N2 physisorption was determined on a Quantachrome Nova 100e apparatus at 77K. Temperature-programmed reduction (TPR) measurements were carried out in a U-shaped quartz reactor, using a 5% H2 /He gas flow of 50 cm3 ·min−1 , a thermal conductivity detector (TCD) and about 10 mg of catalyst. 2.3. Activity test The selective hydrogenation of benzene was performed in a 1 L autoclave lined the hastelloy. The autoclave was charged with 280 mL of H2 O containing a share of Ru-Ce(III, x) catalyst, and 49.2 g of ZnSO4 ·7H2 O, then heating commenced with H2 pressure of 5 MPa and stirring rate of 800 r/min. 140 mL of benzene was fed and the stirring rate was elevated to 1400 r/min to exclude the diffusion effect when the temperature reached 423 K. A small amount of reaction mixture was sampled every 5 min and sent for gas chromatographic analysis with a FID detector, and the benzene conversion and cyclohexene selectivity were calculated. At the end of the reaction, the organic phase was removed using a separating funnel. The slurry containing the catalyst was reused in accordance with the above hydrogenation procedures without any addition. After the reaction the organic was removed, the solid sample was washed with distilled water until no Zn2+ was detected and then was vacuum-dried for characterization. The samples after reaction corresponding to Ru-Ce(III, x) and Ru-Ce(IV, x) were denoted as Ru-Ce(III, x) AH and Ru-Ce(IV, x) AH, respectively, where AH stood for After Hydrogenation. 3. Results and discussion 3.1. Catalyst characterization Figure 1 shows TEM image and the crystallite size distribution of Ru-Ce(III, 0.19) catalyst. It is found that the catalyst particles consist of polycrystalline conglomerates of spherical and ellipsoidal crystallites with the crystallite size mainly distributing around 3.8 nm. Figure 2(a) shows the XRD patterns of the catalysts with different Ce/Ru molar ratios, which were prepared using the Ce precursors with different valences. The XRD patterns of all the catalysts display the diffractional peaks of metallic Ru (JCPDS: 01−070−0274), indicating that the Ru in the catalysts were mainly present in metallic Ru. The crystallite sizes were calculated from the Scherrer equation and were listed in Table 1. It is concluded that the Ru crystallite sizes are in the range of 4.4−5.3 nm, which were similar to those derived from the TEM result. This implies that the addition of the Ce species has a negligible effect on the Ru crystallites. Besides, the XRD patterns of the catalysts prepared using the Ce precursors with different valences all show the diffractional
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peaks of CeO2 (JCPDS: 00−004−0593), indicating that the Ce species in all catalysts prepared using the Ce precursors with different valences mainly existed as CeO2 . This suggests that the valences of the Ce precursors have little effect on the valences of the Ce species in the catalysts, since it is easy for the oxide or hydroxide of Ce(III) to be oxidized to the oxide or hydroxide of Ce(IV) in the precipitation process. Figure 2(b) shows the XRD patterns of different catalysts after hydrogenation. It is interesting to find that the diffractional peaks of CeO2 disappeared and the diffractional peaks of the (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt (JCPDS: 01−078−0247) were observed on the XRD patterns of Ru-Ce(III, 0.19), RuCe(IV, 0.21), and Ru-Ce(III, 0.25) catalysts. All these indicated that the promoter CeO2 on the surface of the cat-
alyst had reacted with the reaction modifier ZnSO4 and H2 O to form an insoluble (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt, which was shown in Reaction (1). The intensity of the diffractional peak at 2θ = 11.2o of this salt increases with the CeO2 loading, indicating the increase of the amount of the (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt formed. However, the diffractional peaks of the (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt are not found on the XRD patterns of Ru-Ce(IV, 0.16) catalyst due to its low amount confirmed by the XRF results below. The crystallite sizes of different catalysts after hydrogenation are in the range of 4.6−5.3 nm, similar to those of the corresponding catalysts. This suggests that serious agglomeration of the catalysts did not happen in the hydrogenation processes.
3CeO2 + 8ZnSO4 + 12H2O −→ 2(Zn(OH)2 )3 (ZnSO4 )(H2 O)3 ↓ +3Ce(SO4 )2
Figure 1. TEM image (a) and the crystallite size distribution of Ru-Ce(III, 0.19) catalyst (b)
Figure 2. XRD patterns of different catalysts and their corresponding samples after hydrogenation
(1)
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The XRD results revealed the existence of the (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt. Figure 3 shows the AES Zn LMM spectrum of Ru-Ce(III, 0.19) AH. It is worthy of noting here that all the spectra were recorded after Ar+ sputtering for 1 min to avoid the interruptions of the surface oxidation of catalysts. As can be seen, the kinetic energy (KE) of Zn LMM of Ru-Ce(III, 0.19) AH is 984.5 eV, this contribution being previously assigned to the oxidized Zn [20,21]. This finding suggests that the Zn is present on the surface of Ru-Ce(III, 0.19) AH mainly as oxidized Zn even under the reaction conditions of 150 ◦ C and H2 pressure of 5.0 MPa, which is consistent with the XRD results that the Zn species are mainly present in the (Zn(OH)2)3 (ZnSO4)(H2 O)3 salt. Unfortunately, AES measurements do not allow discerning any additional contribution from metallic Zn (commonly appearing in the 991−995 eV range). This indicates that the chemisorbed Zn2+ on Ru surface could not be reduced to metallic Zn. Moreover, Table 2 shows that the pH values of the aqueous solutions after hydrogenation in the presence of ZnSO4 at room temperature are below 6.32, indicating the acidity of aqueous phase due to the hydrolysis of ZnSO4 . It is well known that increasing temperature favors the hydrolysis. This means that the acidity of liquid phase is much stronger at the reaction temperature of 423 K due to the increase of hydrolysis degree of ZnSO4 . As we know, metallic Zn can hardly exist in the acid solution, which is consistent with no clear evidences of the presence of metallic Zn on the surface of Ru catalysts observed in the AES results.
noted that different catalysts had the same Ru contents. Thus the molar ratios of Ce/Ru, Zn/Ru and S/Ru reflect the compositions of the catalysts after hydrogenation. The Ru catalyst after hydrogenation in the absence of ZnSO4 only contains Ru element. However, the Zn and S elements are found on Ru catalyst after hydrogenation in the presence of ZnSO4 . Moreover, the molar ratio of Zn/Ru is much higher than that of S/Ru, indicating that the chemisorbed Zn species might not be ZnSO4 . AES results have confirmed that the Zn2+could not be reduced on Ru surface. Thus we propose that the Zn2+ are predominately present in the (Zn(OH)2)3 (ZnSO4)(H2 O)3 salt that is formed by the hydrolysis of ZnSO4 . However, the XRD pattern of Ru catalyst after hydrogenation did not show the diffractional peaks of the (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt due to its low amount beyond the detection limit of the XRD instrument. It is interesting to find that only trace amounts of the Ce are detected after hydrogenation, although the CeO2 loadings of the catalysts increase. The molar ratios of Zn/Ru and S/Ru after hydrogenation increase with the CeO2 loadings of the catalysts. All these indicate that the CeO2 mainly exist on Ru surface and most of the CeO2 on the Ru surface had reacted with the ZnSO4 in the slurry to form (Zn(OH)2)3 (ZnSO4 )(H2 O)3 and Ce(SO4 )2 salts. The (Zn(OH)2)3 (ZnSO4 )(H2 O)3 is a insoluble salt and is readily chemisorbed on the surface of the catalyst. The amount of this salt increases with the CeO2 loadings of the catalysts, which is consistent with the XRD results. The Ce(SO4 )2 is dissolved in the aqueous solution. Table 1. Textural properties and the crystallite sizes of different catalysts and their corresponding samples after hydrogenation Catalyst Ru cat. Ru-Ce(IV, 0.16) Ru-Ce(III, 0.19) Ru-Ce(IV, 0.21) Ru-Ce(III, 0.25) Ru cat. AH Ru-Ce(IV, 0.16) AH Ru-Ce(III, 0.19) AH Ru-Ce(IV, 0.21) AH Ru-Ce(III, 0.25) AH a
BET surface area (m2 /g) 69 75 78 74 72 73 65 58 56 51
Pore volume (cm3 /g) 0.18 0.18 0.18 0.19 0.17 0.18 0.18 0.14 0.13 0.10
Pore diameter (nm) 9.63 9.12 8.53 9.16 8.45 8.44 7.85 7.83 7.73 7.45
Ru crystallite size (nm)a 4.7 4.4 4.8 4.9 5.3 4.6 4.8 4.9 4.8 5.3
measured by the XRD instrument
Figure 3. AES Zn LMM spectrum of Ru-Ce(III, 0.19)
Table 1 shows the texture properties of different catalysts and their samples after hydrogenation. It is found that the BET surface areas, pore volumes and pore diameters of the catalysts change little with the increase of the CeO2 loadings. This suggests that the addition of the CeO2 has little effect on the texture properties of the catalysts. However, the BET surface areas, pore volumes and pore diameters after hydrogenation decrease with the CeO2 loadings of the catalysts. This suggests that the (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt covered some of the Ru surface and entered the channels of the pores. Table 2 shows the compositions of different catalysts and their corresponding samples after hydrogenation. It should be
Table 2. Compositions of different catalysts and their corresponding samples after hydrogenation as well as the pH values of aqueous phase after hydrogenationa Catalyst Ru cat. b Ru cat. Ru-Ce(IV, 0.16) Ru-Ce(III, 0.19) Ru-Ce(IV, 0.21) Ru-Ce(III, 0.25) a
Ce/Ru AH 0 0 0.0017 0.0025 0.0030 0.0027
Zn/Ru AH 0 0.0313 0.2561 0.2908 0.3346 0.4206
S/Ru AH 0 0.0026 0.0187 0.0246 0.0275 0.0396
pH value 7.02 5.53 6.27 6.27 6.32 6.30
Reaction conditions: a share of catalyst, 49.2 g of ZnSO4 ·7H2 O, 280 mL of H2 O, 140 mL of benzene, reaction temperature of 150 ◦ C, H2 pressure of 5.0 MPa and stirring rate of 1400 r/min; b without the addition of 49.2 g ZnSO ·7H O 4 2
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Each TPR profile in Figure 4 for the catalysts shows only one peak shoulder between 50−100 ◦ C, ascribable to the reduction of RuO2 . The absence of any additional peaks indicates that the promoter CeO2 could not be reduced within 300 ◦ C. Although bulk CeO2 reduction is thermodynamically feasible, temperature above 630 ◦ C is required [22]. TPR profiles for the catalysts after hydrogenation also show peak shoulders between 50−120 ◦ C, also attributable to the reduction of RuO2 . Specially, the temperatures of complete reduction for all the catalysts after hydrogenation are much lower than the hydrogenation temperature of 150 ◦ C, indicating only the existence of metallic Ru under the hydrogenation conditions of 150 ◦ C and 5 MPa H2 .
In a word, the TPR results have confirmed that the Ru species mainly exist as metallic Ru in the hydrogenation process. Besides, the Ce species mainly exist as CeO2 on the Ru surface. The XRD results, the AES results and the XRF results have confirmed that the promoter CeO2 could react with the reaction modifier ZnSO4 to form a (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt. This salt is readily chemisorbed on the Ru surface. The chemisorbed salt increased with the CeO2 loadings. Based on these, the structures of Ru-Ce and Ru-Ce AH catalysts were sketched as shown in Figure 5. Therefore, the (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt chemisorbed on the Ru surface might directly influence the performance of Ru catalyst.
Figure 4. TPR profiles of (a) the different catalysts and (b) their corresponding samples after hydrogenation
Figure 5. Structure sketches of Ru-Ce and Ru-Ce AH catalysts
3.2. Catalytic performance Before comparing the selectivity over these catalysts, Carberry number (Ca) and Wheeler-Weisz number (ηφ2 ) were calculated according to previous works [23,24]. It was found that Carberry numbers and Wheeler-Weisz numbers were smaller than 0.05 and 0.1, respectively, indicating that external mass transport limitation and pore diffusion limitation could be neglected. This indicates that the reaction was always under kinetic control. Figure 6(a) and 6(b) shows that benzene conversion decreases and cyclohexene selectivity increases with the CeO2 loading of the catalysts in the presence
of ZnSO4 . Figure 6(c) presents the course of the hydrogenation of benzene over Ru-Ce(III, 0.19) catalyst in the presence of ZnSO4 . It is found that Ru-Ce(III, 0.19) catalyst gave a cyclohexene yield of 57.4%, which is among the best results reported so far [14,15,25]. However, the benzene conversion reached 100% at 5 min and cyclohexene was barely detectable in the absence of ZnSO4 , suggesting that the promoter CeO2 alone could not improve the selectivity to cyclohexene. This indicates that the selectivity to cyclohexene of the Ru catalyst might be closely related to the (Zn(OH)2)3 (ZnSO4)(H2 O)3 salt chemisorbed on Ru surface. Figure 7 shows the relationships between the Zn/Ru or S/Ru molar ratios and benzene conversion as well as cyclohexene selectivity at 5 min. It is found that the benzene conversion monotonically decreased and the selectivity to cyclohexene monotonically increased with the molar ratios of Zn/Ru and S/Ru. This indicates that the (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt chemisorbed on the Ru surface directly improved the selectivity to cyclohexene of Ru catalyst. Based on the catalyst characterization and the previous works, the roles of the (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt in improving the selectivity to cyclohexene of Ru catalyst could be attributed to the following reasons. (1) The chemisorbed
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(Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt could selectively cover the most reactive Ru sites, which could reduce the active sites for the chemisorption of cyclohexene and suppress the further hydrogenation of cyclohexene to cyclohexane [16,17,26]. (2) The (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt chemisorbed on Ru surface caused Ru catalyst to be surrounded by a firm stagnant water layer as shown in Figure 5. The existence of the stagnant water layer on the surface of the catalyst could accelerate the desorption and hinder the re-adsorption of cyclohexene for further hydrogenation to cyclohexane due to the very low solubility of cyclohexene [17,27], resulting in the improvement
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of the selectivity to cyclohexene. (3) The Zn2+ of the chemisorbed (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt could form loosely bound adducts with cyclohexene, which could stabilize the formed cyclohexene on the surface of Ru catalyst and improve the selectivity to cyclohexene of Ru catalyst [14,15]. Therefore, increasing the loading of CeO2 increased the formation of the (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt, resulting in the decrease of activity and the increase of the selectivity to cyclohexene of Ru catalyst. Above all, the chemisorbed (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt formed by the CeO2 reacting with ZnSO4 in the slurry directly enhanced the selectivity to cyclohexene of Ru catalyst.
Figure 7. Relationships between the Zn/Ru or S/Ru molar ratios and benzene conversion as well as cyclohexene selectivity at 5 min
Figure 6. Benzene conversion (a) and cyclohexene selectivity over different catalysts (b), as well as reaction course of benzene hydrogenation (c) over Ru-Ce(III, 0.19) catalyst
Figure 8. Resability of the Ru-Ce(III, 0.19) catalyst for selective hydrogenation to cyclohexene
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Figure 8 shows the reusability of the Ru-Ce(III, 0.19) catalyst. As can be seen, although the benzene conversion slightly decreased with cycle times, the selectivity to cyclohexene increased with cycle times and the cyclohexene yields were kept above 57% in the five cycles. This indicates that the Ru-Ce(III, 0.19) catalyst had good stability and potential for industrial applications. 4. Conclusions The Ce species in the catalysts prepared with different valences of the Ce precursors are all present in CeO2 on the Ru surface. The promoter CeO2 alone could not improve the selectivity to cyclohexene of Ru catalysts. However, most of the CeO2 could react with the reaction modifier ZnSO4 to form a (Zn(OH)2)3 (ZnSO4 )(H2 O)3 salt. This salt chemisorbed on the Ru surface plays a key role in improving Ru catalysts. That is, the promoters in some catalysts could not directly improve the selectivity to the desired product. However, these promoters could react with the reaction modifier to form a new compound that could significantly enhance the selectivity to the desired product. This finding might open a new avenue for selectivity enhancement for other hydrogenation reactions. References [1] Sun H J, Guo W, Zhou X L, Chen Z H, Liu Z Y, Liu S C. Chin J Catal (Cuihua Xuebao), 2011, 32: 1 [2] Lu F, Liu J, Xu J. Prog Chem (Huaxue Jinzhan), 2003, 15: 338 [3] Zanutelo C, Landers R, Carvalho W A, Cobo A J G. Appl Catal A, 2011, 409-410: 174 [4] Ning J B, Xu J, Liu J, Lu F. Catal Lett, 2006, 109: 175 [5] Sun H J, Zhang C, Yuan P, Li J X, Liu S C. Chin J Catal (Cuihua Xuebao), 2008, 29: 441 [6] Liu S C, Liu Z Y, Zhao S H, Wu Y M, Wang Z, Yuan B. J Nat Gas Chem, 2006, 15: 319 [7] Fan G Y, Li R X, Li X J, Chen H. Catal Commun, 2008, 9: 1394
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