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Monolithic Ru-based catalyst for selective hydrogenation of benzene to cyclohexene
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Catalysis Communications 9 (2008) 459–464 www.elsevier.com/locate/catcom
Monolithic Ru-based catalyst for selective hydrogenation of benzene to cyclohexene Yujun Zhao a
a,b
, Jin Zhou a, Jianguo Zhang
a,b
, Shudong Wang
a,*
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China b Graduate University of Chinese Academy of Sciences, Beijing 100039, China Received 25 May 2007; received in revised form 24 July 2007; accepted 25 July 2007 Available online 2 August 2007
Abstract A novel Ru-based cordierite monolithic catalyst was prepared for the selective hydrogenation of benzene to cyclohexene. The catalyst was characterized by elemental analysis, XRD, SEM-EDX and physisorption measurements. The performance of the catalyst was tested in a continuous monolithic fixed bed reactor (MFBR). Compared with particulate catalyst, the monolithic catalyst gave much higher selectivity. Monolithic catalyst with ZrO2–Al2O3 as washcoating was found to be more active than that with Al2O3 as washcoating and a high cyclohexene yield of about 30% was achieved at a relatively lower LHSV. The egg-shell distribution of the active component, the large pores in the walls of cordierite monolith and the Taylor flow pattern formed in the monolith channels were considered to be the crucial reasons. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Ruthenium; Monolithic catalyst; Hydrogenation; Benzene; Cyclohexene; Washcoat
1. Introduction Cyclohexene is commercially important for the production of adipic acid, nylon 6, nylon 66 and many other fine chemicals [1–3]. Selective hydrogenation of benzene to cyclohexene (SHBC) has attracted much attention during the past decades [1–6], due to its simplicity, high yields and economization, compared with other production methods. Nagahara and Ono [3] studied the selective hydrogenation of benzene in a stirred autoclave batch reactor at the temperature range of 413–423 K using nano Ru catalyst suspended in a ZnSO4 aqueous solution. The authors reported a high cyclohexene yield up to 50% at 60% conversion of benzene. This process is industrially carried out by Asahi with a steady yield for cyclohexene of 30– 40% at the selectivity of 80% in a continuous operation *
Corresponding author. Tel.: +86 411 84379052; fax: +86 411 84662365. E-mail address: [email protected] (S. Wang). 1566-7367/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.07.037
mode [7]. In recent years, Ru-based supported catalysts for the SHBC reaction have been reported in the literature. The best catalyst supports are often single or binary hydrophilic metal oxides such as Al2O3, TiO2, ZrO2, SiO2, La2O3 and ZnO [1,2,8–11]. The reaction system and conditions are generally the same as those adopted by Nagahara [3], and the cyclohexene yields are from 10% to 40%. In the tetraphase slurry reaction system, high stirring speed is essential to get high activity and selectivity. Therefore the scaling up of the reactor is limited. Moreover, the entire separation of the small particulate catalysts from the liquid phase is very difficult and costly. To overcome these drawbacks, numerous authors have conducted the hydrogenation in continuous gas-phase catalytic reactor [12–15]. However, the obtained selectivity of cyclohexene was less than 40% at a conversion level of 20%. This was substantially lower than that obtained in the liquid-phase process. Monoliths are structures consisting of a large number of parallel channels, where catalysts are applied and an eggshell catalytic system with a very short diffusion path in
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the catalytic active layer is formed [16]. Monolith catalyst reactors offer the advantages of low pressure drop, large geometrical surface area, easily scaling up, less catalyst attrition and the absence of catalyst separation step [17– 19]. These make monolithic reactors promising for application in the chemical industry. A well-known example is the use for treatment of automotive exhaust gas. Monolithic reactors have been applied in multiphase catalytic reactions to replace slurry and fixed bed reactors [20,21]. Taylor flow mode, under which transportation coefficient is thought to be high, can be easily realized in the monolith channel at a proper gas and liquid flow rate [22]. Thus, the external diffusion resistance is suppressed greatly, favoring its application in multiphase reactions. At present, the most successful industrial application of monolithic catalyst in gas–liquid–solid reaction is the H2O2 production using the anthraquinone process [23]. Although the potential application in the production of cyclohexene was proposed by Wolffenbuttel et al. [24], few investigations [25] about this have been reported. In this paper, the selective liquid-phase hydrogenation on a washcoated monolith supported Ru catalyst in a continuous flow monolith fixed bed reactor (MFBR) is discussed in detail. MFBR, as an alternative reactor for SHBC, can not only overcome the trouble in separating the powder catalyst (less than 1 lm) from the liquid phase in the slurry reactor, but also avoid the high pressure drop associated with the high flow rates in the conventional FBR. The objective of the current investigation was to study experimentally the performance of monolithic catalysts on selective hydrogenation of benzene to cyclohexene in an aqueous medium. 2. Experimental 2.1. Catalyst preparation Various monolithic catalysts were prepared with different oxides as coatings and ruthenium chloride (RuCl3) as the precursor of the active component. A cordierite honeycomb monolith (diameter = 10 mm, length = 22 mm) was used with a cell density of 400 cpsi and a wall thickness of 0.18 mm. It is well-known that bare cordierite monolith is not an ideal catalyst support because of its low BET surface area (<1 m2 g 1). So the cordierite monolith should be coated with an oxide layer of high surface area as the carrier of active component. Here the monolithic substrates were coated with alumina or zirconia–alumina (mole ratio of ZrO2 to Al2O3 was 0.06) mixture layer by slurry-coating method [26]. The slurries were prepared by wet-milling of the materials to be coated. The coated monolith was calcined in air at 1373 K for 2 h and the loading of the washcoat was generally 5–6 wt%. Then, the catalyst was prepared by the wet impregnation technique using ruthenium chloride as the precursor. And the loading of 0.3 wt% Ru on the washcoated monolith was obtained. Some spherical Ru/Al2O3 catalysts (Ø 2 mm) as a reference
were also prepared in the laboratory according to the method of incipient-wetness impregnation. The catalyst was dried in a microwave oven for 3 min. Finally the catalyst was calcined in air at 573 K for 2 h. The samples obtained were labeled as CMAR (Ru/Al2O3/monolith), CMZAR (Ru/ZrO2–Al2O3/monolith) and PAR (Ru/ Al2O3) individually. 2.2. Catalyst characterization BET surface area and average pore diameter of the washcoat layer on the monolith were measured based on nitrogen adsorption measurements at 77 K with a Micromeritics ASAP 2400 instrument. The samples were prepared from Al2O3 slurry and ZrO2–Al2O3 slurry by drying and calcination at the same temperature as that adopted in the preparation of the washcoated monoliths. X-ray diffraction (XRD) was used to examine the bulk structure of the washcoating on the monolithic catalyst. The sample was prepared by the method mentioned above. The XRD data were recorded by a Rigaku D/Max-2500 XRD diffractometer with Cu Ka radiation, operated under 40 kV and 100 mA at 2h scale from 10° to 100° with a scan step size of 0.02°. The Ru content of the catalysts was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). The morphology of the washcoat and the distribution of the active materials on the monolithic catalyst were studied by a Philips XL-30 scanning electron microscopy (SEM) equipped with an energy dispersive X-ray detector (EDX). 2.3. Experimental setup and selective hydrogenation of benzene Selective hydrogenation of benzene with the monolithic catalysts was carried out in a continuous-flow monolith fixed bed reactor (MFBR) as shown in Fig. 1a. The reactor consisted of a stainless steel tube with 210 mm in length and 12 mm in inner diameter. The monolithic catalyst was placed in the middle of the tube. Benzene and water were injected from the top of the reactor through the high pressure pump. Hydrogen and nitrogen flow rate were controlled by the mass flow controllers. The reactor was operated in continuous mode except that the aqueous solution of ZnSO4 was recirculated at a certain flow rate to keep a dynamic balance of Zn on the catalyst surface. The monolithic catalyst was first reduced in situ with 20% H2/N2 at 473 K for 10 h, followed by filling pure hydrogen to the system to 3 MPa and decreasing the temperature to 423 K. Then, 2 h of pretreatment with aqueous solution of ZnSO4 at the flow rate of 1 ml/min was carried out in the hydrogen flow. After the pretreatment, the reactant (benzene > 99.9%) and ZnSO4 solution were pumped with a certain hydrogen flow to start the reaction. Samples were withdrawn at intervals and analyzed by GC-FID.
Y. Zhao et al. / Catalysis Communications 9 (2008) 459–464
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Fig. 1. (a) Experimental setup of the monolith fix-bed reactor system; (b) reaction scheme of benzene hydrogenation on a ceramic monolithic catalyst surrounded by water.
3. Results and discussion The textural characteristics of the washcoated monolith catalyst (CMAR) are observed by SEM as shown in Fig. 2. It can be clearly seen that on the surface of monolithic catalyst, there is an Al2O3 layer with a thickness of ca. 7 lm. The element distribution was monitored by SEM-EDX with a line scan from a to b in Fig. 2. According to the element profiles shown in Fig. 3, there is a transition region for about 2 lm long between the substrate and the coating layer. It indicates that only a small amount of Al2O3 penetrates into the thin wall of cordierite monolith. It also reveals that Ru is mainly distributed within the thin coating layer like an egg-shell for the cordierite monolithic catalysts. According to the nitrogen adsorption measurements, the average pore diameter and BET surface area of Al2O3
Fig. 3. Element profiles in cross-section of the cordierite monolith catalyst washcoated with Al2O3.
Fig. 2. SEM image of cross-section of the cordierite monolithic catalyst washcoated with Al2O3.
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Fig. 4. XRD patterns of the unsupported oxide calcined at 1373 K.
washcoating were 15.1 nm and 32 m2 g 1, respectively, while those of ZrO2–Al2O3 washcoating were 8.8 nm and 90 m2 g 1. The enhanced resistance to sintering and the grain growth of Al2O3 and ZrO2 [27], after the addition of ZrO2 into Al2O3, may be the reason for the high BET surface area. The XRD patterns shown in Fig. 4 exhibit that the characteristic peaks (at about 25.58°, 35.15°, 43.36° and 57.5°) of the a-Al2O3 nearly disappear in ZrO2–Al2O3. While for t-ZrO2, the characteristic peaks of 30.27°, 35.25°, 50.38° and 60.2° are weakened significantly. It indicates that the transformation temperature for a-Al2O3 is increased during the calcination of the ZrO2–Al2O3 composite, so is the formation temperature of t-ZrO2. The small diffraction peaks of 24.05°, 28.18°, 31.47° and 34.17° can be assigned to monoclinic ZrO2, while the broader peaks at about 67.40° should be assigned to h-Al2O3. But the contents of m-ZrO2 and h-Al2O3 are quite lower in ZrO2–Al2O3 composite. The analysis can further prove the high thermal stability of the ZrO2–Al2O3 composite mentioned above. The differences in catalytic performances between the monolithic catalyst (CMAR) and particulate catalyst (PAR) were investigated in fix-bed reactor. As listed in Table 1, CMAR exhibits much higher selectivity (61%) than that of PAR (26%). It can be explained from three aspects: (i) Egg-shell like distribution of Ru in the monolithic catalysts lead to a short diffusion distance for cyclohexene in the catalyst layer, which inhibits the deep hydrogenation of cyclohexene. (ii) A stable water layer
on catalyst surface is considered as the crucial reason for higher selectivity because of its competition adsorption with cyclohexene on the surface active sites [4]. As shown in Fig. 2, there are many large pores (about 3 lm) in the wall of cordierite monolith, which will be filled with water as reservoirs during the pre-treating process with ZnSO4 solution. This can be understood easily from the schematic illustration in Fig. 1b. During the hydrogenation, the water content in the catalyst layer (or coating layer) will decrease, due to its solubility (7.13% at 423.2 K) in the organic phase [28]. However it will be reinforced immediately by the water in those reservoirs locating in the cordierite monolith. In a word, a dynamic balance of water exits in the catalyst layer. Consequently, the stagnant water layer on the catalyst surface is kept stably and the selectivity is enhanced significantly. (iii) Taylor flow in the thin monolithic channel increases the transportation coefficient of cyclohexene from aqueous phase to the organic phase [22]. However, for particulate catalysts, the Ru distributions are much broad and Taylor flow could not be formed during the hydrogenation. Moreover, there are no pores as large as in cordierite monolith. The lower stability of water layer on particulate catalyst probably leads to the low diffusion resistance of benzene, resulting in high catalytic activity, compared to CMAR. The hydrogenation data of ZrO2–Al2O3 washcoated monolithic catalyst (CMZAR) are shown in Table 1. It can be seen that the CMZAR catalyst shows much higher activity than CMAR, which may be attributed to the high surface area of ZrO2–Al2O3 washcoating as mentioned above. Liu et al. [29] reported that high surface area of the support will benefit a high dispersion of Ru after the impregnation. It was also reported that high dispersion of Ru on the support showed positive influence on its catalytic activity [10]. Thus, the Ru dispersion on ZrO2–Al2O3 composite washcoating was higher and the activity of CMZAR was increased considerably compared to that of CMAR. In addition, by the analysis of the physisorption data and the catalytic performances, the large difference in pore size distribution between the two samples seems not to be the crucial factor determining the selectivity. This is not consistent with the conclusion that the large pores of the support favor higher selectivity to cyclohexene [29]. Further investigation of the reason on this fact is currently under way. Anyway, CMZAR showed better performance according to the yield of cyclohexene.
Table 1 Results of benzene hydrogenation on the several catalystsa Sample
Description
Pore sizec (nm)
Surface areac (m2 g 1)
Conversion (%)
Selectivity (%)
Yield (%)
CMAR CMZAR PAR CMZAR-2b
Ru/Al2O3/monolith Ru/ZrO2–Al2O3/monolith Ru/Al2O3 Ru/ZrO2–Al2O3/monolith
15.1 8.8 15.7 8.8
32 90 35 90
7.44 12.13 21.21 68.81
61 62 26 44
4.57 7.48 5.60 30.36
a b c
Reaction conditions: P = 3.0 MPa, T = 423 K, catalyst amount = 3.6 ml, LHSV = 4 h 1, C(ZnSO4) = 0.5%, H2/H2O/benzene = 500:1:1 (v/v). Reaction conditions: P = 4.0 MPa, T = 423 K, catalyst amount = 14.4 ml, LHSV = 0.75 h 1, C(ZnSO4) = 0.5%, H2/H2O/benzene = 1000:5:1 (v/v). Characteristics of the washcoatings on the cordierite monolith or those of the particulate Al2O3 in PAR.
Y. Zhao et al. / Catalysis Communications 9 (2008) 459–464
To obtain a higher yield of cyclohexene for industrial purpose, a larger amount (14.4 ml) of CMZAR catalysts were therefore used at a relatively lower liquid hourly space velocity (LHSV). A high cyclohexene yield of 30% was obtained at a selectivity higher than 44% (CMZAR-2 in Table 1). The catalytic performances in detail are shown in Fig. 5, which shows high stability of the catalyst during the experiment. The high hydrophilicity of the coated cordierite monolith and the stable water layer are considered as the key reason. As a result of the presence of water around the active site, desorption of cyclohexene is enhanced greatly and at the same time, the diffusion of that from the organic phase to the catalytic surface is suppressed. In another aspect, the residence time of cyclohexene on the catalyst surface would not increase as much as the decrease of the LHSV. Therefore the consecutive hydrogenation was inhibited with a higher cyclohexene yield achieved. Nevertheless, it can be imagined that too small LHSV will not benefit the production of cyclohexene because an adequate amount and concentration of benzene in the organic phase is essential to extract the cyclohexene in the aqueous phase [7]. An appropriate range of LHSV evidently exists for getting high yield of cyclohexene. Although the corresponding selectivity was relatively lower than in the literature (about 80%) [7], the advantages of higher efficiency (105 g cyclohexene/gRu h for the monolithic catalyst vs. 45–60 g cyclohexene/gRu h in the literature), simplicity of process and easy scaling up are very attractive from the industrial viewpoint. It can be seen from the earlier work that the presence of an aqueous phase is essential for getting good cyclohexene yields [5]. The additive in the aqueous phase is always ZnSO4, which is considered to be one of the best salt to obtain high selectivity [30]. The effects of water and ZnSO4 on the catalytic performances were studied for the monolithic catalyst CMAR. As shown in Table 2, no cyclohexene can be produced when no water is added in the reaction system and the only product is cyclohexane. When water presented in the reaction system, some amount of cyclohexene is
Fig. 5. Catalyst performance for the selective hydrogenation of benzene to cyclohexene over the monolithic catalyst CMZAR.
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Table 2 Effects of additives on the catalytic performance of monolithic catalyst CMAR No.
Additive
H2/benzene (v/v)
Conversion (%)
Selectivity (%)
Yield (%)
1 2 3
Nothing Water Water + ZnSO4
1000:1a 500:1 500:1
99.64 26.54 7.44
0 25 61
0 6.42 4.57
Reaction conditions: P = 3.0 MPa, T = 423 K, catalyst amount = 3.6 ml, LHSV = 4 h 1. a High activity need enough H2 to stabilize the system pressure.
observed in the products, but the selectivity is only about 20%, which is apparently lower than that obtained under the conditions when the aqueous solution of ZnSO4 is added. On the other hand, the use of ZnSO4 as an additive reduces the catalytic activity considerably. According to the investigations by Struijk et al. [5,31], it was postulated to attribute to several reasons: (i) Water can expel the formed cyclohexene from catalytic surface through competitive adsorption; (ii) Zinc sulphate is chemisorbed on the catalyst surface, thereby making the catalyst surface more hydrophilic and resulting in an increase in the cyclohexene selectivity; (iii) The preferential adsorption of water on strongly bonding ruthenium sites which favors the formation of cyclohexane will benefit the yield of cyclohexene. Although the selectivity on the monolithic catalyst was still lower than that obtained in slurry reactor on particulate Ru catalyst [5] in the presence of ZnSO4, it is interesting to note that the yield of cyclohexene in the presence of only water is significantly higher than that observed on a particulate Ru/Al2O3 catalyst [5] where the selectivity to cyclohexene is less than 10% under the same conditions. This is probably attributed to the higher hydrophilicity of the cordierite monolith and the relatively shorter resident time of cyclohexene in the monolithic reactor. 4. Conclusions This study represents a novel monolithic catalyst for the selective liquid-phase hydrogenation of benzene. The cordierite monolithic catalysts show high selectivity even in the absence of ZnSO4, due to the several possible reasons including the egg-shell distribution of the active component, the Taylor flow pattern formed in the channels and the large pores inside the thin walls which act as water reservoirs keeping a stagnant water layer on the catalytic surface. The monolithic catalysts give much higher selectivity than particulate catalyst in fix-bed reactor. Compared to Al2O3 coated monolithic catalyst, the ZrO2–Al2O3 coated monolithic catalyst is proved to be more efficient with a high yield at equal selectivity. It should be noted that larger amount of catalysts have given much higher yield at a relatively lower LHSV. ZnSO4 added in the aqueous phase can increase significantly the selectivity of monolithic catalyst. Our study also shows that the monolith reactor is
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promising for the selective hydrogenation of benzene to cyclohexene at industrial level. References [1] S.C. Hu, Y.W. Chen, Ind. Eng. Chem. Res. 36 (1997) 5153. [2] S.C. Hu, Y.W. Chen, Ind. Eng. Chem. Res. 40 (2001) 6099. [3] H. Nagahara, M. Ono, M. Konishi, Y. Fukuoka, Appl. Surf. Sci. 121 (1997). [4] J. Struijk, J.J.F. Scholten, Appl. Catal. A 82 (1992) 277. [5] J. Struijk, M. d’Angremond, W.J.M. Lucas-de Regt, J.J.F. Scholten, Appl. Catal. A 83 (1992). [6] F. Dobert, J. Gaube, Catal. Lett. 31 (1995) 431. [7] K. Yamashita, H. Obana, I. Katasuta, US5457251, 1995. [8] E.V. Spinace, J.M. Vaz, Catal. Commun. 4 (2003) 91. [9] J.W. da-silva, A.J.G. Cobo, Appl. Catal. A 252 (2003) 9. [10] C. Milone, G. Neri, A. Donato, M.G. Musolino, L. Mercadante, J. Catal. 159 (1996) 253. [11] L. Rochin, L. Toniolo, Catal. Today 66 (2001) 363. [12] E. Dietzsch, P. Claus, D. Honicke, Top. Catal. 10 (2000) 99. [13] J.A. Don, J.J.F. Scholten, Faraday Discuss. Chem. Soc. 72 (1982) 145. [14] P.J. van der Steen, J.J.F. Scholten, Appl. Catal. 58 (1990) 291. [15] J. Patzlaff, J. Gaube, Chem. Eng. Technol. 21 (1998) 8. [16] P. Bardhan, Curr. Opin. Solid State Mater. Sci. 2 (1997) 577. [17] T.A. Nijhuis, M.T. Kreutzer, A.C.J. Romijn, F. Kapteijn, J.A. Moulijn, Chem. Eng. Sci. 56 (2001) 823.
[18] P.J.M. Lebens, Development and design of a monolith reactor for gas–liquid countercurrent operation, Ph.D. Thesis, Delft University of Technology, Delft, 1999 (in English). [19] P.M. Machado, D.J. Parrillo, R.P. Boehme, R.R. Brokuis, US6005143, 1999. [20] R.K. Edvinsson, A. Cybulski, Catal. Today 24 (1995) 173. [21] S. Irandoust, A. Cybulski, J.A. Moulijn, The use of monolithic catalysts for three-phase reactions, in: A. Cybulski, J.A. Moulijn (Eds.), Structured Catalysts and Reactors, Marcel Dekker, Inc., New York, 1998, pp. 239–265. [22] F. Kapteijn, T.A. Nijhuis, J.J. Heiszwolf, J.A. Moulijn, Catal. Today 66 (2001) 133. [23] R. Edvinsson Albers, M. Nystro¨m, M. Siverstro¨m, A. Sellin, A.C. Dellve, U. Andersson, W. Herrmann, Th. Berglin, Catal. Today 69 (2001) 247. [24] B.M.A. Wolffenbuttel, T.A. Nijhuis, A. Stankiewicz, J.A. Moulijn, Catal. Today 69 (2001) 265. [25] D. Zhao, G.W. Chen, Q. Yuan, Chinese J. Catal. 26 (2005) 824. [26] T.A. Nijhuis, A.E.W. Beers, T. Vergunst, I. Boek, F. Kapteijn, J.A. Moulijn, Catal. Rev. 43 (2001) 345. [27] Y.X. Hao, J.S. Li, X.J. Yang, X. Wang, L.D. Lu, Mater. Sci. Eng. A 367 (2004) 243. [28] C. Tsonopoulos, G.M. Wilson, AIChE J. 29 (1983) 990. [29] S.C. Liu, G. Luo, M.L. Han, Z.G. Li, Chin. J. Catal. 22 (2001) 559. [30] J. Struijk, R. Roene, T. van der Kamp, J.J.F. Scholten, Appl. Catal. 89 (1992) 77. [31] J. Struijk, J.J.F. Scholten, Appl. Catal. 62 (1990) 151.
Catalysis Communications 9 (2008) 459–464 www.elsevier.com/locate/catcom
Monolithic Ru-based catalyst for selective hydrogenation of benzene to cyclohexene Yujun Zhao a
a,b
, Jin Zhou a, Jianguo Zhang
a,b
, Shudong Wang
a,*
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China b Graduate University of Chinese Academy of Sciences, Beijing 100039, China Received 25 May 2007; received in revised form 24 July 2007; accepted 25 July 2007 Available online 2 August 2007
Abstract A novel Ru-based cordierite monolithic catalyst was prepared for the selective hydrogenation of benzene to cyclohexene. The catalyst was characterized by elemental analysis, XRD, SEM-EDX and physisorption measurements. The performance of the catalyst was tested in a continuous monolithic fixed bed reactor (MFBR). Compared with particulate catalyst, the monolithic catalyst gave much higher selectivity. Monolithic catalyst with ZrO2–Al2O3 as washcoating was found to be more active than that with Al2O3 as washcoating and a high cyclohexene yield of about 30% was achieved at a relatively lower LHSV. The egg-shell distribution of the active component, the large pores in the walls of cordierite monolith and the Taylor flow pattern formed in the monolith channels were considered to be the crucial reasons. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Ruthenium; Monolithic catalyst; Hydrogenation; Benzene; Cyclohexene; Washcoat
1. Introduction Cyclohexene is commercially important for the production of adipic acid, nylon 6, nylon 66 and many other fine chemicals [1–3]. Selective hydrogenation of benzene to cyclohexene (SHBC) has attracted much attention during the past decades [1–6], due to its simplicity, high yields and economization, compared with other production methods. Nagahara and Ono [3] studied the selective hydrogenation of benzene in a stirred autoclave batch reactor at the temperature range of 413–423 K using nano Ru catalyst suspended in a ZnSO4 aqueous solution. The authors reported a high cyclohexene yield up to 50% at 60% conversion of benzene. This process is industrially carried out by Asahi with a steady yield for cyclohexene of 30– 40% at the selectivity of 80% in a continuous operation *
Corresponding author. Tel.: +86 411 84379052; fax: +86 411 84662365. E-mail address: [email protected] (S. Wang). 1566-7367/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.07.037
mode [7]. In recent years, Ru-based supported catalysts for the SHBC reaction have been reported in the literature. The best catalyst supports are often single or binary hydrophilic metal oxides such as Al2O3, TiO2, ZrO2, SiO2, La2O3 and ZnO [1,2,8–11]. The reaction system and conditions are generally the same as those adopted by Nagahara [3], and the cyclohexene yields are from 10% to 40%. In the tetraphase slurry reaction system, high stirring speed is essential to get high activity and selectivity. Therefore the scaling up of the reactor is limited. Moreover, the entire separation of the small particulate catalysts from the liquid phase is very difficult and costly. To overcome these drawbacks, numerous authors have conducted the hydrogenation in continuous gas-phase catalytic reactor [12–15]. However, the obtained selectivity of cyclohexene was less than 40% at a conversion level of 20%. This was substantially lower than that obtained in the liquid-phase process. Monoliths are structures consisting of a large number of parallel channels, where catalysts are applied and an eggshell catalytic system with a very short diffusion path in
460
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the catalytic active layer is formed [16]. Monolith catalyst reactors offer the advantages of low pressure drop, large geometrical surface area, easily scaling up, less catalyst attrition and the absence of catalyst separation step [17– 19]. These make monolithic reactors promising for application in the chemical industry. A well-known example is the use for treatment of automotive exhaust gas. Monolithic reactors have been applied in multiphase catalytic reactions to replace slurry and fixed bed reactors [20,21]. Taylor flow mode, under which transportation coefficient is thought to be high, can be easily realized in the monolith channel at a proper gas and liquid flow rate [22]. Thus, the external diffusion resistance is suppressed greatly, favoring its application in multiphase reactions. At present, the most successful industrial application of monolithic catalyst in gas–liquid–solid reaction is the H2O2 production using the anthraquinone process [23]. Although the potential application in the production of cyclohexene was proposed by Wolffenbuttel et al. [24], few investigations [25] about this have been reported. In this paper, the selective liquid-phase hydrogenation on a washcoated monolith supported Ru catalyst in a continuous flow monolith fixed bed reactor (MFBR) is discussed in detail. MFBR, as an alternative reactor for SHBC, can not only overcome the trouble in separating the powder catalyst (less than 1 lm) from the liquid phase in the slurry reactor, but also avoid the high pressure drop associated with the high flow rates in the conventional FBR. The objective of the current investigation was to study experimentally the performance of monolithic catalysts on selective hydrogenation of benzene to cyclohexene in an aqueous medium. 2. Experimental 2.1. Catalyst preparation Various monolithic catalysts were prepared with different oxides as coatings and ruthenium chloride (RuCl3) as the precursor of the active component. A cordierite honeycomb monolith (diameter = 10 mm, length = 22 mm) was used with a cell density of 400 cpsi and a wall thickness of 0.18 mm. It is well-known that bare cordierite monolith is not an ideal catalyst support because of its low BET surface area (<1 m2 g 1). So the cordierite monolith should be coated with an oxide layer of high surface area as the carrier of active component. Here the monolithic substrates were coated with alumina or zirconia–alumina (mole ratio of ZrO2 to Al2O3 was 0.06) mixture layer by slurry-coating method [26]. The slurries were prepared by wet-milling of the materials to be coated. The coated monolith was calcined in air at 1373 K for 2 h and the loading of the washcoat was generally 5–6 wt%. Then, the catalyst was prepared by the wet impregnation technique using ruthenium chloride as the precursor. And the loading of 0.3 wt% Ru on the washcoated monolith was obtained. Some spherical Ru/Al2O3 catalysts (Ø 2 mm) as a reference
were also prepared in the laboratory according to the method of incipient-wetness impregnation. The catalyst was dried in a microwave oven for 3 min. Finally the catalyst was calcined in air at 573 K for 2 h. The samples obtained were labeled as CMAR (Ru/Al2O3/monolith), CMZAR (Ru/ZrO2–Al2O3/monolith) and PAR (Ru/ Al2O3) individually. 2.2. Catalyst characterization BET surface area and average pore diameter of the washcoat layer on the monolith were measured based on nitrogen adsorption measurements at 77 K with a Micromeritics ASAP 2400 instrument. The samples were prepared from Al2O3 slurry and ZrO2–Al2O3 slurry by drying and calcination at the same temperature as that adopted in the preparation of the washcoated monoliths. X-ray diffraction (XRD) was used to examine the bulk structure of the washcoating on the monolithic catalyst. The sample was prepared by the method mentioned above. The XRD data were recorded by a Rigaku D/Max-2500 XRD diffractometer with Cu Ka radiation, operated under 40 kV and 100 mA at 2h scale from 10° to 100° with a scan step size of 0.02°. The Ru content of the catalysts was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). The morphology of the washcoat and the distribution of the active materials on the monolithic catalyst were studied by a Philips XL-30 scanning electron microscopy (SEM) equipped with an energy dispersive X-ray detector (EDX). 2.3. Experimental setup and selective hydrogenation of benzene Selective hydrogenation of benzene with the monolithic catalysts was carried out in a continuous-flow monolith fixed bed reactor (MFBR) as shown in Fig. 1a. The reactor consisted of a stainless steel tube with 210 mm in length and 12 mm in inner diameter. The monolithic catalyst was placed in the middle of the tube. Benzene and water were injected from the top of the reactor through the high pressure pump. Hydrogen and nitrogen flow rate were controlled by the mass flow controllers. The reactor was operated in continuous mode except that the aqueous solution of ZnSO4 was recirculated at a certain flow rate to keep a dynamic balance of Zn on the catalyst surface. The monolithic catalyst was first reduced in situ with 20% H2/N2 at 473 K for 10 h, followed by filling pure hydrogen to the system to 3 MPa and decreasing the temperature to 423 K. Then, 2 h of pretreatment with aqueous solution of ZnSO4 at the flow rate of 1 ml/min was carried out in the hydrogen flow. After the pretreatment, the reactant (benzene > 99.9%) and ZnSO4 solution were pumped with a certain hydrogen flow to start the reaction. Samples were withdrawn at intervals and analyzed by GC-FID.
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Fig. 1. (a) Experimental setup of the monolith fix-bed reactor system; (b) reaction scheme of benzene hydrogenation on a ceramic monolithic catalyst surrounded by water.
3. Results and discussion The textural characteristics of the washcoated monolith catalyst (CMAR) are observed by SEM as shown in Fig. 2. It can be clearly seen that on the surface of monolithic catalyst, there is an Al2O3 layer with a thickness of ca. 7 lm. The element distribution was monitored by SEM-EDX with a line scan from a to b in Fig. 2. According to the element profiles shown in Fig. 3, there is a transition region for about 2 lm long between the substrate and the coating layer. It indicates that only a small amount of Al2O3 penetrates into the thin wall of cordierite monolith. It also reveals that Ru is mainly distributed within the thin coating layer like an egg-shell for the cordierite monolithic catalysts. According to the nitrogen adsorption measurements, the average pore diameter and BET surface area of Al2O3
Fig. 3. Element profiles in cross-section of the cordierite monolith catalyst washcoated with Al2O3.
Fig. 2. SEM image of cross-section of the cordierite monolithic catalyst washcoated with Al2O3.
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Fig. 4. XRD patterns of the unsupported oxide calcined at 1373 K.
washcoating were 15.1 nm and 32 m2 g 1, respectively, while those of ZrO2–Al2O3 washcoating were 8.8 nm and 90 m2 g 1. The enhanced resistance to sintering and the grain growth of Al2O3 and ZrO2 [27], after the addition of ZrO2 into Al2O3, may be the reason for the high BET surface area. The XRD patterns shown in Fig. 4 exhibit that the characteristic peaks (at about 25.58°, 35.15°, 43.36° and 57.5°) of the a-Al2O3 nearly disappear in ZrO2–Al2O3. While for t-ZrO2, the characteristic peaks of 30.27°, 35.25°, 50.38° and 60.2° are weakened significantly. It indicates that the transformation temperature for a-Al2O3 is increased during the calcination of the ZrO2–Al2O3 composite, so is the formation temperature of t-ZrO2. The small diffraction peaks of 24.05°, 28.18°, 31.47° and 34.17° can be assigned to monoclinic ZrO2, while the broader peaks at about 67.40° should be assigned to h-Al2O3. But the contents of m-ZrO2 and h-Al2O3 are quite lower in ZrO2–Al2O3 composite. The analysis can further prove the high thermal stability of the ZrO2–Al2O3 composite mentioned above. The differences in catalytic performances between the monolithic catalyst (CMAR) and particulate catalyst (PAR) were investigated in fix-bed reactor. As listed in Table 1, CMAR exhibits much higher selectivity (61%) than that of PAR (26%). It can be explained from three aspects: (i) Egg-shell like distribution of Ru in the monolithic catalysts lead to a short diffusion distance for cyclohexene in the catalyst layer, which inhibits the deep hydrogenation of cyclohexene. (ii) A stable water layer
on catalyst surface is considered as the crucial reason for higher selectivity because of its competition adsorption with cyclohexene on the surface active sites [4]. As shown in Fig. 2, there are many large pores (about 3 lm) in the wall of cordierite monolith, which will be filled with water as reservoirs during the pre-treating process with ZnSO4 solution. This can be understood easily from the schematic illustration in Fig. 1b. During the hydrogenation, the water content in the catalyst layer (or coating layer) will decrease, due to its solubility (7.13% at 423.2 K) in the organic phase [28]. However it will be reinforced immediately by the water in those reservoirs locating in the cordierite monolith. In a word, a dynamic balance of water exits in the catalyst layer. Consequently, the stagnant water layer on the catalyst surface is kept stably and the selectivity is enhanced significantly. (iii) Taylor flow in the thin monolithic channel increases the transportation coefficient of cyclohexene from aqueous phase to the organic phase [22]. However, for particulate catalysts, the Ru distributions are much broad and Taylor flow could not be formed during the hydrogenation. Moreover, there are no pores as large as in cordierite monolith. The lower stability of water layer on particulate catalyst probably leads to the low diffusion resistance of benzene, resulting in high catalytic activity, compared to CMAR. The hydrogenation data of ZrO2–Al2O3 washcoated monolithic catalyst (CMZAR) are shown in Table 1. It can be seen that the CMZAR catalyst shows much higher activity than CMAR, which may be attributed to the high surface area of ZrO2–Al2O3 washcoating as mentioned above. Liu et al. [29] reported that high surface area of the support will benefit a high dispersion of Ru after the impregnation. It was also reported that high dispersion of Ru on the support showed positive influence on its catalytic activity [10]. Thus, the Ru dispersion on ZrO2–Al2O3 composite washcoating was higher and the activity of CMZAR was increased considerably compared to that of CMAR. In addition, by the analysis of the physisorption data and the catalytic performances, the large difference in pore size distribution between the two samples seems not to be the crucial factor determining the selectivity. This is not consistent with the conclusion that the large pores of the support favor higher selectivity to cyclohexene [29]. Further investigation of the reason on this fact is currently under way. Anyway, CMZAR showed better performance according to the yield of cyclohexene.
Table 1 Results of benzene hydrogenation on the several catalystsa Sample
Description
Pore sizec (nm)
Surface areac (m2 g 1)
Conversion (%)
Selectivity (%)
Yield (%)
CMAR CMZAR PAR CMZAR-2b
Ru/Al2O3/monolith Ru/ZrO2–Al2O3/monolith Ru/Al2O3 Ru/ZrO2–Al2O3/monolith
15.1 8.8 15.7 8.8
32 90 35 90
7.44 12.13 21.21 68.81
61 62 26 44
4.57 7.48 5.60 30.36
a b c
Reaction conditions: P = 3.0 MPa, T = 423 K, catalyst amount = 3.6 ml, LHSV = 4 h 1, C(ZnSO4) = 0.5%, H2/H2O/benzene = 500:1:1 (v/v). Reaction conditions: P = 4.0 MPa, T = 423 K, catalyst amount = 14.4 ml, LHSV = 0.75 h 1, C(ZnSO4) = 0.5%, H2/H2O/benzene = 1000:5:1 (v/v). Characteristics of the washcoatings on the cordierite monolith or those of the particulate Al2O3 in PAR.
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To obtain a higher yield of cyclohexene for industrial purpose, a larger amount (14.4 ml) of CMZAR catalysts were therefore used at a relatively lower liquid hourly space velocity (LHSV). A high cyclohexene yield of 30% was obtained at a selectivity higher than 44% (CMZAR-2 in Table 1). The catalytic performances in detail are shown in Fig. 5, which shows high stability of the catalyst during the experiment. The high hydrophilicity of the coated cordierite monolith and the stable water layer are considered as the key reason. As a result of the presence of water around the active site, desorption of cyclohexene is enhanced greatly and at the same time, the diffusion of that from the organic phase to the catalytic surface is suppressed. In another aspect, the residence time of cyclohexene on the catalyst surface would not increase as much as the decrease of the LHSV. Therefore the consecutive hydrogenation was inhibited with a higher cyclohexene yield achieved. Nevertheless, it can be imagined that too small LHSV will not benefit the production of cyclohexene because an adequate amount and concentration of benzene in the organic phase is essential to extract the cyclohexene in the aqueous phase [7]. An appropriate range of LHSV evidently exists for getting high yield of cyclohexene. Although the corresponding selectivity was relatively lower than in the literature (about 80%) [7], the advantages of higher efficiency (105 g cyclohexene/gRu h for the monolithic catalyst vs. 45–60 g cyclohexene/gRu h in the literature), simplicity of process and easy scaling up are very attractive from the industrial viewpoint. It can be seen from the earlier work that the presence of an aqueous phase is essential for getting good cyclohexene yields [5]. The additive in the aqueous phase is always ZnSO4, which is considered to be one of the best salt to obtain high selectivity [30]. The effects of water and ZnSO4 on the catalytic performances were studied for the monolithic catalyst CMAR. As shown in Table 2, no cyclohexene can be produced when no water is added in the reaction system and the only product is cyclohexane. When water presented in the reaction system, some amount of cyclohexene is
Fig. 5. Catalyst performance for the selective hydrogenation of benzene to cyclohexene over the monolithic catalyst CMZAR.
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Table 2 Effects of additives on the catalytic performance of monolithic catalyst CMAR No.
Additive
H2/benzene (v/v)
Conversion (%)
Selectivity (%)
Yield (%)
1 2 3
Nothing Water Water + ZnSO4
1000:1a 500:1 500:1
99.64 26.54 7.44
0 25 61
0 6.42 4.57
Reaction conditions: P = 3.0 MPa, T = 423 K, catalyst amount = 3.6 ml, LHSV = 4 h 1. a High activity need enough H2 to stabilize the system pressure.
observed in the products, but the selectivity is only about 20%, which is apparently lower than that obtained under the conditions when the aqueous solution of ZnSO4 is added. On the other hand, the use of ZnSO4 as an additive reduces the catalytic activity considerably. According to the investigations by Struijk et al. [5,31], it was postulated to attribute to several reasons: (i) Water can expel the formed cyclohexene from catalytic surface through competitive adsorption; (ii) Zinc sulphate is chemisorbed on the catalyst surface, thereby making the catalyst surface more hydrophilic and resulting in an increase in the cyclohexene selectivity; (iii) The preferential adsorption of water on strongly bonding ruthenium sites which favors the formation of cyclohexane will benefit the yield of cyclohexene. Although the selectivity on the monolithic catalyst was still lower than that obtained in slurry reactor on particulate Ru catalyst [5] in the presence of ZnSO4, it is interesting to note that the yield of cyclohexene in the presence of only water is significantly higher than that observed on a particulate Ru/Al2O3 catalyst [5] where the selectivity to cyclohexene is less than 10% under the same conditions. This is probably attributed to the higher hydrophilicity of the cordierite monolith and the relatively shorter resident time of cyclohexene in the monolithic reactor. 4. Conclusions This study represents a novel monolithic catalyst for the selective liquid-phase hydrogenation of benzene. The cordierite monolithic catalysts show high selectivity even in the absence of ZnSO4, due to the several possible reasons including the egg-shell distribution of the active component, the Taylor flow pattern formed in the channels and the large pores inside the thin walls which act as water reservoirs keeping a stagnant water layer on the catalytic surface. The monolithic catalysts give much higher selectivity than particulate catalyst in fix-bed reactor. Compared to Al2O3 coated monolithic catalyst, the ZrO2–Al2O3 coated monolithic catalyst is proved to be more efficient with a high yield at equal selectivity. It should be noted that larger amount of catalysts have given much higher yield at a relatively lower LHSV. ZnSO4 added in the aqueous phase can increase significantly the selectivity of monolithic catalyst. Our study also shows that the monolith reactor is
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