Catalysis Communications 9 (2008) 2612–2615

Contents lists available at ScienceDirect

Catalysis Communications

journal homepage: www.elsevier .com/locate /catcom

Ru/SBA-15 catalysts for partial hydrogenation of benzene to cyclohexene:Tuning the Ru crystallite size by Ba

Juan Bu, Jian-Liang Liu, Xue-Ying Chen, Ji-Hua Zhuang *, Shi-Run Yan, Ming-Hua Qiao *,He-Yong He, Kang-Nian FanDepartment of Chemistry and Shanghai, Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 18 June 2008Accepted 22 July 2008Available online 3 August 2008

Keywords:BenzeneCyclohexeneRuBaSelective hydrogenation

1566-7367/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.catcom.2008.07.024

* Corresponding authors. Tel.: +86 21 55664679; faE-mail addresses: juanbu@fudan.edu.cn (J. Bu), 07

Liu), xueyingchen@fudan.edu.cn (X.-Y. Chen), jhuaz@sryan@fudan.edu.cn (S.-R. Yan), mhqiao@fudan.edu.fudan.edu.cn (H.-Y. He), knfan@fudan.edu.cn (K.-N. Fa

Ru–Ba/SBA-15 catalysts with different Ba/Ru molar ratios were prepared by the ‘two solvents’ method,which can readily introduce the active components into the channels of SBA-15. Characterizationsrevealed that the crystallite size of Ru was tuned smoothly from 3.6 to 7.5 nm by increasing the Ba/Ruratio from 0.1 to 1.0. In liquid phase hydrogenation of benzene, the maximum yield of cyclohexenewas obtained on the Ru–Ba/SBA-15 catalyst with Ru crystallite size of 5.6 nm, which is interpreted asthe presence of the highest population of the active sites favorable for the production of cyclohexeneon such Ru crystallites.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

Cyclohexene is an important intermediate in the production ofadipic acid and its derivatives such as polyamides and polyesters.Partial hydrogenation of benzene to cyclohexene has attractedmuch attention owing to its atomically economy than the dehydra-tion of cyclohexanol [1,2]. However, it is difficult to obtain a highyield of cyclohexene through this route, because cyclohexane, thecomplete hydrogenation product, is thermodynamically morefavorable [3,4]. Although various catalysts have been designed toimprove the yield of cyclohexene [3–11], the best result reportedby an Asahi patent was obtained on a Ru catalyst in the presenceof ZnSO4, ZrO2, and a pH modifier, with 56% cyclohexene yield at80% selectivity being claimed [1]. However, the catalyst is expen-sive as the noble metal content in the catalyst is up to 90 wt%. Itis highly desirable to develop a cost-effective catalyst with muchlower Ru loading and cut down the usage of the additives, whichwill undoubtedly make the route more economic and environmen-tally friendly.

Regular mesoporous molecular sieves have exhibited great po-tential in catalysis, which is closely related to their unique featuressuch as large surface area and narrow pore size distribution [12].These features offer new possibilities for preparing highly dis-

ll rights reserved.

x: +86 21 65641740.1022044@fudan.edu.cn (J.-L.fudan.edu.cn (J.-H. Zhuang),

cn (M.-H. Qiao), heyonghe@n).

persed metal catalysts. In our previous work [13], we demon-strated that the yield of cyclohexene over the Ru/SBA-15 catalystprepared by the ‘two solvents’ method (Ru/SBA-15) doubled thatover the catalyst prepared by the conventional wetness impregna-tion method [14]. As a continuation of that work, we investigatedthe promoting effects of Sn, Fe, and Ba on the Ru/SBA-15 catalystprepared by the ‘two solvents’ method, and found that Ba wasthe most effective one at the optimized content of these promoters.In this paper, we report on the effects of the content of Ba on theyield of cyclohexene over the Ru/SBA-15 catalyst. The superior cat-alytic performances of the Ba-promoted catalysts (Ru–Ba/SBA-15)were interpreted based on the characterization results.

2. Experimental

The preparation procedure for the Ru–Ba/SBA-15 with a Ba/Rumolar ratio of 0.5 was presented as an example. 3.0 ml of aqueoussolution of RuCl3 (0.4 M) and Ba(NO3)2 (0.2 M) was added gradu-ally to 15.0 ml of cyclohexane containing 1.0 g of SBA-15 at298 K. After being stirred vigorously for 15 min, the dark browncolor of the solution disappeared, while the originally white-col-ored SBA-15 became black, signifying that the ‘two solvents’ meth-od is very efficient in transferring the salts in the aqueous solutionto the mesoporous SBA-15. The black powders were then centri-fuged, dried at 393 K for 6 h, and reduced by 5% H2 in Ar at673 K for 4 h at a ramping rate of 2 K min�1.

The bulk composition was analyzed by inductively coupledplasma-atomic emission spectroscopy (ICP-AES; IRIS Intrepid),

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00

10

20

30

40

Yie

ld o

fC

yclo

hex

en

e (

%)

Ba/Ru (molar ratio)

a

0.00 0.13 0.26 0.39 0.520

10

20

30

40

50

Yie

ld o

fC

yclo

hex

en

e (

%)

CZnSO4(M)

b

Fig. 1. (a) The maximum yield of cyclohexene on Ru–Ba/SBA-15 catalysts againstthe Ba/Ru molar ratio. The ZnSO4 concentration is 0.13 M. (b) The maximum yield ofcyclohexene on the Ru–Ba/SBA-15–0.5 catalyst as a function of the concentration ofZnSO4. Reaction conditions: 1.0 g of catalyst, 50 ml of benzene, 100 ml of water,reaction temperature of 423 K, H2 pressure of 4.0 MPa, and stirring rate of1000 rpm.

20 30 40 50 60 70 80 90

Ru-Ba/SBA-15-1.0

Ru-Ba/SBA-15-0.5

Ru-Ba/SBA-15-0.3

Ru-Ba/SBA-15-0.2

Inte

nsity

(a.u

.)

Ru-Ba/SBA-15-0.1

Ru

Fig. 2. XRD profiles of the Ru–Ba/SBA-15 catalysts.

J. Bu et al. / Catalysis Communications 9 (2008) 2612–2615 2613

verifying that the nominal Ba/Ru molar ratios and loadings werewell retained in the catalysts. The BET surface area (SBET) was mea-sured using N2 physisorption at 77 K on a Micromeritics Tri-Star3000 apparatus. The X-ray diffraction (XRD) patterns werecollected on a Bruker AXS D8 Advance X-ray diffractometer usingCu Ka radiation (k = 0.15418 nm). The tube voltage was 40 kV,and the current was 40 mA. The morphology was observed bytransmission electron microscopy (TEM; JEOL JEM2011). The

Table 1Physicochemical properties of Ru–Ba/SBA-15 catalysts

Ba/Ru (molar ratio) SBET (m2 g�1) Vpore (cm3 g�1) dpore (nm) da (nm)

0.1 501 1.1 7.9 3.60.2 469 1.0 7.6 3.80.3 466 1.0 7.5 4.20.5 459 1.0 7.5 5.61.0 456 1.0 7.4 7.5

a Ru crystallite size calculated based on X-ray line broadening and the Scherrerequation.

Fig. 3. TEM images of Ru–Ba/SBA-15 (Ba/Ru molar ratio = 0.5) catalysts prepared by(a) the ‘two solvents’ method and (b) the wetness impregnation method.

2614 J. Bu et al. / Catalysis Communications 9 (2008) 2612–2615

surface state was detected by X-ray photoelectron spectroscopy(XPS; Perkin Elmer PHI5000C). The spectra were recorded withMg Ka line as the excitation source (hm = 1253.6 eV). The bindingenergy (BE) values were referenced to the Si 2p peak of SiO2 at103.3 eV with an uncertainty of ±0.2 eV.

The hydrogenation reaction was performed in a 500 ml stainlesssteel autoclave equipped with a mechanical stirrer. The reactionconditions were as follows: 1.0 g of catalyst, 50 ml of benzene,100 ml of water, reaction temperature of 423 K, H2 pressure of4.0 MPa, stirring rate of 1000 rpm to eliminate the diffusion limita-tion, and a certain amount of ZnSO4. The products, cyclohexeneand cyclohexane, were analyzed by a GC102 gas chromatographwith a PEG-20M packed column and a TCD detector.

3. Results and discussion

Fig. 1a shows the evolution of the maximum yield of cyclohex-ene over Ru–Ba/SBA-15 catalysts of different Ba/Ru molar ratios.The Ru loading was fixed at 12 wt%. Under the reaction conditionsspecified in Fig. 1, the yield of cyclohexene increased remarkablyfrom 28.8% on Ru/SBA-15 to 41.7% on Ru–Ba/SBA-15 with a Ba/Ru molar ratio of 0.5 (Ru–Ba/SBA-15–0.5). Increasing the Ba/Ru ra-tio to 1.0 led to the drop of the yield of cyclohexene. For compari-son, the optimized Ru–Fe/SBA-15 catalyst gave a maximum yieldof cyclohexene of 31.6%, while Sn was adverse to the formationof cyclohexene irrespective of the amount of Sn.

On the Ru–Ba/SBA-15–0.5 catalyst, the effect of the concentra-tion of ZnSO4 on the yield of cyclohexene was exploited. Fig. 1bshows that the optimized concentration of ZnSO4 was 0.39 M,and the corresponding cyclohexene yield was as high as 50.8%. Itis anticipated that by further adding ZrO2 as disperser and by finelytuning the acidity of the aqueous solution of ZnSO4, a higher cyclo-hexene yield may be resulted [11]. However, the present result hasclearly demonstrated that by properly choosing the preparationmethod and the promoter of the Ru catalyst, a high yield of cyclo-hexene can be directly obtained, which simplifies the operationprocedure and lowering the operation cost.

Small-angle XRD patterns verified the preservation of the regu-lar mesoporous structure of SBA-15 in Ru–Ba/SBA-15 catalysts.Wide-angle XRD patterns shown in Fig. 2 revealed that besidesthe reflection of amorphous silica at 2h = 27.5o, there are reflec-tions from metallic Ru [15]. Moreover, the reflection peaks of Ruwere sharpened with the increment of the Ba content. The Ru crys-

275 280 285 290 295

Inte

nsity

(a.u

.)

Binding Energy (eV)

Ru 3d5/2

Fig. 4. XPS spectra of Ru 3d (a) and Ba 3d and (

tallite sizes were calculated from the Scherrer equation and werelisted in Table 1, from which it is concluded that the Ru crystallitesize can be changed monotonically by increasing the Ba content.The BET surface area and the porosity of the catalysts were alsosummarized in Table 1.

The Ru particles were uniformly dispersed in the SBA-15 chan-nels, as verified by the TEM image of the Ru–Ba/SBA-15–0.5 cata-lyst (Fig. 3a). The Ru particle size of 6.5 nm measured by TEM isconsistent with the crystallite size of 5.6 nm derived from XRD.For a control catalyst prepared by the conventional wetnessimpregnation method with the same Ru and Ba loadings (Ru–Ba/SBA-15-wi-0.5), randomly sized Ru particles were mainly locatedon the exterior of SBA-15 (Fig. 3b), revealing the superiority ofthe ‘two solvents’ method in introducing Ru crystallites to thechannels of SBA-15. Under the reaction condition identical to thatof Ru–Ba/SBA-15–0.5, the maximum yield of cyclohexene over Ru–Ba/SBA-15-wi-0.5 was only 28.9%.

XPS spectrum shown in Fig. 4 revealed that the Ru species inRu–Ba/SBA-15 catalysts was solely in the metallic state with theRu 3d5/2 binding energy (BE) of 280.1 eV [16]. The ascription ofthe Ba species with 3d5/2 BE of 783.6 eV is less straightforward.However, the presence of Ba(NO3)2 can be readily excluded, asthe N 1s signal was not visible in these catalysts. Moreover, thedecomposition of Ba(NO3)2 to BaO in H2 atmosphere and in thepresence of Ru started at about 553 K [17], about 120 K lower thanthe reduction temperature employed in this work. Although forbulk BaO the standard Ba 3d5/2 BE is 779.9 eV [16], Ozensoy et al.found that for BaO thin layers on h-Al2O3/NiAl(100), the Ba 3d5/2

BE can positively shift by several eVs [18]. Thus, we ascribed theBa 3d5/2 peak at 783.6eV to BaO highly dispersed in Ru–Ba/SBA-15 catalysts with large surface areas.

It is well-known that Ba is one of the most effective promotersfor Ru in ammonia synthesis. In that reaction, either the electronic[19,20] or the structural effect [21–24] of BaO has been used to ac-count for activity enhancement. The electronic effect of BaO mightexist in Ru–Ba/SBA-15 catalysts after reduction, but it should benoted that BaO is water-soluble and could be leached away as waterwas present in the reaction system. By intentionally washing the re-duced Ru–Ba/SBA-15 catalysts free from Ba ions using Na2SO4 asthe indicator, the deviation in the yields of cyclohexene over the un-washed and washed catalysts were found to be within ±1%, reveal-ing that the presence of BaO during the hydrogenation process isnot the prerequisite for the improved selectivity to cyclohexene.

775 780 785 790 795 800 805

Inte

nsity

(a.u

.)

Binding Energy (eV)

Ba 3d5/2Ba 3d3/2

b) levels of the Ru–Ba/SBA-15–0.5 catalyst.

J. Bu et al. / Catalysis Communications 9 (2008) 2612–2615 2615

On the other hand, Fig. 2 revealed that with the increment ofthe Ba content, the reflections of metallic Ru were broadenedaccordingly, suggesting that Ba in Ru–Ba/SBA-15 catalysts canmodify the Ru crystallite size. In ammonia synthesis, Dahl et al.found that the B5-type site is responsible for the activity of Ru cat-alysts [25]. The number of such site depends on the Ru crystallitesize for a given crystal morphology, and an optimum size of�2 nm exhibits the highest possible concentration of such site[23]. We analogously suggest that there is an optimum Ru crystal-lite size for partial hydrogenation of benzene to cyclohexene, andon Ru crystallites of ca. 5.6 nm the number of the active sites favor-able for the production of cyclohexene is the highest. Although themechanism underlying the modification effect of Ba on the size ofthe Ru crystallite deserves further investigation, it is remarkablethat it provides a new but simple route to finely tune the crystallitesize of Ru, which has the possibility to be extended to control thecrystallite size of other metals. Moreover, Ba in the form of BaO canbe easily removed from the catalyst by water, which will simplifythe interpretation of the size effect of metal nanoparticles incatalysis.

4. Conclusion

The Ru–Ba/SBA-15 catalyst prepared by the ‘two solvents’method with much lower Ru loading than the industrial catalystexhibited high selectivity to cyclohexene in liquid phase hydroge-nation of benzene using ZnSO4 as the only additive. The yield ofcyclohexene of 50.8% was obtained at the optimized Ba/Ru molarratio of 0.5. The superior catalytic performance of the Ru–Ba/SBA-15–0.5 catalyst is attributed to the suitable crystallite size ofRu tuned by BaO, which possesses the highest population of the ac-tive sites favorable for the production of cyclohexene.

Acknowledgements

This work was supported by the National Basic Research Pro-gram of China (2006CB202502), Shanghai Science and Technology

Committee (06JC14009), the Fok Ying Tong Education Foundation(104022), and the NSF of China (20673025).

References

[1] H. Nagahara, M. Konishi, US Patent 4 734 536 to Asahi Kasei Kogyo KabushikiKaisha, 1988.

[2] L. Ronchin, L. Toniolo, Catal. Lett. 48 (1999) 255.[3] J. Struijk, M. d’Angremond, M. Lucas-de Regt, J.J.F. Scholten, Appl. Catal. A 83

(1992) 263.[4] C. Milone, G. Neri, A. Donato, M.G. Musolino, L. Mercadante, J. Catal. 159 (1996)

253.[5] S.H. Xie, M.H. Qiao, H.X. Li, W.J. Wang, J.F. Deng, Appl. Catal. A 176 (1999) 129.[6] S.C. Hu, Y.W. Chen, J. Chem. Technol. Biotechnol. 76 (2001) 954.[7] S.C. Hu, Y.W. Chen, Ind. Eng. Chem. Res. 40 (2001) 6099.[8] J.Q. Wang, Y.Z. Wang, S.H. Xie, M.H. Qiao, H.X. Li, K.N. Fan, Appl. Catal. A 272

(2004) 29.[9] J.Q. Wang, P.J. Guo, M.H. Qiao, S.R. Yan, K.N. Fan, Acta Chim. Sinica 62 (2004)

1765.[10] J.Q. Wang, P.J. Guo, S.R. Yan, M.H. Qiao, H.X. Li, K.N. Fan, J. Mol. Catal. A 222

(2004) 229.[11] S.C. Liu, Z.Y. Liu, Z. Wang, S.H. Zhao, Y.M. Wu, Appl. Catal. A 313 (2006)

49.[12] A. Corma, Chem. Rev. 97 (1997) 2373.[13] J. Bu, Y. Pei, P.J. Guo, M.H. Qiao, S.R. Yan, K.N. Fan, Stud. Surf. Sci. Catal. 165

(2007) 381.[14] M. Imperor-Clerc, D. Bazin, M.D. Appay, P. Beaunier, A. Davidson, Chem. Mater.

16 (2004) 1813.[15] PDFMaint Version 3.0, Powder Diffraction Database, Bruker Analytical X-ray

Systems GmbH, 1997.[16] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray

Photoelectron Spectroscopy, in: J. Chastain (Ed.), Perkin-Elmer Corporation,USA, 1992.

[17] Z.H. Zhong, K. Aika, J. Catal. 173 (1998) 535.[18] E. Ozensoy, C.H.F. Peden, J. Szanyi, J. Phys. Chem. B 110 (2006) 17009.[19] H.S. Zeng, K. Inazu, K. Aika, J. Catal. 211 (2002) 33.[20] T.W. Hansen, J.B. Wagner, P.L. Hansen, S. Dahl, H. Topsøe, C.J.H. Jacobsen,

Science 294 (2001) 1508.[21] Z. Kowalczyk, S. Jodzis, W. Raróg, J. Zielinski, J. Pielaszek, A. Presz, Appl. Catal. A

184 (1999) 95.[22] W. Raróg, Z. Kowalczyk, J. Sentek, D. Składanowski, J. Zielinski, Catal. Lett. 68

(2000) 163.[23] C.J.H. Jacobsen, S. Dahl, P.L. Hansen, E. Törnqvist, L. Jensen, H. Topsøe, D.V. Prip,

P.B. Møenshaug, I. Chorkendorff, J. Mol. Catal. A 163 (2000) 19.[24] H. Bielawa, O. Hinrichsen, A. Birkner, M. Muhler, Angew. Chem. Int. Edit. 40

(2001) 1061.[25] S. Dahl, E. Törnqvist, I. Chorkendorff, J. Catal. 192 (2000) 381.