Selective hydrogenation of benzene to cyclohexene over monometallic ruthenium catalysts in the presence of CeO2 and ZnSO4 as co-modifiers

Selective hydrogenation of benzene to cyclohexene over monometallic ruthenium catalysts in the presence of CeO2 and ZnSO4 as co-modifiers

JOURNAL OF RARE EARTHS, Vol. 31, No. 10, Oct. 2013, P. 1023 Selective hydrogenation of benzene to cyclohexene over monometallic ruthenium catalysts i...

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JOURNAL OF RARE EARTHS, Vol. 31, No. 10, Oct. 2013, P. 1023

Selective hydrogenation of benzene to cyclohexene over monometallic ruthenium catalysts in the presence of CeO2 and ZnSO4 as co-modifiers SUN Haijie (ᄭ⍋ᵄ)1,2, CHEN Lingxia (䰜‫ޠ‬䳲)2, LI Shuaihui (ᴢᏙ䕝)1, JIANG Houbing (∳८݉)1, ZHANG Yuanxin (ᓴ‫ܗ‬佼)1, REN Baozeng (ӏֱ๲)3, LIU Zhongyi (߬ӆ↙)1,*, LIU Shouchang (߬ᇓ䭓)1 (1. College of Chemistry and Molecular Engnineering, Zhengzhou University, Zhengzhou 450001, China; 2. Institute of Environmental and Catalytic Engineering, Department of Chemistry, Zhengzhou Normal University, Zhengzhou 450044, China; 3. School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001, China) Received 3 April 2013; revised 23 June 2013

Abstract: The monometallic Ru catalysts with the CeO2 without calcination and ZnSO4 as co-modifiers gave a cyclohexene yield of 58.5% at the optimum nominal CeO2/Ru molar ratio of 0.15. Moreover, this catalyst had a good stability. The chemisorbed (Zn(OH)2)3(ZnSO4)(H2O)3 salt on Ru surface, which was formed by the CeO2 reacting with ZnSO4, created the new Ru active sites suitable for the formation of cyclohexene and improved the selectivity to cyclohexene. In addition, the Zn2+ in the aqueous phase could form a stable complex with cyclohexene, stabilizing the cyclohexene in the liquid phase and improving the selectivity to cyclohexene. The calcination treatment of CeO2 was not beneficial for the enhancement of the selectivity to cyclohexene since it is difficult for the CeO2 calcinated to react with ZnSO4 to form the (Zn(OH)2)3(ZnSO4)(H2O)3 salt. Keywords: hydrogenation; benzene; cyclohexene; Ru; CeO2; rare earths

Cyclohexene is commercially important for the production of adipic acid, nylon 6, nylon 66 and many other fine chemicals[1–4]. Selective hydrogenation of benzene to cyclohexene has drawn much attention due to its inexpensive products, lower amounts of undesirable products and simplified operation[5]. However, it is difficult to obtain a high yield of cyclohexene through this route, because cyclohexane, the complete hydrogenation product, is thermodynamically more favorable[6]. The use of modifiers (the so-called reaction or process modifier) is by far a simple and effective strategy for improving the selectivity to cyclohexene of the Ru-based catalysts. ZnSO4 has been regarded as the best modifier. Various catalysts gave high cyclohexene yields in the presence of ZnSO4. Ning et al.[7] prepared a colloidal ruthenium catalyst stabilized by silica through a microemulsion processing and obtained a cyclohexene yield of 42% in the presence of ZnSO4. Fan et al.[8] prepared a Ru-Co-B/-Al2O3 catalyst by a reduction impregnation method and got a cyclohexene yield of 34.8% in the presence of ZnSO4 and ethylenediamine. Liu et al.[9] performed the reaction over a Ru-La-B/ZrO2 catalyst and achieved a cyclohexene yield of 53.7% in the presence of ZnSO4. Liu et al.[10] prepared a Ru-Ce/SBA-15 catalyst by a “two solvents” impregnation method and reached a cyclohexene yield of 53.8% in the presence of ZnSO4.

Then they prepared a Ru-La/SBA-15 catalyst by the same method and obtained a cyclohexene yield of 57.0% in the presence of CdSO4 and ZnSO4[11]. Liu et al.[12] prepared a Ru-Cu/ZnO catalyst by a deposition-precipitation method and reached a cyclohexene yield of 49.4%. Sun et al.[13,14] prepared Ru-Zn catalysts by a co-precipitation method and achieved a cyclohexene yield of 63.6% in the presence of ZnSO4 and diethanolamine. They also prepared a Ru-La catalyst by a co-precipitation method and got a cyclohexene yield of 59.5%[15]. However, because in these catalysts the effects of the strong metal-support interactions[3], the interactions between Ru and the promoters and probably the interactions between the modifier and the promoter and so on all should not be ignored, the roles of the ZnSO4 in enhancing the selectivity to cyclohexene of Ru-based catalyst have not yet been satisfactorily explained up to now. Moreover, it is generally accepted that only the soluble compound could be as the modifiers of the reaction. It is suggested that the Ce is one of the best promoters for hydrogenation catalysts[16,17]. In the present work, we for the first time employed ZnSO4 and the insoluble CeO2 as co-modifiers for the reaction, and found that the combination of them could significantly improve the selectivity to cyclohexene of the monometallic Ru catalyst. This not only provided a simple method to produce cyclohexene

Foundation item: Project supported by National Natural Science Foundation of China (21273205), and the Innovation Fund for Technology Based Firms of China (10C26214104505) * Corresponding author: LIU Zhongyi (E-mail: [email protected]. Tel.: +86-371-67783384) DOI: 10.1016/S1002-0721(12)60397-4

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but also displayed the roles of the ZnSO4 and CeO2 clearly. And the effects of the calcination temperature of CeO2 were also investigated.

280 mL of H2O, 49.2 g of ZnSO4·7H2O, the desired amount of CeO2, 140 mL of benzene, reaction temperature of 423 K, H2 pressure of 5 MPa, stirring rate of 1400 r/min to exclude the diffusion effect when the temperature reached. The reaction products were analyzed using a GC-1690 Gas Chromatograph with a FID detector. At the end of the reaction, the organic phase was removed using a separating funnel. The slurry containing the catalyst was reused, according to the above operations, without any additions. The M Ru catalysts after reaction in the presence of the different CeO2 and ZnSO4 were denoted as M Ru+CeO2(NC)-x P, M Ru+CeO2(C373)-x P and M Ru+CeO2(C473)-x P, where the x denotes the nominal CeO2/Ru molar ratio and P represents the presence of ZnSO4. The M Ru catalysts after hydrogenation in the presence of ZnSO4 and in the absence of ZnSO4 are denoted as M Ru P and M Ru A, where A representes the absence of ZnSO4. The M Ru catalyst after hydrogenation with the nominal CeO2(NC)/Ru molar ratio of 0.15 is denoted as M Ru+CeO2(NC)-0.15 A.

1

Experimental

1.1 Catalyst and CeO2 preparation The monometallic Ru catalyst was prepared according to the following procedure. 300 mL of a 10% (weight percentage, the same below) NaOH solution was added to 0.36 mol/L of a RuCl3·3H2O solution under stirring at 353 K and the resulting mixture was agitated for an additional 4 h at 353 K. 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 423 K 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 monometallic Ru catalysts were obtained. This catalyst was denoted as M Ru catalyst. The CeO2 was prepared according to the following procedure. 100 mL of a 5% NaOH solution was added to 100 mL of a 0.27 mol/L Ce(NO3)3·6H2O solution under stirring at 353 K and the resulting mixture was agitated for an additional 4 h at 353 K. Then the solid was washed until neutrality, subsequently vacuum-dried at 323 K and calcinated at the desired temperature. The CeO2 sample without calcination was denoted as CeO2(NC). The CeO2 samples calcinated at 373 and 473 K were denoted as CeO2(C373) and CeO2(C473), respectively. 1.2 Catalyst characterization X-ray diffraction (XRD) patterns were acquired on a PANalytcal XPert PRO instrument using Cu K (= 0.1541 nm) with a scan range from 5º–90º at a step of 0.03º. The compositions of M Ru catalysts in the presence of the different CeO2 and ZnSO4 after hydrogenation were measured by X-ray fluorescence (XRF) on a Bruker S4 Pioneer instrument. Auger electron spectroscopy (AES) and sputter profiles were taken on a ULVAC PHI-700 Nano-canning Auger system with an on-axis scanning argon ion gun and a CMA energy analyzer. The energy resolution ratio was 0.1%. The background 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. Spectra were recorded after Ar+ sputtering for 1 min to avoid the interruptions of the surface oxidation of catalysts. 1.3 Activity test The selective hydrogenation of benzene was performed in a 1 L autoclave lined the hastelloy. The reaction conditions are as follows: 1.8 g of M Ru catalyst,

2 Results and discussion The XRD patterns of the different CeO2 in Fig. 1(a) show that the intensities of the diffractional peaks of the CeO2 (JCPDS: 00-004-0593) increase with their calcination temperatures, indicating the increase of the crystallinity. The XRD patterns of the M Ru catalyst in Fig. 1(b) only show the diffractional peaks of the metallic Ru (JCPDS: 01-078-0246). It should be noted that only Ru is detected in the M Ru catalyst by the XRF instrument, indicating that the Na+, OH– and Cl– were washed clean. Fig. 1(c) shows that the intensities of the diffractional peaks of the (Zn(OH)2)3(ZnSO4)(H2O)3 salt (JCPDS: 00-078-0247) increase with the decrease of the calcination temperatures of CeO2 when the nominal CeO2/Ru molar ratio is 0.20. Table 1 shows that the Ce/Ru molar ratios of these samples gradually decrease and the Zn/Ru and S/Ru molar ratios gradually increase with the decrease of the calcination temperatures. Moreover, decreasing calcination temperature increases the pH values of the aqueous phase after hydrogenation. This suggests that the amount of the (Zn(OH)2)3(ZnSO4)(H2O)3 salt increases with the decrease of the calcination temperatures, however, the amount of CeO2 decreases. All of these suggests that the CeO2 reacted with ZnSO4 to form a (Zn(OH)2)3(ZnSO4)(H2O)3 salt. The higher the calcination temperatures of CeO2 are, the more difficultly the CeO2 reacts with ZnSO4 to form the (Zn(OH)2)3(ZnSO4) (H2O)3 salt. This results in the decrease of the concentration of Zn2+ and the increase of pH values. The intensities of the diffractional peaks of the (Zn(OH)2)3(ZnSO4) (H2O)3 salt increase with the amount of the CeO2(NC) increasing from 0.15 to 0.20 in Fig. 1(c). Table 1 shows

SUN Haijie et al., Selective hydrogenation of benzene to cyclohexene over monometallic ruthenium catalysts in the …

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Fig. 1 XRD patterns of the different samples (a) CeO2 calcinated at different temperatures; (b) the M Ru catalyst; (c) the M Ru catalyst after reaction in the presence of the different CeO2 and ZnSO4; (d) the M Ru catalyst after reaction with the nominal CeO2(NC)/Ru molar ratio of 0.15 in the absence of ZnSO4 Table 1 Compositions of the M Ru catalysts after hydrogenation under different conditions (mol.%) Condition

Ce/Ru AH* Zn/Ru AH* S/Ru AH*

pH values

M Ru A

0

0

0

7.10

M Ru P

0

0.0213

0.0026

5.53

M Ru+CeO2 (C473)-0.20 P

0.1749

0.1115

0.0071

5.64

M Ru+CeO2 (C337)-0.20 P

0.1129

0.2515

0.0305

5.87

M Ru+CeO2 (NC)-0.15 P

0.0048

0.3670

0.0440

6.27

M Ru+CeO2 (NC)-0.20 P

0.0093

0.5014

0.0603

6.47

M Ru+CeO2 (NC)-0.15 A

0.1424

0

0

7.12

CeO2(NC)/Ru molar ratio. AES Zn LMM spectrum of the M Ru catalyst with the nominal CeO2(NC)/Ru molar ratio of 0.15 after hydrogenation in Fig. 2 shows that the kinetic energy is 987.0 eV, which previously are ascribed to the oxidized Zn[18,19]. This is consistent with the XRD results that the Zn species on the suface of M Ru+CeO2(NC)-0.15 P are present in the (Zn(OH)2)3(ZnSO4)(H2O)3 salt. Table 1 (entry 1) shows that only Ru sepecies are detectable in M Ru A. The Zn and S species are found on the surface of M Ru P (Table 1, entry 2). Moreover, the

* Measured by the XRF instrument, AH denoted as after hydrogenation

only trace amount of Ce species is detectable and the molar ratios of Zn/Ru and S/Ru increase with the amount of CeO2(NC) increasing. The pH values of the aqueous phase also increase with the amount of CeO2(NC) increasing. This indicates that almost all of the CeO2(NC) reacted with ZnSO4. The more the CeO2(NC) is added, the more the (Zn(OH)2)3(ZnSO4)(H2O)3 salt is formed and the higher the pH values of the aqueous phase is. The XRD patterns of M Ru+CeO2(NC)-0.15 A in Fig. 1(d) show the diffractional peaks of CeO2 besides the diffraction peaks of metallic Ru. Table 1 shows that the Ce/Ru molar ratio of this sample is similar to the nominal

Fig. 2 Zn LMM spectrum of M Ru+CeO2(NC)-0.15 P

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Zn/Ru molar ratio of M Ru P is much higher than the S/Ru molar ratio. This promotes us to conclude that the Zn species are also present in the (Zn(OH)2)3(ZnSO4) (H2O)3 salt. However, the diffractional peaks of the (Zn(OH)2)3(ZnSO4)(H2O)3 salt are not found on the XRD patterns of this sample due to its low amount as confirmed by the XRF results. Table 1 also shows that the aqueous phase in the presence of ZnSO4 displays acidity due to the hydrolysis of ZnSO4. Based on these catalyst characterizations above, the mechamism for the formation of (Zn(OH)2)3(ZnSO4) (H2O)3 is proposed. It is well known that it is easy for ZnSO4 to hydrolysis to form Zn(OH)2. Subsequently the Zn(OH)2 could react with ZnSO4 to form the (Zn(OH)2)3(ZnSO4)(H2O)3 salt due to the presence of the abundant ZnSO4 in the aqueous phase. CeO2 might be a basic oxide. It could react with the H+ from the hydrolysis of ZnSO4. This leads to the increase of the degree of the hydrolysis. Thus the amount of the (Zn(OH)2)3(ZnSO4)(H2O)3 salt increased. The reaction equations are described as Eqs. (1), (2) and (3). Zn2++2H2OZn(OH)2+2H+ (1) 3Zn(OH)2+Zn2++SO42–+3H2O (Zn(OH)2)3(ZnSO4)(H2O)3 (2) CeO2+2H+Ce4++2H2O (3) Fig. 3 shows the TEM images of the M Ru catalysts (a) and the M Ru+CeO2(NC)-0.15 P (b). Fig. 3(a) and (b)

reveal that both of the M Ru catalysts (a) and the M Ru+CeO2(NC)-0.15 P (b) consist of spherical and ellipsoidal crystallites, with the Ru crystallite sizes mainly centering on around 4.5 nm. Although the diffractional peaks of the (Zn(OH)2)3(ZnSO4)(H2O)3 salt are detected by the XRD instrument, no images of the (Zn(OH)2)3 (ZnSO4)(H2O)3 salt are observed by the TEM instrument. This suggests that the (Zn(OH)2)3(ZnSO4)(H2O)3 salt uniformly disperses on Ru surface. The benzene conversion reached 100.0% at 5 min and cyclohexene was hardly detectable over M Ru+ CeO2(NC)-0.15 A, indicating that the CeO2 alone could not enhance the selectivity to cyclohexene. Fig. 4(a) and (b) show the benzene conversion of 100.0% and the selectivity to cyclohexene of 0% were obtained over M Ru A, indicating that only cyclohexane was formed on the M Ru surface. The performance of M Ru in the presence

Fig. 3 TEM images of the M Ru catalysts (a) and the M Ru+ CeO2(NC)-0.15 P (b)

Fig. 4 (a) Benzene conversion; (b) Cyclohexane selectivity over the M Ru catalysts in the presence of the different CeO2 and ZnSO4; (c) the reaction course of benzene hydrogenation over M Ru+CeO2(NC)-0.15 P

SUN Haijie et al., Selective hydrogenation of benzene to cyclohexene over monometallic ruthenium catalysts in the …

of only Ce(SO4)2 was also investigated. The selectivity to cyclohexene was found to be 0% and the benzene was fully converted to cyclohexane over the M Ru catalyst with the Ce(SO4)2/Ru molar ratio of 0.15. This indicates that the Ce(SO4)2 formed by the CeO2 reacting with H+ also could not enhance the selectivity of M Ru catalyst. The benzene conversion monotonically decreases and the selectivity to cyclohexene monotonically increases with the molar ratios of Zn/Ru and S/Ru, suggesting that the chemisorbed (Zn(OH)2)3(ZnSO4)(H2O)3 salt improves the selectivity to cyclohexene. The reaction of forming cyclohexene only occurrs on the Ru surface with the chemisorbed (Zn(OH)2)3(ZnSO4)(H2O)3 salt. M Ru+ CeO2(NC)-0.15 P gave a cyclohexene yield of 58.5% at the selectivity to cyclohexene of 73.0% at 20 min, which is among the best results reported so far[10,11]. The beneficial effects of the chemisorbed (Zn(OH)2)3(ZnSO4)(H2O)3 salt are ascribed to the following three reasons. (1) The Zn2+ of the chemisorbed (Zn(OH)2)3(ZnSO4)(H2O)3 salt can cover some of Ru active sites (as confirmed by TEM) and lead to the geometric arrangement of the Ru actives[20,21], which creates the new Ru active sites suitable for the formation of cyclohexene, as shown in Fig. 5 (a). Moreover, this can significantly reduce the Ru active sites for the chemisorption of cyclohexene and suppress the further hydrogenation of cyclohexene to cyclohexane[20]. (2) The chemisorbed (Zn(OH)2)3(ZnSO4)(H2O)3 salt is rich in crystal water. Therefore, the salt chemisorbed on the surface causes the M Ru catalyst to be surrounded by a firm stagnant water layer, as shown in Fig. 5 (b). 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[22], resulting in the improvement of the selectivity to cyclohexene. (3) The Zn2+ of the chemisorbed (Zn(OH)2)3(ZnSO4)(H2O)3 salt could form loosely bound adducts with cyclohexene, which could stabilize the

Fig. 5 Sketches of the roles of the chemisorbed (Zn(OH)2)3 (ZnSO4)(H2O)3 salt (a) the Zn2+ covering some of Ru active sites; (b) the chemisorbed (Zn(OH)2)3(ZnSO4)(H2O)3 salt making the M Ru catalyst be surrounded by a firm stagnant water layer

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formed cyclohexene on the Ru surface and improve the selectivity to cyclohexene of the M Ru catalyst[9,10]. Thus the more the amount of the chemisorbed (Zn(OH)2)3 (ZnSO4)(H2O)3 salt, the more the M Ru surface is covered by this salt, resulting in the decrease of the activity and the increase of the selectivity to cyclohexene for the M Ru catalyst. When the nominal CeO2(NC)/Ru ratio is 0.15, the optimum amount of the (Zn(OH)2)3(ZnSO4) (H2O)3 salt is chemisorbed on the M Ru surface, which creates the highest population of the Ru active sites favorable for the production of cyclohexene. Thus the M Ru catalyst gave the highest cyclohexene yield. However, when further increasing the nominal CeO2(NC)/Ru molar ratio, the chemisorbed (Zn(OH)2)3 (ZnSO4)(H2O)3 salt further increases, resulting in not only the dramatically decrease of the total number of the Ru active sites but also the decrease of the Ru active sites suitable for the formation of cyclohexene, which have been created by the chemisorbed (Zn(OH)2)3(ZnSO4) (H2O)3 salt. Thus the activity dramatically decreases and the selectivity to cyclohexene of the Ru catalyst only slightly increases. Specially, the calcination treatment of CeO2 was not beneficial for the improvement of the selectivity to cyclohexene. This also might be one of the most important reasons why the catalysts calcinated or reduced at high temperature showed the poor selectivity to cyclohexene[23]. In addition, the Zn2+ in the aqueous phase is also reported to be able to form a stable complex with cyclohexene in the liquid phase[11], as shown in Fig. 6. This could accelerate the desorption and hinder the re-adsorption of cyclohexene, resulting in the increase of the selectivity to cyclohexene. The reusability of M Ru+CeO2-0.15 P was also investigated. The catalyst was recycled four times without any addition and the results are shown in Fig. 7. As can be seen, benzene conversion and cyclohexene selectivity were stable, at above 75.7% and 75.0% respectively, and cyclohexene yields remained above 58.0% in the four recycles. This suggested that this catalyst has a good stability and potential for industrial applications.

Fig. 6 Complexes formed by the Zn2+ ions with cyclohexene (a) The complex formed by a Zn ion with one cyclohexene molecule; (b) The complex formed by a Zn ion with two cyclohexene molecules

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Fig. 7 Reusability of M Ru+CeO2-0.15 P for selective hydrogenation of benzene to cyclohexene

3 Conclusions The CeO2 without calcination easily reacted with the ZnSO4 to form a (Zn(OH)2)3(ZnSO4)(H2O)3 salt. The chemisorbed (Zn(OH)2)3(ZnSO4)(H2O)3 salt on the M Ru surface could created the new Ru active sites suitable for the formation of cyclohexene and significantly enhanced the the selectivity to cyclohexene. This finding not only provided a simple method to produce cyclohexene and might open a new avenue for selectivity improvement for other hydrogenation reactions. Acknowledgements: Authors would like to thank the Scientific Research Foundation of Graduate School of Zhengzhou University for financial support.

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