Partial Hydrogenation of Benzene to Cyclohexene on Ru@XO2

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Cite This: Ind. Eng. Chem. Res. 2018, 57, 1961−1967

Partial Hydrogenation of Benzene to Cyclohexene on Ru@XO2 (X = Ti, Zr, or Si) Xue-Lian Yu,† Yan Li,‡ Shuang-Mei Xin,† Pei-Qing Yuan,*,† and Wei-Kang Yuan† †

State Key Laboratory of Chemical Engineering and ‡School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China S Supporting Information *

ABSTRACT: The effect of coating Ru particles with a film of hydrophilic materials on the tetra-phase partial hydrogenation of benzene to cyclohexene was examined. By the sol−gel method, amorphous mesoporous TiO2, ZrO2, and SiO2 were coated on hexagonal Ru particles, by which Ru@XO2 (X = Ti, Zr or Si) catalysts were prepared. During the partial hydrogenation of benzene on Ru@XO2, the diffusion of benzene and cyclohexene dissolved in the water phase onto Ru particles is suppressed by the presence of mesoporous coating film. The diffusion of benzene is retarded further by the hydrogen bonding with the structural OH groups contained in hydrophilic materials. Because of the delicate diffusion behavior of benzene and cyclohexene through coating film, a decreasing rate of benzene conversion but a significantly increasing cyclohexene selectivity was observed in the partial hydrogenation on different Ru@XO2 catalysts. A maximum cyclohexene yield of 52.6 mol % occurs on Ru@TiO2 at the benzene conversion of 85.0%. film theory developed in multiphase reaction systems, a stagnant water layer covering the catalyst surface was suggested.26 The presence of the stagnant water layer guarantees the dispersion of Ru catalysts in the water phase. Furthermore, the mass transfer limitation through the stagnant water layer was suggested to play an important role on cyclohexene selectivity. Sun et al. reported that a high benzene flux through the stagnant water layer favors the desorption of cyclohexene because of competitive adsorption on catalyst surface.27 Struijk et al. proposed that the hydrophilicity of the catalyst surface might be decreased if the diffusion of cyclohexene through the stagnant water layer cannot keep up with its formation.26 Ronchin et al. concluded that the limitation to H2 mass transfer through the stagnant water layer could be beneficial for partial hydrogenation since it helps keeping a moderate H2 coverage and the hydrophilicity of catalyst surface.7,24 It is noteworthy that the nature of the stagnant water layer on Ru-based catalysts is determined simultaneously by various factors, like the mixing intensity of reaction system, the type of additives dissolved in the water phase and the property of catalyst supports. As a result, it is difficult to determine quantitatively the detailed effects of the stagnant water layer on partial hydrogenation. To improve the hydrophilicity of Ru-based catalysts, hydrophilic materials are widely used as the support.17,18,24,28,29 Alternatively, the hydrophilicity of Ru-based catalysts can also

1. INTRODUCTION The partial hydrogenation of benzene to cyclohexene, providing a green synthetic route for caprolactam, is of high economic and environmental interest.1 From a thermodynamic viewpoint, the reaction however is not favored since the Gibbs free energy of cyclohexane is lower than that of cyclohexene by 75 kJ/mol.2 After more than 100 years of laboratory scale research, the tetra-phase partial hydrogenation of benzene, involving a water phase, an organic phase, a gas phase and a ruthenium (Ru) based solid catalyst phase, was industrialized by Asahi Co. in 1990 in Japan.3 The subtleties of the Asahi technology are the introduction of a water phase into the reaction system and the dispersion of Ru-based catalysts in the water phase. The solubility of cyclohexene in water is as low as 21 mol/m3, so the direct hydrogenation of cyclohexene in the organic phase is avoided. Under the tetra-phase catalytic framework, the suppression of the adsorption of cyclohexene onto catalyst surface and the promotion of the desorption of cyclohexene from catalyst surface both are considered to be vital to the improvement on cyclohexene selectivity. It has been found that the addition of organics or inorganics into the water phase might result in the formation of coordinated compounds with cyclohexene, which is adverse to the adsorption of cyclohexene onto Ru catalysts.4−11 By surface doping with heteroatoms, electron deficient Ru species (Ruδ+) are formed.12−20 The desorption of cyclohexene from Ruδ+ is believed to be easier than that from metallic Ru species.21−23 Nearly all the studies on the tetra-phase partial hydrogenation of benzene emphasized the importance of the hydrophilicity of Ru-based catalysts.24,25 Following the two© 2018 American Chemical Society

Received: Revised: Accepted: Published: 1961

November 8, 2017 December 31, 2017 January 30, 2018 January 30, 2018 DOI: 10.1021/acs.iecr.7b04642 Ind. Eng. Chem. Res. 2018, 57, 1961−1967

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Industrial & Engineering Chemistry Research be enhanced by coating a film of hydrophilic materials, through which a stable water layer confined in the framework of coating materials is developed on Ru-based catalysts. The diffusion of reactants involved in partial hydrogenation through the confined water layer should be affected by the thickness, pore structure and surface property of the materials coated. One may reasonably propose that the coating of hydrophilic materials on Ru-based catalysts might have an intricate influence on the performance of partial hydrogenation. Unfortunately, no relevant reports on this issue can be found in the literature up to now. Hereby, three typical hydrophilic materials, that is, TiO2, ZrO2 and SiO2 were coated on Ru particles. For simplicity, these oxides are designated hereafter as XO2. The crystal pattern, morphology and surface properties of Ru@XO2 (X = Ti, Zr, or Si) prepared then were characterized. After that, the tetra-phase partial hydrogenation of benzene to cyclohexene on Ru particles or Ru@XO2 was evaluated. On the basis of the results obtained, the possible mechanism responsible for the consistently increasing cyclohexene selectivity on Ru@XO2 was discussed further.

Evaluation of Catalysts. Due to the complexity of the tetra-phase catalytic system, the performance of Ru@XO2 in partial hydrogenation is determined simultaneously by various factors, like reaction temperature, H2 partial pressure, type of coating materials and the thickness of the coating film. According to preliminary experiments, an optimized reaction condition with the highest yield of cyclohexene can be determined on a given Ru@XO2 catalyst with the specific type and thickness of coating film. In this work, the performance of different Ru@XO2 catalysts was compared at 160 °Cand H2 partial pressure of 7.0 MPa, the optimal condition for Ru@TiO2 prepared. Ru particles or Ru@XO2 containing 1 g of Ru component, 100 mL of deionized water, 14.38 g of ZnSO4·7H2O and 50 mL of benzene were loaded into the Parr autoclave. Purged with H2 at ambient pressure three times, the reactor was sealed and heated. Once the temperature reached 160 °C, H2 at the pressure of 7.0 MPa was introduced into the autoclave. The reaction heat released was removed by coil cooling, by which the reaction temperature was controlled at the precision of ±1 °C. To promote interphase mass transfer, the stirring rate was kept at 1000 rpm during hydrogenation. When reaction time reached a preset value, the stirring was stopped. After cooling to ambient temperature, the reactor was opened and the upper organic phase containing products was sampled. The composition of the product was analyzed on a gas chromatograph GC9800 equipped with a PEG 20000 capillary column of 30 m × 0.32 mm × 0.25 μm. The conversion of benzene (XB), the selectivity of cyclohexene (SHe) and the yield of cyclohexene (YHe) were defined by

2. EXPERIMENTAL SECTION Preparation of Catalysts. Following the precipitation method suggested by Qu et al., Ru particles were prepared first.6 Under vigorous agitation and at the temperature of 25 °C, a 40 mL NaOH solution of 15 wt % was added into a 60 mL RuCl3 solution of 0.6 M. After the precipitation lasting 2 h, black Ru(OH)3 precipitates were collected by filtration. The precipitates collected were washed with deionized water until to pH of 7 and transferred into a Parr 5500 autoclave containing a 150 mL NaOH solution of 5 wt %. At the H2 partial pressure of 6.0 MPa, the stirring rate of 1000 rpm and the temperature of 150 °C, Ru(OH)3 was reduced in the autoclave for 6 h. The Ru particles obtained were washed with deionized water until to pH of 7, dried, and stored under N2 environment. The coating of TiO2, ZrO2, or SiO2 on Ru particles was based on the sol−gel method suggested by Nishikiori et al., Schäfer et al. and Nann et al., respectively.30−32 Tetrabutyl titanate (TBOT), zirconium n-butoxide (TOBZ) and ethyl silicate (TEOS) were used as the precursors of XO2. At the temperature of 25 °C, 1 g of Ru particles, 400 mL of ethanol and a certain amount of XO2 precursor were mixed first in a flask under vigorous agitation. The molar ratios of TBOT, TOBZ and TEOS to Ru are all fixed at 0.1. Then, 2 mL of nitric acid or ammonia, corresponding to the coating of different materials, was introduced. After 2 h of agitation, the Ru@XO2 prepared was collected, washed with ethanol, dried at 90 °C under a N2 environment, and stored under a N2 environment. Characterization of Catalysts. The crystal structure of Ru or Ru@XO2 catalysts was determined on an X-ray diffractometer (XRD, Bruker D8). The morphology of the catalysts was analyzed on a transmission electron microscope (HRTEM, JEM 2100). The surface functional groups of the catalysts were obtained on a Fourier transform infrared spectrometer (FT-IR, Fisher Nicolet 6700). The composition of the catalysts was determined on an inductively coupled plasma optical emission spectrometer (ICP-OES, Agilent 725ES). The electronic state of the catalysts was analyzed on an X-ray photoelectron spectrometer (XPS, ESCALAB 250Xi). The porosimetry data of the catalysts were obtained on a surface area and porosity analyzer (BET, Micromeritics ASAP 2460).

XB =

SHe =

YHe =

C benzene,0 − C benzene, t C benzene,0

Ccyclohexene, t C benzen,0 − C benzene, t

Ccyclohexene, t C benzene,0

100% (1)

100% (2)

100% (3)

where Cbenzene,0 is the initial benzene concentration. Cbenzene,t and Ccyclohexene,t are the concentrations of benzene and cyclohexene in the product at the sampling time of t.

3. RESULTS AND DISCUSSION 3.1. Crystal Structure and Morphology of Ru and Ru@ XO2. The composition of Ru@XO2 prepared was analyzed with ICP-OES first, with the results listed in Table 1. The weight fractions of TiO2, ZrO2 and SiO2 in Ru@XO2 vary narrowly from 2.2 to 2.6 wt %, indicating an extremely low loading of XO2 covering Ru particles. The XRD patterns of Ru particles and Ru@XO2 prepared are illustrated in Figure 1. For comparison purposes, the corresponding data of XO2 particles prepared with the same sol−gel method are also attached. Table 1. Composition of Ru@XO2 Prepared

Ru (wt %) XO2 (wt %) 1962

Ru@TiO2

Ru@ZrO2

Ru@SiO2

97.8 2.2

97.4 2.6

97.8 2.2

DOI: 10.1021/acs.iecr.7b04642 Ind. Eng. Chem. Res. 2018, 57, 1961−1967

Article

Industrial & Engineering Chemistry Research

particles or Ru@XO2. On the images of Ru@XO2, the irregular boundaries with low contrast could be ascribed to the amorphous oxides covering Ru particles. 3.2. Surface Properties of Ru and Ru@XO2. The FT-IR spectra of Ru particles and Ru@XO2 prepared are illustrated in Figure 3. Because of the hydrophilic nature of Ru surface, a

Figure 1. XRD patterns of Ru, XO2 and Ru@XO2.

Diffraction peaks at the 2θ values of 37.7, 43.4, 57.9, 68.1, and 78.0° are observed on the XRD spectrum of Ru particles, which means Ru particles prepared are in hexagonal crystal structure (JCPDS 88−1734).33 In the meantime, broad diffraction peaks centered at the 2θ values of 25, 31, and 23° appear on the XRD spectra of TiO2, ZrO2 and SiO2, respectively.34−37 Accordingly, these oxides prepared based on the sol−gel method are all in amorphous structure. Despite of the difference in coating materials, the XRD spectra of Ru@ XO2 prepared have no evident difference from that of Ru particles. Du et al. reported that on the XRD spectrum of graphene@ZrO2 the broad diffraction peak ascribed to amorphous ZrO2 coating film even at the thickness of 2 nm could be observed.38 Comparing the data presented in Figure 1, one may deduce that the hydrolysis of XO2 precursors and the subsequent decomposition of XO2 occur primarily on the surface of Ru particles. Moreover, the thickness of XO2 film covering Ru particles should be less than a few nanometers. The HRTEM images of Ru particles and Ru@XO2 are illustrated in Figure 2, and the magnified images are presented in Figures S1−S4. The Ru particles prepared with the precipitation method and the Ru@XO2 prepared further with the sol−gel method both are the aggregates of nanoparticles. One may identify clearly the presence of Ru particles by the lattice fringes of hexagonal crystal structure on the images of Ru

Figure 3. FT-IR spectra of Ru and Ru@XO2.

broad peak at the wavenumbers ranging from 3000 to 3600 cm−1 and a peak centered at the wavenumber of 1630 cm−1 appear on the FT-IR spectrum of Ru particles. These IR absorptions are related to the stretching and bending vibration of adsorbed water. After XO2 coating, the IR absorptions corresponding to adsorbed water are strengthened evidently. Meanwhile, the peaks ascribed to structural OH groups are detected. For Ru@TiO2, a peak attributed to the bending vibration of Ti−OH appears at 1040 cm−1.39−41 On the spectrum of Ru@ZrO2, the IR absorptions relating to the vibration of Zr−OH can be found at 1764, 1385, and 1055 cm−1.35,42 As for Ru@SiO2, the IR absorption at 984 cm−1 was suggested to result from the vibration of Si−OH.43,44 By the data presented in Figure 3, not only adsorbed water but also structural OH groups exist in XO2 film coated on Ru particles. Ru@XO2 prepared therefore should be highly hydrophilic. To determine the specific surface area of Ru@XO2, we performed N2 adsorption measurement at −195.85 °C. The adsorption−desorption isotherms measured are illustrated in Figure 4. Ru@TiO2, Ru@ZrO2 and Ru@SiO2 are found to all show type IV isotherms with the hysteresis type of H3 defined by IUPAC, which suggests the presence of mesopores. The average pore volume and pore size of Ru@XO2 then were determined using the Barrett−Joyner−Halenda (BJH) method, with the results listed in Table 2. By the average pore diameter and pore volume varying around 10 nm and 0.12 g/cm3, Ru@ TiO2, Ru@ZrO2 and Ru@SiO2 prepared should share a similar mesoporous structure. It was reported that the deposition of Ru nanoparticles on tetragonal TiO2 or tetragonal ZrO2 has no influence on the electronic structure of Ru.45−47 To verify whether the coating of amorphous XO2 on Ru particles might lead to the formation of Ruδ+, which is characterized by Ru 3p1/2 peak with a binding energy of 485.5 eV, we applied XPS analysis of Ru particles and Ru@XO2, with the results illustrated in Figures S5−S8.

Figure 2. HRTEM Images of Ru and Ru@XO2. 1963


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7.0 MPa and ZnSO4 concentration of 0.5 M in the water phase, the partial hydrogenation of benzene on Ru particles and Ru@ XO2 was evaluated, with the results illustrated in Figure 5.

Figure 4. N2 adsorption−desorption isotherms of Ru@XO2.

Table 2. Textural Properties of Ru and Ru@XO2 Catalysts catalysts

SBET (m2/g)

Vpore (cm3/g)

dpore (nm)

Ru particles Ru@TiO2 Ru@ZrO2 Ru@SiO2

40 53 59 44

0.13 0.12 0.13 0.12

11.9 8.6 8.7 11.2

Figure 5. Yields of cyclohexene and benzene during partial hydrogenation of benzene on Ru or Ru@XO2.

The product distribution of the partial hydrogenation on Ru particles fully reflects the characteristics of tandem reactions. The concentration of benzene in the product decreases along with the extension of reaction time, approaching the equilibrium value of 3.0 mol % eventually. As for cyclohexene, its concentration in the product increases first and decreases then. A maximum cyclohexene yield of 28.6 mol % is obtained at the reaction time of 10 min. At any sampling time, the yield of cyclohexane is always higher than that of cyclohexene, which means the apparent rate constant of the hydrogenation of cyclohexene to cyclohexane is larger than that of the partial hydrogenation of benzene to cyclohexene. Compared with the reaction on Ru particles, a decreasing rate of benzene conversion but an increasing cyclohexene yield occurred simultaneously during the partial hydrogenation on different Ru@XO2. At the reaction time of 15 min, the maximum cyclohexene yields of 52.6, 48.7, and 43.5 mol % were observed on Ru@TiO2, Ru@ZrO2, and Ru@SiO2, respectively. A calculation on cyclohexene selectivity vs benzene conversion then was performed, and the results are illustrated in Figure 6. The cyclohexene selectivity on Ru particles decreases monotonically with the increase in benzene conversion, declining drastically at benzene conversion approaching equilibrium. Such a tendency keeps unchanged on Ru@XO2. Nevertheless, a significant difference in cyclohexene selectivity between the reactions on Ru particles and Ru@XO2 is readily established even at the beginning of partial hydrogenation. Take the reaction on Ru@TiO2 as an example. At the benzene conversion lower than 85.0%, the cyclohexene selectivity on Ru@TiO2 is steadily higher than that on Ru particles by at least 30%. The tendency presented in Figures 5 and 6 can also be observed in the partial hydrogenation run under Asahi reaction conditions, that is, temperature of 150 °C and H2 partial pressure of 6.0 MPa. The corresponding results are listed in Table S2. 3.4. Mechanism of Partial Hydrogenation on Ru Particles. For the tetra-phase partial hydrogenation of benzene on Ru particles, the production of cyclohexene is accomplished

On the XPS spectra of Ru@XO2, the presence of TiO2, ZrO2 or SiO2 in Ru@XO2 is supposed to be identified by the characteristic binding energies listed in Table S1. However, on the XPS spectrum of Ru@TiO2 the peak of Ti 2p overlaps with that of Ru 3p3/2. Ru 3p3/2 is 4-fold degenerate, while Ru 3p1/ 2 is 2-fold degenerate. On the XPS spectrum of Ru particles, the calculated ratio of Ru 3p3/2 peak area to Ru 3p1/2 peak area is 2.1, approaching the theoretical value of 2.0. The corresponding ratio increases to 2.8 on the XPS spectrum of Ru@TiO2, suggesting that Ti 2p contributes partly to the overlapping peak at the binding energy around 461.5 eV. On the XPS spectrum of Ru@ZrO2, the peak at the binding energy of 183.0 eV can be ascribed to Zr 3d. On the XPS spectrum of Ru@SiO2, the peaks at the binding energies of 153.8 and 102.8 eV are related to Si 2s and Si 2p. Ru 3d peaks overlap with the C 1s peak of contaminant carbon, so only Ru 3p peaks were employed to determine the chemical state of Ru. Since Ti 2p peaks overlap partly with Ru 3p3/2 peaks, the spectra of Ru@TiO2 were deconvoluted to resolve their respective contributions. The measured binding energies of Ru 3p3/2 and Ru 3p1/2 in Ru@XO2 are listed in Table 3. The binding energy of Ru 3p3/2 ranging from 461.9 to 462.1 eV and the doublet separation of 3p3/2 to 3p1/2 around 22.2 eV explicitly indicate the metallic state of Ru in Ru@XO2. 3.3. Performance of Partial Hydrogenation on Ru and Ru@XO2. At the temperature of 160 °C, H2 partial pressure of Table 3. Binding Energies of Ru 3p3/2 and Ru 3p1/2 in Ru@XO2 catalysts

Ru 3p3/2 (eV)a

Ru 3p1/2 (eV)b

Ru@TiO2 Ru@ZrO2 Ru@SiO2

462.1 461.9 461.9

484.3 484.2 484.2

a

Reference value of 461.1−462.2 eV.48−51 bReference value of 484.3 eV.49 1964


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hexene through hydrophilic mesopores and the surface hydrogenation of these species on Ru particles.

Figure 6. Cyclohexene selectivity vs benzene conversion during partial hydrogenation of benzene on Ru or Ru@XO2.

sequentially by the dissolution of benzene from the organic phase into the water phase, adsorption of benzene onto Ru particles and surface hydrogenation of benzene to cyclohexene. Cyclohexene formed may either return to the water phase through desorption or be converted further to cyclohexane through in situ hydrogenation. A solubility calculation based on the NRTL activity coefficient model suggests that, at the operating condition applied in this work, the equilibrium fraction of benzene dissolved in the water phase decreases linearly from its initial value of 4.96 × 10−3 to 1.04 × 10−3 at the benzene conversion of 80%. Benzene dissolved in the water phase then can be readily adsorbed onto Ru particles in hcp or fcc configuration with an adsorption heat around −139 kJ/mol.33 By the data presented in Figure 5, Ru particles prepared are highly active for partial hydrogenation since an equilibrium benzene conversion of 97% is obtained only in 15 min. It should be noted that the solubility of cyclohexene in the water phase is much lower than that of benzene. In other words, the desorption of cyclohexene from Ru surface into the water phase should be relatively difficult. At the early partial hydrogenation stage with a higher benzene concentration in the water phase, the desorption of cyclohexene formed therefore becomes the rate-determining step of the whole process of partial hydrogenation, resulting in the accumulation of cyclohexene on catalyst surface. At that time, the undesired in situ direct hydrogenation of cyclohexene to cyclohexane occurs inevitably. As the reaction goes on, the fraction of benzene dissolved in the water phase decreases but that of cyclohexene increases. With the reducing competitive adsorption of benzene, cyclohexene will be readsorbed onto Ru particles with growing priority, responsible for the decreasing cyclohexene selectivity at the middle partial hydrogenation stage. 3.5. Influence of Hydrophilic Material Coating on Partial Hydrogenation. By the tetra-phase partial hydrogenation data presented in Figure 5, cyclohexene yield can be significantly improved by coating hydrophilic materials on Ru particles. The similarities of these coating films covering Ru particles are as follows: a thickness less than a few nanometers, mesoporous structure and perfect hydrophilicity. Figure 7 illustrates schematically the diffusion of benzene and cyclo-

Figure 7. Schematic illustration on diffusion in mesopores and surface hydrogenation on Ru@XO2.

Some studies mentioned the hydrogen bonding between cyclohexene and water; however, no convincing evidence was reported in literatures. By contrast, quantum mechanism based calculation, molecular dynamics simulation and magnetic resonance analysis all have confirmed that the delocalized π electrons of benzene may interact with the electron deficient H atom in OH groups.52−58 During the diffusion of benzene through XO2 coating film, unconventional hydrogen bonding between benzene and X−OH groups contained in hydrophilic materials appears. It was reported that the diffusion of acetonitrile and pyridine in hydrophilic mesopores could be evidently retarded by the hydrogen bonding with structural OH groups.59,60 Apart from the uniform decrease in diffusivity of benzene and cyclohexene in mesoporous framework, the diffusion of benzene will be further selectively retarded by hydrogen bonding. There is a good chance that, at the early partial hydrogenation stage, the rate-determining step of partial hydrogenation transits from cyclohexene desorption to benzene diffusion through hydrophilic coating film. With the decreased rate of benzene conversion, a delicate balance between cyclohexene formation and cyclohexene desorption on catalyst surface could be established. Benefiting from the reduced surface concentration of cyclohexene on Ru particles, a satisfactory cyclohexene selectivity is guaranteed. After the massive conversion of benzene, the readsorption of cyclohexene onto catalyst surface becomes vital to cyclohexene selectivity. At the middle partial hydrogenation stage, the readsorption of cyclohexene dissolved in the water phase onto catalyst surface is effectively prevented by the presence of coating film. Based on the data presented in Figure 6, even at the benzene conversion of 80%, still the cyclohexene selectivities on Ru@TiO2, Ru@ ZrO2 and Ru@SiO2 are all higher than 50%. The ultimate performance of the partial hydrogenation on Ru@XO2 is determined by the interaction between the diffusion behavior of reactants through coating film and the reaction kinetics of surface hydrogenation. In this work, a maximum cyclohexene yield of 52.6 mol % is obtained on Ru@ 1965


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(3) Yamashita, K.; Obana, H.; Katsuta, I. Method for partially hydrogenating a monocyclic aromatic hydrocarbon. Patent EP552809 (A1), 1993. (4) Odenbrand, C. U. I.; Lundin, S. T. Hydrogenation of benzene to cyclohexene on a ruthenium catalyst: Influence of some reaction parameters. J. Chem. Technol. Biotechnol. 1980, 30, 677. (5) Johnson, M. M.; Nowack, G. P. Cyclic olefins by selective hydrogenation of aromatics. J. Catal. 1975, 38, 518. (6) Qu, Y. X.; Fang, C. X.; Qian, C. Y.; He, Q. L.; Wang, J. D. The influence of transition metal ions on the catalytic performance of Ru particles during the liquid phase hydrogenation of benzene to cyclohexene. React. Kinet., Mech. Catal. 2014, 111, 647. (7) Struijk, J.; Moene, R.; van der Kamp, T.; Scholten, J. J. F. Partial liquid-phase hydrogenation of benzene to cyclohexene over ruthenium catalysts in the presence of an aqueous salt solution: II. Influence of various salts on the performance of the catalyst. Appl. Catal., A 1992, 89, 77. (8) Liu, Z. Y.; Sun, H. J.; Wang, D. B.; Liu, S. C.; Li, Z. J. The modifiable character of a novel Ru-Fe-B/ZrO2 catalyst for benzene selective hydrogenation to cyclohexene. Chin. J. Chem. 2010, 28, 1927. (9) Struijk, J.; Scholten, J. J. F. Selectivity to cyclohexenes in the liquid phase hydrogenation of benzene and toluene over ruthenium catalysts, as influenced by reaction modifiers. Appl. Catal., A 1992, 82, 277. (10) Wang, J. Q.; Guo, P. J.; Qiao, M. H.; Fan, K. N. The effect of modifiers on benzene selective hydrogenation over Ru/ZrO2·xH2O,. Fudan Univ. Nat. Sci. 2004, 43, 610. (11) Spinacé, E. V.; Vaz, J. M. Liquid-phase hydrogenation of benzene to cyclohexene catalyzed by Ru/SiO2 in the presence of water-organic mixtures. Catal. Commun. 2003, 4, 91. (12) Xie, S. H.; Qiao, M. H.; Li, H. X.; Wang, W. J.; Deng, J. F. A novel Ru-B/SiO2 amorphous catalyst used in benzene-selective hydrogenation. Appl. Catal., A 1999, 176, 129. (13) Liu, Z.; Xie, S. H.; Liu, B.; Deng, J. F. Benzene-selective hydrogenation to cyclohexene over supported ruthenium boride catalysts prepared by a novel method. New J. Chem. 1999, 23, 1057. (14) Wang, J. Q.; Guo, P. J.; Yan, S. R.; Qiao, M. H.; Li, H. X.; Fan, K. N. Colloidal RuB/Al2O3·xH2O catalyst for liquid phase hydrogenation of benzene to cyclohexene. J. Mol. Catal. A: Chem. 2004, 222, 229. (15) Liu, Z.; Dai, W. L.; Liu, B.; Deng, J. F. The effect of boron on selective benzene hydrogenation to cyclohexene over ruthenium boride powders. J. Catal. 1999, 187, 253. (16) Ronchin, L.; Toniolo, L. Supported Ru catalysts: A study of the influence of supports, promoters and preparative variables on the catalytic activity and selectivity. React. Kinet. Catal. Lett. 2003, 78, 281. (17) Ronchin, L.; Toniolo, L. Selective hydrogenation of benzene to cyclohexene catalyzed by Ru supported catalysts-Influence of the alkali promoters on kinetics, selectivity and yield. Catal. Today 2001, 66, 363. (18) Suryawanshi, P. T.; Mahajani, V. V. Liquid-phase hydrogenation of benzene to cyclohexene using ruthenium-based heterogeneous catalyst. J. Chem. Technol. Biotechnol. 1997, 69, 154. (19) Liu, H. Z.; Liang, S. G.; Wang, W. T.; Jiang, T.; Han, B. X. The partial hydrogenation of benzene to cyclohexene over Ru-Cu catalyst supported on ZnO. J. Mol. Catal. A: Chem. 2011, 341, 35. (20) Schwab, F.; Lucas, M.; Claus, P. Simple selective hydrogenation of benzene to cyclohexene in the presence of sodium dicyanamide. Green Chem. 2013, 15, 646. (21) Mazzieri, V. A.; L’Argentiere, P. C.; Coloma-Pascual, F.; Figoli, N. S. Effect of chlorine on the properties of Ru/Al2O3. Ind. Eng. Chem. Res. 2003, 42, 2269. (22) Mazzieri, V.; Coloma-Pascual, F.; Arcoya, A.; L’Argentiere, P.; Figoli, N. S. XPS, FTIR and TPR characterization of Ru/Al2O3 catalysts. Appl. Surf. Sci. 2003, 210, 222. (23) Mazzieri, V.; Figoli, N.; Pascual, F. C.; L’Argentiere, P. Effect of the support on the selective hydrogenation of benzene over ruthenium catalysts. 1. Al2O3 and SiO2. Catal. Lett. 2005, 102, 79.

TiO2 at the benzene conversion of 85.0%. With the further optimization on the coating film covering Ru particles and operating conditions, hopefully an even better partial hydrogenation performance could be achieved.

4. CONCLUSIONS Amorphous TiO2, ZrO2 and SiO2 were coated on Ru particles, by which Ru@XO2 (X = Ti, Zr or Si) catalysts with the XO2 loading from 2.2 to 2.6 wt % were obtained. The Ru@XO2 catalysts prepared share the similarities of mesoporous structure and perfect hydrophilicity. During the tetra-phase partial hydrogenation of benzene on Ru@XO2, the diffusion of benzene and the diffusion of cyclohexene from the water phase onto Ru particles both are retarded by coating film. The former is further selectively suppressed by the H bonding with the structural OH groups contained in hydrophilic materials. The presence of coating film makes it possible that the ratedetermining step of partial hydrogenation at the early reaction stage is transferred from the desorption of cyclohexene from catalyst surface to the diffusion of benzene through coating film. Furthermore, the readsorption of cyclohexene from the water phase onto Ru particles is restricted at the middle reaction stage. With the reduced surface concentration of cyclohexene and the suppressed readsorption of cyclohexene, a consistently increasing cyclohexene yield up to 52.6 mol % could be observed on Ru@XO2 prepared.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b04642. Figure S1, HRTEM image of Ru particles; Figure S2, HRTEM image of Ru@TiO2; Figure S3, HRTEM image of Ru@ZrO2; Figure S4, HRTEM image of Ru@SiO2; Figure S5, XPS spectrum of Ru particles; Figure S6, XPS spectrum of Ru@TiO2; Figure S7, XPS spectrum of Ru@ ZrO2; Figure S8, XPS spectrum of Ru@SiO2; Table S1, characteristic binding energies of Ti, Zr, and Si elements in corresponding oxides; Table S2, performance of Ru@ XO2 (X = Ti, Zr, or Si) under Asahi conditions (temperature of 150 °C and H2 partial pressure of 6.0 MPa) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: + 862164253529. Fax: + 862164253528 (P.Q.Y.). ORCID

Pei-Qing Yuan: 0000-0003-0797-963X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21376075 and 21676084). REFERENCES

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Partial Hydrogenation of Benzene to Cyclohexene on Ru@XO2

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