Selective Hydrogenation of Benzene to Cyclohexene Over Ru-Zn Catalysts: Investigations on the Effect of Zn Content and Zro2 As the Support and Dispersant

catalysts

Article Selective Hydrogenation of Benzene to Cyclohexene over Ru-Zn Catalysts: Investigations on the Effect of Zn Content and ZrO2 as the Support and Dispersant

Haijie Sun 1 , Zhihao Chen 2,* , Lingxia Chen 1, Huiji Li 1, Zhikun Peng 3,*, Zhongyi Liu 3 and Shouchang Liu 3

1 Institute of Environmental and Catalytic Engineering, College of Chemistry and Chemical Engineering, Zhengzhou Normal University, Zhengzhou 450044, Henan, China; [email protected] (H.S.); [email protected] (L.C.); [email protected] (H.L.) 2 Zhengzhou Tobacco Research Institute of CNTC, Zhengzhou 450001, Henan, China 3 College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, Henan, China; [email protected] (Z.L.); [email protected] (S.L.) * Correspondence: [email protected] (Z.C.); [email protected] (Z.P.); Tel.: +86-371-6767-2762 (Z.C.); +86-158-3810-9080 (Z.P.) Received: 15 October 2018; Accepted: 29 October 2018; Published: 2 November 2018

Abstract: m-ZrO2 (monoclinic phase) supported Ru-Zn catalysts and unsupported Ru-Zn catalysts were synthesized via the impregnation method and co-precipitation method, respectively. The catalytic activity and selectivity were evaluated for selective hydrogenation of benzene towards cyclohexene formation. Catalyst samples before and after catalytic experiments were thoroughly characterized via X-ray diffraction (XRD), X-ray Fluorescence (XRF), transmission electron microscopy (TEM), N2-sorption, X-ray photoelectron spectroscopy (XPS), H2-temperature programmed reduction (H2-TPR), and a contact angle meter. It was found that Zn mainly existed as ZnO, and its content was increased in Ru-Zn/m-ZrO2 by enhancing the Zn content during the preparation procedure. This results in the amount of formed (Zn(OH)2)3(ZnSO4)(H2O)3 increasing and the catalyst becoming more hydrophilic. Therefore, Ru-Zn/m-ZrO2 with adsorbed benzene would easily move from the oil phase into the aqueous phase, in which the synthesis of cyclohexene took place. The generated cyclohexene then went back into the oil phase, and the further hydrogenation of cyclohexene would be retarded because of the high hydrophilicity of Ru-Zn/m-ZrO2. Hence, the selectivity towards cyclohexene formation over Ru-Zn/m-ZrO2 improved by increasing the Zn content. When the theoretical molar ratio of Zn to Ru was 0.60, the highest cyclohexene yield of 60.9% was obtained over Ru-Zn (0.60)/m-ZrO2. On the other hand, when m-ZrO2 was utilized as the dispersant (i.e., employed as an additive during the reaction), the catalytic activity and selectivity towards cyclohexene synthesis over the unsupported Ru-Zn catalyst was lower than that achieved over the Ru-Zn catalyst with m-ZrO2 as the support. This is mainly because the supported catalyst sample demonstrated superior dispersion of Ru, higher content of (Zn(OH)2)3(ZnSO4)(H2O)3, and a stronger electronic effect between Ru and ZrO2. The Ru-Zn(0.60)/m-ZrO2 was reused 17 times without any regeneration, and no loss of catalytic activity and selectivity towards cyclohexene formation was observed.

Keywords: selective hydrogenation; benzene; cyclohexene; Ru; Zn; ZrO2

1. Introduction Selective hydrogenation of benzene towards cyclohexene synthesis has been a signiﬁcant reaction in the ﬁeld of catalysis research [1–5]. This is mainly attributed to the fact that the production of

Catalysts 2018, 8, 513; doi:10.3390/catal8110513 www.mdpi.com/journal/catalysts Catalysts 2018, 8, 513 2 of 17 caprolactam and adipic acid via cyclohexene is environmentally friendlier, more energy preservation, and might result in higher carbon atom economy in comparison to that via cyclohexane [6,7]. The first industrial plant for production of cyclohexene from selective hydrogenation of benzene over unsupported Ru-Zn catalyst was manufactured by Asahi in 1989 [8]. However, some drawbacks for the catalyst, such as high Ru content and the ease of being poisoned, are of great difficulty to overcome. Therefore, the development of supported Ru catalysts with relatively low Ru loading and high Ru dispersion have drawn great interest. Commonly, ZrO2 is selected as a proper support. For instance, Zhou et al. [9] prepared Ru-Zn/ZrO2 catalyst via the deposition-precipitation method, from which a 54% cyclohexene yield was obtained. Furthermore, Ru-Zn/ZrO2 catalyst was prepared via a two-step impregnation method by Yan et al. [1], which gave a 48.5% cyclohexene yield. Moreover, Peng et al. [10] applied a chemical reduction method to synthesize a Ru/m-ZrO2/t-ZrO2 catalyst, from which a 55.3% cyclohexene yield was achieved. Liu et al. [11] also used the chemical reduction method to prepare a Ru-La-B/ZrO2 catalyst, and a 53.2% cyclohexene yield was shown. Other than ZrO2, zeolite (i.e., SBA-15) [12] and γ-Al2O3 [13] were also reported as the catalyst support for selective hydrogenation of benzene over Ru-based catalysts. However, how supports affect the catalytic activity and selectivity has been rarely addressed. A promoter is one of the most effective ways to improve the cyclohexene yield over the Ru-based catalytic system. Zn [14,15], Fe [16,17], Co [18], Mn [19], La [20,21], and Ce [22,23] have been investigated as promoters for Ru catalysts on selective hydrogenation of benzene. Zn has been widely studied and reported among all promoters due to its superior promotion performance [15]. However, the status of Zn (e.g., valence of Zn) in supported Ru-based catalysts has been controversial. Zhou et al. [24] and Wang et al. [25] deemed that in Ru-Zn/ZrO2, Zn existed as metallic Zn covering some of the Ru active sites, with lower selectivity towards cyclohexene formation. They also suggested that the Zn@Ru system could be achieved by doping metallic Zn into Ru, which spontaneously modified the geometric and electronic structure of Ru, and thus improved the catalytic activity and selectivity towards cyclohexene formation. On the other hand, Zhou et al. [9] and Yan et al. [1] demonstrated that Zn existed as ZnO in Ru-Zn/ZrO2. However, the mechanism of ZnO affecting the formation of cyclohexene was not clearly explained. Thus, it is of great significance to investigate the status of Zn in Ru-Zn/ZrO2 and how it affects the catalytic system, which could provide essential guidance for the development of the supported Ru-based catalysts. Based on the previous work, we prepared Ru-Zn/m-ZrO2 catalysts with different Zn content via the impregnation method. In order to reveal the status of Zn in Ru-Zn/ZrO2 and how it affects the catalytic system, all samples were evaluated for selective hydrogenation of benzene towards cyclohexene formation. In addition, unsupported Ru-Zn catalyst with the same content of Ru and Zn was synthesized via the co-precipitation method, which was tested under the same experimental conditions by adding ZrO2 as a dispersant. In comparison to that observed over Ru-Zn/ZrO2, the support effect of ZrO2 was proposed for the catalytic performance of Ru-Zn on the selective hydrogenation of benzene towards cyclohexene generation.

2. Results

2.1. Effect of Zn Content

XRD patterns of Ru-Zn(x)/m-ZrO2 before (a) and after hydrogenation (b) are given in Figure1. In Figure1a, ZrO 2 of the monoclinic phase as well as metallic Ru can be observed for all fresh catalyst samples, indicating that ZrO2 exists as the monoclinic phase and Ru is completely reduced. Notably, when the theoretical molar ratio of Zn to Ru reaches 0.60, characteristic diffraction of ZnO starts to be shown, suggesting that Zn mainly exists as ZnO in Ru-Zn(x)/m-ZrO2. This is consistent with that reported by Zhou et al. [9] and Yan et al. [1]. As long as the molar ratio of Zn to Ru is no higher than 0.47, ZnO reﬂections cannot be detected. This can be rationalized in terms that the amount of ZnO is relatively low, and might not be able to aggregate into a phase. After hydrogenation (Figure1b) on the Catalysts 2018, 8, 513 3 of 17

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other hand, instead of ZnO, reﬂections related to (Zn(OH)2)3(ZnSO4)(H2O)3 can be detected when the molar ratio of Zn to Ru is higher than 0.6. This indicates that ZnO could react with ZnSO4 during the molar ratio of Zn to Ru is higher than 0.6. This indicates that ZnO could react with ZnSO4 during 2 3 4 2 3 2 3 4 2 3 the thereaction reaction to toform form the the (Zn(OH)2))3(ZnSO4)(H2O)O)3 salt. Similarly,Similarly, nono (Zn(OH) (Zn(OH)2)3(ZnSO) (ZnSO4)(H)(H2O)O)3 diffractiondiffraction can can be beobserved observed when when the the molar molar ratio ratio of of Zn Zn to to Ru Ru is is less less than 0.47,0.47, andand thethe same same reason reason couldcould be applied be applied here. here.

(a) (b)

FigureFigure 1. 1.XRDXRD patterns patterns of of Ru-Zn(x)/ZrO Ru-Zn(x)/ZrO2 before2 before (a ()a )and and after after hydrogenation hydrogenation ((bb).).

The composition and texture properties of Ru-Zn(x)/m-ZrO catalysts before and after catalytic The composition and texture properties of Ru-Zn(x)/m-ZrO22 catalysts before and after catalytic experiments,experiments, as well as well as aspH pH values values of of the the slurry slurry after after hydrogenation, hydrogenation, are are listed inin TableTable1 .1. As As can can be be seen,seen, for forthe thefresh fresh catalysts, catalysts, the the specific speciﬁc surface surface ar area,ea, pore pore diameter, diameter, and pore sizesize slightlyslightly decreased decreased with increasing Zn content. This implies that some of the macro pores of m-ZrO2 were blocked by with increasing Zn content. This implies that some of the macro pores of m-ZrO2 were blocked by the the impregnated Ru and ZnO. However, with enhancing the Zn content, pore diameter increased, impregnated Ru and ZnO. However, with enhancing the Zn content, pore diameter increased, while while the speciﬁc surface area and pore volume dropped after the reaction. This might be due to the specific surface area and pore volume dropped after the reaction. This might be due to the fact the fact that (Zn(OH)2)3(ZnSO4)(H2O)3 was generated during the reaction, which covered some of that (Zn(OH)2)3(ZnSO4)(H2O)3 was generated during the reaction, which covered some of the micro the micro pores of m-ZrO2. On the other hand, it is noticed that the molar ratio of Zn to Ru over pores of m-ZrO2. On the other hand, it is noticed that the molar ratio of Zn to Ru over Ru-Zn (x)/m- Ru-Zn (x)/m-ZrO2 after catalytic experiments is higher than that obtained over the fresh catalysts, ZrO2 after catalytic2+ experiments is higher than that obtained over the fresh catalysts, implying that implying that Zn from ZnSO4 or (Zn(OH)2)3(ZnSO4)(H2O)3 was chemisorbed on the catalyst surface. Zn2+ from ZnSO4 or (Zn(OH)2)3(ZnSO4)(H2O)3 was chemisorbed on the catalyst surface. Additionally, Additionally, no obvious variation was noticed for n (Zr)/n (Ru), indicating that m-ZrO2 was barely no obviouslost during variation the catalytic was noticed experiments. for n (Zr)/ Moreover,n (Ru), theindicating pH value that of m-ZrO the slurry2 was after barely the reaction lost during is less the catalyticthan 6,experiments. suggesting thatMoreover, the slurry the pH is acidic. value of This the isslurry mainly after attributed the reaction to the is less hydrolysis than 6, suggesting of ZnSO4. thatMore the slurry importantly, is acidic. the This pHvalue is mainly of the attributed slurry after to the the reaction hydrolysis increased of ZnSO with4. increasingMore importantly, Zn content, the pH demonstratingvalue of the slurry that after the hydrolysis the reaction of increased ZnSO4 is retarded.with increasing This can Zn be content, rationalized demonstrating in terms that that the the hydrolysisformation of of ZnSO (Zn(OH)4 is2) 3(ZnSOretarded.4)(H 2ThisO)3 led can to thebe decreaserationalized in the in concentration terms that of ZnSOthe formation4. of (Zn(OH)TEM2)3(ZnSO images4)(H and2O)3 the led Ru to particlethe decrease size distribution in the concentration of Ru-Zn (0.60)/m-ZrO of ZnSO4. 2 catalysts before (a,b) and after (c,d) catalytic experiments are shown in Figure2. It can be observed from Figure2a,b thatTable Ru is1. uniformlyComposition dispersed and texture on m-ZrO properties2, and theof Ru-Zn(x)/m-ZrO particle size of2 Ru catalysts is around before 4.5 nm.and Afterafter the hydrogenationhydrogenation, reaction, as well as the pH particle values sizeof the of slurry Ru remained after catalytic at 4.5 experiments. nm, indicating that no aggregation of Ru happened during the reaction. Catalyst SBET/(m2·g−1) 1 Vpore/(cm3·g−1) 1 dpore/(nm) 1 nZn/nRu 2 nZr/nRu 2 pH 3 ZrO2 34 0.13 16.0 - - - Ru(0)/ZrO2 31 0.10 10.7 0 5.19 - Ru-Zn(0.06)/ZrO2 30 0.09 9.8 0.06 5.30 - Ru-Zn(0.33)/ZrO2 30 0.09 9.7 0.33 5.26 - Ru-Zn(0.47)/ZrO2 30 0.10 9.6 0.47 5.24 - Ru-Zn(0.60)/ZrO2 30 0.09 9.7 0.60 5.38 - Ru-Zn(0.69)/ZrO2 28 0.08 9.6 0.69 5.26 - Ru-Zn(0.86)/ZrO2 28 0.08 9.1 0.86 5.25 - Ru-Zn(1.02)/ZrO2 29 0.08 9.1 1.02 5.16 - Ru(0)/ZrO2 AH 30 0.11 9.5 0.20 5.21 4.25 Ru-Zn(0.06)/ZrO2 AH 30 0.10 9.8 0.25 5.29 4.25 Ru-Zn(0.33)/ZrO2 AH 30 0.09 9.9 0.37 5.38 5.38

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Table 1. Composition and texture properties of Ru-Zn(x)/m-ZrO2 catalysts before and after hydrogenation, as well as pH values of the slurry after catalytic experiments.

2 −1 1 3 −1 1 1 2 2 3 Catalyst SBET/(m ·g ) Vpore/(cm ·g ) dpore/(nm) nZn/nRu nZr/nRu pH

Catalysts 2018ZrO, 82, x FOR PEER REVIEW34 0.13 16.0 - -4 -of 17 Ru(0)/ZrO2 31 0.10 10.7 0 5.19 - Ru-Zn(0.06)/ZrO2 30 0.09 9.8 0.06 5.30 - Ru-Zn(0.47)/ZrO2 AH 30 0.08 9.8 0.55 5.36 5.38 Ru-Zn(0.33)/ZrO2 30 0.09 9.7 0.33 5.26 - Ru-Zn(0.60)/ZrORu-Zn(0.47)/ZrO2 2 AH 30 30 0.10 0.08 9.6 10.1 0.47 0.68 5.245.32 5.38 - Ru-Zn(0.69)/ZrORu-Zn(0.60)/ZrO2 2 AH 30 28 0.09 0.07 9.7 10.3 0.60 0.86 5.385.30 5.44 - Ru-Zn(0.69)/ZrO2 28 0.08 9.6 0.69 5.26 - Ru-Zn(0.86)/ZrO2 AH 29 0.08 10.6 1.10 5.26 5.82 Ru-Zn(0.86)/ZrO2 28 0.08 9.1 0.86 5.25 - Ru-Zn(1.02)/ZrORu-Zn(1.02)/ZrO2 2 AH 29 29 0.08 0.06 9.1 11.4 1.02 1.25 5.165.31 5.73 - Ru(0)/ZrO2 AH 30 0.11 9.5 0.20 5.21 4.25 1 Determined by N2-sorption; 2 Determined by XRF; 3 Determined by a pH meter at room temperature; Ru-Zn(0.06)/ZrO2 AH 30 0.10 9.8 0.25 5.29 4.25 Ru-Zn(0.33)/ZrOAH: after hydrogenation.2 AH 30 0.09 9.9 0.37 5.38 5.38 Ru-Zn(0.47)/ZrO2 AH 30 0.08 9.8 0.55 5.36 5.38 Ru-Zn(0.60)/ZrO AH 30 0.08 10.1 0.68 5.32 5.38 TEM images2 and the Ru particle size distribution of Ru-Zn (0.60)/m-ZrO2 catalysts before (a,b) Ru-Zn(0.69)/ZrO AH 28 0.07 10.3 0.86 5.30 5.44 and after (c,d) catalytic2 experiments are shown in Figure 2. It can be observed from Figure 2a,b that Ru-Zn(0.86)/ZrO2 AH 29 0.08 10.6 1.10 5.26 5.82 RuRu-Zn(1.02)/ZrO is uniformly2 AHdispersed on 29 m-ZrO2, and0.06 the particle size 11.4 of Ru is 1.25around 4.5 5.31nm. After 5.73 the 1 2 3 hydrogenationDetermined reaction, by N2-sorption; the particleDetermined size of by Ru XRF; remaDeterminedined at 4.5 by nm, a pH indicating meter at room that temperature; no aggregation AH: of Ru happenedafter hydrogenation. during the reaction.

Figure 2.2. TEMTEM imagesimages andand thethe RuRu particleparticle sizesize distributiondistribution ofof Ru-Zn Ru-Zn (0.60)/m-ZrO (0.60)/m-ZrO22 catalysts before ((a,,b)) andand afterafter ((cc,,d)) catalyticcatalytic experiments.experiments.

X-ray photoelectron spectroscopy (XPS) profiles of the Ru-Zn (0.33)/m-ZrO2 catalyst and Ru-Zn (0.60)/m-ZrO2 catalyst after catalytic experiments are presented in Figure 3. It can be observed from Figure 3a that the peak of Ru3d includes Ru3d3/2 and Ru3d5/2, and the former one is partly overlapped with the peak of C1s. Therefore, Ru3d5/2 is selected for further discussion. There are two peaks for Ru3d5/2 with binding energy (BE) of 280.3 eV and 281.4 eV, which are attributed to Ru0 and Ruδ+, respectively [26]. The presence of Ruδ+ implies that some Ru lost electrons. Furthermore, it is found that n (Ruδ+)/n (Ru0) over Ru-Zn (0.60)/m-ZrO2 after hydrogenation (AH) is slightly higher than that

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X-ray photoelectron spectroscopy (XPS) proﬁles of the Ru-Zn (0.33)/m-ZrO2 catalyst and Ru-Zn (0.60)/m-ZrO2 catalyst after catalytic experiments are presented in Figure3. It can be observed from Figure3a that the peak of Ru3d includes Ru3d 3/2 and Ru3d5/2, and the former one is partly overlapped with the peak of C1s. Therefore, Ru3d5/2 is selected for further discussion. There are two peaks for 0 δ+ Ru3d5/2 with binding energy (BE) of 280.3 eV and 281.4 eV, which are attributed to Ru and Ru , respectively [26]. The presence of Ruδ+ implies that some Ru lost electrons. Furthermore, it is found δ+ 0 that n (Ru )/n (Ru ) over Ru-Zn (0.60)/m-ZrO2 after hydrogenation (AH) is slightly higher than that obtained over Ru-Zn (0.33)/m-ZrO AH (i.e., 1.18 vs. 1.09), indicating that more electrons were Catalysts 2018, 8, x FOR PEER REVIEW 2 5 of 17 transferred out of Ru with the enhancement of Zn content. Moreover, as can be seen from Figure3b, the BEobtained of Zn2p over3/2 Ru-Znover Ru-Zn(0.33)/m-ZrO (0.33)/m-ZrO2 AH (i.e.,2 AH1.18 andvs. 1.09), Ru-Zn indicating (0.60)/m-ZrO that more2 AH electrons is 1022.0 were eV and 1021.1transferred eV, respectively. out of Ru However,with the enhancement it is very difﬁcult of Zn co tontent. justify Moreover, whether as can it is be metallic seen from Zn Figure or Zn 3b,2+, since the BE of Zn2p3/2 over Ru-Zn2+ (0.33)/m-ZrO2 AH and Ru-Zn (0.60)/m-ZrO2 AH is 1022.0 eV and 1021.1 the BEs of metallic Zn and Zn for Zn2p3/2 are extremely close [27]. Hence, the kinetic energy (KE) of 2+ Zn LMMeV, respectively. is considered However, to judge it is thevery valence difficult ofto justify Zn. It whether was shown it is metallic that the Zn KE or ofZn Zn, since LMM the for BEs Ru-Zn of metallic Zn and Zn2+ for Zn2p3/2 are extremely close [27]. Hence, the kinetic energy (KE) of Zn LMM (0.33)/m-ZrO2 AH and Ru-Zn (0.60)/m-ZrO2 AH is 988.6 eV and 989.2 eV, respectively (Figure3d). is considered to judge the valence of Zn. It was shown that the KE of Zn LMM for Ru-Zn (0.33)/m- This suggests that the valence of Zn in Ru-Zn/m-ZrO2 AH is positive 2, since the KE of metallic Zn is ZrO2 AH and Ru-Zn (0.60)/m-ZrO2 AH is 988.6 eV and 989.2 eV, respectively (Figure 3d). This 992.1 eV [28]. This is in good agreement with the XRD results. In addition, it is noticed that the BE of suggests that the valence of Zn in Ru-Zn/m-ZrO2 AH is positive 2, since the KE of metallic Zn is 992.1 Zn2peV1/2 [28].over This Ru-Zn is in good (0.60)/m-ZrO agreement with2 AH the is XRD lower resu thanlts. In that addition, observed it is noticed over Ru-Zn that the (0.60)/m-ZrO BE of Zn2p1/2 2 AH (i.e.,over 1021.1 Ru-Zn eV vs.(0.60)/m-ZrO 1022.0 eV),2 AH but is lower the former’s than that KEobserved is higher over than Ru-Zn that (0.60)/m-ZrO obtained2over AH (i.e., the 1021.1 latter. This demonstrateseV vs. 1022.0 that eV), more but the electrons former’s were KE is transferredhigher than that to Zn obtained2+ with over increasing the latter. Zn This content. demonstrates In contrast, 2+ the BEthat of more Zr3d electrons5/2 over were Ru-Zn transferred (0.33)/m-ZrO to Zn 2 withAH increasing and Ru-Zn Zn (0.60)/m-ZrO content. In contrast,2 AH isthe 180.7 BE of eV Zr3d and5/2 180.6 eV, respectively,over Ru-Zn (0.33)/m-ZrO which means2 AH that and the Ru-Zn addition (0.60)/m-ZrO of Zn is2 notAH ableis 180.7 to modifyeV and 180.6 the electronic eV, respectively, structure of which means that the addition of Zn is not able to modify the electronic structure of Zr. However, it Zr. However, it is worth mentioning that the BE of Zr3d5/2 for ZrO2 observed in this work is clearly is worth mentioning that the BE of Zr3d5/2 for ZrO2 observed in this work is clearly lower than that lower than that reported in literature (i.e., 182.2 eV) [29], which might be due to the fact that some of reported in literature (i.e., 182.2 eV) [29], which might be due to the fact that some of the electrons the electrons from Ru were transferred to Zr as well. This reveals that there is a strong electronic effect from Ru were transferred to Zr as well. This reveals that there is a strong electronic effect between between the active component (Ru) and the support (m-ZrO ). the active component (Ru) and the support (m-ZrO2). 2

FigureFigure 3. X-ray 3. X-ray photoelectron photoelectron spectroscopy spectroscopy (XPS)(XPS) profiles proﬁles of of Ru-Zn Ru-Zn (0.33)/m-ZrO (0.33)/m-ZrO2 catalyst2 catalyst and Ru-Zn and Ru-Zn (0.60)/m-ZrO2 catalyst after catalytic experiments. (a) Ru3d3/2 and Ru3d5/2; (b) Zn2p1/2 and Zn2p3/2; (c) (0.60)/m-ZrO2 catalyst after catalytic experiments. (a) Ru3d3/2 and Ru3d5/2;(b) Zn2p1/2 and Zn2p3/2; Zr3d3/2 and Zr3d5/2; (d) Zn LMM. (c) Zr3d3/2 and Zr3d5/2;(d) Zn LMM.

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The hydrophilicity of Ru-Zn(x)/m-ZrO2 after catalytic experiments was further examined, and the corresponding water droplets on each catalyst are illustrated in Figure4. It is obvious that the ◦ water contact angle declined along with raising the Zn content; that is, 139 over Ru/m-ZrO2 AH ◦ versusCatalystsCatalysts 6 2018 2018over, ,8 8, ,x x Ru-Zn FOR FOR PEER PEER (1.02)/m-ZrO REVIEW REVIEW 2 AH. This indicates that the hydrophilicity of the catalysts66 ofof 1717 is drastically improved by increasing the Zn content. The same assumption can be made from Figure5, ininin which which which fewer fewer fewer catalysts catalysts catalysts suspended suspended suspended in in the the organic organicorganic phase phase were were were observed observed observed by by by enhancing enhancing enhancing the the the Zn Zn Zncontent content content of the catalyst. The improvement of the wettability of Ru-Zn(x)/m-ZrO2 catalysts can be rationalized ofof the the catalyst. catalyst. The The improvement improvement of of the the wettability wettability ofof Ru-Zn(Ru-Zn(x)/m-ZrO2 2catalystscatalysts can can be be rationalized rationalized asas follows: follows: (1) (1) When When ZnO ZnO content content was was increased, increased, more more (Zn(OH) (Zn(OH)22))33(ZnSO(ZnSO44)(H)(H22O)O)33 would would be be generated generated as follows: (1) When ZnO content was increased, more (Zn(OH)2)3(ZnSO4)(H2O)3 would be generated and chemisorbed on the Ru surface, which directly led to more Ruδδ++ being formed during the reaction. andand chemisorbed chemisorbed onon the Ru Ru surface, surface, which which directly directly led to led more to Ru more being Ruδ formed+ being during formed the duringreaction. the Since the lone pair of electrons of oxygen in water molecules are easily linked to the empty d orbitals reaction.Since the Since lone the pair lone of electrons pair of electrons of oxygen of in oxygen water molecules in water moleculesare easily linked are easily to the linked empty to d theorbitals empty of Ru, the higher concentration of Ruδδ++ resulted in more water molecules being linked to the Ru d orbitalsof Ru, the of higher Ru, the concentration higher concentration of Ru resulted of Ruδ +inresulted more water in more molecules water being molecules linked being to the linked Ru surface,surface, andand thusthus therethere waswas anan improvementimprovement inin thethe hydrophilicity.hydrophilicity. (2)(2) TheThe chemisorbedchemisorbed to the Ru surface, and thus there was an improvement in the hydrophilicity. (2) The chemisorbed (Zn(OH)(Zn(OH)22))33(ZnSO(ZnSO44)(H)(H22O)O)33 mainlymainly existedexisted asas hydratedhydrated ionsions onon thethe RuRu surface,surface, whichwhich causedcaused thethe (Zn(OH) ) (ZnSO )(H O) mainly existed as hydrated ions on the Ru surface, which caused the formationformation2 3 ofof thethe4 stagnantstagnant2 3 waterwater layer.layer. ThisThis alsoalso leadsleads toto thethe increaseincrease inin hydrophilicityhydrophilicity forfor thethe formation of the stagnant water layer. This also leads to the increase in hydrophilicity for the catalysts. catalysts.catalysts.

FigureFigureFigure 4. 4. 4.Water Water Water contact contact contact angleangle angle of of Ru-Zn(x)/m-ZrO Ru-Zn(x)/m-ZrORu-Zn(x)/m-ZrO2 2after after2 after catalytic catalytic catalytic experiments: experiments: experiments: ( (aa)) Ru/m-ZrO Ru/m-ZrO (a) Ru/m-ZrO2 2AH; AH; ( 2(bbAH;)) 2 2 2 (bRu-Zn(0.06)/m-ZrO)Ru-Zn(0.06)/m-ZrO Ru-Zn(0.06)/m-ZrO 2 AH;AH;2 AH; ((cc)) ( cRu-Zn(0.33)/m-ZrO)Ru-Zn(0.33)/m-ZrO Ru-Zn(0.33)/m-ZrO 2 AH;AH;2 AH; ((dd)) ( dRu-Zn(0.60)/m-ZrORu-Zn(0.60)/m-ZrO) Ru-Zn(0.60)/m-ZrO 2 AH;AH;2 AH; ((ee)) (Ru-ZneRu-Zn) Ru-Zn (0.86)/m-ZrO2 AH; (f) Ru-Zn (1.02)/m-ZrO2 AH. (0.86)/m-ZrO(0.86)/m-ZrO2 2AH; AH;( f(f)) Ru-ZnRu-Zn (1.02)/m-ZrO(1.02)/m-ZrO2 2AH.AH.

OilOil

WaterWater

FigureFigureFigure 5. 5.5. The TheThe suspendsuspendsuspend situationsituationsituation of of Ru-Zn(x)/m-ZrO Ru-Zn(x)/m-ZrO2 2 2catalystscatalystscatalysts inin in thethe the oiloiloil phasephase phase afterafter after catalyticcatalytic catalytic

experiments:experiments:experiments: (( a(aa))) Ru/m-ZrO Ru/m-ZrO2 2AH; 2AH;AH; ( (bb) ()Ru-Zn(0.06)/m-ZrOb Ru-Zn(0.06)/m-ZrO) Ru-Zn(0.06)/m-ZrO2 2AH; AH;2 ( (cAH;c)) Ru-Zn(0.33)/m-ZrO Ru-Zn(0.33)/m-ZrO (c) Ru-Zn(0.33)/m-ZrO2 2AH; AH; ( (dd)) 2Ru-Zn Ru-ZnAH; ( d) Ru-Zn(0.47)/m-ZrO(0.47)/m-ZrO (0.47)/m-ZrO2 2AH; AH; ( (ee2)) Ru-Zn(0.60)/m-ZrO AH;Ru-Zn(0.60)/m-ZrO (e) Ru-Zn(0.60)/m-ZrO2 2AH; AH; ( (ff)) Ru-Zn(0.69)/m-ZrO Ru-Zn(0.69)/m-ZrO2 AH; (f) Ru-Zn(0.69)/m-ZrO2 2AH; AH; ( (gg)) Ru-Zn Ru-Zn2 (0.86)/m-ZrO (0.86)/m-ZrOAH; (g) Ru-Zn2 2 (0.86)/m-ZrOAH;AH; ( (hh)) Ru-Zn Ru-Zn2 AH; (1.02)/m-ZrO (1.02)/m-ZrO (h) Ru-Zn2 2AH. (1.02)/m-ZrOAH. 2 AH.

TheTheThe catalytic catalyticcatalytic activity activityactivity and andand selectivity selectivityselectivity towards towardstowards cyclohexene cyclohexenecyclohexene formation formationformation over Ru-Zn/m-ZrO overover Ru-Zn/m-ZrORu-Zn/m-ZrO2 catalysts22 withcatalystscatalysts different withwith Zn differentdifferent content ZnZn is contentcontent illustrated isis illustratedillustrated in Figure inin6 . FigureFigure It can 6. be6. ItIt clearly cancan bebe seenclearlyclearly that seenseen the thatthat catalytic thethe catalyticcatalytic activity 2 towardsactivityactivity benzene towardstowards conversion benzenebenzene conversionconversion over Ru-Zn/m-ZrO overover Ru-Zn/m-ZrORu-Zn/m-ZrO2 catalysts2 catalysts wascatalysts inhibited waswas inhibitedinhibited by increasing byby increasingincreasing the Zn content; thethe thatZnZn is, content;content; when thatthat the is, molaris, whenwhen ratio thethe molar ofmolar Zn ratio toratio Ru ofof grew ZnZn toto from RuRu grewgrew 0 to fromfrom 1.02, 00 catalytic toto 1.02,1.02, catalyticcatalytic activity activityactivity towards towardstowards benzene conversionbenzenebenzene conversionconversion dropped fromdroppeddropped 99.8% fromfrom to 99.8% 14.9%99.8% toto in 14.9%14.9% 20 min inin of 2020 reaction minmin ofof reactionreaction time. On time.time. the OnOn contrary, thethe contrary,contrary, catalytic selectivitycatalyticcatalytic selectivity towardsselectivity cyclohexene towards towards cyclohexene cyclohexene formation formation formation increased increased increased along with along along Zn with content.with Zn Zn content. content. Notably, Notably, Notably, when the when when molar the molar ratio of Zn to Ru exceeded 0.6, catalytic activity towards benzene conversion was ratiothe of molar Zn to Ruratio exceeded of Zn 0.6,to Ru catalytic exceeded activity 0.6, towardscatalytic benzene activity conversion towards benzene was drastically conversion suppressed was drasticallydrastically suppressedsuppressed (e.g.,(e.g., 88.8%88.8% ofof benzenebenzene conversionconversion overover Ru-ZnRu-Zn (0.60)/m-ZrO(0.60)/m-ZrO22 vs.vs. 21.7%21.7% ofof (e.g., 88.8% of benzene conversion over Ru-Zn (0.60)/m-ZrO2 vs. 21.7% of benzene conversion over benzenebenzene conversionconversion overover Ru-ZnRu-Zn (1.02)/m-ZrO(1.02)/m-ZrO22 afterafter 4040 minmin ofof catalyticcatalytic experiments),experiments), whilewhile nono significantsignificant improvement improvement for for the the selectivity selectivity towards towards cyclohexene cyclohexene synthesis synthesis was was observed observed (e.g., (e.g., 68.0% 68.0% ofof cyclohexenecyclohexene selectivityselectivity overover Ru-ZnRu-Zn (0.60)/m-ZrO(0.60)/m-ZrO22 vs.vs. 88.8%88.8% ofof benzenebenzene conversionconversion overover Ru-ZnRu-Zn (1.02)/m-ZrO(1.02)/m-ZrO22 afterafter 4040 minmin ofof catalyticcatalytic experiments).experiments). WhenWhen nn(Zn)/(Zn)/nn(Ru)(Ru) isis 0.6,0.6, aa 60.9%60.9% cyclohexenecyclohexene yieldyield waswas achieved achieved withinwithin 35 35 min,min, whichwhich isis thethe hihighestghest yieldyield ofof cyclohexenecyclohexene everever reportedreported overover Ru- Ru-

Catalysts 2018, 8, 513 7 of 17

Ru-Zn (1.02)/m-ZrO2 after 40 min of catalytic experiments), while no significant improvement for the selectivity towards cyclohexene synthesis was observed (e.g., 68.0% of cyclohexene selectivity over Ru-Zn (0.60)/m-ZrO2 vs. 88.8% of benzene conversion over Ru-Zn (1.02)/m-ZrO2 after 40 min of catalytic experiments). When n(Zn)/n(Ru) is 0.6, a 60.9% cyclohexene yield was achieved within 35 min, which is the highest yield of cyclohexene ever reported over Ru-Zn/ZrO2 [1,9,25]. Combined with theCatalysts characterization 2018, 8, x FOR PEER results, REVIEW the effect of Zn content can be concluded as follows: If the surface7 of 17 of Ru is hydrophobic, the catalyst mainly stays in the oil phase, in which the adsorbed benzene tends Zn/ZrO2 [1,9,25]. Combined with the characterization results, the effect of Zn content can be to be completely hydrogenated into cyclohexane, since desorption of the formed cyclohexene hardly concluded as follows: If the surface of Ru is hydrophobic, the catalyst mainly stays in the oil phase, proceedsin which in the the oil adsorbed phase. Therefore, benzene tends When to Zn be contentcompletely increases, hydrogenated more(Zn(OH) into cyclohexane,2)3(ZnSO 4since)(H2 O)3 woulddesorption be generated of the and formed chemisorbed cyclohexene on hardly the Ruprocee surface,ds in the improving oil phase. theTherefore, wettability When ofZn the content catalyst. The resultincreases, of this more is that(Zn(OH) the2 hydrophilic)3(ZnSO4)(H2O) catalyst3 would easilybe generated moves and from chemisorbed the oil phase on the into Ru the surface, aqueous phaseimproving and stays the there. wettability It is well of known the catalyst. that the The solubility result of ofthis cyclohexene is that the hydrophilic in water is catalyst weaker easily than that of benzenemoves [from30], thusthe oil the phase synthesized into the aqueous cyclohexene phase and in aqueousstays there. phase It is well would known be that transferred the solubility into the oil phaseof cyclohexene spontaneously. in water The is formedweaker than cyclohexene that of benzene is difficult [30], tothus be the re-adsorbed synthesized on cyclohexene the hydrophilic in Ru surface,aqueous leading phase towould the inhibitionbe transferred of its into further the oil hydrogenation, phase spontaneously. and thus The improvingformed cyclohexene the selectivity is difficult to be re-adsorbed on the hydrophilic Ru surface, leading to the inhibition of its further towards cyclohexene formation. Besides, when the hydrogenation of benzene takes place in the hydrogenation, and thus improving the selectivity towards cyclohexene formation. Besides, when aqueous phase, the generated cyclohexene can be stabilized by the hydrogen bond formed between the hydrogenation of benzene takes place in the aqueous phase, the generated cyclohexene can be cyclohexenestabilized and by water the hydrogen molecules bond [31 ].formed As demonstrated between cyclohexene in Figure and7, two water types molecules of hydrogen [31]. As bond betweendemonstrated cyclohexene in Figure and water7, two moleculestypes of hydrog coulden bebond formed between in cyclohexene the aqueous and phase. water molecules However, as the hydrophilicitycould be formed further in the increases aqueous withphase. increasing However, n as (Zn)/n the hydrophilicity (Ru), there isfurther less retention increases timewith that Ru-Zn/ZrOincreasing2 spends n (Zn)/n in the (Ru), oil there phase. is Thisless retention leads to atime fall inthat benzene Ru-Zn/ZrO adsorption2 spends ofinRu-Zn/ZrO the oil phase.2 catalysts,This and catalyticleads to activitya fall in benzene towards adsorption benzene conversion of Ru-Zn/ZrO declines2 catalysts, (Figure and 6catalytica). Therefore, activity there towards is an benzene optimum conversion declines (Figure 6a). Therefore, there is an optimum molar ratio of Zn to Ru for Ru- molar ratio of Zn to Ru for Ru-Zn/ZrO2 catalysts; that is, n (Zn)/n (Ru) = 0.6 (Figure7, “catalyst B”). WhenZn/ZrO Zn2 catalysts; content isthat very is, insufficient,n (Zn)/n (Ru) as = in0.6 “catalyst (Figure 7, A” “catalyst in Figure B”).7 ,When the generated Zn content cyclohexene is very insufficient, as in “catalyst A” in Figure 7, the generated cyclohexene is likely hydrogenated into is likely hydrogenated into cyclohexane, leading to a relatively low selectivity towards cyclohexene. cyclohexane, leading to a relatively low selectivity towards cyclohexene. In contrast, when Zn content In contrast, when Zn content is excessive (Figure7 “catalyst C”), adsorption of benzene becomes quite is excessive (Figure 7 “catalyst C”), adsorption of benzene becomes quite challenging, which causes challenging,a drastic which decrease causes of catalytic a drastic activity decrease towards of catalyticbenzene conversion. activity towards benzene conversion.

Figure 6. Cont.

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Figure 6. Catalytic activity towards selective hydrogenation of benzene over Ru-Zn/m-ZrO2 catalysts FigureFigure 6. Catalytic 6. Catalytic activity activity towards towards selective selective hydrogenation hydrogenation ofof benzene over over Ru-Zn/m-ZrO Ru-Zn/m-ZrO2 catalysts2 catalysts 3 3 with withdifferent different Zn contentZn content (m (mcat cat= 1.2= 1.2 g, g, m mZnSO4ZnSO4 = = 50.050.0 g, vvH2OH2O = = 280 280 cm cm3, v,benzene vbenzene3 = 140 = 140 cm 3cm, T =, 423T = K,3423 p H2K, pH2 with different Zn content (mcat = 1.2 g, mZnSO4 = 50.0 g, vH2O = 280 cm , vbenzene = 140 cm , T = 423 = 5.0 MPa). (a) Benzene conversion as function of reaction time; (b) Cyclohexene selectivity as function K,= 5.0 pH2 MPa).= 5.0 ( MPa).a) Benzene (a) Benzene conversion conversion as function as function of reaction of reaction time; (b) time; Cyclohexene (b) Cyclohexene selectivity selectivity as function as functionof benzeneof benzene of benzeneconversion; conversion; conversion; (c) (Cyclohexenec) Cyclohexene (c) Cyclohexene yield yield as as yieldfuncfunction as functionof of reaction reaction of time. reactiontime. time.

Figure 7. Two types of hydrogen bond formed between cyclohexene and water molecules, as well as

the hydrogenation scheme both in the oil phase and aqueous phase, over Ru-Zn/ZrO2 catalysts with FigureFiguredifferent 7.7. TwoTwo wettability. typestypes ofof hydrogenhydrogen bondbond formedformed betweenbetween cyclohexenecyclohexene andand waterwater molecules,molecules, asas wellwell asas

thethe hydrogenationhydrogenation scheme scheme both both in in the the oil oil phase phase and and aqueous aqueous phase, phase, over over Ru-Zn/ZrO Ru-Zn/ZrO22 catalystscatalysts withwith differentdifferent2.2. Effect wettability.wettability. of ZrO2 as Support and Dispersant

Figure 8 demonstrates the XRD patterns of the supported Ru-Zn (0.60)/m-ZrO2 catalyst and the 2.2. Effect of ZrO2 as Support and Dispersant 2.2. Effectunsupported of ZrO2 asRu-Zn Support (0.60) and catalyst Dispersant before (a) and after (b) catalytic experiments. As with m-ZrO2 supported samples, metallic Ru and ZnO reflections were observed for the unsupported Ru-Zn FigureFigure8 8 demonstrates demonstrates thethe XRDXRD patternspatterns of the supported supported Ru-Zn Ru-Zn (0.60)/m-ZrO (0.60)/m-ZrO2 catalyst2 catalyst and and the catalyst, which again proves that the valence of Ru and Zn is 0 and positive 2, respectively. Moreover, theunsupported unsupported Ru-Zn Ru-Zn (0.60) (0.60) catalyst catalyst before before (a) and (a) after and (b) after catalytic (b) catalytic experiments. experiments. As with As m-ZrO with2 the characteristic diffraction of (Zn(OH)2)3(ZnSO4)(H2O)3 was also pronounced over the unsupported m-ZrOsupported2 supported samples, samples, metallic metallicRu and RuZnO and reflecti ZnOons reﬂections were observed were observed for the forunsupported the unsupported Ru-Zn sample, indicating that ZrO2 as the support plays no role in the formation of Ru-Zn catalyst, which again proves that the valence of Ru and Zn is 0 and positive 2, respectively. catalyst,(Zn(OH) which2)3 (ZnSOagain4 proves)(H2O)3. that the valence of Ru and Zn is 0 and positive 2, respectively. Moreover, Moreover,the characteristic the characteristic diffraction of diffraction (Zn(OH)2) of3(ZnSO (Zn(OH)4)(H22O))3(ZnSO3 was also4)(H pronounced2O)3 was also over pronounced the unsupported over thesample, unsupported indicating sample, that indicatingZrO2 as that the ZrO support2 as the supportplays no plays role no rolein inthe the formation formation ofof (Zn(OH)(Zn(OH)22))33(ZnSO4)(H)(H2O)O)33. .

Catalysts 2018, 8, 513 9 of 17 Catalysts 2018, 8, x FOR PEER REVIEW 9 of 17

FigureFigure 8. 8. XRDXRD patterns patterns of of the the supported Ru-Zn(0.60)/m-ZrO 2 2catalystcatalyst and and the the unsupported unsupported Ru-Zn Ru-Zn (0.60)(0.60) catalyst catalyst before before (a (a) )and and after after ( (bb)) catalytic catalytic experiments. experiments.

TheThe texture texture properties properties and and compositio compositionn of of the the supported supported Ru-Zn Ru-Zn (0.60)/m-ZrO 2 2catalystcatalyst and and the the unsupportedunsupported Ru-Zn Ru-Zn (0.60), (0.60), as as well well as as the the pH pH values values of of the the slurry, slurry, after after catalytic catalytic experiments experiments are are given given inin Table Table 2.2. It It is is found found that that unsupported unsupported Ru-Zn Ru-Zn (0. (0.60)60) catalyst catalyst demonstrated demonstrated similar similar texture texture properties properties toto that that obtained obtained over over parent parent m-ZrO m-ZrO2 2afterafter the the catalytic catalytic experiment. experiment. This This can can be be attributed attributed to to the the fact fact thatthat the the added added m-ZrO m-ZrO2 waswas 10 times thethe amountamountof of catalyst catalyst used used and and was was mechanically mechanically mixed mixed with with the thecatalyst catalyst during during the the reaction. reaction. Moreover, Moreover, an an increase increa inse thein the molar molar ratio ratio of Znof Zn to Ruto Ru was was also also shown shown for forthe unsupportedthe unsupported Ru-Zn catalyst Ru-Zn after catalyst catalytic experiments,after catalytic indicating experiments, that (Zn(OH) indicating2)3(ZnSO4 )(Hthat2O)3 (Zn(OH)was formed2)3(ZnSO as well.4)(H Interestingly,2O)3 was formed it was asnoticed well. Inte thatrestingly, n (Zn)/n it (Ru) was over noticed Ru-Zn that (0.60)/ZrO n (Zn)/n 2(Ru)is slightly over Ru-Znhigher (0.60)/ZrO than that obtained2 is slightly over higher the unsupported than that sampleobtained after over the reaction;the unsupported that is, 0.68 sample versus after 0.65. Thisthe reaction;implies thatthat m-ZrOis, 0.682 versusas the 0.65. support This beneﬁts implies the that adsorption m-ZrO2 as of the (Zn(OH) support2) 3benefits(ZnSO4 )(Hthe 2adsorptionO)3 on the Ruof (Zn(OH)surface.2) Furthermore,3(ZnSO4)(H2O) the3 on pH the value Ru surface. of the Furthermore, slurry with application the pH value of Ru-Znof the slurry (0.60)/ZrO with application2 is slightly 2+ ofhigher Ru-Zn than (0.60)/ZrO that observed2 is slightly with higher unsupported than that Ru-Zn observed (0.60), with suggesting unsupported that more Ru-Zn Zn (0.60),was chemisorbed suggesting thaton themore catalyst Zn2+ was surface chemisorbed as well. on the catalyst surface as well.

TableTable 2 2. TextureTexture properties properties and and compositio compositionn of of the the supported Ru-Zn(0.60)/ZrO 2 2catalystcatalyst and and the the unsupportedunsupported Ru-Zn Ru-Zn (0.60), (0.60), as as well well as as the the pH pH values values of of the the slurry, slurry, after after hydrogenation. hydrogenation.

2 −1 1 3 −1 1 1 2 2 3 CatalystCatalyst SBETSBET/(m/(m·g2·g−)1) 1 VVporepore/(cm/(cm·3g·g−1) 1 ddporepore/(nm)/(nm) 1 nnZnZn/n/nRuRu 2 nnZrZr/n/nRuRu 2 pHpH 3

Ru-Zn(0.60)/ZrO2 2 3030 0.09 9.7 9.7 0.60 0.60 5.38 5.38 - - Ru-Zn(0.60)/ZrO AH 30 0.08 10.1 0.68 5.33 5.38 Ru-Zn(0.60)/ZrO2 2 AH 30 0.08 10.1 0.68 5.33 5.38 Ru-Zn(0.60) 65 0.19 11.7 0.61 0 - Ru-Zn(0.60) 65 0.19 11.7 0.61 0 - Ru-Zn(0.60)+ZrO2 AH 33 0.12 14.6 0.65 5.35 5.29 Ru-Zn(0.60)+ZrOZrO2 2 AH 34 33 0.120.13 14.6 16.0 0.65 - 5.35 - 5.29 - 1 2 3 DeterminedZrO2 by N2-sorption; Determined34 by XRF;0.13Determined by16.0 a pH meter at- room temperature;- AH:- after hydrogenation. 1 Determined by N2-sorption; 2 Determined by XRF; 3 Determined by a pH meter at room temperature; AH: after hydrogenation. Figure9 shows the TEM images and Ru particle size distribution of the unsupported Ru-Zn (0.60) catalyst before and after the reaction. As can be observed from Figure9a,b, fresh Ru-Zn (0.60) displays Figure 9 shows the TEM images and Ru particle size distribution of the unsupported Ru-Zn a circular or elliptical shape, of which the particle size is around 4.5 nm. In addition, analogous to that (0.60) catalyst before and after the reaction. As can be observed from Figure 9a,b, fresh Ru-Zn (0.60) observed over the supported sample, no obvious change in the particle size of Ru-Zn (0.6) after the displays a circular or elliptical shape, of which the particle size is around 4.5 nm. In addition, catalytic experiment is demonstrated (Figure9c). However, unlike Ru-Zn (0.60)/m-ZrO , it is noticed analogous to that observed over the supported sample, no obvious change in the particle2 size of Ru- that Ru-Zn (0.6) is clearly not uniformly dispersed by adding m-ZrO as the dispersant (Figure9d). Zn (0.6) after the catalytic experiment is demonstrated (Figure 9c). However,2 unlike Ru-Zn (0.60)/m- This suggests that in comparison with using m-ZrO2 as the dispersant, m-ZrO2 could contribute better ZrO2, it is noticed that Ru-Zn (0.6) is clearly not uniformly dispersed by adding m-ZrO2 as the to the dispersion of Ru when it is utilized as the support. dispersant (Figure 9d). This suggests that in comparison with using m-ZrO2 as the dispersant, m- ZrO2 could contribute better to the dispersion of Ru when it is utilized as the support.

Catalysts 2018, 8, 513 10 of 17 Catalysts 2018, 8, x FOR PEER REVIEW 10 of 17

Figure 9. TEMTEM images images and and Ru Ru particle particle size size distributions distributions of the the unsupported unsupported Ru-Zn (0.60) catalyst with

m-ZrO2 asas the the dispersant dispersant before ( a,,b) and after ( c,d) catalytic experiments.

XPS proﬁlesprofiles of of the the unsupported unsupported Ru-Zn Ru-Zn (0.60) (0.60) catalyst catalyst with m-ZrOwith m-ZrO2 as the2 dispersantas the dispersant after catalytic after experimentscatalytic experiments are demonstrated are demonstrated in Figure in 10 Figure. As can 10. be As observed can be observed from Figure from 10 Figurea, two 10a, peaks two related peaks 0 0 δ+ torelated the BE to ofthe Ru3d BE of5/2 Ru3dwere5/2 observed were observed at 280.3 at eV280.3 and eV 281.4 and eV,281.4 which eV, which are attributed are attributed to Ru toand Ru Ru and, respectively.Ruδ+, respectively. This is This consistent is consistent with that with obtained that ob overtained the over corresponding the corresponding supported supported sample (Ru-Znsample (0.60)/m-ZrO(Ru-Zn (0.60)/m-ZrO2). Moreover,2). Moreover, from Figurefrom Figure 10b, the10b, BE the of BE Zn2p of Zn2p3/2 over3/2 over unsupported unsupported Ru-Zn Ru-Zn (0.60) (0.60) is observedis observed to beto be 1021.5 1021.5 eV, eV, which which is slightly is slightly higher higher than than that demonstratedthat demonstrated over Ru-Znover Ru-Zn (0.60)/m-ZrO (0.60)/m-2 (i.e.,ZrO2 1021.1 (i.e., 1021.1 eV). Additionally, eV). Additionally, the KE ofthe Zn KE LMM of Zn over LMM unsupported over unsupported Ru-Zn (0.60) Ru-Zn is slightly (0.60) loweris slightly than thatlower observed than that over observed Ru-Zn (0.60)/m-ZrOover Ru-Zn (0.60)/m-ZrO2; that is, 989.02; that eV versus is, 989.0 989.2 eV eV.versus This 989.2 can be eV. rationalized This can be in termsrationalized that more in terms (Zn(OH) that2 more)3(ZnSO (Zn(OH)4)(H2O)2)33(ZnSOwas chemisorbed4)(H2O)3 was onchemisorbed Ru-Zn (0.60)/m-ZrO on Ru-Zn (0.60)/m-ZrO2 than that on2 unsupportedthan that on unsupported Ru-Zn (0.60), Ru-Zn thus leading(0.60), thus to more leadin electronsg to more being electrons transferred being transferred from Ru to Zn.from This Ru to is inZn. a goodThis agreementis in a good with agreement the XRF results. with the Additionally, XRF results. the BEAdditionally, of Zr3d5/2 overthe unsupportedBE of Zr3d5/2 Ru-Zn over (0.60)unsupported with m-ZrO Ru-Zn2 as (0.60) the dispersantwith m-ZrO was2 as normalized the dispersant to be was 182.0 normalized eV, which to is closebe 182.0 to thateV, reportedwhich is inclose literature to that reported (i.e., 182.2 in eV) literature [29]. This (i.e., suggests 182.2 eV) that [29]. there This issuggests no obvious that there electronic is no effectobvious between electronic the activeeffect between component the (Ru)active and component m-ZrO2 as (Ru) the and dispersant. m-ZrO2 Itasis the therefore dispersant. deemed It is thattherefore no electrons deemed were that transferredno electrons from were Ru transferred to Zr by mechanical from Ru to mixingZr by mechanical of Ru-Zn (0.60) mixing with of m-ZrORu-Zn 2(0.60). with m-ZrO2.

Catalysts 2018, 8, 513 11 of 17 CatalystsCatalysts 2018 2018, 8, ,8 x, x FOR FOR PEER PEER REVIEW REVIEW 1111 of of 17 17

Figure 10. XPS profiles of the unsupported Ru-Zn (0.60) catalyst with m-ZrO2 as the dispersant after Figure 10. XPS profiles proﬁles of the unsupported Ru-Zn (0.60) catalyst with m-ZrO2 as the dispersant after catalytic experiments. (a) Ru3d3/2 and Ru3d5/2; (b) Zn2p1/2 and Zn2p3/2; (c) Zr3d3/2 and Zr3d5/2; (d) Zn catalytic experiments.experiments. ((aa)) Ru3dRu3d3/23/2 andand Ru3d 5/2; ;((bb) )Zn2p Zn2p1/21/2 andand Zn2p Zn2p3/23/2; (c;() cZr3d) Zr3d3/2 3/2andand Zr3d Zr3d5/2; 5/2(d);( Znd) LMM.LMM.Zn LMM.

TheThe H H2-TPR2-TPR-TPR profilesprofiles proﬁles of of Ru-Zn Ru-ZnRu-Zn (0.60)/m-ZrO (0.60)/m-ZrO(0.60)/m-ZrO2 22 andandand unsupportedunsupported unsupported Ru-ZnRu-Zn Ru-Zn (0.60)(0.60) (0.60) afterafter catalytic catalytic experimentsexperiments are are given given in in Figure FigureFigure 11. 1111.. The The redu reductionreductionction peak peakpeak of ofof Ru RuRu over overover Ru-ZnRu-Zn (0.60)/m-ZrO (0.60)/m-ZrO2 2 cancan bebe observedobserved at at around aroundaround 360 360360 K, K,K, which which is is higher higher thanthan than thatthat that shownshown shown over over over unsupported unsupported unsupported Ru-Zn Ru-Zn Ru-Zn (0.60) (0.60) (0.60) (i.e., (i.e., (i.e., 344 344 344 K). K).K).This This This indicates indicates indicates that that thethat interactionthe the interaction interaction between between between Ru and Ru Ru the and and support the the support support (m-ZrO (m-ZrO (m-ZrO2) is stronger2)2 )is is stronger stronger than that than than between that that betweenbetweenRu and theRu Ru and dispersant. and the the dispersant. dispersant. This is in This This agreement is is in in agreement agreement with the with XPSwith theresults. the XPS XPS results. results.

360K360K Ru-Zn(0.60)/ZrO AH Ru-Zn(0.60)/ZrO2 AH 2

344K344K TCD signal/a.u. TCD signal/a.u. Ru-Zn(0.60)+ZrO AH Ru-Zn(0.60)+ZrO2 AH 2

300300 320 320 340 340 360 360 380 380 400 400 420 420 440 440 460 460 480 480 Temperature/K Temperature/K

Figure 11. H2 -temperature-programmed reduction proﬁles of Ru-Zn(0.60)/m-ZrO2 and unsupported FigureFigure 11. 11. H H-temperature-programmed2-temperature-programmed reduction reduction profiles profiles of of Ru-Zn(0.60)/m-ZrO Ru-Zn(0.60)/m-ZrO 2and and unsupported unsupported Ru-ZnRu-Zn (0.60) (0.60) after after catalytic catalytic experiments. experiments.

Catalysts 2018, 8, 513 12 of 17 Catalysts 2018, 8, x FOR PEER REVIEW 12 of 17

The wettability of unsupported Ru-Zn(0.60) after catalytic experiments was also examined, and the water droplet on the catalyst is illustratedillustrated in Figure 1212.. As presented, the contacted angle over ◦ ◦ unsupported Ru-Zn(0.60) is 5252°,, which is higher than that observedobserved overover Ru-Zn(0.60)/ZrORu-Zn(0.60)/ZrO2 (i.e.,(i.e., 40°). 40 ). This demonstrates that hydrophilicityhydrophilicity of Ru-Zn with m-ZrO2 asas the dispersant is weaker than that with m-ZrO22 as the support.support. ThisThis isis mainlymainlydue due to to the the fact fact that that no no electron electron was was transferred transferred from from Ru Ru to δ+ toZr, Zr, causing causing less less Ru Rutoδ+ to be be generated. generated. This This leads leads to theto the fact fact that that less less water water molecules molecules are are linked linked to the to theRu surface,Ru surface, thus thus decreasing decreasing the hydrophilicitythe hydrophilicity of the of catalystthe catalyst surface. surface.

FigureFigure 12. WaterWater contact angle of unsupported Ru-Zn (0.60) after catalytic experiments.

Catalytic activity and selectivity selectivity towards towards cyclohexene cyclohexene formation formation over over unsupported unsupported Ru-Zn Ru-Zn (0.60), (0.60), as well as Ru-ZnRu-Zn (0.60)/m-ZrO(0.60)/m-ZrO22, ,are are presented presented in in Figure Figure 13.13. As As ca cann be be seen, both catalytic activity towards benzene conversion, and selectivity toward towardss cyclohexene, over over unsupported Ru-Zn Ru-Zn (0.60), (0.60), are lower than than that that achieved achieved over over Ru-Zn Ru-Zn (0.60)/m-ZrO (0.60)/m-ZrO2; that2; that is, only is, only 69.5% 69.5% of benzene of benzene conversion conversion and 71.5%and 71.5% of cyclohexene of cyclohexene selectivity selectivity were were obtained obtained over over unsupported unsupported Ru-Zn Ru-Zn (0.60), (0.60), while while Ru-Zn (0.60)/m-ZrO22 gavegave 84.6% 84.6% of of benzene benzene conver conversionsion and and 71.5% 71.5% of of selectivity selectivity to to cyclohexene cyclohexene after 35 min of reaction time.time. ThisThis can can be be attributed attributed to to two two main main reasons: reasons: (1) (1) Ru Ru can can behighly be highly dispersed dispersed on m-ZrO on m-2 whenZrO2 when m-ZrO m-ZrO2 is utilized2 is utilized as the as support,the support, while while the dispersionthe dispersion of Ru of Ru is less is less uniform uniform when when m-ZrO m-ZrO2 is2 isapplied applied as as the the dispersant. dispersant. This This results results in morein more active active Ru Ru sites sites being being available available during during the reaction,the reaction, and andfurther further improves improves the catalytic the catalytic activity activity towards towards benzene benzene conversion. conversion. (2) m-ZrO (2)2 m-ZrOsupported2 supported Ru-Zn (0.60) Ru- Znbeneﬁts (0.60) the benefits adsorption the adsorption of (Zn(OH) of2 )(Zn(OH)3(ZnSO42)(H)3(ZnSO2O)3,4)(H enhancing2O)3, enhancing the hydrophilicity the hydrophilicity of the catalyst of the catalystsurface. surface. Therefore, Therefore, more Ru-Zn more (0.60)/m-ZrO Ru-Zn (0.60)/m-ZrO2 would2 staywould in thestay aqueous in the aqueous phase than phase that than happens that happensfor the unsupported for the unsupported sample, whichsample, helps which the helps desorption the desorption of the generated of the generated cyclohexene cyclohexene and inhibits and inhibitsits further its hydrogenation.further hydrogenation. Hence, theHence, highest the high cyclohexeneest cyclohexene yield of yield 60.9% of was60.9% achieved was achieved over Ru-Zn over (0.60)/m-ZrORu-Zn (0.60)/m-ZrO2, while2, onlywhile 50% only of the50% maximum of the maximum cyclohexene cy yieldclohexene was obtained yield was over obtained unsupported over unsupportedRu-Zn (0.60). Ru-Zn (0.60). The reusability of Ru-Zn (0.60)/m-ZrO2 was investigated under the same reaction conditions without further regeneration (Figure 14). It can be observed that benzene conversion as well as cyclohexene selectivity is maintained above 80% and 70%, respectively, after 17 iterations of the catalytic experiments. Thus, the catalytic system over Ru-Zn (0.60)/m-ZrO2 shows a good reusability, indicating that this catalyst possesses great potential for industrial application in selective hydrogenation of benzene towards cyclohexene production.

Figure 13. Catalytic activity towards selective hydrogenation of benzene over unsupported Ru-Zn (0.60) as well as Ru-Zn(0.60)/m-ZrO2 (m(Ru-Zn (0.60)/m-ZrO2) = 1.2 g, or m(Ru-Zn (0.60)) = 0.2 g and

m(ZrO2) = 1.0 g; mZnSO4 = 50.0 g; vH2O = 280 cm3; vbenzene = 140 cm3; T = 423 K; pH2 = 5.0 MPa). (a) Benzene conversion as function of reaction time; (b) Cyclohexene selectivity as function of benzene conversion; (c) Cyclohexene yield as function of reaction time.

Catalysts 2018, 8, x FOR PEER REVIEW 12 of 17

The wettability of unsupported Ru-Zn(0.60) after catalytic experiments was also examined, and the water droplet on the catalyst is illustrated in Figure 12. As presented, the contacted angle over unsupported Ru-Zn(0.60) is 52°, which is higher than that observed over Ru-Zn(0.60)/ZrO2 (i.e., 40°). This demonstrates that hydrophilicity of Ru-Zn with m-ZrO2 as the dispersant is weaker than that with m-ZrO2 as the support. This is mainly due to the fact that no electron was transferred from Ru to Zr, causing less Ruδ+ to be generated. This leads to the fact that less water molecules are linked to the Ru surface, thus decreasing the hydrophilicity of the catalyst surface.

Figure 12. Water contact angle of unsupported Ru-Zn (0.60) after catalytic experiments.

Catalytic activity and selectivity towards cyclohexene formation over unsupported Ru-Zn (0.60), as well as Ru-Zn (0.60)/m-ZrO2, are presented in Figure 13. As can be seen, both catalytic activity towards benzene conversion, and selectivity towards cyclohexene, over unsupported Ru-Zn (0.60), are lower than that achieved over Ru-Zn (0.60)/m-ZrO2; that is, only 69.5% of benzene conversion and 71.5% of cyclohexene selectivity were obtained over unsupported Ru-Zn (0.60), while Ru-Zn (0.60)/m-ZrO2 gave 84.6% of benzene conversion and 71.5% of selectivity to cyclohexene after 35 min of reaction time. This can be attributed to two main reasons: (1) Ru can be highly dispersed on m- ZrO2 when m-ZrO2 is utilized as the support, while the dispersion of Ru is less uniform when m-ZrO2 is applied as the dispersant. This results in more active Ru sites being available during the reaction, and further improves the catalytic activity towards benzene conversion. (2) m-ZrO2 supported Ru- Zn (0.60) benefits the adsorption of (Zn(OH)2)3(ZnSO4)(H2O)3, enhancing the hydrophilicity of the catalyst surface. Therefore, more Ru-Zn (0.60)/m-ZrO2 would stay in the aqueous phase than that happens for the unsupported sample, which helps the desorption of the generated cyclohexene and inhibits its further hydrogenation. Hence, the highest cyclohexene yield of 60.9% was achieved over CatalystsRu-Zn2018 (0.60)/m-ZrO, 8, 513 2, while only 50% of the maximum cyclohexene yield was obtained13 ofover 17 unsupported Ru-Zn (0.60).

Catalysts 2018, 8, x FOR PEER REVIEW 13 of 17

The reusability of Ru-Zn (0.60)/m-ZrO2 was investigated under the same reaction conditions withoutFigureFigure further 13. 13.Catalytic Catalytic regenerationactivity activity towards (Figurtowardse selective selective14). It can hydrogenation hydrog be observedenationof of benzene thatbenzene benzene over over unsupported unsupported conversion Ru-Zn Ru-Znas well as cyclohexene selectivity is maintained above 80% and 70%, respectively, after 17 iterations of the (0.60)(0.60) as as well well as as Ru-Zn(0.60)/m-ZrO Ru-Zn(0.60)/m-ZrO2 2(m(Ru-Zn (m(Ru-Zn (0.60)/m-ZrO (0.60)/m-ZrO22)) == 1.21.2 g,g, oror m(Ru-Znm(Ru-Zn (0.60)) (0.60)) = = 0.2 0.2 g g and and catalytic experiments. Thus, the catalytic system3 3 over Ru-Zn 3(0.60)/m-ZrO3 2 shows a good reusability, m(ZrOm(ZrO22)) == 1.01.0 g; mZnSO4 = =50.0 50.0 g; g;vH2O vH2O = 280= 280 cm cm; vbenzene; vbenzene = 140 =cm 140; T cm = 423; T K; = 423pH2 = K; 5.0 pH2 MPa).= 5.0 (a MPa).) Benzene (a) indicatingBenzeneconversion that conversion as functionthis catalyst as functionof reaction possesses of time; reaction (b ) greatCyclohexene time; (bpotential) Cyclohexene selectivity for selectivity industrialas function as ofapplication function benzene ofconversion; benzenein selective hydrogenationconversion;(c) Cyclohexene ( cof) Cyclohexenebenzene yield as functiontowards yield asof cyclohexene functionreaction oftime. reaction production. time.

FigureFigure 14. 14.Reusability Reusability of of Ru-Zn Ru-Zn (0.60)/m-ZrO (0.60)/m-ZrO22 forfor selective selective hydrogenation hydrogenation of of benzene, benzene, including including 3 3 33 benzenebenzene conversion, conversion, cyclohexene cyclohexene selectivity, selectivity, and and yield yield (vH2O (vH2O= 280= 280 cm cm, v,benzene vbenzene= =140 140 cm cm,, T T = = 423 423 K, K, pH2pH2= = 5.05.0 MPa,MPa,t treactionreaction == 40 40 min). min).

3.3. Materials Materials and and Methods Methods 3.1. Chemicals 3.1. Chemicals All chemicals were directly used without any further puriﬁcation. RuCl3·3H2O was delivered All chemicals were directly used without any further purification. RuCl3·3H2O was delivered from Sino-Platinum Co., Ltd. (Kunming, China). ZnSO4·7H2O was purchased from the Fuchen from Sino-Platinum Co., Ltd. (Kunming, China). ZnSO4·7H2O was purchased from the Fuchen Chemical Reagent Factory (Tianjin, China). NaOH and benzene were commercially obtained from Chemical Reagent Factory (Tianjin, China). NaOH and benzene were commercially obtained from the Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). m-ZrO2 was synthesized according to the the Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). m-ZrO2 was synthesized according to the literature [32]. Distilled water was applied in all experiments. literature [32]. Distilled water was applied in all experiments. 3.2. Preparation of Catalysts 3.2. Preparation of Catalysts Ru-Zn/m-ZrO2 catalysts were synthesized as follows. First, 0.45 g of RuCl3·3H2O and 0.28 g Ru-Zn/m-ZrO2 catalysts were synthesized3 as follows. First, 0.45 g of RuCl3·3H2O and 0.28 g of of ZnSO4·7H2O were dissolved in 1 cm of deionized water. With continuous stirring, the aqueous ZnSO4·7H2O were dissolved in 1 cm3 of deionized water. With continuous stirring, the aqueous solution was added dropwise onto 1.0 g of m-ZrO2 powder. Subsequently, the solid was dried at 323 K solution was added dropwise onto 1.0 g of m-ZrO2 powder. Subsequently, the solid was dried at 323 for 3 h, and then calcined at 423 K in a Mufﬂe furnace for another 3 h. After that, the calcined sample, K for 3 h, and then calcined at 423 K in a Muffle furnace for another 3 h. After that, the calcined together with 200 cm3 of 5 wt. % NaOH aqueous solution, was transferred into a 1000 cm3 Hastelloy 3 3 sample, together with 200 cm of 5 wt. % NaOH aqueous solution, was transferred−1 into a 1000 cm autoclave at 423 K under 5.0 MPa of hydrogen and a stirring speed of 800 min for 1 h. Ru(OH)3 Hastelloy autoclave at 423 K under 5.0 MPa of hydrogen and a stirring speed of 800 min−1 for 1 h. was generated when the NaOH was mixed with the calcined sample in the autoclave during the Ru(OH)3 was generated when the NaOH was mixed with the calcined sample in the autoclave during the temperature rising procedure. Then the black powder was cooled down to room temperature, washed with distilled water until neutral, and vacuum-dried. Subsequently, 1.2 g of fresh Ru-Zn/m- ZrO2 catalyst was obtained with a theoretical molar ratio of Zn to Ru of 0.06, and was denoted as Ru- Zn (0.60)/m-ZrO2. Additionally, Ru-Zn (x)/m-ZrO2 catalysts were synthesized via the same procedure by modifying the usage of ZnSO4·7H2O, where x refers to the theoretical molar ratio of Zn to Ru. The preparation procedure of Ru-Zn (x)/m-ZrO2 catalysts is illustrated in Scheme 1.

Catalysts 2018, 8, 513 14 of 17 temperature rising procedure. Then the black powder was cooled down to room temperature, washed with distilled water until neutral, and vacuum-dried. Subsequently, 1.2 g of fresh Ru-Zn/m-ZrO2 catalyst was obtained with a theoretical molar ratio of Zn to Ru of 0.06, and was denoted as Ru-Zn (0.60)/m-ZrO2. Additionally, Ru-Zn (x)/m-ZrO2 catalysts were synthesized via the same procedure by modifying the usage of ZnSO4·7H2O, where x refers to the theoretical molar ratio of Zn to Ru. TheCatalysts preparation 2018, 8, x FOR procedure PEER REVIEW of Ru-Zn (x)/m-ZrO2 catalysts is illustrated in Scheme1. 14 of 17

OH- Zn2+ Ru3+ RuCl3 calcination ZnSO4 OH- ZrO2 ZrO impregnation ZrO2 2

ZnO Zn(OH)2 Ru Ru(OH)3

H2 ZrO2 ZrO2

SchemeScheme 1.1. Preparation procedureprocedure ofof Ru-ZnRu-Zn (x)/m-ZrO(x)/m-ZrO2 catalysts.catalysts.

Unsupported Ru-Zn catalyst was prepared using a reported co-precipitation method [8]. [8]. First, First, 3 0.45 g g of of RuCl RuCl3·3H3·3H2O2O precursor precursor and and 0.28 0.28 g of g ZnSO of ZnSO4·7H42·O7H were2O were dissolved dissolved in 100 in cm 1003 of cm distilledof distilled water. water.Then 100 Then cm 1003 of cmNaOH3 of NaOH(5 wt.%) (5 aqueous wt.%) aqueous solution solution was adde wasd added at 353 atK 353with K continuous with continuous stirring stirring for 2 h,for followed 2 h, followed by moving by moving all solids all solids and and solutions solutions into into a 1000 a 1000 mL mL Hastelloy Hastelloy autoclave autoclave for for a a reduction reduction procedure at 423 K with 5.0 MPa of hydrogen and a st stirringirring speed of 800 rpm for 3 h. h. After After reduction, reduction, the sample was cooled, and the fresh catalyst was gained by washing with deionized water to neutral and vacuum-drying. The prepared Ru-Zn catalysts were denoted as Ru-Zn (0.60), where 0.60 stands for the theoretical molar ratio of ZnZn toto Ru.Ru.

3.3. Catalytic Experimental Procedure All hydrogenation reactions took place in a 1000 mL GS-1 type Hastelloy autoclave. In a typical x · −1 hydrogenation reaction,reaction, 1.21.2 g g Ru-Zn Ru-Zn ( (x)/m-ZrO)/m-ZrO22 catalyst,catalyst, 50.050.0 g g ZnSO ZnSO44·7H7H22OO (0.62(0.62 molmol LL−1),), as as well well as 3 280 cm3 distilled water, werewere addedadded ininthe thereactor. reactor. Then Then the the autoclave autoclave was was charged charged using using N N2 to2 to 4 4 MPa MPa 4 4times, times, which which was was followed followed by by puriﬁcation purification with with H2 Hto2 4to MPa 4 MPa for for another another 4 times. 4 times. Then, Then, the reactorthe reactor was −1 3 washeated heated to 423 to K423 under K under 5.0 MPa 5.0 MPa of H2 ofwith H2 with a stirring a stirring speed speed of 800 of min 800 min, followed−1, followed by pouring by pouring 140 cm140 cmof benzene3 of benzene and and modifying modifying the stirring the stirring speed speed to 1400 to 1400 rpm rpm (to get (to rid get of rid the of mass the mass transfer transfer limitation) limitation) [33]. After[33]. After that, that, each each liquid liquid sample sample was takenwas taken from from the reactor the reactor every every 5 min. 5 Allmin. withdrawn All withdrawn samples samples were wereanalyzed analyzed using using GC-FID GC-FID from from the Hangzhou the Hangzhou Kexiao Kexiao Chemical Chemical Instrument Instrument and Equipmentand Equipment Co., Co., Ltd. (Hangzhou,Ltd. (Hangzhou, China). China). As with As thewith evaluation the evaluation of the of unsupported the unsupported Ru-Zn Ru-Zn catalyst, catalyst, 0.2 g of 0.2 catalyst g of catalyst sample sampleas well as as well 1 g ofas 1 m-ZrO g of m-ZrO2 were2 were individually individually added added instead instead of 1.2of 1.2 g ofg of Ru-Zn Ru-Zn (x ()/m-ZrOx)/m-ZrO22 catalyst, and the rest of the procedureprocedure waswas thethe same.same. The catalyticcatalytic activity towards benzene conversion and selectivity towards cyclohexene were calculated using the calibration area normalization method, 2 and the correlation coefﬁcientcoefficient (R2)) of all compounds was higher than 0.99. After each reaction, the organic phasephase was was removed removed via via a separating a separating funnel, funn andel, otherand partsother were parts recharged were recharged into the autoclaveinto the autoclaveto investigate to investigate the reusability the reusability of the catalysts of the through catalysts identical through experimental identical experimental procedures. procedures. Statistical validationStatistical validation was evaluated was evaluated by conducting by conducting catalytic experimentscatalytic experiments over Ru-Zn over (0.60)/m-ZrO Ru-Zn (0.60)/m-ZrO2 in three2 inseparate three separate runs under runs the under same the reaction same conditions,reaction conditions, and the standard and the standard deviation deviation for cyclohexene for cyclohexene yield was yield was calculated at 1.3% after 35 min of reaction time. For the used catalysts that needed to be characterized, samples were filtered and washed until the filtrate became neutral and no Zn2+ could be detected. Then solid samples were dried in Ar flow at 373 K and stored in ethanol, ready for further characterization.

Catalysts 2018, 8, 513 15 of 17 calculated at 1.3% after 35 min of reaction time. For the used catalysts that needed to be characterized, samples were filtered and washed until the filtrate became neutral and no Zn2+ could be detected. Then solid samples were dried in Ar flow at 373 K and stored in ethanol, ready for further characterization.

3.4. Catalysts Characterization X-ray diffraction (XRD) patterns for the fresh and spent catalysts were measured using an X’Pert Pro instrument from Philips (Almelo, The Netherland) at room temperature. The diffracted intensity of Cu-Kα radiation (λ = 0.154 nm) was recorded in the range of 2θ from 5◦ to 90◦, with a step size of 0.03◦. In addition, textural properties were analyzed using the Nova 1000 e-Physisorption Analyzer (Quantachrome Instruments, Boynton Beach, FL, USA). Before measurements, all the samples were evacuated at 523 K under vacuum pressure for 2 h, then the isotherms were taken at 77 K. The specific surface area (SBET) was determined using the Brunauer-Emmett-Teller (BET) model. Furthermore, elemental analysis was conducted via X Ray Fluorescence (XRF) using a S4 Pioneer instrument (Bruker AXS, Karlsruhe, Germany). Additionally, X-ray photoelectron spectroscopy (XPS) using a PHI Quantera SXM instrument from Ulvac-Phi (Kangawa, Japan) was utilized for analyzing the valence state of Ru and Zn on the catalyst surface. Al Kα (Eb = 1486.6 eV) was selected as the source of radiation and the vacuum degree was set to 6.7 × 10-8 Pa. The C1s (Eb = 284.8 eV) line as the binding energy reference was used for calibrating and correcting the energy scale. Furthermore, JEOL JEM 2100 transmission electron microscopy (TEM) combined with an energy dispersive spectrometer (EDS) (Akishima, Tokyo, Japan) was applied to investigate the dispersion of the catalysts, as well as the particle size. To investigate the hydrophilicity of the catalyst surface, a contact angle meter (JC2000 C1, Powereach, Shanghai, China) was used to measure water contact angle values (CAs) at ambient temperature for each sample. Moreover, temperature programmed reduction (TPR) was conducted with an Autosorb-IQ from Quantachrome (Boynton Beach, FL, USA). Typically, prior to reduction, a 10 mg sample was oxidized in flowing synthetic air (flow rate: 30 cm3 min−1) while being heated to 423 K and held for 1 h. After cooling in an Argon stream (flow rate: 30.0 cm3 min−1) to 293 K, the sample was treated for another 2 h. Then an Ar stream containing 10 Vol % H2 was introduced instead (30 cm3 min-1), while being heated to 573 K (10 K min-1) and held for 1 h. The hydrogen consumption was recorded and determined using a standard CuO calibration.

4. Conclusions

m-ZrO2 supported Ru-Zn catalysts, as well as unsupported Ru-Zn catalyst with m-ZrO2 as the dispersant, were evaluated for selective hydrogenation of benzene towards cyclohexene formation. Zn mainly exists as ZnO in Ru-Zn catalysts. Moreover, by increasing the Zn content in the Ru-Zn/m-ZrO2 catalyst, more (Zn(OH)2)3(ZnSO4)(H2O)3 was generated and chemisorbed on the surface of the catalysts. This decreases the catalytic activity towards benzene (99.8% over Ru/m-ZrO2 vs. 14.9% over Ru-Zn (1.02)/m-ZrO2 after 20 min of reaction) and improves the selectivity to cyclohexene formation (68.0% over Ru-Zn (0.06)/m-ZrO2 vs. 88.8% over Ru-Zn (1.02)/m-ZrO2 after 40 min of reaction). When the molar ratio of Zn to Ru reached 0.6, the highest cyclohexene yield of 60.9% was achieved. In addition, when m-ZrO2 was applied as the dispersant instead of being utilized as the support, both catalytic activity towards benzene conversion and selectivity to cyclohexene were suppressed; that is, only 69.5% of benzene conversion and 71.5% of cyclohexene selectivity were obtained over unsupported Ru-Zn (0.60), while Ru-Zn (0.60)/m-ZrO2 gave 84.6% of benzene conversion and 71.5% of selectivity to cyclohexene after 35 min of reaction time. This can be rationalized in terms that m-ZrO2 as the support has a strong interaction with the Ru-Zn catalyst, beneﬁting the dispersion of Ru and the chemisorption of (Zn(OH)2)3(ZnSO4)(H2O)3. Notably, the reusability of Ru-Zn (0.60)/m-ZrO2 was evaluated for selective hydrogenation of benzene, and no obvious decrease in catalytic activity towards cyclohexene formation was observed after 17 reaction iterations, over which at least a 59.3% cyclohexene yield can be achieved. Catalysts 2018, 8, 513 16 of 17

Author Contributions: H.S., Z.C., and Z.P. conceived and designed the experiments; H.L. performed the experiments; Z.L. and S.L. analyzed the data; L.C. contributed reagents, materials, and analysis tools; H.S. wrote the paper. Funding: This research received no external funding. Acknowledgments: This work was supported by the National Nature Science Foundation of China (21273205), the Key Scientific Research Project of Henan Province (18A180018), the Environmental Catalysis Innovative Research Team of Zhengzhou Normal University (702010), and the Student Innovation Program of Zhengzhou Normal University (DCZ2017014). Conflicts of Interest: The authors declare no conflict of interest.

References

1. Yan, X.H.; Zhang, Q.; Zhu, M.Q.; Wang, Z.B. Selective hydrogenation of benzene to cyclohexene over

Ru-Zn/ZrO2 catalysts prepared by a two step impreganation method. J. Mol. Catal. A Chem. 2016, 413, 85–93. [CrossRef] 2. Martins, L.M.D.R.D.S.; Carabineiro, S.A.C.; Wang, J.; Rocha, B.G.M.; Maldonado-Hódar, F.J.; Pombeiro, A.J.L.D.O. Supported Gold Nanoparticles as Reusable Catalysts for Oxidation Reactions of Industrial Signiﬁcance. ChemCatChem 2017, 9, 1211–1221. [CrossRef] 3. Martins, L.M.; de Almeida, M.P.; Carabineiro, S.A.; Figueiredo, J.L.; Pombeiro, A.J. Heterogenisation of a C-Scorpionate FeII Complex on Carbon Materials for Cyclohexane Oxidation with Hydrogen Peroxide. ChemCatChem 2013, 5, 3847–3856. [CrossRef] 4. De Almeida, M.P.; Martins, L.M.D.R.S.; Carabineiro, S.A.C.; Lauterbach, T.; Rominger, F.; Hashmi, A.S.K.; Pombeiro, A.J.L.; Figueiredo, J.L. Homogeneous and heterogenised new gold C-scorpionate complexes as catalysts for cyclohexane oxidation. Catal. Sci. Technol. 2013, 3, 3056–3069. [CrossRef] 5. Carabineiro, S.A.C.; Martins, L.M.D.R.S.; Avalos-Borja, M.; Buijnsters, J.G.; Pombeiro, A.J.L.; Figueiredo, J.L. Gold nanoparticles supported on carbon materials for cyclohexane oxidation with hydrogen peroxide. Appl. Catal. A Gen. 2013, 467, 279–290. [CrossRef] 6. Sun, H.J.; Chen, Z.H.; Li, C.G.; Chen, L.X.; Peng, Z.K.; Liu, Z.Y.; Liu, S.C. Selective Hydrogenation of Benzene to Cyclohexene over Ru-Zn Catalysts: Mechanism Investigation on NaOH as a Reaction Additive. Catalysts 2018, 8, 104. [CrossRef] 7. Sun, H.J.; Chen, Z.H.; Li, C.G.; Chen, L.X.; Li, Y.; Peng, Z.K.; Liu, Z.Y.; Liu, S.C. Selective Hydrogenation

of Benzene to Cyclohexene over Monometallic Ru Catalysts: Investigation of ZnO and ZnSO4 as Reaction Additives as Well as Particle Size Effect. Catalysts 2018, 8, 172. [CrossRef] 8. Nagahara, H.; Ono, M.; Konishi, M.; Fukuoka, Y. Partial hydrogenation of benzene to cyclohexene. Appl. Surf. Sci. 1997, 121–122, 448–451. [CrossRef]

9. Zhou, G.B.; Wang, H.; Pei, Y.; Qiao, M.H.; Sun, B.; Zong, B.N. Pore size effect of Ru-Zn/ZrO2 catalyst on partial hydrogenation of benzene to cyclohexene. Acta Chim. Sin. 2017, 75, 321–328. [CrossRef] 10. Peng, Z.K.; Liu, X.; Li, S.H.; Li, Z.J.; Li, B.J.; Liu, Z.Y.; Liu, S.C. Heterophase-structured nanocrystals as superior supports for Ru-based catalysts in selective hydrogenation of benzene. Sci. Rep. 2017, 7, 39847. [CrossRef][PubMed] 11. Liu, S.C.; Liu, Z.Y.; Wang, Z.; Wu, Y.M.; Yuan, P. Characterization and study on performance of the

Ru–La–B/ZrO2 amorphous alloy catalysts for benzene selective hydrogenation to cyclohexene under pilot conditions. Chem. Eng. J. 2008, 139, 157–164. [CrossRef] 12. Liu, J.L.; Zhu, Y.; Liu, J.; Pei, Y.; Li, Z.H.; Li, H.; Li, H.X.; Qiao, M.H.; Fan, K.N. Discrimination of the roles of

CdSO4 and ZnSO4 in liquid phase hydrogenation of benzene to cyclohexene. J. Catal. 2009, 268, 100–105. [CrossRef] 13. Fan, G.F.; Li, R.X.; Li, X.J.; Chen, H. Effect of organic on partial hydrogenation of benzene. Catal. Commun. 2008, 9, 1394–1397. [CrossRef] 14. Sun, H.J.; Wang, H.X.; Jiang, H.B.; Li, S.H.; Liu, S.C.; Liu, Z.Y.; Yuan, X.M.; Yang, K.J. Eeffect of

(Zn(OH)2)3(ZnSO4)(H2O)5 on the performance of Ru-Zn catalyst for benzene selective hydrogenation to cyclohexene. Appl. Catal. A Gen. 2013, 450, 160–168. [CrossRef] Catalysts 2018, 8, 513 17 of 17

15. Sun, H.J.; Jiang, H.B.; Dong, Y.Y.; Wang, H.X.; Pan, Y.J.; Liu, S.C.; Tang, M.S.; Liu, Z.Y. Effect of alcohols as additives on the performance of a nano-sized Ru-Zn(2.8%) catalyst for selective hydrogenation of benzene to cyclohexene. Chem. Eng. J. 2013, 218, 415–424. [CrossRef] 16. Sun, H.J.; Pan, Y.J.; Jiang, H.B.; Li, S.H.; Zhang, Y.X.; Liu, S.C.; Liu, Z.Y. Effect of transition metals (Cr, Mn, Fe, Co, Ni, Cu and Zn) on the hydrogenation properties of benzene over Ru-based catalyst. Appl. Catal. A Gen. 2013, 464–465, 1–9. [CrossRef] 17. Da-Silva, J.W.; Cobo, A.J.G. The role of the titania and silica supports in Ru-Fe catalysts to partial hydrogenation of benzene. Appl. Catal. A Gen. 2003, 252, 9–16. [CrossRef] 18. Fan, G.Y.; Jiang, W.D.; Wang, J.B.; Li, R.X.; Chen, H.; Li, X.J. Selective hydrogenation of benzene to cyclohexene

over RuCoB/γ-Al2O3 without additive. Catal. Commn. 2008, 10, 98–102. [CrossRef] 19. Sun, H.J.; Jiang, H.B.; Li, S.H.; Wang, H.X.; Pan, Y.J.; Dong, Y.Y.; Liu, S.C.; Liu, Z.Y. Selective hydrogenation of

benzene to cyclohexene over nanocomposite Ru-Mn/ZrO2 catalyst. Chin. J. Catal. 2013, 34, 684–694. [CrossRef] 20. Sun, H.J.; Dong, Y.Y.; Li, S.H.; Jiang, H.B.; Zhang, Y.X.; Liu, Z.Y.; Liu, S.C. The role of La in improving the selectivity to cyclohexene of Ru catalyst for hydrogenation of benzene. J. Mol. Catal. A Chem. 2013, 368–369, 119–124. [CrossRef] 21. Liao, H.G.; Ouyang, D.H.; Zhang, J.; Xiao, Y.J.; Liu, P.L.; Hao, F.; You, K.Y.; Luo, H.A. Benzene hydrogenation over oxide-modiﬁed MCM-41 supported ruthenium-lanthanum catalyst: The inﬂuence of zirconia crystal form and surface hydrophilicity. Chem. Eng. J. 2014, 243, 207–216. [CrossRef] 22. Sun, H.J.; Pan, Y.J.; Li, S.H.; Zhang, Y.X.; Dong, Y.Y.; Liu, S.C.; Liu, Z.Y. Selective hydrogenation of benzene to cyclohexene over Ce-promoted Ru catalysts. J. Energy Chem. 2013, 22, 710–716. [CrossRef] 23. Liu, J.L.; Zhu, L.J.; Pei, Y.; Zhuang, J.H.; Li, H.; Li, H.X.; Qiao, M.H.; Fan, K.N. Ce-promoted Ru/SBA-15 catalysts prepared by a “two solvent” impregnation method for selective hydrogenation of benzene to cyclohexene. Appl. Catal. A Gen. 2009, 353, 282–287. [CrossRef] 24. Zhou, G.B.; Tan, X.H.; Pei, Y.; Fan, K.N.; Qiao, M.H.; Sun, B.; Zong, B.N. Structural and Catalytic

Properties of Alkaline Post-Treated Ru/ZrO2 Catalysts for Partial Hydrogenation of Benzene to Cyclohexene. ChemCatChem 2013, 5, 2425–2435. [CrossRef] 25. Wang, J.Q.; Wang, Y.Z.; Xie, S.H.; Qiao, M.H.; Li, H.X.; Fan, K.N. Partial hydrogenation of benzene to

cyclohexene on a Ru-Zn/m-ZrO2 nanocomposite catalyst. Appl. Catal. A Gen. 2004, 272, 29–36. [CrossRef] 26. Mazzieri, V.A.; L’Argentiere, P.C.; Coloma-Pascual, F.; Fígoli, N.S. Effect of Chlorine on the Properties of

Ru/Al2O3. Ind. Eng. Chem. Res. 2003, 42, 2269–2272. [CrossRef] 27. Ramos-Fernández, E.V.; Ferreira, A.F.P.; Sepúlveda-Escribano, A.; Kapteijn, F.; Rodríguez-Reinoso, F. Enhancing the catalytic performance of Pt/ZnO in the selective hydrogenation of cinnamaldehyde by Cr addition to the support. J. Catal. 2008, 258, 52–60. [CrossRef] 28. Evans, S. Energy calibration secondary standards for X-ray photoelectron spectrometers. Surf. Interface Anal. 1985, 7, 299–302. [CrossRef]

29. Mizuno, N.; Fujii, H.; Igarashi, H.; Misono, M. Formation of lanthanum cobalt oxide (LaCoO3) highly dispersed on zirconium dioxide. J. Am. Chem. Soc. 1992, 114, 7151–7158. [CrossRef] 30. Struijk, J.; d’Angremond, M.; Lucas-de Regt, W.J.M.; Scholten, J.J.F. Partial liquid phase hydrogenation of benzene to cyclohexene over ruthenium catalysts in the presence of an aqueous salt solution I. Preparation, characterization of the catalyst and study of a number of process variables. Appl. Catal. A Gen. 1992, 83, 263–295. [CrossRef] 31. Milone, C.; Neri, G.; Donato, A.; Musolino, M.G.; Mercadant, L. Selective Hydrogenation of Benzene to

Cyclohexene on Ru/γ-Al2O3. J. Catal. 1996, 159, 253–258. [CrossRef] 32. Liu, Z.Y.; Sun, H.J.; Wang, D.B.; Guo, W.; Zhou, X.L.; Liu, Z.Y.; Li, Z.J. Selective Hydrogenation of Benzene to Cyclohexene over Ru-Zn Catalyst with Nanosized Zirconia as Dispersant. Chin. J. Catal. 2010, 31, 150–152. [CrossRef] 33. Sun, H.J.; Chen, L.X.; Huang, Z.X.; Sun, L.L.; Li, Y.Y.; Liu, S.C.; Liu, Z.Y. Effect of reduction medium and reduction temperature on the performance of Ru-Zn catalysts for selective hydrogenation of benzene to cyclohexene. Chem. Ind. Eng. Prog. 2017, 36, 2962–2970.

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