Qi Zhang,Xuhua Yan,Peng Zheng,Zhengbao Wang*

College of Chemical and Biological Engineering,Zhejiang University,Hangzhou 310027,China

1.Introduction

As an important intermediate for producing nylons and fine chemicals,cyclohexene is commercially important.The route of selective hydrogenation of benzene to cyclohexene is of significant industrial interest in terms of inexpensive feedstock,simplified operation and energy-saving.In 1990,Asahi-Kasei Chemical Co.,Ltd.developed the first commercial plant of 60,000 ton cyclohexene per year from the selective hydrogenation of benzene[1].Ru–Zn catalysts were used and the reaction was carried out in a mechanically agitated tetra-phase(oil–water–gas-catalyst)reactor at 150 °C under 5 MPa of H2pressure in the presence of a solution of ZnSO4and suspended ZrO2.

In recent years,several kinds of Ru-based catalysts are developed in the tetra-phase catalytic system[2–19].However,the unsupported Ru–Zn catalyst remains the only one used in industry.Ru–Zn catalysts are mainly prepared by co-precipitation method[1,20,21]and chemical reduction method[22].Zn promoter could modify ruthenium active sites and dramatically improve the selectivity to cyclohexene.Liu's group carried out a lot of research about the Ru–Zn unsupported catalysts[20–25].In 2002,Liuet al.indicated that Ru and Zn existed as a solid solution with Zn atoms dispersing incidentally and disorderly in Ru crystal lattices(3–5 nm)[23].Furthermore,they suggested that the introduction of Zn species had a significant influence on the catalytic performance.Zn species led to the obvious decrease of surface area of Ru–Zn catalysts.As a result,the activity decreased and the selectivity to cyclohexene was enhanced.Zn species formed solid solution with Ru under low Zn content while it formed ZnO phase separately under high Zn content[24–26].Recently,Sunet al.[20–21]proposed that ZnO on the catalyst surface could react with ZnSO4to form a(Zn(OH)2)3(ZnSO4)(H2O)5salt which played a key role in improving the selectivity.The conversion of benzene dropped down slowly while the selectivity to cyclohexene was enhanced dramatically with the addition of Zn content(0–29.1%).In addition,Wanget al.[5]suggested that metallic zinc occupied the most reactive ruthenium sites and donated an electron to ruthenium,therefore it affected its catalytic behavior.

In our previous work,it was reported that hexagonal phases of ZnO appeared when the Zn content was higher than 16.7 wt%in Ru–Zn catalysts[26].It is also reported that different shapes of metal nanoparticles affected the catalytic activity of various organic and inorganic reactions[27].Liaoet al.[28]suggested that the plate-like ZnO crystals gave higher selectivity toward methanol from CO2hydrogenation than rod-shaped ZnO crystals for Cu/ZnO catalysts,because the exposed polar face(002)in plate-like ZnO crystals showed a much stronger material synergy with copper than other crystal facets.Leiet al.[29]also found that the activities of CuO/ZnO catalysts depended strongly on the morphology of ZnO.

In this study,to investigate the effects of ZnO crystals on the catalytic properties of Ru–Zn catalysts,Ru–Zn catalysts with different Zn contents and ZnO morphologies were obtained by adjusting the amount of NaOHin the co-precipitation process.The catalysts of different reduction times were also investigated.The obtained catalysts were characterized by N2physisorption,XRD,ICP-OES,SEM,and Mastersizer,and their catalytic properties were evaluated in the selective hydrogenation of benzene to cyclohexene.

2.Experimental

2.1.Catalyst preparation

A series of Ru–Zn catalysts were prepared by co-precipitation method,according to the procedures described in our previous paper[26].In short,a NaOH solution was quickly added into a solution of RuCl3·xH2O(2.5 g;Shenyang Nonferrous Metal Research Institute,China;Ru content:36 wt%–38 wt%)and ZnCl2(0.60 g;Wako Pure Chemical Industries,Japan)in water(250 ml)being stirred at 80°C and the resulting solution was kept stirring at80°C for2 h and then aged overnight.Atotal150 g of the bottom precipitate and solution were reduced under 5 MPa of H2pressure at 150 °C and a stirring rate of 1000 r·min?1for several hours(1–8 h,typically 3 h)in a 250 ml Teflon-lined autoclave.The resulting Ru–Zn black powders were washed with water until no Cl?could be detected by AgNO3solution and stored in water.Various Ru–Zn catalysts,as shown in Table 1,were prepared using different amounts of NaOH.

2.2.Catalyst characterization

The Ru–Zn catalysts were vacuum-dried at 60 °C prior to characterization.X-ray powder diffraction(XRD)patterns were collected on an Ultima IV diffractometer(Ultima,Rigaku,Japan)using Cu Kαradiation.N2physisorption experiments were performed on a Micromeritics ASAP 2020 system.The total surface area of each catalyst was obtained using the BET equation,and the pore size was determined by BJH desorption analysis.Zn contents in the catalysts were measured on inductively coupled plasma optical emission spectrometer(ICP-OES,Optima 8000,PerkinElmer).Scanning electron microscope(SEM,Hitachi TM-1000)was employed to observe the morphology of the catalysts.Temperature-programmed reduction(H2-TPR)experiments were performed on a PX200 Multi-sorption equipment(Tianjin Golden Eagle Technology Co.,Ltd.,China).About 5 mg of dry unreduced catalyst was added in a U-shaped quartz reactor and it was pretreated at 200 °C for 2 h under Ar stream(20 ml·min?1).Then the reactor was heated from 20 to 350 °C at a rate of 5 °C·min?1with 5%H2–95%Ar stream(20 ml·min?1)and the amount of H2consumed was determined by a thermal conductivity detector(TCD).The particle size of aggregated catalysts was measured on Malvern laser particle size analyzer(Mastersizer 2000).The sample was dispersed in water and pretreated under ultrasound.

2.3.Catalytic testing

The selective hydrogenation of benzene was performed in a 250 ml autoclave(Hastelloy C276,Beijing Century Senlong Experimental Apparatus Co.,Ltd.).The typical reaction conditions are as follows.The autoclave was charged with 70 ml of water,0.12 g of Ru–Zn catalyst(dry base),8.40 g ZnSO4·7H2O,and 0.65 g ZrO2powder(RC-100,Daiichi Kigenso Kagaku Kogyo Co.,Ltd.,Japan).The autoclave was purged five times with hydrogen and then was heated at a stirring(magnetic stirrer)rate of 300 r·min?1and hydrogen pressure of 1.0 MPa.35 ml benzene was saved in a stainless steel tank connected with the autoclave and introduced into the autoclave when the temperature was increased to 150°C.Meanwhile,the H2pressure and the stirring rate were adjusted to 5 MPa and 1200 r·min?1,respectively.The reaction was carried out at150°C for 25–70 min.The products in the organic phase were analyzed by a GC-1690 gas chromatography(Hangzhou Kexiao Chemical Equipment Co.,Ltd.)with a FID detector.The benzene conversion and cyclohexene selectivity were calculated by the GC results.The specific activity of the catalyst(g·g?1·h?1)is defined as the converted benzene amount(g)per hour for 1 g Ru,and the γ40is the specific activity at the benzene conversion of 40%.

3.Results and Discussion

3.1.Effects of NaOH amount

3.1.1.Characterization of catalysts

As shown in Table 1,Ru–Zn catalysts with different Zn contents were prepared by changing the amount of NaOH in the co-precipitation process.The pH values of the solution after co-precipitation(abbreviated as COP solution)and the solution after reduction(abbreviated as RED solution)increased gradually with increasing the NaOH amount for Ru–Zn-1 and Ru–Zn-2,and the pH value of RED solution was close to that of COP solution.However,for catalysts Ru–Zn-3 and Ru–Zn-4,the pH values of the COP solution and the RED solution increased significantly with increasing the NaOH amount,and the pH value of RED solution was higher than that of COP solution.From Table 1,it can also be seen that the Zn content first increased then decreased when increasing the amount of NaOH from 1.5 g to 1.8 g.The highest Zn content of 22.8%was obtained as the NaOH amount reached 1.7 g.SEM technique is a direct method to observe the morphologies of the catalysts.SEM images of catalysts Ru–Zn-1 to Ru–Zn-4 are shown in Fig.1.It can be seen from Fig.1a that ZnO crystals exist as hexagonal wurtzite structure,which was also reported in our previous work[26].The length and width of ZnO pyramids of Ru–Zn-1 are about 10–14 μm and 1–2 μm by measurement,respectively.The average diameters of ZnO crystals are shown in Table 1.ZnO crystals became shorter and thinner and their amounts increased with increasing the amount of NaOH(pH).Itis observed that ZnO crystals were well dispersed and inserted into Ru particles.At higher pH(＞12)after reduction,slim needle-like shaped ZnO rods are produced.

Table 1Physiochemical properties of Ru–Zn catalysts prepared with different NaOH contents

The changes of Zn content and ZnO crystals can be explained by the dissolution of Zn species and growth mechanism of ZnO crystals.According to the literature[30],in the co-precipitation and reduction(hydrothermal)process,the ZnO crystals are considered to formviathree reactions:

Fig.1.SEM images of(a–d)Ru–Zn-1 to Ru–Zn-4 catalysts.

The Zn(OH)2precipitate can form when the pH value is higher than a certain value,and the growth unit of ZnO crystals,Zn(OH)42?can form in the high alkali solution or during the hydrothermal process(e.g.,reduction at 150°C).Then,due to heat convection,diffusion of ions and deregulation movement among molecules and ions in the solution dur-ZnO nucleus is formed and then the ZnO is precipitated[30].According to the literature,the pH variation of precursor solution affects the morphology of ZnO crystals significantly.Vernardouet al.[31]reported that the increase of the pH of solution significantly led to a modification of the ZnO morphology from rod-like to prism-like and flower-like structures.Liet al.[32–34]pointed out that elongated rods with pyramidalshaped and flat ends were grown from neutral solutions while needleshaped ZnO rods with sharp ends were produced from basic solutions.Wahabetal.[35]and Huangetal.[36]found that the solution pH played a decisive role in determining the structure of ZnO.

The needed pH value for the formation of Zn(OH)2precipitate is～5.2,therefore,the OH?ions cannot completely precipitate Zn2+ions as the pH values of COP and RED solutions are ～5 for catalysts Ru–Zn-1 and Ru–Zn-2.This is con firmed by the Zn2+contents in COP and RED solutions.The concentration of Zn2+ions in COP and RED solutions for Ru–Zn-1 was 299 mg·L–1and 455 mg·L–1,respectively.And it was 56 mg·L–1and 136 mg·L–1for Ru–Zn-2,respectively.This indicates that more Zn(OH)2precipitate was formed when a higher NaOH amount(pH value)was added.Therefore,catalysts Ru–Zn-1 and Ru–Zn-2 had lower Zn contents than the loading value,and Ru–Zn-2 had higher Zn content than Ru–Zn-1.It should be noted that part of COP clear solution was removed before reduction process.This also led to the lower Zn content of the final catalysts of Ru–Zn-1 and Ru–Zn-2.The amount of Zn(OH)2precipitates was very low for Ru–Zn-1 because of the low pH,leading to the low concentration of the growth unit(Zn(OH)42?).Therefore,only a few ZnO nuclei and crystals were formed during the reduction process and the crystals are pyramidal-shaped.The amount of Zn(OH)2precipitates was higher for Ru–Zn-2 than Ru–Zn-1 because of the higher NaOH amount,leading to higher concentration of the growth units.Therefore,many ZnO nuclei and needle-shaped crystals were obtained as shown in Fig.1b.

When the alkalinity increases(Ru–Zn-3),the concentration of OH?is high and the Zn2+ions can completely precipitate.The concentration of Zn2+ions in COP and RED solutions is 9.5 mg·L–1and 10.8 mg·L–1,respectively.The super saturation of the growth units in the solution is relatively high.Therefore,for Ru–Zn-3,the highest Zn content(22.1%)was obtained and many needle-shaped ZnO crystals were on the catalyst particles.When the NaOH amount reached 1.8 g(Ru–Zn-4),the pH value of COP solution was～11 and that of RED solution was～12.The following reaction may happen in the solution:

The medial product Zn(OH)2dissolved in the alkali solution and formed Zn(OH)42?complexes in the solution.Therefore,the concentration of Zn2+in the bulk COP solution for Ru–Zn-4 was higher than that for Ru–Zn-3,leading to the lower Zn content of Ru–Zn-4.As the super saturation of the solution was very high and the Zn(OH)42?complexes were surrounded by a large amount of OH?,many nuclei formed,resulting in slim needle-shaped ZnO crystals on Ru particles or in the bulk reduced solution.These slim free ZnO crystals in the solution are prone to be removed by decantation,resulting in the lower Zn content also in the final catalyst.The pH value of RED solution is higher than that of COP solution since OH?ions are released into the solution during dehydration reaction of Zn(OH)42?complexes.

Fig.2 shows the XRD patterns of the above catalysts.The diffraction peaks at 2θ of 38.4°,42.2°,44.0°,58.3°,69.4°,78.4°and 84.7°are readily assigned to metal Ru(Ru,JCPDs 65-1863).Meanwhile,the diffraction peaks at 2θ of 31.8°,34.4°,36.3°,47.5°,56.6°,62.9°,68.0°and 69.1°belong to ZnO(ZnO,JCPDs 36-1451).The three most intense peaks at 31.8°,34.4°,and 36.3°correspond to the(100),(002)and(101)planes of ZnO crystals,respectively.The XRD peak intensities of ZnO crystals for Ru–Zn-1 were weak,being consistent with the Zn content(Table 1)and SEM image(Fig.1a).Basically,the intensity of the ZnO diffraction peaks increased with the increase of Zn content in the catalyst.However,the intensities of(100)and(101)peaks for Ru–Zn-3 were slightly weaker than those for Ru–Zn-2.This may be due to the shorter and thinner ZnO crystals of Ru–Zn-3.When the amount of NaOH was increased to 1.8 g(Ru–Zn-4),the intensities of XRD peaks decreased,which is consistent with the Zn content in the catalyst.The intensity ratio of peak(101)/(100)of ZnO crystals tended to increase as the pH increased,especially when the pH of RED solution was＞10(Ru–Zn-3 and Ru–Zn-4).The results are similar with the results reported in the synthesis of ZnO nano-particles.Singhet al.[37]suggested that ZnO nano-rods transformed into ZnO nano-particles when the alkalinity of solution increased.The trend of XRD peak intensity with increasing pH is almost the same with our results.Aliaset al.[38]synthesized ZnO particles in aqueous solution with pH values ranging from 6(acidic)to 11(alkaline).They found that the highest ZnO intensity peak(2θ =36.16°)appeared at pH 9.

Fig.2.XRD patterns of the Ru–Zn catalysts with different NaOH contents.

The physiochemical properties of Ru–Zn-1 to Ru–Zn-4 catalysts are also shown in Table 1.According to the calculated crystal size by XRD results,Ru crystals were 3–4 nm and Ru–Zn-3 catalyst had relatively big Ru crystal size,3.6 nm.The specific surface areas for the catalysts were 60–70 m2·g?1and first decreased then increased with increasing the NaOH amount.Ru–Zn-3 catalyst had the lowest surface area and minimum pore size,while Ru–Zn-4 had the highest surface area.It could be attributed to the change of ZnO and Ru particles as well as the interaction between them.

Fig.3 shows the H2-TPR pro files of Ru–Zn catalyst precursors before reduction.The reduction temperature for Ru–Zn-1 was 132 °C,with a weak peak shoulder at 151°C.The intensity of the main peak decreased while more peak shoulders appeared between 104 and 169°C when the amount of NaOH was increased.Ru–Zn-3 catalyst had three shoulders and the highest reduction temperature was at 165°C.As reported by Yanet al.[39],the reduction peaks can be attributed to the reduction of Ru(OH)3or RuxOy.After precipitation in the NaOH solution,RuCl3precursors convert to Ru(OH)3and then dehydrated into RuxOyduring drying process.The peak shoulders are the step-by-step reduction of Ru oxides,which has been reported by Liu[21]and conformed with our previous results[26].Addition of Zn species effectively inhibited the reduction of the Ru species[26]thus the reduction peaks shift to higher reduction temperature.It indicates that the interaction between Ru species and Zn species increases,being consistent with the change of Ru–Zn-1 to Ru–Zn-3.However,the reduction peaks of Ru–Zn-4 moved toward low temperature compared with Ru–Zn-3.It indicates that the interaction between Ru and ZnO in Ru–Zn-4 is weaker than that of Ru–Zn-3.

In summary,the Zn content first increased then decreased with increasing the NaOH amount in the co-precipitation process.The ZnO crystals changed from relatively thick pyramidal-shaped crystals to slimneedle-shaped ZnOrods,which determine the interaction between Ru particles and ZnO crystals.At pH of 10.6(RED solution),Ru–Zn catalyst had the highest Zn content and slim needle-shaped ZnO rods,in which Ru particles had the strongest interaction with ZnO crystals.

Fig.3.H2-TPR pro files of Ru–Zn catalysts before reduction.

3.1.2.Catalytic results

The catalytic performances of catalysts Ru–Zn-1 to Ru–Zn-4 are shown in Table 2.The selectivity to cyclohexene was all about 80%because the Zn content was relatively high[26].On the other hand,the activity of Ru–Zn catalyst(γ40) first decreased then increased with the increase of NaOH amount.Ru–Zn-3 catalyst had the lowest activity.The reaction time for Ru–Zn-1 was 25 min at a benzene conversion of 42.6%,while it was 70 min for Ru–Zn-3.As reported in the literature[20–21],the conversion of benzene decreased from 68.1%to 6.8%withthe Zn content of Ru–Zn catalyst increasing from 2.6%to 29.1%.The activity is suppressed with Zn content in the Ru–Zn catalyst since Zn acts as a promoter and poisons the Ru active sites partly.The decrease of the activity of Ru–Zn catalysts from Ru–Zn-1 to Ru–Zn-3 is consistent with the Zn content of the catalyst as shown in Table 1.Therefore,the Zn content is one of factors that determine the activity of Ru–Zn catalysts in this study.However,it is worth noting that the reaction time for Ru–Zn-4(50 min)is 28.5%less than that for Ru–Zn-3 although the Zn content of Ru–Zn-4 catalyst is only 0.7%less than that of Ru–Zn-3 catalyst.And also the reaction time of Ru–Zn-3 was much longer compared with the increase of Zn content from Ru–Zn-2.According to the above characterization of catalysts,it is suggested that the interaction between Ru particles and ZnO crystals plays an important role in the activity.The interaction between Ru and ZnO in Ru–Zn-3 was stronger than those of Ru–Zn-1 and Ru–Zn-2,and also than that of Ru–Zn-4.

Table 2Catalytic performance of Ru–Zn catalysts prepared with different NaOH contents①

As shown in Fig.4,the dispersion state of the Ru–Zn catalysts in water was different.That is,the dispersion of catalysts Ru–Zn-1 to Ru–Zn-3 became worse with the increase of NaOH amount in the coprecipitation process,whereas the dispersion of Ru–Zn-4 increased.As discussed in Section 3.1.1,this was determined by the alkalinity of precipitation and growth of ZnO crystals in the reduction process.For the NaOH amount of 1.7 g,Zn2+and Ru3+ions could be precipitated completely under the almost neutral solution and the alkalinity of reduction solution was mild,therefore,Ru particles and ZnO crystals had the strongest interaction.For the NaOH amount of 1.8 g,the pH of both the COP and RED solutions was higher than 11,therefore,more growth units Zn(OH)42?were in the bulk solution,resulting in needleshaped ZnO crystals in the bulk solution.This leads to the fine dispersion of catalyst particles in water.As shown in Table 2,the dispersion of catalysts in water was consistent with the catalytic activity.

Fig.4.The dispersion state of the catalysts(Ru–Zn-1 to Ru–Zn-4)in water.

To demonstrate the effect of the dispersion of catalysts in water on the catalytic activity,the effect of pretreatment of stirring 0.5 h prior to heating the reaction solution on the catalytic reaction was investigated.The weight-specific activity at a benzene conversion of 40%(γ40,g C6H6·(g Ru)?1·h?1)is given in Fig.5.It is obvious that γ40of all catalysts increased after stirring for 0.5 h at 1200 r·min?1prior to heating compared with that of fresh catalysts without pre-stirring.It is found that the increasing ratio of catalysts Ru–Zn-1 to Ru–Zn-4 is different.As shown in Fig.5,Ru–Zn-3 showed the highest increasing rate of 25%,due to the worst original dispersion in water.The increase of the catalytic activity can be attributed to the better dispersion of catalysts by pre-stirring at a high stirring rate of 1200 r·min?1.

Fig.5.The mass-specific activity at the benzene conversion of 40%(γ40)of(a)fresh catalysts and(b)catalysts stirred for 0.5 h at 1200 r·min?1 prior to heating.(Reaction conditions:0.12 g catalyst,8.40 g ZnSO4·7H2O,0.65 g ZrO2,70 ml H2O,35 ml benzene,5 MPa H2,150 °C,stirring rate of 1200 r·min?1.)

Although Ru–Zn-1 showed the highest catalytic activity,it is difficult to obtain high selectivity to cyclohexene with high reproducibility due to the limited Zn content.Ru–Zn-2 and Ru–Zn-4 had similar catalytic activity,which is higher than that of Ru–Zn-3.However,the free ZnO crystals from the Ru particles are easy to be removed during washing.Therefore,it is concluded that Ru–Zn-2 is the best catalyst.

3.2.Effects of reduction time

In general,crystals grow big with the increase of the growth time.To elucidate the effect of the reduction time on the growth of ZnO crystals in the Ru catalyst particles and further their catalytic performance,catalysts Ru–Zn-5 to Ru–Zn-7 with the same NaOH amount with Ru–Zn-2 were prepared for different reduction times.The detailed preparation and characteristic information are listed in Table 3.

Table 3Effects of the reduction time on the physiochemical properties of Ru–Zn catalysts

The SEM images and XRD patterns of catalysts reduced at different times are presented in Figs.6 and 7,respectively.All the catalysts had similar XRD peak intensities of ZnO crystals,indicating no further growth of ZnO crystals after longer reduction than 1 h.From the SEM images,it can also be seen that there is nearly no difference on the ZnO morphology.All the ZnO crystals with small pyramidal-shaped structures are well-dispersed and are about 6–7 μm in length.Luet al.[40]obtained similar results that the formation and growth ZnO powder proceeded rapidly as the temperature reached 100°C thus prolonging time did not significantly influence the characteristics of ZnO powder during the hydrothermal synthesis of ZnO crystals.They also found that the characteristics of ZnO powder profoundly depended on the pH of the starting solutions.In the reduction system of this study,ZnO crystals formed steadily at 150°C with the existence of Ru particles.

Fig.6.SEM images of catalysts with different reduction times,(a)Ru–Zn-5:1 h,(b)Ru–Zn-2:3 h,(c)Ru–Zn-6:6 h and(d)Ru–Zn-7:8 h.

Fig.7.XRD patterns of the Ru–Zn catalysts with different reduction times.

Fig.8.The particle size distribution of catalysts with different reduction times from the measurement on Malvern laser particle size analyzer.

However,as shown in Table 3,the BET surface area of above catalysts decreased rapidly with increasing the reduction time and the calculated Ru crystal size increased from 3.2 nm to 3.8 nm.The particle size distribution of catalysts dispersed in water is given in Fig.8.It reveals the state of aggregation of the catalyst particles in water.As the reduction time increased,d0.5number became bigger from 15.3 μm to 61.0 μm,indicating that the aggregation of Ru–Zn catalysts occurred with prolonging the reduction time.Therefore,we conclude that the decrease of BET surface area of the catalysts with the increase of reduction time is due to the growth of Ru nanoparticles and aggregation of the catalyst particles.As shown in Table 4,the reaction time achieving the same benzene conversion increased with the increase of reduction time.That means that the activity of Ru–Zn catalysts(γ40)decreases with increasing the reduction time.This is probably due to the decrease of the BET surface area of catalysts,which is due to the growth of Ru nanoparticles and aggregation of the catalyst particles.

Table 4Effects of the reduction time on the reaction properties of Ru–Zn catalysts

4.Conclusions

A series of Ru–Zn catalysts with different Zn contents and ZnO morphologies were prepared by changing the amount of NaOH in the coprecipitation process.With the increase of NaOH amount,the Zn content first increased then decreased and the ZnO crystals changed from relatively thicker pyramidal-shaped crystals to slimmer needle-shaped crystals.The catalyst(Ru–Zn-3)prepared with 1.7 g NaOH,0.6 g ZnCl2and 2.5 g RuCl3·xH2O had the highest Zn content and strongest interaction between ZnO crystals and Ru nanoparticles because the Zn2+ions were completely precipitated by OH?at pH of～6.5 and the growth of ZnO from Zn(OH)2/Zn(OH)42?well occurred at pH of～10.Ru–Zn-3 had the lowest activity in selective hydrogenation of benzene to cyclohexene.It is concluded that the activity of the Ru–Zn catalyst was determined by the Zn content and the interaction between Ru particles and ZnO crystals.That is,the activity decreases with increasing Zn content and stronger interaction between ZnO crystals and Ru nanoparticles.Ru–Zn-1 from the NaOH amount of 1.5 g showed the highest activity,however,it was not very reproducible because of the low Zn content.Ru–Zn-2 from the NaOH amount of 1.6 g showed the proper activity.It is also found that prolonging reduction time leads to the growth of Ru nanoparticles and aggregation of catalyst particles,resulting in the lower BET surface area and lower activity of Ru–Zn catalysts.

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