Industrial & Engineering Chemistry Research

E ?ect of Support Acidity on Liquid-Phase Hydrogenation of Benzene to Cyclohexene over Ru ?B/ZrO 2Catalysts

Gongbing Zhou,?Jianliang Liu,?Xiaohe Tan,?Yan Pei,?Minghua Qiao,*,?Kangnian Fan,?

and Baoning Zong *,??Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials,Fudan University,Shanghai 200433,People ’s Republic of China

?The State Key Laboratory of Catalytic Materials and Reaction Engineering,Research Institute of Petroleum Processing,Beijing 100083,People ’s Republic of China

INTRODUCTION Partial hydrogenation of benzene to cyclohexene has attracted much attention,because cyclohexene has wide applications in organic synthesis as the intermediate of adipic acid,nylon-6,nylon-66,polyamides,polyesters,and other ?ne chemicals.1However,it is di ?cult to acquire a high yield of cyclohexene through this route,as the standard free energy change for cyclohexene formation from benzene hydrogenation is ?23kJ mol ?1,while that for cyclohexane formation is ?98kJ mol ?1.This is the reason that only cyclohexane was obtained in the hydrogenation of benzene for a long time.2Catalytic hydrogenation of benzene to cyclohexene has been carried out in gas 3or liquid phase.4The main advantage of the liquid-phase reaction is that it is accessible to a much higher selectivity to cyclohexene at a high conversion level of benzene.4Among the various metals screened,Ru is the most selective,especially when in combination with ZnSO 4and other inorganic reaction modi ?ers such as NaOH 5,6and CdSO 4.7The properties of the support can exert a signi ?cant impact on the catalytic performance of the Ru catalyst,because the reducibility and dispersity of Ru are in ?uenced by the interaction between the support and the metal.8Many oxides,such as SBA-15,7SiO 2,9,10Al 2O 3,11AlOOH,12ZrO 2,13,14

bentonite,15and ZnO-containing binary oxides,16?18have been employed as supports for Ru,among which ZrO 2is the most e ?ective.For instance,Wang et al.prepared a Ru ?Zn/m -ZrO 2nanocomposite catalyst by coprecipitation of RuCl 3and

ZrOCl 2with ammonia followed by reduction in an ZnSO 4aqueous solution and obtained a cyclohexene yield of 43.4%.13Liu et al.performed the reaction with a Ru ?La ?B/ZrO 2

catalyst and found that the selectivity to cyclohexene could amount to 66%at the conversion of 81%by further adding ZrO 2as the disperser and by ?nely tuning the acidity of the aqueous phase.14However,to the best of our knowledge,the function of the ZrO 2support in partial hydrogenation of benzene has not been explored,and hence scienti ?c studies elucidating the support e ?ects are desired.

There are numerous examples showing that the surface acidic properties of the supports play important roles in determining the catalytic performance by a ?ecting the adsorption and desorption behaviors of the reactants and products.For example,Venezia et al.found that the activities for toluene

hydrogenation and dibenzothiophene hydrodesulfurization on Au ?Pd/SiO 2?Al 2O 3catalysts increased with the concentration of the medium-strength acid sites on the support.19This result is consistent with that of Grzechowiak et al.,who found that the activity for toluene hydrogenation over the Ni/SiO 2?TiO 2catalyst increased with the total number of weak-and medium-strength acid sites of the support.20Chupin et al.

reported that Pt and Pd supported on HFAU zeolite were more active for toluene hydrogenation than on Al

2O 3,because toluene molecules adsorbed on the acid sites of HFAU were easily hydrogenated by hydrogen spilled over from the metal sites.21Yasuda et al.studied the acidity e ?ect on the hydrogenation of aromatics over Pd ?Pt/USY catalysts by varying the SiO 2/Al 2O 3ratio.22The activity and the sulfur tolerance of the Pd ?Pt/USY catalysts both decreased when

increasing the SiO 2/Al 2O 3ratio,which was attributed to the decrease of the amount of electron-de ?cient Pd ?Pt resulting

from the decrease in the Lewis acidity of the supports.

ZrO 2was claimed to display four polymorphs,monoclinic (m ),amorphous (a ),tetragonal (t ),and cubic (c ),23which have di ?erent surface acidic features.Unlike m -ZrO 2on which both Received:May 17,2012

Revised:July 30,2012

Accepted:August 29,2012

Published:September 11,2012

Br?nsted and Lewis acid sites coexist,only Lewis acid sites were present on t-ZrO2.24It has been documented that the crystal phase of ZrO2in?uences the catalytic activity and selectivity in a number of reactions.25?29Li et al.synthesized pure m-and t-ZrO2nanoparticles and used them as supports for MoO x catalysts.In methanol selective oxidation,the MoO x/m-ZrO2 catalyst exhibited higher intrinsic activity than the t-ZrO2-supported one due to the higher reducibility of the former.25 Sulfated m-ZrO2showed catalytic activity lower than sulfated t-ZrO2by about a factor of4in acid-catalyzed n-alkane isomerization.26Li et al.evaluated the catalytic performances of CO hydrogenation on ZrO2catalysts and found that m-ZrO2 favors the synthesis of isobutene,whereas t-ZrO2favors the formation of ethylene and propylene.27Rhodes and Bell reported that the Cu/ZrO2catalyst with ZrO2of the monoclinic polymorph,as compared to that of the tetragonal polymorph,exhibited higher selectivity for methanol synthesis from CO hydrogenation,attributable to the presence of a high concentration of anionic vacancies on the surface of m-ZrO2.28 Such vacancies exposed coordinatively unsaturated Zr cations, which act as Lewis acid sites and enhance the Br?nsted acidity of adjacent Zr?OH groups,thus resulting in stronger and larger CO adsorption capacity on the Cu/m-ZrO2catalyst.He et al.evaluated the catalytic performances of ZrO2-based catalysts for the synthesis of higher alcohols from synthesis gas and found that m-ZrO2exhibited a higher selectivity to iso-butanol than t-ZrO2.29The higher selectivity to iso-butanol over m-ZrO2was probably due to the strong Lewis acidity of Zr4+cations and the strong Lewis basicity of O2?anions of the coordinatively unsaturated Zr4+?O2?pairs on m-ZrO2.

The goal of the present work is to examine the e?ect of the acidity of ZrO2on the catalytic performances of the Ru?B/ ZrO2catalysts for liquid-phase hydrogenation of benzene to cyclohexene.As the acidic properties are di?erent for ZrO2of di?erent polymorphs,our strategy is to prepare monoclinic, amorphous,and tetragonal ZrO2,and use them to support Ru in the same approach.The correlation between the amounts and types of acid sites and the selectivity to cyclohexene is identi?ed with the aid of systematic characterizations and

catalytic evaluations.

■EXPERIMENTAL SECTION

Preparation of ZrO2and Ru?B/ZrO2Catalysts. Monoclinic,amorphous,and tetragonal ZrO2were synthesized according to Jung and Bell.30In brief,monoclinic ZrO2was prepared by dropwise addition of30wt%ammonia to a0.5M aqueous solution of ZrOCl2·8H2O until a pH of1.5,and then re?uxed at373K under ambient pressure for10days.The resulting precipitates were washed thoroughly with deionized water until attaining a pH of7.0,followed by drying at373K in air for24h.The obtained product was denoted herein as ZrO2-M.Amorphous ZrO2(denoted as ZrO2-A)was synthesized in the same procedure as ZrO2-M,except for adding30wt% ammonia to a0.5M aqueous solution of ZrOCl2until a pH of 10.Tetragonal ZrO2(denoted as ZrO2-T)was obtained by calcination of ZrO2-A in air at873K at a heating rate of10K min?1and held for5h.

The Ru?B/ZrO2catalysts were prepared by incipient wetness impregnation followed by chemical reduction.Exactly 1g of ZrO2was impregnated with30mL of an aqueous solution of RuCl3·3H2O(26mM)and stirred for48h.Next,2 mL of an aqueous solution of KBH4(3.0mM)was added to the slurry dropwise at room temperature under mild stirring.The black precipitates were washed thoroughly with deionized water.The obtained catalysts were denoted as Ru?B/ZrO2-M, Ru?B/ZrO2-A,and Ru?B/ZrO2-T depending on the crystallo-graphic form of the ZrO2employed.The Ru loading on these catalysts was ca.8.0wt%relative to ZrO2,and the atomic ratio of Ru to B in Ru?B is ca.85:15as determined by inductively coupled plasma-atomic emission spectroscopy. Characterization.N2physisorption was performed on a Micromeritics TriStar3000apparatus at77K.Before measure-ment,the sample was transferred to the adsorption glass tube and heated at423K under N2for2h.The pore size distribution was calculated from the desorption branch of the isotherm by the Barrett?Joyner?Halenda(BJH)algorithm. Powder X-ray di?raction patterns(XRD)were acquired on a Bruker AXS D8Advance X-ray di?ractometer using Ni-?ltered Cu Kαradiation(λ=0.15418nm).The tube voltage was40 kV,and the current was40mA.The2θangles were scanned from10°to70°at a speed of2°min?1.

The surface morphology and particle size were observed by transmission electron microscopy(TEM)(JEOL JEM2011) operating at200kV.Particle size distribution histograms were constructed by measuring at least300nanoparticles.

X-ray photoelectron spectroscopy(XPS)was performed on a Perkin-Elmer PHI5000C instrument with Mg Kαradiation as the excitation source(hν=1253.6eV).The catalyst,protected by ethanol,was mounted on the sample plate,degassed in the pretreatment chamber at393K for4h in vacuo,and then transferred to the analyzing chamber where the background pressure was<2×10?9Torr.Because the Ru3d3/2peak partly overlaps with the C1s line of contaminant carbon,all binding energy(BE)values were referenced to the Zr3d5/2peak of ZrO2at182.2eV.

The nature of the acid sites on ZrO2was characterized by infrared spectroscopy of adsorbed pyridine(Py-IR)based on the fact that pyridine molecules adsorbed on di?erent acid sites display characteristic vibration bands.The spectra were recorded with4cm?1spectral resolution on a Nicolet Nexus 470spectrometer equipped with a DTGS detector by signal-averaging32scans.The spectrometer was also equipped with a temperature-controlling accessory to provide sequences of temperature-dependent IR spectra.The powdered samples(ca. 20mg)were pressed with KBr into self-supporting wafers of2 cm in diameter.The wafers were placed in an evacuable IR cell with CaF2windows,evacuated at473K for4h,and cooled to 298K.After the background spectrum was recorded,pyridine was introduced and balanced at this temperature for1h. Physisorbed pyridine was removed by evacuation,and then the samples were heated stepwise under vacuum from298to573 K.The spectra were recorded at298,373,423,473,523,and 573K.

The amount and strength of acid sites on ZrO2were monitored by temperature-programmed desorption of NH3 (NH3-TPD).The weighed samples were heated at473K for1 h under Ar and cooled to393K.The NH3/He(10vol.%) mixed gas was introduced by the pulse method at393K until the eluted gas chromatographic peak did not change in intensity as monitored by a thermal conductivity detector (TCD).The gaseous and/or physisorbed NH3was removed by purging with Ar until the signal returned to the baseline.The desorption curve of NH3was acquired by heating from393to 1073K at10K min?1.31The amount of the adsorbed NH3was calculated on the basis of the area under the desorption curve.

H2chemisorption was used to determine the active surface area(S Ru)and dispersion of Ru.The procedure of H2 chemisorption was the same as that of NH3-TPD except that H2and Ar were used instead of10vol%NH3/He and He, respectively,and the adsorption temperature was273K instead of393K.32The active surface area was calculated on the bais of the desorption area with the assumption of a H:Ru stoichiometry of1:1and a Ru surface atomic density of1.63×1019atoms m?2.33

Catalytic Testing.The partial hydrogenation of benzene was carried out in a mechanically stirred500mL-capacity stainless steel autoclave.The autoclave was charged with100 mL of deionized water containing0.7M ZnSO4,1.0g of catalyst,and50mL of benzene,then sealed and purged with H2 four times to expel air.The reaction conditions were temperature of413K,H2pressure of4.0MPa,and stirring rate of1000rpm to exclude the di?usion e?ect.7The reaction process was monitored by taking a small amount of the reaction mixture at intervals,followed by gas chromatographic analysis using a PEG-20M packed column and a TCD detector. Catalytic activity was expressed as both the weight speci?c activity(v benzene)and the turnover frequency(TOF)of benzene.The former was obtained by extrapolating the slope of the concentration?time curve of benzene to zero reaction time and expressed as numbers of moles of converted benzene per min on per gram of catalyst,whereas the latter was expressed as TOF=v benzene×M Ru/(dispersion×L),where M R u and L are molar mass of Ru and loading of the catalyst, respectively.The selectivity to cyclohexene(S HE)was calculated as S HE=n HE/(n HE+n HA),where n HE and n HA are the molar percentages of cyclohexene and cyclohexane, respectively.

■RESULTS AND DISCUSSION

Physicochemical Properties.As listed in Table1,the speci?c surface areas(S BET)of ZrO2-M,ZrO2-A,and ZrO2-T

were153,375,and283m2g?1,respectively,which were comparable to the values reported by Jung and Bell.30They reported that precipitating ZrOCl2at low pH(1.5)with extended re?ux in a boiling ammonia solution at ambient pressure produced ZrO2with S BET of110m2g?1,precipitating at high pH(10)with the same re?ux produced ZrO2with S BET of400m2g?1,and calcining the latter at873K caused a loss in S BET to ca.275m2g?1.It was found that the speci?c surface areas of the ZrO2samples prepared by precipitating from zirconium chloride and ammonia were a?ected by digestion of the hydrous zirconia.34The hydrous oxides digested at373K were thermally more stable than the undigested ones or samples aged in the mother liquor at room temperature. Consequently,the high S BET of the digested hydrous oxides could be retained in the resulting ZrO2.In addition,the pore volume(V p)and average pore diameter(d p)presented in Table 1had an order identical to S BET as ZrO2-M

Figure1shows the XRD patterns of the as-synthesized ZrO2-M,ZrO2-A,and ZrO2-T.The di?raction peaks at2θof17.6°,

24.1°,28.2°,31.5°,34.5°,35.3°,40.7°,45.0°,50.1°,55.4°, 61.4°,and65.7°for ZrO2-M are assigned to monoclinic ZrO2 (JCPDS37-1484).For ZrO2-A,only a broad feature centered at2θof ca.30°was observed,indicating the amorphous structure of this sample.For ZrO2-T,?ve di?raction peaks characteristic of tetragonal ZrO2appeared at2θof30.3°,35.3°, 50.6°,60.2°,and63.0°(JCPDS17-0923).These observations are consistent with previous works.30,34

For ZrO2-M,ZrO2-A,and ZrO2-T,the Lewis and Br?nsted acid sites are Zr4+cations29and hydroxyl groups on the ZrO2 surfaces,respectively.The Py-IR spectra of ZrO2-M,ZrO2-A, and ZrO2-T at298K are compared in Figure2.ZrO2-M

exhibited several bands originating from pyridine adsorbed on di?erent acid sites,including Lewis acid sites-bonded pyridine (L-Py,1613and1450cm?1)and Br?nsted acid sites-bonded pyridine(PyH+,1641and1543cm?1).35The band at1613 cm?1can be assigned to pyridine adsorbed on strong Lewis acid sites.36In addition,bands are expected for hydrogen-bonded pyridine(hb-Py)in the ranges of1440?1447and1590?1600 cm?1similar to those of L-Py,as well as for physically adsorbed

Table1.Physicochemical Properties of the ZrO2Samples and the Corresponding Ru?B/ZrO2Catalysts

sample

S BET

(m2g?1)

V p

(cm3g?1)

d p

(nm)

S Ru(m2

g Ru?1)

dispersion

(%)

ZrO2-M1530.2618.4

Ru?B/ZrO2-M1090.1932.48924.3 ZrO2-A3750.7933.2

Ru?B/ZrO2-A3340.6540.0308.2 ZrO2-T2830.6226.4

Ru?B/ZrO2-T2340.4840.180

21.8Figure1.XRD patterns of(a)ZrO2-M,(b)ZrO2-A,and(c)ZrO2

-T.

Figure2.Py-IR spectra of(a)ZrO2-M,(b)ZrO2-A,and(c)ZrO2-T at 298K.

pyridine (ph-Py)at 1439and 1581cm ?1.35,37,38The detailed assignments of the bands discussed above were labeled in Figure 2for clarity.It is evident that,in principle,discrimination between L-Py and hb-Py is a challenging task from a room temperature Py-IR spectrum.According to Kalevaru et al.,39the thermal stability of the adsorbed pyridine species di ?ers and increases in the order of ph-Py

The bands of PyH +on ZrO 2-M decreased in intensity with the desorption temperature and vanished at 523K,while the bands of L-Py still could be observed even at 573K,indicating that on ZrO 2-M the Lewis acid sites bind more tightly with pyridine than the Br?nsted acid sites.On the basis of the Py-IR spectra of the ZrO 2samples at 373K,the integral band intensities of L-Py and PyH +are calculated and summarized in Table 2.It can be seen from Table 2that the integral band intensities of L-Py and the total integral intensities are 7.1and 11.1for ZrO 2-M,and drastically decreased to 2.6for ZrO 2-A and 1.7for ZrO 2-T,showing that ZrO 2-M had the highest amounts of the Lewis and total acid sites,and ZrO 2-T the lowest.When the di ?erence in the speci ?c surface area is concerned,the area-averaged total amount of acid sites still changes in the sequence of ZrO 2-M >ZrO 2-A >ZrO 2-T (Table 2).Figure 4shows the NH 3-TPD pro ?les of the ZrO 2samples.ZrO 2-M exhibited three desorption peaks at 444,575,and 805K.ZrO 2-A also exhibited three peaks,but at 596,897,and 980K.Only an ill-de ?ned broad peak can be seen with the peak maximum at about 422K for ZrO 2-T.The total amounts of the acid sites on the ZrO 2samples are displayed in Figure 5,which decrease in the order of ZrO 2-M >ZrO 2-A >ZrO 2-T,qualitatively consistent with the tendency of the total integral

band intensities revealed by Py-IR (Table 2).

Taking into account of the results presented in Table 2and Figures 2?5,it is clear that ZrO 2-M,ZrO 2-A,and ZrO 2-T di ?er in the types of the acid sites,the amount of each type of acid site,and the total amount of acid sites.Such distinct di ?erences in acidic property may exert a signi ?cant in ?uence on the catalytic performance of the Ru ?B/ZrO 2catalysts derived

from

Figure 3.Temperature-dependent Py-IR spectra of (a)ZrO 2-M,(b)ZrO 2-A,and (c)ZrO 2-T.Each series was measured at the same

temperature sequence as that of ZrO 2-M.

Table 2.Integral Absorption Band Intensities in the Py-IR Spectra of ZrO 2-M,ZrO 2-A,and ZrO 2-T

sample L-Py a (au)PyH +b (au)total c (au)total d (au)ZrO 2-M 7.1 4.011.10.07ZrO 2-A 2.60 2.60.007ZrO 2-T 1.70 1.70.006a

Values corresponding to the total integral intensities of the absorption bands at 1450cm ?1(Lewis acid sites)and 1613cm ?1(strong

Lewis acid sites)at 373K.b Values corresponding to the total integral intensities of the absorption bands at 1641and 1543cm ?1(Br?nsted acid sites)

at 373K.c Values

corresponding

to the total

integral intensities of the absorption bands of L-Py and PyH +at 373K.

d Values corresponding to th

e total integral intensities o

f the absorption bands of L-Py and PyH +at 373K averaged by the speci ?c

surface areas of the ZrO 2

samples.

Figure

4.NH 3-TPD pro ?les of (a)ZrO 2-M,(b)ZrO 2-A,and (c)

ZrO 2

-T.

Figure 5.Total amount of acid sites on ZrO

2-M,ZrO 2-A,and ZrO 2-T

determined by NH 3-TPD.

these supports in liquid-phase hydrogenation of benzene to cyclohexene.The textural properties of the Ru ?B/ZrO 2-M,Ru ?B/ZrO 2-

A,and Ru ?B/ZrO 2-T catalysts are summarized in Table 1.As compared to their corresponding ZrO 2supports,the Ru ?B/ZrO 2catalysts showed reduced speci ?c surface areas and pore volumes,and increased average pore diameters,suggesting the incorporation of some Ru ?B particles into the small pores.Table 1also shows that the active surface area (S Ru )and

dispersion are the highest for the Ru ?B/ZrO 2-M catalyst,while they are the lowest for the Ru ?B/ZrO 2-A catalyst.Figure 6reveals that the crystallographic forms of the supports were retained after the loading of Ru ?B,although

their intensities were attenuated to some extent.Furthermore,the di ?ractograms of the Ru ?B/ZrO 2catalysts only contain the features of the supports,signifying the high dispersion of the amorphous Ru ?B particles,40which is directly con ?rmed by TEM.In the TEM images of the Ru ?B/ZrO 2-M,Ru ?B/ZrO 2-A,and Ru ?B/ZrO 2-T catalysts (Figure 7),the Ru ?B particles (marked by white arrows in Figure 7a ?c)have narrow particle size distributions in the range of 1?4nm.The average particle sizes of Ru ?B are 2.1,2.5,and 2.3nm for Ru ?B/ZrO 2-M,Ru ?B/ZrO 2-A,and Ru ?B/ZrO 2-T,respectively,which are below the detection limit of the XRD technique.However,for the Ru ?B/ZrO 2-A catalyst,agglomeration of the Ru ?B nano-particles is observed,as highlighted by the circles drawn in Figure 7b,which explains its lowest S Ru and dispersion in Table 1derived from H 2-TPD.Figure 8shows the Ru 3d XPS spectra of the Ru ?B/ZrO 2

catalysts.Because the Ru 3d 3/2peak overlaps the C 1s peak of contaminant carbon,only the Ru 3d 5/2peak is employed to determine the chemical state of Ru.It is found that the Ru species in all three catalysts are mainly in the metallic state with the Ru 3d 5/2BE of 280.1eV.41In addition,the spectrum of the Ru ?B/ZrO 2-A catalyst is slightly broader than those of other two catalysts,which may be due to the formation of some RuO 2

on the surface of the Ru ?B/ZrO 2-A catalyst during sample transfer.Nevertheless,the dominant Ru species on this catalyst is still metallic Ru.The surface chemical state of boron cannot be identi ?ed for these Ru ?B/ZrO 2catalysts,because the B 1s peak is unfortunately totally superimposed by a broad Zr 3d 5/2

feature of the supports.Catalytic Performance.The courses of the hydrogenation

of benzene over these Ru ?B/ZrO 2catalysts are displayed in Figure 9.Only cyclohexene and cyclohexane were detected as products under the present reaction conditions.In Figure 9,

the

Figure 6.XRD patterns of (a)Ru ?B/ZrO 2-M,(b)Ru ?B/ZrO 2-A,and (c)Ru ?B/ZrO 2-T

catalysts.Figure 7.TEM images and the corresponding particle size distribution histograms of the Ru ?B/ZrO 2-M (a)and (d),Ru ?B/ZrO 2-A (b)and

(e),and Ru ?B/ZrO 2-T (c)and (f)

catalysts.Figure

8.Ru 3d XPS spectra of (a)Ru ?B/ZrO 2-M,(b)Ru ?B/ZrO 2-

A,and (c)Ru ?B/ZrO 2-T catalysts.

concentration of benzene decreased and the concentration of cyclohexane increased monotonically with the reaction time.For cyclohexene,there was a maximum concentration at a certain reaction time depending on the type of the Ru ?B/ZrO 2catalyst.Among these catalysts,the Ru ?B/ZrO 2-T catalyst showed the best catalytic performance in terms of the selectivity and yield of cyclohexene.On this catalyst,the concentration of cyclohexene increased much faster than that of cyclohexane at the beginning of the reaction,and reached a maximum yield of 47%at benzene conversion of 83%at a reaction time of ca.15min.The yield of cyclohexene then declined gradually following the known behavior of consecutive reaction.On the Ru ?B/ZrO 2-A catalyst,the maximum yield of cyclohexene was 39%at benzene conversion of 73%at a reaction time of 30min.The Ru ?B/ZrO 2-M catalyst exhibited the lowest yield of cyclo-hexene of 30%at benzene conversion of 72%.However,the reaction proceeded so fast on this catalyst that within only 20min benzene was completely consumed.Table 3summarizes the catalytic results of benzene hydrogenation over these Ru ?B/ZrO 2catalysts.It can be seen that the v benzene over the Ru ?B/ZrO 2-M catalyst is the highest,followed by the Ru ?B/ZrO 2-T catalyst and the Ru ?B/ZrO 2-A catalyst in sequence.According to Table 1,it can be seen that the evolution of v benzene is consistent with the change

of S Ru or dispersion.Higher S Ru or dispersion means larger number of the exposed Ru atoms,thus leading to higher v benzene ,which is in line with the work by Wang et al.40On the basis of the v benzene and the dispersion values,the TOFs of benzene over these catalysts are calculated and listed in Table 3.It is interesting that the TOFs over all three Ru ?B/ZrO 2catalysts are essentially the same,strongly suggesting that the active sites for the hydrogenation of benzene are situated on the Ru ?B nanoparticles and identical in nature,as substantiated by the similar diameter and chemical state of the Ru ?B nanoparticles on these catalysts shown above.As far as the close similarity of the Ru ?B nanoparticles on these Ru ?B/ZrO 2catalysts is concerned,it is reasonable to assume that the di ?erence in the selectivity to cyclohexene is mainly determined by the ZrO 2supports.Researchers have evidenced that the product selectivity was in ?uenced by the support acidity in many selective hydrogenation reac-tions.36,42,43Volckmar et al.found that a high total amount of acid sites and a high amount of strong Lewis acid sites on the Ag/SiO 2?Al 2O 3catalysts induced a low selectivity to allyl

alcohol in acrolein hydrogenation.36Murzin and co-workers found a decrease in the selectivity to unsaturated alcohol with increasing support acidity in cinnamaldehyde

hydrogenation

over Pt-modi ?ed molecular sieves catalysts.42Similar results

were obtained over Ru/Al 2O 3catalysts,43inferring that the hydrogenation of the C C bond is promoted by the acid sites.Thus,it is rational to relate the selectivity patterns observed on the Ru ?B/ZrO 2-M,Ru ?B/ZrO 2-A,and Ru ?B/ZrO 2-T catalysts to the di ?erence in the acidity of the ZrO 2supports.The e ?ect of the acidic property of the support on the hydrogenation of benzene has been investigated on Pt catalysts.In general,both benzene and cyclohexene molecules can adsorb on the acid sites of the supported Pt catalysts and convert to ?nal products by hydrogen spilled over from H 2dissociated on Pt sites.44?49According to Ishikawa et al.,50the spillover hydrogen also existed on the Ru/ZrO 2catalyst.FT-IR characterization revealed that when H 2(D 2)was introduced to the Ru/ZrO 2catalyst,H 2(D 2)dissociatively adsorbed on the

Ru surface and spilled from Ru particles onto the ZrO 2support to form the H 2O (D 2O)-like species,which desorbs as H 2(D 2)but not as H 2O (D 2O).Besides,

Appay et al.found that on the acid sites of the Pt/ZrO 2?SO 4catalyst,the hydrogenation of cyclohexene was always faster than that of benzene at various reaction temperatures,51indicating the more facile hydro-genation of cyclohexene than benzene on acid sites.Considering the consecutive reaction mechanism of benzene hydrogenation on Ru-based catalysts 52and the virtually invariable TOFs on the Ru ?B/ZrO 2-M,Ru ?B/ZrO 2-A,

and

Figure 9.Time courses of benzene hydrogenation over (a)Ru ?B/

ZrO 2-M,(b)Ru ?B/ZrO 2-A,and (c)Ru ?B/ZrO 2-T catalysts.Reaction conditions:1.0g of catalyst,50mL of benzene,100mL of

H 2O,temperature of 413K,H 2pressure of 4.0MPa,and C ZnSO4of 0.07M.(■)Benzene,(▼)cyclohexene,and (▲)cyclohexane.Table 3.Results of the Hydrogenation of Benzene over the Ru ?B/ZrO 2-M,Ru ?B/ZrO 2-A,and Ru ?B/ZrO 2-T

Catalysts a catalyst v benzene (mmol min ?1

g cat ?1)TOF

(s ?1)C BZ b (%)S HE b

(%)Y HE b (%)time b

(min)

Ru ?B/ZrO 2-M 51 4.472423011

Ru ?B/ZrO 2-A 19 4.973533930

Ru ?B/ZrO 2-T 47 4.583564715

a Reaction conditions:1.0g of catalyst,50mL of benzene,100mL of H 2O,temperature of 413K,H 2pressure of 4.0MPa,and C ZnSO4of

0.07M.b Values recorded at the maximum yield of cyclohexene.

Ru ?B/ZrO

2-T catalysts irrespective of the distinct di ?erences in the acidic properties of the supports (Table 3),we suggest that successive hydrogenation of cyclohexene,produced by benzene hydrogenation on Ru ?B nanoparticles,on the acid sites of the ZrO 2supports determines or at least strongly

in ?uences the selectivity to cyclohexene on these catalysts.Notably,Aboul-Gheit et al.observed that the hydrogenation of cyclohexene to cyclohexane was enhanced when the number of the acid sites was increased on the Pt/H-ZSM-5,53,54Ir/H-ZSM-5,55and Re/H-ZSM-5catalysts.56Thus,it is expected that the di ?erence in the selectivity to cyclohexene can be observed over Ru ?B catalysts supported on ZrO 2with di ?erent acidic

properties,and the Ru ?B/ZrO 2catalyst with the lowest amount of acid sites will exhibit the highest selectivity to cyclohexene.As can be seen from Tables 2,3,and Figure 5,the changes in the amount of acid sites on the ZrO 2supports and the selectivity to cyclohexene over these Ru ?B/ZrO 2catalysts

follow an opposite trend.The amount of acid sites on the ZrO 2supports decreased in the sequence of ZrO 2-M >

ZrO 2-A >

ZrO 2-T,while the selectivity to cyclohexene increased in the sequence of Ru ?B/ZrO 2-M

catalysts,further work is needed to di ?erentiate the e ?ects of the Br?nsted acid sites and the Lewis acid sites on the selectivity to cyclohexene,which may be instructive for the design of Ru catalysts with improved selectivity to cyclohexene in partial hydrogenation of benzene.■CONCLUSION Three kinds of ZrO 2(monoclinic,amorphous,and tetragonal)were prepared and used as supports for Ru ?B catalysts.In liquid-phase partial hydrogenation of benzene to cyclohexene,Ru ?B/ZrO 2-M,Ru ?B/ZrO 2-A,and Ru ?B/ZrO 2-T catalysts

exhibited similar TOFs,but the Ru ?B/ZrO 2-T catalyst exhibited better selectivity to cyclohexene than others,and the maximum yield of cyclohexene amounted to 47%.The lower amount of the acid sites on ZrO 2-T is suggested as the

main reason responsible for the superior selectivity of the Ru ?B/ZrO 2-T catalyst,and the absence of the Br?nsted acid sites on ZrO 2-T may be another possible reason.This ?nding opens a new avenue for the searching of supports capable of enhancing the selectivity to cyclohexene in liquid-phase hydrogenation of benzene.■AUTHOR INFORMATION Corresponding Author *Tel.:+86-21-55664679(M.Q.);+86-10-82368011(B.Z.).Fax:+86-21-55665701(M.Q.);(+86-10)-82368011(B.Z.).E-mail:mhqiao@https://www.doczj.com/doc/f013704884.html, (M.Q.);zongbn.ripp@https://www.doczj.com/doc/f013704884.html, (B.Z.).

Notes The authors declare no competing ?nancial interest.

ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (2012CB224804),the NSF of China (21073043),the Science &Technology Commission of Shanghai Municipality (10JC1401800,08DZ2270500),the

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