Salen配体

Zeolite-encapsulated Ru(III)tetrahydro-Schiffbase complex:An efficient heterogeneous catalyst for the hydrogenation ofbenzene under mild conditionsPing Chen,Binbin Fan,Minggang Song,Chun Jin,Jinghong Ma,Ruifeng Li*Key Laboratory of Coal Science and Technology,MOE,Institute of Special Chemicals,Taiyuan University of Technology,79West Yinze Street,Taiyuan 030024,PR ChinaReceived 7January 2006;received in revised form 7April 2006;accepted 7April 2006Available online 18April 2006AbstractA series of Ru(III)tetrahydro-Schiffbase complexes (denoted as Ru[H 4]-Schiffbase with Schiffbase =salen,salpn and salcn,see Scheme 1)were encapsulated in the supercages of zeolite Y by flexible ligand method.The prepared catalysts were characterized by X-ray diffraction,diffuse reflectance UV–Vis spectroscopy,Infrared spectroscopy,elemental analysis,as well as N 2adsorption tech-niques.It was shown that upon encapsulation in zeolite Y,Ru(III)tetrahydro-Schiffbase complexes exhibited higher activity for the hydrogenation of benzene than the corresponding Ru(III)-Schiffbase complexes.This indicates that hydrogenation of the C @N bond of the Schiffbase ligands led to a modification of the coordination environment of the central Ru(III)cations.The stability of the prepared catalysts has also been confirmed against leaching of the complex molecule from the zeolite cavities,as revealed by the result that no loss of catalytic activity was observed within three successive runs with regeneration.Ó2006Elsevier B.V.All rights reserved.Keywords:Benzene hydrogenation;Ru(III)tetrahydro-Schiffbase complex;Encapsulation;Zeolite Y1.IntroductionTransition metal complexes have been extensively used for homogeneous hydrogenation of organic substrates such as benzene due to their high selectivity under mild reaction conditions [1,2].However,the difficult recovery and recy-cling of the catalysts limit their reuse.Therefore,increasing demands have been generated for the preparation of effec-tive heterogeneous catalysts.In this context,immobiliza-tion of metal complex is an attractive strategy.Up to date,several methods,such as covalent bonding of ligands with host materials,entrapment or occlusion of complex molecule in polymer matrices or porous inorganic materi-als by steric hindrance,and adsorption of the complexes on the support via ionic interaction,have been examinedfor the immobilization of transition metal complexes [3].Among them,the physical encapsulation of transition metal complex molecules into the supercages of zeolites gains incomparable advantages for homogeneous and con-ventional heterogeneous catalysts [4–7].Upon encapsula-tion in the cavities of the zeolites with FAU and EMT topological structures,palladium and rhodium salen com-plexes are highly active for the selective hydrogenation of 1,5-cyclooctadiene to cyclooctene [8],particularly chiral palladium salen complex makes it possible of enantioselec-tive hydrogenation of unsaturated organic compounds such as 3-methyl-2-cyclohexenone [9].In addition,palla-dium and nickel salen complexes grafted on mesostruc-tured silicates and delaminated ITQ zeolites also show good catalytic performance in the hydrogenation of imines [10],while Ru(III)-Schiffbase complex anchored on poly-mer exhibits considerably improved catalytic efficiency for the hydrogenation of styrene [11].The catalytic activity1566-7367/$-see front matter Ó2006Elsevier B.V.All rights reserved.doi:10.1016/j.catcom.2006.04.003*Corresponding author.Tel./fax:+863516010112.E-mail address:rfli@ (R.Li)./locate/catcomCatalysis Communications 7(2006)969–973of transition metal Schiffbase complexes in a given process is highly dependent on the environment of metal center and their conformationalflexibility[12].Compared to Schiffbase ligands,the corresponding tetrahydro-Schiffbase ligands possess stronger N-basicity and higherflexibility owing to the hydrogenation of C@N bond[13].It was found that zeolite-encapsulated Cu(II)tetrahydro-salen complex is more active than the encapsulated Cu(II)salen for the oxidation of cycloalkane in the presence of H2O2 [14].To the best of our knowledge,the hydrogenation per-formance of transition metal tetrahydro-salen complex has not been reported yet.Here,we report the preparation of zeolite Y encapsulated Ru(III)tetrahydro-Schiffbase com-plexes by usingflexible ligand method and their catalytic property in an industrially important reaction of benzene hydrogenation(see Scheme1).2.Experimental2.1.Preparation of ligandsSchiffbase L(L=salen,salph and salcn)was prepared by following the procedures reported in Ref.[13,15]. Ligand[H4]L was prepared with the following method [16]:0.01mol Schiffbase L was dissolved in60ml metha-nol,followed by the addition of0.011mol NaBH4at ambi-ent temperature.After2h of stirring,the solvent was removed by distilling under vacuum conditions.The solid product was washed with distilled water and re-crystallized from ethanol.The purity of the ligands was confirmed by IR and1H NMR before coordination to Ru(III)cations.2.2.Preparation of zeolite Y encapsulated Ru(III) complexesZeolite Y was ion-exchanged with RuCl3aqueous solution(0.005mol/l,Liquid/Solid=20ml/g)at room temperature for24h,and then washed with deionized water and dried at100°C overnight.The obtained Ru-Y was mixed with an excessive amount of tetrahydro-Schiffbase ligands(n ligand/n Ru=3:1mol/mol),followed by heat-ing in a sealed glass ampoule for24h under vacuum conditions at temperatures of150,130and130°C for Ru[H4]-salen,Ru[H4]-salpn and Ru[H4]-salcn,respectively. The resultant materials were soxhlet-extracted with acetone to remove the complex molecules on the external surface and the free ligands.The extracted samples were dried at 373K for24h under vacuum conditions.2.3.Characterization of zeolite Y encapsulated Ru(III) complexesRu content was determined by an inductively coupled plasma-atomic emission spectrometer(TJA Atom Scan 16).X-ray power diffraction(XRD)patterns were recorded on a Rigaku Dmax X-ray diffractometer(Ni-filtered, CuK a radiation).Infrared spectra(IR)were measured on a FTIR spectrometer by using the conventional KBr pellet method.Diffuse reflectance(DR)UV–Vis spectra in the range of200–800nm were measured against a halon white reference standard by a Perkin–Elmer Lambda Bio40spec-trophotometer equipped with an integration sphere.The Brunauer–Emmett–Teller(BET)surface area was mea-sured on a NOVA1200instrument.Before the measure-ment,the sample was evacuated at200°C for12h.2.4.Catalytic testHydrogenation of benzene was carried out in a Parr4842 stainless steel batch reactor,which is equipped with a gauge,a thermocouple,an internal mechanic stirring sys-tem,gas inlet valves,liquid sampling valves and a temper-ature-programmed electric heater.In a typical batch,0.36g catalyst and30.0ml(26.4g)benzene were employed.To remove the air,the closed reactor was purged with hydro-gen for three times.When the temperature reached60°C, hydrogen was charged into the reactor.The stirring speed was maintained at a value as high as600rpm in order to eliminate the diffusion problem.The product was analyzed by a GC-9A gas chromatography equipped with aflame ionization detector.3.Results and discussion3.1.Characterization of catalystsNo remarkable difference was observed from the XRD patterns of the zeolitic host before and after complexation as well as the further soxhlet-extraction,indicating that the zeolite framework was not significantly affected by the for-mation of complexes.The BET surface areas of different Ru[H4]L-Y are given in Table1.Clearly,inclusion of Ru(III)Schiffbase com-plex considerably reduced the adsorption capacity and the surface area of the zeolite host.This is indicative of the presence of Ru(III)complexes in the zeolite cavities rather than on the external surface.The surface area,as]970P.Chen et al./Catalysis Communications7(2006)969–973expected,decreased with increasing molecular dimensions of ligands.The surface areas of Ru[H 4]salen-Y,Ru[H 4]-salpn-Y and Ru[H 4]salcn-Y were 500,465and 484m 2/g,respectively,in contrast to 570m 2/g of Ru-Y.Table 1also shows the Ru content in different samples.Although all samples were prepared from the same Ru(III)-exchanged zeolite Y matrix,their Ru content were obviously different,and decreased with the increase in the complex molecule size.Irrespective of its coincidence with our previous results on the encapsulation of Ru(III)-Schiffbase com-plexes in zeolite Y [17],it is different from the inclusion of Mn(III)salen complex in zeolite X and zeolite Y by zeo-lite synthesis method and template synthesis method,respectively [18,19].It was argued that the increase in the complex size could enhance its physical entrapment in zeo-lite supercages,resulting in an increase in the encapsulation efficiency.The contrary results might arise from the differ-ent preparation methods.Concerning the flexible ligand method,an increase in the ligand size would cause its diffu-sion into the zeolite channels difficult,and consequently hindering its access to Ru(III)cations and the formation of complex molecules in the supercages of zeolite Y.IR spectra of Ru-Y and Ru[H 4]L-Y are shown in Fig.1.Although in the low-wavenumber region the bands for the encapsulated complexes were overlapped by the framework vibration of the zeolite matrix,in the range from 1200to 1600cm À1,the bands due to C–N,C @C and atomic ring vibrations could be clearly distinguished for the zeolite-encapsulated Ru(III)tetrohydro-Schiffbase complex sam-ples,while these bands were not observed for the Ru-Y.The encapsulation of Ru(III)tetrohydro-Schiffbase com-plexes in zeolite Y was further confirmed by DR UV–Vis spectroscopy.It is evident in Fig.2that after complexation and further soxhlet-extraction,a relatively intense absorp-tion band could still be seen around 325nm in the DR UV–Vis spectra of all zeolite Y encapsulated Ru(III)tetrohydro-Schiffbase samples.As we know,this band is attributed to ligand-to-metal charge transfer (CT),giving a strong evidence for the formation of Ru(III)tetrahydro-Schiffbase complex molecules inside the cavity of zeolite Y.3.2.Hydrogenation of benzeneTable 2compares the catalytic results for the hydroge-nation of benzene over Ru-Y,RuL-Y and Ru[H 4]L-Y sam-ples.RuL-Y was prepared by the same method as that used for Ru[H 4]L-Y [17].Under the same reaction conditions,RuL-Y and Ru[H 4]L-Y catalysts gave much higher ben-zene conversion than Ru-Y despite that the product was only cyclohexane for all the catalysts.This indicates that the electronic environment of central Ru(III)ions drasti-cally influences the catalytic performance.This also gives another piece of evidence for the successful complexation of Ru(III)ions with the tetrahydro-Schiffbase ligands inside the zeolitic host.Nevertheless,it is worth noting that all Ru[H 4]L-Y catalysts showed higher activity than the corresponding RuL-Y materials,as revealed by the fact that the Ru[H 4]L-Y catalysts gave a TOF being 2–4times as high as that obtained for the RuL-Y samples.At a reac-tion time of 2h,the benzene conversion reached 75.3%with a TOF of 4361over the Ru[H 4]salen-Y catalyst.In contrast,the Rusalen-Y sample only gave a benzene con-version of 19.7%,and thus a TOF of 1155,although it con-tained a comparable number of complex molecules.A further increase in the reaction time to 3h could result in a benzene conversion of 98.3%over the Ru[H 4]salen-YTable 1Ru content and surface area of Ru-Y and Ru[H 4]L-Y samples Catalyst Ru content (wt%)Surface area (m 2/g)Ru-Y1.00571Ru[H 4]salen-Y 0.82500Ru[H 4]salpn-Y 0.57465Ru[H 4]salcn-Y0.44484P.Chen et al./Catalysis Communications 7(2006)969–973971catalyst.Homogeneous catalysis and Ru(III)cations play a minor role since at the same reaction conditions neat Rusalen and Ru-Y with a comparable Ru content to Rusalen-Y both showed a conversion of about 2.5%.It was suggested that hydride complexes as intermediate species or starting materials play a key role in most hydro-genation reactions [20].The required dihydrogen molecule cleavage is proposed to occur as a result of the interaction of a filled metal d orbit with the empty sigma antibonding molecular orbit of H 2,which could weaken the H–H bond.In addition,an electron-rich atmosphere around the metal atom may also facilitate the breaking of the H–H bond [20].Compared to the Schiffbase ligands,the hydrogenated tetrahydro-Schiffbase ligands have stronger N-basicity and more flexibility because of the hydrogenation of C @N bond [13].This makes the metal center electron-rich,pro-moting the appropriate overlap of the filled metal d orbit with the empty sigma antibonding molecular orbit of H 2,and hence favoring the cleavage of H–H bond,which is a crucial step in the hydrogenation reaction.It is noteworthy that although three types of Ru[H 4]L-Y catalysts all showed higher activity than the corresponding RuL-Y ana-logues,the degree substantially decreased with increasing ligand size.The conversion obtained over the Ru[H 4]sa-len-Y at the reaction time of 2h was about 3.8times as high as that obtained on the Rusalen-Y.In contrast,for the Ru[H 4]salcn-Y catalyst,it was only about 1.7times ashigh as that of its corresponding Schiffbase analogue.This indicates that the effect of the C @N bond hydrogenation on the metal center weakens with the ligand size.Effect of the reaction temperature on the hydrogenation of benzene over the Ru[H 4]salen-Y catalyst is illustrated in Fig.3.As expected,the benzene conversion was highly dependent on the reaction temperature.It sharply increased with increasing reaction temperature when the temperature was lower than 60°C.When the reaction was carried out at 60°C for 2h,the benzene conversion was 75.3%in contrast to 19.4%at 50°C.Regardless of this,a further increase in the reaction temperature to 70°C had no such an apparent effect.The influence of hydrogen pressure was investigated at 60°C between 2.0and 4.0MPa.The relationship between the hydrogen pressure and the benzene conversion is shown in Fig.4.Obviously,the benzene conversion obtained at 3.0MPa was higher than those obtained at 2.0andTable 2Catalytic results of different catalysts in benzene hydrogenation a Catalyst Ru content (wt%)Reaction time (h)Conversion (%)TOF b(mol/(mol Æh À1))Ru-Y0.872 2.614748.7611.5Rusalen c 22.5141Rusalen-Y0.81219.71115448.5675.2Ru[H 4]salen-Y 0.82275.34361398.3Rusalpn-Y0.5528.3717420.9644.9Ru[H 4]salpn-Y 0.57223.61967452.0672.9Rusalcn-Y 0.432 6.0663416.3632.3Ru[H 4]salcn-Y 0.44210.21101423.5648.1aReaction conditions:0.36g catalyst,26.4g benzene,60°C,3.0MPaH 2.bThe amount of the converted benzene per mole of Ru(III)complexes per hour.c0.0135g Rusalen,26.4g benzene,60°C,3.0MPa H 2.972P.Chen et al./Catalysis Communications 7(2006)969–9734.0MPa,showing that the optimum hydrogen pressure was 3.0MPa.This is in accordance with the results reported by Wang and co-workers for the partial hydrogenation of benzene to cyclohexene on a Ru–Zn/m-ZrO2catalyst[21]. This was accounted for by assuming a competitive adsorp-tion between hydrogen and benzene on the same active sites on the basis of the slow adsorption theory and the existence of a stagnant waterfilm around the catalysts. However,the effect of hydrogen pressure depends on the reaction system and the catalytic mechanism.Therefore, the reason is not unambiguous yet for the moment,and further studies are in progress.The stability and recycling possibility of the prepared Ru[H4]salen-Y catalyst in benzene hydrogenation were fur-ther investigated.After one reaction run,the catalyst was recovered by the centrifugation of hot reaction mixture so as to avoid the readsorption of possibly leached complex molecules,and further washed with ethanol and dried at 90°C under vacuum conditions.The dried sample was then used for the next run under the same reaction conditions. Fig.5shows the catalytic results of three successive recy-cles.It is clear that the obtained benzene conversion was almost the same within three reaction runs,verifying that Ru[H4]salen-Y is highly stable and can be reused.This is further confirmed by the following experiment.When the reaction was performed for2h,the catalyst was separated from the reaction mixture,and left the reaction for another 2h.It was found that the benzene conversion remained at 75%without a further increase.In contrast,if the catalyst was not separated,the benzene in the reaction mixture was completely converted.This proves that the reaction is cat-alyzed heterogeneously since homogeneous catalysis,as above discussed,plays a negligible role.This is also sup-ported by the ICP analysis that a trace amount of Ru was observed in the reaction liquid.Thus,the possibility of leaching of the complex molecules from the zeolite cav-ities could be excluded.Otherwise,the benzene conversion would decrease because of a reduction in active sites of the encapsulated complex molecules.4.ConclusionsA series of Ru(III)tetrahydro-Schiffbase complexes have been encapsulated in zeolite Y,and show much higher activity than the corresponding Ru(III)Schiffbase ana-logues for the hydrogenation of benzene under mild condi-tions.This results from a modification of the coordination environment of central Ru(III)cations by the hydrogena-tion of C@N bond,and consequently a more H2coordina-tion activation.The prepared catalysts are also highly stable and reusable,as verified by the unchanged activity within three successive reaction runs and no further increase in the benzene conversion with increasing reaction time after removing the catalyst.AcknowledgementThis work is supported by the National Science Founda-tion of China(Nos.20443004and50472083). 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