Synthesis and characterization of Mn–MCM-41andZr–Mn-MCM-41M.Selvaraja,*,P.K.Sinha b ,K.L ee a ,I.Ahn a ,A.Pandurangan c ,T.G.Leea,*aYonsei Center for Clean Technology,Yonsei University,Seoul 120-749,Republic of KoreabCWMF,BARC Facilities,Kalpakkam 603102,IndiacDepartment of Chemistry,Anna University,Chennai 600025,IndiaReceived 14July 2004;received in revised form 30September 2004;accepted 5October 2004Available online 10December 2004AbstractMesoporous silica molecular sieves MCM-41containingmanganese ions (Zr–Mn-MCM-41)with Si/(Zr+Mn)ratio equal to 49,98,147,196,262and 327manganese ions with Si/Mnratio equal to 15,31,46,61,82and 102respec-tively,were synthesized under hydrothermal using cetyltrimethylammonium (CTMA +)surfactants as template in the absence of auxiliary organics.The viz.Zr–Mn-MCM-41and Mn-MCM-41were characterized using several techniques,e.g.ICP-AES,XRD,FTIR,Nitrogen adsorption,TG/DTA,ESR,UV–Vis/DRS,SEM and TEM.ICP-AES studies indicated the content of zirconium and manganese in the mesoporous materials.XRD studies indicated that the materials had the standard MCM-41structure.FTIR studies showed that zirconium and manganese ions were incorporated into the hexagonal mesoporous structure of Zr–Mn-MCM-41and Mn-MCM-41.The thermal stability of the as-synthesized materials was studied using TG/DTA.Nitrogen adsorption was used to determine specific surface area,pore diameter,pore volume and wall thickness in the calcined Zr–Mn-MCM-41and Mn-MCM-41mesoporous molecular sieves.The incorporated of zirconium and ions oxidation state was determined by ESR and UV–Vis/DRS.The morphology of Zr–Mn-MCM-41and Mn-MCM-41was determined by SEM.The inference that the Zr–Mn-MCM-41and Mn-MCM-41mesoporous materials had uniform pore was obtained by TEM studies.The incorporated metal ions as Zr 4+,Mn 2+and Mn 3+in Zr–Mn-MCM-41are coordinated to by tetrahedral,disordered octahedral and tetrahedral environments respectively.The zirconium and manganese ions are homogen-ously dispersed on the inside silica surface of the Zr–Mn-MCM-41,but only the manganese ion is not homogenously dispersed on the inside surface of the Mn-MCM-41under hydrothermal conditions.Thus the Zr–Mn-MCM-41mesoporous molecular sieves are more useful for oxidation of olefin reaction with TBHP,e.g.,in the of trans -stilbene,trans -stilbene-oxide (84.58%)was found to be the highest in the presence of Zr–Mn-MCM-41(49).Ó2004Elsevier Inc.All rights reserved.Keywords:Zr–Mn-MCM-41;Mn-MCM-41;Hydrothermal stability;Thermal stability;Lewis acidity1.IntroductionSince the discovery of MCM-41in 1992by Mobil Sci-entists [1,2],numerous studies concerning the prepara-tion conditions,synthesis mechanism,characterization and use of these materials as catalysts and catalyst sup-ports in various [3–6]have been reported.In-deed,pure siliceous MCM-41can be used in inclusion for the encapsulation of conducting quantum wires,the silicate framework providing the isolating part of the device.But as these are pure silica materials,lacking sufficient acidity,it is difficult to use them directly as catalysts.Metallosilicate1387-1811/$-see front matter Ó2004Elsevier Inc.All rights reserved.doi:10.1016/j.micromeso.2004.10.004*Corresponding authors.Tel.:+82221235751/409941;fax:+8223126401.E-mail addresses:selvarajman25@ (M.Selvaraj),ted-dy.lee@yonsei.ac.kr (T.G.Lee)./locate/micromesoMicroporous and Mesoporous Materials 78(2005)139–149molecular sieves,obtained by isomorphous substitution of Si by metal ions in silicate structures,exhibit catalytic activity in selective oxidation reactions using hydrogen peroxide and alkylhydroperoxides at mild conditions.In recent years,attention has increasingly been directed towards the study of metal-containing mesoporousM41S type molecular sieves with large pores (20–100A˚diameter)suitable for the transformation of bulky or-ganic compounds [1,2,7–13].Metal ions such as Al [14–17],Ti [6,18],Fe[20–25],V [26,27],and Ga[28–30]have been incorporatedinto the MCM-41et al.[31–33]reported the details of synthesis and characterization of bimetal-lic as Zn-Al-MCM-41mesoporous materials.How-ever,hydrothermal stability,hydrophobicity,thermal stability and acid strength in the mesoporous materials increased by the introduction of the incorporation of zinc.Manganese are well known catalysts for epoxidation reactions [34].Recently,many research groups worked on the immobilization of Mn complexes onto the mesoporous material.Although ion-exchange method was applied to Al-MCM-41[35],only lower loading amount can be obtained by this method.Caps and Tsang [36]prepared Mn-MCM-41with a molecular organic chemical vapor deposition (MOCVD)method.Burch et al.[37]prepared surface-grafted manganese–oxo species on the walls of MCM-41channels.Yonemi-tsu et al.[38]firstly developed the template ion-exchange (TIE)method for the synthesis of Mn-MCM-41,and they argued that manganese was highly dispersed in the mesopore of MCM-41and only existed as Mn 2+.But,when the other transition metal ions are incorporated in Mn-MCM-41,the environment struc-ture,morphology and distribution of the Mn in the framework and also hydrothermal stability have not established.In the present study,Zr–Mn-MCM-41and Mn-MCM-41materials with different silicon to metal/metals ratio have been synthesized using hydrother-mal method.The materials characterized by ICP-AES,XRD,FTIR,Nitrogen adsorption,TG/DTA,DR/UV–Vis spectroscopy,ESR,SEM and TEM.Mesostructure,metal incorporation,morphology of Zr–Mn-MCM-41,the site geometry,coordination and distribution of Mn in Zr–Mn-MCM-41when the Zr-ions incorporated in Mn-MCM-41are subsequently investigated.2.Experimental 2.1.MaterialsThe syntheses of Zr–Mn-MCM-41and Mn-MCM-41materials were carried out by hydrothermal method using sodium metasilicate (Na 2SiO 3Á5H 2O),Zirconiumtetraisopropoxide ((Zr–(O–CH–(CH 3)2)4),Manganese (II)acetate ((CH3COO)2Mn),cetyltrimethylammo-niumbromide (C 3)3N +Br)and sulfuric acid (H 2SO 4).The all were purchased from M/s Aldrich &Co.,2.2.Synthesis of Zr–Mn-MCM-41and Mn-MCM-412.2.1.Synthesis of Zr–Mn-MCM-41The Zr–Mn-MCM-41was synthesized according to the previous report [39]using hydrothermal method.For the synthesis of the Zr–Mn-MCM-41(Si/Zr +Mn =49),21.2g (1mol)of sodium metasilicate (44–47%SiO 2)dissolved in 50g of deionised water was with 0.54g (0.015mol)of zirconium (IV)isoprop-and 0.4g (0.015mol)of manganese (II)acetate (dissolved in 10g of deionized water)solution.This mix-ture was stirred for 30min using a mechanical stirrer at a speed of about 250rpm and in order to reduce the pH to 10.8,1N of sulphuric acid was added with continuous stirring for another 30min at a speed of about 250rpm until the gel formation.After that,9.1g (0.25mol)of cetyltrimethylammonium bromide was added drop by drop (30ml h À1)through the dual syringe pump so that the gel was changed into suspension.After further stirring for 1h the resulting synthesis gel of composition 1SiO 2:0.015ZrO 2:0.015MnO:0.25CTMABr:100H 2O,was transferred into Teflon-lined steel autoclave and heated to 165°C for 48h.After cooling to room temper-ature,the material was recovered by filtration,washed with deionised water and ethanol and finally calcined in flowing air at 540°C for 6h.The different Zr–Mn-MCM-41catalysts (Si/Zr +Mn =98,147,196,262and 327)were also synthe-sized in a above similar manner wherein only the ratio of sodium metasilicate,zirconium (IV)isopropoxide and manganese (II)acetate was adjusted and the input in gel molar compositions 1SiO 2:x ZrO2:y MnO:0.25CT-MABr:100H 2O;(x =0.008–0.0025,y =0.008–0.0025).2.2.2.Synthesis of Mn-MCM-41The Mn-MCM-41catalysts (Si/Mn =15,31,46,61,82and 102)were also synthesized in a above similar manner wherein only the ratio of sodium metasilicate and manganese (II)acetate was adjusted without zirco-nium source and the input in gel molar compositions 1SiO 2:x MnO:0.25CTMABr:100H 2O;(x =0.033–0.005).2.3.Characterization2.3.1.ICP-AESThe zirconium and manganese content in Zr–Mn-MCM-41and Mn-MCM-41were recorded using ICP-AES with allied analytical ICAP 9000.140M.Selvaraj et al./Microporous and Mesoporous Materials 78(2005)139–1492.3.2.XRDThe crystalline phase identification and phase purity determination of the calcined Zr–Mn-MCM-41and Mn-MCM-41samples were carried out by XRD(Phi-lips,Holland)using nickelfiltered Cu K a radiation (k=1.5406A˚).The samples were scanned from0.5°to 5°(2h)angle in steps of0.5°,with a count of5s at each point.In order to protect the detector from the high en-ergy of the incident and diffracted beam,slits were used in this work.2.3.3.FTIRInfrared spectra were recorded with a Nicolet Impact 410FTIR Spectrometer in KBr pellet(0.005g sample with0.1g KBr)scan number36,resolution2cmÀ1. The data was treated with Omnic Software.2.3.4.N2-AdsorptionThe surface areas and pore properties of Zr–Mn-MCM-41and Mn-MCM-41samples before and after hydrothermal treatments were analyzed using a NOVA-1000(QUANTACHROME,version5.01)sorp-tometer.The calcined samples were dried at130°C and evacuated overnight for8h inflowing arogn atflow rate of60ml minÀ1at200°C under vacuum.Surface area, pore size,pore volume and wall thickness was obtained from these isotherms using the conventional BET and BJH equations.2.3.5.TG-DTAThe weight loss,dehydration and dehyoxylation for as-synthesized Zr–Mn-MCM-41and Mn-MCM-41 samples was evaluated by a Thermogravimetric-Differ-ential Thermal Analysis(TG-DTA)in a Rheometric scientific(STA15H+)thermobalance.10–15mg of as-synthesized MCM-41was placed in a platinum pan and heated from room temperature to1273K at a heat-ing rate of2K/min in air withflow rate of50ml/min. For comparison experiments,samples were dried at 323K for the same period.The data were collected at 30-s intervals using on-line PC.2.3.6.ESRFor ESR measurements,Zr–Mn-MCM-41and Mn-MCM-41samples were loaded into2mm i.d x150mm length suprasil quartz tubes.ESR spectra were recorded at X-band at either293or77K on a Bruker ESP300 spectrometer.The magneticfield was calibrated with a Varian E-500gaussmeter.The microwave frequency was measured by a Hewlett Packard HP5342A fre-quency counter.2.3.7.UV–visible/Diffuse reflectance spectroscopyThe diffuse reflectance UV–visible spectra were meas-ured with a Perkin-Elmer330Spectrometer equipped with a60mm Hitachi integrating sphere accessory.Powder samples were loaded in a quartz cell with Supra-sil windows,and spectra were collected in the200–800nm wavelength range against quartz standard.2.3.8.SEMThe SEM microscope of a typical sample of MCM-41 materials were obtained on a JEOL2010microscope operated at200KV from a thinfilm dispersed on a holly carbon grid in ethanol solvent.2.3.9.TEMTEM images were recorded using a JEOLJEM-200CX electron microscope operating at200KV with modified spectrum stage with objective lens parameters C s=0.41mm and C c=0.95mm,giving an interpretable point resolution of Ca.0.185nm.Samples for analysis were prepared by crushing the particles between two glass slides and spreading them on a holly carbonfilm supported on a Cu grid.The samples were briefly heated under tungstenfilament light bulb in air before transfer into the specimen chamber.The images were recorded at magnifications of100000·.3.Results and discussion3.1.ICP-AESThe zirconium and manganese ions content in Mn-MCM-41and Zr–Mn-MCM-41have been observed using ICP-AES.The observed zirconium and manga-nese ions content in the samples have been presented in Table1.3.2.XRDFig.1a and b show the X-ray powder diffraction pat-terns of calcined Mn-MCM-41and Zr–Mn-MCM-41 samples having different ratio of Si/Mn and Si/Zr+Mn respectively.The X-ray diffractograms of Zr–Mn-MCM-41and Mn-MCM-41after calcinations in air at540°C for6h, contain,a sharp d100reflection line in the range 2h=2.15–2.30°and 2.03–2.22°respectively.Physico-chemical properties of these mesoporous materials are summarized in Table1.For example,the pore-to-pore distance of Zr–Mn-MCM-41could be determined by the XRD patterns.The XRD patterns of calcined Zr–Mn-MCM-41(49)with characteristic peaks of hexago-nal(p6mm)symmetry and with d100of41.090are shown in Fig.1b.The repeating distance(a0)between pore cen-ters was47.440.The hexagonal unit cell parameter(a0) was calculated using the formula as a0=2d100/p3from d100,which was obtained from the peak in the XRD pat-tern by BraggÕs equation(2d sin h¼k,where k=1.54060for the Cu K a line).The value of a0wasM.Selvaraj et al./Microporous and Mesoporous Materials78(2005)139–149141equal to the internal pore diameter plus one pore wall thickness.These peaks reflections were indexed based on the hexagonal unit cell as described by Beck et al.[2].The XRD peaks shifted to lower 2h values for the rest of the samples.The higher d -spacing and unit cell parameter values observed with increasing metal content suggest an incorporation of metal in the framework locations [40]because of its (Zr–O ($1.71160)and Mn–O ($1.73080))longer bonding length with oxygen than Si–O ($1.5090).However,there is no regular rule in MCM-41as it has an amorphous structure where both bond length and angle may be change.The pertur-bation of d-spacing and unit cell parameter is hypothe-sized to be dependent on the metal substitution stateresulting from the sample preparation method,i.e.,coordination number of metal with OH groups and also along with bond length and bond angle of metal ion on the silica surface.3.3.FTIRInfrared spectroscopy had been used extensively for the characterization of transition-metal cation modified zeolites.The as-synthesized Zr–Mn-MCM-41and Mn-MCM-41samples exhibit absorption bands around 2921and 2851cm À1corresponding to n -C–H and d -C–H vibrations of the surfactant molecules.The broad bands around 3500cm À1may be attributed surfaceTable 1Physicochemical characterization of Zr–Mn-MCM-41and Mn-MCM-41Catalysts Zr content d (wt%)Mn content d (wt%)d -spacing a (A˚)Unit cell parameter aa 0(A ˚)Surface areab (m 2/g)Pore size bD (A ˚)Pore volume b (cm 3/g)Wallthickness c (A ˚)Zr–Mn-MCM-41(49)0.1650.16641.0947.4483530.30.8117.14Zr–Mn-MCM-41(98)0.0830.08240.5646.8386330.20.7916.63Zr–Mn-MCM-41(147)0.0520.05540.1546.3689030.10.7516.26Zr–Mn-MCM-41(197)0.0410.04139.4345.5292429.90.7015.62Zr–Mn-MCM-41(261)0.0320.03138.4044.3494329.80.6914.54Zr–Mn-MCM-41(327)0.0230.02536.8142.50102529.40.6813.1Mn-MCM-41(15)–0.33043.5150.2486332.70.9917.54Mn-MCM-41(31)–0.16542.6149.2088432.60.9816.60Mn-MCM-41(46)–0.11142.2648.7990332.50.9716.29Mn-MCM-41(61)–0.08241.6148.1194532.40.9415.71Mn-MCM-41(82)–0.06240.7147.0099932.30.9114.7Mn-MCM-41(102)–0.05039.7945.94103432.20.8913.74a Values obtained from XRD studies.b Values obtained from N 2-adsorption results.c Wall thickness (t )=Unit cell parameter(a 0)pore size(D ).dThe results obtained from ICP-AES.142M.Selvaraj et al./Microporous and Mesoporous Materials 78(2005)139–149silanols and adsorbed water molecules,while deforma-tional vibrations of adsorbed molecules cause the absorption bands at1623–1640cmÀ1[41](the results of peaks have not shown infigures).The substitution of silicon by zirconium and manganese causes shifts of the lattice vibration bands to lower -pared to the Si-MCM-41,the wave number of the anti-symmetric Si–O–Si vibration band of Mn-MCM-41and Zr–Mn-MCM-41samples decreases to1086and 1080cmÀ1,receptively(Fig.2a and b).Theses shifts should be due to the increase of the mean Si–O distance in the walls caused by the substitution of the small sili-con(radius40pm)by the larger size of Mn2+(radius 83pm)and Zr4+(radius59pm)[42].The observed shifts, which depend as well on the change in the ionic radii as on the degree of substitution,are comparatively small. Therefore,only a low degree of substitution is sug-gested.Interestingly,the wavenumber shifts decrease in the series Zr–Mn-MCM-41>Mn-MCM-41, although the ionic radius of zirconium is smaller than those of manganese.While the wave number of the anti-symmetric Si–O–Zr and Si–O–Mn vibration bands of Zr–Mn-MCM-41and Mn-MCM-41samples de-creases to957and962cmÀ1,receptively(Fig.2a and b).The absorption band at1057and1223cmÀ1are due to asymmetric stretching vibrations of Si–O–Si bridges,while the960to965cmÀ1bands are due to Si–OÀM+(M=Mn and Zr)vibrations in metal incorporated silanols.By the disappearing peaks at 2851and2921cmÀ1,one could conclude that calcination of the original framework was complete and the identity of organic molecule completely disappeared from the calcined M-MCM-41.Upon introduction of higher metal content,most of the bands shifted to higher wave numbers,consistent with their incorporation in lattice positions.Addition-ally,an absorption band in the range962–967cmÀ1as-signed to a stretching vibration of Si–OÀM+linkage was observed.This is generally considered to be a proof of the incorporation of the heteroatom into the frame-work.Cambler et al.[43]have reported similar stretch-ing vibrations of Si–OH groups present at defect sites.3.4.N2-adsorption isothermFig.3a and b show the isotherm of nitrogen adsorp-tion by calcined Mn-MCM-41and Zr–Mn-MCM-41 measured at liquid nitrogen temperature(77K).Three well-defined stages may be identified:(1)A slow increase in nitrogen uptake at low relative pressure,correspond-ing to monolayer-multilayer adsorption on the pore walls;(2)a sharp step at intermediate relative pressures indicative of capillary condensation within mesopores; and(3)a plateau with a slight inclination at high relative pressures associated with multilayer adsorption on the external surface of the crystals[44].A fourth stage,M.Selvaraj et al./Microporous and Mesoporous Materials78(2005)139–149143characterized by a sharp rise in N2uptakefilling all other available pores as the pressure reaches saturation (p/p0=1.0),may be identified in some isotherms.Each isotherm showed type IV character,which is a typical shape for mesoporous MCM-41.The third stage of the isotherm shifts slightly toward higher relative pressure with the incorporation of transition metal(Zr+Mn and Mn).The hysteresis loop broadened with an in-crease in the metal content suggesting some disorder in the pore system arising from metal incorporation.Incor-poration of metal in MCM-41has significant effect on these parameters.The surface area values decreased with increasing metal content.The pore diameter,pore vol-ume and wall thickness increased in Zr–Mn-MCM-41144M.Selvaraj et al./Microporous and Mesoporous Materials78(2005)139–149and Mn-MCM-41with increasing metal content.and are listed in Table1.In the crystalline zeolites,metal incorporation slightly increases the pore size because of its longer bonding length with oxygen than Si–O. However,there is no regular rule in MCM-41as it has an amorphous structure where both bond length and angle may ually,it has been observed that the pore size of MCM-41is decreased after metal incor-poration,but there is no clear explanation for this obser-vation.MCM-41has thin pore walls relative to zeolites so that the incorporated metal cannot be substituted into the silica framework completely.That is,a part of the metal will be exposed on the pore wall surface so that it might have properties similar to impregnated metal complexes on the MCM-41walls.The incorpo-rated metal may interact with surface hydroxyl groups and may contract the pore wall when combined with two or three hydroxyl groups,so that the pore size will be de-creased[45,46].If these metals are incorporated deep within the silica framework,this pore shrinkage might not occur as with condensed surface hydroxyl groups, and the pore size might be increased instead by the in-creased metal-oxygen bond length,as for zeolites.In this study,we obtained an increased pore size after both metal as Zr and Mn,and alone Mn incorporation as shown in Fig.2a and b respectively.This is an unusual result, and strongly suggests that Zr and Mn are incorporated into silica framework according to the above hypothesis. This can be confirmed indirectly by the ease of metal reduction,and such a result has been reported[47].The remarkable improvement of hydrothermal stabil-ity due to the higher metal incorporation is further evi-denced by the measurement of N2adsorption.The surface areas of all the MCM-41materials were mea-sured after they had been treated in boiling water for1 week.The surface areas slightly decreased from863–1034to830–980m2/g in the Mn-MCM-41samples and also slightly decreased from1025-890to967–1002m2/g in Si/(Zr+Mn)ratios of147–327in Zr–Mn-MCM-41 samples,but the surface area does not decrease in other Zr–Mn-MCM-41samples.The Si–O–Zr and Si–O–Mn bonds are relatively stable to further attack from boiling water[39,48];the presence of Mn2+and Zr4+creates a negative charge on the surface of the pore walls,repel-ling OHÀions and therefore preventing the hydrolysis of siloxane bonds and also resulting in an increase in the number of acid sites.The surface area in the Mn-MCM-41and Zr–Mn-MCM-41(Si/Zr+Mn=147–327) samples decreased due to low metal incorporation/non-framework as it does not create sufficient negative charges on the surface of the pore walls.But the surface area of other Zr–Mn-MCM-41samples does not decrease due to repelling OHÀions higher on the pore walls.Hence,it is concluded that Zr is irreversibly incor-porated into the structure of Zr–Mn-MCM-41samples. Thus,the hydrothermal stability is higher in Zr–Mn-MCM-41(49)than that in other Zr–Mn-MCM-41and Mn-MCM-41samples.3.5.Thermal analysisThermogravimetric analysis of the catalysts show dis-tinct weight losses that depend on framework composi-tion.Representative thermograms are given in Fig.4a (Mn-MCM-41)and Fig.4b(Zr–Mn-MCM-41).Gene-rally,when the metal content increases,there is a de-crease in organic content and increase in water content.Three distinct regions of weight loss are noted in the temperature range50–150°C,150–350°C and 350–550°C.Thefirst weight loss($4.6–16.61%)corre-sponds to the desorption and removal of the water and/or ethanol molecules physisorbed on the external surface of the crystallites or occluded in the macropores and mesopores present between the crystallites aggre-gates.A second weight loss($38.58–40.5%),between 150°C and350°C is attributed to the removal of the or-ganic template.Finally,a third weight loss($1.51–1.9%) between350°C and550°C is related to water loss from the condensation of adjacent silanol groups to form siloxane bond[49].The total weight loss at1000°C of the Mn-MCM-41and Zr–Mn-MCM-41samples are in the44.69–59.01%range.However,the distribution of successive weight loss depends on the framework or sub-stituted silicon to metal ratio[50].Thus,weight loss was higher in the low metal contents of MCM-41materials than that in the high metal contents of MCM-41 materials.3.6.ESRThe Q-band ESR spectrum of synthesized Mn-MCM-41at150°C in the presence of suphuric acid exhibits a typical Mn2+spectrum,sextet lines with a hyperfine coupling(A)of81G and g=2.00,as shown in Fig.5a.Low intensity forbidden transitions are evi-dent in room temperature spectra,indicating that the Mn2+ions are rather incorporated.The greater incorpo-ration of the Mn2+species in as-synthesized Mn-MCM-41at150°C can be attributed either to its location within the wall or in the interface region of the polar head groups of the template and the silica walls.How-ever,their high mobility indicates that they are not located within the inorganic walls but are within the pores.This indicates that the Mn2+is more mobile after calcination.The same spectrum was obtained from cal-cined Mn-MCM-41that did not exhibit Mn2+ESR sig-nals prior to calcinations.From the above results,it is inferred that the Mn2+is coordinated to Si(IV)by dis-torted octahedral environments.The preliminary EPR analysis was carried out for the Zr–Mn-MCM-41.A strong resonance at g=2.01with a hyperfine coupling of91g was observed(Fig.5b)whichM.Selvaraj et al./Microporous and Mesoporous Materials78(2005)139–149145could be easily attributed to a distorted octahedral Mn2+species.The hyperfine lines are slightly broader for Zr–Mn-MCM-41than for Zr-MCM-41.The spin Hamilton parameters are also similar to those reported by Piaggio et al.[51]for the homogeneous Mn(II)-salen complex,suggesting the incorporation of the Zr and Mn in the MCM-41materials.3.7.Diffuse reflectance UV/visible spectroscopyThe calcined Mn-MCM-41samples are characterized by reflectance UV–Visible spectroscopy.Two bands at 270and500nm were observed for these samples.The A1g!T2g crystalfiled transitions of Mn2+in Mn2O3 or MnO showed a band at500nm[52].Thus the band at500nm was assigned to Mn2+probably existing on the surface of MCM-41.However,the band at270nm with such a high intensity was not reported in the liter-ature.The charge transfer transition of O2À!Mn2+in Mn3O4in which Mn was disordered octahedrally coor-dinated with oxygen exhibited a band at320nm[52]. We thus tentatively assigned the intense band observed at lower wavelength(270nm)to the charge transfer transition of O2À!Mn3+tetrahedral coordination,i.e in the framework of MCM-41.From the above results, it is inferred that the Mn2+and Mn3+ions may be coor-dinated with Si(IV)by disordered octahedral and tetra-hedral environments respectively.The DR UV–Vis spectra of Zr–Mn-MCM-41were found to be similar for those of Zr-MCM-41[39]and Mn-MCM-41in that the peaks were in the same position and the Zr4+and Mn2+are coordinated to Si(IV)by tet-rahedral and disordered octahedral environments respectively.From the ESR and UV–Vis/DRS results, theoretically,we conclude that the Lewis acidity in Zr–Mn-MCM-41materials are higher than that that in Mn-MCM-41due to the higher electron-pair donated to the inner side silica surface.3.8.SEMAll Mn-MCM-41and Zr–Mn-MCM-41materials are having micellar rod-like shape hexagonal or spherical edges,as shown in the Fig.6a(Mn-MCM-41(31))and Fig.6b(Zr–Mn-MCM-41(49)).Each rod,itself trans-formed into the MCM-41hexagonal-phase mesostruc-ture.Because all materials have been synthesized using cetyltrimethyammonium bromide as surfactant in the liquid crystal template mechanism,Steel et al.[53]pos-tulated that CTMABr surfactant molecules assembled directly into the hexagonal liquid crystal phase upon addition of the silicate species,based on14NMR spectro-scopy.The size of micellar rod like shape and hexagonal phase in Zr–Mn-MCM-41materials are slightly smaller than those in Mn-MCM-41.3.9.TEMThe TEM image of all Mn-MCM-41and Zr–Mn-MCM-41samples(Fig.7a(Mn-MCM-41(31))and Fig.7b(Zr–Mn-MCM-41(49))exhibit ordered hexago-nal arrays of mesopores with uniform pore size[1]. The corresponding electron diffraction pattern also shows reflections contrast in the TEM image of the sam-ple,the distance between mesopores are estimated,in good agreement with the value determined from XRD and N2-adsorption measurements values.Both pore channels and hexagonal symmetry can be clearly identified in the TEM image for the Mn-MCM-41and Zr–Mn-MCM-41samples which indicates that the MCM-41materials have only one uniform phase as inferred from the XRD results.Interestingly,the TEM image viewed down the d110direction shows that pore channels are not straight,they are arc-like.In con-trast,the TEM image recorded along the d110direction of MCM-41showshorizontal.Fig.6.(a)SEM image of Mn-MCM-41(31).(b)SEM image of Mn-MCM-41(49).M.Selvaraj et al./Microporous and Mesoporous Materials78(2005)139–149147。
