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Fe-Ce-MCM-48

Synthesis and characterization of Fe-Ce-MCM-48from silatrane precursor

via sol–gel process

Hussaya Maneesuwan,Rujirat Longloilert,Thanyalak Chaisuwan,Sujitra Wongkasemjit n

The Petroleum and Petrochemical College and Center of Excellence on Petrochemical and Materials Technology,Chulalongkorn University,Bangkok10330,Thailand

a r t i c l e i n f o

Article history:

Received4September2012

Accepted28November2012

Available online12December2012

Keywords:

Bimetallic mesoporous molecular sieves

Fe-Ce-MCM-48

Silatrane

Sol–gel process

a b s t r a c t

A series of Fe(0.01mol)and Ce(0.01to0.09mol)incorporated into MCM-48framework was

successfully synthesized by sol–gel method using cetyltrimethylammonium bromide(CTAB)as a

structural directing agent;and silatrane,FeCl3,and cerium glycolate as silica,iron,and cerium sources,

respectively.X-ray diffraction(XRD)patterns showed well-de?ned order cubic mesoporous structures

while N2adsorption/desorption measurements indicated that the synthesized bimetallic materials had

a BET surface area of up to1225m2/g,large mesopores(3.1nm),mean pore volume,and diameters of

0.83cm2/g and2.89nm,respectively.X-ray?uorescence(XRF)revealed the total metal content of the

?nal product.UV–visible absorption spectra con?rmed that both iron(Fe3t)and cerium(Ce4t)species

highly dispersed in the framework.Scanning electron microscopy(SEM)showed the truncated

octahedron morphology of Fe-Ce-MCM-48.

1.Introduction

The mesoporous materials discovered by Mobil group are

known as the M41S family[1].These materials can overcome

the limitations of microporous materials unable to allow large

reactants to penetrate inside the pores.The main members of

the M41S family are hexagonal MCM-41,cubic MCM-48,and

unstable lamellar MCM-50mesostructures.Among them,MCM-48

is the most attractive material,owing to its three-dimensional,

interconnected channels,providing more advantages—including fast

diffusion and resistance to pore blocking of coming molecules—

over the one-dimensional pores of MCM-41.Moreover,due to its

long-range order,large surface area,and narrow pore size distribu-

tion,MCM-48has been used as an adsorbent,catalyst,and catalyst

support,sensor,as well as an inorganic template for the synthesis of

advanced nanostructure[2–5].However,the pure silica MCM-48

lacks catalytic active sites,and thus,many researchers have

attempted to incorporate heteroatoms(such as Fe,Ce,Cr,V,Ti,

etc.[6–8])into the mesoporous framework to enhance its redox

properties.MCM-48,supporting two or more metal atoms,is very

attractive since one metal can modify the structural and redox

properties of the other.Consequently,bimetallic catalysts usually

improve catalytic activity,selectivity,and stability of the mono-

metallic catalysts.Many reports have shown that Fe-containing

materials have a high activity in phenol hydroxylation[6,9]

and cerium enhances hydrothermal stability[7,10].Generally,in

Contents lists available at SciVerse ScienceDirect

journal homepage:https://www.wendangku.net/doc/d814125832.html,/locate/matlet

Materials

Letters

Fig.1.XRD patterns of MCM-48,Fe-MCM-48,Ce-MCM-48and Fe-Ce-MCM-48.

https://www.wendangku.net/doc/d814125832.html,/10.1016/j.matlet.2012.11.139

n Corresponding author.Tel.:t6622184133;fax:t6622154459.

E-mail address:dsujitra@chula.ac.th(S.Wongkasemjit).

Materials Letters94(2013)65–68

catalytic reactions,the catalyst is exposed to high temperatures or boiling water;therefore,the loss of hydrothermal stability could be a serious barrier for application.Simultaneous incorporation of these two metals on to MCM-48might enhance both its phenol hydro-xylation and hydrothermal properties.In this study,Fe-Ce-MCM-48 loading different iron and cerium contents was hydrothermally synthesized via sol–gel method and characterized using XRD,XRF, N2adsorption/desorption,DRUV,and SEM.

2.Experimental

Materials:Fumed silica(99.8%,SiO2),cerium(IV)hydroxide (Ce(OH)4),and iron(III)chloride hexahydrate(FeCl3á6H2O)from Sigma-Aldrich,USA;hexadecyltrimethyl ammonium bromide (CTAB)from Fluka,Denmark;triethylenetetramine(TETA)from Facai,Thailand;ethylene glycol(EG)from J.T.Baker,USA;triethanolamine(TEA)from QREC,Asia;and acetronitrile and sodium hydroxide(NaOH)from Labscan,Asia,were used without puri?cation.

Synthesis of xFe-yCe-MCM-48:Bimetallic MCM-48materials were synthesized using Wongkasemjit’s method[11].A desired amount of FeCl3á6H2O was dissolved in water.The solution was stirred continuously while adding2M NaOH.The mixture was then slightly heated at501C while adding CTAB,followed by dissolving silatrane precursor,which was synthesized according to the method described elsewhere[12].A required amount of cerium glycolate,prepared according to the method in Ref.[13], was added and stirred for1h.The molar ratio composition of the gel was 1.0SiO2:0.3CTAB:0.5NaOH:62.0H2O:x Fe:y Ce,where 0.01r x,y r0.09.The mixture was autoclaved for16h in

Fig.2.N2adsorption/desorption isotherms of MCM-48and Fe-Ce-MCM-48.

Table1

Textural properties of MCM-48and metal modi?ed MCM-48.

Sample Fe/Si n(mole ratio)Ce/Si n(mole ratio)BET surface

area(m2/g)

pore volume

(cm2/g)

pore

diameter(nm)

a0a(nm)d211(nm)wall thickness b(nm) Gel Product Gel Product

MCM-4800001673 1.07 2.568.57 3.50 1.46

0.01Fe-MCM-480.010.0040012950.98 3.048.65 3.53 1.28

0.01Ce-MCM-48000.010.00414690.91 2.478.57 3.50 1.54

0.03Ce-MCM-48000.030.01013180.82 2.498.45 3.45 1.49

0.05Ce-MCM-48000.050.01812130.76 2.528.40 3.43 1.46

0.07Ce-MCM-48000.070.02811280.76 2.688.72 3.56 1.50

0.09Ce-MCM-48000.090.03011310.74 2.638.52 3.48 1.44

0.01Fe-0.01Ce-MCM-480.010.0050.010.00512140.93 3.078.87 3.62 1.33

0.01Fe-0.03Ce-MCM-480.010.0050.030.01412250.91 2.978.77 3.58 1.35

0.01Fe-0.05Ce-MCM-480.010.0040.050.02010800.80 2.958.74 3.57 1.35

0.01Fe-0.07Ce-MCM-480.010.0050.070.03310700.68 2.558.72 3.56 1.55

a a

?d211(6)1/2.

b Wall thickness?a

/3.0919àpore diameter/2.

n Data were obtained from

XRF.

Fig.3.DRUV–vis spectra of MCM-48,Fe-MCM-48,Ce-MCM-48and Fe-Ce-MCM-48.

H.Maneesuwan et al./Materials Letters94(2013)65–68

Te?on-lined stainless steel vessel

and treated at 1401C.The resultant solid product was ?ltered and washed with distilled water.After drying,the sample was calcined at 5501C for 6h in air at a heating rate of 0.51C/min.Pure MCM-48,Fe-MCM-48,and Ce-MCM-48were also synthesized,using the same method as the bimetallic MCM-48,for comparison.

Characterization:XRD patterns were recorded on a Rigaku X-ray diffractometer with CuK a radiation over the range of 2y ?2–61.XRF was carried out using a PANalytical AXIOS PW 4400.The N 2adsorption/desorption was determined by a Quantasorb JR instrument using the Brunauer–Emmett–Teller (BET)method.Diffuse re?ectance UV–visible (DRUV)spectra were measured on a Shimadzu UV-2550.SEM micrographs were obtained using a Hitachi S-4800.

3.Results and discussion

XRD :The XRD patterns of the calcined samples shown in Fig.1indicate that Ce-MCM-48with a Ce/Si molar ratio from 0.01to 0.09developed patterns consistent with our previously reported MCM-48[11]and JCPDS no.00-051-1592in 2y range of 2–61,being indexed to the [211],[220],[420],and [332]re?ections of the Ia 3d cubic phase of MCM-48.As the content of heteroatoms increased,all the peaks obviously shifted to lower angles,imply-ing the dilation of material structure.The radii of Ce 4t(Paulling radius ?103.4pm)and Fe 3t(Paulling radius ?64pm)are larger than that of Si 4t(Paulling radius ?42pm).There should be an enlargement in the unit cell parameter as the bimetal cations are incorporated,resulting in a larger M–O bond distance.This result is con?rmed by the increase of the d spacing of bimetallic

materials [14–15].Mono-and bimetallic MCM-48containing Fe/Si of more than 0.01M ratios were not successfully achieved,probably due to an imbalance of charge matching [16].Thus,high Fe/Si ratios could not be synthesized while bimetallic samples of x Fe-yCe-MCM-48with x ?0.01and 0.01r y r 0.07provided a cubic morphology of MCM-48.According to the study of Zhao et al.[17]on the formation of cubic phase using g ?V /a 0l ,where g is the local effective surfactant-packing parameter,V is the volume of hydrophobic tail,a 0is the effective headgroup area of the cationic ammonium,and l is the kinetic length of the hydrophobic tail of the surfactant,the cubic phase can be easily formed if g is large.The presence of the anionic species of FeCl 3and Ce(C 2H 4O 2)2metal precursors increases the local effective surfactant-packing parameter (g )value,causing a phase change from hexagonal to cubic.Moreover,Mahoney [16]also found that the unequal distribution of charge density in the electric double layer resulted in a curvature of surface in which the cubic structure prefers a high curvature radius with high g.A com-parison between those two anions,C 2H 4O 2

2à

and Cl à,reveals C 2H 4O 2

2à

to be more effective in reducing the thickness of the double electric layer because of its higher valence state [17].

Therefore,C 2H 4O 2

2à

provides a better assembly between the ammonium cations and the inorganic silicate anions.The XRD results clearly showed that the cubic structure of bimetallic products was successfully obtained at various amounts of Ce with a 0.01Fe/Si molar ratio.

N 2adsorption/desorption isotherms:The N 2adsorption/deso-rption isotherms and pore size distribution curves of the products are shown in Fig.2.All of the samples showed a steep increase in the volume of the adsorbed nitrogen at P/P 0?0.20–0.35,exhi biting the type IV isotherm,which con?rms the mesoporous

Fig.4.FE-SEM images of (a)MCM-48,(b)0.01Ce-MCM-48,(c)0.01Fe-MCM-48,(d)0.01Fe0.01Ce-MCM-48,(e)0.01Fe0.03Ce-MCM-48,(f)0.01Fe0.05Ce-MCM-48,and (g)0.01Fe0.07Ce-MCM-48.

H.Maneesuwan et al./Materials Letters 94(2013)65–6867

structure with a narrow pore size distribution,having an average pore size$2–3nm.The structural parameters of various samples are summarized in https://www.wendangku.net/doc/d814125832.html,pared with pure MCM-48,the incorporation of the heteroatom resulted in higher pore diameter and unit cell(a0).BET surface area decreased with an increase in the amount of incorporated metal.This result could be explained by some destruction of metallic MCM-48pore structure,which is in agreement with the XRD’s results[18].However,a high BET surface area of more than1000m2/g was obtained for all samples.

XRF spectroscopy:The XRF results(Table1)con?rmed the total amounts of Fe and Ce introduced into MCM-48.However,the actual amount of the metal was less than the added amount,due to the solubility of Fe and Ce sources in the medium.

DRUV–v is spectroscopy:DRUV–vis spectra of the synthesized samples are presented in Fig.3.The absorption band is absent for MCM-48,while Fe-MCM-48showed a strong absorption at 200nm,attributed to the charge-transfer transitions involving isolated framework Fe3tin FeO4tetrahedral coordination[10].In addition,there is no absorption band at500–600nm,referring to Fe3toctahedral coordination in extra framework[9].The result suggests that iron indeed exists inside the MCM-48framework.For Ce-MCM-48,they exhibited a weak broad band at200–300nm and an intense band at350nm.These bands corresponded to Ce4ttetrahedral coordination.No extra framework band at405nm was observed[19].As the cerium content increased,the band at 350nm also increased.Fe-Ce-MCM-48exhibited iron and cerium bands at200and350nm,respectively.Both iron and cerium were very well incorporated into the MCM-48framework.

FE-SEM:SEM images of the samples are shown in Fig.4. Ce-MCM-48morphology was quite the same as pure MCM-48 because the anionic C2H4O22àfacilitates the formation of the cubic structure.However,Clàin FeCl3does not seem to support the cubic formation,thus causing more distortion from MCM-48in the Fe-MCM-48morphology.Bimetallic Fe-Ce-MCM-48showed much more distortion since there are two metal atoms and high counter ions in the system.Moreover,as the metal contents in bimetallic materials increased,crystal size trended downward (Fig.4d–g).As described by Vekilov and Kashchiev,at a high metal ratio,there are more ions in the solution able to form many nuclei,resulting in tiny crystal growth[20–21].

4.Conclusions

Fe-Ce-MCM-48mesoporous structures were successfully synthesized using silatrane as a silica source.High iron content in MCM-48could not be achieved while various amounts of Ce in MCM-48were well incorporated.Highly dispersed Fe and Ce in the bimetallic framework at0.01Fe/Si and various cerium contents(Ce/Si?0.01–0.07)was observed and all synthesized samples provided a high surface area and narrow pore size distribution.

Acknowledgments

This research is?nancially supported by the Thailand Research Fund,Ratchadapisake Sompote Endowment Fund,Part of the "Strengthen CU’s Researcher’s Project",and the Center of Excel-lence on Petrochemical and Materials Technology,Chulalongkorn University,Thailand.The authors would like to thank Mr.John M. Jackson for English proofreading.

Appendix A.Supporting information

Supplementary data associated with this article can be found in the online version at https://www.wendangku.net/doc/d814125832.html,/10.1016/j.matlet.2012.

11.139.

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