(Co) Síntesis y caracterización de Co-SBA-15 y su actividad en la epoxidación de estireno con...

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Applied Catalysis B: Environmental 101 (2010) 4553

Contents lists available atScienceDirect

Applied Catalysis B: Environmental

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / a p c a t b

Synthesis and characterization of cobalt-substituted SBA-15 and its high activityin epoxidation of styrene with molecular oxygen

Haitao Cui a,b, Ye Zhang a,, Zegang Qiu a, Liangfu Zhao a,, Yulei Zhu a

a State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR Chinab Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China

a r t i c l e i n f o

Article history:

Received 15 May 2010Received in revised form 1 September 2010

Accepted 3 September 2010

Available online 21 September 2010

Keywords:

Cobalt

SBA-15

Epoxidation

Styrene

Molecular oxygen

a b s t r a c t

Cobalt-substituted SBA-15 (Co-SBA-15x) was successfully synthesized by a simple and effective method

designated as pH-adjusting. Based on series of characterizations, the material still retained a highly

ordered mesostructure of SBA-15 when a certain amount of cobalt was introduced. The cobalt species in

Co-SBA-15x mainly existed in the single-site Co(II) state. Most of them were incorporated into the silica

framework of SBA-15, locating at tetrahedrally coordinated sites. For epoxidation of styrene with O2, Co-

SBA-15x exhibited the highest catalytic activity among the cobalt-based catalysts prepared by different

method with the same cobalt content, and the single-site Co(II) in Co-SBA-15xwas the most active in the

reaction. The influences of reaction conditions such as reaction temperature, catalyst amount, solvent,

solvent amount,reactiontime,oxidant, andoxidant amounton theperformanceof Co-SBA-15xwere also

investigated in detail. Co-SBA-15x possessed excellent stability and recyclability in the reaction.

2010 Elsevier B.V. All rights reserved.

1. Introduction

The epoxidation of styrene has attained great attention recently

because theepoxide is a key intermediatein theproductionof many

fine chemicals and pharmaceuticals. In conventional process, the

epoxide is produced by the reaction of styrene with peracid [1].

However, a number of problemsfrom thismethodhinderits further

application, such as formation of undesirable wastes, high cost and

corrosivity of the peracid. Some environmentally benign methods

have been explored in the past few years, and many attractive cat-

alyticsystemshave been reported forthe reaction. Forinstance, the

epoxidation of olefins withH2O2 overTS-1 [2] orusingO2 combined

with a sacrificial reductant (H2, aldehyde or alcohol) catalyzed by

noble-metal catalyst[36]is thought to be a green process. Nev-

ertheless, from both environmental and economic viewpoints, the

epoxidation of styrene withO2 or air without any co-reductantovereffective catalysts is an attractive and promising method.

Cobalt-based catalysts are widely used in a variety of catalytic

reactions, such as reforming of methane[7]or ethanol[8],ethene

hydroformylation [9], Fischer-Tropsch synthesis [1012], hydro-

genation of aromatics [13] or aldehydes [14] and ethyl acetate

total oxidation [15]. Likewise, several studies on the epoxidation of

Corresponding authors at: Institute of Coal Chemistry, Chinese Academy of Sci-

ences, Taiyuan 030001, PR China. Tel.: +86 351 4041526; fax: +86 351 4041526.

E-mail addresses:[email protected](Y. Zhang),[email protected](L. Zhao).

alkenes with molecular oxygen or air over cobalt-based catalysts

have been carried out recently. Ion-exchanged Co-X and Co-MCM-

41[16],Co-ZSM-5[17],CoOxand CoOx/SiO2[18],etc. all exhibited

better catalytic performance in the reaction. Therefore, cobalt-

based catalysts have aroused great interest of the researchers for

the reaction.

Mesoporous silicas have attracted increasing attentions due to

the large surface area and uniform mesoporous channels since the

researchers at Mobil Corporation reported M41S in the early 1990s

[19]. SBA-15, another type of mesoporous silica, shows broader

application in catalysisbecause of its larger pores, thicker walls and

higher hydrothermal stability compared with M41S [20]. Unfor-

tunately, SBA-15, as a pure silica, is nearly inert for chemical

reactions. The active sites in the molecular sieves are often from

heteroatoms. However, it is very difficult to prepare SBA-15 con-

taining heteroatoms in the framework because of the strong acidic

synthesis condition (pH < 0). Metals always exist in cationic formrather than oxo species under acidic condition and they therefore

cannot be introduced into the framework of SBA-15. Much efforts

have been devoted to the effective introduction of heteroatoms.

According to reported studies, postsynthesisgrafting [2128] and

direct-synthesis [2933] are typical processes for heteroatoms

introduction. Postsynthesis grafting can introduce high-content

heteroatoms into the mesoporous silica. But one drawback is that

the experiment procedure is relatively complicated, and another

is that metal oxides tend to appear on the external surface or in

the pores, resulting in the ordering destruction of mesostructure.

Direct-synthesis is a relatively simple way. However, most of the

0926-3373/$ see front matter 2010 Elsevier B.V. All rights reserved.

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46 H. Cui et al. / Applied Catalysis B: Environmental 101 (2010) 4553

metal ions in the initial gels cannot be effectively introduced into

the mesoporous molecular sieves by this route.

Wu et al. [34] reported an effective and convenient method

denoted as pH-adjusting for the grafting of heteroatoms. In this

way, Al and Ti could be largely introduced into the mesophase.

However, the incorporation of Co is thought to be a bigchallenge in

the fieldof mesoporousmaterialsynthesis [35]. There arenot many

studies on synthesis of Co-SBA-15. Lou et al. synthesized a highly

ordered Co-SBA-15at pH> 2.0using sodiumhydroxideto adjustthe

pH of synthesis gels, and mainly discussed the effect of pH on the

structure in their work[36]. In the present study, we also success-

fully introduced a certain amount of cobalt into the mesoporous

silica SBA-15 by so-called pH-adjusting method. Ammonia was

used to adjust the pH of synthesis system. The prepared Co-SBA-

15 was firstly used for the epoxidation of styrene with O2 and the

results showed high activity due to the effective introduction of

cobalt species and its suitable state of existing.

2. Experimental

2.1. Materials

All chemicals used were of reagent grade, including tri-

block copolymer poly(ethylene oxide)poly(propylene oxide)poly(ethylene oxide) EO20PO70EO20 (P123, Aldrich), HCl solution

(2 M), tetraethyl orthosilicate (TEOS), deionized water, cobalt(II)

nitrate hexahydrate (99%, Co(NO3)26H2O), nickel(II) nitrate

hexahydrate (98%, Ni(NO3)26H2O), ferric nitrate nonahydrate

(98.5%, Fe(NO3)39H2O), anhydrous manganese chloride (99.5%,

MnCl2), tin chloride dihydrate(99%, SnCl22H2O), titanium chlo-

ride (99%, TiCl4), hydrogen peroxide (30%, H2O2), tert-butyl

hydroperoxide solution (65%, TBHP), N,N-dimethylformamide

(99.5%, DMF), styrene (99%), bromobenzene (99%), absolute

ethanol (99.7%), N,N-dimethylacetamide (99%, DMA), dimethyl

sulfoxide (99%, DMSO), toluene (99.5%), pyridine (99.5%), and

cyclohexanone (99.5%).

2.2. Preparation of catalysts

Cobalt-substitutedSBA-15 was synthesized by the pH-adjusting

method according to reference [34]. In our typical run, 2 g P123 tri-

blockcopolymersurfactantwasdissolvedin15gwaterand60g2M

HCl solution, followed by addition of 4.25 g TEOS to the homoge-

neous solution under stirring. The mixture was stirred at 313K for

4 h, and then an appropriate amount of Co(NO3)26H2O was added

to the mixture, followed by additional stirring at 313 K for20 h. The

mixture was then transferred into an autoclave for crystallizing at

373K for 2 days. After the procedure above, the pH value of the

system was adjusted to 7.5 by adding ammonia dropwise at room

temperature and the mixture was crystallized again at 373K for

another 2 days. The resultantmixture wasfiltered,and the obtained

solid was thoroughly washed with deionized water, and dried atroom temperature. Then its surfactants were removed by Soxhlet

extraction. The obtained product is denoted as Co-SBA-15x, where

xis the Si/Co ratio in the initial gel.

For comparison, series of the catalysts such as Fe-SBA-1520,

Ni-SBA-1520, Sn-SBA-1520, Ti-SBA-1520 and Mn-SBA-1520 were

synthesized through the same procedure as shown above except

that Co(NO3)26H2O was replaced by other corresponding het-

eroatom sources. In addition, cobalt-containing MCM-41 was

synthesized by the template-ion exchange method (Co-MCM-41-

TIE) according to Tang et al. [16]. SBA-15 or silica gel (SG) was used

as support to prepare Co/SBA-15 and Co/SG by the conventional

impregnation method. Typically, a certain amount of SBA-15 or

silica gel (surface area of 220 m2/g) powder was immersed in the

aqueous solution containing an appropriate amount of cobalt(II)

nitrate for 8 h at ambient temperature under stirring, and then the

excess solvent was evaporated off at 323K. After drying at 373 K

and calcinating at 823 K for 6 h, the resultant sample is designated

as Co/SBA-15 or Co/SG.

Moreover, in order to investigate the influence of preparation

steps on the epoxidation of styrene with O2, four cobalt-based

catalysts were prepared by adding the same amount of identical

materialsin the initial gels but the preparation stepswere different.

(1) Co-SBA-1520

waspreparedby thepH-adjusting method. (2)The

preparation steps were the same as those of Co-SBA-1520 at first.

But after the first crystallization procedure, the steps are different.

The excess solvent of the produced mixture was evaporated off at

323K.Theproductwasdriedat373Kandcalcinatedat823Kfor6h.

The obtained powder is designated as Co-SBA-1520-1. (3) The sam-

ple was prepared without adjusting pH of synthesis system. The

preparation procedures were also the same as those of Co-SBA-

1520 at first. But after the first crystallization step, the resultant

product was filtered. The obtained solid was thoroughly washed

with deionized water, followed by extracting the template with

Soxhlets extracter, and drying at room temperature. The resultant

powder is denoted as Co-SBA-1520-2. (4) Co-SBA-1520-3 was pre-

pared by the same steps as Co-SBA-1520except that the surfactants

of the former were not removed by Soxhlets extracter.

2.3. Characterizations

The textural properties of the samples were derived from N2adsorption/desorption measurement at 77 K on Micromeritics Tris-

tar 3000. In each case, the sample was outgassed under vacuum at

573Kfor3hbeforeN2adsorption. Pore size distributions for meso-

porous materialswere calculated fromthe desorption isotherms by

the BJH method. Cobalt content in each sample was analyzed with

inductively coupled plasma-atom emission spectroscopy (ICP-AES)

on TJA Atomscan 16 spectrometer. X-ray photoelectron spec-

troscopy (XPS) was measured with a PHI Quantum 2000 Scanning

ESCA Microprobe and monochromatic Al-Kradiation. The bind-

ing energies were calibrated by referencing the C 1s at 284.6 eV.

High resolution transmission electron microscopy (HRTEM) mea-surements were carried out on JEOL JEM-2010 operatingat 200 kV.

X-ray diffraction (XRD) analysis was performed on RIGAKU D/max-

rB X-ray diffractometer. Diffraction patternswere recorded withCu

K radiation (40mA, 40 kV). Temperature-programmed reduction

(TPR) with H2was recorded by a flow system with a TCD detector.

Generally, 0.1g sample was firstly pretreated in a quartz reactor

with a flowing argon at 823 K for 1 h. After cooling to about 313K,

a H2Ar (5% H2) mixture was introduced into the reactor, and the

temperature was raised to 1200 K at a rate of 10 K min1. Raman

spectroscopic measurements were carried out on J.Y. LabRAM

laser micro-Raman spectroscopy. A laser at 514.5 nm was used as

the excitation source. UVvis spectra (UVvis) of the sample was

recorded on a Shimadzu UV-2550 spectrometer. The powdered

samplewas loadedintoa quartzcell, andthe spectra were collectedin the range of 200800nm referenced to BaSO4.

2.4. Catalytic reactions

The epoxidation of styrene with O2was performed in a 250-ml

three-necked flat-bottomedglass flask equipped with a liquid con-

denser. Typically, 8 mmol of styrene, a measured amount of solvent

anda certain amountof catalystwereaddedto thereactor. Themix-

ture was stirred vigorously by a magnetic stirrer and heated to the

desired temperature. ThenO2 at a certain stable flowrate controlled

by a massflow controller wasintroduced intothe liquid by bubbling

at atmospheric pressure. The reactant mixture was stirred vigor-

ously fora specifiedperiod of time. After completionof thereaction,

the liquid products were obtained by centrifugation and they were


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H. Cui et al. / Applied Catalysis B: Environmental 101 (2010) 4553 47

quantitatively analyzed by a gas chromatograph equipped with

a capillary column (DB-5, 30 m0.25mm0.25m) and a FID

detector,with bromobenzeneas an internalstandard. Inmost cases,

the epoxide was obtained as the main product, and benzaldehyde

was mainby-product.Phenylacetaldehyde selectivity was less than

10%. Other products included benzoic acid, phenylacetic acid, etc.

Under the most reaction conditions, the polymerization of styrene

was negligible, so polymers in the reaction products were not con-

sidered. The conversion of styrene and selectivity of the products

were calculated as follows:

Styrene conversion (%) =(initialmoles final moles)

(initial moles) 100%

The product selectivity (%)=(moles of theproduct)

(moles of all products) 100%

Recycling reactions of Co-SBA-1520werecarriedout at373K for

8 h. The catalyst recovered from the reaction mixture by centrifu-

gation, washing with acetone,and drying at 373K for12 h, and then

it was used in the next run under the same reaction conditions.

3. Results and discussion

3.1. Structural characteristics of Co-SBA-15x

Fig. 1shows the XRD patterns of pure silica SBA-15 and Co-

SBA-15xsamples prepared by the pH-adjusting method with Si/Co

ratios of 10100.Although thed spacingof thefirst strong reflection

had some changes from sample to sample, all the samples exhib-

ited three well-resolved diffraction peaks that can be indexed as

the (1 0 0), (11 0), and (20 0) diffractions associated with hexag-

onal symmetry. The results indicate that a certain amount of Co

can be grafted into SBA-15 by the pH-adjusting method without

destroying the structural ordering greatly.

The HRTEM images of Co-SBA-1520also show the well-ordered

hexagonal arrays of mesopores with one-dimensional channels

(Fig. 1S(A) and (B) in the supplement). The result coincides withthe XRD observations, and it further demonstrates that the sam-

ples prepared by the pH-adjusting method still keep the highly

ordered mesostructure when a certain amount of cobalt species

is introduced.

N2 adsorption/desorption isotherms of pure silica SBA-15 and

Co-SBA-15xwith varying cobalt content indicate that all materials

gave typical irreversible type IV adsorption/desorption isotherms

with H1 hysteresis loop, which is characteristic of the mesoporous

materials with 2D-hexagonal structure, as defined by IUPAC[24]

(Fig. 2S in the supplement). The sharp steps occurred at a relative

partial pressure of 0.70.9, corresponding to the capillary conden-

Fig. 1. XRD patterns of (A) SBA-15, (B) Co-SBA-15 100 , (C) Co-SBA-1540 , (D) Co-SBA-

1520 , and (E) Co-SBA-1510 respectively.

Table 1

Textural parameters of SBA-15 and Co-SBA-15x .

Materials Surfac e area

(m2/g)

Pore volume

(cm3/g)

Pore diameter (nm)

SBA-15 537 1.18 8.82

Co-SBA-15100 335 1.08 12.9

Co-SBA-1540 305 1.05 13.8

Co-SBA-1520 298 1.03 13.8

Co-SBA-1510 292 1.01 13.8

sation of N2, which indicates the uniformity of the pores[36].Pore

structure parameters of SBA-15 and Co-SBA-15x calculated from

the desorption branch are shown inTable 1.Compared with pure

silica SBA-15, the introduction of cobalt made the surface area

and pore volume significantly decrease. For Co-SBA-15x, with the

increasing of cobaltcontent, both thesurface area andpore volume

gradually reduced. The micropore area and micropore volume of

the pure silicaSBA-15were84.7m2/gand0.029cm3/g respectively,

but the value of the parameters was reduced once cobalt species

was introduced into the molecular sieves. The micropores contin-

ued to decrease with an increase of cobalt amount. It is proposed

that fillingof themicroporesleadsto theirreduction duringthepH-

adjusting procedure. A similar tendency is also observed by Wu et

al.[34]and Ryoo and Ko[37].The changes for these parameters ofmicropores obviouslybringaboutdecrease of the total surface area

and pore volume of Co-SBA-15x. Another reasonfor decrease of the

surface area andpore volume is that some of cobalt species is incor-

porated into mesoporous channels [16,38]. On theotherhand, pore

diameter had an obvious increase when cobaltwas introduced into

the molecular sieves, and the parameter first became larger and

then the change was notobservable with cobalt content rising(see

Table 1).It shows that cobalt species is incorporated into the silica

framework of SBA-15 because of the longer bond length of Co with

oxygenthan that ofSiO [36]. As cobalt amount increasesin synthe-

sis system, more cobalt ions are introduced into the framework of

themesoporous silica, which makespore diameterfurtherincrease.

However, when cobalt amount continues to increase, more cobalt

ions are incorporated into mesoporous channels rather than onlyframework of SBA-15, as confirmed by the change of surface area

and pore volume with cobalt content[38]and the observations of

UVvis. As a result, pore diameter changes little if cobalt amount is

further increased.

3.2. Influence of synthesis conditions on structure of Co-SBA-15x

As Fig. 2 shows, Co-SBA-15x with an appropriate amount of

cobalt couldretain a highly ordered mesostructure,but higher con-

Fig. 2. XRD patterns of (A) SBA-15, (B) Co-SBA-1540-6.5, (C) Co-SBA-15100 , (D) Co-

SBA-1540-7.5, (E) Co-SBA-1540-8.5, (F) Co-SBA-1510 , (G) Co-SBA-152, and (H) Co-

SBA-1540-9.0 respectively.


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tent of Co2+ in the initial gel would destroy the structural ordering

of the molecular sieves. For example, almost no diffraction peaks

associated with hexagonal symmetry were detected when Si/Co

ratio decreased to 2. Metal ions permeate the interstitial regions

between the silica and block copolymer before the pH is adjusted.

When the system is adjusted from strong acid to neutral, the metal

ions transform to the oxo form and condense with the adjacent

silanols[34]. In the present work, cobalt species is introduced into

the mesoporouswalls similarly. However, moremetal ions in inter-

stitial regions probably affect the interaction of silica and organic

template during the crystallization procedure, resulting in disor-

dered structure[34,37].

Another great effect of the synthesis condition on the struc-

ture of Co-SBA-15x is the adjusted pH value. Fig. 2 also shows

the XRD patterns of Co-SBA-1540 prepared at different pH values

(6.5, 7.5, 8.5 and 9.0). The samples synthesized at pH value lower

than 8.0 showed multiple well-resolved XRD peaks. Based on the

results of Zhao et al. [39],the absence of sufficiently strong elec-

trostatic or hydrogen-bonding interactions at pH 2.07.0 led to

the formation of disordered porous silica. However, in this study,

highly ordered Co-SBA-15xwas successfully synthesized at pH less

than 8.0 by the pH-adjusting method. This is consistent with the

observations of Lou et al. According to their hypothesis, because

sodium hydroxide was added to adjust the pH of synthesis system,the resultant salt (NaCl) helped to improve ordered mesostruc-

ture of SBA-15 [36].Similarly, in our study, the produced NH4Cl

is responsible for effectively improving the ordered structure since

ammonia is used to adjustpH duringthe synthesis. However, when

pH further increased, the XRD peaks were hardly observed for the

sample prepared at pH 9.0(see Fig.2). This indicates that higherpH

value (pH> 9.0)negatively influences the mesostructure of SBA-15,

which is in agreement with the results of Zhao et al. The meso-

porous silica synthesized under strong acidic condition has an

electrically neutral framework[39].Under weakly alkaline condi-

tions, hydrolysis tends to speed up due to the presence of OH

[34].On the other hand, although lower pH is favorable to highly

ordered mesostructure, it results in lowamount of cobaltintroduc-

tion. For instance, when pH was 7.5, 1.3 wt% of Co was introducedinto the mesoporous molecular sieves for Co-SBA-1540, whereas

there was only 0.005 wt% of Co content if pH decreased to 6.5, con-

firmed by ICP-AES analysis shown inTable 3.On the basis of our

studies about synthesis of Co-SBA-15x, the optimal pH for prepa-

ration of Co-SBA-15x is finally determined to be in the range of

7.28.0.

3.3. Coordination environment and oxidation state of cobalt

High-angle XRD patterns of pure silica SBA-15, Co-SBA-1520,Co/SBA-15 and Co3O4 are shown inFig. 3.Only a broad peak due

to the amorphous feature of SBA-15 framework appeared, and no

peaks of the crystalline Co3O4 were observed for Co-SBA-1520.

While Co/SBA-15 with the same cobalt content prepared by theimpregnation method, showed peaks at about 37, 45, 60 and

65, attributed to the crystalline Co3O4. It is concluded that cobalt

species grafted by the pH-adjusting method is highly dispersed in

SBA-15. This is also consistent with the HRTEM observations, indi-

cating that no large particles are located outside the mesopores of

Co-SBA-15x.

The state of cobalt in the samples was further investigated by

Raman spectroscopic measurements (see Fig. 4). The crystalline

Co3O4exhibited four Raman bands at 467, 510, 603 and 670 cm1

due to the vibrations of CoO bonds [16,40]. Co/SG and Co/SBA-

15 showed bands at 660 and 657cm1 respectively, indicating

the presence of some amount of Co3O4 species on the surface

of these materials. In addition, the Raman band at 829cm1 was

also observed for Co/SBA-15, perhaps arising from the vibrations

Fig. 3. High-angle XRD patterns of (A) SBA-15, (B) Co-SBA-15 20 (Co, 2.1wt%), (C)

Co/SBA-15 (Co, 2.1wt%), and (D) Co3O4.

of CoO that strongly interacts with silica. However, no Raman

bands of CoO vibrations appeared for Co-SBA-1520and Co-MCM-

41-TIE, indicating the highly dispersed cobalt species in the two

samples.

The oxidation state of cobalt was investigated by XPS. Fig. 5

exhibits Co 2p XPS spectra of the Co-containing samples. The bind-ing energy of Co 2p3/2of Co-MCM-41-TIE was 781.5 eV, similar to

that of Co-NaX, and the cobalt species with such a binding energy

can be assigned to Co(II) in an isolated state [16,41]. It is inter-

esting to note that Co-SBA-1520 and Co-MCM-41-TIE possessed

almost the same binding energy of Co 2p3/2, suggesting that the

Fig. 4. Raman spectra of (A) Co-SBA-1520 (Co, 2.1 wt%), (B) Co-MCM-41-TIE (Co,

2.1wt%), (C)Co/SG(Co, 2.1wt%), (D)Co/SBA-15 (Co, 2.1wt%), and(E) Co3O4respec-

tively.

Fig. 5. C o 2p XPS spectra of (A) Co-MCM-41-TIE (Co, 2.1 wt%), (B) Co-SBA-1520 (Co,

2.1wt%), (C) Co/SBA-15 (Co, 2.1wt%), and (D) Co3O4respectively.


5/9


Fig. 6. TPR profiles of (A) Co-MCM-41-TIE (Co, 2.1 wt%), (B) Co-SBA-1520 (Co,

2.1wt%), (C) Co/SBA-15 (Co, 2.1wt%), and (D) Co3O4 respectively.

cobalt species in Co-SBA-1520is also mainlyin thesingle-siteCo(II)

state. In addition, Co/SBA-15preparedby the impregnationmethod

exhibited a binding energy of Co 2p3/2 close to that of Co3O4.

The result coincides with the observations of XRD, Raman and

HRTEM.The state of cobalt species of the samples was also investigated

by TPR. AsFig. 6shows, two reduction peaks at 573673K were

mainly found for Co3O4 and Co/SBA-15, probably ascribed to the

stepwise reduction of Co3O4 to CoO and then CoO to Co [42,43].

Co-SBA-1520 and Co-MCM-41-TIE seemed to show the reduction

peaks at almost the same high temperature (about 1044 K), fur-

ther suggesting that the cobalt species in Co-SBA-15xis similar to

that in Co-MCM-41-TIE, and is likely in the single-site Co(II) state.

In addition, for Co/SBA-15 prepared by the impregnation method,

a relatively weak reduction peak was also observed, which is the

reduction of some cobalt species that strongly interacts with the

support[16].

Electronic spectroscopy in UVvis region is considered as an

effective technique for studying the local coordination environ-ment and electronic state of isolated transition metal ions as well as

aggregated transition metal oxides[44].Co-SBA-1520was further

investigated by UVvis spectroscopy (see Fig. 3S in the supple-

ment). The absorbance in the range of 500680nm is characteristic

of Co(II) in an isolated state [45]. The peaks at 530, 580 and 650 nm

for the sample show that the cobalt species in Co-SBA-1520mainly

exists in the state of single-site Co(II), coincident with the results

of XPS and TPR. Moreover, the peaks in the range of 550700nm

are absorbance of tetrahedrally coordinated Co2+ [46].The sample

indicated tetrahedrally coordinated Co2+ in the silica framework

at 580, 650 nm, and most of cobalt species was incorporated into

the framework of SBA-15. This is similar to the result reported by

Lou et al. [36]. In addition, a weak peak at 350 nm is attributed

to absorbance of Co3O4 [36], indicating that a small part of cobaltspecies is introduced into the mesoporous channels in the state of

Co3O4. In this work, the amount of Co3O4 increased with the rise

of cobalt content in the samples.

3.4. Studies of styrene epoxidation with O2catalyzed by various

metal species based on SBA-15

Though epoxidation of styrene withmolecular oxygen is consid-

ered as a promising process for production of the epoxide, it is one

of themost challengingsubjects. Thisis because triplet ground state

of O2 disfavors reactions with singlet organic compounds[17,18].

Therefore, many researchers have pursued new effective catalytic

systems for the reaction. In this study, in order to compare which

metal species was highly active for the reaction, some metal species

that can activate oxygen in the oxidation[4752]were introduced

into SBA-15 with their precursors by the pH-adjusting method.

Table 2summarizes catalytic performances of the catalysts in the

epoxidation of styrene with O2. Co-SBA-1520achieved the highest

styrene conversion (94.1%) and epoxide selectivity (65.5%) under

the same reaction conditions. Other metal species were all inferior

to cobalt for the reaction. According to previous studies, the coor-

dination of DMF to cobalt(II) affected the ability of O2 to bind to

the Co(II) and their redox potential, and this resulted in the activa-

tion of O2in the epoxidation[16,53,54].In conclusion, cobalt is an

effective component for the epoxidation of styrene with O2.

3.5. Investigations of styrene epoxidation with O2over various

cobalt-based catalysts

Table 3shows the effect of preparation steps for the cobalt-

based catalystson theepoxidationof styrene with O2. Co-SBA-1520prepared by the pH-adjusting method showed the best catalytic

performance, with 94.1% of styrene conversion and 65.5% of epox-

ide selectivity. The catalytic activity of Co-SBA-1520-1 dropped

sharply, with 36.7% of the conversion and 52.1% of the epoxide

selectivity. The cobalt species in the sample mainly existed in the

form of Co3O4 from the result of XRD (not shown). In addition, if

the pH of synthesis system was notadjusted, the sample expressed

as Co-SBA-1520-2 showed negligible catalytic activity, with only

3.4% of styrene conversion, ascribed to no cobalt sites in the sam-

ple. It is concluded that cobalt ions in the initial gel are difficult to

be introduced into the mesostructure without adjustingpH, as evi-denced by ICP-AES. When an cobalt source is added into the initial

gels, the cobalt species is dispersed in the synthesis system in the

form of Co2+ because of the strong acidic condition. Co2+ perme-

ates theinterstitial regions between thesilica and block copolymer

before pH adjusting. The cobalt ions are possibly coatedwith silica-

block copolymer mesophase, but have almost no linkage with the

silicates in strong acidic media. As a result, few of cobalt species

can be introduced if the semi-product is filtered before pH adjust-

ing. This phenomenon is in agreement with the mechanism for the

formation of Al-SBA-15 and Ti-SBA-15 proposed by Wu et al. [34].

Moreover, if the surfactants in Co-SBA-1520were not removed, the

sample designated as Co-SBA-1520-3 had only 8.8% of styrene con-

version, much lower than that of surfactant removing. When the

pores are full of template, the reactants in the reaction systemcannot enter the pores to contact with a large number of cobalt

sites in the particles. However, when the template is extracted

from pores, almost all the active sites participate in the reaction.

Table 2

Epoxidation of styrene with O2 catalyzed by various metal species based on SBA-15a .

Catalyst Metal content (wt%) Surface area (m2/g) Pore volume (cm3/g) Pore diameter (nm) Styrene conversion (%) Epoxide selectivity (%)

Fe-SBA-1520 4.1 299 1.04 13.9 12.3 47.1

Ti-SBA-1520 3.3 303 1.06 14.0 5.8 56.8

Ni-SBA-1520 1.9 298 1.02 13.7 5.3 42.7

Mn-SBA-1520 1.2 301 1.04 13.8 9.4 41.0

Sn-SBA-1520 5.5 298 1.06 14.2 3.5 30.3

Co-SBA-1520 2.1 298 1.03 13.8 94.1 65.5

a

Reaction condition: styrene, 8mmol; DMF, 25 g; catalyst, 0.3g; temperature, 373 K; time, 8h; flow rate of O2, 15 ml/min.


6/9


Table 3

Epoxidation of styrene with O2 catalyzed by various cobalt-based catalystsa .

Catalyst Co content (wt%) Styrene conversion (%) Product selectivity (%)

Benab Epoxide Pheac Other

Co-SBA-1520-1 4.5 36.7 30.5 52.1 4.3 13.1

Co-SBA-1520-2 Trace 3.4 57.1 38.9 3.1 0.9

Co-SBA-1520-3 2.0 8.8 43.5 36.7 10.9 9.0

Co-SBA-1540-6.5 0.005 10.9 15.1 63.9 8.0 13.0

Co-SBA-1540-7.5 1.3 88.9 14.6 64.3 8.6 12.5

Co-SBA-1540-9.0 1.28 42.5 21.5 55.6 7.4 15.5

Co-SBA-15100 0.12 46.8 15.6 65.1 7.3 12.0

Co-SBA-1540 1.3 88.9 14.6 64.3 8.6 12.5

Co-SBA-1520 2.1 94.1 14.7 65.5 8.2 11.6

Co-SBA-1510 3.6 93.9 15.2 65.5 7.8 11.5

Co/SBA-15 2.1 43.5 21.5 55.6 7.4 15.5

Co/SG 2.1 18.6 16.2 55.1 8.1 20.6

Co-MCM-41-TIE 2.1 54.2 29.4 57.9 4.9 7.8

Co-MCM-41-TIE 3.6 54.6 28.3 58.3 6.2 7.1

Co(NO3)2 / 1.3 46.2 53.8 0 0

a Reaction condition: styrene, 8mmol; DMF, 25 g; catalyst, 0.3g; Co(NO3)2, 0.11 mmol; temperature, 373 K; time, 8h; flow rate of O2, 15 ml/min.b Benzaldehyde.c Phenylacetaldehyde.

Consequently, the catalytic activity of Co-SBA-1520is much higher

than that of Co-SBA-1520-3. In contrast, if all the cobalt species isanchored to the external surface, the catalytic activity of the sam-

ple will not be affected bythe blockage of the template and the two

data should be the same. But the result is not so, indicating that

most of the cobalt sites are exposed on the inner face of the pores

rather than only on the external surface, and the epoxidation of

styrene with O2mainly occurs on theinternalsurface of pores. This

is consistentwith the result of Wuet al. [34], in which Al inAl-SBA-

1510prepared by the same method was also chiefly situated in the

interior of particles. In conclusion, through comparison, different

processing methods lead to the catalytic materials with significant

different results although the same amounts of identical materials

are added in the initial gels at the beginning of preparation pro-

cess. And some valuable information was also obtained from the

comparison.Table 3also summarizes the catalytic performance of Co-SBA-

1540 prepared at different pH. The sample prepared at pH 7.5

exhibited the highest conversion (88.9%) and epoxide selectivity

(64.3%). If the pH of synthesis system was adjusted to 6.5, the

catalyst obtained only 10.9% of styrene conversion. This further

confirms that lower pH leads to the introduction of less cobalt

into the molecular sieves although it helps to attain well-ordered

mesostructure, and low levels of cobalt sites undoubtedly bring

about low activity. In addition, the ordered mesostructure of SBA-

15 was hardly obtained accordingto the XRD observation when the

pH was adjusted to 9.0. The catalyst showed 42.5% of styrene con-

version, much lower than that of the samplewithpH 7.5, indicating

the sample thatpossesses well-ordered mesostructure of SBA-15 is

muchsuperiorto thatwith disorderedstructure for theepoxidationof styrene with O2.

Table 3 also lists the catalytic performance of various cobalt-

based catalysts in the epoxidation of styrene with O2. Catalytic

activityof the cobalt-based samples in thereactiondecreased in the

sequence Co-SBA-1520 > Co-MCM-41-TIE > Co/SBA-15> Co/SG with

the same cobalt content (2.1wt%). Co(NO3)2 was almost inactive

in the reaction. According to the results of XRD, HRTEM, Raman,

XPS, TPR, and UVvis illustrated above, the cobalt in both Co-SBA-

15x and Co-MCM-41-TIE basically existed in the single-site Co(II)

state. But Co-SBA-1520 achieved much higher activity than Co-

MCM-41-TIE, indicative of the difference in the performance for

the two cobalt-functionalized mesoporous silica. Moreover, Co/SG

mainly aggregated into Co3O4 particles, whereas Co/SBA-15 con-

tained highly dispersed Co3O4 particles and a small part of cobalt

species strongly interacting with support. It is likely that better

dispersion of cobalt species results in higher catalytic activity. Bycomparison, the single-site Co(II) is the most active for the epoxi-

dation of styrene with O2.

As Table 3 shows, cobalt content in Co-SBA-15x exerted an

obvious impact on the catalytic performance in the reaction.

Apparently, the conversion of styrene increased with the rising of

cobalt content: Co-SBA-15200 < Co-SBA-15100 < Co-SBA-1540


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Fig. 7. Effect of reaction temperature on epoxidation of styrene with O 2 over

Co-SBA-1520 , () styrene conversion, () epoxide selectivity, () benzaldehyde

selectivity, () phenylacetaldehydeselectivity, () other selectivity. Reactioncondi-

tion:styrene, 8 mmol;DMF,25 g;catalyst, 0.3g; time,8 h;flowrateof O2, 15ml/min.

Fig. 8. Effect of Co-SBA-1520 amounton epoxidationof styrene with O2, () styrene

conversion, () epoxide selectivity, () benzaldehyde selectivity, () phenylac-

etaldehyde selectivity, () other selectivity. Reaction condition: styrene, 8 mmol;

DMF, 25g; temperature, 373K; time, 8h; flow rate of O2, 15 ml/min.

Theeffect of Co-SBA-1520amounton theepoxidationof styrene

is depicted in Fig.8. The conversion of styrene increasedfrom 69.9%

over 0.05g of Co-SBA-1520 to the maximum (94.1%) with 0.3g

of the catalyst, and then decreased slowly to 90.1% using 0.7 g of

the sample. But the epoxide selectivity kept more or less a con-

stant about 65.0% in the range of catalyst amount from 0.05 g to

0.7 g. According to previous research work, increasing the catalyst

amount,more cobalt siteswould be obtainedto coordinate withthe

solvent (DMF), resulting in the true active sites during the reaction

or acting as a co-reductant [16,18,55].However, the observations

ofFig. 8 indicate that the raise of styrene conversion cannotmerely

depend on the increase of catalyst amount.

Table 4presents the influence of solvent on the styrene epoxi-

dation with O2over Co-SBA-1520at 373 K. It is found that solvent

Table 4

Effect of reaction solvent on epoxidation of styrene with O 2over Co-SBA-1520a .

Solvent Styrene conversion (%) Product selectivity (%)

Be na Epoxide Ph ea Othe r

DMF 94.1 14.7 65.5 8.2 11.6

DMA 95.3 15.0 64.5 8.9 11.5

DMSO 47.9 89.3 4.3 1.2 5.2

Cyclohexanone 50.5 66.3 27.2 2.8 3.7

Pyridine 0 b

Toluene 0

a Reaction condition: styrene, 8mmol; solvent, 25 g; catalyst, 0.3 g; temperature,

373 K; time, 8 h; flow rate of O2, 15 ml/min.b

means none.

Fig. 9. Effect of DMF amount on epoxidation of styrene with O 2 over Co-SBA-

1520 , () styrene conversion, () benzaldehyde selectivity, () epoxide selectivity,

() phenylacetaldehyde selectivity, () other selectivity. Reaction condition:

styrene, 8mmol; catalyst, 0.3g; temperature, 373 K; time, 8 h; oxidant amount, O2,

15 ml/min.

played a great role on the epoxidation reaction. The amides such

as N,N-dimethylformamide (DMF) and N,N-dimethylacetamide

(DMA) were especially efficient in obtaining both highstyrene con-

version and high epoxide selectivity. Although a certain styreneconversion was obtained in dimethyl sulfoxide (DMSO) and cyclo-

hexanone, the epoxide selectivity was low, and the main product

was benzaldehyde. When pyridine or toluene was used as reaction

solvent, no products were detected. Such results are in agreement

with the observations of Tang et al. [16]. Based on their results, the

solvent (DMF) could coordinate with cobalt ions in the molecular

sieves, and such coordination was an important factor for the acti-

vation of O2in the epoxidation reaction. Recently, some studies on

the effect of solvent have been reported [1618,56], but the poten-

tial causes are controversial. Rao et al. proposed that the aprotic

solvents with high dielectric constant were favorable to high selec-

tivity of epoxide, but the observations of Zhan et al. were not so. In

fact, manyfactorsincluding the nature andstructure of the solvents

can affect the results. The effect of solvent on the reaction may bequite complicated.

The effectof DMFamount on theepoxidationof styrene with O2over Co-SBA-1520at 373 K is illustrated inFig. 9.The styrene con-

version rose with the increase of DMF amount when DMF amount

was less than24 g.If DMFamountcontinuedto increase,thestyrene

conversion decreased slowly. The change of the epoxide selectivity

with DMF amount was similar to the trend of the styrene conver-

sion. The epoxide selectivity rose from 45.9% with 8 g of DMF to

the maximal value (65.5%) using 24g of DMF, and then reduced to

50.2% when48 g ofDMF was used. Itshould benotedthat the selec-

tivity of other products (including benzoic acid, phenylacetic acid,

etc.) changed in the opposite direction to that of the epoxide with

DMF amount. These results suggest that an appropriate amount

of solvent should be beneficial for the formation of epoxide andmeanwhile inhibits overoxidation of styrene to acids. Fig. 9indi-

cates the optimal DMF amount is 24 g. When such amount of DMF

is used, both styrene conversion and epoxide selectivity reach the

highest (94.1% and 65.5% respectively) but the selectivity of acids

is lowest.

The influence of reaction time on the epoxidation of styrene

withO2over Co-SBA-1520at 373K was also investigated, as shown

inFig. 10.The styrene conversion increased with the prolonging of

reaction time. When reaction time was less than 8 h, the styrene

conversion rose quickly from 58.3% in 2 h to 94.1% in 8 h, show-

ing the induction period characteristic of radical reaction. Yet the

styrene conversion increased slowly if reaction time was longer

than 8 h. The effect of reaction time on the epoxide selectivity was

not obvious. The epoxide selectivity slowly increased from 56.7%


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Fig. 10. Effect of reaction time on epoxidation of styrene with O2 over Co-

SBA-1520 , () styrene conversion, () epoxide selectivity, () epoxide yield

(epoxide yield= styrene conversionepoxide selectivity100%). Reaction condi-

tion: styrene, 8 mmol; DMF, 25g; catalyst, 0.3 g; temperature, 373K; flow rate of

O2, 15 ml/min.

Table 5

Epoxidation of styrene by various oxidants over Co-SBA-15 20a .

Oxidant Styrene conversion (%) Product selectivity (%)

Bena Epoxide Phea Other

None 0

O2 94.1 14.7 65.5 8.2 11.6

H2O2 50.8 24.4 58.8 2.5 14.3

TBHP 40.7 18.1 72.4 9.4 0

Air 93.9 15.0 65.4 7.8 11.8

a Reaction condition: styrene, 8 mmol; DMF, 25 g; catalyst, 0.3 g; temperature,

373 K; time, 8h; Oxidant amount, O2, 15 ml/min; H2 O2, 10 mmol; TBHP, 10mmol;

air, 40 ml/min.

in 2 h to 65.5% in 8 h, and then began to decrease when reaction

timecontinuouslyincreased. As we know, prolonging reactiontime

makes the substratefully react. Butit will also lead to overoxidation

of styrene to acid and polymerization of styrene at the same time,

which results in the decrease of epoxide selectivity. Therefore, the

optimal reaction time should be 8h from our studies, in which the

highest epoxide yield can be achieved.

Table 5compares the effect of various oxidants on the epoxida-

tionof styrene catalyzedby Co-SBA-1520. No epoxidation of styrene

occurred when the reaction was carried out under the inert atmo-

sphere (N2or Ar) without any oxidants. If O2orair was used as the

oxidant, the highest styrene conversion was obtained. The styrene

conversion was 50.8% when H2O2 was used as the oxidant, which

Fig.11. Effectof flowrateofO 2onepoxidation of styrenewith O2 over Co-SBA-1520 ,

() styrene conversion, () epoxide selectivity, () benzaldehyde selectivity, ()

phenylacetaldehyde selectivity, () other selectivity. Reaction condition: styrene,

8 mmol; DMF, 25 g; catalyst, 0.3 g; temperature, 373 K; time, 8h.

Fig. 12. Recycling investigations of Co-SBA-1520 in epoxidation of styrene with O2,

() styreneconversion,() epoxide selectivity. Reactioncondition: styrene,8 mmol;

DMF, 25g; catalyst, 0.3g; temperature, 373K; time, 8 h; flow rate of O2, 15 ml/min.

was much lower than that with O2 or air, mainly because of the

rapid decomposition of H2O2 into O2 at the initial stage. Though

72.4% of epoxide selectivity was obtained when TBHP wasused,the

styrene conversion was the lowest, and polymerization of styrene

was a serious problem. Through comparison, O2or air is a suitableoxidant for the reaction over the cobalt-based catalyst. This also

indicates that Co-SBA-15xis particularly effective in the activation

of O2for the epoxidation of olefins.

Fig. 11 showsthe influence of flow rate of O2 on the epoxidation

of styrene over Co-SBA-1520at 373 K. The styrene conversion was

77.4% when flow rate was 5 ml/min, and then slowly increased to

94.1% with the rising of flow rate of O2 to 15 ml/min. The styrene

conversion changed little when theflow rate of O2was higher than

15 ml/min.The flowrate hadlittle impact on theproductselectivity,

and the overoxidation was not basically aggravated in the range of

flow rate of O2studied.

3.7. Recycling investigations of Co-SBA-1520in epoxidation of

styrene with O2

Catalyst recycling experiments were performed with repeated

use of Co-SBA-1520at 373 K for 8 h. It can be seen fromFig. 12that

the styrene conversion and the epoxide selectivity kept unchanged

by and large with the reuse of Co-SBA-1520 for seven times. Lit-

tle leaching of cobalt from the catalyst was found in the reaction

mixture by the result of the elemental analyzer, further indicative

of excellent stability and the recyclability of Co-SBA-1520 in the

epoxidation of styrene with O2.

4. Conclusions

Highly ordered cobalt-substitutedSBA-15 was successfully syn-thesized by the pH-adjusting method. The cobalt species in

Co-SBA-15 was mainly in the single-site Co(II) state. Most of them

were incorporated into the silica framework of SBA-15, existing in

tetrahedral coordination. Co-SBA-1520showed excellent catalytic

performance in the epoxidation of styrene with O2. It exhibited

much higher activity than Co-MCM-41-TIE and Co/SBA-15. The

optimized preparation conditions for Co-SBA-15xwere Si/Co ratio

1020 and pH of synthesis system 7.28.0. The material could

obtain the best catalytic performance under the reaction condi-

tions such as reaction temperature 373K, 0.3 g of the catalyst, 24g

of DMF (DMA) as reaction solvent, reaction time 8 h and 15ml/min

of O2(40 ml/min of air) as oxidant if 8 mmol of styrene was added

to the reaction system. Co-SBA-1520 possessed excellent stability

and recyclability in the reaction.


9/9


Acknowledgement

We are grateful for the financial support of the National Key

Technology R&D Program (No. 2007BAB24B04).

Appendix A. Supplementary data

Supplementary data associated with this article canbe found, in

the online version, at doi:10.1016/j.apcatb.2010.09.003.

References

[1] M. Caldarelli, J. Habermann, S.V. Ley, J. Chem. Soc., Perkin Trans. 1 (1999)107110.

[2] Q.-H. Xia, X. Chen, T. Tatsumi, J. Mol. Catal. A: Chem. 176 (2001) 179193.[3] M.P. Kapoor, A.K. Sinha, S. Seelan, S. Inagaki, S. Tsubota, H. Yoshida, M. Haruta,

Chem. Commun. (2002) 29022903.[4] A.K. Sinha, S. Seelan, S. Tsubota, M. Haruta, Angew. Chem. Int. Ed. 43 (2004)

15461548.[5] Y. Liu, K. Murata, M. Inaba, Chem. Commun. (2004) 582583.[6] J. Huang, T. Takei, T. Akita, H. Ohashi, M. Haruta, Appl. Catal. B: Environ. 95

(2010) 430438.[7] E. Ruckenstein, H.Y. Wang, Appl. Catal. A 204 (2000) 257263.[8] A.Karim, Y.Su,J. Sun, C.Yang,J.Strohm,D.King,Y.Wang,Appl. Catal.B:Environ.

96 (2010) 441448.[9] T.A. Kainulainen, M.K. Niemela, A.O.I. Krause, Catal. Lett. 53 (1998) 97101.

[10] K. Okabe, X. Li, M. Wei, H. Arakawa, Catal. Today 89 (2004) 431438.[11] A.Y. Khodalkov,R. Bechara,A. Griboval-Constant,Appl. Catal.A: Gen.254 (2003)

273288.[12] J. Panpranot, S. Kaewkun, P. Praserthdam, J.G. Goodwin, Catal. Lett. 91 (2003)

95102.[13] S.W. Ho, J.M. Cruz, M. Houalla, D.M. Hercules, J. Catal. 135 (1992) 173185.[14] C. Ando, H. Kurokawa, H. Miura, Appl. Catal. A 185 (1999) 181183.[15] T. Tsoncheva, L. Ivanova, J. Rosenholm, M. Linden, Appl. Catal. B: Environ. 89

(2009) 365374.[16] Q. Tang, Q. Zhang, H. Wu, Y. Wang, J. Catal. 230 (2005) 384397.[17] G. Xu,Q.-H. Xia, X.-H.Lu, Q. Zhang,H.-J. Zhan,J. Mol. Catal.A: Chem.266 (2007)

180187.[18] H.-J. Zhan, Q.-H. Xia, X.-H. Lu, Q. Zhang, H.-X. Yuan, K.-X. Su, X.-T. Ma, Catal.

Commun. 8 (2007) 14721478.[19] J.S. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992)

710712.[20] P. Yang, D. Zhao, D. Margolese, B. Chmelka, G. Stucky, Nature 396 (1998)

152154.

[21] M. Morey, S. OBrien, G.D. Stucky, Chem. Mater. (2000) 898911.

[22] R. Mokaya, J. William, Chem. Commun. (1997) 21852186.[23] S. Sumiya, Y. Oumi, T. Uozumi, T. Sano, J. Mater. Chem. (2001) 11111115.[24] Z. Luan, J.Y. Bae, C. Kevan, Chem. Mater. (2000) 32023207.[25] R. Ryoo, S. Jun, J.M. Kim, M.J. Kim, Chem. Commun. (1997) 22252226.[26] Z. Luan, M. Hartmann, D. Zhao, W. Zhou, L. Kevan, Chem. Mater. (1999)

16211627.[27] R. Mokaya, Angew. Chem. 111 (1999) 30793083.[28] Z. Luan, E.M. Maes,P.A.W. vander Heide,D. Zhao, R.S. Czernuszewicz, L. Kevan,

Chem. Mater. (1999) 36803686.[29] Y. Liu, T.J. Pinnavaia, Chem. Mater. (2002) 35.[30] Y. Han, N. Li, L. Zhao, D. Li, X. Xu, S. Wu, Y. Di, C. Li, Y. Zou, Y. Yu, F.-S. Xiao, J.

Phys. Chem. B 107 (2003) 75517556.[31] Y.Han, F.-S. Xiao, S.Wu,Y. Sun,X. Meng, D.Li,S. Lin,J. Phys. Chem. B 105(2001)

79637966.[32] B. Newalkar, J. Olanrewaju, S. Komarneni, Chem. Mater. (2001) 552557.[33] W. Zhang, Q. Lu, B. Han, M. Li, J. Xiu, P. Ying, C. Li, Chem. Mater. (2002)

34133421.[34] S. Wu, Y. Han, Y.-C. Zou, J.-W. Song, L. Zhao, Y. Di, S.-Z. Liu, F.-S. Xiao, Chem.

Mater. (2004) 486492.[35] A. Vinu, M. Hartmann, Chem. Lett. 33 (2004) 588589.[36] Z. Lou, R. Wang, H. Sun, Y. Chen, Y. Yang, Microporous Mesoporous Mater. 110

(2008) 347354.[37] R. Ryoo, C. Ko, J. Phys. Chem. B 104 (2000) 1146511471.[38] Q. Zhang,Y. Wang, S. Itsuki, T. Shisshido, K. Takehira, J. Mol. Catal.A 188(2002)

189200.[39] D. Zhao, Q. Huo, J. Feng, B. Chmelka, G. Stucky, J. Am. Chem. Soc. 120 (1998)

60246036.[40] M.A.Vuurman,D.J. Stufkens,A. Oskam, G. Deo,I.E. Wachs, J. Chem.Soc.,Faraday

Trans. 92 (1996) 32593265.[41] L. Guczi, D. Bazin, Appl. Catal. A 188 (1999) 163174.

[42] S. Sun, N. Tsubaki, K. Fujimoto, Appl. Catal. A 202 (2000) 121131.[43] L.A. Boot, M.H.J.V. Kerkhoffs, A.J. van Dillen, J.W. Geus, F.R. van Buren, B.T. van

der Linden, Appl. Catal. A 137 (1996) 6986.[44] S. Bordiga, R. Buzzoni, F. Geobaldo, C. Lamberti, E. Giamello, A. Zecchina, G.

Petrini, G. Vlaic, J. Catal. 158 (1996) 486501.[45] A. Verberckmoes, M. Uytterhoeven, R. Schoonheydt, Zeolites 19 (1997)

180189.[46] Y. Brik, M. Kacimi, M. Ziyad, F. Verduraz, J. Catal. 202 (2001) 118128.[47] Y. wang, K. Otsuka, J. Catal. 171 (1997) 106114.[48] E. Heracleous, A.A. Lemondou, J. Catal. 237 (2006) 162174.[49] N.G. Valente, L.A. Arra, L.E. Cads, Appl. Catal. A: Gen. 205 (2001) 201214.[50] Y. Liu, K. Murata, M. Inaba, N. Mimura, Appl. Catal. A 309 (2006) 91105.[51] W. Zeng, J. Li, S. Qin, Inorg. Chem. Commun. 9 (2006) 1012.[52] J. Li, W. Ma, Y. Huang, X. Tao, J. Zhao, Y. Xu, Appl. Catal. B: Environ. 48 (2004)

1724.[53] V.T. Yilmaz, Y. Topcu, Thermochim. Acta 307 (1997) 143147.[54] A. Pui, Croat. Chem. Acta 75 (2002) 165173.[55] J. Jiang,R. Li,H. Wang, Y. Zheng,H. Chen, J. Ma,Catal.Lett.120 (2008)221228.

[56] S. Rao, K. Munshi, N. Rao, J. Mol. Catal. A: Chem. 156 (2000) 205211.
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