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Solid State Sciences 13 (2011) 1938e1942

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Solid State Sciences

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Oxovanadium(IV) and dioxomolybdenum(VI) salen complexes tethered ontoamino-functionalized SBA-15 for the epoxidation of cyclooctene

Ying Yanga,b, Shijie Haob, Ying Zhangb,*, Qiubin Kana,**

aCollege of Chemistry, Jilin University, Jiefang Road 2519, Changchun 130023, Jilin, PR ChinabDepartment of Materials Science and Engineering, China University of Petroleum, Changping District, Beijing 102249, PR China

a r t i c l e i n f o

Article history:Received 7 May 2011Received in revised form16 June 2011Accepted 16 August 2011Available online 23 August 2011

Keywords:OxovanadiumDioxomolybdenumTetheringCyclooctene epoxidationSBA-15

* Corresponding author. Tel.: þ86 10 89732273; fax** Corresponding author. Tel./fax: þ86 431 8849914

E-mail addresses: catalyticscience@yahoo.com.cncom (Y. Zhang), catalyticscience@yahoo.com.cn (Q. Ka

1293-2558/$ e see front matter � 2011 Elsevier Masdoi:10.1016/j.solidstatesciences.2011.08.020

a b s t r a c t

Oxovanadium(IV) and dioxomolybdenum(VI) salen complexes were firstly tethered onto amino-functionalized mesoporous SBA-15 materials by a stepwise procedure and were screened as catalystsfor the epoxidation of cyclooctene. The mesoporous structural integrity throughout the tetheringprocedure, the successful tethering of the organometallic complexes, the loadings of metal ions andorganic ligands as well as the catalyst surface constitution and location of active organometallic specieson the SBA-15 support were determined by comprehensive characterization techniques such as XRD, N2

adsorption/desorption, FT-IR, UVevis spectroscopy, ICP-AES, XPS and TG/DTA. Catalytic properties in theepoxidation of cyclooctene demonstrate that both tethered oxovanadium(IV) and dioxomolybdenum(VI)catalysts were more active than their respective homogeneous analogue, and the tethered oxovana-dium(IV) complex showed the best activity (64.3%) with H2O2 as the oxidant and CH3CN as the solvent.

� 2011 Elsevier Masson SAS. All rights reserved.

1. Introduction

Schiff base transition metal complexes have been extensivelystudied because of their potential use as catalysts in awide range ofoxidation reactions [1e4]. Many strategies have been adopted toheterogenize these homogeneous catalysts to isolate metalcomplexes to prevent their dimerization, increase their ruggednessand separability. Schiff base complexes are usually immobilized onpolymeric organic materials such as resins, or polystyrene [5], thensupported on inert porous solid such as alumina [6] and silica orencapsulated in the pores of zeolite-Y [7]. There are certain disad-vantages with polymeric supports due to their vulnerability tosome chemicals and solvents. Encapsulation of metal complexes inporous materials typified by zeolites, leads to size restriction andsevere leaching of catalysts. Mesoporous silica materials have beenwidely used as useful and versatile solid supports to constructvarious hybrid materials in catalysis, enzyme immobilization anddrug delivery due to their large tunable pore dimensions, highsurface areas and great diversity in surface functionalization [8,9].Therefore, covalent anchoring of Schiff base complexes ontoa functionalized siliceous mesoporous material with the large pore

: þ86 10 89733200.0.(Y. Yang), yingzh1977@163.n).

son SAS. All rights reserved.

diameter seems to be promising. The mesoporous SBA-15 materialis more versatile than other mesoporous supports because of itslarger pore diameter permitting easy diffusion of bulky reactantsand products during the liquid-phase oxidations.

Various transition metal compounds (Cu, Fe, Co, Mn, etc.) havebeen reported as active catalysts for the epoxidation of olefins[10e14]. A few reports show that oxovanadium(IV) and dioxomo-lybdenum(VI) based catalytic systems were efficient for the epox-idation of olefins when using H2O2 or tert-butyl hydroperoxide(TBHP) as the oxidant [15e19], since such metals with the highoxidation state are both a Lewis acid and a weak oxidant, leading tohigher selectivity to epoxides. Jia et al. [20] reported that oxodi-peroxomolybdenum modified mesoporous MCM-41 materialswere highly efficient for catalytic epoxidation of cyclooctene. Wealso found that the oxovanadium(IV) complex of 8-hydroxyquinoline templated on SBA-15 exhibited high catalyticactivity and excellent selectivity to epoxides in the epoxidation ofstyrene and cyclooctene [21].

In this work, oxovanadium(IV) and dioxomolybdenum(VI)complexes of Schiff base derived from chloromethyl modified sal-icylaldehyde and ethylenediamine are tethered onto amino-functionalized SBA-15 by a stepwise procedure and are fully char-acterized, and their catalytic properties in the epoxidation ofcyclooctene are also evaluated.

Y. Yang et al. / Solid State Sciences 13 (2011) 1938e1942 1939

2. Experimental

2.1. Preparation of amino-functionalized SBA-15 and chloromethylmodified salen ligand

The mesoporous support SBA-15 was prepared by a literaturemethod [22] and activated by heating at 120 �C for 2 h and thenused to prepare amino-functionalized SBA-15 (APS-SBA-15)following a procedure described in our early work [23]. Thesynthetic procedures for SBA-15 and APS-SBA-15 are depicted inSupporting Information.

Chloromethyl modified salen ligand, N,N0-bis(5-chloromethylsalicylidene)ethylenediamine (CM-salen), wassynthesized by the condensation of ethylenediamine (1 equiv.) and5-chloromethylsalicylaldehyde (5-CM-Sal) (2 equiv.) in CH2Cl2 andpurified following the method described in our early work [24]. 5-CM-Sal was synthesized from salicylaldehyde by the classicalchloromethylation method according to the procedure reported inthe literature [25]. The two procedures are depicted in Scheme S1and the detailed procedures with characterization data are givenin Supporting Information (Figs. S1eS3).

2.2. Preparation of SBA-15 tethered metal complexes

The procedures for the preparation of SBA-15 tethered salenligands as well as their metal complexes are depicted in Scheme 1.In a typical synthesis, 0.8mmol of CM-salenwasmixedwith 0.5 g ofAPS-SBA-15 in dry toluene and the mixture was stirred at refluxtemperature for 24 h. The resulting yellow solid was filtered off,Soxhlet-extracted with CH2Cl2 to remove the untethered speciesand dried in vacuum to obtain SBA-15 tethered salen (salen-SBA-15).

Heterogenized oxovanadium(IV) or dioxomolybdenum(VI)salen complexes (M-salen-SBA-15, M ¼ VO or MoO2) wereprepared by the stirring 0.41 g of salen-SBA-15 with 0.25 mmol ofVO(acac)2 (dissolved in 10 mL of MeOH) or MoO2Cl2(dmf)2 (in10 mL of CHCl3) at room temperature for 12 h. The resulting solidwas filtered, Soxhlet-extracted with CH2Cl2 for 24 h, washed withwater and then dried in vacuum. Anal. found for VOsalen-SBA-15:

Scheme 1. Schematic outlines of syntheses of anchored metal complexes.

C, 12.48; H, 2.23; N, 2.07%. Anal. found for MoO2salen-SBA-15: C,9.08; H, 2.04; N, 1.95%.

2.3. Synthesis of pure metal salen complexes

The salen ligand was prepared basically by following theprocedure reported in the literature [26]. The detailed syntheticprocedure was depicted in Supporting Information. The 1H NMRspectra of salen was shown in Fig. S4 and the spectroscopic datawere also given in Supporting Information.

Metal salen complexes (M-salen, M ¼ VO or MoO2) were alsoprepared for comparison. In a typical synthesis, 2.0 mmol of salenligand was dissolved in 20 mL methanol, followed by the dropwiseaddition of a solution of 2.0 mmol VO(acac)2 in 10 mL methanol atreflux temperature. The resultant solution was stirred and refluxedfor 2 h. After cooling, the solid product was separated by filtrationand denoted as VOsalen. MoO2salen was prepared as follows:2.0 mmol of salen ligand was mixed with 2.0 mmol ofMoO2Cl2(dmf)2 in 20 mL of CH2Cl2, and the mixture was stirred atroom temperature for 24 h and then separated by filtration.

2.4. Catalytic epoxidation

Five mmol of cyclooctene and 5 mL of solvent along witha certain amount of catalyst were added to a 100mL two-neck flaskequipped with a stirrer and a reflux condenser. The mixture washeated to 70 �C and then 5 mmol of oxidant (30 wt.% H2O2 or65 wt.% TBHP) was injected into the solution to start the reaction.The liquid organic products were quantified by using a gas chro-matograph (Shimadzu, GC-8A) fitted with flame detector and HP-5capillary column and identified by a comparison with authenticsamples and GCeMS coupling.

3. Results and discussion

3.1. Structural integrity studies

The powder XRD patterns for SBA-15, VOsalen-SBA-15 andMoO2salen-SBA-15 are depicted in Fig. 1. The SBA-15 sample showsthree peaks, indexed as the (100), (110) and (200) diffraction peaks,associated with the typical two-dimensional hexagonal symmetryof the SBA-15 material (Fig. 1a) [27]. For the hybrid materials, therelative intensities of the prominent diffraction peak (100)decreased after the introduction of bulky organometallic groups.The intensity reduction may be mainly due to contrast matching

Fig. 1. XRD patterns of (a) SBA-15, (b) VOsalen-SBA-15 and (c) MoO2salen-SBA-15.

0.0 0.2 0.4 0.6 0.8 1.0

e

d

c

b

a

0 4 8 12 16 20

Relative pressure (P/P0) Pore diameter (nm)

DV

/d

lo

g(D

) (cm

3

g-1

nm

-1

)

Vo

lu

me ad

so

rb

ed

/cm

3

g

-1 (S

TP

)

e

d

c

b

a

Fig. 2. N2 adsorption/desorption isotherms and pore size distribution profiles of (a)SBA-15, (b) APS-SBA-15, (c) salen-SBA-15, (d) MoO2salen-SBA-15 and (e) VOsalen-SBA-15.

2000 1500 1000 500

545

518951

994

1609

1619

1635

c

b

a

2000 1500 1000 500

402

807

1086

518545

f

e

d

%T

ra

ns

mitta

nc

e

Wavenumbers (cm-1

)

Fig. 3. FT-IR spectra of (a) salen, (b) VOsalen, (c) MoO2salen, (d) SBA-15, (e) VOsalen-SBA-15 and (f) MoO2salen-SBA-15.

b

aA

bs

orb

an

ce

(a

.u

.)

e

d

c

Y. Yang et al. / Solid State Sciences 13 (2011) 1938e19421940

between the silicate framework and organic moieties which arelocated inside the channels of SBA-15 [20]. The N2 adsorption/desorption isotherms of SBA-15 and the hybridmaterials are shownin Fig. 2. Obviously, the hybrid materials maintain the characteris-tics of type IV isotherms and show a uniform pore size distributionin the mesoporous region. As shown in Table 1, the pore diameterdecreases from 6.35 nm for pure silica SBA-15 to 5.09 nm forMoO2salen-SBA-15 and to 5.26 nm for VOsalen-SBA-15. Comparedto the pure silica SBA-15, a pronounced decrease of the BET surfacearea and the pore volume and an increase of the wall thicknessoccurred due to the introduction of dioxomolybdenum(VI) oroxovanadium(IV) complexes. Similar trend has also been observedpreviously [28].

200 300 400 500 600 700 800 200 300 400 500 600 700 800Wavelength (nm)

Fig. 4. UVevis spectra of (a) VOsalen, (b) MoO2salen, (c) salen-SBA-15, (d) VOsalen-SBA-15 and (e) MoO2salen-SBA-15.

90

100

4

5

3.2. Spectroscopic characterization

Fig. 3 shows the FT-IR spectra of the pure ligand and neat metalcomplexes. The salen ligand exhibits a band at 1635 cm�1 due toazomethine (yC]N) stretch (Fig. 3a). This band registers a lowfrequency shift of ca. 16e26 cm�1 in the spectra of neat oxovana-dium (Fig. 3b) and dioxomolybdenum (Fig. 3c) complexes, indi-cating the coordination of azomethine nitrogen to the metal ions[29,30]. The coordination of the phenolic oxygen could not beascertained unequivocally due to the appearance of a broad bandcentred at ca. 3400 cm�1 (not shown). In the lower frequencyregion, bands appeared at ca. 994 and 951 cm�1 can be ascribed toyasym, M]O and ysym, M]O stretches, and bands at ca. 569 and518 cm�1 can be assigned to yM-O and yM-N, respectively [31]. Thesebands are still present upon homogeneous complex anchorage

Table 1Characteristics of support and hybrid materials, specific surface area, SBET (m2 g�1);pore volume, VBJH (cm3 g�1); pore diameter, DBJH (nm); cell parameters, Ao (nm);interplaner spacing, D100 (nm); wall thickness, W (nm); C (M), initial concentrationof metal species (mmol g�1).

Materials C (M) SBET VBJH DBJHa Ao D100 Wb

SBA-15 e 892 1.01 6.35 11.87 10.28 5.52APS-SBA-15 e 411 0.68 6.23 11.41 9.88 5.18Salen-SBA-15 e 409 0.72 5.31 12.37 10.71 7.06MoO2salen-SBA-15 0.4795 276 0.52 5.09 12.37 10.71 7.28VOsalen-SBA-15 0.2324 292 0.55 5.26 11.71 10.14 6.45

a Calculated from the desorption branch.b W ¼ Ao e DBJH (Ao ¼ 2D100/

ffiffiffi

3p

).

although the strong bands at 1086, 807 and 402 cm�1 assigned toSBA-15 framework are dominated. Moreover, in the region of 1600-1300 cm�1, the spectra of hybrid materials shows weak peaks dueto CeO, CeN and aromatic ring vibrations [32], indicating theexistence of salen ligands in the tethered materials (Fig. 3e and f).

100 200 300 400 500 600 700 800

50

60

70

80

DT

A/(u

V/m

g)

TG

/%

-1

0

1

2

3

Temperature (o

C)

Fig. 5. TG/DTA curves of MoO2salen (dot lines) and MoO2salen-SBA-15 (solid lines).

Table 2Surface analysis of supported catalysts by XPS.

Materials Surface concentrationa (at%) Binding energy (eV)

Si O C Cl N Mb N/M Cl 2p N 1s M 3d

VOsalen-SBA-15 10.92 28.06 54.38 0.07 6.00 0.58 (0.3765) 10.3 198.4 399.5 517.7, 524.6MoO2salen-SBA-15 10.90 25.62 53.57 0.05 7.22 1.74 (1.0489) 4.1 198.7 399.2 233.0, 236.2

a 1s of O, C and N, 2p of Si and Cl, 3d of V andMowere used for quantification. Quantification was performed using the sensitivity factors according toWagner (see Ref. [37]).b M ¼ V or Mo, and the data in the parentheses using “mmol/g” as the unit.

Y. Yang et al. / Solid State Sciences 13 (2011) 1938e1942 1941

All data above demonstrate that oxovanadium(IV) and dioxomo-lybdenum(VI) complexes formed inside the SBA-15 channels.

The UVevis spectra of the samples are shown in Fig. 4. Intensen-p*, p-p* ligand charge transfer bands in the range of220e380 nm are present in all cases. The expected Ded transition atca. 560 nm for VOsalen-SBA-15 as well as that at 600 nm for itshomogeneous analogue indicates that oxovanadium(IV) complexeshave been incorporated onto the surface of SBA-15. However, theseis no Ded transition for molybdenum(VI) modified SBA-15.

3.3. TG/DTA studies

TG/DTA analysis results of MoO2salen and VOsalen are depictedin Fig. 5 and Fig. S5. The TG curve recorded in air shows a two-stepweight loss at temperatures ranged from 30 to 800 �C. The weightloss below 400 �C is mainly related to the combustion of iminegroups. Weight loss (29.8% for MoO2salen and 68.1% for VOsalen) inthe temperature ranged from 380 to 600 �C is probably attributedto the combustion of phenyl groups [33]. However, the stabilities offinal residues at ca. 700 �C suggest the formation of MoO3 and V2O5,corresponding to the weight percent of 46.0% and 22.0%, respec-tively. On the other hand, metal complex modified SBA-15 samplesdisplay similar TG/DTA curves to their respective neat complexes(see Fig. 5, FigsS5 and S6), just the third step involves slow weightloss of ca. 6.0% for MoO2salen-SBA-15 or ca. 3.5% for VOsalen-SBA-15 in a wider temperature range (390e600 �C), suggesting thatmetal complexes have been anchored. The loading of salen ligand is0.46 mmol g�1 based on 3.5% of weight loss and the vanadiumcontent is 0.2324 mmol g�1 estimated by ICP-AES. The molar ratioof ligand to vanadium is approximately 2: 1, indicating that allvanadium introduced have coordinated to ligands. Moreover, Nelemental analysis shows that the total content is 1.4786 mmol g�1,suggesting that parts of amine groups remain free in the channels.It is the same case for MoO2salen-SBA-15.

390 395 400 405 410

401.7

399.3

Binding Energy (eV)

In

ten

sity (a.u

.)

b

a

Fig. 6. XPS analysis of N 1s for (a) MoO2salen-SBA-15 and (b) VOsalen-SBA-15.

3.4. XPS measurements

To gain deeper insight into the surface constitution of thecatalysts, the Cl 2p, N 1s and M 2p (M ¼ V or Mo) binding energiesare investigated, and the corresponding spectra are composed andintegrated with the results shown in Table 2. The N 1s spectra ofMoO2salen-SBA-15 and VOsalen-SBA-15 show two peaks withmaxima at 399.3 and 401.7 eV (Fig. 6), which can be assigned to twotypes of nitrogen atoms in the tethered catalyst, the NeH in aminogroups and the C]N in salen ligands [34]. The XPS spectra in the Cl2p region of these catalysts show low-intensity peaks at 198.2 eVdue to trace of unreacted CH2Cl. Such low surface concentrations ofchlorine can be neglected, and hence the salen ligands are bound tothe SBA-15 matrix via producing the nucleophilic substitution onthe both benzyl halides of a ligand moiety (Scheme 1). Neverthe-less, these metal complexes do not anchored homogeneouslythroughout the SBA-15 matrix, since a higher value of metalcontent was obtained by XPS (Table 2) as compared to that of metalcontent estimated by ICP-AES for both VOsalen-SBA-15 andMoO2salen-SBA-15 (Table 1). Hence, the tethered metal species aremainly inhomogeneous located near the pore entrance or on theexternal surface but some on the internal surface.

3.5. Catalytic properties

The catalytic performances of various catalysts are shown inTable 3. As expected, all catalysts are active for the epoxidation ofcyclooctene with nearly 100% of selectivity to epoxycyclooctane.The MoO2salen-SBA-15 catalyst shows the activity with 49.1%conversion after 8 h (TOF 25.6 h�1), while a lower activity with22.5% conversion (TOF 11.7 h�1) is obtained when using H2O2 as theoxidant. As for VOsalen-SBA-15, it shows the highest activity(64.3%) when using H2O2 as the oxidant, while it only shows 25.0%activity when using TBHP as the oxidant. It is clear that MoO2salen-SBA-15 catalyst behaves well with TBHP as the oxidant in CHCl3,while VOsalen-SBA-15 shows best performance with H2O2 as theoxidant in CH3CN, which is in accordance with those reported inliterature [15e21]. Vanadium(IV) centre in CH3CN, or molybde-num(VI) centre in CHCl3 may easily withdraw electron from theperoxidic oxygen making them more susceptible to be attacked by

Table 3Catalytic properties of various catalysts for the epoxidation of cyclooctene.

Materials Oxidant Solvent Conversiona (Epoxide yield)% TOFb (h�1)

MoO2salen-SBA-15 TBHP CHCl3 49.1 (49.1) 25.6MoO2salen-SBA-15 H2O2 CH3CN 22.5 (22.5) 11.7VOsalen-SBA-15 TBHP CHCl3 25.0 (25.0) 26.9VOsalen-SBA-15 H2O2 CH3CN 64.3 (64.3) 69.2MoO2salen TBHP CHCl3 12.5 (12.5) 12.3VOsalen H2O2 CH3CN 22.6 (22.6) 18.8

a Reaction conditions: catalyst 25 mg (2.5 mg for neat catalyst), substrate 5 mmol,solvent 5 mL, oxidant 5 mmol, duration 8 h and temperature 70 �C.

b TOF, h�1: (turnover frequency) moles of substrate converted per mole metal ionper hour.

Y. Yang et al. / Solid State Sciences 13 (2011) 1938e19421942

nucleophilic cyclooctene, since the Lewis acidity of vanadium(IV)and molybdenum(VI) complexes is one of the most importantcharacteristics that determines catalytic performance in olefinepoxidation.

For comparison, the catalytic properties of neat complexes arealso examined. As listed in Table 3, MoO2salen shows 12.5%conversion when TBHP was used in CHCl3, which is much lowerthan that obtained from MoO2salen-SBA-15. Similarly, theVOsalen catalyst shows lower activity as compared to VOsalen-SBA-15 when H2O2 was used in CH3CN. These results suggestthat tethering of oxovanadium(IV) or dioxomolybdenum(VI)complexes onto SBA-15 material probably isolated these neatcomplexes and prevented their dimerization. The enhancedactivity of tethered catalysts may be also ascribed to thehexagonally ordered uniform pores of SBA-15, especially thesynergistic beneficial catalytic interaction between the metalcomplexes and the SAB-15 support. These have been observed inother catalytic systems. For example, Anand et al. [35] found thatSBA-Fe(acac) is superior to Fe(acac) in the activity point view forthe oxidation of styrene with 30% H2O2. González-Arellano et al.[36] also reported that the heterogenized Rh(I) and Ir(I) metalcomplexes with chiral triaza donor ligands showed much higheractivity than that observed under homogeneous conditions forthe hydrogenation reactions.

4. Conclusions

Oxovanadium(IV) and dioxomolybdenum(VI) complexes withsalen ligands have been tethered on SBA-15 matrix via a step-wise procedure, as evidenced by FT-IR, UVevis spectroscopy, N2adsorption/desorption and TG/DTA techniques. XRD and N2adsorption/desorption show the characteristic channel structuresof the support SBA-15 remain intact and accessible after teth-ering of metal complexes. ICP-AES, TG/DTA and XPS datademonstrate the location, anchorage status and loadings of activespecies. Catalytic tests show that the tethered oxovanadium(IV)complex is the most active with H2O2 as the oxidant and CH3CNas the solvent, while the tethered dioxomolybdenum(VI) catalystshows better activity with TBHP as the oxidant and CHCl3 as thesolvent compared to that when using H2O2 as the oxidant andCH3CN as the solvent. It is also found that heterogenizationenhanced the activity of homogeneous catalysts due to site-isolation and cooperative effects between the SBA-15 supportand the metal complexes.

Acknowledgements

This work was supported by the National Basic ResearchProgram of China (2004CB217804) and the National NaturalScience Foundation of China (20673046).

Appendix. Supplementary material

Supplementary data related to this article can be found online atdoi:10.1016/j.solidstatesciences.2011.08.020.

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