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Catalysis Communications 8 (2007) 1507–1510

Selective liquid phase oxidation of aromatics over silica–polymernanocomposite materials

Amit Dubey *, Braj Gopal Mishra

Chemistry Group, Birla Institute of Technology and Science, Pilani 333031, Rajasthan, India

Received 3 November 2006; received in revised form 1 December 2006; accepted 28 December 2006Available online 9 January 2007

Abstract

Catalytic oxidation of different aromatics was carried out in liquid phase over silica–polymer nanocomposites in situ synthesised viafree radical polymerization under mild reaction conditions and the catalytic activity studies indicate higher conversion and selectivitytowards desired product.� 2007 Elsevier B.V. All rights reserved.

Keywords: Nanocomposite; Catalysis; Aromatics oxidation

1. Introduction

Catalytic oxidation of hydrocarbons is widely used forbulk as well for fine chemicals [1]. Significant efforts havebeen devoted during the last two decades to fabricatenew catalysts for better scientific understanding andadvanced technologies because of serious environmentalconcern. Liquid phase catalytic oxidation of aromatics isa challenging area as the products obtained are commer-cially important and the difficulties associated with overoxidation and product selectivity [2]. In particular for cat-alytic oxidation of toluene in liquid phase is of significantinterest in terms of product selectivity (because of the facileoxidation of benzaldehyde, one of the products). Manyefforts have been devoted in the recent times both in gasand liquid phase but the product selectivity remains an areaof challenge so far [3,4]. AMM–Mn (amorphous micropo-rous mixed oxides) [5], modified V2O5/TiO2 catalysts in thegas phase [6], Ti-phosphate supported V2O5 [7], Mg, Cuand Ni containing ternary hydrotalcites in the liquid phase[8–10], Influence of Cl on modified Pt/Al2O3 [11] and Cr-substituted mesoporous materials [12] were employed for

1566-7367/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.catcom.2006.12.028

* Corresponding author.E-mail addresses: amitdubey@bits-pilani.ac.in, amitdubey75@yahoo.

com (A. Dubey).

this challenging reaction in the past using different solventsand oxidants. In continuation, Ce–Mo mixed oxides syn-thesized by coprecipitation method were analysed for theacid–base properties on this challenging reaction [13]. Toachieve better conversion and product selectivity, nanopor-ous solids for different organic conversions [14], TiOx/Sepolite supported vanadium catalysts and Fe-containingcatalysts were also reported [15,16] recently.

Following the discovery of the mesoporous materials, astrong input has been universally acknowledged and findsadvantages over the microporous materials due to theirhigh surface area combined with well defined pore sizes(2–50 nm) and small particle sizes [17]. The wide rangesof these mesoporous silicas allow incorporating the multi-functional organic moiety into their uniform pore sizes.However, the advanced catalytic applications of these mes-oporous materials are limited due to difficulties associatedto control the pore sizes uniformly. During our recentreport [18] we have reported a new synthesis strategy tosynthesize mesoporous polymer–silica (KIT-6 with cubicIa3d symmetry) nanocomposite materials through in situradical controlled polymerization of vinyl monomers (sty-rene and vinyl ferrocene as a functional monomer) insidethe silica mesopores for hydroxylation of phenol. In anextension to the previous work, here we present the liquidphase catalytic activity studies of these silica–polymer

1508 A. Dubey, B.G. Mishra / Catalysis Communications 8 (2007) 1507–1510

nanocomposites over the wide variety of aromatics usingenvironmentally oxidants (O2 or H2O2) and acetonitrileas a solvent and the knowledge obtained was extended indetail for catalytic oxidation of toluene. The advantagesof these materials lie in the fact that all the monomersincluding active functional moiety (vinyl ferrocene in pres-ent study) can be synthesized in one step with greaterstability.

2. Experimental

The synthesis of KIT-6 was done by the methodreported in the literature [19]. The synthesis methodologyfor KIT-polymer composites is same as reported in ourprevious communication [18] except the difference in thecontents of vinyl ferrocene. Typically, for 30 wt% polymerloading, 0.109 g of styrene (80 mol%), 0.0345 g divinylben-zene (20 mol%), 0.0075 g of AIBN, a,a, 9-azoisobutyronit-rile (3% relative to the total vinyl group) and 0.01140 g ofvinyl ferrocene (9:1 wt ratio; styrene:vinyl ferrocene) weredissolved into 2 ml of solvent (dichloromethane). The roleof styrene is to solubilize vinyl ferrocene and to make thenature of the surface more hydrophobic. The sample wassealed in a pyrex tube and subjected to control temperatureprogramming for polymerization. Three samples for differ-ent amounts of vinyl ferrocene (2, 5 and 8 wt%, vinyl ferro-cene:styrene) were synthesized for further study. Thestructural parameters of the samples are given in Table 1.The amount of iron contents were determined by ICP-AAS analysis. The samples were named as x/KIT-PS,where x presents the percentage of Fe present in thesample.

Table 1Catalytic oxidation of different substrates over 0.34% Fe/KIT-PS catalyst

Reactant (s) Product (s) Oxi (sub:oxi) Solvent

Benzene Phenol (1) H2O2 (5:1) Acetonitrile

Benzyl alcohol Benzaldehyde (1) O2 Without solvBenzoic Acid (2)

Cyclohexane Cyclohexanol (1) H2O2 (1:2) AcetonitrileCyclohexanone (2)

Toluene Benzaldehyde (1) H2O2 (1:1) AcetonitrileBenzoic Acid (2)Benzyl alcohol (3)

a Reaction conditions: substrate, 1 g; catalyst wt, 50 mg; (conversion is based

Table 2Structural parameters and the catalytic activity of the samples

Catalyst Surface area (m2/g) Pore volu

KIT-6 (silica) 800.7 0.970.21% Fe/KIT-PS 451.8 0.530.34% Fe/KIT-PS 428.2 0.520.58% Fe/KIT-PS 401.1 0.49

a Reaction Conditions: Toluene, 1 g; catalyst wt, 50 mg; temp, 323 K; time, 2based on the products).

The catalytic oxidation of aromatics (Table 2) was car-ried out in the liquid phase in two neck glass reactor con-nected with the condenser. The desired amount ofreactants, catalysts and solvent are taken in the oil bathat a particular reaction temperature. The calculatedamount of oxidant was added to the continuously stirringreaction solution. The course of the reaction was moni-tored by taking periodically small samples (0.05 cm3) whichwere analyzed by gas chromatography (GC) fitted with SE-30 packed column (2 m · 4 mm i.d.) using flame ionizationdetector. Quantification was done after considering theresponse factors of reactants and products, whose retentiontimes were determined using authentic samples, derivedusing standard mixtures containing both of them. The con-version of toluene (mass balance) was measured using n-decane as the internal standard which showed a variationof 5–7% (than obtained through direct analysis) probablydue to coke formation.

3. Results and discussions

The powder X-ray diffraction (PXRD) pattern (Fig. 1)of the samples showed an increase in the intensity of thecharacteristic peak compared to KIT-6 due to the polymerformation inside the mesopores of silica materials resultingin an increase in the apparent density of the mesopore walls[18–20]. The thermogravimetric analysis indicated that 99%of the monomers were converted to polymers. The detailsof the synthesis and characterization were reported earlier[18]. The catalytic activity studies of different aromatics(Table 2) showed the promise use of these nanocompositesrelatively at mild reaction conditions. The choice of the sol-

Temp (K) Conversiona (wt%) Product selectivity (%)

303 7.2 100

ent 303 100 100 (1)0 (2)

330 7 60 (1)

323 10.7 80 (1)20 (2)0 (3)

on products).

me (ml/g) Pore size (nm) Conversiona (wt%)

8.3 06.0 5.75.2 10.74.7 13.5

4 h; sub: H2O2 = 1:1, solvent-acetonitrile (10 ml) (conversion of toluene is

1 3

cba

d

Inte

nsi

ty (

a.u

.)

2 Theta (degrees)2

Fig. 1. PXRD pattern of (a) KIT-6, (b) 0.21% Fe/KIT-PS, (c) 0.34% Fe/KIT-PS, and (d) 0.58% Fe/KIT-PS.

Table 3Variation of conversion of toluene over different catalysts

Catalyst Conversion(wt%)

Product selectivity (%)

Ph-CHO

Ph-COOH

Ph-CH2OH

KIT-6 0 0 0 0KIT-PS 0 0 0 00.21% Fe/KIT-

PS5.7 100 0 0

0.34% Fe/KIT-PS

10.7 95 5 0

0.58% Fe/KIT-PS

13.5 80 20 0

0.65% Fe/KIT* 8.2 30 50 20

* Indicates the direct grafted sample (conditions as in Table 2).

0

2

4

6

8

10

12

14

5 10 25 50 100 150 200

Catalyst wt. (mg)

Co

nver

sio

n (

wt%

)

Fig. 2. Variation of conversion of toluene with weight of the catalyst0.34% Fe/KIT-PS (conditions as in Table 2).

A. Dubey, B.G. Mishra / Catalysis Communications 8 (2007) 1507–1510 1509

vent and the oxidant for a particular aromatic is based onthe screening of various oxidants and solvents. Based onthe results and the importance of the products, catalyticoxidation of toluene was selected for further studies usingH2O2 as oxidant and acetonitrile as solvent. The catalyticactivity of 0.34% Fe/KIT-PS catalyst indicated around10.7% conversion of toluene (sub:H2O2 molar ratio; 1:1)with 80% selectivity of benzaldehyde and 20% selectivityof benzoic acid. No benzyl alcohol was observed underour experimental conditions. No conversion was observedin the blank runs (without catalyst) indicating that theentire activity is due to the presence of Fe contents thoughtto be the active centers involved in this reaction. The cata-lytic activity studies with different amounts of iron contentsrevealed that the conversion increases with increase in theiron content (Table 3). Furthermore, to compare the activ-ity and the selectivity, the direct grafted sample (withoutmonomer incorporation) with similar iron content was syn-

0 10 15 20 25

0

20

40

60

80

100

Benzoic acid

Benzaldehyde

Sel

ecti

vity

(%

)

Time (h)5

Fig. 3. Variation of conversion and selectivity of toluene with tim

thesized by the method reported earlier [21]. It wasobserved that 8.2% conversion of toluene with only 30%selectivity of benzaldehyde was obtained under similarreaction conditions. This higher selectivity of benzaldehydein polymer derived nanocomposites compared to the directgrafted sample may be attributed to the hydrophobic nat-ure of the surface which enables the selective adsorption

0 10 15 20 250

2

4

6

8

10

Benzaldehyde (wt%)

Benzoic acid (wt %)

Total Conv.(wt%)

Co

nver

sio

n (

wt%

)

Time (h)5

e over 0.34% Fe/KIT-PS catalyst (conditions as in Table 2).

1510 A. Dubey, B.G. Mishra / Catalysis Communications 8 (2007) 1507–1510

of the reactant molecules leading to avoid the over oxida-tion. These results have advantages over some of thehydrotalcites catalysts in terms of lesser requirement of oxi-dant and temperature. The variation of substrate: catalystratio indicated that the conversion increases with theincrease in the weight of the catalyst up to a certain extent(100 mg) and then decreased with further increase in theweight of the catalyst (Fig. 2). This decrease in the conver-sion may be due to the coke formation (the reaction mix-ture turned black with the progress of the reaction) whichblocks the active sites of the catalysts thereby reducingthe activity of the catalyst (Fig. 3). To verify, we have esti-mated the coke formed for the reaction having catalystweight 100 mg and observed around 0.1–0.3 g of coke pergram of catalyst (calculated after taking due considerationof weight loss due to catalyst upon calcination in air). Theoptimum catalyst weight (50 mg) was chosen for furtherstudy. Interestingly, no conversion was observed with5 mg of the catalyst indicating that minimum concentrationof the active centers is required to initiate this reaction. Thereusability of the catalysts was tested for five cycles withalmost same conversion (9.6%) and selectivity whereasthe decrease in the catalytic activity (3.9%) of the directgrafted sample was observed. This is due to the leachingof active Fe-species from the direct grafted catalyst duringthe first run. This was confirmed by the series of leachingstudies experiments based on our earlier reports [18,22].To the best of the literature knowledge, it is generally verydifficult to control the reaction up to benzaldehyde stagedue to its facile oxidation to benzoic acid and the presentresults have more advantage to fabricate new in situ poly-mer generated nanocomposites to obtain the desiredselectivity.

4. Conclusions

In conclusion, we have shown the selective use of thesesilica–polymer nanocomposites for catalytic oxidation ofdifferent aromatics. The introduction of hydrophobicityon the surface reduces the undesired products under themild reaction conditions. The wide variety of applications

can be explored over these polymer nanocomposites bysurface modification.

Acknowledgements

Authors thank the Chemistry Group and institute forproviding help and support to carry this work.

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