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Microporous and Mesoporous Materials 102 (2007) 223–233

Effect of pretreatment conditions on the catalytic activity of nano-sizedTPA/ZrO2 dispersed in SBA-15 channels in acetylation reactions

Dhanashri P. Sawant a, M. Hartmann b, S.B. Halligudi a,*

a Inorganic Chemistry and Catalysis Division, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, Indiab Advanced Materials Science, Institute of Physics, University of Augsburg, Germany

Received 16 January 2006; received in revised form 22 August 2006; accepted 30 December 2006Available online 10 January 2007

Abstract

Acetylation of an aromatic compounds such as veratrole (1,2-dimethoxybenzene), anisole, thioanisole, isobutyl benzene and 2-meth-oxynaphthalene with acetic anhydride has been investigated over a nano-sized 12-tungstophosphoric acid (TPA)/22.4 wt.%ZrO2/SBA-15catalysts in the liquid phase conditions. Catalysts with different (%) TPA and zirconia loadings calcined at different temperatures wereprepared by wet-impregnation and characterized by XRD, N2 adsorption–desorption, FT-Raman, X-ray photoelectron spectroscopy(XPS). The acidic behavior of the catalyst was studied by TPD of ammonia and FT-IR pyridine adsorption. Among the catalysts,15 wt.%TPA/22.4 wt.%ZrO2/SBA-15 calcined at 1123 K had the highest acidity and was the most active catalyst in veratrole acetylationwith acetic anhydride.� 2007 Elsevier Inc. All rights reserved.

Keywords: TPA/ZrO2; SBA-15; Wet-impregnation; Veratrole; Acetylation

1. Introduction

Friedel–Crafts acetylation of aromatic compound is use-ful in the production of aromatic ketones used in the syn-thesis of pharmaceuticals, naproxen, dyes, fragrances andagrochemicals [1–3]. To overcome the difficulties of con-ventional Lewis acid catalysts, an attention has been paidin recent years to the use of solid acid catalysts such as zeo-lites, clays, mesoporous silica in acetylation of aromaticcompounds [4–7]. The first industrial application of a zeo-lite-promoted acylation for the production of an aromaticketone was reported by researchers at Rhone-Poulenc [5].The acetylation of anisole to 4-methoxyacetophenone, anintermediate for paracetamol drug was performed overzeolites [8,9], Nafion-H on silica [10,11], and microencapsu-lated scandium(III) triflate in combination with nitrometh-ane/lithium perchlorate [12]. The acetylation of isobutyl

1387-1811/$ - see front matter � 2007 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2006.12.046

* Corresponding author. Tel.: +91 20 25893300; fax: +91 20 25893567.E-mail address: sb.halligudi@ncl.res.in (S.B. Halligudi).

benzene over beta zeolite to produce 4-isobutyl acetophe-none an intermediate for ibuprofen has been reported byDavis and coworkers [7]. Sulfur containing aromaticketones such as 4-methylthio acetophenone produced byacetylation of thioanisole are potential intermediates forthe synthesis of useful drugs as vioxx (refecoxib, for treat-ing an arthritis) [1,13,14]. Acylation of 2-methoxynaphtha-lene (2-MN) was studied with various zeolites and clays[15] and beta zeolite was better than others. However, theselectivity towards 6-acyl-2-methoxynaphthalene used asan anti-inflammatory drug – naproxen, a desired regiomerwas poor to moderate. Recently, heteropoly acids (HPAs)of the Keggin type are strong Brønsted acids and have beenwidely used as an acid catalysts [16,17]. There are severalreports on the use of HPAs supported on metal oxides suchas silica [18], active carbon [19], acidic ion-exchange resin[20], titania [21], zirconia [22,23] and ZrO2/SBA-15 [24] inacid catalyzed reactions. Metal oxides that exhibit basicproperties like Al2O3 and MgO tend to decompose HPAs,causing a significant decrease in their catalytic activities[25,26]. As the surface areas of the HPAs are very low,

224 D.P. Sawant et al. / Microporous and Mesoporous Materials 102 (2007) 223–233

supporting HPAs on high surface area supports would pro-duce a catalyst with higher activity for reactions involvingnon-polar substrates.

Acetoveratrone (3 0,4 0-dimethoxyacetophenone) is usedin the synthesis of papaverine (1-(3,4-dimethoxybenzyl)-6,7-dimethoxyisoquinoline), an opium-alkaloid antispas-modic. It is prepared by acetylation of veratrole in presenceof hafnium(IV) triflate, scandium triflate in combinationwith lithium per chlorate nitromethane system and carbox-ylic acid or trisubstituted silyl carboxylates in presence oftrifluoromethyl benzoic acid anhydride [27–29]. ZeoliteHY was found to be more active than Hb [30,31] in theabove reaction. HPAs supported over hexagonal mesopor-ous silica (HMS) in presence of 1,2-dichloroethane as a sol-vent has been used for the synthesis of acetoveratrone [32].

We report in this paper, the effect of pretreatment condi-tions on the improved catalytic activity of nano-sized TPA/ZrO2 dispersed in SBA-15 channels in acetylation ofveratrole, anisole, thioanisole, isobutyl benzene and 2-methoxynaphthalene to give respective acetylated products.

2. Experimental

2.1. Materials

Hexadecyltrimethylammonium bromide (Aldrich,>99%), tetraethylorthosilicate, (TEOS) (Aldrich, >99%),triblock copolymer of ethylene oxide (EO) and propyleneoxide (PO), EO20PO70EO20 (Pluronic P123) (Aldrich,Mavg = 5800), zirconium oxychloride ZrOCl2 Æ 8H2O(Merck, 99.5%) and 12-tungstophosphoric acid (TPA)(99.99%) received from Merck were used as such withoutfurther purification in the synthesis of catalysts. Anisole(99.7%), thioanisole (99%), veratrole (99%), 2-methoxy-naphthalene (99%), isobutyl benzene (99%) and aceticanhydride (99%) were purchased from Aldrich and s.d. finechem. Ltd., Mumbai, respectively and used without furtherpurification. Zeolites with (SiO2/Al2O3 ratio), H-beta (30)and H-Y (13.5) recovered by calcining NH4-Y wereobtained from Catalysis Pilot Plant (CPP-NCL, Pune).Zn2+ exchanged K-10 montmorillonite clay prepared asper literature [15] were used in the above reaction for com-parison of the activities in acetylation reactions of with thenew catalysts. All catalysts were in the powder form andwere activated prior to their use in the reaction.

2.2. Synthesis

Pure siliceous SBA-15, MCM-41 and MCM-48 weresynthesized according to literature [33–35]. The gel chemi-cal composition for SBA-15 was 4 g polymer:0.041 molTEOS:0.24 mol HCl:6.67 mol H2O. The molar ratio ofthe synthesis gel composition was 10SiO2:5.4CnH2n+1-(CH3)3NBr:4.24Na2O:1.3H2SO4:480H2O for MCM-41while i.e. for MCM-48 it was 1 M TEOS: 0.25 MNa2O:0.65 M C16H33(CH3)3NBr:0.62 M H2O. The as-syn-thesized mesoporous samples were calcined in air at

813 K for 8 h. TPA/ZrO2/MS (MS refers Mesoporous sup-ports hereafter) composite materials were prepared in twosteps by wet-impregnation by following our earlierreported procedure [24]. After impregnation, the catalystswere dried and calcined in air at 1123 K. Catalysts with dif-ferent TPA loadings were prepared and calcined at 1123 K.TPA (15 wt.%)/ZrO2 (22.4 wt.%)/SBA-15 was preparedand calcined at different calcination temperatures. To studythe effect of zirconia (%) loading over SBA-15, pure sili-ceous SBA-15 was impregnated with an aqueous solutionof ZrOCl2 Æ 8H2O (10–12 ml of distilled water per g of mes-oporous silica support) with different ZrO2:SBA-15 weightratios. The resulting mixture was stirred in a rotary evapo-rator for 2–3 h followed by evaporation to dryness anddried at 373 K for 12 h and powdered well for furtheruse. These solids were used to prepare catalysts containingknown amount of an aqueous solution of TPA (10–12 mldistilled water) per g of dried ZrO2/SBA-15 support bywet-impregnation method and then calcined at 1123 K.

2.3. Characterization

Zr, W and P contents in the resulting solid was deter-mined by inductively coupled plasma–optical emissionspectroscopy (ICP–OES) and X-ray fluorescence spectro-photometer by sequential X-ray photoelectron spectrome-ter (XRF), Rigaku 3070 E Model with Rh target. BeforeICP–OES analysis, 10 mg of sample is digested with a mix-ture of 1–2 ml conc. HF and 5 ml distilled water for 10 minunder reflux condition. Then solution is filtered off and thefiltrate with a known concentration is submitted for furtherinvestigation.

Bruker small-angle X-ray scattering (SAXS) instrumentwith general area detector diffraction (GADDS) usingCu Ka radiation at steps of 0.01�. SAXS pattern of thesamples was obtained in reflection mode using a RigakuDmax 2500 diffractometer and Ni filtered copper radiation.The wide reflections in the X-ray diffractogram at 2h � 30�,50� and 60� characteristics of the ZrO2-t (tetragonal) phasewere detected using X-ray powder diffraction with Cu Karadiation (Rigaku Model D/MAXIII VC, Japan,k = 1.5418 A).

The specific surface area, pore volume and pore size dis-tribution of samples were measured with an Omnisorb100CX (Coulter, USA) system under liquid N2 tempera-ture, using N2 as an adsorbent.

The Raman spectrometer (The Brucker FRA106) isequipped with a Nd:YAG (Neodymium:Yttrium Alumin-ium Garnet) laser that is frequency doubled to 532 nm.The laser was operated at a power level of 20 mW mea-sured at the sample with a power meter. The spectra havebeen collected at room temperature in the wavelengthrange from 0 to 4000 cm�1. The spectral resolution of thespectrometer is 5 cm�1. Sampling times were between 20and 25 min.

XPS spectra are recorded on a VG Microtech Multilab-ESCA 3000 spectrometer equipped with a twin anode of Al

10 20 30 40 50 60 70 80

a

Rel

ativ

e In

tens

ity

(a.u

.)2 Theta (degree)

b

c

d

e

f

g

h

M-ZrO2

T-ZrO2

Fig. 1. XRD patterns of (a) neat TPA, (b) ZrO2, (c) 15 wt.%TPA/ZrO2,wide-angle XRD patterns for 15 wt.% TPA with (d) 10, (e) 22.4, (f) 30, (g)50 and (h) 70 wt.%ZrO2/SBA-15 calcined at 1123 K.

D.P. Sawant et al. / Microporous and Mesoporous Materials 102 (2007) 223–233 225

and Mg. All measurements are made on as received powdersamples using Mg Ka X-ray at room temperature. Basepressure in the analysis chamber was 4 · 10�10 Torr. Mul-tichannel detection system with nine channels is employedto collect the data. The overall energy resolution of theinstrument is better than 0.7 eV, determined from the fullwidth at half maximum of the 4f7/2 core level of gold sur-face. The errors in all the BE values were within ±0.1 eV.The binding energy correction was performed using theC 1s peak of carbon at 284.9 eV as reference.

The total amount of acidity present in the catalyst wasestimated using temperature programmed desorption(TPD) of NH3 on Micromeritics AutoChem 2910 instru-ment. It was carried out by dehydrating 0.1 g of thecatalyst sample at 773 K in dry air for 1 h and thenpurged with helium for 0.5 h. The temperature wasdecreased to 398 K under the flow of helium and then0.5 ml NH3 pulses were supplied to the samples until nomore uptake of NH3 was observed. NH3 was desorbed inHe flow by increasing the temperature to 813 K with aheating rate of 10 �C min�1 measuring NH3 desorptionusing TCD detector.

2.4. Acetylation of aromatic substrates (veratrole, anisole,

thioanisole, isobutyl benzene, 2-methoxynaphthalene) with

acetic anhydride

The liquid-phase acetylation of veratrole with aceticanhydride (Ac2O) was carried out in a 50 ml two-neckedround bottom flask equipped with a condensor, a magneticstirrer, a septum for sample withdrawal and kept inthermostated oil bath. In a typical experiment, 0.09 g cata-lyst was added to a solution of 2.61 g (18.9 mmol) veratroleand 0.39 g (3.82 mmol) acetic anhydride and the reactionmixture was heated to 333 K with constant stirring for2 h. Samples withdrawn periodically were analyzed by Shi-madzu gas chromatograph fitted with a fused megaborecolumn SE-52, HP-%(cross-linked 5% PhMe silicone),30 m in length, 0.53 lm film thickness and a FID. The iden-tity of the products was confirmed by GC–MS (ShimadzuQP 5000) and also by comparing with the authentic stan-dard. Conversion of acetic anhydride was determined onits disappearance in the reaction mixture. Similarly, theacetylation of anisole, thioanisole, isobutyl benzene, 2-methoxynaphthalene with acetic anhydride were conductedand the data on substrate conversions, products selectivi-ties and the reaction conditions are listed in Table 4.

3. Results and discussion

3.1. Catalysts characterization

Fig. 1 shows the wide-angle XRD patterns of neat TPA,bulk ZrO2, 15 wt.%TPA/ZrO2, along with different (%)ZrO2 loadings calcined at 1123 K. As shown in Fig. 1,15 wt.%TPA/ZrO2 calcined at 1123 K shows along withmonoclinic and tetragonal phases of zirconia, WO3 crystal-

lites due to decomposition of TPA. As for bulk ZrO2, crys-tallization of the amorphous phase to the metastabletetragonal phase typically occurred in the range 673–773 K, but after impregnating over SBA-15, the stabiliza-tion of transition of phases shifted to 923 K. Wide-angleXRD of TPA over 22.4 wt.%ZrO2/SBA-15 shows mono-layer coverage up to 15 wt.% loading and above this, itstarted decomposing into WO3 (2h � 23.12�, 23.59�, and24.38�) crystalline peaks as per literature [24]. Similarly,15 wt.%TPA/22.4 wt.%ZrO2/SBA-15 showed monolayercoverage up to 1123 K calcination temperature. After sup-porting SBA-15 with 15 wt.%TPA/22.4 wt.%ZrO2, smallparticles of TPA/ZrO2 formed inside nanochannels ofSBA-15 with the crystallite size of the material calculatedfrom TEM as 3–4 nm. ZrO2 (22.4 wt.%, 5.5–6.5 nm size)i.e. an optimum loading has been inserted into SBA-15by wet impregnation method, which shows higher capacityfor monolayer coverage of TPA [24]. However, above1123 K, it showed decomposition of TPA into WO3 crys-tallites [24]. The XRD patterns of 15 wt.%TPA/22.4 wt.%ZrO2/SBA-15 with different ZrO2 (%) loadingover SBA-15 shows that, monoclinic ZrO2 (m-ZrO2,2h � 28�) phase also appeared when the ZrO2/SBA-15wt. ratio was higher than 0.5.

The nitrogen adsorption–desorption isotherms and poresize distribution at 77 K as shown in Fig. 2 are of type IVfor SBA-15 and for different TPA (%) loadings over22.4 wt.%ZrO2/SBA-15 calcined at 1123 K. A well-definedstep associated with the filling of the mesopores due to cap-illary condensation occurs approximately at P/P0 = 0.54 to

0.0 0.2 0.4 0.6 0.8 1. 0

Vol

ume

adso

rbed

cc/

g ST

P

Relative pressure (P/P )0

g

f

e

d

c

b

a

Fig. 2. BET isotherm (a) SBA-15, (b) 5, (c) 15, (d) 30, (e) 50, (f) 70 and (g)90 wt.%TPA/22.4 wt.%ZrO2/SBA-15 calcined at 1123 K.

Table 1Physicochemical properties of the catalysts calcined at 1123 K

Catalyst Surface area(m2 g�1)

Pore volume(cm3 g�1)

Pore diameter(A)

Different (%) TPA loading over 22.4 wt.%ZrO2/SBA-15

0 426 0.69 71.35 398 0.63 67.7

15 372 0.59 67.730 329 0.57 67.450 328 0.53 67.070 199 0.33 66.790 199 0.31 66.0

15 wt.%TPA/22.4 wt.%ZrO2/SBA-15

923 481 0.63 68.41023 410 0.61 68.11123 372 0.59 67.71173 326 0.47 67.31223 249 0.42 66.81273 212 0.33 66.1

15 wt.% TPA with different (%) ZrO2 loading over SBA-15

10 416.8 0.67 68.622.4 372 0.59 67.730 297.3 0.48 67.150 190.9 0.34 66.670 167.9 0.25 66.0

700 750 800 850 900 950 1000 1050 1100

a

Inte

nsit

y (a

.u.)

Raman shift (cm ) -1

1009

b

Fig. 3. FT-Raman spectra of (a) 22.4 wt.%ZrO2/SBA-15 and(b) 15 wt.%TPA/22.4 wt.%ZrO2/SBA-15 calcined at 1123 K.

226 D.P. Sawant et al. / Microporous and Mesoporous Materials 102 (2007) 223–233

0.59–1.00 for 5–90 wt.% of TPA over 22.4 wt.%ZrO2/SBA-15 catalyst calcined at 1123 K and at P/P0 = 0.69–1.00 inSBA-15. These phenomena can be attributed to the encap-sulation of TPA/ZrO2 inside the channels of SBA-15, andfurthermore, the high loading i.e. higher than 50 wt.%may lead to a partial blocking of the pores. The texturalproperties of 22.4 wt.%ZrO2/SBA-15 with different (%)TPA loadings, 15 wt.%TPA/22.4 wt.%ZrO2/SBA-15 cal-cined at different calcination temperatures and15 wt.%TPA/wt.%ZrO2/SBA-15 with different (%) ZrO2

loadings are presented in Table 1. It is seen that the sur-face area, pore size and pore volume of modified SBA-15decreased as compared with parent SBA-15. As TPA/ZrO2 was well dispersed inside the pores of mesoporouschannels so with increase in TPA and ZrO2 loadingscatalysts shows decrease in surface area, pore volumeand pore diameter. Similarly, in case of 15 wt.%TPA/22.4 wt.%ZrO2/SBA-15 surface area and pore sizedecreases with increase in calcination temperature. The sur-face area of the samples up to 50 wt.% TPA loadings werestill >300 m2 g�1, and pore volume was in the range of0.53–0.63 cm3 g�1, which are sufficient for catalyticfunctions.

FT-Raman spectra of 22.4 wt.%ZrO2/SBA-15 samplewith and without TPA calcined at 1123 K are presentedin Fig. 3. Distinctly, the intensity of the bands around

1009, 992 cm�1 and a weak peak around 905 cm�1assignedto pure TPA [36,37]. As seen in Fig. 3, the bands assignedto TPA are clearly seen for 15 wt.%TPA/22.4 wt.%ZrO2/SBA-15 sample. The broadness and minor shift could bedue to the formation of the species formed by linking theKeggin units to the zirconia support. The Raman bandsof ZrO2 below 700 cm�1 interfere with the tungstate spe-cies, while above 700 cm�1 is free from characteristic bandsof ZrO2.

In order to study the surface stability of TPA/ZrO2 sup-ported over SBA-15 system, we have carried out XPS anal-ysis. The results of XPS analyses for 22.4 wt.%ZrO2/SBA-15 are shown without and with TPA calcined at 1123 K.

Table 2XPS binding energies (eV) and FWHM (eV) values of sample without and with TPA calcined at 1123 K

Sample O 1s Zr 3d5/2 Si 2p W 4f7/2 P 2p

BE FWHM BE FWHM BE FWHM BE FWHM BE FWHM

22.4 wt.%ZrO2/SBA-15 531.2 (521.2) 2.7 183.5 4.7 102.3 2.8 – – – –15 wt.%TPA/22.4 wt.%ZrO2/SBA-15 531.2 (521.1) 3.4 183.5 5.9 102.8 2.9 35.9 (38.0) 3.5 134.0 6.9

D.P. Sawant et al. / Microporous and Mesoporous Materials 102 (2007) 223–233 227

The binding energies and FWHM values of various corelevels (Si 2p, O 1s, Zr 3d5/2, W 4f7/2, P 2p) of this mesopor-ous materials are summarized in Table 2. The spin–orbitsplit doublet of tungsten, W 4f7/2 and W 4f5/2 with fullwidth at half maximum is obtained at 3.5 eV and arelocated at 35.9 and 38.2 eV, respectively. The observedbinding energy value of W (4f7/2) is 35.9 eV, a characteristicof W6+ indicating that it contains W–O bonding [38].Fig. 4A shows the contribution of the O2� 1s at 531.2 eV,which is related to an O–H bonding. This would indicatethat the compound is hydrated. Si 2p binding energy(Fig. 4B) values of silica support i.e. SBA-15 and modifiedSBA-15 with TPA/ZrO2 are remained the same around102–103 eV [39]. The same Si 2p FWHM values for sam-ples 22.4 wt.%ZrO2/SBA-15 i.e. without and with TPAshows that impregnation does not affect the silica supporti.e. SBA-15 as per shown in Table 2. In Fig. 4C,

515 520 525 530 535 540

ba

Cou

nts

(arb

. uni

ts)

Binding Energy (eV)

176 178 180 182 184 186 188 190 192

bCou

nts

(arb

. uni

ts)

Binding Energy (eV)

a

A

C

B

Fig. 4. XPS spectra of (A) O (1s) spectra of (a) 22.4 wt.%ZrO2/SBA-15(a) 22.4 wt.%ZrO2/SBA-15 and (b) 15 wt.%TPA/22.4 wt.%ZrO2/SBA-15, (C)22.4 wt.%ZrO2/SBA-15 and (D) P (2p) spectra of (a) 22.4 wt.%ZrO2/SBA-15

22.4 wt.%ZrO2/SBA-15 catalyst without and with15 wt.% TPA shows similar Zr 3d5/2 binding energy valuesaround 183.5 eV. Table 2 shows the binding energy valuesand FWHM values which shows almost the same FWHMvalues even after TPA loading. The binding energy of theZr 3d5/2 band in the case of modified ZrO2 samples wasfound to be higher than that of the pure ZrO2

(182.5 + 0.1 eV). This shift towards higher side could beattributed to the dispersion of zirconia on the other sup-port oxides and/or the change in the coordination numberof zirconium by the formation of a Zr–O-Support bond asper evidenced in literature [40,41]. The phosphorous signalP5+ 2p at 134.4 eV could be associated with a compoundthat contains P–O bonding as per shown in Fig. 4D. How-ever, high FWHM values are observed due to peak-broad-ening possibility and i.e. due to differential charging of thesample caused by mesoporous silica support.

90 95 100 105 110

b

a

Cou

nts

(arb

. uni

ts)

Binding Energy (eV)

126 128 130 132 134 136 138 140 142 144

Cou

nts

(arb

. uni

ts)

Binding Energy (eV)

D

and (b) 15 wt.%TPA/22.4 wt.%ZrO2/SBA-15, (B) Si (2p) spectra ofZr (3 d5/2) spectra of (a) 22.4 wt.%ZrO2/SBA-15 and (b) 15 wt.%TPA/

and (b) 15 wt.%TPA/22.4 wt.%ZrO2/SBA-15 calcined at 1123 K.

228 D.P. Sawant et al. / Microporous and Mesoporous Materials 102 (2007) 223–233

The amount and the total acidity of the catalysts weredetermined from TPD of ammonia was correlated withcatalytic activities of 22.4 wt.%ZrO2/SBA-15 with different(%) TPA loadings (Fig. 5a), 15 wt.%TPA/22.4 wt.%ZrO2/SBA-15 calcined at different calcination temperatures(Fig. 5b) and 15 wt.%TPA/wt.%ZrO2/SBA-15 with differ-ent (%) ZrO2 loading as per shown in Fig. 5c. It shows thattotal acidity of TPA/22.4 wt.%ZrO2/SBA-15 calcined at1123 K catalyst increases till 15 wt.% and then on furtherincreasing TPA loading it shows decrease so as to catalytic

0 20 40 60 80 100

20

30

40

50

60

70

80

Acetic anhydride

% C

onve

rsio

n ac

etic

anh

ydri

de

(%) loading of TPA900 1000 1100

Acetic anhyd

Calcinatitemperatur

Acidity Acidity

0 20 40 60 80 100 1200

10

20

30

40

50

60

70

80 0 5 15 30 50 70 90

a'

% C

onve

rsio

n ac

etic

anh

ydri

de

Time (min.)0 20 40 60 8

650 750 850 900 950 1000

Time (mi

a

Fig. 5. Effect of TPA loading and acidity on acetic anhydride conversion (a) aAc2O (molar ratio) = 5, catalyst weight = 0.09 g (3 wt.% of total reaction mixanhydride conversion (b) and time on stream study (b 0). Conditions: temper(3 wt.% of total reaction mixture), time = 2 h. Effect of ZrO2 loading and aConditions: temperature = 333 K, veratrole/Ac2O (molar ratio) = 5, catalyst w

activity. It can be concluded as till 15 wt.%TPA, theKeggin unit of heteropoly acid retains its structure andacidity. The decrease in its activity at higher TPA loadingis due to the formation of WO3 crystallites due to decom-position of HPAs [24]. Total acidity data of the catalyst15 wt.%TPA/22.4 wt.%ZrO2/SBA-15 at different calcina-tion temperatures were plotted along with catalytic activ-ity. It shows that till 1123 K, total acidity increases andon further increase in calcination temperature showsdecrease in acidity and also decrease in catalytic activity,

0.20

0.22

0.24

0.26

0.28

0.30

0.32

0.34

0.36

0.38

0.40

0.42

0.44

1200 1300

ride

on e (K)

10 20 30 40 50 60 70

Acetic anhydride

(%) loading of ZrO2

Acidity

Acidity (N

H3 nm

-2)

0 100 120

n.)

b'

0 20 4 0 60 100 120

10 22.4 30 50 70

Time (min.)

c'

80

b c

nd time on stream study (a 0). Conditions: temperature = 333 K, veratrole/ture), time = 2 h. Effect of calcination temperature and acidity on acetic

ature = 333 K, veratrole/Ac2O (molar ratio) = 5, catalyst weight = 0.09 gcidity on acetic anhydride conversion (c) and time on stream study (c 0).

eight = 0.09 g (3 wt.% of total reaction mixture), time = 2 h.

Table 3Activities of different catalysts, recyclability study and leaching study of the catalysts in acetylation of veratrole with acetic anhydride

Catalyst SBET (m2/g) Acidity (mmol g�1) Ac2O (% Conv.)

Comparison of different catalysts

H-b (30)a 540 0.94 53H-Y (13.5)a 530 2.25 63H-Mordenite (20)a 490 0.72 12H-ZSM-5 (60)a 364 0.82 9Zn(II)-exchanged claya – 0.67 2022.4 wt.%ZrO2/SBA-15 426 0.24 3715 wt.%TPA/22.4 wt.%ZrO2/SBA-15 372 0.42 7322.4 wt.%ZrO2/MCM-41 589.8 0.24 915 wt.%TPA/22.4 wt.%ZrO2/MCM-41 516.1 0.33 2522.4 wt.%ZrO2/MCM-48 612.3 0.10 615 wt.%TPA/22.4 wt.%ZrO2/MCM-48 540.5 0.25 1515 wt.%TPA/ZrO2 11 0.02 113.36 wt.%TPA/SBA-15 341 0.29 30

Recyclability of the catalyst

15 wt.%TPA/22.4 wt.%ZrO2/SBA-15 (I) ne ne 7315 wt.%TPA/22.4 wt.%ZrO2/SBA-15 (II) ne ne 72

Leaching study

Filtrateb – – 60

ne = not evaluated.a Except these entries all other catalysts were calcined at 1123 K in air. Conditions: temperature = 333 K, veratrole/Ac2O molar ratio = 5, catalyst

wt. = 0.09 g (3 wt.% of total reaction mixture), time = 2 h.b After 1 h the catalyst was removed and reaction was carried out for further 1 h with the filtrate.

D.P. Sawant et al. / Microporous and Mesoporous Materials 102 (2007) 223–233 229

which is due to decomposition of TPA into WO3 crystal-lites. Similarly, with increasing ZrO2 loading till22.4 wt.% catalyst 15 wt.%TPA/ZrO2/SBA-15 calcined at1123 K shows increase in acidity and thereby catalyticactivity. Among all the catalysts given in Table 3,15 wt.%TPA/22.4 wt.%ZrO2/SBA-15 calcined at 1123 Kshows the highest acidity corresponds to monolayer cover-age of TPA, where the Keggin-like structure is intact, assupported by XRD, 31P CP-MAS NMR [24], FT-Ramanand XPS. These results indicate that TPA/ZrO2 dispersedmore uniformly over large surface area and large pore vol-ume SBA-15 as compared with other supports.

3.2. Acetylation of veratrole

Acetylation of veratrole with acetic anhydride catalyzedby 15 wt.%TPA/22.4 wt.%ZrO2/SBA-15 under selectedreaction conditions gave acetoveratrone (3 0,4 0-dimethoxy-acetophenone) via electrophilic substitution of H3CCO+

to aromatic ring as shown in the Scheme 1.The catalytic activities of different catalysts such as H-

beta, H-Y, H-Mordenite, H-ZSM-5 and Zn(II)-exchangedclay catalysts along with 22.4 wt.%ZrO2/MCM-41 (with

CO

CO 33

3

3333 K+ (CH CO)

H

H H2O

Scheme

and without 15 wt.% TPA), 22.4 wt.%ZrO2/MCM-48 (withand without 15 wt.% TPA) catalysts were evaluated in theacetylation of veratrole with acetic anhydride under identi-cal reaction conditions for comparison and the results arepresented in Table 3. It is seen from the results that neat15 wt.%TPA/ZrO2 gave the lowest conversion, which isdue to its low surface area. It is also observed that3.36 wt.%TPA/SBA-15 calcined at 1123 K shows very lesscatalytic activity in the reaction, since it has very less acid-ity required for acid catalyzed reactions. Among the zeolitecatalysts, H-Y is more active than others due to its highestacidity with large pores gave higher conversion of Ac2Ofollowed by H-beta (strong and medium acid sites). Simi-larly, lower activity of H-ZSM-5 might be explained onthe basis of its smaller pore size compared with the largersize of the products attributed to diffusional limitations.While, H-Mordenites has one-dimensional pore with ellip-tical channels, 6.5 · 7.0 also has the low activity. In thepresent catalyst system, zirconia alone loaded on mesopor-ous silica support i.e. 22.4 wt.%ZrO2/SBA-15 calcined at1123 K shows some catalytic activity, while after load-ing 15 wt.% TPA over it shows two-fold increase in cata-lytic activity (Table 3). So here is the specific role of the

CO

CO 3 3

3

+ CH COOH

H

c

O

CH

A

1.

230 D.P. Sawant et al. / Microporous and Mesoporous Materials 102 (2007) 223–233

composite materials and the effect of zirconia on the acidityfunction of TPA, which enhances the catalytic activity.Therefore, we have chosen 15 wt.%TPA/22.4 wt.%ZrO2/SBA-15 calcined at 1123 K as the catalyst for further inves-tigations on its catalytic performance in the veratroleacetylation with Ac2O.

It is seen that conversion of Ac2O increases with increasein TPA loading up to 15 wt.% and on further increase in

0 4

30

40

50

60

70

80

90

Acetic anhydride

% C

onve

rsio

n ac

etic

anh

ydri

de

Catalyst (wt.%)

0 4

Acetic

Mola(Verat

0 20 40 60 80 100 1200

10

20

30

40

50

60

70

80

90 1wt% 2wt% 3wt% 4wt% 5wt%

a'

% C

onve

rsio

n ac

etic

anh

ydri

de

Time (min.)

1:1 3:1 5:1 7:1 9:1

Time

a

0 20 40 6

2 6 2

Fig. 6. Effect of catalyst weight on acetic anhydride conversion (a) and time(molar ratio) = 5, time = 2 h. Effect of molar ratio on acetic anhydride convercatalyst weight = 0.09 g (3 wt.% of total reaction mixture), time = 2 h. Effect o(c 0). Conditions: veratrole/Ac2O (molar ratio) = 5, catalyst weight = 0.09 g (3

loading of TPA there is decrease in acetic anhydride con-version Fig. 5a and a 0. The results on the effect of calcina-tion temperature on 15 wt.%TPA/22.4 wt.%ZrO2/SBA-15catalyst in veratrole acetylation are shown in Fig. 5b andb 0. There is an increase in Ac2O conversion up to 1123 Kcalcination temperature but on further increase, Ac2O con-version decreased. This observation is supported by thefact that 15 wt.%TPA/22.4 wt.%ZrO2/SBA-15 calcined at

8 0

anhydride

r ratio role:AA)

310 320 330 340 350 360

Acetic anhydride

Temperature(K )

b'

Time (min.) (min.)

313K 323K 333K 343K 353K

c'

b c

0 80 100 120 0 20 40 60 80 100 120

6 1

on stream study (a 0). Conditions: temperature = 333 K, veratrole/Ac2Osion (b) and time on stream study (b 0). Conditions: temperature = 333 K,f temperature on acetic anhydride conversion (c) and time on stream studywt.% of total reaction mixture), time = 2 h.

D.P. Sawant et al. / Microporous and Mesoporous Materials 102 (2007) 223–233 231

1123 K has the maximum concentration of acid sites, whereTPA forms a monolayer coverage over the support. Hence,catalyst calcined at 1123 K was chosen for further study.As shown in Fig. 5c and c 0, which showed that there isan increase in Ac2O conversion up to 22.4 wt.% ZrO2 load-ing and on further increase in loading Ac2O conversiondecreased.

3.3. Effect of reaction parameters on the conversion of acetic

anhydride in acetylation of veratrole

The effect of catalysts concentration shows that withincrease in catalyst concentration from 1 to 5 wt.% showsincrease in Ac2O conversion from 28.8% to 82.1% with100% selectivity for acetoveratrone as a monoacetylatedproduct (Fig. 6a and a 0).

Similarly, the effect of veratrole to Ac2O molar ratio onthe conversion of Ac2O and time on stream study for differ-ent molar ratios are presented in Fig. 6b and b 0. The moleratio of veratrole to Ac2O was varied from 1 to 9, the con-version of Ac2O increased from 36% to 82% with 100%selectivity for a monoacetylated product.

The acetylation of veratrole with Ac2O was carried outin the temperature range 313–353 K, to know the effectof temperature on the conversion of Ac2O. It is seen fromFig. 6c and c 0 that the conversion of Ac2O increases from

Table 4Acetylation of aromatic substrates with acetic anhydride in presence of 15 wt.

Substrate Mole ratio Temp. (�C)

1 Veratrole 5:1 602 Anisole 10:1 903 Thioanisole 10:1 903 Isobutyl benzene 10:1 1004 2-Methoxy naphthalene (Nitrobenzene)a 2:1 1005 2-Methoxy naphthalene (Chlorobenzene)a 2:1 1006 2-Methoxy naphthalene (1,2-DCB)a 2:1 1007 2-Methoxy naphthalene (Benzene)a 2:1 100

a Solvent amount = 20 ml.b Product distribution (%), A and B are from reaction Schemes 1–4, respect

+363 K

XCH

(CH CO) O

3

3 2

where, X = O, S

Scheme

46% to 83% with increase in temperature from 313 to353 K. The maximum conversion of Ac2O (73%) wasobtained at 333 K, at veratrole:Ac2O molar ratio 5 with3 wt.% of catalyst (total reaction mixture). At one particu-lar temperature, first-order rate constants were calculatedfor 1 h of reaction time, and then the constant values of‘k1’ showed that the acetylation of veratrole is a first-orderreaction with respect to Ac2O. The standard equations fora first order series reaction CA=CA0 ¼ e�k1t has been usedfor the determination of rate constant, where CA0 and CA

are the concentration of Ac2O at initial time and at timet, respectively. Activation energy evaluated from Arrheniusgraph was found to be 5.79 kcal mol�1.

3.4. Acetylation of anisole, thioanisole, isobutyl benzene and

2-methoxynaphthalene

The acetylation of all the above substrates with aceticanhydride were conducted using the best 15 wt.%TPA/22.4 wt.%ZrO2/SBA-15 calcined at 1123 K at selected men-tioned reaction conditions and the results are presented inTable 4. Anisole gave 98% p-methoxyacetophenone (a) andremaining is o-methoxyacetophenone (b) (Scheme 2) with61% of Ac2O conversion. While acetylation of thioanisolegave 99.6% p-(methylthio acetophenone) (a) and remainingis o-(methylthio acetophenone) (b) (Scheme 2) isomer with

%TPA/22.4 wt.%ZrO2 calcined at 1123 K

Catalyst wt.% Time (h) % Ac2O Convn. Distribution (%)b

A B

0.09 2 73 100 –0.15 3 61 98 20.15 3 36 99.6 0.40.15 5 30 100 –0.15 8 61 97 30.15 8 50 96 40.15 8 57 97 30.15 8 39 98 2

ively.

+

A B

CH COOH

3

3

3

O

OXCH3 XCH3

C

C

CH

CH

+

2.

+373 K

3

3

3 2

O

CH COOH(CH CO) O

CH

+

C

A

Scheme 3.

232 D.P. Sawant et al. / Microporous and Mesoporous Materials 102 (2007) 223–233

36% of Ac2O conversion. Isobutyl benzene gave 4-isobutylacetophenone (a) as the only product (Scheme 3) at 30%Ac2O conversion.

The data on the acetylation of 2-methoxynaphthalenewith Ac2O under different solvents are presented in Table4. Solvent effect on the conversion is visible, affordinghigher conversion in nitrobenzene. However, the selectivityfor 1-acyl-2-methoxynaphthalene (a) was close to 97% andthe remaining 3% of 6-acyl-2-methoxynaphthalene (b)(Scheme 4) irrespective of the solvents employed, which isin contrast to the classical homogeneous acylation reac-tion. Acylation occurs at 1-position predominantly in car-bon disulphide, and at 6-position in nitrobenzene.Isomeric ratio varies with different transition metal chlo-rides in stoichiometric amounts correlated to their acidstrength [42]. Insufficient acid strength in the present cata-lytic reaction due to the use of small amounts of solid acidsprecludes formation of other isomers accentuated by proti-odeacetylation at moderate temperatures instead favoursexclusive formation of 1-acetyl derivative. The acylationof 2-methoxynaphthalene (2-MN) has been extensivelystudied as a test reaction for zeolites, USY, and ZSM-12[4] and has recently been used with MCM-41 catalysts[43]. Acetylation with acetic anhydride occurs at the 1-posi-tion yielding the kinetically favored 1-acetyl-2-methoxy-naphthalene (1-Ac-2-MN). Although Friedel–Craftsacylation is usually irreversible, the main product 1-Ac-2-MN can undergo protiodeacylation [44] and reacylationthen yields the thermodynamically more stable 2-acetyl-6-methoxynaphthalene (2-Ac-6-MN) which is also producedby migration of the acetyl group [45].

OCH

+373 K

3

3 2

O

(CH CO ) O

C

A

Scheme

3.5. Leaching and recyclability of the catalyst

To check the leaching of TPA into veratrole acetylationreaction medium, the reaction was carried out for 1 h byusing 15 wt.%TPA/22.4 wt.%ZrO2/SBA-15 catalyst cal-cined at 1123 K, under selected reaction conditions. Thereaction was stopped, the catalyst was separated by filtra-tion from hot reaction mixture and then the filtrate wasstirred for 1 h under the same reaction conditions. It wasfound that in the absence of the catalyst, there was no fur-ther increase in the conversion of Ac2O (Table 3), indicat-ing that there is no any leaching of TPA into the reactionmixture. In addition, leaching of TPA into the hot filtratewas tested by inductively coupled plasma-optical emissionspectroscopy, which found the complete absence of P or W.

For reusing the catalyst, the catalyst (15 wt.%TPA/22.4 wt.%ZrO2/SBA-15 catalyst calcined at 1123 K) usedin the first cycle of the reaction was separated by filtration,washed three times with 1,2-dichloromethane, dried in anoven at 373 K for 24 h, and activated at 773 K for 4 h inan air. Then this activated catalyst was used for acetylationof veratrole with Ac2O and this same procedure wasrepeated for the second cycle; the data on the conversionof Ac2O gives almost same acetic anhydride conversion(Table 3). From these results, we can conclude that therewas no appreciable loss in catalytic activity and productselectivity in the two cycles, and that the catalyst can bereused.

4. Conclusions

The results presented in this work demonstrate thatTPA/ZrO2 dispersed uniformly inside the mesopores ofSBA-15 formed nano-sized (3–4 nm) composite materialat a calcination temperature of 1123 K. TPA (15 wt.%)/ZrO2 (22.4 wt.%)/SBA-15 catalyst calcined at 1123 K wasfound to be the most active catalyst in all the acetylationreactions studies under selected liquid phase conditionsunder N2 atmosphere and shows greater catalytic activity(6–7 fold higher) than with the corresponding neat TPA/ZrO2 catalyst. The performance of the supported HPAswas strongly influenced on catalyst pretreatment condi-tions such as effect of loadings of TPA, zirconia and calci-nation temperatures and most importantly on the nature

33

3

O

CH COOH

OCH

+

3 OCH

+

3

CCH

CH

B

4.

D.P. Sawant et al. / Microporous and Mesoporous Materials 102 (2007) 223–233 233

and the characteristics of the mesoporous support and thecatalyst was eco-friendly and cost effective.

Acknowledgment

This work was supported financially by DST, New Del-hi. One of the authors (Dhanashri P. Sawant) acknowl-edges CSIR New Delhi for research fellowship.

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