Fuel Processing Technology 91 (2010) 1288–1295

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Fuel Processing Technology

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Synthesis and characterization of fly ash supported sulfated zirconia catalyst forbenzylation reactions

Chitralekha Khatri a, Manish K. Mishra b, Ashu Rani a,⁎a Department of Pure and Applied Chemistry, University of Kota-324005, Rajasthan, Indiab Department of Chemical Engineering, Faculty of Technology & Shah-Schulman Center for Surface Science and Nanotechnology, Dharmsinh Desai University,Nadiad-387001, Gujarat, India

⁎ Correspondingauthor. Postal address: 2-m-1,RangbariIndia. Tel.: +91 9352619059.

E-mail addresses: chitra_cool81@yahoo.com (C. Khatri

0378-3820/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.fuproc.2010.04.011

a b s t r a c t

a r t i c l e i n f o

Article history:Received 12 October 2009Received in revised form 12 February 2010Accepted 23 April 2010

Keywords:Fly ashSulfated zirconiaSolid acid catalystBenzylation

Synthesis of highly active nano-crystalline, thermally stabilized solid acid catalyst has been reported byloading different weight fractions of sulfated zirconia on chemically activated fly ash through two step sol–gel technique. The catalysts were characterized using powder XRD, FT-IR, N2-adsorption desorption study,CHNS elemental analysis, SEM-EDAX and their acidity were measured by pyridine adsorbed FTIR. Liquidphase benzylation of benzene and toluene with benzyl chloride was studied as test reaction for catalyticactivity of SZF catalysts. A very high conversion of benzene (87%) and toluene (93%) were observed, which isattributed to significant amount of acid site on the catalyst surface. The FTIR study of the pyridine adsorbedsamples reflects the presence of Brønsted as well as Lewis acid sites. The catalyst with 12 wt.% zirconia (SZF-12) was regenerated and reused up to four reaction cycles with equal efficiency as in the first run.

scheme,Kota-324005, Rajasthan,

), ashu.uok@gmail.com (A. Rani).

ll rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Fly ash a by-product recovered from coal based electric power plants[1], is the finest coal combustion residue (0.2–90 μm) with a specificsurface area, typically between 250 and 600 m2/kg [1]. Chemically flyash is an ensemble of compounds such as silica, alumina, ferric oxide,calcium oxide and other metal oxides such as Mn2O3 and TiO2 [2] withinert crystallised phases such as mullite, quartz and magnetite [3].Porous spherical aluminosilicate particles, which are referred to as ashmicrospheres, aluminosilicatemicrospheres, or cenospheres, are amongthe most valuable fly ash components [4]. Main applications of fly ashare in the field of cement and other construction materials [5], agri-culture, metal recovery, water and atmospheric pollution control [6]etc. Nevertheless important advances in the applications of fly ashare its catalytic role in oxidation [7], chlorination [8], synthesis of 2-mercaptobenzothiazole derivatives under microwave irradiation [9]and as solid acid catalyst for esterification reactions, for the synthesis ofaspirin and oil of wintergreen [10].

Zirconia, a versatile material besides being used in structuralceramics and electronic ceramic products [11] is well reported as solidacid due to high acidity induced by sulfation [12]. Several industriallyimportant reactions such as isomerization, Friedel Crafts acylation,alkylation, esterification [13] etc. were catalyzed by sulfated zirconiadue to its high acidity. Silica supported sulfated zirconia were

synthesized by grafting sulfated zirconia on the surface of a silicaaerogel to generate relatively strong Brønsted and Lewis acid sites onthe surface to catalyze n-hexane isomerization reactions [14]. Asreceived fly ash (silica 58%) was chemically activated to increase silicaup to 80% andwas thought to be used for supporting sulfated zirconia.The use of fly ash as support not only reduced the cost of the catalyst italso showed high acidity and catalytic activity for benzylation ofbenzene and toluene. The product diphenylmethane and its deriva-tives are widely used as pharmaceuticals, petrochemicals, cosmetics,dyes, fine chemicals, insulators etc [15]. Previously, several heteroge-neous catalysts such as Fe containing mesoporous silicate catalysts[16], clay (K-10, montmorillonite), H-beta, MFI structured titanosili-cate (TS-1) and heteropoly acids [HPA, namely dodeca-tungstopho-sphoric acid [H3PO4–12WO3·xH2O] supported on clay, H-beta and TS-1 [17], sulfated zirconia [18] and metal modified zeolites [19] wereused effectively for Friedel-Crafts alkylations. The synthesized SZFcatalysts gives a solution to overcome the use of harmful liquid acidsand other costly commercial heterogenous catalysts for industriallyimportant benzylation reaction.

2. Experimental

2.1. Materials

Fly ash [Class F type (SiO2+Al2O3) N70%] produced from com-bustion of bituminous coal, collected from Kota Thermal Power Plant(Rajasthan, India) was used as solid support after acid activation.The components of fly ash are SiO2 (54%), Al2O3 (21%), Fe2O3 (9%),CaO (1.6%), MgO (0.8%), TiO2 (1.3%), Na2O (4.8%), K2O (3.2%) and

Scheme 1. Benzylation of benzene and toluene with benzyl chloride over SZF-12catalyst.

Table 1Characterization of fly ash samples before and after activation and SZF catalysts.

Catalyst Silica(wt.%)

Crystallite size(nm)

Sulfur(wt.%)

BET surface area(m2/g)

Pore size(A)

FA 54 33 0.3 8 29CFA 80 12 1.0 10 26SZF-6 80 11 2.4 7 37SZF-9 80 9 3.6 6 43SZF-12 80 7 5.6 4 51

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trace elements (4.0%). The L.O.I (loss on ignition) was 3 wt.% at 900 °Cfor 4 h. Zirconium propoxide (70 wt.%) was procured from SigmaAldrich, while concentratedH2SO4 (98%), n-Propanol (99.99%), benzene(99.8%), toluene(99.8%) andbenzyl chloride (99%)werepurchased froms. d. fine Chem. Ltd., India and were used as such.

2.2. Catalyst preparation

The fly ash-sulfated zirconia catalysts (SZF-6, SZF-9 and SZF-12)were prepared by loading zirconia 6, 9 and 12 wt.% on activated fly ashby two step sol–gel technique.

The activation of fly ash was performed in a stirred reactor takingfly ash and concentrated H2SO4 acid in the ratio of 1:2 followed bydrying at 110 °C and calcination at 450 °C for 4 h similar to our pre-vious work [10]. Zirconium propoxide (1.08 g for 6 wt.%, 1.62 g for9 wt.% and 2.16 g for 12 wt.%) diluted with 15 ml n-propanol wastaken in glass beaker. The 5 g activated fly ash preheated at 400 °Cfor 4 h was added into it under constant stirring. Water was addeddrop wise in this stirred mixture (water: zirconium propoxide molarratio=4:1) for hydrolysis and polycondensation. The gel thus formedwas aged for 24 h followed by drying at 110 °C for 24 h to removethe solvent from the pores of the gel. In the second step, the driedgel was powdered and sulfated with concentrated H2SO4 solution(15 ml 1 N H2SO4 solution/1 gm powdered dried gel) under stirringfor 1 h. The sulfated gel was filtered and dried at 110 °C for 24 hfollowed by calcination at 550 °C for 2 h in a muffle furnace understatic condition.

2.3. Catalyst characterization

X-ray diffraction pattern were recorded on diffractometer (PhilipsX'pert) using CuKα radiation (λ=1.54056 Å), angle range was be-tween 0 and 80° at a scanning rate of 0.04°s−1. Crystallite size ofthe crystalline phase was determined from the peak of maximumintensity (2θ=26.57) by using Scherrer formula [20] as Eq. (1) witha shape factor (K) of 0.9.

Crystallite size = K:λ = W:cosθ ð1Þ

where, W=Wb−Ws; Wb is the broadened profile width of exper-imental sample and Ws is the standard profile width of referencesilicon sample.

The FT-IR study of the samples was done by FT-IR spectro-photometer (IRPrestige-21, Shimadzu) in DRS (Diffuse ReflectanceSpectroscopy) system by mixing the sample with KBr in 1:20 weightratio. The Brønsted and Lewis acidity of the catalysts were measuredby pyridine adsorbed FT-IR. The sample (0.2 g) was activated at 450 °Cfor 2 h and was exposed to pyridine (25 ml) for 24 h. The spectra ofthe samples were recorded in the range of 400–4000 cm−1 at roomtemperature. Specific surface area, pore volume and pore size in thesamples were determined from N2 adsorption–desorption isothermsat 77 K by NOVA 1000e Surface Area and Pore Size Analyzer using BETand BJH approaches [21]. The sample was degassed under vacuumat 120 °C for 4 h, prior to adsorption measurement to evacuate thephysisorbed moisture and the isotherm was recorded with 20 pointsadsorption and desorption. The sulfur content was analyzed usingCHNS/O elemental analyzer (Perkin-Elmer, 2400). The detailedimaging information about the morphology and surface texture ofthe catalyst was provided by SEM (Philips XL30 ESEM TMP).

2.4. Catalytic activity of (SZF) catalysts

The benzylation of benzene and toluene with benzyl chloride insolvent free liquid phase reactionwas carried by SZF catalyst as shownin Scheme 1.

The benzlyation was performed in liquid phase batch reactorconsisting of 25 ml round bottom flask with condenser in a constanttemperature oil bath with magnetic stirring. A mixture of benzene(0.78 g, 20 mmol) or toluene (0.94 g, 10 mmol) and benzyl chloride(1.1 g, 10 mmol) was taken in round bottom flask. The catalyst(substrate to catalyst ratio=10), activated at 450 °C for 2 h wasadded in the reaction mixture. The reaction mixture was kept on hotoil at different temperatures ranging from 60 to 120 °C and time from30min to 8 h. The reactions were carried out at differentmolar ratio ofsubstrate (benzene or toluene) and benzyl chloride and at differenttemperatures in the range of 60 to 120 °C for time ranging from30 min to 24 h. The kinetics of benzylation reaction was studied usingSZF-12 as it showed highest catalytic activity. After completion ofthe reaction the catalyst was filtered and the reaction mixture wasanalyzed by Gas Chromatography (Dani Master GC) having a flameionization detector and HP-5 capillary column of 30 m length and0.25 mm diameter, programmed oven temperature of 50–280 °C andN2 (1.5 ml/min) as a carrier gas. The conversion of benzene or toluenewas calculated by using weight percent method [10].

2.5. Catalyst regeneration

The used catalyst (SZF-12) was washed with acetone dried in ovenat 110 °C for 12 h followed by activation at 450 °C for 2 h and reusedfor next reaction cycle under similar reaction conditions as earlier.

3. Results and discussion

3.1. Catalyst characterization

The physiochemical properties of the fly ash before and afterchemical activation and after loading Sulfated-Zirconia are summa-rized in Table 1. The silica content in chemically activated fly ash(CFA) is greatly increased from 54 to 80%. The silica content of asreceived fly ash was analyzed by Atomic Absorption Spectrophotom-eter while increased silica after chemical activation is evident fromSEM-EDAX spectra in Fig. 1. The increase in silica content afterchemical activation is due to the loss of significant amount of othercomponents during the H2SO4 treatment.

Fig. 1. SEM-EDAX spectra of activated fly ash (CFA).

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X-ray diffraction pattern of activated fly ash (CFA) in Fig. 2 (a)shows increased amorphous content with crystallite size 12 nm [10].The XRDpatterns of the catalysts SZF-6, SZF-9 and SZF-12 as illustratedin Fig. 2 (b) shows a broad peak at around 22°(2θ) indicating thepresence of amorphous form of zirconia which is formed due to theincorporation of zirconium atoms in the framework of amorphousSiO2 of fly ash [22,23] to form zircon (ZrSiO4) as the major component[11]. The peaks at 50°, 60° and a little sharp peak at 78° 2θ showed thepresence of tetragonal phase of zirconia [23], which increases withincrease in ZrO2 content from 6 to 12 wt.%. while a broad peak at 22°

Fig. 2. (a) XRD of activated fly ash (CFA). (b) XRD of S

(2θ) represents monoclinic phase in all the catalysts. The crystallitesize of SZF-6 containing 6 wt.% zirconia is 11 nm, which decreases to 9and 7 nm respectively for SZF-9 and SZF-12 with increasing zirconiacontent from 9 to 12 wt.% as presented in Table 1. The absence oflarge sharp lines in ZrO2 rich SZF-12 indicates that zirconia crystallitesin this catalyst are either too small to be detected by XRD or they aredispersed through the fly ash network in small amount.

The FT-IR spectrum of fly ash, before and after chemical activationFig. 3, shows a broad band between 3400 and 3000 cm−1, which isattributed to surface –OH groups of –Si–OH and adsorbed water

ZF-6, SZF-9 and SZF-12 after calcination at 550 °C.

Fig. 3. FTIR spectra of fly ash before and after chemical activation.

Fig. 4. (a). FTIR spectra of fly ash supported sulfated-zirconia samples (i) SZFA-6,(ii) SZFA-9 and (iii) SZFA-12. (b) FT-IR spectra of SZF catalysts (i) SZF-6; (ii) SZF-9; and(iii) SZF-12 (magnified). (c) FTIR spectra of pyridine adsorbed fly ash supportedsulfated-zirconia samples (i) SZF-6, (ii) SZF-9 and (iii) SZF-12.

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molecules on the surface. The broadness of the band is due to thestrong hydrogen bonding [10]. The hydroxyl groups do not existsin isolation and a high degree of association is experienced as a resultof extensive hydrogen bonding with other hydroxyl groups. A peakat 1650 cm−1 in the spectra of both the samples is attributed tobending mode (δO–H) of water molecule. The FT-IR spectrum of SZFcatalysts is shown in Fig. 4 (a) shows –OH peaks characteristics ofSiO2 of fly ash and incorporated ZrO2. The SZF catalysts show shiftingof band at 1095 cm−1 towards lower wave number (954 cm−1) withhighest peak intensity for SZF-12 due to the formation of Zr–O–Silinkages [24,25]. However, the ZrO2was also observed to be present asdispersed clusters on SiO2 of fly ash, as is evident from the char-acteristic peak of zirconia at 440 and 525 cm−1 [26]. The FT-IR spectraof SZF catalysts in Fig. 4 (b) shows the bands for sulfate group be-tween 1400 and 1040 cm−1 (1400, 1156, 1040 and 1000 cm−1), forasymmetric and symmetric stretching vibrations of S=O double bond,partially ionized S=O double bond and S–O single bonds [27]. Thebands at ∼1040 is attributed to the asymmetric stretching mode ofvibrations of S–O bonds. The S=O bond is characterized by the band at∼1400 cm−1, which is attributed to asymmetric stretching vibrationof S=O bond. However, in hydrated SZF catalyst, the sulfate is bondedwith water molecule by hydrogen bonding and therefore, S=O bondis partially ionized and does not show the band at ∼1400 cm−1

which is similar as observed in sulfated zirconia [27]. The intensity ofthese bands is very high in SZF-12 which is due to increased zirconia(12 wt.%) and sulfur (5.6 wt.%) content.

The FT-IR spectra, obtained after pyridine adsorption on SZF-6,SZF-9 and SZF-12 catalysts, are presented in Fig. 4 (c), which showsbands in range of 1400–650 cm−1 for adsorbed pyridine at Brønstedand Lewis sites. Two diagnostic peaks characteristic of Brønsted andLewis sites were observed at 1554 cm−1 and 1450 cm−1 respectively.The spectra of pyridine adsorbed on SZF-12 showed an intense peakat 1554 and 1445 cm−1 showing the presence of higher amount ofBrønsted and Lewis acidity in SZF-12 as compared to SZF-6 and SZF-9which is attributed to increase in zirconia content from 6 to 12 wt.%,which enhances the loading of sulfur responsible for increase inconcentration of Brønsted and Lewis acid sites [28].

The morphology of the activated fly ash and synthesized SZFcatalysts is shown in Figs. 5–8. The SEM photograph of pure fly ash isobservedwith hollow cenospheres and irregularly shaped amorphousparticles, agglomerated particles minerals and mineral aggregates.After chemical activation of fly ash shown in Fig. 5 the agglomeratedparticles are scattered conferring the increase in surface area, irreg-ularly shaped amorphous particles are decreased and mineral clustersare increased. With the addition of zirconia, small particles get ag-glomerated to form large particles of size up to 20 μm and more. Suchparticles have different shapes ranging from spherical to irregular.

An increase in zirconia loading from 6 to 12 wt.% is seen to increasein agglomeration thus decreasing the surface area as evident fromFigs. 7–9. The addition of zirconia is reported to increase density of flyash [29], which is also reflected from SEM micrograph.

The surface area of as received and activatedfly ashwere calculatedto be 8 m2/g from respectively as given in Table 1. The chemicalactivation of fly ash results to higher surface area which increasesthe probability of ZrO2 loading. Isotherms shown in Fig. 9 are of typeIV, which are characteristics of mesoporous materials showing thecatalysts are mesoporous. The presence of large mesopores is alsoconfirmed by their low surface area (4–7 m2/g) and higher pore size

Fig. 5. SEM micrograph of activated fly ash (CFA).

Fig. 8. SEM micrograph of SZF-12 catalyst.

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(37–51 cm3/g). The inflection point in the N2 adsorption isotherms inthe samples is not sharp, indicating that the pores are not of uniformsize and have broad distribution. The isotherms possess a broad hys-teresis of type H3, reflecting the presence of large mesopores. A pro-

Fig. 6. SEM micrograph of SZF-6 catalyst.

Fig. 7. SEM micrograph of SZF-9 catalyst.

gressive decrease in surface area and decrease in pore diameter isobserved with increasing zirconia and sulfur content in SZF catalyst.The decrease of BET surface area was related to the blockage of thesmallest pores induced by sulfation. It seems that increase in theamount of zirconia and sulfate groups contribute to the blockage ofmesopores of smallest aperture making them inaccessible of the N2

molecule [30]. This phenomenon is accompanied by the decreasein BET surface area and pore volume. Almost similar evaluation of theBET surface area and pore volumewas also observed earlier for Sulfur/ZrN0.6 in silica supported zirconia catalyst and this behavior wasproduced due to gelation mechanism leading to solids with enhancedpore diameter and reduced small mesopores [30].

The increase in sulfur in activated fly ash from 0.3 to 1.0wt.% isdue to use of sulfuric acid for chemical activation. After zirconiaincorporation and sulfation the total sulfur content in the catalystsincreases from 2 to 5.6% Table 1. The sulfur in the SZF catalyst isbonded with zirconia as covalent and ionic bonds are responsible forthe increased acidity in the catalysts. There was no significant acidityin activated fly ash (CFA) but after loading zirconia acidity increasesdue to enhanced sulfation.

3.2. Proposed model structure for SZF catalyst

The proposed model structure of SZF is shown in Scheme 2 (a) and(b). The zirconia clusters are supported on fly ash surface by Si–O–Zrlinkages and sulfate species are bonded with zirconia surface. Thesulfate species are bondedwith zirconium atom as bidentate chelating

Fig. 9. N2 adsorption desorption isotherm of SZF-6, SZF-9 and SZF-12 catalysts.

Scheme 2. Proposed model structure of sulfated zirconia-fly ash catalyst (SO42−/ZrO2-

Fly ash). (a) Generation of Brønsted and Lewis acid sites due to SO42−/ZrO2–SiO2 group.

(b) Generation of Brønsted due to Si–Zr–OH group.

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ligand [27,31]. The catalyst exhibit both types of acid sites, Brønstedas well as the Lewis sites, on its surface, as confirmed by Pyridine FT-IRspectrum. The sulfate group in association of watermolecules behaveslike ionic sulfate and generates Brønsted acid sites on the surface. Thedesorption of water molecule can convert the ionic sulfate to cova-lently linked sulfate with zirconium atom and generate Lewis acidsites [27,31]. However in our reaction system interconversion ofLewis sites to Brønsted sites may occur which is represented inScheme 2. Other acid sites on the surface are due to Si–OH or Si–O–Zr–OH groups.

3.3. Catalytic activity of SZF catalysts for benzylation reactions

The results of catalytic activity of fly ash-sulfated zirconia catalystsfor solvent free benzylationof benzeneand toluenewith benzyl chlorideare given in Table 2. The catalyst SZF-12 show highest catalytic activitygiving maximum conversion of benzene (87%) and toluene (93%) with

Table 2Catalytic activity of fly ash (FA), chemically activated fly ash (CFA) and sulfatedzirconia-fly ash catalysts (SZF-6, SZF-9 and SZF-12) for benzylation of benzene andtoluene with benzyl chloride.

Catalyst Conversion (%)of Benzenea

Selectivity (%) ofDiphenylmethanea

Conversion (%)of tolueneb

Selectivity (%)p-benzyl tolueneb

FA Nil Nil Nil NilCFA Nil Nil Nil NilSZF-6 47 100 56 99SZF-9 72 100 79 99SZF-12 87 100 93 99

Reaction condition: a Temperature=80 °C; Time=4 h; Molar ratio (benzene/benzylchloride) 1:1; substrate/catalyst ratio=10. b Temperature=110 °C; Time=4 h; Molarratio (toluene/benzyl chloride) 1:1; substrate/catalyst ratio=10.

∼100% selectivity of diphenylmethane and p-benzyl toluene respec-tively. For SZF-6 and SZF-9, conversions of benzene and toluene are 47%and72% respectivelywith∼100% selectivity of diphenylmethane aswellas p-benzyl toluene respectively. The catalytic activity was observed tobe varying with increase in zirconia and sulfur content in the catalyst.The higher catalytic activity of the sample SZF-12 is attributed to higherzirconia (12 wt.%) and sulfur content (5.6 wt.%) in the sample, whichgenerates higher Lewis and Brønsted acid sites.

The catalysts SZF-6, SZF-9 and SZF-12 exhibit lower surface area(7, 6 and 4 m2/g respectively) but they showed higher catalyticactivity for benzylation of benzene and toluene. It shows that surfacearea alone is not responsible for generating catalytic activity over SZFcatalyst. The amount of zirconia and sulfur is found to be responsiblefor the catalytic activity of the catalysts. As benzylation reactions areBrønsted acid catalyzed reactions the catalyst SZF-12 showmaximumconversion and selectivity for both the substrates which correspondsto increased Brønsted acid sites in the reaction system. In the light ofthe above inference, the reaction conditions such as reaction tem-perature, reaction time and molar ratio of reactants for benzylation ofbenzene and toluene with benzyl chloride were studied using onlySZF-12 catalyst.

3.3.1. Effect of temperatureThe effect of reaction temperature was studied at temperatures

ranging from60 to 90 °C for 4 hwhereas that of toluene at temperaturefrom90 to 120 °C for sameperiod of time. The conversion of benzene is68% at 60 °C which increased to 87% at 80 °C for a reaction time of 4 hwith 100% selectivity for diphenylmethane (DPM). Similarly, con-version of toluene was maximum 93% at 110 °C with 99% selectivityof p-benzyl toluene, selectivity was decreased with further increasein temperature which is assigned to the formation of other di or tribenzylated products of benzene and toluene at higher temperatures.

3.3.2. Effect of reaction timeThe optimization of reaction time required to achieve maximum

conversion was carried out at 80 and 110 °C for different time in-tervals ranging from 30 min to 8 h. The conversion of benzene andtoluene was gradually increased with time giving maximum 87% and93% values respectively after 4 h which remained constant till 8 h.

3.3.3. Effect of molar ratio (substrate/benzyl chloride)The effect of the molar ratio of benzene to benzyl chloride and

toluene to benzyl chloride was studied at a reaction temperature 80and 110 °C for 4 h. At the molar ratio of 1:1 maximum conversion ofbenzene and toluene with highest selectivity of their respectiveproducts were obtained (Table 3). With increasing the molar ratio ofbenzene or toluene with benzyl chloride, the conversion as well asselectivity of diphenylmethane (DPM) and p-benzyl toluene wasobserved to be decreased which is attributed to formation of di ortri alkylated products by alkylation reaction of benzylated substrateswith excess benzyl chloride.

3.3.4. MechanismThe surface acid sites (Brønsted and Lewis) of the catalyst produce

carbocations from benzyl chloride as shown in Scheme 3, whichattack on the aromatic substrate and form benzylated product bysimple electrophilic substitution reaction. The higher conversion oftoluene as compared to benzene shows higher reactivity of toluene forbenzlyation. This is assigned to presence of electron donating methylgroup on aromatic nucleus of toluene, which activates the aromaticring for electrophilic substitution reaction.

3.4. Regeneration study

The used SZF-12 catalyst from both the reaction mixtures (benzyla-tion of benzene and toluene) were filtered, washed with acetone and

Scheme 3. Proposed mechanistic pathway of benzylation of benzene with benzyl chloride over SZF-12 catalyst.

Table 3Conversion (%) of benzene and toluene with SZF-12 catalyst at different molar ratios of benzene and toluene with benzyl chloride.

Molar ratio (benzene ortoluene/benzyl chloride)

Conversion %of benzenea

Selectivity % ofdiphenyl-methanea

Selectivity % ofother isomersa

Conversion %of tolueneb

Selectivity % ofp-benzyl tolueneb

Selectivity % ofother isomersb

1:1 87 100 Nil 93 99 11:2 78 81 19 82 71 291:4 65 74 26 70 60 40

Reaction condition: aTemperature=80 °C; Time=4 h; substrate/catalyst ratio=10. bTemperature=110 °C; Time=4 h; substrate/catalyst ratio=10.

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regenerated by heating at 450 °C for 2 h. The regenerated catalystswere used for the next reaction cycles under similar reaction conditionsto first cycle. The catalyst was found efficient up to 4 cycles givingconversion in the range of 87–82%. The catalyst was easily regeneratedby thermal treatmentwithout loss of catalytic activity. The FT-IR spectraof regenerated catalyst after second cycle in Fig. 10 shows similarity

Fig. 10. FTIR spectra of fresh and regeneratedfly ash supported sulfated-zirconia (SZFA-12).

with that of fresh catalyst indicating no change in chemical compositionof catalyst surface. The significant decrease in conversion after fourthcycle is due to the deposition of carbonaceous material on the externalsurface of the used catalyst that may block the active sites present onthe catalyst.

4. Conclusion

The study provides fly ash supported sulfated zirconia as anefficient solid acid catalyst possessing significant amount of acidity.The chemical activation of fly ash by acid leaching results in increasedsilica content and thus surface hydroxyl contents, which are re-sponsible for efficient loading of zirconia on fly ash support. In SZFcatalysts zirconia reacts with silica of fly ash and form Si–O–Zr phasewhich increases with increase in zirconia content in the catalyst. Thecatalyst SZF-12 exhibit high acidity due to high zirconia (12 wt.%) andsulfur (5.6 wt.%) content bonded with zirconia generating Brønstedand Lewis acid sites responsible for higher conversion of benzene andtoluene. The catalyst is also recyclable suggesting that acid sites of thecatalysts are not lixiviated during the reaction. The novelty of thework is that fly ash can replace pure silica after its suitable activationas a solid support for loading of zirconia. The fly ash supportedcatalysts are cost effective, atom efficient and thus finds a novel routeto utilize abundant waste fly ash.

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Acknowledgement

The authors are thankful to Vice-Chancellor Prof. B.M. Sharma forproviding laboratory and analytical facilities. Analytical support wasprovided by Dharmsinh Desai University, Nadiad, Gujarat and SardarPatel UniversityVallabhVidhyanagar Gujarat. Thefinancial support forthe research work was provided by Fly Ash Mission, DST, New Delhi,India.

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