Fuel Processing Technology 91 (2010) 1015–1021

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Fuel Processing Technology

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Fly ash supported calcium oxide as recyclable solid base catalyst for Knoevenagelcondensation reaction

Deepti Jain, Chitralekha Khatri, Ashu Rani ⁎Department of Pure and Applied Chemistry, University of Kota-324005, Rajasthan, India

⁎ Corresponding author. Postal address: 2-m-1, RaRajasthan, India. Tel.: +91 9352619059.

E-mail addresses: deepti_skjain@yahoo.com (D. Jain(C. Khatri), ashu.uok@gmail.com (A. Rani).

0378-3820/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.fuproc.2010.02.021

a b s t r a c t

a r t i c l e i n f o

Article history:Received 29 September 2009Received in revised form 14 January 2010Accepted 26 February 2010

Keywords:Heterogeneous catalysisSolid base catalystKnoevenagel condensation

A new type of solid base catalyst has been prepared by loading of CaO on thermally activated fly ash, with theaim of being used as heterogeneous catalyst for fine chemical production. The prepared fly ash supportedcalcium oxide catalyst (FAC) was characterized by FT-IR spectroscopy, X-ray diffraction analysis, ScanningElectron Microscopy and atomic absorption spectroscopy. The catalytic activity of FAC was evaluated byKnoevenagel condensation of benzaldehyde and ethyl cyanoacetate as model test reaction under optimizedconditions. The catalyst gave very high conversion (87%) of benzaldehyde to desired product ethyl (E)-α-cyanocinnamate with high purity. The catalyst was completely recyclable without significant loss in activityup to three reaction cycles, which confers its stability during reaction unlike commercial catalysts. Moreoverthis catalyst shows a promising future in providing environmentally clean process for the industrial sector.

ngbari scheme, Kota-324005,

), chitrak09@yahoo.in

l rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Base catalyzed condensation reactions are synthetically importantorganic transformation for the preparation of wide variety ofcondensation products, which are key intermediates in the manufac-ture of pharmaceutical and fine chemical intermediates [1]. Homo-geneous bases such as alkaline oxide, alkaline earth oxides, andhydroxides are widely used as catalysts in organic reactions includingisomerizations [2], C–C bond formation [3], additions [4], cyclization,oxidation [5] and condensation [6–8]. Among all condensationreactions, Knoevenagel condensation between benzaldehyde andethyl cyanoacetate is of great interest as this reaction leads to ethyl(E)-α-cyanocinnamate, a well known pharmaceutical and finechemical intermediates [9]. Commercially this reaction was carriedout using various homogeneous base catalysts such as piperidine,amines, ammonia, and ammonium salts, which are corrosive, toxic,nonreusable and also produce neutralizationwaste [10]. An importantstep to phase out the problems associated with use of homogeneousbases is the application of product selective and recyclable solid basecatalyst in such reactions. A number of solid bases such as KF/Al2O3,hydrotalcite, KNH2/Al2O3 and immines/SiO2 have been reported inliterature for catalyzing several condensation reactions [11–14]. Otherheterogeneous catalysts used so far for Knoevenagel condensationreaction are zeolites [15], Al-enriched fluoroapatites and hydroxy-apetites [16], clay, calcined hydrotalcite and Al2O3 [6], Cs-exchanged

NaX faujasite zeolite [17], amino based metal organic framework [18],alkali earth oxide supported on alumina [19] and magnesium oxide[20]. Condensation of benzaldehyde with some methylenic com-pounds viz. ethyl cyanoacetate, ethyl acetoacetate and diethylmalonate has been reported to be catalyzed by a series of lithium,sodium, potassium, rubidium and cesium impregnated on oxidematrices (SiO2, Al2O3, and Nb2O5) [21].

The present work elaborates the synthesis of fly ash supported-CaOcatalyst to have high basicity and catalytic activity for Knoevenagelcondensation reaction to produce ethyl (E)-α-cyanocinnamate. Fly ashis a silica enriched material, containing silica, alumina, ferric oxide,calcium oxide and other metal oxide such as Mn2O3 and TiO2 and inertcrystallite phases such as mullite, quartz and magnetite [22], which isbeing used in the present work as solid support for loading CaO. One ofthe main constituents of fly ash is silica (54%), having insufficientcatalytic activity which was enhanced by loading CaO correspondinglygenerating CaO–SiO2 phase with high basicity and sufficient catalyticactivity. In the present research work fly ash after loading of CaO hasbeen employed as a novel, noncorrosive, efficient and recyclable solidbase catalyst for organic transformations.

2. Experimental procedure

2.1. Material

Calcium carbonate (CaCO3) (98%), benzaldehyde (99.9%) and ethylcyanoacetate (99%) were purchased from s. d. Fine Chem. Ltd., Indiaand were used as such. The coal fly ash (Class F type) used in thisstudy was collected from Kota Super Thermal Power Station (Kota,Rajasthan, India). The components of fly ash are SiO2 (54%),

Scheme 1. The Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate using FAC catalyst.

1016 D. Jain et al. / Fuel Processing Technology 91 (2010) 1015–1021

Al2O3 (21%), Fe2O3 (9%), CaO (1.6%), MgO (0.8%), TiO2 (1.3%),Na2O (4.8%), K2O (3.2%) and trace elements (4.0%). The L.O.I (losson ignition) was found to be 3% on heating fly ash at 900 °C for 3 h.

2.2. Preparation of the catalyst

FAC was prepared by loading of calcium oxide on fly ash support.Fly ash support was calcined at 900 °C for 3 h prior to use. 0.25 gCaCO3 dissolved in hot deionized water was added to 1 g of calcinedfly ash in a 250 ml round bottom flask. The mixture was refluxed at110 °C for 2 days maintaining the temperature. After refluxing, themixture was filtered and the paste obtained was dried at 120 °C for24 h followed by calcination at 700 °C for 2 h in order to get CaO fromCaCO3.

2.3. Characterization

2.3.1. Physicochemical properties of FACThe prepared fly ash supported calcium oxide catalyst (FAC) was

characterized by different analytical techniques such as XRD, FT-IR,SEM and Flame Atomic Absorption Spectrophotometer, whichconfirm the presence of CaO on fly ash. The crystallinity and crystallitesize of the catalyst was analyzed by X-ray powder diffractometer(Philips X'pert) using CuKα radiation (λ=1.54056 Å). The samplewas scanned in 2θ range of 0–80° at a scanning rate of 0.04° s−1. TheFT-IR study of the sample was done by FT-IR spectrophotometer(IRPrestige-21, Shimadzu) in DRS (Diffuse Reflectance Spectroscopy)system by mixing the sample with KBr in 1:20 weight ratio for betterresolution. The spectrum was recorded in the range 400–4000 cm−1

with a resolution of 4 cm−1. The chemical composition of fly ash andamount of Ca present in FAC were analyzed by Flame AtomicAbsorption Spectrophotometer (AA-6300, Shimadzu). The detailed

Fig. 1. FT-IR

imaging information about the morphology and surface texture of thesample was provided by SEM (Philips XL30 ESEM TMP).

2.3.2. Catalytic activityThe catalyst FAC was tested by Knoevenagel condensation of

benzaldehyde and ethyl cyanoacetate shown as Scheme 1. The reactionwas carried out in a liquid phase batch reactor under optimizedconditions.

2.3.2.1. Typical reaction procedure. In the procedure, benzaldehyde(1.5 g) and ethyl cyanoacetate (1.1 g) (molar ratio of benzaldehyde/ethyl cyanoacetate=1.5:1) were taken in a 50 ml round bottom flask,equipped with magnetic stirrer and condenser, immersed in a con-stant temperature oil bath. The solid base catalyst (benzaldehyde/catalyst=5:1), activated at 700 °C for 2 h prior to the reaction, wasadded in the reaction mixture. The reaction mixture was heated atrequired reaction temperature ranging from 40 to 120 °C and timefrom 30 min to 4 h at atmospheric pressure. After the completion ofreaction, the mixture was cooled and filtered to separate the catalystand was analyzed by gas chromatograph (Dani Master GC) having aflame ionization detector and HP-5 capillary column of 30 m lengthand 0.25 mm diameter, programmed oven temperature of 50–280 °Cand N2 (1.5 ml/min) as a carrier gas. The conversion was calculated asfollows:

Conversion wt%ð Þ = 100 × Initial wt%−Final wt%½ �= Initial wt%:

2.3.3. Catalyst regenerationAfter completion of the reaction, catalyst was filtered and washed

thoroughly with acetone and dried in an oven at 110 °C for 12 hfollowed by activation at 700 °C for 2 h in static condition prior to theuse in next reaction cycle.

of FA.

Fig. 2. FT-IR of FAC.

Fig. 3. FT-IR of FAC (in the range 450–1200 cm−1).

Fig. 4. XRD

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3. Results and discussion

3.1. Catalyst characterization

In fly ash only 1.6 wt.% of CaO was detected but the amount of Caincorporated in FAC catalyst was found to be 11.7 wt.% as evaluated byAtomic Absorption Spectrophotometer. The increase in Ca is respon-sible for more CaO–SiO2 phases in FAC resulting in increased basicityand catalytic activity for condensation reaction.

The FT-IR spectrum of fly ash, in Fig. 1 shows a broad band between3400 and 3000 cm−1, which indicates the presence of surface –OHgroups,–Si-OHandabsorbedwatermolecules on the surface. The spectraalso show a broad range of bands from 1055 cm−1 to 1100 cm−1 whichis attributed tomodes of Si–O–Si asymmetric band stretching vibrations.The low frequencymodes at 794 cm−1 are due to the symmetric Si–O–Sistretching vibrations. The FT-IR spectra of FAC catalyst in Fig. 2 show abandat 3740–3742 cm−1which is due to theSi–OHstretching in isolatedSi–OH species [23]. A peak at 1650 cm−1 is due to the bending mode ofwater molecule. The adsorption peak at around 991 cm−1 shows thepresence of Si–O–Ca bond [24] in the catalyst which is an evidence forloadingof CaOonfly ash surface (Fig. 3). This band isdue to the formationof calcium silicate hydrate (C–S–H), a new phase found after loading ofCaO. This phase also indicates theassociated–OHgroupson the surfaceofthe support silica or with the calcium silicate which is evident fromintense band between 3400 and 3640 cm−1. This C–S–H is responsiblefor the increase in basicity of fly ash catalyst. At high temperature up to250 °C themolecularwater is removedwhile structural –OH ions remainassociated in the Ca and SiO2 skeleton up to 700 °C [25]. The lowcoordinatedO−2 ionson theedges/corners in theCaOparticles loadedonfly ash are less stabilized by adjacent Ca+2 ions and could also exhibithigherbasicity in FAC [26]. Abroadand intensebandat 1500 cm−1 andat881 cm−1 is due to the asymmetric stretching of CO3

−2 which remains inFACdue to incomplete decomposition of CaCO3 even at high temperatureup to 700 °C [27].

The XRD pattern of fly ash from Fig. 4 shows the presence of calcitephase at 30°, 40°, and 50°. In Fig. 5 after the loading of CaO on fly ash inFAC, the amorphous silica of fly ash reacts with CaO to form calciumsilicate phase which is observed from the peaks at 31°, 32°, 37°, 47°

of FA.

Fig. 5. XRD of FAC (CS—calcium silicate).

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and 54° [28]. The amorphous nature of the catalyst increases which isevident from the decrease in the intensity of crystalline phases of FAC.

SEM micrograph of fly ash in Fig. 6, shows the presence of hollowcenospheres, irregularly shaped unburned carbon particles, mineralaggregates and agglomerated particles, while the typical SEM imagesof the fly ash supported-CaO catalyst in Fig. 7 show dense particleswith distribution of varying particle size. CaO particles are also clearlyvisible on the external surface of spherical silica particle on fly ashsupport in Fig. 7B, corroborating the conclusion extracted from theXRD pattern [28].

3.2. Catalytic performance

Tomeasure the catalytic performance of FAC catalyst, Knoevenagelcondensation reaction in single step and solvent free condition, wasselected as model test reaction. The condensation of benzaldehydeand ethyl cyanoacetate with FAC catalyst was first carried out at 90 °Cfor 2 h, taking benzaldehyde/ethyl cyanoacetate molar ratio 1.5:1 andbenzaldehyde to catalyst weight ratio of 5. After 2 h the conversion ofbenzaldehyde to ethyl (E)-α-cyanocinnamate was 87%. The reactiondid not proceed in the presence of pure CaO or pure fly ash undersimilar reaction conditions which showed that FAC possessessufficient catalytic activity for the reaction.

Parameters such as reactant molar ratio, amount of catalyst,reaction time and temperature were optimized in order to achieve themaximum catalytic activity and good conversion of desired product.

Fig. 6. SEM micrograph of fly ash.

3.2.1. Effect of reaction timeIn order to check the reactivity of the catalystwith time, the reaction

was carriedout at 90 °C for different reaction times ranging from30 minto 4 h as shown in Fig. 8. It was found that in the first 2 h of the reactionperiod the conversion increases linearly over the catalyst and then levelsoff with increase in time up to 4 h, which may be due to the formationof benzoic acid as other side product. The optimum reaction time was

Fig. 7. A. SEM micrograph of FAC. B. SEM micrograph of FAC (magnified).

Fig. 8. Variation of conversion (%) of benzaldehyde with time at temperature 90 °C.

Table 1Effect of molar ratios of benzaldehyde/ethyl cyanoacetate on conversion (%) ofbenzaldehyde to ethyl (E)-α-cyanocinnamate with FAC catalyst.

Molar ratio % Conversion % Selectivity

1:1 33 472:1 62 521.5:1 87 831:2 31 45

Reaction condition: temperature=90 °C; time=2 h; substrate/catalyst=5; catalystactivation temperature=700 °C.

Table 2Effect of substrate/catalyst weight ratio on conversion (%) of benzaldehyde to ethyl (E)-α-cyanocinnamate with FAC catalyst.

Benzaldehyde/FAC weight ratio % Conversion % Selectivity

10 33 395 87 832 87 83

Reaction condition: temperature=90 °C; time=2 h; benzaldehyde/ethylcyanoacetate=1.5:1; catalyst activation temperature=700 °C.

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found to be 2 h in which FAC catalyst gave high conversion (87%) ofbenzaldehyde to ethyl (E)-α-cyanocinnamate with high purity.

3.2.2. Effect of reaction temperatureOptimization of reaction temperature to give maximum conversion

of ethyl (E)-α-cyanocinnamate was studied at temperature rangingfrom 40° to 120 °C for 2 h taking benzaldehyde/ethyl cyanoacetatemolar ratio of 1.5:1 while benzaldehyde to catalyst weight ratio was 5.Conversion was observed to increase on increasing reaction tempera-tures ranging from 40° to 90 °C as inferred from Fig. 9. The results showthat the maximum conversion (87%) of benzaldehyde to ethyl (E)-α-cyanocinnamate was found at 90 °C, after which conversion remainsalmost steady till 120 °C.

3.2.3. Effect of reactant molar ratioThe effect of benzaldehyde to ethyl cyanoacetate molar ratio on

conversion was monitored at different molar ratios from 2:1 to 1:2 asgiven inTable1. The conversionwas foundmaximumat 1.5:1molar ratiowith 87% conversion of benzaldehyde to ethyl (E)-α-cyanocinnamate. Atthismolar ratio due to high dispersion of catalyst sufficient catalytic sitesare available for reaction while on future increase in concentration ofbenzaldehyde more benzoic acid is formed, which decreases theconversion and selectivity of the main product.

3.2.4. Effect of substrate/catalyst weight ratioThe effect of substrate to catalyst weight ratio on conversion of

benzaldehyde was studied by varying the amount of catalyst underoptimized reaction conditions. As indicated fromTable 2, at lower catalystamount i.e. benzaldehyde/FAC weight ratio 10, only 33% conversion ofbenzaldehyde was observed. On increasing benzaldehyde/FAC weightratio to5, conversionofbenzaldehyde increasedup to87%. The increase inthe conversionwith an increase in the catalystweight can be attributed toan increase in the availability of number of catalytically active sites

Fig. 9. Variation of conversion (%) of benzaldehyde with temperature after 2 h.

required for this reaction. On further increase in the amount of catalystthere is no further change in conversion.

3.3. Proposed mechanism

The surface basic sites of the catalyst produce carbanion from ethylcyanoacetate as shown in Scheme 2, which attack on the benzalde-hyde and form condensation product by simple neucleophillicsubstitution reaction. The high conversion shows higher reactivityof FAC for condensation reaction.

3.4. Reuse of catalyst FAC

To study the stability (reusability) of the catalyst, the solid catalystrecovered after the reaction by filtration was washed with acetoneand calcined at 700 °C under static conditions for 2 h. Thus obtainedcatalyst was reused in the next reaction cycle. The calcium contentestimated in used catalyst was found similar to the fresh FAC catalystwhich confirms that CaO–SiO2 bonds in the fly ash are too strong toprevent lixiviation of calcium from fly ash. The catalyst wassuccessfully repeated three times under similar reaction conditionsgiving high conversion in the range 87–82%, which shows that there isno considerable change in the catalytic activity of the catalysts evenafter three reaction cycles. The conversion was decreased after thethird cycle, which may be due to the deposition of significant amountof carbonaceous material on the external surface of the used catalystthat may block the active sites present on the catalyst [29].

4. Conclusion

A new type of base catalyst has been prepared by loading andsubsequent thermal treatment of calcium oxide supported on fly ash.Fly ash a solid waste having several metal oxides is yet to be utilized incomparable amount to its production. Loading of CaO on fly ashincreases its catalytic activity towards base catalyzed reactions due toincrease in –OH content and formation of C–S–H phase which areresponsible for catalyzing condensation reactions as evident fromhigh conversion of ethyl (E)-α-cyanocinnamate (87%). The catalyst isalso recyclable suggesting that this small amount of catalyst hassufficient stable basic sites for organic synthesis. It is thus confirmedthat under experimental conditions, the leaching and/or the deacti-vation of the catalyst is practically negligible. This investigation bringsinto light the structural aspects of a novel fly ash supported solid base

Scheme 2. The mechanism of Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate using FAC catalyst.

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which is cost effective, environmentally safe, recyclable and can beused in catalytic amount in solvent free organic transformations.

Acknowledgements

The analytical support was provided by DAE CSR UGC, Indore. SEManalysis was carried out at Sardar Patel University Vallabh Vidhyanagar,Anand, Gujarat. The authors are thankful to fly ash mission projectsanctioned by the Department of Science and Technology, New Delhi,for financial support to carry out the research work.

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