Fuel 90 (2011) 2083–2088

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Fuel

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Synthesis and characterization of novel solid base catalyst from fly ash

Deepti Jain, Chitralekha Khatri, Ashu Rani ⇑Department of Pure and Applied Chemistry, University of Kota, Rajasthan 324 005, India

a r t i c l e i n f o

Article history:Received 17 December 2008Received in revised form 23 July 2010Accepted 14 September 2010Available online 22 January 2011

Keywords:Fly ashSolid base catalystClaisen–Schmidt condensation

0016-2361/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.fuel.2010.09.025

⇑ Corresponding author. Address: 2-m-1, Rangbari005, India. Tel.: +91 9352619059.

E-mail addresses: deepti_skjain@yahoo.com (D. Jai(C. Khatri), ashugck@rediffmail.com (A. Rani).

a b s t r a c t

A new type of solid base catalyst was synthesized by chemical and thermal activation of fly ash, collectedfrom Thermal Super Power Station situated in Kota, Rajasthan, India. The chemical activation was carriedout by 50 wt.% NaOH followed by thermal activation at 450 �C. The modified physiochemical property ofsolid base fly ash (SBFA) was determined by X-ray diffraction, FT-IR spectroscopy, Scanning ElectronMicroscopy, N2 adsorption–desorption studies and Flame Atomic Absorption Spectrophotometry. Theresults reveal that the catalyst is nano-crystalline in nature with crystallite size 11 nm and particle sizein the range 840 nm to 6.95 lm. The surface basicity and therefore, catalytic activity in SBFA was origi-nated by increased hydroxyl content as compared to fly ash, suggesting that the catalyst possess highersurface active sites. The basicity of the catalyst was measured by liquid phase, solvent free, single stepcondensation of benzaldehyde with cyclohexanone giving higher conversion (>70%) and selectivity(>80%) of desired product a,a0-dibenzylidenecyclohexanone. This excellent conversion shows that thecatalyst has sufficient basic sites both on the surface and in the bulk, responsible for the catalytic activity.Furthermore, this catalyst may replace conventional environmentally hazardous homogeneous liquidbases making an ecofriendly; solvent free, solid base catalyzed process. The application of fly ash to syn-thesize a solid base catalyst finds a noble way to utilize this abundant waste material.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Base catalysis is an important area of organic synthesis and offundamental industrial importance particularly in fine chemical,petrochemical and pharmaceutical industries. Commercially thebase catalyzed reactions are largely carried out by using homoge-neous bases like NaOH, Ca(OH)2, KOH, etc. [1,2]. These bases areharmful, required in more than stoichiometric amount, high oper-ating costs and serious environmental issues associated with baseneutralization, product separation and purification, corrosion, andwaste generation motivated substantial efforts toward the devel-opment of processes mediated by heterogeneous catalysts [2].The solid bases were found best alternative for solving the abovesaid problems of homogeneous bases as well as they make thereaction more selective [2], as the type of basicity (Brønsted orLewis basic sites) and basic strength of the solid base can be de-signed [3], according to the requirement of the reactions. The solidbases are also of industrial interest because of their low tempera-ture operation, environment friendly nature and higher selectivityof required product with ease of product separation. Solid base cat-alysts such as hydrotalcite, CaO, and KF/Al2O3 [1] are well reported

ll rights reserved.

Scheme, Kota, Rajasthan 324

n), chitra_cool81@yahoo.com

in the literature. In this series we synthesize and introduce a newtype of solid base, generated from fly ash, having catalytic effi-ciency comparable to other solid bases.

Fly ash is a micro spherical particulate byproduct of coal burn-ing power plants, produced approximately 420 million tons peryear globally [4]. There is an urgent and imperative need to adopttechnologies for gainful utilization and safe management of flyashes on sustainable basis [5]. Fly ash consists of silica, alumina,iron oxide, lime, magnesia and alkali in varying amount with someunburnt activated carbon [6,7] and possess large surface area in therange of 40–115 m2/gm [8]. Because fly ash shows pozzolanicproperties, most of this is used as a raw material for the synthesisof cement and other construction materials [9]. Some more advan-tageous use of fly ash are in agriculture, in metal recovery, in waterand atmospheric pollution control [10], etc. Furthermore, fly ashcan be converted into efficient adsorptive material [11], zeolitesand other cementitious material [12] by chemical activation meth-ods. During the alkali activation process of alumino-silicates mate-rial, immediately after the alkali solution comes in the contact withthe raw material, the Si–O–Si, Si–O–Al and Al–O–Al bonds break torelease silicon and aluminum ion into solution, which form highernumber of Si–OH and Al–OH groups, which can condense to formSi–O–Al, and Si–O–Si bonds converting into 3-D alumino-silicategel, reported as zeolites precursor [13]. Prolonged thermal activa-tion converts such gels into well crystallize zeolites of differentshape and sizes [14]. However a suitable change in chemical and

2084 D. Jain et al. / Fuel 90 (2011) 2083–2088

thermal activation method can convert them into amorphousmaterial having high surface area with adsorbed hydroxyl groups.

As a catalyst itself, fly ash has been used for H2 production,deSOx, deNOx, hydrocarbon oxidation, and hydro cracking gas-phase oxidation of volatile organic compounds, aqueous-phaseoxidation of organics, solid plastic pyrolysis, and solvent-freeorganic synthesis [15]. So far the use of fly ash as a solid basecatalyst for organic transformations is unprecedented in the liter-ature. In the present work F-type fly ash, rich in silica and aluminacontent, is selected to convert it into a solid base, entirely differentfrom the zeolites to have sufficient –OH groups to catalyze organictransformations. Recently our laboratory reported fly ash as anactive, reusable and cost effective solid acid catalyst to be usedfor the synthesis of aspirin and oil of wintergreen [16].

The solid base, generated from fly ash in this work is a nano-crystalline, reusable catalyst, having sufficient active basic sitesas the catalytic activity was tested on solvent free condensationof cyclohexanone and benzaldehyde to produce a,a0-dibenzylid-enecyclohexanone, which has large applications in fine chemicalindustries.

2. Experimental procedure

2.1. Materials

Sodium hydroxide (98%), benzaldehyde and cyclohexanonewere purchased from s.d. Fine Chem. Ltd., India and were used assuch. Fly ash (Class F type) was collected from Kota Super ThermalPower Station (Kota, Rajasthan, India).

2.2. Catalyst synthesis

The solid base fly ash catalyst (SBFA) was synthesized by chem-ical activation of fly ash with NaOH concentration 50 wt.%. Thechemical activation was carried out in a stirred reactor taking themixtures of fly ash and NaOH, heated at 110 �C under stirringand aged for 2 days maintaining the temperature. After ageing,the pulp obtained was washed with distilled water to remove lea-ched compounds and excess NaOH. The pulp thus obtained wasdried at 110 �C for 24 h. The chemically activated fly ash was ther-mally stabilized after calcination at 450 �C for 4 h in staticconditions.

2.2.1. Catalytic activity of SBFAThe catalytic performances of the nano-crystalline SBFA was

evaluated by Claisen–Schmidt condensation between cyclohexa-

Scheme 1. The Claisen–Schmidt condensation of cyclo

none and benzaldehyde (Scheme 1) to give a,a0-dibenzylidenecy-clohexanone, as test reaction in a liquid phase batch reactor.

2.2.2. Condensation between cyclohexanone and benzaldehydeThe condensation of cyclohexanone and benzaldehyde using

SBFA was carried out in a liquid phase batch reactor. In the proce-dure, cyclohexanone and benzaldehyde (molar ratio of cyclohexa-none and benzaldehyde = 1:2) were taken in a 50 ml round bottomflask, equipped with magnetic stirrer and condenser, immersed ina constant temperature oil bath. The catalyst (benzaldehyde to cat-alyst weight ratio = 10:1), activated at 450 �C for 2 h prior to thereaction in static air, was added in the reaction mixture. The reac-tion mixture was heated at required reaction temperature rangingfrom 90 to 140 �C and time from 30 min to 4 h at atmosphericpressure. After the completion, the reaction mixture was cooledand filtered to separate the catalyst and was analyzed by gas chro-matograph (Dani Master GC) having a flame ionization detectorand HP-5 capillary column of 30 m length and 0.25 mm diameter,programmed oven temperature of 50–280 �C and N2 (1.5 ml/min)as a carrier gas. The conversion was calculated as follows:

Conversion ðwt:%Þ ¼ 100�½Initial wt:%� Final wt:%�=Initial wt:%

2.2.3. Catalyst regenerationAfter initial use, spent catalyst from the reaction mixture was

recovered by filtration and re-generated for further use. The recov-ered catalyst was washed thoroughly with acetone and dried inoven at 110 �C for 12 h followed by activation at 450 �C for 2 h instatic condition prior to the reaction. Thus, re-generated catalystwas used for further reaction cycles under the similar reactionconditions.

2.3. Characterization

2.3.1. Physicochemical properties of SBFAThe synthesized activated fly ash catalyst (SBFA) was character-

ized by XRD, FT-IR, SEM, N2 adsorption–desorption isotherms andFlame Atomic Absorption Spectrophotometry to examine the phys-icochemical properties.

The crystalline nature and the crystallite size of the sample wasanalyzed by X-ray diffraction study by X-ray powder diffractome-ter (Philips X’pert) using Cu Ka radiation (k = 1.54056 Å). The sam-ple was scanned in 2h range of 0–80� at a scanning rate of 0.04� s�1.Crystallite size of the crystalline phase was determined from thepeak of maximum intensity (2h = 26.231) by using Scherrer for-mula with a shape factor (K) of 0.9 [17] as below:

hexanone and benzaldehyde using SBFA catalyst.

Table 2Characterization of fly ash samples before and after activation.

Catalyst Crystallite size(nm)

Alumina(wt.%)

Silica(wt.%)

BET surface area(m2/g)

FA 33 19 58 80SBFA 11 23 54 116

FA – fly ash sample, SBFA – solid base fly ash catalyst.

D. Jain et al. / Fuel 90 (2011) 2083–2088 2085

Crystallite size ¼ K � k=W � cos h

where W = Wb �Ws; Wb is the broadened profile width of experi-mental sample and Ws is the standard profile width of reference sil-icon sample.

The FT-IR study of the sample was done by FT-IR spectropho-tometer (IRPrestige-21, Shimadzu) in Diffuse Reflectance System(DRS) by mixing the sample with KBr in 1:20 weight ratio. Thespectrum was recorded in the range 400–4000 cm�1 with a resolu-tion of 4 cm�1. Specific surface area in the samples was determinedfrom N2 adsorption–desorption isotherms at 77 K by NOVA 1000eSurface Area and Pore Size Analyzer. BET approach was used to cal-culate surface area [18]. The sample was degassed under vacuumat 120 �C for 4 h, prior to adsorption measurement to evacuatethe physisorbed moisture. The chemical components present inthe fly ash samples was analyzed by Flame Atomic AbsorptionSpectrophotometer (AA-6300, Shimadzu). The detailed imaginginformation about the morphology and surface texture of the sam-ple was provided by SEM (Philips XL30 ESEM TMP).

Fig. 1. XRD of (a) FA and (b) SBFA.

3. Results

3.1. Characterization of fly ash catalyst

The chemical composition of raw fly ash used for the synthesisof SBFA is given in Table 1. The physiochemical properties of thefly ash samples before and after chemical activation are given inTable 2 as determined from flame photometry, which shows thatalumina content in SBFA samples is slightly increased from 19%to 23% (Table 2) while silica percentage is decreased. The increasein alumina content after activation shows the loss of other compo-nents during the chemical activation with higher concentration ofNaOH.

X-ray diffraction pattern of SBFA after calcination at 450 �C(Fig. 1b) shows the presence of crystalline phases in the sample.The chemical activation removes some crystalline componentspresent in the fly ash sample thus lowering the crystallinity ofthe sample and increases the amorphous nature [16]. The XRDpattern shows that the fly ash samples before as well as afteractivation (Fig. 1a and b) contain quartz, mullite and calcite(CaSO4) in large amount [19] The crystallite size of SBFA was11 nm showing the presence of nano-crystalline phase in thesample. SBFA showed increased amorphous nature, differentiatingit with zeolites, which are reported to have significantly increasedcrystalline phase [20].

The FT-IR spectrum of fly ash, before and after chemical activa-tion (Fig. 2i and ii), shows a broad band between 3400 and3000 cm�1, which is attributed to surface –OH groups of –Si–OHand adsorbed water molecules on the surface. The broadness ofthe band is due to the strong hydrogen bonding. The hydroxylgroups do not exists in isolation and a high degree of associationis experienced as a result of extensive hydrogen bonding withother hydroxyl groups. A peak at 1650 cm�1 in the spectra of boththe samples is attributed to bending mode (dO–H) of water mole-cule. The FT-IR spectrum of SBFA, after chemical activation(Fig. 2ii), shows a significant increase in peak intensity of the bandfor –OH group. FT-IR studies clearly showed changes in the inten-sity of IR peaks corresponding to Si–O–Si bending (460 cm�1),

Table 1Chemical composition of the fly ash.

Sample SiO2 (wt.%) Al2O3 (wt.%) Fe2O3 (wt.%) CaO (wt.%) MgO (wt.%) T

FA 58 19 8 0.6 0.6 1

L.O.I. the loss on ignition analysis was carried out at 900 �C for 4 h.

Si–O–Si symmetric stretching (798 cm�1) and T–O–Si (T = Si, Al)asymmetric stretching (913, 1090 and 1160 cm�1 [21]. SBFA showssignals at 996 cm�1, 1081 cm�1 and 1185 cm�1 for vitreous phaseof the unreacted fly ash, quartz and mullite [22].

SEM photographs of the fly ash revealed smooth spherical par-ticles of cenospheres with diameter of 840 nm to 6.95 lm inter-spersed with aggregates of crystalline compounds as shown inFig. 3a. The majority of the particles consisted of solid spheres,mineral aggregates, hollow cenospheres and irregularly shaped un-burned carbon particles with irregularly shaped amorphous parti-cles [23]. After chemical and thermal activation in SBFA thecrystalline and spherical particles break down into amorphousones and get agglomerated as shown in Fig. 3b.

iO2 (wt.%) Na2O (wt.%) K2O (wt.%) Other elements (wt.%) L.O.I. (wt.%)

.3 3.74 2.18 3.0 3

50075010001250150017502000225025002750300032503500375040001/cm

0

20

40

60

80

100

120

140

%T

1124.50

1091.71

896.90

842.89

3765.05

3464.15

1643.35

1105.21

970.19

i

ii

Fig. 2. (i) IR of FA and (ii) IR of SBFA.

Fig. 3. SEM of (a) FA and (b) SBFA.

Fig. 4. Effect of reaction temperature.

2086 D. Jain et al. / Fuel 90 (2011) 2083–2088

3.2. Catalytic performance

The catalytic activity and catalytic performance of the pure flyash and catalyst SBFA after activation was studied by Claisen–Schmidt condensation reaction of benzaldehyde with cyclohexa-none in single step, solvent free condition. The condensation ofbenzaldehyde with cyclohexanone over FA and SBFA was first car-ried out at 120 �C for 2 h taking benzaldehyde with cyclohexanonein 2:1 molar ratio and benzaldehyde to catalyst weight ratio of 10.Pure fly ash did not show any catalytic activity, while the catalystSBFA was found highly active for condensation of benzaldehyde

with cyclohexanone giving high conversion of desired producta,a0-dibenzylidenecyclohexanone (77%) and selectivity (82%) after2 h.

The reaction was carried out with different optimal conditionssuch as different reaction temperature, time, molar ratios of thereactants and recyclability reactions which are discussed below.

3.2.1. Effect of reaction temperatureTo optimize the reaction temperature giving maximum conver-

sion of the reaction of benzaldehyde with cyclohexanone was car-ried out at different temperature ranging from 90 to 140 �C asshown in Fig. 4, the conversion of a,a0-dibenzylidenecyclohexa-none was maximum (77%) at 120 �C within 2 h which decreasedat higher reaction temperature (140 �C). From the above catalyticresults, the optimum reaction temperature was found to be120 �C for high selectivity (82%) and higher conversion.

3.2.2. Effect of reaction timeThe variation of reaction time on the condensation of benzalde-

hyde with cyclohexanone was studied in the range of 30 min to 4 hat 120 �C with 2:1 molar ratio and the results are shown in Fig. 5.

Fig. 5. Effect of reaction time.

D. Jain et al. / Fuel 90 (2011) 2083–2088 2087

The conversion and selectivity of a,a0-dibenzylidenecyclohexa-none increased up to 77% with increasing reaction time up to2 h. Further the reaction time is increased, the conversion re-mained constant, but the selectivity of a,a0-dibenzylidenecyclo-hexanone decreased from 82% to 74% because of the formation ofother side product 2-benzylidenecyclohexanone due to very highcontact time. From the above results, the optimum reaction timewas found to be 2 h for highly selective synthesis ofa,a0-dibenzylidenecyclohexanone.

3.2.3. Effect of reactant molar ratioThe synthesis of a,a0-dibenzylidenecyclohexanone was carried

out at 120 �C with various molar ratios of reactants for 2 h overSBFA catalyst. A decrease in conversion of a,a0-dibenzylidenecyclo-hexanone was observed at 1:1 molar ratio of cyclohexanone tobenzaldehyde. This may be due to insufficient quantity of the

Scheme 2. The mechanistic pathways of Claisen–Schmidt condensation of cyclohexanocatalyst (SBFA).

reactants to react with each other. But, there is an increase in con-version of desired product at 1:2 molar ratio of cyclohexanone tobenzaldehyde at the same reaction conditions. This may be dueto equilibrating of each reactant quantity on the basic sites of thesurface of the catalyst SBFA. The conversion decreases from 32%to 31% on further increasing the molar ratio from 1:3 to 1:5.

3.2.4. Recyclability of the catalystThe spent catalyst from the reaction mixture was filtered,

washed with acetone and re-generated at 450 �C to use for the nextreaction cycles. The catalyst was equally efficient up to four reac-tion cycles for giving similar conversion of the desired producta,a0-dibenzylidenecyclohexanone in the range 77–68%. It showsthat the catalyst is easily re-generated by thermal treatment with-out loss of catalytic activity.

4. Discussion

The chemical activation of the fly ash with higher concentrationof NaOH at high temperature and longer duration significantly lea-ched out some of the components of fly ash and increased the alu-mina content. NaOH based systems are reported to have low SiO2/Al2O3 ratio [24], as found in our case, where reactive Si content isdecreased and Al is increased. It seems that breaking of Si–O–Si,Si–O–Al and Al–O–Al bonds results in release of Si and Al ions intothe solution, which either convert into sodium aluminosilicate orbecome reactive through Si–OH and Al–OH species. The enhancedpercentage of alumina and formation of sodium aluminosilicate isan important factor for the increased in surface hydroxyl groups,responsible for basicity and higher catalytic activity of fly ash cat-alyst. The crystallinity of the activated fly ash catalyst SBFA (Fig. 2)is lower than the fly ash sample before activation (Fig. 1) due to in-crease in amorphous nature attributed to the rise in alumina anddecrease in silica content [25]. The increased surface basicity offly ash catalyst is comparable to other solid base catalyst such aszeolites [13] but the catalytic material formed after activation isentirely different from zeolites. SBFA produced after chemical

ne and benzaldehyde to a,a0-dibenzylidenecyclohexanone over solid base fly ash

2088 D. Jain et al. / Fuel 90 (2011) 2083–2088

and thermal activation of FA during this work, show structural andmorphological difference with zeolites. The SEM micrograph ofSBFA (Fig. 3b) shows several agglomerated amorphous particles,can be easily differentiate from the SEM photographs of zeolites[12], which show a defined regular crystalline shape of zeolitesparticles. The increase in amorphous properties in SBFA is also con-firmed by XRD data. Increase surface hydroxyl groups affect themorphology and particle size and shape through H-bonding. Thehigher BET surface area in SBFA greatly increases the reactioncapacity, which increases the selectivity of the product and largesurface area provides more basic sites for the reaction. Increasein surface area is also due to breaking of large spherical particlesagglomeration of fine particles after activation.

The catalyst gave an excellent conversion of a,a0-dibenzylidene-cyclohexanone showing that it has sufficient basic sites for thereactions. In addition use of catalyst in small quantity (benzalde-hyde to catalyst weight ratio of 10) in the reaction indicates thehigher activity of the catalyst for condensation of cyclohexanoneand benzaldehyde. Sufficient surface basicity in SBFA promotesthe condensation of cyclohexanone to benzaldehyde. The increasedsurface –OH groups (surface –OH groups of –Al–OH and Si–OH) areresponsible for the generation of surface basicity in SBFA and makeit a potential solid base catalyst. The mechanistic pathways for thecondensation of cyclohexanone to benzaldehyde is given inScheme 2, which shows that the catalyst helps in the formationof carbanion intermediates which attach on benzaldehyde forcondensation.

The regeneration study reflects that the fly ash catalyst can bere-generated by simple thermal regeneration method and retainsthe catalytic activity. The re-generated catalyst shows similarcatalytic activity till 4th reaction cycle giving the conversion ofa,a0-dibenzylidenecyclohexanone in the range of 77–68% whichindicates that the sites are not deactivated in the re-generated cat-alyst. The yield was significantly decreased after 4th reaction cycle,which is attributed due to the deposition of carbonaceous materialon the external surface of the used catalyst, which may block theactive sites present on the catalyst [16,26]. This catalytic processfor condensation reaction may lower the cost of production ofthe product in fine chemical industries, as commercial catalyst(zeolites, KF, etc.) can be replaced by SBFA, a solid base generatedfrom waste fly ash.

5. Conclusion

The study provides an efficient solid base catalyst from fly ash,which possesses higher basicity responsible for higher conversionvalues of product. The new application of coal-generated fly ashis investigated upon using it as effective solid base catalyst. Thechemical activation of fly ash by alkaline solution results in in-creased amorphous alumina content and sodium alumino-silicatehaving surface hydroxyl contents, which are responsible for sur-face basicity. SBFA is nano-crystalline in nature with crystallite size11 nm and has sufficient amount of basicity for catalytic activity.This fly ash catalyst serves as potential solid base catalyst forcondensation reactions, which is evident from high conversion ofa,a0-dibenzylidenecyclohexanone (77%). The catalyst is also recy-clable suggesting that small amount of catalyst has stable basicsites for several cycles organic synthesis. The synthesis was donein a solvent free batch process, catalyst was filtered, can be re-gen-erated and reused four times with equal efficiency and provides nowaste in cost effective route. This study reports a new solid basecatalyst for the replacement of traditional liquid bases in importantreactions and introduces a new catalyst for organic chemistry. Thiscatalytic study may explore wide application of fly ash based cat-alysts in fine chemical industries.

Acknowledgements

The authors are thankful to Vice-Chancellor Prof. Kalra for pro-viding laboratory and analytical facilities. Analytical support wasprovided by Dr. Manish Kumar Mishra from Dharmsinh DesaiUniversity, Nadiad, Gujarat and SEM analysis was conducted atSardar Patel University Vallabh Vidhyanagar Gujarat. The financialsupport was provided by University Grants Commission,New Delhi, India.

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