Preparation and characterization of fly ash based mesoporous catalyst for transesterification of soybean oil

Journal of Environmental Chemical Engineering 3 (2015) 906–914

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Preparation and characterization of fly ash based mesoporous catalyst for transesterification of soybean oil Ravi Bhandari a , Vikranth Volli a , M.K. Purkait * ,a a

Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati, 781039 Assam, India

A R T I C L E I N F O

A B S T R A C T

Article history: Received 22 January 2015 Accepted 7 April 2015

In this work, waste coal fly ash was utilized to synthesis different types of zeolites. Conventional alkali fusion step followed by hydrothermal treatment was used for zeolite synthesis. The synthesis conditions were optimized to obtain highly crystalline zeolite. The effect of acid treatment and calcination temperature on zeolite formation was investigated. Further, the effect of addition of sodium aluminate (controlling Si/Al ratio) on zeolitization was also studied. highly crystalline zeolite was obtained at fly ash/NaOH ratio of 1:1.5, fusion temperature of 550  C, hydrothermal temperature of 90  C and 8 h hydrothermal time. The zeolite obtained was characterized for its structural, morphological and compositional properties. It was found that high Si/Al ratio favors zeolite X and higher concentration of aluminum results in the formation of zeolite A. The average particle size of the synthesized zeolites was found to be about 2–5 mm. The maximum surface area of fly ash based zeolite X and A was found to be 727.2 and 24 m2 g 1 respectively. Maximum biodiesel yield of 81.2% was obtained using synthesized catalyst. The calorific value of the synthesized biodiesel was 39.2 MJ kg 1 and was found competent with commercial biodiesel (38–42 MJ kg 1). ã2015 Elsevier Ltd. All rights reserved.

Keywords: Fly ash utilization Biodiesel Fusion time Thermal stability

Introduction Materials with high surface area, tunable pore size and adjustable framework are of great importance especially as catalyst, catalyst supports, thermal insulators, sensors, filters, electrodes and burner materials. Many trails for the synthesis of these materials were performed targeting specific applications [1]. These materials are highly crystalline framework of solids known as zeolites and adsorb both reactants and products. In the present era, the development, characterization and application of these tale material are of great importance [2]. Zeolites are inorganic porous materials with three dimensional networks of tetrahedral silica or alumina anions and are widely used in the field of adsorption, ion-exchange, molecular sieve, and catalyst [3]. The early sources for synthesis of these zeolites are standard chemical reagents. With the advances in technology, many synthetic pathways utilizing industrial waste as starting material were suggested [4–7]. Fly ash is one such material (30–60% SiO2, 10–20% Al2O3, 5–10% Fe2O3, 5–10% MgO and 2–4% CaO) with high contents of silicates and alumina-silicates and upon chemical treatment they can be converted to zeolite-like crystalline materials [8]. The

* Corresponding author. Tel.: +91 361 2582262; fax: +91 361 2582291. E-mail address: [email protected] (M.K. Purkait). http://dx.doi.org/10.1016/j.jece.2015.04.008 2213-3437/ ã 2015 Elsevier Ltd. All rights reserved.

effective utilization of fly ash as raw material for zeolite synthesis and its use as catalyst for transesterification would not only improve ecological balance but also helps in value addition with an inexpensive alternative source. Ojha et al. [9] reported the optimum conditions for the synthesis of zeolite X from fly ash in terms of surface area and crystallinity as: NaOH/fly ash ratio, 1:3; fusion temperature, 550  C and 6 h of hydrothermal treatment. The use of microwave assisted hydrothermal process for synthesis of zeolite Na-P1 from coal fly ash was reported by Inada et al. [10]. It was observed that for hydrothermal treatment, partial microwave irradiation helps zeolite formation when compared to continuous microwave irradiation. Murayama et al. [11] used conventional hydrothermal synthesis from fly ash using KOH and NaOH as alkali source to produce zeolite P, HS (hydroxysodalite) and K-CHA (potassium chabazite). In the production of biodiesel, acids/base (H2SO4, KOH and NaOH) were used as catalysts for transesterification [12]. The purification of products, catalyst separation and catalyst reusability are the problem associated with their use. The application of heterogeneous base catalysts for biodiesel production helps in process simplification along with cost reduction in product purification [13]. Babajide et al. [14] used ion exchanged potassium based fly ash zeolite for biodiesel production from sunflower oil. A yield of 83.53% was obtained at methanol:oil ratio of 6:1, catalyst

R. Bhandari et al. / Journal of Environmental Chemical Engineering 3 (2015) 906–914

amount of 3% (w/w) of oil at 65  C after 8 h. Transesterification of Jatropha seed oil with methanol using K and Na loaded on zeolite Na-X was carried out by Manadee et al. [15]. With 16:1 of methanol:oil molar ratio at 65  C and 2 wt% catalyst, the biodiesel yield of 95.2% was obtained when reaction was carried out for 3 h. Xie et al. [16] performed transesterification of soybean oil using KOH loaded Na-X zeolite as catalyst. Conversion of 85.6% was obtained at 65  C with 10:1 molar ratio of methanol to oil for 8 h with 3 wt% of catalyst. Transesterification of rapeseed oil with methanol by cesium exchanged Na-X faujasite and mixed metal oxides as catalyst was performed by Leclercq et al. [17]. Maximum conversion of 76% was obtained for Cs exchange rate of 43% with methanol to oil molar ratio of 275 for 22 h of reaction time at catalyst concentration of 20 wt%. Intarapong et al. [18] investigated the transesterification of palm oil to methyl ester on a KOH/Na-Y catalyst using a packed-bed reactor. It was found that the KOH/Na-Y created strong basic sites, and the agglomeration was greatly increased by increasing the potassium content. The highest FAME yield of 92.1% was obtained for the 15 wt% K/Na-Y at reaction temperature of at 60  C for 7 h. Supamathanon et al. [19] used potassium loaded (4, 8 and 12 wt%) Na-Y zeolite as catalyst for the transesterification of jatropha oil. The optimum biodiesel yield of 73.4% was obtained at 12 wt% of potassium loading at 65  C for 3 h with methanol to oil molar ratio of 16:1. According to the above mentioned literature, the advantages of working with heterogeneous alkaline catalysts are evident, and therefore zeolite can be used as a heterogeneous catalyst alone or as a support for other metal oxides. Several researchers have reported the optimum process parameters for the synthesis of zeolite from fly ash. But, studies related to the effect of calcination and acid treatment of fly ash on the optimum parameters for zeolites synthesis has not been reported yet. Studies involving the degradation kinetics of soybean oil methyl ester using iso-conversional methods are scant. The main objectives of the present study is to synthesize zeolite from acid treated fly ash after calcination and to study the effect of synthesis condition (NaOH/fly ash ratio, fusion temperature, hydrothermal time and temperature, addition of aluminum source) on the formation of final product based on degree of crystallinity. The synthesized zeolite was characterized (i.e. morphology, physicochemical and phase identification) via SEM, FESEM, FTIR, XRD, XRF and CEC. The catalytic activity was studied by the transesterification of soybean oil after ion-exchanged with potassium precursor. The biodiesel obtained was characterized for it physical properties and degradation kinetics was analyzed using Kissinger, Friedman, Flynn– Wall–Ozawa and modified Coats–Redfern iso-conversional models. Experimental

907

1.5 h to increase its activity during zeolitiation. A homogenous fusion mixture of fly ash and NaOH was prepared in varying NaOH/fly ash ratio of 1:1 to 1:2 (1:1, 1:1.2, 1:1.5 and 1:2) and this mixture was heated in temperatures between 400 and 600  C (400, 500, 550 and 600  C) for 1 h. The fusion mixture was cooled, crushed and mixed in de-ionized water (1:10) and sodium aluminate (10, 20, 30 and 40 wt%) was added. The mixture was stirred overnight and the slurry was crystallized between 90 and 120  C (90, 100, 110 and 120  C) for 4–24 h (4, 8, 12, 24 h). The product was filtered, washed thoroughly until the filtrate pH was in the range of 10–11 and dried at a temperature of 100  5  C. The synthesized zeolite was ion exchanged using potassium acetate as a precursor. Zeolite was dispersed in a 1.0 M solution of potassium acetate with powder to the solution ratio of 1:10 for 24 h at 60– 70  C. The slurry was washed, dried and then calcined at 500  C for 2 h to obtain the ion exchanged zeolite KX. Typically for zeolite synthesis, 5 g of fly ash was fused with 7.5 g of NaOH at 550  C for 1 h. The fusion mixture was mixed in de-ionized water (1:10), and 0.5 g (10%) of sodium aluminate was added. The mixture was stirred overnight and the slurry was crystallized at 90  C for 8 h. Transesterification of soybean oil Transesterification reaction was performed in a 250 ml three necked flask equipped with magnetic stirrer and condenser immersed in a constant temperature oil bath. The reaction was performed for 12 h and product samples were collected after every 1 h. The 3% catalyst (w/w) was mixed with oil and methanol (1:6 molar ratio) mixture and was heated at 65  C. Two phases were separated upper phase was the product – biodiesel – and the lower was the glycerin with the catalyst. The residual solvent and water were evaporated in rotary evaporator maintained at 65  C at 10 rpm. The conversion of the oil to fatty acid methyl esters (FAME) was determined by nuclear magnetic resonance (NMR) and the biodiesel yield (% FAME) was calculated by the method explained by Roschat et al. [20]. Characterization methods Thermogravimetric analysis (TGA) was done using Netzsch/ STA449F3A00 with nitrogen as carrier gas at a constant flow rate of 45 ml min 1 from an initial temperature of 25–950  C at heating rate of 10  C min 1. X-ray fluorescence (XRF) analysis was performed using Philips PW 2404 wavelength dispersive spectrometer fitted with Rh tube. The overall crystalline phases of samples were determined by Bruker D Advance X-ray diffraction system. Radial scans of intensity vs. scattering angle (2u) were recorded from 5 to 50 with scan rate of 0.02 s 1 using an Ni-filtered Cu Ka radiation (l = 1.5406 Å. The surface morphology

Materials The coal fly ash used in this study was collected from NTPC, Visakhapatnam, India. Hydrochloric acid (10%), sodium hydroxide (98%) and sodium aluminate (99.99%) obtained from Sigma– Aldrich Pvt., Ltd., was used in zeolite synthesis. Analytical grade methanol was obtained from Merck. For transesterification, soybean oil was obtained from a local supplier in Guwahati, Assam and was used without any further refinement. Zeolite synthesis and catalyst preparation The fly ash based zeolite was synthesized by alkaline fusion prior to hydrothermal treatment. The calcination of fly ash was done at 850 (10)  C for 2 h to remove unburnt carbon along with other volatile materials. The activity of calcined fly ash was further increased by treating it with hydrochloric acid (10%) at 80  C for

Table 1 Chemical composition of fly ash before and after pretreatment. Component

CaO Fe2O3 K2O MnO P2 O 5 SO3 SrO TiO2 Al2O3 MgO SiO2 Na2O Si/Al molar ratio

Fly ash

Zeolite

Raw

Acid treated

X

A

4.36 6.04 0.84 0.03 0.70 0.04 0.02 1.52 24.59 0.75 59.82 0.73 4.13

1.78 2.92 1.43 0.03 0.07 0.01 0.03 1.23 22.46 0.99 68.33 0.29 4.46

0.69 2.40 0.41 0.04 0.01 0.02 0.05 1.88 23.13 0.25 43.16 22.71 3.16

0.31 2.71 0.29 0.03 0.01 0.02 0.05 1.50 26.1 0.16 38.58 26.8 2.52

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Q

The soybean oil and methyl ester samples were characterized for their physical and chemical properties. The pH values of the samples were measured by using pH Spear from Eutech instruments. Specific gravity of all the samples was measured at room temperature using specific gravity bottle as per American society for testing and materials (ASTM) D 1298-85. The viscosities of the samples were measured at 40  C using rheometer (Rehostress RS 1) from Thermo Electron according to ASTM D 445. Acid value was determined as per ASTM D664-89. Calorific values of the samples were measured using bomb calorimeter according to ASTM D 240-92. 1H NMR spectrum of biodiesel samples were recorded using 600 MHz NMR spectrometer (BRUKER, Advance III HD) using CdCl3 as solvent.

Intensity (a.u.)

Q=Quartz M=Mullite H=Hematite

M

Q

M

HQ

M M M

Q

Results and discussion 0

5

10

15

20

25

30

35

40

45

50

55

2θ (º) Fig. 1. XRD pattern of fly ash.

of fly ash was examined using Sigma/Zeiss FESEM. N2 adsorption and desorption isotherms were recorded at 196  C with Beckmann–Coulter SA 3100 surface area and pore size analyzer. Different characteristic functional groups were identified by using SIMADZU Corp Fourier Transform Infrared spectroscopy (FTIR). The FTIR spectra were collected in the range of 450–4000 cm 1 region with 8 cm 1 resolution. The measurement of particle size distribution in the range 0.02–2000 micron was performed in Malvern Master Sizer 2000 Laser Particle Size Analyzer (LPSA).

Catalyst characterization and optimal process parameters for zeolite synthesis The chemical composition of raw, acid treated fly ash and synthesized X and A zeolite are given in Table 1. From the table it is clear that fly ash used in the present study can be classified as class F type (ASTM C618), since the combined SiO2, Al2O3 and Fe2O3 content was found to be 90.45%. The average SiO2/Al2O3 molar ratio of the raw and acid treated fly ash used in this study was found to be 4.13 and 4.46, respectively. The acid treatment of fly ash helped in the reduction of calcium and iron content from 4.36 wt% to 1.78 wt% and 6.04 wt% to 2.92 wt%, respectively thereby increasing the activity and thermal stability [9]. The synthesized zeolite X and

Fig. 2. XRD pattern for the effect of (A) fly ash: NaOH, (B) crystallization time, (C) fusion and (D) crystallization temperature on zeolite formation.

R. Bhandari et al. / Journal of Environmental Chemical Engineering 3 (2015) 906–914

1:1.2. Therefore, the optimal fly ash/NaOH ratio for the formation of zeolite may be considered as 1:1.5. The effect of hydrothermal time on the formation of resultant zeolites and their crystallinity is represented in Fig. 2(B). It can be seen from the XRD plot that the minimum time required for the complete formation of zeolite is 8 h. No product was obtained at crystallization time of 4 h. It was observed that there was slight decrease in degree of crystallinity of the zeolite with increase in time of crystallization. Therefore, the optimal crystallization time required for the formation of fully crystalline zeolite was found to be 8 h. The effect of fusion and crystallization temperature for a mixture at NaOH/fly ash = 1.5 on zeolite formation is given in Fig. 2(C and D), respectively. It may be seen from the XRD plot that the minimum temperature required for the complete formation of zeolite is 500  C. However, maximum crystallinity of zeolite was obtained at fusion temperature of 550  C. Above 550  C, the crystallinity of the product tends to decrease. Therefore, the optimal fusion temperature for the formation of zeolite is considered as 550  C. It can be seen from Fig. 2(D) that the minimum temperature required for the complete formation of zeolite is 90  C. However, zeolite formation was also observed at crystallization temperature of 120  C but the crystallinity of the product was decreased with the increase of crystallization temperature. Therefore, the optimal crystallization time for the formation of zeolite is 90  C.

A had an Si/Al ratio of 3.16 and 2.52, respectively. The formation of a particular type of zeolite depends on the Si/Al ratio of the raw material. Wang et al. [21] used fly ash with Si/Al ratio of 1.5 to produce zeolite X and A with Si/Al ratios of 1.3 and 1.2 respectively. Franus et al. [22] synthesized zeolite X of Si/Al 1.12 using fly ash with Si/Al of 1.62. Fly ash with Si/Al of 4 was used to synthesize zeolite X and A with Si/Al ratio of 1.7 and 0.9, respectively by Tanaka et al. [23]. The XRD pattern of fly ash depicted the presence of crystalline quartz (SiO2) and mullite (3Al2O3
2SiO2). The major phases were quartz (SiO2) with peaks at 20.86, 26.65 and 36.5 2u and less intense peaks were mullite and hematite [24]. Presence of amorphous glassy phase in fly ash can be observed by a broad hump in the region between 18 and 35 2u, as indicated in Fig. 1. A series of experiments were undertaken to determine the effects of fly ash/NaOH ratio on zeolite formation. The sodium hydroxide added to the fly ash helps in adjusting the sodium content in the starting material and also works as an activator. Mullite and quartz present in the fly ash are the sources of aluminum and silicon, respectively, for zeolite formation. Fig. 2(A) shows the effect of fly ash/NaOH weight ratio (1:1 to 1:2) on the formation of resultant zeolites and their crystallinity. It can be seen from the XRD plot that the minimum fly ash/NaOH ratio required for the complete formation of pure, single phase zeolite X is 1:1.5. However, partially crystalline zeolite was obtained at fly ash/NaOH ratio of 1:1 and

A

AAA A

AA

AA

909

B

A

Intensity (a.u.)

A

A

A A

A

A A A A 30%

X XX X

X X X

X 20%

X XX X

0

8

5

X X X

X

X

% Transmittance (a.u.)

40%

Zeolite A Zeolite X

Fly ash

10%

10 15 20 25 30 35 40 45 50 55 2θ (º)

4000 3500 3000 2500 2000 1500 1000 500

Wavelength (cm-1)

C

D

7

Zeolite A Zeolite X

Zeolite KX

5

Vads( a.u.)

Volume (%)

6

Fly ash

4 3 2

Zeolite A Zeolite X

1 0 0.01

Zeolite KX

Fly ash 0.1

1

10

Particle size ( μ m)

100

1000

0.00

0.25

0.50

0.75

1.00

Ps/Po

Fig. 3. XRD pattern for the effect of (A) Si/Al ratio on zeolite formation, (B) FTIR spectra (C) particle size distribution and (D) adsorption/desorption isotherm of synthesized zeolite.

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The type of zeolite formation was studied by changing the Si/Al ratio with the addition of varying doses of sodium aluminates (10–40 wt%). Zeolite X was formed at Si/Al ratio of 3.16. Addition of 10–20% of sodium aluminate resulted in the formation of highly crystalline zeolite X. With further increase in amount to 30–40%, zeolite A (LTA) was formed. The XRD spectra of the effect of sodium aluminates concentration on the formation of various types of zeolite were shown in Fig. 3(A). The FTIR spectra of fly ash and synthesized zeolites are shown in Fig. 3(B). Presence of quartz was observed at 1053 cm 1 which is associated with T O (T = Si, Al) asymmetric stretching vibrations for both fly ash and zeolite. The broad bands appearing at 800 and 550 cm 1 correspond to quartz and mullite. The band at 420 cm 1 is associated with T O bending vibrations. The presence of O H, H O H stretching vibration was observed at 3475 cm 1 and 3124 cm 1, respectively. The band at 1640 cm 1 can be attributed to O H bending. The band at 1467 cm 1 represents asymmetric stretching of Si/Al O. The main asymmetric stretching of Si O Si is at 982 cm 1 with a shoulder at 1053 cm 1. Symmetric stretching of Si O Si occurs at 756 cm 1 and 675 cm 1. The band at 560 cm 1 is associated with the double 6 rings that connect the sodalite cages. The band at 464 cm 1 represents Si O Si and O Si O bending vibrations. The particle size distribution (PSD) of fly ash and synthesized products are represented in Fig. 3(C). The PSD of fly ash was in the range of 10–110 mm with mean particle size of 27 mm, the smallest

particles approximately 0.4 mm. The PSD of synthesized zeolites were in the range of 0.2–60 mm with average particle diameter of 2–5 mm for zeolite X and zeolite A, respectively. Lee et al. [25] reported that the particle size of zeolite A was 7.5 mm during aging and it increased to 15.7 mm after hydrothermal treatment. The nitrogen adsorption/desorption isotherms of fly ash and the synthesized zeolite are shown in Fig. 3(D). In the case of fly ash isotherm it can be observed that hysteresis loop is of H3 with type III isotherm. In the case of isotherm of zeolite A and zeolite X (Type II isotherms), it can be observed that adsorption increases at relatively low pressure more than in original fly ash sample. Hysteresis loops for zeolites can be classified similarly to fly ash as H3 hysteresis loop, which indicates uniform size slit pores in parallel plates. The BET surface area of fly ash, zeolite X, zeolite A and ion exchanged KX was determined to be 4.9, 727.2, 24 and 735.8 m2 g 1 with total pore volume of 0.08, 0.46, 0.11 and 0.56 cc g 1, respectively. Zeolite X obtained from coal fly ash has much higher specific surface area than zeolite A. This may be due to the fact that the nitrogen molecule (molecule diameter: 0.375– 0.378 nm) cannot enter into the narrow pore of zeolite A, while zeolite X has larger pores in which nitrogen molecules are easily penetrated and adsorbed [26,27]. Fly ash particles are primarily spherical in shape with wide particle size range. In some cases smaller particles are attached to the surface of larger particles, serving as substrates and are either solid or hollow (cenospheres). Fig. 4(A) shows the surface

Fig. 4. FESEM image of (A) fly ash, (B) zeolite X, (C) zeolite KX and (D) zeolite A.

R. Bhandari et al. / Journal of Environmental Chemical Engineering 3 (2015) 906–914

90

550

Zeolite X Zeolite KX

A

B 440

% Transmittance

Conversion(%)

75 60 45 30

330

220 Soybean oil

110 Soybean oil methylester

15 0 5

6

7 8 Time (h)

9

105

10

o

-1

10 C min o -1 20 C min o -1 30 C min

DTG ( % wt min-1)

C 90 75

Weight (%)

911

60 45 30

0 4000 3500 3000 2500 2000 1500 1000 500 Wavelength (cm-1) 32 o -1 10 C min D o -1 20 C min 28 o -1 30 C min 24 20 16 12

15

8 4

0

0 0

100

200 300 400 500 Temperature (oC)

600

700

0

100 200 300 400 Temperature (oC)

500

600

700

Fig. 5. (A) Conversion, (B) FTIR spectra, (C) TGA and (D) DTG curves of biodiesel.

morphology of fly ash. The FESEM image of the synthesized zeolites showed a clear transformation of the spherical particles of the fly ash (A) into octahedral crystals of zeolite X (B), ion exchanged zeolite KX (C) and cubic crystalline structures which is a characteristic of zeolite A (D). However, these cubes had smooth edges and were covered with unconverted amorphous materials. This morphology is similar to that reported in literature [28]. Catalytic activity, characterization and kinetics of soybean oil methylester The transesterification of soybean oil with methanol was performed by using fly ash based zeolite X and KX to study their catalytic activity. The catalytic activities of both the catalysts used in the transesterification reactions reveal maximum ester yields of

63.5% and 81.2%. These are obtained for zeolite X and KX, respectively, after reaction time of 8 h using 6:1 methanol to oil molar ratio at 65  C using 3 wt% catalyst. Decrease in conversion was observed thereafter as shown in Fig. 5(A). From the figure it is clear that the catalytic activity of the ion exchanged fly ash zeolite KX was higher than zeolite X. This improved catalytic activity after ion exchanging zeolite X to KX form was probably due to the presence of more electropositive cations and more basic sites in zeolite KX when compared to zeolite X [16,29]. Wu et al. [30] examined the catalytic activity of CaO supported Na-Y zeolite for the transesterification of soybean oil. A biodiesel yield of 95% was obtained at methano/oil molar ratio 9, reaction temperature 65  C, reaction time 3 h and 3 wt% catalyst concentration. A maximum biodiesel yield of 93.5 wt% was obtained when zeolite X was used as catalyst for transesterification of sunflower oil by Ramos et al.

Table 2 Comparison of % yield of methyl ester with some reported zeolite based catalysts. Oil

Palm Soybean Jatropha Sunflower

Catalyst

KOH/Na-Y Na-X zeolites loaded with KOH Potassium supported on Na-X Na-Y Fly ash/Na-X ion exchanged with potassium Na-X

Reaction conditions Catalyst conc. (wt%)

Methanol/oil molar ratio

Time (h)

Temperature ( C)

Yield (%)

Reference

15 3 16 12 3 10

15:1 10:1 16:1 16:1 6:1 6:1

7 8 3 3 8 7

60 65 65 65 65 60

92.18 85.6 95.2 73.4 83.5 93.5

[18] [16] [15] [19] [14] [31]

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Table 3 Physical properties of soybean oil and its methylester. Property

Present work

Acid value (mg KOH g 1) Specific gravity (gm cc 1) Viscosity @ 40  C (cP) pH Calorific value (MJ kg 1)

Soybean oil

Methyl ester

0.57 0.912 30.61 4.07 37.1

0.1 0.901 6.8 5.2 39.2

ASTM standard

0.08 0.87–0.9 1.9–6.0 – 38–43

D 664-89 D 1298-85 D 445 D240-92

1462 cm 1 implies the presence of the methyl group in the product biodiesel. Presence of low concentration of unsaturation in the product was observed by the small band at 3009 cm 1 ( HC CH stretching). A broad band at 3444 cm 1 represents the presence of water in soybean oil. The thermal stability of the produced biodiesel, degradation and its kinetic behavior was studied using various methods like; modified Coats and Redfern, Friedman, Kissinger and Flynn–Wall– Ozawa. The kinetic analysis used for the thermal conversion of the biodiesel is discussed in our previous work [32]. The kinetic data were estimated by plotting mass loss vs. temperature at 10, 20 and 30  C min 1 heating rates given in Fig. 5(C). All the experiments were performed in nitrogen atmosphere and the thermal stability was determined from the onset temperature (Tonset) which signifies the temperature of start of decomposition. A single stage

[31]. Table 2 shows the comparison of conversions obtained by the use of different types of faujasite zeolite catalysts in transesterification reactions and are found to be comparable with present work. The physico-chemical properties like density, viscosity and acid value of soybean oil and its methyl ester were analyzed as per standard ASTM test methods and are summarized in Table 3. The acid value of biodiesel reduced from 0.57 to 0.1 mg KOH g 1 and calorific value increased to 39.2 MJ kg 1. The infrared spectra of soybean oil and soybean oil methyl ester are shown in Fig. 5(B). The peaks at 1744 cm 1 (the C O vibration) and around 1160–1236 cm 1 (C O vibrations) represent ester peaks. The peak at 1377 cm 1 shows the presence of glycerol in the product biodiesel. The peak at 1118 cm 1 attributed to the C CH2 O vibration indicates the presence of tri glyceride and peak at

-9.6

Biodiesel [24]

-1.5

A

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

B

-2.0

-10.0

-2.5

Ln (dX/dt)

Ln (β / T2 p)

-9.8

-10.2 -10.4

-3.0 -3.5 -4.0

-10.6

-4.5 -10.8 1.41

1.42

1.43 -3

1.44

1.45

1.46

1.2

1.3

-3

10 /T(10/K)

1.5 -3

1.6

1.7

1.8

-3

10 /T(10 /K) -9.2

3.6

0.1

D

0.1

C

0.2

0.2

3.4

0.3

0.3

-9.6

0.4

3.2

0.5

0.6

0.6

0.9

0.7

-10.0

0.8

Ln(β/t 2 )

0.8

2.8

0.4

0.5 0.7

3.0

Ln(β)

1.4

0.9

-10.4

2.6 -10.8

2.4 2.2 1.3

1.4

1.5 -3

1.6 -3

10 /T(10 /K)

1.7

1.8

1.3

1.4

1.5 -3

1.6

1.7

1.8

-3

10 /T(10 /K)

Fig. 6. Iso-conversional plot of (A) Kissinger method, (B) Friedman method, (C) F–W–O method and (D) modified Coats–Redfern method at varying degree of conversion.

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decomposition with temperature ranging from 247 to 536  C for all the heating rates with mass loss ranging from 20 to 90% was observed. The onset temperature at 10  C min 1 was found to be 223  C and it increased to 254  C when the heating rate was increased to 30  C min 1. The onset temperatures increased with increase in heating rate. This is due to the fact that low heating rate favors the early decomposition of poly and mono unsaturated fatty acids resulting in increase of the onset temperature [32]. Fig. 5(D) shows the DTG curves at different heating rates. The peak temperature (Tpeak) at which the maximum mass loss occurred was found to be 414  C and it increased to 437  C shifting the DTG curves to higher temperature zone with increase in heating rate. The thermal decomposition rate was increased with heating rate from 10 to 30  C min 1. This favors the shifting of DTG curves towards higher temperature zone and hence changing the temperatures corresponding to the maximum loss of mass peaks towards higher value. The shifting of DTG curves to higher temperature zone was because of faster release of volatile materials [32]. Zhao et al. [33] performed the similar kind of experiments at 5  C min 1 and found that onset and peak temperatures for soy biodiesel as 193  C and 377  C, respectively, indicating that the values are in agreement with the literature. The activation energy of soybean oil methylester was calculated in the conversion range of 0.2 < X < 0.9 from Kissinger, Friedman, Flynn–Wall–Ozawa and modified Coats–Redfern models and the plots are shown in Fig. 6(A, B, C and D). The activation energy (Ea) of biodiesel was in the range of 142.3–326.8 kJ mol 1 with varying conversion. The average value was found to be 189.9, 235.1, 203.7 and 200.2 kJ mol 1 for Kissinger, Friedman, FWO and modified Coats–Redfern methods, respectively. It may be observed that FWO and modified Coats–Redfern methods fitted well (within 1.5% error) with average regression coefficients as 0.98 and 0.95. But the error percentage of activation energy calculated by Kissinger and Friedman method was found to be much higher (>5%). From the present investigation it may be concluded that the activation energy of soybean oil methyl ester calculated by above mentioned methods in nitrogen atmosphere was quite more than that of soybean oil (150–155 kJ mol 1) [34] and is in agreement with the literature. Conclusion Pure zeolites X and A were selectively produced from Indian coal fly ash by alkali fusion followed by hydrothermal treatment. The treatment conditions and concentrations of the raw materials helped increase in zeolitiation. At Si/Al molar ratio of 3.16 and 2.52 zeolite X and A were formed proving that formation of zeolite type strictly depends on Si/Al ratio. Zeolite X (727.2 m2 g 1) has much larger surface area and total pore volume in comparison to zeolite A (24 m2 g 1) and fly ash (4.9 m2 g 1), which makes it most suitable for catalytic reactions. In the transesterification of soybean oil, biodiesel yield of 81.2% was obtained using 6:1 methanol to oil molar ratio at 65  C for 8 h and 3 wt% catalyst concentration. TGA analysis showed that the biodiesel produced is thermally stable up to 223  C and has single stage decomposition at different heating rates. Kinetic analysis showed that FWO and modified Coats– Redfern methods fitted well with regression coefficients 0.98 and 0.95. The average value of activation energy was found to be 205.2 kJ mol 1. Ion exchanged zeolite KX synthesized from fly ash could be a potential candidate for heterogeneous catalysis in biodiesel production helping in the utilization of large scale industrial waste. Acknowledgement This work is partially supported by a grant from the Fly Ash Unit, Department of Science and Technology (FAU-DST) New Delhi,

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India. Any opinions, findings and conclusions expressed in this paper are those of the authors and do not necessarily reflect the views of FAU-DST, New Delhi.

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