12-Tungstophosphoric Acid Anchored to Zeolite Hβ: Synthesis

Aug 21, 2012 - TGA of the samples were carried out using a Mettler Toledo Star SW 7.01 ... out in a 100 mL batch reactor provided with a double walled...
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12-Tungstophosphoric Acid Anchored to Zeolite Hβ: Synthesis, Characterization, and Biodiesel Production by Esterification of Oleic Acid with Methanol Anjali Patel* and Nilesh Narkhede Department of Chemistry, Faculty of Science, M. S. University of Baroda, Vadodara, 390002, India ABSTRACT: Heterogeneous acid catalysts comprised of 12-tungstophosphoric acid (10-40 %) and zeolite Hβ were synthesized, and 30 % loaded catalyst was characterized by various physicochemical techniques. The use of synthesized catalyst was explored for biodiesel production by esterification of free fatty acid, oleic acid with methanol. The effect of various reaction parameters such as catalyst concentration, acid/alcohol molar ratio, and temperature were studied to optimize the conditions for maximum conversion. The catalyst showed high activity in terms of high conversion (84%) and a high turnover number, 1048. The kinetic study as well as Koros−Nowak test were carried out, and it was found that esterification of oleic acid follows first order kinetics with the calculated activation energy Ea = 45.2 kJ mol−1 and pre-exponential factor A = 5.4 × 104 min−1. The catalyst showed potential of being used as a recyclable catalytic material after simple regeneration without significant loss in conversion. As an application, preliminary studies were carried out for biodiesel production from waste cooking oil and jatropha oil, as feedstock over present catalyst.

1. INTRODUCTION Catalysis by anchored heteropoly acids (HPAs) have been greatly expanded during the past few years from the viewpoint of their variety of structures and compositions. They have found enormous applications in various industrially important classes of reactions such as oxidation, alkylation, acylation, and esterification.1−4 Recently, they have also gained tremendous interest in the synthesis of biodiesel.5−14 Biodiesel is a renewable fuel and consists alkyl esters derived from either transesterification of triglycerides (TGs) or esterification of free fatty acids (FFAs) with low molecular weight alcohols.15−17 Several methodologies are available, which utilize homogeneous, heterogeneous, or biocatalysts for the synthesis of biodiesel. The most used commercial technology for biodiesel production is the transesterification of triglycerides with methanol under basic conditions. Generally, alkaline catalysts are used for biodiesel production, as they are cheaper, but the major issue is the saponification, which is more pronounced in feedstock with large amounts of FFAs. Although enzymatic catalysts are very selective and present high conversions18 using low oil to alcohol molar ratios, they are very expensive and show unstable activities.19 Also, supercritical conditions require high temperature and hence increase the overall cost of biodiesel production processes. Therefore, much attention has been paid to easily reusable solid acid catalysts. Supported heteropoly acids are the best alternative, as they can carry out simultaneously esterification and transesterification.5 The esterification reaction of long chain carboxylic acids such as oleic acid is interesting in the context of biodiesel production, as it is present in major extension in vegetable oils such as soybean, jatropha curcas, sunflower, rapeseed, pongamia, palm, and sea mango. Even though oleic acid is so important in the context of biodiesel production; reports on the use of anchored © 2012 American Chemical Society

heteropoly acids for biodiesel production by esterification of oleic acid are scanty. To our knowledge, only two reports on esterification of oleic acid by anchored 12-tungstophosphoric acid are available. Oliveira, et al.20 reported esterification of oleic acid with ethanol over 12-tungstophosphoric acid supported on to zirconia. The catalyst, (20%) 12-tungstophosphoric acid supported on ZrO2 showed 88% conversion at 4 h reaction with 1:6 (oleic acid/ethanol) molar ratio at 100 °C under nitrogen atmosphere. Recently, we have reported esterification of oleic acid with methanol over 12-tungstophosphoric acid (TPA) supported on to SBA-15.21 The catalyst showed 90% conversion at 40 °C for 4 h of reaction time by keeping oleic acid to methanol molar ratio of 1:40. At the same time, it was also found that there are no reports available for the same over 12-tungstophosphoric acid anchored to zeolite Hβ. Therefore, it was thought of interest to carry out biodiesel production by esterification of oleic acid with methanol over a heterogeneous acid catalyst comprising 12tungstophosphoric acid and Hβ. A series of catalysts containing 10−40 wt % 12tungstophosphoric acid and Hβ was synthesized. The support and catalyst were characterized by various thermal and spectral techniques such as elemental analysis (EDX), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), powder X-ray diffraction (XRD), and surface area measurement (BET (Brunauer−Emmett−Teller) method). The surface morphology of the support and catalyst was studied by scanning electron microscopy (SEM). The acidity values were measured by NH3-TPD analysis. The catalytic activity was evaluated for esterification of oleic acid with methanol. The effects of various reaction parameters such as Received: June 26, 2012 Revised: August 17, 2012 Published: August 21, 2012 6025

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catalyst concentration, acid to alcohol molar ratio, and temperature were studied to optimize the conditions for maximum conversion. Also, the catalyst was regenerated and reused up to four cycles. A detailed kinetic study was also carried out, and it was found that the esterification reaction of oleic acid with methanol follows a first-order dependency on the concentration of oleic acid and catalyst. The influence of temperature on the rate constant was studied, and the activation energy was also calculated. As an application, preliminary study was carried out for biodiesel production from waste cooking oil (WCO), and jatropha oil (JO) over the present catalyst.

Table 1. Elemental Analysis (EDX) elemental analysis by weight % P

W

catalyst

Si

Al

by EDX

theoretical

by EDX

theoretical

Hβ TPA3/Hβ

40.0 28.0

3.5 2.3

0.25

0.21

18.57

17.71

TGA curves of zeolite support and catalyst are shown in Figure 1. A unique weight loss of 13−15% was observed up to

2. EXPERIMENTAL SECTION 2.1. Materials. All chemicals used were of A.R. grade. 12Tungstophosphoric acid was obtained from Loba Chemie, Mumbai. The sodium form of zeolite β with Si/Al ratio 10 was purchased from Zeolites and Allied Products, Bombay, India, and used as received. Oleic acid, methanol and ammonium chloride were purchased from Merck. 2.2. Treatment of the Support (Hβ). Support (Naβ) was converted in to the NH4+ form by conventional ion exchange method22 using a 10 wt %, 1 M NH4Cl aqueous solution. The resulting NH4+ type zeolite was further converted to H+ type by calcination in air at 550 °C for 6 h. 2.3. Synthesis of the Catalyst (TPA Anchored to Hβ). A series of catalysts containing 10−40% of TPA anchored to Hβ were synthesized by impregnation. Hβ (1 g) was impregnated with an aqueous solution of TPA (0.1/10−0.4/40 g/mL of double distilled water) and dried at 100 °C for 10 h. The obtained materials were designated as TPA1/Hβ, TPA2/Hβ, TPA3/Hβ and TPA4/Hβ, respectively. 2.4. Catalyst Characterization. SEM and EDX analyses were carried out using JSM 5910 LV combined with an INCA instrument for SEM-EDX. TGA of the samples were carried out using a Mettler Toledo Star SW 7.01 instrument under nitrogen atmosphere with a flow rate of 2 mL/min and a heating rate of 10 °C/min. The BET surface area measurements were carried out in a Micromeritics ASAP 2010 volumetric static adsorption instrument by nitrogen adsorption at 77 K. FT-IR spectra of samples pressed with dried KBr into discs were recorded in the range of wave numbers 4000−400 cm−1 using a Perkin-Elmer spectrometer. The X-ray powder diffraction (XRD) patterns of the support and catalyst were measured using a Philips X′ pert MPD system in the 2θ range of 1−60° using Cu Kα radiation (λ = 1.54056 Å). NH3 chemisorption studies were carried out using Micromeritics Pulse Chemisorb-2705. 2.5. Reaction Procedure. The esterification of oleic acid (0.01 mol) with methanol (0.2 mol) was carried out in a 100 mL batch reactor provided with a double walled air condenser, Dean−Stark apparatus, magnetic stirrer, and a guard tube. A Dean−Stark apparatus was attached to a round-bottom flask to separate the water formed during the reaction. The reaction mixture was refluxed at 60 °C for 6 h. The obtained products were analyzed on a gas chromatograph (Nucon-5700) using a BP1 capillary column. Products were identified by comparison with the authentic samples. Turnover freuencies (TOFs) are calculated by using following formula. TOF =

Figure 1. DTA-TGA profiles of (a) Hβ and (b) TPA3/Hβ.

250 °C for zeolite support, which is attributed to desorption of physically adsorbed water. No further weight loss was observed beyond 250 °C which indicates zeolite Hβ retains its framework structure up to 600 °C. For the catalyst TPA3/ Hβ, similar weight loss of 10−12% up to 200 °C assigned to adsorbed water was detected. Second weight loss of 1.2−1.5% was observed between 200 and 300 °C due to the loss of water of crystallization of Keggin ion. After 430 °C, gradual weight loss was observed for TPA3/Hβ. This suggests zeolite catalyst is stable up to 430 °C. The FTIR spectra (Figure 2) shows typical bands for TPA at 987 cm−1 and 897 cm−1 corresponding to (νasym WO) and

moles of substrate reacted moles of catalyst × reaction time

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. EDX analysis results are shown in Table 1. Analytically calculated weight percentages of elements were comparable statistically with the results obtained from EDX analysis.

Figure 2. FT-IR spectra of (a) Hβ, (b) fresh TPA3/Hβ, (c) recycled TPA3/Hβ, and (d) TPA. 6026

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stretching vibrations of (W−Oc−W) that correspond to corner sharing oxygen, respectively, and P−O stretching vibration at 1080 cm−1 are clearly observed. The FT-IR spectra for Hβ and TPA3/Hβ (Figure 2) shows large and broad peak appearing in range 1060−1090 cm−1 due to asymmetric stretching vibration O−T−O (νasym), which is sensitive to the silicon and aluminum contents in the zeolite framework. A broad band between 3700 and 3200 cm−1 is assigned as hydrogen bonds of silanol groups, associated with the high defect density of β zeolite and/or as Si−O(H)−Al bridges hydrogen bonded to neighboring oxygens in the zeolite framework.23 A band at 455 cm−1 is characteristic of the pore opening. The typical bands for TPA, at 987 cm−1 and 897 cm−1 corresponding to W−O (νasym) and stretching vibrations of (W−Oc−W) that corresponds to corner sharing oxygens, respectively, are clearly observed in TPA3/Hβ. FT-IR spectra indicate that the TPA anions preserve Keggin unit even after anchoring to support. Figure 3 shows XRD patterns of support Hβ, fresh TPA3/ Hβ, and recycled catalyst TPA3/Hβ. Retention of major

pattern with three stages: monolayer adsorption of nitrogen on the walls of mesopores (P/Po < 0.4), the part characterized by a steep increase in adsorption due to capillary condensation in mesopores with hysteresis (P/Po = 0.4−0.8), and multilayer adsorption on the outer surface of the particles. It was observed that pore diameter was decreased after anchoring TPA on to the support.14 SEM images of Hβ and TPA3/Hβ are shown in Figure 5. The surface morphology of the catalyst is almost identical to that of the support. This indicates that the framework structure of the zeolite support is retained after incorporation of TPA. Also, the dark color of the catalyst compared to support indicates good dispersion of TPA ions on to the supports. TPD of ammonia is a useful method to determine the surface acid sites of solid acids. The amount of desorbed ammonia can be considered as the amount of the acid sites on the sample. The strength of the acid sites also can be determined by desorption temperature. The higher the ammonia desorption temperature is, the stronger the acid sites are. NH3-TPD profiles of the catalyst and the support are illustrated in Figure 6. Acidic amount determined by the amount of desorbed ammonia (peak area) and acidic strength determined by desorption peak position (temperature) are summarized in Table 2. There are two ammonia TPD peaks in the profiles, which means there are weak acid sites and strong acid sites on the samples. On the support, weak acid sites are dominant and strong acid sites just form a little shoulder peak. However, in the case of catalyst, the strong acid sites are more predominant than weak acid sites. Also, the acid strengths are shifted to slightly higher temperatures for the catalyst. 3.2. Catalytic Activity. The esterification of free fatty acid is an equilibrium-limited reaction. To overcome the equilibrium limitation, generally, esterification of free fatty acids is carried out by taking alcohol in excess to favor the forward reaction. The esterification of oleic acid with alcohol over the present catalyst is shown in Scheme 1. The effect of various reaction parameters such as acid/ alcohol molar ratio, amount of catalyst, reaction time, and temperature were studied to optimize the conditions for maximum conversion. To study the effect of % loading (Figure 7) esterification reaction was carried out with TPA1/Hβ, TPA2/Hβ, TPA3/Hβ, and TPA4/Hβ. Both 10 and 20% loaded catalysts showed lower conversions. Also, there was not much difference in the conversion by 30 and 40% loaded catalysts. Therefore, 30% loaded catalyst (TPA3/Hβ) was considered for the further studies. To see the effect of the mole ratio, the esterification reaction was carried out by varying the mole ratio of oleic acid to methanol, with 0.1 g of the catalyst for 6 h at 60 °C. It was observed from Figure 8 that the oleic acid conversion increases with an increase in the oleic acid/methanol ratio and reaches a maximum of 83.7% at the oleic acid/methanol mole ratio of 1:20. With a further increase in the molar ratio, there is only a small increase in conversion. Hence, the molar ratio of 1:20 is optimum for obtaining high conversion products. The effect of the amount of catalyst on oleic acid conversion was investigated. The catalyst amount was varied in the range 0.025−0.150 g. As shown in Figure 9, the oleic acid conversion increased with the increase in a catalytic amount of TPA3/Hβ and reaches a maximum of 83% conversion. However, with a further increase in the amount of catalyst, the oleic acid

Figure 3. X-ray diffraction patterns of (a) Hβ, (b) recycled TPA3/Hβ, (c) fresh TPA3/Hβ.

characteristic peaks in the catalyst confirms the retention of framework structure of zeolite after incorporation of TPA ions. A sharp XRD peak at 2θ equal to 26° corresponds to the characteristic peak of TPA ions was observed in the case of TPA3/Hβ. This confirms the retention of Keggin unit after supporting on to Hβ supercages.24 The data for surface area, pore width, and NH3-TPD are presented in Table 2. Specific surface area and pore diameter strongly decreased for TPA-containing Hβ relative to the starting support. The decrease in surface area is the first evidence of chemical interaction between TPA and Hβ. The nitrogen adsorption isotherms of support and catalyst are displayed in Figure 4. Both samples showed Type (IV) Table 2. Textural Properties of Support Hβ and Catalyst TPA3/Hβ NH3 acidity, mmol/g

acid strength, °C

sample

surface area (m2/g)

pore width (nm)

weak

strong

weak

strong

Hβ TPA3/Hβ

587.25 439.42

2.48 1.71

0.677 0.697

0.358 0.745

195 198

389 393 6027

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Figure 4. Nitrogen sorption and pore size distribution of (a) Hβ and (b) TPA3/Hβ.

Figure 5. SEM images of (a) Hβ, (b) fresh TPA3/Hβ, and (c) recycled TPA3/Hβ.

conversion remains constant. The maximum conversion reaches 83% with 0.1 g of catalyst. The effect of reaction time on conversion of oleic acid was investigated. It was observed (Figure 10) that the oleic acid conversion increases with an increase in reaction time. The oleic acid conversion was 84% in 6 h. After 6 h, no significant increase in conversion was observed. The effect of reaction temperature on oleic acid conversion was studied, and it was found that with an increase in reaction temperature % conversion increases (Figure 11). At 60 °C, 84% conversion of oleic acid was achieved in 6 h of reaction time.

The optimized conditions for esterification of oleic acid over TPA3/Hβ are as follows: mole ratio of acid to alcohol 1:20; amount of catalyst 0.1 g; reaction temperature 60 °C, and reaction time 6 h. The control experiment with Hβ support was carried out under optimized conditions. It can be seen from the Table 3 that Hβ is not much active toward the esterification of oleic acid indicating catalytic activity is due to TPA. Thus, we were successful in synthesis of a heterogeneous catalyst and in overcoming the traditional problems of homogeneous catalyst. 6028

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Figure 9. Effect of amount of catalyst. Reaction conditions: mole ratio, 1:20; reaction temperature, 60 °C; reaction time, 6 h.

Figure 6. NH3-TPD curves of (a) Hβ and (b) TPA3/Hβ.

Scheme 1. Esterification of Oleic Acid with Methanol over TPA3/Hβ

Figure 10. Effect of reaction time. Reaction conditions: mole ratio, 1:20; amount of catalyst, 0.1 g; reaction temperature, 60 °C.

Figure 11. Effect of reaction temperature. Reaction conditions: amount of catalyst, 0.1 g; mole ratio, 1:20; reaction time, 6 h.

Table 3. Control Experiments over Support and Catalysta Figure 7. Effect of % loading of TPA onto Hβ. Reaction conditions; mole ratio of oleic acid (0.01 mol)/methanol (0.2 mol), 1:20; amount of catalyst, 0.1 g; reaction temperature, 60 °C; reaction time, 6 h.

catalyst

% yield

Hβ TPAb TPA3/Hβ

23 89 84

a

Amount of catalyst, 0.1 g; mole ratio of alcohol/oleic acid, 20; reaction temperature, 60 °C; reaction time, 6 h. bcatalyst quantity of 23 mg.

3.3. Heterogeneity Test. Any leaching of the active species from the support makes the catalyst unattractive, and hence, it is necessary to study the stability as well as leaching of TPA from the support. Rigorous proof of heterogeneity can be obtained only by filtering the catalysts at the reaction temperature before completion of the reaction and testing the filtrate for activity.25 A test was carried out by filtering the catalyst from the reaction mixture at 60 °C after 4 h, and the filtrate was allowed to react up to 6 h. The reaction mixture of 6 h and the filtrate

Figure 8. Effect of mole ratio. Reaction conditions: amount of TPA3/ Hβ, 0.1 g; reaction temperature, 60 °C; reaction time, 6 h. 6029

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acid under present optimized conditions with TPA3/SBA-15 showed 75% conversion whereas the present reported catalyst showed 84% conversion. This study reveals the advantage of zeolite based support (zeolite β) over other metal oxides for esterification of FFA, which demands high porosity as well as strong acidity. 3.6. Kinetic Study. A detailed study on the kinetic behavior was carried out for esterification of oleic acid over TPA3/Hβ. In all the experiments, reaction mixtures were analyzed at a fixed interval of time using gas chromatography. Esterification of oleic acid with methanol was carried with a 1:20 molar ratio; since methanol was taken in large excess, the rate law is expected to follow first-order dependence. The plot of ln Co/C vs time (Figure 13) shows a linear relationship of oleic acid consumption with respect to time.

were analyzed by gas chromatogram. There was no change in the % conversion, indicating the present catalyst falls into category C.25 On the basis of these results, it can be concluded that there is no leaching of TPA species from the support, and the present catalyst is truly heterogeneous in nature. 3.4. Recycling of the Catalyst. The catalyst was recycled in order to test its activity as well as stability. The catalyst was separated from the reaction mixture only by simple filtration; the first washing was given with methanol to remove unreacted oleic acid, then the subsequent washings were done by conductivity water and then dried at 100 °C, and the recovered catalyst was charged for the further run. No appreciable decrease in the conversion of oleic acid using a regenerated catalyst was observed for four cycles (Figure 12). FT-IR spectra

Figure 12. % conversion of methyl oleate with recycled catalyst. Reaction conditions: amount of catalyst, 0.1 g; acid to alcohol mole ratio, 1:20; reaction temperature, 60 °C; reaction time, 6 h. Figure 13. First-order plot for esterification of oleic acid over TPA3/ Hβ.

of the used catalyst (Figure 2) shows retention of typical bands of the fresh catalyst, which suggest the catalyst is unchanged even after four cycles of the catalytic reaction. XRD patterns of the reused catalyst (Figure 3) showed retention of major characteristic peaks of the catalyst. SEM micrograph of the reused catalyst (Figure 5) shows surface morphology was unchanged. 3.5. Comparison with the Reported Catalysts. It is observed from Table 4 that carbon coated alumina28 (CCA) gives high conversion but it consumes high alcohol content whereas CAT 4 catalyst requires high alcohol, high amount of catalyst, and long reaction time. Better conversion was obtained by M19S4 clay,30 but it requires high reaction temperature of 160 °C and excess alcohol content. TPA2/ZrO220 shows considerable activity at high temperature under nitrogen atmosphere. TPA3/SBA-15 shows high conversion (90%) under mild conditions, but the methanol/oleic acid mole ratio is doubled over the present catalyst. Esterification of oleic

With an increase in reaction time, there is a gradual and linear decrease in the oleic acid concentration over TPA3/Hβ catalyst. These observations indicate the esterification of oleic acid follows first-order dependence with respect to time. This was further supported by the study of the effect of catalyst concentration on the rate of esterification of oleic acid. The catalyst concentration was varied from 2 × 10−3 to 13 × 10−3 mmol at a fixed substrate concentration of 10 mmol and at a temperature of 60 °C. It can be observed from Figure 14 that the rate of reaction increases linearly with an increase in the catalyst concentration. 3.7. Estimation of Activation Energy (Ea). The graph of ln k vs 1/T was plotted (Figure 15) and the value of activation energy (Ea) was determined from the plot. The value of Ea and

Table 4. Comparison of Conversion of Oleic Acid with Reported Catalyst

a

catalyst

reaction conditionsa

conversion %

remarks

ref

CMK-3-873-SO3H protonated-nafion 6% WO3/USY carbon coated alumina carbon based CAT 4 M19S4 clay TPA2/ZrO2 TPA3/SBA-15 TPA3/Hβ

0.43:10:80:10 1.25:10:80:10 0.28:6:200:2 0.05:66:65:6 0.42:60:65:24 0.05:60:160:4 0.1:6:100:4 0.1:40:40:4 0.1:20:60:6

52 59 80 90 94 95 88 90 84

low conversion low conversion very high temperature high alcohol to oleic acid ratio high catalyst and alcohol content, long reaction time high temperature, high alcohol to acid ratio high temperature, use of nitrogen atmosphere high mole ratio mild reaction conditions, easy regeneration

26 26 27 28 29 30 20 21 this work

Reaction conditions = amount of catalyst (g): mole ratio of alcohol/oleic acid: reaction temperature °C: reaction time (h). 6030

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Table 5. Koros−Nowak Test for Heat and Mass Transfer Limitationsa catalyst

amount of active TPA, mg

TOF, min−1

TPA1/Hβ TPA2/Hβ TPA3/Hβ TPA4/Hβ

9.1 16.6 23.0 28.6

2.27 2.28 2.35 2.30

a

Reaction conditions: mole ratio oleic acid, 1/20; amount of catalyst, 0.1 g; reaction temperature, 60 °C; reaction time, time at which the same oleic acid conversion was observed.

Figure 14. Plot of reaction rate vs catalyst concentrations.

oils (Scheme 2). One example of both edible and nonedible oil was taken as a model experiment for transesterification reaction over TPA3/Hβ. Waste cooking oil (WCO) and jatropha oil (JO) are an attractive low cost feedstock which can help in improving the economic feasibility of biodiesel production. The typical reaction of transesterification was carried out in a 100 mL round reactor, provided with thermometer, mechanical stirring, and condenser. The oil sample (WCO or JO) was taken with methanol in a 1:8 w/w ratio and followed by catalyst addition (6 wt. % of oil), and then, the reaction mixture was held at 60 °C for 20 h with stirring at 800 rpm in order to keep the system uniform in temperature and suspension. After the reaction is completed, the mixture was rotary evaporated at 50 °C to separate the methyl esters. The conversion of FFA in the WCO/JO to biodiesel was calculated by means of the acid value (AV) of the oil layer with the following equation.34

the pre-exponential factor (A) were determined using the Arrhenius equation.

⎛ AVOL ⎞ ⎟ × 100 Conversion (%) = ⎜⎜1 − AVWCO/JO ⎟⎠ ⎝

Figure 15. Arrhenius plot for determination of activation energy.

The rate constant, k for the esterification of oleic acid with methanol by using TPA3/Hβ at 60 °C was found to be 4.8 × 10−3 min−1. The pre-exponential factor (A) and activation energy (Ea) were found to be 5.4 × 104 min−1 and 45.2 kJ mol−1, respectively. It is important to recognize whether the reaction rate is diffusion limited/mass transfer limited or it is truly governed by the chemical step where the catalyst is being used to its maximum capacity. It is reported that the activation energy for diffusion limited reactions is as low as 10−15 kJ mol−1, and reactions whose rate is governed by a truly chemical step usually show activation energy excess of 25 kJ mol−1.31 In the present case, the observed activation energy is 45.2 kJ mol−1, and hence, the rate is truly governed by the chemical step. 3.8. Koros−Nowak Test. Koros and Nowak proposed an elegant experimental test to identify heat or mass transfer limitations in measurements of catalytic rates.32,33 If the observed TOFs (turnover frequencies) are the same, it can be concluded that, under tested conditions, rates are not subjected to heat or mass transfer limitations. In the present case, rate measurements were carried on the catalyst with similar dispersion but different active species loading (in the present case TPA). The comparison was done at the same conversion of oleic acid. As shown in Table 5, the observed TOFs are the same at different TPA loadings, and hence, the rates are not heat and mass transfer limited. 3.9. Transesterification of Waste Cooking Oil and Jatropha Oil. Transesterification is a universal and established method for biodiesel production from vegetable and other fatty

Where, AV and OL refer to acid value and oil layer, respectively. The conversion of FFAs in WCO and JO is 83.9% and 92.6%, respectively. The superiority lies in getting very high conversions of FFAs for biodiesel production under mild conditions from waste cooking oil and jatropha oil over the present catalyst.

4. CONCLUSIONS FT-IR and XRD spectra show that TPA remains intact even after anchoring to Hβ. SEM images shows that TPA is uniformly dispersed on the entire pores of the zeolite Hβ. BET analysis shows decrease in both pore diameter and surface area which indicates that TPA anions goes inside the channels of the support. The present catalyst exhibits excellent activity for the esterification of oleic acid with methanol under mild conditions. Also, the catalyst shows potential for being used as a recyclable catalytic material after simple regeneration without any significant loss in the conversion. Kinetic study shows that esterification of oleic acid follows first-order rate law. Studies also reveal that the catalyst can be used for biodiesel production from WCO and JO without any pretreatment. Hence, the present catalyst can be employed as an efficient environmentally benign catalyst, for the feedstocks that are rich in FFAs where simultaneous esterification of FFAs and transesterification of TGs using heterogeneous solid acid catalyst provide an alternative single step process for biodiesel production. 6031

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Scheme 2. Biodiesel Production from Transesterification of Triglyceride over TPA3/Hβ

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +91- 265 2795552. E-mail: aupatel_chem@yahoo. com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful to UGC (39-837/2010 (SR)) for the financial support. One of the authors, Mr. Nilesh Narkhede, is also thankful to UGC, New Delhi, for the award of Research Fellowship.

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dx.doi.org/10.1021/ef301126e | Energy Fuels 2012, 26, 6025−6032