Synthesis of Biodiesel from Vegetable Oil Using Supported Metal

Mar 27, 2014 - (1) Inedible vegetable oil, mostly produced from tree-borne seeds, is an attractive low-cost feedstock for biodiesel production. The oi...
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Synthesis of Biodiesel from Vegetable Oil Using Supported Metal Oxide Catalysts Dheerendra Singh,† Rohidas Bhoi,‡ Anuradda Ganesh,† and Sanjay Mahajani*,‡ †

Department of Energy Science and Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India Department of Chemical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India

‡

S Supporting Information *

ABSTRACT: Biodiesel is known for its less polluting, renewable, and biodegradable properties. It is conventionally produced by the alkali-catalyzed transesterification of triglycerides. The use of a heterogeneous catalyst can make the production process costeffective and environmentally friendly. In this work, we evaluated two catalysts, ZnO/zeolite and PbO/zeolite, for the synthesis of biodiesel using jatropha oil as a feedstock. Both catalysts exhibit reasonably good activity for the reaction of interest and are reusable under the reaction conditions. The leaching of metal ions during the course of the reaction is minimized by using zeolite as a support. The catalysts were characterized by X-ray diffraction, N2 adsorption, transmission electron microscopy, scanning electron microscopy, and temperature-programmed desorption/temperature-programmed reduction. The PbO/zeolite catalyst performed better than the ZnO/zeolite catalyst when sunflower oil, which is free of fatty acids, was used as the feedstock. However, ZnO/zeolite was more active when jatropha oil, which contains substantial amounts of free fatty acids (>10% w/w), was used as the feedstock.

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INTRODUCTION Biodiesel, a mixture of fatty acid alkyl esters (FAAEs), is an attractive biofuel because of its renewability and environmental benefits.1 Inedible vegetable oil, mostly produced from treeborne seeds, is an attractive low-cost feedstock for biodiesel production. The oils/fats mainly consist of triglycerides (TGs), formed by ester bonding of long-chain fatty acids to glycerol molecules. Transesterification is a reversible reaction that occurs via displacement of glycerol by alcohols in TGs, producing FAAEs (i.e., biodiesel) as the main product and glycerol as the byproduct. This process requires excess alcohol (molar ratio >3:1) to drive the reversible reaction in the forward direction to obtain a higher FAAE yield and achieve phase separation of glycerol. Alkaline catalysts (NaOH/KOH) are commonly used for this reaction because their reaction rates are higher than those of their acidic counterparts.2 Most developed countries use edible oils such as soybean, canola, sunflower, rapeseed, and palm oil to produce biodiesel. These oils contain low amounts of free fatty acids (FFAs) (98% pure), zinc nitrate, and sodium bicarbonate, all of which were analytical reagent grade, were procured from Merck Chemicals India, Ltd. The zeolites, ZSM-5 (Si/Al ratio = 38) and β-zeolite (CP814T and CP811C), were supplied by Zeolyst International. CP811C and CP814T represent the two types of β-zeolites having Si/Al ratios of 300 and 40, respectively. To characterize the jatropha oil, it was subjected to homogeneous catalysis, first using sulfuric acid for esterification and then using KOH for transesterification. Both reactions were performed by adding excess methanol, with a methanol to FFA molar ratio of 20:1 for esterification and a methanol to oil molar ratio of 10:1 for transesterification. Highpressure liquid chromatography (HPLC) analysis (described later) was performed to make sure that almost complete conversion of the oil to the corresponding FAMEs was realized. The purified biodiesel sample was then diluted in isopropanol and injected into a gas chromatography−mass spectrometry instrument (HP model GCD-1800A) to determine the ester composition and back-calculate the corresponding amounts of TGs and fatty acids present in the jatropha oil. This allowed us to calculate the average molecular weight of the oil. Refined sunflower oil purchased from the local market was also used in a few runs to evaluate the performance of the different catalysts. Table 2 shows the 2744

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composition of the various TGs/fatty acids in these oils that were used as feedstocks. Catalyst Preparation. The 25 wt % ZnO/zeolite28 used here was prepared by the hydrothermal impregnation precipitation (HIP) method, as reported for ZnO/MCM-41.29 The notation “25% ZnO/ zeolite” represents 25 wt % ZnO and 75 wt % zeolite. Zinc nitrate (1 M solution), as a precursor for zinc, was mixed with zeolite in a ZnO to zeolite weight ratio of 1:3. Urea, as a precipitating agent, was added to the mixture with a urea to zinc molar ratio of 2:1. The mixture was stirred at 85 °C under nitrogen at a pressure of 4 bar for 10 h in a 300 mL Teflon-lined stainless steel autoclave. The mixture was filtered, and the solid obtained was dried at 110 °C for 14 h. The catalyst was calcined from 110 to 500 °C at a rate of 10 °C·min−1 and maintained at 500 °C for 3 h. A ZnO loading of 24.86% was confirmed by energy-dispersive Xray (EDX) spectrometry and inductively coupled plasma atomic emission spectroscopy (ICP-AES). A similar process was used to prepare the 25% ZnO/alumina. Furthermore, 25% PbO/alumina and 25% PbO/zeolite catalysts were also synthesized by a similar method using Pb(NO3)2 as a precursor. Reaction Procedure. Biodiesel synthesis was carried out in a 300 mL batch autoclave (model 4843, Parr Instrument Company; maximum temperature 350 °C, maximum pressure 199 bar). Excess methanol was used to ensure a higher biodiesel yield. At a reaction temperature of 200 °C, the autogenous pressure in the autoclave was close to 37 bar. After transesterification, the reaction mixture was centrifuged to separate the solid catalyst. The reaction mixture comprising two immiscible liquid phases was distilled to recover the excess methanol. The residue was centrifuged; glycerol formed the heavy lower layer, and the mixture of biodiesel and unconverted oil formed the top layer. Gradient-elution reversed-phase liquid chromatography was used to quantitatively analyze the compounds present in the upper layer. A known weight of the top layer of the reaction mixture was collected in vials and dissolved in an isopropanol/hexane (5:4 v/v) mixture. HPLC was used to separate fatty acids, monoglycerides (MGs), methyl esters, diglycerides (DGs), and TGs using a combination of eluents. Eluent A was water, eluent B was acetonitrile, and eluent C was a 5:4 (v/v) mixture of 2-propanol and n-hexane. A 30 min ternary gradient with two linear gradient steps was employed: 30% A + 70% B in 0 min, 100% B in 10 min, 50% B + 50% C in 20 min, and isocratic elution with 50% B + 50% C for the last 10 min.30 The solvent flow rate was maintained at 1 mL/min, and the injection volume was 20 μL. Catalyst Characterization. The catalyst was characterized by various techniques, including X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), temperature-programmed desorption/temperature-programmed reduction (TPD/TPR) and N2 adsorption by the Brunauer−Emmett− Teller (BET) method. For XRD, the catalyst powder was analyzed using an X’Pert diffractometer (PANalytical). The data were collected over a 2θ range of 5−90° using Cu Kα radiation (λ = 1.5418 Å) at 40 kV and 40 mA. The BET surface area was obtained by N2 adsorption−desorption isotherms measured at −196 °C on an ASAP-2020 instrument (V3.01H, Micromeritics). Approximately 0.2 g of powder was first degasified at 400 °C for 400 min and then used for the analysis. TEM analysis was performed on a high-resolution field-emission transmission electron microscope (JEM-2100F, JEOL) using an accelerating voltage of 200 kV. A well-dispersed solution was prepared by adding a small amount of catalyst powder to ethanol and sonicating it for 10−15 min. One drop of the dispersed solution was placed on a TEM grid and dried under an IR lamp for 30 min. To determine the surface morphology, SEM analysis was performed (JSM-7600F, JEOL). The catalyst powder was directly sprinkled over the carbon tape, and images were taken under the best operating conditions. TPD analysis of the catalyst was performed on an AutoChem II instrument (2920 V3.03, Micromeretics) using 10% ammonia in helium as a basic probe molecule. TPR analysis was performed on a TPDRO instrument (Thermo 1100, Thermo Scientific). Metal oxide leaching was studied using ICP-AES (ARCOS, M/s. Spectro, Germany). Reaction samples were analyzed by HPLC (Agilent technologies, 1200 series, USA) using a C18 column (ZORBAX ODS 4.6 mm × 250 mm, 5 μm).

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RESULTS AND DISCUSSION XRD Analyses. ZnO/Alumina and ZnO/Zeolite. Figure 1a shows the XRD patterns of ZnO/alumina and ZnO/zeolite at a

Figure 1. (a) XRD patterns of ZnO/alumina and ZnO/zeolite at 25 wt % ZnO loading. (b) XRD patterns of PbO, PbO/γ-alumina, and PbO/ zeolite at 25 wt % PbO loading. T denotes the tetragonal phase and β the β-PbO phase.

ZnO loading of 25 wt % and a calcination temperature of 500 °C. The diffraction peaks of ZnO obtained from the ZnO/alumina and ZnO/zeolite were at 2θ = 31.9, 34.7, 36.4, 47.7, 56.8, and 63.10°, which correspond to the (100), (002), (101), (102), (110), and (103) lattice planes, respectively. The XRD patterns of ZnO were consistent with the values in the JCPDS database (no. 36-1451). The XRD patterns of ZSM-5 and β-zeolite in the ZnO/ZSM-5 and ZnO/β-zeolite catalysts were consistent with their standard peaks. Figure 1a shows the two separate phases (ZnO and zeolite). The average crystallite size of ZnO in the ZnO/ZSM-5 catalyst as estimated by the Scherrer equation was 22.15 nm. PbO/γ-Alumina and PbO/Zeolite. Figure 1b shows the XRD patterns of pure PbO, PbO/γ-alumina, and PbO/zeolites at a PbO loading of 25 wt % and a calcination temperature of 500 °C. The XRD pattern of PbO in the synthesized PbO/γ-alumina were consistent with the pure PbO XRD pattern, corresponding to ICDD PDF card no. 72-0093. The synthesized PbO in this 2745

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case occurs in two phases: the tetragonal α-phase and the βphase. The diffraction peaks (tetragonal phase, corresponding to ICDD PDF card no. 72-0093) at 2θ = 29.1, 34.4, 35.9, 38.1, 45.8, 60.1 and 66.7 correspond to the (111), (210), (120), (002), (300) (222), and (131) lattice planes, respectively. The diffraction peaks at 2θ = 17.9, 32.1, 48.8, and 54.9, represent the β-PbO phase, corresponding to ICDD PDF card no. 030561. The XRD pattern for the zeolite-supported catalysts did not show any relevant peak positions corresponding to the standard PbO. This suggests that while the PbO in PbO/γ-alumina is crystalline in nature, its crystallinity declines drastically when it is in the zeolite-supported form. This was also confirmed by the TEM images (see Figure 3b). The reason for this finding is unknown. Surface Area Measurements. The BET surface areas (SBET) of the as-supplied ZnO, ZSM-5, and β-zeolite (CP814T) and of ZnO/ZSM-5, ZnO/β-zeolite, PbO/ZSM-5, and PbO/βzeolite (25 wt % ZnO and PbO) were determined using a BET apparatus and are shown in Table 3. The nature of the type IV

The ZnO particle size was 14−26 nm, and the average particle size was 19.54 nm (Figure 2a inset). The ZnO particles were larger than the average pore diameter of zeolite (2.3 nm from BET analysis); hence, much of the ZnO was deposited on the external surface of the zeolite. A similar distribution of ZnO over the support was observed in the case of ZnO/β-zeolite. The histogram in the Figure 3a inset shows that the particle size of PbO was 2.9−6.8 nm with an average particle size of 4.2 nm. Selected-area electron diffraction (SAED) of the TEM image (Figure 3b) showed no ring pattern, confirming the amorphous nature of PbO in agreement with the XRD results as mentioned above (Figure 1b). PbO/ZSM-5 also exhibited a uniform distribution, as shown in Figure 3. A similar distribution of PbO over the support was observed in the PbO/β-zeolite case. SEM and EDX Analyses. Figure 4a,b shows SEM images of ZnO/ZSM-5 and PbO/ZSM-5, respectively. It was evident from the SEM analysis that the shapes of the zeolite (support) particles were nonuniform, and the particle size distribution was large with sizes varying from 50 to 300 nm. The particle size of the zeolite support was much larger than the average particle size of either ZnO (19.5 nm) or PbO (4.2 nm) measured by TEM analysis. Elemental mapping is useful for determining the catalyst distribution in the textural context. Figure 5 shows the EDX elemental maps for ZnO/alumina, ZnO/zeolite, and PbO/ zeolite. Figure 5a shows that the Zn metal particles in ZnO/ alumina were locally present as agglomerates, which led to low metal distribution. On the other hand, in the case of ZnO/ZSM-5 (Figure 5b), Zn was well-distributed over the external surface of the support. EDX mapping further supported the HRTEM results of the spatial distribution of ZnO over the zeolitic support. A similar uniform distribution of PbO over ZSM-5 was observed with the PbO/ZSM5 catalyst (Figure 5c). Metal−Support Interactions As Determined by TPD/ TPR. The catalysts used for the TPD and TPR analyses were pretreated with 5% H2/He at a flow rate of 20 mL/min and temperature ramp rate of 20 °C/min up to 350 °C. The temperature was then raised to 400 °C and maintained there for 60 min under argon at a flow rate of 20 mL/min. This pretreatment ensured that the metal oxide on the zeolite surface was free from impurities such as chlorides or nitrates. TPD Analyses. For the TPD analyses of ZSM-5 and ZnO/ ZSM-5, the samples were first heated to 600 °C, and the ammonia pulses were introduced at room temperature until saturation. The temperature was varied from 50 to 600 °C at a rate of 5 °C/min. The TPD results are shown in Figure 6. Two distinct peaks were observed in the case of ZSM-5, corresponding to weak and strong acid sites. The zeolite contains both Brønsted (H form) and Lewis acid sites as a result of charge imbalances induced by the trivalent aluminum. ZnO is amphoteric in nature; it can behave as either an acid or a base. As shown in Figure 6, the strong-acid peak of ZSM-5 was not observed in the TPD results for the ZnO/ZSM-5 sample. This indicates that being basic in nature, ZnO is adsorbed on the strong acid sites by the donation of an electron pair to electrondeficient ZSM-5. Alumina, on the other hand, in it is both α and γ forms, is relatively neutral in nature and thus interacts less with ZnO. Hence, there are stronger metal−support interactions in the case of zeolite than in the case of alumina. These results were also confirmed by leaching tests on the catalysts. The TPR of PbO on alumina and zeolite also shows similar behavior. Figure 6 shows that after ZnO deposition, the strong acid sites of ZSM-5 disappear and only the weak acid sites on the uncovered surface of ZSM-5 are present.

Table 3. Textural Properties of ZnO, Zeolite, ZnO/Zeolite, and PbO/Zeolite Samples textural properties sample

SBET (m2/g)

pore volume (cm3/g)

average pore size (Å)

ZnO ZSM-5 β-zeolite (CP814T) 25% ZnO/ZSM-5 25% PbO/ZSM-5 25% ZnO/β-zeolite 25% PbO/β-zeolite

3.35 400.98 623.47 297.31 232.09 422.50 432.75

0.0132 0.2333 0.3435 0.1827 0.1419 0.2655 0.2674

157.44 23.28 22.03 24.58 24.45 25.93 24.72

isotherm did not change even after the catalyst was loaded on the zeolite support. The surface areas of ZSM-5 and β-zeolite were 400.98 and 623.47 m2/g, respectively, whereas the surface areas of 25% ZnO/ZSM-5 and 25% ZnO/β-zeolite were 297.31 and 422.5 m2/g, respectively. The drop in the surface area and pore volume possibly could be explained by the reduction in the zeolite content of the catalyst (from 100% to 75%). Alternatively, blockage of pores due to loading of the metal oxide on the support could have led to the decrease. The BET surface area of pure zinc oxide is 3.35 m2/g; hence, the zeolite is the main contributor to the total surface area of the catalyst. A similar reduction in surface area was also observed in the case of the PbO/ZSM-5 and PbO/β-zeolite (CP814T) catalysts. The catalyst is present mainly on the external surface of the zeolite support. Hence, the reduction in the pore surface area of the support is not relevant here since the main role of the zeolite is to hold the ZnO particles and prevent them from leaching. The average pore sizes of the different zeolites and their supported metal oxide catalysts remain largely unchanged, and the catalyst activity was independent of the type of microporous support chosen for the esterification/transesterification reaction. TEM Analyses. TEM imaging was used to determine catalyst morphology and particle size. TEM images of ZnO/ZSM-5 are shown in Figure 2a,b. The ZnO was well-distributed on the external surface of the zeolite. On the other hand, in the case of ZnO/alumina, the particle sizes of ZnO and alumina were comparable, as shown in Figure 2c,d. 2746

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Figure 2. TEM images of (a, b) the ZnO/ZSM-5 catalyst and (c, d) ZnO/alumina. The inset in (a) is the ZnO particle size distribution, and the inset in (b) is the SAED pattern.

Figure 3. TEM images of PbO/ZSM-5.TEM images of PbO/ZSM-5. The inset in (a) is the PbO particle size distributionand the inset in (b) is the SAED pattern.

desorption peaks were not realized in either case. TPR was used to determine the effect of the precursors as well as the types of interactions between the active precursor and the support. TPR of the ZnO catalyst with 5% H2/He up to 850 °C did not result in any reduction peaks. It is evident from the literature that ZnO is reduced above 1400 °C, but because of instrumental limitations, we were unable to confirm this behavior. In the case of PbO, unlike ZnO, a reduction peak was observed under the identical reduction conditions. Figure 7 shows the TPR profiles of PbO, PbO/alumina, and PbO/ZSM-5. The observed peak for pure PbO was sharp, while it was broad for the supported catalysts, PbO/Al2O3 and PbO/ZSM-5. The shift in the reduction peak in response to higher temperature occurred in the following order: PbO < 25% PbO/alumina < 25% PbO/ZSM-5 = 30% PbO/

Metal−support interactions are highly influenced by the acidic or alkaline nature of the support and the nature of the metal precursors. As the alkalinity of the support increases, the metal ionization potential decreases, that is, a shift to lower binding energy occurs because of the electrostatic Coulombic interaction between the support material and the metal particle. Mojet et al.31 confirmed a decrease in the ionization potential of the metal valence orbitals with increasing support alkalinity by IR, X-ray photoelectron spectroscopy, and X-ray absorption near edge structure analyses. TPR Analyses. Pulsed chemisorption with CO and H2 gas was performed for ZnO and PbO, but the adsorption was negligible. The peak area for the pulse showed a negligible deviation, so it was difficult to proceed with desorption. As expected, the 2747

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Figure 4. SEM images of (a) ZnO/ZSM-5 and (b) PbO/ZSM-5.

Figure 5. Elemental mapping of the catalysts: (a) ZnO/alumina; (b) ZnO/ZSM-5; (c) PbO/ZSM-5.

indicated that the PbO leaching was as low as 10 ppm (Table 4), which is lower than that of ZnO/zeolite. The minimal leaching of ZnO and PbO in the solution may be attributed to the relatively strong metal−support bonding, as explained in the discussion of the TPD/TPR analyses. Catalyst Reusability. ZnO/ZSM-5 has excellent reusability. Batch experiments were performed using the same catalyst without any pretreatment. The conversion remained similar even after five cycles (Figure 8). Furthermore, the stability of the ZnO/zeolite catalyst was examined for a longer time to explore its industrial potential. The reaction was performed in continuous mode using a fixed-bed reactor (FBR) packed with catalyst pellets (6 mm ID × 8 mm long prepared on a pelletizer using 2 T of mechanical pressure). The continuous reactor setup consisted of high-pressure feed pumps, a preheater, and the FBR, which was heated externally using an electrical heater. A backpressure regulator on the outlet valve helped maintain the desired pressure in the reactor. Oil and methanol in the desired molar ratio were pumped using high-pressure pumps. The two

ZSM-5. The shift in the reduction peak is attributed to the metal−support interaction. ZnO and PbO Leaching. Metal oxide leaching was studied using ICP-AES. Analysis of metal leaching in the product mixture is very important since catalysts with less leaching last longer. The reaction product (i.e., biodiesel) was burned, and the ash that was left behind was dissolved in 6 mL of perchloric acid (78%). The leaching characteristics of ZnO and ZnO/zeolite were studied, and the results are shown in Table 4. The catalyst used in the autoclave was in powdered form. The data in Table 4 show that the leaching of ZnO in free form was more than that of the supported catalysts. Also, the leaching of ZnO/ZSM-5 was much lower than that of the other supported catalysts (ZnO/αalumina and ZnO/γ-alumina). The extent of metal leaching in the glycerol phase was also measured, and it was found that almost all of the leached metal was present in the nonpolar phase.32 Leaching of active species in the reaction medium is mainly caused by weak metal−support interactions.33 ICP-AES analysis of PbO supported on zeolite 2748

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Figure 8. Recyclability of the ZnO/ZSM-5 catalyst (jatropha oil to methanol molar ratio, 1:40; catalyst loading, 1.0 wt %; reaction temperature, 200 °C). Figure 6. Ammonia TPD profiles for (top) ZSM-5 and (bottom) ZnO/ ZSM-5.

reactants were mixed together, and the mixture was passed through the preheater, where it was heated to 150 °C. The preheated mixture was then passed through a tubular FBR containing ZnO/ZSM-5 catalyst in pellet form. The reaction was carried out for 300 h in the FBR, and the product was analyzed using ICP-AES as described earlier. Oil and methanol in the desired molar ratio (1:6) were mixed together and preheated to 150 °C before being sent to the reactor. The reactor was maintained at 200 °C and 40 bar and was packed with 36.5 g of catalyst. An analysis of the reaction product is shown in Figure 9 as a function of time. The metal leaching was initially high (∼26 ppm), but it eventually decreased to 4−5 ppm and remained constant thereafter.

Figure 7. TPR profiles of PbO, PbO/alumina, and PbO/ZSM-5 with 5% H2 in helium.

Table 4. Comparison of Zn/Pb Leaching in Biodiesel on Different Supports sample

Zn/Pb (ppm)

blank (perchloric acid) ZnO powder ZnO/γ-alumina ZnO/α-alumina ZnO/ZSM-5 PbO powder PbO/γ-alumina PbO/β-zeolite

not detected >1238.15 920.575 614.033 127.523 >3400 >464 9.29

Figure 9. Zn leaching and percent TG conversion in a fixed-bed reactor at different times (reaction temperature, 200 °C; jatropha oil to methanol molar ratio, 1:6).

The total amount of metal leached was calculated on the basis of the oil flow rate and the total reaction time. The amount that was leached in 300 h was around 0.8 wt % of the initial amount. No effect of leaching on the TG conversion was observed up to a reaction time of 300 h. In this continuous reaction, an excess of methanol (6:1 molar ratio of methanol to oil) was used. This excess methanol can be removed by vaporization and recycled back to the reactor with fresh methanol. The economics cannot 2749

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be worked out in detail without process design studies, which are the subject of future work. Performance Evaluation. These runs were performed at high molar ratio of methanol to oil (30:1). The idea of using excess methanol is to reach close to the thermodynamic yield in less time. All of the reactions performed in this study were conducted at a sufficiently high speed of agitation (500−600 rpm) with a small enough catalyst particle size (>1 μm) to ensure that they lie in the kinetic regime. The zeolite used in this study had a high Si/Al ratio and was thus weakly acidic, thereby showing no catalytic activity; the metal oxide (i.e., ZnO or PbO) was the only active species in the catalyst. It can be seen from Table 4 that the leaching of metal in the reaction mixture was very low compared with the total catalyst loading used for the reaction. Hence, the effect of homogeneous catalysis due to the leached metal oxide can be considered to be insignificant. This was also confirmed experimentally by performing independent experiments on the reaction mixture with leached metal oxide in the absence of solid catalyst and analyzing the product compositions by HPLC. At equilibrium, the DG and TG peaks were below the detectable limits in the HPLC chromatogram, and only the peaks of biodiesel and small amount of MG were seen. The conversion was calculated on the basis of TG consumption. ZnO/ZSM-5. General Reaction Course. The reactions were conducted in a Parr autoclave, and samples were withdrawn at regular time intervals. Figure 10 shows the trends in the weight percents of the different components [i.e., TG, DG, MG, FFA, and biodiesel (BD)] with respect to time. At 200 °C, BD reached 94.2 wt % in 60 min.

Figure 10. General course of the reaction on ZnO/ZSM-5 (jatropha oil to methanol molar ratio, 1:30; temperature, 200 °C; catalyst loading, 0.50 wt %; 500 rpm).

Figure 11. (a) Effect of ZnO/ZSM-5 catalyst loading on the reaction rate (jatropha oil to methanol molar ratio, 1:20; temperature, 200 °C; 500 rpm). (b) Effect of temperature on the reaction kinetics (jatropha oil to methanol molar ratio, 1:20; catalyst loading, 0.50 wt %; 500 rpm). (c) Effect of jatropha oil to methanol molar ratio on the reaction kinetics (temperature, 200 °C; catalyst loading, 0.50 wt %; 500 rpm).

Effect of Reaction Parameters. a. Catalyst Loading. The catalyst loading was in the range of 0.0625−1.0 wt %. As shown in Figure 11a, the reaction rate increased with increasing catalyst loading up to 0.5 wt % but became insensitive to further increases in catalyst loading. This is a commonly observed trend in most solid-catalyzed reactions; above a certain loading, the number of available sites becomes comparable to that required for the given number of substrate molecules.34 b. Temperature. The reactions were performed over the temperature range from 180 to 210 °C. Figure 11b shows the weight percents of biodiesel at different temperatures, indicating that the reaction rate increased with an increase in temperature.

At 210 °C, the reaction was very fast and reached equilibrium in 40 min. c. Molar Ratio. The use of a large quantity of methanol in the reaction promotes better conversion of oil to biodiesel. Figure 11c shows that the effect is insensitive to a change in molar ratio beyond a certain limit. A molar ratio of 1:20 was sufficient to maximize the reaction rate and approach quantitative conversion in 1 h. ZnO/ZSM-5 versus ZnO/Alumina. Figure 12 compares the performances of the different ZnO-supported catalysts (ZnO/αalumina, ZnO/γ-alumina, and ZnO/ZSM-5). The BET surface areas and average particle sizes of α-alumina and γ-alumina were 2750

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Figure 12. Comparison of ZnO/ZSM-5 and ZnO/α,γ-alumina (jatropha oil to methanol molar ratio, 1:30; temperature, 200 °C; catalyst loading, 0.50 wt %; 500 rpm).

144.85 and 149.23 m2/g and 20−200 nm and 98 μm, respectively. The ZnO/α-alumina showed slightly better catalytic activity than ZnO/γ-alumina and ZnO/ZSM-5. However, as mentioned before, the leaching in the aluminasupported catalyst is much greater than that of the ZnO/zeolite catalyst. PbO/ZSM-5. As expected, the reaction rate increased with an increase in reaction temperature over the range from 160 to 200 °C (Figure 13). Most of the nonedible vegetable oils contain

Figure 14. (a) General course of the reaction for ZnO/β-zeolite vs PbO/β-zeolite (sunflower oil to methanol molar ratio, 1:30; temperature, 200 °C; catalyst loading, 0.5 wt %; 600 rpm). (b) General course of the reaction for ZnO/ZSM-5 vs PbO/ZSM-5 (sunflower oil to methanol molar ratio, 1:30; temperature, 200 °C; catalyst loading, 0.5 wt %; 600 rpm).

100% for PbO/ZSM-5 at 30 min. Furthermore, the two catalysts were tested for jatropha oil (FFAs, 10%; MGs, 5.89%; DGs, 15.7%; TGs, 68.3%). Figure 15a shows that the ZnO/ZSM-5 catalyst exhibited more catalytic activity than PbO/ZSM-5. This may be due to the higher activity of ZnO/ZSM-5 in the esterification reaction. Figure 15b shows the weight percent of FFAs during the course of the reaction catalyzed by both ZnO/ ZSM-5 and PbO/ZSM-5. The reaction rate was higher in the presence of ZnO/zeolite than PbO/zeolite. We also studied the reactions of oleic acid with methanol in the presence of ZnO/β-zeolite, with PbO/β-zeolite, and without catalyst (Figure 16). The reaction rates in the presence of the ZnO/β-zeolite and PbO/β-zeolite catalysts were significantly higher than that without a catalyst. Figure 16 shows that the catalytic activity for esterification is higher for ZnO/β-zeolite than for PbO/β-zeolite. This is one of the reasons why the reaction rate for jatropha oil in the case of ZnO/β-zeolite was higher than that in the case of PbO/β-zeolite. More work is necessary to investigate the exact reason for this behavior of the two catalysts. The PbO/zeolite catalyst appears to be a better catalyst for the transesterification reaction of oil that is free of fatty acid (e.g., sunflower oil), whereas the ZnO/zeolite catalyst may be the preferred choice for the esterification of fatty acids (e.g., oleic

Figure 13. PbO/ZSM-5 performance at different temperatures (molar ratio of methanol to sunflower oil + 5% FFAs, 30:1; temperature, 200 °C; catalyst loading, 1 wt %; 500 rpm).

FFAs, and the content typically varies in the range 5−15%. Hence, in order to check whether the catalyst would work in the presence of FFAs, the reactions were performed on sunflower oil to which 5% FFAs had been added. ZnO/Zeolite versus PbO/Zeolite. ZnO/zeolite and PbO/ zeolite were prepared using two different zeolites as supports, ZSM-5 (Si to Al ratio = 30) and β-zeolite (CP814T, Si to Al ratio = 38). Figure 14 compares the results for the β-zeolite and ZSM5 using sunflower oil (FFAs, 0.2%; MGs, 0.0%; DGs, 2.1%; TGs, 97.7%) as the feedstock. The effect of the support was less significant. The reaction rate for the PbO catalyst was significantly higher than that for the ZnO catalyst. The TG conversion was close to 2751

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5 compares the performances of the various catalysts reported in the literature using jatropha oil as a feedstock. It is evident that Table 5. Performances of Various Heterogeneous Catalysts for Biodiesel Synthesis Using Jatropha Oil as the Feedstock catalyst sulfated tin oxide KNO3/Al2O3 Mg alkoxides (hydrotalcite) alumina-modified Mg− Zn La2O3−ZnO and Li-CaO + Fe2(SO4)3 CaZnO and CaMgO ZnO/zeolite PbO/zeolite

performance

reaction conditionsa

FAME yield 97 wt % oil conversion 84% oil conversion 89% FAME yield 94%

180 °C, 15:1, 2 h, 3% w/w, 360 rpm 70 °C, 12:1, 6 h, 6% w/ w, 600 rpm 70 °C, 12:1, 6 h, 6% w/ w, 600 rpm 182 °C, 11:1, 6 h, 8.68% w/w, high rpm 60 °C, 6:1, 3 h, 5 wt %, 300 rpm 65 °C, 15:1, 6 h, 4 wt %

FAME yield 30% and 100% oil conversion 83% FAME yield 93.8% FAME yield >90%

200 °C, 30:1, 1 h, 1.0 wt %, 600 rpm 200 °C, 30:1, 1 h, 0.5 wt %, 600 rpm

ref 35 36 37 38 39 40 this work this work

a

The reaction conditions include temperature, methanol to oil molar ratio, reaction time, catalyst loading, and speed of agitation.

the performance of our catalyst is comparable to that of other reported catalysts. Furthermore, none of the reported catalysts were tested for their leaching behavior. Physicochemical Properties of FAMEs. The physicochemical properties of the fatty acid methyl esters prepared from jatropha and sunflower oils are reported in Table S1 in the Supporting Information. The biodiesel thus-prepared from metal oxide catalysts meets the ASTM standard and can be used in commercial applications.

Figure 15. (a) General course of the reaction for ZnO/ZSM-5 vs PbO/ ZSM-5 (jatropha oil to methanol molar ratio, 1:30; temperature, 200 °C; catalyst loading, 0.5 wt %; 600 rpm). (b) Esterification of free fatty acids from jatropha oil (jatropha oil to methanol molar ratio, 1:30; temperature, 200 °C; catalyst loading, 0.5 wt %; 600 rpm).

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CONCLUSION We investigated zeolites as supports for ZnO and PbO catalysts. The HIP method for catalyst preparation ensured good distribution of the metal oxide on the support. The particle sizes of ZnO and PbO over zeolite were 19.5 and 4.2 nm, respectively, according to TEM analysis. Both catalysts were promising for feedstocks with or without FFAs. Almost complete TG conversion occurred within 30 min at a reaction temperature of 200 °C. More importantly, Pb and Zn leaching was much less because of the strong interaction between the metal oxide and the support, as observed by TPD/TPR analyses of the catalysts. A detailed kinetic study of the ZnO/ZSM-5 catalyst in batch mode as well as continuous mode will further enhance our understanding of the exact mechanisms involved in the catalysis of this reaction.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 16. Esterification of oleic acid with ZnO/β-zeolite, with PbO/βzeolite, and without catalyst (temperature, 200 °C; oleic acid to methanol molar ratio, 1:10; catalyst loading, 0.7 wt %, 600 rpm).

Physicochemical properties of the FAME prepared from jatropha and sunflower oils. This material is available free of charge via the Internet at http://pubs.acs.org.

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acid) and the combination of esterification/transesterification of oil containing FFAs (e.g., jatropha oil). The conversion of jatropha oil and the FAME yield on ZnO/ zeolite and PbO/zeolite were approximately 100% and 93.8%, respectively at 200 °C, with an oil to methanol molar ratio of 1:30 after 1 h of reaction time with a catalyst loading of 1.0 wt %. Table

AUTHOR INFORMATION

Corresponding Author

*Tel.: +91 22 25767246. Fax: +91 22 25726895. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 2752

dx.doi.org/10.1021/ef500045x | Energy Fuels 2014, 28, 2743−2753

Energy & Fuels

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Article

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ACKNOWLEDGMENTS The authors acknowledge Tata Consulting Engineers Limited (TCE), Mumbai, for offering financial support of this work and Ms. Rohini Tanksale for assisting us in the analysis of the reaction products.

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dx.doi.org/10.1021/ef500045x | Energy Fuels 2014, 28, 2743−2753