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Synthesis of fatty acid methyl esters via the methanolysis of palm oil over Ca3.5xZr0.5yAlxO3 mixed oxide catalyst
Renewable Energy 66 (2014) 680e685
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Synthesis of fatty acid methyl esters via the methanolysis of palm oil over Ca3.5xZr0.5yAlxO3 mixed oxide catalyst H. Amani, Z. Ahmad, B.H. Hameed* School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 April 2013 Accepted 9 January 2014 Available online 11 February 2014
Novel mixed metal oxide catalyst Ca3.5xZr0.5yAlxO3 was synthesized through the coprecipitation of metal hydroxides. The textural, morphological, and surface properties of the synthesized catalysts were characterized via BrunauereEmmetteTeller method, X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, and energy-dispersive X-ray spectroscopy. The catalytic performance of the as-synthesized catalyst series was evaluated during the transesterification of cooking palm oil with methanol to produce fatty acid methyl esters (FAME). The influence of different parameters, including the calcination temperature (300e700 C), methanol to oil molar ratio (6:1e25:1), catalyst amount (0.5e6.5 wt%), reaction time (0.5e12 h) and temperature (70e180 C), on the process was thoroughly investigated. The metal oxide composite catalyst with a Ca:Zr ratio of 7:1 showed good catalytic activity toward methyl esters. Over 87% of FAME content was obtained when the methanol to oil molar ratio was 12:1, reaction temperature 150 C, reaction time 5 h and 2.5 wt% of catalyst loading. The catalyst could also be reused for over four cycles. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Transesterification Heterogeneous catalyst Palm oil Methyl ester
1. Introduction It is coming up that the production of biodiesel from natural low-carbon sources increases energy security in this era. Biodiesel is an alternative to fossil diesel and has been classified as an ecofriendly fuel due to its reduced harmful effects on human health and the environment [1]. Transesterification of vegetable oils with methanol for biodiesel production proceeds through three consecutive reversible reactions, namely, converting triglyceride to diglyceride, diglyceride to monoglyceride, and the latter to methyl esters. This process also produces the valuable co-product glycerol, which has numerous applications in foods, cosmetics, and pharmaceutical sectors [2e4]. Transesterification processes facilitated by homogeneous catalysts have serious technical drawbacks, such as the difficult separation and purification of the produced biodiesel [5]. To address these challenges, heterogeneous catalyzed transesterification reactions are performed, in which a solid catalyst that is insoluble in methanol is used to easily separate the products. This technique also enhances recycling and reusability of catalyst and entails lower generation of wastewater during production thus, results in lower production cost [6,7]. High activity, selectivity, and water tolerance are important properties of
* Corresponding author. Fax: þ60 45941013. E-mail address: [email protected] (B.H. Hameed). 0960-1481/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2014.01.008
heterogeneous catalysts which depend on the amount and strength of their active acid and basic sites. Different heterogeneous catalysts for biodiesel production have been proposed, including basic, acidic, and acidebase heterogeneous catalysts. Basic catalysts can be subdivided based on the type of metal oxides and derivatives. The subdivision of acidic catalysts depends on their acidic active sites [8]. Calcium oxide is one of the commonly used heterogeneous base catalysts in transesterification reactions. This catalyst can be synthesized from low-cost sources such as calcium hydroxide and limestone. Also, it has less adverse environmental impacts [9]. Several researchers have reported the usage of CaO as a heterogeneous catalyst in transesterification reactions. Wijaya et al. [10] used waste capiz (Amusium cristatum) shell as catalyst for biodiesel production of palm oil with methanol, and obtained 93% yield of biodiesel within 6 h of reaction. Monica et al. reported that the soluble substances of CaO affect transesterification products [11]. Therefore, CaO loading on carriers or supports can improve its stability as a catalyst. Among the several common supports of CaO are aluminum oxide [12], silica [13], and activated carbon [14]. CaO supported on Al2O3 and KF/CaeAl reportedly improves the catalytic activity of CaO in transesterification reactions [12,15]. On the other hand, zirconium oxide as a heterogeneous acid catalyst for transesterification of different feedstocks has been studied because of its strong surface acidity [16,17]. Patel et al. reported that the application of sulfated zirconia produces promising results in the
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esterification of oleic acid under mild conditions, with a maximum methyl oleate yield of 90%. This catalyst can also be used for producing biodiesel from waste cooking oil and Jatropha oil [18]. In the present work, heterogeneous acidebase catalyst Ca3.5xZr0.5yAlxO3 (0.1 x 0.5, 0.5 y 2.5), which is a novel composite of three metal oxides, was synthesized. The catalytic activity was tested during the transesterification of palm oil (Elaeis guineensis), which is one of the most affordable oils in Malaysia. To our knowledge and based on available information from the literature, the activity of the combined composite oxide catalyst has not been tested in the transesterification of cooking palm oil. The effects of process parameters such as catalyst calcination temperature, catalyst amount, alcohol to oil molar ratio, reaction time, and temperature on the transesterification reaction and Ca:Zr ratio on the process were investigated to identify the highest conversion that can be achieved. The catalyst produced was characterized, and its reusability was tested.
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the elemental composition was analyzed using an energy dispersive X-ray detector (EDX) mounted on the microscope. The crystallographic structures of the mixed oxide was recorded by XRD using a Philips PW 1710 diffractometer, with Cu-Ka radiation used to check the structure and unit cell of the catalyst before and after transesterification reaction. Fourier transform infrared spectrophotometer (FTIR) was used to determine the active surface functional groups. FTIR transmission measurements were performed using the standard KBr wafer technique and spectrum was recorded in the range 4000e400 cm1. 2.4. Transesterification of palm oil with methanol
Palm oil (cooking grade) was purchased from the supermarket at Nibong Tebal, Malaysia. Analytical grade Ca(NO3)2.4H2O, Al(NO3)3. 9H2O, ZrOCl2.8H2O, 25% NH4OH and 65% HNO3 solution used to synthesize the catalysts were purchased from Merck company, Malaysia. Methanol (99.9% HPLC gradient grade) used for the transesterification reactions was purchased from Merck (Malaysia). Reference standards that were used as an internal standard for gas chromatography analysis (methyl myristate, methyl oleate, methyl palmitate, methyl stearate, and methyl linoleate and methyl heptadecanoate) were purchased from SigmaeAldrich company, Malaysia. Also, n-Hexane (96%) used as a solvent for Gas Chromatography analysis was purchased from Merck (Malaysia). All the chemicals used were analytical reagent grade.
The activity of CaZrAlO and other synthesized catalysts were tested in the transesterification of palm oil with methanol. All reactions were conducted in a 250 mL stainless steel Teflon-lined autoclave batch reactor equipped with a magnetic stirrer and proportionaleintegralederivative temperature controller. In a typical batch run, 80 mL palm oil, 40 mL methanol, and 1.77 g catalyst were placed inside a reactor at 150 C and stirred at 500 rpm to avoid mass transfer limitations. The reaction was performed for 5 h. At the end of the experiment, the stirrer and heater were switched off and the reactor was quickly cooled to room temperature. The catalyst was separated from the product mixture via centrifugation. Product and by-product were discharged in a glass bottle and allowed to settle. Two layers formed, with fatty acid methyl esters (FAME) in the top layer and glycerol in the bottom layer. The top layer (FAME) was analyzed via gas chromatography using a Shimadzu GC-2010 model equipped with flame ionization detectors (FID-2010 plus) and split/splitless injection unit (SPL2010 plus). A capillary column (Nukol) with 15 m length, 0.53 mm internal diameter, and 0.5 mm film thickness was used. Methyl heptadecanoate was used as an internal standard. The sample (1 mL) was injected, and then FAME content was calculated following the standard procedure described by the EN14103 application note [19,20].
2.2. Catalyst preparation
2.5. Catalyst reusability test
The heterogeneous catalyst, with empirical formula Ca3.5xZr0.5yAlxO3 (0.1 x 0.5, 0.5 y 2.5) with Ca:Zr mass ratio 7:1 and Ca:Al ratio 3.5:1 was prepared by the co-precipitation method. The catalyst preparation was based on 20 g of mixture of the salts. For a particular batch, 50 mL of mixed salt solution containing 0.18 M of ZrOCl2.8H2O, 1.19 M of Ca(NO3)2.4H2O and 0.21 M of Al(NO3)3.9H2O were prepared in 250 mL conical flask with vigorous magnetic stirring at 300 rpm. The metals were precipitated using 10 mL equivalent to 5.2 M of NH4OH. The mixture was aged at 80 C for 5 h until a completely homogenized mixture was obtained. Also, the basic strength was maintained in the range of 8e9. The solution was filtered and dried at oven temperature of 80 C for 12 h. This was followed by calcination at 500 C for 5 h to obtain solid catalyst. Similarly, individual metal oxides precipitates were prepared under the same treatment conditions and activity tested in order to allow for comparison of the element combination effect during the transesterification reaction.
The reusability of solid catalyst is an important factor to determine its economic and environmental application. After the first run the catalyst was filtered from the product and thoroughly washed with n-hexane to remove any adhered oil. The catalyst was dried in oven at 80 C for 12 h and was reused in second run and repeated for the next runs for FAME synthesis.
2. Materials and methods 2.1. Materials
2.3. Catalyst characterization The surface area, pore volume, and pore size distribution of catalyst were estimated by the data from nitrogen adsorption at 77 K using Micromeritics ASAP 2020 surface area and porosity analyzer by BrunauereEmmetteTeller (BET) method. The particle microstructures were studied by a Zeiss Supra 55 VP PGT/HKL model Field Emission Scanning Electron Microscopy (FESEM) and
3. Results and discussion 3.1. Catalyst characterization The X-ray diffraction (XRD) pattern of the synthesized catalyst (Ca1.75Zr0.25Al0.5) is presented in Fig. 1. The fresh catalyst showed characteristic peaks corresponding to the monoclinic structure of calcium aluminum oxide (CaAl4O7), which occurred at 0.98 e85 and over a wide spectrum range. Peaks with strong intensities were observed at 51 and 61, which correspond to the orthorhombic structure of kappa-alumina and tetragonal structure of ZrO2. The other peaks were weakly dispersed. A structure at 30.5 was also observed, which represents the crystal phase of the monoclinic structure of calcium zirconium oxide (CaZrO9). The catalyst had a pattern similar to each of the contributing oxide. A similar spectrum was observed for the used catalyst, but the additional peaks were observed at 17, 26 , and 28.5 . Other similar peaks for the used catalyst were represented by weak spectra. The differences observed in these spectra and structure may have resulted from the
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Fig. 1. X-ray diffraction pattern of CaZrAlO catalyst.
catalytic activity during transesterification. The observed peaks for the as-synthesized catalyst were consistent with those in previous studies [21e24]. The composition of the catalyst was verified using an energy-dispersive X-ray spectrometer mounted on a microscope. The elemental analysis revealed that the sample contained 25.38 wt% Ca, 28.03 wt% Zr, 11.28 wt% Al, and 35.31 wt% O. This result is in good agreement with the XRD diffractograms. The FESEM images of a typical catalyst (Ca:Zr ¼ 7:1) that was synthesized in this study are presented in Fig. 2. The images for both fresh (Fig. 2(a)) and used (Fig. 2(b)) catalysts revealed the porous nature of the catalyst for the fresh samples. The observed structural pattern may be ascribed to the high activity of the fresh samples. For the used catalyst, the image shows a dense surface partially covered by large triglyceride molecules after four consecutive cycle runs. The Fourier transform infrared spectroscopy analysis of the fresh mixed oxides and used catalysts is shown in Fig. 3. The peaks for both spectra are assigned based on existing data from literature [25,26]. The spectra of both catalysts are generally similar, except for some new peaks in the used catalyst. These peaks occur at 1624, 1543, and 1458 cm1, and are attributed to the presence of symmetric and asymmetric CO2 stretches, which represent the CeO or C]O functional groups of the carboxylates. The peaks at the band ranging from 3852 cm1 to 2374 cm1 for (A) and 3735 cm1 to 2852 cm1 for (B) indicate the presence of an interlayer of water molecules bonded to OH ions, which is dominant and does not exist in isolation. The band broadening and decrease in absorption frequency confirm the presence of inter- and intra-molecular hydroxyl functional groups. This tends to be a function of the degree and strength of the hydrogen bonding, which is a characteristic of hydroxides with large shifts. Thus, a decrease in infrequency was
a
b
Fig. 2. FTIR of CaZrAlO: (a) fresh and (b) used catalyst.
Fig. 3. FESEM images of CaZrAlO: (a) fresh and (b) used catalyst.
observed. The OH group assigned to the peaks is consistent with the results obtained by Olutoye and Hameed [27]. The broad shoulder at the bandwidth of 1637 cm1 is assigned to medium-weak Ca2þ ions and other anions such as CaeOH. The bandwidth of 1384 cm1 corresponds to the very strong nitrate ion stretching vibrations. The peaks at the bandwidth of 825 and 876 cm1 are assigned to the weak nitrate ion stretching and CaeOeCa, respectively. Furthermore, the peaks at 1050 cm1 for (A) with a slight shift and broad shoulder at 1027 cm1 for spectrum (B) represents M-OH deformation (where M represents Ca, Zr or Al metal ions). The bandwidth region in the range of 592 cm1e519 cm1 for both spectra is assigned to M-O bending vibration. The textural properties of catalysts are important features that determine their performance. Table 1 shows the characteristics of the catalyst used for the transesterification reaction. The external surface area of the as-synthesized catalyst (Ca1.75Zr0.25Al0.5) was observed. Catalytic reactivity and its relationship to external surface area were also investigated. In this study, alumina acted as support and improved the surface area of the catalyst by forming a composite structure with other metal oxides. High catalytic performance was achieved when the Ca:Zr ratio was modified into 7:1,
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Table 1 BET surface area, total pore volume and average pore diameter of the synthesized catalysts. Catalyst
Ca1.75Zr0.25Al0.5
Ca1.75Zr0.25
BET area (m2/g) Total pore volume (cm3/g) Average pore size (nm)
91.4 0.19 85.2
72.9 0.06 35.0
which partly resulted from the enhanced surface properties compared with that of the other catalyst used in the same process (Ca1.75Zr0.25). Thus, the combined effects may have contributed to the high activity of the synthesized catalyst. The pore size in this catalyst falls under the mesopore group based on the International Union of Pure and Applied Chemistry classification [28]. 3.2. Performance of different Ca:Zr ratio
Fig. 5. Effect of temperature calcination on FAME content, reaction temperature ¼ 150 C, reaction time ¼ 5 h, methanol/oil molar ratio ¼ 12:1, and catalyst loading ¼ 2.5 wt%.
The effects of the interaction between the metals of the synthesized catalyst were studied to obtain further insights into its performance during transesterification. The activities of the 2.5 wt.% catalyst loading with different Ca:Zr ratios at 150 C are presented in Fig. 4. The catalytic activities of the independent oxides of CaO and ZrO during transesterification are 61% and 3% FAME content, respectively, which clearly reveal that the synergetic effect of the three metal composite catalysts (Ca3.5xZr0.5yAlxO3) is better under the treatment conditions. The catalyst containing 2.5 wt.% Ca1.75Al0.5 and Ca1.75Zr0.25 results in 76% and 63% FAME content, respectively, after 5 h. The catalyst containing 2.5 wt.% Ca1.75Zr0.25Al0.5 at a Ca:Zr ratio of 7:1 achieves the highest conversion (87%) after 5 h. Thus, the combination of Al and Zr with a basic component (Ca) forms a complex mixture with methanol. This catalyst breaks the triglyceride bonds, thereby resulting in higher FAME content in the mixture.
in a specific range. The phase transformation may have resulted from the increased basicity of the composite catalyst, which is favorable for the alkali transesterification of low-free fatty acid oil feedstock. Over 86% FAME content was obtained when the catalyst was treated at 500 C. Beyond this temperature, the catalytic activity decreases, which may be caused by the decomposition of the synergetic structure of the synthesized catalyst. At this condition (>500 C), the disordered structure formed is no longer active in transesterification, thus reducing the catalytic activity. This result is in accordance with that obtained by Tang et al. [29], who reported that the heat treatment of calcium aluminate on Fe3O4 catalyst is active at the calcination temperature of 600 C. However, the catalytic activity decreases beyond this temperature. Therefore, 500 C is the optimal calcination temperature used in the subsequent experimental runs.
3.3. Effect of reaction conditions 3.3.1. Heat treatment effect on CaZrAlO and test of activity The composite catalyst Ca3.5xZr0.5yAlxO3 was subjected to different calcination temperatures ranging from 300 C to 700 C for 5 h. The catalytic activity on the transesterification of palm oil with methanol was examined. As shown in Fig. 5, the catalytic activity increases with increased calcination temperature. The increased catalytic activity may be due to the phase transition of the three metal components as the temperature increased. This result is consistent with that obtained from the XRD analysis, in which the crystalline formation of CaZrO and CaAlO was observed
Fig. 4. FAME content for catalyst with different Ca:Zr ratios.
3.3.2. Effect of reaction time Reaction time is an important factor that reflects the transesterification reaction rate. The mass transfer of heterogeneous catalyst systems results in slow reaction within a short reaction time. Thus, FAME content was assumed to be low in the first 1 h of reaction. However, Fig. 6 shows that the FAME content for catalyst gradually increases within the first 2 h of reaction time and thereafter remains nearly constant (87%) at a reaction time of 5 h. Furthermore, when the reaction time was further increased to 12 h, no significant change in the FAME content was observed. The FAME content only slightly decreases (77%) when the reaction
Fig. 6. Effect of reaction time on FAME content, reaction temperature 150 C, methanol/oil molar ratio ¼ 12:1, and catalyst loading 2.5 wt%.
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time is extended. This observation may be due to the backward shift in the reaction, which is a reversible process. The reaction complex formed at this extended period no longer affects the FAME content [30]. 3.3.3. Effect of catalyst loading The effect of catalyst dosage on FAME content was investigated, and the amount of catalyst was varied in the range of 0.5 wt% to 6.5 wt% (catalyst to oil). Fig. 7 shows that the FAME content gradually increases (over 86%) when the catalyst to oil weight ratio increases from 0.5 wt% to 2.5 wt%. FAME content decreases with further addition of the catalyst. The observed trend may be ascribed to the mixing resistance of the triglyceride molecules, product, and solid catalyst. The viscosity increases as the catalyst loading increases [14]. Thus, 2.5 wt% is the optimal catalyst loading to obtain high FAME content. 3.3.4. Effect of methanol to oil molar ratio Transesterification reactions stoichiometrically require 3 mol of methanol to 1 mol of triglyceride [31]. To shift the reaction forward, an excess of methanol to oil ratio is needed [32]. In this study, at 12:1 methanol to oil ratio the FAME content increased up to a maximum value of 87% (Fig. 8). When the alcohol to oil ratio was further increased, the FAME content decreased, which may have resulted from the dilution effect caused by the excess methanol. Therefore, separation was difficult, which reduced the catalyst concentration. This result is consistent with that obtained by Felipe et al. [33], who reported that excess ethanol in the esterification of waste coconut oil aggravates the difficulty in the separation and decrease of FAME yield. 3.3.5. Effect of reaction temperature The effect of reaction temperature was investigated in the range of 70 Ce180 C. During the preliminary runs, the FAME content was lower than 50% when the reaction temperature was below 70 C. The FAME content increased to more than 76% at a higher temperature of 120 C. As shown in Fig. 9, the maximum FAME content was achieved at 150 C. Based on the kinetic performance, the reaction rate was expected to increase with the increase in temperature because an enhancement of collision among molecules lowers the activation energy barrier, thereby resulting in enhanced product conversion. Several authors have reported similar effects of temperature on FAME content. Olutoye and Hameed reported that increasing the temperature from 150 C to 182 C improves the FAME conversion [34]. Aderemi and Hameed reported that increasing the temperature from 170 C to 190 C
Fig. 7. Effect of catalyst loading on FAME content, reaction temperature ¼ 150 C, reaction time ¼ 5 h, and methanol/oil molar ratio ¼ 12:1.
Fig. 8. Effect of methanol to oil molar ratio on FAME content, reaction temperature ¼ 150 C, catalyst loading ¼ 2.5 wt%, and reaction time ¼ 5 h.
results in a minimal decrease in product yield [35]. However, in the current study, the FAME content decreased beyond 150 C, which indicated that temperature inhibits catalytic activity during transesterification. Thus, 150 C is the suitable operating temperature. 3.3.6. Catalyst reusability studies for palm oil transesterification The reusability was examined after the first run. After the first run, the catalyst was filtered from the product and washed with nhexane until all adhered oil and glycerol were removed. Then, the catalyst was dried at 80 C for 12 h and placed in contact with fresh methanol and palm oil for the next run. The result of the reusability test is shown in Fig. 10. The performance decreased with the number of experimental runs. For example, 87% of FAME content was obtained for the first run. The second run resulted in 77% FAME content. This result may be due to material loss during the experimental runs, which resulted in fewer active sites in the catalyst for subsequent runs. The EDX analysis of the used catalyst shows the weight percent of the components of the catalyst after the fourth cycle (Ca ¼ 21.37 wt.%, Zr ¼ 27.82 wt.%, Al ¼ 14.60 wt.%, and O ¼ 36.22 wt.%). Based on the comparison of the fresh and used catalysts, the component that decreased during the reaction runs was calcium. 4. Conclusion The mesostructured mixed oxide catalyst Ca3.5xZr0.5yAlxO3 was synthesized using the coprecipitation method, and then used in the transesterification of palm oil. The catalytic activity was closely
Fig. 9. Effect of reaction temperature on FAME content, reaction time ¼ 5 h, methanol/ oil molar ratio ¼ 12:1, and catalyst loading ¼ 2.5 wt%.
H. Amani et al. / Renewable Energy 66 (2014) 680e685
Fig. 10. Catalytic activity of Ca1.75Zr0.25Al0.5 in successive reaction runs (reaction temperature ¼ 150 C, reaction time ¼ 5 h, methanol/oil molar ratio ¼ 12:1, and catalyst loading ¼ 2.5 wt%).
related to its molecular structure and distribution of basic and acidic sites. The catalyst showed high activity toward FAME synthesis at a Ca:Zr ratio of 7:1. The optimum conditions were as follows: methanol to oil molar ratio 12:1, catalyst loading 2.5 wt.%, temperature 150 C and 5 h reaction time. Over 87% yield was obtained when the catalyst was calcined at 500 C for 5 h. The catalyst was easily separated from the product mixture and reusable for four cycles. Acknowledgment The authors acknowledge the research grants provided by the Universiti Sains Malaysia, under Research University (RU) grant (Project No:814126) and Postgraduate Research Grant Scheme (PRGS) (Project No: 1001/PJKIMIA/8045027) that resulted in this article. References [1] Moser BR. Influence of extended storage on fuel properties of methyl esters prepared from canola, palm, soybean and sunflower oils. Renew Energy 2011;36:1221e6. [2] Janulis P. Reduction of energy consumption in biodiesel fuel life cycle. Renew Energy 2004;29:861e71. [3] Jitrwung R, Verett J, Yargeau V. Optimization of selected salts concentration for improved biohydrogen production from biodiesel-based glycerol using Enterobacter aerogenes. Renew Energy 2013;50:222e6. [4] Boey PL, Maniam GP, Hamid SA. Performance of calcium oxide as a heterogeneous catalyst in biodiesel production. Chem Eng J 2011;168:15e22. [5] Macario A, Giordano, Onida B, Cocina D, Tagarelli A, Giuffre AM. Biodiesel production process by homogeneous/heterogenous catalytic system using an acid-base catalyst. Appl Catal A 2010;378:160e8. [6] Zanette AF, Barella RA, ergher SBCP, Treichel H, liveria DO, Mazutti MA, et al. Screening, optimization and kinetics of jatropha curcas oil transesterification with heterogeneous catalysts. Renew Energy 2011;36:726e31. [7] Hasheminejad M, Tabatabaei M, Mansourpanah Y, Khatamifar M, Javani A. Upstream and downstream strategies to economize biodiesel production. Bioresour Technol 2011;102:461e8. [8] Helwani Z, Othman MR, Aziz N, Fernando WJN, Kim J. Technologies for production of biodiesel focusing on green catalytic techniques. Fuel Process Technol 2009;90:1502e14.
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Synthesis of fatty acid methyl esters via the methanolysis of palm oil over Ca3.5xZr0.5yAlxO3 mixed oxide catalyst H. Amani, Z. Ahmad, B.H. Hameed* School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 April 2013 Accepted 9 January 2014 Available online 11 February 2014
Novel mixed metal oxide catalyst Ca3.5xZr0.5yAlxO3 was synthesized through the coprecipitation of metal hydroxides. The textural, morphological, and surface properties of the synthesized catalysts were characterized via BrunauereEmmetteTeller method, X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, and energy-dispersive X-ray spectroscopy. The catalytic performance of the as-synthesized catalyst series was evaluated during the transesterification of cooking palm oil with methanol to produce fatty acid methyl esters (FAME). The influence of different parameters, including the calcination temperature (300e700 C), methanol to oil molar ratio (6:1e25:1), catalyst amount (0.5e6.5 wt%), reaction time (0.5e12 h) and temperature (70e180 C), on the process was thoroughly investigated. The metal oxide composite catalyst with a Ca:Zr ratio of 7:1 showed good catalytic activity toward methyl esters. Over 87% of FAME content was obtained when the methanol to oil molar ratio was 12:1, reaction temperature 150 C, reaction time 5 h and 2.5 wt% of catalyst loading. The catalyst could also be reused for over four cycles. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Transesterification Heterogeneous catalyst Palm oil Methyl ester
1. Introduction It is coming up that the production of biodiesel from natural low-carbon sources increases energy security in this era. Biodiesel is an alternative to fossil diesel and has been classified as an ecofriendly fuel due to its reduced harmful effects on human health and the environment [1]. Transesterification of vegetable oils with methanol for biodiesel production proceeds through three consecutive reversible reactions, namely, converting triglyceride to diglyceride, diglyceride to monoglyceride, and the latter to methyl esters. This process also produces the valuable co-product glycerol, which has numerous applications in foods, cosmetics, and pharmaceutical sectors [2e4]. Transesterification processes facilitated by homogeneous catalysts have serious technical drawbacks, such as the difficult separation and purification of the produced biodiesel [5]. To address these challenges, heterogeneous catalyzed transesterification reactions are performed, in which a solid catalyst that is insoluble in methanol is used to easily separate the products. This technique also enhances recycling and reusability of catalyst and entails lower generation of wastewater during production thus, results in lower production cost [6,7]. High activity, selectivity, and water tolerance are important properties of
* Corresponding author. Fax: þ60 45941013. E-mail address: [email protected] (B.H. Hameed). 0960-1481/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2014.01.008
heterogeneous catalysts which depend on the amount and strength of their active acid and basic sites. Different heterogeneous catalysts for biodiesel production have been proposed, including basic, acidic, and acidebase heterogeneous catalysts. Basic catalysts can be subdivided based on the type of metal oxides and derivatives. The subdivision of acidic catalysts depends on their acidic active sites [8]. Calcium oxide is one of the commonly used heterogeneous base catalysts in transesterification reactions. This catalyst can be synthesized from low-cost sources such as calcium hydroxide and limestone. Also, it has less adverse environmental impacts [9]. Several researchers have reported the usage of CaO as a heterogeneous catalyst in transesterification reactions. Wijaya et al. [10] used waste capiz (Amusium cristatum) shell as catalyst for biodiesel production of palm oil with methanol, and obtained 93% yield of biodiesel within 6 h of reaction. Monica et al. reported that the soluble substances of CaO affect transesterification products [11]. Therefore, CaO loading on carriers or supports can improve its stability as a catalyst. Among the several common supports of CaO are aluminum oxide [12], silica [13], and activated carbon [14]. CaO supported on Al2O3 and KF/CaeAl reportedly improves the catalytic activity of CaO in transesterification reactions [12,15]. On the other hand, zirconium oxide as a heterogeneous acid catalyst for transesterification of different feedstocks has been studied because of its strong surface acidity [16,17]. Patel et al. reported that the application of sulfated zirconia produces promising results in the
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esterification of oleic acid under mild conditions, with a maximum methyl oleate yield of 90%. This catalyst can also be used for producing biodiesel from waste cooking oil and Jatropha oil [18]. In the present work, heterogeneous acidebase catalyst Ca3.5xZr0.5yAlxO3 (0.1 x 0.5, 0.5 y 2.5), which is a novel composite of three metal oxides, was synthesized. The catalytic activity was tested during the transesterification of palm oil (Elaeis guineensis), which is one of the most affordable oils in Malaysia. To our knowledge and based on available information from the literature, the activity of the combined composite oxide catalyst has not been tested in the transesterification of cooking palm oil. The effects of process parameters such as catalyst calcination temperature, catalyst amount, alcohol to oil molar ratio, reaction time, and temperature on the transesterification reaction and Ca:Zr ratio on the process were investigated to identify the highest conversion that can be achieved. The catalyst produced was characterized, and its reusability was tested.
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the elemental composition was analyzed using an energy dispersive X-ray detector (EDX) mounted on the microscope. The crystallographic structures of the mixed oxide was recorded by XRD using a Philips PW 1710 diffractometer, with Cu-Ka radiation used to check the structure and unit cell of the catalyst before and after transesterification reaction. Fourier transform infrared spectrophotometer (FTIR) was used to determine the active surface functional groups. FTIR transmission measurements were performed using the standard KBr wafer technique and spectrum was recorded in the range 4000e400 cm1. 2.4. Transesterification of palm oil with methanol
Palm oil (cooking grade) was purchased from the supermarket at Nibong Tebal, Malaysia. Analytical grade Ca(NO3)2.4H2O, Al(NO3)3. 9H2O, ZrOCl2.8H2O, 25% NH4OH and 65% HNO3 solution used to synthesize the catalysts were purchased from Merck company, Malaysia. Methanol (99.9% HPLC gradient grade) used for the transesterification reactions was purchased from Merck (Malaysia). Reference standards that were used as an internal standard for gas chromatography analysis (methyl myristate, methyl oleate, methyl palmitate, methyl stearate, and methyl linoleate and methyl heptadecanoate) were purchased from SigmaeAldrich company, Malaysia. Also, n-Hexane (96%) used as a solvent for Gas Chromatography analysis was purchased from Merck (Malaysia). All the chemicals used were analytical reagent grade.
The activity of CaZrAlO and other synthesized catalysts were tested in the transesterification of palm oil with methanol. All reactions were conducted in a 250 mL stainless steel Teflon-lined autoclave batch reactor equipped with a magnetic stirrer and proportionaleintegralederivative temperature controller. In a typical batch run, 80 mL palm oil, 40 mL methanol, and 1.77 g catalyst were placed inside a reactor at 150 C and stirred at 500 rpm to avoid mass transfer limitations. The reaction was performed for 5 h. At the end of the experiment, the stirrer and heater were switched off and the reactor was quickly cooled to room temperature. The catalyst was separated from the product mixture via centrifugation. Product and by-product were discharged in a glass bottle and allowed to settle. Two layers formed, with fatty acid methyl esters (FAME) in the top layer and glycerol in the bottom layer. The top layer (FAME) was analyzed via gas chromatography using a Shimadzu GC-2010 model equipped with flame ionization detectors (FID-2010 plus) and split/splitless injection unit (SPL2010 plus). A capillary column (Nukol) with 15 m length, 0.53 mm internal diameter, and 0.5 mm film thickness was used. Methyl heptadecanoate was used as an internal standard. The sample (1 mL) was injected, and then FAME content was calculated following the standard procedure described by the EN14103 application note [19,20].
2.2. Catalyst preparation
2.5. Catalyst reusability test
The heterogeneous catalyst, with empirical formula Ca3.5xZr0.5yAlxO3 (0.1 x 0.5, 0.5 y 2.5) with Ca:Zr mass ratio 7:1 and Ca:Al ratio 3.5:1 was prepared by the co-precipitation method. The catalyst preparation was based on 20 g of mixture of the salts. For a particular batch, 50 mL of mixed salt solution containing 0.18 M of ZrOCl2.8H2O, 1.19 M of Ca(NO3)2.4H2O and 0.21 M of Al(NO3)3.9H2O were prepared in 250 mL conical flask with vigorous magnetic stirring at 300 rpm. The metals were precipitated using 10 mL equivalent to 5.2 M of NH4OH. The mixture was aged at 80 C for 5 h until a completely homogenized mixture was obtained. Also, the basic strength was maintained in the range of 8e9. The solution was filtered and dried at oven temperature of 80 C for 12 h. This was followed by calcination at 500 C for 5 h to obtain solid catalyst. Similarly, individual metal oxides precipitates were prepared under the same treatment conditions and activity tested in order to allow for comparison of the element combination effect during the transesterification reaction.
The reusability of solid catalyst is an important factor to determine its economic and environmental application. After the first run the catalyst was filtered from the product and thoroughly washed with n-hexane to remove any adhered oil. The catalyst was dried in oven at 80 C for 12 h and was reused in second run and repeated for the next runs for FAME synthesis.
2. Materials and methods 2.1. Materials
2.3. Catalyst characterization The surface area, pore volume, and pore size distribution of catalyst were estimated by the data from nitrogen adsorption at 77 K using Micromeritics ASAP 2020 surface area and porosity analyzer by BrunauereEmmetteTeller (BET) method. The particle microstructures were studied by a Zeiss Supra 55 VP PGT/HKL model Field Emission Scanning Electron Microscopy (FESEM) and
3. Results and discussion 3.1. Catalyst characterization The X-ray diffraction (XRD) pattern of the synthesized catalyst (Ca1.75Zr0.25Al0.5) is presented in Fig. 1. The fresh catalyst showed characteristic peaks corresponding to the monoclinic structure of calcium aluminum oxide (CaAl4O7), which occurred at 0.98 e85 and over a wide spectrum range. Peaks with strong intensities were observed at 51 and 61, which correspond to the orthorhombic structure of kappa-alumina and tetragonal structure of ZrO2. The other peaks were weakly dispersed. A structure at 30.5 was also observed, which represents the crystal phase of the monoclinic structure of calcium zirconium oxide (CaZrO9). The catalyst had a pattern similar to each of the contributing oxide. A similar spectrum was observed for the used catalyst, but the additional peaks were observed at 17, 26 , and 28.5 . Other similar peaks for the used catalyst were represented by weak spectra. The differences observed in these spectra and structure may have resulted from the
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Fig. 1. X-ray diffraction pattern of CaZrAlO catalyst.
catalytic activity during transesterification. The observed peaks for the as-synthesized catalyst were consistent with those in previous studies [21e24]. The composition of the catalyst was verified using an energy-dispersive X-ray spectrometer mounted on a microscope. The elemental analysis revealed that the sample contained 25.38 wt% Ca, 28.03 wt% Zr, 11.28 wt% Al, and 35.31 wt% O. This result is in good agreement with the XRD diffractograms. The FESEM images of a typical catalyst (Ca:Zr ¼ 7:1) that was synthesized in this study are presented in Fig. 2. The images for both fresh (Fig. 2(a)) and used (Fig. 2(b)) catalysts revealed the porous nature of the catalyst for the fresh samples. The observed structural pattern may be ascribed to the high activity of the fresh samples. For the used catalyst, the image shows a dense surface partially covered by large triglyceride molecules after four consecutive cycle runs. The Fourier transform infrared spectroscopy analysis of the fresh mixed oxides and used catalysts is shown in Fig. 3. The peaks for both spectra are assigned based on existing data from literature [25,26]. The spectra of both catalysts are generally similar, except for some new peaks in the used catalyst. These peaks occur at 1624, 1543, and 1458 cm1, and are attributed to the presence of symmetric and asymmetric CO2 stretches, which represent the CeO or C]O functional groups of the carboxylates. The peaks at the band ranging from 3852 cm1 to 2374 cm1 for (A) and 3735 cm1 to 2852 cm1 for (B) indicate the presence of an interlayer of water molecules bonded to OH ions, which is dominant and does not exist in isolation. The band broadening and decrease in absorption frequency confirm the presence of inter- and intra-molecular hydroxyl functional groups. This tends to be a function of the degree and strength of the hydrogen bonding, which is a characteristic of hydroxides with large shifts. Thus, a decrease in infrequency was
a
b
Fig. 2. FTIR of CaZrAlO: (a) fresh and (b) used catalyst.
Fig. 3. FESEM images of CaZrAlO: (a) fresh and (b) used catalyst.
observed. The OH group assigned to the peaks is consistent with the results obtained by Olutoye and Hameed [27]. The broad shoulder at the bandwidth of 1637 cm1 is assigned to medium-weak Ca2þ ions and other anions such as CaeOH. The bandwidth of 1384 cm1 corresponds to the very strong nitrate ion stretching vibrations. The peaks at the bandwidth of 825 and 876 cm1 are assigned to the weak nitrate ion stretching and CaeOeCa, respectively. Furthermore, the peaks at 1050 cm1 for (A) with a slight shift and broad shoulder at 1027 cm1 for spectrum (B) represents M-OH deformation (where M represents Ca, Zr or Al metal ions). The bandwidth region in the range of 592 cm1e519 cm1 for both spectra is assigned to M-O bending vibration. The textural properties of catalysts are important features that determine their performance. Table 1 shows the characteristics of the catalyst used for the transesterification reaction. The external surface area of the as-synthesized catalyst (Ca1.75Zr0.25Al0.5) was observed. Catalytic reactivity and its relationship to external surface area were also investigated. In this study, alumina acted as support and improved the surface area of the catalyst by forming a composite structure with other metal oxides. High catalytic performance was achieved when the Ca:Zr ratio was modified into 7:1,
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Table 1 BET surface area, total pore volume and average pore diameter of the synthesized catalysts. Catalyst
Ca1.75Zr0.25Al0.5
Ca1.75Zr0.25
BET area (m2/g) Total pore volume (cm3/g) Average pore size (nm)
91.4 0.19 85.2
72.9 0.06 35.0
which partly resulted from the enhanced surface properties compared with that of the other catalyst used in the same process (Ca1.75Zr0.25). Thus, the combined effects may have contributed to the high activity of the synthesized catalyst. The pore size in this catalyst falls under the mesopore group based on the International Union of Pure and Applied Chemistry classification [28]. 3.2. Performance of different Ca:Zr ratio
Fig. 5. Effect of temperature calcination on FAME content, reaction temperature ¼ 150 C, reaction time ¼ 5 h, methanol/oil molar ratio ¼ 12:1, and catalyst loading ¼ 2.5 wt%.
The effects of the interaction between the metals of the synthesized catalyst were studied to obtain further insights into its performance during transesterification. The activities of the 2.5 wt.% catalyst loading with different Ca:Zr ratios at 150 C are presented in Fig. 4. The catalytic activities of the independent oxides of CaO and ZrO during transesterification are 61% and 3% FAME content, respectively, which clearly reveal that the synergetic effect of the three metal composite catalysts (Ca3.5xZr0.5yAlxO3) is better under the treatment conditions. The catalyst containing 2.5 wt.% Ca1.75Al0.5 and Ca1.75Zr0.25 results in 76% and 63% FAME content, respectively, after 5 h. The catalyst containing 2.5 wt.% Ca1.75Zr0.25Al0.5 at a Ca:Zr ratio of 7:1 achieves the highest conversion (87%) after 5 h. Thus, the combination of Al and Zr with a basic component (Ca) forms a complex mixture with methanol. This catalyst breaks the triglyceride bonds, thereby resulting in higher FAME content in the mixture.
in a specific range. The phase transformation may have resulted from the increased basicity of the composite catalyst, which is favorable for the alkali transesterification of low-free fatty acid oil feedstock. Over 86% FAME content was obtained when the catalyst was treated at 500 C. Beyond this temperature, the catalytic activity decreases, which may be caused by the decomposition of the synergetic structure of the synthesized catalyst. At this condition (>500 C), the disordered structure formed is no longer active in transesterification, thus reducing the catalytic activity. This result is in accordance with that obtained by Tang et al. [29], who reported that the heat treatment of calcium aluminate on Fe3O4 catalyst is active at the calcination temperature of 600 C. However, the catalytic activity decreases beyond this temperature. Therefore, 500 C is the optimal calcination temperature used in the subsequent experimental runs.
3.3. Effect of reaction conditions 3.3.1. Heat treatment effect on CaZrAlO and test of activity The composite catalyst Ca3.5xZr0.5yAlxO3 was subjected to different calcination temperatures ranging from 300 C to 700 C for 5 h. The catalytic activity on the transesterification of palm oil with methanol was examined. As shown in Fig. 5, the catalytic activity increases with increased calcination temperature. The increased catalytic activity may be due to the phase transition of the three metal components as the temperature increased. This result is consistent with that obtained from the XRD analysis, in which the crystalline formation of CaZrO and CaAlO was observed
Fig. 4. FAME content for catalyst with different Ca:Zr ratios.
3.3.2. Effect of reaction time Reaction time is an important factor that reflects the transesterification reaction rate. The mass transfer of heterogeneous catalyst systems results in slow reaction within a short reaction time. Thus, FAME content was assumed to be low in the first 1 h of reaction. However, Fig. 6 shows that the FAME content for catalyst gradually increases within the first 2 h of reaction time and thereafter remains nearly constant (87%) at a reaction time of 5 h. Furthermore, when the reaction time was further increased to 12 h, no significant change in the FAME content was observed. The FAME content only slightly decreases (77%) when the reaction
Fig. 6. Effect of reaction time on FAME content, reaction temperature 150 C, methanol/oil molar ratio ¼ 12:1, and catalyst loading 2.5 wt%.
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time is extended. This observation may be due to the backward shift in the reaction, which is a reversible process. The reaction complex formed at this extended period no longer affects the FAME content [30]. 3.3.3. Effect of catalyst loading The effect of catalyst dosage on FAME content was investigated, and the amount of catalyst was varied in the range of 0.5 wt% to 6.5 wt% (catalyst to oil). Fig. 7 shows that the FAME content gradually increases (over 86%) when the catalyst to oil weight ratio increases from 0.5 wt% to 2.5 wt%. FAME content decreases with further addition of the catalyst. The observed trend may be ascribed to the mixing resistance of the triglyceride molecules, product, and solid catalyst. The viscosity increases as the catalyst loading increases [14]. Thus, 2.5 wt% is the optimal catalyst loading to obtain high FAME content. 3.3.4. Effect of methanol to oil molar ratio Transesterification reactions stoichiometrically require 3 mol of methanol to 1 mol of triglyceride [31]. To shift the reaction forward, an excess of methanol to oil ratio is needed [32]. In this study, at 12:1 methanol to oil ratio the FAME content increased up to a maximum value of 87% (Fig. 8). When the alcohol to oil ratio was further increased, the FAME content decreased, which may have resulted from the dilution effect caused by the excess methanol. Therefore, separation was difficult, which reduced the catalyst concentration. This result is consistent with that obtained by Felipe et al. [33], who reported that excess ethanol in the esterification of waste coconut oil aggravates the difficulty in the separation and decrease of FAME yield. 3.3.5. Effect of reaction temperature The effect of reaction temperature was investigated in the range of 70 Ce180 C. During the preliminary runs, the FAME content was lower than 50% when the reaction temperature was below 70 C. The FAME content increased to more than 76% at a higher temperature of 120 C. As shown in Fig. 9, the maximum FAME content was achieved at 150 C. Based on the kinetic performance, the reaction rate was expected to increase with the increase in temperature because an enhancement of collision among molecules lowers the activation energy barrier, thereby resulting in enhanced product conversion. Several authors have reported similar effects of temperature on FAME content. Olutoye and Hameed reported that increasing the temperature from 150 C to 182 C improves the FAME conversion [34]. Aderemi and Hameed reported that increasing the temperature from 170 C to 190 C
Fig. 7. Effect of catalyst loading on FAME content, reaction temperature ¼ 150 C, reaction time ¼ 5 h, and methanol/oil molar ratio ¼ 12:1.
Fig. 8. Effect of methanol to oil molar ratio on FAME content, reaction temperature ¼ 150 C, catalyst loading ¼ 2.5 wt%, and reaction time ¼ 5 h.
results in a minimal decrease in product yield [35]. However, in the current study, the FAME content decreased beyond 150 C, which indicated that temperature inhibits catalytic activity during transesterification. Thus, 150 C is the suitable operating temperature. 3.3.6. Catalyst reusability studies for palm oil transesterification The reusability was examined after the first run. After the first run, the catalyst was filtered from the product and washed with nhexane until all adhered oil and glycerol were removed. Then, the catalyst was dried at 80 C for 12 h and placed in contact with fresh methanol and palm oil for the next run. The result of the reusability test is shown in Fig. 10. The performance decreased with the number of experimental runs. For example, 87% of FAME content was obtained for the first run. The second run resulted in 77% FAME content. This result may be due to material loss during the experimental runs, which resulted in fewer active sites in the catalyst for subsequent runs. The EDX analysis of the used catalyst shows the weight percent of the components of the catalyst after the fourth cycle (Ca ¼ 21.37 wt.%, Zr ¼ 27.82 wt.%, Al ¼ 14.60 wt.%, and O ¼ 36.22 wt.%). Based on the comparison of the fresh and used catalysts, the component that decreased during the reaction runs was calcium. 4. Conclusion The mesostructured mixed oxide catalyst Ca3.5xZr0.5yAlxO3 was synthesized using the coprecipitation method, and then used in the transesterification of palm oil. The catalytic activity was closely
Fig. 9. Effect of reaction temperature on FAME content, reaction time ¼ 5 h, methanol/ oil molar ratio ¼ 12:1, and catalyst loading ¼ 2.5 wt%.
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Fig. 10. Catalytic activity of Ca1.75Zr0.25Al0.5 in successive reaction runs (reaction temperature ¼ 150 C, reaction time ¼ 5 h, methanol/oil molar ratio ¼ 12:1, and catalyst loading ¼ 2.5 wt%).
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