Energy Fuels 2010, 24, 4748–4755 Published on Web 02/23/2010
: DOI:10.1021/ef901307w
Efficient Esterification and Transesterification of Used Cooking Oil Using 12-Tungstophosphoric Acid (TPA)/Nb2O5 Catalyst† K. Srilatha,‡ T. Issariyakul,§ N. Lingaiah,‡ P. S. Sai Prasad,‡ J. Kozinski,§ and A. K. Dalai*,§ ‡
Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad, Andhra Pradesh 500607, India, and §Catalysis and Chemical Reaction Engineering Laboratories, Department of Chemical Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, Saskatchewan S7N5A9, Canada Received November 6, 2009. Revised Manuscript Received January 28, 2010
12-Tungstophosphoric acid (TPA, 5-30 wt %) supported on niobia (Nb2O5) catalysts were prepared, and their efficacy as solid acid catalysts for the transesterification of used cooking oil (with high free fatty acids) with methanol was investigated. The catalysts prepared were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), NH3-temperature-programmed desorption (TPD), laser Raman, Brunauer-Emmett-Teller (BET) surface area, and scanning electron microscopy (SEM) techniques. Among the catalysts, 25 wt % TPA/Nb2O5 was the most promising one with the highest ester yield and having the ability to catalyze transesterification and esterification reactions simultaneously. Further, reaction parameters, such as catalyst weight, methanol/oil mole ratio, and reaction temperature, were also optimized with the most active catalyst, i.e., 25 wt % TPA/Nb2O5. It was found that, at 200 °C, 18:1 alcohol/oil molar ratio, and 3 wt % catalyst loading, a maximum ester yield of 92 wt % could be obtained. The catalyst was recycled and reused with negligible loss in activity. The kinetic studies suggested that the reaction follows the first-order kinetics.
catalysts is in their low corrosive effect and negligible environmental problems, which have recently attracted considerable attention. Biodiesel production via heterogeneous acid catalysis has been published by a few researchers. Solid acid catalysts, such as WO3/ZrO2,5 SO4-2/TiO2-SiO2,6 12-tungstophosphoric acid (TPA)/ZrO2,7 H3PW12O40/Ta2O5,8 carbohydrate-derived solid acid,9 cerium trisdodecyl sulfate,10 zinc stearate immobilized on silica gel,11 and propylsulfonicacid-functionalized mesoporous silica12 exhibited high catalytic activity for simultaneous esterification and transesterification reactions. All of these catalysts reported until now have strong acid sites, which make them the ideal candidates. The Keggin-type heteropolyacids (HPAs) are of interest as catalysts because they are strong Brønsted acids and are known for their efficient catalysis of reactions in both homoand heterogeneous phases over cleaner processes.13,14 The disadvantages of HPAs as catalysts lie in their low thermal stability, low surface area (1-10 m2/g), and high solubility in
Introduction Alternative fuels for diesel engines are of utmost importance because of diminishing petroleum reserves and the environmental concerns posed by the exhaust gases from petroleum-fueled engines.1 Earlier reports suggested that biodiesel holds higher promise as an alternative diesel engine fuel because it is renewable, biodegradable, and nontoxic.2 Alkali-catalyzed transesterification of vegetable oils with short-chain alcohols, such as methanol, is usually adopted for biodiesel production.3 However, the high cost of biodiesel is the major obstacle for its commercialization. Used cooking oil (UCO) is an economical choice for biodiesel production, because of its availability and low cost.1,4 However, the relatively higher amounts of free fatty acids (FFAs) in the feedstock result in the production of undesirable saponified products in the presence of the alkali catalyst, which in turn decreases the biodiesel yield.1 Homogeneous acid catalysts do not exhibit measurable susceptibility to low amounts of FFAs in oils, are difficult to recycle and operate at high temperatures, and give rise to serious environmental and corrosion problems.2 The significance of solid acid catalysts over homogeneous acid
(5) Suwannakarn, K.; Lotero, E.; Ngaosuwan, K.; Goodwin, J. G., Jr. Ind. Eng. Chem. Res. 2009, 48, 2810–2818. (6) Peng, B. X.; Shu, Q.; Wang, J. F.; Wang, G. R.; Wang, D. Z.; Han, M. H. Process Saf. Environ. Prot. 2008, 86, 441–447. (7) Kulkarni, M. G.; Gopinath, R.; Meher, L. C.; Dalai, A. K. Green Chem. 2006, 8, 1056–1062. (8) Xu, L.; Wang, Y.; Yang, X.; Yu, X; Guo, Y.; Clark, J. H. Green Chem. 2009, 10, 746–755. (9) Lou, W. Y.; Zong, M. H.; Duan, Z. Q. Bioresour. Technol. 2008, 99, 8752–8758. (10) Ghesti, G. F.; de Macedo, J. L.; Parente, V. C. I.; Dias, J. A.; Dias, S. C. L. Appl. Catal., A 2009, 355, 139–147. (11) Jacobson, K.; Gopinath, R.; Meher, L. C.; Dalai, A. K. Appl. Catal., B 2008, 85, 86–91. (12) Mbaraka, I. K.; McGuire, K. J.; Shanks, B. H. Ind. Eng. Chem. Res. 2006, 45, 3022–3028. (13) Vazquez, P. G.; Blanco, P. G.; Caceres, C. V. Catal. Lett. 1999, 60, 205–215. (14) Pizzio, L. R.; Cacares, C. V.; Blanco, M. N. Appl. Catal., A 1998, 167 (2), 283–294.
† This paper has been designated for the Bioenergy and Green Engineering special section. *To whom correspondence should be addressed: Department of Chemical Engineering, 57 Campus Drive, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5A9, Canada. Telephone: þ1-306-9664771. Fax: þ1-306-966-4777. E-mail:
[email protected]. (1) Canakci, M. Bioresour. Technol. 2007, 98, 183–190. (2) Lotero, E.; Liu, Y.; Lopez, D. E.; Suwannakarn, K.; Bruce, D. A.; Goodwin, J. G., Jr. Ind. Eng. Chem. Res. 2005, 44, 5353–5363. (3) Encinar, J. M.; Gonzalez, J. F.; Sabio, E.; Ramiro, M. J. Ind. Eng. Chem. Res. 1999, 38, 2927–2931. (4) Kulkarni, M. G.; Dalai, A. K. Ind. Eng. Chem. Res. 2006, 45, 2901–2913.
r 2010 American Chemical Society
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: DOI:10.1021/ef901307w
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polar media. HPAs can be made eco-friendly insoluble solid acid catalysts with high thermal stability and high surface area by supporting them on suitable supports. The support provides HPAs an opportunity to be dispersed over a large surface area, which increases catalytic activity. Various supports, such as ZrO2,7 tantalum oxide,8 niobia,15 clays,16 etc., for HPAs have already been published. Research on niobium compounds in catalytic applications is a recent advancement in heterogeneous catalysis.17,18 In general, the present research focuses on the study of the catalytic performance of niobia impregnated with metal oxides.19 It has also been reported that niobia-promoted catalysts exhibit the strong metal-support interaction (SMSI) effect because it is reducible over a wide temperature range.20 Niobium oxide can be used as a support, promoter, and solid acid catalyst. However, scanty information is available in the literature on the use of niobia for transesterification reactions. The particular properties of the niobium acidity and redox properties were the point of interest for us to use it as a support for HPAs. The current work discusses the activity of the solid acid catalyst, TPA supported on niobia, for the transesterification of UCO containing high FFAs with methanol. The effect of various reaction parameters on the ester yield, such as catalyst loading, reaction temperature, and molar ratio of methanol/ oil, were investigated. A kinetic model was also developed on the basis of the experimental data.
Table 1. Fatty Acid Composition of UCO fatty acid
composition (wt %)
myristic (C14:0) palmitic (C16:0) palmitoleic (C16:1) stearic (C18:0) oleic (C18:1) linoleic (C18:2) linolenic (C18:3) arachidic (C20:0) eicosenoic (C20:1) behenic (C22:1)
0.42 8.64 0.91 5.92 51.12 11.68 2.98 0.59 1.02 0.31
chromatograph using a thermal conductivity detector (TCD). In a typical experiment, about 0.1 g of the oven-dried sample was taken in a quartz tube. Prior to TPD studies, the catalyst sample was treated at 300 °C for 1 h by passing pure helium (99.9%, 50 mL/min). After pretreatment, the sample was saturated with anhydrous ammonia (10% NH3-90% He mixture gas) at 100 °C at a flow rate of 50 mL/min for 1 h and was subsequently flushed with He at the same temperature to remove physiosorbed ammonia. The process was continued until a stabilized baseline was obtained in the gas chromatograph. Then, the TPD analysis was carried out from ambient temperature to 800 °C at a heating rate of 15 °C/min. The amount of NH3 evolved was calculated from the peak area of the already calibrated TCD signal. All TPD studies were made on the calcined samples. Brunauer-Emmett-Teller (BET) surface areas of the catalyst samples were calculated from N2 adsorption data acquired on an Autosorb-1 instrument (Quantachrome, Boynton Beach, FL) at liquid N2 temperature. The Raman spectra of the samples were collected with a Horiba-Jobin Yvon Lab Ram-HR spectrometer equipped with a confocal microscope, 2400/900 grooves/mm gratings, and a notch filter. The visible laser excitation at 532 nm (visible/green) was supplied by a Yag doubled diode pumped laser (20 mW). The scattered photons were directed and focused onto a single-stage monochromator and measured with a UV-sensitive LN2-cooled charge-coupled device (CCD) detector (Horiba-Jobin Yvon CCD-3000 V). Scanning electron microscopy (SEM) of the catalysts was obtained in a Hitachi S-520 electron microscope at an accelerated voltage of 10 kV. Samples were mounted on aluminum stubs using double-adhesive tape and were goldcoated in a Hitachi HUS-5GB vacuum evaporator. The size distribution of the catalyst was determined using standard-sized sieves (150-20 μm). A known amount of catalyst was accurately weighed and placed in the top sieve. This was then sieved down through the tower of sieves with decreasing sieve diameters. The amount of catalyst in each sieve was weighed. This procedure was repeated several times to ensure reproducibility of the results. Reaction Procedure. The properties of UCO are as follows: saponification value = 193.1 mg of KOH/g; iodine value = 97.5 g of iodine/100 g; molecular weight = 870 g/mol; water content = 0.03 wt %; FFA = 8%; and acid value = 16 mg of KOH/g. Fatty acid composition of UCO is presented in Table 1. Prior to the reaction, fine solid particles were filtered from UCO. No other pretreatment was performed. Transesterification of UCO was carried out in a 500 cc Parr reactor (Parr Instrument Co.). In a typical reaction, 0.115 mol of UCO and 2.069 mol of methanol were taken with a catalyst loading of 0.015 g/cm3 (3 wt % of oil). The initial sample was taken when the reaction mixture attained the desired temperature, after which agitation commenced. Samples were taken at periodic intervals of 1-20 h. To ensure that at the reaction temperature the reactants were in the liquid phase, the reactor was pressurized to 600 psig with nitrogen gas. Reproducibility of results was ensured by experimental triplicates at optimal conditions. Analysis. The products (3 mL was withdrawn each time) were analyzed for the percentage of tri-, di-, and monoglycerides,
Experimental Section Materials. UCO with 8 wt % FFA content was obtained from Saskatoon Processing Co., Saskatoon, Saskatchewan, Canada. Niobium oxide hydrate was supplied from CBMM, Brazil (HY-340, AD/1447). Methanol, tetrahydrofuran (THF), anhydrous sodium sulfate, and TPA were purchased from Alfa Aesar, Ward Hill, MA. Reference standard chemicals, methyl oleate, triolein, diolein, and monoolein, were purchased from Sigma-Aldrich, St. Louis, MO. Catalyst Preparation. The TPA-supported niobia catalysts with different TPA loadings were prepared by the impregnation method according to the procedure reported by us previously.15 The niobium oxide used in the present study was prepared by calcination of niobium pentoxide hydrate in air at 500 °C for 4 h to yield Nb2O5. A series of TPA/Nb2O5 catalysts with TPA loadings varying from 5 to 30 wt % were prepared by wet impregnation of TPA on the Nb2O5 support. The samples were dried at 120 °C for 12 h and subsequently calcined at the desired temperature for 2 h. Catalyst Characterization. Powder X-ray diffraction (XRD) patterns of the catalysts were recorded on a Siemens D-5000 diffractometer using nickel-filtered Cu KR radiation, with a scan speed of 2° min-1 and a scan range of 2-80°. Fourier transform infrared (FTIR) spectra of catalysts were recorded on a Biorad Excalibur spectrometer using the KBr disk method. Temperature-programmed desorption (TPD) of ammonia was carried out on a laboratory-built apparatus equipped with a gas (15) Srilatha, K.; Lingaiah, N.; Devi, B. L. A. P.; Prasad, R. B. N.; Venkateswar, S.; Sai Prasad, P. S. Appl. Catal., A 2009, 365, 28–33. (16) Bokade, V. V.; Yadav, G. D. J. Mol. Catal. A: Chem. 2008, 285, 155–161. (17) Tanabe, K. Catal. Today 2003, 78, 65–77. (18) Aranda, D. A. G.; Goncalves, J. A.; Peres, J. S.; Ramos, A. L. D.; Melo, C. A. R., Jr.; Antunes, O. A. C.; Furtado, N. C.; Taft, C. A. J. Phys. Org. Chem. 2009, 22, 709–716. (19) Pereira, E. B.; Pereira, M. M.; Lam, Y. L.; Perez, C. A. C.; Schmal, M. Appl. Catal., A 2000, 197, 99–106. (20) Hu, Z.; Kunimori, K.; Uchijima, T. Appl. Catal. 1991, 69, 253– 268.
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Table 2. Surface Area and Acid Strength Distribution Data of TPA/ Nb2O5 Catalysts aciditya (104 mol/g of catalyst) catalyst
surface area (m2/g)
weak
moderate/strong
Nb2O5 5 wt % TPA/Nb2O5 10 wt % TPA/Nb2O5 15 wt % TPA/Nb2O5 20 wt % TPA/Nb2O5 25 wt % TPA/Nb2O5 30 wt % TPA/Nb2O5
55.0 53.2 50.5 48.1 45.2 41.3 41.0
2.80 1.10 0.54 0.90 1.10 1.60 1.90
0.66 1.10 1.20 2.30 2.10
a
Data from ref 15.
and the ester content was determined by gel permeation chromatography (GPC) using Hewlett-Packard 1100 series highperformance liquid chromatography (HPLC) with a refractive index detector and two Phenogel (5 μm, 100A, 300 7.80 mm) columns in series protected with a guard column. The data were collected by Chem Station software, Agilent Technologies. THF was used as a mobile phase at 1 mL/min for 25 min. The operating parameters were as follows: injection volume, 5 μL; column temperature, 25 °C; and detector temperature, 35 °C. Reference standard chemicals, including methyl oleate, triolein, diolein, and monoolein, were used for the calibration. The percentage of FFA content was determined from the acid value following the American Oil Chemists’ Society (AOCS) method Te 1a-64.
Figure 1. Raman spectra of (a) Nb2O5, (b) 5 wt % TPA/Nb2O5, (c) 10 wt % TPA/Nb2O5, (d) 15 wt % TPA/Nb2O5, (e) 20 wt % TPA/Nb2O5, (f) 25 wt % TPA/Nb2O5, and (g) 30 wt % TPA/Nb2O5 (all samples were calcined at 300 °C for 2 h, except support Nb2O5).
stretching modes of νW-Ocentral in Keggin ions, respectively.21 In addition to these bands, a band is observed at 917 cm-1, which can be assigned to the asymmetric stretching mode of νW-O-W.22,23 Raman bands between 300 and 600 cm-1 are attributed to the stretching and deformation modes of W-O-W bridges of Keggin ions.23 On the whole, the Raman spectra indicate the preservation of the Keggin structure of TPA and some other bands, which interfere with bands of the support. Apart from these bands, the spectra of all have an intense band at 700 cm-1, typical for niobium oxides, because of vibrations of Nb-O-Nb bridges. The weak band around 825 cm-1 can be assigned to the symmetric stretching mode of surface NbdO, while Raman bands between 200 and 300 cm-1 can be assigned to the bending modes of Nb-O-Nb linkages.24,25 A blue shift on the WdO band observed in the spectra, from that reported in the literature,23,24 can be attributed to the increased bond order and suggested to occur because of the strength of the interaction or structural changes of WOx species.21,22 The blue shift and intensity changes of the Raman bands of the support suggest that the structures of the surface niobium oxide species are dependent upon the TPA content, which leads to altered acidity of the catalyst, as evident from NH3-TPD analysis of the catalysts (Table 2).26 Intensities of Raman bands at 1024, 1008, and 220 cm-1 corresponding to Keggin ions became more pronounced in catalysts with high TPA loading (25-30 wt %) than that with low TPA loading (5-20 wt %), indicating the existence of the ion clusters on the support and attainment of bulk nature.21 SEM pictures of 5-30 wt % TPA/Nb2O5 catalysts calcined at 300 °C for 2 h are shown in Figure 2. Figure 2 clearly indicates an increase in the size of the catalysts with respect to the high amount of TPA. SEM images are a visible reconfirmation of the aforementioned phenomenon of
Results and Discussion Catalyst Characterization. TPA (5-30 wt %)/Nb2O5 catalysts were characterized by XRD, FTIR, NH3-TPD, laser Raman, SEM, and BET surface area. In detail, XRD, FTIR, and NH3-TPD characterization results of 5-30 wt % TPA/ Nb2O5 catalysts were already discussed in a previous paper.15 In the context of the present work, a few salient features of these results are mentioned. Crystallinity and textural patterns of the 5-30 wt % TPA/Nb2O5 catalysts calcined at 300 °C for 2 h shown by XRD data reveal the welldispersion of TPA Keggin ions up to 25 wt %, beyond which it attains bulk nature. The FTIR analysis also established the preservation of the Keggin structure. NH3-TPD analysis of 5-30 wt % TPA/Nb2O5 catalysts calcined at 300 °C for 2 h showed the presence of more moderate and strong acid sites at 25 wt % of TPA loading (Table 2). XRD and FTIR analyses of 25 wt % TPA/Nb2O5 catalyst calcined at different temperatures revealed the decomposition of TPA above a calcination temperature of 500 °C. Even the NH3-TPD patterns show the disappearance of desorption peaks in catalysts calcined beyond 500 °C. The BET results of 5-30 wt % TPA/Nb2O5 catalysts along with support Nb2O5 are shown in Table 2. It was observed that the surface area decreased with increasing TPA loading; this can be due to the pore blockage of the surface with TPA. The Raman technique is known to be well-suited for observing TPA Keggin structures and supports from their vibrational information. Raman spectra of 5-30 wt % TPA/ Nb2O5 catalysts calcined at 300 °C for 2 h along with support Nb2O5 are depicted in Figure 1. It shows the characteristic bands of Keggin ions on the niobia support at 1024, 1008, and 220 cm-1, which are attributed to the symmetric and asymmetric stretching modes of νWdOterminal and symmetric
(22) Devassy, B. M.; Halligudi, S. B. J. Catal. 2005, 236, 313–323. (23) Teague, C. M.; Li, X.; Biggin, M. E.; Lee, L.; Kim, J.; Gewirth, A. A. J. Phys. Chem. B 2004, 108, 1974–1985. (24) Nowak, I.; Misiewicz, M.; Ziolek, M.; Kubacka, A.; Corberan, V. C.; Sulikowski, B. Appl. Catal., A 2007, 325, 328–335. (25) Gao, X.; Bare, S. R.; Weckhuysen, B. M.; Wachs, I. E. J. Phys. Chem. B 1998, 102, 10842–10852. (26) Li, Y.; Yan, S.; Qian, L.; Yang, W.; Xie, Z.; Chen, Q.; Yue, B.; He, H. J. Catal. 2006, 241, 173–179.
(21) Ranga Rao, G.; Rajkumar, T. Catal. Lett. 2008, 120, 261–273.
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methanol. The yield of methyl esters as a function of the TPA content is presented in Figure 4. It was found that the ester yield increased with the TPA content from 5 to 25 wt %, and there is no significant effect on the ester yield for a further increase in TPA loading. The higher activity of 25 wt % TPA/Nb2O5 catalysts compared to others could be due to the higher acidic nature of the catalyst. In solid acid-catalyzed esterification and transesterification reactions, the acidity of the catalyst is a matter of great importance and it is reported that the catalyst with high accessible surface acid sites shows the optimum performance.27,28 From the acid strength distribution data of TPA/Nb2O5 catalysts (Table 2), it is evident that 25 and 30 wt % TPA/ Nb2O5 catalysts possess more or less similar acidity. The constancy of the acidic nature beyond 25 wt % can be ascertained to the attainment of the bulk nature beyond this point, which is clearly depicted in SEM pictures (Figure 3). The agglomeration of particles beyond 25 wt % TPA is responsible for constant surface area values of 25 and 30 wt % TPA/Nb2O5 catalysts (Table 2). From the characterization data, it can be deduced that the agglomeration of particles diminishes the number of accessible acid sites available for reaction, which in turn decreases the ester yield. TPA (25 wt %)/Nb2O5 was chosen over other catalysts because it showed high activity and better acidity. An earlier report on esterification15 shows a detailed evaluation of the 25 wt % TPA/Nb2O5 catalyst calcined at different temperatures (300-650 °C) and their effect on the esterification of palmitic acid as well as sunflower fatty acids with methanol. The 25 wt % TPA/Nb2O5 catalyst calcined at 400 °C for 2 h showed better activity for esterification because of its high acidity compared to other catalysts, as observed from NH3-TPD analysis. Intact Keggin ions of TPA on the support and their interactions are reported to be responsible for high acidity, and hence, they were used for further experiments on transesterification. To establish the effect of external mass-transfer limitations during the transesterification reaction, experiments were conducted at different speeds of agitation using 25 wt % TPA/Nb2O5 catalyst, so that the true intrinsic kinetics can be developed. The stirrer speed, beyond which there was no effect on the reaction rate, was considered to be the minimum speed of agitation required to eliminate external diffusion effects. Results shown in Figure 5 reveal that the external diffusion control was negligible for stirrer speeds greater than 600 rpm. For all of the reaction kinetic studies reported here, a stirrer speed of 600 rpm was used. Because catalyst loading is an important criterion that needs to be optimized for better yield of esters, its effect on yield was studied at a reaction temperature of 200 °C and 18:1 methanol/UCO molar ratio. The catalyst concentration was varied over a range of 0.005-0.025 g/cm3 on the basis of the total volume of the reaction mixture. Figure 6 shows the effect of the catalyst concentration on the yield. The yield is proportional to the catalyst concentration of 25 wt % TPA/ Nb2O5, which is obvious because of the proportional increase in the number of active sites. However, beyond a catalyst concentration of 0.015 g/cm3, there was no significant increase in the yield, and hence, all further experiments
Figure 2. Scanning electron micrographs of (a) 5 wt % TPA/Nb2O5, (b) 10 wt % TPA/Nb2O5, (c) 15 wt % TPA/Nb2O5, (d) 20 wt % TPA/Nb2O5, (e) 25 wt % TPA/Nb2O5, and (f) 30 wt % TPA/Nb2O5 (all samples were calcined at 300 °C for 2 h, except support Nb2O5). Table 3. Size Distribution of 25 wt % TPA/Nb2O5 Catalyst Calcined at 400 °C for 2 h diameter range (μm)
mass fraction
>100 80-100 63-80 45-63 32-45 20-32
