Nanocrystalline Lithium Ion Impregnated Calcium Oxide As

Feb 12, 2010 - Potassium Ion Impregnated Calcium Oxide as a Nanocrystalline Solid Catalyst for Biodiesel Production from Waste Cotton Seed Oil. D. Kum...
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Energy Fuels 2010, 24, 2091–2097 Published on Web 02/12/2010

: DOI:10.1021/ef901318s

Nanocrystalline Lithium Ion Impregnated Calcium Oxide As Heterogeneous Catalyst for Transesterification of High Moisture Containing Cotton Seed Oil Dinesh Kumar and Amjad Ali* School of Chemistry and Biochemistry, Thapar University, Patiala 147004 India Received November 9, 2009. Revised Manuscript Received January 16, 2010

The present work demonstrates the application of nanocrystalline Liþ impregnated CaO as a heterogeneous catalyst for transesterification of used cottonseed oil with a higher amount of moisture. Catalysts with different alkali metal ions were prepared by the wet impregnation method, and their basic strengths were measured by Hammett indicators and a maximum was found in the case of lithium carbonate impregnated CaO. Further characterization of the lithium impregnated catalyst by powder X-ray diffraction and transmission electron microscopy studies supports the formation of a nanosized catalyst with a particle size of about 50 nm. The same catalyst has been chosen for studying the transesterification reaction of used cotton seed oil with methanol. The variables used for the transesterification were impregnated alkali metal ion, percentage of Liþ impregnation, catalyst concentration, reaction temperature (35-65 °C), oil to methanol molar ratio, reaction time (0.5-8 h), free fatty acid content (up to 6 wt %), moisture content (up to 15 wt %), and mixture of free fatty acid and moisture content. Reaction parameters have been optimized to achieve the least reaction period for the completion of the reaction. Complete transesterification of used cotton seed oil with methanol required 45 min in the presence of lithium carbonate impregnated CaO nanocatalyst at 65 °C when moisture and free fatty acid contents present in oil were 0.26 and 0.31 wt %, respectively. The same catalyst was found to be effective for the transesterification of cotton seed oil even in the presence of 15 wt % water though it took 2.5 h for the completion of the reaction.

use of vegetable oil.6 Biodiesel can be prepared from a variety of triglycerides including vegetable oils, animal fats, and waste greases in the presence of homogeneous or heterogeneous catalyst.7-11 Homogenous alkali catalyst has been used frequently at the industrial scale for biodiesel production as it catalyzes the reaction at faster rate and requires mild reaction conditions. However, the same catalyst yielded biodiesel and glycerol contaminated with catalyst and leads to the formation of soap if free fatty acids (FFAs) and moisture contents are greater than 0.5% and 0.3% (w/w), respectively.12,13 More recently, there has been an increased research activity directed toward the development of heterogeneous catalysts which has several advantages, viz., formation of uncontaminated products, is recyclable, has low sensitivity toward FFAs and moisture content, and does not corrode the reaction vessel.14,15 A variety of heterogeneous catalysts for transesterification reactions were reported in the literature, including

1. Introduction Depleting fossil fuel reserves and increasing environmental concerns are the main cause for the search of renewable fuels, such as biodiesel, in the recent past. Combustion of fossil fuels causes the emission of air pollutants viz., NOx, SOx, CO, CO2, particulate matter, and volatile organic compounds (VOCs), out of which CO2 is the main culprit for global warming.1 Hence, there is a need to substitute the fossil based fuels with renewable and eco-friendly one. Vegetable oils have long been considered a potential diesel fuel substitute although their use has been hindered by economic and technical difficulties. High viscosity and high molecular weight of vegetable oils leads to the poor fuel atomization, incomplete combustion, carbon deposition on the injector, and fuel build up in the lubricant oils causing serious engine fouling.2-4 Transesterification of vegetable oils with short carbon chain alcohols in the presence of catalyst (acid or base) leads to the formation of glycerol and fatty acid alkyl esters, commonly known as biodiesel.5 Viscosity of biodiesel (4-6 cSt) is less than vegetable oil (35-40 cSt) and hence could overcome most of the problems associated with the

(6) Allen, C. A. W.; Watts, K. C.; Ackman, R. C.; Pegg, M. J. Fuel 1999, 78, 1329–1336. (7) Ma, F.; Hanna, M. F. Bioresour. Technol. 1999, 70, 1–15. (8) De, B. K.; Bhattacharya, D. K. Lipid-Fett 1999, 101, 404–406. (9) Alcantara, R.; Amores, J.; Canoira, L.; Fidalgo, E.; Franco, M. J.; Navarro, A. Biomass Bioenergy 2000, 18, 515–527. (10) Meher, L. C.; Vidya Sagar, D.; Naik, S. N. Renewable Sustainable Energy Rev. 2006, 10, 248–268. (11) Vicente, G.; Martinez, M.; Aracil, J. Bioresour. Technol. 2004, 92, 297–305. (12) Haas, M. J. Lipid Technol. 2004, 16, 7–11. (13) Canakci, M.; Gerpen, J. V. Trans. ASAE 1999, 42, 1203–1210. (14) Clark, J. H.; Macquarrie, D. J. Chem. Soc. Rev. 1996, 25, 303– 310. (15) Tanabe, K.; Misono, M.; Ono, Y.; Hattori, H. Stud. Surf. Sci. Catal. 1989, 51, 27–213.

*To whom correspondence should be addressed. Tel: +91-1752393832. Fax: þ91-175-2364498, 2393005. E-mail: amjadali@thapar. edu, [email protected]. (1) Klass, L. D. Biomass for Renewable Energy, Fuels, and Chemicals; Academic Press: New York. 1998; pp 1-2. (2) Pryde, E. H. Vegetable oil fuel standards. Vegetable Oil Fuels: Proceedings of the International Conference on Plant and Vegetable Oils Fuels; ASAE: Fargo, ND, 1982; pp 101-105. (3) Sridharan, R.; Mathai, I. M. J. Sci. Ind. Res. 1974, 33, 178–187. (4) Encinar, J. M.; Gonzalez, J. F.; Rodriguez, J.; Tejedor, A. Energy Fuels 2002, 16, 443–450. (5) Wagner, L. E. Master’s Thesis, Kansas State University, Manhattan, KS, 1983. r 2010 American Chemical Society

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immobilized enzymes, calcium carbonate, alkali-earthmetal compounds,25 sulphated zirconia,26 tin compounds supported in ion-exchange resins,27,28 alkylguanidines heterogenized on organic polymers,29,30 zeolites,31,32 alumina loaded with alkali-metal salt,33,34 and calcium oxide loaded with lithium.35-37 Recent studies have also found functionalized mesoporous silica, such as tin-oxide-modified mesoporous SBA-15,38,39 titanium-grafted mesoporous silica,40-43 and magnesium supported MCM-4144,45 effective for catalyzing transesterification reactions using a variety of triglycerides. However, for complete (>98%) biodiesel conversion from vegetable oils, most of the heterogeneous catalyst requires high temperature and pressure conditions and a longer duration of reaction. All these factors lead to an increase in the production cost of biodiesel.20,46 A major problem for the commercialization of biodiesel is its production cost which should be at par with mineral diesel. Production cost can be reduced by using low-quality cheap

feedstock, using heterogeneous catalyst, and by reducing the time period of the transesterification reaction in order to reduce the energy consumption during the reaction. With these objectives kept in mind, the present work demonstrates the preparation of Liþ impregnated calcium oxide as a nanocrystalline heterogeneous catalyst for biodiesel production from used cottonseed oil. The reaction parameters have been optimized to obtain the complete transesterification of oil in the minimum possible time to reduce the energy consumption for the reaction. 2. Experimental Section 2.1. Materials and Methods. Used cooking oil was collected from local restaurants located in Patiala. NaOH, KOH, LiNO3, KNO3, NaNO3, Li2CO3, K2CO3, Na2CO3, CaO, palmitic acid, and silica gel for thin layer chromatography (TLC), of reagent grade quality, were purchased from Loba Chemie, India, and used as such without further purification. Methanol (99.8%) used in the present study was obtained from Merck, India, and methyl oleate (99%) used as a biodiesel standard was procured from Sigma-Aldrich. Free fatty acid value, saponification, and the iodine value of the used cottonseed seed oil were determined by following the methods as reported in the literature,47 and values were found to be 0.31% (w/w), 190.4 mg of KOH/g, and 94.72, respectively. The moisture content was determined by the Karl Fisher titrimetric method and was found to be 0.26% (w/w). Alkali metal ion concentrations in CaO were determined by ESICO microprocessor flame photometer model 1382E. X-ray diffraction (XRD) data for powder samples were collected on Panalytical’s X’Pert Pro with Cu KR radiation. The samples were scanned in the range of 2θ = 5-70° at the scanning speed of 2°/ min. The surface areas of the catalyst were determined by using the adsorption/desorption method at 77 K by the standard Brunauer-Emmett-Teller (BET) method using Micromeritics Tristar 3000 equipment. All samples were degassed at 473 K for 90 min under nitrogen atmosphere to remove the physisorbed moisture from the catalysts. Field emission scanning electron microscopy (FESEM) was performed on TECNAI G220, S-TWIN to collect the SEM images of the catalysts, and transmission electron microscopy (TEM) was performed on QUANTA 200 FEG to record TEM images. Fourier transform-nuclear magnetic resonance (FT-NMR) spectra of biodiesel and vegetable oil were recorded on a Bruker Avance-II (400 MHz) spectrophotometer. The basic strengths of the catalysts (H_) were determined by using Hammett indicators: neutral red (H_ = 6.8), bromthymol blue (H_ = 7.2), phenolphthalein (H_ = 9.3), Nile blue (H_ = 10.1), tropaeolin-O (H_ = 11.1), 2,4-dinitroaniline (H_ = 15.0), and 4-nitroaniline (H_ = 18.4). 2.2. Preparation of Catalyst. The nanocrystalline alkali metal ion impregnated CaO catalyst was prepared by the wet impregnation method as reported in the literature36 with a slight modification. In a typical preparation, 10 g of calcium oxide was suspended in 40 mL of deionized water, and to this 10 mL of aqueous alkali metal salt solution of appropriate concentration was added, e.g., while preparing 1.5 wt % lithium ion impregnated calcium oxide, 0.798 g of Li2CO3 was used. The slurry was stirred for 2 h, then evaporated to dryness, and heated at 120 °C for 24 h. A series of alkali metal ions (Liþ, Naþ, and Kþ) impregnated catalysts using different salts of alkali metal ions (K2CO3, Na2CO3, Li2CO3, KNO3, NaNO3, and LiNO3) with varying metal concentrations (wt %) ranging from 0.5 to 5.0 wt % (metal ion/CaO) were prepared and characterized by flame photometry, X-ray powder diffraction (XRD), BET surface area measurement, a Hammett indicator test, and FESEM and TEM techniques.

(16) Staubmann, R.; Ncube, I.; Gubitz, G. M.; Steiner, W.; Read, J. S. J. Biotechnol. 1999, 75, 117–126. (17) Clapes, P.; Torres, J. L.; Adlercreutz, P. Bioorg. Med. Chem. 1995, 3, 245–255. (18) Carrea, G.; Riva, S. Angew. Chem., Int. Ed. 2000, 39, 2226–2254. (19) Gupta, M. N.; Verlag, B.; Berlin, B. B. Biochemistry 2002, 67, 1405. (20) Fukuda, H.; Kondo, A.; Noda, H. J. Biosci. Bioeng. 2001, 92, 405–416. (21) Briand, D.; Dubreucq, E.; Galzy, P. Biotechnol. Lett. 1991, 16, 813–818. (22) Shah, S.; Sharma, S.; Gupta, M. N. Energy Fuels 2004, 18, 154– 159. (23) Mukesh, D.; Banerji, A. A.; Newadkar, R.; Bevinakatti, H. S. Biotechnol. Lett. 1993, 15, 77–82. (24) Suppes, G. J.; Bockwinkel, K.; Lucas, S.; Bots, J. B.; Mason, M. H.; Heppert, J. A. J. Am. Oil Chem. Soc. 2001, 78, 139–145. (25) Arzamendi, G.; Argui~ narena, E.; Campo, I.; Zabala, S.; Gandia, L. M. Catal. Today 2008, 133, 305–313. (26) Charusiri, W.; Yongchareon, W.; Vitidsant, T. Korean J. Chem. Eng. 2006, 23, 349–355. (27) Abreu, F. R.; Alves, M. B.; Macedo, C. C. S.; Zara, L. F.; Suarez, P. A. Z. J. Mol. Catal. A: Chem. 2005, 227, 263–267. (28) Cardoso, A. L.; Neves, S. C. G.; da Silva, M. J. Energies 2008, 1, 79–92. (29) Schuchardt, U.; Vargas, R. M.; Gelbard, G. J. J. Mol. Catal. A: Chem. 1995, 99, 65–70. (30) Schuchardt, U.; Vargas, R. M.; Gelbard, G. J. J. Mol. Catal. A: Chem. 1996, 109, 37–44. (31) Suppes, G. J.; Dasari, M. A.; Doskocil, E. J.; Mankidy, P. J.; Goff, M. J. Appl. Catal., A 2004, 257, 213–223. (32) Brito, A.; Borges, M. E.; Otero, N. Energy Fuels 2007, 21, 3280– 3283. (33) D’Cruz, A.; Kulkarni, M. G.; Meher, L. C.; Dalai, A. K. J. Am. Oil Chem. Soc. 2007, 84, 937–943. (34) Ebiura, T.; Echizen, T.; Ishikawa, A.; Murai, K.; Baba, T. Appl. Catal., A 2005, 283, 111–116. (35) Macleod, C. S.; Harvey, A. P.; Lee, A. F.; Wilson, K. Chem. Eng. J. 2008, 135, 63–70. (36) Watkins, R. S.; Lee, A. F.; Wilson, K. Green Chem. 2004, 6, 335– 340. (37) Alonso, D. M.; Mariscal, R.; Granados, M. L.; Maireles-Torres, P. Catal. Today 2009, 143, 167–171. (38) Liu, Z.; Chen, H.; Huang, W.; Gu, J.; Bu, W.; Hua, Z.; Shi, J. J. Mater. Res. 2006, 21, 655–663. (39) Ramaswamy, V.; Shah, P.; Lazar, K.; Ramaswamy, A. V. Catal. Surv. Asia 2008, 12, 283–309. (40) Freedman, B.; Butterfield, R. O.; Pryde, E. H. J. Am. Oil Chem. Soc. 1986, 63, 1375–1380. (41) Diasakou, M.; Louloudi, A.; Papayannakos, N. Fuel 1998, 77, 1297–1302. (42) Darnoko, D.; Cheryan, M. J. Am. Oil Chem. Soc. 2000, 77, 1263– 1267. (43) Srinivas, D.; Srivastava, R.; Ratnasamy, P. Catal. Today 2004, 96, 127–133. (44) Li, E.; Rudolph, V. Energy Fuels 2008, 22, 145–149. (45) Barrault, J.; Bancquart, S.; Pouilloux, Y. C. R. Chim. 2004, 7, 593–599. (46) Feuge, R. O.; Grose, T. J. Am. Oil. Chem. Soc. 1949, 26, 97–102.

(47) Plummer, D. T. An Introduction to Practical Biochemistry; Tata McGraw-Hill: New Delhi, India, 1988; pp 195-197.

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Kumar and Ali Table 1. Comparison of the Basic Strengths of Alkali Metal Ion Impregnated CaO with That of Pure CaO catalyst type

basic strength (H_)

CaO LiC-CaO-1.5 LiN-CaO-1.5 NaC-CaO-1.5 NaN-CaO-1.5 KC-CaO-1.5 KN-CaO-1.5

9.8 < H_ < 10.1 15.0 < H_ < 18.4 11.1 < H_ < 15.0 10.1 < H_ < 11.1 10.1 < H_ < 11.1 10.1 < H_ < 11.1 10.1 < H_ < 11.1

Figure 1. Comparison of BET surface areas of pure CaO with that of Li, Na, and K ion impregnated CaO.

The catalysts prepared in such a manner were designated as MA-CaO-XX, where MA represent the impregnated alkali metal salt and XX represent the percentage of impregnation in CaO, e.g., LiC-CaO-1.5 and LiN-CaO-1.5 represent CaO catalysts formed by impregnation of 1.5% (wt %) of Liþ using Li2CO3 and LiNO3 salts, respectively. 2.3. Transesterification of Used Cottonseed Oil. In a typical transesterification reaction, used cottonseed oil and methanol in a 1:12 molar ratio and 5 wt % of catalyst were taken in a two neck round-bottom flask. The reaction mixture was heated up to 65 °C with continuous stirring until the completion of the reaction. The samples have been withdrawn from the reaction mixture after every 5 min with the help of a glass capillary and diluted with hexane to perform thin layer chromatographic (TLC) studies. The completion of the reaction and primary characterization of the product was done by the TLC technique using hexane/ ethyl acetate/acetic acid (90:9:1) as the mobile phase and silica gel as the stationary phase. Biodiesel shows a higher mobility than vegetable oil with the selected solvent system, and complete conversion of vegetable oil to biodiesel was supported by the disappearance of the vegetable oil spot on the TLC plate. Further biodiesel so produced was characterized by 1H and 13 C NMR spectroscopy, and the 1H NMR technique was also used for the quantification of biodiesel as described in the literature.48,49

Figure 2. Comparison of powder XRD patterns of commercially available CaO with LiC-CaO-1.5, NaC-CaO-1.5, and KC-CaO-1.5.

3.1. Catalyst Characterization. The basic strength of the alkaline earth metal oxides and hydroxides increase in the order of Mg < Ca < Sr < Ba,36 and CaO has been chosen for the present work becasue it is inexpensive and less toxic. Further, the basic strength of CaO can be enhanced by impregnating it with alkali metal ions. The actual impregnated amount of alkali metal ions was determined by the flame photometric method. The surface area of Li, Na, and K carbonate impregnated (1.5 wt %) calcium oxide catalysts were measured by the BrunauerEmmett-Teller (BET) method. Alkali metal ion impregnated CaO catalysts show high surface area in comparison with pure CaO, and the highest surface area was found in the case of lithium impregnated CaO as shown in Figure 1.

The basic strengths of the catalysts were determined by using the Hammett indicators50 and summarized in Table 1. The basic strength of LiC-CaO-1.5 was found to be maximum, 15.0 < H_ < 18.4, among the prepared catalysts, probably due to the formation of strong basic sites on the CaO surface. Hence, the LiC-CaO-1.5 catalyst is expected to show the highest catalytic activity toward the transesterification reaction of vegetable oil. Powder X-ray diffraction studies of CaO and Li, Na, and K carbonate impregnated (1.5 wt %) CaO have been performed, and comparisons of XRD patterns are given in Figure 2. The intense peaks at 2θ ∼ 37.34, 53.93, and 32.23 correspond to the d-values of 2.39, 1.69, and 2.75, respectively, of calcium oxide, while peaks at 2θ ∼ 34.18, 18.16, and 47.18 correspond to d-values of 2.63, 4.92, and 1.92, respectively, of calcium hydroxide as shown in Figure 2. The particle size of the LiC-CaO-1.5 catalyst was determined by the Debye-Scherrer method51 using powder XRD data and found to be about 70 nm. Scanning electron microscopic images of pure CaO shows that it has clusters of ∼5 μm sized particles. Impregnation of the same with Liþ (1.5 wt %) using Li2CO3 leads to the formation of impregnated CaO particles in hexagonal and oval shapes as shown in Figure 3. The average size of particles by FESEM studies were found to be ∼4 μm. TEM analysis of the same particles shows that these particles are the clusters of further smaller particles with an average size of ∼50 nm and hence supports the presence of catalyst in the nanoparticle form as shown in Figure 4. The characterization of impregnated CaO catalysts reveals that lithium carbonate impregnated CaO possesses the

(48) Gelbard, G.; Bres, O.; Vargas, R. M.; Vielfaure, F.; Schuchardt, U. F. J. Am. Oil Chem. Soc. 1995, 72, 1239–1241. (49) Knothe, G. J. Am. Oil Chem. Soc. 2001, 78, 1025–1028.

(50) Wan, T.; Yu, P.; Wang, S.; Luo, Y. Energy Fuels 2009, 23, 1089– 1092. (51) Qadri, S. B.; Skelton, E. F.; Hsu, D.; Dinsmore, A. D.; Yang, J.; Gray, H. F.; Ratna, B. R. Phys. Rev. B 1991, 60, 9191–9193.

3. Results and Discussion

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Figure 3. Comparison of SEM images of (a) CaO with (b) LiC-CaO-1.5.

Figure 4. TEM image of LiC-CaO-1.5.

Figure 5. Comparison of the 1H NMR spectra of (a) cotton seed oil with that of (b) biodiesel.

highest surface area and basic strength and is expected to show highest activity toward the transesterification reaction. When transesterification reactions were performed in the presence of Liþ, Naþ, and Kþ impregnated CaO, the lithium ion impregnated CaO took the least time for the completion of the reaction and hence chosen for further studies. 3.2. Transesterification Reactions and Biodiesel Characterization. Transesterification reactions of used cotton seed oil with methanol (1:12 molar ratio) were performed in the presence of prepared catalysts (5 wt %, catalyst/oil) at 65 °C. Prepared catalysts were used to carry out the transesterification reactions of used cotton seed oil in order to determine the least reaction time for the completion of the transesterification reaction. During the course of study, the following parameters have been varied (i) impregnated alkali metal ion, (ii) impregnated lithium ion concentration, (iii) catalyst concentration, (iv) reaction temperature, (v) oil/ methanol molar ratio, (vi) moisture content (1-15 wt %), (vii) free fatty acid content (0.5-6 wt %), and (viii) mixture of moisture and free fatty acid contents. Biodiesel produced in the transesterification reaction was characterized by 1H and 13C NMR spectroscopy. The proton NMR spectrum of used cottonseed oil shows a multiplet at 4.1 and 5.2 ppm due to the presence of glyceridic protons

along with other hydrocarbon-proton peaks at their normal positions.52 The appearance of a new peak at 3.6 ppm due to -OCH3 protons and disappearance of the glyceridic protons support the formation of biodiesel as shown in Figure 5. The proton NMR spectra has also been used to confirm the completion of the reaction (>98% conversion of vegetable oil to biodiesel) by following the methods as reported in the literature.48,49 The carbon-13 NMR spectrum of cottonseed oil shows two peaks at 62.2 and 69.0 ppm due to the glyceridic carbon atoms along with other hydrocarbon-carbons peaks at their normal positions. Disappearance of these two peaks and appearance of a new peak at 51.5 ppm due to the -OCH3 carbon supports the formation of the methyl ester of cotton seed oil as shown in Figure 6. 3.2.1. Effect of Alkali Metal Ion on Catalyst Activity. In order to test the effect of impregnated alkali metal ions on the activity of the catalyst, six different catalysts, viz., LiC-CaO1.5, LiN-CaO-1.5, KN-CaO-1.5, KC-CaO-1.5, NaN-CaO1.5, and NaC-CaO-1.5 were prepared. The basic strengths of the prepared catalysts were determined by Hammett indicators and found to be maximum (15.0-18.4) for nanocrystalline LiC-CaO-1.5. The basic strength of the alkali metal ion impregnated catalysts decreases with the increase of alkali metal ion size. Smaller size ions are expected to insert easily

(52) Vigli, G.; Philipidis, A.; Spyros, A.; Dais, P. J. Agric. Food Chem. 2003, 51, 5715–5722.

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Figure 6. Comparison of 13C NMR spectra of (a) cotton seed oil with that of (b) biodiesel.

Figure 8. Effect of lithium ion concentration in CaO on the time required for complete transesterification of used cottonseed oil to biodiesel.

Figure 7. Effect of impregnated alkali metal ion on reaction time required for the complete conversion of used cottonseed oil to biodiesel.

Figure 9. Effect of catalyst concentration on the time required for complete transesterification of used cottonseed oil to biodiesel.

in the framework of CaO to create oxygen gaps, which are responsible for the strong basic strength.53 Thus, with an increase in the size of the alkali metal ion in CaO, the basic strength as well as the catalytic activity toward transesterification decreases. Thus, the reaction proceeds at a faster rate in the case of LiC-CaO-1.5 while other catalyst shows relatively lower activity toward transesterification of used cottonseed oil as summarized in Figure 7. LiC-CaO-1.5 was selected for further studies because it was the highest in activity toward the transesterification reaction. 3.2.2. Effect of Impregnated Lithium Ion Concentration. To determine the optimum amount of lithium ion impregnation in CaO, a series of catalysts by varying the amount of lithium ion from 0.5 to 5 wt % (metal ion/CaO) in CaO were prepared. Transesterification of used cottonseed oil was performed with methanol (1:12 molar ratio) at 65 °C in the presence of prepared catalysts. The reaction time required for the complete transesterification was found to decrease

from 3 h to 45 min as the amount of lithium ion in CaO was increased from 0.5 to 1.5 wt %. However, a further increase in Liþ ion concentration does not reduce the reaction time as shown in Figure 8, and hence, LiC-CaO-1.5 catalyst was used for transesterification reactions to optimize other parameters for achieving the minimum time for the completion of the transesterification reaction. 3.2.3. Effect of Catalyst Concentration. A series of transesterification reactions of used cotton seed oil with methanol (1:12 molar ratio) at 65 °C was performed in the presence of nanocrystalline LiC-CaO-1.5 by varying its concentration from 1 to 8 wt % (catalyst/oil) in order to find the optimum catalyst concentration. Time required for the complete conversion of vegetable oil to biodiesel decreases from 7 h to 45 min as the catalyst concentration was increased from 1 to 5 wt %. A further increase in the catalyst concentration does not reduce the reaction time significantly as shown in Figure 9. The transesterification reactions were further studied with a catalyst concentration of 5 wt % of oil for optimization of the other parameters.

(53) Barrault, J.; Pouilloux, Y.; Clacens, J. M.; Vanhove, C.; Bancquart, S. Catal. Today 2002, 75, 177–181.

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Figure 11. Effect of the oil/methanol molar ratio on the time required for complete transesterification of used cottonseed oil to biodiesel.

Figure 10. Effect of reaction temperature on the time required for complete transesterification of used cottonseed oil to biodiesel.

3.2.4. Effect of Reaction Temperature. A series of transesterification reactions was conducted in the presence of 5 wt % (catalyst/oil) nanocrystalline LiC-CaO-1.5 to find the optimum temperature for the transesterification reaction. The time required for the complete transesterification of vegetable oil to biodiesel decreases from 6 h to 45 min as the temperature of the reaction was increased from 35 to 65 °C. A further increase in the reaction temperature does not reduce the reaction time significantly as shown in Figure 10. 3.2.5. Effect of Methanol/Oil Molar Ratio. The effect of the methanol/oil molar ratio on the transesterification reaction is one of the important parameters which affects the methyl ester yield as well as cost of the biodiesel production. The theoretical minimum oil to methanol molar ratio should be 1:3 for the complete conversion of vegetable oil to biodiesel. However, with transesterification being a reversible reaction, usually such reactions performed with an excess of methanol shifts the equilibrium in the forward direction to achieve a maximum methyl ester yield. Heterogeneous catalysts usually catalyzed the transesterification reaction at a slower rate and took more time for the completion of the reaction. The use of higher molar ratios of oil to alcohol (viz., 1:15, 1:40, and even 1:275) to improve the yield of transesterified product in less time have been reported in the literature54-56 when reactions were catalyzed by heterogeneous catalysts. To determine the optimum oil/methanol molar ratio, the reactions were performed with 1:3 to 1:18 molar ratios at 65 °C using 5 wt % of LiC-CaO-1.5 catalyst. The rate of transesterification reaction increases as the oil/methanol ratio was increased from 1:3 to 1:12, and the reaction was found to be completed in 45 min when a 1:12 ratio was taken. A further increase in the oil/methanol ratio does not increase the reaction rate significantly, and complete conversion still takes 45 min as shown in Figure 11. 3.2.6. Effect of Moisture Content. The presence of water (>0.3%) in vegetable oil, used for the transesterification reaction, leads to the formation of soap instead of biodiesel

Figure 12. Effect of moisture content on the time required for complete transesterification of used cottonseed oil to biodiesel.

in the presence of homogeneous catalyst.13 Used cotton seed oil taken for the present studies was found to have 0.26% moisture content, and the transesterification reaction of the same using NaOH or KOH as a catalyst leads to the saponification reaction. However, the same reaction when catalyzed by nanocrystalline LiC-CaO-1.5 yielded the complete conversion of oil to biodiesel. In order to determine the highest moisture resistance of LiC-CaO-1.5 catalyst, transesterification reactions were performed in the presence of 1-35 wt % (water/oil) water. The same catalyst was found to be effective for the complete transesterification of cotton seed oil in 2.5 h even when 15 wt % of moisture content was present in the reaction mixture as shown in Figure12. Further addition of water (>15%) to the reaction mixture decreases the product yield and leads to saponification of the oil. 3.2.7. Effect of Free Fatty Acid Contents. The presence of free fatty acid contents (>0.5 wt % of oil) leads to the saponification instead of the transesterification in the presence of homogeneous catalyst.12 Used cottonseed oil taken

(54) Xie, W.; Peng, H.; Chen, L. J. Mol. Catal., A 2005, 246, 24–32. (55) Furuta, S.; Matsuhashi, H.; Arata, K. Catal. Commun. 2004, 5, 721–723. (56) Leclercq, E.; Finiels, A.; Moreau, C. J. Am. Oil Chem. Soc. 2001, 11, 1161–1165.

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Kumar and Ali Table 2. Effect of Moisture and Free Fatty Acid Content When Present Together on the Reaction Time Required for the Complete Transesterification of Cottonseed Oil in the Presence of the LiCCaO-1.5 Catalyst wt % S. no.

moisture

FFAs

reaction time (h)

conversiona

1 2 3 4 5 6

1 2 3 5 5 6

1 2 3 3 2 2

2 2.5 8 8 4 8

complete complete saponification saponification complete saponification

a Complete conversion of cottonseed oil to biodiesel was determined by TLC and proton NMR spectroscopy, and saponification was visualized due to the formation of solid soap.

reaction mixture leads to the formation of soap instead of the transesterified product. 4. Conclusions

Figure 13. Effect of free fatty acid content on the time required for complete transesterification of used cottonseed oil to biodiesel.

The present work demonstrates the preparation of a series of alkali metal ion (Li, Na, and K) impregnated CaO catalysts and investigation of their catalytic activities toward transesterification of used cottonseed oil (with 0.26, 0.31, wt % moisture and FFA contents, respectively). Calcium oxide impregnated with Li2CO3 was found to have the highest basic strength as supported by Hammett indicator tests and exist in the nanocrystalline form as supported by TEM and powder XRD studies. The same catalyst (LiC-CaO-1.5) took 45 min for the complete transesterification of used cottonseed oil with methanol (1:12 molar ratio) at 65 °C and 6 h at room temperature (35 °C). Further, the prepared catalyst was found to be effective for the complete transesterification of used cottonseed oil at 65 °C with the oil/methanol molar ratio being 1:12 even in the presence of (i) 15 wt % moisture in 2.5 h, (ii) 6 wt % of free fatty acid in 2.5 h, (iii) 5 wt % moisture and 2 wt % free fatty acid content in 4 h. In short, the prepared nanocatalyst has the potential to convert the cheap feedstock, having a higher amount of moisture and FFAs, into biodiesel to reduce its production cost. Presently, studies are focused in our lab to test the efficiency of the same catalyst for the transesterification of other vegetable oils and animal fats.

for the present studies was found to have 0.31% free fatty acid content, and the transesterification reaction of the same using NaOH or KOH as a homogeneous catalyst leads to the formation of soap instead of biodiesel. However, the same reaction when catalyzed by nanocrystalline LiC-CaO-1.5 yielded the complete conversion of oil to biodiesel in 45 min. In order to determine the maximum tolerance of prepared catalyst for free fatty acid contents, FFAs (1-6 wt % of oil) in the form of palmitic acid were added in the reaction mixture while performing the transesterification reaction of used cotton seed oil. Catalyst LiC-CaO-1.5 was found to be effective for the complete transesterification of cottonseed oil in 2.5 h even when 6 wt % of free fatty acid content was present in the reaction mixture as shown in Figure 13. Further addition of free fatty acid content (>6%) to oil leads to the saponification of oil. 3.2.8. Effect of Mixture of Moisture Content and Free Fatty Acid. Most of the low-quality feedstock contains a high amount of moisture and free fatty acid content. To study the combined effect of moisture and free fatty acid content on the activity of prepared catalyst (LiC-CaO-1.5), both were added together during the transesterification reaction of used cottonseed oil. When 2 wt % moisture and the free fatty acid content of each were added together, LiC-CaO-1.5 nanocatalyst took 2.5 h for the completion of the transesterification reaction. The moisture content when increased up to 5 wt % in the presence of 2 wt % of free fatty acid took 4 h for the completion of the reaction as shown in Table 2. A further increase of either the moisture or FFA content in the

Acknowledgment. A.A. acknowledges the financial support from the DST, New Delhi (Grant No. SR/FTP/CS-30/2007), and SCBC, Thapar University for providing the necessary research facility. D.K. acknowledges the JRF fellowship from DST. We acknowledge SAIF (Punjab University) for the powder XRD and NMR, IIC (IIT Roorkee) for the FESEM and TEM, and Kunash Instruments for the surface area studies.

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