Energy Fuels 2010, 24, 634–645 Published on Web 10/27/2009
: DOI:10.1021/ef900780q
Transesterification of triglycerides with methanol catalyzed by heterogeneous zinc hydroxy nitrate catalyst. Evaluation of variables affecting the activity and stability of catalyst. A. Zie-ba, A. Pacuza, and A. Drelinkiewicz* Institute of Catalysis and Surface Chemistry Polish Academy of Sciences, 30-239 Krak ow, Niezapominajek 8, Poland Received July 23, 2009. Revised Manuscript Received October 8, 2009
Zinc hydroxy nitrate Zn5(OH)8(NO3)2 2 H2O (Zn-5) was proved to be active heterogeneous catalyst in the transesterification of triglycerides with methanol and it retained its activity in the next catalytic cycles. Systematic studies were performed to characterize catalytic performance of Zn-5 in methanolysis which was carried out under mild conditions (temperature 50-60 °C and atmospheric pressure). Reaction was studied for triacetin (glycerol triacetate), a model triglyceride molecule and for vegetable oil, castor oil {(triglyceride of 12-hydroxy-9-octadecenoic acid (ricinoleic acid)}. The homogeneously catalyzed reaction by KOH was performed for comparison as well. The effects of methanol to triglyceride molar ratio, catalysts loading, reaction temperature, and precontacting Zn-5 catalyst with reactants (methanol or triglycerides) were investigated. The influence of experimental variables on the course of triacetin methanolysis and in particular on the content of partial glycerides (diacetin, monoacetin) was considered. The next investigated aspect referred to an influence of cosolvents. A correlation between polarity of cosolvents and the content of partial glycerides was found proving a change of adsorption properties of Zn-5 catalyst due to the presence of cosolvent. The Zn-5 catalyst underwent partial but reversible deactivation during the first reaction cycle due to the accumulation of glycerides/glycerol evidenced by the FTIR technique. By simple washing with THF/methanol, complete regeneration was achieved resulting in stable activity of Zn-5 catalyst in recycling use.
catalyst may undergo various kinds of changes. For instance, deposition of organic materials in the case of CaO-ZnO mixed oxide catalyst6 and formation of calcium diglyceroxide in the case of CaO-based catalysts7 were observed. Moreover, noncalcined Mg-Al-CO3 hydrotalcite catalysts exhibited thermal decomposition due to the temperature of reaction8 and base sites of hydrotalcite catalysts were partially poisoned by acids formed in hydrolysis of methyl esters.9 However, the main problem is the leaching of catalytic active alkaline species. In extreme cases, the leaching may lead to predominant homogeneous catalysis. Recently, Mg-Al layered double hydroxides (LDHs)9-12 were found to be effective catalysts for transesterification of vegetable oils. High activity of LDHs-based catalysts without any sign of catalyst leaching was reported by Liu et al.13 for poultry fat methanolysis. Similarly, Li et al.14 observed that mixed oxide catalysts
1. Introduction Biodiesel fuel, an alternative to classic diesel fuel, belongs with ecological fuels because it consists of methyl esters of fatty acids derived from vegetable oils or animal fats. Methyl esters are formed through a transesterification of fatty acids triglycerides with methanol (methanolysis) (Scheme 1). The reaction has traditionally been catalyzed by homogeneous catalysts, such as K- or Na- alkoxides or -hydroxides. However, the application of heterogeneous catalysts is more desirable from economic, technological, and environmental points of view. A variety of solid bases have already been tested for bioester synthesis and their performance was reviewed in number of papers.1-5 The catalysts included several classes of alkaline materials, like inorganic oxides and hydroxides, impregnated oxides, zeolites, base form of titanosilicate ETS10, functional resins, and alkoxy-derivatives of alkali earth metals. Although numerous solids with basic characteristics were recognized to be very efficient for synthesis of biodiesel, the performance of recycled catalysts is still far from being satisfactory. In the course of reaction, the alkaline solid
(6) Ngamcharussrivichai, Ch.; Totarat, P.; Bunyakiat, K. Appl. Catal., A 2008, 341, 77–85. (7) Kouzu, M.; Hidaka, J.; Komichi, Y.; Nakano, H.; Yamamoto, M. Fuel 2009, 88, 1983–1990. (8) Barakos, N.; Pasias, S.; Papayannakos, N. Bioresour. Technol. 2008, 99, 5037–5042. (9) Xi, Y.; Davis, R. J. J. Catal. 2008, 254, 190–197. (10) Xie, W.; Peng, H.; Chen, L. J. J. Mol. Catal. A: Chem. 2006, 246, 24–32. (11) Cantrell, D. G.; Gillie, L. J.; Lee, A. F.; Wilson, K. Appl. Catal., A 2005, 287, 183–190. (12) Tittabut, T.; Trakarnpruk, W. Ind. Eng. Chem. Res. 2008, 47, 2176–2181. (13) Liu, Y.; Lotero, E.; Goodwin, J. G., Jr; Mo, X. Appl. Catal., A 2007, 331, 138–148. (14) Li, E.; Xu, Z. P.; Rudolph, V. Appl. Catal., B 2009, 88 (1-2), 42– 49.
*To whom correspondence should be addressed. E-mail: ncdrelin@ cyf-kr.edu.pl. (1) Helwani, Z.; Othman, M. R.; Aziz, N.; Kim, J.; Fernando, W. J. N. Appl. Catal., A 2009, 363, 1–10. (2) Ma, F.; Hanna, M. A. Biores. Technol. 1999, 70, 1–15. (3) Vicente, G.; Matrinez, M.; Aracil, J. Biores. Technol. 2004, 92, 297–305. (4) Di Serio, M.; Tesser, R.; Pengmei, Lu.; Santacesaria, E. Energy Fuels 2008, 22 (2), 207–218. (5) Zabeti, M.; Daud, W. M. A. W.; Aroua, M. K. Fuel Proc. Technol. 2009, 90, 770–777. r 2009 American Chemical Society
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Energy Fuels 2010, 24, 634–645
: DOI:10.1021/ef900780q
Zie-ba et al.
presence of OH group at C-12 carbon, castor oil exhibits unique chemical and physical properties, such as good solubility in methanol and methyl esters formed after its transesterification.20 Similarly, triacetin is also readily soluble in methanol and methyl esters. Thus, during methanolysis of both tested triglycerides, there was a single liquid-phase system. No separate phases of methanol and triglycerides appeared and the mass-transfer effects resulting from the presence of two phases (oil-methanol) encountered in the transesterification reaction of natural oils are, in fact, negligible. Moreover, by using a triacetin as the model compound we were able to gain some insight into reactivity of Zn-5 catalyst for partial glycerides formation.
Scheme 1. Transesterification of Triglyceride with Methanol
2. Experimental Section 2.1. Catalysts Preparation. Zinc hydroxy nitrate Zn5(OH)8(NO3)2 2 H2O (abbreviated as Zn-5) (specific surface area 23 m2/g, porosity 0.13 cm3/g, average pore diameter 11.1 nm) was prepared as described before.17,21 The salt was obtained by dropwise addition of 50 cm3 of 0.75 M aqueous sodium hydroxide to 20 cm3 of 3.5 M aqueous zinc nitrate at room temperature with constant stirring. The obtained white precipitate was filtered, washed with deionized water and dried overnight at 50 °C. 2.2. Characterization of Catalysts. The specific surface areas of samples were calculated from the nitrogen adsorptiondesorption isotherms at 77 K in an Autosorb-1, Quantachrome apparatus. Prior to the measurements, the samples were preheated and degassed under vacuum at 473 K for 2 h. FTIR spectra were recorded using Bruker-Equinox 55 spectrometer and standard KBr pellets technique. Morphology of samples was studied by means of field emission scanning electron microscope JEOL JSM-7500 F. 2.3. Catalytic Tests. The transesterification of triglycerides, triacetin (Fluka), and castor oil (Microfarm, Poland) with methanol was carried out in a 100 cm3 glass reactor at atmospheric pressure following the procedure reported in our previous papers.17,22 Reactor was equipped with a reflux condenser, magnetic stirrer, and a tube for sampling the solution. In typical procedure initial molar ratio of methanol: triglycerides (triacetin or castor oil) was 29:1, and the content of catalyst was 5 wt. % relative to the mass of triglyceride. In catalytic experiment, triacetin or castor oil, methanol, and internal standard (toluene or eicosane) were introduced to the reactor, heated up to a given temperature and then the catalyst was added. Typically, methanolysis was carried out for 3 h and the samples were withdrawn at appropriate time intervals. The transesterification of triacetin with methanol (1:29 molar ratio) was performed using 2.6 cm3 of triacetin, 16.2 cm3 of methanol, and 0.15 g of Zn-5 catalyst at 50 °C (typical procedure). In the course of catalytic tests the samples of reaction mixture were periodically withdrawn and analyzed by GC method (PE Clarus 500 equipped with a flame ionization detector, capillary column Elite-5 MS: 30 m 0.25 mm 0.25 μm coating) using toluene as internal standard. The response factors for triacetin, diacetin (Fluka), monoacetin (Acros Organics), and methyl acetate (Fluka) were determined through multipoint calibrations of standards. In the present studies, the triacetin conversion (CTG), and the yields of methyl
obtained by calcination of Mg-Co-Al-La-LDHs maintained their activity in recycling use in methanolysis of canola oil at temperature of 200 °C, whereas Oku et al.15 reported high content of Mg and Al in the products after 24 h of methanolysis of triolein performed at 150 °C. Moreover, encouraging results were obtained for zinc hydroxy nitrate Zn5(OH)8(NO3)2 2 H2O which is another interesting class of solid base belonging to layered hydroxide salts.16,17 The structure of Zn5(OH)8(NO3)2 2 H2O consists of infinite brucite-like layers.18,19 The activity of recycled Zn5(OH)8(NO3)2 2 H2O remained unchanged in esterification of lauric acid with methanol/ethanol, but it slightly decreased in the presence of water.16 This catalyst offered promising activity also in the transesterification of triglycerides with methanol.16,17 As already described, Zn5(OH)8(NO3)2 2 H2O salt (abbreviated as Zn-5) is the double layered hydroxides and its structure- morphology is very sensitive to the thermal treatment.18,19 Therefore, our previous studies focused on thermal decomposition of Zn-5 and its impact on the activity in methanolysis of triglycerides.17 They showed that thermal treatment of Zn-5 at temperature higher than 140 °C reduced its activity remarkably. Structural changes, e.g., the collapse of lamelar structure of original Zn-5 was identified (BET, XRD, FTIR, SEM techniques) as the reason of reduced activity. By a simple regeneration of spent catalyst its activity was shown to be preserved in the next cycles of reaction. In the present work systematic studies are performed to characterize catalytic reactivity of Zn-5 salt for transesterification of triglycerides with methanol. Catalytic tests are performed using wide range of operating conditions (catalyst loading, initial molar ratio of methanol to triglycerides, reaction temperature). The effects responsible for deactivation/ regeneration of original Zn-5 salt as well as the influence of cosolvent are also examined. Here, methanolysis is studied for triacetin (glycerol triacetate), a model triglyceride molecule and for natural oil, castor oil. The main constituent of castor oil is triglyceride of 12-hydroxy-9-octadecenoic acid (ricinoleic acid). Due to the (15) Oku, T.; Nonoguchi, M.; Moriguchi, T. PCT application No WO2005/021697, March 10, 2005. (16) Cordeiro, C. S.; Arizaga, G. G. C.; Ramos, L. P.; Wypych, F. Catal. Commun. 2008, 9, 2140–2143. (17) Zieba, A.; Pacuza, A.; Serwicka, E. M.; Drelinkiewicz, A. Fuel submitted. (18) Biswick, T.; Jones, W.; Pacuza, A.; Serwicka, E.; Podobi nski, J. J. Solid State Chem. 2007, 180, 1171–1179. (19) Auffredic, J. P.; Louer, D. J. Solid State Chem. 1983, 46, 245–252.
(20) Plentz Menghetti, S. M.; Meneghetti, M. R.; Wolf, C. R.; Silva, E. C.; Lima, G. E. S.; Silva, L. L.; Serra, T. M.; Cauduro, F.; Oliveira, L. G. Energy Fuels 2006, 20, 2262–2265. (21) Newman, S. P.; Jones, W. J. Solid State Chem. 1999, 148, 26–40. (22) Zieba, A.; Matachowski, L.; Lalik, E.; Drelinkiewicz, A. Catal. Lett. 2009, 127, 183–194.
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Energy Fuels 2010, 24, 634–645
: DOI:10.1021/ef900780q
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ester (YES) were calculated as follows: NTG, 0 -NTG CTG ¼ 100% NTG, 0 YES ¼
Table 1. Transesterification of Triacetin and Castor Oil with Methanol at Various Methanol/Triglyceride Molar Ratiosa methanol: triglyceride molar ratio
NES 100% 3 NTG, 0
where NTG,0 and NTG are the moles of triacetin initially present in the reactor and the moles of triacetin remaining at time t, respectively; NES is the number of moles of methyl ester in the reactor at time t. The transesterification of castor oil with methanol was performed using 6 g of castor oil, 7.6 cm3 of methanol (29:1 molar ratio) and 0.3 g of Zn-5 catalyst (typical procedure). The progress of reaction was monitored by means of HPLC and GC techniques following the procedure reported in our previous paper.22 The HPLC analysis showed that our castor oil was entirely composed of triglyceride of ricinoleic acid (87%), traces of di- and monoglycerides of ricinoleic acid, glycerol, and free ricinoleic acid (below 0.1 wt. %). From the GC analysis of methyl-esters of fatty acids (ricinoleic 87.44 wt. %, linoleic 5.05 wt.%, oleic 3.88 wt.%, stearic 1.4 wt.%, palmitic 1.28 wt.%, linolenic 0.56 wt.%, and others 0.39 wt.%) the average molecular weight of the castor oil was calculated to be 928 g/mol, being in agreement with data reported by other authors.20,23 Apart from the dominating triglycerides of ricinoleic acid, low amount of triglycerides of other fatty acids was present in our castor oil as well, making determination of conversion data more complex. Therefore, in the discussion of catalytic results the yield of methyl esters formed in methanolysis of castor oil is taken into account, identically to the procedure widely used in the case of vegetable oils. As described before22 the yield of methyl esters formed in methanolysis of castor oil was expressed in terms of the percentage of methyl esters produced.
6: 1 12: 1 18: 1 24: 1 29: 1 35: 1 a
triacetin
castor oil
conversion [%]
yield of methyl ester [ % ]
yield of methyl ester [ % ]
53.2 59.0 64.8 69.7 73.2 76.0
29.9 35.0 40.5 45.9 51.6 48.7
7.9 11.9 15.5 18.7 20.9 21.1
Data obtained after 3 h of reaction (catalyst content 5 wt.%, 50 °C).
by increasing the amount of methanol the transetserification is faster process. In order to determine the maximum conversion of triacetin, the reaction was performed at high methanol to oil ratios, i.e., 12:1, 29:1, and 35:1. The obtained products distribution profiles are displayed in Figure 1. The maxima corresponding to triacetin conversion of 68% at the methanol/triacetin = 12:1 ratio, and 83% at the ratio of 29:1 were achieved. Beyond the molar ratio of 29:1, the addition of methanol had no further effect. Therefore, methanol to triacetin molar ratio of 29: 1 was used in all further catalytic experiments. As Figure 1a and b show, in the presence of Zn-5 salt the content of diacetin reaches a plateau after a short reaction time. After the plateau, the content of diacetin practically does not decrease. Very low amounts of monoacetin are observed only when the conversion of triacetin is relatively high (50-60%). In addition, the monoacetin level is very low, not exceeding ca. 8-10% in all cases throughout the whole recorded reaction. As the consequence, the yields of methyl ester are definitively lower than the corresponding conversions of triacetin. This is evidenced by the relationship between the yield of methyl ester and the conversion of triacetin (Figure 1c). At a given conversion of triacetin, the yield of methyl ester attains higher level at higher excess of methanol, i.e., at molar ratio of 29:1. However, the maximum yields of methyl ester reach 55 and 38% at methanol/ triacetin molar ratio of 29:1 and 12:1, respectively, whereas the corresponding maximum conversions of triacetin are 83 and 68%. The effect of reaching plateau for diglyceride was also observed by Cantrell et al.11 in methanolysis of tributyrin catalyzed by Mg-Al hydrotalcites. It was reported that the rates of tributyrin conversion, and the formation of methyl ester and diglyceride were both first order in triglyceride concentration during the initial stage of reaction (
