3268
Ind. Eng. Chem. Res. 2009, 48, 3268–3270
Epoxidation of Methyl Oleate Using Heterogeneous Catalyst Paulo A. Z. Suarez,*,†,‡ Mı´rian S. C. Pereira,‡ Kenneth M. Doll,† Brajendra K. Sharma,†,§ and Sevim Z. Erhan† FIO Research Unit, NCAUR/USDA/ARS, 1815 North UniVersity Street, Peoria, Illinois 61604, Instituto de Quı´mica - UniVersidade de Brası´lia, CP 4478, 70919-970 Brası´lia-DF, Brazil, and Department of Chemical Engineering, PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802
In this work, we studied the catalytic activity of commercial alumina, and laboratory-synthesized alumina doped with Lewis acid metals, in the epoxidation of methyl oleate with aqueous hydrogen peroxide. It was observed that the reaction yields increased when the amount of catalyst, the quantity of hydrogen peroxide, the concentration of the hydrogen peroxide solution, or the surface area of the catalyst was increased. Reaction yields decreased when the surface acidity of the alumina was modified by doping with Lewis acid metals. 1. Introduction A variety of synthetic processes have been reported, utilizing epoxidized soybean oil and corresponding epoxy fatty ester systems, as intermediates to obtain industrially significant materials, for example, biobased polymers, emollients, chemical solvents, fuel additives, and lubricants. One of the reasons for the use of vegetable oils or their derivatives in these applications is their inherent biodegradability, which reduces their environmental impact.1 Vegetable oils, especially soybean oil, are relatively inexpensive, and their production has been increasing in recent years. At the same time, petroleum prices are volatile and not likely to stabilize in the near future. The use of oleochemicals in fuels is well-known, and several countries around the world are encouraging the use of biodiesel and biobased materials. For instance, the U.S. government has encouraged the use of biobased products2 through various regulations and tax incentives, causing a dramatic increase in the production and use of biobased fuels in recent years.3,4 Another example is the Brazilian Government, who has launched a BD program that aims to mandate the use of biobased fuels. Under the program, all diesel consumed after January 2008 is at least a B2 (blend of 2% BD in fossil diesel), and B5 diesel use is mandatory after January 2013.5 However, vegetable oils and biodiesel have oxidative stability and cold flow properties that limit their use as fuels or lubricants. One potential way to change these properties is through chemical derivatization of the olefinic groups of the oleochemical. Indeed, some attempts have already been done to transform the double bonds by hydrogenation6 or epoxidation, followed by epoxy ring opening using carboxylic acids7 or alcohols.8 It is worth mentioning that the last strategy seems to be very efficient to improve the cold temperature properties and oxidative stability of biodiesel, producing oleochemicals with great potential to be used as lubricant additives. Several studies have been done to develop active catalytic systems for the epoxidation of different olefins using hydrogen peroxide. Recently, catalysts based on iron,1 manganese,1,9 titanium,10 tungsten,11 and rhenium12 have been described for this reaction. More recently, it was shown that both commercial alumina and alumina synthesized by the sol-gel method are * To whom correspondence should be addressed. E-mail: psuarez@ unb.br. † FIO Research Unit. ‡ Universidade de Brası´lia. § Pennsylvania State University.
inexpensive and efficient catalysts for the epoxidation of different olefins and that the activity is highly affected by the acidity of the catalyst.13 Regarding the epoxidation of fatty acid methyl esters with hydrogen peroxide using alumina as catalyst, a conversion of 95% and epoxidation selectivity >97% were obtained after 24 h.14 It was also shown that the catalyst could be reused. Indeed, in these very elegant works, Schuchardt et al.13,14 prepared different pure γ-aluminas with distinct surface texture and acidity and found that strong Al-OH Bronsted acid sites are responsible for hydrogen peroxide decompositions. The main goals of this work were to study the influence of different reactions conditions in the catalytic activity of alumina for the epoxidation of methyl oleate with aqueous hydrogen peroxide and the presence of Lewis acid sites in the catalyst. Thus, we studied the catalytic activity of commercial alumina using different amounts of hydrogen peroxide (we varied the volume and the hydrogen peroxide aqueous solution concentration) and also different amounts of catalyst. We also investigated the catalytic activity of different aluminas doped with Ti, Zn, and Sn, which were prepared in our research groups and have different surface areas and Lewis acidity. 2. Experimental Section 2.1. Materials. Reagent grade zirconium(IV) sulfate, titanium(IV) chloride, tin(II) chloride, zinc(II) sulfate, aluminum nitrate, sodium carbonate, and sodium hydroxide were obtained from commercial sources (Aldrich, Milwaukee, WI) and were used as received without further purification. Al2O3 (Commercial) was obtained from Aldrich and used without further treatment. Hydrogen peroxide (in 30% or 50% concentration) and ethyl acetate (HPLC grade) were acquired from Aldrich (Milwaukee, WI), while methyl oleate was obtained by Nuchek Prep Inc. and used without further purification. 2.2. Catalysts Preparation. Al2O3 (Laboratory),15 (Al2O3)0.8(SnO)0.1(ZnO)0.1,15 (Al2O3)0.8(TiO2)0.1(ZrO2)0.1,16 and (Al2O3)0.8(TiO2)0.216 were synthesized according to methods previously described. A total of four 100 mL aqueous solutions of the desired metal salts were prepared: (i) hydrated aluminum nitrate (25 mmol of Al3+); (ii) hydrated aluminum nitrate (20 mmol of Al3+), SnCl2 (2.5 mmol), and ZnSO4 (2.5 mmol); (iii) hydrated aluminum nitrate (20 mmol of Al3+), TiCl4 (2.5 mmol), and Zr(SO4) (2.5 mmol); and (iv) hydrated aluminum nitrate (20 mmol of Al3+) and TiCl4 (5 mmol). Thus, each solution was slowly added to a 100 mL aqueous solution of sodium carbonate (9.3 g of NaCO3 in 100 mL of H2O), under magnetic
10.1021/ie801635b CCC: $40.75 2009 American Chemical Society Published on Web 03/04/2009
Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009 3269
stirring. The mixture was stirred at room temperature for 30 min and then kept in a refrigerator overnight. The resulting four precipitates were isolated by filtration, washed several times with distilled water, and dried in a vacuum desiccator over silica gel. Next, the precipitates were thermally activated at 500 °C for 4 h, yielding: (i) Al2O3 (laboratory); (ii) (Al2O3)0.8(SnO)0.1(ZnO)0.1; (iii) (Al2O3)0.8(TiO2)0.1(ZrO2)0.1; and (iv) (Al2O3)0.8(TiO2)0.2. 2.3. Catalysts Characterization. Specific surface areas of the solids were determined by the BET method,17 using nitrogen adsorption isotherms in Quantachrome NOVA 2200e instrument, using a N2 adsorption isotherm at -195 °C. The mesopore size distributions were obtained from the adsorption branch of N2 with a cylindrical pore model, according to the BJH model. Prior to analysis, samples were outgassed under vacuum at 150 °C for 8 h. The acid properties of the solids were evaluated by the thermal ammonia desorption method (NH3-TPD) in a CHEMBET-3000 (American Quantachrome Co.) instrument. Next, 0.2 g of each solid was treated at 500 °C for 50 min under continuous helium flux (85 cm3 min-1). The temperature was then dropped to 150 °C under an NH3 flux. The TPD profile was then obtained from 150 to 800 °C using a heating rate of 15 °C min-1 under helium flux using a thermal detector. 2.4. Epoxidation Reactions. All reactions were carried out in a round-bottom three-necked flask equipped with a condenser. In a typical reaction, 20 mL of ethyl acetate, the desired amount of catalyst, and 2.5 g of methyl oleate were added to the flask and heated under magnetic agitation until gentle reflux was achieved. Next, the desired amount of hydrogen peroxide in 30% or 50% water solution was added. The reaction was kept under magnetic stirring and gentle reflux (approximately 80 °C) for 6 h. The final liquid solution was separated from the catalyst by simple decantation and analyzed by gas chromatography performed on a Hewlett-Packard (Loveland, CO) 5890 GC system equipped with a 6890 series injector and an FID detector. A J and W DB-1 column (15 m × 320 µm) was used with a helium flow rate of approximately 0.9 mL min-1. The temperature program used started at 180 °C, held for 2 min, then increased to 280 at 5 °C min-1 and held for 5 min. A 1 µL injection was used and an inlet temperature of 250 °C and a detector temperature of 280 °C. HP Chemstation software was used for data collection and processing. The retention times of both substrate and product were verified by injection of commercial samples. Compound concentrations were determined by direct comparison of starting material and product peak areas. 3. Results and Discussion Table 1 summarizes the main results obtaining using commercial Al2O3 under different reaction conditions. As is clear from entry 1 of Table 1, no reaction takes place without assistance by an aluminum oxide catalyst. As can be depicted from Table 1, using the same amount of hydrogen peroxide, it was observed in most cases that the reaction yield is considerably enhanced when the amount of catalyst was increased (for instance, compare entries 3, 6, and 9 or 11, 13, and 14). However, a linear relationship between the reaction yield and the catalyst amount was not observed, and in some cases, no differences were observed when the catalyst amount was increased (compare entries 15, 16, and 17). This is probably a kinetic effect showing that pseudozero reaction order was achieved when catalyst amounts above 600 mg were added. However, it becomes clear from our results that before achieving the pseudozero order conditions, the reaction is influenced by
Table 1. Epoxidation Using Commercial Al2O3 under Different Reaction Conditionsa entry
cat. amount (mg)
H2O2 (30%, mL)
H2O2 (50%, mL)
reaction yield (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
0 300 300 300 600 600 600 900 900 900 300 300 600 900 300 600 900
0 2.1 4.3 6.5 2.1 4.3 6.5 2.1 4.3 6.5 0 0 0 0 0 0 0
2.6 0 0 0 0 0 0 0 0 0 1.3 2.6 1.3 1.3 3.9 3.9 3.9
0.0 9.0 12.5 16.0 12.0 17.5 28.0 17.0 19.0 28.0 24.0 32.5 39.0 40.0 40.0 54.0 54.0
a
Temperature 80 °C; methyloleate 2.5 g; time 6 h.
Figure 1. Products and yields obtained during attempts to recycle the catalyst in the epoxidation reaction (catalyst 300 mg; H2O2 (50%) 3.9 mL; temperature 80 °C; methyloleate 2.5 g; time 6 h).
the amount of catalyst. This shows that the catalytic system probably achieves its equilibrium under the conditions used in these reactions. On the other hand, it is also clear from Table 1 that using the same catalyst amount, the reaction yield is considerably increased when the amount of hydrogen peroxide is increased (see entries 5, 6, and 7 or 13 and 16). It is also worth mentioning that using the same molar amount of hydrogen peroxide and amount of catalyst, the reaction yield is also increased by using a higher concentration hydrogen peroxide solution (for example, compare entries 2 and 11 or 6 and 16). At the end of the reaction described in entry 15 of Table 1, the catalyst was recovered by centrifugation, washed three times with hexane, and dried at 100 °C overnight. It was reused using the same reaction conditions. This procedure was repeated four times, and the results are shown in Figure 1. For the two first changes of methyl oleate, similar reactions yields were obtained (approximately 40%). However, a drastic decrease in yield was observed for the third (around 15%) and fourth (around 11%) substrate charges, showing a clear deactivation of the catalyst. To study the influence of the presence of Lewis acid metals in the alumina, and the surface area effect on catalytic activity, we decided to prepare alumina doped with different metals. Thus, we prepared alumina doped with tin, zinc, zirconium, and titanium by coprecipitation of carbonates of aluminum and these metals followed by thermal activation. We also prepared pure alumina by the same method. The chemical and spectroscopic characterization of the different solids was published elsewhere
3270 Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009 Table 2. Epoxidation Reaction Using Different Aluminum Oxidesa
entry
catalyst
18 19 20 21 22
Al2O3 (commercial) Al2O3 (laboratory) (Al2O3)0.8(SnO)0.1(ZnO)0.1 (Al2O3)0.8(TiO2)0.1(ZrO2)0.1 (Al2O3)0.8(TiO2)0.2
surface surface acidity reaction area (m2/g) (NH3-TPD, mV) yield (%) 167 120 2 29 8
5 3 57 120 110
33.0 21.0 9.0 2.0 0.5
Catalyst 300 mg; H2O2 (50%) 2.6 mL; temperature 80 °C; methyloleate 2.5 g; time 6 h. a
by our research group.15,16 In Table 2, the surface area and acidity of the different catalysts used in this work are presented. The commercial alumina showed the highest surface area (167 m2 g-1), considerably higher than the surface area of the alumina prepared by our method (120 m2 g-1). As is clear from the data presented in Table 2, the surface area of the solids dramatically dropped to 29 m2 g-1 or less when the alumina was doped. The pore size distributions of the catalysts varied from 2730 to 6079 nm, which means that the solids can be classified as mesoporous materials. However, it was observed that the surface acidity of the solids is strongly increased when doped with titanium and zirconium and mildly increased when doped with tin and zinc. Table 2 also shows that the activity of the catalyst is strongly dependent upon the surface properties of the catalyst. Better reaction yields were observed at higher surface areas for similar acidity solids (see entries 18 and 19 of Table 2). For comparable surface areas, increasing the acidity (compare entries 20 and 22 of Table 2) significantly decreased the reaction yield. In very elegant work, Schuchardt et al.13 prepared different pure γ-aluminas with distinct surface texture and acidity. They claim that strong Al-OH Bronsted acid sites are also active catalysts for hydrogen peroxide decompositions. One of the goals of our study was to test Lewis acid sites to find out if they can also diminish the activity of alumina. As we could see here, the presence of strong Lewis acids metals is probably also catalyzing the decomposition of hydrogen peroxide in our systems as well, as demonstrated when alumina was doped with Lewis acid metals and its activity dropped. 4. Conclusion In summary, the epoxidation of fatty acid methyl esters using hydrogen peroxide and alumina is strongly affected by the reaction parameters studied in this work. It was observed that the reaction yields increase when the amount of catalyst, the quantity of hydrogen peroxide, the concentration of the hydrogen peroxide solution, or the surface area of the catalyst is increased. A decrease in the activity is observed when the surface acidity of the alumina is increased by doping it with Lewis acid metals.
Acknowledgment We kindly acknowledge CNPq/CTEnerg and CNPq/UNIVERSAL for partial financial support. We would like to acknowledge Ms. Donna I. Thomas for GC measurements. Professor P.A.Z.S. would also like to thank CNPq for a research fellowship. Literature Cited (1) Guodong, D.; Tekin, A.; Hammond, E. G.; Woo, L. K. Catalytic epoxidation of methyl linoleate. J. Am. Oil Chem. Soc. 2004, 81, 477. (2) Hagstrom, J. USDA to set requirements for agencies to purchase bio-based products. 2005; govexec.com. (3) Gunstone, F. Update on food and nonfood uses of oils and fats. Inform 2007, 18, 573. (4) Pruszko, R. Rendered fats and oils as a biodiesel feedstock. Inform 2006, 17, 431. (5) Pousa, G. P. A. G.; Santos, A. F.; Suarez, P. A. Z. History and policy of biodiesel in Brazil. Energy Policy 2007, 35, 5393. (6) Moser, B. R.; Haas, M. J.; Winkler, J. K.; Jackson, M. A.; Erhan, S. Z.; List, G. R. Evaluation of partially hydrogenated methyl esters of soybean oil as biodiesel. Eur. J. Lipid Sci. Technol. 2007, 109, 17. (7) Doll, K. M.; Sharma, B. K.; Erhan, S. Z. Synthesis of branched methyl hydroxy stearates including an ester from bio-based levulinic acid. Ind. Eng. Chem. Res. 2007, 46, 3513. (8) Moser, B. R.; Erhan, S. Z. Preparation and evaluation of a series of alpha-hydroxy ethers from 9,10-epoxystearates. Eur. J. Lipid Sci. Technol. 2007, 109, 206. (9) De Vos, D. E.; Sels, B. F.; Reynaers, M.; Rao, S. Y. V.; Jacobs, P. A. Epoxidation of terminal or electron-deficient olefins with H2O2, catalysed by Mn-trimethyltriazacyclonane complexes in the presence of an oxalate buffer. Tetrahedron Lett. 1998, 39, 3221. (10) Notari, B. Microporous crystalline titanium silicates. AdV. Catal. 1996, 41, 253. (11) Sato, K.; Aoki, M.; Ogawa, M.; Hashimoto, T.; Panyella, D.; Noyori, R. A halide-free method for olefin epoxidation with 30% hydrogen peroxide. Bull. Chem. Soc. Jpn. 1997, 70, 905. (12) Herrmann, W. A.; Kratzer, R. M.; Ding, H.; Thiel, W. R.; Glas, H. Methyltrioxorhenium/pyrazole-A highly efficient catalyst for the epoxidation of olefins. J. Organomet. Chem. 1998, 555, 293. (13) Rinaldi, R.; Fujiwara, F. Y.; Ho¨lderich, W.; Schuchardt, U. Tuning the acidic properties of aluminas via sol-gel synthesis: New findings on the active site of alumina-catalyzed epoxidation with hydrogen peroxide. J. Catal. 2006, 244, 92. (14) Sepulveda, J.; Teixeira, S.; Schuchardt, U. Alumina-catalyzed epoxidation of unsaturated fatty esters with hydrogen peroxide. Appl. Catal., A: Gen. 2007, 318, 213. (15) Macedo, C. C. S.; Abreu, F. R.; Tavares, A. P.; Melquizedeque, B. A.; Zara, L. F.; Rubim, J. C.; Suarez, P. A. Z. New heterogeneous metaloxides based catalyst for vegetable oil trans-esterification. J. Braz. Chem. Soc. 2006, 17, 1291. (16) Quirino, R. L.; Daher, L. O.; Rubim, J. C.; Suarez, P. A. Z. Synthesis and characterization of alumina doped with TiO2 and ZrO2 and their application in the soybean oil cracking. Submitted for publication. (17) Brunauer, S.; Emmett, P. H.; Teller, E. Adsorption of gases in multimolecular layers. J. Am. Chem. Soc. 1938, 60, 309.
ReceiVed for reView October 27, 2008 ReVised manuscript receiVed January 5, 2009 Accepted February 3, 2009 IE801635B
