Energy Fuels 2010, 24, 5257–5263 Published on Web 08/23/2010
: DOI:10.1021/ef100884b
To Cyclopropanate or Not To Cyclopropanate? A Look at the Effect of Cyclopropanation on the Performance of Biofuels Alexandre Langlois and Olivier Lebel* Department of Chemistry and Chemical Engineering, Royal Military College of Canada, P.O. Box 17000, Station Forces, Kingston, Ontario K7K 7B4, Canada Received July 9, 2010. Revised Manuscript Received August 5, 2010
The cyclopropanated derivatives of three diesel-compatible biofuels;limonene, turpentine, and canola oil methyl ester;were prepared. Their heats of combustion were both calculated using theoretical methods and measured experimentally, and it was found that for all three compounds the specific energy (in kJ/g) was similar to that of the starting materials, while the energy density (in kJ/mL) was 3-4% higher, which is slightly lower than that for the predicted values. For canola oil methyl ester, the effect of cyclopropanation on the accelerated oxidation stability was also studied, and similar trends in viscosity were observed for both the starting material and the cyclopropanated product, while no degradation of the cyclopropane groups was observed and only residual alkene groups had shown any sign of reaction, hinting that the increase in the viscosity observed with unsaturated fatty esters upon oxidation may not be directly related to autoxidation of the fatty acid chains.
temperatures near or below 0 °C during the winter.5 However, canola oil based FAME is derived from crops usually used for food, making other alternatives more desirable. Furthermore, (1) the energetic yield of FAME is approximately 9% lower than that of petrodiesel because of the 11% oxygen content of FAME due to the ester groups (even though combustion of biodiesel is usually more complete),6 and (2) fuels rich in polyunsaturated fatty acid chains tend to undergo facile oxidation, causing an important increase in the viscosity of the fuel.7 Terpenes constitute another class of compounds isolated from vegetable sources that can be used as biodiesel fuels. In particular, turpentine and limonene are especially appealing because they are hydrocarbons with self-ignition temperatures, boiling points, densities, cetane ratings, and viscosities similar to those of petrodiesel.8,9 For these reasons, both of these compounds have already been used as additives in diesel.9 Furthermore, both compounds are not edible and are readily available as byproducts from other processes: turpentine from the lumber industry and limonene from orange juice manufacture.10 For certain applications, including aviation fuels and fuels for racing engines or military vehicles, higher energetic yields can confer advantages that can potentially outweigh higher costs. For example, for military vehicles, more energetic fuels
Introduction Both Henry Ford and Rudolf Diesel had proposed at the beginning of the 20th century the use of fuels from vegetable sources.1 One century later, the concept has not only survived but also tremendously grown in popularity.2 Because they can be readily obtained from sustainable sources, biofuels constitute an appealing alternative to petroleum-based fuels, which will eventually deplete in the future if the current usage is maintained. Among vegetable sources to be used as fuels, the ones that are obtained from waste sources are highly desirable because they do not compete with crop lands or crops destined for consumption and because they provide an outlet to use the waste material for practical purposes.3 Aside from ethanol, the most widespread biofuel is fatty acid methyl ester (FAME) biodiesel. It can be derived from several sources, but depending on the location, animal-fatbased FAMEs are not always compatible with the climate, especially in winter, because of their relatively high cloud point.4 For these reasons, FAMEs derived from vegetable oils, such as soybean in the United States, rapeseed in Europe, or canola in Canada, are often used in countries experiencing
*To whom correspondence should be addressed. E-mail: Olivier.
[email protected]. (1) (a) Kovarik, B. Automot. Hist. Rev. 1998, 32, 7–27.(b) Knothe, G., Van Gerpen, J., Krahl, J., Eds. The Biodiesel Handbook; AOCS Publishing: Champaign, IL, 2005. (2) (a) Wyman, C. E., Ed. Handbook on Bioethanol: Production and Utilization; Taylor & Francis: Washington, DC, 1996.(b) Srivastava, A.; Prasad, R. Renewable Sustainable Energy Rev. 2000, 4, 111–133. (c) Vasuvedan, P. T.; Briggs, M. J. Ind. Microbiol. Biotechnol. 2008, 35, 421–430. (3) (a) Zhang, Y.; Dube, M. A.; McLean, D. D.; Kates, M. Bioresour. Technol. 2003, 90, 229–240. (b) Brethauer, S.; Wyman, C. E. Bioresour. € C-ınar, C.; Technol. 2010, 101, 4862–4874. (c) G€ur€u, M.; Koca, A.; Can, O.; S-ahin, F. Renewable Energy 2010, 35, 637–643. (4) (a) Ma, F.; Clements, L. D.; Hanna, M. A. Ind. Eng. Chem. Res. 1998, 37, 3768–3771. (b) Marulanda, V. F.; Anitescu, G.; Tavlarides, L. L. Energy Fuels 2010, 24, 253–260. (5) Ma, F.; Hanna, M. A. Bioresour. Technol. 1999, 70, 1–15. r 2010 American Chemical Society
(6) Lang, X.; Dalai, A. K.; Bakhshi, N. N.; Reaney, M. J.; Hertz, P. B. Bioresour. Technol. 2001, 80, 53–62. (7) (a) Knothe, G. J. Am. Oil Chem. Soc. 2002, 79, 84–854. (b) Sendzikien_e, E.; Makarevicien_e, V.; Janulis, P. Pol. J. Environ. Stud. 2005, 14, 335–339. (c) McCormick, R. L.; Ratcliff, M.; Moens, L.; Lawrence, R. Fuel Process. Technol. 2007, 88, 651–657. (d) Farahani, M.; Page, D. J. Y. S.; Turingia, M. P.; Tucker, B. D. J. Energy Resour. Technol. 2009, 131, 041801/1–041801/6. (8) ASTM D975-10. Standard Specification for Diesel Fuel Oils, 2010, www.astm.org. (9) (a) Kaplan, C.; Hakki Alma, M.; Tutus-, A.; C-etinkaya, M.; Karaosmanoglu, F. Pet. Sci. Technol. 2005, 23, 1333–1339.(b) de Avila, M. T.; Feitosa, M. V.; Modolo, D. L.; de Mello Innocentini, M. D. Braz. Pedido PI0501714-9A, 2007. (10) (a) Mikami, K. Aromatopia 2000, 43, 14–18. (b) Erman, M. B.; Kane, B. J. Chem. Biodiversity 2008, 5, 910–919.
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Scheme 1
Scheme 3
Table 1. Densities of Studied Biofuels and Their Cyclopropanated Derivatives compound
density (g/mL)
limonene 1 turpentine 2 canola oil methyl ester 3 (90% conversion)
0.8405 ( 0.0004 0.8879 ( 0.0005 0.8625 ( 0.0005 0.9002 ( 0.0005 0.8834 ( 0.0005 0.9083 ( 0.0007
Synthesis of Cyclopropanated Biofuels. Cyclopropanation of turpentine and limonene was carried out using the procedure of Friedrich and Lewis12 involving the reaction of the starting material with zinc metal and dibromomethane in the presence of catalytic amounts of cuprous chloride and acetyl chloride (Scheme 1). This method proved to be more costefficient than both the typical Simmons-Smith conditions and the dichlorocarbene addition-reduction sequence. Cyclopropanated (þ)-limonene [(R)-4-(1-methylcyclopropyl)1-methylbicyclo[4.1.0]heptane, 1] and turpentine (2) were then purified by distillation under reduced pressure. Cyclopropanated canola oil methyl ester (3) was prepared in a similar fashion (Scheme 2), both for cost issues and because
methyl esters are stable under these conditions. However, only approximately 90% conversion was achieved, and the mixture could not be satisfactorily purified into their respective cyclopropanated and noncyclopropanated fractions. Chromatographic methods proved to be highly inefficient given that the starting material is a mixture itself and the effect of the reaction on the polarity of the components is negligible. On the other hand, the boiling point of the components at 20 Torr is above 200 °C, and attempts at distillation caused thermal degradation of the cyclopropane groups.13 From NMR data of the distillate, a significant portion of the cyclopropane rings have undergone a ring-opening rearrangement to give the corresponding elongated unsaturated fatty acid chains (a proposed mechanism involving a 1,2-hydride shift is shown in Scheme 3), as evidenced by an increase in the vinylic and allylic signals but not in the biallylic and methyl signals (liquid chromatography-mass spectrometry has confirmed that the mass distributions of the crude product and the distillate are the same). Furthermore, only one vinylic set of signals is observed at the same chemical shift as that in canola oil methyl ester (δ 5.31 in CDCl3), which corresponds to a cis-alkene with CH2 groups on both sides. The specific gravities of all compounds used were measured using a pycnometer and are reported in Table 1. It can be noticed that the densities for all of the compounds reported herein are similar to those of diesel fuels, and cyclopropanated compounds are slightly denser than their alkene counterparts. The values observed for canola oil methyl ester, turpentine, and limonene agree with the typical reported values, which can slightly change depending on the source or composition in the case of biodiesel and turpentine. Heat of Combustion Calculations. The heats of combustion of both the starting materials and the cyclopropanated compounds were first estimated by calculating the heats of formation of the respective components by theoretical methods using the T1 thermochemical recipe of the Spartan 06 software.14 This method, while slightly less accurate than the G3 recipe, allows one to perform calculations on larger molecules (including fatty acid esters) with quicker computing times. The high and low heat of combustion values were
(11) (a) Knowlton, J. W.; Rossini, F. D. J. Res. Natl. Bur. Stand. 1949, 43, 113–115. (b) Savos'kin, M. V.; Kapkan, L. M.; Vaiman, G. E.; Vdovichenko, A. N.; Gorkunenko, O. A.; Yaroshenko, A. P.; Popov, A. F.; Mashchenko, A. N.; Tkachev, V. A.; Voloshin, M. L.; Potapov, Yu. F. Russ. J. Appl. Chem. 2007, 80, 31–37. (12) (a) Friedrich, E. C.; Lewis, E. J. J. Org. Chem. 1990, 55, 2491– 2494. (b) Friedrich, E. C.; Niyati-Shirkhodaee, F. J. Org. Chem. 1991, 56, 2202–2205.
(13) (a) Chambers, T. S.; Kistiakowsky, G. B. J. Am. Chem. Soc. 1934, 56, 399–405. (b) Banks, M.; Cadogan, J. I. G.; Gosney, I.; Hodgson, P. K. G.; Jack, A. G. C.; Rodger, D. R. J. Chem. Soc., Chem. Commun. 1989, 1033– 1034. (c) Parziale, P. A.; Berson, J. A. J. Am. Chem. Soc. 1990, 112, 1650– 1652. (d) Fan, K.-N.; Li, Z.-H.; Wang, W.-N.; Huang, H.-H.; Huang, W. Chem. Phys. Lett. 1997, 277, 257–263. (14) Ohlinger, W. S.; Klunzinger, P. E.; Deppmeier, B. J.; Hehre, W. J. J. Phys. Chem. A 2009, 113, 2165–2175.
Scheme 2
can translate into the possibility either (1) to carry less fuel to achieve the same amount of work or (2) to travel a longer distance with the same amount of fuel. For these purposes, both FAMEs and terpenes contain groups that can be further functionalized. The alkene groups of unsaturated fatty esters, turpentine components, and limonenes are suitable for the introduction of strained rings such as cyclopropane groups to increase the energetic content (cyclopropane possesses a strain energy of 27.5 kJ/mol relative to cyclohexane).11 Furthermore, cyclopropanation is expected to help prevent reactions at the biallylic positions of unsaturated fatty esters (thus preventing degradation over time), while maintaining the bent conformation that results in lower cloud points than those for saturated fatty esters. Results and Discussion
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Table 2. Low and High Heats of Combustion of Studied Biofuels and Their Cyclopropanated Derivatives, Calculated Using the T1 Thermochemical Recipe of Spartan 06a heat of combustion (kJ/mol)
specific energy (kJ/g)
energy density (kJ/mL)
compound
low
high
low
high
low
high
limonene 1 R-pinene β-pinene camphene turpentine 2-methanopinane 2-ethylene-6,6-dimethylbicyclo [3.1.1]heptane 3-ethylene-2,2-dimethylbicyclo [2.2.1]heptane 2 methyl stearate methyl oleate methyl linoleate methyl linolenate canola oil methyl ester methyl 9-methanooctadecanoate methyl 9,12-dimethanooctadecanoate methyl 9,12,15-trimethanooctadecanoate 3 (100% conversion) 3 (90% conversion)
-5869 -7176 -5898 -5909 -5846 -5894 -6553 -6558 -6493 -6581 -11322 -11196 -11073 -10944 -11151 -11841 -12362 -12879 -12028 -11940
-6262 -7665 -6290 -6302 -6238 -6287 -6994 -6999 -6934 -7025 -12235 -12065 -11898 -11725 -12005 -12758 -13284 -13805 -12947 -12853
-43.08 -43.68 -43.29 -43.37 -42.91 -43.26 -43.61 -43.65 -43.21 -43.59 -37.93 -37.76 -37.60 -37.42 -37.70 -38.13 -38.33 -38.50 -38.20 -38.14
-45.96 -46.66 -46.17 -46.25 -45.79 -46.15 -46.54 -46.58 -46.15 -46.53 -40.99 -40.69 -40.40 -40.09 -40.59 -41.09 -41.19 -41.27 -41.12 -41.07
-36.27 -38.88 -37.01 -37.26 -36.00 -37.21
-38.70 -41.53 -39.48 -39.73 -38.42 -39.69
-39.23
-41.88
-33.00 -33.43 -33.49 -33.18
-35.57 -35.92 -35.88 -35.72
-34.72
-37.37
a
Densities from the literature were used for known compounds, while the measured densities were used for compounds 1-3.
because the addition of cyclopropane groups also increases the mass of the compounds. However, the values per unit of volume are approximately 4% higher for the cyclopropanated compounds because of their higher density. For all three compounds, the values measured were actually lower than predicted relative to their respective precursors, possibly because of less complete combustion. While the specific energies of both limonene and turpentine are similar to that of diesel, cyclopropanation does not increase the energetic content by unit of mass significantly. However, dicyclopropanated limonene, and in particular cyclopropanated turpentine, releases more energy by unit of volume than even diesel-type fuels because of their relatively high density. On the other hand, while the introduction of cyclopropane groups to biodiesel results in a 3.3% increase in the energy density, the values are still far below that of petrodiesel. Accelerated Oxidation of Biodiesel. One of the challenges for long-term storage of biodiesel is to prevent oxidation reactions that can degrade the fuel and cause an increase of the viscosity of the fuel, which can cause damage to the engine.7 While it has been shown that the increase in the viscosity upon long-term storage for canola FAME is negligible, fatty acid sources richer in polyunsaturates might not be as stable. It is believed that oxidation causes polymerization of fatty acid chains, which, in turn, causes a sharp increase in the viscosity.16 These reactions occur primarily at the allylic positions of the unsaturated fatty acid chains, which are more prone to homolytic cleavage of a C-H bond and a subsequent reaction with oxygen to generate a hydroperoxide that can then participate in other reactions,17 and the reaction rate is greatly enhanced at the biallylic positions of polyunsaturated chains.18 For these reasons, fatty acid sources that are rich in polyunsaturates are much more prone
calculated from the respective heats of formation under standard pressure and temperature of the chemical species involved in the combustion reaction. For biodiesel and turpentine, the heats of combustion of the individual components were calculated individually. The values were then averaged using approximative compositions. A composition of 61% methyl oleate, 21% methyl linoleate, 11% methyl linolenate, and 7% methyl stearate was used for canola oil based biodiesel, while a composition of 85% R-pinene and 5% each of β-pinene, camphene, and limonene was used for turpentine. The results are summarized in Table 2 in kJ/mol, kJ/g (specific energy), and kJ/mL (energy density). For the cyclopropanated biodiesel, the values were reported for both 90% and 100% conversion. While the calculations estimate the heat of combustion per mole of the cyclopropanated products to be higher by 8% for canola oil and 11% per cyclopropane group for limonene and turpentine, the expected increase in the specific energy is only on the order of 1% for all three fuels. However, because the density of the cyclopropanated fuels is higher, this would translate to an increase in the energy density of approximately 7% for limonene, 5.5% for turpentine, and 5% for canola (4.6% for 90% conversion), which is appreciable, especially for limonene and turpentine, which already show high energy densities. Heat of Combustion Measurements. The heats of combustion for the starting materials and the cyclopropanated products were then measured using a bomb calorimeter. The results are reported in Table 3 and are expressed in kJ/ mol, kJ/g, and kJ/mL. The values for diesel are included for reference. There is a good agreement between the calculated and experimental values. For all compounds, the experimental values fall within the range between the high and low calculated values, and the values obtained for biodiesel, turpentine, and limonene are consistent with previously reported values.6,15 While the difference of the energetic yield by mole between noncyclopropanated and cyclopropanated compounds is significant, the difference by unit of mass is negligible
(16) Muizebelt, W. J.; Nielen, M. W. F. J. Mass Spectrom. 1996, 31, 545–554. (17) (a) Porter, N. A. Acc. Chem. Res. 1986, 19, 262–268. (b) Porter, N. A.; Caldwell, S. E.; Mills, K. A. Lipids 1995, 30, 277–290. (18) (a) Gunstone, F. D.; Hilditch, T. P. J. Chem. Soc. 1946, 1022– 1025.(b) Frankel, E. N. Lipid Oxidation; The Oily Press: Dundee, Scotland, 1998.
(15) Hawkins, J. E.; Eriksen, W. T. J. Am. Chem. Soc. 1954, 76, 2669– 2671.
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Table 3. Heats of Combustion of Studied Biofuels and Their Cyclopropanated Derivatives, Measured by Bomb Calorimetrya compound
heat of combustion (kJ/mol)
specific energy (kJ/g)
energy density (kJ/mL)
limonene 1 turpentine 2 canola oil methyl ester 3 (90% conversion) diesel8
-6092 ( 89 -7184 ( 38 -6115 ( 56 -6723 ( 59 -11652 ( 154 -12194 ( 145
-44.72 ( 0.65 -43.73 ( 0.23 -44.89 ( 0.41 -44.74 ( 0.39 -39.30 ( 0.52 -39.27 ( 0.46 -45.41
-37.56 ( 0.55 -38.92 ( 0.20 -38.61 ( 0.35 -40.27 ( 0.35 -34.58 ( 0.46 -35.74 ( 0.42 -38.60
a
Values for petrodiesel are included for reference.
Figure 1. Kinematic viscosity of canola oil methyl ester, cyclopropanated derivative 3, and compound 3 after distillation as a function of the accelerated oxidation time. Error bars are shown.
to oxidation, and while fish oil would be a desirable alternative to vegetable oils because it is a waste product from the fish industry, it is rich in polyunsaturated fatty acids, including docosahexaenoic acid.19 Because cyclopropanation effectively rids the molecules of alkene groups, and thus of allylic CH2 groups, it would be reasonable to think that oxidation would be slower and would cause a much lesser increase in the viscosity on cyclopropanated FAME than on regular FAME biodiesel. Samples of canola oil methyl ester and its cyclopropanated derivative were submitted to an accelerated oxidation test, in which the compounds were exposed to air while heated at 95 °C under constant stirring for various periods of time. The kinematic viscosities of the compounds following oxidation were then measured and are reported as a function of time in Figure 1. It can be seen that cyclopropanation does not seem to show any beneficial effect to hinder the oxidation and polymerization of fatty acid chains. The initial viscosity of the cyclopropanated sample was 10.0 mm2/s, and the sample reached a viscosity above 45 mm2/s after 72 h of heating. Under the same conditions, a canola biodiesel sample went from 4.62 to 40 mm2/s in the same time period. In contrast, the maximum allowable viscosity according to the Canadian Biodiesel Association is 6 mm2/s.20 The 1H NMR spectra of both canola biodiesel (Figure 2) and its cyclopropanated derivative 3 (Figure 3) were recorded after various periods of accelerated oxidation. For biodiesel, it can be observed that the peaks attributed to vinylic, biallylic, and allylic protons get depleted as the sample is heated under
Figure 2. 1H NMR spectra of canola oil FAME submitted to accelerated oxidation under air at 95 °C for various periods of time.
aerobic conditions. These results, along with the appearance of degradation product signals at 2.5, 2.6, and 2.8 ppm are consistent with previous studies on soybean oil based biodiesel,21 although in our case, a small conjugated diene peak could be observed at 6.1 ppm. However, while cyclopropanated biodiesel shows the same consumption of the remaining vinylic and allylic protons from unreacted alkene moieties, no change was observed for the cyclopropane groups themselves. These observations, nonetheless, raise important questions about the nature of the degradation products causing the increase in the viscosity and the kinetics of their formation. It is widely believed that oxidation at elevated temperatures causes in a first step hydrogen abstraction in
(19) Lin, C.-Y.; Li, R.-J. Fuel Process. Technol. 2009, 90, 130–136. (20) ASTM D6751-09a, Standard Specification for Biodiesel Fuel Blend Stock (B100) for Middle Distillate Fuels, 2010, www.astm.org.
(21) Knothe, G. Eur. J. Lipid Sci. Technol. 2006, 108, 493–500.
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of accelerated oxidation. While this value is still above the regulations for biodiesel, the presence of a plateau shows that the compounds responsible for the increase in the viscosity have been completely reacted. The NMR spectra at various oxidation times are shown in Figure 4. It can be observed that while biallylic hydrogen atoms are not present in a detectable amount, the allylic and vinylic signals decrease as a function of the oxidation time in a fashion similar to that of the two other samples, even after the viscosity has reached a plateau. Obviously, the fact that the allylic signal continues to decrease even when the viscosity has reached a plateau, coupled with the absence of a biallylic signal, suggests that the increase in the viscosity may not be directly related to autoxidation of the fatty acid chains. Additionally, the viscosity reaching a plateau suggests that most of the compounds responsible for the increase in the viscosity have been removed during the cyclopropanation-distillation sequence. Of course, going through cyclopropanation, which requires excess amounts of reagents, followed by thermal ring opening, is obviously not a very costeffective strategy to prevent oxidation of the fuel. However, (1) cheaper and more efficient cyclopropanation methods and (2) large-scale purification methods capable of removing unreacted alkenes from the mixture and trace contaminants such as glycerides and sterol glycosides might successfully solve this issue and allow one to obtain a FAME biodiesel with a slightly higher energetic yield and a greatly increased resistance to both autoxidation and viscosity increase.
Figure 3. 1H NMR spectra of cyclopropanated canola oil FAME 3 submitted to accelerated oxidation under air at 95 °C for various periods of time.
Conclusion We have shown that cyclopropanation of terpenes or unsaturated fatty esters increases their energy density by approximately 4%, although the energy released by unit of mass is unchanged when compared to their noncyclopropanated derivatives. Cyclopropane moieties have been demonstrated to be more resistant to accelerated oxidation than alkene groups, as evidenced by NMR spectrometry, but despite this fact, the kinematic viscosity still follows a trend similar to that of canola oil methyl ester, despite the fact that the cyclopropanated sample contained only 10% unreacted alkene groups. These observations suggest that autoxidation of allylic C-H groups on unsaturated fatty acid chains does not seem directly correlated to the increase in the viscosity observed upon accelerated oxidation. Whether this increase in the viscosity is due to the slow polymerization of degradation products or to contaminants present in trace amounts will be further probed in future studies.
Figure 4. 1H NMR spectra of cyclopropanated canola oil FAME 3 distilled under vacuum submitted to accelerated oxidation under air at 95 °C for various periods of time.
allylic (primarily biallylic) CH2 groups followed by isomerization and then oxygen insertion to form the corresponding hydroperoxides, which eventually lead to polymerization by reaction with the alkene groups of other molecules.16-18 However, because cyclopropane rings do not seem to participate in these reactions, only a small fraction of the molecules actually degrade in the cyclopropanated sample. On the other hand, the biodiesel sample actually showed a significant decrease of the vinylic, allylic, and biallylic signals over 72 h, but the viscosity followed a trend similar to that of the cyclopropanated sample. These observations rather tend to support that either (1) only a small fraction of the oxidized products polymerize and the kinetics of polymerization are slow and independent of the concentration of the reactants or (2) the degradation of other compounds present in trace amounts in the biodiesel, such as mono-, di-, or triglycerides or sterol glycosides,22 is actually responsible for the majority of this increase in the viscosity. This will be confirmed in future studies. However, cyclopropanated biodiesel submitted to vacuum distillation after synthesis has shown a much higher resistance to oxidation, as can be evidenced by the viscosity increasing linearly to reach a plateau of 15 mm2/s after 48 h
Experimental Section (þ)-Limonene (FCC grade), copper(I) chloride, and acetyl chloride were purchased from Aldrich and used without further purification. Zinc (mossy) was purchased from Aldrich and powdered in a ball mill. Turpentine was purchased from Home Depot, and anhydrous ether was purchased from Caledon and used as is from a freshly opened bottle. Canola FAME was obtained from the Department of National Defence. All reactions were carried out under a nitrogen atmosphere with a flow of 25 mL/min in a presealably dried 2 L round-bottomed flask. Mixing was performed by a football-shaped magnetic stirring bar (38 mm 16 mm 16 mm) and a stirring plate with a spinning speed of 900 rpm. In each case, the mixture was heated to reflux (35 °C) for 18 h using a heating mantle heated to approximately 55 °C using a variable heating controller (120 V and 10 A) and a water-jacketed condenser kept at approximately 18 °C with cold water to prevent loss of the solvent. NMR spectra were recorded
(22) Yu, L.; Lee, I.; Hammond, E. G.; Johnson, L. A.; Van Gerpen, J. H. J. Am. Oil Chem. Soc. 1998, 75, 1821–1824.
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: DOI:10.1021/ef100884b
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methane (142 mL, 352 g, 2.02 mol), and acetyl chloride (5.75 mL, 0.0810 mol). The reaction yielded 102 g of cyclopropanated canola oil methyl ester 3 (0.326 mol, 64%) in which approximately 10% of the double bonds were unreacted. Attempts to separate the material by chromatography or distillation were unsuccessful to separate the cyclopropanated and noncyclopropanated fractions: bp 225-230 °C (dec, 20 Torr); IR (neat) 3059, 2994, 2926, 2855, 1748, 1460, 1440, 1382, 1365, 1321, 1308, 1253, 1203, 1172, 1118, 1023, 884, 850, 827, 776, 732 cm-1; 1H NMR (300 MHz, CDCl3) δ 5.31 (m, 0.3H), 3.63 (s, 3H), 2.27 (t, 2H), 1.97 (m, 1.1H), 1.59 (m, 4H), 1.28 (m, 26H), 1.11 (m, 2H), 0.85 (t, 3H), 0.61 (m, 3H), -0.36 (m, 1H); 13C NMR (75 MHz, CDCl3) δ 174.2, 129.9, 129.6, 51.3, 34.0, 31.9, 30.2, 30.1, 29.8, 29.6, 29.4, 29.3, 29.3, 29.1, 28.8, 28.7, 28.6, 28.0, 27.8, 27.2, 27.1, 24.9, 22.7, 16.0, 15.9, 14.0, 11.0, 10.9. Specific Gravity Measurements. The measurements were performed using a clean, dry pycnometer with a capillary stopper. The pycnometer was tared, and then the volume of the pycnometer was first measured by performing 12 measurements with water, each time recording the temperature. The volume was extrapolated from the average of the densities of the measurements with water. These measurements were used to calculate the volume of the pycnometer. The densities of fuels 1-3 were measured in a similar fashion by performing at least 10 measurements for each compound. The densities of (þ)-limonene, turpentine, and canola oil methyl ester were also measured following the same procedure to validate the measured results. Heat of Combustion Calculations. Calculations were performed using the Wave function Spartan 06 software. Heats of formation were first calculated using the T1 thermochemical recipe.14 Heats of combustion were then calculated by calculating first the stoichiometry of the combustion reaction [CxHy þ (x þ y/4)O2 f xCO2 þ (y/2)H2O] and then the differences between the heats of formation of the products and reactants. Heats of formation used were -393.51 kJ/mol for CO2, -241.83 kJ/ mol for H2O gas, and -285.83 kJ/mol for liquid H2O. Adiabatic Bomb Calorimetry. Measurements were performed with a Parr 1241 adiabatic oxygen bomb calorimeter. The sample was placed in a capsule, and then a fuse of 10.0 cm ((0.1 cm) was attached to the electrodes while in contact with the sample. Afterward, 1 mL of distilled water was dispensed in the bomb with a volumetric pipet. The bomb was closed and filled with oxygen at a maximal pressure of 30 atm. The calorimeter switch was set to the “purge” position to fill the atmosphere inside the bomb with oxygen. A total of 2 L of distilled water was poured in the water bath by using a volumetric flask, the bomb was placed in the water bath, and the electrodes were plugged by taking care not to remove any water. The calorimeter was sealed and switched to “run” mode. After the temperature was allowed to equalize between the water bath and the jacket, the temperature of the water bath was recorded to (0.01 °C using the water bath thermometer. The ignition button was pressed and held for 5 s. The highest temperature reached by the jacket was recorded. The bomb was taken out of the water bath, and the residual gas was released slowly (typically over 1 min). Then, the bomb was opened and examined to find evidence of incomplete combustion, in which case the test was discarded. Afterward, the interior of the bomb was washed with distilled water and the washings were collected in a beaker. The unburned pieces of fuse were strained, their lengths were recorded, and the bomb washings were titrated with a standard solution of NaOH of around 0.1000 M using bromothymol blue as the pH indicator. After at least five successful tests, the energy equivalent of the calorimeter was calculated. The apparatus was first calibrated using benzoic acid (approximately 1.0000 ( 0.0.0005 g). For measurements on fuels, samples ranged from approximately 0.59 to 0.73 g. Accelerated Oxidation. A total of 7.0 mL of sample was dispensed in a 50 mL beaker. A magnetic stirrer was added, and the beaker was placed in an oil bath kept at 95 °C with
on a Varian Mercury 300 MHz or a Bruker Avance 400 MHz spectrometer. FTIR spectra were recorded on a Perkin-Elmer Spectrum GX spectrometer using KBr windows. Cyclopropanation of (þ)-Limonene: 4-(1-Methylcyclopropyl)1-methylbicyclo[4.1.0]heptane (1). In a round-bottomed flask equipped with a magnetic stirrer and a water-jacketed condenser were successively added powdered zinc metal (165 g, 2.53 mol), copper(I) chloride (30.1 g, 0.304 mol), and dibromomethane (47.3 mL, 117 g, 0.675 mol) in dry ether (250 mL). Acetyl chloride (5.75 mL, 0.0810 mol) was then added to the mixture via a syringe, and then the mixture was heated to obtain a gentle reflux. (þ)-Limonene (81.8 mL, 68.9 g, 0.506 mol) in 160 mL of dry ether was then added over 10 min, and then a second portion of dibromomethane (94.6 mL, 234 g, 1.35 mol) diluted with 85 mL of ether was added over 30 min. The mixture was then refluxed for 18 h and then placed in an ice bath, and 350 mL of saturated aqueous NH4Cl was slowly poured into the flask with constant stirring, which caused the aqueous layer to become blue because of the formation of copper(II) tetraammine complexes. The solids, which include zinc oxide formed during the workup along with unreacted zinc metal, were then removed by filtration, and the flask and precipitate were washed three times with hexanes and aqueous NH4Cl. The filtrate was then recovered and placed in an extraction funnel, and then both layers were separated. The aqueous layer was extracted twice with hexanes, and then the combined organic extracts were successively extracted with three portions of 1 M aqueous NaOH and saturated aqueous NaCl to remove the remaining water-soluble reaction components. The organic extracts were recovered, dried over Na2SO4, and filtered, and the volatiles, including the remaining unreacted dibromomethane, were removed under reduced pressure. The crude product was purified by distillation under vacuum. A short forerun of starting materials and monocyclopropanated derivatives was first collected, followed by the desired product 1 as a mixture of diasteromers (43.5 g, 0.288 mol, 57%): bp 87-92 °C (20 Torr); IR (neat) 2991, 2950, 2937, 2923, 2865, 1453, 1430, 1386, 1352, 1331, 1304, 1267, 1250, 1223, 1196, 1172, 1155, 1091, 1033, 1013, 979, 962, 938, 922, 881, 871, 854, 830, 803, 783, 766, 756, 677 cm-1; 1H NMR (300 MHz, CDCl3) δ 2.0-0.8 [m, 13H, the CH3 peaks can be found at 1.024, 1.016, 0.87, and 0.82 ppm (approximately 1.5H each)], 0.8-0.0 (m, 7H, cyclopropyl CH and CH2); 13C NMR (75 MHz, CDCl3) δ 43.3, 39.4, 32.0, 31.6, 27.9, 27.9, 27.4, 27.0, 26.8, 26.7, 24.8, 20.8, 19.2, 18.8, 18.3, 18.1, 18.0, 16.0, 14.5, 12.7, 12.5, 12.3, 12.2. Cyclopropanation of Turpentine (2). Compound 2 was prepared from turpentine (68.9 g, 0.506 mmol) by the same procedure as that for compound 1 using the following quantities of reagents: powdered zinc metal (165 g, 2.53 mol), copper(I) chloride (30.1 g, 0.304 mol), dibromomethane (142 mL, 352 g, 2.02 mol), and acetyl chloride (5.75 mL, 0.0810 mol). The crude product was purified by distillation under vacuum. The remaining starting material was first collected, followed by compound 2. A total of 35.9 g of cyclopropanated turpentine 2 was thus obtained (0.238 mol, 47%): bp 75-80 °C (20 Torr); IR (neat) 2987, 2950, 2916, 2899, 2865, 1494, 1453, 1386, 1369, 1331, 1277, 1264, 1230, 1209, 1172, 1152, 1125, 1111, 1091, 1033, 1020, 959, 942, 905, 888, 861, 820, 793, 762, 701 cm-1; 1H NMR (300 MHz, CDCl3) δ 2.35-0.8 (m, 15H, the CH3 peaks of 2-methanopinane can be found at 1.26, 1.08, and 1.04 ppm), 0.75-0.05 (m, 3H, cyclopropyl CH and CH2); 13C NMR (75 MHz, CDCl3) δ 51.6, 51.2, 50.2, 48.6, 48.1, 47.1, 45.4, 41.5, 40.9, 40.7, 40.6, 37.9, 37.2, 36.8, 36.4, 35.7, 31.4, 31.2, 29.7, 28.9, 28.0, 27.3, 27.0, 26.9, 26.6, 26.3, 25.4, 24.7, 24.4, 24.2, 24.1, 23.4, 22.9, 21.8, 21.0, 20.8, 20.2, 19.6, 18.9, 18.3, 18.1, 18.0, 16.2, 15.9, 11.7. Cyclopropanation of Canola Oil Methyl Ester (3). Compound 3 was prepared from canola oil methyl ester (150 g, 0.507 mmol) by the same procedure as that for compound 1 using the following quantities of reagents: powdered zinc metal (165 g, 2.53 mol), copper(I) chloride (30.1 g, 0.304 mol), dibromo5262
Energy Fuels 2010, 24, 5257–5263
: DOI:10.1021/ef100884b
Langlois and Lebel
ing the same procedure as that with 5.00 mL of sample at 40.0 ( 0.1 °C. For each sample, one premeasurement and five flow-time measurements were recorded.
constant stirring to maximize the exposure to ambient air. After the desired amount of time, the beaker was taken out and the sample was transferred in a screw-top vial. Kinematic Viscosity. A Cannon-Fenske viscometer with a constant of approximately 0.01 mm2/s2 was first standardized using 5.00 mL of a S20 viscosity reference standard solution. The viscometer was placed in the water bath of a Lauda PVS1 automatic viscometer system, comprised of a Lauda Clear-View thermostat (D 15 KP) and an Edition 2000 temperature controller (Lauda, Lauda-K€ onigshofen, Germany), kept at 50.0 ( 0.1 °C and allowed to equilibrate for 20 min, and then one premeasurement and nine flow-time measurements were recorded. The process was repeated at 80.0 ( 0.1 °C, and then the measured viscosities were compared to the viscosity of the standard to determine the capillary constant of the viscometer. The measurements were performed on biodiesel samples follow-
Acknowledgment. We are grateful to DGLEPM of the Department of National Defence for funding. We also thank Dr. Danny Page and Kommy Farahani from RMC, Dan Emil Dulgheru from DGLEPM, and Dr. Michael Lam and Aneliia Krasteva from the Quality Engineering Test Establishment. Supporting Information Available: NMR spectra of compounds 1-3, DFT calculations for dihedral angle scans of compounds 17a-17i and for hydrogen-bonding energies of dimers of compounds 2, 4, 8b, and 9, and 1H and 13C NMR spectra for all compounds reported therein. This material is available free of charge via the Internet at http://pubs.acs.org.
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