Biodiesel Derived from a Model Oil Enriched in Palmitoleic Acid

Feb 4, 2010 - The issues of energy availability and security have caused intense efforts around the world to search for alternatives to reduce the dep...
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Energy Fuels 2010, 24, 2098–2103 Published on Web 02/04/2010

: DOI:10.1021/ef9013295

Biodiesel Derived from a Model Oil Enriched in Palmitoleic Acid, Macadamia Nut Oil Gerhard Knothe* National Center for Agricultural Utilization Research, Agricultural Research Service, United States Department of Agriculture, Peoria, Illinois 61604 Received November 10, 2009. Revised Manuscript Received January 20, 2010

Numerous vegetable oils, animal fats, or other feedstocks have been investigated to obtain biodiesel, defined as the monoalkyl esters of vegetable oils and animal fats. While biodiesel is competitive with petrodiesel, technical problems facing biodiesel include cold flow and oxidative stability. Most biodiesel fuels largely contain five fatty acids, palmitic, stearic, oleic, linoleic, and linolenic fatty acids, in varying amounts in their fatty acid profiles. Dependent upon the major fatty acids present, biodiesel from different feedstocks faces these technical problems with varying severity. As previous work has indicated, enrichment of other fatty acids, such as decanoic or palmitoleic acid, in the fatty acid profile may be advantageous to address the technical issues facing biodiesel. In this work, an oil moderately enriched in palmitoleic acid (approximately 16-20%), macadamia nut oil, was selected for producing the corresponding biodiesel fuel and investigating its fuel properties. Methyl esters of macadamia nut oil were prepared by conventional transesterification with sodium methoxide. Fuel properties, such as cetane number, kinematic viscosity, oxidative stability, cold flow, as well as lubricity, are discussed in light of biodiesel standards. The approximately 15% content of saturated fatty esters in macadamia nut oil affects cold flow. The 1H nuclear magnetic resonance (NMR) spectrum of macadamia methyl esters is also reported.

Most common vegetable oil feedstocks, including commodity oils, such as palm, soybean, and rapeseed/canola, possess a fatty acid profile consisting mainly of the five common fatty acids, palmitic, stearic, oleic, linoleic, and linolenic fatty acids. Even “alternative” oils, such as jatropha oil,5 have such fatty acid profiles. Thus, the biodiesel fuels derived from these oils display the aforementioned problems with varying severity depending upon the exact amounts of these fatty acids. Solving these problems has proven difficult because of opposing effects caused by the different fatty acids. For example, while unsaturated esters display good cold flow properties, oxidative stability is poor. On the other hand, saturated esters are oxidatively stable but possess poor cold flow properties. There are five approaches to addressing these problems,6 which are the use of additives, the use of alcohols other than methanol to produce monoalkyl esters, the use of physical procedures, the use of feedstocks with inherently different fatty acid profiles, and the use of feedstocks with genetically altered fatty acid profiles. It has been discussed that fatty acid profiles enriched in some acids, such as decanoic or palmitoleic acid [9(Z)-hexadecenoic acid; C16:1], may impart overall favorable properties to a biodiesel fuel,7 especially cold flow. Related work also discusses identifying an ideal biodiesel composition.8 It was recently reported that a vegetable oil (cuphea) enriched in

Introduction The issues of energy availability and security have caused intense efforts around the world to search for alternatives to reduce the dependence upon petroleum as a major energy source. Biofuels play a prominent role in these efforts. Among these fuels is biodiesel,1,2 which is defined as the fatty acid monoalkyl esters of vegetable oils or animal fats3 or waste oils. Biodiesel is a technically competitive and environmentally friendly alternative to conventional petrodiesel for use in compression-ignition (diesel) engines. Biodiesel is biodegradable, renewable, and nontoxic, possesses inherent lubricity, contains little to no sulfur, has a relatively high flash point and a positive energy balance, and in comparison to petrodiesel, reduces most regulated exhaust emissions. However, some technical issues have beset biodiesel. These issues are poor cold flow properties and/or storage stability as well as slightly elevated NOx exhaust emissions in comparison to petrodiesel, with the latter affecting especially older engines not equipped with newer exhaust emission reduction technologies. Two standards, ASTM D67513 in the United States and EN 142144 in Europe, serve as guidelines for biodiesel fuel quality.

*To whom correspondence should be addressed: USDA/ARS/ NCAUR, 1815 N. University St., Peoria, IL 61604. Telephone: (309) 681-6112. Fax: (309) 681-6524. E-mail: [email protected]. (1) The Biodiesel Handbook; Knothe, G., Krahl, J., Van Gerpen, J., Eds.; American Oil Chemists' Society (AOCS) Press: Champaign, IL, 2005. (2) Mittelbach, M.; Remschmidt, C. Biodiesel;The Comprehensive Handbook; Martin Mittelbach: Graz, Austria, 2004. (3) American Society for Testing and Materials (ASTM). ASTM Standard D6751. Standard Specification for Biodiesel Fuel Blend Stock (B100) for Middle Distillate Fuels; ASTM: West Conshohocken, PA, 2009. (4) European Committee for Standardization. Standard EN 14214. Automotive fuels;Fatty acid methyl esters (FAME) for diesel engines;Requirements and test methods, 2009. This article not subject to U.S. Copyright. Published 2010 by the American Chemical Society

(5) Foidl, N.; Foidl, G.; Sanchez, M.; Mittelbach, M. Jatropha curcas L. as a source for the production of biofuel in Nicaragua. Bioresour. Technol. 1996, 58, 77–82. (6) Knothe, G. Improving biodiesel fuel properties by modifying fatty ester composition. Energy Environ. Sci. 2009, 2, 759–766. (7) Knothe, G. “Designer” biodiesel: Optimizing fatty ester composition to improve fuel properties. Energy Fuels 2008, 22, 1358–1364. (8) Pinzi, S.; Garcia, I. L.; Lopez-Gimenez, F. J.; Luque de Castro, M. D.; Dorado, G.; Dorado, M. P. The ideal vegetable oil-based biodiesel composition: A review of social, economical and technical implications. Energy Fuels 2009, 23, 2325–2341.

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decanoic acid indeed possesses overall favorable fuel properties with a cloud point from -9 to -10 °C.9 An oil with 32.8% erucic acid as a major component also containing linoleic (22.4%), linolenic (11.8%), oleic (11.1%), 11(Z)-eicosenoic (8.6%), and palmitic (3.1%) acids as well as minor amounts of other species, pennycress oil, also showed a lower cloud point (around -10 °C), although the kinematic viscosity is higher.10 However, with the exception of Zanthoxylum bungeanum (a Chinese spice) seed oil (ZBSO), which contains from about 5.211 to 10.8%12 palmitoleic acid, an oil containing palmitoleic acid in amounts higher than the traces observed in most common vegetable oils has not yet been studied as a source of biodiesel. The fatty acid profile of ZBSO was given as 10.3% C16:0, 1.1% C18:0, 10.8% C16:1, 35.9% C18:1, 24.8% C18:2, and 15.9% C18:3.12 The macadamia tree belongs to the Proteaceae family, most species of which are indigenous to Australia and South Africa13 but have also been introduced in locations such as Mexico, Hawaii, and California. Numerous species of this family possess oils enriched in palmitoleic acid.14 Palmitoleic acid has been reported to be enriched in land flora containing greater amounts of palmitoleic acid, and it is possible that members of the Proteaceae family are “survivors from one of the more primitive types characteristic of the original flora of the Australian continent”.13 Macadamia oil apparently has benefits for human metabolism when included in the diet, such as lowering cholesterol levels and favorably modifying the risk factors for coronary heart disease.15,16 The fact that the oil of macadamia nuts is enriched in palmitoleic acid was apparently first reported by Bridge and Hilditch,13 who reported 20.4 wt % (22 mol %) in the oil of Macadamia ternifolia. A report on different macadamia nut cultivars in New Zealand discussed a range of about 17-34% palmitoleic acid, with oleic acid in the range of 40.5-59% (total monounsaturated

fatty acid chains around 80%, with approximately 3.5% polyunsaturated fatty acid chains).17 Other literature report fatty acid compositions for macadamia nut oil falling into this range.18-20 Thus, macadamia nut oil contains more palmitoleic acid than the Z. bungeanum seed oil mentioned above. Besides macadamia and the aforementioned Z. bungeanum, other sources of oils enriched in palmitoleic acid have been reported in the literature, including marine cyanobacteria with two species, Phormidium sp. NKBG 041105 and Oscilatoria sp. NKBG 091600, exhibiting about 54.5% palmitoleic acid in the fatty acid profile,21 a mutant of sunflower Helianthus annuus,22,23 a linseed variety, with the oil containing up to 4% palmitoleic acid,24 the fungus Linderina pennispora, with approximately 37% palmitoleic acid,25 and sea buckthorn (Hippophae rhamnoides L.) oil.26-28 Palmitoleic acid is practically absent in the seed oil of sea buckthorn but 12.1-39.0% in the oil of the pulp/peel and 8.9-31.0% of that in the whole berries.27 Distillation at reduced pressure of methyl esters of sea buckthorn oil gave a fraction highly enriched (82.1%) in palmitoleic acid.29 Macadamia nut oil is probably the oil with the highest palmitoleic acid content that is commercially available. In this work, two macadamia nut oils were selected as feedstocks for biodiesel containing above average amounts of palmitoleic acid (approximately 16-20%) to serve as a model for other feedstocks that could potentially be altered in a fashion as to contain more palmitoleic acid. Besides this possibility, microorganisms can be genetically engineered to provide desired fatty acid products from the fatty acid biosynthetic pathway.30 It must be noted here, however, that macadamia nut oil is only serving as a model oil in this work because macadamia nut oil itself is not a realistic feedstock for biodiesel because of issues such as supply and price (also avoiding any food versus fuel discussion) and that the amounts of palmitoleic acid in macadamia nut oil are generally lower than would be desirable for enrichment through means such as genetic breeding. Oils with high enrichment of palmitoleic acid are currently not commercially available.

(9) Knothe, G.; Cermak, S. C.; Evangelista, R. L. Cuphea oil as source of biodiesel with improved fuel properties caused by high content of methyl decanoate. Energy Fuels 2009, 23, 1743–1747. (10) Moser, B. R.; Knothe, G.; Vaughn, S. F.; Isbell, T. A. Production and evaluation of biodiesel from field pennycress (Thlaspi arvense L.) oil. Energy Fuels 2009, 23, 4149–4155. (11) Zhang, J.; Jiang, L. Acid-catalyzed esterification of Zanthoxylum bungeanum seed oil with high free fatty acids for biodiesel production. Bioresour. Technol. 2008, 99, 8995–8998. (12) Yang, F.-X.; Su, Y. Q.; Li, X.-H.; Zheng, Q.; Shi, R. C. Studies on the preparation of biodiesel from Zanthoxylum bungeanum seed oil. J. Agric. Food Chem. 2008, 56, 7891–7896. (13) Bridge, R. E.; Hilditch, T. P. The seed fat of Macadamia ternifolia. J. Chem. Soc. 1950, 2396–2399. (14) Badami, R. C.; Patil, K. B. Structure and occurrence of unusual fatty acids in minor seed oils. Prog. Lipid Res. 1981, 19, 119–153. (15) Hiraoka-Yamamoto, J.; Ikeda, K.; Negishi, H.; Mori, M.; Hirose, A.; Sawada, S.; Onobayashi, Y.; Kitamori, K.; Kitano, S.; Tashiro, M.; Miki, T.; Yamori, Y. Serum lipid effects of a monounsaturated (palmitoleic) fatty acid-rich diet based on macadamia nuts in healthy, young Japanese women. Clin. Exp. Pharmacol. Physiol. 2005, 31, S37–S37. (16) Garg, M. L.; Blake, R. J.; Wills, R. B. H.; Clayton, E. H. Macadamia nut consumption modulates favourably risk factors for coronary artery disease in hypercholesterolemic subjects. Lipids 2007, 42, 583–587. (17) Kaijser, A.; Dutta, P.; Savage, G. Oxidative stability and lipid composition of macadamia nuts grown in New Zealand. Food Chem. 2000, 71, 67–70. (18) Rodrigues, C. E. C.; Silva, F. A.; Marsaioli, A., Jr.; Meirelles, A. J. A. Deacidification of Brazil nut and macadamia nut oils by solvent extraction. J. Chem. Eng. Data 2005, 50, 517–523. (19) Venkatachalam, M.; Sathe, S. K. Chemical composition of selected edible nut seeds. J. Agric. Food Chem. 2006, 54, 4705–4714. (20) Lı´ sa, M.; Holcapek, M.; Bohac, M. Statistical evaluation of triacylglycerol composition in plant oils based on high-performance liquid chromatography-atmospheric pressure chemical ionization mass spectrometry data. J. Agric. Food Chem. 2009, 57, 6888–6898.

(21) Matsunaga, T.; Takeyama, H.; Miura, Y.; Yamazaki, T.; Furuya, H.; Sode, K. Screening of marine cyanobacteria for high palmitoleic acid production. FEMS Microbiol. Lett. 1995, 133, 137–141. (22) Salas, J. J.; Martı´ nez-Force, E.; Garces, R. Biochemical characterization of a high-palmitoleic acid Helianthus annuus mutant. Plant Physiol. Biochem. 2004, 42, 373–381. (23) Salas, J. J.; Moreno-Perez, A. J.; Martı´ nez-Force, E.; Garces, R. Characterization of the glycreolipid composition of a high-palmitoleic acid sunflower mutant. Eur. J. Lipid Sci. Technol. 2007, 109, 591–599. (24) Rowland, G. G.; McHughen, A.; Gusta, L. V.; Bhatty, R. S.; MacKenzie, S. L.; Taylor, D. C. The application of chemical mutagenesis and biotechnology to the modification of linseed (Linum usitatissimum L.). Euphytica 1995, 85, 317–321. (25) Konova, I. V.; Galanina, L. A.; Kochkina, G. A.; Pan’kina, O. I. Fatty acids in the species of several zygomycete taxa. Microbiology 2002, 71, 639–647. (26) Johansson, A. K.; Korte, H.; Yang, B.; Stanley, J. C.; Kallio, H. P. Sea buckthorn berry oil inhibits platelet aggregation. J. Nutr. Biochem. 2000, 11, 491–495. (27) Yang, B.; Kallio, H. P. Fatty acid composition of lipids in sea buckthorn (Hippophae rhamnoides L.) berries of different origins. J. Agric. Food Chem. 2001, 49, 1939–1947. (28) Pchelkin, V. P.; Kuznetsova, E. I.; Tsydendambaev, V. D.; Vereshchagin, A. G. Distribution of unusual fatty acids in the triacylglycerols of sea buckthorn mesocarp oil. Russ. J. Plant Physiol. 2006, 53, 346–354. (29) Klaas, M. R. G.; Meurer, P. U. A palmitoleic acid ester concentrate from seabuckthorn pomace. Eur. J. Lipid Sci. Technol. 2004, 106, 412–416. (30) Keasling, J. D.; Hu, Z.; Somerville, C.; Church, G.; Berry, D.; Friedman, L.; Schirmer, A.; Brubaker, S.; del Cardayre, S. B. Production of fatty acids and derivatives thereof. WO/2007/136762, Nov 29, 2007 (http://www.wipo.int/pctdb/en/wo.jsp?WO=2007136762).

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Experimental Section

Table 1. Fatty Acid Profile of the Two Macadamia Nut Oils Used in This Work

Reagents. Methanol, sodium methoxide, anhydrous magnesium sulfate, and fatty acid methyl standards were analyticalreagent-grade and purchased from Sigma-Aldrich (St. Louis, MO). The two refined commercial macadamia nut oils used in the present work were produced by Macadamia Oils of Australia (Alstonville, New South Wales, Australia) and NOW Foods (Bloomingdale, IL). The oils are referred to as MNO-1 and MNO-2, and the methyl esters synthesized from both sources of macadamia nut oil are referred to as MacME-1 and MacME-2, respectively, in the order of producers given in the previous sentence. The fatty acid profiles of the two macadamia nut oils were determined by gas chromatography (GC) using a Varian 34 CX (Palo Alto, CA) gas chromatograph, equipped with a flame ionization detector and a Supelco (Bellefonte, PA) SP-2380 capillary column (30 m  0.25 mm inner diameter, 0.2 μm film thickness). The oven temperature ramp program was 150 °C for 15 min, 150-210 °C at 2 °C/min, and then ballistic heating to 220 °C with a 5 min hold. Retention times were verified against authentic samples of individual pure fatty acid methyl esters. All relative percentages determined by GC for each fatty acid methyl ester sample are the means of triplicate runs. Additional determination of the fatty acid profile by 1H nuclear magnetic resonance (NMR) spectroscopy was performed on a Bruker (Billerica, MA) Avance 500 spectrometer operating at 500 MHz with CDCl3 as the solvent. NMR spectra were processed using Spinworks software (Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, Canada; http://www.umanitoba.ca/chemistry/nmr/spinworks/). Transesterification of Macadamia Nut Oil. The macadamia nut oils used for producing biodiesel had acid values of 1.13 and 0.335 mg of KOH/g, respectively, obviating the need for acid pretreatment.31 Methanolysis of macadamia nut oil was conducted by a standard procedure employing a 6:1 molar ratio of methanol/vegetable oil (scale: 100 g of macadamia nut oil) for 1 h at 60 °C with 1 wt % NaOCH3 as the catalyst. After completion of the reaction, the mixture was cooled to room temperature without agitation, leading to separation of two phases: the lower glycerol and the upper methyl ester phases. After separation of the two phases by decantation, most excess methanol was removed from the upper MacME layer at 80 °C. The remaining catalyst was then removed through several water washes by slowly dripping the product into the water in a separatory funnel and permitting it to rise above the water. After separation of the phases, residual water was removed by treatment with anhydrous MgSO4, followed by filtration. Property Determination. Cetane numbers were determined as derived cetane numbers using an ignition quality tester (IQT) as described previously.32 The IQT (ASTM D6890) is approved as an alternative to the traditional cetane number standard (ASTM D613) in the biodiesel standard ASTM D6751. Kinematic viscosity was obtained with Cannon-Fenske viscometers employing the standard ASTM D445 as described previously.33 Oxidative stability measurements were carried out with a Rancimat (Metrohm, Herisau, Switzerland; equipped with software for statistical evaluation) using the standard EN 14112.

amount (%)a fatty acid

MNO-1

MNO-2

tetradecanoic (myristic, 14:0) hexadecanoic (palmitic, 16:0) 9(Z)-hexadecenoic (palmitoleic, 16:1) octadecanoic (stearic, 18:0) 9(Z)-octadecenoic (oleic, 18:1) 11(Z)-octadecenoic (asclepic, 18:1) 9(Z),12(Z)-octadecenoic (linoleic, 18:2) eicosanoic (arachidic, 20:0) 8(Z)-eicosenoic (20:1) docosanoic (behenic, 22:0)

0.64 8.49 15.88 3.57 58.69 3.79 2.13 2.68 2.57 0.88

0.80 8.88 20.19 3.30 55.32 3.99 1.79 2.32 2.58 0.67

a

Minor components not listed were detected to give 100%.

Lubricity was investigated using a high-frequency reciprocating rig (HFRR) lubricity tester following the method ASTM D6079 as described in the literature.34 Cloud and pour point determinations were conducted with a Phase Technology (Richmond, British Columbia, Canada) cloud, pour, and freeze point analyzer. Heteroelements (Na, K, Ca, Mg, S, and P) were determined with a Perkin-Elmer (Norwalk, CT) 5500 inductively coupled plasma-optical emissions spectrometer. Free and total glycerol content of the methyl esters were determined by GC on an Agilent Technologies (Santa Clara, CA) 7890A gas chromatograph following the procedure described in the standard ASTM D6584. Acid values were determined with American Oil Chemists’ Society (AOCS) method Cd3d-63. Moisture was determined by coulometric Karl Fischer titration with a Metrohm 831 KF coulometer (Metrohm USA, Tampa, FL).

Results and Discussion Fatty Acid Profile and Properties of Macadamia Nut Oil. The fatty acid profiles of the macadamia nut oils used here as determined by GC are given in Table 1 and agree with prior literature on macadamia nut oil,17 which gives ranges of 8.4-11.1% palmitic acid, 16.9-33.8% palmitoleic acid, 1.5-3.2% stearic acid, 40.5-59% oleic acid, 2.6-3.6% asclepic acid [11(Z)-octadecenoic acid], 2.6-4.5% linoleic acid, 1.3-2.1% arachidic acid, and 1.4-2.6% eicosenoic acid. Other literature references give similar values for the fatty acid profile of macadamia nut oil. Thus, oleic acid is the major fatty acid in macadamia nut oil, followed by palmitoleic acid. Together with smaller amounts of other fatty acids with one double bond, the monounsaturated fatty acids in the macadamia nut oils used here comprise slightly over 80% of the fatty acid profile. The GC results were confirmed by 1 H NMR using a method described in the literature,35 which showed a total content of saturated fatty acids of 15.3%, with about 83.1% monounsaturated fatty acid chains and 1.58% C18:2. The 1H NMR spectrum of macadamia oil methyl esters is shown in Figure 1, and a summarizing evaluation of the spectrum is given there. The properties of MNO-1 and MNO-2 are given in Table 2. Slight differences in the properties of MNO-1 and MNO-2 can be observed, which may be ascribed to the varying fatty acid profiles and different amounts of other constituents, such as tocopherols as naturally occurring antioxidants affecting oxidative stability. Transesterification and Product Analysis. Conventional transesterification with sodium methoxide was successfully

(31) Canakci, M.; Van Gerpen, J. Biodiesel production from oils and fats with high free fatty acids. Trans. Am. Soc. Agric. Eng. 2001, 44, 1429–1436. (32) Knothe, G.; Matheaus, A. C.; Ryan, T. W., III. Cetane numbers of branched and straight-chain fatty esters determined in an ignition quality tester. Fuel 2003, 82, 971–975. (33) Knothe, G.; Steidley, K. R. Kinematic viscosity of biodiesel fuel components and related compounds. Influence of compound structure and comparison to petrodiesel fuel components. Fuel 2005, 84, 1059– 1065. (34) Knothe, G.; Steidley, K. R. Lubricity of components of biodiesel and petrodiesel. The origin of biodiesel lubricity. Energy Fuels 2005, 19, 1192–1200.

(35) Knothe, G.; Kenar, J. A. Determination of the fatty acid profile by 1H-NMR spectroscopy. Eur. J. Lipid Sci. Technol. 2004, 106, 88–96.

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and 57.05, respectively, from the two macadamia oil feedstocks (Table 3), exceeding the minimum cetane numbers of 47 and 51 prescribed in the American biodiesel standard ASTM D6751 and the European biodiesel standard EN 14214, respectively. For a comparison, the cetane number of methyl palmitoleate is 56.59 or 51.0 in two tests,7,32 the cetane number of methyl palmitate is around 85, the cetane number of methyl oleate is in the range of 56-59, and the cetane number of methyl linoleate is around 38. Because of the high amounts of methyl oleate (and enrichment of methyl palmitoleate), the cetane number of MacME can be expected to be in the range observed. The heat of combustion of MacME (not determined experimentally during the course of this work) is likely largely comparable to that of other biodiesel fuels consisting mainly of C16-C18 esters. For orientation, the heat of combustion of methyl oleate, the major component of MacME, is 40 092 kJ/kg (calculated from data and the equation in ref 36). While neither of the biodiesel standards ASTM D6751 and EN 14214 contains a specification regarding the heat of combustion, the European standard EN 14213 for use of biodiesel as a heating oil prescribes a minimum heat of combustion of 35 000 kJ/kg. Exhaust emissions as generated in an engine test were not determined during the course of this work; however, previous results from the literature37 can be used to estimate the effect of macadamia-oil-derived biodiesel on exhaust emissions. Because of the high content of methyl oleate in MacME, the effect of MacME on regulated exhaust emissions [nitrogen oxides (NOx), particulate matter (PM), hydrocarbons (HCs), and carbon monoxide] versus soybean methyl ester (SME) or petrodiesel would likely be similar to that of technical-grade methyl oleate (77% methyl oleate in ref 37). In a heavy-duty exhaust emissions test with a 2003 model year diesel engine,37 technical-grade methyl oleate increased NOx exhaust emissions by 6.2% versus the reference petrodiesel fuel used but reduced PM by 72.9%, HC by 54.6%, and CO by 49%. However, the effect of technicalgrade methyl oleate would likely be offset slightly for MacMe because of the presence of methyl palmitoleate, which could cause a slight increase of HC and CO versus SME, similar to the effect of chain length observed when comparing methyl palmitate versus methyl laurate and hexadecane versus dodecane,37 with a major effect on NOx and PM appearing unlikely. Cold Flow. Both MacME fuels displayed cloud and pour points >0 °C (Table 3). Although the total amount of saturated esters in MacME is similar, approximately 15%, to that of SME, MacME display higher cloud and pour points. These elevated values are likely due to the small amounts of C20 and C22 saturated fatty esters (melting points of 46.4 and 53.2 °C for methyl eicosanoate and methyl docosanoate, respectively)38 comprising the 15% saturated esters in MacME versus SME, the saturated esters of which are almost exclusively methyl palmitate and methyl stearate. Thus, the presence of some methyl palmitoleate in MacME,

Figure 1. 1H NMR spectrum of macadamia nut oil methyl esters (MacME). The spectrum displays typical features of methyl esters of vegetable oils [-CHdCH- protons at 5.4-5.5 ppm, CH3OCO- at 3.65 ppm, CH3O-CO-CH2- at about 2.30 ppm, CH2-CHdCH- at 2.05 ppm, CH3O-CO-CH2-CH2- at 1.6 ppm, -(CH2)n- at 1.2-13.3 ppm, and -(CH2)m-CH3 at 0.85 ppm]. The absence of polyunsaturated fatty acids is shown by the lack of signals at 2.8 ppm (-CHdCH-CH2-CHdCH-) and about 0.90 ppm (-CHdCH-CH2-CH3). Table 2. Properties of the Macadamia Nut Oils Used in This Work property kinematic viscosity (40 °C, mm2/s) cloud point (°C) pour point (°C) oxidative stability (h) acid value (mg of KOH/g) water (%) sodium (μg/g, ppm) potassium (μg/g, ppm) calcium (μg/g, ppm) magnesium (μg/g, ppm) phosphorus (μg/g, ppm) sulfur (μg/g, ppm)

MNO-1

MNO-2

39.24 1 -6 7.31 1.13 0.058 0.86 10.14 19.61 18.02 34.17 4.13

38.80 -2 -8 30.22 0.335 0.035 0 1.01 2.35 2.04 40.10 0.08

applied to prepare the methyl esters (biodiesel) of the two macadamia nut oils. The washed and dried fuels were analyzed for acid value, moisture, and free and total glycerol. The biodiesel fuels obtained from both macadamia nut oils met all of the corresponding specifications in the ASTM D6751 and EN 14214 biodiesel standards (Table 3). The biodiesel fuels prepared from both macadamia oils also met the standard specifications for the heteroelements Na, K, Ca, Mg, and P. With the exception of Mg (perhaps because of drying with magnesium sulfate; therefore, this may not be a problem if another drying method is used) and S, which were present in higher amounts in the biodiesel fuels than in the parent oils, all heteroelements were either reduced or occurred at similar levels in the biodiesel fuels compared to the parent oils. Properties of Macadamia Nut Oil Methyl Esters. The fuel properties of biodiesel can be distinguished as being caused by the major components, i.e., the alkyl esters, or by minor components, such as impurities. This work focuses on the properties largely determined by the ester components. These properties are also summarized in Table 3 together with the relevant specifications from the biodiesel standards ASTM D6751 and EN 14214 and discussed below for each individual property. The other properties and specifications in standards are often affected by issues such as storage, quality of the feedstock, and completeness of the transesterification reaction. Among these other issues is the extent of the transesterification reaction, i.e., content of acylglycerols and alcohol, which has been addressed above. Cetane Number and Combustion. The cetane number (as the derived cetane number) of MacME was found to be 59.5

(36) Handbook of Chemistry and Physics, 80th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1999. (37) Knothe, G.; Sharp, C. A.; Ryan, T. W., III. Exhaust emissions of biodiesel, petrodiesel, neat methyl esters, and alkanes in a new technology engine. Energy Fuels 2006, 20, 403–408. (38) Knothe, G.; Dunn, R. O. A comprehensive evaluation of the melting points of fatty acids and esters determined by differential scanning calorimetry. J. Am. Oil Chem. Soc. 2009, 86, 843–856.

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Table 3. Properties of Macadamia Nut Methyl Esters with a Comparison to Standards property cetane number kinematic viscosity (40 °C, mm2/s) cloud point (°C) pour point (°C) oxidative stability (h) free glycerol (mass %) total glycerol (mass %) acid value (mg of KOH/g) water content (vol %) sodium (μg/g, ppm) potassium (μg/g, ppm) calcium (μg/g, ppm) magnesium (μg/g, ppm) phosphorus (μg/g, ppm) sulfur (μg/g, ppm) a

MacME-1

MacME-2

59.5 4.57 7.0 5 1.97 0.0025 0.107 0.08 0.033 0.43 2.09 0 67.8 0.25 5.06

57.05 4.41 4.5 1 2.06 0.009 0.125 0.055 0.04 0.55 0.08 0 68.69 0 1.06

ASTM D6751

EN 14214

47 min 1.9-6.0 report b 3 min 0.02 max 0.24 max 0.5 max 0.05 max Na and K combined: 5 max

51 min 3.5-5.0 a a 6 min 0.02 max 0.25 max 0.5 max 0.05 max Na and K combined: 5 mg/kg max

Ca and Mg combined: 5 max

Ca and Mg combined: 5 mg/kg max

0.001 mass % max 0.0015 mass % max

4 mg/kg 10 mg/kg max

Not specified. EN 14214 uses time- and location-dependent values for the cold-filter plugging point (CFPP) instead. b Not specified.

despite its lower melting point (-34 °C)7,38 compared to methyl oleate (-20 °C), has no significant influence on the CP or PP, although it may be noted that MacME-2, which contains slightly more C16:1 than MacME-1, displayed a slightly lower CP and PP than MacME-1. This observation coincides with the literature that amounts and the nature of higher melting saturated esters are responsible for the overall cold flow properties of biodiesel.39 Coinciding observations are methyl esters derived from Moringa oleifera oil, containing around 4% eicosanoic and 7% docosanoic acid besides over 70% oleic acid, exhibiting a CP of 18 °C,40 and biodiesel from Z. bungeanum seed oil containing 10.3% C16:0 and 1.1% C18:0 methyl esters, displaying a CP of -6 °C.12 Thus, as confirmed here for MacME, decreasing the amounts of higher melting saturated fatty esters inherently present in a feedstock is likely the best method for improving cold flow properties. It may be noted that the CP is the parameter contained in the biodiesel standard ASTM D6751, while the European standard EN 14214 prescribes the cold-filter plugging point (CFPP). The CP can be correlated with tests, such as the CFPP, and is more stringent as it relates to the temperature at which the first solids form in the liquid fuel.41 Kinematic Viscosity. The kinematic viscosity at 40 °C of the two MacME fuels was determined to be in the range of 4.4-4.6 mm2/s (Table 3). This result agrees with the kinematic viscosity values of the neat esters comprising MacME, which are33 methyl palmitate (4.38 mm2/s), methyl palmitoleate (3.67 mm2/s), methyl oleate (4.51 mm2/s), methyl asclepate (4.29 mm2/s), methyl linoleate (3.14 mm2/s), and methyl eicosenoate (5.77 mm2/s). The kinematic viscosity of MacME is also within the range of viscosity specified in the American biodiesel standard ASTM D6751 and the European biodiesel standard EN 14214 (Table 3). It may also be noted that the kinematic viscosity determined for MacME-2 is slightly lower than that of MacME-1, which coincides with the fatty acid profile of MacME-1 possessing slightly higher amounts of lower viscosity methyl palmitoleate compared to MacME-2.

Table 4. Lubricity of Macadamia Nut Methyl Esters by HFRR wear scar (μm) sample

x

y

average

ULSD MacME-1 MacME-2 2% MacME-1 in ULSD 2% MacME-2 in ULSD

535 172 179 254 303

509 136 158 210 275

522 154 168 232 289

Oxidative Stability. The oxidative stability of MacME as determined by the Rancimat method EN 14112 is around 2 h (Table 3). Thus, MacME does not meet the oxidative stability requirement in both ASTM D6751 and EN 14214 standards, which prescribe a minimum of 3 and 6 h, respectively. This issue could be addressed by the addition of antioxidants to MacME. For comparison, saturated fatty esters have induction times >24 h (experiments terminated after this time) in the Rancimat test, while the induction time of methyl oleate is 2.79 h, the induction time of methyl linoleate is 0.94 h, and the induction time of methyl palmitoleate is 2.11 h.7 The oxidative stability of MacME is largely determined by its components with reduced oxidative stability, an observation that was predicted in the literature.42 The oxidative stability of MacME is considerably reduced in comparison to the parent oil, an effect also observed for other oils and their alkyl ester derivatives. Possible explanations for the reduced stability of the methyl esters versus the parent oils are that antioxidants naturally present in MNO are either deactivated through the transesterification process and/or removed by the subsequent purification or separation procedures. Lubricity. The lubricity of biodiesel is not only determined by the component alkyl esters but also by minor constituents, such as monoacylglycerols and free fatty acids,33,43,44 although the latter species likely do not a play a role here (low acid value of MacME; see text above). Two tests for each MacME fuel using the HFRR lubricity tester gave average ball wear scars (averages of the two tests per fuel given in Table 4) well below the maximum value limits of 520

(39) Imahara, H.; Minami, E.; Saka, S. Thermodynamic study on cloud point of biodiesel with its fatty acid composition. Fuel 2006, 85, 1666–1670. (40) Rashid, U.; Anwar, F.; Moser, B. R.; Knothe, G. Moringa oleifera oil: A possible source of biodiesel. Bioresour. Technol. 2008, 99, 8175–8179. (41) Dunn, R. O.; Bagby, M. O. Low-temperature properties of triglyceride-based diesel fuels: Transesterified methyl esters and petroleum middle distillate/ester blends. J. Am. Oil Chem. Soc. 1995, 72, 895– 904.

(42) Knothe, G.; Dunn, R. O. Dependence of oil stability index of fatty compounds on their structure and concentration and presence of metals. J. Am. Oil Chem. Soc. 2003, 80, 1021–1026. (43) Hillion, G.; Montagne, X.; Marchand, P. Methyl esters of plant oils used as additives or organic fuel. Ol., Corps Gras, Lipides 1999, 6, 435–438 (in French). (44) Barbour, R. H.; Rickeard, D. J.; Elliott, N. G. Understanding diesel lubricity. SAE Tech. Pap. 2000-01-1918, 2000.

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: DOI:10.1021/ef9013295

Knothe

and 460 μm prescribed in the petrodiesel standards ASTM D975 and EN 590, respectively (details of the wear scars generated by the lubricity tests are given in Table 4). A low lubricity ultra-low sulfur diesel (ULSD) fuel gave average wear scar values of 526 and 532 μm. Adding 2% MacME to this ULSD fuel lead to wear scars (Table 4) also well below the limits in petrodiesel standards. Thus, MacME-1 and MacME-2 display excellent lubricity, which is in accordance with the results on lubricity for biodiesel derived from other oils or fats.34,43,44

oxidative stability, and others, were determined. Generally, the fuel properties of macadamia methyl esters are comparable to those of other biodiesel fuels. The results also show that palmitoleic esters would likely have to be present at higher levels and saturated esters, especially the high-melting species, such as C20 and C22, would likely have to be present at lower levels in a feedstock to achieve fuel property enhancement in terms of cold flow. Investigating biodiesel fuels with even higher amounts of palmitoleic esters and lower amounts of high-melting esters is therefore of interest.

Conclusions

Acknowledgment. The author thanks Kevin R. Steidley and Kim Ascherl (USDA/ARS/NCAUR) for excellent technical assistance, Barrett Mangold (Southwest Research Institute, San Antonio, TX) for cetane testing, and Dr. Karl Vermillion (USDA/ARS/NCAUR) for obtaining the NMR spectra.

Biodiesel was prepared from macadamia nut oil by sodiummethoxide-catalyzed transesterification with methanol. Fuel properties, such as the cetane number, kinematic viscosity,

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