Coriander seed oil methyl esters as biodiesel fuel: Unique fatty acid composition and excellent oxidative stability

biomass and bioenergy 34 (2010) 550–558

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Coriander seed oil methyl esters as biodiesel fuel: Unique fatty acid composition and excellent oxidative stability5 Bryan R. Moser*, Steven F. Vaughn United States Department of Agriculture, Agricultural Research Service, National Center for Agricultural Utilization Research, 1815 N. University St, Peoria, IL 61604, USA

article info

abstract

Article history:

Coriander (Coriandrum sativum L.) seed oil methyl esters were prepared and evaluated as an

Received 23 October 2009

alternative biodiesel fuel and contained an unusual fatty acid hitherto unreported as the

Received in revised form

principle component in biodiesel fuels: petroselinic (6Z-octadecenoic; 68.5 wt%) acid. Most of

21 December 2009

the remaining fatty acid profile consisted of common 18 carbon constituents such as linoleic

Accepted 26 December 2009

(9Z,12Z-octadeca-dienoic; 13.0 wt%), oleic (9Z-octadecenoic; 7.6 wt%) and stearic (octade-

Available online 29 January 2010

canoic; 3.1 wt%) acids. A standard transesterification procedure with methanol and sodium methoxide catalyst was used to provide C. sativum oil methyl esters (CSME). Acid-catalyzed

Keywords:

pretreatment was necessary beforehand to reduce the acid value of the oil from 2.66 to

Biodiesel

0.47 mg g1. The derived cetane number, kinematic viscosity, and oxidative stability (Ran-

Coriandrum sativum L.

cimat method) of CSME was 53.3, 4.21 mm2 s1 (40  C), and 14.6 h (110  C). The cold filter

Fatty acid methyl esters

plugging and pour points were 15  C and 19  C, respectively. Other properties such as acid

Methanolysis

value, free and total glycerol content, iodine value, as well as sulfur and phosphorous

Petroselinic acid

contents were acceptable according to the biodiesel standards ASTM D6751 and EN 14214.

Transesterification

Also reported are lubricity, heat of combustion, and Gardner color, along with a comparison of CSME to soybean oil methyl esters (SME). CSME exhibited higher oxidative stability, superior low temperature properties, and lower iodine value than SME. In summary, CSME has excellent fuel properties as a result of its unique fatty acid composition. Published by Elsevier Ltd.

1.

Introduction

Biodiesel is defined as the monoalkyl esters of long-chain fatty acids prepared from vegetable oils, animal fats, or other lipids [1,2]. Advantages of biodiesel over conventional petroleum diesel fuel (petrodiesel) include derivation from renewable feedstocks, displacement of imported petroleum, superior lubricity and biodegradability, lower toxicity, essentially no sulfur content, higher flash point, and a reduction in most exhaust emissions. Disadvantages include inferior oxidative

and storage stability, lower volumetric energy content, reduced low temperature operability, and higher oxides of nitrogen exhaust emissions [2,3]. Biodiesel must be satisfactory according to accepted fuel standards (Table 1) such as ASTM D6751 [1] in the United States or the Committee for Standardization (CEN) standard EN 14214 [4] in Europe before combustion in diesel engines. Feedstock availability for biodiesel production varies considerably according to geography and climate. Thus, rapeseed/canola oil is principally used in Europe, palm oil

5 Disclaimer: Product names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may also be suitable. * Corresponding author. Tel.: þ1 309 681 6511; fax: þ1 309 681 6524. E-mail addresses: [email protected] (B.R. Moser), [email protected] (S.F. Vaughn). 0961-9534/$ – see front matter Published by Elsevier Ltd. doi:10.1016/j.biombioe.2009.12.022

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biomass and bioenergy 34 (2010) 550–558

Table 1 – Physical properties of coriander and soybean oil methyl esters with comparison to biodiesel fuel standards. Units

a

Yield MWcalc Gardner color AV Free glycerol Total glycerol Low temperature: CP PP CFPP Oxidative stability: IP, 110  C OT Kinematic viscosity: 40  C Wear scar, 60  C Sulfur Phosphorous DCN Heat of combustion: HHV LHV IV a b c d e f

Biodiesel standards

Fatty acid methyl esters

ASTM D6751

EN 14214

Coriander

Soybean

Mass% g mol1 – mg g1 Mass% Mass%

– – – 0.50 max 0.020 max 0.240 max

– – – 0.50 max 0.020 max 0.250 max

94 294.80 10 0.10 (0.01)b 0.005 0.119

98 292.45 1 0.04 (0.03) 0.010 0.156

 

C C  C

Report – –

– – Variabled

n/dc 19 (0) 15 (1)

0 (1) 2 (1) 4 (0)

h C

3.0 min –

6.0 min –

14.6 (0.7) 205.2 (1.1)

5.0 (0.1) 171.6 (1.5)

mm2 s1 mm ppm Mass%

1.90–6.00 –e 15 max 0.001 max 47.0 min

3.50–5.00 –e 10 max 0.0004 max 51.0 min

4.21 (0.01) 167 (3) 4 0.0000 53.3

4.12 (0.01) 136 (4) 1 0.0000 54.0f

– – 120 max

40.10 (0.07) 37.50 89

39.85 (0.05) 37.38 133



MJ kg1 MJ kg1 g I2 100 g1

– – –

Yield (mass%) ¼ mass of observed product divided by the maximum theoretical value. Number in parenthesis represents the standard deviation (n 1⁄4 3). Not determined. Variable by location and time of year. Maximum wear scar values of 520 and 460 mm are specified in petrodiesel standards ASTM D975 and EN 590. From reference [41].

predominates in tropical countries, and soybean oil and animal fats are primarily used in the United States [2,3]. However, the combined supply of these fats and oils is sufficient to displace only a small percentage of petrodiesel at current usage levels. Consequently, alternative feedstocks for biodiesel production have attracted considerable attention, as evidenced by recent reports on jatropha (Jatropha curcas L.) [5], wild mustard (Brassica juncea L.) [6], field pennycress (Thlaspi arvense L.) [7], moringa (Moringa oleifera L.) [8], and camelina (Camelina sativa L.) [9] oils, among numerous others [3]. Vegetable oils are generally composed of five common fatty acids (FA): palmitic (hexadecanoic), stearic (octadecanoic), oleic (9Z-octadecenoic), linoleic (9Z,12Z-octadecadienoic), and linolenic (9Z,12Z,15Z-octadecatrienoic) acids [3,10]. Other FA such as erucic (13Z-docosenoic) acid may be found in plant oils from the Brassicaceae family, of which wild mustard [6] and field pennycress [7] are examples. Biodiesel prepared from vegetable oils containing a significant percentage of less common FA are largely unreported, with the exceptions of capric (decanoic) acid-containing cuphea (Cuphea viscosissima  C. lanceolata) [11] oil and lauric (dodecanoic) acid-containing coconut [12], palm kernel [12], and babassu [13] oils. Fuel properties of biodiesel are largely dependant on the FA composition of the lipid from which it was prepared [3,10]. As a result, biodiesel fuels with different

FA compositions have different fuel properties and may serve as models for other oils with similar FA profiles. Additionally, such oils may guide the genetic modification of existing oilseed crops for optimum biodiesel fuel properties [14]. A plant of commercial significance that contains a vegetable oil with an unusual FA profile is coriander. Coriander (Coriandrum sativum L.), also known as cilantro, is an annual herb belonging to the Apiaceae family that is widely cultivated but is indigenous to southwestern Asia and North Africa [15]. All parts of the plant are edible with the fresh leaves and dried seeds most commonly used as culinary ingredients. The yield of C. sativum seeds is reported to be around 954 kg ha1 [16]. The seeds, which contain 26–29 wt% vegetable oil [17], are also used in perfumery, cosmetic, and medicinal applications [18]. The primary FA constituent in C. sativum oil (CSO) that comprises 31–75% of the FA profile is petroselinic (9Z-octadecenoic) acid, which is an uncommon isomer of oleic acid and is found at high levels in a restricted range of seed oils mostly from the Apiaceae family [19]. Other FA of significance in CSO includes linoleic and oleic acids, along with lesser amounts of stearic and palmitic acids, among others [17]. A volatile essential oil fraction composed of terpenoid and phenolic phytochemicals that have antioxidant and other medicinal properties are also found in coriander and is transported at least in part into the lipid phase during extraction [15,17,18,20–23].

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biomass and bioenergy 34 (2010) 550–558

The objective of the current study was to prepare and evaluate C. sativum oil methyl esters (CSME) as a potential biodiesel fuel. Using standard methods, the following fuel properties were determined: low temperature properties, oxidative stability, cetane number, sulfur content, free and total glycerol content, kinematic viscosity, acid value (AV), phosphorous content, lubricity, heat of combustion, Gardner color, iodine value (IV), FA profile, and tocopherol content. Comparison to ASTM D6751 and EN 14214 as well as to soybean oil methyl esters (SME) were further objectives. Soybean oil methyl esters were chosen for comparison as a result of their common use as biodiesel fuel in the United States as well as their typical composition of the five common FA discussed previously.

2.

Materials and methods

2.1.

Materials

Coriander seeds (cv. Santo) were purchased from Johnny’s Selected Seeds (Winslow, ME), which were ground in a coffee grinder and oil was extracted with hexane for 24 h in a Soxhlet apparatus. Hexane was removed by rotary evaporation (10 mbar, 25  C) to provide C. sativum oil (CSO). Refined, bleached, and deodorized (RBD) soybean oil (SBO) was purchased from KIC Chemicals, Inc. (New Platz, NY). Tocopherol standards (97% purity) were obtained from Matreya, LLC (Pleasant Gap, PA). Fatty acid ethyl ester standards were purchased from Nu-Chek Prep, Inc. (Elysian, MN). All other chemicals and reagents were obtained from Sigma– Aldrich Corp (St. Louis, MO) and used as received.

2.2.

Fatty acid profile

Derivatization of CSO to the FA ethyl esters was required to resolve petroselinate and oleate, since they co-elute as methyl esters [24]. Preparation of the ethyl esters was accomplished using potassium hydroxide catalyst as described previously [24]. The esters were separated using a Hewlett–Packard 5890 Series II GC (Palo Alto, CA) equipped with an FID, an HP series 7673 autosampler/injector, and an SP2380 column (30 m  0.25 mm i.d., 0.20 mm film thickness). Carrier gas was He at 1.0 mL min1. The oven temperature ramped from 170  C to 190  C at 4  C min1, followed by an increase to 265  C at 30  C min1, which was held for 2.5 min. The injector and detector temperatures were 250  C. Peaks were identified (triplicates, means reported) by comparison to the retention times of reference standards.

2.3.

Tocopherol content

Tocopherols were quantified by HPLC according to American Oil Chemists’ Society (AOCS) official method Ce 8-89 [25]. Samples diluted in hexane to a concentration of 50– 100 mg mL1 were analyzed for tocopherol content by a Varian (Palo Alto, CA) HPLC Pro-Star model 230 pump, model 410 autosampler, and model 363 fluorescence detector using excitation and emission wavelengths of 290 and 330 nm, respectively. The mobile phase consisted of hexane:

2-propanol (99.5:0.5 v/v) pumped at a rate of 1 cm3 min1. Samples were injected by autosampler using the full loop option (100 mm3), and tocopherols were separated using an ˚ , 250 mm  4.6 mm Inertsil (Varian), silica column (5 mm, 150 A i.d.). Tocopherol peaks were identified (triplicates, means reported) by comparison to the retention times of reference standards. A mixture of a, b, g, and d-tocopherol standards was injected to verify HPLC response. Samples were quantified using external standard curves.

2.4.

Acid-catalyze d pretreatment of coriander oil

Acid-catalyzed pretreatment of CSO with an initial AV of 2.66 mg KOH g1 was accomplished in a 500 mL three-necked round bottom flask connected to a reflux condenser and a mechanical magnetic stirrer set to 1200 rpm. Initially, CSO (225 g, 250 mL, 0.256 mol) and methanol (88 mL, 2.15 mol, 35 vol%) were added to the flask, followed by drop-wise addition of sulfuric acid (conc., 2.50 mL, 0.045 mol, 1.0 vol%). The contents were heated at reflux for 4 h. Upon cooling to room temperature (rt), the phases were separated. The oil phase was washed with distilled water until a neutral pH was achieved, followed by rotary evaporation (20 mbar; 30  C) to remove residual methanol. Finally, treatment with magnesium sulfate (MgSO4) provided CSO (209 g, 93 wt%) with a final AV of 0.47 mg KOH g1.

2.5.

Methanolysis

Methanolysis of CSO was conducted in a 1 L three-necked round bottom flask connected to a reflux condenser and a mechanical magnetic stirrer set at 1200 rpm. Initially, CSO (436 g, 0.50 mol) and methanol (122 mL; 3.0 mol; 6:1 mol ratio) were added and heated to 60  C (internal reaction temperature monitored by digital temperature probe), followed by the addition of sodium methoxide catalyst (0.50 wt% with respect to CSO). After reacting for 1.5 h the mixture was equilibrated to rt and the lower glycerol phase was removed by gravity separation (>2 h settling time), followed by removal of methanol from the ester phase by rotary evaporation (10 mbar; 30  C). Crude methyl esters were washed with distilled water until a neutral pH was obtained and dried with MgSO4 to yield CSME (94 wt%). SME was prepared in a similar fashion (98 wt%). Refer to Figs. 1 and 2 for the 1H NMR spectra of CSME and SME, respectively.

2.6. 1

Characterization and physical properties

H NMR data were recorded using a Bruker AV-500 spectrometer (Billerica, MA) operating at 500 MHz using a 5-mm broadband inverse Z-gradient probe in CDCl3 (Cambridge Isotope Laboratories, Andover, MA) as solvent. Oxidation onset temperature (OT,  C) was determined (triplicates, means reported) by pressurized differential scanning calorimetry (PDSC) using a DSC 2910 thermal analyzer from TA Instruments (Newcastle, DE). Typically, a 2 mm3 sample, resulting in a film thickness of <1 mm, was placed in an aluminum pan hermetically sealed with a pinhole lid and oxidized with pressurized (1.38 MPa) dry air (Gateway Airgas, St. Louis, MO) in the module with a heating

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B C H H

H3C F

C E H HH H O H H D

HAH

OCH3 B

F C A

PM

5.2

4.8

4.4

4.0

3.6

3.2

2.8

2.4

D

E

2.0

1.6

1.2

0.8

0.4

1

Fig. 1 – H NMR (500 MHz) spectrum of CSME. Methyl petroselinate is shown for peak identification. The multiplet at d 1.2–1.4 ppm corresponds to methylene protons not otherwise identified on the FA backbone.

rate of 10  C min1 from 50 to 350  C. A computer-generated plot of heat flow (W g1) versus temperature ( C) was used to mathematically determine OT. Gross heat of combustion (higher heating value; HHV; MJ kg1) data was collected (triplicates, means reported) utilizing a model C-2000 IKA (Wilmington, NC) analytical calorimeter in isoperibol mode (30  C) following ASTM D4809. A model C-5012 halogen-resistant decomposition vessel was used and was pressurized (30 mbar) with dry oxygen (99.6%; Gateway Airgas, St. Louis, MO). Net heat of combustion (lower heating value; LHV; MJ kg1) was calculated according to the equation listed in ASTM D4809 (LHV ¼ HHV  [0.2122  mass% H]). In the case of SME, methyl linoleate was used to calculate the mass percentage of hydrogen (11.64%) whereas methyl petroselinate (12.24%) was used for CSME, as these were the principle FA components in these fuels (Table 2). Prior to

acquisition of data, HHVs of reference standards (benzoic acid and hexadecane) were measured and found to agree closely with literature values. Physical property data collected according to standard methods were performed in triplicate with mean values reported. Specific gravity (SG) was determined at 25 and 40  C by AOCS Cc 10a-25 using a Kimax 25 cm3 gravity pycnometer from Kimble Chase Life Science and Research Products (Vineland, NJ). Iodine value (IV, g I2 100 g1) was calculated from the FA profile according to AOCS Cd 1c-85. Gardner color was measured on a Lovibond 3-Field Comparator from Tintometer, Ltd. (Salisbury, England) using AOCS Td 1a-64. Sulfur (S, ppm) and phosphorous (P, mass%) were measured by Magellan Midstream Partners, L.P. (Kansas City, KS) according to ASTM standards D5453 and D4951, respectively. Derived cetane number (DCN) was determined by

B

A

A

H3C

G

F

H H H H

HH O

HH

HH

HH

D

C

HH

D

OCH3

E

G

D A

5.5

E

C 5.0

4.5

4.0

3.5

3.0

2.5

B

2.0

F 1.5

1.0

0.5

Fig. 2 – 1H NMR (500 MHz) spectrum of SME. Methyl linoleate is shown for peak identification. Note the presence of the bis-allylic protons (protons labeled as C) in this case versus the spectra in Figs. 1 and 2. The multiplet at d 1.3–1.4 ppm corresponds to methylene protons not otherwise identified on the FA backbone.

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Table 2 – Fatty acid profiles (wt%) of coriander and soybean oils. Fatty acida C16:0 C16:1 c9 C18:0 C18:1 c6 C18:1 c9 C18:1 c11 C18:2 c9, 12 C18:3 c9, 12, 15 Unknown (sum) S Satc S Monounsatd S Polyunsate

Coriander

Soybean

b

5.3 (0.5) 0.3 (0.2) 3.1 (0.1) 68.5 (0.8) 7.6 (0.2) 1.0 (0.1) 13.0 (0.1) – 1.2 8.4 77.4 13.0

10.5 (0.1) – 4.1 (0.1) – 22.6 (0.1) 1.5 (0) 53.6 (0.2) 7.7 (0.1) 0 14.6 24.1 61.3

a For example, C18:1 9c signifies an 18 carbon fatty acid chain with one cis double bond located at carbon 9 (9Z-octadecenoic acid; oleic acid). b Number in parenthesis represents the standard deviation (n ¼ 3). c S Sat ¼ C16:0 þ C18:0. d S Monounsat ¼ C16:1 þ C18:1. e S Polyunsat ¼ C18:2 þ C18:3.

Southwest Research Institute (San Antonio, TX) utilizing an Ignition Quality TesterÔ (IQT) following ASTM D6890. The following data was collected (triplicates, means reported) using equipment described previously [6,7,9,26]: AV (mg KOH g1): AOCS Cd 3d-63; cold filter plugging point (CFPP,  C): ASTM D6371; induction period (IP, h): EN 14112; kinematic viscosity (y, mm2 s1): ASTM D445; lubricity: ASTM D6079; viscosity index (VI): ASTM D2270; free and total glycerol: ASTM D6584; cloud point (CP,  C): ASTM D5773; pour point (PP,  C): ASTM D5949. For a greater degree of accuracy PP was measured with a resolution of 1  C instead of the specified 3  C increment. Average calculated molecular weight (MWcalc, g mol1) was determined by a weighted average method utilizing the FA profiles depicted in Table 2. To avoid providing values that were artificially low, unknown constituents were assumed to be petroselinic and oleic acids in the cases of CSO and SBO, and the corresponding methyl esters.

[17,19,21]. In addition, a volatile essential oil fraction was detected by GC and comprised 6.3 wt% of crude CSO. The chemical composition of the essential oil is reported elsewhere and is dominated by linalool, with other phytochemicals such as g-terpinene, camphor, geranyl acetate, geraniol, limonene, a-pinene, and borneol also found in significant quantities, along with numerous other minor constituents [15,18,22,23,27]. The average MW of CSO calculated from the FA profile was 880.43 g mol1 and the AV was measured as 2.66 mg KOH g1 (Table 3). The Gardner color of crude CSO was 11 (1 is lightest, 18 is darkest), which was significantly more opaque than commercial RBD SBO (1, Table 3). The low temperature properties (Table 3) of CSO were considerably better than SBO (PP: 9  C) with a PP of 26  C. Since CP (SBO: 7  C) is an optical method and requires translucent samples for determination, CP was not measured as a result of the opaqueness of crude CSO. Oxidative stability (Table 3) of CSO was quantified by Rancimat (EN 14112) and PDSC methods through measurement of IP (21.0 h; EN 14112; 110  C) and OT (214.9  C; PDSC) and was found to be superior to SBO (8.3 h and 181.7  C, respectively). Kinematic viscosity (Table 3) was determined at 25, 40, and 100  C and yielded results for CSO of 25.13, 15.84, and 4.91 mm2 s1, respectively. The calculated VI of CSO was higher than that found for SBO (228) with a value of 273. Specific gravity (Table 3) values of 0.886 and 0.877 were obtained for CSO at 25 and 40  C, respectively, and were lower than the corresponding values for SBO. As expected, the specific gravities

Table 3 – Physical properties of coriander and soybean oils. Coriander

Oil content Gardner color MWcalc AV

wt% – g mol1 mg g1

26–29b 11 880.43 2.66 (0.03)d/ 0.47 (0.01)e

18–22c 1 873.31 0.03 (0.03)

Low temperature: CP PP



n/df 26 (1)

7 (1) 9 (1)

h C

21.0 (0.1) 214.9 (2.7)

8.3 (0.2) 181.7 (0.3)

mm2 s1 mm2 s1 mm2 s1

25.13 (0.09) 15.84 (0.04 4.91 (0.03)

52.11 (0.09) 31.49 (0.03) 7.67 (0.01)

273

228

0.886 (0.001) 0.877 (0.001) 163 (2)

0.919 (0.001) 0.910 (0.001) 124 (2)

Oxidative stability: IP, 110  C OT

3. 3.1.

Results and discussion Composition and properties of coriander oil

The primary FA detected in CSO was petroselinic acid (68.5 wt%; Table 2), with linoleic (13.0 wt%) and oleic (7.6 wt%) acids constituting most of the remaining FA profile. Minor constituents included palmitic (5.3 wt%), stearic (3.1 wt%), and vaccenic (11Z-octadecenoic; 1.0 wt%) acids, with a trace amount of palmitoleic (0.3 wt%) acid also identified. Coriander oil was characterized by a high percentage of monounsaturated FA (77.4 wt%) largely as a result of the combined petroselinic and oleic acid content. Polyunsaturated FA comprised 13.0 wt% of CSO, with saturated FA (8.4 wt%) constituting the remaining content. These results are in accordance with previous reports on the FA profile of CSO

Soybeana

Units

Kinematic viscosity: 25  C 40  C 100  C Viscosity index Specific gravity: 25  C 40  C Wear scar, HFRR, 60  C



C C



–

– – mm

a Refined, bleached, and deodorized. b From reference [17]. c From references [46,47]. d Number in parenthesis represents the standard deviation. e Before (2.66) and after (0.47) acid-catalyzed pretreatment. All other data is for crude (non-treated) CSO. f Not determined.

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biomass and bioenergy 34 (2010) 550–558

and kinematic viscosities of CSO and SBO decreased with increasing temperature. The wear scar generated by CSO according to the high-frequency reciprocating rig (HFRR) lubricity method (ASTM D6079, 60  C) was 163 mm (Table 3), which was longer than that found for SBO (124 mm). The primary tocopherol in CSO was a-tocopherol (196 ppm), with g- and d-tocopherols (18 and 15 ppm, respectively) found at significantly lower concentrations. The overall tocopherol content of CSO was 229 ppm, which was lower than the previously reported contents of crude (1300–1600 ppm) and RBD (757 ppm) SBO [28,29].

3.2.

Preparation of coriander oil methyl esters

Homogenous base-catalyzed transesterification of CSO afforded CSME in high yield (94 wt%) employing classic conditions described previously [3,5–9,11,30,31]. The AV of crude CSO (2.66 mg KOH g1; Table 3) was prohibitively high for direct methanolysis. Previous studies determined that FFA content greater than 0.50 wt% (AV of 1.0 mg KOH g1) was detrimental to the yield of FAME produced by homogenous base-catalyzed transesterification [5,31]. Free fatty acids react with homogenous base catalysts such as sodium methoxide to form soap (sodium salt of FA) and methanol (or water in the case of sodium hydroxide), thus irreversibly quenching the catalyst and reducing product yield [3,32]. Therefore, sulfuric acidcatalyzed pretreatment of crude CSO with methanol was conducted prior to base-catalyzed transesterification to reduce the AV to 0.47 mg KOH g1 (Table 3) utilizing reaction conditions described previously [7,8]. Soybean oil methyl esters were prepared directly from RBD SBO in excellent yield (98 wt%). The lower yield of CSME versus SME was likely a result of the higher AV of acid-pretreated CSO in comparison to RBD SBO. Optimization of CSME yield using techniques such as response-surface methodology was considered beyond the scope of the current study. The methyl esters prepared from CSO satisfied the specifications for free and total glycerol content contained in the biodiesel standards ASTM D6751 and EN 14214 with values of 0.005 and 0.119 mass%, respectively (Table 1). The average MW of CSME calculated from the FA profile was 294.80 g mol1 (Table 1), which was similar to SME (292.45 g mol1). As a result of its preparation from crude CSO, the Gardner color of CSME (10; Table 1) was considerably darker than that found for SME (1). The essential oil content was reduced from 6.3 wt% in crude CSO to less than 1 wt% in CSME. Such a reduction was attributed to the volatile nature of the relatively low MW essential oil constituents, as both acid-catalyzed pretreatment and methanolysis required elevated reaction temperatures and application of vacuum (3 kPa) during purification. Preparation of CSME resulted in a reduction in tocopherol content of 14% (Table 4), as the amount after methanolysis was reduced to 191 ppm. For comparison, the amounts in SME prepared from crude and RBD SBO were reported as 1301 [33] and 757 ppm [30], respectively. Retention of tocopherol content was not surprising, as they are non-polar and should partition into hydrophobic materials such as biodiesel during purification. Previous studies also reported that tocopherol content was largely retained after transesterification [6,7]. Retention of tocopherols is beneficial, as they inhibit oxidation [33].

Table 4 – Tocopherol content (ppm) of coriander oil, along with the corresponding methyl esters. Oil a-Tocopherol b-Tocopherol d-Tocopherol g-Tocopherol S Tocopherolsb

196 (5)a 0 15 (1) 18 (1) 229 (5)

Methyl esters 162 0 13 16 191

(4) (1) (1) (4)

a Number in parenthesis represents the standard deviation from the reported mean (n ¼ 3). b S Tocopherols ¼ total tocopherol content.

The 1H NMR spectrum of CSME (Fig. 1) was qualitatively similar to spectra of FAME reported elsewhere [7,8,34]. The 1H NMR spectrum of SME (Fig. 2) differed subtly from the spectrum of CSME. For example, the spectrum of SME contained a bis-allylic proton signal at 2.8 ppm (labeled as C in Fig. 2) that was essentially absent from the spectrum of CSME. Another more subtle difference was the presence of a small triplet at 1.0 ppm in the spectrum of SME (left of the peak labeled as G in Fig. 2) that was absent in the spectrum of CSME. This triplet in SME was the signal for the terminal methyl protons of methyl linolenate, which was absent in CSME. Lastly, the spectrum of CSME contained a larger number of minor unlabelled signals than the spectrum of SME. These minor signals, such as those around 1.1 ppm and 3.2–3.4 ppm in Fig. 1, were attributed to the presence of a small amount of essential oil constituents.

3.3.

Oxidative stability

The Rancimat method (EN 14112) is listed as the oxidative stability specification in ASTM D6751 and EN 14214. A minimum IP (110  C) of 3 h is required for ASTM D6751, whereas a more stringent limit of 6 h or greater is specified in EN 14214. The IP of CSME easily satisfied these specifications with a value of 14.6 h (Table 1). The IP of CSME was unusually high, especially when compared to SME (5.0 h). Palm oil methyl esters, which are widely regarded as being stable to oxidation as a result of their high content of saturated and low content of polyunsaturated FAME, yielded an IP of 10.3 h in a previous study [30]. An important factor that contributed to the high oxidative stability of CSME was the absence of trienoic (C18:3) FAME. The rate of autoxidation of FAME is dependant on the presence of double bonds that are separated by allylic methylene positions (AMP; labeled as C in Fig. 1, for instance), with bisallylic methylene positions (BAMP; labeled as C in Fig. 2) even more susceptible to oxidation. Polyunsaturated FAME are particularly vulnerable to autoxidation due to the presence of BAMP, as evidenced by the reported relative rates of oxidation for the unsaturates: 1 for ethyl oleate (2 AMP), 41 for ethyl linoleate (2 AMP and 1 BAMP), and 98 for ethyl linolenate (2 AMP and 2 BAMP) [35]. The IP of methyl esters of oleic, linoleic, and linolenic acids were reported as 2.5, 1.0, and 0.2 h, respectively [36]. Methyl petroselinate (IP 3.5 h) was more stable to oxidation than methyl oleate [36]. As seen in Table 2, SBO contained trienoic FA (7.7 wt%) whereas CSME did not. Additionally, CSO and SBO had combined polyunsaturated

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contents of 13.0 and 61.3 wt%, respectively. However, FA profile alone cannot explain the high oxidative stability of CSME, since the primary FAME constituents of CSME had reported IP values significantly below those observed for CSME. It is unlikely that tocopherol content was a significant contributing factor to the oxidative stability of CSME, as it contained a lower concentration (Table 4) than reported for the less oxidatively stable RBD SME [30]. It is speculated that the low level of essential oil constituents in CSME acted as inhibitors of oxidation, as they have been previously reported to possess such activity [20–23]. Oxidative stability results obtained by PDSC corroborated those collected by the Rancimat method, as indicated by OT values of 205.2 and 171.6  C for CSME and SME. The OT is defined as the temperature at which a rapid increase in the rate of oxidation is observed at constant pressure (1378.95 kPa). Higher OT values indicate greater stability to oxidation. As was the case with the IP data, CSME was more stable to oxidation than SME.

3.4.

Low temperature operability

The low temperature properties of CSME were measured by CFPP, and PP. As seen in Table 1, CSME provided CFPP and PP values of 15 and 19  C, respectively. The CP of CSME was not determined because the sample was too opaque for measurement (Gardner color of 10; Table 1). The values obtained for SME were 0 (CP), 4 (PP), and 2  C (CFPP), respectively. The enhanced low temperature operability of CSME versus SME may be attributed to its lower content of saturated FAME (see Table 2). Previous studies have determined that relatively small levels of saturated FAME have a disproportionate effect on low temperature properties of biodiesel as a result of their considerably higher melting points [30,37]. Melting point (mp) decreases with increasing double bond content in otherwise similar FAME, as indicated by the mp of methyl esters of stearic (C18:0; 37.7  C), oleic (C18:1; 20.2  C), linoleic (C18:2; 43.1  C), and linolenic (C18:3; 57  C) acids [38].

3.5.

Other physical properties

The kinematic viscosity of CSME was 4.21 mm2 s1 (Table 1) at 40  C, which was satisfactory according to the specified ranges in ASTM D6751 (1.9–6.0 mm2 s1) and EN 14214 (3.5– 5.0 mm2 s1). The value obtained for SME (4.12 mm2 s1) was satisfactory according to the biodiesel standards and concurred with several previous studies [6,9,26,30,33,34,39,40]. The maximum allowable limit specified in ASTM D6751 and EN 14214 for AV is 0.50 mg KOH g1. The AV of CSME was significantly below the specified maximum limit with a value of 0.10 mg KOH g1 (Table 1). The AV of SME was satisfactory as well with a value of 0.04 mg KOH g1. The DCN of CSME was above the minimum limits of 47 and 51 specified in ASTM D6751 and EN 14214, respectively, with a value of 53.3 (Table 1). For comparison, a previous study reported the DCN of SME as 54.0 [41]. Results generated by ASTM D6890 (DCN) generally correlate with CN determination by ATSM D613. The ASTM D6890 method is now approved as an alternative to the more traditional CN method (ASTM D613) specified in ASTM D6751 [1].

The sulfur content of CSME was 4 ppm (Table 1), which was satisfactory according to the specified maximum allowable limits in ASTM D6751 and EN 14214 of 15 and 10 ppm, respectively. SME was essentially free of sulfur (1 ppm). In addition, phosphorous content is limited in ASTM D6751 and EN 14214 to maximum values of 0.001 and 0.0004 mass%, respectively. Neither CSME nor SME contained phosphorous (0.0000 mass%; Table 1). Although ASTM D6751 does not contain an IV specification, EN 14214 limits IV to a maximum value of 120 g I2 100 g1. This restriction disqualifies SME as biodiesel in the neat form in Europe, as it had an IV in excess of 120 (133; Table 1). However, CSME was well below the maximum limit with a value of 89. The low content of polyunsaturated FAME was responsible for the low-IV of CSME, especially in comparison to SME. Blending low-IV FAME with biodiesel that has an unacceptably high IV is a method by which IV can be lowered [30]. CSME may have use in this regard. Lubricity (ASTM D6079) is not specified in ASTM D6751 or EN 14214 but is included in the petrodiesel standards ASTM D975 and EN 590 with maximum prescribed wear scars (60  C) of 520 and 460 mm, respectively. Fuels with poor lubricity may cause failure of diesel engine parts that rely on lubrication from fuels, such as fuel pumps and injectors [2]. The wear scar generated by CSME according to ASTM D6079 (60  C) was 167 mm (Table 1). As expected, the lubricity of CSME was considerably below the maximum limits contained in the petrodiesel standards, which was in agreement with several previous studies indicating that biodiesel possessed inherent lubricity [6–9,11,26,30,40,42]. The wear scar generated by SME (133 mm) was also satisfactory according to the petrodiesel standards. The wear scar produced by unadditized petrodiesel (<15 ppm S) was reported as 525–550 mm [9,11,26,43]. Blending with biodiesel is a method by which the lubricity of petrodiesel may be improved [2,3,9,11,26,42,43]. Although not specified in either ASTM D6751 or EN 14214, heat of combustion is important because it influences fuel efficiency and consumption. The gross heat of combustion (higher heating value; HHV) of CSME was determined by bomb calorimetry (ASTM D4809) to be 40.10 MJ kg1, which resulted in a calculated net heat of combustion (lower heating value; LHV) of 37.50 MJ kg1. The corresponding values for SME agreed closely with previous reports [44,45] with values of 37.38 MJ kg1 (LHV) and 39.85 MJ kg1 (HHV). Energy content of biodiesel generally increases with increasing chain length and decreases with increasing unsaturation [3,43]. The higher results obtained for CSME may be attributed to its lower content of polyunsaturated FAME, as well as its lower content of shorter-chain FAME (C16), as seen in Table 2.

4.

Conclusions

Coriander (C. sativum L.) seed oil methyl esters were evaluated as an alternative biodiesel fuel and were prepared in 94 wt% yield by a standard transesterification procedure with methanol and sodium methoxide catalyst. Acid-catalyzed pretreatment was necessary beforehand to reduce the AV of the oil from 2.66 to 0.47 mg KOH g1. Coriander oil contained

biomass and bioenergy 34 (2010) 550–558

a high level of petroselinic acid (68.5 wt%) hitherto unreported as the principle FA component in biodiesel fuels. The DCN, kinematic viscosity, and IP values of CSME were 53.3, 4.21 mm2 s1 (40  C), and 14.6 h (110  C), respectively, all of which satisfied the ASTM D6751 and EN 14214 biodiesel fuel standards. The CFPP and PP values were 15 and 19  C, respectively. Other fuel properties of CSME such as AV, free and total glycerol content, IV, as well as sulfur and phosphorous contents were satisfactory according to ASTM D6751 and EN 14214. Additional properties such as heat of combustion and lubricity were measured and found to be comparable to SME. In summary, CSME possessed superior oxidative stability and low temperature properties, along with a lower IV, when compared to SME. Consequently, CSME appears to have attractive fuel properties as a result of its unique FA composition.

Acknowledgement The authors acknowledge Dr. Terry A. Isbell for determination of the fatty acid profile of coriander oil. Ms. Benetria N. Banks and Mr. Ray K. Holloway are acknowledged for excellent technical assistance, along with Dr. Karl Vermillion for acquisition of NMR data.

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