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Current Green Chemistry, 2015, 2, 392-398
Reforming Biodiesel Fuels via Metathesis with Light Olefins Sean P. Mc Ilratha, Barada P. Dasha, Michael J. Topinkaa and Douglas A. Klumppa,* a
Department of Chemistry and Biochemistry, Douglas A. Klumpp, Northern Illinois University, DeKalb, Illinois, 60115 United States Abstract: Fatty-acid methyl esters (FAME) are considered a renewable source of raw materials for the chemical industry, including their use in biodiesel fuel mixtures. In the following manuscript, we describe the cross-metathesis reactions of a corn oil-derived FAME mixture with isobutylene and other branched light olefins. Cross-metathesis reactions are also reported with styrene. Cross-metathesis reactions were done with methyl oleate as a reference. These reforming reactions were done in an effort to increase the volatility of the fuel mixtures, as well as introduce branching or the phenyl group into the hydrocarbon chains. The resulting metathesis products contain high percentages of biorenewable carbon (46-100%). The chemistry represents a new approach to the preparation of green fuels - as a biorenewable stream is merged with high volume petroleum-derived streams.
Keywords: Biodiesel, biorenewable, catalysis, fatty-acid methyl esters, olefin metathesis, reforming. 1. INTRODUCTION Biodiesel fuels are important products generated from plant oils, animal fats, waste materials and other sources [16]. Fatty acid methyl ester (FAME) mixtures are considered an alternative to fossil fuels and moreover they can potentially serve future energy needs arising from dwindling fossil fuel resources [7]. As a transportation fuel, biodiesel blends have the advantage of being biorenewable, biodegradable, and almost carbon neutral [7, 8]. However, biodiesel fuels have several disadvantages over petroleum derived diesel fuels. Most biodiesel fuels exhibit relatively high density and viscosity, while also having decreased volatility. These factors can have severe effects on fuel performance, especially at low temperatures [9-12]. Mixtures of fatty acid methyl esters also have less energy content (ca. 5-11%) than petroleum-based diesel fuels. Properties such as density, volatility, and viscosity depend on molecular weight, length of the carbon chain, and branching [13-17]. It is also known that fuel mixtures tend to have greater energy content with increasing degrees of unsaturation and aromatic components. Combustion performance is also enhanced by incorporation of branching into the hydrocarbon chain of fuels [15-17]. Plant oils and animal fats are triglycerides composed of straight-chain fatty acids and this limits their potential as feedstock for high-quality fuels [14]. Unsaturated fatty acids and their derivatives have been modified by chemical conversions at the carbon-carbon double bond. Chemical modifications have for example utilized hydrogenation, hydroformylation, oxidation, hydrosilylation, hydroboration, and olefin metathesis, to prepare novel derivatives of these oils [18-24]. Olefin metathesis is a particularly *Address correspondence to this author at the Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois, 60115, United States; Tel: (815)753-1959; Fax: (815)753-4802; E-mail:
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useful method for incorporating new functionality and modifying the structures of unsaturated substrates [25] Both unsaturated triglycerides and the esters of unsaturated fatty acids have been used in the olefin metathesis reactions [24], and cross-metathesis has been done using a variety of olefins such as ethylene, methyl acrylate, 2-butene, 1-hexene, and cis-2butene-1,4-diyl diacetate etc [26-30]. These previous studies suggested that FAME biodiesel might be catalytically reformed using cross-metathesis and branched olefins or aromatic olefins. High-volume petrochemicals such as isobutylene and styrene are attractive reagents for this chemistry, because they are relatively inexpensive and these olefins would incorporate branching and aromatic rings into the biofuel mixture. In the following manuscript, we report our studies of the cross-metathesis reactions of FAME biodiesel with branched light olefins and styrene. The catalytic reforming reactions are found to incorporate branching and aromatic groups into the hydrocarbon chains. Gas chromatography analysis also indicates that the reformed fuel has a decreased average molecular weight and increasing volatility. 2. MATERIALS AND METHOD 2.1. General Consumer-grade corn oil was used in the preparation of FAME biodiesel fuel. All olefins, solvents, methyl oleate, and the Grubbs 2nd generation catalyst (Grubbs 2) were purchased from commercial suppliers and used as received. Samples were analyzed using commercial GC-FID and GCMS instruments equipped with capillary DB-5 columns. The metathesis product distributions were obtained from peak area integration obtained from the GC-FID instrument. FID relative response factors were evaluated for pure compounds (methyl stearate, methyl oleate, and 1-octene) against an external standard (naphthalene). Molecular weights were determined from mass spectra and the components were identified with reference to a NIST mass-spectra library. © 2015 Bentham Science Publishers
Reforming Biodiesel Fuels via Metathesis with Light Olefins
Current Green Chemistry, 2015, Vol. 2, No. 4
Reactions with isobutylene were performed in a mechanically stirred, high-pressure reactor equipped with a glass-lined reactor vessel. Reaction vessels were flushed with argon prior to their use. 2.2. Preparation of FAME Biodiesel Corn oil (100 mL) is placed in a separatory funnel with methanol (15 mL) and NaOH solution (10 M, 1 mL) is added to it. This mixture is shaken for several minutes, after which, it is washed with brine (3x). The clear organic layer is collected and dried over MgSO4 and filtered through a short pack of silica gel.
3. RESULTS AND DISCUSSION 3.1. FAME Mixture and Light Olefins A representative FAME biodiesel mixture was prepared using base-catalyzed transesterification of corn oil with methanol. In accord with other preparations of the corn oilderived FAME, analysis of the product oil showed a mixture of primarily methyl palmitate (1), methyl oleate (2), methyl linoleate (3), and methyl stearate (4) (Fig. 1). O OCH3
2.3. Cross-metathesis Reactions of Methyl Oleate
1, Methyl palmitate
Procedure A: Methyl oleate (0.2 g, 0.67 mmol), the olefin substrate are combined (1.0 mmol of 3-methyl-1butene, 4-methyl-1-pentene, or 3,3-dimethyl-1-butene; 0.67 mmol of styrene; 0.33 mmol isoprene) and dissolved in CH2Cl2 (2 mL). To this solution Grubbs 2 catalyst (11.45 mg, 2 mol%, ~50 ppm) is added to the solution with stirring. After stirring 6 hr at 25°C, the mixture is filtered through a pack of chromatography grade silica gel. Procedure B: Isobutylene (ca. 10 mL) is condensed into a cooled (-78°C) flask. Methyl oleate (1.0 g, 3.4 mmol) is dissolved in CH2Cl2 (10 mL) placed into the glass-lined pressure reactor and the solution is cooled to -78 °C. The isobutylene solution is then added to the pressure vessel, along with Grubbs 2 catalyst (57 mg, 2 mol%, ~50 ppm). The solution is warmed to 25°C in the sealed vessel and stirred for 6 hrs. The mixture is then filtered through a pack of chromatography grade silica gel. 2.4. Cross-metathesis Reactions of Corn Oil FAME Procedure A: Corn oil biodiesel (4 mL, ca. 12.3 mmol) and the olefin substrate are combined (18.5 mmol of 3-methyl-1butene, 4-methyl-1-pentene, or 3,3-dimethyl-1-butene; 12.3 mmol of styrene; 6.2 mmol isoprene). To this mixture Grubbs 2 catalyst (24 mg, 6 ppm) is added to the solution with stirring. After stirring 6 hr at 25°C, the mixture is filtered through a pack of chromatography grade silica gel. Procedure B: Isobutylene (ca. 10 mL) is condensed into a cooled (-78°C) flask. Corn oil biodiesel (10 mL, 30.7 mmol) is added to the cold isobutylene and placed into the glass-lined pressure reactor. Grubbs 2 catalyst (60 mg, 6 ppm) is added and the vessel is sealed. The solution is stirred for 6 hrs and then filtered through a pack of chromatography grade silica gel. Catalyst:
OCH3 2, Methyl oleate
3, Methyl linoleate
Cl
OCH3 57.0%
O OCH3 4, Methyl stearate
2.2%
Fig. (1). Major components and relative amounts of fatty acid methyl esters from a corn biodiesel mixture.
As a typical biodiesel fuel, this mixture has an average molecular weight around 295 g/mol and it is composed of C16 and C18 fatty acid esters. Over 80% of the FAME contains unsaturation - a necessity for catalytic reforming using olefin metathesis. A convenient cross-metathesis catalyst is the Grubbs 2nd generation catalyst (5, Grubbs 2; Fig. 2), as it possesses good solubility, high catalytic activity, and it is stable to air and moisture. Grubbs 2 has previously been used in the ethenolysis of FAME mixtures [25b]. In order to incorporate branching into the hydrocarbon chain, cross-metathesis reactions are done with branched, light olefins (A-E), while aromatic structures (phenyl groups) are incorporated into the fuel by cross-metathesis with styrene (F).
Mes
B, 3-methyl-1butene
C, Isoprene
Ph Cl
26.1%
O
A, Isobutylene N
10.9%
O
Olefins:
Mes N
393
Ru PCy3
5, Grubbs 2 D, 3,3-dimethyl-1butene
Fig. (2). Catalyst and olefins used in the reforming reaction of FAME biodiesel.
E, 4-methyl-1pentene
F, Styrene
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Mc Ilrath et al.
methyl-1-pentene E, a small amount of the olefin dimer D1 is formed in the reaction. With the exception of product 8, the cross-metathesis products all contain branching in the hydrocarbon chain. Moreover, analysis indicates that the average molecular weight has decreased and the volatility has increased. For example with 3-methyl-1-butene B, the average molecular weight decreases from 296 g/mol (methyl oleate, 2) to about 190 g/mol. The increased volatility of the reformed mixture is evident from the decreased average retention times in the GC analysis. Whereas methyl oleate 2 has a retention time of 13.5 min, the average retention time of the 3-methyl-1-butene B mixture is at about 9 minutes. The best yields of cross-metathesis products were obtained from 3-methyl-1-butene B (9,10B 61%) and 4-methyl-1pentene E (9,10E 39%).
3.2. Cross-metathesis with Methyl Oleate Methyl oleate (2) is a major component of the cornderived FAME, as well as in FAME mixtures from canola, sunflower, soybean, and other oils. Our initial experiments sought to established product distributions from crossmetathesis reactions between pure methyl oleate and olefins A-F. Methyl oleate is known to undergo a self-metathesis reaction in the presence of Grubbs 2 and other metathesis Catalysts [29]. This green chemistry has been used to generate feedstock for the oleochemical industry [23, 31, 32] In accord with previous studies, we obtained the stereoisomeric mixture of products 6 (9-octadecene) and 7 (dimethyl 9octadecenedioate) from the self-metathesis reaction, as well as unreacted methyl oleate (2) (Scheme 1). In the crossmetathesis reaction of methyl oleate and isobutylene A, several notable observations can be made. The methyl oleate self-metathesis products (6 and 7) are formed in the conversion, but significantly, cross-metathesis products 8, 9A, and 10A are also formed in significant quantities. Both 9A and 10A have a branched hydrocarbon chains intheir structure. It can also be inferred that ethylene is produced in the reaction, however volatile product gases were not analyzed. An unidentified product (U1, 152 g/mol) is also formed as a minor product. Cross-metathesis reactions between methyl oleate 2 and branched alkene B, D, and E gave similar product distributions (Scheme 1 and Table 1). Although self-metathesis products 6 and 7 are formed in each case, the crossmetathesis products (8 and 9B,D,E and 10B,D,E) are formed as significant components of the mixtures. In the case of 4-
The effect of time on the cross-metathesis was also examined. Using olefin E and general procedure A, it was found that a consistent mixture of products was obtained within 15 minutes. Little change in product composition was observed with extended reaction times. A significant amount of ethylene is produced at the initial stages of the reaction but little is produced as the mixture reaches equilibrium. In the cross-metathesis reaction of isoprene C and methyl oleate 2, about 70% of the methyl oleate remained unreacted (Table 1). Small amounts of the desired cross-metathesis products (9,10C) were obtained. Isoprene has previously been used in cross-metathesis reactions with simple olefins and Grubbs 2 catalyst, however, relatively low yields were also obtained in these reactions [33]. O
O 14
O
15 7
5
7
O
O
O O
6
13
O
6
Grubbs-II, RT 6h, CH2Cl2
O
O
O
O
6 Grubbs-II, RT 6h, CH2Cl2
O
5
O
O
12
18
5
O
O
19
7
D2
O
O
O
17
16
Grubbs-II, RT 6h, CH2Cl2
7
O
Grubbs-II, RT 6h, CH2Cl2
5
O 2
O
O
O
O
6
Grubbs-II, RT 6h, CH2Cl2
Grubbs-II, RT 6h, CH2Cl2
Grubbs-II, RT 6h, CH2Cl2 Self Metathesis
8
7
10 O
7 O 9
O
O 6
O
O
5 O
O 6
(Scheme 1). Products of metathesis of methyl oleate with light olefins.
D1
O
O
5
O
11
5
O
Reforming Biodiesel Fuels via Metathesis with Light Olefins
Table 1.
Current Green Chemistry, 2015, Vol. 2, No. 4
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Composition of metathesis products of methyl oleate and corn biodiesel. Metathesis Products of Methyl Oleate
Metathesis Products of Corn Biodiesel
Olefin
c
Self Metathesis
Isobutylene (A)
3-methyl-1butene (B)
Isoprene (C)
3,3-dimethyl-1butene (D)
4-methyl-1pentene (E)
Styrene (F)
Yield (%)
Product
Mol.Wt.a
Yield (%)b
Product
Mol.Wt.a
Yield (%)b
252 296 340
18.3 53.4 28.3
14 15 6 13
168 210 252 254
7.7 19.4 14.7 7.6
1 3 2 4
270 294 296 298
8.3 10.5 8.2 1.8
8 9A U1 10A 6 7 2
140 168 152 212 252 340 296
3.0 11.4 4.7 14.6 15.7 14.8 34.8
11A 8 U1 14 9A U4 15 10B
126 140 152 168 168 180 210 212
6.3 3.0 4.1 5.7 8.6 1.0 13.8 8.2
6 13 1 3 2 4 7
252 254 270 294 296 298 340
1.9 3.0 10.6 2.7 7.0 1.8 3.4
8 U1 9B 10B 6 2
140 152 182 226 252 296
11.5 13.6 28.8 32.9 3.1 6.4
11B 8 U1 14 9B 15 10B
140 140 152 168 182 210 226
13.3 7.4 4.4 4.7 15.7 9.0 6.5
U2 6 13 1 2 3 4
222 252 254 270 296 294 298
4.6 4.9 4.4 6.4 2.7 1.0
