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Metathesis of methyl soyate with ruthenium catalysts
Fuel 85 (2006) 393–395 www.fuelfirst.com
Metathesis of methyl soyate with ruthenium catalysts RonaldA. Holser*, KennethM. Doll, SevimZ. Erhan Food and Industrial Oils Research Unit, USDA-ARS-NCAUR, 1815 North University Street, Peoria, IL 61604, USA Received 9 December 2004; accepted 28 July 2005 Available online 24 August 2005
Abstract Three ruthenium catalysts were investigated for the metathesis reaction of methyl soyate. Dichlorotris (triphenylphosphine) ruthenium II and bis (tricyclohexyl phosphine) benzylidine ruthenium (IV) dichloride displayed no reactivity at 40 8C and atmospheric pressure. However, ruthenium [1,3-bis-(2,4,6-trimethylphenyl)-2-imidazolidinylidene] dichloro (phenylmethylene) tricyclohexylphosphine, Grubbs second generation catalyst, exhibited high activity at these mild conditions. After 2 h, 46% of the unsaturated methyl esters in a commercial methyl soyate were metathesized. This suggests a method to modify the structure of methyl soyate to improve diesel fuel performance properties. q 2005 Elsevier Ltd. All rights reserved. Keywords: Methyl soyate; Metathesis; Biodiesel
1. Introduction Oilseed crops such as soybean and canola provide a renewable source of triglycerides that can be converted to the corresponding fatty acid esters for use as an alternative diesel fuel [1,2]. The combustion and flow properties of such a fuel are determined primarily by the fatty acid composition of the original oil while the alcohol moiety exhibits a minor influence [3,4]. In addition, the alcohol used for transesterification is typically selected based on cost and availability. Methanol is a common choice, although, ethanol may be economical if a fermentation process is located nearby. The structure of the alkyl moiety presents greater opportunity to alter the fuel properties and thereby improve performance. Methods such as pyrolysis, catalytic cracking, and hydroformylation are available to modify alkyl structures and have been applied to vegetable oils [5,6]. These techniques are routinely applied in the petrochemical industry but require elevated temperatures and pressures. Olefin metathesis reactions offer another approach to achieve structural modification of unsaturated fatty acid esters [7,8]. Metathesis catalysts were originally developed * Corresponding author. Tel.: C1 309 681 6111; fax: C1 309 681 6340. E-mail address: [email protected] (R.A. Holser).
0016-2361/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2005.07.018
by the petroleum industry although applications to vegetable oils are noted [9,10]. These versatile catalysts promote reactions at the unsaturated sites of the reactants with minimal influence on substituent groups [11,12]. While the early metathesis catalysts were difficult to prepare and handle the recently developed Ru-complex catalysts have greater stability [13,14]. The interest in these newer catalysts is to convert a fraction of the unsaturated fatty esters in methyl soyate, e. g. oleate, linoleate, and linolenate, to alkenes and unsaturated diesters. Based on structure– property considerations such a reformulation of the fuel is expected to improve the viscosity and lubricity without decreasing storage stability or adversely effecting biodegradability [15,16].
2. Experimental 2.1. Materials Methyl soyate was obtained from Soygold (Ag Environmental Products, LLC, Omaha, NE, USA). Methyl oleate, methyl linoleate, and methyl linolenate standards of greater than 99% purity were supplied by (Nu-check Prep., Inc., Elysian, MN, USA). Ruthenium catalysts A, dichlorotris (triphenylphosphine) ruthenium II, CAS 15529-49-4; and B, bis (tricyclohexyl phosphine) benzylidine ruthenium (IV) dichloride, CAS 172222-30-9, were obtained from Strem Chemicals, Inc. (Newburyport, MA, USA). Grubbs second
R.A. Holser et al. / Fuel 85 (2006) 393–395
generation catalyst, C, ruthenium, [1,3-bis-(2,4,6-trimethylphenyl)-2-imidazolidinylidene] dichloro (phenylmethylene) tricyclohexylphosphine, CAS 246047-72-3, was obtained from Sigma-Aldrich (St Louis, MO, USA). All chemicals were used as received. 2.2. Procedure Reactions were performed by placing 10-g samples of the substrate, e. g. methyl oleate, methyl linoleate, methyl linolenate, or biodiesel into tared glass vials with a small magnetic stir bar. Catalyst was added at 0.1 wt% of the substrate while the vials were contained under nitrogen in an atmospheric chamber (NPS Corp., Green Bay, WI, USA) to limit exposure to air. The vials were sealed using screw-caps with septa and removed from the atmospheric chamber. The sealed vials were heated to 40 8C on a laboratory hot plate and stirred continuously for the duration of the reaction. A 0.2-mL aliquot of each reaction mixture was taken periodically by syringe for analysis. These samples were either analyzed immediately or frozen to quench the reaction until analysis could be performed. 2.3. Analysis Aliquots of the reaction mixtures were analyzed for alkenes using an Agilent 6890 gas chromatograph equipped with flame ionization detectors and the automatic liquid sampler (Agilent, Palo Alto, CA, USA). Separations were achieved using a DB-1 column measuring 15 m!0.32 mm ID!0.25 mm film thickness. Helium carrier gas was set to produce a total flow of 10 mL/min. The inlet was maintained at 200 8C and 1-mL injections were made with a 10:1 split ratio. The oven was programmed to an initial temperature of 50 8C for 1 min, ramped at 5 8C/min to 150 8C, then ramped at 15 8C/min to 255 8C with a final 5 min hold. The detector temperature was set to 280 8C. Data were collected and analyzed with Chemstation software (Agilent, Palo Alto, CA, USA). Analytes were identified by their retention time. Esters were analyzed on a Hewlett-Packard 5890 series II gas chromatograph equipped with flame ionization detectors and the HP 6890 model automatic liquid sampler. Separations were obtained using a BPX-70 column measuring 30 m!0.32 mm ID!0.25 mm film thickness. The inlet was maintained at 200 8C and 2-mL injections were made with a 10:1 split ratio. The helium carrier gas was set to produce a total flow of 5 mL/min. The oven was programmed to an initial temperature of 120 8C for 2 min, ramped to 180 8C at 3 8C/min, then ramped to 290 8C at 15 8C/min, with a final 2 min hold. The detector temperature was set to 280 8C. Data were collected and analyzed with Chemstation software. Structural confirmation of products was performed using the Agilent 6890 gas chromatograph equipped
with the model 5973 mass spectral detector. Onemicroliter injections were made splitless onto an HP5 ms column measuring 30 m!0.25 mm ID!0.5 mm film thickness. Helium carrier gas was used. The oven was programmed to an initial temperature of 120 8C for 2 min, ramped to 180 8C at 3 8C/min, held at 180 8C for 15 min, then ramped to 290 8C at 15 8C/min, with a final 2 min hold. The detector was operated in scan mode with the source heated to 230 8C and the quadrapole heated to 150 8C. Data were collected and analyzed by Chemstation software and identified by comparison to library spectra of standard compounds.
3. Results and discussion The metathesis reactions of methyl oleate, methyl linoleate, methyl linolenate, and methyl soyate with Ru catalysts were analyzed for the conversion of the esters over an 8–80-h interval. The reaction mixtures were sampled hourly over the first several hours with subsequent samples taken at longer intervals to obtain equilibrium conversions. No change in reactant concentration or product formation was detected with catalyst A even after 80 h at 40 8C for any of the methyl esters. Similarly, no detectable conversion of methyl esters was observed with catalyst B at these conditions. In contrast, catalyst (C) exhibited nearly complete conversion of the individual methyl esters within the 1st hour. The time series data for the reaction of biodiesel with catalyst (C) is shown in Fig. 1. The composition of the starting material was 54% linoleate, 23% oleate, 11% palmitate, 7% linolenate, and 5% stearate. After 2 h of reaction time the oleate, linoleate, and linolenate components of the methyl soyate were converted 30, 50, and 70%, respectively. Because the saturated esters do not participate in the metathesis reactions they proved quite useful as internal standards for the analyses. Assuming similar conversion rates are obtained at a larger scale, 40 kg
Substrate concentration ratio (C/C0)
394
1.2 C18:1 C18:2 C18:3
1.0 0.8 0.6 0.4 0.2 0.0 0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
Reaction time (hr)
Fig. 1. Conversion of the unsaturated methyl esters in biodiesel with 0.1 wt% Grubbs second generation catalyst (C).
R.A. Holser et al. / Fuel 85 (2006) 393–395
of material from a 100-kg batch of methyl soyate would be converted to the corresponding alkene and diester products after 2 h. The relative rates of conversion of the methyl esters can be seen from the curves in Fig. 1 and follow the expected trend with the rate increasing with substrate unsaturation. The more highly unsaturated esters also produce a greater number of products and as the reaction progresses secondary metathesis reactions increase this number. While all the possible products generated from the selfmetathesis, cross-metathesis, and secondary reactions can be described, it is difficult to predict the product distribution as a function of time without detailed knowledge of the reaction rates. A simplifying assumption can be made to limit the number of reactions under consideration for short reaction times when the number of products formed is relatively small. The separation and recovery of the catalyst from the product remains problematic. The cost of the catalyst is significant and for economic reasons needs to be regenerated. The requirements for efficient process design and product quality also indicate this is necessary. Ideally, a heterogeneous catalyst would be desired as the catalyst could be easily separated by filtration or centrifugation from a batch reactor system or immobilized in a packed bed for continuous reaction. However, it is not clear that the high catalytic activity currently exhibited would be maintained by conventional supported catalyst preparation technologies [17]. Alternatively, the catalyst may be redesigned to exploit solubility effects that could allow the catalyst to be separated from the product phase by liquid–liquid extraction or washing.
395
4. Conclusion The second generation Ru-complex metathesis catalysts provide a method to modify the structure of methyl soyate. The conversion of the unsaturated esters, oleate, linoleate, and linolenate occurred rapidly at 40 8C. This catalytic approach requires significantly milder conditions than alternative methods such as practiced on petroleum feedstocks. The recovery and regeneration of the catalyst will be necessary for economic feasibility and remains an active area of research.
References [1] Clark SJ, Wagner L, Schrock MD, Piennaar PG. J Am Oil Chem Soc 1984;61:1632–8. [2] Chang DY, Van Gerpen JH, Lee I, Johnson LA, Hammond EG, Marley SJ. J Am Oil Chem Soc 1996; 73(11):1549–1555. [3] Freedman B, Bagby M. J Am Oil Chem Soc 1989;66:1601–5. [4] Allen C, Watts K, Ackman R, Pegg M. Fuel 1999;78:1319–26. [5] Pryde EH, Frankel EN, Cowan JC. J Am Oil Chem Soc 1972;49:451. [6] Pryde EH. J Am Oil Chem Soc 1984;61:419–25. [7] Boelhouwer C, Mol JC. J Am Oil Chem Soc 1984;61:425–30. [8] Mol JC. Topics in Cat 2004;27:97–104. [9] Mol JC. J Mol Cat A: Chem 2004;213:39–45. [10] Refvik MD, Larock RC, Tian Q. J Am Oil Chem Soc 1999;76(1):93–8. [11] Rufvik MD, Larock RC. J Am Oil Chem Soc 1999;76(1):99–102. [12] Sanford MS, Love JA, Grubbs RH. J Am Chem Soc 2001;123: 6543–54. [13] Grubbs RH. Tetrahedron 2004;60:7117–40. [14] Lynn DM, Kanaoka S, Grubbs RH. J Am Chem Soc 1996;118: 784–90. [15] Grunberg L, Nissan AH. Nature 1949;164:799–800. [16] Albahri TA. Ind Eng Chem Res 2003;42:657–62. [17] Dias EL, Nguyen ST, Grubbs RH. J Am Chem Soc 1997;119: 3887–97.
Metathesis of methyl soyate with ruthenium catalysts RonaldA. Holser*, KennethM. Doll, SevimZ. Erhan Food and Industrial Oils Research Unit, USDA-ARS-NCAUR, 1815 North University Street, Peoria, IL 61604, USA Received 9 December 2004; accepted 28 July 2005 Available online 24 August 2005
Abstract Three ruthenium catalysts were investigated for the metathesis reaction of methyl soyate. Dichlorotris (triphenylphosphine) ruthenium II and bis (tricyclohexyl phosphine) benzylidine ruthenium (IV) dichloride displayed no reactivity at 40 8C and atmospheric pressure. However, ruthenium [1,3-bis-(2,4,6-trimethylphenyl)-2-imidazolidinylidene] dichloro (phenylmethylene) tricyclohexylphosphine, Grubbs second generation catalyst, exhibited high activity at these mild conditions. After 2 h, 46% of the unsaturated methyl esters in a commercial methyl soyate were metathesized. This suggests a method to modify the structure of methyl soyate to improve diesel fuel performance properties. q 2005 Elsevier Ltd. All rights reserved. Keywords: Methyl soyate; Metathesis; Biodiesel
1. Introduction Oilseed crops such as soybean and canola provide a renewable source of triglycerides that can be converted to the corresponding fatty acid esters for use as an alternative diesel fuel [1,2]. The combustion and flow properties of such a fuel are determined primarily by the fatty acid composition of the original oil while the alcohol moiety exhibits a minor influence [3,4]. In addition, the alcohol used for transesterification is typically selected based on cost and availability. Methanol is a common choice, although, ethanol may be economical if a fermentation process is located nearby. The structure of the alkyl moiety presents greater opportunity to alter the fuel properties and thereby improve performance. Methods such as pyrolysis, catalytic cracking, and hydroformylation are available to modify alkyl structures and have been applied to vegetable oils [5,6]. These techniques are routinely applied in the petrochemical industry but require elevated temperatures and pressures. Olefin metathesis reactions offer another approach to achieve structural modification of unsaturated fatty acid esters [7,8]. Metathesis catalysts were originally developed * Corresponding author. Tel.: C1 309 681 6111; fax: C1 309 681 6340. E-mail address: [email protected] (R.A. Holser).
0016-2361/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2005.07.018
by the petroleum industry although applications to vegetable oils are noted [9,10]. These versatile catalysts promote reactions at the unsaturated sites of the reactants with minimal influence on substituent groups [11,12]. While the early metathesis catalysts were difficult to prepare and handle the recently developed Ru-complex catalysts have greater stability [13,14]. The interest in these newer catalysts is to convert a fraction of the unsaturated fatty esters in methyl soyate, e. g. oleate, linoleate, and linolenate, to alkenes and unsaturated diesters. Based on structure– property considerations such a reformulation of the fuel is expected to improve the viscosity and lubricity without decreasing storage stability or adversely effecting biodegradability [15,16].
2. Experimental 2.1. Materials Methyl soyate was obtained from Soygold (Ag Environmental Products, LLC, Omaha, NE, USA). Methyl oleate, methyl linoleate, and methyl linolenate standards of greater than 99% purity were supplied by (Nu-check Prep., Inc., Elysian, MN, USA). Ruthenium catalysts A, dichlorotris (triphenylphosphine) ruthenium II, CAS 15529-49-4; and B, bis (tricyclohexyl phosphine) benzylidine ruthenium (IV) dichloride, CAS 172222-30-9, were obtained from Strem Chemicals, Inc. (Newburyport, MA, USA). Grubbs second
R.A. Holser et al. / Fuel 85 (2006) 393–395
generation catalyst, C, ruthenium, [1,3-bis-(2,4,6-trimethylphenyl)-2-imidazolidinylidene] dichloro (phenylmethylene) tricyclohexylphosphine, CAS 246047-72-3, was obtained from Sigma-Aldrich (St Louis, MO, USA). All chemicals were used as received. 2.2. Procedure Reactions were performed by placing 10-g samples of the substrate, e. g. methyl oleate, methyl linoleate, methyl linolenate, or biodiesel into tared glass vials with a small magnetic stir bar. Catalyst was added at 0.1 wt% of the substrate while the vials were contained under nitrogen in an atmospheric chamber (NPS Corp., Green Bay, WI, USA) to limit exposure to air. The vials were sealed using screw-caps with septa and removed from the atmospheric chamber. The sealed vials were heated to 40 8C on a laboratory hot plate and stirred continuously for the duration of the reaction. A 0.2-mL aliquot of each reaction mixture was taken periodically by syringe for analysis. These samples were either analyzed immediately or frozen to quench the reaction until analysis could be performed. 2.3. Analysis Aliquots of the reaction mixtures were analyzed for alkenes using an Agilent 6890 gas chromatograph equipped with flame ionization detectors and the automatic liquid sampler (Agilent, Palo Alto, CA, USA). Separations were achieved using a DB-1 column measuring 15 m!0.32 mm ID!0.25 mm film thickness. Helium carrier gas was set to produce a total flow of 10 mL/min. The inlet was maintained at 200 8C and 1-mL injections were made with a 10:1 split ratio. The oven was programmed to an initial temperature of 50 8C for 1 min, ramped at 5 8C/min to 150 8C, then ramped at 15 8C/min to 255 8C with a final 5 min hold. The detector temperature was set to 280 8C. Data were collected and analyzed with Chemstation software (Agilent, Palo Alto, CA, USA). Analytes were identified by their retention time. Esters were analyzed on a Hewlett-Packard 5890 series II gas chromatograph equipped with flame ionization detectors and the HP 6890 model automatic liquid sampler. Separations were obtained using a BPX-70 column measuring 30 m!0.32 mm ID!0.25 mm film thickness. The inlet was maintained at 200 8C and 2-mL injections were made with a 10:1 split ratio. The helium carrier gas was set to produce a total flow of 5 mL/min. The oven was programmed to an initial temperature of 120 8C for 2 min, ramped to 180 8C at 3 8C/min, then ramped to 290 8C at 15 8C/min, with a final 2 min hold. The detector temperature was set to 280 8C. Data were collected and analyzed with Chemstation software. Structural confirmation of products was performed using the Agilent 6890 gas chromatograph equipped
with the model 5973 mass spectral detector. Onemicroliter injections were made splitless onto an HP5 ms column measuring 30 m!0.25 mm ID!0.5 mm film thickness. Helium carrier gas was used. The oven was programmed to an initial temperature of 120 8C for 2 min, ramped to 180 8C at 3 8C/min, held at 180 8C for 15 min, then ramped to 290 8C at 15 8C/min, with a final 2 min hold. The detector was operated in scan mode with the source heated to 230 8C and the quadrapole heated to 150 8C. Data were collected and analyzed by Chemstation software and identified by comparison to library spectra of standard compounds.
3. Results and discussion The metathesis reactions of methyl oleate, methyl linoleate, methyl linolenate, and methyl soyate with Ru catalysts were analyzed for the conversion of the esters over an 8–80-h interval. The reaction mixtures were sampled hourly over the first several hours with subsequent samples taken at longer intervals to obtain equilibrium conversions. No change in reactant concentration or product formation was detected with catalyst A even after 80 h at 40 8C for any of the methyl esters. Similarly, no detectable conversion of methyl esters was observed with catalyst B at these conditions. In contrast, catalyst (C) exhibited nearly complete conversion of the individual methyl esters within the 1st hour. The time series data for the reaction of biodiesel with catalyst (C) is shown in Fig. 1. The composition of the starting material was 54% linoleate, 23% oleate, 11% palmitate, 7% linolenate, and 5% stearate. After 2 h of reaction time the oleate, linoleate, and linolenate components of the methyl soyate were converted 30, 50, and 70%, respectively. Because the saturated esters do not participate in the metathesis reactions they proved quite useful as internal standards for the analyses. Assuming similar conversion rates are obtained at a larger scale, 40 kg
Substrate concentration ratio (C/C0)
394
1.2 C18:1 C18:2 C18:3
1.0 0.8 0.6 0.4 0.2 0.0 0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
Reaction time (hr)
Fig. 1. Conversion of the unsaturated methyl esters in biodiesel with 0.1 wt% Grubbs second generation catalyst (C).
R.A. Holser et al. / Fuel 85 (2006) 393–395
of material from a 100-kg batch of methyl soyate would be converted to the corresponding alkene and diester products after 2 h. The relative rates of conversion of the methyl esters can be seen from the curves in Fig. 1 and follow the expected trend with the rate increasing with substrate unsaturation. The more highly unsaturated esters also produce a greater number of products and as the reaction progresses secondary metathesis reactions increase this number. While all the possible products generated from the selfmetathesis, cross-metathesis, and secondary reactions can be described, it is difficult to predict the product distribution as a function of time without detailed knowledge of the reaction rates. A simplifying assumption can be made to limit the number of reactions under consideration for short reaction times when the number of products formed is relatively small. The separation and recovery of the catalyst from the product remains problematic. The cost of the catalyst is significant and for economic reasons needs to be regenerated. The requirements for efficient process design and product quality also indicate this is necessary. Ideally, a heterogeneous catalyst would be desired as the catalyst could be easily separated by filtration or centrifugation from a batch reactor system or immobilized in a packed bed for continuous reaction. However, it is not clear that the high catalytic activity currently exhibited would be maintained by conventional supported catalyst preparation technologies [17]. Alternatively, the catalyst may be redesigned to exploit solubility effects that could allow the catalyst to be separated from the product phase by liquid–liquid extraction or washing.
395
4. Conclusion The second generation Ru-complex metathesis catalysts provide a method to modify the structure of methyl soyate. The conversion of the unsaturated esters, oleate, linoleate, and linolenate occurred rapidly at 40 8C. This catalytic approach requires significantly milder conditions than alternative methods such as practiced on petroleum feedstocks. The recovery and regeneration of the catalyst will be necessary for economic feasibility and remains an active area of research.
References [1] Clark SJ, Wagner L, Schrock MD, Piennaar PG. J Am Oil Chem Soc 1984;61:1632–8. [2] Chang DY, Van Gerpen JH, Lee I, Johnson LA, Hammond EG, Marley SJ. J Am Oil Chem Soc 1996; 73(11):1549–1555. [3] Freedman B, Bagby M. J Am Oil Chem Soc 1989;66:1601–5. [4] Allen C, Watts K, Ackman R, Pegg M. Fuel 1999;78:1319–26. [5] Pryde EH, Frankel EN, Cowan JC. J Am Oil Chem Soc 1972;49:451. [6] Pryde EH. J Am Oil Chem Soc 1984;61:419–25. [7] Boelhouwer C, Mol JC. J Am Oil Chem Soc 1984;61:425–30. [8] Mol JC. Topics in Cat 2004;27:97–104. [9] Mol JC. J Mol Cat A: Chem 2004;213:39–45. [10] Refvik MD, Larock RC, Tian Q. J Am Oil Chem Soc 1999;76(1):93–8. [11] Rufvik MD, Larock RC. J Am Oil Chem Soc 1999;76(1):99–102. [12] Sanford MS, Love JA, Grubbs RH. J Am Chem Soc 2001;123: 6543–54. [13] Grubbs RH. Tetrahedron 2004;60:7117–40. [14] Lynn DM, Kanaoka S, Grubbs RH. J Am Chem Soc 1996;118: 784–90. [15] Grunberg L, Nissan AH. Nature 1949;164:799–800. [16] Albahri TA. Ind Eng Chem Res 2003;42:657–62. [17] Dias EL, Nguyen ST, Grubbs RH. J Am Chem Soc 1997;119: 3887–97.