Article pubs.acs.org/EF
Reactive Extraction of Triglycerides as Fatty Acid Methyl Esters using Lewis Acidic Chloroaluminate Ionic Liquids Patrick M. Bollin and Sridhar Viamajala* Department of Chemical and Environmental Engineering, The University of Toledo, 2801 West Bancroft Street, Toledo, Ohio 43606, United States S Supporting Information *
ABSTRACT: This study investigated the in situ production of fatty acid methyl esters (FAMEs) directly from lipid-containing soy flour using a Lewis acidic 1-ethyl-3-methylimidazolium chloroaluminate [EMIM]Cl·2AlCl3 (N = 0.67) ionic liquid (IL). The system also contained (i) dichloromethane, added as a cosolvent to reduce IL viscosity and (ii) methanol. The chloroaluminate, methanol, and dichloromethane formed a homogeneous reaction medium that facilitated a relatively rapid rate of FAME formation. IL, methanol, and cosolvent requirements, along with reaction temperature and time needed for high FAME yields, were determined. The highest biomass concentration that could be processed without negatively impacting product yields was also assessed. The results show that FAME yields of >90% can be achieved in 4 h at 110 °C with solids concentrations as high as 20% (w/v). Further, carbohydrates associated with the postreaction residues remained chemically unmodified, which would allow for additional sugar-based coproduct generation. A strategy for recovery of IL is also reported.
1. INTRODUCTION Concerns over rapidly depleting fossil energy sources and escalating greenhouse gas emissions have resulted in an increased interest in alternative energy. Biofuels are a renewable alternative to fossil fuels and are compatible with existing automobile- and fuel- infrastructure. However, first-generation biofuels, such as biodiesel from oil seeds, are unlikely to replace a significant fraction of the U.S. petroleum demand without adversely affecting food production due to the low yields (per unit land area) of feedstocks from traditional agriculture.1 As a result, alternative feedstocks such as microalgae are now being explored for biodiesel production.1−3 Studies reported in the literature suggest that oil productivities (per unit area) from microalgae can be nearly 8−20 times higher than those of terrestrial plants.1,4 While several microalgal species have the potential to accumulate lipids such as triglycerides, the traditional methods of oil recovery used for oil seeds (such as mechanical “pressing”) are not applicable to microalgae due to the microscopic sizes of these feedstocks and the relatively tough cell walls.5 As a result, oil from these feedstocks must be recovered by other cell disruption methods (e.g., sonication) or chemical means (e.g., solvent extraction) or combinations of the two.6,7 If biodiesel is the final desired product, fatty acid methyl esters (FAMEs) may be more easily obtained from cellular triglycerides through in situ transesterification where oleaginous biomass is directly reacted with a mixture of methanol and catalyst without prior solvent extraction.5,8,9 Base-catalyzed transesterification that occurs at lower temperatures may be used, but trace amounts of water and free fatty acids in the biomass would promote saponification, contaminating the product and consuming the catalyst.9,10 Acid catalyzed in situ transesterification, such as reactions carried out in acidified methanol alone5,9 or in the presence of other solvents such as chloroform8 or ionic liquids,11 is likely a better approach. © 2012 American Chemical Society
However, reaction media containing mineral acids are corrosive and recovery of mineral acid catalysts is also difficult. As an alternative, solid-supported acids, which are noncorrosive and have potential for recovery, may be used.12 However, solid catalysts are less suitable for in situ transesterification applications, since there is a low probability of direct contact of solid catalyst particles with oil droplets embedded within biomass and reactions are expected to be slow and incomplete. Even for transesterification of vegetable oils, where intimate contact of the catalysts with reactants is possible, studies have shown that heterogeneous catalysts have relatively low activities resulting in reactions requiring high temperatures and long reaction times.10,12 Chloroaluminates, such as those synthesized by reacting 1ethyl-3-methylimidazolium chloride ([EMIM]Cl) with AlCl3 exhibit behavior of powerful Lewis acids when AlCl3 molar concentrations are >0.50 due to the formation of the Lewis acidic [Al 2Cl 7]− and [Al 3Cl 10 ]− anions.13 In addition, chloroaluminates are noncorrosive and have the potential to be recovered and reused.14 These ionic liquids (ILs) are especially well suited for performing Lewis acid catalyzed reactive extractions due to their liquid state, which would allow the chloroaluminate to penetrate into biomass, access cellular lipids, and catalyze their transesterification in situ. In addition, unlike traditional polar coordinating solvents that form complexes on the active site of many homogeneous catalysts and block activity, chloroaluminates provide a noncoordinating, yet highly polar, environment to both reactants and catalyst species, thus allowing for more efficient catalysis.15,16 In general, [EMIM]Cl·xAlCl3 systems are well studied and have been shown to promote the same reactions that are Received: June 30, 2012 Revised: September 13, 2012 Published: September 13, 2012 6411
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The vials were then put in an oil bath that had been preheated to the reaction temperature. The bath was placed on a heated stir-plate, and its temperature was held constant throughout the duration of the reaction using a thermocouple-aided temperature controller. Temperatures were also periodically verified with an external thermometer. The reaction mixture was stirred at 125 rpm for the entire duration using a triangular stir-bar designed for mixing viscous fluids. All experiments were performed at least in duplicate. Reported values are averages of results from replicate experiments and error bars indicate one standard deviation from mean values. At the end of the reaction period, the vials were removed from heat, quenched and allowed to cool to room temperature. The samples were then centrifuged at 5000 rpm (4700 × g) for 15 min at 20 °C using a Sorvall Legend X1R centrifuge (Thermo Fisher Scientific, Waltham, MA). Solubilized FAMEs and glycerides were recovered from the supernatant through liquid−liquid extraction with hexane. These extractions were performed by adding 5 mL of hexane per mL of supernatant and stirring the mixture at 125 rpm at room temperature for 3 h. Solids (postreaction residues) were washed with deionized water, dried in an oven for 24 h at 40 °C and retained for thermogravimetric analysis (TGA) and for Fourier transform infrared (FTIR) spectroscopy. 2.4. Lipid Determination. Total glyceride content (including mono-, di-, and triglycerides) of biomass (fresh soy flour and postreaction residues) was determined through extraction of the lipids using a solvent mixture containing equal volumes of hexane, tetrahydrofuran, and chloroform, similar to procedures previously reported.19 20 mg of dry samples (ovendried for 24 h at 60 °C) were sonicated (Model S-450A, Branson Ultrasonics Corporation, Dunbury, CT) in 5 mL of the solvent mixture at a power setting of approximately 150 W for 1 min in 10 s intervals. The extract was filtered and analyzed for glycerides using gas chromatography (GC) as described in Sections 2.5.1 and 2.5.2. Total FAME content of soy flour was determined using an analytical in situ transesterification method, similar to a previously described protocol.20,21 In brief, 100 mg of dry soy flour was incubated with 5 mL of acidified methanol (containing 5% (v/v) H2SO4) for 1 h at 90 °C in a sealed serum vial to convert cellular glycerides to FAMEs. Thereafter, to extract FAMEs, 15 mL hexane was added to the serum vials and the mixture was incubated at 90 °C for 15 min. 1 mL of the hexane phase (containing extracted FAMEs) was carefully removed, filtered, further diluted in hexane as required (to ensure measured concentrations stayed within the calibration range) and placed in GC vials for analysis. 2.5. Analytical Methods. 2.5.1. Reaction Product Identification. Products from chloroaluminate-based in situ transesterification reactions were identified using a HewlettPackard 6890 GC equipped with a 5973 series mass selective detector (GC-MS) and an autosampler (Agilent Technologies, Santa Clara, CA). A Supelco PTE-5 (30 m × 0.25 mm × 0.25 μm film thickness) fused silica capillary column supplied by Sigma-Aldrich was used to separate the analytes. Sample sizes of 1 μL were injected. The GC carrier gas was He (1.4 mL min−1). The temperature program was as follows: constant temperature of 40 °C for 4 min followed by temperature ramp to 280 at 5 °C min−1 and, finally, a constant temperature of 280 °C for 15 min. The injector and MS temperatures were maintained at 270 °C. Chemical compounds corresponding to each chromatogram peak were identified by using the NIST98
traditionally carried out with AlCl3, without the disadvantage of the low solubility of AlCl3 in many solvents.17 Although ILs can be prepared with several Lewis acidic metal salts, the work of Liang et al.14 has demonstrated that AlCl3 is more effective than Fe, Zn, Sn, and Mg halides for the transesterification of soy oil. Liang et al.14 also showed that [EMIM]+ was the most appropriate cation and found that activity was lower when cationic species containing more carbon atoms were used. In this study, in situ transesterification of cellular triglycerides to FAMEs using [EMIM]Cl·2AlCl3 as a catalyst was assessed. A cosolvent (dichloromethane) was added to the reaction system to lower IL viscosity and improve mass transfer and reaction rates. The effects of temperature, cosolvent (dichloromethane), and reactant (soy flour and methanol) ratios were systematically assessed to determine reaction conditions that resulted in highest rates and yields. Overall, our results suggest that >90% of the soy flour lipids can be recovered as FAMEs under appropriate reaction conditions. To our knowledge, this is the first report of in situ transesterification of biomass lipids using Lewis acidic chloroaluminate catalysts.
2. EXPERIMENTAL SECTION 2.1. Materials. [EMIM]Cl, purity >98%, was obtained from Tokyo Chemical Industry (TCI America, Portland, OR). Greater than 98% purity AlCl3, hexane, and dichloromethane were purchased from Sigma-Aldrich (Allentown, PA). FAMEs were quantified using a FAME-10 standard mixture (containing methyl palmitate, methyl linolenate, methyl linoleate, methyl oleate, and methyl stearate in addition to other FAMEs) from Sigma-Aldrich. Monoolein, diolein, and triolein standards were also obtained from Sigma-Aldrich. Soy flakes were obtained from Shiloh Farms (New Holland, PA), which produces organically grown and processed soy products. The flakes were ground into flour using a laboratory Wiley mill (Model 4, Thomas Scientific, Swedesboro, NJ) equipped with a 40 mesh screen. Milled biomass was further sieved through an 80 mesh screen, and the flour retained on this screen (particle size −40 +80 mesh) was used. 2.2. Chloroaluminate Synthesis. Vendor-supplied [EMIM]Cl was first melted and dehydrated in a rotary evaporator for 3 h at 85 °C under vacuum at 0.15 bar. The molten [EMIM]Cl was then removed from the rotary evaporator and kept under a constant stream of N2 flowing at 4 psi. AlCl3 was added to the [EMIM]Cl in a molar ratio of 2:1 to make a eutectic mixture of chloroaluminate containing AlCl3 at a mole fraction of 0.67 (commonly referred to as n0.6718). Adding AlCl3 to the [EMIM]Cl results in an exothermic reaction. To prevent overheating, the addition of AlCl3 was carried out in a uniform stepwise fashion while keeping the mixture in an ice bath with stirring. After AlCl3 addition was complete, the mixture was removed from the ice bath, allowed to reach room temperature, and then heated to 60 °C, increasing the solubility of the AlCl3 in [EMIM]Cl. The entire chloroaluminate synthesis was carried out in an inert and dry atmosphere maintained through a constant stream of N2 flowing through the headspace of the chloroaluminate synthesis vials. After 1 h, the chloroaluminate was removed from the heat source and sealed under N2 in a gastight vial. 2.3. Transesterification Reaction. In situ transesterification reactions were carried out in 10 mL borosilicate glass vials. Dry soy flour (oven-dried for 24 h at 60 °C) was first added to dry vials and subsequently a mixture of [EMIM]Cl·2AlCl3, methanol, and dichloromethane was aliquoted into the vial. 6412
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consistent with previously reported results for soybean lipids and validate our lipid analysis methods.22 3.1. Impact of the Cosolvent. Reactions carried out in ILs can be hampered due to inefficient mixing as a result of the high viscosity of this reaction medium. The mixing characteristics are worse when solids are present due to the formation of viscous slurries. In this study, an n0.67 chloroaluminate was used. At this concentration, the IL has low viscosity (approximately 12− 14 cP23) and melting point (−96 °C13), while high Lewis acidity is also maintained due to the abundance of Al2Cl7− anions.24 Lewis acidity is higher at greater AlCl3 concentrations, but IL viscosity and melting point also significantly increase. In spite of the relatively low viscosity of the n0.67 IL, our preliminary tests in reactions that contained only IL, methanol, and biomass showed poor conversion and yield (50%). However, at the end of 2 h, FAME production was lower in ethyl acetate suggesting that transesterification is relatively slower in this cosolvent system. The improved FAME yields observed in the presence of a nonreacting cosolvent suggest that the cosolvent serves to 6413
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disperse the IL and mitigate mass transfer limitations. Further, the presence of the nonpolar cosolvents within the polar mixture of EMIM[Cl] and methanol also likely improved access to the hydrophobic triglycerides. Previous studies have used methanol alone as both the reactant and cosolvent to decrease IL viscosity.11,25 However, as observed by Young et al.,11 this approach may require the addition of methanol in large excess of stoichiometric requirements (methanol to triglyceride ratios in the excess of 1500:1). In this study, although reactions containing chloroform and dichloromethane showed similar FAME production, dichloromethane is likely a better cosolvent choice, since it is easier to separate and recover from spent reaction media due to its lower boiling point than chloroform. Dichloromethane is also considered to be the least toxic of the chlorohydrocarbons.26 In subsequent tests, dichloromethane was used as the cosolvent. 3.2. Effect of Catalyst, Methanol, and Cosolvent Amounts on Transesterification Yields. 3.2.1. Catalyst Requirement. The optimal catalyst requirement was determined through experiments performed in the presence of varying amounts of chloroaluminate in the reaction media that also contained 1 mL dichloromethane, 1 mL methanol, and approximately 205 mg soy flour. As can be seen in Figure 2a, FAME production steadily improved when chloroaluminate was increased from 0.02 to 0.10 mL. However, increasing the IL volume further to 0.25 mL did not result in significant improvement in the production of FAMEs. When a much higher amount of chloroaluminate (0.5 mL) was added, reaction yields decreased significantly. In absence of the catalyst, the reaction simply did not proceed. At low catalyst concentrations, sufficient reactive sites may not be present to rapidly produce FAMEs. On the other hand, since transesterification is reversible,10 high catalyst concentrations may have resulted in sufficient reactive sites for the reverse reaction to simultaneously occur,14 resulting in low product yields. It is also possible that, at high IL loadings, the higher viscosity of the reaction medium caused slower reactions and lower yields. Based on these results, it can be concluded that the optimum chloroaluminate volume is approximately 0.10 mL for the reactors used in this study. 3.2.2. Methanol Requirement. Transesterification is generally carried out in the presence of excess methanol to decrease the rates of the reverse reaction and thereby achieve faster conversions and higher FAME yields.10 To test for the effect of methanol on in situ transesterification yields, experiments were performed at 90 °C for 4 h with varying amounts of methanol (0−1.25 mL) added to reaction mixtures containing 0.1 mL chloroaluminate, 1.0 mL dichloromethane, and approximately 330 mg soy flour. From Figure 2b, it can be seen that FAME yields increased when methanol volumes of up to 1 mL were used for the reaction. However, yields decreased when higher amounts of methanol (1.25 mL) were added, possibly due to excess dilution of catalyst. Young et al.11 also tracked the effect of methanol addition on the yield of FAMEs and observed improved product formation in the presence of increasing amounts of methanol (up to a methanol to triglyceride molar ratio of 1500:1). However, FAME formation was not lowered when higher amounts of methanol (up to a methanol to triglyceride molar ratio of 3000:1) were added, suggesting that the catalytic activity of the Bronsted acid used in their IL medium was not negatively impacted by excess dilution.
Figure 2. Impact of (a) chloroaluminate, (b) methanol, and (c) dichloromethane on the production of FAMEs. Results are reported in terms of percent lipids recovered relative to the amount of biomass initially present. The dashed line indicates the maximum expected lipid recovery based on lipid content of fresh biomass. Error bars indicate one standard deviation from average values.
3.2.3. Dichloromethane Requirement. In addition to catalyst and methanol, experiments to determine the optimum cosolvent volumes required for FAME production were also performed. In these tests, mixtures of 0.1 mL chloroaluminate, 1 mL methanol, and 330 mg soy flour were incubated for 4 h at 90 °C in the presence of varying amounts of dichloromethane. The results of these experiments (Figure 2c) showed that the reaction yields were low in the absence of the cosolvent, likely due to the high viscosity of the reaction media (as discussed in Section 3.1). The addition of dichloromethane improved FAME production. Although similar FAME concentrations were observed in all reactions containing dichloromethane, yields were statistically higher (based on t tests at a significance level of 0.05) for reactions carried out in the presence of 0.5 mL dichloromethane, suggesting that this is the optimal cosolvent volume for the reactors used in this study. 3.3. Effect of Temperature and Biomass Concentration on Transesterification Rates. Experiments to determine the effects of temperature were performed in media containing 0.1 mL chloroaluminate, 1 mL dichloromethane, 1 mL methanol, and approximately 330 mg soy flour. As seen from Figure 3a, FAME production rates increased 6414
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containing 5, 10, 15, and 20% (w/v) biomass. FAME yields decreased to 90% of cellular glycerides can be converted to FAMEs via chloroaluminate catalyzed in situ transesterification. The method proposed here is potentially scalable, since the catalyst and cosolvents form a homogeneous phase and the reactions can be carried out at relatively high biomass loadings (20% (w/v)) without loss of reaction efficiency. The catalyst is also likely recoverable (in addition to methanol and the cosolvent that could be recovered by distillation methods), resulting in low material inputs to the process. Unmodified starch residues and solubilized protein or amino acid methyl esters, if recovered, could be valuable coproducts. 6417
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