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Butoxylation of Butyl Biodiesel: Reaction Conditions and Cloud Point Impact Paul C. Smith,* Brian K. O’Neill, Yung Ngothai, and Q. Dzuy Nguyen School of Chemical Engineering, UniVersity of Adelaide, Adelaide, South Australia 5005 ReceiVed February 25, 2009. ReVised Manuscript ReceiVed May 7, 2009
A property of biodiesel that currently restricts its use to blends of 20% or less is its relatively poor lowtemperature properties (cloud point). Alkoxylation of the unsaturated fraction of biodiesel offers the potential benefit of reduced cloud point without compromising ignition quality or oxidation stability. In addition, the butoxylation of butyl biodiesel improves the renewable nature of biodiesel by substituting fossil-derived methanol with biobutanol. Butyl biodiesel derived from canola oil was epoxidized via the in situ peroxyacetic acid method for 6 h, resulting in a conversion of 46% of the unsaturated esters. Epoxy butyl biodiesel was butoxylated with n-butanol with sulfuric acid catalyst without the use of solvents. Conversion and selectivity for butoxy butyl biodiesel were optimized by examining reaction conditions including: temperature, reaction time, catalyst concentration, and molar ratio of alcohol to epoxy biodiesel. The optimal conditions for the butoxylation of epoxy butyl biodiesel were: 80 °C, 2 wt % sulfuric acid, and a 40:1 molar ratio of n-butanol over a period of 1 h. Conversion of epoxy butyl biodiesel was 100%, and selectivity for butoxy biodiesel was 87.0%. The cloud point (CP) of butyl biodiesel was -3 °C, as was the cloud point of butoxy butyl biodiesel produced under the aforementioned optimal conditions. Butoxylation of butyl biodiesel, at the conversion rate of 46%, therefore had no discernible impact on cloud point. To determine the impact of a higher conversion of unsaturated ester to butoxy ester, butyl biodiesel was subjected to 30 h of epoxidation, resulting in the conversion of 93% of the oleic portion. Subsequent butoxylation at the optimal conditions resulted in a butoxy content of 74%. The cloud point of this material was 2 °C, representing an increase of 5 K over the original butyl biodiesel CP. Blends of the high conversion batch of butoxy biodiesel showed that cloud point was virtually unchanged at concentrations below 35% and then increased 1 K every 8 wt % to approximately 70 wt % butoxy biodiesel. The loss of unsaturated ester, due to conversion to butoxy ester, appears to have a significant effect on cloud point only after approximately one-third of the unsaturated ester is converted. Butoxy biodiesel is therefore able to prevent the earlier onset of crystallization due to the decrease in unsaturated content, but only at lower concentrations.
Introduction Biodiesel has been widely accepted as an additive for fossilderived diesel fuel for use in compression ignition engines. B20 (20% biodiesel/petroleum diesel blend) reduces life-cycle petroleum consumption by 19% and lifecycle CO2 emissions by 16%.1 Other benefits of the use of biodiesel have been widely recognized and include: high cetane number (CN); high lubricity (even in blends of 1-2%); and significant reductions in emissions of sulfur oxides, hydrocarbons, particulates, polycylic aromatic hydrocarbons, and carbon monoxide. A property of biodiesel that currently restricts its use to blends of 20% or less is its relatively poor low-temperature properties. Neat biodiesel can solidify in fuel lines or clog filters when utilized in cold ambient conditions. Although the cloud point (CP) of petroleum diesel is reported to be -16 °C, biodiesel typically has a CP of 0 °C. This limits its application to ambient temperatures above freezing.2-4 Waste cooking oils and tallows have the potential to significantly lower the final price of biodiesel due to their * To whom correspondence should be addressed. Phone: +61 8 8303 3093; fax: +61 8 8303 4373; e-mail:
[email protected]. (1) Harrow, G. E85 and Biodiesel Deployment, NREL/PR-540-42312. In Clean Cities ActiVity; National Renewable Energy Laboratory: Golden, CO, USA, 2007. (2) Chandler, J. E.; Horneck, F. G.; Brown, G. I. The Effect of Cold Flow Additives on Low-Temperature Operability of Diesel Fuels. In SAE Technical Paper Series; Warrendale, 1992.
low cost when compared to virgin vegetable oils. Oils such as palm oil are also seen as beneficial because they may not directly compete with food crops and therefore may not place pressure on food prices. However, the highly saturated nature of these oils means that the CP of biodiesel derived from such feedstocks can be as high as 17 °C. Hence, the use of virgin vegetable oils with high levels of unsaturation is favored in terms of coldtemperature properties. Despite the higher purchase cost, the final product possesses a reduced cetane number and reduced oxidation stability due the proportion of unsaturated ester. Alkoxylation of the unsaturated fraction of biodiesel offers the potential benefit of reduced cloud point without compromising ignition quality or oxidation stability. In addition, the butoxylation of butyl biodiesel improves the renewable nature of biodiesel by substituting fossil derived methanol with biobutanol. Swern, et al. described a method for preparing hydroxy-ethers from 9,10-epoxystearic acid with various alcohols, including n-butanol.5 A large excess of the alcohol was used and the sulfuric acid catalyst was added dropwise to the solution, which was cooled to 30 °C. The solution was heated on a steam bath (3) Knothe, G.; Krahl, J.; Van Gerpen, J. The Biodiesel Handbook, First ed.; AOCS Press: Champaign, IL, 2004. (4) Dunn, R. O.; Shockley, M. W.; Bagby, M. O. J. Am. Oil Chem. Soc. 1996, 73 (12), 1719–1728. (5) Swern, D.; Billen, G. N.; Scanlan, J. T. J. Am. Chem. Soc. 1948, 70 (3), 1226–1228.
10.1021/ef9001633 CCC: $40.75 2009 American Chemical Society Published on Web 05/19/2009
Butoxylation of Butyl Biodiesel
for 2 h before neutralization with bicarbonate and purification by vacuum distillation. As the ester of 9,10-dihydroxystearic acid was always obtained as a byproduct, the reaction product was further purified by crystallization fractionation in acetone. It was noted that the hydroxy-ethers did not solidify when stored for long periods at 0 °C. Moser and Erhan epoxidised isopropyl oleate (IPO) via the in situ peroxyformic acid method at room temperature over a period of 12-14 h to produce approximately 95% epoxidised IPO (EIPO).6 R-Hydroxy ethers of IPO were prepared by subjecting a 1 M solution of EIPO with a number of alcohols in the presence of an acid catalyst (H2SO4), again at room temperature for 24-240 h. As the chain length of the alcoholic adduct increased above a floor of 3 carbons, a dramatic improvement in low-temperature properties was observed. For instance, the decyl ether had a CP of -23 °C compared with a CP of -9 °C for unmodified isopropyl soyate. It must be stressed however that these were the cloud points of essentially pure compounds and are therefore more closely associated with melting points. Further work by Moser and Erhan in 2007 included the epoxidation of alkyl oleates and subsequent alkoxy addition with acid catalyst, or the epoxidation of oleic acid followed by the simultaneous esterification and etherification in the presence of an acid catalyst. They produced alkoxy esters with both heterogeneous and homogeneous ester and ether groups. The best low-temperature properties were shown by the 2-ethylhexyl ethers of 2-ethylhexyl 9(10)-(2-ethylhexyl)-10(9)hydroxystearate with CPs and PPs of -26 and -29 °C, respectively.7 The authors found that as the alcohol chain length increased, the reactivity decreased. However, the CP of the product was lowered. The optimal sulfuric acid concentration was 10 wt % at a temperature of 60 °C. Temperatures exceeding 60 °C improved the reaction rate but resulted in unwanted sidereactions. Prior work by these authors assessed the synthesis of alkoxylated biodiesel from canola oil under mild conditions, with reasonable residence times and without the use of organic solvents.8 Canola oil was transesterified with methanol, ethanol, and n-butanol. Epoxidation was performed via either the in situ peroxyformic acid or peroxyacteic acid method with 30% H2O2. Selectivity for epoxy biodiesel over the vicinal dihydroxy byproduct was maximized for methyl biodiesel without resorting to organic solvents either during synthesis or purification. The expoxidized material was then alkoxylated with the same alcohol as the ester headgroup under acid conditions. Cloud point for methyl and ethyl biodiesel increased from -2 to 4 °C, and from -3 to 0 °C, respectively. The cloud point of butyl biodiesel reduced from -3 to -4 °C, which was not significant given the accuracy of the test method. Selectivity for butoxy was 81.7%, and conversion of the unsaturated portion was 43%. It was thought that significant quantities of vicinal dihydroxy and keto biodiesel produced during the alkoxylation step negated much of the expected improvement in cloud point. This study focused on the butoxylation of epoxy butyl biodiesel produced from canola oil to determine the optimal reaction conditions for maximizing conversion while limiting byproduct formation. Parameters examined were temperature, reaction time, catalyst concentration, and molar ratio of alcohol to epoxy biodiesel. Conversion and selectivity were monitored by gas chromatog(6) Moser, B. R.; Erhan, S. Z. J. Am. Oil Chem. Soc. 2006, 83 (11), 959–963. (7) Moser, B. R.; Erhan, S. Z. Eur. J. Lipid Sci. Technol. 2007, 109, 206–213. (8) Smith, P. C.; Ngothai, Y.; Nguyen, Q. D.; O’Neill, B. K. Fuel 2009, 88, 605–612.
Energy & Fuels, Vol. 23, 2009 3799 Table 1. Effect of Temperature on Selectivity for Butoxy Butyl Biodiesel and By-product Content after 6 h of Reaction Time temperature (°C)
glycol (wt %)
ketone (wt %)
selectivity (%)
40 60 80 100
1.3 1.3 0.6 0.0
7.5 4.3 5.7 7.1
79.4 86.8 85.1 83.3
raphy (GC). The impact of butoxylation on cloud point of biodiesel was determined. Materials and Methods Reagents. Canola oil (Crisco, Meadow Lea Foods) was purchased from a supermarket. Free fatty acid (FFA) content was determined according to ASTM D 5555-95 and was found to be 0.2 mg KOH/g oil, equivalent to 0.1 wt % FFA. n-Butanol (ACS, ISO, Reag. Ph Eur, Merck KGaA); potassium hydroxide (pellets GR for analysis, Merck Pty. Limited); sulfuric acid 96% (Merck KGaA); formic acid (ACS, Sigma-Aldrich); acetic acid (ACS, Sigma-Aldrich); sodium sulfate (Analytical reagent, Chem-Supply); sodium bicarbonate (ACS reagent, Sigma-Aldrich); pyridine (ACS reagent, Ph Eur, Merck KGaA); n-heptane (Reag. Ph Eur, Merck KGaA); hydrogen peroxide (solution 30%, puriss., stabilized, Riedel-de Haen, Sigma-Aldrich); ethyl oleate (Sigma-Aldrich); oleic acid (Technical grade, 90%, Sigma-Aldrich); N-methyl-N-(trimethylsilyl)trifluoroacetamide (for synthesis, Merck KGaA); methyl margarate (reference substance for GC, Merck KGaA); and tricaprin (MP Bimedicals, Inc.) were used without further purification. Biodiesel Synthesis. The well established base-catalyzed transesterifaction of canola oil was performed in a glass reaction kettle on a hotplate/stirrer with feedback temperature control. Canola oil was preheated to 80 °C. The combined n-butanol (molar ratio of 7:1) and catalyst (2 wt % potassium hydroxide) were added at room temperature. A two-stage process, each with 4 h batch time and the purification method outlined below, was performed to ensure >99% conversion. Purification steps included a first phase separation to remove the bulk of the glycerol followed by neutralization with sulfuric acid and repeated water washes to remove residual glycerol and alcohol. The top organic phase was dried over anhydrous sodium sulfate followed by filtration. Epoxidation of Biodiesel. Epoxidation was performed at 60 °C for 6 h on a temperature controlled hotplate/stirrer with in situ generated peroxyacetic acid (oxygen carrier) and sulfuric acid catalyst. The molar ratio of hydrogen peroxide to biodiesel was 2:1, and the sulfuric acid concentration was 2 wt %. Residual acid and peroxide were neutralized with sodium bicarbonate solutions, followed by several water washes and drying over anhydrous sodium sulfate. Butoxylation of Epoxy Biodiesel. Oxirane ring-opening and subsequent addition of n-butanol was performed in a glass reaction kettle on a hotplate/stirrer with feedback temperature control. Sulfuric acid was chosen as the catalyst and the reaction was monitored for the batch time of 6 h. Residual catalyst and alcohol were removed by repeated water washes and phase separation followed by drying over anhydrous sodium sulfate and filtration. Reaction conditions for the optimization exercise are listed in chronological order in Tables 1-3. For the initial temperature experiments, the molar ratio of butanol was fixed at 10:1, as was the acid concentration at 2 wt %. The optimal reaction temperature was then chosen based on the conversion rate and butoxy selectivity. Catalyst concentration was then varied at the optimal temperature. Finally, the alcohol molar ratio was varied at the optimal temperature and catalyst concentration. Samples were taken periodically and placed into vials containing 1 mL of saturated sodium bicarbonate solution and 3 mL of heptane. Samples were shaken to neutralize the acid and allowed to stand at room temperature before transferring an aliquot of the top heptane layer to vials for GC analysis. Analytical Methods. Biodiesel purity was confirmed by gas chromatographic analysis, in accordance with The European
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Table 2. Effect of Catalyst Concentration on Selectivity for Butoxy Butyl Biodiesel and By-product Content after 6 h of Reaction Time catalyst concentration (wt %)
glycol (wt %)
ketone (wt %)
selectivity (%)
1 2 5 10
0.9 1.3 6.9 9.7
5.1 4.3 2.6 2.6
85.8 86.8 77.5 71.2
Table 3. Effect of Molar Ratio of Butanol on Selectivity for Butoxy Butyl Biodiesel and By-product Content after 6 h of Reaction Time butanol molar ratio
glycol (wt %)
ketone (wt %)
selectivity (%)
5 10 20 40
1.3 1.3 1.0 0.7
4.1 4.3 4.3 4.5
87.3 86.8 87.7 87.9
Standard EN14105, with a Perkin-Elmer Clarus 500 with flame ionization detector (FID). Glycerol and mono-, di-, and triglyceride concentrations were determined following derivatization with N-methyl-N-(trimethylsilyl)trifluoroacetamide (MSTFA). A 15 m Perkin-Elmer Elite-5HT capillary column was used with H2 as the carrier. Aliquots of 1 µL were injected onto the column via a split/ splitless injector at 300 °C. Hydrogen flow rate was 2 mL · min-1, and the detector temperature was 380 °C. The oven program was 1 min at 50 °C, 15 °C · min-1 to 180 °C, 7 °C · min-1 to 230 °C, 20 °C · min-1 to 380 °C, maintained for 10 min. The fatty acid profile of biodiesel produced from canola oil was determined by gas chromatography-mass spectrometry (GC-MS) on a Perkin-Elmer Clarus 500 with an electron ionization mass spectrographic detector. The capillary column was a 30 m PerkinElmer Elite-5MS with 0.25 mm id, and the carrier gas was helium at a rate of 0.9 mL · min-1. The oven temperature program was based on the program reported by Wilson et al.9 and was: 100 °C for 3 min, 25 °C · min-1 to 170 °C, 2 °C · min-1 to 260 °C, maintained for 5 min. The fatty acid profile was dominated by oleic ester (>73%) and linoleic/linolenic at 18%. Total unsaturated alkyl ester was 93 wt %. The epoxidized and alkoxylated biodiesel content were determined using the same GC parameters, and peaks were identified as previously described by Smith, et al.8 Cloud point was determined visually as the appearance of a distinct clouding of a 50 mL sample in a 150 mL test jar. Equipment and method were according to ASTM D 2500-02 using a Julabo F34 water bath containing a propylene glycol solution set at -18 °C. Replicates of cloud point tests on the same material were performed to determine the repeatability of the method, with measurements generally accurate to within 1 K of each other, allowing for meaningful comparisons to be made.
Results and Discussion Epoxidation of Butyl Biodiesel. The mixed fatty acid nature of canola oil-derived biodiesel means that a number of different epoxy compounds are possible. These included multiepoxy esters and epoxy esters ranging from palmitoleic to nervonic. The most likely byproduct is vicinal dihydroxy compounds (glycol). Epoxides and glycol may also be present at one or more of the available double bonds for polyunsaturated esters. As the bulk of the unsaturated portion of biodiesel derived from canola oil is butyl oleate, conversion of butyl oleate to epoxy biodiesel was monitored by the reduction in area of the butyl oleate peak, referenced to the butyl palmitate peak. Selectivity for epoxy butyl biodiesel was determined to be 100% for all batches. Epoxidation of butyl biodiesel for 6 h resulted in a conversion (XB) of 46% of the available unsaturated portion. This was determined from the extent of conversion of the oleate (C18:1) fraction of biodiesel. Conversion of C18:1 was multi-
plied by the percentage of unsaturated ester (93 wt %) in the original biodiesel to arrive at an epoxy biodiesel content of 43% for the bulk epoxy biodiesel used for subsequent butoxylation. Although conversion of unsaturated ester was far from complete, a batch time of 6 h was deemed as the maximum for a commercially viable process and would provide a satisfactory basis for the butoxylation optimisation exercise. Butoxylation of Epoxy Butyl Biodiesel. Conversion of epoxy butyl biodiesel was monitored via GC-MS over the 6 h period, for the optimisation batches, by disappearance of the epoxy butyl stearate peak. The main byproduct of this reaction are the aforementioned glycol and the keto form as a result of rearrangement of oxirane, as reported by Rios et al.10 Mass spectra of the major peaks confirmed these identities, as detailed by Smith et al.8 Butoxy selectivity (SA) was calculated from the relative areas of the butoxy (mA), glycol (mG), and ketone (mK) peaks. SA )
(
)
mB × 100 mB + mG + mK
(1)
Glycol and keto content in the butoxy biodiesel were calculated from the selectivity and conversion of unsaturated biodiesel (XB) as demonstrated for ketone: × 0.93 × X ( ( ketoneketone + glycol ))
mK(%) ) (1 - SA) × 1 -
B
(2) Temperature had a dramatic effect on conversion (Figure 1). At 100 °C, all epoxy was converted within 10 min at the initial conditions of 2 wt % sulfuric acid and 10:1 molar ratio of butanol to biodiesel. At the lowest temperature of 40 °C, conversion was not complete after 6 h, but at 60 °C it was complete within 4 h. Selectivity was dependent on temperature and increased sharply from a low of 79.4% at 40 °C to a maximum of 86.8% at 60 °C (Table 1). At 80 °C, selectivity was slightly lower, and 100 °C was much lower, due mainly to the increase in keto formation at this elevated temperature. Glycol formation was influenced by temperature and was nonexistent at 100 °C. Keto formation, however, mirrored the overall selectivity and was at a minimum at 60 °C. An optimal reaction temperature of 60 °C was chosen based on overall selectivity with the possibility of further reducing glycol and improving overall selectivity for subsequent batches. A clear positive trend is evident for catalyst concentration on conversion of epoxy to butoxy (Figure 2). Even at the lowest catalyst concentration of 1 wt %, 100% conversion was achieved within 4 h, whereas at 10 wt % conversion was complete in 1 h. Selectivity for butoxy butyl biodiesel displayed a similar trend to the temperature data with a slight increase from 1 to 2 wt %, then a rapid decline as catalyst concentration increased (Table 2). The glycol content was strongly dependent on catalyst concentration and increased significantly at >2 wt %. Keto content exhibited an inverse relationship with catalyst concentration but plateaued at g5 wt %. The catalyst optimization exercise therefore confirmed that a concentration of 2 wt % was optimal and was maintained for the alcohol ratio batches. Molar ratio of n-butanol had a significant positive effect on conversion of epoxy to butoxy (Figure 3). Butoxylation was complete within 1 h at a molar ratio of 40:1 and was barely (9) Wilson, R.; Smith, R.; Wilson, P.; Shephard, M. J.; Reimersma, R. A. Anal. Biochem. 1997, 248, 76–85. (10) Rios, L. A.; Weckes, P. P.; Schuster, H.; Hoelerich, W. F. Appl. Catal. A 2005, 284, 155–161.
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Figure 1. Effect of temperature on conversion of epoxy butyl biodiesel. Temperature: b, 40 °C; 9, 60 °C; 2, 80 °C; +, 100 °C.
Figure 2. Effect of catalyst concentration on conversion of epoxy butyl biodiesel. Catalyst concentration: b, 1%; 9, 2%; 2, 5%; +, 10%.
Figure 3. Effect of molar ratio of butanol on conversion of epoxy butyl biodiesel. Butanol molar ratio: b, 5:1; 9, 10:1; 2, 20:1; +, 40:1.
complete after 6 h at a molar ratio of 5:1. Selectivity was relatively unchanged for all molar ratios of butanol but was highest at 40:1 (Table 3). Glycol content reduced significantly with increasing molar ratio of butanol whereas keto content
increased slightly. A molar ratio of 40:1 for butanol to biodiesel was chosen as optimal, producing a selectivity of 87.9%. This can be contrasted with the figure of 81.7% for the process described in the earlier work.8 Given the marginal impact of
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Table 4. Effect of Higher Temperature and Catalyst Concentration on Selectivity for Butoxy Butyl Biodiesel and By-product Content after 6 h of Reaction Time temperature (°C)
catalyst concentration (wt %)
glycol (wt %)
ketone (wt %)
selectivity (%)
80 80 100 100
2 5 2 5
0.5 2.4 0.2 0.0
4.0 3.8 4.9 5.0
89.4 85.4 87.8 88.3
Table 5. Impurity Profile and Cloud Point Results Forvarious Butoxy Butyl Biodiesel Batches butoxy selectivity (%) glycol (wt %) ketone (wt %) cloud point (°c) 83.6 80.6 83.0 87.0 83.4
3.8 3.1 1.6 0.6 0.0
2.8 5.2 5.6 4.9 7.1
-4 -3 -3 -3 -5
alcohol ratio on selectivity, a lower ratio could be chosen for an industrial-scale process to reduce the costs associated with a larger working volume and the corresponding cost increases due to the requirement to recover the excess alcohol. As the required residence time increases with the reduction in alcohol ratio, an economic trade-off would need to be considered. In an attempt to further optimize the reaction conditions, four batches were produced at a 40:1 molar ratio of n-butanol, but at the two higher temperatures and at catalyst concentrations of 2 and 5 wt % (Table 4). The higher temperature of 100 °C resulted in reduced glycol content but increased keto content, as seen earlier. The higher catalyst concentration did not alter keto content but appeared to have a large impact on glycol content in the case of the 80 °C batch, which is consistent with the data presented in Table 2. From the results presented in Table 4, a larger batch at the optimal conditions of 80 °C and a catalyst concentration of 2 wt % was produced in order to determine the impact on biodiesel cloud point. The impurity profile of the larger batch was slightly lower than that for the smaller one with a selectivity of 87.0%. Impact on Cloud Point. The cloud point of butyl biodiesel was -3 °C, as was the cloud point of butoxy butyl biodiesel produced under the aforementioned optimal conditions. Butoxylation of butyl biodiesel, at the conversion rate of 46%, therefore had no impact on cloud point. To determine the impact of a higher conversion of unsaturated ester to butoxy ester, a batch of butyl biodiesel was subjected to 30 h of epoxidation, resulting in the conversion of 93% of the oleic portion. Subsequent butoxylation at the optimal conditions resulted in 100% conversion of epoxy and a selectivity for butoxy of 84.6%. Since the fraction of canola oil that is unsaturated is 93%, the butoxy content was calculated to be 74%. The glycol content was very low as for the lower conversion batches at 0.5%, but keto content was significant at almost 13%. The cloud point of this material was 2 °C, representing an increase of 5 K over the original butyl biodiesel. One may suspect that the relatively high content of byproduct may have influenced the cloud point of the butoxy butyl biodiesel. Data presented in Table 5 are for butoxy batches produced under various conditions from the same 43% epoxy butyl biodiesel. All batches had similar overall selectivities but a range of byproduct contents, from 0% glycol and 7.1% keto to approximately equal amounts of both. There is no discernible trend in cloud point with respect to either glycol, keto content, or both. A review of the literature for the relative melting points for the methyl esters reveals that methyl-9,10-dihydroxystearate has a melting point of 103 °C compared to methyl-9(10)-keto-
stearate at 46-48 °C.11 This can be contrasted with the melting point of methyl stearate at 39 °C. This suggests that the dihydroxy-esters will have a much greater impact on cloud point than the keto-esters. Even the batches in Table 5 with significant quantities of glycol had essentially the same cloud point as for the original butyl biodiesel. It is therefore unlikely that the cloud point of the butoxy butyl biodiesel batches were significantly affected by the byproduct content since the glycol content was always very low. Blends of the high conversion batch of butoxy biodiesel with butyl biodiesel were prepared to determine the cloud point trend across a range of fractions from 2 to 74 wt % (Figure 4). Cloud point was virtually unchanged at concentrations below 35% and then increased 1 K every 8 wt % to approximately 70 wt % butoxy biodiesel. The loss of unsaturated ester, due to conversion to butoxy ester, appears to have a significant effect on cloud point only after approximately one-third of the unsaturated ester is converted. Butoxy biodiesel is therefore able to prevent the earlier onset of crystallization due to the decrease in unsaturated content but only at lower concentrations. Once the proportion of unsaturated material had decreased by approximately onethird, crystallization temperature rose linearly. The major fatty acid contained in canola oil is oleic acid at 73 wt %, which has a cis- double bond at the 9,10 carbons of the fatty chain. The result is a fatty acid with a 30° bend in the middle of the tail-group. The nonlinear conformation of the unsaturated esters is what ordinarily restricts the molecules from approaching each other close enough for crystals to nucleate, resulting in a lower melting point than the saturated equivalent (stearate). Epoxidation of a cis-unsaturated fatty acid ester is known to produce a cis-epoxy fatty acid ester.12 Butoxylation under these relatively mild conditions results in the addition of only one molecule of n-butanol at either the 9 or 10 position and a hydroxyl group on either the 10 or 9 position in a transconformation. Thus, opening of the oxirane ring results in the conversion of a cis-isomer to a straight-chain trans-isomer, in this case a trans-R-hydroxy ether of the fatty acid ester. Therefore, butoxylation results in the linearization of molecules that were originally nonlinear. To investigate the impact of linearization of the ester molecules, without the complication of saturated fatty esters, an identical treatment to that for canola oil of oleic acid was performed. Technical grade oleic acid was subjected to esterification with butanol followed by epoxidation and alkoxylation under the aforementioned optimal conditions for the canola oilbased biodiesel. The conversion of butyl oleate to epoxy stearate was identical to that for biodiesel at 46%. The conversion to butoxy butyl stearate was complete, and the selectivity was 88.9%. The cloud point for butyl oleate was -18 °C, whereas the cloud point for butoxy butyl stearate was -12 °C, demonstrating that butoxylation results in a net increase in cloud point, even in the absence of saturated ester. To determine whether the increase in cloud point for butoxylated oleic acid was due only to the linearization of the molecule, butyl stearate was substituted for butoxy butyl stearate. Stearic acid was esterified with butanol and was blended with butyl oleate to produce the same relative fractions as that for the butoxylated butyl oleate (including the byproduct, which is also a linear molecule). If the cloud point impact of butoxylation was simply due to the linearization of the unsaturated portion of biodiesel, then the cloud point of a mixture of 46% butyl stearate and (11) Markley, K. Fatty Acids: Their Chemistry, Properties, Production, and Uses, Second ed.; Interscience Publishers: New York, 1967; Vol. Part 1. (12) Swern, D. Fatty Acids: Their Chemistry, Properties, Production, and Uses, Second ed.; Interscience Publishers: New York, 1967; Vol. Part 2.
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Figure 4. Cloud point of butoxy butyl biodiesel from 2 to 74 wt %.
54% butyl oleate would be similar to that for the 46% butoxy butyl oleate. The cloud point of the aforementioned blend of butyl oleate and butyl stearate was actually +18 °C. This represents an increase in CP of 30 K compared to the increase in CP of 4 K for the butoxy butyl oleate. The oleate/stearate blend also did not contain any of the keto or glycol product that is found in the butoxy butyl oleate, providing further evidence that the byproduct does not adversely influence CP. The reason for the failure to significantly alter the crystallization habit of biodiesel is likely to be the short length of the butoxy side-chain. Substitution of a second butoxy group on the other side of the chain may be more effective at interrupting the orderly alignment of the ester head-groups by providing more effective steric hindrance between the ester functional groups. Conclusion Optimal conditions for the butoxylation of epoxy butyl biodiesel were: 80 °C, 2 wt % sulfuric acid, and a 40:1 molar ratio of butanol over a period of 1 h. Conversion of epoxy butyl biodiesel was 100% and selectivity for butoxy biodiesel was 87.0%. The cloud point of butoxy butyl biodiesel (conversion
of 46% of unsaturated portion) was identical to that for butyl biodiesel. Butoxy butyl biodiesel at a conversion of 93% of unsaturated portion had a cloud point 5 K higher than that for butyl biodiesel. Blends of the high conversion batch of butoxy biodiesel showed that cloud point was virtually unchanged at concentrations below 35% and then increased 1 K every 8 wt % to approximately 70 wt % butoxy biodiesel. The loss of unsaturated ester, due to conversion to butoxy ester, appears to have a significant effect on cloud point only after approximately one-third of the unsaturated ester is converted. Butoxy biodiesel is therefore able to prevent the earlier onset of crystallization due to the decrease in unsaturated content but only at lower concentrations. Once the proportion of unsaturated material was lowered by approximately one-third, crystallization temperature rose linearly. Butyl biodiesel can be butxoylated under mild conditions and reasonable reaction times, but this process does not offer an improvement in biodiesel cloud point. The length of the butoxy side-chain is not sufficiently long to cause disruption to the normal crytallisation mechanism for fatty esters. EF9001633
