Article pubs.acs.org/EF
Effect of Different Alcohols and Palm and Palm Kernel (Palmist) Oils on Biofuel Properties for Special Uses Claudia Cristina Cardoso Bejan,*,† Vinicius Guilherme Celante,‡ Eustáquio Vinicius Ribeiro de Castro,§ and Vânya Márcia Duarte Pasa∥ †
Departamento de Química, Universidade Federal Rural de Pernambuco, Rua Dom Manoel de Medeiros, s/n, Dois Irmãos, 52171-900 Recife, Pernambuco, Brazil ‡ Instituto Federal de Educaçaõ , Ciência e Tecnologia do Espírito Santo, Campus Aracruz, Avenida Marobá, 248, Marobá, 29192-733 Aracruz, Espírito Santo, Brazil § Departamento de Química, Universidade Federal do Espírito Santo, Avenida Fernando Ferrari, 845, Goiabeiras, 29060-900 Vitoria, Espírito Santo, Brazil ∥ Fuel Laboratory, Departamento de Química, Instituto de Ciências Exatas (ICEx), Universidade Federal de Minas Gerais, Avenida Antônio Carlos, 6627, Pampulha, 31270-901 Belo Horizonte, Minas Gerais Brazil ABSTRACT: The properties of biodiesel are determined by its oleaginous composition and the alcohol used in the transesterification. This study was performed using palm and palmist oils and methyl, ethyl, isopropyl, and benzyl alcohols to investigate their influence on the cold flow properties, density, and viscosity of the resulting biofuel. The products were also characterized by 1H nuclear magnetic resonance (NMR) and high-performance liquid chromatography (HPLC) analyses. Biofuel produced from palmist oil had different properties compared to biofuel synthesized from palm oil because of its lower average chain length and lower content of unsaturation. Biodiesel produced with palmist oil and isopropyl alcohol had the lowest values of cold filter plugging point (CFPP) (−16 °C) and density (860 kg/m3). The use of benzyl alcohol yielded a biofuel with high viscosity and density values that do not adhere to any international specifications for biodiesel.
1. INTRODUCTION There are many different definitions and specifications for biodiesel in the world. The Brazilian (Resoluçaõ 14-ANP) and the U.S. (ASTM D6751) specifications define biodiesel as alkyl esters of long-chain fatty acids derived from vegetable oils and animal fats. According to the resolutions, the type of alcohol used for the transesterification reaction is not specified. The European biodiesel specification (EN 14214) is more restrictive and applies the definition of biodiesel only to monoalkyl esters made with methanol, the fatty acid methyl esters (FAMEs). Studies of biofuel using superior alcohols, including benzyl alcohols, have rarely been performed, possibly because of the restrictive specifications cited. Despite this alcohol restriction imposed by the European specification for biodiesel, perhaps because of the toxicity and also the fact that methanol is not a renewable commodity, the transesterification reaction has been performed with different alcohols, such as ethanol and isopropanol, with the aim to improve the cold flow properties of biodiesel fuel.1,2 Ethanol has been widely studied, especially in Brazil and Spain,3 because it is renewable, with low environmental impact, and can be produced from agricultural resources.4−6 Although isopropyl alcohol is used instead of ethanol to further improve the cold flow properties of biodiesel in soybean transesterification, this alcohol is significantly more expensive, thereby causing some restrictions to its use on a large scale.1 Moreover, to the best of our knowledge, there is no report in the literature on the use of benzyl alcohol in biofuel production, despite its low cost. In the present study, the use of benzyl © 2014 American Chemical Society
alcohol was investigated with the aim to evaluate the effect of the aromatic ring on the biofuel properties. The study is motivated by the fact that many aromatic additives are used in biodiesel fuel as a pour point depressant and thermal stabilizer because these aromatics are known to be more thermally stable than paraffin.7 The overall properties of the biodiesel fuel also depend upon the individual characteristics of the fatty esters present at the triacylglyceride content of the vegetable oil or animal fat used during the transesterification process. Structural parameters of some fatty esters, such as chain length, degree of unsaturation, and branching, are important to biodiesel properties. Hoekman et al. discussed that the use of feedstock composed primarily of a medium-chain fatty acid, such as lauric acid, or unsaturated fatty acids, such as oleic or linoleic acid, is adequate to improve the cold flow properties of biodiesel fuel.8 Palm oil is one of the most abundant oils in the world, with a production of 4.2 tons ha−1 year−1, which is higher than the production of soybean (0.3 tons ha−1 year−1).9 The fruit from the palm tree can be divided into mesocarp and endocarp. The oil produced from the endocarp is named palmist (or palm kernel) and is composed of medium-chain fatty acids, such as lauric acid (C12:0), whereas palm oil is extracted from the mesocarp is composed primarily of palmitic (C16:0) and oleic (C18:1) acids in approximately a 1:1 ratio. While palm oil has Received: April 7, 2014 Revised: June 25, 2014 Published: June 27, 2014 5128
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Table 1. Fatty Acid Composition of Palmist Oil (Pt), Palm Oil (Pm), and Their Respective Methyl (M), Ethyl (E), Isopropyl (I), and Benzyl (B) Esters, as the Total Ester Content Quantified by 1H NMR palmist
a
palm
FA
Pt
PtM
PtE
PtI
C8:0 C10:0 C12:0 C14:0 C16:0 C16:1 C18:0 C18:1n9c C18:2n6c C18:3n6 C20:0 C20:1 saturated unsaturated ester contenta (1H NMR) (%) FAME content (1H NMR) (%)a total ester content (1H NMR) (%)
2.03 2.36 44.61 14.10 10.75
2.91 3.17 47.73 16.04 9.75
2.27 12.38 0.70
1.60 16.44 2.07
2.94 3.08 45.16 15.65 9.92 0.18 1.97 18.19 2.41
2.93 3.43 48.51 15.12 8.61 0.11 1.89 16.48 2.74
78.72 20.78 87.0 12.0 99.0
80.49 19.33 75.0 17.7 92.7
0.09 1.84 76.21 14.92
81.20 18.51 99.3 99.3
PtB
Pm
PmM
PmE
PmI
PmB
2.35 40.76 16.59 11.50
0.60 0.82 41.59
0.53 0.92 41.17
3.30 21.99 3.50
5.07 44.05 7.43 0.20 0.24
4.46 44.10 8.52 0.12
0.46 0.84 40.15 0.69 4.73 44.30 8.51 0.16
0.44 0.84 40.22 0.67 4.37 44.74 8.45 0.08
0.39 0.78 39.01 0.28 5.29 45.46 8.58 0.22
74.50 25.49 75.0 11.3 86.3
48.32 51.68
47.08 52.74 99.6
46.18 53.66 84.0 14.0 98.0
45.87 53.94 74.0 19.0 93.0
45.47 54.54 77.0 18.0 95.0
99.6
These ester contents are referred to the amount of ester obtained from the superior alcohols, apart from the FAME presented at the mixture.
been widely used and studied for biodiesel applications,10 information about the fuel properties of biodiesel derived from palmist oil11−15 or its use in blends16 is still limited. Although biodiesel has environmental benefits over conventional diesel fuel, certain problems that have persisted with biodiesel are its properties at low temperatures, something that can be changed using the appropriate feedstock or additives. Because of the low-temperature properties, the use of biodiesel with a high cold filter plugging point (CFPP) value in cold countries may cause wax crystals, gels, and insoluble compounds to clog vehicle engines when fuel passes through the filtration system.17 We found no report in the literature on the comparison of the cold flow properties of biofuels derived from palm and palmist oils produced using different alcohols, such as the methyl, ethyl, isopropyl, and benzyl alcohols, studied here. In addition to the analysis of the cold flow properties, a comparative study on other important properties, such as kinematic viscosity and density, for all of the products is also presented. These results show the differences in the biofuel properties depending upon the oil and the alcohols used and are compared to regulatory limits imposed by current international legislations for biodiesel.
(6:1 molar ratio of methyl, ethyl, isopropyl, or benzyl alcohol) and sodium methoxide as the catalyst (3.3 g, 1%, w/w). The mixture was stirred at 65 °C for the methanol preparation and at 85 °C for the preparation with other alcohols. After 1 h, the resulting upper alkyl or benzyl layer was separated and washed with warm water (40 °C) and dried over Na2SO4. The remaining alcohol was evaporated under reduced pressure. For the reaction performed with benzyl alcohol, an extra step was adopted to remove the remaining alcohol after washing the ester layer with water. The reaction product was washed with 10 mL of methanol up to 5 times, as necessary, and then the excess methanol was removed by distillation under reduced pressure. 2.3. Analytical Methods. The CFPP, density, and kinematic viscosity of the samples were determined in triplicate according to ASTM D6371, ASTM D4052, and ASTM D445, respectively. In addition, the ester content of the methyl ester from palmist and palm oils was determined by GC according to EN 14103:2011, which was adapted for the use of methyl heptadecanoate as an internal standard, using a Shimadzu GC-2010 gas chromatograph with an AOC-5000 auto injector and a DB-Wax 30 m × 0.32 mm × 0.250 μm capillary column. Helium was used as the carrier gas (2 mL/min). To determine the profile of the fatty acids from the palm and palmist oils as well as that of their respective esters produced with methyl (M), ethyl (E), isopropyl (I), and benzyl (B) alcohols, the sample was previously hydrolyzed, followed by a methylation step.18 Their respective methyl esters were characterized using a HP5890 gas chromatograph with a flame ionization detector. The HP-INNOWax (HP) (30 m × 0.25 mm × 0.20 μm) capillary column was used with a temperature gradient of 100 °C for 1 min, followed by an increase of 7 °C/min up to 240 °C, a split of 1:50, and a detector temperature of 250 °C. Hydrogen was used as the carrier gas (2 mL/min), and the injection volume was 1 μL. The identification of the peaks was performed by comparison to the retention time, using the standard fatty acid methyl esters SUPELCO37. HPLC was used to detect the presence of the fatty acid (FA), monoacylglycerides (MG), diacylglycerides (DG), triacylglycerides (TG), and the alkyl and benzyl esters using a Shimadzu LC-20AT chromatography system [SIL-20A HT automatic sampler, DGU-20A5 degasser, CTO-20A column oven, SPD-M20A UV−vis detector, SIL20HT pump, and Shimadzu CLC-ODS C18 column (M) (25 × 4.6 mm)] at 40 °C, with a flow rate of 1 mL/min and a detector with the wavelength set to 205 nm. The samples were diluted with a 10% solution of 5:4 (v/v) isopropyl alcohol/hexane. HPLC-grade methanol was used in line A, and a solution of 5:4 (v/v) isopropyl alcohol/
2. EXPERIMENTAL SECTION 2.1. Materials. Typical refined, bleached, and deodorized palm and palmist oils were supplied by the Agropalma Group and used without further purification. Reagent-grade methanol (99.8%), ethanol (99.5%), isopropyl alcohol (99.5%), and sodium methoxide in methanol (30%) were acquired from VETEC. Benzyl alcohol (99.0%) was purchased from SYNTH, and CDCl3 containing 1% tetramethylsilane (TMS) (D, 99.8%) was purchased from CIL. Highperformance liquid chromatography (HPLC)-grade methanol (J. T. Baker), isopropyl alcohol (J. T. Baker), and hexane (Mallinckodt) were used as eluents for HPLC analysis. A solution of BF3 in methanol (Sigma-Aldrich) was used for the methylation step of the fatty acids during the preparation of the samples for gas chromatography with flame ionization detection (GC−FID) analysis. 2.2. Biodiesel Synthesis. Alkyl and benzyl esters were prepared by transesterification of approximately 100 g of the oils with alcohol 5129
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Figure 1. Overlay of the 1H NMR spectra (200 MHz, in CDCl3) of palmist oil and its respective methyl (PtM), ethyl (PtE), isopropyl (PtI), and benzyl (PtB) esters. hexane was used in line B. A linear gradient was applied from 100 to 50% A and 50% B in 15 min at 40 °C with a flow rate of 1 mL/min and a wavelength of 205 nm. The elution was performed isocratically with 100% B for an additional 8 min. The crystallization onset temperature (Tco) was measured using a differential scanning calorimeter (DSC Q200 V24.4 Build 116). Samples of approximately 15 mg were weighed into aluminum pans. Samples were cooled from +25 to −75 °C at 20 °C/min and maintained until the heat flow stabilized. Then, samples were heated back to +25 °C at 20 °C/min and maintained until the heat flow stabilized. Finally, under a N2 flow of 50 mL/min, the samples were cooled from +25 to −75 °C at a rate of 1 °C/min and the cooling curves were obtained. The Tco was determined as the high temperature end of the highest freezing transition peak from the DSC curve, which is the highest temperature at which the sample begins to freeze. 1 H nuclear magnetic resonance (NMR) was used to determine the yield of the TG conversion into esters using a 200 MHz Bruker Avance DPX 200 spectrometer. The samples were dissolved in CDCl3, and TMS was used as the reference.
biofuel prepared with superior alcohol has a disadvantage of time and energy demands when using different solvents and distillation process.1,19 However, instead of using the distillation process, especially for the case of the benzyl ester, an alternative process of washing the final product with methanol was preferable because exposure of the biodiesel to high temperatures can affect its oxidation stability.20,21 Table 1 shows the FA composition data of palmist (Pt) and palm (Pm) oils and their respective esters produced with methyl (M), ethyl (E), isopropyl (I), and benzyl (B) alcohols, abbreviated as follows: palmist methyl ester (PtM), palmist ethyl ester (PtE), palmist isopropyl ester (PtI), palmist benzyl ester (PmB), palm methyl ester (PmM), palm ethyl ester (PmE), palm isopropyl ester (PmI), and palm benzyl ester (PmB). The physical and chemical properties of the biofuel samples are partially influenced by the FA composition profiles, and their composition showed a slight difference compared to their respective feedstock (Table 1). The biofuel derived from palmist oil was composed primarily of medium-length saturated chains (primarily laureate, C12:0), whereas the palm-derived biofuel was primarily composed of long saturated chains (palmitate, C16:0, and oleate, C18:1). This difference in composition implies the presence of distinct characteristics and properties of each biofuel. The content of the methyl esters from palm and palmist oils was 97.9 and 95.1%, respectively. This result was obtained using GC, as proposed by EN 14103 specifications. However, this method is intended only for FAME and not for higher alcohols, and thus, it was not used for the other ester samples, obtained from ethyl, isopropyl, and benzyl alcohols. For this reason, 1H
3. RESULTS AND DISCUSSION 3.1. Biofuel Composition. The transesterification conditions of the reactions were the same, independent of the oil and the alcohol used, except for the reflux temperature when using methyl alcohol because of the lower boiling point of its mixture. For the reactions processed with isopropyl and benzyl alcohols, their excess was removed using special conditions because of their higher boiling points, which are above 100 °C at 1 atm. The excess of isopropyl alcohol was removed under a reduced pressure at 90 °C for more than 1 h, while the removal of the excess benzyl alcohol required an extra step of washing with methanol to remove the excess of the alcohol before the evaporation step. The special treatment required for purifying 5130
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NMR spectroscopy was used to quantify the relationship between the converted esters and their respective TG, inclusive of the FAME for comparative purposes. Figure 1 shows the 1H NMR spectra (200 MHz) of the palmist and its respective esters with different alcohols. The percentage yields for the TG conversion into esters (% C) were obtained directly from the peak area (I) of the selected signals from the 1H NMR spectra, as proposed in the following equations, depending upon the alkyl or benzyl ester. The subscript associated with the percent yield refers to the alcohol used in the transesterification reaction and indicated as methyl (M), ethyl (E), isopropyl (I), or benzyl (B) alcohol. The values are presented in Table 1 % CM = 100(2Ia /3IαCH2)
(1)
% C E = 100(Ib/IαCH2 + 2Ia /3IαCH2)
(2)
% C I = 100(2Ic/IαCH2 + 2Ia /3IαCH2)
(3)
% C B = 100(Id /IαCH2 + 2Ia /3IαCH2)
(4)
where Ia is the area of the three methoxy hydrogens in the region of 3.66 ppm (singlet),22 Ib is the area of the two αethoxy hydrogens at 4.07−4.17 ppm (multiplet),23 Ic is the area of the secondary hydrogens of the isopropoxy at 4.90−5.09 ppm (multiplet), and Id is the area of the two CH2-benzylic hydrogens in the region of 5.10 ppm (singlet). Note that, in eqs 2−4, the conversion of the oil into esters was considered as the sum of the ester content obtained with the corresponding alcohol (integration of the signals b−d) with the methyl ester content (integration of signal a at 3.6 ppm) because these esters were also formed using sodium methoxide as the catalyst. For the purpose of comparison, the ester content obtained for the FAME from palm and palmist oils was of 99.6 and 99.3%, respectively, using the 1H NMR method and 97.9 and 95.1%, respectively, using the GC method. Because it has shown a good relation between the methods, the results of ester conversion for the others ester using the 1H NMR method was considered. Analysis of the data obtained by 1H NMR spectra revealed a reduction in the total ester content as the length of the carbon chain of the alkoxy group increased. Because the minimum ester content proposed by EN 14214 is 96.5%, the results showed that the isopropyl and benzyl esters do not meet the standard specifications. Furthermore, Table 1 shows that the amount of FAME present as a contaminant in the ester samples obtained from the superior alcohol, especially for the PtI and PmI, is significant. This contamination reaches almost 19% (mol/mol) of FAME because of sodium methoxide used as the catalyst in a methanol solution (about 30 wt %). Such results can be attributed to the lower reactivity of isopropyl alcohol because of its lower acidity (pKa = 16.6) compared to methanol (pKa = 15.5) and ethanol (pKa = 15.9) and its greater steric hindrance.24,25 Although the acidity of benzyl alcohol (pKa = 15.4) is similar to that of methanol, its higher steric hindrance may also be responsible for the lower ester conversion for both palm and palmist oils. Figure 2 presents the HPLC chromatograms of palm and palmist oils and their respective ester samples with the different alcohols. By analyzing the HPLC chromatograms of the palmist oil, we can notice the presence of its TG through the peaks scattered between 11 and 22 min (Figure 2a), whereas for palm oil, the TG is present as four major peaks between 18 and 21 min. The presence of a small amount of DG in the palm oil is
Figure 2. Overlay of HPLC chromatograms of (a) palmist (Pt) and (b) palm (Pm) oils and their respective methyl (M), ethyl (E), isopropyl (I), and benzyl (B) esters.
indicated by the peaks in the region between 10 and 14 min (Figure 2b). The absence of those peaks in the ester chromatograms is evidence that there is neither TG nor DG in the samples. However, especially for the PmI, we observed the presence of MG or free fat acid (FFA) represented by the peaks located in the region with a retention time between 3 and 5 min. For the PtM, three peaks between 4.5 and 7.5 min are highlighted, which can be attributed to the respective methyl esters of C12:0, C14:0, and C18:1, according to GC analysis (Table 1), whereas PmM has two main peaks between 5.2 and 7.6 min because of the presence of C16:0 and C18:1. For the PtM, the presence of DG should be detected through peaks at the region between 8 and 11 min, while the MG and FFA should be found through peaks at the region between 3 and 4.5 min. The absence of those peaks indicates that those species are not considerably present in the sample. The higher number of peaks or the presence of broader elution bands on the chromatograms of the esters synthesized with ethyl, isopropyl, 5131
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The relationship between the wax crystallization and the close packing of the molecules can also explain the reduction in the crystallization temperature encountered with the replacement of the straight-chain methyl or ethyl moieties with branched-chain alcohol moieties, such as isopropyl, perhaps because of the reduction of intermolecular associations.19 This effect is in agreement with the finding that the CFPP value of biodiesel derived from palm oil is reduced with increasing the length of the carbon chain of the alkyl alcohol moiety. We observed the same tendency for the biodiesel derived from palmist oil, except for the CFPP value of ethyl ester. The strong London interactions among the aromatic rings of the benzyl ester and their consequent close packing may be responsible for the higher CFPP value compared to their respective alkyl esters, from both palm and palmist oils. The higher CFPP of the benzyl ester compared to the alkyl esters shows an opposite behavior to what was expected because additives of the aromatic nature are used to decrease the CFPP of biodiesels.7 The cold properties of biodiesel fuel can be evaluated by standard methods, such as cloud point (CP), pour point (PP), and CFPP. Certain studies have presented the evaluation of cold flow properties using a differential scanning calorimetry (DSC) apparatus, multipurpose equipment that has the advantage of using a small sample.7 DSC curves exhibit exothermic peaks for fuel crystallization, which, in turn, show a correlation between its maximum temperature peak and the CFPP value.27 Foon et al.28 used DSC to investigate the winterization of FAME of palm oil and their separated fractions, showing that their crystallinity depends upon the degree of unsaturation. In this study, the DSC analysis showed two major exothermic peaks for fuel crystallization. The first peak, occurring at a higher temperature, can be denoted as the starting point of the overall crystallization process related to the medium-length saturated methyl esters (primarily C16:0), and the second peak is related to the long-chain unsaturated methyl esters (primarily C18:1). Foon et al. also showed that the position of the first peak has a strong dependence upon the degree of unsaturation, decreasing from +20.5 to −23.2 °C as the percentage of total saturation also decreased from 95.9 to 9.9%.28 The second peak position remained virtually unchanged at approximately −48.4 ± 2.2 °C, regardless of the percentage of saturated methyl esters. In agreement with Foon et al.,28 the DSC curves for the biofuels derived from both palm and palmist oils obtained in the present study presented two main peaks, independent of the alkoxy group (Figure 4). However, because the saturated portion of the biofuel derived from palmist oil was composed primarily of short-chain fatty acids (C12:0) (Table 1), its Tco value was lower than that of the biofuel derived from palm oil,27 which was composed primarily of a palmitate ester (C16:0). For the biofuel derived from both palm and palmist oils, the tendency of reducing the Tco obtained by the DSC analysis is in agreement with the reduction in the temperature of the CFPP (Figure 3).27 Using DSC, we observed that the ethyl ester of palmist oil was not an exception to the rule, as occurred during the CFPP measurements. Lee et al.29 revealed that there was a typical Tco shift to a lower temperature from the methyl, ethyl, and isopropyl moieties in the biodiesel produced from soybean oil (primarily C18:1) to +5.2, +1.4, and −6.0 °C, respectively. The alkyl esters of palm and palmist oils in this study followed the same
and benzyl alcohols is due to the presence of methyl biodiesel in all of the samples, as previously evidenced in the 1H NMR spectra (Figure 1). This result occurred because the commercial catalyst used was prepared in a methanol solution. In addition, we observed that the higher the molar mass of the alcohol employed in the transesterification reactions, the longer the retention time of the respective esters (panels a and b of Figure 2). For a better view of the higher retention time of the superior alcohol, the respective peaks found for the methyl esters are highlighted, while we use a dashed line to distinguish on the chromatogram the retention time for the main FAME peaks present as a contaminant. 3.2. Biofuel Properties. 3.2.1. Cold Properties. The cold flow temperature is one of most important properties for determining the suitability of biodiesel for use in countries that experience very low temperatures. However, because of large seasonal and geographic temperature variability, neither the U.S. nor European biodiesel standards have established specifications for a lower limit for the CFPP.8 Figure 3 presents the CFPP values for the biofuel samples synthesized with various alcohols. The palmist biofuels
Figure 3. Tco and TCFPP relationship of the biofuels of palm (Pm) and palmist (Pt) oils and their respective methyl (M), ethyl (E), isopropyl (I), and benzyl (B) esters.
presented lower cold flow properties compared to palm biofuels because of their lower average chain length of the fatty acid, as presented in Table 1. The temperature of wax crystallization has a high correlation with the CFPP value8 and is related to the close packing of the molecules. Therefore, the higher ordered molecules present the poorest cold flow properties of the biofuel because of the stronger intermolecular forces of attraction. In addition, the length of the FA chain has a marked influence on the crystallization temperature of biodiesel, with the longest chain showing the highest CFPP value.19,26 Because of the natural cis configuration of the unsaturated FA chains, the spatial arrangement of the molecules disrupts its crystal packing ability, thereby lowering its crystallization temperature.26 Although the palm biofuel presented a higher content of unsaturated compounds (Table 1), which could reduce its CFPP value,8 this parameter does not appear to be relevant in our study. The palm and palmist oils in this study showed that the effect of the average chain length appears to be more prominent than the effect of the unsaturation content. 5132
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The relative difference in intensity between the two main exothermic events of these DSC curves (Figure 4) is accentuated more for the biofuel derived from palmist oil than with the biodiesel derived from palm oil, independent of the alkoxy group. The sizes of those peaks represent crystallization enthalpy and are related to the amount of the components in a mixture, which are in accordance with the composition of the biofuel shown in Table 1, showing that the biofuel derived from palmist oil has more saturated esters than the biofuels derived from palm oil.27,28,30 The results obtained in our study showed that the best alternative to obtain a biofuel with low cold flow properties is to use the palmist oil. None of the palm esters, even with the use of isopropyl alcohol, yielded biofuel with negative crystallization temperatures. Probably the solidification temperature for the ethyl and isopropyl esters of palm and palmist oils should be lower than the temperatures presented in this study if the FAME contamination could be reduced. One alternative for that would be through the transesterification reaction using the alkoxide prepared with their respective alcohols, instead of sodium methoxide in methanol solution used here. 3.2.2. Kinematic Viscosity. The alkyl esters of palm and palmist oils adhere to the 1.9−6.0 mm2 s−1 viscosity limit specification (Table 2) set forth by the ASTM D6751. However, the more restrictive EN 14214 specification of 3.5− 5.0 mm2 s−1 for kinematic viscosity at 40 °C would exclude PtM and PmI. The benzyl esters of palm and palmist oils do not adhere to any of the specifications, presenting high values of viscosity. These high values can be one of the reasons that benzyl esters are not included as biodiesel fuels in the U.S. and European specifications. In agreement with the literature,4,26 biodiesel with longer FA chain lengths shows higher viscosities, which explains the higher viscosity for the respective biofuel from palm oil compared to that from palmist oil, independent of the alkoxy moieties (Table 2). Although Hoekman et al.8 and Demirbas31 have shown that the viscosity decreases with an increasing degree of unsaturation, Rodrigues et al. have concluded that one double bond increases the viscosity,26 whereas two or three double bonds reduce the viscosity in a comparison of chains with the same carbon number. This last observation can also explain the higher viscosity of the biofuel derived from palm oil compared to that from palmist oil because of its higher concentration of monounsaturated FAs. The same tendency was observed, in which the biofuel viscosity increased with an increasing length of the linear chain of the alcohol moieties in a homologous series of compounds because of the higher molecular interaction but with a lesser effect.24,26,32 Another reason for the small difference among the kinematic viscosities of the biofuels obtained from the same oil but with different alcohols can also be attributed to a reasonable amount of FAME contamination (Table 1) of the ester obtained with superior alcohol.
Figure 4. DSC curves for the biofuel of (a) palmist (Pt) and (b) palm (Pm) oils and their respective methyl (M), ethyl (E), isopropyl (I), and benzyl (B) esters. The arrows in the curves indicate the Tco point.
tendency in which the Tco shifted to a lower temperature from the methyl, ethyl, and isopropyl moieties. However, we can consider that the presence of FAME as a contamination (Table 1) of the esters produced with superior alcohol, using the same oil, can be responsible for the small differences between their Tco. The difference at the position of the second major exothermic peaks for fuel crystallization of the alkyl ester was more evident while changing the linear alkoxy to the isopropyl ester, for both esters of palm oil (−40, −41, and −56 °C for PmM, PmE, and PmI, respectively) and palmist oil (−54, −57, and −62 °C for PtM, PtE, and PtI, respectively). The close packing because of the strong London interactions among the aromatic rings of the benzyl ester, used to explain its high CFPP value, can also be applied to justify the increase in the Tco value for PmB and PtB compared to the respective biodiesel with the isopropyl moiety, even though they have a higher molecular weight (Figure 4).
Table 2. Density and Kinematic Viscosity of Palmist (Pt) and Palm (Pm) Methyl (M), Ethyl (E), Isopropyl (I), and Benzyl (B) Esters palmist ester viscosity (mm2/s) density (kg/m3)
palm ester
PtM
PtE
PtI
PtB
PmM
PmE
PmI
PmB
3.12 870.51
3.52 866.54
3.79 861.13
6.31 926.04
4.44 871.57
4.93 866.17
5.42 863.24
7.86 917.94
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the palm and palmist ester properties in light of kinematic viscosity, density, and CFPP, it became evident that the oil composition presents a more significant effect on the biofuel fuel properties than the nature of the alcohol used in the transesterification process. This finding can be attributed to the greater effect of the smaller size of the chain at the palmist ester, especially for cold properties. Alkyl esters of palmist oil showed the best CFPP value with negative temperatures, especially for the isopropyl esters.
The tendency of increasing the viscosity while increasing the alkoxy group may be the reason for the greater difference observed for the biofuel with benzyl moieties (PtB and PmB) compared to the biofuel with alkyl moieties because of their stronger intermolecular interaction. 3.2.3. Density. Although the ASTM D6751 standard does not include a specification for density, the European EN 14214 standard includes a density specification in the range of 860− 900 kg m−3, which includes all of the alkyl esters of palm and palmist oils and excludes the benzyl esters (Table 2). A study performed by Refaat26 has shown that density increases with a decreasing chain length; moreover, Demirbas31 found an inverse relationship between the density and viscosity. According to these relationships, we expected a higher density for the biofuel derived from palmist oil compared to the biofuel from palm oil. However, Refaat also affirmed that density increases with an increasing number of double bonds,26 which leads to the opposite behavior of the previous expectation. Therefore, the biofuel from palm oil would present a higher density value according to its higher content of unsaturated compounds, whereas the biofuel from palmist oil would present a higher density according to its shorter FA chain composition. In this case, one effect overrides the other, and therefore, biofuel derived from both palm and palmist oils presented similar densities for the same alkoxy group. These overriding effects may explain the finding by Hoekman et al.,8 in which there was no significant correlation between the density and the FA chain length of biodiesel. A previous study reported that a constant feedstock with a varying chain length of the alcohol moieties showed a decrease in the density of biodiesel as the linear carbon number increased.32 This behavior was also observed with biofuel of both palm and palmist oils. However, Lang et al.24 observed that density also decreases with the degree of branching in the alcohol moiety chain in the following order: methyl ∼ isopropyl > ethyl > 1-butyl. Together with that, the reasonable amount of FAME contaminant from the isopropyl esters of palm and palmist oils should contribute to this behavior of the similarity of density between the methyl and isopropyl esters. Nevertheless, we did not observe this tendency with the isopropyl biodiesel, although its density was lower than the biodiesels with methyl and ethyl moieties. In the case of the benzyl ester, although it has a higher number of carbons, it showed a decrease in the density of biofuel compared to the alkyl esters, because of the smaller molecular volume occupied by this compound, which is attributed to the stronger intermolecular interactions.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +55-81-33206370. E-mail: claudiacardoso75@ gmail.com. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the Universidade Federal Rural de Pernambuco (UFRPE) for the REUNI Fellowship and CNPq and FINEP for financing this study.
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4. CONCLUSION Of all of the normalized methods used for biodiesel production, only the FAME can be considered according to the European specification (EN 14214); however, esters obtained with higher alcohol are also considered as an alternative for a biofuel. In addition to that, transesterification of palm and palmist oils with superior alcohol as ethyl, isopropyl, and benzyl alcohols was performed. Their composition and some of their properties were compared to their standard FAME. 1H NMR analysis was used as an alternative to the international specification, to determine the ester content, which showed that methyl and ethyl esters offered the highest conversion. The isopropyl and benzyl esters also resulted in a good conversion but did not reach the minimum content of 96.5% imposed by the EN 14214. Moreover, benzyl esters appear inadequate as biodiesel fuel because of the high viscosity and density. In comparison of 5134
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Energy & Fuels
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