The synthesis and application of a hybrid catalyst for the esterification of free fatty acids (FFA) in Jatropha oil is reported. Three catalysts, namely silica sulfuric acid, silica supported boron trifluoride and a combination of the two in the weight ratio of 1:1, the hybrid catalyst, were investigated. Jatropha oil samples with a wide range of FFA values i.e. 6.64 to 45.64% were prepared and utilized for the experimental work. This study revealed that silica sulfuric acid and silica supported boron trifluoride were not very effective when used independently. However, a strong synergistic effect was noted in the catalytic activity of the hybrid catalyst which reduced the FFA value from 45.64 to 0.903% with a conversion efficiency of 98%. Reusability of the catalyst was also tested and the results were promising in up to three cycles of use when used with lower amounts of FFA (6.64%) in the oil. Under the influence of the catalyst, the reaction was found to follow first order kinetics. Activation energy was calculated to be 45.42 KJ·mol−1 for 2 wt.% of hybrid catalyst. The products were analyzed by FT-IR and NMR spectroscopic techniques and the results are reported.
The most convenient feedstock for fatty acid alkyl esters is plant oils and animal fat. It is reported that more than 95% of biodiesel is being produced from edible oils (Lee
Although there is not much work reported for solid acid catalysts for the oil esterification process, some notable catalysts reported include Nafionl resins, sulphonated zirconia, suphonated sacharides and tungton oxides (Otadi
For the catalyst in an esterification reaction, Brönsted acids such as sulfuric acid, hydrochloric acid, phosphoric acid and Lewis acids such as BF3 and TiCl4 can be used. Sulfuric acid is the most commonly employed catalyst but despite good conversion, its use also presents many problems. A common hurdle is its native acid nature which severely hampers its ease of application. Relatively poor efficiency at lower temperatures and a darkening of the color of the product at higher temperatures are common deficiencies of sulfuric acid use. In addition, removal of the liquid catalyst from the bulk product requires several washing steps (Borges and Díaz
The current study focuses on the usage of heterogeneous catalysts based on BF3 and SO3 groups attached to silica to esterify very high amounts of FFA to methyl esters. The catalytic activity of these catalysts has been studied for various processing conditions. The effects of methanol concentration as well as various FFA contents in oil have also been described. Kinetics and thermodynamic investigations are also reported.
Jatropha oil was obtained from BATC Development Bhd., Malaysia. Methanol, Sodium sulfate, Silica and hexanes were Analytical Reagent (AR) grades from Fisher Scientific. Boron trifluoride - diethyl etherate complex (50% BF3), formic acid, sodium bicarbonate and chlorosulfonic acid were obtained from Merck. All materials were used as received without further purification.
Silica supported-BF3 (SSB) was prepared according to Wilson and Clark (Wilson and H. Clark
The silica sulfuric acid (SSA) catalyst was prepared according to Khalifi
An amount of SSA and SSB was mixed in a 1:1 (weight ratio) to make the hybrid catalyst (HC).
The crude Jatropha oil had an FFA value of 6.64% (as oleic acid). A portion of the oil was converted to its fatty acids by saponification with NaOH and subsequent treatment with sulfuric acid. The sulfuric acid was removed with repeated washes using distilled water until the pH of the washings was neutral. The fatty acids were dried at 105 °C. Jatropha oil was spiked with different amounts of fatty acids to obtain FFA values of 9.25, 15.47, 27.85, 35.01 and 45.64% with corresponding acid values of 18.41, 30.79, 55.42, 69.67 and 90.88 mg KOH·g−1 of oil, respectively. In addition, the crude oil with a FFA value of 6.64% (acid value 13.21 mg KOH·g−1 oil) was also used in this study.
In a 300 mL three-neck round-bottom flask, equipped with a magnetic stirrer, coil condenser and thermometer, 50 g of filtered and dried Jatropha oil were transferred and heated to the required temperature (30–65 °C) with stirring at a fixed speed of 600 rpm. In a 250 mL beaker, the required amount of methanol (1–20 moles with respect to FFA moles of oil) and catalyst (0.5–3.0 wt.%) were mixed and heated to the required temperature (30–65 °C). Then the methanol-catalyst mixture was transferred to the oil bearing flask and stirring was continued for 30–180 min. Then the mixture was transferred to a separatory funnel to separate the methanol and oil methyl esters. After 30 minutes, the two layers were separated and the oil +FAME layer was washed with warm (55 °C) water till the washings were neutral to pH. The treated oil was dried over anhydrous sodium sulfate.
Biodiesel purity was confirmed in accordance with ASTM D 6584-00 with a Shimadzu GC 2010 system with fitted FID-2010. The fatty ester profile was determined by GC-MS with an Agilent 7890A GC System coupled with an Agilent 5975C inert XL EI/CI MSD with Triple-Axis Detector. The capillary column was BP5, 30 m × 250 µm × 0.25 µm. The oven temperature program was as follows: 3 min at 100 °C, 25 °C·min−1 to 170 °C, 2 °C·min−1 to 230 °C, 20 °C·min−1 to 250 °C and kept at 250 °C for 10 minutes (Wilson, Smith et al.
The esterification progress was monitored according to the AOCS Official Method Cd 3a–63. FT-IR spectra were recorded using a Perkin Elmer Spectrum One FT-IR spectrometer equipped with a ZnSe 45o HATR assembly. An average of 30 scans was used with a spectral resolution of 4 cm−1 for the range of 4000 - 400 cm−1 wave number.
The NMR analysis was performed on a Bruker Ultrashield 400 at 400 MHz and 100 MHz to determine the 1H NMR and 13C NMR spectra, respectively. The solvent used was chloroform-d.
The FFA value was determined according to the AOCS Official Method Cd 3a-63. FFA conversion was calculated as:
Where, FFAi refers to initial FFA value (%) and FFAt is the FFA value (%) at a specified time. The reported values are mean values of at least three observations.
The fatty acid composition of Jatropha oil is presented in
Fatty acid composition of Jatropha oil
Fatty acids | Common acronym | Composition (wt.%) |
---|---|---|
Palmitic acid | C16:0 | 13.5 |
Palmitoleic acid | C16:1 | 0.9 |
Stearic acid | C18:0 | 7.1 |
Oleic acid | C18:1 | 44.3 |
Linoleic acid | C18:2 | 32.8 |
Others | – | 1.4 |
The esterification reaction of fatty acids with alcohols requires a catalyst to proceed, as shown below. R-COOH + CH3OH ⇔ R-COOCH3 + H2O (1)
As the methanol is not soluble in Jatropha oil and fatty acids (Zhou
The stirring speed is important in heterogeneous systems for better mixing and facilitation in a mass transfer process. It becomes an important factor at lower reaction temperatures. However, at higher temperatures, particularly near the boiling of methanol, the boiling can provide sufficient agitation required for the reaction. It has been shown by several studies that a stirring speed beyond 600 rpm has no significant effect on the conversion (Mbaraka
The esterification reaction is highly dependent on the reaction temperature. Generally for heterogeneous catalysts, the reaction temperature was studied in the range of 45–65 °C because a higher temperature will require a pressurized setup for synthesis due to the low boiling point of methanol (Shahid and Jamal
Effect of temperature on FFA conversion
It can be observed that the hybrid catalyst (mixture of two catalysts) showed better catalytic activity even at a low reaction temperature. The conversion steadily increases with an increase in temperature and 98.2% FFA conversion was obtained at 65 °C. The effect of individual catalysts SSA and SSB is somewhat lower than that of the hybrid catalyst and a conversion of 92.2% and 66.4% was achieved for SSA and SSB, respectively. The silica supported boron trifluoride is found to possess a lower efficiency compared to silica sulfuric acid. This can be due to the higher apparent acidity of the SSA. The synergistic effect of the hybrid catalyst is probably associated with the additive effect between the very strong Lewis acid capability of BF3 and superior Brönsted acidity of SO3.
Reaction time is one of the most important factors affecting the esterification reaction. It is also connected with the reaction temperature. At higher temperatures, relatively short reaction times are sufficient for a maximum conversion. The effect of reaction time with three heterogeneous catalysts is shown in
Effect of reaction time on FFA conversion
Methanol is the major reagent in the reaction. As esterification reaction is a reversible reaction, a high molar excess of methanol is required for driving the reaction forward. In many studies a large excess of methanol has been employed, such as 1:50 and 1:60 [10,28]. The effect of the amount of methanol is shown in
Effect of methanol amount on conversion of FFA
The role of high molar excess of methanol in this context is further discussed in the
Catalyst amount is also an important parameter in the esterification reaction and varying the amount of catalyst shows a noticeable effect (
Effect of catalyst amount on conversion of FFA
The plot shows the maximum conversion (98.21%) for HC at 2 wt.%, and there was no significant effect found for the amount of catalyst exceeding 2%. The SSB and SSA showed a different trend and were lower in performance when compared to the hybrid catalyst even at 3 wt.% loading. SSB converted 69.2% FFA at 3 wt.% and SSA converted 93.6% FFA at 3 wt.% amount.
The cumulative effect of varying the amount of FFA and methanol was also studied. A reaction was conducted at 65 °C with 2 wt.% of hybrid catalyst. It was found that the amount of methanol required for maximum conversion was strongly dependent on the initial FFA value of oil. At lower FFA values (6.64 and 9.25%), a relatively lower FFA:methanol molar ratio (1:6) is sufficient but at higher FFA values, a very high molar ratio of methanol is required (1:15–1:20). The lower molar excess of methanol such as 1:10 and 1:6 are not sufficient for maximum conversion at FFA values of 35.01 and 45.64%. This effect for the hybrid catalyst is presented in
Effect of initial FFA amount and methanol / FFA mole ratio on FFA conversion
The hybrid catalyst was evaluated for re-usability in two types of oil samples with FFA values of 6.64 and 45.64%. For each type of oil, the same portion of catalyst was used each time. The catalysts for each experiment were used for four times. After each use, before the next experiment, each catalyst was washed and dried as described above. Results are presented in
Reusability of catalyst with oils having different FFA values
Experiments showed that the efficiency of the catalyst was drastically affected when used in the oil with high FFA values (45.64%). The efficiency of the catalyst was noted to be 97.5% when it was new and used for first time in this experiment. In the next experiment, the same catalyst exhibited 66.8% efficiency, and a significant decrease in catalytic activity was observed. In the next two tests, third and fourth cycle, the efficiency further decreased to 31.2% and 18.3% respectively. However, the catalyst was found relatively less effected and maintained its high effectiveness when used with the oil with low (6.64%) FFA. For low FFA oil, catalyst efficiency was high (58.3%) even for the fourth cycle of usage.
The substantial effect on catalyst efficiency with oil of high amounts of FFA was probably due to the production of high amounts of water during the reaction. The catalyst probably lost its one or both effective groups in water at high temperatures and its efficiency drastically reduced in subsequent cycles.
A very small amount of triglycerides can be converted to methyl esters by using an acid catalyst. Therefore, base catalyzed transesterification was performed by the method discussed earlier, using sodium methoxide. It was noted that the pre-treated oil was found to be suitable for the transesterification reaction and a high conversion of oil to methyl esters, 93.12%, was achieved. A few important properties of the obtained methyl esters are presented in
Properties of Methyl esters
Properties | Jatropha biodiesel |
---|---|
|
99.1% |
|
112 ppm |
|
86 ppm |
|
321 ppm |
|
183ppm |
|
0.8271 g·mL−1 |
|
4.11mm2· s−1 |
|
2 °C |
|
0 °C |
|
99.27 g I2·100g−1 |
|
0.18 mg KOH·g−1 |
|
162 ppm |
FT-IR spectra were recorded for the crude oil, oil with FFA and produced biodiesel. A transmittance band for carbonyl group in fatty acids appears at 1711 cm−1(a) and for methyl esters (and oils) at 1746 cm−1 (Guillén and Cabo
FT-IR spectra showing the course of FFA conversion reaction
Oil: 3007 cm−1(=C-H cis stretch), 2923, 2852(CH2 stretch), 1746 (ester C=O), 1711 (acid C=O), 1654 (-C=C- stretch),1460 (-C-H bend from CH2, CH3),1419 (=C-H bend), 1376 (CH3sym bend), 1240, 1165, 1118 (ester CO stretch),1099 (C-O stretch), 970 (-HC=CH- bend721 (CH2 rocking).
Methyl esters: 3006 cm−1 (=C-H
NMR spectroscopy is a more powerful technique than FT-IR spectroscopy in terms of the structural elucidation of molecules. There are several obvious differences in the spectra of oil and methyl esters. In the 1H NMR spectra, glyceryl protons appear at 4.1 and 4.3 ppm in the form of typically shaped closely spaced peaks (both quintets). The presence of these peaks in a spectrum is a confirmed indication of the presence of glycerides. In the methyl esters, these peaks are absent. Therefore, it is a quick test for the presence of glycerides in the sample.
A comparison of the 1H NMR spectra of Jatropha oil and methyl esters is shown in
Comparison of 1H NMR spectra of treated Jatropha oil and its methyl esters
The details of all peaks are presented below:
Jatropha oil (treated and contains very low FFA):
The esterification reaction of FFA with a large excess of methanol in oil is reported to be of first order (Berrios
Effect of reaction time at different reaction temperatures on FFA conversion
Plot between ln [FFA]t/ [FFA]i and reaction time (s)
where [FFA]i and [FFA]t are the FFA values at the start and at a particular time, respectively.
ln[FFA]t/ [FFA]i and time (s) were plotted for all temperatures and a linear regression of each curve was used to calculate the value of K. The results are presented in
Reaction rate constants at different temperatures
Temperature (K) | Rate constant K (s−1) | Correlation coefficient (R2) |
---|---|---|
303 | 1.09×10−4 | 0.9876 |
313 | 1.74×10−4 | 0.9894 |
323 | 2.94×10−4 | 0.9969 |
333 | 4.95×10−4 | 0.9937 |
338 | 7.30×10−4 | 0.9965 |
The activation energy (Ea) was calculated by using the Arrhenius equation:
where A is the frequency factor, Ea is activation energy (kJ·mol−1), T is temperature (K), and R is gas constant (J·K−1·mol−1).
The plot between ln K and 1/T is used to calculate the energy of activation (Ea) (Sagir, Tan et al.
Plot between ln
In this study three solid catalysts namely silica sulfuric acid (SSA), silica supported boron trifluoride (SSB) and a hybrid catalyst (HC),which was a 50-50 mixture of SSA and SSB, were evaluated for the esterification of high FFA in Jatropha curcas oil. Experimental results showed that the catalytic activity of the hybrid catalyst was superior to that of SSA and SSB alone. A very strong synergistic effect was noted in catalytic activity when the hybrid (mixed) catalyst was used, and an initial FFA value of 45.64% was reduced to 0.903%, with 98% efficiency. The reaction conditions were optimized for the oil FFA:methanol mole ratio of 1:15, a reaction temperature of 65 °C and a reaction time of 100 minutes. Furthermore, while using 2 wt.% of hybrid catalyst for oil samples with varying FFA values from 6.64 to 45.64%, the FFA conversion to methyl esters was more than 98%. The catalyst was found re-usable for three times for the oil with low FFA (6.64%) values and was found relatively less attractive for re-using in the case of oil with a high FFA (45.64%) value. The catalytic conversion reaction of FFA to methyl esters with this catalyst was found to follow the first order kinetics. Activation energy was calculated to be 45.42 KJ.mol−1for 2 wt.% of the hybrid catalyst. It is concluded that the studied hybrid catalyst can be used successfully to esterify natural oils to methyl esters for both high and low amounts of FFA and with good reusability potential for relatively low FFA oils.
The authors gratefully acknowledge the financial support of PETRONAS Research Sdn. Bhd. (PRSB) through PRF Project 158200042 and the use of EOR Centre of Excellence facilities.