- Email: [email protected]
Synthesis of methyl esters and triacetin from macaw oil (Acrocomia aculeata) and methyl acetate over γ-alumina
Industrial Crops & Products 124 (2018) 84–90
Contents lists available at ScienceDirect
Industrial Crops & Products journal homepage: www.elsevier.com/locate/indcrop
Synthesis of methyl esters and triacetin from macaw oil (Acrocomia aculeata) and methyl acetate over γ-alumina
T
Jéssica S. Ribeiroa, Dian Celantea, Leoni N. Brondania, Débora O. Trojahna, Camila da Silvab, ⁎ Fernanda de Castilhosa, a b
Department of Chemical Engineering, Federal University of Santa Maria (UFSM), Av. Roraima 1000, Santa Maria, Rio Grande do Sul 97105-900, Brazil Department of Technology, Maringa State University (UEM), Av. Ângelo Moreira da Fonseca 180, Umuarama, Paraná 87506-370, Brazil
A R T I C LE I N FO
A B S T R A C T
Keywords: Interesterification Glycerol-free Triacetylglycerol Esterification
Glycerol excess in the market and high cost of common feedstock have encouraged the investigations of new technologies for biodiesel production. In this context, the use of macaw oil (Acrocomia aculeata) for biodiesel production through interesterification and esterification reactions with methyl acetate as acyl acceptor was evaluated, using heterogeneous catalyst γ-alumina (γ-Al2O3). Experiments were performed at different reaction temperatures (225–300 °C), catalyst contents (0–10% relation to oil mass), oil to methyl acetate molar ratios (1:10–1:40) and reaction times in a batch reactor to determine the effect of these variables in FAME (fatty acid methyl esters) and triacetin production. Catalyst γ-Al2O3 improved FAME and triacetin contents in comparison with no catalyzed reaction. Reaction at 300 °C, with 2% of catalyst in relation to oil mass and molar ratio of 1:20, in 60 min provided the best results. The catalyst could be recovered and reused for at least 6 cycles without significant activity loss. The results allowed to point out that the process showed to be promising for industrial biodiesel production with no glycerol and was financial attractive, since it used a low cost feedstock.
1. Introduction Biodiesel is as a strategic source of renewable energy with great potential to replace petro based diesel and reduce environmental pollution levels (Doná et al., 2013; Maddikeri et al., 2013). Although alkaline transesterification is industrially the most used process to produce biodiesel, it has some drawbacks such as the glycerol production as byproduct, which is difficult to be purified and to be sold due to market saturation, and the FFA content restriction in feedstocks (lower than 0.5 wt%) (Casas et al., 2010; Saka and Isayama, 2009; Marousek et al., 2015; Marousek and Kwan, 2013). Interesterification of triglycerides with methyl acetate as the acyl acceptor has been studied to produce FAME and triacetylglycerol (triacetin) as byproduct instead of glycerol (Casas et al., 2011a; Tavares et al., 2017). Triacetin is considered a fuel additive and its mixture with FAME does not present harmful effects on biodiesel properties and thus, no additional separation steps to recovery this byproduct are required. Besides that, due to its mutual solubility, triacetin may be added to biodiesel up to 10% by weight and the blended biodiesel still meets the quality standards (Casas et al., 2010; Goembira and Saka, 2015, 2013; Xu et al., 2005). Additionally, FFA present in feedstock reacts with
⁎
methyl acetate to form FAME and acetic acid and thus improves FAME yield rather than promoting the saponification reaction (Marx, 2016). Thus, interesterification process has potential to maximize the use of biodiesel feedstocks, due to the possible use of product and byproduct as an alternative fuel (Goembira and Saka, 2015). Chemical interesterification has lower operational costs and can be performed at mild reaction conditions, unlike enzymatic and supercritical reaction, respectively (Casas et al., 2011b). However, there is only a report available in literature concerning homogeneous catalysis in interesterification reaction (Casas et al., 2011b). Additionally, the only report available on heterogeneous catalysis is a catalyst screening developed by Ribeiro et al. (2017), where it was pointed out γ-alumina as the most suitable acid heterogeneous catalyst for this reaction from macaw oil and methyl acetate. Alumina exists in metastable forms and the γ transition form is the most important of them, because it presents good catalytic properties as high porosity, high specific surface area, large pore size and pore volume and is appropriate for biodiesel synthesis (Trueba and Trasatti, 2005; Zabeti et al., 2009). Final cost of this biofuel is mainly affected by the raw material that represents around 70–95% of the total value (Maddikeri et al., 2013). Thus, the use of other feedstocks, like crude macaw oil (Acrocomia
Corresponding author. E-mail address: [email protected] (F. de Castilhos).
https://doi.org/10.1016/j.indcrop.2018.07.062 Received 4 May 2018; Received in revised form 11 July 2018; Accepted 24 July 2018 0926-6690/ © 2018 Elsevier B.V. All rights reserved.
Industrial Crops & Products 124 (2018) 84–90
J.S. Ribeiro et al.
sample was scanned over the angular range 5–80° (2θ) with a step size of 0.03° and a scan rate of 1 s per step. Textural properties were determined by a well known nitrogen adsorption-desorption procedure at 77 K in an analyser (Micromeritics, model ASAP 2020). Specific surface area (S) was determined by Brunauer, Emmett and Teller (BET) method and total pore volume (VP) by Barrett-Joyner-Halenda (BJH) method. Catalyst composition was confirmed with X-Ray Fluorescence (XRF) using Bruker Tiger S8 apparatus.
aculeata) than refined oils have been studied. Macaw palm has high productive potential (4–6 ton oil/ha), is found more frequently in Brazilian cerrado, is resistant to pests and temperature variations and can grow in low-rainfall areas (César et al., 2015; Nobre et al., 2014). Its high FFA content makes it inappropriate for conventional alkaline transesterification method (César et al., 2015; Navarro-Díaz et al., 2014). Although all these desirable features, interesterification reaction from macaw oil and methyl acetate has been assessed just in a few reports, in supercritical fluids (Doná et al., 2013) and, more recently, in a catalyst screening performed by Ribeiro et al. (2017). Additionally, there is no report about quantification of the intermediates formed during interesterification reaction of macaw oil and methyl acetate. In this context, complementary to the first report of Ribeiro et al. (2017), which study four different catalysts at the same reaction conditions to establish the best one, this work purposes to perform a broad investigation on FAME and triacetin production from macaw oil and methyl acetate using γ-alumina as catalyst in a batch reactor. It was done by changing operational conditions and carrying out kinetic experiments to evaluate process variables influence in a range of reaction temperature, catalyst concentration and oil to methyl acetate molar ratio. Additionally, for the best condition, intermediates compounds of interesterification reaction were quantified, clarifying reaction kinetic. Catalyst reutilization was studied in experimental condition of highest yield.
2.3. Apparatus and experimental procedure A stainless steel batch reactor of 500 mL (PARR 4575) was used to perform the reaction of macaw oil with methyl acetate over γ-alumina. The reactor was equipped with temperature controller and pressure and rotation indicators. Once charged with macaw pulp oil, methyl acetate and γ-alumina, the reactor was closed, the temperature was set and the stirring was switched on about 600 rpm. Reaction time started when the reactor started to heat and the temperature was monitored over the time. Samples were collected in 15, 30, 45, 60, 90, 120, 150 and 180 min of reaction. Each sample was centrifuged in a Fanem Excelsa Baby 206-R apparatus, for catalyst separation, and the liquid phase was filtered using 0.45 μm syringe filters to remove remaining particulates. Methyl acetate excess (boiling point 57 °C) and acetic acid (boiling point 119 °C) from esterification of FFA were removed by rotaevaporation with a vacuum rotaevaporator (Buchi RII) (Saka and Isayama, 2009). In this work, the temperature ranged between 225 °C and 300 °C, the catalyst content varied between 2 and 10% (relation to oil mass) and the oil to methyl acetate molar ratio between 1:10 and 1:40. Based on triplicate experiments done at 250 °C, 5% (m/m) of catalyst and methyl acetate to oil molar ratio of 1:20, the overall experimental error was found to be less than 3% on FAME and triacetin yield. Effect of the main reaction variables was evaluated and for the experimental condition of highest yield, catalyst reusability was studied. For reusing investigation, after reaction time, a sample was withdrawn for FAME and triacetin quantification. Reaction medium was centrifuged in order to separate solid catalyst, with no washing. Thereafter, macaw oil and methyl acetate were fed to a new run (Saba et al., 2016). This procedure was repeated for 8 cycles (of 1 h each).
2. Experimental section 2.1. Materials Crude macaw pulp oil (Cocal Brazil Company, Brazil) and methyl acetate (ReagentPlus® 99%, Sigma-Aldrich Brazil) were used in methyl esters synthesis. Macaw oil was filtered by a coarse cloth filter at room temperature to remove beads from extraction process and kept stored in a HDPE (high-density polyethylene) container at room temperature to protect from light and air. Oil characterization was reported by Ribeiro et al. (2017) and is presented in Table 1. Aluminum nonhydrate nitrate (ACS reagent, 98%), sodium hydroxide (Microprills, 98.5%), heptane (p.a. 99%), pyridine (p.a. 99.5%), methyl heptadecanoate (analytical standard), tricaprin (analytical standard), FAME (palmitate, stearate, oleate, linoleate and linolenate methyl esters analytical standard) and triacetin (analytical standard) were supplied by Sigma-Aldrich (Brazil).
2.4. Quantification of products and intermediates compounds Prior to analysis, 20 mg of the treated sample collected from the reactor were dissolved in heptane in a 1 mL volumetric flask. To this solution 500 μL of methyl heptadecanoate and 100 μL of tricaprin were added, both internal standards with concentration of 10 g L−1 in pyridine. After that, 1 μL of final solution was injected in a Shimadzu GCMS-QP2010 Ultra gas chromatograph equipped with mass spectrometer, FID and capillary column (ZB-5HT 15 m × 0.32 mm × 0.10 μm). Oven temperature was programmed to first increase until 70 °C, holding 2 min, then to 190 °C at 15 °C min−1 and finally to 380 °C at 7 °C min−1, holding 4 min. Helium was used as carrier gas and the injector and detector temperatures were 380 °C and 390 °C, respectively, with a split ratio of 1:60. FAME and triacetin peaks were identified by comparison with the relative retention times of FAME and triacetin standards. Intermediates peaks were identified using mass spectrometry and GCMS Solution software package by comparing the spectrum obtained with a database of different libraries. Fig. 1 provides an example of a chromatogram from a reaction sample where all final and intermediates compounds from interesterification reaction, free fatty acids and internal standards can be seen properly separated. Quantification of FAME, triacetin and intermediates contents were obtained by peak area integration of the corresponding compounds and calculated using Eq. (1) (Saba et al., 2016).
2.2. Catalyst preparation and characterization Alumina (Al2O3) in γ transition form was synthesized from hydrolysis of aluminum nonahydrate nitrate, followed by precipitation with sodium hydroxide (Lu et al., 2009). The resulting gel was dried for 20 h at 100 °C, and calcined in a muffle furnace at 500 °C at a rate of 2 °C min−1, for 6 h. Crystal structures were identified by X-ray Powder Diffraction (XRD) collected in a X-ray diffractometer manufactured by Rigaku analyser (USA, model Miniflex) using Cu Kα radiation. The Table 1 Chemical properties of macaw oil. Property
Measured value
Acid value (mg KOH/g) Water content (% m/m) Convertibility (% m/m) Fatty acid profile (% m/m) Palmitic acid (C16:0) Stearic acid (C18:0) Oleic acid (C18:1) Linoleic acid (C18:2) Linolenic acid (C18:3) Others
123.85 ± 0.20 0.81 ± 0.01 61.60 ± 1.41 13.05 ± 0.66 2.37 ± 0.06 74.18 ± 1.85 4.82 ± 0.58 2.66 ± 2.29 2.92 ± 1.61
COMPOUND (% m /m) = 85
∑A C × VIS × IS × 100 AIS W
(1)
Industrial Crops & Products 124 (2018) 84–90
J.S. Ribeiro et al.
Fig. 1. Example of chromatogram of a sample from interesterification reaction with oil to methyl acetate molar ratio of 1:20, 2% (m/m) catalyst, 300 °C and reaction time of 60 min. Chromatogram’s abscissa presents retention time (minutes).
Besides different reaction rates can be verified, it is worthwhile to point out that equilibrium conditions in FAME content was achieved in all temperatures and in, approximately, 150 min. It was necessary 180 min for reaction at 225 °C to reach equilibrium FAME contents, while for 250 °C and 275 °C it was reached in 120 min and 90 min, respectively. In reaction at 300 °C could also be noted a decrease of FAME content after 120 min, which can be a result of FAME thermal degradation as related by literature (Campanelli et al., 2010; Goembira and Saka, 2014; Marousek et al., 2013a). Thermal degradation can also be occurring according to triacetin behaviour shown in Fig. 2a, mainly at high temperatures. Campanelli et al. (2010) checked the thermal stability of triacetin and reported that this compound is particularly vulnerable to thermal decomposition if exposed to high temperatures for a long time. Besides thermal degradation effect, high temperatures are associated to high energy demands. Goembira and Saka (2013, 2014, 2015) studied the conversion of rapeseed oil using supercritical methyl acetate and reported that, even with 10% of acetic acid as additive and at 300 °C, FAME and triacetin yields of 25% and 0% were achieved in 45 min of reaction. Good results were just achieved using supercritical methyl acetate at least at 350 °C and 20 MPa or aqueous acetic acid as additive. Campanelli et al. (2010) investigated the conversion of four different oils (soybean oil, sunflower oil, Jatropha curcas oil and waste soybean oil) with supercritical methyl acetate at 20 MPa, with oil to methyl acetate molar ratio 1:42 and FAME and triacetin yields have not achieved 5% at 300 °C. Reasonable results were just obtained for temperature of 345 °C. Doná et al. (2013) reported macaw oil conversion with supercritical methyl acetate at 300 °C and 325 °C with a FAME yield over 80% at 325 °C, 20 MPa, oil to methyl acetate mass ratio 1:5 and 45 min of reaction. Thus, the use of macaw oil with methyl acetate over γ-Al2O3 is a promising method, since it requires less energy to achieve good conversion to methyl esters. Fig. 2c shows the temperature profile as a function of the reaction time, obtained by reactor monitoring. It can be seen that thermal equilibrium at temperatures of 225, 250 and 275 °C was reached around 70 min and that 55 min were required to reach equilibrium at 300 °C. Comparing with concentration profiles in Fig. 2a and b, it can be verified that heating dynamics is faster than reaction dynamics, since thermal equilibrium is achieved in a shorter time than chemical equilibrium. This is an interesting result, since it assures that heating process is not limiting reaction performance. However, it is important to
where, ∑ A is the total peak area of the compound (FAME, triacetin or intermediate), AIS is the proper internal standard area, CIS is the internal standard concentration (10 mg mL−1), VIS is the volume of the proper internal standard solution (in mL) and W is the sample weight (in mg). For FAME and triacetin content calculation methyl heptadecanoate was the internal standard considered, while for intermediate content calculation tricaprin was the internal standard considered. Since crude macaw oil presents compounds not convertible to alkyl esters, the maximum theoretical ester content in final product is 61.6% (m/m) (convertibility), and to evaluate the real efficiency of the process, the conversion efficiency was calculated using Eq. (2) (Gonzalez et al., 2013).
conversion efficiency (% m /m) =
alkyl ester content × 100 convertibility
(2)
3. Results and discussion 3.1. Characterization of γ-alumina No diffraction peaks were observed for γ-Al2O3 synthesized, indicating that this catalyst present amorphous structure, as described in literature (Samain et al., 2014). The catalyst presented specific area of 246.01 m² g−1 and pore volume of 0.32 cm³ g−1, near to that pointed by Samain et al. (2014) of 272.61 m² g−1 and 0.24 cm³ g−1. Thus, the catalyst was satisfactory synthesized and XRF showed that the catalyst presents no contamination. 3.2. Effect of temperature Fig. 2a and b shows triacetin and FAME content profiles at different temperatures, respectively. It could be observed a smaller reaction rate at 225 °C, while the highest reaction rate was at 300 °C. The reaction rates at 250 °C and 275 °C were close and intermediate. High reaction temperatures generally results in higher reaction rates and higher ester and triacetin contents (Goembira and Saka, 2013). In 60 min, the reaction conducted at 300 °C achieved 53.19% (m/m) and 2.01% (m/m) of FAME and triacetin content, respectively, which means 89.6% (m/m) of conversion efficiency, while at 275 °C it reached 35.66% (m/m) and 1.23% (m/m), at 250 °C it reached 31.80% (m/m) and 0.66% (m/m), and for 225 °C it reached 11.55% (m/m) and 0.15% (m/m). 86
Industrial Crops & Products 124 (2018) 84–90
J.S. Ribeiro et al.
reaction. 3.3. Effect of catalyst concentration The effect of catalyst concentration was evaluated through addition of 2, 5 and 10% (m/m) of γ-alumina to reactions at 250 °C and 300 °C, with oil to methyl acetate molar ratio 1:20, as shown the results in Fig. 3. In addition, a reaction without catalyst was conducted for comparative effect. At 250 °C, 10% (m/m) of γ-alumina were needed to achieve 51.38% (m/m) and 1.11% (m/m) of FAME and triacetin content, in 90 min of reaction (85.2% (m/m) of conversion efficiency), while in 60 min these yields were 46.67% (m/m) and 0.98% (m/m), (77.2% (m/m) of conversion efficiency). Besides that, it is remarkable that an increment in catalyst concentration positively affected FAME and triacetin content at this temperature, since in 60 min, reaction without catalyst resulted in 7.53% (m/m) and 0% (m/m) of these compounds, while reactions with 2% (m/m) and 5% (m/m) of the catalyst resulted in FAME and triacetin contents of 24.3% (m/m) and 0.31% (m/m) and 31.8% (m/m) and 0.66% (m/m), respectively. On the other hand, at 300 °C, 2% (m/m) of γ-alumina was enough to reach 53.87% (m/m) and 2.10% (m/m) of FAME and triacetin contents in 90 min of reaction (90.86% (m/m) of conversion efficiency), while in 60 min these values were 48.96% (m/m) and 1.84% (m/m), corresponding to 82.46% (m/m) of conversion efficiency. At this temperature, catalyst addition didn’t show great effect through FAME and triacetin contents, since the increase of catalyst to 10% (m/m) resulted only in 52.39% (m/m) and 1.71% (m/m) in 60 min. Additionally, it can be pointed out that, at this temperature, catalyst effect was more pronounced in lower reaction times. After these considerations, reaction at 300 °C using 2% (m/m) of γalumina was chosen as the best condition, since requires less time reaction to achieve a satisfactory FAME and triacetin content, spending a smaller amount of catalyst. 3.4. Effect of oil to methyl acetate molar ratio According to interesterification reaction stoichiometry, 3 mols of methyl acetate are needed to convert 1 mol of triglycerides, thus, in order to shift the reaction to product side, an excessive amount of methyl acetate should be applied, mostly oil to methyl acetate molar ratio of 1:42 (Goembira and Saka, 2013). On the other hand, due to the high amount of FFA, part of FAME content is produced from esterification reaction, which stoichiometry is 1 mol of methyl acetate to convert 1 mol of FFA, also, to proceed the reaction to product side, an excess of reagent must be applied, but in this case it corresponds to oil to methyl acetate molar ratio around 1:3 or 1:6 (Park et al., 2011; Marousek et al., 2013b). Fig. 4 presents the effect of oil to methyl acetate molar ratios of 1:10, 1:20 and 1:40, at 300 °C, with 2% (m/m) of catalyst. While for molar ratio 1:10 the chemical equilibrium was reached in 60 min, with 46.83% (m/m) and 1.41% (m/m) of FAME and triacetin contents, respectively, for molar ratio of 1:20 it stabilized in 90 min, with 53.87% (m/m) and 2.10% (m/m) of FAME and triacetin content, and for a molar ratio of 1:40 it stabilized in 120 min with 55.80% (m/ m) and 2.51% (m/m) contents. Additionally, it can be observed that molar ratio has little influence in reaction rate, which is expected since the higher the molar ratio the greater the reagents dilution and smaller the reaction rate. Different from that required for low FFA content oils, where oil to methyl acetate molar ratio of 1:42 is needed to achieve the best conditions (Campanelli et al., 2010), higher molar ratios than 1:40 were not necessary to reach the best conditions in this case. This is probably due to the high FFA amount in macaw oil and consequent esterification reaction that requires less amounts of solvent to shift the equilibrium reaction to product side, according to its stoichiometry. Campanelli et al. (2010) investigated supercritical
Fig. 2. FAME (a) and triacetin (b) content and temperature (c) profiles in reaction medium from macaw oil and methyl acetate in oil to methyl acetate molar ratio of 1:20 with 5% (m/m) of catalyst at different temperatures: 225 °C (▲), 250 °C (■), 275 °C (♦) and 300 °C (●). Experimental data were linked for better visualization.
point out that in 60 min of reaction, 11.55, 31.80, 35.66 and 53.10% (m/m) of FAME where obtained (Fig. 2b), at 225 °C, 250 °C, 275 °C and 300 °C, respectively, indicating that during reactor heating, a significant esters production can occur. It can be noted from these results that ester yield in heating step increases with set point temperature, and so this information consists in important information for batch operation, and besides that, from the knowledge of these authors, is not yet reported in literature. Considering the results obtained in this work, temperature of 300 °C presented the best results for a short time 87
Industrial Crops & Products 124 (2018) 84–90
J.S. Ribeiro et al.
Fig. 3. FAME and triacetin content profiles in reaction medium from macaw oil and methyl acetate in oil to methyl acetate molar ratio of 1:20 at (a) 250 °C and (b) 300 °C at different catalyst contents: 0% (▲), 2% (●), 5% (■) and 10% (♦). Experimental data were linked for better visualization.
above 300 °C. Fig. 5 shows the content profiles of initial compounds present in macaw oil, intermediates and products formed during the reactions with methyl acetate at 300 °C, with oil to methyl acetate molar ratio 1:20 and 2% (m/m) of catalyst. Macaw oil is mostly composed by FFA and diglycerides. Interesterification reaction occurs mainly in the first 60 min of reaction (Fig. 5a), when it could be observed a more pronounced decrease in mono-, di- and triglycerides and an increase in intermediates monoacetinmonoglycerides (MAMG), monoacetindiglycerides (MADG) and diacetinmonoglycerides (DAMG) indicating conversion of macaw oil to methyl esters and triacetin. Initially, production of MADG stands out due to glycerides presents on reaction medium. Then, from 60 min, conversion of MADG to DAMG results in a decrease of MADG content and increase in DAMG content. However, DAMG content decrease is not observed over time suggesting that the conversion to triacetin is the limiting step in interesterification reaction, as reported by Casas et al. (2011b). FFA content in macaw oil also reacts with methyl acetate to produce FAME and acetic acid (evaporated after sample collect). Fig. 5b shows the consumption of FFA over time and consequent FAME content increasing. Although it can be observed interesterification reaction occurrence because of triacetin production and over FAME content based on FFA content, mostly of FAME produced comes from esterification reaction, since macaw oil contains mainly FFA in its composition. Additionally, it could be observed from Fig. 5 that 90 min were necessary to reach reaction equilibrium but 60 min were sufficient to reach compound contents close to the equilibrium, without more energy expenditure. Thus, temperature reaction of 300 °C, 2% (m/m) of catalyst, oil to methyl acetate molar ratio of 1:20 and 60 min of reaction were chosen as the best reaction conditions.
interesterification reaction for various oils with low FFA content and oil to methyl acetate molar ratio of at least 1:42 was required to shift reaction equilibrium and achieve maximum yields, while an increase in molar ratio to 1:59 did not affect neither the yields nor the kinetics of the process. Casas et al. (2011a, 2011b) studied chemical interesterification reaction of sunflower oil and methyl acetate and observed that triglyceride content was fully consumed with oil to methyl acetate molar ratio of 1:18, while a molar ratio of 1:50 was needed to remove monoacetindiglycerides. However, even with an impractical molar ratio of 1:100, diacetinmonoglycerides were not completely eliminated. Thus, they concluded that the conversion of diacetinmonoglycerides to triacetin is the limiting step of the reaction and 1:50 was the best oil to methyl acetate molar ratio in their studies. In this study oil to methyl acetate molar ratio of 1:20 was chosen as the best condition to proceed the investigation of reaction parameters, since an increase in molar ratio to 1:40 has not represented a great increase in FAME and triacetin contents, besides representing high reagent consumption. 3.5. Effect of reaction time For the conditions that provided the best FAME and triacetin contents in Figs. 3–6 (i.e., temperature of 300 °C, oil to methyl acetate molar ratio of 1:20 and 2% (m/m) of catalyst) the effect of reaction time was evaluated. Reactions achieved equilibrium within 120 min, and from Figs. 3 and 5b after a long time of reaction at 300 °C, it can be observed a light decreased in FAME and triacetin contents, probably due to thermal degradation (Abdala et al., 2014; Campanelli et al., 2010; Vieitez et al., 2011; Marousek et al., 2012). Vieitez et al. (2011) studied the stability of ethyl esters from soybean oil exposed to high temperatures and notice that severe conditions of reaction cause the occurrence of important degradation processes of the lipid material, suggesting that the process should not be performed at temperatures 88
Industrial Crops & Products 124 (2018) 84–90
J.S. Ribeiro et al.
Fig. 4. FAME and triacetin content profiles in reaction medium from macaw oil and methyl acetate over 2% (m/m) of γ-alumina at 300 °C in different oil to methyl acetate molar ratios: 1:10 (●), 1:20 (■) and 1:40 (▲). Experimental data were linked for better visualization.
Fig. 5. Content profiles of (a) triglycerides (□), diglycerides (○), monoglycerides (Δ), monoacetinmonoglycerides (▲), monoacetindiglycerides (■) and diacetinmonoglycerides (●) and (b) FFA (◊), FAME (●) and triacetin (▲) in reaction medium at 300 °C, oil to methyl acetate molar ratio of 1:20 and 2% (m/m) catalyst. Experimental data were linked for better visualization.
3.6. Catalyst reuse Catalyst reuse was evaluated at 300 °C, with 2% (m/m) of catalyst and oil to methyl acetate molar ratio of 1:20 for 60 min. The catalyst was recovered and reused for three more runs under the same conditions. Fame and triacetin contents are shown in Fig. 6 for each run. In the first run, with fresh catalyst, FAME and triacetin content achieved was 50.3% (m/m) and 1.52% (m/m), respectively, corresponding to 84.12% of conversion efficiency. After the second cycle the FAME content reached 49.45% (m/m), which is not significantly different considering the experimental error of 1.68%, while triacetin content increased to 2.11% (m/m). The increase in triacetin content could be related to acetic acid adsorbed in catalyst surface due to esterification reaction that reduces thermal degradation of triacetin (Campanelli et al., 2010). In the following cycles, FAME content remained at similar level as, with 49.4, 49.1, 48.9 and 44.1% (m/m) of ester content. From sixth level, there is a more pronounced decay in ester content. Values of catalytic efficiency decay are presented in parentheses for each cycle, showing that a high decay occurs from seventh cycle, with a catalytic activity loss of 12.47%. These results confirm that γ-alumina could be used for at least six cycles without a great loss of activity.
Fig. 6. Reusability of γ-alumina on interesterification and esterification reactions from macaw oil and methyl acetate at 300 °C, 2% (m/m) catalyst and oil to methyl acetate molar ratio 1:20, for 60 min.
confirmed that this process showed to be a promising method for FAME and triacetin production. Besides, it could be verified that esterification took place primarily in relation to interesterification reaction due to high acid value of the oil. Conversion of DAMG to triacetin was the
4. Conclusion In this work, heterogeneous catalyst using γ-Al2O3 was investigated for biodiesel production from macaw oil and methyl acetate. The results 89
Industrial Crops & Products 124 (2018) 84–90
J.S. Ribeiro et al.
limiting step of interesterification reaction. The best condition was 300 °C, 2% (m/m) of catalyst, oil to methyl acetate molar ratio of 1:20 and 60 min of reaction, reaching 48.96% and 1.84% of FAME and triacetin content, which corresponds to 82.46% of conversion efficiency. Catalyst could be reused for six cycles without significant loss of activity. Biodiesel production from this chemical route has great potential for industrial applications and should be more investigated in continuous processes, since it showed to be promising for reducing cost.
of waste cooking oil and methyl acetate for biodiesel and triacetin production. Fuel Process. Technol. 116, 241–249. https://doi.org/10.1016/j.fuproc.2013.07.004. Marousek, J., Kwan, J.T., 2013. Use of pressure manifestations following the water plasma expansion for phytomass disintegration. Water Sci. Technol. 67, 1695–1700. https://doi.org/10.2166/wst.2013.041. Marousek, J., Itoh, S., Higa, O., Kondo, Y., Ueno, M., Suwa, R., Tominaga, J., Kawamitsu, Y., 2012. The use of underwater high-voltage discharges to improve the efficiency of Jatropha curcas L. biodiesel production. Biotechenol. Appl. Biochem. 59, 541–546. https://doi.org/10.1002/bab.1045. Marousek, J., Itoh, S., Higa, O., Kondo, Y., Ueno, M., Suwa, R., Tominaga, J., Kawamitsu, Y., 2013a. Enzymatic hydrolysis enhanced by pressure shockwaves opening new possibilities in Jatropha Curcas L. processing. Chem. Technol. Biotech. 88, 1650–1653. https://doi.org/10.1002/jctb.4014. Marousek, J., Itoh, S., Higa, O., Kondo, Y., Ueno, M., Suwa, R., Tominaga, Y., Tominaga, J., Kawamitsu, Y., 2013b. Pressure schockwaves to enhance oil extraction from Jatropha curcas L. Biotech. Biotechnol. Equip. 27, 3654–3658. https://doi.org/10. 5504/BBEQ.2012.0143. Marousek, L., Haskova, S., Marouskova, A., Myskova, K., Vanickova, R., Vachal, J., Vochozka, M., Zeman, R., Zak, J., 2015. Financial and biotechnological assessment of new oil extraction technology. Energy Sources Part A Recov. Util. Environ. Eff. 37, 1723–1728. https://doi.org/10.1080/15567036.2015.1048391. Marx, S., 2016. Glycerol-free biodiesel production through transesterification: a review. Fuel Process. Technol. 151, 139–147. https://doi.org/10.1016/j.fuproc.2016.05.033. Navarro-Díaz, H.J., Gonzalez, S.L., Irigaray, B., Vieitez, I., Jachmanián, I., Hense, H., Oliveira, J.V., 2014. Macauba oil as an alternative feedstock for biodiesel: characterization and ester conversion by the supercritical method. J. Supercrit. Fluids 93, 130–137. https://doi.org/10.1016/j.supflu.2013.11.008. Nobre, D.A.C., Trogello, E., Borghetti, R.A., de S. David, A.M.S., 2014. Macaúba: palmeira De Extração Sustentável para biocombustível. Colloq. Agrar. 10, 92–105. https://doi. org/10.5747/ca.2014.v10.n2.a112. Park, J.Y., Wang, Z.M., Kim, D.K., Lee, J.S., 2011. Effects of water on the esterification of free fatty acids by acid catalysts. Renew. Energy 36, 889–891. https://doi.org/10. 1016/j.renene.2010.08.025. Ribeiro, J.S., Celante, D., Simões, S.S., Bassaco, M.M., Da Silva, C., De Castilhos, F., 2017. Efficiency of heterogeneous catalysts in interesterification reaction from macaw oil (Acrocomia aculeata) and methyl acetate. Fuel 200, 499–505. https://doi.org/10. 1016/j.fuel.2017.04.003. Saba, T., Estephane, J., El, B., El, M., Khazma, M., El, H., Aouad, S., 2016. Biodiesel production from refined sunflower vegetable oil over KOH/ZSM5 catalysts. Renew. Energy 90, 301–306. https://doi.org/10.1016/j.renene.2016.01.009. Saka, S., Isayama, Y., 2009. A new process for catalyst-free production of biodiesel using supercritical methyl acetate. Fuel 88, 1307–1313. https://doi.org/10.1016/j.fuel. 2008.12.028. Samain, L., Jaworski, A., Edén, M., Ladd, D.M., Seo, D.-K., Garcia-Garcia, F.J., Haussermann, U., 2014. Structural analysis of highly porous γ-Al2O3. J. Solid State Chem. 217, 1–8. https://doi.org/10.1016/j.jssc.2014.05.004. Tavares, G.R., Gonçalves, J.E., dos Santos, W.D., da Silva, C., 2017. Enzymatic interesterification of crambe oil assisted by ultrasound. Ind. Crops Prod. 97, 218–223. https://doi.org/10.1016/j.indcrop.2016.12.022. Trueba, M., Trasatti, S.P., 2005. γ-Alumina as a support for catalysts: a review of fundamental aspects. Eur. J. Inorg. Chem. 3393–3403. https://doi.org/10.1002/ejic. 200500348. Vieitez, I., Da Silva, C., Alckmin, I., De Castilhos, F., Oliveira, J.V., Grompone, M.A., Jachmanián, I., 2011. Stability of ethyl esters from soybean oil exposed to high temperatures in supercritical ethanol. J. Supercrit. Fluids 56, 265–270. https://doi. org/10.1016/j.supflu.2010.10.033. Xu, Y., Du, W., Liu, D., 2005. Study on the kinetics of enzymatic interesterification of triglycerides for biodiesel production with methyl acetate as the acyl acceptor. J. Mol. Catal. B Enzym. 32, 241–245. https://doi.org/10.1016/j.molcatb.2004.12.013. Zabeti, M., Wan Daud, W.M.A., Aroua, M.K., 2009. Activity of solid catalysts for biodiesel production: a review. Fuel Process. Technol. 90, 770–777. https://doi.org/10.1016/j. fuproc.2009.03.010.
Acknowledgements The authors wish to thank CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) for scholarships and CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) for financial support. References Abdala, A.C.D.A., Colonelli, T.A.D.S., Trentini, C.P., Oliveira, J.V., Cardozo-Filho, L., Da Silva, E.A., Da Silva, C., 2014. Effect of additives in the reaction medium on noncatalytic ester production from used frying oil with supercritical ethanol. Energy Fuels 28, 3122–3128. https://doi.org/10.1021/ef402253e. Campanelli, P., Banchero, M., Manna, L., 2010. Synthesis of biodiesel from edible, nonedible and waste cooking oils via supercritical methyl acetate transesterification. Fuel 89, 3675–3682. https://doi.org/10.1016/j.fuel.2010.07.033. Casas, A., Ruiz, J.R., Ramos, M.J., Pérez, Á., 2010. Effects of triacetin on biodiesel quality. Energy Fuels 24, 4481–4489. https://doi.org/10.1021/ef100406b. Casas, A., Ramos, M.J., Pérez, Á., 2011a. New trends in biodiesel production: chemical interesterification of sunflower oil with methyl acetate. Biomass Bioenergy 35, 1702–1709. https://doi.org/10.1016/j.biombioe.2011.01.003. Casas, A., Ramos, M.J., Pérez, Á., 2011b. Kinetics of chemical interesterification of sunflower oil with methyl acetate for biodiesel and triacetin production. Chem. Eng. J. 171, 1324–1332. https://doi.org/10.1016/j.cej.2011.05.037. César, A.D.S., Almeida, F.D.A., De Souza, R.P., Silva, G.C., Atabani, A.E., 2015. The prospects of using Acrocomia aculeata (macaúba) a non-edible biodiesel feedstock in Brazil. Renew. Sustain. Energy Rev. 49, 1213–1220. https://doi.org/10.1016/j.rser. 2015.04.125. Doná, G., Cardozo-Filho, L., Silva, C., Castilhos, F., 2013. Biodiesel production using supercritical methyl acetate in a tubular packed bed reactor. Fuel Process. Technol. 106, 605–610. https://doi.org/10.1016/j.fuproc.2012.09.047. Goembira, F., Saka, S., 2013. Optimization of biodiesel production by supercritical methyl acetate. Bioresour. Technol. 131, 47–52. https://doi.org/10.1016/j.biortech.2012. 12.130. Goembira, F., Saka, S., 2014. Effect of additives to supercritical methyl acetate on biodiesel production. Fuel Process. Technol. 125, 114–118. https://doi.org/10.1016/j. fuproc.2014.03.035. Goembira, F., Saka, S., 2015. Advanced supercritical methyl acetate method for biodiesel production from pongamia pinnata oil. Renew. Energy 83, 1245–1249. https://doi. org/10.1016/j.renene.2015.06.022. Gonzalez, S.L., Sychoski, M.M., Navarro-Díaz, H.J., Callejas, N., Saibene, M., Vieitez, I., Jachmanián, I., Da Silva, C., Hense, H., Oliveira, J.V., 2013. Continuous catalyst-free production of biodiesel through transesterification of soybean fried oil in supercritical methanol and ethanol. Energy Fuels 27, 5253–5259. https://doi.org/10. 1021/ef400869y. Lu, C.L., Lv, J.G., Xu, L., Guo, X.F., Hou, W.H., Hu, Y., Huang, H., 2009. Crystalline nanotubes of γ-AlOOH and γ-Al2O3: hydrothermal synthesis, formation mechanism and catalytic performance. Nanotechnology 20 (215604), 1–9. Maddikeri, G.L., Pandit, A.B., Gogate, P.R., 2013. Ultrasound assisted interesterification
90
Contents lists available at ScienceDirect
Industrial Crops & Products journal homepage: www.elsevier.com/locate/indcrop
Synthesis of methyl esters and triacetin from macaw oil (Acrocomia aculeata) and methyl acetate over γ-alumina
T
Jéssica S. Ribeiroa, Dian Celantea, Leoni N. Brondania, Débora O. Trojahna, Camila da Silvab, ⁎ Fernanda de Castilhosa, a b
Department of Chemical Engineering, Federal University of Santa Maria (UFSM), Av. Roraima 1000, Santa Maria, Rio Grande do Sul 97105-900, Brazil Department of Technology, Maringa State University (UEM), Av. Ângelo Moreira da Fonseca 180, Umuarama, Paraná 87506-370, Brazil
A R T I C LE I N FO
A B S T R A C T
Keywords: Interesterification Glycerol-free Triacetylglycerol Esterification
Glycerol excess in the market and high cost of common feedstock have encouraged the investigations of new technologies for biodiesel production. In this context, the use of macaw oil (Acrocomia aculeata) for biodiesel production through interesterification and esterification reactions with methyl acetate as acyl acceptor was evaluated, using heterogeneous catalyst γ-alumina (γ-Al2O3). Experiments were performed at different reaction temperatures (225–300 °C), catalyst contents (0–10% relation to oil mass), oil to methyl acetate molar ratios (1:10–1:40) and reaction times in a batch reactor to determine the effect of these variables in FAME (fatty acid methyl esters) and triacetin production. Catalyst γ-Al2O3 improved FAME and triacetin contents in comparison with no catalyzed reaction. Reaction at 300 °C, with 2% of catalyst in relation to oil mass and molar ratio of 1:20, in 60 min provided the best results. The catalyst could be recovered and reused for at least 6 cycles without significant activity loss. The results allowed to point out that the process showed to be promising for industrial biodiesel production with no glycerol and was financial attractive, since it used a low cost feedstock.
1. Introduction Biodiesel is as a strategic source of renewable energy with great potential to replace petro based diesel and reduce environmental pollution levels (Doná et al., 2013; Maddikeri et al., 2013). Although alkaline transesterification is industrially the most used process to produce biodiesel, it has some drawbacks such as the glycerol production as byproduct, which is difficult to be purified and to be sold due to market saturation, and the FFA content restriction in feedstocks (lower than 0.5 wt%) (Casas et al., 2010; Saka and Isayama, 2009; Marousek et al., 2015; Marousek and Kwan, 2013). Interesterification of triglycerides with methyl acetate as the acyl acceptor has been studied to produce FAME and triacetylglycerol (triacetin) as byproduct instead of glycerol (Casas et al., 2011a; Tavares et al., 2017). Triacetin is considered a fuel additive and its mixture with FAME does not present harmful effects on biodiesel properties and thus, no additional separation steps to recovery this byproduct are required. Besides that, due to its mutual solubility, triacetin may be added to biodiesel up to 10% by weight and the blended biodiesel still meets the quality standards (Casas et al., 2010; Goembira and Saka, 2015, 2013; Xu et al., 2005). Additionally, FFA present in feedstock reacts with
⁎
methyl acetate to form FAME and acetic acid and thus improves FAME yield rather than promoting the saponification reaction (Marx, 2016). Thus, interesterification process has potential to maximize the use of biodiesel feedstocks, due to the possible use of product and byproduct as an alternative fuel (Goembira and Saka, 2015). Chemical interesterification has lower operational costs and can be performed at mild reaction conditions, unlike enzymatic and supercritical reaction, respectively (Casas et al., 2011b). However, there is only a report available in literature concerning homogeneous catalysis in interesterification reaction (Casas et al., 2011b). Additionally, the only report available on heterogeneous catalysis is a catalyst screening developed by Ribeiro et al. (2017), where it was pointed out γ-alumina as the most suitable acid heterogeneous catalyst for this reaction from macaw oil and methyl acetate. Alumina exists in metastable forms and the γ transition form is the most important of them, because it presents good catalytic properties as high porosity, high specific surface area, large pore size and pore volume and is appropriate for biodiesel synthesis (Trueba and Trasatti, 2005; Zabeti et al., 2009). Final cost of this biofuel is mainly affected by the raw material that represents around 70–95% of the total value (Maddikeri et al., 2013). Thus, the use of other feedstocks, like crude macaw oil (Acrocomia
Corresponding author. E-mail address: [email protected] (F. de Castilhos).
https://doi.org/10.1016/j.indcrop.2018.07.062 Received 4 May 2018; Received in revised form 11 July 2018; Accepted 24 July 2018 0926-6690/ © 2018 Elsevier B.V. All rights reserved.
Industrial Crops & Products 124 (2018) 84–90
J.S. Ribeiro et al.
sample was scanned over the angular range 5–80° (2θ) with a step size of 0.03° and a scan rate of 1 s per step. Textural properties were determined by a well known nitrogen adsorption-desorption procedure at 77 K in an analyser (Micromeritics, model ASAP 2020). Specific surface area (S) was determined by Brunauer, Emmett and Teller (BET) method and total pore volume (VP) by Barrett-Joyner-Halenda (BJH) method. Catalyst composition was confirmed with X-Ray Fluorescence (XRF) using Bruker Tiger S8 apparatus.
aculeata) than refined oils have been studied. Macaw palm has high productive potential (4–6 ton oil/ha), is found more frequently in Brazilian cerrado, is resistant to pests and temperature variations and can grow in low-rainfall areas (César et al., 2015; Nobre et al., 2014). Its high FFA content makes it inappropriate for conventional alkaline transesterification method (César et al., 2015; Navarro-Díaz et al., 2014). Although all these desirable features, interesterification reaction from macaw oil and methyl acetate has been assessed just in a few reports, in supercritical fluids (Doná et al., 2013) and, more recently, in a catalyst screening performed by Ribeiro et al. (2017). Additionally, there is no report about quantification of the intermediates formed during interesterification reaction of macaw oil and methyl acetate. In this context, complementary to the first report of Ribeiro et al. (2017), which study four different catalysts at the same reaction conditions to establish the best one, this work purposes to perform a broad investigation on FAME and triacetin production from macaw oil and methyl acetate using γ-alumina as catalyst in a batch reactor. It was done by changing operational conditions and carrying out kinetic experiments to evaluate process variables influence in a range of reaction temperature, catalyst concentration and oil to methyl acetate molar ratio. Additionally, for the best condition, intermediates compounds of interesterification reaction were quantified, clarifying reaction kinetic. Catalyst reutilization was studied in experimental condition of highest yield.
2.3. Apparatus and experimental procedure A stainless steel batch reactor of 500 mL (PARR 4575) was used to perform the reaction of macaw oil with methyl acetate over γ-alumina. The reactor was equipped with temperature controller and pressure and rotation indicators. Once charged with macaw pulp oil, methyl acetate and γ-alumina, the reactor was closed, the temperature was set and the stirring was switched on about 600 rpm. Reaction time started when the reactor started to heat and the temperature was monitored over the time. Samples were collected in 15, 30, 45, 60, 90, 120, 150 and 180 min of reaction. Each sample was centrifuged in a Fanem Excelsa Baby 206-R apparatus, for catalyst separation, and the liquid phase was filtered using 0.45 μm syringe filters to remove remaining particulates. Methyl acetate excess (boiling point 57 °C) and acetic acid (boiling point 119 °C) from esterification of FFA were removed by rotaevaporation with a vacuum rotaevaporator (Buchi RII) (Saka and Isayama, 2009). In this work, the temperature ranged between 225 °C and 300 °C, the catalyst content varied between 2 and 10% (relation to oil mass) and the oil to methyl acetate molar ratio between 1:10 and 1:40. Based on triplicate experiments done at 250 °C, 5% (m/m) of catalyst and methyl acetate to oil molar ratio of 1:20, the overall experimental error was found to be less than 3% on FAME and triacetin yield. Effect of the main reaction variables was evaluated and for the experimental condition of highest yield, catalyst reusability was studied. For reusing investigation, after reaction time, a sample was withdrawn for FAME and triacetin quantification. Reaction medium was centrifuged in order to separate solid catalyst, with no washing. Thereafter, macaw oil and methyl acetate were fed to a new run (Saba et al., 2016). This procedure was repeated for 8 cycles (of 1 h each).
2. Experimental section 2.1. Materials Crude macaw pulp oil (Cocal Brazil Company, Brazil) and methyl acetate (ReagentPlus® 99%, Sigma-Aldrich Brazil) were used in methyl esters synthesis. Macaw oil was filtered by a coarse cloth filter at room temperature to remove beads from extraction process and kept stored in a HDPE (high-density polyethylene) container at room temperature to protect from light and air. Oil characterization was reported by Ribeiro et al. (2017) and is presented in Table 1. Aluminum nonhydrate nitrate (ACS reagent, 98%), sodium hydroxide (Microprills, 98.5%), heptane (p.a. 99%), pyridine (p.a. 99.5%), methyl heptadecanoate (analytical standard), tricaprin (analytical standard), FAME (palmitate, stearate, oleate, linoleate and linolenate methyl esters analytical standard) and triacetin (analytical standard) were supplied by Sigma-Aldrich (Brazil).
2.4. Quantification of products and intermediates compounds Prior to analysis, 20 mg of the treated sample collected from the reactor were dissolved in heptane in a 1 mL volumetric flask. To this solution 500 μL of methyl heptadecanoate and 100 μL of tricaprin were added, both internal standards with concentration of 10 g L−1 in pyridine. After that, 1 μL of final solution was injected in a Shimadzu GCMS-QP2010 Ultra gas chromatograph equipped with mass spectrometer, FID and capillary column (ZB-5HT 15 m × 0.32 mm × 0.10 μm). Oven temperature was programmed to first increase until 70 °C, holding 2 min, then to 190 °C at 15 °C min−1 and finally to 380 °C at 7 °C min−1, holding 4 min. Helium was used as carrier gas and the injector and detector temperatures were 380 °C and 390 °C, respectively, with a split ratio of 1:60. FAME and triacetin peaks were identified by comparison with the relative retention times of FAME and triacetin standards. Intermediates peaks were identified using mass spectrometry and GCMS Solution software package by comparing the spectrum obtained with a database of different libraries. Fig. 1 provides an example of a chromatogram from a reaction sample where all final and intermediates compounds from interesterification reaction, free fatty acids and internal standards can be seen properly separated. Quantification of FAME, triacetin and intermediates contents were obtained by peak area integration of the corresponding compounds and calculated using Eq. (1) (Saba et al., 2016).
2.2. Catalyst preparation and characterization Alumina (Al2O3) in γ transition form was synthesized from hydrolysis of aluminum nonahydrate nitrate, followed by precipitation with sodium hydroxide (Lu et al., 2009). The resulting gel was dried for 20 h at 100 °C, and calcined in a muffle furnace at 500 °C at a rate of 2 °C min−1, for 6 h. Crystal structures were identified by X-ray Powder Diffraction (XRD) collected in a X-ray diffractometer manufactured by Rigaku analyser (USA, model Miniflex) using Cu Kα radiation. The Table 1 Chemical properties of macaw oil. Property
Measured value
Acid value (mg KOH/g) Water content (% m/m) Convertibility (% m/m) Fatty acid profile (% m/m) Palmitic acid (C16:0) Stearic acid (C18:0) Oleic acid (C18:1) Linoleic acid (C18:2) Linolenic acid (C18:3) Others
123.85 ± 0.20 0.81 ± 0.01 61.60 ± 1.41 13.05 ± 0.66 2.37 ± 0.06 74.18 ± 1.85 4.82 ± 0.58 2.66 ± 2.29 2.92 ± 1.61
COMPOUND (% m /m) = 85
∑A C × VIS × IS × 100 AIS W
(1)
Industrial Crops & Products 124 (2018) 84–90
J.S. Ribeiro et al.
Fig. 1. Example of chromatogram of a sample from interesterification reaction with oil to methyl acetate molar ratio of 1:20, 2% (m/m) catalyst, 300 °C and reaction time of 60 min. Chromatogram’s abscissa presents retention time (minutes).
Besides different reaction rates can be verified, it is worthwhile to point out that equilibrium conditions in FAME content was achieved in all temperatures and in, approximately, 150 min. It was necessary 180 min for reaction at 225 °C to reach equilibrium FAME contents, while for 250 °C and 275 °C it was reached in 120 min and 90 min, respectively. In reaction at 300 °C could also be noted a decrease of FAME content after 120 min, which can be a result of FAME thermal degradation as related by literature (Campanelli et al., 2010; Goembira and Saka, 2014; Marousek et al., 2013a). Thermal degradation can also be occurring according to triacetin behaviour shown in Fig. 2a, mainly at high temperatures. Campanelli et al. (2010) checked the thermal stability of triacetin and reported that this compound is particularly vulnerable to thermal decomposition if exposed to high temperatures for a long time. Besides thermal degradation effect, high temperatures are associated to high energy demands. Goembira and Saka (2013, 2014, 2015) studied the conversion of rapeseed oil using supercritical methyl acetate and reported that, even with 10% of acetic acid as additive and at 300 °C, FAME and triacetin yields of 25% and 0% were achieved in 45 min of reaction. Good results were just achieved using supercritical methyl acetate at least at 350 °C and 20 MPa or aqueous acetic acid as additive. Campanelli et al. (2010) investigated the conversion of four different oils (soybean oil, sunflower oil, Jatropha curcas oil and waste soybean oil) with supercritical methyl acetate at 20 MPa, with oil to methyl acetate molar ratio 1:42 and FAME and triacetin yields have not achieved 5% at 300 °C. Reasonable results were just obtained for temperature of 345 °C. Doná et al. (2013) reported macaw oil conversion with supercritical methyl acetate at 300 °C and 325 °C with a FAME yield over 80% at 325 °C, 20 MPa, oil to methyl acetate mass ratio 1:5 and 45 min of reaction. Thus, the use of macaw oil with methyl acetate over γ-Al2O3 is a promising method, since it requires less energy to achieve good conversion to methyl esters. Fig. 2c shows the temperature profile as a function of the reaction time, obtained by reactor monitoring. It can be seen that thermal equilibrium at temperatures of 225, 250 and 275 °C was reached around 70 min and that 55 min were required to reach equilibrium at 300 °C. Comparing with concentration profiles in Fig. 2a and b, it can be verified that heating dynamics is faster than reaction dynamics, since thermal equilibrium is achieved in a shorter time than chemical equilibrium. This is an interesting result, since it assures that heating process is not limiting reaction performance. However, it is important to
where, ∑ A is the total peak area of the compound (FAME, triacetin or intermediate), AIS is the proper internal standard area, CIS is the internal standard concentration (10 mg mL−1), VIS is the volume of the proper internal standard solution (in mL) and W is the sample weight (in mg). For FAME and triacetin content calculation methyl heptadecanoate was the internal standard considered, while for intermediate content calculation tricaprin was the internal standard considered. Since crude macaw oil presents compounds not convertible to alkyl esters, the maximum theoretical ester content in final product is 61.6% (m/m) (convertibility), and to evaluate the real efficiency of the process, the conversion efficiency was calculated using Eq. (2) (Gonzalez et al., 2013).
conversion efficiency (% m /m) =
alkyl ester content × 100 convertibility
(2)
3. Results and discussion 3.1. Characterization of γ-alumina No diffraction peaks were observed for γ-Al2O3 synthesized, indicating that this catalyst present amorphous structure, as described in literature (Samain et al., 2014). The catalyst presented specific area of 246.01 m² g−1 and pore volume of 0.32 cm³ g−1, near to that pointed by Samain et al. (2014) of 272.61 m² g−1 and 0.24 cm³ g−1. Thus, the catalyst was satisfactory synthesized and XRF showed that the catalyst presents no contamination. 3.2. Effect of temperature Fig. 2a and b shows triacetin and FAME content profiles at different temperatures, respectively. It could be observed a smaller reaction rate at 225 °C, while the highest reaction rate was at 300 °C. The reaction rates at 250 °C and 275 °C were close and intermediate. High reaction temperatures generally results in higher reaction rates and higher ester and triacetin contents (Goembira and Saka, 2013). In 60 min, the reaction conducted at 300 °C achieved 53.19% (m/m) and 2.01% (m/m) of FAME and triacetin content, respectively, which means 89.6% (m/m) of conversion efficiency, while at 275 °C it reached 35.66% (m/m) and 1.23% (m/m), at 250 °C it reached 31.80% (m/m) and 0.66% (m/m), and for 225 °C it reached 11.55% (m/m) and 0.15% (m/m). 86
Industrial Crops & Products 124 (2018) 84–90
J.S. Ribeiro et al.
reaction. 3.3. Effect of catalyst concentration The effect of catalyst concentration was evaluated through addition of 2, 5 and 10% (m/m) of γ-alumina to reactions at 250 °C and 300 °C, with oil to methyl acetate molar ratio 1:20, as shown the results in Fig. 3. In addition, a reaction without catalyst was conducted for comparative effect. At 250 °C, 10% (m/m) of γ-alumina were needed to achieve 51.38% (m/m) and 1.11% (m/m) of FAME and triacetin content, in 90 min of reaction (85.2% (m/m) of conversion efficiency), while in 60 min these yields were 46.67% (m/m) and 0.98% (m/m), (77.2% (m/m) of conversion efficiency). Besides that, it is remarkable that an increment in catalyst concentration positively affected FAME and triacetin content at this temperature, since in 60 min, reaction without catalyst resulted in 7.53% (m/m) and 0% (m/m) of these compounds, while reactions with 2% (m/m) and 5% (m/m) of the catalyst resulted in FAME and triacetin contents of 24.3% (m/m) and 0.31% (m/m) and 31.8% (m/m) and 0.66% (m/m), respectively. On the other hand, at 300 °C, 2% (m/m) of γ-alumina was enough to reach 53.87% (m/m) and 2.10% (m/m) of FAME and triacetin contents in 90 min of reaction (90.86% (m/m) of conversion efficiency), while in 60 min these values were 48.96% (m/m) and 1.84% (m/m), corresponding to 82.46% (m/m) of conversion efficiency. At this temperature, catalyst addition didn’t show great effect through FAME and triacetin contents, since the increase of catalyst to 10% (m/m) resulted only in 52.39% (m/m) and 1.71% (m/m) in 60 min. Additionally, it can be pointed out that, at this temperature, catalyst effect was more pronounced in lower reaction times. After these considerations, reaction at 300 °C using 2% (m/m) of γalumina was chosen as the best condition, since requires less time reaction to achieve a satisfactory FAME and triacetin content, spending a smaller amount of catalyst. 3.4. Effect of oil to methyl acetate molar ratio According to interesterification reaction stoichiometry, 3 mols of methyl acetate are needed to convert 1 mol of triglycerides, thus, in order to shift the reaction to product side, an excessive amount of methyl acetate should be applied, mostly oil to methyl acetate molar ratio of 1:42 (Goembira and Saka, 2013). On the other hand, due to the high amount of FFA, part of FAME content is produced from esterification reaction, which stoichiometry is 1 mol of methyl acetate to convert 1 mol of FFA, also, to proceed the reaction to product side, an excess of reagent must be applied, but in this case it corresponds to oil to methyl acetate molar ratio around 1:3 or 1:6 (Park et al., 2011; Marousek et al., 2013b). Fig. 4 presents the effect of oil to methyl acetate molar ratios of 1:10, 1:20 and 1:40, at 300 °C, with 2% (m/m) of catalyst. While for molar ratio 1:10 the chemical equilibrium was reached in 60 min, with 46.83% (m/m) and 1.41% (m/m) of FAME and triacetin contents, respectively, for molar ratio of 1:20 it stabilized in 90 min, with 53.87% (m/m) and 2.10% (m/m) of FAME and triacetin content, and for a molar ratio of 1:40 it stabilized in 120 min with 55.80% (m/ m) and 2.51% (m/m) contents. Additionally, it can be observed that molar ratio has little influence in reaction rate, which is expected since the higher the molar ratio the greater the reagents dilution and smaller the reaction rate. Different from that required for low FFA content oils, where oil to methyl acetate molar ratio of 1:42 is needed to achieve the best conditions (Campanelli et al., 2010), higher molar ratios than 1:40 were not necessary to reach the best conditions in this case. This is probably due to the high FFA amount in macaw oil and consequent esterification reaction that requires less amounts of solvent to shift the equilibrium reaction to product side, according to its stoichiometry. Campanelli et al. (2010) investigated supercritical
Fig. 2. FAME (a) and triacetin (b) content and temperature (c) profiles in reaction medium from macaw oil and methyl acetate in oil to methyl acetate molar ratio of 1:20 with 5% (m/m) of catalyst at different temperatures: 225 °C (▲), 250 °C (■), 275 °C (♦) and 300 °C (●). Experimental data were linked for better visualization.
point out that in 60 min of reaction, 11.55, 31.80, 35.66 and 53.10% (m/m) of FAME where obtained (Fig. 2b), at 225 °C, 250 °C, 275 °C and 300 °C, respectively, indicating that during reactor heating, a significant esters production can occur. It can be noted from these results that ester yield in heating step increases with set point temperature, and so this information consists in important information for batch operation, and besides that, from the knowledge of these authors, is not yet reported in literature. Considering the results obtained in this work, temperature of 300 °C presented the best results for a short time 87
Industrial Crops & Products 124 (2018) 84–90
J.S. Ribeiro et al.
Fig. 3. FAME and triacetin content profiles in reaction medium from macaw oil and methyl acetate in oil to methyl acetate molar ratio of 1:20 at (a) 250 °C and (b) 300 °C at different catalyst contents: 0% (▲), 2% (●), 5% (■) and 10% (♦). Experimental data were linked for better visualization.
above 300 °C. Fig. 5 shows the content profiles of initial compounds present in macaw oil, intermediates and products formed during the reactions with methyl acetate at 300 °C, with oil to methyl acetate molar ratio 1:20 and 2% (m/m) of catalyst. Macaw oil is mostly composed by FFA and diglycerides. Interesterification reaction occurs mainly in the first 60 min of reaction (Fig. 5a), when it could be observed a more pronounced decrease in mono-, di- and triglycerides and an increase in intermediates monoacetinmonoglycerides (MAMG), monoacetindiglycerides (MADG) and diacetinmonoglycerides (DAMG) indicating conversion of macaw oil to methyl esters and triacetin. Initially, production of MADG stands out due to glycerides presents on reaction medium. Then, from 60 min, conversion of MADG to DAMG results in a decrease of MADG content and increase in DAMG content. However, DAMG content decrease is not observed over time suggesting that the conversion to triacetin is the limiting step in interesterification reaction, as reported by Casas et al. (2011b). FFA content in macaw oil also reacts with methyl acetate to produce FAME and acetic acid (evaporated after sample collect). Fig. 5b shows the consumption of FFA over time and consequent FAME content increasing. Although it can be observed interesterification reaction occurrence because of triacetin production and over FAME content based on FFA content, mostly of FAME produced comes from esterification reaction, since macaw oil contains mainly FFA in its composition. Additionally, it could be observed from Fig. 5 that 90 min were necessary to reach reaction equilibrium but 60 min were sufficient to reach compound contents close to the equilibrium, without more energy expenditure. Thus, temperature reaction of 300 °C, 2% (m/m) of catalyst, oil to methyl acetate molar ratio of 1:20 and 60 min of reaction were chosen as the best reaction conditions.
interesterification reaction for various oils with low FFA content and oil to methyl acetate molar ratio of at least 1:42 was required to shift reaction equilibrium and achieve maximum yields, while an increase in molar ratio to 1:59 did not affect neither the yields nor the kinetics of the process. Casas et al. (2011a, 2011b) studied chemical interesterification reaction of sunflower oil and methyl acetate and observed that triglyceride content was fully consumed with oil to methyl acetate molar ratio of 1:18, while a molar ratio of 1:50 was needed to remove monoacetindiglycerides. However, even with an impractical molar ratio of 1:100, diacetinmonoglycerides were not completely eliminated. Thus, they concluded that the conversion of diacetinmonoglycerides to triacetin is the limiting step of the reaction and 1:50 was the best oil to methyl acetate molar ratio in their studies. In this study oil to methyl acetate molar ratio of 1:20 was chosen as the best condition to proceed the investigation of reaction parameters, since an increase in molar ratio to 1:40 has not represented a great increase in FAME and triacetin contents, besides representing high reagent consumption. 3.5. Effect of reaction time For the conditions that provided the best FAME and triacetin contents in Figs. 3–6 (i.e., temperature of 300 °C, oil to methyl acetate molar ratio of 1:20 and 2% (m/m) of catalyst) the effect of reaction time was evaluated. Reactions achieved equilibrium within 120 min, and from Figs. 3 and 5b after a long time of reaction at 300 °C, it can be observed a light decreased in FAME and triacetin contents, probably due to thermal degradation (Abdala et al., 2014; Campanelli et al., 2010; Vieitez et al., 2011; Marousek et al., 2012). Vieitez et al. (2011) studied the stability of ethyl esters from soybean oil exposed to high temperatures and notice that severe conditions of reaction cause the occurrence of important degradation processes of the lipid material, suggesting that the process should not be performed at temperatures 88
Industrial Crops & Products 124 (2018) 84–90
J.S. Ribeiro et al.
Fig. 4. FAME and triacetin content profiles in reaction medium from macaw oil and methyl acetate over 2% (m/m) of γ-alumina at 300 °C in different oil to methyl acetate molar ratios: 1:10 (●), 1:20 (■) and 1:40 (▲). Experimental data were linked for better visualization.
Fig. 5. Content profiles of (a) triglycerides (□), diglycerides (○), monoglycerides (Δ), monoacetinmonoglycerides (▲), monoacetindiglycerides (■) and diacetinmonoglycerides (●) and (b) FFA (◊), FAME (●) and triacetin (▲) in reaction medium at 300 °C, oil to methyl acetate molar ratio of 1:20 and 2% (m/m) catalyst. Experimental data were linked for better visualization.
3.6. Catalyst reuse Catalyst reuse was evaluated at 300 °C, with 2% (m/m) of catalyst and oil to methyl acetate molar ratio of 1:20 for 60 min. The catalyst was recovered and reused for three more runs under the same conditions. Fame and triacetin contents are shown in Fig. 6 for each run. In the first run, with fresh catalyst, FAME and triacetin content achieved was 50.3% (m/m) and 1.52% (m/m), respectively, corresponding to 84.12% of conversion efficiency. After the second cycle the FAME content reached 49.45% (m/m), which is not significantly different considering the experimental error of 1.68%, while triacetin content increased to 2.11% (m/m). The increase in triacetin content could be related to acetic acid adsorbed in catalyst surface due to esterification reaction that reduces thermal degradation of triacetin (Campanelli et al., 2010). In the following cycles, FAME content remained at similar level as, with 49.4, 49.1, 48.9 and 44.1% (m/m) of ester content. From sixth level, there is a more pronounced decay in ester content. Values of catalytic efficiency decay are presented in parentheses for each cycle, showing that a high decay occurs from seventh cycle, with a catalytic activity loss of 12.47%. These results confirm that γ-alumina could be used for at least six cycles without a great loss of activity.
Fig. 6. Reusability of γ-alumina on interesterification and esterification reactions from macaw oil and methyl acetate at 300 °C, 2% (m/m) catalyst and oil to methyl acetate molar ratio 1:20, for 60 min.
confirmed that this process showed to be a promising method for FAME and triacetin production. Besides, it could be verified that esterification took place primarily in relation to interesterification reaction due to high acid value of the oil. Conversion of DAMG to triacetin was the
4. Conclusion In this work, heterogeneous catalyst using γ-Al2O3 was investigated for biodiesel production from macaw oil and methyl acetate. The results 89
Industrial Crops & Products 124 (2018) 84–90
J.S. Ribeiro et al.
limiting step of interesterification reaction. The best condition was 300 °C, 2% (m/m) of catalyst, oil to methyl acetate molar ratio of 1:20 and 60 min of reaction, reaching 48.96% and 1.84% of FAME and triacetin content, which corresponds to 82.46% of conversion efficiency. Catalyst could be reused for six cycles without significant loss of activity. Biodiesel production from this chemical route has great potential for industrial applications and should be more investigated in continuous processes, since it showed to be promising for reducing cost.
of waste cooking oil and methyl acetate for biodiesel and triacetin production. Fuel Process. Technol. 116, 241–249. https://doi.org/10.1016/j.fuproc.2013.07.004. Marousek, J., Kwan, J.T., 2013. Use of pressure manifestations following the water plasma expansion for phytomass disintegration. Water Sci. Technol. 67, 1695–1700. https://doi.org/10.2166/wst.2013.041. Marousek, J., Itoh, S., Higa, O., Kondo, Y., Ueno, M., Suwa, R., Tominaga, J., Kawamitsu, Y., 2012. The use of underwater high-voltage discharges to improve the efficiency of Jatropha curcas L. biodiesel production. Biotechenol. Appl. Biochem. 59, 541–546. https://doi.org/10.1002/bab.1045. Marousek, J., Itoh, S., Higa, O., Kondo, Y., Ueno, M., Suwa, R., Tominaga, J., Kawamitsu, Y., 2013a. Enzymatic hydrolysis enhanced by pressure shockwaves opening new possibilities in Jatropha Curcas L. processing. Chem. Technol. Biotech. 88, 1650–1653. https://doi.org/10.1002/jctb.4014. Marousek, J., Itoh, S., Higa, O., Kondo, Y., Ueno, M., Suwa, R., Tominaga, Y., Tominaga, J., Kawamitsu, Y., 2013b. Pressure schockwaves to enhance oil extraction from Jatropha curcas L. Biotech. Biotechnol. Equip. 27, 3654–3658. https://doi.org/10. 5504/BBEQ.2012.0143. Marousek, L., Haskova, S., Marouskova, A., Myskova, K., Vanickova, R., Vachal, J., Vochozka, M., Zeman, R., Zak, J., 2015. Financial and biotechnological assessment of new oil extraction technology. Energy Sources Part A Recov. Util. Environ. Eff. 37, 1723–1728. https://doi.org/10.1080/15567036.2015.1048391. Marx, S., 2016. Glycerol-free biodiesel production through transesterification: a review. Fuel Process. Technol. 151, 139–147. https://doi.org/10.1016/j.fuproc.2016.05.033. Navarro-Díaz, H.J., Gonzalez, S.L., Irigaray, B., Vieitez, I., Jachmanián, I., Hense, H., Oliveira, J.V., 2014. Macauba oil as an alternative feedstock for biodiesel: characterization and ester conversion by the supercritical method. J. Supercrit. Fluids 93, 130–137. https://doi.org/10.1016/j.supflu.2013.11.008. Nobre, D.A.C., Trogello, E., Borghetti, R.A., de S. David, A.M.S., 2014. Macaúba: palmeira De Extração Sustentável para biocombustível. Colloq. Agrar. 10, 92–105. https://doi. org/10.5747/ca.2014.v10.n2.a112. Park, J.Y., Wang, Z.M., Kim, D.K., Lee, J.S., 2011. Effects of water on the esterification of free fatty acids by acid catalysts. Renew. Energy 36, 889–891. https://doi.org/10. 1016/j.renene.2010.08.025. Ribeiro, J.S., Celante, D., Simões, S.S., Bassaco, M.M., Da Silva, C., De Castilhos, F., 2017. Efficiency of heterogeneous catalysts in interesterification reaction from macaw oil (Acrocomia aculeata) and methyl acetate. Fuel 200, 499–505. https://doi.org/10. 1016/j.fuel.2017.04.003. Saba, T., Estephane, J., El, B., El, M., Khazma, M., El, H., Aouad, S., 2016. Biodiesel production from refined sunflower vegetable oil over KOH/ZSM5 catalysts. Renew. Energy 90, 301–306. https://doi.org/10.1016/j.renene.2016.01.009. Saka, S., Isayama, Y., 2009. A new process for catalyst-free production of biodiesel using supercritical methyl acetate. Fuel 88, 1307–1313. https://doi.org/10.1016/j.fuel. 2008.12.028. Samain, L., Jaworski, A., Edén, M., Ladd, D.M., Seo, D.-K., Garcia-Garcia, F.J., Haussermann, U., 2014. Structural analysis of highly porous γ-Al2O3. J. Solid State Chem. 217, 1–8. https://doi.org/10.1016/j.jssc.2014.05.004. Tavares, G.R., Gonçalves, J.E., dos Santos, W.D., da Silva, C., 2017. Enzymatic interesterification of crambe oil assisted by ultrasound. Ind. Crops Prod. 97, 218–223. https://doi.org/10.1016/j.indcrop.2016.12.022. Trueba, M., Trasatti, S.P., 2005. γ-Alumina as a support for catalysts: a review of fundamental aspects. Eur. J. Inorg. Chem. 3393–3403. https://doi.org/10.1002/ejic. 200500348. Vieitez, I., Da Silva, C., Alckmin, I., De Castilhos, F., Oliveira, J.V., Grompone, M.A., Jachmanián, I., 2011. Stability of ethyl esters from soybean oil exposed to high temperatures in supercritical ethanol. J. Supercrit. Fluids 56, 265–270. https://doi. org/10.1016/j.supflu.2010.10.033. Xu, Y., Du, W., Liu, D., 2005. Study on the kinetics of enzymatic interesterification of triglycerides for biodiesel production with methyl acetate as the acyl acceptor. J. Mol. Catal. B Enzym. 32, 241–245. https://doi.org/10.1016/j.molcatb.2004.12.013. Zabeti, M., Wan Daud, W.M.A., Aroua, M.K., 2009. Activity of solid catalysts for biodiesel production: a review. Fuel Process. Technol. 90, 770–777. https://doi.org/10.1016/j. fuproc.2009.03.010.
Acknowledgements The authors wish to thank CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) for scholarships and CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) for financial support. References Abdala, A.C.D.A., Colonelli, T.A.D.S., Trentini, C.P., Oliveira, J.V., Cardozo-Filho, L., Da Silva, E.A., Da Silva, C., 2014. Effect of additives in the reaction medium on noncatalytic ester production from used frying oil with supercritical ethanol. Energy Fuels 28, 3122–3128. https://doi.org/10.1021/ef402253e. Campanelli, P., Banchero, M., Manna, L., 2010. Synthesis of biodiesel from edible, nonedible and waste cooking oils via supercritical methyl acetate transesterification. Fuel 89, 3675–3682. https://doi.org/10.1016/j.fuel.2010.07.033. Casas, A., Ruiz, J.R., Ramos, M.J., Pérez, Á., 2010. Effects of triacetin on biodiesel quality. Energy Fuels 24, 4481–4489. https://doi.org/10.1021/ef100406b. Casas, A., Ramos, M.J., Pérez, Á., 2011a. New trends in biodiesel production: chemical interesterification of sunflower oil with methyl acetate. Biomass Bioenergy 35, 1702–1709. https://doi.org/10.1016/j.biombioe.2011.01.003. Casas, A., Ramos, M.J., Pérez, Á., 2011b. Kinetics of chemical interesterification of sunflower oil with methyl acetate for biodiesel and triacetin production. Chem. Eng. J. 171, 1324–1332. https://doi.org/10.1016/j.cej.2011.05.037. César, A.D.S., Almeida, F.D.A., De Souza, R.P., Silva, G.C., Atabani, A.E., 2015. The prospects of using Acrocomia aculeata (macaúba) a non-edible biodiesel feedstock in Brazil. Renew. Sustain. Energy Rev. 49, 1213–1220. https://doi.org/10.1016/j.rser. 2015.04.125. Doná, G., Cardozo-Filho, L., Silva, C., Castilhos, F., 2013. Biodiesel production using supercritical methyl acetate in a tubular packed bed reactor. Fuel Process. Technol. 106, 605–610. https://doi.org/10.1016/j.fuproc.2012.09.047. Goembira, F., Saka, S., 2013. Optimization of biodiesel production by supercritical methyl acetate. Bioresour. Technol. 131, 47–52. https://doi.org/10.1016/j.biortech.2012. 12.130. Goembira, F., Saka, S., 2014. Effect of additives to supercritical methyl acetate on biodiesel production. Fuel Process. Technol. 125, 114–118. https://doi.org/10.1016/j. fuproc.2014.03.035. Goembira, F., Saka, S., 2015. Advanced supercritical methyl acetate method for biodiesel production from pongamia pinnata oil. Renew. Energy 83, 1245–1249. https://doi. org/10.1016/j.renene.2015.06.022. Gonzalez, S.L., Sychoski, M.M., Navarro-Díaz, H.J., Callejas, N., Saibene, M., Vieitez, I., Jachmanián, I., Da Silva, C., Hense, H., Oliveira, J.V., 2013. Continuous catalyst-free production of biodiesel through transesterification of soybean fried oil in supercritical methanol and ethanol. Energy Fuels 27, 5253–5259. https://doi.org/10. 1021/ef400869y. Lu, C.L., Lv, J.G., Xu, L., Guo, X.F., Hou, W.H., Hu, Y., Huang, H., 2009. Crystalline nanotubes of γ-AlOOH and γ-Al2O3: hydrothermal synthesis, formation mechanism and catalytic performance. Nanotechnology 20 (215604), 1–9. Maddikeri, G.L., Pandit, A.B., Gogate, P.R., 2013. Ultrasound assisted interesterification
90