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Measurement of Dielectric Properties and Microwave-Assisted Homogeneous Acid-Catalyzed Transesterification in a Monomode Reactor Evandro L. Dall’Oglio,*,† Paulo T. de Sousa Jr.,† Deibnasser C. Campos,† Leonardo Gomes de Vasconcelos,† Alan Cândido da Silva,† Fabilene Ribeiro,† Vaniomar Rodrigues,† and Carlos Alberto Kuhnen‡ †

Departamento de Química, Universidade Federal do Mato Grosso, Av. Fernando Corrêa da Costa s/n, Coxipó, Cuiabá-MT CEP 78090-600, Brazil ‡ Departamento de Física, Universidade Federal do Santa Catarina, Campus Universitário Trindade, Florianópolis-SC CEP 88040-970, Brazil S Supporting Information *

ABSTRACT: Microwave heating technology is dependent on the dielectric properties of the materials being processed. The dielectric properties of H2SO4, H3PO4, ClSO3H, and H3CSO3H were investigated in this study using a vector network analyzer in an open-ended coaxial probe method at various temperatures. Phosphoric and sulfuric acids presented higher loss tangents in the frequency range 0.3−13 GHz, reflecting greater mobility of the ions and counterions. The acids were employed as catalysts in microwave-assisted homogeneous transesterification reactions for the production of methylic and ethylic biodiesel. The effects of catalyst concentration, alcohol to oil molar ratio, and irradiation time on biodiesel conversions were investigated. The results showed a significant reduction in the reaction time for microwave-assisted transesterification reactions as compared to times for conventional heating. Also, despite its higher loss tangent, it was observed that H3PO4 leads to lower conversion to biodiesel, which can be explained by its lower carbonyl protonation capacity.

1. INTRODUCTION Climate change (CC) is in the top of the international political agenda and its effects are felt in many countries, affecting mainly the poorer population. To mitigate this phenomenon, many alternatives have been discussed, mainly the ones related to reduce the anthropogenic causes of CC. Among those, there has been increasing interest in developing alternative energy sources, such as solar energy, hydrogen cells, wind power, and biofuels, to provide solutions for the environmental problems associated with fossil fuels. Biodiesel, which is synthesized from vegetable oil, is a realistic alternative to diesel fuel because it may provide a fuel from renewable resources and has lower emissions than petroleum diesel.1−5 Biodiesel consists of simple alkyl esters of fatty acids that are derived from renewable energy sources such as animal fats and vegetable oils. The transesterification process is a well-known chemical reaction and is widely used to obtain this important liquid fuel. Biodiesel is, therefore, obtained after three equivalent, consecutive, and reversible, acid- or basic-catalyzed reactions, where triglyceride (vegetable oils or animal fats) reacts with alkyl alcohols affording monoalkyl esters and glycerol. By doing so, the triglyceride (TG) is converted to a diglyceride (DG), then to a monoglyceride (MG) and finally to glycerol (GL) and monoalkyl ester. At each stage one molecule of alkyl ester is © 2015 American Chemical Society

produced. From the theoretical point of view, DFT calculations were recently reported for the alkaline- and acid-catalyzed methanolysis and ethanolysis of butyric and pentylic acid triglycerides in the gas phase6 and in solution considering solvent effects and including microsolvation.7 Many recent experimental studies have been aimed at enhancing the biodiesel production yield8−10 along with diminishing the reaction time and making use of cheaper raw materials. This includes the use of different homogeneous catalyst systems and heterogeneous catalyzed reactions where other variables affecting the ratios, reaction time, reaction temperature and addition of cosolvent were extensively studied. The alkaline-catalyzed transesterification process is usually the method adopted for large-scale biodiesel production because it is well-known that alkaline metal alkoxides and hydroxides are much more effective as catalysts than acids and in all cases, methanol, produced mostly from nonrenewable sources, is the most effective alcohol.1 Alkaline conditions are not suitable for biodiesel production from residual waste vegetable oils and animal fats, which are promising alternative raw materials Received: May 21, 2015 Revised: July 10, 2015 Published: August 5, 2015 8971

DOI: 10.1021/acs.jpca.5b04890 J. Phys. Chem. A 2015, 119, 8971−8980

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The Journal of Physical Chemistry A

0.05 wt %, respectively, were used. Residual oils and fats (ROFs) were obtained from restaurants, bars, and cafeterias. After filtration, the ROFs were sent to a tapered evacuated heating system for dehumidification at 90 °C and low pressure (120 mmHg). AOCS Ca 5a-40 of 1997 and the total polar compounds standard ISO 8420 2002 were used for the quality control of the ROFs and the free acidity, respectively. Average values of 1.58% acidity and 21.65% total polar compounds were obtained for the ROFs. 2.2. Equipment. The measurements to determine the dielectric properties were performed with an open-ended coaxial probe (HP 85070B, Agilent, Palo Alto, CA, USA) connected to a network analyzer (HP 8753C, Agilent, Palo Alto, CA, USA), in a 101-point frequency sweep from 300 MHz to 13 GHz. The network analyzer was controlled by the Hewlett−Packard 85070B dielectric probe kit software (Agilent, Palo Alto, CA, USA) and calibrated using the threepoint method (short-circuit, air and water at the measurement temperature). Measurements were taken in triplicate. The probe was mounted at the top of the cells used for the calibration and for the measurements. Water, as the reference liquid, was maintained in the jacketed cell of a 50 mL glass sample holder connected to a thermostated bath for temperature control. For the measurement of the isolated components of the reaction (the acids, alcohols, oil, etc.) the respective sample and the reference liquid were maintained in jacketed cells at the same temperature in independent thermostated baths. For the measurements taken in the reaction mixture a specific cell was built to provide the basic features of a conventional reflux reaction system: mechanical stirring, heat exchange, and temperature control. The jacketed cell consists of a 250 mL glass sample holder, connected to a thermostated bath and coupled to a stirrer and a digital thermometer. The cell was sealed during the measurements to avoid alcohol loss due to evaporation. Measurements were not carried out for HCl because it is a gas and is available only in liquid solutions. In addition, due to inherent difficulties, in the case of ClSO3H, measurements were performed only at 70 °C (high reactivity with air leading to calibration problems). The transesterification reactions carried out under microwave irradiation to test the various homogeneous acid catalysts were performed in the monomode reactor. The apparatus consists of a Teflon reactor, working as a monomode resonant cavity supplied with electromagnetic irradiation by a magnetron valve emitting at 2.45 GHz with a nominal power of 800 W, equipped with a high voltage system that supplies the magnetron, a condenser, and cooling coil coupled with a thermal bath and a mechanical stirrer. To maximize the energy transfer between the source and the sample, a system consisting of a Watt meter and a mechanical tuner is included in the waveguide line. Nine calorimetric measurements were carried out (each one in triplicate) to determine the effective power emitted by the magnetron and absorbed by the sample (40 mL of water irradiated for 20 s) in the monomode reactor, which gave 700 W (average over nine measurements) as the effective power absorbed by the sample. Other control sensors (e.g., temperature, pressure, etc.) were used and for microwave generation the system was equipped with a Programmable Logic Controller (WEG; TPW-03/30HR-A/8AD). This allows experiments to be performed with emission of irradiation continuously or in cycles, for example, a 30/10 cycle involves 30 s of emission and 10 s without emission of irradiation.

considering environmental and economic issues. The catalytic activity of acid catalysts is not strongly affected by the presence of free fat acids (FFAs) in the feedstock, and they must be employed if a high content of FFAs is present, as in some lowcost raw materials or in waste frying oils. Although the homogeneous acid-catalyzed reaction provides an increase in the biodiesel yield, it is much slower than the alkaline-catalyzed reaction and requires higher temperatures and pressures.1,2 Microwave irradiation has been used as an alternative heating system to overcome the characteristic reaction drawbacks associated with conventional heating in the homogeneous or heterogeneous base- or acid-catalyzed transesterification of triacylglycerides, such as long reaction time and higher temperatures.11−13 In this regard, the application of microwave heating has been reported as a fast and easy method to obtain biodiesel by employing alkaline- or acid-catalyzed reactions.14−19 Heterogeneous or homogeneous catalytic alcoholysis of triglycerides induced by microwaves may be a viable alternative technology for ethylic biodiesel production, especially in Brazil, one of the world’s largest producers of renewable sugar cane bioethanol.20 Microwave heating can be enhanced by controlling variables such as the power level, the frequency of the applied field, and the initial temperature of the sample. However, microwave dielectric heating is a macroscopic effect of the interaction of electromagnetic fields in the microwave region with continuous media characterized by their intrinsic dielectric properties,21,22 which requires empirical measurements for both simple and complex materials, mainly for mixtures such as reaction solutions. Although the dielectric properties of most pure solvents and some of their mixtures are widely available in the literature,23−29 data on the dielectric properties of vegetables oils, animal fats, or even their mixtures with methylic or ethylic alcohols are scarce.30 In a previous paper we reported31 the dielectric constant (ε′) and loss factor (εef″ ) of the pure liquids methanol, ethanol, glycerin, and sulfuric acid, of vegetable oils (Brazil nut and soybean), and of the reaction mixtures during the acid-catalyzed transesterification reaction. Another recent study has addressed the measurement of the dielectric properties of the reaction mixture in the alkaline-catalyzed transesterification of soybean oil.32 In an effort to better understand the microwave-assisted acidcatalyzed transesterification reactions, in this study the dielectric properties of the acid catalysts H2SO4, H3PO4, ClSO3H, and H3CSO3H were determined. The dependence of the results on the field frequency was studied in the range 0.3− 13 GHz at different temperatures. Also, the effect of various homogeneous acid catalysts was investigated in the optimization of the microwave-assisted transesterification of refined soybean and maize oils and residual oils and fats (ROFs) for the production of methylic and ethylic biodiesel in a monomode reactor.

2. EXPERIMENTAL SECTION 2.1. Materials. Ethanol (99% Synth) and methanol (99.8% FM) were purchased from Tedia. The homogeneous acid catalysts tested were H2SO4 (95%−99%, Spectrum), HCl (produced in laboratory scale from NaCl-H2SO4 reaction, and stored as 0.1 M ethanolic or methanolic solution), H3PO4 (85%, Sigma), ClSO3H (98%, Sigma), and H3CSO3H (99.8%, Fluka). Commercial soybean, maize, and Brazil nut oils, with water and free fatty acid (FFA) contents of around 0.11 and 8972


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Figure 1. (a) Relative dielectric constant, ε′, and (b) relative dielectric loss, εef″ , for sulfuric acid at various temperatures; loss tangent values for (c) studied acids at 70 °C and for (d) Brazil nut oil, methanol, glycerol, ethanol, and ethanol−Brazil nut oil reaction mixtures (90:1 and 6:1 molar ratios) using 3% and 5% (V/VT) acid concentration (70 °C). *Methanol at 60 °C.

were monitored by analyzing the biodiesel using 1H NMR spectroscopy, as described in the literature for methanolysis33 and ethanolysis,34 and the results were recorded on a Varian Mercury 300 MHz spectrometer (data available in the Supporting Information).

2.3. Transesterification Process. The transesterification reactions were carried out with the following homogeneous acid catalysts: H2SO4, H3PO4, H3CSO3H ClSO3H, and HCl. The reactions were induced by dielectric microwave heating for different concentrations of the catalysts, with different alcohol/ oil molar ratios and varying the irradiation time to find the catalyst with the best catalytic activity under microwave heating and the optimum reaction conditions. The catalyst−alcohol−oil mixture was poured into the monomode reactor and stirred to ensure complete mixing as part of the catalyst activation and irradiated by microwaves. The reaction temperatures were 80− 84 and 66−70 °C for the ethanolysis and methanolysis, respectively. At the end of the reaction the mixture was distilled to remove excess alcohol under reduced pressure at 50 °C. The biodiesel was separated from the glycerol in a decanting funnel, where the glycerol was drained. The product was then neutralized with NaHCO3 and washed with 20% water (V/ V) to achieve a pH of 7. Concerning the purity of the biodiesel, the ABNT NBR 14448 (Brazil), ASTM D664 (USA), and EN 14104 (European) international standards allow a maximum acidity of 0.8 mg KOH/g of biodiesel, which is equivalent to an acidity in oleic acid of lower than 0.4%. All transesterification reactions were performed in triplicate, and the results shown are the average of three values. The transesterification reactions

3. RESULTS AND DISCUSSION 3.1. Dielectric Properties. Parts a and b of Figure 1 show the relative dielectric constant and dielectric loss of sulfuric acid as a function of the frequency (0.3−13 GHz, logarithmic scale) at various temperatures. Sulfuric acid is a liquid with polar molecules with a permanent dipole moment and the dielectric constant of polar liquids is frequency-dependent, showing a distinct decrease as the frequency increases, as can be verified from the data in Figure 1a. Similar results have been reported in the literature for polar solvents, such as liquid water, ethanol, and methanol.24−28,31,32 Measurements for the liquid acids H3PO4, ClSO3H, and H3CSO3H show the same trend for the dielectric constant (data available in the Supporting Information). To the best of our knowledge this is the first report on the frequency and temperature dependence of the dielectric properties of H2SO4, H3PO4, ClSO3H, and H3CSO3H. The results at various temperatures demonstrate that the rate of the decrease in the dielectric constant is lower at higher 8973


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range. It should be noted that glycerol has a high loss tangent in the frequency range studied, except for very high and very low frequencies, and as a byproduct of the transesterification reaction its high dielectric loss must be accounted for when the irradiation time exceeds a certain value in the microwaveassisted transesterification reaction, as we shall see below. Table 1 provides the numerical values for the dielectric parameters at 2.45 GHz for the acids H2SO4, H3PO4, ClSO3H,

temperatures (Figure 1a). This can be understood by noting that at higher temperatures the kinetic energy of molecules is high; hence, the response to the varying electric field is faster and the viscosity is lower (molecules further apart), which enhances the realignment process, increasing the dipolar rotation of the molecules and therefore the dielectric constant of the material compared to those at lower temperatures. As can be seen in Figure 1b, very high values for the loss factor are achieved at low frequencies; however, this trend changes markedly, showing a steady decrease as the frequency increases. Similar results for the behavior of the loss factor were found for methanesulfonic, phosphoric, and chlorosulfonic acids (data not shown). These acids have a very high loss factor due to the high concentration of ions and counterions in their liquid phase. For example, for sulfuric acid the ionic species chiefly responsible for the high conductivity of liquid sulfuric acid are principally H3SO4+ and HSO4−, through autoprotolysis, with a small contribution from small amounts of H3O+ and HS2SO7− through an ionic self-dehydration mechanism. The high loss factor of liquid sulfuric acid is mainly associated with the high mobility of the ions and counterions (ionic conductivity) and the dielectric relaxation process has a negligible effect on the absorption of electromagnetic energy by the liquid. Clearly, the same kind of mechanism occurs in the case of H3PO4, ClSO3H, and H3CSO3H liquid acids and, consequently, as the temperature increases, the mobility of the ions and the conductivity increases, leading to an increase in the dielectric loss of such substances, in particular H3PO4, where there is a greater concentration of ionic species due to the presence of water (15%). The loss tangent is defined by tan δ = ε″ef/ε′ and is an important parameter in describing the dielectric response of materials in terms of, for example, the penetration depth (Dp), that is, the distance at which the amplitude of the electrical field is damped to 1/e = 0.369 of its initial value at the surface of the material.21 The loss tangents for H2SO4, H3PO4, ClSO3H, and H3CSO3H liquid acids at 70 °C in the frequency range 0.3−13 GHz are shown in Figure 1c. Phosphoric and sulfuric acids have higher loss tangents across the entire frequency range, reflecting the high mobility of the ions and counterions compared to the case of the other acids investigated in this study. The high values for the loss tangent obtained for the acids and the values for the biodiesel precursors, Brazil nut oil, ethanol, methanol, for glycerol (a byproduct of the transesterification reaction), and for the reaction mixtures with ethanol at Brazil nut oil molar ratios of 90:1 and 6:1 and H2SO4 acid concentrations of 3% and 5% (V/VT) can be observed in Figure 1d (note the change in the scale relative to Figure 1c). The measurements were taken at 70 °C and in the case of the reaction mixtures the measurements were performed in the first 20 min, that is, with negligible conversion to ethyl esters. Measurements taken during the transesterification reaction have been previously reported by our group,31 and the reactions with conventional heating require longer times to proceed. For a molar ratio of 90:1 and 5% H2SO4 a reaction time of 2 h was required to reach 100% conversion whereas, for a molar ratio of 6:1 and 3% H2SO4, 100% conversion was achieved in 12 h.31 As shown in Figure 1d, the loss tangent of the Brazil nut oil is negligible, whereas high values were obtained for the reaction mixtures with a high molar ratio and high acid concentration. Even with a molar ratio of 6:1 the loss tangent increases appreciable as the acid concentration increases from 3% to 5%, being higher than the loss tangent for methanol and ethanol in a wide frequency

Table 1. Relative Dielectric Constant, Relative Dielectric Loss, and Loss Tangent at Various Temperatures for Pure Acids at 2450 MHz sample sulfuric acid

methanesulfonic acid

phosphoric acid

chlorosulfonic acid

T (°C)

ε′

ε″ef

Tgδ

10 20 30 40 50 60 70 20 30 40 50 60 70 20 30 40 50 60 70 70

18.9 25.7 29.1 30.6 31.9 31.5 29.5 22.2 26.6 28.7 27.4 30.5 29.1 25.0 23.9 27.7 28.3 30.9 28.4 23.0

85.5 93.1 112.2 127.4 136.4 164.6 192.2 40.3 46.3 52.9 59.1 53.7 65.3 62.1 86.4 115.8 132.5 160.1 185.9 34.9

4.5 3.6 3.9 4.2 4.3 5.2 6.5 1.8 1.7 1.8 2.2 1.8 2.2 2.5 3.6 4.2 4.7 5.2 6.5 1.5

and H3CSO3H at various temperatures. Sulfuric acid has a higher dielectric loss and for all acids the loss factor increases with the temperature as the mobility of ions and counterions increases. Water has a loss tangent of 0.05 at 70 °C, verifying the high values displayed in Table 1, because, even in the case of ClSO3H, the values are around 30 times greater than that for the water value. The high loss factors exhibited by liquid acids (Table 1) originate from the high mobility of the ions and counterions and the dielectric relaxation process has a negligible effect on the absorption of electromagnetic energy. This means that the dielectric loss is mainly due to the ionic conductivity, σ (ε″ef ≃ σ/ωε0), which allows us to describe the penetration depth in terms of the conductivity because for high dielectric loss the penetration depth21 reduces to Dp ≃ c/ (2πεef″ )1/2f ≃ c(ε0/fσ)1/2 (where c is the speed of light and f the frequency in Hz). The linear dependence of the penetration depth of the acids on 1/σ1/2 can be observed in Figure 2a,b for sulfuric and methanesulfonic acids, respectively, at three allocated frequencies, for industrial, scientific, and medical (ISM) applications and at various temperatures. Similar results were found for phosphoric acid (data not shown). As shown in Figure 2, high electromagnetic energy absorption due to the ionic conductivity leads to low penetration depth values for the acids and a decrease in their values as the temperature increases. The influence of alcohol and acid concentration on dielectric properties of reaction mixtures can be observed by analyzing the behavior of the 8974


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Figure 2. Dependence of penetration depth (cm) at various temperatures on the inverse of the square root of the conductivity (S/m) for (a) sulfuric acid and (b) methanesulfonic acid.

%.1,8,9,35,36 The reactions were carried out using methanol and ethanol with an alcohol/oil molar ratio of 6:1, which is lower than the values described in many reports in the literature applying conventional heating, where ratios of 9:1 up 30:1 are commonly used. Furthermore, the values of 6:1 and 9:1 are also the molar ratios adopted in microwave-assisted alkaline transesterification reactions.1 The reactions were performed in the monomode reactor with an irradiation time of 20 min in 30 cycles of 40/10, that is, 40 s of emission followed by 10 s without emission (concerning this choice for the cycles see section 3.3). The results obtained using maize oil for the ethyl and methyl esters conversions for each catalyst are summarized in Table 2.

penetration depth. Small penetration depths of 0.80 cm (at 70 °C and 2.45 GHz) were obtained also for the reaction mixtures with an ethanol to oil molar ratio of 90:1 and high acid concentration (5% V/VT H2SO4). As shown in Figure 1d, the ionic contribution to the loss factor reduces considerably on decreasing the molar ratio to 6:1 and the acid concentration to 3% V/VT, leading to an increase in the penetration depth (at 70 °C and 2.45 GHz) to 5.3 cm. These results for the dielectric properties make it clear that data on parameters such as the dielectric constant, loss factor, and loss tangent as well as the penetration depth are critical for the design of microwaveassisted transesterification reactions, and also for the development of numerical methods for biodiesel production processes. In the next section transesterification reactions were performed in a monomode reactor to obtain and to compare the catalytic activity of some acids under dielectric instead of conventional heating. 3.2. Screening of Some Acid Catalysts. Acid catalysis with esters under conventional heating requires a very strong acid due to the weakly basic nature of the carbonyl group. Until now most studies have focused on sulfuric acid,1,35,36 but other acids, such as H3PO4,3 HCl,4,5 and organic sulfonic, have also been used.1,36 For example, in the pioneering work of Freedman et al.35 the transesterification kinetics of soybean oil with butanol, using sulfuric acid as the catalyst, was reported. Sulfuric acid is also a very promising catalyst for homogeneous acid-catalyzed transesterification under microwave heating.1,11,18 However, to find an optimum acid catalyst for the microwave-assisted transesterification process, screening of the catalytic activity of five homogeneous acid catalysts (H2SO4, H3PO4, H3CSO3H, ClSO3H, and HCl) in the maize and soybean oil transesterification was performed in this study. The same reaction conditions were employed for each catalyst; hence, direct comparisons are possible. Considering that the use of high acid concentrations raises questions concerning corrosion at an industrial level, biodiesel transport, and increased levels of effluent generated in the purification step,1,2 the experiments were performed with 3% (V/VT) catalyst. In general, the acid concentrations used in acidcatalyzed transesterification reactions are in the range 1−5 wt

Table 2. Catalytic Activity of Homogeneous Acid Catalysts in the Conversion of Maize Oil to Ethyl and Methyl Esters Induced by Microwave Irradiation in a Single Monomode Reactora conversion (%) catalysts

ethyl

methyl

pKab

H2SO4 HCl H3PO4 H3CSO3H ClSO3H

82.79 78.69 3.83 89.56 93.11

55.49 20.15 7.87 11.75 53.32

−6.0 −7.0 2.1 −1.9 −6.0

a

Reaction conditions: ethanol/oil molar ratio 6:1; catalyst amount 3% (V/VT); reaction time 20 min with cycles of 40/10 (see text). Mechanical stirring 280 rpm. bReference 37.

It should be noted that the reaction conditions were not optimized to maximize the reaction yield for each different catalyst, but the procedure adopted provided a way to compare the activities of the acid catalysts employed. From the loss tangents reported in Table 1 it can be expected that all acids act as strong catalysts under microwave irradiation, leading to high conversions in all cases. As can be observed in Table 2, high conversions were obtained (except for H3PO4) and the difference between the numerical values for the conversion demonstrates that the catalytic activity of these acids differs 8975


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The Journal of Physical Chemistry A appreciably under microwave irradiation. According to reports in the literature, the reaction times range from 12 to 60 h, when sulfuric,1,4,32,35,36 hydrochloric,5 or phosphoric acids3 are employed as catalysts under conventional heating. As shown in Table 2, the reaction times are considerably reduced under microwave heating, verifying that dielectric heating is an efficient and fast method of biodiesel production.12−19 Reports on the conventional or microwave-assisted acid-catalyzed transesterification using ClSO3H and H3CSO3H as catalysts were not found in the literature. Concerning the use of HCl and chlorosulfonic acid as catalysts, it must be stressed that chlorination or sulfonation products were not observed in the reactions carried out in the present work (using an open reactor). But in reactions carried out in a closed reaction vessel sulfonation products were observed. These experiments are in progress and results will eventually be published elsewhere. Phosphoric acid exhibits very low activity in carbonyl protonation in acid catalysis employing ethanol or methanol, which may be related to its higher pKa value relative to the other catalysts, as shown in Table 2. In contrast to microwave heating, with conventional heating, the butanolysis of low-cost feedstock has been reported using 5 wt % (oil) of H3PO4, high conversion rates being achieved with reaction times varying from 24 to 10 h depending on the alcohol to oil molar ratio and acid concentration.3 The high loss factor of H3PO4 (Table 1) helps to explain the almost null conversion of oil to ethylic biodiesel using phosphoric acid because high absorption of electromagnetic energy implies that high local temperatures easily appear with microwave heating,21,22 that is, instantaneous localized superheating, which leads to side reactions between the alcohol and the acid, leading to undesirable byproducts. In the ethanolysis with phosphoric acid, GC/MS analysis was performed (data available as Supporting Information) where ethene was detected, indicating a greater affinity of this acid for ethanol dehydration than for oil transesterification. In the methanolysis with phosphoric acid, a slightly higher conversion was obtained in comparison with that from ethanolysis (Table 2), which can be related to the fact that the dehydration of methanol is very difficult to occur under normal conditions.38−40 The conversion of refined maize oil to ethyl and methyl esters according to the irradiation time, using 2% (V/VT) of ClSO3H and employing cycles of 40/10, is shown in Figure 3. A high conversion is achieved after 15 min, indicating that the transesterification reaction proceeds faster than it does under conventional heating. Ethanolysis proceeds faster than methanolysis. This may be related to the high loss tangent of ethanol relative to methanol (Figure 1d). Methanol at 60 °C has a loss tangent of 0.31 whereas 0.60 and 0.50 are the values for ethanol at 60 and 70 °C, respectively, implying higher absorption of microwave energy by reaction mixtures with ethanol, increasing the conversion to ethyl esters. On the contrary, the low conversion in the methanolysis could be attributed to the high volatility of methanol relative to ethanol, which decreases the concentration of methanol in the mixture, hence lowering the conversion to methyl esters. This can be quantified by observing that the enthalpies of vaporization at boiling point are 38.56 and 35.21 kJ mol−1 for ethanol and methanol, respectively. These energies indicate that intermolecular forces are weaker in methanol, explaining its higher volatility relative to ethanol at close to their respective boiling points, which, in turn, may be related to the low conversion observed in methanolysis under microwave irradiation.

Figure 3. Conversion of refined maize oil to biodiesel and irradiation time (cycles 40/10) using ClSO3H as the catalyst (2% V/VT) employing methanol and ethanol. Alcohol to oil molar ratio 6:1.

Furthermore, the low conversion in methanolysis under microwave heating may be related to the influence of a microwave irradiation field on the vapor−liquid equilibrium, as recently reported by Gao et al.41 The experimental results for benzene/ethanol and DOP/isooctanol binary mixtures illustrate that the microwave heating could strongly affect the vapor−liquid equilibrium of the binary mixture when the difference between the dielectric properties of the two substances is large.41 Their experiments showed that increasing the microwave power increases the shift in the vapor−liquid equilibrium. In the experiments of Gao et al.41 the emitted power/mass sample ratio (Watt per gram) was varied from approximately 0.30 to 2.5 W/g. In the study reported herein, the experiments in Table 2 were performed with approximately 22 W/g of emitted power/mass sample ratio and for these mixtures the differences between the dielectric properties of oil, alcohol, and acid are large, as observed in the previous section. Therefore, the low conversion in methanolysis (Table 2) could be associated with different dielectric properties of the mixtures, leading to significant changes in the vapor−liquid equilibrium under microwave irradiation. Research on vapor−liquid equilibrium measurements carried out in reaction mixtures under microwave heating is currently in progress. From Table 2 it can be noted that for all of the acid catalysts employed, with the exception of H3PO4, ethanolysis gave higher conversions than methanolysis under the same reaction conditions. Sulfuric acid, ClSO3H, and H3CSO3H showed high catalytic activity with microwave heating, giving conversions above 90%. Considering that sulfuric acid has a higher loss tangent (Table 1) and has been used in acid-catalyzed transesterification under conventional heating, we choose sulfuric acid to investigate the effects of the parameters that influence the conversion, in the search for the optimum reaction conditions. 3.3. Effect of Amount of Acid Catalyst on Biodiesel Conversion. On the basis of the results shown in Table 2, we investigated in greater detail the microwave-assisted transesterification of ROFs to biodiesel using sulfuric acid as the 8976


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The Journal of Physical Chemistry A catalyst. First, we focused on the effects of the catalyst concentration on the biodiesel conversion. Figure 4 shows the

Figure 5. Conversion of maize oil to biodiesel with irradiation time by employing a cycle of 40/10 at various H2SO4 concentrations. Reaction conditions: ethanol/oil molar ratio of 6:1.

Figure 4. Conversion to biodiesel by applying different H2SO4 concentrations in the transesterification of ROFs. Reaction conditions: ethanol or methanol/oil molar ratio 6:1; reaction time 60 s, continuous emission.

3.4. Effect of Alcohol to Oil Molar Ratio and Irradiation Cycles on Biodiesel Conversion. From the theoretical point of view, the transesterification reaction requires three moles of alcohol for each mole of oil. However, to drive the reaction toward completion, the molar ratio should be higher than that of the stoichiometric ratio (transesterification reactions behave as pseudo-first-order reactions). In fact, the molar ratio of alcohol to oil is an important factor affecting the conversion efficiency and, consequently, the biodiesel production cost. Before the effect of the alcohol to oil molar ratio on the oil to biodiesel conversion using microwave heating was investigated, the transesterification reactions of soybean oil were performed using various cycles of microwave irradiation, to find the optimum cycle of emission and nonemission. The results for the conversion of soybean oil to ethyl ester using a 12:1 ethanol to oil molar ratio and 1.5% (V/VT) of H2SO4 employing different cycles and a total irradiation time of 40 min in each case are shown in Table 3. It is clear that the 40/10, 15/15, and 30/20 cycles achieved higher conversions in relation to the other cycles. These results show that there is a degree of flexibility regarding the selection of an irradiation cycle that allows high conversions to be achieved, where the total times are different but the conversions are similar. Experiments were performed by

effect of different concentrations of catalyst on the conversion of ROFs to biodiesel (from 1.0% up to 5.0%). In all of the reactions the alcohol to oil molar ratio was maintained at 6:1 and the irradiation times were 60 s (continuous emission) employing the monomode reactor without mechanical stirring. It can be observed from Figure 4 that, as in conventional heating,1−5,35,36 the microwave-assisted reaction rate in acidcatalyzed processes also increases with the use of a greater amount of catalyst but, in contrast to conventional heating, ethanolysis affords higher conversion when compared with methanolysis under the same reaction conditions, as observed in the transesterification of refined maize oil (Figure 3) and ROFs (Figure 4). The influence of the amount of catalyst on the transesterification of maize oil can be observed in Figure 5, where an ethanol to oil molar ratio of 6:1 was used with various acid concentrations. Microwave irradiation was performed with cycles of 40/10. The conversion increased markedly as the content of sulfuric acid increased, achieving 100% in less than 20 min by employing 3% (V/VT) of H2SO4. For the transesterification of grease with methanol under conventional heating, a rate enhancement has been reported36 with higher concentrations of H2SO4 and methyl ester. The yield increased from 72.7 to 95.0% as the catalyst concentration was increased from 1 to 5 wt %. However, it was noted during these experiments that any excess addition of sulfuric acid darkens the color of the biodiesel. In the acid-catalyzed production of biodiesel from waste frying oil, Zheng et al.9 found that to prevent scorching of the oil by the sulfuric acid, its concentration must be kept in the range 1.5−3.5% (V/VT) in the total oil−methanol−acid mixture. In addition to corrosion problems, it is known that large quantities of acid catalyst can promote ether formation through alcohol dehydration, requiring additional amounts of NaHCO3 during the catalyst neutralization process.36 In light of all of these aspects and on the basis of the results shown in Figures 4 and 5, a sulfuric acid concentration of 3% was employed in all subsequent transesterification reactions.

Table 3. Conversion of Soybean Oil to Ethyl Esters Employing Different Cycles with Emission and without Emission of Microwave Irradiation in a Single Monomode Reactora no. of cycles

emission time (s)

without emission (s)

total time (min)

conversion (%)

60 160 120 160 80

40 15 20 15 30

10 15 30 25 20

50 80 100 107 67

90.96 100.00 87.00 87.02 92.17

a

Reaction conditions: ethanol/oil molar ratio 12:1; catalyst 1.5% (V/ VT); irradiation time, 40 min.

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Figure 6. (a) Conversion through ethanolysis of soybean (molar ratio 12:1) and maize (molar ratio 6:1) oils to biodiesel with irradiation time (cycles 15/15 and 40/10) and using 1.5% (V/VT) of H2SO4. (b) Conversion of soybean oil to biodiesel with ethanol to oil molar ratio, employing 3% (V/ VT) and 3% (V/Voil) of H2SO4, irradiation time 40 min and cycle 15/15. Mechanical stirring at 280 rpm. Emitted power/mass ratio (ep/ms) in Watt per gram.

sample mass increased continuously as the molar ratio increased. This reduction in the ep/ms ratio clearly contributes to reducing the conversion in the experiments using 3% of H2SO4 (V/VOil) together with a reduction in the acid relative to the sample volume. Figure 6b shows that, despite the reduction in the ep/ms ratio in the experiments maintaining the acid concentration constant, the conversion decreases slightly, indicating that on increasing the molar ratio the transesterification under microwave heating increases markedly. Clearly, a better quantitative analysis of the influence of the alcohol/oil molar ratio can be achieved with a microwave system that allows a variation in the power emitted by the magnetron valve (this research is currently in progress). The biodiesel conversion increased with irradiation time and catalyst concentration, as can be observed in Figures 5 and 6, but for longer irradiation times the conversion decreased, which is an indication of an optimum irradiation time. After this point the degradation of both biodiesel and glycerol begins. This can be attributed to the chemical transformation of glycerol in the presence of sulfuric acid submitted to a high electromagnetic energy density inside the reactor, leading to acrolein, aliphatic ethers, and other byproducts.42 Irradiation time as an important and limiting factor in microwave-assisted biodiesel production has also been noted in relation to alkaline-catalyzed transesterification. In the work of Refaat et al.19 on the alkaline transesterification of refined and waste vegetable oils with a methanol to oil molar ratio of 6:1 and 1% of KOH, a conversion of 99.63% was obtained with an irradiation time of 120 s and for reaction times of over 2 min a considerable decrease in the biodiesel conversion was observed. The reaction conditions with microwave heating can be compared with earlier findings for the conventional acidcatalyzed alcoholysis of some vegetable oils. In previous studies by Freedman et al.35 on the acid-catalyzed (using 1% of sulfuric acid) methanolysis, ethanolysis, and butanolysis of vegetable oils, an optimum molar ratio of 30:1 was observed. Later, in this regard, Canakci and van Gerpen36 noted the need for a large excess of methanol (15:1−35:1) when sulfuric acid was used as

avoiding cycles with longer times without emission, and Figure 6a shows the ethanolysis of maize oil with a cycle of 40/10 and an alcohol to oil molar ratio of 6:1 and of soybean oil with a cycle of 15/15, molar ratio of 12:1 and H2SO4 concentration of 1.5% (V/VT). In both sets of experiments the electromagnetic power emitted by the source was 700 W, and as the molar ratio was increased from 6:1 to 12:1, the emitted power to mass sample (ep/ms) ratio decreased by 20%, explaining the differences in the curves in Figure 6a, particularly for short irradiation times. As observed for longer irradiation times, despite the considerable reduction in ep/ms, the reaction carried out with a molar ratio of 12:1 achieved a higher conversion than that with a molar ratio of 6:1, demonstrating that, as expected, increasing the molar ratio enhances the transesterification reaction as in conventional heating. As mentioned above, the reaction time is 12 h via conventional heating using 3% (V/VT) of H2SO4 with a molar ratio of 6:1, whereas, as shown in Figure 6a, applying microwave heating with 1.5% (V/VT) of H2SO4 and the same molar ratio the reaction time is reduced to 40 min. The effect of the alcohol to oil molar ratio on the transesterification of soybean oil can be observed in Figure 6b employing 3% H2SO4 (V/VT) and 3% H2SO4 (V/VOil). All experiments were performed with an irradiation time of 40 min and cycle of 15/15. It was observed that for molar ratios between 6:1 and 9:1 higher conversions were achieved for these two acid concentrations, but the conversion to biodiesel for molar ratios of 12:1 and higher and with 3% (V/VT) H2SO4 decreases, whereas with 3% H2SO4 (V/VOil) a steady decrease with low conversion at a molar ratio of 60:1 was observed. This is explained by the fact that in this case the acid concentration relative to the total volume of the sample decreased continuously whereas in the other experiments (3% of H2SO4 V/VT) the acid concentration remained constant. It should be noted that in both sets of experiments the emitted power/mass sample (ep/ms) ratio did not remain constant but instead it decreased, because the power emitted by the microwave system had an average value of 700 W while the 8978



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the catalyst. In a study reported by Zheng9 on the kinetics of the conventional acid-catalyzed transesterification of waste frying oil in excess methanol for biodiesel production, the optimum reaction conditions (conversion >99%) were found to be an oil−methanol−acid molar ratio of 1:245:3.8, a stirring rate of 400 rpm, pressure in the range of 266−293 kPa, temperatures of 70 and 80 °C, and a reaction time of 4 h. Therefore, it is clear that microwave-assisted acid-catalyzed transesterification is a very promising approach to converting refined oil and ROFs into biodiesel because, in contrast with conventional heating, there is no need for a large excess of methanol/ethanol to convert the free fatty acids in waste oil to methyl/ethyl esters in the first few minutes under the conditions reported herein. Furthermore, considering the difficulties inherent to the alkaline-catalyzed process, such as soap formation and the fact that the performance of the acid catalyst is not strongly affected by the presence of FFAs in the feedstock, the above results on biodiesel conversion show that microwave acid-catalyzed transesterification is an easier and faster method to obtain biodiesel than conventional heating or the microwave alkaline-catalyzed transesterification of refined and waste vegetable oils using a scientific or adapted domestic microwave oven and employing NaOH and KOH as catalysts.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b04890. 1 H NMR spectra and related discussion for the respective conversions. GC/MS analysis. Measured dielectric parameters for acids, pure liquids, and mixtures (PDF)

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REFERENCES

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4. CONCLUSIONS The measurement of the dielectric properties showed that H2SO4, H3PO4, ClSO3H, and H3CSO3H liquid acids have a higher loss tangent compared to those of the biodiesel precursors and, as a consequence, the loss factors of reaction mixtures are strongly affected by the catalyst and alcohol concentrations. Phosphoric and sulfuric acids presented higher loss tangents across the entire frequency range, reflecting a greater mobility of their ions and counterions. Under microwave heating, high conversions were obtained in the transesterifications employing H2SO4, ClSO3H, H3CSO3H, and HCl as catalysts. H3PO4, however, in spite of its higher loss tangent, afforded lower conversion in both ethylic and methylic transesterification reactions.

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AUTHOR INFORMATION

Corresponding Author

*E. L. Dall’Oglio. E-mail: [email protected]. Tel./ Fax: (55) 65 3615 8798/8799. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank CNPq (National Council for Scientific and Technological Development − Brazil), FINEP (Funding Agency for Studies and Projects-Brazil), CPP (Pantanal Research Centre) and CAPES (Coordination for the Improvement of Higher Education - Brazil) for financial support. 8979


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