Ionic liquid and microwave irradiation synergism for efficient biodiesel synthesis from waste cooking oil

Abstract Ionic liquid (IL) and microwave irradiation synergism was successfully employed in the catalytic conversion of waste cooking oil (WCO) into biodiesel: 1-butyl-3-methylimidazolium hydrogen sulfate ([Bmim]HSO4) was used as an IL catalyst, and tested parameters influencing the biodiesel conversion were reaction time and reaction temperature, catalyst amount, and the methanol:oil molar ratio. A biodiesel conversion of 93.4% was achieved in a reaction time of 4 h, at a reaction temperature of 150°C, using a methanol:oil molar ratio of 28:1, and 10 wt% of [Bmim]HSO4. A comparison was made between conventional and microwave methods. While high conversion was achieved after 4 h with the microwave method, high conversion was achieved after 6 h with the conventional method. A kinetic study was also carried out for biodiesel conversion, and the activation energy and pre-exponential factor were found to be 73.30 kJ/mol and 1.36 × 107min−1, respectively. Finally, the properties of the produced biodiesel were assessed, and it was found to be compatible with fuel specifications based on the American Society for Testing and Materials (ASTM) D6751 standard.


Introduction
Nowadays, energy consumption is constantly increasing due to population growth, rapid industrialization, and energy-intense economic activities. Fossil sources primarily supply this increasing energy necessity. But the continuous consumption of fossil sources places great pressure on the environment, fossil energy reserves, and the energy industry, so it is of great importance to develop alternative, green fuel sources [1][2][3]. An alternative fuel should be readily available and environmentally friendly [4]. Biodiesel is an important renewable, environmentally friendly, clean, and promising green fuel that could be an alternative to fossil fuels [5]. But biodiesel cannot compete with petroleum diesel because of its high raw material and production costs. The use of edible oils as a biodiesel raw material is a critical factor that increases production costs. Instead of using these oils, it may reduce biodiesel costs to use cheaper waste cooking oils (WCOs) [6,7]. Edible oils are used by people worldwide for cooking in restaurants or homes, and discarding these oils into the environment causes environmental problems. In the case of further use, WCOs have the potential to damage human health due to changes in their structure during cooking [8,9]. One of the ways to dispose of WCOs is to convert them to biodiesel. Thus, the use of WCOs not only decreases the cost of biodiesel production but also diminishes the damage caused by WCOs to the environment.
During the use of edible oils, the increased hydrolysis of triglycerides with the effect of temperature and water causes an increase in the free fatty acid (FFA) content in the oil [10,11]. Therefore, oils used more than once have high FFA content. WCOs with high FFA content cannot be efficiently converted to biodiesel by transesterification using base catalysis due to the associated soap formation. Therefore, strong homogeneous catalysts such as sulfuric and hydrochloric acid are needed to carry out the catalytic conversion of WCOs with high FFA into biodiesel [2]. However, these homogeneous catalysts have various disadvantages, such as being difficult to recover from the reaction medium, causing corrosion in the equipment used, creating wastewater, and producing acidic pollution [12][13][14]. To overcome these hurdles, heterogeneous catalysts, which have the advantages of reduced corrosion, easy recovery and regeneration, can be used for biodiesel synthesis [15]. Thus, it may be possible to reduce the current high biodiesel manufacturing budget [16]. However, heterogeneous catalysts also have some disadvantages such as low catalytic activity and deactivation [15]. For these reasons, efforts have been made to find or produce environmentally friendly, efficient, and new catalysts to be used to synthesize biodiesel. Recently, biodiesel synthesis has been performed using various ionic liquids (ILs) as alternative green catalysts, and good results have been obtained [15,[17][18][19][20][21]. Compared to conventional homogeneous catalysts, ILs cause less corrosion, generate less wastewater, and are easy to recover and reusable. Thus, it may be possible to reduce the cost of biodiesel and prepare biodiesel with a more environmentally friendly method [19,21].
In general, the conventional heating method has been used as the heating method in commercial biodiesel production. But conventional heating has some disadvantages such as the inability to provide homogeneous heating, long reaction time, formation of by-products, and low yield [22,23]. Therefore, in recent years, various alternatives such as supercritical, microwave, and ultrasonic methods have been developed and used for biodiesel production [24]. By using these methods, it is possible to reduce the reaction time and energy requirements, minimize the consumption of catalysts and reduce the amount of alcohol, which could effectively reduce biodiesel production costs. In the last year, with the use of the microwave method in chemical reactions, the interest in its use in biodiesel synthesis has also increased. Unlike the conventional method, the microwave method provides a shorter reaction time, gives highpurity products by consuming less energy, and provides chemical reactions that are safer, more comfortable, and greener [19,22,25].
Microwave irradiation is absorbed by polar and ionic components, resulting in rapid heating. ILs have excellent microwave absorbance due to their special polarization structure containing anion-cation pairs [26,27]. Since ILs have high microwave absorption, IL and microwave irradiation may have a positive synergy that will enable biodiesel synthesis to occur in a shorter time, unlike the conventional method.
The present study aimed to produce biodiesel from WCO using alternative green methods. For this purpose, biodiesel production from WCO was carried out via a transesterification reaction using the microwave method and an IL catalyst. Parameters affecting biodiesel conversion, such as catalyst amount, reaction time, temperature, and methanol:oil molar ratio, were investigated and are discussed. In addition, reaction kinetics, catalyst reusability, characterization of biodiesel, and a comparison of the conventional and microwave methods were carried out. Finally, some essential properties of biodiesel produced are specified and evaluated against the American (D6751) standard.

Materials
WCO was supplied from the central cafeteria of Mersin University. After the suspended particles and food residues were removed by filtration with a filter paper, the WCO was dried in an air oven at 110 C for 2 h. The properties and fatty acid composition of the WCO used are listed in Table 1. ILs ([Bmim]HSO 4 ) were purchased from Sigma-Aldrich Co. Other chemicals were purchased from Merck Co. All the used chemicals were of analytical grade and used without further purification.

Microwave-assisted transesterification reaction
The transesterification reactions were performed in highpressure polytetrafluoroethylene reactors using a microwaveaccelerated reaction system (Milestone, Italy). The reaction temperature was monitored using a temperature sensor in a control vessel during the reaction. A certain amount of methanol, WCO, and the IL catalyst was put into the reactor, which was then capped. The reaction was carried out under the following conditions: reaction time of 0.5 to 6 h, reaction temperature of 60-200 C, 5:1 to 31:1 molar ratio of methanol to WCO, and catalyst amount of 1-25 w/w% of oil.

Conventional transesterification reaction
Conventional transesterification reactions were carried out in a 100 mL closed glass reactor using an oil bath as a heating system. Reactions were performed under the following conditions: methanol:oil molar ratio 28:1, 150 C, 10 wt% catalyst, and three different reaction times (2,4 and 6 h) at a mixing speed of 500 rpm.

Separation and purification
After the reactions were complete, the reaction mixture was transferred to a separatory funnel and left overnight for phase separation. After phase separation occurred, the lower and upper phases were separated by decantation. The upper phase contains biodiesel with unused methanol, while the lower phase contains glycerol, water, and IL. The separated upper biodiesel phase was washed three times with warm deionized water to remove contaminants such as residual IL catalyst and glycerol. After washing, the product was dried at 80 C overnight, thus obtaining crude biodiesel. The product was then passed over silica to give the final product, biodiesel. In Figure 1, the crude biodiesel obtained by the transesterification reaction and the final product after purification are shown together.
Biodiesel conversion was calculated from the following equation using the 1 H NMR technique [28][29][30]. Each  experiment was conducted thrice, and the mean values were taken. Conversion where A ME and A CH2 are the integration values of the methoxy signal at 3.6 ppm and a-methylene signal at 2.3 ppm, respectively.

Biodiesel analysis
Thin-layer chromatography (TLC) was performed on Merck silica gel plates (60 F-254) using a mobile phase with the composition hexane:diethyl ether:acetic acid 9:1:0.1 (v/v/v). WCO and the obtained biodiesel were analyzed by 1 H NMR spectroscopy using a BRUKER 500 NMR instrument. Attenuated total reflection-Fourier transformed Infrared spectra (ATR-FT-IR) of biodiesel and WCO were performed using a Perkin-Elmer Spectrum 100. Each sample was scanned 16 times over a wave number range from 600 to 4000cm À1 and at a spectral resolution of 4cm À1 .

Recovery of [Bmim]HSO 4
[Bmim]HSO 4 was recycled to examine its reusability. After the lower and upper phases were separated, methanol was first removed from the reaction mixture by heating. Then, the remaining mixture containing [Bmim]HSO 4 and glycerol was washed with ethyl acetate and n-hexane three times and then dried in a vacuum. The mixture was cooled and then centrifuged for 30 min at 8000 rpm [13,14]. Thus, the [Bmim]HSO 4 was recovered for reuse in subsequent cycles.

Reaction parameters
Methanol:oil molar ratio Stoichiometrically, 3 moles of alcohol and 1 mole of triglyceride are required for biodiesel conversion of triglyceride [31,32]. Because of the reversible nature of the transesterification reaction, high methanol concentration shifts the equilibrium to the right, namely toward methyl ester formation [33]. Therefore, excess alcohol is used in the transesterification reaction relative to what is required stoichiometrically. On the other hand, the amount of methanol varies with the catalyst used. For instance, when basic catalysts are used, the methanol:oil molar ratio is generally low (e.g. 6:1), but when acidic catalysts are used, this ratio rises above 15:1 [34,35]. As shown in Figure 2, biodiesel conversion increases with the methanol:oil molar ratio. Particularly after the methanol:oil molar ratio reached 25, the biodiesel conversion was found to be above 90%. As the amount of methanol increases, the collision frequency of the molecules will increase, thus increasing the conversion. In addition, the increase in the amount of methanol causes an increase in the contact area between the reactants, increasing the reaction rate [13]. However, further increasing the methanol:oil molar ratio did not affect the biodiesel conversion. Hence, the molar ratio of methanol:oil was held at 28:1 to investigate the effect of other parameters.

Reaction time
Reactions were conducted for different durations to determine the effect of time on the biodiesel conversion, and the results are depicted in Figure 3. As the figure shows, the biodiesel conversion is initially low and increases with increasing reaction time. It was observed that conversion was less than 90% after 0.5, 1, 2 and 3 h, but after 4 h, the conversion increased to over 90% and remained almost constant at 4, 5 and 6 h. Low conversion during short reaction times may be caused by incomplete transesterification reactions between methanol and triglyceride [36]. After 4 h, the conversion reached equilibrium and remained constant.

Reaction temperature
Temperature can affect biodiesel conversion differently depending on the catalyst used. In general, biodiesel synthesis is performed at temperatures as low as 65 C when homogeneous basic catalysts are used, whereas the temperature is generally high when acidic catalysts are used [37]. Here, transesterification reactions were carried out in the range of 80-200 C to investigate the effect of temperature on biodiesel conversion, and the results are depicted in Figure 4. While the biodiesel conversions obtained in experiments at 80, 100 and 120 C were low, significant increases (above 90%) were observed in the conversion after 140 C. The highest biodiesel conversion (93.5%) was achieved at 150 C. Therefore, the optimum reaction temperature was determined to be 150 C. The increase in reaction temperature facilitates both molecular collision and mixing of the reactants.
Increasing the reaction temperature also causes the viscosities of both the WCO and the IL catalyst to decrease [38]. For these reasons, IL mixes well with all reactants used, resulting in higher biodiesel conversion. Additionally, the transesterification reaction was done in a closed vessel, and thus mass loss due to the evaporation of methanol did not occur. Since the reaction was carried out in a confined space, liquid-phase oil could react with gas-phase methanol. At high temperatures, increased collision frequency between these two reagents significantly increases the reaction kinetics of transesterification [39,40].
Moreover, further temperature increases did not cause a further increase in biodiesel conversion. For example, at 180 C, the conversion fell to 92.2%, while at 200 C, it decreased to 81.8%. At high temperatures, it is also possible that some of the oil will deteriorate under the influence of temperature, which may be a factor that causes the conversion to decrease [41].

Catalyst amount
A catalyst is required for the transesterification reaction, and the amount of catalyst affects the biodiesel conversion. In this study, [Bmim]HSO 4 in amounts ranging from 1 to 25 wt% were used to investigate the effect of IL catalyst amount. When the amount of IL was increased to 10%, the biodiesel conversion increased significantly; it remained almost constant between 10 and 25%, as shown in Figure 5. This result indicates that triglyceride does not turn into fatty acid methyl ester (FAME) when an insufficient amount of IL catalyst is used. Thus, the IL catalyst amount should be increased to 10% to complete the reaction. Initially, as the amount of IL was increased, the biodiesel conversion increased as more reactants interacted with the acid portions of the IL [42]. But after a certain point, increasing the amount of IL did not affect the biodiesel conversion.
[Bmim]HSO 4 is a high-viscosity IL, and thus increasing the amount of this IL will cause the viscosity of the reaction medium to increase. Mixing problems occur with increased viscosity, and as a result, mass transfer and reaction rate will be affected [2,13,43]. Therefore, using large amounts of catalyst did not increase efficiency. Since using large amounts of IL will also increase the biodiesel cost, the optimum IL catalyst amount is taken as 10%.

Comparison of conventional and microwave methods
By comparing the microwave method and conventional method, the effect of different heating methods on biodiesel production was investigated. The transesterification reaction was carried out using three different reaction times (2, 4 and 6 h) and the conventional method. Other conditions were as follows: temperature of 150 C, methanol:oil molar ratio 28:1, and catalyst amount of 10 wt%. As shown in Figure 6, the reaction reached equilibrium after 4 h with the microwave method, while with the conventional method, the reaction reached the highest efficiency after 6 h. The conversion amounts obtained by the microwave method at the end of the 2nd and 4th reaction times were about 17-19% higher than by the conventional method. Because microwave irradiation does not have enough energy to do so, it cannot break chemical bonds and therefore does not cause a change in chemical bonds at the molecular level [44,45]. Therefore, the transesterification reaction carried out using the microwave method proceeds through the same mechanism as conventional heating.
However, localized superheating caused by microwaves due to dipole moment and ionic conduction accelerates the reaction more than conventional heating [45,46]. Microwave irradiation acts on polar molecules and ions,  resulting in rapid heating. Two species can absorb microwave irradiation in the reaction medium during the transesterification reaction. These are methanol and IL. Because methanol has a high dipole moment, it is selectively heated under microwave irradiation, which could lead to the rapid formation of microzones with temperatures much higher than that of the reaction mixture [47][48][49]. Thus, methanol heats up fast, quickly reaches the boiling point, and provides energy for the transesterification reaction. Moreover, ILs can rapidly and effectively absorb microwave energy due to their polar and ionic nature [50]. Thus, the reaction medium heats up rapidly, accelerating the reaction. Microwave irradiation can transfer energy directly to the reactant such that the energy transfer is more effective, causing the reaction medium to heat up quickly [51,52]. Unlike microwave heating, in the conventional heating method, the reaction mixture is heated by the inward transfer of heat from the surface, resulting in a longer reaction time and higher energy consumption [49,53].

Recycling of [Bmim]HSO 4
Although homogeneous catalysts are effective in biodiesel production, difficulties in recycling and reuse are a disadvantage of these catalysts. When they are not recycled and reused, they cause both increased cost of biodiesel production and the formation of hazardous wastes. ILs are expensive compounds, and when they are used as catalysts, reusing them after the reaction provides a significant advantage. To examine the recyclability of the IL catalyst, four-cycle transesterification of WCO was performed. As   Figure 7 shows, a decrease in biodiesel conversion toward the 4th use was observed. When the IL was used for the first time, the conversion was 93.5%, while in the 4th use, this ratio decreased to 85.4%. In the fourth use of the IL, an 8.1% reduction in conversion was observed. This result showed that the activity of the catalyst after recycling is generally preserved and has reusability. It is difficult to achieve the same results for most conventional homogeneous acid catalysts, such as sulfuric acid. It has been demonstrated in the literature [17,54] that ILs used as catalysts can be recovered and reused. In addition, the FT-IR spectrum of the recovered [Bmim]HSO 4 was recorded and compared with that of fresh [Bmim]HSO 4 (see Supplementary material, Figure S1). The FT-IR spectrum of the recovered [Bmim]HSO 4 was almost identical to that of fresh [Bmim]HSO 4 . As a result, ILs can be recycled and reused, and thus, the cost of biodiesel production can be expected to decrease with their use.

H NMR analysis
The producced biodiesel and the WCO were characterized by 1 H NMR spectroscopy, and their spectra are depicted in Figure S2 (Supplementary material). The formation of a single new intense peak at 3.66 ppm in the 1 H NMR spectrum of biodiesel and the disappearance of the glycerol moiety signals at 4.1-4.3 ppm in the 1 H NMR spectrum of the WCO confirm the conversion of WCO into biodiesel [55][56][57]. On the other hand, the peaks at 0.88, 1.31, 1.61, and 2.3 ppm originated from terminal methyl, methylene, b-carbonyl  and a-methylene protons, respectively [55,58,59]. The peaks at 2.04, 2.3, 2.75, and 5.36 ppm are due to allylic, bis-allylic, and olefinic hydrogens, respectively [60].

FT-IR analysis
FT-IR spectroscopy is a sensitive method applied in determining functional groups. The FT-IR spectra of WCO and biodiesel are depicted in Figure S3 (Supplementary material). As the figure shows, the two spectra are almost the same. The small peak at 3008 cm À1 is assigned a cis olefinic-H double bond (¼C-H) [61]. The peaks at %2923, 2855, and 1375 cm À1 are assigned CH 3 , -CH 2 , and C-H functional groups and indicate the presence of alkane moieties [62]. The sharp and strongest peaks in the range 1750-1740 cm À1 originate from the carbonyl group (C ¼ O), indicating the presence of esters in both WCO (glycerol ester) and biodiesel (methyl ester) [61]. In the FT-IR spectrum of WCO, the peaks at 1238, 1157, and 1095 cm À1 , and in the FT-IR spectrum of biodiesel, the peaks at 1244, 1196, and 1170 cm À1 are due to the C-O stretching bands of triglyceride and ester molecules, respectively. The signal at approximately 721 cm À1 in the FT-IR spectrum of both WCO and biodiesel is due to -CH2-rocking of the fatty acid chains [13,58,62]. The FT-IR spectra of WCO and biodiesel are almost identical; nevertheless, a few changes due to the substitution of the glycerol by the methoxy group have been observed. The disappearance of some peaks in the FT-IR spectrum of the WCO, the shifting of some peaks, and the formation of some new peaks in the biodiesel spectrum indicate that the WCO was converted into biodiesel. The peaks at 1375 and 1157 cm À1 in the FT-IR spectrum of the WCO shifted to 1363 and 1170 cm À1 in the biodiesel, respectively. While the peak at 1095 cm À1 in the spectrum of WCO disappeared, the formation of new peaks in the spectrum of biodiesel at 1437 and 1196 cm À1 is a clear indicator of biodiesel formation. These new peaks observed in biodiesel originated from the -OCH 3 stretching that occurred due to the transesterification reaction [60,63].

TLC analysis
TLC is a simple and rapid method to monitor the transesterification reaction and control the purity of the biodiesel.
TLC analyses of WCO, synthesized biodiesel, and commercial biodiesel were performed, and the results from these three samples were compared (see Supplementary material, Figure S4). The WCO was separated into four components monoglyceride, diglyceride, FFA, and triglycerideat different positions (Rf ¼ 0.1, 0.18, 0.31, and 0.70). After the transesterification reaction, the spot caused by triglyceride disappeared and a new spot at a different position (Rf ¼ 0.87) appeared. This new spot at 0.87 Rf, also observed in commercial biodiesel, is due to FAMEs. The TLC analyses indicated close to complete conversion of WCO into its methyl esters.

Properties of the WCO biodiesel
Some physicochemical properties of the WCO biodiesel were analyzed to determine whether they meet international standards, and the results are given in Table 2, together with the American Society for Testing and Materials (ASTM) D6751 standard. As shown, the physicochemical properties of the WCO biodiesel conform with the specifications required by the ASTM D6751 standard. For example, the kinematic viscosity, a critical fuel property, was found to be 4.25 mm 2 /s, and this indicates that WCO biodiesel can be used as a suitable fuel for existing engines. Furthermore, the high flash point of the WCO biodiesel (found to be 163 C) indicats it is safe for transport, handling, and storage purposes without any risk of flammability [20,28,61].

Kinetic study
A kinetic study was conducted to determine the rate constant and activation energy for the transesterification The overall stoichiometric reaction can be written as: where TG is triglyceride, DG is diglyceride, MG is monoglyceride, and FAME is fatty acid methyl ester. Generally, monoglyceride and diglyceride formation steps in kinetic studies are neglected, thus simplifying the three steps into a single step [63,64]. Thus, the rate expression of the transesterification reaction shown in Equation (5) can be written as: where [TG] is the concentration of TG, [MeOH] is the concentration of methanol, and k 0 is the equilibrium rate constant. As shown in Equation (6), the transesterification reaction should follow a fourth-order reaction rate law since the rate depends on the concentrations of both methanol and TG. However, since excess methanol is used in the reaction, the methanol concentration is assumed to remain constant. Therefore, it can be assumed that the methanol concentration does not affect the reaction kinetics, and only TG will be included in the rate expression [62]. In this situation, the kinetics of the transesterification reaction obeys the pseudo first order, as shown by Equation (7) [65,66]: where k is the modified rate constant and k ¼ k 0 .
[MeOH] 3 . Therefore, the formula for the pseudo-first-order kinetics might be expressed by Equation 8 [67,68]: where X refers to biodiesel conversion at time t and k is the reaction rate constant. Corresponding plots of À ln(1-X) vs. reaction time (t) of 60-240 min at various temperatures are shown in Figure 8. Thus, from these graphs, the reaction rate constants were found to be 0.0024, 0.0047, and 0.0071 min À1 at 120, 130, and 140 C, respectively. The linearity of the plots and the high value of R 2 (above 0.9) support the interpretation that the transesterification reaction followed the pseudo-firstorder kinetic model [63]. Then, the Arrhenius equation given in Equation (9) was used to determine the activation energy: where E a , A, R, and T refer to the activation energy (J.mol À1 ), pre-exponential factor (min À1 ), universal gas constant (8.314 Â 10 À3 kJ.K À1 .mol À1 ), and reaction temperature (Kelvin), respectively.
To determine E a and A, ln k was plotted against 1/T as depicted in Figure 9. As the figure shows, there is a relationship of ln k against 1/T with a high regression coefficient (R 2 ¼ 0.985). Using this graph, the E a and A were determined to be 73.30 kJ/mol and 1.38 Â 10 7 min À1 , respectively.
Many studies examining the kinetics of the transesterification reaction catalyzed by IL catalysts have been reported in the literature. Some of the activation energies calculated in these studies were 19.24 kJ/mol [13], 56.12 kJ/ mol [42], 62.60 kJ/mol [68], 72.81 kJ/mol [69], 86.48 kJ/mol [70], 98.80 kJ/mol [71] and 122.93 kJ/mol [14]. As can be seen, the E a calculated in the current study is within the range of the reported values.

Conclusions
In this work, IL catalyst and microwave synergism were used to convert WCO to biodiesel. Experimental parameters such as reaction time, temperature, methanol:oil molar ratio, and catalyst amount affecting biodiesel conversion were studied. The suggested reaction conditions determined were 150 C reaction temperature, 28:1 methanol:oil molar ratio, 10% catalyst amount, and 4 h reaction time. A biodiesel conversion of 93.4% was obtained under these conditions. A comparison of conventional and microwave methods was also made, and it was confirmed that the microwave method was more effective and reduced the transesterification reaction time. Moreover, this transesterification followed the pseudo-first-order reaction mechanism, and the activation energy was 73.30 kJ/mol. The main properties of the synthesized biodiesel were determined and found to comply with the ASTM D6751 standard.