Optimized Conversion of Waste Cooking Oil to Biodiesel Using Calcium Methoxide As Catalyst Under Homogenizer System Conditions

energies

Article Optimized Conversion of Waste Cooking Oil to Biodiesel Using Calcium Methoxide as Catalyst under Homogenizer System Conditions

Ming-Chien Hsiao 1,2, Shuhn-Shyurng Hou 2,3,*, Jui-Yang Kuo 1 and Pei-Hsuan Hsieh 4

1 Department of Environmental Engineering, Kun Shan University, Tainan 71070, Taiwan; [email protected] (M.-C.H.); [email protected] (J.-Y.K.) 2 Green Energy Technology Research Center, Kun Shan University, Tainan 71070, Taiwan 3 Department of Mechanical Engineering, Kun Shan University, Tainan 71070, Taiwan 4 Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan; [email protected] * Correspondence: [email protected]; Tel.: +886-6-205-0496

Received: 4 September 2018; Accepted: 29 September 2018; Published: 1 October 2018

Abstract: Although many types of heterogeneous catalysts have been applied to the transesterification reaction, some of them are unsuitable for industrial applications due to their high price and the extra preparation required to synthesize them. Calcium methoxide is a low cost, strong base with high catalytic activity and is thus commonly used in the biofuels synthesis process during the transesterification reaction. The objective of this study was to determine the optimized conversion in the transesterification reaction of waste cooking oil (WCO) for biodiesel production by using a homogenizer with a calcium methoxide catalyst. It was shown that the optimal reaction conditions ◦ are a methanol-to-oil molar ratio of 6:1, 4 wt % Ca(OCH3)2, a reaction temperature of 65 C, a rotation speed of 7000 rpm, and a reaction time of 90 min. The conversion rate under these conditions reached 90.2%. Ca(OCH3)2 thus has potential as a catalyst for industrial use. In addition, with a homogenizer system, the reaction time for synthesizing calcium methoxide catalyst can be reduced by half compared to that for conventional water-bath heating. In addition, the large amount of waste water required in the oil-water separation step can be reduced by using calcium methoxide instead of a homogeneous catalyst, significantly reducing manufacturing costs.

Keywords: biodiesel; transesteriﬁcation; waste cooking oil; homogenizer; calcium methoxide

1. Introduction Biodiesel, also called fatty acid methyl esters (FAME), can be derived from a variety of vegetable oils, animal fats, and used cooking oil [1–3]. Converting waste cooking oil (WCO) into biodiesel has become increasingly popular due to its economic and environmental benefits. Research has shown that biodiesel can be produced using base-catalyzed transesterification [4–6]. Biodiesel is regarded as an alternative energy source for diesel generators and other machines [7–9]. In Taiwan, the scale of the first demonstration factory for handling WCO has reached 3000 m3. WCO have also been used in commercial programs since October 2004 [10]. Sodium oxide and potassium oxide are commonly used as catalysts in the process of alkaline methanolysis for producing biodiesel [11]. Using a homogenous catalyst makes the mixture blend uniformly and shortens the reaction time. However, there are some problems in separating lipid catalysts and biodiesel [12]. Also, a large amount of water is required to balance the pH value of the waste liquid after the reaction, which greatly increases manufacturing costs.

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Heterogeneous catalyzed biodiesel production has become the preferred route because it is environmentally benign and needs no water washing, and product separation is relatively easy [13]. Various kinds of heterogeneous compound have been shown to be efficient catalysts. Examples include ETS-10 zeolites [14], modified zeolites [14,15], alkaline earth metal oxides [16,17], zinc oxide modified with alkali earth metals [18–20], Na/NaOH/γ-Al2O3 heterogeneous base catalyst [21], a mixture of alkali and alkaline metal, hydrotalcite [22–24], rare earth/lanthanide elements [25,26], ion exchange resin [27], and zirconia [28,29]. Bases derived from calcium have the most potential due to their low cost. Liu et al. [30] carried out an experiment on producing biodiesel under conventional heating for 2 h. Calcium methoxide was used as the catalyst in their study. A 98% biodiesel yield was obtained. Calcium methoxide has high catalytic activity and strong basicity. It is an alternative heterogeneous solid base catalyst. Moreover, it has a long catalyst lifetime, retaining its activity after 20 uses. However, the reaction requires a lot of time and energy under conventional heating. Their work examined several heterogeneous calcium compounds as a catalyst to produce methyl esters of WCO with the assistance of ultrasound. Large-scale homogenizers are used in industry. Using a homogenizer significantly shortens the reaction time [31,32]. However, few studies have investigated the transesterification reaction of WCO for biodiesel production using a high-speed homogenizer [33]. In the present study, a homogenizer is used to quickly synthesize calcium methoxide. Then, the synthesized calcium methoxide is used as a catalyst to convert WCO. The optimal parameters are determined for this transesterification reaction.

2. Experimental Section

2.1. Chemicals

Sodium methoxide (CH3OH; purity: 99.8%) was purchased from Nihon Shiyaku Reagent (Nihon Shiyaku Industries, Taipei City, Taiwan). Methyl laurate and hexane were purchased from Fluka (Uni-Onward Corp., New Taipei City, Taiwan). Calcium oxide (CaO) was obtained from Riedel-De Haen (Uni-Onward Corp.) WCO was collected from fast-food restaurants.

2.2. Catalyst Preparation

The Ca(OCH3)2 catalyst synthesized with a traditional water-bath heating system was prepared as follows: commercial CaO (5.0 g) was added into an Erlenmeyer flask with methanol (100 mL) under a reflux system at a temperature of 65 ◦C and a rotation speed of 700 rpm for 2 h. After stirring, the solution was placed in a centrifuge at 5000 rpm for 3 min to remove the methanol on the CaO surface. ◦ Ca(OCH3)2 was obtained after 1 h under a vacuum drying process at a temperature of 105 C. The Ca(OCH3)2 catalyst used in the experiment with a homogenizer system was processed as follows: commercial CaO (5.0 g) was added into an Erlenmeyer flask with methanol (100 mL) under a homogenizer system, and then turned into a closed reflux system at a temperature of 65 ◦C and a rotation speed of 700 rpm for 30 to 60 min. After the reaction, the solution placed in a centrifuge at 5000 rpm for 3 min to remove the methanol on the CaO surface. The Ca(OCH3)2 was obtained after a vacuum drying process at a temperature of 105 ◦C for 1 h.

2.3. Catalyst Characterization X-ray diffraction (XRD) was employed to observe the crystallography of the catalysts. XRD patterns of the samples were recorded on a D8 diffractometer (Bruker, Billerica, MA, USA) using Cu Kα radiation. The patterns were recorded at a 2θ scanning rate of 0.04◦/s in the 2θ range of 0◦ to 60◦. A PKI Spectrum GX Fourier transform infrared (FTIR) spectrophotometer (Wellesley, MA, USA) was used to identify the groups of CaO. The spectra of the samples were recorded in the range of 400–4000 cm−1 with a resolution of 4 cm−1. The standard KBr technique was used for sample preparation. Energies 2018, 11, 2622 3 of 12

Energies 2018, 11, x FOR PEER REVIEW 3 of 11 2.4. Transesteriﬁcation Reaction Procedure 2.4. Transesterification Reaction Procedure The experimental setup is shown in Figure1. A homogenizer synthesis reactor (Hsiangtai H-M-300,The Hsiangtai experimental Co., setup Ltd., is New shown Taipei in Figure City, 1. A Taiwan) homogenizer equipped synthesis with reactor a thermostatic (Hsiangtai H-M- tank and a condenser300, Hsiangtai was used Co., forLtd., the New homogenizer Taipei City, reactions. Taiwan) equipped The process withwas a thermostatic carried out tank under and various a operationcondenser conditions. was used The for operatingthe homogenizer parameters reactions. include The reactionprocess was temperature, carried out amount under various of catalyst, operation conditions. The operating parameters include reaction temperature, amount of catalyst, reaction time, methanol-to-oil molar ratio, and rotation speed. A conventional heating system reaction time, methanol-to-oil molar ratio, and rotation speed. A conventional heating system (HTS- (HTS-1003, Laboratory & Medical Supplies Co., Ltd., Tokyo, Japan) equipped with a mechanical stirrer 1003, Laboratory & Medical Supplies Co., Ltd., Tokyo, Japan) equipped with a mechanical stirrer and and aa condensercondenser (LC-10, Hi-point Hi-point Co., Co., Ltd., Ltd., Kaohsiun Kaohsiungg City, City, Taiwan) Taiwan) was was used used for the for theconventional conventional heatingheating reactions. reactions.

FigureFigure 1. 1.The The experimental setup. setup.

The acidThe acid value value (AV (AV) and) and saponification saponification valuevalue ( SV)) were were determined determined using using a standard a standard American American Oil ChemistsOil Chemists0 Society′ Society (AOCS) (AOCS) titrimetry titrimetry method.method. AVAV waswas measured measured as asfollows: follows: the oil the (5.0 oil g) (5.0 was g) was addedadded into into an alcohol/etheran alcohol/ether solution solution (1:1;(1:1; v/v, 15 1500 mL) mL) in in a a250 250 mL mL Erlenmeyer Erlenmeyer flask; flask; then, then, a few a few drops of 1% phenolphthalein indicator was added into the solution. Finally, titration was conducted drops of 1% phenolphthalein indicator was added into the solution. Finally, titration was conducted with 0.1 N potassium hydroxide solution. SV was measured as follows: the oil (2.0 g) was added into with 0.1 N potassium hydroxide solution. SV was measured as follows: the oil (2.0 g) was added into a potassium hydroxide/alcohol solution (1:1; v/v, 25 mL) in a 250 mL Erlenmeyer flask. The solution a potassiumwas refluxed hydroxide/alcohol with heat for 1 h. solution Then, a few (1:1; drops v/v, 25of 1% mL) phenolphthalein in a 250 mL Erlenmeyer indicator were flask. added The into solution was refluxedthe solution. with Finally, heat for titration 1 h. Then, was acond fewucted drops with of 1% 0.5 phenolphthalein N hydrochloric acid. indicator The AV were of WCO added was into the solution.calculated Finally, as titration[33]: was conducted with 0.5 N hydrochloric acid. The AV of WCO was calculated as [33]: 5.61 × V 5.61= × V NaOH AV AV= NaOH ,, (1) (1) WW wherewhereVNaOH VNaOHis the is volume the volume of the of the sodium sodium hydroxide hydroxide titrant titrant (mL)(mL) and and WW isis the the oil oil weight weight (g). (g). The SVTheof SV WCO of WCO was was calculated calculated as as [ 33[33]:]: −× × (B= −()56.10.5BSS) × 56.1 × 0.5 SV =SV , ,(2) (2) WW where S is sample titration amount (mL); B is the blank titration amount (mL), and W is the oil weight (g). Energies 2018, 11, 2622 4 of 12

The molecular weight (MW) of WCO was calculated from its SV and AV [33,34] as:

3 MW = 56.1 × 1000 × , (3) (SV − AV) where SV and AV are in units of mg KOH/g. The AV, SV, and MW of the WCO used here are 0.062 mg KOH/g, 223.558 mg KOH/g, and 753.03, respectively. Therefore, the WCO moles can be obtained by dividing the mass by the molecular weight (MW). The WCO was kept at a constant mass (mole) when running the experiments with different methanol to oil ratios. A 100 g sample of WCO, as a reactant, was put into a 500 mL reactant tank. A homogenizer was used to promote the uniformity of the reaction in the system. The experiment was carried out at temperatures ranging from 50 to 70 ◦C, a catalyst content of 1 to 5 wt % (based on the weight of WCO), reaction times of 30 to 90 min, various methanol-to-oil molar ratios (4:1, 6:1, 8:1, and 10:1), and various rotation speeds (1000, 3000, 5000, 7000, and 9000 rpm). After the reaction, the upper layer was biodiesel and the lower layer was glycerol. The material (biodiesel) was dried in an oven at 105 ◦C for 6 h.

2.5. Analytical Methods The analytical method used to determine the content of FAME followed the Taiwan CNS-15051 standard. Similar studies using this method were found [33,35–38]. The biodiesel sample was analyzed with a Clarus 600 GC (PerkinElmer, Shelton, CT, USA) equipped with a capillary column (SPBTM-WAX, 30 m × 0.75 m × 1.0 µm) and a flame ionization detector (FID). The FAME standards for GC calibration were methyl myristate (C14:0), methyl palmitate (C16:0), methyl palmitoleate (C16:1), methyl heptadecanoate (C17:0 used as the internal standard [37]), methyl stearate (C18:0), methyl oleate (C18:1), methyl linoleate (C18:2), methyl linolenate (C18:3), methyl arachidate (C20:0), methyl eicosapentaenoate (C20:5), and methyl behenate (C22:0). Their retention times were used to identify and confirm the chromatogram FAME peaks obtained from the samples. In this experiment, 50 mg of the sample was mixed evenly with 1 mL of the internal standard, and a small portion of the sample (1 µL) was injected under the following conditions: the injector temperature was 280 ◦C with a split ratio of 1:20; nitrogen was used as the carrier gas with a flow rate of 45 mL/min; the air flow rate was 450 mL/min, and the temperature of the detector was 300 ◦C. The oven temperature was initially set at 210 ◦C, held for 4 min, and then increased to 240 ◦C at a rate of 4 ◦C/min, and held there for 8 min. Finally, the peak areas after gas chromatography analysis were compared, and the conversion of WCO to biodiesel was calculated following the standard method EN 14103, which is defined as follows:

(∑ A) − A A × V C = EI × EI EI × 100%, (4) AEI m where ΣA is the total peak area from FAME C14:0 to C24:1; AEI is the peak area of the internal standard (methyl heptadecanoate); CEI is the concentration of the internal standard (mg/mL); VEI is the volume of the internal standard (mL), and m is the mass of the sample (mg) to be analyzed. The data on conversion rate were obtained by averaging three individual measurements, and the standard deviations were shown with error bars.

3. Results and Discussion

3.1. Characterization of the Catalyst

The XRD patterns of Ca(OCH3)2 synthesized for 2 h using the conventional water-bath heating system and Ca(OCH3)2 synthesized for 30 or 60 min with a homogenizer are shown in Figure2. The obvious peaks shown in Figure2a at 32.1 ◦ and 37.5◦ (2θ) are consistent with CaO (JCPDS 37-1497). The commercial CaO used in this study was very pure and therefore did not contain Energies 2018, 11, 2622 5 of 12 Energies 2018, 11, x FOR PEER REVIEW 5 of 11

CaCOThe commercial3. Accordingly, CaO inused the in XRD this patterns study was of the very commercial pure and CaO therefore (Figure did2a), not a prominent contain CaCO peak3. (indicatedAccordingly, with in athe black XRD square) patterns in the of 37–38the commercial◦ (2θ) range CaO is not (Figure observed. 2a), a As prominent shown in peak Figure (indicated2b,c, the intensitywith a black of the square) reﬂections in the decreased, 37–38° (2θ indicating) range is thatnot theobserved. CaO reacted As shown completely in Figure with 2b,c, methanol the intensity in the synthesisof the reflections reaction. Baseddecreased, on Equation indicating (5), CaOthat wasthe consumedCaO reacted by methanol completely either with in partmethanol or completely in the insynthesis this reaction. reaction. The peakBased corresponding on Equation to (5), the CaOCaO phases was consumed disappeared by after methanol the reaction. either Additionally, in part or thecompletely CaO might in this have reaction. contacted The atmospheric peak corresponding air and reacted to the CaO with phases H2O and disappeared CO2 through after chemisorption. the reaction. Eventually,Additionally, very the small CaO might amounts have of contac Ca(OH)ted2 and atmospheric CaCO3, were air and produced reacted throughwith H2O hydroxylation and CO2 through and thechemisorption. carbonation Eventually, reaction, respectively very small [ 39amounts]. Hence, of inCa(OH) the other2 and two CaCO spectra3, were shown produced in Figure through2b,c, thehydroxylation intensity of and the the peak carbonation corresponding reaction, to therespecti CaCOvely3 phases [39]. Hence, is very in low.the other However, two spectra as shown shown in Figurein Figure2d, 2b,c, the reaction the intensity was notof the complete peak corresponding after 30 min in to the the homogenizer CaCO3 phases system. is very Thelow. reﬂections However, atas 32.1shown◦ and in 37.5Figure◦ (2θ 2d,) indicate the reaction that the was CaO not had comple not totallyte after reacted. 30 min Also, in athe slightly homogenizer more prominent system. peak The ◦ inreflections the 37–38 at 32.1°range and corresponding 37.5° (2θ) indicate to the CaCOthat the3 phases CaO had was not observed. totally reacted. For the Also, Ca(OCH a slightly3)2 catalyst more synthesizedprominent peak using in the the conventional 37–38° range method corresponding or the homogenizer to the CaCO system,3 phases the was most observed. remarkable For peak the ◦ forCa(OCH Ca(OCH3)2 catalyst3)2 was synthesized at 11 (2θ). using The peakthe conventional of Ca(OCH 3method)2 is similar or the to homogenizer that shown insystem, reference the most [40]. Aremarkable comparison peak of thefor mainCa(OCH peaks3)2 betweenwas at 11° Figure (2θ).2 b,cThe indicated peak of Ca(OCH that there3)2 was is similar only a slightto that difference shown in inreference the peak [40]. area. A Thiscomparison suggests of that the the main homogenizer peaks between system Figure can synthesize2b,c indicated Ca(OCH that there3)2 more was rapidly only a andslight effectively difference than in conventionalthe peak area. water-bath This sugge heating.sts that The the reaction homogenizer time for system the traditional can synthesize method ◦ wasCa(OCH double3)2 more that for rapidly the homogenizer and effectively method. than conventional In addition, the water-bath diffraction heating. peak at The 18 reaction(2θ) is a time feature for ◦ ofthe Ca(OH) traditional2 and method that at was 29.2 double(2θ) is that a feature for the of homogenizer CaCO3. Both method. of them In are addition, byproducts the diffraction in Equation peak (5). Thisat 18° might (2θ) beis a resultfeature of of oxidation Ca(OH) during2 and that synthesis at 29.2° [40 (2].θ) is a feature of CaCO3. Both of them are byproducts in Equation (5). This might be a result of oxidation during synthesis [40]. CaO + 2CH3OH → Ca(OCH3)2 + H2O (5) CaO + 2CH3OH → Ca(OCH3)2 + H2O (5)

The infraredinfrared (IR)(IR) spectra spectra of of the the Ca(OCH Ca(OCH3)23)synthesized2 synthesized for for 2 h 2 using h using the the conventional conventional system system and Ca(OCHand Ca(OCH3)2 synthesized3)2 synthesized for 30 for or 60 30 min or with60 min the homogenizerwith the homogenizer are shown inare Figure shown3. Thein Figure characteristic 3. The −1 absorptioncharacteristic between absorption 1050 between and 1085 1050 cm andindicates 1085 cm the−1 indicates presence the of presence the C–O of in the the C–O alcohol. in the The alcohol. other −1 characteristicThe other characteristic absorption absorption between 3700 between and 3580 3700 cm and 3580represents cm−1 represents the existence the ofexistence the strong of the and strong wide −1 –OHand wide mode. –OH This mode. peak correspondsThis peak corresponds to the –OH modeto the of–OH water mode [41]. of For water the absorption[41]. For the at absorption 1080 cm inat Figure1080 cm3a,b,-1 in the Figure bands 3a,b, are almostthe bands the same.are almost However, the same the peak. However, intensity the is weakerpeak intensity in Figure is3 weakerc. This is in attributedFigure 3c. toThis the is incomplete attributed reactionto the incomplete in Equation reac (5).tion Furthermore, in Equation Granados (5). Furthermore, et al. [17] pointedGranados out et the al. −1 −1 important[17] pointed feature out the between important 2800 feature and 3000 between cm 2800and 1480and 3000 cm cmis− observed1 and 1480 as cm –C–H,-1 is observed also one as of –C– the characteristicH, also one of peaks the characteristic of the CH3 in peaks methanol. of the CH3 in methanol.

Figure 2.2. XRD patterns of (a)(a) commercialcommercial CaO,CaO, (b)(b) Ca(OCHCa(OCH33))22 synthesized for 2 h using conventional water-bathwater-bath heating,heating, andand Ca(OCHCa(OCH33))22 synthesized using a homogenize homogenizerr for (c) 60 min and (d) 30 min.

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Figure 3. IR analysis of (a) Ca(OCH3)2 synthesized using conventional water-bath heating for 2 h and FigureFigure 3. 3.IR IR analysis analysis of of (a) (a) Ca(OCH Ca(OCH33))22 synthesized using using conventional conventional water-bath water-bath heating heating for for 2 h 2 and h and Ca(OCH3)2 synthesized using a homogenizer (b) 60 min and (c) 30 min. Ca(OCHCa(OCH3)23)synthesized2 synthesized using using aa homogenizerhomogenizer (b) 60 60 min min and and (c) (c) 30 30 min. min.

3.2.3.2.3.2. Effects EffectsEffects of ofof Rotation RotationRotation Speed SpeedSpeed on onon the thethe Conversion ConversionConversion RateRate of of Biodiesel Biodiesel TheTheThe previous previousprevious experimental experimentalexperimental resultsresultsresults showshow that that thethe reactionreaction reaction ofof of CaOCaO CaO andand and methanolmethanol methanol usingusing using thethe the homogenizerhomogenizerhomogenizer system systemsystem for forfor 6060 min min gavegave gave thethe the bestbest best conversionconversion conversion forfor for Ca(OCHCa(OCH Ca(OCH33))22 catalyst.catalyst.3)2 catalyst. ThisThis productproduct This product waswas wasusedused used asas asthethe the basebase base solidsolid solid catalystcatalyst catalyst inin subsequentsubsequent in subsequent experiments.experiments. experiments. TheTheThe effects effectseffects of of rotationof rotationrotation speed speedspeed on on theon thethe conversion conversionconversion rate rara oftete the ofof biodieselthethe biodieselbiodiesel are shown areare shownshown in Figure inin FigureFigure4. As 4.4. shown, AsAs theshown,shown, rotation thethe speed rotationrotation influenced speedspeed theinfluencedinfluenced reaction. thethe The reactionreaction experiment.. TheThe wasexperimentexperiment carried outwaswas with carriedcarried various outout with rotationwith variousvarious speeds (1000,rotationrotation 3000, speedsspeeds 5000, 7000,(1000,(1000, and 3000,3000, 9000 5000,5000, rpm), 7000,7000, a methanol-to-oil andand 90009000 rpm),rpm), ratio aa methanol-to-oilmethanol-to-oil of 6:1, a reaction ratioratio temperature ofof 6:1,6:1, aa reactionreaction of 65 ◦ C, temperaturetemperature ofof 6565 °C,°C, 44 wtwt %% Ca(OCHCa(OCH33))22 catalystcatalyst synthesizedsynthesized usingusing thethe homogenizerhomogenizer systemsystem forfor 6060 4 wt % Ca(OCH3)2 catalyst synthesized using the homogenizer system for 60 min, and a reaction time of min, and a reaction time of 60 min. The reactants did not react completely at low rotation speed (1000 60min, min. and The a reactants reaction didtime not of 60 react min. completely The reactants at low did rotation not react speed completely (1000 rpm); at low therefore, rotation thespeed conversion (1000 rpm); therefore, the conversion rate was relatively low, only 12.4%. As the rotation speed was raterpm); was relativelytherefore, low,the conversion only 12.4%. rate As thewas rotation relative speedly low, was only increased, 12.4%. As the the conversion rotation ratespeed increased was increased,increased, thethe conversionconversion raterate increasedincreased approximatelyapproximately linearlylinearly forfor 10001000 toto 70007000 rpm.rpm. ThisThis increasingincreasing approximately linearly for 1000 to 7000 rpm. This increasing trend is due to the increase in the number of trendtrend isis duedue toto thethe increaseincrease inin thethe numbernumber ofof colliscollisionsions betweenbetween particles.particles. IncreasingIncreasing ofof thethe rotationrotation collisions between particles. Increasing of the rotation speed enhanced the contact area between reactants speedspeed enhancedenhanced thethe contactcontact areaarea betweenbetween reactantsreactants andand mademade thethe WCO,WCO, methanol,methanol, andand catalystcatalyst mixmix and made the WCO, methanol, and catalyst mix evenly. When the rotation speed was increased to 7000 evenly.evenly. WhenWhen thethe rotationrotation speedspeed waswas increasedincreased toto 70007000 rpm,rpm, thethe conversionconversion raterate waswas 76.7%.76.7%. FurtherFurther rpm, the conversion rate was 76.7%. Further increasing the rotation speed increased the conversion rate. increasingincreasing thethe rotationrotation speedspeed increasedincreased thethe conversiconversionon rate.rate. However,However, consideringconsidering thethe problemproblem ofof thethe However,homogenizerhomogenizer considering overheating,overheating, the problem thethe optimaloptimal of the homogenizerrotationrotation speespeedd overheating, waswas 70007000 rpm.rpm. the optimal Therefore,Therefore, rotation inin thethe speed subsequentsubsequent was 7000 rpm.experiments,experiments, Therefore, wewe in theusedused subsequent aa fixedfixed rotationrotation experiments, speedspeed (7000(7000 we used rprpm)m) a toto fixed determinedetermine rotation thethe speed optimaloptimal (7000 conditionsconditions rpm) to determine forfor thethe thehighesthighest optimal conversionconversion conditions forbyby theadjustingadjusting highest the conversionthe methanol-to-oilmethanol-to-oil by adjusting molarmolar the methanol-to-oilratio,ratio, catalystcatalyst loading, molarloading, ratio, reactionreaction catalyst loading,temperature,temperature, reaction andand temperature, reactionreaction time.time. and reaction time.

100100

8080

6060

4040 Conversion rate (%) Conversion rate (%) 2020

00 10001000 3000 3000 5000 5000 7000 7000 9000 9000 RotationRotation speedspeed (rpm)(rpm)

FigureFigureFigure 4. 4.4.Effect EffectEffect of ofof rotation rotationrotation speed speedspeed on ononthe thethe transesteriﬁcation transesterifictransesterificationation reaction. reaction. ReactionReaction Reaction condition:condition: condition: methanol/oilmethanol/oil methanol/oil molar ratio 6:1, catalyst amount 4 wt %, reaction temperature 65 °C, reaction time 60 min, rotation molarmolar ratio ratio 6:1, 6:1, catalyst catalyst amount amount 44 wtwt %,%, reactionreaction temperaturetemperature 65 65 °C,◦C, reaction reaction time time 60 60 min, min, rotation rotation speed 1000–9000 rpm. speedspeed 1000–9000 1000–9000 rpm. rpm.

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3.3. Effects of Methanol-to-Oil Molar Ratio on thethe ConversionConversion RateRate ofof BiodieselBiodiesel The effects of the methanol-to-oil molar ratio on the conversion rate are shown in Figure 55.. Experiments werewere carriedcarried outout with with various various methanol-to-oil methanol-to-oil molar molar ratios ratios (4:1, (4:1, 6:1, 6:1, 8:1, 8:1, and and 10:1), 10:1), 4 wt4 wt % ◦ Ca(OCH% Ca(OCH3)2 3catalyst,)2 catalyst, a reactiona reaction temperature temperature of 65of 65C, °C, a reaction a reaction time time of of 60 60 min, min, and and a rotation a rotation speed speed of 7000of 7000 rpm rpm to to investigate investigate the the inﬂuence influence of of the the molar mola ratior ratio on on the the conversion conversion rate. rate. As As mentionedmentioned above,above, thethe WCOWCO waswas keptkept atat aa constantconstant molemole whenwhen runningrunning thethe experimentsexperiments withwith variousvarious methanol-to-oilmethanol-to-oil molar ratios.ratios. FigureFigure5 5 shows shows that that the the conversion conversion rate rate of of the the biodiesel biodiesel increased increased with with the the methanol-to-oil methanol-to- molaroil molar ratio ratio for for ratios ratios of 4:1of 4:1 to to 10:1. 10:1. When When the the methanol-to-oil methanol-to-oil molar molar ratio ratio was was 4:1,4:1, thethe reactionreaction diddid not reach aa balance,balance, causingcausing thethe conversionconversion raterate toto bebe low.low. TheThe conversionconversion raterate waswas 53.5%.53.5%. When the methanol-to-oil ratio was increased to 6:1,6:1, thethe conversionconversion raterate waswas 76.8%.76.8%. When thethe methanol-to-oilmethanol-to-oil ratioratio waswas further further increased increased to 8:1to and8:1 10:1,and the10:1, conversion the conversion rate decreased. rate decreased. These results These were results attributed were toattributed excess methanolto excess promotingmethanol promoting the reverse the reaction reverse and reaction diluting and thediluting concentration the concentration of the catalyst. of the Thatcatalyst. is, the That results is, the can results be explained can be explained by the fact by thatthe fa glycerolct that glycerol is soluble is soluble in alcohol. in alcohol. Hence, Hence, an excess an ofexcess methanol of methanol can increase can increase the concentration the concentration of glycerol of glycerol in the in reaction the reaction mixture, mixture, which which can shiftcan shift the equilibriumthe equilibrium to the to the reactant reactant side side [36 [36].]. Furthermore, Furthermore, glycerol glycerol and and biodiesel biodiesel are are miscible miscible duedue toto thethe use ofof excessexcess methanolmethanol [[33].33]. In this situation, the opportunity for the catalyst and oil particlesparticles to collide decreased. decreased. The The reaction reaction could could not not be carried be carried out outefficiently, efﬁciently, and andthus thusthe conversion the conversion was lower was lower[33,36,42]. [33,36 The,42 ].optimal The optimal methanol-to-oil methanol-to-oil ratio was ratio 6:1. was 6:1.

Conversion rate (%) rate Conversion 50

40 46810 Methanol-to-oil molar ratio Figure 5. Effect of methanol-to-oil molar ratio on the tr transesteriﬁcationansesterification reaction. Reaction conditions: catalystcatalyst amount:amount: 4 wt %; rotation speed: 7000 rpm; reaction temperature:temperature: 65 ◦°C;C; reaction time: 60 min; methanol-to-oil molarmolar ratio:ratio: 4:1–10:1.

3.4. Effects of Amount of Catalyst on the Conversion Rate ofof BiodieselBiodiesel The effectseffects of of the the amount amount of catalyst of catalyst on the on conversion the conversion rate are rate shown are in shown Figure 6in. The Figure experiment 6. The was carried out with various amounts of Ca(OCH ) (1, 2, 3, 4, and 5 wt %) synthesized using the experiment was carried out with various amounts 3of2 Ca(OCH3)2 (1, 2, 3, 4, and 5 wt %) synthesized homogenizerusing the homogenizer system for system 60 min, for a methanol-to-oil 60 min, a methanol-to-oil ratio of 6:1, aratio reaction of 6:1, time a reaction of 60 min, time a temperatureof 60 min, a ◦ oftemperature 65 C, and of arotation 65 °C, and speed a rotation of 7000 speed rpm. When of 7000 the rpm. amount When of the catalyst amount was of increased catalyst fromwas increased 1 wt % to 4from wt %, 1 thewt % opportunity to 4 wt %, of the collisions opportunity between of thecollisions catalyst between and reactant the catalyst particles and increased, reactant promoting particles theincreased, formation promoting of the biodiesel. the formation Gryglewicz of the biodiesel. [41] found Gryglewicz that alkaline-earth [41] found metal that alkoxides alkaline-earth are slightly metal solublealkoxides in alcoholsare slightly when soluble they arein alcohols more than when 0.04 they wt % are in themore liquid. than Therefore,0.04 wt % alcoholin the liquid. can be Therefore, catalyzed notalcohol only can by be free catalyzed alkoxylate not ionsonly butby free also alkoxylate by solid alkoxylate,ions but also which by solid can alkoxylate, be regarded which as abducts. can be Anregarded alkoxide as abducts. can be introduced An alkoxide directly can be intointroduced the reaction directly system. into the Alkoxides reaction system. have been Alkoxides found tohave be highlybeen found catalytically to be highly active catalytically in such cases. active Ca(OCH in such3)2 has cases. strong Ca(OCH basicity.3)2 has When strong the catalystbasicity. was When added the atcatalyst 5 wt %, was the added conversion at 5 ofwt the %, biodieselthe conversion decreased of bytheabout biodiesel 10%. decreased This can possibly by about be 10%. attributed This tocan a possibly be attributed to a mixing problem involving the reactants and the solid catalyst. The excess amount of catalyst led to an increase in the viscosity of the reaction mixture, resulting in the poor

Energies 2018, 11, 2622 8 of 12 mixing problem involving the reactants and the solid catalyst. The excess amount of catalyst led to Energies 2018, 11, x FOR PEER REVIEW 8 of 11 an increase in the viscosity of the reaction mixture, resulting in the poor diffusion of the reactants in thediffusion methanol–oil–catalyst of the reactants systems,in the methanol–oil–cat thus causing thealyst lower systems, conversion thus causing rate [21, 42the–44 lower]. Therefore, conversion the optimalrate [21,42–44]. catalyst Therefore, content was the 4 optimal wt %. catalyst content was 4 wt %.

50 Conversion rate (%) rate Conversion 40

30 12345 Amount of Ca(OCH ) catalyst (wt%) 3 2

Figure 6.6. Effect ofofcatalyst catalyst amount amount on on the the transesterification transesterification reaction. reaction. Reaction Reaction conditions: conditions: methanol-to-oil methanol- molarto-oil molar ratio: 6:1;ratio: rotation 6:1; rotation speed: speed: 7000 rpm; 7000 reaction rpm; re temperature:action temperature: 65 ◦C; reaction 65 °C; reaction time: 60 time: min; catalyst60 min; amount:catalyst amount: 1–5 wt %. 1–5 wt %.

3.5. Effects of Temperature onon thethe ConversionConversion RateRate ofof BiodieselBiodiesel The effects of of temperature temperature on on the the conversion conversion rate rate of ofbiodiesel biodiesel are areshown shown in Figure in Figure 7. The7. ◦ Theexperiment experiment was carried was carried out with out temperat with temperaturesures of 50 to of70 50°C, to4 wt 70 %C, Ca(OCH 4 wt %3)2 Ca(OCHcatalyst synthesized3)2 catalyst synthesizedusing the homogenizer using the homogenizersystem for 60 systemmin, a methanol-to-oil for 60 min, a methanol-to-oil ratio of 6:1, a reaction ratio of time 6:1, aof reaction60 min, ◦ timeand a of rotation 60 min, andspeed a rotation of 7000 speed rpm. ofThe 7000 conversion rpm. The conversionrates at 50 ratesand at55 50 °C and were 55 22.8%C were and 22.8% 29.1%, and ◦ 29.1%,respectively. respectively. When Whenthe temperature the temperature was wasincreased increased to 60 to 60°C, C,the the conversion conversion rate rate of of the the biodiesel ◦ increasedincreased toto 50.9%.50.9%. AtAt a temperaturetemperature ofof 65 °C,C, the conversion rate waswas 76.7%. Note that the conversion ◦ ◦ rate reachedreached aa maximummaximum atat 6565 °C C with with increasing increasing temperature temperature and and then then decreased decreased slightly slightly after after 65 65C. ° ◦ ThisC. This might might be be related related to to the the boiling boiling point point of of the the methanol methanol (64.7 (64.7 °C).C). When When thethe reactionreaction temperaturetemperature waswas aboveabove thethe critical point (boiling point), methanol vaporized easily in the reaction tank, inhibiting thethe reaction between between methanol, methanol, WCO, WCO, and and the the catalyst. catalyst. More More specifically, speciﬁcally, this this phenomenon phenomenon could could be beattributed attributed to a to large a large amount amount of ofmethanol methanol vaporized vaporized in the in the gas gas phase phase as aswell well as asa sharp a sharp decrease decrease in inmethanol methanol in inthe the liquid liquid phase phase at a at higher a higher temperatur temperaturee than than the the boiling boiling point point of methanol of methanol [44,45]. [44,45 In]. Inaddition, addition, too too high high of ofa reaction a reaction temperature temperature wi willll increase increase the the risk risk of of saponification saponiﬁcation [44]. [44]. Accordingly, Accordingly, ◦ thethe optimaloptimal reactionreaction temperaturetemperature waswas 6565 °C.C.

3.6. Effects of Reaction Time on the Conversion Rate of Biodiesel 80 The effects of the reaction time on the conversion rate of the biodiesel are shown in Figure8. The experiment was carried out with various reaction times (30, 45, 60, 75, and 90 min), a methanol-to-oil ◦ ratio of 6:1, a reaction temperature60 of 65 C, 4 wt % Ca(OCH3)2 catalyst, and a rotation speed of 7000 rpm. Figure8 shows that the reactants did not efficiently undergo transesterification in 30 min; the conversion rate was below 20%. When the reaction time was increased to 60 min, the conversion rate increased significantly to 76.7%. The reason for this increase is the increase in reaction time. The reactants could be 40 mixed much more evenly. Further increasing the reaction time from 60 to 90 min led to gradual increase in the conversion rate. When theConversion (%) rate reaction time was 90 min, the conversion rate reached 90.2%. The optimal reaction time was 90min. In summary, the optimal reaction conditions are a methanol-to-oil molar ratio of 20 ◦ 6:1, 4 wt % Ca(OCH3)2, a reaction temperature of 65 C, a rotation speed of 7000 rpm, and a reaction time of 90 min. 50 55 60 65 70 o Reaction temperature ( C) Figure 7. Effect of reaction temperature on the transesterification reaction. Reaction conditions: methanol-to-oil molar ratio: 6:1; catalyst amount: 4 wt %; rotation speed: 7000 rpm; reaction time: 60 min; reaction temperature: 50–70 °C.

Energies 2018, 11, x FOR PEER REVIEW 8 of 11 diffusion of the reactants in the methanol–oil–catalyst systems, thus causing the lower conversion rate [21,42–44]. Therefore, the optimal catalyst content was 4 wt %.

50 Conversion rate (%) rate Conversion 40

30 12345 Amount of Ca(OCH ) catalyst (wt%) 3 2 Figure 6. Effect of catalyst amount on the transesterification reaction. Reaction conditions: methanol- to-oil molar ratio: 6:1; rotation speed: 7000 rpm; reaction temperature: 65 °C; reaction time: 60 min; catalyst amount: 1–5 wt %.

3.5. Effects of Temperature on the Conversion Rate of Biodiesel The effects of temperature on the conversion rate of biodiesel are shown in Figure 7. The experiment was carried out with temperatures of 50 to 70 °C, 4 wt % Ca(OCH3)2 catalyst synthesized using the homogenizer system for 60 min, a methanol-to-oil ratio of 6:1, a reaction time of 60 min, and a rotation speed of 7000 rpm. The conversion rates at 50 and 55 °C were 22.8% and 29.1%, respectively. When the temperature was increased to 60 °C, the conversion rate of the biodiesel increased to 50.9%. At a temperature of 65 °C, the conversion rate was 76.7%. Note that the conversion rate reached a maximum at 65 ° C with increasing temperature and then decreased slightly after 65 ° C. This might be related to the boiling point of the methanol (64.7 °C). When the reaction temperature was above the critical point (boiling point), methanol vaporized easily in the reaction tank, inhibiting the reaction between methanol, WCO, and the catalyst. More specifically, this phenomenon could be attributed to a large amount of methanol vaporized in the gas phase as well as a sharp decrease in methanol in the liquid phase at a higher temperature than the boiling point of methanol [44,45]. In Energiesaddition,2018 too, 11 ,high 2622 of a reaction temperature will increase the risk of saponification [44]. Accordingly,9 of 12 the optimal reaction temperature was 65 °C.

Energies 2018, 11, x FOR PEER REVIEW 9 of 11

60 3.6. Effects of Reaction Time on the Conversion Rate of Biodiesel The effects of the reaction time on the conversion rate of the biodiesel are shown in Figure 8. The experiment was carried out 40with various reaction times (30, 45, 60, 75, and 90 min), a methanol-to-oil

3 2 ratio of 6:1, a reaction temperatureConversion (%) rate of 65 °C, 4 wt % Ca(OCH ) catalyst, and a rotation speed of 7000 rpm. Figure 8 shows that the reactants did not efficiently undergo transesterification in 30 min; the conversion rate was below 20%.20 When the reaction time was increased to 60 min, the conversion rate increased significantly to 76.7%. 50The reason 55 for this 60 increase 65 is the 70increase in reaction time. The reactants could be mixed much more evenly. Further increasing the reaction time from 60 to 90 min Reaction temperature (oC) led to gradual increase in the conversion rate. When the reaction time was 90 min, the conversion rate reachedFigure 90.2%. 7.7. Effect The optimalof reaction reacti temperatureon time was onon the the90min. transesteriﬁcationtran sesterificationIn summary, reaction.reaction. the optimal Reaction reaction conditions: conditions are amethanol-to-oil methanol-to-oil molar molar ratio: ratio: ratio 6:1; 6:1; of catalyst catalyst6:1, 4 wtamount: amount: % Ca(OCH 4 4wt wt %;3 %;) 2rotation, rotationa reaction speed: speed: temperature 7000 7000 rpm; rpm; reaction of reaction 65 °C, time: a time: rotation 60 speed60min; of min; 7000reaction reaction rpm, temperature: and temperature: a reaction 50–70 50–70 time °C.◦ C. of 90 min.

100

40 Conversion rate (%)rate Conversion 20

0 30 45 60 75 90 Reaction time (min) Figure 8.8. Effect ofof reactionreaction time time on on the the transesteriﬁcation transesterification reaction. reaction. Reaction Reaction conditions: conditions: methanol-to-oil methanol-to- molaroil molar ratio: ratio: 6:1; 6:1; catalyst catalyst amount: amount: 4 wt4 wt %; %; rotation rotation speed: speed: 7000 7000 rpm; rpm;reaction reaction temperature:temperature: 65 ◦°C;C; reaction time:time: 30–90 min.

4. Conclusions

This studystudy investigated investigated synthesizing synthesizing Ca(OCH Ca(OCH3)2 using3) a2 homogenizerusing a homogenizer system for a transesterification system for a reaction.transesterification Calcium methoxidereaction. Calcium was synthesized methoxide using was asynthesized homogenizer using system a homogenizer for 60 min. system Compared for 60 to conventionalmin. Compared water-bath to conventional heating, the water-bath reaction time heatin wasg, decreasedthe reaction by half.time Thewas chemical decreased composition by half. The of thechemical catalysts composition was examined of the using catalysts XRD and was IR examined spectroscopy. using The XRD WCO and underwent IR spectroscopy. a transesteriﬁcation The WCO reactionunderwent with a transesterification the catalyst synthesized reaction usingwith the a homogenizer catalyst synthesized system using to reduce a homogenizer the reaction system time. Theto reduce optimal the conditions reaction time. for this The experiment optimal conditions were a methanol-to-oil for this experiment molar ratiowere of a 6:1,methanol-to-oil 4 wt % Ca(OCH molar3)2 ◦ catalyst,ratio of 6:1, a reaction4 wt % Ca(OCH temperature3)2 catalyst, of 65 a reactionC, a rotation temperature speed of 65 7000 °C, rpm,a rotation and aspeed reaction of 7000 time rpm, of 90and min. a reaction Under time these of conditions,90 min. Under the conversionthese condit rateions, of the the conversion biodiesel rate reached of the 90.2%. biodiesel In addition, reached the90.2%. large In amountaddition, of the waste large water amount required of waste in the wate oil-waterr required separation in the stepoil-water could separation be reduced step by usingcould calciumbe reduced methoxide by using instead calcium of methoxide a homogeneous instead catalyst, of a homogeneous signiﬁcantly catalyst, lowering significantly manufacturing lowering costs. Homogenizersmanufacturing cancosts. increase Homogenizers scale and can effectively increase shortenscale and the effectively reaction time.shorten The the proposed reaction method time. The is highlyproposed competitive method is with highly conventional competitive homogenous with conventional catalysts homogenous and the heating-stirring catalysts and method. the heating- stirring method.

Funding: This research was funded by the Ministry of Science and Technology, Taiwan, under grant number MOST 107-2637-E-168-001.

Energies 2018, 11, 2622 10 of 12

Author Contributions: J.-Y.K. and P.-H.H. performed the experiments and analyzed the results. M.-C.H. contributed to the structure, organization, and presentation of the work. S.-S.H. contributed ideas, designed experiments, analyzed results, prepared the manuscript, and help with editing the manuscript. Funding: This research was funded by the Ministry of Science and Technology, Taiwan, under grant number MOST 107-2637-E-168-001. Acknowledgments: The authors would like to thank H.P. Wang of the Department of Environmental Engineering, National Cheng Kung University, Tainan, Taiwan, for his assistance in the experiments. Conﬂicts of Interest: The authors declare no conﬂicts of interest.

References

1. Demirbas, A. Biodiesel production from vegetable oils via catalytic and non-catalytic supercritical methanol transesterification methods. Prog. Energy Combust. Sci. 2005, 31, 466–487. [CrossRef] 2. Graboski, M.S.; McCormick, R.L. Combustion of fat and vegetable oil derived fuels in diesel engines. Prog. Energy Combust. Sci. 1998, 24, 125–164. [CrossRef] 3. Bozbas, K. Biodiesel as an alternative motor fuel: Production and policies in the European Union. Renew. Sustain. Energy Rev. 2008, 12, 542–552. [CrossRef] 4. Mittelbach, M.; Enzelsberger, H. Transesterification of heated rapeseed oil for extending diesel fuel. J. Am. Oil Chem. Soc. 1999, 76, 545–550. [CrossRef] 5. Alcantara, R.; Amores, J.; Canoira, L.; Fidalgo, E.; Franco, M.J.; Navarro, A. Catalytic production of biodiesel from soybean oil, used frying oil and tallow. Biomass Bioenergy 2000, 18, 515–527. [CrossRef] 6. Mittelbach, M.; Gangl, S. Long storage stability of biodiesel made from rapeseed and used frying oil. J. Am. Oil Chem. Soc. 2001, 78, 573–577. [CrossRef] 7. Murayama, T.; Fujiwara, Y.; Noto, T. Evaluating waste vegetable oils as a diesel fuel. Proc. Inst. Mech. Eng. Part D J. Automob. Eng. 2000, 214, 141–148. [CrossRef] 8. Dorado, M.P.; Ballesteros, E.; Arnal, J.M.; Gomez, J.; Lopez Gimenez, F.J. Testing waste olive oil methyl ester as a fuel in a diesel engine. Energy Fuels 2001, 17, 1560–1565. [CrossRef] 9. Tashtoush, G.; AlWidyan, M.I.; AlShyoukh, A.O. Combustion performance and emissions of ethyl ester of a waste vegetable oil in a water-cooled furnace. Appl. Therm. Eng. 2003, 23, 285–293. [CrossRef] 10. Hsien, H.T. Green energy-biodiesel demonstration plant. Energy Mag. 2004, 12, 22–24. (In Chinese) 11. Meher, L.C.; Sagar, D.V.; Naik, S.N. Technical aspects of biodiesel production by transesterification—A review. Renew. Sustain. Energy Rev. 2006, 10, 248–268. [CrossRef] 12. Ghadge, S.V.; Raheman, H. Biodiesel production from mahua (Madhuca indica) oil having high free fatty acids. Biomass Bioenergy 2005, 2, 601–605. [CrossRef] 13. Semwal, S.; Arora, A.K.; Badoni, R.P.; Tuli, D.K. Biodiesel production using heterogeneous catalysts. Bioresour. Technol. 2011, 102, 2151–2161. [CrossRef][PubMed] 14. Suppes, G.J.; Dasari, M.A.; Doskocil, E.J.; Mankidy, P.J.; Goff, M.J. Transesterification of soybean oil with zeolite and metal catalysts. Appl. Catal. A 2004, 257, 213–223. [CrossRef] 15. Jitputti, J.; Kitiyanan, B.; Rangsunvigit, P.; Bunyakiat, K.; Attanatho, L.; Jenvanitpanjakul, P. Transesterification of crude palm kernel oil and crude coconut oil by different solid catalysts. Chem. Eng. J. 2006, 116, 61–66. [CrossRef] 16. Wang, L.; Yang, J. Transesterification of soybean oil with nano-MgO or not in supercritical and subcritical methanol. Fuel 2007, 8, 328–333. [CrossRef] 17. Granados, M.L.; Poves, M.; Alonso, D.M.; Mariscal, R.; Galisteo, F.C.; Moreno-Tost, R. Biodiesel from sunflower oil by using activated calcium oxide. Appl. Catal. B 2007, 73, 317–326. [CrossRef] 18. Yang, Z.; Xie, W. Soybean oil transesterification over zinc oxide modified with alkali earth metals. Fuel Process. Technol. 2007, 88, 631–638. [CrossRef] 19. Xie, W.; Yang, Z. Ba-ZnO catalysts for soybean oil transesterification. Catal. Lett. 2007, 117, 159–165. [CrossRef] 20. Albuquerque, M.C.G.; Jiménez-Urbistondo, I.; Santamaría-González, J.; Mérida-Robles, J.M.; Moreno-Tost, R.; Rodríguez-Castellón, E.; Rodríguez-Castellón, E.; Jiménez-López, A.; Azevedo, D.C.S.; Cavalcante, C.L., Jr.; et al. CaO supported on mesoporous silicas as basic catalysts for transesterification reactions. Appl. Catal. A 2008, 334, 35–43. [CrossRef] Energies 2018, 11, 2622 11 of 12

21. Kim, H.J.; Kang, B.S.; Kim, M.J.; Park, Y.M.; Kim, D.K.; Lee, J.S. Transesteriﬁcation of vegetable oil to biodiesel using heterogeneous base catalyst. Catal. Today 2004, 93, 315–320. [CrossRef] 22. Cantrell, D.G.; Gillie, L.J.; Lee, A.F.; Wilson, K. Structure-reactivity correlations in MgAl hydrotalcite catalysts for biodiesel synthesis. Appl. Catal. A 2005, 287, 183–190. [CrossRef] 23. Gao, L.; Teng, G.; Xiao, G.; Wei, R. Biodiesel from palm oil via loading KF/Ca-Al hydrotalcite catalyst. Biomass Bioenergy 2010, 34, 1283–1288. [CrossRef] 24. Dias, A.P.S.; Bernardo, J.; Felizardo, P.; Correia, M.J.N. Biodiesel production over thermal activated cerium modiﬁed Mg-Al hydrotalcites. Energy 2012, 41, 344–353. [CrossRef]

25. Li, X.; Lu, G.; Guo, Y.; Wang, Y.; Zhang, Z.; Liu, X. A novel solid superbase of Eu2O3/Al2O3 and its catalytic performance for the transesteriﬁcation of soybean oil to biodiesel. Catal. Commun. 2007, 8, 1969–1972. [CrossRef]

26. Thitsartarn, W.; Kawi, S. An active and stable CaO-CeO2 catalyst for transesterification of oil to biodiesel. Green Chem. 2011, 13, 3423–3430. [CrossRef] 27. Shibasaki-Kitakawa, N.; Honda, H.; Kuribayashi, H.; Toda, T.; Fukumura, T.; Yonemoto, T. Biodiesel production using anionic ion-exchange resin as heterogeneous catalyst. Bioresour. Technol. 2007, 98, 416–421. [CrossRef][PubMed] 28. Sun, H.; Ding, Y.; Duan, J.; Zhang, Q.; Wang, Z.; Lou, H. Transesterification of sunflower oil to biodiesel on ZrO2 supported La2O3 catalyst. Bioresour. Technol. 2010, 101, 953–958. [CrossRef][PubMed] 29. Wan Omar, W.N.N.; Amin, N.A.S. Biodiesel production from waste cooking oil over alkaline modified zirconia catalyst. Fuel Process. Technol. 2011, 92, 2397–2405. [CrossRef] 30. Liu, X.; Piao, X.; Wang, Y.; Zhu, S.; He, H. Calcium methoxide as a solid base catalyst for the transesterification of soybean oil to biodiesel with methanol. Fuel 2008, 87, 1076–1082. [CrossRef] 31. Huppertz, T. Homogenization of Milk-Other Types of Homogenizer (High-Speed Mixing, Ultrasonics Microfluidizers, Membrane Emulsification). In Encyclopedia of Dairy Sciences, 2nd ed.; Fuquay, J.W., Fox, P.F., McSweeney, P.L.H., Eds.; Elsevier: Oxford, UK; Academic Press: Oxford, UK, 2011; pp. 761–764. 32. Saheki, A.; Seki, J.; Nakanishi, T.; Tamai, I. Effect of back pressure on emulsification of lipid nanodispersions in a high-pressure homogenizer. Int. J. Pharm. 2012, 422, 489–494. [CrossRef][PubMed] 33. Hsiao, M.C.; Kuo, J.Y.; Hsieh, P.H.; Hou, S.S. Improving biodiesel conversions from blends of high-and low-acid-value waste cooking oils using sodium methoxide as a catalyst based on a high speed homogenizer. Energies 2018, 11, 2298. [CrossRef] 34. Anastopoulos, G.; Zannikou, Y.; Stournas, S.; Kalligeros, S. Transesterification of vegetable oils with ethanol and characterization of the key fuel properties of ethyl esters. Energies 2009, 2, 362–376. [CrossRef] 35. Hsiao, M.C.; Lin, C.C.; Chang, Y.H.; Chen, L.C. Ultrasonic mixing and closed microwave irradiation-assisted transesterification of soybean oil. Fuel 2010, 89, 3618–3622. [CrossRef] 36. Hsiao, M.C.; Lin, C.C.; Chang, Y.H. Microwave irradiation-assisted transesterification of soybean oil to biodiesel catalyzed by nanopowder calcium oxide. Fuel 2011, 90, 1963–1967. [CrossRef] 37. Kuan, I.; Kao, W.C.; Chen, C.L.; Yu, C.Y. Microbial biodiesel production by direct transesterification of Rhodotorula glutinis biomass. Energies 2018, 11, 1036. [CrossRef] 38. Chen, K.S.; Lin, Y.C.; Hsu, K.H.; Wang, H.K. Improving biodiesel yields from waste cooking oil by using sodium methoxide and a microwave heating system. Fuel 2012, 38, 151–156. [CrossRef] 39. Al-Zaini, E.O.; Olsen, J.; Nguyen, T.H.; Adesina, A. Transesterification of waste cooking oil in presence of crushed seashell as a support for solid heterogeneous catalyst. SAE Int. J. Fuels Lubr. 2011, 4, 139–157. [CrossRef] 40. Kouzu, M.; Kasuno, T.; Tajika, M.; Yamanaka, S.; Hidaka, J. Active phase of calcium oxide used as solid base catalyst for transesterification of soybean oil with refluxing ethanol. Appl. Catal. A 2008, 334, 357–365. [CrossRef] 41. Gryglewicz, S. Alkaline-earth metal compounds as alcoholysis catalysts for ester oils synthesis. Appl. Catal. A 2000, 192, 23–28. [CrossRef] 42. Maneerung, T.; Kawi, S.; Dai, Y.; Wang, C.H. Sustainable biodiesel production via transesterification of waste cooking oil by using CaO catalysts prepared from chicken manure. Energy Convers. Manag. 2016, 123, 487–497. [CrossRef] 43. Teo, S.H.; Islam, A.; Yusaf, T.; Taufiq-Yap, Y.H. Transesterification of Nannochloropsis oculata microalga's oil to biodiesel using calcium methoxide catalyst. Energy 2014, 78, 63–71. [CrossRef] Energies 2018, 11, 2622 12 of 12

44. Li, F.J.; Li, H.Q.; Wang, L.G.; Cao, Y. Waste carbide slag as a solid base catalyst for effective synthesis of biodiesel via transesteriﬁcation of soybean oil with methanol. Fuel Process. Technol. 2015, 131, 421–429. [CrossRef] 45. Birla, A.; Singh, B.; Upadhyay, S.N.; Sharma, Y.C. Kinetic studies of production of biodiesel from waste frying oil using heterogeneous catalysts derived from snail shell. Bioresour. Technol. 2012, 106, 95–100. [CrossRef] [PubMed]

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