SiO2 Beads Decorated with SrO Nanoparticles for BiodieselProduction from Waste Cooking Oil Using Microwave IrradiationAlex Tangy,† Indra Neel Pulidindi,† and Aharon Gedanken*,†,‡

†Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel‡Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan

*S Supporting Information

ABSTRACT: Energy sources are necessary for human existence, comfort, and progress. Limited crude petroleum resources andincreasing awareness of the environmental impacts of using fossil fuels motivate the search for new energy sources and alternatefuels. Herein, a low cost, fast, and green methodology for the synthesis of a hybrid solid base catalyst, strontium oxide coatedmillimetric silica beads (SrO@SiO2), is designed for the transesterification of cooking oil into biodiesel in a domestic microwaveoven. The cost reduction is due to the effective utilization of the catalyst by the homogeneous dispersion of the active sites on thesilica beads and their reusability. The catalyst synthesis process was optimized with respect to the amount of glass beads,microwave irradiation time, calcination time, and calcination temperature. Several methods for synthesizing SrO by minimizingenergy consumption were investigated, and an optimized process for designing SrO@SiO2 was developed. The SrO@SiO2catalyst produced under optimum conditions was characterized by TGA, XRD, FTIR, ICP, SEM, and TEM. XRD analysisindicated peaks typical of SrO alone. ICP analysis indicated 41.3 wt % deposition of SrO on silica beads. The novel solid basecatalyst thus generated was used for the transesterification of waste cooking oil. Conversion values as high as 99.4 wt % in 10 sirradiation were observed from 1H NMR analysis using this composite catalyst, indicating the feasibility of economical biodieselproduction from cooking oil waste in a very short time.

1. INTRODUCTION

Demand for alternate energy sources is increasing exponen-tially. Population explosion and depleting fossil fuel reservesprompt vigorous research into alternate fuel sources. Thetransportation sector is currently dependent solely on fossilfuels such as petrol and diesel.1 Alternate fuels are necessary inorder to meet future transportation demands. Biodiesel is arenewable, biodegradable, environmentally friendly, and non-toxic fuel which has attracted considerable attention in pastdecades. Moreover, the gravimetric energy density of biodiesel(∼41 MJ/kg) is close to that of both gasoline (∼46.4 MJ/kg)and diesel (∼46.2 MJ/kg).2 One of the ways to reduce costs inthe field of transportation is to develop an economically viableprocess for the production of biodiesel which could serve as analternative to the current fossil based fuels.3,4

Biodiesel is a promising fuel with the potential to substitutefossil based fuels without requiring major modifications in thephysical structure of the engine in transportation vehicles.Biodiesel also emits fewer hydrocarbons and CO2 thanconventional transportation fuels.5 Moreover, the sulfurcontent of biodiesel is negligible, and, therefore, additionaldesulfurization is not required.6 Better lubricity, no aromaticcontent, and the possibility of use in diesel engines without anymodifications are other advantages of using biodiesel.7

Moreover, biodiesel is the only alternate fuel that has obtainedthe clearance to be used as a transesterification fuel based onthe 1990 Clean Air Act amendments.8 The use of biodiesel canbe a solution to the problem of environmental pollution. Owingto these advantages, biodiesel is a promising alternative topetroleum-based fuels.

Biodiesel is defined as a mixture of monoalkyl esters of long-chain fatty acids derived from natural, renewable feedstock,such as vegetable oil or animal fats.9 Vegetable oils have to bemodified to be suitable substitutes for petroleum diesel. Thereare four major techniques to convert vegetable oils to biodiesel:dilution,10 microemulsion,11 pyrolysis (thermal cracking),12−14

and transesterification.15−18 Transesterification is the mostsuitable method for producing an environmentally friendly andsafe fuel from unprocessed vegetable oil. In this process,triglycerides react with short-chain alcohols in the presence of abase or an acid catalyst, to obtain fatty acid methyl esters(FAME) and glycerol as a byproduct.19

A variety of feedstock containing fatty acids, such asvegetable oils or animal fats, have been evaluated for theproduction of biodiesel. The utilization of edible oil asfeedstock gives rise to certain concerns, such as food crises.Moreover, the price per liter of vegetable oil is higher than theprice of gasoline. Therefore, great efforts have been devotedsince 2006 to use nonedible oils or waste cooking oils asfeedstock for biodiesel production.20−22

Cooking oil is regarded as an environmental waste. It isgenerated in tons worldwide. Various strategies have beendeveloped for converting cooking oil to biodiesel.23−25 Use ofcooking oil as a feedstock for biodiesel production is beneficialfrom both energy use and environmental viewpoints. However,as cooking oils contain high amounts of free fatty acidscompared to edible oils, the undesired side reactions

Received: February 2, 2016Revised: March 27, 2016

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© XXXX American Chemical Society A DOI: 10.1021/acs.energyfuels.6b00256Energy Fuels XXXX, XXX, XXX−XXX

(hydrolysis of triglycerides) are accelerated. As a result, thetransesterification reaction is greatly affected, and theconversion of waste cooking oils to biodiesel becomes moredifficult.26,27

The use of a catalyst is crucial for accelerating thetransesterification reaction. Sodium and potassium hydroxideare commonly used as homogeneous base catalysts,28 but thereare many obstacles to their utilization: the hydroxides producesoap by neutralizing the free fatty acid in waste oils and bytriglyceride saponification.29 The soap formation is anundesirable surface reaction because it partially consumes thecatalyst, decreases the biodiesel yield, and complicates theseparation and purification steps.30,31 The removal of thesecatalysts is technically difficult and adds extra cost to the finalproduct. Acidic catalysts are also being used for thetransesterification reaction.32−35 However, despite the increasein the yield of the biodiesel, the acid-catalyzed reaction is muchslower than the alkali catalyzed reaction and also requireshigher temperatures and pressures.36,21

In contrast, the use of a heterogeneous solid catalystfacilitates its separation from the liquid products, allows itsreusability, and reduces soap formation, resulting in a moreenvironmentally friendly process. Moreover, the biodieselobtained is purified under mild conditions. In an attempt toreduce the production cost, a variety of catalysts, namely, anion-exchange resin,37,38 alkaline earth metal oxides,39−41 mixedmetal oxides of alkaline earth group elements,42 rare earthmetal,43 zeolites,44 spinels,45,46 and perovskites47 are beingtested for the industrial production of biodiesel. The commongoals are always to reduce the catalyst amount and lower theoverall energy requirements of the process. The commonproblem associated with the heterogeneous biodiesel produc-tion process is its slow reaction rate due to poor surface contactbetween triglycerides and alcohol during the reaction becauseof their reciprocal immiscibility.30 Among the solid catalysts,alkaline earth metal oxides have higher basicity and lowersolubility in alcohol and produce higher biodiesel yield. Theorder of activity among the alkaline earth oxide catalyst is BaO> SrO > CaO > MgO.48 Refaat49 has discussed the reactionmechanism in detail. BaO is toxic and soluble in methanol,50

affecting the quality of the biodiesel produced51 and is thereforenot suitable for biodiesel production. SrO, despite its lowersurface area52 and the partial solubility of the metal ion in thereaction medium,53 exhibits excellent catalytic performance forthe transesterification process, accelerating the transesterifica-tion reaction from hours to seconds when using microwaveirradiation.54,55 Its high activity is mainly due to its alkalinityand basic sites.Microwave irradiation is a well-known method for

accelerating and enhancing chemical reactions because it carriesthe energy directly to the reactant.57,58 Thus, microwaveirradiation is a potential route to accelerate the trans-esterification reaction in the presence of SrO based catalysts.55

Patil et al. demonstrated the advantage of using microwaveirradiation for the transesterification of Camelina sativa oil usingthe SrO catalyst. The yield of biodiesel (80 wt %) that could beachieved using microwave irradiation in a short duration of 4min requires 180 min using conventional heating (using a hotplate heater with a magnetic stirrer).56 Even though precisetemperature control and uniformity in temperature distributionare common problems in a modified domestic microwave oven,the problems could be surmounted by the use of advanced

microwave systems with precise temperature control which iscurrently available.Even though, SrO powder has been demonstrated to be an

active catalyst in a batch process, for its industrial adoptabilitysuch a catalyst needs to be used as a coating on the inert solidmaterial. Such a supported catalyst in millimeter dimensionsnot only eliminates the mass flow constraints and pressuredrops but also reduces the cost of the catalyst drastically. Thecost reduction is due to the effective utilization of the catalystby the homogeneous dispersion of the active sites on the glassbeads. Moreover, such a catalyst contributes to the increase ofmass and heat transfer, improves the contact between the liquidmedium and the catalyst surface, and facilitates its separationfrom the products.59 Also, depositing the catalyst on solidsupports can help prevent possible health risks caused byinhalation of fine powders. In addition, catalysts supported onspherical beads can offer shape-dependent advantages such asminimizing the abrasion of the catalyst in the reactionenvironment.60

The objective of the current work is to successfully design aheterogeneous solid base catalyst comprising of SrO depositedon silica beads (SrO@SiO2) that can be used as a potentialcatalyst for the conversion of waste cooking oil to biodieselunder microwave irradiation conditions. In brief, the currentreport is the first step toward constructing a semi-industrialpilot plant for flowing cooking oil through an MW ovencontaining a fixed catalyst. The paper is focused on theevaluation of preparation, properties, and utilization of thecatalyst (SrO@SiO2) for transesterification reaction.

2. EXPERIMENTAL SECTION2.1. Materials and Methods. The waste vegetable cooking oil was

obtained without charge from a restaurant near Bar-Ilan University andfiltered through a USA standard testing sieve of mesh size 250 μm toremove residues and impurities. The acid value of the cooking oil wasdetermined by the titrimetric method61 and was found to be 3.6 mgKOH/g. This value is substantially lower than the value (17.41 mgKOH/g) reported by Patil et al.62 To avoid the saponification of freefatty acids (FFA) and the hydrolysis during the transesterificationreaction, FFA and water were removed from the cooking oil before thetransesterification reaction. The cooking oil was mixed with a basicsolution of potassium hydroxide (KOH) to remove the FFA in theform of soap. The soap was separated from the oil content bycentrifugation.54 Then, the cooking oil was heated at 110 °C toevaporate the water. Even though the acid catalyzed esterificationprocess is well-known for the removal of FFA, usually, higher acidconcentrations, as well as longer reaction times, are required.63 Toavoid these complications a KOH-based pretreatment method wasadopted for the removal of FFA from waste cooking oil. Moreover,since the subsequent transesterification reaction is also a base catalyzed(SrO@SiO2) reaction, a base (KOH) catalyzed pretreatment was usedfor the removal of FFA.

Strontium nitrate (Sr(NO3)2) (≥99.0%) was used as the precursorand was purchased from Sigma-Aldrich. Sodium carbonate (Na2CO3)and silica gel (particles with sizes ranging from 1 to 3 mm and from 3to 6 mm) were purchased from Sigma-Aldrich. Methanol andisopropyl alcohol were purchased from Bio Lab and were used asreceived. SrO coated silica beads (SrO@SiO2), used as catalysts, weresynthesized by irradiation in a domestic microwave oven (DMWO).The transesterification reaction was conducted under DMWOirradiation. The DMWO was operated at 2.45 GHz in a batchmode. The output of the domestic microwave reactor was 1100 W.The microwave oven was operated at 70% power (cycle mode of 21 son and 9 s off), a cycle mode function provided by the DMWO’smanufacturer. The reaction temperature attained as a result ofmicrowave irradiation was measured using a Pyrometer (Fluke, 65

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Infrared thermometer) after the irradiation was completed. Thereaction temperature was found to be 333 K. The microwave oven wasmodified, so as to have a provision for the distillation column passingthrough the MW oven (for enhanced safety of operation) and with astirring facility during the reaction. The modification was performed byreplacing the bottom part of the oven by a rounded aluminum plate.The plate was carefully attached to the framework in such a way as toallow for magnetic stirring (Figure S1).2.2. Preparation of SrO@SiO2. 2.2.1. Synthesis of SrO@SiO2

Using Microwave Irradiation. The deposition of SrO on SiO2 gelconsists of dissolving equal molar amounts of Sr(NO3)2 and Na2CO3in water under vigorous stirring. Optimized catalyst preparation wasobtained by dissolving 4.23 g of Sr(NO3)2 and 2.11 g of Na2CO3 into100 mL of water taken in a 250 mL round-bottom (RB) flask at roomtemperature. Ten mL of ethylenediamine (C2H8N2, EDA) were thenadded. EDA was successfully used elsewhere64 as a chelating andcapping agent for the synthesis of SrCO3 nanoparticles (NPs) undermicrowave irradiation. Subsequently, 6 g of SiO2 gel was added to thesolution. The contents were then irradiated in a DMWO for 1, 3, and5 min. After having been cooled, the residual solid mass was separatedby centrifugation from the supernatant, washed with EtOH threetimes, and subjected to drying under vacuum. The material (Sr(CO3)2deposited on SiO2 beads) was subjected to calcination at differenttemperatures for varying calcination times to determine the optimalconditions for the decomposition of Sr(CO3)2 and in situ depositionof SrO NPs on SiO2 beads.Further optimization of the catalyst preparation was done by

varying the ratio of the Sr precursor to the amount of silica beads. Toeffectively utilize the strontium precursor Sr(NO3)2, keeping theSr(NO3)2 and Na2CO3 amounts constant (4.23 and 2.11 g), theamount of silica beads (SiO2) is steadily increased by an increment of2 g to 6, 8, and 10 g.2.2.2. Characterization of SrO@SiO2. The precise decomposition

temperature of SrCO3, a reaction intermediate for the generation ofSrO from Sr(NO3)2, was deduced from the thermogravimetric analysis(TGA). The TGA curves were recorded using a Q500 Thermogravi-metric Analyzer (TGA) in the temperature range of 25−1000 °C inthe air atmosphere at a heating rate of 10 °C/min. Powder X-raydiffraction (PXRD) analyses were conducted to probe the crystallo-graphic nature of the catalyst (SiO2@SrO). XRD patterns werecollected using a Bruker AXS Advance powder X-ray diffractometer(Cu Kα radiation; λ = 0.154178 nm) operating at 40 kV/30 mA with a0.02 step size in the range of 10−80° (2θ). The phases were identifiedusing the power diffraction file (PDF) database (JCPDS, InternationalCentre for Diffraction Data). The crystallite size was estimated fromXRD patterns choosing the most intense signal, finding the full widthat half maxima and substituting the parameters λ and θ (in radians) inthe Scherrer’ equation: L = 0.9λ/B cos θ, where L is the crystallite size,λ is the X-ray wavelength, B is the line broadening, and θ is the Braggangle in radians. FT-IR spectra were recorded in KBr pallet mode on aNicolet (Impact 410) FT-IR spectrophotometer under atmosphericconditions. The samples were scanned in the range of 400 and 4000cm−1. The imaging and morphology of SrO@SiO2 were obtainedusing a high-resolution scanning electron microscopy (HR-SEM)having a JEOL-JSM 700F instrument and an LEO Gemini 982 fieldemission gun SEM (FEG-SEM) and by using environmental scanningelectron microscope (ESEM) having the FEI QUANTA 200F device.Elemental analysis was carried out using Energy-dispersive X-rayanalysis (EDAX) in conjunction with the HR-SEM instrument.Transmission electron microscope (TEM) images of SrO particleswere taken with JEM-1400, JEOL to visualize their morphology.Samples for TEM were prepared by making a suspension of theparticles in isopropyl alcohol, using water-bath sonication. The crystalstructure of the SrO was determined by selected area electrondiffraction (SAED) crystallographic analysis. To evaluate the exactamount of SrO deposited on silica beads ICP analysis was used.Typical methodology for this is comprised of taking a known amountof SrO@SiO2 in concentrated HNO3 and stirring at 50 °C on amagnetic stirring base for 1 h so as to dissolve the SrO coated on thesilica beads. Subsequently, the silica beads were separated from the

filtrate using Whatman (150 MM Φ) filter paper. The filtrate wasanalyzed for Sr2+ ions using an inductively coupled plasma (ICP)spectrometer (Ultima 2, Jobin Yvon Horiba). Specific surface areaanalysis of the catalyst (SrO@SiO2) prepared under optimal reactionconditions was carried out using a Nova 3200e Quantachromeanalyzer.

2.3. Evaluation of Catalytic Activity of SrO@SiO2. Thetransesterification reactions were performed by a DMWO equippedwith a condenser and carried out in a 50 mL round-bottom flask. Atypical batch process of the transesterification reaction comprises oftaking 15 g of cooking oil, 4 mL of MeOH, and 0.5 g of catalyst, SrO@SiO2, and irradiating the content in a microwave oven for 10 s at 70%(cycle mode of 21 s on and 9 s off) power. This means that the actualirradiation time is only 7 s during the reaction. First, SrO@SiO2 wasdispersed in MeOH with high magnetic stirring to ensure a gooddispersion of the catalyst into the MeOH. The cooking oil wassubsequently added, and the mixture was irradiated for 10 s. At the endof the reaction, the temperature of the mixture was measured by apyrometer and was found to be 60 °C. The mixture was thencentrifuged, and three distinguished layers were observed: the top layerwas composed of Fatty Acid Methyl Esters (FAME) and excessMeOH, the middle one was SrO@SiO2, and the bottom layer wasglycerol. Then, the top layer was extracted, and the excess MeOH wasremoved by a rotary evaporator. The catalyst was then separated inorder to recycle it and study the catalyst activity and stability. To eachsample, cooking oil and MeOH were added in the same amounts usedfor the initial reaction.

The FAME product was analyzed by 1H NMR spectroscopy(Bruker Avance 300 spectrometer). Chloroform (CDCl3) was used asa solvent for 1H NMR sample preparation. The conversion wascalculated directly from the integrated areas of the methoxy group inthe fatty acid methyl esters (FAME) at 3.65 ppm (singlet) and of theα-carbonyl methylene protons present in the triglyceride derivatives at2.26 ppm (triplet).54,65 Eq 1 was used to estimate the conversion ofthe waste cooking oil to FAME

= ×I IConversion (%) [2 /3 ] 100Me CH2 (1)

The conversion ratio of the oil to the resultant fatty acid methylester was obtained by dividing IMe (the integration value of the protonsof the methyl esters) by ICH2 (the integration value of the methyleneprotons). The factors 2 and 3 were derived from the fact that themethylene carbon possesses two protons and the methyl carbon hasthree attached protons.66,67

3. RESULTS AND DISCUSSION

3.1. Strategy for the Deposition of SrO on Silica Beads(SrO@SiO2). 3.1.1. TGA Analysis for Evaluating theAppropriate Calcination Temperature for the Conversionof SrCO3@SiO2 to SrO@SiO2. SrCO3@SiO2 is generated afterthe microwave irradiation of Sr(NO3)2 and Na2CO3 taken in

Figure 1. XRD of SiO2@SrCO3 prepared with Sr(NO3)2 as Srprecursor using microwave irradiation.

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water and EDA in the presence of millimetric silica beads for 5min. Deposition of a pure strontianite SrCO3 phase (JCPDFfile no. 84-1778) on silica beads was confirmed by the XRDanalysis (Figure 1) of powder scratched from the SiO2 beadsusing a forceps and a spatula. The SiO2 beads coated withSrCO3 (SrCO3@SiO2) could not be used as such for XRDanalysis, as the analysis required fine powder and the silicabeads coated with SrCO3 are spheres of millimetric size. Assuch the crystal structure of SrCO3 was found to be

orthorhombic (space group, Pmcn) with the lattice parameters,a = 5.10 Å, b = 8.40 Å, and c = 6.02 Å.To determine the conversion temperature of SrCO3@SiO2

to SrO@SiO2, the TGA of SrCO3 (by scratching the surface ofthe coated silica beads) was measured in air at a heating rate of10 °C/min (Figure 2). A three stage weight loss is observed.The first one, in the range of 180−210 °C was attributed to theevaporation of trapped water molecules. The second weightloss, at approximately 400 °C, is probably due to the

Figure 2. TGA curve of SrCO3 powder.

Figure 3. XRD pattern of SrCO3 deposited on SiO2 (SrCO3@SiO2) ina short microwave irradiation of 30 s.

Figure 4. XRD pattern of the material obtained after calcination ofSrCO3@SiO2 at 900 °C for 4 h in air.

Figure 5. FT-IR spectra of SiO2@SrCO3 and SrO@SiO2.

Figure 6. SEM images (A and B), EDS spectrum (inset B), andelemental mapping (C and D) of SrO@SiO2.

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decomposition of Sr(OH)2. The Sr(OH)2 phase could be thetransient intermediate formed in situ during the TGA analysis inthe air atmosphere containing moisture as this phase is notobserved in the starting material, SrCO3 (Figure 1). The lastweight loss, occurring at 850 °C, is attributed to thedecomposition of SrCO3 to SrO with the release of CO2.Beyond 900 °C, the weight remained constant. Strontiumcarbonate is usually decomposed upon heating to temperatureshigher than 900 °C.68 Therefore, a calcination temperature of900 °C was subsequently used for the decomposition of theSrCO3 phase on SiO2 to produce the SrO@SiO2 catalyst.3.1.2. Determination of the Optimum Time of Microwave

Irradiation for Coating SiO2 Beads with SrCO3. Microwaveirradiation offers an elegant pathway for the conversion ofSr(NO3)2 to SrCO3 in the presence of NaCO3 and EDA in anaqueous medium. The SrCO3 generated in situ during thereaction was deposited on the millimetric silica beads whichwere present in the reaction vessel along with the Sr precursor.Different irradiation times (30 s, 1, 3, and 5 min) were set forthe deposition process. It was observed that even a shortirradiation time would be sufficient for effectively depositingSrCO3 on silica beads. The XRD pattern typical of SrCO3(JCPDF file no. 84-1778) was observed in the case of the

SiO2@SrCO3 catalyst obtained after 30 s of microwaveirradiation (Figure 3). In contrast, similar deposition of theSrCO3 phase on silica beads could not be achieved by stirring atroom temperature even after 1 h without microwave radiation.This signifies the potential of microwave volumetric heatingfacilitating the strong adhesion of SrCO3 on SiO2 beads. Thiscould be due to the surface etching of silica beads causingsurface roughness and adsorption sites required for thedeposition of SrCO3 on the SiO2 surface. In addition, in thepresence of microwave irradiation, the collisions between theSrCO3 particles, and surface binding sites of SiO2 particlesmight be strong enough to cause the adhesion of SrCO3 on theSiO2 surface.The crystallite size values of SrCO3 deposited on silica beads

after 30 s MW irradiation was 12.7 nm. A calcination time for 2h at 900 °C in the air could not convert all the SrCO3 on thesilica beads to SrO as can be seen in Figure S2 where intensesignals of unreacted SrCO3 are observed. The SrCO3@SiO2

Figure 7. TEM images of the SrO@SiO2 catalyst.

Figure 8. Pictorial depiction of (A) the HR-SEM image of SrO@SiO2,(B) the SEM image of the upper leaf side indicating the nanotubules ofwax, and (C) lotus leaves exhibiting extraordinary hydrophobicity.

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material obtained after the microwave irradiation for 30 s, 1, 3,and 5 min was calcined at 900 °C in air for 3 h, and thecrystallite sizes of the resulting SrO particles on the SiO2surface (JCPDF file no. 06-0520; cubic structure with a latticeconstant value of 5.16 Å; Fm3m) were found to be 61.6, 57.5,47.5, and 68.1 nm, respectively, as deduced from the XRD byScherrer analysis (Figure S3).The relatively large crystallite size of the SrO particles

deposited on the silica surface, in comparison to the sizeexpected based on the precursor SrCO3 particles on the SiO2surface, could be due to their agglomeration, caused by the highcalcination temperature (900 °C). In addition to the SrO phase,minor impurities like the Sr(OH)2 and SrCO3 were alsoobserved in the material SrO@SiO2 after calcination (FigureS2). The presence of such impurities could be due to thereaction of the SrO particles with atmospheric moisture, H2O,and CO2, implying the hygroscopic nature of SrO. Interestingly,when the SiO2@SrCO3 samples were calcined for 4 h, theresulting material showed the XRD pattern with the exclusiveSrO phase (JCPDF file no. 06-0520) (Figure 4). Thus, theoptimum time of calcination is 4 h under air at 900 °C. Atypical XRD pattern of SrO@SiO2 (crystallite size of SrO − 55nm) shown in Figure 4 was similar to that of commercial SrOwith a crystallite size of 60 nm (Figure S4). Therefore, thedeveloped methodology is an effective way to coat silica beadswith SrO.3.2. FT-IR Analysis of the Effectiveness of SrO Coating

on SiO2 Beads. Absorption bands typical of normal modes ofvibration of free planar CO3

2− ions bound to Sr2+ in SrCO3were observed at 1473, 1075, 856, and 700 cm−1 for theSrCO3@SiO2 material obtained by the microwave irradiationmethod.69 After calcination of the aforementioned material, thestretching and deformation peaks of CO bonds of carbonatewere drastically suppressed with a new band appearing at 592cm−1 denoting the Sr−O bond stretching. This indicates theeffectiveness of the methodology developed for the depositionof SrCO3 on silica beads and also its subsequent decompositionto SrO on the surface of the silica beads (see Figure 5).

3.2.1. Morphology and Chemical Composition of theSrO@SiO2 Catalyst. The SEM images, as well as the EDSspectrum and the elemental mapping recorded on the SrO@SiO2 catalyst, are shown in Figure 6. It is interesting to notethat the spherical morphology of the millimetric silica bead isretained even after microwave irradiation for 30 s followed bycalcination at a high temperature in air for 3 h at 900 °C(Figure 6 (A)). The deposition of SrO on the SiO2 surface wasconfirmed from the EDS spectrum (Figure 6 (B)). Moreover,the uniform distribution of SrO on the surface of SiO2 can beenvisaged from the presence of the elements Sr and Othroughout the surface of silica beads (Figure 6 (C) and (D)).Raja et al. observed such homogeneous distribution of SrO onthe mesoporous carbon surface (CMK-3).70 This analysisreveals the potential of the methodology designed for thepreparation of SrO coated silica beads in an innovative andgreen methodology.In agreement with the XRD analysis, the TEM image of SrO

deposited on SiO2 beads shows agglomerates of SrO nano-particles (Figure 7 (A)). The TEM image of the SrOnanoparticles on the SiO2 surface was also taken at highermagnification as depicted in Figure 7(B). As a fine powdersample is required for the TEM analysis, SrO particlesscratched from the SiO2 beads of the SrO@SiO2 surfaceusing a forceps and a spatula were used for recording the TEMimages. For further examination of the morphology of the SrOparticles deposited on the silica surface, the selected areaelectron diffraction pattern (SAED) was recorded (inset Figure7(B)). The average particle size of SrO nanoparticles was 56.8nm. The SAED obtained from the SrO particles exhibits diffuseand hollow concentric rings of bright spots. Such a ring patternis generated by the diffraction of transmitted electrons throughthe nanocrystal with different orientations. The ring patternobserved corresponds to the polycrystalline nature of the SrOmaterial with aggregates of SrO particles.

3.3. Parameters Affecting the SrO Loading on SiO2Beads. 3.3.1. Influence of Ratio of Sr Precursor and SilicaBeads on the Deposition of SrO on SiO2 Beads. The amount

Figure 9. Reusability study of SiO@SiO2 and its effect on the conversion of waste cooking oil.

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of silica beads has been varied from 2 to 10 g while keeping theamounts of Sr(NO3)2 (4.23 g) and Na2CO3 (2.11 g) constant.The size of the SiO2 beads was 1−3 mm, and the microwaveirradiation time was 1 min. The amount of SrO deposited onthe SiO2 surface was found to be 20.8, 33.5, 5.2, and 1.4 wt %when the initial amount of SiO2 beads in the reaction mediumwas 2, 6, 8, and 10 g, respectively. Thus, 6 g of SiO2 beads wasfound to be the optimum amount for the effective utilization ofthe Sr precursor resulting in the highest loading of SrO.Moreover, the amount of SrO loading on the silica beads wasfound to have a significant effect on the conversion of wastecooking oil to FAME. Conversion values of 97.6, 99.2, 95.1, and71.2 wt % were observed with the SrO@SiO2 catalyst loadedwith 20.8, 33.5, 5.2, and 1.4 wt % SrO.When the amount of SiO2 beads in the reaction medium is

low (<6 g) the probability of SrCO3 particles adhering to thebeads is lower and most of the SrCO3 particles remainsuspended in the aqueous medium owing to the lower numberof effective collisions between the SrCO3 particles and SiO2beads. The higher the amount of the silica beads the probabilityof deposition of the SrCO3 particles on the silica surface ishigher upon the action of microwave irradiation owing to thelarger surface area offered by the silica surface. So an increase inSrO loading from 20.8 to 33.5 wt % with an increase in thebead content from 2 to 6 g is observed; but beyond 6 g of theSiO2 beads, owing to the presence of excess silica bead contentin the reaction medium, the effective microwave radiationexperienced by individual SrCO3 particles could be lower dueto the shielding effect offered by the excessive presence of thebeads.71 This, in turn, might result in the precipitation of theSrCO3 particles in the reaction medium rather than beingcoated on the surface of the bead. A pictorial representation ofthe process of deposition of SrCO3 onto the silica beads undermicrowave irradiation is shown in Figure S5.3.3.2. Influence of Diameter of SiO2 Beads on SrO

Deposition. After examining the optimum ratio (1.42) ofSiO2 beads (6 g) to Sr(NO3)2 (4.23 g) for the effectivedeposition of SrO, the size effect of the silica beads was alsostudied. When the size of the SiO2 beads was varied from 1 to 3mm to 3−6 mm with other reaction conditions being constant,the wt % loading of SrO was found to increase from 33.5 to41.3 indicating a further enhancement in the utilization of Sr.Such an improvement in the SrO deposition could be due tomore surface area of the bigger silica beads (3−6 mm) availableas binding sites for SrO. The BET analysis of the catalyst withoptimum SrO loading (41.3 wt % SrO on SiO2) was analyzedand was observed to be 0.2 m2/g. The low specific surface areavalue is due to the bigger particle size (3−6 mm) of thenonporous silica beads comprising the catalyst support. Theactivity of the catalyst (41.3 wt % SrO deposited on SiO2beads) thus generated under optimum reaction conditions (1min microwave irradiation, 6 g beads, 3−6 mm diameter, 4.23 gSr(NO3)2, and 2.11 g Na2CO3) was evaluated for biodieselproduction from waste cooking oil.3.4. Catalytic Activity of SrO@SiO2 (41.3 wt %) for

Transesterification of Waste Cooking Oil to Biodiesel.The solid base catalyst, SiO2@SrO, thus generated was used forthe transesterification of waste cooking oil. A typical digitalimage of the transesterification product is shown in Figure S6.The transesterification reaction product obtained after themicrowave irradiation was analyzed by 1H NMR (Figure S7). Aconversion value of 99.4 wt % of waste cooking oil to FAME

was observed using the novel composite catalyst indicating thesuccessful design of the catalyst.Of significance is the use of a modest mass (wt %) ratio of

the catalyst, SrO@SiO2 (0.5 g), to cooking oil (15 g) to achievealmost complete conversion of triglycerides to FAME.Moreover, since the loading of SrO on SiO2 is 41.3 wt %, 0.5g of the SrO@SiO2 catalyst corresponds to 0.2065 g of theactive component SrO which is, in fact, a lower amount of thecatalyst used for the reaction compared to previous reports.54,55

Catalysts with lower loadings of the active component (SrO)on silica beads resulted in lower conversion values.The reaction continued further even after the irradiation time

of 10 s as the temperature remained stable owing to the use of acondenser. Therefore, the reaction product was collected fromthe reaction vessel in the microwave oven after 15 min. Toprove the efficiency of DMWO and its acceleration of thereaction rate, the same reaction was done at room temperatureunder stirring, and equivalent conversion of the waste cookingoil was achieved after 45 min. The high conversion oftriglycerides in such conditions is due to the nanoscale SrOparticles that offer high surface area and larger active sites forthe catalytic conversion of waste cooking oil.

3.4.1. Structure−Activity Relationship of the SrO@SiO2Catalyst. HR-SEM analysis was performed to elucidate theobserved high activity of the SrO@SiO2 catalyst at such modestloading (41.3 wt %) of SrO coated on silica beads. A typicalHR-SEM image of the SrO@SiO2 prepared under an optimalreaction (Figure 8 (A)) unraveled a structure−propertycorrelation. In brief, nanometric tubules of SrO were formedon the silica bead surface under the microwave irradiationconditions followed by calcination at high temperature (900°C) in the air. The average length and width of the nanotubulesof SrO were 139 and 50 nm. Moreover, the structural featuresat the nanometric level were analogous to that observed in alotus leaf, probably making it hydrophobic, as depicted inFigure 8 (B and C). The hydrophobic property of the SrO@SiO2 catalyst is only a hypothesis based on the observedstructural features, and quantitative measurements have notbeen carried out.The nanotubules present on the SrO@SiO2 could be

attributed to the observed high activity of the catalyst even ata modest loading of 41.3 wt % of SrO. Such nanotubules mayact as a repelling agent for the water molecules if any arepresent in the cooking oil and promote transesterification of thecooking oil more effectively.

3.4.2. Reusability of the SrO@SiO2 Catalyst for FAMEProduction. Pretreatment of the waste cooking oil has asignificant effect on the catalyst activity and its reusability.SrO@SiO2 exhibited sustainable activity for 10 consecutivecycles of the transesterification reaction of pretreated wastecooking oil. Even after 10 repeated runs, the catalyst activityonly decreased from 99.4 to 95 wt % (Figure 9).From an economic viewpoint, the catalyst cost is a major

factor involving biodiesel production relative to waste cookingoil or methanol, the two principal reactants. The stability andsustained activity of the catalyst are of great importance for theindustrial application of the catalyst. Therefore, the presentcomposite catalyst offers an innovative pathway for productionand exploitation of a reusable solid base catalyst (SrO@SiO2)for biodiesel production.Without the pretreatment by SrO@SiO2, the conversion

value drastically decreased from 99.4 to 78.6 wt %. Such a lossin the catalytic activity owing to the presence of FFA was also

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observed by Viola et al.72 Pretreatment of the cooking oil allowsthe removal of the free fatty acid (FFA) and the water content,thereby hindering the saponification and hydrolysis of thetriglycerides. This is another reason for the sustainable activity(Figure 9) observed in the case of SrO@SiO2 for thetransesterification reaction.

4. CONCLUSIONThe novel solid base catalyst (SrO@SiO2) was synthesized byfabrication and deposition of SrO nanoparticles on millimetricsilica beads (SrO@SiO2). The catalyst showed excellent activityfor the conversion of waste cooking oil to FAME in just 10 s.Moreover, the catalyst could be easily separated and reused for10 consecutive reaction runs without any significant activityloss. Its high activity was attributed to the unique morphologyof SrO particles on the silica bead surface at the nanometriclevel, resulting in well dispersed active sites for the trans-esterification reaction. This low cost and reusable catalyst willbe advantageous for the transformation of waste cooking oil.Further high throughput could be obtained using continuousflow microwave irradiation. Initial results obtained forcontinuous microwave irradiation processing are promisingwith a conversion value close to 100% in a single cycle for 1 Lof oil and methanol. Thus, fast and efficient biodieselproduction based on the solid catalyst together with continuousmicrowave irradiation could be an important process for wastecooking oil transformation. Doubtlessly, the scale-up of thisprocess will involve several scientific and engineeringconstraints, a subject of future endeavor.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.energy-fuels.6b00256.

Image of the modified microwave oven (Figure S1),XRD pattern of SiO2@SrCO3 catalysts (Figures S2 andS3) and of SrO commercial (Figure S4), pictorialrepresentation of the catalyst preparation (SrO@SiO2)under microwave irradiation (Figure S5) and of thetransesterification product (Figure S6), and a representa-tive 1H NMR spectrum of FAME product from wastecooking oil (Figure S7) (PDF)

■ AUTHOR INFORMATIONCorresponding Author*Phone: 972-3-5318315. Fax: 972-3-7384053. E-mail:gedanken@mail.biu.ac.il.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSGedanken thanks the Israeli Ministry of Science, Technologyand Space for the research grant (206712) for supporting thiswork. Grateful thanks are due to India-Israel cooperativescientific research grant (203768) for supporting this research.

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