Article pubs.acs.org/EF
SiO2 Beads Decorated with SrO Nanoparticles for Biodiesel Production from Waste Cooking Oil Using Microwave Irradiation Alex 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 and increasing awareness of the environmental impacts of using fossil fuels motivate the search for new energy sources and alternate fuels. Herein, a low cost, fast, and green methodology for the synthesis of a hybrid solid base catalyst, strontium oxide coated millimetric silica beads (SrO@SiO2), is designed for the transesterification of cooking oil into biodiesel in a domestic microwave oven. The cost reduction is due to the effective utilization of the catalyst by the homogeneous dispersion of the active sites on the silica 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 minimizing energy consumption were investigated, and an optimized process for designing SrO@SiO2 was developed. The SrO@SiO2 catalyst produced under optimum conditions was characterized by TGA, XRD, FTIR, ICP, SEM, and TEM. XRD analysis indicated peaks typical of SrO alone. ICP analysis indicated 41.3 wt % deposition of SrO on silica beads. The novel solid base catalyst thus generated was used for the transesterification of waste cooking oil. Conversion values as high as 99.4 wt % in 10 s irradiation were observed from 1H NMR analysis using this composite catalyst, indicating the feasibility of economical biodiesel production from cooking oil waste in a very short time.
1. INTRODUCTION
Biodiesel is defined as a mixture of monoalkyl esters of longchain fatty acids derived from natural, renewable feedstock, such as vegetable oil or animal fats.9 Vegetable oils have to be modified to be suitable substitutes for petroleum diesel. There are 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 most suitable method for producing an environmentally friendly and safe fuel from unprocessed vegetable oil. In this process, triglycerides react with short-chain alcohols in the presence of a base 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 as vegetable oils or animal fats, have been evaluated for the production of biodiesel. The utilization of edible oil as feedstock gives rise to certain concerns, such as food crises. Moreover, the price per liter of vegetable oil is higher than the price of gasoline. Therefore, great efforts have been devoted since 2006 to use nonedible oils or waste cooking oils as feedstock for biodiesel production.20−22 Cooking oil is regarded as an environmental waste. It is generated in tons worldwide. Various strategies have been developed for converting cooking oil to biodiesel.23−25 Use of cooking oil as a feedstock for biodiesel production is beneficial from both energy use and environmental viewpoints. However, as cooking oils contain high amounts of free fatty acids compared to edible oils, the undesired side reactions
Demand for alternate energy sources is increasing exponentially. Population explosion and depleting fossil fuel reserves prompt vigorous research into alternate fuel sources. The transportation sector is currently dependent solely on fossil fuels such as petrol and diesel.1 Alternate fuels are necessary in order to meet future transportation demands. Biodiesel is a renewable, biodegradable, environmentally friendly, and nontoxic fuel which has attracted considerable attention in past decades. 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 in the field of transportation is to develop an economically viable process for the production of biodiesel which could serve as an alternative to the current fossil based fuels.3,4 Biodiesel is a promising fuel with the potential to substitute fossil based fuels without requiring major modifications in the physical structure of the engine in transportation vehicles. Biodiesel also emits fewer hydrocarbons and CO2 than conventional transportation fuels.5 Moreover, the sulfur content of biodiesel is negligible, and, therefore, additional desulfurization is not required.6 Better lubricity, no aromatic content, and the possibility of use in diesel engines without any modifications are other advantages of using biodiesel.7 Moreover, biodiesel is the only alternate fuel that has obtained the clearance to be used as a transesterification fuel based on the 1990 Clean Air Act amendments.8 The use of biodiesel can be a solution to the problem of environmental pollution. Owing to these advantages, biodiesel is a promising alternative to petroleum-based fuels. © 2016 American Chemical Society
Received: February 2, 2016 Revised: March 27, 2016 Published: April 1, 2016 3151
DOI: 10.1021/acs.energyfuels.6b00256 Energy Fuels 2016, 30, 3151−3160
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microwave systems with precise temperature control which is currently available. Even though, SrO powder has been demonstrated to be an active catalyst in a batch process, for its industrial adoptability such a catalyst needs to be used as a coating on the inert solid material. Such a supported catalyst in millimeter dimensions not only eliminates the mass flow constraints and pressure drops but also reduces the cost of the catalyst drastically. The cost reduction is due to the effective utilization of the catalyst by the homogeneous dispersion of the active sites on the glass beads. Moreover, such a catalyst contributes to the increase of mass and heat transfer, improves the contact between the liquid medium and the catalyst surface, and facilitates its separation from the products.59 Also, depositing the catalyst on solid supports can help prevent possible health risks caused by inhalation of fine powders. In addition, catalysts supported on spherical beads can offer shape-dependent advantages such as minimizing the abrasion of the catalyst in the reaction environment.60 The objective of the current work is to successfully design a heterogeneous solid base catalyst comprising of SrO deposited on silica beads (SrO@SiO2) that can be used as a potential catalyst for the conversion of waste cooking oil to biodiesel under microwave irradiation conditions. In brief, the current report is the first step toward constructing a semi-industrial pilot plant for flowing cooking oil through an MW oven containing a fixed catalyst. The paper is focused on the evaluation of preparation, properties, and utilization of the catalyst (SrO@SiO2) for transesterification reaction.
(hydrolysis of triglycerides) are accelerated. As a result, the transesterification reaction is greatly affected, and the conversion of waste cooking oils to biodiesel becomes more difficult.26,27 The use of a catalyst is crucial for accelerating the transesterification reaction. Sodium and potassium hydroxide are commonly used as homogeneous base catalysts,28 but there are many obstacles to their utilization: the hydroxides produce soap by neutralizing the free fatty acid in waste oils and by triglyceride saponification.29 The soap formation is an undesirable surface reaction because it partially consumes the catalyst, decreases the biodiesel yield, and complicates the separation and purification steps.30,31 The removal of these catalysts is technically difficult and adds extra cost to the final product. Acidic catalysts are also being used for the transesterification reaction.32−35 However, despite the increase in the yield of the biodiesel, the acid-catalyzed reaction is much slower than the alkali catalyzed reaction and also requires higher temperatures and pressures.36,21 In contrast, the use of a heterogeneous solid catalyst facilitates its separation from the liquid products, allows its reusability, and reduces soap formation, resulting in a more environmentally friendly process. Moreover, the biodiesel obtained is purified under mild conditions. In an attempt to reduce the production cost, a variety of catalysts, namely, anionexchange resin,37,38 alkaline earth metal oxides,39−41 mixed metal oxides of alkaline earth group elements,42 rare earth metal,43 zeolites,44 spinels,45,46 and perovskites47 are being tested for the industrial production of biodiesel. The common goals are always to reduce the catalyst amount and lower the overall energy requirements of the process. The common problem associated with the heterogeneous biodiesel production process is its slow reaction rate due to poor surface contact between triglycerides and alcohol during the reaction because of their reciprocal immiscibility.30 Among the solid catalysts, alkaline earth metal oxides have higher basicity and lower solubility in alcohol and produce higher biodiesel yield. The order of activity among the alkaline earth oxide catalyst is BaO > SrO > CaO > MgO.48 Refaat49 has discussed the reaction mechanism in detail. BaO is toxic and soluble in methanol,50 affecting the quality of the biodiesel produced51 and is therefore not suitable for biodiesel production. SrO, despite its lower surface area52 and the partial solubility of the metal ion in the reaction medium,53 exhibits excellent catalytic performance for the transesterification process, accelerating the transesterification reaction from hours to seconds when using microwave irradiation.54,55 Its high activity is mainly due to its alkalinity and basic sites. Microwave irradiation is a well-known method for accelerating and enhancing chemical reactions because it carries the energy directly to the reactant.57,58 Thus, microwave irradiation is a potential route to accelerate the transesterification reaction in the presence of SrO based catalysts.55 Patil et al. demonstrated the advantage of using microwave irradiation for the transesterification of Camelina sativa oil using the SrO catalyst. The yield of biodiesel (80 wt %) that could be achieved using microwave irradiation in a short duration of 4 min requires 180 min using conventional heating (using a hot plate heater with a magnetic stirrer).56 Even though precise temperature control and uniformity in temperature distribution are common problems in a modified domestic microwave oven, the problems could be surmounted by the use of advanced
2. EXPERIMENTAL SECTION 2.1. Materials and Methods. The waste vegetable cooking oil was obtained without charge from a restaurant near Bar-Ilan University and filtered through a USA standard testing sieve of mesh size 250 μm to remove residues and impurities. The acid value of the cooking oil was determined by the titrimetric method61 and was found to be 3.6 mg KOH/g. This value is substantially lower than the value (17.41 mg KOH/g) reported by Patil et al.62 To avoid the saponification of free fatty acids (FFA) and the hydrolysis during the transesterification reaction, FFA and water were removed from the cooking oil before the transesterification reaction. The cooking oil was mixed with a basic solution of potassium hydroxide (KOH) to remove the FFA in the form of soap. The soap was separated from the oil content by centrifugation.54 Then, the cooking oil was heated at 110 °C to evaporate the water. Even though the acid catalyzed esterification process is well-known for the removal of FFA, usually, higher acid concentrations, as well as longer reaction times, are required.63 To avoid these complications a KOH-based pretreatment method was adopted 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 used for the removal of FFA. Strontium nitrate (Sr(NO3)2) (≥99.0%) was used as the precursor and was purchased from Sigma-Aldrich. Sodium carbonate (Na2CO3) and silica gel (particles with sizes ranging from 1 to 3 mm and from 3 to 6 mm) were purchased from Sigma-Aldrich. Methanol and isopropyl alcohol were purchased from Bio Lab and were used as received. SrO coated silica beads (SrO@SiO2), used as catalysts, were synthesized by irradiation in a domestic microwave oven (DMWO). The transesterification reaction was conducted under DMWO irradiation. The DMWO was operated at 2.45 GHz in a batch mode. The output of the domestic microwave reactor was 1100 W. The microwave oven was operated at 70% power (cycle mode of 21 s on and 9 s off), a cycle mode function provided by the DMWO’s manufacturer. The reaction temperature attained as a result of microwave irradiation was measured using a Pyrometer (Fluke, 65 3152
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filtrate using Whatman (150 MM Φ) filter paper. The filtrate was analyzed for Sr2+ ions using an inductively coupled plasma (ICP) spectrometer (Ultima 2, Jobin Yvon Horiba). Specific surface area analysis of the catalyst (SrO@SiO2) prepared under optimal reaction conditions was carried out using a Nova 3200e Quantachrome analyzer. 2.3. Evaluation of Catalytic Activity of SrO@SiO2. The transesterification reactions were performed by a DMWO equipped with a condenser and carried out in a 50 mL round-bottom flask. A typical batch process of the transesterification reaction comprises of taking 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 actual irradiation time is only 7 s during the reaction. First, SrO@SiO2 was dispersed in MeOH with high magnetic stirring to ensure a good dispersion of the catalyst into the MeOH. The cooking oil was subsequently added, and the mixture was irradiated for 10 s. At the end of the reaction, the temperature of the mixture was measured by a pyrometer and was found to be 60 °C. The mixture was then centrifuged, and three distinguished layers were observed: the top layer was composed of Fatty Acid Methyl Esters (FAME) and excess MeOH, the middle one was SrO@SiO2, and the bottom layer was glycerol. Then, the top layer was extracted, and the excess MeOH was removed by a rotary evaporator. The catalyst was then separated in order to recycle it and study the catalyst activity and stability. To each sample, cooking oil and MeOH were added in the same amounts used for the initial reaction. The FAME product was analyzed by 1H NMR spectroscopy (Bruker Avance 300 spectrometer). Chloroform (CDCl3) was used as a solvent for 1H NMR sample preparation. The conversion was calculated directly from the integrated areas of the methoxy group in the fatty acid methyl esters (FAME) at 3.65 ppm (singlet) and of the α-carbonyl methylene protons present in the triglyceride derivatives at 2.26 ppm (triplet).54,65 Eq 1 was used to estimate the conversion of the waste cooking oil to FAME
Infrared thermometer) after the irradiation was completed. The reaction temperature was found to be 333 K. The microwave oven was modified, so as to have a provision for the distillation column passing through the MW oven (for enhanced safety of operation) and with a stirring facility during the reaction. The modification was performed by replacing the bottom part of the oven by a rounded aluminum plate. The plate was carefully attached to the framework in such a way as to allow 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 gel consists of dissolving equal molar amounts of Sr(NO3)2 and Na2CO3 in water under vigorous stirring. Optimized catalyst preparation was obtained by dissolving 4.23 g of Sr(NO3)2 and 2.11 g of Na2CO3 into 100 mL of water taken in a 250 mL round-bottom (RB) flask at room temperature. Ten mL of ethylenediamine (C2H8N2, EDA) were then added. EDA was successfully used elsewhere64 as a chelating and capping agent for the synthesis of SrCO3 nanoparticles (NPs) under microwave irradiation. Subsequently, 6 g of SiO2 gel was added to the solution. The contents were then irradiated in a DMWO for 1, 3, and 5 min. After having been cooled, the residual solid mass was separated by centrifugation from the supernatant, washed with EtOH three times, and subjected to drying under vacuum. The material (Sr(CO3)2 deposited on SiO2 beads) was subjected to calcination at different temperatures for varying calcination times to determine the optimal conditions for the decomposition of Sr(CO3)2 and in situ deposition of 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. To effectively utilize the strontium precursor Sr(NO3)2, keeping the Sr(NO3)2 and Na2CO3 amounts constant (4.23 and 2.11 g), the amount of silica beads (SiO2) is steadily increased by an increment of 2 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 of SrO from Sr(NO3)2, was deduced from the thermogravimetric analysis (TGA). The TGA curves were recorded using a Q500 Thermogravimetric Analyzer (TGA) in the temperature range of 25−1000 °C in the air atmosphere at a heating rate of 10 °C/min. Powder X-ray diffraction (PXRD) analyses were conducted to probe the crystallographic nature of the catalyst (SiO2@SrO). XRD patterns were collected using a Bruker AXS Advance powder X-ray diffractometer (Cu Kα radiation; λ = 0.154178 nm) operating at 40 kV/30 mA with a 0.02 step size in the range of 10−80° (2θ). The phases were identified using the power diffraction file (PDF) database (JCPDS, International Centre for Diffraction Data). The crystallite size was estimated from XRD patterns choosing the most intense signal, finding the full width at half maxima and substituting the parameters λ and θ (in radians) in the 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 Bragg angle in radians. FT-IR spectra were recorded in KBr pallet mode on a Nicolet (Impact 410) FT-IR spectrophotometer under atmospheric conditions. The samples were scanned in the range of 400 and 4000 cm−1. The imaging and morphology of SrO@SiO2 were obtained using a high-resolution scanning electron microscopy (HR-SEM) having a JEOL-JSM 700F instrument and an LEO Gemini 982 field emission gun SEM (FEG-SEM) and by using environmental scanning electron microscope (ESEM) having the FEI QUANTA 200F device. Elemental analysis was carried out using Energy-dispersive X-ray analysis (EDAX) in conjunction with the HR-SEM instrument. Transmission electron microscope (TEM) images of SrO particles were taken with JEM-1400, JEOL to visualize their morphology. Samples for TEM were prepared by making a suspension of the particles in isopropyl alcohol, using water-bath sonication. The crystal structure of the SrO was determined by selected area electron diffraction (SAED) crystallographic analysis. To evaluate the exact amount of SrO deposited on silica beads ICP analysis was used. Typical methodology for this is comprised of taking a known amount of SrO@SiO2 in concentrated HNO3 and stirring at 50 °C on a magnetic stirring base for 1 h so as to dissolve the SrO coated on the silica beads. Subsequently, the silica beads were separated from the
Conversion (%) = [2IMe/3ICH2] × 100
(1)
The conversion ratio of the oil to the resultant fatty acid methyl ester was obtained by dividing IMe (the integration value of the protons of the methyl esters) by ICH2 (the integration value of the methylene protons). The factors 2 and 3 were derived from the fact that the methylene carbon possesses two protons and the methyl carbon has three attached protons.66,67
3. RESULTS AND DISCUSSION 3.1. Strategy for the Deposition of SrO on Silica Beads (SrO@SiO 2). 3.1.1. TGA Analysis for Evaluating the Appropriate Calcination Temperature for the Conversion of SrCO3@SiO2 to SrO@SiO2. SrCO3@SiO2 is generated after the microwave irradiation of Sr(NO3)2 and Na2CO3 taken in
Figure 1. XRD of SiO2@SrCO3 prepared with Sr(NO3)2 as Sr precursor using microwave irradiation. 3153
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Figure 2. TGA curve of SrCO3 powder.
Figure 5. FT-IR spectra of SiO2@SrCO3 and SrO@SiO2. Figure 3. XRD pattern of SrCO3 deposited on SiO2 (SrCO3@SiO2) in a short microwave irradiation of 30 s.
Figure 4. XRD pattern of the material obtained after calcination of SrCO3@SiO2 at 900 °C for 4 h in air.
Figure 6. SEM images (A and B), EDS spectrum (inset B), and elemental mapping (C and D) of SrO@SiO2.
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 of the coated silica beads) was measured in air at a heating rate of 10 °C/min (Figure 2). A three stage weight loss is observed. The first one, in the range of 180−210 °C was attributed to the evaporation of trapped water molecules. The second weight loss, at approximately 400 °C, is probably due to the
water and EDA in the presence of millimetric silica beads for 5 min. Deposition of a pure strontianite SrCO3 phase (JCPDF file no. 84-1778) on silica beads was confirmed by the XRD analysis (Figure 1) of powder scratched from the SiO2 beads using a forceps and a spatula. The SiO2 beads coated with SrCO3 (SrCO3@SiO2) could not be used as such for XRD analysis, as the analysis required fine powder and the silica beads coated with SrCO3 are spheres of millimetric size. As such the crystal structure of SrCO3 was found to be 3154
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Figure 7. TEM images of the SrO@SiO2 catalyst.
decomposition of Sr(OH)2. The Sr(OH)2 phase could be the transient intermediate formed in situ during the TGA analysis in the air atmosphere containing moisture as this phase is not observed in the starting material, SrCO3 (Figure 1). The last weight loss, occurring at 850 °C, is attributed to the decomposition of SrCO3 to SrO with the release of CO2. Beyond 900 °C, the weight remained constant. Strontium carbonate is usually decomposed upon heating to temperatures higher than 900 °C.68 Therefore, a calcination temperature of 900 °C was subsequently used for the decomposition of the SrCO3 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. Microwave irradiation offers an elegant pathway for the conversion of Sr(NO3)2 to SrCO3 in the presence of NaCO3 and EDA in an aqueous medium. The SrCO3 generated in situ during the reaction was deposited on the millimetric silica beads which were present in the reaction vessel along with the Sr precursor. Different irradiation times (30 s, 1, 3, and 5 min) were set for the deposition process. It was observed that even a short irradiation time would be sufficient for effectively depositing SrCO3 on silica beads. The XRD pattern typical of SrCO3 (JCPDF file no. 84-1778) was observed in the case of the
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 of wax, and (C) lotus leaves exhibiting extraordinary hydrophobicity.
SiO2@SrCO3 catalyst obtained after 30 s of microwave irradiation (Figure 3). In contrast, similar deposition of the SrCO3 phase on silica beads could not be achieved by stirring at room temperature even after 1 h without microwave radiation. This signifies the potential of microwave volumetric heating facilitating the strong adhesion of SrCO3 on SiO2 beads. This could be due to the surface etching of silica beads causing surface roughness and adsorption sites required for the deposition of SrCO3 on the SiO2 surface. In addition, in the presence of microwave irradiation, the collisions between the SrCO3 particles, and surface binding sites of SiO2 particles might be strong enough to cause the adhesion of SrCO3 on the SiO2 surface. The crystallite size values of SrCO3 deposited on silica beads after 30 s MW irradiation was 12.7 nm. A calcination time for 2 h at 900 °C in the air could not convert all the SrCO3 on the silica beads to SrO as can be seen in Figure S2 where intense signals of unreacted SrCO3 are observed. The SrCO3@SiO2 3155
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Figure 9. Reusability study of SiO@SiO2 and its effect on the conversion of waste cooking oil.
3.2.1. Morphology and Chemical Composition of the SrO@SiO2 Catalyst. The SEM images, as well as the EDS spectrum and the elemental mapping recorded on the SrO@ SiO2 catalyst, are shown in Figure 6. It is interesting to note that the spherical morphology of the millimetric silica bead is retained even after microwave irradiation for 30 s followed by calcination at a high temperature in air for 3 h at 900 °C (Figure 6 (A)). The deposition of SrO on the SiO2 surface was confirmed from the EDS spectrum (Figure 6 (B)). Moreover, the uniform distribution of SrO on the surface of SiO2 can be envisaged from the presence of the elements Sr and O throughout the surface of silica beads (Figure 6 (C) and (D)). Raja et al. observed such homogeneous distribution of SrO on the mesoporous carbon surface (CMK-3).70 This analysis reveals the potential of the methodology designed for the preparation of SrO coated silica beads in an innovative and green methodology. In agreement with the XRD analysis, the TEM image of SrO deposited on SiO2 beads shows agglomerates of SrO nanoparticles (Figure 7 (A)). The TEM image of the SrO nanoparticles on the SiO2 surface was also taken at higher magnification as depicted in Figure 7(B). As a fine powder sample is required for the TEM analysis, SrO particles scratched from the SiO2 beads of the SrO@SiO2 surface using a forceps and a spatula were used for recording the TEM images. For further examination of the morphology of the SrO particles deposited on the silica surface, the selected area electron diffraction pattern (SAED) was recorded (inset Figure 7(B)). The average particle size of SrO nanoparticles was 56.8 nm. The SAED obtained from the SrO particles exhibits diffuse and hollow concentric rings of bright spots. Such a ring pattern is generated by the diffraction of transmitted electrons through the nanocrystal with different orientations. The ring pattern observed corresponds to the polycrystalline nature of the SrO material with aggregates of SrO particles. 3.3. Parameters Affecting the SrO Loading on SiO2 Beads. 3.3.1. Influence of Ratio of Sr Precursor and Silica Beads on the Deposition of SrO on SiO2 Beads. The amount
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 the crystallite sizes of the resulting SrO particles on the SiO2 surface (JCPDF file no. 06-0520; cubic structure with a lattice constant value of 5.16 Å; Fm3̅m) were found to be 61.6, 57.5, 47.5, and 68.1 nm, respectively, as deduced from the XRD by Scherrer analysis (Figure S3). The relatively large crystallite size of the SrO particles deposited on the silica surface, in comparison to the size expected based on the precursor SrCO3 particles on the SiO2 surface, could be due to their agglomeration, caused by the high calcination temperature (900 °C). In addition to the SrO phase, minor impurities like the Sr(OH)2 and SrCO3 were also observed in the material SrO@SiO2 after calcination (Figure S2). The presence of such impurities could be due to the reaction 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, the resulting material showed the XRD pattern with the exclusive SrO phase (JCPDF file no. 06-0520) (Figure 4). Thus, the optimum time of calcination is 4 h under air at 900 °C. A typical XRD pattern of SrO@SiO2 (crystallite size of SrO − 55 nm) shown in Figure 4 was similar to that of commercial SrO with a crystallite size of 60 nm (Figure S4). Therefore, the developed methodology is an effective way to coat silica beads with SrO. 3.2. FT-IR Analysis of the Effectiveness of SrO Coating on SiO2 Beads. Absorption bands typical of normal modes of vibration of free planar CO32− ions bound to Sr2+ in SrCO3 were observed at 1473, 1075, 856, and 700 cm−1 for the SrCO3@SiO2 material obtained by the microwave irradiation method.69 After calcination of the aforementioned material, the stretching and deformation peaks of CO bonds of carbonate were drastically suppressed with a new band appearing at 592 cm−1 denoting the Sr−O bond stretching. This indicates the effectiveness of the methodology developed for the deposition of SrCO3 on silica beads and also its subsequent decomposition to SrO on the surface of the silica beads (see Figure 5). 3156
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was observed using the novel composite catalyst indicating the successful 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 achieve almost complete conversion of triglycerides to FAME. Moreover, since the loading of SrO on SiO2 is 41.3 wt %, 0.5 g of the SrO@SiO2 catalyst corresponds to 0.2065 g of the active component SrO which is, in fact, a lower amount of the catalyst 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 a condenser. Therefore, the reaction product was collected from the reaction vessel in the microwave oven after 15 min. To prove the efficiency of DMWO and its acceleration of the reaction rate, the same reaction was done at room temperature under stirring, and equivalent conversion of the waste cooking oil was achieved after 45 min. The high conversion of triglycerides in such conditions is due to the nanoscale SrO particles that offer high surface area and larger active sites for the catalytic conversion of waste cooking oil. 3.4.1. Structure−Activity Relationship of the SrO@SiO2 Catalyst. HR-SEM analysis was performed to elucidate the observed high activity of the SrO@SiO2 catalyst at such modest loading (41.3 wt %) of SrO coated on silica beads. A typical HR-SEM image of the SrO@SiO2 prepared under an optimal reaction (Figure 8 (A)) unraveled a structure−property correlation. In brief, nanometric tubules of SrO were formed on the silica bead surface under the microwave irradiation conditions followed by calcination at high temperature (900 °C) in the air. The average length and width of the nanotubules of SrO were 139 and 50 nm. Moreover, the structural features at the nanometric level were analogous to that observed in a lotus leaf, probably making it hydrophobic, as depicted in Figure 8 (B and C). The hydrophobic property of the SrO@ SiO2 catalyst is only a hypothesis based on the observed structural features, and quantitative measurements have not been carried out. The nanotubules present on the SrO@SiO2 could be attributed to the observed high activity of the catalyst even at a modest loading of 41.3 wt % of SrO. Such nanotubules may act as a repelling agent for the water molecules if any are present in the cooking oil and promote transesterification of the cooking oil more effectively. 3.4.2. Reusability of the SrO@SiO2 Catalyst for FAME Production. Pretreatment of the waste cooking oil has a significant effect on the catalyst activity and its reusability. SrO@SiO2 exhibited sustainable activity for 10 consecutive cycles of the transesterification reaction of pretreated waste cooking oil. Even after 10 repeated runs, the catalyst activity only 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 cooking oil or methanol, the two principal reactants. The stability and sustained activity of the catalyst are of great importance for the industrial application of the catalyst. Therefore, the present composite catalyst offers an innovative pathway for production and 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 loss in the catalytic activity owing to the presence of FFA was also
of silica beads has been varied from 2 to 10 g while keeping the amounts 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 microwave irradiation time was 1 min. The amount of SrO deposited on the 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 medium was 2, 6, 8, and 10 g, respectively. Thus, 6 g of SiO2 beads was found to be the optimum amount for the effective utilization of the Sr precursor resulting in the highest loading of SrO. Moreover, the amount of SrO loading on the silica beads was found to have a significant effect on the conversion of waste cooking oil to FAME. Conversion values of 97.6, 99.2, 95.1, and 71.2 wt % were observed with the SrO@SiO2 catalyst loaded with 20.8, 33.5, 5.2, and 1.4 wt % SrO. When the amount of SiO2 beads in the reaction medium is low (
