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May 19, 2017 - Solar-Heated Sustainable Biodiesel Production from Waste Cooking. Oil Using a Sonochemically Deposited SrO Catalyst on Microporous...
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Solar-Heated Sustainable Biodiesel Production from Waste Cooking Oil Using a Sonochemically Deposited SrO Catalyst on Microporous Activated Carbon Betina Tabah, Anjani P. Nagvenkar, Nina Perkas, and Aharon Gedanken* Department of Chemistry and Institute for Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat Gan 5290002, Israel S Supporting Information *

ABSTRACT: A novel catalyst, SrO on microporous activated carbon (SrO/C), was synthesized by sonochemical deposition of SrO on carbon. The SrO/C demonstrated high catalytic performance in solar-heated transesterification reactions of canola oil, soybean oil, and waste cooking oil (WCO). The catalytic activity increased more than 5-fold (corresponding to 81% less catalyst compared to pristine SrO), using soybean oil as a feedstock. Similar enhancement was also observed with canola oil and WCO, where the catalytic activity improved more than 4-fold, corresponding to 76% less catalyst compared to pristine SrO. A yield of 98.5 wt % fatty acid methyl esters (FAME) was obtained from WCO in 60 min using a 1:6 oil:methanol molar ratio and 7.1 wt % SrO/C catalyst (24% SrO loading) at 46 °C (solar heating). The SrO/C catalyst was used for four consecutive transesterification reactions of WCO without any significant decrease in its catalytic activity (only 3% decrease in FAME yield and less than 5 ppm leaching). The results confirm the stability and sustained activity of the SrO/C catalyst, which is of great importance for industrial applications. The proposed method can significantly minimize the cost of biodiesel production by harnessing solar thermal energy. Performing the reaction without any additional energy consumption for heating, using a carbon-supported catalyst and low-cost feedstock, make the current biodiesel production a simple, economically worthwhile, environmentally-friendly, and industrially appealing process.

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INTRODUCTION Petroleum diesel continues to be a major fuel, with an annual global consumption of 934 million tons; however, it is estimated that petroleum will be depleted by 2050.1,2 Recently, due to decreasing fossil fuel reserves and increasing atmospheric pollution created by petroleum-based fuels, many countries have directed research toward the utilization of alternative fuels (biofuels) such as bioethanol, biodiesel, and biogas.2,3 Biodiesel is defined as a mixture of fatty acid methyl esters (FAME) obtained through the transesterification of triglycerides from renewable sources.4 Biodiesel is considered a promising substitute for conventional petroleum-based diesel mainly because it is biodegradable, nontoxic, energy efficient, and environmentally benign. In comparison to conventional diesel, biodiesel causes reduced emission of CO, SO2, particulates, and unburned hydrocarbons into the atmosphere.4−8 Biodiesel can be blended with diesel and used in current diesel engines with no further modification.6,9,10 Currently, the main disadvantage of the biodiesel industry is the high production cost due to the high prices of the feedstock and catalysts.6,11 Typical raw materials for biodiesel production are canola (rapeseed), soybean, palm, and sunflower oils.12−14 These vegetable oils produce very promising diesel fuel substitutes; however, they contribute 60−75% of the total cost, determining the final cost of the production.1,2,15,16 The waste cooking oil (WCO) collected from industrial, commercial, and residential disposals is a low-cost, readily available, and nonedible feedstock that can serve as a potential solution for the economics of the biodiesel industry.2,15,17 Considering the large amount of WCO produced every day throughout the world, as well as the harmful disposal methods that contaminate © XXXX American Chemical Society

the environmental water through drainage, the conversion of WCO into biodiesel is a highly efficient and economical process.2,15,18,19 Aside from cheap raw materials, a high reaction yield is also crucial for cost-effective biodiesel production.20 Thus, the search for an appropriate solid catalyst that can improve the esterification of free fatty acids (FFAs) and the transesterification of triglycerides is of great interest for the biodiesel industry. The selection of an effective catalyst is essential especially in the case of feedstocks with high water and FFA content (such as WCO and nonedible oils), which strongly affect the behavior of conventional homogeneous catalysts.7 Heterogeneous catalysts have been proposed as an alternative to homogeneous catalysts for transesterification reactions due to their higher biodiesel yields, simpler and cheaper separation from the reaction medium, and reusability.6,7,12 Heterogeneous catalysis is considered to be an economically and ecologically important green process, as it requires neither catalyst recovery nor aqueous treatment steps (reduced water effluent).21 Many metal oxides, such as alkali-earth metal oxides, transition metal oxides, mixed metal oxides, and supported metal oxides, have been studied as heterogeneous solid catalysts for the transesterification process of oils.7,8 The order of activity among various alkaline earth metal-oxide catalysts (BaO > SrO > CaO > MgO) shows that BaO has the highest catalytic activity.22−24 However, BaO is noxious and can be dissolved in methanol; therefore, it is not a suitable catalyst for biodiesel Received: April 2, 2017 Revised: May 18, 2017 Published: May 19, 2017 A

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Energy & Fuels production.22−24 SrO exhibits excellent catalytic performance and is insoluble in methanol, oil, and biodiesel.22−24 Thus, SrO is one of the most suitable catalysts for transesterification reactions from both the economic and technical aspects, due to its stability and long catalyst lifetime.1,21−24 The major drawback of heterogeneous catalysts is their deactivation with time due to poisoning, coking, sintering, and, most importantly, leaching.7,12,25 The leaching of active species to the reaction mixture reduces the activity and reusability of a solid catalyst, which increases the production cost. The leaching also has a negative effect on the amount of washing water in the industry, as well as product contamination.5,7 The anchoring of a metal-oxide catalyst to a solid support can significantly reduce the leaching.6 Structure promoters or catalyst supports can provide more specific surface area and pores for active species, where they can be anchored and react with large triglyceride molecules.26,27 Previous studies in the literature confirm that the type of support strongly affects the activity and the leaching of the heterogeneous catalysts.5 Aluminum oxide and zinc oxide show favorable support properties for alkali catalysts used in transesterification reactions.26−29 Activated carbon has emerged as an attractive alternative to conventional refractory oxide supports because of its good loading capacity, high surface area, porous structure, and low cost.30,31 Activated carbon has proven to be highly effective as a catalyst support in liquid and vapor phase processes such as heterogeneous transesterification reactions.3 Its large surface area, inert carbon skeleton, and controllable pore structure allow the catalyst to disperse largely and effectively.17,26 Activated carbon can also be used under high pressure and temperature reaction conditions without any change to its surface characteristics.26,30,32 Recently, a novel SrO-carbon dot (C-dot) catalyst was prepared by using metal oxide (SrO) as a host matrix for C-dots. The SrO-functionalized C-dots exhibited higher activity than commercial SrO in the transesterification of Chlorella vulgaris under microwave irradiation due to their higher surface area and unique structural features.33 In this study, microporous activated carbon was used as a support for the metal oxide catalyst (SrO) to increase the surface area and the number of accessible active reactant sites in transesterification reactions. Several methods such as thermal pretreatment (calcination), hydrothermal synthesis, physical mixing, incipient wetness impregnation, and precipitation are described in the literature for the preparation of solid catalysts.7 These techniques have several drawbacks such as complicated procedures involving many stages, nonhomogenous particle distribution, and weak anchoring of the particles to the substrate surface.34 In the current study, we introduce a novel heterogeneous solid-base catalyst, namely, sonochemically deposited SrO on microporous activated carbon (SrO/C). Sonochemical irradiation has been proven as an effective technique for the synthesis of nanophased materials and for the deposition and insertion of nanoparticles on/into mesoporous ceramic and polymer supports, fabrics, and glass.34 The mechanism of deposition involves microjet formation after the collapse of the acoustic bubble. Ultrasound irradiation was applied as a “throwingstone” technique for the deposition of SrO on activated carbon where the microjets push the newly formed nanoparticles onto the substrate surface at such a high speed that they are embedded into the substrate.34 This technique results in homogeneous distribution of SrO nanoparticles on the support surface. Under the ultrasound irradiation, the nanoparticles in the solution are directly thrown to the surface of the support,

strongly adhering to the substrate. This strong anchoring minimizes the leaching of the particles from the surface, which makes the sonication a very promising and superior method for the industry.34 Transesterification is mostly performed by conventional heating at 60−70 °C. Recently, various alternative heating sources such as microwave dielectric heating, sonication, and heating under reflux were tested for transesterification reactions.1,22−24,35,36 An ideal heating source for transesterification should complete the reaction in a short time and be energy efficient. Moreover, in order to reach industrial quantities, the heating source should be suitable for industrial applications.1,37 In most of the biodiesel-production processes reported in the literature, reaction temperatures are attained by the use of electricity, which results in a considerable increase in the production cost.38 Renewable energy sources such as solar, wind, geothermal, and biomass energy can be applied to supplement the power to biorefineries, to lower the production cost and improve the net energy ratio. Among these sources, solar energy, with the advantages of abundance and availability, is an excellent candidate for use in biofuel production.39 Solar energy may be an important alternative energy source and provides a solution to meet the growing energy demand, even if only a small portion of this source is harnessed for heating applications.40,41 The current study uses renewable solar thermal energy as a heating source to power the production of biodiesel. The solar energy cumulative flux, measured by the Israel Meteorological Service during 2015, was, on average, 5.3 kWh/m2 per day, which is about 2.8% higher than the 1965− 2014 multiyear average. In 2016, similar solar irradiance was measured across Israel, continuing the trend of above-average irradiance records.42 Using solar thermal energy for biodiesel production has economic and environmental advantages, as it reduces much of the electricity costs associated with the process, making a cost-effective sustainable green production. Several factors that influence the quality of the produced biodiesel such as the reaction time, the amount of the catalyst, the loading of the catalyst on the support, and the type of feedstock were optimized in this study. Moreover, reusability and the leaching of the catalyst were studied in detail to determine the lifetime of the catalyst. This study introduces a simple, economically worthwhile, environmentally-friendly, and industrially appealing biodiesel production process using WCO, a novel heterogeneous solid-base SrO/C catalyst, and solar thermal energy. The labeled transesterification reaction is represented in Scheme 1.

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EXPERIMENTAL SECTION

Materials. Strontium oxide (99.5% metal-based, SrO 97%, cat. no. 088220) was purchased from Alfa Aesar (Massachusetts, USA), and microporous activated carbon was purchased from EnerG2 Tech-

Scheme 1. Solar-Energy-Driven Base-Catalyzed (Heterogeneous Solid-Base SrO/C Catalyst) Transesterification of Vegetable Oils

B

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0.75 g of SrO or SrO/C) were mixed in a 50 mL Erlenmeyer flask and placed on a magnetic stirring plate on the roof of the Chemistry Department building at Bar-Ilan University, Ramat Gan, Israel. All the reactions were performed between 10 a.m. and 4 p.m. at ambient pressure under direct sunlight, using solar radiation as the heating source. The reaction temperature, measured on the surface of the reaction vessel, was monitored as a function of time, and the average temperature was found to be 46 °C. Hourly values of the solar radiation and reaction temperature are presented in Figure S1 (see the Supporting Information). All experiments were conducted in triplicate. Fatty Acid Methyl Ester (FAME) Product Analysis. The product of the transesterification reaction was centrifuged for 15 min at 10,000 rpm. After the centrifugation, the product was separated into three layers: biodiesel (top phase), catalyst (middle phase), and glycerol (bottom phase, the byproduct) (see Supporting Information Figure S2 for product separation after centrifugation). The biodiesel was analyzed by 1H NMR spectroscopy and GC-MS for FAME content and composition. 1H NMR spectra were recorded on a 300 MHz spectrometer (Bruker Avance DPX-300). The chemical shifts were referenced to the solvent (CDCl3, singlet at 7.26 ppm), and the conversion percentage of oil to biodiesel (FAME yield wt %) was calculated directly from the integrated areas of triglyceride and FAME signals (see Supporting Information eq S1).15,44 The signals chosen for integration were those of the methoxy groups in the FAME (singlet at 3.63 ppm) and those of the α-methylene protons in all triglyceride derivatives of the oil (triplet centered at 2.26 ppm) (see Supporting Information Figure S3 for a representative 1H NMR spectrum). The composition of FAME contained in the biodiesel was analyzed by GCMS (Varian 431-GC, 220-MS) using a VF-5 ms column (see Supporting Information Table S1 for GC-MS conditions and details). The biodiesel phase was carefully removed and kept in vacuum (at room temperature) to remove any residual methanol. The presented components were identified by comparing the retention time and mass spectra with a library data of mass spectra compounds. Methyl hexanoate was used as an analytical standard, and FAME composition was quantified by an internal standard method (see Supporting Information Figure S4 for a representative gas chromatogram). The calibration curve was built using samples with a fixed mass of internal standard and various masses of biodiesel dissolved in constant volume. In addition, the conversion percentage of oil to biodiesel (FAME yield wt %) was also calculated by GC-MS analysis (see Supporting Information eq S2) to confirm the results of the 1H NMR spectroscopy.15 Leaching Analysis. ICP analysis was used to determine the amount of ions leached out of the catalysts (SrO and SrO/C) during the transesterification reaction. The analysis was performed in aqueous homogeneous media using doubly distilled water. The leached-out metal ions were phase transferred from the oil to the aqueous phase. A portion of 1 mL of biodiesel was added to 250 mL of doubly distilled water in a 500 mL Erlenmeyer flask and incubated on a shaker for 48 h at 50 °C to enhance the transfer of the ions in the aqueous phase. After incubation, aliquots of the solutions were centrifuged to separate the two phases. One mL of the aqueous phase was diluted 10 times in doubly distilled water and analyzed by ICP to check the quality of the biodiesel according to the quality standards set by the European Committee for Standardization (EN 14214).

nologies (Washington, USA). Anhydrous methanol (99.8%, cat. no. 136806) and n-hexane (AR-b, cat. no. 829052100), used for sample preparation for gas-chromatography−mass-spectrometry (GC-MS) analysis, were purchased from Bio-Lab Ltd., Israel. Deuterated CDCl3 (D, 99.8%, cat. no. DLM-7-100S), used as a solvent in NMR analysis, was purchased from CIL Inc., USA. Methyl hexanoate (>99.8%, Cat. No. 21599), used as an internal standard for GC-MS analysis, was purchased from Sigma-Aldrich, Israel. All chemicals were of analytical grade and used as received. Commercial fresh oils (soybean and canola) were purchased from a local supermarket, and WCO (mixture of various used liquid vegetable oils) was collected from a restaurant near Bar-Ilan University. Waste Cooking Oil (WCO). The collected WCO was contaminated with solid particles; therefore, it was filtered before the transesterification process to remove all food residues (USA standard testing sieve (250 μm) and Whatman filter paper circles, 150 mm Ø, grade 1, 11 μm pore size). The filtered WCO was directly used as feedstock for biodiesel production without any pretreatment. The acid value of the WCO was determined by a standard acid−base titration method using a phenolphthalein indicator.15,43 Catalyst Preparation and Characterization. Synthesis of SrO/C by Sonication. SrO was deposited on activated carbon by the sonochemical method. The activated carbon (2 g) and SrO (0.5 g) were dispersed in 60 and 20 mL of ethanol, respectively, by stirring the solutions for 2 h. The two solutions were then mixed and sonicated for 1 h using a high-intensity ultrasonic Ti-horn (20 kHz frequency and 100 W/cm2 power, Sonics&Materials, Inc. model VCX750). After the sonication, the solvent was evaporated from the reaction mixture by stirring on a hot plate. The catalyst (SrO/C) was then dried overnight in an oven at 100 °C. Characterization of the Catalysts. Powder X-ray diffraction (XRD) analysis of the catalysts was performed on a Bruker D8 Advance X-ray diffractometer using Cu Kα radiation (λ = 1.5418 Å). The phases were identified using the power diffraction file (PDF) database (JCPDS, International Center for Diffraction Data). X-ray photoelectron spectroscopy (XPS) analysis was performed with an Omicron 95 nanotechnology XPS system (X-ray source: Al Kα, 1486.6 eV). The surface area was determined from nitrogen (N2) adsorption−desorption isotherms measured at liquid nitrogen temperature using a Nova 3200e Quantachrome Analyzer. The specific surface area was calculated from the linear part of the Brunauer− Emmett−Teller (BET) plot. The Fourier transform infrared (FTIR) spectra of the catalysts were recorded using a Nicolet 6700 FT-IR spectrometer (Thermo Scientific, USA) under atmospheric conditions. Thermogravimetric analysis (TGA) was performed on a TGA/DSC1 analyzer (Mettler Toledo, USA) over the temperature range of 25− 1,000 °C at a 10 °C/min heating rate under argon with ∼15 mg of samples. Inductively coupled plasma (ICP) measurements were performed using a ICP-OES spectrometer (Ultima 2, Jobin Yvon Horiba) to determine the amount of SrO (wt %) deposited on the activated carbon. The samples for the ICP analysis were prepared by dissolving a weighed amount of the catalyst in 20 mL of 0.5 M HNO3 by heating. The solution was then filtered and diluted to 50 mL in doubly distilled water and analyzed by ICP. The morphology of the catalysts was studied using an environmental scanning-electron microscope (ESEM, FEI QUANTA 200 F, FEG 250). A drop of a sample dispersed in isopropyl alcohol was placed on silica wafers mounted on sample holders and dried in air at room temperature. The samples were carbon-coated prior to imaging. Elemental analysis was performed using energy-dispersive X-ray analysis (EDX, Genesis XM 4) in conjunction with the ESEM instrument. The particle size of the pristine SrO catalyst was determined by transmission electron microscopy (TEM, JEM-1400, JEOL), with a microscope operated at an accelerating voltage of 120 kV. The sample was prepared by dispersing SrO in isopropyl alcohol and placing a drop of this suspension on a carbon-coated copper grid. The grid was dried in a vacuum chamber prior to the measurements. Transesterification Reactions. For a typical transesterification reaction, the oil (7 g of WCO/soybean oil/canola oil), the methanol (2.02 mL, molar ratio of 1:6 oil:methanol), and the catalyst (0.0625 to

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RESULTS AND DISCUSSION Optimization of Transesterification Reaction Conditions Using SrO as a Catalyst. In order to optimize the biodiesel production and meet the requirements of the EN 14214 standards for FAME yield (minimum 96.5 wt %), various preliminary experiments were conducted. All the transesterification reactions were performed with 7 g of fresh canola oil, a 1:6 molar ratio of oil:methanol, and 0.5 g of SrO catalyst, under identical conditions. The molar ratio of oil to methanol was kept at 1:6, based on our previous optimization studies, and the amount of catalyst was chosen arbitrarily to be 0.5 g (7.1 wt C

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Energy & Fuels % catalyst).15,22 The reaction time was varied between 10 and 120 min. The FAME yield after 60 min met the EN 14214 standards with 96.5 wt % oil conversion to biodiesel (see Supporting Information Figure S5). Therefore, 60 min was set as the optimum reaction time for further transesterification reactions. The FAME yield (wt %) was also optimized as a function of the catalyst amount. Various amounts of SrO catalyst were tested (0.0625, 0.125, 0.250, 0.375, 0.500, 0.625, and 0.750 g) for transesterification of canola oil in 60 min. The reactions with 0.500 g of SrO showed the optimum FAME conversion (96.5 wt %), meeting the EN 14214 standards (see Supporting Information Figure S6 and Table S2). Identical experiments were conducted using soybean oil and WCO to compare the biodiesel production process and FAME yield. The optimum catalyst amount for the transesterification of WCO and soybean oil were found to be 0.500 g (7.1 wt % catalyst) and 0.625 g (8.9 wt % catalyst) of SrO, with 98.3 and 96.8 wt % FAME yield, respectively (see Supporting Information Table S2). Sonochemical Deposition of SrO Catalyst on Activated Carbon (SrO/C). The activated carbon was used as a support for the SrO catalyst to increase the amount of active catalytic centers. Various amounts of SrO were deposited on carbon to optimize the loading procedure. Considering the high surface area and porosity of activated carbon, initially, a small amount of SrO (0.0625 g, 12.5 wt % loading) was deposited on 0.5 g of carbon by sonication. The SrO loading determined by ICP analysis was 11 wt %. Using this catalyst for the transesterification reaction of canola oil resulted in 88 wt % FAME yield, which does not meet the minimum requirements of the EN 14214 standards. Therefore, the deposition of SrO on carbon was increased to 25 and 50 wt % (0.125 and 0.250 g SrO, respectively). The loadings were found to be 24 and 48 wt % by ICP analysis. The FAME yield of the reactions using canola oil and these catalysts were 97.3 and 97.8 wt %, respectively. Since both reactions meet the requirements of the EN 14214 standards for biodiesel, 24 wt % SrO deposition on carbon was selected as the optimum loading amount for further transesterification reactions. Characterization of the Catalysts (SrO and SrO/C). The catalysts were characterized by X-ray diffraction measurements (Figure 1). The XRD pattern of the high-surface activated carbon confirms its amorphous nature. Commercial SrO shows

high crystallinity, with characteristic diffraction peaks at 2θ = 30.09, 34.88, 50.16, 59.61 and 62.55°, corresponding to the (111), (200), (220), (311), and (222) reflection planes of the cubic structure of the SrO (space group, Fm3m ̅ (225)). The data matches the JCPDS Powder Diffraction File No. 75-0263. The additional weak peaks observed in the diffractogram are attributed to the hydroxides of strontium present in the catalyst (JCPDS Powder Diffraction File No. 71-2365). Therefore, the catalyst is, most probably, a mixture of SrO and Sr(OH)2. Our observation is consistent with the results of Koberg et al., who reported a considerable amount of Sr(OH)2 in the diffractogram of a commercial SrO catalyst.22 The XRD pattern of the sonochemically synthesized SrO/C catalyst confirms the successful deposition of SrO on microporous activated carbon. The crystalline nature of SrO was maintained in the SrO/C catalyst, with slight shifts in the observed peaks. The resulting shifts can be attributed to the high acoustic energy of ultrasonication, causing disorganization of the SrO lattice and introducing defects and dislocations.45 The additional peaks observed in the diffractogram are attributed to the hydroxides of strontium, indicating again that the catalyst is a mixture of SrO and Sr(OH)2. The broad peak centered around 2θ = 45° confirms the presence of carbon as a support for the SrO/C catalyst. The XPS spectra of the SrO and SrO/C catalysts are presented in Figure 2. In the 3d region, the Sr2+ exhibits two characteristic peaks at binding energies of 131.5 and 133.4 eV, originating from the 3d5/2 and 3d3/2 shells of the atom, respectively. These Sr peaks are also visible in the SrO/C catalyst with reduced intensity and a slight shift compared to the pristine SrO (Figure 2a). Similarly, the O (1s) peak present in the SrO/C catalyst has a decreased intensity (with respect to the SrO) due to 24% deposition of the SrO on the activated carbon (Figure 2b). The binding energy shifts observed in the SrO/C catalyst can be attributed to the disrupted lattice of the SrO caused by the high ultrasonic energy during the sonication process. These results are consistent with the XRD data, where shifts were also observed in the diffraction peaks of the SrO/C catalyst. The results of the BET measurements (surface area, pore volume, and pore width) are reported in Table 1. The high surface area of the activated carbon was confirmed as 1,180 m2/ g, whereas the commercial SrO has a very small surface area of 0.42 m2/g. Ultrasound-assisted deposition of SrO on the carbon support formed a new SrO/C catalyst with a substantially larger surface area of 435 m2/g. The larger surface area of the SrO/C (compared to the pristine SrO) indicates that the resulting catalyst has more available active sites on the surface, which facilitate the adsorption of more reactant molecules and enhance the reaction yield. Nitrogen adsorption−desorption isotherms for the activated carbon and SrO/C catalyst are shown in Figure 3. According to the shape of the isotherms, the obtained SrO/C catalyst has a porous structure. The reduction in pore volume of SrO/C (compared to activated carbon) may be due to partial blocking of the carbon pores by SrO nanoparticles. The increase in pore diameter of SrO/C (from 21.6 to 27.6 Å) may be a result of sonochemical irradiation. The widening of the pores may contribute to the activation of reagents, making the catalytic process more effective. The thermal stability and purity of the samples were examined by analysis (Figure 4). The commercial activated carbon showed 7% gradual weight loss, 3% at 450 °C and 4%

Figure 1. XRD patterns of activated carbon, commercial SrO, and the SrO/C catalyst. D

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Figure 2. XPS spectra of (a) Sr 3d level and (b) O 1s level of commercial SrO and SrO/C catalyst.

Table 1. BET Measurements of Carbon, SrO/C, and Commercial SrO

a

catalyst

BET surface area (m2/g)

pore volumea (cm3/g)

carbon SrO/C SrO

1,180 435 0.42

0.640 0.300 0.008

pore widtha (Å) 21.6 27.6 below detection limit

Measured at P/Po = 0.99.

Figure 3. Nitrogen adsorption−desorption isotherms of the SrO/C catalyst compared to the activated carbon support. Figure 4. TGA (black) and DTA (red) curves of (a) activated carbon, (b) commercial SrO, and (c) SrO/C catalyst.

above 750 °C, corresponding to the evaporation of impurities (Figure 4a). Pristine SrO showed about 8% weight loss at 450 °C due to the decomposition of the Sr(OH)2 to SrO at this E

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Figure 5. FTIR spectra of (a) commercial SrO, (b) activated carbon, (c) fresh SrO/C catalyst, (d) SrO/C catalyst after transesterification reaction, (e) clean commercial glycerol, (f) crude glycerol (byproduct), and (g) biodiesel (FAME).

(Figure 5g). Some base catalysts such as KOH cause a reduction in the quality of the glycerol byproduct due to the dissolution of the catalyst into the glycerol phase.22 The SrO catalysts overcome this problem by not appearing in the glycerol phase, thereby leading to a higher quality of glycerol with fewer impurities. The absence of distinct SrO/C peaks in the FTIR spectrum of the crude glycerol (Figure 5f) confirms that the catalyst did not interfere with glycerol phase. The morphology study by ESEM indicates the flaky shape of the SrO grains (Figure 6a). The average particle size of SrO

temperature (Figure 4b). This weight loss corresponds to 54 wt % Sr(OH)2 in the commercial powder of SrO. The same observation was reported by Koberg et al., who observed an appreciable amount of Sr(OH)2 in the commercial SrO catalyst after exposure to air for a few minutes during the reaction preparation.22 The SrO/C catalyst was observed to be thermally stable until 450 °C. The weight loss (∼4%) at 450 °C is attributed to the decomposition of the Sr(OH)2 to SrO and to the evaporation of some impurities in the activated carbon. The subsequent weight loss around 800 °C can be attributed to the desorption of CO2 (absorbed from the air by the catalyst) from the catalyst surface (Figure 4c). A similar observation was noted by Tangy et al., who reported a CO2 evolution from the catalyst above 800 °C.36 The presence of impurities (adsorbed moisture and CO2 from the air) in the SrO/C does not affect its catalytic performance. The FAME yield using this catalyst meets the requirements of the EN 14214 standards for biodiesel. Moreover, the observation by Kwon et al. confirms that transesterification can be enhanced in the presence of CO2.47 Our results are also in accordance with the literature, where the CaO and SrO catalysts were used for biodiesel production and the formation of Ca(OH)2 and Sr(OH)2 due to moisture also catalyzed the reaction and, therefore, did not cause any significant effect in the FAME yield of the transesterification reactions.15,22,46,48 The catalysts were further characterized by FTIR measurements (Figure 5). The IR spectrum of the commercial SrO shows a characteristic metal-oxide peak at ∼600 cm−1, attributed to the stretching vibration of the Sr−O bond (Figure 5a).49 The peak at 3,500 cm−1 indicates the presence of Sr(OH)2 in the catalyst.22,46 The CC stretching vibrations for the activated carbon can be observed over the range of 1,200−1,600 cm−1 (Figure 5b).49 The peak at ∼600 cm−1 in the SrO/C catalyst spectrum indicates the presence of SrO on the carbon surface, and the peak at 3,500 cm−1, which represents OH vibrations, corresponds to the presence of Sr(OH)2 in the catalyst (Figure 5c). Additional peaks observed in the SrO/C catalyst after the transesterification reaction (at ∼1,500 and 3,000 cm−1) can be attributed to an insignificant amount of residual triglyceride molecules (Figure 5d). The FTIR spectrum of the crude glycerol, the byproduct of the transesterification reaction catalyzed by the SrO/C, matches the spectrum of clean commercial glycerol (Figures 5e and 5f), with some additional peaks arising from the residues of FAME

Figure 6. Environmental scanning-electron micrographs of fresh (a) commercial SrO, (b) activated carbon, (c) SrO/C catalyst, and (d) EDX spectrum of SrO/C catalyst.

nanoparticles measured by TEM analysis is 2.9 ± 0.1 nm (see Supporting Information Figure S7). The ESEM micrographs of activated carbon before and after the sonochemical deposition of SrO exhibit an obvious difference in the surface morphology (Figures 6b and 6c). The well-covered surface of the SrO/C catalyst (surface area = 435 m2/g, average particle size = 7.7 ± 1.7 μm) is a result of the homogeneous deposition of SrO on the carbon support (surface area = 1180 m2/g, average particle size = 9.2 ± 1.7 μm), giving the catalyst a coarse appearance compared to the smooth carbon surface that does not have any F

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Table 2. GC-MS Analysis of the Biodiesel Produced from Canola Oil, Soybean Oil, and WCO Using the SrO and SrO/C Catalysts under Optimum Reaction Conditionsa composition (wt %) FAME component

saturated

monounsaturated

polyunsaturated

a

hexadecanoic acid methyl ester (Z); palmitic acid methyl ester, C16:0 pentadecanoic acid, 14-methyl-; methyl ester, C16:0-iso octadecanoic acid methyl ester; stearic acid methyl ester, C18:0 eicosanoic acid methyl ester; arachidic acid methyl ester, C20:0 9-octadecenoic acid methyl ethyl ester (E); elaidic acid methyl ester, C18:1-trans(n9) 9-octadecenoic acid methyl ester (Z); oleic acid methyl ester, C18:1-cis(n9) 11-eicosenoic acid methyl ester (Z); C20:1-cis(n9) 8,11-octadecadienoic acid methyl ester, C18:2 9,12-octadecadienoic acid methyl ester; linoleic acid methyl ester, C18:2(n6) ∑saturated ∑monounsaturated ∑polyunsaturated

canola + SrO

canola + SrO/C

soybean + SrO

soybean + SrO/C

WCO + SrO

WCO + SrO/C

ND

ND

ND

ND

4.4

11.7

5.7 2.4

6.8 3.1

6.9 5.2

8.1 6.1

7.8 5.7

ND 7.2

1.1

1.3

0.8

0.9

1.5

1.0

10.1

12.7

19.5

26.7

25.6

30.1

49.8

50.3

9.8

7.4

11.2

9.2

2.5 9.6 18.8

2.4 6.6 16.8

ND 11.5 46.3

ND 8.7 42.1

ND 8.2 35.6

ND 6.2 34.6

9.2 62.4 28.4

11.2 65.4 23.4

12.9 29.3 57.8

15.1 34.1 50.8

19.4 36.8 43.8

19.9 39.3 40.8

ND: not detected, ∑saturated = C16:0 + C18:0 + C20:0, ∑monounsaturated = C18:1 + C20:1, and ∑polyunsaturated = C18:2.

of the new SrO/C catalyst increase the number of active sites and overcome the mass transfer limitation caused by insufficient diffusion of the reactant molecules. Due to the catalyst support, more triglyceride molecules can be adsorbed on active sites, producing a higher FAME yield. Buasri et al. reported 49.3 wt % FAME yield from WCO at 50 °C using activated carbon as a support for CaO in a 7-h transesterification reaction.17 Zhang et al. reported 92.6 wt % FAME yield from soybean oil at 55 °C using carbon-supported Ni and Na2SiO3 in a 100 min reaction.3 Baroutian et al. studied the transesterification of palm oil using carbon-supported KOH at 50 °C for 60 min and achieved 30 wt % FAME yield.26 We achieved more than 96.5 wt % FAME yield (meeting the EN 14214 standards for biodiesel) in 60 min with a carbonsupported SrO catalyst at 46 °C using solar radiation as the source of heating. The control experiments were also conducted at 50 °C using a conventional heating source, and similar conversion values (97.5 wt % FAME yield) were obtained. The work of Zhang et al., with 7 wt % catalyst, agrees with our optimum catalyst amount (7.1 wt %); however, their reaction yielded lower biodiesel conversion (92.6 wt %) and was performed at a 10 °C higher reaction temperature using a conventional heating method.3 In contrast, Baroutian et al. used a large catalyst amount within the range of 10−58 wt % and achieved a much lower FAME yield (30 wt %), compared to our study at a 20 °C higher reaction temperature via conventional heating.26 To summarize, past attempts to enhance biodiesel yield and efficiency using regular heating did not approach our enhancement utilizing solar thermal energy, making our production method cost-effective, sustainable, and green. The FAME Content and Composition of Biodiesel. The FAME content and composition of biodiesel obtained from each oil feedstock using SrO and SrO/C catalysts under optimum reaction conditions are summarized in Table 2. The composition is expressed as the weight percent of each FAME component with respect to the total FAME amount measured by GC-MS analysis. Since each feedstock has a unique chemical composition, the biodiesel produced from various feedstocks

deposition. The morphology of the carbon in the SrO/C catalyst was retained, with a uniform distribution of SrO on its surface, resulting in a larger surface area (435 m2/g) compared to pristine SrO (0.42 m2/g). The EDX spectrum shows the composition of the selected surface area of the SrO/C catalyst with a semiquantitative analysis of the elements (Figure 6d). The selected area is mainly composed of Sr, O, and C. No other impurity elements were detected. Evaluation of the Catalytic Activity. The optimum reaction parameters (the reaction time and the catalyst amount) obtained with commercial SrO were tested to evaluate the performance of the SrO/C catalyst. All transesterification reactions were conducted with 0.5 g of SrO/C catalyst (7.1 wt % catalyst with 24 wt % SrO loading) under solar heating for 60 min. The FAME yield (wt %) was quantified, and the products meet the EN 14214 standards for biodiesel, with 97.5, 96.5, and 98.5 wt % FAME conversion, using canola oil, soybean oil, and WCO, respectively. The FAME yield of the transesterification reactions was also calculated using GC-MS analysis. The yield determined from the GC-MS analysis matches the 1H NMR spectroscopy results, confirming the reliability of the methodology (see Supporting Information Table S3 for comparison). Although the reported FAME yields of the SrO/C catalyst are similar to those of the commercial SrO catalyst (more than 96.5 wt % for all oils), the amount of SrO used in the SrO/C catalyst was only 0.12 g (1.7 wt %), corresponding to 24% of the pristine SrO catalyst (0.5 g) used without a carbon support. The results indicate that using carbon as a support for the SrO catalyst enhances the catalytic activity 4.2-fold, with 76% less catalyst amount using canola oil and WCO. Similar enhancement was observed using soybean oil, which improves the catalytic activity 5.2-fold, with 81% less catalyst amount. The substantial decrease in the amount of SrO in the reaction (by using the SrO/C catalyst) is due to the high specific surface area of the activated carbon and the effective distribution of SrO on the surface of the support by the sonochemical method. Furthermore, the deposition of SrO on the carbon support increased the porosity of the catalyst (pore volume of SrO: 0.008 cm3/g, SrO/C: 0.3 cm3/g).17,26 These superior features G

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Energy & Fuels has different fuel properties.50 Important diesel properties that are directly influenced by the FAME composition include low temperature operability, oxidative and storage stability, kinematic viscosity, exhaust emission, cetane number, and energy content.50−52 Table 2 shows that, regardless of the feedstock, FAMEs with C16−18 hydrocarbons are the most abundant methyl esters in each biodiesel produced, constituting more than 96% of all FAMEs. The low percentage of methyl esters with a long carbon chain (more than 18 carbons) is a desirable trait for biodiesel, as it leads to a lower viscosity value.1,22 The limit set by the EN 14214 specifications is a max of 1 wt % polyunsaturated methyl esters with more than four double bonds. Our results show no polyunsaturated FAMEs with more than two double bonds, meeting the EN 14214 standards. To reduce the cost of the final product and shorten the production process, the transesterification reactions of WCO were performed without any esterification reaction. After eliminating the contaminants by filtration, the acid value of the WCO was calculated at 3.6 mg KOH/g oil (corresponds to 1.8 wt % FFA content) by standard titration.15,43 The acid value of commercial canola oil is in the range of 0.07−0.5 mg KOH/g oil (corresponds to 0.04−0.25 wt % FFA content), and soybean oil is between 0.12 and 0.6 mg KOH/g oil (corresponds to 0.06−0.3 wt % FFA content).53−55 The conversion of WCO to biodiesel results in a similar FAME yield compared to fresh vegetable oils. Thus, our study demonstrates that WCO can be successfully used as a feedstock for biodiesel production without a pretreatment step. A similar observation was reported by other authors who studied the transesterification of WCO with an acid value up to 5.1 mg KOH/g oil (corresponding to 2.6 wt % FFA content).15,43,56 Moreover, the performance and exhaust-emission tests of biodiesel derived from WCO and other vegetable oils show similar results, which again confirms that WCO is a promising low-cost and environmentally-friendly feedstock for biodiesel production.2,12,57 Table 2 demonstrates the total percent composition of saturated, monounsaturated, and polyunsaturated FAMEs obtained from each feedstock. The content and composition of the FAMEs obtained from each feedstock are in line with similar studies in the literature, which report 6−10% of saturated, 62−67% of monounsaturated, and 26−29% of polyunsaturated FAMEs for canola oil; 6−17% of saturated, 23−38% of monounsaturated, and 45−62% of polyunsaturated FAMEs for soybean oil; and 15−20% of saturated, 29−39% of monounsaturated, and 41−55% of polyunsaturated FAMEs for WCO.12,15,35,50,58−60 The presence of a higher percentage of saturated fatty acids is known to impart good oxidation resistance to biodiesel.1,22,24,60,61 Moreover, biodiesel that obtains high levels of monounsaturated FAMEs has been reported to have the optimal balance between cold flow properties and oxidative stability, which makes it a superior fuel.50,60 According to Table 2, all types of biodiesel obtained with the SrO/C catalyst have higher saturated and monounsaturated FAMEs compared to the biodiesel obtained with the SrO catalyst (using the same feedstock). This shows that the biodiesel produced using the SrO/C catalyst has higher oxidative stability and improved low-temperature (cold-flow) performance, which are among the essential fuel properties for the biodiesel industry. The mesoporous structure of the carbon support in the SrO/C catalyst is beneficial to achieving the selective hydrogenation of the double bond that results in the production of less polyunsaturated FAMEs.62

Reusability of the Catalyst. One of the most important parameters to evaluate the catalytic performance is the number of cycles for which the catalyst can be reused under the selected reaction conditions. After each transesterification reaction, the catalyst (SrO or SrO/C) was separated by centrifugation to analyze the catalytic activity and stability. The catalysts were recycled without any prior activation or regeneration (with residues of crude glycerol). The FAME yields obtained with SrO and SrO/C indicate that both catalysts can be used for at least four consecutive reaction cycles without any significant loss (up to 4% decrease in FAME yield) in catalytic activity (Figure 7). Despite the similar reported FAME yields and the

Figure 7. Reusability of SrO and SrO/C catalysts in transesterification reaction of waste cooking oil. The error bars represent the standard deviation of three repeated measurements.

number of recycles of SrO/C and SrO, the amount of SrO deposited in the SrO/C catalyst was only 0.12 g (1.7 wt %), corresponding to 76% less catalyst amount compared to pristine SrO without a carbon support (0.5 g, 7.1 wt %). Therefore, the SrO/C catalyst exhibits superior reusability performance compared to commercial SrO. Fresh SrO/C catalyst showed 98.3 wt % FAME yield during transesterification reactions of WCO. At the end of the fourth cycle, only ∼3% decrease was observed in FAME yield, corresponding to 95.6 wt % conversion. Another 7.4% decrease in FAME yield was observed in the fifth cycle (88.2 wt %). Using the same catalysts further up to six cycles resulted in 25.5% reduction in FAME yield (73 wt %) (Figure 7). The reusability results confirm the stability and sustained activity of the catalyst, which is of great importance to the biodiesel industry. Similar results were obtained by Koberg et al., who reported at least four cycles of reusability with the commercial SrO catalyst during the transesterification of soybean oil.22 Our results with the SrO/C catalyst are also in accordance with various previous studies, where different supported catalysts were recycled up to four times for biodiesel production.3,17,20,26 To our knowledge, among the studies that use activated carbon as a support for the catalyst, our results with SrO/C are the best, showing only 3% decrease in FAME yield after the fourth cycle. Buasri et al. and Baroutian et al. tested the reusability of CaO/AC and KOH/AC catalysts and observed 24 and 37% decrease in FAME yield, respectively, after the fourth cycle.17,26 Other studies performed by Baroutian et al. and Fan and Zhang reported only up to three cycles of reusability for KOH/AC H

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Figure 8. ESEM images of SrO/C catalyst after transesterification reaction of waste cooking oil: (a) first cycle and (b) fifth cycle with (c) EDX spectrum.

and K2CO3/AC catalysts, respectively.63,64 Zhang et al. tested the reusability of carbon-supported nickel and sodium silicate catalyst (Na2SiO3@Ni/C) up to five cycles and reported 17% decrease in FAME yield compared to 10% decrease in the current study after five reaction cycles using SrO/C catalyst.3 The deactivation of heterogeneous catalysts after several cycles of transesterification reactions can be attributed to the partial erosion of SrO from the surface of the carbon support and the adsorption of the reaction byproducts on the active sites of the catalyst.3,7,20,35 The decrease in the FAME yield can also be due to some loss of the catalyst during recycling.24 Moreover, FFAs present in WCO destroy the activity of the catalyst faster than those present in fresh oil and decrease the number of recycles.15 The loss in catalytic activity in subsequent reaction cycles also corresponds to some changes in the morphology of the reused catalyst revealed by ESEM micrographs in Figure 8 (also see Supporting Information Figures S8 and S9). The surface of the fresh SrO catalyst presents a flaky discrete-particle appearance, whereas after the first, third, and fifth cycles, gradual degradation was observed in the quality of the catalyst (see Supporting Information Figure S8). The reduction in the quality and yield is due to the remains of oil, FFAs, crude glycerol, or other impurities on the surface of the reused catalysts.46 The product is possibly strongly attached to the catalyst active sites, which blocks its activity for the adsorption of new reactant molecules.35 A similar phenomenon was observed in ESEM micrographs of the reused SrO/C catalyst (Figures 8 and S9). The EDX spectrum of the SrO/C catalyst after the fifth transesterification cycle with WCO (Figure 8c) supports these observations, showing a decrease in Sr (from 57.65 to 25.49 wt %) and O (from 17.59 to 7.38 wt %) content compared to the fresh catalyst (Figure 6d). This reduction is due to the partial erosion of the strontium catalyst from the surface of the carbon support, which is probably the main reason for the decrease in the biodiesel yield. The weight ratio between Sr and O in the used catalyst was kept the same as in the freshly prepared catalyst (Sr/O = 3.4). This ratio corresponds to the composition of the active phase of the commercial SrO containing 54 wt % Sr(OH)2 and 46 wt % SrO, calculated from the TGA analysis (Sr/O = 3.6). Leaching Analysis. Leaching of the catalyst is one of the major problems in the biodiesel industry, as extensive leaching not only lowers the reaction yield and the quality of the product but also causes product contamination.7 In addition, leaching

reduces the sustainability and reusability of the catalyst while increasing the operational cost as a result of catalyst replacement.7,65 Group II metal concentration in biodiesel is limited by strict regulations. According to the EN 14214 standards and the quality standards set by the American Society for Testing and Materials (ASTM D6751), the maximum allowed concentration of all group II metals is 5 ppm.8,66 Here, the total amount of group II metal ions leached out of the catalysts (SrO and SrO/C) was analyzed by ICP and found to be 9.8, 10.3, and 10.8 ppm after the first cycle of the transesterification reaction with the SrO catalyst using WCO, canola oil, and soybean oil, respectively. The same analysis shows 4.3, 2.8, and 3.1 ppm of leached-out group II metal ions using the SrO/C catalyst with WCO, canola oil, and soybean oil, respectively. The leached ion concentrations using the pristine SrO catalyst exceed the upper limit of the EN standards, making the FAME product unsuitable for the biodiesel industry. In contrast, the leaching measurements using the SrO/C catalyst show less than 5 ppm of Sr2+ leaching, which is within the limit of the EN standards. The reduction in the concentration of the leached-out ions using the SrO/C catalyst is a result of the immobilization of SrO on the carbon support by sonochemical deposition. Therefore, the novel SrO/C catalyst overcomes one of the major drawbacks of the SrO catalyst and produces an industrially suitable quality standard biodiesel. Our results are in accordance with the literature studying the CaO catalyst (in the absence and presence of a support) for biodiesel production. Kouzu et al. reported 200 ppm of Ca2+ ion leaching using pristine CaO, whereas the leaching was greatly reduced to 31 ppm by Zabeti et al. using alumina as a support for the CaO catalyst.6,8 The amount of leaching is also affected by the type of the support matrix for the catalyst and can be reduced further by using different supports.5 Several reports studied the leaching of K+ ions in transesterification reactions using a KOH catalyst deposited on various supports.5,20,26 More than 50% of K+ ion leaching was reported using alumina as a support for the KOH catalyst.20 Replacing the alumina support with activated carbon (KOH/C) substantially decreases the leaching of the catalyst to ∼3 ppm, which is consistent with our results (2.8−4.3 ppm with the SrO/C catalyst) using the same support matrix.26 The degree of leaching directly affects the number of runs for which the catalyst can be reused.7 Leaching of the catalysts was also tested after the second and third cycles of the I

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Energy & Fuels transesterification reaction of the WCO. The concentration of group II metal ions leached using pristine SrO as a catalyst was 2.4 and 0.5 ppm for the second and third cycles, respectively. Similarly, analysis shows that the leached ion concentrations using the SrO/C catalyst is 2.9 and 1.5 ppm for the second and third cycles, respectively. The reduction in the amount of leaching in subsequent cycles can be attributed to the presence of less catalyst in the reaction due to the loss of catalyst at each cycle. The same observation was reported by Zabeti et al., who reported that leaching of the catalyst active species into the biodiesel phase decreased (from 31 to 12 ppm) with the increase in the number of reaction cycles.6 It is important to emphasize that the SrO/C catalyst can be reused for four consecutive transesterification reactions of WCO without any significant decrease in its catalytic activity, resulting in only 4% decrease in FAME yield and less than 5 ppm leaching.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +972-3-5318315. Fax: +972-3-7384053. E-mail: [email protected]. ORCID

Aharon Gedanken: 0000-0002-1243-2957

■

Author Contributions

B.T. and A.P.N. contributed equally to the work.

CONCLUSIONS A carbon-supported reusable heterogeneous solid-base catalyst was successfully synthesized by sonochemical deposition. The novel SrO/C catalyst significantly enhances the catalytic activity and exhibits superior features compared to the commercial SrO in solar-heated transesterification reactions of soybean oil, canola oil, and WCO. A FAME yield of 98.5 wt % was obtained in 60 min from the transesterification of WCO (a mixture of various used liquid vegetable oils) with a 1:6 oil:methanol molar ratio and 7.1 wt % catalyst (24% SrO loading) at 46 °C. The WCO was used as received (after filtration) without any esterification pretreatment. Using activated carbon as a support for SrO, the catalytic activity was enhanced 4.2-fold, corresponding to 76% less catalyst compared to pristine SrO without a carbon support. The same SrO/C catalyst was used for four consecutive transesterification reactions of WCO without any significant decrease in its catalytic activity (only 3% decrease in FAME yield and less than 5 ppm leaching). Using the carbon-supported SrO/C catalyst, performing the reaction without any energy consumption for heating (only solar radiation), and using WCO, which is much cheaper than edible vegetable oils, as a feedstock, make the current biodiesel production process simple, economically worthwhile, environmentally-friendly, and industrially appealing.

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S3, comparison of FAME yields (wt %) calculated from 1 H NMR and GC-MS analysis; Figure S8, ESEM micrographs of fresh SrO catalyst and SrO catalyst after cycles of transesterification reaction of waste cooking oil; Figure S9, ESEM micrographs of fresh SrO/C catalyst and SrO/C catalyst after cycles of transesterification reaction of waste cooking oil (PDF)

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the India-Israel cooperative scientific research grant (grant no. 3-11692) and the Israel Ministry of Science, Technology and Space (grant no. 3-8793).

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b00932. Figure S1, variation in average solar radiation and reaction temperature; Figure S2, centrifuged product of solar-heated transesterification reaction of waste cooking oil; Figure S3, 1H NMR spectrum of FAMEs; eq S1, FAME yield (wt %) calculation from 1H NMR spectroscopy; Table S1, GC-MS analyzer details; Figure S4, gas chromatogram of FAMEs obtained from solarheated transesterification reaction of waste cooking oil; eq S2, FAME yield (wt %) calculation from GC-MS analysis; Figure S5, effect of reaction time on FAME yield (wt %) of transesterification reaction of canola oil; Figure S6, effect of SrO catalyst amount on the conversion of canola oil to biodiesel; Table S2, FAME yield (wt %) from transesterification of different oil feedstocks; Figure S7, TEM image of SrO catalyst; Table J

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DOI: 10.1021/acs.energyfuels.7b00932 Energy Fuels XXXX, XXX, XXX−XXX