Article pubs.acs.org/IECR
Synthesis of Epoxidized Canola Oil Using a Sulfated-SnO2 Catalyst Asish K. R. Somidi, Rajesh V. Sharma, and Ajay K. Dalai* Catalysis and Chemical Reaction Engineering Laboratories, Department of Chemical and Biological Engineering, University of Saskatchewan, Saskatoon, SK, Canada S7N 5A9 ABSTRACT: In the present investigation, a heterogeneous sulfated-SnO2 catalyst was synthesized and used for the epoxidation of unsaturation present in the canola oil. The physicochemical properties were studied to measure the surface and bulk properties of prepared SnO2 and sulfated-SnO2 samples. Process optimization studies were performed for parameters such as catalyst loading, ethylenic unsaturation in canola oil to hydrogen peroxide molar ratio, ethylenic unsaturation in canola oil to acetic acid molar ratio, and temperature. Sulfated-SnO2 demonstrated promising catalytic activity with 100% conversion of unsaturation in canola oil to epoxy canola oil in 6 h at optimum process conditions. Based on the experimental results and kinetic data, a Langmuir−Hinshelwood-Hougen-Watson type mechanism was proposed, and the reaction followed pseudo first order. Calculated energy of activation was 17.75 kcal/mol. The tribological properties of epoxy canola oil such as lubricity property in terms of wear scar length, kinematic viscosities, viscosity index, oxidative induction time, cloud point, and pour point were measured.
1. INTRODUCTION
improvement in lubricity, oxidative stability, and viscosity which are important characteristics for ideal lubricant. This research work emphasizes on the synthesis of epoxidized canola oil, which is a base oil for biolubricant synthesis. Commercially available epoxidized vegetable oils are produced by in situ epoxidation process using peroxy acids as oxidizers catalyzed by mineral acids.8 Also, literature reports on the epoxidation of vegetable oils are mostly involved with the use of mineral acids, enzymes, and ion exchange resins as catalysts. Dinda et al.9 reported on the epoxidation of cottonseed oil using hydrogen peroxide as an oxidant catalyzed by sulfuric acid. Cai et al.10 also worked on the synthesis of epoxidized soybean oil, corn oil, and sunflower oil using peroxyacetic acid catalyzed by sulfuric acid. Mungroo et al.11 reported on the synthesis of epoxy canola oil using hydrogen peroxide as an oxidant and acetic acid as peroxy carrier. Under optimized conditions, 81% conversion of ethylenic unsaturation to oxirane was obtained at 7 h using sulfuric acid as a catalyst and about 90% conversion with Amberlist IR120H resin as a catalyst. Sun et al.12 worked on the epoxidation of Sapindus mukorossi (soap nut) oil using hydrogen peroxide as oxidant and stearic acid as peroxy carrier catalyzed by immobilized Candida antartica lipase B enzyme. Vlcek and Petrovic et al.13 studied on the optimization of chemo enzymatic epoxidation of soybean oil. Though homogeneous acid catalysts favor epoxidation reactions, they may lead to many side reactions such as oxirane ring-opening to diol, dimer formation, and hydroxyl ester formations.14 In the case of enzymatic catalysis, deactivation of enzymes is observed due to the presence of hydrogen peroxide. To avoid the limitations with mineral acids,
Increase in the consumption of fossil fuels not only shows adverse effects on the environment but also faces challenges in meeting the demand in the future. It is necessary to find an alternate potential source which is renewable, eco-friendly, and cheaper to produce. Since vegetable oils are produced abundantly from the natural resources, development of novel biobased fuels and chemicals from these feedstocks would serve as an alternate source to substitute fossil fuels.1 Vegetable oils comprise of unsaturated fatty acids attached to a triglyceride skeleton. Though, polar long chain fatty acids in vegetable oils render high strength and stability to the molecules,2 the presence of unsaturation in fatty acids leads to poor rancidity, poor lubrication, and ease to chemical degradation which in turn limits their application as substitutes to mineral oils.3 Hence, chemical modification of unsaturated fatty acids into useful and stable saturated compounds would help in producing new value added biobased products. Among several plant oils, canola oil also finds its importance in the production of new value added products. This oil contains triglyceride molecules with high oleic content, indicating the presence of high unsaturation. Major unsaturated fatty acids are oleic acid (64%), linoleic acid (22%), and αlinoleic acid (8%).4 Oxygen transfer reaction (epoxidation) is one of the most fundamental reactions to remove the unsaturation in olefins. The general processes of olefin epoxidation include the transfer of an oxygen atom from peroxide to the double bond.5 Epoxidation of olefins can produce many stable products and the presence of an active oxirane ring helps to investigate many chemical transformations.6 Sharma et al.7 reported on the synthesis of a canola oil derived biolubricant, which is a saturated triglyceride ester. This process involves two steps (i) synthesis of epoxy canola oil using hydrogen peroxide as oxidant and (ii) epoxy ring-opening and esterification with acetic anhydride. Chemical modification of double bonds into esters showed outstanding © XXXX American Chemical Society
Special Issue: Ganapati D. Yadav Festschrift Received: February 4, 2014 Revised: April 8, 2014 Accepted: April 14, 2014
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introduced through one of the sides of the flask to measure the temperature of the reaction mixture. Calculated amounts of canola oil, acetic acid, and a catalyst were added initially and stirred at desired temperatures for 30 min. Then, the required amount of 30 wt % hydrogen peroxide was added dropwise to the reaction mixture. Uniform stirring was maintained during the reaction time, samples were collected for the oxirane content, and iodine value analysis was performed at regular intervals. Prior to the analysis, each sample is collected and extracted with ethyl acetate to separate the oil and water mixture. Ethyl acetate in the oil sample is degassed by using rota evaporator, and anhydrous sodium sulfate is added to it to adsorb traces of water.11 2.4. Method of Analysis. The oxirane oxygen content of each sample was measured as per the AOCS Cd 9-57 method by volumetric titration against 0.1 N HBr solution in glacial acetic acid, and the iodine value which measures percentage unsaturation was determined using the AOCS Cd 1-25 method. Also, product was confirmed with spectroscopic studies such as FTIR, 1H NMR, and 13C NMR. Oxirane oxygen content and percentage unsaturation are measured as follows
enzymes, and resins, heterogeneous metal oxide catalysts are preferred. Also, use of heterogeneous catalysts would be more favorable with respect to ease of separation, reusability, and environmental safety.15 However, studies on the heterogeneous catalytic activity of metal oxides for epoxidation of vegetable oils are very limited. Campanella et al.16 used heterogeneous Ti-SiO2 for the catalytic epoxidation of soybean oil and soybean methyl esters using hydrogen peroxide as oxidant. It is found that titanium on amorphous silica support is highly effective for epoxidation of unsaturated fatty acids using hydrogen peroxide as oxidant. Mohamed et al.17 reported on the catalytic activity of tungsten for the epoxidation of sunflower oil. Sulfated-SnO2 is known to have strong surface acidity and comes under the class of super solid acid. It was used in most of the acid catalyzed reactions such as esterification and transesterification reactions.18 There was no literature report available on the use of sulfated-SnO2 as a catalyst for epoxidation of unsaturation in vegetable oils. Also, Sharma et al.7 reported that sulfated metal oxide surface can act as both solid acid and oxidative catalysts. Based on its acidic properties it was selected for epoxidation of unsaturation in canola oil. The main objective of this research work is to synthesize epoxidized canola oil using sulfated tin oxide (sulfated-SnO2) as a heterogeneous catalyst. Based on our knowledge this is the first report on the use of sulfated-SnO2 for the epoxidation of unsaturation in canola oil. Other objectives of this research work are to determine the surface reaction mechanism, process parameter optimization, kinetic studies and development of kinetic model to find the order of reaction and apparent activation energy of the reaction, and evaluation of tribological properties of epoxy canola oil to measure and compare its lubricity properties with the ISO VG [viscosity grade] 46 specifications of light gear oil lubricant.
Oxirane oxygen =
mL of HBr consumed × 0.1 × 1.60 mass of sample, g (1)
Iodine value (%unsaturation) =
(B − S) × N × 12.69 mass of sample, g (2)
where B = volume of titrant, mL of blank, S = volume of titrant, mL of sample, and N = normality of Na2S2O3 2.5. Catalyst Characterization Methods. Both SnO2 and sulfated-SnO2 samples were characterized by the following methods to determine and confirm the bulk and surface properties of the catalysts. For FT-IR analysis, a small amount of sample (5−10 mg) is mixed with KBr crystals (90−95 mg) and thoroughly grounded to form fine powder. Pellets were made from this powder using 6 mm dye under 10 ton of pressure. Further, IR analysis was carried out between frequency range 4000−400 cm−1 using a PerkinElmer spectrophotometer with 8 cm−1 resolution.19,20 A Renishaw Raman InVia microscope (spectra-physics model 163) was used to obtain the Raman spectra of the samples. An argon ion laser beam with the wavelength 514 nm, spot size of 1−2 μm, and 3.6 mW power was used. The XRD patterns of the samples were determined on a Rigaku Diffractometer using CuKα radiation in the range 10−80° with a scan rate of 2°/min. The specific surface area, pore volume, and pore size distributions were measured with a Micromeritics ASAP 2000 instrument using low temperature (77 K) N2 adsorption−desorption isotherms. The Brunauer−Emmett−Teller (BET) method was used to measure the specific surface area; average pore diameter and total pore volume were calculated using BJH adsorption and desorption isotherms.7 The thermal analysis of SnO2 and sulfated-SnO2 were investigated on TGA-Q500 series (TA Instruments). Weighed samples were heated from 25 to 800 °C to study the weight loss attributed to decomposition of these materials. Also, this study was made to investigate the decomposition of sulfate species at reaction and calcinations temperatures.20 The NH3-Temperature-Programmed Desorption (NH3-TPD) of both the samples was analyzed by TPD/ TPR-2720 Micromeritics (USA) instrument to investigate acidic strengths of the samples. 0.1 g of sample was collected
2. EXPERIMENTAL SECTION 2.1. Chemicals and Reagents. Canola oil was supplied by Loblaws Inc. (Montreal, Canada). The sources of other chemicals are as follows: glacial acetic acid (100%) from EMD Chemicals Inc. (Darmstadt, Germany) and methylene chloride ACS grade, sulfuric acid, sodium thiosulfate, tin(II) chloride dihydrate 98%, and ammonium hydroxide ACS reagent from Sigma-Aldrich (St. Louis, MO, USA). Hydrogen peroxide GR grade (30 wt %) and ethyl acetate from EMD Chemicals Inc. (Darmstadt, Germany) and Wij’s solution from VWR (San Diego, CA, USA) were procured. 2.2. Catalyst Synthesis. Tin oxide (SnO2) and sulfatedSnO2 were prepared as per the following method.18 100 g of tin(II) chloride dihydrate was dissolved in 2 L of distilled water with continuous stirring at 30 °C followed by the dropwise addition of 25% ammonium hydroxide solution until pH was 8−10. After the formation of a white precipitate, the solution was aged for 2 h at 30 °C. The precipitate was filtered and washed with distilled water until pH 7 and dried at 120 °C for 12 h. Ten g of tin hydroxide powder is then treated with the 300 mL of 1 M H2SO4 solution to obtain sulfated-SnO2. Obtained gel was then dried at 120 °C for 12 h and calcined at 550 °C for 4 h. Also, synthesis of tin oxide was made by the calcination of tin hydroxide gel at the same conditions. 2.3. Experimental Setup and Procedure. All experiments were carried out in a 100 mL three necked roundbottom flask equipped with a magnetic stirrer and placed in a temperature controlled oil bath. The center neck of the flask is connected to a reflux condenser, and a thermometer was B
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in a quartz tube and purged with helium at 200 °C for 2 h to remove impurities on the surface and then cooled down to room temperature by passing helium. About 3 wt % NH3 in N2 was passed through the sample at a flow rate of 30 mL/min for 3 h, and physisorbed NH3 was removed by passing helium at 100 °C for 1 h. The analysis was carried out between 100 to 800 °C with a 10 °C/min ramp rate.7 SEM and TEM analysis were made to identify the morphological features of the samples. Surface micrographs were obtained on JEOL 840A Scanning Electron Microscopy and Transmission Electron Microscope images from Philips CM (10) 1990. For TEM analysis, samples were finely grounded and dispersed in ethanol using ultrasonic bath. Images were collected on a 200 mesh carbon coated copper grid. 2.6. Tribological Properties of Epoxidized Canola Oil. As per ASTM D97-11 and ASTM D2500-11 standards, cloud point and pour point of canola oil and epoxy canola oil are measured. The instrument used for this method was K46100 Cloud point and Pour point Apparatus [Koehler Instrument Company, Inc., USA]. The test sample was introduced into a test vessel and kept inside the cooling chamber. Observations were made for every 3 °C drop in the temperature. Cloud point temperature was reported when there was any hazy appearance in the liquid, and pour point temperature was recorded when the test sample was found to be immobile after a tilt. Using a DV-II+ viscometer [Brookfield, USA], dynamic viscosities of the canola oil and epoxy canola oil were measured at temperatures ranging from 26.5 °C−100 °C as per the ASTM D2983-03 standard method. From the obtained dynamic viscosities, kinematic viscosities and viscosity index were calculated as per the ASTM D2270-10 standard. As per the AOCS CD 12-57-1981 method on oxidative stability, induction time was calculated. A Metrohm 743 Rancimat apparatus [Metrohm, Canada] was used to measure the oxidative property of the test samples. Test samples were heated to 112 °C in a test vessel and supplied with air at 12 L/h to get oxidized. Oxidized compounds were detected by a pulse in the collector vessel consisting of milli-pore water and electrodes. Induction time was measured at the point of time where there is a rapid increase in the oxidation. High Frequency Reciprocating Rig [HFRR] apparatus was used to measure the lubricity property of the oil samples. As per ASTM D6079, ASTM D7688 methods wear scar of the test samples were measured. 5% of the test sample was added to 2 mL of standard diesel and moved against the metal surface at 50 Hz for 75 min. Using an optical microscope wear scar diameter of the test metal sample was measured.
Figure 1. FT-IR spectra: (A) SnO2 and (B) sulfated-SnO2.
Figure 2. Raman spectra: (A) SnO2 and (B) sulfated-SnO2.
interactions between SO4−2 and SnO2 molecules.21 FT-IR and Raman spectroscopic analysis confirmed the presence of sulfate linkage (acidity) in the catalyst which is important for promoting the oxidation reactions. BET surface area measurement of both the samples exhibited type IV isotherm with hysteresis loop. Specific surface areas of SnO2 and sulfatedSnO2 were found to be 16 m2/g and 38 m2/g, respectively. An increase in the BET surface area of sulfated-SnO2 is due to the formation of sulfate (SO 4 −2 ) linkage. BJH pore size distributions also elucidate that accumulation of sulfates on the oxide surface increased its pore volume from 0.1 to 0.2 cm3/g and resulted in the decrease of average pore diameter to 17 nm. Table 1 summarizes the BET surface area and BJH pore size results. As anticipated, for bulky molecular transformation the porous catalyst is needed, and hence sulfation improves the pore volume and is expected to increase the conversion. The obtained XRD patterns of SnO2 and sulfated-SnO2 were represented in Figure 3. The peaks corresponding to 2θ =
3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. The FT-IR spectra of SnO2 and sulfated-SnO2 samples (Figure 1) are interpreted as follows. The absorption broad band centered at 627 cm−1 indicates O−Sn−O stretching, and the band at 3430 cm−1 is due to the presence of hydroxyl groups on the surface of the samples. The bands at 1209 cm−1 1151, and 1010 of sulfatedSnO2 are due to the asymmetric and symmetric stretching frequencies of sulfate ions.19,20 Figure 2 illustrates the Raman spectra of SnO2 and sulfated-SnO2 molecules. Raman shifts obtained at 634 cm−1, 704, and 478 cm−1 correspond to rotational and vibrational modes of O−Sn−O molecules. Raman intensities of sulfated-SnO2 were observed to be less compared to SnO2 spectra because of a decrease in polarizability and crystalline size of sulfated-SnO2 due to the strong
Table 1. BET and BJH Results of SnO2 and Sulfated-SnO2 catalyst SnO2 sulfatedSnO2 C
BET surface area (m2/g)
pore volume (cm3/g)
avg. pore diameter (nm)
16 38
0.1 0.2
23 17
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Figure 3. XRD peak pattern of (A) SnO2 and (B) sulfated-SnO2.
26.5°, 33.8°, 37.9°, 51.8°, 64.9°, 71.5° represent the diffractions from planes (110), (101), (200), (211), (112), (202) of SnO2 molecules which confirms the tetragonal crystal phase.20 Addition of sulfates did not result in any crystalline phase change. The approximate crystalline sizes of SnO2 and sulfatedSnO2 were calculated using Scherrer’s equation, davg = (0.8 × λ)/ (β × Cos θ), where davg is the average crystal size, β is the fwhm (full width half max) of the reflection peak, and θ is the Bragg’s angle. The calculated average crystalline size of sulfatedSnO2 was found to be 18.8 and 25 nm for SnO2. Addition of sulfate ions resulted in the decrement of crystalline size, because bulky sulfate groups prevent the agglomeration of SnO2 particles during calcination.20 The crystalline spacing in sulfated-SnO2 was measured using Bragg’s equation (dspacing= (λ)/(2 × Sin θ) and was found to be 0.47 nm. This data is in agreement with the report by Jogalekar et al.22 The thermal analysis shown in Figure 4 elucidates the percentage weight loss
Figure 5. NH3-TPD profiles of (A) SnO2 and (B) sulfated-SnO2.
SnO2 acidic strength area, it can be confirmed that sulfatedSnO2 is more acidic. Sharma et al.7 reported that sulfation of metal oxide surface increases the catalytic oxidative activity by 3-fold, and, hence, it can be concluded that sulfation is the key to increase the conversion of canola oil to epoxidized canola oil. Figure 6 represents SEM images that show the surface morphology of the samples, and Figure 7 represents TEM images of both the samples. SEM confirmed that there is no change in external morphology of the catalyst after sulfation. From TEM micrographs, average grain sizes of SnO2 and sulfated-SnO2 particles were measured to be in the range of 15−25 nm which are in accordance with XRD data.
4. PROCESS PARAMETER OPTIMIZATION STUDY 4.1. Effect of External Mass Transfer and Intraparticle Mass Transfer Resistances. In any heterogeneous catalytic process, the overall rate of reaction proceeds through rate of mass transfer of the reactants from bulk phase to the surface of the catalyst, rate of surface reaction, and rate of desorption of products from the active sites. Since canola oil molecules are complex, viscous, and bulky, a liquid film interphase may form between the catalyst surface and bulk fluid phase resulting in external mass transfer resistance. This film can also hinder diffusion of other canola oil molecules to access into the pores of the catalyst leading to internal mass transfer resistance.23 Influence of external mass transfer resistance on sulfated-SnO2 was investigated by changing the rate of mixing at different speeds ranging from 600 to 1000 rpm. Table 2 illustrates the effect of speed of agitation on percentage conversion of canola oil to epoxidized canola oil based on iodine value measurement. All the reactions were carried out at 70 °C for 6 h with 5 g of canola oil, 6.5 g of 30 wt % hydrogen peroxide, and 4 g of acetic acid. The influence of rpm on the conversion beyond 1000 rpm showed no effect. Hence, 1000 rpm is considered as optimum. Also, theoretical calculations were performed to confirm the absence of inter- and intraparticle mass transfer resistances. Sherwood number and Wilke-chang equations were used to calculate the mass transfer coefficients and investigate the interparticle mass transfer resistances. Considering canola oil molecules as limiting reactants, the value of diffusivity of canola oil (DCO) was calculated from the Wilke-chang equation DCO = 117.3 × 10‑18 × (Ψ × MPAA)0.5 × T/(μ × VCO0.6)24 and was found to be 1.7 × 10−10 m2/s. Where Ψ = 1 (association factor of peracetic acid), MPAA is the molecular weight of peracetic acid, T is reaction temperature in K (343 K), μ is the viscosity
Figure 4. TGA analysis of (A) SnO2 and (B) sulfated-SnO2.
of SnO2 and sulfated-SnO2 samples between the temperature range 25−800 °C. SnO2 was found to be very stable at applied temperatures, and percentage change in the weight was observed to be negligible (4.1
-−5 −15 3.8
141 15 9 60
>90 −10 -
252
189
-
respectively by minimum dilution of 5% vol/vol of canola oil and epoxidized canola oil. From Table 5, it can be observed that kinematic viscosities of epoxidized canola oil are high which meet the ISO VG46 specifications of light gear oil lubricant. It shows that the removal of unsaturation improved the viscosity properties of canola oil and reflects its effect on lubricity between surfaces. By the addition of 5% vol/vol epoxidized canola oil to standard diesel sample the wear scar length was reduced confirming the importance of treated canola oil in reducing friction between the surfaces. Hence, this study proves that epoxidized canola oil has good lubricity properties and can be used as an additive to diesel-fuel type engines to reduce friction, wear, and tear. Oxidative stability of oil samples is measured by the Rancimat method.28 This method determines a parameter known as oxidative induction time, which is the elapsed time between the start of the test and the moment at which a sudden increase in the conductivity is detected due to the onset of decomposition under isothermal conditions. The oxidative induction time of canola oil was found to be 3.8 and 60 h for epoxidized canola oil. In our published report,7 saturated canola triglyceride esters formed by epoxy ring-opening and esterification with acetic anhydride showed an oxidative induction time of 56 h. A high oxidative induction time of epoxidized canola oil confirms that the removal of unsaturation in canola oil improved its oxidative stability which is important for a potential lubricant.
6. CONCLUSIONS Sulfated-SnO2 was found to be the most active, selective, and stable catalyst for the epoxidation of unsaturation in canola oil. A kinetic model was developed for this reaction, and it follows the LHHW type mechanism. Compared to canola oil, epoxidized canola oil demonstrated excellent lubricity properties with high oxidative stability, viscosity, and lubricity due to the absence of unsaturation in the molecules. Since the product is derived from natural vegetable oil, it is renewable, biodegradable, and eco-friendly. Therefore, it can be considered as a potential source for lubricity applications.
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AUTHOR INFORMATION
Corresponding Author
*Phone: 306 966 4771. Fax: 306 966 4777. E-mail: ajay.dalai@ usask.ca. Notes
The authors declare no competing financial interest. I
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ACKNOWLEDGMENTS The funding for this research was supported by the “Saskatchewan Agricultural Development Fund and SaskCanola”, Canada.
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NOMENCLATURE CO substrate species - canola oil HP hydrogen peroxide AA acetic acid PAA peracetic acid ECO product species−epoxidized canola oil DCO diffusion coefficient CO in PAA (m2/s) MPAA molecular weight of peracetic acid VCO molar volume of canola oil kcCO mass transfer coefficient for canola oil WCOr mass transfer flux Dp characteristic length (m) Rp particle radius (θ) porosity of the catalyst K1 equilibrium constant for peracetic acid formation (L/ mol) K2 equilibrium constant for the adsorption of CO on the catalyst surface (L/mol) K3 equilibrium constant for the adsorption of PAA on the catalyst surface (L/mol) KSR equilibrium constant for surface reaction (L/mol) KECO equilibrium constant for desorption of ECO on the catalyst surface (mol/L) rCO rate of reaction (mol/g cat·h) Ct total active sites t time (h) ρ density of the catalyst particle (g/cm3) τ tortuosity μ viscosity of reaction mixture (kg/m·s)
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REFERENCES
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