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FTIR spectroscopy analysis for monitoring biodiesel production by heterogeneous catalyst
Vibrational Spectroscopy 105 (2019) 102990
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FTIR spectroscopy analysis for monitoring biodiesel production by heterogeneous catalyst
T
Morgana Rosset, Oscar W. Perez-Lopez* Laboratory of Catalytic Processes―PROCAT, Department of Chemical Engineering, Federal University of Rio Grande do Sul (UFRGS), Rua Ramiro Barcelos, 2777. CEP, 90035-007, Porto Alegre, RS, Brazil
A R T I C LE I N FO
A B S T R A C T
Keywords: Biodiesel Transesterification Heterogeneous catalysis Basic strength FTIR spectroscopy
Layered double hydroxides, mixed oxides, and pure oxides based on magnesium and calcium were evaluated as catalysts in the methyl transesterification of soybean oil in a batch reactor. The catalysts were prepared by continuous co-precipitation and characterized by specific surface area, thermogravimetric analysis, temperatureprogrammed desorption of CO2, and X-ray diffraction. The reactions were performed at 65 °C, methanol-to-oil molar ratio 9:1, catalyst dosage of 1 wt.%, and reaction time of 240 min. The biodiesel analysis was carried out by Fourier Transform Infrared spectroscopy (FTIR) and Gas Chromatography (GC). Calcium-based catalyst, pure oxide (CaO) and mixed oxide (CaAl), demonstrated higher activity for methyl transesterification than magnesium-based catalyst. Considering both groups of catalysts, the activity for transesterification was: pure oxide > mixed oxide > > LDH. These results were related to the strength of the basic sites. FTIR and GC results were similar, with the advantage of FTIR analysis being simple, fast, and economical.
1. Introduction Biodiesel is a biofuel made from renewable biomass for use in engines, and can partially or totally replace fossil fuels, like diesel. The biodiesel or fatty acid alkyl ester can be produced from the transesterification reaction of triglycerides using alcohol in the presence of a catalyst. Due to the reversibility of the reaction, an excess of alcohol is used to shift the equilibrium for the products side [1,2]. Several species of vegetable oils and animal fats, such as soybean, canola, castor oil, cotton, bovine tallow, in addition to residual oil can be employed as a raw material in the production of biodiesel. The biodiesel produced in the industry is obtained by the homogeneous transesterification reaction of triglycerides using basic catalysts, such as sodium and potassium hydroxides, carbonates, and alkoxides [1,3]. However, in the conventional industrial process, removal of the catalyst is technically difficult and a large amount of wastewater is produced to separate and to clean the catalyst and the product [4,5]. According to Lam et al. [6], the advantages of the application of heterogeneous catalysts are due to performance improvements in the presence of free fatty acids, unmixing the catalysts with biodiesel, and recovering and reuse of it. Moreover, it reduces the costs of separation since it eliminates the biodiesel washing step and the maintenance problems of the equipment because these catalysts are not corrosive. The transesterification with heterogeneous catalysts has been ⁎
extensively investigated in recent years and several materials have been studied as catalysts for biodiesel production. Catalysts with basic properties include pure oxides (MgO, CaO), mixed oxides (MgFe, CaAl, LiAl), supported (Li/CaO, Ca/Al2O3, Cs/SiO2) and layered double hydroxides (CeMgAl, MgAlFe, ZnAl, CaAl, MgAl) [7–9]. Among these, calcium oxide is widely studied as a heterogeneous catalyst because it ensures high basicity, low solubility, and easy manipulation [10,11]. Zeng et al. [12] achieved a conversion of 90.5% for transesterification of canola oil with MgAl hydrotalcites as catalyst, with the molar ratio 6:1 of methanol/oil, 1.5 wt.% catalyst, temperature 65 °C and 4 h of reaction. Jeon et al. [13] utilized mesoporous MgO as a heterogeneous solid catalyst to produce biodiesel from canola oil, the volume ratio of methanol to canola oil was 20:3, the MgO catalyst loading was 3 wt.%, and the transesterification reaction was performed at 190 °C for 2 h. Lu et al. [14] studied CaFeAl layered double oxides in biodiesel production, where the catalytic activity reached more than 90% biodiesel yield, under the conditions of 12:1 methanol/oil, temperature of 60 °C, 60 min of reaction and 6 wt.% of catalyst. Besides, Sousa et al. [15] showed that the soybean oil transesterification catalyzed with CaO yielded greater than 93% (wt/wt) methyl esters, 4 h reflux, methanolto-oil molar ratio of 12 and 3 wt.% catalyst. Castro et al. [16] used the model of transesterification reaction between methyl acetate and ethanol under mild reaction conditions: ethanol/methyl acetate molar ratio 6:1, catalyst concentration of 4 wt.%, and temperature of 50 °C.
Corresponding author. E-mail address: [email protected] (O.W. Perez-Lopez).
https://doi.org/10.1016/j.vibspec.2019.102990 Received 29 June 2019; Received in revised form 30 September 2019; Accepted 8 November 2019 Available online 09 November 2019 0924-2031/ © 2019 Elsevier B.V. All rights reserved.
Vibrational Spectroscopy 105 (2019) 102990
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the mixture of Ca(NO3)2.4H2O and Al(NO3)3.9H2O and solution "B" containing NaOH. To obtain CaO, solution "A" contains only Ca (NO3)2.4H2O. For magnesium containing samples, the solution "A" was composed of the mixture of Mg(NO3)2.6H2O and Al(NO3)3.9H2O and solution "B" by the mixture of NaOH and Na2CO3, whereas for the synthesis of MgO the solution "A" is composed of Mg(NO3)2.6H2O. Then solution A and solution B were slowly mixed with constant stirring at a temperature of 50 °C. The pH of the mixture was maintained at 10 (magnesium-based) and 12 (calcium-based). After coprecipitation, the material was crystallized under stirring for 1 h at 50 °C. The precipitate formed was filtered and washed with deionized water to remove the ions, and then dried in oven at 80 °C at night. To obtain the oxides the solids were calcined in a tubular quartz reactor with synthetic air at a flow rate of 50 mL min-1 at 600 °C for 6 h for Mg-based samples, and at 800 °C for 4 h for Ca-based samples. The catalysts were named LDHCaAl (hydrocalumite) and LDH-MgAl (hydrotalcite) for uncalcined samples, calcined CaAl and MgAl mixed oxides, and calcined pure oxides CaO and MgO, respectively.
According to the authors, MgAl mixed oxide was inactive for transesterification. On the other hand, the Ca addition to the oxides considerably increased the amount and strength of basic sites of Ca/MgAl, greatly improving their catalytic performance. Nayebzadeh et al. [17,18] studied the application of basic catalyst KOH/Ca12Al14O33 in the transesterification reaction via microwave irradiation to produce biodiesel from canola oil. They were able to achieve conversions ranging from 83% to 99%, depending on the operating conditions as methanol-to-oil molar ratio, catalyst dosage and reaction time. Layered double hydroxides (LDH), also known as hydrotalcites, are 3+ x+ nanionic clays that have general formula [M2+ Ax/n⋅yH2O, 1-x Mx (OH)2] 2+ 2+ 2+ where M corresponds to divalent (Mg , Ca or Zn ) and trivalent (Al3+ or Fe3+) metallic cations, An- represents an inorganic or organic anion (CO32- or OH-) and x is the molar ratio of cations [19]. The hydrocalumite has a similar structure to hydrotalcite, but Ca and Al are octahedrally coordinated in the layers. The thermal treatment of layered double hydroxides above 350 °C causes the layered structure to collapse, thereby obtaining the mixed oxides [20]. The combination and ratio of metallic ions influence the structure and chemical properties of the LDH and their respective mixed oxides [21]. Various techniques of analysis have been developed to evaluate the transesterification reaction. Chromatography techniques are usually used because they offer comprehensive perception during transesterification process and detailed information required for quality control of the product. However, recently Nuclear Magnetic Resonance (NMR) spectroscopy and Fourier Transform Infrared (FTIR) spectrometry have been employed for monitoring transesterification [22]. FTIR spectroscopy is considered a powerful technique that can be used to determine the chemical structure and composition of various materials, including biological samples [23]. Although the FTIR method is less sensitive than Gas Chromatography (GC) for quantifying minor components, by correlation with existing GC or other analytical data, biodiesel fuel quality can be assessed through the FTIR method. The FTIR method is easier and faster to use than GC [24]. Analyzing the spectra corresponding to biodiesel, the feedstock oil, and samples at intermediate conversion a method to control transesterification can be established [25]. Other studies in the literature also report the use of FTIR analysis for the determination and quantification of compounds. Nugrahani et al. [26] developed and validated a method for the simultaneous content determination of caffeine, paracetamol, and acetosal. The authors also report that this method exhibits some advantages as being faster, straightforward, efficient, economical, and environmentally friendly when compared to analytical methods such as high-performance liquid chromatography (HPLC). On the other hand, when applied to the analysis of biodiesel and its fuel blends, infrared spectroscopy can be used for adulteration control, monitoring of the biodiesel content in fuel blends, monitoring of the transesterification efficiency and classification [27]. Considering that few studies compared the FTIR and GC techniques, the present work aims to evaluate the use of two groups of basic heterogeneous catalysts, calcium-based and magnesium-based materials, in the methyl transesterification of soybean oil, applying the FTIR and GC methods for monitoring the transesterification reaction. The layered double hydroxides (LDH-CaAl and LDH-MgAl), their corresponding mixed oxides (CaAl and MgAl), and the pure oxides (CaO and MgO) were used as catalysts.
2.2. Catalysts characterization Thermogravimetric (TG) coupled with differential thermal analysis (DTA) was obtained in a TA Instruments, model SDT Q600, using 10 mg of sample under 100 mL min-1 of synthetic air flow and heating rate of 10 °C min-1 [29]. The X-ray diffraction patterns (XRD) were obtained through the powder method with a Bruker D2-Phaser diffractometer using Cu Kα radiation (λ = 1.5406 Å), 0.02° step, in the interval from 5 to 70° [28]. The specific surface area measurements of the catalysts were performed by nitrogen adsorption at −196 °C by the dynamic method. The sample amount used was approximately 100 mg. The specific area value was determined by the BET method [28]. Temperature-programmed desorption of CO2 (CO2-TPD) profiles were obtained in order to verify the presence of basic sites. The analyses were performed with 100 mg of calcined sample. First, the sample was degassed at 100 °C under He flow. Then it was saturated with CO2 (30 mL min-1) over a period of 60 min [28]. Desorption curves were recorded with a thermal conductivity detector (TCD), using a heating rate of 10 °C min-1. 2.3. Transesterification reactions The efficiency of uncalcined and calcined catalysts was evaluated in the transesterification reactions of soybean oil (locally purchased) with methyl alcohol (> 99%). The runs were performed in a bench apparatus, containing a glass reactor, operating in batch mode. Initially, the catalysts were pre-activated in the reactor by mixing with methanol under agitation for 30 min at room temperature. After this period, the reactor was loaded with soybean oil and heating was started. The operating conditions were 1 wt.% of catalyst relative to soybean oil, and methanol/soybean oil molar ratio 9:1. An excess of alcohol was used to shift the reaction equilibrium to the products and facilitate the separation of biodiesel/glycerol phases. When the reaction mixture reached 65 °C, the reaction was carried out during 4 h. At the end of the reaction, the mixture was immediately filtered by removing the solid catalyst and interrupting the reaction. The liquid mixture was maintained to stand for 1 h in a funnel of separation. The upper phase is composed of biodiesel and methanol and the bottom phase is a mixture of glycerin, methanol, and unreacted oil. The supernatant phase was subsequently analyzed. The standard biodiesel was prepared by homogeneous basic reaction in the Laboratory of Catalytic Processes (PROCAT), with the mixture of NaOH, methanol and soybean oil. The biodiesel content was determined by Fourier Transform Infrared spectroscopy (FTIR) in a Perkin-Elmer FTIR/NIR Frontier equipment. All infrared absorption spectra were obtained in the medium infrared region, in the range between 800 and 2000 cm-1. The
2. Material and methods 2.1. Catalysts preparation A set of layered double hydroxide type precursors of MgAl and CaAl with molar ratio 4:1 were synthesized by coprecipitation method as described previously [28,29]. Pure oxides of MgO and CaO were also prepared following this procedure. For calcium containing samples, the following aqueous solutions were prepared: Solution "A" composed of 2
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Fig. 1. Thermogravimetric analysis of the prepared samples: a) LDH-MgAl, b) MgO, c) LDH-CaAl, and d) CaO.
decomposition (200−300 °C). The third and fourth events can be attributed to the dehydroxylation and total decomposition of anion carbonates (350−420 °C and 500−600 °C) [33]. These losses were evidenced by the presence of endothermic peaks in the DTA curves. Fig. 1(d) shows the thermogram for the CaO sample. The DTA profile indicates the existence of two endothermic effects near 450 °C and 650 °C, which correspond to the removal of water and Ca(OH)2 molecules, and to the decomposition of CaCO3 in CaO and CO2, respectively [34,35]. Fig. 2(a) shows the diffraction patterns for the magnesium-based samples. The main diffraction peaks of the LDH-MgAl sample coincided with the standard diffractogram of hydrotalcite, similar to that reported by Yuan et al. [36]. Intense reflections were observed at low angles (11°, 24°, and 35°) and less intense reflections at high angles (38°, 46°, and 60°), these correspond to the hydrotalcite structure (JCPDS 41–1428). The diffractograms of the MgAl and MgO calcined samples presented predominance of cubic MgO (JCPDS 04-0829) [37]. The characteristic reflections of hydrotalcites disappeared in MgAl sample after calcination, indicating that the thermal treatment at 600 °C promotes the collapse and the decomposition of the hydrotalcite structure [38]. Fig. 2(b) illustrated the X-ray diffraction patterns of the calciumbased samples. The LDH-CaAl sample showed characteristic diffraction peaks of the hydrocalumite structure (JCPDS 50-0652) [39]. However, the presence of peaks corresponding to calcium hydroxide also was observed. The diffractogram of the CaO sample confirmed the formation of cubic CaO oxide, whereas CaAl showed the destruction of the hydrocalumite phase and the formation of a mixed oxide of Ca and Al, corresponding to Ca12Al14O33 (JCPDS-70-2144) and cubic CaO [40]. Table 1 presents the specific surface area (SBET) results obtained for the synthesized catalysts. The LDH samples had a specific area slightly higher than their mixed oxides, which can be attributed to the sintering
universal attenuated total reflectance (UATR) technique was used, which a multiple reflecting zinc selenide crystal was used. Each spectrum was obtained with a resolution of 4 cm-1 and 32 scans. Mixtures containing biodiesel and soybean oil were prepared in volumetric percentages in the range of 0–100% in 10% intervals for the construction of the reference curve of biodiesel by FTIR spectroscopy. The analyses were performed in triplicate. The percentage of biodiesel was calculated from the calibration curve. The fatty ester contents in biodiesel were also determined by GC. The analyses of GC were performed according to EN 14103, revision 2003, in a gas chromatograph GC 2010 Shimadzu. The quantification was made by internal standardization with methyl heptadecanoate (C17:0), one-point calibration and heptane solvent. 3. Results and discussion 3.1. Catalysts characterization The profiles of the thermogravimetric analysis for the Mg-based samples are shown in Fig. 1(a) and Fig. 1(b) for LDH-MgAl and MgO, respectively. Both samples showed two peaks of weight loss (black curve). The first loss in the range 100−250 °C, it can be attributed to the elimination of water from the interlayer, resulting from desorption of the OH- ions. The second loss between 350 and 470 °C may be due to the dehydroxylation and anions CO32- decomposition [30,31]. The presence of endothermic peaks in the DTA curves confirms these losses (blue curve), which is in agreement with typical results for hydrotalcites [32]. The total weight loss for the MgO and LDH-MgAl samples were 46.2% and 58.7%, respectively. For the LDH-CaAl sample (Fig. 1(c)), were identified four weight loss events. The first is related to the loss of physically adsorbed water (70−180 °C). The second indicates the start of dehydroxylation and 3
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revealed two peaks of desorption at low temperatures, which correspond to weak (ca. 200 °C) and medium (ca. 350 °C) basic sites. According to Di Serio et al. [41] and Albuquerque et al. [42], the desorption band from 127 to 427 °C could be assigned to the interaction of CO2 with basic sites of weak and medium strengths, mainly associated with Mg2+–O2− pairs. However, the band between 427 and 577 °C can be associated to Ca2+–O2− pairs, for which the basic strength is very high. The addition of Al caused a decrease in the total amount of sites, in comparison with the pure oxide [43,44]. These observations are in agreement with the results obtained in the present work, since both Ca and Mg samples showed a decrease in the amount of basic sites with the addition of Al. 3.2. Transesterification reactions The FTIR method was employed to determine the percentage of biodiesel in the transesterification reaction of the soybean oil. Samples were analyzed by Infrared mid-range in the region extending from 800 to 2000 cm-1 (MIR), covering the absorption bands characteristic of biodiesel (methyl ester) and soybean oil. According to Siatis et al. [45] and Mahamuni and Adewuyi [46] there are some regions, such as 1425−1447 cm-1, and 1188−1200 cm-1, where there is a slight difference in the spectra of soybean oil and biodiesel. These peaks are present in the biodiesel spectra but not in the oil spectra. There are other regions such as 1370−1400 cm-1 and 1075−1111 cm-1, that are present in soybean oil spectra but absent in biodiesel spectra. According to these authors, there are other regions such as 1700−1800 cm-1 corresponding to the C]O bond and 2800-3000 cm-1 corresponding to the CH stretching mode of olefins. The spectra of the biodiesel and soybean oil mixtures are shown in Fig. 3, where B0 is zero per cent biodiesel and B100 is 100% biodiesel [46]. From these data spectra, a calibration curve was constructed (Fig. 4), which related the integrated area of the band between 1427 and 1445 cm-1 with the biodiesel concentration. The conversion to methyl esters was obtained from the calibration equation of the biodiesel/soybean oil blends. Fig. 5 show the infrared spectra of the different catalysts. CaO and CaAl were the only catalysts that showed the peaks in the 1427−1445 cm-1 band, demonstrating that the samples with strong basic sites favor the transesterification reaction. The density and strength of basic sites can be verified through the data of the CO2-TPD in Table 1. Figs. 6 and 7 show the values of biodiesel yield, obtained by the FTIR and GC methods, for the different Ca-based or Mg-based catalysts, respectively. After 4 h of reaction, it was obtained a conversion at 99% and 95% for CaO and CaAl, respectively (Fig. 6). On the other hand, the LDH-CaAl sample provided a conversion of less than 10%. For the Mgbased catalysts (Fig. 7), a lower conversion values were obtained, reaching around 12%. Among them, the MgO catalyst presented a slightly better performance than the others. These values are in agreement with those reported in the literature. Concerning mixed oxides, Meng et al. [47] reported that calcined CaAl showed the highest activity with > 94% yield of fatty acid methyl esters when applied to
Fig. 2. XRD patterns of the synthesized catalyst: a) magnesium-based and b) calcium-based samples.
of the material during calcination, since localized overheating modifies the structure, resulting in surface loss. Moreover, it was demonstrated that the specific area of the catalysts with magnesium is higher than those of the calcium, independent of the phase considered. The results of basicity in the calcined samples and the maximum CO2-desorption temperature peaks (Tmax) are shown in Table 1. The quantification and strength of the sites were obtained by deconvolution using the Gaussian functions. The calcium-containing samples showed only one desorption peak at high temperature (Tmax above 400 °C), corresponding to strong basic sites. It is noticed that CaO exhibit a higher desorption temperature than CaAl, indicating a higher strength of basic sites. On the other hand, the magnesium-containing samples
Table 1 Specific surface area, density and strength of basic sites of the catalysts. Catalysts
CaO CaAl LDH-CaAl MgO MgAl LDH-MgAl
SBET(m2 g-1)
74 60 77 84 102 105
Density of basic sites (mmol g-1)
Temperature Max. (°C) 1st Peak
2nd Peak
– – n.d. 199 204 n.d.
– – n.d. 340 360 n.d.
3rd Peak 654 438 n.d. – – n.d.
4
Weak
Medium
Strong
Total
– – n.d. 0.03 0.03 n.d.
– – n.d. 0.12 0.08 n.d.
0.15 0.12 n.d. – – n.d.
0.15 0.12 n.d. 0.15 0.11 n.d.
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Fig. 7. Conversion of soybean oil to biodiesel by FTIR and GC method of analysis magnesium-based catalyst. Fig. 3. Infrared spectra of blends of biodiesel and soybean oil for calibration, where B0 is 0% biodiesel and B100 is 100% biodiesel. In detail, the range selected for analysis.
Hojjat [49] reported the application of different basic nanocatalysts for the transesterification of used cooking oil. They reported that the calcium oxide sample was the most appropriate, with high activity and stability in the biodiesel production. Gao et al. [50] reported that the layered double hydroxides (uncalcined Ca-Al-HT and Mg-Al-HT) present no catalytic activity for transesterification reaction. Sun et al. [51] described that the precursor Mg3Al1 presented low yield for biodiesel (approximately 18%), which occurred due to low alkalinity. Comparing the different phases of the Ca-based catalysts it can be noticed that the activity decreases as follows: CaO > CaAl > > LDHCaAl. The difference in biodiesel yield between CaO and CaAl may be related to the higher strength of the basic sites of CaO, since the difference in the amount of sites was small (Table 1). The same sequence was verified for Mg-based catalysts. Therefore, considering both groups, it can be generalized that the activity for methyl esters was: pure oxide > mixed oxide > LDH. Comparing the results obtained by the two methods (FTIR and GC) utilized to determine the methyl ester, as shown in Figs. 6 and 7, it was observed that the simplified method of FTIR analysis presented very close values; the smallest difference was 0.10% and the maximum was 1.30% compared to those obtained by the conventional gas chromatography. However, it is noteworthy that FTIR analysis is much simpler, faster, and more economical, since it is not necessary a sample preparation and it is not required internal standards. The CaO catalyst which provided a conversion close to 99% was the one that had the strong basic sites. On the other hand, comparing the biodiesel yield and CO2-TPD results of the CaO/CaAl with MgO/MgAl catalysts demonstrates that the nature of the basic sites is more important for the transesterification reaction than the amount of basic sites. The Ca-based catalyst showed stronger basic sites whereas the Mgbased catalyst presented weak and medium basic sites. The conversion results demonstrated that the activity for transesterification is independent of the specific area, however is related to the strength of basic sites present in the catalyst. Under the pre-established conditions, the calcium-based catalysts provided the highest conversions than the other catalysts tested. The results are in agreement with the results in the literature, because the transesterification activity is associated with the basicity—sites of medium and strong basicity are indicated to be the main active sites in transesterification [52].
Fig. 4. Calibration curve.
Fig. 5. Infrared spectrum of biodiesel obtained with the different catalysts.
4. Conclusion The CaO and CaAl catalysts presented significant activity in the reaction of transesterification of soybean oil, under the established conditions, reaching high conversions, close to 90%. Samples with magnesium presented showed poor yield to methyl ester. These results are mainly related to the strength of the basic sites present. In general, the activity for transesterification was: pure oxide > mixed oxide > > LDH for both groups of catalysts. The specific surface area of the catalysts showed no influence on the transesterification reaction. The infrared spectroscopy method employed for the determination of the methyl ester content has proved to be an alternative to
Fig. 6. Conversion of soybean oil to biodiesel by FTIR and GC method of analysis calcium-based catalyst.
the transesterification of rapeseed oil at 65 °C for 3 h and methanol:oil molar ratio of 15:1. Kawashima et al. [48] obtained yields of approximately 90% to methyl ester at 60 °C for 3 h using CaO. Nayebzadeh and 5
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chromatographic methods. The FTIR analysis is much simpler, faster, economical, and required no sample preparation or internal standards.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors acknowledge CAPES for the financial support granted to carry out this work. The authors also appreciate the analytical support by Chromatographic Analysis Laboratory from CIENTEC (Foundation for Science and Technology of Rio Grande do Sul State), Porto Alegre, Brazil. References [1] F. Ma, M.A. Hanna, Biodiesel production: a review 1, Bioresour. Technol. 70 (1999) 1–15. [2] J.M. Encinar, J.F. González, A. Rodríguez-Reinares, Ethanolysis of used frying oil. Biodiesel preparation and characterization, Fuel Process. Technol. 88 (2007) 513–522. [3] G. Knothe, Monitoring a progressing transesterification reaction by fiber-optic near infrared spectroscopy with correlation to 1 H nuclear magnetic resonance spectroscopy, J. Am. Oil Chem. Soc. 77 (2000) 489–493. [4] D.Y.C. Leung, X. Wu, M.K.H. Leung, A review on biodiesel production using catalyzed transesterification, Appl. Energy 87 (2010) 1083–1095. [5] I.M. Atadashi, M.K. Aroua, A.R. Abdul Aziz, N.M.N. Sulaiman, The effects of catalysts in biodiesel production: a review, J. Ind. Eng. Chem. 19 (2013) 14–26. [6] M.K. Lam, K.T. Lee, A.R. Mohamed, Homogeneous, heterogeneous and enzymatic catalysis for transesteri fi cation of high free fatty acid oil (waste cooking oil) to biodiesel : a review, Biotechnol. Adv. 28 (2010) 500–518. [7] Z. Helwani, M.R. Othman, N. Aziz, J. Kim, W.J.N. Fernando, Solid heterogeneous catalysts for transesterification of triglycerides with methanol : a review, Appl. Catal. A Gen. 363 (2009) 1–10. [8] K. Ramachandran, T. Suganya, N.N. Gandhi, S. Renganathan, Recent developments for biodiesel production by ultrasonic assist transesterification using different heterogeneous catalyst : a review, Renewable Sustainable Energy Rev. 22 (2013) 410–418. [9] M.R. Avhad, J.M. Marchetti, A review on recent advancement in catalytic materials for biodiesel production, Renewable Sustainable Energy Rev. 50 (2015) 696–718. [10] D.M. Marinković, M.V. Stanković, A.V. Veličković, J.M. Avramović, M.R. Miladinović, O.O. Stamenković, V.B. Veljković, D.M. Jovanović, Calcium oxide as a promising heterogeneous catalyst for biodiesel production: current state and perspectives, Renewable Sustainable Energy Rev. 56 (2016) 1387–1408. [11] R. Shan, C. Zhao, P. Lv, H. Yuan, J. Yao, Catalytic applications of calcium rich waste materials for biodiesel : current state and perspectives, Energy Convers. Manage. 127 (2016) 273–283. [12] H. Zeng, Z. Feng, X. Deng, Y. Li, Activation of Mg – Al hydrotalcite catalysts for transesterification of rape oil, Fuel. 87 (2008) 3071–3076. [13] H. Jeon, D.J. Kim, S.J. Kim, J.H. Kim, Synthesis of mesoporous MgO catalyst templated by a PDMS-PEO comb-like copolymer for biodiesel production, Fuel Process. Technol. 116 (2013) 325–331. [14] Y. Lu, Z. Zhang, Y. Xu, Q. Liu, G. Qian, CaFeAl mixed oxide derived heterogeneous catalysts for transesterification of soybean oil to biodiesel, Bioresour. Technol. 190 (2015) 438–441. [15] F.P. Sousa, G.P. Reis, C.C. Cardoso, W.N. Mussel, V.M.D. Pasa, Performance of CaO from different sources as a catalyst precursor in soybean oil transesterification: kinetics and leaching evaluation, J. Environ. Chem. Eng. 4 (2016) 1970–1977. [16] C.S. Castro, L.C.F. Garcia, J.M. Assaf, The enhanced activity of Ca/MgAl mixed oxide for transesterification, Fuel Process. Technol. 125 (2014) 73–78. [17] H. Nayebzadeh, N. Saghatoleslami, M. Haghighi, M. Tabasizadeh, Influence of fuel type on microwave-enhanced fabrication of KOH/Ca12Al14O33 nanocatalyst for biodiesel production via microwave heating, J. Taiwan Inst. Chem. Eng. 75 (2017) 148–155, https://doi.org/10.1016/j.jtice.2017.03.018. [18] H. Nayebzadeh, M. Haghighi, N. Saghatoleslami, M. Tabasizadeh, E. Binaeian, Influence of carbon source content on the structure and performance of KOH/ Ca12Al14O33–C nanocatalyst used in the transesterification reaction via microwave irradiation, J. Alloys. Compd. 743 (2018) 672–681, https://doi.org/10.1016/ j.jallcom.2018.01.357. [19] F. Cavani, F. Trifirò, A. Vaccari, Hydrotalcite-type anionic clays: preparation, properties and applications, Catal. Today 11 (1991) 173–301. [20] W. Yang, Y. Kim, P.K.T. Liu, M. Sahimi, T.T. Tsotsis, A study by in situ techniques of the thermal evolution of the structure of a Mg–Al–CO3 layered double hydroxide, Chem. Eng. Sci. 57 (2002) 2945–2953. [21] W.T. Reichle, Synthesis of anionic clay minerals (mixed metal hydroxides, hydrotalcite), Solid State Ion. 22 (1986) 135–141. [22] T. Issariyakul, A.K. Dalai, Biodiesel from vegetable oils, Renewable Sustainable Energy Rev. 31 (2014) 446–471. [23] A. Bouyanfif, S. Liyanage, E. Hequet, N. Moustaid-Moussa, N. Abidi, Review of FTIR microspectroscopy applications to investigate biochemical changes in C. Elegans,
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Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec
FTIR spectroscopy analysis for monitoring biodiesel production by heterogeneous catalyst
T
Morgana Rosset, Oscar W. Perez-Lopez* Laboratory of Catalytic Processes―PROCAT, Department of Chemical Engineering, Federal University of Rio Grande do Sul (UFRGS), Rua Ramiro Barcelos, 2777. CEP, 90035-007, Porto Alegre, RS, Brazil
A R T I C LE I N FO
A B S T R A C T
Keywords: Biodiesel Transesterification Heterogeneous catalysis Basic strength FTIR spectroscopy
Layered double hydroxides, mixed oxides, and pure oxides based on magnesium and calcium were evaluated as catalysts in the methyl transesterification of soybean oil in a batch reactor. The catalysts were prepared by continuous co-precipitation and characterized by specific surface area, thermogravimetric analysis, temperatureprogrammed desorption of CO2, and X-ray diffraction. The reactions were performed at 65 °C, methanol-to-oil molar ratio 9:1, catalyst dosage of 1 wt.%, and reaction time of 240 min. The biodiesel analysis was carried out by Fourier Transform Infrared spectroscopy (FTIR) and Gas Chromatography (GC). Calcium-based catalyst, pure oxide (CaO) and mixed oxide (CaAl), demonstrated higher activity for methyl transesterification than magnesium-based catalyst. Considering both groups of catalysts, the activity for transesterification was: pure oxide > mixed oxide > > LDH. These results were related to the strength of the basic sites. FTIR and GC results were similar, with the advantage of FTIR analysis being simple, fast, and economical.
1. Introduction Biodiesel is a biofuel made from renewable biomass for use in engines, and can partially or totally replace fossil fuels, like diesel. The biodiesel or fatty acid alkyl ester can be produced from the transesterification reaction of triglycerides using alcohol in the presence of a catalyst. Due to the reversibility of the reaction, an excess of alcohol is used to shift the equilibrium for the products side [1,2]. Several species of vegetable oils and animal fats, such as soybean, canola, castor oil, cotton, bovine tallow, in addition to residual oil can be employed as a raw material in the production of biodiesel. The biodiesel produced in the industry is obtained by the homogeneous transesterification reaction of triglycerides using basic catalysts, such as sodium and potassium hydroxides, carbonates, and alkoxides [1,3]. However, in the conventional industrial process, removal of the catalyst is technically difficult and a large amount of wastewater is produced to separate and to clean the catalyst and the product [4,5]. According to Lam et al. [6], the advantages of the application of heterogeneous catalysts are due to performance improvements in the presence of free fatty acids, unmixing the catalysts with biodiesel, and recovering and reuse of it. Moreover, it reduces the costs of separation since it eliminates the biodiesel washing step and the maintenance problems of the equipment because these catalysts are not corrosive. The transesterification with heterogeneous catalysts has been ⁎
extensively investigated in recent years and several materials have been studied as catalysts for biodiesel production. Catalysts with basic properties include pure oxides (MgO, CaO), mixed oxides (MgFe, CaAl, LiAl), supported (Li/CaO, Ca/Al2O3, Cs/SiO2) and layered double hydroxides (CeMgAl, MgAlFe, ZnAl, CaAl, MgAl) [7–9]. Among these, calcium oxide is widely studied as a heterogeneous catalyst because it ensures high basicity, low solubility, and easy manipulation [10,11]. Zeng et al. [12] achieved a conversion of 90.5% for transesterification of canola oil with MgAl hydrotalcites as catalyst, with the molar ratio 6:1 of methanol/oil, 1.5 wt.% catalyst, temperature 65 °C and 4 h of reaction. Jeon et al. [13] utilized mesoporous MgO as a heterogeneous solid catalyst to produce biodiesel from canola oil, the volume ratio of methanol to canola oil was 20:3, the MgO catalyst loading was 3 wt.%, and the transesterification reaction was performed at 190 °C for 2 h. Lu et al. [14] studied CaFeAl layered double oxides in biodiesel production, where the catalytic activity reached more than 90% biodiesel yield, under the conditions of 12:1 methanol/oil, temperature of 60 °C, 60 min of reaction and 6 wt.% of catalyst. Besides, Sousa et al. [15] showed that the soybean oil transesterification catalyzed with CaO yielded greater than 93% (wt/wt) methyl esters, 4 h reflux, methanolto-oil molar ratio of 12 and 3 wt.% catalyst. Castro et al. [16] used the model of transesterification reaction between methyl acetate and ethanol under mild reaction conditions: ethanol/methyl acetate molar ratio 6:1, catalyst concentration of 4 wt.%, and temperature of 50 °C.
Corresponding author. E-mail address: [email protected] (O.W. Perez-Lopez).
https://doi.org/10.1016/j.vibspec.2019.102990 Received 29 June 2019; Received in revised form 30 September 2019; Accepted 8 November 2019 Available online 09 November 2019 0924-2031/ © 2019 Elsevier B.V. All rights reserved.
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the mixture of Ca(NO3)2.4H2O and Al(NO3)3.9H2O and solution "B" containing NaOH. To obtain CaO, solution "A" contains only Ca (NO3)2.4H2O. For magnesium containing samples, the solution "A" was composed of the mixture of Mg(NO3)2.6H2O and Al(NO3)3.9H2O and solution "B" by the mixture of NaOH and Na2CO3, whereas for the synthesis of MgO the solution "A" is composed of Mg(NO3)2.6H2O. Then solution A and solution B were slowly mixed with constant stirring at a temperature of 50 °C. The pH of the mixture was maintained at 10 (magnesium-based) and 12 (calcium-based). After coprecipitation, the material was crystallized under stirring for 1 h at 50 °C. The precipitate formed was filtered and washed with deionized water to remove the ions, and then dried in oven at 80 °C at night. To obtain the oxides the solids were calcined in a tubular quartz reactor with synthetic air at a flow rate of 50 mL min-1 at 600 °C for 6 h for Mg-based samples, and at 800 °C for 4 h for Ca-based samples. The catalysts were named LDHCaAl (hydrocalumite) and LDH-MgAl (hydrotalcite) for uncalcined samples, calcined CaAl and MgAl mixed oxides, and calcined pure oxides CaO and MgO, respectively.
According to the authors, MgAl mixed oxide was inactive for transesterification. On the other hand, the Ca addition to the oxides considerably increased the amount and strength of basic sites of Ca/MgAl, greatly improving their catalytic performance. Nayebzadeh et al. [17,18] studied the application of basic catalyst KOH/Ca12Al14O33 in the transesterification reaction via microwave irradiation to produce biodiesel from canola oil. They were able to achieve conversions ranging from 83% to 99%, depending on the operating conditions as methanol-to-oil molar ratio, catalyst dosage and reaction time. Layered double hydroxides (LDH), also known as hydrotalcites, are 3+ x+ nanionic clays that have general formula [M2+ Ax/n⋅yH2O, 1-x Mx (OH)2] 2+ 2+ 2+ where M corresponds to divalent (Mg , Ca or Zn ) and trivalent (Al3+ or Fe3+) metallic cations, An- represents an inorganic or organic anion (CO32- or OH-) and x is the molar ratio of cations [19]. The hydrocalumite has a similar structure to hydrotalcite, but Ca and Al are octahedrally coordinated in the layers. The thermal treatment of layered double hydroxides above 350 °C causes the layered structure to collapse, thereby obtaining the mixed oxides [20]. The combination and ratio of metallic ions influence the structure and chemical properties of the LDH and their respective mixed oxides [21]. Various techniques of analysis have been developed to evaluate the transesterification reaction. Chromatography techniques are usually used because they offer comprehensive perception during transesterification process and detailed information required for quality control of the product. However, recently Nuclear Magnetic Resonance (NMR) spectroscopy and Fourier Transform Infrared (FTIR) spectrometry have been employed for monitoring transesterification [22]. FTIR spectroscopy is considered a powerful technique that can be used to determine the chemical structure and composition of various materials, including biological samples [23]. Although the FTIR method is less sensitive than Gas Chromatography (GC) for quantifying minor components, by correlation with existing GC or other analytical data, biodiesel fuel quality can be assessed through the FTIR method. The FTIR method is easier and faster to use than GC [24]. Analyzing the spectra corresponding to biodiesel, the feedstock oil, and samples at intermediate conversion a method to control transesterification can be established [25]. Other studies in the literature also report the use of FTIR analysis for the determination and quantification of compounds. Nugrahani et al. [26] developed and validated a method for the simultaneous content determination of caffeine, paracetamol, and acetosal. The authors also report that this method exhibits some advantages as being faster, straightforward, efficient, economical, and environmentally friendly when compared to analytical methods such as high-performance liquid chromatography (HPLC). On the other hand, when applied to the analysis of biodiesel and its fuel blends, infrared spectroscopy can be used for adulteration control, monitoring of the biodiesel content in fuel blends, monitoring of the transesterification efficiency and classification [27]. Considering that few studies compared the FTIR and GC techniques, the present work aims to evaluate the use of two groups of basic heterogeneous catalysts, calcium-based and magnesium-based materials, in the methyl transesterification of soybean oil, applying the FTIR and GC methods for monitoring the transesterification reaction. The layered double hydroxides (LDH-CaAl and LDH-MgAl), their corresponding mixed oxides (CaAl and MgAl), and the pure oxides (CaO and MgO) were used as catalysts.
2.2. Catalysts characterization Thermogravimetric (TG) coupled with differential thermal analysis (DTA) was obtained in a TA Instruments, model SDT Q600, using 10 mg of sample under 100 mL min-1 of synthetic air flow and heating rate of 10 °C min-1 [29]. The X-ray diffraction patterns (XRD) were obtained through the powder method with a Bruker D2-Phaser diffractometer using Cu Kα radiation (λ = 1.5406 Å), 0.02° step, in the interval from 5 to 70° [28]. The specific surface area measurements of the catalysts were performed by nitrogen adsorption at −196 °C by the dynamic method. The sample amount used was approximately 100 mg. The specific area value was determined by the BET method [28]. Temperature-programmed desorption of CO2 (CO2-TPD) profiles were obtained in order to verify the presence of basic sites. The analyses were performed with 100 mg of calcined sample. First, the sample was degassed at 100 °C under He flow. Then it was saturated with CO2 (30 mL min-1) over a period of 60 min [28]. Desorption curves were recorded with a thermal conductivity detector (TCD), using a heating rate of 10 °C min-1. 2.3. Transesterification reactions The efficiency of uncalcined and calcined catalysts was evaluated in the transesterification reactions of soybean oil (locally purchased) with methyl alcohol (> 99%). The runs were performed in a bench apparatus, containing a glass reactor, operating in batch mode. Initially, the catalysts were pre-activated in the reactor by mixing with methanol under agitation for 30 min at room temperature. After this period, the reactor was loaded with soybean oil and heating was started. The operating conditions were 1 wt.% of catalyst relative to soybean oil, and methanol/soybean oil molar ratio 9:1. An excess of alcohol was used to shift the reaction equilibrium to the products and facilitate the separation of biodiesel/glycerol phases. When the reaction mixture reached 65 °C, the reaction was carried out during 4 h. At the end of the reaction, the mixture was immediately filtered by removing the solid catalyst and interrupting the reaction. The liquid mixture was maintained to stand for 1 h in a funnel of separation. The upper phase is composed of biodiesel and methanol and the bottom phase is a mixture of glycerin, methanol, and unreacted oil. The supernatant phase was subsequently analyzed. The standard biodiesel was prepared by homogeneous basic reaction in the Laboratory of Catalytic Processes (PROCAT), with the mixture of NaOH, methanol and soybean oil. The biodiesel content was determined by Fourier Transform Infrared spectroscopy (FTIR) in a Perkin-Elmer FTIR/NIR Frontier equipment. All infrared absorption spectra were obtained in the medium infrared region, in the range between 800 and 2000 cm-1. The
2. Material and methods 2.1. Catalysts preparation A set of layered double hydroxide type precursors of MgAl and CaAl with molar ratio 4:1 were synthesized by coprecipitation method as described previously [28,29]. Pure oxides of MgO and CaO were also prepared following this procedure. For calcium containing samples, the following aqueous solutions were prepared: Solution "A" composed of 2
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Fig. 1. Thermogravimetric analysis of the prepared samples: a) LDH-MgAl, b) MgO, c) LDH-CaAl, and d) CaO.
decomposition (200−300 °C). The third and fourth events can be attributed to the dehydroxylation and total decomposition of anion carbonates (350−420 °C and 500−600 °C) [33]. These losses were evidenced by the presence of endothermic peaks in the DTA curves. Fig. 1(d) shows the thermogram for the CaO sample. The DTA profile indicates the existence of two endothermic effects near 450 °C and 650 °C, which correspond to the removal of water and Ca(OH)2 molecules, and to the decomposition of CaCO3 in CaO and CO2, respectively [34,35]. Fig. 2(a) shows the diffraction patterns for the magnesium-based samples. The main diffraction peaks of the LDH-MgAl sample coincided with the standard diffractogram of hydrotalcite, similar to that reported by Yuan et al. [36]. Intense reflections were observed at low angles (11°, 24°, and 35°) and less intense reflections at high angles (38°, 46°, and 60°), these correspond to the hydrotalcite structure (JCPDS 41–1428). The diffractograms of the MgAl and MgO calcined samples presented predominance of cubic MgO (JCPDS 04-0829) [37]. The characteristic reflections of hydrotalcites disappeared in MgAl sample after calcination, indicating that the thermal treatment at 600 °C promotes the collapse and the decomposition of the hydrotalcite structure [38]. Fig. 2(b) illustrated the X-ray diffraction patterns of the calciumbased samples. The LDH-CaAl sample showed characteristic diffraction peaks of the hydrocalumite structure (JCPDS 50-0652) [39]. However, the presence of peaks corresponding to calcium hydroxide also was observed. The diffractogram of the CaO sample confirmed the formation of cubic CaO oxide, whereas CaAl showed the destruction of the hydrocalumite phase and the formation of a mixed oxide of Ca and Al, corresponding to Ca12Al14O33 (JCPDS-70-2144) and cubic CaO [40]. Table 1 presents the specific surface area (SBET) results obtained for the synthesized catalysts. The LDH samples had a specific area slightly higher than their mixed oxides, which can be attributed to the sintering
universal attenuated total reflectance (UATR) technique was used, which a multiple reflecting zinc selenide crystal was used. Each spectrum was obtained with a resolution of 4 cm-1 and 32 scans. Mixtures containing biodiesel and soybean oil were prepared in volumetric percentages in the range of 0–100% in 10% intervals for the construction of the reference curve of biodiesel by FTIR spectroscopy. The analyses were performed in triplicate. The percentage of biodiesel was calculated from the calibration curve. The fatty ester contents in biodiesel were also determined by GC. The analyses of GC were performed according to EN 14103, revision 2003, in a gas chromatograph GC 2010 Shimadzu. The quantification was made by internal standardization with methyl heptadecanoate (C17:0), one-point calibration and heptane solvent. 3. Results and discussion 3.1. Catalysts characterization The profiles of the thermogravimetric analysis for the Mg-based samples are shown in Fig. 1(a) and Fig. 1(b) for LDH-MgAl and MgO, respectively. Both samples showed two peaks of weight loss (black curve). The first loss in the range 100−250 °C, it can be attributed to the elimination of water from the interlayer, resulting from desorption of the OH- ions. The second loss between 350 and 470 °C may be due to the dehydroxylation and anions CO32- decomposition [30,31]. The presence of endothermic peaks in the DTA curves confirms these losses (blue curve), which is in agreement with typical results for hydrotalcites [32]. The total weight loss for the MgO and LDH-MgAl samples were 46.2% and 58.7%, respectively. For the LDH-CaAl sample (Fig. 1(c)), were identified four weight loss events. The first is related to the loss of physically adsorbed water (70−180 °C). The second indicates the start of dehydroxylation and 3
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revealed two peaks of desorption at low temperatures, which correspond to weak (ca. 200 °C) and medium (ca. 350 °C) basic sites. According to Di Serio et al. [41] and Albuquerque et al. [42], the desorption band from 127 to 427 °C could be assigned to the interaction of CO2 with basic sites of weak and medium strengths, mainly associated with Mg2+–O2− pairs. However, the band between 427 and 577 °C can be associated to Ca2+–O2− pairs, for which the basic strength is very high. The addition of Al caused a decrease in the total amount of sites, in comparison with the pure oxide [43,44]. These observations are in agreement with the results obtained in the present work, since both Ca and Mg samples showed a decrease in the amount of basic sites with the addition of Al. 3.2. Transesterification reactions The FTIR method was employed to determine the percentage of biodiesel in the transesterification reaction of the soybean oil. Samples were analyzed by Infrared mid-range in the region extending from 800 to 2000 cm-1 (MIR), covering the absorption bands characteristic of biodiesel (methyl ester) and soybean oil. According to Siatis et al. [45] and Mahamuni and Adewuyi [46] there are some regions, such as 1425−1447 cm-1, and 1188−1200 cm-1, where there is a slight difference in the spectra of soybean oil and biodiesel. These peaks are present in the biodiesel spectra but not in the oil spectra. There are other regions such as 1370−1400 cm-1 and 1075−1111 cm-1, that are present in soybean oil spectra but absent in biodiesel spectra. According to these authors, there are other regions such as 1700−1800 cm-1 corresponding to the C]O bond and 2800-3000 cm-1 corresponding to the CH stretching mode of olefins. The spectra of the biodiesel and soybean oil mixtures are shown in Fig. 3, where B0 is zero per cent biodiesel and B100 is 100% biodiesel [46]. From these data spectra, a calibration curve was constructed (Fig. 4), which related the integrated area of the band between 1427 and 1445 cm-1 with the biodiesel concentration. The conversion to methyl esters was obtained from the calibration equation of the biodiesel/soybean oil blends. Fig. 5 show the infrared spectra of the different catalysts. CaO and CaAl were the only catalysts that showed the peaks in the 1427−1445 cm-1 band, demonstrating that the samples with strong basic sites favor the transesterification reaction. The density and strength of basic sites can be verified through the data of the CO2-TPD in Table 1. Figs. 6 and 7 show the values of biodiesel yield, obtained by the FTIR and GC methods, for the different Ca-based or Mg-based catalysts, respectively. After 4 h of reaction, it was obtained a conversion at 99% and 95% for CaO and CaAl, respectively (Fig. 6). On the other hand, the LDH-CaAl sample provided a conversion of less than 10%. For the Mgbased catalysts (Fig. 7), a lower conversion values were obtained, reaching around 12%. Among them, the MgO catalyst presented a slightly better performance than the others. These values are in agreement with those reported in the literature. Concerning mixed oxides, Meng et al. [47] reported that calcined CaAl showed the highest activity with > 94% yield of fatty acid methyl esters when applied to
Fig. 2. XRD patterns of the synthesized catalyst: a) magnesium-based and b) calcium-based samples.
of the material during calcination, since localized overheating modifies the structure, resulting in surface loss. Moreover, it was demonstrated that the specific area of the catalysts with magnesium is higher than those of the calcium, independent of the phase considered. The results of basicity in the calcined samples and the maximum CO2-desorption temperature peaks (Tmax) are shown in Table 1. The quantification and strength of the sites were obtained by deconvolution using the Gaussian functions. The calcium-containing samples showed only one desorption peak at high temperature (Tmax above 400 °C), corresponding to strong basic sites. It is noticed that CaO exhibit a higher desorption temperature than CaAl, indicating a higher strength of basic sites. On the other hand, the magnesium-containing samples
Table 1 Specific surface area, density and strength of basic sites of the catalysts. Catalysts
CaO CaAl LDH-CaAl MgO MgAl LDH-MgAl
SBET(m2 g-1)
74 60 77 84 102 105
Density of basic sites (mmol g-1)
Temperature Max. (°C) 1st Peak
2nd Peak
– – n.d. 199 204 n.d.
– – n.d. 340 360 n.d.
3rd Peak 654 438 n.d. – – n.d.
4
Weak
Medium
Strong
Total
– – n.d. 0.03 0.03 n.d.
– – n.d. 0.12 0.08 n.d.
0.15 0.12 n.d. – – n.d.
0.15 0.12 n.d. 0.15 0.11 n.d.
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Fig. 7. Conversion of soybean oil to biodiesel by FTIR and GC method of analysis magnesium-based catalyst. Fig. 3. Infrared spectra of blends of biodiesel and soybean oil for calibration, where B0 is 0% biodiesel and B100 is 100% biodiesel. In detail, the range selected for analysis.
Hojjat [49] reported the application of different basic nanocatalysts for the transesterification of used cooking oil. They reported that the calcium oxide sample was the most appropriate, with high activity and stability in the biodiesel production. Gao et al. [50] reported that the layered double hydroxides (uncalcined Ca-Al-HT and Mg-Al-HT) present no catalytic activity for transesterification reaction. Sun et al. [51] described that the precursor Mg3Al1 presented low yield for biodiesel (approximately 18%), which occurred due to low alkalinity. Comparing the different phases of the Ca-based catalysts it can be noticed that the activity decreases as follows: CaO > CaAl > > LDHCaAl. The difference in biodiesel yield between CaO and CaAl may be related to the higher strength of the basic sites of CaO, since the difference in the amount of sites was small (Table 1). The same sequence was verified for Mg-based catalysts. Therefore, considering both groups, it can be generalized that the activity for methyl esters was: pure oxide > mixed oxide > LDH. Comparing the results obtained by the two methods (FTIR and GC) utilized to determine the methyl ester, as shown in Figs. 6 and 7, it was observed that the simplified method of FTIR analysis presented very close values; the smallest difference was 0.10% and the maximum was 1.30% compared to those obtained by the conventional gas chromatography. However, it is noteworthy that FTIR analysis is much simpler, faster, and more economical, since it is not necessary a sample preparation and it is not required internal standards. The CaO catalyst which provided a conversion close to 99% was the one that had the strong basic sites. On the other hand, comparing the biodiesel yield and CO2-TPD results of the CaO/CaAl with MgO/MgAl catalysts demonstrates that the nature of the basic sites is more important for the transesterification reaction than the amount of basic sites. The Ca-based catalyst showed stronger basic sites whereas the Mgbased catalyst presented weak and medium basic sites. The conversion results demonstrated that the activity for transesterification is independent of the specific area, however is related to the strength of basic sites present in the catalyst. Under the pre-established conditions, the calcium-based catalysts provided the highest conversions than the other catalysts tested. The results are in agreement with the results in the literature, because the transesterification activity is associated with the basicity—sites of medium and strong basicity are indicated to be the main active sites in transesterification [52].
Fig. 4. Calibration curve.
Fig. 5. Infrared spectrum of biodiesel obtained with the different catalysts.
4. Conclusion The CaO and CaAl catalysts presented significant activity in the reaction of transesterification of soybean oil, under the established conditions, reaching high conversions, close to 90%. Samples with magnesium presented showed poor yield to methyl ester. These results are mainly related to the strength of the basic sites present. In general, the activity for transesterification was: pure oxide > mixed oxide > > LDH for both groups of catalysts. The specific surface area of the catalysts showed no influence on the transesterification reaction. The infrared spectroscopy method employed for the determination of the methyl ester content has proved to be an alternative to
Fig. 6. Conversion of soybean oil to biodiesel by FTIR and GC method of analysis calcium-based catalyst.
the transesterification of rapeseed oil at 65 °C for 3 h and methanol:oil molar ratio of 15:1. Kawashima et al. [48] obtained yields of approximately 90% to methyl ester at 60 °C for 3 h using CaO. Nayebzadeh and 5
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chromatographic methods. The FTIR analysis is much simpler, faster, economical, and required no sample preparation or internal standards.
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