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Synergy between sodium carbonate and sodium titanate nanotubes in the transesterification of soybean oil with methanol
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Synergy between sodium carbonate and sodium titanate nanotubes in the transesterification of soybean oil with methanol Mark E. Martínez-Klimov, Pamela Ramírez-Vidal, Pedro Roquero Tejeda, Tatiana E. Klimova
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Laboratorio de Nanocatálisis, Departamento de Ingeniería Química, Facultad de Química, Universidad Nacional Autónoma de México (UNAM), Cd. Universitaria, Coyoacán, Ciudad de México, 04510, Mexico
A R T I C LE I N FO
A B S T R A C T
Keywords: Sodium titanate nanotubes Sodium carbonate Synergetic effect Transesterification reaction Soybean oil Biodiesel
Sodium titanate nanotubes (STN) modified by the addition of sodium carbonate (3–10 wt. %) were tested as catalysts for the transesterification of soybean oil with methanol giving good results. Prepared catalysts were characterized by N2 physisorption, X-ray diffraction, FT-IR, temperature-programmed desorption of CO2, scanning and transmission electron microscopy. The STN catalysts modified with sodium carbonate were significantly more active in the transesterification reaction than the reference catalyst containing 5 wt. % of Na2CO3 on alumina. The most active catalyst, STN with 10 wt. % of Na2CO3, resulted in the 97% yield of methyl esters at a short reaction time (30 min) in mild conditions (80 °C). High activity of the prepared catalysts was attributed to a synergetic effect between the support and the deposited sodium carbonate leading to a noticeable increase in the amount of strong basic sites able to generate methoxide anions required for the transformation of triglycerides to methyl esters. According to the FT-IR characterization, the above synergy was attributed to the presence of different types of carbonate species in the alumina and STN-supported catalysts. Upon reutilization, Na2CO3-containing STN catalysts showed a decrease in their activity due to leaching of the active phase in the reaction media.
1. Introduction Nowadays, a clear increase in the requirements for high quality gasoline and diesel fuels is observed all over the world. The use of only petroleum-derived fuels cannot satisfy this demand, since fossil fuel resources are limited and petroleum price continually rises. One promising approach to mitigate global energy problems is the use of biofuels, such as bioethanol and biodiesel, as abundant and environmentally friendly energy sources. Biodiesel is a non-petroleum based, alternative diesel fuel that is relatively clean burning, non-toxic, biodegradable and renewable [1–4]. The most common method for producing biodiesel is via transesterification of vegetable oils and animal fats with short-chain alcohols (mainly methanol) in the presence of homogeneous base catalysts (NaOH, KOH or methoxides) leading to the formation of fatty acid methyl esters (FAME) [3,5]. In this case, biodiesel production includes different steps of separation and purification, which, in addition to the difficulty of removal of the homogeneous catalyst after the reaction for its possible reutilization, results in a high cost of the obtained biodiesel [6]. Different approaches were proposed to improve the efficiency of the production of biodiesel. Due to the immiscible nature of methanol and oil, the transesterification reaction
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can be accelerated by increasing the interfacial contact area using vigorous stirring, ultrasonic or hydrodynamic cavitation [7] or by using of novel Pickering interfacial catalysts (PICs) for transestrification which combine the advantages of Pickering emulsion and heterogeneous catalysts, enhancing the liquid-liquid biphasic reactions [8,9]. The replacement of the conventional homogeneous catalysts by highlyactive heterogeneous ones is also an environmentally friendly option that allows simplification of the existing processes [10]. Many different solids have been tested in transesterification, such as unsupported sodium and potassium carbonates and phosphates [10], as well as a number of supported alkali and alkaline earth metals, metal oxides, salts and hydroxides: K2CO3 supported on alumina [11] or alumina/ silica [12], Na/NaOH/γ-Al2O3 [13], KNO3/Al2O3 [14], K2CO3, NaOH and KOH supported on MgO [15–17], sodium and potassium phosphates and carbonates fixed to the silica and alumina supports [6], CaO/Al2O3 [18], etc. Most of these catalysts are based on the employment of alkali or alkaline earth metal compounds having strong basic sites. It was noted that in some cases, the deposition of the active basic salt (sodium or potassium phosphate) on the support's surface results in the destruction of its catalytic activity [6]. Also, some of the supports (silica, for instance) were found to be unstable in the
Corresponding author. E-mail address: [email protected] (T.E. Klimova).
https://doi.org/10.1016/j.cattod.2019.08.027 Received 10 April 2019; Received in revised form 31 July 2019; Accepted 27 August 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Mark E. Martínez-Klimov, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.08.027
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2 h, then cooled to room temperature and flushed with a CO2/He gas mixture for 30 min followed by purging of the remaining CO2 with He flow. The CO2 desorption was performed from room temperature to 600 °C at a constant heating rate of 10 °C/min. The amount of desorbed CO2 was determined by mass spectrometry following the MS intensity for CO2 (m/e = 44) as a function of temperature. Basicity was estimated based on the amount of desorbed CO2 assuming that one CO2 molecule was desorbed from one basic site present on the catalyst's surface. The temperature required to release adsorbed CO2 molecules characterized the base strength.
transesterification reactions due to partial dissolution produced by strongly basic species [6]. Another problem of the supported metal salts and hydroxides is leaching of the active sites from the catalyst's surface upon reaction conditions. Therefore, a development of novel stable heterogeneous base catalysts for biodiesel synthesis nowadays continues being an actual task. Recently, in our group, sodium titanate nanotubes (STN) were tested as catalysts for biodiesel production from vegetable oil and methanol giving good results (97–100 % yields of fatty acid methyl esters at 8 h reaction time, methanol reflux temperature, methanol:oil molar ratio of 40:1, 1–2 wt. % of the catalyst) [19]. The advantages of these solids are that they are stable in basic reaction medium, active in mild conditions and do not require thermal pre-treatment before the transesterification reaction [19,20]. In the present work, we modified sodium titanate nanotubes by the addition of different amounts of sodium carbonate in order to increase their basicity. Prepared catalysts were characterized and tested in the transesterification of soybean oil with methanol. The possible reutilization of the catalysts was also investigated.
2.3. Catalytic tests The transesterification reactions were performed in a 100 mL stainless steel batch reactor under constant stirring (600 rpm), at 80 °C and 30 min reaction time. Soybean oil (27 g) and methanol (19.9 g, CH3OH to soybean oil molar ratio = 20:1) were added together with 0.5 g of catalyst (1 wt. %) into the reactor. After the reaction, the solid catalyst was separated from the reaction mixture by centrifugation (5000 rpm, 5 min). The biodiesel yield was calculated on the basis of 1H NMR characterization of the obtained reaction product [22]. Kinematic viscosity of the obtained biodiesel was also determined after complete removal of residual methanol, using Cannon-Fenske viscosimeters at 40 °C according to the standard ASTM D445 method [23].
2. Experimental 2.1. Catalyst preparation Sodium titanate nanotubes (STN) were prepared by the Kasuga method [21] using TiO2 anatase nanopowder as a precursor. In a typical synthesis, 10 g of TiO2 precursor were added to 300 mL of a 10 M NaOH solution in a Teflon-lined autoclave. The obtained mixture was maintained at 140 °C for 20 h upon magnetic stirring. After the hydrothermal reaction was over, the obtained white product (Na2Ti3O7∙xH2O, sodium titanate nanotubes) was filtered in vacuum, washed several times with deionized water to eliminate the excess of non-reacted sodium hydroxide and dried at 120 °C for 12 h. Catalysts with Na2CO3 loadings of 3, 5 and 10 wt. % were prepared by incipient wetness impregnation using aqueous solutions of sodium carbonate. After the impregnation of sodium salt, the catalysts were dried at 120 °C for 12 h. Hereinafter, the catalysts will be denoted as x-STN, where x is the nominal Na2CO3 loading (wt. %) in the samples. For comparison purposes, the reference catalyst containing 5 wt. % of Na2CO3 impregnated on an alumina support (5-Al2O3) was also prepared.
3. Results and discussion 3.1. Characterization of x-STN catalysts Chemical composition of the STN and x-STN catalysts was determined by SEM-EDX (Table 1). Sodium content in the starting sodium titanate nanotubes was 8.99 wt. %. The x-STN catalysts had higher Na loadings (10.12–13.16 wt. %) than the STN support due to the addition of different amounts of sodium carbonate. Based on the amount of extra-sodium in the x-STN catalysts (excess of Na compared to the Na content in the starting support), the amount of Na2CO3 in the samples was estimated. The experimentally obtained values were close to the theoretically expected ones (3, 5 and 10 wt. %), which confirms that the prepared catalysts had the expected chemical composition. The morphology of the prepared samples was inspected by HRTEM. The micrographs obtained for the STN support, 3-STN and 5-STN catalysts are shown in Fig. 1. The presence of randomly distributed nanotubular structures can be observed in the images of all samples. The nanotubes had approximately 3–6 nm internal diameters, 9–12 nm external diameters and 200–500 nm lengths. The incorporation of sodium carbonate to the STN material did not produce changes in its morphology. In addition, no particles formed by the agglomerated Na2CO3 were detected. Nitrogen adsorption-desorption isotherms of the STN support and x-
2.2. Catalyst characterization The STN support and x-STN catalysts were characterized by N2 physisorption, powder X-ray diffraction (XRD), Fourier-transformed infrared spectroscopy (FT-IR), temperature-programmed desorption of CO2 (CO2-TPD), scanning and transmission electron microscopy. Nitrogen adsorption-desorption isotherms were measured with an ASAP 2020 automatic analyzer (Micromeritics) at liquid N2 temperature (-197.5 °C). Prior to the experiments, the samples were degassed at 250 °C for 6 h. Specific surface areas (SBET) were calculated by the BET method, the total pore volume (Vp) was determined by nitrogen adsorption at a relative pressure of 0.98, pore diameters (DP) and pore size distributions were obtained from the desorption isotherms by the BJH method. The X-ray powder diffraction patterns were recorded between 4° and 80° (2θ) on a Bruker D8 Advance diffractometer using CuKα radiation (λ =1.5406 Å). The FT-IR spectra of the supports and catalysts were recorded in an absorbance mode with a Varian 640-IR spectrometer equipped with a PIKE accessory. The chemical composition of the synthesized materials was determined by SEM-EDX using a JEOL 5900 L V microscope with OXFORD ISIS equipment. High-resolution transmission electron microscopy (HRTEM) characterization was performed on a JEOL 2010 microscope operating at 200 kV (resolving power 1.9 Å). The solids were ultrasonically dispersed in heptane and the suspension was collected on carbon-coated grids. Results from the CO2–TPD were obtained using a U-shaped quartz reactor. The samples put in the reactor were first pre-treated in He flow at 350 °C for
Table 1 Chemical composition and textural characteristics of x-STN catalysts. Sample
Chemical compositiona Na content (wt. %)
STN 3-STN 5-STN 10-STN a
8.99 10.12 11.05 13.16
Textural characteristicsb
Amount of added Na2CO3 (wt. %) Theoretical
Experimental
– 3.0 5.0 10.0
– 2.6 4.8 9.6
SBET (m2/ g)
Vp (cm3/g)
Dp (nm)
144 146 119 126
0.336 0.350 0.292 0.324
6.5 6.5 6.5 6.5
As determined by SEM-EDX. SBET, BET surface area; VP, total pore volume; DP, mesopore diameter determined from the pore size distribution calculated from the desorption isotherm by the BJH method. b
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Fig. 1. HRTEM micrographs of (a, b) STN support, (c) 3-STN and (d) 5-STN catalysts.
Fig. 3 shows the X-ray diffraction patterns of the STN support and xSTN catalysts with different Na2CO3 loadings. In all diffractograms, four main diffraction peaks were observed at 10.5, 24.3, 28.4 and 48.1° (2θ), which correspond to (001), (201), (111) and (020) crystal planes of monoclinic layered sodium trititanate (Na2Ti3O7, JCPDS-ICDD card 31–1329) [26]. In the catalysts with 3–10 wt. % of Na2CO3 no signals corresponding to sodium carbonate were observed indicating a good dispersion of the deposited sodium salt. Fig. 4 and Table 2 show results obtained by temperature-programmed desorption of CO2 (CO2–TPD) used for the characterization of the amount and strength of basic sites in the prepared catalysts. All catalysts presented four signals of CO2 desorption at different temperatures (Fig. 4), which indicates the presence of four types of basic sites of different strengths. The addition of sodium carbonate to the sodium titanate nanotubes resulted in an increase in the total amount of basic sites and, especially, in the amount of strong basic sites (CO2 desorption above 310 °C, Table 2). Thus, the amount of strong basic sites increased from 42.6 μmol/g in the STN material to 152.2 μmol/g in the 10-STN catalyst (more than three-fold increase). The results obtained for the reference 5-Al2O3 catalyst (5 wt. % of Na2CO3 supported on Al2O3) are also presented in Table 2 for comparison. It can be observed that the amount of strong basic sites in the 5-STN catalyst was more than twice larger than in the 5-Al2O3 material with the same Na2CO3 loading (113.7 μmol/g versus 46.8 μmol/g, respectively). This points out to the importance of the material used as a support for the deposition of the basic sodium salts. A higher basicity of the 5-STN compared to the 5-Al2O3 can be due to the combination of the basic sites of the proper STN solid and of the deposited sodium carbonate
STN catalysts are shown in Fig. 2 (a). All synthesized materials showed the type IV adsorption isotherm with a noticeable hysteresis loop, which can be interpreted as a combination of H1 and H3 types. The shape of the isotherms of the STN material and x-STN catalysts and of their hysteresis loops were similar to those reported previously for titanate nanotubes prepared from a small crystalline size titania [24]. The type H1 hysteresis is characteristic of mesoporous materials having uniform nearly cylindrical channels; whereas the H3 one is associated with solids consisting of aggregates or agglomerates of particles forming slit shaped pores of non-uniform size [24]. The pore size distribution curves obtained from the desorption branches of the isotherms of the STN and x-STN samples are shown in Fig. 2 (b). All synthesized materials had two principal peaks of pore distributions; the first one with the main maximum at 6.5 nm and a small intensity contribution at 3 nm can be ascribed to the inner spaces of the nanotubes, while the second one located between 30 and 100 nm corresponds to the free spaces between nanotubes and their aggregates [25]. All prepared catalysts with and without sodium carbonate showed similar pore size distributions, which confirms that the nanotubular structure of the STN material was preserved after the impregnation of different amounts of Na2CO3. Table 1 shows the textural characteristics of the synthesized STN support and x-STN catalysts. All the samples had specific surface areas between 119 and 146 m2/g, pore volumes around 0.3 cm3/g and similar mesopore sizes. The main size of the internal pores of the nanotubes was 6.5 nm. A slight decrease in the specific textural characteristics (SBET and VP) was observed after the incorporation of 5 and 10 wt.% of Na2CO3 that can be attributed to an increase in the catalysts’ density. 3
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Fig. 4. CO2 temperature-programmed desorption (CO2-TPD) profiles of STN support and x-STN catalysts.
Table 2 Characteristics of basic sites of the x-STN catalysts determined by CO2-TPD. Sample
Amount of basic sites (μmol/g) Weak (30-130 °C)
Medium (130-220 °C)
Mediumstrong (220-310 °C)
Strong (310580 °C)
Total amount
STN 3-STN 5-STN 10-STN
65.5 71.1 77.5 92.4 (30-180 °C)
19.2 23.7 31.3 33.6 (180-280 °C)
8.0 11.4 13.1 24.9 (280-370 °C)
135.3 199.6 235.6 303.1
Al2O3 5-Al2O3
102.5 273.0
15.1 54.0
3.4 25.0
42.6 93.4 113.7 152.2 (370600 °C) 1.5 46.8
122.5 398.8
carbonate species in the prepared catalysts, the FT-IR characterization was performed. Fig. 5 (a) shows FT-IR spectra of the STN support and the corresponding x-STN catalysts. The spectrum of the sodium titanate nanotubes showed the following signals: the broad band from about 800 cm−1 till 400 cm−1 assigned to the TieOeTi skeletal frequency region (only a part of this signal can be seen in Fig. 5 (a)) [27], the vibrational mode at 894 cm−1 ascribed to the stretching of the TieO bonds from a distorted TiO6 octahedron, whose oxygen is unshared [28], and the signal at 1366 cm−1 attributed to Na-O vibrations due to the presence of sodium ions intercalated into the titanate nanotubular structure [29]. In addition, two more IR peaks were observed: the first one located at 1637 cm−1 is generally assigned to a water molecule bending vibration [27,30], while the second broad signal around 3000 3600 cm−1 can be attributed to the OeH symmetrical and asymmetrical stretching vibrations, indicating the presence of surface Ti−OH hydroxyl groups and physisorbed water [27,29–31]. The position of the maximum of the peak observed in the ν(OH) region was located at 3157 cm−1, which corresponds to the stretching mode of the interlamellar water molecules present in layered sodium titanates [32]. The above IR results are in line with the chemical composition of sodium titanate nanotubes synthesized in the present work, which, considering the experimentally-determined Na content of ∼ 9 wt. %, can be described as NaxH2-xTi3O7∙nH2O with n ≈ 1 [24]. The FT-IR spectrum of the unsupported bulk Na2CO3 (Fig. 5 a) showed two main absorption bands at 1423 and 876 cm−1, which is in agreement with previously reported results for dry Na2CO3 (< 1H2O) [33]. The above vibrations can be attributed to asymmetric CO stretching and out-of-plane
Fig. 2. Nitrogen adsorption-desorption isotherms (a) and pore size distributions (b) of STN support and x-STN catalysts.
Fig. 3. Powder XRD patterns of STN support and x-STN catalysts.
species or to some differences in the type of the carbonate species supported on alumina and titanate materials. In order to inquire into the characteristics of the supported
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HCO3- + OH- producing bicarbonate species that can form dimers upon drying. On the other hand, this hydrolysis also results in the formation of hydroxyl anions, similar to those of NaOH because of a high Na content in the STN, with higher basicity (pKB = 0) than that of the starting Na2CO3 (pKB = 3.7) [6]. The FT-IR results showed that the above species are predominant in the 3-STN and 5-STN catalysts, while they coexist with some bulk Na2CO3 in the 10-STN sample. Probably, the formation of hydroxyl anions is responsible for an increase in the number of strong basic sites in the x-STN catalysts with increasing sodium carbonate loading. The spectra of the reference 5-Al2O3 catalyst and γ-Al2O3 support are also shown in Fig. 5 (b) for comparison. In the spectrum of the 5Al2O3 catalyst, two well-defined peaks were observed at 1404 and 1570 cm−1 and a shoulder at ∼1090 cm−1. Two of these peaks correspond well with the presence of free ionic CO32- adsorbed on the alumina surface: 1404 cm−1 (asymmetric stretch) and 1090 cm−1 (symmetric stretch) [36,37]. The peak at 1570 cm−1 can be due to mono- or bidentate carbonate species coordinated to Al3+ cations [34,36,38]. These species could be formed through the interaction of the basic carbonate ions with the Lewis acid sites of the alumina support, leading to a decrease in the amount of strong basic sites (neutralization) and the formation of a significant amount of weak basic sites in the 5-Al2O3 catalyst as it was detected by the CO2-TPD characterization (Table 2). Therefore, the FT-IR characterization of the prepared samples showed the presence of different types of carbonate species on the STN and γalumina supports.
3.2. Catalytic activity in the transesterification reaction The synthesized x-STN catalysts were tested in the transesterification of soybean oil with methanol. Table 3 presents the yields of fatty acid methyl esters (FAME) obtained with different catalysts upon the same reaction conditions (80 °C, 30 min reaction time, CH3OH to soybean oil molar ratio = 20:1, 1 wt. % of catalyst). Also, the kinematic viscosity of the purified product obtained in each reaction is shown. The unmodified STN catalyst resulted in 14.0% FAME yield, while the catalysts containing sodium carbonate, 3-STN, 5-STN and 10-STN, gave biodiesel yields of 27.2, 66.0 and 97.1%, respectively. Therefore, the modification of the sodium titanate nanotubes with Na2CO3, as expected, resulted in a noticeable improvement of their catalytic activity in transesterification. The 10-STN catalyst showed the best catalytic activity among all tested catalysts, leading to a reaction product with a kinematic viscosity of 4.5 mm2/s that satisfies biodiesel specifications [39]. The FAME yield obtained with this catalyst was almost seven times higher than that obtained with the starting STN material. For comparison purposes, unsupported Na2CO3 and alumina-supported Na2CO3 (5-Al2O3) were also evaluated in the same reaction (Table 3). Unsupported sodium carbonate had a very low activity (∼1% of FAME yield) that can be due to its small surface area and low solubility in the reaction medium, while the 5-Al2O3 catalyst gave higher biodiesel yield
Fig. 5. FT-IR spectra of: (a) STN support, x-STN catalysts and unsupported sodium carbonate used as a precursor for the catalysts’ preparation; (b) γ-Al2O3 and 5-Al2O3 references. The IR peaks of the STN support are marked with an asterisk (*).
deformation, respectively, of the free (symmetrical) carbonate ion [34]. When this precursor was deposited on the SNT material, new absorption bands at 1372 and 980 cm−1 were observed in the spectra of x-STN catalysts, whose intensity increased with the increase in the Na2CO3 loading in the catalysts from 3 to 10 wt. %. On the other side, the characteristic signals of the bulk Na2CO3 almost disappeared, being the peak with the maximum at 1423 cm−1 detected only in the spectrum of the 10-STN catalyst with the largest sodium carbonate loading. Therefore, it can be concluded that the states of CO32- in bulk Na2CO3 and the prepared x-STN catalysts with small sodium carbonate loadings are different. According to literature [34,35], the absorbance band at 1372 cm−1 can be attributed to monodentate Ti-O−CO2- species or to bicarbonate ions forming hydrogen-bonded dimers in the solid state. This assumption seems to be reasonable considering that the presence of water even in the dried STN material can induce hydrolysis of the deposited sodium carbonate according to the equation: CO32- + H2O ↔
Table 3 Catalytic activity of the x-STN catalysts in transesterification of soybean oil with methanol. Reaction conditions: 0.5 g of catalyst; 27 g of soybean oil; methanol to soybean oil molar ratio 20:1; temperature 80 °C, reaction time 30 min. Sample
Conversion to FAME (%)a
Kinematic viscosity (mm2/ s)b
STN 3-STN 5-STN 10-STN 5-Al2O3 Unsupported Na2CO3
14.0 27.2 66.0 97.1 13.3 1.1
21.1 16.2 6.9 4.5 22.0 30.1
a b
5
Determined by 1H NMR. FAME - fatty acid methyl esters (biodiesel). Kinematic viscosity of soybean oil was 32.6 mm2/s.
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should be related with the increment in the amount of strong basic sites in these systems as a result of the interaction of Na2CO3 with STN. In addition, high Na+ content in the x-STN catalysts could also facilitate the transesterification reaction making easier the approaching of the triglyceride molecules to the catalyst’s surface and by increasing the reaction rate due to the increment of the positive charge on the carbon atom of the C]O group, both because of the possible interaction of the carbonyl oxygen with the sodium cations of the catalyst. Finally, the particular structure of the nanotubular sodium titanate material makes the active basic sites more easily available for the interaction with large triglyceride molecules. This is because the basic sites are located not only in the internal pores of the nanotubes, but also a significant amount of them (half or more) are on the external surface of the nanotubes, exposed for the interaction with the reactant molecules. On the contrary, in the γ-alumina-supported catalysts, the active sites are mostly located inside the ink-bottle shaped pores and only a very small amount of them on the external surface of the catalyst particles. The combination of all above factors could explain why the STN material has an advantage compared to the conventional γ-alumina in the preparation of basic catalysts for the transesterification of large triglyceride molecules.
(13.3%) that could be attributed to a better dispersion of Na2CO3 on the alumina surface and an increase in the amount of the accessible basic sites. Nevertheless, a comparison of the activity of the 5-Al2O3 and 5STN catalysts with the same Na2CO3 loading showed that the change of the alumina support to STN resulted in a five-time increase in the FAME yield (from 13 to 66%). Such high activity of the x-STN catalysts evidences the presence of a synergetic effect between sodium carbonate and the STN material. It could be mentioned that in the case of the 5Al2O3 catalyst, the active basic phase (Na2CO3) was deposited on an almost inert material with a very low amount of strong basic sites (Table 2) used as a support. Therefore, an increase in the activity of this catalyst could be due mainly to an increase in the dispersion of the deposited sodium carbonate salt. In addition, according to the FT-IR results, some decrease in the amount of strong basic active sites could be expected in this catalyst because of the interaction of CO32− anions with Lewis acid sites of the alumina support leading to the formation of mono- and bidentate carbonate species coordinated to Al3+ cations (a kind of neutralization of strong basic active sites of the Na2CO3 active phase due to the interaction with the acidic alumina support). On the contrary, the x-STN catalysts represent the combination of two basic materials, sodium carbonate and sodium titanate nanotubes, both of which are active for the transesterification of triglycerides. The interaction between these two materials resulted in the appearance of different types of basic sites in the x-STN catalysts compared to the 5Al2O3 one. The formation of strongly-basic OH- groups was assumed from the FT-IR characterization results that can be promoted by the presence of physisorbed and structural water in the sodium titanate nanotubes. From the above, it can be concluded that the presence of strong basic sites in the catalysts and their amount is the most important characteristic for the transesterification activity of the catalysts. The above trends in the catalytic activity can be explained based on the mechanism of the transesterification of triglycerides with methanol in basic catalysis conditions. Namely, the role of the basic catalyst is the deprotonation of methanol with the formation of methoxide anions, which are the active species that attack the triglyceride molecules at the carbonyl group [40]. Previously [6], it was shown that only strong basic salts (Na3PO4, K3PO4, Na2CO3, etc.) were able to deprotonate methanol and catalyze the conversion of triglycerides to FAME (biodiesel), while less strong bases (Na2HPO4, NaHCO3, etc.) were not catalytically active. Therefore, we assumed that in our catalysts only strong basic sites are active for the transesterification of soybean oil with methanol. A comparison between the amount of strong basic sites in the catalysts and the biodiesel yield (Fig. 6) showed a clear correlation between these two parameters confirming the above supposition. Therefore, the high catalytic activity of the x-STN catalysts in the transesterification
3.3. Reusability of the synthesized x-STN catalysts The possibility of the reutilization of heterogeneous basic catalysts in several reaction cycles is an important issue for biodiesel production at an industrial level. In the present work, we selected the most active catalyst (10-STN) to test its reusability. This catalyst was re-used three times without any further purification and activation. In these experiments, after each run the catalyst was separated from the reaction mixture by centrifugation and re-used in the consecutive run with the fresh portions of soybean oil and methanol maintaining the reaction conditions described previously (Table 3, title). It was observed that biodiesel yield decreased in the successive cycles from 97.1% in the first run to 38.5% in the third one. For comparison purposes, we performed a similar experiment with the reference 5-Al2O3 catalyst which showed a very low catalytic activity in the third run (around 1–2 % biodiesel yield). In both cases, the decrease in the biodiesel yield upon reutilization of the same catalyst in several reaction runs can be only partially attributed to some loss of the catalyst during its separation and reutilization. The physical loss of the catalyst between two consecutive reactions was not larger than ∼ 5%. In order to inquire into the reasons of the catalysts’ deactivation, the characterization of the recycled 10STN and 5-Al2O3 catalysts was performed. Fig. 7 shows XRD patterns of the fresh and used 10-STN catalysts. It can be observed that the crystalline structure of the 10-STN catalyst did not suffer any change after recycling and no new crystalline phase was detected. Therefore, no destruction of the catalyst or agglomeration of the active phase took place upon recycling. On the other hand, the SEM-EDX characterization of the reused catalysts showed that reutilization of the same catalyst in several reaction cycles resulted in a decrease in its sodium content. Thus, for the 10-STN catalyst, sodium content decreased from 13.16 wt. % in the fresh catalyst (Table 1) to 12.01 wt. % in the catalyst re-used in three catalytic cycles. Regarding the reference 5-Al2O3 catalyst, sodium content also decreased upon recycling from 2.13 to 1.73 wt. %. Based on these results, we assumed that the principal reason for the deactivation of our catalysts prepared with sodium carbonate was the leaching of the catalytically active phase (Na2CO3) in the reaction media. This is in line with the available literature reports about the utilization of pure and supported Na2CO3 and K2CO3 salts as catalysts for biodiesel synthesis [6,10,12,41]. Thus, it was reported that unsupported sodium and potassium carbonate and phosphate salts are slightly soluble in the transesterification reaction mixture, mainly in polar alcohols (in an excess of methanol and methanol/glycerol solutions) that complicates the complete catalyst’s recovery after the transesterification process [6,10]. To diminish this problem and
Fig. 6. Correlation between the amount of strong basic sites in the x-STN catalysts and the obtained biodiesel (FAME) yields. Results obtained with the reference 5-Al2O3 catalyst are also shown for comparison purposes. 6
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Acknowledgements Financial support by DGAPA-UNAM, México (PAPIIT grant number IN-115218) is gratefully acknowledged. The authors thank I. Puente Lee, C. Salcedo Luna and L. Pedro Hernández for performing electron microscopy, powder XRD and NMR characterizations, respectively, and Gabriela Díaz and Antonio Gómez-Cortés from Instituto de Física, UNAM for help with the CO2–TPD experiments. References [1] D. Laforgia, V. Ardito, Bioresour. Technol. 51 (1995) 53–59. [2] H.J. Kim, B.S. Kang, M.J. Kim, Y.M. Park, D.K. Kim, J.S. Lee, K.Y. Lee, Catal. Today 93-95 (2004) 315–320. [3] H. Fukuda, A. Kondo, H. Noda, J. Biosci. Bioeng. 92 (2001) 405–416. [4] P.T. Vasudevan, B. Fu, Waste Biomass Valoriz. 1 (2010) 47–63. [5] A. Hayyan, M.Z. Alam, M.E.S. Mirghani, N.A. Kabbashi, N.I.N.M. Hakimi, Y.M. Siran, S. Tahiruddin, Bioresour. Technol. 101 (2010) 7804–7811. [6] S.L. Britton, J.Q. Bond, T.W. Root, Energy Fuels 24 (2010) 4095–4096. [7] J. Ji, J. Wang, Y. Li, Y. Yu, Z. Xu, Ultrasonics 44 (2006) 411–414. [8] J. Tang, Q. Zhang, K. Hu, S. Cao, S. Zhang, J. Wang, J. Catal. 368 (2018) 190–196. [9] B. Yang, L. Leclercq, J.-M. Clacens, V. Nardello-Rataj, Green Chem. 19 (2017) 4552–4562. [10] K. Malins, Fuel Process. Technol. 179 (2018) 302–312. [11] P.E. Gama, E.R. Lachter, R.A.S. San Gil, A.V. Coelho, I.A. Sidi, R.L. Poubel, A. da Costa Faro Jr., A.L.F. deSouza, Quim. Nova 38 (2015) 185–190. [12] I. Lukić, J. Krstić, D. Jovanović, D. Skala, Bioresour. Technol. 100 (2009) 4690–4696. [13] H.J. Kim, B.S. Kang, M.J. Kim, Y.M. Park, D.K. Kim, J.S. Lee, K.Y. Lee, Catal. Today 93–95 (2004) 315–320. [14] W. Xie, H. Peng, L. Cheng, Appl. Catal., A-Gen 300 (2006) 67–74. [15] X. Liang, S. Gao, H. Wu, J. Yang, Fuel Process. Technol. 90 (2009) 701–704. [16] O. Ilgen, A.N. Akin, Energy Fuels 23 (2009) 1786–1789. [17] M. Manríquez-Ramírez, R. Gómez, J.C. Hernández-Cortez, A. Zúñiga-Moreno, C.M. Reza-San Germán, S.O. Flores-Valle, Catal. Today 212 (2013) 23–30. [18] M. Zabeti, W.M.A.W. Daud, M.K. Aroua, Fuel Process. Technol. 91 (2010) 243–248. [19] P. Hernández-Hipólito, M. García-Castillejos, E. Martínez-Klimova, N. JuárezFlores, A. Gómez-Cortés, T.E. Klimova, Catal. Today 220-222 (2014) 4–11. [20] P. Hernández-Hipólito, N. Juárez-Flores, E. Martínez-Klimova, A. Gómez-Cortés, X. Bokhimi, L. Escobar-Alarcón, T.E. Klimova, Catal. Today 250 (2015) 187–196. [21] T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Adv. Mater. 11 (1999) 1307–1311. [22] G. Knothe, J. Am. Oil Chem. Soc. 77 (2000) 489–493. [23] American Society for Testing Materials (ASTM), Standard D445. Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids, PA, ASTM, West Conshohocken, 2006. [24] E. Morgado Jr, M.A.S. de Abreu, G.T. Moure, B.A. Marinkovic, P.M. Jardim, A.S. Araujo, Chem. Mater. 19 (2007) 665–676. [25] E. Morgado Jr, M.A.S. de Abreu, O.R.C. Pravia, B.A. Marinkovic, P.M. Jardim, F.C. Rizzo, A.S. Araújo, Solis State Sci. 8 (2006) 888–900. [26] R.A. Ortega-Domínguez, J.A. Mendoza-Nieto, P. Hernández-Hipólito, F. GarridoSánchez, J. Escobar-Aguilar, S.A.I. Barri, D. Chadwick, T.E. Klimova, J. Catal. 329 (2015) 457–470. [27] S. Mozia, E. Borowiak-Paleń, J. Przepiórski, B. Grzmil, T. Tsumura, M. Toyoda, J. Grzechulska-Damszel, A. Morawski, J. Phys. Chem. Solids 71 (2010) 263–272. [28] B.C. Vaina, O.P. Ferreira, A.G. Souza Filho, A.A. Hidalgo, J. Mendes Filho, O.L. Alves, Vib. Spectrosc. 55 (2011) 183–187. [29] L. Zhang, X. Wang, H. Chen, F. Jiang, Clean (Weinh) 42 (2014) 947–955. [30] Q. Chen, G.H. Du, S. Zhang, L.-M. Peng, Acta Crystallogr. Sect. B Struct. Sci. B58 (2002) 587–593. [31] H. Li, K. Zheng, X. Xu, H. Zhao, Y. Song, Y. Sheng, Q. Huo, H. Zou, Powder Technol. 228 (2012) 277–283. [32] M. Shirpour, J. Cabana, M. Doeff, Energy Environ. Sci. 6 (2013) 2538–2547. [33] R.A. Niquist, R.O. Kagel, Infrared Spectra of Inorganic Compounds, Academic Press, Inc, USA, 1971. [34] G. Busca, V. Lorenzelli, Mater. Chem. 7 (1982) 89–126. [35] T. Klimova, D.A. Solís Casados, J. Ramírez, Catal. Today 43 (1998) 135–146. [36] T. Yamaguchi, Y. Wang, M. Komatsu, M. Ookawa, Catal Surv. Jap. 5 (2002) 81–89. [37] T.J. Toops, D.B. Smith, W.P. Partridge, Appl. Catal. B: Environ. 58 (2005) 245–254. [38] D.M. Alonso, R. Mariscal, R. Moreno-Tost, M.D. Zafra Poves, M. López Granados, Catalan J. Commun. Cult. Stud. 8 (2007) 2074–2080. [39] ASTM D6751 – biodiesel blends stock specification (B100). [40] F.A. Carey, Organic Chemistry, 3rd ed., The McGraw-Hill Companies, Inc, New York, 1996. [41] R. Malhotra, A. Ali, Renew. Energy 133 (2019) 606–619.
Fig. 7. Powder XRD patterns of the fresh 10-STN catalyst and the same catalyst after three consecutive reaction runs.
stabilize the above mentioned alkali salts, it was proposed to fix them to a surface of the silica or alumina supports [6]. However, this attempt was not successful since a leaching of the active sites was observed in both cases of silica- and alumina-supported samples. Similar results (potassium leaching) were also obtained in work [12] for the heterogeneous K2CO3 catalysts supported on an alumina/silica material synthesized by the sol–gel method. In the recent work [41], the preparation of the Na/ZnO/SBA-15 catalysts with improved stability was reported. The 10-Na/ZnO/SBA-15 catalyst was capable of yielding complete conversion of virgin cotton seed oil to biodiesel in the first three catalytic runs decreasing the activity in the fourth and fifth cycles due to the partial dissolution of the catalyst. However, it is worth to mention that in this case, the catalyst recovered after each run was subjected to the strict regeneration process including several runs of washing with hexane and methanol to eliminate polar and non-polar contaminants, followed by drying and calcination, which was not performed with the catalysts prepared in the present work. 4. Conclusions Sodium titanate nanotubes modified by different amounts of sodium carbonate showed high activity in the transesterification of soybean oil with methanol. The most active catalyst, 10-STN, resulted in the 97% yield of methyl esters at a short reaction time (30 min) in mild conditions (80 °C). High activity of the STN catalysts containing Na2CO3 was attributed to a synergetic effect between the support and the deposited sodium salt, which leads to a noticeable increase in the amount of strong basic sites able to catalyze the transformation of triglycerides to methyl esters governed by methoxide anions. A comparison of two catalysts with the same sodium carbonate loading supported on γ-alumina and STN, showed that the 5-STN catalyst was about 5 times more active than the 5-Al2O3 one, that was attributed to the presence of different types of supported carbonate species in these samples. However, upon reutilization, both the synthesized Na2CO3-containing STN catalysts and the reference 5-Al2O3 sample showed a decrease in their activity due to a leaching of the active phase in the reaction media.
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Synergy between sodium carbonate and sodium titanate nanotubes in the transesterification of soybean oil with methanol Mark E. Martínez-Klimov, Pamela Ramírez-Vidal, Pedro Roquero Tejeda, Tatiana E. Klimova
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Laboratorio de Nanocatálisis, Departamento de Ingeniería Química, Facultad de Química, Universidad Nacional Autónoma de México (UNAM), Cd. Universitaria, Coyoacán, Ciudad de México, 04510, Mexico
A R T I C LE I N FO
A B S T R A C T
Keywords: Sodium titanate nanotubes Sodium carbonate Synergetic effect Transesterification reaction Soybean oil Biodiesel
Sodium titanate nanotubes (STN) modified by the addition of sodium carbonate (3–10 wt. %) were tested as catalysts for the transesterification of soybean oil with methanol giving good results. Prepared catalysts were characterized by N2 physisorption, X-ray diffraction, FT-IR, temperature-programmed desorption of CO2, scanning and transmission electron microscopy. The STN catalysts modified with sodium carbonate were significantly more active in the transesterification reaction than the reference catalyst containing 5 wt. % of Na2CO3 on alumina. The most active catalyst, STN with 10 wt. % of Na2CO3, resulted in the 97% yield of methyl esters at a short reaction time (30 min) in mild conditions (80 °C). High activity of the prepared catalysts was attributed to a synergetic effect between the support and the deposited sodium carbonate leading to a noticeable increase in the amount of strong basic sites able to generate methoxide anions required for the transformation of triglycerides to methyl esters. According to the FT-IR characterization, the above synergy was attributed to the presence of different types of carbonate species in the alumina and STN-supported catalysts. Upon reutilization, Na2CO3-containing STN catalysts showed a decrease in their activity due to leaching of the active phase in the reaction media.
1. Introduction Nowadays, a clear increase in the requirements for high quality gasoline and diesel fuels is observed all over the world. The use of only petroleum-derived fuels cannot satisfy this demand, since fossil fuel resources are limited and petroleum price continually rises. One promising approach to mitigate global energy problems is the use of biofuels, such as bioethanol and biodiesel, as abundant and environmentally friendly energy sources. Biodiesel is a non-petroleum based, alternative diesel fuel that is relatively clean burning, non-toxic, biodegradable and renewable [1–4]. The most common method for producing biodiesel is via transesterification of vegetable oils and animal fats with short-chain alcohols (mainly methanol) in the presence of homogeneous base catalysts (NaOH, KOH or methoxides) leading to the formation of fatty acid methyl esters (FAME) [3,5]. In this case, biodiesel production includes different steps of separation and purification, which, in addition to the difficulty of removal of the homogeneous catalyst after the reaction for its possible reutilization, results in a high cost of the obtained biodiesel [6]. Different approaches were proposed to improve the efficiency of the production of biodiesel. Due to the immiscible nature of methanol and oil, the transesterification reaction
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can be accelerated by increasing the interfacial contact area using vigorous stirring, ultrasonic or hydrodynamic cavitation [7] or by using of novel Pickering interfacial catalysts (PICs) for transestrification which combine the advantages of Pickering emulsion and heterogeneous catalysts, enhancing the liquid-liquid biphasic reactions [8,9]. The replacement of the conventional homogeneous catalysts by highlyactive heterogeneous ones is also an environmentally friendly option that allows simplification of the existing processes [10]. Many different solids have been tested in transesterification, such as unsupported sodium and potassium carbonates and phosphates [10], as well as a number of supported alkali and alkaline earth metals, metal oxides, salts and hydroxides: K2CO3 supported on alumina [11] or alumina/ silica [12], Na/NaOH/γ-Al2O3 [13], KNO3/Al2O3 [14], K2CO3, NaOH and KOH supported on MgO [15–17], sodium and potassium phosphates and carbonates fixed to the silica and alumina supports [6], CaO/Al2O3 [18], etc. Most of these catalysts are based on the employment of alkali or alkaline earth metal compounds having strong basic sites. It was noted that in some cases, the deposition of the active basic salt (sodium or potassium phosphate) on the support's surface results in the destruction of its catalytic activity [6]. Also, some of the supports (silica, for instance) were found to be unstable in the
Corresponding author. E-mail address: [email protected] (T.E. Klimova).
https://doi.org/10.1016/j.cattod.2019.08.027 Received 10 April 2019; Received in revised form 31 July 2019; Accepted 27 August 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Mark E. Martínez-Klimov, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.08.027
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2 h, then cooled to room temperature and flushed with a CO2/He gas mixture for 30 min followed by purging of the remaining CO2 with He flow. The CO2 desorption was performed from room temperature to 600 °C at a constant heating rate of 10 °C/min. The amount of desorbed CO2 was determined by mass spectrometry following the MS intensity for CO2 (m/e = 44) as a function of temperature. Basicity was estimated based on the amount of desorbed CO2 assuming that one CO2 molecule was desorbed from one basic site present on the catalyst's surface. The temperature required to release adsorbed CO2 molecules characterized the base strength.
transesterification reactions due to partial dissolution produced by strongly basic species [6]. Another problem of the supported metal salts and hydroxides is leaching of the active sites from the catalyst's surface upon reaction conditions. Therefore, a development of novel stable heterogeneous base catalysts for biodiesel synthesis nowadays continues being an actual task. Recently, in our group, sodium titanate nanotubes (STN) were tested as catalysts for biodiesel production from vegetable oil and methanol giving good results (97–100 % yields of fatty acid methyl esters at 8 h reaction time, methanol reflux temperature, methanol:oil molar ratio of 40:1, 1–2 wt. % of the catalyst) [19]. The advantages of these solids are that they are stable in basic reaction medium, active in mild conditions and do not require thermal pre-treatment before the transesterification reaction [19,20]. In the present work, we modified sodium titanate nanotubes by the addition of different amounts of sodium carbonate in order to increase their basicity. Prepared catalysts were characterized and tested in the transesterification of soybean oil with methanol. The possible reutilization of the catalysts was also investigated.
2.3. Catalytic tests The transesterification reactions were performed in a 100 mL stainless steel batch reactor under constant stirring (600 rpm), at 80 °C and 30 min reaction time. Soybean oil (27 g) and methanol (19.9 g, CH3OH to soybean oil molar ratio = 20:1) were added together with 0.5 g of catalyst (1 wt. %) into the reactor. After the reaction, the solid catalyst was separated from the reaction mixture by centrifugation (5000 rpm, 5 min). The biodiesel yield was calculated on the basis of 1H NMR characterization of the obtained reaction product [22]. Kinematic viscosity of the obtained biodiesel was also determined after complete removal of residual methanol, using Cannon-Fenske viscosimeters at 40 °C according to the standard ASTM D445 method [23].
2. Experimental 2.1. Catalyst preparation Sodium titanate nanotubes (STN) were prepared by the Kasuga method [21] using TiO2 anatase nanopowder as a precursor. In a typical synthesis, 10 g of TiO2 precursor were added to 300 mL of a 10 M NaOH solution in a Teflon-lined autoclave. The obtained mixture was maintained at 140 °C for 20 h upon magnetic stirring. After the hydrothermal reaction was over, the obtained white product (Na2Ti3O7∙xH2O, sodium titanate nanotubes) was filtered in vacuum, washed several times with deionized water to eliminate the excess of non-reacted sodium hydroxide and dried at 120 °C for 12 h. Catalysts with Na2CO3 loadings of 3, 5 and 10 wt. % were prepared by incipient wetness impregnation using aqueous solutions of sodium carbonate. After the impregnation of sodium salt, the catalysts were dried at 120 °C for 12 h. Hereinafter, the catalysts will be denoted as x-STN, where x is the nominal Na2CO3 loading (wt. %) in the samples. For comparison purposes, the reference catalyst containing 5 wt. % of Na2CO3 impregnated on an alumina support (5-Al2O3) was also prepared.
3. Results and discussion 3.1. Characterization of x-STN catalysts Chemical composition of the STN and x-STN catalysts was determined by SEM-EDX (Table 1). Sodium content in the starting sodium titanate nanotubes was 8.99 wt. %. The x-STN catalysts had higher Na loadings (10.12–13.16 wt. %) than the STN support due to the addition of different amounts of sodium carbonate. Based on the amount of extra-sodium in the x-STN catalysts (excess of Na compared to the Na content in the starting support), the amount of Na2CO3 in the samples was estimated. The experimentally obtained values were close to the theoretically expected ones (3, 5 and 10 wt. %), which confirms that the prepared catalysts had the expected chemical composition. The morphology of the prepared samples was inspected by HRTEM. The micrographs obtained for the STN support, 3-STN and 5-STN catalysts are shown in Fig. 1. The presence of randomly distributed nanotubular structures can be observed in the images of all samples. The nanotubes had approximately 3–6 nm internal diameters, 9–12 nm external diameters and 200–500 nm lengths. The incorporation of sodium carbonate to the STN material did not produce changes in its morphology. In addition, no particles formed by the agglomerated Na2CO3 were detected. Nitrogen adsorption-desorption isotherms of the STN support and x-
2.2. Catalyst characterization The STN support and x-STN catalysts were characterized by N2 physisorption, powder X-ray diffraction (XRD), Fourier-transformed infrared spectroscopy (FT-IR), temperature-programmed desorption of CO2 (CO2-TPD), scanning and transmission electron microscopy. Nitrogen adsorption-desorption isotherms were measured with an ASAP 2020 automatic analyzer (Micromeritics) at liquid N2 temperature (-197.5 °C). Prior to the experiments, the samples were degassed at 250 °C for 6 h. Specific surface areas (SBET) were calculated by the BET method, the total pore volume (Vp) was determined by nitrogen adsorption at a relative pressure of 0.98, pore diameters (DP) and pore size distributions were obtained from the desorption isotherms by the BJH method. The X-ray powder diffraction patterns were recorded between 4° and 80° (2θ) on a Bruker D8 Advance diffractometer using CuKα radiation (λ =1.5406 Å). The FT-IR spectra of the supports and catalysts were recorded in an absorbance mode with a Varian 640-IR spectrometer equipped with a PIKE accessory. The chemical composition of the synthesized materials was determined by SEM-EDX using a JEOL 5900 L V microscope with OXFORD ISIS equipment. High-resolution transmission electron microscopy (HRTEM) characterization was performed on a JEOL 2010 microscope operating at 200 kV (resolving power 1.9 Å). The solids were ultrasonically dispersed in heptane and the suspension was collected on carbon-coated grids. Results from the CO2–TPD were obtained using a U-shaped quartz reactor. The samples put in the reactor were first pre-treated in He flow at 350 °C for
Table 1 Chemical composition and textural characteristics of x-STN catalysts. Sample
Chemical compositiona Na content (wt. %)
STN 3-STN 5-STN 10-STN a
8.99 10.12 11.05 13.16
Textural characteristicsb
Amount of added Na2CO3 (wt. %) Theoretical
Experimental
– 3.0 5.0 10.0
– 2.6 4.8 9.6
SBET (m2/ g)
Vp (cm3/g)
Dp (nm)
144 146 119 126
0.336 0.350 0.292 0.324
6.5 6.5 6.5 6.5
As determined by SEM-EDX. SBET, BET surface area; VP, total pore volume; DP, mesopore diameter determined from the pore size distribution calculated from the desorption isotherm by the BJH method. b
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Fig. 1. HRTEM micrographs of (a, b) STN support, (c) 3-STN and (d) 5-STN catalysts.
Fig. 3 shows the X-ray diffraction patterns of the STN support and xSTN catalysts with different Na2CO3 loadings. In all diffractograms, four main diffraction peaks were observed at 10.5, 24.3, 28.4 and 48.1° (2θ), which correspond to (001), (201), (111) and (020) crystal planes of monoclinic layered sodium trititanate (Na2Ti3O7, JCPDS-ICDD card 31–1329) [26]. In the catalysts with 3–10 wt. % of Na2CO3 no signals corresponding to sodium carbonate were observed indicating a good dispersion of the deposited sodium salt. Fig. 4 and Table 2 show results obtained by temperature-programmed desorption of CO2 (CO2–TPD) used for the characterization of the amount and strength of basic sites in the prepared catalysts. All catalysts presented four signals of CO2 desorption at different temperatures (Fig. 4), which indicates the presence of four types of basic sites of different strengths. The addition of sodium carbonate to the sodium titanate nanotubes resulted in an increase in the total amount of basic sites and, especially, in the amount of strong basic sites (CO2 desorption above 310 °C, Table 2). Thus, the amount of strong basic sites increased from 42.6 μmol/g in the STN material to 152.2 μmol/g in the 10-STN catalyst (more than three-fold increase). The results obtained for the reference 5-Al2O3 catalyst (5 wt. % of Na2CO3 supported on Al2O3) are also presented in Table 2 for comparison. It can be observed that the amount of strong basic sites in the 5-STN catalyst was more than twice larger than in the 5-Al2O3 material with the same Na2CO3 loading (113.7 μmol/g versus 46.8 μmol/g, respectively). This points out to the importance of the material used as a support for the deposition of the basic sodium salts. A higher basicity of the 5-STN compared to the 5-Al2O3 can be due to the combination of the basic sites of the proper STN solid and of the deposited sodium carbonate
STN catalysts are shown in Fig. 2 (a). All synthesized materials showed the type IV adsorption isotherm with a noticeable hysteresis loop, which can be interpreted as a combination of H1 and H3 types. The shape of the isotherms of the STN material and x-STN catalysts and of their hysteresis loops were similar to those reported previously for titanate nanotubes prepared from a small crystalline size titania [24]. The type H1 hysteresis is characteristic of mesoporous materials having uniform nearly cylindrical channels; whereas the H3 one is associated with solids consisting of aggregates or agglomerates of particles forming slit shaped pores of non-uniform size [24]. The pore size distribution curves obtained from the desorption branches of the isotherms of the STN and x-STN samples are shown in Fig. 2 (b). All synthesized materials had two principal peaks of pore distributions; the first one with the main maximum at 6.5 nm and a small intensity contribution at 3 nm can be ascribed to the inner spaces of the nanotubes, while the second one located between 30 and 100 nm corresponds to the free spaces between nanotubes and their aggregates [25]. All prepared catalysts with and without sodium carbonate showed similar pore size distributions, which confirms that the nanotubular structure of the STN material was preserved after the impregnation of different amounts of Na2CO3. Table 1 shows the textural characteristics of the synthesized STN support and x-STN catalysts. All the samples had specific surface areas between 119 and 146 m2/g, pore volumes around 0.3 cm3/g and similar mesopore sizes. The main size of the internal pores of the nanotubes was 6.5 nm. A slight decrease in the specific textural characteristics (SBET and VP) was observed after the incorporation of 5 and 10 wt.% of Na2CO3 that can be attributed to an increase in the catalysts’ density. 3
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Fig. 4. CO2 temperature-programmed desorption (CO2-TPD) profiles of STN support and x-STN catalysts.
Table 2 Characteristics of basic sites of the x-STN catalysts determined by CO2-TPD. Sample
Amount of basic sites (μmol/g) Weak (30-130 °C)
Medium (130-220 °C)
Mediumstrong (220-310 °C)
Strong (310580 °C)
Total amount
STN 3-STN 5-STN 10-STN
65.5 71.1 77.5 92.4 (30-180 °C)
19.2 23.7 31.3 33.6 (180-280 °C)
8.0 11.4 13.1 24.9 (280-370 °C)
135.3 199.6 235.6 303.1
Al2O3 5-Al2O3
102.5 273.0
15.1 54.0
3.4 25.0
42.6 93.4 113.7 152.2 (370600 °C) 1.5 46.8
122.5 398.8
carbonate species in the prepared catalysts, the FT-IR characterization was performed. Fig. 5 (a) shows FT-IR spectra of the STN support and the corresponding x-STN catalysts. The spectrum of the sodium titanate nanotubes showed the following signals: the broad band from about 800 cm−1 till 400 cm−1 assigned to the TieOeTi skeletal frequency region (only a part of this signal can be seen in Fig. 5 (a)) [27], the vibrational mode at 894 cm−1 ascribed to the stretching of the TieO bonds from a distorted TiO6 octahedron, whose oxygen is unshared [28], and the signal at 1366 cm−1 attributed to Na-O vibrations due to the presence of sodium ions intercalated into the titanate nanotubular structure [29]. In addition, two more IR peaks were observed: the first one located at 1637 cm−1 is generally assigned to a water molecule bending vibration [27,30], while the second broad signal around 3000 3600 cm−1 can be attributed to the OeH symmetrical and asymmetrical stretching vibrations, indicating the presence of surface Ti−OH hydroxyl groups and physisorbed water [27,29–31]. The position of the maximum of the peak observed in the ν(OH) region was located at 3157 cm−1, which corresponds to the stretching mode of the interlamellar water molecules present in layered sodium titanates [32]. The above IR results are in line with the chemical composition of sodium titanate nanotubes synthesized in the present work, which, considering the experimentally-determined Na content of ∼ 9 wt. %, can be described as NaxH2-xTi3O7∙nH2O with n ≈ 1 [24]. The FT-IR spectrum of the unsupported bulk Na2CO3 (Fig. 5 a) showed two main absorption bands at 1423 and 876 cm−1, which is in agreement with previously reported results for dry Na2CO3 (< 1H2O) [33]. The above vibrations can be attributed to asymmetric CO stretching and out-of-plane
Fig. 2. Nitrogen adsorption-desorption isotherms (a) and pore size distributions (b) of STN support and x-STN catalysts.
Fig. 3. Powder XRD patterns of STN support and x-STN catalysts.
species or to some differences in the type of the carbonate species supported on alumina and titanate materials. In order to inquire into the characteristics of the supported
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HCO3- + OH- producing bicarbonate species that can form dimers upon drying. On the other hand, this hydrolysis also results in the formation of hydroxyl anions, similar to those of NaOH because of a high Na content in the STN, with higher basicity (pKB = 0) than that of the starting Na2CO3 (pKB = 3.7) [6]. The FT-IR results showed that the above species are predominant in the 3-STN and 5-STN catalysts, while they coexist with some bulk Na2CO3 in the 10-STN sample. Probably, the formation of hydroxyl anions is responsible for an increase in the number of strong basic sites in the x-STN catalysts with increasing sodium carbonate loading. The spectra of the reference 5-Al2O3 catalyst and γ-Al2O3 support are also shown in Fig. 5 (b) for comparison. In the spectrum of the 5Al2O3 catalyst, two well-defined peaks were observed at 1404 and 1570 cm−1 and a shoulder at ∼1090 cm−1. Two of these peaks correspond well with the presence of free ionic CO32- adsorbed on the alumina surface: 1404 cm−1 (asymmetric stretch) and 1090 cm−1 (symmetric stretch) [36,37]. The peak at 1570 cm−1 can be due to mono- or bidentate carbonate species coordinated to Al3+ cations [34,36,38]. These species could be formed through the interaction of the basic carbonate ions with the Lewis acid sites of the alumina support, leading to a decrease in the amount of strong basic sites (neutralization) and the formation of a significant amount of weak basic sites in the 5-Al2O3 catalyst as it was detected by the CO2-TPD characterization (Table 2). Therefore, the FT-IR characterization of the prepared samples showed the presence of different types of carbonate species on the STN and γalumina supports.
3.2. Catalytic activity in the transesterification reaction The synthesized x-STN catalysts were tested in the transesterification of soybean oil with methanol. Table 3 presents the yields of fatty acid methyl esters (FAME) obtained with different catalysts upon the same reaction conditions (80 °C, 30 min reaction time, CH3OH to soybean oil molar ratio = 20:1, 1 wt. % of catalyst). Also, the kinematic viscosity of the purified product obtained in each reaction is shown. The unmodified STN catalyst resulted in 14.0% FAME yield, while the catalysts containing sodium carbonate, 3-STN, 5-STN and 10-STN, gave biodiesel yields of 27.2, 66.0 and 97.1%, respectively. Therefore, the modification of the sodium titanate nanotubes with Na2CO3, as expected, resulted in a noticeable improvement of their catalytic activity in transesterification. The 10-STN catalyst showed the best catalytic activity among all tested catalysts, leading to a reaction product with a kinematic viscosity of 4.5 mm2/s that satisfies biodiesel specifications [39]. The FAME yield obtained with this catalyst was almost seven times higher than that obtained with the starting STN material. For comparison purposes, unsupported Na2CO3 and alumina-supported Na2CO3 (5-Al2O3) were also evaluated in the same reaction (Table 3). Unsupported sodium carbonate had a very low activity (∼1% of FAME yield) that can be due to its small surface area and low solubility in the reaction medium, while the 5-Al2O3 catalyst gave higher biodiesel yield
Fig. 5. FT-IR spectra of: (a) STN support, x-STN catalysts and unsupported sodium carbonate used as a precursor for the catalysts’ preparation; (b) γ-Al2O3 and 5-Al2O3 references. The IR peaks of the STN support are marked with an asterisk (*).
deformation, respectively, of the free (symmetrical) carbonate ion [34]. When this precursor was deposited on the SNT material, new absorption bands at 1372 and 980 cm−1 were observed in the spectra of x-STN catalysts, whose intensity increased with the increase in the Na2CO3 loading in the catalysts from 3 to 10 wt. %. On the other side, the characteristic signals of the bulk Na2CO3 almost disappeared, being the peak with the maximum at 1423 cm−1 detected only in the spectrum of the 10-STN catalyst with the largest sodium carbonate loading. Therefore, it can be concluded that the states of CO32- in bulk Na2CO3 and the prepared x-STN catalysts with small sodium carbonate loadings are different. According to literature [34,35], the absorbance band at 1372 cm−1 can be attributed to monodentate Ti-O−CO2- species or to bicarbonate ions forming hydrogen-bonded dimers in the solid state. This assumption seems to be reasonable considering that the presence of water even in the dried STN material can induce hydrolysis of the deposited sodium carbonate according to the equation: CO32- + H2O ↔
Table 3 Catalytic activity of the x-STN catalysts in transesterification of soybean oil with methanol. Reaction conditions: 0.5 g of catalyst; 27 g of soybean oil; methanol to soybean oil molar ratio 20:1; temperature 80 °C, reaction time 30 min. Sample
Conversion to FAME (%)a
Kinematic viscosity (mm2/ s)b
STN 3-STN 5-STN 10-STN 5-Al2O3 Unsupported Na2CO3
14.0 27.2 66.0 97.1 13.3 1.1
21.1 16.2 6.9 4.5 22.0 30.1
a b
5
Determined by 1H NMR. FAME - fatty acid methyl esters (biodiesel). Kinematic viscosity of soybean oil was 32.6 mm2/s.
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should be related with the increment in the amount of strong basic sites in these systems as a result of the interaction of Na2CO3 with STN. In addition, high Na+ content in the x-STN catalysts could also facilitate the transesterification reaction making easier the approaching of the triglyceride molecules to the catalyst’s surface and by increasing the reaction rate due to the increment of the positive charge on the carbon atom of the C]O group, both because of the possible interaction of the carbonyl oxygen with the sodium cations of the catalyst. Finally, the particular structure of the nanotubular sodium titanate material makes the active basic sites more easily available for the interaction with large triglyceride molecules. This is because the basic sites are located not only in the internal pores of the nanotubes, but also a significant amount of them (half or more) are on the external surface of the nanotubes, exposed for the interaction with the reactant molecules. On the contrary, in the γ-alumina-supported catalysts, the active sites are mostly located inside the ink-bottle shaped pores and only a very small amount of them on the external surface of the catalyst particles. The combination of all above factors could explain why the STN material has an advantage compared to the conventional γ-alumina in the preparation of basic catalysts for the transesterification of large triglyceride molecules.
(13.3%) that could be attributed to a better dispersion of Na2CO3 on the alumina surface and an increase in the amount of the accessible basic sites. Nevertheless, a comparison of the activity of the 5-Al2O3 and 5STN catalysts with the same Na2CO3 loading showed that the change of the alumina support to STN resulted in a five-time increase in the FAME yield (from 13 to 66%). Such high activity of the x-STN catalysts evidences the presence of a synergetic effect between sodium carbonate and the STN material. It could be mentioned that in the case of the 5Al2O3 catalyst, the active basic phase (Na2CO3) was deposited on an almost inert material with a very low amount of strong basic sites (Table 2) used as a support. Therefore, an increase in the activity of this catalyst could be due mainly to an increase in the dispersion of the deposited sodium carbonate salt. In addition, according to the FT-IR results, some decrease in the amount of strong basic active sites could be expected in this catalyst because of the interaction of CO32− anions with Lewis acid sites of the alumina support leading to the formation of mono- and bidentate carbonate species coordinated to Al3+ cations (a kind of neutralization of strong basic active sites of the Na2CO3 active phase due to the interaction with the acidic alumina support). On the contrary, the x-STN catalysts represent the combination of two basic materials, sodium carbonate and sodium titanate nanotubes, both of which are active for the transesterification of triglycerides. The interaction between these two materials resulted in the appearance of different types of basic sites in the x-STN catalysts compared to the 5Al2O3 one. The formation of strongly-basic OH- groups was assumed from the FT-IR characterization results that can be promoted by the presence of physisorbed and structural water in the sodium titanate nanotubes. From the above, it can be concluded that the presence of strong basic sites in the catalysts and their amount is the most important characteristic for the transesterification activity of the catalysts. The above trends in the catalytic activity can be explained based on the mechanism of the transesterification of triglycerides with methanol in basic catalysis conditions. Namely, the role of the basic catalyst is the deprotonation of methanol with the formation of methoxide anions, which are the active species that attack the triglyceride molecules at the carbonyl group [40]. Previously [6], it was shown that only strong basic salts (Na3PO4, K3PO4, Na2CO3, etc.) were able to deprotonate methanol and catalyze the conversion of triglycerides to FAME (biodiesel), while less strong bases (Na2HPO4, NaHCO3, etc.) were not catalytically active. Therefore, we assumed that in our catalysts only strong basic sites are active for the transesterification of soybean oil with methanol. A comparison between the amount of strong basic sites in the catalysts and the biodiesel yield (Fig. 6) showed a clear correlation between these two parameters confirming the above supposition. Therefore, the high catalytic activity of the x-STN catalysts in the transesterification
3.3. Reusability of the synthesized x-STN catalysts The possibility of the reutilization of heterogeneous basic catalysts in several reaction cycles is an important issue for biodiesel production at an industrial level. In the present work, we selected the most active catalyst (10-STN) to test its reusability. This catalyst was re-used three times without any further purification and activation. In these experiments, after each run the catalyst was separated from the reaction mixture by centrifugation and re-used in the consecutive run with the fresh portions of soybean oil and methanol maintaining the reaction conditions described previously (Table 3, title). It was observed that biodiesel yield decreased in the successive cycles from 97.1% in the first run to 38.5% in the third one. For comparison purposes, we performed a similar experiment with the reference 5-Al2O3 catalyst which showed a very low catalytic activity in the third run (around 1–2 % biodiesel yield). In both cases, the decrease in the biodiesel yield upon reutilization of the same catalyst in several reaction runs can be only partially attributed to some loss of the catalyst during its separation and reutilization. The physical loss of the catalyst between two consecutive reactions was not larger than ∼ 5%. In order to inquire into the reasons of the catalysts’ deactivation, the characterization of the recycled 10STN and 5-Al2O3 catalysts was performed. Fig. 7 shows XRD patterns of the fresh and used 10-STN catalysts. It can be observed that the crystalline structure of the 10-STN catalyst did not suffer any change after recycling and no new crystalline phase was detected. Therefore, no destruction of the catalyst or agglomeration of the active phase took place upon recycling. On the other hand, the SEM-EDX characterization of the reused catalysts showed that reutilization of the same catalyst in several reaction cycles resulted in a decrease in its sodium content. Thus, for the 10-STN catalyst, sodium content decreased from 13.16 wt. % in the fresh catalyst (Table 1) to 12.01 wt. % in the catalyst re-used in three catalytic cycles. Regarding the reference 5-Al2O3 catalyst, sodium content also decreased upon recycling from 2.13 to 1.73 wt. %. Based on these results, we assumed that the principal reason for the deactivation of our catalysts prepared with sodium carbonate was the leaching of the catalytically active phase (Na2CO3) in the reaction media. This is in line with the available literature reports about the utilization of pure and supported Na2CO3 and K2CO3 salts as catalysts for biodiesel synthesis [6,10,12,41]. Thus, it was reported that unsupported sodium and potassium carbonate and phosphate salts are slightly soluble in the transesterification reaction mixture, mainly in polar alcohols (in an excess of methanol and methanol/glycerol solutions) that complicates the complete catalyst’s recovery after the transesterification process [6,10]. To diminish this problem and
Fig. 6. Correlation between the amount of strong basic sites in the x-STN catalysts and the obtained biodiesel (FAME) yields. Results obtained with the reference 5-Al2O3 catalyst are also shown for comparison purposes. 6
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Acknowledgements Financial support by DGAPA-UNAM, México (PAPIIT grant number IN-115218) is gratefully acknowledged. The authors thank I. Puente Lee, C. Salcedo Luna and L. Pedro Hernández for performing electron microscopy, powder XRD and NMR characterizations, respectively, and Gabriela Díaz and Antonio Gómez-Cortés from Instituto de Física, UNAM for help with the CO2–TPD experiments. References [1] D. Laforgia, V. Ardito, Bioresour. Technol. 51 (1995) 53–59. [2] H.J. Kim, B.S. Kang, M.J. Kim, Y.M. Park, D.K. Kim, J.S. Lee, K.Y. Lee, Catal. Today 93-95 (2004) 315–320. [3] H. Fukuda, A. Kondo, H. Noda, J. Biosci. Bioeng. 92 (2001) 405–416. [4] P.T. Vasudevan, B. Fu, Waste Biomass Valoriz. 1 (2010) 47–63. [5] A. Hayyan, M.Z. Alam, M.E.S. Mirghani, N.A. Kabbashi, N.I.N.M. Hakimi, Y.M. Siran, S. Tahiruddin, Bioresour. Technol. 101 (2010) 7804–7811. [6] S.L. Britton, J.Q. Bond, T.W. Root, Energy Fuels 24 (2010) 4095–4096. [7] J. Ji, J. Wang, Y. Li, Y. Yu, Z. Xu, Ultrasonics 44 (2006) 411–414. [8] J. Tang, Q. Zhang, K. Hu, S. Cao, S. Zhang, J. Wang, J. Catal. 368 (2018) 190–196. [9] B. Yang, L. Leclercq, J.-M. Clacens, V. Nardello-Rataj, Green Chem. 19 (2017) 4552–4562. [10] K. Malins, Fuel Process. Technol. 179 (2018) 302–312. [11] P.E. Gama, E.R. Lachter, R.A.S. San Gil, A.V. Coelho, I.A. Sidi, R.L. Poubel, A. da Costa Faro Jr., A.L.F. deSouza, Quim. Nova 38 (2015) 185–190. [12] I. Lukić, J. Krstić, D. Jovanović, D. Skala, Bioresour. Technol. 100 (2009) 4690–4696. [13] H.J. Kim, B.S. Kang, M.J. Kim, Y.M. Park, D.K. Kim, J.S. Lee, K.Y. Lee, Catal. Today 93–95 (2004) 315–320. [14] W. Xie, H. Peng, L. Cheng, Appl. Catal., A-Gen 300 (2006) 67–74. [15] X. Liang, S. Gao, H. Wu, J. Yang, Fuel Process. Technol. 90 (2009) 701–704. [16] O. Ilgen, A.N. Akin, Energy Fuels 23 (2009) 1786–1789. [17] M. Manríquez-Ramírez, R. Gómez, J.C. Hernández-Cortez, A. Zúñiga-Moreno, C.M. Reza-San Germán, S.O. Flores-Valle, Catal. Today 212 (2013) 23–30. [18] M. Zabeti, W.M.A.W. Daud, M.K. Aroua, Fuel Process. Technol. 91 (2010) 243–248. [19] P. Hernández-Hipólito, M. García-Castillejos, E. Martínez-Klimova, N. JuárezFlores, A. Gómez-Cortés, T.E. Klimova, Catal. Today 220-222 (2014) 4–11. [20] P. Hernández-Hipólito, N. Juárez-Flores, E. Martínez-Klimova, A. Gómez-Cortés, X. Bokhimi, L. Escobar-Alarcón, T.E. Klimova, Catal. Today 250 (2015) 187–196. [21] T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Adv. Mater. 11 (1999) 1307–1311. [22] G. Knothe, J. Am. Oil Chem. Soc. 77 (2000) 489–493. [23] American Society for Testing Materials (ASTM), Standard D445. Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids, PA, ASTM, West Conshohocken, 2006. [24] E. Morgado Jr, M.A.S. de Abreu, G.T. Moure, B.A. Marinkovic, P.M. Jardim, A.S. Araujo, Chem. Mater. 19 (2007) 665–676. [25] E. Morgado Jr, M.A.S. de Abreu, O.R.C. Pravia, B.A. Marinkovic, P.M. Jardim, F.C. Rizzo, A.S. Araújo, Solis State Sci. 8 (2006) 888–900. [26] R.A. Ortega-Domínguez, J.A. Mendoza-Nieto, P. Hernández-Hipólito, F. GarridoSánchez, J. Escobar-Aguilar, S.A.I. Barri, D. Chadwick, T.E. Klimova, J. Catal. 329 (2015) 457–470. [27] S. Mozia, E. Borowiak-Paleń, J. Przepiórski, B. Grzmil, T. Tsumura, M. Toyoda, J. Grzechulska-Damszel, A. Morawski, J. Phys. Chem. Solids 71 (2010) 263–272. [28] B.C. Vaina, O.P. Ferreira, A.G. Souza Filho, A.A. Hidalgo, J. Mendes Filho, O.L. Alves, Vib. Spectrosc. 55 (2011) 183–187. [29] L. Zhang, X. Wang, H. Chen, F. Jiang, Clean (Weinh) 42 (2014) 947–955. [30] Q. Chen, G.H. Du, S. Zhang, L.-M. Peng, Acta Crystallogr. Sect. B Struct. Sci. B58 (2002) 587–593. [31] H. Li, K. Zheng, X. Xu, H. Zhao, Y. Song, Y. Sheng, Q. Huo, H. Zou, Powder Technol. 228 (2012) 277–283. [32] M. Shirpour, J. Cabana, M. Doeff, Energy Environ. Sci. 6 (2013) 2538–2547. [33] R.A. Niquist, R.O. Kagel, Infrared Spectra of Inorganic Compounds, Academic Press, Inc, USA, 1971. [34] G. Busca, V. Lorenzelli, Mater. Chem. 7 (1982) 89–126. [35] T. Klimova, D.A. Solís Casados, J. Ramírez, Catal. Today 43 (1998) 135–146. [36] T. Yamaguchi, Y. Wang, M. Komatsu, M. Ookawa, Catal Surv. Jap. 5 (2002) 81–89. [37] T.J. Toops, D.B. Smith, W.P. Partridge, Appl. Catal. B: Environ. 58 (2005) 245–254. [38] D.M. Alonso, R. Mariscal, R. Moreno-Tost, M.D. Zafra Poves, M. López Granados, Catalan J. Commun. Cult. Stud. 8 (2007) 2074–2080. [39] ASTM D6751 – biodiesel blends stock specification (B100). [40] F.A. Carey, Organic Chemistry, 3rd ed., The McGraw-Hill Companies, Inc, New York, 1996. [41] R. Malhotra, A. Ali, Renew. Energy 133 (2019) 606–619.
Fig. 7. Powder XRD patterns of the fresh 10-STN catalyst and the same catalyst after three consecutive reaction runs.
stabilize the above mentioned alkali salts, it was proposed to fix them to a surface of the silica or alumina supports [6]. However, this attempt was not successful since a leaching of the active sites was observed in both cases of silica- and alumina-supported samples. Similar results (potassium leaching) were also obtained in work [12] for the heterogeneous K2CO3 catalysts supported on an alumina/silica material synthesized by the sol–gel method. In the recent work [41], the preparation of the Na/ZnO/SBA-15 catalysts with improved stability was reported. The 10-Na/ZnO/SBA-15 catalyst was capable of yielding complete conversion of virgin cotton seed oil to biodiesel in the first three catalytic runs decreasing the activity in the fourth and fifth cycles due to the partial dissolution of the catalyst. However, it is worth to mention that in this case, the catalyst recovered after each run was subjected to the strict regeneration process including several runs of washing with hexane and methanol to eliminate polar and non-polar contaminants, followed by drying and calcination, which was not performed with the catalysts prepared in the present work. 4. Conclusions Sodium titanate nanotubes modified by different amounts of sodium carbonate showed high activity in the transesterification of soybean oil with methanol. The most active catalyst, 10-STN, resulted in the 97% yield of methyl esters at a short reaction time (30 min) in mild conditions (80 °C). High activity of the STN catalysts containing Na2CO3 was attributed to a synergetic effect between the support and the deposited sodium salt, which leads to a noticeable increase in the amount of strong basic sites able to catalyze the transformation of triglycerides to methyl esters governed by methoxide anions. A comparison of two catalysts with the same sodium carbonate loading supported on γ-alumina and STN, showed that the 5-STN catalyst was about 5 times more active than the 5-Al2O3 one, that was attributed to the presence of different types of supported carbonate species in these samples. However, upon reutilization, both the synthesized Na2CO3-containing STN catalysts and the reference 5-Al2O3 sample showed a decrease in their activity due to a leaching of the active phase in the reaction media.
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