Fuel 87 (2008) 3071–3076
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Activation of Mg–Al hydrotalcite catalysts for transesterification of rape oil Hong-yan Zeng *, Zhen Feng, Xin Deng, Yu-qin Li Institute of Biotechnology, College of Chemical Engineering, University of Xiangtan, Hunan 411105, PR China
a r t i c l e
i n f o
Article history: Received 3 July 2007 Received in revised form 31 March 2008 Accepted 1 April 2008 Available online 23 April 2008 Keywords: Rape oil Transesterification Mg–Al hydrotalcite
a b s t r a c t Mg–Al hydrotalcites with different Mg/Al molar ratios were prepared and characterized by powder X-ray diffraction (XRD), Fourier-transform infrared spectra (FTIR), thermogravimetric apparatus and differential thermal analysis (TGA-DTA) and scanning electron micrograph (SEM). It was confirmed by XRD that the materials had hydrotalcite structure. The hydrotalcite catalyst calcined at 773 K with Mg/Al molar ratio of 3.0 exhibited the highest catalytic activity in the transesterification. In addition, a study for optimizing the transesterification reaction conditions such as molar ratio of the methanol to oil, the reaction temperature, the reaction time, the stirring speed and the amount of catalyst, was performed. The optimized parameters, 6:1 methanol/oil molar ratio with 1.5% catalyst (w/w of oil) reacted under stirring speed 300 rpm at 65 °C for 4 h reaction, gave a maximum ester conversion of 90.5%. Moreover, the solid catalyst could be easily separated and possibly reused. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction The increasing demand for energy and environmental awareness has prompted a lot of researches into ways of producing alternative fuels from renewable resources that are environmentally more acceptable. Biodiesel is an interesting alternative because it produces favorable effects on the environment, such as decreases in acid rain and in greenhouse effect caused by combustion. Due to these factors and to its biodegradability, the production of biodiesel is considered an advantage to that of fossil fuels. In addition to these, it also shows a decrease in the emission of CO2, SOx and unburned hydrocarbons during the combustion process. China, as many countries, does not have enough reserves of fossil resources, which implies a dependency on petroleum imports in order to provide for the demand of petrol and diesel fuel in the transport sector. The supply of part of the demand with biodiesel would contribute to decreasing this dependency. Different methods of synthesizing biodiesel had been proposed [1–3]. The most common is the catalytic transesterification reaction of vegetable oils with a short-chain alcohol, usually methanol. This reaction was well studied and established for soybean, sunflower and rapeseed oils, using homogeneous acids and alkalis as catalysts [2,4–7]. However, these traditional catalytic systems have some technological problems, being that the acid system is associated with corrosion and the basic one with emulsification, separation difficulties and side reaction such as decomposition and polymerization which would also take place during distillation
after the reaction. To eliminate these problems, attention was focused on to develop heterogeneous catalysts [8–10]. The use of heterogeneous catalysts makes separation of the product easier and produces neither corrosion nor emulsion. Knifton and Duranleau [11] used free organic phosphines supported on partially cross-linked polystyrene for the reaction. And solid base catalysts were also widely explored [10,12,13]. Recently, Kim et al. [10] prepared a solid superbase of Na/NaOH/c-Al2O3 that showed almost the same catalytic activity under the optimized reaction conditions as that found with the conventional homogeneous NaOH catalyst. In the methanolysis experiments using Mg–Al hydrotalcite catalysts for biodiesel, the best ester conversions of soybean oil and glyceryl tributyrate were below 80% [14,15]. So it is important to increase the ester conversion for the reduction of production cost. In the present work, calcined Mg–Al hydrotalcites were adopted for methanolysis of rape oil to methanol. The catalytic efficiency was studied regarding the methyl ester conversion. The structure characterization of Mg–Al hydrotalcites was studied with XRD, FTIR, TGA-DTA and SEM. The transesterification process of rape oil to methanol using Mg–Al hydrotalcite catalysts was studied and the catalyst could be easily separated and possibly reused. In the optimized reaction conditions, the highest ester conversion was 90.5% using the Mg–Al hydrotalcite catalyst. 2. Experimental 2.1. Preparation of the hydrotalcites
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0016-2361/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2008.04.001
Hydrotalcites with various Mg/Al molar ratios were prepared by coprecipitation at high supersaturation. In the method, two
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solutions, A and B, were first heated to 40 °C, respectively. Then they were added simultaneously to a beaker under vigorous stirring. Solution A (200 ml) was prepared by mixing saturated solutions of Mg and Al metal nitrates in the desired molar ratios. Solution B was prepared by dissolving 14.0 g sodium hydroxide (0.35 mol) and 15.9 g sodium carbonate (0.15 mol) in 200 ml deionized water. After 2 h reaction, the precipitates were aged at 65 °C in a thermostatic bath. The resulting product was filtered, washed thoroughly with deionized water until the filtrate showed no presence of NaOH and subsequently dried at 90 °C for 24 h. Part of the samples was calcined at 673–1073 K for 7 h in a muffle furnace for further characterization analysis and catalytic activity study. For convenience, the hydrotalcites thus obtained with different Mg/Al molar ratios of 1.0, 3.0, 4.0 and 6.0 were designated as MAH-1, MAH-3, MAH-4 and MAH-6. 2.2. Characterization of the hydrotalcites XRD (Rigaku D/MAX-3C), FTIR (Perkin-Elmer Spectrum One B), TGA/DTA (ShimadzeDT-40) and SEM (JEOL JSM-6360LV). 2.3. Transesterification reaction Refined rape oil and an appropriate volume of methanol with calcined Mg–Al hydrotalcite catalysts (0.5–2.5%) were placed into a 500 ml three-angel necked flask equipped with reflux condenser and Teflon stirrer (100–500 rpm). The reaction mixture was blended for a period time at 55–75 °C temperature under atmospheric pressure. Molar ratio of methanol to oil was taken at 3– 9:1. After reaction, the methanol was recovered by a rotary evaporator in vacuum at 45 °C. Subsequently, the hydrotalcite catalyst was separated by filtration and the ester layer was separated from the glycerol layer in a separating funnel. The fined ester layer was dried over sodium sulfate and analyzed by gas chromatography on a Perkin-Elmer GC-200 chromatograph with FID detector, equipped with a stainless steel packed column (3mm 2 m Silar 9 cp). The oven temperature program consisted of: start at 160 °C (2 min), ramp at 1.5 °C min 1 to 215 °C (10 min). Undecanoic acid methyl ester was used as the internal standard. In parallel fashion, each reaction was repeated three times under the same experimental conditions in order to determine the change in transesterification product composition over time.
2.4. Statistical analysis All the experiments were carried out four times in order to determine the variability of the results and to assess the experimental errors. Thus, when the two experimental procedures have been carried out for each catalyst, both of them were repeated three times. In this way, the arithmetical averages and the standard deviations were calculated for all the results. 3. Results and discussion 3.1. Catalyst characterization The XRD patterns of the 90 °C dried samples (Fig. 1a) showed sharp and symmetric peaks which gave clear indication that the samples were well crystallized and the peaks corresponding to (0 0 3), (0 0 6), (0 0 9), (0 1 5), (0 1 8), (1 1 0) and (1 1 3) planes were characteristic of clay mineral (hydrotalcite) having a layered structure [16]. The peaks at 11.34° and 22.9° were assigned to the (0 0 3) and (0 0 6) reflections, respectively, and could be used to calculate the basal spacing between the layers, d. The peak 60.5° was assigned to the (1 1 0) reflections and could be used to calculate the unit cell dimension, a, where a = 2d110. The results of these calculations were shown in Table 1. In agreement with earlier measurements by Cavani et al. and others [15–18], the unit cell dimension a increased with Mg2+ content (Table 1) and the intensity of the corresponding peaks decreased as the Mg/Al molar ratio increased (Fig. 1a). During calcination, the decomposition of hydrotalcite resulted in formation of mixed Mg–Al oxides phases. This fact was confirmed by the XRD patterns of the hydrotalcite samples (MAH-3) calcined at 673 K, 773 K, 873 K and 1073 K, which were shown in Fig. 1b. By 673 K, all the hydrotalcite reflections were gone, with the exception of a broadened and shifted peak which might have evolved from the hydrotalcite (0 0 9) (Fig. 1b). The only other peaks present at 673 K were the major reflections of MgO which were broadened due to poor crystallization or small particle size, or both, which confirmed the results of Mackenzie et al. [19]. All the XRD patterns of the mixed oxides (calcined hydrotalcites) exhibited the typical features of a mixed oxide of Mg(Al)O type [19,20]. It was especially noteworthy that the intensities of these peaks were the highest for 773 K sample indicating the presence of well crystallized MgO in this sample. For 873 K and 1073 K,
Fig. 1. XRD patterns of hydrotalcite samples aged for 12 h. (a) Uncalcined samples with various Mg/Al molar ratios; (b) MAH-3 samples calcined at 673–1073 K.
H.-y. Zeng et al. / Fuel 87 (2008) 3071–3076 Table 1 Calculation of lattice parameter and basal plane for uncalcined samples as a function of Mg content Sample
(0 0 3) reflection, 2h (°)
d003 (Å)
(0 0 6) reflection, 2h (°)
d006 (Å)
(1 1 0) reflection, 2h (°)
d110 (Å)
a (Å)
MAH-1 MAH-3 MAH-4 MAH-6
11.34 11.36 11.07 10.97
7.8 7.8 7.9 8.0
22.87 22.89 22.41 21.94
3.9 3.8 3.9 4.0
60.46 60.47 60.17 59.73
1.50 1.52 1.54 1.55
3.00 3.04 3.08 3.10
the peaks, which appeared at 2h 20°, could suggest that traces of Al-containing amorphous phases were present in these samples [21]. For all the calcined samples, the characteristic reflections observed clearly at 2h 43° and 63° corresponded to a MgO-like phase (periclase) or magnesia–alumina phase [20], while the peaks of Al2O3 phase were very small, indicating that Al3+ cations were dispersed in the structure of MgO without the formation of spinel species. The FTIR spectra of the catalysts prepared under different conditions were presented in Fig. 2. For all the uncalcined hydrotalcites with different Mg/Al molar ratios (Fig. 2a), the first band was found at 3498–3467 cm 1 (m1-OH 1) and it corresponded to the OH mode, caused by the interlayer water molecules and hydroxyl groups in the brucite-like layers [22]. The weak band observed in the 1646–1640 cm 1 region (d-HOH) was due to the H2O from the interlayer water [22,23]. The 1384 cm 1 peak (m2-CO3 2 ) corresponded to carbonate [24]. The peaks at 868–827 cm 1 (m3-CO3 2 ) stood for covalent carbonate [23,24], but appeared only in sample MAH-1. In the low energy ranges of the spectra 704–637 cm 1 (m4MgO or Al–O) and around 506 cm 1 (m5- Mg–O or Al–O), the peaks were attributed to the presence of Mg–O and Al–O bands [25]. After 773 K calcination of Mg–Al hydrotalcite (Fig. 2b), there was a significant decrease in the intensity of water and carbonate characteristic peaks due to the removal of water and CO2 vapors. The spectra of calcined samples were different from the previous spectra. The (m1-OH 1) mode appeared around 3400 cm 1 which was about 85 cm 1 lower than that of uncalcined samples, and a shoulder at 1680–1636 cm 1 corresponded to characteristic vibrations of water. Calcination at 773 K induced dehydration, dehydroxylation and decarbonation leading to the formation of mixed oxides of MgO and Al2O3 and the FTIR spectra of calcined samples reflected these changes. A broadly intensified band around 1420– 1385 cm 1 corresponded with non-interlayer carbonate. The amount of carbonate remaining strongly depended on the Mg content since the basicity of surface oxygen in Mg-containing samples
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was higher leading to more stable carbonate species [26]. There were no peaks appearing at 868–827 cm 1. It was obvious that the OH 1 of water was still present at 773 K such as shown by the peaks encompassing 3400 cm 1 band. The TGA-DTA peaks obtained from the thermal analysis studies of the samples were presented in Fig. 3. The analysis exhibited a gradual weight loss process from about 293 K to 1123 K, with two main endothermic effects on one below 523 K and the other below 723 K, respectively. For all the hydrotalcite samples, the first large endothermic effect, between 473 K and 523 K (14.49– 20.04 wt.%), involved interlayer water loss; the second endothermic effect, between 673 K and 698 K (27.80–33.41 wt.%), corresponded to the loss of OH 1 groups and the decomposition of CO3 2 in the brucite-like layers [27]. In order to determine the morphology and particle size distribution of Mg–Al hydrotalcite, we selected the hydrotalcite sample MAH-3 observed with SEM. It could be observed clearly from SEM image of uncalcined hydrotalcite in Fig. 4 that the MAH-3 formed well-developed, thin flat crystals with various edges indicting the layered structure (Fig. 4a). The flat crystals with particle sizes in range of 3–120 lm were observed by SEM and probably consisted of Mg–Al hydrotalcite crystals. Moreover, there were also many bar-like particles (Fig. 4b) and curved sheets (Fig. 4a). The length of these bars was 13–50 lm and the diameter 2–8 lm. Also, there was some tendency for platelets to aggregate in the bar and platy sheet manner, as illustrated by the micrographs in Fig. 4a and b. As originally suggested by De Roy et al. [28], the plate–plate overlapping of crystallites gave rise to interfaces that could accommodate extrinsic surface water, as well as other adsorbates. It was apparent from Fig. 4c that the platy sheets could give rise to pores in the 300–900 Å size range. Clearly, it was the textural features that contributed to the pore water content of Mg–Al hydrotalcite compositions. To visualize the influence of aging time on the crystallinity, the XRD patterns were obtained for the MAH-3 samples aged at 65 °C for various times (Fig. 5). The crystallinity of hydrotalcite depended upon aging which increased crystallinity and allowed time for the classical hydrotalcite shape to form [29]. In Fig. 5, the samples showed typical XRD patterns of well-crystallized hydrotalcite. From the sharpness and intensity of peaks, it was understood that better crystallinity occurred as the aging time was prolonged [30]. In the peaks corresponding to (0 0 3) reflection, the sample aged for 12 h had the most high layer spacing d003 (data not shown) and sharp peak, but the sample aged for 18 h had the most intense peak of all the four samples (Fig. 5). The sample aged for 12 h was the most optimally crystalline though there was little difference in the intensity of (0 0 3) reflection peak between of 12 h and 18 h samples.
Fig. 2. IR spectra of hydrotalcite samples aged for 12 h. (a) Uncalcined samples with various Mg/Al molar ratios; (b) samples calcined at 773 K with various Mg/Al molar ratios.
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Fig. 3. TGA-DTA curves of uncalcined samples aged for 12 h with various Mg/Al molar ratios.
Fig. 4. SEM images of uncalcined MAH-3 sample aged for 12 h. (a) 1000; (b) 1.500; (c) 10,000.
3.2. Catalytic activity measurements
Fig. 5. XRD patterns of MAH-3 samples at different aging times.
The variation in conversion level to hydrotalcite catalysts calcined at 773 K for 7 h with different Mg/Al molar ratios was studied. The conversion of the methyl ester for 4 h reaction was improved from 75.0% to 90.2%, when the molar ratio of Mg/Al was increased from 1.0 to 3.0. Interestingly, the acid values (MAH-1 and 3: 0.20 and 0.16 mg[KOH]g[oil] 1, respectively) and saponification values (MAH-1 and 3: 164.4 and 163.7 mg[KOH] g[oil] 1, respectively) of the methyl ester were the lowest when Mg/Al molar ratio was 3.0. As a result, the conversion of methyl ester showed a maximum with Mg/Al molar ratio of 3.0. Qualitatively similar trends were also reported by others [14,15]. The increased basicity could be expected to correlate with an increase of the catalyst activity. This could be due to the presence of much stronger basic sites (super basic) in the catalyst with 3.0, probably corresponding always to the isolated O2 but locating in a particular position of the surface [31]. In addition, the ester conversions (MAH-4 and 6: 80.2% and 67.0%, respectively) decreased and the acid values (MAH-4 and 6: 0.22 and 0.21 mg[KOH]g[oil] 1, respectively) and saponification values (MAH-4 and 6: 164.9 and 164.6 mg[KOH] g[oil] 1, respectively) were enhanced, when the Mg/Al molar ratio was greater than 3.0. The effect of calcination temperature on the catalytic activity was investigated, and the results showed that the catalytic activity was affected significantly by temperature of calcination. With increasing calcination temperature to 773 K, the ester conversion increased gradually and reached the maximum of 90.5% (uncalcined: 60.0%,
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673 K: 86.0%). However, when the temperature was higher than 773 K, the conversion dropped considerably (873 K: 79.0%, 1073 K: 68.5%). Temperature 773 K was probably the optimum temperature for calcination of MAH-3 sample. Another important issue concerning use of calcined hydrotalcite was its reusability and stability under optimal reaction conditions. To gain insight into this issue, we performed six consecutive reactions using of the same catalyst after filtration and washing with methanol for removing the ester and glycerol attaching to the catalyst. The catalyst could be recycled at least three times in the transesterification although the catalytic activity decreased with the reuse of the catalyst (1st: 90.5%, 2nd: 90.0%, 3rd: 88.3%, 4th: 81.7%, 5th: 77.8%, 6th: 61.1% for 4 h reaction). The catalyst could be recycled without appreciable loss in activity and kept over 88.0% in the three recycle experiments. After three uses, the activity of the catalyst decreased sharply. Taking all the information obtained from the Mg–Al hydrotalcite characterization into account, we could finally conclude that the calcined Mg–Al hydrotalcite with an Mg/Al molar ratio of 3.0, aged for 12 h at 773 K calcination was determined to be the optimum catalyst that could give the best crystallinity (Fig. 1b) and catalytic activity for the transesterification reaction. 3.3. Transesterification reaction The molar ratio of methanol to vegetable oil was one of the most important variables that affect ester formation because the conversion and the viscosity of produced ester depended on it. The stoichiometric molar ratio of methanol to oil is 3.0. But when mass transfer is limited due to problems of mixing, the mass transfer rate seems to be much slower than the reaction rate, so the conversion can be elevated by introducing excess amount of the reactant methanol to shift the equilibrium to the right-hand side. Higher molar ratios result in greater ester conversions in a shorter time. Krisnangkura and Simamaharnnop [32] transesterified palm oil at 70 °C in an organic solvent with sodium methoxide as a catalyst and found that the conversion increased with increasing molar ratios of methanol to palm oil. And high molar ratios such as 9.0 or 15.0 were used in the literatures [10,14]. In our experiments, the ester conversions increased considerably (methanol/oil molar ratio = 3.0: 61.2%, 4.0: 66.8%, 5.0: 79.3%, 6.0: 90.2%, 7.0: 90.8%, 9.0: 90.4%) with increasing the methanol loading amount. The optimum molar ratio of methanol to rape oil was found to be 6.0. Beyond the molar ratio of 6.0, the excessively added methanol had no enhance significantly on the ester conversion. In addition, the conversion increased sharply with reaction time, then reached a plateau value representative of a nearly equilibrium conversion after 4 h reaction (data not shown). A nearly maximum conversion of 90.8% was obtained after 4 h reaction time. When increasing the amount of catalyst, the slurry (mixture of catalyst and reactants) becomes too viscous giving rise to a problem of mixing and a demand of higher power consumption for adequate stirring. On the other hand, when the catalyst amount is not sufficient, maximum conversion can not be reached. In most cases, sodium hydroxide or potassium hydroxide had been used in the process of alkaline methanolysis, both in concentrations ranging from 0.5% to 1.5% w/w of oil [1,33]. In our works, the reaction profiles indicated that the ester conversion increased with the increase of catalyst amount from 0.5% to 1.5% (0.5% w/w of oil: 68.2%, 1.0: 79.5%, 1.5: 90.5%). However, the conversion decreased with further increase of catalyst amount (2.0: 85.0%, 2.5: 83.7%), which was possibly due to mixing problem of reactants, products and solid catalyst [14]. The maximum ester conversion reached to 90.5% when 1.5% catalyst was added. Mixing is very important for the transesterification of rape oil, because rape oil and methanol solution are immiscible and the
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reactants and the solid catalyst are separated in the heterogeneous system. Generally, a more vigorous stirring speed causes better contact among the reactants and solid catalyst, resulting in the increase of reaction rate. Our experiment results showed there was no reaction without stirring and stirring had a significant effect on transesterification of oil and methanol (100 rpm: 19.5%, 200: 60.2%, 300: 90.2%, 400: 90.2%, 500: 90.3%). Adding solid catalyst to the reactants while stirring facilitated the chemical reaction, the reaction started quickly. It quickly established a very stable emulsion of oil, MeOH and catalyst. The ester conversion increased rapidly with an increase of stirring speeds from 100 to 500 rpm. When the stirring speed was over 300 rpm, there was no significant enhancement in the conversions (about 90.2%). The effect of reaction temperature on the ester conversion was studied with the catalyst at six temperatures, i.e. 50, 55, 60, 65, 70 and 75 °C. The results showed the ester conversion at different reaction temperatures. Transesterification proceeded slowly at 50 °C, where the conversion was 19.6% in a 4 h reaction. Lower temperatures resulted in a drop of the ester conversion because only a small amount of molecules was able to get over the required energy barrier. The ester conversion increased up to 90.4% in 4 h reaction by increasing the reaction temperature to 65 °C (55 °C: 31.8%, 60 °C: 70.2%). Thus, the optimum temperature for the preparation of the ester was found to be 65 °C, which was near the boiling point of anhydrous methanol. The conversion fell to about 80.0% in the temperature range of 70–75 °C (70 °C: 80.9%, 75 °C: 80.0%), probably because the molar rate of methanol to oil decreased when methanol reactant volatilized into gas phase above 65 °C, the boiling point of pure methanol. 4. Conclusion For the transesterification of rape oil with methanol, calcined Mg–Al hydrotalcite was found effective as solid base catalyst. The activity of the catalysts was correlated with calcination temperature, aging time and Mg/Al molar ratio. The catalyst with an Mg/ Al molar ratio of 3.0, aged for 12 h at 773 K calcination possessed the best catalytic activity for the transesterification. The reaction conditions for the system were optimized to maximize the methanol ester conversion (about 90.5%). Optimum reaction conditions were obtained with methanol to rape oil molar ratio of 6.0, 1.5% catalyst (w/w of oil) and 300 rpm stirring speed for 4 h reaction at 65 °C. The catalyst brings advantages such as high catalytic activity, easy separation of the catalyst by simple filtration, possible recycling of the catalyst and use of non-toxic and inexpensive catalysts. It is probable that the solid base catalyst becomes a practical alternative to soluble bases. References [1] Vicente G, Martínez M, Aracil J. Integrated biodiesel production: a comparison of different homogeneous catalysts systems. Bioresour Technol 2004;92:297–305. [2] Fukuda H, Kond A, Noda H. Biodiesel fuel production by transesterification of oils. J Biosci Bioeng 2001;92:405–16. [3] Stavarache C, Vinatoru M, Nishimura R, Maeda Y. Fatty acids methyl esters from vegetable oil by means of ultrasonic energy. Ultrason Sonochem 2005;12:367–72. [4] Antolín G, Tinaut FV, Briceño Y, Castaño V, Pérez C, Ramírez AI. Optimisation of biodiesel production by sunflower oil transesterification. Bioresour Technol 2002;83:111–4. [5] Lang X, Dalai AK, Bakhshi NN, Reaney MJ, Hertz PB. Preparation and characterization of bio-diesel from various bio-oil. Bioresour Technol 2001;80:53–62. [6] Al-Widyan MI, Al-Shyoukh AO. Experimental evaluation of the transesterification of waste palm oil into biodiesel. Bioresour Technol 2002;85:253–6. [7] Freedman B, Pryde EH, Mounts TL. Variables affecting the yields of fatty esters from transesterified vegetable oils. J Am Oil Chem Soc 1984;61:1638–43. [8] Mehera LC, Kulkarnib MG, Dalaib AK, Naika SN. Transesterification of karanja (Pongamia pinnata) oil by solid basic catalysts. Eur J Lipid Sci Technol 2006;108:389–97.
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