The use of heterogeneous catalysts in the transesterification reaction of vegetable oils has getting emphasis in recent years, mainly by the alternative of obtaining clean fuel derived from renewable sources. Metal oxide such as MgO or CaO, supported ones like and zeolites are often applied in transesterification process. Among them, hydrotalcite has shown potential as catalysts on this reaction due to their physicochemical properties, such as: thermal stability, porosity, specific surface area, memory effect, basicity, acidity and anion exchange capacity. This work studies the catalytic performance of the calcium and aluminum based hydrotalcite in the transesterification reaction of soybean oil in methanol and ethanol. The hydrotalcite samples treated at 450?C were impregnated with KOH. The efficiency of impregnated and non- impregnated samples was compared and the non-impregnated one showed the best performance. This property was attributed to the higher availability of active sites used in the transesterification process.
Biodiesel is a mixture of ethyl or methyl esters of fatty acids derived from natural sources (vegetable oil or animal fat). This mixture has basically the same specification of mineral diesel, particularly in terms of viscosity and density. The transesterification reaction is the most widely used route for obtaining biodiesel and it is constituted of a set of organic reactions. In a transesterification reaction the ester group (R'-COOR'') is replaced by an alkoxy radical (OH), which is derived from an alcohol. Equation (1) shows an example of a transesterification reaction; where a triglyceride molecule reacts with methanol/ethanol producing methyl/ethyl ester and glycerol. The presence of a catalyst (usually an acid or strong base) significantly accelerates the balance of reaction [
The use of heterogeneous catalysts in the transesterification reaction has been intensely applied due to its reusability and easy separation from the reaction system [
The layered double hydroxides (LDHs) can be represented by the following general formula:
where:
M2+ = divalent metal cation;
M3+ = trivalent metal cation;
Am‒ = anion interspersed with charge m−;
n = mols.
The ratio of the di and trivalent cations in HDLs (M2+/M3+) can vary from 1 to 8 and x corresponds to a range between 0.20 and 0.33. This ratio determines the charge density on each layer and has great influence on the material properties such as crystallinity [
The hydrotalcite precursors were prepared from aqueous solutions containing Ca (NO3)2∙6H2O and Al(NO3)3∙9H2O. The atomic ratio, y = Ca2+/Al3+ was equal to 2. An aqueous solution of 1.5 mol.L−1 of K2CO3 was used as a precipitating agent. These two solutions were maintained at 55˚C and mixed in a reactor keeping pH of 10.0 ± 0.4 during all process. The precipitate was kept at 40˚C for 24 hours under slow agitation. Then, it was separated by vacuum filtration and washed with deionized water up to neutral pH. The precursor was calcined at 450˚C and named as 2Ca-Al calcined. The impregnation of the 2Ca-Al precursor was carried out in a roto-evaporator, keeping low pressure and 70˚C until the water is completely eliminated. After impregnation steps, the catalysts were dried at 90˚C for 24 hours and calcined at 450˚C for 4 h. A series of catalysts named as 10 K/2Ca-Al and 20 K/2Ca-Al, where K correspond to KOH and 10 and 20 represents the weight percentage of K impregnated in the catalyst.
Samples were thermally treated at 300˚C for 3 hours in order to remove chemicals adsorbed The surface area and pore volume were analyzed in an equipment BEL, model Belsorp II, using nitrogen adsorption at 196˚C and relative pressures (P/P0) ranging from 0.05 to 0.35. The pore distribution was determined using the BJH method from the adsorption/desorption isotherms and the specific area by BET method.
X-ray diffraction was performed using the powder method in an SEISERT equipment, model 1001, radiation CuKα (l = 1.54178 Å) and nickel filter. Crystalline phases were identified using JCPDS (Joint Committee of Powder Diffraction Standards) [
The catalytic performance was evaluated through the decomposition of isopropanol, using a fixed bed reactor (quartz), with a continuous flow of reagents at atmospheric pressure. The activation of the catalyst was performed in situ at 200˚C for 2 h at oxygen (AGA, 99.9999%) atmosphere. The reactor was fed with isopropanol (99.7%, Merck) using a pump Thermo Separation Products, Model Spectra P100, at 0.05 mL∙min−1 of flow, diluted in a mixture of He and O2 (molar ratio 3/1) at a flow rate of 74 mL∙min−1. The reaction was carried out in the range of 170˚C to 310˚C, using a catalyst mass of 100 mg and a residence time factor (W/FA0) equal to 2.4 g.h/molIsop. Gases released by the catalyzed reaction were analyzed on line by a chromatograph Varian GC-3350, equipped with a thermal conductivity detector (TCD) and provided with Carbowax 20 M packed column, operating with a heating temperature between 30˚C and 150˚C. Helium was used as carrier gas in the column.
The conversion (XA), selectivity (Sp) and specific activity (mols of converted products) were calculated using Equations (3)-(5). It was based on the input and output concentration and assuming a differential reactor:
where:
nI :mols of isopropanol consumed;
nIo: mols of isopropanol fed;
NCP: mols of carbon atoms formed;
NCI: mols of carbon atoms in isopropanol;
FAo: mols of isopropanol fed per minute;
WA: catalyst mass;
Sg: catalyst specific area (m2×g−1).
The catalysts were activated in an oven at 200˚C for 24 h and the transesterification reactions were conducted in a spherical reactor (500 mL) equipped with a mechanical stirring. The amount of catalyst used in each experiment was 10% relative in mass of soybean oil. The experiments were performed in fixed conditions of temperature: 68.5˚C (methanol) and 78.5˚C (ethanol) and molar ratio of 1 to 30 (oil/alcohol). After the reaction the catalyst was separated by vacuum filtration. Then, distilled water was added to filtrate and homogenate. The upper phase was separated and washed twice with distilled water. Then, the sample was centrifuged and trace of methanol and ethanol were removed by distillation. The biodiesel was finally dried with anhydrous sodium sulfate and stored at 4˚C for further analysis.
Aliquots of biodiesel were diluted in deuterated chloroform (CDCl3, Aldrich) and analyzed in a spectrometer model Varian MERCURY 300 MHz operating in the region from 4.4 to 4.0 ppm. The absorption signals of chloroform were used as internal reference for the standard scale. The values of chemical shifts are expressed in units (ppm), coupling constants (J) and Hertz (Hz).
The quantification of methyl ester group of the biodiesel was performed by proton nuclear magnetic resonance (1H NMR) and based on the technique proposed by Gelbart et al. (1995) [
where:
A1 = area under hydrogen methoxyl group;
A2 = area under methylene protons of α-carbonyl;
Cm (%) = conversion of oil to methyl esters.
The ethanolysis conversion evaluation was based on the signals present in the region of 4.05 to 4.35 ppm of 1H NMR spectrum. The resonance peak of the etoxyl hydrogen atoms of the ethyl esters is split into a quartet. The area under of the first and fourth peak is
where:
Ac4 = area of the component fourth peak;
Add+ee = area of all signals between 4.35 and 4.05 ppm;
EE(%) = percentage of ethyl esters in vegetable oils.
the presence of basal planes of LDH-Ca/Al only when the samples were prepared at room temperature or calcined at temperature lower than 400˚C. The samples 2Ca-Al uncalcined and 2Ca-Al calcined also presents peaks at 29˚, 47˚ and 48˚ (JCPDS 72-1652) related to the formation of CaCO3 crystals (calcite). This formation is a result of Ca2+ agglomeration, which is known by its intense peaks at 2θ equal to 29˚, 47˚ and 48˚. The loss of the layered structure promoted the formation of great amount of CaCO3 and a less abundance of Al2O3, which can co-exist in the 2Ca-Al calcined and uncalcined samples. The formation of oxides like CaO, Al2O3 and intercalated carbonates occurs by the calcination of LDH‑Ca/Al and the incorporation of Ca2+, Al3+ and
Catalyst | SBET (m2∙g−1) | Vp (cm3∙g−1) |
---|---|---|
2Ca-Al calcined | 6.60 | 0.07 |
10 K/2Ca-Al | 2.40 | 0.03 |
20 K/2Ca-Al | 1.70 | 0.01 |
decreases the values of surface area and specific pore volume as showed in
The decomposition reaction of isopropanol promotes the formation of acetone and propylene and diisopropyl ether. Acetone is formed by a dehydrogenation reaction, while propylene and diisopropyl ether are obtained by a dehydration one. Isopropanol decomposition occurs by three different mechanisms: E1, E2 and E1B. Propylene is formed by E1 mechanism using the strong acid sites of the catalyst. However, depending on acid-base properties of the catalysts the dehydration or dehydration of isopropanol may also happen via E1B or E2 mechanisms. The E1B mechanism can form propylene and acetone. The formation of propylene by E1B mechanism requires acid and basic sites with unbalanced forces, namely weak acid sites of Lewis and strong bases of Bronsted. However, the acetone formation occurs on strong base sites provided by the highest density of CaO pairs of the catalyst samples. The mechanism E2 promotes the formation of propylene and diisopropyl ether with acidic sites of Lewis and basic sites with medium or strong forces. However, it was reported the dehydration decomposition reaction of isopropanol is performed by the Bronsted acid groups on the MgO sites and samples with great
Catalyst | Specific Catalytic Activity (10−4 mol∙m−2∙min−1) | ||||||
---|---|---|---|---|---|---|---|
Propylene | Diisopropyl Ether | ||||||
250˚C | 280˚C | 300˚C | 250˚C | 280˚C | 300˚C | ||
2Ca-Al Calcined | 0.110 | 0.440 | 1.400 | 0.002 | 0.140 | 0.660 | |
10 K/2Ca-Al | 0.770 | 2.110 | 4.200 | 0.010 | 0.680 | 1.890 | |
20 K/2Ca-Al | 0.420 | 2.010 | 6.130 | 0.020 | 1.000 | 2.720 | |
amount of aluminum ions are more propylene formation selective due to the high density of Al3+O2− groups on the surface [
The method proposed by Garcia (2006) [
Sample | SBET (m2∙g−1) | Vp (cm3∙g−1) | Conversion (%) |
---|---|---|---|
2Ca-Al calcined | 1.03 | 0.68 | 100 |
10 K/2Ca-Al | 1.00 | 0.70 | 95 |
20 K/2Ca-Al | 1.29 | 0.96 | 90 |
signals indicating an elevated conversion to ethyl ester.
According to the
The calcined hydrotalcites were considered effective as catalysts of soybean oil both with ethanol as methanol. When the reaction was carried out with the 2Ca-Al catalyst the conversion of soybean oil was 94% with ethanol and 100% with methanol. Although, when the impregnated catalysts were used the yields were much lower to the ethanolysis when compared with the methanolysis. The conversion for ethanolysis reached a maximum of 58% while the methanolysis kept the conversion between 90% - 95%. The catalytic performance of the catalysts is related to the acid properties determined in isopropanol decomposition reaction. The higher acid activity on the impregnated catalysts was unfavorable to the transesterification reaction. This was more pronounced on ethanolysis. Besides the higher acid activity of these catalysts the low conversion is also associated to the lower reactivity of ethanol and the formation of azeotropic system which is hard to disassociate and the lower surface area
Sample | SBET (m2∙g−1) | Vp (cm3∙g−1) | Conversion (%) |
---|---|---|---|
2Ca-Al calcined | 1.06 | 7.58 | 94 |
10 K/2Ca-Al | 1.00 | 12.90 | 58 |
20 K/2Ca-Al | 1.00 | 40.35 | 20 |
of the impregnated compounds. The catalysts were also characterized with XRD, BET and isopropanol decomposition reaction. The XRD analysis showed the presence of calcite (CaCO3) observed in the Ca-Al = 2 catalyst, resulting in low surface area and pore volume as characterized on BET. The isopropanol decomposition reaction promotes the formation of propylene and diisopropyl ether by Al3+O2− sites present on the catalysts surface which probably is the responsible for their catalytic activities. Besides high conversion obtained, the use of this heterogeneous catalysts has the advantage of its easy separation from the reaction mixture which can be of commercial interest.