1. INTRODUCTION:

Biodiesel is produced from fat or oil through a chemical process called transesterification which is a compulsory step in the production of biodiesel [1]. Heterogeneous catalysts are promising and advantageous for biodiesel production since they are able to be operated in continuous processes to give a high quality of products. Apart from that they are reusable, environmental friendly, and more effective than homogeneous acid/base catalysts and enzymes. Besides, the use of heterogeneous catalysts does not produce soaps through free fatty acid neutralization ortriglyceride saponification. Finally, they can be designed to give higher activity, selectivity, and longer catalyst life times[2]. Hydrotalcite Mg₆Al₂(OH)₁₆CO₃.4H₂O is one of the remarkable catalysts for tranesterification because of its basic characteristic properties [8-12].

However, it is quite inert to triglycerides with high free fatty acid due to saponification reaction, forming emulsifiers and creating water. This catalytic system also has a high affinity for glycerol[11]. Therefore, the concentration of glycerol on the catalytic surface prevents catalytic systems from simultaneous exposure to methanol and triglycerides. A large number of research works have demonstrated Mg-Al hidrotalcite, which was calcined at high temperatures to form MgO phase, as catalyst for tranesterification because it has absolutely strong base properties, even stronger than the initial hydrotalcite phase.

Other solid super acid catalysts for transesterification are SO₄^2-/TiO₂, SO₄^2-/ZrO₂thanks to their strong acidity, resistance to equipment corrosion, ease of re-refining and subsequent use, and environmentally friendly impacts. Furthermore, they can catalyze methyl transesterification reaction of oil with high free fatty acid [3]. However, the disadvantages of this kind of catalyst system are low surface area, small capillary diameter, and difficult diffusion of triglyceride molecules(sizefrom2to4nm) into the catalyst capillary as well.

This work shows a novel method to synthesize theMgO-γAl₂O₃-TiO₂-SO₄^2- catalytic system with average pore size, large surface area, and strong acid (base) active sites on the surface in order to accelerate the transesterification reaction in the presence of triglyceride. Then, the catalytic ability evaluation was performed through the transesterification of waste cooking oil by methanol on the purpose of biodiesel preparation.

2.EXPERIMENTAL:

2.1. Synthesis of Materials

2.1.1. Synthesis ofγ-Al₂O₃

γ-alumina oxide support was synthesized by sol-gel method. At first, Al(OH)₃ was dissolved in NaOH solution and heated for 30 minutes. The mixture was filtered to remove the undissolved solid to have a solution of sodium aluminate as a filtrate, which was followed by adding sodium alginate into the solution. After that, H₂SO₄ 0,1M solution was dropwise added to the solution during being stirred and heated at 90^oC until a maximum precipitation was attained (about 12 hours and the pH of the filtrate was 6.5). In the next step, the alumina gel was isolated from the reaction mixture and calcined at 500^oC for 4 hours with the heating rate of 5°C/min to produce porous γ-alumina powder.

2.1.2. Synthesis of MgO-γ-Al₂O₃/TiO₂-SO₄^2-

1 ml of Ti(OC₃H₇)₄was mixed with60ml of ethanol to form solution A. Solution B is a mixture of 20ml ofethanoland20 mlHNO₃10% solution. Solution B was added drop wise into solution A to get solution C, and then 10g γ-Al₂O₃ was added into solution C in order to adsorbTi⁴⁺ on the capillary of γ-Al₂O₃ and was kept stirring for 2 hours. Then, the pH was adjusted to 5 using NH₄OH to have precipitate in thecapillariesofγ-Al₂O₃, followed by heatingat100^oCfor 24 hours. Finally, the formed precipitation was filtered, dried at 80^oC for 12 hours, and calcined at 450^oC for 4hours to get γ-Al₂O₃/TiO₂. Anion SO₄^2-was integrated by adding (NH₄)₂SO₄ 1 M into the mixture and continuously stirring for 2 hours. The mixture was filtered to receive a solid form of γ-Al₂O₃/TiO₂-SO₄^2-, which was calcined at 450^oC for 3 hours.

In the next step, theγ-Al₂O₃/TiO₂-SO₄^2-sample was added to a liter of solution containing Al³⁺ 0.001 M, Mg²⁺ 0.003M.Asolution of NaOH 0.0084 M, Na₂CO₃ 0.0025 M was then slowly poured into the above mixture to keep the solution pH in the range of 9-10. After that, the solution was stirred, heated at 65^oC for 12 hours, and washed with distilled water until the filtrate pH was 7. The final product was dried for 24 hours at 80^oC and then finally calcined at 450^oC for 5 hours to have catalyst system MgO-γ-Al₂O₃/TiO₂-SO₄^2-.

2.2. Characterization

Characterization of the catalyst structure was performed with powder X-ray diffraction (XRD). The patterns were obtained on a Shimadzu diffractometer model XRD600 with a monochromatic radiation source of CuKα at 40k V and filament current of 30mA. Measurements were made with a diffraction angle 2θ from 20° to 70° with a step of 0.03° at a speed of 1.2^omin^-1. FTIR spectra were using Shimadzu spectrophotometer. The basic strength of the hydrotalcite synthesized was obtained by temperature-programmed- desorption of CO₂(CO₂-TPD) using a Micromeritics Instrument -AutoChem 2920. The speciﬁc surface area was measured by the BET method on a Gemini VII 2390 V1.02 using nitrogen adsorption isotherms at 77 K. Temperature programmed desorption of NH₃ (NH₃-TPD) was performed on a Micromeritics Instrument Corporation -AutoChem II 2920 V4.01 and AutoChem II 2920 V3.03.

2.3. Reaction testing

Transesterification of waste cooking oil with methanol was carried out at 120^oC on autoclave. The reaction mixture was 1:1 in volume of methanol to oil and the catalyst used accounted for 20 %oil mass. After the reaction, MgO-Al₂O₃/TiO₂-SO₄^2- was separated from the reaction medium by vacuum filtration and the final product was evaporated to remove the excessive methanol. Then, the mixture was transferred to a decanting funnel for phase separation. After 24 hours, the glycerol phase was separated and the ester phase (biodiesel) was collected. After purification, the quality of biodiesel product was evaluated by measuring their viscosity on an Analis. The synthesized biodiesel was mixed with waste cooking oil to have a range of mixture containing100%, 95%, 90%, 85% biodiesel by mass, respectively, and then measured their kinematic viscosity values at 40^oC in order to set up the standard curve for percentage of biodiesel. Biodiesel synthesized was collected to measure viscosity, thereby deducing the reaction yields.

Composition of biodiesel was determined by Gas Chromatography Mass Spectrometry (GC-MS) analysis on GC-MS Hewlett HP 6800 gas chromatograph equipped with Hewlett HP5973 detector, column separation HP -5MS5% PEPH crosslinked siloxane in size of 30m×0.32micrometers.

3.RESULTS AND DISCUSSION:

3.1. X-ray diffraction powder

Figure1(a) and 1(b) shows that both spectrum contain the diffraction peaks at angle 2q ~38.5^o, 46^oand 67^o, corresponding to the [311], [400], and [440] planes of γ-Al₂O₃materials. However, the diffraction peaks of MgO-γ-Al₂O₃/TiO₂-SO₄^2-are not as clear as those of pureγ-Al₂O₃, suggesting that the crystals of the aluminum oxide are reduced after being modified. Diffraction diagram (1b) shows the diffraction peaks of 2θ~25.4°, 37.8°, 48.05° and 53.91° corresponding to the [101], [004], [200], and [105] planes of anatase TiO₂ phase [3,4,5]. These results can be explained by the incorporation model proposed by Chen et al. [12]suggesting that the dispersed metal cations are incorporated into the surface vacant sites for the support with their accompanying anions sitting on the top for extra charge compensation, resulting in the formation of M-O-M linkages.

Fig 1: XRD pattern of catalysts samples:

(a): γ- Al₂O₃; (b): MgO- γ- Al₂O₃/TiO₂-SO₄^2-

3.2. IR spectrum of MgO-γ-Al₂O₃/TiO₂-SO₄^2-

The IR spectrum of MgO-γ-Al₂O₃/TiO₂-SO₄^2-sample owns absorption bands at 3444.87cm^-1characterizing for oscillation of the -OH group on the material surface due to water vapor absorption, at 1384.89 cm^-1for oscillations of the symmetric S=O resulting from the interaction of TiO₂ with SO₄^2- to form super acid Ti(SO₄)₂ [6].Especially, there are oscillations at the frequency of586.36cm^-1which is characteristic of the MgO phase[4]. It is interesting that while XRD result of MgO-γ-Al₂O₃/TiO₂-SO₄^2-shows low characteristic diffraction peaks of MgO phase and the absence of diffraction peaks for the Ti(SO₄)₂, the result of IR spectrum of the prepared sample shows strong acidic Ti(SO₄)₂phase and strong basic MgO phase formed.

3.3. NH₃-TPDof g-Al₂O₃and MgO- g-Al₂O₃/TiO₂-SO₄^2-

NH₃-TPD was employed to evaluate the amount and strength of acid formed on the catalyst surface. Table 1 shows that MgO-g-Al₂O₃/TiO₂-SO₄^2-contained strong acid sites that were 7.1times stronger than theg-Al₂O₃ and the medium acid sites was 7.9 times higher than those of theg-Al₂O_3.These result from the production of phase SO₄^2-/TiO₂super acid. The result is consistent with those of infrared spectroscopy.

According to Xing chen Wang (Fig 3), SO₄^2- is linked with TiO₂through the S-O-Ti bond. The decrease of electron density of Ti⁴⁺ resulting from an electronic attraction of S cause increase in the Lewis acidity of Ti⁴⁺. At the same time, polarity rise of the surface OH groups caused by the affinity of S leads to Bronsted acidity of the OH group. After integrating TiO₂ into g-Al₂O₃support, they combine with SO₄^2- to form TiO₂-SO₄^2- super acid system. The acid site activates the carbonyl group easily to form carbocations. This is an important stage in a transesterification reaction.

Fig 2: IR spectrum of MgO-γ-Al₂O₃/TiO₂-SO₄^2-

Table1: NH₃ –TPD results of : (a) g-Al₂O₃and (b): MgO- g-Al₂O₃/TiO₂-SO₄^2-

Peak Number	g-Al₂O₃		MgO-g-Al₂O₃/TiO₂-SO₄^2-
Peak Number	Temperature at Maximum (^oC)	Quantity (cm³/g STP)	Temperature at Maximum (^oC)	Quantity (cm³/g STP)
1	200.6	3.50	210.3	9.17
2	278.5	9.67	347.7	36.53
3	519.6	1.67	397.8	58.85
4			509.5	10.97
5			546.6	0.88

Table 2: CO₂ –TPD results of: (a)g-Al₂O₃and (b): MgO- g-Al₂O₃/TiO₂-SO₄^2-

Peak Number	(a): g-Al₂O₃		(b): MgO-g-Al₂O₃/TiO₂-SO₄^2-
Peak Number	Temperature at Maximum (^oC)	Quantity (cm³/g STP)	Temperature at Maximum (^oC)	Quantity (cm³/g STP)
1	220.2	3.78	252.7	48.14
2	381.8	8.01	407.0	52.56
3	488.7	1.20	502.1	20.64

Table 4: EDX results of MgO-g-Al₂O₃/TiO₂-SO₄^2-

Times	C	O	Mg	Al	S	Ti
1	28.73	47.55	0.26	18.31	1.73	3.31
2	28.77	46.75	0.24	18.86	1.61	3.58
3	28.10	47.57	0.26	18.54	1.81	3.72

Fig 3: Bronsted and Lewis acid site in sulfated Titania

3.4. CO₂- TPD of g-Al₂O₃and MgO- g-Al₂O₃/TiO₂-SO₄^2-

CO₂-TPD was employed to estimate the amount of strong base sites formed on the catalyst surface. The results given in table 2 show that strong base sites and medium base sites of MgO-g-Al₂O₃/TiO₂-SO₄^2-are17.2 and 8.5 times higher than the g-Al₂O₃, respectively. The cause of increasing number of base site is from the production of MgO phase. According toZhen Fang, Hydrotalcite surface basicity is from basic sites of medium (Mg-O pairs) and strong (O^2- anions).

3.5. BET surface area

Table 3 shows that the BET surface area of g-Al₂O₃sample and MgO-g-Al₂O₃/TiO₂-SO₄^2-sample are 316.24 and 261.85m²/g, respectively; and the average size of pore diameter of g-Al₂O₃and MgO-g-Al₂O₃/TiO₂-SO₄^2-samples are 121.5Å and 95.5Å, correspondingly. These prove that after modification, some elements such as Mg, Ti, Slocated in the heart of capillaries and fastened in the capillary. The MgO-g-Al₂O₃/TiO₂-SO₄^2-material has pore diameter which is mainly distributed between 60 and100Å. These sizes are suitable for triglyceride molecules (size20-40Å) to occupy and exposure to methanol, as well as a catalyticsite

Table 3. Speciﬁc surface area and pore size of: (a) g-Al₂O₃, (b) MgO-g-Al₂O₃/TiO₂-SO₄^2-

Materials Textual characteristic

g-Al₂O₃

MgO-g-Al₂O₃/

TiO₂-SO₄^2-

BET surface area(m²/g)

316.24

261.85

Adsorption average pore width (Å)

121.5

95.5

3.4. 3.6. EDX analysis

The composition of MgO-g-Al₂O₃/TiO₂-SO₄^2-was determined by energy-dispersive method (EDX). The analysis was performed at 3 different points on the catalytic material surface. As seen in Table4, the dispersion of Mg, Al, O, Ti, S in catalysts was evenly distributed at different points on the γ-Al₂O₃support.

MgO-g-Al₂O₃/TiO₂-SO₄^2-catalytic system has high specific surface area and integrates a large amount of strong acid sites and strong base sites dispersing evenly in the capillary. Capillary diameter that is primarily from 60-100Å is capable of attracting and locating the triglyceride molecule, methanol in the heart of capillary so that they can easily contact with the catalytic site. As a result, the reaction rate is increased. Therefore, it is a suitablecatalystsystemfortransesterificationreactionoftriglycerideswithalcoholtoproducebiodiesel.

3.5. Catalytic ability of the system MgO-g-Al₂O₃/TiO₂-SO₄^2-

The reaction was carried out at the given conditions: temperature of 120 °C, the ratio of waste cooking oil: methanol of 1:1 in volume, reaction time of 3, 4, 5 hours, respectively. The catalyst used accounts for 20 % of waste cooking oil mass. The efficiency of the tranesterification reaction carried out at varying conditions is shown in Table 5. These results are inferred from the linear equation showing the dependence of the product viscosity on percentage of biodiesel as shown in Figure 4: y = - 8.7x + 137.28; where x is viscosity of the product and y is the percentage of biodiesel. The reaction yield is about 98.1% after a 4 hour reaction. The composition of biodiesel analyzed by GC-MS method is shown in table 6 with 11 main components found.

Fig 4. concentration of biodiesel depends on the viscosity

Table 5.Reaction yields results

Reaction time	Viscosity (cSt)	Reaction efficiency(%)
3 hours	5.3	91.2
4 hours	4.5	98.1
5 hours	4.47	98.4

Table 6.Components ofthe biodiesel product

RT (min)	Library/ID	Area %	Quality
6.32	Octanoic acid, methyl ester	0.26	91
10.58	Nonanoic acid, methyl ester	0.94	91
14.06	Tetradecanoic acid, methyl ester	1.71	97
16.63	9- Hexadecenoic acid, methyl ester	0.31	98
16.70	9- Hexadecenoic acid, methyl ester	1.24	99
17.09	Hexadecanoic acid, methyl ester	24.28	97
18.58	Heptandecanoic acid, methyl ester	0.26	96
19.74	9,12- Octadecadienoic acid (Z,Z), methyl ester	3.73	99
19.94	9-Octadecenoic acid (Z), methyl ester	43.35	99
20.29	Octadecanoic, acid, methyl ester	18.58	97
22.94	Oxiraneoctanoic, 3- octyl, methyl ester	0.55	90
23.06	11- Eicosenoicacid, methyl ester	1.18	99
23.46	Eicosanoicacid, methyl ester	0.69	99

4. CONCLUSION:

g-Al₂O₃support with surface area of 316.24 m²/g and pore diameters mainly distributed in the region of 110 ÷ 160 Å. TiO₂ were successfully integrated in the capillaries of g-Al₂O₃, then sulfated in order to generate a large amount of strong acid sites with NH₃ desorption temperature is 509.5^oC and 546.6^oC. The final catalyst was also successfully integrated with MgO phase creating a large amount of strong base sites with a CO₂ desorption temperature of 502.1°C. A transesterification between methanol and waste cooking oil (the volume ratio of methanol to oil of 1:1) carried out at 120^oC for 4 hours withMgO-γ-Al₂O₃/TiO₂-SO₄^2-playing a role as a catalyst shows high reaction yield up to 98.1%.

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Received on 20.03.2016 Modified on 25.03.2016

Asian J. Research Chem. 9(3): March, 2016; Page 145-149

DOI: 10.5958/0974-4150.2016.00024.9