Metakaolinite as a catalyst for biodiesel production ... - Springer Link

Front. Chem. Sci. Eng. 2012, 6(4): 403–409 DOI 10.1007/s11705-012-1224-2

RESEARCH ARTICLE

Metakaolinite as a catalyst for biodiesel production from waste cooking oil Jorge RAMIREZ-ORTIZ (✉)1, Merced MARTINEZ2, Horacio FLORES3 1 Academic Unit of Chemical Sciences, Autonomous University of Zacatecas, Zacatecas 98160, México 2 University of Guanajuato, Guanajuato 36050, México 3 Potosino Institute of Scientific and Technological Research, San Luis Potosí 78216, México

© Higher Education Press and Springer-Verlag Berlin Heidelberg 2012

Abstract The use of metakaolinite as a catalyst in the transesterification reaction of waste cooking oil with methanol to obtain fatty acid methyl esters (biodiesel) was studied. Kaolinite was thermally activated by dehydroxylation to obtain the metakaolinite phase. Metakaolinite samples were characterized using X-ray diffraction, N2 adsorption-desorption, simultaneous thermogravimetric analyse/differential scanning calorimetry (TGA/DSC) experiments on the thermal decomposition of kaolinite and Fourier-transform infrared spectrometer (FTIR) analysis. Parameters related to the transesterification reaction, including temperature, time, the amount of catalyst and the molar ratio of waste cooking oil to methanol, were also investigated. The transesterification reaction produced biodiesel in a maximum yield of 95% under the following conditions: metakaolinite, 5 wt-% (relative to oil); molar ratio of oil to methanol, 1∶23; reaction temperature, 160°C; reaction time, 4 h. After eight consecutive reaction cycles, the metakaolinite can be recovered and reused after being washed and dried. The biodiesel thus obtained exhibited a viscosity of 5.4 mm2∙s–1 and a density of 900.1 kg∙m–3. The results showed that metakaolinite is a prominent, inexpensive, reusable and thermally stable catalyst for the transesterification of waste cooking oil. Keywords biodiesel, metakaolinite, transesterification, waste cooking oil

Received July 13, 2012; accepted October 18, 2012

1

Introduction

Biodiesel is a fuel formed by mono-alkyl esters of longchain fatty acids derived from vegetable oils or animal fats, and it is designated as B100 according to the specifications of ASTM International1). Compared to petroleum diesel, biodiesel reduces emissions of particulate matter and carbon monoxide; it contains neither aromatic hydrocarbons nor sulfur. Biodiesel is also non-toxic and highly biodegradable. Most of the industrial processes for the production of biodiesel use a basic homogeneous catalyst, such as sodium hydroxide or potassium hydroxide [1,2]. At present, an important issue in the production of biodiesel is the cost of production because, if used as feedstock, refined vegetable oil represents 70%–95% of the total production cost [3]. This catalytic approach has certain advantages, which include a low-temperature reaction, high biodiesel yields in short periods, good availability and low costs. However, the use of these catalysts is limited to refined vegetable oils that contain less than 0.5 wt-% free fatty acids (FFA), whereas waste cooking oil (WCO) contains between 1.5% and 15% FFA by weight [4]. The use of oils with a high FFA content leads to high catalyst consumption and increases the cost of biodiesel purification. Hence, using basic homogeneous catalysts with WCO poses a few drawbacks, which include the washing of biodiesel with water to remove the catalyst and the inclusion of neutralization steps. Acid catalysis process is one of the most important processes in biodiesel synthesis; this reaction is usually catalyzed by Brønsted acids like H2SO4 and HCl in liquid phase. The present tendency is to replace these catalysts by solid acid catalysts. The development of heterogeneous catalysts has been a relatively recent area of research in the synthesis of biodiesel and has arisen because of the inherent

E-mail: [email protected] 1) American Society for Testing Materials (ASTM) International. http://www.astm.org/digital_library/mnl/pages/mnl11645m.htm accessed (July 2012)

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Front. Chem. Sci. Eng. 2012, 6(4): 403–409

advantages of heterogeneous catalysts, such as their insensitivity to the presence of FFA, lower toxicity, the possibility of catalyst recovery and reuse, their minimized corrosion capacity and the ease with which they are separated from reaction [5]. Kaolinite is a clay mineral. Its chemical formula is Al2Si2O5(OH)4, which corresponds to a two-layered crystal structure composed of silicon–oxygen tetrahedral layers joined to alumina octahedral layers in an alternating fashion. Kaolinite has a 1 : 1 sheet structure composed of SiO4 tetrahedral sheets and Al(O, OH)6 octahedral sheets with pseudo-hexagonal symmetry. The sheets are created from planes, which are occupied as O6 – Si4 – O4 – (OH)2 – Al4 – (OH)6. Kaolinite crystals can be decomposed to a partially disordered structure through a dehydroxylation reaction followed by a smaller shrinkage of the dimensions of the sample and an increase in the porosity. The result of dehydroxylation is a new phase called metakaolinite, which exhibits a short-range-ordered crystalline structure [6]. The use of 1∶1 group clay minerals as catalyst is largely restricted to kaolinite, and even then its use is also restricted as support to the actual catalyst or as precursor to other catalyst ceramic materials or zeolites. In two papers [7,8], metakaolinite has been used successfully for the transesterification of WCO to fatty acid methyl esters (FAME). The transesterification and transthiolesterification of βcetoesters with a variety of alcohols and thiols and the selective protection of carbonyl functions with various protecting groups catalyzed by natural kaolinitic clay have been reported [9]. Acid-activated kaolinite was used as a catalyst in the esterification of carboxylic acids mixed with xylene and various different alcohols, as reported [10]. The esterification of oleic acid with several short-chain alcohols over a solid acid catalyst prepared from Amazon flint kaolin has also been reported [7]. In addition, the same authors have reported a comparative study between esterification catalysts prepared from kaolins [8]. In this study we report the use of metakaolinite as a solid acid catalyst for the production of biodiesel from waste cooking oil with methanol. Parameters related to the transesterification reaction, including temperature, time, the amount of catalyst and the oil:methanol molar ratio, were investigated.

2

Experiment

2.1

Materials

Kaolinite, phenolphthalein, potassium hydroxide and potassium nitrate were purchased from Fluka (SigmaAldrich Química, S.A. de C.V. México), triethanolamine, isopropyl alcohol, anhydrous methanol and oleic acid, were purchased from J.T. Baker (Performance Materials S.A. de C.V. México). Waste cooking oil was obtained

from domestic frying oil with the trade name “Cristal” vegetable corn oil. 2.2

Thermal treatment of kaolinite

Kaolinite was used as received and was thermally activated to form the metakaolinite phase. The calcination was performed under static air in a programmable furnace for 10 h at 800°C, with a heating rate of 10°C$min–1 from room temperature to the calcination temperature [11]. 2.3

Catalyst characterization

The N2 adsorption-desorption isotherms were obtained at liquid-nitrogen temperature using an ASAP 2010 from Micrometrics Instruments. Before each measurement, the samples were outgassed at 130°C for 2 h. The specific surface area, the porous area, the porous volume and the pore size distribution were obtained, respectively, using the Brunauer-Emmett-Teller (BET), t-plot and Barrett-JoynerHalenda (BJH) methods. The samples for powder X-ray diffraction analysis were prepared by placing the powdered material onto a neutral glass slide with a thin film of silicon grease orienting the sample by hand-pressing. X-ray diffraction patterns were obtained using a Bruker D8 Advance diffractometer (Bruker AXC Inc., Madison, WI, USA) equipped with a CuKα radiation source operated at 25 mA and 35 kV; the 2θ scanning speed was 2.5°C$min–1. A Varian 640-IR Fourier-transform infrared spectrometer (FTIR) was used to identify the surface functional groups of the kaolinite and the metakaolinite. The spectra were recorded in a range of 4000–500 cm–1. Thermal analysis measurements were performed in 150 μL platinum crucibles on a TA Instruments SDT Q600 simultaneous thermogravimetric analyse/differential scanning calorimetry (TGA/DSC) under a nitrogen atmosphere flowing at 50 mL∙min–1. The heating rate was 10°C min–1, and the temperature range was 30°C– 1000°C. Particle size distribution (PSD) of powdered samples was measured on device of ultrasonic attenuation AcoustoSizer II (Colloidal Dynamics) with distilled water as the dispersant at 28°C. The method is based on the measure ultrasound generated by the particles. The acidity of catalyst was determined by titration acidbase using an AR25 Research Accumet potentiometer following the method reported [12]. Density and viscosity of WCO and biodiesel samples were determined using the picnometer method at room temperature and using an Ostwald viscometer at 40°C, respectively. 2.4

Free fatty acid determination

The acid value was determined using a method reported by Kardash and Tur’yan [13] to prepare the titration reagent by mixing 500 mL deionized water, 500 mL isopropyl alcohol and 7 mL triethanolamine. Place 100 mL of

Jorge RAMIREZ-ORTIZ et al. Metakaolinite as a catalyst for biodiesel

reagent into a conical flask on a magnetic mixer, add 0.5 mL of phenolphthalein solution to obtain pink color of the reagent. Add an oil test portion according to range of FFA content to the reagent. Mix the reagent on the stirrer until colorless and then, keeping on stirring, begin titration with 0.1 mol∙L–1 KOH aqueous solution. The titration is finished at the appearance of the first permanent pink color, which is supposed to persist for 10 s. The amount of KOH consumed was registered and the acid value was calculated as follows: multiplying the volume of KOH spent by the concentration of the KOH solution and by 56.1 then divided by the sample weight. The FFA content in the waste cooking oil was 0.45 wt-%. 2.5

Transesterification reactions

Waste cooking oil was allowed to dry on a hot plate with stirring at 140°C for 3 h to evaporate the water. The oil was subsequently filtered by suction to remove solids, such as solid food pieces and sediment. Prior to the experiments, the metakaolinite was dried at 120°C in an oven and stored there until used. The metakaolinite test was performed in a Parr 2000 mL Bolted Closure Stirred Reactor (Parr Instrument Company, model 4522, Moline, Il, USA), fitted with a mechanical stirrer, temperature control, and sample outlet. In a typical experiment, WCO was mixed with methanol and with 3 wt-% of metakaolinite (related to oil). The reaction mixture was kept under constant stirring (627 r∙min–1) at 160°C for 2 h. After the reaction was complete, the reactor was cooled to room temperature and the mixture of FAME, glycerin and methanol was filtered by suction to remove the metakaolinite. The mixture was then kept in a decanter funnel at room temperature for overnight to ensure separation of the phases. The bottom phase consisted of glycerol, and the top phase contained the FAME dissolved in methanol. A rotary evaporator was used to recover the excess methanol, and the obtained FAME was analyzed by gas chromatography. The yield of biodiesel was calculated as the mass ratio of FAME obtained after the reaction to the initial WCO mass multiplied by 100. The transesterification reactions were carried out batch by batch. A blank reaction without a catalyst was also performed as a control experiment. 2.6

Simultaneous esterification and transesterification

The feedstock for simultaneous esterification and transesterification of WCO containing 10% of pure oleic acid (OA) was prepared manually by mixing 90 parts of WCO and 10 parts of OA by mass [14]. The reactions were carried out in identical procedure and reaction conditions (temperature 160°C, 3 wt-% catalyst, 1∶31 and 1∶23 mixture WCO + OA/methanol molar ratios) as used for transesterification reactions in the same Parr 2000 mL Bolted Closure Stirred Reactor.

2.7

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Analysis of fatty acid methyl esters

For the quantification of reaction products, the samples were analyzed by a gas chromatography (GC) using an Agilent 6890N equipped with a capillary column (50 m 0.32 mm, 1.00 µm film; DB-WAX). Methyl heptadecanoate (10 mg internal standard) was dissolved in 1 mL nheptane to prepare the standard solution. Approximately 250 mg of sample was dissolved in 5 mL standard solution for GC analysis. Nearly 1 µL sample was injected into the GC. Helium was used as the carrier. The split ratio was 80∶1. The column temperature was programmed from 210°C holding 10 min, with a heating rate of 10°C min–1, to 250°C holding 10 min. The injector temperature was set at 250°C and detector set at 300°C. FAME content for each sample was done using European Standard EN14103 [15]. 2.8

Catalyst reuse

To test the catalyst reuse, repeated reaction runs were performed at the optimum conditions: reaction time, 2 h; oil/methanol molar ratio, 1∶31; catalyst, 3 wt-%; temperature, 160°C. After each one of the eight consecutive reaction cycles, the catalyst was recovered by vacuum filtration and reused after washed with methanol and then with n-hexane to remove possible oil and glycerol adhered to catalyst surface, and dried in oven to 110°C.

3

Results and discussion

3.1

Catalyst characterization

As shown in Fig. 1 for TGA/DSC analysis, the first endothermic weight loss (2.56%) was observed at below approximately 200°C due to moisture loss [16]. The major endothermic weight loss of the kaolinite sample was observed at 400°C–750°C. The minimum weight loss occurs above 523°C, which can be attributed to endothermic dehydroxylation that produces disordered metakaolinite (Al2Si2O7). Dehydroxylation continued above 900°C to form mullite, as indicated by an exothermic peak at 993°C. The formation of mullite may be attributed to the gradual oxolation of the metakaolinite, which resulted in a total weight loss of 12.29%. The results indicate that the SiO4 sheets in metakaolinite persist but in a distorted form, whereas the octahedral sheets are profoundly altered, although some short-range order is preserved. The exothermic peak at 993°C can be ascribed to the formation of the crystalline phase [17]. To confirm the disappearance of the kaolinite diffraction peaks after the thermal treatment, the XRD patterns of raw kaolinite and metakaolinite were compared in Fig. 2. As evident from the XRD pattern in Fig. 2(a), the major mineral constituents of the kaolinite were kaolinite and

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Fig. 1 TGA/DSC data for parent kaolinite

quartz. The characteristic diffraction peaks for kaolinite (2θ & 12.31°, 20.38° and 24.66°) disappeared, whereas the diffraction peaks assigned to quartz (2θ & 21.13°, and 26.44°) remained unchanged Fig. 2(b). The XRD analysis revealed a complete breakdown of the crystalline structure of the metakaolinite compared to the parent kaolinite and a structural water loss that facilitated the transformation of AlO6 octahedra into tetra- and penta-coordinated Al units; thus, the reactivity of the metakaolinite was at a maximum when the content of AlO6 was at a minimum [18]. In addition to XRD measurements, FTIR spectroscopy was used to confirm the transformation of the kaolinite during calcination. The FTIR spectra of the kaolinite and the metakaolinite are shown in Fig. 3. As shown in Fig. 3(a), the FTIR spectra for kaolinite show characteristic Al – O – H bands at 3686 cm–1, 3668 cm–1, 3649 cm–1 and 3617 cm–1; Al – OH at 909 cm–1; Si – O at 1024 cm–1 and 1003 cm–1 and Si – O – Al at 583 cm–1. The absence of detectable Al – O – OH bands at 909 cm–1 and the absence of the four Al – O – H bands at 3686 cm–1, 3668 cm–1, 3649 cm–1 and 3617 cm–1 were evident in the spectra of dehydroxylated kaolinite, as shown in Fig. 3(b). The absence of the band at 909 cm–1 and the appearance of a new band at 794 cm–1 can be attributed to the change from octahedral coordination of Al3+ in kaolinite to tetrahedral coordination in metakaolinite. The band at 1050 cm–1 is assigned to amorphous SiO2 [19]. The specific surface area, the average pore diameter and the pore volume of the metakaolinite catalyst were 10 m2∙g–1, 13.0 nm and 30 mm3∙g–1, respectively. The value of acid sites was 0.01 meq∙g–1. The determination of these parameters is a very important because they are closely associated with the catalytic activity. Catalysts for the production of biodiesel are required to contain large interconnected pores that minimize the diffusional limitations of molecules with a long alkyl chain [20]. A type III (IUPAC) classification N2 adsorption-desorption isotherm and the pore size distribution curves (not shown) of the metakaolinite indicated that this material was macroporous

Fig. 2 XRD patterns of (a) parent kaolinite and (b) metakaolinite phase K = kaolinite; Q = quartz

Fig. 3 FTIR spectra of parent kaolinite before (a) dehydroxylation and (b) metakaolinite phase

[21]. It can be seen from Fig. 4, the particle size distribution of the metakaolinite is homogeneous. Table 1 show that 85% (in volume) of the particles are smaller than 0.7 µm and have a volume average diameter of 0.2 µm. 3.2

Transesterification reaction

The transesterification of triglycerides with methanol using a solid acid catalyst is well known to be reversible and slow. An excess of methanol is required to force the reaction toward the formation of FAME. As shown in Fig. 5 for the four experiments, the high yield at 1 h was obtained for the molar ratio 1∶31 of oil/methanol and the maximum conversion of WCO into biodiesel was 92.5%,


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Fig. 4 Average size distribution of the metakaolionite (◆) vs. attenuation differential fraction

which was obtained after 2 h of reaction. An hour later, the yield dropped to 85% and was almost unchanged at 87% for another hour. The decrease in the yield may be due to the fact that when the ratio of oil/methanol was too high, the large excess of methanol will cause flooding of the active sites, as reported by Shu et al. [4]. Therefore, the increased molar ratio hindered the conversion of the triglycerides at the active sites. In the presence of 3 wt-% of catalyst, at 160°C, and with an oil/methanol molar ratio of 1∶23, biodiesel was obtained in 67% yield after 2 h. With one additional hour of reaction time, the yield increases to 77%, and a maximum yield of 94.5% of biodiesel could be achieved in 4 h. When the transesterification was performed with 5 wt-% of catalyst, and an oil/methanol molar ratio of 1∶23 at 160°C, the yield was 45% at 2 h. The yield increased to 75% at 3 h and reached a maximum of 95% at 4 h. For the transesterification reaction of WCO and methanol, as the amount of catalyst was increased from 3 wt-% to 5 wt-%, there was an effect only after 3 h of reaction and the production of biodiesel also increased, attaining the highest yields of 94.5% and 95% at 4 h. The experimental temperatures utilized were 140°C and 160°C, which were outcomes of preliminary investigations reported elsewhere [22]. The normal behavior for the catalyzed reactions is that the reaction rate increases with temperature: the higher the reaction temperature, the shorter the time to reach the maximum yield, which indicates that transesterification is an endothermic reaction. As Fig. 5 shows in the curve for a catalyst concentration of 3 wt-% and an oil/methanol molar ratio of 1∶31, biodiesel reached a yield of 92.5% at 160°C after 2 h, whereas a yield of 58.5% at 140°C after 2 h. This difference in yield suggests a strong influence of the molar ratio of oil/ methanol on the transformation of fatty acids with a rapid increase in transesterification products. According to Phan and Phan [23], the oil/methanol ratio is one of the most important factors affecting the yield of biodiesel. Since the excess of methanol could interfere with the separation of

Fig. 5 Yield of FAME and parameters related to the transesterification reaction, including temperature, time, the amount of catalyst and the molar ratio of oil to methanol

ester product from by-products by increasing solubility of glycerol. Consequently, part of the diluted glycerol remained in the ester phase, leading to the formation of foam and therefore the apparent loss of ester product. The effect of the reaction time was studied using two different oil/methanol molar ratios of 1∶21 and 1∶31 at 140°C and 160°C, respectively. A steady increase was observed in the yield of biodiesel after 2 h of reaction with a maximum yield of 92.5% for a 1∶31 molar ratio, a temperature of 160°C and a catalyst content of 3 wt-%. Running the reaction beyond this time adds to the cost of biodiesel synthesis. Finally, when the transesterification reaction was performed in the absence of catalyst at 160°C, the yields were 22% after 2 h and 29% after 4 h for oil/methanol molar ratios of 1∶23 and 1∶31, respectively. The results showed that FFA can act as a weak acid to catalyze the transesterification of triglycerides, as reported by Wang et al. [24]. Loading either 3% or 5% catalyst gave a similar yield of FAME when the other experimental conditions (160°C, 1∶23 molar ratio, 4 h) was the same. This is probably due to poor mixing of the slurry that formed at a higher catalyst content. The amount of catalyst did not appear to affect the final yield since the two reactions has almost the same final yield. The following major compounds were found in the FAME (wt-%): palmitic acid (C16∶0–11.70), stearic acid (C18∶0–2.30), oleic acid (C18∶1–32.40), and linoleic acid (C18∶2–51.0). The total saturated and unsaturated FAME content was 14% and 83.4% (w/w), respectively. The viscosity and density of FAME were 5.4 mm2∙s–1 and 900.1 kg∙m–3, respectively. These values were in agreement with requirements of ASTM D6751 biodiesel standard [25], which gave the value ranges of

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1.9 mm2∙s–1 to 6.0 mm2∙s–1 for viscosity and 870 to 900 kg∙m–3 for density. 3.3

Simultaneous esterification and transesterification

The acid value of the mixture WCO + OA was 33 mg KOH/g oil, which is higher than the levels typically found in WCO. Figure 6 shows two different experiments on yield of the simultaneous esterification and transesterification

Fig. 7 Acid value content as a function of time for simultaneous esterification and transesterification of WCO containing 10 wt-% OA

Fig. 6 Yield of FAME as a function of time for simultaneous esterification and transesterification of WCO containing 10 wt-% OA

Figure 7 shows the variation of the acid value with the mixture WCO + OA /methanol molar ratio at a catalyst concentration of 3 wt-%. This result corroborates those obtained by Marchetti and Errazu [26], where the reaction of transesterification of a mix of refined sunflower oil with pure oleic acid and anhydrous ethanol to produce ethyloleate was carried on using sulfuric acid as catalyst. 3.4

Catalyst reuse

The yield data for reusability of eight consecutive reaction cycles of metakaolinite as catalyst are presented in Fig. 8. The yield is almost constant, even in the last (eighth) cycle of the reaction performed under the same experimental conditions (160°C; oil/methanol molar ratio: 1:31), indicating that the metakaolinite can act as a reusable catalyst under these conditions for long term use due to its good stability.

Fig. 8 Stability of the metakaolinite as catalyst

2) The metakaolinite catalyst can be reused because it exhibited only a slight loss of activity even after eight consecutive reaction cycles. 3) The use of the metakaolinite catalyst clearly offers compelling environmental benefits because it is natural, inexpensive and readily available worldwide. Acknowledgements The authors thank Veridiana Reyes Zamudio for the TGA/DSC analysis.

References 4

Conclusions

1) The catalytic activity of metakaolinite in the transesterification reaction of waste cooking oil with methanol was demonstrated by the high yields of biodiesel obtained.

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