transesterification of waste frying oils using znal2o4 as ...compression ignition. the astm...

Procedia Engineering 42 ( 2012 ) 1928 – 1945

1877-7058 © 2012 Published by Elsevier Ltd.

doi: 10.1016/j.proeng.2012.07.589

20th International Congress of Chemical and Process Engineering CHISA 2012 25 – 29 August 2012, Prague, Czech Republic

Transesterification of waste frying oils using ZnAl2O4 as heterogeneous catalyst

C. T. Alves a,c, a*, A. S. de Oliveiraa, S. A. V. Carneiroa, R. C. D. Santosc, S. A. B. Vieira de Meloa, H. M. C. Andradeb, F. C. Marquesc, E. A. Torresa

aFederal University of Bahia, Chemical Engineering Dept., R. Prof. Aristides Novis, 02, 40210-630, Salvador, Brazil bFederal University of Bahia, General and Inorganic Chemistry Dept., R. Barão de Geremoabo, 147, 40170-115, Salvador, Brazil

c University of Birmingham, School of Chemical Engineering, Edgbaston, Birmingham, B15 2TT, United Kingdom

Abstract

In this paper, biodiesel production from waste frying oils was investigated with zinc aluminate (ZnAl2O4) as a heterogeneous catalyst, synthesised by the combustion of urea with aluminium nitrate and zinc nitrate. The transesterification was studied at temperatures between 60 and 200 ºC, in an alcohol:oil molar ratio of 40:1, 2 h reaction time, stirring at 700 rpm and different catalyst amounts (from 1% to 10% based on oil weight). The biodiesel yield was analyzed in a gas chromatographer. Synthesised and recovered catalyst were characterized for their physical and textural properties. Thermogravimetric analysis (TGA) and Differential scanning calorimetry (DSC) have been used to study the thermal behavior of the zinc aluminate spinel. Powder X-ray diffraction (XRD) was used to determine the spinel-type structure, which was confirmed by Raman and Fourier Transform Infrared (FTIR) Spectroscopy. The particle and pore sizes were measured by transmission electron microscopy (TEM) and He pycnometry respectively. The specific surface area was determined by BET surface area analysis to be 17.3084 ± 0.0804 m2.g-1, and the density 4,61 g.cm-3. Residual glycerin and biodiesel were also observed in the recovered catalysts. Optimal yields above 95% were measured for both methylic and ethylic routes. Moreover, the same catalyst was utilized for three reaction cycles without significant loss of biodiesel yield at the best reaction conditions, which indicates the feasibility of the proposed method. © 2012 Published by Elsevier Ltd. Selection under responsibility of the Congress Scientific Committee (Petr Kluson) Keywords: Biodiesel; heterogeneous catalysis; zinc aluminate; waste frying oils

* Corresponding author. Tel.: +55-71-3283-9808; fax: +55-71-3283-9801. E-mail address: [email protected].

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1929 C. T. Alves et al. / Procedia Engineering 42 ( 2012 ) 1928 – 1945

1. Introduction

People are concerned about the negative impact of fossil fuels on the climate and their serious consequences for the world economy, society and environment. In order to meet the growing demand for energy in a sustainable way, whilst minimizing the emission of pollutants and causing the least possible impact to the environment, there is growing motivation to develop technologies based on renewable energy sources that can partially replace the use of fossil fuels.

Renewable energy is considered to be an important resource in many countries around the world and biomass and its derivatives are the most common form of such energy. In relation to diesel oil, biodiesel produced from vegetable oils and animal fats is a potentially renewable substitute [1-4]. In recent years, biodiesel synthesis methods have been extensively studied by researchers, particularly interested in the environmental benefits arising from the consumption of this biofuel.

Biodiesel is a renewable and biodegradable fuel to be used in internal combustion engines with compression ignition. The ASTM (American Society for Testing and Materials) defines biodiesel as monoalkyl esters of fatty acids, derived from renewable lipid feedstocks, such as vegetable oils or animal fats [5]. These fatty acid esters have physicochemical properties similar to mineral diesel specifications and can therefore partially or completely replace petroleum-based diesel. The use of biodiesel in diesel engines, as a pure fuel, or blended in any proportion to fossil-diesel, requires no engine changes and maintains their durability and reliability [6-8].

The proposed biofuel is produced by the transesterification reaction of a vegetable oil, or animal fat, with a short-chain alcohol, under supercritical conditions, or in the presence of catalysts, such as enzymes, or chemical compounds of alkali or acid nature, generating glycerol as a co-product [1,7]. In principle, any type of feedstock containing free fatty acids or triglycerides, such as vegetable oils and animal fats, can be converted into biodiesel. However, the final products must meet stringent quality requirements before being accepted as biodiesel. A generic transesterification reaction can be produced in presence of a suitable acid or basic catalyst, using any type of a triglyceride source with short-chain alcohols.

Methanol is the transesterificant agent most currently used, due mainly to its high polarity and short chain, which facilitates the process of breaking the double bonds of triglycerides molecules. Moreover, compared with ethanolysis, the advantage of using the methylic route is the smaller amount of alcohol required for the reaction, reduced reaction time and lower temperatures. However, the use of ethanol as a transesterificant agent makes the process independent of non-renewable raw materials, since it is an alcohol produced from vegetable sugars.

As a source of triglycerides, waste frying oils (WFO) are an attractive alternative to biodiesel production because they are non-expensive, readily available as raw materials, and the cost of the oil is about 85% of the total operating cost of the process. Furthermore, the cost of WFOs vary from free to 60% less than virgin vegetable oils, depending on the source, availability and quantity [9-11]. In addition, reducing domestic and commercial oil effluents eliminates the detrimental environmental effects related to their inadequate discharge into rivers and springs [12-14].

The transesterification reaction can be performed using various types of catalysts, such as acid (sulfuric, sulfonic, phosphoric and hydrochloric acids), or alkali (potassium and sodium hydroxides, metal alkoxides or hydrotalcites), but heterogeneous catalysts are preferred, in order to avoid neutralization and separation steps [15-24], and to recover and reuse the catalyst. Many researchers are working to replace the homogeneous catalysts by solid catalysts in the transesterification reaction, and study the kinetics of the heterogeneously acid/base catalyzed process, aiming to evaluate their potential to industrial applications [20,25]. Depending on the nature of the triglycerides source, the use of heterogeneous catalysts is strictly recommended, which is the case for waste frying oils.


WFOs have a great amount of free fatty acids (FFA) that enable the occurrence of saponification reactions when using a homogeneous catalyst. Soap formation reduces the yield of the transesterification reaction and hinders the biodiesel purification step. An alternative to avoid saponification is adopting a heterogeneous catalyst, such as zinc aluminate, which has a rich catalytic activity and simplicity of production. Zinc aluminate (ZnAl2O4) is an oxide with a normal spinel AB2O4 structure and is commonly used as a catalyst or support in many catalytic reactions, such as cracking, dehydrogenation, acetylation and transesterification [21-22,26-27]. Zinc aluminate consists of an arrangement of closed packing CFC atoms Fd3m, presenting tetrahedral interstices, A, and octrahedral interstices, B, occupied by the bivalent and trivalent ions respectively [27]. Powders with nanosized particles of zinc aluminate may be prepared by several methods, including hydrothermal synthesis, coprecipitation and sol-gel techniques [28-32].

The synthesis of nanosized powders by combustion reactions is a promising technique for the preparation of ZnAl2O4. This method is simple and is based on a very fast and exothermic chemical reaction to form the material. A key feature of the process is that the necessary heat governing the reaction is supplied by the reaction itself and, therefore, the combustion method is self-sustaining after starting the reaction. Another important feature is that the combustion reaction reaches temperatures high enough to achieve powder formation in a short time, with the release of a great amount of gases. This method does not include many steps and produces high-purity and chemically-homogeneous powders, often consisting of nanosized particles [33-34].

The present work aimed to produce biodiesel using WFOs, ethanol or methanol as starting materials, in the presence of a ZnAl2O4 catalyst, prepared by the combustion reaction method. The experiments were conducted in a batch reactor and the influence of the following reaction parameters were investigated: temperature, alcohol:WFO molar ratio, catalyst ratio, and catalyst reuse.

2. Materials and methods

2.1. Materials

The waste frying oils (WFO) were donated from commercial establishments such as restaurants, hospitals and others from the city of Salvador, Brazil. Before being fed to the reactor for biodiesel production, WFO was pre-treated to remove undesirable impurities for the transesterification reaction, such as suspended particulate materials, water and free fatty acids. Analytical grade reagents were used: anhydrous ethanol (99.9% of purity) and methanol (99.9% of purity), purchased from Synth®. Ethanol and methanol were used in excess in relation to stoichiometric conditions of the reaction, because the transesterification reaction is reversible. For the synthesis of zinc aluminate via the combustion reaction method, analytical grade reagents were employed, namely aluminum and zinc nitrates, as starting materials, and urea as fuel. All these products were obtained from Synth®.

2.2. Equipment

The experiments were performed in a Parr stainless steel batch reactor of 1000 mL capacity, equipped with interal pressure gauge, temperature and stirring controllers. The biodiesel produced was analyzed by gas chromatography in a GC 3800A Varian Chrompack, with a 30 m long capillary column, Ultimetal VF 5HT (30 m x 0.32 mm x 0.10 μm), He (3 mL.min-1) as carrier gas, and a flame ionization detector. The column temperature program was: initial temperature of 50 ºC, first rate of 15 ºC.min-1 up to 180 ºC, second rate of 7 ºC.min-1 up to 230 ºC and third rate of 30 ºC.min-1 up to 380 ºC with holding time of 10 minutes.


2.3. Pre-treatment and characterization of WFO

The as-received WFO was filtered to remove the suspended particulate matter, neutralized with potassium hydroxide and water, to decrease the acidity and to remove remaining impurities - usually solids residues - and finally the oil was dried at 120 ºC to eliminate the excess water. The volume of oil recovered after pretreatment was about 85%. The fatty acid composition of the pre-treated WFO was determined by gas chromatography (GC-3800A Varian Chrompack) and the results are shown in Table 1.

Table 1. Fatty acid composition of the waste frying oil (WFO)

Fatty Acid Nomenclature Composition (%)

Sample 01 Sample 02

C16:0 Palmtic Acid 11.33 11.48

C18:0 Estearic Acid 3.53 3.53

C18:1ω9 cis Oleic Acid 22.71 21.73

C18:1ω9 trans Elaidic Acid 1.49 1.43

C18:2ω6 cis Linoleic Acid 54.82 55.67

C18:2ω6 trans Linolelaidic Acid 0.18 0.16

C18:3ω3 Linolenic Acid 5.62 6.00

C20:0 Eicosanoic Acid 0.32 0.00

2.4. Catalyst Synthesis and Characterization

The amounts of metal nitrates and urea were calculated based on the stoichiometry of the total reagents’ oxidizing and reducing valences (as indicated in the literature), which served as the numerical coefficients for the stoichiometric balance, to maximize the energy released [33-34]. The batches were placed in a vitreous silica basin, homogenized and directly heated in the furnace to a temperature of 400 ºC, until ignition took place, producing the zinc aluminate in a foam form. The thus obtained material was heated at 500 ºC for a further 20 minutes, in order to eliminate any volatile products, and then sieved in the range of 0.149-0.105 mm.

The synthesized and recycled powders were characterized by thermogravimetric analysis, TGA (Netzsch TG 209 F1, with air flowing at 50 mL/min and a heating rate of 10 °C/min up to 120 °C, keeping this temperature for 30 min. Then a heating rate of 10 °C/min was used to reach 1000 °C, and this temperature was kept for 20 min), and differential scanning calorimetry (DSC PYRIS 1 from Perkin Elmer. The sample was subjected to heating in Ar gas flow of 25mL/min, from 50 °C to 400 °C at a rate of 40 °C/min, remaining for 30 seconds at this temperature, and then cooled under the same rate to 50 °C) to study the thermal behavior and stability of the catalyst.

Powder X-ray diffraction (Shimadzu 6000 diffractometer, Cukα with a Ni filter and scanning rate of 2° 2θ.min-1, in a 2θ range of 20-60º), Raman (WiTec Alpha 300R confocal Raman microscope operating at 0.3 W single frequency 785 nm diode laser and an Acton SP2300 triple grating monochromator/spectrograph over the wavenumber range 0 – 3,000 cm-1 at a mean resolution of 3 cm-1. Mean spectra were composed of 10 accumulations, acquiring individual spectra using an integration time of 1 s) and Fourier Transform Infrared Spectroscopy (using a Perkin Elmer Spectrum BX Spectrometer) were used to determine the spinel-type structure. The particle size and pore size were measured by TEM


(XL30 ESEM FEG with 20.0kV) and He pycnometry (using an AccuPyc II 1340 Pycnometer from Micromeritics operating with a chamber volume of 1 cm3. The chamber and sample were purged with helium gas until all remaining air was removed from the chamber. The values reported are the mean of 30 consecutive measurements) respectively. BET specific surface area and the pore volume of the catalysts were determined from N2 adsorption isotherms collected in a Micromeritics ASAP 2010 system and NH3-TPD and CO2-TPD (Temperature Programmed Desorption of ammonia or carbon dioxide) profiles were collected in the temperature range of 20 – 800 ºC, at a heating rate of 10 ºC.min-1, using a Micromeritics 2720 system.

2.5. Transesterification reaction

The reaction runs were carried out in a Parr batch reactor for 2 hours, using an alcohol:oil molar ratio of 40:1, temperature range from 60 °C to 200 ºC, catalyst:oil ratio of 1-10 wt%, under 700 rpm stirring speed. The oil, alcohol and catalyst were weighed and introduced into the reactor, and stirring only started once the reaction temperature was reached. At the end of the two hours reaction time, the reactor was cooled down and the samples were withdrawn at room temperature. The insoluble catalyst was recovered by filtration, vacuum dried in an oven at 50 ºC, sieved and reused in subsequent reaction cycles. The reaction mixture was introduced into a separation funnel, where the glycerin was decanted, and the excess alcohol was recovered using a rotary evaporator. The biodiesel samples were characterized as follows.

2.6. Biodiesel Characterization

The yield of ethyl and methyl esters were determined by a gas chromatographic procedure. The biodiesel aliquots were diluted with n-heptane, using 8000 μg/mL tricaprin in pyridine solution and a 1000 μg/mL 1,2,4-butanetriol solution in pyridine as internal standards. The samples obtained were analyzed by GC (CP 3800A Varian Chrompack) according to DIN EN14105 [35].

3. Results and discussion

3.1. Preparation and Characterization of ZnAl2O4

The overall reaction that represents the synthesis of the ZnAl2O4 catalysts is given by the self-combustion method [33]. Fig. 1 shows the representative TGA and DTGA curves of the precursor (Fig.1a) and pure powder (Fig.1b) zinc aluminate samples prepared by combustion method, and the reused catalyst after the transesterification reaction from ethanol (Fig.1c) and methanol (Fig.1d). Fig.1a indicates that the combustion method simulated inside the oven was complete and Fig.1b indicates two peaks observed in the DTGA curve and proved by weight loss in the TGA curve: the first broad peak is associated with the removal of physically adsorbed molecular water [36], while the second one results from the removal of constitutional water from zinc aluminate. The TGA/DTGA curves of the zinc aluminate recycled from the transesterifications reaction from ethanol (Fig.1c) and methanol (Fig.1d) shows three steps which are attributed to the evaporation of the ethyl and methyl esters [37]. The yield of OGR into ethyl and methyl esters was proved by the gas chromatographic results (> 95%). The DSC curve (Fig.2) from precursor ZnAl2O4 powder showed 2 steps: the first is endothermic peak between 275 ºC and 315 ºC associated with the removal of physically adsorbed molecular water, also indicated in the TGA/DTGA curves, and the second peak was identified as exothermic peak at 399,717 °C indicating that the combustion reaction was completed.


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Fig. 1. TGA analysis of the precursor (a), pure (b), recycled from the transesterification reaction using ethanol (c) and recycled from the transesterification reaction using methanol (d) ZnAl2O4 prepared by the combustion reaction method.

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The X-ray diffraction (XRD) patterns collected for the zinc aluminate powder correspond to the characteristic peaks of the cubic spinel-phase ZnAl2O4, namely the peaks at 31.3, 36.9, 44.9, 55.6, 59.4, 65.2, 74.1 and 77.3º 2θ [27]. These XRD patterns are in accordance with the JCPDS card (JCPDS Nº 05-0669) and also indicate that the product is a single-phase and high purity material. This is a feature of the combustion reaction for the synthesis of oxide powders. Better crystallization of the oxide powders is attained when high temperatures are reached during the combustion reaction.

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The specific surface area of the as prepared powder was determined to be 17.3084 ± 0.0804 m2.g-1 and the pore volume 0.023 cm3.g-1. The surface area and porosity of zinc aluminate are strongly affected by the preparation method and by the calcination temperature. High surface area has been reported for samples prepared by sol-gel or hydrothermal methods, but lower surface area may be expected for zinc aluminate prepared by the combustion reaction as a consequence of the very high temperatures attained during the combustion synthesis [33]. The N2 physisorption isotherms and the BJH pore size distribution in the ZnAl2O4 powder are shown in Fig.4. Type IV isotherms and a hysteresis loop of type H2 are usually assigned to mesoporous materials with a complex structure, where network effects are important. The bimodal pore size distribution also confirmed a low contribution of micropores [27-28].


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Fig. 4. N2 physisorption isotherms and the BJH pore size distribution in the ZnAl2O4 powder.

The particle size of the synthesised mesoporous ZnAl2O4 nanoparticles was investigated by transmission electron microscopy (TEM). Fig 5 shows the TEM micrographs of the zinc aluminate powder prepared by the combustion reaction. The micrographs indicate that ZnAl2O4 powders consists of mesopore particles attributed to the high temperatures obtained during the combustion reaction and to the excess expulsion of gases during synthesis, which promotes the formation of “holes” between the particles, as seen in the images [29,34,38-39].


Fig. 5. TEM micrographs of ZnAl2O4 powders particles prepared by the combustion method.

The zinc aluminate spinel is usually considered to be a neutral solid but the presence of typical Lewis acid sites of different strengths, and the distribution of surface acidity, have been reported and seem to depend on the preparation method and sample treatment [29,40].

In order to assess the balance of surface acidity and basicity on the zinc aluminate spinel prepared by combustion reaction, the number and distribution of the surface acid and base sites on the sample were determined by NH3-TPD and CO2-TPD, as respectively shown in Fig. 6. Quantitative analyses of the TPD profiles indicated that the total acidity was 370 μmol g-1 and the total basicity was 258 μmol g-1. In both cases, the low temperature peak (< 200 ºC) was assigned to the contribution of physisorption and corresponded to < 1% of both acid and base sites. On the other hand, the strong sites (> 400 ºC) corresponded to nearly 75% of both acid and base sites. These findings indicate that the acidity of the sample prepared by combustion reaction is somewhat lower than that of γ-alumina in opposition to zinc aluminate spinel prepared by hydrothermal methods [29]. Thus, in spite of the acid character indicated by the balance of acid and base sites, the catalyst used in this study showed similar amounts of strong acid and base sites and this feature may be useful for the transesterification of WFO, a mixture of triglycerides with very low acid values.


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3.2. Transesterification reaction experiments

3.2.1. Influence of the reaction temperature and catalyst:oil ratio

In order to investigate the influence of the temperature on the transesterification of WFO with ethanol and methanol, 2-hour reaction runs were carried out, in duplicate, in the range of 60-200 ºC, for different catalyst:oil ratio (1-10 wt% of the initial amount of oil), alcohol:oil molar ratio equal to 40:1, under stirring (700 rpm). The yields of ethyl and methyl esters were plotted as a function of the reaction temperature and are shown in Fig.7. The ester yields are high at temperatures higher than 100 ºC. The highest yields were attained using 10 wt% catalyst, over the temperature range for both ethanol and methanol but, in general, the influence of the catalyst ratio is more significant at lower reaction temperatures. In general, the yields increased with the increase of temperature and catalyst ratio. In fact, high methyl ester yields have been previously reported in the transesterification of rapeseed oil at 200 ºC, using 12.5 wt% ZnAl2O4:oil and 27:1 methanol: oil ratio [21].

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Fig. 7. Yield of Ethyl (a) and Methyl (b) esters produced from the transesterification reaction


3.2.2. Influence of the alcohol:WFO molar ratio

In spite of the stoichiometric alcohol:oil molar ratio in the transesterification reaction, an excess of alcohol is generally used to shift the reaction equilibrium towards the products, in order to achieve high yield of esters and to enhance solubility of the alcohol in the oil. One of the most important variables affecting the yield of ester is the molar ratio of alcohol to oil employed.

The overall process of transesterification reaction is a sequence of three consecutive and reversible reactions, in which diglycerides and monoglycerides are formed as intermediates. The stoichiometric reaction requires 1 mole of a triglyceride and 3 moles of alcohol to obtain 3 moles of fatty alkyl ester and 1 mole of glycerol. However, when the molar ratio of alcohol to oil is increased, the reverse reaction is negligible, because the glycerol formed is not miscible in the ester formed, leading to a two-phase system. The frequency of the collisions between the product molecules is considerably reduced, preventing the reverse reaction. Most of the researches have used higher molar ratios, up to 45:1, particularly when the triglyceride contained large amounts of free fatty acids [4,41-44].

Pugnet et al. [21] reported that high yields could be obtained using methanol:rapseed oil molar ratio = 27, at 200 ºC, in the presence of ZnO based heterogeneous catalysts. The influence of the alcohol:oil molar ratio was investigated in the range of alcohol:WFO = 6-65, at 200 ºC. The results are collected in Fig. 8 and indicate that the ratio alcohol:WFO = 40:1 is the most adequate for the transesterification of WFO, either with methanol or ethanol.

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Fig. 8. Effect of the molar ratio of alcohol to oil in the biodiesel yield

3.2.3. Catalyst Reuse

The reuse of catalyst is an important issue for industrial applications. Fig. 9 shows the FTIR spectra of fresh and reused ZnAl2O4 catalyst. The spectrum collected for the fresh powder obtained by combustion reaction showed two strong bands around 670 e 560 cm-1, assigned to Al–O stretching and O–Al–O bending vibrations of AlO6 groups in the spinel structure, and two bands between 450 and 700 cm-1, assigned to the vibration modes of octahedral and tetrahedral groups [27,45]. These results can also be


shown in the Raman microscopy (Fig. 10) that indicated two strong bands between 800 and 300 cm-1, indicating the presence of AlO6 groups.

In the present study, ZnAl2O4 was recovered from the reaction mixture by vacuum filtration and dried in a vacuum oven at 50 °C, without use of any solvent to extract adsorbed organic matter from the catalyst before reuse. Thus, bands assigned to the main products of the transesterification reaction could be observed along with those of the catalyst and they increased after each reaction cycle. A strong band between 3600 – 3200 cm-1, assigned to OH stretching; two bands between 3000 – 2850 cm-1, assigned to alkane groups; two bands at 1465 and 1375 cm-1, assigned to CH3 and CH2, indicate the presence of glycerol in the recovered catalysts [45]. On the other hand, the bands between 1750 – 1730 cm-1 and 1300 – 1000 cm-1, respectively corresponding to C=O and C-O stretching modes, suggest presence of ester groups in the porous catalyst. In addition, vibration modes assigned to carboxylic acids (RCOOH) suggest that the WFO starting material was not completely converted to ester.

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Fig. 10. Raman microscopy of reused zinc aluminate powder

The reuse of the ZnAl2O4 catalyst was investigated in three reaction cycles with methanol and ethanol, at 100, 150 and 200 ºC, for different catalyst ratios. The results are presented in Figures 7 and 8, respectively. At low temperatures, the yields of esters were low, probably due to the difference in the miscibility between alcohol and oil, mainly for ethanol, which has a longer chain. Between 60 ºC and 70 ºC, the formation of four phases as oil, liquid alcohol, gaseous alcohol and catalyst may have occurred, which hindered the mass transfer in catalyst pores, decreasing the formation of esters. For high temperatures, namely above the boiling point of alcohol, the miscibility factor becomes less significant due to the better miscibility of gaseous alcohol and oil.

In general, lower yields were obtained in the third reaction cycle and the decrease was more significant at 100 ºC, using both alcohols and over the investigated range of catalyst ratios. These facts may be due to either a thermal effect, or to the mass loss during the procedure to recover the catalyst. At higher temperatures, desorption of products from the catalyst surface is favored and the lower viscosity of WFO contributes to reduce mass transfer limitations. Leaching of zinc from zinc aluminate catalysts prepared by solid state reaction, impregnation and co-precipitation have been reported [21-22,26-27]. In spite of this, no zinc depletion could be determined by EDX analyses of the catalysts after three reaction cycles and no significant homogeneous reaction was observed in a side test, even after the first reaction run. Thus, it seems reasonable that some of the fine catalyst powder obtained by the combustion reaction method could be retained in the products or in the glycerol. Therefore the use of larger catalyst particles may be envisaged to overcome this issue, allowing higher yields in a higher number of reaction cycles.


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Zinc Aluminate (%)

Fresh Cat. rec. Cat. rec2.

0 1 2 3 4 5 6 7 8 9 10

80

85

90

95

100


Yiel

d (%

)

Zinc Aluminate (%) (a) (b)

0 1 2 3 4 5 6 7 8 9 10

80

85

90

95

100


Yiel

d (%

)

Zinc Aluminate (%) (c)

Fig. 12. Methyl ester yields at (a) 100 °C; (b) 150 °C; and (c) 200 ºC, Methanol:WFO molar ratio = 40:1, 1-10% ZnAl2O4 catalyst: fresh and recovered after the 1st and 2nd reaction cycles.


4. Conclusions

Methyl and ethyl esters have been successfully produced by the transesterification of waste frying oil using a ZnAl2O4 catalyst prepared by the combustion reaction method. A single-phase and high-purity spinel material was obtained, showing a bimodal pore size distribution. In spite of the acid character indicated by the balance of acid and base sites, similar amounts of strong acid and base sites were determined on the catalyst surface. High ester yields (above 95%) were obtained using alcohol:WFO molar ratio of 40:1, at temperatures equal to, or greater than, 150 ºC, using 1-10 wt% catalyst:WFO ratio. The catalyst was recovered and used in three reaction cycles, but either the increase of carbon deposits or loss of fine catalyst particles during the recovery steps could account for the yield decrease after the third reaction cycle. Finally, the results presented herein demonstrate that the transesterification of waste frying oil (WFO) with ethanol, in the presence of the ZnAl2O4 heterogeneous catalyst, is a sustainable alternative to produce biodiesel using renewable and waste raw materials.

Acknowledgements

This work was supported by CNPq – “National Counsel of Technological and Scientific Development”, a body of the Brazilian Ministry for Science and Technology.

The Confocal Raman Microscope, Helium Pycnometer, and Particle Size Analyser used in this research were obtained through Birmingham Science City: Innovative Uses for Advanced Materials in the Modern World (West Midlands Centre for Advanced Materials Project 2), with support from Advantage West Midlands (AWM) and part funded by the European Regional Development Fund (ERDF).

The authors are grateful for the collaboration with the staff in the Schools of Chemical Engineering and Metallurgy and Materials at the University of Birmingham for the following: conduct experiments using the Parr reactor and prepare catalysts with the facilities in the Supercritical Fluids Technology group; carry out TPD and Infrared analysis in the Catalysis and Reaction Engineering laboratory, facilitated by Dr Joe Wood and Dr Jiawei Wang; perform BET Surface Area analysis at the Interdisciplinary Research Centre (IRC) in Materials Processing, facilitated by John Wedderburn; obtain TEM micrographs at the Centre for Electron Microscopy, with help from Paul Stanley.

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