Advances on the development of novel heterogeneous catalysts for transesterification of triglycerides in biodiesel

Chemical Eng. Dept., ISEL From the SelectedWorks of João F Gomes 2010 Advances on the development of novel heterogeneous catalysts for transesterification of triglycerides in biodiesel João F Gomes Available at: https://works.bepress.com/joao_gomes/35/

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Fuel 89 (2010) 3602 3606 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Short communication Advances on the development of novel heterogeneous catalysts for transesterification of triglycerides in biodiesel J.F. Puna a, J.F. Gomes a,b, *, M. Joana N. Correia c, A.P. Soares Dias d, J.C. Bordado b a ISEL Instituto Superior de Engenharia de Lisboa, Chemical Engineering Department/CEEQ, R. Cons. Emídio Navarro 1, 1959-007 Lisboa, Portugal b IST Instituto Superior Técnico/UTL, Institute of Biotechnology and Bioengineering, Av. Rovisco Pais, 1049-001 Lisboa, Portugal c IST Instituto Superior Técnico/UTL, Centre for Chemical Process, Av. Rovisco Pais, 1049-001 Lisboa, Portugal d IST Instituto Superior Técnico/UTL, Institute of Science and Engineering of Materials and Surfaces, Av. Rovisco Pais, 1049-001 Lisboa, Portugal article info abstract Article history: Received 17 December 2009 Received in revised form 7 April 2010 Accepted 20 May 2010 Available online 3 June 2010 Keywords: Biodiesel Heterogeneous catalysts Transesterification This paper describes experimental work done towards the search for more profitable and sustainable alternatives regarding biodiesel production, using heterogeneous catalysts instead of the conventional homogenous alkaline catalysts, such as NaOH, KOH or sodium methoxide, for the methanolysis reaction. This experimental work is a first stage on the development and optimization of new solid catalysts, able to produce biodiesel from vegetable oils. The heterogeneous catalytic process has many differences from the currently used in industry homogeneous process. The main advantage is that, it requires lower investment costs, since no need for separation steps of methanol/catalyst, biodiesel/catalyst and glycerine/catalyst. This work resulted in the selection of CaO and CaO modified with Li catalysts, which showed very good catalytic performances with high activity and stability. In fact FAME yields higher than 92% were observed in two consecutive reaction batches without expensive intermediate reactivation procedures. Therefore, those catalysts appear to be suitable for biodiesel production. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Biodiesel, a renewable fuel, can be used in conventional diesel engines, pure or mixed with conventional diesel, without significant modifications of the engine. This fuel has higher oxygen content than petroleum diesel and its use in diesel engines have shown great reduction in the emission of particulate matter, carbon monoxide, sulphur, polyaromatics, hydrocarbons, smoke and noise. In addition, burning of vegetable-oil based fuel does not contribute to net atmospheric CO 2 [1]. Biodiesel is mostly produced through a transesterification reaction between a lipid source (vegetable oils and animal fats) and an alcohol (low molecular weight such as methanol and ethanol) yielding a mixture of long chain esters and a valuable co-product, glycerol. In a previous paper [2], the authors pointed out that an important drawback related with the use of homogeneous catalysts in biodiesel production is that, they have to be neutralized after the end of the reaction, thus producing salt streams. Moreover, if the oil contains free fatty acids, they react with the catalyst to form soaps as unwanted by-products, hence requiring more expensive * Corresponding author at: ISEL Instituto Superior de Engenharia de Lisboa, Chemical Engineering Department/CEEQ, R. Cons. Emídio Navarro 1, 1959-007 Lisboa, Portugal. E-mail address: jgomes@deq.isel.ipl.pt (J.F. Gomes). separation processes [3]. Therefore, there is currently a drive towards the development of industrial processes for biodiesel production using solid catalysts. The key benefit of using heterogeneous catalysts is that no polluting by-products are formed and the catalysts do not mix with biodiesel and can be recovered and reused allowing the operation in continuous reactors instead batch reactors used with homogeneous catalysis. In addition to lower separation costs, less maintenance is needed as these catalysts are not corrosive [4]. 2. Experimental 2.1. Catalyst preparation Based on previous work reported in the literature concerning the use of heterogeneous catalysts [2,5 14], a pre-screening comprising some solid catalysts was made in order to select catalysts that could show promising performances in terms of methyl ester (biodiesel) yield, reaction conversion, potentiality and sustainability, as well as simplicity and reduction of costs preparation. This pre-screening comprised several (around 20) solid basic catalysts, some prepared by precipitation and others by impregnation (wet technique) of support materials. Different precursor/support ratios were used. For the time being, the influence of the precursor content on the yield of the reaction was not studied be- 0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.05.035

J.F. Puna et al. / Fuel 89 (2010) 3602 3606 3603 cause the definition of the optimum ratio is to take place at a later stage, as well the influence of calcination temperature. The catalysts were prepared by coprecipitation or wet impregnation. The preparation procedures, coprecipitation and wet impregnation, are schematized in Fig. 1. All the used reagents were p.a. grade (except NaOH) acquired from Aldrich. The preparation of the catalysts was always carried out at room temperature. The precursors salts of the alkaline and alkaline earths metals were always nitrate salts except for the CaO B. The CaO B sample was prepared according to Huaping et al. [12] and calcium carbonate was used instead calcium nitrate. Samples were dried at 120 C overnight and calcinated in a muffle during 5 h. Table 1 resumes the prepared catalysts as well as the respective preparation method used. Table 1 Prepared methods, calcination conditions and FAME yields for tested catalysts. Catalyst Calcination Preparation method FAME content (%) K/Al 2 O 3 575 C, 3 h Impregnation 92.0 Mg/Zr 2:1 575 C, 5 h Coprecipitation 35.1 MgO B 700 C, 5 h Impregnation 13.5 Al 2 O 3 650 C, 5 h Commercial 2.1 CaO B 900 C, 2 h Calcination of the carbonate salt 83.7 Sr/MgO 650 C, 5 h Impregnation 1.1 MgAl 5:8 500 C, 5 h Coprecipitation 79.1 MgCa 3:8 800 C, 5 h Coprecipitation 91.9 Sr/CaO 900 C, 2 h Impregnation 88.7 Ca/Al 2 O 3 450 C, 4 h Impregnation 94.8 Li/CaO 575 C, 5 h Impregnation 88.9 CaO 575 C, 3 h Commercial 93.3 2.2. Raw oil pre-treatment All tests were made using soybean oil from a local producer. The raw oil was subjected to a pre-treatment consisting of the addition of 0.05 (wt.%) of H 3 PO 4, for gum removal, followed by neutralization with NaOH solution, 8%, until an acidity index below 0.5 mg KOH/g oil is obtained. Then, the oil was centrifuged at 8000 rpm, during 15 min, and washed first with a citric acid solution (ph 3.5; 50/700 ml of oil) and again with distilled water (50/700 ml of oil) until the ph of washing water reach 5.5. The obtained oil is centrifuged again and dried at 80 C under vacuum (200 mbar) in a rotary evaporator for 30 min [15]. 2.3. Transesterification laboratory tests All the catalytic tests were carried out using the same experimental conditions, chosen so that maximum conversion in terms of methyl esters is obtained. Thus, the reaction was carried out aqueous solution of M a m+ nitrate salts aqueous solution of M b n+ nitrate salts aqueous solution of M a m+ nitrate or carbonate (CaO B, MgO) salts aqueous solution of M a m+ + M b n+ vigorous stirring Support material MgO CaO Al 2 O 3 vigorous stirring Ammonium or sodium carbonate with NH 3 or KOH solution, dropwise under vigorous stirring M a m+ + M b n+ precipitate aqueous suspension support material+ M a m +precipitate Filtration Washing Drying Calcination Filtration Washing Drying Calcination Catalyst Catalyst COPRECIPITATION WET IMPREGNATION Fig. 1. Catalyst preparation procedures: coprecipitation and wet impregnation.

3604 J.F. Puna et al. / Fuel 89 (2010) 3602 3606 in a 500 ml round bottom reactor equipped with a condenser and a mechanical stirrer. The methanolysis were carried out at methanol reflux temperature (60 C) using a water bath with temperature control. Typically, 100 g soybean oil was transferred to the reactor and heated until the desired temperature was reached. At this point a mixture of methanol and of the catalyst was added to the oil and the transesterification reaction began for 5 7 h. For each test, 5% (w/w, oil basis) of catalyst and a ratio of 12:1 of methanol:oil was used. 2.4. Reaction product purification After the reaction, the catalyst was recovered by hot vacuum filtration, washed with methanol and dried eventually for further use. The liquid two phases obtained, the glycerol-rich phase and the methyl esters (biodiesel) phase, were separated in a decantation funnel. After separation, the biodiesel phase, containing the methyl esters, unreacted oil, methanol and other impurities, was washed three times with 15 ml of distilled water, 10 ml of 1.5% nitric acid solution, and 20 ml of distilled water, respectively. It was centrifuged at 8000 rpm for 15 min, filtered and finally dried at 80 90 C for 30 min under vacuum using a rotary evaporator. The glycerine phase is not subjected to any further treatment. 2.5. Determination of methyl esters content As generally accepted, fatty acids methyl esters (FAME) content of the biodiesel phase was used as a measure of the catalytic activity [16]. The quality of the crude biodiesel, after purification samples was evaluated by measuring their density and final content of fatty acids methyl esters (FAME). Actually, the esters content is an indicator of the degree of conversion of the transesterification reaction, whereas the density of biodiesel is influenced by its methyl esters content, the type of feedstock used in its production and the presence of contaminants (namely, methanol) [17,18]. Thus, the European Norm EN 14214 specifies the minimum value of 96.5% for the FAME content and the density between 860 and 900 kg/m 3. In this work, these properties were determined using near-infrared spectroscopy (NIR), according to the procedure described elsewhere [17,18]. The near-infrared diffuse absorbance spectra of the biodiesel samples were acquired using an ABB BOMEM MB160 (Zurich, Switzerland) spectrometer equipped with an InGaAs detector and a transflectance probe from SOLVIAS (Basel, Switzerland). CaO Li/CaO Ca/Al2O3 Sr/CaO MgCa 3:8 MgAl 5:8 Sr/MgO CaO B Al 2 O 3 MgO Mg/Zr 2:1 K/Al 2 O 3 Density (Kg/m 3 ) 840 860 880 900 920 940 FAME Density 0 20 40 60 80 100 FAME content (%) Fig. 2. FAME content and density (by NIR) for the diesel phases obtained using the prepared catalysts (T =60 C, W catalyst /W oil = 5%, methanol/oil = 12:1 M ratio, reaction time = 5 h). the carbonate and hydroxyl species remain on catalyst surface thus reducing the catalytic activity. An analogous effect was also reported for Al 2 O 3 catalysts [21]. The main results in Fig. 2 showed that the catalytic behaviour for triglycerides methanolysis can be Table 2 Catalytic performances for reused catalysts (batch #2). Catalyst Reactivation treatment after batch #1 FAME content (%) Density (kg/m 3 ) K/Al 2 O 3 None 0.5 920 Mg/Zr 2:1 None 1.9 920 Ca/Al 2 O 3 None 1.1 921 MgAl 5:8 None 0.4 919 Sr/CaO 575 C, 4 h 1 920 CaO B 575 C, 4 h 2.8 918 Li/CaO None 92.7 890 CaO None 96.5 884 3. Results The methanolysis of triglycerides (TG) presents three steps, consecutive and reversible, reaction mechanism. Diglycerides (DG) and monoglycerides (MG) are intermediates species, while GL represents glycerol [19]: TG þ CH 3 OH DG þ RCOOCH 3 DG þ CH 3 OH MG þ RCOOCH 3 MG þ CH 3 OH GL þ RCOOCH 3 A large methanol excess is generally used in order to increase the rate of the global reaction and to displace the equilibrium towards the reaction products. The main catalytic performances for the prepared catalysts (Table 1) are resumed in the Fig. 2. In the tested conditions only the Al 2 O 3 and Sr/MgO samples presented negligible catalytic activities, what can be attributable to the low calcination temperature. Actually recent result reported a strong influence on the calcination temperature for Sr/MgO catalysts [20]. If the calcination temperature it is not enough high, Density (Kg/m 3 ) 925 920 915 910 905 900 895 890 885 880-2 18 38 58 78 98 118 FAME content (%) Fig. 3. Density versus FAME content (estimated by NIR) for the obtained diesel phases.

J.F. Puna et al. / Fuel 89 (2010) 3602 3606 3605 Fig. 4. NIR spectra of two biodiesel samples obtained by different methods: the grey spectrum refers to a biodiesel sample (>96.5% of FAME) produced using a basic homogeneous catalyst process. The red spectrum refers to a biodiesel sample (89% of FAME) produced using Li/CaO (0.4:1; 575 C) heterogeneous catalyst. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) enhanced by incorporating alkaline metals and alkaline earth metals in the samples composition, especially K, Li, Ca and Sr. In order to evaluate the stability of the prepared catalysts, after the first batch reaction test (batch #1), some of them were used to perform a second batch catalytic test (batch #2). The main catalytic results for the batch #2, and the reactivation procedures, after the batch #1, are in Table 2. Only the calcium based catalysts showed high stability in the reaction medium. This finding agrees with the recent results of Alonso et al. [22]. The other tested catalysts showed severe decay of the catalytic activity after the batch #1. As expected [18] the density of the diesel phases (FAME + unreacted oil), including data from batch #1 and batch #2, decreases raising the FAME content (Fig. 3). In analogous conditions the density the raw oil was 920 kg/m 3 whereas the pure FAME (obtained by homogenous basic catalysis, unpublished results) displays 885 kg/m 3. Fig.4 shows two NIR spectra of biodiesel samples obtained, respectively, with heterogeneous catalyst (Li/CaO) and with homogeneous catalysis. It can be easily noticed the similarity of both spectra. 4. Conclusions From the presented results it can be inferred that CaO and CaO modified with Li are promising catalysts. In fact, these catalysts showed high catalytic activity and stability with biodiesel yields higher than 93% for both reaction batches. So, they will be further studied in detail. 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