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

Transcription

1 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:

2 This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier s archiving and manuscript policies are encouraged to visit:

3 Fuel 89 (2010) Contents lists available at ScienceDirect Fuel journal homepage: 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, Lisboa, Portugal b IST Instituto Superior Técnico/UTL, Institute of Biotechnology and Bioengineering, Av. Rovisco Pais, Lisboa, Portugal c IST Instituto Superior Técnico/UTL, Centre for Chemical Process, Av. Rovisco Pais, Lisboa, Portugal d IST Instituto Superior Técnico/UTL, Institute of Science and Engineering of Materials and Surfaces, Av. Rovisco Pais, 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, Lisboa, Portugal. 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 /$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi: /j.fuel

4 J.F. Puna et al. / Fuel 89 (2010) 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 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 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 C, 4 h Impregnation 94.8 Li/CaO 575 C, 5 h Impregnation 88.9 CaO 575 C, 3 h Commercial 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] 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.

5 3604 J.F. Puna et al. / Fuel 89 (2010) 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 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 C for 30 min under vacuum using a rotary evaporator. The glycerine phase is not subjected to any further treatment 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 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 ) FAME Density 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 Mg/Zr 2:1 None Ca/Al 2 O 3 None MgAl 5:8 None Sr/CaO 575 C, 4 h CaO B 575 C, 4 h Li/CaO None CaO None 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 ) FAME content (%) Fig. 3. Density versus FAME content (estimated by NIR) for the obtained diesel phases.

6 J.F. Puna et al. / Fuel 89 (2010) 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. However, studies using other alkaline and alkaline earths oxides catalysts unsupported and supported over materials such as alumina, magnesia, zirconia and titania and, also, over materials obtained by thermal activation of hydrotalcites (Mg Al) and other mixed oxides (e.g. Mg Ca) are going also to be carried out. Acknowledgements The authors would also like to thank to Eng. Pedro Felizardo for the calculations of the biodiesel properties from its NIR Spectrum. The authors would like, also, acknowledge to Professor José Cardoso Menezes of the Centre for Biological and Chemical Engineering of Instituto Superior Técnico for the use of the NIR spectrometer ABB BOMEM MB160. References [1] Balat M, Balat H. Recent trends in global production and utilization of bioethanol fuel. Appl Energy 2009;86(11): [2] Gomes JF, Puna JF, Bordado JC, Correia MJN. Development of heterogeneous catalysts for transesterification of triglycerides. React Kinet Catal Lett 2008;95(2): [3] Pelkmans L et al. European biofuels strategy. Int J Env Stud 2007;64(3): [4] Thuijl EV. An overview of biofuel technologies, markets and policies in Europe. Energy Research Centre of the Netherlands; [5] Santos A. Heterogeneous catalysts for biodiesel production methanolysis of soy oil over. Hidrotalcytes of magnesium and aluminium changed. Master Thesis on Environmental Engineering. IST/UTL, Lisbon; [6] Albuquerque M et al. MgM (M = Al and Ca) oxides as basic catalysts in transesterification processes. Appl Catal A: Gen 2008;347(2): [7] Benjapornkulaphong S et al. Al 23 -supported alkali and alkali earth metal oxides for transesterification of palm kernel oil and coconut oil. Chem Eng J 2009;145(3): [8] Park Y et al. The heterogeneous catalyst system for the continuous conversion of free fatty acids in used vegetable oils for the production of biodiesel. Catal Today 2008;131: [9] MacLeod C et al. Evaluation of the activity and stability of alkali-doped metal oxide catalysts for application to an intensified method of biodiesel production. Chem Eng J 2008;135: [10] Albuquerque M et al. CaO supported on mesoporous silicas as basic catalysts for transesterification reactions. Appl Catal A: Gen 2008;334: [11] Xie W et al. Alumina-supported potassium iodide as a heterogeneous catalyst for biodiesel production from soybean oil. J Mol Catal A: Chem 2006;255:1 9. [12] Huaping et al. Preparation of biodiesel catalyzed by solid super base of calcium oxide and its refining process. Chin J Catal 2006;27(5): [13] Damião C. Alkaline heterogeneous catalysts for biodiesel production by transesterification of vegetable oils, with earth alkaline metals over MgO. Master Thesis on Environmental Engineering. IST/UTL, Lisbon; [14] Sree R et al. Transesterification of edible and non-edible oils over basic solid Mg/Zr catalysts. Fuel Process Technol 2009;90: [15] Felizardo P et al. Production of biodiesel from waste frying oils. Waste Manage 2006;26: [16] Granados M, Poves MD, Alonso DM, Mariscal R, Cabello F, Galisteo, et al. Biodiesel from sunflower oil by using activated calcium oxide. Appl Catal B: Environ 2007;73(3 4):

7 3606 J.F. Puna et al. / Fuel 89 (2010) [17] Felizardo P et al. Monitoring biodiesel fuel quality by near infrared spectroscopy. J Near Infrared Spectrosc 2007;15(97). [18] Baptista P, Felizardo P, Menezes José C, Correia MJN. Multivariate near infrared spectroscopy models for predicting the iodine value, CFPP, kinematic viscosity at 40 C and density at 15 C of biodiesel. Talanta 2008;77:144. [19] Vicente G, Martínez M, Aracil J, Esteban A. Kinetics of sunflower oil methanolysis. Ind Eng Chem Res 2005;44: [20] Dias AP, Bernardo J, Felizardo P, Correia MJN. Biodiesel by soybean oil methanolysis over strontium modified magnesia. Appl Catal B: Environ submitted for publication. [21] Zabeti M, Daud W, Aroua M. Optimization of the activity of CaO/Al 2 O 3 catalyst for biodiesel production using response surface methodology. Appl Catal A: Gen 2009;366(1): [22] Alonso DM, Mariscal R, Granados ML, Maireles-Torres P. Biodiesel preparation using Li/CaO catalysts: activation process and homogeneous contribution. Catal Today 2009;143(1 2). [In: International symposium on catalysis for clean energy and sustainable chemistry, on occasion of the 60th birthday of Prof. Jose L.G. Fierro, 15 May p ].