TRANSESTERIFICATION OF RAPESEED OIL TO BIODIESEL USING HETEROGENEOUS BASE CATALYSTS

Transcription

1 TRANSESTERIFICATION OF RAPESEED OIL TO BIODIESEL USING HETEROGENEOUS BASE CATALYSTS Inara Liepina, Maija Strele, Svetlana Chornaja, Mara Jure Riga Technical University Faculty of Material Science and Applied Chemistry Department of Chemical Tehnology of Biologically Active Compounds Abstract. Methyl esters of fatty acids usually are produced by transesterification of triglycerides with methanol using homogenous base catalysts; more often these are potassium or sodium hydroxides and methoxides. The yields of biodiesel are high using these catalysts. However, the conventional method has several serious drawbacks besides transesterification saponification takes place, removal of catalysts and soap is technically difficult, large amount of wastewater is produced to separate and purify biodiesel. Therefore it is expected that in the near future homogenous catalysts will be replaced with environmentally more friendly heterogeneous catalysts. Our investigations were devoted to synthesis of biodiesel using heterogeneous catalysts. This method leads to simplification of catalyst s separation procedure catalyst can be removed from methyl esters of fatty acids by filtration and obtained raw glycerol contains less admixtures. Investigated catalysts were KI and KF loaded on alumina, KF on ZnO, KF on crude clays from Serpukhov and Priekule, KF on activated (with 10% H 2 SO 4 ) clays from Priekule. Alkali metal salts were loaded onto solid support by an impregnation method from aqueous solution, dried and calcined at high temperatures prior to each reaction. Crude clays from Serpukhov and Priekule were used as catalysts for transesterification of rapeseed oil. Different conditions (amount of potassium salt and calcination temperature) were examined to prepare the catalysts; influence of transesterification conditions (catalyst s loading, the ratio of methanol to oil, reaction time) on conversion of raw materials to biodiesel and raw glycerol, as well as quality of these products were investigated. Key words: transesterification, rapeseed oil, methyl esters, biodiesel, heterogeneous catalyst, clay. Introduction The amount of petroleum is limited in nature and decreasing fast during the last years. Due to this price of fossil fuel increases. The role of vegetable oils as an alternative fuel or as a raw material for production of biodiesel is growing. However, direct use of oils for injection diesel engines is problematic due to their high viscosity and low volatility. The viscosity of fossil diesel is about ten times lower than that of vegetables oils. Different methods exist to reduce the viscosity of vegetable oils: mixing of oil with fossil diesel, emulsification, pyrolysis, cracking and transesterification. The last one appears to be the best and the most often used method. Alkyl esters of fatty acids, commonly known as biodiesel, are nontoxic, biodegradable and serve to replace fossil fuel. Triglyceride molecules are transformed into smaller esters as a result of transesterification reaction. The properties of these esters, e.g., heating capacity, cetane number and viscosity, are similar to those of fossil fuel. The methyl esters of fatty acids are usually produced by transesterification of triglycerides with methanol using dissolved sodium or potassium hydroxides in alcohol as homogenous catalysts. Though the yield of conversion is very high, the production of biodiesel using homogenous catalysis is connected with several problems: it is technically difficult to remove catalyst large amount of waste water is produced in order to separate and purify biodiesel; purification of glycerol the by-product of biodiesel production is complicated. Therefore, conventional homogenous catalysts are expected to be replaced with environmentally more friendly heterogeneous catalysts in the near future. Usually, transesterification with heterogeneous catalyst can be carried out at high temperature, pressure and large excess of methanol. Heterogeneous catalysis has several advantages the most important one is easier separation of catalyst from reaction mixture by filtration or centrifugation. Two types of heterogeneous alkali catalysts are used: solid alkali and alkaline-earth metal carbonates and oxides [1, 2] and alkali metal salts or hydroxides on support, e.g., KI/Al 2 O 3, KF/Al 2 O 3, KOH/Al 2 O 3, KF/ZnO [3, 4]. Transesterification reactions of fats and oils in the presence of CaCO 3 are 135

2 realized at high temperatures (up to 200 ºC) and pressure and excess of alcohol [1]. Reactions using KF/ZnO, KI/Al 2 O 3 and KF/Al 2 O 3 have been carried out at reflux temperature of methanol at atmospheric pressure [3, 4]. Na/NaOH/ -Al 2 O 3 and Li/CaO are quite expensive and it is complicate to prepare these catalysts [2]. Montmorillonites, zeolites and anionic ion-exchange resins have been used as heterogeneous catalysts, too [2, 5, 6]. Zeolites faujasite NaX and titanosilicate structure-10 have been found as the most effective heterogeneous catalysts for transesterification of vegetable oils with methanol at high temperature [2]. Most often soybean oil was used for transesterification in above mentioned experiments. The aim of our research is to find out the possibilities to use local and cheap raw materials as heterogeneous catalysts for synthesis of biodiesel. Materials and methods Unrefined rapeseed oil, produced by Iecavnieks Ltd. and commercially available refined rapeseed oil Risso produced in Belgium, were used in our experiments. The composition of fatty acids of these oils were following (the first number is for oil produced by Iecavnieks Ltd., the second for Risso): palmitic acid 5.2% and 4.1%, stearic acid 2.7% and 0.9%, oleic acid 68.1% and 52.0%, linoleic acid 17.5% and 31.7%, linolenic acid 6.9% and 11.3%. Preparation of KI/Al 2 O 3 and KF/Al 2 O 3 catalysts Al 2 O 3 was used as a support. The supported catalysts were prepared by an impregnation method mixing 24 h Al 2 O 3 with 0.7 mol/l aqueous solution of KI or KF. The resulting solids were dried in air for 24 h at 120 ºC. Prior to the reaction, catalysts were calcined at 500 ºC or 600 ºC for 5 h. We investigated the catalytic activity of 35% KI/Al 2 O 3 and 12.5% KF/Al 2 O 3. Preparation of KF/ZnO catalyst ZnO was utilized as a support. The supported catalyst was prepared by an impregnation method mixing 24 h ZnO with mol/l aqueous solution of KF. The resulting solid was dried in air for 24 h at 120 ºC. Prior to the reaction, catalyst was calcined at 500 ºC or 600 ºC for 5 h. The experiments were done with 15% KF/ZnO. Preparation of clays as catalysts We carried out experiments with clays from Priekule (Latvia) and Serpukhov (Russia). Clays were thermally treated, used crude or activated with 10% H 2 SO 4. Typical clay minerals of Latvian quaternary and Devon system s clays are hydromicas, illites, kaolinites, montmorillonites. The composition of clay from Priekule is following: SiO %, Al 2 O %, Fe 2 O %, CaO 9.19%, MgO 3.46% [7]. The analysis of clay from Serpukhov provided such results: SiO %, Al 2 O %, Fe 2 O %, TiO %, CaO 8.24%, MgO 7.20%, Na 2 O 0,09%, K 2 O 1.23%. The crude clays were dried in thermostat at ºC temperature while the weight was constant. Dried clays were sifted out through the sieve 063. The resulting materials were heated 6 h under air at 200 ºC, in order to prepare thermaly treated clays. Activated clays were prepared by treating of crude clays with 10% sulfuric acid at 100 ºC for 1 h (molar ratio of clay and sulfuric acid 1:5). After this workup clays were filtrated and washed until neutral reaction (methylorange was used as an indicator), then dried in thermostat at ºC temperature while the weight was constant. The dried product was ground and sifted out through the sieve 063. Size of catalyst particles was mm. Preparation of KF/clay catalysts Clay supported catalysts were prepared by an impregnation method mixing clays 24 h with 0.7 mol/l aqueous solution of KF. The resulting solids were dried 24 h under air at 120 ºC. Prior to the reaction, catalysts were calcined at 500 ºC or 600 ºC for 5 h. We used 12.5% KF/clay (both crude and activated (with 10% H 2 SO 4 ) clays) from Priekule, as well as 12.5% Serpukhov. Transesterification reaction Transesterification reaction was realized in two-necked round glass flask equipped with condenser and magnetic stirrer. Flask was charged with rapeseed oil and then catalyst and anhydrous methanol were added. The amounts of methanol and catalyst are presented in Tables 1 5. Each mixture was 136

3 stirred and heated at boiling temperature of methanol. When the reaction was finished the mixture was filtrated to separate catalyst, methanol was evaporated on a rotary evaporator and glycerol was removed. Kinematic viscosity at 40 o C was determined for synthesized rapeseed oil methyl esters () according to the standard LVS EN Purity of produced biodiesel was characterized by content of mono-, di- and triglycerides and glycerol. The samples were analyzed using a Hewlett-Packard 5890 gas chromatograph equipped with column Restek 15 m MXT-5 and flame ionization detector. The composition of crude glycerol was detected using an Agilent 1100 HPLC equipped with Zorbax Carbohydrate column ( mm) and refractive index detector. Flow rate of mobile phase (acetonitrile:water = 70:30) was 1.2 ml/min. Results and discussion We have studied the effect of different heterogenous catalysts KI/Al 2 O 3, KF/Al 2 O 3, KF/ZnO and clays, which were obtained from Priekule (Latvia) and Serpukhov (Russia) deposits onto biodiesel synthesis. The clays as catalysts were prepared at different conditions used crude or activated by treatment with sulfuric acid; some experiments were carried out employing clay as support loaded with KF. The advantage of heterogeneous catalysis is relatively easy preparation of the catalyst, too. We tested as heterogeneous catalysts 35% KI loaded on Al 2 O 3, 15% KF on ZnO and 12.5% KF on alumina. They were calcined at 500 ºC or 600 ºC. Acidity of such catalysts according literature data [3, 4] has to be about The assayed clay catalysts were prepared at following conditions 12.5% KF loaded on clay from Serpukhov, 12.5% KF on both crude and activated (pre-treatment with 10% H 2 SO 4 ) clays from Priekule. All those samples were calcined at 600 ºC. Rapeseed oil was transesterified with methanol at boiling temperature of alcohol. Tables 1 5 represent the yield of conversion, kinematic viscosity of synthesized biodiesels, the molar ratios of oil and methanol, the amount of catalyst. Experiments were carried out with both refined and unrefined rapeseed oil. We used kinematic viscosity at 40 o C to characterize the total amount of esters in biodiesel. Kinematic viscosity of biodiesel containing 96.5% of is 5.0 mm 2 /s; kinematic viscosity of rapeseed oil 38.0 mm 2 /s. One of the most important advantages of heterogeneous catalysis in biodiesel synthesis is fact, that crude glycerol is considerably purer than glycerol obtained by homogeneous catalysis (the total amount of glycerol is 40 50%). The composition of crude glycerol was analyzed for sample which was obtained using reaction conditions described in Table 1, experiment 3 the total amount of glycerol in this case was 94%. Table 1. Transesterification of refined rapeseed oil using KI/Al 2 O 3 and KF/Al 2 O 3, duration of reaction 8 h, Yield, Kinematic viscosity activation methanol: oil % (m/m) at 40 ºC, mm 2 /s temperature, ºC 1 KI/Al 2 O : KI/Al 2 O : KF/Al 2 O : Our experiments showed that the necessary amount of KI/Al 2 O 3 and KF/Al 2 O 3 (if the molar ratio oil:methanol is 15:1) and KF/ZnO (if the molar ratio oil:methanol is 12:1) for transesterification reaction is 5 weight. Results obtained with these catalysts are presented in Tables 1, 2 and 5. Table 2 shows the impact of storage time on the efficiency of catalysts. Our experiments demonstrate that it is possible to realize transesterification of unrefined oil at the conditions of heterogeneous catalysis kinematic viscosity (5.07 mm 2 /s) of obtained in 4 th experiment (Table 5) with KF/Al 2 O 3 catalyst is close to requirements of biodiesel standard LVS EN S.K. Karmee et al. [5] showed possibilities to apply montmorillonites as transesterification reaction catalysts. Reactions were carried out in autoclave at 120 ºC temperature. 137

4 Table 2. Transesterification of refined rapeseed oil using KF/ZnO, duration of reaction 9 h, Kinematic Yield, Activation Storage methanol:oil viscosity at 40 ºC, % (m/m) mm 2 temperature, ºC time, days /s : : : : : : : : Table 3. Transesterification of refined rapeseed oil in presence of clays, Duration of Kinematic viscosity reaction, h methanol: oil at 40 ºC, mm 2 /s 1 Clay from Serpukhov : Clay from Priekule : Actived clay from Priekule : Table 4. Transesterification of refined rapeseed oil using clays as a support (catalyst activation temperature 600 ºC) Serpukhov Serpukhov Priekule KF/activated clay from Priekule NR no reaction, Duration of reaction, h methanol: oil Yield, % (m/m) Kinematic viscosity at 40 ºC, mm 2 /s : : :1 NR :1 NR Table 5. Transesterification of unrefined rapeseed oil using KI/Al 2 O 3, KF/Al 2 O 3 and KF/ZnO, Duration Kinematic activation of methanol: Yield, % viscosity at temperature, reaction, h oil (m/m) 40 ºC, mm 2 /s ºC 1 KF/ZnO : KF/ZnO : KI/Al 2 O :1 NR KF/Al 2 O : We tried to realize transesterfication of rapeseed oil at atmospheric pressure using clays from Latvia and Serpukhov as catalysts (nonactivated or activated with acid). Thermal treatment encourages alumosilicate dehydration and dehydroxylation. Treatment with diluted inorganic acid promotes cation exchange with H + and concentration of acidic centres increases on catalyst surface. If clay is treated with concentrated acid, the aluminum ions drain out due to increase of the size of pores. According to this the degree of activation regulates acid properties of catalyst and optimizes the structure of active surface [8]. 138

5 The kinematic viscosity (at 40 ºC) of synthesized biodiesel was mm 2 /s, if clay scooped in Serpukhov was employed as a support for KF. Amount of catalyst used in experiment was 5% or 6 weight. Unfortunatelly, acceptable results (ester content, kinematic viscosity) have not been reached, though in some cases (Table 4, experiment 2) these parameters were not far from biodiesel standard requirements; disappointing results were obtained with Latvian clays. Optimization of transesterification conditions using KF on support of clay is needed. Obviously, it should be possible to use also some Latvian clays as heterogenous catalysts for synthesis of biodiesel. Conclusions We found out the optimal conditions for synthesis of methyl esters of rapeseed oil employing KF/Al 2 O 3, KI/Al 2 O 3 and KF/ZnO as heterogeneous catalysts. It is possible to realize transesterification of rapeseed oil (both unrefined and refined) at atmosphere pressure at the boiling temperature of methanol during 8 9 hours. Methanol should be used in large excess (12 15 times) comparing with oil, desirable amount of catalyst is 5 weight. Storage of these catalysts (even several days) lead to partial loss of activity. We found out that it is possible to use clays from Latvian and Russian deposits as heterogeneous catalysts for transesterification of oil our experiments demonstrated that Serpukhov clay impregnated with KF could be promising catalyst. References 1. Suppes G.J., Buchwinkel K., Lucas S., Botts J.B., Mason M.H., Heppert J.A. Calcium carbonate catalyzed alcoholysis of fats and oils. J. Am. Oil. Chem. Soc., 2001, 78(2), pp Suppes G.J., Dasari M.A., Doskocil E.J., Mankidy P.J., Golf M.J. Transesterification of soybean oil with zeolite and metal catalysts. Appl. Catal., 2004, 257, pp Xie W., Li H. Alumina-supported potassium iodide as a heterogenous catalyst for biodiesel production from soybean oil. J. Mol. Catal., 2006, 255, pp Xie W., Huang X. Synthesis of biodiesel from soybean oil using heterogenous KF/ZnO catalyst. Catal. Lett., 2006, 107(1-2), pp Karmee S.K., Chadha A. Preparation of biodiesel from crude oil of Pongamia pinnata. Biores. Technol., 2005, 96, pp Shibasaki-Kitakawa N., Honda H., Kuribayashi H., Toda T., Fukumura T., Yonemoto T. Biodiesel production using anionic ion-exchange resin as heterogenous catalyst. Biores. Technol., 2007, 98, pp Sedmalis U. Latvijas izplat t k s miner l s izejvielas un to izmantošanas iesp jas. Latv. m. Žurn., 1997, 2, lpp. 8. ebedevs A., Stonkus V., Leite L., Gudriniece E., Ruplis A., Fleišers M., Lukevics E. Latvijas m lu aktivit te 1,4-but ndiola dehidrat cijas reakcij. Latv. m. Žurn., 1998, 2, lpp. 139