Journal of Scientific & Industrial Research Vol. 73, February 2014, pp. 129-133
Preparation, Characterization and Application of Zeolite-based Catalyst for Production of Biodiesel from Waste Cooking Oil Maryam Hassani1, Ghasem D Najafpour1*, Maedeh Mohammadi1 and Mahmood Rabiee2 1
Faculty of Chemical Engineering, 2Faculty of Mechanical Engineering Noushirvani University of Technology, 47148, Iran Received 08 October 2012; revised 24 September 2013; accepted 28 November 2013
Zeolite-based catalyst was prepared from a fine powder and kaolinite by pelletization method and used to synthesize fatty acid methyl esters (FAME) known as biodiesel from waste cooking oil (WCO) containing high amounts of free fatty acids (FFA). The prepared catalyst was characterized by Thermogravimetric analysis (TG/DTA), X-ray Diffraction (XRD), Fourier Transform Infrared (FTIR) spectroscopy and Brunauer-Emmett-Teller (BET) surface area analysis. The zeolite-based catalyst was employed to simultaneously catalyze the esterification of fatty acids and transesterification of triglycerides present in the waste oil feedstock to biodiesel. The condition for biodiesel synthesis was optimized in terms of reaction temperature (50-85 oC), methanol/ WCO molar ratio (2.6-6.0) and reaction time (2-10 h). Maximum triglyceride conversion of 46 % was achieved at the near optimum conditions. These conditions were defined at reaction temperature of 70 oC, methanol/ WCO molar ratio of 5.1 and reaction time of 6 h. Keywords: Waste cooking oil, Biodiesel, Zeolite-based catalyst, Transesterification
Introduction Finding an alternative fuel resource for diesel is an imperious task for humans1,2. Biodiesel is a sustainable fuel alternative whose production has recently become a priority for many countries and would play an important role in diesel industry. Biodiesel offers advantages such as being environmentally friendly fuel (less CO2 emissions, almost zero CO and sulfur emissions), nontoxic and biodegradability with low viscosity. Biodiesel possesses good lubricity and is renewable in comparison to traditional petroleum diesel3,4. Furthermore, this fuel can be used in blend form and very small proportion of it is relatively effective in reducing particulate emissions from engine5. Biodiesel has been derived from various renewable lipid sources such as vegetable oils or animal fats6. A literature survey reveals that this lipid feedstock can be derived from canola7, palm oil8, jatropha9, soybean10, sunflower11, rapeseed12 and coconut13. Production of biodiesel from highly pure oils used for cooking purposes and methanol or ethanol as alcohol in presence of homogeneous catalyst such as sodium or potassium hydroxides, carbonates or alkoxides contributes to the increase of the total manufacturing cost of biodiesel14. Biodiesel produced from this method has a higher cost compared to fossil fuels; approximately 1.5 times _____________ * Author for correspondence Email:
[email protected]
higher15. One of the effective methods for reducing the cost of biodiesel production is the use of low quality oils which are not suitable for human consumption and readily available such as waste cooking oil (WCO), animal fat and tall oil4 as well as soap stock (by-product of vegetable oil refinery)15. These oils have low value and there is no concern for their availability; they can be regarded as attractive feedstock for production of biodiesel16. Replacing homogeneous catalysts by heterogeneous catalysts is another alternative to reduce biodiesel production costs. Homogeneous catalytic systems create serious downstream problems such as difficulties in removal and separation of catalysts after the reaction and production of huge amount of wastewater. Zeolites have been widely used as industrial heterogeneous catalysts because they are inexpensive and environmentally benign. They offer generous surface area and high porosity17. However, application of zeolites as catalysts for transesterification reaction is limited by the narrow pore size of zeolites and diffusion limitations for adsorption of triglyceride on the active sites of zeolites. The molecular size of FFA is comparably lower than that of triglyceride, thus, zeolites are more effective for esterification than transesterification reactions17. Hence, development of mesoporous zeolites would provide great opportunity to avoid the diffusion limitation and improvement of transesterification of vegetable oils.
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In this study, a mesoporous zeolite-based catalyst was developed to carry out the transesterification reaction. Waste cooking oil (WCO) was used as a worthless and abundant feedstock to produce biodiesel. The effect of several operation parameters on conversion of triglyceride was considered and the near optimum conditions to yield maximum FAME were defined. Materials and Methods Pretreatment of the waste cooking oil
WCO was collected from the university canteen. Solid particles were removed from WCO by filtration. The sample WCO containing water and other impurities was centrifuged at 15000 rpm and dried for 72 h. Then, the WCO was mixed with n-hexane (1:3 oil/ hexane, volume ratio) to remove the remaining impurities. After 72 h, the oil was separated from n-hexane in a Rotavapour. The composition of fatty acids in WCO was determined by GC-MS as summarized in Table 1. Catalyst preparation and characterization
Fine zeolite powder was supplied by Hexagon Synergy (M) Sdn Bhd (Kuala Lumpur, Malaysia). The chemical composition of the powder was determined by X-ray Fluorescence (XRF) analysis using a Philips PW 1480 spectrometer (Table 2). In preparation of zeolite-based catalyst poly vinyl alcohol (PVA) and kaolinite were used as binder. In each batch, 200 g of fine powder was used. The slurry was obtained by mixing the fine zeolite powder, PVA and kaolinite with proportion of 75; 15 and 10 wt%. The slurry was mixed by a mechanical stirrer for 3 to 5 min, and then dried in an oven at 60 C over night. The dried mixture was ground using a pestle and mortar to a fine powder of 0.03 mm. The powder was Table 1 Fatty acid, FFA and water content of the WCO Fatty acid components
(%)
C 14: 0 C 16: 0 C 16: 1 C 18: 0 C 18: 1 C 18: 2 C 18: 3 FFA content Water content
0 28.91 1.45 1.26 26.51 27.44 4.60 9.85 0.03
rehydrated by a few droplets of water and pressed one-dimensionally under a force higher than 20 tons to produce thin pellets (80 mm diameter and 2 mm thickness). The pellets were then calcined in a muffle furnace in the presence of air at 700 C for 3 h. The calcined pellets were kept in desiccator for further characterization and transesterification experiments. Thermal stability of the synthesized zeolite-based catalyst was determined by Thermogravimetric analysis (TGA) carried out using a TG/DTA analyzer (PL-STA-1640). Crystal structure of the developed catalyst was investigated by wide angle X-ray diffraction (XRD) pattern taken using a Philips 1887 diffractometer equipped with Cu Kα radiation at 40 kV and 30 mA in the scanning angle 2θ of 20–80°. The detection of surface functional groups of the catalyst was carried out using Fourier Transform Infrared Spectroscopy (FTIR) analysis (Shimadzu FTIR-8300). N2 adsorption-desorption isotherms were obtained at 77 K using Micromeritics Gemini series surface area analyzer. The surface area was measured from the adsorption-desorption isotherm using Brunauer-EmmettTeller (BET) equation. Pore size distribution was obtained from Barrett-Joyner-Halenda (BJH) method. Synthesis of biodiesel
The simultaneous transesterification and esterification of the WCO containing 9.85 wt% FFA was carried out in the presence of prepared zeolite-based catalyst in a round-bottom flask equipped with a reflux condenser, temperature controller and mechanical stirrer. The flask was immersed into a water bath while the temperature was controlled in the range of 50–85 oC. Initially the reactor was loaded with 25 g of WCO and variable amounts of methanol. Then, fixed amount of 0.65 g catalyst was added to the reaction vessel while stirring speed was fixed at 600 rpm. Upon completion of the transesterification reaction, the excess methanol was vaporized. The catalyst was removed from the reaction vessel by filtration and the produced mixture was centrifuged. Upon the centrifugation, two phases were formed, i.e. FAME in the upper layer and glycerol in the lower layer. The glycerol phase was separated from the FAME layer in a separating funnel. Finally, for purification of the achieved FAME phase, the water washing method was used to separate the glycerol from the FAME–glycerol mixture and to obtain pure FAME.
Table 2 XRF analysis of the fine powder used as catalyst Compound
SiO2
Al2O3
Fe2O3
CaO
Na2O
TiO2
MgO
K2O
P2O5
SO3
MnO
L.O.I
wt %
85.3
6.92
0.90
0.75
0.74
0.48
0.47
0.21
0.029
0.003
0.001
3.78
HASSANI et al: APPLICATION OF ZEOLITE-BASED CATALYST FOR COOKING OIL
The products were analyzed using GC-MS spectrometer (Agilent 5973) equipped with a capillary column (HP-5); Wiley libraries were used as reference databases. Results and Discussion Catalyst characterization
Physio-chemical properties of the prepared zeolite-based catalyst were studied using several analyses before being implemented in transesterification reaction. Fig. 1 shows the TG/DTA curves obtained for the catalyst under air. A minor weight loss was observed in TG curve in the temperature range of 25-250 oC which was due to the dehydration of the sample. The main weight loss in TG curve started at around 250 °C and continued to 500 °C which was attributed to the oxidation of organic phase and dehydroxylation of the zeolite. The two exothermic peaks observed on the DTA curve at 279 and 424 °C represent the two-stage oxidation of organic matter (H and C) and the formation of H2O and CO2. The thermal degradation profiles of the catalyst further confirm that the applied calcinations temperature was high enough to remove the organic impurities. The XRD pattern of the developed catalyst is shown in Fig. 2. The XRD spectrum exhibited a well-
resolved diffraction pattern with prominent peaks at 2θ= 21.85, 26.80 and 35.97. All the three peaks represent crystals with the same chemical formula but different crystal structures. The first peak that is taller than others is assigned to the crystal structure cristobalite and the second peak with moderate intensity corresponds to quartz and the third peak is associated with tridymite structure with a probability close to certainty. Also, three broad, but low peaks appeared at 2θ= 50.29, 60.10 and 68.48 that have same chemical formula. These peaks show patterns for crystal phase quartz, tridymite and cristobalite, respectively. Fig. 3 depicts the FTIR spectrum of the zeolitebased catalyst. The sharp peaks with strong absorption at 478 and 1102 cm−1 are related to deformation bands SiOSi18 and the perpendicular SiO vibration bands in kaolinite19, respectively. The peaks found at 791 and 3416 cm−1 could be assigned to bands SiOSi19,20 and OH21. Vibration bands of the internal (inner) carbonate groups20 and AlOH vibration band22 are represented by the peaks located at 1400 and 1625 cm−1, respectively. The peak appeared at 625 cm−1 is associated with coupled outof-plane vibration band of Al-O and Si-O18 and the peak at 2358 cm−1 is assigned to CO223. The SiO groups exist in raw materials, AlOH groups were created due to the mixing of H2O with zeolite and carbonate groups and CO2 were formed as a result of calcination. The textural properties of the prepared catalyst were studied by N2 adsorption adsorption–desorption isotherms (data not shown). According to the IUPAC classification, the isotherm exhibited a type IV curve
Fig.1TG/DTA curves of zeolite-based catalyst
Fig. 2XRD pattern for the zeolite-based catalyst
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Fig. 3FTIR spectrum of the zeolite-based catalyst
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with a hysteresis loop which is an indication of mesoporous material. The adsorption- desorption was not a reversible process which led to the appearance of a hysteresis loop between the curves of adsorption and desorption. While the capillary condensation in the mesopores occurs at a higher relative pressure than capillary evaporation, the hysteresis loop is observed24,25. The pore size distribution (PSD) which represents the structural homogeneity of the developed catalysts confirmed the mesoporous structure (2< dP< 50 nm) of the catalyst. Based on the analysis of the results, a BET surface area of 61.9 m2/g with mean pore diameter of 23.414 nm and total pore volume of 0.32 cm3/g was achieved for the prepared catalyst. Effect of reaction parameters Effect of reaction temperature
Reaction temperature is one of the most influencing parameters on the rate of reaction. The effect of temperature on the conversion of triglyceride in biodiesel production using zeolite-based catalyst was examined at different temperatures from 50 to 85 oC with initial WCO of 25 g, catalyst loading of 2.6 w/w%, methanol/WCO molar ratio of 5.1 and reaction time of 6 h. The results of present investigation are projected in Fig. 4 (a). Increase in the reaction temperature to 70 oC improved the triglyceride conversion after which, a decreasing trend was observed in the conversion. Maximum triglyceride conversion of 46.2 % was achieved at 70 oC. Although high reaction temperatures enhance the reaction rate to produce more FAME, however, increase of the temperature to above 70 oC which exceeds the boiling point of methanol (65 oC) adversely affects the conversion. In this case, evaporation of methanol to the condenser and condensation into the reaction vessel causes an unstable reaction condition.
Effect of methanol/WCO molar ratio
The yield of FAME production is affected by the methanol/oil molar ratio. Theoretically, for transesterification of vegetable oil, three moles of methanol is required per mole of triglyceride. Since the transesterification is a reversible reaction, the mole of methanol in the reaction mixture must be excess enough to shift the equilibrium towards the formation of FAME. In the present work, the methanol/WCO molar ratio was optimized by varying this molar ratio in the range of 2.6-6.0 at a fixed WCO amount of 25 g and the reaction temperature of 70 oC. The results are shown in Fig. 4 (b). The conversion of triglyceride noticeably enhanced from 26 to 48% with increase of the methanol/WCO ratio from 2.6 to 5.1, afterward, the conversion almost leveled off. With increase of the methanol/WCO ratio from 5.1 to 6, only a slight improvement in the conversion of triglyceride was observed. The excess amount of methanol is favorable for the conversion of triglyceride to monoglyceride. However, monoglyceride significantly affects the solubility of glycerol in FAME, which are naturally immiscible, and lead to the glycerolysis of FAME and reduction of triglyceride conversion26. Based on the achieved results and the presented discussion, the methanol/ WCO molar ratio of 5.1 was selected for further experiments. Effect of reaction time
The influence of reaction time on the conversion of triglyceride was studied in the range of 2 to 10 h with an increment of 2 h and the results are shown in Fig. 4 (c). As it was noted from the results, increase of the reaction time to 6 h considerably improved the conversion of triglyceride to around 46 %. Further increase in the reaction time did not show notable increase in the conversion. Such observation may be due to the catalyst deactivation resulted from accumulation of reactants or products in the porosity
Fig. 4Influence of (a) reaction temperature, (b) methanol/ WCO molar ratio and (c) reaction time on the conversion of triglycerid
HASSANI et al: APPLICATION OF ZEOLITE-BASED CATALYST FOR COOKING OIL
of catalyst in long reaction times. This leads to the loss of active sites on the catalyst surface and reduces the activity of catalyst. Therefore, the reaction time of 6 h was defined as the suitable reaction time for the synthesis of FAME. Conclusion Zeolite-based catalyst was prepared from zeolite powder, kaolinite and PVA and characterized by TG/DTA, XRD, FTIR and BET techniques. The ability of the developed catalyst to catalyze the transesterification reaction was assessed in some sets of batch experiments. WCO containing high amount of FFA which was much cheaper than feed grade oil was used as the substrate for biodiesel synthesis. The transesterification reaction condition was optimized in terms of reaction temperature, methanol/WCO molar ratio and reaction time to improve the conversion of triglyceride to FAME. Maximum triglyceride conversion of 46% was attained at the near optimum condition. Acknowledgement The authors gratefully acknowledge the research committee of postgraduate studies, Noushirvani University for the support and facilities provided in Biotechnology research center. References
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