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ZSM5 catalysts
Renewable Energy 90 (2016) 301e306
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Biodiesel production from refined sunflower vegetable oil over KOH/ ZSM5 catalysts Tony Saba a, Jane Estephane a, *, Bilal El Khoury b, Maroulla El Khoury a, Mahmoud Khazma a, Henri El Zakhem a, Samer Aouad b a b
Department of Chemical Engineering, University of Balamand, P.O. Box 100, Tripoli, Lebanon Department of Chemistry, University of Balamand, P.O. Box 100, Tripoli, Lebanon
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 August 2015 Received in revised form 30 November 2015 Accepted 1 January 2016 Available online 13 January 2016
ZSM5 zeolite was impregnated with different KOH loadings (15 wt.%, 25 wt.% and 35 wt.%) to prepare a series of KOH/ZSM5 catalysts. The catalysts were calcined at 500 C for 3 h and then characterized by N2 adsorptionedesorption and X-ray diffraction (XRD) techniques. The catalysts were tested in the transesterification reaction in a batch reactor at 60 C and under atmospheric pressure. It was found that KOH/ ZSM5 with 35 wt.% loading showed the best catalytic performance. The best reaction conditions in the presence of KOH/ZSM5 (35 wt.%) were determined while modifying the catalyst to oil ratio and the reaction time. The highest methyl ester yield (>95%) was obtained for a reaction time of 24 h, a catalyst to oil ratio of 18 wt.%, and a methanol to oil molar ratio of 12:1. The properties of produced biodiesel complied with the ASTM specifications. The catalytic stability test showed that 35KOH/ZSM5 was stable for 3 consecutive runs. Characterization of the spent catalyst indicated that a slight deactivation might be due to the leaching of potassium oxides active sites. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Biodiesel Transesterification KOH ZSM5 Reusability
1. Introduction The worldwide economic growth and sustainable development are leading to an increase in energy demand. Fossil fuels (oil, gas and coal) are providing almost 80% of the world energy requirements [1]. However, fossil fuel reserves are shrinking rapidly due to increased growth in world population and industrialization [2]. Moreover, the global warming due to greenhouse gases emissions prompted the efforts to explore new sources of energy. Therefore, biodiesel production is receiving considerable attention nowadays because biodiesel is a promising alternative fuel to conventional diesel [3], is renewable, biodegradable, and produces less CO, hydrocarbons and particulates emissions than petroleumbased diesel [4,5]. Biodiesel and conventional diesel properties are very similar; however biodiesel has higher cetane number than conventional diesel, virtually no sulfur and no aromatics and high flash point [5e7]. Biodiesel, also known as Fatty Acid Methyl Esters (FAME), can be
* Corresponding author. Department of Chemical Engineering, Faculty of Engineering, University of Balamand, P.O.Box 100, Tripoli, Lebanon. E-mail address: [email protected] (J. Estephane). http://dx.doi.org/10.1016/j.renene.2016.01.009 0960-1481/© 2016 Elsevier Ltd. All rights reserved.
obtained by a transesterification reaction of triglycerides (in oil or fats) with methanol by using a catalyst [8]. This reaction gives glycerol as a by-product. In general, homogeneous base catalysts such as NaOH or KOH are the most suitable catalysts for transesterification reactions [6] since they are able to catalyze these reactions faster than acid catalysts [9,10]. However, homogeneous catalysts have some drawbacks, e.g. they produce large amount of waste water [11], low-quality glycerol and they cannot be reused [12]. Moreover, side reactions such as saponification can occur, imposing difficulties in the separation and purification of biodiesel [13]. On the other hand, heterogeneous catalysts are less corrosive, more environmentally friendly, safer, cheaper and they can be easily recovered, regenerated and reused [14,15]. Therefore, the use of heterogeneous catalysts for the production of biodiesel is gaining great interest. In general, heterogeneous basic catalysts are catalytically more active than acid catalysts, thus they require lower temperatures and shorter reaction times [16,17]. For instance, the alkali metals and their hydroxides supported on SBA-15 are highly active catalysts in the transesterification reaction [18]. In addition, CaO base catalysts give high yields of biodiesel, but they are highly sensitive to air [19]. It has been recently shown that lithium-based catalysts, in particular LiAlO2 and Li4SO4, are highly active, air-insensitive and can be reused for many cycles [20e22].
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Moreover, KOH supported on Alumina, NaY zeolite [6], mordenite [23], bentonite [24], MgO [25] showed excellent catalytic activity in the production of biodiesel. However, it has been clearly documented in literature that the heterogeneous catalysts for biodiesel production deactivate or loose slightly their activity when used for several consecutive runs. This can be due to the leaching of the active phase in solution, or to the adsorption of hydrocarbons on the active sites [26e28]. To assess whether the KOH/ZSM5 catalysts perform well in the transesterification of sunflower oil with methanol, we tested a series of KOH/ZSM5 catalysts in the reaction and we evaluated the effect of active phase loading on the yield of biodiesel production. This paper discusses the influence of active phase content in KOH/ZSM5 catalysts, the catalyst to oil ratio and the reaction time on the biodiesel production yield. The catalyst reusability is also investigated for several consecutive runs as it represents a key advantage for industrial biodiesel production. 2. Methods 2.1. Materials Refined sunflower vegetable oil (Plein Soleil) was purchased from a local market and was analyzed based on density at 25 C (ASTM D4052), kinematic viscosity (ASTM D445), free fatty acid content (AOCS Ca-5a-40), and water content (AOCS Aa 3e38). ZSM5 zeolite (CBV2314) was obtained from Zeolyst Company. Methanol (purity 99.8%), n-hexane, potassium hydroxide (85%, pellets), and methyl heptadecanoate were supplied by SigmaeAldrich. 2.2. Catalyst preparation
EVO apparatus. The spent catalyst (~10 mg) was placed in an alumina crucible and introduced in the apparatus. The sample was heated from room temperature up to 600 C (5 C/min) under an air flow of 50 mL/min. 2.4. Transesterification reaction All experiments were carried out in a round bottom flask batch reactor equipped with a magnetic stirrer, controlled temperature (60 C), and a water-cooled condenser. The desired amounts of catalyst and methanol were added to the reactor and mixed together; the mixture was heated up to 60 C. Subsequently, the sunflower oil preheated at 60 C was added to the mixture under a stirring rate of 500 rpm. The transesterification reaction was carried out for all the catalysts with a methanol-to-oil molar ratio (MOMR) of 12:1, a temperature of 60 C and a stirring rate of 500 rpm. The best reaction conditions were determined while varying: the amount of KOH impregnation, the catalyst-to-oil mass ratio (CTOR, wt.%) and the reaction time. At the end of the reaction, the catalyst was recovered from the mixture by means of vacuum filtration. A rotary evaporator was used to remove the excess of methanol from the filtrate. The residue was then centrifuged for 20 min at 3000 rpm to separate the biodiesel (FAME) from glycerol. The possibility to reuse the catalyst was also studied in order to check its ability to provide the same catalytic activity after many cycles. A Gas Chromatography (Agilent 7890A series) equipped with a flame ionization detector (FID) and an HP-5 (30 m 0.32 mm) column was used to determine the FAME content. According to the European regulated procedure EN14103, 250 mg of the ester layer were dissolved in 5 mL of the internal standard methyl heptadecanoate C17 solution (10 g/L of C17 in n-hexane). The FAME content was calculated using Eq. (1):
ZSM5 zeolite was used as support to prepare a series of KOH/ ZSM5 catalysts with varying KOH loadings according to the wet impregnation method [6,25,35]. Appropriate amounts of KOH (1.76 g, 3.9 g and 6.33 g for 15 wt.%, 25 wt.% and 35 wt.% respectively) were dissolved in 50 mL of distilled water. Each aqueous solution of KOH was slowly added to 10 g of the ZSM5 support. The resulting slurry was then stirred (200 rpm) at 80 C for 2 h. After impregnation, the catalysts were dried at 80 C overnight and then calcined at 500 C for 3 h at a heating rate of 0.5 C/min. KOH/ZSM5 catalysts with different active phase loadings (15, 25 and 35 wt. %) were labeled 15KOH/ZSM5, 25KOH/ZSM5 and 35KOH/ZSM5, respectively.
where, SA is the total peak area of methyl esters, AIS is the area of the internal standard (C17), CIS is the internal standard concentration (10 mg/mL), VIS is the volume of the internal standard solution (in mL), and W is the sample weight (in mg) [29]. The physical properties, of the biodiesel produced in this work were determined according to the ASTM standard (Density at 15 C: ASTM D.4052; kinematic viscosity: ASTM D.445; flash point: ASTM D.93).
2.3. Catalyst characterization
3. Results and discussion
A Thermo-Electron QSurf M1 apparatus was used to determine the specific surface areas of the calcined catalysts, using the BET method. Prior to analysis, the samples were treated at 120 C under helium flow for 30 min. X-ray diffraction (XRD) experiments were performed on a Panalytical powder X-ray diffractometer. The diffraction patterns were recorded over an angular range of 5 < 2Q < 80 with a stepsize of (2Q) ¼ 0.02 . By comparing the XRD patterns to the ICDD files, the crystalline phases were identified. X-ray Fluorescence (XRF) spectrometry was performed using a Thermo Scientific ARL 9900 Series. The potassium (K) content in the fresh and spent catalysts was measured by XRF. FTIR Spectra of the samples were recorded on a Perkin Elmer spectrometer in the range of 500e4000 cm1 with a resolution of 2 cm1. Before analysis, the samples were diluted with KBr powder and then pelletized. The simultaneous differential scanning calorimetry/thermogravimetry (DSC/TG) analyses were carried out on a Setaram Labsys
3.1. Characterization of vegetable oil
P FAME ð%Þ ¼
A CIS VIS 100 AIS W
(1)
Table 1 shows the properties of the sunflower oil used in this study and its free fatty acid content. This oil showed the required properties for a basic transesterification reaction. 3.2. Catalyst characterization The BET surface area of the support ZSM5 and the various
Table 1 Properties of the sunflower oil. Properties
Sunflower oil
Density at 25 C (Kg/m3) Kinematic Viscosity at 40 C (cSt) Water Content (wt.%) FFA Content (wt.%)
920.5 53.87 0.09 0.22
T. Saba et al. / Renewable Energy 90 (2016) 301e306
3.3. Catalytic activity of KOH/ZSM5 catalysts 3.3.1. Effect of KOH loading and catalyst to oil ratio on FAME yield The effect of KOH loading (wt.%) on FAME yield was studied. The transesterification reactions were carried out with the ZSM5 support, 15KOH/ZSM5, 25KOH/ZSM5 and 35KOH/ZSM5 catalysts. The experimental conditions were the following: methanol-to-oil molar ratio of 12:1, reaction temperature of 60 C, stirring rate of 500 rpm and a reaction time of 2 h. Moreover, in order to determine the effect of catalyst amount on FAME yield, the CTOR was varied between 3 and 9 wt.%. Fig. 3 clearly shows that the amount of KOH
Fig. 1. XRD patterns of the different freshly calcined catalysts.
4000 3500 3000 2500 2000 1500 1000 -1 Wavenumber (cm )
544
1224 1084
1640
3651
ZSM5
792
1003
758
3425
35KOH/ZSM5
Transmittance (a.u.)
impregnated catalysts were determined. The support presented the highest surface area with a value of 290 m2 g1. Upon impregnation with KOH, the specific surface areas decreased drastically to 6 m2 g1 for the 15KOH/ZSM5 and even lower values (non detectable by the apparatus) for the 25KOH/ZSM5 and 35KOH/ ZSM5 catalysts. This decrease is probably due to the filling of the pores and the deposition of potassium species on the pores openings due to the high amount of active phase used. Fig. 1 shows the XRD patterns recorded for the various catalysts. The presence of intense diffraction peaks in the XRD profile of ZSM5 zeolite (ICDD 00-057-0145) shows the high crystallinity of the support. The diffraction peaks of the 15KOH/ZSM5 sample are less intense than those of the support indicating that the zeolite crystallinity decreased upon KOH impregnation [30]. Moreover, as the KOH loading on ZSM5 was increased up to 25 wt.% and 35 wt.%, a significant loss of crystallinity has occurred. These results are in accordance with previous studies [23,31]. Moreover, no KOH or K2O diffraction peaks are observed on the XRD patterns of the prepared catalysts. This might be due to the presence of highly dispersed and/or amorphous K2O particles. The FTIR spectrum of the 35KOH/ZSM5 catalyst calcined at 500 C is shown in Fig. 2. The bands at 3425 cm1 and 1640 cm1 are assigned to the OeH stretching and bending of adsorbed water, respectively [18,32e34]. The band at 3650 cm1 in the FTIR spectrum of the ZSM5 is related to the hydroxyl groups of Si(OH)Al. The band at 1084 cm1 is assigned to the internal asymmetric stretching vibration of SieOeT (T ¼ Si or Al), while the band at 1224 cm1 is assigned to the external asymmetric stretching. The band at 792 cm1 is related to the symmetric stretching vibrations of external linkage, the band at 544 cm1 is characteristic of the double ring external linkage vibrations, while the band at 440 cm1 is assigned to O-T bending vibrations [34].
303
500
Fig. 2. FTIR spectra of the freshly calcined ZSM5 and 35KOH/ZSM5 catalysts.
loaded on ZSM5 affected the conversion of sunflower oil into Fatty Acid Methyl Esters. The ZSM5 support used alone is inactive in the transesterification reaction. However, by increasing the loading amount of KOH from 15 wt.% to 35 wt.%, a significant increase in FAME yield was observed. For instance, by using a CTOR of 3wt.%, the FAME yield increased from 12.4% to 46.4% by varying the loading amount of KOH from 15 to 35 wt.%. Moreover, by using a CTOR of 6wt.% and 9wt.%, the FAME yield increased from 22.5% to 73.7%, and from 41.0% to 76.8% respectively, by varying the KOH loading from 15 to 35 wt.%. This increase in FAME yield is attributed to the higher K2O active phase content in the catalyst [6,24]. It is to note that due to the high active phase loading and the small surface areas of the solids, the catalytic reaction is mainly taking place at the surface of the catalyst while a smaller conversion would probably occur inside the pores. It can also be inferred from Fig. 3 that the increase in the CTOR led to an increase in the FAME yield. The highest FAME yield (76.8%) was obtained in the presence of the 35KOH/ZSM5 catalyst and a CTOR of 9 wt.%. Therefore, finding the best reaction conditions in the presence of 35KOH/ ZSM5 catalyst was done by modifying the catalyst to oil ratio and the reaction time. 3.3.2. Effect of catalyst to oil ratio (wt.%) on FAME yield In order to determine the optimum conditions for the 35KOH/ ZSM5 catalyst on the transesterification of sunflower oil, different
Fig. 3. Effect of catalyst to oil ratio on FAME yield over the different catalysts (CH3OH/ oil ¼ 12:1; T ¼ 60 C; P ¼ 1 atm; t ¼ 2 h/Standard deviation values near symbols).
T. Saba et al. / Renewable Energy 90 (2016) 301e306
3.3.3. Effect of reaction time on FAME yield Reaction time plays a significant role in the transesterification of triglycerides [36]. Fig. 4(b) clearly shows that the FAME yield increased from 83.2% to 95.1% as the reaction time was increased from 2 to 24 h. In particular, the results showed that within the time range 2e6 h, the biodiesel yield increased up to 93.6%. With a further increase in the reaction time, there wasn't any significant change in the biodiesel yield as the equilibrium conversion had been attained. It was clearly documented in literature that the transesterification reaction is slow at the beginning due to a poor dispersion and mixing of alcohol into the oil resulting in lower FAME yield. Afterward, the reaction proceeds rapidly, the yield reaches a maximum and remains constant [37,38]. The optimum conditions for the transesterification of sunflower oil in the presence of KOH/ZSM5 (35 wt.%) were a catalyst to oil ratio of 18 wt.%, a methanol to oil ratio of 12:1 and a reaction time of 6 h. Therefore, these conditions were adopted in the study of the reusability of the catalyst. 3.4. Reusability of the catalyst Reusability is one of the main important features for the
st
1
75.9
83.1 76.2
90.9 88.6
91.8 93.3 2
nd
3
rd
th
4
After regeneration
Number of runs Fig. 5. FAME yield versus the number of runs over the 35KOH/ZSM5 catalyst in two different experiments; W: with washing and WW: without washing (Catalyst/ oil ¼ 18 wt.%; CH3OH/oil ¼ 12:1; T ¼ 60 C; P ¼ 1 atm; t ¼ 6h).
commercialization of a heterogeneous catalyst. In this work, the reuse of 35KOH/ZSM5 catalyst was studied by recovering and reusing this catalyst in four consecutive runs, where the first run designates the reaction with the fresh 35KOH/ZSM5 catalyst. Two different experiments were adopted. In the first experiment WW, the catalyst was only filtered and then fed into another run. However, in the second experiment W, the catalyst was filtered, recovered, washed several times with methanol, dried at 110 C for 2 h and reused for the next run. Moreover, after the 4th run in the experiment W, the spent catalyst was regenerated at 500 C (heating rate of 0.5 C/min) for 3 h before reuse. Fig. 5 shows the FAME yield as a function of the number of runs in the two experiments; W (with washing) and WW (without washing). For the first three runs in both experiments, the 35KOH/ ZSM5 catalyst exhibited good stability and kept its catalytic performance, where the FAME yield decreased only by 2.9% in experiment WW, and by 5.8% in experiment W. By reaching the 4th run, the 35KOH/ZSM5 catalyst showed an increase in the deactivation. For instance, the decrease in biodiesel yield after the 4th run in experiment WW was about 11.2%. However, in experiment W it was about 19%. Moreover, the catalyst regeneration for the reusability of the spent catalyst was studied. After regeneration of the spent
(b)
80
93.7
24
93.8
4 6 8 10 12 Reaction time (hours)
93.6
95.1
40
94.1
60
87.2
FAME yield (%)
81.5
81.9
83.2
80.1
76.7
73.7
40
76.8
60
46.4
FAME yield (%)
40
0
80
20
60
20
100
(a)
WW W
80
83.2
100
100
93.6 94.1
CTORs were adopted (3, 6, 9, 12, 15, 18, 21, and 24 wt.%). All the other reaction conditions were fixed and the transesterification reaction was carried out with a MOMR of 12:1, stirring rate of 500 rpm, temperature of 60 C, and a reaction time of 2 h. Fig. 4(a) shows that the FAME yield is greatly influenced by the CTOR at low ratios. As the catalyst to oil ratio was increased from 3 to 6 wt.%, a remarkable increase in the FAME yield (from 46.4% to 73.7%) was observed. Soetaredjo et al. [24] showed that an increase in the catalytic conversion of triglycerides into biodiesel is mainly due to an increase in the number of basic active species (K2O). By further increasing the CTOR, the FAME yield increased, and the highest yield (83.2%) was obtained with a CTOR of 18 wt.%. An increase in the catalyst concentration (CTOR over 18 wt.%) did not increase the conversion; conversely a slight decrease in the FAME yield was observed. The higher amount of catalyst in the same amount of oil and methanol resulted in a poor mixing due to the higher viscosity of the mixture of reactants and catalyst. Therefore, this increase in mass transfer resistance in the multi-phase system was behind the lower conversions [6,35]. The CTOR of 18 wt.% at 2 h over the 35KOH/ZSM5 catalyst were used to study the effect of reaction time on biodiesel yield.
FAME yield (%)
304
20 0
0 3
6 9 12 15 18 21 24 Catalyst to oil ratio (wt.%)
2
Fig. 4. FAME yield versus (a) catalyst to oil ratio (t ¼ 2h) and (b) reaction time (Catalyst/oil ¼ 18 wt.%) over the 35KOH/ZSM5 catalyst.
T. Saba et al. / Renewable Energy 90 (2016) 301e306
catalyst following the 4th run, no increase in the catalytic activity of the regenerated catalyst was detected as the FAME yield was almost the same as in the 4th run. The reduction in FAME yield is most probably due to the leaching and poisoning of the basic sites during the transesterification reaction [23,24,35]. In fact, the XRF results obtained for the spent 35KOH/ZSM5 catalyst showed that 4.8% of the K2O active phase were leached after the 1st run and 6.2% after the 4th run in the experiment WW. In the experiment W, 3.8% of K2O active phase were leached from the surface after the 1st run and 9.4% after the 4th run. These results explain the decrease in FAME yield with the number of runs as shown in Fig. 5. The small amount of K2O leached from the surface does not compromise the stability of the 35KOH/ZSM5 catalyst under reaction conditions. In addition, this small amount is not comparable to the quantities added for homogeneously catalyzed transesterification reaction. Moreover, ZSM5 support could play an important role in maintaining the catalytic activity of the catalyst similar to the work of Noiroj el al. [6], where they showed that the K species supported on the zeolite (NaY) support where strongly bounded to the zeolite matrix compared to the Al2O3 support, suggesting that the support strongly influences the activity of the catalyst for the transesterification reaction. The FTIR spectra of the spent 35KOH/ZSM5 catalyst after the 1st run in experiment W (S1W) and in experiment WW (S1WW) are shown in Fig. 6(a). The FTIR spectrum of S1WW contains absorption bands present in the spectra of the produced biodiesel and glycerol (Fig. 6(a)). This indicates the adsorption of glycerol and fatty acid methyl esters on the surface of the unwashed catalyst. However, washing the reused catalysts with methanol removes all the adsorbed species as shown in the FTIR spectrum of S1W which is identical to the spectrum of the fresh 35KOH/ZSM5 catalyst. This washing did not restore the activity of the catalyst, as it continued to decrease since the first run (Fig. 5). The DSC analysis was performed on the spent catalysts (washed and unwashed) after the 1st run and are presented in Fig. 6(b). The DSC curve of the S1WW sample shows exothermic peaks
305
Table 2 Properties of the produced biodiesel. Property
Value
Density at 15 C (Kg/m3) Kinematic Viscosity at 40 C (cSt) Flash Point ( C)
885 4.7 94
corresponding to the oxidation of organic deposits (glycerol, methyl esters and glycerides) present on the surface of the catalyst. An accompanying weight loss of 13.5 wt.% was observed. Conversely, no oxidation peaks were observed in the DSC curve of the S1W sample. Therefore, as the washed catalyst deactivated faster than the unwashed catalyst, the main reason behind the catalyst deactivation is most probably the leaching of the basic active species (K2O). 3.5. Properties of the synthesized biodiesel The properties of the produced biodiesel from refined sunflower oil at optimal conditions using 35KOH/ZSM5 catalyst were determined (Table 2). The values of density, kinematic viscosity, flash point of the synthesized biodiesel were found to comply with ASTM standard specifications. 4. Conclusions The present work shows that KOH supported on ZSM5 zeolite are efficient catalysts for biodiesel production via transesterification of sunflower vegetable oil. Various parameters such as KOH loading, catalyst to oil ratio and reaction time greatly influence the transesterification reaction. Among the different catalysts used, ZSM5 loaded with 35 wt.% of KOH showed a very high catalytic activity (>93% of FAME yield) at the optimum determined reaction conditions: methanol to oil molar ratio of 12:1, catalyst amount of 18 wt.%., and a reaction time of 6 h. The reusability of 35KOH/ZSM5 catalyst was studied under these conditions and the latter remained stable for 3 consecutive runs. The slight decrease in
Fig. 6. (a) FTIR spectra of the S1W, S1WW, biodiesel and glycerol (b) DSC of the S1W and S1WW catalysts.
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catalytic activity after the 4th run was attributed to the leaching of potassium active species from the surface. The strong interaction between the zeolite support and the K2O active phase inhibited lthe leaching process even for very high active phase contents. The properties of produced biodiesel are found to be in good accordance with ASTM standard requirements. Finally, the 35KOH/ZSM5 catalyst could be successfully adopted for the transesterification of vegetable oils in industrial applications. References [1] M. Asif, T. Muneer, Energy supply, its demand and security issues for developed and emerging economies, Renew. Sust. Energy Rev. 11 (2007) 1388e1413. [2] M. Farroq, A. Ramli, A. Naeem, Biodiesel production from low FFA waste cooking oil using heterogeneous catalyst derived from chicken bones, Renew. Energy 76 (2015) 362e368. [3] M.H.M. Ashnani, A. Johari, H. Hashim, E. Hasani, A source of renewable energy in Malaysia, why biodiesel? Renew. Sust. Energy Rev. 35 (2014) 244e257. [4] H.V. Lee, J.C. Juan, Y.H. Taufiq-Yap, Preparation and application of binary acidebase CaOeLa2O3 catalyst for biodiesel production, Renew. Energy 74 (2015) 124e132. [5] M. Canakci, Combustion characteristics of a turbocharged DI compression ignition engine fueled with petroleum diesel fuels and biodiesel, Bioresour. Technol. 98 (2006) 1167e1175. [6] K. Noiroj, P. Intarapong, A. Luengnaruemitchai, S. Jai-In, A comparative study of KOH/Al2O3 and KOH/NaY catalysts for biodiesel production via transesterification from palm oil, Renew. Energy 34 (2009) 1145e1150. [7] T. Danuthai, S. Jongpatiwut, T. Rirksomboon, S. Osuwan, D.E. Resasco, Conversion of methylesters to hydrocarbons over an H-ZSM5 zeolite catalyst, Appl. Catal. A Gen. 361 (2009) 99e105. [8] I. Choedkiatsakul, K. Ngaosuwan, S. Assabumrungrat, S. Mantegna, G. Cravotto, Biodiesel production in a novel continuous flow microwave reactor, Renew. Energy 83 (2015) 25e29. [9] M. Tariq, A. Saqib, K. Nasir, Activity of homogeneous and heterogeneous catalysts, spectroscopic and chromatographic characterization of biodiesel: A review, Renew. Sust. Energy Rev. 16 (2012) 6303e6316. [10] B. Freedman, R.O. Butterfield, E.H. Pryde, Transesterification kinetics of soybean oil, J. Am. Oil Chem. Soc. 63 (1986) 1375e1380. [11] H.J. Kim, B.-S. Kang, M.J. Kim, Y.M. Park, D.K. Kim, J.S. Lee, Y.K. Lee, Transesterification of vegetable oil to biodiesel using heterogeneous base catalyst, Catal. Today 93 (2004) 315e320. [12] J. Lee, S. Saka, Biodiesel production by heterogeneous catalysts and supercritical technologies, Bioresour. Technol. 101 (2010) 7191e7200. [13] D.Y.C. Leung, X. Wu, M.K.H. Leung, A review on biodiesel production using catalyzed transesterification, Appl. Energy 87 (2010) 1083e1095. [14] V.B. Veljkovi c, I.B. Bankovi c-Ili c, O.S. Stamenkovi c, Purification of crude biodiesel obtained by heterogeneously-catalyzed transesterification, Renew. Sust. Energy Rev. 49 (2015) 500e516. [15] A.Z. Abdullah, N. Razali, H. Mootabadi, B. Salamatinia, Critical technical areas for future improvement in biodiesel technologies, Environ. Res. Lett. (2007) 2, 034001. [16] M. Hara, Environmentally benign production of biodiesel using heterogeneous catalysts, ChemSusChem 2 (2009) 129e135. [17] W. Xie, X. Yang, M. Fan, Novel solid base catalyst for biodiesel production: Mesoporous SBA-15 silica immobilized with 1, 3-dicyclohexyl-2-octylguanidine, Renew. Energy 80 (2015) 230e237.
[18] T.M. Albayati, A.M. Doyle, Encapsulated heterogeneous base catalysts onto SBA-15 nanoporous material as highly active catalysts in the transesterification of sunflower oil to biodiesel, J. Nanopart Res. 17 (2015) 109, http://dx.doi.org/10.1007/s11051-015-2924-6. [19] J.X. Wang, K.T. Chen, S.T. Huang, C.C. Chen, Application of Li2SiO3 as a heterogeneous catalyst in the production of biodiesel from soybean oil, Chin. Chem. Lett. 22 (2011) 1363e1366. [20] Y.M. Dai, J.S. Wu, C.C. Chen, K.T. Chen, Evaluating the optimum operating parameters on transesterification reaction for biodiesel production over a LiAlO2 catalyst, Chem. Eng. J. 280 (2015) 370e376. [21] J.X. Wang, K.T. Chen, J.S. Wu, P.H. Wang, S.T. Huang, C.C. Chen, Production of biodiesel through transesterification of soybean oil using lithium orthosilicate solid catalyst, Fuel Process Technol. 104 (2012) 167e173. [22] Y.M. Dai, K.T. Chen, C.C. Chen, Study of the microwave lipid extraction from microalgae for biodiesel production, Chem. Eng. J. 250 (2014) 267e273. [23] P. Intarapong, S. Iangthanarat, P. Phanthong, A. Luengnaruemitchai, S. Jai-In, Activity and basic properties of KOH/mordenite for transesterification of palm oil, J. Energy Chem. 2 (2013) 690e700. [24] F.E. Soetaredjo, A. Ayucitra, S. Ismadji, A.L. Maukar, KOH/bentonite catalysts for transesterification of palm oil to biodiesel, Appl. Clay Sci. 53 (2011) 341e346. [25] V. Mutreja, S. Singh, A. Ali, Biodiesel from mutton fat using KOH impregnated MgO as heterogeneous catalysts, Renew. Energy 36 (2011) 2253e2258. [26] D.M. Alonso, R. Mariscal, R. Moreno-Tost, M.D.Z. Poves, M.L. Granados, Potassium leaching during triglyceride transesterification using K/g-Al2O3 catalysts, Catal. Commun. 8 (2007) 2074e2080. [27] M. Bender, Economic feasibility review for community-scale farmer cooperatives for biodiesel, Bioresour. Technol. 70 (1999) 81e87. [28] Z. Wen, X. Yu, S.T. Tu, J. Yan, E. Dahlquist, Biodiesel production from waste cooking oil catalyzed by TiO2eMgO mixed oxides, Bioresour. Technol. 101 (2010) 9570e9576. [29] E. Rashtizadeh, F. Farzaneh, Z. Talebpour, Synthesis and characterization of Sr3Al2O6 nanocomposite as catalyst for biodiesel production, Bioresour. Technol. 154 (2014) 32e37. [30] J. Estephane, S. Aouad, S. Hany, B. El Khoury, C. Gennequin, H. El Zakhem, J. El Nakat, A. Aboukaïs, E. Abi Aad, CO2 reforming of methane over Ni-Co/ZSM5 catalysts. Aging and carbon deposition study, Int. J. Hydrogen Energy 40 (2015) 9201e9208. [31] W. Xu, S. Ji, W. Quan, J. Yu, One-Pot synthesis of dimethyl carbonate over basic zeolite catalysts, Mod. Res. Catal. 2 (2013) 22e27. [32] T.M. Albayati, A.M. Doyle, SBA-15 supported bimetallic catalysts for enhancement isomers production during n-heptane decomposition, Int. J. Chem. React. Eng. 12 (2014) 1e10. [33] M. Feyzi, G. Khajavi, Investigation of biodiesel production using modified strontium nanocatalysts supported on the ZSM-5 zeolite, Ind. Crops Prod. 58 (2014) 298e304. [34] J. Zheng, Q. Zeng, Y. Yuming, Y. Wang, J. Ma, B. Qin, X. Zhang, W. Sun, R. Li, The hierarchical effects of zeolite composites in catalysis, Catal. Today 168 (2011) 124e132. [35] M. Agarwal, G. Chauhan, S.P. Chaurasia, K. Singh, Study of catalytic behavior of KOH as homogeneous and heterogeneous catalyst for biodiesel production, J. Taiwan Inst. Chem. E 43 (2012) 89e94. [36] B. Freedman, E.H. Pryde, T.L. Mounts, Variables affecting the yields of fatty esters from transesterified vegetable oils, J. Am. Oil Chem. Soc. 61 (1984) 1638e1643. [37] D.Y.C. Leung, Y. Guo, Transesterification of neat and used frying oil: optimization for biodiesel production, Fuel Process Technol. 87 (2006) 883e890. [38] O.J. Alamu, M.A. Waheed, S.O. Jekayinfa, T.A. Akintola, Optimal transesterification duration for biodiesel production from nigerian palm kernel oil, Agric. Eng. Int. CIGR Ejournal IX (2007).
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Biodiesel production from refined sunflower vegetable oil over KOH/ ZSM5 catalysts Tony Saba a, Jane Estephane a, *, Bilal El Khoury b, Maroulla El Khoury a, Mahmoud Khazma a, Henri El Zakhem a, Samer Aouad b a b
Department of Chemical Engineering, University of Balamand, P.O. Box 100, Tripoli, Lebanon Department of Chemistry, University of Balamand, P.O. Box 100, Tripoli, Lebanon
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 August 2015 Received in revised form 30 November 2015 Accepted 1 January 2016 Available online 13 January 2016
ZSM5 zeolite was impregnated with different KOH loadings (15 wt.%, 25 wt.% and 35 wt.%) to prepare a series of KOH/ZSM5 catalysts. The catalysts were calcined at 500 C for 3 h and then characterized by N2 adsorptionedesorption and X-ray diffraction (XRD) techniques. The catalysts were tested in the transesterification reaction in a batch reactor at 60 C and under atmospheric pressure. It was found that KOH/ ZSM5 with 35 wt.% loading showed the best catalytic performance. The best reaction conditions in the presence of KOH/ZSM5 (35 wt.%) were determined while modifying the catalyst to oil ratio and the reaction time. The highest methyl ester yield (>95%) was obtained for a reaction time of 24 h, a catalyst to oil ratio of 18 wt.%, and a methanol to oil molar ratio of 12:1. The properties of produced biodiesel complied with the ASTM specifications. The catalytic stability test showed that 35KOH/ZSM5 was stable for 3 consecutive runs. Characterization of the spent catalyst indicated that a slight deactivation might be due to the leaching of potassium oxides active sites. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Biodiesel Transesterification KOH ZSM5 Reusability
1. Introduction The worldwide economic growth and sustainable development are leading to an increase in energy demand. Fossil fuels (oil, gas and coal) are providing almost 80% of the world energy requirements [1]. However, fossil fuel reserves are shrinking rapidly due to increased growth in world population and industrialization [2]. Moreover, the global warming due to greenhouse gases emissions prompted the efforts to explore new sources of energy. Therefore, biodiesel production is receiving considerable attention nowadays because biodiesel is a promising alternative fuel to conventional diesel [3], is renewable, biodegradable, and produces less CO, hydrocarbons and particulates emissions than petroleumbased diesel [4,5]. Biodiesel and conventional diesel properties are very similar; however biodiesel has higher cetane number than conventional diesel, virtually no sulfur and no aromatics and high flash point [5e7]. Biodiesel, also known as Fatty Acid Methyl Esters (FAME), can be
* Corresponding author. Department of Chemical Engineering, Faculty of Engineering, University of Balamand, P.O.Box 100, Tripoli, Lebanon. E-mail address: [email protected] (J. Estephane). http://dx.doi.org/10.1016/j.renene.2016.01.009 0960-1481/© 2016 Elsevier Ltd. All rights reserved.
obtained by a transesterification reaction of triglycerides (in oil or fats) with methanol by using a catalyst [8]. This reaction gives glycerol as a by-product. In general, homogeneous base catalysts such as NaOH or KOH are the most suitable catalysts for transesterification reactions [6] since they are able to catalyze these reactions faster than acid catalysts [9,10]. However, homogeneous catalysts have some drawbacks, e.g. they produce large amount of waste water [11], low-quality glycerol and they cannot be reused [12]. Moreover, side reactions such as saponification can occur, imposing difficulties in the separation and purification of biodiesel [13]. On the other hand, heterogeneous catalysts are less corrosive, more environmentally friendly, safer, cheaper and they can be easily recovered, regenerated and reused [14,15]. Therefore, the use of heterogeneous catalysts for the production of biodiesel is gaining great interest. In general, heterogeneous basic catalysts are catalytically more active than acid catalysts, thus they require lower temperatures and shorter reaction times [16,17]. For instance, the alkali metals and their hydroxides supported on SBA-15 are highly active catalysts in the transesterification reaction [18]. In addition, CaO base catalysts give high yields of biodiesel, but they are highly sensitive to air [19]. It has been recently shown that lithium-based catalysts, in particular LiAlO2 and Li4SO4, are highly active, air-insensitive and can be reused for many cycles [20e22].
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Moreover, KOH supported on Alumina, NaY zeolite [6], mordenite [23], bentonite [24], MgO [25] showed excellent catalytic activity in the production of biodiesel. However, it has been clearly documented in literature that the heterogeneous catalysts for biodiesel production deactivate or loose slightly their activity when used for several consecutive runs. This can be due to the leaching of the active phase in solution, or to the adsorption of hydrocarbons on the active sites [26e28]. To assess whether the KOH/ZSM5 catalysts perform well in the transesterification of sunflower oil with methanol, we tested a series of KOH/ZSM5 catalysts in the reaction and we evaluated the effect of active phase loading on the yield of biodiesel production. This paper discusses the influence of active phase content in KOH/ZSM5 catalysts, the catalyst to oil ratio and the reaction time on the biodiesel production yield. The catalyst reusability is also investigated for several consecutive runs as it represents a key advantage for industrial biodiesel production. 2. Methods 2.1. Materials Refined sunflower vegetable oil (Plein Soleil) was purchased from a local market and was analyzed based on density at 25 C (ASTM D4052), kinematic viscosity (ASTM D445), free fatty acid content (AOCS Ca-5a-40), and water content (AOCS Aa 3e38). ZSM5 zeolite (CBV2314) was obtained from Zeolyst Company. Methanol (purity 99.8%), n-hexane, potassium hydroxide (85%, pellets), and methyl heptadecanoate were supplied by SigmaeAldrich. 2.2. Catalyst preparation
EVO apparatus. The spent catalyst (~10 mg) was placed in an alumina crucible and introduced in the apparatus. The sample was heated from room temperature up to 600 C (5 C/min) under an air flow of 50 mL/min. 2.4. Transesterification reaction All experiments were carried out in a round bottom flask batch reactor equipped with a magnetic stirrer, controlled temperature (60 C), and a water-cooled condenser. The desired amounts of catalyst and methanol were added to the reactor and mixed together; the mixture was heated up to 60 C. Subsequently, the sunflower oil preheated at 60 C was added to the mixture under a stirring rate of 500 rpm. The transesterification reaction was carried out for all the catalysts with a methanol-to-oil molar ratio (MOMR) of 12:1, a temperature of 60 C and a stirring rate of 500 rpm. The best reaction conditions were determined while varying: the amount of KOH impregnation, the catalyst-to-oil mass ratio (CTOR, wt.%) and the reaction time. At the end of the reaction, the catalyst was recovered from the mixture by means of vacuum filtration. A rotary evaporator was used to remove the excess of methanol from the filtrate. The residue was then centrifuged for 20 min at 3000 rpm to separate the biodiesel (FAME) from glycerol. The possibility to reuse the catalyst was also studied in order to check its ability to provide the same catalytic activity after many cycles. A Gas Chromatography (Agilent 7890A series) equipped with a flame ionization detector (FID) and an HP-5 (30 m 0.32 mm) column was used to determine the FAME content. According to the European regulated procedure EN14103, 250 mg of the ester layer were dissolved in 5 mL of the internal standard methyl heptadecanoate C17 solution (10 g/L of C17 in n-hexane). The FAME content was calculated using Eq. (1):
ZSM5 zeolite was used as support to prepare a series of KOH/ ZSM5 catalysts with varying KOH loadings according to the wet impregnation method [6,25,35]. Appropriate amounts of KOH (1.76 g, 3.9 g and 6.33 g for 15 wt.%, 25 wt.% and 35 wt.% respectively) were dissolved in 50 mL of distilled water. Each aqueous solution of KOH was slowly added to 10 g of the ZSM5 support. The resulting slurry was then stirred (200 rpm) at 80 C for 2 h. After impregnation, the catalysts were dried at 80 C overnight and then calcined at 500 C for 3 h at a heating rate of 0.5 C/min. KOH/ZSM5 catalysts with different active phase loadings (15, 25 and 35 wt. %) were labeled 15KOH/ZSM5, 25KOH/ZSM5 and 35KOH/ZSM5, respectively.
where, SA is the total peak area of methyl esters, AIS is the area of the internal standard (C17), CIS is the internal standard concentration (10 mg/mL), VIS is the volume of the internal standard solution (in mL), and W is the sample weight (in mg) [29]. The physical properties, of the biodiesel produced in this work were determined according to the ASTM standard (Density at 15 C: ASTM D.4052; kinematic viscosity: ASTM D.445; flash point: ASTM D.93).
2.3. Catalyst characterization
3. Results and discussion
A Thermo-Electron QSurf M1 apparatus was used to determine the specific surface areas of the calcined catalysts, using the BET method. Prior to analysis, the samples were treated at 120 C under helium flow for 30 min. X-ray diffraction (XRD) experiments were performed on a Panalytical powder X-ray diffractometer. The diffraction patterns were recorded over an angular range of 5 < 2Q < 80 with a stepsize of (2Q) ¼ 0.02 . By comparing the XRD patterns to the ICDD files, the crystalline phases were identified. X-ray Fluorescence (XRF) spectrometry was performed using a Thermo Scientific ARL 9900 Series. The potassium (K) content in the fresh and spent catalysts was measured by XRF. FTIR Spectra of the samples were recorded on a Perkin Elmer spectrometer in the range of 500e4000 cm1 with a resolution of 2 cm1. Before analysis, the samples were diluted with KBr powder and then pelletized. The simultaneous differential scanning calorimetry/thermogravimetry (DSC/TG) analyses were carried out on a Setaram Labsys
3.1. Characterization of vegetable oil
P FAME ð%Þ ¼
A CIS VIS 100 AIS W
(1)
Table 1 shows the properties of the sunflower oil used in this study and its free fatty acid content. This oil showed the required properties for a basic transesterification reaction. 3.2. Catalyst characterization The BET surface area of the support ZSM5 and the various
Table 1 Properties of the sunflower oil. Properties
Sunflower oil
Density at 25 C (Kg/m3) Kinematic Viscosity at 40 C (cSt) Water Content (wt.%) FFA Content (wt.%)
920.5 53.87 0.09 0.22
T. Saba et al. / Renewable Energy 90 (2016) 301e306
3.3. Catalytic activity of KOH/ZSM5 catalysts 3.3.1. Effect of KOH loading and catalyst to oil ratio on FAME yield The effect of KOH loading (wt.%) on FAME yield was studied. The transesterification reactions were carried out with the ZSM5 support, 15KOH/ZSM5, 25KOH/ZSM5 and 35KOH/ZSM5 catalysts. The experimental conditions were the following: methanol-to-oil molar ratio of 12:1, reaction temperature of 60 C, stirring rate of 500 rpm and a reaction time of 2 h. Moreover, in order to determine the effect of catalyst amount on FAME yield, the CTOR was varied between 3 and 9 wt.%. Fig. 3 clearly shows that the amount of KOH
Fig. 1. XRD patterns of the different freshly calcined catalysts.
4000 3500 3000 2500 2000 1500 1000 -1 Wavenumber (cm )
544
1224 1084
1640
3651
ZSM5
792
1003
758
3425
35KOH/ZSM5
Transmittance (a.u.)
impregnated catalysts were determined. The support presented the highest surface area with a value of 290 m2 g1. Upon impregnation with KOH, the specific surface areas decreased drastically to 6 m2 g1 for the 15KOH/ZSM5 and even lower values (non detectable by the apparatus) for the 25KOH/ZSM5 and 35KOH/ ZSM5 catalysts. This decrease is probably due to the filling of the pores and the deposition of potassium species on the pores openings due to the high amount of active phase used. Fig. 1 shows the XRD patterns recorded for the various catalysts. The presence of intense diffraction peaks in the XRD profile of ZSM5 zeolite (ICDD 00-057-0145) shows the high crystallinity of the support. The diffraction peaks of the 15KOH/ZSM5 sample are less intense than those of the support indicating that the zeolite crystallinity decreased upon KOH impregnation [30]. Moreover, as the KOH loading on ZSM5 was increased up to 25 wt.% and 35 wt.%, a significant loss of crystallinity has occurred. These results are in accordance with previous studies [23,31]. Moreover, no KOH or K2O diffraction peaks are observed on the XRD patterns of the prepared catalysts. This might be due to the presence of highly dispersed and/or amorphous K2O particles. The FTIR spectrum of the 35KOH/ZSM5 catalyst calcined at 500 C is shown in Fig. 2. The bands at 3425 cm1 and 1640 cm1 are assigned to the OeH stretching and bending of adsorbed water, respectively [18,32e34]. The band at 3650 cm1 in the FTIR spectrum of the ZSM5 is related to the hydroxyl groups of Si(OH)Al. The band at 1084 cm1 is assigned to the internal asymmetric stretching vibration of SieOeT (T ¼ Si or Al), while the band at 1224 cm1 is assigned to the external asymmetric stretching. The band at 792 cm1 is related to the symmetric stretching vibrations of external linkage, the band at 544 cm1 is characteristic of the double ring external linkage vibrations, while the band at 440 cm1 is assigned to O-T bending vibrations [34].
303
500
Fig. 2. FTIR spectra of the freshly calcined ZSM5 and 35KOH/ZSM5 catalysts.
loaded on ZSM5 affected the conversion of sunflower oil into Fatty Acid Methyl Esters. The ZSM5 support used alone is inactive in the transesterification reaction. However, by increasing the loading amount of KOH from 15 wt.% to 35 wt.%, a significant increase in FAME yield was observed. For instance, by using a CTOR of 3wt.%, the FAME yield increased from 12.4% to 46.4% by varying the loading amount of KOH from 15 to 35 wt.%. Moreover, by using a CTOR of 6wt.% and 9wt.%, the FAME yield increased from 22.5% to 73.7%, and from 41.0% to 76.8% respectively, by varying the KOH loading from 15 to 35 wt.%. This increase in FAME yield is attributed to the higher K2O active phase content in the catalyst [6,24]. It is to note that due to the high active phase loading and the small surface areas of the solids, the catalytic reaction is mainly taking place at the surface of the catalyst while a smaller conversion would probably occur inside the pores. It can also be inferred from Fig. 3 that the increase in the CTOR led to an increase in the FAME yield. The highest FAME yield (76.8%) was obtained in the presence of the 35KOH/ZSM5 catalyst and a CTOR of 9 wt.%. Therefore, finding the best reaction conditions in the presence of 35KOH/ ZSM5 catalyst was done by modifying the catalyst to oil ratio and the reaction time. 3.3.2. Effect of catalyst to oil ratio (wt.%) on FAME yield In order to determine the optimum conditions for the 35KOH/ ZSM5 catalyst on the transesterification of sunflower oil, different
Fig. 3. Effect of catalyst to oil ratio on FAME yield over the different catalysts (CH3OH/ oil ¼ 12:1; T ¼ 60 C; P ¼ 1 atm; t ¼ 2 h/Standard deviation values near symbols).
T. Saba et al. / Renewable Energy 90 (2016) 301e306
3.3.3. Effect of reaction time on FAME yield Reaction time plays a significant role in the transesterification of triglycerides [36]. Fig. 4(b) clearly shows that the FAME yield increased from 83.2% to 95.1% as the reaction time was increased from 2 to 24 h. In particular, the results showed that within the time range 2e6 h, the biodiesel yield increased up to 93.6%. With a further increase in the reaction time, there wasn't any significant change in the biodiesel yield as the equilibrium conversion had been attained. It was clearly documented in literature that the transesterification reaction is slow at the beginning due to a poor dispersion and mixing of alcohol into the oil resulting in lower FAME yield. Afterward, the reaction proceeds rapidly, the yield reaches a maximum and remains constant [37,38]. The optimum conditions for the transesterification of sunflower oil in the presence of KOH/ZSM5 (35 wt.%) were a catalyst to oil ratio of 18 wt.%, a methanol to oil ratio of 12:1 and a reaction time of 6 h. Therefore, these conditions were adopted in the study of the reusability of the catalyst. 3.4. Reusability of the catalyst Reusability is one of the main important features for the
st
1
75.9
83.1 76.2
90.9 88.6
91.8 93.3 2
nd
3
rd
th
4
After regeneration
Number of runs Fig. 5. FAME yield versus the number of runs over the 35KOH/ZSM5 catalyst in two different experiments; W: with washing and WW: without washing (Catalyst/ oil ¼ 18 wt.%; CH3OH/oil ¼ 12:1; T ¼ 60 C; P ¼ 1 atm; t ¼ 6h).
commercialization of a heterogeneous catalyst. In this work, the reuse of 35KOH/ZSM5 catalyst was studied by recovering and reusing this catalyst in four consecutive runs, where the first run designates the reaction with the fresh 35KOH/ZSM5 catalyst. Two different experiments were adopted. In the first experiment WW, the catalyst was only filtered and then fed into another run. However, in the second experiment W, the catalyst was filtered, recovered, washed several times with methanol, dried at 110 C for 2 h and reused for the next run. Moreover, after the 4th run in the experiment W, the spent catalyst was regenerated at 500 C (heating rate of 0.5 C/min) for 3 h before reuse. Fig. 5 shows the FAME yield as a function of the number of runs in the two experiments; W (with washing) and WW (without washing). For the first three runs in both experiments, the 35KOH/ ZSM5 catalyst exhibited good stability and kept its catalytic performance, where the FAME yield decreased only by 2.9% in experiment WW, and by 5.8% in experiment W. By reaching the 4th run, the 35KOH/ZSM5 catalyst showed an increase in the deactivation. For instance, the decrease in biodiesel yield after the 4th run in experiment WW was about 11.2%. However, in experiment W it was about 19%. Moreover, the catalyst regeneration for the reusability of the spent catalyst was studied. After regeneration of the spent
(b)
80
93.7
24
93.8
4 6 8 10 12 Reaction time (hours)
93.6
95.1
40
94.1
60
87.2
FAME yield (%)
81.5
81.9
83.2
80.1
76.7
73.7
40
76.8
60
46.4
FAME yield (%)
40
0
80
20
60
20
100
(a)
WW W
80
83.2
100
100
93.6 94.1
CTORs were adopted (3, 6, 9, 12, 15, 18, 21, and 24 wt.%). All the other reaction conditions were fixed and the transesterification reaction was carried out with a MOMR of 12:1, stirring rate of 500 rpm, temperature of 60 C, and a reaction time of 2 h. Fig. 4(a) shows that the FAME yield is greatly influenced by the CTOR at low ratios. As the catalyst to oil ratio was increased from 3 to 6 wt.%, a remarkable increase in the FAME yield (from 46.4% to 73.7%) was observed. Soetaredjo et al. [24] showed that an increase in the catalytic conversion of triglycerides into biodiesel is mainly due to an increase in the number of basic active species (K2O). By further increasing the CTOR, the FAME yield increased, and the highest yield (83.2%) was obtained with a CTOR of 18 wt.%. An increase in the catalyst concentration (CTOR over 18 wt.%) did not increase the conversion; conversely a slight decrease in the FAME yield was observed. The higher amount of catalyst in the same amount of oil and methanol resulted in a poor mixing due to the higher viscosity of the mixture of reactants and catalyst. Therefore, this increase in mass transfer resistance in the multi-phase system was behind the lower conversions [6,35]. The CTOR of 18 wt.% at 2 h over the 35KOH/ZSM5 catalyst were used to study the effect of reaction time on biodiesel yield.
FAME yield (%)
304
20 0
0 3
6 9 12 15 18 21 24 Catalyst to oil ratio (wt.%)
2
Fig. 4. FAME yield versus (a) catalyst to oil ratio (t ¼ 2h) and (b) reaction time (Catalyst/oil ¼ 18 wt.%) over the 35KOH/ZSM5 catalyst.
T. Saba et al. / Renewable Energy 90 (2016) 301e306
catalyst following the 4th run, no increase in the catalytic activity of the regenerated catalyst was detected as the FAME yield was almost the same as in the 4th run. The reduction in FAME yield is most probably due to the leaching and poisoning of the basic sites during the transesterification reaction [23,24,35]. In fact, the XRF results obtained for the spent 35KOH/ZSM5 catalyst showed that 4.8% of the K2O active phase were leached after the 1st run and 6.2% after the 4th run in the experiment WW. In the experiment W, 3.8% of K2O active phase were leached from the surface after the 1st run and 9.4% after the 4th run. These results explain the decrease in FAME yield with the number of runs as shown in Fig. 5. The small amount of K2O leached from the surface does not compromise the stability of the 35KOH/ZSM5 catalyst under reaction conditions. In addition, this small amount is not comparable to the quantities added for homogeneously catalyzed transesterification reaction. Moreover, ZSM5 support could play an important role in maintaining the catalytic activity of the catalyst similar to the work of Noiroj el al. [6], where they showed that the K species supported on the zeolite (NaY) support where strongly bounded to the zeolite matrix compared to the Al2O3 support, suggesting that the support strongly influences the activity of the catalyst for the transesterification reaction. The FTIR spectra of the spent 35KOH/ZSM5 catalyst after the 1st run in experiment W (S1W) and in experiment WW (S1WW) are shown in Fig. 6(a). The FTIR spectrum of S1WW contains absorption bands present in the spectra of the produced biodiesel and glycerol (Fig. 6(a)). This indicates the adsorption of glycerol and fatty acid methyl esters on the surface of the unwashed catalyst. However, washing the reused catalysts with methanol removes all the adsorbed species as shown in the FTIR spectrum of S1W which is identical to the spectrum of the fresh 35KOH/ZSM5 catalyst. This washing did not restore the activity of the catalyst, as it continued to decrease since the first run (Fig. 5). The DSC analysis was performed on the spent catalysts (washed and unwashed) after the 1st run and are presented in Fig. 6(b). The DSC curve of the S1WW sample shows exothermic peaks
305
Table 2 Properties of the produced biodiesel. Property
Value
Density at 15 C (Kg/m3) Kinematic Viscosity at 40 C (cSt) Flash Point ( C)
885 4.7 94
corresponding to the oxidation of organic deposits (glycerol, methyl esters and glycerides) present on the surface of the catalyst. An accompanying weight loss of 13.5 wt.% was observed. Conversely, no oxidation peaks were observed in the DSC curve of the S1W sample. Therefore, as the washed catalyst deactivated faster than the unwashed catalyst, the main reason behind the catalyst deactivation is most probably the leaching of the basic active species (K2O). 3.5. Properties of the synthesized biodiesel The properties of the produced biodiesel from refined sunflower oil at optimal conditions using 35KOH/ZSM5 catalyst were determined (Table 2). The values of density, kinematic viscosity, flash point of the synthesized biodiesel were found to comply with ASTM standard specifications. 4. Conclusions The present work shows that KOH supported on ZSM5 zeolite are efficient catalysts for biodiesel production via transesterification of sunflower vegetable oil. Various parameters such as KOH loading, catalyst to oil ratio and reaction time greatly influence the transesterification reaction. Among the different catalysts used, ZSM5 loaded with 35 wt.% of KOH showed a very high catalytic activity (>93% of FAME yield) at the optimum determined reaction conditions: methanol to oil molar ratio of 12:1, catalyst amount of 18 wt.%., and a reaction time of 6 h. The reusability of 35KOH/ZSM5 catalyst was studied under these conditions and the latter remained stable for 3 consecutive runs. The slight decrease in
Fig. 6. (a) FTIR spectra of the S1W, S1WW, biodiesel and glycerol (b) DSC of the S1W and S1WW catalysts.
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