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Transesterification of soybean oil catalyzed by Sr-doped cinder
Energy Conversion and Management 95 (2015) 272–280
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Transesterification of soybean oil catalyzed by Sr-doped cinder Sadia Nasreen a, Hui Liu a,⇑, Romana Khan b, Xiao-chan Zhu a, Dejan Skala c a
State Key Laboratory of Biogeology and Environmental Geology and School of Environmental studies, China University of Geosciences, Wuhan 430074, PR China Department of Environmental Sciences, COMSATS Institute of Information Technology, Abbottabad, Pakistan c Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, Belgrade 11000 R, Serbia b
a r t i c l e
i n f o
Article history: Received 7 November 2014 Accepted 2 February 2015 Available online 27 February 2015 Keywords: Biodiesel Strontium Solid base catalyst Subcritical condition Transesterification
a b s t r a c t The Strontium doped cinder was prepared using the wet impregnation method and analyzed as catalyst for biodiesel synthesis. Different procedure for cinder impregnation were investigated (temperature and duration of catalyst calcinations, the concentration of SrCl2 solution) and optimal condition was established: impregnation 20 g of cinder 2–5 mm particles with 0.2 M SrCl2 solution followed by calcinations at 1000 °C for 4 h. The Sr-cinder activity was tested at 90–200 °C using 1–5% mass of catalyst with different molar ratio of methanol to soybean oil (from 9 to 36). The maximum triglycerides (TG) conversion of 99.0% with the Fatty Acid Methyl Esters (FAME) yield of 97.1% was obtained by using 4% catalyst at 180 °C, for 1 h, and methanol/oil molar ratio 24:1. Influence of free fatty acid (FFA) and water in soybean oil on catalyst activity was analyzed, too. The catalyst could be used for 14 times with TG conversion and FAME yield above 90% and 80% respectively. The Sr-doped cinder catalysts before and after transesterification were characterized using BET surface area, basic strength, X-ray diffraction (XRD), scanning electron microscopy (SEM) and ICP-AAS. Results showed that the formation of SrAl2Si2O8 and Sr5Al8O17 complexes should be the main reason for the catalytic activity of prepared catalyst. Slow decrease of catalyst activity during its repeated use is result of Sr–Al-glycerolate formation in reaction between Sr–Al complexes and glycerol. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Increased environmental problem associated with energy consumption and continuous decline of limited crude oil reserves were focused by scientific researchers to find alternative potential sources of substantial energy production [1,2]. As compare to other conventional fuel, diesel fuel is most efficient as transportation energy and most commonly used allover the world. Biodiesel which is a mixture of fatty acid methyl esters (FAME) might be produced from light alcohols (methanol and ethanol) and triglycerides from plant oils or animal fats [3–5]. It is renewable, nontoxic and biodegradable alternative to diesel fuel obtained from fossil fuel [6,7]. Problems associated with use of homogeneous (acid H2SO4, HCl; or base NaOH, KOH) catalyst for biodiesel synthesis [8], like slow process of transesterification (acid catalysis), purification of product and catalyst neutralization (acid and base catalyst), use of raw triglycerides containing water and free fatty acids (base catalysis) are limiting factors for economical industrial ⇑ Corresponding author. Tel.: +86 15927501778; fax: +86 27 87436235. E-mail address: [email protected] (H. Liu). http://dx.doi.org/10.1016/j.enconman.2015.02.006 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.
production of biodiesel [9,10]. Many metal oxides as heterogeneous catalysts were also explored for biodiesel synthesis and some of these catalysts have shown very good catalytic performance [11,12]. Some metal complexes, active metals loaded on supports, zeolites and ezymes were also tested in the recent past [1,13,14]. Currently, majority of heterogeneous catalysts used in producing biodiesel are based on either oxides of alkali or alkaline earth metals as precursors combined with some inert or less active support which usually have a large surface area [15]. Studies in which CaO [16], SrO [17,18], ZnO–La2O3 [19], Znaluminate [20] Mg-hydrocalcite [21,22] and KOH [23,24] were supported and used for biodiesel have shown promising effect [25]. Among the well-known solid base catalysts, SrO has emerged as an attractive one for biodiesel production. SrO as strong base can catalyze not only transesterification of vegetable oil [18,26] but some other chemical reactions such as: oxidative coupling of methane [27], selective oxidation of propane [28], oxidation of vinyl chloride emission [29]. SrO compounds are non-toxic with high basic strength and according to some authors they are not soluble in methanol, vegetable oils or FAME [18,30]. To improve specific surface area and activity of SrO it has been loaded on
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S. Nasreen et al. / Energy Conversion and Management 95 (2015) 272–280 Table 1 Review of recently published research about SrO (pure) or in combination with some other methals or supports. Catalyst
SrO
Sr3Al2O6
Sr/ZrO2
Sr(NO3)2/ZnO
Sr/MgO
CaO loaded with Sr(NO3)2
Hydroxyapatite loaded with Sr(NO3)2
Methanol:oil Reaction temperature (°C) Reaction time (min) Calcination temperature (°C) Catalyst amount (%) Reusability TG conversion (%) Method of analysis
6:1 65 30 700 3 10 95 Diffusion
25:1 60 60 – 1.3 15 95.7 Sol gel
29:1 115.5 90 900 2.7 – 80 Wet impregnation
12:1 65 300 – 5 – 95.7 Co-precipitation
12:1 65 30 600 5 – 93 Wet impregnation
12:1 750 120 750 3.5 3 97.3 Sol gel
9:1 70 300 600C 5.6 1 85 Wet impregnation
various supports such as alumina [30], zinc oxide and zirconia [31–33], hydroxyapatite [18] and results of these investigation is shown in Table 1. Data presented in Table 1 show that high conversion of triglycerides (80-97%) could be obtained using SrO as pure compound or in combination with other supports or metal-oxide. Relative short reaction time (less than 2 h), different molar ratio of methanol and oil (6–29) at low or moderate temperature (65– 115 °C), with 1.3–5 wt% of catalyst were tested. Most of heterogeneous catalysts (Table 1) were applied as powder or even there is no information about the catalyst granulation. If powder must be used than separation of biodiesel and glycerol, after evaporation excess of methanol, could be even more difficult than neutralization and removal some homogeneous catalysts [34,35]. Supported solid base catalysts with well-distributed pores can improve basic strength and reduce the leaching of precursor. In such conditions, utilization of some cheep solid support like cinder can reduce amount of precursor and improve its dispersion onto surface of catalyst giving much better its catalytic behavior. Cinder used in this study as support for SrO is a solid waste from coal burning process. It is usually in the form of fine particles having the meso pore size and relatively small pore volume. It was already shown that cinder supported by K2CO3 has been successfully applied for biodiesel production [36,37]. The goal of this study was also to show that cinder, mainly composed of SiO2, Al2O3 and CaO, even in some cases K2O and MgO [43], can stabilize the precursor (SrO) and improve the catalytic performance in the base-catalyzed reaction of vegetable oil transesterification. Different conditions of Sr-doped cinder catalyst preparation and its testing for transesterification of soybean oil were optimized taking into account catalytic efficiency and stability. Characteristics of fresh and catalyst after repeated use were analyzed measuring the catalyst base strength, the specific surface area (BET), using scanning electron microscopy with analysis of elements composition at catalyst surface (SEM-EDS), powder X-ray Diffraction (XRD). Leaching of active metal (Sr) in biodiesel and glycerol were characterized by ICP-AAS. 2. Experimental 2.1. Materials Raw soybean oil was purchased from Baifu Oils & Fat Co. Limited (Wuhan, China). The physical and chemical properties of the soybean oil are listed in Table 2. HPLC grade methanol was purchased from Tedia Company Inc. (Fairfield, OH, USA) and Mallinckrodt Baker, Inc. (Phillipsburg, New Jersey, USA), respectively. Analytical grade SrCl26H2O was purchased from Aopu Chemical Company (Wuhan, China).
Table 2 Physical-chemical properties of the soybean oil used for the experiment. Parameters
Values
Water content (wt%) Acid value (mgKOH/g) Density (g/cm3) Saponification number Fatty acid components (%) C16:0 C16:1 C18:0 C18:1 C18:2 C18:3 Others
0.12 0.2 0.925 192 16.5 – 2.0 24.8 51.8 2.6 0.6
2.2. Catalyst preparation The Sr-doped cinder catalysts were prepared by wet impregnation method. Cinder with diameter grades of 2.0–5.0 mm were used after drying and sieving. SrCl2 solutions were prepared with various concentrations by dissolving certain amount of SrCl26H2O into deionized water. Typically 20 g of freshly dried cinder was added into 500 mL SrCl2 solutions and left overnight in an oven at 50 °C. After 24 h the excess of water was evaporated by slow heating at 70 °C, and then wet solid was dried overnight in an oven at 50 °C and finally calcined in the muffle furnace at a certain temperature (600–1200 °C) for a certain time (1–5 h). 2.3. Transesterification reaction A 500 mL stainless steel autoclave (PCF0.5-10 JianBang Chemical Machinery Co. Limited, China) with a stirrer, an internal cooling pipe, a temperature sensor inside, and an external heating jacket, was used as batch reactor for transesterification of soybean oil with methanol. The importance of mixing intensity for biodiesel synthesis has been reported quite well in the past. It has more influence in the period of slow rate of methanolysis process which is governed by the rate of triglyceride mass transfer to the catalyst surface. It has been shown, that excessive mixing can deactivate the catalyst and reduce the conversion of triglyceride and yield of FAME as well as the low intensity of mixing which enable only limited contact of reactants influencing decay of TG conversion and FAME yield for certain time of reaction [38,39]. Experiments performed in this study was realized in such way that soybean oil, methanol, and catalyst were put into the reactor and then heated to the pre-set temperature and stirred at a constant speed of 200 rpm. The beginning of methanolysis was counted from the moment when temperature inside reactor was reached the preset reaction temperature. After some reaction time, the reaction mixture was cooled down and transesterification reaction was
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stopped by the flowing water through the internal cooling pipe and then, the autoclave was opened and the reaction mixture poured into a beaker. 2.4. Analysis of triglyceride conversion Triglycerides (TG), Fatty acid methyl ester (FAME) and intermediate products, monoglycerides (MG) and diglycerides (DG), were analyzed using Agilent 1100 Series with a C18 column (40 °C, particle size 5 lm, 200 4.6 mm I.D.) and UV/Visible detection at 205 nm [40]. A linear gradient from 100% methanol to 50% methanol + 50% isopropanol–hexane (5:5, v/v) in 25 min was employed. TG conversion was calculated as the (percent) diminution of the sum of the areas of TG HPLC peaks. The FAME yield was calculated as the fraction (percent) of the sum of the areas of all FAME HPLC peaks relative to that of samples with maximum conversion. 2.5. Catalyst characterization The basic strength of the samples (H_) was determined using Hammett indicator [36]. About 100 mg of the sample was shaken with 5 mL cyclohexane and 1 mL of Hammett indicators-benzene solution (0.1%, w/w) and then left to equilibrate for 2 h when no further color changes were observed. The following Hammett indicators were used and the corresponding H_ values are as follows: 4-nitroaniline (H_ = 18.4), 2, 4-dinitroaniline (H_ = 15.0), phenolphthalein (H_ = 9.8). The basic strength is defined as being stronger than the weakest indicator, which exhibits a color change, and weaker than the strongest indicator that showed no color change. BET surface areas of the samples were determined according to the multipoint N2 adsorption-desorption method using an instrument of SSA-4200 Surface Area & Pore Size Analyzer (Beijing Builder Electronic Technology Co., Ltd., China). Prior to measurements, all the samples were out gassed overnight under vacuum at 373 K. XRD was performed on a D/MAX-RB powder X-ray diffractometer (Rigaku Corporation, Japan) at room temperature. Cu Ka radiation (k = 0.15418 nm), with a step size of 0.02° in the 2h range from 5° to 65°, was used in all the samples. The data were processed with the X’Pert HighScore Plus software. The peaks were identified using the Powder Diffraction File (PDF) database created by International Centre for Diffraction Data (ICDD). The sample morphology was characterized at room temperature by a Quanta 200 SEM system (FEI Company, Netherlands).
reaction. Fig. 1 shows that the calcined cinder resulted in only 59.1% TG conversion and 36.9% FAME yield, while calcined SrCl2 resulted in 89.7% TG conversion and 73.3% FAME yield, respectively. However, TG conversion and FAME yield were dramatically enhanced to 93.5–99.8% and 89.4–94.4%, respectively, by impregnating cinder into increasing concentration of SrCl2 solutions. The highest transesterification efficiency was obtained for the catalyst prepared by using 0.2 M SrCl2, further increase of SrCl2 only slightly increase its activity. 3.1.2. Effect of temperature and duration of calcination In order to determine the optimum calcination conditions a series of catalysts with various calcination temperatures (600–1200 °C) and duration (1–5 h) were prepared and applied for transesterification of soybean oil with methanol. Fig. 2 show that TG conversion and FAME yield increase to 99.1% and 96.6%, respectively, when the catalyst was calcined at 1000 °C. A higher calcination temperature did not increase TG conversion. Thus, taking into account the energy consumption for calcinations process, 1000 °C was selected as appropriate one. Enough duration of calcination is usually required to obtain a complete decomposition of metal precursors and its oxidation [41]. In order to investigate the effect of calcination duration on TG conversion and FAME yield a series of catalysts calcined for 1–5 h at 1000 °C were prepared and used for heterogeneous transesterification reaction. Fig. 3 showed that TG conversion increased from 88.5% to 99.0% and FAME yield increased from 73.0% to 94.4%, respectively, when the time of catalyst calcination extended from 1 to 4 h. Further increase time of calcination did not show that a higher TG conversion could be obtained. In consideration of energy consumption, the following condition for catalyst preparation was established: calcination at 1000 °C for 4 h. 3.1.3. Effect of catalyst amount Catalyst amount is an important parameter which mainly affects the conversion of triglycerides to methyl esters. Generally, catalysts supported by natural material need a higher amount to gain satisfactory efficiency because the presence of precursor is relatively less compared to pure active metal oxide [41]. As shown in Fig. 4, the TG conversion increased with increasing the catalyst
3. Results and discussion 3.1. Effects of catalyst preparation and transesterification reaction conditions on TG conversion and FAME yield TG conversion and FAME yield were investigated with a goal to find optimal procedure for catalyst preparation (concentration of impregnating SrCl2 solution, duration and temperature of catalysts calcination), and transesterification reaction conditions (catalyst amount used for transesterification, methanol to oil molar ratio, transesterification reaction temperature and time). 3.1.1. Effect of SrCl2 impregnating solution concentration In order to study the effect of the concentration of SrCl2 impregnating solution on catalytic activity, a series of catalysts with fixed amount of cinder (20 g) impregnated in 500 mL SrCl2 solutions with various concentrations from 0.1 to 0.3 M were prepared and calcined at 1000 °C for 4 h. The fresh cinder and SrCl2 were also calcined at the same conditions. The catalysts, together with calcined cinder and SrCl2 were employed to catalyze the transesterification
Fig. 1. TG conversion and FAME yield of the transesterification reaction catalyzed by catalysts prepared in impregnating SrCl2 solutions with concentration of 0.1 M (catalyst 1), 0.2 M (catalyst 2), and 0.3 M (catalyst 3), and calcined cinder and SrCl2. Catalyst preparation conditions: calcination temperature = 1200 °C; calcination time = 5 h. Reaction conditions: catalyst amount = 5%; molar ratio of methanol to oil = 36:1; reaction temperature = 200 °C; reaction time = 1 h.
S. Nasreen et al. / Energy Conversion and Management 95 (2015) 272–280
Fig. 2. Effects of calcination temperature on TG conversion and FAME yield. Catalyst preparation conditions: calcination time = 5 h; concentration of impregnating SrCl2 solutions = 0.2 M. Reaction conditions: catalyst amount = 5%; molar ratio of methanol to oil = 36:1; reaction temperature = 200 °C; reaction time = 1 h.
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Fig. 4. Effects of catalyst amount on TG conversion. Catalyst preparation conditions: calcination temperature = 1000 °C; calcination time = 4 h; concentration of impregnating SrCl2 solutions = 0.2 M. Reaction conditions: molar ratio of methanol to oil = 36:1; reaction temperature = 200 °C; reaction time = 1 h.
Fig. 3. Effects of calcination time on TG conversion. Catalyst preparation conditions: calcination temperature = 1000 °C/4 h; concentration of impregnating SrCl2 solutions = 0.2 M. Reaction conditions: catalyst amount = 5%; molar ratio of methanol to oil = 36:1; reaction temperature = 200 °C; reaction time = 1 h.
Fig. 5. Effects of methanol/oil molar ratio on TG conversion and FAME yield. Catalyst preparation conditions: calcination temperature = 1000 °C; calcination time = 4 h; concentration of impregnating SrCl2 solutions = 0.2 M. Reaction conditions: catalyst amount = 4%; reaction temperature = 200 °C; reaction time = 1 h.
amount from 1% to 4%. Only a little bit higher TG conversion was detected if 5% of catalyst was used for biodiesel synthesis. FAME yield of 94.4% was obtained when 4% catalyst is used. Disadvantage of increased amount of catalyst for heterogeneous biodiesel synthesis is caused by higher biodiesel production cost. Further reaction tests were performed using 4% catalyst.
unused alcohol recovery [42]. As shown in the Fig. 5, TG conversion and FAME yield considerably increased with increase of methanol/ oil molar ratio from 9:1 to 18:1; TG conversion and FAME yield was 98.8% and 87.7%, respectively, when 18:1 molar ratio of methanol/ oil was used in the transesterification reaction (Fig. 5). Increase of methanol/oil molar ratio to 24:1 and 30:1 showed a little effect on TG conversion but had considerably effects on FAME yield (increased from 88.4% to 91.6%). Further increase of methanol/oil molar ratio more than 30:1 showed very limited effect on TG conversion and FAME yield.
3.1.4. Effect of methanol to oil molar ratio Amount of methanol and its molar ratio to soybean oil is one of the most important factors influencing the TG conversion. In order to suppress the reversible reaction between glycerol and FAME, which leads to lower yield of FAME, a higher molar ratio of methanol and oil is usually required. Unfortunately, increasing the alcohol amount in initial reaction mixture will increase the cost of
3.1.5. Effect of transesterification reaction temperature and time Temperature has direct influence on reaction rate and thus on duration of transesterification and overall energy consumption.
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Fig. 6. Effects of transesterification reaction temperature on TG conversion and FAME yield. Catalyst preparation conditions: calcination temperature = 1000 °C; calcination time = 4 h; concentration of impregnating SrCl2 solutions = 0.2 M. Reaction conditions: catalyst amount = 4%; reaction time = 1 h.
Fig. 8. Effects of repeated use of the catalyst on TG conversion and FAME yield. Catalyst preparation conditions: calcination temperature = 1000 °C; calcination time = 4 h; concentration of impregnating SrCl2 solutions = 0.2 M. Reaction conditions: catalyst amount = 4%; reaction temperature = 180 °C; reaction time = 1 h.
to catalyze transesterification reaction, using methanol:oil molar ratio of 24:1 and 4% catalyst amount, at 180 °C for 1 h. 3.2. Reusability of the Sr-dopped cinder catalyst
Fig. 7. Effects of reaction time on TG conversion and FAME yield. Catalyst preparation conditions: calcination temperature = 1000 °C; calcination time = 4 h; concentration of impregnating SrCl2 solutions = 0.2 M. Reaction conditions: catalyst amount = 4%; reaction temperature = 180 °C.
The effects of reaction temperature from 90 to 200 °C on transesterification efficiency were investigated. The results are presented in Fig. 6. It was observed that TG conversion increased from 63.3% to 99.7% and FAME yield increased from 54.7% to 91.4% when reaction temperature increased from 90 to 180 °C. Further increase of temperature from 180 to 200 °C did not considerably change TG conversion, while FAME yield slightly increased from 91.4% to 92.5%. Complete transesterification of triglyceride requires enough time as shown in Fig. 7; TG conversion remarkably increased from 82.9% to 99.7%, and FAME yield from 77.1% to 90.6%, when the reaction time prolonged from 30 to 60 min. Further extension of time did not change either TG conversion or FAME yield. Therefore, 60 min is enough for the transesterification reaction. In summary, considering highest transesterification efficiency (TG Conversion 99% and FAME yield 97.1%) the cinder impregnated with 0.2 M SrCl2 solution calcined at 1000 °C for 4 h was suggested
From economic and industrial point of view, the reusability of catalyst is of great importance. A highly stable catalyst can be several times reused and thus a long catalyst life directly influences the cost of biodiesel production. After one run, without any regeneration, the catalyst was separated from reaction mixture and reused in the next run of transesterification. The results shown in Fig. 8 indicated that TG conversion and FAME yield were above 95.6% and 90.7%, respectively, after 6th cycles. A significant decrease of catalyst activity was detected after 7th repeated use while TG conversion was dropped to 89.0%. Collected catalyst after 7th cycles was again calcinated at 1000 °C for 4 h (reactivation) and reused at same operating conditions. It was found that after such regeneration the catalyst activity was again increased; TG conversion was 94.6% and FAME yield 91.6%, respectively. Further use of the regenerated catalyst gradually reduced TG conversion to 82.9% and FAME yield to 72% from 8th to 14th cycles. Obviously catalyst was slowly deactivated and many reasons could be responsible for such effect. Mostly, it is followed by the change of accessible basic sites at catalyst surface, the loss of active metal precursor or decrease of catalyst surface area. All of these steps lead to decrease of accessible active sites on the catalyst surface and their crystalline morphology [43]. Other reason could be the adsorption of other species and blockage of active sites by impurities from reaction medium (mainly by glycerol). Adsorbed glycerol at active catalyst sites might be easily removed by calcinations as shown with performed experiments of catalyst regeneration after its 7th use. The fact that catalyst activity could be only partly recovered after regeneration indicated that there were also other reasons for deactivation besides surface occupation by organic species. 3.3. Effect of water and free fatty acid (FFA) present in soybean oil As mentioned before, the water and FFA content in oil, negatively effects transesterification performed with homogeneous base catalyst [44]. One of the goals of this study was to show in which
S. Nasreen et al. / Energy Conversion and Management 95 (2015) 272–280
Fig. 9. Effects of FFA on TG conversion and FAME yield. Catalyst preparation conditions: calcination temperature = 1000 °C; calcination time = 4 h; concentration of impregnating SrCl2 solutions = 0.2 M. Reaction conditions: catalyst amount = 4%; reaction temperature = 180 °C; reaction time = 1 h.
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Fig. 11. ICP-AAS of biodiesel for determination of Sr content.
0.5 wt% water. Further gradual decrease of TG conversion and FAME yield was observed for soybean oil containing more water. TG conversion of 64.1% and FAME yield of 54% was determined in the case of soybean oil containing 2.5 wt% of water. These results also indicate that the Sr-cinder catalyst is also sensitive to the presence of water in the oil, which suggest that water must be removed before transesterification. 3.4. Leaching of Sr in biodiesel and glycerol
Fig. 10. Effects of water on TG conversion and FAME yield. Catalyst preparation conditions: calcination temperature = 1000 °C; calcination time = 4 h; concentration of impregnating SrCl2 solutions = 0.2 M. Reaction conditions: catalyst amount = 4%; reaction temperature = 180 °C; reaction time = 1 h.
extent presence of water and FFA in soybean oil influence the rate of biodiesel synthesis with Sr-cinder heterogeneous base catalyst. To investigate the effect of FFA and water on triglycerides conversion, initial soybean oil was prepared by adding 1.0%, 1.5%, 2.0% and 2.5% of oleic acid and water, respectively. As shown in Fig. 9, increased amount of FFA in oil obviously affected the TG conversion and FAME yield. When 1 wt% FFA was added into the oil, TG conversion slightly decreased from 99.7% to 96.6%, however, FAME yield decreased remarkably from 90.6% to 78.9%. Addition of more FFA up to 2.0 wt% further decrease of TG conversion and FAME yield to 73.8% and 63.9% was detected, while sharp decrease was observed by adding 2.5 wt% FFA. These results indicate that the Sr-dopped cinder catalyst is sensitive to presence of larger amount (>2 wt%) of FFA in the oil. Water in soybean oil showed obvious adverse impact on TG conversion and FAME yield (Fig. 10). The TG conversion of the transesterification decreased from 99.7% to 97.6% and FAME yield decreased from 90.6% to 81.1% in the case of soybean oil containing
Stability of catalyst in biodiesel and glycerol is an important data for biodiesel synthesis. The presence of some inorganics in biodiesel can compromise the fuel quality and enhance the emission of pollutants. The leaching of Sr contents for 5 repeated batch reaction products (biodiesel and glycerol) were analyzed using atomic absorption spectrophotometer (AAS) (Fig. 11). It was found that the average Sr content in glycerol phase is 0.058 mg/L while the average Sr content in biodiesel phase is 0.246 mg/L; Sr content is the highest in the reactions product after 1st use of catalyst and gradually began to decrease in corresponding repeated batch cycle products. The Sr content in glycerol was 0.009 mg/L after 5th cycle and 1/3 of the value detected in biodiesel. Taking into account that detection limits of heavy metals from 0.05 to 0.7 mg/L are acceptable (government policy and legislations worldwide), obtained data indicate that collected biodiesel must be treated for Sr removal. However in Packer et al. study permissible limit of Sr in biodiesel synthesized by soybean was 5 mg/L [45]. Compared to that information, presence of Sr in biodiesel of these investigation is much lower which show that Sr-cinder catalyst has better characteristic compared to pure SrO. Furthermore, catalyst prepared in this study has improved surface areas and porosity compared to the unsupported SrO or Sr supported at MgO or Al2O3 [46,18,47]. 3.5. Catalyst characterization To understand the mechanism of catalyst activity and its deactivation, the sample of the fresh catalyst and the catalyst after being reused for 14 cycles were characterized measuring their basicity, BET surface area, as well as performing XRD, SEM-EDS and ICP-OES. 3.5.1. Basic strength Fresh cinder showed weak basicity, its basic strength donated as (H_) was <9.8. However, calcinated SrCl2 could also change the
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Fig. 12. SEM pictures of fresh catalyst (A1 and A2), and the catalyst after being reused for 14 cycles (B1 and B2).
color of phenolphthalein (H_ = 9.8) from colorless to purple, but failed to convert 2,4-dinitroaniline (H_ = 15.0) from yellow to mauve, so the basic strength could be tentatively denoted as 9.8 < H_ < 15.0. However, the basic strength of the fresh catalysts was identified as 15 < H_ < 18.0, because it could change the color of phenolphthalein and 2, 4-dinitroaniline from colorless to purple and from yellow to mauve respectively, but its fail to change the color of nitroaniline. The base strength of fresh catalyst is an important factor on its high activity for transesterification reaction. The basic strength of the catalyst after its reuse for 14th time was detected as H_ < 15.0. Several logical assumptions can be posted to explain the decrease of basic strength. One is that different compounds at catalyst surface and in bulk are formed after repeated use of catalyst. The second one can be attributed to the change of physical properties of available basic sites and change of their physical structure thus influencing Sr-cinder activity. 3.5.2. BET surface area The primary requirement of an ideal solid catalyst for biodiesel synthesis is existence of larger (meso- and macro-) pores that would minimize diffusion limitations of large molecules of acylglycerols with long alkyl chains of fatty acids [36]. According to the multipoint N2 adsorption-desorption result, the BET surface area, the average pore sizes, and pore volume of the fresh catalyst was 0.1480 m2 g 1, 16.90 nm, and 0.0011 cm 3 g 1, respectively. Such small specific surface area might be caused by calcination at high temperature and sintering [48]. Comparing with fresh prepared catalyst, the BET surface areas and pore volume of the catalyst after its use are decreased to even smaller values 0.0198 m2 g 1 and 0.0006 cm 3 g 1, while the average pore size increased to 62.26 nm. Thesse results indicated that some small pores were blocked after use of catalyst, causing decrease of BET surface area and pore volume and loss of basic active sites for transesterification. 3.5.3. Scanning electron microscope (SEM) Cinder after calcination at high temperature was white and yellow with a lot of pores. The SEM results of fresh and used catalysts
Fig. 13. X-ray diffraction patterns of cinder, fresh and used catalyst. SrAl2Si2O8 (N), Sr5Al8O17 ( ), SiO2 (H), Al2O3(k).
are illustrated in Fig. 12. The fresh catalyst appears as filiform. After used, the catalyst morphology hardly changed. This filiform morphology of the catalyst is very different from that of cinder [36], which suggests that cinder react with strontium creating some new compound.
3.5.4. X-ray diffraction (XRD) X-ray diffractions of cinder, fresh and used catalysts were performed to determine the existential state of strontium in the catalysts (Fig. 13). According to the PDF of ICDD the XRD patterns of fresh cinder exhibited the typical characteristic peaks of crystalline phases such as Al2O3 and SiO2 [36]. The fresh catalyst had characteristic diffraction peaks at 2h = 13.62°, 15.29°, 19.36°, 21.61°, 23.69°, 25.14°, 25.95°, 27.25°, 27.67°, 35.14°, which correspond to SrAl2Si2O8 [49,50]. Diffraction peaks at 2h = 32.53°, 41.80°,
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S. Nasreen et al. / Energy Conversion and Management 95 (2015) 272–280 Table 3 Content of different elements for fresh and used catalyst. Catalyst
Sr%
Al%
Si%
Ni%
Na%
Ca%
K%
Mg%
Ti%
Fe%
Other elements
Fresh catalyst Used catalyst
56.59 51.25
14 13.69
16.78 16.52
1.06 1.29
1.03 1.84
5.13 4.89
0.62 0.54
0.91 0.81
0.98 0.91
1.85 1.66
1.05 6.6
44.14°, were attributed to Sr5Al8O17 [51]. The newly formed compounds demonstrated that cinder reacted strongly with Sr at high temperature, and this newly formed SrAl2Si2O8 and Sr5Al8O17 complexes have basic and catalytic activity for transesterification reaction. Similar XRD pattern of used catalyst was determined. This fact indicates that the crystalline form of the catalyst was only slightly changed in the transesterification process. 3.6. Effect of SrO supported on cinder The amounts of strontium loaded on cinder were checked by ICP-OES. The results for the analysis of the fresh and used Sr-cinder catalyst are shown in Table 3. The Strontium, Aluminium and Silicium are the elements of primary interest in this analysis. It can be seen that the amount of Strontium is changed from 56.59% (fresh catalyst) to 51.25% (after 14 time used catalyst). It is decrease of Sr of only 9.4% which is higher to decrease of Al (app. 2.2%) and Si (app. 1.5%). Namely, detected amount of Al was 14% and 13.69% (in fresh and used Sr-cinder catalyst) while detected amount of Si for the same samples of catalyst was 16.78 and 16.52, respectively. This data indicate that presence of SrO at cinder used as support in this study is lower to pure SrO or SrO supported on MgO [18,52,53] 4. Conclusion Experiments performed in this study showed that cinder dopped with strontium exhibited excellent catalytic activity and relatively good reusability in the transesterification reaction of soybean oil with methanol. However, FFA and water content in the oil must be controlled so as to guarantee the long life and good catalyst activity. This catalyst is potential to be used for the continuous production of biodiesel under higher temperature. The main active compounds in the catalyst are SrAl2Si2O8 and Sr5Al8O17, which are responsible for the basic strength and catalytic activity. They are relatively stable in the transesterification process. However, the blockage of the small pores in the catalyst can cause the loss of the active sites and then the deactivation of the catalyst was occured. Acknowledgements This study was supported by the International S&T Cooperation Program of China (Grant No. 2013DFG92250), the ‘‘New Century Excellent Talent of Ministry of Education of China’’ project (Grant No. NCET-09-0713), and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan). References [1] Liu W, Yin P, Zhang J, Tang QH, Qu RJ. Biodiesel production from esterification of free fatty acid over PA/NaY solid catalyst. Energy Convers Manage 2014;82:83–91. [2] Marchetti JM, Errazu AF. Technoeconomic study of supercritical biodiesel production plant. Energy Convers Manage 2008;49:2160–4. [3] Gole VL, Gogate PR. A review on intensification of synthesis of biodiesel from sustainable feed stock using sonochemical reactors. Chem Eng Process 2012;53:1–9.
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Transesterification of soybean oil catalyzed by Sr-doped cinder Sadia Nasreen a, Hui Liu a,⇑, Romana Khan b, Xiao-chan Zhu a, Dejan Skala c a
State Key Laboratory of Biogeology and Environmental Geology and School of Environmental studies, China University of Geosciences, Wuhan 430074, PR China Department of Environmental Sciences, COMSATS Institute of Information Technology, Abbottabad, Pakistan c Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, Belgrade 11000 R, Serbia b
a r t i c l e
i n f o
Article history: Received 7 November 2014 Accepted 2 February 2015 Available online 27 February 2015 Keywords: Biodiesel Strontium Solid base catalyst Subcritical condition Transesterification
a b s t r a c t The Strontium doped cinder was prepared using the wet impregnation method and analyzed as catalyst for biodiesel synthesis. Different procedure for cinder impregnation were investigated (temperature and duration of catalyst calcinations, the concentration of SrCl2 solution) and optimal condition was established: impregnation 20 g of cinder 2–5 mm particles with 0.2 M SrCl2 solution followed by calcinations at 1000 °C for 4 h. The Sr-cinder activity was tested at 90–200 °C using 1–5% mass of catalyst with different molar ratio of methanol to soybean oil (from 9 to 36). The maximum triglycerides (TG) conversion of 99.0% with the Fatty Acid Methyl Esters (FAME) yield of 97.1% was obtained by using 4% catalyst at 180 °C, for 1 h, and methanol/oil molar ratio 24:1. Influence of free fatty acid (FFA) and water in soybean oil on catalyst activity was analyzed, too. The catalyst could be used for 14 times with TG conversion and FAME yield above 90% and 80% respectively. The Sr-doped cinder catalysts before and after transesterification were characterized using BET surface area, basic strength, X-ray diffraction (XRD), scanning electron microscopy (SEM) and ICP-AAS. Results showed that the formation of SrAl2Si2O8 and Sr5Al8O17 complexes should be the main reason for the catalytic activity of prepared catalyst. Slow decrease of catalyst activity during its repeated use is result of Sr–Al-glycerolate formation in reaction between Sr–Al complexes and glycerol. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Increased environmental problem associated with energy consumption and continuous decline of limited crude oil reserves were focused by scientific researchers to find alternative potential sources of substantial energy production [1,2]. As compare to other conventional fuel, diesel fuel is most efficient as transportation energy and most commonly used allover the world. Biodiesel which is a mixture of fatty acid methyl esters (FAME) might be produced from light alcohols (methanol and ethanol) and triglycerides from plant oils or animal fats [3–5]. It is renewable, nontoxic and biodegradable alternative to diesel fuel obtained from fossil fuel [6,7]. Problems associated with use of homogeneous (acid H2SO4, HCl; or base NaOH, KOH) catalyst for biodiesel synthesis [8], like slow process of transesterification (acid catalysis), purification of product and catalyst neutralization (acid and base catalyst), use of raw triglycerides containing water and free fatty acids (base catalysis) are limiting factors for economical industrial ⇑ Corresponding author. Tel.: +86 15927501778; fax: +86 27 87436235. E-mail address: [email protected] (H. Liu). http://dx.doi.org/10.1016/j.enconman.2015.02.006 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.
production of biodiesel [9,10]. Many metal oxides as heterogeneous catalysts were also explored for biodiesel synthesis and some of these catalysts have shown very good catalytic performance [11,12]. Some metal complexes, active metals loaded on supports, zeolites and ezymes were also tested in the recent past [1,13,14]. Currently, majority of heterogeneous catalysts used in producing biodiesel are based on either oxides of alkali or alkaline earth metals as precursors combined with some inert or less active support which usually have a large surface area [15]. Studies in which CaO [16], SrO [17,18], ZnO–La2O3 [19], Znaluminate [20] Mg-hydrocalcite [21,22] and KOH [23,24] were supported and used for biodiesel have shown promising effect [25]. Among the well-known solid base catalysts, SrO has emerged as an attractive one for biodiesel production. SrO as strong base can catalyze not only transesterification of vegetable oil [18,26] but some other chemical reactions such as: oxidative coupling of methane [27], selective oxidation of propane [28], oxidation of vinyl chloride emission [29]. SrO compounds are non-toxic with high basic strength and according to some authors they are not soluble in methanol, vegetable oils or FAME [18,30]. To improve specific surface area and activity of SrO it has been loaded on
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S. Nasreen et al. / Energy Conversion and Management 95 (2015) 272–280 Table 1 Review of recently published research about SrO (pure) or in combination with some other methals or supports. Catalyst
SrO
Sr3Al2O6
Sr/ZrO2
Sr(NO3)2/ZnO
Sr/MgO
CaO loaded with Sr(NO3)2
Hydroxyapatite loaded with Sr(NO3)2
Methanol:oil Reaction temperature (°C) Reaction time (min) Calcination temperature (°C) Catalyst amount (%) Reusability TG conversion (%) Method of analysis
6:1 65 30 700 3 10 95 Diffusion
25:1 60 60 – 1.3 15 95.7 Sol gel
29:1 115.5 90 900 2.7 – 80 Wet impregnation
12:1 65 300 – 5 – 95.7 Co-precipitation
12:1 65 30 600 5 – 93 Wet impregnation
12:1 750 120 750 3.5 3 97.3 Sol gel
9:1 70 300 600C 5.6 1 85 Wet impregnation
various supports such as alumina [30], zinc oxide and zirconia [31–33], hydroxyapatite [18] and results of these investigation is shown in Table 1. Data presented in Table 1 show that high conversion of triglycerides (80-97%) could be obtained using SrO as pure compound or in combination with other supports or metal-oxide. Relative short reaction time (less than 2 h), different molar ratio of methanol and oil (6–29) at low or moderate temperature (65– 115 °C), with 1.3–5 wt% of catalyst were tested. Most of heterogeneous catalysts (Table 1) were applied as powder or even there is no information about the catalyst granulation. If powder must be used than separation of biodiesel and glycerol, after evaporation excess of methanol, could be even more difficult than neutralization and removal some homogeneous catalysts [34,35]. Supported solid base catalysts with well-distributed pores can improve basic strength and reduce the leaching of precursor. In such conditions, utilization of some cheep solid support like cinder can reduce amount of precursor and improve its dispersion onto surface of catalyst giving much better its catalytic behavior. Cinder used in this study as support for SrO is a solid waste from coal burning process. It is usually in the form of fine particles having the meso pore size and relatively small pore volume. It was already shown that cinder supported by K2CO3 has been successfully applied for biodiesel production [36,37]. The goal of this study was also to show that cinder, mainly composed of SiO2, Al2O3 and CaO, even in some cases K2O and MgO [43], can stabilize the precursor (SrO) and improve the catalytic performance in the base-catalyzed reaction of vegetable oil transesterification. Different conditions of Sr-doped cinder catalyst preparation and its testing for transesterification of soybean oil were optimized taking into account catalytic efficiency and stability. Characteristics of fresh and catalyst after repeated use were analyzed measuring the catalyst base strength, the specific surface area (BET), using scanning electron microscopy with analysis of elements composition at catalyst surface (SEM-EDS), powder X-ray Diffraction (XRD). Leaching of active metal (Sr) in biodiesel and glycerol were characterized by ICP-AAS. 2. Experimental 2.1. Materials Raw soybean oil was purchased from Baifu Oils & Fat Co. Limited (Wuhan, China). The physical and chemical properties of the soybean oil are listed in Table 2. HPLC grade methanol was purchased from Tedia Company Inc. (Fairfield, OH, USA) and Mallinckrodt Baker, Inc. (Phillipsburg, New Jersey, USA), respectively. Analytical grade SrCl26H2O was purchased from Aopu Chemical Company (Wuhan, China).
Table 2 Physical-chemical properties of the soybean oil used for the experiment. Parameters
Values
Water content (wt%) Acid value (mgKOH/g) Density (g/cm3) Saponification number Fatty acid components (%) C16:0 C16:1 C18:0 C18:1 C18:2 C18:3 Others
0.12 0.2 0.925 192 16.5 – 2.0 24.8 51.8 2.6 0.6
2.2. Catalyst preparation The Sr-doped cinder catalysts were prepared by wet impregnation method. Cinder with diameter grades of 2.0–5.0 mm were used after drying and sieving. SrCl2 solutions were prepared with various concentrations by dissolving certain amount of SrCl26H2O into deionized water. Typically 20 g of freshly dried cinder was added into 500 mL SrCl2 solutions and left overnight in an oven at 50 °C. After 24 h the excess of water was evaporated by slow heating at 70 °C, and then wet solid was dried overnight in an oven at 50 °C and finally calcined in the muffle furnace at a certain temperature (600–1200 °C) for a certain time (1–5 h). 2.3. Transesterification reaction A 500 mL stainless steel autoclave (PCF0.5-10 JianBang Chemical Machinery Co. Limited, China) with a stirrer, an internal cooling pipe, a temperature sensor inside, and an external heating jacket, was used as batch reactor for transesterification of soybean oil with methanol. The importance of mixing intensity for biodiesel synthesis has been reported quite well in the past. It has more influence in the period of slow rate of methanolysis process which is governed by the rate of triglyceride mass transfer to the catalyst surface. It has been shown, that excessive mixing can deactivate the catalyst and reduce the conversion of triglyceride and yield of FAME as well as the low intensity of mixing which enable only limited contact of reactants influencing decay of TG conversion and FAME yield for certain time of reaction [38,39]. Experiments performed in this study was realized in such way that soybean oil, methanol, and catalyst were put into the reactor and then heated to the pre-set temperature and stirred at a constant speed of 200 rpm. The beginning of methanolysis was counted from the moment when temperature inside reactor was reached the preset reaction temperature. After some reaction time, the reaction mixture was cooled down and transesterification reaction was
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stopped by the flowing water through the internal cooling pipe and then, the autoclave was opened and the reaction mixture poured into a beaker. 2.4. Analysis of triglyceride conversion Triglycerides (TG), Fatty acid methyl ester (FAME) and intermediate products, monoglycerides (MG) and diglycerides (DG), were analyzed using Agilent 1100 Series with a C18 column (40 °C, particle size 5 lm, 200 4.6 mm I.D.) and UV/Visible detection at 205 nm [40]. A linear gradient from 100% methanol to 50% methanol + 50% isopropanol–hexane (5:5, v/v) in 25 min was employed. TG conversion was calculated as the (percent) diminution of the sum of the areas of TG HPLC peaks. The FAME yield was calculated as the fraction (percent) of the sum of the areas of all FAME HPLC peaks relative to that of samples with maximum conversion. 2.5. Catalyst characterization The basic strength of the samples (H_) was determined using Hammett indicator [36]. About 100 mg of the sample was shaken with 5 mL cyclohexane and 1 mL of Hammett indicators-benzene solution (0.1%, w/w) and then left to equilibrate for 2 h when no further color changes were observed. The following Hammett indicators were used and the corresponding H_ values are as follows: 4-nitroaniline (H_ = 18.4), 2, 4-dinitroaniline (H_ = 15.0), phenolphthalein (H_ = 9.8). The basic strength is defined as being stronger than the weakest indicator, which exhibits a color change, and weaker than the strongest indicator that showed no color change. BET surface areas of the samples were determined according to the multipoint N2 adsorption-desorption method using an instrument of SSA-4200 Surface Area & Pore Size Analyzer (Beijing Builder Electronic Technology Co., Ltd., China). Prior to measurements, all the samples were out gassed overnight under vacuum at 373 K. XRD was performed on a D/MAX-RB powder X-ray diffractometer (Rigaku Corporation, Japan) at room temperature. Cu Ka radiation (k = 0.15418 nm), with a step size of 0.02° in the 2h range from 5° to 65°, was used in all the samples. The data were processed with the X’Pert HighScore Plus software. The peaks were identified using the Powder Diffraction File (PDF) database created by International Centre for Diffraction Data (ICDD). The sample morphology was characterized at room temperature by a Quanta 200 SEM system (FEI Company, Netherlands).
reaction. Fig. 1 shows that the calcined cinder resulted in only 59.1% TG conversion and 36.9% FAME yield, while calcined SrCl2 resulted in 89.7% TG conversion and 73.3% FAME yield, respectively. However, TG conversion and FAME yield were dramatically enhanced to 93.5–99.8% and 89.4–94.4%, respectively, by impregnating cinder into increasing concentration of SrCl2 solutions. The highest transesterification efficiency was obtained for the catalyst prepared by using 0.2 M SrCl2, further increase of SrCl2 only slightly increase its activity. 3.1.2. Effect of temperature and duration of calcination In order to determine the optimum calcination conditions a series of catalysts with various calcination temperatures (600–1200 °C) and duration (1–5 h) were prepared and applied for transesterification of soybean oil with methanol. Fig. 2 show that TG conversion and FAME yield increase to 99.1% and 96.6%, respectively, when the catalyst was calcined at 1000 °C. A higher calcination temperature did not increase TG conversion. Thus, taking into account the energy consumption for calcinations process, 1000 °C was selected as appropriate one. Enough duration of calcination is usually required to obtain a complete decomposition of metal precursors and its oxidation [41]. In order to investigate the effect of calcination duration on TG conversion and FAME yield a series of catalysts calcined for 1–5 h at 1000 °C were prepared and used for heterogeneous transesterification reaction. Fig. 3 showed that TG conversion increased from 88.5% to 99.0% and FAME yield increased from 73.0% to 94.4%, respectively, when the time of catalyst calcination extended from 1 to 4 h. Further increase time of calcination did not show that a higher TG conversion could be obtained. In consideration of energy consumption, the following condition for catalyst preparation was established: calcination at 1000 °C for 4 h. 3.1.3. Effect of catalyst amount Catalyst amount is an important parameter which mainly affects the conversion of triglycerides to methyl esters. Generally, catalysts supported by natural material need a higher amount to gain satisfactory efficiency because the presence of precursor is relatively less compared to pure active metal oxide [41]. As shown in Fig. 4, the TG conversion increased with increasing the catalyst
3. Results and discussion 3.1. Effects of catalyst preparation and transesterification reaction conditions on TG conversion and FAME yield TG conversion and FAME yield were investigated with a goal to find optimal procedure for catalyst preparation (concentration of impregnating SrCl2 solution, duration and temperature of catalysts calcination), and transesterification reaction conditions (catalyst amount used for transesterification, methanol to oil molar ratio, transesterification reaction temperature and time). 3.1.1. Effect of SrCl2 impregnating solution concentration In order to study the effect of the concentration of SrCl2 impregnating solution on catalytic activity, a series of catalysts with fixed amount of cinder (20 g) impregnated in 500 mL SrCl2 solutions with various concentrations from 0.1 to 0.3 M were prepared and calcined at 1000 °C for 4 h. The fresh cinder and SrCl2 were also calcined at the same conditions. The catalysts, together with calcined cinder and SrCl2 were employed to catalyze the transesterification
Fig. 1. TG conversion and FAME yield of the transesterification reaction catalyzed by catalysts prepared in impregnating SrCl2 solutions with concentration of 0.1 M (catalyst 1), 0.2 M (catalyst 2), and 0.3 M (catalyst 3), and calcined cinder and SrCl2. Catalyst preparation conditions: calcination temperature = 1200 °C; calcination time = 5 h. Reaction conditions: catalyst amount = 5%; molar ratio of methanol to oil = 36:1; reaction temperature = 200 °C; reaction time = 1 h.
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Fig. 2. Effects of calcination temperature on TG conversion and FAME yield. Catalyst preparation conditions: calcination time = 5 h; concentration of impregnating SrCl2 solutions = 0.2 M. Reaction conditions: catalyst amount = 5%; molar ratio of methanol to oil = 36:1; reaction temperature = 200 °C; reaction time = 1 h.
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Fig. 4. Effects of catalyst amount on TG conversion. Catalyst preparation conditions: calcination temperature = 1000 °C; calcination time = 4 h; concentration of impregnating SrCl2 solutions = 0.2 M. Reaction conditions: molar ratio of methanol to oil = 36:1; reaction temperature = 200 °C; reaction time = 1 h.
Fig. 3. Effects of calcination time on TG conversion. Catalyst preparation conditions: calcination temperature = 1000 °C/4 h; concentration of impregnating SrCl2 solutions = 0.2 M. Reaction conditions: catalyst amount = 5%; molar ratio of methanol to oil = 36:1; reaction temperature = 200 °C; reaction time = 1 h.
Fig. 5. Effects of methanol/oil molar ratio on TG conversion and FAME yield. Catalyst preparation conditions: calcination temperature = 1000 °C; calcination time = 4 h; concentration of impregnating SrCl2 solutions = 0.2 M. Reaction conditions: catalyst amount = 4%; reaction temperature = 200 °C; reaction time = 1 h.
amount from 1% to 4%. Only a little bit higher TG conversion was detected if 5% of catalyst was used for biodiesel synthesis. FAME yield of 94.4% was obtained when 4% catalyst is used. Disadvantage of increased amount of catalyst for heterogeneous biodiesel synthesis is caused by higher biodiesel production cost. Further reaction tests were performed using 4% catalyst.
unused alcohol recovery [42]. As shown in the Fig. 5, TG conversion and FAME yield considerably increased with increase of methanol/ oil molar ratio from 9:1 to 18:1; TG conversion and FAME yield was 98.8% and 87.7%, respectively, when 18:1 molar ratio of methanol/ oil was used in the transesterification reaction (Fig. 5). Increase of methanol/oil molar ratio to 24:1 and 30:1 showed a little effect on TG conversion but had considerably effects on FAME yield (increased from 88.4% to 91.6%). Further increase of methanol/oil molar ratio more than 30:1 showed very limited effect on TG conversion and FAME yield.
3.1.4. Effect of methanol to oil molar ratio Amount of methanol and its molar ratio to soybean oil is one of the most important factors influencing the TG conversion. In order to suppress the reversible reaction between glycerol and FAME, which leads to lower yield of FAME, a higher molar ratio of methanol and oil is usually required. Unfortunately, increasing the alcohol amount in initial reaction mixture will increase the cost of
3.1.5. Effect of transesterification reaction temperature and time Temperature has direct influence on reaction rate and thus on duration of transesterification and overall energy consumption.
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Fig. 6. Effects of transesterification reaction temperature on TG conversion and FAME yield. Catalyst preparation conditions: calcination temperature = 1000 °C; calcination time = 4 h; concentration of impregnating SrCl2 solutions = 0.2 M. Reaction conditions: catalyst amount = 4%; reaction time = 1 h.
Fig. 8. Effects of repeated use of the catalyst on TG conversion and FAME yield. Catalyst preparation conditions: calcination temperature = 1000 °C; calcination time = 4 h; concentration of impregnating SrCl2 solutions = 0.2 M. Reaction conditions: catalyst amount = 4%; reaction temperature = 180 °C; reaction time = 1 h.
to catalyze transesterification reaction, using methanol:oil molar ratio of 24:1 and 4% catalyst amount, at 180 °C for 1 h. 3.2. Reusability of the Sr-dopped cinder catalyst
Fig. 7. Effects of reaction time on TG conversion and FAME yield. Catalyst preparation conditions: calcination temperature = 1000 °C; calcination time = 4 h; concentration of impregnating SrCl2 solutions = 0.2 M. Reaction conditions: catalyst amount = 4%; reaction temperature = 180 °C.
The effects of reaction temperature from 90 to 200 °C on transesterification efficiency were investigated. The results are presented in Fig. 6. It was observed that TG conversion increased from 63.3% to 99.7% and FAME yield increased from 54.7% to 91.4% when reaction temperature increased from 90 to 180 °C. Further increase of temperature from 180 to 200 °C did not considerably change TG conversion, while FAME yield slightly increased from 91.4% to 92.5%. Complete transesterification of triglyceride requires enough time as shown in Fig. 7; TG conversion remarkably increased from 82.9% to 99.7%, and FAME yield from 77.1% to 90.6%, when the reaction time prolonged from 30 to 60 min. Further extension of time did not change either TG conversion or FAME yield. Therefore, 60 min is enough for the transesterification reaction. In summary, considering highest transesterification efficiency (TG Conversion 99% and FAME yield 97.1%) the cinder impregnated with 0.2 M SrCl2 solution calcined at 1000 °C for 4 h was suggested
From economic and industrial point of view, the reusability of catalyst is of great importance. A highly stable catalyst can be several times reused and thus a long catalyst life directly influences the cost of biodiesel production. After one run, without any regeneration, the catalyst was separated from reaction mixture and reused in the next run of transesterification. The results shown in Fig. 8 indicated that TG conversion and FAME yield were above 95.6% and 90.7%, respectively, after 6th cycles. A significant decrease of catalyst activity was detected after 7th repeated use while TG conversion was dropped to 89.0%. Collected catalyst after 7th cycles was again calcinated at 1000 °C for 4 h (reactivation) and reused at same operating conditions. It was found that after such regeneration the catalyst activity was again increased; TG conversion was 94.6% and FAME yield 91.6%, respectively. Further use of the regenerated catalyst gradually reduced TG conversion to 82.9% and FAME yield to 72% from 8th to 14th cycles. Obviously catalyst was slowly deactivated and many reasons could be responsible for such effect. Mostly, it is followed by the change of accessible basic sites at catalyst surface, the loss of active metal precursor or decrease of catalyst surface area. All of these steps lead to decrease of accessible active sites on the catalyst surface and their crystalline morphology [43]. Other reason could be the adsorption of other species and blockage of active sites by impurities from reaction medium (mainly by glycerol). Adsorbed glycerol at active catalyst sites might be easily removed by calcinations as shown with performed experiments of catalyst regeneration after its 7th use. The fact that catalyst activity could be only partly recovered after regeneration indicated that there were also other reasons for deactivation besides surface occupation by organic species. 3.3. Effect of water and free fatty acid (FFA) present in soybean oil As mentioned before, the water and FFA content in oil, negatively effects transesterification performed with homogeneous base catalyst [44]. One of the goals of this study was to show in which
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Fig. 9. Effects of FFA on TG conversion and FAME yield. Catalyst preparation conditions: calcination temperature = 1000 °C; calcination time = 4 h; concentration of impregnating SrCl2 solutions = 0.2 M. Reaction conditions: catalyst amount = 4%; reaction temperature = 180 °C; reaction time = 1 h.
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Fig. 11. ICP-AAS of biodiesel for determination of Sr content.
0.5 wt% water. Further gradual decrease of TG conversion and FAME yield was observed for soybean oil containing more water. TG conversion of 64.1% and FAME yield of 54% was determined in the case of soybean oil containing 2.5 wt% of water. These results also indicate that the Sr-cinder catalyst is also sensitive to the presence of water in the oil, which suggest that water must be removed before transesterification. 3.4. Leaching of Sr in biodiesel and glycerol
Fig. 10. Effects of water on TG conversion and FAME yield. Catalyst preparation conditions: calcination temperature = 1000 °C; calcination time = 4 h; concentration of impregnating SrCl2 solutions = 0.2 M. Reaction conditions: catalyst amount = 4%; reaction temperature = 180 °C; reaction time = 1 h.
extent presence of water and FFA in soybean oil influence the rate of biodiesel synthesis with Sr-cinder heterogeneous base catalyst. To investigate the effect of FFA and water on triglycerides conversion, initial soybean oil was prepared by adding 1.0%, 1.5%, 2.0% and 2.5% of oleic acid and water, respectively. As shown in Fig. 9, increased amount of FFA in oil obviously affected the TG conversion and FAME yield. When 1 wt% FFA was added into the oil, TG conversion slightly decreased from 99.7% to 96.6%, however, FAME yield decreased remarkably from 90.6% to 78.9%. Addition of more FFA up to 2.0 wt% further decrease of TG conversion and FAME yield to 73.8% and 63.9% was detected, while sharp decrease was observed by adding 2.5 wt% FFA. These results indicate that the Sr-dopped cinder catalyst is sensitive to presence of larger amount (>2 wt%) of FFA in the oil. Water in soybean oil showed obvious adverse impact on TG conversion and FAME yield (Fig. 10). The TG conversion of the transesterification decreased from 99.7% to 97.6% and FAME yield decreased from 90.6% to 81.1% in the case of soybean oil containing
Stability of catalyst in biodiesel and glycerol is an important data for biodiesel synthesis. The presence of some inorganics in biodiesel can compromise the fuel quality and enhance the emission of pollutants. The leaching of Sr contents for 5 repeated batch reaction products (biodiesel and glycerol) were analyzed using atomic absorption spectrophotometer (AAS) (Fig. 11). It was found that the average Sr content in glycerol phase is 0.058 mg/L while the average Sr content in biodiesel phase is 0.246 mg/L; Sr content is the highest in the reactions product after 1st use of catalyst and gradually began to decrease in corresponding repeated batch cycle products. The Sr content in glycerol was 0.009 mg/L after 5th cycle and 1/3 of the value detected in biodiesel. Taking into account that detection limits of heavy metals from 0.05 to 0.7 mg/L are acceptable (government policy and legislations worldwide), obtained data indicate that collected biodiesel must be treated for Sr removal. However in Packer et al. study permissible limit of Sr in biodiesel synthesized by soybean was 5 mg/L [45]. Compared to that information, presence of Sr in biodiesel of these investigation is much lower which show that Sr-cinder catalyst has better characteristic compared to pure SrO. Furthermore, catalyst prepared in this study has improved surface areas and porosity compared to the unsupported SrO or Sr supported at MgO or Al2O3 [46,18,47]. 3.5. Catalyst characterization To understand the mechanism of catalyst activity and its deactivation, the sample of the fresh catalyst and the catalyst after being reused for 14 cycles were characterized measuring their basicity, BET surface area, as well as performing XRD, SEM-EDS and ICP-OES. 3.5.1. Basic strength Fresh cinder showed weak basicity, its basic strength donated as (H_) was <9.8. However, calcinated SrCl2 could also change the
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Fig. 12. SEM pictures of fresh catalyst (A1 and A2), and the catalyst after being reused for 14 cycles (B1 and B2).
color of phenolphthalein (H_ = 9.8) from colorless to purple, but failed to convert 2,4-dinitroaniline (H_ = 15.0) from yellow to mauve, so the basic strength could be tentatively denoted as 9.8 < H_ < 15.0. However, the basic strength of the fresh catalysts was identified as 15 < H_ < 18.0, because it could change the color of phenolphthalein and 2, 4-dinitroaniline from colorless to purple and from yellow to mauve respectively, but its fail to change the color of nitroaniline. The base strength of fresh catalyst is an important factor on its high activity for transesterification reaction. The basic strength of the catalyst after its reuse for 14th time was detected as H_ < 15.0. Several logical assumptions can be posted to explain the decrease of basic strength. One is that different compounds at catalyst surface and in bulk are formed after repeated use of catalyst. The second one can be attributed to the change of physical properties of available basic sites and change of their physical structure thus influencing Sr-cinder activity. 3.5.2. BET surface area The primary requirement of an ideal solid catalyst for biodiesel synthesis is existence of larger (meso- and macro-) pores that would minimize diffusion limitations of large molecules of acylglycerols with long alkyl chains of fatty acids [36]. According to the multipoint N2 adsorption-desorption result, the BET surface area, the average pore sizes, and pore volume of the fresh catalyst was 0.1480 m2 g 1, 16.90 nm, and 0.0011 cm 3 g 1, respectively. Such small specific surface area might be caused by calcination at high temperature and sintering [48]. Comparing with fresh prepared catalyst, the BET surface areas and pore volume of the catalyst after its use are decreased to even smaller values 0.0198 m2 g 1 and 0.0006 cm 3 g 1, while the average pore size increased to 62.26 nm. Thesse results indicated that some small pores were blocked after use of catalyst, causing decrease of BET surface area and pore volume and loss of basic active sites for transesterification. 3.5.3. Scanning electron microscope (SEM) Cinder after calcination at high temperature was white and yellow with a lot of pores. The SEM results of fresh and used catalysts
Fig. 13. X-ray diffraction patterns of cinder, fresh and used catalyst. SrAl2Si2O8 (N), Sr5Al8O17 ( ), SiO2 (H), Al2O3(k).
are illustrated in Fig. 12. The fresh catalyst appears as filiform. After used, the catalyst morphology hardly changed. This filiform morphology of the catalyst is very different from that of cinder [36], which suggests that cinder react with strontium creating some new compound.
3.5.4. X-ray diffraction (XRD) X-ray diffractions of cinder, fresh and used catalysts were performed to determine the existential state of strontium in the catalysts (Fig. 13). According to the PDF of ICDD the XRD patterns of fresh cinder exhibited the typical characteristic peaks of crystalline phases such as Al2O3 and SiO2 [36]. The fresh catalyst had characteristic diffraction peaks at 2h = 13.62°, 15.29°, 19.36°, 21.61°, 23.69°, 25.14°, 25.95°, 27.25°, 27.67°, 35.14°, which correspond to SrAl2Si2O8 [49,50]. Diffraction peaks at 2h = 32.53°, 41.80°,
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S. Nasreen et al. / Energy Conversion and Management 95 (2015) 272–280 Table 3 Content of different elements for fresh and used catalyst. Catalyst
Sr%
Al%
Si%
Ni%
Na%
Ca%
K%
Mg%
Ti%
Fe%
Other elements
Fresh catalyst Used catalyst
56.59 51.25
14 13.69
16.78 16.52
1.06 1.29
1.03 1.84
5.13 4.89
0.62 0.54
0.91 0.81
0.98 0.91
1.85 1.66
1.05 6.6
44.14°, were attributed to Sr5Al8O17 [51]. The newly formed compounds demonstrated that cinder reacted strongly with Sr at high temperature, and this newly formed SrAl2Si2O8 and Sr5Al8O17 complexes have basic and catalytic activity for transesterification reaction. Similar XRD pattern of used catalyst was determined. This fact indicates that the crystalline form of the catalyst was only slightly changed in the transesterification process. 3.6. Effect of SrO supported on cinder The amounts of strontium loaded on cinder were checked by ICP-OES. The results for the analysis of the fresh and used Sr-cinder catalyst are shown in Table 3. The Strontium, Aluminium and Silicium are the elements of primary interest in this analysis. It can be seen that the amount of Strontium is changed from 56.59% (fresh catalyst) to 51.25% (after 14 time used catalyst). It is decrease of Sr of only 9.4% which is higher to decrease of Al (app. 2.2%) and Si (app. 1.5%). Namely, detected amount of Al was 14% and 13.69% (in fresh and used Sr-cinder catalyst) while detected amount of Si for the same samples of catalyst was 16.78 and 16.52, respectively. This data indicate that presence of SrO at cinder used as support in this study is lower to pure SrO or SrO supported on MgO [18,52,53] 4. Conclusion Experiments performed in this study showed that cinder dopped with strontium exhibited excellent catalytic activity and relatively good reusability in the transesterification reaction of soybean oil with methanol. However, FFA and water content in the oil must be controlled so as to guarantee the long life and good catalyst activity. This catalyst is potential to be used for the continuous production of biodiesel under higher temperature. The main active compounds in the catalyst are SrAl2Si2O8 and Sr5Al8O17, which are responsible for the basic strength and catalytic activity. They are relatively stable in the transesterification process. However, the blockage of the small pores in the catalyst can cause the loss of the active sites and then the deactivation of the catalyst was occured. Acknowledgements This study was supported by the International S&T Cooperation Program of China (Grant No. 2013DFG92250), the ‘‘New Century Excellent Talent of Ministry of Education of China’’ project (Grant No. NCET-09-0713), and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan). References [1] Liu W, Yin P, Zhang J, Tang QH, Qu RJ. Biodiesel production from esterification of free fatty acid over PA/NaY solid catalyst. Energy Convers Manage 2014;82:83–91. [2] Marchetti JM, Errazu AF. Technoeconomic study of supercritical biodiesel production plant. Energy Convers Manage 2008;49:2160–4. [3] Gole VL, Gogate PR. A review on intensification of synthesis of biodiesel from sustainable feed stock using sonochemical reactors. Chem Eng Process 2012;53:1–9.
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