Article pubs.acs.org/EF
Preparation of Novel Ethylene Glycol Monomethyl Ether Fatty Acid Monoester Biodiesel Using Calcined Sodium Silicate Fenglan Fan,† Lihua Jia,*,† Xiangfeng Guo,*,†,‡ Xiaopeng Lu,‡ and Juan Chen† †
College of Chemistry and Chemical Engineering, Qiqihar University, Qiqihar 161006, People’s Republic of China Key Laboratory of Fine Chemicals, College of Heilongjiang Province, Qiqihar University, Qiqihar 161006, People’s Republic of China
‡
ABSTRACT: A series of ethylene glycol monomethyl ether (EGME) fatty acid monoesters were prepared by the transesterification of different fatty acid methyl esters (FAMEs) with EGME. A solid basic catalyst, namely, calcined sodium silicate, was used. Various parameters, such as the calcination temperature, amount of catalyst, molar ratio of FAME/EGME, reaction temperature, and time on the yield of EGMEML (ML = methyl laurate), were optimized; the reusability of calcined sodium silicate was also examined. Calcined sodium silicate was also used as a catalyst for examining the catalytic activity of soybean oil biodiesel with EGME. Thermogravimetry, X-ray diffraction, carbon dioxide temperature-programmed desorption, Fourier transform infrared spectroscopy, and scanning electron microscopy were employed to characterize the properties of calcined sodium silicate. The results indicated that calcined sodium silicate was effective for the synthesis of novel biodiesel by FAMEs, with EGME as reactants. Centrifugation and decantation were used to separate the solid basic catalyst from the reaction system easily. The separated catalyst can be directly used in the next round of reactions for at least 3 cycles and gave a satisfied yield. A maximum yield of EGME fatty acid monoester of above 90.0% was obtained under the optimal reaction conditions. Furthermore, the reaction kinetic of the transesterification of ML with EGME was investigated. It revealed that the reaction follows second-order kinetics; the activation energy Ea and pre-exponential factor A were 50.05 kJ mol−1 and 1.07 × 104 L min−1 mol−1 by the calculation, respectively. Koros−Nowak tests were designed and conducted, and it was proven that the heat and mass transfer were not limited by the reaction rate.
1. INTRODUCTION Global energy demands are increasing as a result of population growth, industrial and agricultural development, and increased transportation use.1 It is a fact that petroleum fuel resources are being depleted; more attention has therefore been paid to solve the problem of the fuel shortages. Developing alternative renewable energy, such as biosource, wind, solar energy, etc., has become a common view all over the world.2 Biodiesel has similar physical properties to those of diesel fuels; therefore, biodiesel has been recognized worldwide as major renewable energy source for supplementing declining fossil fuel resources.3 A currently used biodiesel, namely, fatty acid methyl esters (FAMEs), is obtained from the reaction of methanol and triglycerides with catalysts.4 Many studies show that the use of such biodiesel containing certain amounts of oxygen atoms can significantly reduce emissions of volatile organic compounds, particulate matter, SO2, CO, and unburned hydrocarbons (HCs) compared to commercially available diesel.5,6 Such biodiesel is therefore considered to be green fuel for diesel engines.7 However, traditional biodiesel contains only one ester group, and one ester only contains two oxygen atoms; the oxygen content for common biodiesel is quite low.8 To fully evaluate the potential of biodiesel as an alternative fuel, the introduction of ether groups into the molecules has been attempted. Gao et al.8 recorded a synthetic method of novel biodiesel, ethylene glycol n-propyl ether palm oil monoester (EGPEPOM), in which sodium was used as a catalyst to accelerate the transesterification of refined palm oil and ethylene glycol n-propyl ether. Using the EGMEPOM fuel, © 2013 American Chemical Society
the start of combustion was higher and the peak cylinder pressure was lower than that of the traditional diesel fuels. The brake thermal efficiency of EGPEPOM was also changed positively. The emissions of smoke, brake-specific CO, HC, and NOx were decreased by 37.5, 66.6, 27.1, and 23.7%, respectively. Jiang et al.9 revealed a novel biodiesel of rapeseed oil monoester of ethylene glycol monomethyl ether (EGME), using a commercial refined rapeseed oil and EGME as raw materials, with 0.6% KOH as a catalyst. The crude products had to be neutralized with dilute HCl solution. The cetane number of rapeseed oil monoester of EGME is much higher, and it owns a shorter ignition delay compared to traditional biodiesel. When a diesel engine is pumped with EGME biodiesel as fuel instead of 0 diesel, the smoke and CO emissions of engine-out decreased by 25.0−75.0 and 50.0%, respectively, and unburned HC emissions are reduced obviously. Li et al.10 reported a synthetic route for a novel biodiesel consisting of EGME cottonseed oil monoester (EGMECOM) by transesterification of refined cottonseed oil with EGME as raw materials, using 1 vol % KOH as a catalyst. The results showed that EGMECOM has a comparatively high oxygen content and high cetane number. The high oxygen content has positive effects on the reduction of both CO and HC emissions. In comparison to diesel fuel, the smoke, NOx, CO, and HC emissions were reduced by a maximum of 50.0, 50.0, 20.0, and 55.6%, respectively, using EGMECOM. A series of tests has proven Received: March 28, 2013 Published: August 1, 2013 5215
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were also characterized by TG/differential thermogravimetric (DTG) analysis using a Mettler TC-10 thermobalance (Diamond TG/DTG, Perkin-Elmer), which was conducted in an air atmosphere, and a heating rate of 20 °C/min was chosen to increase the temperature from room temperature to 1000 °C. Powder XRD patterns were recorded on a Bruker D8 Advance (Germany) diffractometer, using Cu Kα radiation (λ = 1.5418 Å) at 40 kV and 50 mA. The peaks were identified using the Powder Diffraction File database created by International Centre for Diffraction Data. SEM images were recorded using a S-3400 microscope (Hitachi). The accelerating voltage was 20 kV. 2.3. Reaction Procedures. The reaction equation is shown in Scheme 1. Desired amounts of FAME, EGME, and catalyst (on the
that the oxygen content of the fuel affected the reduction in engine-out smoke emissions. The above studies suggest that biodiesel can be further developed by introducing ether groups to enhance their oxygen content to promote the biodiesel performance. The main catalysts used can be classified on the basis of their form in the transesterification reaction, i.e., homogeneous or heterogeneous catalyst.11 There are technological problems with homogeneous basic catalytic systems, such as the production of caustic wastewater, which is a serious environmental threat, and difficulties in removing the catalyst after reaction.12,13 In general, heterogeneously catalyzed production processes have fewer unit operations, with no neutralization process required.11,14 Moreover, the simple methods of filtration, decantation, or centrifugation can be easily used to separate the solid catalyst from the reaction system and then used in the next round.15 As is well-known to all, solid base as one kind of environmentally benign catalyst used in transesterification has become an important alternative promising route. Various solid bases that have already been tested for biodiesel synthesis have been reviewed in a number of papers.11,16 Although numerous solid basic catalysts have been found to be very efficient for biodiesel synthesis, their use as catalysts for the synthesis of novel biodiesel has received little attention until now. Sodium silicate is an important fine chemicals and is used in a wide range of applications, such as foam ceramics,17 concrete,18 synthesis of mesoporous silica materials,19 and oil recovery.20 Recently, it has also been used as a solid basic material and proven to be an effective heterogeneous basic catalyst in biodiesel production.21,22 In this study, calcined sodium silicate is used as a catalyst in the synthesis of a series of EGME fatty acid monoesters by transesterification of FAME with EGME. The crude products do not need to be neutralized and are easily separated from the liquid products. An EGME biodiesel was also synthesized. This type of biodiesel has one ester and one ether group compared to the traditional biodiesel. The catalyst characterizations were probed using thermogravimetry (TG), carbon dioxide temperature-programmed desorption (CO2-TPD), X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR). The reaction kinetics was performed simultaneously. The processing parameters of the reaction, such as the calcination temperature of sodium silicate, mass amount of catalyst, reaction temperature, molar ratio of FAME/EGME, and reaction time, were optimized. Recycling of the calcinated sodium silicate was also conducted.
Scheme 1. Transesterification Equation of FAME with EGME
basis of the weight of FAME) were put into a glass flask with three necks, which is equipped with a thermometer and water-cooled condenser. Then, nitrogen was introduced into the system, and the nitrogen atmosphere was kept until the end of the reaction. Then, the reactants were stirred, and the system was raised to the temperature for a certain time. After the reaction ended, the reaction mixture was centrifuged to separate the catalyst for reuse. The separated crude products were purified in a vacuum to remove excess EGME and methanol remaining in the ester phase. The residue was analyzed by gas chromatography (GC) using a GC9800(N) furnished with a flame ionization detector and fitted with a OV-17 (30 m × 0.25 mm × 0.25 μm) capillary column; the temperatures of the oven, injector, and detector were 250, 280, and 250 °C, respectively. The products were then analyzed by GC based on the peak area normalization method. The yield (Y) and turnover frequency (TOF) were calculated by the following equations: mw Y = 1 × 100% m2 where m1 is the product actual mass (g), m2 is the theoretical calculated mass of the target product (g), and w is the mass concentration of the target product determined by GC
TOF =
mol actual mcatfm t
where molactual is the mole amount of target product, mcat is the mass amount of the catalyst (g), t is the desired reaction time (min), and f m is the amount of basic sites of the calcinated sodium silicate (mmol/g).
3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. Thermal analysis of the catalyst was performed to gain information about the nature of the sodium silicate. There were two weight-loss steps in Figure 1. The first weight-loss step appeared in the range from 30 to 170 °C and is mostly attributed to the loss of adsorbed water (about 36 wt %). There was no significant weight loss shown in the TG curve above 170 °C, and the weight loss was less than 10%. This is ascribed to the loss of water trapped in the framework and residual water of crystallization. The results indicated that the high-temperature calcination did not lead to decomposition, crystallization, and melting of the sodium silicate. Figure 2 shows the CO2-TPD profiles of the sodium silicates, which were calcinated at different temperatures. There were three desorption peaks in Figure 2. They appeared at about 200 °C, from 400 to 600 °C, and above 700 °C, which were ascribed to the interactions between CO2 and the weak basic
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Commercially available sodium silicate hydrate was first dehydrated at 200 °C for 15−20 min, then calcined at 200−500 °C for 2 h, and ground into a fine powder carefully. After that, the catalyst was sealed in a dryer for use. 2.2. Catalyst Characterization. The basicity of the catalysts was studied with CO2-TPD. The experiments were performed using a Quantachrome CHEMBET-3000 apparatus equipped with a thermal conductivity detector.23 First, the calcined sodium silicate sample of 0.010 g was put into a quartz tube, with He of 100 mL/min used to sweep foreign matters of the catalyst at 300 °C for 60 min, and then the sample was cooled to 100 °C. Second, 100 mL/min He + 5 vol % CO2 was introduced into the system for 60 min. Third, physisorbed CO2 was desorption from the catalyst by flushing it at 100 °C in He of 90 mL/min. Finally, a rate of 10 °C/min was adopted for removed of chemisorbed CO2 from the catalyst from 100 to 820 °C. The catalysts 5216
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Figure 3. XRD patterns of (A) sodium silicate hydrate and (B) sodium silicate calcined at 300 °C for 2 h.
Figure 1. TG/DSC curves of sodium silicate.
(Figure 3B), the characteristic diffraction peaks of anhydrous Na2SiO3 appeared at 2θ angles of 16.8°, 25.0°, 29.4°, 34.9°, 37.2°, 48.1°, 52.0°, 64.2°, and 65.8° (JCPDS file number 00016-0818). The reason for this difference is that the combined water of sodium silicate was mostly eliminated. The results showed that the thermal treatment does not only remove the structural water molecules and enhance the crystallization but also promotes the phase transform from hydrated sodium silicates to anhydrous silicates. Infrared (IR) spectroscopy was used to investigate the effect of the thermal treatment on the structure of sodium silicate. An absorption band at ∼3300 cm−1 was observed from Figure 4A, Figure 2. CO2-TPD profiles of the sodium silicate calcinated at (A) 200 °C, (B) 300 °C, (C) 400 °C, and (D) 500 °C.
site, moderate basic site, and super basic site,23 respectively. Here, the strongest peak presented at above 700 °C; therefore, the super basic sites were the main contribution for basicity. The peaks could be attributed to the decomposition of structural carbonate.24 As may be seen from Table 1, the basicity and increase of the thermal treatment temperature of the catalyst show a typical Table 1. Data of the Basicity versus Calcinated Temperatures of Sodium Silicate calcined temperature (°C)
basicity (mmol of CO2/g)
200 300 400 500
0.316 0.767 0.794 0.509
Figure 4. IR spectra of (A) sodium silicate hydrate and (B) calcined sodium silicate at 300 °C for 2 h.
which was ascribed to the OH stretching vibration in hydroxyl groups.26 The bands at ∼465, 530, 841, and 890 cm−1 are ascribed to the different forms of the stretching vibration of O− Si−O.22,27 The absorption bands at around 714 and 990 cm−1 could be attributed to the stretching of Si−O and Si−O−Na, respectively.22,28 After calcination at 400 °C for 2 h (Figure 4B), the intensity of the absorption band at 3300 cm−1 decreased; it was believed that the thermal treatment removed most of the water adsorbed on the catalyst surface, resulting in a decrease of the intensity of the absorption peak. Furthermore, after the thermal treatment, the reduction of the intensity of the Si−O−Na stretching peak of ∼990 cm−1 was observed, the peak underwent a shift to lower frequency, and a new Si−O−Si stretching band of ∼890 cm−1 appeared, which imply that tetrahedral SiO44− was altered from a low-polymerization state
volcano-shape character. It was in line with the study by Xie et al.,25 in which potassium loaded on alumina was used as a catalyst for the synthesis of common biodiesel. The total basicity of the catalyst treated at 300 °C for 2 h is the highest. However, the basicity did not vary much with an increasing thermal treatment temperature from 300 to 400 °C. Considering the energy savings and cost of preparation, we choose the catalyst calcined at 300 °C for the following work. Figure 3 presents the XRD patterns of sodium silicate hydrate and the thermal-treated sodium silicate. The noncalcined sample exhibited the Na2SiO3·5H2O phase [Joint Committee on Powder Diffraction Standards (JCPDS) file number 00-016-0839]. After calcination at 300 °C for 2 h 5217
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Figure 5. SEM images of (A) sodium silicate hydrate and (B) calcined sodium silicate at 300 °C for 2 h.
changes of the catalytic activity are in agreement with the change of basicity with the increase of the calcination temperature. As shown in Figure 7A, the yield of EGMEML increased with an increasing amount of catalyst. The yield of 91% was found when the catalyst amount of 5.0 wt %, which was based on the mass of ML, was added. With the further increase of the catalyst amount, the yield increased slightly. From the stoichiometric equation, it is obvious that the ratio of 1 mol of EGME and 1 mol of ML would yield 1 mol of EGMEML and 1 mol of methanol. It is well-known that transesterification is a reversible reaction and a higher molar ratio of EGME/ML than the stoichiometric value of 1:1 would benefit the progress of the positive reaction. As shown in Figure 7B, the yield increased from 70.6 to a maximum of 91.0% when the molar ratio of ML/ EGME was increased from 1:1 to 1:2. However, the yield did not vary much with an increasing molar ratio of ML/EGME from 1:2.5 to 1:3. In addition, the effect of different reaction temperatures on the yield of EGMEML was also studied (Figure 7C). The results indicated that a higher temperature leads to a higher yield and the yield of EGMEML reached 91.0% at 120 °C. As the temperature further enhanced to 130 °C, the yield was slightly increased. The yield of EGMEML was also affected by the reaction time. Figure 7D shows the variations in the EGMEML yield with the reaction time in the range of 5.0−7.0 h. The results displayed that the maximum yield of EGMEML of 91.0% was obtained for a reaction time around 6 h. With a further increase of the reaction time, the yield was slightly improved. After transesterification, the reaction mixture was allowed to stand for 12 h at room temperature. Centrifugation and decantation was successively used to separate the catalyst from the mixture, and the catalyst was kept in the flask and directly used in the next round of reactions. It is noted that the biodiesel yields of 91, 90.4, and 90.3% were obtained as the calcined sodium silicate was used for three consecutive reactions. The pH of the mixed reaction solution was the same before and after the reaction. It implies that the calcined sodium silicate is insoluble in the mixture of FAME and EGME. The XRD pattern was employed to investigate the stability of the catalyst (Figure 8). Before the XRD measurements, the sample was first centrifuged and then the used catalyst was separated from the mixture by decantation, washed 3 times with ethanol, and dried at 80 °C for 4 h, followed by treatment in a muffle furnace at 300 °C for 2 h. XRD patterns showed that the crystal structure of the catalyst was almost unchanged, the intensity of diffraction peaks was not greatly changed (curves
to a high-polymerization state and the crystallization of sodium silicate became more ordered. Some sodium ions were wrapped in the crystal interior; therefore, the intensity of the absorption peak of Si−O−Na (∼990 cm−1) weakened. The morphological characteristics of the sodium silicate were recorded by SEM (Figure 5). The surfaces of the catalyst particles before calcination are rugged and full of bar-like protrusions (Figure 5A). However, the surfaces of the calcinated catalyst have large numbers of small pores after thermal treatment at 300 °C for 2 h (Figure 5B). These phenomena show that the calcination resulted in the release of water and CO2 of the catalyst by a cratering mechanism. 3.2. Transesterification Reaction. 3.2.1. Reaction of Methyl Laurate (ML) with EGME. ML, a typical FAME, was used in the transesterification with EGME for optimization of reaction conditions; the product is denoted by EGMEML. The thermal treatment temperature is an important factor to the catalytic activity of a catalyst. The effect of the calcination temperature on the catalytic performance of sodium silicate is shown in Figure 6. The yield of EGMEML initially increased
Figure 6. Influence of different calcination temperatures at (a) 200 °C, (b) 300 °C, (c) 400 °C, and (d) 500 °C of sodium silicate on the yield, molar ratio of ML/EGME of 1:2, mass amount of catalyst of 5.0 wt %, reaction time of 6 h, and reaction temperature of 120 °C.
with an increasing calcination temperature. At 200 °C, the yield of EGMEML was less than 40%. Upon increasing the calcination temperature to 300 °C, a yield of 91.0% was attained; as the calcination temperature further increased to 400 °C, the yield was slightly increased. However, as the calcination temperature increased above 400 °C, the yield of EGMEML was reduced obviously. It can be seen that the 5218
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Figure 7. Influence of reaction conditions on the yield of EGMEML: (A) molar ratio of EGME/ML of 1:2, reaction time of 6 h, and reaction temperature of 120 °C, (B) mass amount of catalyst of 5.0 wt %, reaction time of 6 h, and reaction temperature of 120 °C, (C) mass amount of catalyst of 5.0 wt %, molar ratio of ML/EGME of 1:2, and reaction time of 6 h, and (D) mass amount of catalyst of 5.0 wt %, molar ratio of ML/ EGME of 1:2, and reaction temperature of 120 °C.
Table 2. Yields and TOFs of Different FAMEs in the Transesterification Reactiona reactant
TOF (mol min−1 mol−1)
yield (%)
CH3(CH2)6COOCH3 CH3(CH2)8COOCH3 CH3(CH2)10COOCH3 CH3(CH2)12COOCH3 CH3(CH2)14COOCH3 CH3(CH2)16COOCH3 CH3(CH2)7CHCH(CH2)7COOCH3
0.328 0.304 0.297 0.259 0.224 0.191 0.126
74.2 80.8 91.0 89.8 86.4 81.7 53.3
a
Reaction conditions: molar ratio of FAME/EGME of 1:2, mass amount of catalyst of 5 wt %, reaction time of 6 h, and reaction temperature of 120 °C.
Figure 8. XRD patterns of the catalyst: (A) fresh, (B) after the first round, (C) after the second round, and (D) after the third round.
the reaction. The data indicated that, with an increasing molecular weight of FAMEs, the TOF reduced and the maximum TOF of 0.328 mol min−1 mol−1 was reached for the reaction of methyl caprylate with EGME. A maximum yield of 91.0% was obtained for the transesterification of ML and EGME, but under the same conditions, only a 53.3% yield and 0.126 mol min−1 mol−1 TOF were obtained for the reaction of methyl oleate with EGME. This was mainly because the viscosity of the reaction system was relatively high, which might lead to a low yield and TOF. 3.2.3. Transesterification Reaction of Biodiesel with EGME. A traditional biodiesel is a mixture of FAMEs. The above results show that sodium silicate is an effective heterogeneous catalyst for the reaction of FAMEs with EGME. Therefore, the reaction
A−C of Figure 8), but when the catalyst was reused after 3 times, the diffraction peaks became weaker (curves D of Figure 8), which implied that the crystalline degree decreased. The use of calcined sodium silicate can therefore significantly decrease the cost of novel biodiesel production, because of its long catalytic lifetime and good stability. 3.2.2. Transesterification of Seven Different FAMEs with EGME. A series of fatty acid esters of EGMEs were prepared by the transesterification of seven different FAMEs with EGME, using calcined sodium silicate as the catalyst. Table 2 shows the yield and TOF for the different FAMEs as raw material used in 5219
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Table 3. Results of the Common Catalysts in the Transesterification catalyst
catalyst amount (wt %)
temperature (°C)
EGME/ML (mol mol−1)
time (h)
yield (%)
KF KOH CaO non-calcined sodium silicate
2 5 5 5
120 120 120 120
2:1 2:1 2:1 2:1
6 6 6 6
8.4 48.6 73.4 29.7
of soybean biodiesel with EGME was also performed in which the calcined sodium silicate was used as a catalyst. As expected, the novel biodiesel yield of 90.0% was obtained when the soybean biodiesel/EGME molar ratio was 1:10, the mass amount of catalyst was 5.0 wt %, the reaction temperature was 120 °C, and the reaction time was 6 h. All of results make it clear that the calcinated sodium silicate is a highly effective catalyst for the synthesis of the novel biodiesel. 3.2.4. Common Homogeneous and Heterogeneous Catalysts in the Synthesis of Novel Biodiesel. To demonstrate the high catalytic activity of calcined sodium silicate, some traditional homogeneous and heterogeneous solid base catalysts were used for the transesterification. The results of the transesterification catalyzed by KF, KOH, CaO, non-calcined sodium silicate, etc. are listed in Table 3. It can be seen that the catalytic activity of KF was the weakest, the catalytic activity of KOH was medium, and the catalytic activity of CaO was the strongest among them (Table 3). CaO showed reasonable reaction activity in the transesterification. However, the fact that calcium oxide was difficult to separate from the product mixture was demonstrated from the experimental process, which corresponds to the literature.29 3.3. Reaction Kinetics. The kinetic behavior of the reaction of ML with EGME was explored over calcined sodium silicate.
Figure 9. Plot of (A) −log(dcA/dt) versus −log(cA) (□) and (B) −log(dcB/dt) versus −log(cB) (○).
ML (A) + EGME (B) → products
In the reaction, the rate equation is −dcA /dt = k[cA ]a [c B]b
3.3.1. Order and Rate of the Transesterification. As everyone knows, for a reaction, if the concentration of any one reactant is very high or there is a large excess of any one reactant compared to the other reactants, the rate of the reaction would rely on the concentration of the reactant, with its concentration being lower. Here, EGME was first taken in large excess (cA,0 ≪ cB,0); the graph of −log(dcA/dt) versus −log(cA) was plotted (curve A of Figure 9). The plot of −log(dcA/dt) versus −log(cA) shows a linear relationship, and the rate of grade was 0.975, close to 1; this indicates that the grading number (a) of reactant A was 1. Second, ML was taken in large excess (cB,0 ≪ cA,0); the plot of −log(dcB/dt) versus −log(cB) (curve B of Figure 9) also shows a linear relationship, and the rate of grade was 1.033, close to 1; this indicates that the grading number (b) of reactant B was 1. These observations indicate that the reaction rate formula is more likely to follow second-order dependence of the reaction of ML with EGME and cA,0 ≠ cB,0; therefore, −dcA/dt = kcAcB. Figure 10 shows the relationship between the reaction rates versus the concentration of the calcined sodium silicate. We find the increases of the rate of reaction as the concentration of calcined sodium silicate was gradually altered from 12.37 to 24.75 mmol/L (Figure 10) with a fixed ML/EGME ratio of 1:2, reaction time of 6 h, and temperature of 120 °C. 3.3.2. Activation Energy. To estimate the activation energy of the reaction of ML and EGME, the change of ln k versus 1/T
Figure 10. Effect of the concentration of calcined sodium silicate on the rate of reaction of ML with EGME.
was discussed and the scatter plot was shown in Figure 11. On the basis of the Arrhenius equation, the value of activation energy Ea and the pre-exponential factor A were calculated using the data of Figure 11. A and Ea for the reaction of ML with EGME were 1.07 × 104 L mol−1 min−1 and 50.05 kJ mol−1, respectively, and the rate constant (k) was found to be 2.50 × 10−3 L mol−1 min−1 at 120 °C. According to the literature, if Ea is in the range of 10−15 kJ mol−1, reactions are generally limited by diffusion, and if Ea is in excess of 25 kJ mol−1, the reactions were usually governed by a truly chemical step.30 In this issue, the activation energy of 50.05 kJ mol−1 is obtained; therefore, the rate of the reaction of FAMEs with ML is dominated by the chemical step. 3.3.3. Koros−Nowak Test. In the catalytic reaction, heat or mass transfer limitation might affect the catalytic rates. A graceful experimental method was proposed to authenticate the effect of heat or mass transfer on the rate by Koros and Nowak,31 in which, if the same TOFs are observed, heat or 5220
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the Science Research Project of Key Laboratory of Fine Chemicals of College of Heilongjiang Province of China (JX201203).
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Figure 11. Plot of ln k versus 1/T for the kinetics of ML with EGME.
mass transfer would not affect the reaction rates under tested conditions. As shown in Table 4 the observed TOFs are not Table 4. Koros−Nowak Test for Heat- and Mass-Transfer Limitationsa catalyst amount (wt %)
TOF (mol min−1 mol−1)
3.5 4.0 4.5 5.0
0.819 0.818 0.818 0.820
a
Reaction conditions: molar ratio of FAME/EGME of 1:2, reaction temperature of 120 °C, and yield of 40%.
significant changes for the different amounts of catalyst; therefore, the rates of the reaction of FAMEs with ML are not limited by heat and mass transfer.
4. CONCLUSION In the present work, a solid basic catalyst, sodium silicate calcined, was used to catalyze the transesterification of FAMEs with EGME to produce novel EGME fatty acid monoester biodiesel. The results showed that this solid basic catalyst has excellent catalytic activity and good stability in the reaction of FAMEs with EGME. It is insoluble in both EGME and methyl ester and can be easily separated from the reaction system. The second-order rate law of the transesterification of ML with EGME was obtained from the kinetic study. Use of the solid basic catalyst in the transesterification reaction therefore has significant benefits for developing an environmentally benign and economically produced novel biodiesel.
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AUTHOR INFORMATION
Corresponding Author
*Telephone/Fax: +86-0452-2742573 (L. Jia); +86-04522742563 (X. Guo). E-mail:
[email protected] (L. Jia); xfguo@ 163.com (X. Guo). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21176125), the Science Research Project of the Ministry of Education of Heilongjiang Province of China (2012TD012, 12511Z030, 12521594, and GYGG-201108), and 5221
dx.doi.org/10.1021/ef401514e | Energy Fuels 2013, 27, 5215−5221
