Biodiesel Production via Transesterification of Soybean Oil Catalyzed

Article pubs.acs.org/EF

Biodiesel Production via Transesterification of Soybean Oil Catalyzed by Superhydrophobic Porous Poly(ionic liquid) Solid Base Bin Jiang,† Yumei Wang,† Luhong Zhang,† Yongli Sun,† Huawei Yang,† Baoyu Wang,† and Na Yang*,† †

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People’s Republic of China S Supporting Information *

ABSTRACT: In this work, the preparation and catalytic application of novel superhydrophobic porous poly(ionic liquid) (PIL) solid base catalyst have been reported. The PIL was synthesized by radical polymerization with subsequent anion exchange providing alkali catalysis. The as-prepared catalyst possesses favorable wetting properties in the reaction of biodiesel production. Various characterization techniques were utilized to characterize the as-prepared PIL catalyst, including nuclear magnetic resonance (NMR), Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS) spectra, scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), nitrogen adsorption− desorption isotherms, thermogravimetric analyses (TG), and static water contact angle measurement. In addition, the catalytic performance of the PIL catalyst was evaluated by the transesterification of soybean oil with methanol to produce biodiesel. The effects of various reaction parameters on the oil conversion ratio were systemically investigated by three-level and three-factorial central composite design of response surface methodology to obtain the optimum reaction conditions. The catalyst was reused for five cycles without a significant decrease in activity, and the activity could be recovered by anion exchange. Moreover, the properties of the prepared biodiesel were within the standards of EN14214 and ASTM D6751.

1. INTRODUCTION Biodiesel fuel, composed of monoalkyl esters, has attracted extensive interest due to its biodegradability, low-sulfur content, and renewability. As an ecofriendly fuel, it has the potential to replace traditional fossil fuels.1−3 Generally, the biodiesel fuel is obtained by transesterification reaction, with animal fats or vegetable oils reacting with methanol using various catalysts. In terms of the transesterification reaction, base catalysis is considered as the most efficient way to enhance the kinetic rate of biodiesel synthesis, leading to relatively shorter reaction time and milder reaction temperature.4 Conventional homogeneous alkaline catalysts, such as NaOH and KOH, are frequently adopted in industrial biodiesel production. However, the utilization of homogeneous catalysts causes many unneglected disadvantages, especially difficulty in the separation and reuse of catalysts. The products’ purification and catalyst separation will result in energy wastage and large amounts of discharged wastewater.5 To overcome the problems mentioned above, heterogeneous catalysts are therefore attracting increasing widespread attention in the biodiesel production field. Various heterogeneous catalysts have been utilized, such as alkaline earth oxides,6,7 mixed metal oxides,8−10 zeolites,11,12 hydrotalcites,13 supported hydroxides and oxides,14,15 and hybrid materials.16−18 However, the high affinity of the traditional catalysts for water or moisture makes the active sites easily poisoned by the contaminants in raw materials or in air, such as H2O and CO2, thus severely reducing the service life of catalyst.2,19 The active sites are also easily occupied through the strong adsorption of polar byproducts, such as glycerol. In addition, the alkaline groups are also easily corroded and dissolved by methanol.20−23 All of these negative effects will undoubtedly weaken the reusability of catalysts. Consequently, © XXXX American Chemical Society

it is of significance to seek for a novel heterogeneous catalyst with controllable interfacial properties, in order to enhance the reusability of these catalysts.28 Recently, ionic liquids (ILs), molten salts that exist as liquids below a threshold temperature (usually around 100 °C), have been employed because of their high ion conductivity, nonvolatility, and chemical and thermal stability, as well as finely tunable properties.24,25 However, their widespread utilization is still limited by the difficulty in their recovery. On the other hand, recent advances in poly(ionic liquids) (PILs) bring various concepts to the design of catalysts. The expanding research interest in PILs not only brings unique properties of ILs to polymers but also creates exceptional properties, such as mechanical stability, processability, durability, and controlled wettability.24,26−29 Up to now, some pioneer work has been reported in many fields, such as gas separation,30−32 catalysts,33−36 and polyelectrolytes.37,38 Wu et al.39 synthesized Brønsted acidic PIL catalyst for biodiesel production where Fe3O4 particles acted as hard template. Liu et al.40 synthesized functionalized PIL acid catalyst for transesterification of tripalmitin, where [SO3CF3] anion of ILs acted as an active site. Zhang et al.41,42 successfully synthesized PIL solid base catalyst, which showed good activity for Knoevenagel condensations. But the limited affinity for the reactants and bad stability in air atmosphere hindered its practical application in the field of biodiesel production. To the best of our knowledge, no work about PIL solid base catalyst with tunable wetting properties toward reactant and product has been reported, Received: February 13, 2017 Revised: April 4, 2017 Published: April 19, 2017 A

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ethanol before being used. Activated calcium oxide (99%), sodium hydroxide (99%), diethyl ether, ethyl acetate, methanol (chromatographic grade), glycerin, 2-propanol, n-hexane, methyl salicylate (10 mg/mL), benzenecarboxylic acid, dibromothymolsulfonphthalein, phenolphthalein, 2,4-dinitroaniline, 4-nitroaniline, and potassium hydroxide were provided by Tianjin Chemical Factory. All chemicals were of analytical grade and used as purchased, unless otherwise noted. 2.2. Synthesis of Monomer and Cross-Linker. 1-Octyl-3vinylimidazolim bromide ([C8VIm][Br]) and 1,4-butanediyl-3-bis-lvinylimidazolium dibromide ([BVD]) were prepared as monomer and cross-linker, respectively. The synthetic routes to [C8VIm][Br] and [BVD] are shown in Scheme 1. [C8VIm][Br] was prepared according to the previous literature method.43 In a typical procedure, n-octyl bromide (0.025 mol) and 1vinylimidazole (0.025 mol) dissolved in 30 mL of methanol were stirred for 72 h at 60 °C under nitrogen protection. The reaction mixture was cooled down to room temperature and washed with diethyl ether and ethyl acetate three times, respectively. The final product was dried in a vacuum at room temperature for 24 h. The [BVD] was synthesized by a similar method as the preparation of the monomer. 1,4-Dibromobutane and slightly more (2 equiv) of 1vinylimidazole were stirred for 72 h at room temperature.44 The structural features of [C8VIm][Br] and [BVD] were identified by 1H NMR and 13C NMR (VARIAN INOVA 500 MHz), and the 1H and 13C NMR spectra are shown in Figure S1, parts a, b, and c, respectively, of the Supporting Information (SI). The results (1H NMR 400 MHz, DMSO, dppm) were as follows. 1-Octyl-3-vinylimidazolim bromide ([C8VIm][Br]): 1H NMR δ 8.30 (s, 1H), 80.19 (s, 1H), 7.37−7.33 (dt, 1H), 6.04−6.00 (m, 1H), 5.41−5.38 (d, 1H), 4.23−4.20 (t, 2H), 1.84−1.78 (d, 2H), 1.24−1.21 (m, 11H), 0.83−0.81 (t, 3H) ppm. 1,4-Butanediyl-3-bis-l-vinyl imidazolium dibromide ([BVD]): 1H NMR δ 8.257 (s, 2H) 7.80 (s, 1H), 7.31−7.35 (dd, 2H), 5.97−6.01 (dd, 2H), 5.41−5.43 (dd, 2H), 4.29 (s, 4H), 1.86 (s, 4H), 1.22−1.21 (d, 2H) ppm. 2.3. Synthesis of Poly(ionic liquid) Solid Base (BPIL) via Emulsion Polymerization. The BPIL catalyst was synthesized according to the reported reference with slight modification.41 [C8VIm][Br] (1.80 g, 6.25 mmol), [BVD] (0.50 g, 1.25 mmol), V50 (0.025 g, 0.05 mmol), and AIBN (0.025 g, 0.15 mmol) dissolved in a mixture of water (10 mL) and 2-propanol (5 mL) were stirred magnetically in a three-necked flask at room temperature for 2 h, and the resultant clear solution was mixed with 60 g of 1% CTAB hexane solution in the flask. Subsequently, the obtained mixture was vigorously stirred and ultrasonicated for 30 min in an ice bath to obtain a stable emulsion. The reaction was then kept for 16 h at 60 °C with nitrogen gas bubbling. At the end of the reaction, the resultant mixtures were cooled to room temperature and then poured into 2propanol. The precipitated poly(ionic liquid) (PIL-Br) was first isolated by decantation and then separated by centrifugation. Finally, the obtained product PIL-Br was dried in vacuum at 50 °C for 24 h. The degree of cross-linking of PIL-Br (the ratio of monomer to cross-linker) was determined by elemental analysis, which was performed on a CHN Vario Micro cube elemental analyzer. Given

which is very important for the enhancement of catalytic activities and the recyclability of the catalysts applied in biodiesel production from soybean oil through transesterification. Toward the preparation of heterogeneous base catalyst with enhanced activity and stability, we consider it important to fabricate catalyst possessing hydrophobic and favorable wetting properties toward the reactant and product. Herein, a novel superhydrophobic porous poly(ionic liquid) solid base as the heterogeneous catalyst for biodiesel production was synthesized through radical polymerization and anion exchange. The wetting properties of the catalyst with regard to the reactant and product were tested by contact angle measurement, and other characteristics were further evaluated by relative methods. The catalytic performance of the novel catalyst was also investigated in the transesterification reaction. The response surface methodology (RSM) method was used to optimize the reaction conditions.

2. EXPERIMENTAL SECTION 2.1. Materials. Commercial soybean oil purchased from a local food store was pretreated before being utilized as the feedstock for the transesterification reaction. Water content (0.03%) and the acid value (AV) (0.079 mg KOH/g) of the refined raw oil were determined by a Karl Fischer moisture titrator (DCS-800) and by the method of titration with a standard alkali, respectively. The molecular weight was calculated to be 877 g/mol. The composition of soybean oil was determined by gas chromatography−mass spectrometry (GC−MS) analysis through analyzing the fatty acid methyl esters (FAMEs), which were completely transformed from the fatty acid compositions contained in soybean oil, and the results are listed in Table 1.

Table 1. Fatty Acid Composition of Refined Soybean Oil fatty acid (index)

composition (wt %)

palmitic acid (C16:0) stearic acid (C18:0) oleic acid (C18:1)

13.25

linoleic acid (C18:2) linolenic acid (C18:3)

46.24

5.52 26.57

fatty acid (index) eicosanoic acid (C20:0) eicosenoic acid (C20:1) docosanoic acid (C22:0) tetracosanoic acid (C24:0)

composition (wt %) 0.63 0.32 0.59 0.23

5.45

1-Vinylimidazole, 1,4-dibromobutane, n-octyl bromide, and hexadecyltrimethylammonium bromide (CTAB) were obtained from Shanghai Chemical Reagents Co. 2,2′-Azobis(2-methylpropionamide) dihydrochloride (V50) and azobis(isobutyronitrile) (AIBN), purchased from Aladdin (Shanghai, China), were recrystallized from

Scheme 1. Synthetic Route of Poly(ionic liquid) Solid Base Catalyst

B

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of the methyl ester peak of the FAMEs (biodiesel) product was the singlet peak at 3.7 ppm (Figure S2b, SI). In addition, comparing the signal peak at 1−2 ppm from the obtained product to that from unreactive material oil, it was proved that no side reaction occurred. Quantitively, the conversion ratio of the reaction was determined by the ratio of the peak areas of the protons in characteristic groups, including the methoxy group in the obtained biodiesel methyl ester with a significant singlet at 3.7 ppm (A1) and the α-carbonyl methylene group that appeared in both raw oil and biodiesel methyl ester with a significant triplet at 2.3 ppm (A2). All materials were tested by 1H NMR analysis using CDCl3 as solvent. The conversion ratio of the transesterification reaction was calculated according to eq 1 based on the area (A) of the characteristic signals

that all the nitrogen and carbon content are from the polymer, the degree of cross-linking was calculated to be 1.2 from the following elemental analysis results: C, 50.03%; N, 11.9%; H, 6.01%. The experiment was repeated three times, and the change in the degree of cross-linking for each batch experiment product was within ±5%. For the preparation of BPIL, the obtained product PIL-Br was treated with 0.2 mol/L methanol solution of KOH at room temperature for 48 h to implement the process of anion exchange. Then the resultant mixture was centrifuged and washed with hot water until the filtrate was neutral. The experiment was repeated three times to achieve the largest degree of ion exchange. Finally, the obtained BPIL was dried in vacuum at 50 °C for 24 h. 2.4. Catalyst Characterizations. The basic strength and basicity of the BPIL catalyst was evaluated using the Hammett indicator method. Anhydrous methanol solution of benzenecarboxylic acid (0.02 mol/L) was taken as the titrant. Phenolphthalein (H− = 9.8), 2,4dinitroaniline (H− = 15.0), and 4-nitroaniline (H− = 18.4) were taken as Hammett indicators. Meanwhile, methanol was adopted as solvent of the basic indicators in order to provide a similar surface basicity to the real reaction conditions. Hammett indicators were dissolved in methanol. When we evaluate the basic strength (H−) of BPIL catalyst, about 0.05 g samples were shaken with a moderate methanol solution of Hammett indicators and left until the color did not change. The basic strength was determined according to the color changes of different Hammett indicators. In addition, a different distribution of basic sites as well as the total basicity of the catalyst was obtained according to the quantity of titrant45,46 Static contact angles (CA) on the catalyst surface were tested at ambient temperature by the sessile drop method using an optical contact angle and interface tension meter (SL200 KS, KINO). X-ray photoelectron spectroscopy (XPS) was performed on a Thermo ESCALAB 250 with Al Kα radiation. The binding energies were calibrated using the C 1s peak at 284.8 eV. Fourier transform infrared (FT-IR) spectra was recorded on a Bio-Rad FTS 6000 spectrometer using an anhydrous KBr pellet as standard. The morphology of catalyst was observed by scanning electron microscopy (SEM) (S-4800) and transmission electron microscopy (TEM) (JEM-2100) with an accelerating voltage of 400 kV. Small-angle power X-ray diffraction (XRD) was carried out using Cu Kα radiation (40 kV, 20 mA) over the 2θ range of 5°−90°. Nitrogen adsorption−desorption isotherms were measured using a Micromeritics ASAP 2020 M system, and the samples were outgassed for 10 h at 120 °C before measurements. The pore-size distribution for BPIL was calculated using the Barrett−Joyner−Halenda (BJH) model. The themogravimetric analysis (TGA) were carried out with a TGA/ DSC in flowing N2 by heating the sample from room temperature to 600 °C at a rate of 10 °C/min. 2.5. Transesterification of Soybean Oil with Methanol. All transesterification reactions of soybean oil with methanol were carried out in a 100 mL round-bottom, three-necked flask equipped with a water-cooled reflux condenser and a magnetic stirrer. In order to reproduce the data, each experiment was implemented in triplicate and the error was within ±5%. In a typical run, 10 g soybean oil and the desired amounts of methanol and catalysts were added into the round-bottom flask placed in oil bath, and the reaction was carried out at the fluxing temperature of methanol (65 °C) for the required time with constant stirring (300 rpm). At the end of the reaction, the obtained mixture was filtered to recover the solid catalyst, and the unreacted methanol in filtrate was removed by a rotary evaporator in vacuum at 65 °C for 15 min. The residual materials were then transferred into a funnel and settled for certain time to obtain an efficient delamination. The upper layer product containing biodiesel and less glycerin was taken and washed with saturated NaCl solution to remove residual glycerin. Finally, the treated product was dried with anhydrous Na2SO4 and then filtered to obtain the pure product. 2.6. Product Analysis. The transesterification reaction was analyzed using 1H NMR (Brouker 500 MHz) according to the literature.44,47 Qualitatively, the characteristic peak of triglyceride (material oil) appears at 4.2 ppm (double peak), and the feature signal

conversion ratio (%) = (2A1/3A 2) × 100%

(1)

where A1 and A2 are the areas of the methoxy peak (3.7 ppm singlet) and methylene peak (2.3 ppm triplet) protons, respectively. The yield of biodiesel is estimated by eq 2

yield =

weight of obtained product × FAME% × 100% weight of refined soybean oil

(2)

where FAME% is the concentration of fatty acid methyl ester (FAME) analyzed by gas chromatography (GC). The compositions and quantities of methyl esters (FAME) produced under the optimal reaction conditions were determined by flame ionization detection−gas chromatography (FID-GC-6890) with an HP-5 capillary column (30 m × 0.32 mm, 0.25 μm) according to the standard EN14103. Methyl heptadecanoate (10 mg/mL) was adopted as an nternal standard to determine the amounts of products, and n-hexane was utilized as diluent. The chromatographic conditions were as follows: The injector and detector temperature were set at 250 °C. A split model was adopted with a split ratio of 50:1, nitrogen was used for carrier gas, and the injection volume was 1 μL. The oven temperature was first set at 120 °C for 2 min and then programmed from 120 to 250 °C at a rate of 5 °C min−1 and then kept there for 30 min. A typical chromatographic result is shown in Figure S3 (SI). The error of the conversion, determined using 1H NMR, and the yield, estimated using GC, was less than 4%, which corresponds to the previous study.47 2.7. Properties of Biodiesel Analysis. The density was measured by a digital density meter (DMA 500A) on the basis of standard ASTM D341. According to standard ASTM D445, the viscosity of the biodiesel sample was determined by a digital viscometer (Brookfild DV1). The flash point (closed cup) was measured on the basis of the standard using a flash point tester (TP411). The acid number (AN) of the produced biodiesel sample was tested on the basis of standard ASTM D664 using an automation titration analyzer (BSZ-600). The calorific value was evaluated using an automatic calorimeter and the cetane number was also tested on the basis of ASTM D613 using a cetane number tester. All of the analysis was done in triplicate.

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. The basic strength and basicity of the fresh, recycled, and regenerated catalyst samples were determined by the Hammett indicator method, and the results are summarized in Table 2. All samples turned 2,4dinitroaniline (H− = 15.0) from yellow into mauve but failed to change the color of 4-nitroaniline (H− = 18.4) into its conjugate form. Thus, the basic strength with H− could be taken as in the range of 9.8−15.0. In addition, the turnover frequency was also measured, and the results are shown in Table S1 (SI). XPS was performed to characterize the composition and chemical state of the elements in the catalyst. As shown in Figure 1a, the catalyst was mainly composed of C, N, and O and no K element remained on the surface of BPIL catalyst. A slight amount of element Br residue (1.04 atom %) is possibly C

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FT-IR spectra of ionic liquid monomer [C8Vim][Br] and ionic liquid polymer before (PIL-Br) and after ion exchange (BPIL) are illustrated in Figure 2a−c, respectively. As shown in

Table 2. Textural Characteristics and the Basic Properties of PIL Catalyst BET surface area (m2/g)

pore volume (cm3 g−1)

av pore diameter (nm)

103

0.32

35 basicity (mmol/g)

basic strength (H−) fresh recycled regenerated

9.8−15.0 9.8−15.0 9.8−15.0

H− = 9.8−15.0

total basicity (mmol/g)

3.67 3.17 3.66

3.67 3.52 3.66

H− = 7.2−9.8 0 0.35 0

owing to the encapsulation of polymer matrix. It was confirmed, through the atomic ratio of O and Br, that the degree of anion exchange is more than 90%, which was in accordance with the basicity measurement through titration with a methanol solution of benzoic acid (0.02 M). Both of them suggested that the anion exchange was successfully obtained. The C 1s peaks at 284.8 and 286.2 eV were associated with C−C and C−N, respectively (Figure 1b), and N 1s peaks at 401.46 and 398.85 eV were assigned to the C−N bond (Figure 1c) of the quaternized N of the imidazole rings. These data confirmed the existence of an imidazole ring in the sample.40,48 The O 1s gives a single peak at around 531.5 eV (Figure 1d), which corresponded to O atoms in OH−.49 The XPS results further demonstrated that the imidazole rings were not destroyed during the process of preparation and the hydroxyl anions were successfully exchanged into the porous polymers.

Figure 2. FT-IR spectra of (a) monomer [C8Vim][Br], (b) PIL-Br, (c) BPIL, (d) recycled catalyst, and (e) regenerated catalyst.

Figure 2, the characteristic adsorption peaks at around 1630, 1570, and 2927 cm−1 were attributed to CC, CN, and C− H stretching vibrations of the imidazole ring, respectively.44,50 The adsorption peaks around 1650 cm−1 from [C8Vim][Br] disappeared in PIL and proved that the monomer completely polymerized. Comparing FT-IR spectra in parts b and c of

Figure 1. XPS spectra of (a) wide-scan survey, (b) C 1s, (c) N 1s, and (d) O 1s. D

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Energy & Fuels Figure 2, no detectable change was observed, which reconfirmed the stability of the polymer under the strong alkaline conditions. The morphologies of PIL catalyst were studied by SEM and TEM, and the results are shown in Figure 3. It was evident that

Figure 3. (a) SEM image and (b) TEM image of BPIL catalyst. Figure 5. (a) N2 adsorption−desorption isotherms and (b) pore size distribution of BPIL catalyst.

the BPIL catalyst possessed a layer structure composed of spherical colloidal particles with rough and loose surfaces. This structure could enlarge the surface area and facilitate the diffusion of reactants and the transfer of reaction heat. The structural properties of BPIL catalyst were also characterized by XRD and N2 sorption−desorption isotherms, and the results are indicated in Figures 4 and Figure 5,

Figure 6. TGA curve of BPIL catalyst.

adsorbed on the surface of BPIL catalyst than the common heterogeneous catalyst and further evidenced its superior hydrophobicity.42 In addition, the decomposition temperature of BPIL catalyst was about 230 °C, far higher than the refluxing temperature of methanol, which confirmed that the catalyst was stable enough for the transesterification reaction. 3.2. Catalyst Wettability. In order to evaluate the wettability of BPIL catalyst, the static contact angles of reagent, product, and water on the surface of catalyst were measured, respectively. As shown in Figure S4d (SI), the static contact angle of water was measured to be 152°, indicating that the BPIL catalyst was superhydrophobic. The superhydrophobicity of BPIL catalyst was closely related to its rough surface, unique porous structure, nonhydrophilic organic framework, and the long chains of the ionic liquid monomer.51 Furthermore, the superhydrophobic feature could prevent the H2O and CO2 from poisoning the catalyst and further prolong its service lifetime. In contrast, the contact angles of methanol (Figure S4a, SI) and soybean oil (Figure S4b, SI) were 0° and 13°, respectively, exhibiting the extremely high affinity of catalyst for the reactants.40 The excellent oleophilicity for methanol and soybean oil was beneficial for the conversion of soybean oil to methyl esters. Interestingly, the angle of glycerol was as high as 147° (Figure S4c, SI), demonstrating that BPIL had good

Figure 4. XRD pattern of BPIL catalyst.

respectively. As shown in Figure 4, a wide diffraction peak centered at 2θ = 25° was observed in the XRD pattern, suggesting that this polymer is a carbon material and that the carbon matrix was amorphous.56 Additionally, a weaker diffraction peak of 2θ below 10° might verify the existence of a layered structure, which was consistent with the result of TEM in Figure 4. It was observed from Figure 5 that the PIL catalyst showed a type-IV curve with a sharp capillary condensation step at p/p0 of 0.8−0.95. Moreover, the textural properties of the sample were also determined by the BET method. The result of the nitrogen adsorption−desorption isotherms is listed in Table 2, indicating that the PIL catalyst provided a relatively adequate contact area with reactants. To investigate the thermal stabilities of BPIL catalyst, TGA was carried out from room temperature to 600 °C at a rate of 10 °C/min under a nitrogen atmosphere. As shown in Figure 6, the BPIL catalyst exhibited a slight weight loss before 230 °C. The result indicated that less water from moisture in air was E

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Energy & Fuels antiwetting properties for glycerol and thus favorably repelled glycerol byproduct from the surface of catalyst after the transesterification reaction. In sum, the BPIL catalyst demonstrated good superhydrophobicity, high affinity for reactant, and incompatibility with the byproduct glycerol, which endowed the catalyst with excellent catalytic activity for the transesterification reaction of soybean oil with methanol. 3.3. Influence of Experimental Conditions on the Conversion. In order to eliminate the mass-transport limitation, including external and internal of the catalyst, the effect of stirring speed and the particle size of catalyst on conversion was tested, respectively. As shown in Figure S5 (SI), when the stirring speed was less than 300 rpm, the conversion was obviously influenced by stirring speed. When the stirring rate increased from 300 to 500 rpm, the conversion slightly changed from 76.3% to 76.5%. The results proved that the effect of external mass transport on the reaction could be eliminated when the experiments were carried out at the stirring speed of 300 rpm. Therefore, the agitation speed was fixed at 300 rpm in our study. To evaluate the effect of the internal mass transfer on the reaction, the BPIL catalyst was sieved into five fractions having different particle size ranges, and each fraction was used to catalyze the reaction at a stirring speed of 300 rpm. As shown in Figure S6 (SI), the conversion increased with the decrease in particle size of the catalyst. When the particle size was less than 150 μm, the conversion changed slightly, only increasing from 76.4% to 76.5%. This result indicated that the effect of internal mass transport limitation could be neglected when the particle size of catalyst was less than 150 μm. Therefore, the mass transfer limitations could be eliminated to the greatest degree under the reaction conditions in our study. The molar ratio of methanol/oil is one of the key parameters influencing the transesterification reaction. In order to ensure the complete reaction of the transesterification, the lowest molar ratio of methanol to soybean oil is 3:1 according to the stoichiometric ratio of methanol to glycerol. A series of molar ratios of methanol to soybean oil were selected from 3:1 to 25:1 with other parameters remaining constant. As illustrated in Figure S7 (SI), the oil conversion to methyl esters considerably increased when the molar ratio of methanol to soybean oil increased from 3:1 to 20:1. It was concluded that the increase of the methanol to soybean oil molar ratio could shift the reversible transesterfication to the right, thus increasing the conversion efficiency. However, a slight decrease in the product yield was observed upon further increasing the molar ratio of methanol to soybean oil. It was probably because the excessive methanol diluted the concentration of oil and catalyst in the reaction system. The influence of catalyst loading was investigated when the molar ratio of methanol to soybean oil was fixed at 20:1. As depicted in Figure S7 (SI), when 2.0% of the catalyst amount was added to the reaction system, only 37.0% product yield was achieved, which was probably ascribed to the insufficient catalytic sites for the transesterification reaction. The product yield significantly increased upon increasing the catalyst amount from 2.0 to 10.0 wt %, but the product yield increased slightly by further increasing the catalyst dosage to 12.0 wt %. It is obvious that the catalytic active sites are directly proportional to the catalyst dosage. Considering the economy of reaction, 10.0 wt % of catalyst dosage was selected for the following transesterification reaction.

Furthermore, the influence of reaction time on product yield is also studied, and the result is shown in Figure S8 (SI). As expected, the oil conversion ratio gradually increased with the increase of reaction time. It should be noted that the product yield slightly decreased when the reaction time was more than 10 h. This was possibly due to fact that the redundant time was more favorable for the reverse reaction than the positive reaction of transesterification. 3.4. Optimization of Reaction Conditions. In order to optimize the reaction conditions for transesterification of soybean oil with methanol, a three-level and three-factorial central composite design (CCD) of response surface methodology (RSM) was used to design the experiment with DesignExpert software version 8.0.6. On the basis of the study of single factor experiments, molar ratio of methanol to oil (x1), catalyst amount (wt %) (x2), and reaction time (x3) were taken as the independent variables, while the yield of biodiesel (Y) was chosen as the response to determine the optimal parameters. The range and coded level of the independent variables are shown in Table 3. The other operating parameters Table 3. Range and Coded Level for Variables of Experimental Design range and levels factor code

symbol

−α

−1

0

+1

+α

molar ratio of methanol to oil catalyst concn (wt %) reaction time (h)

x1

11.59

15

20

25

28.41

x2 x3

3.95 3.95

6 6

9 9

12 12

14.05 14.05

were kept constant: the stirring speed of 300 rpm and reaction temperature of 65 °C. All experiments were carried out three times and the error was within ±5%. Equation 3, a full quadratic model used to elucidate the effect of coded variables on the response (biodiesel yield), was established using Design Expert 8.0.6 software52,53 Y = β0 + β1x1 + β2x 2 + β3x3 + β12x1x 2 + β13x1x3 + β23x 2x3 + β11x12 + β22x 2 2 + β33x32 + ε

(3)

where Y is taken as the response (biodiesel yield %) while β0 is the intercept coefficient (offset); x1, x2, and x3 are the three dependent variables; β1, β2, and β3 are coefficients of the linear terms; β12, β13, and β23 are the interaction coefficient; β11, β22, and β33 are the coefficients of quadratic terms; and ε is the error. A total of 20 runs were performed, and the coded factors, the experimental and predicted values for the biodiesel yield, are listed in Table 4. The data were then analyzed using Design-Expert software version 8.0.6, and a second-order response surface model was fitted by analysis of variance (ANOVA). The significance of each parameter was determined by F-test for analysis of variance at the confidence level of 95%. A P value less than 0.5 indicated that the factor effected the reaction significantly at the confidence level of 95%.54 On the basis of the P value result (probability value) listed in Table 5 from CCD, the obtained quadratic polynomial equation model (eq 4) for biodiesel yield was as follows: F

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Energy & Fuels Table 4. Center Composition Design Matrix Together with the Experimental and Predicted Response Values independent variables (coded) methanol/oil molar ratio, x1

standard run 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

15.00 25.00 15.00 25.00 15.00 25.00 15.00 25.00 11.59 28.41 20.00 20.00 20.00 20.00 20.00 20.00 20.00 20.00 20.00 20.00

yield of biodiesel (%)

catalyst amount x2 (wt %)

reaction time, x3 (h)

experimental

predicted

residual

6.00 (−1) 6.00 (−1) 12.00 (+1) 12.00 (+1) 6.00 (−1) 6.00 (−1) 12.00 (+1) 12.00 (+1) 9.00 (0) 9.00 (0) 3.95 (−1.682) 15.05 (+1.682) 9.00 (0) 9.00 (0) 9.00 (0) 9.00 (0) 9.00 (0) 9.00 (0) 9.00 (0) 9.00 (0)

6.00 (−1) 6.00 (−1) 6.00 (−1) 6.00 (−1) 12.00 (+1) 12.00 (+1) 12.00 (+1) 12.00 (+1) 9.00 (0) 9.00 (0) 9.00 (0) 9.00 (0) 3.95 (−1.682) 14.05 (+1.682) 9.00 (0) 9.00 (0) 9.00 (0) 9.00 (0) 9.00 (0) 9.00 (0)

66.5 ± 2.3 71.9 ± 1.4 73.6 ± 2.2 91.3 ± 1.9 73.3 ± 2.2 74.8 ± 1.4 88.9 ± 2.3 96.5 ± 1.5 73.0 ± 2.2 92.4 ± 1.3 64 ± 2.4 93.5 ± 1.9 75.5 ± 2.1 93.2 ± 0.9 94.6 ± 0.7 94.9 ± 1.1 92.8 ± 1.2 95.8 ± 0.8 96.2 ± 1.4 96.5 ± 1.3

61.78 74.19 74.80 90.77 78.05 90.72 83.53 99.74 70.07 94.14 73.59 92.13 71.74 92.97 94.66 94.66 94.66 94.66 94.66 94.66

1.83 −1.16 −0.38 −0.27 −0.95 −0.85 −0.06 −3.05 −0.85 2.58 0.08 1.65 −0.60 2.33 −0.58 −0.28 −2.38 0.62 1.02 1.32

(−1) (+1) (−1) (+1) (−1) (+1) (−1) (+1) (−1.682) (+1.682) (0) (0) (0) (0) (0) (0) (0) (0) (0) (0)

the feasibility of the quadratic regression model. The Adeq precision, illustrating the signal-to-noise ratio, was 24.88, much higher than the minimum requirement of 4, indicating that the noise did not cause any error in the response surface,56 and the value of Adeq precision suggested that the model could be used to navigate the design space. As shown in Table 5, all the linear teams and quadratic teams were clearly significant, because their F values were very high and their corresponding P values were less than 0.01%. The 3D response plots generated from the model are presented in Figure 7. The 3D response plots not only elucidated the effects of the independent variables on the response of the biodiesel yield but also predicted the maximum values by the surface confined in the smallest ellipse in the contour diagram.53 Then the optimum conditions for obtaining a maximum yield of biodiesel were obtained on the basis of the fitted model. The predicted value of the biodiesel yield was 97.2% under the suggested conditions: 21.9 for the molar ratio of methanol to oil, 9.15 wt % for catalyst amount, and the 9.33 h for reaction time. Meanwhile, the reaction was carried out three times under the above conditions and the experimental value of 96.3% was obtained. Compared with the predicted value, the suggested optimal conditions were suitable for the reaction, with only 0.9% error. 3.5. Influence of Water on the Reaction. The moisture significantly affected the biodiesel production, especially for the alkaline-catalyzed reaction. Generally, the water existing in the feedstock over 0.3 wt % results in the formation of soaps instead of biodiesel. Soap formation leads to the higher purification costs and lower biodiesel yield, since soap moves the biodiesel production to the glycerol phase.57 Therefore, the effect of water content on the catalytic activities was investigated under the optimal conditions. As shown in Figure 8, the yield of biodiesel was maintained over 90% when the water content in the oil was less than 1.5 wt %. However, when the water content was more than 1.5 wt %, the yield decreased sharply. This might result from the fact that the water

Y = 95.18 + 4.751x1 + 8.3x 2 + 4.3x3 + 2.3x1x 2 − 1.75x1x3 + 1.35x 2x3 − 4.72x12 − 6.12x 2 2 − 4.14x32 (4)

Table 5. Analysis of Varianance (ANOVA) for the Response Surface Quadratic Modela source of variation

sum of squares

degree of freedom (DF)

mean square

F value

model

2525.68

9

280.63

68.40

x1 x2 x3 x1x2 x1x3 x2x3 x12 x22 x32 residual lack of fit

307.72 941.83 263.32 42.32 24.50 14.58 320.96 539.03 246.52 41.03 31.80

1 1 1 1 1 1 1 1 1 10 5

307.72 941.83 263.32 42.32 24.50 14.58 320.96 539.03 246.52 4.10 6.36

75.00 229.55 64.18 10.31 5.97 3.55 78.23 131.38 60.08

pure error cor total

9.23 2566.71

5 19

1.85

3.44

P value Prob > F