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Magnetic Material Grafted Poly (Phosphotungstate-Based Acidic Ionic Liquid) As Efficient and Recyclable Catalyst for Esterification of Oleic Acid Zuowang Wu, Chong Chen, Lei Wang, Hui Wan, and Guofeng Guan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b02906 • Publication Date (Web): 02 Feb 2016 Downloaded from http://pubs.acs.org on February 6, 2016
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Magnetic Material Grafted Poly (Phosphotungstate-Based Acidic Ionic Liquid) As Efficient and Recyclable Catalyst for Esterification of Oleic Acid
Zuowang Wu, Chong Chen, Lei Wang, Hui Wan*, Guofeng Guan* State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, Nanjing 210009, P. R. China
ABSTRACT A magnetically recyclable catalyst was prepared by embedding magnetic nanoparticle in poly (phosphotungstate-based ionic liquid) matrix. The obtained hybrid material was characterized by XRD, FT-IR, SEM, TEM, TGA, and VSM. The results indicated that Fe3O4 was successfully implanted in multi-layer poly (ionic liquid) and the catalyst was possessed of open active sites (see SEM) to reactants and high magnetization value. The combination of poly (ionic liquid) matrix with magnetic nanoparticle enhanced the structure stability and favored the separation performance. Esterification of oleic acid with ethanol was conducted in the presence of prepared catalyst and the Box-Behnken response surface methodology was applied for maximizing the oleic acid conversion by optimizing process variables at 90 oC. Under the optimum conditions (reaction time, 5h; catalyst amount, 13 wt%; alcohol / acid molar ratio, 12:1), the conversion of oleic acid reached 93.4 %. Besides, the catalyst presented fine reusability after six runs and could be facilely recovered in magnetic field based on its superparamagnetism property.
1. INTRODUCTION Esterification is one of the most applied reactions in organic chemistry, with the significance in the industrial synthesis of fragrances, polymers, and paints, etc. Conventional catalysts for esterification can be divided into homogeneous and heterogeneous phase. The homogeneous ones,1 like sulfuric acid and phosphoric acid, possess properties of fine catalytic activity and selectivity but accompanied by the problems of equipment corrosion, tedious separation process, and environment pollution, etc. Heterogeneous catalysts2, 3 like resins, zeolites, and supported mineral acids are fabricated to overcome the above drawbacks. The heterogeneous ones4 perform well in the separation process. Whereas, the problems of deactivation and mass transfer resistance also restrict their applications in some degree. As the catalyst is a key component of esterification reaction, the development of high-efficient and recyclable catalysts has always been concerned to meet the standards of green chemistry. Recently, with the unique physicochemical properties like negligible vapor pressure, fine chemical and chemical stability, wide electrochemical potential window, ionic liquids (ILs) are considered as environment-friendly green solvents and catalysts.5, 6 Due to the great variety of anion-cation pairs and the diversity in the side chains of the cations, ionic liquids have been tailored for special applications in the fields of organic synthesis, catalysis, and separation processes, etc. For instance, the acidity of ionic liquids can be adjusted7, 8 by alkane sulfonic acid or carboxylate acid groups in the side chains of the cations or using AlCl4- or HSO4- as anions. As the most important representatives of ionic liquids, acidic ionic liquids are expected to act as green alternatives to the traditional acid catalysts to realize clean production.9-11 It has been reported heteropolyanions-based acidic ionic liquids show well performance as acidic catalysts.12 Based on the facile adjustability and functionalization of ionic liquids, supported ionic liquids (SILs) are prepared by depositing ionic liquids on the surface of solid supports, using methods of physisorption, condensation, polymerization, and sol-gel, etc.13 Solid materials like molecular sieves,14,
nanoparticles,19-21 and mental organic frameworks22 can be used as the supports. Particularly, the magnetic Fe3O4 is considered as a promising support with the advantages of easy modification, efficient separation in magnetic field, and lower process cost.23 Supported ionic liquids maintain the advantages of ionic liquids and supports and also derive new performances and novel properties due to the synergistic effect of ionic liquids and supports,24 which are helpful in reducing the cost and overcoming the viscosity of ionic liquids, increasing the number of accessible active sites of the catalysts, and expanding the applications of ionic liquids.25, 26 As for the supported ionic liquid catalysts, the loading amount of ionic liquids is vital feature which determines the catalytic performance. Impregnating ionic liquids into substrates can afford a large loading amount but the active components leach seriously. Immobilizing ionic liquids by means of active groups (like silane coupling agents27,
28
) is widely applied, which offers firm combination between the ionic
liquids and supports but affords low ionic liquids loading amount. Presently, grafting poly(ionic liquid)s onto supports, with repeating monomer ionic liquid units in polymeric backbone, are supposed to be a solution to possessing more active sites and well stability at the same time.29 In this work, magnetic nanoparticle grafted poly (phosphotungstate-based ionic liquid) catalyst (Fe3O4@PILPW) is prepared by embedding magnetic Fe3O4 in multi-layer poly(ionic liquid) for the esterification of oleic acid with ethanol. The catalyst is synthesized via insetting 3-(trimethoxysilyl) propylmethacrylate (MPS) modified Fe3O4 nanoparticle in the copolymer formed by free radical initiation copolymerization of ionic liquid monomer [SO3H-PVIM][HSO4] and cross-linker divinyl benzene (DVB), followed by the anion-exchanging with H3PW. To improve the conversion and efficiency of reaction, response surface methodology (RSM)30 is applied to optimize the process parameters of reaction time, molar ratio of alcohol to oleic acid, and catalyst amount for esterification reaction. The application of RSM allows gathering lots amounts of information in the case of carrying out small number of experiments. At the same time, the effects of individual variables and their combinations of interactions on the response can also be observed. 3
2. EXPERIMENTAL SECTION 2.1. Chemical reagents and instruments Ethylene glycol (99%) , ethanol (99.8%), 1-vinylimidazole (99%), sulfuric acid (95～98%), iron(II) chloride tetrahydrate (98%), iron (III) chloride anhydrous (98%) phosphotungstic acid, 1,3-propanesultone (99.5%), sodium acetate anhydrous (99%), sodium citrate (99%), oleic acid (85%), 3-(trimethoxysilyl)propylmethacrylate (MPS), diethyl ether, phosphotungstic acid, ammonium hydroxide (25~28%), divinyl benzene (DVB), azodiisobutyronitrile (AIBN, 99%) were obtained from Aladdin Industrial Corporation and used without further purification unless otherwise stated. X-ray diffraction (XRD) patterns were recorded on a Smartlab diffractometer. Fourier-transform infrared (FT-IR) spectra were measured by a Nicolet-6700 spectrometer using anhydrous KBr as dispersing agent. Scanning electron microscopy (SEM) images were gained by a S4800 Field-Emission Scanning Electron Microscope. Transmission electron microscopy (TEM) images were taken with a JEM-2100 (HR), and the samples were daubed onto carbon film supported on copper grids for analysis. Thermogravimetry (TG) analysis was carried out with a STA409 instrument in dry air at a heating rate of 10 K/min. The magnetic hysteresis loops were measured by a 7407 vibrating sample magnetometer. 2.2 Synthesis of ionic liquid monomer The ionic liquid monomer 1-vinyl-3-(3-sulfopropyl) imidazolium hydrogen sulfate [SO3H-PVIM][HSO4] was synthesized as shown in Figure 1 step 1. Firstly, 1-vinylimidazole (0.05 mol) was dissolved in ethyl acetate (100 ml) in a 250 ml round-bottom flask setting in ice-water bath. Then equimolar 1,3-propanesultone was added dropwise under continuous stir and the mixture was stirred for 24 h. The generated white precipitate was washed with ethyl acetate (20 ml, 3 times) and dried in vacuum. The obtained solid was dissolved in deionized H2O (10 ml) in a 50 ml round-bottom flask setting in ice-water bath and equimolar H2SO4 was added dropwise. Then the mixture was heated up to 50 oC in N2 atmosphere under continuous stir for 12 h. Rotary evaporation was applied to remove the water and 4
ethyl acetate (10 ml, 3 times) was used to remove the impurities. The obtained product was dried in vacuum at 60 oC for 6 h and a viscous wine-colored liquid was obtained, namely, [SO3H-PVIM][HSO4]. 2.3 Synthesis of vinyl functionalized magnetic nanoparticle The magnetic Fe3O4 particle was prepared using a solvothermal method.31 Briefly, FeCl3 (16 mmol), trisodium citrate (3.2 mmol), and sodium acetate (12 mmol) were added successively in ethylene glycol (120 mL) and stirred for 1 h. The formed yellow liquid was sealed in a Teflon-lined stainless-steel autoclave (200 ml capacity) and kept at 200 oC for 10 h. The upper clear liquid was poured out and the lower sediment was washed with deionized water and ethanol for 3 times respectively. The obtained black product was dried in vacuum at 60 oC for 6 h, giving the Fe3O4 nanoparticle. Fe3O4 (1.0 g) was dispersed in the mixture of ethanol (50 ml), ammonium hydroxide (2 ml), and 3-(trimethoxysilyl) propylmethacrylate (MPS, 10 mmol). The mixture was stirred at room temperature for 48 h. The product was collected by a magnet and washed with ethanol (20 ml, 3 times) and then dried in vacuum. Then vinyl functionalized Fe3O4 was prepared, with Fe3O4@MPS in short (see Figure 1, step 2). 2.4 Synthesis of catalyst Fe3O4@MPS (0.25 g), ionic liquid monomer (1.6 g), and DVB (0.65 g) were set in a flask containing 30 ml ethanol. The mixture was treated with ultrasonic for 15 min and deoxygenated with N2 for another 15 min. Then AIBN (0.02 g) was added and the mixture was heated up to 70 oC in N2 atmosphere. After 12 h, the formed precipitate, abbreviating as Fe3O4@PIL, was magnetically collected, washed with ethanol (50 ml, 3 times), and dried in vacuum at 60 oC. The obtained powder and phosphotungstic acid (3.0 g) were dispersed in ethanol/H2O (50 ml, v/v 1:1), and then the mixture was stirred at 60 oC for 12 h. A magnet
was
used
to
separate
the
product.
The
Fe3O4
grafted
poly
(phosphotungstate-based ionic liquid) catalyst was obtained after washing and drying， abbreviating as Fe3O4@PILPW (see Figure 1, step 3). 5
2.5 Esterification reaction The experiments of esterification of oleic acid with ethanol were performed in a 50ml round bottom flask equipped with a reflux condenser and a thermocouple at normal pressure. The desired amounts of oleic acid, ethanol, and as-prepared catalyst Fe3O4@PILPW were mixed in the reactor which had been adjusted to the desired temperature with an accuracy of ±0.1 oC. The reaction was stirred continuously at a speed of 500 rpm with an accuracy of ±1 rpm. The experiments were conducted at a reaction time range of 2-6 h, ethanol to oleic acid molar ratio of 6-14, catalyst amount of 5-15 (wt % of oleic acid), and reaction temperature of 90 oC. When the reaction was carried out for a given time, the reaction flask was immersed in cool water to stop the reaction. A magnet was put at the bottom of the reaction flask and the reaction mixture was divided into two layers. The upper layer was the crude ethyl oleate for analysis and the lower layer was the catalyst, which was processed for further reuse. The reaction conversion was analyzed by a titration procedure.32 A weighted amount of sample (weighed by analytical balance with an accuracy of ±0.1mg) was dissolved in ethanol and a few drops of phenolphthalein were added as the indicator. A 0.02mol/L KOH solution was used to perform the titration. The amount of KOH consumed was registered (with an accuracy of ±0.1ml) and the conversion was calculated using equation 1: ݔୡ୭୬୴. =
ି
(Eq. 1)
Where xୡ୭୬୴. was the conversion of oleic acid; ai was the initial acidity of the mixture and at was the acidity at the ‘‘t” time. All the experiments were done in triplicate and the average values obtained with random errors and standard deviations were presented. The predicted values calculated from the measured variables were presented with propagation of errors (POE). 2.6 Experimental design As the objective of the present research was to study the combined effects of by 6
response surface methodology (RSM)33, the Box-Behnken experimental design was chosen to design the experiments and estimate the relationship between the conversion of oleic acid with reaction time, catalyst amount, and molar ratio of alcohol to oleic acid for the purpose of optimizing the esterification of oleic acid. As the reaction was carried out in normal pressure, meanwhile, the mixture of oleic acid and ethanol was not an azeotropic system, the bubble point temperature of the mixture would vary with the change of molar ratio of ethanol to oleic acid which could be a few degrees higher than the boiling point of ethanol, the reaction temperature for oleic acid esterification was set as 90 oC.34, 35 A design with three independent variables (reaction time (X1), catalyst amount (X2), and molar ratio of alcohol to oleic acid (X3)) leading to seventeen sets of experiments was performed. The three variables were tested at levels by associated plus signs (+1) with high levels, zero (0) with center value, and minus signs (-1) with low levels. The conversion of oleic acid (Y) was chosen to be the response as a dependent variable. The independent factors and their levels, coded values, and real values were listed in Table 1.
The following model equation was used to analyze the interaction of response and variables and predict the optimum value. The quadratic equation model was expressed according to Eq.(2): 3
3
3
Y = λ0 + ∑ λi xi + ∑ λii xi2 + ∑ λij xi x j i =1
i =1
Where Y was the response function by the model; regression coefficients ( λ0 was the constant term,
(Eq. 2)
i＜j
λ0 , λi, λii
and λij were the
λi was liner effect term, λii
was
squared effect term, and λij was interaction effect term). The above coefficients would be estimated from the experimental results. The coefficient of determination (R2) could be used to evaluate the accuracy and general ability of the second order multiple regression model. The fitted polynomial equation was also expressed in the form of 3D response surface with the purpose of visualizing the relationships between the response and variables and to conclude the optimum conditions by Design Expert. 7
3. RESULTS AND DISCUSSION 3.1 Characterizations of catalyst The powder X-ray diffraction (XRD) patterns were recorded to identify the crystal structures of Fe3O4 and Fe3O4@PILPW, and the results were listed in Figure 2 as follows. The indices (111), (200), (311), (400), (422), (511), (440) matched well with the characteristic peaks of the inverse cubic spinel structure of Fe3O4. As for Fe3O4@PILPW, the pattern obviously consisted of two sets of diffraction peaks. The broad peak around 22.5° was indexed to the amorphous silica phase formed by the introduction of MPS in the process of synthesizing vinyl functionalized magnetic nanoparticle and the diffraction peaks at high angle (2θ>30) matched well with the cubic phase of Fe3O4. To further research the properties of Fe3O4@PILPW catalyst, XRD analysis for recovered catalyst was conducted (Figure 2c). The result was consistent with that of the fresh one, indicating the fine stability of Fe3O4@PILPW. The
FT-IR
spectra
of
Fe3O4,
Fe3O4@MPS,
ionic
liquid
monomer
[SO3H-PVIM][HSO4], H3PW, and Fe3O4@PILPW were shown in Figure 3. The Fe-O stretching vibration was observed around 580cm-1. Bands around 1750cm-1 and 1088cm-1 were respectively assigned to the stretching vibration of carbonyl groups and Si-O-Si, indicating that the surface of Fe3O4 was successfully modified by MPS. Curve c showed characteristic peaks at 1190 and 1042 cm-1 (S=O asymmetric and symmetric stretching vibrations), 1458 cm-1 (C-H asymmetric bending vibration), 1558 and 1654cm-1 (C=N and C=C stretching vibrations of the imidazole ring) and 886cm-1 (S-O stretching vibrations). Curve d showed four characteristic bands of H3PW at 1080cm-1 (P-Oa), 893cm-1 (W-Ob-W), 800cm-1 (W-Oc-W) and 982cm-1 (W-Od). In the spectrum of the Fe3O4@PILPW (Figure 3e), similar peaks in (Figure 3a, b, c, d) could be observed. These features indicated that sulfoacid functionalized ionic copolymeric cation and heteropoly anion were well reserved and Fe3O4 was successfully anchored to the polymer. FT-IR analysis for recovered catalyst was conducted to compare with the fresh ones and the results shown that the characteristic peaks of recovered catalyst (Figure 3f) were consistent with that of the fresh one. The 8
peak intensities of recovered Fe3O4@PILPW decreased to some extent, which might be due to the slight deactivation of the catalyst. The typical SEM and TEM images of Fe3O4, Fe3O4@PILPW, and recovered Fe3O4@PILPW were presented in Figure 4 a-f, respectively. The images of Fe3O4 presented monodispersed sphere. According to the histogram of Fe3O4 particle distribution (inset of Figure 4a), the Fe3O4 nanoparticle was in a relatively narrow size distribution with a diameter of 220~280nm and the average diameter of Fe3O4 was 247.6nm (~250nm). The SEM and TEM images displayed in Figure 4c and d revealed that the Fe3O4@PILPW was composed of Fe3O4 nanoparticles which were partly or fully embedded in poly (phosphotungstate-based ionic liquid). It could be clearly observed that grey poly (ionic liquid) layer had grown on the surface of Fe3O4, thus the Fe3O4 nanoparticle was firmly anchored onto the polymer. The images of recovered Fe3O4@PILPW had no visible change when compared with the fresh one, indicating that the catalyst had a fine stability. The TGA curves of Fe3O4 (a), Fe3O4@MPS (b), and Fe3O4@PILPW were shown in Figure 5. In all samples, the weight losses within 200 oC were the remove of adsorbed water molecules. Figure 5b showed a higher weight loss than bare Fe3O4 due to the decomposition of propylmethacrylate component. As shown in the curve of Fe3O4@PILPW, the weight loss was nearly 41.5% in the temperature range 300-500 o
C, which was attributed to the decomposition of the organic moiety of the poly (ionic
liquid) layers and followed with the complete collapse of PW12O403- to P2O5 and WO3.36 The magnetic properties of bare Fe3O4 and Fe3O4@PILPW were tested by VSM and the saturation magnetization values were 50.0 emu/g and 12.5 emu/g, respectively. As shown in Figure 6, the samples exhibited near-zero coercivity and remanence at room temperature, indicating a superparamagnetic characteristic. The saturation magnetization value of Fe3O4 had a significant drop after the immobilization of poly 9
(phosphotungstate-based ionic liquid). Even though, the prepared Fe3O4@PILPW catalyst showed excellent magnetic sensitivity. A complete separation of Fe3O4@PILPW could be achieved in a short time when a magnet was set at the bottom of the reaction flask.
3.2 Experimental results and RSM analysis 3.2.1 RSM experiments and fitting the model A Box-Behnken design (BBD) was employed to design and analyze the experiments. Design plan including three input variables for each experiment at fixed reaction temperature of 90 oC. The seventeen sets of experiments including twelve factorial points and five center points were performed and analyzed. The actual levels of the variables for the experiments in the design were determined and presented in Table 2. All the experiments were done in triplicate and the average values obtained with random errors and standard deviations were also presented in Table 2. As shown in Table 2, the random error values for each set of experiment were within ±1.5 % and the standard deviations were within 1.4 %, indicating a fine reproductivity. The relationship between the oleic acid conversion and the variables was achieved based on Eq.2 and the predicted oleic acid conversion values with POE values were shown in Table 2. According to the results, the POE values were within 2.0% and the experimental and predicted values matched well, indicating that the statistical model was adequate for predicting the oleic acid conversion.
The standard analysis of variance (ANOVA) was conducted to investigate statistical significance of each effect and the results were listed in Table 3. According to ANOVA, The model F-value of 376.95 implied the model was significant. There was only a 0.01% chance that a "model F-Value" this large could occur due to noise. Besides, the model had no lack of fit at 97.1 % level of significance. P values were used to check the effects of interactions among variables and values of "Prob > F" less than 0.05 indicated model terms were significant. In this case, X1, X2, X3, X1X3, X2X3, 10
X12, X22, and X32 were significant model terms. The model adequacies were checked by R2, adjusted R2and predicted R2. The value of R2 was 0.998, indicating that the model could explain 99.8% of the variability. The predicted R2 of 0.971 was in reasonable with the adjusted R2 of 0.995. As a result, the established model was statistically good.
According to the regression result, a best fitting-model for interaction of the oleic acid conversion and process variables was established. The second-order quadratic equation in terms of coded factors was shown as Eq. (3). Y=87.25+9.15X1+8.47X2+5.35X3+0.44X1X2+3.44X1X3-1.13X2X3-11.10X12-5.9 8X22-7.70X32
(Eq. 3)
Where Y was the oleic acid conversion; X1, X2, and X3 were the coded values of process variables reaction time, catalyst amount, and alcohol / acid molar ratio, respectively. As showing in Eq. 3, the process variables had linear and quadratic effects on the oleic acid conversion. The coefficient of X1 (9.15) was larger than others, which meant reaction time had the strongest effect on the response. Compared with other variables, the effect of alcohol / acid molar ratio (X3) was the weakest with a coefficient of 5.35. 3.2.2 Model Verification and Optimization Residual analysis of response surface design was illustrated graphically in Figure 7, Figure 8 and Figure 9 to check the adequacy of the fitted models before proceeding to optimization. As for the normal probability plot of the residuals, data points should be approximately linear. A non-linear pattern indicated non-normality in the error term. As shown in Figure 7, the normal probability plots of the internally studentized residuals were linear, meeting the demands. Figure 8 displayed the plots of the residuals versus predicted response for the oleic acid conversion. The random data points scattering across the horizontal line of residuals suggested that the proposed models were adequate. 11
Besides, Figure 9 showed the compare of predicted values obtained from regression model with the actual values from experimental studies for oleic acid conversion and all the design points were distributed along the diagonal line. If the points were above or below the line, it meant the data were overestimated or underestimated. The residuals between predicted and actual values were within the range of ±1 %. This comparative result indicated a negligible experimental error and the experimental data was in well agreement with the values predicted by the developed models. With this simple graphical method, the adequacies of the developed model was confirmed. Since the response of oleic acid conversion was influenced by three variables, adjusting the levels of the three input variables was essential to obtain the maximum benefit. As errors in input variables (reaction time, catalyst amount, and alcohol / acid molar ratio) would transmit to the response of oleic acid conversion, thus, in examining the propagation of error (POE), conditions were sought to minimize the transmitted variation of inputs.37 The effects of variable interaction on POE was illustrated in Figure 10. Among the three images, Figure 10b showed the least nonparallel curvatures when compared with Figure 10a and c, suggesting a strong interaction between the variables of reaction time and alcohol / acid molar ratio for POE. 3.2.3 Response surfaces The 3D surfaces and contour plot of the model (Eq. 3) for the oleic acid conversion was plotted as a function of two process variables while the last one was kept at level zero, which would be convenient to understand the interactions between two experimental factors and find out the optimum levels from the plots. The effects of the three variables on the conversion of oleic acid at 90 oC were displayed in Figure 11. 12
The effects of interaction between reaction time and catalyst amount on oleic acid conversion was shown in Figure 11a. At a constant catalyst amount, the conversion of oleic acid increased when the reaction time was prolonged to 5 h and decreased slightly with further increase of time up to 6h. Figure 11b depicted the effects of interaction between reaction time and alcohol / acid molar ratio. At any designated quantity of reaction time, the oleic acid conversion increased with the rise of alcohol / acid molar ratio from about 6:1 to 12:1. A slight decrease occurred when the alcohol / acid molar ratio was larger than 12:1, which was speculated that excess ethanol molecules may flood the active sites of the catalyst, hindering the protonation of oleic acid at the active sites. The circular contour line indicated that the interaction effect of the reaction time and alcohol / acid molar ratio was significant, which was matched with the P-value (F
Model X1 X2 X3 X1X2 X1X3 X2X3 X1 2 X2 2 X3 2 Residual Lack of Fit Pure Error
