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Esterification of Oleic Acid with Glycerol in the Presence of Supported Zinc Oxide as Catalyst Dheerendra Singh,† Prafull Patidar,‡ Anuradda Ganesh,† and Sanjay Mahajani*,‡ †

Department of Energy Science and Engineering, Indian Institute of Technology, Bombay, Powai, Mumbai, Maharashtra 400076, India ‡ Department of Chemical Engineering, Indian Institute of Technology, Bombay, Powai, Mumbai, Maharashtra 400076, India S Supporting Information *

ABSTRACT: The work deals with development of a new catalyst ZnO/zeolite and its performance evaluation for the industrially important reaction of esterification of oleic acid with glycerol. It is a reaction that takes place in liquid−liquid mode due to partial miscibility of glycerol and oleic acid. The reaction kinetics and product distribution over the developed catalyst are investigated under different conditions. The effects of different parameters such as catalyst loading, mole ratio, and temperature are studied. Higher temperatures and simultaneous water removal increase the reaction rate significantly. It is interesting to note that the selectivity toward monoglyceride for a given conversion remains unaffected with respect to temperature and mole ratio. A simplified kinetic model, that considers mono- and diesterification as a combination of series and parallel irreversible reactions, is proposed to explain the kinetic data.

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choice of many investigators.4,10,11,13 Sánchez et al.6 reported basic ultra stable Y-zeolite catalyst (with Si/Al ratio 2.4 and surface area 576 m2/g) and obtained 90% selectivity toward monoglyceride. Among the acid catalysts, heteropoly acids (e.g., silicotungstic acid) have been successfully tested for this reaction.12 Efforts have also been made to use surfactant assisted clays to overcome the immiscibility problems.9 Metal oxides/salts or their modified versions have also been explored as catalysts for this reaction by several investigators. Guner et al.5 proposed sulfated iron oxide catalysts, whereas Bombos et al.7 reported use of zinc oxide, ferrous oxide, and stannous oxide as catalysts; zinc oxide showed the best performance among all the catalysts. Macierzanka et al.8 studied the esterification kinetics for four individual fatty acids (C12:0, C14:0, C16:0, and C18:0), in the presence of zinc carboxylates of these acids. It was observed that the activity toward monoglyceride is higher than that obtained with sodium and potassium carboxylates. The reaction was carried out at reduced pressure (800 hPa) to remove water that was formed during the course of the reaction. The synthesis of glycerol monostearate in solvent medium and with basic oxide was studied by Pouilloux et al.14 The basic catalysts, such as Na2CO3, MgO, and ZnO, were studied for esterification of stearic acid; among all the catalysts, MgO showed better catalytic activity because of its higher basicity and higher surface area. However, the selectivity toward monoglyceride is higher in case of ZnO compared to other catalysts. This could be due to the specific amphoteric properties of ZnO. The basicity of ZnO is lower than MgO, which suppresses the diglyceride formation. They used solvents such as bis(2-methoxyethyl)

INTRODUCTION Glycerol is obtained as a byproduct in synthesis of biodiesel. Its production is about 10 wt % of the biodiesel formed and can be considered as a good value addition if converted to the useful products. It is an important raw material in the pharmaceutical and food industries. Monoglyceride being a good surfactant has a wide range of applications as emulsifier in food, pharmaceutical, and cosmetic industries. Monoglyceride can be synthesized by alternate routes, viz., glycerolysis of triglyceride using enzyme or base catalysts,1,2 trans-esterification of methyl ester with glycerol,3 or esterification of fatty acid with glycerol.4−14 In this work, we investigate the esterification route which is expected to be more selective and cost-effective compared to the other options. A general reaction scheme is given as

The various catalysts that are reported for the esterification of glycerol and the respective performances are summarized in Table S1, Supporting Information. Since the molecules involved in the reaction are relatively bulky, mesoporous materials, either as supports or as a catalysts, have been the © 2013 American Chemical Society

Received: Revised: Accepted: Published: 14776

May 22, 2013 August 20, 2013 September 26, 2013 September 26, 2013 dx.doi.org/10.1021/ie401636v | Ind. Eng. Chem. Res. 2013, 52, 14776−14786

Industrial & Engineering Chemistry Research

Article

Figure 1. XRD of β-zeolite, ZnO, and ZnO/β-zeolite.

International, USA. The ammonium form of zeolite can be converted into the H form when calcined at 500 °C for 10 h. Catalyst Preparation. Twenty-five percent ZnO/zeolite is prepared by the hydrothermal impregnation precipitation (HIP) method reported by Lu et al.16 for ZnO/MCM-41. A 1 M solution of zinc nitrate as a precursor for zinc is mixed with zeolite in a wt ratio of 1:3 (ZnO/zeolite). Urea as a precipitating agent was added to the mixture with atomic ratio of urea to zinc as 2:1. The mixture was stirred at 85 °C under nitrogen pressure of 4 kg cm−2 for 10 h in a 300 mL Teflon-lined stainless steel autoclave. The mixture was filtered, and the solid obtained was dried at 110 °C for 14 h. The catalyst was calcined from 110 to 500 °C at a rate of 10 °C min−1 and was maintained at 500 °C for 3 h. ZnO loading of 24.86% was confirmed by EDX which considers 5−6 different regions for elemental measurement, and an average value is reported. The value is further confirmed by ICP-AES analysis of Zn in the catalyst. Apparatus and Procedure. The reactions are carried out in a benchtop stirred reactor (Parr Instrument Company, USA) coupled with modular temperature controller to maintain the desired temperature and the stirring speed. The reactor has an inside diameter of 2.5 in. and a capacity of 300 mL. It is equipped with a four-blade stirrer of 1.38 in. diameter. In a typical run, the reactor is charged with 60 g of oleic acid, 80 g of glycerol, and 2.76 g of catalyst. The samples, approximately 2 mL each, are withdrawn at specific time intervals, and 10 mL of n-hexane is added to separate the organic layer from glycerol. The mixture is centrifuged to separate the catalyst powder. The top organic layer was separated from the sample and analyzed on HPLC after hexane is evaporated. The parameters like temperature, mole ratio, and catalyst loading are varied over a wide range to generate the kinetic data. Product Analysis. The conversion of FFA is calculated by titration using KOH solution in iso-propanol. The wt %s of FFA, MG, DG, and TG are determined by HPLC. A known

ether (diglyme) to increase the miscibility of glycerol and stearic acid, thereby obtaining 63.3% selectivity at about 82.5% conversion. From the studies reported in literature, zinc oxide catalyst is found to be the most promising catalyst for esterification of oleic acid with glycerol. However, zinc oxide in the powder form has a tendency to leach out with the reaction mixture. This is a serious problem from the point of view of downstream processing and environmental regulations. Moreover, oxide powder, as against the supported catalyst, cannot be easily converted to the form of pellets for their use in continuous reactors. In our earlier studies, we have found that, out of alumina, silica, and zeolites, zeolites work as the most promising support materials due to the less leaching of active metal in the reaction medium, without compromising the catalytic activity (Singh et al.15). The catalyst, in that work, was used at higher temperatures (up to 200 °C) for biodiesel synthesis, wherein the reaction mixture contains similar components, and it was found that the catalyst is reasonably stable for a longer period. It may be noted that glycerol and oleic acid are partially miscible and we have found that the reaction takes place preferentially in the oleic acid rich phase, and hence, the hydrophobic catalysts are more suitable for this reaction. One can conveniently vary Si/Al ratio in zeolite to manipulate the hydrophobicity of the catalyst. Zinc oxide supported on βzeolite with high silica content is hydrophobic and shows significant activity for this reaction. The present work demonstrates the applicability of this catalyst for the said reaction through systematic experiments and modeling work on the reaction kinetics.

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EXPERIMENTAL STUDIES Materials and Catalyst. Glycerol (>98%) and oleic acid (65−88%) are obtained from Merck India Ltd. Zeolite (CP 814T surface area 620 m2/g) is obtained from Zeolyst 14777

dx.doi.org/10.1021/ie401636v | Ind. Eng. Chem. Res. 2013, 52, 14776−14786


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Figure 2. TEM imaging of (a) zeolite; (b) ZnO/zeolite.

Figure 3. (a) SEM imaging, (b) elemental mapping, and (c) EDX of the catalyst ZnO/zeolite (metal indicated in elemental mapping by the following colors: yellow-Zn, red-Si, and blue-Al).

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Figure 4. N2 adsorption−desorption isotherms of (a) β-zeolite and (b)ZnO/β-zeolite.

respectively. The XRD patterns of ZnO obtained are consistent with the values in the database of JCPDS 36-1451. The XRD patterns of ZnO/β-zeolite consist of both ZnO and β-zeolite diffraction peaks. The average crystallite size of the samples calculated by the Scherrer equation is 22.15 nm. Transmittance Election Spectroscopy. TEM images of zeolite and ZnO/zeolite are shown in Figure 2. It can be seen that ZnO is well distributed on the external surface of the zeolite. The histogram of ZnO particles is shown in Figure 2b. The particle size varies over a range of 14−26 nm, average being 20 nm, which is close to the one determined by XRD. It may be noted that the ZnO particles are bigger than the average pore diameter of zeolite (2.2 nm from BET analysis), and hence, much of the ZnO sits on the external surface of the zeolite. Scanning Electron Microscopy. Figure 3b,c shows the elemental mapping and EDX results of ZnO/zeolite. In Figure 3b, the elements Zn, Si, and Al are represented by yellow, red, and blue colors, respectively. It shows that Zn is well distributed over the external surface. EDX mapping further supports the HR-TEM results of uniform spatial distribution of ZnO over the zeolitic support. Surface Area Measurement. The surface areas of βzeolite, supplied by Zeolyst International, and ZnO/β-zeolite 25% w/w ZnO are determined using N2 adsorption− desorption isotherms. The results are tabulated in Table S2, Supporting Information. Figure 4 indicates a type IV isotherm with a hysteresis loop associated with capillary condensation in mesopores, that suggested the coexistence of micropores and mesopores. The surface area of β-zeolite (CP300) is found to be 623.47 m2 g−1, and the surface area of 25% ZnO/zeolite is 327.71 m2 g−1. The decrease in surface area and pore volume (from 0.34 to 0.21 cm3 g−1) is mainly due to the reduction of zeolite amount in the catalyst (from 100% to 75%). The surface area of pure zinc oxide is less than 2 m2 g−1 (from BET); hence, the surface area of catalyst mainly comes from the pores of the zeolite support. The tuning of pore size is applicable mainly in the case of mesoporous supports which house the catalytic species inside the pores. In this case, since the catalyst particles reside on the external surface, tuning of the pore volume and area is not expected to offer any benefit. General Course of the Reaction. Esterification of oleic acid with glycerol in the absence of catalyst is a slow reaction and needs catalyst to achieve substantial rates. The stirring

weight of the top layer is dissolved in 5:4 (v/v) iso-propanolhexane mixture. The components, monoglycerides, fatty acid, diglycerides, and triglycerides, are separated on a C18 column (4.6 mm ID × 250 mm length). The injection volume is 20 μL, and the flow rate and temperature are maintained at 1 mL/min and 45 °C, respectively, to help elute all the components in about 30 min. Glycerol solubility in oleic acid is analyzed using a gas chromatograph (Nucon Ltd., India) at 220 °C on an AT1000 packed column. The injector and detector (FID) are maintained at 220 and 230 °C, respectively. In all the runs, the amount of carbon, calculated on the basis of the products formed, accounts for more than 90% of the oleic acid determined by the titration method. The selectivity of a given product is calculated as the ratio of the total number of moles of the product formed to the moles of FFA converted. Catalyst Characterization. The X-ray diffraction analysis of the powder catalyst is performed using a Philips X’-pert diffractometer. The X-ray diffraction data are recorded by using Cu Kα radiation (λ = 1.5418 Å) at 40 kV and 40 mA. The intensity data are collected over a 2θ range of 5−80°. The samples are analyzed on a high resolution field emission transmission electron microscope (HR-TEM, JEOL-2100F) using an accelerating voltage of 200 kV. For TEM analysis, a well-dispersed solution was prepared by adding a little amount of catalyst powder in ethanol and sonicating it for 10−15 min. One drop of the dispersed solution was taken on the TEM grid and dried under an IR lamp for 30 min; images are taken under the best operating conditions. For SEM analysis, a fieldemission gun scanning electron microscope (FEG-SEM, JEOL7600F) with a resolution of about 1 nm was used to study the surface morphology of the samples. Powder sample was directly sprinkled over carbon tape, and the images are taken under the best operating conditions. Surface area was obtained by N2 adsorption−desorption isotherms measured at −196 °C on an instrument ASAP 2020 V3.01 H (Micromeritics, USA). About 0.2 g of powder sample was used for the analysis; catalyst is degasified at 350 °C for 400 min before its use.

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RESULTS AND DISCUSSION X-ray Diffraction. Figure 1 shows the XRD results of βzeolite and ZnO alone and ZnO/β-zeolite. The diffraction peaks of ZnO, obtained in pure ZnO and ZnO/β-zeolite, at 2θ = 31.9, 34.7, 36.4, 47.7, 56.8, and 63.10 correspond to the lattice planes (100), (002), (101), (102), (110), and (103) 14779



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speed (rpm) is varied over a range of 400−700 rpm, and there is no significant effect on reaction rate. Hence, all the experiments are performed at 700 rpm to ensure minimal external mass transfer limitations. To determine the potential of the catalyst, the reactions are performed under otherwise similar conditions for five cases, i.e., in the absence of catalyst, in the presence of Amberlyst-35, in the presence of only βzeolite, in the presence of pure ZnO, and in the presence of ZnO supported on zeolite. The results are shown in Figure 5a− c. It can be seen that the reaction takes place even in the absence of catalyst but zeolite alone does not show any catalytic activity. ZnO supported on zeolite shows a significant rise in the rate, which is comparable with pure ZnO. The results indicate that the zeolite supported catalyst is equally active as ZnO powder. Hence, because of its other engineering benefits, mentioned before, they are promising catalysts for the commercial production of glycerol esters. Hence, a systematic kinetic study is performed using 25% ZnO/β-zeolite as a catalyst, and the results are used for the estimation of relevant kinetic parameters. The results also indicate that both mono- and diglyceride concentrations increase with time. Interestingly, in spite of being an intermediate product, monoglyceride concentration does not go through a maximum, which is an indication of DG being formed also through a parallel reaction, directly from glycerol. On a conversion vs selectivity plot, all the catalysts show a similar trend. Locale of Reaction. The reaction mixture at any given time contains two liquid phases because glycerol and oleic acid are partially miscible. The reaction may occur in either of the phases or at the interface depending on the solubility of the components and catalyst wettability. We performed systematic experiments specifically to identify the locale of reaction in a four-neck glass reactor with temperature sensor, stirrer, condenser, and sample port. The glycerol and oleic acid are contacted at 100 °C for 36 h. The purpose of contacting oleic acid and glycerol at the elevated temperature (e.g., 100 °C) is to ensure sufficient concentration of glycerol in the oleic acid phase for the reaction to occur and obtain a sufficient amount of product, allowing us to examine the presence of the reaction in the oleic acid rich phase. At equilibration, the layers are separated and independently reacted at 150 °C and 0.5 wt % catalyst loading. The reactions are performed under continuous flow of nitrogen gas to remove water formed during the course of esterification. It is observed that the oleic acid solubility in glycerol is very low and below the detection limit of HPLC. As expected, the esterification reaction did not occur in the glycerol phase. On the other hand, reaction takes place in the oleic acid phase; the conversion of oleic acid and product distribution with respect to time are shown in Figure 6a,b. The reaction in the oleic acid-rich phase shows a maximum conversion of 12%. The conversion was attributed to the presence of glycerol in oleic acid. It is seen in Figure 6b that the mole fraction of MG remains almost constant; DG formation goes through a maximum while the formation of TG increases gradually with time. It may be noted that the reaction is performed in a single liquid phase as against the other kinetic runs performed in liquid−liquid mode. In this case, only a limited amount of glycerol is available in the oleic acid phase for the reaction to occur, and hence, DG probably undergoes subsequent esterification to give TG. Hence, we can also conclude that, though the reaction takes place in organic phase,

Figure 5. (a) Conversion of oleic acid using different catalysts and without catalyst (Gly/OA mole ratio 4:1; reaction temperature 150 °C; zeolite, Amberlyst 35, and ZnO loading 2.0 wt % each). (b) Mole fraction of the component using different catalysts and without catalyst (Gly/OA mole ratio 4:1; reaction temperature 150 °C; zeolite, Amberlyst 35, and ZnO loading 2.0 wt % each). (c) Conversion of OA vs selectivity of MG, DG, and TG using different catalysts and without catalyst (Gly/OA mole ratio 4:1; reaction temperature 150 °C; zeolite, Amberlyst 35, and ZnO loading 2.0 wt % each). 14780



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organic phase, i.e., rich in fatty acid. Glycerol being a polar compound extracts water during the course of the reaction and helps shift the reaction in the forward direction. Figure 6c shows the increase in water concentration during esterification reaction in both the phases. The amount of water formed during the reaction is calculated from oleic acid conversion whereas the concentrations in both the phases are obtained by the Karl Fisher titration. From the results, we can conclude that water is mainly present in the glycerol rich phase. In other words, glycerol helps extract water from the oleic acid phase during the course of the reaction to improve the performance significantly. At longer reaction times (>240 min), the slight decrease in the concentration of water in the glycerol-rich phase is probably due to simultaneous vaporization of water. Leaching Test. As mentioned before, the main objective of using zeolite as a support for ZnO is to reduce the extent of leaching and problems associated with it, such as catalyst deactivation and pollution. The motivation comes from our earlier study on the applicability of ZnO for biodiesel synthesis by transesterification of triglycerides. Table 1 shows the results Table 1. Results of ICP Analysis of Reaction Mixture at the End of 6 h in organic (oleic acid rich) phase

Zn (ppm)

ZnO/zeolite (150 °C, 0.5 wt % cat loading) ZnO/zeolite(150 °C, 1.0 wt % cat loading) ZnO/zeolite (160 °C, 0.5 wt % cat loading) ZnO (150 °C, 0.5 wt % cat loading) in aqueous (glycerol-rich) phase

663.05 826.753 796.877 >2986.86 Zn (ppm)

ZnO (150 °C, 0.5 wt % cat loading) ZnO/zeolite (150 °C, 0.5 wt % cat loading)

61.512 52.396

of the ICP analysis performed by contacting the catalyst with both oleic acid and glycerol under the reaction conditions of interest. It is evident that there is a substantial reduction in the extent of leaching when zeolite is used as a support. Though it needs further investigation, the probable reason for this is more metal support interaction of ZnO with zeolite as compared to other supports. ZnO is amphoteric in nature, which means it can behave as either acid or base. As reported by Singh et al.,15 ammonia TPD of zeolite shows two types of acidic sites. The peak corresponding to strong acidic sites is not observed in the supported sample, which means that, ZnO being basic in nature, is adsorbed on strong sites. Alumina in either form, alpha or gamma, is relatively less acidic and thus interacts less with ZnO. All in all, there is more metal support interaction in the case of zeolite than alumina. Interestingly, the catalyst activity on zeolite support remains unaltered even in the presence of metal−support interactions. Effect of Catalyst Loading. The experiments are performed at three different catalyst loadings, i.e., 0.25, 0.5, and 1.0 wt %. Figure 7a shows that the conversion of oleic acid increases with an increase in catalyst loading only up to 0.5% (w/w), and thereafter, the rate is insensitive to the change in catalyst loading. Figure 7b shows the trend in selectivity. At lower catalyst loading (0.25%), the selectivity toward monoglyceride is relatively low compared to that at higher catalyst loadings. The optimum catalyst loading therefore lies close to 0.5% w/w. The insensitivity to the change in catalyst loading beyond a particular limit for liquid phase reactions, even in the absence of mass transfer limitations, is well-known and reported in the literature for other reactions (Talwalkar et

Figure 6. (a) Conversion of oleic acid when reaction is conducted in the oleic acid-rich phase (reaction temperature 150 °C; catalyst loading 0.5 wt %). (b) Mole fraction of component in oleic acid phase (reaction temperature 150 °C; catalyst loading 0.5 wt %). (c) Distribution of water in glycerol and oleic acid phases during the course of the reaction.

the product distribution is strongly influenced by the presence of a separate glycerol phase in the reactor. As mentioned before, the reaction takes place in liquid− liquid mode due to the miscibility gap between glycerol and oleic acid. The catalyst being hydrophobic prefers to stay in the 14781



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Figure 7. (a) Conversion of oleic acid at different catalyst loadings (reaction temperature 150 °C and mole ratio Gly/OA 4:1). (b) Selectivity vs conversion at different catalyst loadings (reaction temperature 150 °C and mole ratio Gly/OA 4:1).

Figure 8. (a) Conversion of oleic acid at different temperature (Gly/ OA mole ratio 4:1; catalyst loading 0.5 wt %). (b) Conversion of OA vs selectivity of MG and DG at different temperatures (Gly/OA mole ratio 4:1; catalyst loading 0.5 wt %).

al.17 and Moreau et al.18). The probable reason is that the number of sites increases so much that, some of the sites, at any given time, remain unused from the catalysis point of view. For studying the effect of other parameters like temperature and mole ratio, the optimum, i.e., 0.5 wt %, catalyst loading was used to generate the data for kinetic modeling. Reactions are performed at sufficiently high RPM (700 rpm) using powder ZnO/zeolite catalyst (less than 1 μm) to eliminate the mass transfer resistances. Effect of Temperature. The reactions are performed over a temperature range of 140−160 °C. Figure 8a shows the conversion of oleic acid at different temperatures showing an increase in the rate of the reaction with an increase in temperature. Figure 8b shows the selectivity vs conversion plot at different temperatures. It is evident that the selectivity at a given conversion neither gets adversely affected nor does it increase, which means that one should perform the reaction at as high of a temperature as possible to exploit the rise in rate. The formation of triglyceride is found to be insignificant (mole fraction 600 rpm) for 2.5 h and then allowed to settle for 30 min until clear phase separation is obtained. During this period, it is found that about 60% OA is converted to MG, DG, and TG by self-catalysis; hence, the solubility of glycerol measured is in the mixture and not in pure OA. From the top layer, a sample is withdrawn and mixed with methanol. It is then analyzed to determine the solubility of glycerol. As shown in Figure 12b, the glycerol solubility is below the detectable limit up to 80 °C and then rises with temperature. The solubility data (4.85 wt % at 170 °C) is consistent with the data obtained for the reaction mixture, wherein 4−5 wt % glycerol is observed in the organic layer. This data is used later in the kinetic modeling to estimate the intrinsic rate constants of the individual reactions. Kinetic Modeling. Several researchers have undertaken studies to obtain a better understanding of the reaction pathway of glycerol esterification with different fatty acids. On the basis of their studies, reaction pathway could be modeled either as a set of consecutive reactions or as a set of irreversible parallel reactions. The irreversibility is due to simultaneous removal of water from the organic phase to the glycerol phase. The studies in the literature for the kinetic model of glycerol esterification are summarized in Table S3, Supporting Information. Our attempt to fit the data in a model based on both the reaction schemes failed. The selectivity vs conversion plots (Figures 8b and 9b) show that selectivity of MG slightly decreases with an increase in conversion at the cost of rise in DG selectivity. This observation suggests that a consecutive

Figure 9. (a) Conversion of oleic acid at different mole ratios of Gly/ OA (reaction temperature 150 °C; catalyst loading 0.5 wt %). (b) Conversion of OA vs selectivity of MG and DG at different mole ratios of Gly/OA (reaction temperature 150 °C; catalyst loading 0.5 wt %).

Effect of Water Removal. It has been reported that the reaction, being reversible, gets benefited by simultaneous removal of water during the course of the reaction. Hence, the reaction is performed in a glass reactor at 150 °C, and nitrogen is passed at a flow rate of 200 mL/min to strip off the water formed during the reaction. The fatty acid conversion and selectivity for MG and DG, with and without water removal, are shown in Figure 11a,b. It can be seen that there is a slight increase in the rate if we remove water from the reaction although the difference is not significant and it is further lower for reaction time less than 3 h. This may be attributed to the fact that the reaction temperature is much higher than the boiling point of water, and hence, the water formed in the reaction escapes with nitrogen flow during the course of the reaction. As expected, the conversion of oleic acid is found to increase with water removal (Figure 11a). However, the selectivity, as shown in Figure 11b, remains unaffected even with simultaneous removal of water. The mole fraction of TG in both cases is less than 0.05. Solubility of Glycerol. The oleic acid-rich phase is analyzed for glycerol concentration during the course of the reaction, and as shown in Figure 12a, the solubility (4−5 wt %) is found to remain almost constant throughout the reaction. 14783



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Figure 12. (a) Glycerol solubility (wt %) in the organic layer at different time intervals (reaction tempearture 170 °C; Gly/OA mole ratio 3:1; catalyst loading 0.5 wt %). (b) Solubility of glycerol in the organic mixture at different temperatures.

Figure 11. (a) Conversion of oleic acid with and without water removal (reaction temperature 150 °C; Gly/OA mole ratio 4:1; catalyst loading 0.5 wt %). (b) Conversion of oleic acid vs selectivity of MG, DG, and TG both for water removal and without water removal reaction (reaction temperature 150 °C; Gly/OA mole ratio 4:1; catalyst loading 0.5 wt %).

reaction of MG to DG also takes place simultaneously. However, consecutive reaction model alone is also not adequate as its characteristic peak in the intermediate composition is not evident in any of the runs. Hence, we propose a combination of both series and parallel reactions. Esterification of glycerol with oleic acid proceeds with water formation. The water formed during the reaction is mainly present in the glycerol phase as shown earlier in Figure 6c, whereas the esterification reaction occurs in the organic phase that is mainly composed of oleic acid and esters at any given time. Because glycerol is only sparingly soluble in the organic phase, as shown in Figure 12b, and oleic acid solubility in glycerol is not significant, esterification of glycerol with oleic acid may be considered as irreversible reactions, over a large range of conversion. Following are some more assumptions made to formulate the concentration-based kinetic model for this reaction (Figure 13): (1) Formation of triglyceride is very small and can be neglected for the modeling exercise. (2) It is observed that conversion is not affected by an increase in catalyst loading beyond a certain optimum value. The kinetic model developed here is based on the data collected at the optimum catalyst loading (0.5% w/w). The rates of formation of monoglyceride and diglyceride are given by eqs 1 and 2.

Figure 13. Reaction scheme considered for the development of the kinetic model.

rMG =

n dxMG = k f1xOAxGLY − k f2xMGxGLY VOrg dt

(1)

rDG =

n dx DG = k f3(xOA )2 xGLY + k f2xMGxGLY VOrg dt

(2)

where 14784



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Table 2. Parameters of the Proposed Kinetic Modela

a

s. no.

reaction

kf0 (kmol min−1 lit−1)

E (kJ kmol−1)

SSE

1 2 3

OA + Gly → MG + water MG + Gly → DG + water 2OA + Gly → DG + water

240053.7 ± 225865 21285844 ± 12570152 390003.1 ± 384143

44998.03 ± 3397.82 66039.53 ± 7144.66 52869.03 ± 3883.37

0.00073 0.00073 0.00073

The values reported are for a 95% confidence interval.

k f1 = k f10exp( −Ef1/RT)

for monoglyceride in the conversion range of 60−90%. The product distribution, at a given conversion, is not very sensitive to temperature, mole ratio, and catalyst loading beyond a certain limit (>0.5% w/w). A kinetic model, which combines series and parallel reaction pathways, is proposed, and the predictions are in reasonably good agreement with the experimental results.

k f2 = k f2 0exp( −Ef2 /RT) k f3 = k f30exp( −Ef3/RT)

xGLY is the solubility of glycerol in the organic phase and is assumed to be constant. The kinetic parameters of the model are estimated by minimizing the sum of squares of error between the calculated values of mole fractions of different components and that observed through the experiments. min ϕ =

∑ samples

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ASSOCIATED CONTENT

* Supporting Information S

Table S1, Table S2, Table S3, and kinetic data in tabular form. This material is available free of charge via the Internet at http://pubs.acs.org.

(χi ,cal − χi ,exp )2 (3)

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MATLAB function “nlinfit”, which is based on Levenberg− Marquardtalgorithm, is used to perform the regression analysis. The estimated values of the relevant parameters with standard error for a 95% confidence limit are given in Table 2. The parity plot for mole fraction of OA, MG, DG, and glycerol for different runs performed at different temperatures and at optimal catalyst loading is shown in Figure 14. The predictions of series-parallel reactions model agree well with the experimental data generated up to 80% conversion of oleic acid.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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NOMENCLATURE Ef = activation energy of forward reaction, kJ kmol−1 kf0 = Arrhenius pre-exponential factor for forward rate constant, kmol min−1 lit−1 kf = forward reaction rate constants, kmol kg−1 min−1 n = initial molar holdup, kmol ri = rate of the reaction of species i, kmol min−1 R = ideal gas constant, kJ kmol−1 K−1 t = time, min T = temperature, K χi = mole fraction of species i in the liquid phase VOrg = volume of organic phase, lit

Abbreviations

Figure 14. Parity plot showing comparison of predicted and experimental values of the mole fractions of different components in all the experiments.

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CONCLUSION The synthesized ZnO supported zeolite catalyst shows promising results for monoglyceride production. The characterization result suggests a well distributed ZnO on the external surface of the zeolite with ZnO particle size of 20−25 nm and BET surface area of 221 m2 g−1. The leaching of Zn is significantly reduced when zeolite is used as a support. Esterification of oleic acid gave selectivity as high as 70−80%

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MG = monoglyceride DG = diglyceride TG = triglyceride OA = oleic acid FFA = free fatty acid Gly = glycerol XRD = X-ray diffraction TEM = transmittance electron microscopy SEM = scanning electron microscopy EDX = energy-dispersive X-ray spectroscopy BET = Brunauer−Emmett−Teller HPLC = high-performance liquid chromatography FID = flame ionization detector GC = gas chromatograph SSE = sum of squares of errors

REFERENCES

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