TiO2

TiO2

G Model JIEC 3411 No. of Pages 10 Journal of Industrial and Engineering Chemistry xxx (2017) xxx–xxx Contents lists available at ScienceDirect Jour...

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G Model JIEC 3411 No. of Pages 10

Journal of Industrial and Engineering Chemistry xxx (2017) xxx–xxx

Contents lists available at ScienceDirect

Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec

Production of diacylglycerols by esterification of oleic acid with glycerol catalyzed by diatomite loaded SO42/TiO2 Zhen Zhanga,b , Huihua Huanga,** , Xiang Mac , Guanghui Lib , Yong Wangb,* , Guo Sunb , Yinglai Tengb , Rian Yanb , Ning Zhangb , AiJun Lib a b c

College of Food Science and Engineering, South China University of Technology, 381 Wushan Road, Guangzhou 510641, China Guangdong Saskatchewan Oil seed Joint Laboratory, Department of Food Science and Engineering, Jinan University, Guangzhou 510632, China Research School of Chemistry, The Australian National University, Canberra, ACT 2612, Australia

A R T I C L E I N F O

Article history: Received 3 January 2017 Received in revised form 21 March 2017 Accepted 2 May 2017 Available online xxx Keywords: Diatomite-loaded SO42–/TiO2 Solid acid catalyst Molecular distillation Diacylglycerols Esterification

A B S T R A C T

A new and efficient technique is described for the production of diacylglycerols (DAGs) by the esterification of oleic acid with glycerol catalyzed by diatomite-loaded SO42/TiO2. DAGs show some potential health benefits compared to triacylglycerols, and also can be used to produce the novel industrial plasticizer epoxy fatty acid methyl ester in material science. Diatomite-loaded SO42/TiO2 catalyst was prepared and characterized, and the selected conditions for the synthesis of DAGs were determined to be: reaction time = 6.0 h, temperature = 210  C, catalyst loading = 0.1% of the oleic acid weight, and mass ratio of oleic acid to glycerol = 2:1. Under these conditions, DAGs yield reached 59.6% with a purity of 69.6% after a one-stage molecular distillation. Diatomite-loaded SO42/TiO2 as a solid catalyst could be recycled and reused with high catalytic efficiency. Under the same conditions, diatomite-loaded SO42/TiO2 showed a better catalytic performance than the commercial solid acid SO42/ZrO2-Al2O3. Based on this, a two-step reaction method for the production of DAGs was performed and provided a yield similar to the one-step method (58.3% vs. 59.6%), but with a shorter reaction time (4 h vs. 6 h). It is concluded that a two-step reaction method could be a better alternative to the one-step production of DAGs in the presence of diatomite-loaded SO42/TiO2. © 2017 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

Introduction Diacylglycerols (DAGs) are from edible oils and widely used as emulsifiers in food and pharmaceutical industries due to their beneficial physiochemical properties and nutritional values [1–3]. Recently, DAGs have been used for the production of hydroxylated epoxy esters, which represent a new category of safe plasticizers for the polymer industry [4].

Abbreviations: MAG, monoacylglycerol; DAG, diacylglycerol; TAG, triacylglycerol; FFA, free fatty acid; TFA, trans-fatty acids; GC, gas chromatography; X-RD, Xray diffraction; IR, infrared spectroscopy; SEM, scanning electron microscope; FAME, fatty acid methyl ester. * Corresponding author. ** Corresponding author at. Department of Food Science and Engineering, College of Science and Engineering, Jinan University, 601 Huangpu Ave West, Guangzhou 510632, China. Fax: +86 20 85226630. E-mail addresses: [email protected] (H. Huang), [email protected] (Y. Wang).

Catalytic production methods of DAGs can be classified into chemical and enzymatic approaches, and the products of these reactions are purified through high-vacuum distillation. Enzymatic reactions usually occur under mild conditions [5,6]. For instance, Watanabe et al. [7] reported an effective lipase-catalyzed esterification, with the highest DAGs content being 70%. However, drawbacks of enzymatic approaches include limitations inherent to the equipment used, long reaction time, and high cost enzymes [8]. Chemical methods including esterification and glycerolysis are commonly utilized in the production of DAGs. It has been reported that DAGs could be produced through the continuous chemical glycerolysis of fats and oils at high temperatures (220–250  C) using inorganic alkaline catalysts in a nitrogen gas atmosphere [9]. Solid acids such as zeolites, resins of the type Naftion-M, zirconium sulfate on silica, and Amberlyst-15, have been reported to catalyze the esterification of oleic acid [10]. Alternatively, sulfated titania (SO42/TiO2) is also a capable solid acid catalyst for esterification [11]. As solid-catalysts can be easily separated by filtration from the reaction system and be recycled with no corrosive effects on the reaction equipment and little

http://dx.doi.org/10.1016/j.jiec.2017.05.001 1226-086X/© 2017 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

Please cite this article in press as: Z. Zhang, et al., Production of diacylglycerols by esterification of oleic acid with glycerol catalyzed by diatomite loaded SO42/TiO2, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.05.001

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environmental pollution, in recent years they become very attractive for both research and industrial applications [12]. With the rapid development and commercialization of biodiesel, the co-product, glycerol is currently produced in a huge amount, which makes the price of glycerol to decline very sharply. Consequently, it is of potential economic benefits to convert lowcost glycerol into value-added chemicals or materials [13]. As a result, a variety of catalytic processes have been developed for the

valorization of glycerol, by hydrogenolysis, reforming, etherification, esterification, oxidation, dehydration, and so on [14–16]. The reaction route of production of DAGs by esterification is proposed in Scheme 1a, and the side reaction of glycerolysis and transestrification of acylglycerols is given in Scheme 1b. Sulfated titania (SO42/TiO2), the solid acid, shows both Brönsted acid and Lewis acid activity when catalyzing [11] and the mechanisms of catalysis for both activities are clarified in Scheme 1c and d

Scheme 1. (a) Production of acylglycerols by esterification of oleic acid with glycerol. (b) Glycerolysis and transesterification of acylglycerols as side reaction. (c) Esterification reaction pathway over Brönsted acid sites (H+). (d) Esterification reaction pathway over Lewis acid sites (A+).

Please cite this article in press as: Z. Zhang, et al., Production of diacylglycerols by esterification of oleic acid with glycerol catalyzed by diatomite loaded SO42/TiO2, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.05.001

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Oleic acid (industrial grade) was provided by Masson Food Technology Co., Ltd. (Guangzhou, China). TiCl4 (>99%), NH4OH (28%–30%), glycerol (>99%), acetone (>99.5%), diatomite powder, and H2SO4 (>99%) were purchased from Fuyu Chemical Co., Ltd. (Tianjin, China), SO42–/ZrO2-Al2O3, was procured from Taide Chemical Scientific Co., Ltd. (Zibo, Shandong, China).

toglycerol (5:1, 10:3, 5:2, 2:1 and 5:3), the loading of catalyst (0.1%, 0.2%, 0.3%, 0.4% and 0.5%, based on the oleic acid mass), reaction temperature (170, 180, 190, 200 and 210  C) and reaction time (1.5, 3, 4.5, 6 and 7.5 h). The DAGs yield and the content of free fatty acids (FFA) in the final product of each trail have been analyzed. In a typical procedure, oleic acid, glycerol and diatomite-loaded SO42/TiO2 (150.0 g of reaction substrates) were added in a 250 mL round-bottom flask. The reaction was conducted under vacuum pressure (2000 Pa) using a water ring pump inserted through a glass condenser with circulating tap water. The flask was heated in a thermostatic oil bath. A mechanical impeller with a plastic paddle rotating at 200 r/min was used to stir the reaction mixture. After reaction, the reaction mixture was cooled and settled into two layers. The remaining glycerol and catalyst on the bottom phase were recycled. The upper phase comprising acylglycerols were then filtered and analyzed by GC. According to the selected conditions above, comparison tests with other two kinds of solid acid catalysts (SO42/TiO2 and SO42/ ZrO2-Al2O3) were performed. A two-step reaction was also conducted as follows according to the selected conditions: after the first-step reaction followed the typical procedure for 2 h, the reaction mixture was cooled and settled into two layers. The upper oily phase, which was composed of FFA and acylglycerols, was collected for the second-step esterification with glycerol at the same mass ratio. The catalyst at the lower phase was collected after filtration, carbonation, and re-calcination, and then was reused to catalyze the esterification of the oily phase from the first step with the unreacted glycerol from the first step and some newly added glycerol for the other 2 h.

Diatomite-loaded SO42/TiO2 preparation

Catalyst reuse

TiCl4 (3.0 g) was dissolved in deionized water to prepare a 3.0% solution, and a diatomite powder (33.0 g) carrier was added to the solution under agitation. NH4OH (28.0%–30.0%) was introduced to the solution to adjust the pH to 9–10 and precipitate metal oxides. The precipitate formed in the solution was allowed to stand for 24 h, then filtered and washed with deionized water to remove the chloride. The washed precipitate was dried at 110  C for 12 h and then impregnated in H2SO4 (>99%) for 24 h. Excess sulfuric acid was removed by filtration. The remaining solid was dried at 110  C for 8 h and then calcined in a muffle furnace at 550  C for 3 h to obtain the final solid acid, which is expressed as diatomite-loaded SO42/TiO2. SO42/TiO2 were prepared using the same process without being loaded in diatomite.

After reaction, the filtered catalyst was carbonated and recalcined at 550  C for 1 h to obtain the recycled catalyst for reuse test. The total acidity of the catalyst before and after 5 times of reaction was also recorded by titration method [20,21]. Sulphurelement content in the catalyst before and after reaction was determined by the elemental analyzer (Vario EL cube, Elementar, Germany).

respectively. In short, the BrENTITY NOT DEFINED !!!nsted acid sites act as an H+ donor whereas the Lewis acid sites accept the electron pair [17]. Solid acid catalysts have already been used for biodiesel production [18]. In our previous study, a glycerol esterification of free fatty acid from waste cooking oil catalyzed by SO42/ZrO2-Al2O3 was applied to lower the free fatty acid content for biodiesel production [19]. In this work, TiCl4 was used to prepare diatomite-loaded SO42/ TiO2 as a solid acid catalyst for DAGs production. X-ray diffraction (X-RD), infrared spectroscopy (IR) and scanning electron microscopic (SEM) techniques were applied to characterize the structures and properties of diatomite-loaded SO42/TiO2. The conditions for DAGs production and the following product purification by molecular distillation were investigated. The content of trans-fatty acids (TFAs) in DAGs was analyzed by gas chromatography (GC). Recycled and reused tests of diatomiteloaded SO42/TiO2 in esterification were also studied. Based on the results, a two-step method for the production of DAGs has been proposed to achieve similar yield of the one-step method but within a shorter time. Experimental Materials

Catalyst characterization The diatomite-loaded SO42/TiO2 was characterized by X-RD and recorded on an MSAL XD-2 diffract meter (Bragg Science and Technology Co., Ltd., Beijing, China), using Cu Ka radiation (l = 0.15418 nm) at 36 kV and 20 mA from 15 to 70 . The surface morphology and particle size of the catalysts were observed by field emission SEM (ZEISS, Germany), and IR spectroscopy was carried out in a 640-IR Spectrometer (Varian, USA). The spectra were obtained using the transmission method over 32 scans from 4000 to 400 cm1 with a resolution of 1 cm1. Thin film samples were used for qualitative IR investigation using the KBr pellet technique. DAGs production The production of DAGs was performed under different operating conditions, including the mass ratio of oleic acid

Product purification One-stage molecular distillation was performed using MD80 molecular distillation equipment (Handway Technology Co., Ltd., Guangzhou, China) to remove the monoacylglycerols (MAGs) [22]. Approximately 50 g acylglycerols were placed into the head tank and dehydrated at 80  C until the vacuum reached 30 Pa. The distillation conditions were: evaporator temperature = 190  C, vacuum pressure = 0.1 Pa, and knifing rate = 300 r/min. The acylglycerols were separated into two layers, with the light phase being the MAGs and the heavy phase containing DAGs and triacylglycerols (TAGs). The light phase was analyzed by GC. All determinations were performed in duplicate and the mean values were reported. Fatty acids analysis The fatty acid composition of the products was analyzed as fatty acid methyl esters (FAMEs) by GC using a capillary column (CPSil88, 100 m  0.250 mm i.d., 0.2 mm in film thickness, Agilent Technologies Inc., Palo Alto, CA, USA). The oil sample was converted into FAMEs by trans-esterification using the acidic method as previously described [23]. All determinations were performed in duplicate and the mean values were reported.

Please cite this article in press as: Z. Zhang, et al., Production of diacylglycerols by esterification of oleic acid with glycerol catalyzed by diatomite loaded SO42/TiO2, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.05.001

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Acylglycerols analysis Analysis of the esterification products was carried out by a GC system equipped with a capillary column (DB-1HT, 15 m  0. 250 mm i.d., 0.1 mm in film thickness, Agilent Technologies Inc., Palo Alto, CA, USA). Acylglycerol yields were expressed as the percent content of the corresponding peak area response compared to the total peak area, using FID. All determinations were performed in triplicate, and the means  standard deviations were reported. Statistical analysis Analyses for MAGs, DAGs, TAGs, and FFA were done in triplicate, with data reported as the means  standard deviations. One-way ANOVA was performed using SPSS 16 statistical software (SPSS Inc., Chicago, IL). Differences were considered to be significant at p  0.05, according to Duncan’s Multiple Range Test. TFA analysis of oleic acid materials were performed before and after esterification, and differences were considered to be significant at a = 0.05, according to the Independent-Samples T Test. Results and discussion Characterization of diatomite-loaded SO42/TiO2 Fig. 1 shows the X-RD diffractograms of SO42/TiO2 and diatomite-loaded SO42/TiO2, with changes to diffraction angle. The wide peaks that appear at 2u  26 , 38 , 48 , 55 , and 66 correspond to the anatase crystalline phase [24,25]. No wide Bragg peaks corresponding to the TiO2 appear in the region 15–50 . However, by carefully analyzing the X-RD patterns of diatomiteloaded SO42–/TiO2, a peak between 22 and 27 centered at 25 can be observed. The absence of intense Bragg peaks corresponding to the TiO2 may contribute to the broadening of peaks originating from the small sizes of the TiO2 nanocrystals and the interference from the intense reflection corresponding to the a-cristobalite form of silica in the initial diatomite. The observation of the small diffraction peaks at 28.3 and 32.2 , which might be attributed to SO42/TiO2, is consistent with the observation of others [26]. Besides, as to SO42/ZrO2-Al2O3, the disappearance of Al2O3 characteristic peaks around 46 and 67 could be ascribed to calcinations. Diffraction peaks at around 23 and 37 refer to ZrAlO2 [27].

Fig. 1. X-RD spectrum of the catalysts (Before and After reaction).

The morphology of SO42/TiO2 (Fig. 2a-1) diatomite-loaded SO42/TiO2 (Fig. 2b-1) and SO42/ZrO2-Al2O3 (Fig. 2c-1) was observed by SEM under 10,000 magnification, respectively. As can be seen in Fig. 2b-1, after SO42/TiO2 loading, the surface of diatomite has changed. Thus, it is obvious that the loading of SO42/TiO2 posed influences on the interfacial properties of diatomite with some SO42/TiO2 spherical crystal loaded on diatomite surface. The loading of SO42/TiO2 had effects on surface morphology, possibly because the large surface area of diatomite, porous structure and high mechanical strength provide a strong adsorption capacity for the catalyst loading [28]. On the other hand, the SEM images of SO42/ZrO2-Al2O3 (Fig. 2c-1) demonstrate the homogeneous clusters of well-developed riceshape crystals with clear edges which confirm the in situ formation of nanostructures of Zr on the Al-O surface. The IR absorption spectra for TiO2 (Pattern A), SO42/TiO2 (Pattern B), diatomite (Pattern C) and the diatomite-loaded SO42/ TiO2 (Pattern D) are illustrated in Fig. 3. Compared with TiO2 (Pattern A), four new bands on the spectra for SO42/TiO2 (Pattern B) between 1230 cm1 and 980 cm1 attributed to vibrational modes for bidentate sulfate ions are clearly apparent [29,30]. The bands at 1046 cm1 and 990 cm1 could be assigned to asymmetric and symmetric S O stretching vibrations, respectively [30,31]. Two bands appear at 1225 cm1 and 1138 cm1 and are related to asymmetric and symmetric stretching of S¼O vibrations, respectively [30]. The bands at 1628 cm1 and 3404 cm1 are associated with the bending and stretching vibrations of the OH group of water molecules on the surface of the solid and with a terminal OH group, which are characteristics of TiO2. The band at around 562 cm1 corresponds to TiO bending vibrations [30,32]. These results confirm that in SO42/TiO2, SO42appears as sulfate bonded on the titania surface. However, some characteristic peaks of SO42/TiO2 disappears when it is loaded onto diatomite (Pattern C, D). This may possibly because diatomite, as a carrier represents a large proportion of the catalyst, presents strong absorption and hence makes these characteristics of SO42/TiO2 peaks invisible. Production of DAGs by esterification The DAGs yield was investigated under different catalytic process parameters including mass ratios of oleic acid to glycerol, reaction temperature, catalyst loading, and reaction time of esterification in order to establish the optimum reaction conditions for the esterification process. The esterification of oleic acid with glycerol using diatomite-loaded SO42/TiO2 as a solid acid catalyst is a liquid-liquid-solid reaction (three phase system) in which the mass transfer rate of reactant molecules between the oleic acid-glycerol-catalyst phases is very slow. The conversion rate is normally found to increase with reaction time due to an increase in the miscibility of the oleic acid into glycerol [33]. The reaction time was varied within 1.5–7.5 h, while keeping other reaction process parameters constant at reaction temperature = 200  C, mass ratio of oleic acid to glycerol = 2:1, and diatomite-loaded SO42/TiO2 loading = 0.3% (oleic acid mass). It can be noted that the content of FFA was high (22.8%), with approximately 50.2% DAGs in the acylglycerols phase at 1.5 h. Over the next 4.5 h, the content of DAGs increased to 56.4% with a low FFA content (5.6%) as shown in Fig. 4(a). The DAGs remained virtually unchanged until 6 h, and all other curves can be characterized generally as a trade-off between TAGs and FFA. The longer reaction time, the more TAGs was generated resulted from the reaction among FFA with MAGs and DAGs (Scheme 1a). In fact, relatively longer reaction time might be in favor of TAGs production, resulting in undesirable yield of TAGs, since TAGs is not easy to separate from DAGs in purification step. Additionally, a longer reaction time would result in slowly reducing DAGs yield,

Please cite this article in press as: Z. Zhang, et al., Production of diacylglycerols by esterification of oleic acid with glycerol catalyzed by diatomite loaded SO42/TiO2, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.05.001

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Fig. 2. SEM micrograph of the catalysts (10.00k): (a-1) SO42/TiO2; (a-2) SO42/TiO2 after reaction; (b-1) diatomite-loaded SO42/TiO2; (b-2) diatomite-loaded SO42/TiO2 after reaction; (c-1) SO42/ZrO2-Al2O3; (c-2) SO42/ZrO2-Al2O3 after reaction.

possibly due to the strengthened probability of glycerolysis reaction between DAGs and glycerol and the transesterification reaction between MAGs and DAGs (Scheme 1b). Similar phenomenon had been reported by Nayebzadeh et al. [34] who indicated longer time of esterification results in a decrease in target products yield when several kinds of reactions coexist. Therefore, 6 h was set for production of DAGs with a relatively low FFA content and also for the further study for other parameters. A higher temperature would reduce the viscosities and increase the mass transfer of the reactants, subsequently increase the rate of the reaction. The effect of temperature on the DAGs production using diatomite-loaded SO42/TiO2 as a solid acid catalyst was examined by varying the reaction temperatures from 170  C to 210  C while other process parameters were kept constant at reaction time = 6 h, mass ratio of oleic acid to glycerol = 2:1, and diatomite-loaded SO42/TiO2 loading = 0.3% (oleic acid mass). It was found that the FFA consistently decreased, but the oscillating behavior of DAGs and TAGs at higher reaction temperature was observed and shown in Fig. 4(b). For more clear explanations on these phenomena, main reaction and side reaction route have been proposed in the revised Scheme 1a and b, which include esterification, glycerolysis and transeseterifcation. For estertification, it is much easier to be initialized at lower temperatures as compared to glycerolysis of acylglycerols and transesterification among acylglycerols in the acid-catalyzed system [35]. DAGs and TAGs are the intermediate products of these reactions and their contents would be oscillated at different temperatures which favor these reactions to different extents. This can be explained by the fact that esterification is an equilibrium process to generate TAGs and at 170–180  C energy is not enough to break down the ester bond in the TAGs molecules, so TAGs content would increase, whilst increase of the reaction temperatures to 190  C would improve the reaction from TAGs glycerolysis to DAGs. An elevated temperature would increase the probability of molecular collisions which favor the esterification rate and subsequently decrease the content of FFA. Therefore higher temperature helps the activation of carboxylic/carbonyl groups in FFA and generated acylglycerols by protonation and the glycerol nucleophilic attack on the

carboxylic/carbonyl groups [36]. However, a higher reaction temperature gives more energy and also increases the tendency toward glycerol polymerization, especially at 220  C [37]. So temperatures higher than that were not tested and 210  C was selected as the optimal reaction temperature. The effect of diatomite-loaded SO42/TiO2 loading amount on DAGs production is depicted in Fig. 4(c). Under the other fixed reaction conditions, i.e. at reaction temperature = 210  C, mass ratio of oleic acid to glycerol = 2:1, and reaction time = 6 h. The yield of DAGs was the highest (59.6%) when 0.1% diatomite-loaded SO42/ TiO2 was loaded. The oscillating behavior of DAGs and TAGs from 0.1%, 0.2% and 0.3% loading of diatomite-loaded SO42/TiO2 may be due to the fact that increasing the catalyst concentration from 0.1% to 0.3% in the system leads to an increase in the total number of active sites and this results in improvement of the transesterification between MAGs and DAGs to generate more TAGs. An excessive loading of the catalyst has a tendency to decrease DAGs production, and this is because the reaction takes place at the active sites formed in the catalyst. When the active sites are totally occupied by the reactants, additional catalysts are no longer needed to enhance the conversion [38,39]. In addition, the FFA content was found to be stable at 5% at all loading quantities of diatomite-loaded SO42/TiO2. Hence taking into consideration the cost and catalytic efficiency, the loading amount of diatomiteloaded SO42/TiO2 was set at 0.1%. The stoichiometric ratio for esterification reaction, theoretically, requires one mole of glycerol per two mole of FFA. However, the process experimentally requires an excess of glycerol in order to push the equilibrium reaction forward as the reaction process is a reversible reaction [40]. Fig. 4(d) shows the effect of various mass ratios of oleic acid to glycerol under the other fixed reaction process at reaction time = 6 h, reaction temperature = 210  C, and diatomite-loaded SO42/TiO2 loading = 0.1% (oleic acid mass). It should be noticed that the catalyst concentration fixed with oleic acid mass in the whole reaction system was slightly decreased when glycerol mass ratio was increased. The increase of mass ratio from 5:1 to 2:1 resulted in the decrease of TAGs yield. This can be explained that the excessive glycerol and their more OH groups

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Fig. 3. IR spectrum of the catalysts: (A) TiO2; B: SO42/TiO2; (C) diatomite; (D) diatomite-loaded SO42/TiO2.

increased the production yield of MAGs and DAGs. The content of DAGs reached 57.3% when the mass ratio of oleic acid to glycerol increased to 2:1 and the content of FFA was stabilized at 6% with different mass ratios of oleic acid to glycerol. Afterwards, there was a decline in the production yield with mass ratios higher than 2:1. This can be explained by that the relative catalysts loading amount is actually decreased in the mixture while the system glycerol amount increase, and an additional amount of glycerol favors to drive the reversible side of the reaction and to reform MAGs. This consequently increases the viscosity of the reaction system and thus inhibits the diffusion of glycerol and oleic acid between the inside and outside of the diatomite-loaded SO42/TiO2, resulting in decrease of the reaction rate. Oscillating behavior of DAGs esterification production may derive from the coexistence of several kinds of reactions (Scheme 1a and b) in the reaction system. That is to say, heterogeneous esterification between glycerol and FFA as well as homogeneous esterification between acylglycerols (MAGs, DAGs) and FFA would take place within the same reaction time.

Also, possible heterogeneous glycerolysis between acylglycerols (DAGs, TAGs) and glycerol, and homogeneous transesterifications among acylglycerols (MAGs, DAGs and TAGs) complicate the reaction system. All these different reactions are reversible and have different equilibrium rate under different reaction conditions. As a result, time, temperature, catalyst loading amount and molar ratios of substrates could affect the equilibrium rate of these reactions, thus the oscillating contents of DAGs and TAGs were observed. According to the results of the above single-factor tests, the selected conditions for DAGs production are as follows: reaction time = 6 h, reaction temperature = 210  C, mass ratio of oleic acid to glycerol = 2:1, loading amount of diatomite-loaded SO42/TiO2 = 0.1%. Under the selected conditions, the DAGs yield was 59.6% with a content of FFA < 6%. Ropero-Vega et al. [11] also reported that sulfated titanias showed very high activity for the esterification of fatty acids with ethanol in a mixture of oleic acid (79%). Conversions up to 82.2% of the oleic acid and selectivity to ester of 100% were recorded after 3 h of reaction at 80  C. All the results

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Fig. 4. (a)(a) Effects of reaction time on the production of DAGs (Conditions: reaction temperature = 200  C, mass ratio of oleic acid to glycerol = 2:1, and diatomite-loaded SO42/TiO2 loading = 0.3% (oleic acid mass); (b) effects of reaction temperature on the production of DAGs (Conditions: reaction time = 6 h, mass ratio of oleic acid to glycerol = 2:1, and diatomite-loaded SO42/TiO2 loading = 0.3% (oleic acid mass); (c) effects of diatomite-loaded SO42/TiO2 loading amount on the production of DAGs (Conditions: reaction temperature = 210  C, mass ratio of oleic acid to glycerol = 2:1, and reaction time = 6 h); (d) effects of mass ratios of oleic acid to glycerol on the production of DAGs (Conditions: reaction time = 6 h, reaction temperature = 210  C, and diatomite-loaded SO42/TiO2 loading = 0.1%, oleic acid mass). a Values in the same line with different letters are significantly (p  0.05) different, n = 3

suggest that solid acid catalyst is promising to be used in the esterification. Comparison testing The yield under the selected process conditions (6 h of reaction time, reaction temperature at 210  C, 2:1 of the mass ratio of oleic acid to glycerol, 0.1% of the loading amount of catalyst) has been significantly improved by using the catalyst synthesized in this work as compared to the other sulphated metal oxides, like SO42/TiO2 and SO42/ZrO2-Al2O3 (see Table 1). Obviously, diatomite-loaded SO42/TiO2 presented a better performance, which was nearly 20% higher than others.

Analysis of fatty acids before and after esterification The composition of fatty acids before and after esterification was analyzed by GC as FAMEs. The identity of the selected fatty acids and esterification products under the given reaction conditions are shown in Fig. 5. Differences are considered to be significant at a = 0.05. All levels of significance are greater than 0.05, indicating that there is no significant difference between the mean contents of FAMEs before and after esterification, according to the Independent-Samples T Test. Due to TFA in oils has become a hot issue in China, this study could serve as a road sign indicating that it is a safe and efficient way for DAGs-products production, since no TFA had been detected in DAGs produced under higher reaction temperature (210  C).

Table 1 Yield of the DAGs catalyzed by solid acid catalysts by GC analysis.a Solid acid catalyst

Diatomite-loaded SO42/TiO2

SO42/TiO2

SO42/ZrO2-Al2O3

DAGs yield (wt.%)

59.1  0.6 a

41.4  1.0 b

39.9  0.7 b

a

The means  standard deviation with different letters denote significant difference at p < 0.05 (n = 3).

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Fig. 5. Kinds and contents of fatty acids before and after esterification by GC analysisa,b. a Values are means of two replicates, a = 0.05. bAbbreviations used: 14:0, myristic acid; 16:0, palmitic acid; 16:1t, trans-palmitoleic acid; 16:1c, palmitoleic acid; 18:0, stearic acid; 18:1t, trans-oleic acid; 18:1c, oleic acid; 18:2t, trans-linoleic acid; 18:2c, linoleic acid; 18:3t, trans-linolenic acid; 18:3c, linolenic acid; 20:0, arachidic acid.

Purification of DAG products

Two-step reaction method

The DAGs produced under the selected conditions were purified by molecular distillation. DAGs with a purity of 69.6% were obtained after the one-stage molecular distillation, as shown in Fig. 6 and Table 2.

Recycled and reused experiments of diatomite-loaded SO42/ TiO2 were also tested under the same selected conditions. After catalyst being reused for 5 times, the DAGs yield was higher at around 50% (Table 3). The results reveal that the DAGs yield slight decreased for 4 and 5 trials by 12.5% and 10.0%, respectively. This could be probably attributed to the blockage of active centers of catalyst by TAGs product or catalyst leaching [41,42]. Even in the case that the exact nature of the acid sites on the sulfated titania is not known, we can still expect that both Lewis and Brönsted acid sites can be developed on sulfated titania according to the model presented in Fig. 7 [11]. The XRD experiments were carried out for the recycled catalyst, and the result shows that solid acid catalyst maintained its structure after 5 runs as shown in Fig. 1. This is an indication of the stability of the Lewis structure in catalyst and durability for the 5 cycles in esterification. It could be supposed that esterification have limit influences on the solid acid catalysts’ main crystalline structure.

Fig. 6. GC spectrum of the DAGs:a-before distillation; b-after distillation as yields shown in Table 2.

Fig. 7. Schematic representation of the Brönsted and Lewis acid sites present in the sulfated titania [11].

Please cite this article in press as: Z. Zhang, et al., Production of diacylglycerols by esterification of oleic acid with glycerol catalyzed by diatomite loaded SO42/TiO2, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.05.001

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Table 2 Yield of the DAGs before and after distillation by GC analysis.a

Crude DAGs (under selected conditions for glycerolysis) The heavy phase (removed MAGs) a

FFA (wt.%)

MAGs (wt.%)

DAGs (wt.%)

TAGs (wt.%)

4.9 3.0

20.2 2.9

59.6 69.6

15.3 25.5

Values are means of two replicates.

Table 3 Yield of the DAGs catalyzed by recycled and reused diatomite-loaded SO42/TiO2 by GC analysis.a Diatomite-loaded SO42/TiO2 recycled and reused times

1st

2nd

3rd

4th

5th

DAGs yield (wt.%)

59.6

56.3

50.3

47.1

49.6

a

Values are means of two replicates.

However, the acidities before and after reaction were 0.88 mmol/g and 0.15 mmol/g respectively, which means Brönsted acid sites were decreased. The decrease in acidity of diatomiteloaded SO42/TiO2 might affect the esterification of FFA, since both the Lewis and Brönsted acid sites promote esterification (Scheme 1c and d) [43]. It might be considered that esterification would lead to the deactivation owing to the loss of Brönsted acid sites. While the sulphur-element content before and after reaction was were 0.69% and 0.18% respectively, indicating the leached SO42 species from the catalyst. Some researchers also reported that SO42 species was not the main cause for the decline in the catalytic activation of different sulfonated catalysts [44]. In this work catalyst XRD results confirmed that the catalytic deactivation of the prepared solid acid catalyst is not resulted from SO42 species. Park et al. [45] reported to produce biodiesel from high acid value oil by using a recycled solid acid catalyst, and the decrease inactivity can be considered as minor catalyst loss during the processing. Rano [46] supposed the decline in recyclability of solid acid catalyst occluding of the active acid sites was ascribable to accumulation of carbonaceous. Further investigations should be performed in the future studies in order

to better understand the surface chemistry of this catalyst under different conditions. According to the single-factor tests, 56% DAGs with less than 6% FFA can be obtained at the reaction time of 6 h under the one-step method (the selected conditions) (Fig. 4(a)). From Fig. 4(a), it could be observed that the content of DAGs could reach 50% and 56% at the reaction times of 1.5 h and 3 h, respectively, while the content of FFA was high (more than 10%). Therefore, to obtain acceptable contents of DAGs and FFA within a shorter reaction time, a timeefficient two-step method was proposed based on the selected conditions, and a reaction time of 2 h was introduced in each step (total reaction time 4 h) to test the new method. Thus, the high residue of FFA from the first step can be further consumed by using recycled catalyst. The yield of DAGs from esterification under different reaction methods is illustrated in Fig. 8. Compared to the one-step method, it is clear that the two-step method can provide similar DAGs yields and FFA contents but in a shorter reaction time. These interesting results may provide an alternative method to the traditional one-step production of DAGs. To fully compare the oneand two-step methods, it is suggested that the reaction kinetics of both methods should be studied in the future work.

Fig. 8. Yield of the DAGs obtained using different reaction methods as analyzed by GCa. a Values in the same row with different letters are significantly (p  0.05) different, n = 3.

Please cite this article in press as: Z. Zhang, et al., Production of diacylglycerols by esterification of oleic acid with glycerol catalyzed by diatomite loaded SO42/TiO2, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.05.001

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Conclusion Compared with SO42/ZrO2-Al2O3, a kind of commercial solid acid catalyst, diatomite-loaded SO42/TiO2 proved to be a capable, recyclable and stable catalyst for the synthesis of DAGs from oleic acid and glycerol. Based on the single-factor tests for esterification, the selected conditions for DAGs production are as follows: reaction temperature = 210  C, diatomite-loaded SO42/TiO2 loading = 0.1% (based on the mass of oleic acid), oleic acid to glycerol mass ratio = 2:1, reaction time = 6 h. No TFAs were found in the DAGs produced in this study. After a one-stage molecular distillation, DAGs with a purity of 69.6% were obtained. The two-step method for DAGs production could be an alternative approach to the conventional production of DAGs by using the recycled catalyst. Acknowledgements The financial support from the National Natural Science Foundation of China (Grant 31371785 and Grant 31671781), the Program for New Century Excellent Talents in University (grant NCET-12-0675), the Department of Science and Technology of Guangdong Province under Grant 2012B091100035 and 2013B090800009, and the Bureau of Science and Information of Guangzhou under grant 2014Y2-00192 are gratefully acknowledged. We acknowledge Prof. William W. Riley from the International School, Jinan University for polishing the English of this manuscript. References [1] A.V. Paula, G.F.M. Nunes, L. Freitas, H.F. de Castro, J.C. Santos, J. Mol. Catal. B: Enzym. 65 (2010) 117. [2] O. Morita, M.G. Soni, Food. Chem. Toxicol. 47 (2009) 9. [3] D.J. Hu, J.M. Chen, Y.M. Xia, J. Ind. Eng. Chem. 19 (2013) 1457. [4] M. Basri, M.A. Kassim, R. Mohamad, A.B. Ariff, J. Mol. Catal. B: Enzym. 85–86 (2013) 214. [5] D.H. Lee, J.M. Kim, H.Y. Shin, S.W. Kang, S.W. Kim, Biotechnol. Bioprocess Eng. 11 (2006) 522. [6] Y. Watanabe, Y. Yamauchi-Sato, T. Nagao, T. Yamamoto, K. Ogita, Y. Shimada, J. Mol. Catal. B: Enzym. 27 (2004) 249. [7] T. Watanabe, M. Sugiura, M. Sato, N. Yamada, K. Nakanishi, Process Biochem. 40 (2005) 637. [8] R.U. Hess, A. Bornscheuer, T. Scheper, Enzyme Microb. Technol. 17 (1995) 725. [9] B. Cheirsilp, P. Jeamjounkhaw, A. H-Kittikun, J. Mol. Catal. B: Enzym. 59 (2009) 206. [10] J.C. Juan, J. Zhang, M.A. Yarmo, Appl. Catal. A 332 (2007) 209.

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Please cite this article in press as: Z. Zhang, et al., Production of diacylglycerols by esterification of oleic acid with glycerol catalyzed by diatomite loaded SO42/TiO2, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.05.001