Production of Structured Lipids Containing Medium-Chain Fatty Acids

Feb 3, 2016 - Production of Structured Lipids Containing Medium-Chain Fatty Acids by Soybean Oil Acidolysis Using SBA-15-pr-NH2–HPW Catalyst in a He...
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Production of Structured Lipids Containing Medium-Chain Fatty Acids by Soybean Oil Acidolysis Using SBA-15-pr-NH2−HPW Catalyst in a Heterogeneous Manner Wenlei Xie* and Pengtao Hu School of Chemistry and Chemical Engineering, Henan University of Technology, Zhengzhou 450001, P. R. China ABSTRACT: The Keggin-type H3PW12O40 (HPW) was successfully anchored onto the surface of amino-functionalized SBA-15 silica by means of protonating of amino-functionalized silica with the phosphotungstic acid. The as-prepared heterogeneous catalysts were fully characterized at the current research by using small-angle XRD, FT-IR, TEM, SEM, solid-state NMR, and nitrogen porosimetry measurement. The characterization results showed that the functionalized SBA-15 materials still retained the ordered mesoporous structure of SBA-15 silica with high surface area, and meanwhile the primary Keggin structure of heteropolyanions could be also remained intact upon the immobilization of the HPW. By using this solid catalyst, the acidolysis of soybean oil with caprylic or capric acid was carried out in a batch reactor to produce structured lipids containing mediumchain fatty acids. The influence of reaction parameters, such as the substrate ratio, reaction temperature, catalyst loading, and reaction time, on the acidolysis reaction was investigated for the optimization of acidolysis processes. It was shown that the high incorporations of caprylic or capric acid of 58.7% or 61.9% was achieved, respectively, at 150 °C within 10 h when the mediumchain fatty acids/oil molar ratio of 10:1 and catalyst loading of 9 wt % were employed. The solid catalyst could be recovered easily and reused for several cycles with little loss of activity due to the chemical interaction between the amino groups and the HPW anions.

INTRODUCTION In recent years, the increasing interest in the nutritional effect of foods has spurred the research on the preparation of healthy lipids, by altering the original specific triacylglycerol (TAG) compositions of fats and oils.1,2 Structured lipids (SLs) are defined as TAGs that are produced chemically or enzymatically by incorporation of new fatty acids (FAs) and/or varying their positional distribution in the glycerol backbone.3,4 In the past few years, much attention has been paid to the preparation of SLs containing medium-chain FAs, since they provide the physicochemical and nutritional characteristic of fats, but with approximately half of the calories of typical edible oils.5,6 These SLs are absorbed more efficiently and rapidly as compared to long-chain TAGs, owing to their solubilizations in the aqueous phase of the intestinal contents.6 Generally, the low-calorie SLs can be produced by acidolysis reactions between vegetable oils and medium-chain FAs using lipase catalysts, either in solvent or in solvent-free media. The enzymatic acidolysis processes are advantageous because they can be conducted under milder reaction conditions and do not catalyze undesirable side reactions.7 So far, several researchers have reported the utilization of lipase as a biocatalyst to incorporate mediumchain FAs into different source oils.7,8 However, the practical utilization of lipase is hampered by its high cost, the lack of long-term stability, and the difficulty in its separation, recycling, and reusing. As such, the fabrication of a new type of efficient catalysts with a good stability for the acidolysis reaction has been an emergent and challenge field of scientific research. Homogeneous catalysts are generally impractical for industrial applications due to the problems of corrosion, pollution, difficult separation, and nonreusability. Quite recently, there has been considerable growth of interest in © 2016 American Chemical Society

the development of heterogeneous acid catalysts, since they are suitable options economically and industrially owing to their easy recovering, possible recycling of the catalyst, and simple operation procedures.9−11 It seems that an ideal solid catalyst should have the following merits, such as large surface areas, high concentration of active sites, and high catalytic stability.12 Hence, the immobilization of homogeneous catalysts on the surface of porous materials can be thought of as a feasible method to form efficient heterogeneous catalysts with improved catalytic performances.13,14 Up to now, several heterogeneous acid catalysts, including sulfated zirconia, tungstated zirconia, and sulfonated polystyrene compounds, have been developed for the transesterification of vegetable oils for the production of biodiesel.15,16 However, to our best knowledge, there is no report on the application of heterogeneous acid catalysts for the acidolysis reaction of vegetable oils until lately, despite their excellent catalytic activities in a variety of organic transformations. In general, the support plays an important role in determining the nature of the heterogeneous catalysts. Various kinds of supports have been employed for the preparation of heterogeneous catalysts, such as polymeric, organic and inorganic materials.17,18 More recently, mesoporous molecular sieves have emerged as an increasingly important class of materials especially in their applications in heterogeneous catalysis. As a new advanced material, SBA-15 silica is of great interest as a promising catalyst support due to its promising feature such as high surface areas, high thermal stability, Received: November 17, 2015 Published: February 3, 2016 637

DOI: 10.1021/acs.oprd.5b00381 Org. Process Res. Dev. 2016, 20, 637−645

Organic Process Research & Development


techniques such as XRD, FT-IR, 31P MAS NMR, 29Si MAS NMR, SEM, TEM, and nitrogen adsorption−desorption techniques. By using this solid catalyst, the acidolysis reaction, in which the medium-chain FAs (caprylic or capric acid) were used as acyl donors and soybean oil was used as the source of glycerol backbone and long chain FAs, was carried out in a solvent-free system producing SLs containing medium chain FAs. The FA profiles of the acidolysis products were determined by gas chromatography (GC) analysis. The acidolysis variables, such as the molar ratio of substrate, reaction temperature, catalyst loading, reaction time, and reusability of the catalyst were studied in the present study.

uniform and tunable pore sizes, organic solvent tolerance, and great diversity in surface functionalization.19,20 Heteropoly acids, especially 12-tungstophosphoric acid (H3PW12O40, HPW) with a Keggin-type structure, have received much attention in the past decades thanks to their higher acidic strength, low corrosivity, and relatively high thermal stability.21,22 HPW is an important super Brönsted acid that exhibits excellent catalytic activities in various acidcatalyzed reactions.23 Unfortunately, HPW in bulk state has a low specific surface area which restricts the accessibility to the acidic sites, and it can usually act homogeneously in the catalytic reaction, thereby limiting the catalytic activity. To address the drawbacks, several studies have been focused on the dispersion of Keggin-structure HPW onto appropriate porous materials such as silica, zeolite, and polymer to improve its surface area, and consequently, its catalytic activity and lifetime.24−26 Although these supported HPW materials displayed high catalytic activities, the loaded HPW on the surface of the support can be easily rinsed out, especially in polar solvents, thus leading to severe leaching of the active ingredient from the solid supports. For example, the HPW supported SBA-15 silica had high catalytic activities in the isopropylation of naphthalene with isopropanol, but it could be deactivated drastically due to the leaching of active components in the reaction media, thus yielding some homogeneously catalytic activity.27 Patel et al. reported the esterification of FAs for biodiesel production over the HPW loaded on SBA-15 silica.28 However, for the supported HPW catalyst, the leaching of tungsten species in the reaction media is generally observed so as to hinder the advantage of its heterogeneous nature. The leached catalytic species in the reaction mixture may result in the catalytic activity as a homogeneous contribution eventually decreasing the stability of the solid catalyst. Accordingly, the stability of the supported HPW catalysts is still not satisfactory owing to the weak interaction between the HPW and the support. In view of this, it is of great significance to enhance the dispersion of active species and the interaction between the active species and the supports for developing a more stable catalyst. It was reported that HPW was chemically immobilized on porous carbon by forming a positive charge on the support with surface modifications.29,30 Since the physically adsorbed HPW on the support can leach out easily, the immobilization through chemical bonds using functionalized silica has been become a feasible strategy. In this respect, the amino-modified SBA-15 silica can be employed as catalyst supports due to its high surface area and better affinity to anchor firmly the HPW via ionic bonds, allowing to obtain high dispersion of the HPW with minimal leaching and boosting the reusability of heterogeneous catalysts. Due to the increased interest and demand for practical application, the environmentally and easy-separable solid acid catalysts are highly desirable from a green chemistry point of view. With such considerations in mind, the main objective of the current research is to produce low-calorie SLs by acidolysis of soybean oil with caprylic or capric acid catalyzed by solid acids in a heterogeneous manner. For this purpose, the SBA-15 silica was first amino-functionalized by grafting of 3-aminopropyltriethoxysilane (APTES), and then the HPW, as the stronger acid in the Keggin-structure series of heteropoly acids, was readily anchored onto the amino-functionalized SBA-15 materials through chemical bonds to produce the HPW supported SBA-15 catalyst. The characterization of as-prepared solid acid catalyst was performed by means of various

EXPERIMENTAL SECTION Materials. Refined soybean oil used in this study was purchased from the local supermarket (Zhengzhou, China). The FA profile of the soybean oil, as determined by gas chromatography (GC) analysis, was as follows: 12.5% palmitic acid, 2.9% stearic acid, 23.1% oleic acid, 54.6% linoleic acid, 6.9% linolenic acid, and traces of other FAs. Caprylic acid (C8:0, octanoic acid, ≥ 98%) and capric acid (C10:0, decanoic acid, ≥ 98%) were obtained from Sinopharm Chemical Reagent Corporation (Sanghai, China). Pluronic copolymer P123 (EO20PO70EO20), 3-aminopropyltriethoxysilane (APTES, ≥ 99.8%), and tetraethylorthosilicate (TEOS, ≥ 98%) were purchased from Sigma-Aldrich (St. Louis, Missouri, USA). The Keggin type 12-tungstophosphoric acid was purchased from Sinopharm Chemical Reagent Company. Other commercially available materials used were of either analytical or chromatographical grades. Catalyst Preparation. Mesoporous SBA-15 silica was prepared by route using the block copolymer Pluronic P123 and tetraethylorthosilicate under acidic conditions according to a hydrothermal method described previously.31 Figure 1 shows the schematic procedure for the surface amino-modification of SBA-15 silica and then the subsequent

Figure 1. Synthesis of SBA-15-pr-NH2-HPW solid catalysts.

immobilization of HPW on the amino-functionalized SBA-15 silica, involving the protonating of the amino-functionalized SBA-15 silica with the HPW. To achieve this, the aminofunctionalized SBA-15 silica, designated as SBA-15-pr-NH2, was initially prepared through surface modification by using APTES as the silane agent to introduce amino groups. In a typical assay, 10 g of SBA-15 silica was dispersed in 250 mL of dry toluene and ultrasonicated for 20 min; after that 5.1 g of APTES was added dropwise into the above suspension solution. Thereafter, the modification reaction was carried out at 110 °C for 24 h with constant stirring under nitrogen atmosphere to yield the amino-functionalized SBA-15 silica. After cooling to room temperature, the solid was then separated from the solution by filtration, washed sequentially with toluene and ethanol, and finally dried under vacuum at 60 °C for 12 h. In the next step, 5 g of SBA-15-pr-NH2 was dispersed in 100 mL of an aqueous solution containing HPW (5 g) with 638

DOI: 10.1021/acs.oprd.5b00381 Org. Process Res. Dev. 2016, 20, 637−645

Organic Process Research & Development


min with a split ratio of 1:20. The injector and detector temperatures were set at 260 and 300 °C, respectively. The FAMEs were identified by comparing their retention times with those of the reference standards. The incorporation of caprylic or capric acid was defined as the wt % of medium-chain FAs into the TAGs of soybean oil. The FA composition at the sn-2 position of each sample was evaluated by pancreatic lipase hydrolysis on the basis of IUPAC standard method.33 The lipase catalyst could hydrolyze the TAGs at sn-1,3 positions to produce 2-monoacylglycerol (2MAG). In brief, the sample (10 mg) was mixed with 2 mL of Tris-HCl buffer solution (1 mol/L, pH 7.6), 0.5 mL of 0.05% sodium cholate solution, and 0.2 mL of 2.2% calcium chloride solution. Thereafter, the resulting mixture was vortexed vigorously to emulsify the sample. The pancreatic lipase (20 mg) was then added and incubated in a water bath at 37 °C for 2 min and vortexed vigorously for 1 min. Subsequently, 1 mL of 6 mol/L HCl solution and 4 mL of diethyl ether were added to break off the hydrolysis reaction. This mixture was centrifuged for 3 min, and the upper diethyl ether layer containing hydrolysates was collected and dried by anhydrous sodium sulfate. Thereafter, the hydrolysates were spotted on thin-layer chromatography (TLC) plates and then developed with a solvent system of hexane/diethyl ether/formic acid mixture (70:30:1, v/v/v). The bands corresponding to 2-MAG were scraped from TLC plates and extracted with diethyl ether, methylated, and analyzed by GC analysis as described previously.

vigorous stirring. Afterword, the resultant mixture was ultrasonicated for 20 min and then allowed to stir vigorously at 60 °C for 24 h. The solid product was filtered, washed with distilled water, and finally dried under reduced pressure at 100 °C for 24 h. The solid acid catalyst, designed as SBA-15-prNH2-HPW, was obtained for the acidolysis reaction. Catalyst Characterization. Small-angle X-ray power diffraction (XRD) patterns were collected on a Rigaku D/ MAX-3B powder X-ray diffractometer with the Cu Kα radiation source of wavelength 0.154 nm at 40 kV and 20 mA. Fourier transform infrared (FT-IR) spectra were recorded on a Shimadzu IR-Prestige-21 spectrometer using a KBr pellet technique. Scanning electron microscope (SEM) measurements were carried out with a field-emission microscope (JEOL, JSM6390LV) using an accelerating voltage of 15 kV. Transmission electronic microscopy (TEM) micrographs were taken with a JSM-6390LV transmission electronic microscope with an accelerating voltage of 200 kV. The N2 adsorption−desorption isotherms were measured at liquid nitrogen temperature (−196 °C) on a Quantachrome NOVA 1000e instrument. The Brunauer−Emmett−Teller (BET) method was used to determine the specific surface area, while the Barrett−Joyner−Halenda (BJH) method based on the adsorption isotherm was employed to estimate the total pore volume and the pore size distribution. The magic-angle spinning (MAS) solid state NMR study was carried out on the Varian Infinity-plus 400 spectrometer under ambient conditions. The 29Si NMR spectra were recorded at 79.47 MHz using a 5 mm rotor probe with kaolin as an external standard. The 31P NMR spectra were collected at 121.35 MHz using a 4 mm rotor probe with ADP as an external standard. The acid strength of the solid catalyst was measured with Hammett indicators, including anthraquinone (H0 = −8.2), pnitrotoluene (H0 = −11.35), and p-nitrochlorobenzene (H0 = −12.7). The acid concentration of different acid sites was determined by n-butylamine titration method.32 Acidolysis Procedures. Various substrate blends of soybean oil with caprylic or capric acid were prepared according to different molar ratios. The acidolysis between soybean oil and caprylic or capric acid was carried out in batch reactors. In a typical run, 34.2 g of soybean oil was mixed with 57.6 g of caprylic (C8:0) acid or 68.8 g of capric (C10:0) acid, and then the formulated blend was previously dried under reduced pressure at 80 °C to remove the moisture present in the feedstocks. Thereafter, 9 wt % of the catalyst (8.24 g for caprylic acid or 9.24 g for capric acid) was added into the substrate mixture, and the acidolysis reaction to incorporate caprylic or capric acid was performed under continuously stirring (approximately at 600 rpm) at 150 °C for 10 h. After completion of the reaction, the solid catalyst was filtered, and then the acidolysis product was dissolved in hexane (1:1, by volume). Furthermore, the hexane phase was washed with 0.8 mol/L potassium hydroxide in 50% ethanol to eliminate the residual medium-chain FAs. Afterword, the separated hexane layer was washed with 50% ethanol until it was neutral, and finally dried over anhydrous sodium sulfate. The hexane solvent was removed under reduced pressure at 45 °C, and the final product was employed for subsequence analyses. Analytic Methods. The total FA composition of soybean oil and acidolysis product was determined after converting FA residues into their corresponding fatty acid methyl esters (FAMEs) by an Agilent gas chromatograph (model 6890N).33 Nitrogen was used as the carrier gas, flowing at rate of 1.2 mL/

RESULTS AND DISCUSSION Catalytic Activity of the Solid Catalyst. Aminofunctionalization is one of the most commonly employed methods to modify the silica materials for providing terminal amino groups and further forming chemical bonds with functional groups on the surface of the modified materials. In order to improve the stability of the solid acid catalyst, the SBA15 support was first surface-modified with APTES to provide sites for the immobilization of H3PW12O40, and then the HPW was chemically immobilized on the amino-functionalized SBA15 silica as a charge compensating component, affording SBA15-pr-NH2-HPW catalyst. HPW was chosen as the acid tethered on the amino-functionalized SBA-15 silica owing to its strong superacidity. As shown in Figure 1, the HPW can be efficiently anchored onto the framework surface of the mesoporous SBA-15 silica because of the strong interaction between the amino group and the HPW through ionic bonds, in which the HPW anions were attached firmly on the surface of the SBA-15 silica. This binding between the HPW and the support could inhibit the HPW leakage that has generally occurred in most of the supported heterpolyacid catalyst, allowing the solid catalyst to be a genuine heterogeneous nature with high stability and good recyclability. Hence, the immobilization of HPW on the amino-functionalized SBA-15 silica gives more stability of the solid acid catalysts. Moreover, as comparison with the parent HPW, the high surface area of the mesoporous SBA-15 silica could lead to high concentrations of catalytic sites and are also favorable for the mass transfer. Anyway, no acidolysis reaction occurred without the solid acid catalyst under the reaction conditions employed here, suggesting that there was no catalytic contribution from the fatty acid substrates even though they have been reported to be used as an acidic catalyst.34 In the present study, the catalytic activity of the solid catalyst was evaluated in the acidolysis of 639

DOI: 10.1021/acs.oprd.5b00381 Org. Process Res. Dev. 2016, 20, 637−645

Organic Process Research & Development


that the FA composition at sn-2 position of the produced SLs was greatly varied after the catalytic acidolysis reaction. The contents of caprylic or capric acid were significantly enhanced to 31.8% or 32.7% at the sn-2 position, respectively, by using SBA-15-pr-NH2-HPW catalyst. Such positional analysis results further demonstrated the solid catalyst could be used to incorporate the medium-chain FAs into the soybean oil. In order to investigate the HPW loading on the catalytic activity of the solid catalyst, different SBA-15-pr-NH2-HPW catalysts with various HPW loadings were prepared and tested for the acidolysis reactions. The SBA-15-pr-NH2-HPW catalyst showed a strong strength of acid sites with H0 in the range of −8.2 to −11.35. In Table 2, the acid concentration was determined as mmol of acidic sites per gram of catalyst, by titration method.32 From the data presented in Table 2, it was found that with increasing HPW amount from 30% to 50%, the acid concentration was increased accordingly; meanwhile, the corresponding incorporation of medium chain FAs was also enhanced upon increasing the HPW loading. When the HPW amount was further increased to 70%, no significant increase in the acid concentration of the solid catalyst together with the incorporation of medium chain FAs was observed as indicated in Table 2. In view of the results, the catalytic performance of the catalyst is strongly dependent on the acid properties. Evidently, the optimum HPW loading for the catalyst preparation was 50%, and thus the SBA-15-pr-NH2-HPW catalyst with 50% HPW loading was chosen for the subsequent study. The literature lacks information on the acidolysis of soybean oil with FAs using a solid acid catalyst. For the comparison purpose, the acidolysis was performed by using 1 wt % of H2SO4 as a catalyst, and the obtained results showed that the solid catalyst exhibited comparable activities to the commonly used acid catalyst as shown in Table 2. Catalyst Characterization. The amino-modification of SBA-15 silica and the immobilization of HPW were evidenced by FT-IR spectra. The FT-IR spectra of SBA-15, SBA-15-prNH2, and SBA-15-pr-NH2-HPW samples are presented in Figure 2. For the SBA-15 silica, the asymmetric, symmetric stretching and bending vibration of the condensed Si−O−Si silica network were observed at around 1100 cm−1, 770 cm−1, and 450 cm−1, respectively, while the broad IR band located at about 3400 cm−1 could be largely responsible for the surface OH stretching vibration.22 Besides, the characteristic IR band found at 947 cm−1 for the unmodified SBA-15 silica was

soybean oil with caprylic or capric acid, and the obtained results are listed in Table 1. As observed, soybean oil showed major FA Table 1. Total and sn-2 Positional Composition (%) of FAs for Soybean Oil and Acidolysis Products after the Reaction Catalyzed by the SBA-15-pr-NH2-HPW Catalysta FA composition at sn-2 position

total FA composition



after acidolysis (C8:0)

C8:0 C10:0 C16:0 C18:0 C18:1 C18:2 C18:3

0.0 0.0 12.5 2.9 23.1 54.6 6.9

58.7 0 11.3 2.2 13.4 14.4 0

after acidolysis (C10:0) 0 61.9 10.9 2.1 14.7 10.4 0


after acidolysis (C8:0)

after acidolysis (C10:0)

0.0 0.0 1.2 1.1 24.5 67.1 6.1

31.8 0 1.1 0.9 20.6 45.4 0.0

0 32.7 1.0 0.9 21.2 43.8 0.0


Reaction conditions: medium-chain FA/soybean oil molar ratio 10:1; catalyst loading 9 wt %; reaction temperature 150 °C, and reaction time 10 h. SO-Soybean oil.

profiles of 54.6% linoleic acid, 23.1% oleic acid, and 12.5% palmitic acid, with small amounts of linolenic acid (6.9%) and stearic acid (2.9%). As expected, the parent SBA-15 silica and the amino-functionalized SBA-15 silica exhibited no appreciable activities to the reaction, since no incorporation of caprylic or capric acid was observed by using them as a catalyst (Table 2). However, after the acidolysis with SBA-15-pr-NH2-HPW catalyst, the produced SLs displayed significant variations of FA compositions when compared with the original soybean oil. As indicated in Table 1, over the solid acid catalyst, the caprylic or capric acid present in the SLs was found to be enhanced significantly, and unsaturaturated FAs were decreased greatly. Obviously, as the two medium-chain FAs were added individually as acyl donors, the acidolysis product was characterized by the combination of medium-chain and longchain acyl residues into a triacylglycerol molecule. The medium-chain FAs were incorporated into the TAGs at the expense of unsaturaturated FAs, implying that the studied catalyst can catalyze the acidolysis of soybean oil with the medium-chain FAs. As shown in Table 1, the sn-2 position of the soybean oil mainly contained oleic acid (24.5%) and linoleic acid (67.1%), with a small amount of linolenic acid (6.1%), palmitic acid (1.2%), and stearic acid (1.1%). However, it was noteworthy

Table 2. Catalytic Activities, Surface Acidities, and Textural Properties of the SBA-15-pr-NH2-HPW Catalysts with Different HPW Loadingsa acid distribution (mmol/g) samples H2SO4 SBA-15 SBA-15-pr-NH2 30% SBA-15-prNH2-HPW 50% SBA-15-prNH2-HPW 70% SBA-15-prNH2-HPW a

H0⩽ −8.2

H0⩽ −11.35

acid concentration (mmol/g)

surface area (m2/g)

pore volume (cm3/g)

pore size diameter (nm)

incorporation of caprylic acid (%)

incorporation of capric acid (%)

1.06 0.62 0.51

8.01 7.41 7.04

47.8 0 0 27.4

50.2 0 0 29.6




680.7 325.2 284.9

















Reaction conditions: medium-chain FA/soybean oil molar ratio 10:1; catalyst loading 9 wt %; reaction temperature 150 °C, and reaction time 10 h. 640

DOI: 10.1021/acs.oprd.5b00381 Org. Process Res. Dev. 2016, 20, 637−645

Organic Process Research & Development


Figure 2. Fourier transform infrared spectra for samples. (a) SBA-15; (b) SBA-15-pr-NH2; (c) SBA-15-pr-NH2-HPW; (d) HPW.

Figure 3. Small-angle X-ray power diffraction patterns for samples.

functionalization and loading of the HPW as shown in Figure 3, implying the decrease in the pore sizes mostly also due to the occupation of pores with the amino group and HPW.37 Remarkably, despite such a change of the diffraction peaks, the resolved XRD peaks showed that the prepared solid catalyst remained mesoporous characteristics of the siliceous SBA-15 after the surface amino-modification and the immobilization of HPW. Figure 4 presents the typical SEM micrographs of SBA-15 and SBA-15-pr-NH2-HPW catalyst. As clearly illustrated in the SEM image, the SBA-15 silica consisted of short rod-shaped particles based on elongated wheat-like macrostructures in SEM micrograph.22,38 Moreover, the SBA-15-pr-NH2-HPW sample displayed a similar wheat-like macro-structure to the parent SBA-15 silica. Obviously, even after the immobilization of HPW on the SBA-15-pr-NH2 sample, the SBA-15-pr-NH2HPW catalyst still remained the wheat-like macrostructures aggregated with rope-like domains, implying that there is no morphology change of the mesoporous particles after the HPW loading. The morphology property of the solid catalyst was also characterized by TEM techniques. The TEM images of SBA-15 and SBA-15-pr-NH2-HPW sample are shown in Figure 5. For the SBA-15 silica, the TEM image exhibited the hexagonal array of uniform channels with a typical honeycomb-like appearance of solid materials.39 In the case of SBA-15-pr-NH2-HPW sample, as it was seen from the TEM image, similar morphology was also observed, thus further suggesting that the loading of the HPW onto the SBA-15 silica had no distinct influence on the morphology of the mesoporous material. The ordered mesoporous structure of the solid catalyst can ensure efficient reactant diffusion and catalytic sites accessibility during the reaction, giving a positive influence on the catalytic activity. Figure 6 depicts the nitrogen adsorption−desorption isotherms of SBA-15, SBA-15-pr-NH2, and SBA-15-pr-NH2HPW samples. For the three samples, according to IUPAC nomenclature, the adsorption isotherms showed representative irreversible type IV with a H1-type hysteresis loop at high relative pressure, signifying that the materials possessed highly ordered p6 mm mesoporous structures with narrow pore size distributions.37,39 Moreover, a steep increase in adsorption at a relative pressure of about 0.65 could be attributed to capillary nitrogen condensation with uniform pore dimensions and high

attributed to the bending vibration of framework Si−OH group in SBA-15 silica.22,31 As for the SBA-15-pr-NH2 sample, from the comparison it was noticed that a new IR weak peak at 632 cm−1 due to N−H bending was observed and the IR peaks situated at 1630 and 1508 cm−1 could be ascribed to symmetrical NH2 bending, which verified that the NH2 functional moieties were anchored on the SBA-15 silica.35 The primary structure of unsupported HPW could be identified by the four characteristic IR bands located at 1082 cm−1, 984 cm−1, 892 cm −1 and 810 cm−1, which were originated from the stretching vibration modes of the tetrahedral P−O bonds, terminal WO bonds, and two types of bridging W−O−W bands of the Keggin unit.36 In the IR spectrum of SBA-15-prNH2−HPW sample, there were a broad IR band at 2700−3400 cm−1 and a weak IR band at around 1520 cm−1, which were assignable to the −NH3+ stretching vibration, showing that the amino groups in SBA-15-pr-NH2-HPW samples were protonated. Moreover, the characteristic IR absorption bands owing to the Keggin unit of HPW for the SBA-15-pr-NH2-HPW sample were also observed. As compared with the unsupported HPW, the 1082 and 810 cm−1 bands for the SBA-15-pr-NH2HPW sample were not clearly identified due to the overlapping with the broad IR bands of Si−O−Si stretching vibrations, while the 984 and 892 cm−1 bands attributed to WO and W−O−W bonds were clearly observed, as illustrated in Figure 2.22 Therefore, it can be inferred that Keggin-type H3PW12O40 has been successfully anchored to the amino-functionalized SABA-15 support, and the primary Keggin structure of HPW anions remains almost unchangeable on the SBA-15 support. Figure 3 shows low angle XRD patterns of SBA-15, SBA-15pr-NH2, and SBA-15-pr-NH2-HPW samples. All of the three samples exhibited three XRD peaks at 2θ of about 0.91°, 1.56°, and 1.76° in the low-angle region, corresponding to the (100), (110), and (200) reflections, respectively, which suggested that these materials consisted of two-dimensional hexagonal mesoporous structure with p6mm space group symmetry.26,31 For the SBA-15-pr-NH2-HPW catalyst, the three diffraction peaks were decreased in intensity to some extent as comparison with the SBA-15 silica, probably attributing to the filled pores of the SBA-15 silica with the HPW. Moreover, the characteristic diffraction peaks were found to be shifted after amino 641

DOI: 10.1021/acs.oprd.5b00381 Org. Process Res. Dev. 2016, 20, 637−645

Organic Process Research & Development


Figure 4. Scanning electron microscopy images of samples: (a) SBA-15; (b) SBA-15-pr-NH2-HPW.

the SBA-15 silica, the BET surface, average pore size, and pore volume of the SBA-15-pr-NH2 sample were decreased to 325.3 m2/g, 7.41 nm, and 0.62 cm3/g, respectively. As for the SBA15-pr-NH2-HPW catalyst, a surface area of 274.5 m2/g and a pore volume of 0.47 cm3/g were determined, while the pore size distribution was narrow with a mean pore diameter of 6.85 nm. Evidently, the SBA-15-pr-NH2-HPW catalyst displayed some reduction of surface area, pore volume and pore diameter as compared to the SBA-15 silica. The decrease in these parameters upon the loading of HPW could be explained by the deposition of the Keggin anions on the surface of SBA-15 channels. Moreover, the pore size distribution curves revealed that the three samples had narrow pore diameter distributions, and the pore size was decreased after the amino-functionalization and loading of the HPW as shown in Figure 7, mostly due

Figure 5. Transmission electron microscopy images of samples: (a) SBA-15; (b) SBA-15-pr-NH2-HPW.

Figure 6. Nitrogen adsorption/desorption isotherms of samples.

Figure 7. Pore size distribution profiles of samples.

ordering of the material. It was also seen that the onset of the capillary condensation step occurred at decreased relative pressures for the SBA-15-pr-NH2-HPW sample, which showed that the pore diameter of SBA-15-pr-NH2-HPW catalyst was reduced in comparison with the SBA-15 silica, attributing to the introduction of Keggin-type HPW to the SBA-15 silica. The overall mesoporosity was determined by nitrogen porosimetry measurements. BET surface area and pore-size distribution were calculated using nitrogen adsorption at −196 °C, and the results are listed in Table 2. The pure SBA-15 possessed the high measured BET surface area (680.7 m2/g) and pore size (8.01 nm) as well as the large pore volume (1.06 cm3/g), which were in accordance with the previous data reported in the literature.38,39 After amino-functionalization of

to the fact that the pores were blocked by the HPW species. Overall, in spite of such a change in the textural parameters, the solid catalyst still remained the mesoporous structure after the amino-functionalization and the HPW loading, which is favorable for the accessibility of the catalytic sites. The incorporation of HPW into the amino-functionalized SBA-15 silica was further confirmed by 29Si MAS NMR and 31P MAS NMR techniques. Figure 8 shows the 29Si MAS NMR spectra of SBA-15 and SBA-15-pr-NH2-HPW samples. As observed, the SBA-15 exhibited a strong broad NMR peak from −90 ppm to −120 ppm with the middle peak at −108.2 ppm, which was the characteristic peak of the silica framework.23 For SBA-15-pr-NH2-HPW sample, the characteristic NMR peak of the silicon framework was observed at −109.8 ppm. Moreover, a weak new peak at −58.1 ppm was appeared mostly owing to 642

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It could be seen from Figure 10 that the percentage incorporation of caprylic or capric acid was increased from 19.3% to 58.7% or from 22.6% to 61.9% by increasing the substrate molar ratio from 4:1 to 10:1, respectively. The maximum incorporation for both medium-chain FAs was observed at a substrate molar ratio of 10:1. However, beyond the molar ratio of 10:1, no obvious further increase in the incorporation of medium-chain FAs was found. Accordingly, the molar ratio of 10:1 is chosen as an appropriate ratio for acidolysis reaction catalyzed by this solid catalyst. The reaction time was set from 4 to 12 h to investigate the influence on the incorporation of medium-chain FAs. After 4 h of reaction, the extent of incorporated caprylic or capric acid was 15.2% or 18.1%, respectively. As indicated in Figure 10, the percentage incorporation of caprylic or capric acid was improved considerably when the reaction time increased from 4 to 10 h and then kept almost constant after 10 h of reaction. By drawing on the results, it can be inferred that the proper acdiolysis time is 10 h. To study the effect of catalyst loading on the incorporation of medium-chain FAs, the acidolysis reaction was carried out with different amounts of the catalyst. As shown in Figure 10, when the catalyst amount increased from 3 wt % to 9 wt %, the caprylic or capric acid incorporation was shown to increase drastically from 28.2% to 58.7% or 28.4% to 61.9%. This observation was mostly due to the availability of more acidic sites for the acidolysis reaction. Further increase in the catalyst loading from 9 wt % to 11 wt %, however, led to a lower percentage incorporation of medium-chain FAs, which was probably resulted from a mixing problem involving reactants, products and solid catalyst. The influence of reaction temperature on the percentage incorporation of medium-chain FAs is also illustrated in Figure 10. For these trials, the catalyst loading and the substrate molar ratio remained constant at 9 wt % and 10:1, respectively. One can see that the reaction temperature has a considerable influence on the incorporation of caprylic or capric acid. With a rise in the reaction temperature from 90 to 150 °C, the incorporation extents of caprylic or capric acid appeared to be substantially increased from 6.5% to 58.7% or from 8.2% to 61.9%, respectively, and the maximum incorporation for both medium-chain FAs was attained at a reaction temperature of 150 °C. However, with further increase in reaction temperature, the percentage incorporation was increased slightly. Accordingly, in the present investigation, the reaction temperature is selected as 150 °C. In addition, for all of the acidolysis parameters studied, the incorporation extent of capric acid was higher than that of caprylic acid, showing that capric acid was incorporated into the TAGs more easily than caprylic acid. Similar results were also reported in previous literature.43 Reusability of the Solid Catalyst. In order to evaluate the reusability of the catalyst, the solid catalyst was separated by filtration after completion of the reaction and subsequently washed thoroughly with methanol and cyclohexane. The stability of the solid catalyst was investigated in successive runs under the optimized reaction conditions. The results are illustrated in Figure 11. As clearly observed from this figure, the catalytic activity was not significantly decreased as the catalyst was reused from the run 1 to 5. The good stability of the catalyst is due to the firm attachment of HPW on the aminofunctionalized SBA-15 silica via ionic bonds, which can prevent the leakage of the heteropoly acids. The amino-surface modification of SBA-15 silica seems to be essential to provide

Figure 8. 29Si MAS NMR spectra of samples: (a) SBA-15; (b) SBA15-pr-NH2-HPW.

the silicon species of organo-silanes on the surface of SBA-15 silica.40 Figure 9 depicts the 31P MAS NMR spectra of HPW

Figure 9. 31P MAS NMR spectra of samples: (a) HPW; (b) SBA-15pr-NH2-HPW.

and SBA-15-pr-NH2-HPW samples. As indicated in this figure, the pure HPW displayed a sharp and intense resonance peak at −15.4 ppm, corresponding to the structural phosphorus PO4 within the Keggin units.41,42 The SBA-15-pr-NH2-HPW sample showed an additional broad NMR peak at around −13 ppm and a sharp peak at −15.4 ppm. Most probably, the new peak at −13 ppm was originated from the PO4 unit within the dehydrated HPW anions located near the support surface. The appearance of the characteristic peak indicated that the immobilization of HPW did not destroy the Keggin structure of HPW. All together, the immobilization of HPW on SBA-15 silica was successful, and there was a chemical binding interaction between the HPW and the NH2 group on the functionalized SBA-15 materials. As already mentioned, this strong interaction between the Keggin unit and the aminofunctionalized SBA-15 silica is expected to enhance the stability of the solid composite catalyst. Influence of Acidolysis Parameters. The influence of substrate molar ratio on the acidolysis reaction was investigated. 643

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Figure 10. Influence of acidolysis parameters on the incorporation of medium-chain FAs into soybean oil over the SBA-15-pr-NH2-HPW catalyst.

catalyst could be reused for five times without significant loss of activity, revealing that no leaching of the catalytically active species occurred during the liquid phase reaction. Therefore, the solid catalyst works as a recyclable and efficient catalyst in the acidolysis of vegetable oils for the production of SLs. The high catalytic stability can be explained due to the strong chemical interaction between the Keggin unit and the SBA-15 support.

CONCLUSION An environmentally friendly solid acid catalyst, namely, SBA15-pr-NH2-HPW, was prepared at the present work by immobilization of HPW onto the amino-functionalized SBA15 silica. The obtained solid catalysts were characterized by various techniques, including XRD, FT-IR, TEM, SEM, 31P MAS NMR, 29Si MAS NMR, and nitrogen adsorption− desorption techniques. It was shown that the HPW was chemically immobilized onto the mesoporous SBA-15 material through the ionic binding to aminosilane groups, and the ordered mesoporous structure remained almost unchanged after the loading of the HPW. By using this solid catalyst, the production of SLs containing medium-chain FAs from soybean oil was successfully achieved by the acidolysis reaction in a solvent-free system. Further, the solid catalyst could be easily separated from the reaction mixture and reused for several times without obvious loss of catalytic activity. Thus, the solid catalyst shows potential as the efficient and stable heterogeneous catalysts in the acidolysis of vegetable oils.

Figure 11. Catalytic activity over five reaction cycles. Reaction conditions: medium-chain FA/soybean oil molar ratio 10:1; catalyst loading 9 wt %; reaction temperature 150 °C, and reaction time 10 h.

anchoring sites for the immobilization of the HPW. No HPW species could be detected in the liquid reaction mixture in terms of tungsten percentage by atomic absorption spectroscopy. Leaching experiments were also carried out to further confirm that the solid catalyst is truly heterogeneous in nature. For this purpose, the catalyst was filtered out from the reaction mixture after 5 h of reaction, and the acidolysis was allowed to continue with the remaining filtrate for another 5 h. No further incorporation of the medium-chain FAs was observed, suggesting that there was no considerable leaching of the active components into the solution during the course of the acidolysis reaction. In addition, the SEM and TEM images of the recovered catalysts displayed no obvious change in the morphology of the solid catalyst in comparison with the fresh catalyst (figure not shown here). Based on the results, the


Corresponding Author

*E-mail address: [email protected] (W.Xie). Tel.: +86 371 67756302. Fax: +86 371 67756718. 644

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The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by research grants from the National Natural Science Foundation of China (Grant No. 21276066, 21476062), and the Plan for Scientific Innovation Talent of Henan Province (144200510006).

ABBREVIATIONS APTES, 3-aminopropyltriethoxysilane; FA, fatty acid; C8:0, caprylic acid; C10:0, capric acid; SO, soybean oil; TAG, triacylglycerol; 2-MAG, 2-monoacylglycerol; TLC, thin-layer chromatography; TEOS, tetraethylorthosilicate; HPW, 12tungstophosphoric acid; FT-IR, Fourier transform infrared; GC, gas chromatography; 31P CP MAS NMR, 31P crosspolarization magic angle spinning nuclear magnetic resonance; 29 Si CP MAS NMR, 29Si cross-polarization magic angle spinning nuclear magnetic resonance; XRD, X-ray diffractometer; SEM, Scanning electron microscopy; TEM, Transmission electron microscopy


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