Preparation of Low Calorie Structured Lipids Catalyzed by 1, 5, 7

Mar 31, 2014 - (1, 2) Low-calorie structured lipid (LCSL) is a family of structured lipid that ... of fats, but with approximately half of the calorie...
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Preparation of Low Calorie Structured Lipids Catalyzed by 1,5,7Triazabicyclo[4.4.0]dec-5-ene(TBD)-functionalized Mesoporous SBA15 Silica in a Heterogeneous Manner Wenlei Xie* and Cong Qi School of Chemistry and Chemical Engineering, Henan University of Technology, Zhengzhou 450052, P. R. China ABSTRACT: 1,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD, a strong bicyclic guanidine base) functionalized SBA-15 material has been found to be an efficient solid catalyst for the interesterification between tributyrin and methyl stearate in a solvent-free system for the production of low-calorie structured lipid (LCSL). The solid base catalyst was characterized by using small-angle X-ray scattering, Fourier transform infrared spectra, thermo gravimetric analysis, scanning electron microscopy, transmission electron microscopy, nitrogen adsorption−desorption, and elemental analysis techniques. The obtained LCSL was analyzed by reverse-phase high-performance liquid chromatography for triacylglycerol composition. The influence of various reaction parameters, such as the substrate ratio, reaction temperature, and reaction time, on the interesterification reaction was investigated systematically. More than 90% LCSL was obtained at 80 °C within 1 h when the methyl stearate/tributyrin molar ratio of 2:1 was employed. The obtained solid catalyst could be recovered easily and reused for several recycles with a negligible loss of activity. By using the solid base catalyst, an eco-friendly more benign process for the interesterification reaction in a heterogeneous manner was developed. KEYWORDS: heterogeneous catalyst, interesterification, structured lipid, SBA-15

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INTRODUCTION Most native vegetable oils have limited applications in their original forms. The interesterification of vegetable oils can be used to obtain fats with desired functional and nutritional properties, by altering the original specific triacylglycerol (TAG) composition of the blend components, which can widen their commercial use.1,2 Low-calorie structured lipid (LCSL) is a family of structured lipid that provides the desired physical and nutritional characteristic of fats, but with approximately half of the calorie of typical edible oil. Generally, the LCSL is composed of two types of TAGs; one contains two short-chain and one long-chain acyl residues (here designated as SSL), and another contains two long-chain and one shortchain acyl residues (here designated as SLL). Due to the lower caloric content of short-chain acyl residues, the LCSL contains lower caloric contents than the naturally occurring TAGs and accordingly is intended for use in coatings, baking chips, baked products, or as cocoa butter substitutes. The most common process used for the production of LCSL is through the interesterification reaction, which is carried out in the presence of a catalyst.2−5 In general, the interesterification can be conducted chemically or enzymatically. Enzymatic interesterification (EIE) processes are advantageous owing to their selectivity, milder reaction condition and ease of product recovery.6−8 However, the application of EIE reactions is limited by the high cost associated with the lipase used. Chemical interesterification (CIE) is a cheaper process that is tried-and-true on an industrial scale, as it has been around for a long time and relevant industrial procedure and equipment are readily available.8,9 Hence, in a practical application point of view, CIE reaction seems to be the most attractive method in the edible oil industry. © 2014 American Chemical Society

Industrially, the interesterification process is carried out by homogeneous base catalysts to improve the physicochemical properties of oils and produce distributed fatty acid residues among TAG molecules.10 Sodium hydroxide and sodium alkoxide are the most preferred choice of catalysts for the CIE processes.10−12 Although these homogeneous bases are highly efficient and of low cost, they can cause the problem of tedious purification of the product, and the undesired wastewater is inevitably generated. For this reason, the heterogenization of active catalysts is always an appealing option for practical application from both commercial and environmental point of view. More recently, the development of heterogeneous catalysts has attracted increasing attention because of the easy recovering, possible recycling of the catalyst and simple operation procedures.13 Up to now, a variety of different heterogeneous base catalysts have been investigated, including supported alkali or alkaline earth metals, basic zeolites, hydrotalcites, zeolites, and ion-exchange resins.13−16 Supported organic bases, such as guanidines immobilized on polymers or encapsulated in zeolite cages, have been used as solid catalysts for biodiesel production from vegetable oils.17,18 However, to our knowledge, the heterogeneous base catalysts have rarely been reported to be employed for the CIE reaction so far,19 despite their proven catalytic efficiency in a variety of other organic reactions.16,17 Therefore, from both environmental and economical point of views, it is of interest to develop environmentally friendly and high efficient catalysts for the interesterification reaction. Received: Revised: Accepted: Published: 3348

December March 29, March 30, March 31,

4, 2013 2014 2014 2014

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other materials used were of either analytical or chromatographical grades. Catalyst Preparation. Mesoporous silica SBA-15 was prepared according to a hydrothermal approach using Pluronic P123 triblock copolymer as a template under acidic conditions.20 In a typical synthesis, 10 g of Pluronic copolymer P123 was first dissolved in 75 mL of double distilled water and 300 mL of 2 mol/L HCl solution at room temperature, and then stirred at 313 K until a homogeneous solution was obtained. Thereafter, 21 g of tetraethylorthosilicate (TEOS) was added under stirring to the P123 solution, and the stirring was continued for 24 h at 40 °C. Subsequently, the solution was taken in a Teflon-lined autoclave and heated at 100 °C for 48 h. After cooling to room temperature, the solid product was filtered, then washed thoroughly with deionized water, and finally dried in air at room temperature. The SBA-15 powder was obtained by calcining the material in air at 550 °C for 6 h to remove the template. The mesoporous SBA-15 silica was used as a support material for preparation of the solid catalyst. The organic base TBD can be allowed to immobilize onto the surface of SBA-15 silica through the condensation reactions of TBDorganosilane with the surface silanol group. Initially, the TBDorganosilane was prepared by the reaction of 1,3-glycidyloxypropyltrimethoxysilane with TBD base. Typically, 0.6 g of TBD was added to a solution of 1,3-glycidyloxypropyl-trimethoxysilane (4 mL) in dry DMF (10 mL), and after this the resulting mixture was continued to stir for 48 h at room temperature under nitrogen atmosphere. Thereafter, the solution was poured into a flask containing dry toluene (15 mL) and SBA-15 (1.2 g, previously heated at 350 °C for 10 h), after which the mixture was refluxed for 48 h under nitrogen atmosphere. The solid base catalyst thus obtained was filtered under vacuum, washed carefully with toluene and methanol, and extracted with a Soxhlet apparatus for 24 h using a 1/1 diethyl ether/methylene chloride mixture. Finally the solid catalyst was dried at 60 °C for 12 h under vacuum prior to use. Catalyst Characterization. Small-angle X-ray scattering (SAXS) measurements were performed on a Bruker AXS Nanostar instrument with the Cu Kα radiation using an acceleration voltage of 40 kV and a current of 30 mA. Fourier transform infrared spectroscopy (FT-IR) was recorded for functional group detection in the 400−4000 cm−1 range on a Shimadzu IR-Prestige-21 spectrometer using the usual KBr pellet method. Thermogravimetric measurements (TG-DTA) were conducted on a TA Instrument TG 2050 thermogravimetric analyzer with a heating speed of 20 °C/min under air atmosphere with a flow rate of 100 mL/ min. Scanning electron microscope (SEM) measurements were carried out with a field-emission microscope (JEOL, JSM-6390LV) using an accelerating voltage of 15 kV. Transmission electronic microscopy (TEM) images were taken using a JSM-6390LV transmission electronic microscopy at an accelerating voltage of 200 kV. The elemental analysis for carbon, hydrogen and nitrogen contents of the samples to measure the TBD loading in the solid catalyst were carried out on a Carlo-Erba 1106 elemental analyzer. The textural properties of the solid catalysts were measured by nitrogen adsorption−desorption isotherm method at liquid nitrogen temperature (−196 °C) using a Quantachrome NOVA 1000e instrument. The specific surface area was calculated by using the Brunauer−Emmett−Teller (BET) method. The total pore volume and the pore size distribution were assessed according to the Barrett− Joyner−Halenda (BJH) method based on the adsorption isotherm. Interesterification Procedures. A certain amount of tributyrin and methyl stearate mixture was charged into a 50 mL round-bottom flask. After raw materials were melted absolutely and dried under reduced pressure at 80 °C, the CIE reaction was carried out in solventfree system by the addition of 1.5 wt % of TBD-functionalized SBA-15 silica as a catalyst. The interesterification of tributyrin with methyl stearate was allowed to proceed under reduced pressure at 80 °C in a water bath with magnetic stirring (∼750 rpm). After completion of the reaction, the interesterified product was filtered, and then employed for subsequent analysis.

Mesoporous molecular sieves have attracted significant attention especially in their application in catalysis. Among the mesoporous silica materials, SBA-15 silica is emerged as a promising catalyst support in heterogeneous catalysis owing to its well-ordered structure, nanosized channel, large surface area, and sufficient silanol group for surface modification.20 The immobilization of organic base groups on the surface of the mesoporous material by grafting or co-condensation can be recognized as a promising approach to create efficient solid base catalysts with improved catalytic properties as compared to conventional homogeneous catalysts.20−22 This approach can allow the catalyst to be separated easily from the reaction mixture with no wastewater originating from the catalyst neutralization step. TBD (1,5,7-triazabicyclo[4.4.0]dec-5-ene) is a strong bicyclic guanidine base (pKb = 25) widely utilized as a homogeneous base catalyst for various chemical reactions.23 Mesoporous silica materials could be organically functionalized through grafting reactions in which the organic base is anchored onto the surface of the pores with the preservation of a pore host structure. Kalita et al. covalently grafted TBD onto MCM-41 nanoparticles and proved that the TBD-functionalized MCM-41 material is efficient for Michael-addition reactions under solvent-free conditions.24 However, this kind of heterogeneous base catalysts is seldom used for the CIE reaction.19 Due to the increased interest and demand for practical application, the environmentally friendly benign and easy-separable solid base catalysts are highly desirable. In the present contribution, for the purpose of providing a highly efficient and green procedure for the interesterification reaction, the guanidine base TBD was covalently bound onto the surface of the mesoporous SBA-15 silica to form the heterogeneous catalyst SBA-15-pr-TBD. Thereafter, this hybrid organic−inorganic material was tested for the interesterification of tributyrin and methyl stearate in solvent-free system for the production of LCSL. For this purpose, the TBD base was previously linked to 1,3glycidyloxypropyl-trimethoxysilane by nucleophilic addition to the epoxide moiety. The resulting TBD-organosilane compound was then anchored to the mesoporous SBA-15 silica by reaction of the trimethoxysilyl group with the surface silanol. The as-prepared solid base catalyst was characterized by using small-angle X-ray scattering (SAXS), Fourier transform infrared (FT-IR) spectra, thermo gravimetric analysis (TG-DTA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), nitrogen adsorption−desorption and elemental analysis techniques. By using this solid base catalyst, the LCSL was produced in a solvent-free system by interesterification of tributyrin and methyl stearate in a batch reactor. The TAG compositions of the interesterified products were determined by high-performance liquid chromatography (HPLC). Moreover, the interesterification parameters, including the molar ratio of substrate, reaction temperature, reaction time, and reusability of the solid catalyst were investigated systematically in the present investigation.

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MATERIALS AND METHODS

Materials. Tributyrin (≥98%) and methyl stearate (≥98%) were obtained from Sinopharm Chemical Reagent Corporation (Sanghai, China). Pluronic copolymer P123 (EO20PO70EO20, average molecular weight was 5800), 1,3-glycidyloxypropyl-trimethoxysilane (≥99.8%), 1,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD, ≥98%) and tetraethylorthosilicate (TEOS, ≥98%) were purchased from Sigma-Aldrich. All the 3349

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Analyses of TAG Composition. Reversed-phase high-performance liquid chromatography (HPLC) was used to determine the TAG composition of the interesterified product. A commercially packed Genesis C18 HPLC column (15 cm length × 4.6 mm) was used to separate the TAGs. The protocol employed for the mobile phase involved linear elution gradients for a dichloromethane/acetonitrile (HPLC grade) system from 35% acetonitrile increasing to 55% dichloromethane in 45 min. The mobile phase flow rate was 1.2 mL/ min. The column temperature was held at 40 °C. The TAG species were detected by an Alltech 500eVaporative light scattering detector. Individual peaks were identified tentatively according to Han et al.5 HPLC analysis was run in duplicate for each sample of the interesterified product.

peak for (110) and (200) reflection was detected mostly likely owing to the decrease in the scattering contrast between the SBA-15 support walls and the filled pores with the organic moieties, which showed that the organic component was embedded into the channel of the support.24−26 Moreover, the (100) peak of the functionalized SBA-15 silica was shown to shift to higher 2-theta value, revealing the reduction of pore size because of the incorporation of organic components into the pore channel of the support.27 By drawing on the results, the ordered meso-structure of the siliceous SBA-15 is preserved after the incorporation of organic groups. Figure 2 represents the FT-IR spectra of SBA-15 silica and SBA-15-pr-TBD sample. In the case of nonfunctionalized SBA-

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RESULTS AND DISCUSSION Catalyst Characterization. Surface functionalization has been employed to prepare TBD-functionalized mesoporous SBA-15 material. The TBD base can be covalently tethered onto the framework surface of the mesoporous SBA-15 silica due to the presence of silanol groups, which can form strong chemical bonds with TBD-containing organosilane precursors.22 In the present study, the solid base catalyst was prepared by two steps. The TBD base was first reacted with 3trimethoxysilylpropoxymethyloxirane in anhydrous DMF to afford a TBD-containing organosilane by nucleophilic addition of TBD to the epoxide moiety. Then, in the second step, the resulting solution was allowed to react with the SBA-15 silica forming the solid base catalyst. The obtained solid catalyst was identified by FT-IR spectroscopy, SAXS, TG-DTA analysis, nitrogen adsorption−desorption, and elemental analysis techniques. Figure 1 shows the SAXS patterns of SBA-15 silica and functionalized SBA-15 silica. The SAXS pattern of siliceous

Figure 2. Fourier transform infrared spectra for samples. (a) SBA-15 and (b) SBA-15-pr-TBD.

15 silica, there was a broad IR absorption band centered at 3460 cm−1 mostly ascribed to the stretching vibration of framework hydroxyl groups and adsorbed water molecules, and the peak located at 1635 cm−1 was principally attributable to the bending mode of adsorbed water present in the SBA-15 material.25 Moreover, a weak band in the mid-infrared region at 950 cm−1 for the SBA-15 support could be mainly responsible for Si−OH groups in SBA-15 silica.27 Besides, three characteristic absorption bands around 1080 cm−1, 780 cm−1, and 470 cm−1 were the typical vibration modes of mesoporous framework Si−O−Si attributed to the condensed silica net in the samples.27,28 By comparing the IR spectrum of the TBDfunctionalized SBA-15 material, the IR characteristic peaks of surface hydroxyl groups at 3460 and 950 cm−1 were observed to decrease significantly in intensity after the organofunctionalization, thereby suggesting that the surface hydroxyl groups reacted with the anchored functional groups to form the functionalized SBA-15 material. In comparison with the SBA-15 support, new IR absorption bands at 2942 and 2876 cm−1 due to the CH2 asymmetric and symmetric vibration respectively were appeared for the SBA-15-pr-TBD sample.24,25 Additionally, the characteristic peaks at 1587 and 675 cm−1 for the TBDfunctionalized SBA-15 material could be ascribed to the CN stretching vibration and N−H bending vibration of TBD,18,24 and these IR peaks were not found in the IR spectrum of the nonfunctionalized SBA-15 silica. On the basis of the results, the TBD has been successfully anchored on the surface of the SBA15 material. The thermal behavior of SBA-15-pr-TBD is illustrated in Figure 3. As indicated in this figure, the small amount of mass

Figure 1. Small angle X-ray photoelectron spectroscopy patterns for samples: (a) SBA-15 and (b) SBA-15-pr-TBD.

SBA-15 exhibited an intense peak at 0.91° corresponding to the (100) reflection, and two low-intensity peaks at 1.56° and 1.76° that could be indexed as (110) and (200) reflections.19,25 The (100) reflection was characteristic for p6mn symmetry, evidencing the formation of a well formed uniform hexagonal lattice.24 For the functionalized SBA-15 material, the (100) peak was also observed in the SAXS pattern, however the (110) and (200) peaks were not clearly registered as evident from Figure 1. This observation suggested that the long-range ordering of the mesoporous structure remained unaltered even after incorporation of TBD into the SBA-15 silica. No obvious 3350

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Figure 5. The TEM images confirmed the presence of a honeycomb-like hexagonal array through the material for SBA15 silica and TBD-functionalized SBA-15 silica.25 The slight change in TEM image was observed after the functionalization of SBA-15 silica, thus implying that the incorporation of TBD into the SBA-15 silica does not destroy the mesopore structure of SBA-15 silica. This result is in good agreement with the other characterization technique results. The C, H and N elemental analysis results for the solid base catalyst are summarized in Table 1. According to the results, the loading amount of TBD on the SBA-15 silica was calculated to be 2.35 mmol/g, which is in line with the result obtained from TG-DTA measurements. The nitrogen adsorption−desorption isotherms of SBA-15 silica and SBA-15-pr-TBD catalyst are presented in Figure 6. For the two samples, the adsorption isotherm could be classed as type IV isotherm with H1-type hysteresis loop according to IUPAC classification, which was the typical behaviors of highly ordered mesoporous molecular sieves with narrow pore size distributions.25,27 Moreover, the adsorption branch of isotherm displayed a sharp inflection at a relative pressure of about 0.65. This is characteristic of capillary condensation with uniform pore dimensions and high-ordering of the material. This phenomenon suggested that after the functionalization with the organic base the SBA-15 material could still maintain its mesoporous structure. The textural properties of the samples are listed in Table 2. BET surface area and pore-size distribution were calculated using nitrogen adsorption at −196 °C. For pure SBA-15 silica, the mean pore size calculated from the N2 adsorption− desorption isotherm was 6.65 nm, and the measured surface area and pore volume were determined to be 807 m2/g and 1.25 cm3/g, respectively, which are in accordance with the value reported in the literature.19,24 As indicated in Table 2, for the SBA-15-pr-TBD catalyst, a surface area of 312 m2/g and a pore volume of 0.43 cm3/g were determined, and the pore size distribution was narrow with a mean pore diameter of 4.45 nm. Obviously, the SBA-15-pr-TBD catalyst showed some loss of surface area and a pronounced reduction in pore volume and pore diameter after the organofunctionalization, implying that the organic base was incorporated into the SBA-15 support. The decrease of these parameters upon the functionalization of SBA-15 silica could be expected since the partial filling of functionalized TBD base inside the mesopores would increase the average wall thickness, thus decreasing the pore diameter and BET surface area. Although such a change in the textural

Figure 3. Thermo gravimetric curves of SBA-15-pr-TBD catalyst.

loss (about 8.7%) below 200 °C could be apparently originated from the removal of physically adsorbed water. Meanwhile, an endothermic event between 0 and 200 °C was also observed in the DTA curve. With further increasing the temperature, as shown in Figure 3, a significant mass loss in the temperature range of 200−750 °C (about 54.2%) and a strong endothermic event at about 500 °C were appeared. This main mass loss could be principally ascribed to the thermal decomposition of the organic moieties in the solid base catalyst. On the basis of the mass loss (200−750 °C), the amount of functional groups bound on the SBA-15 material could be estimated to be 2.23 mmol/g. Beyond a temperature of 750 °C, almost no obvious mass loss in the TG curve could be observed, revealing that the tethered organic moiety was decomposed totally at this temperature. SEM techniques can be employed to study the morphology feature of the SBA-15-pr-TBD catalyst. The typical SEM micrograph of SBA-15-pr-TBD sample is shown in Figure 4. As can be seen, the SBA-15 silica displayed curved faceted and smooth rods based on elongated wheat-like macrostructures in SEM images.19,24 The TBD-functionalized SBA-15 silica exhibited a similar wheat-like morphology to the nonfunctionalized SBA-15 silica. No significant variation in the surface morphology occurred after the modification of SBA-15 silica except that the domains were more densely packed. The morphology property of the solid base catalyst was also characterized by TEM techniques. The TEM images of SBA-15 silica and TBD-functionalized SBA-15 silica are shown in

Figure 4. Scanning electron microscopy images of samples: (a) SBA-15 and (b) SBA-15-pr-TBD. 3351

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Figure 5. Transmission electron microscopy images of samples: (a) SBA-15 in top view; (b) SBA-15 in side view; (c) SBA-15-pr-TBD in top view; (d) SBA-15-pr-TBD in side view.

that contains three long-chain acyl residues), suggesting that the interesterification proceeded smoothly by using the solid base catalyst. As indicated in Figure 7, the content of SLL was increased obviously with the reaction time in the first 1 h, and the increase became very slow beyond 1 h. However, upon prolonging the reaction time to 5 h, the content of SSL was gradually decreased and then remained almost constant beyond the reaction duration of 5 h. As observed, the LCSL content was shown to increase with increasing the reaction time from 0.5 to 1 h, and no remarkable alteration was found between 1 and 3 h. However, there was a decrease trend in the LCSL content beyond 3 h. In light of the results, it can be concluded that the suitable reaction time required for obtaining a good LCSL yield is 1 h. Reaction temperature is another variable that can affect the interesterification reaction. The effect of reaction temperature on the interesterification reaction was investigated, and the results are illustrated in Figure 9. In absence of any solvent, the reaction temperature needs to be above the melting point of methyl stearate (38 °C), so that the interesterification reaction can be conducted by using the solid catalyst. As seen in Figure 8, when the reaction temperature was increased from 50 to 80 °C, the SLL content also increased from 33.7 to 47.3%. However, with subsequent increase in reaction temperature, the SLL content was dropped slightly. Besides, the SSL content was shown to decrease as the reaction temperature was increased from 50 to 80 °C, but no further variation in SSL content was observed beyond 80 °C. Moreover, the LCSL content of the interesterified product was gradually increased to 91.5% with

Table 1. Results for C, H, and N Elemental Analysis

a

sample

C (wt %)

H (wt %)

N (wt %)

SBA-15 SBA-15-pr-NR3OH

NFa 16.92

NFa 2.85

NFa 1.76

NF stands for not found.

properties, the functionalized SBA-15 silica still remained the typical mesoporous structure after the organofunctionalization reaction. Influence of Interesterification Parameters. In the present study, the interesterification can substantially alter the TAG profiles of the starting oil blend, and the LCSL is produced by the interesterification of tributyrin and methyl stearate. The interesterified product was characterized by the combination of short-chain and long-chain acyl residues into a triacylglycerol structure. As shown in Figure 7, two new types of TAGs (SSL and SLL) were produced, representing the incorporation of stearoyl groups into the new type of TAGs. By varying the fatty acid composition and the ratio of SSL/SLL, the properties of the LCSL product can be improved accordingly. As expected, there was no catalytic activity for the interesterification reaction to be observed for the SBA-15 support. However, after incorporation of TBD into the support, the SBA-15-pr-TBD catalyst displayed comparable activities toward the reaction. Figure 8 shows the TAG compositions on the solid base catalyst as a function of reaction time. At a reaction time of 0.5 h, the interesterified product contained 46.5% of SSL, 37.8% of SLL, and 3.4% of LLL (triacylglycerol 3352

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Figure 8. Evolution of the triacylglycerol distribution with reaction time during the interesterification reaction. Reaction conditions: methyl stearate/tributyrin molar ratio 2:1, reaction temperature 80 °C. SSL, triacylglycerol that contains two short-chain and one long-chain acyl residues; SLL, triacylglycerol that two long-chain and one shortchain acyl residues; LLL, triacylglycerol that contains three long-chain acyl residues; LCSL, low-calorie structured lipid.

Figure 6. Nitrogen adsorption/desorption isotherms and pore size distribution profiles of samples: (a) SBA-15 and (b) SBA-15-pr-TBD.

Table 2. Textual Characteristics of SBA-15 and SBA-15-prNR3OH Catalyst sample

SBETa (m2/g)

Vpb (cm3/g)

DBJHc (nm)

SBA-15 SBA-15-pr-NR3OH

807 312

1.25 0.43

6.65 4.45

a BET surface area. bPore volume. cAverage pore diameter from BJH desorption.

Figure 9. Evolution of the triacylglycerol distribution with reaction temperature during the interesterification reaction. Reaction conditions: methyl stearate/tributyrin molar ratio 2:1, reaction time 1 h. SSL, triacylglycerol that contains two short-chain and one long-chain acyl residues; SLL, triacylglycerol that two long-chain and one shortchain acyl residues; LLL, triacylglycerol that contains three long-chain acyl residues; LCSL, low-calorie structured lipid.

increasing the reaction temperature to 80 °C, and thereafter was diminished slightly at a temperature of 90 °C. Accordingly, in the present investigation, the proper temperature for the interesterification reaction is 80 °C. The interesterification reaction is generally employed to produce structured lipids that have the improved functionality by rearranging the fatty acids on the glycerol backbone. For different mixtures of substrates, the interesterification was carried out by using the solid catalyst at a temperature of 80 °C. As shown in Figure 10, for all the experimental trials involving a variety of molar ratios of methyl stearate to tributyrin, the rapid depletion of tributyrin species occurred affording new SSL and SLL of TAGs simultaneously. With increasing methyl stearate/ tributyrin molar ratio to 2:1, the content of SLL triacylglycerol in the interesterified product increased, while the SSL content

Figure 7. High performance liquid chromatography for the triacylglycerol compositions of the interesterified product. SSS, Tributyrin; SSL, triacylglycerol that contains two short-chain and one long-chain acyl residues; SLL, triacylglycerol that two long-chain and one short-chain acyl residues; LLL, triacylglycerol that contains three long-chain acyl residues. 3353

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Figure 11. Low-calorie structured lipid content of the interesterified product over several catalyst cycles. Reaction conditions: methyl stearate/tributyrin molar ratio 2:1, reaction temperature 80 °C, reaction time 1 h. LCSL, low-calorie structured lipid.

Figure 10. Evolution of the triacylglycerol distribution with tributyrin/ methyl stearate ratio during the interesterification reaction. Reaction conditions: reaction temperature 80 °C, reaction time 1 h. SSL, triacylglycerol that contains two short-chain and one long-chain acyl residues; SLL, triacylglycerol that two long-chain and one short-chain acyl residues; LLL, triacylglycerol that contains three long-chain acyl residues; LCSL, low-calorie structured lipid.

structure remained almost unchanged, which was favorable for improving the contact between the catalytically active component and reactants. Further, the solid catalyst is applicable to the interesterification reaction as a stable and highly active catalyst. Over this catalyst, the LCSL, composed of stearic acid and butyric acid, was successfully prepared in a heterogeneous manner. In the previous work, Han et al. reported that the interesterification of tributyrin and methyl stearate was conducted using a commercially immobilized lipase, Lipozyme RM IM.5 Compared to the immobilized lipase, the SBA-15-pr-TBD catalyst shows higher activities for the interesterification reaction owing to the shorter reaction time employed and lower catalyst loading.

diminished. In this case, with higher methyl stearate/tributyrin molar ratio than 2:1, the SLL content was declined slightly and the SSL content remained nearly constant. In all of these trials, the LLL triacylglycerol was observed to increase on increasing the methyl stearate/tributyrin molar ratio. From Figure 9, it was observed that the content of LCSL was initially ascended from 84.7 to 91.5% with the increase in methyl stearate/tributyrin molar ratio from 0.5:1 to 2:1. However, beyond the molar ratio of 2:1, the lower content of LCSL was observed. Given the present results, the appropriate molar ratio of methyl stearate to tributyrin is found to be 2:1, with the LCSL content of 91.5%. Reusability of the Catalyst. The reusability is an important aspect to evaluate the feasibility of a heterogeneous catalyst for its application in industrial processes, which makes it economic and preferable over homogeneous one. In order to study the catalyst reusability, consecutive reaction runs were carried out using the SBA-15-pr-TBD catalyst. At the end of the interesterification, the catalyst was recovered by filtration, washed with cyclohexane and ethanol, and dried at 120 °C overnight in a vacuum oven. The recovered catalyst was used for the next cycle, and this process was repeated for several times under the optimized reaction conditions as described above. The experimental results are illustrated in Figure 11. It was indicated that the LCSL yield was almost kept stable without significant loss of its catalytic activity (91.5% in the first cycle versus 85.8% in the fifth cycle) for up to five times of use, thus indicating the good stability of the catalyst. However, the catalytic activity was obviously decreased as the catalyst was used more than five times, mostly owing to the accumulation of impurities on the catalyst surface. In light of the aforementioned results, a heterogeneous catalyst, SBA-15-pr-TBD, was prepared by reactions of TBDorganosilane with the mesoporous SBA-15 material. By using this solid base catalyst, the interesterification between tributyrin and methyl stearate in a solvent-free system was performed for the production of LCSL. All the characterization results indicated the successful anchoring of organic base TBD on the framework channel of mesoporous SBA-15 materials. After the functionalization of SBA-15 silica, the ordered mesoporous

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86 371 67756302. Fax: +86 371 67756718. Funding

This work was financially supported by research grants from the National Natural Science Foundation of China (Project No. 21276066), the Plan for Scientific Innovation Talent of Henan Province (144200510006) and the Program for Innovative Research Team in Universities of Henan Province in China (2012IRTSTHN009). Notes

The authors declare no competing financial interest

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ABBREVIATIONS USED BET, Brunauer−Emmett−Teller; BJH, Barrett−Joyner−Halenda; CIE, chemical interesterification; DMF, N,N-dimethylformamide; EIE, enzymatic interesterification; FT-IR, Fourier transform infrared; HPLC, high-performance liquid chromatography; LCSL, low-calorie structured lipid; LLL, triacylglycerol that contains three long-chain acyl residues; SAXS, smallangle X-ray scattering; SEM, scanning electron microscopy; SSL, triacylglycerol that contains two short-chain and one longchain acyl residues; SLL, triacylglycerol that two long-chain and one short-chain acyl residues; TAG, triacylglycerol; TBD, 1,5,7triazabicyclo[4.4.0]dec-5-ene; TEM, transmission electron 3354

dx.doi.org/10.1021/jf405434a | J. Agric. Food Chem. 2014, 62, 3348−3355

Journal of Agricultural and Food Chemistry

Article

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microscopy; TEOS, tetraethylorthosilicate; TG-DTA, thermo gravimetric analysis; TLC, thin-layer chromatography

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dx.doi.org/10.1021/jf405434a | J. Agric. Food Chem. 2014, 62, 3348−3355