Article pubs.acs.org/IECR
Highly Selective Glycerol Esterification over Silicotungstic Acid Nanoparticles on Ionic Liquid Catalyst Wan N. R. W. Isahak,† Zatil A. C. Ramli,† M. Ismail,‡ and Mohd A. Yarmo*,† †
School of Chemical Sciences and Food Technology, Faculty of Science and Technology, and ‡Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Malaysia ABSTRACT: The aim of this work was to study the production, characterization, and performance of ionic liquid (IL) on silicotungstic acid (STA-IL) via the sol−gel method as catalysts in the esterification of oleic acid (OA) and purified glycerol, as well as to examine the effects of IL on heterogeneous catalysts. The STA-IL catalyst showed a relatively high surface area of 88.36 m2/g due to the presence of nanoporous silica. XPS analyses showed the significant formation of W−O−Si, W−O−W, and Si− O−Si bonding. The STA-IL catalyst enabled a conversion of 96% of OA with a significantly higher monoolein selectivity and yields of 96% and 92.2%, indicating that it is the more efficient solid acid catalyst. The presence of IL serves as PTC or medium, enhancing reaction synergistic effect. In addition, the ease of separation of the STA-IL catalyst due to its insolubility in the product phase can improve the recyclability. mesoporous materials,5 zeolitic molecular sieves,6,7 and solid cationic resins.8,9 In another work, the beta-zeolite catalyst gave the conversion of fatty acids above 20% at the optimum conditions of glycerol:LA molar ratio of 1:1 at 100 °C for 24 h.10 For heteropoly acid catalyst such as phosphotungstic or silicostungstic acid, the activity and stability of the catalysts depend on the structure and the type of the central atom along with the metal.11,12 The silicotungstic acid bulk (STAB) is impregnated onto different supports, such as polymers and silica,13 to achieve high surface area and stability in polar solvents. Ionic liquids (IL) are salts that are liquid at low temperature and when in the molten form are composed wholly of ions.14 Their versatile properties make ILs good solvents and catalysts in chemical reactions. In a previous study, ILs were used as catalysts in organic reactions. For example, Deng et al. (2001)15 first reported the ionic liquid 1-butylpyridium chloridealuminum(III) chloride as a green catalytic reaction medium for the esterification of glycerol with acetic acid. A conversion of 99.9% has been achieved at a lower reaction temperature of 30 °C. The slightly higher selectivity toward monoacetin of 43.7% was shown by an IL system as compared to the 42.3% of the sulfuric acid system. In this Article, we discuss the synthesis, characterization, and activities of the silicotungstic acid-silica sol−gel ionic liquid template (STA-IL) and compare it to sulfuric acid, silicotungstic acid bulk (STAB), and silicotungstic acid-silica sol−gel (STA-SG). The ionic liquid (IL) 1,2-dimethyl imidazolium tetrafluoroborate (DMIM·BF4) (shown in Scheme 1) was selected in this study because of its good properties as a reaction medium and a phase transfer catalyst (PTC) as reported by Jiang et al. (2007).16 Moreover, this Article
1. INTRODUCTION Currently, crude glycerol is widely used around the world as a combustion material due to abundant sources from biodiesel production. The uses of crude glycerol were expanded into many other high-quality products, such as pharmaceuticals, foods, and engine lubricants.1 To date, glycerol modification into glycerol monoester (GME) as a lubrication material was based on the biosources and was not practiced in industry. The nature of the polar headgroup and the structure of the hydrocarbon tail of GME result in its use as a friction reducer.2 Figure 1 shows the reaction of GMO production via the acidcatalyzed esterification of glycerol and fatty acids.3,4 Recently, many works related to the heterogeneous catalytic reaction have been reported, such as glycerol esterification with lauric acid (LA) and oleic acid (OA) catalyzed by functionalized
Received: Revised: Accepted: Published:
Figure 1. Schematic of the esterification reaction of glycerol and oleic acid to produce GMO and GDO. © 2014 American Chemical Society
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recyclability studies, the catalysts were washed by organic solvent, and fresh IL was added for the same loading percent (2.5 wt %).
Scheme 1. Structure of Ionic Liquid DMim·BF4
3. RESULTS AND DISCUSSION 3.1. Physical Surface Analysis (BET). The BET analyses showed that STAB had a BET surface area of 0.98 m2/g, whereas STA-SG and STA-IL had surface areas of 460.11 and 88.36 m2/g, respectively. The STA-SG catalyst gave a relatively higher surface area after the sol−gel technique was applied. This could suggest that the STA-IL catalyst was covered by IL, reducing the surface area but at the same time functioning as a phase transfer catalyst (PTC) to achieve higher catalytic activity. The isotherm plots in Figure 2 show that the STAB (H4SiW12O40) consists of macroporous material, while STA-SG
indicates that such a developed catalyst could be used as part of an efficient and clean catalytic technology for the esterification of glycerol and oleic acid.
2. MATERIALS AND METHODS 2.1. Synthesis of the Catalyst. The catalyst was prepared according to the methods of Izumi et al. (1999)17 with some modification. The ionic liquid DMIM·BF4 was synthesized at lab scale.15 In this study, a mixture of water (2.0 mol), 1butanol (0.2 mol), silicotungstic acid (5.0 × 10−4 mol), and ionic liquid (1.9 × 10−3 mol or 2.5 wt % over bulk STA and silica) was added to tetraethyl orthosilicate (TEOS) (0.2 mol) and stirred at 80 °C for 3 h. Next, the hydrogel obtained was dehydrated slowly at 80 °C for 1.5 h. The obtained dried gel was extracted in a Soxhlet apparatus with methanol as a solvent for 72 h and was then dried overnight. The silicotungstic acid immobilized silica was dried at 100 °C for 3 h to use as a catalytic material and was characterized using BET, FTIR, TEM, and XPS methods. 2.2. Characterization of the Catalyst. The Brunauer, Emmett, and Teller (BET) analysis of the STAB, STA-SG, and STA-IL catalysts was applied using a Micromeritics model ASAP 2010, and the physical nitrogen adsorption was performed at the liquid nitrogen temperature of 77 K. The FTIR analysis was conducted using a PerkinElmer GX model. The wavenumber was recorded in the range of 450−4000 cm−1 with 1 cm−1 resolution. The samples were prepared on the basis of the solid KBr method. The XRD method was performed by using a Bruker AXS D8 Advance with an X-ray radiation source of Cu Kα (40 kV, 40 mA) to record the 2θ diffraction angle from 10° to 60° at wavelength (λ = 0.154 nm). The TEM analysis was performed using a Philips CM12 transmission electron microscope with an electron gun at 200 kV. The X-ray photoelectron spectroscopy (XPS) measurements of the STAB, STA-SG, and STA-IL catalysts were performed on an XPS Axis Ultra from Kratos equipped with monochromatic Al Kα radiation. The samples were analyzed at an analysis chamber pressure of approximately 1 × 10−10 Pa. The spectra were referenced with respect to the C 1s line at 284.5 eV. The TGADTG analysis was performed using a Mettler Toledo instrument for a temperature range of 30−900 °C with a heating rate of 10 °C/min under nitrogen flow. Approximately 5−20 mg of sample was used for analysis. 2.3. Esterification Reaction. The esterification process between purified glycerol (from a palm oil transesterification source)18,19 and oleic acid was performed in a batch reactor with STAB, STA-SG, and STA-IL as the catalysts. The reaction was performed at 100 °C for 8 h and was connected to a pump system to remove water during the reaction. The products were separated from the unreacted reactants through centrifugation and were analyzed by HPLC. Analysis was conducted using DIONEX C18 HPLC column type (250 mm × 4.6 mm × 5 mm) with acetonitrile and acetone as mobile phase. The gradient program was: 0−1 min (acetone (60%):acetonitrile (40%)), 2−15 min (acetone (90%):acetonitrile (10%)). This analysis was running for 15 min at flow rates of 1 mL/min by using an evaporative light scattering detector (ELSD). For the
Figure 2. Nitrogen gas isotherms (adsorption−desorption) of (a) STAB, (b) STA-SG, and (c) STA-IL.
and STA-IL mostly consist of mesoporous materials. The graphs in Figure 2a−c clearly show that the adsorption isotherms of the STAB, STA-SG, and STA-IL catalysts belong to type II, type I, and type II, respectively. An inherent property of type I isotherms is that adsorption is limited to the completion of a single monolayer of adsorbate at the adsorbent surface. Type I isotherms are observed for the adsorption of gases on microporous soilds whose pore sizes are not much larger than the molecular diameter of the adsorbate. Complete filling of these narrow pores corresponds to the completion of a molecular monolayer. Type II adsorption isotherms indicate an indefinite multilayer formation after completion of the monolayer and are found in adsorbents with a wide distribution of pore sizes. Near the first point of inflection (point A), a monolayer is completed, following which adsorption occurs in successive layers.20 In addition, the STAB samples show the hysteresis of H4. The STA-SG and STA-IL shaped demonstrate H3 and H1, respectively. A type H1 hysteresis curve represents cylindrical pores that are uniform in size and shape. An H3 type hysteresis curve represents small slit-shaped pores with less uniform sample.21 Meanwhile, an H4 type hysteresis curve represents small pore size slits and more uniform shape. This may explain why the STA-IL sample, which consists of an H1 hysteresis 10286
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curve, results in a higher yield as well as higher selectivity due to uniform cylindrical-shaped pores. 3.2. FTIR Analysis. The typical FTIR spectrum of STAB obtained at 1100 cm −1 (Si−O−Si), 968 cm−1 (W−O d terminal), 903 cm−1 (W−Ob−W edge shared), and 719 cm−1 (W−Oc−W corner shared) corresponded to the primary structure [SiW12O40]4− of the catalyst22 as seen in Figure 3.
Figure 4. XRD diffractograms of STAB, STA-SG, and STA-IL.
that STA-SG and STA-IL had higher surface areas as compared to STAB. These characteristics were also affected by the addition of silica from TEOS in the sol−gel technique, which increased the porosity. Furthermore, the micrograph of the STA-IL catalyst showed that IL covered the STA with silica nanoparticles. 3.5. Surface Analysis by XPS. The XPS investigation of the binding energies (BE) and surface compositions of the silica supported STAB was investigated in detail. The XPS wide scan spectra of STAB, STA-SG, and STA-IL are shown in Figure 6. The photoelectron peaks in the XPS spectra for STAB, STASG, and STA-IL showed the presence of C 1s, O 1s, Si 2p, and W 4f, as expected. The percentage of mass concentration (%) for C 1s, O 1s, Si 2p, and W 4f in the STAB, STA-SG, and STA-IL catalyst is summarized in Table 1. The high percentage of W interaction in STA-IL led to good activity and selectivity of the main product as compared to the other catalysts. From Figure 7a, it was found that the STAB has Si 2p binding energies at 102.5 and 103.3 eV, respectively. The values of the Si 2p binding energies of STA-SG are 103.0, 103.7, and 104.5 eV (Figure 7b), indicating the formation of W−O−Si, Si−O−Si, and SiOH2+, respectively. Meanwhile, the STA-IL catalyst showed Si 2p components at binding energies of 103.1, 103.8, and 105.1 eV (Figure 7c), which represented the formation of W−O−Si, Si−O−Si, and SiO−NH2 binding. Some chemical interaction occurred between the catalyst and ionic liquid in the STA-IL catalyst toward the aminated amine (NH2−SiO2) binding formation. This phase may be due to silica gel and nitrogen atom of imidazolium group of IL during catalyst synthesis as described in section 3.2 on FTIR analysis. Meanwhile, the catalyst surface is rich in silanol groups (SiOH2+) and bonded to the NH+, which is available on imidazolium groups in IL. The Si 2p binding energy of 103.7 eV, representing SiO2, was in agreement with the binding energy of silica found in the literature.25−28 The O 1s XPS narrow scan spectrum recorded from the bulk STA is shown in Figure 8a and contains two distinct chemical states of O 1s. This showed that the main (90.74%, 531.5 eV) and intermediate (9.26%, 532.8 eV) peaks were the contributions of the presence of W−O−W and W−O−Si bonds, respectively.26 The O 1s spectrum recorded from the STA-SG sample (Figure 8b) was different from the STAB (Figure 8a). The spectra consisting of the main signal at 532.9
Figure 3. FTIR spectra of STAB, STA-SG, and STA-IL.
In the STA-SG and STA-IL catalysts, the Si−O−Si bend was detected at 1100 cm−1 at a strength mode. It clearly demonstrated that the Si−O−Si bend had a significant composition in STA-SG and STA-IL, which affected the Si and O from TEOS. This peak was absent for STAB catalyst because it was an originating result of the presence of silica gel. The presence of −CH3 and −CH2 bends at 2920 and 2780 cm−1, respectively, represented the alkyl group in IL. This IL formed by a number of alkyl groups and salt forms a weak bond with the silica and silicotungstic acid. In STA-IL catalyst, a little bend corresponding to SiO2−NH2 interaction was measured at 1560 cm−1. This phase may be due to silica gel and nitrogen atom of imidazolium group of IL. 3.3. Catalyst Crystallinity by XRD. From the XRD analysis, the STAB samples generated multiple peaks, indicating that the sample was forming crystalline compounds. However, the STA-SG and STA-IL produced via the sol−gel technique indicate the presence of amorphous phase resulting from silica compounds. In Figure 4, there is a broader peak detected at 28° representing the SiO2 phase based on silica gel during catalyst synthesis, which functions as a support of STA-SG and STA-IL catalysts.23,24 In addition, the XPS analysis predicts that there are nearly 50% Si−O−Si bonds in the STA and STA-SG-IL, as will be discussed in section 3.5. 3.4. Surface Morphology by TEM. From Figure 5, the TEM analysis shows the morphology of the catalysts. The STAB was arranged in the silica on the basis of the TEOS used to synthesize the STA-SG catalyst. In the STA-SG and STA-IL catalysts, the distribution of the catalyst in silica phase was depicted at magnification of 35 000 times. The STA-SG catalyst was smaller in size, in the range of 3.5−5.5 nm, as compared to STAB, in the range of 17−20 nm, whereas the STA-IL catalyst consists of particles in the range of 5.0−7.0 nm. This finding indicates that the slightly larger size was affected by the IL appearance. From the TEM analysis, it was noted that the uniformly shaped and smaller sized STA-SG and STA-IL particles were in agreement with the BET characterization results that showed 10287
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Figure 5. TEM micrographs for STAB, STA-SG, and STA-IL at different magnifications.
absorption of water can inhibit the catalytic activity in the esterification reaction. The W 4f XPS spectra recorded from the bulk STA (Figure 9a) were composed of the spin−orbit doublet with binding energies for the W 4f7/2 and W 4f5/2 of 36.8 and 39.0 eV, respectively, with ΔBE = 2.13 eV. These values are typical of the presence of W (VI).12 The W 4f spectra recorded from the STA-SG are less well-resolved than those of the STAB and STA-IL’s (Figure 9b). The W 4f XPS also fitted on the basis of two different W contributions: a spin orbit doublet at 35.5 eV (W 4f7/2 component), which accounted for the bigger area of the total spectra, and a second doublet at 37.7 eV (W 4f5/2 component), accounting for the remaining area. The major component in the STA-SG and STA-IL had a binding energy that was the same value as the STAB, but the minor component, appearing at the lower binding energy, may represent the partial decomposition of STAB on the silica surface and the formation of an oxide of the type WOx, such as WO2, in which W has an oxidation state lower than VI. There was no any chemical interaction between IL and STAB or silica, which can be analyzed by the XPS technique. This is because IL only acts as a reaction medium or phase transfer catalyst (PTC) that enhances the synergistic effect of the reaction. On the basis of previous research findings by Isahak et al. (2011),23 Newman et al. (2006),26 and Newman et al. (2005),29 it can be suggested that there is an interaction between (H3SiW12O40)− and the silanol groups at the silica surface to give ion pairs in the form of (SiOH2+)(H3SiW12O40), as in the reaction below:
Figure 6. Wide scans for STAB, STA-SG, and STA-IL.
Table 1. Percentage of Mass Concentration (%) for C 1s, O 1s, Si 2p, and W 4f in the Catalysts type of catalysts
C 1s
O 1s
Si 2p
W 4f
STAB STA-SG STA-IL
7.11 2.33 27.35
23.79 61.19 36.19
0.48 33.89 20.20
68.62 2.59 5.99
eV could be associated with the Si−O−Si bond and much weaker signals at 532.1 and 533.7 eV, which might be representative of W−O−W and adsorbed water, SiOH2+. In the STA-IL catalyst, three peaks were found that represent W− O−W, Si−O−Si, and SiO2 at binding energies of 531.7, 532.8, and 534.1 eV (refer to Figure 8c), respectively. There was no detection of the SiOH2+ phase in STA-IL because the
Si−OH + H4SiW12O40 → [Si−OH 2]+ [H3SiW12O40 ]− 10288
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Figure 7. Narrow scans for Si 2p (a) STAB, (b) STA-SG, and (c) STA-IL.
Figure 8. Narrow scans for O 1s (a) STAB, (b) STA-SG, and (c) STA-IL.
3.6. Thermal Stability by TGA/DTG. For the STA bulk catalyst, there were two regions with weight loss of 8% and 12% at 90 and 210 °C, respectively, as depicted in Figure 10a. From
the DTG, two loops were observed, representing the elimination of absorbed water and crystalline water.30 The STA-SG catalyst (Figure 10b) has one region representing a 10289
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Figure 9. Narrow scans for W 4f (a) STAB, (b) STA-SG, and (c) STA-IL.
weight loss of 15% at 90 °C. The DTG analysis shows a loop for the elimination of the adsorbed water. There was no water in the crystal structure form reported in the STA-SG catalyst. From Figure 10c, the analyses of the STA-IL catalyst showed that three regions of weight loss were associated with the coadsorbed water, crystalline water, and IL degradation at temperatures of 60, 270, and 380 °C, respectively. In the STAIL catalyst, a higher temperature is required to remove the presence coadsorbed water in the catalyst crystalline structure. In addition, it was found that the addition of IL in the catalyst increased the thermal stability of the STA-IL catalyst. However, the fresh IL showed a degradation phenomena at around 320 °C as depicted in Figure 10d. There were two regions of weight loss measured, which represent crystal water removal and IL phase degradation. 3.7. Acidity Level. Figure 11 shows the TPD-NH4 for the STAB and catalyst modified by ionic liquid, STA-IL. From this experiment, it was found that the STA-IL showed a slightly lower acidity level (63.5 mmol/g) as compared to STAB (65.6 mmol/g). For the STA-IL sample, as detected at the higher desorption temperature of 589 °C, the single peak shown corresponded to moderately strong acidity sites. The high acidity level of the catalysts can result in better reaction efficiency and selectivity. 3.8. Esterification Reaction. The activities of the catalysts were evaluated on the basis of three main parameters: the reaction time, OA:glycerol molar ratio, and type of catalyst. At optimum reaction conditions, after 8 h of reaction, the STAB catalyst gives a slightly higher conversion of OA of 98.0%, as compared to the STA-SG and STA-IL conversions of 94.2% and 96.4%, respectively (refer to Figure 12a). However, a depletion of the OA conversion of approximately 1.5% was shown after 9 h of reaction. The backward reaction occurs due
to catalyst inhibition mechanism, which may contribute to the longer response time during the reaction.23 The effect of different catalyst loads, ranging from 2 to 7 wt %, showed significant increment of the OA conversion, as depicted in Figure 12b. For catalytic reaction by STA-IL using 2 wt % catalyst loading, the OA conversion was 75.0%. However, the higher catalyst loading of 7 wt % gave the highest oleic acid conversion of 96.4%. The molar ratio of oleic acid to glycerol had a significant effect on the selectivity of GMO, as shown in Figure 12c. A reaction temperature of 100 °C was chosen to study the other parameters on the basis that the higher temperature would shift the reaction equilibrium to produce more side products, such as GDO and acrolein, but at a lower selectivity for GMO. However, the OA:glycerol molar ratio of 6:1 yielded higher selectivity of GMO of up to 96.4% at 100 °C for 8 h, as depicted in Figure 12c. Hence, the stronger parameter promoted greater desired product. In other words, a higher molar ratio of OA increased the chances for higher GMO production because OA reacts more easily with the glycerol compound. Nanoporous-silica in STA-IL forms a narrow structure and cylindrical pore type. The presence of nanoporous-silica can enhance the formation of certain product with simplest structure; in this case, the GMO production showed a significant increase as compared to the STAB catalyst. On the other hand, the addition of supported IL produces a synergistic effect that contributes a higher selectivity level of GMO as a main product. Functional groups in cations or in the supports for IL can combine with the anions of supported IL, resulting in a synergistic effect on the monoolein formation. Synergistic catalytic effects of IL catalytic system may play an important role of promoting the esterification reaction of glycerol. 10290
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Figure 11. Acidity level by TPD-NH4 experiment: (a) STAB and (b) STA-IL.
Figure 10. TGA-DTG analysis for (a) STA bulk, (b) STA-SG, (c) STA-IL, and (d) fresh IL.
In Table 2, the STA-IL catalyst showed relatively higher yields of GMO (main product) and GDO (byproduct) of 92.2% and 3.8%, respectively. No other products were formed after the reaction was complete. The higher oleic acid to glycerol molar ratio gave the opportunity to produce GMO over the STA-IL catalyst at a maximum catalyst loading of 7 wt % and 8 h of reaction. This would be explained by the excess oleic acid directly increasing the ability of the reaction for only one hydroxyl group of glycerol.24 Even the H2SO4 catalyst gives higher conversion of OA of up to 100%, and several products
Figure 12. Catalytic activity in the esterification reaction. (a) Effect of various catalysts at 100 °C, OA:glycerol ratio = 6:1 and 7 wt %, (b) effect of catalyst loading on the OA conversion at 100 °C, OA:glycerol ratio = 6:1 for 8 h, and (c) effect of the OA:glycerol molar ratio on the selectivity of the STA-IL catalyst.
other than GMO and GDO were yielded at 14.8%. In terms of the higher selectivity toward GMO, STA-IL gives a better 10291
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Table 2. Performance of the Catalysts in the Esterification Reaction As Compared to the Homogeneous Catalyst, Sulfuric Acid selectivity (%)
yield (%)
type of catalyst
conversion (%)
GMO
GDO
others
GMO
GDO
others
H2SO4 IL (DMIM·BF4) STA bulk STA-SG STA-IL STA-IL, first recycle STA-IL, second recycle
100.0 58.3 98.0 94.2 96.4 94.6 91.3
81.6 94.6 87.9 95.0 96.0 95.2 94.3
3.6 5.4 8.1 5.0 4.0 4.8 5.7
14.8
81.6 54.9 86.1 89.3 92.2 89.5 86.1
3.6 3.1 7.9 4.7 3.8 4.5 5.2
14.8 n.d. 3.9 n.d. n.d. n.d. n.d.
4.0 n.d. n.d. n.d. n.d.
Table 3. Physical Properties of GMO from Esterification and Comparison with Other Commercial Lubrication Products on the Market properties density (kg/m3@g/L) viscosity (mm2/s) at 100 °C flash point (°C) cloud point (°C) a
gliserol monooleate + 4% dioleate (GMO from this work)
EMERY 2421a (gliserol monooleate)
945
Vanlube 887b 1-[di(4-octylphenyl) aminomethyl] tolutriazole
948
11.2
963
10
17.76
230
242
182.2
20
18
15
standard test methods ASTM D1298-99 ASTM D445-03 ASTM D92-02b ASTM D97-02
Henkel Corp. (GMO used for additional antifriction). bR.T. Vanderbilt Co.
4. CONCLUSIONS The purified glycerol based on the palm oil source can be esterified by oleic acid to produce GMO with the potential to be used as a lubrication additive. The H2SO4, STAB, STA-SG, and STA-IL catalysts gave conversions of 100%, 98%, 94%, and 96%, respectively. Although there were no significant differences among the three catalysts in terms of the conversion capability, the STA-IL catalyst had the advantage of enabling a higher selectivity of GMO of 96% as compared to 89.9% and 81.6% using STAB and H2SO4, respectively. The yield of GMO produced by the STA-IL catalytic system was 92.2%, which is higher than that of the other catalysts. This indicated that STAIL has better thermal stability because the addition of DMIM· BF4 ionic liquid functions as a reaction medium and phase transfer catalyst (PTC). Additionally, the catalytic activity and higher number of active sites contribute to the higher selectivity and yield of glycerol monooleate (GMO) as the main product.
reaction due to the presence of IL, which improved the liquid− liquid phase reaction of the polar (glycerol) and nonpolar (OA) substances. At the same time, the IL functions as a phase transfer catalyst (PTC) and a good reaction medium that is highly miscible and promotes interaction between raw materials with higher selectivity to form glycerol monooleate (GMO).16 The highly selective reaction using single IL may be affected by the coordination ability of the anion, which could affect the catalytic activity of IL in the phase transfer reaction. A low coordination ability of the anion could weaken the strength of the combination between the cation and the anion, which facilitated the ionic exchange and increased the catalytic activity of the IL.16,31 Furthermore, in Table 2, after the second recycle of the STA-IL catalyst, a less significant loss of OA conversion was observed of 91.3%. However, the selectivity to GMO showed a 1.7% loss after the second catalyst recycle. This may be due to the presence of additional fresh IL as a pretreatment method during recyclability studies, which gives a good selectivity to the focused products. 3.9. Physical Properties and Possible Uses in Lubrication. To ensure that the monoester of the glycerol produced has potential for use as an antifriction agent inside engines, several tests to study its physical properties were performed based on the international standard methods. Comparison of the physical properties of the product was based on the reports of the Henkel Corp. and Vanderbilt Co. through a patent filed by Addagarla and Callis (1999).32 A commercial lubrication product, Vanlube 887, had characteristics different from those of the two other products (GMO from this work and Emery 2421) because it is composed of different lubricants with the addition of some other specialty chemicals, such as anticorrosion and viscosity modifiers. Information for the physical testing an some characterization parameters, as compared to previous studies, is shown in Table 3.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +603-89214083. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We wish to thank Universiti Kebangsaan Malaysia (UKM) for funding this project under research grant number LRGS/BU/ 2011/USM-UKM/PG/02, DPP-2013-056, and the Centre of Research and Innovation Management (CRIM) for the instruments.
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REFERENCES
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dx.doi.org/10.1021/ie501110m | Ind. Eng. Chem. Res. 2014, 53, 10285−10293
