Ti/SiO2 as a Nanosized Solid Catalyst for the Epoxidation of FattyAcid Methyl Esters and TriglyceridesDinesh Kumar and Amjad Ali*

School of Chemistry and Biochemistry, Thapar University, Patiala 147 004, India

*S Supporting Information

ABSTRACT: The chemical method has been employed to prepare a series of titanium-loaded silica (1.7 wt % Ti on SiO2),followed by calcination in the temperature range of 550−950 °C. Diffuse reflectance solid-state ultraviolet−visible spectroscopysupported the incorporation of TiO2 on the silica support and existence of titanium in distorted tetrahedral and octahedralenvironments. The surface morphology and particle size of Ti/SiO2 were observed by field emission scanning electronmicroscopic and transmission electron microscopic techniques and later support the formation of 20−100 nm sized Ti/SiO2particles. Prepared Ti/SiO2 has been employed as a solid catalyst for the epoxidation of fatty acid methyl esters, derived fromused cotton seed oil, under two different reaction conditions, viz., (i) H2O2 and formic acid and (ii) H2O2, acetonitrile, andbutyronitrile in the presence of ultraviolet radiation. To demonstrate the versatility of the prepared Ti/SiO2 catalyst, it was alsosuccessfully employed for the epoxidation of a variety of substrates, viz., methyl oleate, used cotton seed oil, and fatty acid methylesters obtained from soybean, Karanja, and Jatropha oils. Under optimized reaction conditions in both methods, the preparedTi/SiO2 catalyst afforded a high epoxide yield (100%) even at room temperature (35 °C). The catalyst was amenable to recoveryand recycling for at least five consecutive reaction cycles with the partial loss of activity after every successive run. Finally, thesynthesized epoxidized fatty acid methyl esters have been tested as lubricity improver additives for diesel fuel.

1. INTRODUCTIONVegetable oils and fatty acid methyl esters (FAMEs) derivedfrom vegetable oils possess certain excellent frictional proper-ties, e.g., good lubricity, low volatility, high viscosity index,solvency for lubricant additives, and easy miscibility with otherfluids. However, a high degree of multiple −CC−unsaturations in the fatty acid chain of many vegetable oilscauses poor thermal and oxidative stability and limited theirapplication as lubricants to a modest range of temperatures.1−3

Epoxidized methyl oleate and FAMEs possess a lot ofcommercial importance because they have been used aslubricants,4−9 plasticizers in polymers10 and wood impregna-tion,11 stabilizers in chlorine-containing resins,12 cosmetics,13

and pharmaceuticals,14 and biofuel additives.15

Epoxidized fatty acid compounds are obtained at industrialscale mainly by the peracid process.15−20 In this process, theperacid is obtained in situ by reacting carboxylic acid (usuallyacetic acid) with hydrogen peroxide in the presence of a catalyst(mineral acids). There exist several drawbacks in the peracidprocess, viz., (i) the low selectivity to epoxidized productsbecause of acid-catalyzed oxirane ring opening, (ii) thecumbersome separation of acidic byproducts, and (iii) thehandling of strong acids, which is dangerous and causesproblems of corrosion. Thus, in recent years, emphasis has beengiven to develop the catalytic processes that could overcomethe above-mentioned disadvantages using more sustainablecompounds and technologies, viz., application of greenerchemical H2O2 or dioxygen and a solid catalyst in place ofmineral acids.21−27

The incorporation of titanium on an amorphous silicasupport has been reported in the literature28−30 as a catalyst forthe epoxidation of −CC− present in the fatty acid chain with

the help of hydrogen peroxide. In the recent past, titanium-substituted zeolites has been used as a catalyst, including TS-1,31 Ti-β,32,25 and Ti-incorporated mesoporous silica,24,25,33,34

for the epoxidation of olefins with hydrogen peroxide.Furthermore, the ease in catalyst recovery and the use ofacid-free reaction conditions make these catalytic systemspromising alternatives for the epoxidation of oleochemicals andsubstrates derived from renewable sources.35 The literature-reported24,25,33,34 methods involving Ti/SiO2 catalysts eitherrequired a high temperature, high catalyst amount, and longreaction duration to achieve high conversions and/or requiredan inert atmosphere while catalyzing the reaction.In the present work, the titanium-supported silica has been

prepared in nanosized form and characterized by powder X-raydiffraction (XRD), scanning electron microscopy (SEM),energy-dispersive analysis of X-rays (EDAX), transmissionelectron microscopy (TEM), ultraviolet−visible (UV−Vis)diffuse reflectance, and Brunauer−Emmett−Teller (BET)surface area measurement studies. Prepared Ti/SiO2 wasemployed as a solid catalyst for the epoxidation of vegetableoils, FAMEs, and methyl oleate at room temperature (35 °C).The epoxidation reactions have been performed under twodifferent reaction conditions, viz., (i) H2O2 and formic acid and(ii) H2O2, acetonitrile, and butyronitrile in the presence of UVradiation.

Received: November 24, 2011Revised: April 10, 2012Published: April 10, 2012

Article

pubs.acs.org/EF

© 2012 American Chemical Society 2953 dx.doi.org/10.1021/ef300127c | Energy Fuels 2012, 26, 2953−2961

2. EXPERIMENTAL SECTION2.1. Materials and Methods. Titanium isopropoxide (99%),

methyl oleate (99%), and silica (grade 62; surface area, 300 m2 g−1;pore volume, 1.15 cm3 g−1; pore size, 150 Å; and particle size, 75−250μm) were procured from Sigma-Aldrich. Toluene, acetonitrile,butyronitrile, formic acid, and hydrogen peroxide of analytical gradewere purchased from S. D. Fine Chemicals, India. Used cotton seed oil(10 fryings) was collected from local restaurants, and soybean, Karanja,and Jatropha oils were purchased from local shops located in Patiala,India. The free fatty acid (FFA) value, saponification, and iodine valueof the used cotton seed, soybean, Karanja, and Jatropha oils weredetermined by following the methods as reported in the literature,36

and the moisture contents was determined by the Karl Fishertitrimetric method (Table 1).The surface areas of the catalysts were determined using the

adsorption/desorption method at 77 K by the standard BET methodusing Micromeritics Tristar 3000 equipment. Prior to analysis, allsamples were degassed at 473 K for 90 min under a nitrogenatmosphere to remove the physisorbed moisture from the catalysts.Panalytical’s X’Pert Pro with Cu Kα radiation has been used forcollecting powder XRD data. The samples were scanned in the rangeof 2θ = 5−80° at the scanning speed of 2°/min. Diffuse reflectancesolid-state (DRS) UV−Vis spectra were recorded on Shimadzu 2450diffuse reflectance spectrophotometer using BaSO4 as a referencematerial. Titanium analysis has been performed on a GBC A32AAatomic absorption spectrophotometer (AAS).Field emission scanning electron microscopy (FESEM) was

performed on Tecnai G220 S-Twin to collect the SEM images ofthe catalysts, and TEM was performed on Quanta 200 FEG to recordTEM images. For SEM studies, powdered samples were mounted tospecimen stub using carbon tape and sputtered with gold coating tominimize the electrostatic charging. Before TEM analysis, the catalyst-powdered sample was ultrasonicated in diethyl ether and a drop ofwell-dispersed suspension was placed on the copper grid, dried in air,and analyzed by TEM.The photochemical reactor equipped with Philips mercury vapor

lamp H38JA-100/DX (125 W) as a source of UV radiation has beenused for the epoxidation reactions of various substrates.The Fourier transform infrared (FTIR) and Fourier transform-

nuclear magnetic resonance (FT-NMR) spectra were recorded on

Thermo Scientific Nicolet iS10 and Bruker Avance-II (400 MHz)spectrophotometers, respectively. Electron impact mass spectrum(EI−MS) of the epoxide derivative of methyl oleate was recorded on aWaters Micromass Q-ToF Micro mass spectrophotometer.

High-frequency reciprocating rig (HFRR) TE 77 semi-dry transferunit based on American Society for Testing and Materials (ASTM)D6079 has been employed to test the lubricity improver property ofFAMEs and their epoxide derivatives. In a typical lubricity test, a 4000ppm (0.4 wt %) dose of desired additive was added to the base dieselfuel of 458 μm lubricity to prepare the test fuel. A steel ball and steeldisk were completely submerged in the test fuel. The steel ball wasmoved back and forth while making contact on the stationary disk for75 min, and the wear scar generated on the former was measured inmicrometers with the help of an optical microscope. The wear scardiameter (WSD) was considered to be a measure of the lubricatingproperty, and the smaller the WSD, the better the lubricating ability ofthe additive.

2.2. Ti/SiO2 Catalyst Preparation. The titanium-loaded silica(Ti/SiO2) was prepared by the chemical method by following theliterature-reported procedure,28,29 with slight modification. A two-neckround-bottom flask equipped with a water-cooled condenser, oil bath,and magnetic stirrer has been charged with toluene (75 mL) andtitanium isopropoxide (0.50 g) and heated at the boiling point oftoluene (110 °C). To this suspension, 5 g of silica was added, andresulting mixture was stirred at the same temperature for 4 h. Theprecipitates formed were isolated by filtration, washed twice with hottoluene, subsequently dried at 100 °C, and finally calcined at desiredtemperatures (550, 750, and 950 °C) for 6 h. Ti/SiO2 thus obtainedwas subsequently characterized by BET surface area measurement,DRS UV−Vis, powder XRD, FESEM, and TEM studies. Titaniumloading in silica was found to be 1.7 wt %, and Ti/SiO2, henceprepared, was designated as Ti/SiO2-T, where T represent thecalcination temperature.

2.3. Preparation of FAMEs. In the present study, used cottonseed, soybean, Karanja, and Jatropha oil-derived FAMEs have beenused as the substrate for epoxidation. The FAMEs from these oils wereprepared by following the same procedure and catalyst (Li/CaO) asreported earlier by our group.37 All transesterification reactions wereperformed in a two-neck round-bottom flask equipped with a water-cooled condenser, an oil bath, and a magnetic stirrer. In a typicaltransesterification reaction, used cotton seed oil and methanol (1:12

Table 1. Chemical Analysis of Vegetable Oils Used as Feedstock in the Present Study

sample number feedstock FFA value (wt %) moisture content (wt %) saponification value (mg of KOH/g) iodine value (mg of I2/g)

1 used cotton seed oil 1.8 0.3 190.4 101.72 Karanja oil 4.2 0.35 185.0 96.83 Jatropha oil 8.4 0.38 195.5 101.54 soybean oil 0.2 0.27 190.0 125.4

Scheme 1. Epoxidation of FAMEs Derived from Used Cotton Seed Oil, in the Presence of Ti/SiO2-750 Catalyst, by FollowingMethod I (Formic Acid and Hydrogen Peroxide for 3 h at 35 °C) or Method II (Acetonitrile and Butyronitrile for 6 h under UVLight at 35 °C)

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molar ratio) were heated for 45 min at 65 °C in the presence of 5 wt %(with respect to oil) Li/CaO catalyst. After the completion of thereaction, the upper FAME layer was separated from the lower glycerollayer with the help of a separating funnel and excess methanol fromthe FAMEs was recovered with the help of a rotary evaporator. The 1HNMR technique has been used to test the purity of the FAMEsproduced,37 and the same technique supports >99% conversions of oilto FAMEs.The same experimental procedure and catalyst have also been

employed to yield the FAMEs from soybean, Karanja, and Jatrophaoils.2.4. Epoxidation of Used Cotton Seed Oil-Derived FAMEs.

Cotton seed oil consists of ∼70% unsaturated (−CHCH−) fattyacids, including ∼52% polyunsaturated (linoleic and linolenic acids)and ∼18% monounsaturated (oleic acid). The unsaturated bonds havebeen epoxidized by employing prepared Ti/SiO2 as a solid catalystunder two different experimental conditions as given below.2.4.1. Method I. A 100 mL, two-neck round-bottom flask equipped

with a water-cooled condenser, an oil bath, and a magnetic stirrer wascharged with used cotton seed oil-derived FAMEs (10 g), formic acid(1.8 g), 50% hydrogen peroxide (2.6 g), and Ti/SiO2-750 (500 mg).The resulting reaction mixture was stirred for 3 h at 35 °C (Scheme1). To monitor the progress of the reaction, the samples from thereaction mixture have been withdrawn with the help of a glass dropperafter regular time intervals and subjected to FTIR and 1H NMRanalyses. After the completion of the reaction, the solid catalyst wasseparated by filtration and 60 mL of hexane was added to the filtrate.The liquid phase thus obtained was kept in a separating funnel toseparate the upper organic layer, containing the epoxidized product,from the lower aqueous layer. The former was washed with water,dried over anhydrous sodium sulfate, and finally rotary-evaporated toyield the pure epoxidized FAMEs.2.4.2. Method II. A 50 mL, two-neck round-bottom flask equipped

with a magnetic stirrer and an oil bath was placed in a photochemicalreactor, which could provide the UV radiation of 125 W. Used cottonseed oil-derived FAMEs (10 g), acetonitrile (3.6 g), butyronitrile (3 g),50% hydrogen peroxide (2.6 g), and Ti/SiO2-750 (1 g) were added tothe round-bottom flask (Scheme 1), and the reaction mixture wasstirred in the presence of UV radiation (125 W) for 6 h at 35 °C undera dioxygen atmosphere. The progress of the reaction was followed byFTIR and 1H NMR techniques, and finally, epoxidized FAMEs wereisolated from the reaction mixture by adopting the same experimentalprocedure as described in the case of method I.To show the versatility of the prepared Ti/SiO2 catalyst, it was also

employed for the epoxidation of a variety of substrates (methyl oleate,used cotton seed oil, and FAMEs obtained from used cotton seed,soybean, Karanja, and Jatropha oils) using both methods.Finally, all epoxidized products were quantified and characterized by

the 1H NMR technique as described in the literature.38 Epoxidizedmethyl oleate was additionally characterized by EI−MS, besides FTIRand 1H NMR techniques.Fatty acid epoxide of used cotton seed oil: FTIR (cm−1), 914

(νepoxy), 871 (νepoxy), and 840 (νepoxy).1H NMR (CDCl3, ppm) δ, 4.3−

4.1 (m, −OCH2−), 3.2−3.1 (m, −CHOCH−), 2.3 (m, −CH2−CO−), 2.0 (m, −CH2−(CH2)n−CH−), 1.6−1.25 (m, −(CH2)n−),and 0.87 (m, −CH2−CH3).Fatty acid epoxide of used cotton seed oil FAMEs: FTIR (cm−1),

914 (νepoxy), 871 (νepoxy), and 840 (νepoxy).1H NMR (CDCl3, ppm) δ,

3.6 (s, −OCH3), 3.2−3.1 (m, −CHOCH−), 2.3 (m, −CH2−CO−),2.0 (m, −CH2−(CH2)n−CH−), 1.6−1.25 (m, −(CH2)n−), and 0.87(m, −CH2−CH3).Fatty acid epoxide of methyl oleate: FTIR (cm−1), 914 (νepoxy), 871

(νepoxy), and 840 (νepoxy).1H NMR (CDCl3, ppm) δ, 3.6 (s, −OCH3),

3.2−3.1 (m, −CHOCH−), 2.3 (m, −CH2−CO−), 2.0 (m, −CH2−(CH2)6−CH−), 1.6−1.25 (m, −(CH2)n−), and 0.87 (m, −CH2−CH3). EI−MS (m/z) (intensity (%), fragment), 313.3 (15, M), 299.3(17, M−CH3), 281.3 (24, M−OCH3), 263.3 (24, M−OOCH3), 245.3(20, M−(CH2)4CH3), 209.2 (5, M−(CH2)3COOCH3), 175.2 (4, M−C(CH2)5COOCH3), 155.2 (3, M−(CH2)7COOCH3), and 109.2 (5,M−CHOCH(CH2)7COOCH3).

3. RESULTS AND DISCUSSION3.1. Catalyst Characterization. 3.1.1. BET Surface Area

Analysis. The surface areas of pure SiO2 and Ti/SiO2 weremeasured by the BET method. The surface areas of pure SiO2and 1.7 wt % titanium-loaded silica (Ti/SiO2) and calcined at550 °C were found to be 300.0 and 291.7 m2/g, respectively.The surface area of Ti/SiO2 was found to reduce marginally(from 291.7 to 277.7 m2/g) when the calcination temperaturewas increased from 550 to 750 °C. Hence, titanium loading onsilica or an increase in the calcination temperature up to 750 °Cdoes not have any significant impact on the porosity of thesilica. However, a sharp decrease in the surface area (from 277.7to 172.2 m2/g) of Ti/SiO2 was observed when the calcinationtemperature was further increased from 750 to 950 °C, asshown in Figure 1. The decrease in the surface area upon

calcination at 950 °C could be due to the partial collapse of theporous silica structure at this calcination temperature.39

3.1.2. Powder XRD Analysis. The structural analysis of Ti/SiO2, prepared at different calcination temperatures (550−950°C), has been performed by powder XRD analysis. Thecomparison of the powder XRD patterns of commerciallyavailable SiO2 to Ti/SiO2 calcined at different temperatures isshown in Figure 2. The XRD pattern of commercial SiO2 showspeaks at 2θ values of 9.6°, 13.7°, and 21.7° (Figure 2a) tosupport the presence of SiO2 in the monoclinic form [JointCommittee on Powder Diffraction Standards (JCPDS) 89-1666].The powder XRD patterns of Ti/SiO2 calcined at 550, 750,

or 950 °C were found to be almost identical, as shown in curvesb−d of Figure 2. Besides the silica peaks, the appearance ofadditional peaks at 2θ values of 25°, 37.8°, 47.8°, 54.7°, and62.7° support the immobilization of titania (JCPDS 84-1286)over the silica support. However, the broad nature of thesepeaks rules out the possibility of any large phase aggregation ofTiO2 particles over the silica support.

3.1.3. FESEM and TEM Studies. The size, shape, and surfacemorphology of Ti/SiO2 were observed by SEM analysis, andthe same study reveals that Ti/SiO2 calcined in the temperaturerange of 550−950 °C exists as 20−50 μm sized particleswithout having any regular geometry, as shown in panels a−c ofFigure 3. Hence, the calcination temperature was not found to

Figure 1. Comparative BET surface area values of SiO2 and Ti/SiO2.

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affect the macrostructure of the catalyst particles significantly.The presence of ∼1.8 wt % titanium, as supported by theFESEM−EDAX study, also indicates the titanium immobiliza-tion on the silica support.The actual impact of the calcination temperature was found

to be more prominent on the Ti/SiO2 microstructure, asobserved by TEM analysis and shown in panels d−f of Figure 3.Ti/SiO2 calcined at 550 and 950 °C was found to exist as oval-

shaped particles with an average size of ∼20 nm, while the sameparticles when calcined at 750 °C were found to exist asnanorods of ∼100 nm length, as shown in Figure 3e. Thus, theTEM study clearly demonstrates that the microstructure of Ti/SiO2 was found to be affected by the calcination temperature,which, in turn, was found to affect the activity of the catalyst.

3.1.4. DRS UV−Vis Analysis. The coordination environmentof titanium in silica was observed by recording the DRS UV−Vis spectra in the wavelength range of 200−550 nm and usingBaSO4 as the reference material. The peak observed at 250 nmin DRS of Ti/SiO2-550 as shown in Figure 4b supports the

existence of hydrated titanium in a distorted tetrahedralcoordination environment, and a shoulder at ∼305 nmsupports the presence of titanium in an octahedral coordinationenvironment.34,40 As the calcination temperature of Ti/SiO2was increased from 550 to 950 °C, the intensity of the peak at305 nm was found to increase, and at 950 °C, the peaks at 250and 305 nm became almost equally intense, as shown in Figure4d. This observation supports the presence of equallypopulated titanium in tetrahedral and octahedral geometriesat 950 °C calcination temperature. However, the absence of anypeak in the range of 370−410 nm rules out the presence of freeTiO2 in Ti/SiO2 material.

29,40

3.2. Epoxidation Reactions. The epoxidation reactions ofmethyl oleate, cotton seed oil, and FAMEs were performed inthe presence of the Ti/SiO2 catalyst by employing variouspermutation and combinations of the reaction condition andreagents. However, of these employed methods, only two, viz.,methods I and II, were found to yield the completelyepoxidized products. The rate of reaction was found to bedifferent in both methods, and hence, time required for thecomplete epoxidation of FAMEs was found to be 3 and 6 h inmethods I and II, respectively. This difference in the reactionrate could be due to the different catalytic pathways followed byTi/SiO2 under two different reaction conditions employed. Inmethod I, formic acid and H2O2 could form the performicacid42,43 in the presence of Ti/SiO2, and in method II, the Ti-η2-peroxide could be generated on the surface of Ti/SiO2 in thepresence of UV light.44

The formation of the epoxide product was initiallycharacterized by the FTIR technique. The disappearance of

Figure 2. Comparative powder XRD patterns of (a) SiO2, (b) Ti/SiO2-550, (c) Ti/SiO2-750, and (d) Ti/SiO2-950 (◆, silica; □,titanium oxide on the silica support).

Figure 3. Comparative FESEM images of (a) Ti/SiO2-550, (b) Ti/SiO2-750, and (c) Ti/SiO2-950 and (d−f) respective TEM images.

Figure 4. Comparative DRS UV−Vis spectra of (a) silica, (b) Ti/SiO2-550, (c) Ti/SiO2-750, and (d) Ti/SiO2-950.

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the band at 3005 cm−1, because of −CC−, in the FTIRspectra of epoxidized products (spectra b, d, and f of Figure5A) also supports the consumption of double bonds during theepoxidation reaction.41 Further, the appearance of new bands at914, 871, and 840 cm−1 in the FTIR spectra of epoxidizedproducts (spectra b, d, and f of Figure 5B) supports theconversion of −CC− into the corresponding epoxidegroup.6,41

The 1H NMR spectra of used cotton seed oil, used cottonseed oil-derived FAMEs, and methyl oleate (spectra a, c, and eof Figure 6) show a multiplet at 5.3 ppm because of the protons

attached with unsaturated carbons (−CHCH−), besidesother hydrocarbon peaks at their regular positions. In the samespectra of respective epoxidized products (spectra b, d, and f ofFigure 6), the peak at 5.3 ppm was no longer found, to supportthe consumption of the −CHCH− group during theepoxidation reaction. Further, the appearance of a multipletin the range of 3.1−3.2 ppm, because of epoxide protons,supports the formation of respective epoxidized products.Epoxidized methyl oleate has also been characterized by mass

spectrometry, and in the EI−MS (Figure 7) of epoxidized

methyl oleate, the molecular ion peak at m/z 313.3 supportsthe formation of epoxidized methyl oleate. The base peak in thesame spectrum was observed at m/z 281 because of the loss ofthe methoxy (−OCH3) fragment from epoxidized methyloleate.

3.3. Catalytic Activity. To compare the catalytic activity ofprepared Ti/SiO2, used cotton seed oil-derived FAMEs wereemployed as the substrate under two different reactionconditions, as described in section 3.4, but using the desiredamount of catalyst at the desired reaction temperature.Blank experiments, following method I, in the absence of acid

and/or catalyst or in presence of pure silica led to a ∼10% yieldof the epoxidized product even after a 12 h reaction period.Similarly, in the case of method II, blank experiments in the

absence of one nitrile at a time or UV or UV−Vis radiation

Figure 5. Comparison of FTIR patterns of (a) used cotton seed oil, (b) epoxidized derivative of used cotton seed oil, (c) used cotton seed oil-derived FAMEs, (d) epoxidized derivative of used cotton seed oil-derived FAMEs, (e) methyl oleate, and (f) epoxidized methyl oleate. The spectrain panels A and B represent the magnification of the 2650−3180 and 800−1000 cm−1 regions, respectively.

Figure 6. Comparative 1H NMR spectra of (a) used cotton seed oil,(b) epoxidized derivative of used cotton seed oil, (c) used cotton seedoil-derived FAMEs, (d) epoxidized derivative of used cotton seed oil-derived FAMEs, (e) methyl oleate, and (f) epoxidized methyl oleate.

Figure 7. EI−MS spectrum of epoxidized methyl oleate.

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(dark condition) led to a ∼10% yield of the epoxidized product.These experiments suggest that the noncatalytic pathway is notsignificant in both methods and UV radiation as well as bothnitriles are necessary, in method II, to yield the completelyepoxidized product.To optimize the conditions for the epoxidation reaction,

during the course of the study, the following parameters hasbeen varied: (i) the calcination temperature of catalyst, (ii) thecatalyst concentration, and (iii) the reaction temperature.Further, to show the versatility of the Ti/SiO2 catalyst, it hasbeen employed under optimized conditions for the epoxidationof a variety of substrates, viz., used cotton seed oil, methyloleate, and FAMEs derived from used cotton seed, soybean,Karanja, and Jatropha oils.3.3.1. Effect of the Calcination Temperature. To study the

effect of the calcination temperature on the catalytic activity, aseries of Ti/SiO2 was prepared by varying the calcinationtemperature in the range of 550−950 °C. The epoxidationreaction was performed at 35 °C by employing 5 and 10 wt %of the catalysts with respect to FAMEs in methods I and II,respectively. The epoxide yield and turnover frequency (TOF)were found to increase as the calcination temperature wasincreased from 550 to 750 °C. However, a further increase inthe calcination temperature was found to decrease the epoxideyield and TOF significantly, as shown in Table 2. Hence, a 750

°C calcination temperature was found to be suitable to achievethe maximum catalytic activity, and the same (Ti/SiO2-750)was selected for optimizing other parameters for theepoxidation of used cotton seed oil-derived FAMEs. Thepartial collapse of the porous silica structure,39 as supported bythe surface area measurement study, could be the reason for thelesser catalytic activity of Ti/SiO2 at high calcination temper-atures.3.3.2. Catalyst Concentration. To find the optimum catalyst

concentration, a series of epoxidation reactions of used cottonseed oil-derived FAMEs was performed in the presence ofnanocatalyst, Ti/SiO2-750, by varying its concentration in therange of 1−15 wt % (catalyst/FAMEs) at 35 °C. The epoxideyield was found to increase as the catalyst concentration wasincreased from 1 to 5 wt % in method I and from 1 to 10 wt %in method II. A further increase in the catalyst concentrationdoes not influence the epoxide yield significantly, as shown inFigure 8. Hence, the epoxidation reactions were carried out byemploying 5 and 10 wt % of the catalyst in methods I and II,respectively, for optimizing the other parameters.3.3.3. Reaction Temperature. The reaction temperature not

only affects the yield but also the overall production cost of thefinal product. To determine the optimum reaction temperaturefor the Ti/SiO2-750 catalyst, a series of epoxidation reactionswas performed by employing methods I and II and varying thereaction temperature from 20 to 60 °C. Under both methods,

the yield of the epoxidized product was found to increase as thetemperature of the reaction was increased from 20 to 35 °C. At35 °C, the complete epoxidation of FAMEs was achieved in 3and 6 h under methods I and II, respectively, as shown inFigure 9. A further increase in the reaction temperature (>35

°C) does not reduce the reaction time significantly, and hence,epoxidation reactions were performed at 35 °C in the presentstudy.In summary, the present study demonstrates the epoxidation

of FAMEs under ambient conditions, and the developedprocess could also be economically viable because of its lesserenergy demand.

3.3.4. Variation of Feedstock. To demonstrate the efficacyof the prepared Ti/SiO2 catalyst for the epoxidation of a varietyof feedstock, two different sets of substrates, viz., triglycerides(used cotton seed oil) and FAMEs (methyl oleate and FAMEsderived from used cotton seed, soybean, Karanja, and Jatrophaoils), have been used. All epoxidation reactions were conductedin the presence of 5 and 10 wt % catalysts (with respect to

Table 2. Effect of the Calcination Temperature on theEpoxide Yield and TOF of Ti/SiO2-Catalyzed Epoxidation ofUsed Cotton Seed Oil-Derived FAMEs (Reaction Durationof 3 h for Method I and 6 h for Method II)

epoxide yield (%) TOF (h−1)

catalyst method I method II method I method II

Ti/SiO2-550 70 73 43.9 11.4Ti/SiO2-750 100 100 62.7 15.6Ti/SiO2-950 80 85 50.2 13.3

Figure 8. Effect of the catalyst concentration on the epoxide yield(reaction duration of 3 h for method I and 6 h for method II).

Figure 9. Effect of the reaction temperature on the epoxide yield(reaction duration of 3 h for method I and 6 h for method II).

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substrate) in the case of methods I and II, respectively. Asevident from Table 3, the catalyst was found to complete the

epoxidation of all of the substrates employed, under bothexperimental conditions. However, the complete epoxidation oftriglyceride was found to be relatively slow, and hence, Ti/SiO2required more time (5 and 10 h in methods I and II,respectively) than the complete epoxidation of FAMEs.3.3.5. Reusability of the Solid Catalyst. Reusability of the

solid catalyst is one of the most important criteria to make itviable at an industrial scale. To study the reusability of Ti/SiO2-750, the epoxidation of cotton seed oil-derived FAMEs hasbeen performed by following methods I and II. In bothmethods, the catalyst was recovered from the reaction mixtureby filtration, washed with hexane, and finally calcined at 750 °C.The recalcination is necessary to reactivate the catalyst byremoving the high-molecular-weight sticky substrates from thecatalyst surface. The catalyst thus recovered and reactivated wasused 7 consecutive times by following the same experimentalconditions and regeneration methods. The reused catalyst wasfound to complete the epoxidation of FAMEs in five successivecatalytic runs (Table 4). TOF and, hence, the catalytic activitywere found to reduce after every successive run.To analyze the possible reasons for the partial loss of the

catalyst activity, recovered catalyst was subjected to FTIR,powder XRD, and Ti analysis by AAS. The FTIR spectrum ofthe reactivated catalysts upon recalcination at 750 °C showsneither any adsorbed organic molecule nor any shift in the Si−O band.

The comparison of the XRD patterns of fresh and used Ti/SiO2 is shown in Figure S1 of the Supporting Information. Inthe XRD pattern of reused and recalcined Ti/SiO2, the originalpeak at 2θ = 9.6° was found to shift at 8.6°, while the peak at13.7° was no longer found after recalcination. Further, thepresence of relatively sharp peaks in the XRD pattern ofrecalcined Ti/SiO2 at 25°, 37.8°, 47.8°, and 54.7° also indicatesthe partial phase aggregation of titania. The difference observedbetween the XRD patterns of fresh and reused Ti/SiO2supports the changes in the Ti/SiO2 structure uponrecalcination.Further, the titanium analysis in fresh and used Ti/SiO2

suggests the loss of 0.15 wt % titanium after every successivecatalytic run. Hence, the gradual loss of the catalytic activity,after every successive run and recalcination, could be attributedto the structural changes in the catalyst and the partial loss of Tifrom Ti/SiO2.To show the efficacy of the Ti/SiO2-catalyzed epoxidation

process developed in the present work, it has been compared toa few literature-reported methods of a similar type. As evidentfrom Table 5, the reaction conditions optimized in the presentstudy are clearly more advantageous than the literature-reported counterparts because the former (i) yieldedcompletely (100%) epoxidized FAMEs, (ii) required milderreaction conditions, viz., a 35 °C reaction temperature withoutusing an inert atmosphere, (iii) needed a lesser amount ofcatalyst, (iv) required lesser time for the completion of thereaction, (v) could be used for the epoxidation of triglyceridesas well as their FAMEs, and (vi) has catalyst reusability.

3.4. Diesel Fuel Lubricity Improver Study. Because ofenvironmental protection and human health reasons, the sulfurcontent of diesel fuels was reduced to 50 ppm in 2005 and thelimit has been further tightened to <10 ppm46 by the regulatorybodies. Desulfurization reduces or eliminates the inherentlubricity of mineral oil-based diesel fuel, which is essential forproper functioning of vital engine components, such as fuelpumps and injectors. Thus, it would be desirable to add anappropriate lubricity improver additive in diesel fuel to attainthe prescribed lubricity.6,47 FAMEs and their epoxidederivatives have been reported in the literature as lubricityimprovers for diesel fuel.6−8 Such additives being renewableand biodegradable have the additional advantage over conven-tional mineral oil-based lubricity additives. To reduce thefriction, epoxidized FAMEs form a protective film on the metalsurface by polymerization through active oxygen sites of theepoxy group.6

In the present study, used cotton seed oil-derived FAMEsand their epoxide have been used as lubricity improver additivesfor the conventional diesel fuel. In a typical experiment, 4000ppm (0.4 wt %) epoxidized FAMEs (EFAMEs) or simpleFAMEs have been added to the diesel fuel having an inherentlubricity of 458 μm. As evident from Table 6, a significantdecrease in WSD (from 458 to 115 μm) was observed whenEFAMEs were used as lubricity improver additives in dieselfuel. However, when simple FAMEs were employed as lubricityimprovers, the WSD was found to reduce merely from 458 to398 μm. Hence, epoxidized FAMEs were found to act as betterlubricity improver additives for diesel fuel than simple FAMEs.

4. CONCLUSIONSIn the present work, a series of 1.7 wt % Ti-loaded SiO2 wasprepared by a chemical method followed by calcination at threedifferent temperatures, viz., 550, 750, and 950 °C. The catalysts

Table 3. Effect of the Variation of Feedstock on the Ti/SiO2-750-Catalyzed Epoxidation Reactiona

method I method II

substratereaction time

(h)epoxide yield

(%)reaction time

(h)epoxide yield

(%)

MO 3 100 6 100CFAMEs 3 100 6 100SFAMEs 3 100 6 100KFAMEs 3 100 6 100JFAMEs 3 100 6 100UCSO 5 100 10 100

aMO, methyl oleate; CFAMEs, used cotton seed oil-derived FAMEs;SFAMEs, soybean oil-derived FAMEs; KFAMEs, Karanja oil-derivedFAMEs; JFAMEs, Jatropha oil-derived FAMEs; and UCSO, usedcotton seed oil.

Table 4. Recyclability of the Ti/SiO2-750 Catalyst for theEpoxidation of Used Cotton Seed Oil-Derived FAMEs byEmploying Methods I and II

method I method II

catalyticcycle

reactiontime (h)a

epoxideyield (%)

TOF(h−1)

reactiontime (h)a

epoxideyield (%)

TOF(h−1)

1 3.0 100 62.7 6.0 100 15.62 3.5 100 53.8 7 100 13.43 4.5 100 41.8 8.5 100 11.04 5.75 100 32.7 10 100 9.45 7.5 100 25.1 12 100 7.86 10.0 67 12.6 15 64 5.07 10.0 25 4.7 15 21 1.6

aOnly partial conversion was observed, even up to 5th catalytic cycle,if reaction was stopped before the mentioned time.

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were further analyzed by DRS UV−Vis spectroscopy, BETsurface area, powder XRD, FESEM, and TEM techniques. TheBET surface area, FESEM, and TEM analyses of Ti/SiO2 revealthat particle size, shape, and surface area of the later was foundto be affected by the calcination temperature. Ti/SiO2 calcinedat 750 °C was found to exist in nanorod form, while the othersexist in nanoparticle forms. The catalytic activity of the selectedcatalyst (Ti/SiO2-750) toward the epoxidation of used cottonseed oil and FAMEs derived from used cotton seed, soybean,Karanja, and Jatropha oils and methyl oleate has been testedusing two different methods. Under optimized reactionconditions, the catalyst Ti/SiO2-750 required only 3 h for theepoxidation of cotton seed oil-derived FAMEs when formicacid and hydrogen peroxide were used and 6 h whenacetonitrile, butyronitrile, and hydrogen peroxide were usedin the presence of UV radiation. The catalyst has been recycledand reused 5 times for the complete epoxidation of cotton seedoil-derived FAMEs. The cotton seed oil-derived FAMEs andtheir epoxide derivative were finally tested as an additive toimprove the lubricity of the diesel fuel. The addition ofepoxidized FAMEs in diesel fuel was found to improve itslubricity (from 458 to 115 μm) to a greater extent than simpleFAMEs (from 458 to 398 μm).

■ ASSOCIATED CONTENT*S Supporting InformationComparative powder XRD patterns of fresh and recycled Ti/SiO2 (Figure S1). This material is available free of charge viathe Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Telephone: +91-175-2393832. Fax: +91-175-2364498 and/or+91-175-2393005. E-mail: amjadali@thapar.edu and/or amjad_2kin@yahoo.com.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

We acknowledge the financial support from the Council ofScientific and Industrial Research (CSIR) and Department ofScience and Technology (DST), New Delhi, India. We thankthe Sophisticated Analytical Instrumentation Facility (SAIF)(Punjab University) for NMR, powder XRD, and TEM and theMaterials Testing, Engineering and Research (MATTER)Laboratory (Thapar University) for FESEM studies. We alsothank all five reviewers and the Associate Editor for theirvaluable comments and suggestions.

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Table 5. Comparison of the Epoxidation Process Developed in the Present Study to That of the Literature-Reported MethodsUsing Ti/SiO2 Catalysts

catalyst feedstock reaction time (h) reaction temperature (°C) conversion (%) catalyst/substrate ratio catalytic cycles reference

Ti/MCM-41 FAMEs 24 90a 98 1:160b NRc 24Ti/MCM-41 FAMEs 24 90a 98 1:230b 2 45Ti/MCM-41 FAMEs 24 90 100 1:80b 2 35Ti/MCM-41 methyl oleate 24 90a 95 1:160b NRc 24Ti/MCM-41 methyl oleate 8 50 64.6 10d NRc 25Ti/MCM-41 methyl oleate 24 85a 96 90d 2 34Ti/SiO2-750 FAMEs or methyl oleate 3e 35 100 5d,e 7 prf

6g 35 100 10d,g 7 prf

Ti/SiO2-750 triglyceride 5e 35 100 5d,e 7 prf

10g 35 100 10d,g 7 prf

aInert atmosphere. bMolar ratio. cNR = not reported. dWeight percent. eMethod I. fpr = present report. gMethod II.

Table 6. Effect of Cotton Seed Oil-Derived FAMEs andTheir Corresponding EFAMEs on Diesel Fuel Lubricity

sample number additive samples additive dose (wt %) WSD (μm)

1 458a

2 FAMEs 0.4 3983 EFAMEs 0.4 115

aBase fuel.

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