Transesterification of Low-Quality Triglycerides over a Zn/CaOHeterogeneous Catalyst: Kinetics and Reusability StudiesDinesh Kumar† and Amjad Ali*

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

*S Supporting Information

ABSTRACT: Zinc-doped (0.25−7 wt %) calcium oxide (Zn/CaO) has been prepared in nanocrystalline form by a simple wetchemical method followed by calcination up to 950 °C. The structural analysis has been investigated by powder X-ray diffraction(XRD), whereas the surface morphology and average particle size of Zn/CaO were determined by scanning electron andtransmission electron microscopic studies, respectively. The catalytic activity of the prepared Zn/CaO toward thetransesterification of cotton seed oil with methanol was found to be a function of its calcination temperature, crystallite size,and basic strength. A pseudo-first-order kinetic model was applied to evaluate the kinetic parameters for the transesterification ofwaste cotton seed oil with methanol, and a first-order rate constant (k) and activation energy (Ea) were found to be 0.10 min−1

and 43 kJ mol−1, respectively. The catalyst, Zn/CaO, was amenable to recovery and recycling for at least five consecutive reactioncycles. The Koros−Nowak criterion test has been employed to demonstrate that measured catalytic activity was independent ofthe influence of transport phenomenon. Further, Zn/CaO was also found as an efficient catalyst for the completetransesterification of a variety of triglycerides (having up to 8.4 wt % free fatty acids), such as virgin cotton seed oil, soybean oil,waste cotton seed oil, castor oil, karanja oil, jatropha oil, and mutton fat. Thus, the present work demonstrates the application ofhigh free fatty acid containing waste or non-edible oils as feedstock, without any pre-treatment, for biodiesel production.

1. INTRODUCTION

A continuously growing energy demand, depleting fossil fuelresources, global warming, and environmental pollution are thediverse reasons for searching for alternate and renewable energyresources. Biodiesel, chemically known as fatty acid methylesters (FAMEs), has been globally accepted as a renewablealternate to the fossil-based diesel fuel.1 Transesterification oftriglycerides (vegetable oil or animal fat) with methanol in thepresence of a catalyst (chemical or bio) leads to the formationof FAMEs and glycerol as a byproduct.2 Moreover, FAMEs notonly provide an eco-friendly alternative to the conventionaldiesel fuel but could also be employed as a substrate for thepreparation of diesel fuel additives,3 viz., lubricity and cetanenumber improvers.Homogenous base catalysts, such as hydroxides and

methoxides of sodium or potassium, are frequently used forthe transesterification reaction at an industrial scale.2 However,the same catalysts are non-recyclable and non-green and lead tothe formation of sodium- or potassium-ion-contaminatedbiodiesel and glycerol.2,4 Additionally, same catalysts producesoap instead of biodiesel and become deactivated if moistureand/or free fatty acid (FFA) contents in feedstock are >0.3and/or >0.5 wt %, respectively.5,6 Thus, the use of ahomogeneous catalyst required refined and costly feedstocksfor biodiesel production, which not only increase biodieselproduction cost but also resulted in a fuel versus food situation.Despite several advantages over conventional diesel fuel,

biodiesel could not obtain the desired commercial success inmany countries, including India, because of the non-availabilityof adequate feedstock. The same problem could be pacified byemploying non-edible oils (e.g., karanja and jatropha), animalfat, or waste cooking oils as feedstock for biodiesel

production.7−10 Such feedstocks being less expensive thanedible oils have the potential to reduce the overall biodieselproduction cost. However, these oils usually contain high FFA(up to 12 wt %) and moisture (∼3 wt %) contents, andtherefore, a homogeneous base catalyst could not be employedfor their transesterification.11

To circumvent the above-mentioned problems associatedwith the application of low-quality feedstock for biodieselproduction, research has been directed for the development ofheterogeneous catalysts in the recent past.12,13 Heterogeneouscatalysts have several advantages over a homogeneous catalyst,including easier catalyst operation and separation, recyclability,and high moisture and FFA content resistance.12,13

A commercial biodiesel plant based on Esterfip-H technologyhas been set up by the French Institute of Petroleum (IFP) touse the mixed oxides of Zn and Al as a heterogeneous catalystfor the transesterification of triglycerides.14 The sametechnology does not require catalyst recovery and biodieselwashing with water and yielded >98% biodiesel yield.4,14

However, because of the relatively low activity under moderatereaction conditions, the same catalyst demanded a high reactiontemperature (200−250 °C) and high pressure to obtain aspecified FAME yield.14,15

Calcium oxide is one of the most frequently literature-reported heterogeneous catalysts for the transesterificationreaction, owing to its non-toxicity, ease of availability, and lowercost.13,16−19 The main problem associated with commerciallyavailable CaO is its moisture sensitivity and lesser activity

Received: January 15, 2013Revised: June 16, 2013Published: July 1, 2013

Article

pubs.acs.org/EF

© 2013 American Chemical Society 3758 dx.doi.org/10.1021/ef400594t | Energy Fuels 2013, 27, 3758−3768

toward the transesterification reaction. The activity of pureCaO could be enhanced by either preparing it in nanocrystal-line form17 or impregnating it with lithium,12,19−21 potassium,22

or lanthanum ions.23 Although modified CaO was found toshow better activity, it was not stable and suffered the leachingof Ca and/or active species into the reaction media. Mixedoxides of CaO/CeO2 have also been prepared via the co-precipitation method and employed as a reusable catalyst forthe transesterification of palm oil to yield >90% FAMEs.24 Kimet al. reported25 the preparation of CaO-supported La2O3 orCeO2 and their application in a flow reactor for thetransesterification of soybean oil. In both studies, the effect ofmoisture and FFA contents on catalytic activity has not beenreported. It is crucial to study the effect of these factors oncatalytic activity because low-quality feedstocks usually havehigh moisture and FFA contents.Among transition-metal oxides, ZnO is frequently stud-

ied26−28 as a heterogeneous catalyst for the transesterificationreaction. Incomplete conversion (<96.5%) of feedstock intoFAMEs, even at a high reaction temperature, is the majordrawback associated with the ZnO-catalyzed transesterificationreactions. The activity of ZnO has been reported to improveupon impregnation with Li, K, or Sr.29−31 A maximum (96.3%)conversion of soybean oil into corresponding methyl esters in a3 h reaction period was obtained in the presence of a Li/ZnOcatalyst when a 12:1 methanol/oil molar ratio was used.However, the same catalyst was found to be deactivated evenwhile catalyzing the first reaction cycle because of the leachingof Li from the ZnO support.31 Ngamcharussarivichai et al.reported14 the preparation of CaO·ZnO by the co-precipitationmethod by varying the Ca/Zn atomic ratio in the range of0.25−4.0. The prepared CaO·ZnO has been employed as aheterogeneous catalyst for the transesterification of palm kerneloil and reused up to 3 cycles after being washed with a mixtureof NH4OH/CH3OH to yield ∼90% methyl ester contents.It is evident from the literature review that CaO- and ZnO-

based mixed oxides either show incomplete conversion(<96.5%) or suffer the leaching of metal ions or requiredhigh temperature and high methanol/oil molar ratio to obtain asatisfactory FAME yield. To the best of our knowledge, there isno reported work available in the literature dealing with thepreparation of Zn-doped CaO as a heterogeneous catalyst forthe transesterification of triglycerides.Thus, the objective of the present work is to prepare a

reusable catalyst that could be effective for the single-steptransesterification of even high moisture and FFA containinglow-quality feedstock, such as waste cooking and non-edibleoils. To achieve the target, Zn-doped CaO (Zn/CaO) has beenprepared in nanosized form by a simple chemical method andcharacterized by powder X-ray diffraction (XRD), scanningelectron microscopy−energy-dispersive X-ray (SEM−EDX),transmission electron microscopy (TEM), Hammett indicators,and Brunauer−Emmett−Teller (BET) surface area measure-ment studies. Reaction parameters have been optimized toattain the optimum catalytic activity of the prepared Zn/CaOcatalyst toward the transesterification of waste cotton seed oiland to study the kinetics of the reaction.

2. EXPERIMENTAL SECTION2.1. Materials and Methods. Zinc acetate, nitrates of Mn, Fe, Co,

Cu, Ni, Zn, and Cd, calcium oxide of reagent-grade quality, andalcohols (99.8% purity) have been procured from Merck, India, andused as such without further purification. Waste cotton seed oil (WO),

soybean oil (SO), mutton fat (MF), castor oil (CS), virgin cotton seedoil (CO), karanja oil (KO), and jatropha oil (JO) were purchased fromlocal shops located in Patiala, India.

The FFA, saponification, iodine value, and moisture content of theSO, CO, MF, CS, WO, KO, and JO were determined by following theliterature-reported32 methods (Table 1).

XRD data for powder samples were collected on PANalytical’sX’Pert Pro with Cu Kα radiation. The samples were scanned in therange of 2θ = 5−80° at the scanning speed of 2°/min. The catalystsurface areas were determined by N2 adsorption−desorption at 77 Kby the standard BET method using a Micromeritics Tristar 3000equipment. Prior to analysis, all samples were degassed at 473 K for 90min under a nitrogen atmosphere to remove the physisorbed moisture.SEM−EDX and TEM were performed on JEOL JSM 6510LV andHITACHI 7500 to record the SEM and TEM images, respectively.Zinc and calcium in FAMEs and glycerol have been quantified on aGBC A32AA atomic absorption spectrophotometer (AAS). Fouriertransform−nuclear magnetic resonance (FT−NMR) spectra ofFAMEs and vegetable oils were recorded on a Bruker Avance-II(400 MHz) spectrophotometer. The basic strengths of the catalysts(pKBH+) were determined by the Hammett indicator test using thefollowing indicators: neutral red (pKBH+ = 6.8), bromthymol blue(pKBH+ = 7.2), phenolphthalein (pKBH+ = 9.3), nile blue (pKBH+ =10.1), tropaeolin-O (pKBH+ = 11.1), 2,4-dinitroaniline (pKBH+ = 15.0),and 4-nitroaniline (pKBH+ = 18.4).

2.2. Catalyst Preparation. The nanocrystalline Zn/CaO wasprepared by the wet chemical method, and in a typical preparation,CaO (10 g) was suspended in 40 mL of deionized water. To this, 10mL of an aqueous solution of zinc acetate of a desired concentrationwas added. The concentration of the zinc acetate was varied to obtaina 0.25−7 wt % Zn2+ concentration in CaO. The slurry thus obtainedwas stirred for 3 h, then air-dried, and finally calcined in a mufflefurnace at varying temperatures of 150−950 °C for 12 h. The Zn/CaOthus prepared was characterized by powder XRD, BET surface areameasurement, a Hammett indicator test, SEM−EDX, and TEMtechniques.

The prepared catalysts were designated as x-Zn/CaO-T, where xand T represent the Zn concentration (wt %) and calcinationtemperature, respectively. A series of the M/CaO (where M = Mn, Fe,Co, Cu, Ni, and Cd) has also been prepared by employing the samemethod but using aqueous solutions of metal nitrates of an appropriateconcentration in place of zinc acetate.

2.3. Transesterification of Vegetable Oils. All transesterifica-tion reactions were performed in a 100 mL, two-neck, round-bottomflask equipped with a water-cooled reflux condenser, oil bath, andmagnetic stirrer. In a typical transesterification reaction, 10 g ofvegetable oil was mixed with desired molar concentrations of methanoland catalyst and stirred (500 rpm) at a desired temperature to achievethe complete conversion (>99%) of feedstock into correspondingFAMEs. To monitor the progress of the reaction, the sample from thereaction mixture has been withdrawn after every 15 min with the help

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

feedstockFFA value(wt %)

moisturecontent(wt %)

saponification value(mg of KOH/g)

iodinevalue

waste cottonseed oil

1.8 0.3 190.4 101.7

karanja oil 4.2 0.3 185.0 96.8jatropha oil 8.4 0.4 195.5 101.5castor oil 1.8 0.3 180.3 90.7virgin cottonseed oil

0.1 0.3 188.7 102.6

virginsoybean oil

0.2 0.3 190.0 125.4

mutton fat 0.9 0.3 192.4 46.1

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of a glass dropper and centrifuged to separate the catalyst from thesupernatant. The FAME yield in the latter was calculated by protonnuclear magnetic resonance (NMR) spectroscopy following theliterature-reported method33,34 as given below

= ×I Ipercent FAME yield (%) {2 /3 } 100(methoxy) (methylene)

where I(methoxy) and I(methylene) are the areas of the methoxy (3.6 ppm)and methylene (2.33 ppm) protons, respectively, in 1H NMR spectraof FAMEs. An error of ±2% was observed when the FAME yield wasquantified by this method.Methyl esters of waste cotton seed oil: 1H NMR (CDCl3, ppm) δ:

5.34 (m, −CHCH−), 3.6 (s, −OCH3), 2.77 (m, −CHCH−CH2−CHCH−), 2.3 (m, −CH2−CO−), 2.03 [m, −CH2−(CH2)n−], 1.6−1.25 [m, −(CH2)n−], 0.88 (m, −CH2−CH3).

13CNMR (CDCl3, ppm) δ: 174.09 (−CO−CH2−), 129.9 (−CHCH−), 77.1 (CDCl3), 51.2 (−OCH3), 34.1 (−CO−CH2−), 31.9 (ω3,−CH2−), 29.66−29.08 (−CHCH−CH2−, −CH2−), 27.2 (−CHCH−CH2−CHCH−), 25.6−24.80 (−CO−CH2−CH2−), 22.70,22.47 (ω2, −CH2−), and 14.16 (ω1, −CH3).Similar patterns were observed in the proton NMR spectrum of

FAMEs obtained from the transesterification of other feedstocks (seeFigure S1 of the Supporting Information).

3. RESULTS AND DISCUSSION3.1. Catalyst Characterization. The basic strength

(pKBH+) of the pure CaO was found to be in the range of

9.8−10.1, and the same was found to increase gradually from11.1−15 to 15−18.4 as the Zn2+ concentration in CaO wasincreased from 0.25 to 1.5 wt % (calcined at 550 °C), as givenin Table 2. A further increase in the Zn2+ concentration (up to7 wt %) or calcination temperature (up to 950 °C) was notfound to increase the basic strength of Zn/CaO. Theenhancement of the catalyst basic strength is due to theformation of Lewis base sites (−O−) on the CaO surface uponZn doping followed by calcination above 550 °C (see section3.2.2).As evident from Table 2, the specific surface area of 1.5-Zn/

CaO was found to increase from 3.96 to 10.95 m2/g as thecalcination temperature was increased from 150 to 550 °C. A

further increase in the calcination temperature was found toreduce the specific surface area because of the sintering of Zn/CaO particles. The sintering of the Zn/CaO particles, at highcalcination temperatures (>550 °C), is further supported by theincrease (from 23 to 75 nm) in the crystallite size, as given inTable 2.The comparison of XRD patterns of commercial CaO and

Zn/CaO prepared by varying zinc concentrations (0.25−7 wt%) at 550 °C calcination temperature is shown in Figure 1. Thecommercial CaO exhibited the characteristic reflections ofcalcium oxide cubic form (JCPDS 821691). The XRD of Zn/CaO shows typical diffraction patterns of CaO in the cubicphase, and no distinct peak corresponding to the ZnO phasewas observed as long as the Zn2+ concentration remains lessthan 6 wt %. This could be attributed to the formation of Zn/CaO in a single cubic phase, in which Zn2+ has substituted Ca2+

sites. However, a further increase in the Zn2+ concentration(≥7 wt %) in CaO leads to the formation of ZnO in ahexagonal phase (JCPDS 891397), as also supported by thelow-intensity peaks at 2θ = 34.4°, 36.2°, 47.6°, 56.7°, and 62.7°.Therefore, a 6 wt % of Zn2+ in CaO forms a saturated solidsolution, and a further increase in the Zn2+ concentration leads

Table 2. Effect of the Calcination Temperature and ZnConcentration on the Basic Strength and Crystallite Size ofZn/CaO

catalyst type basic strength (pKBH+)BET surfacearea (m2/g)

averagecrystallitesize (nm)a

CaO 9.8 < pKBH+ < 10.1 3.96 ± 0.01 1041.5-Zn/CaO-150 11.1 < pKBH+ < 15.0 4.49 ± 0.01 271.5-Zn/CaO-350 11.1 < pKBH+ < 15.0 5.08 ± 0.02 271.5-Zn/CaO-550 15.0 < pKBH+ < 18.4 10.95 ± 0.01 231.5-Zn/CaO-750 15.0 < pKBH+ < 18.4 9.17 ± 0.04 301.5-Zn/CaO-950 11.1 < pKBH+ < 15.0 2.52 ± 0.01 750.25-Zn/CaO-550 10.1 < pKBH+ < 11.1 NDb 320.5-Zn/CaO-550 11.1 < pKBH+ < 15.0 ND 291-Zn/CaO-550 11.1 < pKBH+ < 15.0 ND 282-Zn/CaO-550 15.0 < pKBH+ < 18.4 ND 253-Zn/CaO-550 15.0 < pKBH+ < 18.4 ND 274-Zn/CaO-550 15.0 < pKBH+ < 18.4 ND 295-Zn/CaO-550 15.0 < pKBH+ < 18.4 ND 306-Zn/CaO-550 15.0 < pKBH+ < 18.4 ND 257-Zn/CaO-550 15.0 < pKBH+ < 18.4 ND 27

aOn a (200) plane by the Debye−Scherrer method.35 bND = notdetermined.

Figure 1. Comparative powder XRD patterns of CaO and Zn/CaOwith a varying Zn2+ concentration (0.25−7 wt %) (∗, calcium oxide;●, zinc oxide).

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to the formation of two distinct phases of Zn/CaO andCaO·ZnO.36

To study the effect of the calcination temperature on the Zn/CaO structure, 1.5 wt % Zn2+-doped CaO has been calcined inthe range of 150−950 °C temperature, and XRD patterns ofthe same are compared in Figure 2. The XRD patterns of Zn/CaO calcined up to 350 °C supported the formation ofhexagonal Ca(OH)2 as a major phase (JCPDS 841275) andCaO as a minor phase. A phase change, because of thedecomposition of Ca(OH)2 into CaO, initiated at 450 °C, assupported by the appearance of diffraction patterns for both ofthe phases (Figure 2). A further increase in the calcinationtemperature (≥550 °C) leads to the formation of pure CaO inthe cubic phase (JCPDS 821691). Similar XRD patterns wereobserved for Zn/CaO calcined up to 950 °C to indicate that anincrease in the calcination temperature (from 550 to 950 °C)has not initiated any chemical or structural change in Zn/CaO.The size, shape, and morphology of the Zn/CaO particles

were observed by SEM, and the same study reveals that 1.5-Zn/CaO-550 exists as a hexagonal, cubic, and irregular-shapedparticle of 2−5 μm size, as shown in Figure 3a. This variation inshape may be due to the heavy clustering of the Zn/CaOnanoparticles during their preparation and calcination.

Qualitative analysis by the SEM−EDX study supports thepresence of 1.4 wt % Zn2+ in 1.5-Zn/CaO-550. TEM analysisreveals that Zn/CaO particles are the clusters of further smallerparticles with an average size of ∼13 nm (see Figure S2 of theSupporting Information) in oval or spherical shape, as shown inFigure 3b. Thus, powder XRD as well as the TEM studysupports the formation of nano structures of Zn/CaO.The Zn/CaO characterization studies reveals that 1.5 wt %

Zn2+-doped CaO nanoparticles have the smallest size, possesshigher basic strength and larger surface area than the rest of thesamples, and hence, are expected to show higher activity towardthe transesterification reaction.

Figure 2. Comparative powder XRD patterns of Zn/CaO, calcined inthe temperature range of 150−950 °C (∗, calcium oxide; ⧫, calciumhydroxide).

Figure 3. (a) SEM and (b) TEM images of 1.5-Zn/CaO-550.

Figure 4. Effect of various transition-metal ions on the catalytic activity(reaction conditions: methanol/WO molar ratio, 9:1; catalyst amount,5 wt % of oil; temperature, 65 °C; and reaction duration, 45 min).

Figure 5. Effect of the Zn2+ concentration in CaO on the FAME yield(reaction conditions: methanol/WO molar ratio, 9:1; catalyst amount,5 wt % of feedstock; temperature, 65 °C; and reaction duration, 45min).

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3.2. Catalytic Activity. To optimize the reaction conditionsfor the optimum catalytic activity, transesterification reactionshave been carried out at a fixed stirring speed of 500 rpm byvarying one parameter at a time out of the following: (i) dopedmetal ion (Mn, Fe, Co, Cu, Ni, Zn, and Cd) in CaO, (ii) Zn2+

concentration in CaO, (iii) calcination temperature of thecatalyst, (iv) catalyst amount with respect to oil, (v) methanol/oil molar ratio, (vi) reaction temperature, (vii) alcohol carbonchain length, and (viii) amount of FFA and moisture contentspresent in feedstock.Additionally, the reusability of the catalyst and homogeneous

contribution in catalytic activity has also been evaluated. Toconfirm the reproducibility of the optimized reactionconditions, the transesterification reactions were repeated atleast 10 times.

3.2.1. Effect of Doped Metal Ions on the Catalytic Activity.Significant reports13,19,21,22 are available in the literatureregarding the activation of CaO through alkali metal iondoping. However, such catalysts show poor stability and alsosuffer from the leaching of active species in reaction medium.Not many reports were found in the literature regarding the use

Figure 6. Effect of the calcination temperature on the FAME yield(reaction conditions: methanol/WO molar ratio, 9:1; catalyst amount,5 wt % of feedstock; temperature, 65 °C; and reaction duration, 45min).

Figure 7. Effect of the moisture contents on the time required for thecomplete transesterification of waste cotton seed oil (reactionconditions: methanol/WO molar ratio, 9:1; catalyst amount, 5 wt %of oil; and temperature, 65 °C).

Figure 8. Effect of FFA on the time required for the completetransesterification of a variety of feedstocks (reaction conditions:methanol/feedstock molar ratio, 9:1; catalyst amount, 5 wt % offeedstock; and temperature, 65 °C) (acronyms: CO, virgin cotton seedoil; WO, waste cotton seed oil; SO, soybean oil; MF, mutton fat; CS,castor oil; KO, karanja oil; and JO, jatropha oil; with correspondingFFA values written in parentheses).

Figure 9. Effect of the alcohol carbon chain length on the catalyticactivity of Zn/CaO toward the transesterification of waste cotton seedoil (reaction conditions: alcohol/WO molar ratio, 9:1; catalystamount, 5 wt % of oil; and temperature, 65 °C) (acronyms: methanol,MeOH; ethanol, EtOH; 1-propanol, PrOH; 1-butanol, BtOH; 1-pentanol, PtOH; 1-hexanol, HxOH; 1-heptanol, HtOH; 1-octanol,OtOH; 1-nonanol, NnOH; 1-decanol, DeOH; 1-undecanol, UnOH;and 1-dodecanol, DoOH).

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of transition-metal ions for CaO activation. To evaluate theeffect of transition-metal ion doping on CaO activity, a series ofM/CaO (M = Mn, Fe, Co, Cu, Ni, Zn, and Cd) was preparedby doping 1.5 wt % of each metal ion. Among the preparedcatalysts, Zn/CaO was found to show better catalytic activity(Figure 4) toward the transesterification reaction of WO underoptimized reaction conditions. The activity of solid basecatalysts toward transesterification reactions was found to be afunction of their basic strength,13,19,21,22 and in the case of M/CaO, the maximum basic strength was observed for Zn/CaO(see Table S1 of the Supporting Information). Thus, theformation of stronger basic sites in Zn/CaO among theprepared catalysts, as a result of Zn doping, could be the reasonbehind the higher activity of Zn/CaO. Moreover, Zn-dopedCaO was found to show better resistance toward moisture anddid not lose activity, unlike pure CaO, even after exposure toatmospheric moisture.

Figure 10. KN criterion plot of fw (g−1) versus r (mol h−1 g−1) for the

transesterification reaction performed by Zn/CaO catalysts preparedby varying the Zn2+ concentration in the range of 0.25−1.5 wt %.

Figure 11. Plot of −ln(1 − Xme) versus kt at different reactiontemperatures for the 1.5-Zn/CaO-550-catalyzed transesterification ofWO (reaction conditions: methanol/WO molar ratio, 9:1; catalyst, 5wt % of oil; and temperature, 35−65 °C).

Table

3.Com

parisonof

the1.5-Zn/CaO

-550-Catalyzed

Transesterification

toThatof

aFewLiterature-ReportedCatalystsa

catalyst

catalyst

amount

(wt%)

reactio

ntemperature

(°C)

MeO

H/oil

molar

ratio

reactio

nduratio

n(h)

FAMEyield

(wt%)

reusability

(num

berof

cycles)

metalleaching

inFA

MEs/glycerol(ppm

)FF

A/m

oisture

tolerance(w

t%)

activity

with

otheroils/alcohols

reference

CaO

(calcinedat

700°C

)10

64.5

18:1

3.5

58.0

NR

NR/N

RNR/N

RIL,C

R,R

R,IC,C

P/NR

17

CaO

/MgO

1064.5

18:1

1.8

92.0

3NR/N

R1.1/0.5

IL,C

R,R

R,IC,C

P/NR

17CaO

−CeO

2/La

2O3

865

10:1

3.0

90.0

200hb

194−

5/NR

NR/N

RSO

/NR

25CaO

−CeO

2c5

8520:1

3.0

95.0

18159−

5/NR

NR/N

RRP/NR

24Ca/Znmixed

oxide

1060

30:1

3.0

94.8

3NR/N

RNR/N

RPK

/NR

15La/C

aO5

5820:1

1.0

94.3

NR

NR/N

R3.6/10

SO,S

C,C

P,WC/N

R23

Al/Znmixed

oxide

NR

200−

250

NR

NR

98.0

NR

<2/nil

NR

NR/N

R14

and

151.5-Zn/CaO

-550

565

9:1

0.75

99.0

55/20

8.4/4.3

SO,C

O,M

F,WO,C

S,KO,

JO/upto

a12

carbon

chain

PM

aIL,industriallard;

CR,crude

rapeseed

oil;RR,refinedrapeseed

oil;IC,industrialcoconut

oil;CP,

crudepeanut

oil;SC

,crude

soybeanoil;PO

,crude

palm

oil;RP,

refinedpalm

oil;PK

,palm

kerneloil;

WC,w

astecookingoil;SO

,soybean

oil;CO,cottonseed

oil;MF,muttonfat;WO,w

astecotton

seed

oil;CS,castor

oil;KO,karanjaoil;JO

,jatrophaoil;NR,not

reported;and

PM,present

manuscript.

bThe

reusability

studyhasbeen

performed

inacontinuous

reactorfor200h.

c The

reactio

nisperformed

inaParrreactorunderpressure.

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3.2.2. Effect of the Zn2+ Concentration and CalcinationTemperature on the Catalytic Activity. As seen from Table 2,an increase in the Zn2+ concentration was found to influencethe basic strength of Zn/CaO, which, in turn, could affect thecatalytic activity later. Figure 5 suggests that up to 1.5 wt %increase in the Zn2+ concentration was found to enhance theactivity of the Zn/CaO catalyst. However, a further increase inthe Zn2+ concentration has neither influenced the basic strengthnor activity of Zn/CaO. Hence, the activity of the Zn/CaOcatalyst was found to follow the same trend as that of basicity(Table 2). These results are in line with the literaturereports13,19,21,22 that base sites on heterogeneous catalysts actas the active center for the transesterification reaction.To determine the optimum calcination temperature for the

better catalytic activity, a series of catalysts was prepared bycalcining 1.5 wt % Zn-doped CaO in the temperature range of150−950 °C. The FAME yield was found to increase from 38to 99% (±2%) as the calcination temperature of Zn/CaO wasincreased from 150 to 550 °C. As supported by the XRD study,below 550 °C, the Brønsted base [Ca(OH)2] is the dominatingphase, while at 550 °C or above, the Lewis base (CaO) is thepredominating phase in Zn/CaO. Lewis bases, being morebasic than the corresponding Brønsted bases, have beenreported to show better activity toward the transesterificationreaction.37 Moreover, Zn/CaO at 550 °C calcination temper-ature had a higher BET surface area, thus allowing for a higherLewis base concentration at the catalyst surface. Nevertheless, afurther increase in the calcination temperature was found todecrease the FAME yield significantly, as shown in Figure 6. Ata higher calcination temperature (>550 °C), the lesser activitycould be attributed to the reduction in basic strength as well assurface area of the catalyst because of the sintering of the Zn/CaO particles (Table 2). Thus, 1.5 wt % Zn-doped CaOcalcined at 550 °C (1.5-Zn/CaO-550) was found to be moreeffective than its other counterparts and, hence, employed foroptimizing the reaction parameters for the transesterification ofWO.3.2.3. Effect of the Catalyst Amount, Methanol/Oil Molar

Ratio, and Reaction Temperature. To find the optimumcatalyst concentration, a series of transesterification reactions ofWO with methanol (1:9 molar ratio) were performed in thepresence of 1−10 wt % (with respect to oil) of 1.5-Zn/CaO-550 for 45 min. The FAME yield was found to increase as thecatalyst concentration was increased from 1 to 5 wt % (seeFigure S3 of the Supporting Information). A 5 wt % catalystconcentration required 45 min for the complete conversion of

WO into corresponding FAMEs. However, a higher catalystloading (≥5 wt %) was not found to change the FAME yieldsignificantly. This could be due to the fact that, at a highercatalyst loading, the reaction mixture becomes more viscous,which could resist the mass transfer in the liquid−liquid−solidsystem.38

The effect of the methanol/oil molar ratio on thetransesterification reaction is one of the important parametersthat affects the methyl ester yield as well as the cost of thebiodiesel production. The use of higher molar ratios ofmethanol/oil (even up to 275:1) in the case of heterogeneouscatalysts was found to improve the methyl ester yield becauseexcess alcohol not only promotes the transesterification rate butalso removes product molecules from the catalyst surface and,thus, regenerates the catalytic sites.17,39−41 To determine theoptimum methanol/oil molar ratio for 1.5-Zn/CaO-550, thereactions were performed by varying the methanol/oil molarratio from 3:1 to 18:1 for 45 min. The FAME yield increasesfrom 22 to 99% upon an increasing methanol/oil molar ratiofrom 3:1 to 9:1, and a further increase in the latter were notfound to influence the FAME yield significantly (see Figure S4of the Supporting Information).Heterogeneous catalysts, because of the phase difference

from reagents, usually required a high temperature and pressureand longer reaction period to yield the significant conversion.4

The same reaction conditions demand costlier and complicatedreactor design, which lead to an increase in the biodieselproduction cost.2,4 In the present study, the FAME yield wasfound to increase regularly as the reaction temperature wasincreased from 35 to 65 °C (see Figure S5 of the SupportingInformation), and a further increase in the reaction temperaturewas not found to influence the FAME yield significantly.Although the catalyst was found to be more effective at 65 °C,however, even at room temperature (35 °C), completeconversion of WO into corresponding FAMEs was achievedin 5 h of reaction duration.On the basis of the above-mentioned experiments, a 9:1

methanol/oil molar ratio, 5 wt % catalyst concentration (withrespect to oil), 65 °C reaction temperature, and 45 minreaction duration have been established as optimum reactionconditions for the 1.5-Zn/CaO-550-catalyzed transesterificationof WO.

3.2.4. Effect of the Moisture and FFA Contents on theCatalytic Activity. The presence of >0.3 wt % moisture and/or>0.5 wt % FFA contents in feedstock leads to the formation ofsoap instead of biodiesel in the presence of a homogeneous

Table 4. Physicochemical Properties of Biodiesel Fuel Derived from WO, JO, and KO in Comparison to EN Standards

parameters CBD KBD JBD EN 14214 test method

ester content (%) 99 ± 2 99 ± 2 99 ± 2 ≥96.5 1H NMR

flash point (°C) 110 115 100 100−170 ASTM D93pour point (°C) 4 5 1 ASTM D2500kinematic viscosity (40 °C) 3.9 5.0 4.5 1.9−6.0 ASTM D445calorific value (MJ/kg) 40 35 39 IS 1350 P:2ash (%) 0.02 0.02 0.01 ≤0.02 ASTM D874density at 31 °C (g/mL) 0.86 0.89 0.87 0.86−0.89 IS 1448 P:32water (%) 0.05 0.05 0.5 ≤0.5 ASTM D2709iodine value 98 94 99 <120 EN 14111acid number (mg of KOH/g) 0.1 0.5 0.3 ≤0.8 ASTM D664total glycerol (%, m/m) 0.14 0.16 0.21 ≤0.25 ASTM D6584Zn/Ca (±0.5 ppm) NFa/5 NF/5 NF/5 <5 (total metal) ASTM D1318

aNF = not found.

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catalyst.5,6 The WO employed in the present study was foundto have 0.3 wt % moisture and 1.8 wt % FFA contents, and thetransesterification reaction of the same using NaOH or KOH asa homogeneous catalyst leads to the saponification reaction.Similarly, use of pure CaO as a catalyst yielded only ∼10%conversion under optimized reaction conditions. However, thesame reaction when catalyzed by nanocrystalline 1.5-Zn/CaO-550 yielded the complete conversion of oil to FAMEs. Todetermine the maximum moisture resistance of the heteroge-neous catalyst, the transesterification reactions of WO wereperformed by adding up to 4.0 wt % (water/oil) of water in thereaction mixture. Zn/CaO was found to be effective for thecomplete transesterification of WO, having up to 4.3 wt % ofmoisture contents, as shown in Figure 7. Nonetheless, anincrease in the water concentration in the reaction mixture wasfound to reduce the catalytic activity, as supported by therequirement of a longer reaction duration for the completetransesterification of oil. This could be attributed to the changeof stronger Lewis basic sites (−O−) into weaker Brønsted basicsites (−OH) because of the reaction between water andcatalyst.37 A further increase in moisture contents (>4.3 wt %),in the reaction mixture, results in a high degree of catalystdeactivation, and hence, only ∼20% FAME yield could beachieved.The presence of high FFA contents in feedstock can

neutralize the base sites present on the catalyst surface toresult in lower catalytic activity. To determine the maximumFFA tolerance of Zn/CaO, transesterification reactions of CO,SO, MF, WO, CS, KO, and JO (having 0.1−8.4 wt % FFA)were performed with methanol. As depicted from Figure 8, thecatalyst was able to complete the transesterification of oilhaving up to 8.4 wt % FFA. Moreover, the catalyst activity wasnot found to be affected as long as the FFA concentration infeedstock remains below 1.8 wt %. A further increase in FFAwas found to reduce the catalytic activity, as depicted by themore time requirement for the complete conversion of KO andJO into corresponding FAMEs.Thus, the prepared Zn/CaO catalyst not only shows better

moisture and FFA resistance but was also found to be effectivefor the transesterification of a variety of low-quality feedstock ina single step without any pre-treatment.3.2.5. Effect of the Alcohol Carbon Chain Length on the

Catalyst Activity. Transesterification of triglycerides have beenfrequently performed with methanol for the production ofbiodiesel at an industrial scale. However, methanol is not onlypoisonous but also a byproduct of the fossil fuel refiningprocess. Hence, biodiesel thus produced will not be having100% renewable carbon. A greener substitute of methanolcould be ethanol, because the latter is mainly produced fromrenewable agriculture products, such as sugar and starch. Thus,fatty acid ethyl esters (FAEEs) produced from the ethanolysisof vegetable oils or animal fat would be completely renewable.42

To test the efficacy of 1.5-Zn/CaO-550 toward theethanolysis of WO, a 9:1 ethanol/oil molar ratio in thepresence of 5 wt % catalyst at 65 °C has been employed. Thecatalyst was found to be effective even for the ethanolysis andyielded 99% FAEEs in 1 h. However, a higher reaction duration(1 h) for the ethanolysis in comparison to the methanolysisreaction (0.75 h) suggests that the catalyst activity is influencedby the inherent carbon chain length of the alcohols.To investigate further the influence of the alcohol carbon

chain length on the catalytic activity, a series of reactions hasbeen performed in the presence of alcohols of varying carbon

chain length (3−12). As depicted from Figure 9, the reactiontime increases sequentially as the alcohol carbon chain lengthincreases from 1 to 8. A further increase in the alcohol carbonchain length (from 9 to 12) was not found to influence thereaction rate significantly. Nevertheless, in the case of all of thealcohols, Zn/CaO catalyzed the complete alcoholysis of WOinto corresponding fatty acid alkyl esters (see Figure S6 of theSupporting Information). The influence on Zn/CaO activitycould be attributed to the (i) reduction in acidic strength ofalcohols and (ii) change in alcohol solubility, upon increasingthe carbon chain length.To the best of our knowledge, this is the first report when a

heterogeneous catalyst was successfully employed not only forthe transesterification of a variety of triglycerides but also for avariety of alcohols as well.

3.2.6. Reusability Study. The reusability of the heteroge-neous catalyst is an important advantage over the homogeneouscatalyst because the same could reduce the overall processingcost of a chemical reaction. To test the reusability of 1.5-Zn/CaO, transesterification of the WO was performed withmethanol under optimized reaction conditions. After thecompletion of the reaction, Zn/CaO was recovered from thereaction mixture by filtration, washed with hexane, and calcinedat 550 °C. The catalyst hence recovered and regenerated wasemployed for 7 successive catalytic cycles under the sameexperimental and regeneration methods. As shown in Figure S7of the Supporting Information, the reused catalyst was alsofound to yield >99% FAME yield in five successive catalyticruns. However, after the sixth cycle and onward, only partialconversion was achieved, even after 6 h of reaction period (seeFigure S7 of the Supporting Information).The gradual loss of the catalytic activity could be due to (i)

the blockage of active sites because of the adsorbed organicmolecule, (ii) structural changes occurring in the catalyst duringthe regeneration process, and (iii) the partial leaching of theactive species from the catalyst.The deposition of the adsorbed organic species on the

catalyst support could partially deactivate the catalyst becauseof the blockage of catalyst active sites.14 The Fourier transforminfrared (FTIR) spectrum (see Figure S8 of the SupportingInformation) of the recovered and recalcined catalyst did notshow vibrations corresponding to any adsorbed organicmolecules to support that FAMEs or glycerol have not beenaccumulated on the recovered and regenerated Zn/CaOsurface.To evaluate the structural changes that occurred in Zn/CaO,

the XRD patterns of the fresh and regenerated Zn/CaO arecompared (see Figure S9 of the Supporting Information). TheXRD pattern of regenerated Zn/CaO (after 5 cycles) supportsthe formation of mixed phases, consisting of hexagonalCa(OH)2 as the major phase (2θ = 18.1°, 28.7°, 29.4°, 34.2°,47.2°, and 51.0°; JCPDS 841275) and cubic CaO (2θ = 32°and 54.4°; JCPDS 821691), hexagonal ZnO (2θ = 36.3°, 56.2°,and 62.8°; JCPDS 891397), and hexagonal CaCO3 (2θ =43.04° and 64.3°; JCPDS 881811) as minor phases. Thus, uponrepeated use and recalcination, CaO and ZnO becomeseparated, and because of intrinsic moisture sensitivity, CaOreacts with atmospheric moisture easily. As a result of this,strong Lewis basic sites (−O−) become converted to weakBrønsted sites (−OH), and consequently, catalytic activity islost.The concentration of metal ions (Ca2+ and Zn2+) in FAMEs

and glycerol leached from the catalyst while catalyzing the

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reaction has been analyzed by the atomic absorption spectros-copy (AAS) technique. The low zinc concentration in thereaction medium suggests that zinc has a strong interactionwith CaO supports43 and, hence, resulted in either no or minor(8 ppm) lixiviation of Zn2+ in FAMEs and glycerol, respectively.On the other hand, a low calcium concentration was found inFAMEs (5 ppm) as well as in glycerol (120 ppm). Thus,gradual loss of the catalytic activity could be attributed to the(i) structural changes occurring in the catalyst and (ii) partialloss of Zn from the catalyst, upon successive reuse.As evident from the metal analysis, there is partial leaching of

Ca2+ and Zn2+ in the reaction mixture and leached metalspecies could catalyze the reaction similar to a homogeneouscatalyst.44 Hence, it is noteworthy to quantify the homoge-neous contribution in the activity of a catalyst to be claimed as aheterogeneous catalyst.To investigate whether the leached metal ion has catalyzed

the reaction, 1.5-Zn/CaO-550 (1 g) has been stirred vigorouslywith refluxing methanol (10 mL) for 45 min. After a stipulatedtime, the catalyst was removed by filtration and recoveredmethanol was mixed with WO (9:1 molar ratio) and heatedagain at 65 °C for 45 min. Under mentioned reactionconditions, not more than 5% FAME yield was obtained, andthis was similar to the yield obtained in a blank experimentcarried out in the absence of the catalyst. Hence, it is safe toassume that leached metal ions have not catalyzed the reactionto a significant extent and heterogeneous Zn/CaO is mainlyresponsible for the catalytic activity.3.2.7. Koros−Nowak (KN) Criterion Test. The KN criterion

test modified by Madon−Boudart has been performed toestablish that the measured catalytic activity is independent ofthe influence of transport phenomena.45,46 The reaction rates inthe kinetic regime should be proportional to the concentrationof the active material (Zn2+ in the present case). Hence, a seriesof the Zn/CaO catalyst was prepared by varying the Zn2+

doping in CaO from 0.25 to 1.5 wt % (for example, 0.25-Zn/CaO-550, 0.50-Zn/CaO-550, 1.0-Zn/CaO-550, and 1.5-Zn/CaO-550) but following the identical experimental conditions.The prepared catalysts were employed (5 wt % of oil) for thetransesterification of WO with methanol (1:9 molar ratio) at 35and 65 °C reaction temperature. The KN criterion has beenexplained by plotting the reaction rate (r) in mol h−l g−1 ofcatalyst versus the weight of Zn2+ ( fw) in g−1 of CaO, as shownin Figure 10.The value of the slope was calculated from the graph shown

in Figure 10 and found to be 0.94 and 0.96 at 35 and 65 °C,respectively. Because the calculated values of the slope are closeto unity, hence, it could be concluded that transesterification ofcotton seed oil in the presence of the Zn/CaO catalyst obeyedthe KN criterion and the rate of transport has not influencedthe reaction rates.3.3. Kinetic Study. The transesterification of triglycerides

in the presence of excess methanol has been reported47,48 tofollow a pseudo-first-order kinetic model as given in eq 1

− − =X ktln(1 )me (1)

where Xme is the fraction of FAME content at time t (min).The kinetics of the Zn/CaO-catalyzed transesterification of

WO has been studied at a 9:1 methanol/oil molar ratio in thetemperature range of 35−65 °C.Figure 11 shows the linear nature of −ln(1 − Xme) versus t

(time) plots to maintain that the Zn/CaO-catalyzed reactionfollows pseudo-first-order kinetics. The rate constants were

calculated from these plots and found to be 0.10, 0.04, 0.02, and0.01 min−1 at 65, 55, 45, and 35 °C, respectively.The Arrhenius model49 was employed to estimate the

activation energy (Ea) and pre-exponential factor (A) for thesame reaction as given in eq 2

= − +k E RT Aln / lna (2)

where R is the gas constant (8.31 J K−1 mol−1) and T is thereaction temperature in Kelvin.The values of Ea and A, calculated from 1/T versus ln k plot

(see Figure S10 of the Supporting Information), were found tobe 43 kJ mol−1 and 1.65 × 107 min−1, respectively. Thecalculated activation energy (43 kJ mol−1) for the WOtransesterification was found within the range (26−84 kJmol−1) reported for the transesterification of various vegetableoils (see Table S2 of the Supporting Information).23,47,49−55

3.4. Comparison of the Zn/CaO Activity with FewLiterature-Reported Similar Catalysts. To show theefficacy and advantages of 1.5-Zn/CaO-550 over literature-reported similar catalysts, a comparison between two has beenmade and shown in Table 3. As evident from the comparison,the use of 1.5-Zn/CaO-550 as a solid catalyst is moreadvantageous than its literature-reported counterparts becausethe former (i) yielded a higher FAME yield (99 ± 2%), (ii)required less methanol/oil molar ratio (9:1), (iii) neededshorter reaction duration (0.75 h for WO) at 65 °C, (iv) wasreusable (up to 5 catalytic runs), (v) leached lesser metal inFAMEs and glycerol, (vi) demonstrated high FFA and moisturetolerance, and (vii) was effective for the transesterification of avariety of triglycerides with alcohols having up to a 12 carbonatom chain.

3.5. Physicochemical Properties of Prepared FAMEs.FAMEs obtained from the transesterification of triglyceridesshould satisfy EN 14214 or ASTM standards before they couldbe employed in diesel engines as biodiesel. To study thephysicochemical properties, transesterification of WO, JO, andKO with methanol were carried out in the presence of 1.5-Zn/CaO-550 under optimal reaction conditions. Upon completionof the reaction, the catalyst was filtered out and the liquid phasewas kept in a separating funnel for 12 h to separate the upperFAME layer from the lower glycerol layer. FAMEs thusobtained were rotary-evaporated to remove the excessmethanol, washed, and finally studied for a few physicochemicalproperties. The values of studied properties of prepared FAMEswere found within the acceptable limits of EN 14214, as givenin Table 4.

4. CONCLUSIONThe catalyst, 1.5-Zn/CaO-550, has been prepared by a wetchemical method in nanocrystalline form as revealed by TEMand powder XRD studies. The same catalyst was found to havethe highest surface area and basic strength, among the preparedcatalysts, as supported by the BET surface area and Hammettindicator analysis, respectively. The Zn/CaO nanocatalyst wasfound to be efficient even at room temperature (35 °C) forcomplete transesterification reactions of triglycerides withmethanol. Under optimized reaction conditions (methanol/oilmolar ratio, 9:1; catalyst concentration, 5 wt %; reactiontemperature, 65 °C; and reaction duration, 45 min), Zn/CaO-catalyzed transesterification of WO was found to followpseudo-first-order kinetics, and the apparent first-order rateconstant and activation energy for the same reaction werefound to be 0.10 min−1 (at 65 °C) and 43 kJ mol−1,

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respectively. Further, the catalyst could be employed for 5catalytic cycles without significant loss in activity. The catalystwas found to be effective even for the transesterification of low-quality feedstock having up to 4.3 wt % moisture and 8.4 wt %FFA contents. The lixiviation study supported negligiblehomogeneous contribution in catalytic activity, and the KNtest demonstrates that activity is independent of the influenceof transport phenomenon. Few physicochemical properties ofthe FAMEs derived from the WO, JO, and KO have also beenstudied, and the same was found within the limits of EN 14214specifications.

■ ASSOCIATED CONTENT*S Supporting InformationComparative proton NMR spectrum of triglycerides and theirrespective FAMEs, histogram showing particle size distribution,effect of the catalyst concentration on the FAME yield, effect ofthe methanol/oil molar ratio on the FAME yield, effect of thereaction temperature on the FAME yield, comparative protonNMR spectrum of fatty acid alkyl esters derived from CO withvarious alcohols, reusability studies of 1.5-Zn/CaO-550, FTIRspectra of fresh and recycled Zn/CaO, XRD patterns of freshand recycled Zn/CaO, and Arrhenius plot for Zn/CaO-catalyzed transesterification of WO (Figures S1−S10, respec-tively) and comparison of the basic strength of M/CaO (M =Mn, Fe, Co, Cu, Ni, Zn, and Cd) and comparison of catalyticactivities of a few literature-reported catalysts with the presentreport (Tables S1 and S2, respectively). This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Telephone: +91-175-2393832. Fax: +91-175-2393005. E-mail:amjadali@thapar.edu and/or amjad_2kin@yahoo.com.Present Address†Dinesh Kumar: Department of Chemistry, Sri Guru GranthSahib World University, Fatehgarh Sahib 140 406, India.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe acknowledge the Council of Scientific and IndustrialResearch (CSIR) [Grant 01(2503)/11/EMR-II] and theDefence Research and Development Organisation (DRDO)[Grant ERIP/ER/1103933/M/01/1453] for the financialsupport. We are also thankful to the Sophisticated AnalyticalInstrumentation Facility (SAIF, Panjab University, Chandigarh,India) for powder XRD, NMR, and TEM, Matter Lab (ThaparUniversity, Patiala, India) for SEM, and Kunash Instruments,Thane, India, for surface area analysis. We are also thankful tothe Associate Editor, both the reviewers, and Dr. B. K.Chudasama for their valuable comments and suggestions.

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