Fuel 203 (2017) 488–500

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Fuel

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Full Length Article

Efficient mesoporous SO42�/Zr-KIT-6 solid acid catalyst for green diesel

production from esterification of oleic acid

http://dx.doi.org/10.1016/j.fuel.2017.04.0900016-2361/� 2017 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: Dept. of Chemistry, CEG, Anna University, Chennai600025, India.

E-mail address: baskaralingam@gmail.com (P. Baskaralingam).

S. Gopinath, P. Sahaya Murphin Kumar, K.A. Yasar Arafath, K.V. Thiruvengadaravi, S. Sivanesan,P. Baskaralingam ⇑Anna University, Chennai 600025, India

h i g h l i g h t s

� Hydrothermal synthesis andcharacterization of mesoporousSulphated Zr-KIT-6 (SZK) solid acidcatalyst.

� Biodiesel produced from oleic acidand jatropha oil with the conversionof 96% and 85% at 120 �C.

� The conformational mechanism foresterification and transesterificationproposed.

� SZK is an environmentally benigncatalyst and cost effective.

� The produced green diesel meetsstandard petro-diesel properties andcan be used as blend for commercialuse.

g r a p h i c a l a b s t r a c t

Esterification of oleic acid by Sulphated Zr-KIT-6 (20).

a r t i c l e i n f o

Article history:Received 2 January 2017Received in revised form 18 April 2017Accepted 20 April 2017Available online 6 May 2017

Keywords:MesoporousTransesterificationSulphationEsterificationBiodieselKIT-6 (Korean Institute of Technology-6)

a b s t r a c t

Highly ordered mesoporous sulphated Zr-KIT-6(x) [x = Si/Zr] solid acid catalysts were successfully syn-thesized by hydrothermal method. Different weight fractions of zirconium were loaded into fixed gelin the acidic medium and then sulphated to get sulphated Zr-KIT-6(x) catalysts. X-ray diffraction(XRD), surface area analysis (Brunauer-Emmett-Teller equation) and microscopic techniques (SEM andTEM) results suggest the incorporation of Zr4+ into the silica KIT-6 framework by isomorphous substitu-tion between Zr4+ and Si4+ ions. Diffuse reflection spectroscopy (DRS-UV) results reveal that Zr atoms arehighly dispersed in +4 oxidation state. During the experiments it was observed that sulphated Zr-KIT-6(20) showed the highest catalytic conversion for esterification of oleic acid (96%) and transesterificationof jatropha oil (85%) at an optimized reaction conditions of 120 �C, 20:1 methanol to reactant molar ratio,4 wt% catalyst loading and 6 h reaction time. The ester yields of 95% and 80% obtained from oleic acid andjatropha oil respectively. The used catalysts could be recycled up to three cycles with significant activityand stability.

� 2017 Elsevier Ltd. All rights reserved.

1. Introduction

Renewable and clean energy sources are currently in highdemand due to environmental challenges such as shrinking fossil

S. Gopinath et al. / Fuel 203 (2017) 488–500 489

fuel reserves, climate change and worsening quality of crude oil[1]. Biodiesel, a renewable energy source due to its sustainablesupply, carbon neutrality, biodegradability and non-toxicity haspeculiar environmental benefits. Biodiesel is comprised of mono-alkyl esters of long chain fatty acids synthesized from alcoholand oil feedstocks including vegetable oils, animal fats, and cookedwaste oils [2]. Non-edible vegetable oils are regarded as promisingfeedstocks for biodiesel production. In this regard, jatropha oil is arobust aspirant and an ultimate source of triglycerides [3]. Esteri-fication of long chain carboxylic acids such as oleic acid is also afascinating compound in the perspective of biodiesel production,as it is present as major component in various vegetable oils suchas jatropha, sunflower, rapeseed, soybean, pongamia and palm [4].Among the several processes, liquid-phase esterification and trans-esterification of free fatty acids and non-edible oils using heteroge-neous catalysts have been attracted considerable attention toproduce biodiesel [5] (Fig. 1). Liquid phased reactions gives highlevel of conversion of fatty acids and triglycerides to their analo-gous methyl esters in shorter reaction times [6]. The developmentof solid acid catalysts which are active and stable towards one-stepsimultaneous esterification and transesterification is of great need.Many types of heterogeneous catalysts such as heteropoly acids,ion exchange resins, zeolites, sulphated metal oxides, etc., havebeen developed to meet requirements as described [7]. Heteroge-neous catalysts have the advantage of reusability and easy separa-tion without need for washing from reaction mixture overtraditional homogenous catalysts (H2SO4, NaOH & KOH) and low-ers the extent of poisons produced [8]. With a good heterogeneouscatalyst, both esterification and transesterification can be carriedout in a single step, which can considerably decrease the cost ofproduction. Due to the blockage of active sites by adsorbed inter-mediates, some solid acid catalysts have been deactivated afterfew runs [9]. In this context, porous cubic Ia3d mesostructuredKIT-6 (Korean Institute of Technology) has proved as popular can-didate for biodiesel synthesis which could provide in-pore accessi-bility of catalytic acid sites compared to MCM-41 and SBA-15materials [10]. KIT-6 mesoporous molecular sieves exhibit weakacidity and are catalytically inactive. In order to enable them, var-ious metal ions have been incorporated into KIT-6 [11]. Zirconiumbased materials have attracted significant interest in the recentpast as a catalytic support and as a base material for the prepara-tion of solid acids due to their strong acidity. They have beenwidely applied for various types of reactions such as dehydration,hydrogenation and hydroxylation [12]. Zr has also been success-fully incorporated into mesoporous MCM-41, MCM-48, HMS,SBA-15 and KIT-6 mesoporous materials [13]. Among these,

Fig. 1. Esterification and tran

KIT-6 is considered to be particularly beneficial due to decreaseddiffusion resistances in its framework.

Modified mesoporous silica can be obtained either by directsynthesis or by post-modification and there has been much efforttaken to the incorporation of various metals (M = La, Ga, Sn, Al,Fe, Zn, Zr, etc.) into the silica frameworks [14]. Among the solidacid catalysts, sulphated zirconia showed superior performancein biodiesel production [15]. Kiss et al. reported the esterificationof dodecanoic acid with 2-ethylhexanol catalyzed by sulphated zir-conia which exhibited best conversion of 70% at 160 �C [16]. Fur-thermore, Shao et al. reported biodiesel production oversulphated mesoporous TiO2-silica catalysts which are synthesizedby the sol gel process [17]. In view of the above, Zr was selectedto be incorporated in KIT-6 material and the acidity profile to beimproved by grafting the surface with anions like sulphate or tung-state. Cyril Pirez et al. reported that KIT-6 mesoporous sulfonic acidcatalysts showed best performance in propanoic and hexanoicfatty acids esterification. Jimenez-Lopez et al. reported SO4

2�/ZrO2

supported with mesoporous MCM-41, which showed high yieldof ethyl ester and good recyclability in ethanolysis of sunfloweroil. Using H2SO4, ZrO2 can be sulphated to become sulphated zirco-nia, SO4

2�/ZrO2 (Miao and Gao, 1997). Jitputti et al. (2006) reportedthat SO4

2�/ZrO2 produced promising results in transesterification ofpalm kernel oil and crude coconut oil with methyl ester yieldreaching as high as 90.3% and 86.3%, respectively [18]. Conven-tional sulphated zirconia has been usually prepared by two step(precipitation-impregnation) method with the sulfating agent ofsulfuric acid or (NH4)2SO4. Sang et al. reported the highest yieldof fatty acid esters (98%) in transesterification of soybean oil withmethanol at 120 �C using mesoporous sulphated zirconia [19]. Thiseventually indicates that modification of surface acidity is the keyfactor in obtaining high conversion of triglycerides. For this pur-pose, sulphation was selected as a means to functionalize the Zr-KIT-6 to introduce acidic sulphate groups without the risk of poreblockage or leaching of the grafted sulphate groups. Nevertheless,there seems to be no reports on the synthesis of sulphated Zr-KIT-6 mesoporous materials for the production of biodiesel.

The present work deals with the development of ordered sul-phated Zr-KIT-6 mesoporous catalysts by one-step hydrothermalsynthesis under acidic medium using SiO2/P123/Bu-OH/HCl/H2Ogel. A series of sulphated Zr-KIT-6(x) catalysts with varying Si/Zrcontent were synthesized by the direct injection of zirconium oxy-chloride octa hydrate into the fixed stock gel and the subsequentimpregnation of sulphate groups using sulfuric acid. The catalyticactivities of the synthesized catalysts were evaluated by esterifica-tion of oleic acid and transesterification of jatropha oil with

sesterification reactions.

490 S. Gopinath et al. / Fuel 203 (2017) 488–500

methanol. The stability and recyclability of catalysts which areimportant for practical catalytic operations have also been discussed.

2. Materials and methods

2.1. Materials

Jatropha oil (free fatty acid content 4.25 wt%) was obtainedfrom local farm and used without further treatment. Oleic acid(99%), methanol (99.9%), triblock co polymer P123 [poly block(ethylene glycol)-poly block (propylene glycol) block-poly (ethy-lene glycol)], ZrOCl2�8H2O (98% purity), sulfuric acid were obtainedfrom sigma aldrich and used as such.

2.2. Catalyst preparation

A series of mesoporous sulphated Zr-KIT-6(x) catalysts weresynthesized using PEO20PPO70PEO20 (Pluronic P123, Aldrich) asa template by following the methods of Ryoo et al. which is shownin Fig. 2 [20].

2.2.1. Synthesis of Zr-KIT-6Zr-KIT-6(x) (x = Si/Zr molar ratio) catalysts with different Si/Zr

molar ratios of 20, 50, and 100 were synthesized using zirconyloxy chloride octa-hydrate and tetraethyl orthosilicate. The Zrsource used in this synthesis has slower hydrolysis rate, hencethe silicate network forms with higher amount of Zr4+ ions. In atypical synthesis, 5.0 g of triblock copolymer pluronic P123 wasdissolved in 180 mL of 0.5 M HCl at 35 �C. Then, 5.0 g of n-butanol was added and stirred for 1 h at the same temperature.Finally, 10.6 g of tetraethyl orthosilicate and required amounts ofzirconium precursor were added to the mixture and stirred wellfor another 18 h. The mixture was then transferred to an autoclavelined with Teflon and heated at 100 �C for 24 h. The solid productobtained was filtered off followed by mild washing and dried at100 �C overnight. The dried catalyst was powdered well and cal-cined at 550 �C for 5 h in flowing air.

2.2.2. Sulphation of Zr-KIT-6Sulphation for the as synthesized plain KIT-6 and Zr-KIT-6 (x)

catalysts [x (Si/Zr) = 20, 50 & 100] were carried out by impregnat-ing the sulphate groups using H2SO4 at room temperature. 1 g ofcatalyst was added into a 50 mL beaker containing 20 mL of 1 MH2SO4. The reaction mixture was stirred for 3 h followed by ultra-sonication. The product was filtered off and washed with distilledwater followed by drying at 100 �C overnight to get a fluffy powderwhich was calcined at 300 �C for 3 h. The calcined samples weredesignated as SK, SZK-20, SZK-50 and SZK-100 respectively.

2.2.3. Reaction methodThe esterification of oleic acid with methanol was carried out in

an autoclave reactor equipped with a magnetic stirrer-oil bath setup, thermocouple, cooling coil and temperature controller in order

Fig. 2. Schematic diagram of synthetic strat

to keep the system uniform in temperature and suspension. Ester-ification is a reversible reaction, consequently an excess quantity ofmethanol is commonly required to enhance the conversion. Aftercompletion, the reaction mixture was centrifuged to separate thesolid catalyst. The catalyst was recovered at the end of each runand washed thoroughly with hexane to remove the methyl estersand dried at 100 �C for 3 h and finally calcined at 300 �C for 3 h.The regenerated catalyst can be reused for subsequent runs. Thesupernatant layer was examined by titration with 0.1 N KOH usingphenolphthalein indicator to determine the acid value (AV) asshown in the following equation.

AV ¼ ml of KOH � N � 56:1Weight of Sample ðgÞ

N – Normality of KOH solution.From the acid value, the conversionof oleic acid and jatropha oil can be calculated by the followingequation,

Conversionð%Þ ¼ 1� AVðbdÞAVðrtÞ

� �� 100

where, AV(bd) is acid value of biodiesel, and AV(rt) refers to acidvalues of reactant [oleic acid and jatropha oil].The yield of producedmethyl esters can be calculated by the following equation,

Yieldð%Þ ¼ mass of methyl estermass of feedstock

� �� 100

3. Characterization

The catalyst was characterized by various techniques includingX-ray diffraction (XRD), N2 adsorption by Brunauer-Emmett-Teller(BET) method, Fourier Transform Infrared Spectroscopy (FTIR), Dif-fused Reflectance Spectroscopy (DRS-UV–vis), Scanning ElectronMicroscopy (SEM), Transmission Electron Microscopy (TEM) andTemperature-Programmed Desorption (NH3-TPD). For XRD analy-sis, the catalyst powder was analyzed using an X’Pert diffractome-ter (PANalytical). The data were collected over a 2h range of 5–90�using Cu Ka radiation (k = 1.5418 Å) at 40 kV and 40 mA. The BETsurface area was obtained by N2 adsorption-desorption isothermsmeasured at �196 �C on an ASAP-2020 instrument (V3.01H, nMicromeritics). DRS-UV–Vis spectra was recorded using JascoV650 Spectrophotometer. FTIR spectra was recorded on a PerkinElmer IR spectro photometer in the range of 400–4000 cm�1. Todetermine surface morphology, SEM-EDAX analysis was performedand the images were obtained using a Hitachi SU-4800 SEM instru-ment. The catalyst powder was directly sprinkled over the carbontape and images were taken under the best operating conditions.TEM analysis was performed on a high-resolution field-emissiontransmission electron microscope (JEM-2100F, JEOL) using anaccelerating voltage of 200 kV. A well dispersed solution was pre-pared by adding a small amount of catalyst powder to ethanol andsonicating for 10–15 min. One drop of dispersed solution was

egy of mesoporous Sulphated Zr-KIT-6.

Fig. 3. Diffuse reflectance UV–Vis spectra of Sulphated Zr-KIT-6(x) samples.

S. Gopinath et al. / Fuel 203 (2017) 488–500 491

placed on a TEM grid and dried under IR lamp for 30 min. X-rayPhotoelectron Spectroscopy was obtained from ESCALAB 250,Thermo Scientific Ltd. Approximately 0.2 g of powder was firstdegasified at 400 �C for 400 min and then used for XPS analysis.TPD analysis of catalyst was performed on Micromeritics Chemi-sorb 2750 TPD/TPR using 10% ammonia in helium as a basic probemolecule. Metal oxide leaching and elemental composition wasstudied using ICP-OES (Perkin Elmer Optima 5300 DV). Reactionsamples were also analyzed by JEOL GCMATE II GC–MS with highresolution data system. Thermal stability and degradation behaviorof catalysts were determined by TGA with a TG-DTA: SII 6300EXSTAR thermo-analyzer, under a N2 flow of 60 mL/min with aheating rate of 10 �C/min from 0 to 1000 �C. The products wereconfirmed by 1H NMR spectroscopy, recorded on SPECMAN_ ASC-II (ACD) and Bruker 500 MHz spectrometer.

4. Results and discussion

In this work, direct co-precipitation method was used and Zr4+

species was successfully incorporated into three-dimensional (3D)body centered cubic Ia3d KIT-6 mesostructured framework. Thetextural properties of sulphated Zr-KIT-6(x) catalysts, such as sur-face area and average pore diameter are presented in Table 1.

4.1. XRD analysis

In small angle XRD pattern of sulphated Zr-KIT-6 (x) [where xrepresents Si/Zr = 20, 50,100] catalysts, well-resolved intensepeaks in the low angle region of 2h = 0.5� and shoulder peaks at�1.02� were found (Fig. S-1a). The peaks are indicative of (211)and (220) Bragg reflections of a well ordered three-dimensional(3-D) mesoporous structure with body centered cubic Ia3d sym-metry [21]. The above observation was also confirmed from TEMresults. The intensity of above mentioned peaks did not vary muchbetween the catalysts suggesting that heteroatom has no detri-mental effect and there must be a chemical interaction betweenthe host KIT-6 and guest Zr4+ions. On the contrary, a minimalincrease in unit cell parameter (a0) was noticed with an increasein Zr content indicating successful incorporation of Zr4+ in theKIT-6 framework [22]. No separate crystal phase of ZrO2 wasobserved which indicates that Zr4+ was finely dispersed insidethe channels of KIT-6. It is further confirmed from broad diffractionpeaks observed at 2h values around 20–30 which corresponds toamorphous silica Fig. S-1(b).

4.2. BET analysis

Nitrogen adsorption-desorption isotherms of sulphated Zr-KIT-6(x) catalysts are shown in Fig. S-2(a). The adsorption of samples

Table 1Structural properties of Sulphated Zr-KIT-6 (x) samples.

Catalysta Si/Zrb Zrb a0c D(wt%) (nm) (

SK 0 0 24.40 7ZK 23 6.2 24.49 7SZK-(20) 24 8.9 24.50 7SZK-(50) 49 3.8 24.48 7SZK-(100) 92 1.6 24.47 7

a Numbers in parenthesis represents Si/Zr ratio in synthesis gel.b ICP-OES analysis.C a0 = d211/p (h2 + k2 + l2)�1/2.d DP,BJH = BJH adsorption pore diameter.e VP,BJH = total pore volume measured at P/P0 > 0.4.f SBET = specific surface area.g NH3-TPD measurements.

increase gradually as relative pressure increases at a high relativepressure (P/P0 > 0.4). All the catalysts displayed Langmuir type IVisotherms with a H4-type hysteresis loop (IUPAC) and this typeis often associated with a typical ordered mesoporous material[23]. A sharp inflexion observed at P/P0 value between 0.8 and0.9 is due to capillary condensation within uniform pores. As Si/Zr ratio increases, a small decrease in the surface area and pore vol-ume was observed due to incorporated Zr4+. When Zr4+ substituteSi4+, the bond length of Zr–O–Si increases compared to Si–O–Siwhich affects pore volume and surface area [24]. BET surface area,pore size and pore volume of SZK (20) are 410.87 m2/g, 7.59 nmand 1.75 cm3/g respectively. It is further evident that blockagesin the mesopores were considerably low. The above results confirmthat Zr4+ are well dispersed in KIT-6 which are in good agreementwith XRD results.

Pore size distribution can be determined from the desorptionisotherms according to the BJH (Barrett-Joyner-Halenda) method.Curves are displayed in Supplementary file Fig. S-2(b), presentinga characteristic mesoporous structure. The curves exhibit porestructures with a little wider distribution in the mesoporous rangefrom 4 to 10 nm and also shows that all samples have a consider-ably high surface area greater than 400 m2/g. The highly orderedpore structure was also evidenced from TEM analysis (discussedlater) of catalytically active SZK (20) catalyst (Fig. 6). The estimatedpore size from the intensity profile across TEM micrographs(�7 nm) was found to be consistent with that estimated from N2

sorption analysis.

p (BJH)d Vp (BJH)

e SBETf Total acidityg

nm) (nm) (m2/g) (NH3 mmol/g)

.70 1.79 438.09 0.13

.65 1.72 408.90 0.49

.59 1.75 410.87 0.87

.42 1.78 427.11 0.43

.36 1.80 430.56 0.29

Fig. 4. (a) FTIR spectra (b) Pyridinyl-FTIR spectra of Sulphated Zr-KIT-6(x) samples.

Fig. 5. SEM images of Sulphated Zr-KIT-6 (20).

492 S. Gopinath et al. / Fuel 203 (2017) 488–500

4.3. DRS-UV analysis

Diffuse-reflectance UV–vis spectroscopy was carried out todetermine Zr ions incorporated in the mesoporous silica matrixand the spectra are shown in Fig. 3 in which an absorption bandin the range of 205–215 nm is observed. The sharp peak around215 nm could be attributed to charge-transfer transition corre-sponding to excitation of electrons from O2 valence band (2p) toconduction band (4d) of isolated Zr4+ ions of Si–O–Zr linkage intetrahedral configuration [11]. Intensity of this peak decreaseswith lower Zr contents and at higher Zr contents it becomesbroader corresponding to the formation of Zr–O–Zr linkages. Dis-tinct broad bands of low intensity at �280 nm and �330 nm rangecan be attributed to highly distributed Zr4+ [11]. Absence of bulkZrO2 further evidenced from the absence of absorption peak at230 nm indicates the fine dispersion of Zr4+ over the catalyst. Thissuggests that most of the Zr is incorporated as Zr4+ ions into theframework.

4.4. FTIR

Fig. 4a shows FTIR spectra of sulphated Zr-KIT-6(x) samplesanalyzed in the region of 500 cm�1 to 4000 cm�1. A sharp peakwith high intensity at 3450 cm�1 corresponds to O–H bond vibra-tion of Si–O–H groups. The samples also exhibit peaks at 805 cm�1

attributed to Si–O–Si stretching vibration and at 957 cm�1

assigned to Si–O–H symmetric stretching vibration. These resultsconfirm the presence of silanol groups [25]. In addition, newabsorption band appears around 964 cm�1 which is assigned toasymmetric stretching vibration of SiO4 tetrahedron in Zr–O–Sihetero linkages [26]. A sharp peak at 1094 cm�1 indicating S@Ostretching vibration which may be regarded as characteristic bandsof SO4

2� which has been successfully impregnated on the meso-porous framework [27]. The band at 1094 cm�1 is probably attrib-uted to the interaction between SO4

2- and Zr4+. The central metalion Zr4+ acts as Lewis sites and the inductive effect of S@O stronglyinfluences its acid strength. Bronsted acid sites may be due to

Fig. 6. TEM images of Sulphated Zr-KIT-6 (20).

S. Gopinath et al. / Fuel 203 (2017) 488–500 493

weakening of O–H bond by the inductive effect of neighboring sul-phate groups [28]. Peaks around 457 cm�1 and 604 cm�1 indicatesthe presence of Zr–O–Zr stretching vibration which in turn con-firms the presence of Zr4+ [29]. The band at 1639 cm�1 may beattributed to surface silanol groups and adsorbed water molecules[28]. Strong Brønsted acid sites evidenced by peaks at 3880 cm�1

and 3965 cm�1 corresponds to –OH stretching vibration whichare linked to disulfate species and trace amounts of physisorbedwater [29]. Pyridinyl – FTIR spectra of adsorbed pyridine on sul-phated Zr-KIT-6(x) samples are shown in Fig. 4b. Noticeable bandsat 1446 cm�1 and 1598 cm�1 can be assigned to the Lewis acid (L)sites whereas the weak band at 1427 cm�1 is attributed to a com-bination of Brønsted and Lewis (B + L) acid sites [30]. These resultsconfirm that sulphated Zr-KIT-6(x) samples contain predominantlyLewis acid sites. As for adsorbed pyridine, the other two peaks(besides the peak at 1446 cm�1) at 1450 cm�1 and 1532 cm�1 havealso been found. The former peak is due to co-ordinatively boundpyridine to unsaturated surface Zr4+ (Lewis acidity) while the latterpeak is attributed to the pyridinium ions formed due to the proto-nation of Bronsted sites [31]. This results implies that regardless ofthe presence of alkyl groups both Bronsted and Lewis acidity arefound. The contribution of the Lewis acidity is due to Zr4+ whilethe Bronsted acidity is due to sulphate group functionalization.

4.5. SEM-EDAX analysis

Among all the catalysts, the surface morphology of active SZK(20) was revealed by SEM analysis (Fig. 5). It is evident that SZK(20) presented mesoporous structure which has been identicalwith the results obtained from BET. SEM images are comparedwith the morphology of KIT-6 (Fig. 5b) indicates that the porosityand surface smoothness was not affected [32]. Furthermore, EDAXmeasurements are carried out to determine the chemical composi-tion of the catalyst (Fig. S-3). The quantitative analysis of differentelements shows that SZK (20) contained 74.9 wt% Si, 23.63 wt% Zr,

and 2.47 wt% S confirming that Si/Zr ratio in the calcined sampleswas close to that in the synthesis mixture.

4.6. TEM analysis

TEM analysis was carried out to determine surface morphologyand the images of SZK (20) are shown in Fig. 6. It is clearly seenthat the long range order with 3D cubic network highlights theinterconnectivity in pore structure (cubic Ia3d) with ordered meso-porous structure of SZK (20) [33]. In addition, the long range order,cubic nature and mesoporous framework have been retained evenafter the incorporation of Zr4+ and SO4

2� groups. Selected-area elec-tron diffraction (SAED) pattern (Fig. 6c) is in good agreement withXRD results evidences the absence of bulk ZrO2 which further con-firms the fine dispersion of Zr4+.

4.7. XPS analysis

Oxidation state and chemical composition of SZK (20) are givenin Fig. 7 with insets. The presence of +4 oxidation state of Zr in theKIT-6 framework was identified from Zr 3d doublet (5/2 & 3/2)with binding energy value of 178.1 eV & 182.8 eV. O 1s bindingenergy of 532.8 eV (Fig. 7b) indicates that the oxygen atoms existas O2� species. The Si 2p core level spectrum was identified atbinding energy value of 103.2 eV which is characteristic of Si4+ spe-cies in the silicate frame work [34]. Moreover, sulphation increasesthe binding energy of Zr 3d3/2 and O1s of SiO2 to 182.8 and532.8 eV. This upward shift of binding energy could be explainedby the increase of effective positive charge of Zr4+. Also, additionof sulphate ions cause a delocalization of electrons from bulkSiO2 [35].

4.8. NH3-TPD analysis

NH3–Temperature programmed desorption (NH3-TPD) of sul-phated Zr-KIT-6(x) materials are shown in Fig. 8. Acid sites distri-

Fig. 8. NH3-Temperature programmed desorption (NH3-TPD) of Sulphated Zr-KIT-6(x) catalysts.

494 S. Gopinath et al. / Fuel 203 (2017) 488–500

bution of the catalysts was determined from the amount of ammo-nia desorbed at different temperatures. Generally, acid sites areclassified into weak (<200 �C), medium (200 �C–300 �C) and strong(>350 �C). The sulphated Zr-KIT-6(x) materials display a significantdesorption of ammonia in 100–400 �C region due to the presenceof acid sites with varying strengths. The sharp peaks found in therange of 204 �C and 238 �C have been attributed to the combina-tion of Bronsted and Lewis acid sites [36]. In general, total acidityof the materials increase with an increase in Zr content which indi-cates heterogeneity of acidic surface. Among all the catalysts, SZK(20) showed highest total surface acidity of 0.87 mmol/g (Table 1)which shows the presence of more number of acidic sites in thecatalyst. This can be due to dispersion of active sites (Zr4+) on thesupport and strong interaction between Zr4+ and KIT-6 support[37]. The incorporation of Zr4+ in the frame work generated Lewisacid sites whereas sulphation created Bronsted acidity. Due to theremoval of proton from silanol groups coordinated with Zr–O–Zrlinkages, Bronsted acid sites might be generated potentially [38].Further these Bronsted and Lewis acid sites were confirmed fromboth NH3-TPD measurements and pyridine-adsorbed FTIR spec-trum. Desorption of ammonia shows shoulder peak in the rangeof 387 �C which has been attributed to the fine dispersion of strongacid sites. Weak and strong acid sites due to the large amount ofdesorption of NH3 was observed >350 �C which reveals that strongacidity was generated on the surface of the materials in a propaga-tive manner. However total acid sites in the case of sulphated Zr-KIT-6(x) catalysts was found to be higher than that of unsupportedKIT-6 (SK) and non-sulphated ZK.

4.9. Thermogravimetric analysis (TGA)

TGA analysis of SZK (20) catalyst is illustrated in Fig. 9 and theweight loss found agree well with those expected for the decompo-sition of sulphate groups. The thermogravimetric curve showstwo-stage weight loss, first one due to elimination of hydratedwater (100–200 �C) and the second major weight loss around300–750 �C is attributed to the decomposition of sulphate ions

Fig. 7. (a) Survey, high-resolution (b) O 1s, (c) Si 2p, (d) Z

[14].These results suggest that sulphate groups are stable up to200 �C.

4.10. Inductively coupled plasma-optical emission spectra (ICP-OES)

To verify the elemental composition and the effect of Zr on thecatalytic activity, ICP-OES was recorded. The SZK (20) catalyst con-tained Si/Zr ratio in the range of 24 (Table 1) which is consistentwith the theoretical value Si/Zr = 20 and the leaching of Zr wasfound to be less than 10 ppm. The minimal leaching may be attrib-uted to the relatively strong metal-support bonding which resultsin an excellent catalytic activity as explained in NH3-TPDmeasurements.

4.11. Catalytic activity

Catalytic activity of sulphated Zr-KIT-6(x) catalysts was evalu-ated from esterification of oleic acid with methanol, under hetero-

r 3d, (e) S 2p XPS spectra of Sulphated Zr-KIT-6 (20).

Fig. 9. TGA profile of Sulphated Zr-KIT-6 (20).

S. Gopinath et al. / Fuel 203 (2017) 488–500 495

geneous conditions. For comparison, transesterification of jatrophaoil was also carried out with methanol. The effect of different reac-tion parameters such as reaction temperature, reactant to alcoholmolar ratio, reaction time and catalyst loading were studied tooptimize the reaction conditions for esterification and transesteri-fication reactions. To eliminate adsorbed moisture, the catalystswere activated at 250 �C before start up the reaction. The followingorder of the catalysts is observed due to higher total acidic value(NH3-TPD method) and higher concentration of Bronsted andLewis acid sites as determined from Pyridinyl-FTIR respectively[SZK (20) > ZK > SZK (50) > SZK (100) > SK]. From the above results,SZK (20) has been chosen to optimize the reaction conditions forboth esterification and transesterification reactions.

Fig. 10. (a) Effect of reaction temperature (b) Effect of oleic acid-methano

4.11.1. Effect of temperatureOptimization of reaction parameters is an important stage for

commercial applications in order to increase the yield and mini-mize the cost of biodiesel. In this context, reaction temperaturehas been found to be the most important aspect. To study the effectof reaction temperature on esterification, the temperature was var-ied from 60 �C to 120 �C with oleic acid to alcohol molar ratio of1:3, 1 wt% of SZK (20) catalyst with respect to oleic acid weight,and a reaction time of 3 h. Fig. 10a shows that with an increasein the reaction temperature the percentage conversion alsoincreases. This is in accordance with the previous reports thatthe reaction time was shorter with higher reaction temperature[39]. At lower reaction temperature ranges of 60 �C and 80 �C,the percentage conversion was relatively low. However, whenthe reaction temperature increases from 100 �C to 120 �C, 65% ofmethyl oleate was obtained. From the results, 120 �C was chosenas optimum reaction temperature. The vapour pressure of metha-nol increases when there was an increase in reaction temperaturewhich simultaneously increases the conversion. This designatesthat the vapor pressure of the solvent plays a dynamic role in con-version of fatty acid.

4.11.2.. Effect of oleic acid–methanol molar ratioThe oleic acid-to-methanol molar ratio was varied from 1:3 to

1:20 at a reaction temperature of 120 �C for 3 h using 1 wt% ofSZK (20) catalyst with respect to oleic acid. Fig. 10b shows the con-version of oleic acid with increasing amount of methanol. The per-centage conversion reached a maximum of 75% at an acid-alcoholmolar ratio of 1:20. This shows that increasing the amount ofmethanol would drive the equilibrium towards the product side[40]. There was no significant increase in conversion with furtherincrease in the methanol amount.

l molar ratio (c) Effect of catalyst amount (d) Effect of reaction time.

496 S. Gopinath et al. / Fuel 203 (2017) 488–500

4.11.3. Effect of catalyst amountThe effect of catalyst loading on esterification of oleic acid was

studied at 120 �C with an acid-methanol mole ratio of 1:20 and 3 hreaction time (Fig. 10c). It reveals that the conversion increases sig-nificantly from 75% to a maximum of 83% with an increase of cat-alyst loading from 1 to 4 wt%. The initial improvement could beattributed to the availability of the catalytic active sites. Further-more, the water produced would compete with the reactants forcatalytic active sites. The limitation imposed by the reaction equi-librium whereby more catalyst would increase only the speed ofthe reaction but not the percentage conversion needs to be consid-ered [41]. Moreover, in a three-phase system (acid/alcohol/cata-lyst), a higher dosage of catalyst tends to increase viscosity of thereaction mixture, resulting in mass-transfer limitations for thereactants to reach the catalytic sites [30]. However with anincrease in the catalyst loading beyond 4 wt%, the catalytic systemmight be saturated with the maximum number of active sites.Therefore, the catalyst loading was optimized to 4 wt%.

4.11.4. Effect of timeTo investigate the effect of reaction time, esterification reaction

was carried out at different reaction times of 3–6 h. The molar ratioof oleic acid-methanol was fixed at 1:20 and the reaction temper-ature was set at 120 �C with catalyst loading of 4 wt%. Fig. 10dshows that the conversion increases with reaction time and no fur-ther increase was found beyond 6 h. This was due to the limitationimposed by the equilibrium of the reaction, whereby increasedreaction time would only increase the speed of reaction but notpercentage conversion [42]. Reaction time is an important factorthat reflects on esterification reaction rate. The mass transfer ofheterogeneous catalyst systems results in slower rate for shorterreaction time. Thus, the conversion was assumed to be low in first3 h of reaction. However, the reaction rate gradually increaseswhile the conversion reaches nearly 96% at a reaction time of 6 h.

4.11.5. Effect of Si/Zr ratioThe effect of Si/Zr ratio in the sulphated Zr-KIT-6(x) catalysts

was studied from the esterification of oleic acid and transesterifica-tion of jatropha oil with methanol at optimized reaction condi-tions. The catalytic activity presented in Fig. 11a shows that theconversion increases as the Si/Zr ratios increases. In esterification,the use of SZK (20) results in the maximum conversion of 96%,which was almost threefold that of unsupported sulphated KIT-6(SK) (39%). In addition, SZK (20) catalyst produced the ester yieldof 95% and 80% from oleic acid and jatropha oil respectively. This

Fig. 11. (a) Effect of Si/Zr ratio in transesterification and esterification (b) effect of alcomolar ratio – 1:20; T – 120 �C; catalyst loading – 4 wt%, reaction time – 6 h.

is probably because of higher acidic nature of Zr4+ ions due to highelectronegative oxygen in the Si–O–Zr linkages as Zr possess moreelectron deficiency to accept the lone pair electrons from bothmethoxide and reactant (FFA & TG) molecules. This acidity profilewith reactants accessible mesopores in the silica framework wasconfirmed from pyridine adsorption, DRS-UV and BET experimentsrespectively. The higher catalytic activity of SZK (20) could beattributed to the fact that acidic sites are more approachable bythe reactants during the course of reaction [22]. Since, SZK (20)gave the best performance.

The following major compounds were found in jatropha oil:palmitic acid, stearic acid, oleic acid, linoleic acid, linolenic acid,which are in agreement with the results presented by Berchmansand Hirata [3]. Additionally, the water content (wt%, Karl Fischertitration method, DL 50, Mettler-Toledo) of jatropha oil biodieseland methyl oleate were determined to be approximately 0.024and 0.02 respectively [43]. The produced biodiesel was character-ized for their physical properties using American Society for Test-ing and Materials (ASTM D6751) and European standard (EN14214) methods [44]. As shown in Table 2, the density of producedbiodiesel was comparable to the density of petro-diesel value(0.83 g/cm3). Viscosity and pour point are within the specifiedrange of ASTM values suggesting improved fuel injection property,better storage and transportation characteristics in low tempera-ture environment [45].

To evaluate the effect of alcohol hydrocarbon chain on esterifi-cation reaction, different alcohols such as ethanol, n-propanol andbutanol are used with oleic acid and SZK (20) catalyst. Highermethyl ester conversion (96%) was observed when compared toethyl (85%), propyl (74%) and butyl (67%) esters (Fig. 11b). Thiscan be attributed to the steric hindrance of alcohols towards elec-trophilic site as the size of alcohol hydrocarbon chain increases [4].

4.12. 1H NMR studies

1H NMR study was carried out to confirm the methyl esters pro-duced from both oleic acid and jatropha oil. In Supplementaryimages S-4 (a, b), the characteristic singlet peak of methoxy pro-tons of ester functionality at 3.65 and 3.643 ppm, a doublet belong-ing to a-CH2 proton at 2.28 and 2.261 ppm is observedrespectively. These two distinct peaks confirm methyl ester pre-sent in biodiesel. Terminal methyl protons (C–CH3) peak at 0.8–0.9 and 0.958 ppm, intense signal of methylene protons of longchain ester at 1.26–1.31 and 1.132 ppm are observed. A signal at1.60–1.63 and 1.503 ppm corresponds to b-carbonyl methylene

hol carbon chain on oleic acid conversion. Reaction conditions: reactant to alcohol

Table 2Physicochemical properties of methyl oleate and jatropha oil methyl ester.

Properties Testmethod

Petro-Diesel

MO MO-10

MO-20

MO-30

JMO JMO-10

JMO-20

JMO-30

PMO CIME JMO PO NO CO

Density (15 �C,g/cm3)

D6890 0.83 0.872 0.859 0.853 0.85 0.865–0.880 0.84 0.853 0.865 0.868 0.881 – 0.88 0.871 0.918

Viscosity(mm2 s�1)

D445 2.98 4.86 4 3.6 3.1 4.82–5.55 4.2 4 3.8 4.18 5.21 4.43 4.5 4.63 10.8

Flash point (�C) D93 74 176 111 105 101 172–190 161 169 173 130.5 126.5 166.1 – – 186Pour point (�C) D2500 �12 �15 �13 �12 �12 �6 0 �2 0 6 2 – �15 – �27Heat of

combustion(Mj/kg)

D4809 42.85 38.7 40.1 40.8 41.3 39.2 40.5 41.2 41.8 – – – – – –

Iodine number(mg/g)

D664 60–80 96 76 74 71 92–105 74 72 70 51.22 80.48 99.7 – – –

Acid number(mg/g)

D664 0.35 0.4 0.37 0.39 0.4 0.06–0.5 0.31 0.39 0.41 0.22 0.41 0.42 0.3 – –

Cetane value D6980 49 54.2 53.3 52.6 51.9 51–55.9 50.1 52.6 54 – – – – 53.5 78Calorific value

(Mj/kg)EN14214 45 42.7 43.5 43.9 44.2 38.43–41 39.2 40.5 41.9 – – – – 41 43.72

Copper corrosionstrip

D130 1a 1a 1a 1a 1a 1a 1a 1a 1a 1a 1b 1a – – 1b

Cold filterplugging point(�C)

D6371 �8 3 3 4 4 4 0 3 4 4 0 �2 �16 – –

Ref Thisstudy

Thisstudy

Thisstudy

Thisstudy

Thisstudy

Thisstudy

Thisstudy

Thisstudy

[44] [44] [46] [47] [48] [49]

MO –methyl oleate, MO10, 20, 30 – different percentages of methyl oleate blended with petro-diesel, JOM- jatropha oil methyl ester, JOM 10, 20, 30 – different percentages ofjatropha oil methyl ester blended with petro-diesel, PME – palm oil methyl ester, CIME – Calophyllum inophyllum methyl esters, PO – pongamia oil methyl ester, NO – neemoil methyl ester, CO – castor oil methyl ester.

S. Gopinath et al. / Fuel 203 (2017) 488–500 497

protons (CH2–CH2–COO–Me) and the multiplet at 5.29–5.38 and5.266 ppm due to olefinic protons (–CH@CH–) are also observed[50].

4.13. Sulphation of the catalyst

Suwannakarn et al. reported SO42�/ZrO2 was found to be an

active catalyst in transesterification of tricaprylin with n-butanol,ethanol and methanol. Sun et al. reported a solvent-free route tosynthesize sulphated ZrO2 which was very active for cellulosede-polymerization reaction. The metal atom present in sulphatedZr-KIT-6 acts as Lewis acid sites while the protons on the surfacehydroxyl groups acts as Brønsted acid sites as shown in Fig. 12.The M–O–S bond in the catalyst leads to the coordination of S@Obonds with –OH groups causing enhanced acidity. Since, theBrønsted acidic strength is sensitive to the metal type supportedin the catalyst, the lower electronegativity Zr causes the protonto be released more easily results in stronger acidic strength[43,51]. It is worth to point out that the mesoporous structure ofsulphated Zr-KIT-6(x) with high surface area is very constructivein improving the acidity profile.

Fig. 12. Acidic sites of Sulphated Zr-KIT-6.

4.14. Leaching test

To evaluate the leaching of sulphate groups from the catalyst, asmall portion of used catalyst was employed for Si/Zr molar ratiomeasurements by EDAX as well as the product mixtures by AAS.Analysis of used catalyst showed a sulphate loss below 3 ppm com-pared to fresh catalyst. Catalyst leaching was also tested by dis-solving fresh catalyst with water and there was no sudden dropin pH level. This indicates that the esterification reactions occurredvia heterogeneous catalytic route and not because of H2SO4 andHSO4

� species in the medium as a result of sulphate group hydrol-ysis. This observation strongly suggest the strong interactionbetween sulphate groups and Zr -KIT-6 silica.

4.15. Catalyst reusability

To check stability and reusability nature, the catalysts wererecovered by filtration after the completion of reaction. The recov-

Fig. 13. XRD Pattern of spent catalyst Sulphated Zr-KIT-6(20).

498 S. Gopinath et al. / Fuel 203 (2017) 488–500

ered catalysts were washed with water and acetone to removetraces of ester followed by drying at 100 �C for 12 h. The catalyticperformance of the reactivated catalyst was investigated by ester-ification of oleic acid under optimized reaction conditions. It hasbeen observed that the conversion obtained during second cyclewas 90%. In the successive runs, activity of the regenerated catalystdropped and the conversion obtained in third run was 82%. Garciaet al. reported a quick deactivation of sulphated zirconia when onlysmall amounts of water and it was suggested that the smallamount of sulphate leaching was not only the cause for catalystrapid deactivation. According to Kiss et al., the catalyst deactiva-tion was due to hydrolysis of sulphated zirconia in aqueous media,

Fig. 14. Heterogeneity test. Reaction conditions: Catalyst – 4 wt%; oil-methanolmole ratio – 1:20; T – 120 �C; Time – 6 h.

Fig. 15. Reaction mechanism of ester

but there was no leaching of sulphate groups when only a smallamount of water was present in transesterification phase [52]. Inthis work, small amount of sulphate species has been found to bedissolved into the polar media (aqueous layer) and not into thenon-polar media (organic layer). It is due to that hydrogen sulphateand di alkyl sulphate formed by methanol and sulfuric acid are sol-uble in the polar media but hardly soluble in the aliphatic hydro-carbons [53]. This type of sulphate leaching was anotherimportant reason for the catalyst deactivation during the subse-quent test runs. After each cycle, the catalyst was recovered andanalyzed by ICP technique to determine zirconium content. Itwas found that the loss of zirconium occurred to an extent of lessthan 0.5 wt% after the third cycle. No loss of Zr was observed in thefirst three runs indicating the stability of the catalyst upon recycle.The conversion of oleic acid in the successive cycles are 96%, 90%,82% and 70% respectively (Fig. S-5). This results suggest that SZKcatalysts can be reused without significant loss in activity towardsthe production of biodiesel.

4.16. XRD analysis of spent catalyst

XRD analysis of spent catalyst is shown in Fig. 13 from which nodistinguishable pattern was observed for fresh and spent catalyst.The 2h peak at 0.5� is unchanged for fresh and spent catalyst indi-cating the stability of SZK (20) even after the completion of reac-tion. The diffraction patterns for ZrO2 has not been observed forspent catalyst which confirms the absence of Zr4+ loss due to thestrong interaction between Zr4+ ions and KIT-6 [54]. From theabove results, the SZK catalyst system can be considered for com-mercial use with significant stability.

4.17. Heterogeneity test

The heterogeneity of SZK (20) catalyst was tested accordinglywith hot filtration test. Esterification of oleic acid was carried out

ification and transesterification.

S. Gopinath et al. / Fuel 203 (2017) 488–500 499

at 120 �C, as shown in Fig. 14. After 60 min, the catalyst was recov-ered from the reaction mixture in a hot condition and then thereaction was carried out for 6 h. The conversion was only about6% during 60 min run and it was raised only up to 15% in 6 h,whereas during the catalytic run of 6 h, 83% conversion wasobtained. This results suggest that the reaction was carried outtruly with heterogeneous sulphated Zr-KIT-6 (20) catalyst [55].

4.18. Proposed mechanism

The reaction mechanism of esterification and transesterificationinvolves the interaction of carbonyl oxygen of free fatty acids ormono-glycerides with acidic sites of the catalyst which producecarbocation (Fig. 15). Further, the nucleophilic CH3O� attacks thecarbocation to produce a tetrahedral intermediate which elimi-nates water or R0-OH molecules to form one mole of methyl ester(R0-COOCH3). In case of transesterification, the mechanism can beextends to di- and triglycerides [56]. Excess of methanol in boththe esterification and transesterification always favors for the max-imum biodiesel yield.

5. Conclusion

Sulphated Zr-KIT-6 (x) catalysts were successfully synthesizedand methyl ester was produced by esterification of oleic acid andtransesterification of jatropha oil. The BET surface area and poresize were found to be in the range of 408–438 m2/g and 7.36–7.70 nm respectively. Zr4+ ions were successfully incorporated intothe framework of KIT-6 via one-step hydrothermal synthesis andidentified using the DRS-UV and NH3-TPD techniques. Improvedmass transport aroused via 3D interconnected pore network andassociated enhanced accessible to the catalytic active sites havebeen observed. Accessibility of pores became the rate-determining step as the hydrothermal aging at 100 �C aided poreexpansion of the catalyst up to 7 nm brought an enhanced catalyticactivity. Further optimization of reaction conditions and a detailedstudy of reaction mechanism are currently in progress. Structuralintegrity and a high degree of pore ordering were revealed fromcomplementary N2 sorption and TEM analysis. In sharp contrast,Zr incorporation and sulphation displayed a remarkable amountof acidity resulted an excellent activity towards esterification andtransesterification. Thus, the present sulphated Zr-KIT-6 catalystcan be employed as an efficient environment benign catalyst forsimultaneous esterification and transesterification to producegreen diesel. Moreover, catalysts with different functionalizationand strong acid sites in the KIT-6 framework will play an activerole in the production of green fuels.

Acknowledgements

This research did not receive any specific grant from fundingagencies in the public, commercial, or not-for-profit sectors.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.fuel.2017.04.090.

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