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TransesterificationofcrotonmegalocarpusoiltobiodieseloverWO3supportedonsilica...

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DOI:10.1016/j.cej.2017.02.049

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Chemical Engineering Journal 316 (2017) 882–892

Contents lists available at ScienceDirect

Chemical Engineering Journal

journal homepage: www.elsevier .com/locate /ce j

Transesterification of croton megalocarpus oil to biodiesel over WO3

supported on silica mesoporous-macroparticles catalyst

http://dx.doi.org/10.1016/j.cej.2017.02.0491385-8947/� 2017 Elsevier B.V. All rights reserved.

⇑ Corresponding author.E-mail address: sugeng@utm.my (S. Triwahyono).

M.A.A. Aziz a, K. Puad b, S. Triwahyono b,⇑, A.A. Jalil a,c, M.S. Khayoon b, A.E. Atabani d, Z. Ramli b, Z.A. Majid b,D. Prasetyoko e, D. Hartanto e

aDepartment of Chemical Engineering, Faculty of Chemical and Energy Engineering, Universiti Teknologi Malaysia, UTM, 81310 Johor Bahru, Johor, MalaysiabDepartment of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, UTM, 81310 Johor Bahru, Johor, MalaysiacCentre of Hydrogen Energy, Institute of Future Energy, Universiti Teknologi Malaysia, UTM, 81310 Johor Bahru, Johor, Malaysiad Energy Division, Department of Mechanical Engineering, Faculty of Engineering, Erciyes University, 38039 Kayseri, TurkeyeDepartment of Chemistry, Faculty of Mathematics and Natural Sciences, Institut Teknologi Sepuluh Nopember, Keputih, Sukolilo, Surabaya 60111, Indonesia

h i g h l i g h t s

� WO3/SMP catalyst was prepared fortransesterification of crotonmegalocarpus oil.

� Introduction WO3 on SMP generatedBrønsted and intensified Lewis acidsites.

� WO3/SMP catalyst possessed intra-and interparticles pores.

� 2 wt% WO3 loaded on SMP exhibitedthe highest activity of catalyst.

� A high FAME yield of 96% wasachieved under optimum reactionconditions by RSM.

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

SMP

Fatty acid Methyl ester

a r t i c l e i n f o

Article history:Received 5 November 2016Received in revised form 18 January 2017Accepted 8 February 2017Available online 10 February 2017

Keywords:Silica mesoporous-macroparticles (SMP)WO3 catalystTransesterificationCroton megalocarpus oilResponse surface methodologyLewis acid sites

a b s t r a c t

The transesterification of croton megalocarpus oil with methanol to fatty acid methyl ester (FAME) wascarried out using WO3 supported on silica mesoporous-macroparticles (WO3/SMP) as a heterogeneousacid catalyst. The silica mesoporous-macroparticles (SMP) and WO3/SMP were synthesized by sol-geland impregnation method, respectively. The catalysts were characterized with XRD, FTIR, N2

adsorption-desorption, SEM and TEM. The presence of WO3 gave a negative effect on the crystallinityand surface area of the SMP as evidenced by XRD and N2 adsorption-desorption studies, respectively.Pyridine adsorbed FTIR spectroscopy showed that the concentration of Brønsted and Lewis acid siteswas dependent on the WO3 loading on SMP. 2 wt% of WO3 loading on SMP (2WO3/SMP) exhibited thehighest intensity of Lewis acid sites which is vital in transesterification reaction. Under the optimumreaction condition determined through response surface methodology (RSM), 2 wt% WO3 loading,4.5 wt% catalyst amount, 9:1 methanol to oil molar ratio, 45 min reaction time and 343 K reaction tem-perature yielded a 96% of biodiesel product. The highest catalytic activity of 2WO3/SMP may be attributedto the high Lewis acid sites content and the presence of both intra- and interparticle pores of the catalystthat facilitated and enhanced the transport of reactants and products during the reaction.

� 2017 Elsevier B.V. All rights reserved.

Table 1Physical properties of Croton megalocarpus oil.

M.A.A. Aziz et al. / Chemical Engineering Journal 316 (2017) 882–892 883

1. Introduction

Recent years, global warming issues and crises of diesel fuelshad triggered the use of biodiesel as engine fuels. Biodiesel canbe defined as liquid fuel with similar combustion properties butbetter exhaust gas emission quality as compared to the petroleumdiesel [1]. It can be synthesized either by transesterification withlower alcohol or by esterification of fatty acid [2]. Edible oils arethe main resources for biodiesel production which they come fromthe vegetables feedstock. However, these oils are not suitable forfuel usage due to the competent with human feedstock. Besides,the increase in feedstock’s price make the edible oils are not suit-able in biodiesel production [3]. Therefore, production of biodieselfrom non-edible feedstock is attracting more attention than in thepast [4]. Croton megalocarpus oil is an example of non-edible oilwhich obtained from the seeds of croton megalacorpus plant andcan be used for biodiesel production [5].

Transesterification process of biodiesel can be carried out byusing catalytic homogenous or heterogeneous reaction. In thiscase, heterogeneous catalysts have been paid more attention dueto reusability, easier catalyst and product separation, reductionin the amount of wastewater produce and less sensitive to thepresence of water in feedstock [5]. Heterogeneous catalysts arenon-corrosive and environmentally benign catalysts that are eco-logically and economically an important in catalysis filed systemswith no disposal problems. In addition, the heterogeneous catalystcan perform simultaneous esterification on free fatty acid (FFA)and transesterification of triglycerides, and therefore the pretreat-ment of low-cost feedstock is no longer required to eliminate theFFA. This single-step method can be a potential process for biodie-sel preparation from low-grade oils by simplifying the procedure.

Currently, several solid acid catalysts, including zeolite, silica-bonded sulfuric acid, sulfated ZrO2, SnO2/SiO2, CeO2-Al2O3, WO3/AlPO4 and Zn/Al2O4 catalysts have been developed for the prepara-tion of biodiesel [6–10]. WO3-based materials comprise anotherinteresting of acid solids, first reported as a strongly acidic systemby Hino et al. [11]. While for the support of the metal oxide, mostof the catalytic reactions have been successfully studied using puremesoporous or heteroatom-doped silica support of diverse activephases [12–14]. Silica is more preferred to be used as a supportbecause it can provide high surface area that can increase the dis-persion of the metal. Besides, the bonding formation of supportand metal dispersion could increase the acidity/basicity of the cat-alyst and give a high catalytic performance.

Therefore, a series of WO3/SMP catalyst was prepared in thiswork, and then tested on the transesterification of megalocarpusoil with methanol. The effects WO3 loading amount on the catalyticactivity were investigated in regards to the production of fatty acidmethyl ester (FAME). The catalyst was characterized by powder X-ray diffractometry (XRD), N2 adsorption-desorption, scanning elec-tron microscopy (SEM), transmission electron microscopy (TEM)and pyridine adsorbed FTIR spectroscopy. Moreover, the transes-terification parameters, including the amount of catalyst, the molarratio of methanol to oil, the reaction time and reaction tempera-ture were studied and optimized by response surface methodology(RSM). Furthermore, the proposed mechanism of the reaction wasalso discussed.

Physical properties Value

Free fatty acid 5.04Dynamic viscosity at 40 �C (mpa.s) 28.086Kinematic viscosity at 40 �C (mm2/s) 30.852Density at 40 �C (g/cm3) 0.9104Kinematic viscosity at 100 �C (mm2/s) 7.3562Viscosity index 217.8Absorbance (abs) 0.051Transmission (%T) 89Refractive index 1.4743

2. Experimental

2.1. Catalyst preparation

Silica mesoporous-macroparticles (SMP) were prepared by sol-gel method according to the procedures of Zhang et al. [15]. Thesurfactant cetytrimethylammonium bromide (CTAB; Merck), ace-

tone (ACN; QRec) and NH4OH solution (QRec) were dissolved inthe water with the following molar compositions of CTAB:ACN:NH4OH:H2O = 14:1.4:150:20. After vigorous stirring for 20 min at298 K, 2.8 mL of tetraethyl orthosilicate (Merck) was added to givea white suspension solution. The mixture was kept under continu-ous stirring for 2 h at room temperature. The as-synthesized SMPwas dried at 333 K overnight followed by calcination at 823 K for6 h to remove the surfactant. WO3/SMP catalysts with differentWO3 loadings (1, 2, 3.5 and 5 wt%) were prepared by wet impreg-nation method. The aqueous solution of (NH4)6[H2W12O40�nH2O]was impregnated onto SMP at 333 K, and was then dried overnightat 383 K followed by calcination at 823 K for 3 h.

2.2. Catalyst characterization

Powder X-ray diffraction (XRD) was carried out on a BrukerAdvanced D8 using Cu Ka (k = 1.5418 Å) radiation in the range of2h = 2 � 40�. The nitrogen adsorption-desorption isotherm analysiswas determined with multiple-point Brunauer, Emmett, and Teller(BET) gas adsorption measurements using the SA 3100 SurfaceAnalyzer (Beckman Coulter) at 77 K. Prior to the measurement,the sample was evacuated at 573 K for 1 h.

Fourier Transform Infrared (FTIR) measurements were carriedout with Agilent Cary 640 FTIR spectrometer. The catalysts wereperformed using the KBr method with a scan range of 400–4000 cm�1. In the pyridine adsorption measurement, 2 Torr of pyr-idine was adsorbed on activated samples at 423 K for 15 min fol-lowed by outgassing at 423 K for 30 min [16]. All spectra wererecorded at room temperature with a spectral resolution of5 cm�1 with five scans.

The transmission electron microscopy (TEM) analysis was car-ried out using a JEOL JEM-2100F. The samples were ultrasonicallydispersed in acetone and deposited on an amorphous, porous car-bon grid. The surface morphology of the samples were performedusing a JEOL JSM-6701F scanning electron microscopy with anaccelerating voltage of 15 kV.

2.3. Reaction of transesterification

Table 1 shows the properties of croton megalocarpus oil. In atypical transesterification reaction, croton megalocarpus oil andmethanol in 1:7 M ratio and 4 catalyst/oil wt% of catalyst weretaken in a three neck round-bottom flask. The three-neckedround-bottom flask of 50 mL fitted with a water-cooled condenserand thermometer. Agitation was performed simultaneously by amechanical stirrer. The catalyst was first activated by dispersingit in methanol at room temperature with constant stirring for30 min. After the catalyst activation, a required amount of crotonmegalocarpus oil was added to the reactor and the reaction wascarried out under the identified reaction conditions. Next, reactionsolutions were cooled to room temperature before allowed to beseparated in separating funnel for 1 day before analysis.

884 M.A.A. Aziz et al. / Chemical Engineering Journal 316 (2017) 882–892

The FAME content of the prepared biodiesel sample was quan-tified by 6090 N Agilent gas chromatography equipped with a silicacapillary column (Nukol 15 m � 0.53 mm � 0.5 lm); a splitlessinjection unit with FID detector according to the standard testmethods EN 14103 [17]. Methyl heptadecanoate was used as theinternal standard. The FAME yield was calculated with the follow-ing Eq. (1):

Cð%Þ ¼ ðPAÞ � AelAel

þ Cel� Velm

� 100% ð1Þ

where C is FAME yield,P

A is the total peak area from the conver-sion, Ael is the peak area of internal standard, Cel is the concentra-tion of the internal standard solution, Vel is the volume of theinternal standard solution and m is the mass of sample.

2.4. Experimental design and optimization

In this study, statistical analysis of FAME yield was performedusing Statsoft Statistica 7.0 software. The face-centered centralcomposite design (FCCCD) was used to study the interaction ofprocess variables and to predict the optimum process conditionfor FAME yield by applying RSM. Independent variables consideredimportant were reaction temperature (X1), reaction time (X2), cat-alyst dosage (X3), and methanol to oil ratio (X4). The range andcoded level of the FAME yield process variable studied are listedin Table 2. The independent variables were coded to (�1, 1) inter-val where the low and high levels were coded as �1 and +1, respec-tively. According to FCCCD, the total number of experimentsconducted is 30 with 24 factorial points, 8 axial points and 2 repli-cates at the center points. The FAME yield was taken as theresponse of the design experiment. The experimental design andcorresponding results of the responses are listed in Table 3. The fullquadratic model for FAME yield is given as the following Eq. (2):

Yi ¼ bo þ b1X1 þ b2X2 þ b3X3 þ b4X4 þ b12X1X2 þ b13X1X3

þ b14X1X4 þ b23X2X3 þ b24X2X4 þ b34X3X4 þ b11X21

þ b22X22 þ b33X

23 þ b44X

24 ð2Þ

where Yi is the predicted response i whilst X1, X2, X3, and X4 are thecoded form of independent variables. The terms bo is the offsetterm; b1, b2, b3, and b4 are the linear terms; b11, b22, b33, and b44are the quadratic terms; and b12, b13, b14, b23, b24, b34 are the inter-action terms.

The equation model was tested with the analysis of variance(ANOVA) with 5% level of significant. The ANOVA was used tocheck significance of the second order models and it is determinedby F-value. Generally, the calculated F-value should be greater thantabulated F-value to reject the null hypothesis, where all theregression coefficients are zero. The calculated F-value is definedas the following Eq. (3):

F � value ¼ MSSSRMSSSE

ð3Þ

where MSSSR and MSSSE are mean of square regression and mean ofsquare residual. The MSSSR and MSSSE were obtained by dividingsum of squares (SSR) and sum of residual (SSE) over degree of free-

Table 2Coded levels for independent variables used in the experiment design.

Independent variables Symbol Unit

Reaction temperature X1 KReaction time X2 MinCatalyst dosage X3 %Methanol to oil Ratio X4 mol/mol

dom (DF), respectively. Meanwhile, tabulated F-value was obtainedfrom F distribution based on DF for regression and residual, respec-tively at a specific level of significance, a-value [18].

3. Results and discussion

3.1. Characterization of catalysts

Fig. 1A shows the small-angle XRD patterns for SMP and WO3/SMP catalysts. SMP showed a significant peak at 2h = 2.1�, whichindexed as (100), indicating the presence of ordered mesoporoussilica [15]. This is similar to the result reported by Zhang et al. inthe synthesis of mesoporous silica nanoparticles where they useof acetone as surfactant to controls the morphology of the silicastructural ordering [15]. Their results revealed the synthesis of sil-ica mesoporous-macroparticles using acetone led to a disorderedof mesostructure of silica particles. In the present work, the intro-duction of WO3 with the loading amount of 1, 2, 3.5 and 5 wt%WO3

onto SMP was significantly decreased and eliminated the (100)peak. This could be attributed to the highly dispersed of WO3 onthe SMP which degraded the structural order of silicamesoporous-macroparticles. Similar observation was reported byCecilia et al. that show the incorporation of WO3 into mesoporousSBA-15 framework destructed the order of the mesoporous struc-ture [19]. The presence of WO3 on the SMP support was demon-strated in the wide-angle XRD (20–40�) in Fig. 1B. The diffractionpeaks at 2h = 23.06�, 23.13� and 26.91� are corresponded to themonoclinic phase (002), (020) and (200) of WO3, respectively.These peaks were more intense with increasing WO3 loading from1 to 2 wt% corresponding to increase in the monoclinic phase ofWO3 on the SMP support. While, the peak intensities are almostconstant for loading of 2–5 wt%. These results demonstrated thatthe interaction between WO3 on the SMP is very strong due tothe formation of crystalline WO3 in the support.

Fig. 2 shows the SEM images of SMP, WO3 and WO3/SMP cata-lysts. The SEM image of SMP showed the spherical shape of silicaparticles, while the TEM image showed the solid spheres. Theseresults are in good agreement with the XRD result. Besides, theSEM image of SMP showed non-uniform size of particle due tothe effect of acetone that controls the morphology of SMP duringthe synthesis. Fig. 2B shows the monoclinic structure of WO3 thatis in accord with the XRD result. The SEM image of 2WO3/SMPshowed the spherical shape of silica particle and metal dispersionon the support. There was a small change in the surface morphol-ogy in the SEM image of 2WO3/SMP which showed two shapetypes which are spherical and monoclinic structure. It indicatedthat there is metal which well dispersed on support caused achanging in the structure, while another metal destructed thestructure of the support.

Fig. 3 shows the N2 adsorption desorption isotherms and poresize distribution of SMP and WO3/SMP catalysts with differentloading amount of WO3. The results were analyzed by applyingnon-local density functional theory (NLDFT) method. The NLDFTmethod is applicable to micro-mesoporous materials [20]. It allowscalculating the specific cumulative surface area (i.e., specific sur-

Coded level

�1 0 +1

333 348 36345 68 903 4.5 64 7 10

Table 3Experimental design and results of the response surface design.

Std order Run order Temperature (K) Time (min) Catalyst dosage (%) Methanol to oil ratio (mol/mol) FAME Yield (%)

1 11 333 45 3 4 502 9 363 45 3 4 623 5 333 90 3 4 544 20 363 90 3 4 505 13 333 45 6 4 466 4 363 45 6 4 547 2 333 90 6 4 468 18 363 90 6 4 549 3 333 45 3 10 6710 14 363 45 3 10 4411 8 333 90 3 10 6112 1 363 90 3 10 3813 15 333 45 6 10 6814 17 363 45 6 10 4815 7 333 90 6 10 5716 19 363 90 6 10 4017 6 348 68 4.5 7 9018 16 348 68 4.5 7 9019 10 348 68 4.5 7 9220 12 348 68 4.5 7 8821 27 333 68 4.5 7 7922 31 363 68 4.5 7 6123 26 348 45 4.5 7 9024 22 348 90 4.5 7 8125 28 348 68 3 7 6026 23 348 68 6 7 5827 25 348 68 4.5 4 6828 21 348 68 4.5 10 8029 30 348 68 4.5 7 8230 24 348 68 4.5 7 82

Fig. 1. (A) Small-angle and (B) wide-angle XRD patterns of (a) SMP, (b) 1WO3/SMP,(c) 2WO3/SMP, (d) 3.5WO3/SMP, and (e) 5WO3/SMP.

M.A.A. Aziz et al. / Chemical Engineering Journal 316 (2017) 882–892 885

face area as a function of pore size) over the complete range ofmicro- and mesopores. For all catalysts, all isotherms were typeIV adsorption isotherms with type H1 hysteresis loops, which indi-cated an agglomeration of uniform spheres with a complex meso-porous solid [21]. A sharp uptake at low relative pressure indicatedthe presence of microporosity. In addition, an increased uptake atrelative pressures of P/P0 = 0.2–0.4 was due to the presence ofmesoporosity. The first step at a relative pressure of 0.2–0.4 wasdue to the presence of intraparticle pores, while the second stepat P/P0 = 0.9–1.0 was due to the presence of interparticle pores

[22–24]. These results confirmed the permanence of the meso-porous phase in parallel with the microporous phase in the WO3/SMP. Besides, it is noteworthy that the second step at higher partialpressure was only observed after the introduction of WO3 on SMP.By contrast, the decrease of the step at high partial pressure of3.5WO3/SMP could be attributed to the fact that the WO3 particlesblocked the interparticle pores of SMP. The pore size distribution ofSMP showed a narrow pore size which in the range of 3–6 nm. ForWO3/SMP, the mesoporosities were increased with increasingamounts of WO3 loading, indicating an improvement in the WO3

interaction between the possible formations of monoclinic on thesolid sphere of SMP.

The summary data on surface areas and total pore volumes ofall catalysts are listed in Table 4. In all cases, it can be seen thatthe surface area and total pore volume decreased considerablyafter the introduction of WO3, suggesting that a portion of theWO3 particles were dispersed in the pores of the supports and/orpartially blocking the access to the mesopores of SMP. In accordto this, Yang et al. reported WO3-containing hexagonal meso-porous silica (W-HMS) which gave a lower surface area comparedto the parent HMS due to the tungsten heteroatoms were embed-ded into the lattice of the HMS bulk [25]. While, the pore size of thecatalysts were decreased as increasing the WO3 loading. This ismay be due to the generation of bigger pore from the agglomera-tion of WO3 particles near to the mouth of the SMP pores.

FTIR spectra of SMP, WO3 and WO3/SMP catalysts were shownin Fig. 4. For SMP WO3/SMP catalysts, a broad band was observedaround 1371 cm�1 which corresponding to the asymmetric ofSiAOASi. Besides, two bands at 794 and 458 cm�1 are attributedto the symmetric stretching and bending vibration of SiAOASi,respectively. The broad absorption band at 3455 cm�1 is relatedto the SiAOH on the surface, which provides opportunities of form-ing hydrogen bond. For bare WO3 catalyst (Fig. 4f), typical bandswere observed at 879 and 755 cm�1 which corresponding to the(W@O) and stretching vibrations of (WAOAW), respectively [26].

100 nm

10 µm

100 nm

50 nm

50 nm

50 nm

A

B

C

Fig. 2. TEM (left) and SEM (right) images of (A) SMP (B) WO3 and (C) 2WO3/SMP.

886 M.A.A. Aziz et al. / Chemical Engineering Journal 316 (2017) 882–892

The absorption band at 794 cm�1 has been widely used to charac-terize the incorporation of transition metal atoms in the silicaframework. FTIR spectra of WO3/SMP with different weight loadingshowed no additional band upon introduction of WO3 on the SMP.However, the stretching vibration of SiAO at 794 cm�1 is affectedby the neighboring metal atom of W@O and WAOAW whichcaused the peak intensified. This indicated the incorporation ofthe WO3 into the silica framework of SMP. Jeroen et al. alsoobserved the appearance of the peak of W@O and WAOAW afterintroduction of WO3 on the support sample [27].

Pyridine was used as probe molecule to evaluate the acidity ofcatalysts, particularly in the observation of the Brønsted and theLewis acid sites [28]. Fig. 5A shows the Fourier transform infrared(FTIR) spectra of pyridine adsorbed on SMP and WO3/SMP cata-lysts. For SMP, only one band was observed at 1447 cm�1, whichare ascribed to pyridine coordinately bonded to weak surface Lewisacid site [28]. For WO3/SMP, two significant absorbance bandsarose at around 1450 and 1542 cm�1, which are ascribed to thepyridine species adsorbed on Lewis and Brønsted acid sites respec-tively. The introduction of WO3 on the SMP substantially generatedBrønsted acid sites and intensified Lewis acid sites. The introduc-tion of WO3 on the SMP shifted the band ascribed to the Lewis acidsites at 1447 cm�1 to a higher frequency at 1450 cm�1, suggestingthat there was an interaction of W6+ with silica atom [28,29].Fig 4B shows the Gaussion deconvolution area of SMP and WO3/SMP catalysts at the bands 1450 and 1542 cm�1. In general, thepeak areas of Lewis acid sites increased as increasing of WO3 load-ing on SMP. While, the peak areas of Brønsted acid sites showed notrend as increasing of theWO3 loading. These results indicated thatthe incorporation of WO3 into SMP framework increased and gen-erated Lewis acid site and Brønsted acid site, respectively. 2WO3/SMP exhibited the highest Lewis acid sites which may be due to

the formation of monolayer WO3 on the surface of SMP, while3.5WO3/SMP possessed the highest Brønsted acid sites whichmay be due to the higher agglomeration and the presence of bulkWO3 on the SMP surface. Whereas, bare SMP possessed only Lewisacid sites which corresponding to the presence of electron pairacceptor sites from SiAOH groups.

3.2. Catalytic performance

The catalytic performance of all catalysts for biodiesel produc-tion from transesterification of croton megalocarpus oil withmethanol is shown in Fig. 6. Fig. 6A shows the production of FAMEas a function of time over WO3/SMP catalysts at 338 K with metha-nol to oil molar ratio of 7. All catalysts showed a similar trendwhich fast reaction rate was observed at and below 30 min andit started to slow after 30 min of reaction time. There is no muchdifferent of FAME yield after 60 min of reaction time indicatingthe catalysts were completely deactivated. In general, 2WO3/SMPexhibited the highest activity followed by 5WO3/SMP, 3.5WO3/SMO and 1WO3/SMP. Fig. 6B shows the comparison of FAME yieldagainst WO3 loading at reaction time of 60 min. The bare SMP sup-port was not very active for the reaction as only 20% of FAME yieldwas obtained. The addition of WO3 increased the activity of SMPwhich may be corresponded well with the acid strength of the cat-alysts. 2WO3/SMP exhibited the highest FAME yield while 1WO3/SMP gave the lowest FAME yield with 86% and 55%, respectively.Further increased of the WO3 content decreased slightly the FAMEyield product which may be due to the altering of the catalystsstructure or decreasing the acidity of the catalysts. However, itwas noteworthy that the FAME yield of bare WO3 catalyst was con-siderably higher than that of the 1WO3/SMP catalyst which may bedue to the high acidity properties of the WO3 metal.

Fig. 3. (A) N2 adsorption-desorption isotherm and average pore width of (A) SMP, (B) 1WO3/SMP, (C) 2WO3/SMP, (D) 3.5WO3/SMP and (E) 5WO3/SMP.

Table 4Physical properties of the catalysts.

Catalyst BET surface area(m2/g)

Total pore volume(cm3/g)

Average pore size(nm)

SMP 579 0.4249 2.941WO3/SMP 495 0.3319 2.682WO3/SMP 384 0.3051 3.183.5WO3/SMP 320 0.2615 3.275WO3/SMP 229 0.1904 3.32

M.A.A. Aziz et al. / Chemical Engineering Journal 316 (2017) 882–892 887

Fig. 6C shows the dependence activity of WO3/SMP upon thespecific surface area and fraction of Lewis and Brønsted acid sites.The results showed a direct correlation between the fraction ofLewis acid sites and the FAME yield of transesterification of croton

megalocarpus oil. The ratio of FAME yield to fraction of Lewis acidsites did not change much for the WO3 loading below 5 wt% indi-cating that the activity of the catalysts is clearly depend on thefraction of Lewis acid sites. While the dependence activity ofWO3/SMP catalyst on the surface area and fraction of Brønsted acidsites was not observed for any range of the WO3 loading in thisexpriment.

In addition to the Lewis acid sites, the presence of intra- andinterparticle porosity may be determined the catalytic perfor-mance of WO3/SMP. Lewis acid sites are vital for the catalytic activ-ity due to the lack of electrons and can activate substrates whichare rich in electrons such as carbonyl group in the reaction. Fre-quently, Lewis acid-base adducts is the key intermediates in theacid-catalyzed reaction [30]. Whereas, the presence of both type

0031 004220031004000

Tran

smitt

ance

[%]

(a)

(b)

(c)

(d)

(e)

(f)

34551371

794458

879 755

Fig. 4. FTIR spectra of (a) SMP, (b) 1WO3/SMP, (c) 2WO3/SMP, (d) 3.5WO3/SMP, (e)5WO3/SMP and (f) WO3.

888 M.A.A. Aziz et al. / Chemical Engineering Journal 316 (2017) 882–892

of pores facilitated and enhanced the transport of reactants andproducts during the reaction as well as diminishing of the diffusionlimitation. This may be increased the reaction rate of conversion oftriglyceride to the desired products of FAME.

3.3. Proposed mechanism of transesterification process

The reactions of transesterification are based on the Lewis andBrønsted acid site of the catalyst. The formation of Lewis andBrønsted acid sites over the WO3/SMP were confirmed by pyridineadsorbed FTIR spectroscopy. This result indicated that the incorpo-ration of WO3 into SMP framework has intensified the Lewis and

1420147015201570

Abs

orba

nce

Wavenumber [cm-1]

0.2

(e)

(a)

(b)

(c)

(d)

A

Fig. 5. (A)IR spectra pyridine adsorbed on (a) SMP, (b) 1WO3/SMP, (c) 2WO3/SMP, (d)deconvulation area of the catalysts.

Brønsted acid sites [28]. Therefore, a mechanism of croton megalo-carpus oil transesterification catalyzed by WO3/SMP was proposedas shown in Fig. 7. The transesterification proceeds via several con-secutive steps. The first step started with the adsorption of metha-nol (CH3OH) and oil (triglyceride) onto the surface of WO3/SMP.This step may be accelerated by strong stirring conditions in thereactor. Triglyceride is adsorbed on the W6+ surface site which actsas Lewis acid sites. The methanol is adsorbed on the lattice oxygenatom (Lewis basic sites) on the surface of the catalyst and formsoxygen anion. The protonation of carbonyl group of the ester leadsto the carbocation. The nucleophilic attack of alcohol to the carbo-cation produces a tetrahedral intermediate. This intermediateeliminates glycerol to form a new ester and to regenerate thecatalyst.

3.4. RSM analysis

RSM is a method to determine the optimum condition of theprocess, and it allows users to gather large amount of informationfrom a small number of experiments. It is also possible to observethe relationships between variables and responses, and has beensuccessfully applied for a wide range of chemical reactionsinvolves more than one response [31]. Based on the RSM analysis,the quadratic model for FAME yield is presented in Eq. (4) asfollows:

Y ¼ 83:69� 9X1 � 4:5X2 � 1X3 þ 6X4 � 0:8125X1X2

þ 1:0625X1X3 � 6:6875X1X4 þ 0:0625X2X3 � 1:4375X2X

þ 1:1875X3X4 � 11:5354X21 þ 3:9645X2

2 � 22:5354X23

� 7:5354X24 ð4Þ

0

0.780.63

1.05 0.991.09

2.39

3.44

3.063.24

Dec

onvu

latio

n ar

ea

BronstedLewis

B

3.5WO3/SMP and (e) 5WO3/SMP, after activated at 423 K (B) show the Gaussion

Fig. 6. (A) FAME yield of transesterification reaction of crotonmegalocarpus oil as a function of time over WO3/SMP catalysts at 338 K with methanol to oil molar ratio of 7. (B)Effect of type of catalyst in the FAME yield production at 338 K, methanol to oil molar ratio of 7 and 60 min of reaction time. (C) Normalized FAME yield (results on Fig. 6B)against specific surface area [10�2%/(m2/g)] and fraction of Lewis acid sites [%/LAS] and Brønsted acid sites [%/BAS] of the catalysts.

R= alkyl group of fatty acid R1= alkyl ester of triglyceride

W+= acid site on the catalyst surface

Fig. 7. Proposed mechanism of transesterification of Croton Megalocarpus oil.

M.A.A. Aziz et al. / Chemical Engineering Journal 316 (2017) 882–892 889

The coefficient of determination, (R2) for FAME yield optimiza-tion using RSM was 0.9972, indicating 99.72% of the variability inthe data is accounted by the model from Eq. (4). According to Haa-land [32], the empirical model is adequate to explain most the vari-ability in the asset reading which should be at least 0.75 or greater.Table 5 shows the analysis of variance (ANOVA) of the regressionparameters for the predicted response surface quadratic model.As the model value (F = 115.35) exceeds the table value(F = 3.452), it can be concluded that the model obtain from Eq.

(4) give a good prediction at 5% level of significance. Fig. 8 showsthe t-distribution values in a Pareto chart and the correspondingp-value of the variables in Eq. (4). The p-value serves as a tool tocheck the significance of each coefficient. The corresponding coef-ficient with a smaller p-value or a t-value with a greater magnitudedonates more significance into the model. The largest effect ofFAME yield is the quadratic term of catalyst dosage (X3

2), whichhas the smallest p-value (0.0000) and the largest t-value(�473.6837) at the 95% significance level. In addition, the linear

Table 5ANOVA for analysis of variance and adequacy of the quadratic model.

Sources Sum of squares (SS) Degree of freedom Mean square (MS) F-value F0.05

X32 1289.822 1 1289.8215 473.8637 0.00001

X1X4 715.5625 1 715.5625 262.8884 0.00001X12 337.9600 1 337.9600 124.1621 0.00001

X1 162 1 162 59.51671 0.0001Regression 6907.533 22 313.9788 115.3517 –Residual 19.05347 7 2.7219 – –Total 6926.586 29 – – –

Fig. 8. Pareto chart and p-value of FAME yield.

890 M.A.A. Aziz et al. / Chemical Engineering Journal 316 (2017) 882–892

term for the reaction temperature and molar ratio (X1X4), reactiontemperature (X1), and the quadratic term of reaction temperature(X1

2), could be regarded as significant factors in affecting the FAMEyield, owing to the large t-value of �262.8884, �124.1621and�115.3517, respectively. The rest of factors could be consideredless significant in affecting the FAME yield, as their p = values werehigher than 0.05.

The response surfaces and contour plot are generally used toevaluate the relationships between parameters and to predict theresult under given conditions. However, it is complicated to ana-lyze the interaction between parameters in this study, owing tothe presence of many interaction terms. Instead, the response sur-faces and contour plot were used for optimizing the conditions ofthe FAME yield over 2WO3/SMP. There are 4 parameters that hadbeen used and 6 RSM 3-D plots were constructed for the FAMEyield. They were plotted as a function of two of the factors whilethe others were maintained constant at their mean levels. Theinteraction between the corresponding variables was negligiblewhen the contour of response surface was circular. On the con-trary, the interactions between the relevant variables were signif-icant when the contour of response surface was elliptical. It isinteresting to note that all the contour plots in Fig. 9 were ellipticalindicating the significant interaction effects between the parame-ters studied.

Fig. 9A shows the response surface plot, demonstrating theeffects of the reaction temperature and catalyst dosage on FAMEyield. From the analysis of the response surface plot, the reactiontemperature exhibited a more significant influence on the responsesurface in comparison to catalyst dosage, which also can beexplained by the Pareto chart (Fig. 8) showing larger t-value ofreaction temperature (�59.5167) as compared to the catalystdosage (�0.7348). An increase in the reaction temperature resulted

to an increase in the FAME yield, passing through a maximumaround 350 K and decrease at higher temperature. This could beattributed to the increase in the number of catalytic active siteswhich accelerated the forward reaction in specific temperatureand decrease at higher temperature. This behavior could be dueto the state of activation of these catalysts at certain temperature[33].

Fig. 9B represents the effect of reaction time and catalyst dosageon the FAME yield. The FAME yield was significantly affected byreaction time. The significant effect of reaction time on the FAMEyield can be explained by the larger t-value of reaction time(14.8792) as compared to the reaction catalyst (�0.7348). At a con-stant catalyst dosage 4.5 wt%, it was clear that increases of thereaction time evidently increase in the FAME yield, reach the max-imum around 58–62%. At this condition, the catalyst lowered theactivation energy, which thus promotes transesterification towardthe products, and increasing of the catalyst dosage implies moreactive sites for catalyzing of the reaction [34]. However, at longerreaction time and larger amount of the catalyst dosage led to aslightly decrease in the FAME yield. This could be due to the rever-sible reaction whereby the larger amount of catalysts contributedto the larger surface area that allows rapid reversible reaction ofthe FAME back to the triglyceride, resulted in the lowering of con-version. This result showed a similar trend with that of using seamango oil as a reactant [35]. In addition, exceeding catalyst dosagewould increase the viscosity of the reaction mixture and interferein the mass transfer between the catalyst and reactants [36].

The effects of molar ratio and catalyst dosage on the FAME yieldare shown in Fig. 9C. The result indicated that the increment ofmolar ratio slightly affected in the FAME yield. In addition, theeffect of different molar ratios of methanol to Croton Megalocarpusoil was studied. The FAME yields significantly increase withincreasing catalyst dosage and slightly decrease after reached themaximum. It shows that the catalyst achieved maximum activatedat the catalyst dosage of 4.5 wt%. The result obviously showed thatthe reaction had an optimum molar ratio of oil to methanol toobtain a higher FAME production. The conversion of oil wasenhanced with an increasing amount of the methanol, becausethe excess amount of the methanol provided more opportunityfor the reactant molecules to collide and then shifted the reactionequilibrium towards production of the biodiesel. However, thedecrease of conversion was observed when the catalyst dosageexceeded 5 wt% .This result might be attributed to the saturationof the catalytic surface with the methanol at the high concentra-tion of the methanol, which deactivated the catalyst whereby thetransesterification process was inhibited [37].

Fig. 9D represents the effect of reaction temperature and molarratio on the FAME yield. From the analysis of the response surfaceplot, reaction temperature exhibited more significant influence onthe response surface in comparison to the molar ratio, which alsocan be explained by the Pareto chart (Fig. 8) showing larger t-value of reaction temperature (�59.9831) as compared to themolar ratio (�26.4519). An increase in the reaction temperatureresulted to a decrease in the FAME yield. This is because the tem-

Fig. 9. Response surface plot of the combined (A) catalyst dosage and temperature, (B) catalyst dosage and time, (C) catalyst dosage and molar, (D) temperature and molar (E)time and molar and (F) time and temperature on FAME yield.

M.A.A. Aziz et al. / Chemical Engineering Journal 316 (2017) 882–892 891

perature should be not more than 338 K due to the boiling point ofmethanol at 333.7 K. Higher temperature may contribute to thenegative impact on the FAME yield. Leung et al. reported thathigher temperature than 323 K had a negative impact on FAMEyield, but on the contrary, it had a positive effect for waste oil withhigher viscosity [38]. Fig. 8E shows the effect of the time and molarratio on the FAME yield. No significant changes were observed inthe response surface plot, which indicating the interaction of thetime and molar ratio was not significant. Moreover, the effects ofthe time and reaction temperature on the FAME yield are shown

in Fig 9F. The result indicated that the interactions of the timeand reaction temperature are not significant.

Form the analysis of the response surface plot, all of the param-eters studied were found affected to the FAME yield. However, ithas appeared that the reaction temperature is the dominant factorin the FAME yield percentage. The reaction temperature exhibiteda significance influence on the response surface in comparison toothers parameters, which can be clarified by larger t-value(�59.9831) from the pareto chart. This is may be due to the effectof high reaction temperature which could rapidly break the struc-ture of triglyceride molecules. The optimum FAME yield predicted

892 M.A.A. Aziz et al. / Chemical Engineering Journal 316 (2017) 882–892

from the response surface analysis is 96% at the reaction tempera-ture of 345 K, reaction time of 45 min, molar ratio of 1:9 and 4.5 wt% of the catalysts dosage.

4. Conclusion

Investigation on WO3/SMP with various WO3 loading is carriedout for the transesterification of Croton megalocarpus oil which is anonedible feedstock. WO3 loading on SMP give a negative effect onthe crystallinity and surface area of the catalysts. However,Brønsted and Lewis acid sites were markedly influenced by WO3

loading in which 2 wt% of WO3 loading on SMP (2WO3/SMP) gavehighest intensity of Lewis acid sites which is vital in transesterifi-cation reaction. Under the optimum reaction condition determinedthrough response surface methodology (RSM), 2 wt% WO3 loading,4.5 wt% catalyst amount, 9:1 methanol to oil ratio, 45 min reactiontime and 343 K reaction temperature gave a 96% of biodiesel con-version. High catalytic activity of 2WO3/SMP is may be attributedto the high Lewis acid sites content and the presence of bothintra- and interparticle pores of the catalyst that facilitated andenhanced the transport of reactants and products during thereaction.

Acknowledgements

This work supported by the Ministry of Higher EducationMalaysia through Fundamental Research Grant Scheme no 4F781and the Universiti Teknologi Malaysia through Research UniversityGrant no 00M67. The authors would also like to acknowledgeErciyes University for the Scientific Research Projects Unit ofErciyes University, Turkey, for the financial support under thegrant number FOA-2015-5817 and FOA-2015-5790.

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