Transesterification of croton megalocarpus oil to biodiesel over WO3 supported on silica mesoporous-macroparticles catalyst

Chemical Engineering Journal 316 (2017) 882–892

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Transesterification of croton megalocarpus oil to biodiesel over WO3 supported on silica mesoporous-macroparticles catalyst 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 a

Department of Chemical Engineering, Faculty of Chemical and Energy Engineering, Universiti Teknologi Malaysia, UTM, 81310 Johor Bahru, Johor, Malaysia Department of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, UTM, 81310 Johor Bahru, Johor, Malaysia Centre of Hydrogen Energy, Institute of Future Energy, Universiti Teknologi Malaysia, UTM, 81310 Johor Bahru, Johor, Malaysia d Energy Division, Department of Mechanical Engineering, Faculty of Engineering, Erciyes University, 38039 Kayseri, Turkey e Department of Chemistry, Faculty of Mathematics and Natural Sciences, Institut Teknologi Sepuluh Nopember, Keputih, Sukolilo, Surabaya 60111, Indonesia b c

h i g h l i g h t s

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


WO3/SMP catalyst was prepared for

transesterification of croton megalocarpus oil.
Introduction WO3 on SMP generated Brønsted and intensified Lewis acid sites.
WO3/SMP catalyst possessed intraand interparticles pores.
2 wt% WO3 loaded on SMP exhibited the highest activity of catalyst.
A high FAME yield of 96% was achieved under optimum reaction conditions by RSM.

a r t i c l e

i n f o

Article history: Received 5 November 2016 Received in revised form 18 January 2017 Accepted 8 February 2017 Available online 10 February 2017 Keywords: Silica mesoporous-macroparticles (SMP) WO3 catalyst Transesterification Croton megalocarpus oil Response surface methodology Lewis acid sites

⇑ Corresponding author. E-mail address: [email protected] (S. Triwahyono). http://dx.doi.org/10.1016/j.cej.2017.02.049 1385-8947/Ó 2017 Elsevier B.V. All rights reserved.

Fatty acid

Methyl ester

SMP

a b s t r a c t The transesterification of croton megalocarpus oil with methanol to fatty acid methyl ester (FAME) was carried out using WO3 supported on silica mesoporous-macroparticles (WO3/SMP) as a heterogeneous acid catalyst. The silica mesoporous-macroparticles (SMP) and WO3/SMP were synthesized by sol-gel and 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 crystallinity and 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 sites was dependent on the WO3 loading on SMP. 2 wt% of WO3 loading on SMP (2WO3/SMP) exhibited the highest intensity of Lewis acid sites which is vital in transesterification reaction. Under the optimum reaction 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 temperature yielded a 96% of biodiesel product. The highest catalytic activity of 2WO3/SMP may be attributed to the high Lewis acid sites content and the presence of both intra- and interparticle pores of the catalyst that facilitated and enhanced the transport of reactants and products during the reaction. Ó 2017 Elsevier B.V. All rights reserved.

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1. Introduction Recent years, global warming issues and crises of diesel fuels had triggered the use of biodiesel as engine fuels. Biodiesel can be defined as liquid fuel with similar combustion properties but better exhaust gas emission quality as compared to the petroleum diesel [1]. It can be synthesized either by transesterification with lower alcohol or by esterification of fatty acid [2]. Edible oils are the main resources for biodiesel production which they come from the vegetables feedstock. However, these oils are not suitable for fuel usage due to the competent with human feedstock. Besides, the increase in feedstock’s price make the edible oils are not suitable in biodiesel production [3]. Therefore, production of biodiesel from non-edible feedstock is attracting more attention than in the past [4]. Croton megalocarpus oil is an example of non-edible oil which obtained from the seeds of croton megalacorpus plant and can be used for biodiesel production [5]. Transesterification process of biodiesel can be carried out by using catalytic homogenous or heterogeneous reaction. In this case, heterogeneous catalysts have been paid more attention due to reusability, easier catalyst and product separation, reduction in the amount of wastewater produce and less sensitive to the presence of water in feedstock [5]. Heterogeneous catalysts are non-corrosive and environmentally benign catalysts that are ecologically and economically an important in catalysis filed systems with no disposal problems. In addition, the heterogeneous catalyst can perform simultaneous esterification on free fatty acid (FFA) and transesterification of triglycerides, and therefore the pretreatment of low-cost feedstock is no longer required to eliminate the FFA. This single-step method can be a potential process for biodiesel preparation from low-grade oils by simplifying the procedure. Currently, several solid acid catalysts, including zeolite, silicabonded sulfuric acid, sulfated ZrO2, SnO2/SiO2, CeO2-Al2O3, WO3/ AlPO4 and Zn/Al2O4 catalysts have been developed for the preparation of biodiesel [6–10]. WO3-based materials comprise another interesting of acid solids, first reported as a strongly acidic system by Hino et al. [11]. While for the support of the metal oxide, most of the catalytic reactions have been successfully studied using pure mesoporous or heteroatom-doped silica support of diverse active phases [12–14]. Silica is more preferred to be used as a support because it can provide high surface area that can increase the dispersion of the metal. Besides, the bonding formation of support and metal dispersion could increase the acidity/basicity of the catalyst and give a high catalytic performance. Therefore, a series of WO3/SMP catalyst was prepared in this work, and then tested on the transesterification of megalocarpus oil with methanol. The effects WO3 loading amount on the catalytic activity were investigated in regards to the production of fatty acid methyl ester (FAME). The catalyst was characterized by powder Xray diffractometry (XRD), N2 adsorption-desorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and pyridine adsorbed FTIR spectroscopy. Moreover, the transesterification parameters, including the amount of catalyst, the molar ratio of methanol to oil, the reaction time and reaction temperature were studied and optimized by response surface methodology (RSM). Furthermore, the proposed mechanism of the reaction was also discussed.

2. Experimental 2.1. Catalyst preparation Silica mesoporous-macroparticles (SMP) were prepared by solgel method according to the procedures of Zhang et al. [15]. The surfactant cetytrimethylammonium bromide (CTAB; Merck), ace-

tone (ACN; QRec) and NH4OH solution (QRec) were dissolved in the water with the following molar compositions of CTAB:ACN: NH4OH:H2O = 14:1.4:150:20. After vigorous stirring for 20 min at 298 K, 2.8 mL of tetraethyl orthosilicate (Merck) was added to give a white suspension solution. The mixture was kept under continuous stirring for 2 h at room temperature. The as-synthesized SMP was dried at 333 K overnight followed by calcination at 823 K for 6 h to remove the surfactant. WO3/SMP catalysts with different WO3 loadings (1, 2, 3.5 and 5 wt%) were prepared by wet impregnation method. The aqueous solution of (NH4)6[H2W12O40nH2O] was impregnated onto SMP at 333 K, and was then dried overnight at 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 Bruker Advanced D8 using Cu Ka (k = 1.5418 Å) radiation in the range of 2h = 2  40°. The nitrogen adsorption-desorption isotherm analysis was determined with multiple-point Brunauer, Emmett, and Teller (BET) gas adsorption measurements using the SA 3100 Surface Analyzer (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 carried out with Agilent Cary 640 FTIR spectrometer. The catalysts were performed using the KBr method with a scan range of 400– 4000 cm1. In the pyridine adsorption measurement, 2 Torr of pyridine was adsorbed on activated samples at 423 K for 15 min followed by outgassing at 423 K for 30 min [16]. All spectra were recorded at room temperature with a spectral resolution of 5 cm1 with five scans. The transmission electron microscopy (TEM) analysis was carried out using a JEOL JEM-2100F. The samples were ultrasonically dispersed in acetone and deposited on an amorphous, porous carbon grid. The surface morphology of the samples were performed using a JEOL JSM-6701F scanning electron microscopy with an accelerating voltage of 15 kV. 2.3. Reaction of transesterification Table 1 shows the properties of croton megalocarpus oil. In a typical transesterification reaction, croton megalocarpus oil and methanol in 1:7 M ratio and 4 catalyst/oil wt% of catalyst were taken in a three neck round-bottom flask. The three-necked round-bottom flask of 50 mL fitted with a water-cooled condenser and thermometer. Agitation was performed simultaneously by a mechanical stirrer. The catalyst was first activated by dispersing it in methanol at room temperature with constant stirring for 30 min. After the catalyst activation, a required amount of croton megalocarpus oil was added to the reactor and the reaction was carried out under the identified reaction conditions. Next, reaction solutions were cooled to room temperature before allowed to be separated in separating funnel for 1 day before analysis.

Table 1 Physical properties of Croton megalocarpus oil. Physical properties

Value

Free fatty acid Dynamic viscosity at 40 °C (mpa.s) Kinematic viscosity at 40 °C (mm2/s) Density at 40 °C (g/cm3) Kinematic viscosity at 100 °C (mm2/s) Viscosity index Absorbance (abs) Transmission (%T) Refractive index

5.04 28.086 30.852 0.9104 7.3562 217.8 0.051 89 1.4743

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The FAME content of the prepared biodiesel sample was quantified by 6090 N Agilent gas chromatography equipped with a silica capillary column (Nukol 15 m  0.53 mm  0.5 lm); a splitless injection unit with FID detector according to the standard test methods EN 14103 [17]. Methyl heptadecanoate was used as the internal standard. The FAME yield was calculated with the following Eq. (1):

Cð%Þ ¼

ð

P

3. Results and discussion 3.1. Characterization of catalysts

AÞ  Ael Cel  Vel þ  100% Ael m

ð1Þ

P where C is FAME yield, A is the total peak area from the conversion, Ael is the peak area of internal standard, Cel is the concentration of the internal standard solution, Vel is the volume of the internal standard solution and m is the mass of sample. 2.4. Experimental design and optimization In this study, statistical analysis of FAME yield was performed using Statsoft Statistica 7.0 software. The face-centered central composite design (FCCCD) was used to study the interaction of process variables and to predict the optimum process condition for FAME yield by applying RSM. Independent variables considered important were reaction temperature (X1), reaction time (X2), catalyst dosage (X3), and methanol to oil ratio (X4). The range and coded level of the FAME yield process variable studied are listed in Table 2. The independent variables were coded to (1, 1) interval where the low and high levels were coded as 1 and +1, respectively. According to FCCCD, the total number of experiments conducted is 30 with 24 factorial points, 8 axial points and 2 replicates at the center points. The FAME yield was taken as the response of the design experiment. The experimental design and corresponding results of the responses are listed in Table 3. The full quadratic model for FAME yield is given as the following Eq. (2):

Y i ¼ bo þ b1 X 1 þ b2 X 2 þ b3 X 3 þ b4 X 4 þ b12 X 1 X 2 þ b13 X 1 X 3 þ b14 X 1 X 4 þ b23 X 2 X 3 þ b24 X 2 X 4 þ b34 X 3 X 4 þ b11 X 21 þ b22 X 22 þ b33 X 23 þ b44 X 24

ð2Þ

where Yi is the predicted response i whilst X1, X2, X3, and X4 are the coded form of independent variables. The terms bo is the offset term; b1, b2, b3, and b4 are the linear terms; b11, b22, b33, and b44 are the quadratic terms; and b12, b13, b14, b23, b24, b34 are the interaction terms. The equation model was tested with the analysis of variance (ANOVA) with 5% level of significant. The ANOVA was used to check significance of the second order models and it is determined by F-value. Generally, the calculated F-value should be greater than tabulated F-value to reject the null hypothesis, where all the regression coefficients are zero. The calculated F-value is defined as the following Eq. (3):

F  v alue ¼

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

MSSSR MSSSE

ð3Þ

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

Fig. 1A shows the small-angle XRD patterns for SMP and WO3/ SMP catalysts. SMP showed a significant peak at 2h = 2.1°, which indexed as (1 0 0), indicating the presence of ordered mesoporous silica [15]. This is similar to the result reported by Zhang et al. in the synthesis of mesoporous silica nanoparticles where they use of acetone as surfactant to controls the morphology of the silica structural ordering [15]. Their results revealed the synthesis of silica mesoporous-macroparticles using acetone led to a disordered of mesostructure of silica particles. In the present work, the introduction of WO3 with the loading amount of 1, 2, 3.5 and 5 wt% WO3 onto SMP was significantly decreased and eliminated the (1 0 0) peak. This could be attributed to the highly dispersed of WO3 on the SMP which degraded the structural order of silica mesoporous-macroparticles. Similar observation was reported by Cecilia et al. that show the incorporation of WO3 into mesoporous SBA-15 framework destructed the order of the mesoporous structure [19]. The presence of WO3 on the SMP support was demonstrated in the wide-angle XRD (20–40°) in Fig. 1B. The diffraction peaks at 2h = 23.06°, 23.13° and 26.91° are corresponded to the monoclinic phase (0 0 2), (0 2 0) and (2 0 0) of WO3, respectively. These peaks were more intense with increasing WO3 loading from 1 to 2 wt% corresponding to increase in the monoclinic phase of WO3 on the SMP support. While, the peak intensities are almost constant for loading of 2–5 wt%. These results demonstrated that the interaction between WO3 on the SMP is very strong due to the formation of crystalline WO3 in the support. Fig. 2 shows the SEM images of SMP, WO3 and WO3/SMP catalysts. The SEM image of SMP showed the spherical shape of silica particles, while the TEM image showed the solid spheres. These results are in good agreement with the XRD result. Besides, the SEM image of SMP showed non-uniform size of particle due to the effect of acetone that controls the morphology of SMP during the synthesis. Fig. 2B shows the monoclinic structure of WO3 that is in accord with the XRD result. The SEM image of 2WO3/SMP showed the spherical shape of silica particle and metal dispersion on the support. There was a small change in the surface morphology in the SEM image of 2WO3/SMP which showed two shape types which are spherical and monoclinic structure. It indicated that there is metal which well dispersed on support caused a changing in the structure, while another metal destructed the structure of the support. Fig. 3 shows the N2 adsorption desorption isotherms and pore size distribution of SMP and WO3/SMP catalysts with different loading amount of WO3. The results were analyzed by applying non-local density functional theory (NLDFT) method. The NLDFT method is applicable to micro-mesoporous materials [20]. It allows calculating the specific cumulative surface area (i.e., specific sur-

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

Reaction temperature Reaction time Catalyst dosage Methanol to oil Ratio

Symbol

X1 X2 X3 X4

Unit

K Min % mol/mol

Coded level 1

0

+1

333 45 3 4

348 68 4.5 7

363 90 6 10

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M.A.A. Aziz et al. / Chemical Engineering Journal 316 (2017) 882–892 Table 3 Experimental 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 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

11 9 5 20 13 4 2 18 3 14 8 1 15 17 7 19 6 16 10 12 27 31 26 22 28 23 25 21 30 24

333 363 333 363 333 363 333 363 333 363 333 363 333 363 333 363 348 348 348 348 333 363 348 348 348 348 348 348 348 348

45 45 90 90 45 45 90 90 45 45 90 90 45 45 90 90 68 68 68 68 68 68 45 90 68 68 68 68 68 68

3 3 3 3 6 6 6 6 3 3 3 3 6 6 6 6 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 3 6 4.5 4.5 4.5 4.5

4 4 4 4 4 4 4 4 10 10 10 10 10 10 10 10 7 7 7 7 7 7 7 7 7 7 4 10 7 7

50 62 54 50 46 54 46 54 67 44 61 38 68 48 57 40 90 90 92 88 79 61 90 81 60 58 68 80 82 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.

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

[22–24]. These results confirmed the permanence of the mesoporous phase in parallel with the microporous phase in the WO3/ SMP. Besides, it is noteworthy that the second step at higher partial pressure was only observed after the introduction of WO3 on SMP. By contrast, the decrease of the step at high partial pressure of 3.5WO3/SMP could be attributed to the fact that the WO3 particles blocked the interparticle pores of SMP. The pore size distribution of SMP showed a narrow pore size which in the range of 3–6 nm. For WO3/SMP, the mesoporosities were increased with increasing amounts of WO3 loading, indicating an improvement in the WO3 interaction between the possible formations of monoclinic on the solid sphere of SMP. The summary data on surface areas and total pore volumes of all catalysts are listed in Table 4. In all cases, it can be seen that the surface area and total pore volume decreased considerably after the introduction of WO3, suggesting that a portion of the WO3 particles were dispersed in the pores of the supports and/or partially blocking the access to the mesopores of SMP. In accord to this, Yang et al. reported WO3-containing hexagonal mesoporous silica (W-HMS) which gave a lower surface area compared to the parent HMS due to the tungsten heteroatoms were embedded into the lattice of the HMS bulk [25]. While, the pore size of the catalysts were decreased as increasing the WO3 loading. This is may be due to the generation of bigger pore from the agglomeration of WO3 particles near to the mouth of the SMP pores. FTIR spectra of SMP, WO3 and WO3/SMP catalysts were shown in Fig. 4. For SMP WO3/SMP catalysts, a broad band was observed around 1371 cm1 which corresponding to the asymmetric of SiAOASi. Besides, two bands at 794 and 458 cm1 are attributed to the symmetric stretching and bending vibration of SiAOASi, respectively. The broad absorption band at 3455 cm1 is related to the SiAOH on the surface, which provides opportunities of forming hydrogen bond. For bare WO3 catalyst (Fig. 4f), typical bands were observed at 879 and 755 cm1 which corresponding to the (W@O) and stretching vibrations of (WAOAW), respectively [26].

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A

100 nm

50 nm

B

10 µm

50 nm

C

100 nm

50 nm

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

The absorption band at 794 cm1 has been widely used to characterize the incorporation of transition metal atoms in the silica framework. FTIR spectra of WO3/SMP with different weight loading showed no additional band upon introduction of WO3 on the SMP. However, the stretching vibration of SiAO at 794 cm1 is affected by the neighboring metal atom of W@O and WAOAW which caused the peak intensified. This indicated the incorporation of the WO3 into the silica framework of SMP. Jeroen et al. also observed the appearance of the peak of W@O and WAOAW after introduction of WO3 on the support sample [27]. Pyridine was used as probe molecule to evaluate the acidity of catalysts, particularly in the observation of the Brønsted and the Lewis acid sites [28]. Fig. 5A shows the Fourier transform infrared (FTIR) spectra of pyridine adsorbed on SMP and WO3/SMP catalysts. For SMP, only one band was observed at 1447 cm1, which are ascribed to pyridine coordinately bonded to weak surface Lewis acid site [28]. For WO3/SMP, two significant absorbance bands arose at around 1450 and 1542 cm1, which are ascribed to the pyridine species adsorbed on Lewis and Brønsted acid sites respectively. The introduction of WO3 on the SMP substantially generated Brønsted acid sites and intensified Lewis acid sites. The introduction of WO3 on the SMP shifted the band ascribed to the Lewis acid sites at 1447 cm1 to a higher frequency at 1450 cm1, suggesting that 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 cm1. In general, the peak areas of Lewis acid sites increased as increasing of WO3 loading on SMP. While, the peak areas of Brønsted acid sites showed no trend as increasing of the WO3 loading. These results indicated that the incorporation of WO3 into SMP framework increased and generated 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, while 3.5WO3/SMP possessed the highest Brønsted acid sites which may be due to the higher agglomeration and the presence of bulk WO3 on the SMP surface. Whereas, bare SMP possessed only Lewis acid sites which corresponding to the presence of electron pair acceptor sites from SiAOH groups. 3.2. Catalytic performance The catalytic performance of all catalysts for biodiesel production from transesterification of croton megalocarpus oil with methanol is shown in Fig. 6. Fig. 6A shows the production of FAME as a function of time over WO3/SMP catalysts at 338 K with methanol to oil molar ratio of 7. All catalysts showed a similar trend which fast reaction rate was observed at and below 30 min and it started to slow after 30 min of reaction time. There is no much different of FAME yield after 60 min of reaction time indicating the catalysts were completely deactivated. In general, 2WO3/SMP exhibited the highest activity followed by 5WO3/SMP, 3.5WO3/ SMO and 1WO3/SMP. Fig. 6B shows the comparison of FAME yield against WO3 loading at reaction time of 60 min. The bare SMP support was not very active for the reaction as only 20% of FAME yield was obtained. The addition of WO3 increased the activity of SMP which may be corresponded well with the acid strength of the catalysts. 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 FAME yield product which may be due to the altering of the catalysts structure or decreasing the acidity of the catalysts. However, it was noteworthy that the FAME yield of bare WO3 catalyst was considerably higher than that of the 1WO3/SMP catalyst which may be due to the high acidity properties of the WO3 metal.

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

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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 4 Physical properties of the catalysts. Catalyst

BET surface area (m2/g)

Total pore volume (cm3/g)

Average pore size (nm)

SMP 1WO3/SMP 2WO3/SMP 3.5WO3/SMP 5WO3/SMP

579 495 384 320 229

0.4249 0.3319 0.3051 0.2615 0.1904

2.94 2.68 3.18 3.27 3.32

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

megalocarpus oil. The ratio of FAME yield to fraction of Lewis acid sites did not change much for the WO3 loading below 5 wt% indicating that the activity of the catalysts is clearly depend on the fraction of Lewis acid sites. While the dependence activity of WO3/SMP catalyst on the surface area and fraction of Brønsted acid sites was not observed for any range of the WO3 loading in this expriment. In addition to the Lewis acid sites, the presence of intra- and interparticle porosity may be determined the catalytic performance of WO3/SMP. Lewis acid sites are vital for the catalytic activity due to the lack of electrons and can activate substrates which are rich in electrons such as carbonyl group in the reaction. Frequently, Lewis acid-base adducts is the key intermediates in the acid-catalyzed reaction [30]. Whereas, the presence of both type

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Brønsted acid sites [28]. Therefore, a mechanism of croton megalocarpus oil transesterification catalyzed by WO3/SMP was proposed as shown in Fig. 7. The transesterification proceeds via several consecutive steps. The first step started with the adsorption of methanol (CH3OH) and oil (triglyceride) onto the surface of WO3/SMP. This step may be accelerated by strong stirring conditions in the reactor. Triglyceride is adsorbed on the W6+ surface site which acts as Lewis acid sites. The methanol is adsorbed on the lattice oxygen atom (Lewis basic sites) on the surface of the catalyst and forms oxygen anion. The protonation of carbonyl group of the ester leads to the carbocation. The nucleophilic attack of alcohol to the carbocation produces a tetrahedral intermediate. This intermediate eliminates glycerol to form a new ester and to regenerate the catalyst.

794 1371

(a)

458

3455

Transmittance [%]

(b) (c) (d) (e)

(f) 879

4000

3100

2200

1300

755

400

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

of pores facilitated and enhanced the transport of reactants and products during the reaction as well as diminishing of the diffusion limitation. This may be increased the reaction rate of conversion of triglyceride to the desired products of FAME. 3.3. Proposed mechanism of transesterification process

RSM is a method to determine the optimum condition of the process, and it allows users to gather large amount of information from a small number of experiments. It is also possible to observe the relationships between variables and responses, and has been successfully applied for a wide range of chemical reactions involves more than one response [31]. Based on the RSM analysis, the quadratic model for FAME yield is presented in Eq. (4) as follows:

Y ¼ 83:69  9X 1  4:5X 2  1X 3 þ 6X 4  0:8125X 1 X 2 þ 1:0625X 1 X 3  6:6875X 1 X 4 þ 0:0625X 2 X 3  1:4375X 2 X

The reactions of transesterification are based on the Lewis and Brønsted acid site of the catalyst. The formation of Lewis and Brønsted acid sites over the WO3/SMP were confirmed by pyridine adsorbed FTIR spectroscopy. This result indicated that the incorporation of WO3 into SMP framework has intensified the Lewis and

þ 1:1875X 3 X 4  11:5354X 21 þ 3:9645X 22  22:5354X 23  7:5354X 24

ð4Þ

Bronsted Lewis

A 0.2

B 3.44 3.24

(e) (d) (c)

Deconvulation area

Absorbance

3.06

2.39

1.09

1.05 0.99 0.78

(b)

0.63

(a) 0

1570

1520 1470 1420 Wavenumber [cm -1]

Fig. 5. (A)IR spectra pyridine adsorbed on (a) SMP, (b) 1WO3/SMP, (c) 2WO3/SMP, (d) 3.5WO3/SMP and (e) 5WO3/SMP, after activated at 423 K (B) show the Gaussion deconvulation area of the catalysts.

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

889

Fig. 6. (A) FAME yield of transesterification reaction of croton megalocarpus 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 [102%/(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.

The coefficient of determination, (R2) for FAME yield optimization using RSM was 0.9972, indicating 99.72% of the variability in the data is accounted by the model from Eq. (4). According to Haaland [32], the empirical model is adequate to explain most the variability in the asset reading which should be at least 0.75 or greater. Table 5 shows the analysis of variance (ANOVA) of the regression parameters 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 shows the t-distribution values in a Pareto chart and the corresponding p-value of the variables in Eq. (4). The p-value serves as a tool to check the significance of each coefficient. The corresponding coefficient with a smaller p-value or a t-value with a greater magnitude donates more significance into the model. The largest effect of FAME yield is the quadratic term of catalyst dosage (X23), which has the smallest p-value (0.0000) and the largest t-value (473.6837) at the 95% significance level. In addition, the linear

890

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Table 5 ANOVA 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

X23

1289.822 715.5625 337.9600 162 6907.533 19.05347 6926.586

1 1 1 1 22 7 29

1289.8215 715.5625 337.9600 162 313.9788 2.7219 –

473.8637 262.8884 124.1621 59.51671 115.3517 – –

0.00001 0.00001 0.00001 0.0001 – – –

X1X4 X21 X1 Regression Residual Total

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

term for the reaction temperature and molar ratio (X1X4), reaction temperature (X1), and the quadratic term of reaction temperature (X21), could be regarded as significant factors in affecting the FAME yield, owing to the large t-value of 262.8884, 124.1621and 115.3517, respectively. The rest of factors could be considered less significant in affecting the FAME yield, as their p = values were higher than 0.05. The response surfaces and contour plot are generally used to evaluate the relationships between parameters and to predict the result under given conditions. However, it is complicated to analyze the interaction between parameters in this study, owing to the presence of many interaction terms. Instead, the response surfaces and contour plot were used for optimizing the conditions of the FAME yield over 2WO3/SMP. There are 4 parameters that had been used and 6 RSM 3-D plots were constructed for the FAME yield. They were plotted as a function of two of the factors while the others were maintained constant at their mean levels. The interaction between the corresponding variables was negligible when the contour of response surface was circular. On the contrary, the interactions between the relevant variables were significant when the contour of response surface was elliptical. It is interesting to note that all the contour plots in Fig. 9 were elliptical indicating the significant interaction effects between the parameters studied. Fig. 9A shows the response surface plot, demonstrating the effects of the reaction temperature and catalyst dosage on FAME yield. From the analysis of the response surface plot, the reaction temperature exhibited a more significant influence on the response surface in comparison to catalyst dosage, which also can be explained by the Pareto chart (Fig. 8) showing larger t-value of reaction temperature (59.5167) as compared to the catalyst dosage (0.7348). An increase in the reaction temperature resulted

to an increase in the FAME yield, passing through a maximum around 350 K and decrease at higher temperature. This could be attributed to the increase in the number of catalytic active sites which accelerated the forward reaction in specific temperature and decrease at higher temperature. This behavior could be due to the state of activation of these catalysts at certain temperature [33]. Fig. 9B represents the effect of reaction time and catalyst dosage on the FAME yield. The FAME yield was significantly affected by reaction time. The significant effect of reaction time on the FAME yield can be explained by the larger t-value of reaction time (14.8792) as compared to the reaction catalyst (0.7348). At a constant catalyst dosage 4.5 wt%, it was clear that increases of the reaction time evidently increase in the FAME yield, reach the maximum around 58–62%. At this condition, the catalyst lowered the activation energy, which thus promotes transesterification toward the products, and increasing of the catalyst dosage implies more active sites for catalyzing of the reaction [34]. However, at longer reaction time and larger amount of the catalyst dosage led to a slightly decrease in the FAME yield. This could be due to the reversible reaction whereby the larger amount of catalysts contributed to the larger surface area that allows rapid reversible reaction of the FAME back to the triglyceride, resulted in the lowering of conversion. This result showed a similar trend with that of using sea mango oil as a reactant [35]. In addition, exceeding catalyst dosage would increase the viscosity of the reaction mixture and interfere in the mass transfer between the catalyst and reactants [36]. The effects of molar ratio and catalyst dosage on the FAME yield are shown in Fig. 9C. The result indicated that the increment of molar ratio slightly affected in the FAME yield. In addition, the effect of different molar ratios of methanol to Croton Megalocarpus oil was studied. The FAME yields significantly increase with increasing catalyst dosage and slightly decrease after reached the maximum. It shows that the catalyst achieved maximum activated at the catalyst dosage of 4.5 wt%. The result obviously showed that the reaction had an optimum molar ratio of oil to methanol to obtain a higher FAME production. The conversion of oil was enhanced with an increasing amount of the methanol, because the excess amount of the methanol provided more opportunity for the reactant molecules to collide and then shifted the reaction equilibrium towards production of the biodiesel. However, the decrease of conversion was observed when the catalyst dosage exceeded 5 wt% .This result might be attributed to the saturation of the catalytic surface with the methanol at the high concentration of the methanol, which deactivated the catalyst whereby the transesterification process was inhibited [37]. Fig. 9D represents the effect of reaction temperature and molar ratio on the FAME yield. From the analysis of the response surface plot, reaction temperature exhibited more significant influence on the response surface in comparison to the molar ratio, which also can be explained by the Pareto chart (Fig. 8) showing larger tvalue of reaction temperature (59.9831) as compared to the molar ratio (26.4519). An increase in the reaction temperature resulted to a decrease in the FAME yield. This is because the tem-

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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.

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

in Fig 9F. The result indicated that the interactions of the time and reaction temperature are not significant. Form the analysis of the response surface plot, all of the parameters studied were found affected to the FAME yield. However, it has appeared that the reaction temperature is the dominant factor in the FAME yield percentage. The reaction temperature exhibited a significance influence on the response surface in comparison to others parameters, which can be clarified by larger t-value (59.9831) from the pareto chart. This is may be due to the effect of high reaction temperature which could rapidly break the structure of triglyceride molecules. The optimum FAME yield predicted

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

[13]

4. Conclusion

[14]

Investigation on WO3/SMP with various WO3 loading is carried out for the transesterification of Croton megalocarpus oil which is a nonedible feedstock. WO3 loading on SMP give a negative effect on the 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) gave highest intensity of Lewis acid sites which is vital in transesterification reaction. Under the optimum reaction condition determined through response surface methodology (RSM), 2 wt% WO3 loading, 4.5 wt% catalyst amount, 9:1 methanol to oil ratio, 45 min reaction time and 343 K reaction temperature gave a 96% of biodiesel conversion. High catalytic activity of 2WO3/SMP is may be attributed to the high Lewis acid sites content and the presence of both intra- and interparticle pores of the catalyst that facilitated and enhanced the transport of reactants and products during the reaction.

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