Esterification of Free Fatty Acids in Crude Palm Oil

International Journal of Chemical Reactor Engineering 2014; 12(1): 1–12

Kamchai Nuithitikul* and Worrapat Hasin

Esterification of Free Fatty Acids in Crude Palm Oil Using Sulfated Cobalt–Tin Mixed Oxide Catalysts Abstract: In this study, sulfated tin oxide was modified with cobalt oxide resulting in sulfated cobalt–tin mixed oxide (SO42–/Co2O3–SnO2). For the first time, the catalytic activity of SO42–/Co2O3–SnO2 for the esterification reaction of free fatty acids (FFA) in crude palm oil to produce methyl esters has been investigated. The effects of amount of Co and calcination temperature were studied. The properties of SO42–/Co2O3–SnO2 were determined with N2 adsorption, X-ray diffraction and X-ray fluorescence analysis, Fourier transform infrared spectroscopy, thermogravimetric analysis and potentiometric titration. The esterification was carried out in a stirred-tank reactor equipped with a reflux condenser. The reaction conditions (methanol/oil ratio, catalyst size, catalyst loading and reaction time) were optimized. The results confirm that SO42–/Co2O3–SnO2 is a promising catalyst in the production of methyl esters from FFA in crude palm oil. The addition of Co improved the reusability of sulfated tin oxide. Keywords: sulfated cobalt–tin mixed oxide, biodiesel, esterification, free fatty acid, crude palm oil DOI 10.1515/ijcre-2013-0146

1 Introduction Biodiesel is a renewable energy that gains a lot of attentions from both academics and industries because of the continuously decreased amount of limited oil reserves and the growing environmental concerns. The World Energy Forum has predicted that oil reserves will be exhausted in less than another ten decades [1]. The use of biodiesel can significantly reduce the emission of carbon monoxide, carbon dioxide and smoke. Since the transportation sector accounts for about 30% of the

*Corresponding author: Kamchai Nuithitikul, Department of Chemical and Process Engineering, School of Engineering and Resources, Walailak University, Nakhonsithammarat 80161, Thailand, E-mail: [email protected]; [email protected] Worrapat Hasin, Department of Chemical Engineering, King Mongkut’s University of Technology North Bangkok, Bangkok, 10800, Thailand, E-mail: [email protected]

world’s total energy consumption [2], the use of biodiesel as a fuel in diesel engines has become more important. The main disadvantage regarding biodiesel production is the high cost of oil feedstocks [3]. Therefore, low-quality oils containing large amounts of free fatty acids (FFA) such as crude palm oil are preferred for the production. However, biodiesel production from lowquality oils needs two consequent processes (esterification and transesterification, respectively) in order to avoid soap formation and obtain enough yield of biodiesel. In the esterification process, an acid catalyst is required to convert FFA to methyl esters. Typically, amount of FFA must be reduced to be less than 1 wt% or 2 mg KOH/g [4–7] before the transesterification process with a homogeneous base catalyst (e.g. KOH) can take place efficiently. Therefore, the search for suitable solid acid catalysts used in the esterification process is carried out. This not only helps prevent soap formation but also simplifies the biodiesel production process since heterogeneous catalysts are easily separated and recycled. Solid acid catalysts are less hazardous and corrosive than homogeneous acid catalysts such as H2SO4. It can be said that acid heterogeneous catalysts are an alternative group to minimize environmental damage and reduce biodiesel cost [8]. With respect to H2SO4, the use of solid acid catalysts eliminates the wastewater disposal issue and they can be recovered. Sulfated metal oxides are known to possess mediumto-strong acid sites [9], be stable to air and heat [10] and widely used in a number of important industrial processes [11]. Compared to current liquid acids and halogen-based solid acids, sulfated metal oxides are more friendly to the environment. With the greater requirement of renewable energy and awareness of environmental protection, sulfated metal oxides are therefore promising catalysts in biodiesel production. However, sulfated metal oxides are reported to be unstable to moisture. Leaching of sulfates can occur in water, leading to the deactivation of sulfated metal oxides [12]. In the family of sulfated metal oxides, sulfated tin oxide (SO42–/SnO2) is an interesting catalyst. It has stronger acidity than sulfated zirconia [13–15], which has been reported to possess the highest acid strength [16]. Unlike sulfated zirconia, sulfated tin oxide is not commercially

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K. Nuithitikul and W. Hasin: Esterification of Free Fatty Acids by SO42–/Co2O3–SnO2

available [17]. Moreover, the synthesis cost of tin oxide is cheaper than that of zirconium oxide. Therefore, sulfated tin oxide is an interesting candidate for the esterification of FFA. According to Lam et al. [14], studies concerning the usage of sulfated tin oxide catalysts in biodiesel production are very limited. Furuta et al. [18] have shown the high activity of sulfated tin oxide in the esterification of n-octanoic acid with methanol in a fixed-bed reactor. However, high reaction temperature (>100°C) and vaporized methanol were required. Similarly, Khder et al. [19] have reported the esterification of acetic acid with amyl alcohol in vapor phase at 150– 250°C in a fixed-bed reactor containing sulfated tin oxide catalyst. The yield of amyl acetate was found to depend on the preparation conditions (i.e. calcination temperature) of the sulfated tin oxide catalyst. Finally, the esterification reaction between oleic acid and ethanol was carried out under reflux at 80°C in the presence of sulfated tin oxide catalyst [20]. Its catalytic activity was determined by the preparation conditions (amount of incorporated sulfates and calcination temperature). Recently, the activity of sulfated tin oxide doped with aluminum oxide for the esterification of FFA was studied [21]. In this study, sulfated tin oxide was modified with cobalt oxide resulting in sulfated cobalt–tin mixed oxide (SO42–/Co2O3–SnO2) catalyst. The activity of this catalyst in esterification of FFA in crude palm oil was investigated for the first time. Cobalt–tin mixed oxides were prepared with co-precipitation method by which the amounts of Co were varied. The mixed oxides were further impregnated with sulfate ions, and the influence of calcination temperature was investigated. The activities of sulfated cobalt–tin mixed oxides were tested in the esterification reaction of FFA in a practical oil (crude palm oil), rather than model acids. Palm oil is an abundant resource in Southeast Asian countries; therefore, it is an economic and commercially feasible feedstock for biodiesel production. In the esterification study, the reaction variables such as methanol/oil ratio, catalyst size, catalyst loading and reaction time were optimized.

2 Materials and methods 2.1 Catalyst preparation Sulfated cobalt–tin mixed oxide (SO42–/Co2O3–SnO2) was prepared by adjusting a two-step route proposed

by Furuta et al. [18]. In the first step, the mixture of cobalt and tin hydroxides was synthesized with co-precipitation method. Ten grams of SnCl4·5H2O (98% purity supplied by Acros Organics) and the required amounts of CoCl2·6H2O (98% purity supplied by QREC) which corresponded to 2, 4, 8 and 10 mol% Co/(Sn þ Co) were dissolved in 300 mL of distilled water. Thirty-three percent of ammonia solution was added to the mixed solution with stirring until the pH was 8. The precipitated product was separated, suspended in 4 wt% CH3COONH4, filtered and dried at 105°C for 24 h, yielding Co(OH)3–Sn(OH)4. In the second step, 2 g of the mixture of cobalt and tin hydroxides was immersed in 30 mL of H2SO4 solution (1 mol/L) which corresponded to 0.03 mol of sulfate. The gel was stirred for 1 h, filtered, dried at 105°C for 2 h and calcined at different temperatures (300–600°C) for 3 h. The obtained catalysts were denoted as SO42–/ Co2O3–SnO2 (x%), where the number x represented the loaded amount of Co. For comparison, sulfated tin oxide (SO42–/SnO2) was prepared in the same way without the addition of CoCl2·6H2O.

2.2 Catalyst characterization The BET specific surface areas, pores volumes and average pores sizes of SO42–/Co2O3–SnO2 catalysts were determined by N2 adsorption at –196°C with Quantachrome Autosorb-1. The crystallinity of the catalysts was determined by powder X-ray diffraction (XRD) with JEOL JDX 3530. The samples were recorded in the 2θ range of 5–80° with a step time of 1 s and a step size of 0.02° (2θ). The quantitative analysis of cobalt oxide in SO42–/Co2O3–SnO2 catalysts was performed with X-ray fluorescence (XRF) using Philips PW2400 model. The acidity of sulfated cobalt–tin mixed oxide catalysts was determined by non-aqueous potentiometric titration [19]. The samples (0.05 g) were stirred in 10 mL of acetonitrile for 3 h. The suspension then was titrated with n-butylamine in acetonitrile (0.1 N) at 0.05 mL/ min. The electrode potential variation was measured with Orion 420A using a non-aqueous electrode. Lewis and Brönsted acid sites of SO42–/Co2O3–SnO2 catalysts were determined by Fourier Transform Infrared (FTIR) spectroscopy (Nexus 670) using pyridine as a probe molecule. Thermogravimetric analysis (TGA) was conducted on a Mettler-Toledo thermogravimetric analyzer (TGA/DSC 1). The samples were heated from 40 to 1,000°C at a heating rate of 10°C/min under N2 atmosphere.



2.3 Catalytic test for the esterification of FFA The esterification of FFA in crude palm oil to produce methyl esters was performed in a stirred-tank reactor (a glass round bottom flask) equipped with a reflux condenser. The flask was immersed in a water bath which was heated and maintained at 80°C. Twentyfive milliliters of crude palm oil (10 wt% of FFA) was added into the flask, followed by a mixture of methanol (15–60 mL) and prepared SO42–/Co2O3–SnO2 (0.25–2 g). The reaction was initiated by stirring (500 rpm) and stopped after a specific reaction time. The product mixture was filtered, washed with 100 mL of water for at least three times and evaporated. The FFA contents in crude palm oil before and after the reaction were determined by titration (ASTM D 5555). The conversion of FFA (XFFA) was defined as:

XFFA ð%Þ ¼

½FFA0 ½FFAt 100 ½FFA0

ð1Þ

[FFA]0 and [FFA]t were the FFA contents in crude palm oil before and after the reaction, respectively. After the catalytic test of SO42–/Co2O3–SnO2 synthesized from different Co loadings and calcination temperatures, the best catalyst was then used in the optimum study of the reaction variables including methanol/oil ratio, catalyst size, catalyst loading and reaction time.

3 Results and discussion 3.1 Properties of cobalt–tin mixed oxides The XRD patterns of SO42–/Co2O3–SnO2 (4% and 10% Co) calcined at 450°C compared with SO42–/SnO2 calcined at the same temperature are shown in Figure 1. All samples were found to exhibit three well-distinguished peaks at 27, 34 and 52°. These peaks indicate the presence of tetragonal phase of SnO2 crystallites. Sulfate treatment on tin oxide has been reported to stabilize the tetragonal structure [22]. Moreno et al. [20] have observed similar XRD patterns of sulfated tin oxides. It is important to note that all the SO42–/Co2O3–SnO2 samples did not show any diffraction peak of cobalt oxides, and their XRD patterns were indifferent with the SO42–/SnO2 sample. These imply that there is no phase change when sulfated tin oxide is modified with Co. It is likely that atoms of Co are incorporated into the vacancy sites in the structure of SnO2. Toledo-Antonio et al. [23] reported that SnO2 prepared at high calcination temperature (i.e. 500°C) still contained hydroxyls in its structure. The hydroxyls caused a structure poorly crystallized with considerable amount of tin vacancy sites. Our result of the XRF analysis evidenced the presence of Co2O3 but in very small amount in comparison to SnO2: the SO42–/Co2O3–SnO2 (2%) sample consisted of 97.3% SnO2, 0.8% Co2O3 and 1.0% SO3. Table 1 provides the BET surface areas, pores volumes and mean pores diameters of SO42–/Co2O3–

Intensity

(c)

(b)

(a)

0

10

20

30

40

50

60

3

70

80

2θ (°) Figure 1 XRD patterns of (a) SO42–/SnO2; (b) SO42–/Co2O3–SnO2 (4%) and (c) SO42–/Co2O3–SnO2 (10%)


4


Table 1

Properties of SO42–/Co2O3–SnO2 and SO42–/SnO2 calcined at 450°C

Sample

BET surface area (m2/g)

Pore volume (cm3/g)

Mean pore size (nm)

Ei (mV)

Number of acid sites (meq./g)

101.1 102.2 103.1 103.4

0.073 0.078 0.081 0.085

3.4 3.3 3.1 3.0

þ 587 þ 503 þ 492 þ 486

1.52 0.80 0.69 0.60

SO42–/SnO2 SO42–/Co2O3–SnO2 (2%) SO42–/Co2O3–SnO2 (4%) SO42–/Co2O3–SnO2 (10%)

SnO2 samples calcined at 450°C. It was found that all the synthesized SO42–/Co2O3–SnO2 had mesoporous structure (pore size >2 nm). Mesopores are necessary for the diffusion of large sized molecules of FFA prior to the adsorption and surface reaction to take place. With the increased amount of Co loaded to SO42–/ SnO2, the BET surface area and pore volume were found to slightly increase whereas the average pore size slightly decreased. Therefore, for SO42–/Co2O3– SnO2, the addition of Co has only marginal effects in the BET surface area, pore volume and pore diameter. This is in agreement with Khalaf et al. [24] who have observed a slight impact of Cu addition on the specific surface area of Cu-modified SnO2. The addition of Co to sulfated tin oxide caused decreases in the acid strength and, more significantly, the number of acid sites. The SO42–/Co2O3–SnO2 (2%) sample had Ei ¼ þ 503 mV and the number of acid sites ¼ 0.80 meq./g, compared to the SO42–/SnO2 sample which had Ei ¼ þ 587 mV and the number of acid sites ¼ 1.52 meq./g. With the greater loaded amount of

Co, both the acid strength and the number of acid sites decreased. However, the synthesized SO42–/Co2O3–SnO2 had very strong acid sites based on the classification of the strength of the acid sites: very strong when Ei > 100 mV, strong when 0 < Ei < 100 mV, weak when –100 < Ei < 0 mV and very weak when Ei < –100 mV [19]. The very strong acid sites of SO42–/Co2O3–SnO2 are responsible for their catalytic activities in the esterification reaction of FFA. Figure 2 provides the FTIR spectra of pyridine adsorbed on SO42–/Co2O3–SnO2 (2%) and SO42–/SnO2 samples. The SO42–/Co2O3–SnO2 sample possessed both Lewis and Brönsted acid sites. The peaks at 1,444 and 1,540 cm−1 were assigned to pyridine adsorbed on Lewis and Brönsted acid sites, respectively. The lower intensity of the peak at 1,540 cm−1 indicated weak Brönsted acid sites in comparison to Lewis acid sites. This is due to the presence of S ¼ O covalent bonds that act as electron-withdrawing species, leading to stronger Lewis acid strength of Sn4 þ [18, 19]. The peak at 1,479 cm−1 was assigned to

Transmittance (a.u.)

(a)

(b)

Lewis

1,300

1,350

1,400

1,450

Brönsted Brönsted and Lewis 1,500

1,550

Wavenumber (cm–1) Figure 2 Pyridine-adsorbed FTIR spectra of (a) SO42–/SnO2 and (b) SO42–/Co2O3–SnO2 (2%)



range of 40–200°C was mainly attributed to the physisorbed water. Interpreted from the ratio of weight loss, the introduction of Co into sulfated tin oxide increased amount of physisorbed water. The weight losses of the SO42–/SnO2, SO42–/Co2O3–SnO2 (4%) and SO42–/Co2O3– SnO2 (10%) samples were 5.1, 5.6 and 7.2%, respectively. At the higher temperature range of 600–800°C, the weight loss was mainly due to the decomposition of the strongly attached-on-surface sulfate complexes. The weight losses of the SO42–/SnO2, SO42–/Co2O3–SnO2 (4%) and SO42–/Co2O3–SnO2 (10%) samples during 600–800°C were similar: 3.0, 3.5 and 3.3%, respectively.

pyridine adsorbed on both Lewis and Brönsted acid sites. In relative to the SO42–/SnO2 sample, the weaker absorption bands of the SO42–/Co2O3–SnO2 sample at 1,444 and 1,479 cm−1 indicated the smaller number of Lewis acid sites. This corresponded to the acidity measurement results obtained by non-aqueous potentiometric titration (see Table 1). The TGA for SO42–/SnO2, SO42–/Co2O3–SnO2 (4%) and SO42–/Co2O3–SnO2 (10%) samples was carried out. Figure 3(a) shows the weight losses of the three samples, which are observed as peaks on the DTG profile in Figure 3(b). All samples similarly showed two distinct regions of weight loss. The first weight loss in the

(a) 102 100 98

TGA (%)

96 94 92 90

I II

88

III 86 0

200

400

600

800

1,000

T (°C)

(b) I

DTG (a.u.)

II

III 0

200

400

600

800

5

1,000

T (°C) Figure 3 (a) TGA and (b) DTG profiles of (I) SO42–/SnO2; (II) SO42–/Co2O3–SnO2 (4%) and (III) SO42–/Co2O3–SnO2 (10%)



6

3.2 Catalytic activity of sulfated cobalt–tin mixed oxides

attack of alcohol molecule to give the corresponding ester. Both Lewis and Brönsted acid sites appeared in the structure of SO42–/SnO2 [25].

3.2.1 Effect of loaded amount of Co The activities of SO42–/Co2O3–SnO2 catalysts at different amounts of Co for the esterification of FFA in crude palm oil are shown in Figure 4. The conversion of FFA was found to drop dramatically when 2% Co was added to SO42–/SnO2 catalyst. This was strongly related to a significant drop in the number of acid sites of SO42–/Co2O3– SnO2 (2%) in comparison to SO42–/SnO2 (see Table 1). When the greater amount of Co was added (from 2 to 10%), the conversion of FFA further decreased but with a lesser extent. The decrease in the conversion corresponded to the decreases in both the acid strength and the number of acid sites of the SO42–/Co2O3–SnO2 catalysts. As previously reported in Table 1, the number of acid sites and acid strength of the SO42–/Co2O3–SnO2 catalysts were in the following order: SO42–/Co2O3–SnO2 (2%) > SO42–/Co2O3–SnO2 (4%) > SO42–/Co2O3–SnO2 (10%). However, the catalytic activity of SO42–/Co2O3– SnO2 was more related to the number of acid sites than the acid strength because the decrease in the number of acid sites was more significantly than the decrease in the acid strength (see Table 1). It is generally known that the surface acidity of metal oxide can be increased when sulfates are attached to its surface because of the increase in numbers of Lewis and Brönsted acid sites. Khder et al. [19] reported that Lewis and Brönsted acid sites were accessed by the carbonyl group of carboxylic acid molecule to produce electrophile, followed by the nucleophilic

3.2.2 Effect of calcination temperature The activities of SO42–/Co2O3–SnO2 (2%) catalysts calcined at different temperatures for the esterification of FFA in crude palm oil are reported in Figure 5. The SO42–/ Co2O3–SnO2 calcined at 450°C showed the highest activity. This was due to its greatest number of acid sites (0.80 meq. n-butylamine/g) and highest acid strength (Ei ¼ 503 mV). Both the number and strength of acid sites of SO42–/Co2O3–SnO2 (2%) were found to decrease with the calcination temperature above 450°C resulting in its lower activity. High calcination temperature causes the loss of sulfate complexes attached to the metal oxide surface; as a result, Lewis acid sites become weaker. In contrast, crystallization of metal oxide occurs insufficiently at low calcination temperature [14], causing lower catalytic activity.

3.2.3 Reusability study The study in reusability of SO42–/Co2O3–SnO2 (2%) catalyst was performed with repeated runs of the esterification. After each run (2 h of reaction), the catalyst was separated from the reaction mixture by filtration, washed with hexane and methanol, dried at 105°C for 2 h and calcined at 450°C for 1 h. The catalyst then was reused in the reaction. Figure 6 provides the results. The

100 90

Conversion of FFA (%)

80 70 60 50

0 %Co

40

2 %Co

30

4 %Co

20

8 %Co 10 %Co

10 0 0

1

2 Reaction time (h)

3

4

Figure 4 Effect of loaded amount of Co of SO42–/Co2O3–SnO2 calcined at 450°C. [Reaction condition: 45 mL CH3OH, 0.5 g catalyst]



7

90 80


70 60 50 40 300°C 400°C 450°C 500°C 600°C

30 20

Ei (mV) 259 352 503 420 295

No. of acid sites (meq./g) 0.26 0.40 0.80 0.53 0.33

10 0 0

1

2

3

4

Reaction time (h) Figure 5 Effect of calcination temperature of SO42–/Co2O3–SnO2 (2%). [Reaction condition: 45 mL CH3OH, 0.5 g catalyst]

90 Fresh

80

1st


70

2nd 3rd

60 50 40 30 20 10 0 SO42–/Co2O3–SnO2 (2%)

SO42–/SnO2

Figure 6 Reusability of SO42–/Co2O3–SnO2 (2%) and SO42–/SnO2 samples calcined at 450°C. [Reaction condition: 45 mL CH3OH, 1 g catalyst, 2 h reaction time]

activity of SO42–/Co2O3–SnO2 did not decrease appreciably even after three cycles: the conversion decreased from 64 to 53%. The activity of SO42–/SnO2 also decreased with the number of recycles but with a

greater extent (from 80 to 47%). This implied that the addition of Co could decrease the deactivation of SO42–/SnO2 catalyst although the activity of fresh SO42–/Co2O3–SnO2 is lower than fresh SO42–/SnO2.


8


Deactivation of the sulfated catalysts can be attributed to poisoning and leaching of the active sites. Washing the used catalysts with hexane and methanol followed by calcining at 450°C could reduce the effect of poisons depositing on the catalyst surface. Therefore, the main reason for the loss in activities of both catalysts is leaching of the active sites (sulfate complexes). It is expected that surface sulfate complexes become more stable with the addition of cobalt oxide as previously found in the case of Al2O3-promoted sulfated tin oxide [21, 26] and Fe2O3-promoted sulfated tin oxide [17]. Moreover, Co atoms incorporated into the vacancy sites of SnO2 structure might improve the poorly crystallized structure of SnO2 and hence preventing the leaching of sulfate complexes. Toledo-Antonio et al. [23] found the hydroxyls remained from the preparation of tin oxide and this phenomenon caused a structure poorly crystallized with a large number of tin vacancy sites. Although the conversion of FFA with fresh SO42–/ Co2O3–SnO2 catalyst was lower than fresh SO42–/SnO2 catalyst, the conversion could be increased by varying the reaction conditions. Since a commercial heterogeneous catalyst needs to be reused several times without a significant drop in its activity, SO42–/Co2O3–SnO2 could be a better catalyst than SO42–/SnO2 for the esterification of FFA to produce methyl esters. Washing of impurities with solvents (hexane and methanol) and calcination of used SO42–/Co2O3–SnO2 catalyst at 450°C were not enough to completely recover its activity. This is in agreement with the previous study of Zhai et al. [27] that the activity of Fe2O3-doped sulfated tin oxide catalysts could not be completely recovered by calcination at 500°C for 1 h. Deactivation of SO42–/ Co2O3–SnO2 catalyst is due to the leaching of sulfate complexes which act as electron-withdrawing species to increase the Lewis acid strength. The acid strength and number of acid sites of the SO42–/Co2O3–SnO2 sample after three cycles decreased to 215 mV and 0.30 meq./g, respectively. We further performed TGA on three SO42–/Co2O3– SnO2 (2%) samples: (I) fresh SO42–/Co2O3–SnO2, (II) used SO42–/Co2O3–SnO2 after washing with hexane and methanol and drying at 105°C for 2 h and (III) regenerated SO42–/Co2O3–SnO2 (by further calcining the used SO42–/ Co2O3–SnO2 at 450°C for 1 h). A comparison in TGA and DTG curves of fresh, used and regenerated SO42–/Co2O3– SnO2 samples was made as shown in Figures 7(a) and 7(b), respectively. The weight loss of sulfate complexes at the high temperature range of 600–800°C of fresh SO42–/Co2O3–SnO2 sample was significantly higher than

the other two samples. This suggested that the fresh SO42–/Co2O3–SnO2 catalyst had the greatest number of the strongly attached-on-surface sulfate complexes. This proved that leaching of sulfate complexes occurred. Regeneration of SO42–/Co2O3–SnO2 by calcination at high temperature (450°C) could recover the activity to some extent. The DTG curve of the used SO42–/Co2O3– SnO2 sample showed different peaks at the temperature ranges of 370–540°C and 540–610°C. These might represent the remaining of impurities (e.g. components in oil) attached to the surface of washed and dried SO42–/Co2O3– SnO2 catalyst. Therefore, washing with solvents (hexane and methanol) and drying were not enough to recover the activity of SO42–/Co2O3–SnO2 catalyst. Leaching of sulfates remained the problem.

3.3 Optimization of reaction variables To determine the optimum conditions for the esterification of FFA in crude palm oil catalyzed by SO42–/Co2O3– SnO2 (2%) calcined at 450°C, the effects of amount of methanol, catalyst size, catalyst loading and reaction time were investigated.

3.3.1 Effect of amount of methanol The effect of amount of methanol on the conversion of FFA catalyzed by SO42–/Co2O3–SnO2 (2%) calcined at 450°C is shown in Figure 8. When the amount of methanol used in the reaction was increased from 15 to 45 mL, the conversion of FFA was found to considerably increase. Since esterification is a reversible reaction, using excess amount of methanol promotes the forward reaction. However, there was no difference in the conversion of FFA when the amount of methanol was increased from 45 to 60 mL. This was probably due to the formation of water produced from the esterification reaction. The adsorption of methanol and FFA on the active sites of the catalyst is necessary for the reaction to happen. The mixing of oil and methanol causes a non-homogeneous phase. Since FFA are more soluble in methanol than oil, the greater amount of methanol promotes the mass transfer of FFA to methanol phase, leading to the increased adsorption of FFA and methanol on the active sites of the catalyst. However, when too much methanol is used, the large amount of water produced from the reaction can strongly adsorb on the active sites. A similar result was reported by Shu et al. [28]. The negative effect of water in esterification reaction is due to that water tends to



9

(a) 100

TGA (%)

98 96 94 92 II 90

III I

88 0

200

400

600

800

1,000

800

1,000

T (°C)

(b)

III

DTG (a.u.)

II

I

0

200

400

600 T (°C)

Figure 7 (a) TGA and (b) DTG profiles of (I) fresh SO42–/Co2O3–SnO2; (II) used SO42–/Co2O3–SnO2 and (III) regenerated SO42–/Co2O3–SnO2

weaken the electrophilicity of positive carbonyl ion, preventing the reacting species to reach the active sites [29]. Moreover, separation becomes more difficult with large amount of methanol due to the formation of more stable emulsion and increased phase miscibility. Based on our result, the optimal amount of methanol was 45 mL, equivalent to the methanol/oil molar ratio of 42.

3.3.2 Effect of catalyst size To investigate the influence of mass transfer, particularly intraparticle diffusion, the synthesized SO42–/Co2O3–SnO2 (2%) catalyst having the average particle size about 20 μm was grounded to a smaller size (~10 μm) and tested for the esterification of FFA. As shown in Figure 9, the conversion of FFA was found to be insignificantly

different among the two sizes of the SO42–/Co2O3–SnO2 catalysts. This implied that the overall reaction rate was not controlled by mass transfer. Typically, the overall reaction rate of a heterogeneous catalysis is determined by the rates of three consequent steps: the external mass transfer, internal mass transfer (or pore diffusion) and surface reaction. Decreasing the catalyst size reduces the influences of external mass transfer and, more importantly, internal mass transfer since molecules of reactants can quickly access to the active sites within the pores of catalyst.

3.3.3 Effect of catalyst loading The effect of loading amount of catalyst on the conversion of FFA is presented in Figure 10. The amount of



10

100 90


80 70 60 50 40 15 mL, molar ratio of 14:1 30 mL, molar ratio of 28:1 45 mL, molar ratio of 42:1 60 mL, molar ratio of 57:1

30 20 10 0 0

1

2

3

4 5 Reaction time (h)

6

7

8

Figure 8 Effect of amount of methanol catalyzed by 0.5 g of SO42–/Co2O3–SnO2 (2%)

90 80


70 60 50 40

10 μm

30

20 μm

20 10 0 0

1

2 Reaction time (h)

3

4

Figure 9 Effect of catalyst size. [Reaction condition: 45 mL CH3OH, 0.5 g SO42–/Co2O3–SnO2 (2%)]

SO42–/Co2O3–SnO2 (2%) was varied from 0.25 to 2 g, corresponding to 1.1–8.9 wt% of the initial oil mass. The conversion of FFA was found to significantly increase with the amount of catalyst up to 1 g (4.5 wt % of the crude palm oil) owing to the more active sites available for the reactants to adsorb and react. However, when the amount of catalyst was further increased to 2 g, the conversion of FFA slightly increased. This was because that diffusion of reactant molecules becomes more significant in controlling the overall reaction rate.

The optimal loading amount of catalyst in this study was 1 g or 4.5 wt% of the oil, similar to a previous study in which the optimal catalyst/oil ratio of 4 wt% was found in the esterification of FFA in waste frying oil with an acidified SiO2 catalyst [30]. The results in Figures 8 and 10 suggest that the reaction time of 4 h was enough for the esterification of FFA to reach the equilibrium as long as other reaction parameters (i.e. methanol/oil ratio, amount of catalyst) were kept optimized. The equilibrium conversion of FFA in crude palm oil catalyzed by SO42–/Co2O3–SnO2 was about 89%.



11

100 90


80 70 60 50 40

0.25 g 0.5 g 1g 2g

30 20 10 0 0

1

2

3

4 5 Reaction time (h)

6

7

8

Figure 10 Effect of loading amount of SO42–/Co2O3–SnO2 (2%). [Reaction condition: 45 mL CH3OH]

4 Conclusions SO42–/Co2O3–SnO2 was found to be a promising catalyst in the production of methyl esters from the esterification of FFA in crude palm oil. SO42–/Co2O3–SnO2 exhibited both Lewis and Brönsted acid sites but the former was prominent. SO42–/Co2O3–SnO2 was more stable than SO42–/SnO2 during the reusability process. The greater the amount of Co added to SO42–/SnO2, the lower the catalytic activity. The optimum calcination temperature to prepare SO42–/Co2O3–SnO2 was 450°C. In the esterification study, the optimum methanol/oil molar ratio was 42, catalyst loading 4.5 wt% and reaction time 4 h. Acknowledgments: This work was supported by ENCON Fund, Energy Policy & Planning Office, Ministry of Energy, Thailand.

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