Silica-Supported Tin Oxides as Heterogeneous Acid Catalysts for

ARTICLE pubs.acs.org/IECR

Silica-Supported Tin Oxides as Heterogeneous Acid Catalysts for Transesterification of Soybean Oil with Methanol Wenlei Xie,* Hongyan Wang, and Hui Li School of Chemistry and Chemical Engineering, Henan University of Technology, Zhengzhou 450052, P. R. China ABSTRACT: In this work, the SnO2/SiO2 materials with various Sn loadings ranging from 1 to 16 wt % were prepared and used as heterogeneous acid catalysts for soybean oil transesterification to produce biodiesel. The catalyst with 8 wt % Sn loading and calcined at 873 K exhibited the best catalytic activity, giving an oil conversion of 81.7%. The dispersed amorphous SnO2 species on the silica surface are considered to be active sites for the transesterification reaction. The effect of reaction parameters such as catalyst amount, reaction temperature, reaction time, and methanol-to-oil ratio was investigated to optimize the transesterification conditions. It is shown that the free fatty acid (FFA) and water have no significant influence on the catalytic activity to the transesterification reaction. In addition, the heterogeneous acid catalyst was found to have high activities toward the esterification of FFAs. Furthermore, the catalyst could be recovered with a better reusability.

1. INTRODUCTION Due to depletion of fossil fuels and environmental concerns, biodiesel fuels, consisting of methyl esters of long chain fatty acids, have attracted much attention as a promising alternative fuel with significantly lower exhaust emissions of particulate matter and green-house gases such as carbon monoxide, carbon dioxide, and sulfur oxides.1 Generally, biodiesel can be produced by the transesterification of vegetable oils with methanol, which is carried out in the presence of base or acid catalyst. In the conventional biodiesel process, homogeneous base catalysts, such as potassium hydroxide, sodium hydroxide, or alkoxides, are the most preferred choice of catalysts because the rate of transesterification reactions catalyzed by alkaline catalysts is much faster under moderate operating conditions than that of reactions catalyzed by the acid catalysts.2,3 However, these homogeneous catalysts suffer from various drawbacks, including sensitivity to moisture and free fatty acid (FFA) content and the need to deal with the waste from the neutralization processes, though they are highly efficient and of low cost. The FFA present in the reactants can react with the alkaline catalysts to form unwanted soap byproduct, and these generated soaps could lead to the formation of emulsions, which inhibit the separation of biodiesel from the reaction mixture and ultimately lowers the biodiesel yield. Additionally, the removal of these base catalysts after the reaction is technically difficult, and a large amount of wastewater is generated in separating and cleaning the products. Therefore, the alkalicatalyzed procedure requires the use of higher purity vegetable oils with less than 0.5 wt % FFA and anhydrous conditions.3
5 At present, the major hurdle in the commercialization of biodiesel production is the high cost of raw materials used, since the high price of virgin oil can contribute potentially as much as 70% of total costs of biodiesel. For this reason, an effective way of reducing the cost of raw material is to employ low-quality oils (such as waste cooking oils and nonedible oils), which are cheaply available and can be regarded as attractive feedstocks for biodiesel production.6 However, the low-cost oils usually contain large amounts of FFAs and water. As mentioned previously, the presence of high percentages of FFAs makes the low-cost oils r 2011 American Chemical Society

unsuitable for the industrial process based on alkaline catalysis. Therefore, homogeneous acid catalysts have the potential to replace alkali catalysts, since they show less sensitive to FFAs and able to conduct esterification and transesterification simultaneously.7,8 As a result, biodiesel produced by the transesterification of low-cost waste oils using an acid catalyst could make it more cost-competitive with petroleum diesel as far as the economic benefit for biodiesel production was concerned. Among the homogeneous catalysts, sulfuric acid and hydrochloric acid are the most common acid catalysts used for the transesterification process. However, several drawbacks such as high reaction temperature, slow reaction rate, difficulty in separation, reactor corrosion, and the incapability for reuse have depreciated the utilization of the liquid acid catalyst for biodiesel production.8
10 In recent years, the recyclable solid acid catalysts have attracted much attention because of their potential to replace corrosive homogeneous acid catalysts in industrial processes.2 Heterogeneous acid catalysts can be easily separated from the reaction mixture by centrifugation or filtration and do not require neutralization processes, thus providing a more environmentally benign process and reducing the cost of processing.4 In particular, they have the ability to simultaneously catalyze the transesterification of triglycerides and the esterification of FFAs.11 Thus, the utilization of solid acid catalysts for the biodiesel preparation is expected to simplify significantly the manufacturing process when the low-cost oils are employed as feedstcocks and allows for good practices of catalyst recycling. There are several reports about the use of heterogeneous acid catalysts to produce biodiesel, including zeolites,12 La/zeolite β,13 tungstated zirconia,14 sulfated zirconia,15,16 sulfated zirconia
alumina (SZA),17 sulfated tin oxide,18 heteropoly acids,11,19 vanadyl phosphate,20 sulfonated polystyrene compounds,21 and carbonbased solid acid catalyst.22 Received: October 2, 2011 Accepted: November 27, 2011 Revised: November 25, 2011 Published: November 28, 2011 225

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The silica used as a support presents significant advantageous features owing to its high specific surface area, large pore size, and high thermal stability. In the present contribution, the SnO2/ SiO2 catalysts were prepared and tested as a heterogeneous acid catalyst for the transesterification of soybean oil with methanol. The catalytic performance of the catalyst in the transesterification reaction was investigated regarding the conversion to methyl esters, and more attention was paid to the effect of calcination temperatures and Sn loadings. Besides, the experimental variables such as the amount of catalyst, methanol/oil molar ratio, reaction temperature, reaction time and the presence of FFAs and water were investigated to optimize the transesterification conditions. Moreover, the catalysts were characterized by various physicochemical techniques such as powder X-ray diffractometry (XRD), thermogravimetric and differential thermogravimetric analysis (TG
DTG), and scanning electron microscopy (SEM). Further, the reusability of the catalyst was checked and the catalytic activity of the catalyst toward the esterification reaction was also investigated in terms of the FFA conversion to methyl esters.

different amounts of methanol, and the catalyst (5 wt % referred to the initial oil weight) were incorporated together inside the reactor. To minimize mass transfer limitations, the reactants were stirred at 350 rpm. The reactor temperature was raised to the required value, and after running the reaction for the desired duration, the reactor was allowed to cool to room temperature. Afterward, the solid catalyst was separated from the product mixture by filtration, and the excess methanol was recovered under reduced pressure prior to the subsequent analyses. Experiments were performed by varying reaction parameters such as methanol/oil ratio, reaction temperature, reaction time, and catalyst amount. The conversion of soybean oil to methyl esters was determined by measuring hydroxyl content on the transesterified soybean oil as previously described by us in the literature.23 2.5. Esterification Reaction of FFAs. In the current work, oleic acid was used as a model free fatty acid and was deliberately added into the soybean oil. The liquid-phase esterfication of oleic acid present in the feedstocks with methanol over the catalyst was carried out in a stainless steel high-pressure vessel using 5 wt % of catalyst in the presence of 10 wt % FFA. The acid values of the reaction mixtures were determined by titration of the aliquots, taken out from the reaction mixture at different time intervals with an alkali solution. The conversion of FFAs in the reactant to methyl esters can be determined according to the following equation.

2. EXPERIMENTAL SECTION 2.1. Materials. Commercial edible-grade soybean oil was obtained from the local market. According to GC (Shimadzu DC-9A) analysis, the fatty acid profiles of the soybean oil used were as follows: palmitic acid, 12.3%; stearic acid, 5.8%; oleic acid, 26.5%; linoleic acid, 49.4%; and linolenic acid, 5.9%. The acid value was less than 0.1 mg KOH g
1, and the average molar mass of triacylglycerols present in the soybean oil was 874 g mol
1, calculated from the saponification index (SV = 192.6 mg KOH g
1). All other employed materials were of analytical grade and were used as received without further purification. 2.2. Catalyst Preparation. The SnO2/SiO2 catalysts with various Sn loadings were prepared by an incipient wetness impregnation of powdered silica (SiO2) with an acetone solution containing an appropriate amount of dibutyltin dilaurate. Commercial silica was used as the support. After impregnation, the obtained sample was dried in an oven at 373 K, and finally, the solid was calcined at designed temperature for 5 h under air atmosphere in a muffle furnace before use for the transesterification reaction. The concentration of the precursor solution was adjusted so as to yield the desired Sn loadings in the final catalysts. The tin content in the SnO2/SiO2 catalysts was varied from 1 to 16 wt % based on the Sn loading. 2.3. Catalyst Characterization. Powder X-ray diffraction (XRD) measurements were conducted on a Rigaku D/MAX3B powder X-ray diffractometer using a radiation source of Cu Kα (λ = 0.154 nm) at 40 kV and 20 mA over a 2θ range of 20°
70° at a scanning speed of 5° min
1. The XRD diffraction patterns thus obtained were compared with references from the Power Diffraction File (PDF) database (JCPDS, International Centre for Diffraction Data) to identify the phase present in the samples. TG
DTG was carried out on a STA409PC thermal analyzer operating under a flow of air at a 20 K min
1 heating rate up to 1273 K. The surface morphology of the catalysts was studied by scanning electron microscopy using a JEOL JSM-6700F instrument. 2.4. Transesterification Procedures. All the transesterification reactions were carried out in a 250 mL stainless steel highpressure autoclave reactor fitted with a temperature controller and mechanical stirrer. In a typical assay, 16.0 g of soybean oil,

FFA conversion ð%Þ ¼

initial acid value
final acid value initial acid value

ð1Þ

3. RESULTS AND DISCUSSION 3.1. Screening of the Catalyst. A screening of different catalysts was performed under identical reaction conditions to identify the most promising solid acid catalysts for the transesterification reaction. The best catalyst was selected on the basis of maximum conversion to methyl esters. The experimental results are listed in Table 1. From this table it was seen that the selected support (Al2O3, ZrO2, and SiO2) showed low activities, with conversions below 30%. However, when the metal oxides were loaded on the supports and activated at a high temperature, the catalytic activity of the formed solid acid catalyst was improved in the transesterification reaction. For example, over SnO2/ γ-Al2O3, a soybean oil conversion of 68.5% was obtained, and a conversion of 64.3% was observed with SnO2
TiO2/SiO2 catalyst. Obviously, the Sn-based catalysts and the ZrO2-supported catalysts showed higher catalytic activities. In particular, SnO2/ SiO2 solid exhibited superior catalytic activity, giving a conversion of 81.6% after 5 h of reaction at a reaction temperature of 453 K, probably because of the presence of strong acid sites on the surface of the catalyst (entry 14 in Table 1). It is expected that the incorporation of tin oxides to SiO2 leads to the formation of SnO2/SiO2 catalyst with an enhanced acidity in comparison with the silica. On the basis of the above results, the SnO2/SiO2 catalyst showed potential for use as a heterogeneous acid catalyst and therefore was chosen for the subsequent study. 3.2. Catalyst Characterization. The XRD patterns for 8 wt % SnO2/SiO2 calcined at different temperatures are illustrated in Figure 1. For the samples calcined at temperatures below 873 K (curves a
c in Figure 1), there was only a broad diffraction peak centered at 2θ angles of 22°, due to the amorphous silica, to be observed in the XRD patterns, and no diffraction peak corresponding 226

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Table 1. Catalytic Activities of Different Catalystsa entry

catalysts

Tp (K)

conversion (%)

1

γ-Al2O3

873

17.8

2

ZrO2

873

10.5

3

SiO2

873

22.1

4

B2O3/γ-Al2O3

873

43.6

5

MoO3/γ-Al2O3

873

48.7

6

SnO2/γ-Al2O3

873

68.5

7

TiO2/γ-Al2O3

873

58.8

8 9

B2O3/ZrO2 MoO3/ZrO2

873 873

61.7 64.6

10

SnO2/ZrO2

873

58.6

11

TiO2/ZrO2

873

63.3

12

B2O3/SiO2

873

47.2

13

MoO3/SiO2

873

51.6

14

SnO2/SiO2

873

81.6

15

TiO2/SiO2

773

61.2

16 17

SnO2
TiO2/SiO2 SnO2
MoO3/SiO2

873 873

64.3 62.1

Figure 2. XRD patterns for samples with different Sn loadings: (a) 2 wt %, (b) 4 wt %, (c) 8 wt %, (d) 12 wt %, and (e) 16 wt %. The calcination temperature of supported catalysts was 873 K. (Δ) SiO2, (2) SnO2..

of the original one. This phenomenon shows that the SnO2 species is not changed during the transesterification processes. Notably, the changes of XRD diffraction peaks with the calcination temperature for the catalyst are in agreement with the changes in the catalytic activity toward the transesterification reaction, as illustrated in the following section. In addition, all the catalysts maintained the amorphous structure of the support, since their XRD patterns showed broad diffraction peaks at 2θ angles of 22°, similar to the amorphous silica. Accordingly, SnO2 amorphous species probably can be considered as catalytically active sites for the transesterification reaction. Figure 2 shows the XRD patterns of the catalyst with different Sn loadings. As presented in Figure.2, when the Sn loading was below 8 wt %, the major features of the XRD pattern basically belonged to the amorphous silica support, and no characteristic peak of crystalline SnO2 or another tin compound was detected by the XRD techniques, indicating the good dispersion of tin compounds on the silica.24,27 As the amount of Sn loading increased to 12 wt %, the defined XRD peaks at 2θ angles of 26.7°, 33.9°, 37.9°, 71.8°, 54.8°, 61.8°, and 65.8° appeared on the XRD patterns (curve e in Figure 2). These characteristic XRD peaks were well-matched with the rutile cassiterte phase of SnO2 (JCPDS 41-1445), and their intensities further grew with increasing the Sn loading to 16 wt %, which suggests that a residual crystalline phase of SnO2 is formed on the support at these loadings.25 Besides, it is also shown that there is no crystallite SnO2 species in the catalyst when the Sn loading amount is less than 8 wt %. Evidently, the XRD-undetectable phase of SnO2 may have dispersed onto the support, which is important for the generation of active sites in the SnO2/SiO2 catalysts. When the Sn loading amount increases beyond a borderline, i.e., above its dispersion capacity, the XRD SnO2 peaks could be detected, revealing that tin is dispersed in the form of an amorphous species at lower Sn loading and the SnO2 with crystal structure appears at higher Sn loading. Indeed, for those catalysts loaded with tin oxides at more than the spontaneous dispersion capacity on the silica, the residual bulk crystalline phase of SnO2 remains on the composite, which reduces the catalytic activity of the catalyst, since the crystalline SnO2 exhibits low activities for the transesterification reaction. In other words, the catalysts that contain only amorphous SnO2 are catalytically superior to the other catalysts that contain both amorphous and crystallite SnO2. The thermal analysis of the catalyst was performed in an attempt to gain information about the nature of the catalysts. As shown in Figure 3, the weight loss was less than 1.5% upon

a

Reaction conditions: methanol/oil molar ratio, 25:1; catalyst amount, 5 wt %; reaction temperature, 453 K; reaction time, 5 h.

Figure 1. XRD patterns for 8 wt % SnO2/SiO2 calcined at different temperatures: (a) not calcined, (b) 573 K, (c) 873 K, (d) 1073 K, and (e) recovered catalyst; (Δ) SiO2, (2) SnO2..

to SnO2 could be detected by XRD measurements, suggesting that tin compounds are highly dispersed on the silica at calcination temperatures below 873 K.24 As expected, the incorporation of tin compounds into the silica and subsequent thermal treatment could result in a remarkable increase in the number of acid sites and hence the catalytic activity. However, when the calcination temperature increased to 1073 K, the XRD pattern was completely different from the catalyst calcined at lower temperatures. For the sample calcined at 1073 K, apart from the diffraction peaks owing to the silica, some new diffraction peaks at 2θ angles of 26.7°, 33.9°, and 54.8°, which are the characteristic peaks for the cassiterite SnO2 phase (curve d in Figure 1), emerged in the XRD pattern.25 Such a result indicates that the local crystalline SnO2 is formed on the surface of the silica upon calcination at a temperature of 1073 K. Since the 1073 K-calcined catalyst was found to have lower activities, the amorphous SnO2 probably appeared to be catalytically better for the transesterification reaction than the SnO2 crystallites. This finding was also reported in the literature.26 Besides, the XRD pattern of the recovered catalyst exhibited very similar diffraction peaks to those 227

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Figure 3. TG
DTG curves of noncalcined 8 wt % SnO2/SiO2 sample.

Figure 5. Influence of calcination temperature on the conversion to methyl esters. Reaction conditions: catalyst amount, 5 wt %; reaction time, 5 h; reaction temperature, 453 K; methanol/oil molar ratio, 24:1.

Figure 4. SEM images of samples: (a) SiO2 and (b) 8 wt % SnO2/SiO2. Figure 6. Influence of loading amount of Sn on the conversion to methyl esters. Reaction conditions: catalyst amount, 5 wt %; reaction time, 5 h; reaction temperature, 453 K; methanol/oil molar ratio, 24:1.

heating to 423 K, and the total weight loss value was 21%. The low-temperature weight loss below 423 K in the TG profile could be assigned to the loss of residual solvent or water trapped in the silica framework. Besides, The TG curve of the original sample showed a significant weight loss in a wide temperature between 453 and 873 K. This weight loss, accompanied by a large DTG peak centered at 543 K, is mainly attributed to the progressively thermal decomposition of supported dibutyltin dilaurate, producing tin oxides when the temperature is raised. After 873 K there was no change in the weight loss value in the TG curve. SEM analyses were used to elucidate the morphological characteristics of the solid catalysts. The SEM photographs of the samples are presented in Figure 4. The morphology of the silica and SnO2/SiO2 samples showed well-ordered particles with a size of about 2
12 μm and nearly polygonal and irregular in shape with defined edges. As shown in Figure.4, no significant morphology change was observed, except small particles (maybe SnO2 particles) on the SnO2/SiO2 catalyst that were not seen on the silica. On the basis of the results, after loading of tin compounds, the silica retains its structure and the tin species are distributed upon the silica. 3.3. Influence of Preparation Conditions on the Catalyst Performance. The transesterifications catalyzed by various SnO2/ SiO2 catalysts calcined at different temperatures were carried out using 5 wt % catalysts, and the results are illustrated in Figure 5. It is shown that the calcination temperature is of importance in determining the final activity of the catalysts. In the absence of

calcination, the catalyst was not particularly active. After calcination the catalyst showed considerable activities toward the transesterification reaction. With increasing the calcination temperature from 473 to 873 K, the conversion to methyl esters was gradually increased. At 873 K, the conversion of soybean oil was attained a maximum value of 81.7% over the catalyst. The enhancement of the catalytic activity for the solid catalyst is most likely caused by the different decomposition extents of the precursor compound upon increasing the calcination temperature and subsequently the different amounts of the thus generated active species. However, as the calcination temperature increased to 1073 K, it can be seen that the conversion was substantially reduced. Presumably, the decrease in the catalytic activity at higher temperatures largely results from the formation of crystalline SnO2, as was demonstrated by the XRD measurements. Therefore, the optimal calcinatiion temperature is set at 873 K. The conversions to methyl esters over the catalysts with different Sn loadings are presented in Figure 6. As can be observed from this figure, when the Sn loading rose from 1 to 8 wt %, the conversion to methyl esters was gradually increased, and the best conversion was reached at the Sn loading of 8 wt %. However, with the further enhancement in the Sn loading from 8 to 16 wt %, a diminishing trend in the conversion was noticed, probably due to the formation of crystallites SnO2 that could cover the surface 228

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Figure 7. Effect of methanol/oil molar ratio on the conversion to methyl esters. Reaction conditions: catalyst amount, 5 wt %; reaction time, 5 h; reaction temperature, 453 K.

Figure 8. Effect of reaction time on the conversion to methyl esters. Reaction conditions: methanol/oil molar ratio, 24:1; reaction temperature, 453 K; catalyst amount, 5 wt %.

active sites of the catalyst. As a result, the catalytic activity of the catalyst is largely dependent on the Sn loading, and the proper Sn loading is necessary for the catalyst to get high catalytic activity. Thus, it may then be suggested that the amorphous SnO2 species formed during the calcination processes could be considered to be the mainly catalytically active phase for the reaction. Accordingly, the SnO2/SiO2 catalyst with the Sn loading of 8 wt % is the most suitable catalyst for the transesterification reaction. 3.4. Influence of Reaction Parameters on the Oil Conversion. The reaction temperature is one of the important variables affecting the transesterification reaction. Usually, higher reaction temperature can give higher reaction rate in the transesterification reaction, especially by using acid catalysts.2 The effect of reaction temperature was investigated with this catalyst by varying the reaction temperature in the range of 393
493 K. The results achieved here showed that the soybean oil conversion was even less than 60%, though the reaction was run at 393 K. However, it could be noted that the conversion to methyl esters increased steadily from 56.5% to 70.8% to 81.7% upon increasing the reaction temperature from 393 to 423 to 453 K. The increased conversion with reaction temperature might be not only due to the effect of the increase in reaction rate by increasing temperature but also due to the improvement of the solubility of methanol in soybean oil. However, the further increase in the reaction temperature gave an insignificant increase in conversion to methyl esters. Hence, it could be inferred that the optimum operating temperature for the transesterification reaction is 453 K. In the reaction sequence, triglycerides are converted stepwise to diglyceride, monoglyceride, and finally glycerol accompanied with the formation of methyl esters. Stoichiometrically, the transesterification reaction requires 3 mol of methanol for each mole of triglyceride. However, the excess methanol is commonly employed for the transesterification reaction in order to shift the reaction equilibrium toward the desired product side.28 In general, the higher methanol/oil molar ratio is required for the acidcatalyzed process to obtain a higher reaction rate, in comparison with the base-catalyzed process.4 The effect of methanol/oil molar ratio was investigated using this solid acid catalyst, and the results are shown in Figure 7. With the increase in methanol/oil molar ratio, the conversion was gradually enhanced and a maximum conversion after reaction for 5 h was achieved over the catalyst at the methanol/oil molar ratio of 24:1. However, there was no significant increase in the conversion to be observed with

increasing methanol addition beyond the molar ratio of 24:1. From the results, the optimum molar ratio of methanol/oil for the reaction is approximately 24:1. The excess methanol can be recovered from the reaction product by simple distillation, and the byproduct of glycerol in the process can be gravitationally separated by decantation without any complex process. The influence of catalyst loadings was also investigated by using different catalyst loadings between 1 and 7 wt %. The transesterification reactions were set at a methanol/oil molar ratio of 24:1, a temperature of 453 K, and a reaction time of 5 h. As the catalyst loading increased from 1 to 3 to 5 wt %, the corresponding conversion to methyl esters was increased from 40.8 to 68.4 to 81.7 and reached a plateau thereafter when the catalyst loading was further increased beyond 5 wt %. The optimum catalyst loading for obtaining high conversion to methyl esters was found to be 5 wt %. As the catalyst loading increases, more catalytically active sites are available to facilitate the reaction to occur, which allows the conversion to methyl esters to increase. From the above results, a catalyst loading of 5 wt % was chosen for subsequent studies. To examine the effect of reaction time on the conversion to methyl esters, experiments were performed by using different reaction times under the reaction conditions of 453 K and 24:1 ratio of methanol to oil. The reaction time was varied within a range from 1 to 9 h. From the results shown in Figure 8, it was observed that when the reaction time extended to 5 h, the conversion to methyl esters was increased and reached the maximum value of 81.7%, achieving the equilibrium of the reaction. The further increase in reaction time to 9 h resulted in the small increase in the conversion to 83.6%. Therefore, the suitable reaction time for the transesterification reaction is 5 h. The methodology based on acid-catalyzed transesterification has raised a lot of interest because it allows for the preparation of biodiesel by using low-cost oils as feedstocks. To study the influence of FFA and water on the activity of the catalysts, the simulated waste oils with different amounts of FFA and water were prepared by deliberately adding oleic acid or water. Additional experiments were performed with the simulated waste oils under the optimized reaction conditions. When the FFA amount was 1%, 3%, 5%, 7%, and 10% (based on refined soybean oil weight), the conversions to methyl esters were 80.1%, 78.9%, 78.2%, 77.1%, and 75.7%, respectively. Obviously, the conversion went down slightly with the increase in the FFA amount within the 229

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the silica, leading to the improvement of the catalyst stability. However, when the catalyst was further used for a fifth cycle, the conversion decreased remarkably to 54.3%. The loss of the catalytic activity is probably owing to the original active sites being blocked partially by adsorbed intermediate or product species, which diminishes the amount of readily available catalytic sites. In general, the acid-catalyzed transesterification usually involves high reaction temperature, long reaction time, and low conversion rate. In the present research, the SnO2/SiO2 catalyst shows higher efficiency in biodiesel production when low-cost feedstocks are employed. Over the catalyst, the soybean oil conversion of 81.7% and the FFA conversion of 94.6% are obtained under appropriate operating conditions. It is shown that the soybean oil and FFA can be converted to biodiesel in a one-step process catalyzed by the solid catalyst. Notably, the conversions achieved in this work are relatively low and are not of commercial interest, and thus, for the industrial production of biodiesel, the activity of the catalyst needs to be improved considerably. Alternatively, another stage of transesterification after glycerol separation would be expected to complete the reaction if the solid catalyst is to be practically employed for the biodiesel production.

Figure 9. Effect of reaction time on the conversion of FFA to methyl esters. Reaction conditions: methanol/oil molar ratio, 24:1; reaction temperature, 453 K; catalyst amount, 5 wt %; reaction time, 5 h.

range studied, and the conversion of soybean oil could retain 75.7% even with 10% of FFA. It is reported that during the transesterification processes, the activity of solid catalysts is adversely affected by the presence of water.4 By using the SnO2/SiO2 catalyst, the effect of the water amount on the catalytic activity was studied. The conversions were 81.2%, 80.4%, 78.7%, 75.4%, 72.1%, and 63.2%, respectively, when the water content was 0.1%, 0.3%, 0.5%, 0.7%, 1%, and 1.5%. Clearly, the oil conversion declined with the increase in water content. Indeed, the water content of less than 1.0% could lead to the conversion above 70% after 5 h of reaction. However, up to a water content higher than 1.0%, the conversion to methyl esters decreased rapidly. With water content of 1.5%, the soybean oil conversion was low and only attained 63.2%. Therefore, the water content in oil had better be less than 1.0%, which is higher than the allowable water content of the base catalyst employed. In order to examine the catalytic activity of the catalyst toward the esterification of FFAs, the esterification of oleic acid present in the oils with methanol over the catalysts was carried out in a stainless steel high-pressure vessel, using 5 wt % of catalyst at a reaction temperature of 453 K. With the SnO2/SiO2 catalyst, both the esterification and transesterification could be conducted in a single process. As illustrated in Figure 9, a FFA conversion of 93.8% was attained after 3 h. When the reaction time increased to 5 h, the conversion of FFA to methyl esters over the catalyst was increased steadily and reached as high as 94.1%, followed by a slight increase upon a further increase of reaction time. By drawing on the results, the solid catalyst also exhibited catalytic activities toward the esterification of FFAs with methanol. 3.5. Reusability of the Catalyst. The reutilization of heterogeneous catalyst is an important aspect that makes it economic and preferable over a homogeneous one. For the reusability investigation, the used catalysts were separated by filtration and washed with petroleum ether and methanol. Before reuse in the second reaction cycle, the recovered catalysts were dried at 393 K and subsequently used for new reactions under the optimum operating conditions. When the solid catalyst was used for one, two, three, and four cycles, the conversion of soybean oil was declined slightly from 80.2%, 78.3%, and 74.6% to 67.5%. Thus, the catalyst retained a higher catalytic stability even after continuous use up to four cycles. It might be hypothesized that the Sn is linked to the surface of the silica by a Si
O
Sn chemical bond arising due to the stronger interactions between SnO2 and

4. CONCLUSIONS The SnO2/SiO2 catalysts containing different amounts of Sn loading were prepared by impregnation with dibutyltin dilaurate and thermal activation at higher temperature for the production of biodiesel. The maximum activity is found for the catalyst with 8 wt % Sn loading and calcination at 873 K, which could catalyze the transesterification reaction of soybean oil even containing a large amount of FFA. The soybean oil conversion of 81.7% was obtained after 5 h of reaction, when a methanol/oil molar ratio of 24:1 and 5 wt % of catalyst loading were employed. The amorphous SnO2 on the silica support is shown to be catalytically active sites for the transesterification reaction. Besides, the catalyst also exhibits high catalytic activities in the esterification reaction of FFA, with a FFA conversion of 94.6%. Further, the SnO2/SiO2 catalyst could be easily recovered and reused without serious loss in the catalytic activity for four cycles. ’ AUTHOR INFORMATION Corresponding Author

*Telephone: +86-371-67756302. Fax: +86-371-67756718. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was financially supported by research grants from National Science Foundation of China (project No. 21076062) and Program for Science & Technology Innovation Talents in Universities of Henan province in China (HASTIT). ’ REFERENCES (1) Sharma, Y. C.; Singh, B.; Upadhyay, S. N. Advancements in development and characterization of biodiesel: A review. Fuel 2008, 87, 2355. (2) Semwal, S.; Arora, A. K.; Badoni, R. P.; Tuli, D. K. Biodiesel production using heterogeneous catalysts. Bioresour. Technol. 2011, 102, 2151. (3) Leung, D. Y. C.; Wu, X.; Leung, M. K. H. A review on biodiesel production using catalyzed transesterification. Appl. Energy 2010, 87, 1083. 230

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dx.doi.org/10.1021/ie202262t |Ind. Eng. Chem. Res. 2012, 51, 225–231