Production of biodiesel by esterification of oleic acid with ethanol over organophosphonic acid-functionalized silica

Bioresource Technology 110 (2012) 258–263

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Production of biodiesel by esterification of oleic acid with ethanol over organophosphonic acid-functionalized silica Ping Yin ⇑, Lei Chen, Zengdi Wang, Rongjun Qu ⇑, Xiguang Liu, Shuhua Ren School of Chemistry and Materials Science, Ludong University, Yantai 264025, PR China

a r t i c l e

i n f o

Article history: Received 17 July 2011 Received in revised form 17 January 2012 Accepted 19 January 2012 Available online 28 January 2012 Keywords: Biodiesel Esterification Oleic acid Organophosphonic acid-functionalized silica Response surface methodology

a b s t r a c t Esterification of oleic acid with ethanol catalyzed by organophosphonic acid-functionalized silica SG–T–P was optimized using response surface methodology (RSM). The interactive effect of catalyst to FFA weight ratio and molar ratio of alcohol to acid were more significant than that of reaction temperature. The optimum values for maximum conversion ratio obtained by a Box-Behnken center-united design reached 77.02 ± 0.62% when the reaction was carried out at 112 °C for 10 h with a molar ratio of alcohol to oleic acid of 8.8:1 and a content of 14.5 wt.% triethylenetetramine bis(methylene phosphonic acid)- functionalized silica catalyst SG–T–P. The research results show that SG–T–P is a potential catalyst for biodiesel production that can adsorb water from the reaction mixture at the same time. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Biodiesel, which consists of methyl or ethyl esters of fatty acids, is gaining increasing interests because of the advantages of cleaner engine emissions, biodegradable, renewable and superior lubricating property (Lee and Saka, 2010; Alcantara et al., 2000; Kawashima et al., 2009), which makes it an excellent substitute or additive to conventional diesel fuels. Methyl and ethyl esters derived from vegetable oil or animal fat, known as biodiesel, have good potential as alternative diesel fuel. Ideally, such oils and fats should not contain more than 1% free fatty acids (FFAs) since saponification of these FFAs reduces the yield of fatty acid alkyl esters (FAAEs). Recycled or waste oil and byproducts of the refining of vegetable oils, some non-edible oils, animal fats and oils can contain higher levels of FFAs, and crude mahua oil and tobacco seed oil contain about 20% and 17% FFAs, respectively (Shashikant and Hifjur, 2006; Veljkovic et al., 2006). Therefore, an esterification step is required using homogeneous acid-catalyzed, supercritical, enzymatic or heterogeneous catalyst processes (Edgar et al., 2005; Saka and Kusdiana, 2001; Madras et al., 2004; Kamini and Iefuji, 2007). Many acid heterogeneous catalysts such as super-solid acid (SO2 4 =SnO2 and SO2 =ZrO ) (Jitputti et al., 2006), heteropolyacid (Oliveira et al., 2 4 2010), metal phosphate (Serio et al., 2007), acid ion exchange resin (Qu et al., 2009), mesoporous SnO2/WO3 (Sarkar et al., 2010), ⇑ Corresponding authors. Tel.: +86 535 6696162; fax: +86 535 6697667. E-mail addresses: [email protected] (P. Yin), [email protected] (R. Qu). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2012.01.115

Amberlyst (Park et al., 2010), and Amazon flint kaolin activated by sulfuric acid (Nascimento et al., 2011) display outstanding catalytic activities. In order to be able to implement cleaner and economically improved processes for biodiesel production, novel acid heterogeneous catalysts are still being developed. The esterification reaction is an equilibrium reaction, and methyl/ethyl esters yield can be increased by removing water from the reaction mixture. Therefore, it would be very useful to develop solid acid catalysts that could adsorption water at the same time. It is well known that silica is widely used as inorganic solid matrix in inorganic–organic composite materials due to its excellent mechanical and thermal stability, and its unique large surface area. On the surface of active silica gel, there are a large number of silanol groups, which could react with silane coupling reagents that act as precursors for further immobilization of organic ligands. Modified silica materials have excellent performance in chromatography, adsorption, and catalysis (Wang et al., 2005; Ohta et al., 2001; Zhang et al., 2009); however, there are no data on esterification of free fatty acids with ethanol catalyzed by organophosophonic acid-modified silica as well as on the effects of molar ratio of ethanol to free fatty acid, catalyst amount and reaction temperature. Thus, in the present work, the esterification of FFA oleic acid with ethanol using organophosphonic acid-functionalized silica catalyst (triethylenetetramine bis(methylene phosphonic acid)- functionalized silica catalyst SG– T–P) was investigated using response surface methodology (RSM) to determine the optimum catalyst amount, molar ratio of ethanol to oleic acid and reaction temperature.

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2. Experimental

2.5. Experimental design and optimization by RSM

2.1. Materials and methods

Response surface methodology (RSM) was employed to analyze the operating conditions of esterification to obtain a high percent conversion. The experimental design was carried out by three chosen independent process variables at three levels (Table 1). The studied factors were: catalyst amount, molar ratio of alcohol to acid and temperature. For each factor, the experimental range and the central point are shown in Table 1, the percent conversion of oleic acid (c%) was the responses of the experimental design. A BoxBehnken center-united design was employed to design the experiments. The software of Minitab (Minitab Inc, USA) and the software TableCurves software (Systat Software Inc. USA) were used for regression and graphical analyses. The maximum conversion ratio values were taken as the responses of the design experiment. Statistical analysis of the model was performed to evaluate the analysis of variance (ANOVA). The Box-Behnken design of three factors consisted of 15 experiments. SG–T–P-catalyzed esterification reactions were carried out using ethanol and oleic acid. In order to search for the optimum reaction conditions for biodiesel synthesis, experiments were conducted according to the central composite design experimental plan (Table 2). Coefficients of the single-response model were evaluated by regression analysis and tested for their significance (Kalavathy et al., 2009). Insignificant coefficients were eliminated stepwise on the basis of their P-values after testing the coefficients. Therefore, the best-fitting model was determined by regression and stepwise elimination. A model equation was used to predict the optimum value and subsequently to elucidate the interaction between the factors. The quadratic equation model for predicting the optimal point was expressed according to Eq. (3) (Kim and Akoh, 2007):

Silica gel (SG) is of chromatographic grade (80–100 mesh size) and obtained from Qingdao Silicon Create Fine Chemical Co. Ltd., Shandong Province of China, was activated with nitric acid (HNO3:H2O = 1:1) at a refluxing temperature of 112 °C for 3 h, and hydrochloric acid (HCl:H2O = 1:1) at room temperature for 6 h. The activated gel was filtered through a Buchner funnel, washed thoroughly with distilled water till acid-free, and calcined in a muffle oven at 160 °C for 10 h. Toluene was redistilled just before use. 3-chloropropyltrimethoxysilane (CPTS) (Jianghan Chemicals Factory, Jinzhou, China), triethylenetetramine (TETA) (Shanghai Chemical Factory of China) and the other reagents were used without further purification. Porous structure parameters were determined with an automatic physisorption analyzer, ASAP 2020, (Micromeritics Instruments Corporation, USA) utilizing the BET (Brunauer–Emmet–Teller) and BJH (Barrett–Joyner–Halenda) methods (Das et al., 2007) involving N2 adsorption at 77 K. 2.2. Synthesis of SG–T–P Under a nitrogen atmosphere, a mixture of 25.0 mL of triethylenetetramine and 15.0 mL of CPTS was stirred at 80 °C into 150 mL of ethanol for 12 h. The reaction mixture was distilled until free of ethanol and 15.0 g of activated silica gel and 150 mL toluene were added. The mixture was stirred at 110 °C for 12 h, filtered through a Buchner funnel and the filtrate was transferred to a Soxhlet extraction apparatus for reflux-extraction in ethanol for 24 h. The solid product (SG-TETA) was dried under vacuum at 50 °C for 48 h, and 10.0 g of SG-TETA were added to 95 mL ethanol, incubated at room temperature for 12 h, then 2.5 g of paraformaldehyde, 6.9 g of phosphorous acid and 2.9 mL of hydrochloric acid were added. After being refluxed at 90 °C for 12 h, the reaction mixture was filtered through a Buchner funnel, and the solid catalyst (SG–T–P) was washed thoroughly with distilled water and dried under vacuum for 48 h at 50 °C. 2.3. Esterification Reactions were carried out under batch reaction conditions using a 250-ml flask fitted with a stirrer, a thermometer and a reflux condenser at 90, 100, 110 and 120 °C. A typical reaction mixture contained oleic acid (32 mL), ethanol and the solid catalyst SG–T–P. The molar ratio of ethanol to oleic acid was 6:1, 8:1, 10:1 and 12:1, and the quantity of catalysts was 7.0, 8.4, 9.8, 11.2, 12.6, and 14.0 wt.% (wt of catalysts/wt of oleic acid). The experiments were conducted for 10 h with stirring. 2.4. Analytical procedure The esterification reaction between oleic acid and alcohol can be represented as catalyst

Oleic acidðAÞ þ alcoholðBÞ $ ethyl oleateðCÞ þ waterðDÞ

ð1Þ

The amount of unreacted oleic acid in product mixture was obtained from its acid value (AV), which was determined by titration method (Marchetti and Errazu, 2008). The conversion of oleic acid was calculated according to the following equation:

x ¼ ð1  AV1 =AV0 Þ  100%

ð2Þ

where AV0 and AV1 are the acid values of feed and products, respectively.

Y ¼ k0 þ

4 X i¼1

ki xi þ

4 X

kii x2i þ

i¼1

3 4 X X

kij xi xj

ð3Þ

i¼1 j¼iþ1

where k0 , ki , kii and kij are regression coefficients (k0 is constant term, ki is linear effect term, kii is squared effect term, and kij is interaction effect term), and Y is the predicted response value. The optimum values of the selected variables were obtained by solving the regression equation using Matlab6.5 software (MathWorks Inc. USA).

2.6. Statistical analysis All data were analyzed with the assistance of Minitab, and significant second-order coefficients were selected by regression analysis with backward elimination. The fit of the model was evaluated by coefficients of determination and a test for lack of fit, which was performed by comparing mean square lack of fit to mean square experimental error, from the analysis of variance (ANOVA).

Table 1 Coded levels for independent factors used in the experimental design. Factors

Catalyst to FFA weight ratio (%) Molar ratio of alcohol to acid Temperature (°C)

Symbol

X1 X2 X3

Coded levels 1

0

+1

11 6:1 100

13 8:1 110

15 10:1 120

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respectively. It was clear that the amount of catalyst had a positive effect on the conversion ratio of oleic acid, and the conversion ratio at the reaction time of 10 h increased with increasing catalyst amount and became constant at a catalyst amount above 12.6 wt.%. The results, presented in Fig. 1a, confirmed that the reaction is catalyst amount-limited, and increasing the catalyst amount increased the reaction rate and consequently reduced the time to achieve a high conversion ratio. Fig. 1b represents the relationships between the conversion ratio and reaction time at various ethanol/oleic acid molar ratios, 12.6 wt.% SG–T–P catalyst to oleic acid and 100 °C with stirring. The reaction had a faster initial reaction rate and reached a higher final conversion for lower values of molar ratio of ethanol to oleic acid than for higher values of molar ratio of ethanol to oleic acid. The conversion ratio depended largely on the ethanol/oleic acid molar ratio, and the conversion increased with increasing mole ratio of the reactants. However, the conversion ratio decreased with the increase in the mole ratio from 10:1 to 12:1. The conversion ratio with the ethanol/oleic acid molar ratio of 6:1, 8:1, 10:1 and 12:1 for 10 h was 64.62, 75.31, 71.20 and 70.37%, respectively. This outcome was likely due to chemisorption of alcohol onto the Bronsted acid sites (Usha Nandhini et al., 2006). Temperature is one of the important variables for acid-catalyzed esterification because the rate of reaction is strongly influenced by the reaction temperature. The effect of the reaction temperature was examined from 90 to 120 °C with ethanol/oleic acid molar ratio 8:1 and 12.6 wt.% of SG–T–P catalyst to oleic acid is shown in Fig. 1c. The conversion ratio increased with increasing temperature (Fig. 1 c). In addition, the time for the conversion ratio to reach the steady state became shorter with the increase in temperature. Since this reaction is a reversible reaction, the oleic acid conversion increased with increasing temperature. As a result, the higher temperature has the highest reaction rate and higher conversion. Fig. 1 displays that the initial reaction rate (first 6 h) was high, then the reaction rate decreased. In the beginning of the esterification, the FFA oleic acid phase was free from ethanol and water.

Table 2 Experimental design and results of the response surface design. No.

X1

X2

X3

Conversion/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

1 1 1 1 1 1 1 1 0 0 0 0 0 0 0

1 1 1 1 0 0 0 0 1 1 1 1 0 0 0

0 0 0 0 1 1 1 1 1 1 1 1 0 0 0

Experimental

Fitted value

60.92 74.66 69.03 76.50 63.32 75.37 67.00 76.84 67.23 73.87 70.21 75.75 75.30 75.63 75.58

60.56 74.47 69.22 76.86 63.44 75.32 67.05 76.72 67.47 73.56 70.53 75.51 75.50 75.50 75.50

3. Results and discussion 3.1. Effect of catalyst amount, molar ratio of alcohol to acid and temperature on esterification Fig. 1a shows the relationships between the conversion ratio and reaction time at various catalyst amounts, ethanol/oleic acid molar ratio 8:1 and 100 °C. The effect of the catalyst amount was examined from 7.0 wt.% to 14.0 wt.% of SG–T–P to oleic acid. As seen in Fig. 1a, the reactions had higher initial reaction rates than later rate, and reached steady state at a stirring time of about 10 h. The conversion ratio increased with increasing catalyst amount, which could be attributed to the reason that more SG–T–P catalyst would provide more active reaction sites. The conversion ratio with the catalyst amount of 7.0, 8.4, 9.8, 11.2, 12.6 and 14.0 wt.% for 10 h was 62.67, 63.89, 68.92, 71.43, 75.32 and 75.68%,

(a)

60 A B C D E F

40 20 0

0

(b)

80 Oleic acid conversion/%

Oleic acid conversion/%

80

2

4

6 Time/h

8

Oleic acid conversion/%

80

60

20 0

10

A B C D

40

0

2

4

6 Time/h

8

10

(c)

60 A B C D

40 20 0

0

2

4

6 Time/h

8

10

Fig. 1. Effect of experimental factors on the conversion ratio of oleic acid. (a) Catalyst amount (catalyst to FFA weight ratio) (7.0%, plot A; 8.4%, plot B; 9.8%, plot C; 11.2%, plot D; 12.6%, plot E; 14.0%, plot F) on esterification. Reaction conditions: 100 °C, molar ratio of alcohol to acid, 8:1; (b) Molar ratio of alcohol to acid (6:1, plot A; 8:1, plot B; 10:1, plot C and 12:1, plot D). Reaction conditions: 100 °C, 12.6 wt.% catalyst; (c) Temperature (90 °C, plot A; 100 °C, plot B; 110 °C, plot C; and 120 °C, plot D). Reaction conditions: 12.6 wt.% catalysts, 8:1 M ratio of alcohol to acid.

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Therefore, the direct reaction could occur at the interface of oleic acid and ethanol. As the esterification reaction progressed, ethyl esters were formed (Luneca et al., 2011) and the FFA phase included oleic acid, ethyl esters, ethanol and water. Because ethanol and water are partially soluble in ethyl esters, the reaction rate might be reduced further. 3.2. RSM experiments and fitting the models In order to improve the conversion of acid and the efficiency of the work, the response surface methodology (RSM) is presented to indicate the parameters for an optimized synthesis process. It allows the user to gather large amounts of information from a small number of experiments (Zhang et al., 2008), and it is also possible to observe the effects of individual variables and their combinations of interactions on the response. At first, a Box-Behnken center-united design was employed to design the experiments, and the results obtained after running the 15 trials for the statistical design are shown in Table 2. All of the 15 designed experiments were performed and the results were multi-regression analyzed. Table 2 also presented the experimental value of oleic acid conversion and the fitting value of oleic acid conversion, and then the results indicated a good fit. Table 3 lists the regression coefficients of the established model equation and the results of the analysis of variance (ANOVA). ANOVA indicated that the model was highly significant as the Fmodel value (275.25) was very high at P < 0.001. The value of the determination coefficient (R2) and the predicted relevant coefficient of the model was 0.9980 and 0.9701, respectively, which indicated that the model was suitable to represent the real relationships among the selected reaction parameters. In this case, the value of the determination coefficient (R2 = 0.9980) indicated that the sample variation of 99.80% for the esterification reaction was attributed to the independent variables and only 0.20% of the total variations was not explained by the model, and a higher value of the correction coefficient (R = 0.9990) justified an excellent correlation between the independent variables. Moreover, the insignificant lack-of-fit test (Fmodel = 7.27) also indicated that the model was suitable to represent the experimental data using the designed experimental data.

Table 3 Coefficients of the model and ANOVA. Terms

Coefficients

Standard error

t-Stat

P-value

Intercept X1 X2 X3 X1  X1 X2  X2 X3  X3 X1  X2 X1  X3 X2  X3

75.5033 5.3875 2.7663 1.2512 3.1792 2.0467 1.6917 1.5675 0.5525 0.2750

0.2241 0.1372 0.1372 0.1372 0.2020 0.2020 0.2020 0.1941 0.1941 0.1941

336.945 39.261 20.159 9.118 15.740 10.133 8.375 8.077 2.847 1.417

P < 0.001 P < 0.001 P < 0.001 P < 0.001 P < 0.001 P < 0.001 P < 0.001 P < 0.001 0.036 0.216

Source ANOVA Regression Linear Square Interaction Residual error Lack of fit Pure error Total R2 Q2

Degrees of freedom

Sum of squares

Mean sum of squares

F

P

9 3 3 3 5

373.165 305.943 55.870 11.352 0.753

41.463 101.981 18.623 3.784 0.151

275.25 676.99 123.63 25.12

P < 0.001 P < 0.001 P < 0.001 0.002

3 2 14 0.9980 0.9701

1.334 0.063 373.919

0.230 0.032

7.27

0.123

261

Using the designed experimental data, the polynomial model for the conversion of oleic acid was regressed by considering the significant terms and was shown as follows:

Y ¼ 75:50 þ 5:39X 1 þ 2:77X 2 þ 1:25X 3  3:18X 21  2:05X 22  1:69X 23  1:57X 1 X 2  0:55X 1 X 3  0:28X 2 X 3

ð4Þ

where Y is the response (the conversion ratio of oleic acid), and X1, X2 and X3 are the coded values of the test values, the quantity of catalysts, the molar ratio of ethanol to oleic acid and reaction temperature, respectively. The significance of each coefficient was determined by t-values and P-values (Table 3). The larger the magnitude of the t-value and smaller the P-value, the more significant is the corresponding coefficient (Khuri and Cornell, 1987). So, the variable with the largest effect was catalyst to FFA oleic acid weight ratio. The linear effects of catalyst amount and ethanol/oleic acid molar ratio are more significant than those of reaction temperature. Moreover, the quadratic effect of catalyst to oleic acid weight ratio is more significant than those of molar ratio of alcohol to acid and reaction temperature (X 21 has the highest absolute value of t-value in all the quadratic items). The interactive effect of catalyst to oleic acid weight ratio and molar ratio of alcohol to acid might be significant to some extent (X1 and X2 has the highest absolute value of t-value and the smallest P-value in all the interactive items).The great importance of catalyst amount in the conversion to ethyl ester was also emphasized by Yuan et al. (2008). The response surfaces and contour plot for the above mentioned model for oleic acid conversion are generally used to evaluate the relationships of parameters, and the graphical representation of the regression equation. In general, 2D plots can provide information on the influence of the main process variables in a chemical reaction; however, a reactor can perform differently at different levels of process variables for most chemical reactions. Therefore, 3D plots present the overall behavior in a better way. 3D response surface plots are displayed in Fig. 2, and show oleic acid conversion as function of two variables, while keeping other variables at the zero level. Fig. 2 (1) shows the effects of catalyst to FFA weight ratio, molar ratio of alcohol to acid and their reciprocal interaction on oleic acid conversion. Increasing quantities of SG–T–P catalyst brought about a high conversion ratio, but excess amounts of catalyst led to a decline in the conversion ratio. Moreover, it was noted that the conversion ratio of oleic acid increased with increasing ethanol/oleic acid molar ratios, and then decreased. The effects of catalyst quantities, temperature and their reciprocal on oleic acid conversion are shown in Fig. 2 (2). Oleic acid conversion increased with the increase in temperature, and it was clear that when temperature was fixed at one level, the change in oleic acid conversion showed a parabolic pattern with catalyst to oleic acid weight ratio. Fig. 2 (3) depicts the effects of molar ratio of alcohol to acid and reaction temperature on oleic acid conversion. The interaction between the corresponding variables was negligible when the contour of the response surface was circular. On the contrary, the interactions between the relevant variables were significant when the contour of the response surfaces was elliptical. Fig. 2 (3) indicates that the interaction of molar ratio of alcohol to acid and temperature were not obvious. 3.3. Optimization of reaction conditions The optimum values of the selected variables were obtained by solving the regression Eq. (4) and the optimal conditions for ethyl ester production of esterification of oleic acid with ethanol estimated by the model equation were X1 = 0.73, X2 = 0.38, and X3 = 0.22. The theoretical conversion ratio was Y = 78.14% under optimal conditions (catalyst to FFA weight ratio: 14.5 wt.%, molar

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ratio of alcohol to acid: 8.8:1, reaction temperature: 112 °C). To confirm the prediction, three independent experiments were conducted under the established optimal conditions. The average conversion ratio reached 77.02 ± 0.62% and was close to the predicted value. Thus, response surface methodology with appropriate experimental design can be effectively applied to optimize the process of factors in this esterification synthesis of oleic acid with ethanol over SG–T–P catalyst. The esterification of FFA with ethanol usually results in lower yields than with methanol, and the reaction system is more affected by the presence of water as water and ethanol can form an azeotropic mixture. Luneca et al. (2011) reported an increase in the yield of ethyl ester by 15.7% using an adsorption system. When water was removed from the reaction mixture by molecular sieves, the reaction conversion ratio (77.85%) did not increase under the above-mentioned optimal conditions (catalyst to FFA weight ratio: 14.5 wt.%, molar ratio of alcohol to acid: 8.8:1, reaction temperature: 112 °C). The reason probably is that the SG–T– P catalyst has a porous structure and has adsorbed water. In order to verify the point that SG–T–P can adsorb water, measurements of the porous structure of triethylenetetramine bis(methylene phosphonic acid)- functionalized silica catalyst SG–T–P were carried out. Fig. 3 shows that the nitrogen adsorption–desorption isotherms for SG–T–P organophosphonic acid-functionalized silica catalyst is a type IV according to the IUPAC classification (Sing et al., 1985) with a hysteresis loop representative of mesopores. The adsorbed volume increased steeply at medium relative pressure (p/p0) indicating capillary condensation of nitrogen within the uniform mesoporous structure, and the two lines are approximately parallel, indicating that the pores of silica have a uniform radius and are open. The BET surface area and the BJH desorption cumulative volume of pores and BJH desorpion average pore diameter of SG–T–P were 192.41 m2/g, 0.65 cm3/g, and

Quantity adsorbed(cm3/g STP)

Fig. 2. The interactions and response surfaces of the process variables on the conversion ratio of oleic acid.

500 400 300

SG-T-P-1 Adsorption Desorption

200 100 0

0.0

0.2 0.4 0.6 0.8 Relative Pressure P/P0

1.0

Fig. 3. Nitrogen adsorption–desorption isotherms of SG–T–P.

42.80 Å, respectively. Therefore, the SG–T–P catalyst has the catalytic properties for esterification of free fatty acid oleic acid with ethanol and can also adsorb water from the reaction mixture.

4. Conclusions Esterification of free fatty acid oleic acid with ethanol over organophosphonic acid-functionalized silica SG–T–P was successful. Response surface methodology showed that the most important experimental factor affecting the esterification reaction was the amount of catalyst. Under the optimal conditions, the predicted value of the conversion ratio of oleic acid could reach 78.14%. The present catalyst exhibits catalytic activity, and also adsorbs water from the reaction system at the same time. Therefore, SG–T–P can be a potential catalyst for biodiesel production as it reduces equipment needs and cost. Further work is underway to improve

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