Bioresource Technology 110 (2012) 258–263

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Bioresource Technology

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

Ping Yin ⇑, Lei Chen, Zengdi Wang, Rongjun Qu ⇑, Xiguang Liu, Shuhua RenSchool 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 2011Received in revised form 17 January 2012Accepted 19 January 2012Available online 28 January 2012

Keywords:BiodieselEsterificationOleic acidOrganophosphonic acid-functionalizedsilicaResponse surface methodology

0960-8524/$ - see front matter � 2012 Elsevier Ltd. Adoi:10.1016/j.biortech.2012.01.115

⇑ Corresponding authors. Tel.: +86 535 6696162; faE-mail addresses: yinping426@163.com (P. Yin), ro

a b s t r a c t

Esterification of oleic acid with ethanol catalyzed by organophosphonic acid-functionalized silica SG–T–Pwas optimized using response surface methodology (RSM). The interactive effect of catalyst to FFA weightratio and molar ratio of alcohol to acid were more significant than that of reaction temperature. The opti-mum values for maximum conversion ratio obtained by a Box-Behnken center-united design reached77.02 ± 0.62% when the reaction was carried out at 112 �C for 10 h with a molar ratio of alcohol to oleicacid of 8.8:1 and a content of 14.5 wt.% triethylenetetramine bis(methylene phosphonic acid)- function-alized silica catalyst SG–T–P. The research results show that SG–T–P is a potential catalyst for biodieselproduction that can adsorb water from the reaction mixture at the same time.

� 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 cleanerengine emissions, biodegradable, renewable and superior lubricat-ing property (Lee and Saka, 2010; Alcantara et al., 2000; Kawashimaet al., 2009), which makes it an excellent substitute or additive toconventional diesel fuels. Methyl and ethyl esters derived fromvegetable oil or animal fat, known as biodiesel, have good potentialas alternative diesel fuel. Ideally, such oils and fats should not con-tain more than 1% free fatty acids (FFAs) since saponification ofthese FFAs reduces the yield of fatty acid alkyl esters (FAAEs). Recy-cled or waste oil and byproducts of the refining of vegetable oils,some non-edible oils, animal fats and oils can contain higher levelsof FFAs, and crude mahua oil and tobacco seed oil contain about 20%and 17% FFAs, respectively (Shashikant and Hifjur, 2006; Veljkovicet al., 2006). Therefore, an esterification step is required usinghomogeneous acid-catalyzed, supercritical, enzymatic or heteroge-neous catalyst processes (Edgar et al., 2005; Saka and Kusdiana,2001; Madras et al., 2004; Kamini and Iefuji, 2007). Many acid het-erogeneous catalysts such as super-solid acid (SO2�

4 =SnO2 andSO2�

4 =ZrO2) (Jitputti et al., 2006), heteropolyacid (Oliveira et al.,2010), metal phosphate (Serio et al., 2007), acid ion exchange resin(Qu et al., 2009), mesoporous SnO2/WO3 (Sarkar et al., 2010),

ll rights reserved.

x: +86 535 6697667.ngjunqu@sohu.com (R. Qu).

Amberlyst (Park et al., 2010), and Amazon flint kaolin activated bysulfuric acid (Nascimento et al., 2011) display outstanding catalyticactivities. In order to be able to implement cleaner and economicallyimproved processes for biodiesel production, novel acid heteroge-neous catalysts are still being developed. The esterification reactionis an equilibrium reaction, and methyl/ethyl esters yield can be in-creased by removing water from the reaction mixture. Therefore, itwould be very useful to develop solid acid catalysts that couldadsorption water at the same time. It is well known that silica iswidely used as inorganic solid matrix in inorganic–organic compos-ite materials due to its excellent mechanical and thermal stability,and its unique large surface area. On the surface of active silicagel, there are a large number of silanol groups, which could reactwith silane coupling reagents that act as precursors for furtherimmobilization of organic ligands. Modified silica materials haveexcellent performance in chromatography, adsorption, and cataly-sis (Wang et al., 2005; Ohta et al., 2001; Zhang et al., 2009); how-ever, there are no data on esterification of free fatty acids withethanol catalyzed by organophosophonic acid-modified silica aswell as on the effects of molar ratio of ethanol to free fatty acid, cat-alyst amount and reaction temperature. Thus, in the present work,the esterification of FFA oleic acid with ethanol using organophos-phonic acid-functionalized silica catalyst (triethylenetetraminebis(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 ethanolto oleic acid and reaction temperature.

P. Yin et al. / Bioresource Technology 110 (2012) 258–263 259

2. Experimental

2.1. Materials and methods

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 hydro-chloric acid (HCl:H2O = 1:1) at room temperature for 6 h. Theactivated gel was filtered through a Buchner funnel, washed thor-oughly with distilled water till acid-free, and calcined in a muffleoven at 160 �C for 10 h. Toluene was redistilled just before use.3-chloropropyltrimethoxysilane (CPTS) (Jianghan Chemicals Fac-tory, Jinzhou, China), triethylenetetramine (TETA) (ShanghaiChemical Factory of China) and the other reagents were used with-out further purification. Porous structure parameters were deter-mined 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 triethyl-enetetramine and 15.0 mL of CPTS was stirred at 80 �C into 150 mLof ethanol for 12 h. The reaction mixture was distilled until free ofethanol and 15.0 g of activated silica gel and 150 mL toluene wereadded. The mixture was stirred at 110 �C for 12 h, filtered througha Buchner funnel and the filtrate was transferred to a Soxhletextraction apparatus for reflux-extraction in ethanol for 24 h. Thesolid product (SG-TETA) was dried under vacuum at 50 �C for48 h, and 10.0 g of SG-TETA were added to 95 mL ethanol, incu-bated at room temperature for 12 h, then 2.5 g of paraformalde-hyde, 6.9 g of phosphorous acid and 2.9 mL of hydrochloric acidwere added. After being refluxed at 90 �C for 12 h, the reactionmixture was filtered through a Buchner funnel, and the solid cata-lyst (SG–T–P) was washed thoroughly with distilled water anddried under vacuum for 48 h at 50 �C.

2.3. Esterification

Reactions were carried out under batch reaction conditionsusing a 250-ml flask fitted with a stirrer, a thermometer and a re-flux condenser at 90, 100, 110 and 120 �C. A typical reaction mix-ture contained oleic acid (32 mL), ethanol and the solid catalystSG–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). Theexperiments were conducted for 10 h with stirring.

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

Factors Symbol Coded levels

�1 0 +1

Catalyst to FFA weight ratio (%) X1 11 13 15Molar ratio of alcohol to acid X2 6:1 8:1 10:1Temperature (�C) X3 100 110 120

2.4. Analytical procedure

The esterification reaction between oleic acid and alcohol canbe represented as

Oleic acidðAÞ þ alcoholðBÞ $catalystethyl oleateðCÞ þwaterðDÞ ð1Þ

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

x ¼ ð1� AV1=AV0Þ � 100% ð2Þ

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

2.5. Experimental design and optimization by RSM

Response surface methodology (RSM) was employed to analyzethe operating conditions of esterification to obtain a high percentconversion. The experimental design was carried out by three cho-sen independent process variables at three levels (Table 1). Thestudied factors were: catalyst amount, molar ratio of alcohol to acidand temperature. For each factor, the experimental range and thecentral point are shown in Table 1, the percent conversion of oleicacid (c%) was the responses of the experimental design. A Box-Behnken center-united design was employed to design the experi-ments. The software of Minitab (Minitab Inc, USA) and the softwareTableCurves software (Systat Software Inc. USA) were used forregression and graphical analyses. The maximum conversion ratiovalues were taken as the responses of the design experiment. Statis-tical analysis of the model was performed to evaluate the analysis ofvariance (ANOVA). The Box-Behnken design of three factors con-sisted of 15 experiments. SG–T–P-catalyzed esterification reactionswere carried out using ethanol and oleic acid. In order to search forthe optimum reaction conditions for biodiesel synthesis, experi-ments were conducted according to the central composite designexperimental plan (Table 2).

Coefficients of the single-response model were evaluated byregression analysis and tested for their significance (Kalavathyet al., 2009). Insignificant coefficients were eliminated stepwiseon the basis of their P-values after testing the coefficients. There-fore, the best-fitting model was determined by regression andstepwise elimination.

A model equation was used to predict the optimum value andsubsequently to elucidate the interaction between the factors.The quadratic equation model for predicting the optimal pointwas expressed according to Eq. (3) (Kim and Akoh, 2007):

Y ¼ k0 þX4

i¼1

kixi þX4

i¼1

kiix2i þ

X3

i¼1

X4

j¼iþ1

kijxixj ð3Þ

where k0, ki, kii and kij are regression coefficients (k0 is constantterm, ki is linear effect term, kii is squared effect term, and kij isinteraction effect term), and Y is the predicted response value.The optimum values of the selected variables were obtained bysolving the regression equation using Matlab6.5 software (Math-Works Inc. USA).

2.6. Statistical analysis

All data were analyzed with the assistance of Minitab, and sig-nificant second-order coefficients were selected by regressionanalysis with backward elimination. The fit of the model was eval-uated by coefficients of determination and a test for lack of fit,which was performed by comparing mean square lack of fit tomean square experimental error, from the analysis of variance(ANOVA).

Table 2Experimental design and results of the response surface design.

No. X1 X2 X3 Conversion/%

Experimental Fitted value

1 �1 �1 0 60.92 60.562 1 �1 0 74.66 74.473 �1 1 0 69.03 69.224 1 1 0 76.50 76.865 �1 0 �1 63.32 63.446 1 0 �1 75.37 75.327 �1 0 1 67.00 67.058 1 0 1 76.84 76.729 0 �1 �1 67.23 67.4710 0 1 �1 73.87 73.5611 0 �1 1 70.21 70.5312 0 1 1 75.75 75.5113 0 0 0 75.30 75.5014 0 0 0 75.63 75.5015 0 0 0 75.58 75.50

260 P. Yin et al. / Bioresource Technology 110 (2012) 258–263

3. Results and discussion

3.1. Effect of catalyst amount, molar ratio of alcohol to acid andtemperature on esterification

Fig. 1a shows the relationships between the conversion ratioand reaction time at various catalyst amounts, ethanol/oleic acidmolar ratio 8:1 and 100 �C. The effect of the catalyst amount wasexamined from 7.0 wt.% to 14.0 wt.% of SG–T–P to oleic acid. Asseen in Fig. 1a, the reactions had higher initial reaction rates thanlater 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 catalystwould provide more active reaction sites. The conversion ratiowith 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%,

0 2 4 6 8 100

20

40

60

80

Ole

ic a

cid

conv

ersi

on/%

Time/h

A B C D E F

0 2 40

20

40

60

80

Ole

ic a

cid

conv

ersi

on/%

T

(a)

(c)

Fig. 1. Effect of experimental factors on the conversion ratio of oleic acid. (a) Catalyst amD; 12.6%, plot E; 14.0%, plot F) on esterification. Reaction conditions: 100 �C, molar ratio oplot C and 12:1, plot D). Reaction conditions: 100 �C, 12.6 wt.% catalyst; (c) Temperature12.6 wt.% catalysts, 8:1 M ratio of alcohol to acid.

respectively. It was clear that the amount of catalyst had a positiveeffect on the conversion ratio of oleic acid, and the conversion ratioat the reaction time of 10 h increased with increasing catalystamount and became constant at a catalyst amount above12.6 wt.%. The results, presented in Fig. 1a, confirmed that the reac-tion is catalyst amount-limited, and increasing the catalyst amountincreased the reaction rate and consequently reduced the time toachieve a high conversion ratio.

Fig. 1b represents the relationships between the conversion ra-tio 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 higherfinal conversion for lower values of molar ratio of ethanol to oleicacid than for higher values of molar ratio of ethanol to oleic acid.The conversion ratio depended largely on the ethanol/oleic acidmolar ratio, and the conversion increased with increasing mole ra-tio of the reactants. However, the conversion ratio decreased withthe increase in the mole ratio from 10:1 to 12:1. The conversion ra-tio with the ethanol/oleic acid molar ratio of 6:1, 8:1, 10:1 and 12:1for 10 h was 64.62, 75.31, 71.20 and 70.37%, respectively. This out-come was likely due to chemisorption of alcohol onto the Bronstedacid sites (Usha Nandhini et al., 2006).

Temperature is one of the important variables for acid-catalyzedesterification because the rate of reaction is strongly influenced bythe reaction temperature. The effect of the reaction temperaturewas examined from 90 to 120 �C with ethanol/oleic acid molar ratio8:1 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. 1c). In addition, the time for the conversion ratio to reach the steadystate became shorter with the increase in temperature. Since thisreaction is a reversible reaction, the oleic acid conversion increasedwith increasing temperature. As a result, the higher temperaturehas 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 esterifica-tion, the FFA oleic acid phase was free from ethanol and water.

0 2 4 6 8 100

20

40

60

80

Ole

ic a

cid

conv

ersi

on/%

Time/h

A B C D

6 8 10ime/h

A B C D

(b)

ount (catalyst to FFA weight ratio) (7.0%, plot A; 8.4%, plot B; 9.8%, plot C; 11.2%, plotf alcohol to acid, 8:1; (b) Molar ratio of alcohol to acid (6:1, plot A; 8:1, plot B; 10:1,

(90 �C, plot A; 100 �C, plot B; 110 �C, plot C; and 120 �C, plot D). Reaction conditions:

P. Yin et al. / Bioresource Technology 110 (2012) 258–263 261

Therefore, the direct reaction could occur at the interface of oleicacid and ethanol. As the esterification reaction progressed, ethylesters were formed (Luneca et al., 2011) and the FFA phase in-cluded oleic acid, ethyl esters, ethanol and water. Because ethanoland water are partially soluble in ethyl esters, the reaction ratemight be reduced further.

3.2. RSM experiments and fitting the models

In order to improve the conversion of acid and the efficiency ofthe work, the response surface methodology (RSM) is presented toindicate the parameters for an optimized synthesis process. It al-lows the user to gather large amounts of information from a smallnumber of experiments (Zhang et al., 2008), and it is also possibleto observe the effects of individual variables and their combina-tions of interactions on the response.

At first, a Box-Behnken center-united design was employed todesign the experiments, and the results obtained after runningthe 15 trials for the statistical design are shown in Table 2. All ofthe 15 designed experiments were performed and the results weremulti-regression analyzed. Table 2 also presented the experimen-tal value of oleic acid conversion and the fitting value of oleic acidconversion, and then the results indicated a good fit. Table 3 liststhe regression coefficients of the established model equation andthe results of the analysis of variance (ANOVA). ANOVA indicatedthat the model was highly significant as the Fmodel value (275.25)was very high at P < 0.001. The value of the determination coeffi-cient (R2) and the predicted relevant coefficient of the model was0.9980 and 0.9701, respectively, which indicated that the modelwas suitable to represent the real relationships among the selectedreaction parameters. In this case, the value of the determinationcoefficient (R2 = 0.9980) indicated that the sample variation of99.80% for the esterification reaction was attributed to the inde-pendent variables and only 0.20% of the total variations was notexplained by the model, and a higher value of the correction coef-ficient (R = 0.9990) justified an excellent correlation between theindependent variables. Moreover, the insignificant lack-of-fit test(Fmodel = 7.27) also indicated that the model was suitable to repre-sent the experimental data using the designed experimental data.

Table 3Coefficients of the model and ANOVA.

Terms Coefficients Standard error t-Stat P-value

Intercept 75.5033 0.2241 336.945 P < 0.001X1 5.3875 0.1372 39.261 P < 0.001X2 2.7663 0.1372 20.159 P < 0.001X3 1.2512 0.1372 9.118 P < 0.001X1 � X1 �3.1792 0.2020 �15.740 P < 0.001X2 � X2 �2.0467 0.2020 �10.133 P < 0.001X3 � X3 �1.6917 0.2020 �8.375 P < 0.001X1 � X2 �1.5675 0.1941 �8.077 P < 0.001X1 � X3 �0.5525 0.1941 �2.847 0.036X2 � X3 �0.2750 0.1941 �1.417 0.216

Source Degrees offreedom

Sum ofsquares

Mean sum ofsquares

F P

ANOVARegression 9 373.165 41.463 275.25 P < 0.001Linear 3 305.943 101.981 676.99 P < 0.001Square 3 55.870 18.623 123.63 P < 0.001Interaction 3 11.352 3.784 25.12 0.002Residual

error5 0.753 0.151

Lack of fit 3 1.334 0.230 7.27 0.123Pure error 2 0.063 0.032Total 14 373.919R2 0.9980Q2 0.9701

Using the designed experimental data, the polynomial model forthe conversion of oleic acid was regressed by considering the sig-nificant terms and was shown as follows:

Y ¼ 75:50þ 5:39X1 þ 2:77X2 þ 1:25X3 � 3:18X21 � 2:05X2

2

� 1:69X23 � 1:57X1X2 � 0:55X1X3 � 0:28X2X3 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 cat-alysts, the molar ratio of ethanol to oleic acid and reaction temper-ature, respectively.

The significance of each coefficient was determined by t-valuesand P-values (Table 3). The larger the magnitude of the t-value andsmaller the P-value, the more significant is the corresponding coef-ficient (Khuri and Cornell, 1987). So, the variable with the largesteffect was catalyst to FFA oleic acid weight ratio. The linear effectsof catalyst amount and ethanol/oleic acid molar ratio are more sig-nificant than those of reaction temperature. Moreover, the qua-dratic effect of catalyst to oleic acid weight ratio is moresignificant than those of molar ratio of alcohol to acid and reactiontemperature (X2

1 has the highest absolute value of t-value in all thequadratic items). The interactive effect of catalyst to oleic acidweight ratio and molar ratio of alcohol to acid might be significantto some extent (X1 and X2 has the highest absolute value of t-valueand the smallest P-value in all the interactive items).The greatimportance of catalyst amount in the conversion to ethyl esterwas also emphasized by Yuan et al. (2008).

The response surfaces and contour plot for the above mentionedmodel for oleic acid conversion are generally used to evaluate therelationships of parameters, and the graphical representation ofthe regression equation. In general, 2D plots can provide informa-tion on the influence of the main process variables in a chemicalreaction; however, a reactor can perform differently at differentlevels of process variables for most chemical reactions. Therefore,3D plots present the overall behavior in a better way. 3D responsesurface plots are displayed in Fig. 2, and show oleic acid conversionas function of two variables, while keeping other variables at thezero level. Fig. 2 (1) shows the effects of catalyst to FFA weight ra-tio, molar ratio of alcohol to acid and their reciprocal interaction onoleic acid conversion. Increasing quantities of SG–T–P catalystbrought about a high conversion ratio, but excess amounts of cat-alyst led to a decline in the conversion ratio. Moreover, it wasnoted that the conversion ratio of oleic acid increased with increas-ing ethanol/oleic acid molar ratios, and then decreased. The effectsof catalyst quantities, temperature and their reciprocal on oleicacid conversion are shown in Fig. 2 (2). Oleic acid conversion in-creased with the increase in temperature, and it was clear thatwhen temperature was fixed at one level, the change in oleic acidconversion showed a parabolic pattern with catalyst to oleic acidweight ratio. Fig. 2 (3) depicts the effects of molar ratio of alcoholto acid and reaction temperature on oleic acid conversion. Theinteraction between the corresponding variables was negligiblewhen the contour of the response surface was circular. On the con-trary, the interactions between the relevant variables were signif-icant when the contour of the response surfaces was elliptical.Fig. 2 (3) indicates that the interaction of molar ratio of alcoholto acid and temperature were not obvious.

3.3. Optimization of reaction conditions

The optimum values of the selected variables were obtained bysolving the regression Eq. (4) and the optimal conditions for ethylester production of esterification of oleic acid with ethanol esti-mated by the model equation were X1 = 0.73, X2 = 0.38, andX3 = 0.22. The theoretical conversion ratio was Y = 78.14% underoptimal conditions (catalyst to FFA weight ratio: 14.5 wt.%, molar

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

0.0 0.2 0.4 0.6 0.8 1.00

100

200

300

400

500

Qua

ntity

ads

orbe

d(cm

3/g

STP)

Relative Pressure P/P0

SG-T-P-1 Adsorption Desorption

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

262 P. Yin et al. / Bioresource Technology 110 (2012) 258–263

ratio of alcohol to acid: 8.8:1, reaction temperature: 112 �C). Toconfirm the prediction, three independent experiments were con-ducted under the established optimal conditions. The average con-version ratio reached 77.02 ± 0.62% and was close to the predictedvalue. Thus, response surface methodology with appropriateexperimental design can be effectively applied to optimize the pro-cess of factors in this esterification synthesis of oleic acid with eth-anol over SG–T–P catalyst.

The esterification of FFA with ethanol usually results in loweryields than with methanol, and the reaction system is more af-fected by the presence of water as water and ethanol can forman azeotropic mixture. Luneca et al. (2011) reported an increasein the yield of ethyl ester by 15.7% using an adsorption system.When water was removed from the reaction mixture by molecularsieves, the reaction conversion ratio (77.85%) did not increase un-der the above-mentioned optimal conditions (catalyst to FFAweight ratio: 14.5 wt.%, molar ratio of alcohol to acid: 8.8:1, reac-tion temperature: 112 �C). The reason probably is that the SG–T–P catalyst has a porous structure and has adsorbed water. In orderto verify the point that SG–T–P can adsorb water, measurements ofthe porous structure of triethylenetetramine bis(methylene phos-phonic acid)- functionalized silica catalyst SG–T–P were carriedout. Fig. 3 shows that the nitrogen adsorption–desorption iso-therms for SG–T–P organophosphonic acid-functionalized silicacatalyst is a type IV according to the IUPAC classification (Singet al., 1985) with a hysteresis loop representative of mesopores.

The adsorbed volume increased steeply at medium relativepressure (p/p0) indicating capillary condensation of nitrogen with-in the uniform mesoporous structure, and the two lines areapproximately parallel, indicating that the pores of silica have auniform radius and are open. The BET surface area and the BJHdesorption cumulative volume of pores and BJH desorpion averagepore diameter of SG–T–P were 192.41 m2/g, 0.65 cm3/g, and

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

4. Conclusions

Esterification of free fatty acid oleic acid with ethanol overorganophosphonic acid-functionalized silica SG–T–P was success-ful. Response surface methodology showed that the most impor-tant experimental factor affecting the esterification reaction wasthe amount of catalyst. Under the optimal conditions, the predictedvalue of the conversion ratio of oleic acid could reach 78.14%. Thepresent catalyst exhibits catalytic activity, and also adsorbs waterfrom the reaction system at the same time. Therefore, SG–T–Pcan be a potential catalyst for biodiesel production as it reducesequipment needs and cost. Further work is underway to improve

P. Yin et al. / Bioresource Technology 110 (2012) 258–263 263

its catalytic activity by synthesizing composite catalysts usingSG–T–P to make the process commercially feasible.

Acknowledgements

We greatly appreciate the support provided by the NationalNatural Science Foundation of China (51102127 and 51073075), theNature Science Foundation of Shandong Province (2009ZRB01463),and the Foundation of Innovation Team Building of Ludong Univer-sity (08-CXB001).

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