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Biodiesel Synthesis by Simultaneous Esterification and Transesterification Using

Oleophilic Acid Catalyst

Yi-Shen Lien, Li-Shan Hsieh, and Jeffrey C. S. Wu*

 Department of Chemical Engineering, National Taiwan UniVersity, Taipei, Taiwan 10617 

Solid-acid catalysts can perform transesterification and esterification simultaneously so that free fatty acids(FFAs) in oil can be converted into biodiesel to avoid the disposal of biomaterial. A carbon catalyst wasprepared by pyrolyzing glucose at 400 °C under a N2 stream. The catalyst was further sulfated usingconcentrated sulfuric acid. Transesterification of soybean oil and methanol was carried out at 150 °C and 1.7MPa in a pressurized autoclave. More than 90% biodiesel yield was achieved within 2 h with a molar ratioof methanol to soybean oil of 30:1. The total biodiesel yield for the mixture of soybean oil and palmitic aciddecreased to 85% when 20 wt % palmitic acid was used. A rate equation based on the Langmuir -Hishelwoodmechanism was established to describe the kinetic behavior of transesterification. The adsorption equilibriumconstant of soybean oil was higher than those of the other species, implying an oleophilic surface of thesulfated carbon catalyst.

Introduction

Biodiesel is one of the most popular biofuels because it iscompletely compatible with fossil diesel and the synthesis

process is commercially available. Biodiesel can be produced

by the transesterification of oil under ambient pressure at around

60 °C as shown in eq 1. In the reaction, 1 mol of triglyceride

reacts with 3 mol of methanol under the help of catalysts, such

as NaOH. The main product is fatty acid methyl ester (FAME),

known as biodiesel, and the byproduct is glycerol. Usually,

excess methanol is applied to enhance the conversion of 

triglyceride. The separation of excess methanol is done simply

by distillation, and methanol is recycled to the feed. Because

FAME is immiscible with glycerol, the former can be recovered

by decanting.1,2

The traditional synthesis of biodiesel used homogeneous

catalyst, NaOH, in a liquid-phase reaction. Although this method

is simple and fast, the neutralization/washing step requires the

consumption of acid, and the generation of acid/base waste

becomes an environmental problem. Furthermore, when raw oil

contains high free fatty acid (FFA) content (>4%), FFAs mustbe removed before transesterification. Otherwise, the saponifica-

tion of FFAs occurs and produces undesired soap.3 The base

catalyst, NaOH, is also consumed or deactivated by the

saponification as shown in eq 2.4 Meher et al. suggested an

alternative way to solve the FFA problem by using an acid

catalyst. The esterification of FFA by methanol can be carried

out before the transesterification of oil as shown in eq 3.5 Thus,

valuable FFAs can be completely converted to biodiesel without

being wasted.

Using heterogeneous catalysts, such as solid-acid or base

catalysts, is an attractive route for biodiesel production because

there is no aqueous waste. A process of biodiesel synthesis is

shown in Figure 1. A direct transesterification can be performed

when the FFA content is less than 4% in the oil feed.

Alternatively, pre-esterification can be carried out by acid

catalyst to convert FFAs (>4%) to FAME, followed by

transesterification by a base catalyst. Thus, the preliminary

separation of FFA in the oil feed is eliminated, and the yield of 

biodiesel is increased. However, the two-step process is rathercomplicated and can be further simplified. The esterification

and transesterification can be carried out simultaneously in a

one-step process by using acid catalyst.6,7 Even though the

esterification of FFAs with methanol to FAME is fast on acid

catalyst, the transesterification is very slow on a regular acid

catalyst. Thus, a superacid catalyst is necessary to give a

reasonable transesterification rate comparable to that on base

catalyst.8

Our objective was to prepare a highly acidic carbon catalyst

to replace the conventional NaOH process in biodiesel produc-

tion.9 We followed the method reported by Toda et al. to

* To whom correspondence should be addressed. Tel.: +886-2-2363-1994. Fax: +886-2-2362-3040. E-mail: cswu@ntu.edu.tw.

RCOOH + CH3OH f RCOOCH3 + H2O (3)

Figure 1. Two-step processes of biodiesel production.

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synthesize sulfated carbon-base catalysts.10 The carbon support

was prepared by the carbonization of glucose to form small

polycyclic aromatic carbon rings that provided anchoring points

for sulfonite groups (-SO3H), supplied by sulfuric acid. In

addition, the carbon support is oleophilic, which means oil can

be easily adsorbed on the catalyst surface as compared with

the conventional hydrophilic oxide supports. Thus, the rate of 

transesterification can be enhanced as a result of increasing oil

concentration on the catalyst surface.

Experimental Section

The carbon support was prepared by the pyrolysis of glucose.

Twenty grams of glucose was pyrolyzed at 400 °C for 15 h

under a N2 flow. After the carbon had been pulverized, 200

mL of 98% sulfuric acid was added, and the mixture was heated

at 150 °C for 15 h under a N 2 atmosphere. The sulfated carbon

was washed with 1 L of deionized water at 80 °C for 30 min

and then separated by centrifugation. The washing process was

repeated seven times to remove residual sulfuric ion.10 The

surface area of catalyst was measured by Ar adsorption and

calculated by the Brunauer-Emmett-Teller (BET) method. The

strength of acidity was examined using color-producing indica-

tors, anthraquinone ( H o ) -8.2) and p-nitrotoluene ( H o )

-11.35). Because the coloration of the indicator cannot be

observed by visual inspection due to the black carbon, the color

change of indicator was determined by a diffuse reflectance

UV-vis spectrometer (Varian Cary 100). Prior to the UV-vis

measurement, the catalyst (0.2 g) was mixed with BaSO 4 (1.0

g, a reference material for UV-vis spectroscopy) and heated

at 80 °C for 1 h to remove any adsorbed moisture. The acidic

density of the catalyst was measured by acid-base back-titration

using NaOH and HCl aqueous solutions. The detailed procedure

for acidity measurement was reported in the literature.11

Soybean oil (Uni-President Co., Yungkang City, Taiwan) was

purchased from the supermarket and used without pretreatment.

The transesterification of soybean oil was carried out in apressurized autoclave (model 4560, Parr) under 1.7 MPa

pressure at 130-150 °C for 6 h. Approximately 100 g of 

soybean oil was charged into the autoclave for each batch

reaction. The molar ratio of methanol to soybean oil ranged

from 6 to 30. The catalyst loading was 1 -3 wt % soybean oil.

The influence of FFAs was studied by adding palmitic acid into

the soybean oil at concentrations ranging from 5 to 20 wt %.

The products were analyzed by a gas chromatograph (HP-6890,

Agilent) equipped with a 30-m-long HP-Innowax column and

a flame ionization detector (FID). A 0.5-mL aliquot of sample

was withdrawn and diluted with 25 mL of isopropanol for gas

chromatography (GC) analysis each hour during the reaction.

The biodiesel yield is defined as the amount of FAMEs formedin the transesterification reaction. The peak areas of FAMEs

were lumped as total FAME. Therefore, the biodiesel yield was

calculated as the weight percentage of total FAMEs relative to

soybean oil.

Results and Discussion

The specific surface area of the carbon support was 0.31 m2 /g

as determined by Ar adsorption. The carbon support was

nonporous with a mean particle diameter of 38.7 µm. The acidic

densities of sulfated catalysts are listed in Table 1. Fresh catalyst

had an acidic density of 2.73 mmol of SO3H/g. The acidic

density of the catalyst was only slightly reduced after three

consecutive transesterifications at 60 °C. To examine the lossof acidic density at high temperature and pressure, the catalysts

were tested in glycerol solution under two conditions. The acidic

loss was not serious at high temperature and ambient pressure

(Table 1). However, a significant acidity loss was found under

a high pressure of 1.7 MPa and at a temperature of 150 °C (see

Table 1). The catalyst was heated at 150 °C for 15 h under

ambient pressure (0.1 MPa) in the preparation procedure.

Therefore, the catalyst should be stable at the reaction temper-

ature of 150 °C. However, the leaching of SO3H groups could

be significant under harsh conditions of high pressure (i.e., 1.7

MPa). A similar acidity loss due to SO 3H leaching was also

observed on sulfated carbon catalyst by Mo et al.9

Figure 2 shows the correlation between FAME (i.e., biodiesel)

yield and reaction time for different catalyst loadings at 150

°C. A FAME yield of higher than 90% was obtained within2 h of reaction time for 3 wt % catalyst loading. It is noted that

the production rate of FAME decreased with decreasing catalyst

loading. Nevertheless, near 90% FAME yield was still achieved

in 6 h for 1 wt % catalyst loading. Figure 3 shows the influence

of the methanol/oil ratio on the correlation between FAME and

reaction time for 1 wt % catalyst loading at 150 °C. Apparently,

the FAME yield decreased with decreasing ratio of methanol

to oil. Theoretically, 3 mol of methanol is sufficient to convert

1 mol of triglyceride as shown in eq 1. However, because

methanol and soybean oil are not miscible, a lower-than-

expected conversion rate resulted. To increase the utilization

of oil, a much higher methanol-to-oil ratio is usually used to

obtain a better dispersion, thus increasing the conversion of oil.Afterward, methanol can be easily separated by vaporization

for recycling. The temperature effect on transesterification is

shown in Figure 4. The FAME yield increased with temperature,

as expected. The temperature was limited to 150 °C because

the vapor pressure of methanol at this temperature is 1.4 MPa

and it is under such a vapor pressure that liquid-phase methanol

could be maintained in the reactor.

Figure 5 shows the conversion of pure palmitic acid in

esterification using 1 wt % catalyst loading at 60 °C. Note that,

Table 1. Acidic Density of Catalysts under Different Conditions

conditionsacidic density

(mmol of SO3H/g)

fresh 2.73

catalyst used for three consecutivetransesterifications at 60 °C

2.51

catalyst treated at 150 °C under ambientpressure

2.66

catalyst treated at 150 °C under a pressure of 1.7 MPa

1.18

Figure 2. Effect of catalyst loading on FAME yield, for methanol/oil )

30:1, at 150 °C.

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at a low temperature of 60 °C, a conversion of near 60% can

be achieved in less than 4 h. However, the transesterificationof soybean oil was negligible at 60 °C in our experiment (not

shown), which justifies the conclusion that the rate of esterifi-

cation must be much higher than the rate of transesterification

using a solid-acid catalyst. Figure 6 shows the esterification

conversion of palmitic acid mixed with soybean oil at 150 °C.

As shown in Figure 6, the conversion achieves 100% in less

than 1 h for 5-20 wt % palmitic acid in soybean oil. The rate

of esterification is much higher than the rate of transesterification

in the mixture, which suggests that palmitic acid is easily

adsorbed and reacts quickly on acid sites. Figure 7 shows the

total FAME yield of the mixture of palmitic acid and soybean

oil at 150 °C. The total FAME yield is the sum of palmitic and

soybean oil, indicating that the simultaneous esterification of 

FFAs and transesterification of oil can be carried out on the

solid-acid catalyst. Apparently, no significant hindrance in the

reaction rate is observed even at a high content of palmitic acid

in soybean oil.

A Langmuir-Hinlshelwood rate equation was derived based

on simple elemental steps including adsorption, surface reaction,

and desorption. Both soybean oil and methanol adsorbed on

the same active sites. The rate-limiting step was assumed to bethe surface reaction. The rate constant and equilibrium constants

Figure 3. Effect of methanol-to-oil ratio on FAME yield, for 1 wt % catalyst,at 150 °C.

Figure 4. Effect of temperature on FAME yield, for 1 wt % catalyst andmethanol/oil ) 30:1.

Figure 5. Esterification of pure palmitic acid at 60 °C, for methanol/palmiticacid ) 30:1 and 1 wt % catalyst.

Figure 6. Conversion of palmitic acid to biodiesel in soybean oil, for molarmethanol/oil ) 30:1 and 1 wt % catalyst, at 150 °C.

Figure 7. Effect of palmitic acid on biodiesel yield, for molar methanol/oil) 30:1 and 1 wt % catalyst, at 150 °C.

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in eq 4 are listed in Table 2 and were estimated by nonlinear

regression based on experimental data.

In eq 4, C O, C M, C F, and C G are the concentrations of soybean,

methanol, FAME, and glycerol, respectively. k  is the rate

constant; K  is the reaction equilibrium constant; and K 1, K 2,

K 3, and K 4 are the adsorption equilibrium constants of soybean

oil, methanol, FAME, and glycerol, respectively.

The correlation coefficients ( R2) in Table 2 are all close to 1,

meaning good statistical consistency of experimental data. The

enthalpy of transesterification was calculated to be 14.9 kcal/ mol from chemical equilibrium constants (K ) at temperatures

ranging from 130 to 150 °C, indicating an endothermic reaction.

The adsorption equilibrium constant of soybean oil, K 1, was

found to range from 9.71 to 6.83 at temperatures of 130-150

°C. The value was much higher than those of the other three

adsorption equilibrium constants for methanol (K 2), FAME (K 3),

and glycerol (K 4), indicating a strong affinity of oil on the

catalyst. This implies an oleophilic property of the surface of 

carbon-based catalyst. Moreover, the byproduct, hydrophilic

glycerol, could also be desorbed quickly from the catalyst

surface so that more active sites could be made available for

oil molecules. Therefore, the rate of transesterification was

enhanced by the high concentration of oil as well as the high

turnover of active sites on the catalyst surface.

Conclusions

The conventional alkali aqueous-phase process has several

drawbacks for biodiesel production, such as the generation of 

liquid acid/base waste in the neutralization step. In addition,

FFAs must be removed before the transesterification of oil

because FFAs consume base catalyst (e.g., NaOH) as a result

of saponification. Solid-acid catalyst can be easily separated

from the products and reused without generating aqueous acid/ 

base waste. Extra energy can also be saved on product separation

by using a solid catalyst as compared with the aqueous NaOH

process. An additional advantage of solid-acid catalyst is toachieve esterification simultaneously, thus eliminating the

preliminary separation of FFAs from the oil feed. The disposal

of FFAs is not only expensive but also a loss of biomaterial. In

this study, oleophilic acidic carbon catalysts were prepared and

successfully applied to the synthesis of biodiesel. A >90% yield

of FAME from soybean oil was achieved within 2 h at 1.7 MPaand 150 °C. The simultaneous process of esterification and

transesterification will greatly improve the efficiency of biodiesel

production when waste cooking oil and animal fat are used as

the raw materials. Unlike in the aqueous NaOH batch process,

a solid-acid catalyst can also be used in a packed-bed reactor

so that the continuous production of biodiesel is feasible.

Acknowledgment

Financial support by the Ministry of Economic Affairs,

Taiwan, under Grant 98-EC-17-A-09-S1-019 is gratefully

acknowledged.

Literature Cited

(1) Ma, F. R.; Hanna, M. A. Biodiesel production: A review. Bioresour.

Technol. 1999, 70 (1), 1.(2) Gerpen, J. V. Biodiesel processing and production. Fuel Process.

Technol. 2005, 86  (10), 1097.(3) Freedman, B.; Pryde, E.; Mounts, T. Variables affecting the yields

of fatty esters from transesterified vegetable oils. J. Am. Oil Chem. Soc.

1984, 61 (10), 1638.(4) Lotero, E.; Liu, Y. J.; Lopez, D. E.; Suwannakarn, K.; Bruce, D. A.;

Goodwin, J. G. Synthesis of biodiesel via acid catalysis. Ind. Eng. Chem.

 Res. 2005, 44 (14), 5353.(5) Meher, L. C.; Sagar, D. V.; Naik, S. N. Technical aspects of biodiesel

production by transesterificationsA review. Renewable Sustainable Energy

 ReV. 2006, 10 (3), 248.(6) Lupez, D. E.; Goodwin, J. G., Jr.; Bruce, D. A.; Furuta, S.

Esterification and transesterification using modified-zirconia catalysts. Appl.Catal. A: Gen. 2008, 339 (1), 76.

(7) Yan, S. L.; Salley, S. O.; Ng, K. Y. S. Simultaneous transesterificationand esterification of unrefined or waste oils over ZnO-La2O3 catalysts.

 Appl. Catal. A: Gen. 2009, 353 (2), 203.(8) Furuta, S.; Matsuhashi, H.; Arata, K. Biodiesel fuel production with

solid superacid catalysis in fixed bed reactor under atmospheric pressure.Catal. Commun. 2004, 5 (12), 721.

(9) Mo, X.; Lopez, D. E.; Suwannakarn, K.; Liu, Y.; Lotero, E.;Goodwin, J. G.; Lu, C. Q. Activation and deactivation characteristics of sulfonated carbon catalysts. J. Catal. 2008, 254 (2), 332.

(10) Toda, M.; Takagaki, A.; Okamura, M.; Kondo, J. N.; Hayashi, S.;Domen, K.; Hara, M. Green chemistrysBiodiesel made with sugar catalyst.

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 ReceiVed for reView September 23, 2009 ReVised manuscript receiVed  January 19, 2010

 Accepted  January 23, 2010

IE901496H

Table 2. Kinetic Constants of Rate Equation a

temperature (°C) k  (mol-1 L gcat-1 h-1) K K 1 (mol-1 L) K 2 (mol-1 L) K 3 (mol-1 L) K 4 (mol-1 L) R2

130 1.20 4.17 9.71 0.06 1.34 0.64 0.91

140 1.28 5.04 8.64 0.03 0.70 0.48 0.93

150 1.38 5.14 6.83 0.01 0.46 0.23 0.97

a Where k  is the rate constant; K  is the reaction equilibrium constant; and K 1, K 2, K 3, and K 4 are the adsorption equilibrium constants of soybean oil,methanol, FAME, and glycerol, respectively.

rate )

k (C OC M -C FC G

K  )(1 + K 1C O + K 2C M + K 3C F + K 4C G)

2(4)

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