ORIGINAL PAPER

Sulfonic acid functionalized solid acid: an alternative eco-friendlyapproach for transesterification of non-edible oils with high freefatty acids

Deepali A. Kotadia • Saurabh S. Soni

Received: 21 May 2013 / Accepted: 22 June 2013 / Published online: 13 August 2013

� Springer-Verlag Wien 2013

Abstract A highly efficient procedure has been devel-

oped for the synthesis of biodiesel from non-edible oil

using a supported acidic ionic liquid catalyst. The effects of

reaction time, temperature, and other reaction parameters

on the reaction were investigated. The supported ionic

liquid catalyst was very efficient and gave an approxi-

mately 95 % yield when the reaction was carried out under

the optimized conditions. The process represents a simple,

ecologically safer route to transesterification with high

product quality, as well as product recovery and catalyst

recycling.

Keywords Supported ionic liquid �Heterogeneous catalysis � Biodiesel � Fatty acids �Transesterification

Introduction

With crude fossil fuel prices at nearly an all-time high,

biodiesel has emerged as one of the most promising

renewable energy sources to replace current petroleum-

derived diesel [1, 2]. Biodiesel possesses particular

advantages of being a renewable, biodegradable, and non-

toxic fuel which can be easily produced through transe-

sterification [3]. However, current commercial usages of

refined vegetable oils/first-generation alternative biofuels

have proven controversial owing to high feedstock cost and

their priority use as food resources [4, 5]. The biodiesel

producers are now more focused towards so-called second-

generation non-food biomass sources like cellulose, algae,

or non-edible plant oils from the Euphorbiaceae family,

notably castor and jatropha oil [6], which do not compete

with the traditional agronomies. The economic evaluation

has shown that biodiesel production from non-edible oils is

very profitable provided the by-products of biodiesel pro-

duction can be sold as valuable products [1, 2]. The

transesterification reaction (Scheme 1) is carried out by

base (homogeneous or heterogeneous catalysts), acid

(homogeneous or heterogeneous catalysts), and enzyme

catalysis [1, 2]. The base-catalyzed transesterification

reaction is the most used process, but suffers from major

drawbacks like sensitivity to moisture and free fatty acids

(FFAs). The base catalyst tends to react with the FFAs to

form soaps which lowers the biodiesel yield and inhibits

the separation of the esters from glycerol [7]. In addition, it

binds to the catalyst; thus, more catalyst is required to

ensure complete transesterification, and this involves a

higher process cost. Usually this problem is overcome

through a two-step process; a previous acid-catalyzed

esterification of FFA followed by base-catalyzed transe-

sterification of triglycerides. Alternatively homogeneous

acid catalysts have been proposed to promote simultaneous

esterification and transesterification in a single catalytic

step, thus avoiding the pre-conditioning step when using

low cost feedstock with high FFA content. However,

inorganic acids used are corrosive in nature and the reac-

tion requires use of high alcohol to oil ratios and high

reaction times [8]. The use of heterogeneous catalysts

could be an attractive solution [9, 10], as they have the

potential to replace liquid acids, eliminating separation,

corrosion, and environmental problems. Heterogeneous

Electronic supplementary material The online version of thisarticle (doi:10.1007/s00706-013-1041-4) contains supplementarymaterial, which is available to authorized users.

D. A. Kotadia � S. S. Soni (&)

Department of Chemistry, Sardar Patel University,

Vallabh Vidyanagar, Gujarat 388120, India

e-mail: soni_b21@yahoo.co.in

123

Monatsh Chem (2013) 144:1735–1741

DOI 10.1007/s00706-013-1041-4

catalysts possess many advantages like insensitivity to

FFA, simultaneous esterification and transesterification,

they eliminate the need for a washing step, and they allow

the easy separation of the catalyst, resulting in lower

product contamination, easy regeneration and recycling of

catalyst, and reduced corrosion problems [11]. The use of

heterogeneous catalysts for biodiesel production from dif-

ferent feedstock has been studied by many researchers

[12–23]. Development of efficient and low cost heteroge-

neous catalysts for transesterification of low cost vegetable

oils can lead to a much lower total production cost of

biodiesel. Heterogeneous catalysts can be grouped as basic,

acidic, or biological (enzymatic) types. Selection of a

catalyst from these groups depends on the type of feedstock,

operating conditions, required catalyst activity, cost, and

availability [24]. Solid acids favor both esterification and

transesterification reactions simultaneously for biodiesel

with high FFA such as non-edible oils. The ideality of het-

erogeneous catalyst lies in the fact that they should have an

interconnected system of large pores, a moderate to high

concentration of strong acid sites, and a compatible surface.

In recent years, ionic liquids (ILs) have become pow-

erful alternatives to conventional molecular organic

solvents owing to their particular properties, such as

undetectable vapor pressure and the ability to dissolve

many organic and inorganic substances [25]. In addition,

ILs are readily recycled and tunable to specific chemical

tasks. One type is Brønsted acidic ILs (BAILs). The acidic

nature of these ILs as catalyst has been exploited for many

other important organic reactions, which proceed with

excellent yields and selectivities and demonstrate the great

potential of these ILs in catalytic technologies for chemical

production [26–28]. Recently, immobilization process

involving acidic ILs on solid supports has been designed.

On the basis of economic criteria, it is desirable to mini-

mize the amount of IL utilized in a process and to make

separation processes easier as most of the acidic ILs are

soluble in methanol making recovery of IL difficult. Sup-

ported acidic ionic liquid catalysis involves immobilization

of functional moieties with IL-like structures onto a surface

of a porous high area support material [29]. The use of

IL-like functionalities as modifiers in support systems is

very interesting because ideally they will tune the nature of

materials by transferring the IL properties at molecular and

nanoscale level [30–32]. A number of major benefits arise

from solid catalysts, e.g., high surface area, availability of

active sites, ease of separation, and recycling reproduc-

ibility. In this context, we have previously synthesized and

characterized benzimidazolium-based Brønsted acidic

supported ionic liquid catalyst (SILC) and used it as a

catalyst for the synthesis of 1-(amidoalkyl)naphthols [33].

The bright prospects of the application of SILC in synthesis

spurred us to investigate their ability to catalyze the

transesterification of non-edible plant oils. To our knowl-

edge, the use of immobilized –SO3H functionalized ionic

liquid as a solid catalyst for the synthesis of biodiesel has

not been reported before. It can be expected that the

immobilization of IL on silica should provide not only

considerable activity towards the transesterification reac-

tion but also improve the stability of the catalyst. In the

present work, the catalytic activity of SILC was evaluated

for the synthesis of biodiesel from castor oil, jatropha oil,

and neem oil with high acid value. The influence of various

reaction parameters such as reaction temperature, time,

methanol to oil ratio, and catalyst loading was also studied.

Results and discussion

The SILC was prepared by a process reported earlier by us

[33] (Fig. 1). Sulfonic acid functionalized SILC was syn-

thesized via a two-step procedure; the first step involves the

covalent attachment between silica gel and 3-chloro-

propyltriethoxysilane which is treated with benzimidazole

salt to give 3-(1-benzimidazolyl)propyl silica. The second

step involves condensation of 3-(1-benzimidazolyl)propyl

silica with 1,3-propanesultone, which is further treated

with one equivalent of HCl to give sulfonic acid func-

tionalized SILC. The resulting material is a white-colored

free-flowing powder and its acidic site loading was found

to be 0.588 mmol/g [33].

We opted to use SILC as a catalyst for transesterification

owing to its stability, excellent catalytic activity, and

importantly the presence of the hydrophilic functional

group –SO3H. The incorporation of –SO3H groups on the

silica surface results in the increased hydrophilicity of the

CH2

CH

CH2

O

O

O

C

C

C

O

O

O

R1

R2

R3

3 CH3OH

CH2

CH

CH2

OH

OH

OH

CH3O

CH3O

CH3O

C

C

C

O

O

O

R1

R2

R3

Triglyceride Methanol Methyl Ester Glycerol

Scheme 1

O

O

O

SiN N

SO3H

Cl

SILC =

Fig. 1 Sulfonic acid functionalized benzimidazolium-based sup-

ported ionic liquid catalyst (SILC)

1736 D. A. Kotadia, S. S. Soni

123

silica material, and therefore hydrophilic molecules (such

as methanol) can more easily get into the interior of the

silica bulk and react more easily with the hydrophobic

reactants (triglyceride and FFAs). Furthermore, upon

addition of sultone, larger numbers of sulfonic acid sites

are introduced and these interior acid sites are generally

adopted as catalytically active sites which can catalyze the

transesterification of triglycerides and the esterification of

FFAs [13].

The activity of SILC was tested for the transesterifica-

tion of castor oil with high acid value. In a typical reaction,

castor oil (10 mmol), methanol (120 mmol), and SILC

(3 wt%) were taken in the reaction vessel and allowed to

react for 6–8 h at 70 �C. The catalyst shows good to

excellent activity towards biodiesel synthesis. The pre-

liminary optimization was achieved by studying the

influence of various reaction parameters like reaction

temperature, time, methanol to oil molar ratio, and amount

of catalyst loading. To study the influence of reaction

temperature on the yield of FAME, experiments using

SILC were conducted at various temperatures from 50 to

90 �C. As shown in Fig. 2, with the increase in reaction

temperature from 50 to 70 �C, there is an increase in

FAME yield; however, further increasing the reaction

temperature has no particular effect on FAME yield. In

general, higher temperature usually can lead to a higher

yield as it makes molecules move more actively, which

increases the collision probability of molecules of oil and

methanol, thus accelerating the reaction more easily.

Therefore, yield of FAMEs increases with temperature

(Fig. 2). However, much higher temperature is not advised,

because the boiling point of methanol is 64 �C; thus,

methanol volatilized and became less available to react

when the temperature was greater than 70 �C. Thus, 70 �C

was chosen as the optimum reaction temperature for the

synthesis of biodiesel from castor oil.

The molar ratio of methanol to oil is the most important

variable affecting the yield of FAME. Although the stoi-

chiometric molar ratio of methanol to triglyceride is 3:1,

for transesterification reactions higher molar ratios are used

to enhance the solubility and to increase the contact

between the triglyceride and methanol molecules [34]. The

effect of methanol to oil ratio was investigated using SILC

and the results are shown in Fig. 3. With the increase in

methanol to oil ratio, the conversion of oil to FAME was

enhanced steadily and reached 87.5 % at the molar ratio of

12:1. However, there was no obvious increase in the con-

version with increasing methanol addition beyond the

molar ratio of 12:1. Thus, the optimum molar ratio of

methanol to castor oil for the reaction was taken as 12:1.

The SILC loading amount plays a crucial role in

determining the yield of FAME and this can be illustrated

by carrying out the transesterification at 70 �C with dif-

ferent amounts of SILC varying from 2 to 5 wt%. From the

results (Fig. 4) it can be seen that the yield of FAME

increases from 46.2 to 94.9 % for 1–3 wt% SILC. On

further increasing the amount of SILC to 4 wt% , little

increase in FAME yield (95.3 %) was observed; therefore,

3 wt% SILC was considered suitable for the transesterifi-

cation reaction. The active component of the catalyst is the

sulfonic acid group (–SO3H). With the increase in the

amount of catalyst, the number of acidic sites also

increases, which leads to an increase in the FAME yield.

The sulfonic acid groups are principally located on the

porous surface of silica, where they are accessible for

adsorption and catalytic reaction processes [29].

The effect of reaction time on the yield of FAME was

examined at 70 �C with 3 wt% of SILC loading for 12:1

methanol to castor oil ratio. The yield of FAME increased

50 60 70 80 900

20

40

60

80

100

Reaction temperature/°C

FA

ME

yie

ld/%

Fig. 2 Effect of reaction temperatures on FAME yield

3:1 6:1 9:1 12:1 15:1 20:10

20

40

60

80

100

FA

ME

yie

ld/%

Methanol/oil ratio

Fig. 3 Effect of methanol to oil ratio on FAME yield

Sulfonic acid functionalized solid acid 1737

123

steadily from 47.2 to 94.2 % from 2 to 6 h, respectively,

and remains steady afterwards up to 8 h reaction time

(Fig. 5). A slight decrease in FAME yield was observed for

10 h reaction time which can be explained as the transe-

sterification reaction is a reversible process with an

equilibrium time around 6 h, which was therefore taken as

the ideal time for the transesterification reaction of castor

oil with methanol.

In order to explore the catalytic activities of SILC for

the transesterification reaction, two other non-edible veg-

etable oils, viz., jatropha and neem oil, with high FFAs

were studied and the results are shown in Table 1. The oils

used are raw oils with varying acid values and FFA com-

position which lets us study the behavior of SILC towards

these oils. The result showed that SILC was very efficient

for all the oils with high acidity. The longer reaction time

for jatropha and neem oil is required because of slow

mixing and dispersion of methanol in oil. On the other

hand castor oil and methanol are readily miscible and thus

need less time (about 6 h) to reach equilibrium. Both the

oil and the FFA could be transformed to biodiesel under

these conditions. The catalyst has high activities for

transesterification of oil as well as esterification of the FFA.

One way of reducing the biodiesel production costs is to

use the less expensive feedstock containing FFAs. Bio-

diesel produced from these oils showed similar

performance to that of biodiesel produced from fresh

vegetable oils. Our investigation shows that SILC could be

a good candidate for biodiesel production from vegetable

oils with high FFAs content because of its high activity for

the conversions of FFAs.

The catalyst SILC was recycled to study its reusability

by performing several runs under the optimized reaction

conditions determined above. After the first run the catalyst

was filtered, washed with methanol, acetone, dried and

used in next run. The recycling data for SILC (Fig. 6)

shows decrease in FAME yield from 94.9 to 82.8 % after 5

runs, from the results it is clear that SILC is found to be a

promising catalyst for transesterification of castor, jatro-

pha, and neem oil.

After optimizing the reaction conditions and testing the

recyclability of the catalyst, we next attempted to understand

the possible reaction mechanism for the transesterification

1 2 3 40

20

40

60

80

100F

AM

E y

ield

/%

Catalyst/%

Fig. 4 Effect of amount of catalyst on FAME yield

2 4 6 8 100

20

40

60

80

100

FA

ME

yie

ld/%

Reaction time/h

Fig. 5 Effect of reaction time on FAME yield

Table 1 Transesterification reaction of different oils with varying

acid values

Entry Oil Acid value Conventional methoda

Yield/% Time/h

1 Castor oil 10.8 94.9 6

2 Jatropha oil 13.5 95.7 7

3 Neem oil 16.6 94.4 7

a Oil (10 mmol), methanol (120 mmol), SILC (3 wt% ), 70 �C

Fig. 6 Recyclability of SILC for transesterification of castor oil

1738 D. A. Kotadia, S. S. Soni

123

reaction in the presence of SILC (Scheme 2). In case of

SILC, the sulfonic acid functionalized part is the most active

acidic site and will participate in the transesterification

reaction which will activate the triglycerides by protonation

of a carbonyl oxygen. The catalyst–substrate interaction is

essential for protonation of a carbonyl oxygen which in turn

increases the electrophilicity of the adjoining carbon atom,

making it more susceptible to nucleophilic attack. In a

stepwise manner, this will give diglycerides, monoglyce-

rides, and finally glycerol accompanied with the liberation of

an ester at each step with regeneration of SILC. Although the

reaction mechanism for the heterogeneous acid-catalyzed

transesterification is shown to be in principle similar to the

homogeneously catalyzed one [35], there is an important

difference that concerns the relationship between the surface

nature and the catalyst activity. As we discussed earlier the

incorporation of –SO3H groups increases the hydrophilicity

of the silica material, and therefore hydrophilic molecules

(such as methanol) can more easily get into the interior of

silica bulk and can react more easily with the hydrophobic

reactants (triglyceride and FFAs). Also, the larger the num-

ber of sulfonic acid groups present, the higher the catalytic

activity observed for the transesterification reaction.

The performance of SILC catalyst for the oils used for

biodiesel synthesis in the present work is compared with

that of other catalysts used for the same oils, viz. jatropha,

castor, and neem oil, and the results are summarized in

Table 2. The different solid acid catalysts used for

transesterification of these oils show comparable results, all

achieving about 95 % FAME yield. The reaction condi-

tions for solid acid catalyst derived from lignin [20] for

jatropha oil and sulfated zirconia catalyst [21] for neem oil

were mild, whereas those specified for SiO2/H2SO4 acidic

silica gel [22] and H2SO4/C [23] require higher amounts of

catalyst for the effective conversion of FAMEs. In the

present study SILC shows effective conversion of the non-

edible oils into FAMEs under moderate conditions giving

approximately 95 % yield for all the three oils. In com-

parison with other catalysts employed for biodiesel

synthesis, SILC showed a higher activity in terms of its

short reaction time and mild conditions. The higher yields

indicate the effectiveness of the active sites on the catalyst

surface. No leaching of sulfonic acid groups has been

observed and the catalyst can be reused effectively for five

runs.

Conclusion

The supported ionic liquid catalyst comprising benzimi-

dazolium-based acidic ionic liquid immobilized on silica

support is effective in the transesterification of various non-

edible oils, viz. castor, jatropha, and neem oil, with high

content of free fatty acids. The catalyst was very efficient

OO

OOH

OH

Si N N S

O

OH

O

CH2

HC

OR1

H2C

OCOR2

OCOR3

O

Cl OO

OOH

OH

Si N N S

O

O

O

CH2

HC

OR1

H2C

OCOR2

OCOR3

OH

Cl

OO

OOH

OH

Si N N S

O

O

O

CH2

HC

OR1

H2C

OCOR2

OCOR3

OH

HO

R

OO

OOH

OH

Si N N S

O

O

O

CH2

HC

O

O

H2C

OCOR2

OCOR3OH

R1

R

H

HC

H2C

OH

OH

H2C OH R1

O

OR

R2

O

OR

R3

O

OR

ROH

-H

Activation of triglyceride by protonation of carbonyl oxygen using SILC

Nucleophilic attack by alcohol on electrophilic carbon

OO

OOH

OH

Si N N S

O

OH

OCl

RegeneratedOO

OOH

OH

Si N N S

O

O

O

CH2

HC

O

O

H2C

OCOR2

OCOR3OH

R1

R

H

ClCl

Cl

Scheme 2

Sulfonic acid functionalized solid acid 1739

123

for the reaction giving approximately 95 % yield under the

optimum reaction conditions. This process has many

advantages, such as operational simplicity, no saponifica-

tion, high yields, and reusability, which make it a potential

promising green process.

Experimental

Castor oil and neem oil (crude oil) were obtained from Anand

Oil, Anand, Gujarat, India. Jatropha oil was obtained on

request from Anand Agriculture University, Anand, Gujarat,

India. Silica gel 230–440 mesh (0.037–0.063 mm) was

purchased from Spectrochem, India and was used for the

preparation of the support. (3-Chloropropyl)triethoxysilane,

benzimidazole, and 1,3-propanesultone were purchased

from Sigma-Aldrich, India and were used without further

purification. Other chemicals were commercially available

and used without further purification. The transesterified

products obtained were analyzed GC, 1H NMR, and IR (see

Supplementary Material).

Benzimidazolium-based sulfonic acid functionalized

solid support (SILC)

A detailed synthetic procedure for benzimidazole-based

sulfonic acid functionalized solid support (SILC, Fig. 1)

was reported by us earlier [33].

3-Chloropropyl silica

A mixture of 5.0 g silica and 5 cm3 (3-chloropropyl)tri-

ethoxysilane (42.5 mmol) in toluene was stirred at room

temperature and then refluxed for 24 h. After completion of

the reaction, the mixture was cooled, the product was

filtered and repeatedly washed with toluene, and dried

under reduced pressure to produce 3-chloropropyl silica

(4.90 g).

3-(1-Benzimidazolyl)propyl silica

Sodium hydride (0.48 g, 10 mmol, 50 % in mineral oil)

was added to a solution of 1.17 g benzimidazole

(10 mmol) in dry benzene and stirred at room temperature

for 3 h under nitrogen atmosphere to give sodium

benzimidazole. Then 5.00 g 3-chloropropyl silica was

added and refluxed under a nitrogen atmosphere for 24 h.

The resulting product was filtered and washed with ethanol

and dried under vacuum to give 3-(1-benzimidazolyl)pro-

pyl silica (4.89 g).

Sulfonic acid functionalized solid support

3-(1-Benzimidazolyl)propyl silica (4.00 g, 3 mmol of

benzimidazole group) was suspended in toluene and

0.38 g 1,3-propanesultone (3.1 mmol) was added. The

reaction mixture was stirred under reflux for 6 h and then

cooled, filtered, and washed with toluene. Concentrated

hydrochloric acid (3 mmol, 36 % w/w) was added and the

mixture was allowed to stand at room temperature for 24 h.

The obtained material was washed with ether and dried

under vacuum to give sulfonic acid functionalized sup-

ported ionic liquid catalyst (SILC).

Synthesis of fatty acid methyl esters (FAMEs)

A weighed amount of oil (9.32 g, 10 mmol of castor oil),

3.84 g methanol (120 mmol), and 3 wt% SILC were mixed

in a flask equipped with a reflux condenser, thermometer,

and a magnetic stirrer. The mixture was heated at 70 �C for

the time shown in Table 1. The progress of the reaction

was monitored by TLC and on completion the excess

methanol was distilled off under vacuum and the reaction

mixture was transferred to a separating funnel for complete

separation of biodiesel from glycerol and SILC. A three-

phase system was formed comprising the top biodiesel

layer, middle glycerol layer, and SILC settled at the bot-

tom. The top layer was separated and analyzed using a gas

chromatograph equipped with a flame-ionization detector

(FID) detector. The solid catalyst (residue) was washed

with acetone, dried, and reused for the next run. The syn-

thesized biodiesel was characterized by IR, GC, and 1H

NMR (see Supplementary Material). The biodiesel content

was analyzed using a gas chromatograph (Perkin Elmer,

Auto system XL) equipped with a PE-WAX (length 30 m,

inner diameter 250 lm, film thickness 0.25 lm) capillary

column and a FID connected to an Intergraph. Nitrogen

was used as carrier gas. The column temperature was

Table 2 Comparison of different catalysts for the transesterification of jatropha, castor, and neem oils

Entry Catalyst Reaction conditions Yield/% References

1 Solid acid derived from lignin Jatropha oil, 1:12 molar ratio, 5 wt% catalyst, 318 K, 1.5 h 95.2 [20]

2 Sulfated zirconia Neem oil, 1:9 molar ratio, 1 wt% catalyst, 65 �C, 2 h 94 [21]

3 SiO2/H2SO4 acidic silica gel Castor oil, 1:6 molar ratio, 10 wt% catalyst, 60 �C, 3 h [95 [22]

4 H2SO4/C Castor oil, 1:12 molar ratio, 5 wt% catalyst, 338 K, 60 min (MW) 94 [23]

5 SILC Castor oil, 1:12 molar ratio, 3 wt% catalyst, 70 �C, 6 h 94.9 This work

Jatropha oil, 1:12 molar ratio, 3 wt% catalyst, 70 �C, 7 h 95.7

Neem oil, 1:12 molar ratio, 3 wt% catalyst, 70 �C, 7 h 94.4

1740 D. A. Kotadia, S. S. Soni

123

220 �C, the temperatures of the injector and detector were

250 �C, and the oven temperature was kept at 70 �C for

5 min.

Acknowledgments The authors thank Anand Agriculture Univer-

sity, Anand, Gujarat, India for providing jatropha oil on request;

SICART, Vallabh Vidyanagar, India for GC analysis; and DAK

thanks UGC-Delhi for financial assistance in terms of a meritorious

fellowship.

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Sulfonic acid functionalized solid acid 1741

123