Biodiesel synthesis from the esterification of free fatty acids and alcohol catalyzed by long-chain Bronsted acid ionic liquid

Article abstract of DOI:10.1039/c2cy20714a

A long-chain Bronsted acid ionic liquid (IL), 3-(N,N- dimethyldodecylammonium)propanesulfonic acid p-toluenesulfonate ([DDPA][Tos]), was prepared and characterized by FT-IR, ¹H NMR, ¹³C NMR, UV/vis and TGA. The H₀ (Hammett function) value of the IL was also determined. The IL as catalyst was applied to the catalytic synthesis of biodiesel from free fatty acids (FFAs). The influencing factors, such as the type and amount of catalyst, reaction time, molar ratio of fatty acid to methanol and reaction temperature, were investigated. The results indicated that the long-chain Bronsted acid IL showed high catalytic activity and fair reusability. Esterification was accomplished under the following optimized conditions: molar ratio of alcohols to FFA at 1.5:1, mole fraction of ionic liquid at 10%, 60 °C, and 3 h. The products could be separated from the catalyst system by liquid-liquid biphase separation at room temperature with good yields of 92.5% to 96.5%. The catalyst could be reused nine times after the removal of water and alcohol. Therefore, the long-chain Bronsted acid IL has good potential for the synthesis of biodiesel from low-cost feedstocks such as waste oil and woody plant oils.

Full text of DOI:10.1039/c2cy20714a

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Biodiesel synthesis from the esterification of free fatty acids and alcohol
catalyzed by longꢀchain Brønsted acid ionic liquid
1
,2
1,
1
1
1
Leqin He , Shenjun Qin *, Tao Chang , Yuzhuang Sun , and Xiaorui Gao
1
Key Laboratory for Resource Exploration Research of Hebei Province, Hebei University of Engineering,
Handan, Hebei, China, 056038
2
School of Chemical Engineering, Hebei University of Technology Tianjin, China , 300130
*Corresponding author: chemlab@hebeu.edu.cn (S. Qin) Tel: +86ꢀ310ꢀ8578759; Fax: +86ꢀ310ꢀ6025698
Abstract: A longꢀchain Brønsted acid ionic liquid_(IL), 3ꢀ(N,Nꢀdimethyldodecylammonium)propanesulfonic acid
1
13
pꢀtoluenesulfonate ([DDPA][Tos]), was prepared and characterized by FTꢀIR, H NMR, C NMR, UV/vis and
TGA. The H (Hammett function) value of the IL was also determined. The IL as catalyst was applied to the
0
catalytic synthesis of biodiesel from free fatty acids (FFAs). The influencing factors, such as the type and amount
of catalyst, reaction time, molar ratio of fatty acid to methanol and reaction temperature, were investigated. The
results indicated that the longꢀchain Brønsted acid IL showed high catalytic activity and fair reusability.
Esterification was accomplished under the following optimized conditions: molar ratio of alcohols to FFA at 1.5:1,
mole fraction of ionic liquid at 10%, 60 °C, and 3 h, through which the products could be separated from the
catalyst system by liquid–liquid biphase separation at room temperature with good yields of 92.5% to 96.5%. The
catalyst could be reused nine times after the removal of water and alcohol. Therefore, the longꢀchain Brønsted
acid IL has good potential for the synthesis of biodiesel from lowꢀcost feedstocks such as waste oil and woody
plant oils.
Key words: esterification, biodiesel, longꢀchain IL, methyl oleate, Hammett acidity
1
. Introduction
Biodiesel is defined as the monoalkyl esters of longꢀchain fatty acids prepared from plant oils, animal fats or

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1
other lipids . As an alternative to fossil fuels, biodiesel must be technically feasible, economically competitive and
2
environmentally acceptable . To reduce the production costs and to facilitate competitiveness with petroleum
3
,4
diesel, lowꢀcost feedstocks, such as nonꢀedible oils and waste frying oils, can be used as raw materials .
However, the high free fatty acid (FFA) content of lowꢀcost feedstocks may be a major hindrance to biodiesel
5–7
production because of the use of dominant industrial basic catalysts . The higher FFA content of feedstock is
known to reduce biodiesel yield significantly when using alkalineꢀcatalysis technology, because of the
8
consumption of catalyst and difficulties involved in separating and purifying the product . Therefore, a twoꢀstep
pretreatment process for the removal of FFAs by acid esterification, and the subsequent base transesterification to
4
biodiesel are typically used to convert such oils to biosiesel . Liquid inorganic acid catalysts and solid acid
catalysts are widely used for the synthesis of esters. However, homogeneous acids facilitate corrosion in the
reaction vessel and thus need to be neutralized with large amounts of base after the reaction, a process that is
environmentally unfriendly. Solid acid catalysts also have such disadvantages as low activity, easy deactivation,
and adsorption of products, which result in their limited application. Hence, the replacement of current
esterification protocols with a more environmentally benign process involving the use of ionic liquids (ILs)
9
,10
appeared to be an area worthy of investigation
.
Room temperature ionic liquids (RTILs) are organic salts that can be designed with different cations, anions
10
and substitutes, which have controllable chemical and physical properties . With the rapid development of RTILs,
11–15
various types of ILs are used for esterification as reaction medium or as catalyst with satisfactory yield
.
Reactants can generally be dissolved in ionic liquids, whereas esters are insoluble. Thus, these components can be
separated through simple decantation. To date, various forms of ILs have been synthesized and used as a
1
6–18
19
preꢀcatalyst for biodiesel production
. Acidic ILs, such as Nꢀmethylꢀ2ꢀpyrrolidonium methyl sulfonate ,
20
21
dicationic ILs and 1ꢀalkylꢀ3ꢀmethylimidazolium cations ILs , have also been used as catalysts for the
preparation of biodiesel from longꢀchain FFAs with very good yields. Although extensive work has been

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conducted in this area, the disadvantages of the aboveꢀmentioned catalytic system are evident, such as high
temperature, long reaction time and large catalyst amount. Longꢀchain ILs have recently been tapped as a new
generation of ionic solvents, given that several longꢀchain ILs demonstrated an amphiphilic nature and thus
2
2,23
24
representing a new class of cationic surfactants with peculiar properties . Iborra described the use of long
alkyl chain ILs based on imidazolium cations as a medium, which increased the efficiency of biodiesel production.
As already seen, various types of longꢀchain Brønsted acid ILs have been used for biodiesel synthesis as a
1
6,18,21
catalyst
, but the ILs with imidazole as the cation are relatively expensive, which hinders their industrial
applications. Thus, it is necessary to synthesize less expensive ILs, which can be used straightforwardly with the
simple procedure.
In this work, we prepared a novel longꢀchain Brønsted acid IL, [DDPA][Tos], the structure and solubility of
which are similar to those of the phaseꢀtransfer catalyst. The catalytic activity of esterification and the reuse
performance of the longꢀchain Brønsted acid IL were examined. For comparison, known ILs with similar
structures were prepared (see Scheme 1) and used for the synthesis of biodiesel. The results indicate that the
longꢀchain Brønsted acid IL showed high catalytic activity and fair reusability. To gain an understanding of the
acidity–activity relationship, the Hammett method was introduced to evaluate the acidity of ILs. An IL containing
a long carbon chain is capable of efficiently promoting esterification because of the contribution of ‘phase
transfer’, in which the long carbon chain facilitate mass transmission in the reaction system (see Figure 1).
Therefore, an efficient and environmentally friendly catalyst was provided for the synthesis of biodiesel from
lowꢀcost feedstocks such as waste oils.
X-
-
P-CH (C H )SO
3
3
6
4
R
N
N
SO H
N
SO H
3
3
I: R=CH ; X =HSO -, [MPSIM][HSO4]
-
VI:[DOPA][Tos]
3
3
4
-
-
II: R=CH
; X =H
2
PO
4
,
[MPSIM][H
2
PO
4
]
-
P-CH (C H )SO
3
3
6
4
-
-
III:R=CH ; X =CH SO , [MPSIM][CH SO ]
IV:R=CH
3
3
3
3
3
-
-
3
; X =P-CH
3
(C
6 4 3
H )SO , [MPSIM][Tos]
N
3
SO H
-
-
V: R=C H ; X =P-CH (C H )SO ,[BPSIM][Tos]
4
9
3
6
4
3
VII: [DDPA][Tos]
Scheme1. Structure of the acidic IL

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Figure1. Illustration of esterifation catalyzed by [DDPA][Tos]
2
2
. Experimental
.1 Materials
N,NꢀDimethyldodecylamine(97%) was purchased from Hangzhou Dayang chemical Co., Ltd.,
N,NꢀDimethyloctylamine(95%), 1ꢀmethylimidazole(99%), 1ꢀtertylimidazole, pꢀToluenesulfonic acid monohydrate,
Methane sulfonic acid and 1,3ꢀpropansulate were obtained from the AladdinꢀReagent Co. Shanghai. Other
chemicals(AR) were obtained from Tianjin Kemio Fine Chemical Institute and were used directly.
2
.2 Catalyst characterization
Melting points were determined with XTꢀ4 melting point apparatus. FTꢀIR spectra were recorded by ATR using a
ꢀ1
ꢀ1
1
13
Bruker vector 22 spectrophotometer in the range from 600 cm to 4000 cm , The H NMR and C NMR spectra
were measured by using a Bruker ACꢀP 300 spectrometer in D O and DMSOꢀd. Thermal decomposition profiles
2
were collected through TGA (Diamond TG/DTA/DSC, 10 °C/min heating rate under nitrogen). GC analyses were
performed on a HewlettꢀPackard model 6890 GC coupled to a hydrogen ion FID.
2
2
.3 Catalyst preparation
.3.1 Preparation of the zwitterion
The classic procedure was based on the synthesis of 3ꢀ(N,Nꢀdimethyldodecylammonium)propanesulfonate.
,3ꢀpropanesultone (5.0 ml, 0.06 mol) was dissolved in 15 ml ethyl acetate, and N,NꢀDimethyldodecylamine (15.5
1
ml, 0.06 mol) was dropped slowly into the previous solution. The mixture was stirred for 3 h at 55°C to 60 °C,
after which the reaction mixture was filtered to obtain the white precipitate. The precipitate was washed with ethyl
2
acetate and diethyl ether twice and then dried at 100 °C for 10 h, yielding nꢀDodecMe NꢀPS as a white power.

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1
(
yield: 90%). mp:244 °C, FTꢀIR(ATR):3058, 3032, 2953, 2917, 1657, 1457, 1165, 1035, 929,752,720, 619; H
NMR(400MHz, D O): δ 0.813(t, 3H, CH ꢀCꢀ), 1.229(s,18H, ꢀCH ꢀ), 1.689(s, 2H, CꢀCH ꢀCꢀN), 2.105ꢀ2.147(pent,
2
3
2
2
2
2
5
2
H, NꢀCꢀCH
2
ꢀCꢀS), 2.862(t, 2H, CH
2
3 2
ꢀS), 3.052(s, 6H, NꢀCH ), 3.228ꢀ3.270(pent, 2H, NꢀCH ), 3.394ꢀ3.436(pent,
13
2 2
H, NꢀCH ), C NMR(100MHz, D O): δ 13.878, 18.354, 22.221, 22.671, 26.113, 29.101ꢀ29.878, 32.001, 47.412,
0.872, 61.927, 63.929
.3.2 IL preparation
The classic procedure was based on the synthesis of 3ꢀ(N,Nꢀdimethyldodecylammonium)propanesulfonic acid
pꢀtoluenesulfonate ([DDPA][Tos]), which was obtained by mixing the zwitterion with an equal mole of
pꢀtoluenesulfonic acid monohydrate at room temperature and stirring for 5 h at 110 °C without solvent. A white
emulsion was then obtained as the surfactant property of the product. After the reaction, the white viscous
emulsion was dried under vacuum at 100 °C for 30 min, after which the emulsion was cooled to obtain a white
solid product (yield >95%). mp: 86 °C, FTꢀIR(ATR):3738, 3446, 30470, 2923, 2851, 1700, 1602, 1485, 1156,
1
1
007, 815, 681, 614; H NMR(400MHz, DMSOꢀd): 0.857(t, 3H, CH
3
ꢀCꢀ), 1.249(s,18H, ꢀCH
2
ꢀ), 1.637(s, 2H,
2 2 3 2
CꢀCH ꢀCꢀN), 1.937ꢀ1.978(pent, 2H, NꢀCꢀCH ꢀCꢀS), 2.294(s, 3H, ArꢀCH ), 2.487(t, 2H,CH ꢀS ), 2.988(s, 6H,
3 2 2
NꢀCH ), 3.205ꢀ3.247(pent, 2H, NꢀCH ), 3.352ꢀ3.394(pent, 2H, NꢀCH ), 7.141(d, 2H, ArꢀH), 7.500(d, 2H, ArꢀH);
1
3
C NMR(100MHz, DMSOꢀd): 13.927, 18.695, 20.770, 21.647, 22.090, 25.780, 28.531ꢀ29.025, 31.293, 47.657,
9.946, 61.875, 62.927, 125.474, 128.525, 138.230, 144.687, 144.728; UVꢀvis(λmax): 280nm
.4 Acidity measurements of ILs
4
2
For comparing the acidities of ILs with different anions and cations, spectroscopic measurements were used to
determine the Hammett acidity of ILs. Typical experimental procedure was as follows: The ILs and the indicator
4
ꢀnitroaniline(pKa=0.99) were dissolved in ethanol at concentrations of 35 mmol/L and 54 ꢁmol/L, respectively.
The solutions were shaken vigorously and then left to stand for 6 h. Their Uv/Vis spectra were recorded on an
Perkin Elmer Lambda 35 UV/vis spectra at room temperature.

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2
.5 General procedure for biodiesel synthesis
A certain amount of FFAs, methanol and IL were added to a flask charged with a reflux condenser and a magnetic
stirring apparatus. Esterification was typically conducted for a length of time at a specific temperature with
vigorous stirring. After the reaction, the reactor was cooled to room temperature, and two phases were formed.
The IL could be found at the bottom of the flask, which was immiscible with the ester of the upper phase. Thus,
the biodiesel could be simply separated by decantation, and the catalyst was easily recyclable after removing
water and methanol. The concentration of the product was directly measured by GC. The conversion of aliphatic
acid was calculated based on the acid number, according to the equation below:
1 2 1
Conversion(%)=[(a -a )×100]/a
The acid number (the mass of KOH in milligrams that is required to neutralize 1 g of chemical substance) was
determined by KOH titration using phenolphalein as indicator. Here, a is the acid number (mg KOH/g) of FFAs,
1
and a is the acid number(mg KOH/g) of the crude reaction mixture of the upper phase after vaporization of water
2
and methanol in vacuum.
3
3
.
Results and discussion
.1 Acidity of ILs
The Hammett acidity function can effectively express the acidity strength of an acid in organic solvents and can be
calculated by the equation below:
+
H
0
=pK(Iaq)+lg([I]
s
/[IH ]
s
)
+
s s
where "I" represents the indicator base (mainly substituted nitroaniline), whereas [IH ] and [I] are the respective
molar concentrations of the protonated and unprotonated forms of the indicator. According to Lambert–Beer's law,
+
the value of [I]
s
/[IH ]
s
can be determined and calculated through the UV/vis spectrum. In our experiment, The
maximum absorbance of the unprotonated form of 4ꢀnitroaniline was observed at 371 nm in ethanol. With the
increase in IL acidity , the absorbance of the unprotonated form of the basic indicator decreased, whereas the

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protonated form of the indicator could not be observed because of its small molar absorptivity and its location.
+
Thus, [I] /[IH ] can be determined from the differences in measured absorbance after the addition of ILs, based on
s
s
which H
0
can be calculated. The results obtained are listed in Table 1.
0
Table 1 Calculation and comparison of H values of different ILs in ethanol
Entry
ILs
blank
A
max
[I](%)
100
[IH](%)
0
H
0
1
2
3
4
5
6
7
8
0.6556
0.4488
ꢀ
ꢀ
[MPSIM][HSO4]
68.46
ꢀ
31.54
ꢀ
1.33
ꢀ
a
2
[MPSIM][H PO
4
]
[MPSIM][CH
3
SO
3
]
0.4649
0.4861
0.4794
0.4527
0.4543
70.91
74.15
73.12
69.05
69.29
29.08
25.85
26.88
30.95
30.71
1.37
1.45
1.42
1.34
1.34
[MPSIM][Tos]
[BPSIM][Tos]
[DOPA][Tos]
[DDPA][Tos]
a: No protonation.
As shown in Table 1, the Brønsted acidities of the ILs with different anions were determined (entries 2–5), we can
conclude that the acidities of these ILs decreased in the following order: [MPSIM][HSO ]> [MPSIM][CH SO ]>
4
3
3
2 4
[MPSIM][Tos]> [MPSIM][H PO ], which is in accordance with the acidities of the anions. The effect of variation
of the cations in the ILs on their acidity was examined (entries 5–8), which suggests that the Hammett acidity of
these ILs follows the order: [DDPA][Tos]= [DOPA][Tos]> [BPSIM][Tos]> [MPSIM][Tos]. The results show that
the length of the carbon chain in the cations can also affect the acidities of ILs.
3
.2 TGA result
To characterize the thermal degradation of [DDPA][Tos], TGA was performed and the result is shown in (Figure
S5, ESI). A multiꢀstep decomposition process was observed for the SO Hꢀfunctionalized quaternary ammonium
3
IL. Three weight loss regions were observed from 30°C to 600 °C. From room temperature to 240 °C, the weight
loss was 7% because of the removal of the water and organic solvent. From 240 °C to 340 °C, the weight loss was
ꢀ
3
5%, which was ascribed to the decomposition of PTSA . From 340 °C to 410 °C the weight loss was 53%, which
was ascribed to the decomposition of the amphiphilic cation. The weight of IL sharply decreased above 340 °C.
Therefore, the thermal decomposition temperature of [DDPA][Tos] was found to be 340 °C. Thus, [DDPA][Tos]

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was found to be sufficiently stable to be the catalyst in biodiesel synthesis.
3
.3 Catalyst screening for biodiesel synthesis
Oleic acid was used as the substrate to simplify the investigation of the screening and subsequent processes given
that this acid is a key component of nonꢀedible oil for biodiesel production. Methanol was used as an alcohol
substrate for its low cost. Anions and cations of ILs were varied to observe their effects on and contributions to the
catalysts. The catalytic performances of different ILs are listed in Table 2.
Table 2 Effect of different ILs on esterification
Entry
Catalyst
Time (h)
Conversion (%)
1
2
3
4
5
6
7
[MPSIM][HSO4]
3
3
3
3
2
1
1
89.7
17.4
94.9
95.5
96.0
96.6
97.9
2
[MPSIM][H PO
4
]
[MPSIM][CH
3
SO
3
]
[MPSIM][Tos]
[BPSIM][Tos]
[DOPA][Tos]
[DDPA][Tos]
Reaction conditions: n (methanol): n (oleic acid):n (catalyst)= 2:1:0.2, temperature: 80°C
ꢀ
ꢀ
ꢀ
ꢀ
4 2 4 3 3 3 6 4 3
Initially, the activities of ILs containing different anions, such as HSO , H PO , CH SO , and pꢀCH C H SO ,
were compared to select the better anion (entries 1–4). Although the cations of ILs were the same, various yields
of Methyl oleate were obtained, which indicates that the anions of ILs had a decisive influence on the
2 4
performance of the catalysts. Among these ILs, [MPSIM][H PO ] facilitated very poor conversion because of its
weak acidity (entry 2), ILs with organic anions showed higher catalytic activity for the esterification of oleic acid
compared with inorganic anions (entries3,4 vs. entry 1), which disagrees with the acidity order of the Brønsted
ꢀ
acidic intensities of these ILs. The difference can be attributed to the fact the bulky anion ꢀPTSA in the IL can
ꢀ
promote contact between the reactant and catalyst. When the anions of these ILs were the same PTSA , the ILs
with a longer carbon chain had higher catalytic activity (entries 4–7). Thus cations also had an important influence
on the performance of the catalysts. Given the poor solubility of the product as well as the presence of long chain
FFAs in the ILs, which results in the easy separation of the catalytic system, the substrate and catalyst transfer
becomes less efficient. Thus, stirring is required to mitigate the liquid–liquid massꢀtransfer limitations. In addition,

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the main reason why the ILs containing a long carbon chain could efficiently promote esterification is the
contribution of ‘phase transfer’ in which the long carbon chain facilitates mass transmission in the reaction system.
Thus，[DDPA][Tos] was selected as the most efficient catalyst for the reaction.
3
3
.4 Catalytic performances of [DDPA][Tos] for the esterification of oleic acid
.4.1 Effect of the catalyst amount on esterification
100
9
9
9
9
8
6
4
2
0
2
4
6
8
10
12
14
16
18
20
Amounts of catalyst,mol%
Figure 2. Effect of the amount of [DDPA][Tos] on oleic acid conversion
Reaction conditions: n (methanol):n (oleic acid)=2:1, reaction temperature=80 °C, reaction time=3 h
Figure 2 clearly shows that an increase of the amount of catalyst from 2.5% to 10% resulted in an increase in the
conversion of oleic acid. We thus conclude that the number of active sites for the reaction is inadequate when the
catalyst amount is low. However, with an excessive amount of catalyst, the conversion of oleic acid increased only
slightly. Additionally, subsequent increases could result in a sideꢀreaction in which the product turns orange. Thus,
considering the reaction rate and the cost of the catalyst, the optimum molar ratio of catalyst to oleic acid was
found to be 10%.
3
.4.2 Effect of reaction time on esterification

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98
97
96
95
94
93
0
1
2
3
4
5
Reaction time,h
Figure 3. Effect of reaction time on esterification
Reaction conditions: n (methanol):n (oleic acid):n (catalyst)=2:1:0.1, reaction temperature=80 °C
The effect of reaction time on the conversion of oleic acid was likewise investigated. Figure 3 shows that
[DDPA][Tos] was very efficient for the reaction, with the conversion reaching 93.0% in only 0.5 h. However, it
2
1
3 3
was reported that the same reaction catalyzed by [MBSIM][CF SO ] only got a conversion of 80% (reaction 6 h
at 80°C). Initially, an increase in the conversion could be observed with an increase in the reaction time. A further
increase in reaction time could not improve the conversion significantly because of the equilibrium of
esterification with a conversion of 97.1% in 3 h. On the other hand, the conversion of oleic acid decreased when
the reaction time lasted for more than 3 h. This observation can be explained by the fact that the esterification of
oleic acid is a reversible reaction with an equilibrium time of around 3h, which was therefore taken as the ideal
time for the reaction.
3
.4.3 Effect of the amount of methanol on esterification
Table 3 Effect of the ratio of methanol to oleic acid on oleic acid conversion
Entry
Ratio of methanol to oleic acid
Conversion (%)
1
2
3
4
5
1:1
1.5:1
2:1
95.0
96.6
96.0
95.8
94.7
3:1
4:1
Reaction conditions: n (oleic acid):n (catalyst)=1:0.1, reaction temperature=80 °C, reaction time=3 h
An excess of reactant methanol is necessary for the esterification of FFA, which can increase the rate of
methanolysis. The effects of methanol amount are shown in Table 3. Since the esterification of oleic acid is
reversible, one would expect that increasing the amount of methanol would shift the reaction equilibrium toward

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the product. The highest conversion of oleic acid achieved was 96.6% with a methanol to oleic acid mole ratio of
.5:1 in 3 h (entry 2). A further increase of this mole ratio did not result in an increase in the conversion probably
1
because oleic acid and IL became excessively diluted with excess methanol (entries 2–5). Additionally, the use of
excess methanol would not facilitate the workup procedure.
3
.4.4 Effect of reaction temperature on oleic acid conversion
9
9
9
9
9
8
8
8
8
8
6
4
2
0
8
6
4
2
20
30
40
50
60
o
70
80
Temperture, C
Figure 4. Effect of reaction temperature on oleic acid conversion
The effect of reaction temperature on esterification is shown in Figure 4. When the reaction temperature was
5 °C, 83.7% of the oleic acid was converted into methyl oleate in 3 h. The conversion of oleic acid was found to
2
increase with temperature from 25 °C to 60 °C. Further increases in temperature had no significant effect on
conversion. The selectivity of methyl oleate decreased at 80 °C, at which the reaction system exhibited an
orange color. Considering conversion and selectivity, the optimum reaction temperature is 60 °C. It was reported
1
7
4
that the Brønsted acid IL [BMIM][HSO ] showed a satisfactory conversion (91.2%) for esterifaction of FFAs ,
but it needed a higher temperature (120°C). Thus, [DDPA][Tos] could benefit the esterification of FFAs in view of
the low energy consumption.
Based on the above experiments, the optimum conditions were obtained as follows: reaction temperature, 60 °C;
methanol to oleic acid molar ratio: 1.5:1; amount of IL, 10 mol%; and reaction time, 3 h. Under these conditions,
the conversion of free oleic acid was 96.5%. Esterification could be performed at mild conditions, and the ester

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produced could be isolated conveniently with high yield and purity. However, the ILs Nꢀmethylꢀ2ꢀpyrrolidonium
1
9
20
methyl sulfonate and dicationic ILs used for eaterification of oleic acid need a reaction time for 8 h and 6 h at
0°C to reach the same conversion, respectively. Therefore, an efficient catalyst was provided for the synthesis of
biodiesel from FFAs.
.5 Reusability of [DDPA][Tos] for esterification
7
3
100
80
60
40
20
0
1
2
3
4
5
6
7
8
9
Run
Figure 5. Reusability of [DDPA][Tos] for esterification with methanol
One property of the catalyst is reusability. Thus the recycling performance of this longꢀchain IL was investigated
by performing several iterations under the optimum reaction conditions. After completion of the reaction, the
product was isolated from the catalytic system by decantation. The catalyst was then recovered by centrifugation
and washed with petroleum ether to remove the organic ester. The recovered activities were investigated carefully
(
see Figure 5). The results show that the catalyst exhibited high stability during the reaction process and that
conversion remained unchanged even after nine recycling instances. Compared with traditional catalysts, efficient
recycling performance is an attractive property of the proposed catalysts from the environmental protection and
economic perspectives.
3
.6 Catalytic performance of [DDPA][Tos] for the esterification of other FFAs and short-chain alcohols
Table 4 Result of esterification for different FFAs
Entry
Fatty acids
Oleic acid
Oleic acid
Oleic acid
Alcohol
Methanol
Ethanol
Conversion
96.5
1
2
3
94.6
Propanol
93.2

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4
5
6
Oleic acid
Linoleic acid
Palmitic acid
Butanol
Methanol
Methanol
92.5
95.8
95.2
Reaction conditions: n (methanol):n (FFAs):n (catalyst)=1.5:1:0.1, 60 °C, 3 h
To investigate the scope and limitation of [DDPA][Tos] in terms of the preparation of fatty acid alkyl ester (FAME)
related to biodiesel production, linoleic acid and palmitic acid, which are also key components in waste oils for
biodiesel production, as well as alcohols were tested as the substrates. The results are listed in Table 4. Excellent
conversion rates were obtained in all cases (entries1–6). Notably, neither the length of the alkyl chains of alcohols
nor that of fatty acids had a significant effect on the conversion of fatty acids (entries1–4). Our investigation
showed that [DDPA][Tos] could be a good candidate for biodiesel production from nonꢀediable oils.
4
. Conclusions
In conclusion, the longꢀchain IL [DDPA][Tos] has been determined to be an efficient catalyst for biodiesel
production from FFAs. The catalyst was efficient for the reaction with a yield of 96.5% under the optimum
reaction conditions. The catalyst can be easily synthesized and is thermally stable and nonꢀvolatile. Furthermore,
the longꢀchain Brønsted acidicꢀsurfactantꢀcombined IL will have a good catalytic activity for the
transesterification of immiscible triglyceride and alcohol, with potential to transform the reaction system into a
turbid emulsion, which could promote mass transfer. So the proposed process has many advantages such as
operational simplicity, low cost of the catalyst used, high yield thus proving its potential as green process.
Acknowledgements
We acknowledge the financial support by the National Natural Science Foundation of China (No. 41102095),
the Science Foundation of Hebei province (No. B2012402006), Handan Municipal Science Foundation
(No.1221104084ꢀ7) and Science and technology plan projects of education department of Hebei province (No.
Q2012030 )

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