Br?nsted acid surfactant-combined dicationic ionic liquids as green catalysts for biodiesel synthesis from free fatty acids and alcohols

Article abstract of DOI:10.1016/S1872-2067(15)60852-X

Quaternary ammonium Br?nsted acid surfactant-combined dicationic ionic liquids (BASDILs) based on zwitterionic 1,2-bis[N-methyl-N-(3-sulfopropyl)-alkylammonium]ethane betaines and various anions were prepared and characterized. BASDILs possess properties similar to those of phase-separated catalysts and were applied to the catalytic synthesis of biodiesel from free fatty acids and alcohols. Several factors were investigated and the results indicated that [C₁₂Sb][p-CH₃C₆H₄SO₃] was the optimal catalyst, with good catalytic performance and reusability under mild conditions.

Full text of DOI:10.1016/S1872-2067(15)60852-X

Chinese Journal of Catalysis 36 (2015) 982–986
a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / c h n j c
Article
Brönsted acid surfactant-combined dicationic ionic liquids as green
catalysts for biodiesel synthesis from free fatty acids and alcohols
a
a,b
a
a
a,
b,#
Tao Chang , Leqin He , Xiaojing Zhang , Mingxia Yuan , Shenjun Qin *, Jiquan Zhao
a
Key Laboratory for Resource Exploration Research of Hebei Province, Hebei University of Engineering, Handan 056038, Hebei, China;
School of Chemical Engineering, Hebei University of Technology, Tianjin 300130, China
b
A R T I C L E I N F O
A B S T R A C T
Article history:
Quaternary ammonium Brönsted acid surfactant-combined dicationic ionic liquids (BASDILs) based
on zwitterionic 1,2-bis[N-methyl-N-(3-sulfopropyl)-alkylammonium]ethane betaines and various
anions were prepared and characterized. BASDILs possess properties similar to those of
phase-separated catalysts and were applied to the catalytic synthesis of biodiesel from free fatty
acids and alcohols. Several factors were investigated and the results indicated that
Received 29 January 2015
Accepted 27 March 2015
Published 20 July 2015
[
C
12Sb][p-CH
3
C
6
H
4
SO ] was the optimal catalyst, with good catalytic performance and reusability
3
Keywords:
Biodiesel
Esterification
Brönsted acidity
Surfactant-combined dicationic ionic
liquid
under mild conditions.
©
2015, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
Phase separated catalyst
1.
Introduction
Because of environmental concerns and the ongoing deple-
methods with more environmentally benign processes.
Various ionic liquids (ILs) have been used as catalysts for
preparing biodiesel from long-chain FFAs and can be easily
tion of fossil fuels, biodiesel has attracted considerable atten-
tion during the past decade as an alternative, green and re-
newable fuel [1]. Biodiesel is generally produced from vegeta-
ble oils or animal fats by transesterification of triglycerides
recycled [12–14]. In particular, SO
3
H-functionalized ILs have
been successfully used as catalysts and reaction media for bio-
diesel production, resulting in excellent performance and high
yields [15–20]. Among these, the dicationic ILs have received
significant attention [21] because of their higher stability as
compared with their monocationic analogues. Fang et al. [22]
used halogen-free dicationic acidic ILs in the synthesis of bio-
diesel from FFAs and alcohols, while Zhang et al. [23] prepared
three dicationic basic ILs for biodiesel production from soy-
bean oil. Aghabarari et al. [24,25] reported the use of acidic ILs
based on bentonite as catalysts for the esterification of oleic
acid with short-chain alcohols. However, the relatively high
[2–4] or by esterification of free fatty acids (FFAs) with
short-chain aliphatic alcohols [5–9]. However, the mineral acids
that are employed for this purpose in industrial processes
cannot be recycled and have other disadvantages [10]. In addi-
tion, the rate of the esterification reaction with heterogeneous
catalysts is adversely affected by mass transfer limitations re-
sulting from the heterogeneous reaction conditions [11].
Hence, it would be beneficial to replace existing esterification
*
#
Corresponding author. Tel/Fax: + 86-310-8577750; E-mail: qinsj528@hebeu.edu.cn
Corresponding author. Tel/Fax: +86-22-60202926; E-mail: zhaojq@hebut.edu.cn
This work was supported by the Science Foundation of Hebei Province (B2012402001, D2014402046), Program for One Hundred Innovative Talents
in Universities of Hebei Province (BR2-204), and the Science and Technology Foundation of Universities of Hebei Province (Q2012030).
DOI: 10.1016/S1872-2067(15)60852-X | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 36, No. 7, July 2015

Tao Chang et al. / Chinese Journal of Catalysis 36 (2015) 982–986
983
cost of ILs containing the imidazolium cation hinders their in-
dustrial applications. Thus, a more efficient, simple and biode-
gradable catalyst need to be developed.
In the present work, we prepared quaternary ammonium
Brönsted acid surfactant-combined dicationic ILs (BASDILs,
Scheme 1) and explored their application as green catalysts for
biodiesel production from FFAs and alcohols. To gain an un-
derstanding of the acidity-activity relationship, the Hammett
method was also employed to evaluate the acidity of the
BASDILs.
according to an experimental design. Upon reaction comple-
tion, the reactor was cooled to room temperature and phase
separation was observed. Excess methanol was evaporated
under vacuum and the IL settled to the bottom of the flask be-
cause it was immiscible with the ester, which formed the upper
layer. Thus, the biodiesel could be separated by simple de-
cantation and the catalyst was easily recycled by removal of
water. The conversion of oleic acid was determined by ac-
id–base titration [18].
3.
Results and discussion
2.
Experimental
3.1. Acidity of BASDILs
2.1. Synthesis of BASDILs
The Hammett acidity function (H
the strength of an acid in an organic solvent, and the H
of the different BASDILs are summarized in Table 1. These data
allow an assessment of the effect of the BASDIL structure on H .
0
) can effectively express
BASDILs were synthesized according to our previously de-
scribed protocol [26]. The zwitterionic 1,2-bis[N-methyl-N-
3-sulfopropyl)-alkylammonium]ethane betaines (CnSbs) were
0
values
(
0
prepared by the reaction of N,N’-dimethylethylenediamine with
n-alkyl bromide, followed by reaction with 1,3-propanesul-
fonate. Zwitterion acidification was subsequently accomplished
by mixing the zwitterions with p-toluenesulfonic acid monohy-
drate.
The results demonstrate that the acidity increased with elonga-
tion of the hydrocarbon chain (entries 2 and 3) and plateaued
when the number of carbon atoms exceeded eight (entries 4, 5
and 9). Moreover, the acidity of BASDILs with different anions
decreased in the order [C12Sb][Tos] > [C12Sb][HSO
12Sb][CH SO ] > [C12Sb][Br] (entries 5–8), in accordance with
the acidity of the anionic counterparts.
4
] >
[C
3
3
2.2. Measuring the acidity of BASDILs
The Brönsted acidity of the ILs was determined by employ-
3.2. The catalytic performance for the esterification of oleic
acid with methanol
ing Hammett functions (H ) together with UV-visible spectros-
0
copy data acquired on a Perkin Elmer Lambda 35 UV-Vis spec-
trophotometer, following a previously reported procedure
To simplify the screening process, oleic acid was used as a
model substrate, since it is a major and key component of low
quality feedstock. The catalytic performance of different
[
18,27,28]. In this process, the BASDIL and the indicator
-nitroaniline (pK = 0.99) were both dissolved at respective
concentrations of 35 and 110 μmol/L in anhydrous ethanol.
4
a
Table 1
0
H values of different BASDILs in ethanol.
2.3. Esterification reaction
Entry
BASDILs
blank
[I] (%)
100
[IH] (%)
0
3.47
22.85
21.97
22.37
17.03
12.37
4.93
H
—
0
1
2
3
4
5
6
7
8
9
The esterification reactions were carried out in a 10-ml tube
[C
[C
4
Sb][Tos]
Sb][Tos]
96.53
77.15
78.03
77.63
82.97
87.64
95.06
78.61
2.43
1.52
1.54
1.53
1.67
1.84
2.28
1.55
with a reflux condenser. Portions of the catalyst (0.020–0.316
mmol) and of oleic acid (7.9 mmol, 2.23 g) were transferred
into the reactor and preheated prior to the addition of the al-
cohol. Upon reaching the desired reaction temperature (25–70
8
[C10Sb][Tos]
[C Sb][Tos]
12
[C12Sb][HSO
4
]
3
[C12Sb][CH SO ]
3
°C), methanol (0.32–1.28 ml) was added into the reactor and
[C12Sb][Br]
[C14Sb][Tos]
the reaction was initiated. The reaction mixture was vigorously
stirred at a constant rate for all runs. The oleic acid-methanol
molar ratio, reaction time and temperature were all varied
21.39
Condition for UV-Vis spectra measurements: solvent anhydrous etha-
nol, indicator 4-nitroaniline at 110 μmol/L, BASDIL 35 mmol/L, 25 C.
o


SO H
1: n = 4, X = p-CH (C H )SO , [C Sb][Tos];
3 6 4 3 4
3



2: n = 8, X = p-CH (C H )SO , [C Sb][Tos];
3 6 4 3 8
2
X


3
: n = 10, X = p-CH (C H )SO , [C Sb][Tos];
3 6 4 3 10
N
CnH
2n+1


H2n+1Cn
N
4a: n = 12, X = p-CH (C H )SO , [C Sb][Tos];
3
6
4
3
12


4
b: n = 12, X = HSO4
,
[C Sb][HSO ];
12 4


4
c: n = 12, X = CH SO
,
[C Sb][CH SO ];
12 3 3
3
3
HO3S


4
d: n = 12, X = Br ,
[C Sb][Br];
12


5
: n = 14, X = p-CH (C H )SO , [C Sb][Tos];
3
6
4
3
14
Scheme 1. Structures of the BASDILs.

984
Tao Chang et al. / Chinese Journal of Catalysis 36 (2015) 982–986
9
9
8
8
7
7
6
6
5
5
5
0
5
0
5
0
5
0
5
0
9
9
8
8
7
7
6
6
5
0
5
0
5
0
5
0
[
C
4
Sb][Tos]
[C Sb][Tos]
8
[C
10Sb][Tos]
[C12Sb][Tos]
[
C
12Sb][HSO
12Sb][CH SO
C12Sb][Br]
4
]
[C
3
3
]
[
[C
14Sb][Tos]
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Amount of catalyst (mol%)
1
2
3
4
5
6
Fig. 3. Effect of the amount of [C12Sb][Tos] on oleic acid conversion.
Reaction conditions: oleic acid 7.9 mmol, methanol 15.8 mmol, 4 h, 60
°C.
Time (h)
Fig. 1. Conversion of oleic acid versus reaction time using different
BASDILs. Reaction conditions: catalyst 2 mol%, oleic acid 7.9 mmol,
methanol 15.8 mmol, 60 °C.
3
.3. Effect of the catalyst amount on the esterification reaction
The amount of catalyst is an important factor in esterifica-
tion efficiency. When using a low catalyst loading, the number
of available active sites was insufficient to promote the reaction
BASDILs is shown in Fig. 1. The catalytic activity of ILs contain-
ing Tos associated with different cations followed the order
−
4 8
[C Sb][Tos] < [C Sb][Tos] < [C10Sb][Tos] < [C12Sb][Tos] ≈
(
Fig. 3). Increasing the amount of catalyst from 0.25 to 4 mol%
[C14Sb][Tos]. These results suggest that acidity is not the sole
resulted in a higher conversion of oleic acid, with the maximum
yield of 93.5% occurring with a catalyst loading of 2 mol%.
Further increasing the catalyst amount did not substantially
improve the conversion. Therefore, a sufficient number of ac-
tive sites were available for the reaction to occur when the cat-
alyst loading was 2 mol%.
factor influencing the efficiency of the catalytic process.
12Sb][Tos] was found to dissolve well in methanol to form a
homogeneous mixture, thereby accelerating the reaction (Fig.
, (a)). We also observed that, upon reaction completion, the
time required to achieve phase separation increased with
elongation of the hydrocarbon chain. The activities of ILs with
different anions were examined and were found to decrease in
[
C
2
3.4. Effect of temperature on esterification reaction
the following order: [C12Sb][Tos]
>
3 3
[C12Sb][CH SO ] >
[
C
12Sb][HSO ] > [C12Sb][Br]. This finding was attributed to the
4
Temperature is another important variable affecting the
enhanced contact between the reactant and the catalyst result-
ing from the bulky Tos anion in the IL, suggesting that the
rate of acid-catalyzed esterification reactions and its effect was
examined by performing experiments in the range of 25–70 °C.
The results are summarized in Fig. 4. As expected, because of
the increase in the equilibrium constant, the conversion of oleic
acid increased with temperature, reaching a maximum at 60 °C.
Further increases in the temperature resulted in reduced con-
version because of methanol evaporation. The observed in-
−
presence of a sufficiently acidic proton on the anionic compo-
nent of the IL played an important role in activating the reac-
tion. Thus, balancing the requirements for a fast reaction rate
with low catalyst costs, [C12Sb][Tos] proved to be the most
efficient catalyst. As shown in Fig. 1, the conversion increased
until equilibrium was achieved after 4 h, and no significant
improvements were obtained by further prolonging the reac-
tion time.
95
90
85
80
75
70
(a)
(b)
Oleic acid
Methanol
IL catalyst
Biodiesel
IL catalyst
20
30
40
50
60
70
o
Temperature ( C)
Fig. 2. Photographs of the esterification of oleic acid and methanol
catalyzed by [C12Sb][Tos]. (a) The full catalytic system in the tube dur-
ing the reaction; (b) Phase separation after the reaction.
Fig. 4. Effect of the reaction temperature on the esterification. Reaction
conditions: oleic acid 7.9 mmol, methanol 15.8 mmol, [C12Sb][Tos] 2
mol%, 4 h.

Tao Chang et al. / Chinese Journal of Catalysis 36 (2015) 982–986
985
crease in the conversion with temperature resulted both from
the effect of temperature on the reaction rate and also from the
significant improvement in the mass transfer between the re-
actant and the catalyst. Thus, the optimal esterification tem-
perature was 60 °C.
1
00
9
3.1
93.0
92.0
92.3
92.1
92.5
8
6
4
2
0
0
0
0
0
3.5. Effect of the amount of methanol on the esterification
reaction
An excess of methanol contributed to the esterification of
oleic acid by improving the reaction rate. Various metha-
nol/oleic acid ratios between 1:1 and 4:1 were tested, and the
results are summarized in Table 2. The highest conversion of
oleic acid (93.5%) after 4 h was achieved with a molar ratio of
1
2
3
4
5
6
Reaction cycle
Fig. 5. Reusability of [C12Sb][Tos] for esterification with methanol.
1.5:1. Further increases in the molar ratio did not result in im-
proved conversion, probably because excess methanol overly
diluted the reaction mixture and hampered the workup proce-
dure. Taking into account the energy consumption and the
yield, the optimal molar ratio of methanol to oleic acid was
Table 3
Results for the esterification of different FFAs and alcohols.
Fatty acid
Oleic acid
Oleic acid
Oleic acid
Lauric acid
Palmitic acid
Stearic acid
Alcohol
Methanol
Ethanol
Propanol
Methanol
Methanol
Methanol
Time (h)
Conversion (%)
4
4.5
6
3
3
93.5
95.7
93.3
92.7
93.9
94.3
1.5:1.
3.6. Reusability of [C12Sb][Tos] for esterification with methanol
3
The recycling performance of [C12Sb][Tos] was investigated
Reaction conditions: [C12Sb][Tos] 2 mol%, molar ratio of alcohol:acid
1.5:1, 60 °C.
by performing several repeat reaction trials under optimum
conditions. Upon completion, the reaction mixture was brought
to room temperature and two phases were formed (Fig. 2, (b)).
The product was isolated from the catalytic system by decanta-
tion. The catalyst was then recovered by centrifugation and
washed with petroleum ether to remove the organic ester. The
results are presented in Fig. 5, showing that there was negligi-
ble decline in the catalytic performance after six successive
runs. Compared with traditional catalysts, the possibility of
efficiently recycling [C12Sb][Tos] is of interest from environ-
mental and economic perspectives.
4.
Conclusions
Various BASDILs were synthesized and applied as green
catalysts for biodiesel production from FFAs and alcohols. The
results indicate that the activity of BASDILs is dependent not
only on the properties of the anion, but also on those of the
cation. [C12Sb][Tos] exhibited the best catalytic activity and
biphasic behavior in the esterification reaction. BASDILs con-
taining a long carbon chain were found to efficiently promote
esterification because of a ‘phase transfer’ effect, in which the
long carbon chains facilitate mass transfer in the reaction sys-
tem.
3.7. Results of esterification for different FFAs and alcohols
To investigate the scope and limitations of [C12Sb][Tos] in
terms of the preparation of fatty acid alkyl esters, different
FFAs and alcohols were examined (Table 3). Excellent conver-
sion rates were obtained in all cases. Notably, the alkyl chain
lengths of both the alcohol and the fatty acid did not have a
significant effect on the conversion. Hence, our investigation
shows that [C12Sb][Tos] could be a good candidate for biodiesel
production from non-edible oils.
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Tao Chang et al. / Chinese Journal of Catalysis 36 (2015) 982–986
Graphical Abstract
Chin. J. Catal., 2015, 36: 982–986 doi: 10.1016/S1872-2067(15)60852-X
Brönsted acid surfactant-combined dicationic ionic liquids as green catalysts for biodiesel synthesis from free fatty acids and
alcohols
Tao Chang, Leqin He, Xiaojing Zhang, Mingxia Yuan, Shenjun Qin *, Jiquan Zhao*
Hebei University of Engineering; Hebei University of Technology
SO3H
FFAs
BASDILs
Biodiesel
BASDILs
Cooling
Alcohols
BASDILs
N
CnH2n+1
H2n+1Cn
N

2
X
Recyling
HO3S
During the reaction
After the reaction
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