Novel acidic ionic liquid polymer for biodiesel synthesis from waste oils

Article abstract of DOI:10.1016/j.apcata.2013.01.036

The novel solid acidic ionic liquid polymer (PIL) has been synthesized through the copolymerization of acidic ionic liquid oligomers and divinylbenzene (DVB). Its catalytic activities were investigated through the biodiesel synthesis from waste oils. The

Full text of DOI:10.1016/j.apcata.2013.01.036

Applied Catalysis A: General 455 (2013) 206–210
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Novel acidic ionic liquid polymer for biodiesel synthesis from
waste oils
Xuezheng Liang∗
Institute of Applied Chemistry, Shaoxing University, Shaoxing 312000, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The novel solid acidic ionic liquid polymer (PIL) has been synthesized through the copolymerization of
acidic ionic liquid oligomers and divinylbenzene (DVB). Its catalytic activities were investigated through
the biodiesel synthesis from waste oils. The results showed that the PIL was very efficient for both the
transesterification of triglycerides and esterification of free fatty acids with the total yield over 99.0%. The
catalytic activities were quite high with just one-step to complete the reactions simultaneously under
very mild conditions. The PIL owned the advantages of high activities, high hydrophobic surface, high
acidity and high stability.
Received 28 December 2012
Received in revised form 28 January 2013
Accepted 31 January 2013
Available online xxx
Keywords:
Solid acidic ionic liquid polymer
Hydrophobic
© 2013 Elsevier B.V. All rights reserved.
Biodiesel synthesis
1. Introduction
synthesis of homoallylic alcohols and esterifications [11,12].
However, the expensive and toxic reagents were used for the
Ionic liquids (ILs) were widely used in various areas for the
excellent properties such as high conductivity, negligible volatile
pressure, wide electrochemical window and strong dissolution
ability [1]. Many efficient procedures using the ILs as catalysts or
solvents were developed [2–5]. Sulfonic acid groups functionalized
ionic liquids were one of the most useful functional ILs, which were
very effective for many acids-catalytic reactions and showed even
higher activities than the homogeneous catalysts such as sulfuric
acid [6–8]. Although acidic ILs owned high activities for various
reactions, such as the esterification, acetalization and alkylation,
the ILs also contain some drawbacks such as certain solubility with
some organic compounds, especially the polar molecules, which
not only made the catalyst loss, but also added the purification dif-
ficulty. Furthermore, the high viscosity increased the mass transfer
resistance and limited their industrial application. Much attempt
was made to solve the problems including adjusting the molecular
structure and melt point. The immobilization of ILs became a
good choice. Various supports and linkages were employed for the
purpose. Silica owned high surface and hydroxyl groups for the
anchorage of ionic liquid. The acidic ionic liquids were successfully
immobilized on silica and showed high activities for acetalization
and hydrolysis of cellulose [9,10]. Polystyrene (PS) owned the
aromatic rings and double bonds and used as the support. Various
acidic ionic liquids were immobilization on PS using different
coupling agents and showed high activities for the expeditious
immobilization, which further added the catalyst cost. Further-
more, the stability of these supported ILs should be concerned
and the recycled catalytic activities usually dropped quickly. The
acid sites covered on the support surface, which fell off easily
from the surface and decrease the activities. Here, the novel solid
acidic polymeric ionic liquid from Brønsted acidic ionic liquid
[SO3H (CH2)3VIm]HSO4 and divinylbenzene (DVB) was presented
(Scheme 1). The cheap divinylbenzene was directly copolymerized
with the double bonds of ILs, which avoided the use of expensive
coupling reagents and reduced the cost. In order to form the ions
clusters and enhance the ions interaction, the IL monomer was
polymerized first to form the oligomer. Then, the oligomers were
copolymerized with DVB. Poly DVB supplied the high hydrophobic
BET surface [13], which raised the efficiency of mass transfer and
prevent the acid sites releasing. Biodiesel was well-known as
the renewable replacement for the mineral diesel fuel [14,15].
Biodiesel fits well with the standard as the green energy. The nitro-
gen and sulfur content was quite low, which decreased the SOx
and NOx emission greatly [16]. Biodiesel contained high oxygen
content, which made the unburnt hydrocarbons and particulate
matter very little during the combustion process [17,18]. Biodiesel
is generally produced by transesterification of vegetable oils with
short-chain alcohols. Various vegetable oils (e.g. soybean oil and
rapeseed oil) can be employed as the raw material for the biodiesel
production. However, vegetable oils are not favored as the raw
materials for the high cost. Waste oils, such as used frying oil, trap
grease and soapstock (byproduct of a vegetable oil refinery) that
are available cheaply, can be considered as feedstocks for biodiesel.
Various acidic ILs were used for the biodiesel synthesis. FFA from
∗
Tel.: +86 575 88345681; fax: +86 575 88345681.
E-mail address: liangxuezheng@126.com
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.01.036

X. Liang / Applied Catalysis A: General 455 (2013) 206–210
207
THF, RT
N
SO3
+
O
O
N
72 h
S
O
H2SO4
N
SO3H
+
HSO4
SO H
3
3
N
HSO4
3SO3H
Ethanol
AIBN
N
HSO4
SO3H
HSO4
3
N
HO3S
3 N
HSO4
HO3S
3N
HSO4
Scheme 1. The synthesis route of the novel acidic ionic liquid polymer.
the soapstock was used as the raw materials and the ILs such
as [NMP][CH3SO3], [SO3H (CH2)3VPy]HSO4 and Dicationic Ionic
Liquids were used for the esterification [19–21]. The ILs showed
high activities for esterification with the average yield of 96%. The
byproduct water affected the reaction equilibrium and further
increasing the yield was quite difficult even if the harsh reaction
conditions were applied. As to the vegetable oil, ILs showed high
activities for transesterification with the conversion over 98%
[22,23]. In order to reducing the production cost, waste oils were
used as the raw materials. The base catalysts were not suitable
for the waste oils high FFA content. The two steps including
acid-catalyzed pre-esterification and base-catalyzed transesteri-
fication were developed [24,25]. In order to simplify the reaction
procedure, the acid-catalyzed one-step synthetic processes were
studied [26,27]. However, the reactions often carried out at high
temperature (>180 ◦C) with the relatively low yields. The novel
efficient one-pot synthesis of biodiesel from waste oils using the
solid acidic ionic liquid polymer (PIL) were developed. The results
showed that the novel PIL was very efficient for the reactions with
the yield over 99% under mild conditions.
solution. After stirring at 70 ◦C for 4 h, DVB (1.30 g, 10 mmol) and
AIBN (0.01 g) were added to the mixture and stirred for another
4 h. Then, the mixture stand for 12 h at 80 ◦C and the white
organic gel was formed. The gel was dried at room tempera-
ture overnight and ground to powders. The solid was washed
with hot acetone and water until no acidity detected in the fil-
trate. The PIL was obtained after drying at 120 ◦C overnight in an
oven.
2.2. The procedure for the biodiesel synthesis
The fried cooking oil was used as the raw material. The oil was
obtained directly from the restaurant. Dehydration under reduced
temperature and decolorization with active carbon were taken to
remove the water and solid residues. The acidity of the waste oil
was 45 mgKOH g−1. The fatty acid content in both FFA and triglyce-
rides were analyzed trough the totally methanol-esterification of
them (Tables 1 and 2). According to the fatty acid content, the free
fatty acid content was about 22 wt.%. The molar ratio of methanol
to waste oil was calculated by treating 3 mol of FFA as 1 mol of
triglyceride. The mixture of oil, methanol and catalyst was trans-
ferred to a round-bottom flask and stirred at 70 ◦C for the specified
period. The process was monitored by GC analysis. The acidity
of the reaction mixture was measured through the neutralization
titration during the reaction process. The catalyst was recycled by
filtering. After the removal of methanol and water through distil-
lation, the filtrate formed two phases. The top layer was biodiesel,
and the lower was glycerol and a small amount of glyceride. The
biodiesel was collected for chromatographic analysis. Quantitative
analysis of the extract solution was carried out on a temperature-
programmed Shimadzu (GC-14C) gas chromatograph according to
the method provide by Alcantara et al. [28]. Here, other analy-
sis methods such as NMR, glycerol titration and hydroxyl value
were also used for the analysis, and the results showed that the GC
analysis fits well with other methods with the difference within
1%. Furthermore, pure biodiesel was collected under the optimal
reaction conditions and the isolated yield fits well with the GC
analysis. The total yield of biodiesel was calculated from the con-
centration of methyl esters analyzed by GC with the following
equation.
2. Experimental
All organic reagents were commercial products of the highest
purity available (>98%) and used for the reactions without further
purification.
2.1. Synthesis of the novel acidic ionic liquid polymer
4-Vinylpyridine (10.5 g, 0.1 mol), 1,3-propane sulfonate (12.2 g,
0.1 mol) and 20 ml tetrahydrofuran were mixed and stirred for 72 h
at room temperature. Then, the white solid zwitterion was formed.
The white solid zwitterion was filtrated and washed repeatedly
with ether. After dried in vacuum (110 ◦C, 1.33 Pa), the white solid
zwitterion was obtained in good yield (91%). Equimolar amount
of sulfuric acid was added to the above-obtained zwitterion and
the mixture was stirred for 4 h at 60 ◦C to form the ionic liquid
monomer. 1H NMR for the zwitterion (400 MHz, D2O, TMS): ı 2.25
(m, 2H), 2.85 (t, J = 7.6 Hz, 2H), 4.32 (t, JH–H = 7.2 Hz, 2H), 5.34 (m,
1H), 5.71 (m, 1H), 7.05 (m, 1H), 7.69 (d, 2H), 8.99 (d, 2H). IR(KBr):
1049 cm−1 and 924 cm−1
(
SO3H), 1166 cm−1 (C N).
(weight of biodiesel produced/Mw of biodiesel) × biodiesel conc.
Monomer (3.14 g, 10 mmol), 20 mL ethanol, 2 mL H2O, and 0.01 g
Azobisisobutyronitrile (AIBN) were mixed together to form the
Yield (%) =
× 100
(weight of oil/Mw of oil) × 3

208
X. Liang / Applied Catalysis A: General 455 (2013) 206–210
Table 1
The fatty acid content in free fatty acid.
Fatty acid
Palmitic acid (C16:0)
25
Stearic acid (C18:0)
32
Oleic acid (C18:1)
28
Linolenic acid (C18:3)
8
Linoleic acid (C18:2)
7
Content (wt.%)
Table 2
The fatty acid content in triglycerides.
Fatty acid
Palmitic acid (C16:0)
21
Stearic acid (C18:0)
29
Oleic acid (C18:1)
32
Linolenic acid (C18:3)
8
Linoleic acid (C18:2)
10
Content (wt.%)
Table 3
The effect of the methanol amount on the reaction.
3. Results and discussion
Methanol (g)
Yield (%)a,b
1.16
84.5
1.75
93.8
2.33
97.6
2.91
99.1
3.49
98.5
3.1. Characterization of the acidic ionic liquid polymer
The reaction conditions: waste oil 5 g, catalyst 50 mg, 70 ◦C, 12.0 h.
The yield was calculated on GC using methyl laurate as internal standard.
a
b
The acidity of the novel solid acidic PIL was 4.3 mmol g−1, which
was determined through the neutralization titration. The PIL owned
much higher acidity compared to common solid acids. The acidity
was in accordance with the equimolar of IL and DVB usage, which
could also be adjusted easily through the molar ratio. The more
acidic monomer, the higher acidity obtained. On the other hand,
the hydrophobic BET surface decreased with the DVB content. The
are reversible, one would expect that increasing the amount of
methanol would shift the reaction equilibrium toward the prod-
ucts [23]. Compared to the pure rapeseed oil, the methanol amount
was more [13]. The esterification of free fatty acids would produce
water and the byproduct water would cause the hydrolysis of the
ester product, which reduced the yield. More methanol promoted
the reaction equilibrium forward to obtain the high conversion. On
the other hand, too much methanol may cause the dilute effect and
decrease yield instead [21]. So 2.91 g methanol was chosen as the
optimal value with the molar ratio of n(methanol)/n(waste oil) = 15.
Then, the effect of reaction time on the yield was investigated
(Fig. 3). It can be seen from Fig. 3 that the PIL was very efficient for
both the transesterification and esterification. The high total yield
of 99.1% was achieved after 12 h, which was very high for the one-
step biodiesel synthesis from waste oil [26,27]. The esterification
of the FFA took place faster at first for the low steric hindrance
of carbonyl groups and could interact with the active sites eas-
ily. The hydrophobic surface further benefited the mass transfer
and separated the byproduct water from the active sites, which
promoted the reactions. On the other hand, the initial transes-
terification took place more slowly compared to rapeseed oil for
high steric hindrance and the active catalytic sites occupied by FFA
first. It is generally known that the transesterification reaction was
carried out through three steps. The triglycerides transformed to
diglycerides, monoglycerides and glycerol. According to the FFA
conversion, the byproduct water content reached the value of 1.3%,
which would cause the hydrolysis of the ester compounds. The
PIL owned the BET surface of 523 m2 g−1
.
The IR spectrum (Fig. 1) of the PIL showed the sulfonic acid
group absorbability at 1049 cm−1, which confirmed the acid groups.
FT–IR spectrum also showed that the PIL contained resident func-
tionalities including C C (1250 cm−1), Ar H (3150 cm−1), and C
O
(1740 cm−1) and OH (3400 cm−1).
The scanning electron microscope (SEM) image of PIL showed
that the resulting particles were irregular spheres structure with
the particle sizes about 0.2–0.5 ␮m as depicted in Fig. 2. The
particles connected with each other without obvious boundary.
The particles own the uniform structure, which indicated the
copolymerization occurred well during the synthetic process. The
cross-linked structure made the recovery of the PIL quite simple
without specified operation except for filtration.
3.2. The catalytic activities for the biodiesel synthesis
The molar ratio of the waste oil and methanol was investigated
first (Table 3). The methanol amount is useful for the reaction
at first. Since the reactions involved in biodiesel production
3500
3000
2500
2000
1500
1000
wavenumber(cm-1)
Fig. 1. The IR spectrum of the PIL.
Fig. 2. The SEM images of the PIL.

X. Liang / Applied Catalysis A: General 455 (2013) 206–210
209
100
100
80
60
40
20
0
80
60
40
20
0
1
2
3
4
5
6
Run times
2
4
6
8
10
12
14
Reaction time/h
Fig. 4. The recycled activity of PIL.
Fig. 3. The effect of reaction time on the reaction.
from Table 5 that novel PIL was the most efficient catalyst of all. For
the Lewis acid ionic liquid [23], besides the high catalyst amount
and long reaction time, the IL was sensitive to water. However, the
esterification of FFA undoubtedly produced the byproduct water,
which would decompose the catalyst. The low yield was obtained
and the IL could not be recycled. Furthermore, the hydrolytic prod-
ucts from IL made the purification of biodiesel difficult. As to the
traditional SO3H functionalized ionic liquid [SO3H-Bmim][HSO4],
the acid sites interacted well with the reactants especially the polar
methanol and water [22]. The byproduct water from the esterifi-
cation of FFA interacted well the acid sites and the water content
was high in the end of the reaction, which cause the hydrolysis of
the ester products. Therefore, the conversion of FFA decreased. Fur-
thermore, the polar compounds such as water and glycerol caused
the IL some soluble with the reaction mixture, which was difficult
to separate. The traditional homogeneous catalysts H2SO4 owned
relatively low activities. Although H2SO4 owned strong acidity, H+
was quite efficient for the all the reactions including esterifica-
tion, transesterification and hydrolysis reactions, which would be
harmful to the yield. There were still some carbonized products
emerged during the process, which made the biodiesel deep color.
H3PO4 with low acidity owned low activities. The traditional ionic
exchange resins (Amberlyst-15, acidity 0.8 mmol g−1) owned much
lower activity for the low acidity. The novel solid acidic PIL owned
high hydrophobic surface and the IL could embed in the pores,
which could separate the water easily from the catalyst. Therefore,
the acid sites hardly interacted with water and the hydrolysis was
effectively stopped. Therefore, the novel PIL owned much higher
activities for the biodiesel synthesis from waste oils with high FFA
content. Although the cost of the PIL was higher than the traditional
Amberlyst-15, the catalyst owned much higher activities and could
be recycled with high activities, which would reduce the product
cost greatly. Therefore, the novel PIL should be one of the best
choices for the operational simplicity, user-friendly catalyst and
for the scaling up purpose.
diglycerides and monoglycerides owned both the hydrophobic long
aliphatic carbon chain and the hydrophilic hydroxyl, which could
bring the byproduct water to the acid sites. Therefore, the transes-
terification and hydrolysis became the competitive processes. The
total yield changed little after 12.0 h, which indicated the equilib-
rium of the reaction. The converse reaction occurred after 12.0 h,
which made the yield reduced instead.
The catalyst amount was also very important (Table 4). There
were not enough active sites for the reaction when the catalyst
amount was low. The yield increased with catalyst amount at first.
Then, the yield remained the unchanged when 0.05 g catalyst was
applied used. Both the esterification and transesterification were
the typical reversible reactions and the catalyst could catalyze both
the forward and converse reactions. Too much acidic sites would
also accelerate the converse reaction instead and reduced the yield.
Therefore, the optimal amount was 50 mg.
3.3. The recycled activity of the catalyst
One property of the PIL was the reusability. After reactions,
the solid catalyst could be recovered by filtration. The recov-
ered activities were investigated carefully (Fig. 4). For those acidic
ILs immobilized on silica and PS, the recycled catalytic activities
dropped quickly [9–12]. The novel PIL owned high stability and the
recycled catalytic activities were still very high for the reactions.
The yield of 99% was obtained even after the PIL had been recycled
for 6 times. The PIL owned high hydrophobic surface and the acid
sites entered to the pores of the DVB polymers, which separated
the polar compounds such as water and glycerol effectively from
the acid sites and prevent the acidic sites releasing. In addition, the
element analysis of the filtrate showed no sulfur existed, which
further confirm the stability of the PIL.
3.4. Comparative study on the catalytic activities of different
catalysts
Table 5
The comparison of different catalysts.
A comparative study on the catalytic activities of the PIL with
the other catalysts was carried out (Table 5). It can be concluded
Catalyst
Catalyst amount (mg) Reaction time (h) Yield (%)a,b
Novel PIL
50
60
60
70
150
500
12
24
16
18
24
18
99.1
78.8
94.5
89.7
79.8
81.4
[Et3NH]Cl AlCl3
[SO3H-Bmim][HSO4]
H2SO4
H3PO4
Amberlyst-15
Table 4
The effect of the catalyst amount on the reaction.
Catalyst amount (g)
Yield (%)a,b
0.01
95.4
0.03
97.2
0.05
99.1
0.07
98.7
0.09
98.5
a
a
The reaction conditions: waste oil 5 g, methanol 2.91 g, 70 ◦C, 12.0 h.
The yield was calculated on GC using methyl laurate as internal standard.
The reaction conditions: waste oil 5 g, methanol 2.91 g, 70 ◦C.
The yield was calculated on GC using an internal standard.
b
b

210
X. Liang / Applied Catalysis A: General 455 (2013) 206–210
Table 6
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Entry
Value
Density (15 ◦C) (g cm−3
)
0.87
4.2
Viscosity (40 ◦C) (mm2 s−1
Sulfur content (%)
)
1.8 mg kg−1
200
Water content (mg kg−1
Methanol content (%)
Biodiesel content (%)
Glycerol content (%)
)
0.05
>99.1
0.01
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The novel solid acidic PIL has been synthesized through the
copolymerization of acidic ionic liquid oligomers and DVB. The
PIL showed high activities for the one-pot biodiesel synthesis from
the waste oil with high FFA content with the total yield over 99%.
High acidity, operational-simplicity and high stability were the key
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