Lewis Acidic Ionic Liquid [Bmim]FeCl4 as a High Efficient Catalyst for Methanolysis of Poly (lactic acid)

Article abstract of DOI:10.1007/s10562-017-2138-x

Abstract: Methanolysis of poly (lactic acid) (PLA) was studied using Lewis acidic ionic liquid [Bmim]FeCl₄ as a catalyst and the qualitative analysis indicated that the main product was methyl lactate. Compared with ionic liquid [Bmim]Cl, FeCl₃ and other traditional catalysts, the Lewis acidic ionic liquid showed much better catalytic activity in the methanolysis of PLA. Significantly, the conversion of PLA and the yield of methyl lactate were 99.3 and 94.6% respectively with low dosage of catalyst (the molar ratio of [Bmim]FeCl₄ to LA unit is 0.0025:1). The lewis acidic ionic liquid [Bmim]FeCl₄ could be recycled up to 6 times and FT-IR results showed that the spectrum of the reused catalyst was similar to that of the fresh one. Moreover, a possible mechanism of the methanolysis of PLA catalyzed by [Bmim]FeCl₄ was proposed. The kinetics of PLA methanolysis was also investigated under the optimum conditions and the results indicated that it was a first-order kinetic reaction with activation energy of 21.28?kJ?mol^?1. Graphical Abstract: The methanolysis of poly (lactic acid) (PLA) was studied using Lewis acidic ionic liquid [Bmim]Fe_xCl_3x+1 (x>1) as catalyst. The effects of reaction temperature, reaction time, ionic liquid dosage and methanol dosage on methanolysis results were examined. The reusability of ionic liquid in the methanolysis of PLA was investigated, and the results showed that it could be recycled up to 6 times with no apparent decrease in the conversion of PLA and yield of methyl lactate. The reaction mechanism and kinetics of PLA methanolysis catalyzed by [Bmim]Fe_xCl_3x+1 were also investigated. Moreover, a possible catalysis mechanism of the methanolysis of PLA was proposed. [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-017-2138-x

Catal Lett
DOI 10.1007/s10562-017-2138-x
Lewis Acidic Ionic Liquid [Bmim]FeCl4 as a High Efficient
Catalyst for Methanolysis of Poly (lactic acid)
Huiqing Liu1 · Ruiyang Zhao1 · Xiuyan Song1 · Fusheng Liu1 · Shitao Yu1 ·
Shiwei Liu1 · Xiaoping Ge1
Received: 6 April 2017 / Accepted: 4 July 2017
© Springer Science+Business Media, LLC 2017
Abstract Methanolysis of poly (lactic acid) (PLA) was
studied using Lewis acidic ionic liquid [Bmim]FeCl4 as a
catalyst and the qualitative analysis indicated that the main
product was methyl lactate. Compared with ionic liquid
[Bmim]Cl, FeCl3 and other traditional catalysts, the Lewis
acidic ionic liquid showed much better catalytic activity in
the methanolysis of PLA. Significantly, the conversion of
PLA and the yield of methyl lactate were 99.3 and 94.6%
respectively with low dosage of catalyst (the molar ratio
of [Bmim]FeCl4 to LA unit is 0.0025:1). The lewis acidic
ionic liquid [Bmim]FeCl4 could be recycled up to 6 times
and FT-IR results showed that the spectrum of the reused
catalyst was similar to that of the fresh one. Moreover, a
possible mechanism of the methanolysis of PLA cata-
lyzed by [Bmim]FeCl4 was proposed. The kinetics of PLA
methanolysis was also investigated under the optimum
conditions and the results indicated that it was a first-order
kinetic reaction with activation energy of 21.28 kJ mol−1.
recycled up to 6 times with no apparent decrease in the
conversion of PLA and yield of methyl lactate. The reac-
tion mechanism and kinetics of PLA methanolysis cata-
lyzed by [Bmim]FexCl3x+1 were also investigated. Moreo-
ver, a possible catalysis mechanism of the methanolysis of
PLA was proposed.
Graphical Abstract The methanolysis of poly (lactic
acid) (PLA) was studied using Lewis acidic ionic liquid
[Bmim]FexCl3x+1 (x>1) as catalyst. The effects of reac-
tion temperature, reaction time, ionic liquid dosage and
methanol dosage on methanolysis results were examined.
The reusability of ionic liquid in the methanolysis of PLA
was investigated, and the results showed that it could be
Keywords Poly (lactic acid) · Methanolysis · Lewis
acidic ionic liquid · Methyl lactate
1 Introduction
Poly (lactic acid) (PLA) is also known as polylactide. As a
kind of biocompatible and biodegradable polymer, it pos-
sesses many unique properties, such as nontoxicity and
nonirritant, excellent glossiness, transparency, breathabil-
ity and oxygen permeability [1, 2]. Compared with other
polymers, PLA can be depolymerized into carbon dioxide
and water by microorganism without the assistance of cat-
alyst, thus PLA is identified as a sustainable polymer [3].
Recently, PLA has been widely used in package, medicine,
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-017-2138-x) contains supplementary
material, which is available to authorized users.
* Fusheng Liu
liufusheng63@sina.com
1
College of Chemical Engineering, Qingdao University
of Science and Technology, 53 Zhengzhou Road,
Qingdao 266042, China
Vol.:(0123456789)
1 3

H. Liu et al.
textile, etc. Meanwhile, the extreme increase of PLA pro-
duction and application also leads to the increase of waste
PLA amount. Although PLA can be biodegraded com-
pletely in nature, the degradation period is too long and the
degradation products cannot be recycled and reused, which
will result in a huge waste of resources. Therefore, a recy-
cling process must be established for the preservation of
resources and the protection of the global environment.
Several methods for chemical recycling of PLA to obtain
valuable products have been reported, including thermal
pyrolysis [4–8], alcoholysis [9, 10], and hydrolysis [11–14].
Thermal pyrolysis can take place both in the absence of
[15] and in the presence of [16–18] a catalyst, however
the reaction needs higher reaction temperature which may
cause many side reactions. Due to the insolubility of PLA
in methanol or water, reported alcoholysis and hydrolysis
methods also require higher temperature [19] and higher
pressure [14] as well as large quantities of concentrated
acids [20] or bases [12, 21] which cannot be reused. These
methods are associated with other disadvantages, such
as equipment corrosion, tedious workup procedures and
environmental problems. Therefore, their applications are
limited.
catalyze the methanolysis of PLA. The structure of Lewis
ionic liquid was characterized and the reaction conditions
were optiumized. Under the optimal reaction conditions,
the Lewis acidic ionic liquid showed higher catalytic
activity. Moreover, the recycling of catalyst, the mecha-
nism and kinetics of the reaction were investigated.
2 Experimental
2.1 Materials and Instruments
Pure PLA pellets (3.0 mm length and 2.5 mm diameter)
were obtained from Shenzhen ESUN Industrial Co. Ltd.
China. The weight-average and number-average molecu-
lar weights were about 220,500 and 159,500 respectively.
N-methylimidazole (MIM, 99%) was supplied from Zhe-
jiang Kaile Chemicals Co. Ltd., FeCl3 was purchased
from Shanghai Shanpu Chemical Co. Ltd. Other materi-
als were commercially available and used without further
purification.
Infrared (IR) spectra were recorded using a Nicolet
510P FT-IR spectrometer in the range of 4000–400 cm−1
using KBr optics and liquid film. Raman spectra were
recorded using a Thermo Scientific DXR Raman Micro-
scope. Mass spectrum was measured by Bruker Maxis
Quadruple Ultrahigh Resolution Time-of-Flight mass
instrument (Q-TOF MS). TGA curves were measured by a
NETZSCH STA 409 PC/PG thermogravimetric analyzer
in an atmosphere of N2 with a flow rate 25 mL min−1 over
a temperature range from room temperature to 600 °C
at a heating rate of 20 °C min−1. The average molecular
weights were determined by GPC (Waters-515) equipped
with a differential refraction detector and an ultrasty-
ragel column, using tetrahydrofuran (THF) as mobile
phase at a flow rate of 1.0 mL min−1. Calibration of
GPC was carried out with standard polystyrene samples.
UV–Vis absorption spectra were recorded on a UV-3100
spectrophometer.
Recently, methanolysis of PLA is one of the most widely
studied processes for PLA depolymerization. However,
it is very inert in the absence of a catalyst. Traditionally,
the methanolysis of PLA was carried out in the presence of
montmorillonite K10 [10], zinc acetate [22], concentrated
sulfuric acid or p-toluenesulfonicacid [23], FeCl3 [24], etc.
However, these catalysts are toxic and/or corrosive and/or
result in severe pollution. Therefore, the development of
an environmentally benign and highly efficient catalyst for
PLA methanolysis is still important.
More recently, our group used ionic liquids as solvents
and/or catalysts in some polymers depolymerization to
recover corresponding monomers, such as polycarbon-
ate (PC), poly(ethylene terephthalate) (PET), and some
good results have been obtained [25–28]. In our previ-
ous research, ionic liquid 1-methyl-3-butylimidazolium
acetate ([Bmim]OAc) [29] and acidic functionalized
ionic liquid 1-methyl-3-(3-sulfopropyl)-immidazolium
hydrogen sulfate ([HSO3-pmim][HSO4]) [30] were syn-
thesized successfully and used as catalysts for methanoly-
sis of PLA. The conversions of PLA were over 97% and
the yields of methyl lactate were 92 and 88.7% respec-
tively. However, the optimum dosage of ionic liquid was
too much (2 wt% based on polymer). Because FeCl3 is a
good catalyst for PLA methanolysis [24] and ionic liquid
[Bmim]Cl may enhance the solubility of PLA in reaction
mixture, Lewis acidic ionic liquid [Bmim]FeCl4 which is
the reaction product of FeCl3 and [Bmim]Cl should be a
more effective catalyst for PLA methanolysis. Therefore,
in this paper, [Bmim]FeCl4 was synthesized and used to
2.2 Synthesis of Lewis Acidic Ionic Liquid [Bmim]
FeCl4
The ionic liquid, 1-methyl-3-butylimidazolium chloride
([Bmim]Cl), was synthesized according to the procedures
described in the literatures [31, 32]. A given amount of
anhydrous FeCl3 was added into [Bmim]Cl and the result-
ing mixture was stirred at 30°C under N2 atmosphere for
5 h to obtain Lewis acidic ionic liquids [Bmim]FeCl4 with
different FeCl3 molar fraction, and it was a brown green
viscous liquid.
1 3

Lewis Acidic Ionic Liquid [Bmim]FeCl4 as a High Efficient Catalyst for Methanolysis of Poly…
2.3 Methanolysis of PLA
that the peaks up to m/z 139.1 and 197.8 were responding
to the positive ion [Bmim]+ and negative ion [FeCl4]− of
[Bmim]FeCl4 in Fig. 1a, b, correspondingly. The Raman
spectra of [Bmim]FeCl4 and [Bmim]Cl were shown in
Fig. 2. Comparing with the Raman spectrum of [Bmim]
Cl, the spectrum of [Bmim]FeCl4 showed a strong band
at 330 cm−1, which was assigned to the totally symmetric
Fe–Cl stretch viation of [FeCl4]− [33]. Thus, all above char-
acteristic results indicated that the synthesized ionic liquid
was [Bmim]FeCl4.
PLA (w1), methanol and catalyst (w2) were added into a
50 mL autoclave with a magnetic stirrer and a thermom-
eter. The methanolysis reactions were carried out at the
given temperature for the prescribed time under autogenous
pressure. When the reaction was finished, the mixture of
the reaction was cooled to room temperature, then the unre-
acted methanol was recovered by atmospheric distillation
and the product (methyl lactate) (w3) was collected by vac-
uum distillation. The residue (w4) which was mainly com-
posed of catalyst and undepolymerized PLA was directly
reused as catalyst.
3.2 Qualitative Analysis of Methanolysis Product
The conversion of PLA and yield of methyl lactate were
The obtained product was characterized by FT-IR and the
result was shown in Fig. 3. From Fig. 3, it can be clearly
deifined by the following equations (1) and (2).
(
)
w1 − w4 − w2
Conversion of PLA =
× 100%
(1)
(2)
w1
w3
w1
M(LA unit)
Yield of Methyl lactate =
×
M(Methyl lactate)
where M (LA unit) and M (methyl lactate) are the molar
masses of the repeating units of PLA and methyl lactate,
respectively.
a
3 Results and Discussion
b
3.1 Characterization of Ionic Liquid
200
300
400
500
600
700
800
Raman shift / cm-1
The obtained [Bmim]FeCl4 was characterized by MS and
Raman spectroscopy. It was clear from the mass spectra
Fig. 2 Raman spectra of [Bmim]FeCl4 (a) and [Bmim]Cl (b)
a
b
139.1
197.8
100
200
300
400
500
100
150
200
250
300
350
400
450
500
m / z
m / z
−
Fig. 1 Q-TOF mass spectra of [Bmim]FeCl4, a positive ion [Bmin]+, b negative ion FeCl4
1 3

H. Liu et al.
Table 1 Catalytic activities of different catalysts in methanolysis of
PLA
D
Entry
Catalysts
PLA conver-
sion/%
Methyl
lactate
yield/%
1
2
3
4
5
6
7
Blank
[Bmim]Cl
FeCl3
[Bmim]FeCl4
H2SO4
ZnAc
0
0
42.1
92.8
42.3
39.3
58.6
0
0
39.4
87.5
30.9
34.5
53.5
E
NaAc
n(CH3OH):n(LAunit)=5:1; T=110°C; t=3 h; n(Catalyst):n(LA
unit)=0.0025:1
4000
3500
3000
2500
2000
1500
1000
[Bmim]FeCl4 was an excellent catalyst for methanolysis of
PLA.
-1
Wave numbersꢀcm ꢁ
The influences of FeCl3 molar fraction in [Bmim]FeCl4
on PLA methanolysis were investigated and the results
were shown in Table 2. It could be seen that the conversion
of PLA and methyl lactate yield increased gradually with
increasing the molar fraction of FeCl3 in ionic liquid. This
could be attributed to the increasing in acidic strength of
catalyst caused by the increasing of FeCl3 amount in this
reaction. It is well known that the methanolysis of PLA
is a transesterification reaction, and the stronger acidity
of catalyst facilitates the reaction. When the molar frac-
tion of FeCl3 was 0.67, the PLA conversion and product
yield reached 89.4 and 83.7% respectively. Whereas fur-
ther increasing the amount of FeCl3, there was no apparent
increasing in PLA conversion and product yield.
Fig. 3 IR spectra of obtained product (b) and standard methyl lactate
sample (a)
seen that there are a –OH band at 3445 cm−1, a C=O
band of ester at 1725 cm−1, two alkyl C–H bands at 2890
and 2950 cm−1, and two C–O bands of ester at 1225 and
1130 cm−1. The IR spectrum of the obtained product is
almost the same as that of the standard sample. The result
of FT-IR indicated that the obtained product was methyl
lactate in the methanolysis of PLA. Moreover, the obtained
product was analyzed by UV–Vis spectroscopy and com-
pared it with a standard monomer (as shown in Fig. S1).
The results showed that the absorption peak of obtained
product was consistent with that of the standard methyl lac-
tate, which validates that the methanolysis product was not
dyed by the Fe-containing ionic liquid.
3.4 Influences of Reaction Conditions on PLA
Methanolysis
The influences of reaction conditions on PLA methanolysis
were examined and the results were shown in Table 3. It
3.3 Selection of the Catalysts
The catalytic activities of different catalysts obtained under
the same conditions were shown in Table 1. From Table 1,
the PLA methanolysis could not take place in the pres-
ence of [Bmim]Cl (Entry 2) or in the absence of any cata-
lyst (Entry 1). Compared with FeCl3 and other traditional
catalysts, both the PLA conversion and methyl lactate yield
catalyzed by [Bmim]FeCl4 were the highest. It was indi-
cated that there was a synergistic effect between [Bmim]
Cl and FeCl3. Moreover, the methanolysis was a reaction
between a rigid PLA macromolecule and a liquid methanol
molecule, only the PLA macromolecules which was dis-
solved or swelled in the reaction mixture could react with
methanol. And the ionic liquid [Bmim]Cl in [Bmim]FeCl4
may improve the dissolving or swelling of PLA. Therefore,
Table 2 Effects of FeCl3 molar fractionsin [Bmim]FeCl4 on metha-
nolysis of PLA
FeCl3 molar fraction/x
PLA conversion/%
Methyl
lac-
tateyield/%
0.40
0.50
0.60
0.63
0.67
0.75
74.1
78.3
84.5
87.8
89.4
89.9
67.2
70.1
77.5
80.3
83.7
84.1
n(CH3OH):n(LA unit)=5:1; n(catalyst):n(LA unit)=0.0025:1;
T=120°C; t=2 h; x=n(FeCl3)/[n(FeCl3)+n([Bmim]Cl)]
1 3

Lewis Acidic Ionic Liquid [Bmim]FeCl4 as a High Efficient Catalyst for Methanolysis of Poly…
Table 3 Effects of reaction
Entry
Temperature/°C
Time/ h
n(CH3OH):n(LA
unit)
n([Bmim]
FeCl4):n(LA
unit)
PLA con-
version/%
Methyl
lactate
yield/%
conditions on methanolysis of
LA
1
2
3
4
5
6
7
8
9
100
110
120
120
120
120
120
120
120
3.0
3.0
3.0
2.5
2.0
3.0
3.0
3.0
3.0
5.0:1
5.0:1
5.0:1
5.0:1
5.0:1
3.0:1
7.0:1
5.0:1
5.0:1
0.0025:1
0.0025:1
0.0025:1
0.0025:1
0.0025:1
0.0025:1
0.0025:1
0.0010:1
0.0020:1
79.1
92.8
99.3
93.1
89.2
69.2
90.7
76.2
95.6
73.3
87.5
94.6
85.3
79.8
64.4
84.5
69.5
89.4
could be seen that with the increasing of reaction tempera-
ture, the PLA conversion and methyl lactate yield increased
dramatically firstly. The PLA conversion increased from
79.1 to 99.3% when the temperature increased from 100
to 120°C (Entries 1–3). The PLA was almost completely
depolymerized. Both PLA conversion and methyl lac-
tate yield increased gradually with the extending of reac-
tion time (Entries 3–5). When reaction time was 3 h, PLA
conversion and methyl lactate yield reached their maxi-
mums and kept stable with further increasing of reaction
time. It could be seen from Table 3 (Entries 3, 6–7) that
PLA conversion and methyl lactate yield increased gradu-
ally with increasing methanol dosage. And their maxima
of 99.3 and 94.6% were obtained when n(CH3OH):n(LA
unit) was 5.0:1 (Entry 3). However, a further increase of
n(CH3OH):n(LA unit) (Entry 7) resulted in decrease of
PLA conversion and methyl lactate yield. This result could
be explained by the decrease of concentration of catalyst in
the reaction mixture caused by too much methanol dosage.
The decrease of catalyst concentration was unfavorable for
improving the reaction. The effects of catalyst dosage on
PLA conversion and methyl lactate yield were also exam-
ined (Table 3, Entries 3, 8–9). With the increase of cata-
lyst dosage, the conversion of PLA and methyl lactate yield
increased significantly. When n([Bmim]FeCl4):n(LA unit)
was increased to 0.0025:1 (Entry 3), the PLA conversion
reached 99.3%, meaning that the PLA methanolysis was
almost finished.
PLA conversion
120
Methyl lactate yield
100
80
60
40
20
0
1
2
3
4
5
6
Reuse time
Fig. 4 Reusability results of [Bmim]FeCl4 in methanolysis of PLA
a
b
3.5 Reusability of Ionic Liquid [Bmim]FeCl4
The reusability of ionic liquid [Bmim]FeCl4 was investi-
gated under the conditions of temperature 120°C, time 3 h,
n(CH3OH):n(LA unit)=5.0:1 and n([Bmim]FeCl4):n(LA
unit)=0.0025:1. As shown in Fig. 4, there was no obvious
decrease in PLA conversion and methyl lactate yield after
the ionic liquid was reused for six times, which indicated
the excellent reusability of the catalyst. Figure 5 showed
4000
3500
3000
2500
2000
1500
1000
500
Wave numbers / cm-1
Fig. 5 IR spectra of fresh [Bmim]FeCl4 (a) and reused one (b)
1 3

H. Liu et al.
the IR spectra of fresh [Bmim]FeCl4 and reused one. It can
be clearly seen that the spectra contain a aromatic hetero-
cycte C–H stretching vibration at 3149 cm−1, an alkyl C–H
at 2968 cm−1, a C=C stretching vibration at 1625 cm−1 and
a C=N stretching vibration at 1595 cm−1. The TG curves
of fresh and reused [Bmim]FeCl4 were shown in Fig. S2. It
could be seen that there was less change both between the
two IR spectra and between the two TG curves, indicating
there was no change in chemical structure of the ionic liq-
uid after reused for 6 times. Therefore, [Bmim]FeCl4 was
an effective and reusable catalyst for the methanolysis of
PLA.
3.6 Mechanism of PLA Methanolysis Catalyzed
by [Bmim]FeCl4
In order to study the mechanism of PLA methanolysis, the
IR spectra of PLA residues (which were mainly composed
of intermediate products, i.e. oligomers) obtained at dif-
ferent conversions were shown in Fig. S3. Compared with
pure PLA, there was an obvious absorption peak at about
3500 cm−1 which belongs to the hydroxyl resulted from the
reaction of PLA methanolysis when the conversion of PLA
was 38.9%, and the peak became more intensive with the
increase of PLA conversion.
Scheme 1 Possible mechanism of PLA methanolysis catalyzed by
[Bmim]FeCl4
PLA macromolecules were dissolved or swelled in the
reaction mixture under the given temperature, the [Bmim]+
cation interacted with the carbonyl oxygen (C=O) in PLA
macromolecules dissolved or swelled, which increased
the electropositivity of the carbonyl carbon atom and was
advantageous to the attack of methanol. Meanwhile, the
anion [FeCl4]− interacted with the hydrogen in –OH group
of methanol, and the oxygen atom in –OH group of metha-
nol attacked the carbonyl carbon atom, forming a transition
state of a six membered ring [31]. Afterwards, the hydrogen
left the –OH group of methanol, the electrons on the oxy-
gen in –OCH(CH3)CON transferred to form C=O and the
acyl-oxygen cleaved forming the –OCH(CH3)CON inter-
mediate which combined with the hydrogen left from the
–OH group of methanol to form a oligomer HOCH(CH3)
CON, meanwhile a oligomer MOCH(CH3)COOCH3 was
also formed. These transfer processes repeated, the oligom-
ers formed became smaller and smaller until methyl lactate
monomer was formed.
Table 4 showed the molecular weights of PLA residues
obtained at different conversions. It was found that the
molecular weight of residue decreased gradually with the
increase of PLA conversion. The TG curves of the resi-
dues obtained at different conversions were shown in Fig.
S4. The results in Table 4 and Fig. S4 demonstrated that
with the conducting of methanolysis, PLA macromolecules
became to be oligomers of smaller molecular weights. The
oligomers reacted further with methanol to generate methyl
lactate product. Moreover, the products obtained at differ-
ent PLA conversion were characterized by IR (Fig. S5). It
could be seen that the IR spectra of the products obtained
at PLA conversions of 38.9% and 77.6% were almost the
same as that of pure methyl lactate, illustrating that PLA
was depolymerized into methyl lactate and there was no
oligomer or other by-products in the obtained product.
Based on the above results and references [31, 34], a
possible reaction mechanism of PLA methanolysis cata-
lyzed by [Bmim]FeCl4 was proposed in Scheme 1. Firstly,
3.7 Reaction Kinetics of PLA Methanolysis Catalyzed
by [Bmim]FeCl4
Table 4 Molecular weights of the residues obtained at different con-
versions
The reaction kinetics was studied under the optimum condi-
tions obtained in Sect. 3.4. Under the optimum conditions,
the molar ratio of CH3OH to LA unit was 5:1. Moreover,
the reaction only occurred when solid PLA was dissolved
in the reaction mixture, because the reaction is one between
solid polymer macromolecule and liquid small molecule.
PLA conversion/% Mn
Mw
Mw/Mn
0
38.9
77.6
159,566
2930
2722
200,533
3622
2966
1.256
1.236
1.089
1 3

Lewis Acidic Ionic Liquid [Bmim]FeCl4 as a High Efficient Catalyst for Methanolysis of Poly…
Table 5 Linear regression results of the data in Fig. 6
And the concentration of LA units in the mixture would
be very low because the PLA cannot dissolve completely
in the system. Therefore, it can be assumed that CH3OH is
in a great excess. Furthermore, the reaction order was usu-
ally considered to be first-order in literatures in studies of
chemical depolymerization kinetics of polymers [30, 31,
35]. Therefore, the PLA methanolysis catalyzed by [Bmim]
FeCl4 under the optimum conditions was initially assumed
to be controlled by first-order kinetic Eq. (3).
Reaction
Linear regressive equation
Linearity
correla-
tion
temperature/°C
100
110
120
125
y=1.08519+0.88562x
y=−0.43543+1.02343x
y=−0.25324+1.24285x
y=0.0449+1.3564x
0.99993
0.99806
0.99924
0.99363
d(CLA
)
= −kCLA
(3)
dt
temperatures was proportional to the reaction time, and
that this methanolysis was a first order kinetic reaction.
The methanolysis rate of PLA catalyzed by [Bmim]FeCl4
was proportional to LA unit concentration. The slope
values of straight lines in Fig. 7 were 0.88562, 1.02343,
1.24285 and 1.3564 h−1, which were the rate constants
of the methanolysis reaction at 100, 110, 120 and 125 °C,
respectively.
where k is the reaction rate constant, CLA is the concentra-
tion of LA unit at time t.
CLA = CLA0(1 − X)
(4)
where X is the conversion of PLA; therefore Eq. (3) can be
written as follows:
dX
= k(1 − X)
(5)
Based on the reaction rate constants above, the reac-
tion activation energy (Ea) could be calculated by Eq. (7).
dt
Equation (5) was integrated against time to give Eq. (6).
Ea
ln k = ln A −
(7)
1
RT
ln
= kt
(6)
1 − X
where A is pre-exponential factor, R was the is constant
(8.31 J k−1 mol−1), T is temperature in Kelvin. Figure 7
showed the Arrhenius plot of the reaction. The activation
energy for this reaction calculated from the slope of Arrhe-
nius plot was 21.28 kJ mol−1 which was lower than that of
methanolysis of PLA catalyzed by [Bmim][Ac] [29].
To study the reaction kinetics of PLA methanoly-
sis catalyzed by [Bmim]FeCl4, the influences of reac-
tion time on PLA methanolysis at different reaction
temperatures were investigated under the conditions of
n(CH3OH):n(LA unit) = 5.0:1 and n([Bmim]FeCl4):n(LA
unit) = 0.0025:1. The results were shown in Fig. 6, and
the linear regression data were shown in Table 5. All
of the linear correlative coefficients in Table 5 were
higher than 0.99, indicating that ln1/(1–X) at different
0.3
0.2
2.5
2.0
1.5
0.1
1.0
100 ć
110 ć
0.0
120 ć
125 ć
0.5
0.0
-0.1
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
2.50
2.52
2.54
2.56
2.58
2.60
2.62
2.64
2.66
2.68
Methanolysis time / h
103/T(K-1)
Fig. 6 Effect of reaction temperature on methanolysis rate of PLA
Fig. 7 Arrhenius plot of rate constant of PLA methanolysis catalyzed
catalyzed by [Bmim]FeCl4
by [Bmim]FeCl4
1 3

H. Liu et al.
9. Grala A, Ejfler J, Jerzykiewicz LB, Sobota P (2011) Dalton
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4 Conclusions
10. Okamoto K, Toshima K, Matsumura S (2005) Macromol Biosci
5:813
An easy and feasible method for PLA methanolysis to
recover methyl lactate using Lewis acidic ionic liquid
[Bmim]FeCl4 as catalyst was developed. The reaction could
take place under mild conditions without any traditional
acid or base catalyst. The conversion of PLA and yield
of methyl lactate were 99.3 and 94.6% respectively under
the following optimized conditions: n(methanol):n(LA
unit)=5:1, n([Bmim]FeCl4):n(LA unit)=0.0025:1, reac-
tion time 3 h and reaction temperature 120°C. The catalyst
could be reused up to six times, without apparent decrease
in the PLA conversion and methyl lactate yield. A possi-
ble mechanism of PLA methanolysis catalyzed by [Bmim]
FeCl4 was proposed. Moreover, the kinetics of this reaction
was investigated. The results showed that the PLA metha-
nolysis catalyzed by [Bmim]FeCl4 was a first order reaction
and the activation energy was 21.28 kJ mol−1. In summary,
[Bmim]FeCl4 is a potential catalyst in methanolysis of PLA
to recover methyl lactate.
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Acknowledgements This research was supported financially by
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174:872
the National Natural Science Foundation of China (No. 51673106),
the Program of Science
& Research of Shandong Province
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(2016GSF116005), Natural Science Foundation of Shandong Prov-
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Shandong.
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