Synergistic catalysis of imidazole acetate ionic liquids for the methanolysis of spiral poly(ethylene 2,5-furandicarboxylate) under a mild condition

Article abstract of DOI:10.1039/d0gc04019c

Alcoholysis is a potential method for the degradation of polyesters to their precursor building blocks, which can then be polymerized again to form new qualified polyester materials. This method therefore makes recycling possible in a permanent polymer-monomer-polymer loop. In this study, a series of metal-free tetrabutylphosphonium-based ionic liquids (ILs) were prepared as catalysts for the methanolysis of poly(ethylene 2,5-furandicarboxylate) (PEF), which is well known as an ideal alternative to the petroleum-based plastic poly(ethylene terephthalate) (PET). It was found that anions with stronger electronegativity were more efficient for the methanolysis of PEF. By adjusting the structures of the anions and cations of ILs, 1-butyl-3-methylimidazole acetate ([BMIm][OAc]) exhibited excellent catalytic activity in the methanolysis of PEF. The optimization of reaction parameters enabled the [BMIm][OAc]-catalyzed methanolysis of PEF with nearly 100% PEF degradation and a 77.6% dimethyl furan-2,5-dicarboxylate (DMFD) yield at 130 °C for 30 min. Furthermore, [BMIm][OAc] could keep high-activity after using for six times under the optimum conditions. Assisting with¹H NMR, FT-IR and density functional theory (DFT) calculations, an in-depth study was performed on the feasible synergetic catalytic mechanism of [BMIm][OAc] for the methanolysis of PEF, induced by hydrogen bonds formed between reactants and [BMIm][OAc]. Finally, on the basis of experimental results and scanning electron microscopy (SEM), it was confirmed that PEF with the spiral motif could be degraded under milder conditions than PET. Therefore, this strategy of using adjustable cation and anion structures in ILs provides an opportunity to develop other more efficient ILs for the alcoholysis of furan-based polymersviaa green and mild pathway.

Full text of DOI:10.1039/d0gc04019c

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PAPER
Synergistic catalysis of imidazole acetate ionic
liquids for the methanolysis of spiral poly(ethylene
2,5-furandicarboxylate) under a mild condition†
Cite this: Green Chem., 2021, 23,
1871
Xiaoling Qu,a,b Guangyuan Zhou,b Rui Wang,b Bolei Yuan,a Min Jiang*b and
a
Jun Tang
*
Alcoholysis is a potential method for the degradation of polyesters to their precursor building blocks,
which can then be polymerized again to form new qualified polyester materials. This method therefore
makes recycling possible in a permanent polymer–monomer–polymer loop. In this study, a series of
metal-free tetrabutylphosphonium-based ionic liquids (ILs) were prepared as catalysts for the methanoly-
sis of poly(ethylene 2,5-furandicarboxylate) (PEF), which is well known as an ideal alternative to the pet-
roleum-based plastic poly(ethylene terephthalate) (PET). It was found that anions with stronger electro-
negativity were more efficient for the methanolysis of PEF. By adjusting the structures of the anions and
cations of ILs, 1-butyl-3-methylimidazole acetate ([BMIm][OAc]) exhibited excellent catalytic activity in
the methanolysis of PEF. The optimization of reaction parameters enabled the [BMIm][OAc]-catalyzed
methanolysis of PEF with nearly 100% PEF degradation and a 77.6% dimethyl furan-2,5-dicarboxylate
(DMFD) yield at 130 °C for 30 min. Furthermore, [BMIm][OAc] could keep high-activity after using for six
times under the optimum conditions. Assisting with 1H NMR, FT-IR and density functional theory (DFT)
calculations, an in-depth study was performed on the feasible synergetic catalytic mechanism of
[BMIm][OAc] for the methanolysis of PEF, induced by hydrogen bonds formed between reactants and
[BMIm][OAc]. Finally, on the basis of experimental results and scanning electron microscopy (SEM), it was
confirmed that PEF with the spiral motif could be degraded under milder conditions than PET. Therefore,
this strategy of using adjustable cation and anion structures in ILs provides an opportunity to develop
other more efficient ILs for the alcoholysis of furan-based polymers via a green and mild pathway.
Received 27th November 2020,
Accepted 30th January 2021
DOI: 10.1039/d0gc04019c
rsc.li/greenchem
date, several chemical recycling strategies have been put
forward, including pyrolysis,14,15 hydrolysis,16–18 aminolysis,19
Introduction
The development of polymer recovery strategies promises not alcoholysis,20–22 and glycolysis.23–25 These strategies are widely
only to provide a solution to the end-of-use issue for polymer used in the chemical degradation of polycarbonate (PC), poly
materials, but also offers a closed-loop method to realize a (lactic acid) (PLA), poly(3-hydroxybutyrate) (PHB), and poly-
recycled materials economy.1,2 A broad variety of approaches3–6 ethylene terephthalate (PET). In contrast, there is a great
has been sought to dispose of several hundred million metric potential for development in the alcoholysis of polyesters since
tons of synthetic polymers. These include physical strategies,3 it is not only possible to obtain the initial monomer as a raw
biological degradation,7–9 and chemical recycling.10,11 Among material for the production of new virgin plastic, but also to
these approaches, chemical recycling, meaning the conversion produce other valuable chemicals through a relatively mild
of waste polymers into valuable chemicals through chemical and green chemical process.20,21,26–28
methods, has gained widespread attention from both “sustain-
With respect to the alcoholysis of polyester, a great number
able society” and “green chemistry” points of view.12,13 To of catalysts have been reported, including oxides,29,30 hydrox-
ides,31 metal salts,32 halides33 and ionic liquids (ILs). ILs are
regarded as potential alternatives to traditional metal-based
catalysts because of their excellent tunabilities, low toxicities,
aDepartment of Polymer Science, College of Chemistry, Jilin University, Changchun,
130012 Jilin, China. E-mail: chemjtang@jlu.edu.cn
good reusability, and absence of transition or heavy
bDivision of Energy Materials (DNL 22), Dalian Institute of Chemical Physics.
metals.34–36 They have recently been extensively studied in the
Chinese Academy of Sciences, Dalian, 116023, China. E-mail: jiangmin@dicp.ac.cn
alcoholysis of PET,37,38 PC,20,26,28 PHB,22 and PLA.27,30 Nunes
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
et al.39 studied the depolymerisation of PET in supercritical
d0gc04019c
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ethanol in the presence of 1-butyl-3-methylimidazolium tetra- alcohol to make a nucleophilic attack at the carbonyl of the
fluoroborate ([BMIm][BF4]). The use of [BMIm][BF4] and super- polyester. However, PET still needs to be fully degraded for
critical ethanol represented an extremely promising combi- several hours at a high temperature. To reveal the process and
nation for PET depolymerisation with a conversion of 98 wt% characteristics of the alcoholysis of PEF, it is necessary to
from PET to diethylterephthalate. However, harsh reaction understand the physical and chemical roles of the furan ring
conditions (high temperature and pressure) were needed. Liu within the polymer chain, and provide an in-depth study on
et al.22 reported that 1-(3-sulfonic acid)-propyl-3-methyl- the catalytic mechanism in the trace amounts of ionic liquids.
imidazole ferric chloride ([MImPS]FeCl4) exhibited excellent Additionally, with PEF as the potential substitute for PET, it is
catalytic activity and reusability in the methanolysis of the bio- of great significance to investigate the difference of alcoholysis
based polymer PHB. However, the monomer produced was between PEF and PET.
easily stained by Fe-containing IL. The methanolysis of PC over
To this end, we conducted a series of methanolysis of PEF
[BMIm][Cl] and [BMIm][OAc] was reported by Liu et al.40,41 catalyzed by various ILs with different cations and anions, and
The process could be performed under moderate conditions a comparative study on the methanolysis of PEF and PET was
(105 or 90 °C). However, large amounts of ILs were needed to carried out to reveal the influence of the molecular chain
act together as a solvent, at high cost. Therefore, efficient alco- structure on the alcoholysis reaction. To explore the effect of
holysis strategies for each polymer need different alcoholysis the anion on the catalytic activity, diverse phosphonium car-
processes and different catalytic systems, which mainly boxylate-based ILs were prepared through a one-step neutraliz-
depend on the structural characteristics of the polymer and ation reaction (Scheme 1). By tuning the structures of the
the catalytic activity of the catalyst. Therefore, it is of great sig- anions and cations, it was found that [BMIm][OAc] represents
nificance and challenge to explore the alcoholysis of other a remarkable catalytic activity in the methanolysis of PEF. The
polyester materials with the potential for industrial application structure and purity of the main product were determined
by using ILs as catalysts under mild and green conditions.
using a series of characterization methods. Furthermore, a
Poly(ethylene 2,5-furandicarboxylate) (PEF), a promising plausible synergetic catalysis mechanism of the [BMIm][OAc]-
100% renewable polyester is fabricated from 2,5-furandicar- catalyzed methanolysis of PEF was proposed on the basis of
1
boxylic acid (FDCA) from plant biomass.42 Due to the simi- experimental results, H NMR and IR spectra, and DFT calcu-
larities in the structural, chemical, and physical properties of lations. Finally, SEM analysis was performed in order to under-
FDCA to those of terephthalic acid (PTA),43,44 PEF is an ideal stand the contribution of the furan ring to the alcoholysis of
alternative to PET, the giant of the plastic industry (global PEF. To the best of our knowledge, the study on the methano-
annual production >50 MT).45–47 Interestingly, the inherent lysis of PEF with ILs as catalysts has not been reported.
angle (129.4°) between the carboxylates of FDCA48 gives PEF a
spiral motif and a nonlinear architecture.49 The spiral motif
endows the polyester with unique properties, like high
stiffness, chemical stability, and excellent barrier properties
(13–19 and 6–11 times higher barriers to CO2 and O2), which
are even superior to those of its fossil-based counterpart,
Experimental section
Chemicals
PEF (powder, 80–100 mesh, Mn 3.90–4.10 × 104 g mol−1, PDI,
2.12), PET (powder, 80–100 mesh, Mn 2.00 × 104 g mol−1, PDI,
1.94) methanol (99.0%, China National Medicines).
Tetrabutylphosphonium hydroxide solution (40 wt% in H2O,
Alfa Aesar), acetate acid (99.5%), propionic acid (99 + %),
L-(+)-lactic acid (90% in water), 4-(hydroxymethyl)benzoic acid
(98%), and 5-hydroxymethyl-2-furancarboxylic acid (98%) were
purchased from Innochem. 2-Furoic acid (98%, Acros), benzoic
acid (99.5%, Alfa Aesar). Tetra-n-butyl quaternary acetate
([N4444][OAc], 98%) was purchased from Shanghai Chengjie
PET.43,50 These properties make PEF highlight the advantages
compared with PET, especially in the field of food packaging,
such as bottles and films. However, chemical resistance, which
makes PEF especially attractive, also causes disadvantages, like
limited or no biodegradability.9,51 Furthermore, the conse-
quent accumulation after its commercialization will definitely
bring great pollution and harm to the ecological environment
and human health. Therefore, it is necessary and significant to
study the recycling of PEF. Unfortunately, at present, most of
the studies about PEF focus on its synthesis, properties and
modification.42,47,49,52–54
It was reported that the synergic interaction of the ILs’
anions and cations endowed it with excellent catalytic activity
in the alcoholysis of PET.20,55 Among the reported anions of
the ILs investigated, carboxylate anions stand out due to their
potential to form hydrogen bonds with the hydroxyl groups of
the monomer alcohol.18,26,56 The cations of the ILs, such as
1,8-diazabicyclo[5.4.0]undec-7-ene ([HDBU]+),26 choline,55
imidazole,57 and phosphonium,58 can activate carbonyl groups
by electrostatic interaction with the oxygen of the carbonyl in
the polyester. It then becomes more favorable for the activated Scheme 1 The structures of the investigated cations and anions in ILs.
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Chemical Co., Ltd. 1-Butyl-3-methylimidazolium acetate were recycled from the H2O phase by rotary evaporation. EG
([BMIm][OAc], 98%), 1-butyl-3-methylimidazolium chloride could be recovered by vacuum distillation, and then the separ-
([BMIm]Cl)
and
1-butyl-3-methylimidazolium
bromide ated [BMIm][OAc] catalyst was reused for the next run under
([BMIm]Br) were purchased from Lanzhou Institute of the optimum reaction condition. The crude product was then
Chemical Physics, CAS. Zinc oxide (ZnO, 99.99%, Alfa Aesar), dissolved in hot ethanol under reflux (80 °C) to remove the
zinc acetate (Zn(CH3COO)2, 99.99%, Alfa Aesar), 1,8-diazabicy- unreacted PEF by hot filtration. The ethanol solution was stati-
clo[5.4.0]undec-7-ene (DBU, 99%, Aladdin). Pure dimethyl cally cooled at room temperature to collect the precipitated
furan-2,5-dicarboxylate (DMFD) was purchased from dimethyl furan-2,5-dicarboxylate (DMFD). The conversion of
Innochem as standard samples.
PEF and the obtained product yield were calculated using the
following formulas:
Synthesis of tetrabutylphosphonium-based ionic liquids
WPEF ꢀ W1
PEF conversion ¼
ꢁ 100%
ꢁ 100%
Tetrabutylphosphonium-based ILs were synthesized according
to the reported procedures,26,59–61 as follows. A tetrabutyl-
phosphonium hydroxide ([P4444]+[OH]−) aqueous solution
(40 wt% in H2O, 0.01 mol) was neutralized with equimolar
acetic acid (0.01 mol) by stirring at room temperature for 12 h.
The resulting ([P4444][OAc]) IL was washed several times with
diethyl ether after the removal of water by vacuum rotary evap-
oration, and then dried under vacuum for at least 24 h at
70 °C. The synthetic methods and purification procedures of
other tetrabutylphosphonium-based ILs were the same as
those of [P4444][OAc]. Similarly, [DBU][OAc] was prepared using
a one-step neutralization reaction from DBU (0.01 mol) and
acetic acid (0.01 mol) in aqueous solution by stirring at room
temperature for 12 h. The purification method was similar to
that for the tetrabutylphosphonium-based ILs. The 1H NMR,
13C NMR, and 31P NMR data of the synthetized ILs are listed in
the electronic ESI.† The IL cations and anions investigated are
listed in Scheme 1.
WPEF
WDMFD ꢀ MDMFD
YieldDMF
¼
W
PEF=MPEF
where WPEF is the initial weight of PEF, W1 is the weight of the
unreacted PEF, WDMFD is the weight of the obtained product,
and MDMFD and MPEF are the molecular weights of DMFD and
the PEF repeat unit, respectively.
Characterization
1H NMR, 13C NMR, and 31P NMR measurements were carried
out on a Bruker Avance 400 MHz and 500 MHz NMR spectro-
meter. The materials to be tested were dissolved in the corres-
ponding deuterium reagents with tetramethylsilane (TMS) as
the internal reference. FTIR was carried out on a Nicolet 380
spectrometer (Thermo Fisher Scientific, USA). MS were
obtained on a microTOF instrument (Bruker, Germany) by
electrospray ionization mass spectrometry (ESI-MS). The XRD
test was performed57 using a Smartlab (9) (Rigaku, Japan) in
the range of 2θ = 2–80°. The purity of the product was deter-
mined by external reference method of HPLC on an
ACCHROM GC-S6000 LC at room temperature, 1 mg mL−1
samples were prepared by dissolving in CHCl3 and the pure
DMFD was the reference sample. SEM images of PEF/PET
before and after glycolysis were obtained using a JEOL 5410
system operating at 5 kV. The crystallization behavior of PEFs
and PET were recorded with differential scanning calorimetry
(DSC) on a TA-Q 200 thermal analyzer, and calibrated with an
indium standard. The PEF samples (6.0 0.5 mg) were heated
to 240 °C at a heating rate of 10 °C min−1, and kept for 5 min
in order to erase the thermal history. The melt was cooled to
General procedure for the alcoholysis of PEF
The alcoholysis of PEF is shown in Scheme 2. The reaction was
carried out in a 20 mL pressure bottle with magnetic stirring.
Typically, W g of PEF powder (80–100 mesh), 5 eq. of CH3OH,
and 0.25 mol% (mole ratio of methanol) of IL were charged
into the pressure bottle at room temperature. Taking
[BMIm][OAc] (Mn = 198.26 g mol−1) for example, the mass of
reactants (PEF and CH3OH) and ILs were 2 g, 10 g and 0.15 g,
respectively. Then, the reactor was heated to the required
130 °C and maintained at that temperature for a certain time.
Upon completion of the reaction, the reactor was cooled
quickly in an ice water bath to quench the reaction. The
obtained mixture was washed several times with deionized
H2O to separate ILs from the solid crude product by filter. The
filtrate composed of furan-based oligomers and EG was con-
centrated by rotary evaporation. After 4 hours in the refriger-
ator at minus 10 °C, the furan-based oligomers precipitated
and could be separated from the H2O phase. The ILs and EG
25 °C, and then heated to 240 °C at 10 °C min−1
.
Results and discussion
Analysis of the main product
A series of tetrabutylphosphonium-based ILs were synthesized,
and their structures were determined using 1H NMR, 13C
NMR, and 31P NMR. All of the detailed characterization results
are provided in ESI (Fig. S1–S22†). The spectra obtained
matched the intended chemical structures well. The methano-
lysis of PEF catalyzed by ILs was thoroughly investigated for
the first time. In order to identify the structure and purity of
the main product in the methanolysis of PEF catalyzed by the
Scheme 2 Methanolysis of PEF catalyzed by ILs.
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prepared ILs (taking [P4444][OAc] for example), various charac- 12.335 min, which indicates that the purity of the main
terization methods were used, including NMR, FT-IR, XRD, product was high.
ESI-MS, and HPLC. Upon comparing the NMR, FT-IR, and
Screening of catalysts with diverse cations and anions
XRD of the main product obtained from the methanolysis of
PEF (obtained DMFD) with those of pure DMFD (purchased To study the influence of the ILs’ structures on the catalytic
from Innochem and regarded as the standard sample) (Fig. 1 activity, the effects of the anion structures in the tetrabutyl-
(a–c)), it was found that the obtained DMFD had the same phosphonium-based ILs on the catalytic efficiency for the
structure as the pure DMFD sample. For example, in the 1H methanolysis of PEF were investigated first under the same
NMR spectrum, single peaks at 3.86 ppm and 7.44 ppm corre- reaction conditions. The results are shown in Table 1. It was
sponded to two aromatic protons of the furan ring and six found that the rate of PEF degradation was very slow without a
protons of the two methyls, respectively. The peak at 3.32 ppm catalyst, and the alcoholysis reaction of PEF was accelerated
corresponded to the proton of H2O. The 13C NMR spectrum, when a tetrabutylphosphonium-based IL was added. The order
with the main peaks at δ = 158.3, 146.5, 119.5, and 52.8 ppm, of the catalytic activities was [P4444][OAc] > [P4444][Pro] >
is shown in Fig. S24.† The FT-IR spectrum for the obtained [P4444][BAc] > [P4444][LAc] > [P4444][HMBAc] > [P4444][FAc] >
DMFD was in good accordance with that of the pure DMFD. As [P4444][HMFAc]. [P4444][OAc] exhibited the most efficient cata-
shown in Fig. 1(b), the signals observed at 1584, 1436, and lytic activity among the investigated ILs. An almost complete
1382 cm−1 corresponded to the stretching vibration frequen- degradation of PEF (99.2%) and a yield of the main product
cies of the furan ring skeleton. The absorption peak at about (DMFD) of above 75% were achieved in a shorter time
3118 cm−1 could be attributed to the C–H stretching vibration (130 min vs. 145–270 min) with [P4444][OAc] as the catalyst.
of the furan ring. The strong stretching vibration peak of the However, [P4444][BF4] displayed little catalytic activity. The con-
carbonyl in DMFD appeared at 1721 cm−1. The characteristic version of PEF was less than 35%, and no product was
peaks in the XRD pattern of the obtained DMFD were consist- obtained at 130 °C for 4.5 h when using [P4444][BF4] as the
ent with those of the pure DMFD, as shown in Fig. 2(c). It catalyst. In contrast, ILs containing carboxylate showed better
further illustrates that the structure of the obtained catalytic activity under the same reaction conditions. This is
DMFD was the same as that of the pure DMFD. ESI-MS was mainly attributed to the strong hydrogen-bond-accepting
performed and the patterns showed that the peak with the ability (HBA) of the carboxylate.26,62
highest intensity, appearing at m/z = 207.2, was consistent with
To determine how the anions influenced the catalytic per-
the molecular weight of DMFD ionized by Na+. Moreover, the formance, the electronegativity and structure of the anions
purity of the obtained DMFD was checked using HPLC. As were investigated. The electrostatic potential fit (ESP) charges
shown in Fig. S24,† there was one sharp single peak at of all the oxygen atoms in the carboxyls (–COO) of the opti-
Fig. 1 (a) 1H NMR patterns, (b) FT-IR spectra, (c) XRD patterns of the obtained DMFD and the pure DMFD, (d) ESI-MS spectrum of the obtained
DMFD.
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Fig. 2 Effects of different reaction parameters on PEF methanolysis: (a) temperature (reaction conditions: PEF (2.0 g), methanol (10.0 g), catalyst
(0.25 mol% (CH3OH), reaction time (30 min)); (b) m(CH3OH)/m(PEF) (reaction conditions: catalyst (0.25 mol% (CH3OH), reaction time (30 min at
130 °C)); (c) amount of catalyst (reaction conditions: PEF (2.0 g), methanol (8.0 g), reaction time (30 min at 130 °C)); (d) reaction time (reaction con-
ditions: PEF (2.0 g), methanol (8.0 g), catalyst (0.15 mol% (CH3OH)), reaction temperature (130 °C)).
Table 1 Effects of the anion structures of the ILs on the catalytic
performancea
of the anion, the stronger the hydrogen bonds formed between
–COO and methanol, and so the more facile the attack of the
PEF by the activated methanol. Thus, it made the transesterifi-
cation process easier. A greater electronegativity is therefore
beneficial to the PEF methanolysis reaction. However,
although the electronegativities of [P4444][HMBAc] and
[P4444][HMFAc] were greater than that of [P4444][LAc] (entries 6,
8 vs. 4, charge, −0.74, −0.76 vs. −0.72), the catalytic activities
of the former were inferior to that of the latter, with lower conver-
sions (98.8%, 47.5% vs. 99.3%) and yields (73.1%, 19.7% vs.
73.8%). This is mainly ascribed to the steric hindrance of the aro-
matic benzene and furan ring. In addition, the influence of the
hydroxyl group on the catalytic performance was investigated. It
is worth noting that the ILs containing hydroxyl groups in the
anions, such as [P4444][LAc], [P4444][HMBAc] and [P4444][HMFAc]
(entries 4, 6 and 8), were less efficient (longer reaction time, lower
conversion of PEF, or lower yield of the main product) than
[P4444][OAc], [P4444][BAc] and [P4444][FAc] (entries 2, 5 and 7),
Entry Cat.
Chargeb tR c (min) Con. d (%) Yielde (%)
f
1
2
3
4
5
6
7
8
9
—
—
270
130
145
270
270
270
270
270
270
18.5%
99.2%
98.7%
99.3%
99.1%
98.8%
56.5%
47.5%
32.0%
—
[P4444][OAc]
[P4444][Pro]
[P4444][LAc]
[P4444][BAc]
[P4444][HMBAc] −0.74
[P4444][FAc] −0.76
[P4444][HMFAc] −0.76
[P4444][BF4]
−0.84
−0.78
−0.72
−0.75
75.1%
73.1%
73.8%
75.1%
73.1%
30.1%
19.7%
—
—
a Reaction conditions: 2 g of PEF powder (80–100 mesh), 10 g of
CH3OH, and 0.25 mol% (mole ratio of methanol) of IL, reacting for a
certain amount of time at 130 °C. b ESP charges of all the oxygen
atoms in the carboxyl group of the optimized anion structure of the IL.
c Reaction time for the methanolysis of PEF. d Conversion of PEF.
e Yield of the main product. f Methanolysis of PEF without catalyst.
mized anion structures in the ILs were calculated using the respectively. This is probably due to the –OH in these ILs forming
Merz–Kollmann (MK) method. The ESP charge distribution of intramolecular hydrogen bonds with –COO, inhibiting the acti-
the anions provides a good representation of the electrostatic vation of the hydroxyl groups in methanol.
potential, and the charges on the oxygen atoms of –COO are
Overall, it is concluded that [P4444][OAc], with good thermal
listed in Table 1. Table 1 shows that the anions with greater stability, high electronegativity, low steric hindrance, and no
electronegativity tended to be more effective. The order of the intramolecular hydrogen bonding, displayed excellent catalytic
electronegativity of [P4444][OAc], [P4444][Pro] and [P4444][BAc] activity. Therefore, [P4444][OAc], as the ideal catalyst, was
was opposite that of the charges (entries 2, 3 and 5, charge, chosen for further investigation.
−0.84 < −0.78 < −0.75), which was in good agreement with the
Next, the catalytic activities of the cations, including
order of the catalytic performance ([P4444][OAc] > [P4444][Pro] > [P4444]+, [N4444]+, [BMIm]+, and [HDBU]+, combined with the
[P4444][BAc]). This is because the greater the electronegativity optimum anion [OAc]−, were studied (Table 2, entries 2–5).
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Table 2 Effects of the cation structures of the ILs on the catalytic
When the temperature was over 130 °C, taking 140 °C as an
example, the conversion of PEF and the yield of the product
were 99.0% and 78.1%, respectively. It seems that the increas-
ing tendency was retarded. From a practical, energy-saving
standpoint, a temperature of 130 °C was chosen to be applied
to the other chosen reactions.
performancea
Entry Cat.
Chargeb tR c (min) Con.d (%) Yielde (%)
f
1
2
3
4
5
6
—
—
270
130
120
30
70
210
18.5%
99.2%
99.0%
99.2%
99.3%
98.9%
—
[P4444][OAc]
[N4444][OAc]
[BMIm][OAc]
[HDBU][OAc]
[BMMIm][OAc]
−0.84
−0.80
−0.78
−0.61
—
75.1%
73.9%
77.6%
75.6%
73.2%
Fig. 2(b) expresses the effect of the CH3OH/PEF mass ratio
(m(CH3OH)/m(PEF)) on the methanolysis reaction. When the
amount of CH3OH was increased from 2 : 1 to 4 : 1, the PEF con-
version increased from 83.0% to 99.1%, and the yield of DMFD
increased from 51.9% to 77.6%. Nevertheless, further increasing
the CH3OH/PEF mass ratio seemed to be disadvantageous,
resulting in a decrease in the yield of DMFD (75.1% vs. 77.6%).
This may be attributed to the dilution effect of CH3OH on the
concentration of [BMIm][OAc]. To obtain a high DMFD yield, a
CH3OH/PEF mass ratio of 4 : 1 was optimal for this reaction.
The effect of the catalyst amounts on the conversion of PEF
and the yield of DMFD was explored, and the results are
shown in Fig. 2(c). When the amount of [BMIm][OAc] was
increased from 0 to 0.15 mol% of CH3OH, there was a sharp
increase in the conversion of PEF (4.5 to 99.1%) and the yield
of DMFD (0 to 77.6%). After that, the increasing rate became
gradually stable. The tendency is consistent with the effect of
temperature on the reaction. Due to the hydrogen bonds of the
anion and cation formed with the reactants, more catalyst
results in more possibilities for interactions with the reactants.
Therefore, more catalyst results in an accelerated methanolysis
reaction and a reduced equilibrium time.57 A catalyst amount
of 0.15 mol% of CH3OH was chosen to be applied in the other
reactions because the yield of DMFD was high enough and the
conversion of PEF was at its maximum.
Furthermore, the effect of the reaction time on the PEF
degradation and DMFD yield were examined using the
[BMIm][OAc] catalyst (Fig. 2(d)). The degradation of PEF was
smooth with time. A PEF degradation of 99.2% and a DMFD
yield of 77.6% were obtained within 30 min. Further prolong-
ing the reaction time seems to be of little significance. This
phenomenon can be attributed to the equilibrium between the
DMFD monomers and oligomers. Therefore, the reaction time
of 30 min was preferred.
Under the optimum conditions (m(CH3OH)/m(PEF) = 4/1,
the catalyst amount being 0.15 mol% of CH3OH, reacting for
30 min under 130 °C), an excellent conversion of PEF (99.2%)
and a satisfactory yield of DMFD (77.6%) were obtained. The
catalytic activities of several representative imidazole ILs24,39
and metal catalysts30,38,66 (Table 3, entries 2–6), which have
been reported to be effective in polyester degradation, were
also investigated. The performance of [BMIm][OAc] was better
than that of the reported imidazole ILs. Importantly, its cata-
a Reaction conditions: 2 g of PEF powders (80–100 mesh), 10 g of
CH3OH and 0.25 mol% (mole ratio of methanol) of ILs, reacting for a
certain time under 130 °C. b The ESP charges of all the oxygen atoms
in the carboxyl group of the optimized anion structure of the ILs. c The
reaction time of the methanolysis of PEF. d The conversion of PEF.
e The yields of the main products. f Methanolysis of PEF without
catalyst.
The thermal stabilities of the screened ILs were checked using
thermogravimetric analysis (TGA). As shown in Fig. S27,† all of
them exhibited good thermal stability (Td 162–306 °C). As
shown in Table 2, when [OAc]− was combined with different
cations, the electronegativity of its –COO group varied.
Although the electronegativities of [P4444][OAc] and
[N4444][OAc] were greater than those of [BMIm][OAc] and
[HDBU][OAc] (−0.84, −0.80 vs. −0.78, −0.61), the catalytic
activities of the latter were higher than those of the former. It
is speculated that [BMIm]+ and [HDBU]+, which have been
reported to have hydrogen-bond donor sites,63,64 were able to
form hydrogen bonds with the ester carbonyl of PEF. The acti-
vated carbonyl carbon was then susceptible to nucleophilic
attack by methanol. Table 2 shows that [BMIm]+ combined
with the optimum anion [OAc]− displayed better catalytic
activity than the other combination. The almost complete
degradation of PEF was achieved at 130 °C, and the yield of
the main product (DMFD) was above 75% in merely 30 min
when [BMIm][OAc] was used as a catalyst. It is well known that
the proton at the C2 position of the imidazolium ring, which
has hydrogen-bond-donating ability (HBD), is a hydrogen-
bond donor site.65 When the hydrogen atom at the C2 position
of [BMIm]+ was substituted for a methyl group (Table 2, entry
6), the catalytic activity of [BMMIm][OAc] was greatly reduced.
To achieve the required conversion of PEF and yield of DMFD,
a longer reaction time (210 min vs. 30 min) was needed. This
indicates that C2–H on [BMIm]+ was the catalytic active site of
[BMIm]+. Furthermore, the combination of [BMIm]+ and
[OAc]−, which could achieve a synergistic effect, realized an
optimal catalytic performance.
Optimization of the reaction parameters
The effect of temperature on the conversion of PEF and the lytic activity was even better than that of ZnO and was close to
yield of DMFD was investigated, and the results are shown in that of Zn(OAc)2, with a PEF conversion of 99.2% (vs. 99.8%
Fig. 2(a). It can be concluded that temperature plays a critical for Zn(OAc)2) and a DMFD yield of 77.6% (vs. 76.1% for
role in the methanolysis of PEF. The PEF conversion was Zn(OAc)2). This further shows that the synergic interaction of
clearly increased in the range of 46.0%–99.2% with an elevated the anion and cation in [BMIm][OAc] enables it to have
temperature from 100 °C to 130 °C, and the yields of DMFD highly efficient catalytic activity in the PEF methanolysis
were increased with a similar trend from 19.3% to 77.6%. reaction.
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Table 3 Comparison of the catalytic activity of [BMIm][OAc] with those
of the reported catalysts for the methanolysis of PEFa
which is consistent with the concept of modern green and sus-
tainable chemistry.
Entry
Cat.
Con.b (%)
Yieldc (%)
Possible reaction mechanism
1
2
3
4
5
6
[BMIm][OAc]
[BMIm]Cl
[BMIm]Br
[BMIm]BF4
ZnO
99.2%
1.7%
6.5%
21.0%
3.8%
99.8%
77.6%
—
—
10.7%
—
76.1%
It is well known that the alcoholysis of polyesters is a typical
transesterification reaction. Based on the experimental results
and a previous study,67 the excellent catalytic activity of
[BMIm][OAc] was mainly attributed to the hydrogen bonds
forming between the reactants and the cation and anion. A
possible synergetic catalytic mechanism involving the cation
and anion was proposed (Scheme 3). The C-2 hydrogen of
Zn(OAc)2
a Reaction conditions: m(CH3OH)/m(PEF) = 4/1, i.e., 2 g of PEF
powders (80–100 mesh), 8 g of CH3OH, catalyst amount 0.15 mol% of
CH3OH, reaction time 30 min, temperature 130 °C. b Conversion of
PEF. c Yields of main products.
Furthermore, [BMIm][OAc], with excellent stability in the
1
reaction system (as shown in the H NMR data in Fig. S28†),
can be easily recycled from the filtrate. Therefore, the re-
usability of the [BMIm][OAc] catalyst was examined at 130 °C
for 0.5 h under optimum condition. As shown in Fig. 3(a),
there was no significant loss of activity after six runs, which
indicated that [BMIm][OAc] had durable high activity for the
methanolysis of PEF. Moreover, the last recycled [BMIm][OAc]
1
was characterized via H NMR and FT-IR to contrast the struc-
tural changes with pristine [BMIm][OAc]. As can be seen from
Fig. 3(b), the structure of the recycled [BMIm][OAc] was essen-
tially similar to its pristine form. A slight absorption peak at
1719.4 cm−1 was observed, which corresponded to the stretch-
ing vibration of the carbonyl group in the residual furan-based
1
oligomer. Additionally, from H NMR (as shown in Fig. S29†),
a signal peak appearing at 3.40 ppm was detected, which cor-
responded to the protons of the methylene groups in the
residual EG. Calculated by 1H NMR of the recycled catalyst
(Fig. S30†), the mole ratio of [BMIm][OAc] to EG was 2.41 and
the mass ratio was 7.70. In order to reduce the interference of
EG, the amount of recycled catalyst was calculated from 1H
NMR until the weight of [BMIm][OAc] became equal to the
initial weight of fresh [BMIm][OAc] (0.15 mol% to methanol).
From the results, the residue of the furan oligomer and EG did
not obviously affect the recycled ILs’ catalytic activity.
Therefore, [BMIm][OAc] displayed great advantage in terms of
the activity and catalyst reusability under mild conditions,
Scheme 3 A possible mechanism for the methanolysis of PEF using
[BMIm][OAc] as the catalyst.
Fig. 3
(a) Catalytic recyclability and (b) FT-IR characterization of the [BMIm][OAc] catalyst. Conditions: PEF 2.0 g, m(CH3OH)/m(PC) = 4 : 1,
[BMIm][OAc] 0.15 mol% of CH3OH, 130 °C, 30 min.
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Fig. 4 1H NMR spectra of the [BMIm][OAc] and DMFD mixture with
different molar ratios.
[BMIm]+ was expected to prompt the electrophilic activation of
the carbonyl of the PEF through a hydrogen bond, similar to
previous reports in the literature.63,68 To verify this specu-
lation, pure DMFD was treated as a model compound, and the
interactions between DMFD and [BMIm][OAc] were studied
using 1H NMR and DFT. As shown in Fig. 4, the chemical shift
of the C-2 hydrogen of [BMIm]+ moved from 9.98 to 9.67 ppm,
and then further to 9.51 ppm, when the molar ratio of
[BMIm][OAc] to DMFD was 1 : 0, 1 : 1, and 1 : 3, respectively.
The chemical shift moving towards the high-field direction
indicated that an intermolecular hydrogen bond was formed
between the C-2 hydrogen of [BMIm]+ and the carbonyl oxygen
atom of DMFD. In this way, the carbonyl carbon of DMFD
could be activated and a carbonyl carbocation was formed
through electron transfer, which was conducive to nucleophilic
attack by methanol.
Fig. 5 Structures of (a) DMFD, DMFD and [BMIm]+; (b) BHEF, BHEF
and [BMIm]+ at the level of DFT-B3LYP. Dark grey: C; red: O; white: H;
blue: N.
To further illustrate the interaction between [BMIm]+ and
DMFD, the length of the hydrogen bond formed between them
was calculated by DFT. From Fig. 5(a), the hydrogen bond was
formed between the C-2 hydrogen of [BMIm]+ and the carbonyl
oxygen atom of DMFD, with a bond length of 2.188 Å. As
expected, the bond length of the carbonyl in DMFD changed
from 1.203 Å to 1.215 Å, which confirms that the ester carbonyl
was activated by the hydrogen bond. Similarly, the length of
the hydrogen bond between [BMIm]+ and bis(2-hydroxyethyl)
furan-2,5-dicarboxylate (BHEF), containing the repeat unit of
PEF, was also investigated, as shown in Fig. 5(b). The hydrogen
bond (of length 2.180 Å) formed between the C-2 hydrogen of
[BMIm]+ and the carbonyl oxygen atom of BHEF activated the
ester carbonyl of BHEF, making the bond length of the carbo-
nyl in BHEF change from 1.202 Å to 1.213 Å. This demon-
strates that an intermolecular hydrogen bond could form
between the C-2 hydrogen of [BMIm]+ and the carbonyl oxygen
atom of PEF, thus activating the ester carbonyl of PEF. This is
beneficial for the nucleophilic attack on PEF by methanol.
Fig. 6 (a) 1H NMR and (b) FTIR spectra of the [BMIm][OAc] and CH3OH
mixture with different molar ratios.
In addition, the interaction between CH3OH and
1
[BMIm][OAc] was further investigated via H NMR and FT-IR. and the signal peak was shifted to 4.84 ppm when the molar
In the 1H NMR spectrum of CH3OH (Fig. 6(a)), the chemical ratio of [BMIm][OAc] to CH3OH was 1 : 1. Further increasing
shift of the hydrogen of the hydroxyl in CH3OH was 4.12 ppm, the amount of [BMIm][OAc], the signal peak of the hydrogen
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Table 4 Comparison of the methanolysis of PEF and PETa
of the hydroxyl in CH3OH could no longer be detected,
suggesting that a strong hydrogen bond was formed between
the hydrogen of OH in methanol and the oxygen of –COO in
the acetate anion. Furthermore, the activation of CH3OH was
proved by FT-IR. As shown in Fig. 6(b), when the molar ratio of
[BMIm][OAc] to CH3OH was increased to 1 : 1, the –OH
Entry
Polyester
tR b (h)
TR c (°C)
Con. d (%)
Yield e (%)
1
2
PEF
PET
0.5
4.0
130
150
99.2%
88.5%
77.6%
41.7%
a Reaction conditions: m(CH3OH)/m(PEF/PET) = 4/1, amount of cata-
vibration of CH3OH in the FT-IR spectrum became broader, lyst of 0.15 mol% of CH3OH (i.e.,
2 g of PEF/PET powders
(80–100 mesh), 8 g of CH3OH, 74.3 mg of [BMIm][OAc]). b Reaction
time. c Reaction temperature. d Conversion of PEF/PET. e Yields of the
main products DMFD and DMT.
and there was a redshift of 101.6 cm−1, from 3362.9 to
3261.3 cm−1. It is worth noting that the –OH vibration band
nearly disappeared when more [BMIm][OAc] was added. This
further evidences that the change in the wavenumber was
induced by strong hydrogen bonds forming between the hydro-
gen of the hydroxyl group in CH3OH and oxygen of the acetate has a linear structure and a compact stacking of polymer
anion. The above results indicate that CH3OH could be acti- chains, which severely impeded the degradation. Accordingly,
vated well by [BMIm][OAc], enhancing the electronegativity of two distinct characteristics could be observed in the alcoholy-
the hydroxyl oxygen, and making methanol more conducive to sis of PEF compared with that of PET: (1) milder reaction con-
nucleophilic attack of the ester group in PEF.
ditions, and (2) a higher degradation rate of PEF and product
Therefore, it is reasonable to speculate that the hydrogen yield.
bonds formed between the reactants and [BMIm][OAc] pro-
Furthermore, scanning electron microscopy (SEM) was
moted the nucleophilic attack of activated CH3OH on the acti- carried out to reveal the morphologies of the original poly-
vated PEF, and played an important role in accelerating the esters and the residual polyesters after degradation. The image
PEF alcoholysis process.
Taking account of the above analysis and experimental PEF was relatively smooth. However, the surface of the residual
results, plausible alcoholysis mechanism using the PEF after degradation for 15 min, as shown in Fig. 7(b),
presented in Fig. 7(a) shows that the surface of the original
a
[BMIm][OAc] catalyst was proposed, as shown in Scheme 3. seemed to exhibit a porous structure. This implies that metha-
Initially, PEF was partially dissolved or swelled in the reaction nol permeated into the PEF matrix, and the surfaces of the
medium, and the alcohol was activated by the hydrogen bond non-depolymerized PEF were covered in obvious cracks. The
formed with the anion of [BMIm][OAc], as mentioned in Fig. 6. morphology of the residual PET, obtained after degradation
Simultaneously, the cation [BMIm]+ underwent a hydrogen for 30 min at 150 °C, is presented in Fig. 7(e). The surface of
bond interaction with the oxygen of the carboxyl group in PEF, the residual PET was rougher than that of the original PET
which was susceptible to carbonyl carbocation formation (Fig. 7(d)). Although harsh conditions were used for the
through electron transfer. Then, nucleophilic attack of the car- methanolysis of PET, the morphology of the residual PET did
bonyl carbocation of PEF by the activated alcohol became not change significantly, in contrast with that of the residual
more favorable. This process resulted in the fragmentation or PEF. As expected, PET promoted a relatively rapid crystalliza-
disconnection of the long molecular chain of PEF, thus break- tion due to the highly symmetric and planar structure of the
ing into smaller oligomers. The oligomers were further benzene ring, which exhibited π–π stacking in the polymer
attacked by the activated alcohol to generate trimers, dimers, chain.48 In contrast, PEF, with a spiral motif and a diverse non-
and finally the DMFD monomer and ethylene glycol (EG). linear architecture caused by an inherent angle (129.4°)
Therefore, it is the synergistic effect between the cation and between the carboxylates of the furan ring, underwent an
anion that might endow the excellent catalytic activity of inferior crystallization.49
[BMIm][OAc].
To further illustrate the effect of crystallization on the
methanolysis of PEF, crystalline PEF was obtained after
heating for 3 h at 170 °C. The crystallinity of crystalline PEF
Comparison on methanolysis of PEF and PET
The prominent effect of different aromatic structures on the and PEF without treatment was qualitatively discussed by non-
alcoholysis of PEF and PET can be better understood from the isothermal crystallization. As listed in Fig. 6f, a broad cold
alcoholysis conditions, the conversion of the two polyesters, crystallization peak (at 171.8 °C) and an obvious melting peak
and the yield of main products (Table 4). It shows that the (at 211.8 °C) of crystalline PEF were observed in the DSC curve.
alcoholysis of PEF occurred more easily than that of PET. A In contrast, PEF without treatment did not display a cold crys-
PEF degradation of 99.2% and a DMFD yield of 77.6% were tallization peak, but only one melting peak. This indicates that
obtained within only 30 min at 130 °C. By comparison, with a the crystalline PEF had a greater crystallinity than that of the
higher temperature (150 °C) and a longer reaction time (4 h), a untreated PEF. Correspondingly, the morphology image of the
PET degradation of 88.5% and a dimethyl terephthalate (DMT) residual crystalline PEF after degradation for 15 min under
yield of 41.7% were achieved. The faster degradation of PEF 130 °C is presented in Fig. 6c. By comparing the SEM images
compared with PET is mainly due to its inherent structural of Fig. 7(b) and (c), the untreated PEF with weak crystallization
irregularity caused by the distortion and asymmetry of the showed more obvious cracks than that of crystalline PEF. This
furan ring in PEF.69 However, PET, with a planar benzene ring, shows that the high crystallinity inhibited the degradation of
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Fig. 7 SEM spectra of the (a) original PEF, (b) residual PEF and (c) crystalline PEF (heated for 3 h at 170 °C) after 15 min under 130 °C, (d) original PET
and (e) residual PET after 30 min under 150 °C; (f) DSC curves of PEFs.
PEF. The observed decrease in the methanolysis of PEF with CH3OH) through the hydrogen bonds formed between the
increasing crystallinity is fully consistent with studies on the reactants and [BMIm][OAc], and a feasible synergistic catalytic
hydrolysis of PEF and PET.8,9,51 Therefore, it can be concluded mechanism was proposed. IL catalysts exhibit the remarkable
that it was easier to perform the alcoholysis of PEF than that advantages of environmental friendliness, mild synthesis con-
of PET, mainly owing to the weak crystallization induced by ditions, and high catalytic activity. It is therefore a good strat-
the distortion and asymmetry of the furan ring.
egy to design more efficient and economical ILs to be applied
To summarise, PEF is a biobased polyester that is not only in the polymer recycling industry via a mild and sustainable
easily prepared, but also readily recycled under mild con- pathway.
ditions due to the special spiral motif induced by the distorted
furan ring.
Conflicts of interest
There are no conflicts to declare.
Conclusions
In this study, a series of tetrabutylphosphonium-based IL cata-
lysts without metals were developed for catalyzing the metha-
nolysis of PEF. The electronic and steric effects, and the
Acknowledgements
This work was supported by the Science and Technology
Development Project of Jilin Province (China, No.
20180201069SF) and National Natural Science Foundation of
China (Grant No. 52073284).
electronegativity of ILs, were the key factors affecting the cata-
lytic activity of ILs. The catalytic activity of ILs was enhanced
when the electronegativity of the anion increased.
[BMIm][OAc] exhibited outstanding catalytic activity for the
methanolysis of PEF, and can achieve comparable perform-
ance with the reported metal-based catalysts. The investigation
of the reaction parameters in this work indicates that tempera-
ture played an important role in the alcoholysis reaction. With
an increase in temperature, the rate of alcoholysis of PEF accel-
erated significantly. Under the optimum conditions
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