Polyvinyl alcohol-potassium iodide: An efficient binary catalyst for cycloaddition of epoxides with CO2

Article abstract of DOI:10.1016/j.mcat.2018.02.007

In this study, we have for the first time demonstrated that polyvinyl alcohol (PVA) and potassium iodide (KI) can form an efficient catalytic system for synthesis of cyclic carbonates from epoxide and CO₂. The catalytic reaction happens in solvent-free conditions. A synergetic effect occur between PVA and KI which considerably increases the reaction yield. This binary catalytic system is mainly suitable for mono-substituted terminal epoxides. In the optimized reaction condition, over 90% reaction yield can be achieved. The binary catalyst is reusable and can be recycled at least five times without significant loss of the catalytic activity. The PVA hydrolysis degree affect the catalytic activity as well. A possible mechanism of synergetic effect of the binary system was proposed. PVA and KI may form a non-toxic, low cost, recyclable, highly-efficiency catalyst for fixing CO₂ through cycloaddition with epoxides.

Full text of DOI:10.1016/j.mcat.2018.02.007

MolecularCatalysis449(2018)25–30
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Polyvinyl alcohol-potassium iodide: An efficient binary catalyst for
cycloaddition of epoxides with CO2
Haibo Changa,⁎, Qingshuo Lia, Xuemin Cuia, Hongxia Wangb, Congzhen Qiaoa, Zhanwei Bua,
Tong Linb,⁎
a
Institute of Functional Polymer Composites, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, 475004, China
Institute for Frontier Materials, Deakin University, Geelong, VIC 3216, Australia
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
In this study, we have for the first time demonstrated that polyvinyl alcohol (PVA) and potassium iodide (KI) can
form an efficient catalytic system for synthesis of cyclic carbonates from epoxide and CO2. The catalytic reaction
happens in solvent-free conditions. A synergetic effect occur between PVA and KI which considerably increases
the reaction yield. This binary catalytic system is mainly suitable for mono-substituted terminal epoxides. In the
optimized reaction condition, over 90% reaction yield can be achieved. The binary catalyst is reusable and can
be recycled at least five times without significant loss of the catalytic activity. The PVA hydrolysis degree affect
the catalytic activity as well. A possible mechanism of synergetic effect of the binary system was proposed. PVA
and KI may form a non-toxic, low cost, recyclable, highly-efficiency catalyst for fixing CO2 through cycloaddition
with epoxides.
Carbon dioxide
Cyclic carbonates
Polyvinyl alcohol
Potassium iodide
Synergetic effects
1. Introduction
reaction. Despite of the studies, most of the HBDs are either toxic, ex-
pensive, unrecyclable or involving using organic solvents. Non-toxic,
In recent decades, conversion of carbon dioxide into usable mate-
rials has gained much attention [1–4]. A coupling reaction between
epoxide and CO2 has been considered as an efficient method for the
chemical fixation of CO2 [2–4], and the reaction products, cyclic car-
bonates, have show wide application potential in chemical synthesis,
catalysis, and electrochemistry [5,6]. Since the epoxide-CO2 reaction is
required to proceed via catalysis, a considerable effort have been made
to develop high efficiency catalysts for this reaction. Catalysts such as
metal oxides [7], alkali metal halides [8], ionic liquids (ILs) [9–13],
transition metal complexes [14–16], and metal-organic frameworks
(MOFs) [17] have been reported, and potassium iodide (KI) is the most
promising one because of the abundance and low price. However, KI
alone typically has low catalytic activity, and a co-catalyst is commonly
used to improve the catalytic efficiency [18–37]. It has been reported
that hydrogen bond donors (HBDs), such as tetraethylene glycol [22],
β-cyclodextrin [23], cellulose [24], lignin [25], C60 fullerenol [26],
pentaerythritol [27], sugarcane bagasse [28], hydroxyl-functionalized
imidazoles [29], triethanolamine [30], amino alcohols [31], poly-
dopamine [32], formic acid [33], amino acids [34,35] and tannic acid
[36] can be used as KI co-catalysts for epoxide-CO2 coupling. HBDs can
activate the ring-opening of epoxides by forming hydrogen bond with
the oxygen atom of epoxide, through which to promote the coupling
cheap, recyclable, readily available and environmentally benign co-
catalysts are highly desirable, but much less reported in research lit-
erature.
Polyvinyl alcohol (PVA) is a linear polymer with enriched hydroxyl
groups. PVA is prepared by hydrolysis of poly(vinyl acetate). It is a non-
toxic, low price polymer with high excellent biocompatibility and
biodegradability [37]. PVA has wide applications in the fields of textile,
papermaking, adhesives, food, biomedical, pharmaceutical and bio-
catalysis [38–41]. The abundant hydroxyl groups, which can be served
as HBDs, make PVA an ideal co-catalyst candidate with KI to accelerate
epoxide-CO2 coupling reaction. However, the co-catalysis ability to of
PVA has not been reported to date.
In the study, we have for the first time demonstrated that PVA-KI is
an excellent catalyst system for the epoxide-CO2 coupling reaction in
solvent-free condition. PVA-KI can promote the coupling reaction in
relatively mild conditions. We further showed that the residual acetate
groups in the PVA had a positive effect on the yield and PVA of
87–89 mol% hydrolysis degree combined with KI showed the best cat-
alytic efficiency. A synergetic effect happened between PVA and po-
tassium halides. Effects of various parameters on their catalytic activity
were examined. A possible mechanism of synergetic effect of the binary
system was proposed.
⁎
Corresponding authors.
E-mail addresses: changhaibo@henu.edu.cn (H. Chang), tong.lin@deakin.edu.au (T. Lin).
https://doi.org/10.1016/j.mcat.2018.02.007
Received 11 December 2017; Received in revised form 5 February 2018; Accepted 12 February 2018
Availableonline22February2018
2468-8231/©2018ElsevierB.V.Allrightsreserved.

H. Chang et al.
MolecularCatalysis449(2018)25–30
2. Experimental section
Table 1
Catalysis results for different substances.a
2.1. Chemicals
Entry
Cat.
Co-Cat.
PC yield (%)b
TOFc
CO2 with a purity of 99.99% was obtained commercially. All the
epoxides and potassium halides (all analytical grade) were purchased
from the Aladdin Chemicals and were used as received. PVA1788
(polymerization degree: 1700; degree of hydrolysis: 87–89 mol%) and
PVA1799 (polymerization degree: 1700; degree of hydrolysis:
98–99 mol%) were obtained from the Aladdin Chemicals. PVAs were
ground into powder and the sizes of PVAs powder below 500 mesh was
obtained through sieves. PVA1788 and PVA1799 have surface area of
0.995 m2/g and 1.407 m2/g (Fig. S1), respectively.
1
2
3
4
5
6
7
8
9
KI
None
KI
KBr
KCl
KBr
KCl
KI
PVA1788
PVA1788
None
PVA1788
PVA1788
None
None
PVA1799
PVA1799
93
Trace
8
19
10
23.3
–
2.0
4.8
2.5
1.0
–
4
Trace
83
Trace
20.8
–
None
a
Reaction conditions: PO (8.3 g, 143 mmol), potassium halide (1.0 mol% relative to
PO),PVA(593 mg) g),CO2 (initial pressure 1.5 MPa), 120 °C, and 4 h.
b
Isolated PC Yield.
TOF: mole of synthesized PC per mole of KI per hour.
2.2. Characterizations
c
The 1H and 13C NMR spectra were recorded on a Bruker DPX-400
spectrometer using deuterated chloroform (CDCl3) as the solvent and
tetramethylsilane as an internal standard. LC–MS spectra were recorded
on an AmaZon SL spectrometer. X-ray diffraction (XRD) patterns were
recorded on a Bruker D8 Advance X-ray diffractometer equipped with
nickel monochromatized Cu Kα radiation (λ = 1.5406 Ǻ). The crys-
tallinity degree of PVA was estimated by the ratio of the crystalline
signal areas to the total area of the diffractogram. Scanning electron
microscopy (SEM) was undertaken using a JSM-7610F scanning electro-
microscope at 5 kV accelerating voltage. The specific surface area of
PVA1788 and PVA1799 powder were measured by N2 isothermal ad-
coupling reaction and the product propylene carbonate (PC).
Table 1 summarizes the results. For KI-PVA1788, the reaction yield
was 93%, whereas the reaction catalyzed by KI alone had a very low
yield, only 8%. When PVA was used alone as catalyst, no matter which
sample used, only trace amount of PC was obtained, indicating very low
catalysis activity. These results suggest that PVA1788 and KI forms an
efficient catalytic system for cycloaddition of PO with CO2 and there
exists a synergistic effect between the two catalyst components.
When KBr or KCl was combined with PVA1788 for the reaction, the
yields were very low although the yields were slightly higher than that
using KBr or KCl alone. This can be explained by the difference in nu-
cleophilicity and leaving ability. I− has stronger nucleophilicity and
sorption/desorption
(77 K)
using
Autosorb-IQ-MP-C
system
(Quantachrome, USA).
leaving
ability
when
compared
with
Cl−
and
Br−
[23,25,28,30,32,34,36].
2.3. Cycloaddition reactions of epoxides with CO2
When PVA1799 was used as a co-catalyst with KI, the reaction yield
was 83%, 10% lower than that using PVA1788 in the same condition.
This indicates that lower hydrolysis degree of PVA could lead to higher
The cycloaddition reactions were carried out in a 100 mL stainless
steel reactor equipped with a magnetic stirrer. In the typical procedure,
the desired amounts of potassium halides and PVA1788 were placed
into the reactor and purged with CO2 three times. Then 143 mmol ep-
oxide was added into the reactor and the reactor was sealed. CO2 was
introduced into the reactor until desired pressure reached.
Subsequently, the reactor was heated to the reaction temperature. After
a certain time, the autoclave was cooled to ambient temperature in a
water bath, and the excess CO2 was vented slowly. Dichloromethane
(CH2Cl2) was added into the reaction mixture. The solid catalyst was
separated from the reaction mixture by centrifugation. It was rinsed
three times with CH2Cl2 and dried in vacuum. The recycled catalyst
was used directly for the next round of experiment. Isolated yields were
obtained by distillation or silica gel column chromatography using a
mixture consisting of petroleum ether and ethyl acetate as an eluent.
The products were confirmed by 1H and 13C NMR spectra.
reaction yield. For PVA1788, which has
a hydrolysis degree of
87–89 mol%, there still acetate groups in the polymer (around
11–13 mol%). However, the un-hydrolysis acetate remained in
PVA1799 was only 1–2 mol%. The 10% more acetate group content for
PVA1788 might increase the CO2 content in PVA due to the CO2-phi-
licity of poly(vinyl acetate), hence contributing to the interaction with
reagents and facilitating the cycloaddition.
It is known that PVA is a semi-crystalline polymer and its hydrolysis
degree have a significant effect on the degree of crystalline [37]. The
crystalline region of a semi crystalline polymer is hard to access during
chemical reaction. To examine the effect of crystalline content on the
coupling reaction, we measured the crystallinity of the two PVA ma-
terials using XRD. As expected, PVA1799 had higher degree of crys-
tallinity (40.2%) than PVA1788 (31.7%) (Fig. S2). To verify the effect
of crystallinity on the coupling reaction, we used an isothermally an-
nealing method to treat PVA1788 at 120 °C for 10 h. After treatment,
the degree of crystalline increased to 47.8%, which is higher than that
of PVA1799. When the annealed PVA1788 and KI were used to co-
catalyze the reaction, the PC yield was 84%, which is similar to that of
KI-PVA1799 in the same condition. Therefore, the effect of PVA hy-
drolysis degree on the PO-CO2 coupling reaction should be stemmed
from the effect on PVA crystallinity.
The effect of PVA1788 and KI amount on the coupling reaction was
examined. As shown in Fig. 1(a), when keeping KI/PO ratio unchanged,
while changing the PVA1788/KI ratio, increasing the PVA1788 amount
could initially increase the PC yield. 93% PC yield reached when KI/PO
molar ratio was 1:100 and PVA1788/KI mass ratio was 2.5:1. The PC
yield remained almost unchanged when further increasing the
PVA1788 amount. When KI/PO molar ratio was changed to 1.5:100,
while PVA1788/KI mass ratio was kept at 2.5:1, the PC yield was only
slightly higher than that of the KI/PO molar ratio at 1:100. Thus, the
optimum amount of catalyst was KI content at 1 mol% (based on the
3. Results and discussion
Three potassium halides (i.e. KCI, KBr and KI) and two PVA materials
(i.e. PVA1788 and PVA1799) were used to examine the catalytic activity,
and propylene oxide (PO) was used as epoxide model to react with CO2.
The main difference between the two PVA materials is in the hydrolysis
degree. Both PVAs have a polymerization degree of 1700. PVA1788 has a
slightly lower hydrolysis degree than PVA1799. Scheme 1 shows the
Scheme 1. Cycloaddition reaction between CO2 and PO.
26

H. Chang et al.
MolecularCatalysis449(2018)25–30
Fig. 1. Effect of (a) reagent, and catalyst condition, (b) reaction temperature, (c) CO2 pressure, (d) reaction time on the yield of PC. (in a-d 143 mmol PO is used, in b-d the molar ratio of
KI/PO is 1:100 and the weight ratio of PVA1788/KI is 2.5:1). The reaction temperature for a, c, and d was 120 °C, and reaction time for a-c was 4 h.
PO) and the mass ratio of PVA1788: KI at 2.5:1.
pentaerythritol and MCM-41.
Fig. 1(b) shows the effect of reaction temperature on reaction yield.
Reaction temperature showed a large effect on the reaction yield. The
yield was 46% at 100 °C, but rapidly increased to 93% at 120 °C. Fur-
ther increasing the reaction temperature to 140 °C led to the slight
decrease in the yield. The optimal temperature was chosen as 120 °C.
Fig. 1(c) shows the effect of CO2 pressure on PC yield. When the
reaction was carried out at 120 °C, the PC yield increased with in-
creasing the CO2 pressure in the lower pressure range (1–1.5 MPa) and
reached a maximum at 1.5 MPa. The yield then gradually decreased
with further increasing the pressure from 1.5 MPa to 3.0 MPa. Similar
effect of CO2 pressure on coupling reaction yield was also reported for
other catalytic systems [9,18–20,23,24,28,30]. The initial increase of
the PC yield was related to the increase in CO2 concentration within the
reaction system. However, the excess CO2 at higher pressure may re-
duce the PO concentration in the vicinity of the catalyst, which was not
favorable to the reaction.
Reusability of the catalytic system was examined by undertaking the
cycloaddition reaction in the optimized condition for multiply cycles. In
each cycle, catalysts were reused by extracting with CH2Cl2 and then
drying. Fig. 2 shows the results. The PC yield declined slightly with the
reaction cycles.
Fig. 3(a) show the FTIR spectra of the fresh PVA 1788 and the 5th
reused PVA 1788. New absorption bands appeared after 5 cycle reused
catalyst. The absorption band around 1640 cm−1 corresponded to the
carbon–carbon double bonds, presumably originated from dehydration of
PVA during the thermal process. A new shoulder absorption at 1800 cm−1
was assigned to the stretching vibration of cyclic carbonate from PC. Also,
after 5 cycle of use, the mass of PVA increased by about 2.3%. This was
Fig. 1(d) shows the influence of reaction time on PC yield. It can be
seen that the reaction proceeds rapidly within the initial 4 h, attaining a
PC yield of 93%. Further prolonging the reaction time to 6 h the reac-
tion may be stopped. Thus, 4 h was considered as the appropriate time
for this chemical reaction.
PVA-KI was compared with other catalyst systems reported for PO-
CO2 cycloaddition reaction. As shown in Table S1, PVA-KI can catalyze
the PO-CO2 cycloaddition reaction in relatively mild conditions when
compared with β-cyclodextrin, sugarcane bagasse, lignin, Mg(OH)Cl
and MCM-41. Moreover, PVA has a comparable condition to catalyze
this cycloaddition reaction when compared to L-tryptophan, lecithin,
cucurbit[6]uril, cellulose, C60 fullerenol, Fe3O4@Fe(OH)3, MOF-5,
Cp2TiCl2, pentaerythritol, triethanolamine, and polydopamine. In ad-
dition, KI used in the KI-PVA catalyst system had a much smaller
content than that used with β-cyclodextrin, sugarcane bagasse, lignin,
lecithin, cucurbit [6] uril and cellulose, tetraethylene glycol, Mg(OH)Cl,
Fig. 2. Reusability of KI-PVA. Reaction conditions: PO (143 mmol), CO2 (initial
pressure = 1.5 MPa), T = 120 °C, reaction time = 3 h, KI/PO = 1 mol%, PVA1788/
KI = 2.5:1(weight ratio).
27

H. Chang et al.
MolecularCatalysis449(2018)25–30
Fig. 3. FTIR spectra (a) and TGA curves (b) of the fresh and 5th-reused PVA1788.
attributed to the absorption of the reaction product into the PVA particles.
Fig. 3(b) shows the TGA curves of fresh PVA and the 5th-reused
PVA. A large difference in TGA curve can be seen between the two
samples. The lower thermal stability for the 5th reused PVA than the
fresh PVA verifies the existence of cyclic carbonate. In addition, the 5th
reused PVA showed larger participle size (see the result in Figs. S3 and
S4). Previous papers reported by other researchers have indicated that
residual cyclic carbonate led to swelling of polymeric particles [42,43].
The increased particle size could lead to reduction in specific surface
area, hence the reaction PC yield. Moreover, the absorbed residual
carbonate product on the catalyst surfaces may affect the interaction
between the catalyst and epoxides, which also might result in loss of
catalytic activity [44].
determined by these two reverse effect.
To find out the source of the reduction in catalytic reaction yield
when using the recycled PVA, we washed the 5th-reused PVA with
CH2Cl2 for three times and fifteen times, the effect of CH2Cl2 washing
cycles on catalytic reaction performance. Because residual cyclic car-
bonate is soluble in CH2Cl2 whereas PVA is not. The different CH2Cl2
washing cycles leads to different cyclic carbonate contents in PVA. FTIR
spectra also confirm that the absorption at 1800 cm−1 for the 15-cycle
washed PVA is slightly weaker than that of the 3-cycle (Fig. S5). The
catalytic reaction result shows that the three-cycle washed PVA showed
higher PC yield (68%) than the fifteen-cycle washed one (65%). This
confirms residual cyclic carbonate has a slightly positive effect on the
catalytic activity and the reduction of the overall reaction yield is ori-
ginated from the increase of PVA particle size.
It was established that residual cyclic carbonate in PVA can improve
the solubility of KI in the system. In this case, the 5th reused PVA
should lead to higher catalytic activity. However, the reduced specific
surface area could also lead to reduction in reaction yield [42,43]. The
overall reaction yield of the reused PVA/KI system should be
To explore the catalysis scope, we also carried out cycloaddition of
CO2 with other epoxides using the PVA/KI binary catalyst in the opti-
mized reaction condition. The chemical structure of the epoxides used
and the reaction results are list in Table 2. It is obvious that the catalytic
Table 2
Catalysis coupling of CO2 with other epoxides.a
Entry
1
Substrate
Product
Time (h)
4
Yield (%)b
93.0
TOFc
23.4
2
3
6
9
99.0
96.0
24.8
24.0
4
8
96.1
24.0
5
6
4
97.0
99.0
24.3
24.8
17
7
15
Trace
–
a
Reaction conditions: epoxides(143 mmol), KI(237.2 mg), PVA1788 (593 mg), CO2 (initial pressure 1.5 MPa).
Isolated yield.
TOF: mole of synthesized PC per mole of KI per hour.
b
c
28

H. Chang et al.
MolecularCatalysis449(2018)25–30
mainly suitable for terminal epoxides. In the optimized reaction con-
dition, over 90% reaction yield can be achieved. The binary catalyst is
reusable and can be recycled at least five times without significant loss
the catalytic activity. The PVA hydrolysis degree affect the catalytic
activity as well. PVA and KI may form a non-toxic, low cost, recyclable
and highly-efficiency catalyst for fixing CO2 through cycloaddition with
epoxides.
Acknowledgments
This work was supported from National Natural Science Foundation
of China (NSFC) program (21204018, U1204518), National Science and
Technology support program (2013BAB11B02), and Natural Science
Foundation of Henan Province (15A150033).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.mcat.2018.02.007.
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