Heterogeneous cyclization of sorbitol to isosorbide catalyzed by a novel basic porous polymer-supported ionic liquid

Article abstract of DOI:10.1016/j.mcat.2018.07.019

In this study, heterogeneous cyclization of sorbitol to isosorbide under basic condition was realized for the first time with a novel porous polymer-supported ionic liquid as catalyst. These polymer-supported ILs were synthesized through the suspension polymerization of 4-vinylbenzyl chloride and divinylbenzene, followed by a quaternization reaction. As compared to those of non-porous, the porous polymers had high specific surface area and large number of active sites. Consequently, they exhibited excellent catalytic activity in the cyclization of sorbitol with dimethylcarbonate (DMC) to isosorbide. As a result, a high conversion of sorbitol (99%) was achieved with 83% yield of isosorbide under optimized conditions. Importantly, the catalysts could be easily separated by decantation and reused for five times without obvious loss of catalytic activity.

Full text of DOI:10.1016/j.mcat.2018.07.019

Molecular Catalysis 457 (2018) 59–66
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Heterogeneous cyclization of sorbitol to isosorbide catalyzed by a novel
basic porous polymer-supported ionic liquid
Yao-Feng Wang , Bao-Hua Xua,b, Yi-Ran Dua,b, Suo-Jiang Zhanga,b,⁎
a
a
Beijing Key Laboratory of Ionic Liquids Clean Process, Key Laboratory of Green Process and Engineering, State Key Laboratory of Multiphase Complex Systems, Institute
of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China
b
School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Isosorbide
Supported ionic liquid
Mesoporous
Solid base
In this study, heterogeneous cyclization of sorbitol to isosorbide under basic condition was realized for the first
time with a novel porous polymer-supported ionic liquid as catalyst. These polymer-supported ILs were syn-
thesized through the suspension polymerization of 4-vinylbenzyl chloride and divinylbenzene, followed by a
quaternization reaction. As compared to those of non-porous, the porous polymers had high specific surface area
and large number of active sites. Consequently, they exhibited excellent catalytic activity in the cyclization of
sorbitol with dimethylcarbonate (DMC) to isosorbide. As a result, a high conversion of sorbitol (99%) was
achieved with 83% yield of isosorbide under optimized conditions. Importantly, the catalysts could be easily
separated by decantation and reused for five times without obvious loss of catalytic activity.
Heterogeneous catalysis
1. Introduction
isosorbide. Alternatively, the less explored base catalysis in the pre-
sence of stoichiometric amount of dimethyl carbonate functioning as
Isosorbide is a versatile platform molecule with wide applications as
carboxymethylating agent for hydroxyl group provides an attractive
choice since it can be conducted under mild conditions with high effi-
ciency [24]. Recently, such a conversion was achieved with a high yield
in the presence of catalytic amounts of 1,8-diazabicyclo[5.4.0]undec-7-
ene (DBU) [25]. To the best of our knowledge, the application of basic
heterogenous system for synthesis of isosorbide has not been reported
yet.
As one type of the potential and powerful supports, polymers have
several advantages, such as easy preparation, structural tunability and
fast recovery. The polymer-supported catalysts have been widely used
in heterogeneous transition metal catalysis [26–29] and organocatalysis
[30,31]. The polymers with uniform size and controlled pore structures
enable themselves to be used to develop fixed-bed flow reactors for
continuous flow processes [32,33]. On the other hand, supported ILs
which achieve the advantages of combining the ILs and heterogenous
catalysis, have attracted growing interest [34–36]. For instance, sup-
ported acidic ILs have been employed as catalysts in the bulk and fine
chemical industries [37,38]. Among these, the polymer supported
acidic IL catalyst exhibits high selectivity in the dehydration of sorbitol
to 1,4-anhydro-D-sorbitol [39]. Meanwhile, supported basic ILs show
good catalytic activity for base-catalyzed reactions. Examples include
Knoevenagel reaction [40], Henry reaction [32], transesterification
reaction [41,42] and cascade reaction [43]. In most cases, the active
a monomer or building block for biogenic polymer, functional mate-
rials, pharmaceutical molecules, and new solvents [1–8]. Various
homogeneous catalysts have been developed to promote transformation
of sorbitol to isosorbide, and this reaction is standing as the final step of
industrial process for isosorbide from starch [9–13]. In this regard, the
direct acid catalysis has dominated the field, which involves two step-
wise protonation of the regioselective hydroxyl group, and each fol-
lowed by cyclization with the carbon atom [14,15]. Recently, much
effort has been made towards addressing the catalyst recyclability by
grafting the reactive Brφnsted acid into the skeleton of ionic liquids
(
ILs) [16]. However, serious drawbacks of environmental pollution
arising from significant handling risk, corrosive nature of the catalyst
and stability issues of the products are presented under this protocol. In
comparison with these homogeneous catalysts, heterogeneous catalysts
are industrially preferred for ease of separation and reuse. Therefore,
solid acids with various structures are chosen to promote the transfor-
mation from sorbitol to isosorbide [17–23]. However, the catalytic
system is still lack of practical value because of the requirement for high
reaction temperature, long reaction time, abundant catalyst loading,
reduced pressure condition and limited selectivity of product as well.
Many studies have confirmed that neither homogeneous acid catalysts
nor solid acids are ideal choices for achieving the green production of
⁎
Corresponding author.
E-mail address: sjzhang@ipe.ac.cn (S.-J. Zhang).
https://doi.org/10.1016/j.mcat.2018.07.019
Received 16 March 2018; Received in revised form 19 July 2018; Accepted 19 July 2018
Available online 03 August 2018
2468-8231/ © 2018 Elsevier B.V. All rights reserved.

Y.-F. Wang et al.
Molecular Catalysis 457 (2018) 59–66
sites of the supported basic IL catalysts are basic anions, such as hy-
droxy, acetate or bicarbonate anions. The anions are introduced into
the catalyst through an ion-exchange process. However, deactivation of
the catalyst often occurs during the reaction imposed by the slow ex-
change of the catalytic anion with other less reactive anions [32].
We speculated that the nitrogen tricyclic group is more stable and
efficient than the basic anion as an activator for DMC. A novel porous
polymer supported IL with nitrogen tricyclic group was designed as
heterogenous catalyst for the conversion of sorbitol with DMC.
Compared with traditional basic anion type supported basic IL, this
catalyst will be much more stable. Furthermore, the porous structure
will ensure higher degree of exposure of active sites to the reactants.
In this article, we demonstrate a facile two-step synthetic method to
prepare polymer-supported basic IL catalyst and the first heterogenous
cyclization of sorbitol with DMC under basic condition. In the prepared
catalyst, the catalytic sites were connected to catalyst matrix through
covalent bond rather than basic anions such as hydroxyl. In addition,
the catalyst with mesopores had much higher loading amount of active
ILs moieties than nonporous one. By launching this novel basic
polymer-supported IL catalyst, isosorbide could be obtained with 83%
yield at 140 °C. The catalyst could be reused for five times with 7%
decrease in yield of isosorbide.
(BET) equation and the Barrett-Joyner-Halenda (BJH) model. Fourier
transform infrared (FT-IR) spectra were recorded on a Nicolet380 FT-
IR. The thermal decomposition of catalysts was analyzed by thermo-
gravimetry/differential thermal analysis (TG/DTA, DTG-60H,
Shimadzu, Japan). The experiment was performed at a temperature
ramp rate of 10 °C/min in a nitrogen atmosphere. Transmission electron
microscopy (TEM) was performed on a HT-7700 electron microscope
(Hitachi, Japan) with an acceleration voltage of 80 kV. The cross-sec-
tional specimen for TEM observation was prepared by Leica EM UC7
Ultramicrotome. Scanning electron microscope (SEM) experiments
were performed on a SU-8020 microscope (Hitachi, Japan). Surface
morphology of catalysts was characterized by a scanning electron mi-
croscope (SEM). X-ray photoelectron spectroscopy (XPS) was conducted
using an ESCALAB 250Xi spectrometer. The charging effect was cor-
rected by referencing the binding energy of C1s at 284.8 eV. Nuclear
magnetic resonance (NMR) spectra were recorded on a Bruker ASCEND
1
spectrometer ( H, 600 MHz). Chemical shifts were given with reference
to the solvent resonance.
2.4. Catalyst performance evaluation and product analysis
The reaction was carried out in a sealed glass tube. Sorbitol
(
200 mg, 1.1 mmol, 1 equiv.), PVDD (50 mg), DMC (0.79 mL,
2
. Experimental
.1. Materials
-Vinylbenzyl chloride (VBC), divinylbenzene (DVB, mixtures of
8.78 mmol, 8 equiv.) and methanol (0.4 mL) were added in a glass tube
with a stir bar. The mixture was heated to 140 °C and reacted for 14 h.
After that, the reaction was cooled to room temperature and the mix-
ture was filtered to recover the catalyst. The filtrate was condensed and
analyzed by high performance liquid chromatography (HPLC;
ThermoFisher UltiMate3000) equipped with UV and refractive index
2
4
isomers, 78∼80% grade) were purchased from Alfa Aesar Co. and
distilled under vacuum before use. Azobisisobutyronitrile (AIBN), poly
(RI) detectors and a Rezex RCM-Monosaccharide column (300
×
(
[
vinyl alcohol) (PVA, degree of polymerization: 2080), 1,4-diazabicyclo
2.2.2]octane (DABCO), dimethyl carbonate (DMC), sorbitol were
7.8 mm). The column was operated at 80 °C by a column heater and the
operating temperature of detectors was 40 °C. Distilled water was used
as the eluent at a flow rate of 0.6 mL/min. Sorbitol conversion and
isosorbide yield were calculated according to the following formula:
bought from Adamas Reagent, Ltd. Solvents (AR grade) were obtained
from Beijing Chemical Works.
C
sorbitol% = (moles of reacted sorbitol / moles of initial sorbitol) ×
2
.2. Preparation of catalysts
100%
Y
isosorbide% = (moles of carbon in the produced isosorbide / moles
Step 1：Synthesis of mesoporous copolymers
In a typical synthesis process of mesoporous copolymers of poly
of initial sorbitol)
×
100%
The crude product was purified by recrystallization from ethyl
acetate. The NMR spectra was shown in Figure S1 and the NMR data
correspond to the reported values [25].
(
VBC-DVB), AIBN (122 mg, 0.65 mmol, 0.1 equiv.) were charged into a
25 ml three-necked flask with a mechanical stirrer and reflux con-
1
denser. Then VBC (1.0 g, 6.5 mmol, 1 equiv.), DVB (2.5 g, 19.5 mmol, 3
equiv.) and porogen (3.5 mL) were added successively and stirred
regularly until the mixture became uniform, followed by addition of
PVA (1 wt.%, 20 mL). The polymerization was conducted at 70 °C under
2.5. Procedure for recycling the catalyst
The reactions were repeatedly conducted under identical experi-
mental conditions as mentioned above. After each run, the resulted
solution of products could be separated easily by decantation. The
catalyst was washed with methanol and then dried in the oven at 60 °C
for 1 h, then used directly without regeneration for next cycle.
300 rpm for another 12 h. The raw product poly(VBC-DVB) was col-
lected and washed for five times with water and methanol to remove
the dispersant. The PVB beads were obtained upon drying under va-
cuum at 60 °C for 12 h.
Step 2：Synthesis of poly(VBC-DVB)-DABCO (PVDD)
The target catalyst was obtained by grafting DABCO on the polymer
beads through quaternization reaction. In a 100 ml three-necked flask
equipped with a reflux condenser and magnetic stir bar, the poly(VBC-
DVB) beads (3 g) were reacted with DABCO (1.45 g, 13 mmol, 2.0
equiv.) in 1,4-dioxane (50 mL) at 100 °C for 24 h. Then the suspension
was filtered and washed with 1,4-dioxane and methanol until the
DABCO could not be detected in the final elution. Finally, the PVDD
was obtained upon drying at 60 °C for 12 h.
3. Results and discussion
3.1. Preparation of porous polymer-supported IL catalysts
The basic polymer-supported IL catalysts were prepared through
suspension polymerization between VBC and DVB, followed by nu-
cleophilic addition reaction involving the resulting polymeric beads
and DABCO as shown in Scheme 1. A variety of solvents that are in-
sensitive to polymerization conditions, such as n-hexane, hexadecane
and toluene, were employed as the porogen to synthesize poly(VBC-
DVB) beads, thereby the influences on the pore structure of poly(VBC-
DVB) were investigated (Table 1, entries 1∼3). It is well known that
toluene is a thermo-dynamically good solvent for linear polystyrene
during the polymerization. However, alkanes can dissolve the mixture
of monomers but not the copolymer that was generated in situ. In ad-
dition, poly(VBC-DVB) with a high ratio of VBC and DVB was also
2.3. Characterization
Elemental analysis was performed for carbon, hydrogen and ni-
trogen by using an Elemental vario EL cube. The nitrogen sorption
isotherms at the temperature of liquid nitrogen were measured using a
Micromeritics ASAP2460 system. The specific surface area and the pore
size distribution were calculated using the Brunauer-Emmett-Teller
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Y.-F. Wang et al.
Molecular Catalysis 457 (2018) 59–66
Scheme 1. Synthesis of poly(VBC-DVB)-DABCO (PVDD).
Table 1
Table 2
Preparation of porous polymer-supported IL catalysts.
Porosity of the polymer-supported IL catalysts.
2
a
3
b
D
p
(nm)c
Entry
Sample
Porogen
VBC/DVB/Porogen
Entry
Sample
S
BET (m /g)
V (cm /g)
1
2
3
4
5
PVDD-1
PVDD-2
PVDD-3
PVDD-4
PVDD-5
n-hexane
hexadecane
toluene
toluene
–
1/3/3.5
1/3/3.5
1/3/3.5
2/3/4
1
2
3
4
5
PVDD-1
PVDD-2
PVDD-3
PVDD-4
PVDD-5
271.1
162.6
287.2
155.9
1.2
0.44
0.53
0.29
0.17
0.002
7.1
15.6
5.1
3.8
3.4
1/3/0
a
BET surface area.
Total pore volume.
BJH mesopore diameter calculated from the adsorption branch.
b
c
compared to poly(VBC-DVB), PVDD-1∼4 showed obvious bands at
−
1
1
058 and 3426 cm , of which the former was assigned to the presence
of CeN bond. These results demonstrated that the introduction of the
DABCO functional group into the polymer skeleton upon quaternization
was successful.
The PVDD-1∼5 were further characterized by XPS technology
(
Fig. 2). Two characteristic peaks of N1s were observed (Fig. 2A). The
peak at 399.6 eV is ascribed to the tertiary amine N and the peak at
02.3 eV is attributed to the quaternary ammonium N. The assignment
4
of amino groups was in good agreement with previous studies [44]. The
Cl2p spectrum is the expected doublet due to the spin orbit coupling
(
Fig. 2B). The Cl2p1/2 peak is found at 198.7 eV, and the Cl2p3/2 is
−1
found at 197.2 eV. This doublet was assigned to the Cl of IL moieties
on PVDDs according to literature [45,46]. No chlorine signals due to
covalent C-Cl bonds identified by XPS were detectable in any PVDDs.
The XPS spectra further confirmed the successful quaternization of
DABCO with benzyl chloride moiety on poly(VBC-DVB).
Fig. 1. Comparisons of FT-IR spectra of PVDD-1∼4, poly(VBC-DVB) and
DABCO.
The specific surface area and porosity of the polymer-supported ILs
were characterized by nitrogen adsorption isotherms. The results were
listed in Table 2. There was significant difference in specific surface
areas between non-porous (PVDD-5) and porous (PVDD-1∼4) polymer-
supported ILs. The pore structure of PVDD was significantly affected by
the types of porogen used, with the SBET value decreasing in the fol-
lowing order: toluene > n-hexane > hexadecane. The highest SBET
prepared (entry 4). Moreover, the PVDD-5 was synthesized without
porogen as a comparison (entry 5).
3.2. Characterization of catalysts
FT-IR spectra of poly(VBC-DVB) and PVDD were shown in Fig. 1. As
Fig. 2. XPS spectra of N1s (A) and Cl2p (B) for PVDD-1 (a), PVDD-2 (b), PVDD-3 (c), PVDD-4 (d) and PVDD-5 (e).
61

Y.-F. Wang et al.
Molecular Catalysis 457 (2018) 59–66
Fig. 3. Nitrogen adsorption-desorption isotherms (A) and pore size distribution curves (B) of basic porous polymer-supported IL catalysts.
Fig. 4. TEM images of the sliced specimen: PVDD-1 (A), PVDD-2 (B), PVDD-3 (C), PVDD-4 (D).
Table 3
The loading of basic site in PVDD-1∼5.
[49,50]. As the IL units were the active species, large amount of loading
of the reactive site for quaternization in porous polymer was tested, by
increasing the molar ratio of VBC (PVDD-4). The adsorption/desorption
isotherms of PVDD-4 were similar to PVDD-3, which suggested the
same porogen (toluene) in the polymerization. However, higher VBC/
Samples
N element content
wt.%)
Loading amount
(mmol/g)
(
PVDD-1
PVDD-2
PVDD-3
PVDD-4
PVDD-5
3.00
2.68
3.74
5.44
0.80
1.07
0.96
1.33
1.94
0.28
3
3
DVB ratio led to a decrease of pore volume (0.17 cm /g vs. 0.29 cm /g).
2
In addition, the SBET value decreased from 287.2 to 155.9 m /g, which
was attributed to a lower concentration of DVB incorporated into the
poly(VBC-DVB). This result is consistent with the observation in the
synthesis of mesoporous poly(ionic liquid)-based copolymer reported
by Yuan and co-workers [51].
In order to further analyze the porosity inside the porous polymer-
supported ILs, the sliced specimen of PVDD was observed by TEM
2
value was 287.2 m /g denoted as PVDD-3 when toluene was used as the
3
porogen. In comparison, the largest pore volume was 0.53 cm /g found
in PVDD-2 when hexadecane was used as the porogen. Besides, ni-
trogen (N ) adsorption/desorption isotherm of PVDD-1∼3 showed the
2
type IV, III and II isotherms, respectively, with hysteresis loops in the
relative pressure between 0.45 and 0.9, indicating the formation of
(
Fig. 4). The TEM images with scale bar 500 nm and 50 nm confirmed
the pores were well distributed in PS/IL networks. Furthermore, TEM
images were in good agreement with the results of BET analysis. PVDD-
1, PVDD-3 and PVDD-4 possessed mesopores. And pore structure of
2
mesopores [47–49] (Fig. 3). On the other hand, the N adsorption/
PVDD-2 was dominated by large mesopores and macropores.
desorption isotherms of PVDD-1 and PVDD-3 in the high relative
pressure between 0.9 and 1.0 become flat and close themselves, sug-
gesting no macropore existence in the catalysts. As for PVDD-2 with
largest pore volume, the sharp uptake at high relative pressure between
To quantify the loading of active ILs moieties that were introduced
into the resulted PVDD, the PVDD-1∼5 were analyzed by elemental
analysis (Table S1). The results revealed that the nitrogen mass fraction
in porous PVDD-1∼4 were 3.00%, 2.68%, 3.74%, 5.44%, indicating
that the loading of basic site in PVDD-1∼4 were 1.07 mmol/g,
0.9 and 1.0, together with the pore size distribution, suggested that its
pore structure was dominated by large mesopores and macropores
0.96 mmol/g, 1.33 mmol/g and 1.94 mmol/g, respectively (Table 3).
62

Y.-F. Wang et al.
Molecular Catalysis 457 (2018) 59–66
Fig. 5. The whole and magnified SEM images of PVDD-1 (A), PVDD-2 (B), PVDD-3 (C), PVDD-4 (D), PVDD-5 (E).
Scheme 2. The cyclization of sorbitol with DMC catalyzed by PVDD. Reaction
conditions: sorbitol (200 mg, 1.1 mmol, 1 equiv.), PVDD-1∼5 (50 mg), DMC
(0.79 mL, 8.78 mmol, 8 equiv.) and methanol (0.4 mL).
magnifications, the surfaces of PVDD-1 and PVDD-2 were much
rougher compared to PVDD-3∼5.
The thermal stability of PVDDs was investigated by thermogravi-
metric analysis (Fig. 6). The thermogravimetric curves showed that the
degradation temperature of PVDD-1∼2 and PVDD-3∼4 were about
200 °C and 250 °C, respectively. In addition, the weight loss percentages
of PVDD-1∼4 were about 10%, 9%, 13% and 22%, respectively, at the
first stage of weight loss. It was caused mainly by the loss of DABCO
group accompanied with breakup of CeN bond. According to the re-
sults, it could be deduced that the loading of ILs moieties in PVDD-1∼4
were 0.89 mmol/g, 0.80 mmol/g, 1.16 mmol/g and 1.96 mmol/g, re-
spectively. These loading of ILs obtained from TG analysis were found
to be in good agreement with those from elemental analysis.
Fig. 6. Thermogravimetric curves of PVDDs.
However, the nitrogen mass fraction in non-porous PVDD-5 was 0.8%
and the basic site loading which only distribute over the surface is
0.28 mmol/g.
The morphology of the synthesized PVDD-1∼5 was characterized
by SEM as shown in Fig. 5. All of them exhibited spherical structures
with the diameters in the range of 250∼400 μm. At high
63

Y.-F. Wang et al.
Molecular Catalysis 457 (2018) 59–66
Fig. 7. The catalytic performance of PVDD-1∼5. Reaction conditions: sorbitol
Fig. 10. Conversion of sorbitol and yield of isosorbide in recycle tests under
optimized conditions: sorbitol (200 mg, 1.1 mmol, 1 equiv.), PVDD-4 (50 mg),
DMC (0.79 mL, 8.78 mmol, 8 equiv.) and methanol (0.4 mL), 140 °C, 14 h.
(200 mg, 1.1 mmol, 1 equiv.), PVDD-1∼5 (50 mg), DMC (0.79 mL, 8.78 mmol,
8
equiv.) and methanol (0.4 mL), 120 °C, 12 h.
3.3. Catalytic performance
The formation of isosorbide from sorbitol with PVDD-1 as the cat-
alyst was initially conducted in refluxing DMC under atmospheric
pressure in the reactor with a condenser. However, the conversion of
sorbitol was less than 5% after 24 h. It was speculated that the starting
materials did not contact efficiently with the solid catalyst since the
melting point of sorbitol (95 °C) was higher than the specific reaction
temperature (90 °C/1 atm). Consequently, the reaction was moved into
a sealed reactor and the temperature was increased to 120 °C, thereby
providing isosorbide with 26% yield in 36 h. To our delight, the yield
was increased to 43% when methanol (0.4 mL) was used as an additive.
The by-product in the absence of methanol was isolated and the effect
of methanol will be discussed later. After the initial examination, the
reaction of sorbitol and DMC catalyzed by PVDD in presence of me-
thanol was chosen as the model reaction for further optimization
(
Scheme 2).
The catalytic performance of PVDD-1∼5 were evaluated by using
Fig. 8. The influence of reaction temperature. Reaction conditions: sorbitol
200 mg, 1.1 mmol, 1 equiv.), PVDD-4 (50 mg), DMC (0.79 mL, 8.78 mmol, 8
equiv.) and methanol (0.4 mL), 12 h.
the catalytic cyclization of sorbitol as shown in Fig. 7. Sorbitol was
reacted with DMC (8 equiv.) in the presence of methanol (0.4 mL) over
the catalysts (25 wt.%) for 12 h under 120 °C. Evidently, sorbitol was
significantly converted (more than 89%) in the presence of porous
PVDD-1∼4, but most sorbitol retained with PVDD-5 under identical
conditions. Moreover, various kinds of intermediates rather than the
desired isosorbide were obtained with PVDD-1 and PVDD-2. In com-
parison, the yield of isosorbide was obviously improved in the case of
either PVDD-3 (yield: 37%) or 4 (yield: 58%). The result suggested that
the yield of isosorbide increased with the increasing of the basic site
loading of ILs moieties in PVDD-1∼5 which was determined by ele-
mental analysis. An approximated linear positive correlation between
catalytic activities of PVDD and their quantity of basic sites was shown
in Figure S2. Therefore, the PVDD-4 was selected as the optimal catalyst
for subsequent experiments.
(
The influence of reaction temperature was also examined by using
PVDD-4 as the catalyst (Fig. 8). The variation in the conversion of
sorbitol PVDD was found less than 5% within the range of temperature
scope, with a higher temperature being slightly favorable. The yield of
isosorbide was less than 20% at relatively lower temperatures, such as
100 °C and 110 °C. Instead, intermediate of unrecognized structure was
mainly obtained under these conditions. In contrast, the yield was
greatly improved at relatively higher temperatures. It was increased
from 58% to 73% with the increase of temperature from 120 to 140 °C.
The suitable temperature was set as 140 °C.
Fig. 9. The effect of reaction time on the yield of isosorbide. Reaction condi-
tions: sorbitol (200 mg, 1.1 mmol, 1 equiv.), PVDD-4 (50 mg), DMC (0.79 mL,
8.78 mmol, 8 equiv.) and methanol (0.4 mL), 140 °C.
The effect of reaction time was also examined with PVDD-4 at
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Y.-F. Wang et al.
Molecular Catalysis 457 (2018) 59–66
Scheme 3. Proposed mechanism of cyclization of sorbitol catalyzed over the PVDD catalyst.
been carried out for five times at 140 °C for 14 h (Fig. 10). The PVDD-4
showed good recyclability and stability after recycling for five times
under the optimized reaction conditions. The reaction required 5 h to
reach almost full conversion of sorbide (95%–98%) for each cycle.
Complete conversion was observed at 6 h in each cycle. In particular,
the yield of isosorbide decreased from 83% to 77% after five cycles. In
addition, there was almost no difference in FT-IR spectra before and
after reuse (Figure S3). Herein, it could be concluded that the porous
polymer-supported ILs could serve as stable and efficient catalysts for
the cyclization of sorbitol to isosorbide.
3.5. Mechanism of cyclization reaction
In the previous homogeneous cyclization, reaction of diols with
DMC in a stoichiometric excess of a strong base or catalyzed by basic
catalyst led to the formation of related 5-membered cyclic ethers
[24,25]. This transformation encompasses both methoxycarbonylation
via BAc2 mechanism, i.e., formation of the intermediate, and cyclization
via the BAl2 mechanism, i.e., alkylation reaction. The basic reagent or
catalyst was capable of enhancing the reactivity of DMC and hydroxyl
group. In the present work, the catalytic reaction path of cyclization
over basic polymer-supported IL was consistent with the reported
homogenous reactions. As shown in Scheme 3, DMC was activated by
Fig. 11. Structure of the cyclic carbonates byproduct A.
1
40 °C. As shown in Fig. 9, almost full conversion of sorbitol was
achieved after reaction for 12 h. A 83% yield of isosorbide was obtained
after reaction for additional 2 h. With prolonging the reaction time, the
yield was decreased owing to the increase of by-products, which were
observed by HPLC. Indeed, the use of catalyst PVDD-4 with methanol
0.4 mL) as an additive at 140 °C within 14 h provided isosorbide with
3% yield, which was also established as the optimal conditions.
PVDD and adduct I was formed. Sorbitol reacted with adduct I via BAc
2
(
8
mechanism resulting carbonate intermediate II. The intermediate II
could also be activated by PVDD forming the adduct III. The methoxide
group served as a base deprotonated the 4-hydroxyl group of the in-
3.4. Recycling experiments
termediate III. And then subsequent intramolecular cyclization by BAl2
mechanism leading to the formation of 1,4-sorbitan IV. The 1,4-sor-
bitan underwent another similar cyclization progress affording the
The experiments on catalyst recycling using the PVDD-4 have also
Scheme 4. Reversible reaction in the presence
of methanol.
65

Y.-F. Wang et al.
Molecular Catalysis 457 (2018) 59–66
product isosorbide.
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(
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