Anion exchange: A novel way of preparing hierarchical porous structure in poly(ionic liquid)s

Article abstract of DOI:10.1039/c6cc10158e

Hierarchical porous poly(ionic liquid)s (PILs) with high specific surface area were firstly synthesized via anion exchange. The exchange of bulky salicylate and its dimers/clusters in PILs by other smaller anions increased the specific surface area and fabricated a hierarchical porous structure. The high specific surface area and hierarchical porous structure prompted a high degree of exposure of the active sites and made the heterogeneous PIL catalysts contact with substrates sufficiently, enhancing their catalytic activity.

Full text of DOI:10.1039/c6cc10158e

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
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Anion Exchange: A Novel Way for Preparing Hierarchal Porous
Structure of Poly(ionic liquid)s. †
Received 00th January 20xx,
Accepted 00th January 20xx
Li Qin, Binshen Wang, Yongya Zhang, Li Chen and Guohua Gao*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Hierarchical porous poly(ionic liquid)s (PILs) with high specific monomers, which generates hierarchical meso-macroporous
8
surface area were synthesized via anion exchange firstly. The structure. Electrostatic complexation approach exploits
bulky salicylate and its dimers/clusters of PILs exchanged by other electrostatic complexation between cationic PILs and
smaller anions increased specific surface area and fabricated carboxylate anions in ammonia-containing solvent, triggers the
hierarchical porous structure. The high specific surface area and in situ interpolyelectrolyte complexation and forms a three
11,14
hierarchical porous structure prompted a high degree of exposure dimension net structrue.
Solvothermal/ionothermal
of the active sites and made the heterogeneous PIL catalysts method relies on the free radical self-polymerization of
contact with substrates sufficiently, enhancing their catalytic monomeric ILs by using nonaqueous organic solvents or
9
,15
activity.
another conventional ILs as the solvents, respectively. Post-
modification method is via quaternization of the nitrogen
atom per monomeric unit of non-ionic porous co-polymers
such as divinylbenzene and vinylimidazole to introduce the
Poly(ionic liquid)s (PILs), which possess an ionic liquid (IL)
moiety in the repeating unit of the polymer chains, refer to a
subclass of polyelectrolytes. Combining the intrinsic polymeric
1
0,12,16
ionic liquid functionality in the polymeric matrix.
Despite
nature with specific properties stemming from ionic liquids
1
ILs), PILs have been widely applied as promising materials in
the significant advances of the synthesis of porous PILs in the
past few years, the methods of fabricating porous PILs are still
limited.
(
2
3
the field of catalysts, polymer electrolytes, carbon materials,
4
5
thermoresponsive materials, separation and adsorption
6
materials. In order to increase the degree of exposure of the
Anions occupy the interstitial voids in the process of
polymerization of IL monomers. Thus theoretically, the bulky
anions of PILs exchanged by smaller ones can release the
occupied space and form porous structure. To the best of our
knowledge, fabricating porous PILs via anion exchange has not
been reported yet. Herein we describe a novel methodology
for synthesis of porous PILs by anion exchange (Fig. 1). The
salicylate was chosen as primary anion as it can form
dimers/clusters through hydrogen bond between hydroxyl and
active sites in PILs and accelerate the interfacial mass and
energy exchange, porous PILs have received increasing
7
-12
attention in catalysis.
Many synthetic methods of porous PILs, which are aimed at
increasing the specific surface area and fabricating pore
structure of PILs, have been reported. These methods can be
roughly classified into the following strategies: hard template,
1
7
soft
template,
electrostatic
complexation,
carboxyl to generate different size of anions. Thus, by
solvothermal/ionothermal method and post-modification.
Hard-templating method employs acid/base to etch the
additive inorganic components (usually silica), which fill the
interstitial voids of porous PILs, and forms an inverse opal and
7
,13
well-ordered macro-mesoporous structure.
Soft templating
method utilizes the solvent extraction to remove the organic
tri-block polymer template (P123) after polymerization of IL
*
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of
Chemistry and Molecular Engineering, East China Normal University, North
Zhongshan Road 3663, Shanghai 200062, China, Fax: (+)86-21-62233323,
e-mail: ghgao@chem.ecnu.edu.cn
†Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI:10.1039/x0xx00000x
Fig.1 Schematic illustration of synthesis of porous poly(ionic liquid)s by
anion exchange.
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Scheme 1 Synthetic routes of poly(ionic liquid)s.
exchanging salicylate or its dimers/clusters to other small . The successful anion exchange also was supported by Fourier
anions, the obtained PILs may exhibit high specific surface area transform infrared spectroscopy (Fig. S3 and S4, ESI†). Aꢀer
-
-
the Sal exchanged by OAc , the C=O double bond stretching
and hierarchical porous structure.
-1
The typical procedure for preparation of porous PILs is as absorption peak at 1590 cm (salicylate) red shifted to 1578
-1
-
following:
disalicylate (BVImSal, 0.04 mol, 20.74 g) and divinylbenzene bromide), the C=O double bond absorption of salicylate
DVB, 0.08 mol, 10.41 g) were copolymerized using 2,2’- stretching peak disappeared. Thermogravimetric (TG) analysis
azobisisobutyronitrile (AIBN, 5 wt%, 1.64 g) as an initiator in results showed that all PILs exhibited the weight loss at
1,4-Butanediyl-3,3’-bis-1-vinylimidazolium cm (acetate). While the Sal exchanged by halides (chloride,
(
o
o
methanol at reflux for 24 h. Poly(bisvinylimidazolium-base temperatures between 250–380 C, and 410–500 C, which are
disalicylate) (PBVImSal) was obtained in a yellow powder. The mainly attributed to the decomposition of the imidazoliums
anion exchange of PBVImSal was carried out by stirring in NaCl, and DVB respectively (Fig. S5a and b, ESI†). The DSC analysis
NaBr, NaOAc saturated aqueous solutions respectively, at showed that all PILs did not have the glass transitions in the
o
room temperature several times until the salicylate was temperature from -50 to 200 C (Fig. S6a, ESI†). The chemical
exchanged completely (Scheme 1). The obtained porous PILs composition of PILs was determined by CHN analysis (Table S1,
with different anions were denoted as AE-PIL-X, where AE ESI†) and the contents of imidazoliums of PILs according to
-
-
-
represented anion exchange and X represented Cl , Br , OAc , nitrogen contents were 1.6–2.1 mmol/g of PILs.
respectively. In comparison, the nonporous PILs with different The argon adsorption isotherms were measured at 77K to
-
-
-
anions (Cl , Br and OAc ) were directly synthesized by the characterize the specific surface area, micropore area and pore
copolymerization of corresponding 1,4-butanediyl-3,3’-bis-1- size distribution. The specific surface area of PBVImSal was
2
and 15.1 m /g (Table 1, entry 1). After anion exchange by NaCl,
vinylimidazolium
monomers
(Detail
synthesis
characterizations of ILs were supplied in ESI, Fig. S1 and S2, NaBr and NaOAc, all the specific surface area and micropore
ESI†) with DVB under identical conditions (denoted as DS-PIL- area of AE-PIL-X (X= Cl, Br and OAc) were increased
-
X, where DS represented direct synthesis and X represented Cl significantly (Table 1, entries 2-4). The order of specific surface
-
-
Br , OAc , respectively).
2
2
area was AE-PIL-Cl (227.5 m /g) > AE-PIL-Br (180.6 m /g) > AE-
,
2
The as synthesized PILs were characterized by NMR spectra. PIL-OAc (129.2 m /g), which was opposite to the order of the
-
C MAS NMR spectra of porous PILs and the C NMR anions size (Cl < Br < OAc , Table S2, ESI†). Thereinto, the
1
3
13
-
-
2
spectrum of BVImSal were presented in Fig. 2. Fully micropore areas are 50.7, 28.7 and 23.8 m /g, respectively.
polymerization of carbon-carbon double bond of N-vinyl was These results indicated that after exchanged by other anions,
-
-
-
confirmed by the disappearance of peak at 108 ppm (Fig. 2a the smaller anion (Cl , Br and OAc ) of porous PILs, the higher
and 2b). When salicylate anions were exchanged by acetates, specific surface area was obtained. The argon sorption
carbonyl carbon downfield shifted from 173 to 177 ppm and isotherms of AE-PIL-X exhibited a clear type-IV isotherm with
phenolic hydroxyl carbon peak at 163 ppm disappeared (Fig. hysteresis loop of type H3 at the P/P0 range from 0.3 to 0.99
-
-
2
c). When salicylate anions were exchanged by halides (Cl , Br (Fig. S7, ESI†). The pore size distribuꢁon curves (Fig. S8, ESI†)
, the peaks at 173 ppm and 163 ppm disappeared (Fig. 2d and displayed that the AE-PIL-X were mainly composed of 0.6-1.0
)
-
e). These results suggested that the Sal in PILs had been nm micropores and 2-10 nm mesopores. The formations of
2
-
-
-
completely exchanged by other anions such as Cl , Br and OAc micropores and mesopores were attributed to the exchange of
salicylate and its dimers/clusters by the smaller anions,
respectively. In comparison, those nonporous PILs (DS-PIL-X)
2
showed low specific surface areas around 17.9-25.6 m /g. The
pore size distributions of the DS-PIL-X indicated no
microporous structure and few 10-20 nm pores which might
form by particle packing (Fig. S8, ESI†). The morphologies of
AE-PIL-X and DS-PIL-X were visualized by SEM images (Fig. 3a-f).
AE-PIL-X and DS-PIL-X were composed of around 50-200 nm
round-shaped granules which were agglomerated by smaller
particles. The result indicated that the particle size of porous
PILs prepared by anion exchange was same to that of
nonporous PILs synthesized directly. TEM images of the typical
1
3
13
Fig. 2 C NMR spectrum of a) BVImSal; C MAS NMR spectra
sample of AE-PIL-Cl provided a distinct mesoporous structure
of b) PBVImSal; c) AE-PIL-OAc; d) AE-PIL-Cl and e) AE-PIL-Br.
(Fig. 3g and h). All the above features showed a significant
2
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Table 1 The content of imidazolium and BET of PILs.
reaction of styrene oxide and CO
which may be attributed to
2
a
b
c
d
Im
S
BET
S
micropor
2
V
the larger width of the epoxide (Table S3, ESI†). The
comparison of the yields for all products clearly revealed that
bulky substrate was more difficult to access the active sites of
nonporous PILs. Given similar active sites in both AE-PIL-X and
DS-PIL-X, the increase of catalytic activity can be ascribed to
the increasing specific surface area and hierarchical porous
structure generated from anion exchange. High specific
surface area prompted a high degree of exposure of the active
sites and made active sites contact with substrates sufficiently.
Meanwhile, the fast mass transport in hierarchical pores may
also have positive effects on enhancing the catalytic activity.
In order to further demonstrate the positive correlation
between catalytic activities of PILs and their specific surfaces
Entry
PILs
2
3
(
mmol/g)
(m /g)
(m /g)
(cm /g)
1
2
3
4
5
6
7
PBVImSal
AE-PIL-Cl
1.9
1.9
1.6
15.1
227.5
180.6
129.2
25.6
4.4
50.7
28.7
23.8
─
0.03
0.20
0.20
0.14
0.03
0.03
AE-PIL-Br
AE-PIL-OAc
DS-PIL-Cl
1.6
1.6
2.1
1.7
DS-PIL- Br
DS-PIL-OAc
18.2
─
17.9
1.9
0.02
b
a
The mole amount of imidazolium was calculated by nitrogen content. BET
c
d
surface area was calculated by Ar isotherm. t-plot micropore area. Total
pore volume.
effect of the anion exchange on increasing specific surface
area and creating hierarchical porous structure.
2
0
area, the N-heterocyclic carbomethoxylation reactions
catalyzed by AE-PIL-Cl and DS-PIL-Cl in the presence of
dimethyl carbonate (DMC) were investigated (Fig. 5). When
using indole as substrate, AE-PIL-Cl gave higher yields (89%)
than that of DS-PIL-Cl (68%). Under identical conditions,
pyrrole and carbazole as substrates also gave the similar
results. The differences of catalytic activities between high
specific area porous PILs and nonporous PILs were greater
Further study was focused on the relationship between
specific surface area and catalytic activity of PILs. The
cycloaddition reaction of CO
2
and epoxides using ILs or PILs as
catalysts, which was extensively studied by other groups and
1,18
1
our precious work
was chosen as a model reaction
(Detailed studies on reaction conditions and proposed reaction
course were supplied in Fig S9-11). When 1,2-epoxyhexane
was used as substrate, AE-PIL-Cl displayed higher catalytic
activity than that of DS-PIL-Cl. And AE-PIL-OAc and AE-PIL-Br
also showed more superior catalytic activity than that of DS-
PIL-OAc and DS-PIL-Br (Fig. S12, ESI†). Theoretically, the
catalytic activity of bromide-based ILs is more efficient than
(
8%< 21% <32%) with the increase of the sizes of substrates
pyrrole < indole < carbazole) (Table S3, ESI†). All results
(
revealed that increasing specific surface area of PILs and
fabricating hierarchical porous structure by anion exchange
can improve the accessibility of reactants to active sites and
get more superior catalytic activity.
that of chloride-base ILs in the reactions of epoxides and CO
because bromide promotes ring opening of epoxides owing to
2
In summary, we have successfully developed a novel
synthetic method to hierarchical porous poly(ionic liquid)s via
exchanging the salicylate and its dimers/clusters of PILs to
other smaller anions. The obtained PILs have structurally
stable nanoparticle morphology and hierarchical porous
structure with the high specific surface area. Besides, the
hierarchical porous PILs show superior catalytic activity in the
cycloaddition reactions of CO2 with epoxides and N-
heterocyclic carbomethoxylation reactions. And a positive
correlation between catalytic activities of PILs and their
specific surface area was obtained. High specific surface area
prompted a high degree of exposure of the active sites and the
fast mass transport in hierarchical pores improved the
accessibility of reactants to active sites, enhancing catalytic
1
9
its good nucleophilicity and leaving ability. It should be noted
that AE-PIL-Cl gave slightly higher yields than those of AE-PIL-
2
Br due to the higher specific surface area (227.5 m /g > 180.6
2
m /g) (Fig. S13, ESI†). When propylene oxide, epichlorohydrin,
1,2-epoxyoctane, 1,2-epoxydodecane and styrene oxide were
used as reaction substrates, the superior catalytic activity of
AE-PIL-Cl was also observed comparing with DS-PIL-Cl (Fig. 4).
The differences of activity of AE-PIL-Cl and DS-PIL-Cl were in
order of propylene oxide (6%) < epichlorohydrin (8%) < 1,2-
epoxyhexane (19%)
<
1,2-epoxyoctane (21%), 1,2-
epoxydodecane (21%), which is consistent with the chain
length of the epoxides. The relatively greater difference of
yields (25%) between AE-PIL-Cl and DS-PIL-Cl was found in the
Fig. 4 The activity of AE-PIL-Cl and DS-PIL-Cl in cycloaddition reactions of CO2
Fig. 3 SEM images of a) AE-PIL-Cl; b) AE-PIL-Br; c) AE-PIL-OAc; d) DS-PIL-Cl;
e) DS-PIL-Br; f) DS-PIL-OAc; TEM images of AE-PIL-Cl g) 50 nm; h) 20 nm.
2
and epoxides . Reaction condition: Substrate (5 mmol) CO (1 MPa), catalyst
o
1 mol% based on imidazolium), 140 C, 6 h.
(
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