Microporous polyisocyanurate and its application in heterogeneous catalysis

Article abstract of DOI:10.1002/chem.200801570

The synthesis and catalytic application of microporous polyisocyanurate (PICU) composed of rigid carbon and nitrogen networks was reported. Porous PICU was synthesized by dissolving 1 mmol of monomer in 5 ml of N,N'- dimethylformamide (DMF) in a pressure

Full text of DOI:10.1002/chem.200801570

COMMUNICATION
DOI: 10.1002/chem.200801570
Microporous Polyisocyanurate and Its Application in Heterogeneous
Catalysis
[
a]
Yugen Zhang,* Siti Nurhanna Riduan, and Jackie Y. Ying*
Microporous and mesoporous materials, such as zeolites,
activated carbon, silica and metal organic frameworks
have not been reported. In this study, novel microporous
PICUs were derived by cyclotrimerization of diisocyanate
(
MOFs) are widely used in catalysis, gas adsorption, storage
using N-hetero ACHTUNGTERNNUNcG yclic carbene (NHC) as catalyst. The unique
microporous PICU demonstrated excellent potential in reac-
tions such as selective oxidation.
[1]
and separations. The recent development of polymer-
based microporous materials may provide new opportunities
in hydrogen storage and heterogeneous catalysis, as organic
NHCs have been widely used as organocatalysts in many
[2]
[11]
materials have certain advantages over other materials.
important transformations.
Recently, it was found that
Microporous polymeric materials possess unique surface
properties that can be tailored to facilitate chemoselective
NHCs can efficiently catalyze cyclotrimerization of isocya-
nates to form a planar six-membered heterocyclic ring struc-
ture. This reaction was adapted in this study to prepare
[2,3]
[12]
adsorption, separation and catalysis.
Although hard and
soft templates have been widely used in the synthesis of
porous materials, a bottom-up approach facilitates the tai-
loring of porous materials with tailored porous structure and
porous polymer networks by replacing simple isocyanates
with diisocyanates. Scheme 1 illustrates that a porous C,N
organic framework could be woven by NHC organocata-
lysts. When rigid aryl diisocyanates were used, rigid polymer
frameworks were achieved. Diisocyanate B was employed in
this study, and various NHCs were examined in the synthesis
of PICU networks. It was found that the NHCs of 1,3-bis-
mesitylimidazol-2-ylidene (IMes) and 1,3-bis-(2,6-diisopro-
pylphenyl)imidazol-2-ylidene (IPr) did not work for this syn-
thesis. NHCs of 1,3-bis-mesityl-4,5-dihydroimidazol-2-yli-
dene (SIMes) and 1,3-bis-(2,6-diisopropylphenyl)-4,5-dihy-
droimidazol-2-ylidene (SIPr) showed low activities. In con-
trast, NHCs with more flexible substituents, 1,3-bis-tert-
butyl-4,5-dihydroimidazol-2-ylidene (SItBu) displayed very
high activities for this synthesis.
Typically, porous PICU was synthesized by dissolving
1 mmol of monomer in 5 mL of N,N’-dimethylformamide
(DMF) in a pressure flask, and 0.02 mmol of SItBu NHC
was added. The reaction flask was closed and heated to
808C for 24 h. The reaction can be conducted at tempera-
tures ranging from 258C to 1508C; a longer period of time
would be needed for the completion of the reaction at lower
temperatures. The polymer product was collected by filtra-
tion, washed, and dried in a vacuum oven. In all syntheses,
quantitative yields were obtained. Owing to the low solubili-
ty of the starting materials and product, the reaction was es-
[4]
[
2,3,5]
surface chemistry.
ported the synthesis of conjugated microporous poly(arylene
ethynylene) (PAE) by using palladium-catalyzed Sonoga-
Cooper and co-workers recently re-
ACHTUNGTRENNUNG
[6]
shira–Hagihara crossing-coupling reaction. The PAE net-
works bridge the gap between covalent organic frameworks
[
2f,g,k,l]
(
COFs)
and polymers with intrinsic microporosity
or hypercrosslinked polymers (HCPs).
[
2a,3]
[2c,d]
(
PIMs)
Micro-
porous polymers have also been applied in a wide range of
[3a]
applications, such as, gas storage and adsorption, gas sepa-
[7]
[8]
ration membrane, and heterogeneous catalysis.
We are interested in the design and synthesis of novel co-
valent organic frameworks with different functional groups
and controlled pore size so that the properties of porous
polymers can be designed for the desired applications.
Herein we report the synthesis and catalytic application of
microporous polyisocyanurate (PICU) composed of rigid
carbon and nitrogen networks. Although macroporous poly-
ACHTUNGTRENNUNGi socyanurates with foam-like structure have been stud-
[
9,10]
ied,
microporous and mesoporous polyisocyanurates
[
a] Dr. Y. Zhang, S. N. Riduan, Prof. J. Y. Ying
Institute of Bioengineering and Nanotechnology
3
1 Biopolis Way, The Nanos, Singapore 138669 (Singapore)
Fax : (+65)6478-9020
E-mail: jyying@ibn.a-star.edu.sg
ygzhang@ibn.a-star.edu.sg
sentially conducted under
a heterogeneous condition.
Unlike conventional heterogeneous catalysts, the NHC cata-
lysts in this case might migrate onto the polymer network
and initiate the next catalytic cycle. Photoacoustic Fourier
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801570.
Chem. Eur. J. 2009, 15, 1077 – 1081
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1077

crosslinked organic polymers
would be difficult, if not impos-
[2f,6,13]
sible.
We did not attain
highly crystalline C,N-frame-
work products, but generated
microcrystalline polymer net-
works. The significantly disor-
dered layered structure of poly-
isocyanurates was illustrated by
transmission electron microsco-
py (TEM) (see the Supporting
Information). The two-dimen-
sional rigid organic sheets
might only extend to limited
size, and stacked in layered
structures with short-range or-
dering. The interlayer distance
was estimated by TEM to be
~
3.57 ꢁ. This value was slightly
larger than graphite (3.35 ꢁ),
and Yaghiꢂs COF-1 (3.33 ꢁ)
and COF-5 (3.46 ꢁ); it might
be the result of twisting be-
tween phenyl rings and C N
3
3
[14]
six-membered rings. This new
material represents nice
Scheme 1. Synthesis of PICU.
a
bridge between PIMs and
COFs.
transform infrared (PA-FTIR) spectroscopy provided a
direct method for monitoring the progress of isocyanate
The stability and porosity of PICU were confirmed by N2
adsorption analysis. The as-synthesized polymers were de-
gassed at 1008C for 16 h, and their nitrogen-adsorption iso-
therms were collected at 77 K. PICU showed an isotherm
typical of a microporous material (Figure 2). The Brunauer–
Emmett–Teller (BET) surface area of PICUs are in the
conversion to the polymer product. A weak peak at
À1
~
2250 cm was observed during the synthesis, owing to the
vibration of the isocyanate group (see the Supporting Infor-
mation, Figure S1). This peak disappeared from the spectra
of the polymer product obtained at high temperature, while
À1
2
À1
3
À1
2
À1
a new peak emerged at 1700 cm , owing to the presence of
range of 320 m g (pore volume 0.26 cm g ) to 569 m g
3
À1
the -NC(O)N- group. The microstructure and morphology
of polymer materials were elucidated by using scanning
electron microscopy (SEM) (Figure 1). The particulate-
based structure was formed at standard synthesis conditions.
Thermal gravimetric analysis (TGA) indicated that the net-
work structure of the polymers derived remained stable up
to 3008C (see the Supporting Information).
(pore volume 0.35 cm g ) depending on their synthesis con-
ditions, which were higher than many layered materials (e.g.
graphite and clay), and were in the range of many zeolites,
microporous carbons and other microporous poly-
[2,3,15,16]
mers.
However, it was less porous as compared to
2
À1
3
À1 [2f]
Yaghiꢂs COF-5 (1590 m g and 0.998 cm g ); this could
be due to its short-range order and slipped organic sheets
that resulted in some inaccessible pores. The PICU was do-
minated by micropores of 2–20 ꢁ (from Horvath–Kawazoe
analysis), with an average pore diameter of 5 ꢁ.
It is widely held that owing to the requirement of micro-
scopic reversibility for crystallization, synthesis of crystalline
These novel materials possessed microporous network
structure and isocyanurate functional units. They were ex-
pected to provide acceptor sites for both metal coordination
and hydrogen-bonding interactions. PICU allowed for the
phenol absorption from an aqueous solution of 2 mmol of
phenol/L (see the Supporting Information). It demonstrated
a superb absorption capacity of 6 mmol of phenol/g, which
was higher than that of hexaazatrinaphthylene-based PIM-
À1
À1 [3a]
AHCTUNGTRENNUNG
this new material as a catalyst support and an absorbent to
concentrate reactants in aqueous solution. If successfully
Figure 1. SEM images of PICU synthesized in DMF at a) 808C and
b) 258C.
1078
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Chem. Eur. J. 2009, 15, 1077 – 1081

Microporous Materials in Heterogeneous Catalysis
COMMUNICATION
[
a]
Table 1. Selective oxidation of benzyl alcohol to benzaldehyde.
[
b]
[c]
Entry
Catalyst
H
2
O
2
[equiv]
TON
Selectivity [%]
1
2
3
4
5
6
7
8
9
1
Fe/PICU
Fe/PICU
Fe/PICU
Fe/PICU
Fe/PICU
Fe (2%)/PICU
1.5
1.2
1.0
1.2
1.2
1.2
1.2
1.0
1.5
1.0
105
101
100
101
100
39
29
32
12
30
98
99
99
99
98
95
51
97
66
35
[
[
[
d]
e]
f]
FeCl
2
[
[
g]
g]
[17]
Fe
Fe
Fe
2
2
2
O
O
O
3
3
3
(50 nm)
(50 nm)
(3-5 nm)
[17]
[
g]
[17]
0
[
1
a] Typical reaction conditions: 20 mg of Fe/PICU (0.005 mmol of Fe),
mmol of benzyl alcohol, and 1–1.5 equivalents of H (30 wt% in
water) in 1 mL of H O. [b] Number of moles of benzaldehyde per mole
2 2
O
2
of catalyst. [c] Selectivity based on alcohol conversion, and determined
by gas chromatography-mass spectrometry (GC-MS) with external stan-
dard. [d] Seventh run with recycled catalyst from Entry 2. [e] Eighth run
with recycled catalyst from Entry 2. [f] 2 mol% of Fe catalyst was used.
[
2 2
g] Reaction conditions: 1 mol% of catalyst, neat, H O (30 wt% in
[
17]
water) was added continuously over 12 h.
Figure 2. Nitrogen adsorption–desorption isotherm and pore size distribu-
tion of PICU.
demonstrated, such dual functionality would allow selected
reactions to be run in water instead of environmentally un-
friendly organic solvents. The oxidation of benzyl alcohol
with H O over iron oxide was investigated as a probe reac-
2
2
[
15,16]
tion.
Iron-based catalysts are inexpensive, environmen-
tally benign, relatively non-toxic, and of interest for both
[17–20]
bench-scale and industrial processes.
PICU supported
iron catalyst (Fe/PICU) was tested in oxidation reactions in
aqueous solution under mild reaction conditions.
Fe/PICU was prepared by suspending PICU in hot DMF
À1
solution of FeCl . FeCl /PICU (Fe loading=0.25 mmolg )
2
2
was activated by H O in hot water before it was used as ox-
2
2
idation catalyst. In the selective oxidation of benzyl alcohol
to benzaldehyde, Fe/PICU showed excellent selectivity and
[17]
activity as compared to other Fe catalysts (see Table 1).
Fe/PICU provided a conversion of 51% (TONꢀ100) with
benzaldehyde selectivity of 99% in water at a catalyst load-
ing of 0.5 mol%. The activity and selectivity of Fe/PICU
Figure 3. TEM images of Fe/PICU a,b) before reaction, c) after the first
run, and d) after the fifth run.
were higher than the homogeneous FeCl catalyst (TON=
2
2
9 with 51% selectivity) (Table 1, Entry 7), and free Fe O
after the fifth run (Figure 3d). This illustrated the ultrahigh
dispersion of iron catalyst on PICU initially, and the slow
aggregation to form nanoclusters only after multiple runs. It
demonstrated that the microporous PICU framework with
functional groups helped to stabilize iron catalyst, prevent-
ing the formation of large iron oxide particles that would
result in reduced activity.
2
3
nanoparticulate catalysts (3–5 nm or 50 nm) in neat benzyl
alcohol system (Table 1, Entries 8–10).
PICU also displayed excellent recyclability. It could be used
for at least 8 cycles without any reduction in activity and se-
lectivity (Table 1, Entries 4–5). Control experiment with
PICU (no iron salt) gave less than 5% conversion.
[17]
Remarkably, Fe/
Transmission electron microscopy (TEM) showed that no
particles were observed in the Fe/PICU catalyst before reac-
tion (Figure 3a and b) and after the first run (Figure 3c).
Highly dispersed nanoparticles of ~1 nm were noted only
The superior activity and selectivity of PICU could be at-
tributed to its microporosity and excellent absorption ca-
pacity. In heterogeneous catalysis, ultrafine clusters dis-
persed on a suitable support would provide a higher surface
Chem. Eur. J. 2009, 15, 1077 – 1081
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1079

Y. Zhang, J. Y. Ying and S. N. Riduan
area than free nanoparticles of larger sizes. In addition, the
concentration of reactants was an important factor in deter-
General procedure for oxidation reaction: Fe/PICU (20 mg, 0.005 mmol
of Fe) was suspended in a reaction vial with water (1 mL). The substrate
(
2 2
1 mmol) and H O (1–2 mmol, 30 wt% in water) were then added to the
[21]
mining the catalytic activity. Benzyl alcohol could be con-
centrated in the micropores of PICU around the active Fe
sites to further promote the catalytic conversion. High selec-
tivity was achieved in the presence of the H O dissolved
vial. The vial was capped and stirred at 758C on a hot plate for 12 h. The
reaction mixture was next cooled to room temperature, and an external
standard (chlorobenzene) was added. The reaction mixture was extracted
by ethyl acetate, and quantitatively analyzed by GC with calibration
curve.
2
2
and diluted in water. We further demonstrated that Fe/
PICU offered high activity and selectivity in the oxidation
of secondary benzyl alcohols to ketones and of aryl olefins
to benzaldehyde (see SI for details).
Recycling experiment for oxidation reaction: After extraction with ethyl
acetate, the reaction mixture was further washed with ether. After re-
moving the remaining organic solvent under vacuum, the reaction mix-
ture was used for the next run.
In conclusion, a new method has been developed to syn-
thesize functional microporous covalent network with NHC
as the organic catalyst. The new PICU material possesses
urea-type rigid functional framework, high surface area and
a permanent microporous structure. It forms a layered struc-
ture with short-range order. PICU shows excellent proper-
ties in supporting metal complexes and metal particles, and
has a high absorption capacity for phenol in water. Fe/PICU
catalyst offers excellent activity, selectivity and recyclability
in the oxidation of benzyl alcohol to benzaldehyde with
H O in water. This bifunctional system successfully concen-
Acknowledgements
This work was supported by the Institute of Bioengineering and Nano-
technology (Biomedical Research Council, Agency for Science, Technol-
ogy and Research, Singapore). The authors thank Dr. Jun Yang and Dr.
Yu Han for their assistance with TEM/SEM studies, and Dr. H. Kaper
for the assistance with BET studies.
2
2
trates the organic substrates in the PICU micropores, and
stabilizes the dispersion of active iron species against aggre-
gation. This study demonstrates the potential of a new class
of microporous polymers in the catalysis of organic reactions
in water.
Keywords: alcohol oxidation · heterogeneous catalysis ·
microporous polymers · organocatalysis · polyisocyanurate
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ACHTUNGTRENNUNGm er. After the necessary dilutions, the concentrations of the remain-
2
2
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.
1080
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Chem. Eur. J. 2009, 15, 1077 – 1081

Microporous Materials in Heterogeneous Catalysis
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Received: August 1, 2008
Revised: November 15, 2008
Published online: December 18, 2008
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