A "click-based" porous organic polymer from tetrahedral building blocks

Article abstract of DOI:10.1039/c0jm03483e

A thermally and chemically stable "click-based" porous organic polymer with high surface area was synthesized from two tetrahedral building blocks.

Full text of DOI:10.1039/c0jm03483e

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www.rsc.org/materials | Journal of Materials Chemistry
A ‘‘click-based’’ porous organic polymer from tetrahedral building blocks†
Prativa Pandey,‡ Omar K. Farha,‡ Alexander M. Spokoyny, Chad A. Mirkin, Mercouri G. Kanatzidis,
*
Joseph T. Hupp and SonBinh T. Nguyen
Received 15th October 2010, Accepted 12th November 2010
DOI: 10.1039/c0jm03483e
A thermally and chemically stable ‘‘click-based’’ porous organic
polymer with high surface area was synthesized from two tetrahe-
dral building blocks.
Highly crosslinked, microporous organic polymers1–7 have recently
received significant attention as potential materials for a variety of
applications ranging from gas storage8–10 and gas separation,11,12 to
chemical catalysis,13 chemical sensing,14 and capture of improvised
chemical threats such as ammonia.15 Depending on the design,
synthetic strategy, and properties, these materials have been classified
into several subsets including amorphous hyper-crosslinked polymers
(HCPs),16,17 polymers of intrinsic microporosity (PIMs),12,18 conju-
gated microporous polymers (CMPs),19 porous organic polymers
(POPs),11,20 porous organic frameworks (POFs),21 and (crystalline)
covalent organic frameworks (COFs).1 Regardless of the degree of
crystallinity, the pore properties of these materials can often be
modulated based on the choice of the building blocks. Notably,
organic materials with high surface areas can be made from all-planar
building blocks2,21 as well as from combinations of non-planar
‘‘struts’’.1,11 Herein, we report the synthesis of a highly stable,
microporous ‘‘click-based’’ porous organic polymer (POP) via
copper-catalyzed alkyne–azide coupling (CuAAC) between two rigid
tetrahedral building blocks (Td + Td).22
Scheme 1 Top: synthesis of ‘‘click-based’’ POP C1–D1 from monomers
C1 and D1 using copper-catalyzed alkyne–azide coupling. Bottom:
synthesis of model compound (tetrakis(4-(4-phenyl-1H-1,2,3-triazol-1-yl)-
phenyl)methane; MC).
(440 m2 gꢁ1), further optimization of the synthesis was carried out at
the higher temperature (Table S1, entries 3 and 6 and Fig. S2 in the
ESI†). The yield for POP C1–D1 (Table 1, entry 1a) was almost
quantitative, suggesting that crosslinking was nearly complete with
no soluble oligomers. However, care must be taken in isolating the
product as a fraction in fine particulates can be lost if the filtration is
carried out over filter paper (Table 1, entries 1b–5, see also Table S1
entries 1b–6 in the ESI†). Loss of this component during the filtration
process can result in a slight decrease of the specific surface area of the
overall product (1360 m2 gꢁ1 vs. 1440 m2 gꢁ1, Table 1 cf. entries 1a and
1b) but does not significantly affect the micropore and total pore
volumes.
Because the progress of CuAAC depends on the availability of CuI
intermediates, which in turn depends on the amount of sodium
ascorbate reducing agent, we optimized the porosity of the product
POP C1–D1 by adjusting the amount of sodium ascorbate between
10 and 70 mol%, while keeping the amount of copper catalyst
constant (10 mol%). Surprisingly, the surface area of POPs C1–D1
systematically decreased with increasing amount of sodium ascorbate
in the reaction mixture (Table 1 and Fig. S1 in the ESI†). Treatment
of these POPs via Soxhlet extraction (to remove any sodium ascor-
bate possibly present in the pores) did not result in improvement in
the original surface areas (Table S2 in the ESI†). Thus, we attribute
the aforementioned ascorbate-induced lowering of surface area to an
increase in catalyst availability: at higher loadings of ascorbate more
CuI is available, allowing for more crosslinking to occur within the
porous network and thus lowering surface areas. This hypothesis is
supported by a progressive decrease in micropore volume and total
pore volume as the amount of ascorbate is increased.
Alkyne–azide ‘‘click’’ chemistry has been widely employed in the
syntheses of both linear and crosslinked polymers with diverse
applications ranging from drug delivery23–25 to electronics.26,27 Our
groups have applied this strategy in the modification of porous
materials such as MOFs28,29 and nanobins.30 Given the aromatic,
rigid nature of the 1,2,3-triazole group that results from alkyne–azide
coupling, we reasoned that highly crosslinked organic polymers
synthesized using this strategy would possess permanent micropo-
rosity and be attractive materials for gas storage. In addition, the high
stability of the 1,2,3-triazole linkage could give ‘‘click-based’’ POPs
thermal- and chemical-stability advantages over other polymers
obtained via alternative bond-construction strategies.
The CuAAC reaction between tetrakis(4-ethynylphenyl)methane
(C1) and tetrakis(4-azidophenyl)methane (D1) (Scheme 1) occurs
readily in dimethylformamide (DMF) at both room temperature and
ꢀ
100 C to give several versions of POP C1–D1 as brown powders.
Since the surface area of the POP synthesized at 100 ꢀC (1260 m2 gꢁ1)
was much higher than that for the version synthesized at 25 ꢀC
Department of Chemistry and the International Institute for
Nanotechnology, Northwestern University, 2145 Sheridan Road,
Evanston, IL, 60208-3113, USA. E-mail: stn@northwestern.edu
† Electronic supplementary information (ESI) available: Detailed
instrumentation,
experimental
characterization. DOI: 10.1039/c0jm03483e
procedures,
materials,
and
‡ These authors contributed equally to this work.
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Table 1 Surface-area and pore properties of ‘‘click’’ POP C1–D1 synthesized by varying the amount of sodium ascorbate
Entrya
Sodium ascorbate (mol%)
Specific surface area (m2 gꢁ1
)
Micropore volume (cm3 gꢁ1
)
Total pore volume (cm3 gꢁ1
)
Yield (%)
1ab
1bc
2c
10
10
20
30
50
70
1440
1360
1320
1260
1140
1090
0.56
0.53
0.51
0.49
0.44
0.42
0.76
0.74
0.73
0.67
0.64
0.52
>99
67
62
64
81
77
3c
4c
5c
a
All reactions were carried out in DMF at 100 ꢀC using 10 mol% CuSO4$5H2O and at 0.04 M solution of azide functional group (1 : 1 alkyne-to-azide
functional group ratio). b Obtained using workup procedure II. c Obtained using workup procedure I.
Thermogravimetric analysis (TGA) of dried POP C1–D1 shows
that this microporous material is thermally stable, losing only 20 wt%
of its mass below 500 ꢀC (Fig. S8 in the ESI†). The thermal stability
of this uncapped material approaches those of the most stable metal-
organic framework analogues containing carborane31–33 or Zr
building blocks.34 The powder X-ray diffraction spectrum of POP
C1–D1 is featureless, implying (as expected) that it is amorphous.
Consistent with the anticipated chemical stability of the 1,2,3-tri-
azole linkage,35 POP C1–D1 is remarkably stable. Immersing C1–D1
in 6 N NaOH (aq) or 6 N HCl (aq) at 60 ꢀC for 8 h leads to essentially
no change in internal surface area (Table S3 in the ESI†). Increasing
the immersion temperature to 80 ꢀC and time to 24 h leads to only
slight decreases (10–12%) in surface area (Table S3 in the ESI†). This
acid- and base- stability rivals that of zeolitic imidazolate frame-
works36 and greatly exceeds that of imine-linked POPs having
Fig. 1 FTIR spectra of the starting materials (C1 and D1), POP C1–D1
comparable initial surface areas;21 the surface areas of the latter
(Table 1, entry 1b), and model compound (MC).
rapidly decrease following exposure to water. Its chemical stability is
also superior to those of diimide-linked POPs,11,20 which are stable in
The FTIR spectrum of POP C1–D1 (Table 1, entry 1b) compares
relatively dilute aqueous acids (0.1 N) but not in bases.
well with that of a model compound (MC, Scheme 1). A broad
N]N stretch can be observed at 1610 cmꢁ1 (Fig. 1), together with
a weak C]CH band at 3029 cmꢁ1, consistent with the formation of
the expected triazole ring. However, IR stretches from unreacted
High uptake of nitrogen at low pressure, as observed in the N2
adsorption–desorption isotherms for the various preparations of
POP C1–D1 (Fig. 3, see also Fig. S1 in the ESI†), suggests that these
materials are mainly microporous. However, the isotherms of POPs
azide could still be observed at 2114 cmꢁ1, suggesting that the azide–
prepared with smaller amounts of sodium ascorbate (10–30 mol%)
alkyne coupling is incomplete. Interestingly, a strong and broad
stretch at ꢂ3300 cmꢁ1 is also observed, suggesting the presence of
tend to exhibit more hysteresis than those prepared with larger
amounts (50–70 mol%). The slopes of the isotherm adsorption
significant amounts of physisorbed water. The solid state 13C
branches for the latter materials are also less steep than those for the
CPMAS NMR spectrum of POP C1–D1 (Table 1, entry 1b) matches
former. Particularly interesting is the presence of a pronounced
well with that of MC, further supporting the formation of cyclic
triazole linkages (Fig. 2).
Fig. 3 N2 adsorption–desorption isotherms at 77 K for two versions of
Fig. 2 13C CPMAS NMR spectra of the model compound (MC) and
POP C1–D1 (Table 1, entry 1b).
POP C1–D1 (Table 1, entries 1b and 5). A complete set of isotherms for
all samples can be found in Fig. S1 in the ESI†.
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hysteretic step in the N2 isotherm desorption branch for the POPs
prepared with low amounts (10–30 mol%, Table 1, entries 1a–3) of
sodium ascorbate (Fig. 3, see also Fig. S1 in the ESI†). In contrast,
there is no hysteretic step in the N2 isotherm desorption branch for
the POPs prepared by using higher amounts (50–70 mol%, Table 1,
entries 4 and 5) of sodium ascorbate (Fig. 3, see also Fig. S1 in the
ESI†).
For the samples of POP C1–D1 whose isotherms contain no
hysteretic desorption step (Fig. 3 and Table 1, entries 4 and 5), the
parallel, almost-horizontal nature of the two branches of the isotherm
implies the presence of only narrow slit-like pores.37,38 In contrast, the
samples whose isotherms contain a hysteretic desorption step (Fig. 3
and Table 1, entries 1a–3) likely possess more complex pore
geometry.37,38 The near-horizontal desorption behavior in the
0.5 < P/Po < 1 region and the abrupt switch into a slightly steeper
desorption behavior in the P/Po < 0.5 region (for N2 adsorption at
77 K, this switch often occurs near the P/Po ¼ 0.42 point37) indicate
that the pores are not all slit-like. The upward-sloping trend in the
adsorption branch of these isotherms, which becomes steeper at
higher pressures, has been attributed by others to the presence of
pores of different shapes and sizes (mainly microporous with some
higher micropores and lower mesopores).37
Fig. 5 (a) H2 adsorption–desorption isotherm for POP C1–D1 (Table 1,
entry 1a) at 77 K, up to ca. 1 bar. (b) Isosteric heat of H2 adsorption of
POP C1–D1 (Table 1, entry 1a). Measurements were carried out at 77 K
and 87 K.
1 bar (Fig. 5b). These values are typical for H2 adsorption in
micropores of materials that can offer only London dispersion-type
interactions.
In summary, we have shown that a permanently microporous POP
can be assembled readily from a complementary pair of tetrahedral
building blocks (Td + Td) using CuAAC chemistry. The resulting
material is thermally stable, as well as chemically stable, easily
surviving exposure to highly acidic and basic solutions. The excellent
stability of POP C1–D1 suggests more generally that cyclic-triazole-
linked POP materials can serve as excellent engineering materials
under a wide range of conditions. At 1 bar, the micropores of this
material have reasonably good ambient-temperature adsorption of
CO2 and cryogenic adsorption of H2. We are actively exploring
structure–property relationships for these types of materials and
efforts to turn them into high-surface-area and morphologically well-
defined nano- and microparticle structures, similar to infinite coor-
dination polymer particle analogues.42–46 Such capabilities will allow
researchers to tune the properties of this novel class of materials for
optimal performance in many practical applications, including gas
storage, catalysis, and biological probe design.
Despite the aforementioned indications of complex pore geometry,
non-local density functional theory (NLDFT) pore size distribution
analysis of POP C1–D1 prepared using 10 mol% sodium ascorbate
(Table 1, entry 1b) indicates a mainly microporous material with
a narrow pore-size distribution centered around a primary pore width
˚
˚
of 9.2 A (Fig. 4). This width is much smaller than expected (ꢂ21 A)
for an idealized diamond-like single network formed by simply
‘‘clicking’’ together monomers C1 and D1. This disparity constitutes
strong evidence for interpenetration in the hyper-crosslinked
networks constituting POP C1–D1.
Given the high surface area and narrow pore-size distribution of
POP C1–D1, we explored its potential to adsorb the gases CO2 and
H2. The NLDFT surface area obtained for POP C1–D1 (Table 1,
entry 1a and Fig. S7 in the ESI†) based upon adsorption of CO2 at
273 K is 1080 m2 gꢁ1. This POP has a reasonably good H2 adsorption
capacity at 77 K and 1 bar (1.6 wt%, Fig. 5a), comparable to a range
of MOFs,39 carbon materials,8,40 and microporous polymers10,41
featuring similar or higher surface areas. The isosteric heat of
hydrogen adsorption for this material, which is an approximate
measure of the strength of its interaction with H2, was found to be
7.9 kJ molꢁ1 at low surface coverage, decreasing to 4.0 kJ molꢁ1 at
We gratefully acknowledge the US DOE-EERE (Grant No. DE-
FG36-08GO18137/A001), DTRA, AFOSR, and the NSF-NSEC for
financial support. Instruments in the Northwestern University Inte-
grated Molecular Structure Education and Research Center
(IMSERC) were purchased with grants from NSF-NSEC, NSF-
MRSEC, the Keck Foundation, the state of Illinois, and North-
western University. We thank Dr Alexandros P. Katsoulidis for
helpful discussions.
Notes and references
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