Thiazolothiazole-linked porous organic polymers

Article abstract of DOI:10.1039/c4cc07255c

Thiazolothiazole-linked porous organic polymers have been synthesized from a facile catalyst-free condensation reaction between aldehydes and dithiooxamide under solvothermal conditions. The resultant porous frameworks exhibit a highly selective uptake of CO₂ over N₂ under ambient conditions. This journal is

Full text of DOI:10.1039/c4cc07255c

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Thiazolothiazole-linked porous organic polymers†
Xiang Zhu,a Chengcheng Tian,a Tian Jin,a Jitong Wang,a Shannon M. Mahurin,b
Cite this: Chem. Commun., 2014,
50, 15055
Wenwen Mei,a Yan Xiong,a Jun Hu,a Xinliang Feng,*c Honglai Liu*a and Sheng Dai*b
Received 16th September 2014,
Accepted 3rd October 2014
DOI: 10.1039/c4cc07255c
www.rsc.org/chemcomm
Thiazolothiazole-linked porous organic polymers have been that involve expensive transition-metal-based catalysts and extra
synthesized from
facile catalyst-free condensation reaction purification efforts may limit the scale-up preparation.8 Facile
a
between aldehydes and dithiooxamide under solvothermal condi- and cost-effective preparation processes are the key to fully
tions. The resultant porous frameworks exhibit a highly selective realize the potential of POPs for practical applications.
uptake of CO2 over N2 under ambient conditions.
Recently, a new biheterocyclic system based on thiazolothiazole
(TzTz) has been increasingly studied owing to the steadily growing
Porous organic polymers (POPs) have emerged as a new class of interest in its potential applications in electronic and optoelectronic
advanced porous materials because of their potential applica- devices.9 Despite the importance of the TzTz group that contains
tions in heterogeneous catalysis, gas separation and storage, N-heterocycle units that may facilitate gas binding and thereby enable
energy, and electronic devices.1 The intriguing issue of such potential application in gas separation within porous platforms,
materials is the salient advantages of their functionalities. One so far it has never been introduced into porous polymer networks.
effective approach is to integrate task-specific groups like basic It is believed that the incorporation of TzTz into porous polymeric
N-heterocycle units and acidic motifs into the purely organic architectures can not only expand the range of applications of this
framework using covalent bonds.2 As such, a rapidly increasing biheterocyclic system, but also extend the library of POPs.
number of designed structures, synthetic methods and applica-
Herein, we contribute to the rising array of POPs by presenting a
tions for this kind of porous solids have been intensively studied, new synthesis method to generate TzTz-linked porous organic
yielding a wide variety of POPs including crystalline polymers polymers (TzTz-POPs). The key to this novel preparation strategy
such as covalent organic frameworks3 (COFs), covalent triazine lies in a catalyst-free solvothermal process. The resultant conjugated
frameworks (CTFs),4 amorphous polymers such as polymers of porous polymers exhibit good porosities with a surface area of
intrinsic microporosity (PIMs)5 and porous aromatic frameworks up to 488 m2 gÀ1. We further demonstrate that the TzTz moieties
(PAFs).6 Remarkable examples of this sort are seen in the porous integrated in the networks give rise to highly selective uptake of
polymer networks (PPNs). Grafting an ultrahigh-surface- CO2 over N2 under ambient conditions.
area porous polymer network (PPN-6) with task-specific groups
As shown in Scheme 1, 1,3,5-triformylbenzene (M1) with
like sulfonic acid, lithium sulfonate, polyamines or sulfonate three aldehyde groups, was polymerized with dithiooxamide
ammonium leads to excellent CO2 adsorption capacities as through a one-pot catalyst-free procedure to demonstrate the
well as high CO2/N2 selectivity.7 However, the synthesis of such feasibility of the strategy. CO2-philic TzTz units can be facilely
attractive materials as well as many other conventional POPs formed in situ. This synthesis was inspired by the recent
discovery of the TzTz-based Cu2+ colorimetric and fluorescent
sensor.10 Multiple attempts to generate porous polymers under
conventional reflux conditions failed, and the materials were
a State Key Laboratory of Chemical Engineering and Department of Chemistry,
East China University of Science and Technology, Shanghai, 200237, China.
obtained with negligible porosities. Therefore, we utilized
E-mail: hlliu@ecust.edu.cn; Fax: +86 021-64252921; Tel: +86 21 6425 2921
b Chemical Science Division, Oak Ridge National Laboratory, 1 Bethel Valley Road,
Oak Ridge, TN 37831, USA. E-mail: dais@ornl.gov; Fax: +1 865 576 5235;
Tel: +1 865 576 7307
c Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz,
Germany. E-mail: feng@mpip-mainz.mpg.de; Fax: +49 6131 379 350;
Tel: +49 6131 379 150
a solvothermal method, which has been widely considered as
the most promising methodology to construct COFs,1a to pre-
pare TzTz-POPs with good porosities. In addition, tetrakis(4-
formylphenyl)methane (M2), which is expected to produce a
three-dimensional (locally diamond-like) network, was further
selected as the tetrahedral building block to show the features
of this new class of TzTz-POPs.11 Both TzTz-POPs were isolated
† Electronic supplementary information (ESI) available: Experimental section,
Fig. S1–S5, and Table S1. See DOI: 10.1039/c4cc07255c
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Fig. 2 Nitrogen adsorption isotherms measured at 77 K.
Scheme 1 The synthesis of thiazolothiazole-linked POPs.
analysis and higher than that of TzTz-POP-2 (7.7%). Thermal
gravimetric analysis (TGA) shows that both polymers possess
good thermal stability (Fig. S2, ESI†). Moreover, as shown by
the powder X-ray diffraction (XRD) analysis, TzTz-POPs were
obtained as amorphous solids though solvothermal conditions
were utilized for the polymerizations (Fig. S3, ESI†). It is worth
noting that preparation of crystalline POPs requires enormous
efforts in the screening of diverse reaction conditions.14
in good yields and were insoluble in common organic solvents.
(Experimental details can be found in the ESI.†)
The structure of TzTz-POPs was unambiguously unraveled at
the molecular level by 1H–13C cross-polarization magic-angle
spinning (CP-MAS) 13C NMR spectra (Fig. 1). TzTz-POPs showed
two peaks at ca. 132 ppm and 128 ppm, corresponding to the
aromatic carbons from the building blocks. Most importantly,
the existence of an in situ generated thiazolothiazole ring
within the networks can be confirmed by the presence of sp2
carbons (B150 and 168 ppm). Additionally, for TzTz-POP-1, a
carbonyl peak attributed to unreacted aldehyde groups, was
also observed at 192.7 ppm. The aliphatic carbon from the
tetrahedral center inside TzTz-POP-2 was located at 64.9 ppm.
The Fourier transform infrared (FT-IR) spectra of TzTz-POPs
(Fig. S1, ESI†) further confirmed the high-efficiency of condensation.
Absorption peaks at ca. 1600 cmÀ1 can be ascribed to the stretching
vibration of the carbon–nitrogen double bond (–CQN–) of the
thiazolothiazole ring.12 The absence of a CQO stretching band
(at ca.1700 cmÀ1) in the spectra of TzTz-POP-2 indicates con-
sumption of the tetrahedral monomer (M2).13 The nitrogen
content of the TzTz-POP-1 is found to be 11.3% by elemental
The porous nature of TzTz-POPs was evaluated by nitrogen
adsorption–desorption isotherms measured at 77 K (Fig. 2).
The fully reversible isotherms show rapid uptake at low relative
pressures indicating that both polymers are mainly micro-
porous. The increase in the nitrogen adsorption at a high relative
pressure above 0.9 may arise in part from interparticulate
porosity associated with the meso- and macrostructures of the
samples and interparticulate voids. The desired high micro-
porosity is successfully created using the tetrahedral building
block (M2). The BET (Brunauer–Emmett–Teller) surface area of
TzTz-POP-2 calculated over a relative pressure (P/P0) range from
0.01 to 0.1 is 488 m2 gÀ1 and higher than that of TzTz-POP-1
(299 m2 gÀ1, Table 1). Though the achieved surface areas are lower
than those of the reported benzimidazole-linked POPs,15 they are
comparable to some classical zeolites and POPs.12 The transmis-
sion electron microscopy (TEM) images of TzTz-POPs also revealed
their porous nature (Fig. S4, ESI†). In addition, the pore size
distribution (PSD) analysis of TzTz-POPs, performed using nonlocal
density functional theory (NLDFT), suggests primary narrow micro-
pores of the polymeric networks (Fig. S5A and B, ESI†). To gain a
better understanding of micropores inside TzTz-POPs, we further
used CO2 gas as the adsorbate to calculate their PSDs.16 According
to NLDFT pore size distribution curves (Fig. S5C and D, ESI†), both
TzTz-POPs show narrow micropore peaks at around 0.3–0.7 nm.
Clearly, these narrow ultra-micropores (o7 Å) are essential and
advantageous for potential gas separation.17 The total pore volumes
of TzTz-POP-1 and TzTz-POP-2 estimated from the amount of
nitrogen gas adsorbed at P/P0 = 0.99, are 0.38 and 0.59 cm3 gÀ1
,
respectively. These results from PSD studies further confirm
that the solvothermal method may provide good porosities for
TzTz-linked porous networks.
Fig. 1 Solid state 13CNMR of thiazolothiazole-linked POPs.
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Table 1 Characteristics of thiazolothiazole-linked POPs
Surface area
CO2 uptakeb/
CO2/N2 selectivity
(initial slope, 273 K)
Polymers
(SBET)/m2 gÀ1
Vtotala/cm3 gÀ1
mmol gÀ1 (273/298 K)
Qst/kJ molÀ1
TzTz-POP-1
TzTz-POP-2
299
488
0.38
0.59
1.9/1.3
2.5/1.5
54
35
32
30
a
b
Total volume calculated at P/P0 = 0.99. Measured at a pressure of 1 bar.
In light of the narrow ultra-microporosities of TzTz-POPs the isosteric heats of adsorption for TzTz-POPs were calculated
and abundant N-heterocycle units within the networks, we next by fitting the CO2 adsorption isotherms measured at 273 and
investigated their CO2 uptake capacities. The CO2 adsorption 298 K and applying a variant of the Clausius–Clapeyron equation
was measured up to 1 bar at both 273 K and 298 K (Fig. 3A and B). (see ESI†).22 At low adsorption values, both TzTz-POPs show high
Interestingly, TzTz-POP-1 and TzTz-POP-2 show uptake capacities adsorption heats (Table 1 and Fig. 3C), exceeding those reported
of 1.9 mmol gÀ1 (8.4 wt%) and 2.5 mmol gÀ1 (11.0 wt%), for many other POPs (Table S1, ESI†) such as the amine integrated
respectively, at 273 K. Though the uptake capacities are lower than CMP (CMP-1-NH2, 29.5 kJ molÀ1) and the benzimidazole-linked
those of some reported porous adsorbents (e.g., carbonaceous POP (BILP-4, 28.7 kJ molÀ1).19 Accordingly, the existence of TzTz
solids and porphyrin based POPs),18 they are comparable to those groups within the networks plays an important role in their
of many other porous materials. And for TzTz-POP-2, it is worth exceptionally high CO2 affinity. However, the heats still remain
mentioning that the CO2 adsorption capacity is not only higher below the energy of chemisorptive processes (440 kJ molÀ1),23
than those of previously reported conjugated microporous poly- indicating strong interactions of the polarizable CO2 molecules
mers (CMPs) which were prepared by cross-coupling reactions through dipole–quadrupole interactions with the networks, and
involving expensive transition-metals,19 but it is also comparable also the inherent ultra-microporosities of TzTz-POPs, which are
to that of the ‘‘knitting’’ approach-derived hypercrosslinked pyrrolic more favourable for CO2 desorption.
POP (Py-1, 2.7 mmol gÀ1).20 It is important to note that large
Selectivity is another important parameter for POP-based
amounts of solvents are usually required for the removal of metal- CO2 adsorbents since the industrial emission stream also
based Lewis acid catalysts which are extensively used in the synthesis comprises B70% N2.20 To examine the separation abilities of
of such ‘‘knitted’’ POPs. Clearly, our catalyst-free approach is a more TzTz-POPs, N2 sorption experiments were carried out at 273 K
facile route for the preparation of task-specific POP-based CO2 and 298 K, respectively. Attributed to the incorporation of TzTz
adsorbents. Previous work showed that chemical functional groups moieties, the selectivity for CO2 over N2 was calculated to be
may have a large effect on CO2 uptake besides the contribution of 54 and 35 for TzTz-POP-1 and TzTz-POP-2 at 273 K by the ratios
the surface area and the pore volume.21 In addition, a good cycling of the initial slopes of CO2 and N2 adsorption isotherms (see
of CO2 uptake for TzTz-POP-2 was also observed (Fig. S6, ESI†), Fig. 3D and Fig. S7, ESI†), respectively. Though this value is lower
indicating good desorption during each regeneration cycle.7b than that reported for azo-linked N2-phobic POPs,23 TzTz-POPs show
To provide a better understanding of the CO2 storage properties, better CO2/N2 selectivity than those reported for many other POPs
like polycarbazole (CPOP-1, 25).24 For POPs based CO2 capture
materials, upon increasing the temperature the selectivity usually
decreases rapidly due to smaller uptake of gases. Interestingly, at
298 K, the CO2/N2 selectivities of TzTz-POP-1 and TzTz-POP-2 remain
at a good level and reach 34 and 23, respectively. Accordingly, the
TzTz-rich units within purely organic networks may account for
such high CO2/N2 selectivities. The reason can be ascribed to
the enhanced dipole–quadrupole interaction between the large
quadrupole moment of CO2 molecules (À1.4 Â 10À39 C m2) and
the polar sites associated with N-doped sites. Therefore, the
high selectivities of TzTz-POPs toward CO2 over N2 may make
them promising materials for potential CO2 separation.
In conclusion, we have demonstrated a one-pot catalyst-free
synthesis of new thiazolothiazole-linked porous organic polymers
from a simple condensation reaction between aldehydes and dithio-
oxamide under solvothermal conditions. The resultant materials
display good porosities with a surface area of up to 488 m2 gÀ1
.
Attributed to the narrow ultra-microporosities and abundant
N-heterocycle units within the networks, TzTz-POPs exhibit a
highly selective uptake of CO2 over N2 under ambient conditions.
XZ, JH and HLL thank the National Basic Research Program of
China (2013CB733501), the National Natural Science Foundation
Fig. 3 (A) and (B) CO2 and N2 adsorption isotherms of TzTz-POPs
measured at 273 K and 298 K, respectively. (C) The isosteric heat of
adsorption for TzTz-POPs. (D) The selectivity of TzTz-POP-1 for CO2 over
N2 obtained from the initial slope method.
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