Rational skeletal rigidity of conjugated microporous polythiophenes for gas uptake

Haining Liu a, Qing Li b, Qiqi Li b, Wang Jin b, Xiaoming Li b, Abdul Hameed *a and Shanlin Qiao *b
aCAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for Excellence in NanoscienceNational Center for Nanoscience and Technology, Beijing 100190, P. R. China
bInstitute of Chemical Industry and Pharmaceutical Engineering, Hebei University of Science and Technology, Shijiazhuang 050000, China

Received 28th July 2017 , Accepted 31st August 2017

First published on 1st September 2017


Three microporous polythiophenes, P-TTT, P-THIDT and P-DTBDT, with increasing rigidity of their cores were designed and prepared by FeCl3 catalyzed polymerization. The step by step strengthening of the rigidity of these polymers, stacking morphologies (planar, three-dimensional mesh pattern, and spherical), and Brunauer–Emmett–Teller (BET) surface areas have been extensively improved (thienyl core is 58 m2 g−1, phenyl core is 869 m2 g−1 and BDT fused ring is 1140 m2 g−1). At the same time, the carbon dioxide uptake capacity has been enhanced from 2.1 wt% to 12.1 wt% at 1.1 bar and 273 K temperature, accompanied by an increase of the heat adsorption from 17 to 37 kJ mol−1. Molecular-level models were developed that showed the same trend, which was similar to the experimental results. The rational skeletal rigidity can counteract the nanostructural packing and avoid the nanopores collapsing in organic microporous polymers.


Introduction

Porous organic polymers (POPs) have shown considerable potential in gas storage and separation due to their intrinsic properties, such as their large specific surface areas, high chemical stabilities, and low skeleton density.1–5 A wide range of POPs have been reported, including hyper cross-linked polymers (HCPs),6–8 polymers of intrinsic microporosity (PIMs),9–11 conjugated microporous polymers (CMPs)12–14 and covalent organic networks (COFs).15–17 Among these polymers, CMPs are defined as a type of special microporous polymer, which has a unique feature in the skeleton, where the stiff pore skeleton and conjugated electronic system are together in one bulk,18–20 and exhibits potential applications in gas uptake, catalysis, and the optoelectronic field.

In the past few years, great efforts have been taken in the synthesis of novel CMPs with specific chemical and physical properties for gas capture and sequestration. Y. Xu’s group reported phenylene-based CMPs of the A6 + Mx (x = 2, 3, 4, 6) type, where the BET surface area can be tunable from 571 to 1115 m2 g−1 and the polymer exhibits the highest CO2 adsorption capacity of 1218 mg g−1 at 318 K temperature and 60 bar pressure.21 S. Inagaki reported 9,9′-spirobifluorene-based CMPs prepared by FeCl3-mediated oxidative polymerization, which show CO2 uptake from 9.2–23.7 wt% at 273 K and CH4 from 1.1–3.2 wt%.22 R. Yang’s group reported the first (G-1st) and second (G-2nd) generation dendrimer-like polycarbazole CMPs P-G1(2)-T, and P-G1(2)-Fo show CO2 adsorption capacity up to 18.05 wt% and CH4 to 2.60 wt% at 273 K and 1 bar.23 The design of novel CMPs and understanding of the structure–performance relationships seems to be a difficult topic. The reaction conditions and monomer steric configuration are critical factors affecting the organic porous networks.24 The design of the microporous polymers follows the listed aspects. (1) Preparing porous polymers often requires expensive catalysts and extreme conditions, which give low yield on a large scale.25 The reported FeCl3 oxidative coupling polymerization is cost-effective and a single monomer is used,26,27 which avoids the use of other functional groups (such as halo groups, boric acid, and alkyne) that are required for covalent bonding, then the properties inherited from the monomers will be fully retained and give a vital clue to understand the relationship between the structures and performances clearly. (2) The invalid stacking of the rigid conjugated backbone is beneficial for the formation of porous polymers with permanent porosity and high physicochemical stability.28–31 More stiff, small monomers crosslinking into the amorphous/crystal network backbone can avert the micropores collapsing and becoming blocked. (3) As a star functional group in the field of organic photovoltaics, the fused and rigid thiophene ring has high conjugated electron density and facile oxidative polymerization is inherent, which makes it a potential candidate for use in conjugated porous polymers. Currently, thiophene-based porous (polyarylenethiophene) polymers have the highest Brunauer–Emmett–Teller (BET) specific surface area, which is up to 1520 m2 g−1.32 J. Schmidt reported a polythiophene microporous polymer with a BET area up to 1060 m2 g−1, which can be envisaged for application in organic electronic devices.33 A. Palma-Cando and U. Scherf reported electrochemically prepared polythiophene microporous thin films with high BET surface areas up to 2135 m2 g−1, which have potential applications in the high sensitivity sensor field.34–37

Based on the above, we use thiophene as an elementary unit (Fig. 1, the chemical environments of identical thiophenes are marked with the same number), trying step by step to increase the monomer rigidity and rationally promote the skeleton rigidity of the target polythiophene polymers. Research on the relationship between the monomer rigidity and porous structure of the final polymers will be attractive work.


image file: c7py01268c-f1.tif
Fig. 1 (a–c) The thiophene based monomers TTT, THIDT and DTBDT (two kinds of reacted thiophene marked with 2 and 3, respectively). (d–f) The monomer structures are illustrated with the torsion angle measurements. (g–i) The HOMOs.

In this work, the conjugated polythiophenes P-TTT, P-THIDT and P-DTBDT with permanent porosity were elaborately synthesized utilizing FeCl3-catalyzed oxidative coupling by a single monomer (Scheme 1). The three thiophene-based building units show obvious differences in the molecular rigidity. The gas adsorption test shows that the polythiophenes with more rigid skeletons will have larger BET surfaces and higher gas uptake capacity under the same synthetic conditions. These results provide us an important clue that the rational rigidness degree is necessary to construct a high-performance microporous polymer.


image file: c7py01268c-s1.tif
Scheme 1 The synthetic route towards the polythiophene networks of P-TTT, P-THIDT and P-DTBDT.

Experimental

Materials

All the reagents and solvents were obtained from J & K, Aldrich and Acros Chemical Co. and were used as received unless otherwise specified. Anhydrous tetrahydrofuran (THF) and chloroform were distilled over sodium/benzophenone and calcium hydride under N2 prior to use. The monomers TTT, THIDT and DTBDT were prepared according to the reported process.38 The microporous polythiophenes P-TTT, P-THIDT and P-DTBDT were synthesized by FeCl3 oxidative coupling from their corresponding monomers, respectively. The syntheses of the monomers are found in the ESI, and a representative polymer preparation for P-DTBDT is given as an example.
P-DTBDT. A solution of monomer DTBDT (200 mg, 0.51 mmol) dissolved in 30 mL of anhydrous chloroform was dropwise transferred to a suspension of ferric chloride (662 mg, 4.08 mmol) in 20 mL of anhydrous chloroform. The solution mixture was stirred for 24 h at room temperature under nitrogen protection, and then 100 mL of methanol was added to the above reaction mixture. The resulting precipitate was collected by filtration and washed with methanol and concentrated hydrochloric acid solution. After being extracted in a Soxhlet extractor with methanol for 24 h, and then with tetrahydrofuran for another 24 h of extraction, the desired polymer was collected and dried in a vacuum oven at 80 °C overnight (95% in yield). Anal. calcd for C18H12O2S4: C, 55.64; H, 3.11; found: C 54.20, H 4.94.

Results and discussion

The monomer's rigidity is an important property for building an organic microporous polymer. The strong interaction forces in the bulk lead to more efficient nanostructure packing and the elimination of free volume, especially when the adsorbate is removed from the pore surface and the enhanced rigid skeleton can counteract the newly produced strong surface tension effectively. As shown in Fig. 1, TTT was composed of thiophene core 1 and branched with four thiophenes 2 and 3. The two flanked thiophenes 3 have a large bonding angle and are far away from each other, which could retain the freedom of polymerization. As for THIDT, benzene core 1 also has two kinds of substituted thiophene 2 and 3. The four thiophenes 3 bonding with flexible methenyl and the diamond structure encourage inefficient packing in all directions, thus increasing the amount of microporosity formation. Compared with the aforementioned two monomers, DTBDT features a large three-ring core 1, benzo[1,2-b:4,5-b′]-dithiophene (BDT), and the high degree of heterocyclic-fusing in the core makes the core possess the largest rigidity. The steric inhibition of the two hydroxyl groups twist the plane of core 1 and make DTBDT transform to a 3D configuration. The monomer's rigidity was characterized by the effective angle torsion at the core, as illustrated in Fig. 1d–f. It can be concluded that the order of rigidity of the monomer structures is TTT < THIDT < DTBDT. The enhanced rigidity can contribute to resist the strong attractive interactions and maintain a higher degree of porosity. All of the three building units have two kinds of coupling thiophene 2 and 3, and the final polymerization sites will easily occur at the α-conjugated sites (Fig. 1g–i, the HOMO is mainly located at the α-conjugated site, and the high HOMO will be easily oxidized).39 In TTT, thiophene 2 has a small torsion angle and larger degree of conjugated site than thiophene 3, so under the oxidative conditions the α-site thiophene 2 will participate in the coupling reactions.40 In the THIDT unit, there are two kinds of thiophene in distinct electronic environments, thiophene 2 is conjugated and thiophene 3 is alkyl linked (activity of 2 > activity of 3). In DTBDT, all of the thiophenes 2 and 3 are independent in the electronic state (non-conjugated). The priority ranking of the reaction activity and molecular rigidity may influence the reaction and morphology of the obtained polythiophenes.

The three polythiophenes are obtained by straightforward FeCl3 oxidative polymerization at room temperature. The crude polymers were washed with methanol and concentrated hydrochloric acid, then Soxhlet extraction with methanol and tetrahydrofuran removed the residual FeCl3. The high yield could be attributed to the advantages of the FeCl3 catalyzed oxidative reactions and easily oxidised α-site thiophene. The resulting polymers are characterized by ATR-IR (Fig. S1a–c) and 13C CP/MAS NMR spectroscopy (Fig. 2a–c). The signal at 2950–3100 cm−1 is assigned to the C–H characteristic stretching vibration in the thiophene ring and 1660–1700 cm−1 belongs to the stretching vibration of C[double bond, length as m-dash]C of the thiophene units. In addition, the peaks at 727–750 cm−1 are related to the in-plane C–S–C of the thiophene ring. The 13C CP-MAS NMR spectroscopy chemical shift signals of 120–150 ppm are typically related to the thiophene and benzene of the building blocks. The signals at 58–62 ppm are attributed to the aliphatic carbon, which originates from two parts of chemical shift. One is from the original hydroxyl substituted carbon, and the other contribution is from the side reaction formed carbon (tertiary alcohols can be polymerized under Friedel–Crafts conditions with FeCl3, which provides alternate crosslinking pathways for the final polymer).


image file: c7py01268c-f2.tif
Fig. 2 (a–c) The solid state magic angle spinning 13C CP/MAS NMR spectra.

To further investigate the morphologies, powder X ray diffraction (P-XRD, Fig. S3) was used and the results show the three polycarbazole networks without any crystalline phase that will be assigned to the amorphous aggregation. The scanning electron microscopy images (SEM, Fig. 3a–f) demonstrate that the three polythiophenes present obviously different aggregated morphologies. P-TTT, with the smallest steric hindrance, displays layer by layer stacking morphology (lateral size more than 1 μm, Fig. 3a). The lamellar morphology was contributed by the lower rigidity and consequent quasi-coplanar configuration. Fig. 3b reveals that P-THIDT adopts three dimensional mesh patterns, which are ∼100 nanometers in size. Fig. 3c shows that the P-DTBT network comprises aggregates of small solid sub-micron spheres. The main reason is that the priority ranking of the reaction activity, molecular rigidity and dimension of steric configuration make the oligomer/polymer present a different morphology. The normalized absorption spectra show obvious differences in Fig. S2, which is according to the conjugated systems, analysed above. Meanwhile, the polythiophenes exhibit excellent heat resistance with high decomposition temperatures (Td10%) up to ca. 512 °C for P-TTT, 385 °C for P-THIDT and 407 °C for P-DTBDT, which show excellent thermal stability (Fig. S4).


image file: c7py01268c-f3.tif
Fig. 3 The field-emission scanning electron microscopy images of the polythiophene networks. (a & b) P-TTT, (c & d) P-THIDT and (e & f) P-DTBDT.

The porosities of P-TTT, P-THIDT and P-DTBDT were measured by adsorption analysis using N2 as the probe molecule at 77 K. Fig. 4a shows the N2 adsorption–desorption isotherms for the three networks. P-TTT shows the lowest N2 uptake at the whole pressure range compared to those of P-THIDT and P-DTBDT. The sorption profiles of P-THIDT and P-DTBDT exhibit type I nitrogen gas sorption isotherms with H3 hysteresis loops according to IUPAC classification,41 revealing that the materials consist of micro- and mesopores, respectively. Both polythiophenes exhibited a steep rise in the uptake at a relative pressure (P/P0) of less than 0.01, indicative of the presence of substantial micropore structures. At the middle pressure, both the adsorption isotherms show a slow increase, which means the existence of a small mesoporous structure. The large increase in the adsorption isotherms of the three networks at high pressure may be caused by the collapse of the loose nano-particulate stockpiling that yielded the macroporosity structure. A BET model is adopted to calculate the apparent surface area. The results suggested that P-TTT possesses a tiny surface area of 58 m2 g−1. The BET surface area of P-TTT is smaller than that of polycarbazole, which has similar propeller-like configuration,27 the main reason may be due to the higher conformational flexibility of thiophene than that of large rigidity carbazole. In order to decrease the conformational flexibility, a more rigid phenyl ring was introduced into the building block to form a new monomer, THIDT. The BET surface area of P-THIDT can increase to 869 m2 g−1. To further enhance the rigidity of the system, a fused ring benzo[1,2-b:4,5-b′]dithiophene (BDT) was selected as the core. The BET area of P-DTBDT was improved to 1140 m2 g−1. A comparison of the pore size distribution (PSD) was calculated using quenched solid density functional theory (QSDFT), as shown in Fig. 4b.42 QSDFT is a multicomponent DFT which has been demonstrated to improve the accuracy of the pore distribution significantly compared with NL-DFT.43 P-TTT shows a large pore width distribution (Table 1), while the analysis of P-THIDT and P-DTBDT shows that the samples mainly consisted of micropores (pore size less than 2 nm). The results of the N2 adsorption isotherm indicate that increasing the rigidity of the polythiophene skeleton can avoid pore collapse at a dry state and change the pore distribution.


image file: c7py01268c-f4.tif
Fig. 4 (a) The nitrogen adsorption–desorption isotherms of the polythiophene networks measured at 77 K. (b) The pore size distribution was calculated by application of quenched solid density functional theory (QSDFT).
Table 1 The porosity structural parameters of the polythiophene networks
Networks S BET[thin space (1/6-em)]a, m2 g−1 S langmuir, m2 g−1 V mic[thin space (1/6-em)]b, cm3 g−1 V total[thin space (1/6-em)]b, cm3 g−1 PSD (Å)
a The BET surface area calculated based on the nitrogen adsorption-branch isotherm using the relative pressure range P/P0 = 0.07–0.17. b The micropore volume and total pore volume were determined at P/P0 = 0.1 and P/P0 = 0.99, respectively.
P-TTT 58 94 1.4–5.3
P-THIDT 869 1083 0.34 0.69 0.56, 1.0
P-DTBDT 1140 1425 0.44 0.91 0.54, 0.96


To compare the polythiophene networks for gas adsorption, the CO2 adsorption isotherms were collected at 273 K and 298 K and 1.1 bar, as shown in Fig. 5a and Fig. S5 at 273 K. The CO2 uptake of P-TTT is 2.1 wt%, which is the lowest uptake capacity due to the lowest surface area and negligible pore volume. In the case of P-THIDT, at 1.1 bar, the CO2 uptake is 9.0 wt% at 273 K and 5.6 wt% at 298 K. P-DTBDT exhibited CO2 uptake of 12.1 wt% at 273 K/1.1 bar and 7.2 wt% at 298 K/1.1 bar, respectively, which shows the highest CO2 adsorption ability among the three conjugated polymers. The improvement of the CO2 adsorption capacity coincides well with the level of the BET areas. In addition, the incorporation of a hydroxyl group may also contribute to the gas adsorption ability, which has been previously reported before.44 The CO2 adsorption capacity of P-DTBDT is also comparable to that of other types of porous materials under similar conditions, such as TFM-1 (SBET = 738 m2 g−1, 7.6 wt%),45 COF-102 (SBET = 3620 m2 g−1, 8.6 wt%),46 and ZIF-69 (SBET = 1220 m2 g−1, 13.5 wt%)47 at 273 K and 1.0 bar. From the adsorption isotherms of CO2 at 273 K and 298 K, the isosteric enthalpies Qst of P-TTT, P-THIDT and P-DTBDT at zero loading are calculated, based on the Clausius–Clapeyron equation, to be 17 kJ mol−1, 29 kJ mol−1 and 37 kJ mol−1, respectively. The heats of adsorption with increasing quantities of adsorbed CO2 of the P-THIDT network can retain the same level as that of P-DTBDT despite the relatively modest surface area. It is predicted that P-TTT showed the lowest CO2 adsorption capacity. At the whole loading range, P-DTBDT shows higher isosteric heat than P-TTT and P-THIDT, which could be attributed to the narrower pore size and consequently higher surface adsorption potential.


image file: c7py01268c-f5.tif
Fig. 5 (a) The CO2 adsorption isotherms of the polythiophene networks at 273 K and 298 K. (b) The CO2 variation of the gas isosteric enthalpies with the adsorbed amount.

The uptake capacity of the three polythiophenes for methane is also studied. From the CH4 physisorption isotherms (273 K and 298 K, Fig. 6a and Fig. S6), we can find an increasing trend in the methane loading capacity (0.37 wt%, 1.35 wt% and 1.02 wt% for P-TTT, P-THIDT and P-DTBDT at 273 K, respectively). The isosteric heats of the polythiophenes were found to be 47 and 25 kJ mol−1 at zero loading (P-TTT has very low CH4 uptake, so the isosteric heats are not further considered) (Fig. 6b). It is interesting that the results of CH4 adsorption have fallen out of step with the CO2 trend, the main reason is the flexible alkyl group in the polymer skeleton that can intensely increase the CH4 uptake, and this phenomenon has been verified in a flexible conjugated system in our lab. As the loading increased, the isosteric heat drops by about 50% during the whole loading range, the main reason is that as the adsorption layers slacken the adsorption potential in turn weakens.


image file: c7py01268c-f6.tif
Fig. 6 (a) The CH4 adsorption isotherms of the polythiophene networks at 273 K and 298 K. (b) The CH4 variation of the gas isosteric enthalpies with the adsorbed amount.

Besides gas storage, the gas selectivity is very important for potential applications in gas separation. Considering the outstanding gas adsorption performance and high porosities of P-THIDT and P-DTBDT, the gas selective adsorption behaviour was calculated and is shown in Fig. 7. In this work, the CO2/CH4 selectivity was estimated using the ratios of the Henry law constants calculated from the initial slopes of the single component gas adsorption isotherms at the low pressure range, since this method is widely used for microporous organic polymers.48 The calculated CO2/CH4 selectivities of P-THIDT and P-DTBDT at 273 K are 8.7 and 31.0 mmol g-1, while they are 19.0 and 17.6 mmol g-1 for the two polymers at 298 K, respectively.


image file: c7py01268c-f7.tif
Fig. 7 The initial gas uptake slopes of the polythiophenes for CO2/CH4 at (a) 273 K and (b) 298 K.

In order to deeply understand the nature of the polymer pore structure and monomer configuration, molecular-level models were constructed to describe the three conjugated thiophene networks (Fig. 8 and 9). The three amorphous cell models were constructed from a combination of the two different link styles of the second generation networks (dendrimers) in ideal forms, such as G2-TTT-α and G2-TTT-β. In the amorphous cell, the seeded build unit that the dendrimers used was based on the number of linking sites (G2-TTT-α[thin space (1/6-em)]:[thin space (1/6-em)]G2-TTT-β: is 50%[thin space (1/6-em)]:[thin space (1/6-em)]50%; G2-THIDT-α[thin space (1/6-em)]:[thin space (1/6-em)]G2-THIDT-β is 33.3%[thin space (1/6-em)]:[thin space (1/6-em)]66.7%; G2-DTBDT-α[thin space (1/6-em)]:[thin space (1/6-em)]G2-DTBDT-β: is 50%[thin space (1/6-em)]:[thin space (1/6-em)]50%). When the ideal covalent bond links are formed in the seeds and through the target polythiophene, the structure is locked in a configuration favouring this expanded state, such that the pores are preserved when the solvent is removed.29


image file: c7py01268c-f8.tif
Fig. 8 The molecular models of the second generation dendrimers constructed from the three thiophene based monomers in different covalently linked structures (the linking site is the α-H of thiophene 2 and thiophene 3).

image file: c7py01268c-f9.tif
Fig. 9 (a), (c), and (e) The simulated polythiophene networks P-TTT, P-THIDT and P-DTBDT. The amorphous cells of the three polythiophenes were constructed by second generation dendrimers. (b), (d), and (f) The density map of the adsorbed CO2 molecules in the simulated microporous structure (the Connolly surfaces are highlighted in blue, which represents the pore volume within the structures, and CO2 and CH4 are adsorbed into the amorphous imaging in red and green, respectively).

To construct the amorphous stacking of the three polythiophenes, each build unit seed was fully relaxed and the bulk density kept constant, and the bulk density of the microporous networks was set at 0.8 g cm−1, this value agrees well with the measured bulk densities for a series of reported microporous hyper-crosslinked polystyrenes, which fall in the range of 0.71–0.91 g cm−3.49 The compared surfaces were created using the atom, volumes and surface tool in a fine resolution, the grid is 0.25 Å, and the Connolly radius using N2 kinetic radius is 1.82 Å. Polythiophene P-TTT shows apparent layer stacking in the amorphous cell, which is in keeping with the SEM images, the main reason is the steric configuration and less rigidity of monomer TTT.

The calculated surfaces of the three polythiophenes are 1824.02, 2421.3 and 3196.49 m2 g−1. The simulation shows more invalid space in the top and bottom of the P-TTT amorphous cell, plus the pen’s plain and flexible feature, and a reduced BET surface was obtained in the lab. The simulated steric configuration in accord with the SEM images is shown in Fig. 3a and b, which indicate that P-TTT may show layer by layer stacking in the bulk polymer. The atomistic simulation results were compared with the experiments to date and found to be much higher, but follow the same trend, and the overestimated surface area may be assigned to non-interconnected pockets of occluded free volume.50 A snapshot of the CO2 and CH4 sorption density map is calculated at 273 K, the total pressure is 1 bar (the fractional pressure of CO2[thin space (1/6-em)]:[thin space (1/6-em)]CH4 is 0.5[thin space (1/6-em)]:[thin space (1/6-em)]0.5 bar) as shown in Fig. 9b, d and f. The simulated density distribution maps show that the three polythiophenes have different adsorption selectivity abilities for CO2 and CH4 at 273 K. Polythiophene P-DTBDT more easily uptakes CO2 than CH4, this trend is according to the experimental values (P-THIDT and P-DTBDT at 273 K are 8.7 and 31.0 mmol g-1).

The simulated adsorption isotherms of the three polythiophenes are shown in Fig. 10a, c, and e, at 273 K, 298 K and 1 bar, and the corresponding adsorption distribution energies are calculated based on the amorphous cell shown in Fig. 10b, d, and f. The uptake abilities of simulated CO2 and CH4 were consistent with the experimental data, P-DTBT > P-THIDT > P-TTT. The simulation energy is lower than the experimental values of the calculated isosteric adsorption heats because the simulation does not predict higher energy binding sites than −15 kJ mol−1, which is consistent with the physical adsorption process at a simulated given temperature and other reported similar organic microporous polymers. Comparing the simulation sorption energies, the three polythiophenes have similar chemical compositions, and by varying the steric configuration and rigidity of the backbone the adsorption ability was fully changed.


image file: c7py01268c-f10.tif
Fig. 10 (a), (c), and (e) The simulated CO2 and CH4 sorption isotherms at 273 K and 298 K. (b), (d), and (f) The simulated distribution of CO2 and N2 sorption energies in the polymer network at 298 K and 273 K. The y-axis represents a distribution function, which is a measure of the probability of sorbate molecules at a given sorption energy.

Conclusions

In summary, three polythiophene microporous polymers were designed and prepared by oxidative polymerization at room temperature. Gas adsorption measurement demonstrated that improving the skeleton rigidity of the polymers will be beneficial for a higher surface area (from 58 to 1140 m2 g−1) and narrower pore size distribution. Among the obtained polymers, P-DTBDT exhibits the best uptake capacity for carbon dioxide (12.1 wt% at 1.1 bar and 273 K) and methane (1.35 wt% at 1.1 bar and 273 K). The recognition of such an important structure–function relationship can certainly promote future development to obtain organic microporous networks with improved gas uptake performance.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was supported by the Natural Science Foundation of Hebei Province (B2016208082), Science and Technology Research Projects in Hebei Universities (YQ2014015), and Five Platform Open Fund Projects of Hebei University of Science and Technology (2014YY25, 82/1182123).

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Footnotes

Electronic supplementary information (ESI) available. See DOI: 10.1039/c7py01268c
These authors contributed equally to this work.

This journal is © The Royal Society of Chemistry 2017