Letter pubs.acs.org/macroletters
Nitrogen-Rich Triptycene-Based Porous Polymer for Gas Storage and Iodine Enrichment Hui Ma,† Jing-Jing Chen,† Liangxiao Tan,‡ Jian-Hua Bu,*,§ Yanhong Zhu,*,† Bien Tan,‡ and Chun Zhang*,† †
Key Laboratory of Molecular Biophysics of the Ministry of Education, College of Life Science and Technology, National Engineering Research Center for Nanomedicine and ‡School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China § Xi’an Modern Chemistry Research Institute, Xi’an, 710065, China S Supporting Information *
ABSTRACT: A new kind of nitrogen-rich triptycene-based porous polymer (NTP) with high surface area of up to 1067 m2 g−1 was synthesized by a modified Yamamoto-type Ullmann cross-coupling reaction. Having structure features of expanded conjugated area and nitrogen atoms doping in polymer networks, the NTP displays extraordinary gas storage ability and excellent iodine adsorption capability.
ith characteristics of long radioactive half-life (1.57 × 107 years) and easy access into the human metabolic system, radiological iodine (129I) in the nuclear waste stream has attracted great attention.1 In order to facilitate the development of nuclear energy, so far, different inorganic porous materials2 and some metal−organic frameworks (MOFs)3 have been used as iodine sorbents for effective enrichment of radiological iodine. Recently, organic microporous polymers (OMPs), a new generation of porous materials, has been gradually applied in the same field because of their fascinating properties such as high surface area, low mass density, easy functionality, and high stability.4 A selection of different building blocks and utilization of different coupling reactions have resulted in indispensable types of OMPs for the application fields of gas storage,5 separations,6 catalysis,7 and sensors,8 such as covalent organic frameworks (COFs),9 hyper-cross-linked polymers (HCPs),10 polymers of intrinsic microporosity (PIMs),11 conjugated microporous polymers (CMPs), 12 and porous aromatic frameworks (PAFs).13 Recently, Zhu14 and Deng15 synthesized charged PAFs and CMP nanotubes possessing extraordinary capability for iodine enrichment, respectively. Inspired by these interesting works, we planned to design a novel polymer with expanded conjugated areas and heteroatoms in networks, which may enhance the binding affinity between the adsorbent and iodine molecules.24 Triptycene and its derivatives have been recognized as promising building blocks in construction of supramolecular host compounds16,17 or OMPs18,19 on account of their threedemensional rigid paddlewheel-like structure. Herein, we
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© XXXX American Chemical Society
synthesized a nitrogen-rich triptycene-based porous polymer (NTP) by modified Yamamoto-type Ullmann cross-coupling reaction. With the features of expanded conjugated area and nitrogen atom doping in polymer networks, the NTP displayed high surface area and excellent iodine adsorption capability. Synthesis of the monomer 1 is outlined in Scheme S1. Starting from 2,3,6,7,14,15-hexaammoniumtriptycene hexachloride 2,20 its reaction with 4,4′-dibromobenzyl in the presence of potassium acetate afforded the monomer 1 in a good yield of 80%. As shown in Scheme 1, the synthesis of NTP was carried out by the nickel(0)-catalyzed Yamamototype Ullmann cross-coupling reaction. Typically, a mixture of Scheme 1. Synthesis of NTPa
a
Reagents and conditions: 1,5-cyclooctadiene, bis(1,5-cyclooctadiene) nickel(0), 2,2′-bipyridyl, DMF, 85 °C, 96 h.
Received: July 24, 2016 Accepted: August 22, 2016
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DOI: 10.1021/acsmacrolett.6b00567 ACS Macro Lett. 2016, 5, 1039−1043
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ACS Macro Letters
g−1 (Langmuir surface area was 1439 m2 g−1) (Figure S7). The isotherm displayed a steep nitrogen gas uptake at low relative pressure (P/P0 < 0.001), which was indicative of adsorption into micropores. A sharp rise at the medium- and high-pressure region (P/P0 = 0.8−1.0) implied the presence of macropores in the NTP network. The hysteresis was associated with the irreversible uptake of gas molecules in the pores (or through pore entrances), which was probably attributed to network swelling. The pore size distribution was calculated applying NLDFT and confirmed the presence of a primary micropore and a spot of meso- and macropore, which was similar to that of other star triptycene-based microporous polymers (STPs).19 The gas uptake capacities of NTP for H2 and CO2 were evaluated. The hydrogen sorption and desorption measurements were carried out at 77 K. As shown in Figure 3c, NTP could absorb 1.58 wt % H2 at 1.0 bar, which was larger than that of most OMPs. The CO2 sorption properties at 273 and 298 K of NTP were investigated under low pressure. As shown in Figure 3d, NTP shows reversible CO2 uptake with nearly no hysteresis between the absorption and desorption isotherms, implying that CO2 is reversibly physisorbed. At 1.0 bar, NTP showed the high CO2 uptakes of 15.2 wt % at 273 K, which was much higher than most of the OMPs as well as some recent molecular cages.21 The isosteric enthalpy (Qst) of NTP toward CO2 was 26 kJ mol−1 calculated from the adsorption isotherms at 273 and 298 K in terms of the Clausius−Clapeyron equation (Figure S8).22 Using the slopes at low pressure in the Henry’s law region for both CO2 and N2 at 273 K, the CO2/N2 selectivity of 18 was calculated for NTP (Figure S9). With the good porous properties, NTP could be used as adsorbent for iodine enrichment. To evaluate the adsorption capability of iodine vapor, the NTP powder was exposed to excess iodine vapor in a closed system at 75 °C and ambient pressure. As time went on, with an apparent color change, NTP reached the adsorption equivalent after 48 h. The equilibrium iodine uptake was measured to be 180 wt % (Figure S10). Although it was not the highest one ever reported (276 wt %),14 the uptake was higher than some metal−organic frameworks (MOFs)23 and porous aromatic frameworks (PAFs)14 and even could be comparable to some recent CMPNs15 (Table S1), while commercial activated carbon (granular active charcoal) displayed only 30 wt % iodine vapor uptake capability under the same conditions. The high iodine sorption capacity of NTP is considered to be attributed to expanded conjugated areas14 and heteroatom24 doping in the polymer networks, and the high charge density on these sites may improve the sorption capacity for iodine. Moreover, the iodine sorption of NTP is reversible. The captured iodine could be released from polymer networks by immersing the iodineloaded NTP in polar organic solvents, for example, ethanol. As shown in Figure S11, the color of the solution changed from colorless to dark brown as time went on, which suggested that the NTP can be used as recyclable absorbents for iodine enrichments. To prove this, we investigated the cycling performance of NTP for iodine vapor adsorption. In that case, iodine-loaded NTP was purified by Soxhlet extraction in methanol for 2 days and dried in vacuum for the next iodine vapor adsorption cycle. As shown in Figure S12, NTP exhibited almost the same performance under five cycles. FT-IR analysis with no significant spectral changes of NTP after each cycle confirmed that the structure of NTP was stable in the process of iodine enrichment (Figure S13).
monomer 1,1,5-cyclooctadiene, bis(1,5-cyclooctadiene) nickel(0), and 2,2′-bipyridyl in dehydrated DMF was heated at 85 °C under argon for 96 h. After reaction, concentrated hydrochloric acid was added to the reaction mixture. The precipitated solid was filtrated and washed successively with CHCl3, THF, and H2O and purified by Soxhlet extraction in methanol for 2 days. After drying in vacuum the product NTP was obtained as yellow powder with 95.8% yield. The amount of residual nickel in NTP was determined as 0.063 wt % by inductively coupled plasma mass spectrometry (ICP-MS), and the amount of bromine left in the network was 0.07 wt % measured by an energy-dispersive spectrometer (EDS) (Figure S3), which indicated a complete reaction to some extent. The product polymer was found to be insoluble in any common organic solvent. FT-IR analysis and 13C cross-polarization magic-angle spinning (CP/MAS) NMR experiments were used to confirm the formation of NTP. As shown in Figure S4, the disappearance of C−Br bonds in FT-IR spectra of NTP compared with that of its precursor monomer 1 demonstrated the success of C−C coupling. The 13C CP/MAS NMR spectrum of NTP displayed five kinds of carbon signals with the chemical shifts of 52, 119, 124, 126, 138, and 145 ppm, which could be assigned to the methylidyne bridge carbon (a) and the aromatic carbons (h, g, c, and b, d, e, f, i, respectively (Figure 1).
Figure 1. Cross-polarization (CP) 13C MAS NMR spectrum of NTP.
The results of field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) displayed that NTP adopted rough sphere shape with particle size ∼100 nm (Figure 2). Powder X-ray diffraction (PXRD)
Figure 2. SEM (a) and TEM (b) images of NTP. Scale bar: 1 μm (a) and 500 nm (b).
pattern of NTP (Figure S5) showed a broad peak and indicated its amorphous nature. The thermogravimetric analysis (TGA) of NTP (Figure S6) showed that the material was stable up to 500 °C under nitrogen, and the slightly mass drop before 100 °C was attributed to trapped solvent within the micropore structure. A typical N2 adsorption−desorption isotherm of NTP was measured at 77 K as shown in Figure 3a. Brunauer−Emmett− Teller (BET) specific surface area was calculated to be 1067 m2 1040
DOI: 10.1021/acsmacrolett.6b00567 ACS Macro Lett. 2016, 5, 1039−1043
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ACS Macro Letters
Figure 3. Nitrogen sorption isotherm at 77 K (a), pore size distribution calculated (b), H2 adsorption and desorption isotherm at 77 K (c), and CO2 adsorption and desorption isotherm of NTP (d). In (a), (c), and (d), filled symbols denote gas adsorption, and empty symbols denote desorption.
Figure 4. UV−vis adsorption spectra of the aqueous solutions of I-3 in the presence of NTP at different intervals. The initial concentration of the I-3 solution is 0.2 mmol L−1 (a), adsorption rates of I-3 on NTP. The insets show the corresponding photographs (b), the Langmuir isotherm model for I-3 on NTP (c), and the Freundlich isotherm model for I-3 on NTP (d).
Moreover, NTP also can be used as iodine absorbents in organic solution. When NTP was immersed in an n-hexane solution of iodine (1 mmol L−1) in a small sealed vial at room temperature, it could be found that the purple solutions of iodine fade slowly to very pale red and finally to colorless (Figure S14), which suggested that the iodine was encapsulated into the NTP networks to successively generate iodine-loaded systems in solution.
To estimate the adsorption kinetics of NTP for iodine in aqueous solution, UV−visible (UV−vis) adsorption spectra experiments were employed to measure the change of the maximum adsorption of I-3. As treatment time continued, the adsorption intensities of I-3 aqueous solutions became weaker (Figure 4a). As shown in Figure 4b, more than 99% of I-3 could be removed from water within 30 min at room temperature. To further investigate the adsorption behavior of NTP, two isotherm equations, the Langmuir isotherm model (Figure 1041
DOI: 10.1021/acsmacrolett.6b00567 ACS Macro Lett. 2016, 5, 1039−1043
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ACS Macro Letters
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4c) and the Freundlich isotherm model (Figure 4d), were used to fit the equilibrium data of I-3 adsorption. The simulation results displayed that the correlation coefficient of the Langmuir isotherm model (RL2, 0.998) was higher than that of the Freundlich isotherm model (RF2, 0.950), indicating that they could be well fitted by the Langmuir isotherm model. Considering the pore size distribution of NTP (Figure 3b), we could speculate that I-3 could only enter and bind in the mesoporous, thus the adsorption process was homogeneous, which was consistent with the Langmuir model. Moreover, fitted by the Langmuir isotherm model, the maximum adsorption capacity of NTP for I-3 from aqueous solution was thus calculated as 429 mg g−1. In summary, triptycene-based porous polymer NTP with surface area of up to 1067 m2 g−1 was synthesized by the modified Yamamoto-type Ullmann cross-coupling reaction and displayed excellent gas storage capabilities. Moreover, NTP shows high adsorption capacities for iodine with a maximum uptake of 180 wt %. Combining the hierarchical porous structure with the features of high surface area and high thermal stability, NTP might be the ideal adsorbent for applications in iodine enrichment.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00567. Detailed experimental procedures, 1H NMR and 13C NMR of monomer 1, FT-IR, XRD, TGA, and initial gas uptake slopes of NTP, heat of adsorption for CO2, recycle test of NTP in iodine vapor adsorption, and photographs of iodine release and enrichment (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21672078, 81573013) and the Fundamental Research Funds for the Central Universities (HUST 2015TS086). We thank the Analytical and Testing Center of Huazhong University of Science and Technology for related analysis.
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REFERENCES
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