Microsoft Word - CJOC.201400494E.doc* E-mail: hanbh@nanoctr.cn; Tel.: 0086-010-82545576 Received July 22, 2014; accepted October 9, 2014; published online November 7, 2014. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201400494 or from the author. Chin. J. Chem. 2015, 33, 131—136 © 2015 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 131
DOI: 10.1002/cjoc.201400494
Yi Cui, Yanchao Zhao, Tao Wang, and Baohang Han*
National Center for Nanoscience and Technology, Beijing 100190, China
A series of benzimidazole-linked porous polymers are obtained by the condensation reaction between the o-aminobenzol end groups of building blocks (2,3,6,7,10,11-hexaaminotriphenylene, 3,3'-diaminobenzidine or 1,2,4,5-benzenetetraamine) and the aldehyde groups of building blocks [terephthalicaldehyde, 4,4'-biphenyl- dicarboxaldehyde, 1,3,5-tris(4-acetylphenyl)benzene or 1,3,5-tris(4-formylbiphenyl)amine] in one-pot synthesis without employing any catalyst or template. The existence of the imidazole ring in the obtained polymers could be identified by Fourier transform infrared and solid-state 13C CP/MAS NMR spectroscopy. The sphere-shaped mor- phology of the obtained polymers is observed through scanning electron microscopy. The polymers possess Brunauer-Emmett-Teller specific surface area values over 600 m2•g–1, showing hydrogen storage (up to 1.6 wt%, at 77 K and 1×105 Pa) and carbon dioxide capture (up to 12.6 wt%, at 273 K and 1×105 Pa) properties. Such poly- mers would possess good performance in the applications of gas storage and separation.
Keywords benzimidazole, porous polymers, hydrogen storage, carbon dioxide capture
Introduction Highly porous polymeric materials possess potential
in a myriad of applications in diverse technological ar- eas, such as gas storage and molecule separation.[1] Low density polymers are at the center of the research inter- est, since low density is the most desirable characteristic of any gas storage materials intended for mobile appli- cations. There are three main kinds of microporous polymer networks, (i) hyper-cross-linked polymers (HCP);[2,3] (ii) crystalline microporous polymers: 2D and 3D covalent organic framework (COF)[4] and metal- organic framework (MOF),[5] with well-controlled pore sizes; and (iii) polymers of intrinsic microporosity (PIM).[6,7] In general, MOF possesses the highest sur- face area and specific pore volume.[8] However, MOFs suffer from the chemical instability, mainly due to the nature of a coordination bond,[9] or high toxicity for some MOFs due to chromium in MIL-101,[10] which may affect the potential applications of these materials. In the meanwhile, COFs have to be synthesized in strictly reversible process under thermodynamic control, which is unsuitable for practical application. A disad- vantage of the so far mentioned other microporous polymers is the disordered structure of the frameworks, resulting in a broad pore size distribution. Therefore, both high specific surface area and specific pore sizes are needed for porous polymers for the applications of gas storage and molecule separation. By choosing the
organic building blocks of light elements to achieve the exquisite control over the pore size, through green and simple synthesis procedure, to obtain porous organic polymers with high surface area and high thermal and chemical stability, is the trend of the synthesis of porous materials.[11-13]
As the hydrogen storage and carbon dioxide capture are the two important applications for materials in en- ergy and environment issues, porous materials for re- versible hydrogen storage, based on the physisorption, with exceptional storage capacities is the essential topic. In the meanwhile, capture and sequestration of carbon dioxide from fossil fuel power plants is gaining wide- spread interest as a potential method for controlling greenhouse gas emissions.[14]
As a polymer with a high thermal and chemical sta- bility, poly(benzimidazole) (PBI), which possesses a high thermal and chemical stability, shows a large vari- ety of applications including electronic and automotive components, structural resins, and fire resistant materi- als.[15,16] Recently, a series of porous benzimidazole- linked polymers have been synthesized for gas storage and separation.[17-25] Herein, we describe a convenient and simple synthesis of a series of porous benzimida- zole-linked polymers with Brunauer-Emmett-Teller (BET) surface area values up to 637 m2•g–1, by the con- densation reaction between the o-aminobenzol end groups of building blocks (2,3,6,7,10,11-hexaaminotri- phenylene, 3,3'-diaminobenzidine or 1,2,4,5-benzene-
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tetraamine) and the aldehyde groups of building blocks [terephthalicaldehyde, 4,4'-biphenyl-dicarboxaldehyde, 1,3,5-tris(4-acetylphenyl)benzene or 1,3,5-tris(4-formyl- biphenyl)amine] in one-pot synthesis without employ- ing any catalyst or template. By choosing the organic building blocks with o-aminobenzol end groups and the aldehyde group above, porous polymers (PBIx-y) with different pore size and specific surface area values have been prepared. The obtained polymers show hydrogen storage (up to 1.6 wt%, at 77 K and 1×105 Pa) and carbon dioxide capture (up to 12.6 wt%, at 273 K and 1 ×105 Pa) properties. Besides the usual applications known for high surface area materials, these polymers might open new possibilities for advanced applications in the field of hydrogen storage and carbon dioxide capture.
Experimental Materials
Preparation of porous benzimidazole-linked poly- mers (PBIx-y)
A mixture of A1 (25 mg, 0.047 mmol) and B1 (9.5 mg, 0.071 mmol) was suspended in DMSO (6.00 mL). After ultrasonicaion for 0.5 h, the mixture was degassed by at least three freeze-pump-thaw cycles. The tube was frozen at 77 K (liquid nitrogen bath) and evacuated to high vacuum and flame-sealed. The reaction mixture was heated at 150 for 72 h yielding a black brown solid. This solid, denoted by PBI1-1, was filtrated and washed with acetone, dichloromethane, and ethanol, subsequently. The product was dried in vacuo at 120 for more than 12 h (75% yield).
Similar to the preparation of PBI1-1, A1 (25 mg, 0.047 mmol) and B2 (14.7 mg, 0.071 mmol) were re- acted in DMSO at 150 to afford PBI1-2 in 70% yield.
By using the similar process above, PBI2-3 was pre- pared in DMF at 150 by using A2 (20.6 mg, 0.096 mmol) and B3 (25 mg, 0.064 mmol) as the building blocks in 76% yield. PBI3-3 was also prepared in DMF at 150 by using A3 (21.7 mg, 0.077 mmol) and B3 (20 mg, 0.051 mmol) in 75% yield. By using B4 as one building block, PBIx-4 were prepared. PBI1-4 was pre-
pared in DMSO at 120 by using A1 (25 mg, 0.047 mmol) and B4 (26 mg, 0.047 mmol) as the building blocks in 90% yield. PBI2-4 was prepared in DMF at 150 by using A2 (14.4 mg, 0.067 mmol) and B4 (25 mg, 0.045 mmol) as the building blocks in 78% yield. PBI3-4 was prepared in DMF at 150 by using A3 (19 mg, 0.067 mmol) and B4 (25 mg, 0.045 mmol) as the building blocks in 76% yield.
Instrumental characterization Solid-state 13C CP/MAS NMR measurements were
performed on a Bruker Anence III 400 spectrometer. Thermogravimetric analysis (TGA) was performed on a Pyris Diamond thermogravimetric/differential thermal analyzer by heating the samples at 10 •min–1 to 800 in the atmosphere of nitrogen. Infrared (IR) spectra were recorded in KBr pellets using a Spectrum One Fourier transform infrared (FT-IR) spectrometer (Perkin-Elmer Instruments Co. Ltd, USA). The sample was prepared by dispersing the polymers in KBr and compressing the mixtures to form disks, and fifteen scans were signal-averaged. Scanning electron micros- copy (SEM) observation was carried out by using a Hitachi S-4800 microscope (Hitachi Ltd., Japan) at an accelerating voltage of 6.0 kV. SEM samples were pre- pared by dropping an ethanol suspension of polymers on a silicon wafer, and then dried. Nitrogen and hydrogen adsorption-desorption experimentations were conducted at 77 K using an ASAP 2020 MC surface area and porosity analyzer (Micromeritics, USA). The obtained nitrogen adsorption-desorption isotherms were evalu- ated to give the pore parameters, including BET specific surface area, pore size, and pore volume. The pore size distribution was calculated from the adsorption branch by the density functional theory (DFT) method. Carbon dioxide uptake experimentation was performed by using a Micromeritics TriStar II 3020 accelerated surface area and porosity analyzer at 273 K. Before measurement, the samples were degassed in vacuo at 120 for more than 12 h.
Results and Discussion As shown in Figure 1, PBIx-y materials are obtained
by a condensation reaction between the o-aminobenzol end groups of building blocks (Ax) and the aldehyde group of building blocks (By) in one-pot synthesis, which is based on the imidazole ring-forming reac- tion.[15,16] The mixture reacts under high vacuum condi- tion in a sealed tube to form the benzimidazole-linked polymers in 70%90% yield without employing any catalyst or template. The PBIx-y materials are insoluble in water and common organic solvents, such as di- chloromethane, ethanol, acetone, tetrahydrofuran, DMSO, and DMF. The obtained polymers were ana- lyzed by means of IR spectroscopy, XRD, solid-state 13C CP/MAS NMR spectroscopy, and SEM.
As shown in Figure 2A, PBI1-4 shows aggregated
Benzimidazole-Linked Porous Polymers: Synthesis and Gas Sorption Properties
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particles of ca. 300 nm in size. PBI3-4 could be seen as larger aggregated particles than PBI1-4 as in Figure 2B, which are ca. 500 nm in size. However, Figure 2C re- veals that PBI2-3 formed more regular spheres of ca. 200 nm in size. The different morphology of the polymers demonstrates that the building block is one of the de- termining factors for the morphology of the obtained polymers.
As shown in Figure S1 (Supporting Information), the FT-IR spectra of PBI1-4, PBI1-1 and PBI2-3 are similar to each other. The bands at ca. 3400 cm–1 corresponds to NH stretching, a characteristic feature of imidazole NH absorptions.[15] After the imidazole ring formed, the CO peak intensity at ca. 1760 cm−1 is absent. The intense band appearing at ca. 1450 cm−1 corresponds to CN stretching of the imidazole ring.[16,30] The broad band at ca. 1620 cm−1 is presumably due to the overlap of CC and CN stretching bands.
The formation of imidazole ring in PBIx-y materials
is also examined by solid-state 13C CP/MAS NMR spectroscopy. Due to the analogous structure, the 13C chemical shifts of signals are similar to each other. Therefore, we take PBI1-4 and PBI2-3 as the examples as shown in Figure S2 (Supporting Information). Re- markably, the peaks at δ 153 are observed in the two obtained polymers, which correspond to NC(Ph)N in benzimidazole units, indicating the successful build-up of benzimidazole-linked polymeric network.[17] The five signals between δ 150 and 110 can be ascribed to the carbon of aromatic rings of the building blocks. In addi- tion, no signal originating from unreacted or terminal aldehyde group could be detected. Figure S3 (Support- ing Information) shows the XRD patterns of PBI1-4, PBI2-3, and PBI3-3. The broad peak at around 2θ20° indicates the amorphous characteristics of the PBIx-y.
The polymers show good thermal stability demon- strated by the thermogravimetric analysis. We take PBI1-4 and PBI2-3 as the examples as shown in Figure
NH2
NH2
+ OHC
B1
B2
B3
B4
Figure 1 Synthesis of the PBIx-y and structure of the building blocks.
Figure 2 SEM images of (A) PBI1-4, (B) PBI3-4 and (C) PBI2-3.
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S4 (Supporting Information). The first region occurs at temperature below 125 and is attributed to loss of adsorbed water (ca. 5%). After that there is a wide pla- teau region. The main decomposition starts at ca. 400 , which accounts for approximately 32% of the total mass for PBI2-3 and 38% for PBI1-4.
By choosing different building blocks and solvents, PBIx-y materials are obtained with different morphology and gas sorption properties. To determine the porosity, the PBIx-y materials are investigated by nitrogen adsorp- tion-desorption at 77 K. The obtained BET specific sur- face area results are listed in Table 1. The polymers were activated at 120 in vacuo for 12 h before the tests. It has been noticed that the PBI1-4 prepared using A1 and B4 generally possesses the highest surface area value compared with the other PBIx-y. The reason for the lowest surface area of PBI1-3 might be the same number of reaction sites of A1 and B3. PBI2-4 possesses lower value compared with PBI1-4 and PBI3-4 due to a higher conformational flexibility of the biphenyl linker.[19,31] PBI3-3 might be the short monomer of A3.[32,33] In addition, this polymer has also been synthe- sized by El-Kaderi and coworkers,[18] which possesses much higher porosity (BILP-5, 599 m2•g–1). Therefore, the synthetic procedure adopted here seems not suitable for that specific network. The reaction condition should be optimized in the further experiments. The results of nitrogen adsorption and desorption isotherms for PBI1-4 and PBI2-3 are shown in Figure 3A. PBI1-4 shows a high gas uptake at p/p0 less than 0.05 and a flat course at higher relative pressure, indicating a significant micro- porosity in the obtained polymer. PBI2-3 shows a steep upward-sloping trend of the isotherm at p/p0 more than 0.80, indicating the presence of meso- and macrostruc- tures in the obtained polymer, which reveals the pres- ence of interparticle porosity. The pore size distribution calculated by the original DFT method of the PBIx-y materials is shown in Figure S5 (Supporting Informa- tion). There are one steep peaks locating at ca. 0.6 in the micropore area with some content of the mesopore for the PBIx-y materials. The PBI1-4 and PBI2-3 are tested for hydrogen adsorption capacity at 1×105 Pa and 77 K. As a comparison, the BET specific surface area value, pore size distribution, hydrogen uptake, and CO2 ad- sorption of PBI1-4 and PBI2-3 are listed in Table 2. As shown in Figure 3B, PBI1-4 possesses hydrogen uptake of 1.2 wt%, which possesses the highest BET specific area value among the PBIx-y materials. However, PBI2-3 possesses a higher 1.6 wt% hydrogen uptake. The rea- son may be that PBI2-3 possesses a larger pore volume than PBI1-4, although PBI1-4 possesses a larger BET specific area value. Moreover, pore size is also an im- portant factor in determining the performance of physi- sorption-based storage systems as it governs the rela- tionship between pore volume and surface area and sig- nificantly affects adsorption enthalpy.[34] The pore size of PBI2-3 is 0.66 nm, which is more consistent to the optimum micropores (ca. 0.7 nm) for improving the
hydrogen uptake of an adsorbent significantly.[35,36] In addition, PBI2-1, PBI2-2, PBI3-1, and PBI3-2 have not been synthesized here, since the A2, A3, B1, and B2 are linear monomers, from which porous polymers with relatively low BET surface area values could be ob- tained in common consideration for preparation of or- ganic porous polymers. Therefore, the surface area val- ues of the organic polymer are related to the steric con- figuration, length, the number of reaction sites, and flexibility of the monomers.
Table 1 Specific surface area values of PBIx-y
Specific surface areaa/(m2•g–1)
PBI1-1 533
PBI1-2 252
PBI1-3 8
PBI1-4 637
PBI2-3 433
PBI2-4 60
PBI3-3 47
PBI3-4 414 a Nitrogen adsorption-desorption experimentations were conducted at 77 K. Specific surface area was calculated from the nitrogen adsorption isotherm using the BET method.
The carbon dioxide adsorption isotherms for PBI1-4 and PBI2-3 were collected at 273 K, and the results are plotted in Figure 3C and Table 2. As seen from the iso- therm data, PBI1-4 and PBI2-3 possess the maximum carbon dioxide adsorption of 12.6 and 8.5 wt% at 273 K (1×105 Pa), respectively. PBI1-4 possesses a higher carbon dioxide adsorption than PBI2-3, which is in good agreement with its higher BET specific surface area. Moreover, the nitrogen atom in B4 would also improve the carbon dioxide adsorption. This relatively high car- bon dioxide capture capacity of the PBIx-y materials might be attributed to the high affinity towards the po- larizable carbon dioxide through a strong hydrogen bonding and/or dipole-quadrupole interactions of the imidazole ring.[37,38]
Conclusions A series of benzimidazole-linked porous polymers
have been obtained by the condensation reaction be- tween the o-aminobenzol end groups and the aldehyde groups of the building blocks in one-pot synthesis without employing any catalyst or template. The exis- tence of the imidazole ring in the obtained polymers can be identified by FT-IR and solid-state 13C CP/MAS NMR spectroscopy. The obtained polymers possess large BET specific surface area over 600 m2•g–1, with hydrogen storage capacity (up to 1.6 wt%, at 77 K and 1×105 Pa) and carbon dioxide uptake (up to 12.6 wt%, at 273 K and 1×105 Pa) properties. Therefore, by
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Table 2 Porosity parameters measured by nitrogen sorption, hydrogen uptake, and CO2 adsorption capacity of PBI1-4 and PBI2-3
Specific surface area/(m2•g–1) Pore size/nm Pore volume/(cm3•g–1) Hydrogen uptakea/wt% CO2 adsorptionb/wt%
PBI1-4 637 0.62 0.28 1.2 12.6
PBI2-3 433 0.66 0.61 1.6 8.5 a Hydrogen gravimetric uptake capacities at 77 K measured at hydrogen equilibrium pressure of 1×105 Pa. b Carbon dioxide gravimetric uptake capacities at 273 K and 1×105 Pa.
Figure 3 (A) Nitrogen sorption isotherms of PBI1-4 and PBI2-3. The isotherms have been offset by 100 units for the purpose of clarity. (B) Gravimetric hydrogen adsorption isotherms for PBI1-4 and PBI2-3. (C) Gravimetric carbon dioxide adsorption isotherms for PBI1-4 and PBI2-3.
choosing the organic building blocks with o-amino- benzol end groups and the aldehyde groups, porous polymers with different pore size and specific surface area values could be prepared for gas adsorption and molecular separation. Such materials would possess excellent performance in the applications of gas storage or separation and catalysis in the future.
Acknowledgements The financial support of the National Science Foun-
dation of China (Nos. 21374024 and 61261130092) and the Ministry of Science and Technology of China (Nos. 2014CB932200 and 2011CB932500) is acknowledged.
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