CommunicationMacromolecular

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Synthesis and Characterization of Functional Thienyl-Phosphine Microporous Polymers for Carbon Dioxide Capture

Xianghui Chen , Shanlin Qiao , Zhengkun Du , Yuanhang Zhou , Renqiang Yang *

A novel kind of functional organic microporous polymer is designed by introducing polar organic groups (P=O and P=S) and electron-rich heterocyclic into the framework to obtain high carbon dioxide capture capacity. The estimated Brunauer–Emmett–Teller (BET) surface areas of these polymers are about 600 m 2 g − 1 and the highest CO 2 uptake is 2.26 mmol g − 1 (1.0 bar/273 K). Interestingly, the polymer containing P=O groups shows greater CO 2 capture capacity than that containing P=S groups at the same temperature. In addition, these polymers show high isosteric heats of CO 2 adsorption (28.6 kJ mol − 1 ), which can be competitive with some nitrogen-rich networks. Therefore, these microporous polymers are prom-ising candidates for carbon dioxide capture.

1. Introduction

Carbon dioxide capture and storage is currently a very active research area because of their great effect on the global warming and utilizing in the purifi cation of natural gas. [ 1 ] For the past decades, porous materials are under investigation for carbon dioxide capture relying on their large specifi c surface areas, such as zeolitic tetrazolate frameworks and metal–organic frameworks. [ 1–4 ] Recently, microporous organic polymers (MOPs) are also reported

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X. Chen, S. Qiao, Z. Du, Y. Zhou, Prof. R. Yang Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China E-mail: yangrq@qibebt.ac.cn X. ChenUniversity of Chinese Academy of Sciences, Beijing 100049, China

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on CO 2 sorption. MOPs are tailored using covalently linked molecular building blocks to possess high surface area and better physicochemical stability, which make them highly attractive in CO 2 capture and storage. [ 5 , 6 ] To our knowledge, the highest CO 2 uptake among MOPs is porous benzimi-dazole-linked polymers (BILP-4) with CO 2 adsorption up to 5.3 mmol g − 1 (1.0 bar/273 K). [ 7 ]

Several strategies have been used to enhance CO 2 capture property, including precise control of pore size, open metal-site, extraframework cations, and modifi ca-tion of functional groups. [ 1 , 8–10 ] Functional microporous polymers containing amines, carboxylic acids, hydroxyl groups, and triazine rings have been synthesized and showed attractive CO 2 adsorption. [ 1 , 11 , 12 ] In 2011, a new porous electron-rich covalent organonitridic framework centered on hexachlorocyclotriphosphazene was selected as potential sorbents for carbon dioxide capture, which showed high uptake (up to 3.49 mmol g − 1 at 1.0 atm/273 K)

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Scheme 1 . Synthetic route for the TTPP networks. Reagents and conditions: Pd(PPh 3 ) 4 , CuI, i-Pr 2 NH, DMF, 80 ° C, 72 h.

and excellent gas selectivity. [ 13 ] Then, Tan and co-workers [ 14 ] reported three hypercrosslinked micro-porous aromatic heterocyclic polymers (Th-1, Py-1, Fu-1) and Th-1 exhibited the highest CO 2 adsorption capacity (2.88 mmol g − 1 , at 1.0 bar/273 K) among them. It shows that the incorporation of such electron-rich heterocyclic into the network has the potential to increase the gas cap-ture capacity, because lone pair electrons of heteroatoms can enhance the interaction between the adsorbents and adsorbates.

The high surface area is a critical parameter for MOPs application. Zhu and co-workers [ 15 ] synthesized tetrahe-dral network (PAF-1) with an unprecedented BET surface area of 5640 m 2 g − 1 . Subsequently, a series of conjugated microporous polymers (CMPs) based on tetrahedral mon-omers centered on silicon, adamantane, and germanium centers was prepared and their surface areas were from 1100 to 3200 m 2 g − 1 . [ 16 ]

In this communication, a tetrahedral monomer cen-tered on phosphorus atom and a rigid molecular linker (1,3,5-triethynylbenzene) were designed to form tris(5-thienyl)phosphine polymers (TTPPs). Combined the polar group (P=O, P=S) and electron-rich thienyl group, these polymers exhibit high CO 2 uptake with its isosteric heat of 28.6 kJ mol − 1 and good selectivity for CO 2 over CH 4 and

Figure 1 . Scanning electron microscopy (SEM) images for a) O-TTPP and b) S-TTPP with a scale bar of 500 nm.

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N 2 . It is worth pointing out that the polymer modifi ed by P=O group shows greater CO 2 capture capacity than that containing P=S group because of its larger micropore volume and more polar P=O group. It indicates that it is a promising method to obtain high adsorption capacity through selecting some functional polar groups.

2. Results and Discussion

Two novel functional porous polymers were synthesized by Sonogashira–Hagi-

hara cross-coupling reaction from tris(2-bromo-5-thienyl)phosphine derivatives and 1,3,5-triethynylbenzene to give the corresponding networks as shown in Scheme 1 . [ 17 ] The polymer containing P=O group is named O-TTPP and its sulfur analogue is called S-TTPP. The molar ratio of ethynyl to bromine functionalities in the monomer feed was set at 1.5:1, which referred to the reports of CMPs synthesized by Cooper and co-workers. [ 18 , 19 ]

The structures of TTPPs are characterized by FT-IR spectroscopy (Figure S1, Supporting Information). The signal around 1205 cm − 1 shows the presence of –P=X (X=O, S) groups, and a peak at 2200 cm − 1 with relative low intensity is observed, which is assigned to the –C≡C–stretching. [ 20 ] A triple bond (–C≡C–H) stretching mode at 3303 cm − 1 is also found, indicating the presence of ter-minal alkynyl group. The structures of the polymers are also characterized by 13 C solid-state NMR spectra and 31 P solid-state NMR spectra. As shown in Figure S2 (Sup-porting Infomation), ethynylene C ar -C≡C–C ar units are observed at ca. 95 ppm and the resonances at ca. 82 ppm can be ascribed to the –C≡CH end groups. The thiophene rings exhibit a broad peak at ca. 135 ppm and the benzene ring signals are observed at ca. 124 ppm. Additionally, the 31 P NMR spectra (Figure S3, Supporting Information) have confi rmed the existence of phosphorus with signals at

0.17 and 9.5 ppm for O-TTPP and S-TTPP, respectively. Additionally, thermogravi-metric analysis shown in Figure S4 (Supporting Information) refl ects that the two polymers have high thermal stability ( T dec > 400 ° C) and the scan-ning electron microscopy (SEM) images show that both of the two polymers are consisted of regular spherical particles (Figure 1 ).

The nitrogen adsorption–desorp-tion isotherms are measured at 77 K (Figure 2 ) and their porous proper-ties are summarized in Table 1 . The

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Figure 2 . a) Nitrogen adsorption/desorption isotherms for TTPPs. b) Pore size distribution curves calculated by NL-DFT. c) CO 2 adsorption properties. d) The isosteric heat of CO 2 adsorption for TTPPs. For clarity, curves in (a) and (b) have been shifted vertically.

estimated BET surface areas and total pore volumes of O-TTPP and S-TTPP are 593 and 606 m 2 g − 1 , 0.32 and 0.91 cm 3 g − 1 , respectively. The isotherms reveal a rapid gas adsorption at low relative pressures indicating a perma-nent microporous nature and is refl ected in nonlocal den-sity functional theory (NL-DFT) cumulative pore volume plots (Figure S5, Supporting Information). [ 21 ] However, the isotherms exhibit a relative great increased nitrogen uptake at P / P 0 > 0.85, especially for S-TTPP, which can be ascribed mostly to interparticulate voids as shown in Figure 1 . [ 18 ] In the Figure 2 b, the NL-DFT pore size dis-tribution for O-TTPP confi rms a primary pore width of 0.54 nm, while the dominant pore width of S-TTPP are 0.8 and 1.88 nm, which are larger than O-TTPP. To estimate the contribution of microporosity to the total porosity in

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Table 1. Surface properties and gas uptake for the TTPP Networks.

Polymer S BET [m 2 g − 1 ] a)

V Micro [cm 3 g − 1 ] b)

V Total [cm 3 g − 1 ] c)

V Micro /

O-TTPP 593 0.21 0.32 0.6

S-TTPP 606 0.16 0.91 0.1

a) Surface area calculated from N 2 isotherm; b) The micropore volume

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the networks, the value of V Micro / V Total is calculated, the ratio of the pore volume obtained using the t-plot method to the pore volume at P / P 0 = 0.99. It is surprising that the ratio of S-TTPP is only 0.17 (Table 1 ), almost 1/4 of the value of its analogue O-TTPP (0.65) and lower than that of most reported MOPs. [ 16 , 20 , 21–24 ]

CO 2 adsorption is an important property for porous material and the adsorption data for TTPPs are col-lected at 273 K and 298 K, at pressures up to 1.0 bar (Figure 2 c). At 1.0 bar/273 K, the CO 2 capture capacity of O-TTPP is 2.26 mmol g − 1 and S-TTPP reaches a value of 1.81 mmol g − 1 . These values are slightly higher than CMP-1-(OH) 2 (1.8 mmol g − 1 ), even though the latter has strong polar group (-OH), larger surface area and micro-volume (1043 m 2 g − 1 and 0.40 cm 3 g − 1 , respectively). [ 13 ]

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V Total CO 2 uptake [mmol g − 1 ]

CH 4 uptake [mmol g − 1 ]

273 K 298 K 273 K 298 K

5 2.26 1.56 1.14 0.63

7 1.81 1.04 0.89 0.47

derived using the t-plot method; c) Total pore volume at P/P 0 = 0.99.

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The high CO 2 capture property is expected to arise from dipole–quadrupole interactions and/or Lewis acid–Lewis base interactions between the electron-rich aromatic constituents and the electron poorer carbon dioxide molecules. [ 13 ] It is interesting that O-TTPP shows higher CO 2 adsorption over 25% at 273 K and 51% at 289 K than S-TTPP. One possible explanation is that the framework with P=O group has stronger dipole–quadrupole interac-tions than S-TTPP. Another reason seems to be related to the lower V Micro /V Total of S-TTPP (only 0.17) and the exist-ence of ultrafi ne pores ( < 0.7 nm) in O-TTPP, which would enhance CO 2 uptake at lower pressures effectively. As a result, the large micropore volume and strong polar func-tional group incorporated into O-TTPP play crucial roles in the CO 2 capture property.

To determine the binding affi nity of polymer for CO 2 , the isosteric heat of adsorption ( Q st ) is calculated using the following Clausius–Clapeyron equation. [ 12 ]

LnP = − Q0(nm)/ RT + C(nm) (at constantnm) (1)

Here P and T are the pressure and temperature at the equilibrium state, R is the universal gas constant. C is an integration constant which is a function of n m only.

As shown in Figure 2 d, the highest Q st values for O-TTPP and S-TTPP are 28.6 and 28.0 kJ mol − 1 , respec-tively, which are higher than those of polycarbazole CPOP-1 (27 kJ mol − 1 ) and polybenzimidazole BILP (19.7–26.5 kJ mol − 1 ) but lower than nitrogen-rich network PECONF-2 (31 kJ mol − 1 ). [ 7 , 13 , 25 ] The Q st of O-TTPP decreases to 19.4 kJ mol − 1 with increased CO 2 loading, which indi-cates that this material is a heterogeneous adsorbent. [ 26 ] However, the Q st values for S-TTPP remain nearly constant over a large range of adsorbate loading (only dropped by 7.6%). This suggests that a signifi cantly higher CO 2 gas uptake can probably be achieved only by increasing the micropore volumes of S-TTPP. [ 13 ]

The methane sorption characteristics of the TTPPs are also explored. As shown in Figure S6 (Supporting Infor-mation), at 1.0 bar/273 K, methane uptake of O-TTPP and S-TTPP is 1.14 and 0.89 mmol g − 1 , respectively. The methane uptake of O-TTPP is comparable to benzimidazole-linked polymer BILP-7 (1.18 mmol g − 1 ) at the same condition. [ 7 ] Additionally, the selectivity of TTPPs toward CO 2 over CH 4 and N 2 is estimated using the ratios of the Henry law con-stants and these constants can be calculated from the ini-tial slopes of single-component adsorption isotherms at 273 K and 298 K (Figure S6, Supporting Information). [13] The CO 2 /CH 4 and CO 2 /N 2 of O-TTPP at 273 K are 4.3 and 50.7, respectively, however, when the temperature rises to 298 K, the selectivity decreases to 2.8 and 20.5 (Figure S7, Supporting Information). As shown in Table S1 (Supporting Information), S-TTPP shows almost the same gas selec-tivity as O-TTPP at the same temperature. It indicates

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that modifi cation of the polymers by different functional groups (P=O or P=S) has no effect on CO 2 selectivity and they are promising candidates for methane purifi cation.

3. Conclusion

In summary, a new kind of phosphorus-functionalized microporous polymers was synthesized via Sonogashira–Hagihara cross-coupling reaction. These polymers show high CO 2 uptake accompanied with high isosteric heat of adsorption. Meanwhile, these polymer networks also present a relative high CO 2 /CH 4 selectivity (4.3) and CO 2 /N 2 selectivity (50.7). It is interesting that O-TTPP shows greater gas capacity than S-TTPP, however, the CO 2 /CH 4 and CO 2 /N 2 selectivity of the two polymers are almost the same. The obtained phosphorus-containing microporous polymers may have potential application in gas storage and separation.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements : This work was supported by the Ministry of Science and Technology of China (2010DFA52310), National Natural Science Foundation of China (21274161, 51173199), Chinese Academy of Sciences (KGCX2-YW-399 + 9-2), Department of Science and Technology of Shandong Province (2010GGC10345), Shandong Provincial Natural Science Foundation (ZR2011BZ007), and Qingdao Municipal Science and Technology Program (11-2-4-22-hz).

Received: April 9, 2013; Revised: May 23, 2013; Published online: June 12, 2013; DOI: 10.1002/marc.201300328

Keywords: functionalization of polymers; gas capture property; heat capacity; thienyl-phosphine networks

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