Conjugated Microporous Polymers with Extended π-Structures for

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Conjugated Microporous Polymers with Extended π‑Structures for Organic Vapor Adsorption Si-Jie Yang,†,‡ Xuesong Ding,*,† and Bao-Hang Han*,†,‡ †

CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Conjugated microporous polymers (CMPs), which are organic porous materials with π-conjugated skeletons, have attracted considerable attention in recent years owing to their distinct properties. Here, two sp2 carbon CMPs with analogous structures, namely, BO-CMP-1 and BO-CMP-2, were prepared through palladium-catalyzed Suzuki−Miyaura cross-coupling reaction. BOCMP-1 and BO-CMP-2 possess high surface areas and can be further modulated by postoxidation reaction to form networks with enhanced rigidity, as denoted by oBO-CMP-1 and oBO-CMP-2, respectively. The oxidation process transforms biolefin benzoquinone blocks within the skeletons into tetrabenzocoronene segments and endows the resultant oBO-CMPs with extended π-structures. The postoxidation provides a feasible approach to obtain CMPs with large π-systems. This approach prevents the disadvantages of direct synthesis by using monomers with extended π-structures, which often suffers low polymerization degree from strong π−π stacking of the monomers. The N2 and CO2 adsorption analysis confirms the effective changes in the parent materials during the oxidation process, suggesting that the approach can well modulate the porosities and adsorption properties of CMPs. Furthermore, owing to their aromatic nature, BO-CMPs and oBO-CMPs exihibit good affinity toward aromatic molecules, showing high uptake of benzene and toluene. For example, BO-CMP-2 captures 112.5 wt % toluene and 95.0 wt % benzene at 298 K and P/P0 = 0.99, indicating their promising capability to remove toxic organic vapors.

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encapsulation of solvents or any other organic chemicals,11,12 heterogeneous catalysis,13−16 light emission,17 and chemosensing,18 in which gas adsorption and separation have been intensively studied. The gas adsorption performance of microporous polymers is influenced by numerous factors, such as pore size distribution, specific surface area (SSA), and the physicochemical nature of pore walls.19 Therefore, approaches to control polymer topology and pore morphology need to be considered during the design process. The surface area, pore size, and pore volume of recently developed metal−organic frameworks (MOFs) and COFs can be conveniently tuned by altering the length and geometry of rigid organic struts in a predictable manner due to their definite crystalline structures.5,6,20,21 However, most COFs and MOFs suffer from moderate stability. CMPs possess good long-term stability with robust covalent bonding though they are amorphous and lack of longrange order because the precipitation process is kinetic controlled; however, several studies have proven that it is still possible for CMPs to tune their microporosity.22,23 For example, Cooper’s group designed and synthesized several

INTRODUCTION Porous organic polymers (POPs), emerging as a new kind of adsorbent materials, have attracted considerable research interest in the recent decade.1,2 Compared with inorganic microporous materials, POPs show potential advantages because these materials are totally composed of light elements and thus present low density. Moreover, POPs are available for a wide range of synthetic methods, yielding high surface areas, tunable pore sizes, and diverse functionalization. These polymers exhibit good chemical and physical stability and can maintain excellent performance even under harsh conditions owing to robust covalent bonding and rigid building blocks. POPs can be classified into several categories, including hypercross-linked polymers (HCPs),3 polymers of intrinsic microporosity (PIMs),4 covalent organic frameworks (COFs),5,6 and conjugated microporous polymers (CMPs).7 CMPs are a distinct class of POPs composed of π-conjugated fashioned building blocks. Its diversity is derived from the various structural skeletons of building blocks coupled with extensive reaction types available.7 Because of the flexibility of changing the π-units with tunable sizes, geometries, or functional groups, various CMPs could be designed and synthesized according to practical requirements. Therefore, CMPs are considered as potential materials in various applications, such as gas adsorption and separation,8−10 © XXXX American Chemical Society

Received: November 28, 2017 Revised: January 5, 2018

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DOI: 10.1021/acs.macromol.7b02515 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Preparation Process of BO-CMPs and oBO-CMPs

were purchased from Sinopharm Chemical Reagent Co., Ltd. All commercially available chemicals and solvents were used directly. The monomer 9,10-bis(dibromomethylene)-9,10-dihydroanthracene (BBMA) was synthesized following the reported procedure.33 Preparation of BO-CMP-1. BBMA (260 mg, 0.5 mmol), 1,4phenylenediboronic acid (166 mg, 1.0 mmol), and K2CO3 (1.5 g) were placed in a two-necked round-bottom flask (50 mL), in which water (5 mL) and 1,4-dioxane (30 mL) were charged. After ultrasonication for minutes until homogeneous, the mixture was degassed by N2 flow. Then, Pd(PPh3)4 (200 mg) was rapidly added into the flask in one portion. The mixture was then heated slowly to reflux under N2 protection and continuously stirred for 3 days. The precipitate was filtrated, washed with water, THF, and DCM, and then subjected to Soxhlet extraction for 24 h in THF. The precipitate was subsequently dried at 100 °C under vacuum for 24 h to obtain BOCMP-1 as a dark red powder with a yield of 95%. Elemental analysis for C28H16, calculated: C, 94.34%; H, 5.66%. Found: C, 87.42%; H, 4.93%. Preparation of oBO-CMP-1. BO-CMP-1 (80 mg) was suspended in 1.0 mL of toluene in a two-necked round-bottom flask (100 mL). FeCl3 (2.2 g, 13.5 mmol) was dissolved by 3.0 mL of CH3NO2, which was degassed by N2 flow for 10 min. The FeCl3 solution was added to the flask, and the mixture was stirred in an inert atmosphere at ambient temperature for 3 days. The precipitate was filtrated, washed with CH3NO2, 2 M HCl, CHCl3, THF, and methanol, and then dried at 100 °C under vacuum for 12 h to obtain oBO-CMP-1 as a dark brown powder at 95% yield. Elemental analysis for C28H8, calculated: C, 97.66%; H, 2.34%. Found: C, 90.21%; H, 2.57%. Preparation of BO-CMP-2 and oBO-CMP-2. BO-CMP-2 was obtained as yellow-green powder with a yield of 99% through a similar procedure as that of BO-CMP-1 by using 4,4′-biphenyldiboronic acid as the linker. Elemental analysis for C40H24, calculated: C, 95.21%; H, 4.79%. Found: C, 90.63%; H, 5.04%. oBO-CMP-2 was synthesized using the same procedure as oBO-CMP-1 with a yield of 96%. Elemental analysis for C40H16, calculated: C, 96.75%; H, 3.25%. Found: C, 92.21%; H, 3.05%.

poly(aryleneethynylene) (PAE) networks from CMP-0 to CMP-5 by varying the strut size of monomers, and results showed that the porosity and hydrogen adsorption behavior of these polymers can be controlled in a “quantized” way.24 Wang’s group managed to engineer porosity and surface functionalization of a carbazole-modified polytriazine framework via prefunctionalization of pore walls by introducing three different appended functional groups.25 However, these methods require a large amount of time and energy for monomer design and synthesis. Furthermore, the polymerization problems must be considered for monomers with different functional groups. Several reports proposed another method to control material structures, that is, by modulating the reaction medium, in which the selected organic solvents can significantly influence surface area and pore volume.26 However, this method is not universal because different reaction systems show distinct conditions. In addition to the methods mentioned above, on the established networks is also a good alternative to modulate the pore structures and prevent the polymerization problems as well. To date, few studies have paved the way through postsynthesis amidation, nitration, oxidation, and click reactions.27−31 On the basis of this context, we herein designed and synthesized two CMPs, in which bisolefin anthraquinone segments were further converted into larger π-systems by employing intramolecular oxidative cyclodehydrogenation in CH3NO2 solution using FeCl3 as the Lewis acid and oxidant.32 The postmodulation can effectively alter the polymers’ porosity while affecting gas sorption performance.

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EXPERIMENTAL SECTION

Materials. 1,4-Phenylenediboronic acid and 4,4′-biphenyldiboronic acid were purchased from Macklin. Tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) was bought from Energy Chemical. Potassium carbonate (K2CO3), 1,4-dioxane, tetrahydrofuran (THF), dichloromethane (DCM), and other commonly used organic solvents B

DOI: 10.1021/acs.macromol.7b02515 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. FT-IR spectra of BO-CMPs, oBO-CMPs, and related starting materials.

Figure 2. (a) Nitrogen sorption isotherms for polymers. (b) Pore size distribution profiles for polymers calculated using the NLDFT method.

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strong π−π stacking, and porous materials with high polymerization degree are usually difficult to form. This postreaction takes place after polymer formation and confers networks with extended π-blocks afterward. This strategy prevents the disadvantages of direct polymerization of large π-monomers. Compared with those of the parent polymers, the TGA curves (Figure S1, Supporting Information) show enhanced stability of oBO-CMPs due to the increasingly rigid skeletons formed after oxidation. The chemical structures of these series of polymers were characterized by Fourier transform infrared (FT-IR) spectroscopy measurement (Figure 1). The stretching bands at 2910− 3050 and 1620 cm−1 originate from the aromatic C−H and aromatic C−C vibration modes, respectively. After polymerization, the C−Br stretching mode of the starting monomer BBMA at 635 and 613 cm−1 vanishes, clarifying complete polycondensation. The stretching vibration band at 1486 cm−1 observed in BO-CMP-1 and BO-CMP-2, corresponding to the CC bond in tetrabromobisolefin monomer, vanishes after the oxidation reaction caused by the cyclization between CC and neighboring phenyl rings. The detailed molecular structure of the polymers was investigated by 13C solid-state NMR spectroscopy measurement (Figure S2). The resonance peaks at 135 and 129 ppm for BO-CMP-1 and 139 and 126 ppm for BO-CMP-2 correspond to substituted and nonsubstituted sp2 carbons, respectively. No significant change was observed in the 13 C solid-state NMR spectra between BO-CMPs and oBOCMPs possibly because the olefin carbon is indistinguishable from nonsubstituted aromatic carbons. The elemental analysis

RESULTS AND DISCUSSION The [4 + 2] polycondensation process is depicted in Scheme 1. The tetrabromobisolefin monomer BBMA reacts with M1 (1,4phenylenediboronic acid) or M2 (4,4′-biphenyldiboronic acid) to obtain BO-CMP-1 and BO-CMP-2, respectively, through one-step Suzuki−Miyaura reaction that is an effective crosscoupling reaction for aryl−aryl bond formation.34 This procedure refers to a mild Pd-catalyzed process occurring between an organo-boron reagent with an organic halide in the presence of a base. Here, K2CO3 was used as the base, and a small amount of water was added to aid the dissolution of K2CO3. A long reaction time was applied to obtain highly polymerized products. The obtained powder is insoluble in commonly used organic solvents such as DMF and THF and remains stable even when exposed to acid or base solution. The thermogravimetric analyses (TGA) (Figure S1, Supporting Information) of these two polymers under nitrogen demonstrate no evident weight loss until 500 °C. These results indicate the cross-linked property and high chemical and thermal stability of the polymers. As reported in many literatures, appropriate aryl-substituted olefin or neighbored aromatic precursors can be further cyclized through an intramolecular oxidative dehydrogenation reaction.35−37 Here, to utilize the specific structure of the polymers, FeCl3 was selected as oxidant, and a mixture of toluene and CH3NO2 was used as the solvent for cyclodehydrogenation of BO-CMPs skeletons. Compounds with large π-structures are usually poorly soluble in solvents due to C

DOI: 10.1021/acs.macromol.7b02515 Macromolecules XXXX, XXX, XXX−XXX

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for the surface areas reduce. Interestingly, as shown in Table 1, the ratio of the micropore surface area and the micropore volume derived by t-plot method to the SSA and total pore volumes of oBO-CMPs exceeds that of the parent polymers after oxidation, suggesting the increased microporosity. We also investigated the CO2 adsorption performance of the four polymers at 273 and 293 K (Figure S9). At 273 K and 1.0 bar, BO-CMP-1, BO-CMP-2, oBO-CMP-1, and oBO-CMP-2 display CO2 uptake of 7.9, 10.6, 5.1, and 7.5 wt %, respectively. The CO2 isosteric enthalpies (Qst) of the four polymers are considerably high, with the highest result of 36.5 kJ mol−1 obtained for BO-CMP-1, demonstrating good affinity between CO2 and the aromatic micropore walls.44 Both initial ratio of slope method and ideal adsorbed solution theory (IAST) were used to evaluate the selectivity of CO2 over N2 and CH4, suggesting good CO2 selectivity owing to the domination of micropores within their structures. The corresponding data are summarized in Table S1. Volatile organic compounds (VOCs), which exhibit high vapor pressure at ambient conditions, are included as the most common air pollutants emitted from petrochemical manufacturing, pharmaceutical industry, and related industries.45 Some common VOCs like benzene and its derivatives are extremely harmful to human health and environment. Therefore, materials and technologies for removing these organic pollutants from air are critical. Physical adsorption using porous materials as adsorbents has been proven to be a promising approach to solve this issue.46 Several porous adsorbents, such as mesoporous silicas,47 activated carbons,48,49 and MOFs,50 have been extensively studied in recent years. POPs, emerging as fully organic materials with rich pores and rigid covalent bonds, have been regarded as one of the most promising VOC adsorbents in recent years.51,52 The four polymers in this study are good candidates to remove VOCs owing to their relatively high surface area and the permanent porosity. The sorption isotherms of benzene, toluene, and cyclohexane were measured at 298 K. As shown in Figure 3, steep initial uptake at low relative pressure is observed for BO-CMPs and oBO-CMPs of benzene and toluene, and adsorption increase continued even at the saturated pressure, thus suggesting high affinity between the aromatic molecules with the π-conjugated networks. By contrast, the cyclohexane isotherms exhibit a slight increase at low relative pressure and plateau at higher pressure regions, and uptake is close to saturation at P/P0 = 0.99. The BO-CMPs are naturally aromatic because of the introduction of π-rich blocks, and the π-systems of oBO-CMPs extend further after oxidation. Because of the aromatic rings, the networks prefer π-rich benzene and toluene rather than aliphatic cyclohexane without π-electrons. It is noted that BO-CMP-2 exhibits a relatively high cyclohexane uptake among BO-CMPs and oBO-CMPs, possibly because of its high surface area and large pore volume (0.72 cm3 g−1). The uptakes of benzene, toluene, and cyclohexane for four CMPs at 298 K and P/P0 = 0.99 are listed in Table 2. The benzene uptake for BO-CMP-2 is 95.0 wt %, which is higher than those for other porous organic polymers with similar surface areas, such as PSN-D,53 CE-1,19 surpass PAF-5,54 and PSN-TAPM,53 which present relatively large surface areas (see the comparison in Table S3). For benzene, the adsorption capacities in the four polymers follow the order BO-CMP-2 > oBO-CMP-2 > BO-CMP-1 > oBO-CMP-1, and this trend is consistent with the order of their total pore volumes. Similarly, the uptake capacities of the four polymers toward cyclohexane

of BO-CMPs and oBO-CMPs reveals that experimental data of C and H contents are close to the theoretical calculations (see the Experimental Section). The broad peaks in powder X-ray diffraction (PXRD) patterns of these polymers reveal that the series of polymers are amorphous and devoid of long-distance order (Figure S3). Field emission scanning electron microscopy (FE-SEM) images reveal irregular submicrometer particles (Figure S4) before and after oxidation, thereby suggesting amorphous morphologies. Raman spectroscopy measurement represents that peaks at around 1350 and 1600 cm−1, which are often attributed to D and G bands for graphene-like materials,36,38,39 are more distinguishable after oxidation (Figure S5), further indicating the occurrence of graphitized segments during the cyclodehydrogenation. It is worth mentioning that the UV−vis reflectance spectra (Figure S6) show significant structural changes, where the oBO-CMPs with increased π-conjugation produce broader adsorption spectra in the long-wavelength region compared with that of BO-CMPs. This result coincides with the previously UV−vis data associated with the graphitization process.29,40 Furthermore, apparent color changes were observed by naked-eye during the transformation of BO-CMP-1 (dark red) and BO-CMP-2 (yellow) to oBOCMP-1 (dark brown) and oBO-CMP-2 (brown). The N2 sorption isotherm measurements at 77 K (Figure 2a) were carried out to examine the porosity of BO-CMP-1 and BO-CMP-2. The isotherms present a combination of type I and type IV sorption models according to IUPAC classification.41 The abrupt rise below the relative pressure of 0.10 indicates the presence of permanent micropores. Hysteresis existing in the medium range of relative pressure between adsorption and desorption branches results from the swelling of flexible porous polymers or restricted N2 access to the narrowly opened pores.42 The porosity data of all four polymers are listed in Table 1. The Brunauer−Emmett−Teller (BET) SSA value of Table 1. Pore Structure Parameters of the CMPs and oCMPs Obtained by N2 Adsorption materials

SBETa (m2 g−1)

Smicrob (m2 g−1)

RSc (%)

Vtotald (cm3 g−1)

Vmicroe (cm3 g−1)

RVf (%)

BO-CMP-1 oBO-CMP-1 BO-CMP-2 oBO-CMP-2

440 390 1030 540

160 200 460 300

36 51 44 55

0.33 0.31 0.72 0.36

0.06 0.08 0.20 0.12

18 26 28 33

a

SSA calculated from the nitrogen adsorption branch by using the BET method in the relative pressure range (P/P0 = 0.01−0.10). b Micropore surface area calculated from the nitrogen adsorption branch by using the t-plot method. cRatio of Smicro to SBET. dTotal pore volume at P/P0 = 0.99. eMicropore volume calculated from the nitrogen adsorption branch using the t-plot method. fRatio of Vmicro to Vtotal.

BO-CMP-2 is calculated as 1030 m2 g−1, which is considerably higher than that of BO-CMP-1 (440 m2 g−1), probably owing to the longer biphenyl units.43 The BET SSA values of oBOCMP-1 and oBO-CMP-2 are 540 and 390 m2 g−1, respectively. The experimental pore size distributions of the polymers were calculated from the adsorption branches of the nitrogen sorption isotherms by nonlocal density functional theory (NLDFT). Figure 2b shows that the size of most pores in BO-CMP-1 and BO-CMP-2 is located at the micropore region and is lower than 1 nm. After oxidation, the sorption isotherm type and pore size distributions show no notable change, except D

DOI: 10.1021/acs.macromol.7b02515 Macromolecules XXXX, XXX, XXX−XXX

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Figure 3. Organic vapor sorption isotherms for BO-CMPs and oBO-CMPs at 298 K.

Table 2. Adsorptive Capacities of BO-CMPs and oBO-CMPs for Various Organic Vapors at 298 K and P/P0 = 0.99 C6H6

C7H8

C6H12

materials

wt %

mmol g−1

wt %

mmol g−1

wt %

mmol g−1

BO-CMP-1 oBO-CMP-1 BO-CMP-2 oBO-CMP-2

59.4 42.0 95.0 71.6

7.61 5.37 12.16 9.17

68.9 47.1 112.5 56.1

7.48 5.11 12.21 6.09

28.5 27.8 74.8 34.0

3.26 3.31 8.88 4.04

alternative method to adjust the network topology and pore morphology of porous polymers. In terms of the π-rich structure, these materials possess good affinity to aromatic VOCs; among the materials, BO-CMP-2 can uptake 112.5 and 95.0 wt % toluene and benzene, respectively. All of the four polymers exhibit good uptake capacities toward benzene and toluene compared with aliphatic cyclohexane, and the VOCs adsorption performance largely depends on the pore volumes of the polymers. These results indicate the potential application of these polymers in removing volatile organic vapors.

also follow this order, meaning that the organic vapor capacities of the polymers mainly depend on pore volumes. Although benzene and toluene are aromatic, the number of absorbed benzene molecule surpasses that of toluene in the comparison of uptakes by the same polymer. The possible explanation is that the kinetic diameter of toluene (6.1 Å) is larger than that of benzene (5.9 Å); thus, toluene is inaccessible to small pores and channels. This result further confirms the extensive existence of ultramicropores within the series of polymers.

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CONCLUSIONS In summary, we successfully synthesized two novel POPs (BOCMP-1 and BO-CMP-2) through the Suzuki−Miyaura coupling reaction. The partial region can be further cyclized by an intramolecular oxidative dehydrogenation reaction due to the specific structure in which the olefin bonds are neighbored by two phenyl rings. The postmodulation process on the established networks is direct and simple, providing a good alternative to obtain CMPs with large π-structure, which prevents the low polymerization degree problem resulting from extended strong π−π stacking of π-structure monomer in direct synthesis. This process alters the structure, porosity, and even gas adsorption behavior of the parent polymers, thus offering an

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02515. Details of characterization methods for preparing BOCMPs and oBO-CMPs, corresponding BET SSA, and SEM images, PXRD patterns, TGA curves, solid-state 13 C/CP-MAS NMR spectra, Raman spectra, isosteric heats of adsorption for CO2, and the corresponding data of CO2 uptake and selectivity (PDF) E

DOI: 10.1021/acs.macromol.7b02515 Macromolecules XXXX, XXX, XXX−XXX

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]; phone +86 10 8254 5708 (X.D.). *E-mail [email protected]; phone +86 10 8254 5576 (B.H.H.). ORCID

Bao-Hang Han: 0000-0003-1116-1259 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This paper is dedicated to Prof. Jin-Pei Cheng on the occasion of his 70th birthday. The financial support of the National Science Foundation of China (Grants 21674026 and 21474027) is acknowledged.

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DOI: 10.1021/acs.macromol.7b02515 Macromolecules XXXX, XXX, XXX−XXX