Solid-State Synthesis of Conjugated Nanoporous Polycarbazoles

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Solid-State Synthesis of Conjugated Nanoporous Polycarbazoles Xiang Zhu,† Chengcheng Tian,*,† Tian Jin,† Katie L. Browning,‡ Robert L. Sacci,‡ Gabriel M. Veith,‡ and Sheng Dai*,†,§ †

Department of Chemistry, The University of Tennessee, Knoxville, Tennessee 37996-1600, United States Materials Science and Technology Division and §Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States

‡

S Supporting Information *

ABSTRACT: A novel solid-state synthetic approach has been developed for the generation of conjugated nanoporous polymer networks. Using mechanochemical-assisted oxidative coupling polymerization, we demonstrated a rapid and solvent-free synthesis of conjugated polycarbazoles with high porosities and promising CO2 storage abilities. This innovative approach constitutes a new direction for the development of novel nanoporous polymer frameworks through sustainable solid-state assembly pathways, and may open up new possibilities for the rational design and synthesis of nanoporous materials for carbon capture.

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able solid-state assembly pathways, and may open up new possibilities for the rational design and synthesis of nanoporous materials for carbon capture. As part of our ongoing research program on porous polycarbazoles,22−27 we first initiated a MC-assisted OCP of 1,3-bis(N-carbazolyl)benzene (M1, Scheme 1) with an attempt of solvent-free generation of nanoporous conjugated polycarbazoles. Previously, Han et al. pioneered a solvent-mediated oxidative polymerization strategy and reported a family of polycarbazoles with high Brunauer−Emmett−Teller (BET) specific surface areas and outstanding gas storage abilities.28−30 However, unsustainable chlorinated solvents such as dichloromethane and chloroform, have been widely used.31−33 In contrast, the solid-state assemble approach could provide an alternative route that allows a rapid and solvent-free construction of nanoporous carbazolic frameworks. In our attempt at solid-state OCP promoted by ball-milling, M1 (200 mg) and the mass of 2.5 equiv of FeCl3 (500 mg) were mixed in a zirconium oxide milling jar with a stainless steel ball and then subjected to vibrational ball-milling in a Retsch mixer-mill 400 at 30 Hz for 30 min. The obtained black solid was then thoroughly washed with methanol to remove FeCl3. High synthetic yield of 83% was achieved after drying at 120 °C under vacuum. The yellow powder (PCz-1-A) is insoluble in common organic solvents, such as chloroform and dimethylformamide, and exhibits high thermal stability (Figure S1). The powder X-ray diffraction (PXRD) pattern indicates its

he interest in conjugated nanoporous polymers (CNPs), driven by their versatile application in gas separation, heterogeneous catalysis, sensing and electrochemical energy storage, has inspired an extensive search for efficient synthetic methods that could lead to materials with perfect porosities and functionalities.1−13 A wide variety of CNPs have been well developed via conventional organic synthesis approaches.14 Many compounds nevertheless involve drastic synthetic conditions such as costly noble metal-based catalysts, high temperatures, aromatic solvents, and inert atmosphere, significantly impeding their scale-up and potential industrial implementation. As such, the development of new methodologies for facile and sustainable synthesis of CNPs is of great interest, importance, and urgency. Recently, mechanochemical (MC)-assisted solid-state assemble approach has been demonstrated as a versatile alternative technique that enables the synthesis of nanoporous organic frameworks,15 like metal− organic frameworks (MOFs)16 and covalent organic frameworks (COFs),17 through a straightforward and solvent-free route. However, the direct generation of CNPs through application of mechanochemisty is rarely reported and very few reactions were attempted.18−21 In this work, we report a novel MC-assisted synthetic approach to create a new family of highly porous conjugated polymers. The key to our success lies in the use of FeCl3promoted solid-state oxidative coupling polymerization (OCP), which gives rise to a rapid and solvent-free polymerization of carbazolic scaffolds. The resultant conjugated networks exhibit highly porous nature with promising CO2 storage abilities. This innovative approach constitutes a new direction for the development of novel nanoporous polymers through sustain© XXXX American Chemical Society

Received: July 2, 2017 Accepted: September 12, 2017

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DOI: 10.1021/acsmacrolett.7b00480 ACS Macro Lett. 2017, 6, 1056−1059

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ACS Macro Letters

this new solid-state assemble approach. Recnetly, Borchardt et al. developed a solid-state Friedel−Crafts Alkylation between aromatic monomers and cyanuric chloride and a new family of hyperbranched polymers were successfully generated.20 Higher BET surface area was achieved for PCz-1-A. We reasoned that the difference of the intrinsic linking efficiency between the oxidative coupling reaction and Friedel−Crafts reaction may result in the different porosities. The structure of PCz-1-A was then revealed at the molecular level by solid state 13C crosspolarization magic-angle spinning (CP/MAS) NMR technique (Figure S3). The structure features match well with that of previously reported conjugated nanoporous polycarbazole.24 The resolved resonance peak at 140.1 ppm originates from the substituted phenyl carbons bonded to the carbazole nitrogen.24 Other aromatic carbons within the skeleton can be supported by the two peaks at 124.6 and 109.9 ppm. Accordingly, MCassisted OCP is an efficient methodology for a rapid and sustainable generation of highly porous conjugated polycarbazole. Inspired by this success, we speculated that the quantity of FeCl3 during the ball-milling process could play a significant effect on the synthesis yield and porosity for the resulting materials. We performed a detailed investigation into the synthetic parameter space governing solid-state OCP. With the same milling time (0.5 h), a significant improvement in isolated yield as a function of FeCl3 amount is observed (Table 1); that is, the use of 1.25 mass equiv of FeCl3 affords a relative low 52% yield, whereas a much higher 94% isolated yield was obtained at 5 mass equiv of FeCl3. The BET surface areas of these compounds maintain very well. Furthermore, we tried to decrease the milling time from 0.5 to 0.25 h to improve the reaction efficiency. By this revised protocol, the new network PCz-1-D was obtained with similar isolated yield, whereas BET surface area was significantly decreased to be 309 m2 g−1. As a result, the success of achieving high synthesis yield and porosity for PCz-1-A is mainly due to the association of appropriate quantity of FeCl3 and ball-milling time, enabling an efficient solid-state assemble. Additionally, PCz-1 was also prepared under a larger scale, five times than that of PCz-1-A. Interestingly, the properties of the new PCz-1-Five maintain well under such a larger synthetic condition (SABET = 839 m2 g−1, Figure S4; yield = 79%). To further examine the versatility of this new approach, we sought to implement this solvent-free polymerization toward a wide variety of carbazolic building blocks (Scheme 1), for example, propeller-like scaffold, 1,3,5-tri(9-carbazolyl)-benzene (M6).28 As expected, all the obtained conjugated polycarbazoles show predominant porosities including high BET surface areas and total pore volume (Figure S5 and Table 2). To our delight, M6-derived network PCz-6 exhibits the largest BET surface area of 935 m2 g−1 among all new PCzs. The 13C CP/

Scheme 1. Mechanochemical Solid-State Synthesis of Nanoporous Conjugated Polycarbazoles

amorphous structure feature (Figure S2). Additionally, 0.27 wt % Fe derived from the catalyst was found to exist within the framework, as evidenced by the inductively coupled plasma atomic emission spectrometry (ICP-OES) analysis.26 Subsequently, we performed the N2 adsorption−desorption isotherms of PCz-1-A at 77 K to assess its porous nature (Figure 1). To our delight, the BET surface area calculated over a

Figure 1. N2 adsorption−desorption isotherms of PCz-1 series recorded at 77 K.

relative pressure is 707 m2 g−1 and the total pore volume is estimated to be 0.45 cm3 g−1 (Table 1). Significantly, this BET value is larger than that prepared through solvent-mediated process (SABET = 483 m2 g−1),24 suggesting a good feasibility of

Table 2. Structural Parameters of PCzs Prepared Through the Solid-State Assemble Approach

Table 1. Effects of Amounts of FeCl3 and Grinding Time on N2 Adsorption Property sample

FeCl3 (mass eq)

time (h)

yield (%)

SABET (m2 g−1)

Vtotal (cm3 g−1)

PCz-1-A PCz-1-B PCz-1-C PCz-1-D

2.5 1.25 5 2.5

0.5 0.5 0.5 0.25

83 52 94 84

707 723 678 394

0.45 0.45 0.42 0.27 1057

PCzs

yield (%)

SABET (m2 g−1)

Vtotal (cm3 g−1)

Vmicro (cm3 g−1)

PCz-1-A PCz-2 PCz-3 PCz-4 PCz-5 PCz-6

83 60 86 83 64 82

707 780 601 582 902 935

0.45 0.47 0.49 0.46 0.56 0.52

0.15 0.18 0.10 0.10 0.20 0.28

DOI: 10.1021/acsmacrolett.7b00480 ACS Macro Lett. 2017, 6, 1056−1059

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ACS Macro Letters

low adsorption values was calculated to be 27.1 kJ mol−1 (Figure S11), suggesting a strong physical interaction between the polarizable CO2 molecules and framework PCz-6.41 Pragmatically, this means the CO2 sorption isotherms of PCz-6 are effectively reversible (Figure 2), affording a lower regeneration cost compared with conventional amine solutions.35 In summary, a novel solid-state synthetic approach has been developed for the generation of conjugated nanoporous polymer networks. We demonstrated a rapid and solvent-free synthesis of polycarbazoles with high porosities and CO2 storage abilities through the application of mechanochemicalassisted oxidative coupling polymerization. This innovative approach constitutes a new direction for the development of nanoporous polymers through sustainable solid-state assembly pathways, and could open up new possibilities for the rational design and synthesis of nanoporous materials for carbon capture.

MAS NMR spectra confirms its chemical structure (Figure S6), which matches well with previously reported framework.28 Fe containing is measured to be 0.35 wt % from the ICP analysis. The X-ray photoelectron spectroscopy indicates a high Ndoping of 4.4 at. % on the surface of the framework. Pyridinic (398.9 eV, 18.4%) and pyrrolic nitrogen (400.3 eV, 81.6%) were the two different nitrogen species identified by XPS (Figure S7), which may serve as the binding sites for CO2 uptake through electrostatic interaction.34 The incorporation of more N-containing sites in CNPs has previously been demonstrated to improve CO2 adsorption capacities.35 We then evaluated the CO2 adsorption performance of PCzs. PCz-6 was used as a model adsorbent due to possessing the largest BET surface area and micropore volume as well as being inherently rich in N-doped CO2-philic sites. The CO2 uptake isotherms were carefully measured up to 1 bar at both 273 and 298 K. As shown in Figure 2, PCz-6 has a high CO2 storage

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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.7b00480. Experimental section, Figures S1−S12, and Table S1 (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Figure 2. CO2/N2 adsorption and desorption curves of PCz-6 at both 273 and 298 K (up to 1 bar).

Xiang Zhu: 0000-0002-3973-4998 Gabriel M. Veith: 0000-0002-5186-4461 Sheng Dai: 0000-0002-8046-3931

ability of 92.3 cm3 g−1 (4.1 mmol g−1). Though inferior to that of the best performing porous polymeric adsorbents,12,36−38 like conjugated triazine frameworks (HAT-CTF-450−600, 6.3 mmol g−1)35 and benzimidazole-linked porous polymer adsorbents (BILP-4, 5.34 mmol g−1),39 the capacity readily outperforms many other nanoporous polymer adsorbents mainly due to its permanent microporous nature (Figure S8, Table S1).40 Due to its lower BET surface area, the obtained CO2 uptake for PCz-6 is smaller than that of conventional carbazolic adsorbents (Table S1).23,25,28 N2 adsorption on PCz6 at 273 K was also recorded to examine the separation abilities of PCz-6 toward CO2 over N2. The CO2/N2 selectivity for PCz-6 was calculated to be ca. 11 at 273 K and 1 bar from the ratio of the initial slopes (Figure S9), which can be further supported by the Ideal Adsorbed Solution Theory (IAST) approach (Figure S10). Despite this, selective separation performance is inferior to that of conventional carbazolic adsorbents (like CPOP-129 and P-PCz-325), this newly developed solid-state polymerization methodology holds great promise for time-efficient, green and scalable synthesis of these new materials. At the more industrially relevant temperature of 298 K, PCz-6 still bears high CO2 storage ability, and 54.8 cm3 g−1 (2.44 mmol g−1) CO2 can be adsorbed. To provide a better understanding of the CO2 adsorption in PCz-6 framework, we have also calculated the isosteric heats of adsorption (Qst) by fitting the CO2 adsorption isotherms at 273 and 298 K and applying a variant of the Clausius−Clapeyron equation.28 Qst at

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was supported financially by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy. G.M.V., K.L.B., and R.L.S. were supported by the Department of Energy Office of Science, Division of Materials Sciences and Engineering.

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