Synthesis of Benzobisoxazole-Linked Two-Dimensional Covalent

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Synthesis of Benzobisoxazole-Linked Two-Dimensional Covalent Organic Frameworks and Their Carbon Dioxide Capture Properties David A. Pyles, Jonathan W. Crowe, Luke A. Baldwin, and Psaras L. McGrier* Department of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio 43210, United States S Supporting Information *

ABSTRACT: Developing novel synthetic strategies to construct crystalline polymeric materials with excellent chemical stability and high carbon capture capacity has become a challenging process. Herein, we report the synthesis of two novel 2D benzobisoxazole-linked covalent organic frameworks (BBO−COFs) utilizing C3-symmetric formyl- and C2symmetric o-aminophenol-substituted molecular building blocks. The BBO−COFs exhibit excellent water stability, high surface areas, and great CO2 uptake capacities. This general synthetic method affords the opportunity to prepare ordered BBO-based polymeric materials for carbon capture, chemical sensing, and organic electronic applications. hydrogen-bonded imine, 2 5 − 2 7 and β-ketoenamine linkages,28,29 have lagged behind in gas adsorption performance on account of the limited synthetic options available to enhance their selectivity and chemical stability without compromising the surface area of the materials. Such features are greatly needed to make COFs useful for practical CCS applications. Polybenzoxazoles30 are a class of heterocyclic polymers that exhibit excellent thermal stability and superb chemical resistance. Recently, Yavuz and co-workers demonstrated that benzoxazole units can be incorporated into amorphous COPs and utilized for selective CO2 capture.31 However, the synthesis consists of a multistep process, which involves annealing a silylprotected prepolymer at 400 °C in its final step to form the polymer networks. It would be advantageous to design a synthetic protocol that would not only eliminate the need for multiple synthetic steps and high temperature annealing, but also permit the formation of ordered benzoxazole-based porous materials. Such investigations would be significant in advancing the chemical stability of crystalline polymeric systems. Herein, we report the cyanide-catalyzed32 synthesis of two 2D benzobisoxazole-linked (BBO) COFs utilizing C3-symmetric formyl- and C2-symmetric o-aminophenol-substituted molecular building blocks (Scheme 1). We demonstrate this method can be used to construct ordered BBO polymeric networks with high surface areas and excellent water stability. The BBO−COFs also display respectable CO2 uptake and selectivity performance. BBO−COF 1 and BBO−COF 2 were synthesized by first reacting 1,3,5-triformylbenzene (TFB) and 1,3,5-tris(4formylphenyl)benzene (TFPB), respectively, with 2,5-diami-

A

nthropogenic carbon dioxide (CO2) is a greenhouse gas that is believed to be responsible for global warming and rising sea levels.1 CO2 emissions that result from the worldwide usage of fossil fuels are predicted to surpass 45 billion tonnes by the year 2040.2 As a consequence, there has been an increased effort in developing efficient carbon capture and storage (CCS) technologies to help lower the amount of CO2 that is released into the atmosphere.3 For decades, aqueous amine solutions have been widely utilized to remove CO2 from flue gas streams and natural gas by chemically trapping CO2 as a carbamate. Although this process is efficient, high temperatures are typically required to release the CO2 and facilitate solvent regeneration, which often results in a large energy penalty and corrosion.4 Covalent organic frameworks (COFs), an advanced class of porous crystalline materials, have emerged as a possible solution to help address these issues on account of their permanent porosity, low densities, and high thermal stabilities.5−7 These features make COFs useful for applications related to gas storage/separation,8,9 catalysis,10,11 optoelectronics,12−15 and energy storage.16 Since COFs are typically constructed by the diligent selection of various rigid building blocks, favorable binding sites can be incorporated into the pore wall of the materials to enhance their selectivity and uptake capacity for CO2. For example, it has been shown that the incorporation of amino groups into the pore wall of amorphous covalent organic polymers (COPs),17 porous organic polymers (POPs),18−20 porous polymer networks (PPNs),21 porous polymer frameworks (PPFs),22 and porous organic frameworks (POFs)23 has a profound impact on CO2 uptake and selectivity by enhancing their Lewis acid/base interactions. While these interactions have led to high CO2 uptake capacities for amorphous polymeric systems, many crystalline two-dimensional (2D) and three-dimensional (3D)24 COFs, with the exception of some containing © XXXX American Chemical Society

Received: June 23, 2016 Accepted: September 1, 2016

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DOI: 10.1021/acsmacrolett.6b00486 ACS Macro Lett. 2016, 5, 1055−1058

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ACS Macro Letters Scheme 1. Synthesis of BBO−COFs 1 and 2

Figure 1. Nitrogen isotherms at 77 K for BBO−COF 1 (a) and BBO− COF 2 (b) and NLDFT pore size distributions of BBO−COF 1 (c) and BBO−COF 2 (d).

rapid uptake at very low relative pressure (P/P0 < 0.01), which is indicative of a microporous material. Applying the Brunauer− Emmett−Teller (BET) model over the low pressure region (0.01 < P/P0 < 0.16) of the isotherm provided a surface area of 891 m2 g−1. The total pore volume for BBO−COF 1 calculated at P/P0 = 0.889 was 0.42 cm3 g−1. BBO−COF 2 also exhibited a type I isotherm. Employing the BET model over the lower pressure (0.01 < P/P0 < 0.18) range afforded a surface area of 1106 m2 g−1, while the total pore volume calculated at P/P0 = 0.994 provided a value of 0.55 cm3 g−1. It should be noted that the surface areas of BBO−COFs 1 and 2 are higher than the amorphous polymer networks COP-93 and COP-94,31 which also contain benzoxazole linkages. In addition, boiling both BBO−COFs in water for 24 h had minimal effect on their surface areas, which highlights their excellent water stability (Figure S25, SI). Nonlocal density functional theory (NLDFT) was used to approximate the pore size distributions of BBO− COFs 1 and 2 employing a cylindrical pore-oxide surface model, which yielded average pores sizes of 1.34 and 1.84 nm, respectively. We believe that the observed pore sizes of BBO− COFs 1 and 2 are slightly lower than the predicted values of 1.8 and 3.3 nm, respectively, on account of significant offsets between the stacking layers.34,35 This could explain why BBO− COF 2, in particular, does not exhibit an isotherm that is characteristic of a mesoporous material (Figure 1b). The crystallinity of BBO−COFs 1 and 2 were evaluated using powder X-ray diffraction (PXRD). Taking the difference in the observed pore size distributions into consideration, we first modeled BBO−COFs 1 and 2 using a P6 hexagonal unit cell in which the stacking layers were offset by 5 and 15 Å, respectively (Figure 2). The crystal structures were simulated using the Reflex module of the Materials Studio 7.0 software. BBO−COF 1 displays an intense peak at 5.29 followed by smaller peaks at 8.91 and 10.29, which correspond to the (100), (110), and (200) planes, respectively. Pawley refinement of the experimental PXRD data provided unit cell parameters of a = b = 19.821 Å and c = 3.4 Å (residuals Rp = 2.25%, Rwp = 3.33%). In comparison, BBO−COF 2 exhibited an intense peak at 2.89 followed by smaller peaks at 4.95 and 5.7, which also correspond to the (100), (110), and (200) planes, respectively. The unit cell was also refined to afford parameters of a = b = 35.796 Å and c = 3.4 Å (residuals Rp = 2.45%, Rwp = 3.31%). Both of the simulated patterns were in good agreement with the experimental data. However, we also considered eclipsed bnn (P6/mmm) and staggered gra (P63/mmc) stacking layers

no-1,4-benzenediol dihydrochloride (DABD) in DMF for 3 h at −15 °C to ensure slow formation of the phenolic iminelinked intermediate and avoid rapid precipitation. Afterward, the mixture was slowly brought to room temperature overnight before adding 1 equiv. of sodium cyanide dissolved in 0.2 mL of methanol to the mixture. The solution was then stirred under air at 130 °C for 4 days. BBO−COFs 1 and 2 were obtained by filtration and washed with acetone to yield light brown crystalline solids that were insoluble in common organic solvents. The BBO−COFs were purified by soaking them in methanol and acetone over a 48 h period to remove unreacted monomers and then dried under vacuum. The formation of the BBO linkage is believed to proceed through the following three-step mechanism: (1) the formation of a phenolic iminelinked intermediate, (2) subsequent addition of cyanide to the imine to induce ring closure and formation of a benzoxazoline intermediate, and finally (3) aerobic oxidation of the benzoxazoline intermediate under air to promote the formation of the BBO linkage.32 BBO−COFs 1 and 2 were characterized by Fourier transform infrared (FT-IR) and 13C cross-polarization magic angle spinning (CP-MAS) spectroscopies. The FT-IR spectra of BBO−COF 1 revealed stretching modes at 1651 (CN) and 1118 cm−1 (C−O), while BBO−COF 2 exhibited stretching modes at 1642 and 1117 cm−1, which are both indicative of the formation of the benzoxazole ring (Figures S1 and S3, Supporting Information (SI)). The formation of the BBO linkage was further confirmed by solid-state 13C CP-MAS NMR displaying resonances at 161.0, 147.9, and 140.0 ppm for BBO−COF 1. BBO−COF 2 displayed similar resonances at 164.4 and 141.1 ppm (Figure S12, SI). Thermogravimetric analysis (TGA) indicated that BBO−COF 1 maintained ∼80% of its weight up to 350 °C, whereas BBO−COF 2 retained ∼95% of its weight up to the same temperature (Figures S13 and S14, SI). Scanning electron microscopy (SEM) images revealed a uniform morphology for both materials (Figures S23 and S24, SI). The permanent porosities of BBO−COFs 1 and 2 were evaluated using nitrogen gas adsorption isotherms at 77 K (Figure 1). BBO−COF 1 exhibits a type I isotherm displaying a 1056

DOI: 10.1021/acsmacrolett.6b00486 ACS Macro Lett. 2016, 5, 1055−1058

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ACS Macro Letters

Figure 3. CO2 (a, b) and N2 (c, d) adsorption (filled) and desorption (open) isotherms for BBO−COF 1 (top) and BBO−COF 2 (bottom). All isotherms were measured at 273 and 295 K.

mg g−1 for N2 at 273 and 295 K, respectively. The higher CO2 uptakes for BBO−COF 1 could be attributed to the pore wall containing a higher wt % of nitrogen heteroatoms (see elemental analysis, SI, p S4), which can lead to additional Lewis acid/base interactions at low pressure.19 The CO2 isotherms for BBO−COFs 1 and 2 are completely reversible and exhibit very little hysteresis. These features could make BBO−COFs attractive candidates for practical CCS applications, especially if they can be post-synthetically modified with additional Lewis basic sites to improve their CO2 uptake capacities at low pressure.26,27 The CO2/N2 selectivities for BBO−COFs 1 and 2 were calculated using initial slope calculations18 in the pressure range of 0−0.1 bar to provide values of 35.5 and 30.5 at 273 K and 35 and 31 at 295 K, respectively (Figures S19−S22, SI). The isoteric heats of adsorption (Qst) were calculated using the virial method to assess the binding affinity of CO2 to the materials. BBO−COFs 1 and 2 exhibited Qst values of 30.2 and 27.8 kJ mol−1, respectively, at zero coverage (Figures S17 and S18, SI). These values are comparable to other COFs containing Lewis basic nitrogen- or oxygen-rich heteroatoms.36 The Qst values further demonstrate that the incorporation of BBO units into the pore wall of COFs can significantly enhance their CO2 binding affinity. In conclusion, we have demonstrated the first example of utilizing cyanide as a catalyst to construct ordered 2D BBObased COFs. The BBO−COFs exhibit excellent water stability and great CO2 uptake capacities. In addition to their potential usage in generating high performance CCS scaffolds, we also believe that this general synthetic method could be useful for developing BBO−COFs for sensory37 and organic electronic38 applications. These research endeavors are currently being pursued in our laboratory.

Figure 2. Indexed experimental (blue) and Pawley refined (red) patterns of BBO−COF 1 (top) and BBO−COF 2 (bottom) compared to the simulated hexagonal unit cell (green) with offsets of 5 and 15 Å, respectively.

for both BBO−COF 1 and BBO−COF 2 (Figures S7, S8, S10, and S11, SI). Surprisingly, the bnn simulated structures were almost identical to the offset models. Since the experimental pore size distributions for BBO−COF 1 and BBO−COF 2 deviate from their theoretical values, we believe that the models in which the adjacent layers are slipped by 5 and 15 Å of their unit cells, respectively, provides a more accurate depiction of their solid-state packing. The reason for these significant offsets is unclear at the moment, but further optimization to help alleviate this issue is currently ongoing. The simulated gra models did not match the experimental data. We believe that the formation of the ordered BBO networks is attributed to the van der Waals interactions between the π-systems of the adjacent TFB and TFPB organic linkers, which is significant enough to stabilize the stacking layers upon formation of the imine-linked intermediate and BBO linkage.33 In order to evaluate the influence of the BBO linkage on the uptake and selectivity of CO2 over N2, we measured gas adsorption isotherms at 273 and 295 K from 0 to 1.2 bar (Figure 3). BBO−COF 1 exhibited rapid CO2 uptake at low pressures reaching capacities of 150.6 mg g−1 at 273 K and 91.9 mg g−1 at 295 K. In comparison, the N2 uptake values were considerably lower at 273 and 295 K reaching capacities of 5.12 and 2.90 mg g−1, respectively. BBO−COF 2 exhibited uptake capacities of 112.3 and 69.9 mg g−1 for CO2, and 5.03 and 3.06

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00486. Synthetic procedures, FT-IR, solid-state 13C NMR, TGA PXRD, and SEM (PDF). 1057

DOI: 10.1021/acsmacrolett.6b00486 ACS Macro Lett. 2016, 5, 1055−1058

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

P.L.M. acknowledges the National Science Foundation (NSF) and Georgia Tech Facilitating Academic Careers in Engineering and Science (GT-FACES) for a Career Initiation Grant and funding from The American Chemical Society Petroleum Research Fund (55562-DNI7) and The Ohio State University.

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DOI: 10.1021/acsmacrolett.6b00486 ACS Macro Lett. 2016, 5, 1055−1058