Synthesis of highly porous borazine-linked polymers and their application to H₂, CO₂, and CH₄ storage

Abstract

The synthesis of highly porous borazine-linked polymers (BLPs) and their gas uptakes are reported. BLPs exhibit high surface areas up to 2866 m² g⁻¹ and can store significant amounts of H₂ (1.93 wt%) and CO₂ (12.8 wt%) at 77 K and 273 K, respectively at 1.0 bar with respective isosteric heats of adsorption of 6.0 and 25.2 kJ mol⁻¹.

---

Recently there has been great interest in the design and synthesis of highly porous organic architectures due to their multifaceted potential use in applications that include storage, separation, conductivity, and catalysis.¹ The chemical composition, physical and textural properties are dictated during synthesis that allow for materials with enhanced properties relevant to their respective applications. With the exception of microcrystalline covalent-organic frameworks (COFs),^2,3 these polymeric materials are amorphous yet can possess considerable porosity and well-defined cavities which render them highly attractive especially in adsorptive gas storage.⁴ Such desirable traits are imparted into organic materials through the use of rigid building blocks that direct the growth of polymer networks without the aid of templating agents.^1–3 In addition to customized porosity, polymerization processes can lead to pore wall functionalization that significantly enhance gas uptake and selectivity as we have demonstrated recently for benzimidazole-linked polymers.⁵ Alternative methods for improved gas uptake (i.e.hydrogen) by porous architectures can also be accessed by the use of polarizable building units that increase hydrogen-framework interactions.⁶ Along this line, we sought after the inclusion of borazine (B₃N₃) as a functionalized and polarizable building block into porous organic polymers.⁷Borazine is isostructural to the boroxine units found in COFs prepared by boronic acid self-condensation reactions² and has been mainly used for the fabrication of BN-based ceramics or in organic optoelectronics.^8–10 However, up to date, the use of borazine for the preparation of porous polymers for gas storage remains fairly undeveloped.

We report herein on the synthesis and characterization of a new class of highly porous borazine-linked polymers and investigate their performance in gas (H₂, CO₂, CH₄) storage application under low pressure and cryogenic conditions. The synthesis of BLP-1(H) and BLP-12(H) was performed by thermolysis of 1,4-phenylenediamine and tetra-(4-aminophenyl)methane borane adducts in monoglyme to afford the corresponding polymers as white powders in good yields (Scheme 1).

Scheme 1 Synthesis of BLP-1(H) and BLP-12(H) from in situthermal decomposition of arylamine-borane adducts.

BLPs were isolated under an inert atmosphere followed by degassing at 120 °C/1.0 × 10⁻⁵ torr for 16 h. Scanning electron microscopy (SEM) on as-prepared materials revealed a homogeneous morphology of spherical particles ∼150 nm for BLP-1(H), and platelet particles ∼200 nm for BLP-12(H) indicating phase purity (Fig. S13–S14, ESI). Powder X-ray diffraction studies on as-prepared and activated materials indicated that both polymers are amorphous. The chemical composition and connectivity of the building units were investigated by spectral and analytical methods which included FT-IR, solid-state ¹¹B and ¹³C CP-MAS, and elemental analysis. The formation of the borazine ring was first established by FT-IR studies (Fig. S1–S8, ESI) which revealed a significant depletion of the amine bands around 3420 cm⁻¹ and the formation of characteristic bands consistent with borazine; 2560 cm⁻¹ (B–H), 1400 cm⁻¹ (B–N stretch).^7,11 The intensity of the B–H band which appears as a broad intense band at 2400 cm⁻¹ in the amine-borane adducts is significantly reduced upon borazine formation. We have also collected solid-state ¹¹B and ¹³C CP-MAS (Fig. S9–S12, ESI) to establish the connectivity and coordination number of boron and to verify the inclusion of intact building units into the framework of BLPs. The ¹¹B signals for both polymers appear as very broad peaks centered around 10.8 ppm for BLP-1(H) and 10.1 ppm for BLP-12(H). These measurements are consistent with reported values for borazine-containing polymers (0 to 40 ppm)¹² and are in sharp contrast to tetra-coordinated boron sites found in borane-amine adducts or cycloborazanes (0 to −45 ppm).¹³ Moreover, the ¹³C CP-MAS signals which appear as broad peaks in the aromatic range support the incorporation of intact phenyl and tetraphenylmethane cores into the networks of BLP-1(H) and BLP-12(H), respectively.

Given the fact that borazine is isostructural to the boroxine unit found in COFs such as COF-1 and COF-102, we anticipated BLPs to be thermally stable and highly porous. Therefore we subjected BLPs to thermogravimetric analysis and nitrogen porosity measurements. The TGA traces of the as-prepared materials undergo substantial weight loss, presumably due to pore evacuation of solvent molecules and unreacted adducts, upon heating under nitrogen atmosphere and remain stable up to 400 °C (Fig. S15–S16, ESI). This is a typical behavior of porous materials in general and has been reported for porous covalent frameworks.² Based on TGA studies and the thermal stability of BLPs we activated both materials by degassing at 120 °C and 1 × 10⁻⁵ torr for 16 h prior to nitrogen porosity measurements. The Type I nitrogen isotherms (Fig. 1) are consistent with permanent microporous materials that are characterized by a sharp uptake at P/P_o = 10⁻⁴ to 10⁻², while the final rise in the nitrogen uptake is due to condensation in intermolecular cavities created by the packing of BLP particles. The Brunauer-Emmett-Teller (BET) model was applied to each isotherm for P/P_o between 0.05 and 0.15 and resulted in surface areas of 1360 and 2244 m² g⁻¹ for BLP-1(H) and BLP-12(H), respectively. The Langmuir model (P/P_o = 0.05–0.30) gave surface area values of 1744 and 2866 m² g⁻¹. The surface area of BLP-1(H) is considerably higher than related covalent nets: COF-1 (711 m² g⁻¹),^2aCTF-1 (791 m² g⁻¹),^3a and halogenated BLPs (503–1364 m² g⁻¹),⁷ while the surface area of BLP-12(H) is among the highest for organic polymers. The pore volumes (Vp) calculated at P/P_o = 0.90 were 0.69 cm³ g⁻¹ for BLP-1(H) and 1.08 cm³ g⁻¹ for BLP-12(H). Pore size distributions were estimated using density functional theory (DFT) calculations which revealed narrow pore-size distributions centered at about 12.7 Å.

Fig. 1 Gas uptake isotherms for BLP-1(H) and BLP-12(H); adsorption (filled) and desorption (empty).

Once the porosity of BLPs had been established, we considered their performance in gas storage. Hydrogen is among the leading candidates for future use in automotive applications because of its abundance, renewability, and clean aspects.¹⁴ Physisorbed hydrogen storage is highly attractive because of the rapid uptake and release of hydrogen.¹⁵ However, the weak interactions between hydrogen molecules and pore walls necessitate the use of low temperatures and elevated pressure conditions to offset the usually low heat of adsorption. Therefore, developing new materials with enhanced storage properties to meet targets set by the US Department of Energy for 2015 (5.5 wt% hydrogen and 0.040 kg hydrogen/L) remains a considerable challenge.¹⁶ We collected hydrogen isotherms at 77 K and 87 K (Fig. 1) and calculated the hydrogen isosteric heats of adsorption (Q_st) using the virial method.¹⁷ The H₂ uptake for BLP-1(H) (1.33 wt%) is lower than that of BLP-12(H) (1.93 wt%) yet both are higher than our previously reported halogenated BLPs (0.68–1.30 wt%).⁷ Moreover, despite their amorphous nature, the performance of BLPs in hydrogen storage is very comparable with other organic polymers of similar surface areas. For example, under similar conditions the analogous crystalline COF-1^4a and CTF-1³ store 1.28 and 1.55 wt% of H₂, respectively, the hydrogen uptake by BLPs is also similar to those of purely organic polymers such as hypercrosslinked polymer networks synthesized by the self-condensation of bischloromethyl monomers (0.89–1.69 wt%),¹⁸nitrogen-linked nanoporous networks of aromatic rings (0.01–0.85 wt%),¹⁹ and polymers of intrinsic microporosity (PIMs) which recently have been reported to be among the best organic polymers for hydrogen uptake (0.74–1.83 wt%).²⁰ In contrast, activated carbon such as PICACTIF-SC, AX-21, and zeolite-templated show a noticeably higher uptake (1.90, 2.40, and 2.60 wt%, respectively) due to their ultrafine pores.²¹ The Q_st values for hydrogen at low coverage were found to be 6.8 kJ mol⁻¹ (BLP-1(H)) and 6.0 kJ mol⁻¹ (BLP-12(H)) and drop at higher loading to reach 5.5 and 4.8 kJ mol⁻¹, respectively (Fig. S27, S29, ESI). These Q_st values are similar to those reported for organic polymers such as POFs (5.8–8.3),^4g,h 2D COFs (6.0–7.0 kJ mol⁻¹),^4apolyimide networks (5.3–7.0 kJ mol⁻¹),^4e,f PPNs (5.5–7.6 kJ mol⁻¹),⁴ⁱ and BILP-1 (7.9 kJ mol⁻¹)⁵ and are much higher than values reported for 3D COFs: COF-102 (3.9 kJ mol⁻¹), COF-103 (4.4 kJ mol⁻¹), and the carbon-based porous aromatic framework PAF-1 (4.6 kJ mol⁻¹).^4c As expected, these Q_st values fall below those of the halogenated BLPs (7.1–7.5 kJ mol⁻¹)⁷ which is most likely due to the higher polarization level of the borazine rings in halogen decorated BLPs.

We have also investigated the performance of BLPs in low pressure CH₄ and CO₂ storage at 273 K and 298 K (Fig. 1). Interest in storing CH₄ stems from its potential in automotive applications due to its abundance and low carbon footprint, whereas CO₂ capture or separation from flue gases or coal-fired plants is highly attractive due to environmental and economical reasons.^22,23 The maximum CO₂ uptake at 273/298 K was 7.4/4.1 wt% (BLP-1(H)) and 12.8/7.4 wt% (BLP-12(H)) with Q_st values of 25.3 kJ mol⁻¹ and 25.2 kJ mol⁻¹, respectively. The uptake capacities of BLPs exceed values reported for other organic polymers including diimide based porous organic polymers (POPs) (9.1 and 6.6 wt%),²⁴ and unfunctionalized MOFs which include MIL-53 (9.6 wt%), IRMOF-1 (4.7 wt%), ZIF-100 (4.3 wt%) and MOF-177 (3.5 wt%).^22b,c However, CO₂ uptakes are lower than those of amine functionalized MOFs or organic polymers and molecules such as HKUST-1 (17.9 wt%), Cu–BTC (17.9 wt%) and BioMOF-11 (26.4 wt%),²⁵ and BILP-1 (18.8 wt%).⁵ At zero coverage, the Q_st values 25.3 kJ mol⁻¹ and 25.2 kJ mol⁻¹ are higher than the values reported for COFs,^4aimine-linked organic cages²⁶ or diimide polymers,^4e,f and lower than those of CO₂-selective MOFs⁸ or ZTFs¹⁰ which generally feature –NH₂ or –OH functionalized pores. The maximum uptake for CH₄ at 273 K was 13.7 cm³/g for BLP-1(H) and 18.1 cm³/g for BLP-12(H) (Fig. 1) with respective Q_st values of 16.7 kJ mol⁻¹ and 17.0 kJ mol⁻¹ at low coverage. A very recent study on the use of COFs illustrated their potential in methane uptake where heat of adsorption depended strongly on pore size and spanned a range of 8 to 25 kJ mol⁻¹, with higher values for narrower pores.²³ Both materials reported in this study have relatively similar pore apertures (∼12.7 Å) that resemble those of microporous 2D and 3D COFs.^4a The CH₄ storage capacity and isosteric heat of adsorption of BLPs are in line with those of COFs and organic polymers under similar conditions.^4a,27 Noteworthy are the higher CO₂ uptake and binding affinity over CH₄ and H₂ by BLPs which may arise due to a more favorable interaction between the polarizable CO₂ molecules and borazine rings.

In conclusion, we have demonstrated the synthesis of highly porous borazine-linked polymers by thermolysis of arylamine-borane adducts and investigated their performance under ambient pressure and cryogenic conditions. The hydrogen sorption experiments indicate that BLPs have high hydrogen storage capacities at low pressure and display somewhat higher hydrogen isosteric heat of adsorption; however, higher pressure conditions are required to fully explore the potential of BLPs in gas storage applications.

We are grateful to the U. S. Department of Energy, Office of Basic Energy Sciences (DE-SC0002576) for generous support of this project. H.M.E. acknowledges support of the Donors of the American Chemical Society Petroleum Research Fund ACS-PRF (48672-G5).

Notes and references

1. (a) N. B. McKeown and P. M. Budd, Macromolecules, 2010, 43, 5163; (b) J. X. Jiang and A. I. Cooper, Top. Curr. Chem., 2010, 293, 1.
2. (a) A. P. Côté, A. I. Benin, N. W. Ockwig, M. O'Keeffe, A. J. Matzger and O. M. Yaghi, Science, 2005, 310, 1166; (b) H. M. El-Kaderi, J. R. Hunt, J. L. Mendoza-Cortés, A. P. Côté, R. E. Taylor, M. O'Keeffe and O. M. Yaghi, Science, 2007, 316, 268; (c) A. P. Côté, H. M. El-Kaderi, H. Furukawa, J. R. Hunt and O. M. Yaghi, J. Am. Chem. Soc., 2007, 129, 12914.
3. (a) P. Kuhn, M. Antonietti and A. Thomas, Angew. Chem., Int. Ed., 2008, 47, 3450; (b) P. Kuhn, A. Forget, D. Su, A. Thomas and M. Antonietti, J. Am. Chem. Soc., 2008, 130, 13333.
4. (a) H. Furukawa and O. M. Yaghi, J. Am. Chem. Soc., 2009, 25, 8876; (b) B. S. Ghanem, M. Hashem, K. D. M. Harris, K. J. Msayib, M. Xu, P. M. Budd, N. Chaukura, D. Book, S. Tedds, A. Walton and N. B. McKeown, Macromolecules, 2010, 43, 5287; (c) T. Ben, H. Ren, S. Ma, D. Cao, J. Lan, X. Jing, W. Wang, J. Xu, F. Deng, J. M. Simmons, S. Qiu and G. Zhu, Angew. Chem., Int. Ed., 2009, 48, 9457; (d) R. M. Kassab, K. T. Jackson, O. M. El-Kadri and H. M. El-Kaderi, Res. Chem. Intermed., 2011, 37, 747; (e) Z. Wang, B. Zhang, H. Yu, L. Sun, C. Jiaob and E. Liua, Chem. Commun., 2010, 46, 7730; (f) O. K. Farha, A. M. Spokoyny, B. G. Hauser, Y.-S. Bae, S. E. Brown, R. Q. Snurr, C. A. Mirkin and J. T. Hupp, Chem. Mater., 2009, 21, 3033; (g) P. Pandey, A. P. Katsoulidis, I. Eryazici, Y. Wu, M. G. Kanatzidis and S. T. Nguyen, Chem. Mater., 2010, 22, 4974; (h) A. P. Katsoulidis and M. G. Kanatzidis, Chem. Mater., 2011, 23, 1818; (i) W. Lu, D. Yuan, D. Zhao, C. I. Schilling, O. Plietzsch, T. Muller, S. Bräse, J. Guenther, J. Blümel, R. Krishna, Z. Li and H.-C. Zhou, Chem. Mater., 2010, 22, 5964.
5. M. G. Rabbani and H. M. El-Kaderi, Chem. Mater., 2011, 23, 1650.
6. J. L. Belof, A. C. Stern, M. Eddaoudi and B. Space, J. Am. Chem. Soc., 2007, 129, 15202.
7. T. E. Reich, K. T. Jackson, S. Li, P. Jena and H. M. El-Kaderi, J. Mater. Chem., 2011, 21, 10629.
8. (a) V. Salles, S. Bernard, J. Li, A. Brioude, S. Chehaidi, S. Foucaud and P. Miele, Chem. Mater., 2009, 21, 2920; (b) S. Duperrier, S. Bernard, A. Calin, C. Sigala, R. Chiriac, P. Miele and C. Balan, Macromolecules, 2007, 40, 1028.
9. I. H. T. Sham, C.-C. Kwok, C.-M. Che and N. Zhu, Chem. Commun., 2005, 3547.
10. (a) A. Wakamiya, T. Ide and S. Yamaguchi, J. Am. Chem. Soc., 2005, 127, 14859; (b) S. Yamaguchi and A. Wakamiya, Pure Appl. Chem., 2006, 78, 1413.
11. (a) A. Kaldor and R. F. Porter, Inorg. Chem., 1971, 10, 775; (b) J. S. Li, C. R. Zhang, B. Li, F. Cao and S. Q. Wang, Inorg. Chim. Acta, 2011, 366, 173; (c) K. T. Moon, D. S. Min and D. P. Kim, J. Ind. Eng. Chem., 1997, 3, 288.
12. (a) C. Gervais and F. Babonneau, J. Organomet. Chem., 2002, 657, 75; (b) C. Gervais, E. Framery, C. Duriez, J. Maquet, M. Vaultier and F. Babonneau, J. Eur. Ceram. Soc., 2005, 25, 129.
13. (a) C. Gervais, F. Babonneau, J. Maquet, C. Bonhomme, D. Massiot, E. Framery and M. Vaultier, Magn. Reson. Chem., 1998, 36, 407; (b) J. Li, C. Zhang, B. Li, F. Cao and S. Wang, Eur. J. Inorg. Chem., 2010, 1763; (c) X. Chen, X. Bao, J.-C. Zhao and S. G. Shore, J. Am. Chem. Soc., 2011, 133, 14172.
14. L. Schlapbach and A. Zuttel, Nature, 2001, 414, 353.
15. (a) L. J. Murray, M. Dinca and J. R. Long, Chem. Soc. Rev., 2009, 38, 1294; (b) H.-C. Zhou, Chem. Commun., 2010, 46, 44; (c) J. Germain, J. M. J. Fréchet and F. Svec, Small, 2009, 5, 1098.
16. U.S. Department of Energy, Energy Efficiency and Renewable Energy. http://www1.eere.energy.gov/hydrogenandfuelcells/storage/current_technology.html (accessed Mar 25, 2011).
17. J. L. C. Rowsell and O. M. Yaghi, J. Am. Chem. Soc., 2006, 128, 1304.
18. C. D. Wood, B. Tan, A. Trewin, H. J. Niu, D. Bradshaw, M. J. Rosseinsky, Y. Z. Khimyak, N. L. Campbell, R. Kirk, E. Stockel and A. I. Cooper, Chem. Mater., 2007, 19, 2034.
19. J. Germain, F. Svec and J. M. J. Fréchet, Chem. Mater., 2008, 20, 7069.
20. B. S. Ghanem, M. Hashem, K. D. M. Harris, K. J. Msayib, M. C. Xu, P. M. Budd, N. Chaukura, D. Book, S. Tedds, A. Walton and N. B. McKeown, Macromolecules, 2010, 43, 5287.
21. (a) N. Texier-Mandoki, J. Dentzer, T. Piquero, S. Saadallah, P. David and C. Vix-Guterl, Carbon, 2004, 42, 2744; (b) Z. X. Yang, Y. D. Xia and R. Mokaya, J. Am. Chem. Soc., 2007, 129, 1673.
22. (a) D. M. D'Alessandro, B. Smit and J. R. Long, Angew. Chem., Int. Ed., 2010, 49, 2; (b) S. Q. Ma and H. C. Zhou, Chem. Commun., 2010, 46, 44; (c) S. Keskin, T. M. van Heest and D. S. Sholl, ChemSusChem, 2010, 3, 879.
23. J. L. J. Mendoza-Cortés, S. S. Han, H. Furukawa, O. M. Yaghi and W. A. Goddard, J. Phys. Chem. A, 2010, 114, 10824.
24. O. K. Farha, Y.-S. Bae, B. G. Hauser, A. M. Spokoyny, R. Q. Snurr, C. A. Mirkin and J. T. Hupp, Chem. Commun., 2010, 46, 1056.
25. J. An, S. J. Geib and N. L. Rosi, J. Am. Chem. Soc., 2010, 132, 38.
26. Y. Jin, B. A. Voss, A. Jin, H. Long, R. D. Noble and W. Zhang, J. Am. Chem. Soc., 2011, 133, 6650.
27. M. G. Schwab, A. Lennert, J. Pahnke, G. Jonschker, M. Koch, I. Senkovska, M. Rehahn and S. Kaskel, J. Mater. Chem., 2011, 21, 2131.

---

Footnote

† Electronic Supplementary Information (ESI) available: Experimental procedures, characterization methods, and gas sorption studies. See DOI: 10.1039/c1py00374g/

---