Heteroatom-doped porous carbon microspheres with ultramicropores for efficient CH₄/N₂ separation with ultra-high CH₄ uptake

The separation of methane from the mixture of methane/nitrogen is an imperative and challenging process, which mitigates global warming and fully utilizes coalmine methane. Herein, a range of porous carbon microspheres (PCMs) were prepared through carbonization of a polymer named poly(cyclophosphazene-co-4,4′-sulfonyldiphenol (PZS) at different temperatures. The pore size distribution for the PCMs ranged from 0.3 to 0.6 nm. Among them, PZS-900 (the carbonization temperature is 900 °C) exhibits excellent CH₄ capture performance with a CH₄ uptake of 42.22 cm g⁻¹ at 298 K and 100 kPa, which exceeds all the porous carbon materials. Importantly, PZS-900 also displays high CH₄/N₂ selectivity (4.64) because of low N₂ uptake (14.88 cm g⁻¹). Such CH₄/N₂ separation performance was further confirmed by dynamic breakthrough experiments. The mechanism for selective CH₄/N₂ separation was clarified through Raman spectroscopy and contact angle experiments. The difference in the adsorption of CH₄ and N₂ may be due to influence of polarizability in the carbon material resulting in different van der Waals force interactions. Also, the hydrophilicity of carbon materials contributed to the improved performance of CH₄/N₂ separation. Overall, this research demonstrates that the serials of PCMs have a significant potential for the efficient capture of methane from coal bed gas in practice.