Ultramicroporous carbon materials are realized by exploiting nitrogen-doping and alkali ions activation strategies on classical carboxylic phenolic resins. Through tuning the nitrogen-doped content as well as activation strength and size of the alkali ions, carbon materials present pore sizes that are controlled at the sub-Angstrom level from 5.8 to 7.7 angstrom with precision between 0.1 and 0.4 angstrom. The fine-tuning of the ultramicropore size using melamine as the sacrificial templating agent improves the practicality of the carbon materials for the recovery of SF6 which are larger than conventional molecules such as CO2. Resultantly, the optimized carbon material at 1 bar and 25 degrees C saw a > 16-fold increase in SF6 adsorption from 0.39 to 6.55 mmol g(-1) while the SF6/N-2 selectivity reached as high as 78.8. We attribute this to the fine-tuning of ultramicropores, leading to optimizable pore sizes that allow easy access to the large surface areas of carbon materials by the SF6 molecules. Furthermore, owing to its high adsorption performance, a 1.5-fold higher breakthrough SF6 adsorption capacity alongside a 2-fold increase in SF6/N-2 selectivity were realized under dynamic breakthrough conditions. The stability and recyclability of the carbon materials were also demonstrated with a long-term performance evaluation of up to 10 consecutive adsorption-desorption cycles. (C) 2019 Elsevier Ltd. All rights reserved.