Efficient hydrogen/carbon dioxide separation by magnifying their diffusion difference in carbon molecular sieve membranes

Membranes with ultramicropores have shown great promise in gas separations. However, achieving high separation selectivity for gas with similar kinetic diameters, such as hydrogen (H₂) and carbon dioxide (CO₂), remains a challenge due to the difficulty in precisely controlling ultramicropores distribution. Herein, we developed a polybenzoxazine polymer-derived carbon molecular sieve (CMS) membrane with uniformly concentrated ultramicropores of 3.5 Å to separate H₂ and CO₂. This membrane demonstrated a high selectivity of 67.6, with a H₂ permeance of 118.7 GPU, outperforming most reported samples. The isosteric heat of adsorption (<em>Q</em>_st) for CO₂ in these ultramicropores reached 43.1 kJ mol^-1, approximately twice that of physical adsorption, indicating that CO₂ were effectively confined within the concentrated ultramicropores. Permeation tests revealed that the activation energy for CO₂ permeation in the ultramicropores was 11.3 kJ mol^-1, one order of magnitude higher than that for H₂ (1.3 kJ mol^-1). This significant difference in activation energy magnifies the difference of diffusion rate between H₂ and CO₂. The distinct behavior between the free surface diffusion of H₂ and the activated surface diffusion of CO₂ is the key to achieving a high H₂/CO₂ separation performance. This discovery presents a promising approach for separating H₂ and CO₂ using CMS membranes.