Porous borocarbonitrides (denoted BCN) were prepared through pyrolysis of the polymer stemmed from dehydrocoupled ethane 1,2-diamineborane (BH3NH2CH2CH2NH2BH3, EDAB) in the presence of F-127. These materials contain interconnected pores in the nanometer range with a high specific surface area up to 511 m2 · g−1. Gas adsorption of CO2 demonstrated an interesting uptake (3.23 mmol · g−1 at 0 °C), a high CO2/N2 selectivity as well as a significant recyclability after several adsorption–desorption cycles. For comparison’s sake, a synthesized non-porous BCN as well as a commercial BN sample were studied to investigate the role of porosity and carbon doping factors in CO2 capture. The present work thus tends to demonstrate that the two-step synthesis of microporous BCN adsorbent materials from EDAB using a bottom-up approach (dehydrocoupling followed by pyrolysis at 1100 °C) is relatively simple and interesting.