Herein, three boranes are studied as potential precursors of boron nitride (BN) and their carbon dioxide adsorption capacity is evaluated. The selected boranes are ammonia borane (AB, NH3BH3), lithium amidoborane (LiAB, LiNH2BH3), and sodium amidoborane (NaAB, NaNH2BH3), and they are pyrolyzed at four different temperatures. All the characterization techniques show the formation of BN from the three boranes. However, LiAB and NaAB form hexagonal BN (h-BN) at a lower temperature in comparison with AB. The insertion of the alkali metal inside the BN structure is also evidenced. These materials show interesting results when they are exposed to a CO2 atmosphere. The materials are able to adsorb CO2 at nearly ambient conditions, and the best results are achieved with samples that do not present a crystalline structure. AB treated at 600 °C uptakes 34 cm3 g−1 (66.8 mg g−1) at 1.5 bar CO2 and 30 °C. Considering that the materials developed in this study are nonporous, the results are promising, opening new perspectives as potential sorbents for CO2.