Ammonia borane (AB) has been extensively studied as a solid-state hydrogen storage material. On the other hand, its reactivity with CO2 is still unclear, especially in the solid state. By carefully controlling the CO2 pressure and temperature, AB efficiently reduces a large amount of CO2 without solvent or catalyst. 40 mmol of CO2 reacts with one mole of AB at 0.5 MPa and 60 degrees C. The mechanism was investigated by NMR and DFT calculation. The reaction proceeds through the formation of diammoniate of diborane (DADB) as an intermediate, followed by the reduction and fixation of CO2 with BH4- to give triformatoborohydride ([HB(OCHO)(3)](-)). Aldehyde is then transferred from B to N, yielding formamide as the main final product. The N-formylation of secondary amine can also be achieved without solvent. Finally, the pyrolysis of the product between AB and CO2 produces N-doped amorphous carbon, opening the door to new clean CO2 valorisation pathways.