Incorporation of nanoparticles has been considered as an efficient method for enhancing the adsorption performance of metal–organic frameworks (MOFs). Alkali metal compounds possess outstanding affinity to acidic CO₂. In this study, a robust self-conversion strategy is reported for improving the carbon capture performance of MOFs, through directly transforming partial metal centers to basic carbonate (BC) nanoparticles. Based on the hydrolysis of coordination bonds induced by water impurity in solvents and the decarboxylation of linkers under thermal and alkaline conditions, the self-loading of BC in MOFs can be realized by solvent vapor-assisted thermal treatment. Since water impurity causes limited self-conversion and excess organic solvent can purify MOFs, the BC-MOF materials maintain good crystallinity and even show superior porosity. Owing to the increased specific surface areas, open metal sites, and alkalinity of BC, the prepared MOF composites exhibit substantially improved CO₂ capture performance with good balance between capacity and selectivity. For example, after self-conversion with ethanol solvent, the CO₂ adsorption capacity and CO₂/N₂ (15 : 85) selectivity at 298 K and 100 kPa increase from 3.7 mmol g⁻¹ and 11.4 to 5.8 mmol g⁻¹ and 29.2, respectively.