The development of advanced porous carbon-based materials is a burgeoning field, especially for CO₂ capture and utilization. In this research, renewable, microporous granular nitrogen-doped carbons were prepared by KOH activation of melamine modified walnut shells making use of two different protocols at a relatively low pyrolysis temperature (650 °C). The microporous carbons had a 0.6–1.5 nm micropore size and exhibited high surface areas of 949–2847 m² g⁻¹. Furthermore, the synthesized microporous carbons manifested a good capacity to capture CO₂ owing to the existence of a large number of narrow micropores and nitrogen-bearing functionalities. The equilibrium CO₂ capture capacities of these carbons at 1 atm were respectively in ranges of 4.1–6.6 and 2.7–4.0 mmol g⁻¹ at 0 °C and 25 °C. Even in a CO₂/N₂ mixture, it exhibited a high selectivity for the capture of CO₂ with a separation factor of 19. Thanks to its excellent CO₂ adsorption capability, the incorporation of Ag(0) nanoparticles makes it serve as an efficient catalyst for the conversion of propargylic alcohols with CO₂ into carbonates via a cycloaddition reaction under mild conditions. The sorbent has a multitude of advantages including the ease of its synthesis, high CO₂ uptake capacity, low cost, regenerability, and selectivity, which demonstrate that great potential is manifested by nanoporous nitrogen-doped carbon for CO₂ capture and utilization.