Spherical nitrogen-containing polymer and microporous carbon materials have been synthesized by using hexamethylenetetramine as nitrogen source and one of the carbon precursors under solvothermal conditions, without using any surfactant or toxic reagent such as formaldehyde. The synthesis strategy is user-friendly, cost-effective, and can be easily scaled up for production. The microporous carbon spheres exhibit high surface areas of 528–936 m² g⁻¹ with a micropore size of 0.6–1.3 nm. The synthesized microporous carbons show a good capacity to store CO₂, which is mainly due to the presence of nitrogen-containing groups and a large amount of narrow micropores (<1.0 nm). At 1 atm, the equilibrium CO₂ capture capacities of the obtained microporous carbons are in the range of 3.9–5.6 mmol g⁻¹ at 0 °C and 2.7–4.0 mmol g⁻¹ at 25 °C.