(660g) Hierarchical, Flower-like Carbon Microspheres for CO2 Capture

Among classes of porous sorbents with promise for use in mitigating carbon dioxide (CO₂) emissions, carbons remain attractive due to their economical synthesis, high CO₂ uptake/selectivity, and easy regeneration. Here, we report a facile and scalable imidization-induced precipitation route to intrinsically nitrogen-doped carbon microspheres (~6 µm) bearing a unique, hierarchically structured flower-like morphology. Simple tuning of solution precursor composition offers a handle for tailoring morphology and mesostructure among core (dense)/shell (petal) structures, petaled lobes, and extended petal structures, bearing a direct functional influence on molecular accessibility of the thin (ca. 10-20 nm) highly anisotropic microporous petals. The resulting carbons have tunable specific surface area spanning 750-2064 m²/g and total pore volumes ranging from 0.38-1 cm³/g, achieved through activation by CO₂ at 800-900 °C. Detailed study of the textural properties and surface chemistry of these materials as a function of activation temperature and duration reveals that CO₂ uptake is strongly correlated to the volume of the narrowest micropores (diameters < 1 nm) rather than to specific nitrogen content. Ultramicropore volume, and thus CO₂ uptake, increases as a function of activation until devolving into larger micro- and meso-pores, with the optimal materials in this study yielding high CO₂ uptake at 1 bar of 6.44 mmol/g at 0 °C and 4.5 mmol/g at 25 °C. These intrinsically nitrogen-doped carbons, derived from the simple processing of cheap precursors show competitive CO₂/N₂ selectivity as estimated by the Ideal Adsorbed Solution Theory (IAST), fast uptake kinetics, low isosteric heats of adsorption (20-30 kJ/mol), and, thereby, easy regeneration by both temperature and pressure-swing. Ultimately, these materials show promise as efficient and cost-effective CO₂ sorbents and, given their tunable pore and morphological hierarchy, strong promise for efficient and selective processing of bulky molecules.