Facile synthesis of flower-like carbon microspheres for carbon dioxide capture

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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 capacity, attractive CO2/N2 selectivity, and ease of regeneration. Here, we report a facile and scalable strategy to synthesize carbon microspheres (∼6 μm) bearing a unique, hierarchically structured flower-like morphology. These flowery carbons have specific surface areas as large as 2064 m2/g and total pore volumes ranging as high as 1 cm3/g, achieved through activation by CO2 at 900 °C. Detailed study of the textural properties and surface chemistry of these materials as a function of the activation time, primarily, and temperature, secondarily, reveals that CO2 capacity is strongly correlated to the volume of the ultra-micropores (diameters <1 nm) rather than to surface area, total pore volume, or specific N or O functionality. Ultra-micropore volume increases as a function of activation until devolving into larger micro- and meso-pores, with the optimal materials in this study showing high CO2 uptake at 1 bar spanning 4.5 mmol/g (25 °C) to 6.44 mmol/g (0 °C) and competitive CO2/N2 selectivity, as estimated by the Ideal Adsorbed Solution Theory (IAST), without any specific heteroatom doping. These carbons, derived from the simple processing of cheap precursors, also offer low isosteric heats of adsorption (20–30 kJ/mol), thereby, enabling facile temperature and pressure-swing-based cyclic regeneration. Taken together with their hierarchical structure, these materials show promise as efficient and cost-effective CO2 sorbents.