Selective CO₂ Sequestration with Monolithic Bimodal Micro/Macroporous Carbon Aerogels Derived from Stepwise Pyrolytic Decomposition of Polyamide-Polyimide-Polyurea Random Copolymers

Abstract

Polymeric aerogels (PA-xx) were synthesized via room-temperature reaction of an aromatic triisocyanate (tris(4-isocyanatophenyl) methane) with pyromellitic acid. Using solid-state CPMAS ¹³C and ¹⁵N NMR, it was found that the skeletal framework of PA-xx was a statistical copolymer of polyamide, polyurea, polyimide, and of the primary condensation product of the two reactants, a carbamic-anhydride adduct. Stepwise pyrolytic decomposition of those components yielded carbon aerogels with both open and closed microporosity. The open micropore surface area increased from <15 m² g^-1 in PA-xx to 340 m² g^-1 in the carbons. Next, reactive etching at 1,000 °C with CO₂ opened access to the closed pores and the micropore area increased by almost 4× to 1150 m² g^-1 (out of 1750 m² g^-1 of a total BET surface area). At 0 °C, etched carbon aerogels demonstrated a good balance of adsorption capacity for CO₂ (up to 4.9 mmol g^-1), and selectivity toward other gases (via Henry's law). The selectivity for CO₂ versus H₂ (up to 928:1) is suitable for precombustion fuel purification. Relevant to postcombustion CO₂ capture and sequestration (CCS), the selectivity for CO₂ versus N₂ was in the 17:1 to 31:1 range. In addition to typical factors involved in gas sorption (kinetic diameters, quadrupole moments and polarizabilities of the adsorbates), it is also suggested that CO₂ is preferentially engaged by surface pyridinic and pyridonic N on carbon (identified with XPS) in an energy-neutral surface reaction. Relatively high uptake of CH₄ (2.16 mmol g^-1 at 0 °C/1 bar) was attributed to its low polarizability, and that finding paves the way for further studies on adsorption of higher (i.e., more polarizable) hydrocarbons. Overall, high CO₂ selectivities, in combination with attractive CO₂ adsorption capacities, low monomer cost, and the innate physicochemical stability of carbon render the materials of this study reasonable candidates for further practical consideration.