Direct access to primary amines and particle morphology control in nanoporous CO2 sorbents

Introduction-Chemical tuning of nanoporous, solid sorbents for an ideal CO2 binding requires unhindered amine functional groups on the pore walls. Although common for soluble organics, post-synthetic reduction of nitriles in... [ view full abstract ]

Introduction-Chemical tuning of nanoporous, solid sorbents for an ideal CO₂ binding requires unhindered amine functional groups on the pore walls. Although common for soluble organics, post-synthetic reduction of nitriles in porous networks often fail due to the insufficient and irreversible metal hydride penetration. Here, we synthesized a nanoporous network with pendant nitrile groups, microsphere morphology and in large scale. The hollow microspheres were easily decorated with primary amines through in situ reduction by widely available boranes. CO₂ capture capacity of the modified sorbent was increased up to four times of the starting nanoporous network with a high heat of adsorption (98 kJ/mol). Surface area can be easily tuned between 1 and 354 m²/g. Average particle size (~50 µm) is also quite suitable for CO₂ capture applications where processes like fluidized bed require spheres of micron sizes.

Methods-Polymer beads were synthesized by suspension polymerization. The solution containing monomers and porogen was sonicated and the mixture was introduced to the aqueous phase. The polymer is referred to as Covalent Organic Polymer 122 (COP-122). Nitrile functional groups were reduced by 1.0 M BH₃ in THF under N₂ atmosphere, resulting in complete conversion which is indexed as (COP-122-G1).

Results-Chemisorption of CO₂ by amine groups is evident in COP-122-G1 from the hysteretic isotherm that rapidly increases at low partial pressures. The original COP-122 is mainly a physisorptive solid, hence low capacity with negligible hysteresis. Conversion of the nitrile groups clearly increases the binding, leading to a dominant chemisorptive mode.

Discussion-In summary, acrylonitrile-divinylbenzene copolymers with nanoporous nature have been designed and synthesized for effective and cheap post-combustion CO₂ capture. Surface area and pore volumes can be altered by employing different types of porogens, while highest observed surface area was 354 m²/g. The hollow core inside of each single particle shows potential for acting as a carrier for various cargos, such as drugs. Amine functionalization of COP-122 particles make them promising candidates for CO₂ capture operations. Furthermore, this material is easy to handle because of its large particle size and non-corrosive nature, providing low attrition potential.