Hybrid aminopolymer-silica materials for efficient CO₂ adsorption

The present work focuses on the development of a new eco-efficient chemical method for the polymerization of aziridine to hyperbranched polyethyleneimine (PEI) into mesoporous silica by using compressed CO₂ as a solvent, reaction medium and catalyst. PEI was in situ grafted into MCM-41 and silica gel substrates, with pore diameters of 3.8 and 9.0 nm, respectively. The optimal polymerization conditions were found by varying the reaction pressure (1.0-10 MPa), temperature (25-45°C) and time (20-400 min). The thermal stability analysis indicated that aminopolymer chains were covalently attached on the amorphous silica surface. The described compressed CO₂ route for the synthesis of high amine content hybrid products (6-8 mmol_N g^-1) is a very fast method, with processing times in the order of few minutes even at very low working pressures (1.0 MPa), being a step forward in the design of efficient hybrid aminopolymer nanocomposites for CO₂ capture. The adsorptive behavior of the prepared hybrid materials was experimentally established by recording the N₂ (-196°C) and CO₂ (25, 50 and 75°C) adsorption isotherms. Results were compared to molecular simulation studies performed using Grand Canonical Monte Carlo for either N₂ or CO₂ adsorbed on amino modified MCM-41, thus helping to elucidate the predominant PEI configuration present in the functionalized materials.