Role of Amine Functionality for CO₂ Chemisorption on Silica

The mechanism of CO₂ adsorption on primary, secondary, and bibasic aminosilanes synthetically functionalized in porous SiO₂ was qualitatively and quantitatively investigated by a combination of IR spectroscopy, thermogravimetry, and quantum mechanical modeling. The mode of CO₂ adsorption depends particularly on the nature of the amine group and the spacing between the aminosilanes. Primary amines bonded CO₂ preferentially through the formation of intermolecular ammonium carbamates, whereas CO₂ was predominantly stabilized as carbamic acid, when interacting with secondary amines. Ammonium carbamate formation requires the transfer of the carbamic acid proton to a second primary amine group to form the ammonium ion and hence two (primary) amine groups are required to bind one CO₂ molecule. The higher base strength of secondary amines enables the stabilization of carbamic acid, which is thereby hindered to interact further with nearby amine functions, because their association with Si–OH groups (either protonation or hydrogen bonding) does not allow further stabilization of carbamic acid as carbamate. Steric hindrance of the formation of intermolecular ammonium carbamates leads to higher uptake capacities for secondary amines functionalized in porous SiO₂ at higher amine densities. In aminosilanes possessing a primary and a secondary amine group, the secondary amine group tends to be protonated by Si–OH groups and therefore does not substantially interact with CO<sub>2