Identification of the Basic Sites on Nitrogen-Substituted Microporous and Mesoporous Silicate Frameworks Using CO2 as a Probe Molecule

Carbon dioxide was shown to identify surface basic properties of nitrogen-substituted microporous and mesoporous silicas, in addition to conventional basic oxides, by a detailed study using isotherm and heat of adsorption measurements as well as by infrared spectroscopy. A hydrogen-bonded weak interaction was primarily observed between CO2 and silanol (Si–OH) and silamine (Si–NH–Si) groups. The heat of adsorption of CO2 demonstrated that the latter adspecies were formed preferentially over the former, although a much higher amount of linear CO2 adspecies were found on SBA-15 mesoporous silica because of the presence of a large quantity of silanol groups on its surface. Carbamate-type chemisorbed adspecies were not detected on silamino sites, whereas carbonate-type adspecies were formed on alkali ion-exchanged zeolites and also residual sodium ions on the surface of silicalite-1. CO2 was shown to be a successful probe molecule for identifying weakly interactive hydrogen-bonding sites, and it has potential ...