Chemisorption of CO2 on diaminated silica as bicarbonates and different types of carbamate ammonium ion pairs

The chemisorption of CO₂ on aminated silica has a rich chemistry, and its details are important to research in relation to CO₂ capture and catalytic chemistry. In this study, such chemisorption was investigated on aminated and diaminated silica with ¹H and ¹³C solid state nuclear magnetic resonance (NMR) spectroscopy under dry and wet conditions. Fast magic-angle spinning (MAS) allowed us to obtain high resolution spectra. Porous silica was modified into a monoaminated version using (3-aminopropyl)triethoxysilane (APTS) and a diaminated one by using 3-(2-aminoethylamino)propyltriethoxysilane (AEAPTS). From the corresponding NMR spectra we could conclude that, under dry conditions, CO₂ chemisorbed as carbamic acid and carbamate ammonium ion pairs and gave similar spectra for both directly-excited ¹³C and under cross-polarization (CP) {¹H}¹³C MAS NMR. Under wet conditions, direct excitation and {¹H}¹³C CPMAS NMR showed that carbamate ammonium ion pairs formed along with bicarbonates (HCO₃⁻). For the wet conditions, the ¹³C and ¹H NMR spectra were especially well resolved, and we could detect two different types of carbamate ammonium ion pairs forming on the diaminated silica. We conclude that carbamate ammonium ion pairs and HCO₃⁻ moieties can be detected by {¹H}¹³C MAS NMR, at least qualitatively, in addition to the more time consuming direct excitation.