Characterization of a Mixture of CO₂ Adsorption Products in Hyperbranched Aminosilica Adsorbents by ¹³C Solid-State NMR

Hyperbranched amine polymers (HAS) grown from the mesoporous silica SBA-15 (hereafter “SBA-15–HAS”) exhibit large capacities for CO₂ adsorption. We have used static in situ and magic-angle spinning (MAS) ex situ ¹³C nuclear magnetic resonance (NMR) to examine the adsorption of CO₂ by SBA-15–HAS. ¹³C NMR distinguishes the signal of gas-phase ¹³CO₂ from that of the chemisorbed species. HAS polymers possess primary, secondary, and tertiary amines, leading to multiple chemisorption reaction outcomes, including carbamate (RnNCOO^–), carbamic acid (RnNCOOH), and bicarbonate (HCO₃^–) moieties. Carbamates and bicarbonate fall within a small ¹³C chemical shift range (162–166 ppm), and a mixture was observed including carbamic acid and carbamate, the former disappearing upon evacuation of the sample. By examining the ¹³C–¹⁴N dipolar coupling through low-field (<i>B</i>₀ = 3 T) ¹³C­{¹H} cross-polarization MAS NMR, carbamate is confirmed through splitting of the ¹³C resonance. A third species that is either bicarbonate or a second carbamate is evident from bimodal <i>T</i>₂ decay times of the ∼163 ppm peak, indicating the presence of two species comprising that single resonance. The mixture of products suggests that (1) the presence of amines and water leads to bicarbonate being present and/or (2) the multiple types of amine sites in HAS permit formation of chemically distinct carbamates