Spectroscopic Characterization of Adsorbed ¹³CO₂ on 3-Aminopropylsilyl-Modified SBA15 Mesoporous Silica

Multiple chemisorption products are found from the interaction of CO₂ with the solid-amine sorbent, 3-aminopropyl silane (APS), bound to mesoporous silica (SBA15) using solid-state NMR and FTIR spectroscopy. We employed a combination of both ¹⁵N{¹³C} rotational-echo double-resonance (REDOR) NMR and ¹³C{¹⁵N} REDOR to determine the chemical identity of these products. ¹⁵N{¹³C} REDOR measurements are consistent with a single ¹³C-¹⁵N pair and distance of 1.45 Å. In contrast, both ¹³C{¹⁵N} REDOR and ¹³C CPMAS are consistent with multiple ¹³C products. ¹³C CPMAS shows two neighboring resonances, whose chemical shifts are consistent with carbamate (at 165 ppm) and carbamic acid. The ¹³C{¹⁵N} REDOR experiments resonant at 165 ppm show an incomplete buildup of the REDOR data to ∼90% of the expected maximum. We conclude this 10% missing intensity corresponds to a ¹³C NMR species that resonates at the identical chemical shift but that is not in dipolar contact with ¹⁵N. These data are consistent with the presence of bicarbonate, HCO₃^-, since it is commonly observed at ∼165 ppm and lacks ¹⁵N for dipolar coupling.