Structure of Chemisorbed CO₂ Species in Amine-Functionalized Mesoporous Silicas Studied by Solid-State NMR and Computer Modeling

Two-dimensional (2D) solid-state nuclear magnetic resonance (SSNMR) experiments on samples loaded with ¹³C-labeled CO₂, “<i>under controlled partial pressures</i>”, have been performed in this work, revealing unprecedented structural details about the formation of CO₂ adducts from its reaction with various amine-functionalized SBA-15 containing amines having distinct steric hindrances (e.g., primary, secondary) and similar loadings. Three <i>chemisorbed</i> CO₂ species were identified by NMR from distinct carbonyl environments resonating at δ_C ≈ 153, 160, and 164 ppm. The newly reported chemisorbed CO₂ species at δ_C ≈ 153 ppm was found to be extremely moisture dependent. A comprehensive ¹H-based SSNMR study [1D ¹H and 2D ¹H–X heteronuclear correlation (HETCOR, X = ¹³C, ²⁹Si) experiments] was performed on samples subjected to different treatments. It was found that all chemisorbed CO₂ species are involved in hydrogen bonds (HBs) with either surface silanols or neighboring alkylamines. ¹H chemical shifts up to 11.8 ppm revealed that certain chemisorbed CO₂ species are engaged in very strong HBs. We effectively demonstrate that NMR may help in discriminating among free and hydrogen-bonded functional groups. ¹³C­{¹⁴N} dipolar-recoupling NMR showed that the formation of carbonate or bicarbonate is excluded. Density functional theory calculations on models of alkylamines grafted into the silica surface assisted the ¹H/¹³C assignments and validated various HB arrangements that may occur upon formation of carbamic acid. This work extends the understanding of the chemisorbed CO₂ structures that are formed upon bonding of CO₂ with surface amines and readily released from the surface by pressure swing