| Abstract |
Two-dimensional (2D) solid-state nuclear magnetic resonance (SSNMR) experiments on samples loaded with BC-labeled CO,, under controlled partial pressures , have been performed in this work, revealing unprecedented structural details about the formation of CO, adducts from its reaction with various amine-functionalized SBA-15 containing amines having distinct steric hindrances (e.g., primary, secondary) and similar loadings. Three chemisorbed CO2 species were identified by NMR from distinct carbonyl environments resonating at be approximate to 153, 160, and 164 ppm. The newly reported chemisorbed CO2 species at delta(c) approximate to 153 ppm was found to be extremely moisture dependent. A comprehensive H-1-based SSNMR study [1D H-1 and 2D H-1-X heteronuclear correlation (HETCOR, X = C-13, Si-29) 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. H-1 chemical shifts up to 11.8 ppm revealed that certain chemisorbed CO, species are engaged in very strong HBs. We effectively demonstrate that NMR may help in discriminating among free and hydrogen-bonded functional groups. C-13\N-14\ 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 H-1/C-13 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 CO, with surface amines and readily released from the surface by pressure swing. |
