25 research outputs found
Structure of Chemisorbed CO<sub>2</sub> 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 <sup>13</sup>C-labeled CO<sub>2</sub>, “<i>under controlled partial pressures</i>”,
have been performed in this work, revealing unprecedented structural
details about the formation of CO<sub>2</sub> 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> CO<sub>2</sub> species
were identified by NMR from distinct carbonyl environments resonating
at δ<sub>C</sub> ≈ 153, 160, and 164 ppm. The newly reported
chemisorbed CO<sub>2</sub> species at δ<sub>C</sub> ≈
153 ppm was found to be extremely moisture dependent. A comprehensive <sup>1</sup>H-based SSNMR study [1D <sup>1</sup>H and 2D <sup>1</sup>H–X
heteronuclear correlation (HETCOR, X = <sup>13</sup>C, <sup>29</sup>Si) experiments] was performed on samples subjected to different
treatments. It was found that all chemisorbed CO<sub>2</sub> species
are involved in hydrogen bonds (HBs) with either surface silanols
or neighboring alkylamines. <sup>1</sup>H chemical shifts up to 11.8
ppm revealed that certain chemisorbed CO<sub>2</sub> species are engaged
in very strong HBs. We effectively demonstrate that NMR may help in
discriminating among free and hydrogen-bonded functional groups. <sup>13</sup>C{<sup>14</sup>N} 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 <sup>1</sup>H/<sup>13</sup>C assignments and
validated various HB arrangements that may occur upon formation of
carbamic acid. This work extends the understanding of the chemisorbed
CO<sub>2</sub> structures that are formed upon bonding of CO<sub>2</sub> with surface amines and readily released from the surface by pressure
swing