Abstract
Chemisorbent materials, based on porous aminosilicas, are among the most promising adsorbents for direct air capture applications, one of the key technologies to mitigate carbon emissions. Herein, a critical survey of all reported chemisorbed CO2 species, which may form in aminosilica surfaces, is performed by revisiting and providing new experimental proofs of assignment of the distinct CO2 species reported thus far in the literature, highlighting controversial assignments regarding the existence of chemisorbed CO2 species still under debate. Models of carbamic acid, alkylammonium carbamate with different conformations and hydrogen bonding arrangements were ascertained using density functional theory (DFT) methods, mainly through the comparison of the experimental 13C and 15N NMR chemical shifts with those obtained computationally. CO2 models with variable number of amines and silanol groups were also evaluated to explain the effect of amine aggregation in CO2 speciation under confinement. In addition, other less commonly studied chemisorbed CO2 species (e.g., alkylammonium bicarbonate, ditethered carbamic acid and silylpropylcarbamate), largely due to the difficulty in obtaining spectroscopic identification for those, have also been investigated in great detail. The existence of either neutral or charged (alkylammonium siloxides) amine groups, prior to CO2 adsorption, is also addressed. This work extends the molecular-level understanding of chemisorbed CO2 species in amine-oxide hybrid surfaces showing the benefit of integrating spectroscopy and theoretical approaches.
Highlights
Given the negative environmental consequences associated with CO2 emissions, a great effort has been placed in discovering and developing CO2 capture solutions, with amine-based solid adsorbents emerging as good sorbent materials especially the case for low concentration, low temperature, moisture-containing applications 5-7
This work extends the molecular-level understanding of chemisorbed CO2 species in amine-oxide hybrid surfaces showing the benefit of integrating spectroscopy and theoretical approaches
The most stable structures of the species modelled in the present work are shown in Figures 1 and S2, and the corresponding 15N and 13C calculated chemical shifts (CSs) are presented in Tables 1 and S1, respectively
Summary
Given the negative environmental consequences associated with CO2 emissions, a great effort has been placed in discovering and developing CO2 capture solutions, with amine-based solid adsorbents emerging as good sorbent materials especially the case for low concentration, low temperature, moisture-containing applications 5-7. An extra peak at 153.3 ppm (Table S20) was observed by our group, which has been attributed to a CO2 species extremely sensitive to the presence of water, appearing only in absolutely anhydrous conditions. This resonance has been assigned to the presence of isolated amines reacting with. A critical survey of all reported chemisorbed CO2 species formed in primary amine-modified mesoporous silicas is made, by performing new solid-state NMR (ssNMR) experiments, assisted by electronic density functional theory (DFT).
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