Unravelling the Structure of Chemisorbed CO Species in Mesoporous Aminosilicas: A Critical Survey.

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 CO species, which may form in aminosilica surfaces, is performed by revisiting and providing new experimental proofs of assignment of the distinct CO species reported thus far in the literature, highlighting controversial assignments regarding the existence of chemisorbed CO 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 C and N NMR chemical shifts with those obtained computationally. CO models with variable number of amines and silanol groups were also evaluated to explain the effect of amine aggregation in CO speciation under confinement. In addition, other less commonly studied chemisorbed CO 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 CO adsorption, is also addressed. This work extends the molecular-level understanding of chemisorbed CO species in amine-oxide hybrid surfaces showing the benefit of integrating spectroscopy and theoretical approaches.