The role of amine surface density in carbon dioxide adsorption on functionalized mixed oxide surfaces

Supported amines are considered as adsorbents to replace aqueous amines for carbon capture and for CO₂ capture/conversion into chemicals. Here, amines are grafted to SiO₂ or Ti-SiO₂ by using aminopropyl triethoxysilane (APTES) or (3-triethoxysilylpropyl)-tert-butylcarbamate (TESPtBC) and then removing the carbamate group introduced by the latter by mild heating to 'deprotect' the amine. Structures are verified by using ¹³Ccross polarization magic angle spinning (CP/MAS) NMR spectroscopy, acid titration, thermogravimetric analysis, and elemental analysis. Diffuse reflectance UV/Visible spectroscopy shows that amines from APTES coordinate directly to Ti cations, whereas Ti cations remain coordinatively unsaturated after grafting of TESPtBC and deprotection. CO₂ chemisorption is studied as a function of amine precursor, average surface density, and the presence of Ti. CO₂ uptake increases from <0.02CO₂ per amine for as-synthesized TESPtBC materials to only approximately 0.05CO ₂ per amine for the isolated amines present after deprotection. In contrast, clustered amines from APTES chemisorb up to approximately 0.35CO ₂ per amine. Cooperative ammonium carbamates form preferentially above an apparent local density of 0.6amines per nm² from APTES, but do not form even up to 0.9 amines per nm² for TESPtBC-derived materials. This suggests that the true local surface density form APTES is underestimated by as much as 150%. CO₂ uptake falls to <0.01CO₂ per amine for ATPES on Ti-SiO₂, but uptake is less affected for the 'protected' TESPtBC precursor. These results show that TESPtBC may be a viable precursor for applications in acid-base cooperative CO₂ conversion catalysts, and that variation in the local amine surface density and the chemistry of the underlying support may account for some of the large variability in reported CO₂ capacities of supported amine materials in literature.