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 13C cross 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 <0.02 CO2 per amine for as-synthesized TESPtBC materials to only approximately 0.05 CO2 per amine for the isolated amines present after deprotection. In contrast, clustered amines from APTES chemisorb up to approximately 0.35 CO2 per amine. Cooperative ammonium carbamates form preferentially above an apparent local density of 0.6 amines 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 <0.01 CO2 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 acidbase 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.