NMR Reveals Two Bicarbonate Environments in SBA15-Solid-Amine CO2 Sorbents

We present a spectroscopic study aimed at a better understanding of solid-amine CO2 sorbent materials that employ amine moieties in mesoporous hosts (here, mesoporous silica SBA15). The materials are exposed to water (or D2O) and isotopically enriched (CO2)-C-13 to examine the chemisorption products found with these compositions under conditions relevant to carbon capture from combustion sources. Bicarbonate species have been only recently characterized by solid-state NMR as a product found under humid carbon capture conditions using tertiary amines. Here, we extend these findings to characterize multiple bicarbonate environments (using low-temperature solid-state NMR) associated with not only tertiary but also secondary and primary amine sites using two-dimensional C-13-H-1 heteronuclear correlation (HETCOR) NMR. The use of D2O provides enhanced resolution in the HETCOR spectra by diluting the protons present and reducing the homonuclear dipolar coupling. One bicarbonate environment is coupled to water present on the walls of the mesoporous silica support, whereas the other bicarbonate is coordinated to water and located in the pore or "mid-channel" while still being coupled to the pendant amine moieties. The identification of bicarbonate in primary and secondary amines, the detection of which was previously obscured by dynamic motion attenuating detection via cross-polarization magic-angle spinning NMR, is presented as well. These findings will help further quantify the presence of both carbamate and bicarbonate in carbon capture materials in the future.