The “Missing” Bicarbonate in CO₂ Chemisorption Reactions on Solid Amine Sorbents

Abstract

We have identified a hydrated bicarbonate formed by chemisorption of ¹³CO₂ on both dimethylaminopropylsilane (DMAPS) and aminopropylsilane (APS) pendant molecules grafted on SBA-15 mesoporous silica. The most commonly used sequence in solid-state NMR, ¹³C CPMAS, failed to detect bicarbonate in these solid amine sorbent samples; here, we have employed a Bloch decay ("pulse-acquire") sequence (with ¹H decoupling) to detect such species. The water that is present contributes to the dynamic motion of the bicarbonate product, thwarting CPMAS but enabling direct ¹³C detection by shortening the spin-lattice relaxation time. Since solid-state NMR plays a major role in characterizing chemisorption reactions, these new insights that allow for the routine detection of previously elusive bicarbonate species (which are also challenging to observe in IR spectroscopy) represent an important advance. We note that employing this straightforward NMR technique can reveal the presence of bicarbonate that has often otherwise been overlooked, as demonstrated in APS, that has been thought to only contain adsorbed CO₂ as carbamate and carbamic acid species. As in other systems (e.g., proteins), dynamic species that sample multiple environments tend to broaden as their motion is frozen out. Here, we show two distinct bicarbonate species upon freezing, and coupling to different protons is shown through preliminary ¹³C-¹H HETCOR measurements. This work demonstrates that bicarbonates have likely been formed in the presence of water but have gone unobserved by NMR due to the nature of the experiments most routinely employed, a perspective that will transform the way the sorption community will view CO₂ capture by amines.