The "Missing" Bicarbonate in CO₂ Chemisorption Reactions on Solid Amine Sorbents

Area of Science:

• Materials Science
• Chemistry
• Spectroscopy

Background:

• Solid amine sorbents are crucial for CO2 capture.
• Chemisorption of CO2 on amines can form various species, including bicarbonate.
• Standard solid-state NMR techniques often fail to detect these bicarbonate species.

Purpose of the Study:

• To develop and apply a novel solid-state NMR method for detecting hydrated bicarbonate species formed during CO2 chemisorption on amine-grafted silica.
• To investigate the role of water in the formation and detection of these species.
• To re-evaluate the CO2 sorption products in amine-based materials.

Main Methods:

• Chemisorption of 13CO2 on SBA-15 silica grafted with aminopropylsilane (APS) and dimethylaminopropylsilane (DMAPS).
• Solid-state Nuclear Magnetic Resonance (NMR) spectroscopy using a Bloch decay (pulse-acquire) sequence with 1H decoupling.
• Preliminary 13C-1H HETCOR measurements to probe proton-bicarbonate coupling.

Main Results:

• Hydrated bicarbonate species were successfully identified using the Bloch decay NMR sequence, which was not possible with standard 13C CPMAS.
• The presence of water was found to facilitate bicarbonate formation and influence its dynamic motion, enabling direct 13C detection.
• Two distinct bicarbonate species were observed upon freezing, indicating different dynamic environments.
• The study revealed the overlooked presence of bicarbonate in APS sorbents, challenging previous assumptions.

Conclusions:

• A straightforward NMR technique (Bloch decay) enables routine detection of previously elusive hydrated bicarbonate species in amine-based CO2 sorbents.
• Water plays a critical role in the formation and NMR detectability of these bicarbonate species.
• This finding necessitates a re-evaluation of CO2 capture mechanisms by amines, particularly in the presence of moisture.