Water-enhanced CO2 capture with molecular salt sodium guanidinate

Abstract

Solid-state amine absorbent materials, including those containing guanidine derivatives, have received tremendous attention as the world combats the challenges of climate change. Although these materials are attractive for their selective CO₂ capture capacity and fast absorption kinetics, the CO₂ absorption mechanism with such materials can be unclear and differs when H₂O is present. In this work, we present a detailed study on a model guanidine-based crystalline salt, sodium guanidinate (NaCN₃H₄), and examine the CO₂ absorption with and without humidity. The simple composition and high crystallinity of NaCN₃H₄ and its carbonate capture products allow us to perform ex situ and in situ PXRD to identify possible reaction pathways. These results are complemented by detailed thermogravimetric analysis and gas studies to derive holistic mechanistic insight. We determine that NaCN₃H₄ exhibits a kinetically controlled CO₂ absorption profile with an absorption capacity of ≈12.3 mmol CO₂/g, assuming a 1 : 1 : 1 reaction between CO₂, H₂O, and NaCN₃H₄. Importantly, humidity is found to significantly lower the activation energy and increase absorption kinetics of CO₂ absorption as the products Na₂CO₃ and (CN₃H₆)₂CO₃ form. Our study demonstrates the advantages and drawbacks of using guanidinate materials for CO₂ capture under flue gas conditions, and provides clarity about how CO₂ capture with similar compounds can be enhanced by humidity.