In situ measurement of magnesium carbonate formation from CO₂ using static high-pressure and -temperature ¹³C NMR

We explore a new in situ NMR spectroscopy method that possesses the ability to monitor the chemical evolution of supercritical CO₂ in relevant conditions for geological CO₂ sequestration. As a model, we use the fast reaction of the mineral brucite, Mg(OH)₂, with supercritical CO₂ (88 bar) in aqueous conditions at 80 C. The in situ conversion of CO₂ into metastable and stable carbonates is observed throughout the reaction. After more than 58 h of reaction, the sample was depressurized and analyzed using in situ Raman spectroscopy, where the laser was focused on the undisturbed products through the glass reaction tube. Postreaction, ex situ analysis was performed on the extracted and dried products using Raman spectroscopy, powder X-ray diffraction, and magic-angle spinning ¹H-decoupled ¹³C NMR. These separate methods of analysis confirmed a spatial dependence of products, possibly caused by a gradient of reactant availability, pH, and/or a reaction mechanism that involves first forming hydroxy-hydrated (basic, hydrated) carbonates that convert to the end-product, anhydrous magnesite. This carbonation reaction illustrates the importance of static (unmixed) reaction systems at sequestration-like conditions.