Metal Carbonation of Forsterite in Supercritical CO₂ and H₂O Using Solid State ²⁹Si, ¹³C NMR Spectroscopy

Abstract

Ex situ natural abundance magic angle spinning (MAS) NMR was used for the first time to study fundamental mineral carbonation processes and reaction extent relevant to geologic carbon sequestration (GCS) using a model silicate mineral forsterite (Mg 2 SiO 4 )+supercritical CO 2 with and without H 2 O. Run conditions were 80 °C and 96 atm. With H 2 O but without CO 2, 29 Si MAS NMR reveals that the reaction products contain only two peaks of similar intensities located at about −84.8 and −91.8 ppm, which can be assigned to surface Q1 and Q2 species, i.e., SiO 4 tetrahedra sharing one and two corners with other tetrahedra, respectively. Using scCO 2 without H 2 O, no reaction is observed within 7 days. Using both scCO 2 and H 2 O, the surface reaction products for silica are mainly Q4 species (−111.6 ppm) accompanied by a lesser amount of Q3 (−102 ppm) and Q2 (−91.8 ppm) species. No surface Q1 species were detected, indicating the carbonic acid formation and magnesite (MgCO 3 ) precipitation reactions are faster than the forsterite hydrolysis process. Thus, it can be concluded that the Mg 2 SiO 4 hydrolysis process is the rate limiting step of the overall mineral carbonation process. 29 Si NMR combined with XRD, TEM, SAED, and EDX further reveals that the reaction is a surface reaction with the Mg 2 SiO 4 crystallite in the core and with condensed Q2, Q3, and Q4 species forming highly porous amorphous surface layers. 13 C MAS NMR unambiguously identified a reaction intermediate as Mg 5 (CO 3 ) 4 (OH) 2 ·5H 2 O, i.e., the dypingite.