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

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 (Mg2SiO4)+supercritical CO2 with and Without H2O. Run conditions were 80 degrees C and 96 atm. With H2O but without CO2, Si-29 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., SiO4 tetrahedra sharing one and two corners with other tetrahedra, respectively. Using scCO(2) without H2O, no reaction is observed within 7 days. Using both scCO(2) and H2O, 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 (MgCO3) precipitation reactions are faster than the forsterite hydrolysis process. Thus, it call be Concluded that the Mg2SiO4 hydrolysis process is the rate limiting step of the overall mineral carbonation process. Si-29 NMR combined with XRD, TEM, SAED, and EDX further reveals that the reaction is a surface reaction with the Mg2SiO4 Crystallite in the core and with condensed Q2, Q3, and Q4 species forming highly Porous amorphous surface layers. C-13 MAS NMR unambiguously identified a reaction intermediate as Mg-5(CO3)(4)(OH)(2)center dot 5H(2)O, i.e., the dypingite.