Time-Resolved Synchrotron Powder X-ray Diffraction Studies on the Synthesis of Li8SiO6 and Its Reaction with CO2

Abstract

Lithium oxosilicate was synthesized via the solid-state method using Li₂O and SiO₂ as starting reactants. In situ synchrotron powder X-ray diffraction (SPRXD) coupled with Rietveld refinement allowed describing the synthesis as a two-step process where Li₂O and SiO₂ react to form Li₄SiO₄ and, at higher temperatures, lithium orthosilicate reacts with the remaining Li₂O to form Li₈SiO₆. Time-resolved measurements allowed determining the temperatures at which each phase transformation occurs as well as the time required to complete the synthesis. The CO₂ capture properties of Li₈SiO₆ in the temperature range from room temperature to 770 °C were studied in detail by time-resolved in situ SPXRD. The crystallographic phases present during Li₈SiO₆ carbonation were identified and quantified via Rietveld analysis. Results showed that, within the temperature range from 200 to 690 °C, Li₈SiO₆ carbonation produces Li₄SiO₄ and Li₂CO₃, while, at temperatures from 690 to 750 °C, a secondary reaction occurs, where previously formed Li₄SiO₄ reacts with CO₂, producing Li₂SiO₃ and Li₂CO₃. These findings allowed proposing a mechanism of reaction for Li₈SiO₆ carbonation in the temperature range that is of interest for high temperature solid-state sorbents.