Evaluation of the formation and carbon dioxide capture by Li₄SiO₄ using in situ synchrotron powder X-ray diffraction...

Carbon capture and storage using regenerable sorbents are an effective approach to reduce CO₂ emissions from stationary sources. In this work, lithium orthosilicate (Li₄SiO₄) was studied as a carbon dioxide sorbent. For a deeper understanding of the synthesis and carbonation mechanism of Li₄SiO₄, an in situ synchrotron radiation powder X-ray diffraction technique was used. The Li₄SiO₄ powders were synthesized by a combination of ball milling of a Li₂CO₃ and SiO₂ mixture followed by a thermal treatment process at low temperature. In situ studies showed that formation of Li₄SiO₄ from the as-milled 2Li₂CO₃-SiO₂ mixture involves decomposition of Li₂CO₃ by reaction with SiO₂via Li₂SiO₃ as an intermediate compound. No evidence of Li₂Si₂O₅ formation was obtained, in spite of thermodynamic predictions. The CO₂ capture by Li₄SiO₄ was evaluated dynamically over a wide temperature range, reaching a maximum weight increase of 34 wt% and good cyclability after about 10 cycles. By thermogravimetric and microstructural analyses in combination with ex situ and in situ measurements, a two step carbonation mechanism and its influence on the final CO₂ capture was clearly elucidated. Under dynamical conditions up to 700 °C, the lower number of Li₂CO₃ nuclei initially formed retards the double shell formation and the nucleation and growth of the Li₂CO₃ particles remains the controlling step up to higher CO₂ capture capacity. Isothermal carbonation at 700 °C favours the formation of a higher number of Li₂CO₃ nuclei that creates a thin carbonate shell. The CO₂ diffusion through this shell is the limiting step from the beginning and further carbonation is hindered as the reaction progresses.