Thermodynamics and kinetics analyses of high CO2 absorption properties of Li3NaSiO4 under various CO2 partial pressures

Abstract

The properties of Li₃NaSiO₄ as a CO₂ absorbent were evaluated by thermogravimetry (TG) and X-ray diffraction, which revealed that the CO₂ absorption and desorption reaction, Li₃NaSiO₄ + CO₂ ↔ LiNaCO₃ + Li₂SiO₃, is reversible. In scanning-type TG under various CO₂ partial pressures (P(CO₂)), mass gain ascribed to CO₂ absorption started from approximately 400 °C, irrespective of P(CO₂). CO₂ desorption was observed at a higher temperature, which was considered the approximate equilibrium temperature of the above-mentioned reaction. A pseudo-Ellingham diagram of the reaction between Li₃NaSiO₄ and CO₂, constructed using the obtained approximate equilibrium temperature under each P(CO₂), showed similar behavior to that between Li₄SiO₄ and CO₂, especially between 10⁻² and 10⁻¹ bar of P(CO₂). Kinetics analysis by isothermal TG using the Jander model revealed that the reaction rate of CO₂ absorption of Li₃NaSiO₄ was higher than that of Li₄SiO₄. The rate-determining step of CO₂ absorption by Li₃NaSiO₄ was the diffusion process at the examined temperatures and P(CO₂), which was different from the rate-determining step of the reaction between Li₄SiO₄ and CO₂ under low P(CO₂).