Doped Li₄SiO₄-based sorbents for CO₂ capture at high temperature

One of the most effective methods to lower power plant carbon dioxide emissions is post-combustion CO₂ capture based on regenerable solid sorbents. Lithium-based ceramics, such lithium orthosilicate (Li₄SiO₄), were discovered to be among these sorbents that could capture CO₂ at high temperaturesand low concentration. Li₄SiO₄ shows significant CO₂ sorption capacity (up to 367 mg/g sorbent), good cyclability and lower regeneration temperatures compared to other sorbents as CaO, zeolites, metal–organic frameworks, and activated carbons. Li₄SiO₄ exhibits the following sorption/desorption reaction: Li₄SiO₄(s) + CO₂(g) ⇆ Li₂CO₃(s) + Li₂SiO₃(s) CO₂ adsorption is generally carried out at 500-700 °C, depending on the CO2 partial pressure. Firstly, the reaction between Li₄SiO₄ and CO₂ occurs leading to the formation of a solid product shell on the particle surface consisting of lithium carbonate, Li₂CO₃, and lithium metasilicate, Li₂SiO₃; then, CO₂, Li ⁺ and O²⁻ diffuse through the formed solid shell. It is assumed that CO₂ diffusion resistance plays a dominant role, particularly under low CO₂ partial pressures. For this reason, various doping methods have been proposed to improve Li₄SiO₄ activity such as the addition of alkali carbonates (K₂CO₃ or Na₂CO₃) that form molten eutectic carbonate mixtures with the product Li₂CO₃ at high sorption temperatures (about 500°C). The resultant molten carbonate shell greatly improves CO₂ diffusion throughout the liquid layer.