Adsorption and desorption equilibrium of Li₄SiO₄-based sorbents for high-temperature CO₂ capture

The reversible CO₂ adsorption/desorption by Li₄SiO₄-based sorbents under high temperature has attracted much attention in recent years due to its potential to capture CO₂ with a high capacity and excellent cyclic stability. Implementation of the approach largely depends on the equilibrium between CO₂ partial pressure and temperature of the sorbents. However, until now no research has been focused on revealing the experimental equilibrium correlation, despite some work alluding to potential effects by precursors or dopants. In this work, typical Li₄SiO₄-based sorbents with various silicon precursors, dopants of Ce/Fe/Na/K, and varying K contents were synthesized, and experimentally tested using thermogravimetry (TG) by changing the CO₂ partial pressure in the range of 0.05–0.5 atm. Results show that experimental equilibrium temperatures are 30–60 °C higher than theoretical values, and pure silicon precursors and dopants of Ce/Fe impose only marginal effects on the adsorption/desorption equilibrium. Interestingly, equilibrium temperatures of sorbents with the presence of K/Na drop off significantly under 0.5 atm of CO₂, and more K-dopant corresponds to a lower turnover temperature. This phenomenon is proposed to be the combined action of Li₄SiO₄ formation and eutectic materials at 500–750 °C. Finally, a correlation formula is established to describe the CO₂ adsorption/desorption equilibrium of Li₄SiO₄-based sorbents by fitting experimental data according to thermodynamic laws.