Optimization of Li4SiO4 synthesis conditions by a solid state method for maximum CO2 capture at high temperature

Abstract

The aim of this research work is to optimize the synthesis of Li₄SiO₄ by a solid state method to maximize CO₂ capture. This includes evaluating the main characteristics of the synthesised material which enhance the CO₂ uptake performance. Starting from Li₂CO₃ and SiO₂ pure reagents, the effect of the sintering process conditions (heating rate, synthesis temperature and holding time) of the previously mixed powders has been studied. The samples were characterized by N₂ physisorption, particle size distribution and X-ray diffraction. The evaluation of the CO₂ uptake performance of the samples has been carried out at 600 °C using a thermobalance under a flow of almost pure CO₂. Unreacted Li₂CO₃ is present at low synthesis temperatures, and its content in the synthesised material decreases when higher temperatures are used, so complete conversion to Li₄SiO₄ is reached at 900 °C. At this temperature, the maximum CO₂ uptake was found to be 20%, although it was yet far from the stoichiometric CO₂ uptake value of 36.7%. The holding time at a synthesis temperature of 900 °C was then varied and a maximum CO₂ uptake of 30.5% was obtained for a holding time of 2 h. Finally, under the optimised synthesis temperature and holding time conditions, the heating rate was varied. A value of 5 °C min⁻¹ was found as the optimum one as the use of either lower or higher heating rates has a negative effect on the CO₂ uptake performance. As crystalline phases, the particle size and BET surface area were very similar among all the prepared samples at 900 °C; the crystal size was identified as the main physical property that could explain the CO₂ uptake performance of the samples, i.e., the lower the crystal size, the higher the CO₂ uptake.