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

The aim of this research work is to optimize the synthesis of Li4SiO4 by a solid state method tomaximize 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 N-2 physisorption, particle size distribution and X-ray diffraction. The evaluation of the CO2 uptake performance of the samples has been carried out at 600 degrees 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 degrees 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 degrees 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 degrees 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 degrees 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.