Lithium Silicate Materials For CO₂ Capture At High Temperatures

In recent years, the greenhouse effect resulting global warming becomes more and more rigorous. The control of CO2 emission, as the biggest contribution to greenhouse gas, has aroused worldwide concern. High-temperature CO2 capture technology can directly capture CO2 from the hot flue gas as well as in-situ remove CO2 generated in the chemical processes to improve product yield. Developing the high-temperature CO2 capture technology is of significance for the control of carbon emissions and the anesis of the greenhouse effect. Li4SiO4 exhibits a good prospect as one of high-temperature CO2 absorbent canditates due to the rapid absorption rate, high absorption capacity, and good cycling performance. In this thesis, a wet ball-milling method and a solvent evaporation induced self-assembly (EISA) method was used to synthesize Li4SiO4 materials, respectively. The CO2 absorption rate capacity of the synthesized Li4SiO4 materials have been greatly improved. The detailed work mainly consists of two parts as follows:1) A wet ball-milling method followed by calcination was adopted to prepare nanocrystalline Li4SiO4 materials by using different silicon and lithium sources. CO2 uptakes of the prepared Li4SiO4 materials were investigated on a thermogravity (TG) analyzer, showing that the optimal Li4SiO4 materials were synthesized with LiOH·H2O as the lithium source and TEOS as the silicon source. X-ray powder diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM) were applied to characterize the structure and the morphology of the as-prepared Li4SiO4 materials. Furthermore, the CO2 absorption property and cycle stability were investigated at different temperatures and CO2 partial pressures.The results showed that Li4SiO4 materials with high crystallinity and uniform size were synthesized by using LiOH·H2O and TEOS as the lithium source and silica source, respectively, through the wet ball-milling method. Absorption equilibrium of 27.9 wt.% was achieved within 10 min at 550 ℃ and a CO2 partial pressure of 0.25 bar. The prepared nanocrystalline Li4SiO4 material kept the original absorption properties after used for 5 capture-regeneration cycles, indicating the good cycle stability. A mixture of 25%CO2-25%N2-50%He was introduced through the Li4SiO4 absorption bed, showing that CO2 can be efficiently captured at 550 ℃. The adsorption capacity showed no significant decant in the presence of 10% humidity.2) A solvent evaporation induced self-assembly (EISA) method was employed to attempt the synthesis of porous Li4SiO4 materials by using CTAB, CTAB+PAA, P123 as template. N2 adsorption-desorption, XRD and SEM were used to characterize the obtained Li4SiO4 materials. CO2 absorption properties were studied on the TGA. The results showed that mesostructured lithium silicate precursor could be obtained by using CTAB+PAA as the template. Though the mesostructure collapsed after calcination, the obtained Li4SiO4 material still exhibits a high absorption equilibrium and a fast absorption-desorption rate. The sample synthesized by EISA at 60 ℃ display CO2 uptakes up to 22.5wt% within 5 min and absorption equilibrium of 28.8 wt% can be available within 10 min. At 600 ℃, complete regeneration can be achieved at N2 atmosphere. After five absorption-desorption cycles, the CO2 absorption properties show no change, indicating the good cycle stability.