Preparation,Modification And CO₂ Adsorption Of Li₄SiO₄ Using KIT-6 As Silicon Source

Global warming caused by the release of large amounts of carbon dioxide（CO₂）from the burning of fossil fuels has become one of the most serious environmental problems at present.Lithium orthosilicate（Li₄SiO₄）has been regarded as a promising CO₂ high-temperature solid adsorbent due to its high theoretical adsorption capacity and low regeneration temperature.However,the low CO₂ concentration in factory flue gas（<20 vol%）leads to the less adsorption rate and poor adsorption capacity of Li₄SiO₄ under this condition.Therefore,improving the adsorption performance of Li₄SiO₄ in the atmosphere of low CO₂ concentration has become the focus of research.In this paper,KIT-6 was used as the silicon source to prepare Li₄SiO₄adsorbent（KIT-6-Li₄SiO₄）,and then modified by metal cation eutectic doping and alkali metal carbonate mechanical doping.The adsorbents were characterized by X-ray diffraction（XRD）,scanning electron microscopy（SEM）,low temperature N₂ adsorption/desorption,and the CO₂adsorption capacity of the adsorbents was investgated.The main conclusions are as follows:（1）KIT-6 mesoporous silica was synthesized by hydrothermal method and KIT-6-Li₄SiO₄high temperature adsorbent was prepared by impregnation precipitation method.Compared with gas phase SiO₂,high purity Li₄SiO₄ could be obtained at lower calcination temperature and shorter calcination duration by KIT-6 as the precursor,and can broaden the effective adsorption temperature range of CO₂.The optimal adsorption capacity of each adsorbent appeared at 600℃.After 5 cycles of adsorption-desorption,the adsorption capacity decreased by 13.3%to 37.4%.（2）Ca²⁺and Na⁺eutectic doped KIT-6-Li₄SiO₄（KL）adsorbents were synthesisd by impregnation precipitation method.The addition of Ca²⁺changed the crystal morphology of Li₄SiO₄ and reduced the grain size.Doped Na⁺was involved in the formation of Li₄SiO₄ and replaced Li⁺in the crystal lattice.The specific surface area and pore volume of Li₄SiO₄ were not the main factors affecting CO₂ adsorption performance.The maximum adsorption capacity（29.17 wt%）was obtained when the doping amount of Ca²⁺was x=0.03,and the maximum adsorption capacity（35.35 wt%）was obtained when the doping amount of Na⁺was x=0.06.The optimal adsorption temperature for both of them was 600℃.After 10 cycles of adsorption/desorption,the adsorption capacity of KL-Ca0.03 decreased by 38.8%,while the adsorption capacity of KL-Na0.06 increased first and then decreased.（3）Na₂CO₃ and K₂CO₃ mechanically doped KIT-6-Li₄SiO₄ adsorbents were prepared by impregnation method.At 600℃,the maximum adsorption capacity of the modified material was 32.59 wt%when the doping amount of Na₂CO₃ was x=5 wt%,and 31.78 wt%when the doping amount of K₂CO₃ was x=10 wt%.Alkali-metal carbonate doping could increase the surface porosity of the modified materials in the process of cyclic adsorption/desorption and improved the recycling performance.（4）The adsorption capacity of KIT-6-Li₄SiO₄-Na0.06-Na₂CO₃（5 wt%）,modified by Na⁺and Na₂CO₃ co-doping,was 36.50 wt%at 600℃,which reached 99.4%of the theoretical adsorption capacity.The co-doping increased the ion diffusion rate at higher temperature and the adsorption capacity of the single-doping modified adsorbents greatly.The two methods had a synergistic effect,which improved the adsorption rate,adsorption capacity,effective adsorption temperature range and cycle stability of the materials.（5）The double exponential model had a high degree of fit for the adsorption of CO₂ by Li₄SiO₄.The higher temperature was beneficial to the surface adsorption of CO₂,and the diffusion process of Li⁺and O^2-was the limiting step for the CO₂ adsorption of Li₄SiO₄.