Preparation Of CaO-based Composites By CO₂ Reduction Method And Its CO₂ Adsorption Properties

Because of the large use of fossil fuels,the main greenhouse gas carbon dioxide(CO2)concentrations have been increasing.On the one hand,carbon dioxide emissions have exacerbated the greenhouse effect;on the other hand,as a source of C1,it is very attractive in organic synthesis.If it can be separated and utilized,it can not only reduce the greenhouse effect but also be used as a raw material to synthesize other compounds and be used in life and industrial production.Among all kinds of carbon dioxide adsorbents,calcium oxide has attracted wide attention due to its advantages of wide material sources,high theoretical adsorption capacity,and recycling.However,because the sintering caused by high temperature will lead to a decrease of adsorption capacity and cycle stability,it is necessary to improve the adsorption capacity and anti-sintering ability of CaO-based adsorbents.If carbon dioxide reacts with alkaline earth metal hydride or metal substance,carbon dioxide can be reduced to clean energy fuel(CH4 and H2)or other products(such as amorphous carbon),and corresponding oxides can be obtained.In this way,not only the utilization of CO2 can be realized,but also the solid adsorbents for CO2 adsorption can be obtained.The main content of this paper is the preparation of CaO-based composites by CO2 reduction method and its CO2 adsorption properties,specific research is as follows:1.Firstly,CaO@C composites were prepared by mechanochemical reaction of CO2 and C aH2 at room temperature.Through the adsorption test,it is found that the CaO@C composite is a kind of CaO-based adsorbent with high performance and sintering resistance.Under the condition of carbonation at 650℃ for 2 h,the carbon dioxide adsorption capacity of CaO@C composites is 42 wt%,which is 255%and 23%higher than the CaO of untreated and ball milling,respectively,and the yield of CH4 in the gas product is 27.3%.After 30 cycles,the activity loss of CaO@C composites is 46%,which is significantly lower than that of ball-milled CaO(60%).The uniform mixing of nanometer CaO and C in CaO@C adsorbents achieves high adsorption capacity,low initial adsorption temperature,fast adsorption rate,and good cycle stability.2.Based on the experiments in the previous chapter,we used the MgH2/CaH2 mixture of 1:1,1:2,1:4,and 1:8 mol/mol to carry out the mechanochemical reaction with CO2 to achieve the selective methanation of CO2 while preparing high-performance CaO-based adsorbents.In these reactions,CO2 is completely converted and CH4 is the only hydrocarbon product in the gas phase.The selectivity of methanation is close to 100%;the yield rate of CH4 is 30%.Besides,the solid product composed of MgO,CaO,and C is a new type of CaO-based adsorbent with high performance.In different proportions of adsorbents,MgO/4CaO/C adsorbents showed a high initial adsorption capacity of 59.3 wt%,and the average activity loss was only 1.6%after 30 cycles.3.Considering the high cost of the metal hydride,to reduce the cost and prepare CaO based adsorbent with improved performance,we choose cheaper single substance Ca and Mg as the synthesis raw materials according to the actual conditions,the reduction reaction with CO2 was carried out by heating method,the adsorption properties of the prepared CaO/C and MgO/4CaO/C samples were tested.Although CaO/C was self-activated in the second adsorption,its maximum adsorption capacity was 12 wt%,and only 2.3 wt%could be adsorbed after 30 cycles,the total inactivation rate was as high as 80%.The adsorption capacity of MgO/4CaO/C reached the highest 24.8 wt%at the first adsorption,then decreased gradually in the following cycle,and tended to be stable at the 8th cycle.After 30 cycles,the adsorption capacity was still maintained at 12 wt%,and the total inactivation rate was 51%.In this chapter,the heating method is used to reduce carbon dioxide and prepare CaO based adsorbent,reusing CO2 by reducing CO2 to amorphous carbon and reducing the cost to a certain extent.Although there are limitations in performance improvement,it can also provide a reference for the preparation of CaO based adsorbent by the heating method.