Adsorption Of CO₂ On Modified Ordered Mesoporous Silicas

The global climate change is the most critical environmental problem and is alsoone of the most complicated challenges facing to the whole mankind. CO₂ emissionfrom fossil fuels combustion contributes to more than 60% of the global climatechange, which has been the main reason that results in the global climate change.Therefore, the novel porous solids to capture CO₂ from flue gas have attractedparticular interest due to its high selectivity and low energy penalty. However, thepresent adsorbents are sensitive to the adsorption/desorption conditions and there stillexit some disadvanges. In this paper, three methods such as grafting, coating andimpregnation were used to prepare the novel adsorbents and to promote the CO₂capture capacity, regeneration, hydrophobicity and thermo stability of the adsorbents.The samples of Hydrolyzed-N exhibited superior equilibrium adsorption capacityas well as adsorption rate to those of the samples of Calcined-N, mainly due to theincrease of silanol group concentration on the surface of the adsorbents by hydrolysis.The samples with finer particle size outperformed the samples with coarse particlesize in the equilibrium adsorption capacity. Fine particle size allowed goodaccessibility for the grafting agent. Hence more grafting agents were anchored.Measurements of the enthalpies of adsorption highlighted that the CO₂ adsorption onthe AEAPS functionalized SBA-16 was a strong interaction, indicating that theinteraction between the amine group and CO₂ isa chemisorption. The maximumamount of carbon dioxide adsorbed at 60℃was 0.727 mmol g^-1 for the AEAPSfunctionalized SBA-16 (hydrolyzed SBA-16 as support) of the particle size range at0.124-0.15 mm., which is 18.1 times that of the SBA-16 support, suggesting thatadsorption of CO₂ on the AEAPS functionalized SBA-16 is mostly a chemisorption.The adsorption enthalpy is much higher than 40 kJ mol^-1, about 70 kJ mol^-1, whichalso suggests it is a chemisorption. The relationship between adsorption capacity andamine content is Y=0.3855X-0.4397 where Y is adsorption capacity (mmol g^-1) and Xis amine content (mmol-N g^-1).In the traditional temperature swing adsorption using zeolites as adsorbents, about 30 % of the total energy is consumed to dehumidify flue gas before adsorption,owing to the competitive adsorption between water vapor and CO₂. High regenerationtemperature may promote desorption of CO₂, however, it may decompose the graftingamino groups. At the same time, high temperature and high humidity of flue gas afterFGD may cause the destruction of the mesoporous silica. Therefore, for the first time,thermal stability, hydrothermal stability and hydrophobicity of the SBA-16 supportsand the modified SBA-16 by silylation were investigated.The amino-functionalized SBA-16 prepared by silylating various amine contentson the mesoporous silica SBA-16 exhibited super hydrophobicity to the SBA-16support. The amino-groups silylated on SBA-16 can be thermally stable up to 200℃whether in He or in air, which is advantageous to the regeneration of the adsorbents.The adsorption capacity remains stable during multiple adsorption/desorptions. Afterhydrothermal treatment for 16 h, the amino-functionalized SBA-16 still displaysstructure stability against boiling water. Hydrothermal stability of theamino-functionalized SBA-16 is related to the extent of silylation, namelyamino-groups contents, which are covalently bonded on the support.The CO₂ adsorbents were also prepared through impregnating on theas-synthesized MCM-41 by three types of amines. The synthesis way not only savesthe energy or extractor to remove the template but also is environmentally friendlydue to the absence of the potential pollutants such as toluene. The sample impregnatedby TETA showed the highest adsorption capacity, about 2.22 mmol g^-1 at 60℃due toits highest amino-groups content among the three samples. The CO₂ adsorptionbehavior was also investigated with the deactivation model, which showed excellentprediction of the breakthrough curves. No obvious effect of SO₂ with theconcentration of 550 mg m^-3 on the adsorbent was found. However, CO₂ capture unitshould be placed after FGD unit to minimize the effect of SO₂ in the potentialcommercial applications, due to the irreversible adsorption of SO₂ on the impregnatedadsorbents.