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Selective CO2 Adsorption and Separation over MOF-derived Porous Carbons

Selective CO2 Adsorption and Separation over MOF-derived Porous Carbons
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Porous carbons possessing high surface area have been one of the most important and conventional porous materials studied so far. They are good candidates for adsorbents because of high surface area, high adsorption capacity and exceptional chemical stability. Hierarchical porous carbons (HPCs) were prepared by direct carbonization of MOFs without additional carbon source in our former work. In this thesis, developed HPC synthesis mechanism will be describe. Also, selective CO2 adsorption on hierarchical porous carbons (HPCs) have been investigated. The first part of the thesis explain developed mechanism of MOF carbonization. Zn4O(bdc)3 was heated at various temperatures and characterized by powder X-ray diffraction (PXRD) and N2 sorption measurements. After heating at 500 °C, the porosity of Zn4O(bdc)3 was totally decomposed to non-porous amorphous and developed newly by decomposition of organic molecules and removal of ZnO from the materials. Therefore, porosity and surface area of resulting HPCs directly depends on Zn contents (Zn/C ratio) of original framework as demonstrated in our former work. In second part, sorption and desorption isotherms the HPCs for CO2, N2 and CO under equilibrium condition were measured at 273 K and 298 K. Interestingly, the adsorption capacity of CO2 is not proportional to the surface area of HPCs. HPC-2, which has very small surface area (1000 m2/g), showed the highest CO2 sorption capacity, which is two times higher than that of commercially available activated carbon, NORIT RB4. And the selectivity of CO2 over CO or N2 estimated by Henry’s constants at 298 K. As expected, HPC-2 shows highest selectivity for CO2 over N2 or CO. In addition, HPC-3 also showed second highest selectivity for CO2 over other gasses. Despite the large surface area (1690 m2/g) of HPC-4, it showed the lowest selectivity for CO2 over other gasses. The higher selectivity may be attributed to hierarchical porous structure. Because micropore volumes of HPC-2 and HPC-3 is 43% and 41%, respectively, which is larger than other HPCs, whereas micropore volume of HPC-4 is only 9%. It should be noted that each pore volume of HPC-2 and HPC-3 is quite similar, whereas pore volume of HPC-4 lean too much towards on meso and macropore. This result suggests that the proper ratio of micropore and meso/macropore volume is important for CO2 selective sorption. In third part, breakthrough measurements over HPCs were conducted using a binary mixture gas composed of 15% CO2 and 85% N2, which is similar to the industrial flue gas stream from power plants and 35% CO2, 65% N2 which is high CO2 concentration flue gas stream at ambient temperature and pressure. As expected, HPC-2 showed the highest breakthrough capacity (37.4 mg/g) and saturation capacity (43.3 mg/g). Especially, HPC-2 showed higher breakthrough capacity than commercially available activated carbon NORIT RB4 for binary mixture gas, which has similar composition to flue gas of power plant.
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