ABSTRACT |

By: Trong Pham Chair: Raul Lobo

Due to the increase in the anthropogenic CO2 emissions, the development of carbon capture and storage technology is of profound importance to minimizing global climate change and preserving our environment. In this thesis, I have investigated the adsorption properties of cation-exchanged high silica zeolites chabazite (CHA, Si/Al=6 and 12) and ZK-5 (KFI, Si/Al=3.7) due to their high surface area, large pore volume, and moderate hydrophilic properties compared to commercial faujasite and linde type A zeolites. Li- and Na-cation-exchanged zeolites displayed high adsorption capacity (~ 5 mmol/g at 303 K and 1 bar pressure), comparable to the commercial zeolites, and high selectivity of CO2 over N2. In general, Li-cation zeolites show the highest capacity for N2 and CO2, but lower selectivity of CO2/N2 compared to Na-cation zeolites. The acidity of both the adsorbate and adsorbents, in addition to the low electric field of proton-exchanged zeolites lead to the low adsorption affinity of these materials with CO2. Large cations K+ occupy more space in the void pore of zeolites, thus leave less volume for CO2 molecules. For zeolite materials to be practical and useful as adsorbents for CO2 capture from flue gas, a number of aspects need to be carefully evaluated including adsorption and working capacities, selectivity, and adsorbent regeneration. We found that Li-, and Na-zeolites having high heats of adsorption (40-50 kJ/mol) performed better in the vacuum swing adsorption process and H- and K-zeolites or zeolites with higher Si/Al ratio (CHA, Si/Al=12) having lower heats of adsorption (30-40 kJ/mol) display properties more suited to pressure swing adsorption. The in situ FTIR CO2 adsorption spectra show that physisorption accounts for the largest fraction of the total CO2 adsorbed. A shift to higher frequency of the asymmetric stretching vibration compared to CO2 gas phase (2349 cm-1) indicated the direct coordination of cations with oxygens of CO2 molecules.
To improve zeolite adsorbents for CO2 capture, we have determined the adsorption sites of CO2 in Si-CHA, cation-exchanged CHA and ZK-5 zeolites using the Rietveld refinements of X-ray and neutron powder diffraction data. The structural refinements indicated that CO2 at equilibrium is located close to zeolite framework oxygens due to dispersion interactions and CO2 coordinates with alkali cations by electrostatic interactions. Two CO2 adsorption sites were identified in both high and pure silicachabazite zeolites. The dispersion interactions between Si-CHA zeolite and CO2 is the majority, and the strength of the adsorption depends on the effective close interaction distances between CO2 molecules and the zeolites (d[C(CO2)-O(zeo)] and d[O(CO2)-O(zeo)] are 3-4 Å). Therefore, CO2 site in the middle of 8MR (site A) having total of 24 close contacts with framework oxygens is more stable than CO2 site in the cage (site B) with only 15 close contacts with framework oxygens in total. Even though site B* in cation-exchanged chabazites is strengthened by electrostatic interactions with zeolite frameworks, but higher occupancies of site A in the 8MRs were identified due to high quadrupole and dispersion interactions between CO2 and 8MR zeolite. The decrease in the hardness of metal cations Li+>Na+>K+>Cu2+ resulted in a decrease in the direct interactions of these cations with adsorbate CO2. The structural analysis of CO2 and H2O co-adsorbed on Cu-CHA-6 showed that water is preferentially adsorbed at the copper sites, while CO2 is located predominantly in the 8MRs.
Our refinement of X-ray diffraction patterns on bare adsorbent ZK-5 showed that Na+ locates in three different sites in D6R and in 8MR, Li+ prefers to locate at the center of the D6R, Mg2+ is located in the hexagonal prism, and larger cation K+ is in the middle of 8-membered rings. The weighted average of T-O distances for the feldspar structure with Si/Al=3.65 is 1.64 Å, which is similar to the values of 1.63 Å -1.64 Å in these refinements. The average of O-T-O angles is 109.5o with small deviations around this value for all cation-exchanged ZK-5 samples in agreement with tetrahedral coordination. At the loading amount of nearly 1CO2/8MR, 3 different CO2 sites are found on Na-ZK-5, 4 CO2 adsorption sites are found on Mg-ZK-5 and Li-ZK-5, and only one site of CO2 was found in the low loading amount of CO2 on K-ZK-5. The studies of the interactions of CO2 with Li, Na, K, and Mg-ZK-5 provide better understanding about the strength of the interactions of CO2 with extra-framework cations in ZK-5. The Rietveld refinement results also suggested that 8-membered ring zeolites have high affinity with CO2 moleules and are potential for CO2 separation, especially in the presence of water. Due to the high water content in flue gases, hydrophobic zeolites need to be developed to have better separation performance. Various types of zeolite frameworks including BEA*, CHA, FER, MFI, and STT were investigated in the adsorption of CO2 and N2 by experimental volumetric adsorption and Grand Canonical Monte Carlo simulations. All siliceous zeolites showed low CO2 adsorption heats (18-28 kJ/mol) due to the lack of the electric fields in the zeolite pores. FER siliceous zeolite with narrow pore openings displayed highest adsorption heats and highest selectivity of CO2/N2 and CHA zeolite has the highest adsorption capacity at ambient conditions of temperature and pressure. The study of adsorption of light hydrocarbons and CO2 in siliceous AEI, CHA, RRO, and STT zeolites led to a conclusion that RRO has highest selectivity of CO2 over CH4 and AEI, CHA, RRO all showed their potentials for the kinetic separation of propylene/propane mixture. The results from the thesis indicated our ability to design and tune the properties of zeolite materials to make these adsorbents better for CO2 separation.

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