Experimental investigations of the structure-property relations of CO2-selective zeolite adsorbents

Pham, Trong D.
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University of Delaware
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. For zeolite materials to be practical and useful as adsorbents for CO 2 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 CO 2 adsorption spectra show that physisorption accounts for the largest fraction of the total CO 2 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 silica chabazite zeolites. The dispersion interactions between Si-CHA zeolite and CO2 is the majority of the adsorption energy, 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. 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. Various types of zeolite frameworks including BEA*, CHA, FER, MFI, and STT were investigated in the adsorption of CO 2 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 CO 2 separation. (Abstract shortened by UMI.)