Effect of pore structure on diffusion of sorbates in zeolites

This thesis describes the application of nuclear magnetic resonance (NMR) techniques to measure the dynamics of sorbates in the constrained geometries presented by zeolite molecular sieve micropores. Molecular simulations have been used to further probe the effect of structural modifications of the zeolite on the siting and energetics of the adsorbed phase. The aim of this research effort has been to understand the relationship between the pore structure of the zeolite and the mobility of sorbates. The issues addressed in this research are relevant to the application of zeolites in shape selective catalysis and separations.; The self-diffusion of simple probe sorbate molecules like methane and ethylene has been studied in zeolites of varying pore architecture using the pulsed field gradient (PFG) NMR technique. Using NMR inversion recovery measurements, we estimated the rate of intercage hopping of xenon in zeolite NaA and found it to decrease with pore crowding. The effect of dealumination on adsorption and diffusion in mordenite was studied using a combination of experiments and grand canonical Monte Carlo simulations. Experimental studies using methane as sorbate indicated that diffusional constraints were relieved by dealumination. Simulations revealed an octahedral lattice of sorption sites for xenon in mordenite which remained virtually unchanged by dealumination.; Diffusion measurements of methane in large crystals of the anisotropic molecular sieve AlPO{dollar}sb4{dollar}-5 established the motion of methane to be unidirectional, but not single-file. Finally, we have carried out multicomponent diffusion measurements in large Y and silicalite crystals. Blocking caused by the presence of strongly coadsorbed molecules like benzene and ethylene was found to strongly suppress the diffusion of the relatively mobile methane in NaY. Excellent agreement was found between the experimental diffusivity data and the prediction based on the effective medium approximation to the percolation theory.