INTERCAGE AND INTRACAGE TRANSPORT OF AROMATICS IN ZEOLITES NAY, HY, AND USY STUDIED BY H-2 NMR

Adsorption and transport characteristics of benzene and p-xylene in three forms of zeolite Y are reported: NaY (Si/Al = 2.65), HY (Si/Al = 3.25), and USY (Si/Al 40). H-2 NMR line-shape studies and both independent and simultaneous analysis of the temperature-dependence of spin-lattice and spin-spin relaxation times of the deuterated sorbates confined within the zeolite frameworks are compared as methods of characterizing sorption phenomena and estimating diffusion coefficients and activation energies to site-hopping motions within the zeolite framework. Both benzene and p-xylene in NaY show evidence of one site-hopping motion characterized by an activation energy of 25.8 and 32.0 kJ mol(-1), respectively. In contrast the same sorbates confined within HY and USY show two different motional regimes. Below 260 K, activation energies on the order of 10-15 kJ mol(-1) are measured, whereas above this temperature larger activation energies of 24-30 kJ mol(-1) are typical. It is suggested that the higher activation energies are associated with intercage jump processes, while the lower activation energies are characteristic of intracage transport. The lack of a low-energy activated translational process in NaY suggests that the sorbed aromatics are tightly bound to the Na+ occurring increases with increase in Si/Al ratio, decrease in sorbate concentration, and decrease in the size of the exchangeable cation.