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 th ree 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 independ ent and simultaneous analysis of the temperature-dependence of spin-la ttice and spin-spin relaxation times of the deuterated sorbates confin ed within the zeolite frameworks are compared as methods of characteri zing sorption phenomena and estimating diffusion coefficients and acti vation energies to site-hopping motions within the zeolite framework. Both benzene and p-xylene in NaY show evidence of one site-hopping mot ion characterized by an activation energy of 25.8 and 32.0 kJ mol(-1), respectively. In contrast the same sorbates confined within HY and US Y show two different motional regimes. Below 260 K, activation energie s on the order of 10-15 kJ mol(-1) are measured, whereas above this te mperature larger activation energies of 24-30 kJ mol(-1) are typical. It is suggested that the higher activation energies are associated wit h intercage jump processes, while the lower activation energies are ch aracteristic of intracage transport. The lack of a low-energy activate d translational process in NaY suggests that the sorbed aromatics are tightly bound to the Na+ occurring increases with increase in Si/Al ra tio, decrease in sorbate concentration, and decrease in the size of th e exchangeable cation.