XE-129 PFG NMR TRACER EXCHANGE MEASUREMENTS ON ZEOLITES

In Xe-129 NMR studies of xenon, sorbed by beds of zeolite crystallites, there may be substantial differences in the chemical shifts of the xenon atoms in the sorbed and in the gaseous phase. It is shown that also under this condition the rate of xenon exchange between the sorbate and the gas phase can be followed by Xe-129 PFG NMR, using the stimulated spin echo sequence. The resulting NMR tracer desorption curve is a function of the time between the second and the third 90 degrees rf pulses rather than, as in conventional H-1 PFG NMR, of the time between the two magnetic field gradient pubes. However, using the primary spin echo sequence, even by applying the identical field gradient program, only intracrystalline diffusion may be monitored. Experimental evidence is given by measuring the transverse nuclear magnetic relaxation times of Xe-129 in zeolite NaX in dependence on the crystal size, and by comparison of the attenuations of the primary and stimulated spin echoes in PFG NMR.