Measurement of intracrystalline diffusion of nitrogen in zeolites NaX and NaCaA using pulsed field gradient n.m.r.

The measurement of intracrystalline diffusion (D-intra) of nitrogen in zeolites by N-15 pulsed field gradient (p.f.g.) n.m.r. is complicated by the small magnetogyric ratio. This causes a large reduction in signal sensitivity, necessitating a significant enhancement of the field gradient pulse intensity in comparison with H-1 p.f.g. n.m.r. Owing to recent progress in measuring techniques, these technical problems were overcome and direct measurement of the intracrystalline self-diffusion coefficients of nitrogen in 12- to 15-mu m crystals of zeolite NaX and NaCaA in a temperature range between 135 and 300 K was completed. The intracrystalline diffusion for both these zeolites was found to be a hundred times slower than the long-range diffusion within the crystal bed. At present, these measurements are not possible for typical A- and X-type zeolites, which have diameters less than 10 mu m. (C) Elsevier Science Inc. 1997