MEASUREMENT OF DIFFUSION IN SMALL 5A ZEOLITE CRYSTALS USING A WALL-COATED CAPILLARY COLUMN

A novel method of measuring diffusion in small (micron-sized) zeolite crystals has been developed based on measurement of the chromatographi c response for a capillary column, the internal surface of which is co ated with an incomplete monolayer of the zeolite crystals. Special tec hniques are needed to prepare such a column; in the commercially avail able wall-coated zeolite columns the surface coverage of the zeolite c rystals is too low to permit reliable diffusion measurements. A theore tical expression giving the HETP (height equivalent to a theoretical p late) as a function of gas velocity, based on a modification of Golay' s analysis for a liquid-coated capillary column, has been derived. As in conventional chromatography, the HETP is governed by the sum of the contributions from axial dispersion and mass transfer resistance, but , in contrast to conventional chromatography, the axial dispersion con tribution for a wall-coated column can be confidently estimated a prio ri. The relative importance of intercrystalline and external diffusion resistances depends infer alia on the ratio of tube to particle diame ters. The advantage of the technique arises because it is possible to make reliable measurements of the intracrystalline diffusivity, even f or relatively fast systems, by using a sufficiently small tube diamete r. The validity of the method is tested by measuring the diffusion of n-butane in small commercial 5A zeolite crystals. In contrast to other macroscopic techniques which, for this system, generally yield rather low apparent diffusivity values, the values we obtain are close to th e self-diffusivities obtained from p.f.g. n.m.r. measurements and the macroscopically measured transport diffusivities for large laboratory synthesized crystals.