EFFECTIVENESS OF WINDOW BLOCKING IN ZEOLITE NAY BY STRONGLY COADSORBED MOLECULES

The effectiveness of window blocking in reducing the diffusion of a so rbate inside a three dimensionally connected zeolite pore system is in vestigated using pulsed field gradient(PFG) NMR. H-1 PFG NMR measureme nts were used to study the diffusion of methane in two binary sorbate systems at varying cosorbate loading: (i) methane and benzene coadsorb ed in NaY, and (ii) methane and ethylene coadsorbed in NaY. In both ca ses (i.e. benzene and ethylene), D-blacking molecule/D-methane was les s than similar to 3 x 10(-2), and we are able to test percolation theo ry predictions. These measurements were compared with previously deter mined variable loading single-component methane self-diffusion coeffic ients in NaX (Karger et al., 1980, J. Chern. Sec. Faraday Trans. 176, 717-737). Large crystals of NaY (similar to 30 mu m) were essential fo r this study to ensure the accuracy of the diffusion measurements. A c onstant base methane loading of -2 molecules per NaY supercage was emp loyed in all the samples used in this study. The methane self-diffusio n coefficient in the presence of coadsorbed benzene and ethylene was r educed in a manner qualitatively consistent with concepts from percola tion theory applied to a diamond lattice (representing the connectivit y of the NaY supercages). Coadsorbed benzene though was more effective in ''blocking'' the methane diffusion. Excellent agreement was found between the measured methane selfdiffusion coefficients in the presenc e of benzene and the prediction based on the effective medium approxim ation (EMA) to the percolation theory. Suggestions are made about the prospects of applying other models to blocking by more mobile cosorbat es like ethylene.