UNEXPECTEDLY LOW TRANSLATIONAL MOBILITY OF METHANE AND TETRAFLUOROMETHANE IN THE LARGE-PORE MOLECULAR-SIEVE VPI-5

Molecular diffusion of methane and tetrafluoromethane in the microporo us material VPI-5 was studied by pulsed field gradient nuclear magneti c resonance (PFG NMR) and NMR exchange experiments. The translational mobility of both molecules in VPI-5 was found to be at least two order s of magnitude smaller than in ZSM-5. This is surprising since the cha nnels in VPI-5 are about two times as wide as those in ZSM-5. The surp risingly small translational mobility in VPI-5 could be caused by a mo re complete stabilization effect or by single-file diffusion. The intr acrystalline mean life time of methane in VPI-5 directly measured in t he NMR tracer exchange experiments was found to be in satisfactory agr eement with the value estimated on the basis of the PFG NMR data on tr anslational mobility for ordinary intracrystalline diffusion. It must be ruled out, therefore, that molecular transportation over length sca les of the order of the crystallite dimensions is controlled by single -file diffusion.