Transport enhancement in binderless zeolite X- and A-type molecular sieves revealed by PFG NMR diffusometry

摘要

Pulsed field gradient (PFG) NMR is applied for probing the rate of mass transfer of water molecules in zeolite molecular sieves (beads) of type 4A and NaX (NaMSX). Water diffusivities in the binderless species are found to notably exceed the diffusivities in the binder-containing beads. Diffusivity enhancement in the binderless species is referred to both the existence of microporous zeolite "bridges" connecting the individual particles (crystallites) of genuine zeolite structure and the notably larger diameters of the transport pores within the binderless beads. Either of these structural features leads to an acceleration of long-range diffusion and, hence, to reduced uptake and release times on the individual beads. Both influences act in parallel. In this case the (apparent) activation energy of long-range diffusion can be expected to be intermediate between the activation energy of intracrystalline diffusion and the isosteric heat of adsorption as experimentally observed.