DIFFUSIVITIES OF N-ALKANES IN SILICALITE BY STEADY-STATE SINGLE-CRYSTAL MEMBRANE TECHNIQUE

A novel experimental technique that measures the diffusive flux throug h a single-crystal membrane (SCM) was developed and tested. Unlike all other macroscopic techniques that depend on a transient response, SCM is used under steady-state conditions, which results in a wide range of applicability from 10(-2) to 10(-11) cm(2)/s. Phenomenological equa tions for the steady-state data analysis were developed. The variation of driving force over the diffusion path is included in the model. As required by thermodynamics, the micropore concentration is given as a function of surface-excess amount adsorbed and gas density. The membr ane configuration measures diffusivity in only one crystallographic di rection. The micropore diffusivities of C-1 to C-10 normal alkanes thr ough silicalite crystal in the z-direction were measured at 30, 50 and 70 degrees C. The activation energies for micropore diffusion are als o reported. The data agree excellently with the other two studies that measure directional diffusivities. Diffusion and adsorption of hexane and heptane in silicalite display structural heterogeneity induced by the comparable lengths of molecules and silicalite channels rather th an diameter of molecule vis-a-vis pore diameter.