Diffusivities of n-alkanes in silicalite by steady-state single-crystal membrane technique



A novel experimental technique that measures the diffusive flux through 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 cm2/s. Phenomenological equations 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 membrane configuration measures diffusivity in only one crystallographic direction. The micropore difusivities of C1 to C10 normal alkanes through silicalite crystal in the z-direction were measured at 30, 50 and 70°C. The activation energies for micropore diffusion are also 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 than diameter of molecule vis-à-vis pore diameter.