INTRACRYSTALLINE DIFFUSION OF BENZENE IN SILICALITE - EFFECT OF STRUCTURAL HETEROGENEITY

The sorption kinetics of benzene in silicalite have been measured gravimetrically using large silicalite crystals of sizes 350 mum x 105 mum x 105 mum and 270 mum x 70 mum x 70 mum in the temperature range 283-343 K. Experiments were performed under conditions that ensured isothermal operation with intracrystalline diffusion control. An analytical expression for the Darken's correction factor (delta In P/delta In a) was derived based on the Hill-de-Boer equation and was used to determine the variation of corrected diffusivity with concentration. The transport diffusivity varied significantly with the adsorbed-phase concentration but the corrected diffusivity was found to be essentially independent of the concentration. However, at temperatures below the surface transition temperature, a maximum in corrected diffusivity was observed at an adsorbed-phase concentration of four molecules per unit cell (uc). This maximum is the direct result of the type IV isotherm exhibited by the silicalite-benzene system below the surface transition temperature. It is proposed that the increase in corrected diffusivity at the critical adsorbed-phase concentration of 4 molecules uc-1 arises from reorientation of benzene molecules resulting in a much more efficient packing in the pore channel system.