ADSORPTION AND TRANSPORT OF N-HEXANE IN SILICALITE-1 BY THE FREQUENCY-RESPONSE TECHNIQUE

The adsorption and mass-transfer behaviour of n-hexane in silicalite h ave bees investigated using a gravimetric balance and the frequency re sponse (FR) method, respectively. Deviations of the isotherms of this system from the type-I (Langmuir model) at low temperatures have been observed and attributed to the immobilisation of n-hexane molecules in the sinusoidal channels of silicalite-1. It was found that the FR spe ctra of the system can be described by a model involving two independe nt diffusion processes or a diffusion-rearrangement model, rather than a non-isothermal diffusion model. The first two models may be two spe cial cases of a model which assumes that three processes occur in the n-hexane-silicalite-1 system, two diffusion processes in the straight and sinusoidal channels, respectively, and a mass exchange process bet ween the two channels. Surface barriers have been observed from the FR data of the system and the development of more complicated models des cribing better the overall processes taking place in the system has be en suggested. Comparisons between the diffusivities and activation ene rgies of n-hexane diffusion in silicalite-1 determined by both FR and pulsed held gradient (PFG) NMR methods are also discussed.