Adsorption and transport of n-hexane in silicalite-1 by the frequency response technique

Abstract

The adsorption and mass-transfer behaviour of n-hexane in silicalite have been investigated using a gravimetric balance and the frequency response (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 spectra of the system can be described by a model involving two independent diffusion processes or a diffusion–rearrangement model, rather than a non-isothermal diffusion model. The first two models may be two special 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 between the two channels. Surface barriers have been observed from the FR data of the system and the development of more complicated models describing better the overall processes taking place in the system has been suggested. Comparisons between the diffusivities and activation energies of n-hexane diffusion in silicalite-1 determined by both FR and pulsed field gradient (PFG) NMR methods are also discussed.