Diffusion of n-butane/iso-butane mixtures in silicalite-1 investigated using infrared (IR) microscopy

Adsorption and diffusion of n-butane/iso-butane mixtures in individual silicalite-1 crystals has been investigated using infrared (IR) microscopy. The equilibrium sorption isotherm for an equimolar gas phase mixture is calculated using Configurational Bias Monte-Carlo simulations. The comparison between simulation results and the experimental data enabled the determination of absolute values of concentration in the IR experiments.n-Butane uptake under presence of iso-butane and, vice versa, iso-butane uptake under presence of n-butane have been studied experimentally. It is shown that the n-butane counter-uptake is not limited by the n-butane diffusivity, but by the availability of free sites, which is, in turn, determined by the mobility of iso-butane. A site-percolation threshold, given by the number of iso-butane molecules blocking the ‘traffic junctions’ of the channel network, has been employed for explaining the occurrence of network regions which are initially inaccessible for n-butane in the counter-uptake process.