Diffusion of C4 hydrocarbons in silicalite-1

Abstract

Information on the molecular dynamics of n-butane and 2-butyne, sorbed in silicalite-1, has been obtained from both sorption kinetics and ¹³C NMR spectral lineshape analysis. The adsorption/desorption rates were measured by the recently developed single-step frequency-response technique. In contrast to n-butane the sorption behaviour of 2-butyne, which is a stiff molecule due to its triple bond, cannot be described by a single diffusion coefficient. For this molecule an initial diffusion-controlled sorption is followed by a sorption process the rate of which is determined by a slower molecular reorientation leading to a denser packing of the sorbed molecules. This result could be considered as a common feature in sorption kinetics. From the ¹³C NMR spectra it can be concluded that sorbed 2-butyne molecules perform a 90° flip motion with a correlation time, τ 1 × 10^–5 s.