Sorption Kinetics of n-Paraffins in MFI-type Zeolites Investigated by micro-FTIR

Abstract

The sorption properties of various n-paraffins (carbon number n(c) = 6...9) in single crystals of zeolite HZSM-5 were studied by means of micro-FTIR spectroscopy. The uptake kinetics are well described by a numerical solution of the diffusion equation taking into account the sample geometry and the time dependence of the sorbate boundary concentration. The temperature dependence of the transport diffusion coefficients follows an Arrhenius law, from which the activation energies were calculated. Whereas n-hexane and n-heptane were desorbed readily after reducing the sorbate partial pressure to zero, the concentrations of n-octane as well as of n-nonane in the zeolite channel system remained at ca. 30 % of the initial loading. Only after a purge step at elevated temperature fully desorption was achieved.