A microscope FTIR mapping study on diffusion of hydrocarbons in single silicalite crystal particles

A simple microscope FTIR mapping method is illustrated for studying anisotropic diffusion in zeolite. This method is based on measurements of evolution of the adsorbate concentration (IR absorbance) profiles in two principal directions of a single zeolite crystal particle of a few ten microns in sizes. A mathematical model is presented and a comparison of its numerical solution with experimental data allows the determination of the diffusivity in each direction in zeolite crystals. Diffusion rates during desorption of n-hexane, n-octane and p-xylene in silicalite particles of 130 x 50 x 50 mu m were experimentally studied with the microscope FTIR line mapping. Negligible changes in adsorbate concentration profiles were observed after the silicalite particle saturated with one of the three hydrocarbons was exposed to air for about 15-44 h at room temperature. Under vacuum conditions and at higher temperatures, the hexane concentration profiles developed within about 0.5 h, from which the averaged diffusivity of hexane in the a and b channels of silicalite was found 1.1 x 10(-10) cm(2)/s (at 80 degrees C), significantly higher than that for the c direction. Due to the intergrown microstructure of the single silicalite particles studied in this work, unusual adsorbate concentration profiles were observed in some cases. These results suggest that the adsorbate may diffuse in or out of the single crystal particle through the intercrystallite gaps connected with the straight b channels of silicalite. (C) 2000 Elsevier Science B.V. All rights reserved.