An FR study of the n- and i-butane diffusion in theta-1, ferrierite and silicalite-1

In the one-, two-, and three-dimensional (1D, 2D, and 3D) micropore system of theta-1, ferrierite, and silicalite-1, respectively, diffusion to the sorption sites was found to control the rate of propane and butane sorption at 373 K and 133 Pa pressure. The diffusivity of propane (C3) and n-butane (n-C4) was about the same and much faster than that of the i-butane (i-C4). In theta-1 and ferrierite, the i-C4 transport was too slow to be characterized by the frequency–response (FR) method. Ten-ring apertures determine the critical pore dimension of each material. In ferrierite, however, the transport diffusivity of the n-alkanes was about 10−15 m2 s−1, several orders of magnitude smaller than that in theta-1 and silicalite-1. Results suggest that normal alkane diffusion proceeds in the 3D pore system of silicalite-1 and single-file diffusion prevails in the micropores of ferrierite and theta-1.