Time-resolved powder diffraction as an analytical tool for diffusion studies in microporous topologies

Investigations on an MFI/p-xylene system by time-resolved X-ray powder diffraction in two selected angular domains show that it is possible to derive kinetic and diffusion parameters describing a spontaneous desorption process. The corresponding value of the overall diffusion coefficient, calculated for the MFI,8p-xylene_(S) → MFI,4p-xylene_(S) + 4p-xylene_(g) reaction, is in the 0.4–0.6 × 10⁻¹⁰ cm² s⁻¹ range. The desorption proceeds via a two-phased system. Accordingly, the only solid species involved in this desorption are the high- and low-coverage XYLII and XYLI phases, containing respectively eight and four p-xylene molecules per unit cell. Similar results are also observed for the MFI/benzene, toluene and pyridine systems, which all present high-coverage phases, with eight molecules per unit cell, desorbing via a two-phased system.