Kinetics of sorption of aromatics in zeolite ZSM-5

The kinetics of sorption of benzene, p-xylene and o-xylene into crystals of zeolite ZSM-5 in the hydrogen form were investigated gravimetrically for well-defined constant boundary conditions and constant temperature (298 K) of the sorbent. Zeolite crystals were mounted on aluminium foil separated in space, the metal foil acting as sample holder and heat sink in the exothermic sorption process. The influence of crystal size on the rate of sorption was studied by comparing two samples of the zeolite differing only in crystal size.

Sorption of benzene into ZSM-5 follows a pattern expected on the basis of the diffusion equation with constant diffusivity. p-Xylene is sorbed initially faster than benzene; its rate of sorption decreases at higher degrees of loading where a hysteresis loop in the isotherm is observed. Sorption of o-xylene proceeds much more slowly than that of p-xylene and does not seem to be controlled by diffusion.