Adsorption-Induced Phase Transition of Zsm-5 Zeolites

The phase transition of high siliceous ZSM-5 zeolites which was induced by the adsorption of p-xylene molecule was revealed through measurements of adsorption isotherms and differential heats of adsorption. A step appeared in each of the adsorption isotherm and an increase in the differential heats of adsorption was also observed at the same adsorption amount, which should correspond to the phase transition. With an increase in the Si/Al ratio, the onset of the transition became clearer and the average heats in the transition region became larger. Moreover, hysteresis appeared on the desorption isotherm on more siliceous ZSM-5. These results suggest that the more siliceous ZSM-5 is more susceptible to the transition. Domain structure based on template molecules may be the origin for the phase transition.