Adsorption induced phase transition of ZSM-5 by p-xylene

Adsorption induced phase transitions of ZSM-5 by p-xylene, taking place between P21/n–Pnma and Pnma–P212121 phases, have been measured in the temperature range 298–352 K. Adsorption isotherms well obey the Langmuir equation. The differential heat of adsorption and difference in thermal entropies between adsorbate and gaseous molecules were determined to be 84 kJ mol-1 and -33 k in the Pnma phase and 80 kJ mol-1 and -31.6 k in the P21/n phase, respectively. Adsorption heats were calorimetrically measured as a function of adsorbed amounts at 298, 333 and 352 K. The heat curves have valleys relating to the phase transition, and their shapes are well explained by statistical thermodynamics. Dependences of the transition pressure upon temperature and types of exchangeable cation in ZSM-5 are quantitatively explained. Observed adsorption–desorption hysteresis was ascribed to a domain structure of the ZSM-5 crystal, but was not quantitatively analyzed.