ADSORPTION INDUCED PHASE-TRANSITION OF ZSM-5 BY P-XYLENE

Adsorption induced phase transitions of ZSM-5 by p-xylene, taking plac e between P2(1)/n-Pnma and Pnma-P2(1)2(1)2(1) phases, have been measur ed in the temperature range 298-352 K. Adsorption isotherms well obey the Langmuir equation. The differential heat of adsorption and differe nce 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 m ol(-1) and -31.6 k in the P2(1)/n phase, respectively. Adsorption heat s 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 thermo dynamics. Dependences of the transition pressure upon temperature and types of exchangeable cation in ZSM-5 are quantitatively explained. Ob served adsorption-desorption hysteresis was ascribed to a domain struc ture of the ZSM-5 crystal, but was not quantitatively analyzed.