日本ゼオライト学会　刊行物 Publication of Japan Zeolite Association

At room temperature, the symmetry of ZSM-5 changes from P2₁/n to P2₁2₁2₁ via Pnma with increasing pressure of p-xylene. The adsorption capacities of these phases are 4,4 and 8 molecules per unit cell, respectively, and the Pnma phase strongly adsorbs p-xylene than the P2₁/n phase. It is interpreted that the phase transitions obey Le Chatlier-Braun's law, that is, that the phase changes so as to moderate the pressure increase by adsorbing much more molecules.

Adsorption isotherms with high-siliceous ZSM-5 (Si/Al=650) are well described by Langmuir model, and adsorption hysteresises observed are explained in terms of a domain structure of the crystal. With increasing Al-content, the hysteresis curve deviates from square form, indicating a complicated structure of domains.

Adsorption heat curves have broad wells assigned to P2₁/n-Pnma transition and sharp ones assigned to Pnma-P2₁2₁2₁, in accordance with the present theory.