P-XYLENE ADSORPTION ON SILICALITE MEASURED BY A QUASI-EQUILIBRIUM GRAVIMETRIC TECHNIQUE

The isotherms of sorption of p-xylene in silicalite in the range 323-353 K have been measured using a quasi-equilibrium gravimetric method. It consists of recording the gravimetric response to a continuous increase of the sorbate pressure generated by an adjustable leak valve device. Experimental conditions have been carefully examined in order to eliminate diffusion and thermal effects. The differential molar heat of adsorption as a function of pore filling has been deduced from the isotherms, exhibiting abrupt drops around four and eight molecules per unit cell. These observations were correlated with X-ray powder diffraction interpretations according to which successive phase transitions depending on the p-xylene loading were observed. A general scheme depicting the full sorption mechanism is proposed.