Comparative study of the hydrophobicity of BEA, HZSM-5 and HY zeolites determined by competitive adsorption

A hydrophobicity study was realized for a series of siliceous and steamed BEA, HZSM-5 and HY zeolites. The hydrophobicity was assessed according to the Weitkamp method by performing water and toluene competitive adsorption experiments. The Weitkamp hydrophobicity indexes (HI) were determined for each sample. The BEA samples were the most hydrophobic samples and there was a linear correlation between HI values and the Brønsted acidity. This was explained by a water preferential adsorption on the Brønsted acidic sites. On the contrary for the HZSM-5 and HY samples the decrease of HI with the Brønsted acidity was not linear. This was explained by a water preferential condensation in the available porous volume of the zeolites instead of water chemisorption on the Brønsted acidic sites. For the highly mesoporous samples the water adsorption was essentially localized in the mesopores so the hydrophobicity of these samples remained almost constant despite important variations of the Brønsted acidity. Due to their microporous structure, the HZSM-5 samples were more hydrophobic than the HY samples for a same value of Brønsted acidity and Si/Al ratio, despite a lower toluene adsorption for HZSM-5 samples because of steric hindrance of toluene. In the case of the competitive adsorption of water and toluene, the values of HI were essentially depending on the localization of water molecules and the global water uptake, despite a noteworthy increase of the toluene uptake.