Molecular spectra and polarity sieving of aluminum deficient hydrophobic H-Y zeolites

Contrary to the common belief that Y zeolites require full dehydration and dealumination to be hydrophobic, some Brönsted acidic, commercial, H-Y zeolites from the CBV 780 and CBV 901 series (a0 ∼ 24.24 Å; Si/Al ∼ 40) have been made that demonstrate stronger hydrophobicity than aluminum free silicalite of the MFI family. Combined adsorption and molecular modeling data suggest that this extremely low water adsorption capacity might be the consequence of an as yet unexplored polarity sieving effect (PSE) on adsorbates having different dipole moments. FTIR and Raman spectroscopic measurements indicate systematic differences between the two families of commercial H-Y zeolites as to the type of hydroxyl groups present as well as the bending and stretching vibrations of T-O-T bonds. These fine structural differences affect the homogeneity of the negative electrostatic potential of the micropores, and that affects the resistance to polar adsorbates entering these pores. It is demonstrated that penetration of the adsorbates becomes increasingly more difficult as their polarity increases and highly polar H2O molecules cannot enter the pores of the most hydrophobic H-Y zeolites at all. The adsorption capacity for nonpolar adsorbates decreases only slightly with increasing hydrophobicity. The adsorption isotherm of N2 shows a similar step on moderately hydrophobic Y materials as that reported for the MFI type silicalite around p/p0 ∼ 0.05, but this step gradually disappears with increasing hydrophobicity of the Y zeolite.