Hydrophobic nano-layer on surface prevents H2O adsorption in moderately aluminum deficient Y zeolite crystals

Experimental and computer simulation sorption and spectroscopic studies were evaluated in-order to be able to answer two puzzling questions. (1) How can a Si/Al similar to 40 ratio commercial H+ exchanged Y zeolite (H-Y), CBV 780, adsorb only nearly as little H2O as a Si/Al similar to 4330 ratio ZSM-5? (2) Why another Si/Al similar to 40 ratio commercial H-Y, CBV 901, adsorbs much less H2O than CBV 780 and even less than the mentioned highly siliceous ZSM-5? These zeolites have not been chemically treated to be hydrophobic, only their patented synthesis and post-treatment conditions differ. Results indicate that the crystallites of CBV 901 are covered with a defect and aluminum free siliceous nano-layer, which prevents any H2O penetration into the bulk. The external surface of CBV 780 crystallites has about the same little amount of Al as its bulk, presumably less than 4 Al per unit cell. Due to the low number of Al-associated Bronsted acidic hydroxyls (BA-OH), on which H2O preferably adsorbs, little water adsorption takes place in the micropores. On the other hand the surface of the large mesopore defects of CBV 780 must be enriched in with Al to get the measured overall Si/Al similar to 40 ratio in this material, which provides enough BA-OH sites for adsorbing H2O molecules as a prelude to mesopore filling, that manifests itself in a Type II sorption isotherm.