Characterization of extra-framework species of steamed and acid washed faujasite by MQMAS NMR and IR measurements Original Research Article

Ultrastable Y zeolites with different degrees of steaming severity were further acid leached. They were characterized by multiple quantum magic angle spinning (MQMAS) NMR and infrared spectroscopy, examining the hydroxyl regions as well as the ring vibration region of probe molecules (pyridine and 2,6 dimethyl-pyridine (2,6-lutidine)). The isotropic chemical shifts of the aluminum species observed through 2D MQMAS technique when compared with the 1D MAS spectra showed unambiguously that the peak in the δ2≈32 ppm region can be attributed to a species with tetrahedral coordination. This species was readily removable by acid leaching and it could be an amorphous silica–alumina. Infrared spectroscopy showed that acid leaching reduced but could not completely remove the Lewis sites. This correlates better with the behavior of the octahedral extra-framework species . Even though, acid leaching exposed slightly more Brønsted sites, no formation of strong Brønsted acidity was observed. However, acid removal of the 32 ppm species led to an increase of silanol groups that absorbed at ∼3744 cm−1. According to recent studies, such hydroxyl groups were poisoned by coke formation during certain hydrocarbon conversion conditions.