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

Ultrastable Y zeolites with different degrees of steaming severity were fur ther acid leached. They were characterized by multiple quantum magic angle spinning (MQMAS) NMR and infrared spectroscopy, examining the hydroxyl regi ons 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 delta (2)approxim ate to 32 ppm region can be attributed to a species with tetrahedral coordi nation. This species was readily removable by acid leaching and it could be an amorphous silica-alumina. Infrared spectroscopy showed that acid leachi ng reduced but could not completely remove the Lewis sites. This correlates better with the behavior of the octahedral extra-framework species. Even t hough, acid leaching exposed slightly more Bronsted sites, no formation of strong Bronsted acidity was observed. However, acid removal of the 32 ppm s pecies led to an increase of silanol groups that absorbed at similar to 374 4 cm(-1). According to recent studies, such hydroxyl groups were poisoned b y coke formation during certain hydrocarbon conversion conditions. (C) 2001 Elsevier Science B.V. AU rights reserved.