FT-IR AND SOLID-STATE NMR INVESTIGATION OF SURFACE HYDROXYL-GROUPS ONDEALUMINATED ZSM-5

H-ZSM-5 (Si/Al = 14.6) was progressively dealuminated by steaming at 3 50 degrees C (1-72 h) and then characterized by transmission infra-red and solid-state NMR spectroscopies (Si-29, Al-27, and H-1). The struc tural hydroxyl groups on the activated zeolite surface were examined b y FT-IR before and after pyridine adsorption in the range 4000-400 cm( -1). The number of Bronsted acid sites decreases as the steaming time increases and Bronsted sites of varying acid strength are shown to be present on the surface. Moderately steamed (25-50 h at 350 degrees C) ZSM-5 contains three distinct types of hydroxyl groups, namely, bridge d Si-OH-Al species of strong bronsted acidity, non-acidic terminal SiO H groups, and weakly acidic hydroxylated aluminum species, which was a lso shown by 1H NMR. Two distinct types of Lewis acid sites were obser ved. The number of Si-OH defects estimated by Si-29 and Al-27 NMR is a t maximum in a sample steamed for 25 h and then decreases due to the h ealing of hydroxyl nests during further dealumination.