FT-IR solid-state NMR investigation of surface hydroxyl groups on dealuminated ZSM-S

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