Base-induced Brönsted Acid Sites on Dealuminated HY Zeolite: A Solid-state NMR Spectroscopy Study

Using trimethylphosphine(TMP) and d(5)-pyridine (deuterated pyridine) as the basic probe molecules, the concentrations of Bronsted acid sites on both HY zeolite and dealuminated HY zeolite have been quantitatively determined using solid-state H-1 and P-31 magic-angle spinning (MAS) NMR. After the adsorption of the probe molecules, the concentration of Bronsted acid sites on the dealuminated HY zeolite increases by about 25%, whereas the parent HY sample remains almost unchanged. The increase in the concentration of Bronsted acid sites is due to the appearance of base-induced Bronsted acid sites in the dealuminated HY zeolite. The terminal SiOH in the vicinity of the aluminum atom is to form a bridging hydroxyl group (SiOHAl) in the presence of the basic probe molecules. The mechanism of formation of the induced Bronsted acid sites has also been discussed.