The nature of active sites on zeolites: IX. Sodium hydrogen zeolite

Abstract

The acidic and catalytic properties of a series of sodium hydrogen Y zeolites have been studied as a function of the zeolitic sodium content. The structural hydroxyl groups were investigated after the zeolite had been calcined at 480 °C. The characteristic hydroxyl groups of hydrogen Y zeolite with frequencies near 3745, 3640, and 3540 cm −1 were observed. The intensity of the 3640-cm −1 band increases almost linearly with decreasing sodium content. The 3540-cm −1 band is very weak until about half of the sodium ions of the zeolite have been exchanged. The intensity of the band then increases rapidly until at low sodium contents, the 3640- and 3540-cm −1 bands are of equal intensities. Evidence for the 3540-cm −1 band type of hydroxyl groups being located in the hexagonal prisms of the structure is discussed. The acidities of the zeolites were measured by means of the infrared spectra of chemisorbed pyridine. After calcination at 480 °C, the zeolites are mostly in the Brönsted acid form. The concentration of Brönsted acid sites increases linearly with decreasing sodium content of the zeolite until 60–70% of the sodium ions have been replaced in the structure, corresponding to sodium ions remaining only in the hexagonal prisms of the structure; the Brönsted acidity then remains approximately constant. The catalytic activity for o-xylene isomerization was measured. The activity increases with decreasing sodium content of the zeolite. The activity per site also increases with decreasing sodium content. The changes in catalytic activity with sodium content are considered in terms of the structure of the zeolite. The hydroxyl group concentration, Brönsted acidity, and catalytic activity all increase in a parallel manner with decreasing zeolite sodium content. Brönsted acid sites are probably the primary catalytic centers. However, because of the known changes of activity with calcination temperature, other types of sites are probably involved, either simultaneously or sequentially, in reactions over zeolites.