Type, stability, and acidity of hydroxyl groups of HNaK-erionites

The concentration and acid strength of the hydroxyl groups of a series of HNaK erionites have been investigated by means of temperature programmed activation (t.p.a.), temperature programmed desorption (t.p.d.) of ammonia, and transmission absorption i.r. spectroscopy. With an increasing degree of NH₄⁺ exchange, primarily Na⁺, at first, and, subsequently, K⁺ cations are replaced. During activation of these materials, NH₃ desorbs in a temperature interval from 540 to 900 K, leading to hydroxyl groups. However, it is impossible to decompose NH₄⁺ quantitatively without dealumination, if the degree of NH₄⁺ exchange is higher than 85%. Five types of hydroxyl groups are present, characterized by i.r. bands at approximately 3741, 3691, 3660, 3618-3608, and 3566 cm^-1. The assignment of these bands is discussed. Only the OH groups represented by the bands at 3660, 3618-3608, and 3566 cm^-1 adsorb ammonia under the conditions employed. The concentration and strength of the bridging hydroxyls (3618-3608 cm^-1), associated with framework Al³⁺, increase with increasing degree of ion exchange.