Accumulation of acid sites on natural clinoptilolite under recurring dehydration

A new ¹H NMR approach was applied to study the influence of recurring dehydration on the acidity of a natural Ca-rich zeolite clinoptilolite. It has been found that thermal cycling progressively increases the rate of the proton exchange between water molecules in the rehydrated state. The observed effect is interpreted as a result of the irreversible accumulation of specific structural defects represented by Brønsted acid sites: each dehydration at 720 K adds to the acid sites in an amount of the order of 10^?3 per unit cell. The number of these defects, detected by NMR in hydrated mineral, is in reasonable agreement with their amount estimated for the dehydrated state with an IR-spectroscopy CO-probe method. A comparison of the results obtained for two distinct zeolite samples shows that the Ca²⁺ ions are of first importance in the dehydration-induced formation of the active acid sites. The barrier for the proton-transfer reaction between the acid sites and H₂O molecules in hydrated clinoptilolite is found to be 46 kJ mol^?1, which is not too different from the value of 54 kJ mol^?1 reported recently for natural chabazite.