Hydroxidgruppen an Zeolithen. I. Zahl und Eigenschaften von Hydroxidgruppen an CaNaY- und MgNaY-Zeolithen unterschiedlichen Austauschgrades

Hydroxide Groups on Zeolites. I. Number and Properties of Hydroxide Groups on CaNaY and MgNaY Zeolites of Different Degree of Ion Exchange The number of hydroxide groups was studied on CaNaY and MgNaY zeolites by D₂ exchange in dependence on the degree of exchange, and their properties were examined by infrared spectroscopy. At these zeolites there exist five types of hydroxide groups which differ by their concentration, their acidic strength, and their thermal stability. The OH-groups are produced mainly by dissociative chemisorption of water at the bivalent cations. The concentration of OH groups shows a characteristic dependence on the degree of ion exchange. There are OH groups which are strongly acid BRÖNSTED centers, capable of protonisation of pyridine, and others which are only capable of weak interactions with pyridine. The number of the acid BRÖNSTED centers increases with the degree of ion exchange analogous to the total number of the hydroxide groups and is highly dependent on the species of the cations exchanged and on the preheating temperature. Compared with CaNaY zeolites, MgNaY zeolites posess a greater number of hydroxide groups which are thermally more stable.