Acidity of MCM-58 and MCM-68 zeolites in comparison with some other 12-ring zeolites

Two novel zeolites MCM-58 and MCM-68 with 12-ring channels and containing the cages in the structure were investigated with respect to their acidic properties and compared with other two 12-ring zeolites, namely Beta and ZSM-12. The main objective was to characterize the accessibility and acid strength of Lewis and Brønsted sites using FTIR spectroscopy. Pyridine adsorption allowed for quantification of Brønsted and Lewis acid centres and pointed out that while in the structure of MCM-58 nearly all Al atoms form acidic SiOHAl bridges, only about half of Al is a source of the Brønsted acidity in MCM-68. The special attention was devoted to the origin and location of the hydrogen-bonded OH groups in the MCM-58 zeolite. The frequency lowering is caused by the across-the-ring interaction with the oxygen atom and not by the interaction with the nearby OH groups, as proved by high temperature activation and deuteration experiments. The location of the 3485 cm−1 OH groups in the five member ring was proposed. Carbon monoxide adsorption allowed proposing the location of the residual K+ cations in the main channels of MCM-58 and in the 61046 cages in MCM-68.