Comparison of the dealumination of zeolites beta, mordenite, ZSM-5 and ferrierite by thermal treatment, leaching with oxalic acid and treatment with SiCl4 by 1H, 29Si and 27Al MAS NMR

The Si/Al ratio of the framework and the number of defect sites in the framework of zeolites were investigated by means of solid state NMR investigations and elemental analysis before and after dealumination of zeolites beta, ferrierite, mordenite and ZSM-5. Dealumination was performed by thermal treatment, complexation by oxalic acid and direct replacement of aluminum by silicon with gaseous silicon tetrachloride. The extent of dealumination decreased in the same order as the number of T-sites in four-rings: beta>mordenite>ZSM-5>ferrierite. Considerable differences arising from the synthesis conditions for samples of the same zeolite type were also observed. 1H MAS NMR showed that the extent of dealumination increased with the number of Brønsted acid sites interacting with the zeolite framework, and 29Si MAS NMR showed that at the same time the number of defect sites in the samples increased. Depending on the synthesis conditions, the framework is interrupted to different extents and, as a result, the framework becomes more flexible, thereby enabling interaction between Brønsted protons and oxygen atoms in the framework. Factors such as the zeolite structure type, the Si/Al ratio of the framework, the crystal size, and the number of Brønsted acid sites interacting with the framework or the number of defect sites influence the dealumination behavior of an individual zeolite sample.