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

The Si/Al ratio of the framework and the number of defect sites in the fram ework of zeolites were investigated by means of solid state NMR investigati ons and elemental analysis before and after dealumination of zeolites beta, ferrierite, mordenite and ZSM-5. Dealumination was performed by thermal tr eatment, complexation by oxalic acid and direct replacement of aluminum by silicon with gaseous silicon tetrachloride. The extent of dealumination dec reased in the same order as the number of T-sites in four-rings: beta > mor denite > ZSM-5 > ferrierite. Considerable differences arising from the synt hesis conditions for samples of the same zeolite type were also observed. H -1 MAS NMR showed that the extent of dealumination increased with the numbe r of Bronsted acid sites interacting with the zeolite framework. and Si-29 MAS NMR showed that at the same time the number of defect sites in the samp les increased. Depending on the synthesis conditions, the framework is inte rrupted to different extents and, as a result, the framework becomes more f lexible, thereby enabling interaction between Bronsted protons and oxygen a toms in the framework. Factors such as the zeolite structure type, the Si/A l ratio of the framework, the crystal size, and the number of Bronsted acid sites interacting with the framework or the number of defect sites influen ce the dealumination behavior of an individual zeolite sample. (C) 2000 Els evier Science B.V. All rights reserved.