The dense-structured zeolites of types ZSM-5 (MFI) and mordenite (MOR) of different SiO 2 /Al 2 O 3 moduli are relatively stable under treatment by liquid water for 72 hours up to 513 K. The open-structured zeolites of types Y in dealuminated modification (FAU) and beta (BEA) undergo strong decomposition in the same range. For these two sample types a mathematical model for the decomposition of the zeolite framework is established that takes into account the influence of modulus as well as temperature. Here the kinetic of the solid phase reaction is a superposition of two different mechanisms described by the Monod equation.

Steaming of NH4Y zeolite at 723 K and 873 K is accompanied by the formation of extra‐framework amorphous aluminosilicate and silica gel in addition to earlier observed extra‐framework aluminum species. Their occurrence is directly associated with the formation of mesopores. Bulk (intracrystalline) mesopores occur inside the crystallite nuclei and surface (intercrystalline) mesopores are located nearby the crystallite surface. Corrosion of the zeolite framework results in a loss of crystallinity and, consequently, decreased catalytic activity of the USY catalysts synthesized. Analysis of the reasons of mesopore formation may help to reduce these disadvantages.

The solubility of commercial and synthesized silica gels in a solution of Tetra-Ethyl-Ammonium-Hydroxide (TEAOH) was investigated at room temperature. The state of parent silica frameworks was characterized by BET and SEM. The structural defects were identified both by the Q n group analysis in 29 Si MAS NMR and IR spectroscopic investigation. It was found that the dissolution rate of the samples shows a tendency for growing up with an increasing BET surface.

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