The acid-resistant property of natural CXN zeolite (type STI) in the hot HCl solution with various concentrations, and the thermal stability of ultra-stable high-silica H-STI-I and H-STI-II zeolites prepared by different procedures, and various cation forms of M-STI-I series and M-STI-II series (M＝Li, Na, K, Ca, Ag) zeolites under the stage-temperature programmed calcinations were investigated with measuring the crystallinity by powder X-ray diffraction (XRD). The composition and the structure properties of H-STI-I and H-STI-II zeolites were characterized with X-ray fluorescence scattering (XRF), ²⁹Si and ²⁷Al MAS NMR. The results suggest that the crystal structure of the natural zeolite remained relatively perfect in hot HCl solution with the concentration lower than 2 mol/L in a long period. Dependent on the species of the cations in the zeolites, the relative crystallinity of M-STI-I series and M-STI-II series zeolites was decreased 2.5%～18% and 0～35% after calcination at 1173 K, respectively. The decrease of the relative crystallinity was in the order of K＜(H, Li, Na)＜(Ca, Ag) form for M-STI-I series zeolites, and in the order of (Na, K)＜(Ca, H, Li)＜Ag form for M-STI-II series ones, respectively. Several factors affecting the thermal stability of the zeolties, such as the framework Si/Al ratio, structural defects, cation forms, and the formation heat of the relative oxides were discussed.

Key words: CXN zeolite, STI zeolite, cation, acid-resistant, thermal stability