Baowang Lu

Direct hydrothermal synthesis and stabilization of high-silica mordenite (Si∶Al = 25) using tetraethylammonium and fluoride ions

The direct crystallization of high-silica mordenite (MOR) zeolite using both tetraethylammonium and fluoride ions was investigated by varying the primary variables of the hydrothermal synthesis such as Si∶Al and H2O∶SiO2 ratios in the starting synthesis mixture. Under well-optimized conditions, highly crystalline MOR zeolites with a Si∶Al ratio of approximately 25 were successfully prepared. From evaluation of the thermal stabilities of the MOR zeolites obtained, it was also found that there is a good relationship between the thermal stability and the bulk Si∶Al ratio of MOR zeolites and that any NaF remaining in the zeolite crystals considerably reduces the thermal stability.

CHARACTERIZATION OF HIGH-SILICA MORDENITES SYNTHESIZED BY VARIOUS DIRECT HYDROTHERMAL SYNTHESIS METHODS

Influences of various additives on the direct synthesis of high-silica mordenite (MOR) zeolite with tetraethylammonium hydroxide (TEAOH) were studied in detail. The addition of NaF as a fluoride source enhanced the nucleation and crystal growth of MOR zeolite. The addition of NH4NO3 as a mineralizer was also effective for the synthesis of high-silica MOR zeolite. The MOR zeolites synthesized with NaF and NH4NO3 were found to have much less framework defects as compared to those synthesized without additives. Under well-optimized conditions, the highly crystalline and pure MOR zeolite with a Si/Al ratio of ≈ 34 was synthesized by adding seed crystals as well as NaF and NH4NO3.

Direct synthesis of high-silica mordenite and its thermal stability

Abstract Direct hydrothermal synthesis of highly crystalline MOR with a Si/Al ratio of ca. 30 was achieved by a combination of tetraethylammonium hydroxide and NaF when Al(NO 3 ) 3 was employed as an aluminum source. It was also found that the crystallinity and the Si/Al ratio of MOR zeolite hardly depend on the addition time of NaF. However, the content of fluorine in the as-synthesized MOR zeolite decreased markedly when NaF was added in the course of crystallization process. The decrease in the fluorine content improved considerably the thermal stability of the as-synthesized MOR zeolite.