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.