Hydrothermal synthesis of high-silica mordenite by dual-templating method

High-silica mordenite (MOR) zeolites were hydrothermally synthesized by a dual-templating method using tetraethylammonium hydroxide (TEAOH) and hexamethyleneimine (HMI) as co-structure-directing agents (SDAs). The MOR zeolites were crystallized readily with the addition of NH₄NO ₃ from the gels that had a Si/Al ratio up to 60. The synthesis resulted in highly crystalline MOR zeolites possessing a Si/Al ratio of 37.1. The chemical coordination states of the Al ions in the MOR zeolites were characterized by ²⁷Al and ²⁹Si NMR MAS spectroscopy. The Al ions occupied dominantly the tetrahedral sites in the framework. The dual-templating effect that the two amines exhibited on the zeolite crystallization was investigated by ¹³C MAS NMR spectroscopy and elemental analyses. Both organic SDA molecules were incorporated simultaneously into the zeolite products, corresponding to 65% of TEA⁺ and 35% of HMI. These two kinds of SDAs played a cooperative role in the nucleation and crystallization of the MOR structure under high silica conditions.