Triple-template system for phosphorus-modified AFX/CHA intergrowth zeolite

Phosphorus-modified zeolite with an intergrowth structure of AFX and CHA (P-AFX/CHA) was synthesized by a triple-template system. The synthesis system involved two nitrogen-containing organic structure directing agents (N-OSDAs, N,N,N-trimethyl-1-adamantammonium hydroxide and 1,1'-(1,4-butanediyl)bis(1-azonia-4-azabicyclo[2,2,2]octane) dibromide) for developing the intergrowth zeolite phase. Tetraethylphosphonium cation (TEP) was also added as a phosphorus-modifying agent. The calcination treatment causes the decomposition and oxidation of TEP to form a phosphorus oxide species that interacts with the zeolite framework, thereby realizing direct-phosphorus modification in the zeolite cavity. We optimized the synthesis condition of P-AFX/CHA, tuning the synthesis gel composition and selecting the counter anion in TEP, and found that tetraethylphosphonium bromide (TEPBr) is useful for effectively incorporating phosphorus into the intergrowth zeolite. 31P magic angle spinning (MAS) NMR confirmed the phosphorus modification via the decomposition of P-OSDA, whereas X-ray diffraction and transmission electron microscopy revealed the intergrowth structure. In addition, 13C dipolar decoupling (DD) MAS NMR, elemental analysis, and other synthesis results suggested the selective modification of phosphorus into the CHA phase in the intergrowth zeolite. Phosphorus modification was effective in improving the thermal stability of P-AFX/CHA. The thermal stability increased with increasing the phosphorus modification degree.