Multiple templating strategy for the control of aluminum and phosphorus distributions in AFX zeolite

Abstract

A multiple templating strategy using both bulky and small organic structure-directing agents (SDAs) was adapted for AFX zeolite synthesis in order to control the distribution of aluminum and phosphorus. An AFX zeolite with an Si/Al ratio of 4.8 was obtained in the presence of a bulky and rigid SDA, N,N,N',N'-tetraethylbicyclo[2.2.2]oct-7-ene-2,3:5,6-dipyrrolidinium cation (TEBOP2+), and co-existing Na+ cations. The use of a tetramethylammonium cation (Me4N+) as the SDA instead of Na+ provided AFX zeolite with a middle Si/Al ratio of 7.9. Elemental analysis and 13C and 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy suggested that TEBOP2+ existed in the wide aft cage in both AFX zeolites, while the higher Si/Al ratio was derived from the lower charge density of Me4N+ occupying the narrow gme cage. We also attempted to control the distribution of phosphorus in phosphorus-modified AFX zeolite. The large tetraethylphosphonium cation (Et4P+) and small tetramethylphosphonium cation (Me4P+) were selectively incorporated into aft and gme cages during the hydrothermal synthesis, respectively. As a result, phosphorus oxide with different polymerization degrees were formed by calcination treatment according to the size of the cage occupied by the phosphorus-modifying agents (P-MAs). This direct phosphorus modification using alkylphosphonium was characterized by elemental analysis and 13C, 27Al, and 31P MAS NMR and two-dimensional (2D) 31P–27Al heteronuclear correlation NMR spectroscopies.