Crystallization Mechanism of Cage-Based, Small-Pore Molecular Sieves: A Case Study of CHA and LEV Structures

Abstract

Here we have investigated the crystallization mechanisms of SSZ-13 and SAPO-34 with the CHA topology and NU-3 and SAPO-35 with the LEV topology using 1H-13C CP MAS NMR and IR spectroscopies. The nucleation of these cage-based, small-pore molecular sieves begins with the formation of large 20-hedral cha or 17-hedral lev cages, with incorporation of organic structure-directing agents (SDAs) alone or together with inorganic cations, in both aluminosilicate and silicoaluminophosphate compositions. The next two steps are the construction of multiple-cha or multiple-lev cages in an appropriate arrangement by sharing 8-rings and their subsequent coupling to form smaller 8-hedral double 6-ring units, leading to viable nuclei of CHA or LEV molecular sieves. The initial formation of the large cages, especially in the presence of large SDA molecules, can in our view be generalized to other cage-based zeolite systems.