Crystallization Mechanism of a Family of Embedded Isoreticular Zeolites

Abstract

We have recently been successful in predicting and synthesizing a series of body-centered cubic zeolites with expanding structural complexity and embedded isoreticular structures, termed the RHO family. Here we propose a plausible formation pathway for ECR-18, ZSM-25, and PST-20, the three members of this zeolite family, in the presence of tetraethylammonium ions as an organic structure-directing agent, based on the C-13 MAS NMR, IR, and CO2 adsorption results obtained from the solid products recovered as a function of time during their crystallization processes, together with the quantum-chemical calculation results. The nucleation of these three zeolites, all of which consist of seven different structural units, begins with the almost simultaneous construction of 26-hedral lta and 14-hedral t-plg cages and their subsequent connection via shared 8-rings in the diagonal direction of the cubic unit cell. As a logical next step, 8-hedral t-oto and 12-hedral t-phi cages are built around the preorganized t-plg cages. The remaining embedded spaces are readily filled up with 10-hedral t-gsm cages, as well as with t-oto and t-phi cages. Finally, 10-hedral d8r and 14-hedral pau cages are alternately constructed along the cubic edges. Over the outer surface of the resulting nuclei of the RHO family zeolites, the crystal growth may occur in a similar manner as described above.