Formation Pathway of AEI Zeolites as a Basis for a Streamlined Synthesis

Zeolites are commercially used crystalline micro porous materials. Understanding their crystallization mechanism and corresponding synthesis development is still of scientific and technological significance. Herein, we investigated the formation process of AEI zeolite using a combination of analysis methods, including X-ray diffractometry, nuclear magnetic resonance, and electrospray-ionization mass spectrometry, and developed a rational synthesis method without the use of a specific silica/alumina source (FAU zeolite). First, the synthesis condition was optimized by selecting a variety of synthesis components: FAU zeolites with different Al contents, colloidal silica, sodium aluminate, and three types of organic structure-directing agents (OSDAs). AEI zeolite preintroduced into the synthesis system (seed crystal) enabled its crystal growth from an easily available amorphous silica/alumina source. Stepwise gel preparation (SGP) through divided compositional control using an amorphous starting material effectively provided AEI zeolite in the presence of two types of OSDAs. A selected synthesis component (SGP with N,N-dimethyl-3,5-dimethylpiperidinium) formed AEI zeolite without even using AEI seed. Although the product contained a trace amount of impurity, it could be used as seed crystal for providing pure-phased AEI zeolite. Finally, from the merged synthetic and analytical results, we proposed that oligomeric aluminosilicate with a low dehydration degree contributes to AEI zeolite nucleation from an amorphous starting material. Furthermore, AEI zeolite catalyst with high activity and durability for emission-gas purification can be prepared by the SGP synthesis strategy. This comprehensive study would help rationally build zeolite synthesis and function design, particularly in a case that strongly depends on the starting material.