The synthesis of high silica CHA (SSZ-13) has been widely investigated due to its high thermal stability and use in selective catalytic reduction of NOx. In conventional hydrothermal synthesis, soluble sources of the inorganic reagents, organic structure directing agent (SDA) and high amounts of hydroxide are used to form CHA. Interzeolite conversion, a route where a crystalline zeolitic source is used to form another zeolite, has displayed faster crystallization rates than conventional synthesis. However, the mechanisms underlying the root causes of differences in rates during interzeolite transformation remain unknown. Using N,N,N-trimethyl-1-adamantammonium (tri-ADA) as the OSDA, we studied CHA crystallization kinetics using FAU reagents with different Si/Al ratio and using non-FAU reagents, at fixed synthesis conditions (SDA/SiO2 0.14, H2O/SiO2 40, OH/SiO2 0.6, SiO2/Al2O3gel 30). Interzeolite conversion was verified by XRD and accompanied by a pH jump indicative of CHA formation. Temperature and SDA amount played crucial factors in slowing down or stopping the interzeolite conversion for certain FAU reagents. Certain FAU zeolite reagents, especially CBV720 (Zeolyst), showed faster rates of interconversion to CHA. To understand these differences in rates, FAU (CBV720) was degraded to an amorphous phase confirmed by XRD and micropore volume. Yet, the degraded FAU showed mostly tetrahedral AlO4 by 27Al MAS NMR, and the degraded FAU showed similar crystallization rates as CBV720. The addition of another ammonium SDA into the synthesis sterically hindered the formation of nuclei and attachment of tri-ADA to form CHA. Our findings indicate that the dissolution of FAU into tetrahedral Al compared to soluble Al is an important factor in determining the crystallization rate, which may explain the faster rate of CBV720.This study provides synthetic strategies to tune crystallization rates for zeolite synthesis design that are used for industrial applications.

Synthetic methods to influence the kinetics of FAU interzeolite conversion to CHA

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