(7g) Fine-Tuning SSZ-39 Zeolite Properties for Catalytic Applications Via Ultrafast Mixed Interzeolite Conversion.

SSZ-39 (AEI framework) zeolite is a promising catalyst candidate [1], however, its synthesis is time intensive and requires fine-tuned reactant compositions [2]. Interzeolite conversion (IZC), which uses crystalline Si and Al sources, offers an alternative approach to synthesize zeolites with optimal features: it provides a faster kinetic route to access zeolites synthesis within hours or even minutes [3].

A mixture of two commercial FAU zeolites (Si/Al=385, 15) is employed as source to achieve complete synthesis of pure AEI. Starting with Si/Al=25, AEI zeolite is synthesized in 30 min in a tubular reactor, using 2wt% milled seeds (synthesized in autoclave, 170 °C, 5 days), at 210 °C, obtaining Si/Al=11. The reactants molar ratio is (TMP⁺=tetramethylpyperidinium): 1SiO₂:25^-1Al:0.25TMP⁺:0.25Na⁺:0.5OH^-:10.5H₂O. It has been observed that only milled seeds can significantly accelerate the synthesis, whereas as-made seeds do not influence the kinetics.

In contrast to other IZC systems, this transformation does not proceed via any amorphous intermediate: XRD patterns show a simultaneous decrease in FAU peaks intensities and formation of AEI peaks. This results in a solid yield that never decreases below 30%, with a final value of 44%.

To unravel the intrinsic mechanism of the FAU-AEI transformation, operando IZC at 210 °C and 190 °C is analyzed via SAXS/WAXS, where the role of the milled seeds is also investigated. Moreover, the fast-synthesized AEI is tested for NH₃-SCR, while the as-made seeds are tested for methanol-mediated CO₂ conversion. Two different crystal sizes are obtained: 500 nm for the fast-synthesized and 3-4 μm for the autoclave-synthesized seeds, respectively, which play an important role in the different catalytic processes. The opportunity to control the Si/Al and the crystal size via mixed IZC and fast crystallization leads towards the synthesis of versatile zeolites which can be fine tuned to be applied in several catalytic processes.

[1]- 10.1021/acs.chemrev.7b00738.

[2]-10.1016/j.apcatb.2023.122395.

[3]-10.1039/d1ra02887a.