Seed-Assisted Synthesis of Hierarchical Zeolites

Zeolites are nanoporous aluminosilicates with unique properties that are utilized in commercial applications ranging from ion exchange to catalysis in the petrochemical industry. The small pores of zeolites impose diffusion limitations that can be mitigated via the synthesis of ultra-small crystals or hierarchical materials possessing markedly reduced diffusion path lengths.¹ Synthesis of these materials often requires the use of an organic structure-directing agent (OSDA), which poses commercial limitations owing to high economic and environmental costs. One alternate approach to either decrease or eliminate organics in zeolite synthesis is the use of seed crystals, which can reduce the total synthesis time, eliminate impurities, and alter particle size.²

Here we describe our findings revealing the impact of synthesis parameters on the kinetics of nucleation and interzeolite transformations in seed-assisted zeolite crystallization. We have examined factors governing parent-daughter relationships by selecting zeolites with common structural features as well as those lacking structural similarity. Our findings expose several limitations to the prevailing hypotheses,³ and highlight the effect of seeds on crystal morphology and crystal growth kinetics.⁴ Moreover, we will discuss an exciting discovery wherein the judicious selection of seeds can produce hierarchical self-pillared pentasil (SPP) zeolites⁵ with intergrown nanosheets without the use of organics. Our findings reveal with unprecedented clarity the nonclassical pathway of SPP formation involving the nucleation of crystals at the exterior surface of amorphous precursor particles (Figure 1A, arrow), leading to SPP crystals (Figure 1B). We also explore the effects of various parameters, such as seed structure, chemical composition, temperature, and time on the physicochemical properties of SPP zeolites and confirm that these materials exhibit improved performance in catalytic reactions compared to conventional ZSM-5 catalysts (Figure 1C and D).

Overall, in-depth analysis of zeolite crystallization enables the improved understanding of underlying mechanisms of formation as a basis to develop novel methods in the rational design of nanoporous materials with tailored physicochemical properties.

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2. Li E., García-Martínez J., (ed.) Mesoporous Zeolites: Preparation, Characterization and Applications. WILEY-VCH (2015).
3. Itabashi et al.; J. Am. Chem. Soc. 134 (2012) 11542-11549.
4. Jain and Rimer; Micropor. Mesopor. Mater. 300 (2020)
5. Zhang et al.; Science (2012) 1684-1687.