(754d) Time-Resolved Dynamics of Intracrystalline Mesoporosity Generation in USY Zeolite

The strong acidity, excellent (hydro)thermal stability, and unique micropore structure of zeolites provide physicochemical properties that are ideal for a wide range of applications. A common objective in zeolite design is the reduction of the diffusion limitations imposed by their narrow micropores (typically <1 nm) and long diffusion path lengths in large crystals that negatively impact performance in a wide range of applications involving bulky molecules.¹ Approaches to address these limitations include the synthesis of zeolites with extralarge pores, reduction of crystal size, and/or hierarchical zeolites that possess high surface areas through the introduction of mesopores.² Among these approaches, surfactant templating has emerged as one of the most effective and versatile strategies for the construction of intracrystalline well-defined mesoporosity in zeolites.^3-4 Despite tremendous efforts to elucidate the mechanisms of surfactant templating in zeolite synthesis, these pathways are not well understood. Here, we will discuss the role of the cationic surfactant cetyltrimethylammonium (CTA) as a mesostructuring agent in the rational design of hierarchical zeolites.

More specifically, in this presentation we will show the dynamics of surface reconstruction that occurs during the surfactant-templating of USY zeolite captured using in situ atomic force microscopy (AFM).⁵ The development of surfactant-templated mesoporosity and the concurrent healing of defects that are characteristic of steamed zeolites occur in less than one hour at room temperature, which emphasizes the low energy barriers needed to reorganize the crystalline structure of this zeolite. We also monitored this transformation by X-ray diffraction, N₂ adsorption, and TEM analysis of ultramicrotomed samples to confirm that the rapid formation of surfactant-templated mesoporosity and the reconstruction of the zeolite crystals occur not only on the surface of the zeolite, but homogeneously throughout the whole zeolite. This process involves a significant and rapid breaking and re-formation of bonds; however, the zeolite does not dissolve during this process as solids recovery at any given time of the treatment is approximately 100% and the concentration of soluble Si or Al species in the liquid is negligible. Parametric analysis revealed that excessive NaOH leads to the partial transformation of zeolite into an amorphous mesoporous solid, while insufficient quantity of base and/or treatment time can lead to an incomplete mesostructuring of the zeolite, which highlights the importance of judiciously selecting the treatment conditions for every given zeolite. Collectively, these findings provide new and unequivocal insights that confirm surfactant templating is a facile and efficient method to induce reconstruction of the crystal and the healing of defects towards the generation of optimal zeolites for catalytic applications.

(1) Choi, M et al. Nature 461 (2009) 246–249

(2) Li E., García-Martínez J., (ed.) Mesoporous Zeolites: Preparation, Characterization and Applications. WILEY-VCH (2015)

(3) Sachse A., García-Martínez J., Chem. Mater. 29 (2017) 3827–3853

(4) Chawla, A et al. Mol. Syst. Des. Eng. 3 (2018) 159-170

(5) Chawla, A et al. Chem. Mater. (2019) In Press DOI: 10.1021/acs.chemmater.9b00435