(580d) Investigation of Crystallization of Siliceous LTA Zeolite Using Raman Spectra

Understanding the formation mechanisms of nanoporous catalysts such as zeolites has remained a tantalizing challenge. Discovering how zeolites form is complicated by the fact that zeolite precursor structures fall into a nanoscale blind spot necessitating characterization methods sensitive to medium-range structures such as rings and larger building units. Raman spectroscopy has emerged as a powerful tool for probing medium-range structures in a variety of materials including disordered silica, zeolites, MOFs, and zeolite precursor solutions. However, despite significant research into the Raman spectra of zeolites, the detailed assignments of such Raman spectra are not completely understood. In this presentation, we report a systematic zeolite synthesis, spectroscopy, and periodic DFT study of several all-silica zeolites to determine the fundamental structural motifs that explain Raman spectral features. We have discovered from normal mode analysis that Raman bands can be assigned to tricyclic bridges – three zeolite rings that share a common Si-O-Si bridge. A precise anti-correlation between Raman frequency and Si-O-Si angle has been determined.¹ Raman study was performed on siliceous LTA synthesized in the presence of F^-. It was discovered that F^--filled double-4-ring (F^-/D4R) and empty D4R exhibit distinct Raman features. The Raman band density of F^-/D4R and empty D4R can be used to quantify the distribution of the two different D4R units in the final LTA zeolite. Understanding the Raman signature of the F^-/D4R units and difference between the F^-/D4R and empty D4R opens an interesting new window for studying the roles of F^- during zeolite formation, control nanopore hydrophobicity and design syntheses of zeolites with new structures and compositions.

1. Wang, T.; Luo, S.; Tompsett, G. A.; Timko, M. T.; Fan, W.; Auerbach, S. M., Critical Role of Tricyclic Bridges Including Neighboring Rings for Understanding Raman Spectra of Zeolites. Journal of the American Chemical Society 2019, 141 (51), 20318-20324.