(580a) Smaller, Faster, Better: Distinguishing the Timescales of Finned Zeolites Via Time-Resolved Amine Titration

The utility of probe molecules in infrared (IR) spectroscopy is ubiquitous in the catalysis field to assess the number and type of sites in zeolites. A challenge remains in determining the accessibility and uniformity of these acid sites within the confined voids of zeolite micropores and mesopores. In this work, we present an alternative approach to compare internal diffusion properties of zeolites using methylpyridines, where distinct diffusion regimes within the confinements are discerned.

ZSM-11 (MEL-type) and ZSM-5 (MFI-type) were synthesized via seeded growth mechanism where rough fin-like features (50 nm) were created on the exterior surface of the seed crystals (400 nm). XRD, SEM, and TEM tomography confirmed the presence and sizes of these fins. Time-resolved 2,4,6-trimethylpyridine (TMPyr) titration of Brønsted acid sites (BAS) within MFI/MEL pores were measured via transmission FT-IR spectroscopy. The temporal titration of TMPyr was quantified by the disappearance of the 3605 cm^-1 peak of vacant BAS and also the appearance of 1565 cm^-1 associated with trimethylpyridinium ion. The distinct TMPyr uptake kinetics for MFI and MEL were described by an exponential diffusion model with time scales in the micropores of both bulk particles, Ï„_seed, and fins, Ï„_fin. When the differential of the titrated BAS over time was plotted, we observed two distinct diffusion regimes (Ï„_seed ~10⁵ s, Ï„_fin ~10² s), indicating the initial fast uptake of TMPyr in the fins relative to the bulk particle. When applied to self-pillared pentasil, hierarchical nanosheets with significantly reduced internal diffusion limitations, we observed only the fast uptake regime.

Time-resolved FT-IR spectroscopy is a powerful technique to elucidate transient diffusion of molecules through zeolite pores by assessing the distinct transport properties of seed and finned MFI/MEL zeolites, a new class of hierarchical material. Future work includes broadening of amine class to probe diffusion regimes through one- to two-dimensional zeolites.