(349f) Chemoselective Characterization of Brønsted Acid Site Accessibility Using Probe Amine Uptake in Infrared Spectroscopy

Infrared (IR) spectroscopy is a powerful technique utilized in the catalysis field to assess the number and type of sites in zeolites via the adsorption of organic probe molecules. A challenge remains in determining the accessibility and uniformity of these acid sites within the confined voids of zeolite micropores and mesopores. We present an alternative approach to compare internal diffusion properties of zeolites using methylpyridines, where distinct regimes within the zeolite 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 BAS within MFI/MEL pores were measured via transmission FT-IR spectroscopy. The temporal titration of TMPyr was quantified by the disappearance of the IR peak at 3605 cm^-1 corresponding to the vacant Brønsted acid sites and also the appearance of 1565 cm^-1 associated with trimethylpyridinium ion. The slow transport time constants associated with the seed particle, Ï„_seed, were ~10⁵ s. Finned zeolites exhibit initial fast uptake kinetics three orders of magnitude lower than the seed with Ï„_fin ~10² s. When the differential of the titrated BAS over time was plotted, we observed two distinct diffusion regimes, indicating the initial fast uptake of TMPyr in the fins relative to the bulk particle. When applied to self-pillared pentasil (SPP), hierarchical nanosheets with significantly reduced internal diffusion limitations, we observed only the fast uptake regime.

The finned synthesis method has also been applied to commercial three-dimensional MFI zeolites, which demonstrate similar distinct diffusion regimes. Future work includes broadening of amine class to probe transport properties through one- to two-dimensional zeolites with various pore sizes and obtain active site distributions of zeotype catalysts.