Understanding Length Scales of Diffusion in Hierarchical Materials

Abstract

The utility of probe molecules, such as pyridine, benzene, and mesitylene, 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. Time-resolved Fourier Transformed Infrared (FTIR) Spectroscopy is a powerful technique to elucidate transient diffusion of organic probe molecules through zeolite pores by assessing the distinct transport properties of seed and finned zeolites, a new class of hierarchical material. In this work, we present an alternative approach to compare internal diffusion properties of zeolites using branched, cyclic amines, where diffusion regimes within the zeolite confinements are discerned. We show that the diffusion time scale for trimethylpyridine and dimethylpyridine through MFI/MEL and FER pores are much slower than for pyridine due to steric hindrance from its methyl groups. This methodology can be further extended to probe diffusion regimes and obtain active site distributions of various zeotype catalysts.