(354b) Self-Diffusion of Heptane inside Aggregates of Porous Alumina Crystallites By Pulsed Field Gradient NMR

Porous alumina has piqued the interest of the research community due to its relevance for industrial applications where it can be used as a catalyst, catalyst support, adsorbent, and template for the fabrication of nanomaterials. Specifically, the ability to fabricate alumina with a high surface area (up to 700 m²/g) and well-defined pore structure where the majority of pores fall into the mesopore range makes it highly attractive in these applications. This has led to various studies focused on synthesizing this material under conditions which control the pore structure and size. Such modifications in the pore structure, specifically variations in porosity, have been shown to directly influence molecular transport properties. Until now, diffusion studies of porous alumina have mostly been performed using macroscopic techniques.

Here we report microscopic studies of n-heptane diffusion in porous alumina samples exhibiting a hierarchy of pore sizes that span the range from micropore to macropore sizes. Diffusion measurements were performed using ¹H and ¹³C pulsed field gradient (PFG) NMR utilizing a high field (17.6 T) and high field gradients (up to 30 T/m) to obtain length scales of displacements in the range of about 1-20 mm; much smaller than previously reported studies. The measured heptane diffusivities inside the crystal aggregates were found to be several times smaller than the diffusivity in the bulk liquid heptane. The diffusivities inside the aggregates were correlated with the aggregate structural characteristics, which include pore volume, pore size, and packing density of the individual crystals in the aggregates.