Hyperpolarized Xenon-129 MRI, Fluorine-19 PFG NMR, SAXS and electron microscopy studies of the impact on structure and gas phase mass transport of introducing controlled macroporosity into mesoporous, alumina, catalyst support pellets

A novel combination of experimental characterization techniques has been used to assess the impact of the controlled addition of macroporosity to mesoporous catalyst supports for use with diffusion-limited, gas-phase reactions. This is the first study to 2D-map the gas-phase uptake across different internal regions within individual mesoporous pellets (or any other inorganic mesoporous media) using hyperpolarized xenon MRI. A porogen-templating method aimed to control the gas-phase mass transport properties of disordered alumina pellets. The degree of success of the pore structure design was examined using electron microscopy and gas sorption, and, for the resultant mass transport properties, using gas-phase PFG NMR and hyperpolarized xenon MRI. TEM showed that the macropore templating led to the creation of a corona of parallel-aligned alumina platelets around the macropore boundaries. The particular impact of this corona on mass transport was studied using PFG NMR, and found to be significant. The MRI showed that individual pellets had very large length-scale heterogeneities in the macroscopic spatial distribution of gas uptake rates. The data from the pore structural characterization, along with percolation theory, was used to develop a structural model to predict the impact of creating additional macroporosity by the particular fabrication method employed, and was successfully validated using the PFG NMR data for gas self-diffusion and MRI mapping of gas uptake rates across individual pellets.