Speaker
Description
The characterization of porous aluminas has been a long-standing problem due to their complex, disordered porous structures, with structural features ranging from the nanometre to the millimetre scale. There is a need to understand how the pore structure influences mass transport, to guide new catalyst designs and optimize catalytic processes. However, the complex, hierarchical nature of pellet structure often means that a so-called “brute force” approach is beyond the reach of current computing power, and methods enabling selection of the key structural features to incorporate into a model are necessary. In this work, the combination of hyperpolarized (hp) 129Xe MRI and NMR cryodiffusometry, has been used to probe the structural-transport relationship of bimodal amorphous gamma alumina support pellets, with different degrees of controlled macroporosity to assess the key void space features determining tortuosity.
NMR cryodiffusometry experiments, using water or cyclohexane as the probe fluid, were performed using both boundary and scanning, and both melting and freezing curves. These experiments were coupled with complementary DSC thermoporometry to corroborate the cryoporometry results, and FIB-SEM experiments to aid the interpretation of the NMR data. A novel hp 129Xe NMR/I technique has been developed to allow the probing of the spatial distribution of rates of gas uptake across individual catalyst pellets, and this to be related to pore structure and, also, catalytic performance.
The self-diffusivity of water as a function of molten fraction has been measured using NMR cryodiffusometry1, and, thence, the relative importance of different aspects of the void space, and various bins within the pore size distribution, to mass transport rates has been investigated. Pore structure models have been fitted to the tortuosity versus molten fraction plots, to infer the basic geometrical character. Hp 129Xe MRI allows for the study of low-density, gas-phase mass-transport, such that diffusion can be measured in the Knudsen diffusional regime, instead of the purely molecular regime, more closely mimicking the transport processes that would occur in gas-phase catalytic reactions2. Whilst NMR cryodiffusometry measures the average self-diffusion coefficient of liquid water over 10s of microns, the hp xenon experiments measures the impact of long-scale structural features on pellet mass transfer. Through the combination of these two techniques, structural and diffusional heterogeneities at different length scales can be measured, and their overall contribution to the observed catalytic activity evaluated.
References:
1. E. Perkins et al., Determination of the percolation properties and pore connectivity for mesoporous solids using NMR cryodiffusometry, Chemical Engineering Science, 63 (2008) 1929–1940
2. G. Pavlovskaya et al., NMR Imaging of Low Pressure, Gas-Phase Transport in Packed Beds Using Hyperpolarized Xenon-129 AIChE, 61 (2015), 4013–4019
Acknowledgements:
I would like to thank IFP Energies nouvelles for sponsoring this research.
