The influence of nanoscale microstructural variations on the pellet scale flow properties of hierarchical porous catalytic structures using multiscale 3D imaging

The total activity, selectivity and lifetime of heterogeneous silica–alumina catalysts depends on the flow of molecules through complex three-dimensional (3D) hierarchical pore structures that can span length scales from micro- to nano-meters. The transport of molecules to and from active sites ultimately determines the catalyst pellets performance. In this study a multiscale tomography (MT) methodology was developed by combining x-ray microtomography (XMT), dual beam focused ion beam tomography (DBFIB) and electron tomography (ET) to compare the flow properties of two hierarchically porous silica–alumina catalysts, one sintered/calcined at 580 °C and the other at 800 °C.The use of MT not only allowed visualization and quantification of the effect of sintering temperature on the pore structures at a number of length scales, but also allowed the influence of this pore structure on the flow effects to be calculated from the nanometer to millimeter scale. The lower temperature calcined catalyst exhibited an open, rounded, interconnected pore structure, while the high temperature one had a finer plate/crack-like pore structure, with ∼15-20% lower porosity. The permeability of both catalysts was calculated via direct meshing of the MT volumes and coupled across all three length scales (∼104 m), giving a good agreement to experimentally measured values.It was concluded that the higher calcining temperature significantly reduces the permeability across all length scales investigated. The very fine and tortuous paths in this high temperature calcined catalyst will be more vulnerable to deactivation by coking, illustrating how MT coupled with flow simulations can give new insights into the processing of sintered catalysts and potentially other functional materials.

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► Two silica–alumina catalysts prepared under calcination temperatures of 800 °C and 580 °C, respectively.
► Multiscale tomography successfully used with computational fluid dynamics (CFD) and compared with experimental data.
► Permeability calculated and scaled upwards through four orders of magnitude in length from nanoscale to pellet scale flow.
► High temperature calcination decreases permeability and mass transport properties of catalyst across all length scales analyzed.
► Multiscale tomography and CFD combined across large length scales can facilitate future catalyst development.