FIB-SEM Tomography Probes the Mesoscale Pore Space of an Individual Catalytic Cracking Particle

Abstract

The overall performance of a catalyst particle strongly depends on the ability of mass transport through its pore space. Characterizing the three-dimensional structure of the macro- and mesopore space of a catalyst particle and establishing a correlation with transport efficiency is an essential step toward designing highly effective catalyst particles. In this work, a generally applicable workflow is presented to characterize the transport efficiency of individual catalyst particles. The developed workflow involves a multiscale characterization approach making use of a focused ion beam-scanning electron microscope (FIB-SEM). SEM imaging is performed on cross sections of 10.000 μm2, visualizing a set of catalyst particles, while FIB-SEM tomography visualized the pore space of a large number of 8 μm3 cubes (subvolumes) of individual catalyst particles. Geometrical parameters (porosity, pore connectivity, and heterogeneity) of the material were used to generate large numbers of virtual 3D volumes resembling the sample’s pore space characteristics, while being suitable for computationally demanding transport simulations. The transport ability, defined as the ratio of unhindered flow over hindered flow, is then determined via transport simulations through the virtual volumes. The simulation results are used as input for an upscaling routine based on an analogy with electrical networks, taking into account the spatial heterogeneity of the pore space over greater length scales. This novel approach is demonstrated for two distinct types of industrially manufactured fluid catalytic cracking (FCC) particles with zeolite Y as the active cracking component. Differences in physicochemical and catalytic properties were found to relate to differences in heterogeneities in the spatial porosity distribution. In addition to the characterization of existing FCC particles, our method of correlating pore space with transport efficiency does also allow for an up-front evaluation of the transport efficiency of new designs of FCC catalyst particles.

A three-dimensional view of structural changes caused by deactivation of fluid catalytic cracking catalysts

“…Similarly, the analyzed particles possess near identical interfacial areas of feed-transporting pores with zeolite elements. If such global metrics were applicable, both samples should possess a similar functional capacity 31 .…”

“…Martinez et al 27 extended these observations to other impurities, including nickel and the zeolite-poisoning vanadium and sodium. Structural studies of the pore network 1 , 15 , 20 , 28 further revealed the clogging of diffusion highways by metal-rich impurity deposits 16 , 29 , 30 and, importantly, the pore network’s persistent integrity even after deactivation 31 . Based on these observations, permanent catalyst deactivation is currently postulated to arise in part from the obstruction or removal of diffusion highways close to the particle exterior 15 , 16 , 32 , 33 .…”

Magnetophoretic Sorting of Single Catalyst Particles

“…[18,19] During the FCC process, the catalyst deactivates irreversibly due to dealumination and the accumulation of metals, mainly Fe, Ni, and V. While Ni and V decrease the yield by promoting for example, coke formation, Fe is supposed to deactivate the particles by creating a shell of reduced porosity on the particles outer surface, either via vitrification of the particle matrix in the presence of Fe and/or co-deposition of silica. [20][21][22][23][24][25][26][27][28][29] Therefore, during operation, to create a stable process efficiency, a fraction of catalyst is being replaced on a daily basis, resulting in a mixture called equilibrium catalyst (ECAT). To investigate their deactivation, FCC ECAT particles are often sorted based on their skeletal density, which is associated with metal loading and age.…”

Correlative Multiscale 3D Imaging of a Hierarchical Nanoporous Gold Catalyst by Electron, Ion and X‐ray Nanotomography

“…This results in a large fraction of accessible gold surface atoms, particularly at kink and step sites, which makes np‐Au very interesting for catalysis and sensing applications . As with all functional materials, it is important to understand physical characteristics such as surface area, porosity, shape and mechanical stability, in order to: (i) define structure–activity relationships; (ii) identify and control synthesis parameters which lead to such structures; (iii) design future catalysts for target applications . When applied to catalysis, analytical methods based on structural imaging or microscopy are therefore required to incorporate large fields of view, high spatial resolution, and to extend across multiple length scales .…”