RESUMO
Fluid catalytic cracking is a chemical conversion process of industrial scale. This process, utilizing porous catalysts composed of clay and zeolite, converts heavy crude-oil fractions into transportation fuel and petrochemical feedstocks. Among other factors iron-rich reactor and feedstream impurities cause these catalyst particles to permanently deactivate. Herein, we report tomographic X-ray absorption spectroscopy measurements that reveal the presence of dissimilar iron impurities of specific localization within a single deactivated particle. Whereas the iron natural to clay in the composite seems to be unaffected by operation, exterior-facing and feedstream-introduced iron was found in two forms. Those being minute quantities of ferrous oxide, located near regions of increased porosity, and impurities rich in Fe3+ , preferentially located in the outer dense part of the particle and suggested to contribute to the formation of an isolating amorphous silica alumina envelope.
RESUMO
Porosity in catalyst particles is essential because it enables reactants to reach the active sites and it enables products to leave the catalyst. The engineering of composite-particle catalysts through the tuning of pore-size distribution and connectivity is hampered by the inability to visualize structure and porosity at critical-length scales. Herein, it is shown that the combination of phase-contrast X-ray microtomography and high-resolution ptychographic X-ray tomography allows the visualization and characterization of the interparticle pores at micro- and nanometer-length scales. Furthermore, individual components in preshaped catalyst bodies used in fluid catalytic cracking, one of the most used catalysts, could be visualized and identified. The distribution of pore sizes, as well as enclosed pores, which cannot be probed by traditional methods, such as nitrogen physisorption and isotherm analysis, were determined.
