Evolution of Heterogeneity in Industrial Selective Oxidation Catalyst Pellets

The selective oxidation of n-butane over vanadium phosphorus oxides to maleic anhydride is a crucial step in the production of unsaturated polyester resins. As the demand for maleic anhydride is steadily increasing, we are urged to develop heterogeneous catalysts of still higher productivity and a longer lifetime. To satisfy this demand, we require knowledge of the conversion process on the molecular level, the structure of the catalyst on the nanometer level, for example, the exposure of active sites in nanoporous and millimeter-sized catalyst bodies or pellets, and an understanding of how these pellets behave within the industrial setting. Here, using scanning X-ray diffraction, we visualize how industrial reactor operation affects commercial porous Raschig-ring-type pellets. The examination of a pristine catalyst pellet and two pellets extracted from an industrial reactor after 4 years of operation, one from the reactor hot-spot zone, revealed a series of structural and compositional modifications that can be linked to a decreasing catalyst performance and a low degree of pore utilization and hence a spatially non-uniform utilization of the catalyst pellets. Specifically, we observe how the suggested differential thermal stress and diffusion limitations induce a radial transformation of the pellets. This process is intensified in the hot-spot zone. We hope these observations reiterate the importance of characterizing heterogeneous catalysts at all length scales to contextualize catalyst performance and provide insights that can help to optimize the design of catalyst bodies for the industrial setting.