Spatiotemporal investigation of the regeneration of a shaped zeolite catalyst extrudate using operando powder X-ray diffraction computed tomography

We have used operando powder X-ray diffraction computed tomography (PXRD-CT) analysed by parametric Rietveld refinements to monitor the regeneration of a deactivated ZSM-5 zeolite based extrudate employed as a catalyst in the methanol-to-hydrocarbon reaction. This investigation thus advances beyond previous studies of pure, powdered zeolites. We follow the coke oxidation by monitoring three XRD based descriptors. Lattice relaxation, monitored as the difference in length of the a- and b-lattice parameters (a-b), occurs first at the core of the extrudate and progresses towards the outer surface. This demonstrates a radial distribution of different coke species, wherein the heaviest, more oxidation resistant coke is matured near the external surface of the extrudate. In contrast, the total non-framework species mass (NFSM) decreased more uniformly throughout the extrudate, suggesting that chemical transformations of the coke species can occur prior to full oxidation. This notion is supported by in situ temperature programmed oxidation, which demonstrates that the regeneration of the ZSM-5/binder composite occurs in two distinct steps. We also observe that irreversible (hydro)thermal damage occurs in the severely coked regions of the extrudate during regeneration. Additionally, we demonstrate that the three different XRD based descriptors carry slightly different and complementary information. The (a-b) descriptor describes lattice distortion induced by rigid coke species. The total non-framework species mass (NFSM) and the unit cell volume (UCV), in contrast, capture all guest species, including volatile compounds and water. However, as a drawback, the UCV is strongly influenced also by thermal effects, whereas the (NFSM) is virtually unaffected by temperature.