HIGH-TEMPERATURE OXYCHLORINATION CATALYSTS - ROLE OF LACL3 AS AN INHIBITOR OF THE SEGREGATION OF ACTIVE SPECIES DURING HEATING-COOLING CYCLES

The effects of heating/cooling cycles on the structure and catalytic properties of CUCl2KCl catalysts supported on Al2O3 and SiO2 have been studied. It is demonstrated that after the catalysts are heated to 673 K and cooled to room temperature, an irreversible segregation of K and Cu occurs. X-ray diffraction (XRD), differential thermal analysis (DTA), electron paramagnetic resonance (EPR), and scanning electron microscopy (SEM) analysis have experimentally confirmed the proposed segregation. This segregation process appears to be the main cause of the deactivation observed on SiO2-supported CUCl2KCl catalysts for the CH4 oxychlorination reaction after a heating/cooling cycle. On γ-Al2O3 the situation is somewhat different. Due to the strong affinity of Cu for the Al2O3 support a surface Cu oxychloride species is stabilized and the segregation process is consequently enhanced. However, the deactivation is not evident on this catalyst because the stabilized surface Cu oxychloride species has its own catalytic activity. On the other hand, when La is added to the CUCl2KCl catalysts a significant improvement in activity, which is maintained over a heating/cooling cycle, is observed. This promoter effect may be ascribed to the action of La as an inhibitor of KCu segregation. © 1990.