Structural Changes of Sulfided Nickel-Tungsten Catalysts during Oxidative Regeneration.

Oxidative regeneration of Ni-W, Al₂O₃ catalyst was investigated to elucidate the best conditions for catalyst regeneration. Catalytic activities of the catalysts regenerated under several different conditions were evaluated by means of model test reactions. Structures of the regenerated catalysts were characterized by X-ray absorption fine structure (XAFS) to clarify the causes of the insufficient recovery of activity. The samples analyzed were the catalysts used for three years in hydrotreating of lubricant fraction of petroleum.<br>The regenerated catalystcalcined at 400°C, gave higher recovery of activity than those calcined at higher temperatures. Calcination in flow of a low concentration of oxygen was found to be favorable to obtain high recovery of activity. Based on the above results, multistep calcination in flow of 2% oxygen was developed as the best procedure for calcination. An XPS analysis exhibited that carbonaceous materials were not completely removed from the multi-step regenerated catalyst. Thus, the recovery of activity was found to depend mainly on the structure changes of the catalyst, during regeneration.<br>No significant differences were observed, in the W L_III-edge XANFS or EXAFS, between the fresh and regenerated catalysts. Detailed curve fitting analysis of the EXAFS of sulfide catalysts, however, elucidated that more aggregated WS₂ structure was formed on the onestep regenerated catalyst.<br>NiO structure was formed in the one-step regenerated catalyst, while NiAl₂O₄ structure was the major species in the fresh and multi-step regenerated catalysts. Ni₃S₂ crystalline species in the used catalyst, which was formed during hydrotreating, was redispersed by multi-step calcination. By one-step rapid calcination, however, excess heat of calcination likely accelerated the aggregation of NiO and WO₃, which finally prevented the formation of active "Ni-W-S" binary sulfide phase. The binary sulfide phase was found to play an important role in hydrogenation activity rather than hydrocracking activity. It was concluded that rapid oxidation should be avoided to recover the catalytic activity.