Hydrodemetallation (HDM) kinetics of Ni-TPP over Mo/Al2O3-TiO2 catalyst

The intrinsic hydrodemetallation (HDM) kinetics of nickel 5,10,15,20-tetraphenylporphyrin (Ni-TPP), was studied over a sulfided Mo catalyst supported on a mixed oxide alumina-titania (Mo/Al2O3-TiO2), with a controlled pore distribution. This work analyses the effects of using an Al2O3-TiO2 support, on the reaction steps of Ni-TPP hydrodemetallation. The HDM reaction was carried out in a batch reactor at 325 °C and 8.3 MPa of total hydrogen pressure, using nujol as solvent. Under these conditions, it was determined that Ni-TPP reacts through a sequential mechanism. In the first step, Ni-TPP is reversibly hydrogenated to nickel 5,10,15,20-tetraphenylchlorin (Ni-TPC); this compound was then hydrogenated to nickel 5,10,15,20-tetraphenylisobacteriochlorin (Ni-TPiB). Finally, the Ni-TPiB reacts via a series of fast reactions, ending in demetallation and ring fragmentation. This mechanism is similar to the previously reported HDM mechanisms for Ni-TPP. However, the controlling step for the HDM rate was the first hydrogenation of Ni-TPP instead of the reported hydrogenolysis step. This change in controlling step was ascribed to the support influence and was explained on the basis of the surface acidity of the catalysts.