Catalytic hydrodemetallation of nickel porphyrins: I. Porphyrin structure and reactivity

Abstract

The kinetics and mechanism of hydrodemetallation reactions and the pattern of metal deposition have been investigated using a model residuum oil, consisting of nickel porphyrins (Ni-etioporphyrin, Ni-tetraphenylporphyrin, and Ni-tetra(3-methylphenyl)porphyrin) dissolved in a mineral oil and reacted over an unsulfided CoMo Al 2O 3 catalyst at 285–345 °C and 4.58–10.09 MPa H 2 (650–1450 psig). The porphyrins react via a sequential mechanism, first involving hydrogenation of peripheral double bonds, followed by the final hydrogenolysis step which fragments the ring and removes the metal. Porphyrin molecular structure exerts, through chemical and steric effects, an impact on this reaction pathway. For Ni-etioporphyrin, there is one hydrogenated reaction intermediate and the rate-limiting step is initial hydrogenation. For Ni-tetra(3-methylphenyl)porphyrin, there are three reaction intermediates and the rate-limiting step is final hydrogenolysis and ring opening. Temperature and hydrogen pressure dependencies of the kinetic parameters have been determined. The rate constants are invariant of nickel concentrations from 16 to 90 ppm. Nickel-deposition profiles generated under diffusion-limited conditions in 1.5-mm extrudates have been successfully modeled using intrinsic kinetic parameters obtained with crushed catalysts and diffusivities on the order of 10 −6 cm 2/sec.