Catalytic hydrodemetallation of nickel porphyrins: reactivity and catalyst surface studies

Abstract

The kinetics and mechanism of hydrodemetallation (HDM) of nickel etioporphyrin (NI-EP) and nickel tetra(3-methylphenyl) porphyrin (Ni-T3MPP) have been studied using alumina supported Group VIII metals, Co-Mo, and Ni-Mo as catalysts, and a white oil as a solvent. The metalloporphyrin solution simulates a metals containing residua, and was demetallized at 1000 psig H and temperatures in the range 300 to 375*C. Over all catalysts, goth porphyrins react via a sequential mechanism, involving initial hydrogenation to activate the porphyrin, followed by terminal hydrogenolysis which cleaves the ring, and deposits nickel onto the catalyst surface. While the global HDM mechanism is the same for both porphyrins, the intricacies of the HDM scheme are intimately linked to the structural differences on the periphery of the tetra-pyrrolic ring. With the Group VIII metals/Y-Al 0 the selectivity of the HDM mechanism (terminal hydrogenolysis riti constant/initial hydrogenation rate constant) has been varied over two orders of magnitude. Group VIII metals favor hydrogenolysis, while the Group VIII metals have a high ;electivity for hydrogenation. The HDM activity of lhe Group VIII metals is comparable to that of Co-Mo.Ni-EP was reacted over a series of Ru-Cu/Al 0 catalysts. The addition of Cu to the Ru system suppresses the rate of ih hydrogenolysis reaction, while having little effect on the hydrogenation reaction. Based on XPS measurements these observations were rationalized by geometrical effects. XPS also indicated that the chemical state of nickel deposited on a catalyst via HDM differs from its state and degree of interaction with the support in a Ni-Mo/Al 2 0 3 catalyst.Oil soluble cobalt and molybdenum napthenates were also dissolved in the white oil, and demetallized over bare T-Al 0 support at temperatures of 290*C and 360*C, and pressures of 500 and 1000 psig H 2 . These in situ synthesized Co-Mo catalysts were subsequently used in Ni-T3MPP HDM. The catalysts which were generated at 360*C, 1000 psig H 2 had a higher Ni-T3MPP HDM activity than the commercially manufactured Co-Mo/A1 2 0 3 . SEM/EDX and AES measurements indicate that a thick carbonaceous overlayer envelopes the Al 0 particles during napthenate demetallation. However this overlayer remaPns porous enough to permit all metals to ultimately deposit on the alumina.Model, non-porous polycrystalline alumina supported Co-Mo catalysts were made with surface compositions in the range 0<[Co/(Co+Mo)]<1 and used for Ni-T3MPP HDM. The catalysts were aged in HDMexperiments Tn a spinning basket reactor at 360*C and 1000 psig H The aged catalysts were scrutinized by a variety of surface spe~troscopic techniques (XPS, AES, SIMS, ISS and SEM/EDX). HDM activity was a maximum in the range 0.5<[Co6{Co+Mo) <0.7. Thf~initia 1 ly oxidic catalysts were reduced during HDM (Mo to Mo , and Co to Co ). Both metallic Ni and NiO were observed on the aged catalysts, however, the latter entity is most probably a result of air contamination, and it is not present in the working HD...