Hydrogenation of nickel and vanadyl tetraphenylporphyrin in absence of a catalyst - A kinetic study

The hydrodemetallation (HDM) reactions of nickel-5,10,15,20-tetraphenylporp hyrin (Ni-TPP) and vanadyl-5,10,15,20-tetraphenylporphyrin (VO-TPP) were st udied in the absence of a catalyst. Ni-TPP as well as VO-TPP is found to demetallate through a reversible seque ntial reaction mechanism in which similar hydrogenated intermediate species are formed, As compared to catalized HDM, reaction rates are low. Hydrogen ation reactions of Ni-TPP are first order with respect to the liquid phase concentration of hydrogen. Hydrogen sulfide promotes the conversion of Ni-T PP. An equilibrium constant for the reversible hydrogenation of Ni-TPP to n ickel-5,10,15,20-tetraphenylchlorin (Ni-TPC) has been estimated as a functi on of temperature. A heat of reaction of -97 kJ/mol was found for the rever sible hydrogenation of Ni-TPP to Ni-TPC, Comparison of the reactivities of a metal-free porphyrin (H-2-TPP), Ni-TPP and VO-TPP revealed that the metals have a stabilizing influence towards hy drogenation and ring fragmentation of porphyrin macrocycles. (C) 2001 Elsev ier Science B.V. All rights reserved.