Mechanism of carbon sp(2)-heteroatom bond cleavage in hydroprocessing of substituted benzenes over unsupported transition metal sulfides

Hydroprocessing of substituted benzenes like aniline; phenol, diphenylsulfi de and chlorobenzene was performed in a batch reactor over unsupported tran sition metal sulfides, namely Co, Ni, Nb, Mo, Ru, Rh, Pd and W sulfides at 280 degrees C and 70 bar of hydrogen pressure. Under these experimental con ditions, diphenylsulfide and chlorobenzene mainly react through initial hyd rogenolysis of the carbon-substituent bond whereas aniline and phenol react through initial hydrogenation of the aromatic ring. Such a behavior was al ready reported for conventional sulfided cobalt- and nickel-molybdenum alum ina supported catalysts. Nevertheless, these new results confirm the prepon derant influence of mesomeric effects on the reactivity of organic models t oward sulfided catalysts. In addition to the results obtained over the supp orted bimetallic sulfides, it was found from quantum chemical calculations that the hydrogenolysis rate constants correlate with the rr-electron densi ty on the carbon bearing the substituent and with the overall calculated pi -electron transfer between the substituents and the benzene ring. It is thu s assumed that hydrogenolysis of carbon sp(2)-substituent bonds results fro m the attach, by a soft nucleophilic species like a hydride ion, on the car bon bearing the substituent. (C) 2000 Elsevier Science B.V. All rights rese rved.