KINETIC INVESTIGATION OF THE DEHYDROGENAT IVE COUPLING OF BENZENE ANDTOLUENE IN A REACTOR WITH A LARGE TEMPERATURE-GRADIENT

Dehydrogenative coupling reactions of benzene and toluene were carried out in a reactor with a large temperature gradient, in the temperatur e range from 1270 K to 1370 K for the former, and from 1000 K to 1150 K for the latter to investigate the characteristics of the reactor, in which thermal diffusion was expected to arise due to the temperature gradient. The reaction of benzene followed a pseudo zero-order kinetic s, and the activation energy was 421 kJ/mol, which was much larger tha n those previously reported (155-289 kJ/mol) for the conventional reac tor which had no significant temperature gradient. The reaction of tol uene also obeyed a pseudo zero-order kinetics. The activation energies for the formation of benzyl radicals and tolyl radicals were 349 kJ/m ol, and 423 kJ/mol, respectively. These values of activation energy we re almost the same as bond dissociation energies of the carbon-hydroge n bond, the cleavage of which led to the initiation of the reaction. T he small difference between the obtained activation energies and the b ond dissociation energies can be accounted for by the assumption that only a few chain reactions occurred in a reactor with a large temperat ure gradient, because of the rapid lose of thermal energies of benzyl, tolyl, and phenyl radicals, produced by the initiation reactions.