Comparison of Bromine-treatment and Iodine-treatment in the Carbonization of Pitches

Abstract

The difference between the effects of bromine and iodine treatment in the carbonization of pitches (coal tar pitch and petroleum pitch) has been investigated by adsorption isotherms, XPS and TG-Mass measurements, in order to understand the stabilization mechanism of pitches by halogen. A replacement reaction between bromine and the hydrogen in the hydrocarbon molecules in the pitches was one of the most dominant reactions of the bromine treatment. This was independent of the chemical structure of the parent pitch and resulted ina high carbon yield both in coal tar pitch and in petroleum pitch. The stepwise dehydrogenation, detected as the removal of HBr caused by cleavage of-C-Br bonds, continued up to high temperatures in the carbonization process. It is suggested that bromine-substituted compounds are thermally stable and enhance the dehydrogenation up to about 600°C.These results are very different from iodine treatment, which promotes dehydrogenative polymerization via the formation of charge-transfer complexes.