Experimental and theoretical studies into the laser pyrolytic formation of chlorinated polycyclic aromatic hydrocarbons and fullerene precursors.

Abstract

Fullerenic materials are likely to play an important role in technologies of the future. To ensure that production techniques will be able to keep up with demand, a thorough understanding of their mechanism of formation, which has thus far proved elusive, is required. Hydrocarbon pyrolysis is a potentially viable fullerene production technique, and the pyrolysis of chlorohydrocarbons has also shown promise. However, decomposition of the latter produces toxic and environmentally hazardous chlorinated polycyclic aromatic hydrocarbons, also formed in industrial waste incinerators, as a byproduct. Close study of the high temperature chemistry of chlorohydrocarbons may aid both the mitigation of hazardous byproducts and implementation of more effective fullerene synthesis techniques. To this end, we have studied the formation mechanisms of dichloromethane degradation products generated via Infrared Laser Powered Homogeneous Pyrolysis. This unique technique is well known for having a non-uniform temperature profile, which has a number of attractive features for this work. The most important of these are the absence of complicating surface catalysed reactions, and the potential for allowing annealing reactions necessary for fullerene growth. Time resolved product yields are monitored via GC-MS and FTIR, and mechanistic deductions are supported heavily by Density Functional Theory calculations and kinetic arguments. Results indicate that the initial growth of chlorinated compounds deviate significantly from the radical-based growth found with hydrocarbons. Facile Cl-loss in important radicals and stabilisation of carbenes by chlorine permits novel C4 and C6 production channels. Conventional channels involving acetylene addition to aromatic radicals are eventually restored in C8--C12 formation, although we do suggest some amendments to the mechanism. Bimolecular polycyclic aromatic hydrocarbon addition reactions may also play an important role. Acenaphthylene (C12H8) congeners also allow for the first studies of the migration of five-membered rings about chlorinated polycyclic aromatic hydrocarbon frameworks, a vital process in fullerene annealing; it is found that the presence of chlorine significantly stabilises transition states, suggesting these reactions are much more facile in heavily chlorinated systems.