Free-radical model for coal conversions. Effect of conversion level and concentration on thermolysis of bibenzyl

Abstract

The product distributions from thermolysis of bibenzyl at 366 °C and 400 °C have been determined as a function of conversion level and of bibenzyl concentration from the dilute gas to the neat liquid. At very low (1–3%) conversion levels, five primary products have been detected: toluene ( 2), stilbene ( 3), 1,2,3-triphenylpropane ( 4), 1,2,3,4-tetraphenylbutane ( 5), and 1,1-diphenylethane ( 6). Increasing the concentration of bibenzyl decreases the ratio 4:5 and increases the amount of rearrangement to 6. This product behaviour is interpreted by a multistep, nonchain, free-radical network involving initial formation of benzyl radicals by homolysis of bibenzyl, formation of 1,2-diphenylethyl radicals by hydrogen atom transfer, and subsequent coupling, disproportionation, and rearrangement of the radicals. At higher conversion levels, the primary product distribution is markedly perturbed because products 4 and 5 are inherently less stable than bibenzyl; additional, secondary products include diphenylmethane and phenanthrene.