Marvin L. Poutsma

Fundamental reactions of free radicals relevant to pyrolysis reactions

Abstract Free-radical mechanisms are ubiquitous for pyrolysis of organic substrates. Background information is supplied to assist in the interpretation of such reactions by dissection into the elementary radical-forming, radical-consuming, and radical-interconverting reactions that form the building blocks of overall mechanisms. Particular attention is given to homolysis, hydrogen abstraction, β-scission–addition, and rearrangement steps, and to bond-breaking and bond-forming processes. Since the thermochemistry of such elementary reactions is dependent on the stability of the radicals involved, the major influences of structural changes on radical stability are reviewed. Kinetics of prototypical elementary reactions and their responses to structural changes are rationalized in terms of their Arrhenius parameters. The fundamental relationships between thermochemical parameters and kinetic parameters are summarized, and the use of these to estimate rate constants for cases where data is not available is demonstrated. Some kinetic consequences of the combination of elementary reactions to produce chain reactions are reviewed. Critical structural and kinetic features that determine the selectivity among product types and the response of reactions to temperature and substrate concentration are indicated.

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.

Carbon-14 tracer study of the fate of tetralin under simulated SRC-1 coal liquefaction conditions

Abstract A 28.4 wt% slurry of Illinois No. 6 coal in Wilsonville recycle solvent containing 0.83% of 1-14C-tetralin as a tracer for hydroaromatic solvent components has been hydroliquefied in a bench-scale, continuous-flow system under simulated SRC-I conditions (454 °C dissolver outlet; 13.9 MPa H2). A combination of solvent fractionation, distillation, g.l.p.c. separation, and radio-assay procedures allow determination of the chemical fate of the labelled tetralin. Of the tetralin which is consumed (47 wt% of the feed), only 85% is converted to the ‘ideal’ dehydrogenation product, naphthalene. The remaining 15% is divided in a 1:2 ratio between conversion to structurally altered C9–C11 hydrocarbons (l-methylindane, indane, methylnaphthalene) and bonding to much heavier coal- and solvent-derived products. These side reactions would be expected to decrease both the solvent donor quality and the inventory. Although the mechanisms for grafting of tetralin-derived fragments to heavy materials cannot yet be described in detail, the chemical processes responsible must be quite indiscriminant towards functional groups because the specific radioactivities of the oils, the asphaltenes, the preasphaltenes, and even the insoluble organic matter are very similar.