Pathways in coal thermolysis : a theoretical and experimental study with model compounds

Abstract

Fundamental aspects of coal thermolysis were investigated, including how the chemical structures of aromatics, hydroaromatics, and alcohols affect their reactivities as hydrogen donors and acceptors in coal processing. The susceptibilities of substructural entities in coals to fragmentation via a number of thermal pericyclic and free radical mechanisms were probed, as were the factors governing relative reactivities within series of such coal model compounds. The theoretical part of the work applied perturbation molecular orbital (PMO) and frontier orbital theories, in conjunction with pi- and pseudo-pi MC's, to the study of model compound reactivity. This enabled prediction of reactivity patterns of H-donors, H-acceptors and coal-like structures as functions of their pi- and o-bond configurations, including heteroatomic effects.

Experimentally, the liquid phase reactions of the coal model compound PhOCH2Ph (Benzyl phenyl ether, BPE) were detailed for the first time in each of the four hydronaphthalene H-donor solvents, (1,4- dihydronaphthalene, or 1,4-DHN), (1,2-DHN), (tetralin), and (trans-decalin), in the temperature range 220*-300*C. The thermolysis of BPE exhibited a pronounced dependence on solvent structure, both with respect to product selectivities and reaction kinetics. The most preferential formation of light products, the most efficient hydrogen transfer, and the fastest kinetics were obtained with 1,4-DHN. In contrast, the overall BPE transformation rates in 1,2-DHN, tetralin and t-decalin were roughly comparable, despite qualitative differences in kinetic detail and heavy product (>C130) formation.

BPE thermolysis pathways were delineated as involving (a) rearrangement, leading to isomerization, (b) hydrogenations, leading ultimately to PhOH and PhCH3 products,and (c) addition reactions, engendering heavy products. Pathways (b) and (c) are competitive and, in each, self-reactions of BPE-derivatives vie against reactions between these and the donor solvent. Of the detailed free radical and pericyclic reaction mechanisms postulated, the latter rationalized many more facets of the BPE results than the former. The theoretical and experimental results were appraised against previous coal thermolysis literature.