Date of Award

1-1-1985

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Abstract

High temperature reactions were done with bibenzyl and related compounds to study C-C bond cleavage under coal liquefaction conditions. Reactions were run in 12-ml stainless steel, 3.7-ml glass, and 8.0-ml aluminum autoclaves designed in our laboratory. The range of temperatures studied was 250-515(DEGREES)C and that of pressure was 0-1800 psi (initial).Mechanisms for the cleavage of the aromatic-aliphatic C-C bonds ((alpha)-cleavage) and the aliphatic-aliphatic C-C bond ((beta)-cleavage) have been postulated. The two sets of conditions studied were: (i) those which included some form of sulfur, and those which excluded sulfur.With H(,2)S present, the (beta)-cleavage to toluene was extensively promoted. The rate of this reaction was dependent on the parameters of temperature, time, H(,2)S concentration, and reactor surface. This (beta)-cleavage is postulated to occur by the decomposition of the (alpha)-sulfenyl bibenzyl radical (109,309) which forms by a S(,H2) radical displacement reaction. Thermal decomposition of the (alpha)-sulfenyl bibenzyl radical by a known (beta)-scission reaction (92,263,298) yields thiobenzaldehyde and a benzyl radical which are reduced to toluene, or polymerized to high molecular weight products. This mechanism has been shown to occur by an autocatalytic radical chain process that is initiated by benzyl or sulfur radicals, and it is consistent with the literature (287,323,324).Stainless steel reactors (or stainless steel shavings in the glass reactors) were shown to competitively remove active sulfur radicals and, thus, hinder the pathway to (beta)-cleavage.With molecular hydrogen present, the (alpha)-cleavage to benzene and ethylbenzene is promoted. The effects on the (alpha)-cleavage can be explained by an increase in the hydrogen atom concentration from hydrogen using bibenzyl as a benzyl radical source (95,112,136).The presence of H(,2)S in the bibenzyl-H(,2) reaction was shown to decrease the yields of (alpha)-products while enhancing the conversion to 1,1-diphenylethane. This was related to a decrease in the gas-phase hydrogen-atom concentration by trapping reactions of H(,2)S (267,268).The rate of (alpha)-cleavage was low under essentially non-reducing conditions in this study; i.e., argon, CO-H(,2)O, CO-H(,2)O-H(,2)S. In this study, the mixture of carbon monoxide and water with and without hydrogen sulfide was found to be essentially non-reducing.

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