The Chemistry of Acid-Catalyzed Coal Depolymerization

Abstract

The following comments can be made with regard to a correlation of the coal depolymerization product yields with the relative reactivities of aliphatic carbon and oxygen in bridge structures in coal: In low-rank coals, particularly in lignites, the cleavage of aliphatic ether and benzyl ether oxygen bonds may contribute significantly to depolymerization. Many of the aliphatic bridges, which participate in depolymerization, may be linked to single phenolic rings. The reactivity of an aliphatic structure linked to a phenolic ring is sufficient to permit its participation in the depolymerization reaction. On the other hand, spectrometic studies indicate that these coals do not contain significant amounts of sufficiently reactive condensed aromatic structures which could participate in depolymerization. In high-volatile bituminous coals (C = 80 to 84%), the breaking of aliphatic bridges is seen as the major means of depolymerization. Many of these bridges may be linked to phenolic rings, others to condensed aromatic rings such as phenanthrene. Ether oxygen bonds play a less important role because most of these bonds are in aromatic ethers which do not react with BF/sub 3/. In higher rank bituminous coals (C > 84%), the depolymerization product yield decreases sharply with increasing rank. It is probable thatmore » this decrease is related to the nature of the aliphatic bridge structures. Among the aliphatic bridge structures, -CH/sub 2/- bridges may play a particularly important role in coal depolymerization. Even if the number of such bridges is relatively small, the probability of their cleavage is high because, in general, both carbon--carbon bonds of the bridge are activated under the reaction conditions. This view is supported by depolymerization studies made with phenol-BF/sub 3/ where the yield of soluble depolymerized products was proportional to the amount of -CH/sub 2/- bridge structures found in the soluble products.« less