Studies of angiospermous woods in Australian brown coal by nuclear magnetic resonance and analytical pyrolysis: new insight into early coalification

Abstract

Many Tertiary coals contain abundant fossilized remains of angiosperms that often dominated some ancient peat-swamp environments; modern analogs of which can be found in tropical and subtropical regions of the world. Comparisons of angiospermous woods from Australian brown coal with similar woods buried in modern peat swamps of Indonesia have provided some new insights into coalification reactions. These comparisons were made by using solid-state 13C nuclear magnetic resonance (NMR) techniques and pyrolysis-gas chromatography-mass spectrometry (py-gc-ms), two modern techniques especially suited for detailed structural evaluation of the complex macromolecules in coal. From these studies, we conclude that the earliest transformation (peatification) of organic matter in angiospermous wood is the degradation of cellulosic components. The efficiency of removal of cellulosic components in the wood varies considerably in peat, which results in variable levels of cellulose in peatified wood. However, the net trend is towards eventual removal of the cellulose. The angiospermous lignin that becomes enriched in wood as a result of cellulose degradation also is modified by coalifications reactions; this modification, however, does not involve degradation and removal. Rather, the early coalification process transforms the lignin phenols (guaiacyl and syringyl) to eventually yield the aromatic structures typically found in brown coal. One such transformation, which is determined from the NMR data, involves the cleavage of aryl ether bonds that link guaiacyl and syringyl units in lignin and leads to the formation of free lignin phenols. Another transformation, which is also determined from the NMR data, involves the loss of methoxyl groups, probably via demethylation, to produce catechol-like structures. Coincident with ether-cleavage and demethylation, the aromatic rings derived from lignin phenols become more carbon-substituted and cross-linked, as determined by dipolar-dephasing NMR studies. This cross-linking is probably responsible for preventing the lignin phenols, which are freed from the lignin macromolecule by ether cleavage and from being removed from the coal by dissolution. Pyrolysis data suggest that the syringyl units are altered more readily than are guaiacyl units, which leads to an enrichment of the guaiacyl units in fossil angiospermous woods. Although many of the coalification reactions noted above occur to some degree in all angiospermous fossil woods examined, some significant differences are observed in the degree of coalification of the fossil woods from the same burial depth in the brown coal. This indicates that the depth and the duration of burial are probably not entirely responsible for the variations in degree of coalification. It is likely that different rates of degradation in peat may have contributed to the variations in the apparent degree of coalification, considering the fact that some woods may have been altered more rapidly at the peat stage than others. Although preliminary, it is clear that a systematic study of botanically related woods in peat and coal leads to a more detailed differentiation of coalification reactions than have previous investigations. The combined use of solid-state 13C NMR and py-gc-ms has facilitated this detailed new insight into coalification of angiospermous wood.