Chemical composition of pseudo-phlobaphinite precursors: implications for the presence of aliphatic biopolymers in vitrinite from coal

Abstract

Pseudo-phlobaphinite cell inclusions in a buried stem of angiospermous wood of Rhizophora mangle from Mangrove Lake, Bermuda have been investigated petrographically and chemically. Optical and scanning electron microscopy reveal that the light wood from inner sections of the stem consists predominantly of cell walls with few cell inclusions, whereas the dark wood from outer portion of the stem is composed of both cell walls and cell inclusions which are believed to represent pseudo-phlobaphinite precursors. 13C NMR spectroscopy reveals the presence of lignin and tannin structures within the cell walls. The wood also displays resonances suggestive of aliphatic components. The NMR results are further supplemented and confirmed by flash pyrolysis gas chromatography and mass spectrometry analyses. The pyrogram of the cell-wall dominated light wood is characterized by peaks related to both syringyl and guaiacyl units. The major pyrolysis products from the dark wood, however, are a homologous series of long-chain n-alkalines, n-alk-l-enes and α, ω- alkadienes extending from chain length C 10 to C 30. These aliphatic components arise from the material associated with the pseudo-phlobaphinite cell inclusions. Py-gc/ms analysis of a humic acid extracted from the sediments surrounding the studied wood consists almost exclusively of a homologous series of straight-chain alkanes, alkenes and alkadienes up to C 30. The striking resemblance between pyrograms of the pseudo-phlobaphinite precursor and humic acid suggests that the possible mechanism for the formation of the pseudo-phlobaphinite precursor migration and deposition of humic acid into cell lumina.