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A study of some clay-catalysed organic hydrogen exchange reactions and their significance to the maturation of sedimentary organic matter

Larcher, Alfons Valentino (1985) A study of some clay-catalysed organic hydrogen exchange reactions and their significance to the maturation of sedimentary organic matter. PhD thesis, Murdoch University.

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Clay-catalysed aromatic and alkyl hydrogen exchange reactions have been studied in a number of organic compounds and their significance to the maturation of organic matter in clay-containing sedimentary rocks has been assessed. In a series of experiments using tritium-labelled substrates, hydrogen exchange was found to occur between the aromatic hydrogens in naphthalenes and a series of homoionic montmorillonites. A mechanism of exchange was proposed Involving the protonation of the naphthalene ring by the addle clay surface forming an adsorbed arenium ion intermediate. Aromatic hydrogen exchange was also observed in the presence of sedimentary rock samples at 138°C, indicating that this reaction may occur in sedimentary environments. This suggests that the isotopic composition of the aromatic hydrogen atoms of sedimentary organic matter may be related to that of the addle water adsorbed on clay surfaces with which the organic matter has been in contact.

In a further series of experiments, the exchange of alkyl hydrogens between tritium-labelled cumenes and aluminium montmorillonite was found to occur at 160°C. The exchange process was proposed to involve adsorbed carbocation-like and alkene-like intermediates and to be promoted by the addle properties of the clay surface. Clay-catalysed alkyl hydrogen exchange was also observed in a fully saturated hydrocarbon, meso-pristane, and in a series of acyclic isoprenoid acids, when these substrates were heated in the presence of deuterated aluminium montmorillonite. Hydrogen exchange in meso-pristane did not result in the isomerisation of the C-6 and C-10 chiral centres, whereas the chiral centres at the C-2 positions in the isoprenoid acids were isomerised. The mechanism of these exchange reactions appeared to be similar to that observed in cumene. The fact that alkyl hydrogen exchange was promoted by clays at 160°C, a temperature which is not unusual in sediments, indicates that this reaction could occur in the sedimentary environment. Additional support for this contention was obtained by the application of clay-catalysed aromatic and alkyl hydrogen exchange reactions to the maturation of sedimentary organic matter. This treatment was able to account for some of the variations observed in the hydrogen isotopic compositions of petroleums and related organic matter. Further, application of the proposed mechanism of clay-catalysed alkyl hydrogen exchange to sedimentary biological marker compounds indicated that some of the observed changes in distribution with maturation were due to these hydrogen exchange reactions.

Changes in the distribution of sedimentary hopane and sterane diastereomers were observed when mixtures of these compounds were heated in the presence of aluminium montmorillonite. In particular, 17B(H),21B(H)-hopanes were found to be removed from the alkane fraction at 75°C while the 17a(H),21B(H)- and 17B(H),21a(H)-hopanes were unaffected by the clay. The observed relative depletion of 17B(H),21(3(H)-hopanes in sedimentary rocks of increasing maturity may therefore be due to these selective catalytic processes, and not to interconversion processes caused by isomerisation at C-17 and at C-21. Higher temperature experiments demonstrated the ability of clay minerals to effect additional changes in the distributions of hopanes and steranes, similar to those observed during sedimentary maturation. Additional support for the catalytic role of sedimentary clay minerals was provided by the analysis of coals and shales from the same sedimentary sequence and of the same maturity. The more mature distributions of biological marker compounds found in the shales was attributed to the higher proportions of catalytic clay minerals present.

During the investigation of the exchange reactions of isoprenoid acids, the gas chromatographic separation of their diastereomeric ester derivatives was studied. The technique developed used commercially available fused-silica columns coated with methylsilicone or 5 percent phenylmethylsilicone stationary phases. Previously reported analysis times were reduced and the need for specially prepared polar-phase capillary columns was eliminated.

In a further study, a homologous series of moretanes (C31-C35) was analysed by GC-MS in samples from a sedimentary Except for the C31 homologues, all members were shown to exist as mixtures of C-22 epimers which could be resolved rock sequence. by GC. For all members of the moretane series, immature sedimentary rocks contained predominantly the 22R epimer, with increasing maturity being accompanied by increasing amounts of the 22S epimer until equilibrium was reached in mature samples. These changes were interpreted in terms of clay-catalysed alkyl hydrogen exchange reactions.

Publication Type: Thesis (PhD)
Murdoch Affiliation: School of Mathematical and Physical Sciences
Supervisor(s): Alexander, Robert, Kagi, Robert and Giles, Dion
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