Search

The combination of TMAH thermochemolysis and TEAAc treatment makes it possible to discriminate between the different forms of mono- and dicarboxylic acids present in the structure of humin and humic acids, that is, “free” uncombined acids, methyl or ethyl esters present as tightly trapped molecules within the matrix, or acids chemically linked to the matrix by ester groups. The results confirm that ester groups are involved in the structure of humin and humic acids. The cross-linking of moieties originating from microbial metabolism or inherited from higher plants is partly ensured by these chemical groups. On the other hand, significant amounts of fatty monocarboxylic acids and linear dicarboxylic acids are present as free acids in the humin of the studied sample. Humin contains also fatty acid methyl esters. Free, uncombined α,ω-dicarboxylic acids were only found in humin.

A simple and rapid method for the enzymatic hydrolysis of cellulose was applied to the humin originating from an acidic anmoor soil from the Plateau de Millevaches. Degradation of cellulose present in humin frees C 9 –C 20 fatty acids, C 12 –C 20 fatty acid methyl esters, C 16 –C 35 n -alkanes without an odd/even predominance and C 16 , C 18 and C 20 n -alkenes. This result shows that cellulose moieties, present in soil humin, are involved in the retention of lipids which cannot be directly extracted with solvents. Lipids freed after cellulose degradation are somewhat different from lipids freed after alkaline hydrolysis, indicating a very heterogeneous structure for this humin.

A multistep alkaline hydrolysis under phase transfer catalysis conditions (PTC) was carried out on humic acid and humin fractions from an acid soil sample (anmoor soil) from the Plateau de Millevaches (France). A large part (87%) of the initially insoluble humin became soluble after a first series of hydrolysis. The main fraction obtained, soluble high molecular-weight products, was still reactive when submitted again to a second series of hydrolysis. Humic acid showed a lower reactivity. Linear dicarboxylic acids were produced in significant amounts from humin, they were short-chained (mainly C 9 ) in the first series and long-chained (C 16 –C 28 ) with an even carbon number preference indicating a plant origin in the second series. Linear C 12 –C 32 monocarboxylic acids and alkanols were freed from humin and humic acids. The short linear fatty acids and alkanols, accompanied with iso - and anteiso -C 15 and C 17 members, denote a bacterial input while long even components, as well as various aromatic acids, originate from plants. The results indicate that ester groups are involved in the cross-linking of macromolecular chains forming the matrix of humin. Aliphatic dicarboxylic acids act as bridges between alkyl chains in the matrix, while fatty acids, alcohols and aromatic acids correspond mainly to monosubstituents of the matrix (bound by ester groups). It is interesting to note that this model resembles the kerogen of many immature sediments.

Quaternary ammonium salt Aliquat 336 and 18-Crown-6 enhance hydrolysis of Timahdit-M kerogen when used as a phase transfer catalyst. The yield of hydrolysis products was higher with Aliquat than with 18-Crown-6, but the identified products were found in lower concentration, the difference being soluble heavy parts from kerogen which cannot be analyzed directly. Both reactions afforded series of aliphatic mono- and dicarboxylic acids, alkylsuccinic acids and alkanols, but differences in distributions were observed. The range of monocarboxylic acids was shorter with Aliquat than with 18-C-6, C14C18 compared to C12C32, but palmitic and stearic acids were dominant in the two cases. C10 and C12 alkyl succinic acids were identified in Aliquat products. Aromatic acids were the major hydrolysis products (59%) with crown ether; only one aromatic acid was found with Aliquat. Hydrocarbons and tertiary amines were obtained from both reactions, they were obviously present in the kerogen as trapped molecules. Many tertiary amines arising from the decomposition of the reagent were obtained with Aliquat. The results give evidence of an important reticulation involving ester groups in the Timahdit-M kerogen. Series of n-, iso- and anteisoalkanes were obtained through HI/LAD treatment of Irati and Aleksinac kerogens, originating probably from glycerol-ether lipids. C27C34 α,β-hopanes were also found to be linked to kerogen (Irati) by ether bonds. Reaction with CF3CO2H (Irati) afforded seven series of branched alkenes, indicating that branched saturated alkyl chains were bound to aromatic moieties of kerogen by ether groups. Acetates of cholesterol and 24-ethylcholesterol were identified in the BF3-Et2O/Ac2O products. Esters of fatty acids (max C16, C18) were also identified. Only alkanes were produced on AII3 reaction. Each reaction afforded the same series of n-alkanes. This would reflect modifications in the kerogen matrix during these various treatments, resulting in the release of tightly trapped molecules.