Structural characterization of humic substances using thermochemolysis with tetramethylammonium hydroxide
1996
Rio, J.C del | Hatcher, P.G.
The focus of this paper centers on a review of the recent development of the TMAH thermochemolysis as a rapid, low-coy, and easily implemented technique for the structural analysis of humic substances. Pyrolysis, or rather chemolysis, in the presence of TMAH has been used for the structural characterization of humic substances from different origins. The procedure methylates carboxylic groups and hydroxyl groups, rendering the chemolytic products more amenable to chromatographic separation. The examination of humic substances with this technique reveals the presence of a series of benzenecarboxylic acid methyl esters as well as long-chain fatty acid methyl esters and dimethyl esters. The methylated structures produced by TMAH differ dramatically from those obtained by conventional pyrolysis, calling into question the recently proposed structure of humic acids which are based mostly on conventional pyrolysis. The procedure consists mainly of a thermally assisted chemolysis rather than true pyrolysis, and consequently is effective at subpyrolysis temperatures. This means that the procedure can also be easily implemented in sealed glass tubes. In general, the studies to date have demonstrated that this technique provides excellent preservation of the original structures containing carboxyl and hydroxyl groups in lignin monomers owing to protection of the functional groups from thermal reactions. The TMAH thermochemolysis also induces beta-ether bond lysis in lignin. This is significant because the procedure shows potential in characterizing lignin-derived compounds in much the same way as the CuO oxidation procedure.
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