Experimental reappraisal of flash pyrolysis and low-temperature thermally assisted hydrolysis and methylation using tetramethylammonium hydroxide for the molecular characterization of humic acids

Abstract

In this study, thermal degradation products of humic acids (HAs) from widely different origins and compositions obtained by either on-line flash pyrolysis at 500°C or off-line, low-temperature (270°C) thermally assisted hydrolysis and methylation (THM) using tetramethylammonium hydroxyde (TMAH) have been compared. Both methods yielded phenols, methoxyphenols, benzenecarboxylic acids, alkanoic acids and hydrocarbons, but their relative amounts differed among the samples and for each sample depending on the degradation method. In relative quantitative terms, and when compared with the results obtained by 13C NMR, it was observed that the use of TMAH led to an ‘aliphatic enhancement’, the extent inversely depending of temperature. Our results indicate that thermal degradations based on reactions at 500 and 270°C should be considered as distinct techniques, providing independent structural information in the analysis of HAs. It is suggested that transesterification and base-catalysed hydrolysis reactions occurring at a sub-pyrolysis temperature could be a successful method of mild, low-temperature alkaline degradation, but it is not sufficient for obtaining stoichiometrically representative information on the molecular assemblages of the HA structure.