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

In this study, thermal degradation products of humic acids (HAs) from widel y different origins and compositions obtained by either on-fine flash pyrol ysis at 500 degreesC or off-line, low-temperature (270 degreesC) thermally assisted hydrolysis and methylation (THM) using tetramethylammonium hydroxy de (TMAH) have been compared. Both methods yielded phenols, methoxyphenols, benzenecarboxylic acids, alkanoic: acids and hydrocarbons, but their relat ive 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 C-13 NMR, it was observed that the use of TMAH led to an 'aliphatic enhancement', the extent inversely depending of temperatu re. Our results indicate that thermal degradations based on reactions at 50 0 and 270 degreesC should be considered as distinct techniques, providing i ndependent structural information in the analysis of HAs. It is suggested t hat transesterification and base-catalysed hydrolysis reactions occurring a t a sub-pyrolysis temperature could be a successful method of mild, low-tem perature alkaline degradation, but it is not sufficient for obtaining stoic hiometrically representative information on the molecular assemblages of th e HA structure. (C) 2001 Elsevier Science B.V. All rights reserved.