Structural features of lignite humic acid in light of NMR and thermal degradation experiments

Structural composition of a lignite humic acid (HA) fraction was studied by means of solid-state 13C NMR, different solution-state 1H NMR pulse techniques and thermally assisted hydrolysis–methylation with TMAH and TMAAc followed-up with pyrolysis–GC–MS experiments. The results verified that certain aliphatic compounds have their special tasks in the complicated structural network of lignite HA material, and aprotic solvents with strong electron-donor powers are needed to release the tightly bound certain aliphatics from the macromolecular network for obtaining a fully dissolved HA solution. The occurrence of the relatively large content of different carboxylic acids as their free-acid forms was surprising. The structural interpretations performed by special 1H NMR pulse techniques verified the complexity of aliphatic moieties, the presence of hydroaromatic carbons, residual lignin derivatives, the abundance of aliphatic and aromatic carboxylic acids, and the ability of aliphatics to form intermolecular bridges between aromatic building blocks.