A new look at lignin amination through the exploration of oxidative ammonolysis of steam-exploded Eucalyptus globulus bark

The D-optimal experimental design was applied to optimize the conditions of the oxidative ammonolysis (OA) process (temperature (T), oxygen pressure (PO2), time (t), and ammonia concentration (ωNH3)) to maximize the yield of water-soluble aminated lignin (ηLN) released from steam-exploded and enzymatically hydrolysed Eucalyptus globulus bark. Based on a reduced 2FI (two-factor interaction model), PO2 and the t were found to be the determining factors, whilst T had only minor effect within the studied range (100–130 °C). Additionally, ωNH3 manifests through its interaction with PO2, evidenced by a negative regression coefficient of the pairwise interaction. Under optimized conditions (130 °C; 3 h; PO2 = 10 bar; ωNH3 = 5%), ηLN was almost 50% based on initial cellolignin. The obtained aminated lignin contained 18.5% of nitrogen and was of low molecular weight (Mw= 980 Da). 15N and 13C-labelling was combined with 1D and 2D NMR to clarify lignin amination patterns. Although most of the amination products were low molecular weight compounds (urea, acetamide, formamide, among others), a significant portion of the lignin itself was aminated, containing phenylamine-type structures (25% of the total) and benzamide-type structures (75% of the total). Oxalic acid was suggested to play an important role as a trigger in amination reactions during OA. This was confirmed by the addition of oxalic acid to Eucalyptus kraft lignin resulting in a substantial increase in the amination extent and ηLN. It has been hypothesized that the chemistry of the carbamoyl radical is responsible for the promoted amination of lignin.