Lignin solubilisation and gentle fractionation in liquid ammonia

We present a simple method for solubilizing lignin using liq. ammonia. Unlike water, which requires harsh conditions, ammonia can solubilize tech. lignins, in particular kraft lignin. A com. pine wood Kraft lignin (Indulin AT) was solubilized instantaneously at room temp. and 7-​11 bars autogenous pressure, while a com. mixed wheat straw​/Sarkanda grass soda lignin (Protobind 1000) was solubilized within 3 h at ambient temp., and 30 min at. 85 °C. Hydroxide salts were not required. Wheat straw, poplar and spruce organosolv lignins, as well as elephant grass native lignin (MWL) were also solubilized, albeit at lower values. Different sequences of solubilization and extn. were tested on the Protobind 1000 lignin. The remaining lignin residues were characterized by FTIR, size exclusion chromatog. (SEC)​, elemental anal. (ICP)​, 2D-​NMR and 31P NMR. Liq. ammonia is not an innocent solvent, as some nitrogen was incorporated in the residual lignin which then rearranged to higher mol. wt. fractions. Nevertheless, the mild solubilization conditions make liq. ammonia an attractive candidate as a solvent for lignin in future biorefinery processes.
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