Depolymerization mechanism of alkali lignin in sub- and supercritical ethanol

The depolymerization process of wheat straw alkali lignin in sub- and supercritical ethanol was investigated with a micro autoclave reactor. The degraded product properties and the depolymerization mechanism of lignin structure were studied by scanning electron microscopy (SEM), gas chromatography/mass spectrometry (GC/MS) and infrared spectroscopy (FT-IR). The experimental results show that the minimum residual char yield (16.5%) is obtained at the condition of ethanol supercritical point (240℃, 7.2 MPa). Under subcritical ethanol conditions, alkali lignin firstly melts and disperses in ethanol as 1.0-2.0 μm diameter of microspheres, then a small amount of ether linkages and benzene ring side chain C_α are broken to form phenols, esters, ketones and acids products. Under supercritical ethanol conditions, the diameter of molten microsphere is significantly reduced, and plenty of ether linkages and benzene ring side chain C_α are continuously broken, meanwhile, the lipid products are subjected to secondary decomposition reaction. The yield of lipid is decreased (11.94%), while the yield of phenolic products from depolymerization is increased (52.14%).