Electrochemistry of Biomass-Derived Materials. I. Characterization, Fractionation, and Reductive Electrolysis of Ethanol-Extracted Explosively-Depressurized Aspen Lignin

The chemical characterization (functional group distribution) and spectroscopic properties (¹³C, ¹H, IR, UV-Vis) of a number of lignins obtained from ethano1 extraction of explosively-decompressed aspen (Populus tremuloides) lignin (EEEDAL) are described. In this process vrood is heated to ~240°C by direct steam and fiberized by sudden decompression. Both mechanical and chemical degradation occur which lead to a decrease of the molecular weight of all of the natural polymers. The lignin fraction is soluble in alcohols; it has a low number average molecular weight (925) and is very reactive. EEEDAL was subjected to fractionation by differential solubility prior to and after electrolyses on mercury cathodes at -2,6 V vs. Ag/AgCl in methanol/tetraethyl-ammonium perchlorate. After recovery of 85-90% of the original weight of material used by precipitation with acid, followed by a series of extractions, the chemical and spectroscopic properties of each fraction were determined. The acid-insoluble fractions are more condensed than the acid-soluble fractions and have lower phenolic OH and lower aliphatic OH contents than the acid-soluble fractions. The acid-soluble fractions phenolic OH content approaches one per phenylpropane (C_g) unit of the lignin. Electrolyses increase the amount of lower molecular weight, high phenolic content, acid-soluble lignins and changes the molecular weight distribution by decreasing the polydispersity of the re - suiting lignins. These low-molecular-weight lignins are chemically reactive (for instance, with formaldehyde) and can be used in the partial replacement of phenol in phenol-formaldehyde thermosetting resins.