Thermochemical Conversion of Wheat Straw Dioxane Lignin in a Sub/Supercritical Dimethyl Carbonate Medium

Objective: The main goal of this study was to investigate the thermochemical conversion of wheat straw dioxane lignin in a sub/supercritical dimethyl carbonate medium. The object of our study was the dioxane lignin isolated from wheat straw. The conversion of wheat straw dioxane lignin in a sub/supercritical dimethyl carbonate medium was studied. Methods: The experiment was performed in an autoclave in a temperature range from 200°C to 350°C. The low-molecular-weight products were extracted from the liquid fraction using hexane; GC-MS was used to study their composition. The solid products were analyzed by IR spectroscopy. Results and Discussion: The low-molecular-weight products consist of esters, aldehydes, ketones, and alkyl aromatic compounds. Thirty-four compounds were identified by GC-MS, including 12 C₆–C₃ compounds, i.e., those whose structures match the generally recognized phenylpropane unit of lignin. The low-molecular-weight products fraction is predominantly comprised of methyl esters of aromatic and fatty acids. More than 95% of the identified low-molecular-weight products are veratric-type compounds. New data on the conversion undergone by the wheat straw dioxane lignin in a sub/supercritical dimethyl carbonate medium were obtained. It was shown that high solubility of dioxane lignin under subcritical conditions may be due to cleavage of intermolecular interaction, including hydrogen bonds between the dioxane lignin macromolecules, as a result of thermal exposure and methylation reactions. Fragmentation of the dioxane lignin macromolecules under supercritical conditions involves the processes of radical cleavage of the alkyl aryl ether bonds, as well as the dealkylation, dehydration, methylation, transesterification, and demetoxylation reactions. Conclusions: New data on the conversion of wheat straw dioxane lignin in a sub/supercritical dimethyl carbonate medium were obtained.