Depolymerization behaviors of naked oat stem cell wall during autohydrolysis in subcritical water.

• In subcritical water, naked oat stem cell wall can be depolymerized orderly. • The order of lignin removal in tissues and cell wall was proposed. • At 190 °C, middle lamella was completely separated, producing a pulp-like residue. • At 210 °C, hemicellulose was almost completely degraded. In this study, the depolymerization of naked oat stem cell wall and the dissolution of its chemical compositions during autohydrolysis in subcritical water (150−210 °C, 0.38–1.85 MPa) were investigated, to explore the potential of naked oat stems as raw materials for green biorefinery. Our results showed that the sequence of lignin removal was from parenchyma cells to sclerenchyma fibers in the tissues, and from the middle lamella to the inner side of the cell wall, and finally to the middle layer of the cell wall. The total lignin removal rate reached the highest value (47.7 %) at 190 °C, and by this time, the cell wall was depolymerized and the autohydrolysis residue was in the form of pulp. At 190 °C, the hemicellulose side chain was degraded by 100 %, and the ash was almost completely removed. When the reaction temperature was further increased to 210 °C, the removal rate of xylose was close to 100 %, and degradation of the cellulose crystal zone began. We found that in moderately subcritical water, all the chemical compositions of naked oat stem were expected to be dissolved in a controllable and orderly manner. Our findings will play a substantial role in solving the problem of lignocellulosic biomass composite cell wall resistance to depolymerization in a single green solvent. [ABSTRACT FROM AUTHOR]