Chemical depolymerization of cork suberin with hydrothermal processes

Abstract

This study presents for the first time results on hydrothermal treatments (autohydrolysis) of cork to assess their chemical effect on its structural components, namely on suberin. Extractive-free cork was autohydrolysed in isothermal conditions with a broad range of severity factors (SF) of 2.1–4.7 (120–180 °C, 60–240 min) determining mass loss and chemical changes in cork, namely on suberin content and composition. Mass loss was small for milder conditions, and increased steadily with severity e.g. 4.0% for SF 3.0, 9.8% for SF 3.8, 14.1% for SF 4.6. However, mass loss of cork by autohydrolysis is much smaller than that of lignocellulosic biomass under comparable treatment severity. Until SF 3.0, little chemical changes were seen, with SF 3.3–4.1, the autohydrolysed cork showed higher dichloromethane (DCM) solubles and suberin, and less polysaccharides, while for the most severe conditions (SF 4.4–4.7), there was a substantial increase of DCM solubles, a downfall in suberin and polysaccharides, and an unaltered lignin content. Cork polysaccharides decreased steadily with SF increase by hydrolysis of hemicelluloses, e.g. 7.1% of cork for SF 3.5% and 3.9% for SF 4.7. Suberin was maintained in cork until SF 4.4 without changes in composition, but decreased subsequently e.g. at SF 4.7, suberin represented 47% of the initial untreated cork (56% in the initial cork) and showed less ω-hydroxy acids and more α,ω-diacids. The amount and composition of suberin and DCM solubles in the autohydrolysed cork under the various reaction severities allow concluding that suberin is stable until 160 °C but depolymerizes at higher temperatures by ester bond cleaving mostly of mono-functional long-chain monomers. Overall cork showed higher hydrothermal stability than lignocellulosic materials.