Characterization of the Arundo Donax L. solid residue from hydrothermal conversion: Comparison with technical lignins and application perspectives

The characterization of the solid residue coming from the acid hydrolysis of giant reed (Arundo Donax L.) herbaceous biomass was performed and compared with the characteristics of two commercial technical lignins, i.e. a soda and a sulphate (kraft) one, in order to ascertain their differences, and thus better address their successive possible applications. The investigated hydrolysis residue derived from an hydrothermal treatment with dilute hydrochloric acid under harsh conditions, addressed to the production of levulinic acid, and contains both lignin and humin units. These last components are not present in the compared lignins that are industrially obtained by different extraction processes. A. Donax L. hydrolysis residue still contains aromatic hydroxyl groups, and carbonyl moieties deriving from lignin and carbohydrates degradation reactions, whilst the aliphatic hydroxyl and methoxyl groups were almost completely degraded. The elemental composition of this residue is similar to that of brown coal, highlighting that biomass carbonization has occurred. The solubilisation of the A. Donax L. hydrolysis residue was investigated in different solvents: only a little amount (∼10 wt%) was dissolved. The condensed nature of the hydrolysis residue was confirmed by thermogravimetric analysis. This approach allows the contemporary production of levulinic acid and the recovery of the A. Donax L. hydrolysis residue, hitherto considered only the waste fraction of the hydrothermal process, thus making this last a promising starting material for future green applications.