Disclosing surface functionality evolution in H2SO4-assisted hydrothermally carbonized lignocellulosic precursors: Deciphering the roles of lignin and cellulose

The development of strategies targeting the deployment of renewable platforms has consistently garnered global attention, including lignocellulosic-derived methods for the production of hydrochars. Such materials have a versatile application repertoire due to their distinctive surface features. Despite numerous attempts to enhance the surface acidity of lignocellulosic-based hydrochars for catalytic purposes by incorporating sulfur-oxygen and carbon–oxygen functional groups, the specific contributions of each lignocellulosic component to the development of these groups remains unclear. This study elucidates the roles of lignin and cellulose in the formation of −SO₃H and carbon–oxygen functional groups on the surface of hydrochars. This was achieved by significantly altering the cellulose-to-lignin molar ratio in the herein selected feedstock before subjecting it to the H₂SO₄-assisted hydrothermal carbonization process. The findings demonstrated that intermediate phenolic compounds – generated through the depolymerization and hydrolysis of lignin – are highly susceptible to sulfonation, leading to increased functionalization yields with −SO₃H groups. Conversely, cellulosic-derived furanic intermediates (mainly furfural and 5-hydroxymethylfurfural) enhance the population of carbon–oxygen functional groups, varying in type, nature, and acid strength. The acidic surface features of the hydrochars were successfully validated in the solvent-free acetalization of glycerol with acetone. The concentration of −SO₃H groups significantly boosted the catalytic performance of the material, achieving nearly full selective conversion of glycerol to solketal within 40 min of reaction, with a turnover frequency as high as 228 h⁻¹ and satisfactory reusability. Therefore, this investigation reveals that lignin primarily facilitates the one-step sulfonation process, while cellulose exclusively contributes to the evolution of carbon–oxygen functionalities on the hydrochars.