Lignin contributions to the nanoscale porosity of raw and treated lignocelluloses as observed by calorimetric thermoporometry

•Calorimetric thermoporometry probes nanoscale pores in lignocellulosic biomass.•Delignification increases porosity mainly in the 10–200 nm range.•Pores <4 nm attributed to lignin aggregates left by acidic treatments.

Thermoporometry probes water in nanoscale confinement, the so-called freezing bound water (FBW). Although well established for characterization of delignified cellulosics, application and interpretation of thermoporometry has been elusive for lignin-containing biomass. Here we show that calorimetric thermoporometry discriminates two types of lignin contributions to the nanoscale porosity of a wide set of raw and treated lignocelluloses. First, the well-known role of lignin as pore filler is observed as delignification-promoted gains in FBW, mainly in pores of 10–200 nm. Second, lignocelluloses submitted to acidic aqueous treatments present an additional FBW contribution, mainly in pores <4 nm. This FBW at smaller dimensions is attributed to surface irregularities of the lignin aggregates left by the acidic treatments. Hence, our findings demonstrate that thermoporometry detects a confined water state associated with restructured lignin, thus contributing to the understanding of processes for biomass valorization.