Topology of Pretreated Wood Fibers Using Dynamic Nuclear Polarization

In the continuously developing field of lignocellulosic biomass, high-yield lignin depolymerization processes are sought to optimize its productivity and profitability. Recently, formaldehyde stabilization during lignin extraction and biomass pretreatment has been found to drastically enhance subsequent lignin upgradeability but can affect cellulose digestibility. The exact role and/or form of formaldehyde on the residual biomass surface is still not fully understood. Here, we use magic angle spinning (MAS) dynamic nuclear polarization (DNP) methods to characterize the components that remain inside the residual cell wall after the lignin extraction process and reveal the topochemistry of the solid residue. The regioselectivity of relayed DNP allows the observation of hyperpolarization in a range of 40–200 nm from the surface of the cell wall for poplar wood materials. That regioselectivity allows us to distinguish between the external secondary cell wall and the inner middle lamellae. In that respect, for the untreated wood, we confirm that there is less lignin in the outer part of the cell wall than deeper inside. In treated wood, we determine that the role of dioxane during the process is to enable the extraction of the modified products from the cell wall. We show that the modified lignins which were not extracted in the absence of dioxane accumulate in a 40 nm region at the surface of the cell wall. Also, using carbon-13 enriched formaldehyde during the process, we show that 1% of the total amount of carbon in the material is assigned to self-polymerization and that no covalent bonds to cellulose are observed.