Structural characterizations of lignins extracted under same severity using different acids

Abstract

As the vital renewable feedstock originated from carbon-neutral resources, due to prominent aromaticity lignin owns the potential to produce high value-added products. Multi-functional lignin valorization demands efficient lignin extraction at milder conditions to keep its structure intact to substitute petroleum-based reactants. Lignin extraction severity (LES) is considered as the primary factor affecting the structure of extracted lignin and ultimately determines its applications. Except for the LES, the selection of suitable reagents for lignin extraction concerned with specific applications is crucially important. To explore the influence of different reagents, this study focused on lignin extraction employing the commonly used strong acids at the same LES. Four lignin preparations were extracted using 80% aqueous dioxane with the addition of H₂O (L1), HCl, H₂SO₄ and HNO₃ (pH = 1.30 ± 0.01 L2, L3 and L4, respectively). Analytical high-sensitive NMR (³¹P and 2D-HSQC) together with other characterizations (FTIR and GPC) were successfully employed and quantified while unveiling the structural heterogeneity among extracted lignin preparations. At the same LES, different reagents yielded lignin with varying structural characteristics and were potentially suitable for different applications.

Keywords: Chemical structural features. 2D-NMR (HSQC); Lignin applications; Lignin valorization; Mild acidolysis lignin (MAL); Tailor-made lignin.