Structural characterization of soluble lignin in the pre-hydrolysis liquor of bamboo-willow dissolving pulp

Abstract

The soluble lignin present in the pre-hydrolysis liquor (PHL) is detrimental for value-added utilization of the PHL from production of kraft-based dissolving pulp. In this paper, the soluble lignin was separated from PHL by activated carbon adsorption and subsequent desorption with methanol. The structural characteristics of the soluble and dioxane lignins (from bamboo-willow material) were analyzed by Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR), and thermogravimetric analysis (TGA). The FTIR and proton-NMR results showed that cleavage of β-O-4 aryl-ether linkages and demethylation occurred in the lignin structure during the pre-hydrolysis. The main linkages between structural monomeric units of the soluble lignin were β-β, β-5, β-1, and β-O-4. Additionally, the TGA results showed that the residual char yield at 800 °C of the soluble and dioxane lignins were 33.4% and 35.3%, respectively. Therefore, the soluble lignin obtained from PHL possessed thermal stability comparable to that of dioxane lignin, and can be used as a renewable source for carbon fiber.