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Abstract
This study focused on the structural differences of lignin after pyridine–acetic acid–water (PAW) and dioxane–acidic water (DAW) purification processes. These structural differences included the S/G ratio, condensed structure, weight-average (MW) molecular weights, β-O-4 linkages and sugar content. The chemical structure of the isolated crude lignin (CL), PAW purified lignin (PPL) and DAW purified lignin (DPL) was elucidated using quantitative 13C NMR, 2D-HSQC NMR spectra, thermogravimetric analysis (TGA), gel permeation chromatography (GPC) and Fourier transform infrared spectroscopy (FTIR). The results showed that the PPL fractions contain fewer condensed structures, higher S/G ratios, more β-O-4 linkages, higher average MW and lower thermal degradation properties compared to the CL and DPL fractions. Furthermore, the PAW process was more selective in removing condensed units and enriching S-type lignin from CL compared to the DAW process. These results provide valuable information for understanding which purification process is more suitable to be applied for lignin.
Highlights
Lignin, which is a dominant aromatic polymer found in nature, is found in most terrestrial plants at a concentration of approximately 15%–40% of the dry weight and provides structural integrity [1].Research and development activities that have been directed towards the commercial production of cellulosic ethanol have created an opportunity to dramatically increase the transformation of lignin to value-added products, including low-cost carbon fibers, engineered plastics and thermoplastic elastomers, polymeric foams, fungible fuels and commodity chemicals [1,2,3].It is well-known that lignin has an amorphous and complex structure
It could be concluded that the pyridine–acetic acid–water (PAW) process was more selective for certain structures of lignin compared to the dioxane–acidic water (DAW) process
The content of xylan in DAW purified lignin (DPL) and PAW purified lignin (PPL) was lower than that in crude lignin (CL), which implies that the hemicelluloses attached to lignin were partially removed during the DAW and PAW processes
Summary
Research and development activities that have been directed towards the commercial production of cellulosic ethanol have created an opportunity to dramatically increase the transformation of lignin to value-added products, including low-cost carbon fibers, engineered plastics and thermoplastic elastomers, polymeric foams, fungible fuels and commodity chemicals [1,2,3]. It is well-known that lignin has an amorphous and complex structure. In the auto-catalyzed ethanol organosolv pretreatment process, the produced lignin tends to have high purity, low molecular mass, high phenolic and carboxyl contents and a condensed G-type structure, while it is free of sulfur and ash [6]
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