Lignins are complex and heterogeneous natural polymers that play essential roles in the development of plant cell walls and affect their use as renewable biomaterials. As highly abundant natural aromatic polymers, isolation and application of lignins from lignocellulosic materials remain most challenging because of their complexity and heterogeneity, as well as their association with cellulose and hemicelluloses. Selective isolation of lignins from biomass therefore still plays an important role both in understanding its biosynthesis and structure and for optimizing the value for potential applications. In this study, we report the structural characterization of alkali-lignins extracted from three kinds of non-woody materials including rice straw, bagasse and bamboo by aqueous alkali pretreatment at various temperatures (20, 80, 120, 140, and 160 °C). For the isolation of alkali-lignins, alkaline extracts were acidified, the precipitates were then extracted with 1,4-dioxane-H2O (96/4, v/v) and purified by cyclohexane-EtOAc (5:1, v/v) precipitation to remove hemicelluloses, p-coumaric acid and other kinds of low molecular weight impurities produced during the treatment. A total of 15 purified alkali-lignin preparations were obtained and analyzed by NMR. Considering the isolated yields, it was found that alkaline extraction was a suitable method for producing lignins from bagasse and rice straw with isolated yields ranged from 28.4% to 55.8% and from 20.71% to 64.97% respectively, but was not particularly efficient for bamboo with yields from 8.86 to 31.08%. From the NMR study, we found that most of the main characteristic linkages in lignins, such as β–O–4-, β–5-, and β–β-linkages, were all well preserved in alkali-lignins. Additionally, β–1 structures (spirodienones) that were originally found in MWLs (milled wood lignins) were observed in alkali-lignins extracted at or below 140 °C by NMR for the first time. The results showed that the temperature for alkaline extraction of bamboo and bagasse should not be higher than 140 °C, whereas the temperature for rice straw should be lower than 120 °C in order to minimize the degradation and/or structural modifications of alkali-lignins, which could be beneficial for their structural characterization. Compared with kraft lignins, non-wood alkali-lignins, structurally more closely resemble their original lignins, and could therefore be more attractive renewable resources for future industrial applications.

Towards a better understanding of alkali-lignins isolated from three kinds of non-woody materials

253rd National Meeting (2017)

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