工业木质素分级分离研究进展

Fractionation of industrial lignin:a review

化学制浆及生物炼制工业产生了大量工业木质素,其结构不均一,影响了进一步高值转化利用。采用分级分离的方法处理工业木质素可获得分子质量分布窄、性能良好的组分,进而实现木质素的高值化。针对工业木质素的不均一性,简要阐述了多种分级分离的方法及其研究进展。分级分离的主要方法包括有机溶剂溶解/沉淀法、低共熔溶剂分离法、改变pH的梯度沉淀法、膜分离法。有机溶剂溶解/沉淀法依据木质素在系列溶剂中溶解度的差异,采用多级溶解法或多级沉淀法得到木质素组分,该法具有操作简单,处理时间短,对设备要求低的特点。低共熔溶剂法可通过设计含不同种类和数量的氢供体和氢受体的低共熔溶剂用于分离,从而得到不同分子质量的高反应活性木质素。改变pH的梯度沉淀法添加酸溶液不断降低pH,沉淀得到不同分子质量的木质素组分。膜分离采用不同截留分子质量的分离膜将木质素溶液分离,可以去除碳水化合物、灰分,得到不同分子质量的木质素。针对上述分级分离方法的研究现状,对今后研究方向进行了展望,以期为木质素的高值化利用提供理论依据。

The chemical pulping and biorefinery industries produce a large amount of lignin, in which the heterogeneous structure of lignin affects its further high-value conversion and utilization. Currently, industrial lignin is mainly used as fuels, thus its value-added utilization is of great significance to the integrated biorefinery of lignocellulose resources and the reduction of waste discharge. In order to reduce the heterogeneity of lignin structure, fractionation is considered as a promising method to obtain lignin fractions with narrow molecular weight distribution and good properties, achieving high value of lignin. In this article, various methods of fractionation and the main research progresses of the lignin fractionation are introduced. Currently, the main fractionation methods are organic solvent dissolution/precipitation, deep eutectic solvent separation, gradient precipitation by changing pH, and membrane separation. The organic solvent dissolution/precipitation method is based on the differences in the solubility of organic solvents for lignin, and multi-stage dissolution/multi-stage precipitation methods are used to obtain lignin fractions. This method has the advantages of simple operation, short processing time, and low equipment requirements. For this process, if the dissolving capacities of solvents vary narrowly, or the adjustment of solvent dissolving capacity is not fine enough, only a few fractions can be obtained and their molecular weight distributions are still wide. Deep eutectic solvents are used for separation by designing different types and amounts of hydrogen donors and hydrogen acceptors to obtain highly reactive lignin with different molecular weights. Gradient precipitation by changing pH is operated by continuously lowering pH with acid to obtain lignin fractions of different molecular weights by precipitation, in which the lignin colloid was destroyed by acid and the resulting lignin flocculated particles are precipitated from the solution. Membrane separation is conducted with different molecular weight cut-offs to separate lignin solution, which can remove carbohydrates and ash to obtain lignin with different molecular weights. The process can be operated continuously without additional reagents, chemicals, and energy consumption. Compared with the acid precipitation, the membrane separation can remove impurities(such as carbohydrates, ash, etc.) from the lignin, and the obtained lignin has a high purity and good thermal performance. Based on the research status of the above fractionation methods, the future research aspects are prospected, in order to provide a theoretical basis for the high-value utilization of lignin.