竹材木质素特性及表征方法研究进展

The Characteristics and Representation Methods of Lignin for Bamboo

竹材作为一种天然纤维增强复合材料,性能优良,可在一定程度上缓解木材资源短缺矛盾。木质素的异质性分布和分子结构是决定竹材性能的因素之一,是竹材解剖和化学基础研究中的热点和难点。竹纤维和基本薄壁组织的木质化过程在竹材的第一个生长季节内逐渐加强。纤维细胞壁呈现宽窄交替的多层结构,木质化过程是从细胞腔向复合胞间层推进。导管和纤维细胞在茎伸长生长完成之前开始木质化,而薄壁组织的木质化过程完成在茎伸长生长之后。竹木质素属于G-S-H型,除有愈创木基(G)和紫丁香基(S)两类单体外,还含有相当数量的对羟苯基型(H)结构,同时其结构中还存在对香豆酸酯键联接。本文对竹材细胞壁结构以及木质素微区分布、分离方法和分子结构的研究进展进行了综述,期望可以为竹材力学性能适应性、加工工艺和高附加值利用提供参考数据。研究竹材木质素在细胞壁的微区分布的方法有荧光显微法、紫外显微分光光度计法、扫描电镜技术、X射线能谱分析法、透射显微镜技术和共聚焦激光扫描显微镜技术等,其中还结合了组织或细胞化学技术和免疫细胞化学技术。共聚焦拉曼显微镜结合光谱成像技术和纳米红外技术为植物细胞壁化学成分微区分布研究提供了强有力的手段。本文概述了木质素的分离方法,主要可以分为:磨木木质素、酶解木质素以及多种化学和物理手段结合的酶解-酸解木质素、两次球磨和两次酶解的残渣木质素制备方法,并评价了不同方法所得木质素的结构和纯度等优缺点。此外,化学预处理因试剂不同,分离出来的木质素有酸木质素、碱木质素、溶剂型木质素等,特别是对甲苯磺酸、离子液体、低共溶剂等绿色溶剂的使用实现了木质素的高效分离。木质素分子结构的研究方法有湿部化学法,包括硝基苯氧化法、高锰酸钾氧化法、衍生后还原降解法、硫代酸解法、裂解气质联用法等。湿部化学法可以获得木质素的结构信息,但是对官能团表征还不能获得完整的信息。光谱法可以对木质素及官能团进行定性和定量分析,可以获得木质素完整的结构信息。光谱法主要包括紫外光谱法、傅里叶红外光谱法和核磁共振光谱法,本文重点综述了碳谱和二维异核单量子相干谱等先进液态核磁共振技术表征木质素的分子结构的最新进展,并提出了该领域今后的发展趋势。

Bamboo is a natural fibre reinforced composites,with excellent performance,which is an alternative to shortage of wood resource to a certain extent.Heterogeneous distribution and molecular structure of lignin is one of the factors that determine the performance,and it is the key and difficult basic research in anatomy and chemistry of bamboo.The lignification of bamboo fiber and parenchyma cell incrementally strengthens du-ring the first growing season.The bamboo fibres exhibit a polylamellate structure,and the lignification advances from the cell wall near lumen to compound middle lamellae.Vessels and fibre cells begin to lignify before stem elongation growth is complete,while lignification of parenchyma is completed after stem elongation.Bamboo lignin are composed of guaiacyl(G)units and syringyl(S)units,and it also contains a considerable amount of p-hydroxyphenyl(H)units having ester bonds between p-coumaric acid.This paper reviewed the current advances of cell wall structure,lignin distribution,separation method and molecular structure of bamboo,then the results will be expected to provide reference data for bamboo’s mechanical property adaptability,processing technology and high value-added utilization.The research on the lignin distribution of bamboo cell wall by means of various fluorescence microscopy,ultraviolet micro-spectrophotometry,scanning electron microscopy and energy dispersive X-ray spectroscopy,transmission microscopy and confocal laser scanning microscopy,coupled with the Wiesner and Maule reaction and immunocytochemistry technology.Confocal raman microscopy combined with spectral imaging technology,and nano-infrared technology provides a powerful means for the study of the lignin topochemistry in bamboo cell wall.It also outlines the separation method of the original lignin,which can be mainly divided into milled wood lignin,cellulolytic enzyme lignin,and a combination of various chemical and physical means of enzymatic acidolysis lignin,residual lignin through two ball milling and two enzymatic hydrolysis,and the advantages of the structure and purity of lignin obtained by different methods were evaluated.In addition,the lignin separated by chemical pretreatment due to different reagents includes acid lignin,alkali lignin,solvent lignin,etc.,especially the use of green solvents such as toluenesulfonic acid,ionic liquid,low cosolvent,etc.,enables efficient separation of lignin.The analytical tools to the elucidation of lignin molecular structure such as wet chemical method,including nitrobenzene oxidation,permanganate oxidation,derivatization followed by reductive cleavage,thioa-cidolysis,and pyrolysis gas chromatography-mass spectrometer(Pyr-GC-MS).Wet chemical method can obtain the lignin structure,but the complete information on the functional group characterization is not available.Spectroscopic analysis can qualitatively and quantitatively analyze the functional groups of lignin,and the complete structural information of lignin can be obtained.The spectroscopy mainly includes ultraviolet spectroscopy,Fourier transform infrared spectroscopy(FTIR)and nuclear magnetic resonance(NMR)techniques.The latest progress in cha-racterizing the molecular structure of lignin by advanced liquid-state nuclear magnetic resonance techniques such as 13C and 2D HSQC NMR method is reviewed.In addition,prospects and future research in its field were proposed.