预处理纤维素超分子结构变化机制研究进展

Research progress in variations of cellulose supramolecular structures via biomass pretreatment

木质纤维生物质高值转化生产清洁能源、生物基化学品和功能材料是可再生能源领域的研究热点。纤维素是木质纤维生物质的主要成分,其高效利用是生物炼制的重点。然而,纤维素的生物转化面临分子链有序组装与结晶而成的超分子结构、微纤丝聚集形成的多尺度网络结构及其与木质素、半纤维素的多种化学交联共同形成的木质纤维素抗降解屏障,阻碍了其产业化发展。生物质预处理是破解植物细胞壁三维网络结构,从而打破木质纤维素抗降解屏障的必要途径。综述了木质纤维生物质预处理对纤维素超分子结构的影响,主要包括晶型转变、结晶度及晶体结构变化等,并阐释了其对后续酶解糖化的增效机理。同时,系统总结了纤维素大分子链构成的纤丝聚集体结构与细胞壁中木质素、半纤维素之间的相互作用,及其在预处理过程中的降解机制;并对木质纤维素高值化利用研究中亟须关注的纤维素超分子结构变化问题进行了总结和展望,以期对植物细胞壁多尺度网络的破解机制进行充分阐释,对其宏观应用提供理论指导。

Lignocellulosic biomass is one of the most widespread renewable materials in nature. To produce clean and sustainable bioenergy, functional materials and bio-chemicals from lignocellulose, the development of biorefinery industry has become the hot topic in recent years. The effective utilization of lignocellulosic biomass can help to relieve the shortage of earth resources, environmental pollution and to achieve the long-term development. Cellulose is the main component of lignocellulosic biomass and the main resource for biorefinery. As a long-chain polysaccharide, cellulose is self-assembled in plant cell walls as crystalline nanofibers from the linear β-(1,4) linked D-glucose polymers, which can be hydrolyzed to monomeric glucose and eventually be converted to biofuels via microbial fermentation or chemical catalysis. However, the complex supramolecular structures of cellulose including allomorphs, crystallinity and crystal structure, the aggregation of cellulose micro-and macro-fibrils, as well as their multi-interactions with other components(lignin and hemicelluloses) in plant cell walls have formed stable lignocellulosic biomass recalcitrance, inhibiting the bioconversion of cellulose. As the result, biomass pretreatment is necessary for disrupting the complex architecture, swelling the micro-and macro-fibrils and exposing the cellulosic components to make it more accessible to water and cellulase enzymes. The physical, chemical or biological pretreatment firstly facilitates the removal of biomass matrix surrounding the cellulose chains, followed by the disruption of crystalline cellulose fibrils(including the decrystallization, allomorphic transformation and fibrillary separation), thus contributing high conversion performance to glucose. This paper reviewed the supramolecular characterizations of cellulose during the pretreatments and the mechanisms for enhancement on the subsequent enzymatic hydrolysis. Different changes of cellulose crystal structure during various kinds of pretreatment methods were summarized, which provide the overall understanding of the relationships of biomass pretreatments and cellulose supramolecular structures. Furthermore, the research progress concerning cellulose aggregations and their chemical linkages with lignin and hemicellulose carbohydrates matrix were reviewed. The current challenges and major problems of cellulose structure research during the lignocellulosic biomass utilization were summarized in the last part, including the analytical methods, efficiency improvements of pretreatments as well as the fundamental linkages of carbohydrate polymers in cell walls, which may help for deeper understanding of cell wall components network and full utilization of lignocellulose.