木质纤维生物质资源高效利用分子技术的开发

木质纤维生物质是地球上最丰富的可再生生物质资源,可为造纸、化工、纺织和生物能源等工业提供重要的原材料。木质纤维生物质主要包括木质素、纤维素和半纤维素三种生物多聚物成分。如何利用分子手段改造这些生物聚合物,提高它们的工业利用率是目前高度关注的问题。综述了近年来木质纤维多聚物在生物合成与改造方面的研究进展,展望了利用分子技术改造植物木质纤维生物质实现其高效利用的前景。     

木质纤维素资源化主要途径及半纤维素优先资源化利用策略

木质纤维生物质是地球上最丰富的可再生资源,可转化为能源、化学品和材料,开发木质纤维生物质有利于废弃物的高值化利用和缓解目前面临的环境污染等问题。木质纤维素主要包括纤维素、半纤维素和木质素,将其主要组分进行高效分离,是实现多元化、高值化生物精炼的基础。基于此,笔者简要总结了目前主要的木质纤维素资源化途径,如基于纤维素资源化、基于半纤维素资源化、基于木质素资源化、基于碳水化合物资源化以及全组分资源化的研究策略。依据半纤维素在植物细胞壁中承担的角色,结合前期的研究基础,提出半纤维素优先原位催化转化的木质纤维素生物炼制新策略,实现半纤维素的高选择性溶出和高效转化,保留结构完整的纤维素和木质素分级转化为小分子化学品和材料,最终实现资源生物量全利用,多元化产品联产的目的。     

木质纤维素生物炼制的研究进展

木质纤维素是一种广泛存在的可再生生物质资源,主要由纤维素、半纤维素和木质素组成。如何更有效地综合利用木质纤维素是当前面临的世界性难题。本文中,笔者梳理了木质纤维素生物化学法转化生产以燃料乙醇为代表的生物基产品,特别是转化过程中关键技术环节的研究现状及难点,深入探讨了木质素的生物转化利用趋势,并综述了合成生物学在这些领域的研究趋势和最新成果。本文力图描绘出木质纤维素生物炼制研究全景,为后续研究提供潜在思路。     

生物质原料预处理、组分分离到结构选择性拆分——生物质原料工程学的发展

在石化资源日益匮乏的严峻形势下,生物质将成为未来新一代生物及化工产业的最理想的替代原料.因此,如何使生物质资源成为生物基能源、生物基化学品和生物基材料的通用原料,成为目前世界各国共同关注的焦点和热点.从生物炼制及生物质科学与工程的发展可以看出,原料预处理是实现生物质高效转化的必要手段,而组分分离-定向转化是原料预处理的进一步提升,它可以实现纤维素、半纤维素和木质素的分别转化,但是仍然存在着原子利用率不高、能耗高、工艺路线复杂等问题.鉴于生物质是一个功能大分子体,要使其成为通用原料,应该根据原料结构特点和产物要求,发展结构化功能高值拆分的转化过程,即原料的选择性结构拆分思路.从原料预处理到组分分离,再到选择性结构拆分,实现了原料工程学的发展与逐步成熟,其最终目的是要真正建立以生物质为通用原料的新型工业技术体系和研究平台.     

木质纤维素预处理及高值化技术研究进展

木质纤维素生物质是地球上最丰富的可再生生物资源。随着化石能源的消耗及环境的污染,以取代石化燃料为目标的由生物质向生物燃料的转化受到了广泛的关注。木质纤维素有很强的天然抗降解屏障,需先通过物理、化学及微生物等手段进行预处理,进而以更低的成本和更高的效率转化为生物燃料及其他高附加值产品。本文在总结酸碱等传统预处理方法优缺点的基础上,综述了各种组合预处理对这些传统预处理方法的改进,以及γ-戊内酯预处理、低共熔溶剂预处理、微生物联合体生态位预处理这些新型预处理技术的研究进展,总结了木质素高值化过程中木质素的保护、解聚、改性的新方法,指出了预处理方法在工业生产中的应用及不足,以期为木质纤维素生物质转化的研究提供参考。     

木质素降解酶及相关基因研究进展

生物质的高效综合利用已成为全球关注的热点问题。生物质的主要成分是木质素、纤维素和半纤维素,其利用的关键是如何去除木质素,从而提高纤维素和半纤维素的得率。其中利用真菌的生物预处理方法因条件温和、无二次污染等优点符合全球经济可持续发展需要,受到研究者的普遍关注。综述了近年国内外真菌分泌的主要木质素降解酶,包括木质素过氧化物酶(Li P)、锰过氧化物酶(Mn P)、漆酶(laccase)和多功能过氧化物酶(VP)的主要特点,总结了木质素降解相关酶的基因工程、基因组学的研究成果,并对其发展前景进行了展望。     

β-木糖苷酶及其在纤维素乙醇生产中的应用研究进展

将木质纤维素类生物质生物转化生产液体燃料,如纤维素乙醇和大宗化学品,对缓解当前人类社会面临的能源和资源危机以及保护环境具有重要意义。半纤维素是木质纤维素类生物质的主要组成成分之一,它的生物降解转化对实现木质纤维素生物炼制意义重大。由于半纤维素糖种类的多样性和半纤维素结构的复杂性,需要一个复杂的半纤维素酶系才能完成对半纤维素的有效降解。除了木聚糖酶等以外,β-木糖苷酶也是半纤维素酶系的主要组分。在半纤维素降解过程中,β-木糖苷酶将木聚糖酶的水解产物木寡糖和木二糖水解为木糖,不仅在木聚糖的彻底降解过程中起着重要作用,而且可以缓解木寡糖对木聚糖酶和纤维素酶的抑制作用。该文综述了目前在β-木糖苷酶方面的研究进展,包括β-木糖苷酶的分类、酶学性质、酶结构及其催化机制、基因的克隆与表达等,并对β-木糖苷酶在纤维素乙醇生产中的应用情况进行了简述。     

大肠杆菌对木质纤维素水解液抑制物的胁迫耐受性

木质纤维素类生物质是前景广阔的化石原料替代品，其生物炼制可生产生物能源、生物基化学品和生物材料等多种产品，可降低碳排放，有助于实现“双碳”目标，因此受到越来越多的关注。然而，木质纤维素生物炼制需要经过预处理、微生物发酵和产物纯化等多个步骤，其中，预处理过程产生的多种化合物抑制微生物的细胞生长和发酵性能，是制约生物转化效率的瓶颈之一。大肠杆菌是木质纤维素生物炼制常用的宿主，被广泛应用于多种化合物的生产，研究其对木质纤维素水解液中抑制物的耐受性，对于提高木质纤维素生物炼制效率具有重要意义。本文首先介绍了木质纤维素的主要成分和基本结构，对木质纤维素的预处理方法以及预处理后水解液中的主要抑制物种类进行了简单阐述；随后，总结了木质纤维素水解液中几类主要抑制物呋喃类、羧酸类和酚类对大肠杆菌细胞的毒性，以及大肠杆菌对上述抑制物的胁迫响应机制和基于机制的菌株改造靶点；最后，综述了提高大肠杆菌对上述抑制物的胁迫耐受性的菌株改造策略，包括随机突变、实验室适应性进化和组学辅助的理性设计等，为利用代谢工程构建用于木质纤维素生物炼制的高效大肠杆菌菌株提供参考。     

超声波对木质纤维素糖化过程影响的研究

将超声波应用在木质纤维素预处理及其酶解糖化过程中,通过SEM、FTIR研究了处理前后纤维素的形态结构和结晶性能,并考察了不同预处理方式对原料 成分的影响和超声波对酶解糖化率的影响。结果表明,超声波作用能有效的破坏纤维素分子中的氢键,降低其结晶程度,而且能有效地提高木质素的脱除率和酶解糖化率。对超声波作用于酶解过程中的机理进行了初步探讨     

木质纤维素的自然生物转化系统及其过程特征与仿生利用

规模化和产业化开发利用木质纤维素类生物质面临着许多科学和技术上的挑战,这些挑战的核心是如何实现木质纤维素生物质的高效分离与有效转化。然而,在自然界中,不同生物系统分别进化出了其独特的木质纤维素降解与转化的生物过程机制,通过采用不同的策略与途径来克服生物质的抗降解屏障。综述了不同自然生物转化系统在降解生物质过程中的策略与过程特征,并着重分析了食木白蚁肠道消化系统在生物质降解过程中高效转化与利用的独特系统特点。向白蚁生物系统学习,利用自然生物系统的启迪及其相关基因与酶资源,结合生物仿生技术可望建立新型的生物质降解工艺,逐渐实现生物质的低能耗、低污染、高效率、全值化利用。     

Pretreatments to enhance the digestibility of lignocellulosic biomass

Lignocellulosic biomass represents a rather unused source for biogas and ethanol production. Many factors, like lignin content, crystallinity of cellulose, and particle size, limit the digestibility of the hemicellulose and cellulose present in the lignocellulosic biomass. Pretreatments have as a goal to improve the digestibility of the lignocellulosic biomass. Each pretreatment has its own effect(s) on the cellulose, hemicellulose and lignin; the three main components of lignocellulosic biomass. This paper reviews the different effect(s) of several pretreatments on the three main parts of the lignocellulosic biomass to improve its digestibility. Steam pretreatment, lime pretreatment, liquid hot water pretreatments and ammonia based pretreatments are concluded to be pretreatments with high potentials. The main effects are dissolving hemicellulose and alteration of lignin structure, providing an improved accessibility of the cellulose for hydrolytic enzymes.     

Endowing non-cellulolytic microorganisms with cellulolytic activity aiming for consolidated bioprocessing

With the exhaustion of fossil fuels and with the environmental issues they pose, utilization of abundant lignocellulosic biomass as a feedstock for biofuels and bio-based chemicals has recently become an attractive option. Lignocellulosic biomass is primarily composed of cellulose, hemicellulose, and lignin and has a very rigid and complex structure. It is accordingly much more expensive to process than starchy grains because of the need for extensive pretreatment and relatively large amounts of cellulases for efficient hydrolysis. Efficient and cost-effective methods for the production of biofuels and chemicals from lignocellulose are required. A consolidated bioprocess (CBP), which integrates all biological steps consisting of enzyme production, saccharification, and fermentation, is considered a promising strategy for reducing production costs.     

Bioethanol production from steam-exploded rice husk by recombinant <Emphasis Type="Italic">Escherichia coli</Emphasis> KO11

Rice husk is one of the most abundant types of lignocellulosic biomass. Because of its significant amount of sugars, such as cellulose and hemicellulose, it can be used for the production of biofuels such as bioethanol. However, the complex structure of lignocellulosic biomass, consisting of cellulose, hemicellulose and lignin, is resistant to degradation, which limits biomass utilization for ethanol production. The protection of cellulose by lignin contributes to the recalcitrance of lignocelluloses to hydrolysis. Therefore, we conducted steam-explosion treatment as pretreatment of rice husk. However, recombinant Escherichia coli KO11 did not ferment the reducing sugar solution obtained by enzymatic saccharification of steam-exploded rice husk. When the steam-exploded rice husk was washed with hot water to remove inhibitory substances and M9 medium (without glucose) was used as a fermentation medium, E. coli KO11 completely fermented the reducing sugar solution obtained by enzymatic saccharification of hot water washing-treated steam-exploded rice husk to ethanol. We report here the efficient production of bioethanol using steam-exploded rice husk.     

Two-stage pretreatment of rice straw using aqueous ammonia and dilute acid

Liberation of fermentable sugars from recalcitrant lignocellulosic biomass is one of the key challenges in production of cellulosic ethanol. Here we developed a two-stage pretreatment process using aqueous ammonia and dilute sulfuric acid in a percolation mode to improve production of fermentable sugars from rice straw. Aqueous NH? was used in the first stage which removed lignin selectively but left most of cellulose (97%) and hemicellulose (77%). Dilute acid was applied in the second stage which removed most of hemicellulose, partially disrupted the crystalline structure of cellulose, and thus enhanced enzymatic digestibility of cellulose in the solids remaining. Under the optimal pretreatment conditions, the enzymatic hydrolysis yields of the two-stage treated samples were 96.9% and 90.8% with enzyme loadings of 60 and 15FPU/g of glucan, respectively. The overall sugar conversions of cellulose and hemicellulose into glucose and xylose by enzymatic and acid hydrolysis reached 89.0% and 71.7%, respectively.     

Pretreatments for Enhanced Enzymatic Hydrolysis of Pinewood: a Review

Pinewood is an abundant source of lignocellulosic biomass that has potential to be used as renewable feedstock in biorefineries for conversion into advanced biofuels and other value-added chemicals. However, its structural recalcitrance, due to the compact packing of its major components, viz. cellulose, hemicellulose and lignin, high lignin content, and high cellulose crystallinity, is a major bottleneck in its widespread use as a biorefinery feedstock. Typical chemical, thermal, and biological pretreatment technologies are aimed at removing lignin and hemicellulose fractions for improving enzyme accessibility and digestibility of cellulose. This review highlights common pine pretreatment procedures, associated key parameters and resulting enzymatic hydrolysis yields. The challenges and limitations are also discussed as well as potential strategies to overcome them, providing an essential source of information to realize pine as a compelling biorefinery biomass source.     

Selective detection of crystalline cellulose in plant cell walls with sum-frequency-generation (SFG) vibration spectroscopy

The selective detection of crystalline cellulose in biomass was demonstrated with sum-frequency-generation (SFG) vibration spectroscopy. SFG is a second-order nonlinear optical response from a system where the optical centrosymmetry is broken. In secondary plant cell walls that contain mostly cellulose, hemicellulose, and lignin with varying concentrations, only certain vibration modes in the crystalline cellulose structure can meet the noninversion symmetry requirements. Thus, SFG can be used to detect and analyze crystalline cellulose selectively in lignocellulosic biomass without extraction of noncellulosic species from biomass or deconvolution of amorphous spectra. The selective detection of crystalline cellulose in lignocellulosic biomass is not readily achievable with other techniques such as XRD, solid-state NMR, IR, and Raman analyses. Therefore, the SFG analysis presents a unique opportunity to reveal the cellulose crystalline structure in lignocellulosic biomass.     

Lignin Structural Alterations in Thermochemical Pretreatments with Limited Delignification

Lignocellulosic biomass has a complex and rigid cell wall structure that makes biomass recalcitrant to biological and chemical degradation. Among the three major structural biopolymers (i.e., cellulose, hemicellulose, and lignin) in plant cell walls, lignin is considered the most recalcitrant component and generally plays a negative role in the biochemical conversion of biomass to biofuels. The conversion of biomass to biofuels through a biochemical platform usually requires a pretreatment stage to reduce the recalcitrance. Pretreatment renders compositional and structural changes of biomass with these changes ultimately governing the efficiency of the subsequent enzymatic hydrolysis. Dilute acid, hot water, steam explosion, and ammonia fiber expansion pretreatments are among the leading thermochemical pretreatments with a limited delignification that can reduce biomass recalcitrance. Practical applications of these pretreatment are rapidly developing as illustrated by recent commercial scale cellulosic ethanol plants. While these thermochemical pretreatments generally lead to only a limited delignification and no significant change of lignin content in the pretreated biomass, the lignin transformations that occur during these pretreatments and the roles they play in recalcitrance reduction are important research aspects. This review highlights recent advances in our understanding of lignin alterations during these limited delignification thermochemical pretreatments, with emphasis on lignin chemical structures, molecular weights, and redistributions in the pretreated biomass.     

评价木质纤维素预处理效果的新方法

以玉米秸秆为代表性纤维质原料,尝试建立一种评估预处理效果的新方法——持水率测定法,即:将试样在室温下浸泡1 h,在分离因数1 000下离心5 min后测定持水率。结果表明:在机理上木质纤维的持水率与可消化性具有一致性,在某种程度上具有正相关性;持水率作为一种简单快捷的新型测定方法,能够用来评估木质纤维素类生物质的预处理效果,不同预处理方法通过打破木质纤维的复杂致密结构,破坏氢键和酯键作用增加其孔径和孔穴,同时使其暴露出更多的游离羟基等亲水性基团,最终增加了木质纤维的持水率。     

Consolidated bioprocessing performance of <Emphasis Type="Italic">Thermoanaerobacterium thermosaccharolyticum</Emphasis> M18 on fungal pretreated cornstalk for enhanced hydrogen production

Background

Biological hydrogen production from lignocellulosic biomass shows great potential as a promising alternative to conventional hydrogen production methods, such as electrolysis of water and coal gasification. Currently, most researches on biohydrogen production from lignocellulose concentrate on consolidated bioprocessing, which has the advantages of simpler operation and lower cost over processes featuring dedicated cellulase production. However, the recalcitrance of the lignin structure induces a low cellulase activity, making the carbohydrates in the hetero-matrix more unapproachable. Pretreatment of lignocellulosic biomass is consequently an extremely important step in the commercialization of biohydrogen, and for massive realization of lignocellulosic biomass as alternative fuel feedstock. Thus, development of a pretreatment method which is cost efficient, environmentally benign, and highly efficient for enhanced consolidated bioprocessing of lignocellulosic biomass to hydrogen is essential.

Results

In this research, fungal pretreatment was adopted for enhanced hydrogen production by consolidated bioprocessing performance. To confirm the fungal pretreatment efficiency, two typical thermochemical pretreatments were also compared side by side. Results showed that the fungal pretreatment was superior to the other pretreatments in terms of high lignin reduction of up to 35.3% with least holocellulose loss (the value was only 9.5%). Microscopic structure observation combined with Fourier transform infrared spectroscopy (FTIR) analysis further demonstrated that the lignin and crystallinity of lignocellulose were decreased with better holocellulose reservation. Upon fungal pretreatment, the hydrogen yield and hydrogen production rate were 6.8 mmol H₂ g^-1 pretreated substrate and 0.89 mmol L^-1 h^-1, respectively, which were 2.9 and 4 times higher than the values obtained for the untreated sample.

Conclusions

Results revealed that although all pretreatments could contribute to the enhancement of hydrogen production from cornstalk, fungal pretreatment proved to be the optimal method. It is apparent that besides high hydrogen production efficiency, fungal pretreatment also offered several advantages over other pretreatments such as being environmentally benign and energy efficient. This pretreatment method thus has great potential for application in consolidated bioprocessing performance of hydrogen production.

     

Room temperature ionic liquids as emerging solvents for the pretreatment of lignocellulosic biomass

Room temperature ionic liquids (RTILs) are emerging as attractive and green solvents for lignocellulosic biomass pretreatment. The unique solvating properties of RTILs foster the disruption of the 3D network structure of lignin, cellulose, and hemicellulose, which allows high yields of fermentable sugars to be produced in subsequent enzymatic hydrolysis. In the current review, we summarize the physicochemical properties of RTILs that make them effective solvents for lignocellulose pretreatment including mechanisms of interaction between lignocellulosic biomass subcomponents and RTILs. We also highlight several recent strategies that exploit RTILs and generate high yields of fermentable sugars suitable for downstream biofuel production, and address new opportunities for use of lignocellulosic components, including lignin. Finally, we address some of the challenges that remain before large-scale use of RTILs may be achieved.