木质纤维素生物质预处理研究现状

预处理是木质纤维素生物质转化为燃料乙醇的关键步骤,综述了现有常见预处理技术的国内外研究现状,同时分析比较了各处理技术的优缺点,并对今后木质纤维素生物质预处理的主要研究方向进行了展望,以期为木质纤维素生物质转化条件的优化提供参考。     

利用木质纤维素生产丁醇的研究进展

随着化石燃料的逐年减少,以生物质为原料的生物能源研究近年来成为能源领域的研究热点,充分利用可再生生物质为发展经济的生物燃料生产工艺提供了一个极好的机会。与燃料乙醇和生物柴油相比,生物丁醇更具有优越性,以可再生木质纤维素生物质为原料进行发酵生产丁醇在近年来被广泛的研究。对于利用可再生生物质为原料生产丁醇,需要解决原料的选择、产品收率低、抑制物对生产菌株毒性等问题。本文对以木质纤维素生物质为原料进行生物丁醇发酵过程中的原料预处理、抑制物对丁醇生产菌的影响,以及水解液的脱毒和耐抑制物菌株的选育等方面进行综述,并对以木质纤维素生产燃料丁醇所面临的机遇与问题进行了简要评述。     

木质纤维素预处理抑制物产生及脱除方法的研究进展

利用纤维素酶将木质纤维素降解成可发酵性糖,然后发酵生产氢气、乙醇、丁醇等生物燃料及高附加值产品,是当今全球研究的热点。预处理是生物质转化过程中至关重要的步骤,而预处理过程中产生的抑制物对木质纤维素后续的酶解和发酵微生物有负面影响。因此了解预处理方法及其过程中产生的抑制物及脱除方法是能否高效转化生物质的基础。文中首先介绍了木质纤维素常用的两类预处理方法即化学法和物理化学法。随后阐述了不同抑制物的产生及其抑制机制,并重点介绍了多种脱毒方法。最后展望了脱除木质纤维素预处理抑制物的研究趋势:应用交联聚乙烯亚胺和金属有机骨架化合物等新型材料脱除抑制物或通过基因工程、代谢工程技术等构建抑制物耐受性菌株等。     

预处理对木质纤维素生物质细胞壁超微结构的影响

预处理是提高木质纤维素生物质向生物燃料转化的有效途径,但生物质的天然抗降解屏障严重阻碍了这一转化的进行,因此全面了解预处理过程中植物细胞壁的微观结构及区域化学变化是实现农林生物质高效转化的基础。本文总结了多种预处理方法对植物细胞壁超微结构影响的研究进展,对生物质科学研究可能有一定的促进和指导作用。     

生物质生物预处理研究进展与展望

木质纤维素生物质分布广、产量大、可再生,用于制备生物基能源、生物基材料和生物基化学品。木质纤维素生物质组成复杂,包含纤维素、半纤维素和木质素等,木质素与半纤维素通过共价键、氢键交联形成独特的“包裹结构”,纤维素含有复杂的分子内与分子间氢键,上述因素制约着其资源化利用。生物预处理以其独特优越性成为生物质研究的重要方面。系统阐述了生物预处理过程中木质素降解和基团修饰对纤维素酶解的影响,纤维素含量及结晶区变化,半纤维素五碳糖利用,微观物理结构的改变。进一步提出了以生物预处理为核心的组合预处理、基于不同功能的多酶协同催化体系、木质纤维素组分分级利用和新型高效细菌预处理工艺是生物预处理未来发展的重要趋势。     

关于微生物燃料电池底物的研究进展

近年来,微生物燃料电池已引起了广泛关注,它将低能量废水和木质纤维素生物质等有机废物转化为电能。在将来,微生物电能将成为一种重要的生物能源,因为微生物燃料电池提供了一种复合有机物和可再生生物能源中提取电能的可行性。人们研究了许多物质,以考察其是否能作为微生物电能转化的底物。这些物质包括人工的和天然废物,以及木质纤维素生物质。尽管现在微生物燃料电池提供的电流和功率较低,但是随着技术的发展和对微生物燃料电池系统的深入了解,微生物燃料电池转化的电流和电力将极大增加,从而向世人提供了一种可以将纤维素生物质和废水直接转化为有用能源的有效方法。本文介绍了迄今为止在微生物燃料电池中用到的各种反应底物,并对它们的应用效率和存在的不足进行了分析。     

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。     

生物转化能源草制取纤维素乙醇的研究进展

随着化石燃料的日益枯竭和环境污染的日益加剧,寻找一种绿色能源以代替化石能源成为当今世界迫在眉睫的任务。清洁燃料当中的生物乙醇具有车用价值,可作为化石能源的替代品而受到研究学者的广泛关注。而草本能源植物的生物转化被认为是生物质能源产业化发展的最有效途径之一。能源草作为木质纤维素原料之一,由于其具有生长快,产量高,抗性强等优势而备受瞩目。详细论述了近期国内外以能源草为底物进行纤维素乙醇的生物转化研究进展,从纤维素原料预处理到乙醇发酵工艺等各方面的进展及存在的问题,并对木质纤维素制取生物质能源的生物转化效率,以及全纤维素组分的多级利用进行了简单阐述,以期找出一条产业化生产纤维素乙醇的最优生产模式。     

运动发酵单胞菌在生物炼制中的研究进展

以木质纤维素生物质为原料的生物炼制技术已成为全球研发的热点和难点。欧盟国家和美国的中长期生物质能源发展路线图中均将木质纤维素生物炼制技术作为重要目标,但是目前整体水平尚处于中试阶段。我国的纤维素类生物质原料非常丰富,将其转化成燃料乙醇及生物基础化学品等具有较大的潜力,但当前要想实现商业化生产,还面临着很多瓶颈问题亟待解决。缺乏能够同时高效利用纤维素类水解物的发酵菌株,已成为纤维素生物质高效与高值转化的关键制约因素。运动发酵单胞菌是目前唯一一种通过ED途径兼性厌氧发酵葡萄糖的微生物,其独特的代谢途径使其成为构建产乙醇工程菌的优选宿主之一;同时由于该菌具有较高的糖利用效率等优点,也是其他生物基化学品生产的重要候选平台微生物,如山梨醇、葡萄糖酸、丁二酸和异丁醇等。本文从该菌的研究历程、分子生物学基础、菌种改良及该菌在生物能源及生物基化学品等生物炼制体系中的应用研究角度进行了综述,并提出该菌可作为纤维素生物质生物炼制系统的新的重要平台微生物。     

基于杨木（Populus sp.）的二代燃料乙醇技术研究进展

木质纤维素类生物质作为一种廉价且储量丰富的可再生原料，可通过预处理、酶解和微生物发酵等过程转化为纤维素燃料乙醇，近几十年来受到世界各国的广泛关注。杨木是一种人工广泛种植的速生硬木，主要用于造纸工业，而伴随产生大量枝桠等废弃物。因其富含纤维素和半纤维素组分，被认为是纤维素乙醇生产的优良木质纤维素原料。聚焦于杨木在纤维素乙醇生产中的应用，介绍了杨木的组成及结构特点，重点综述了杨木在预处理技术、预处理原料的酶解、微生物发酵等方面的研究进展。最后，归纳总结了限制杨木在纤维素乙醇应用中的技术障碍及困难，进而分析提出了相应解决对策并展望了其应用前景。     

Do furanic and phenolic compounds of lignocellulosic and algae biomass hydrolyzate inhibit anaerobic mixed cultures? A comprehensive review

Nowadays there is a growing interest on the use of both lignocellulosic and algae biomass to produce biofuels (i.e. biohydrogen, ethanol and methane), as future alternatives to fossil fuels. In this purpose, thermal and thermo-chemical pretreatments have been widely investigated to overcome the natural physico-chemical barriers of such biomass and to enhance biofuel production from lignocellulosic residues and, more recently, marine biomass (i.e. macro and microalgae). However, the pretreatment technologies lead not only to the conversion of carbohydrate polymers (ie cellulose, hemicelluloses, starch, agar) to soluble monomeric sugar (ie glucose, xylose, arabinose, galactose), but also the generation of various by-products (i.e. furfural and 5-HMF). In the case of lignocellulosic residues, part of the lignin can also be degraded in lignin derived by-products, mainly composed of phenolic compounds. Although the negative impact of such by-products on ethanol production has been widely described in literature, studies on their impact on biohydrogen and methane production operated with mixed cultures are still very limited.     

工业木质素预处理及催化转化苯酚

传统化石能源日益枯竭以及环境污染压力日益加大使得开发以生物质为代表的可替代能源迫在眉睫,木质素作为仅次于纤维素的生物质中第二大主要成分用来制备高附加值化学品是提高生物质资源利用效率的关键。本文将工业木质素进行预处理后采用双液相反应体系将工业木质素转化为苯酚等化合物,使用微波辐照代替传统的加热,考察了预处理方式、温度、时间等条件对产物收率的影响。结果表明,以1-甲基-3-乙基咪唑醋酸盐([EMIM]OAc)处理的工业木质素在微波反应器输出功率为400 W、反应温度为90℃、催化剂为1-甲基-3-胺乙基咪唑四氟硼酸盐([AEMIM]BF4)双液相反应体系中反应60 min苯酚的收率最高可达8.14%。     

Biodelignification of lignocellulose substrates: An intrinsic and sustainable pretreatment strategy for clean energy production

Lignocellulosic biomass (LB) is a promising sugar feedstock for biofuels and other high-value chemical commodities. The recalcitrance of LB, however, impedes carbohydrate accessibility and its conversion into commercially significant products. Two important factors for the overall economization of biofuel production is LB pretreatment to liberate fermentable sugars followed by conversion into ethanol. Sustainable biofuel production must overcome issues such as minimizing water and energy usage, reducing chemical usage and process intensification. Amongst available pretreatment methods, microorganism-mediated pretreatments are the safest, green, and sustainable. Native biodelignifying agents such as Phanerochaete chrysosporium, Pycnoporous cinnabarinus, Ceriporiopsis subvermispora and Cyathus stercoreus can remove lignin, making the remaining substrates amenable for saccharification. The development of a robust, integrated bioprocessing (IBP) approach for economic ethanol production would incorporate all essential steps including pretreatment, cellulase production, enzyme hydrolysis and fermentation of the released sugars into ethanol. IBP represents an inexpensive, environmentally friendly, low energy and low capital approach for second-generation ethanol production. This paper reviews the advancements in microbial-assisted pretreatment for the delignification of lignocellulosic substrates, system metabolic engineering for biorefineries and highlights the possibilities of process integration for sustainable and economic ethanol production.     

Modifying plants for biofuel and biomaterial production

The productivity of plants as biofuel or biomaterial crops is established by both the yield of plant biomass per unit area of land and the efficiency of conversion of the biomass to biofuel. Higher yielding biofuel crops with increased conversion efficiencies allow production on a smaller land footprint minimizing competition with agriculture for food production and biodiversity conservation. Plants have traditionally been domesticated for food, fibre and feed applications. However, utilization for biofuels may require the breeding of novel phenotypes, or new species entirely. Genomics approaches support genetic selection strategies to deliver significant genetic improvement of plants as sources of biomass for biofuel manufacture. Genetic modification of plants provides a further range of options for improving the composition of biomass and for plant modifications to assist the fabrication of biofuels. The relative carbohydrate and lignin content influences the deconstruction of plant cell walls to biofuels. Key options for facilitating the deconstruction leading to higher monomeric sugar release from plants include increasing cellulose content, reducing cellulose crystallinity, and/or altering the amount or composition of noncellulosic polysaccharides or lignin. Modification of chemical linkages within and between these biomass components may improve the ease of deconstruction. Expression of enzymes in the plant may provide a cost‐effective option for biochemical conversion to biofuel.     

A review of biological delignification and detoxification methods for lignocellulosic bioethanol production

Future biorefineries will integrate biomass conversion processes to produce fuels, power, heat and value-added chemicals. Due to its low price and wide distribution, lignocellulosic biomass is expected to play an important role toward this goal. Regarding renewable biofuel production, bioethanol from lignocellulosic feedstocks is considered the most feasible option for fossil fuels replacement since these raw materials do not compete with food or feed crops. In the overall process, lignin, the natural barrier of the lignocellulosic biomass, represents an important limiting factor in biomass digestibility. In order to reduce the recalcitrant structure of lignocellulose, biological pretreatments have been promoted as sustainable and environmentally friendly alternatives to traditional physico-chemical technologies, which are expensive and pollute the environment. These approaches include the use of diverse white-rot fungi and/or ligninolytic enzymes, which disrupt lignin polymers and facilitate the bioconversion of the sugar fraction into ethanol. As there is still no suitable biological pretreatment technology ready to scale up in an industrial context, white-rot fungi and/or ligninolytic enzymes have also been proposed to overcome, in a separated or in situ biodetoxification step, the effect of the inhibitors produced by non-biological pretreatments. The present work reviews the latest studies regarding the application of different microorganisms or enzymes as useful and environmentally friendly delignification and detoxification technologies for lignocellulosic biofuel production. This review also points out the main challenges and possible ways to make these technologies a reality for the bioethanol industry.     

Variation in Cell Wall Composition and Accessibility in Relation to Biofuel Potential of Short Rotation Coppice Willows

Short rotation coppice (SRC) willow is currently emerging as an important dedicated lignocellulosic energy crop in the UK. However, investigation into the variation between species and genotypes in their suitability for liquid transport biofuel processing has been limited. To address this, four traits relevant to biofuel processing (composition, enzymatic saccharification, response to pretreatment and projected ethanol yields) were studied in 35 genotypes of willow including Europe’s leading SRC willow cultivars. Large, genotype-specific variation was observed for all four traits. Significant positive correlations were identified between the accessibility of glucan to enzymatic saccharification before and after pretreatment as well as glucose release and xylose release via acid hydrolysis during pretreatment. Of particular interest is that the lignin content of the biomass did not correlate with accessibility of glucan to enzymatic saccharification. The genotype-specific variations identified have implications for SRC willow breeding and for potential reductions in both the net energy expenditure and environmental impact of the lignocellulosic biofuel process chain. The large range of projected ethanol yields demonstrate the importance of feedstock selection based on an ideotype encompassing the performance of both field biomass growth and ease of conversion.     

Monitoring and Analyzing Process Streams Towards Understanding Ionic Liquid Pretreatment of Switchgrass (Panicum virgatum L.)

Fundamental understanding of biomass pretreatment and its influence on saccharification kinetics, total sugar yield, and inhibitor formation is essential to develop efficient next-generation biofuel strategies, capable of displacing fossil fuels at a commercial level. In this study, we investigated the effect of residence time and temperature during ionic liquid (IL) pretreatment of switchgrass using 1-ethyl-3-methyl imidazolium acetate. The primary metrics of pretreatment performance are biomass delignification, xylan and glucan depolymerization, porosity, surface area, cellulase kinetics, and sugar yields. Compositional analysis and quantification of process streams of saccharides and lignin demonstrate that delignification increases as a function of pretreatment temperature and is hypothesized to be correlated with the apparent glass transition temperature of lignin. IL pretreatment did not generate monosaccharides from hemicellulose. Compared to untreated switchgrass, Brunauer–Emmett–Teller surface area of pretreated switchgrass increased by a factor of ～30, with a corresponding increase in saccharification kinetics of a factor of ～40. There is an observed dependence of cellulase kinetics with delignification efficiency. Although complete biomass dissolution is observed after 3 h of IL pretreatment, the pattern of sugar release, saccharification kinetics, and total sugar yields are strongly correlated with temperature.     

Application of metagenomic techniques in mining enzymes from microbial communities for biofuel synthesis

Feedstock for biofuel synthesis is transitioning to lignocelluosic biomass to address criticism over competition between first generation biofuels and food production. As microbial catalysis is increasingly applied for the conversion of biomass to biofuels, increased import has been placed on the development of novel enzymes. With revolutionary advances in sequencer technology and metagenomic sequencing, mining enzymes from microbial communities for biofuel synthesis is becoming more and more practical. The present article highlights the latest research progress on the special characteristics of metagenomic sequencing, which has been a powerful tool for new enzyme discovery and gene functional analysis in the biomass energy field. Critical enzymes recently developed for the pretreatment and conversion of lignocellulosic materials are evaluated with respect to their activity and stability, with additional explorations into xylanase, laccase, amylase, chitinase, and lipolytic biocatalysts for other biomass feedstocks.     

Laccase applications in biofuels production: current status and future prospects

The desire to reduce dependence on the ever diminishing fossil fuel reserves coupled with the impetus towards green energy has seen increased research in biofuels as alternative sources of energy. Lignocellulose materials are one of the most promising feedstocks for advanced biofuels production. However, their utilisation is dependent on the efficient hydrolysis of polysaccharides, which in part is dependent on cost-effective and benign pretreatment of biomass to remove or modify lignin and release or expose sugars to hydrolytic enzymes. Laccase is one of the enzymes that are being investigated not only for potential use as pretreatment agents in biofuel production, mainly as a delignifying enzyme, but also as a biotechnological tool for removal of inhibitors (mainly phenolic) of subsequent enzymatic processes. The current review discusses the major advances in the application of laccase as a potential pretreatment strategy, the underlying principles as well as directions for future research in the search for better enzyme-based technologies for biofuel production. Future perspectives could include synergy between enzymes that may be required for optimal results and the adoption of the biorefinery concept in line with the move towards the global implementation of the bioeconomy strategy.