作者：谷峰 主编 王旺霞、李恒业 副主编
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利用木质纤维素生物质的另一种方式是生产纳米纤维素。纳米纤维素潜在应用主要可分为纸制品、医用高分子材料、电子器件材料等。当前纳米纤维素主要的市场是纸制品(9400万吨/年)和塑料制品(4700万吨/年)。其中,塑料制品主要用于包装行业(39.4%)、建筑行业(20.5%)、汽车制造业(8.3%)。《木质纤维素预处理及其高值化应用》主要介绍了弱碱性预处理对酶促水解过程中脱木质素和糖转化的影响,对几种典型的木质纤维素原料进行了碳酸钠(NaCO)、绿液(NaCO+NaS)和亚硫酸盐+甲23232醛(NaSO+HCHO)的预处理。同时也从机理方面,研究了木质素结构23对底物酶解糖化的影响。另外,本书还介绍了利用内切葡聚糖酶处理和机械法处理联合制备纤维素纳米纤维(CNF)的相关内容。全书共分四章,Chapter 1、Chapter 2、Chapter 3由谷峰编写,Chapter 4由王旺霞编写,全书由李恒业统稿。《木质纤维素预处理及其高值化应用》可供从事新能源、化工、生物工程、材料及相关学科的研究人员和技术人员阅读,也可供高等院校相关专业师生参考或作为教学参考用书。
不妥之处敬请读者批评指正。编者2019年3月Chapter 1 Introduction 引言1.1 Chemical Composition of lignocellulosic biomass 木质纤维素生物质的化学组成
Lignocellulosic biomass, a chemical complex of cellulose, hemicellulose, lignin, extractives and inorganic components, is the most abundant renewable resource on the Earth.Cellulose provides structural support for the lignocellulose, while the highly branched hemicelluloses remain water.Lignin functions as an adhesive filling in vacant space between these cell wall components, which is also important for mechanical support, water transport, and defense against natural invasion for lignocellulose.Fig.1.1 shows that plant cell wall is built up by several layers of middle lamella(M), primary wall(P), outer layer of the secondary wall(S1), middle layer of the secondary wall(S2), and inner layer of the secondary wall(S3).These layers differ from one another with respect to their structure as well as their chemical composition.And the Table 1.1 shows the Composition of lignocellulose in several sources.Fig.1.1 Main structures and compositions of plant cell wallTable 1.1 Composition of lignocellulose in several sources1.1.1 Cellulose
Cellulose is the main constituent of wood and grass.The cellulose located predominantly the secondary cell wall.It is a homopolysaccharide composed of β-D-glucopyranose units in which are linked together by(1→4)-glycosidic bonds with a degree of polymerization(DP) from 100 to 20000.Cellulose molecules are completely linear and have a strong tendency to form intra- and intermolecular hydrogen bonds.Bundles of cellulose molecules are thus aggregated together in the form of microfibrils, in which highly ordered(crystalline) regions alternate with less ordered(amorphous) regions.The size of microfibril is 2~20 nm in diameter and 100~40000 nm in length.Although the chemical structure of cellulose is understood in detail, its supermolecular state, including its crystalline and fibrillar structure, is still open to debate.Examples of incompletely solved problem areas are the exact molecular weight and polydispersity of native cellulose and the dimensions of the microfibrils.Cellulose is a homopolysaccharide composed of β-D-glucopyranose units which are linked together by(1→4)-glycosidic bonds(Fig.1.2). Cellulose molecules are completely linear and have a strong tendency to form intra- and intermolecular hydrogen bonds.Bundles of cellulose molecules are thus aggregated together in the form of microfibrils, in which highly ordered(crystalline) regions alternate with less ordered(amorphous) regions.Microfibrils build up fibrils and finally cellulose fibers.As a consequence of its fibrous structure and strong hydrogen bonds cellulose has a high tensile strength and is insoluble in most solvents.The physical and chemical behavior of cellulose differs completely from that of starch, which clearly demonstrates the unique influence of stereochemical characteristics.Like cellulose, the amylose component of starch consists of(1→4)-linked D-glucopyranose units, but in starch these units are α-anomers. Amylose occurs as a helix in its solid state and sometimes also in solution. Amylopectin, the other starch component, is also a(1→4)-α-D-glucan but is highly branched.The branched structure accounts for its extensive solubility, since no aggregation can take place.Fig.1.2 Structure of cellulose1.1.2 Hemicellulose
Hemicelluloses were originally believed to be intermediates in the biosynthesis of cellulose.Today it is known, however, that hemicelluloses belong to a group of heterogeneous polysaccharides which are formed through biosynthetic routes different from that of cellulose.In contrast to cellulose which is a homopolysaccharide, hemicelluloses are heteropolysaccharides.Like cellulose most hemicelluloses function as supporting material in the cell walls.Hemicelluloses are relatively easily hydrolyzed by acids to their monomeric components consisting of D-glucose, D-mannose, D-galactose, D-xylose, L-arabinose, and small amounts of L-rhamnose in addition to d-glucuronic acid, 4-O-methyl-d-glucuronic acid, and D-galacturonic acid.Most hemicelluloses have a degree of polymerization of only 50 to 350.Some wood polysaccharides are extensively branched and are readily soluble in water.Typical of certain tropical trees is a spontaneous formation of exudate gums, which are exuded as viscous fluids at sites of injury and after dehydration give hard, clear nodules rich in polysaccharides.These gums, for example, gum arable, consist of highly branched, water-soluble polysaccharides.The amount of hemicelluloses of the dry weight of wood is usually between 20% and 30%.The composition and structure of the hemicellulose in the softwoods differ in a characteristic way from those in the hardwoods.Considerable differences also exist in the hemicellulose content and composition between the stem, branches, roots, and bark.18.104.22.168 Softwood Hemicelluloses
Galactoglucomannans Galactoglucomannans are the principal hemicelluloses in softwoods(about 20%).Their backbone is a linear or possibly slightly branched chain built up of(1→4)-linked β-D-glucopyranose and β-D-mannopyranose units(Fig.1.3). Galactoglucomannans can be roughly divided into two fractions having different galactose contents.In the fraction which has a low galactose content the ratio galactose: glucose: mannose is about 0.1∶1∶4 whereas in the galactose-rich fraction the corresponding ratio is 1∶1∶3.The former fraction with a low galactose content is often referred to as glucomannan.The α-D-galactopyranose residue is linked as a single-unit side chain to the framework by(1→6)-bonds.An important structural feature is that the hydroxyl groups at C2 and C3 positions in the chain units are partially substituted by O-acetyl groups, on the average one group per 3~4 hexose units. Galactoglucomannans are easily depolymerized by acids and especially so the bond between galactose and the main chain.The acetyl groups are much more easily cleaved by alkali than by acid.Fig.1.3 Principal structure of galactoglucomannans
Arabinoglucuronoxylan In addition to galactoglucomannans, softwoods contain an arabinoglucuronoxylan(5%~10%).It is composed of a framework containing(1→4)-linked β-D-xylopyranose units which are partially substituted at C2 by 4-O-methyl-α-D-glucuronic acid groups, on the average two residues per ten xylose units.In addition, the framework contains α-L-arabinofuranose units, on the average 1.3 residues per ten xylose units(Fig.1.4).Because of their furanosidic structure, the arabinose side chains are easily hydrolyzed by acids.Both the arabinose and uronic acid substituents stabilize the xylan chain against alkali-catalyzed degradation.Fig.1.4 Principal structure of arabinoglucuronoxylan
Arabinogalactan The heartwood of larches contains exceptionally large amounts of water-soluble arabinogalactan, which is only a minor constituent in other wood species.Its backbone is built up by(1→3)-linked β-D-galactopyranose units.Almost every unit carries a branch attached to position 6, largely(1→6)-linked β-D-galactopyranose residues but also L-arabinose(Fig.1.5).There are also a few glucuronic acid residues present in the molecule.The highly branched structure is responsible for the low viscosity and high solubility in water of this polysaccharide.Fig.1.5 Abbreviated formula of arabinogalactan.Sugar units: β-D-galactopyranose(Galp), β-L-arabinopyranose(Arap), α-L-arabinofuranose(Araf), and R is β-D-galactopyranose or, less frequently, α-L-arabinofuranose, or a α-L-glucopyranosyluronic acid residue22.214.171.124 Hardwood Hemicelluloses
Glucuronoxylan Even if hemicelluloses in various hardwood species differ from each other both quantitatively and qualitatively, the major component is an O-acetyl-4-O-methylglucurono-β-D-xylan, sometimes called glucuronoxylan.Often the xylose-based hemicelluloses in both softwoods and hardwoods are termed simply xylans.
Depending on the hardwood species, the xylan content varies within the limits of 15%~30% of the dry wood.As can be seen from Fig.1.6, the backbone consists of β-D-xylopyranose units, linked by(1→4)-bonds.Most of the xylose residues contain an O-acetyl group at C2 or C3(about seven acetyl residues per ten xylose units).The xylose units in the xylan chain additionally carry(1→2)-linked 4-O-methyl-α-D-glucuronic acid residues, on the average about one uronic acid per ten xylose residues.The xylosidic bonds between the xylose units are easily hydrolyzed by acids, whereas the linkages between the uronic acid groups and xylose are very resistant.Acetyl groups are easily cleaved by alkali, and the acetate formed during kraft pulping of wood mainly originates from these groups.They are slowly hydrolyzed to acetic acid within a living tree as a result of the acidic nature of especially the heartwood.Fig.1.6 Principal structure of glucuronoxylan
Glucomannan Besides xylan, hardwoods contain 2%~5% of a glucomannan, which is composed of β-D-glucopyranose and β-D-mannopyranose units linked by(1→4)-bonds(Fig.1.7).The glucose:mannose ratio varies between 1∶2 and 1∶1, depending on the wood species.The mannosidic bonds between the mannose units are more rapidly hydrolyzed by acid than the corresponding glucosidic bonds, and glucomannan is easily depolymerized under acidic conditions.Fig.1.7 Principal structure of glucomannan1.1.3 Lignin126.96.36.199 Types of Linkages and Dimeric Structures
Lignin is a phenolic polymer commonly derived primarily from three monolignols, namely, p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol(Fig.1.8).As a biopolymer, lignin is special because of its heterogeneity and lack of a defined primary structure.Its most commonly noted function is the support through strengthening of xylem cells in plants.Fig.1.8 The three primary lignin monomers
In a recent review of lignin, the definition of lignin was given as following:
①Protolignins are biopolymers consisting of phenylpropanoid units with an oxygen atom at the p-positon(as HO or O-C) and with none, one or two methoxyl groups in the o-position to this oxygen atom.
②The phenylpropanoid building units are connected to one another by a series of characteristic linkages(β-Ο-4, β-5, β-β, etc.).There are a series of characteristic end group(e.g.cinnamaldehyde units) that exist in lignin.
③All the types of structural elements detected in protolignins are consistent with those formed by oxidation of the p-hydroxycinnamyl alcohols in vitro.The structural units in protolignins are not linked to one another in any particular order.Lignin is not optical active.
④Protolignins are branched and cross-linked to other cell wall compounds.There are strong indications of the occurrence of linkages between lignin and carbohydrate. There are esters that exist in some types of lignin e.g.grass lignin with p-coumarates and aspen lignin with p-hydroxybenzoates.
It is clear that the phenylpropane units are joined together both with C-O-C(ether) and C-C linkages.The ether linkages dominate; approximately two thirds or more are of this type, and the rest are of the carbon-to-carbon type.Detailed knowledge about the characteristics of these linkages is of great theoretical interest and necessary for a thorough insight into the degradation reactions of lignin in technical processes, such as pulping and bleaching.The dominating bond types are depicted in Fig.1.9 and their approximate proportions in lignin can be seen from Table 1.2.Almost all the phenypropane units in softwood lignin are of the guaiacyl type, but hardwood lignin contains additional syringyl units.In this case the picture is more complex and also less information is available.Fig.1.9 Common linkages between the phenylpropane units.For proportions, see Table 1.2Table 1.2 Proportions of Different Types of Linkages Connecting the aPhenylpropane Units in Lignina Approximative values based on the data of Adler(1977) obtained for MWL from spruce(Picea abies) and birch(Betula verrucosa).b For the corresponding structures, see Fig.1.9c Low values have been reported recently.d Of these structures about 40% are of guaiacyl type and 60% of syringyl type.188.8.131.52 Lignin Formula
Based on the information obtained from studies of biosynthesis as well as analysis of various linkage types and functional groups, structural formulas for lignin have been constructed.The formula presented in its final shape in 1968 by Freudenberg for softwood lignin(spruce) has attained general acceptance, and, indeed, the formulas suggested later for softwood lignin by Adler and by others do not much deviate from it.In addition, formulas for hardwood lignins have been suggested as well.
Adler’s formula is shown in Fig.1.10.This formula consists of 16 phenylpropane units and it represents only a segment of the lignin macromolecule.Most of the linkages are of the same types already shown in Fig.1.10.However, additional details are the glyceraldehyde-2-aryl ether group attached to unit 12 and the β-6 linkage between units 14 and 15.Of course, a model of this limitedsize cannot give a strictly quantitative picture.For example, incorporation of one syringyl unit(13) in the formula has only a qualitative meaning because, in reality, only low amounts(ca.1%) of such units are present in softwood lignin.Likewise, one dimer entity of pinoresinol type(Unit 10 and Unit 11) probably overemphasizes the presence of this substructure.In the Adler’s formula no linkages between lignin and carbohydrates or other wood constituents have been indicated.Fig.1.10 A structural segment of softwood lignin proposed by Adler(1977)184.108.40.206 Lignin-Carbohydrate Bonds
The possible existence of covalent bonds between lignin and polysaccharides has been a subject of much debate and intensive studies.This question is of great interest especially when considering the need to separate polysaccharides from lignin as selectively as possible.Because of other association forces of physical type between lignin and carbohydrates, it has been difficult to verify whether there really are chemical bonds. However, it is obvious and now generally accepted that such chemical bonds must exist, and the term “lignin-carbohydrate complex(LCC)” is used for the covalently bonded aggregates of this type.
Chemical bonds have been reported between lignin and practically all the hemicellulose constituents.There are even indications of lignin and cellulose bonds.These linkages can be either of ester or ether type and even glycosidic bonds are possible.For example, instead of a free benzylic alcohol group, it can be occupied through an ester linkage to a 4-O-methylglucuronic acid group present in xylan or through an ether linkage to an arabinose or mannose unit present in arabinoxylan and glucomannan, respectively(Fig.1.11). The ester linkages are easily cleaved by alkali.Fig.1.11 Examples of suggested lignin-carbohydrate bonds: an ester linkage to xylan through 4-O-methyl glucuronic acid as a bridging group(1), an ether linkage to xylan through an arabinofuranose unit(2), and an ether linkage to galactoglucomannan through a galactopyranose unit(3)
More common and also much more stable than the ester bonds are the ether linkages betweenlignin and carbohydrates.The α-position is even in this case the most probable connection point between lignin and the hemicellulose blocks.In softwood xylans the bridging group can be the arabinose unit(HO-2 or HO-3).In galactoglucomannans, the galactose unit(HO-3) has been proposed to transmit this bridging.There are also indications that lignin in the middle lamella and primary wall of the cell wall is associated with the pectic polysaccharides(galactan and arabinan) through ether linkages.In these cases the primary alcohol groups, that is, HO-6 in galactose units and HO-5 in arabinose units, seem to participate in this bridging.
Although less experimental evidence is available, it has been suggested that even glycosidic linkages are uniting lignin and polysaccharides.In addition to the benzylic alcohol group, which is the most probable connection point, the phenolic hydroxyl group may also be partly occupied through glycosidation.The glycosidic linkages are easily cleaved with acid.220.127.116.11 Classification and distribution of lignin
Lignins can be divided into several classes according to their structural elements.So-called “guaiacyl lignin” which occurs in almost all softwoods is largely a polymerization product of coniferyl alcohol.The “guaiacyl-syringyl lignin”, typical of hardwoods, is a copolymer of coniferyl and sinapyl alcohols, the ratio varying from 4∶1 to 1∶2 for the two monomeric units.An additional example is compression wood, which has a high proportion of phenylpropane units of the p-hydroxyphenyl type in addition to the normal guaiacyl units.The terms “syringyl lignin” and “p-hydroxyphenyl lignin” are sometimes used to denote the respective structural elements even if probably no natural lignins are exclusively composed of these units.
The lignin concentration is high in the middle lamella and low in the secondary wall.Because of its thickness, at least 70% of the lignin in softwoods is, however, located in the secondary wall as shown by quantitative UV microscopy(Fig.1.12).The picture is very similar for the hardwoods although in this case analytical uncertainties are involved because of the more heterogeneous nature of the wood and the presence of both guaiacyl and syringyl units in the lignin.The measurements so far indicate that the lignin located in the secondary wall of hardwood fibers has a high content of syringyl units whereas larger amounts of guaiacyl units are present in the middle lamella lignin.The vessels in birch seem to contain only guaiacyl lignin, whereas syringyl lignin predominates in parenchyma cells.Fig.1.12 Transverse section of a spruce tracheid photographed in UV light(240 nm).The densitometer tracing has been taken across the tracheid wall along the dotted line.S—secondary wall; ML—compound middle lamella; CC—cell corner 18.104.22.168 Isolation of lignin
Before quantitative and qualitative characterize the lignin, a pure sample is desirable.However it is difficult to isolate a pure preparation from lignolcellulosic biomass, especially from the grass materials.It has always been a big problem to isolate a pure lignin sample for analysis, although many procedures have been developed for this purpose. Until now, the most used lignin isolation methods for structural and compositional analysis are milled wood lignin(MWL) and cellulolytic enzyme lignin(CEL).Björkman extracted lignin from finely ball milled wood with aqueous dioxane.The resulting milled wood lignin(MWL) was considered to be a representative source of native lignin and had been extensively used in the elucidation of native lignin structure.However, concerns existed over the similarity between MWL and native lignin based on the low yields(25%~50% of protolignin) and structural alterations due to ball milling.Ball milling reduced the degree of polymerization, created new free phenolic hydroxyl groups through cleavage of β-aryl ether linkages and increased α-carbonyl groups via side chain oxidation.To increase the yield of extracted lignin without altering the structure, the cellulolytic enzymes treatment was introduced to MWL process.Chang et al.utilized cellulolytic enzymes to remove carbohydrates prior to aqueous dioxane extraction of ball milled wood meal.Cellulolytic [7,8]enzyme lignin(CEL) was found to be structurally similar to MWL, but was obtained in higher yield with less degradation, and hence is more representative of the total lignin in wood.