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1、Reaction in Alkaline Cooking During kraft pulping lignin fragmentation is performed by the cleavage of aryl ether bonds and formation of new phenolic groups,which increase the hydrophilicity and solubility of lignin.The reactions of lignin in a kraft cook are complex and still not completely underst
2、ood,but the main reactions leading to the lignin degradation in alkaline conditions have been well reviewed.The most important delignifying reaction in alkaline conditions is the cleavage of-aryl ether linkages,which are the most prominent lignin structures.Reaction in Alkaline CookingIn kraft pulpi
3、ng conditions the reaction of hydrosulphide ions with quinone methides leads to the cleavage of-O-4-linkages,whereas in the absence of hydrosulphide ions,such as in the soda cook,the dominating reaction is the elimination of-hydroxymethyl or-proton,which leads to the formation of formaldehyde and en
4、ol ether structures.In addition to hydrosulphide ions,anthraquinone(AQ)and polysulphide(PS)have also been found to enhance the cleavage of-aryl ether linkages.hydrosulphide 硫氢化物硫氢化物methide 甲基化合物甲基化合物hydroxymethyl 羟甲基羟甲基formaldehyde 甲醛甲醛 aldehyde 醛，乙醛醛，乙醛 form-甲酸，甲酰甲酸，甲酰enol【化化】烯醇烯醇anthraquinone 蒽醌蒽醌
5、 anthrahydroquinone 蒽氢醌蒽氢醌polysulphide 多硫化物多硫化物 poly-表示表示“聚、聚合聚、聚合”之义之义Reaction in Alkaline Cooking The cleavage of non-phenolic-aryl ether bonds is the dominating and rate determining reaction in alkaline lignin degradation.The etherified-aryl ether linkages are cleaved by hydroxide ions via an oxi
6、rane intermediate.This reaction is not affected by sodium sulphide or anthraquinone,but for example-carbonyls have been shown to accelerate the cleavage of-aryl ether linkages in non-phenolic lignin units.oxirane -ane -ene -yne环氧环氧 epoxide 环氧化物环氧化物carbonyl【化化】羰基羰基 carboxyl 羧基羧基 carboxylic 羧酸的羧酸的Reac
7、tion in Alkaline CookingBecause the reaction is intramolecular,the stereostructure also affects the reactivity and the erythro form of the-O-4 structure has been found to be more reactive than the corresponding threo form.erythro form 赤式赤式threo form 苏式苏式Reaction in Alkaline Cooking The phenolic-aryl
8、 ether bonds are cleaved most easily,but since the amount of non-cyclic-aryl ether structures is low compared to the-O-4 structures their cleavage does not lead to the major degradation of lignin.The cleavage of the-ether bonds of phenolic phenyl coumaran structures leads to the formation of alkali
9、stable stilbene structures.The non-phenolic-aryl ether structures have been reported to be stable.In alkaline conditions of pulping,some undesired carbohydrate degradation also takes place.coumaran【化化】香豆满香豆满 hydrate 水合物水合物Reaction in Alkaline CookingOwing to the low degree of polymerisation,the amor
10、phous hemicelluloses are more susceptible to degradation and dissolution,but loss of cellulose cannot be avoided either.The end-wise depolymerisation,i.e.peeling,occurs via reducing end groups creating carboxylic acid derivatives.polymerization 【化化】聚合聚合;聚合反应，聚合作用聚合反应，聚合作用amorphous 无定形的无定形的susceptibl
11、e 易受影响的易受影响的-wise 。方向。方向 clockwiseReaction in Alkaline CookingRandom alkaline hydrolysis of glycosidic bonds occurs to a lesser extent,but at the same time new end groups are formed and secondary peeling may take place.In modified kraft processes PS and AQ can be used,not only to accelerate delignif
12、ication,but also to improve the selectivity.Both PS and AQ prevent the peeling of polysaccharides by oxidising their reducing end groups to alkali stable aldonic acids.Reaction in Alkaline CookingProposed obstacles to delignification The low rate of delignification in the residual phase of pulping h
13、as often been suggested to be a consequence of alkali stable lignin-carbohydrate linkages as well as less reactive condensed lignin structures.The location and accessibility of residual lignin may also have a significant effect on the delignification.condense condensate condenserReaction in Alkaline
14、 CookingLignin-carbohydrate complexes Since lignin and carbohydrate components cannot be separated completely by selective chemical treatments or separation methods,a very intimate association between the residual lignin and carbohydrates has been suggested.These so called lignin-carbohydrate comple
15、xes(LCC)are most likely native,but the formation of LC-linkages during pulping has also been shown to be possible.Reaction in Alkaline Cooking Most of the evidence of covalent LC-bonds is indirect,but lignin and carbohydrates have frequently been proposed to be linked by benzyl ester,benzyl ether an
16、d glycosidic linkages.Ester linkages are known to be readily hydrolysed in alkali,whereas the-ether LC-linkages have been shown to be relatively stable under alkaline pulping conditions.covalence covalent-lyze 构成与带构成与带-lysis 词尾的名词相当的动词词尾的名词相当的动词如如 analyze hydrolyzeReaction in Alkaline Cooking Accord
17、ing to model compound experiments the-ether LC-linkages,which have been suggested to exist between lignin and all types of wood polysaccharides,also retard the cleavage of adjacent-aryl ether linkages,having thus a negative impact on the delignification.The covalent LC-linkages also increase the ext
18、ent of cross-linking in lignin molecules and thus decrease the solubility of degraded lignin.cross-link 交联交联Reaction in Alkaline CookingCondensed lignin structures Condensed aromatic lignin structures have also been suggested to contribute to the incomplete delignification,since the relative amount
19、of mainly C5 substituted guaiacyl structures has been found to increase in residual lignins during kraft pulping and oxygen delignification.This may be a consequence of the formation of new condensed structures as well as the enrichment of original condensed lignin structures into fibres due to thei
20、r less reactive nature.Reaction in Alkaline Cooking Condensed 5-5-biphenyl and 5-O-4 lignin structures have been shown to be less reactive and to accumulate into the fibres during kraft pulping.Recently it was also found that the-O-4 linkages connected to the condensed structures are more stable und
21、er kraft pulping conditions than those connected to non-condensed moieties.However,with model compounds it has been shown that a variety of condensation reactions are possible in alkaline pulping,leading to the formation of more stable carbon-carbon bonds between lignin units.biphenyl【化化】联联(二二)苯苯;二苯
22、基二苯基moiety 一部分，组分一部分，组分Reaction in Alkaline CookingNew condensed lignin structures,e.g.-1 and-5-linkages can be formed by the reactions of quinone methides with carbanions formed in alkaline conditions.Diphenyl methane(DPM)structures have also been suggested to form by the reaction of carbanions wit
23、h formaldehyde released during pulping by the cleavage of hydroxylated-carbon.Formation of DPM structures is more prominent in the absence of HS ions,but it has been shown to occur also during kraft pulping.As already mentioned,condensation reactions with carbohydrates are also possible.carbanion 【化
24、化】负碳离子负碳离子 carbonium ion 正碳离子正碳离子diphenyl【化化】联联(二二)苯苯;二苯基二苯基hydroxylate(使使)羟羟(基基)化化 Reaction in Alkaline Cooking Especially the C5 substituted DPM and 5-5-biphenyl structures but also some C6 substituted lignin structures have been shown to be fairly resistant towards oxygen delignification.The corr
25、esponding resistance of condensed structures has not been observed towards peroxide or chlorine dioxide.Using model compounds it has been shown that the biphenyl structure degrades in oxygen delignification conditions much slower than the corresponding monophenolic structure.Reaction in Alkaline Coo
26、kingThis was suggested to be due to the formation of an intramolecular hydrogen bond after ionisation of the other phenolic hydroxyl group,thus making the electron transfer to oxygen more difficult.However,the formation of condensed 5-5-biphenyl structures by radical coupling reactions during oxygen
27、 delignification have also been shown to be possible.Reaction in Alkaline CookingHeterogeneous structure and distribution of lignin in softwoods The accessibility of lignin as well as its reactivity may vary in different morphological regions due to the heterogeneous structure and distribution of li
28、gnin within the cell wall and within various constituents of the wood.It is well known that in the middle lamella and the primary wall the concentration of lignin is higher than in the secondary wall,which still contains most of the lignin.Ray cells and compression wood are also known to be very lig
29、nin-rich.heterogeneous【化化】不均匀的不均匀的,多相的多相的 homogeneous 均相的均相的morphological 形态的形态的,形态学的形态学的,constituent【化化】组分组分 Reaction in Alkaline CookingFurthermore,a distinction in the chemical structure of lignin also exists which depends on the origin of the lignin.The content of phenolic lignin units has been
30、reported to be higher in the secondary wall lignin than in the primary wall or middle lamella lignin.The molar mass of lignin is also higher in the middle lamella.It has been shown that the etherified lignin units are more condensed than their phenolic counterparts,which is consistent with the resul
31、ts,according to which the lignin structure in the middle lamella is more condensed than in the secondary wall.counterpart n.副本副本,复本复本,配对物配对物,相应物相应物Reaction in Alkaline CookingAlso in ray cells lignin has been found to be more condensed and less phenolic than in the other constituents of wood.Since t
32、he degradation of lignin is mainly driven by the cleavage of aryl ether bonds,whose reaction mechanisms are dependent on whether they are phenolic or non-phenolic,and the condensed lignin structures are supposed to be less reactive,the distribution of those functionalities may have a significant eff
33、ect on the delignification.It has been shown that in kraft pulping conditions the dissolution rate for lignin in the highly lignified middle lamella and ray cells is lower than that for whole wood.Reaction in Alkaline Cooking After kraft pulping,residual lignin is still unevenly distributed and the
34、surface layers of the kraft fibres and the ray cells contain more lignin than the long fibres.It has been suggested that two types of LC-complexes exist in the kraft fibres:a high molar mass lignin-galactan complex,which is assumed to originate from the outer layer of the cell wall or compression wo
35、od,and a lower molar mass lignin-carbohydrate complex,whose origin is mainly the secondary cell wall.galactan【生化生化】半乳聚糖半乳聚糖Reaction in Alkaline CookingThe lignin fraction with the higher molar mass was also found to be slightly more condensed than that with the lower molar mass.In addition to the st
36、ructural differences,the content of transition metal ions detrimental to oxygen and peroxide bleaching has also been found to be higher in ray cells and on the fibre surface.These factors may greatly affect the uniformity of further delignification.It has been reported that kraft pulp without ray ce
37、lls consumed considerably less active chlorine in order to obtain a certain brightness level than the original pulp.fraction 小部分,片断,分数 transition metal过渡金属元素过渡金属元素 Reaction in Alkaline CookingHigher viscosity and lower peroxide consumption have been reported for a pulp,which was peeled before TCF-bleaching in order to remove lignin rich ray cells and fibre surface material.The lower content of transition metals after removal of surface material also results in the reduced degradation of peroxides and formation of detrimental hydroxyl radicals which induce cellulose degradation.