Lignin: Functional Biomaterial with Potential in Surface Chemistry and Nanoscience - The Nanoscience and Technology of Renewable Biomaterials - page 179

1957). Later, Lundquist et al . (1977) modified the purification steps of Bjorkman's
original protocol to reduce the risk of solvolytic reactions. The expected yield of the lat-
ter MWL method is typically about 25% of the Klason lignin content. The MWL yields
can be improved by treating the finely ground wood meal with cellulolytic enzymes
that remove the associated polysaccharides before the solvent extraction and a frac-
tion named Cellulolytic Enzyme Lignin (CEL) was first isolated by Pew (Pew 1957,
Pew and Weyna 1962). By using an enzymatic preparation with even greater cel-
lulolytic and hemicellulolytic activities, Chang et al . (1975) extracted the insoluble
residue obtained after enzymatic hydrolysis successfully with 96% and 50% aqueous
dioxane at higher total yields than MWL and CEL. The content of carbohydrates in
the lignin fraction soluble in 50% dioxane was however twice as high as with the
former methods. Recently, a novel lignin isolation procedure (EMAL) was proposed
(Argyropoulos et al . 2002, Wu and Argyropoulos 2003). Starting with an initial mild
enzymatic hydrolysis of milled wood to remove most of the carbohydrates, followed by a
mild acid hydrolysis stage to cleave the remaining lignin-carbohydrate bonds, significant
improvements in yield and purity are obtained (Guerra et al . 2006a). The molecular
mass of the EMAL is also substantially increased compared to MWL and CEL and the
weight-average molecular mass ( M w ) is in the range of 30-60 kDa dependent on the
wood-species of origin. One other way of increasing the yield is through very extensive
milling. This might improve the yield to around 50% of the Klason lignin. The risk of
introducing severe chemical modifications is however considered overwhelming (Guerra
et al . 2006b).
The extracted MWL's morphological origin has been discussed in the literature. Lai
and Sarkanen (1971) suggested that MWL mainly originates from regions adjacent
to the middle lamella, whereas, for example Whiting and Goring (1981) as well as
Terashima et al . (1992) and Maurer and Fengel (1992) found that MWL from spruce
mostly descents from the secondary wall of the tracheids, which is a general opinion
today.
Regarding ways of isolating residual lignin from chemical pulp fibres methods based
on acidolysis (extraction with 1,4-dioxane under acidic conditions, Gellerstedt et al .
1994) or repeated enzymatic treatment and precipitation similar to the CEL and EMAL
procedures described earlier, are found useful. Moreover, a tuned combination of LiCl
and dimethyl acetamide (LiCl-DMAc) can be used to dissolve both hardwood and soft-
wood kraft pulp fibre components (Westermark and Gustafsson 1994, Sjoholm et al .
1999a-b).The solvent is exceptionally good for cellulose, but lignin can also be dissolved
in lower concentrations. This protocol has mostly been applied for analytical purposes,
e.g. in determining the molecular mass distributions of wood polymers in chemical
pulps by size exclusion chromatography but also in the preparation of cellulose micro-
spheres and model films; however it can be used together with chromatography to isolate
the lignin.
In 2003 Lu and Ralph published a paper that described a method for dissolution
of ball-milled wood in dimethylsulphoxide-tetrabutyl ammonium fluoride and dimethyl
sulphoxide-imidazole binary solvent systems (Lu and Ralph 2003). Recently, further
improvements of this procedure were suggested by Fasching et al . (2008).
Protocols involving dissolution of wood by the use of ionic liquids have lately
been developed (Kilpelainen et al . 2007, Pu et al . 2007). Kilpelainen et al . found that