Carbohydrate Reactions in Dioxygen-Alkali Delignification Processes

The reactions of wood polysaccharides during dioxygen-alkali treatment can be

classified according to Malinen [183] into the following main categories:

_ Stabilization of the reducing end-groups.

_ Peeling reactions starting from the reducing end-groups.

_ Peeling reactions starting from stabilized end-groups.

_ Cleavage of the polysaccharide chain.

Reaction steps involving dioxygen are drawn with thicker lines (bold) and the

numbers given in italic.

Stabilization of the Reducing End-Groups

The rapid stabilization of the reducing end-groups of polysaccharides by transformation

to aldonic acid end-residues has been considered to be one great advantage

of the dioxygen-alkali delignification of wood or pulp [184–186]. Under the conditions

of dioxygen-alkali treatment, oxidation of the glucose unit (1) may proceed

via a 1-hydroperoxy-ketose (2 [187]) and a 2-hydroperoxy-aldose (3) (Scheme 7.15).

The hydroperoxy-group can easily be replaced by a hydroxide anion followed by

dehydration (path I) resulting in a a, b– dicarbonyl (glucosone = d- arabino -hexosulose,

4), which converts into gluconic acid (5) and mannonic acid (6) via benzilic

acid rearrangement (BAR) (see Section 4.2.4.2, Carbohydrate reactions). Glucosone

(d- arabino -hexosulose) end-groups have been suggested to be intermediates

in the formation of aldonic end-residues [188,189], and Theander [185] stated that

the fact that mannonic acid and gluconic acid end-residues are obtained on cellulose

treatment with dioxygen in basic solution is the best support for the view that

glucosone is really an intermediate. Alternatively, the hydroperoxy-intermediates

are split to formic acid (7) and arabinonic acid (8) (path II), the latter being converted

to 3-deoxy-d- glycero –2-keto-pentonic acid (9) and further degraded.

H OH

HO H

H OR

H OH

R1

OH

HO H

H OR

H OH

R1

H OH

O2/OH-

O OH

HO H

H OR

H OH

R1

O

O HO

HO H

H OR

H OH

R1

OH

O

OH

H

+

OH-

I

II

I

1. -HOO-

2. -HOH

O

HO H

H OR

H OH

R1

O

OH

HO H

H OR

H OH

R1

BAR

H O HO O

HO O

HO H

H OR

H OH

R1

II

1 2 3 5

R1 = -H for xylan

BAR = Benzilic Acid Rearrangement R = Polysaccharide chain

H

HO

HO H

H OR

H OH

R1

HO O

+

R1 = -CH2OH for cellulose and glucomannan

- HCOOH

- ROH

+ OH -

HO O

HO

H

H OH

R1

+ OH -

Degradation

products

1 D-Glucose

2 1-Hydroperoxy-ketose

3 2-Hydroperoxy-aldose

4 D- arabino -hexosulose

5 Gluconic acid

6 Mannonic acid

8 Arabinonic acid

9 3-Deoxy-D-glycero-2-keto-pentonic acid

1 D-Xylose

2 1-Hydroperoxy-ketose

3 2-Hydroperoxy-aldose

4 D- threo -pentosulose

5 Xylonic acid

6 Lyxonic acid

8 Threonic acid

9 3-Deoxy-2-keto-tetronic acid

Scheme 7.15 Stabilization of reducing end-residues through

formation of aldonic acids (5) and mannonic acid (6)

(adapted from Malinen [183] and Theander [185]).

In the absence of dioxygen, large amounts of 3-deoxy-pentonic acids are formed

and under oxidative conditions arabinonic, erythronic and mannonic acids are the

major reaction products [190]. A relative composition of aldonic acid residues

from various treatments is shown in Tab. 7.12.

Tab. 7.12 Relative composition (mol. %) of aldonic acid residues

from various treatment (from Ref. [185]).

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