Specific Reaction of Xylans

The side chain substituents in the xylan (either the 4- O -methylglucuronic acid in

hardwood xylans, or the arabinopyranose in some softwood xylans; for structures,

see Chapter 1) significantly hamper the alkaline degradation [73](see above). The

xylan backbone undergoes the peeling reaction until a unit with a substituent in

position 2 or 3 is reached, or a stopping reaction with the formation of xylo-metasaccharinic

acid and xylo-isosaccharinic acids sets in.

Model studies have indicated that at low temperatures the linear backbone of

xylans are subjected to the peeling reaction at a higher rate compared to cellulose

[74], though the peeling reaction was strongly retarded by side chain branches

(4- O -methylglucuronic acid) in position C2 (57) [75].

O

HO

OH

O

O

HO

HO

OH

MeO

O

OH

O

HO

OH

O

O

HO

HO

OH

MeO

O

OH

O

HO

O

OH

OH

O

HO

OH

O

O

HO

HO

O

OH

MeO

+ degradation

Products

57 58 59

Scheme 4.19 Stopping of the peeling reaction at 4- O -methylglucuronic

acid side chains (adopted from Ref. [75]).

Johansson and Samuelson [76,77]demonstrated that the peeling reaction is also

hindered by a rhamnose substituent at C2 or an end group carrying a 4- O -methylglucuronic

acid residue. Since these units are more stable, the peeling reaction

pauses at this point until a temperature is reached at which the substituent is

cleaved and the peeling process can be then resumed [78–81]. Recently, novel

insights into the distribution of uronic acids within xylan showed that uronic

acids are distributed rather randomly in hardwood xylan, but quite regularly in

softwood xylans [82].

Also galacturonic acid end groups stabilize the xylan against peeling at lower

temperature (T > 100 °C), but they are equally degraded at temperatures above

130 °C. 4- O -Methylglucuronic acid residues, partially converted into the corresponding

hexenuronic acid (T > 120 °C) (cf. Scheme 4.20), are believed to stabilize

the xylan towards depolymerization. The hexenuronic acid formed has a higher

stability towards alkali, so that further peeling of the chains is prevented [83]. The

arabinose units in softwood xylans also contribute to the higher alkali stability by

inducing the stopping reaction. However, at elevated temperature these effects are

reduced and the arabinose units are also cleaved [84].

4.2 Kraft Pulping Processes 179

O

HO

OH

O

O

HO

XylO

OH

COOH

OH

O

HO

OH

O

O

HO

XylO

MeO

HOOC

OH

O

HO

OH

O

O

HO

XylO

MeO

COOH

O

HO

OH

O

O

HO

XylO

OH

COOH

O

O

OH

COOH

OH

O

HO

XylO

OH

-MeOH

61 62

Scheme 4.20 Formation of HexA and cleavage of the side chain [83,90].

The hexenuronic acid side chains in xylans undergo elimination of methanol

under alkaline conditions, forming hexenuronic acid residues (i.e., 4-deoxy-L threo -

hex-4-enopyranosyl-uronic acid, 61, 62) [85]. The reaction is promoted with

both increasing alkali concentration and temperature [86,87]. After kraft pulping

only about 12% of the carboxyl groups in accessible xylan are still of the

4- O -methylglucurono-type [88]. Formation of HexA is discussed as a cause for the

stability of xylans during kraft cooking due to prevention of peeling reactions at

the branched unit. Eventually, the hexenuronoxylose is further decomposed to

xylitol 83. HexAs are seen as being partly responsible for the diminished brightness

stabilities of bleached pulps. As under acidic conditions hexenuronic acids

are unstable, an acidic treatment can be used to selectively remove HexA from the

pulp [89].

Hexenuronic acids add to the total carboxyl group content in kraft pulps, and

also to the kappa number. To estimate the actual amount of residual lignin in

pulps, a modified kappa number has been proposed, which selectively disregards

nonlignin fractions [91,92]