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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]
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