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As shown in Fig. 4.156, hemicelluloses are degraded much more rapidly than cellulose
under acidic sulfite cooking conditions. The heterogeneous hydrolysis follows
the order cellulose (1) < mannan (2–2.5) < xylan (3.5–4) < galactan (4–5) [30],
which roughly agrees with the observed rates for monomers. Arabinofuranosidic
bonds are hydrolyzed much more rapidly than glucopyranosidic bonds. Hence,
arabinose residues appear at an early stage of the acid sulfite cook. Glucuronopyranosidic
bonds are comparably stable, so that monomeric 4- O -methyl glucuronic
acids are found at a later cooking stage, when most of the xylan backbone has
been removed. Also the ratio of monomeric uronic acids to xylose in wood does
not change significantly as the cook proceeds [31].
Acetyl groups of xylans and glucomannans are also removed at elevated temperature
(cf. pre-hydrolysis kraft process), but in some cases are found to be rather
stable.
Hardwood xylans are partly stable under acid sulfite conditions due to the presence
of glucuronic acid side chains, which significantly decrease the rate of hydrolysis.
In two-stage processes for softwood, which operate at a somewhat higher pH in
the first stage, an increase in the mannan content in the final pulp was observed;
this was attributed to a co-crystallization of the glucomannan with the cellulose
[32]after having lost most of the side chains.
The conversion degree of aldoses to aldonic acids in acid sulfite and bisulfite
cooks for birch and spruce varies in the following limits [33]: Ara and Gal:
17–51%, Xyl: 12–25%, Man and Glc: 5–12%. The concentration of uronic acids
was found to be small for all liquors. Whilst for the acidic cook the total amount
of carbohydrates was 25–30%, the magnesium bisulfite liquors contained only
2.5% of the carbohydrates as monosaccharides, with most of the dissolved carbohydrates
remaining as polymeric or oligomeric material.
The opposite situation is true if the acidic groups in the pulp are considered.
Larsson and Samuelson [34]investigated the content of uronic and aldonic acid
groups in an unbleached spruce sulfite pulp, cooked according to a two-stage process.
The dominating uronic acids found after total hydrolysis were 4- O -methylglucuronic
acid and 2- O -(4- O -methylglucopyranosiduronic acid)-d-xylose. Only small
amounts of aldonic acids, such as gluconic, xylonic and mannonic acid, besides traces
of arabinonic, ribonic and galactonic acids, were found. A slight demethylation reaction
of 4- O -methylglucuronic acid also occurred during pulping.
With increasing pH of the cooking liquor, the situation changes significantly.
Nelson analyzed acidic groups in pine bisulfite and eucalypt neutral sulfite pulps
[35]. The eucalypt neutral sulfite pulp yielded much larger amounts of acids than
the pine bisulfite pulp, but this may be attributed to a higher xylan content of the
hardwood pulp. The pine bisulfite pulp, however, contained considerably larger
amounts of aldonic acids (GlcA, ManA, XylA) than the pulp cooked under neutral
conditions, and also compared to pulps produced using a two-stage process [34].
This suggests that the bisulfite ion is an effective oxidant for the reducing end
group, although oxidation at the reducing end did not proceed to any significant
extent under acidic conditions.
418 4 Chemical Pulping Processes
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Comparison to Sulfonation Reactions under Conditions of Neutral Sulfite Pulping | | | Dehydration of Carbohydrates to Aromatic Structures |