Hemicelluloses

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