Substrates Rel Rate Substrates Rel. Rate

b-D-Glucose 1.0 Cellobiose 1.5

b-D-Mannose 3.0 Glucosyl-Mannose 1.5

b-D-Galactose 4.8 Pseudocellobiouronic

acid

1.5

b-D-Xylose 5.8 Mannobiose 2.7

Xylobiose 11.0

the soluble cellulose molecule – that is, the glycosidic bond at the nonreducing

end would hydrolyze faster than the internal bonds which would all hydrolyze at

the same rate. Thus, the two kinetic constants would completely describe the

homogeneous hydrolysis of cellulose.

With the exception of the arabinogalactans originating in larch that are b-(1–3)

-linked, the backbones of the hemicelluloses present in both hard- and softwoods

are joined b-(1–4). Their substituents are all easily cleaved under acidic conditions,

leaving residual linear fragments of the b-(1–4) backbone. Only one substituent,

4- O -methyl-a-d-glucuronic acid – which occurs predominantly in hardwoods – is,

however, much more resistant to acid hydrolysis than, for example b-(1–4) glycosidic

bonds. Thus, as the uronic acid content of the hardwood xylan increases, the

overall hydrolysis rate decreases. Xylan from hardwood species such as southern

red oak with a low uronic acid-to-xylose ratio show a high hydrolysis rate constant.

It can be concluded that the rates decrease in the order of increasing uronic acidto-

xylose ratio [4,29]. The heterogeneous backbone of the galactoglucomannan,

prevalent in softwoods, hydrolyzes to expected ratios of various disaccharides and

trisaccharides. The mannans of softwoods are more resistant than the hardwood

xylans (due to the difference in the conformation). The heterogeneous hydrolysis rate

of cellulose is one to two orders of magnitude less than that of the hemicelluloses

which, fortunately, enables the selective removal of hemicelluloses during prehydrolysis.

The specific morphology of the cellulose is considered to be the main reason for

the high resistance towards acid hydrolysis. Apparently, the hydrolysis resistancemay

be ascribed to the highly ordered structure of cellulose. The hydrolytic degradation

of cellulose has been comprehensively reviewed by Klemm et al. [30].

Under acidic conditions and elevated temperatures, pentoses are transformed to

furfural by gradual dehydration reactions. Furfural is highly unstable under the

conditions being formed, and its concentration follows a typical growth-and-decay

curve. Unlike the cellulose–glucose system, the mechanism does not follow a simple

pair of consecutive first-order reactions. Furfural degradation does not com-

328 4 Chemical Pulping Processes

4.2 Kraft Pulping Processes 329

prise a first-order reaction, and it is likely that its decomposition products are also

participants. Furfural is produced during the prehydrolysis of hardwood. At the

start of the reaction, the concentration is low as it depends on the furfural concentration.

The mechanistic scheme and routes of furfural production from xylose

have been extensively studied elsewhere [31–33].

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