Content is almost negligible. Therefore, the main objective of the alkali

Purification processes comprises removal of the residual xylan content.

By comparing the amount of xylan removed from the pulp with the amount

Recovered in both the beta- and gamma-cellulose fractions, it can be concluded

That most xylan is recovered in oligomeric and polymeric structures. The proportion

Of degraded xylan is greater only in the lower NaOH concentration range (up

to 80 g L–1) where the easily degradable fraction is removed. Apart from the minimum

at a NaOH concentration of 100 g L–1 at 20 °C and 140 g L–1 at 50 °C and

C due to the increased dissolution of degraded cellulose, the beta-cellulose

Becomes increasingly enriched with xylan as both the NaOH concentration and

Temperature are raised (Fig. 8.3). This means that the xylan part in the hemicelluloses

Is clearly more resistant to alkaline degradation than the other carbohydrate

Components. The major part of the xylan remains stable even after hot caustic

extraction (100 °C, 0.25 N NaOH, 1–4 h) as exemplified in a study conducted by

Corbett and Kidd [11].

Pulp Purification

0 100 200 300

C 50.C 80.C

Xylan content in Beta-Cellulose [%od]

NaOH concentration [g/l]

Fig. 8.3 Xylan content in beta-cellulose as a function of

NaOH concentration and temperature [12]. Caustic treatment:

5%consistency, 30 min reaction time, NaOH concentrations:

20, 40, 60, 80, 100, 140, 180, 280, and 340 g L–1.

Model compound studies using aldobiouronic (4-O-methyl-b-d-glucuronic acid-

(1→2)-xylose) (4OMeGlcA) and aldotriouronic acid (4-O-methyl-b-d-glucuronic

acid-(1→2′)-xylobiose), confirmed that substitution at position 2 of the terminal,

reducing xylose unit strongly inhibits alkaline degradation [18]. In the absence of

A C-2 substituent, the xylose chain is rapidly shortened according to classical peeling

Pathways, until the next C-2 substituted xylose unit is reached. The results

Explain the observed higher stability of the xylan fraction as compared to the glucan

Fraction isolated from the steeping lye. Thus, the decreased alkaline degradation

Of the xylan isolated from the beta-cellulose fraction can be attributed to the

Presence of side branches consisting of 4-O-methyl-glucuronic acid as detected by

FT-IR-spectra and by MALDI-MS with a 4OMeGlcA:Xylose-ratio of 5:100 at the

maximum [19].

The interaction between aqueous NaOH and cellulose also affects the supramolecular

structure of cellulose. Increasing the NaOH concentration beyond 70–

80 g L–1 at room temperature leads gradually to a change from the native cellulose

I structure into the Na-cellulose I structure. Thereby, the plane distance of the

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