Alkaline Hydrolysis

At elevated temperatures (T > 140 °C), direct chain cleavage of cellulose commences

– a process that is also referred to as alkaline hydrolysis or secondary peeling.

This reaction is mainly responsible for observed yield losses during alkaline

pulping. The reaction mechanism is depicted in Scheme 4.18.

Elevated temperatures facilitate a conformational change of the pyranose ring

from the 4C1 conformer with all hydroxyl groups arranged in equatorial position

to a 1C4 conformation, with all hydroxyl groups arranged axially (52). Starting

from this molecular geometry, the ionized hydroxyl at C2 reacts by intermolecular

nucleophilic attack to form an oxirane intermediate (53) under elimination of the

cellulose chain in position 1. The intermediate oxirane can be opened either by

reaction with hydroxyl ions (formation of a terminal glucose moiety, 43) or again

by an intramolecular reaction with the ionized C6 hydroxyl, yielding an anhydrate

(levoglucosan, 54). The activation energy has been determined to be 150 kJ mol–1

for cotton [68]and 179 kJ mol–1 in Soda-AQ-pulping for spruce cellulose [69].

The ultrastructure of cellulose also plays a crucial role in the reaction under

alkaline conditions, and may also affect the reaction rate [70]. Atalla et al. [71]

demonstrated by raman spectroscopy cellulose II structure is formed in kraft

pulp, while Isogai et al. [72]reported the formation of cellulose IV from amorphous

regions of cellulose I during kraft pulping. Amorphous regions are more

readily attacked than highly ordered domains. Even though the DP is decreased,

the total yield loss during alkaline hydrolysis at elevated temperatures is appreciably

small.

O

CH2OH

O

OH

OR

OR

RO

O

HO

OH

OH

OR

OHRO

O

HO

OH

OH

OH

OHO

H C 2

OH

OR

O

OH

OHO

H C 2

OH

OR

O

OH

-ROR

= cellulose

52 53

Scheme 4.18 Alkaline hydrolysis of glycosidic linkages at elevated temperatures.

178 4 Chemical Pulping Processes