Reaction Path A

Addition ofNucleophiles

In kraft cooking, the nucleophilicity of the hydrosulfide anion is higher as compared

to the hydroxyl ions, which results in an improved delignification behavior

in comparison to soda pulping. Other nucleophiles present in the pulping liquor,

such as the carbon-centered mesomer of phenoxide anions or nucleophilic species

originating from carbohydrates, may also compete for the quinone methide,

finally resulting in condensation reactions rather than fragmentation.

Scheme 4.5 outlines the fragmentation of the b-aryl ether bond by hydrosulfide:

after addition of HS– to the quinone methide, an intramolecular attack at the

neighboring b-carbon (neighboring group participation [5]) causes formation of a

thiiran intermediate 6. Elimination of elemental sulfur (formation of polysulfide)

with concomitant re-aromatization yields coniferyl-type structures (8).

CH2OH

O

OAr

OMe

HS

-

+

CH2OH

O

OAr

OMe

HS

CH2OH

O

OMe

S

CH2OH

O

OMe

S

CH2OH

O

OMe

4 5 6 7

-So

Scheme 4.5 Addition of hydrogen sulfide to quinone methide structures.

Cleavage of phenolic a- and b-aryl-ether linkages proceeds relatively easily. This

reaction has thus been proposed as the major pathway occurring in the initial

phase of delignification in kraft pulping [6](see Section 4.2.5, Kinetics).

Condensation Reactions

The formation of stable carbon–carbon bonds between lignin units is normally

referred to as “condensation”. Such condensation processes lead to lignin structures

which are more difficult to cleave. This applies mainly to the terminal phase

of the kraft cook as well as to the residual lignin structures. The unoccupied

5-position in guaiacyl units is very susceptible to carbon–carbon coupling reactions,

and is less frequent in kraft lignin as compared to the more genuine MWL

(Milled Wood Lignin). The lignin moieties with 5–5′, b-5, 5-O-4 and diphenylmethane

structures (DPM) are considered as condensed units. Condensation reactions

are thought to proceed through addition of a carbon-centered mesomer of

phenoxide anions (donor) (in the carbon-centered resonance form) to a quinone

methide (acceptor), and results in a novel a-5-bond (primary condensation), a

Michael-addition-type reaction [7]. Condensation with formaldehyde leads to

stable diarylmethane units (cf. elimination reactions) (Conclusive evidence for the

outlined condensation reactions during pulping and in residual lignin structures

4.2 Kraft Pulping Processes 167

is still missing [56].) However, it has recently been shown with 2D-NMR techniques

that the amounts of DPMs are very small [below the detection limit for

HSQC experiments (0.05–1%)][8,9], while novel a-5 are shown to be present only

to a minor extent [10,55]. Interestingly, more a-5-units are found in hardwood

than in softwood lignins, and these structures are more abundant in the dissolved

lignin than in the residual one. From the structures of these moieties it is concluded

that condensation occurs after lignin degradation rather than before. Thus,

if condensation really occurs in lignin (it can also be simply an accumulation of

native lignin condensed moieties), the mechanism might be different to that

hitherto comprehended.

Condensed phenolic structures can be analyzed using 13C-NMR, permanganate

oxidation and 31P-NMR [11]. 31P-NMR is a semi-quantitative technique, especially

with regard to condensed moieties, and requires a good resolution of the spectra.

31P-NMR is limited to the analysis phenolic (condensed/non-condensed) moieties

only.

Recently, Gellerstedt et al. [12]proposed a novel concept for the formation of

condensed units in residual lignins based on a one-electron mechanism with elemental

sulfur as the radical initiator. The products of a model study with the observed

sulfur bridges are presented in Scheme 4.6. However, the model does not

explain condensation reactions in soda pulping.

An increase of condensed structures in residual lignins as kraft cooking proceeds

was also confirmed by solid-state NMR. However, it cannot be determined

whether these structures are just enriched during pulping or are formed de novo

[13], which is a general question dealing with condensation in lignin chemistry.

H C 3

OH

CH3

H C 3

OH

CH3

S

S

H C 3

OH

CH3

H C 3

OH

CH3

H C 3

OH

CH3

H C 3

OH

CH3

S

H C 3

OH

CH3

H C 3

OH

CH3

SH

polysulfide/NaOH

165°, 30 min

Scheme 4.6 Model reaction to demonstrate the action of

sulfur as electron-transfer reagent under kraft conditions to

bring about condensation reactions in lignin (from Ref. [12]).

168 4 Chemical Pulping Processes

Дата добавления: 2015-10-21