Structural changes in lignin during kraft cooking
Part 4. Phenolic hydroxyl groups in wood and kraft pulps
Göran Gellerstedt and EvaLisa Lindfors, Swedish Forest Products Research Laboratory,
Stockholm, Sweden
Keywords: Pinus, Alkaline pulping, Kraft pulps, Soda pulps, Lignin content, Phenol groups,
Quantitative analysis, Acetylation, Aminolysis.
SUMMARY: In a series of kraft pulps cooked to different yield levels, the content of phenolic
hydroxyl groups present in the residual lignin has been quantitatively determined by means of
aminolysis. For comparison, a soda pulp and the corresponding wood material were also analysed.
The results obtained demonstrate that during the course of a kraft cook there is an increase in the
content of phenolic hydroxyl groups per unit weight of residual lignin. Throughout the cook the
content of these groups is, however, much lower than the content of phenolic hydroxyl groups
present in the dissolved kraft lignins. It was further found that the residual lignin in a soda pulp
contains a significantly smaller amount of phenolic hydroxyl groups than the lignin in a kraft pulp
at the same degree of delignification.
The importance of these results is discussed with reference to known features of alkaline pulping
processes.
ADDRESS OF THE AUTHORS: Swedish Forest Products Research Laboratory, Box 5604, S-114
86 Stockholm, Sweden.
Numerous studies on the reactions of lignin in alkaline pulping have revealed the fundamental
importance of the presence of free phenolic hydroxyl groups (1, 2). In lignin units containing such
groups, the alkaline conditions prevailing in kraft and soda cooking liquors lead to the formation of
methylene quin-ones. These structures are unstable and are readily attacked by nucleophiles such as
hydrogensulfide ions or various organic carbanions present in the reaction medium. Alternatively,
methylene quinones are stabilized via elimination reactions leading to the formation of stilbene and
styrene structures. The extent and distribution of these reactions determine the structural
modification of lignin, including lignin fragmentation and condensation, taking place during
pulping. In addition to these reactions, the dissolution of lignin in the delignification of wood must
be dependent upon the presence of hydrophilic groups (cf. ref. 2). In kraft and soda cooking these
consist mainly of phenolic hydroxyl groups, minor amounts of carbox-ylic acid groups also being
present (3).
In a previous paper in this series, the content of free phenolic hydroxyl groups present in the
residual lignin in a series of kraft pulps was calculated indirectly (4). These calculations were based
upon the quantities of isolated low molecular mass carboxylic acids formed by oxidative
degradation of phenolic structures present in the residual lignin.
In the present work, the amount of phenolic hydroxyl groups present in the residual lignin of the
same series of kraft pulps has been quantitatively determined by means of aminolysis. In addition,
the corresponding wood material and a soda pulp have been analysed. The analytical procedure
employed involves the selective deacetylation of phenolic acetates using pyrrolidine by adaptation
of a method earlier developed for the analysis of phenolic hydroxyl groups in pure lignin materials
(5, cf. also refs. 6,7).
Results and discussion
Analytical method
The selective determination of phenolic hydroxyl groups by aminolysis utilizes the fact that there is
a large difference in the rates of deacetylation of aromatic and aliphatic acetates in the presence of
pyrrolidine. A rapid and quantitative formation of acetyl-pyrrolidine takes place in an amount,
corresponding to the amount of aromatic acetyl groups originally present in the sample. The method
has been used on acetylated lignin model compounds (6), on milled wood lignin (5) and on various
kraft lignins (5,8). It has also been demonstrated that under more severe reaction conditions the
addition of pyrrolidine makes it possible to eliminate virtually all types of acetyl groups present in a
sample. This can be utilized e.g. for the quantitative determination of the content of acetyl groups in
wood (7).
In solid materials such as wood and pulps, two major difficulties arise, viz. the achievement of a
complete acetylation of all the phenolic hydroxyl groups present and the selective deacetylation of
these in the presence of a vast amount of various aliphatic acetyl groups in both lignin and
carbohydrates. In a separate experiment it was demonstrated that acetylated cellulose gave rise to a
small amount of acetyl-pyrrolidine when treated with pyrrolidine under mild conditions (see
Experimental). Approximately one equivalent of acetylpyrrolidine was shown to be formed from ßglucose pentaacetate whereas the formation of acetylpyrrolidine from sorbitol hexaacetate was
negligible. These results demonstrate that the reducing end groups present in polysaccharides are
able to undergo aminolysis of their acetates at rates which are comparable with those of aromatic
acetates. Unrealistically high values for the content of phenolic hydroxyl groups were indeed found
in aminolysis experiments carried out directly on acetylated wood and pulp samples. It is thus
necessary to eliminate all reducing end groups in the samples by sodium borohydride reduction
prior to acetylation. This reaction step was evaluated on pulp samples using two different methods
of reduction viz. reaction with sodium borohydride in pure water (9) and in aqueous buffer solution
at pH = 9.6—9.8 (10).
After subsequent acetylation and aminolysis reactions, the samples were analysed for their contents
of acetylpyrrolidine. It was found that reduction carried out in a pure water suspension gave by far
the greater reproducibility in the aminolysis reaction. This is assumed to be due to the different
degrees of swelling of the wood and pulp materials in water and in buffer
Table 1. Content of phenolic hydroxyl groups, determined by aminolysis, in the residual lignin in
wood, in kraft pulps and in a soda pulp.
1
Determined as Klason lignin and acid soluble lignin
Assumed relative molecular mass for one phenylpropane unit = 183(12)
3
Mean value of two analyses
4
Mean value of three analyses
5
Soda pulp
2
solutions (cf. ref. 11). Samples present in the aqueous phase in a highly swollen state after the
reduction step were thus found to be easily solvent exchanged into pyridine via acetone while still
maintaining a high accessibility. The subsequent acetylation reaction could then be carried out as a
one-step procedure. Otherwise repeated acetylation was necessary nevertheless leading to an
inferior aminolysis reaction due to difficulties in achieving a reproducible and quantitative
acetylation.
After the various reaction steps had been optimized, the analytical procedure was found to give
acetylpyrrolidine with a high degree of reproducibil-ity (standard deviation ± 3%) between the
experiments. From the amounts of acetylpyrrolidine found in the different wood and pulp samples,
the corresponding frequency of phenolic hydroxyl groups per 100 phenylpropane units was
calculated assuming a relative molecular mass of 183 per phenylpropane unit in the lignin
irrespective of pulp yield (table 1).
The value of 183 used for the relative molecular mass comes from elemental analysis data on native
spruce lignin and is based on 9.0 carbon atoms in the average phenylpropane unit (12). This number
of carbon atoms may be somewhat too high since recent analytical data on the structure of native
spruce lignin implies a certain "deficit" of side chains compared with the number of aromatic rings
(13, 14). For lignins present in pulp fibers the structural data available permit no certain conclusions
to be drawn concerning the composition of the phenylpropane units.
However, it has been shown that during pulping the residual lignin in the fibers undergo various
modification reactions (4, 15, 16). In addition kraft pulps contain small but increasing amounts of
sulfur as the cook proceeds (17). This sulfur can be assumed to be chemically bound to the lignin.
Therefore, the base value of 183 used in the calculations of the phenolic hydroxyl groups can lead
to somewhat erroneous results and may need revision as more analytical data on residual lignins
become available.
Content of phenolic groups
The frequency of phenolic hydroxyl groups present in the wood and pulp samples is plotted as a
function of cooking time in fig. 1 together with values obtained for the corresponding dissolved
kraft lignins (data from ref. 8). Thus, it can clearly be seen that there is a large difference in the
number of phenolic hydroxyl groups present during the kraft cook.
It seems reasonable to assume that in order to make a lignin fragment soluble in the cooking liquor,
a minimum (but rather high) amount of phenolic hydroxyl groups is a necessary prerequisite. When
the kraft cook approaches its maximum cooking temperature, i.e. at the beginning of the bulk
delignifi-cation phase (cf. ref. 4), a slight drop in the content of phenolic hydroxyl groups in the
residual lignin is observed. At this point in the cook a certain change in the chemistry of
delignification is therefore indicated.
Such changes have been observed before. Thus, it has been found that at the beginning of the bulk
Fig. 1. Frequency of phenolic hydroxyl groups as a function of cooking time in wood and in the
residual lignin in kraft pulps (open symbols, values from table 1) and in the corresponding
dissolved kraft lignins (filled symbols, values from ref. 8).
delignification phase a larger amount of vinyl aryl ether structures formed from phenylpropane-ßaryl ether structures is present both in the residual and in the dissolved lignin (15). Furthermore, in
this part of the kraft cook, lignin samples precipitated from the black liquors have been found to
contain rather high amounts of elemental sulfur (4). In pulping experiments it has been
demonstrated that, at the beginning of the bulk delignification phase, a high sulfidity level is of
fundamental importance in order to minimize the amount of residual lignin present at the transition
point between the bulk and the final delignification phases (18).
All these results imply that, during a certain part of a kraft cook, starting approximately when the
bulk delignification phase is reached, the cook shows some of the characteristics expected to play a
dominant role in soda pulping. The observed drop in the content of phenolic hydroxyl groups may
thus be explained by a reduction in the degree of phenylpropane-ß-aryl ether cleavage due in turn to
a decreased sulfidity level at this point of the cook.
Towards the end of the kraft cook the content of phenolic hydroxyl groups was found to level off.
In the indirect analytical method employed previously (see above) a decrease in this content was in
fact calculated (4). This result is not unexpected since the possibility of creating new phenolic
hydroxyl groups should decrease as the cook proceeds both because of a decreasing concentration
of hydroxyl ions in the cooking liquor (8, cf. réf. 2) and because of a decreasing amount of
phenylpropane-ß-aryl ether structures present in the residual lignin (15).
In the present work, a value of 27 phenolic hydroxyl groups per 100 phenylpropane units was found
in the residual lignin of a kraft pulp having a kappa number of 31.4. For a residual lignin, obtained
from a kraft pulp (kappa number 35.6) after enzymatic hydrolysis of the polysaccharides, values of
36 and 38 phenolic hydroxyl groups have recently been published (16). These high values may
possibly arise as a result of the enzymatic treatment which was carried out under weakly acidic
conditions which offer the possibility of acid hydrolysis of residual aryl ether bonds in the lignin.
The content of phenolic hydroxyl groups in wood has been investigated before using ultraviolet
spec-troscopy (19,20, cf. also ref. 21) and pyrolysis in combination with gas chromatography (22)
as experimental techniques. Oxidative degradation and quantitative determination of the resulting
carboxylic acids has also been employed as an indirect method of analysis (4,23). In the present
work, the pine wood used was found to contain 13 phenolic hydroxyl groups per 100
phenylpropane units. This value is in excellent agreement with the most recent data obtained for the
secondary wall and middle lamella lignin in spruce wood (15 and 10 phenolic hydroxyl groups
respectively) (19).
Conclusions
Aminolysis of acetylated wood and pulp samples with pyrrolidine can be used for the quantitative
determination of phenolic hydroxyl groups. The accuracy of the method is, however, critically
dependent upon both the elimination of all reducing sugar end groups and the quantitative
acetylation of the hydroxyl groups. If these reaction steps are properly controlled the method gives
highly reproducible results.
The contents of phenolic hydroxyl groups in the residual lignin in kraft pulps were found to be
much lower than the values earlier found in dissolved kraft lignins. In order to make a lignin
fragment soluble in the cooking liquor a large number of new phenolic hydroxyl groups must
therefore be created during the cook by cleavage of aryl ether bonds. Towards the end of the kraft
cook, such cleavage reactions become less and less abundant so that the residual lignin becomes
more resistant to dissolution in the cooking liquor. During a soda cook, the cleavage of aryl ether
bonds is less efficient than in a kraft cook and this difference is reflected in the low number of
phenolic hydroxyl groups found to be present in the soda pulp lignin.
Experimental
Preparation of samples
The pulp samples were obtained from kraft (soda) cooks of pine wood (30% sulfidity, 18%
effective alkali and 25% effective alkali respectively) and were the same as those previously used.
The preparation and purification of these samples as well as of the wood sample have been
thoroughly described in ref. 4.
Cellulose acetate, ß-D-glucose pentaacetate and sorbitol hexaacetate were obtained as commercial
products (Fluka AG, Sigma Chemical Company).
Analytical procedure
An accurately weighed amount of wood or pulp sample corresponding to approximately 25 mg of
lignin was placed in a centrifuge tube (volume 10 ml) equipped with a Teflon screw cap. (In these
tubes an upper limit of approximately 300 mg of sample could be used.) To the sample was added 4
ml of water (distilled and deionized water was used throughout) and subsequently 50 mg of purified
sodium borohy-dride.
Without closing the screw cap, the mixture was immediately placed in a laboratory centrifuge (Wifug, Sweden) at 1250 r.p.m. for 60 minutes. (This procedure was necessary in order to prevent part
of the sample from escaping from the reaction mixture together with the bubbles of hydrogen being
formed.) Subsequently, a further amount of sodium borohy-dride was added to the reaction mixture
so that the total quantity of sodium borohydride added was equivalent to the amount of sample
used.
After the screw cap had been loosely closed, the reaction mixture was placed in the centrifuge for 2
more hours and then allowed to stand for 2 days with gentle magnetic stirring. To the sample was
then added 3 ml of water, after which the tube was closed and centrifuged at 5700 r.p.m. for 15
minutes. The aqueous solution was removed and the residue washed with 7 ml of water by
homogenization and centrifugation. The washing was carried out three times (until the liquid was
neutral).
Solvent exchange was carried out by first treating the sample with 7 ml of acetone. After
homogenization and centrifugation, which was again carried out three times, 7 ml of pyridine was
added. This procedure was also carried out three times. To the centrifuged residue, 2 ml of pyridine
and 2 ml of acetic anhydride were subsequently added and the reaction mixture was allowed to
stand with gentle stirring for 3 days at room temperature (or alternatively for 20 hours at 40°C). The
reaction mixture was then violently stirred for 30 minutes in the presence of 5 ml of diethyl ether
with the screw cap closed and subsequently centrifuged. The residue was washed three times with 3
x 7 ml of diethyl ether as described above and dried in vacuo overnight at 50°C.
The acetylated sample was suspended in 1.0 ml of dioxane containing 5.36 mg/ml of 1propionylpyrro-lidine (internal standard) and an additional 0.5—1.0 ml of dioxane was added. The
mixture was allowed to homogenize while being stirred for approximately 1 hour. The aminolysis
reaction was started by adding 3 ml of dioxane containing 50 mg/ml of pyrrolidine to the centrifuge
tube, tightly closing the cap and vigorously shaking the tube to homogenize the mixture, which was
then allowed to stand with stirring.
The formation of 1-acetylpyrrolidine was followed as a function of time by gas Chromatographic
analysis on a 2.5 mx 1/8" I.D. glass column filled with 5% Castorwax on HP Chromosorb G, 80—
100 mesh at a column temperature of 180°C, injector 230°C and detector 250°C. A flow rate of 20
ml/min of nitrogen was used. Peak quantification was carried out with an electronic integrator. The
reaction mixture was centrifuged for 3 minutes at 5700 r.p.m. prior to withdrawal of each sample in
order to get a clear solution. The tube contents were then homogenized again by rapid shaking and
subsequent stirring.
The first gas Chromatographic analysis was done after approximately 30 minutes and subsequently
5—6 additional analyses were carried out at intervals of about 15 minutes. The amount of 1acetylpyrrolidine formed and thus the amount of phenolic hy-droxyl groups present was calculated
as described in ref. 5.
In the aminolysis of acetylated cellulose, glucose and sorbitol, approximately 100 mg of sample was
used in each experiment. The reaction conditions were the same as those described above.
In the present work, thoroughly milled samples of wood and pulp have been used throughout. A few
experiments with kraft pulps milled through a 40 mesh screen in a Wiley mill gave similar results.
The quantitative handling of coarse samples is, however, extremely difficult unless a more powerful
centrifuge than the one used in the present work is available.
Acknowledgement
The authors are indebted to Dr. Tommy Iversen and Dr. Per Mànsson for many valuable discussions
during the course of this work.
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(Manuscript received June 6, 1984. Accepted August 1984.)