Heterogeneity of Lignin - American Chemical Society

Chapter 3

Heterogeneity of Lignin Dissolution and Properties of Low-Molar-Mass Components

Downloaded by JOHNS HOPKINS UNIV on September 23, 2013 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0397.ch003

Kaj Forss, Raimo Kokkonen, and Pehr-Erik Sågfors The Finnish Pulp and Paper Research Institute, P.O. Box 136, 00101 Helsinki, Finland

On heating pre-extracted spruce wood meal for 48 h with a 60:40 v/v mixture of dioxane and 0.5 M phosphate buffer pH 6.8, low molar mass lignins (i.e., hemilignins and breakdown products of high molar mass glycolignin) amounting to 24 percent of total lignin are dissolved. By extraction of the solution with n-hexane the dioxane is transferred to the hexane phase together with hexane-dioxane soluble lignins. Removal of dioxane results in precipitation of dark brown hydrophobic lignin. Water soluble lignins remain in solution. When wood is heated first with the buffer solution alone and then with the mixture of dioxane and buffer solution, only small amounts of the hydrophobic lignin dissolve. It seems that during heating with the aqueous buffer solution the hydrophobic lignin, which may form the residual lignin in kraft pulp, becomes irreversibly bound to the fibers. It has earlier been shown that spruce wood (Picea abies) lignin is a group of compounds, consisting of 80-85% polymeric carbohydrate-bound lignin, which we are designating glycolignin and 15-20% of a group of low molar mass lignins, monomers, dimers and oligomers, which we are collectively designating hemilignins (1). The purpose of this work is to elucidate the role of hemilignins and glycolignin in the formation of color in pulp. The aim of the present part of the work was to develop a selective method for dissolution of low molar mass lignins without dissolving the glycolignin. Heating wood with an acid bisulfite solution causes sulfonation of both the hemilignins and the glycolignin. The hemilignins are sulfonated and dissolved before the glycolignin. In all probability, due to its bonding with carbohydrates, the dissolution of glycolignin is strongly affected by the pH 0097-6156y89/0397-0029$06.00A) © 1989 American Chemical Society

In Lignin; Glasser, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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LIGNIN: PROPERTIES A N D MATERIALS


of the c o o k i n g l i q u o r . F o r instance, o n h e a t i n g w o o d w i t h a bisulfite s o l u t i o n of p H 5.5 at 130°C, b o t h h e m i l i g n i n s a n d g l y c o l i g n i n are sulfonated b u t d i s s o l u t i o n is l i m i t e d almost entirely to the h e m i l i g n i n sulfonates. However, b y s u b j e c t i n g the w o o d m a t e r i a l to a subsequent a c i d - c a t a l y z e d h y d r o l y s i s , the g l y c o l i g n i n - c a r b o h y d r a t e bonds are broken a n d the g l y c o l i g n i n sulfonic acids dissolve (1). T h e conclusion t h a t a l l low m o l a r mass lignins are h e m i l i g n i n s is, h o w ever, a n o v e r s i m p l i f i c a t i o n since i t has been s h o w n t h a t , o n h e a t i n g a n aqueous s o l u t i o n of g l y c o l i g n i n sulfonic a c i d acidified w i t h h y d r o c h l o r i c a c i d or sulfur dioxide a n d bisulfite, a m o n o m e r i c sulfonated h y d r o l y s i s p r o d u c t is f o r m e d (2). A c o m p a r i s o n of the rates of d i s s o l u t i o n shows t h a t g l y c o l i g n i n is d i s solved m u c h faster i n the k r a f t process t h a n i n the a c i d bisulfite process (3). I n spite of t h i s , k r a f t p u l p contains more r e s i d u a l l i g n i n t h a n sulfite p u l p a n d is more difficult t o bleach. T h i s observation leads to the view t h a t the so-called r e s i d u a l l i g n i n m a y not originate f r o m undissolved g l y c o l i g n i n b u t f r o m low m o l a r mass l i g n i n s , i.e., h e m i l i g n i n s or b r e a k d o w n p r o d u c t s f r o m g l y c o l i g n i n deposited o n the fibers at a n early stage of the cook. In order to investigate t h i s possibility, a series of k r a f t cooks was p e r f o r m e d . C o t t o n w o o l was placed i n each digester. T h e chlorine n u m b e r , ( I S O 3260), of the c o t t o n wool i n F i g u r e 1 shows t h a t the d e p o s i t i o n of lignins takes place at a n early stage of the cook. These colored l i g n i n s c o u l d not be removed b y w a s h i n g the c o t t o n w o o l w i t h s o d i u m h y d r o x i d e solution. T h i s result s u p p o r t s the hypothesis t h a t the r e s i d u a l l i g n i n i n k r a f t p u l p is formed f r o m some h e m i l i g n i n s or the d e g r a d a t i o n p r o d u c t s of g l y c o l i g n i n . I n sulfite p u l p i n g , s u l f o n a t i o n of the h e m i l i g n i n s a n d the h y d r o l y s i s p r o d u c t of g l y c o l i g n i n m a y prevent these c o m p o u n d s f r o m r e a c t i n g f u r t h e r a n d f r o m b e c o m i n g deposited o n the fibers. In m e c h a n i c a l w o o d p u l p i n g , the g l y c o l i g n i n r e m a i n s u n d i s s o l v e d , whereas some of the h e m i l i g n i n s are dissolved, their s o l u b i l i t i e s i n water bei n g the m a i n l i m i t i n g factor. It is thus possible t h a t some of the h e m i l i g n i n s are also responsible for the y e l l o w i n g of m e c h a n i c a l p u l p . Delignification of Spruce W o o d M e a l on H e a t i n g w i t h W a t e r In order to show the a m o u n t a n d m o l a r masses of lignins t h a t can be d i s solved o n h e a t i n g w o o d w i t h w a t e r , 5 g p o r t i o n s of w o o d m e a l e x t r a c t e d w i t h cyclohexane-ethanol (4) were heated w i t h 100 m L water at 150°C. F i g u r e 2 shows t h a t d e l i g n i f i c a t i o n reaches a m a x i m u m of 8% after 4 h of h e a t i n g . P r o l o n g e d h e a t i n g caused a redeposition of lignins o n the w o o d material. D u r i n g h e a t i n g the p H d r o p p e d below 4 due to the f o r m a t i o n of acetic a c i d . T h e a c i d caused h y d r o l y s i s of hemicelluloses a n d f o r m a t i o n of f u r f u r a l . It is thus possible t h a t the d e l i g n i f i c a t i o n was caused by h y d r o l y t i c cleavage of i n t e r u n i t linkages. F i g u r e 3 shows t h a t the dissolved l i g n i n s



3. FORSS ET AL. Heterogeneity of Lignin


31

32


were m o n o m e r s a n d oligomers w i t h m o l a r masses of less t h a n 1000 g / m o l (7).


Delignification of Spruce W o o d Phosphate Buffer Solution

Meal on Heating with

Neutral

T o test the a s s u m p t i o n t h a t the d e l i g n i f i c a t i o n t a k i n g place w h e n spruce w o o d m e a l is heated w i t h water is a result of a c i d c a t a l y z e d h y d r o l y s i s , w o o d m e a l samples were heated w i t h 0.5 M phosphate buffer s o l u t i o n at p H 6.8 ( F i g . 4). F i g u r e 4 shows t h a t 8% d e l i g n i f i c a t i o n was reached b y h e a t i n g w i t h buffer s o l u t i o n b u t t h a t the rate was slower t h a n w h e n h e a t i n g w i t h w a t e r . F i g u r e 5 shows t h a t , o n h e a t i n g w i t h buffer s o l u t i o n , the l i g n i n s dissolved are also of low m o l a r mass. N o furfural was formed i n t h i s e x p e r i m e n t . It can be concluded t h a t the d i s s o l u t i o n of lignins w i t h water a n d w i t h buffer s o l u t i o n is not the result of a n a c i d - c a t a l y z e d h y d r o l y t i c cleavage of c h e m i c a l bonds. However, as s h o w n b y S a k a k i b a r a et al. (5), a - O - 4 bonds i n m o d e l c o m p o u n d s dissolved i n water-dioxane (1:1) are r e a d i l y cleaved o n h e a t i n g . Delignification of Spruce W o o d M e a l on H e a t i n g w i t h M i x t u r e s of Phosphate Buffer a n d Dioxane Since the reason for the l i m i t e d degree of delignification reached o n h e a t i n g w o o d w i t h water or w i t h buffer s o l u t i o n m a y be the low s o l u b i l i t y of l i g n i n s i n w a t e r , w o o d m e a l was heated w i t h m i x t u r e s of phosphate buffer a n d dioxane ( F i g . 6). F i g u r e 6 shows t h a t , o n h e a t i n g w o o d m e a l w i t h buffer s o l u t i o n for 6 h at 150°C, 6% of the t o t a l l i g n i n was dissolved, b u t o n h e a t i n g w i t h dioxane o n l y 4 % dissolved. However, o n h e a t i n g w i t h a m i x t u r e of buffer a n d dioxane 1 4 % of the l i g n i n dissolved. U s i n g reversed-phase c h r o m a t o g r a p h y ( F i g . 7), i t was possible to show t h a t , o n h e a t i n g w i t h buffer s o l u t i o n , most of the l i g n i n s dissolved were of a h y d r o p h i l i c n a t u r e , e l u t i n g between 0 a n d 30 m i n u t e s , whereas dioxane preferentially dissolved h y d r o p h o b i c l i g n i n s , w h i c h eluted between 30 a n d 80 m i n u t e s . A s h e a t i n g w i t h buffer s o l u t i o n was f o u n d preferentially to dissolve h y d r o p h i l i c molecules, an a t t e m p t was m a d e to delignify b y h e a t i n g w o o d m e a l successively w i t h buffer a n d w i t h a buffer-dioxane m i x t u r e . F i g u r e 8 shows t h a t h e a t i n g w i t h buffer resulted i n a m a x i m u m d e l i g n i f i c a t i o n of about 8% after 8 h , at w h i c h p o i n t some of the dissolved l i g n i n was redeposited o n the w o o d m a t e r i a l . A f t e r 48 h of h e a t i n g w i t h buffer, 5 % of the l i g n i n was i n s o l u t i o n . W h e n the w o o d residue was subsequently heated i n dioxane-buffer, a n a d d i t i o n a l 3 % of the t o t a l l i g n i n dissolved instantaneously. Thereafter almost no l i g n i n was dissolved. However, b y h e a t i n g w o o d m e a l w i t h buffer-dioxane s o l u t i o n , 2 4 % of the l i g n i n was dissolved after 48 h . It c a n be concluded t h a t h e a t i n g w i t h buffer alone i r r e v e r s i b l y b o u n d l i g n i n to the w o o d m a t e r i a l .


FORSS ETAL.

3.

Heterogeneity of Lignin

33


i — ^ 280 nm 1.2n

—

i 10000

RELATIVE RETENTION VOLUME i 1 1500 1000 MOLAR M A S S

i i 5000 3000

F i g u r e 3. L i g n i n s dissolved a n d f u r f u r a l f o r m e d o n h e a t i n g spruce w o o d m e a l w i t h w a t e r for 6 h at 150°C. C o l u m n : Sephadex G - 5 0 . E l u e n t : 0.5 M NaOH.

DELIGNIFICATION,%

1 01

I 0

I 1

I 2

I 3

I 4

I

5

I I 1—I 6 7 8 HEATING TIME AT 1 5 0 C , h o

F i g u r e 4. D e l i g n i f i c a t i o n o n h e a t i n g spruce w o o d m e a l w i t h p h o s p h a t e buffer, p H 6.8, at 150°C.



34


280nm

i 10000

» » 5000 3000

RELATIVE RETENTION i i 1500 1000 MOLAR M A S S

VOLUME

F i g u r e 5. L i g n i n s dissolved o n h e a t i n g spruce wood m e a l w i t h p h o s p h a t e buffer, p H 6.8, for 6 h at 150°C. C o l u m n : Sephadex G - 5 0 . E l u e n t : 0.5 M NaOH.

F i g u r e 6. D e l i g n i f i c a t i o n o n h e a t i n g spruce w o o d m e a l for 6 h at 150°C w i t h 0.5 M phosphate buffer ( p H 6.8) a n d dioxane i n various v o l u m e p r o p o r t i o n s .


FORSS ETAL.


3.

35

Heterogeneity ofLignin

F i g u r e 7. L i g n i n s dissolved o n h e a t i n g spruce w o o d m e a l w i t h p h o s phate buffer 0.5 M ( p H 6.8) a n d w i t h d i o x a n e for 6 h at 150°C. C o l u m n : Spherisorb O D S . E l u t i o n : G r a d i e n t e l u t i o n w i t h p h o s p h a t e buffermethanol.

DELIGNIFICATION,% 251B: DIOXANE/BUFFER 20 15 C: DIOXANE/BUFFER 10 5

u

k-S-x-x—x - S — TTTV — -

^

-.x x

A

xx — — xx *

"Er—

x

x

x to B

Heterogeneity of Lignin - American Chemical Society

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