Modification of Lignin at the 2- and 6-Positions of the Phenylpropanoid

Chapter 26

Modification of Lignin at the 2- and 6-Positions of the Phenylpropanoid Nuclei Gerrit H . van der Klashorst

Downloaded by DICLE UNIV on November 4, 2014 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0397.ch026

Renewable Resources Chemicals Programme, Division of Processing and Chemical Manufacturing Technology, CSIR, P.O. Box 395, Pretoria, South Africa

Lignin model compounds were reacted with formaldehyde in acid medium at positions meta to the aromatic hydroxy groups. Both phenolic and etherified phenolic lignin model compounds were shown to give fast polymerization or hydroxymethylation of the meta positions depending on the conditions used. This reaction differs from the reaction of formaldehyde with phenolic lignin model compounds under alkaline conditions, where the reaction with formaldehyde always occurs at positions ortho/para to the aromatic hydroxy group. The conditions developed for the polymerization and hydroxymethylation were also evaluated on industrial lignin. The reaction of formaldehyde with the meta positions of lignin clearly have considerable potential for the use of lignin, particularly heavily condensed alkali lignin, in polymeric applications. L i g n i n , present as a waste m a t e r i a l i n the spent liquors o f the p u l p i n g i n d u s t r y , constitutes a p o t e n t i a l l y useful raw m a t e r i a l for the p r o d u c t i o n o f various p o l y m e r i c p r o d u c t s . T h i s has repeatedly been d e m o n s t r a t e d i n the past b y i t s a p p l i c a t i o n i n p r o d u c t s r a n g i n g f r o m w o o d adhesives t o plastics (1). I n these applications the l i g n i n is crosslinked t o increase i t s molecular mass or to f o r m a r i g i d three-dimension ally crosslinked s t r u c t u r e . T h e features o n l i g n i n t h a t were u t i l i z e d for these p o l y m e r i z a t i o n or m o d i f i c a t i o n reactions are as follows (compare F i g . 1): 1. 2. 3. 4.

phenolic hydroxy group; aliphatic hydroxy group; u n s u b s t i t u t e d 3- or 5-positions o n C9 u n i t s ; a n d structures t h a t c a n f o r m q u i n o n e m e t h i d e intermediates. 0097-6156/89/0397-0346$06.00/0 © 1989 American Chemical Society

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


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E x a m p l e s where the phenolic h y d r o x y groups were u t i l i z e d i n c l u d e the p r e p a r a t i o n of l i g n i n epoxies b y reaction w i t h e p i c h l o r o h y d r i n (2), ester ifications w i t h b i s - a c i d chlorides (3) a n d c y a n u r i c chloride (4), a n d p o l y m e r i z a t i o n w i t h a z i r i d i n e s (5). T h e a l i p h a t i c h y d r o x y groups have been used as the p o l y o l c o m p o n e n t of urethane resins (6). In m a n y of these a p p l i c a t i o n s p h e n o l i c h y d r o x y groups have been a l k o x y l a t e d to afford more a l i p h a t i c alcohols. T h e u n s u b s t i t u t e d 3- a n d 5-positions o n the C9 u n i t s of a l k a l i l i g n i n have been used i n two types of p o l y m e r i z a t i o n r e a c t i o n s — e l e c t r o p h i l i c d i s placement reactions a n d free r a d i c a l o x i d a t i v e c o u p l i n g reactions. The e l e c t r o p h i l i c displacement reaction approaches i n c l u d e d the use of l i g n i n in p h e n o l formaldehyde resins (7), urea formaldehyde resins (8), a n d the c r o s s l i n k i n g of l i g n i n w i t h d i a z o n i u m salts (9). O x i d a t i v e c o u p l i n g reactions v i a free r a d i c a l m e c h a n i s m s y i e l d e d l i g n i n based adhesives o f h i g h s t r e n g t h In t h i s paper a n a d d i t i o n a l a p p r o a c h t h a t can be used for the m o d i f i c a t i o n or p o l y m e r i z a t i o n of especially a l k a l i l i g n i n is discussed. T h a t is: T h e Modification of Lignin Phenylpropane Units

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6-Positions

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It was s h o w n b y S a r k a n e n et al. t h a t h a r d a n d softwood m o d e l c o m p o u n d s undergo p r o t o d e d e u t e r a t i o n i n acidic m e d i u m preferably o n positions 2 a n d 6 (9). T h e y suggested t h a t the observed 2, 6 - p r o t o d e d e u t e r a t i o n reactions s h o u l d also h o l d for other reagent species. E l e c t r o p h i l e s other t h a n protons were indeed s h o w n to react w i t h l i g n i n m o d e l c o m p o u n d s at the 2- a n d 6-positions i n acidic m e d i a : K r a t z l a n d W a g n e r investigated the reaction of p a r a p h e n o l i c b e n z y l alcohols i n a l k a line a n d acidic solutions (10). W h e n 4 - h y d r o x y - 3 - m e t h o x y b e n z y l a l c o h o l 4 was reacted w i t h the 4 - a l k y l s u b s t i t u t e d p h e n o l 5 under alkaline c o n d i tions, the expected ortho l i n k e d p r o d u c t (6) was isolated. However, under acidic c o n d i t i o n s the methylene linkage formed meta to the phenolic h y d r o x y group ( F i g . 2). Y a s u d a a n d T e r a s h i m a (11) recently showed t h a t l i g n i n under K l a s o n l i g n i n d e t e r m i n a t i o n c o n d i t i o n s afforded m e / a - l i n k e d p r o d u c t s . V a n i l l y l a l c o h o l (4) y i e l d e d after reflux i n 5 % sulfuric a c i d , m d a - l i n k e d p r o d u c t (8) ( F i g . 3). These examples thus clearly show 4 - h y d r o x y - 3 - m e t h o x y p h e n y l a n d 4h y d i O x y - 3 , 5 - d i m e t h o x y p h e n y l a l k a n e s ( t y p i c a l of the l i g n i n s t r u c t u r e ) to be reactive at positions 2 a n d 6 towards e l e c t r o p h i l i c s u b s t i t u t i o n reactions under acidic c o n d i t i o n s . T h e subject m a t t e r of t h i s paper deals w i t h the development of procedures whereby the 2- a n d 6- (i.e., meta) positions are u t i l i z e d for the p o l y m e r i z a t i o n a n d m o d i f i c a t i o n of l i g n i n . Polymerization of Lignin Model Compounds. T h e controlled polymerizat i o n of several l i g n i n m o d e l c o m p o u n d s at the "meta" positions was subseq u e n t l y a t t e m p t e d (13): T h e h a r d w o o d l i g n i n m o d e l 13 was reacted i n acidic aqueous dioxane to afford d i m e r s , t r i m e r s , tetramers or higher oligomers ( F i g . 4 A ) .

American Chemical Society library

St., In 1155 Lignin; 16th Glasser, W.,HM» et al.; DC.Society: 20036 ACS Symposium Series;Washington, American Chemical Washington, DC, 1989.

LIGNIN: PROPERTIES AND MATERIALS


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F i g u r e 2. T h e r e a c t i o n of v a n i l l y l alcohol (4) w i t h a reactive p h e n o l ( £ ) .


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Figure 3. Self-condensation of vanillyl alcohol ( 4 ) and 3,4-dimethoxybenzyl alcohol ( 8 ) in acid (11).




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Ο


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T h e etherified h a r d w o o d l i g n i n m o d e l H reacted at a s i m i l a r rate as the phenolic m o d e l i n d i c a t i n g the etherification of the phenolic g r o u p has a s m a l l effect o n the reaction rate. W h e n t h i s reaction was repeated at 5 5 ° C w i t h a n excess f o r m a l d e h y d e , some m et α-hydroxy m e t h y l a t e d p r o d u c t s were obtained (Fig. 4 B ) . P h e n o l i c a n d etherified softwood l i g n i n m o d e l c o m p o u n d s were also successfully crosslinked at the meta positions. S p e c i a l care was t a k e n t o assign the s t r u c t u r e o f the d i m e r (22) ( F i g . 4 C ) . W i t h N M R - S P I (selec tive p o p u l a t i o n inversion) techniques it was u n e v o c a b l y proved t h a t the m e t h y l e n e linkage was s i t u a t e d at the 6-positions (13).


Mechanism T h e m e c h a n i s m for the reactions o n the 2- a n d 6-positions is based o n t w o factors ( F i g . 5): i n d u c t i o n b y the a l k y l g r o u p ; a n d resonance effects o f the methoxy group. A l k y l groups w i t h o u t a n y o x y - f u n c t i o n o n the α - c a r b o n are electron d o n a t i n g by i n d u c t i o n a n d w i l l enhance the electron density o n the 2-, 4-, a n d 6-positions o f the l i g n i n a l k y l r i n g s . D u r i n g alkaline p u l p i n g a m a j o r p o r t i o n of α-oxy-functions are lost due to condensation reactions, i.e., m e t h y l e n e linkage f o r m a t i o n w i t h the α-carbon as b r i d g e . A l k a l i l i g n i n , a n d especially h i g h l y condensed a l k a l i l i g n i n , can therefore be expected to have a h i g h n u m b e r of ηοη-α-oxy-substituted side chains. A l k a l i l i g n i n c a n therefore be expected to have a h i g h r e a c t i v i t y towards m o d i f i c a t i o n at positions 2 a n d 6. D u r i n g the i n v e s t i g a t i o n o n h a r d - a n d softwood m o d e l c o m p o u n d s i t was observed t h a t the d i m e t h o x y m o d e l c o m p o u n d s ( h a r d w o o d ) were a b o u t five times more reactive t h a n the m o n o m e t h o x y m o d e l c o m p o u n d s . T h i s illustrates the beneficial effect of the presence of the second m e t h o x y g r o u p (by resonance effects). F r o m the results o n the m o d e l c o m p o u n d s i t was clear t h a t : • 1 - a l k y l s u b s t i t u t e d s y r i n g y l a n d g u a i a c y l m o d e l c o m p o u n d s react w i t h f o r m a l d e h y d e i n acidic m e d i u m at positions 2 a n d 6. • T h e formed b e n z y l i c alcohols were u n s t a b l e i n a c i d a n d reacted fast w i t h a second m o d e l c o m p o u n d to f o r m m e t h y l e n e l i n k e d p o l y m e r s . • E t h e r i f i e d phenolic models reacted at the same p o s i t i o n s a n d s i m i l a r rates as d i d the phenolic models. • T h e d i m e t h o x y , i.e., h a r d w o o d , m o d e l c o m p o u n d s reacted faster t h a n d i d the m o n o m e t h o x y m o d e l c o m p o u n d s . Meta

Hydroxymethylation

In the above p a r a g r a p h i t was s h o w n t h a t the l i g n i n C9 u n i t s can be p o l y m e r i z e d at the 2- a n d 6-positions w i t h formaldehyde i n a c i d i c aqueous dioxane. T h e p o l y m e r i z a t i o n reaction proceeds p r e s u m a b l y v i a a h y d r o x y m e t h y l a t e d i n t e r m e d i a t e w h i c h was isolated i n low y i e l d s o n l y w h e n large excesses of formaldehyde were e m p l o y e d . If the h y d r o x y m e t h y l a t i o n o f the meta p o s i t i o n can be achieved i n h i g h y i e l d it w o u l d clearly afford a very


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reactive i n t e r m e d i a t e . T h e h y d r o x y m e t h y l a t i o n of the meta p o s i t i o n was subsequently a t t e m p t e d o n m o d e l c o m p o u n d s (14). T h e h y d r o x y m e t h y l a t i o n of the h a r d w o o d m o d e l (13) was first a t t e m p t e d b y u s i n g a 20 m o l a r excess formaldehyde i n 1 , 1 N H C 1 i n 5 0 % aqueous dioxane (14). Since I S has two reactive sites (2 a n d 6), t h i s means a ten-fold excess of formaldehyde per reactive site. T h e r a t i o of h y d r o x y m e t h y l a t i o n to m e t h y l e n e linkage f o r m a t i o n was d e t e r m i n e d b y H - N M R analysis. 1

Samples were t a k e n at different intervals, n e u t r a l i z e d , e x t r a c t e d a n d a c e t y l a t e d for H N M R analyses. T h e coefficients of the integrals of the a l i p h a t i c (62.0-2.2) a n d the a r o m a t i c (62.3-2.5) acetoxy groups of the p r o t o n N M R s p e c t r a , were t a k e n as the yields of h y d r o x y m e t h y l a t i o n . T h e degree of d i m e r f o r m a t i o n v i a a m e t h y l e n e linkage (crosslinking) was e s t i m a t e d b y the coefficient of the integrals of the p r o t o n N M R s p e c t r a of m e t h o x y groups s i t u a t e d adjacent to the n e w l y f o r m e d b o n d — o f w h i c h the resonances are shifted upfield (63.4-3.7)—and t h a t of the t o t a l m e t h o x y resonances (63.4-4.0). T h e large excess of formaldehyde resulted i n the h y d r o x y m e t h y l a t i o n of 13 i n yields of about 5 0 % h y d r o x y m e t h y l a t i o n . T h i s y i e l d was o b t a i n e d w i t h i n 15 minutes a n d r e m a i n e d constant for over 6 hours. T h e n u m b e r of u n s u b s t i t u t e d a r o m a t i c positions decreased f r o m 0 . 6 / m o d e l c o m p o u n d after 15 minutes to 0.2 after 1 hour reaction t i m e . However, after o n l y 15 minutes about one methylene linkage per m o d e l c o m p o u n d o c c u r r e d , i n d i c a t i n g s u b s t a n t i a l methylene linkage f o r m a t i o n . The rae^a-hydroxymethylation y i e l d was very dependent o n large excess formaldehyde used a n d insensitive to a c i d c o n c e n t r a t i o n a n d t e m p e r a ture. A f t e r extensive e x p l o r a t i o n of the reaction c o n d i t i o n variables i t was discovered t h a t a decrease i n water content of the solvent results i n the increase of h y d r o x y m e t h y l a t i o n yields. E v e n t u a l l y c o n d i t i o n s were devised to achieve close to 1 0 0 % m e i a - h y d r o x y m e t h y l a t i o n w i t h no m e t h y l e n e l i n k age f o r m a t i o n . B o t h softwood a n d h a r d w o o d m o d e l c o m p o u n d s gave o n l y m o n o - m e / a - h y d r o x y m e t h y l a t i o n ( F i g . 6). T h i s is p r o b a b l y due t o the dea c t i v a t i o n b y the electron p u l l i n g effect of the i n t r o d u c e d h y d r o x y m e t h y l group.


1

T h e reactions o n the m o d e l are s u m m a r i z e d i n T a b l e I. T h e s u i t a b i l i t y of the different conditions f o u n d for the me

Modification of Lignin at the 2- and 6-Positions of the Phenylpropanoid

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