Chapter 34
Bleaching of Hydroxypropyl Lignin with Hydrogen Peroxide Charlotte A. Barnett
1,2
1
and Wolfgang G . Glasser
1,2
Department of Wood Science and Forest Products, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 Polymeric Materials and Interfaces Laboratory, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 2
The bleaching of non-phenolic hydroxypropyl lignin (HPL) with hydrogen peroxide in homogeneous phase results in 50-90% loss of color depending on the definition of color. This is more realistically represented by a weighted absorptivity average in the visible range of the spectrum rather than by a fixed wavelength. The bleaching effect varies with the amount of hydrogen peroxide charged; with reaction time; with p H ; and with other parameters. The oxidation reaction is best carried out in aqueous alcohol, at either pH 2 or above 12-13; and with 25-50% hydrogen peroxide on lignin derivative. At 80°C, the reaction is 90% complete after 1-2 hours with hardwood organosolv lignin, but it takes longer with other lignin types. Treatment with H O introduces carboxylic acid functionality. 2
2
T h e t y p i c a l l y d a r k b r o w n color o f isolated l i g n i n p r o d u c t s a n d t h e i r d e r i v a tives constitutes a severe d r a w b a c k t o their u t i l i z a t i o n i n such s t r u c t u r a l m a t e r i a l s as engineering plastic (1). A l t h o u g h n o t o f significant d e t r i m e n t to the p h y s i c a l a n d m e c h a n i c a l properties o f l i g n i n - d e r i v e d m a t e r i a l s , color is perceived as a significant h a n d i c a p t o m a r k e t i n g i n c o m m e r c i a l p o l y m e r markets (1). T h e light a b s o r b i n g characteristics o f isolated lignins exceed those o f l i g n i n i n i t s native state (i.e., w o o d a n d other b i o m a t e r i a l s ) , a n d this has been a t t r i b u t e d t o t h e f o r m a t i o n o f various c h r o m o p h o r i c f u n c t i o n a l groups d u r i n g i s o l a t i o n (2). M o s t p r o m i n e n t l y a m o n g t h e m are c o n j u g a t e d q u i n o n o i d structures w h i c h are c o m m o n l y removed f r o m the l i g n i n 1
0097-6156/89/0397-0436$06.00/0 © 1989 American Chemical Society
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of h i g h y i e l d p u l p s b y the use of h y d r o g e n peroxide (3). A vast l i t e r a ture is available o n t h i s type of " l i g n i n - r e t a i n i n g " b l e a c h i n g of p u l p s , w h i c h is contrasted by the " l i g n i n - r e m o v i n g " b l e a c h i n g technology n o r m a l l y i n v o l v i n g chlorine a n d / o r chlorine dioxide (4). L i g n i n - r e t a i n i n g b l e a c h i n g is k n o w n to be t r a n s i e n t since l i g h t a b s o r b i n g s t r u c t u r a l features reappear u p o n exposure to the elements, especially oxygen a n d U V - r a y s . T h i s has been a t t r i b u t e d t o the ease of quinone regeneration by p h e n o l i c f u n c t i o n a l groups (5). A more p e r m a n e n t r e m o v a l of color has been described b y L i n (6), and b y D i l l i n g a n d Sarjeant (7), for l i g n i n derivatives i n w h i c h the phe n o l i c f u n c t i o n a l i t y has been p a r t i a l l y b l o c k e d . These largely n o n - p h e n o l i c ( a n d s u l p h o n a t e d ) l i g n i n derivatives were bleached i n homogeneous aque ous phase w i t h h y d r o g e n peroxide a n d chlorine dioxide. R e d u c t i o n s i n color of 8 0 - 9 3 % were r e p o r t e d for these water soluble derivatives ( 6 , 7 ) . It was the intent of the present i n v e s t i g a t i o n t o e x a m i n e the one step b l e a c h a b i l i t y of n o n - p h e n o l i c , w a t e r - i n s o l u b l e h y d r o x y p r o p y l l i g n i n ( H P L ) derivatives w i t h h y d r o g e n peroxide i n homogeneous phase (i.e., aqueous ethanol). Experimental Materials. H y d r o x y p r o p y l l i g n i n ( H P L ) derivatives were available f r o m the f o l l o w i n g l i g n i n sources: organosolv (methanol) l i g n i n f r o m spruce, s u p p l i e d b y O r g a n o c e l l , M u n i c h , F R G e r m a n y ; organosolv ( m e t h a n o l ) l i g n i n f r o m red oak, s u p p l i e d b y a n undisclosed i n d u s t r i a l source; organosolv (ethanol) l i g n i n f r o m aspen, s u p p l i e d by R e p a p Technologies, Inc. (formerly the B i o l o g i c a l E n e r g y C o r p . ) , V a l l e y Forge, P A ; kraft l i g n i n f r o m pine ( I n d u l i n A T ) , supplied by Westvaco C o r p . , N . Charleston, S C ; and kraft lignin from hardwood, supplied by Westvaco C o r p . , N . Charleston, S C . A l l lignins were d e r i v a t i z e d w i t h propylene oxide b y b a t c h reaction as r e p o r t e d p r e v i ously ( 8 , 9 ) , a n d the products were separated f r o m h o m o p o l y m e r f r a c t i o n s of propylene oxide b y l i q u i d - l i q u i d e x t r a c t i o n . Methods. A t y p i c a l hydrogen peroxide b l e a c h i n g o p e r a t i o n consists of t r e a t m e n t of a 2 0 % H P L s o l u t i o n i n 6 0 - 7 5 % aqueous e t h a n o l w i t h 2 5 - 5 0 % h y d r o gen peroxide (by weight) o n l i g n i n derivative at 80°C. T h e r e a c t i o n m e d i u m is adjusted to p H 2 b y the a d d i t i o n of H C 1 , or to p H >12-13 b y N a O H . C o l o r loss is d e t e r m i n e d b y U V / V I S absorbance measurements o n s u i t a b l y d i l u t e d samples. A b s o r b a n c e was measured at 25 n m intervals f r o m 325 n m to 500 n m . These values were converted to a b s o r p t i v i t y ( L g"" c m " ) a n d the area under the curve was integrated f r o m 350 n m to 500 n m u s i n g software b y P o l y M a t h C o n t r o l D a t a ( © 1 9 8 4 ) . T h i s value was c o m p a r e d to c o r r e s p o n d ing values o b t a i n e d w i t h the s t a r t i n g m a t e r i a l ( l i g n i n or H P L ) , a n d the color loss was c a l c u l a t e d f r o m 1
Color Loss(%) =
1-
A r e a of bleached s a m p l e A r e a of s t a r t i n g m a t e r i a l
1
χ 100
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S i m i l a r c a l c u l a t i o n s were done u s i n g seven i n d i v i d u a l wavelengths between 350 a n d 500 n m (i.e., at 25 n m intervals) a n d , u s i n g the s u m o f the a b s o r p t i v i t i e s at the 7 wavelengths, color loss was c o m p u t e d f r o m Color Loss(%) = f 1 L
«' + * ' "' "( P ) 1 αχ + 2 a + 2 a . . . + a ( s t a r t i n g m a t e r i a l ) J 2
5 0 % per h a l f - h o u r i n i t i a l l y to between 0.5 a n d 2 % after 8 hours. Significant differences are detected for response rate t o b l e a c h i n g w i t h H 2 O 2 for l i g n i n s f r o m different sources ( F i g . 4). T h i s m a y be a t t r i b u t e d to differences i n c h e m i c a l f u n c t i o n a l i t y a n d other factors. pH. L i g n i n - r e t a i n i n g b l e a c h i n g i n the p u l p a n d paper i n d u s t r y c o m m o n l y employs H 2 O 2 at alkaline p H levels ( 3 , 4 , 1 2 ) . T h i s preference m u s t be a t t r i b u t e d to the s u s p e c t i b i l i t y of the phenoxide anions t o H 2 O 2 . T h e s e ions are absent i n H P L due to the non-phenolic character of t h i s derivative t y p e . T h u s a l l p H levels have been explored i n the c u r r e n t s t u d y . T h e results i n d i c a t e t h a t effective color loss is achieved at b o t h extremes of the p H scale, at p H 2, a n d >12-13 ( F i g . 5). U n d e r alkaline c o n d i t i o n s , the i n i t i a l p H must exceed t h a t of the p K of H 2 O 2 (i.e., 11.6). It is, however, recognized t h a t the p H drops r a p i d l y i n the course of the reaction due a p p a r e n t l y t o the f o r m a t i o n of acidic f u n c t i o n a l i t y w h i c h neutralizes the a l k a l i . O n the a c i d side, effective use of H 2 O 2 requires s t r o n g l y acidic c o n d i t i o n s ( p H 2). A p H o f 1, however, was f o u n d t o result i n less t h a n o p t i m a l b l e a c h i n g , p r o b a b l y because of c h r o m o p h o r e - c r e a t i n g side-reactions. T h e c h e m i s t r y of H 2 O 2 involves the f o r m a t i o n of active reaction species f r o m h y d r o g e n peroxide under the extreme p H c o n d i t i o n s : a
H 0 + [HOf ^ 2
[H OOH]« â 2
A
d
H 0 3
2
B
~
[Η0 ] 2
θ
W h e r e a s the reactive species under acidic c o n d i t i o n s are the [ Η 2 θ Ο Η ] a n d [ Η Ο ] cations, the reactive species under a l k a l i n e c o n d i t i o n s is the [ Η θ 2 ] a n i o n . W h e r e a s the c a t i o n i c o x i d a t i o n species are stronger o x i d a n t s , w i t h o x i d a t i o n p o t e n t i a l r i s i n g w i t h increasing a c i d i t y favoring a r o m a t i c r i n g h y d r o x y l a t i o n b y electrophilic s u b s t i t u t i o n (3,13), the a n i o n i c species m e d i a t e a n u c l e o p h i l i c a t t a c k o n quinones a c c o r d i n g t o the scheme i l l u s t r a t e d i n F i g . 6 ( 3 , 1 0 , 1 1 ) . T h e f o r m a t i o n of new a c i d i c f u n c t i o n a l i t y results i n a r a p i d decline i n p H . T h e effect of H 2 O 2 o n H P L c h e m i s t r y a n d f u n c t i o n a l i t y can be assessed conveniently b y F T I R spectroscopy. It has recently been d e m o n s t r a t e d t h a t q u a n t i t a t i v e s t r u c t u r a l differences between lignins c a n be revealed b y φ
φ
θ
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443
(HR)
F i g u r e 4. R a t e of color loss for H P L derivatives f r o m red oak o r g a n o solv l i g n i n ( A ) , aspen organosolv l i g n i n ( B ) , pine kraft l i g n i n ( C ) , h a r d w o o d kraft l i g n i n ( D ) , a n d spruce organosolv l i g n i n ( E ) . ( O N stands for overnight.)
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F i g u r e 6. Suggested m e c h a n i s m for the reaction of o r t h o - q u i n o n o i d s t r u c tures i n l i g n i n derivatives w i t h hydrogen peroxide under alkaline c o n d i t i o n s (according to G i e r e r , ref. 3).
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t h i s m e t h o d ( 1 4 , 1 5 ) . Differences between bleached a n d unbleached h y d r o x y p r o p y l lignins are i l l u s t r a t e d i n F i g u r e s 7 a n d 8. U s i n g a baselinecorrected a b s o r p t i v i t y s p e c t r u m w h i c h was autoscaled so t h a t the ether peak at 1125 c m was adjusted to 100% a b s o r p t i o n , s p e c t r a l differences between the bleached ( p H 2) a n d the unbleached l i g n i n d e r i v a t i v e are re vealed b y the shaded areas of F i g u r e 7. T h e most a p p a r e n t differences are i n the regions between 1600 a n d 1800 c m " , a n d between 3000 a n d 3600 c m " . M i n o r differences also appear between 1200 a n d 1300 c m " . S u b t r a c t i o n s p e c t r a for a c i d a n d alkaline-bleached l i g n i n derivatives are s h o w n i n F i g ures 8 A a n d 8 B , respectively. T h e a r o m a t i c peak at 1505 c m " was t a k e n as the a b s o r p t i o n s t a n d a r d i n b o t h samples. T h i s enhances s p e c t r a l differences a n d emphasizes s t r u c t u r a l characteristics w h i c h differ i n b o t h samples. T h e p r e p a r a t i o n w h i c h was bleached w i t h H 2 O 2 under a c i d i c c o n d i t i o n s e x h i b i t s a b r o a d peak centered at 1725 c m " , a n d t h i s s t r o n g l y suggests c a r b o n y l a n d c a r b o x y l group f o r m a t i o n ( F i g u r e 8 A ) . T h e s a m p l e treated w i t h [ Η θ 2 ] ions, b y contrast, raises a s t r o n g s i g n a l at 1625 c m " , a n d t h i s reflects changes i n a r o m a t i c i t y . B o t h difference s p e c t r a also d i s p l a y weaker s i g nals at lower wavenumbers, b u t they are less p r o n o u n c e d . T h e y generally s u p p o r t , however, the numerous side reactions w h i c h have been a t t r i b u t e d to h y d r o g e n peroxide a n d i t s d e g r a d a t i o n p r o d u c t s (3). P r o m i n e n t a m o n g t h e m are reactions caused by oxygen a n i o n r a d i c a l a n d h y d r o x y r a d i c a l (3). These results i n d i c a t e t h a t differences exist between the H 2 O 2 b l e a c h i n g c h e m i s t r y d e p e n d i n g o n p H ; a n d t h a t d i f f e r e n c e - F T I R spec troscopy is a useful m e t h o d for revealing the effect o f c h e m i c a l m o d i f i c a t i o n on chemical structure. T h u s , b l e a c h i n g of non-phenolic l i g n i n derivatives is possible at b o t h p H extremes, a n d c o n d i t i o n s m a y be chosen a c c o r d i n g to process conve nience. T h e f o r m a t i o n of new f u n c t i o n a l i t y d u r i n g color r e m o v a l is i n e v i t a b l e , a n d t h i s can be assessed b y F T I R spectroscopy. A c i d i c c o n d i t i o n s appear to result i n the f o r m a t i o n of c a r b o n y l a n d c a r b o x y l groups, a n d alkaline H 2 O 2 is suggested to alter a r o m a t i c character. H y d r o x y l a t i o n a n d h y d r o l y s i s are encountered also. - 1
1
1
1
1
1
θ
1
Various Parameters. O t h e r factors affecting b l e a c h i n g were e x p l o r e d briefly, and p r a g m a t i c decisions were m a d e to determine s t a n d a r d r e a c t i o n c o n d i t i o n s . T h e effect of t e m p e r a t u r e was explored over the range of 20 t o 80°C, a n d results favored elevated t e m p e r a t u r e for s t a n d a r d c o n d i t i o n s . U s i n g a 2 0 % charge of H 2 O 2 at p H 2 for 30 m i n u t e s , color loss increased f r o m 3.5% at 4 0 ° C to 2 9 . 6 % at 8 0 ° C . E t h a n o l was chosen as the solvent for reasons r e l a t i n g to other process factors (i.e., i s o l a t i o n ) . ( C h o i c e of solvent must not ignore possible reaction w i t h H 2 O 2 ) T h e selection of a 2 0 % c o n c e n t r a t i o n level reflects a n effort to produce as concentrated solutions as possible a n d s t i l l p e r m i t homogeneous phase reactions at a l l p H levels. Yield. T h e g r a v i m e t r i c d e t e r m i n a t i o n of l i g n i n y i e l d f o l l o w i n g b l e a c h i n g w i t h h y d r o g e n peroxide poses difficulties t h r o u g h the need to remove i n o r g a n i c reagents. A n a l t e r n a t i v e assay of l i g n i n involves d e t e r m i n a t i o n of U V absorbance at 280 n m . T h i s m e t h o d , however, does not produce i n f o r -
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WRVENUMBER
F i g u r e 7. F T I R s p e c t r a of unbleached H P L a n d of H P L bleached w i t h H 2 O 2 under acidic c o n d i t i o n s ; o v e r l a i d a n d scaled so t h a t the 1125 c m " " peak reflects 1 0 0 % a b s o r p t i o n . S h a d e d areas represent q u a n t i t a t i v e s t r u c t u r a l differences. 1
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Figure 8. acidic (A) spectrum) 1505 c m "
-1
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Subtraction spectra of samples bleached with H2O2 under and alkaline (B) conditions. Unbleached H P L control (normal is shown in (C). (Subtraction spectra were scaled using the peak as unity.)
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m a t i o n o n gravimetric y i e l d b u t rather o n the y i e l d of aromatic (i.e., U V absorbing) c o m p o n e n t . T h i s ignores the expected conversion of a r o m a t i c features i n t o a l i p h a t i c structures t h a t escape detection by U V spectroscopy (at 280 n m ) . Nevertheless, the c o m m o n U V spectroscopic assay produces m e a n i n g f u l results o n the y i e l d of a r o m a t i c components. T h e d a t a of F i g . 9 reveal a loss of a r o m a t i c f u n c t i o n a l i t y i n r e l a t i o n to t i m e ( F i g . 9 A ) a n d H 2 O 2 charge ( F i g . 9 B ) for a n organosolv a n d a k r a f t l i g n i n based H P L at constant p H (2), t i m e (4 hours), a n d t e m p e r a t u r e ( 8 0 ° C ) . A consistent a n d expected loss w i t h t i m e a n d H 2 O 2 charge is revealed. J u d g e d b y U V a b s o r p t i v i t y , 15-20% of the s a m p l e s ' a r o m a t i c character is lost under bleaching c o n d i tions. It is d o u b t f u l t h a t t h i s i m p l i e s t h a t one of five a r o m a t i c s t r u c t u r e s i n l i g n i n has q u i n o n o i d n a t u r e , a n d t h i s m a y more reasonably be e x p l a i n e d w i t h the r e m o v a l of several types of U V a b s o r b i n g constituents. 30 r
A
20
10 Ο Χ
ζ ο
ο
3
2I
4
5
6
7
θ
REACTION T I M E (HR) CO CO
Ο 30 r
Β
Ο ω CM
σ
20
10
0
150 Η 0 2
CHARGE
2
(%
W/W
HPL)
F i g u r e 9. R e l a t i o n s h i p between loss of U V a b s o r p t i v i t y at 280 n m a n d ( A ) r e a c t i o n t i m e a n d ( B ) H 0 charge. P i n e kraft H P L (O) a n d aspen organosolv H P L ( • ). 2
2
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Other Blocking Agents. A l t h o u g h propylene oxide is a convenient a n d ef fective b l o c k i n g agent for l i g n i n , other alternatives are available. A m o n g t h e m , most n o t a b l y , is a c e t y l a t i o n . A c o m p a r i s o n o f t w o organosolv l i g n i n derivatives, f r o m aspen a n d f r o m red o a k , is given i n F i g . 10. T h e d a t a reveal t h a t b o t h c h e m i c a l m o d i f i c a t i o n reactions result i n derivatives w i t h reduced light a b s o r p t i o n characteristics, even w i t h o u t o x i d a t i v e post t r e a t m e n t . P r o p y l e n e oxide a n d acetic a n h y d r i d e produce about i d e n t i c a l color loss values. B l e a c h i n g o f the acetylated r e d o a k s a m p l e resulted i n supe rior values as c o m p a r e d t o the c o r r e s p o n d i n g p r o p o x y l a t e d d e r i v a t i v e s . It appears t h a t b l e a c h i n g w i t h h y d r o g e n peroxide c a n be achieved w i t h n o n phenolic l i g n i n derivatives regardless o f the n a t u r e o f the b l o c k i n g g r o u p . Conclusions 1. A n a m o u n t o f 25-50% h y d r o g e n peroxide per l i g n i n d e r i v a t i v e c o n s t i tutes a reasonable compromise between m a x i m u m color loss a n d effi cient peroxide use. T h i s achieves a color loss o f about 6 0 % o n l i g n i n , if color is defined as the Weighted a b s o r p t i v i t y average i n the range o f 350-500 n m .
6.0
^
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h
LIGNIN
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t ζ
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rr
-OAc
4.0 HPL
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3.0 BI. HPL BlrOAc
2.0 HPL
-OAc
BI. HPL
1.0 BI.-OAc
ASPEN
RED OAK
F i g u r e 10. C o l o r loss o f l i g n i n derivatives i n r e l a t i o n t o m e t h o d o f m o d i f i c a t i o n a n d b l o c k i n g o f phenolic O H groups.
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LIGNIN: PROPERTIES AND MATERIALS
2. W h i l e m o r e t h a n 9 0 % o f color loss is achieved after 30 m i n . i n homogeneous phase a t 80°C w i t h some l i g n i n preparations, others, especially softwood l i g n i n s , take longer. 3. B o t h low ( p H 2) a n d h i g h ( p H 12-13) p H levels are suitable for b l e a c h i n g . F T I R spectroscopy indicates, however, t h a t the two different cond i t i o n s p r o d u c e differences i n the chemistry o f the bleached derivatives. 4. A l t h o u g h g r a v i m e t r i c y i e l d may n o t b e significantly affected b y t h e b l e a c h i n g t r e a t m e n t , a r o m a t i c i t y as i n d i c a t e d b y U V a b s o r p t i o n a t 280 n m declines b y as m u c h as 2 0 % . Acknowledgment T h i s s t u d y was f i n a n c i a l l y s u p p o r t e d b y a grant f r o m a n i n d u s t r y cooperative i n w h i c h the following companies were i n v o l v e d : A R C O C h e m i c a l C o r p o r a t i o n , B i o - R e g i o n a l E n e r g y Associates, A l b e r t a Forest Service, a n d B o r r e g a a r d Industries; a n d the C e n t e r o f Innovative T e c h n o l o g y o f V i r g i n i a . T h i s s u p p o r t is gratefully acknowledged. T h i s is p a r t 18 o f a p u b l i c a t i o n series dealing w i t h engineering plastics f r o m l i g n i n . E a r l i e r parts have been p u b l i s h e d i n J. Appl. Polym. Set., J. Wood Chem. TechnoL, a n d Holzforschung.
Literature C i t e d 1. Niederdellmann, G . , Gewinnung von Lignin aus Holzabfallen und Verweriung fur Hersiellung von Polyurethanhartschaumen, Forschungsbericht 03 V M 619-C 077 (Bayer A G , Leverkusen) an das Bundesministerium fur Forschung und Technologie, 1981, 48 pg. 2. Hon, D. N.-S.; Glasser, W . G . Polym.-Plast. Technol. Eng. 1979, 12(2), 159-179. 3. Gierer, J. Holzforschung 1982, 36(2), 55-64. 4. Loras, V . Chapter 5 in Pulp and Paper Chemistry and Chemical Technology;J.P. Casey, Ed.; J. Wiley & Sons: New York, 1980; Vol. 1, pp. 633-764. 5. Gellerstedt, G . ; Petterson, E . - L . Svensk Papperstidn. 1977, 80(1), 15. 6. Lin, S. Y . U.S. Patent #4,184,845, 1980. 7. Dilling, P.; Sarjeant, P. T. U.S. Patent #4,454,066, 1984. 8. Wu, L . C . - F . ; Glasser, W . G . J. Appl. Polym. Sci. 1984, 29(4), 1111-23. 9. Glasser, W . G . ; Wu, L . C . - F . ; Selin, J.-F. Chapter in Wood and Agricultural Residues: Research on Use for Feed, Fuels, and Chemicals; Soltes, Ed J., Ed.; Academic Press, 1983, pp. 149-166. 10. Spittler, T . D.; Dence, C . W . Svensk Papperstidn. 1977, 80(9), 275-284. 11. Bailey, C . W.; Dence, C . W . Tappi 1969, 52(3), 491-500. 12. Dence, C . W . In Reactions of Hydrogen Peroxide with Lignin: Current Status in Chemistry of Delignification with Oxygen, Ozone and Perox-
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ides; Gratzl, J . S.; Nakano, J . ; Singh, R. P., Eds.; Uni: Tokyo, 1980; pp. 199-205. 13. Levitt, L . S. J. Org. Chem. 1955, 20, 1297-1310. 14. Schultz, T . P.; Nichols, D. D. International Analyst 1987, 1(9), 35-39. 15. Schultz, T . P.; Glasser, W . G . Holzforschung 1986, 40, 37-44. RECEIVED March 17,1989
