Biocatalysis and Biomimetics - American Chemical Society

Chapter 10

Biomimetic Catalytic Oxidation of Lignin Model Compounds 1

Robert DiCosimo and Hsiao-Chiung Szabo

Downloaded by UNIV LAVAL on May 11, 2016 | http://pubs.acs.org Publication Date: April 9, 1989 | doi: 10.1021/bk-1989-0392.ch010

B.P. America Research and Development, Cleveland, OH 44128 The single-electron-transfer oxidation of model compounds representative of the arylglycerol β-aryl ether and 1,2diarylpropane linkages of lignin has been examined by using Co(II), Mn(II), or Co(II)/Μn(II) as catalysts. Catalytic oxidation of 1-(3,4-dimethoxyphenyl)-2-(2methoxyphenoxy)propane-1,3-diol (DMMP) in 80% acetic acid with 500 psi of 4% oxygen in nitrogen and at 170 °C resulted predominantly in products of Cα-Cβ bond cleavage when using Co(II)/Μn(II) as catalyst. Cα-Cβ bond cleavage of DMMP results from an initial single -electron oxidation to produce an intermediate aromatic radical cation; in the absence of oxygen and catalyst, acid-catalyzed β-aryl ether cleavage was the predominant reaction pathway. Dihydroanisoin (DHA) and 1,2-bis(4methoxyphenyl)-propane-1,3-diol (BMPD) were oxidized by stoichiometric quantities of Co(III) to give solely products of Cα-Cβ bond cleavage, but produced only acid-catalyzed dehydration products under reaction conditions necessary for catalytic oxidation. The c h e m i c a l b l e a c h i n g o f paper p u l p i s c u r r e n t l y performed using c h l o r i n e o r c h l o r i n e d i o x i d e , which f o r k r a f t pulp r e s u l t s i n the p r o d u c t i o n o f between 45 and 90 kg of organic waste/ton of pulp, c o n t a i n i n g 4-5 kg o f organically-bound c h l o r i n e / t o n . (1,2.) Bleaching of paper 1

Current address: Central Research and Development Department, Ε. I. du Pont de Nemours and Company, Experimental Station, Wilmington, DE 19880-0328 0097-6156/89/0392-0123$06.00/0 1989 American Chemical Society

Burrington and Clark; Biocatalysis and Biomimetics ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

124

BIOCATALYSIS AND BIOMIMETICS

p u l p whitens t h e p u l p by removal o f l i g n i n o r by d e s t r o y i n g the chromophores of l i g n i n . L i g n i n i s a f t e r c e l l u l o s e , the p r i n c i p a l c o n s t i t u e n t of higher p l a n t s , and i s a highly-branched, s t r u c t u r a l l y i n t r i c a t e polymer comprised o f phenylpropanoid u n i t s . (2) The t o x i c i t y of chemical b l e a c h i n g e f f l u e n t s t o f i s h and other a q u a t i c fauna has been known f o r some time, ( Α / ϋ ) and r e g u l a t i o n s that w i l l l i m i t concentrations of polychlorinated aromatics i n waste streams w i l l make a l t e r n a t i v e s t o the chemical b l e a c h i n g o f paper pulp w i t h c h l o r i n e o r chlorine dioxide increasingly desirable. Lignindegrading enzymes are now being examined as b i o c a t a l y s t s f o r the bleaching of paper pulp, as w e l l as the p u l p i n g of wood. " L i g n i n a s e " i s o l a t e d from the w h i t e - r o t f u n g i Phanerochaete chrysosporium u t i l i z e s hydrogen peroxide t o generate an oxy-heme complex, which degrades l i g n i n by o x i d i z i n g the a r y l groups of l i g n i n o r l i g n i n model compounds by a s i n g l e e l e c t r o n t r a n s f e r . 16-10) The r e l a t i v e l y s t a b l e r a d i c a l c a t i o n s that r e s u l t decompose v i a Cα-Cβ bond cleavage (Figure 1 ) . One p o s s i b l e problem w i t h u s i n g l i g n i n a s e as a b i o c a t a l y s t f o r the degradation o f l i g n i n i n paper pulp i s t h a t the p r o t e i n has an apparent molecular weight of 41,000, (1Û.) and t h i s may l i m i t i t s a b i l i t y t o enter i n t o the h i g h m o l e c u l a r weight c e l l u l o s e - l i g n i n polymer. F a s t e r r a t e s o f l i g n i n degradation might be obtained using much smaller "biomimetic" c a t a l y s t s which f u n c t i o n via a s i m i l a r mechanism.The autoxidation of alkylbenzenes t o a l d e h y d e s and c a r b o x y l i c a c i d s , c a t a l y z e d by a number o f d i f f e r e n t t r a n s i t i o n metals, a l s o proceeds by an i n i t i a l e l e c t r o n t r a n s f e r , r e s u l t i n g i n a one-electron r e d u c t i o n of the metal c a t a l y s t and concomitant formation o f a s u b s t r a t e r a d i c a l c a t i o n (Equations 1 and 2) .(11)


f

M

n+

+ ArCH

[ArCH ]'+ 3

3

— • (n-1)+ M

+ [ArCH3] · +

— • ArCH ' + H+ 2

(1) (2)

The ease o f e l e c t r o n - t r a n s f e r o x i d a t i o n o f aromatic hydrocarbons t o produce r a d i c a l c a t i o n s i s d i r e c t l y r e l a t e d t o the i o n i z a t i o n p o t e n t i a l o f these compounds, w i t h e l e c t r o n - d o n a t i n g s u b s t i t u e n t s such as methoxyl (almost every a r y l group o f l i g n i n has one o r two methoxyl s u b s t i t u e n t s ) lowering the o x i d a t i o n p o t e n t i a l . Examination o f the i o n i z a t i o n p o t e n t i a l s f o r v a r i o u s l y s u b s t i t u t e d a r o m a t i c m o l e c u l e s (JL2.) i n d i c a t e s t h a t o x i d a n t s such as Mn(III) and Co ( I I I ) should be q u i t e e f f i c i e n t i n c a t a l y z i n g the a u t o x i d a t i o n of l i g n i n and l i g n i n model compounds v i a a "biomimetic" mechanism that p a r a l l e l s the o x i d a t i o n of l i g n i n by l i g n i n a s e , and we



10. DICOSIMOANDSZABO

Oxidation of Lignin Model Compounds

F i g u r e 1. Ca-Οβ bond cleavage of a l i g n i n model compound by o x i d a t i o n v i a s i n g l e - e l e c t r o n t r a n s f e r . (Reproduced from Ref. 32. Copyright 1988 ACS)


125

126


now r e p o r t t h e r e s u l t s o f a s t u d y o f t h e a u t o x i d a t i o n o f l i g n i n model compounds by t h e s e c a t a l y s t s y s t e m s .


Results

and D i s c u s s i o n

The l i g n i n model compounds whose o x i d a t i o n s have now been examined b y u s i n g a u t o x i d a t i o n c a t a l y s t s have a l l p r e v i o u s l y been employed a s l i g n i n models i n d e g r a d a t i v e reactions using Phanerochaete chrysosporium, which p r o d u c e s t h e enzyme l i g n i n a s e , o r i n r e a c t i o n s that examined t h e e f f e c t o f k r a f t p u l p i n g o r o t h e r c h e m i c a l o r m i c r o b i a l o x i d a t i o n s on l i g n i n model compounds. (X2zi 12.) T h e s e model compounds r e p r e s e n t t h e a r y l g l y c e r o l β-aryl e t h e r a n d 1 , 2 - d i a r y l p r o p a n e l i n k a g e s o f l i g n i n , w h i c h make up 30%-50% a n d c a . 7%, r e s p e c t i v e l y , o f t h e m a j o r t y p e s o f bonds c o n n e c t i n g the phenylpropanoid units of lignin. Q) Figure 2 depicts the four l i g n i n m o d e l compounds u s e d i n t h i s s t u d y a n d t h e e x p e c t e d p r o d u c t s o f t h e Cα-Cβ bond c l e a v a g e o f t h e s e compounds by their single-electron-transfer oxidation: dihydroanisoin (DHA), 1 , 2 - b i s ( 4 - m e t h o x y p h e n y l ) p r o p a n e 1,3-diol (BMPD), 1-(4-hydroxy-3-methoxyphenyl)-2-(2methoxyphenoxy)propane-1,3-diol (HMMP), a n d l - ( 3 , 4 dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-1,3-diol (DMMP) . S t o i c h i o m e t r i c O x i d a t i o n o f L i g n i n Model Compounds. The s t o i c h i o m e t r i c o x i d a t i o n o f t h e l i g n i n model compounds DHA, BMPD, HMMP, a n d DMMP was f i r s t performed t o determine the ability of single-electron-transfer o x i d a n t s s u c h a s C o ( I I I ) t o p r o d u c e Ca-C$ bond c l e a v a g e o f t h e s u b s t r a t e s ( T a b l e I ) . The d i a r y l p r o p a n e model DHA was completely oxidized to yield 2 equiv of anisaldehyde. The d i a r y l p r o p a n e BMPD p r o d u c e d a t l e a s t 1 e q u i v o f a n i s a l d e h y d e ; f u r t h e r o x i d a t i o n o f t h e Cβ fragment (1-(4-methoxyphenyl)-1,2-ethanediol) produced a d d i t i o n a l anisaldehyde. The a r y l g l y c e r o l β-aryl e t h e r m o d e l compounds HMMP a n d DMMP were a l s o o x i d i z e d b y stoichiometric amounts o f Co ( I I I ) , b u t Ca-C$ b o n d c l e a v a g e was o n l y o b s e r v e d f o r DMMP, w h i c h p r o d u c e d DBA and g u a i a c o l . O x i d a t i o n o f HMMP b y Co ( I I I ) i n a c e t i c a c i d p r o d u c e d no p r o d u c t s o f Cα-Cβ bond c l e a v a g e , a n d no other low-molecular weight (monomeric) p r o d u c t s were observed; i t i s likely that once oxidized to a r e s o n a n c e - s t a b i l i z e d phenoxyl r a d i c a l , d i m e r i z a t i o n t o b i p h e n y l i s t h e p r i m a r y r e a c t i o n pathway. The o x i d a t i o n o f 2 - m e t h o x y - 4 - a l k y l p h e n o l s i s known t o r e s u l t i n o r t h o carbon coupling o f two monomers t o p r o d u c e ο , ο ' dihydroxybiphenyl, which a r e i n t u r n s u b j e c t t o f u r t h e r oxidation.(20)


10. DICOSIMO AND SZABO Table


cmpd

Oxidation ofLignin Model Compounds

I. Stoichiometric Oxidation of Lignin Compounds b y C o ( I I I ) [cmpd] : [Co]

solvent

T(OC)

127

Model

% conv.

% sel.

DHA DHA DHA DHA DHA

1: 1: 1: 1: 1:

0 2 1 0 2

AA AA AA 80% AA 80% AA

25 25 25 25 25

nr

100 53 66 100

100 94 0 62

BMPD BMPD BMPD BMPD

1: 1: 1: 1:

0 2 2 2

AA AA AA 50% AA

25 25 50 50

10 100 100 88

0 97 100 54

HMMP HMMP HMMP HMMP

1: 1: 1: 1:

0 2 0 2

AA AA 50% AA 50% AA

25 25 25 25

1 88 7 73

0 0 0 0

DMMP DMMP DMMP DMMP

1: 1: 1: 1:

0 2 0 2

AA AA 50% AA 50% AA

118 118

2 99 18 40

0 67 0 38

[cmpd] = 1.0 mM, AA = a c e t i c

Catalyst

Regeneration

reflux reflux

—

acid

Using

Peracetic

hoAA.

After

h a v i n g u s e d s t o i c h i o m e t r i c amounts o f e l e c t r o n - t r a n s f e r o x i d a n t s f o r t h e o x i d a t i o n o f DHA, BMPD, a n d DMMP t o CaΟβ b o n d - c l e a v a g e p r o d u c t s , t h e g e n e r a t i o n o f c a t a l y t i c amounts of electron-transfer oxidants was first demonstrated by u s i n g p e r a c e t i c a c i d t o o x i d i z e Co(II) t o C o ( I I I ) i n s i t u ( T a b l e I I ) . F i v e e q u i v a l e n t s o f BMPD were o x i d i z e d w i t h 1 e q u i v o f C o ( I I I ) i n g l a c i a l a c e t i c

Table

I I . Regeneration

C o ( I I I ) (mM) BMPD BMPD DMMP DMMP

O.20 0 O.25 0

[BMPD],

of Co(III) with P e r a c e t i c A c i d

CH3C03H(mM) 1.6 1.0 1.0 1.0

% conv. 100 87 90 8

% sel. 100 30 48 0

[DMMP] =1.0 mM, s o l v e n t = AA, 25 C



OCH 3

Figure 2. Model compounds representing 1/2diarylpropane and arylglycerol β-aryl ether linkages of lignin. (Reproduced from Ref. 32. Copyright 1988 A C S ) . ( C o n t i n u e d on next p a g e . )

BMPD

DHA



Figure

2.

Continued.


Η*

I I

ι

i

Ο

va

ο

Ο •

η ο

Ο

©


130


a c i d a t 25 ° C b y u s i n g 8 e q u i v o f p e r a c e t i c a c i d t o r e g e n e r a t e C o ( I I I ) . A n i s a l d e h y d e was p r o d u c e d w i t h 100% s e l e c t i v i t y a t 100% c o n v e r s i o n o f BMPD. The r e a c t i o n o f 1 e q u i v o f BMPD w i t h 1 e q u i v o f p e r a c e t i c a c i d i n t h e a b s e n c e o f added c a t a l y s t gave o n l y 30% s e l e c t i v i t y t o a n i s a l d e h y d e a t 87% c o n v e r s i o n . The c a t a l y t i c o x i d a t i o n o f DMMP u s i n g p e r a c e t i c a c i d a s t h e c o o x i d a n t was a l s o d e m o n s t r a t e d : 4 e q u i v o f DMMP was o x i d i z e d i n t h e presence o f 1 e q u i v o f c o b a l t ( I I ) a c e t a t e by u s i n g 4 equiv o f p e r a c e t i c a c i d as t h e cooxidant. A 48% s e l e c t i v i t y t o DMB a t 90% c o n v e r s i o n was o b t a i n e d , a n d t h r e e t u r n o v e r s o f C o ( I I ) were a c h i e v e d . DMMP i s v e r y s t a b l e i n t h e p r e s e n c e o f p e r a c e t i c a c i d u n d e r t h e same c o n d i t i o n s : a 92% r e c o v e r y was o b t a i n e d a t 25 °C a f t e r 5 h. B e c a u s e o f t h e expense o f u s i n g o f s t o i c h i o m e t r i c q u a n t i t i e s o f p e r a c e t i c a c i d f o r l i g n i n d e g r a d a t i o n , and t h e l a r g e q u a n t i t i e s o f p e r a c i d t h a t would be r e q u i r e d f o r t h e a p p l i c a t i o n o f t h i s type o f c a t a l y t i c o x i d a t i o n to a process such as paper pulp b l e a c h i n g , an a l t e r n a t i v e method o f c a t a l y s t o x i d a t i o n i s d e s i r a b l e . The most economical way t o g e n e r a t e the desired e l e c t r o n - t r a n s f e r o x i d a n t s i n s i t u would u s e oxygen a s the u l t i m a t e oxidant, and a l k y l p e r o x y o r p e r o x y a c i d i n t e r m e d i a t e s formed d u r i n g t h e r e a c t i o n o f oxygen w i t h t h e Ca-C$ b o n d - c l e a v a g e p r o d u c t s of lignin (in this case, lignin m o d e l compounds) could reoxidize the catalyst. A c i d - C a t a l y z e d D e h y d r a t i o n o f L i g n i n M o d e l Compounds. In addition t o Ca-C$ bond cleavage, both t h e diarylethaneand d i a r y l p r o p a n e d i o l s and the arylglycerol β-aryl e t h e r s are subject to acidc a t a l y z e d d e h y d r a t i o n r e a c t i o n s . DHA a n d BMPD c o u l d be o x i d i z e d w i t h s t o i c h i o m e t r i c amounts o f C o ( I I I ) t o g i v e g o o d t o e x c e l l e n t s e l e c t i v i t y t o Ca-C$ b o n d - c l e a v a g e p r o d u c t s under r e a c t i o n s c o n d i t i o n s t h a t would o t h e r w i s e produce only t h e a c i d - c a t a l y z e d dehydration products desoxyanisoin and trans-4,4 dimethoxystilbene, r e s p e c t i v e l y , b u t t h e s e same model compounds were n o t s t a b l e a t t h e h i g h e r r e a c t i o n temperatures and p r e s s u r e s (>150 ° C , 500 p s i 4% oxygen i n n i t r o g e n ) r e q u i r e d t o o b t a i n c a t a l y t i c r e a c t i o n w i t h oxygen a n d t h e C o ( I I ) , M n ( I I ) , o r Μη(II)/Co(II) c a t a l y s t s . HMMP was u n s u i t a b l e as a model compound s i n c e i t d i d n o t undergo Ca-C$ bond c l e a v a g e when o x i d i z e d b y C o ( I I I ) . In c o n t r a s t , t h e a r y l g l y c e r o l β-aryl e t h e r DMMP c o u l d b e c a t a l y t i c a l l y o x i d i z e d t o give predominantly Ο α ^ β bond-cleavage products under t h e r e q u i r e d high-temperature, highp r e s s u r e r e a c t i o n c o n d i t i o n s , where i n t h e a b s e n c e o f c a t a l y s t a n d oxygen, a c i d - c a t a l y z e d d e h y d r a t i o n a n d βa r y l e t h e r c l e a v a g e were a l s o observed.(21) The s t a b i l i t y o f t h e l i g n i n model compound DMMP u n d e r t h e r e a c t i o n c o n d i t i o n s a n d i n t h e s o l v e n t s t o be 1


10. DICOSIMO AND SZABO

Oxidation ofLignin Model Compounds

131

u s e d f o r c a t a l y t i c o x i d a t i o n s was f i r s t d e t e r m i n e d i n t h e a b s e n c e o f added c a t a l y s t . After heating f o r 3 h at 170 °C a n d under 4% oxygen i n n i t r o g e n i n g l a c i a l a c e t i c acid, no DMMP r e m a i n e d , a n d t h e p r o d u c t s were t h e diacetate l-(3,4-dimethoxyphenyl)-2-(2methoxyphenoxy)propane-1,3-diol diacetate (DMPD, 66% yield), t h e monoacetate 3-(3,4-dimethoxyphenyl)-3h y d r o x y - 2 - ( 2 - m e t h o x y p h e n o x y ) p r o p y l a c e t a t e (DHMA, 2.2% y i e l d ) ( w h e r e a c e t y l a t i o n o f t h e p r i m a r y h y d r o x y l group o f DMMP h a s t a k e n p l a c e ) , 3,4-dimethoxybenzaldehyde

(DMB, 7.6%), 3,4-dimethoxybenzoic a c i d

(DBA, 9.9%), a n d

g u a i a c o l (7.3%). A minor p r o d u c t a l s o formed monoacetate a t t h e secondary h y d r o x y l group. aqueous a c e t i c a c i d , 5% DMMP remained, a n d DMPD Downloaded by UNIV LAVAL on May 11, 2016 | http://pubs.acs.org Publication Date: April 9, 1989 | doi: 10.1021/bk-1989-0392.ch010

DHMA (12%), DMB (20%), dimethoxybenzaldehyde

i s the I n 80%

(5.8%),

(DBA,

16%),

and g u a i a c o l (35%) were p r o d u c e d ; s i m i l a r y i e l d s were o b s e r v e d i n 50% a c e t i c a c i d , e x c e p t f o r g u a i a c o l (56%). The p r o d u c t i o n o f DHMA a n d DMPD a s b y p r o d u c t s was o b s e r v e d i n most r e a c t i o n s o f DMMP w i t h Co ( I I ) o r Μ η ( I I ) / C o ( I I ) (1: 9), oxygen, and a c e t a l d e h y d e (added as a cooxidant f o r the generation o f Mn(III)/Co (III), vide i n f r a ) . DMPD a n d DHMA were c o n s i d e r e d t o be u n r e a c t e d s t a r t i n g m a t e r i a l when d e t e r m i n i n g c o n v e r s i o n s o f DMMP and s e l e c t i v i t y t o p r o d u c t s . Catalytic Oxidations. The e f f e c t o f t h e a d d i t i o n o f oxygen a n d Co, Mn, a n d Mn/Co (1:9) o x i d a t i o n c a t a l y s t s on t h e r e a c t i o n o f DMMP i n 80% a c e t i c a c i d a t 170 OC i s i l l u s t r a t e d by t h e examples l i s t e d i n T a b l e I I I . T a b l e I I I . C a t a l y t i c O x i d a t i o n o f DMMP

a

catalyst

r e a c t i o n gas

conv. (%)

% selectivity DMB DBA g u a i a c o l

Co(II) Mn(II Mn(II)/Co(II) Mn(II)/Co(II)

N2 4% 4% 4% 4% N2

92 77 49 89 79 53

4 26 34 41 29 6

02 0 02 02 2

in in in in

N2 N2 N N2 2

6 21 28 33 42 5

77 49 54 0 9 64

^ R e a c t i o n s were r u n f o r 3 h i n 80% a c e t i c a c i d a t 170 OC a n d 500 p s i o f n i t r o g e n o r o x y g e n / n i t r o g e n , u s i n g 10 mM DMP and 100 mM c a t a l y s t ; Μη(II)/Co(II) r a t i o was 1:9. DMB = 3,4-dimethoxybenzaldehyde, DBA = 3,4dimethoxybenzoic a c i d .

H e a t i n g a 10 mM s o l u t i o n o f DMMP i n 80% a c e t i c a c i d i n t h e absence o f c a t a l y s t o r oxygen r e s u l t s i n a c i d -



132


c a t a l y z e d β-aryl ether cleavage t o produce g u a i a c o l and 3-(3,4-dimethoxyphenyl)-2-oxo-l-hydroxypropane,(21) w i t h very l i t t l e Οα-Οβ bond cleavage. Adding oxygen but no c a t a l y s t leads t o an increase i n Ca-C$ bond cleavage and decrease i n β-aryl e t h e r cleavage, w h i l e adding both oxygen and c a t a l y s t produces the highest s e l e c t i v i t i e s (60-70% combined s e l e c t i v i t i e s f o r DMB and DBA) t o Ca-C$ bond cleavage. Running the r e a c t i o n w i t h c a t a l y s t but no oxygen leads t o a c i d - c a t a l y z e d β-aryl ether cleavage as t h e predominant r e a c t i o n pathway, but conversions ( r e l a t e d t o the r a t e o f r e a c t i o n s ) are much lower than i n t h e absence o f c a t a l y s t , i n d i c a t i n g a p o s s i b l e s t a b i l i z a t i o n o f DMMP t o a c i d - c a t a l y z e d r e a c t i o n s by c h e l a t i o n t o the c a t a l y s t . Acetaldehyde was added t o the r e a c t i o n mixtures as a cooxidant f o r c a t a l y s t r e g e n e r a t i o n ; acetaldehyde i s o x i d i z e d under these r e a c t i o n c o n d i t i o n s t o p e r a c e t i c a c i d , which i s capable o f r e o x i d i z i n g Co(II) t o C o ( I I I ) and M n ( I I ) t o M n ( I I I ) . ( 2 i , 2_3J Increasing the c o n c e n t r a t i o n o f acetaldehyde from 1.0 mM t o 100 mM i n r e a c t i o n mixtures c o n t a i n i n g DMMP (10 mM) and Mn/Co(100 mM) r e s u l t e d i n only a small increase i n the conversion of DMMP and the s e l e c t i v i t y t o DMB and DBA (Figure 3 ) , and a s i m i l a r e f f e c t on conversion and s e l e c t i v i t y was o b t a i n e d when u s i n g Mn(II) as c a t a l y s t . When u s i n g Co(II) as c a t a l y s t , a marked increase i n conversion was observed between reactions run with no added acetaldehyde and those c o n t a i n i n g 1-100 mM acetaldehyde. Of t h e d i f f e r e n t c a t a l y s t used (Co ( I I ) , M n ( I I ) , and Μη(II)/Co(II) (1:9)), the mixed c a t a l y s t Mn/Co (1:9) gave t h e best s e l e c t i v i t i e s and c o n v e r s i o n s i n t h e presence o f added acetaldehyde; a s i m i l a r s y n e r g i s t i c e f f e c t when using Co(II) and Mn(II) f o r the a u t o x i d a t i o n of p-xylene has been p r e v i o u s l y r e p o r t e d . ( 2 A ) Mn(II) alone gave h i g h e r s e l e c t i v i t y t o Cα-Cβ bond-cleavage products than C o ( I I ) , but t h e combination of Μη(II)/Co(II) (1: 9) g e n e r a l l y r e s u l t e d i n t h e h i g h e s t c o n v e r s i o n and s e l e c t i v i t i e s t o t h e d e s i r e d products. The c a t a l y t i c o x i d a t i o n o f 10 mM DMMP u s i n g v a r i o u s c o n c e n t r a t i o n s o f Mn/Co (1:9) (0-500 mM) i n 80% a c e t i c a c i d w i t h 100 mM acetaldehyde a t 170 oc and 500 p s i 4% oxygen i n n i t r o g e n was examined (Figure 4). S e l e c t i v i t y t o products o f Ca-C$ bond cleavage i n c r e a s e d from 39% w i t h no added c a t a l y s t t o 71% with 100 mM Mn/Co (1:9); further increases i n catalyst concentration only produced moderate i n c r e a s e s i n s e l e c t i v i t y a t t h i s p a r t i a l p r e s s u r e o f oxygen. Conversions o f DMMP remained f a i r l y constant up t o Mn/Co c o n c e n t r a t i o n s o f 100 mM, b u t were slightly lower at higher concentrations. I n c r e a s i n g t h e c o n c e n t r a t i o n o f DMMP w h i l e maintaining the concentration of c a t a l y s t , acetaldehyde, and oxygen c o n s t a n t r e s u l t e d i n a decrease i n




20


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Figure 3. S e l e c t i v i t y t o DMB and DBA and conversion o f DMMP , as a f u n c t i o n o f acetaldehyde concentration. Reactions were performed by u s i n g 500 p s i o f 4% O2 i n n i t r o g e n and e i t h e r Μη(II)/Co(II) (1:9, O.10 M; ·, conversion o f DMMP; O , s e l e c t i v i t y , t o DMB and DBA) o r Co(II) (O.10 M; •, conversion; •, s e l e c t i v i t y ) , i n 80% a c e t i c a c i d w i t h DMMP (O.010 M) a t 170 o c f o r 3 h. (Reproduced from Ref. 32. Copyright 1988 ACS)


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F i g u r e 4 . S e l e c t i v i t y t o p r o d u c t s o f Ca-C$ bond cleavage (O) and conversion o f DMMP (·) as a f u n c t i o n of c a t a l y s t concentration. Reactions were performed by using 500 p s i o f 4% O2 i n n i t r o g e n i n 80% a c e t i c a c i d w i t h DMMP (O.010 M) and acetaldehyde (O.10 M) at 170 oc for 3 h; Mn(11)/Co(11) = 1 : 9. (Reproduced from Ref. 32. Copyright 1988 ACS)





135

s e l e c t i v i t y t o t h e Ca-C$ b o n d - c l e a v a g e p r o d u c t s DBA a n d DMB. A concomitant increase i n t h e production o f g u a i a c o l was o b s e r v e d w i t h i n c r e a s i n g c o n c e n t r a t i o n o f DMMP, i n d i c a t i n g t h a t t h e a c i d - c a t a l y z e d d e c o m p o s i t i o n o f DMMP becomes t h e p r e d o m i n a n t r e a c t i o n pathway a s t h e c o n c e n t r a t i o n o f oxygen becomes l i m i t i n g : i n c r e a s i n g t h e p a r t i a l p r e s s u r e o f oxygen i n t h e r e a c t i o n p r o d u c e s an i n c r e a s e i n t h e s e l e c t i v i t y t o DBA a n d DMB a t any g i v e n c o n c e n t r a t i o n o f DMMP. The optimum r a t i o o f r e a c t a n t s f a v o r i n g Ca-cp c l e a v a g e i n 80% a c e t i c a c i d was 10 Mn/Co (1:9) :10 a c e t a l d e h y d e : 1 DMMP u s i n g 10 mM DMMP a n d 8% oxygen i n n i t r o g e n ; a 78% y i e l d o f DMB a n d an 18% y i e l d o f DBA a t 100% c o n v e r s i o n o f DMMP were o b t a i n e d . When t h e c o n c e n t r a t i o n s o f a l l r e a c t a n t s e x c e p t oxygen were increased at this same r a t i o , higher concentrations a g a i n gave lower s e l e c t i v i t i e s t o DMB a n d DBA. The r a t e o f c o n v e r s i o n o f DMMP d e c r e a s e d s l i g h t l y w i t h i n c r e a s i n g c o n c e n t r a t i o n o f DMMP, a l l o t h e r r e a c t i o n p a r a m e t e r s being held constant. The t r a n s i t i o n m e t a l - c a t a l y z e d e l e c t r o n - t r a n s f e r oxidation o f DMMP requires oxygen f o r catalyst reoxidation. A blank check performed by h e a t i n g a s o l u t i o n o f 10:10:1 Mn/Co(1: 9) :acetaldehyde:DMMP(10 mM) i n 80% a c e t i c a c i d and a t 500 p s i o f n i t r o g e n a n d 170 °C f o r 3 h p r o d u c e d s e l e c t i v i t i e s o f 4% DMB a n d 11% DBA a t 47% c o n v e r s i o n , w i t h a 64% s e l e c t i v i t y t o g u a i a c o l ; w i t h no oxygen p r e s e n t , t h e p r e d o m i n a n t r e a c t i o n o f DMMP was a c i d - c a t a l y z e d d e h y d r a t i o n a n d β-aryl e t h e r cleavage. In c o n t r a s t , t h e same r e a c t i o n w i t h 8% o x y g e n i n n i t r o g e n gave 63% s e l e c t i v i t y t o DMB a n d 21% s e l e c t i v i t y t o DBA a t 100% c o n v e r s i o n , and no g u a i a c o l was o b s e r v e d . S i m i l a r r e s u l t s were o b t a i n e d when u s i n g e i t h e r Co ( I I ) or Mn(II) a l o n e as c a t a l y s t s . The c o n v e r s i o n s o f DMMP and y i e l d s o f DMB, DBA, a n d g u a i a c o l were a p p r o x i m a t e l y t h e same when s o l u t i o n s o f DMMP were h e a t e d t o 170 °C i n e i t h e r t h e p r e s e n c e o r absence o f added c a t a l y s t a n d / o r a c e t a l d e h y d e i n t h e absence o f oxygen. U s i n g a r a t i o o f Μ η ( I I ) / C o ( I I ) :acetaldehyde:DMMP o f 10:10:1 a n d 100 mM Mn/Co(1:9), r u n n i n g t h e r e a c t i o n a t 170 ° C a n d 500 p s i o f 4% oxygen i n n i t r o g e n gave h i g h selectivities t o DMB a n d DBA a t a l m o s t complete c o n v e r s i o n o f DMMP i n g l a c i a l , 80%, a n d 50% aqueous acetic acid. In g l a c i a l a c e t i c a c i d , t h e s e l e c t i v i t y t o DMB a n d DBA was 82% a n d 16%, r e s p e c t i v e l y , a t 97% conversion. I n 80% a c e t i c a c i d , t h e s e l e c t i v i t y t o DMB and DBA was 58% a n d 29%, r e s p e c t i v e l y , a t 99% conversion. In 50% a c e t i c a c i d , t h e s e l e c t i v i t y t o DMB and DBA was 58% a n d 24%, r e s p e c t i v e l y , a t 100% conversion. T h e s e same r e a c t i o n s were e x a m i n e d a t temperatures o f 100 ° C , 130 ° C , a n d 150 ° C . Low conversions (10-20%) were obtained when running r e a c t i o n s a t 100 °C o r 130 °C f o r 3-5 h; a t 150 °C, DMMP


136


conversions increased t o 50-60% a t s e l e c t i v i t i e s s i m i l a r t o those obtained at 170 °C.


Conclusions

Under a p p r o p r i a t e reaction conditions, i . e . high c a t a l y s t and oxygen c o n c e n t r a t i o n and low DMMP concentration, the c a t a l y t i c oxidation of the l i g n i n model compound DMMP proceeds almost completely by Cct-cp bond c l e a v a g e o f t h e a r y l g l y c e r o l β-aryl ether. Although DMMP undergoes a u t o x i d a t i o n t o produce some p r o d u c t s o f Ca-C$ bond cleavage i n t h e presence o f oxygen a l o n e , s i g n i f i c a n t i n c r e a s e s i n Ca-Cp bond cleavage a r e produced by t h e a d d i t i o n o f c a t a l y s t s capable o f one e l e c t r o n - t r a n s f e r o x i d a t i o n o f Cct-aryl group. The mechanism of t h i s c a t a l y t i c o x i d a t i o n mimics the o x i d a t i o n o f the same l i g n i n model compound by the enzyme l i g n i n a s e . The development o f a "biomimetic" c a t a l y s t f o r the o x i d a t i v e degradation o f l i g n i n , which does n o t depend on hydrogen p e r o x i d e f o r c a t a l y s t reoxidation (as i s found f o r l i g n i n a s e and heme p r o t e i n s ) , would provide a d i s t i n c t advantage f o r t h e use o f such systems over l i g n i n a s e o r other peroxidedependent m i c r o b i a l o r enzymatic systems. It i s possible that hydroperoxy-lignin intermediates are produced d u r i n g t h e a e r o b i c m i c r o b i a l d e g r a d a t i o n o f l i g n i n , but because the heme i s enzyme-bound, i t i s not r e a d i l y a c c e s s i b l e t o r e o x i d a t i o n by t h e hydroperoxy i n t e r m e d i a t e s ; metal a c e t a t e s such as c o b a l t (II) o r manganese(II) acetate should e a s i l y r e a c t w i t h these same hydroperoxy intermediates and be r e o x i d i z e d . A l s o , the m i c r o b i a l and enzyme systems are c u r r e n t l y l i m i t e d t o temperatures below 40 °C and a r e used i n aqueous systems a t an optimum pH o f 4.5-5.0, while the c a t a l y s t systems t h a t have a l r e a d y been developed f o r t h e s e l e c t i v e o x i d a t i o n o f a l k y l a r o m a t i c s (such as p-xylene t o t e r e p h t h a l i c a c i d ) , and which may be adapted t o the o x i d a t i v e degradation o f l i g n i n , can be run i n organic or a c i d i c o r b a s i c aqueous solvents a t temperatures up t o 200 °C. One disadvantage o f t h i s " b i o m i m e t i c " c a t a l y s t system f o r l i g n i n d e g r a d a t i o n i s t h a t o n l y t h e nonphenolic a r y l g l y c e r o l β-aryl ether DMMP was o x i d i z e d v i a Ca-C$ bond cleavage. Under t h e c o n d i t i o n s f o r catalytic oxidation employed, models o f 1,2diarylpropane s t r u c t u r e s o f l i g n i n (DHA and BMPD) gave p r i m a r i l y a c i d - c a t a l y z e d dehydration products, while the p h e n o l i c a r y l g l y c e r o l β-aryl ether HMMP was o x i d i z e d but d i d not produce products o f Ca-C$ bond cleavage. The enzyme l i g n i n a s e can o x i d i z e these same d i a r y l p r o p a n e s t r u c t u r e s v i a 0α-0βbond cleavage, and as a s u b s t i t u t e f o r the c h l o r i n e b l e a c h i n g o f paper pulp f o r r e s i d u a l l i g n i n removal, t h e enzymatic r e a c t i o n may produce



10. DICOSIMO AND SZABO

Oxidation ofUgnin Model Compounds

137

g r e a t e r amounts o f d e l i g n i f i c a t i o n than catalytic oxidation. However, both the s t u d i e s o f l i g n i n a s e and biomimetic models of l i g n i n a s e have focused p r i m a r i l y on r e a c t i o n s o f a r y l g l y c e r o l β-aryl e t h e r s , which a r e r e p r e s e n t a t i v e o f the major type o f s t r u c t u r e s found i n l i g n i n , but the s t r u c t u r e of r e s i d u a l l i g n i n s remaining i n paper p u l p s a f t e r cooking i s not well-known. (2JL) K r a f t cooking o f pulp i s b e l i e v e d t o degrade β-aryl ether s t r u c t u r e s t o s t y r y l a r y l e t h e r s , d i a r y l e t h e r s , and b i p h e n y l s , which a r e not e a s i l y degraded and a r e removed i n a subsequent chemical b l e a c h i n g step.(2_Êr2X) Rather than breaking carbon-carbon bonds f o r d i s s o l u t i o n of r e s i d u a l l i g n i n , o x i d a t i o n by a biomimetic c a t a l y s t , o r by l i g n i n a s e i t s e l f , may r e s u l t i n further p o l y m e r i z a t i o n of the β-aryl ether degradation products. The treatment o f k r a f t pulp w i t h P. chrysosporium has been reported, and although treatment d i d not r e s u l t i n any b l e a c h i n g , the remaining pulp was e a s i e r t o b l e a c h by conventional c h l o r i n e treatment due t o degradation of some o f the r e s i d u a l l i g n i n . ( 2 8 ) An examination o f the c a t a l y t i c o x i d a t i o n o f r e s i d u a l l i g n i n i n paper p u l p which u t i l i z e s oxygen and Mn/Co a c e t a t e s i n aqueous a c e t i c a c i d needs t o be performed before the e f f i c a c y of such a method can be judged i n comparison t o c h l o r i n e bleaching. Experimental Section General Remarks. Extreme c a u t i o n should be taken when working w i t h p e r o x i d e s o r p e r a c i d s d i r e c t l y o r when employing r e a c t i o n c o n d i t i o n s where p e r o x i d e s o r peracids w i l l be generated i n s i t u . No metal-ware (e.g. s y r i n g e needles) should be employed. Only a l l - g l a s s reaction vessels, gas-tight syringes with Teflon l u e r l o c hubs, and T e f l o n syringe needles and cannulas were used. Dihydroanisoin (1,2-bis(4-methoxyphenyl)ethane1.2- d i o l , DHA) (22.) and 1,2-bis(4-methoxyphenyl)propane1.3- d i o l (BMPD)(2 4) were p r e p a r e d as p r e v i o u s l y d e s c r i b e d , whereas 1-(4-hydroxy-3-methoxyphenyl)2-(2methoxyphenoxy)propane-1,3-diol (HMMP)and l-(3,4dimethoxyphenyl)-2-(2-methoxyphenoxy)-propane-1,3-diol (DMMP) were prepared according t o s l i g h t v a r i a t i o n s o f p u b l i s h e d procedures . (2JL/20.) Cobalt ( I I I ) a c e t a t e was p r e p a r e d by t h e o z o n a t i o n o f c o b a l t ( I I ) a c e t a t e according t o a reported procedure.(2L) Reactions u s i n g oxygen/nitrogen mixtures at g r e a t e r than atmospheric pressure were performed i n P a r r Model 4740 H a s t e l l o y C h i g h p r e s s u r e r e a c t i o n v e s s e l s equipped w i t h g l a s s l i n e r s and T e f l o n - c o a t e d s t i r r i n g b a r s . Product s e l e c t i v i t i e s were c a l c u l a t e d on the b a s i s of product y i e l d s and the amount of substrate reacted (conversion). Y i e l d s o f products and recovered s t a r t i n g m a t e r i a l s were determined q u a n t i t a t i v e l y by HPLC.


138



S t o i c h i o m e t r i c O x i d a t i o n o f L i a n i n Model Compounds w i t h Cobalt(III) Acetate. In a t y p i c a l procedure, a s o l u t i o n o f DMMP (6.8 mg, O.018 mmol) a n d c o b a l t ( I I I ) a c e t a t e (84.3%, 10.5 mg, O.037 mmol) i n 18 mL o f g l a c i a l a c e t i c a c i d was h e a t e d t o r e f l u x i n 20 m i n w i t h s t i r r i n g . One hour l a t e r , t h e c o l o r t u r n e d from g r e e n i s h b l a c k t o pink. The s o l u t i o n was c o o l e d t o room t e m p e r a t u r e a n d v e r a t r o l e (10.2 mg, O.074 mmol) was added a s an i n t e r n a l s t a n d a r d f o r HPLC a n a l y s i s . A 67% s e l e c t i v i t y t o DMB a t 99% c o n v e r s i o n o f DMMP was o b t a i n e d . For reactions where c o b a l t ( I I I ) a c e t a t e s t i l l r e m a i n e d , an a q u e o u s s o l u t i o n o f f e r r o u s s u l f a t e was added d r o p w i s e u n t i l t h e c o l o r o f t h e s o l u t i o n t u r n e d from green t o p i n k ; then v e r a t r o l e was added a s i n t e r n a l s t a n d a r d . C a t a l y t i c O x i d a t i o n o f L i g n i n M o d e l Compounds. In a t y p i c a l procedure, a Pyrex g l a s s l i n e r c o n t a i n i n g a T e f l o n - c o a t e d m a g n e t i c s t i r r i n g b a r , DMMP (8.4 gm, O.025 mmol), manganese ( I I ) a c e t a t e (4.3 mg, O.025 mmol), cobalt(II) acetate ( 3 9 . 8 mg, O.23 mmol), a n d acetaldehyde (14 μL, O.25 mmol) i n 2.5 mL o f 80% a c e t i c acid was p l a c e d i n a 71-mL P a r r Hastelloy C high p r e s s u r e r e a c t i o n v e s s e l (Model 4740). The r e a c t o r was s e a l e d , p u r g e d t h r e e t i m e s b y p r e s s u r i z i n g t o 320 p s i o f N2 a n d t h e n v e n t i n g t o a t m o s p h e r i c p r e s s u r e , a n d t h e n c h a r g e d w i t h 336 p s i o f 4% 02 i n N 2 ) . The r e a c t o r was put i n a heating block preheated a t 170 ° C a n d t h e r e a c t i o n m i x t u r e s t i r r e d f o r 3 h; a t 170 °C, t h e r e a c t o r p r e s s u r e i n c r e a s e d t o 500 p s i . The r e a c t o r was t h e n r a p i d l y c o o l e d t o room t e m p e r a t u r e by p l a c i n g i t i n an ice/water bath and t h e v e s s e l subsequently vented t o atmospheric pressure. V e r a t r o l e (11.2 mg, O.081 mmol) was added t o t h e r e a c t i o n m i x t u r e a s a n HPLC internal standard and t h e mixture was a n a l y z e d t o y i e l d DMB ( 3 7 % ) , DBA (25%), and g u a i a c o l (2.4%) a t 87% c o n v e r s i o n . Literature 1.

2. 3.

4. 5.

Cited

Erickson, K. E.; Kolar, M. C.; Ljungquist, P.O.; Kringstad, K. P. Environ. Sci. Technol. 1985, 19, 1219-1224. Kringstad, K. P.; Lindstrom, K. Environ. Sci. Technol. 1984, 18, 236A-248A. Sarkanen, Κ. V. In Lignins: Occurrence, Formation, Structure and Reactions; Sarkanen, Κ. V., Ludwig, C. H., Eds.; Wiley: New York, 1967; Chapters 3 and 4. Walden, C. C. Water Res. 1976, 10, 639-664 Dence, C. W.; Annergren, G. E. In The Bleaching of Pulp, 3rd rev.ed.; Singh, R. P., Ed.; TAPPI Press: 1979; Chapter 3, pp 74-75.


10.

DICOSIMOANDSZABO

6. 7. 8. 9. 10. 11.


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