biomass electrochemistry anodic oxidation of an organosolv lignin in the presence of tics

8/3/2019 Biomass Electrochemistry Anodic Oxidation of an Organosolv Lignin in the Presence of tics

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JOU RN AL OF APPLIED ELECTR OCH EMISTR Y 19 (1989) 535-539

B i o m a s s e l e c tr o c h e m i st r y : a n o d i c o x i d a t i o n o f a no r g a n o - s o l v l i g n i n i n t h e p r e s e n c e o f n i t r o a r o m a t i c sC A R M E N S M I T H , J A M E S H . P . U T L E YDepartment of Chemistry, Queen Mary College, University of London, London E1 4NS, UKM I R E L L A P E T R E S C U , H A N S V I E R T L E RInstituto de Quimica, Universidade de Sao Paulo, CEP 01498, Sao Paulo, SP, BrazilR ece i v ed 2 7 O c t o b e r 1 98 8

T h e t e m p e r a t u r e r e q u i r e d f o r t h e a n o d i c c o n v e r s i o n o f a n o r g a n o - s o l v s p r u c e l ig n in , i n a q u e o u sa l k a l in e s o l u t i o n , m a y b e l o w e r e d b y th e a d d i t i o n o f n i t r o b e n z e n e o r 1 , 3 - d i n i tr o b e n z e n e . T h ea d d i t i v e s c a u s e s i g n i f i c a n t a n d r e p r o d u c i b l e c h a n g e s i n t h e T a f e l p l o t f o r t h e r e a c t i o n . P r e p a r a t i v e -s c a le e x p e r i m e n t s a r e l e s s r e p r o d u c i b l e a l t h o u g h t h e r a n g e o f p h e n o l i c p r o d u c t s is g e n e r a l ly s i m i l a rt o t h a t p r o d u c e d b y c o n v e n t i o n a l n i t r o b e n z e n e o x i d a t i o n o r b y a n o d i c o x i d a t i o n a t h i g he rt e m p e r a t u r e s .

1 . I n t r o d u c t i o nNit roaromat ics , in a lkal ine so lu t ion , have long beenused as ox idan ts [1 ] fo r o rgan ic subs t r a tes . Fur ther -more , usefu l r eagen ts fo r the ox idat ive degradat ionof l ign ins to lower molec u lar p rod ucts such as van i l linand sy r ingaldehyde inc lude a lkal ine n i t robenzene.T h i s m e t h o d i s o n e o f t h e s t an d a r d p r o ced u r es [ 2]fo r s t ructu ra l e lucidat ion , by degrad at ion , o f l igninsand r e la ted mater ia l s . The cond i t ions ar e severe ,r equ i r ing concen t r a ted a lkal i and e levated tempera-tu res ( ca . 150-200~ and analys i s o f the p ro duc tmix tu re i s compl ica ted by the p resence o f an i l ine ,p h en y l h y d r o x y l am i n e , azo b en zen e , azo x y b en zen ean d 4 - h y d r o x y azo b en zen e ( a p r o d u c t o f rea r r an g e -m en t o f azo x y b en zen e ) . A n o d i c o x i d a t i o n i s anal ternat ive , and bet ter , method fo r the degradat ionof l ign ins [3 ] . The e lec t rochemical m etho d essen-t ia lly opera te s a t ca 170~ wi th a n ickel anod ean d 3 M s o d i u m h y d r o x i d e s o l u t i o n . A t l o w er t em ~pera tu res and m less a lkal ine condmons , ox ldat lonof l ign ins (which are po lyme r ic pheno ls ) r esu l t sn o t i n d eg r ad a t i o n b u t i n f u rt h e r i n t r am o l ecu l a r co n -densat ion .T h e f o r m a t i o n i n t h e n it r o b en zen e m e t h o d o f b y -products which are character i s t ic o f e lec t ro reduct io ia[4 ] o f n i t robenz ene sugges ts tha t the n i t roarom at icco m p o u n d s ac t a s e l ec t r o n t r an s f e r o x i d an t s . I tfo l lows tha t in termedia tes such as n i t rosobenzene,known to be fo rmed in i t ia l ly in n i t robenzene r educ-t ion [4 ] , ma y b e r eox id ized e lec t rochemical ly . S uchoxidat ions have been l i t t l e s tud ied a l though Cauquiset al. [5 ] r epor t tha t n i t rosobenzene g ives a wel l -d e f in ed o x i d a t i o n w av e o n cy c li c v o lt am m e t r y .We repor t here in on exper iments which exp lo re theposs ib i l i ty o f anod ical ly r egenera t ing n i t roaromat icox idan ts in the p resence o f l ign ins and r e la ted com-p o u n d s .

2 . E x p e r i m e n t a l d e t a i l s2.1. MaterialsThe acetone- so lub le sp ruce l ign in (ASPPS) has beendescr ibed e l sewhere [7] . W e are g ra tefu l to D r D . C.Ayres (Q M C) fo r a g if t o f mata i r es ino l (2 ). Borneo l ,cam p h o r an d v an i l l i n w er e o b t a i n ed co m m er c i a l l y .Authen t ic samples o r p roducts , used fo r iden t i f ica t ionand ca l ib ra t ion in HPLC analys i s , were e i ther pur -chased o r syn thes ized by l i t e r a tu re methods .2.2. Electrochemical experimentsSolven ts , e lec t ro ly tes and app ara tu s we re as descr ibedin ear l ie r paper s . The ce l l f o r e lec t ro lyses in aqueousalkal i a t 170 ~ C was cons t ru cted o f s ta in less s tee l andequ ip ped wi th a P TF E l iner (vo lum e 50 cm3); the topw as a l s o l in ed w i th PT FE an d f i t ted w i th O - r i ng r u b -be r seals and a safety valve (50 p.s . i .) This , an d rela tedcell s, ha ve b een fu l ly descr ibed e l swh ere [3 ]. The ano dewa s a nickel gauze c ylinde r (s ize 40 mesh, 31 cm 2) andthe ca thode e i ther a lead rod o r a concen t r ic lead fo i lcylinder .O x i d a t i o n s a t l o w er t em p er a t u r e s ( am b i en t ) w e r e inconven t ional und iv ided g lass ce l l s equ ipped wi th an ickel fo il anod e (18cm 2) and a ca rbon p la te o fequ ivalen t ar ea .Typical p rocedures , inc lud ing the a lkal ine p re-t r ea tment ar e as fo l lows .2.2.1. Nitrobenzene oxidation of A SPPS (or o fmatairesinol). AS PP S or m ata i r es ino l (0 .2 g ) d i s so lvedi n 2 M a q u eo u s s o d i u m h y d r o x i d e ( 1 0 e r a 3 ) w as s ea ledin a s ta in less s tee l au toclav e tu be (vo lume 16 cm 3) andheated a t 170-180~ C wi th shak ing a t 15 r a in in tervalsfo r a to ta l o f be twee n 1 .5 and 4 h . The m ix tu re w asco o l ed , n i t ro b en zen e ( 2.4 g ) ad d ed , t h e au t o c l av e t u b e

002L 89fx/89 $03.00 + .12 9 1989 Chapman and Hall Ltd. 535


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536 CARMEN SMITH, JAMES H. P. UTLEY, MIRELLA PETRESCU AND HANS VIERTLERre-sealed and heating continued at 170-180~ for afurther 3 h. Af ter cooling in ice, the contents wereshaken with ether to remove the unwanted nitro-benzene reduction products. The residual alkalinesolution was acidified (conc. HC1 to pH 3) and theremaining organic products extracted into ether andanalysed (HPLC, details below).2.2 .2 . Anodic oxidat ion of A S P P S(i) High temperature, no additive: lignin (ASPPS,0.25 g) was dissolved in aqueous sodium hydroxide(25 cm 3, 3 M) and introduced into the electrolysis cellprior to sealing. The cell was heated to 170~ andelectrolysis was continued a t 4mAcro 2 for 4h,during which l03 coulombs was passed. The contentswere magnetically stirred. The voltage required wasalways less than 5V, usually 1.8-2.0V. The cellwas then cooled, pressure released and the contentsdecanted off. The contents were then acidified to pH 2with hydrochloric acid. The acid mixture was shakenwith chloroform (3 x 70 x 1) and the chloroformlayer separated off, neutralized with sodium carbonateand dried with sodium sulphate.

Filtration and evaporation yielded a light brownsemi-solid product (0.09g, 36% yield by weight).Analysis of this product by chromatographic methodsshowed that the major products were:

Relative yield (mol %)Vanillic acid 194-Hydroxybenzaldehyde 51Vanillin 174-Hydroxyacetophenone 9Acetovanillone 4(Plus 2 unidentified products)

(ii) High temperature, added nitrobenzene: afterthe usual alkaline pretreatment (ASPPS (0.2g), 2MNaOH (10cm~))nitrobenzene (0.12g) in 2M NaOH(40cm 3) was added and the solution electrolysed at170-180~ and 2mA cm -2 for 3 h in the pressurizedceil. The products were analysed as before.(iii) Ambient temperature added nitroaromatic:after the usual alkaline pretreatment (ASPPS (0.2 g),2M NaO H (20cm3)) a further 50cm 3 of 2M NaOHwas added together with nitrobenzene (0.24g) or 1,3-dinitrobenzene (0.08 g). The stirred electrolyte waselectrolysed at 4mAcro -2 in the undivided celldescribed above and products analysed as before.2.3. Analys isThe mixtures of phenolic compounds were analysedby gradient elution HPLC with u.v. detection ofeluants at 280 nm. For nitrobenzene oxidation it wasfirst necessary to remove 4-hydroxyazobenzene, asignificant and interfering by-product, by chroma-tography as before through silica gel (Kieselgel-60PF). The azobenzene derivative was eluted with petrol

(b.p. 40-60 ~ C)-ethyl acetate (3:1 v/v) and the phe-nolic products with ethyl acetate.

The H PLC detector was calibrated by the analysisof mixtures of authentic samples with a knowncomposition. Suitable conditions were found toinvolve the use of a C-18 microbondapak, reversephase column 25cm x 4mm i.d.) with 1% aqueousNHg(HzPO4)/methanol; in a typical run the solventcomposition would change from 10 to 100% methanolduring 65 min.3 . R e s u l t s a n d d i s c u s s i o n

3.1. MaterialsLignins are mixtures of complex three-dimensionalpolymers of hydroxylated and methoxylated aryl-propane (C9) units. The major structural features areknown [6]. We have chosen to work with a well-characterized [7] organo-solv lignin, the acetone-soluble phenol-pulping spruce (Picea abies) lignin(ASPPS). High field 1H and '3C NMR spectroscopyconfirm [7] that e-hydroxyl and/%O-aryl ether func-tions, with erythro stereochemistry, are prominent inthis material and a convenient representation of themost impor tant repeat units is 1. For comparisons ofreactivity towards the several oxidation regimesexplored, matairesinol (2), borneol (3) and vanillin (4)have been used as substrates. Matairesinol con-tains the easily oxidized phenolic moeity of the ligninbut without the c~- and fl-oxygen functions; borneol isan example of a secondary alcohol which may beoxidized to camphor (5) by alkaline nitrobenzene, at> 150 ~C.

O N e ~ O M oO H( 1 )

( 3)

0ArCH~ O O ~ H O M eA C , , , . . ~ C { A t = }

C H O

OH( 4 ) ( 5 )3.2. Anodic and chemical oxidationThe presence of nitroaromatics, specifically nitro-benzene and 1, 3-dinitrobenzene, causes a dramaticreduction in the temperature at which anodic degra-dation o f the ASPPS lignin occurs. Low-temperature(ca 25 ~ C) anodic oxidation in alkaline solution atnickel results in further condensation in the absence ofnitroaromatic. In contrast, for some experiments inthe presence of nitroaromatic , oxidation proceeded tothe carbon dioxide level; in others, useful conversioninto vanillin (4) and 4-hydroxybenzaldehyde (6, 4-HBA) was achieved. The results of key experimentsare summarized in Table 1; they are presented for


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B I O M A S S E L E C T R O C H E M I S T R Y 5 37

Table I . Ni troaromatic oxidation of rnatairesinot (2) and l ignin (AS PP S) w ith and wi thout coelectrolysisEntry Substrate Conversion Me thod Relative yields (%)a Totalproduct yieldOthers ( w/w% ) b-HB A (6) Vani l lin (4) Phenol

1 M a t a i r e s i n o l ( 2 ) P h N O ~ ( 1 . 8 % ) g - -2 P h N O ~ - ( 8 . 8 % ) g - -3 1 , 3 - D N W ( 5 . 3 % ) g - -4 P h N O 2 , c o e l e c t r o l y s i s ~ ( 1 2 % ) g - -5 P h N O 2 c o e l e c t r o l y s is ~ 'f ( 1 8 % ) g - -6 A S P P S P h N O ~ 3 4 4 1 8 1 70 2 67 P h N O ~ 3 0 3 0 5 3 5 h 1 7 . 58 1 , 3 - D N B e 2 2 2 8 1 3 3 7 1 89 P h N O 2 , c o e l e c t r o l y s i s 1 2 1 8 8 6 2 k 0 . 3 6

1 0 P h N O 2 , c o e l e c t r o l y s i s 3 0 2 3 2 6 2 1 m 2 9 . 61 1 P h N O 2 , c o e l e c t r o l y si s~ 7 74 - 19 n 2 .01 2 P h N O z , c o e l e c t r o l y s i s 1 2 3 7 2 4 p 1 3 . 01 3 P h N O 2 , c o e l e c t r o l y si s~ 1 28 10 61 q 8 .01 4 1 , 3 - D N B , c o e l e c t r o l y s i s 5 5 4 8 6 r 2 . 01 5 1 , 3 - D N B , c o e l e c t r o l y s i s 1 1 3 0 1 2 4 7 s 5 0 . 01 6 1 , 3 - D N B , c o e l e c t r o l y s i s 1 8 1 3 4 1 2 8 2 8 . 0a B y H P L C , r e l a t i v e m o l a r y i e ld s .b E x p r e s s e d a s W / W % b e c a u s e o f u n c e r t a i n t y i n li g n in m o l e c u l a r f o r m u l a .~ S u b s t r a t e ( 0 . 0 5 - 0 .2 g ) , 2 N a q . N a O H , 1 7 0 - 1 8 0 ~ 2 . 5 - 4 h .d P h e n o l , v a n i l li c a c i d , q u a i a c o l a n d a c e t o v a n i l l o n e ( e a c h c a 4 % ) .e A f t e r p r o - t r e a t m e n t ( s e e e x p e r i m e n t a l s e c t i o n ).f S u b s t r a t e ( 0 . 2 g ) , P h N O 2 ( 0 . 2 4 g ) , e l e c t r o l y s i s a s i n ( 1 ).g T r u e y i e l d s , n o t r e l a t i v e .h S y r i n g al d e h y d e ( 4 - h y d r o x y- 3 , 5 - d i m e t h o x y b e n z a l d e h y d e , 2 1 % ) v a n il li c a c i d ( 4 - h y d r o x y - 3 - m e t h o x y b e n z o i c a c id , 3 % ) , g u a i a c o l ( 2 - m e t h -o x y p h e n o l , 4 % ) , 4 - h y d r o x y b e n z o i c a c id ( 6 % ) , a c e t o v a n i l l o n e ( 4 - h y d r o x y - 3 - m e t h o x y a c e t o p h e n o n e , 3 % ) .i V a n i t l ic a c i d ( 1 3 % ) , g u a i a c o l ( 1 0 % ) , s y r i n g a l d e h y d e ( 8 % ) , 4 - h y d r o x y b e n z o i c a c i d (6 % ) .J N o p r e - t r e a t m e n t , s u b s t r a t e ( 0 . 2 g ) , s e a le d c e ll , N i g a u z e a n o d e , P b c a t h o d e , 2 N a q . N a O H , 1 7 0 - 1 8 0 ~ C , 0 . 2 m A c m 2 , 3 h .k M a i n l y p o l y m e r , t r a c es o f 4 - h y d r o x y b e n z o i c a c i d .

A m b i e n t t e m p e r a t u r e , N i f o i l a n o d e , c a r b o n c a t h o d e , 4 m A c m - 2 , 3 h .m G u a i a c o l ( 1 2 % ) , 4 - h y d r o x y b e n z o i c a c i d ( 6 % ) , v a n i l l i c a c i d ( 3 % ) .n P o l y m e r i z a t i o n , v a n i l li c a c i d ( 1 9 % ) .P A c e t o v a n i l l o n e ( 3 % ) , t r a ce s o f 4 - h y d r o x y b e n z o i c a c i d a n d 4 - h y d r o x y a c e t o p h e n o n e .q S y r i n g a l d e h y d e ( 2 1 % ) , a c e t o v a n i l l o n e ( 1 8 % ) , v a n i l li c a c i d ( 1 8 % ) , g u a i a c o l ( 4 % ) .r E v o l u t i o n o f C O 2 o n a c i d i fi c a ti o n , m a j o r p r o d u c t 4 - h y d r o x y b e n z o i c a c i d ( 5 0 % ) a n d a c e t o v a n i ll o n e ( 2 3 % ) .

G u a i a c o l ( 2 7 % ) , 4 - h y d r o x y a c e t o p h e n o n e ( 2 0 % ) .M a i n l y s y r i n g a l d e h y d e ( 9 % ) , 4 - h y d r o x y b e n z o i c a c i d ( 6 % ) a n d g u a i a c o I ( 5 % ) .

ind iv idual exper imen ts to i l lus t r a te the r ange o f r esu lt st o b e ex p ec t ed . T h e p r o b l em s o f r ep r o d u c ib i l it y w i t han inheren t ly he terogeneous sys tem have no t beenso lved . In the e lec t ro lyses i t i s commonly observedt h a t l o w co n v e r s i o n t o d eg r ad a t i o n p r o d u c t s i sacco mp anied by conver s ion o f the l ign in in to a lesss o l ub l e , p r o b ab l y m o r e co n d en s ed , f o r m .Especia l ly no tewor thy f rom the r esu l t s in Tab le 1and fo r add i t ional exper iments i s : ( a ) p re t r ea tment o fthe l ign in w i th 2M N aO H at 180~ i s necessary ,a l t h o u g h t h is d o es n o t i t se l f c au s e b r eak d o w n t o l o wm o l ecu l a r w e i g h t p r o d u c t s ; ( b ) th e p r o p o r t i o n o f 6 : 4is high; (c) 1 , 3-dini trobenzene is a more ef fect iveadd i t ive than n i t robenzene: (d ) the n i t robenzeneox idat ions , w i th and wi thou t coelec t ro lys i s , g iveazoxybenzene (7 ) and 4 -hydroxyazobenzene (8 ) asbyproducts; (e) matairesinol (2) is less ef f icientlyoxidized tha n the l ignin; ( f ) born eol (3) is no t oxidizedby ni tro ben zen e at u p to 80 ~ C; and (g) vanil lin (4) iss ign i f ican t ly demethoxy la ted to g ive 4 -HBA (6) whenvan i ll in is p ro -heated in 2 M N aO H at 180~ C. In ane l ec t r o ch em i ca l ex p e r i m en t p e r f o r m ed w i t h o u t p r o -t r ea t m en t o n l y t r ace s o f t h e d em e t h o x y l a t ed p r o d u c tw er e f o u n d .

]P hC H O

P h N ~ N P h

OHOH ( 7 ) ( 8 )( 6 )

T h e f o r m a t i o n o f azo x y b en zen e (7 ) an d t h e p r o d u c to f i t s r ear r ange me nt (8) ind ica te tha t n i t robe nzeneef f ec ts ox idat ion v ia in it i a l e lec t ron t r ans fer a nd i si t se l f r educed . W hat i s kno wn [4 ] o f n i t roarom at icr ed u c t i o n p a t h w ay s a t h i g h p H s u g g es t s t h e r ed o xcycle g iven in Scheme 1 .

The success fu l exper imen ts summ ar ized in theT ab l e 1 w e r e u n d e r t ak en w i t h th e h o p e t h a t co n -t i n u o u s r eg en e r a ti o n b y o x i d a t i o n o f n i tr o a r o m a t i cw o u l d b e p o s s ib l e , th e f o r m a t i o n o f t h e t o x ic an di n co n v en i en t n i t r o g en o u s b y p r o d u c t s av o i d ed , an dl o w co n cen t r a ti o n s o f o x i d an t em p l o y ed . H o w ev e r , ina s ep a r a t e ex p e r im en t n o ev i d ence w as f o u n d , b y G L Canalys i s , f o r r egenera t ion o f n i t robenze ne by anod icox idat ion o f n i t rosobenzene (9 ) in a lka l ine so lu t ion .

In apro t ic so lven ts , e .g . ace ton i t r i l e and n i t ro -m e t h an e , an o d i c o x i d a t i o n o f n i tr o s o b en zen e ( q ) in t oi t s r ad ica l ca t ion has be en dem ons t r a ted [5 ] . Ou r


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53 8 C A R M E N S M I T H , J A M E S H . P . U T L E Y , M I R E L L A P E T R E S C U A N D H A N S V I E R T L E R+ e

0 2 -e

9 f O " I t+

0 + n ~ ~ 0

2 e ~ 2 H *P h N H O H P h N O

" , " + ( 9 )

P h N . ~ . - . N P h + H z Oo Scheme

+ ,P h - - N - - - O

e x p e r i m e n t s s h o w t h a t , a c c o r d i n g t o c y c l ic v o l t a m -m e t r y a t a p l a t i n u m a n o d e i n C H ~ C N - B u 4 B C 1 0 4(0.1 M), ni t rosobenzene (9) gives a s ingle , i r reversible ,o x i d a t i o n p e a k a t 1 .1 V ( v s A g / A g + ) ; t h i s is c o n s i d e r-a b l y m o r e a n o d i c t h a n t h e c o r r e s p o n d i n g v a l u e (0 .5 V )f o r o x i d a t i o n o f m a t a i r e s i n o l ( 2 ) u n d e r s i m i l a r c o n -d i t i o n s . T h e l a t t e r m a y b e t a k e n a s c h a r a c t e r i s t i c o ft h e o x i d a t i o n b e h a v i o u r o f l i g n i n s .

V o l t a m m e t r i c c u r v e s o b t a i n e d w i t h t h e c e l l , e l e c -t r o d e s a n d e l e c tr o l y t e s u s e d f o r p r e p a r a t i v e e l e c tr o l y -s i s a l s o c o n f i r m t h a t i t i s u n l i k e l y t h a t c o n d i t i o n sf o r r e o x i d a t i o n o f n i t r o s o b e n z e n e ( 9 ) p r e v a il a t t h ere l a t i ve ly l ow cur ren t dens i t i e s used (F ig . 1 ) . Accord -i n g t o F i g . 1 t h e m a j o r e f f e c t o f n i t r o b e n z e n e a d d i t i o ni s t h e d i s p l a c e m e n t o f t h e l i g n i n o x i d a t i o n c u r v e t om o r e p o s i t i v e p o t e n t i a l f o r a g i v e n c u r r e n t d e n s i t y .T h e r e s u l t s d i s p l a y e d i n F i g . 1 w e r e o b t a i n e d , f o rs t i rr e d s o l u t i o n s , u n d e r c o n d i t i o n s i d e n t i c a l ly m a t c h -i n g t h o s e f o r t h e s u c c e s s fu l li g n i n d e g r a d a t i o n s . T h e s ere su l t s we re r ep roduc ib l e fo r seve ra l sepa ra t e expe r i -m e n t s a n d , f o r a g i v e n e l e c t r o l y t e , t h e s a m e c u r v e sr e s u l t e d e it h e r b y v a r y i n g c u r r e n t d e n s i t y a n d m e a s u r -i n g t h e s t e a d y s t a t e p o t e n t i a l o r b y v a r y i n g p o t e n t i a la n d m e a s u r i n g t h e s t e a d y s t a t e c u r r e n t . T h i s p r o o f o fr e p r o d u c i b i l i t y i s i m p o r t a n t b e c a u s e b o t h n i t r o -b e n z e n e a n d 1 , 3 - d i n i t r o b e n z e n e a r e s p a r i n g l y s o l u b l ei n 2 M a q u e o u s s o d i u m h y d r o x i d e a n d , i n t h e a b s e n c eo f l i g n i n , i t i s u n l i k e l y t h a t t h e e l e c t r o l y t e i s h o m o -

120

100 9

ao6 0

~ 4 0

2 0 i

0 i-1 0 1 2L o g / ( / i n m A c m - 2)Fig. 1. Effect of additive on T afel plot. [] 2N Aqueou s NaO H, 92N Aqueous NaO H plus PhNO2, o 2N Aqueous NaO H plusASPPS lignin, a As abo ve, plus PhNOz, - . . . . Current density forpreparative electrolyses.

g e n e o us . T h e o u t c o m e o f p r e p a r a ti v e e x p e r i m e n ts w a sn o t s o p r e d i c t a b l e . H o w e v e r , t h e p h e n o l i c l i g n i n i sr e a d i l y s o l u b l e in 2 M s o d i u m h y d r o x i d e a n d s u c h aso lu t i on i s l i ke ly t o ac t a s a de t e rgen t and so lub i l i z et h e n i t r o a r o m a t i c . T h i s s o l u b i l i z a t i o n w i l l a l s o b ea i d e d b y t h e w e l l - k n o w n [ 8 ] p r o p e n s i t y o f a r o m a t i cn i t r o c o m p o u n d s t o f o r m c o m p l e x e s w i t h e l e c t r o n -r i c h a r o m a t i c s , i n c l u d i n g p h e n o l s .3.3. Mechanistic rationalizationT h e i n e r t n e s s o f b o r n e o l ( 3) i n c o n d i t i o n s w h i c hd e g r a d e l i g n i n s t r o n g l y s u g g e s ts t h a t o x i d a t i o n i s n o ti n d i re c t a n d i n i t i a te d b y H - a t o m a b s t r a c t io n ; t h i s m e c h -a n i s m is k n o w n t o b e i m p o r t a n t f o r t h e o x i d a t i o n o fa l c o h o l s [ 9 ] . M a t a i r e s i n o l ( 2 ) i s n o t c o n v e r t e d i n t ovani l l in (4) as eff ic ient ly as i s the l ignin so e le c t ron t rans -f e r f r o m t h e e l e c tr o n - r ic h 3 - m e t h o x y - 4 - h y d r o x y p h e n y lg roup i s no t t he so l e c r i t e r i on fo r success .

A w o r k i n g h y p o t h e s is f o r th e m e c h a n i s m c a n h o w -e v e r b e d e v e l o p e d f o l l o w i n g d e f i n i ti o n o f t h e r o l e o ft h e e s s e n t i a l a l k a l i n e p r e - t r e a t m e n t a n d a s s u m p t i o nt h a t ~ - h y d r o x y l a t i o n ai d s d e g r a d a t i o n ( m a t a ir e s in o lc f . l ig n i n ). O t h e r e s t a b l i s h e d f e a t u r e s t o b e a c c o m m o -d a t e d i n a n h y p o t h e s i s a r e t h a t t h e b e t t e r o x i d a n t(1 , 3 -d in i t robenze ne ) i s t he mor e e f fec t i ve add i t i ve .Al t e rna t i ve ro l e s fo r t he n i t ro a rom a t i c a re t ha t i t e i t he ri n t r o d u c e s a r e a c t i o n w h i c h c o m p e t e s w e ll w i t h f u r t h e rc o n d e n s a t i o n o r t h a t i t r e ta r d s s u c h c o n d e n s a t i o n .

T h e r e i s m u c h e v i d e n ce [ 1 0] t h a t q u i n o n e m e t h i d e s( e .g . 1 0 ) a re c r u c i a l l y i n v o l v e d i n b o t h t h e f o r m a t i o na n d c l e a v a g e o f l ig n i n s . F u r t h e r m o r e , i n a t l e a s t o n ec a s e [ 1 1 ] r e d u c t i v e c l e a v a g e o f a l i g n in m o d e l c o m -p o u n d h a s b e e n e f f e c t e d b y r e d o x c a t a l y s i s i n v o l v i n gt h e c o r r e s p o n d i n g q u i n o m e t h i d e a n d a n t h r a q u i n o n er a d i c a l a n i o n .

0

R ~ N 2( 1 0 )

T h e s e c o n s i d e r a t i o n s a r e c o m b i n e d i n t h e p a t h w a y sp r o p o s e d i n S c h e m e 2 . T h e s u g g e s t i o n t h a t a l k a l i n ep r e - t re a t m e n t i n v ol v es t h e s - h y d r o x y l f u n c t i o n a n dc o n v e r s i o n i n t o a n e p o x i d e i s s t r o n g l y s u p p o r t e d b yp r e c e d en t . C o n v e r s i o n o f a l i g ni n m o d e l c o m p o u n di n t o t h e c o r r e s p o n d i n g e p o x i d e ( 1 1 ) ( s c h e m e 3 ) ,r e s u l t ed [ 12 ] f r o m r e a c t i o n i n a q u e o u s s o d i u m h y d r o x -i d e ( 2 M ) a t 1 70 ~ C , t h e s a m e c o n d i t i o n s u s e d i n o u re x p e r i m e n t s .

T h e f o r m a t i o n b y a n o d i c o x i d a t i o n o f p h e n o x y lr a d i c a ls ( 12 ) a n d o f p h e n o x o n i u m c a t i o n s ( 13 ) is a l sow e l l e st a b l is h e d [ 13 ]; in d e e d , t h e f o r m a t i o n o f l i g n i n i nn a t u r e i s b e li e v e d t o i n v o l v e c o m b i n a t i o n o f p h e n o x y lr a d i c a l s p r o d u c e d b y e n z y m a t i c o x i d a t i o n . Q u i n o n -me th ides a l so r ea c t [14, 15 ] , un l e ss h igh ly h in de red , t og i ve p o l y m e r s .

F i n a l ly , th e c a r b o n - c a r b o n b o n d c l ea v a g e d r i v e nb y t h e 1 , 3 d i s p o s i t io n o f a c a r b o c a t i o n i c c e n t r e a n do x y g e n h a s g o o d p r e c e d e n t [ 1 5] i n t h e f r a g m e n t a t i o n


5/5

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g i v e n i n s c h e m e 4 . T h e m e c h a n i s t i c s u g g e s t io n s u m -m a r i z e d i n S c h e m e 2 i s t h e r e f o r e c o n s i s t e n t w i t h t h ee v i d e n c e s o f a r o b t a i n e d , e x p l a i n s o x i d a t i o n t o t h ea l d e h y d e l e ve l ( t he p r e d o m i n a n t r e a c t i o n ), a n d i sp o w e r f u l l y s u p p o r t e d b y l i t e r a t u r e p r e c e d e n t .

OH O2 . N a O H /' 7 0 ~ / % ~ . / % %A r OHAr OH

O A r ( 1~ )Scheme 3

+ ArO-

CO2H_ H Anedr

_ - 2e~-H"OH - CO~ ~ H OScheme 4

H o w e v e r , t h e r ol e o f a d d e d n i t r o a r o m a t i c h a s n o tb e e n e x p l a i n e d a n d c a n o n l y b e t h e s u b j e c t o f s p e c u -l a t i o n . T h e d i s p l a c e m e n t o f t h e T a f e l c u r v e s ( F ig . 1 )s u g g e s t s a m o d i f i c a t i o n o f t h e e l e c t r o d e c o n d i t i o n ,u s u a l l y t h e l a s t r e f u ge o f t h e c o n f u s e d e l e c t r o c h e m i s t ,o r i n t e r a c t i o n b e t w e e n t h e l ig n i n a n d a d d i t i v e . I n t h ea b s e n c e o f a d d it i v e t h e n i c ke l a n o d e i s o p e r a t i n gu n d e r c o n d i t i o n s n o r m a l l y a s s o c i a t e d [ 1 6] w i t h N i ( I I I )o x i d e f o r m a t i o n . O x i d a t i o n u n d e r t h e s e c o n d i t i o n s ,h o w e v e r , r a r e l y s t o p s a t t h e a l d e h y d e l e v e l . I n s o m ee x p e r i m e n t s s i g n i f i c a n t o v e r o x i d a t i o n w a s o b s e r v e d ,e v e n t o t h e c a r b o n d i o x i d e l e v e l . T h e s e o b s e r v a t i o n sw o u l d b e c o n s i s t e n t w i t h o x i d a t i o n b y N i ( I I I ) o x id e ;c o m p l e t e o x i d a t i o n t o c a r b o n d i o x i d e w o u l d a l s o b ee x p e c t e d f o l l o w i n g t h e o b s e r v a t i o n [ 1 8 ] t h a t l i g n i n -m o d e l q u i n o n e s a r e o x i d iz e d b y a l k a li n e h y d r o -g e n p e r o x i d e t o m i x t u r e s o f l o w m o l e c u l a r w e i g h tm o n o - a n d d i c a r b o x y l i c a c i d s w h i c h w o u l d c e r t a i n l yd e c a r b o x y l a t e a n o d i c a l l y .

T h e n i t r o a r o m a t i c c o u l d p o s s i b l y i n t e rc e d e i n th ep a t h w a y s g i v e n i n S c h e m e 2 b y f a c i li t a t in g t h e s e c o n de l e c t r o n t r a n s f e r . S u c h a n e x p l a n a t i o n i n v o l v e s t h ef u r t h e r a s s u m p t i o n t h a t p o l y m e r i z a t i o n i s m a i n l y vi ap h e n o x y l r a d i c a l s a n d c l e a v a g e v i a p h e n o x o n i u mc a t io n s . T h e a r o m a t i c n i t r o c o m p o u n d m i g h t b y a c ti v ev i a a d s o r p t i o n o r c o n c e n t r a t i o n a t t h e a n o d e , o r b ys p e ci fi c c o m p l e x a t i o n w i t h t h e p h e n o l i c s u b s t r a t e .S u c h c o m p l e x a t i o n i s w e l l k n o w n [8] f o r p h e n o l s a n dn i t r o a r o m a t i c s a n d w o u l d e x p l a i n t h e r e p r o d u c i b i l i t yo f t h e c u r v e s i n F ig . 1 f o r a s y s t e m w h i c h w o u l do t h e r w i s e b e e x p e c t e d t o b e h e t e r o g e n e o u s ( s e ea b o v e ) . T h e g r e a t e r e f f e c t iv e n e s s o f t h e b e t t e r o x i d a n t( 1 , 3 - d i n i t r o b e n z e n e ) f o l l o w s . A l t e r n a t i v e l y t h e s e c o n de l e c t r o n t r a n s f e r m a y b e f a c i l i ta t e d s i m p l y b e c a u s e t h ea d d i t i v e c a u s e s , a t a g i v e n c u r r e n t d e n s i t y , a s h i f t t om o r e a n o d i c p o t e n t ia l s b y w h a t e v e r m e c h a n i s m . I nt h e s u c c e ss f u l e x p e r i m e n t s t h e a m o u n t o f n it r o -a r o m a t i c a d d i t i v e w a s r o u g h l y e q u i v a l e n t t o t h ea m o u n t o f li g ni n .

AcknowledgementsP a r t o f t h e w o r k d e s c r i b e d w a s s u p p o r t e d b y t h eM i n i s t r y o f A g r i c u l t u r e , F i s h e ri e s , a n d F o o d u n d e rc o n t r a c t M A F F N o . C S A 7 36 . W e a r e a l so g r a te f u l tot h e B r i ti s h C o u n c i l a n d t o th e F u n d a c a o d e A m p a r aa P e sq u i sa d o E s t a d o d e S a o P a u l o ( F A P E S P ) f o rs t u d e n t s h i p s ( t o M . P . ) a n d o t h e r f i n a nc i a l s u p p o r t( J . H . P . U . a n d H . V . ).

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biomass electrochemistry anodic oxidation of an organosolv lignin in the presence of tics

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