Production and Hydrolytic Depolymerization of Ethylene Glycol Lignin

Chapter 17

Production and Hydrolytic Depolymerization of Ethylene Glycol Lignin R. W. Thring , E . Chornet , R. P. Overend , and M . Heitz 1

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Département de Génie Chimique, Faculté des Sciences Appliquées, Université de Sherbrooke, Sherbrooke, Québec J1K 2R1, Canada Conseil National de Recherches du Canada, Ottawa, Ontario K1A 0R6, Canada

Downloaded by UNIV OF PITTSBURGH on February 1, 2016 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0397.ch017

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A prototype solvolytic lignin has been isolated from Populus deltoides using ethylene glycol as solvent in a process development unit. The characteristics of this lignin by elemental analysis, methoxyl content, thermogravimetric analysis, gel permeation chromatography, F T I R , and C N M R , suggest that this is a native-type lignin. Depolymerization of this lignin by alkaline hydrolysis produces reasonable yields of monomeric compounds: at optimum conditions of 2% sodium hydroxide, and a treatment severity corresponding to 300°C and 1 h reaction conditions, a maximum of 11% of identifiable monomers, based on the initial lignin, are produced, of which 9% are catechol and its methyl and ethyl derivatives. 13

H y d r o l y s i s o f l i g n i n i n acidic a n d basic m e d i a has received a t t e n t i o n due t o the r a t h e r few a n d s i m p l e d e g r a d a t i o n p r o d u c t s o b t a i n e d . A c i d - c a t a l y z e d h y d r o l y s i s reactions a p p l i e d t o isolated l i g n i n have been s t u d i e d b y a n u m ber o f workers. L u n d q u i s t (1), for e x a m p l e , s u b j e c t e d B j o r k m a n l i g n i n t o acidolysis a n d o b t a i n e d significant yields o f m o n o m e r i c p r o d u c t s . A r e v i e w o f the w o r k p r i o r t o 1971 has been m a d e b y W a l l i s (2). A l k a l i n e degradations, w h i c h also involve h y d r o l y t i c reactions, c a n s p l i t the l i g n i n a n d y i e l d useful low m o l e c u l a r f r a g m e n t s . T h e O H ~ g r o u p has l o n g been k n o w n t o cleave l i g n i n b o n d s a n d is one o f the t w o p r i m a r y c a t a l y t i c agents ( S is the other) responsible for the d i s s o l u t i o n o f l i g n i n i n the k r a f t process. A l k a l i n e h y d r o l y s i s , as c o m p a r e d t o other m e t h o d s o f l i g n i n d e g r a d a t i o n , has certain advantages. T h e c a t a l y t i c reagents are i n e x p e n s i v e a n d available c o m m e r c i a l l y , a n d require no elaborate m e t h o d o f p r e p a r a t i o n . =

0097-6156/89/0397-0228$06.00/0 © 1989 American Chemical Society

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

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Production of Ethylene Glycol Lignin

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A l s o , the yields o f the m o n o m e r i c p r o d u c t s c a n be significant a n d the p r o d uct s p e c t r u m as a whole is not extensive. It is established (3) t h a t these h y d r o x y l groups w i l l o n l y cleave c e r t a i n C - O - C b o n d s , n a m e l y the f o l l o w i n g : 1. α-aryl ether b o n d s p r o v i d e d t h e y c o n t a i n a free p h e n o l i c O H ~ g r o u p i n the p a r a p o s i t i o n of the α-aryl ether g r o u p or a free alcoholic O H " g r o u p o n the /?-carbon a t o m . 2. / ? - a r y l ethers i f the p h e n o l i c O H ~ g r o u p i n the p a r a p o s i t i o n t o the / ? - a r y l ether side c h a i n is etherified a n d i f there is a free alcoholic O H ~ g r o u p b o u n d t o the a - a n d / o r 7~position(s) o f the p r o p a n e side c h a i n . T h e b e h a v i o r o f m i l l e d w o o d l i g n i n a n d i t s various m o d i f i c a t i o n s t o w a r d s a l k a l i ( s o d i u m h y d r o x i d e ) t r e a t m e n t s agreed well w i t h the results o f m o d e l e x p e r i m e n t s (4). C l a r k a n d G r e e n (5) s t u d i e d the p r o d u c t i o n o f phenols f r o m alkaline h y d r o l y s i s o f k r a f t l i g n i n . T h e l i g n i n was cooked at 260-310°C i n s o l u t i o n s o f s o d i u m h y d r o x i d e a n d s o d i u m sulfide. P r i n c i p a l p r o d u c t s i d e n t i f i e d a n d quantified were g u a i a c o l , catechol, m e t h y l - a n d e t h y l - g u a i a c o l s , m e t h y l a n d e t h y l - catechols, a n d p h e n o l . A m a x i m u m q u a n t i t y o f these, a m o u n t i n g t o 1 1 % o f the l i g n i n , o c c u r r e d w h e n the l i g n i n was cooked i n 4 % N a O H at 3 0 0 ° C for 30 m i n u t e s . C a t e c h o l was f o u n d t o be the m o s t a b u n d a n t m o n o m e r i c p r o d u c t , t h a t i s , a b o u t 5.3% o f the l i g n i n at these o p t i m u m conditions. Demethylation of guaiacyl compounds and degradation of o t h ers t o o k place w h e n s o d i u m s u l p h i d e was i n c l u d e d d u r i n g c o o k i n g , w h i c h resulted i n reduced y i e l d s o f t o t a l p h e n o l i c c o m p o u n d s i d e n t i f i e d . H a g g l u n d a n d E n k v i s t (6) developed a l a b o r a t o r y scale m e t h o d for m a n u f a c t u r i n g m e t h y l sulfide f r o m k r a f t b l a c k l i q u o r b y pressure h e a t i n g after a d d i t i o n o f s o d i u m sulfide. T h i s process was l a t e r t a k e n over b y C r o w n - Z e l l e r b a c h i n the U n i t e d States a n d developed i n p i l o t p l a n t a n d f u l l scale. However, the y i e l d is o n l y about 7% o f the i n i t i a l l i g n i n u t i l i z e d i n the process. E n k v i s t a n d co-workers (7) pressure heated k r a f t b l a c k liquors w i t h a d d i t i o n s o f s m a l l a m o u n t s o f N a S a n d N a O H for 10-20 m i n u t e s at 250290°C i n b a t c h as w e l l as i n s i m p l e continuous a p p a r a t u s . T h e a d d i t i o n o f s o d i u m sulfide was f o u n d t o increase the f o r m a t i o n o f ether-soluble m a t e r i a l . T h e highest y i e l d s o f ether-soluble phenols o c c u r r e d w i t h pressure h e a t i n g at a b o u t 291°C o f spent k r a f t l i q u o r c o n t a i n i n g 20.4% o f o r g a n i c s u b s t a n c e after a d d i t i o n of 3.2% N a S a n d 1.6% N a O H . T h e m a x i m u m y i e l d o f ethersoluble m a t e r i a l was 3 3 % o f the organic substance, w i t h catechol a n d i t s nearest homologues c o m p r i s i n g 5%. A n o t h e r p r o d u c t t h a t is p r o d u c e d c o m m e r c i a l l y f r o m the alkaline h y d r o l y s i s o f l i g n i n is v a n i l l i n . It was f o u n d t h a t the y i e l d c o u l d be i m p r o v e d b y the presence o f o x y g e n d u r i n g the alkaline h y d r o l y s i s r e a c t i o n . A n u m b e r o f c o m m e r c i a l ventures have been based o n t h i s procedure ( 8 , 9 ) . T h e focus o f o u r work is t o p r o d u c e a p r o t o t y p e s o l v o l y t i c l i g n i n , c h a r acterize i t , a n d degrade i t t o lower m o l e c u l a r weight chemicals b y h y d r o l y s i s w i t h s o d i u m h y d r o x i d e . T h e breakage o f a r o m a t i c ether linkages has been s h o w n t o be a d o m i n a n t r e a c t i o n i n alkaline d e l i g n i f i c a t i o n processes. It is therefore o f interest t o use t h i s a p p a r e n t l y s i m p l e a n d p r o m i s i n g a p p r o a c h t o investigate the c h e m i c a l u t i l i z a t i o n o f o u r l i g n i n . 2

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LIGNIN: PROPERTIES AND MATERIALS


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T h e efficiency o f ethylene g l y c o l - w a t e r as a d e l i g n i f y i n g solvent has been d e m o n s t r a t e d b y G a s t a n d P u i s (10). R e s u l t s showed t h a t sufficiently delignified p u l p s c o u l d be o b t a i n e d . A l s o , the l i g n i n s p r o d u c e d showed p r o m i s i n g results as extenders i n phenolic resin adhesives. E t h y l e n e g l y c o l p u l p i n g has been p r e v i o u s l y s t u d i e d i n o u r l a b o r a t o r i e s ( 1 1 , 1 2 ) . W h e n a p p l i e d t o Populus deltoïdes, i t acts as a " p r o t e c t i v e " s o l vent for cellulose w h i c h is o n l y s l i g h t l y d e p o l y m e r i z e d even at t e m p e r a t u r e s as h i g h as 220-230°C. U n d e r these c o n d i t i o n s the l i g n i n c a n be scavenged f r o m the w o o d m a t r i x a n d dissolved i n the ethylene g l y c o l . A f r a c t i o n a t i o n o f the m a i n c o n s t i t u t i v e p o l y m e r s can t h u s be achieved. T h e present w o r k a i m s at u s i n g the l i g n i n f r a c t i o n as feedstock for alkaline d e p o l y m e r i z a t i o n to monomeric products. T h i s p a p e r , t h e n , describes the p r e p a r a t i o n a n d c h a r a c t e r i z a t i o n o f a solvolysis l i g n i n prepared u n d e r c o n d i t i o n s t h a t are very different t o p r e v i o u s s o l v o l y t i c l i g n i n p r e p a r a t i o n s . I n p a r t i c u l a r , n o water, a c i d or base is used i n t h i s process d u r i n g the d e l i g n i f i c a t i o n step. T h e l i g n i n is t h e n a c i d p r e c i p i t a t e d a n d h y d r o l y z e d i n d i l u t e aqueous s o l u t i o n t o p r o d u c e m o n o m e r i c s u b s t i t u t e d phenols.

Experimental Materials. A i r d r i e d w o o d (Populus deltoïdes) was g r o u n d t o pass t h r o u g h a screen o f 0.5 m m mesh size. These "fines" were used i n the subsequent s o l v o l y t i c studies. T h e chemicals used were: ethylene g l y c o l ( p r a c t i c a l grade); s o d i u m h y d r o x i d e ( p r a c t i c a l grade); e t h a n o l ( 9 5 % ) ; d i e t h y l ether. C a l i b r a t i o n - p h e n o l s were purchased f r o m A l d r i c h C h e m i c a l s L i m i t e d a n d A l f a Chemicals Limited. Set-up for Ethylene Glycol Lignin Production. A process development u n i t ( P D U ) , p r e v i o u s l y described b y C h o r n e t a n d c o w o r k e r s (11), was used for the e x p e r i m e n t s . A t y p i c a l p r e p a r a t i o n consists of i n i t i a l l y m i x i n g 1-1.2 k g o f w o o d m e a l w i t h 10 1 o f ethylene g l y c o l . T h e m i x t u r e is allowed t o s t a n d overnight for i m b i b i t i o n t o take place. T o enhance solvent t o s u b s t r a t e p e n e t r a t i o n , the s l u r r y is homogenized at 200°C i n the p r e t r e a t m e n t s e c t i o n o f the P D U . It is t h e n p u m p e d t h r o u g h the t r e a t m e n t section w h i c h consists o f a t u b u l a r reactor at 220°C. T h e p r o d u c t s l u r r y is collected i n a receiver. T h e d e t a i l e d procedure a n d choice o f c o n d i t i o n s above have been p u b l i s h e d elsewhere ( 1 1 , 1 2 ) . T h e p r o d u c t s l u r r y is s u c t i o n filtered, w h i l s t h o t , a n d the residue washed w i t h hot ethylene g l y c o l (at 100°C). T h e l i q u i d p r o d u c t a n d first w a s h i n g are c o m b i n e d a n d stored i n the refrigerator t o be used l a t e r . F o r i s o l a t i n g the l i g n i n f r o m the g l y c o l / h e m i c e l l u l o s e s o l u t i o n , i t was necessary t o use d i l u t e aqueous H C 1 as a p r e c i p i t a t i n g c a t a l y s t . T h e best c o n d i t i o n s f o u n d were for 0 . 0 5 % aqueous H C 1 ( a c i d : black l i q u o r = 3:1) at Τ = 5 0 6 0 ° C . B y t h i s procedure 7 0 - 8 0 % o f the l i g n i n f r o m the w o o d is recovered. Lignin Characterization. E l e m e n t a l a n a l y s i s , T G A , m e t h o x y l content, G P C , F T I R , and C N M R were used t o characterize o u r p r o t o t y p e solvolytic lignin. 1 3



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Typical of most isolated lignins, an elemental analysis of glycol lignin in our laboratories using a Perkin Elmer 240C instrument showed it to contain 61.85% C , 6.25% H , 0.16% N , and 33.83% Ο (by difference). Methoxyl con tent determinations were made according to the method initially proposed by Zeisel (13) and later modified by Haluk (14). Methoxyl and ash content were 20% and 0.11%, respectively. T h e presence of sugars was determined to be less than 1%. A comparison between the thermal degradation rate of glycol lignin and kraft Indulin A T by T G A showed that the former de graded at a faster rate than the latter. T h e maximum rate of weight loss was about twice that of kraft lignin and occurred at about 360° C , compared to 400°C for kraft. G P C analysis of acetylated glycol lignin was carried out on a Varian 5000 Liquid Chromatograph equipped with two P L gel columns (50Àand 500Â) connected in series. Tetrahydrofuran was used as eluent. T h e column set was calibrated with monodisperse polystyrene standards for molecular weight determination. Molecular weight averages for derivatized glycol lignin were calculated to be: M = 986 and M = 4762. n

w

The infrared spectrum of glycol lignin is shown in Figure 1. A 5 D X B Nicolet F T I R spectrometer in diffuse reflectence mode was used. T h e sample was prepared by the K B r disk method. Assignment of absorption bands is based on information from Herget (15) and Winston (16). Infrared spectra of glycol and milled wood lignin from the same wood appeared to be similar. One notable difference was the large carbonyl band at 1738 c m " due to carbohydrates present in the M W L . T h e sample of M W L used was known to contain 6-8% sugars. The C N M R spectrum (90 M H z , 8.4 T , 2 seconds delay) of glycol lignin is presented in Figure 2. DMSO-de was used as solvent. Assignment of the major signals is based on the work of Lapierre et ai (17). T h e spectrum is very similar to that of milled wood lignin except that at the low field there appear to be more pronounced aliphatic peaks. Identification of these were not undertaken because the glycol lignin was isolated from as-received hardwood which was not extractives-free. 1

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Depolymerization. Subsequent depolymerization of this lignin was carried out in a 500 ml magnetically stirred autoclave. A typical procedure for the experiments was to load the autoclave with 5 g of dry lignin (dried at 6 0 ° C overnight), 100 ml solvent, and 0-6 g sodium hydroxide. T h e bomb was sealed, secured onto its support frame, then the gas inlet, outlet, and pressure gauge were connected. After purging the reactor with nitrogen to remove air, the stirrer was set at 500 rpm and switched on. T o start a run, the reactor was immersed in a preheated salt bath and the temperature and time were recorded by computer for subsequent calculation of the reaction ordinate, to be defined later. Typically, the desired reaction temperature ( ± 3 ° C experimental error) was reached within the first ten minutes of heating. After the desired treatment was attained, the reactor was quenched in a cold water bath. The stirrer and computer recording were stopped


In Lignin; Glasser, Wolfgang G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989. F i g u r e 1. I n f r a r e d s p e c t r u m o f g l y c o l l i g n i n .



—ι—

150

—ι—

160

—ι—

170

140 120

—I—

F i g u r e 2.

130

—ι—

1

3

—T— 110

100 PPM

—ι—

90

—r—

80

—r—

70

C N M R spectrum of glycol lignin.

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G guaiacyl S « syringyl CH = c a r b o h y d r a t e

Assignments according to Laplerre et a l . (1982)

60

—r—

50


40

D

30

Ci

i

ι

n

z Ci

S


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when the reactor temperature reached its initial value. The bomb was depressurized, if necessary, removed from its frame, opened, and drained of its contents. After reaction, the slurry product consisted of a liquid mixture together with some insoluble material. This slurry was filtered and the residue thor oughly washed with water. Further separation of the reaction products was carried out as shown in Figure 3. A n aliquot of the liquid was acidified, after ethanol removal (if necessary) with 10% (v/v) HC1 to a p H of 1-2, then filtered. The filtrate was extracted with diethyl ether till the aqueous layer was judged colorless by eye. After removal of the ether by rotary evaporation, the extract was analyzed by capillary gas chromatography. T h e solvolytic treatment and the recovery procedures employed yielded about 75% of the original Klason lignin present in the wood. This percent age could be further optimized but was not attempted in the present work. The recovered lignin was subjected to alkaline depolymerization. Yields presented throughout the paper are expressed as percentages of the recov ered lignin. Material balances during the depolymerization step were within 5% closure for all the experiments reported. Gas Chromatographic Analysis. Ether-soluble fractions from the hydrol ysis runs were analyzed by capillary G C (Hewlett-Packard Model 5890A gas chromatography D B - 5 column of 30 m length, 0.25 m m I.D.; flame ionization detector; 1.5 ml H e / m i n ; split injection 1:100; oven tempera ture programmed from 65° C to 220° C at 3°C/min (hold 140° C for 5 min and 220°C for 10 min). Products were acetylated by adding 1-2 ml acetic anhydride and 2 drops of pyridine to the sample and heating at 60° C for 1 h. The peaks assigned to phenol, o- and p- cresol, guaiacol, 4-methyl, 4-ethyl- , 4-propylguaiacol, catechol, 4-methyl-, 4-ethylcatechol, syringol, vanillin, acetovanillone, syringaldehyde, and acetosyringone, were identical (retention time) to those of pure compounds. Identification of peaks was further confirmed by comparing mass spectral fragmentation patterns of products in the sample to pure compounds by G C / M S . For quantitative estimation of identified monomers, response factors were calculated, using 4-ethyl-resorcinol as an internal standard. T h e rela tive error in the determination of all the compounds was ± 4 % .

Results and Discussion Most of the results obtained from the hydrolysis experiments were analyzed and represented as a function of R a reaction ordinate, previously defined and used (18). It is defined as C 1

(1) where

To =constant (taken to be 100°C) λ =constant (taken to be 14.75°C t =time (in min.).



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HYDROLYSIS PRODUCT MIXTURE

Filtration + Water Wash REACTOR RESIDUE

LIQUIDS1. Ethanol Removal (If necessary) 2. Acidify to pH 1-2 3. Filtration + Wash

RESIDUAL LIGNIN

LIQUID Ether Extraction

AQUEOUS ^ > LAYER

^ORGANIC LAYER

ι

Ether Removal

EXTRACT (GC Analysis) (Ether Fraction) F i g u r e 3. P r o c e d u r e for s e p a r a t i o n o f h y d r o l y s i s p r o d u c t s .



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T h e r e a c t i o n o r d i n a t e is u t i l i z e d here as a means o f c o m b i n i n g the effects o f t e m p e r a t u r e a n d t i m e . I n presenting a n d discussing o u r results, the d a t a is presented o n plots of the d e r i v e d d a t a versus l o g i o R C o n v e r s i o n of l i g n i n to l i q u i d a n d gaseous p r o d u c t s was d e t e r m i n e d as follows: % conversion = ( W i - (W + W )/Wi) x 100 (2) 0

x


where

2

W{ = weight of i n i t i a l d r y l i g n i n (g) W\ = weight of reactor residue (g) W2 = weight of r e s i d u a l l i g n i n (g)

Effect of Solvent. F i g u r e 4 shows the h y d r o l y s i s of g l y c o l l i g n i n i n 3 % s o d i u m h y d r o x i d e u s i n g water o n l y a n d a n e t h a n o l / w a t e r m i x t u r e as s o l vent. A s seen, conversion c a n be a p p r o x i m a t e d b y a l i n e a r increase w i t h severity of t r e a t m e n t , for b o t h cases. A s i m i l a r result was f o u n d i n the a l kaline h y d r o l y s i s o f k r a f t l i g n i n i n 2 % s o d i u m h y d r o x i d e s o l u t i o n b y C l a r k a n d G r e e n (5). W h e n water o n l y is used, the m a x i m u m y i e l d of ether s o l uble m a t e r i a l is about 2 0 % of the o r i g i n a l l i g n i n , w i t h m o s t of the l i g n i n d e c o m p o s i n g t o v o l a t i l e p r o d u c t s at higher t r e a t m e n t severities. H o w e v e r , the q u a n t i t y of ether soluble m a t e r i a l o b t a i n e d u s i n g the s o l vent m i x t u r e was higher a n d reached a m a x i m u m of a p p r o x i m a t e l y 2 5 % of the o r i g i n a l l i g n i n . T h i s suggests t h a t the presence of e t h a n o l enhances the d e g r a d a t i o n of l i g n i n i n a l k a l i n e m e d i a . A l s o , i t appears t h a t for either solvent, there is a c r i t i c a l value o f reaction t e m p e r a t u r e a n d t i m e ( a n d o b v i o u s l y o f R ) b e y o n d w h i c h the q u a n t i t y of ether soluble m a t e r i a l r e m a i n s essentially the same. T h u s , i t can be s a i d t h a t i n u s i n g either solvent, o n l y a f r a c t i o n o f the g l y c o l l i g n i n is h y d r o l y z e d i n t o a n ether soluble m a t e r i a l containing monomeric compounds. T h e s p e c t r a of m o n o m e r i c p r o d u c t s i n the ether e x t r a c t , w h e n either solvent was used i n the r e a c t i o n , were v e r y s i m i l a r . C o m p o u n d s i d e n t i fied were: p h e n o l , o c r e s o l , p-cresol, g u a i a c o l , 4 - m e t h y l - , 4 - e t h y l - , a n d 4 - n p r o p y l - g u a i a c o l , catechol, 4 - m e t h y l - a n d 4-ethyl-catechol, v a n i l l i n , s y r i n g o l , s y r i n g a l d e h y d e , acetovanillone a n d acetosyringone. C o n f i r m a t i o n o f t h e i r i d e n t i t y was c a r r i e d out b y c o m p a r i n g r e t e n t i o n t i m e s a n d mass s p e c t r a l f r a g m e n t a t i o n p a t t e r n s w i t h those of a u t h e n t i c c o m p o u n d s . F i g u r e 5 shows p l o t s of the y i e l d s of p h e n o l , g u a i a c o l a n d s y r i n g o l versus R u s i n g water o n l y a n d a 5 0 / 5 0 ethanol-water m i x t u r e , respectively. F o r the same r e a c t i o n c o n d i t i o n s , the y i e l d s of each of the c o m p o u n d s , especially g u a i a c o l a n d s y r i n g o l , are higher w h e n water o n l y is used. It is apparent t h a t the presence of e t h a n o l i n h i b i t s the d e p o l y m e r i z a t i o n o f g l y c o l l i g n i n t o the m o n o m e r s cited above. T h e complete set of d a t a p o i n t s is s h o w n as a f u n c t i o n o f t e m p e r a t u r e i n F i g u r e 6. T h e e x p e r i m e n t s r e p o r t e d below were c a r r i e d o u t u s i n g water as the solvent. 0

0

Effect of Alkali Concentration. F i g u r e 7 A depicts t h a t the v a r i a t i o n of the conversion o f g l y c o l l i g n i n w i t h s o d i u m h y d r o x i d e c o n c e n t r a t i o n reaches a p l a t e a u at about 6 0 % . A l s o , the ether soluble m a t e r i a l r e m a i n s constant


17.

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237


Water

log (fymin) 10

F i g u r e 4. Effect o f solvent o n conversion a n d ether solubles of g l y c o l l i g n i n depolymerized i n 3% N a O H .



238


ETHANOL/WATER

4.8

5.8

6.5

log (R /min) 10

0

Figure 5. Effect of solvent on monomeric product yields in 3% N a O H .


TURING ET AL.



17.


239



240

Figure 7. Effect of alkali concentration on conversion and product yields (logxoRo = 7.7).



17.

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241

at a maximum of 20% of the initial lignin with increasing concentration of sodium hydroxide. It is interesting to note that an aqueous treatment of glycol lignin without using sodium hydroxide under the same conditions reported in Figure 7A yields an ether soluble fraction of about 10%. The variation of catechol and total monomers identified with N a O H concentration is shown in Figure 7B. A maximum of approximately 11% total phenols and 6% catechol is reached at 2% N a O H with increasing alkaline concentration for the same treatment severity. It should be noted that Clark and Green (5) also obtained a maximum of 11% phenolic compounds when they cooked kraft lignin in 4% N a O H at 300°C for 30 minutes. The quantity of catechol was found to be 5.3% of the initial lignin under these conditions. Figure 7C shows that phenol reaches a minimum of 0.5% and stays constant around 1% with increasing concentration of sodium hydroxide. This indicates that the production of phenol from glycol lignin is relatively unaffected by alkaline hydrolysis. Guaiacol, on the other hand, reaches a maximum at 1% N a O H and then rapidly decreases with increasing concentration of sodium hydroxide. This increase in demethoxylation and/or demethylation of guaiacol with alkaline concentration has also been ascertained by Sarkanen and co-workers (19), who investigated the rate of hydrolysis by sodium hydroxide of methoxyl groups in lignin models and lignin. Catechol Production at Optimum Conditions. It has already been demonstrated that the production of monomeric compounds reaches a maximum when glycol lignin is cooked in 2% N a O H at a treatment severity corresponding to a reaction temperature and time of 300°C and 1 h, respectively. It was then necessary to see if higher yields could be achieved when the severity of treatment (R ) is varied. Figure 8A indicates that conversion and ether soluble material vary in the same manner as when 3% N a O H was used. However, the ether solubles only reach a maximum of 20% at higher treatment severities. The distribution of the identified total phenols and total catechols obtained with increasing reaction ordinate is shown in Figure 8B. The increasing amount of catechols in the total phenols identified implies that these are secondary lignin degradation products, probably originating from primary products such as vanillin, syringol, and syringaldehyde. These were found to occur at lower treatment severities in our experiments. As R is increased, reduction in the yields of all monomeric products occurs, demonstrating the increasing influence of pyrolytic reactions in the lignin to produce volatile liquid and gaseous products. As indicated in the separation scheme in Figure 6, a solid residue was recovered from the liquid hydrolysis products. The carbon-to-oxygen ratio of this material was found to increase rather dramatically with increasing R , as seen in Figure 8C. This ratio can either be taken as a measure of the degree of condensation, i.e., a highly condensed lignin means that it has a large number of interunit carbon-carbon bonds, or it can be interpreted as being the result of extensive demethoxylation. The differing levels of 0

0

0




242

Figure 8. Effect of treatment severity on glycol lignin depolymerization in 2% N a O H . C / O = carbon/oxygen ratio in residual lignins.


17.

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243

d e m e t h o x y l a t i o n are s h o w n as p a r a l l e l lines i n F i g u r e 8 C where the o r i g i n a l l i g n i n ( 2 0 % OCH3) appears as b e i n g progressively d e m e t h o x y l a t e d w i t h i n c r e a s i n g t r e a t m e n t severity. A q u a l i t a t i v e test o n the s o l u t i o n s i n d i c a t e d the presence of m e t h a n o l w h i c h provides further evidence of d e m e t h o x y l a tion.


Conclusions T h i s s t u d y has a g a i n d e m o n s t r a t e d the effectiveness of a l k a l i n e h y d r o l y s i s as a p r i m e m e t h o d of l i g n i n d e g r a d a t i o n t o r e a d i l y identifiable p h e n o l i c products. If, as a c r i t e r i o n of value t o the s t u d y of l i g n i n d e p o l y m e r i z a t i o n b y a l kaline h y d r o l y s i s , the m a x i m u m y i e l d of o x y g e n - b e a r i n g , p h e n y l p r o p a n o i d derivatives is chosen, t h e n the c o n d i t i o n s of such a s t u d y have been o p t i m i z e d here at a t r e a t m e n t severity corresponding t o a r e a c t i o n t e m p e r a t u r e of 300°C for 1 h o u r . U n d e r these c o n d i t i o n s , 2 0 % of the l i g n i n is recovered as ether-solubles of w h i c h 5 5 % is identifiable as m o n o m e r i c d e r i v a t i v e s . T h e rest of t h i s m a t e r i a l p r o b a b l y consists of d i m e r i c - t y p e c o m p o u n d s not identified b y c a p i l l a r y gas c h r o m a t o g r a p h y . F r o m t h i s work i t is e v i d e n t , t h e n , t h a t the h y d r o l y s i s of g l y c o l l i g n i n w i t h s o d i u m h y d r o x i d e does y i e l d a rather significant a m o u n t of catechol a n d i t s m e t h y l a n d e t h y l derivatives. A m a x i m u m q u a n t i t y of these, w h i c h a m o u n t e d to 9 % of the o r i g i n a l l i g n i n , was o b t a i n e d at the c o n d i t i o n s c i t e d above. T h e above results appear consistent w i t h a m o d e l for l i g n i n p r o p o s e d b y P e p p e r et al (20) w h i c h consists of a core s t r u c t u r e t o w h i c h are a t t a c h e d the m o r e r e a d i l y accessible s i d e - c h a i n u n i t s . T h e s e u n i t s comprise t h a t fragment of l i g n i n w h i c h is easily degraded a n d released as low m o l e c u l a r weight identifiable p r o d u c t s b y any d e g r a d a t i o n procedure. I n v i e w of our results, a possible route for l i g n i n d e p o l y m e r i z a t i o n c o u l d consist of p r o d u c i n g a n organosolv l i g n i n b y m e t h o d s analogous t o those used i n our w o r k . T h e l i g n i n thus p r o d u c e d can be d i r e c t l y t r e a t e d u n d e r a l k a l i c o n d i t i o n s at m i l d severities (logio R — 7.7) t o recover a low m o l e c u l a r weight f r a c t i o n i n w h i c h m o n o m e r i c p r o d u c t s are p r e d o m i n a n t . A s far as the ethylene g l y c o l l i g n i n is concerned, i t has been s h o w n t o be a n a t i v e - l i k e l i g n i n w h i c h can be p r o d u c e d a n d recovered b y direct s o l v o l y t i c t r e a t m e n t of the i n i t i a l lignocellulosic s u b s t r a t e . It w o u l d also be possible to remove the hemicelluloses v i a a n a q u e o u s / s t e a m t r e a t m e n t p r i o r to s o l v o l y t i c s e p a r a t i o n of the l i g n i n a n d cellulose. S u c h a n o p t i o n w o u l d f a c i l i t a t e the recovery of the three m a i n c o n s t i t u t i v e fractions of lignocellulosics i n significant y i e l d s . W o r k i n t h i s d i r e c t i o n is n o w underway. 0

Acknowledgments T h e a u t h o r s are i n d e b t e d t o J . B u r e a u , J . P . L e m o n n i e r , J . B o u c h a r d , a n d P . V i d a l for t h e i r t e c h n i c a l s u p p o r t . F i n a n c i a l c o n t r i b u t i o n s of N S E R C , N R C C a n d F C A R are g r a t e f u l l y acknowledged.



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Literature C i t e d 1. Lundquist, K . Acta Chem. Scand. 1973, 21, 2597. 2. Wallis, A . F . A . In Lignins: Occurrence, Formation, Structure and Reactions; Sarkanen, Κ. V . , Ludwig, C . F . , Eds.; Wiley-Interscience: New York, 1971; pp. 345-372. 3. Gierer, J.; Noren, I. Acta Chem. Scand. 1962, 16, 1713. 4. Gierer, J.; Lenz, B.; Noren, I.; Soderberg, S. Tappi 1964, 47(4), 233. 5. Clark. I. T . ; Green, J. Tappi 1968, 51(1), 44. 6. Hagglund, E . ; Enkvist, T . U.S. Patent 2 711 430, 1955. 7. Enkvist, T . ; Turunen, J.; Ashorn, T . Tappi 1962, 45(2), 128. 8. Tomlinson, G . H . ; Hibbert, H . Am. Chem. Soc. 1936, 58, 345. 9. Salvesen, J. R.; Brink, D. L.; Diddaus, D. G.; Owzarski, P. U.S. Patent 2 434 626, 1948. 10. Gast, D.; Puls, J. EEC Proc. 1985, p. 949. 11. Chornet, E . ; Vanasse, C . ; Overend, R. P. Entropie 1986, 130/131, 89. 12. Vanasse, C . M.Sc. Thesis, Univ. of Sherbrooke, Québec, Canada, 1986. 13. Zeisel, S. Monatsh Chem. 1985, 6, 989. 14. Haluk, J. P.; Metche, M . Cell. Chem. Technol. 1986, 20, 31. 15. Herget, H . L . In Lignins: Occurrence, Formation, Structure, and Reactions; Sarkanen, Κ. V . , Ludwig, C . H . , Eds.; Wiley-Interscience: New York, 1971; pp. 267-293. 16. Winston, M . H . Ph.D. Thesis, North Carolina State University, Raleigh, N C , 1987. 17. Lapierre, C . ; Lallemand, J. Y . ; Monties, Β. I. Holzforschung 1982, 36(6), 275. 18. Heitz, M . ; Carrasco, F.; Rubio, M . ; Brown, Α.; Chornet, E . ; Overend, R. P. Biomass 1987, 13, 255. 19. Sarkanen, Κ. V . ; Chirkin, G . ; Hrutfiord, B. F . Tappi 1963, 46(6), 375. 20. Pepper, J. M . ; Steck, W . F . ; Swoboda, R.; Karapally, J. C . Adv. Chem. Ser. 1966, 59, 238. RECEIVED May 29,1989


Production and Hydrolytic Depolymerization of Ethylene Glycol Lignin

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