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ALTERNATIVE PATHWAYS FOR PYROLYSIS OF CELLULOSE

F r ed Sha f i zadeh

Mis sou la, Montana 59812

D epa rtmen t o f C hem i s tr y , U n i v e r s i t y o f Montana

INTRODUCTION

Py r o l ys i s o f b iomass i n vo l ves heter ogeneous s ubs t r a t es and com pl ex r ea c t i o ns .

The s u b s tr a t e inay c o n t a i n v a r i o u s am ounts o f c e l l u l o s e , h e m i c e l l u lo s e s , l i g n i n ,

e x t r a c t i v e s and i n o r g a n i c s w i t h d i f f e r e n t t h e rm a l

p r o p e r t i e s . The r e a c t i o n s i n v o l v e

a l t e r n a t i v e p athw ays a nd c o n s e cu t iv e s e r i e s , w h ic h a r e a f f e c t e d b y p y r o l y t i c c on -

d i t i o n s i n c l u d i n g t h e t i m e a nd te m p er at ur e p r o f i l e , a m b ie n t a tm os ph er e a nd c a t a l y s i s

b y i n o r g a n i c m a t e r i a l s .

t he p r oduc ts a r e h i g h l y dependen t on t h e emp loyed subs t r a t e and py r o l y t i c co nd i t i on .

T h is s i t u a t io n a cc oun ts f o r v a r i a t i o n o f t h e re s u l t s , p a r t i c u l a r l y

on

k i n e t i c

s t u d ie s , w hic h may r an ge f r o m k i n e t i c s o f s p e c i f i c c h e m ic a l r e a c t i o n s t o g l o b a l

k i n e t i c s o r m o d el in g o f an e n t i r e p ro c es s . The l a t t e r may b e b ase d

on

a r a t e

d e t e rm i n i ng ch em ic al s t e p o r e ve n a p h y s i c a l p r o c e ss su ch a s h e a t o r m a t e r i a l t r a n s -

f e r . P y r o l y s i s

o f

c e l l u l o s e i l l u s t r a t e s t h es e p o i n t s . The a l t e r n a t i v e pathways

proposed f o r p y r o l y s i s o f t h i s m a t e r i a l i s s hown b e lo w ( 1,2 ).

C on se qu en tly , c o m p o s it i on , y i e l d a nd r a t e o f f o r m a t i o n o f

F i s s i o n p r o d u ct s , C Hys H2 (3)

Ce l lu los e nhydrosugars , Ta r

har,

H20

C02,

CO

(2 )

1)

Py r o l ys i s a t t en ipe r atu res be l ow

300C

i n v o l v e s d e p o l y m e r i z a ti o n , d e h y d ra t i on ,

r ea rr angem ent and f o r m a t i o n o f ca r boxy l and ca r b ony l g roups , ev o l u t i o n o f CO and CO

developm en t o f f r ee r a d i ca l s and condensa t ion t o cha r .

t he s e r e a c t i o n s a r e a ccom panied b y t h e c o n v e r si o n o f t h e g l y c o s y l u n i t s t o l e v o g l u -

cosan by tr a n s g ly c o s y l at i o n . T h i s r e a c t i o n i s p re ce de d b y t h e a c t i v a t i o n o f t h e

m o l ecu le , p resum abl y t h r ough g l ass t r a n s i t i o n , w h i ch g i ves t h e r e qu i r e d con fo rm a -

t i o n a l f l e x i b i l i t y . A t s t i l l h i gh e r t em p er at ur es (above 500 C), t h e g l y c o s yl

s t r u c t u r e

o f

t h e l e v o gl u co s an o r c e l l u l o s e r a p i d l y b re a ks down t o p r o v i d e a v a r i e t y

o f

low m o l ecu l a r w e i gh t f i s s i o n p r oduc t s , i n c l u d i ng hyd r oca r bons and hydr ogen as w e l l

as

COS CO

and H20 ob ta in ed a t l ower tempe ra tu res .

f u r t h e r r g a c t t o a l t e r t h e c o m po si ti on

o f

t he py r o l ysa te . The seconda r y r e ac t i o n may

p ro ce ed i n t h e gas p ha se a s f u r t h e r d e c o m p o s it i on o f l e v o gl u c o sa n ,

i n

t h e s o l i d phase

a s t h e co n de n sa ti on and c r o s s l i n k i n g o f i n t e r m e d i a t e c h a r s t o n i g h l y c on den se d p o l y -

c y c l i c a ro ma tic s t ru c t ur e s , o r b y i n t e r a c t i o n o f b o t h phases a s g a s i f i c a t i o n o f c h ar

b y r e a c t i o n w i t h HZO and C02 a t h igh tempera tu res t o p roduce CO and H2. n v i e w o f

a l l t he se p o s s i b i l i t i e s t h e k i n e t i c d a ta a re v a l i d o n l y f o r s p e c i f i c che mic al r ea c-

t i o n s o r w e l l d e fi ne d s y s t e m . G e n e r a l i z a ti o n c o u l d

be

m i s l e a d i n g a nd c o n t r o v e r s i a l ,

because o f ma jo r d i f fe ren ce s i n t h e r e s u l t s o b t ai n ed u n de r d i f f e r e n t c o r i d it io n s .

These d i f ferences are shown

i n

t h e f o l l o w in g examples o f p y r o l y s i s o f c e l l u l o s e u n de r

d i f f e r e n t c o n d it io n s .

A t tempe ra tu res above 3OOOC;

The p y r o l y s i s p r o d u ct s c o u l d

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2

KINETICS OF CELLULOSE PYROLYSIS

Global Kinetics

The gl o ba l k i n e t i c s f o r i so t he rm a l e v o l u t i o n o f v o l a t i l e p y r o l y s i s p r o d u ct s

from p u r if i e d c o tt o n l i n t e r c e l l u l o s e , w i t h i n the tempera ture range

o f

275-310C,

has been studied

i n

a i r an d n i tr o g e n

3 ) .

The Arrhenius p lo t o f

the

r e s u l t s o n

f i rs t

o r d e r k i n e t i c s is shown in Figure 1 .

37 and 17 Kcal/mole

i n

a i r and n i t r o g e n . F i gu r e 1

i n d i c a te s a t r a n s i t i o n a t c a.

30 0T which r e f l e c t s t he e x i s t e n c e o f two d i f f e r e r . t pathwa ys . As seen in F igure 2 ,

t h e r a t e o f py r o lys i s m ea sure d by weight

loss

under i so thermal condi t ions , shows

a n i n i t i a l p e r i o d o f a c c e le r a t i on and p r oce e ds much f a s t e r i n a i r t h an i n i n e r t

atmosphere. As t he py r o lys i s t e m pe r a tu r e i s i n c r e a s e d ,

the

i n i t i a t i o n p e r i o d a n d

the d i f f e r e nc e s between pyr o lys i s under n i t r og e n and a i r g r a dua l ly d im in ish a nd

disapp ear a t 310C when py ro ly si s by th e second pathway ta k es ove r .

Kinetics of the lower temperature pathway

involve reduc t ion

i n

m ole c u la r we igh t o r

DP

by bond sc i s s io n , a ppe a ra nc e o f f r e e

r a d i c a l s , e l im ina t ion o f wa t e r , f o r m a t ion o f c a r bony l c a rboxyl a nd hydr ope rox ide

groups i n a i r ) , e vo lu t ion o f c a rbon monoxide and ca r bon d iox i de , a nd f i n a l l y p r o -

duc t ion of a char red res idue .

r a t e s of p y r o l y si s o f c e l l u l o s i c m a t e r i a l s , have been i n d i v i d u a l l y i n v e s t i g a t e d .

Reduction i n

the

de gre e o f po lym e riz a tion o f c e l l u l os e on i sothe rm a l h e a t ing in a i r

or n i t r o g e n a t a temperature within the range of 150-190C has been measured by the

viscosi ty method. The r e s u l t i n g d a t a hav e b een c o r r e l a t e d w i t h r a t e s o f bond s c i s s i o n

and used f o r c a l c u l a t in g the k i ne t i c pa r a m er e r s.

t i o n energy of 21 Kcal/mole fo r bond s ci ss io n i n a i r and 27 Kcal/mole i n n i t r o g e n ,

and i n d i c a t e t h a t a t low t e m p e r a t u r e s a l a r g e r

number

of bonds are broken

i n

a i r

than i n n i t r oge n .

much f a s t e r i n a i r th an

i n

n i t r o ge n , a nd f u r the r m o r e , a c c e le r a t e on c on t inue d he a t ing .

I t

i s

i n s t r uc t ive to c om pa r e

the

i n i t i a l l i n e a r r a t e s f o r the e v o l u t i o n o f t h e s e

gases w i t h the ra te s of bond s c i ss io n obta in ed fo r depolymer iza t ion a t 170OC. s can

be seen in Table I the r a t e o f bond s c i s s i o n i n a i r ap p ro x im a te ly e q u a l s t h e r a t e

o f production of carbon dioxide plus carbon monoxide i n moles

per

g l u c o s e u n i t . In

nitrogen, however , t h e r a t e o f bond s c i s s i o n i s g r e a t e r t h an the rates

o f

carbon

monoxide

and carbon dioxide evolution combined.

The se da ta ga ve a c t iva t ion e ne r g ie s o f

The reac t ions in

the

f i r s t

pa thway, which domina tes a t lower temp era tu res ,

T he se r e a c t i o n s , w hich c o n t r i b u t e t o t h e o v e r a l l

T he se c a l c u l a t i o n s g i v e an a c t i v a -

The

ra te s of produ ction o f carbon monoxide and carbon dio xid e a t 17OOC a r e

T a b le I . I n i t i a l r a t e s o f g l y c o s i d i c bond s c i s s i o n an d c ar bo n

monoxide and carbon dioxide formation a t 170C.

5

Rate

X

1 0 i n a i r

mo le/162 gm

hr

Reaction Rate

l o 5

i n

N

mole/162

gm

hr

Bond Sc is si on 2.7 9.0

CO Evolution 0.6

6.4

C02 Evolution

0.4

2.1

286


3/6

2 .

-3

-

I n

k

4

-

5 -

a9

OB

3

Ob

5

b

%$

:--.

a.

.

.

I

0.4

w woo

100

200 300

Time (m in )

Figure

2 .

F i r s t o r d e r p l o t f o r t h e r e s i d ua l c e l l u l o s e w eig ht n o rm a l iz e d)

ver sus t ime.

a r e s i m i l a r .

P l o t s

a t

310C and 3 2 5 O C f o r a i r and n i t rogen

287


4/6

-4-

On h e a t in g c e l l u l o s e i n a i r , h y d ro p e ro x id e f u n c t i o n s a r e s i m u l ta n e o u s ly form ed

and decomposed, and th e i r c onc en t ra t i on c limbs u n t i l a s teady s t a t e i s reached .

A t

17OOC the s teady s ta te i s reached i n about 100 min .

p er ox id e f u n ct i o n appeared t o f o l l o w f i r s t - o r d e r k i n e t i c s w i t h a r a t e c on st a

t O f

2 .5 min-1 a t 17O0C. From the s teady -s ta te con cen t ra t i on o f 3 0 .

0-

mole/

g

min. When com pared w i t h t h e i n i t i a l r a t e o f bo nd s c i s s i o n i n a i r a t 17OoC ( Ta bl e

I ) ,

i t

i s a p pa re nt t h a t h y d ro p e ro x id e f o rm a t i o n c o u l d make a s i g n i f i c a n t c o n t r i b u -

t i o n t o bond s c i s s io n .

K i n e t i c s o f t h e i n t e r me d i a t e p at hway

Ths p r i m a ry r e a c t i o n i n t h i s pa th wa y i n v o l v e s d e p o ly m e r i z at i o n b y t r a n s g l y c o -

s y l a t i o n f o rm i n g a t a r c o n t a i n i n g a n hy dr os ug ar s ( l e v o g l u c o s a n and i t s i s o me r ),

randomly l i nk ed o l i gosa ccha r ide s and va r ious dehyd ra t i o n and f i s s io n p roduc ts . As

t h e r e a c t i o n pr oc ee ds , t h e r e s i d u e c o n t a i n s some g l y c o s y l u n i t s t h a t have be en

detec ted by CP/MAS 13C-NMR and FTIR, and some char ( 4 ) . The c h a r i s f orm ed p a r t l y

f r o m d i r e c t d e c omp o si ti o n o f c e l l u l o s e as d i s c u s s e d e a r l i e r a nd p a r t l y b y decompo-

s f t i o n o f t h e t a r .

ra p i d l y and ove rshadow the p ro duc t i on o f cha r .

A t

t h i s t i m e , i t shou ld be po in ted

o u t t h a t e v a p or a ti o n o f l e vo g lu co s an and t h e v o l a t i l e p y r o l y s i s p r o d u ct s i s h i g h l y

endo the rmic. Thus, t he i nc reased oven tempera tu re cou ld ra i s e th e ra t e o f hea t

t ra n s fe r b u t n o t n e c e s s a r i l y t h e t e m p er at ur e o f t h e a b l a t i n g s u b s t r a t e w hic h i s

c o o l e d

by

t h e hea t o f evapo ra t i on , es pe c i a l l y unde r vacuum.

h i g h e r te mp er a tu r es , t h e p y r o l y s i s p r o ce s s may be c o n t r o l l e d by t h e r a t e o f h e a t

t r a n s f e r r a t h e r t ha n t h e k i n e t i c s o f th e ch em ic al r e a c t io n . M a t e r i a l t r a n s p o r t

p re se nt s a n o th e r m a j o r o b s t a c l e t o t h e i n v e s t i g a t i o n o f c h em i ca l k i n e t i c s , b eca us e

i f t he p roduc ts o f p r imary rea c t i o ns a re no t removed, they can unde rgo fu r th e r de -

compos it i on rea c t i o ns . Tab le

I 1

shows t h e d i f f e r e n c e betw een t h e y i e l d o f d i f f e r -

e n t p y r o l y s i s p r o d u c t s i n vacuum, wh ich removes th e p r im ary vo l a t i l e p roduc ts , and

n n i t r o g e n a t a t mo s ph e ri c p r e s s u r e wh i ch a l l o w s mo re d e c o mp o s i ti o n o f t h e a n hy d ro -

sugars. I t a l s o shows t h e e f f e c t o f i n o r g a n ic c a t a l y s t s i n ch an gin g t h e n a t u r e o f

the reac t i ons and p roduc ts .

A n al ys is o f t h e p y r o l y s i s p ro d uc ts o f c e l l u l o s e a t

3 O O O C

under n i t rogen and vacuum.

The decomp os i t i on o f t he hydro-

162 g

min

t h e r a t e

of

hyd rope rox ide decompos i t i on

i s

t h e r e f o r e 7 .5 x 10 -

P

ole/162

On r a i s i n g t h e t e m p e r at u re t h e t a r f o r m i n g r e a c t i o n s a c c e le r a t e

I n o t h e r w ords, a t t h e

Table

11.

1.5

Mm Hg,

Cond i t ion A t m . pressure 1 .5 Mm

Hg

5% SbC1,

Char

Tar

34.

2%a

17.8%a 25.8%a

19.1

55.8

32.5

levog lucosan 3.57 28.1 6.68

1 6-anhydro-B-~-g1ucofuranose 0.38 5.7 0.91

hyd ro lyz ab le m at er ia ls 6 .08 20.9 11 a

-glucose t r a ce t r a c e 2.68

aThe pe rcentages a re based on the o r i g i n a l amoun t o f c e l l u lo se .

2 a a


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5

I n Vi ew of t hese cons i der at i ons , t he chem cal k i net i cs of c el l ul os e pyr o l ys i s

have been i nvest i gat ed wi t hi n t he l i m t ed t emper at ur e r ange o f 260- 340C and under

vacuum i n Or der t o obt ai n chem cal l y meani ngf ul dat a

5).

Under t hese condi t i ons ,

t he chem cal k i net i cs o f cel l ul ose pyro l ys i s coul d be r epr esent ed by t he t hr ee

r eact i on model shown bel ow.

k i

cei l ul os e Ac t i ve Cel l ul os e

e e l WA *Char + Gases

d(Wc) = 0.35kc[W]

az

I n t hi s model i t

s

assumed that t he i ni t i at i on r eact i ons d i s cussed ear l i er

l ead t o t he f or mat i on of an act i ve cel l ul ose, whi ch s ubsequent l y decomposes by t wo

compet i t i ve f i r s t or der r eact i ons , one yi el di ng anhydr osugar s ( t r ansgl ycosy l at i on

pr oducts) and t he ot her c har and a gaseous f r act i on.

Par t i al l y pyro l yzed cel l ul ose, i n addi t i on t o t he or i gi nal sugar ( gl ycosyl )

uni t s , cont ai ns new f unct i onal i t i es f or med by dehydr at i on, r ear r angement , decarbonyl -

at i on, decar boxyl at i on and condensat i on.

al i t i es ' nc l ude car bonyl , carboxyl , ar omat i c and a l i phat i c car bons as anal yzed by

CP/MS l J C- NMR

5) .

Af t er compl et e pyro l ys i s o f t he gl ycosyl uni t s ( 5 m n at 400OC)

a re l at i vel y s t abl e char i s l ef t that cont ai ns about 70 ar omat i c and

27

al i phat i c

carbons. On heat i ng at 500C char i s conver t ed t o a hi ghl y condensed and cr ossl i nked

mater i al , cont ai ni ng about

905

pol ycyc l i c ar omat i c car bons .

I n t he i nt ermedi at e char s t hese f unct i on-

289


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6

CONCLUSION

o f r e a c ti o n s which p ro v id e d i f f e re n t p rod u c ts . The k i n e t i cs o f t h ese re a c t i o n s

are h igh ly dependent on the exper imenta l condi t ions.

Py ro l ys i s o f ce l l u l o se p ro ce ed s b y a l t e rn a t i v e pathways i n vo l v i n g a va r i e t y

ACKNOWLEDGMENTS

The au thor i s p leased t o acknowledge the c on t r ib u t i on s o f O r .

A.G.W.

Bradbury,

and the support o f the Na t ion a l Bureau o f S tandards and t he Na t ion a l Sc ience

Foundation.

1.

2.

3.

4.

5.

REFERENCES

F. Shafizadeh, Advan. i n Carbohydr.

.23-

419 (1968).

F.

Shafizadeh, 2. Anal and Appl. Py rol.. 3 283 (1982).

F. Sh afiz ade h and A.G.W. Bradbury, 2. Appl. Polym. x. 3, 1431 (1979).

Y. Sekiguch i , J.S. Frye, and F. Shafizadeh,

J . w.

Polym. Sci . , submi t ted

f o r p u b l i c a t i o n .

A.G.W. Bradbury,

Y.

Sakai, and

F.

Shafizadeh,

2.

3271 (1979).

pol ym N . 2

290

alternative pathways for pyrolysis of cellulose

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