The Chemistry of Mesophase Formation - ACS Publications

1 The Chemistry of Mesophase Formation Harry Marsh and Carolyn S. Latham

Downloaded by MCGILL UNIV on August 23, 2012 | http://pubs.acs.org Publication Date: April 14, 1986 | doi: 10.1021/bk-1986-0303.ch001

Northern Carbon Research Laboratories, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, England

The origins and development of the concept of carbonaceous mesophase, as derived from discotic aromatic nematic liquid crystals, enable applications to be made to industrial processes. The world availability of pitch materials is such that there is an abundance of pitch which produces cokes of l i t t l e commercial value. A major incentive for research into the chemistry of mesophase formation is the commercial up-grading of such pitches and the development of specialized pitches. Structure in cokes is described in terms of optical texture. The importance of viscosity of pyrolyzing pitch in controlling size of optical texture is stressed. Pitch viscosity itself is related to chemical composition of the parent material. Those rich in oxygen and sulphur tend to produce cokes with small sized optical texture; those rich in hydrogen tend to produce cokes with large sized optical texture related to needle coke formation. Generally, cokes with small sized optical texture tend to have higher strength, higher reactivity and higher CTE values. Cokes with larger sized optical texture, e.g. needle cokes, have lower reactivity and CTE values. The important role of transferable hydrogen in carbonizing systems is stressed. Attention is drawn to the fact that inerts or quinoline-insoluble material in pitch may not be inert, but can be chemically and physically active in the carbonization system. Future research and development is outlined. The carbonaceous mesophase i s the i n t e r m e d i a t e m a t e r i a l formed d u r i n g c a r b o n i z a t i o n o f p a r e n t p i t c h and l e a d i n g t o the r e s u l t a n t c o k e . The d i s c o v e r y and development o f the concept o f mesophase over t h e l a s t twenty y e a r s must r e p r e s e n t one of the most s i g n i f i c a n t advances i n carbon s c i e n c e . Mesophase i s a term borrowed from the s c i e n c e of c o n v e n t i o n a l l i q u i d c r y s t a l s and means " i n t e r m e d i a t e p h a s e " . The term "carbonaceous mesophase" i s d i s t i n g u i s h e d from the term n e m a t i c 1

0097-6156/86/0303-0001$08.25/0 © 1986 American Chemical Society

In Petroleum-Derived Carbons; Bacha, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

PETROLEUM-DERIVED CARBONS

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l i q u i d c r y s t a l * the former b e i n g a p o l y m e r i z e d l i q u i d c r y s t a l s y s t e m . Mesophase i s composed of l a m e l l a r m o l e c u l e s the s t r u c t u r e s o f w h i c h are based on the h e x a g o n a l network o f carbon atoms o f the g r a p h i t e lattice. C u r r e n t l y , carbonaceous mesophases can be produced w i t h a range o f s o l u b i l i t i e s , i n e . g . q u i n o l i n e . The term 'mesophase p i t c h w i t h t h e r m o t r o p i c p r o p e r t i e s has been i n t r o d u c e d r e c e n t l y . Here the carbonaceous mesophase i s formed by c o o l i n g the f l u i d i s o t r o p i c p i t c h . P r i o r to the r e c o g n i t i o n o f mesophase, the f o r m a t i o n of a n i s o t r o p i c coke from a f l u i d phase was d e s c r i b e d i n terms o f a c o k i n g p r i n c i p l e (1). As e a r l y as 1944, B l a y d e n , Gibson and R i l e y (2) l o o k e d f o r s i g n i f i c a n t changes i n s t r u c t u r e between n o n - g r a p h i t i z i n g and g r a p h i t i z i n g carbons d u r i n g the c a r b o n i z a t i o n p r o c e s s , u s i n g X - r a y d i f f r a c t i o n techniques. D i f f r a c t i o n p a t t e r n s were i n t e r p r e t e d i n terms o f the concept t h a t l a m e l l a r c o n s t i t u e n t m o l e c u l e s w i t h i n the c a r b o n i z i n g system formed s t a c k e d u n i t s , c a l l e d c r y s t a l l i t e s . A l t h o u g h the X - r a y d i f f r a c t i o n approach showed t h a t e s s e n t i a l d i f f e r e n c e s e x i s t e d between the mechanisms o f f o r m a t i o n o f n o n - g r a p h i t i z i n g and g r a p h i t i z i n g carbons ( d e f i n i t i o n s adopted from the s t u d y o f R o s a l i n d F r a n k l i n (_3)), the method was not s p e c i f i c enough to be a b l e to i d e n t i f y c l e a r l y " t h e c o k i n g p r i n c i p l e " . Wandless ( 4 ) , i n 1971, was a n t i c i p a t i n g the f u t u r e q u i t e c l e a r l y when he wrote t h a t the b a s i c s t r e n g t h of coke i s determined i n the p l a s t i c phase and the v e r y e a r l y stages of r e s o l i d i f i c a t i o n . A l t h o u g h T a y l o r (5) d i s c u s s e d the development of o p t i c a l p r o p e r t i e s o f a n i s o t r o p i c m a t e r i a l formed d u r i n g c a r b o n i z a t i o n i n 1961, and t o g e t h e r w i t h Brooks (6) r e v i e w e d the concept o f l i q u i d c r y s t a l s as an i n t e r m e d i a t e t o coke f o r m a t i o n i n 1968, i t was not u n t i l w e l l i n t o the 1 9 7 0 s t h a t the p o t e n t i a l o f t h i s knowledge was r e a l i s e d . The work o f Brooks and T a y l o r l e d t o a r e s u r g e n c e of r e s e a r c h a c t i v i t y i n t o the c a r b o n i z a t i o n p r o c e s s w i t h the advancement of knowledge l e a d i n g to changes and developments w i t h i n the carbon and g r a p h i t e m a n u f a c t u r i n g i n d u s t r i e s . The t a n t a l i s i n g p r o s p e c t of cheap carbon f i b r e s by s p i n n i n g mesophase was one p o s s i b i l i t y , see F i g u r e 1 (7). A window was opened i n t o the m y s t e r i e s o f the d e l a y e d coker ( 8 ) . The m i c r o s t r u c t u r e o f c a r b o n - c a r b o n f i b r e composites u s i n g p i t c h carbon as the m a t r i x m a t e r i a l became b e t t e r u n d e r s t o o d , see F i g u r e 2 (9). Improved i s o t r o p i c g r a p h i t e s o f h i g h d e n s i t y c o u l d be p r e p a r e d by i s o s t a t i c p r e s s u r i s e d c a r b o n i z i n g t e c h n i q u e s . The t h e o r y o f c o a l c o - c a r b o n i z a t i o n p r o c e s s e s was advanced c o n s i d e r a b l y , and the use o f p i t c h a d d i t i v e s to up-grade e f f e c t i v e c o a l rank was u n d e r s t o o d ( 1 0 ) . A s p e c t s o f coke s t r e n g t h and p h y s i c a l p r o p e r t i e s e . g . the t h e r m a l e x p a n s i v i t i e s o f g r a p h i t e s were more f u l l y u n d e r s t o o d . The r o l e o f q u i n o l i n e i n s o l u b l e s ( Q I ) , b o t h p r i m a r y and s e c o n d a r y , i n p i t c h c a r b o n i z a t i o n s was e x p l a i n e d , l e a d i n g to the manufacture of prime needle cokes from c o a l - t a r p i t c h ( 1 1 , 12) by f i l t r a t i o n of the QI material. A knowledge o f the c o n d i t i o n s o f f o r m a t i o n of mesophase from c o a l l i q u e f a c t i o n p r o d u c t s e n a b l e d a more p r e c i s e c o n t r o l of o p e r a t i n g c o n d i t i o n s thus p r e v e n t i n g , i n t h i s c a s e , the r e t r o g r e s s i v e f o r m a t i o n of mesophase and coke i n p i p e - w o r k l e a d i n g to p l a n t c l o s u r e (13). Improvements became p o s s i b l e i n the manufacture o f baked anodes f o r a l u m i n i u m p r o d u c t i o n as a knowledge o f the s t r u c t u r e o f b i n d e r coke became a v a i l a b l e and the r e l a t i o n s h i p s between p i t c h p r o p e r t i e s and r e s u l t a n t cokes were f u r t h e r c l a r i f i e d . T h i s i n t u r n e n a b l e d p i t c h producers to begin to t a i l o r - m a k e p i t c h m a t e r i a l s r e l a t i v e to


1

f

1

1



1.

MARSH AND LATHAM

F i g u r e 1.

Scanning e l e c t r o n m i c r o g r a p h

mesophase p i t c h carbon f i b e r the

radial

arrangement

3

The Chemistry of Mesophase Formation

of f r a c t u r e surface

of a

e t c h e d w i t h chromic a c i d t o r e v e a l

of c o n s t i t u e n t

lamellar molecules.




Figure 2.

Scanning e l e c t r o n m i c r o g r a p h o f a p o l i s h e d

surface

s u b s e q u e n t l y e t c h e d w i t h chromic a c i d , o f a carbon f i b e r - p i t c h c a r b o n composite

showing the o r i e n t a t i o n o f a c o n s t i t u e n t

m o l e c u l e s of p i t c h carbon p a r a l l e l

lamellar

t o s u r f a c e s of the carbon


fiber.


1.

MARSH AND LATHAM

The Chemistry of Mesophase Formation

5

a d e s i r e d type o f carbon p r o d u c t ( 1 4 ) . T h u s , the decade 1974-1984 has been one o f i n t e n s e i n t e r e s t and development o f knowledge o f many a s p e c t s o f the c o n v e r s i o n of p i t c h m a t e r i a l s , w i t h t h e i r v a r i o u s o r i g i n s , to the s t r u c t u r e s o f the many grades o f cokesD u r i n g t h i s p e r i o d , u s e f u l reviews were p u b l i s h e d . White (15) and Zimmer and White (16) d e s c r i b e the d i s c l i n a t i o n s p r e s e n t i n mesophase m i c r o s t r u c t u r e s and t h e i r r e l a t i o n t o coke p r o p e r t i e s . Lewis and S i n g e r (17) o v e r v i e w the r o l e and importance o f s t a b l e f r e e radicals i n carbonization processes. F i t z e r e t a l (18) p r o v i d e a comprehensive d e s c r i p t i o n o f the c h e m i s t r y i n v o l v e d i n t h e c o n v e r s i o n o f s p e c i f i c o r g a n i c compounds t o c a r b o n . Marsh (19-23) and F o r r e s t and Marsh (24) r e l a t e the c h e m i s t r y o f mesophase f o r m a t i o n t o i t s p r o p e r t i e s and a p p l i c a t i o n s . The j o u r n a l CARBON p u b l i s h e d an i s s u e devoted s p e c i a l l y t o s t u d i e s o f mesophase and i t s a p p l i c a t i o n s ( 2 5 ) . The w o r l d a v a i l a b i l i t y o f p i t c h m a t e r i a l s i s such t h a t t h e r e i s an abundance o f p i t c h w h i c h produces cokes o f l i t t l e i n d u s t r i a l v a l u e . A major i n c e n t i v e i n t o r e s e a r c h o f mesophase i s the commercial u p g r a d i n g of such p i t c h e s . Mesophase and Coke S t r u c t u r e P e t r o l e u m and c o a l are t h e dominant p a r e n t s o u r c e s o f c a r b o n s , cokes and g r a p h i t e s . A p a r t from such e x c e p t i o n s as the c a r b o n i z a t i o n o f s u c r o s e and r e l a t e d m a t e r i a l s , the c a r b o n i z a t i o n systems l e a d i n g t o g r a p h i t i z a b l e carbons ( f o r d e f i n i t i o n s see R e f . 26) a l l pass through a f l u i d phase and a l l produce the l i q u i d crystalTmesophase m a t e r i a l . The cokes produced from such m a t e r i a l s , e . g . f i l l e r coke, n e e d l e coke, s h o t coke, m e t a l l u r g i c a l coke and f l u i d c o k e , may n o t be d i s t i n g u i s h a b l e s t r u c t u r a l l y i n a m e a n i n g f u l way by X - r a y d i f f r a c t i o n . A p o w e r f u l a n a l y t i c a l t o o l i n s t u d i e s o f coke s t r u c t u r e i s t h e p o l a r i z e d l i g h t o p t i c a l microscope ( 1 6 , 21) w h i c h , whether o p e r a t i n g w i t h c r o s s - p o l a r i z e d l i g h t o r making use o f r e f l e c t i o n i n t e r f e r e n c e c o l o u r s ( 2 7 , 2 8 ) , v e r y d e f i n i t e l y c a t e g o r i z e s carbons a c c o r d i n g t o t h e i r o p t i c a l t e x t u r e . T h i s o p t i c a l t e x t u r e i s a measure o f t h e s i z e and c o a l e s c e n c e b e h a v i o u r o f mesophase d e v e l o p i n g from t h e i s o t r o p i c p a r e n t p i t c h and e n a b l e s s t r u c t u r e and o r i e n t a t i o n o f l a m e l l a e w i t h i n t h e mesophase t o be e s t a b l i s h e d . A nomenclature (24) used t o d e s c r i b e o p t i c a l t e x t u r e i n cokes d e r i v e d from p e t r o l e u m and c o a l i s i n T a b l e 1. The m i c r o s c o p i c appearance of o p t i c a l t e x t u r e can be q u a n t i f i e d i n an a r b i t r a r y way (29) by making use o f an o p t i c a l t e x t u r e i n d e x (OTI) c a l c u l a t e d u s i n g t h e f o r m u l a : OTI = E f . x (OTI) = f r a c t i o n of component of o p t i c a l t e x t u r e i n the o v e r a l l appearance o f p o l i s h e d s u r f a c e s o f t h e coke ( O T I ) . = an a r b i t r a r y (29) f a c t o r f o r each r e c o g n i z a b l e component of o p t i c a l t e x t u r e r e l a t e d to the r e l a t i v e s i z e s o f the component as i n T a b l e 1. F o r example, f o r an i s o t r o p i c c a r b o n OTI = 0 ; f o r an a n i s o t r o p i c carbon composed e n t i r e l y o f domains, OTI = 3 0 . A l l o t h e r a n i s o t r o p i c carbons have OTI v a l u e s i n the range 0 - 3 0 b e i n g summation o f (OTI)^ v a l u e s . Examples o f o p t i c a l t e x t u r e s are the micrographs of F i g u r e s 3-6. There i s a l i m i t a t i o n to the use o f o p t i c a l m i c r o s c o p y i n terms of i t s r e s o l u t i o n . The c o r r e l a t i o n o f a n i s o t r o p y ( o p t i c a l a c t i v i t y ) w i t h g r a p h i t i z a b i l i t y and o f i s o t r o p y ( z e r o o p t i c a l a c t i v i t y ) w i t h

where: f .

1



6


Figure 3.

O p t i c a l m i c r o g r a p h o f a coke s u r f a c e showing an o p t i c a l

t e x t u r e o f f i n e - g r a i n e d mosaics

F i g u r e 4.

60 ym, OTI = 30.


1

8


Table I .

Nomenclature t o D e s c r i b e O p t i c a l T e x t u r e i n Cokes


Component o f O p t i c a l Texture seen i n M i c r o s c o p y Isotropic Fine-grained mosaics Medium-grained mosaics Coarse-grained mosaics Supra-mos a i cs S m a l l domains Domains

Abbreviation I

Size No o p t i c a l a c t i v i t y

Optical Texture Index (OTI) 0

Mf

60 ym

Medium-flow anisotropy

MFA

Coarse-flow anisotropy

CFA

Flow domain anisotropy

FD

10 ym w i d t h

7 10 20 30 7 20

30

n o n - g r a p h i t i z a b i l i t y i s e x t r e m e l y u s e f u l b u t f a i l s e . g . when e x a m i n i n g cokes from c o a l s o f N a t i o n a l C o a l Board r a n i ; 600-400. I t must be empha s i s e d t h a t mesophase can a l s o e x i s t when i t i s not v i s i b l e i n the o p t i c a l microscope. U n i t s o f mesophase can e x i s t , i n p i t c h m a t e r i a l s , of s i z e e . g .

The Chemistry of Mesophase Formation - ACS Publications

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