Chemical Characterization and Preparation of the Carbonaceous

2 Chemical Characterization and Preparation of the Carbonaceous Mesophase

Downloaded by UNIV OF SYDNEY on May 3, 2015 | http://pubs.acs.org Publication Date: April 14, 1986 | doi: 10.1021/bk-1986-0303.ch002

Isao Mochida and Yozo Korai Research Institute of Industrial Science, Kyushu University 86, Kasuga, Fukuda, 816, Japan

The development of mesophase carbon fiber provides a major incentive for both basic and practical investiga tions of the chemistry of the carbonaceous mesophase. Recent work on the properties and structural chemistry of the mesophase is surveyed, with emphasis on the chemical structures formed in the pyrolysis of pure compounds, interactions between molecules in the carbonization of practical pitch materials, and the preparation of mesophase pitches. Naphthenic intermediates play useful roles in improving anisotropy and solubility and in reducing viscosity. The growing knowledge of such mechanisms as hydrogenation, oxidation, dealkylation, and solvent fractionation opens increasingly precise approaches to control the molecular order, viscosity, and reactivity of mesophase pitch. An example is the use of hydrogenation to improve the fusibility and solubility of mesophase pitch, and under certain conditions, to prepare "dormant mesophase", an isotropic pitch that becomes anisotropic under stress. The o p t i c a l a n i s o t r o p y observed i n most carbon m a t e r i a l s r e f l e c t s the ordered s t a c k i n g of g r a p h i t e - l i k e m i c r o c r y s t a l l i n e u n i t s t h a t has been recognized t o be e s s e n t i a l i n determining their properties. P i t c h - b a s e d carbon f i b e r , e l e c t r o d e and m e t a l l u r g i c a l c o k e s , and carbons f o r n u c l e a r r e a c t o r s a r e c h a r a c t e r i z e d by t h e i r a n i s o t r o p i c texture since t h i s s t r u c t u r a l f a c t o r i s fundamentally r e l a t e d t o t h e i r m e c h a n i c a l , t h e r m a l , e l e c t r o n i c , and c h e m i c a l properties (1-5). The o p t i c a l a n i s o t r o p y has been shown by Brooks and T a y l o r (5) t o be b u i l t i n t h r o u g h t h e carbonaceous mesophase, t h e l i q u i d c r y s t a l l i n e s t a t e formed d u r i n g t h e l i q u i d - p h a s e c a r b o n i z a t i o n of those o r g a n i c m a t e r i a l s t h a t can be p y r o l y z e d and h e a t - t r e a t e d t o the g r a p h i t i c s t a t e . The mesophase i s t h e c r i t i c a l i n t e r m e d i a t e s t a t e i n which the q u a l i t y of carbon products i s d e t e r m i n e d . The c h e m i s t r y of i t s c h a r a c t e r i z a t i o n , p r e p a r a t i o n , and c o n t r o l i s most r e l e v a n t t o modern c a r b o n i z a t i o n t e c h n o l o g y . 0097-6156/86/0303-0029$06.00/0 © 1986 American Chemical Society

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

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PETROLEUM-DERIVED CARBONS

The t e c h n o l o g y of m e s o p h a s e - p i t c h - b a s e d carbon f i b e r has s t i m u l a t e d the r a p i d development of t h e c h e m i s t r y of mesophase behavior and p r e p a r a t i o n . The c a r b o n i z a t i o n schemes and mechanisms l e a d i n g t o o p t i c a l a n i s o t r o p y v i a t h e mesophase, t h e c o n t r o l of c a r b o n i z a t i o n w i t h emphasis on the p r e p a r a t i o n of s p i n n a b l e mesophase, and the mesophase t r a n s i t i o n and r e a c t i v i t y i n r e l a t i o n t o the s t r u c t u r e of i t s c o n s t i t u e n t m o l e c u l e s a r e summarized i n t h i s p a p e r .


C a r b o n i z a t i o n Schemes and Mechanisms When a r o m a t i c m a t e r i a l s ( i n c l u d i n g c o a l , c o a l t a r , p e t r o l e u m r e s i dues, and bitumens) a r e heated i n i n e r t atmosphere under ambient p r e s s u r e , some of t h e i r components undergo p y r o l y t i c r e a c t i o n s t o produce radical fragments, although many c o n s t i t u e n t s may distill. Some r a d i c a l fragments decompose t o produce l i g h t e r components which a r e a l s o e v o l v e d , but o t h e r s recombine with r e a c t i v e molecules to y i e l d l a r g e r molecules. Such r e a c t i o n s a r e r e p e a t e d u n t i l f i n a l l y s o l i d coke appears i n the r e a c t o r . The v i s c o s i t y of the r e a c t i n g l i q u i d phase i n c r e a s e s because of the increase i n molecular weight. The l a r g e a r o m a t i c m o l e c u l e s i n t e r a c t through π - π e l e c t r o n f o r c e s and segregate from the m a t r i x of s m a l l e r m o l e c u l e s t o produce a n i s o t r o p i c s p h e r e s . The a n i s o t r o p i c spheres grow i n d i a m e t e r , c o a l e s c e t o form broader i s o c h r o m a t i c r e g i o n s i n s p i t e of t h e i n c r e a s i n g v i s c o s i t y , and f i n a l l y s o l i d i f y i n t o a n i s o t r o p i c coke. T h i s s e r i e s of s t e p s l e a d i n g t o o p t i c a l a n i s o t r o p y i s observed i n the quenched mesophase specimens shown i n F i g u r e 1, a l t h o u g h some e f f e c t s due t o quenching may be i n c l u d e d . Hot-stage micrography shows s i m i l a r i n s i t u p r o g r e s s d u r i n g c a r b o n i z a t i o n (6-8). T h i s c a r b o n i z a t i o n mechanism may be d e f i n e d as a " s p h e r e mechanism"· S i n c e the o p t i c a l a n i s o t r o p y r e f l e c t s t h e o r d e r e d s t a c k i n g of a r o m a t i c m o l e c u l e s , the a n i s o t r o p i c spheres and the c o a l e s c e d r e g i o n s (which can be q u i t e v i s c o u s but deformable) a r e i n the l i q u i d c r y s t a l l i n e s t a t e during the c a r b o n i z a t i o n process. Their quenched state can be d e s c r i b e d as " l i q u i d crystal glass" a c c o r d i n g t o D i e f e n d o r f (9). The p r e s e n t ' a u t h o r s found a n o t h e r p y r o l y s i s mechanism l e a d i n g t o o p t i c a l a n i s o t r o p y ( 1 0 ) , i n which no d e f i n i t e l i q u i d phase was o b s e r v a b l e d u r i n g the c a r b o n i z a t i o n . I n some carbonaceous sub s t a n c e s , such as s e m i - a n t h r a c i t e c o a l , o p t i c a l a n i s o t r o p y over broad r e g i o n s develops promptly a t c e r t a i n temperatures from the highly viscous stage. The component l a y e r s appear t o be s t a c k e d , and a r e r e a r r a n g e d by h e a t - t r e a t m e n t t o show o p t i c a l a n i s o t r o p y . Such a mechanism can be called the "preordered layert r a n s f o r m a t i o n mechanism". The r e a c t i o n schemes of c a r b o n i z a t i o n have a l s o been i n v e s t i gated ( 1 1 - 1 5 ) . The m o l e c u l a r s t r u c t u r e of the c a r b o n i z a t i o n i n t e r m e d i a t e s can i n f l u e n c e s t r o n g l y t h e o p t i c a l a n i s o t r o p y of t h e resultant coke. The c a r b o n i z a t i o n i n t e r m e d i a t e s have been r e p o r t e d f o r the p y r o l y s i s of a c e n a p h t h y l e n e , which p r o v i d e s a r a r e example of a t m o s p h e r i c c a r b o n i z a t i o n of a pure o r g a n i c c h e m i c a l ( 1 1 ) . The c a r b o n i z a t i o n scheme i s i l l u s t r a t e d i n F i g u r e 2. The i n t e r m e d i a t e s I I , I I I , and VI a r e proposed based on



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Characterization and Preparation of the Mesophase

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F i g u r e 1. Three s t e p s i n the development of o p t i c a l a n i s o t r o p y v i a the mesophase. (a) N u c l e a t i o n of mesophase s p h e r u l e s . (b) Growth and c o a l e s c e n c e of mesophase. ( c ) Flow a n i s o t r o p y .

F i g u r e 2 . C a r b o n i z a t i o n scheme f o r acenaphthylene Reproduced w i t h p e r m i s s i o n from r e f e r e n c e 12. C o p y r i g h t 1979 IPC B u s i n e s s P r e s s , L t d .



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u l t r a v i o l e t s p e c t r o s c o p y ( U V ) , n u c l e a r magnetic resonance (NMR), and vapor phase osmometry (VPO). Comparison of t h e c a r b o n i z a t i o n r e a c t i v i t y of acenaphthylene ( I ) and d e c a c y c l e n e ( V I ) i n d i c a t e s the c o m p l e x i t y of such r e a c t i o n schemes. Decacyclene always forms as an i n t e r m e d i a t e i n the p y r o l y s i s of a c e n a p h t h y l e n e . In f a c t , d e c a c y c l e n e i s s t a b l e at 500°C, whereas acenaphthylene d e v e l o p s o p t i c a l a n i s o t r o p y a t lower p y r o l y s i s temperatures ( 1 2 ) . These r e s u l t s suggest a r o l e f o r r e a c t i v e i n t e r m e d i a t e s which a r e n o t i n c l u d e d i n the major steps of the r e a c t i o n scheme and may a l s o i m p l y t h a t a minor component can o f t e n a f f e c t t h e f i n a l r e s u l t s o f carbonization. Figure 3 describes r e a c t i o n schemes f o r naphthalene car b o n i z a t i o n c a t a l y z e d by m e t a l l i c potassium o r by aluminum c h l o r i d e ( 1 3 , 1 4 ) ; these c a t a l y s t s produce c o n t r a s t i n g i s o t r o p i c and a n i s o t r o p i c carbons, r e s p e c t i v e l y . The i n t e r m e d i a t e s t r u c t u r e s a r e s i m i l a r e x c e p t f o r more n a p h t h e n i c s t r u c t u r e induced i n t h e A l C l ^ catalyzed carbonization. The r o l e of n a p h t h e n i c structures leading to o p t i c a l anisotropy has been r e c o g n i z e d i n many examples, and t h e i r i n t r o d u c t i o n can improve the a n i s o t r o p i c development, as d e s c r i b e d l a t e r . H i g h e r f u s i b i l i t y , lower m e l t i n g t e m p e r a t u r e , and h i g h e r s o l u b i l i t y of t h e i n t e r m e d i a t e m o l e c u l e s may be o b t a i n e d by t h e f o r m a t i o n of partially naphthenic structures (15)· S i n c e the v i s c o s i t y i n c r e a s e i n t h e l a t e r s t a g e s of c a r b o n i z a t i o n i n f l u e n c e s mesophase development, any f a c t o r influ e n c i n g t h e v i s c o s i t y of t h e c a r b o n i z i n g system may a f f e c t meso phase development. The r a t e ( 1 6 , 1 7 ) and atmosphere ( 18) o f c a r b o n i z a t i o n are important f a c t o r s . C o e x i s t i n g substances a r e a l s o i n f l u e n t i a l even i f they a r e not c a r b o n i z e d ( 19) · The carbonaceous s o u r c e s a r e o f t e n m i x t u r e s of complex components so t h a t t h e i r mutual i n t e r a c t i o n i s important f o r u n d e r s t a n d i n g t h e c a r b o n i z a t i o n b e h a v i o r of p i t c h m a t e r i a l s . The a u t h o r s i n t r o d u c e d "compatibility" (20,21) among the c a r b o n i z i n g components t o describe t h i s s i t u a t i o n . The components perform the i n t e r a c t i o n s of l i q u i d - l i q u i d m i x i n g , s o l v a t i o n , and s o l v o l y s i s and i n f l u e n c e the carbonization by m o d i f y i n g the c a r b o n i z a t i o n r a t e , the i n t e r m e d i a t e s t r u c t u r e s and c o m p o s i t i o n , and the v i s c o s i t y of t h e system ( 2 2 ) . The concept of c o m p a t i b i l i t y i s n o t o n l y u s e f u l i n u n d e r s t a n d i n g c a r b o n i z a t i o n r e a c t i o n s i n p r a c t i c a l m a t e r i a l s , but a l s o t o improve the q u a l i t y of a product by proper a d d i t i v e s . Carbonization reactions are also influenced s t r o n g l y by t h e s t a r t i n g s t r u c t u r e and s t r u c t u r a l d i s t r i b u t i o n s i n c e they govern the c a r b o n i z a t i o n r e a c t i v i t y and t h e c a r b o n i z a t i o n phase ( 2 3 25). A complex m i x t u r e i s more p r o p e r l y d e s c r i b e d by i t s s t r u c t u r a l d i s t r i b u t i o n than by i t s average s t r u c t u r e . C o n t r o l of C a r b o n i z a t i o n t o Mesophase An i m p o r t a n t g o a l i s t o be a b l e t o produce a carbon m a t e r i a l w i t h well-defined properties from a g i v e n carbonaceous precursor. Based on c a r b o n i z a t i o n mechanisms, some concepts c o n c e r n i n g the c o n t r o l of a n i s o t r o p i c development from a g i v e n s t a r t i n g m a t e r i a l can be d e v e l o p e d . There a r e two p r i n c i p a l approaches: 1. Design of c a r b o n i z a t i o n c o n d i t i o n s .




MOCHIDA AND KORAI

Figure 3. Sequences of c o n d e n s a t i o n r e a c t i o n s l e a d i n g t o carbon from naphthalene ( I ) u s i n g p o t a s s i u m or aluminum c h l o r i d e c a t a l y s t s . Reproduced w i t h p e r m i s s i o n from r e f e r e n c e 14. C o p y r i g h t 1976 Pergamon P r e s s , I n c .


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2.

Design of starting material including structural modification. The f i r s t approach i n c l u d e s c a t a l y t i c ( 1 3 - 1 7 ) and p r e s s u r i z e d c a r bonization (23-25)· C a t a l y s t s may be used t o c r e a t e n a p h t h e n i c s t r u c t u r e i n the c o n d e n s a t i o n r e a c t i o n . Heating r a t e i s another important f a c t o r ( 2 6 ) . The a u t h o r s have a l s o emphasized t h e i m p o r t a n c e of c o c a r b o n i z a t i o n ; c a r b o n i z a t i o n r e a c t i o n s tend t o be governed by minor components ( F i g u r e 4 ) . Marsh e t a l . ( 2 7 ) d e f i n e d such a s i t u a t i o n as t h e "dominant p a r t n e r e f f e c t " . By a s u i t a b l e a d d i t i v e , the c a r b o n i z a t i o n r e a c t i o n can be m o d i f i e d t o produce a d e s i r e d o p t i c a l t e x t u r e . The a r o m a t i c i t y and h y d r o g e n d o n a t i n g a b i l i t y of the a d d i t i v e a r e r e c o g n i z e d t o be i m p o r t a n t f o r t h e m o d i f y i n g a b i l i t y of t h e a d d i t i v e ( 2 8 - 3 0 ) . The e x t e n t of m o d i f i c a t i o n i n t h e c o c a r b o n i z a t i o n p r o c e s s i s a l s o i n f l u e n c e d by the p r i n c i p a l c a r b o n i z i n g substances ( 2 8 - 3 1 ) . The a u t h o r s i n t r o d u c e d t h e term " c o c a r b o n i z a t i o n s u s c e p t i b i l i t y " to describe t h i s s i t u a t i o n . The presence of o x y g e n - c o n t a i n i n g f u n c t i o n a l g r o u p s , which i s o f t e n observed i n c o a l - d e r i v e d p i t c h , i s one of the major f a c t o r s which i n f l u e n c e the s u s c e p t i b i l i t y (31). The m o l e c u l a r - s i z e d i s t r i b u t i o n of a r o m a t i c and p a r a f f i n i e components i n t h e f e e d s t o c k i s another f a c t o r ( 2 8 ) . The combined e f f e c t of m o d i f y i n g a b i l i t y and s u s c e p t i b i l i t y on the r e s u l t a n t o p t i c a l texture i s i l l u s t r a t e d schematically i n Figure 4. The second approach i n c l u d e s s e p a r a t i o n of u n d e s i r a b l e com ponents and c h e m i c a l m o d i f i c a t i o n of t h e s t r u c t u r e . Anti-solvent t e c h n i q u e s may be a p p l i e d t o remove q u i n o l i n e - i n s o l u b l e p a r t i c l e s (QI) from c o a l t a r p i t c h p r i o r t o d e l a y e d c o k i n g ( 3 2 ) . V a r i o u s k i n d s of c h e m i c a l m o d i f i c a t i o n , t h e r m a l and c a t a l y t i c t r e a t m e n t s , and t h e i r c o m b i n a t i o n a r e p o s s i b l e ( 3 3 - 3 6 ) . These t r e a t m e n t s may perform d e a l k y l a t i o n , c r a c k i n g , c o n d e n s a t i o n , hydrogénation, and hydrocracking. The a u t h o r s have reported that partial hydrogénation can enhance carbonization yield as w e l l as a n i s o t r o p i c development. P a r t i a l l y hydrogenated pyrene (produced u s i n g L i - e t h y l e n e d i a m i n e ) c a r b o n i z e d under a t m o s p h e r i c p r e s s u r e w h i l e pyrene d i d n o t ( 3 7 ) . Some t y p i c a l hydrogenated pyrenes a r e i l l u s t r a t e d i n F i g u r e 5 . Among these h y d r o p y r e n e s , 1 , 6 - and 1 , 8 d i h y d r o p y r e n e y i e l d carbons by t h e r m a l o l i g o m e r i z a t i o n a t a coke y i e l d of about 20%. O x i d a t i v e p r e t r e a t m e n t a t 150°C was e f f e c t i v e i n i n c r e a s i n g t h e coke y i e l d t o as h i g h as 35% w i t h o u t d e c r e a s i n g the s i z e of o p t i c a l t e x t u r e , w h i l e some n a p h t h e n i c p a r t i a l s t r u c t u r e s u r v i v e d even a f t e r o x i d a t i v e o r o x y g e n a t i v e o l i g o m e r i z a t i o n as shown i n F i g u r e 6 ( 3 3 ) · Based on the c r e a t i o n of n a p h t h e n i c s t r u c t u r e s i n t h e condensation reaction, t h e m o d i f i c a t i o n by aluminum c h l o r i d e i n c r e a s e d carbon y i e l d and improved t h e p o t e n t i a l f o r a n i s o t r o p i c development. O x i d a t i v e pretreatment u s u a l l y impairs a n i s o t r o p i c development a l t h o u g h i t i n c r e a s e s carbon y i e l d . The o x i d i z e d p i t c h i s a l s o m o d i f i e d by aluminum c h l o r i d e ( 3 4 ) ; t h i s may be used t o p r e p a r e a d d i t i v e s (38) and mesophase p i t c h e s ( 3 9 ) . I t s h o u l d be noted t h a t these p r o c e s s e s a l l o w t h e c a t a l y s t t o be r e a d i l y separated ( i n contrast to c a t a l y t i c carbonization) s i n c e the m o d i f i e d product i s s t i l l e i t h e r s o l u b l e i n some s o l v e n t s o r fusible.


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ω u D •u

κ

u

ι dominant

partner


1 0 0 % P i t c h b l e n d 100% Β A

Figure 4. Schematic diagram to illustrate three possible effects of cocarbonization on the development of optical anisotropy. A , a d d i t i v e ; B, p r i n c i p a l carbonizing substance.

F i g u r e 5 . Some t y p i c a l s t r u c t u r e s of hydrogenated pyrenes ( p r e pared u s i n g L i and e t h y l e n e d i a m i n e ) . *H-NMR i d e n t i f i c a t i o n : 1, 3.2 ppm; 2, 6.5 - 7.0 ppm; 3, 7.0 - 8 . 0 ppm; 4, 5.5 - 6 . 0 ppm; 5, 4.0 ppm; 6, 3.0 ppm; 7, 2.5 ppm; 8, 2.0 ppm. Reproduced w i t h p e r m i s s i o n from r e f e r e n c e 3 3 . C o p y r i g h t 1982 Pergamon P r e s s , I n c .


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F i g u r e 6. (a) O l i g o m e r i z a t i o n mechanisms of hydrogenated pyrene: l a , radical-coupling oligomerization; l b , olefinic o l i gomerization; I I , o x i d a t i v e o l i g o m e r i z a t i o n ; I I I , oxygenative o l i g o m e r i z a t i o n (x = o l i g o m e r i z a t i o n i n i t i a t o r ) · (b) M i c r o g r a p h of coke from h y d r o p y r e n e . Reproduced w i t h p e r m i s s i o n from r e f e r e n c e 3 3 . C o p y r i g h t 1982 Pergamon P r e s s , I n c .


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P r e p a r a t i o n of Mesophase

Pitch

Mesophase p i t c h , a p r e c u r s o r f o r h i g h performance carbon f i b e r , must be s p i n n a b l e a t moderate t e m p e r a t u r e s , h i g h l y o r i e n t e d , r e a c t i v e f o r o x i d a t i o n , and of h i g h c o k i n g v a l u e , a l t h o u g h these f a c t o r s are sometimes c o n f l i c t i n g . The g e n e r a l approaches to p r e p a r e such a mesophase p i t c h can be c l a s s i f i e d i n t o t h r e e categories: 1. P r e p a r a t i o n from a p p r o p r i a t e s t a r t i n g s u b s t a n c e s . 2. E x t r a c t i o n or s e p a r a t i o n of the a p p r o p r i a t e f r a c t i o n d u r i n g or a f t e r the p r e p a r a t i o n . 3. M o d i f i c a t i o n of o r d i n a r y mesophase. Acenaphthylene and t e t r a b e n z o p h e n a z i n e (40) are known to be a p p r o p r i a t e s t a r t i n g m a t e r i a l s ; hydrogenated o r c a t a l y t i c a l l y c o n densed a r o m a t i c hydrocarbons and p i t c h e s a r e a l s o s u i t a b l e ( 4 1 ) . In t h e r m a l treatment of p i t c h , i t i s s t i l l a q u e s t i o n whether s h o r t r e s i d e n c e times a t h i g h temperatures o r l o n g r e s i d e n c e times a t low temperatures are p r e f e r a b l e . In any c a s e , the r e a c t i o n s h o u l d be homogeneous. The s o l v e n t may thus be i m p o r t a n t . Examples of the second approach a r e d e s c r i b e d i n p a t e n t s from U n i o n C a r b i d e (42) and Exxon ( 4 3 ) , i n which the s e p a r a t i o n i s made during or after the preparative carbonization. Aromatic hydrocarbons l a r g e enough f o r a n i s o t r o p i c development, but not t o exceed a t h r e s h o l d s i z e , are prepared by the removal of l i g h t e r fractions. By the s o l v e n t a p p r o a c h , heavy f r a c t i o n s can a l s o be e l i m i n a t e d i f they are p r o d u c e d . A last approach is the hydrogénation of mesophase. Hydrogénation u s i n g L i - e t h y l e n e d i a m i n e (44) and hydrogen t r a n s f e r from tetrahydroquinoline (32) have been reported. More c o n v e n t i o n a l hydrogénation i s a l s o p o s s i b l e ( 4 5 , 4 6 ) . The a u t h o r s have r e p o r t e d t h a t the c o m b i n a t i o n of a l k y l a t i o n and hydrogénation of mesophase i n c r e a s e s the y i e l d of s o l u b i l i z e d mesophase (47). Hydrogen t r a n s f e r at h i g h temperature f o r s h o r t c o n t a c t time can be e f f e c t i v e ( 4 8 ) . "Dormant mesophase", i n which the i s o t r o p i c p r e c u r s o r d i s p l a y s a n i s o t r o p y a f t e r s t r e s s i n g , has been prepared by the hydrogénation of mesophase ( 4 4 ) ; t h i s t r a n s f o r m a t i o n may p r o v i d e another example of the p r e o r d e r e d l a y e r t r a n s f o r m a t i o n mechanism. P h y s i c a l mixing (blending) can a l s o i n c r e a s e the f u s i b i l i t y of mesophase, thus i n d i c a t i n g the importance of s t r u c tural distribution. S t r u c t u r e and Phase T r a n s i t i o n of

Mesophase

The m o l e c u l a r s t r u c t u r e and c o n s t i t u t i o n of the anisotropic mesophase sphere have been a n a l y z e d ( 4 9 , 5 0 ) · NMR, UV, I R , and o t h e r modern t e c h n i q u e s a r e a p p l i e d a f t e r the mesophase s o l u b i l i t y is enhanced by non-destructive hydrogénation or reductive alkylation. A model s t r u c t u r e i s i l l u s t r a t e d i n F i g u r e 7 ( 4 9 , 5 0 ) where a r o m a t i c sheets 6 t o 15 Â i n e x t e n t are connected d i r e c t l y o r through methylene bonds t o form l a r g e m o l e c u l e s w i t h m o l e c u l a r weights i n the range of 400 t o 4000. T h i s wide m o l e c u l a r weight d i s t r i b u t i o n i s i m p o r t a n t s i n c e i t may be r e s p o n s i b l e f o r the o r d e r e d s t a c k i n g of s m a l l e r m o l e c u l e s t h r o u g h π - π i n t e r a c t i o n , the e x t e n t of i n s o l u b i l i t y i n q u i n o l i n e , and the h i g h v i s c o s i t y a t elevated temperatures.


37


38


The o p t i c a l t e x t u r e of mesophase and r e s u l t a n t carbons i s observed r e a d i l y by means of a r e f l e c t e d p o l a r i z e d l i g h t m i c r o scope and may be c l a s s i f i e d a c c o r d i n g t o t h e shape and s i z e of t h e isochromatic u n i t s . Such a c l a s s i f i c a t i o n i s u s e f u l t o e v a l u a t e the p r o p e r t i e s of mesophase and carbons such as n e e d l e c o k e s . The mesophase has been d e f i n e d as t h e i n t e r m e d i a t e s t a t e which shows o p t i c a l a n i s o t r o p y and i s q u i n o l i n e - i n s o l u b l e a t room temperature (5,51) ( l i q u i d c r y s t a l g l a s s ) , although i t i s a viscous l i q u i d c r y s t a l d u r i n g the c a r b o n i z a t i o n process ( 6 ) . Recent advances i n the technology of carbon f i b e r have r e v i s e d the d e f i n i t i o n of mesophase. I t s n a t u r e as a l i q u i d c r y s t a l has been emphasized. D i e f e n d o r f proposed a phase diagram of carbonaceous mesophase analogous t o t h a t of conventional nematic l i q u i d c r y s t a l s (9). The schematic phase diagram of F i g u r e 8 summarizes the t h e r m a l b e h a v i o r of t h e mesophase. At present, the a b s c i s s a cannot be p r e c i s e l y d e f i n e d , although D i e f e n d o r f has used the c o n t e n t of m e s o - s p e c i e s and non-mesos p e c i e s a c c o r d i n g t o 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 t h e o r y (9). The r e a l mesophase c o n s i s t s of complex m o l e c u l e s t h a t may i n t e r a c t ; hence the a b s c i s s a may need t o take the s t r u c t u r a l d i s t r i b u t i o n i n t o account. The phase diagram i n F i g u r e 8 c o n s i s t s of i s o t r o p i c l i q u i d , a n i s o t r o p i c l i q u i d c r y s t a l , and l i q u i d c r y s t a l g l a s s . The phase b o u n d a r i e s correspond t o the m e l t i n g and g l a s s t r a n s i t i o n temperatures. The a n i s o t r o p i c l i q u i d c r y s t a l w i l l not o f t e n e x h i b i t t h e i s o t r o p i c l i q u i d s t a t e when the temperature r i s e s s i n c e the p y r o l y s i s r e a c t i o n s u s u a l l y form n o n - f u s i b l e c o k e , as u s u a l l y observed i n needle coke p r o d u c t i o n . The l i q u i d c r y s t a l and l i q u i d c r y s t a l g l a s s may d i s p l a y the v a r i o u s morphologies summarized i n Table I of the paper by Marsh and Latham ( 5 2 ) . I n t e r c o n v e r s i o n among the morphologies of t h e g l a s s i s a l l o w e d v i a i s o t r o p i c l i q u i d and l i q u i d c r y s t a l s t a t e s as shown by the f o l l o w i n g scheme: Anisotropic liquid crystal ( v a r i a b l e morphologies)

t

Isotropic liquid

Anisotropic liquid • c r y s t a l glass ( v a r i a b l e morphologies)

•

prohibited ^

Isotropic

glass

The i n t e r c o n v e r s i o n i n c l u d e s r e c r y s t a l l i z a t i o n and rearrangement by a n n e a l i n g . The i n t e r c o n v e r s i o n p r o c e s s e s a r e r a t e - d e p e n d e n t , and v a r i o u s morphologies (even i s o t r o p y from the p o t e n t i a l l y anisotropic components) may result, depending on the recrystallization rate. I n g e n e r a l , slow r a t e s tend t o e n l a r g e the u n i t s i z e . T h i s a l s o suggests t h a t t h e i s o t r o p i c p i t c h may be spun t o a n i s o t r o p i c f i b e r i f t h e c o o l i n g and e x t e n s i o n p e r m i t a n i s o t r o p i c development. Substances which show o p t i c a l a n i s o t r o p y and a r e s o l u b l e i n q u i n o l i n e or even i n t e t r a h y d r o f u r a n (THF) have been prepared (53). Thus, s o l u b i l i t y and o p t i c a l a n i s o t r o p y a r e now c o n s i d e r e d d i s t i n c t phenomena. T h i s i s i m p o r t a n t s i n c e t h e carbon f i b e r


2.

MOCHIDA AND KORAI


39


Molecular

F i g u r e 7. Models f o r the c o n s t i t u e n t m o l e c u l e s of s p h e r e s . Reproduced w i t h p e r m i s s i o n from r e f e r e n c e 2 0 . C o p y r i g h t 1977 IPC B u s i n e s s P r e s s , L t d .

mesophase

1iquid

glass

F i g u r e 8. phase.

A schematic

phase

diagram f o r the carbonaceous


meso



40

p r e c u r s o r s h o u l d be h i g h l y o r i e n t e d and a t t h e same time s p i n n a b l e ( p l a s t i c ) w i t h o u t d e c o m p o s i t i o n a t the a p p r o p r i a t e temperature p r i o r t o the c a r b o n i z a t i o n . The m o l e c u l e s of h i g h s o l u b i l i t y o r f u s i b i l i t y may d i f f e r c o n s i d e r a b l y i n t h e i r s i z e s and f u n c t i o n a l groups from those i l l u s t r a t e d i n F i g u r e 7. The a u t h o r s have prepared s o l u b l e mesophase p i t c h e s from p e t r o l e u m p i t c h (A240) and c o a l t a r p i t c h e s ( C T P ) , a p p l y i n g e s s e n t i a l l y t h e method r e p o r t e d by the Union C a r b i d e group ( 4 2 ) , and have a n a l y z e d the c h e m i c a l s t r u c t u r e s of the c o n s t i t u e n t m o l e cules. A240 h e a t - t r e a t e d a t 380°C f o r 30 h r . and CTP h e a t - t r e a t e d a t 380°C f o r 25 h r . developed a n i s o t r o p i c c o n t e n t s of more than 90 v o l % . According to the m a t e r i a l balance a considerable p o r t i o n of b o t h the THF S ( T H F - s o l u b l e ) and THFI-PS ( T H F - i n s o l u b l e p y r i d i n e s o l u b l e ) f r a c t i o n s , i n a d d i t i o n t o the P I ( p y r i d i n e - i n s o l u b l e ) f r a c t i o n from both A240 and CTP, e x h i b i t e d o p t i c a l a n i s o t r o p y . However, o n l y t h e THFI-PS f r a c t i o n from A240 e x h i b i t e d a n i s o t r o p y as r e c r y s t a l l i z e d ( F i g u r e 9 ) , a l t h o u g h both P I f r a c t i o n s were anisotropic. Some s t r u c t u r a l parameters f o r t h e THFS and THFI-PS f r a c t i o n s from the mesophase p i t c h e s , a n a l y z e d a c c o r d i n g the Brown-Ladner method ( 5 4 ) , a r e summarized i n Table I . The a r o m a t i c i t y v a l u e s (fa") f o r t h e THFS and THFI-PS f r a c t i o n s from A240 were s i g n i f i c a n t l y s m a l l e r than those from CTP, w h i l e t h e average m o l e c u l a r w e i g h t s of t h e A240 were d e f i n i t e l y l a r g e r than those o f t h e CTP f r a c t i o n s . Both Rnus ( n a p h t h e n i c r i n g number p e r u n i t s t r u c t u r e ) and σ (number of s u b s t i t u t i o n groups per u n i t s t r u c t u r e ) of the A240 f r a c t i o n s were a l s o l a r g e r than those of the CTP f r a c t i o n s . The n a p h t h e n i c and a l k y l groups enhance t h e s o l u b i l i t y and f u s i b i l i t y through more i n t i m a t e i n t e r a c t i o n w i t h s o l v e n t m o l e c u l e s and somewhat l o o s e r i n t e r m o l e c u l a r i n t e r a c t i o n within the mesophase. The h i g h e r m o l e c u l a r weight favors m o l e c u l a r a s s o c i a t i o n of l a y e r e d s t a c k i n g t o e x h i b i t a n i s o t r o p y . Thus, t h e degree of anisotropy of t h e THFI-PS fractions r e c r y s t a l l i z e d from A240 and CTP may r e f l e c t the s t r u c t u r e o f t h e i r components. S m a l l e r m o l e c u l e s from CTP, m o l e c u l e s which do not develop a n i s o t r o p y by t h e m s e l v e s , were l o c a t e d i n l a y e r s o f t h e i n s o l u b l e f r a c t i o n , which d i s p l a y e d a n i s o t r o p y b e f o r e the fractionation. Table I .

S t r u c t u r a l I n d i c e s of THFS and THFI-PS F r a c t i o n s

A240:380°C, 30 h . THFS THFI-PS

fa

Rnus

0.84 0.88

0.86 0.96

σ

AMW

0.1 5.7

0.19 0.19

540 1800

0.0 0.0

0.14 0.12

380 860

CTP: 380° C, 25 h . THFS THFI-PS fa: Rnus: σ: AMW:

aromaticity n a p h t h e n i c r i n g number p e r u n i t s t r u c t u r e number of s u b s t i t u t i o n groups p e r u n i t s t r u c t u r e average m o l e c u l a r weight




MOCHIDA AND KORAI

F i g u r e 9. O p t i c a l micrographs o f r e c r y s t a l l i z e d THFI-PS f r a c t i o n s from a) A240- and b) C T P - d e r i v e d mesophase p i t c h e s .


42



Mesophase R e a c t i v i t y Mesophase i s s u s c e p t i b l e t o c h e m i c a l r e a c t i o n s o t h e r than those induced by p y r o l y s i s . M o d i f i c a t i o n s to enhance f u s i b i l i t y o r s o l u b i l i t y f o r e a s i e r s p i n n i n g (see P r e p a r a t i o n of Mesophase P i t c h ) and to induce t h e r m o s e t t i n g f o r c a r b o n i z a t i o n w i t h o u t d e f o r m a t i o n a r e b o t h p r a c t i c a l s t e p s i n carbon f i b e r m a n u f a c t u r e . The r e a c t i v i t y f o r hydrogénation i s governed both by the r e a c t i v i t y of hydrogénation sources and the s u s c e p t i b i l i t y of the mesophase. G e n e r a l l y , mesophase prepared a t h i g h e r temperature i s more d i f f i c u l t to h y d r o g e n a t e , p r o b a b l y due t o the e x t e n s i v e l y condensed s t r u c t u r e of the c o n s t i t u e n t s (55). Nevertheless s e l e c t i v e hydrogénation i s r e q u i r e d f o r e f f e c t i v e m o d i f i c a t i o n . A l t h o u g h the heterogeneous c a t a l y s t system has seldom been a p p l i e d f o r the hydrogénation of mesophase, i t may be a p p l i c a b l e i f a proper s o l v e n t i s s e l e c t e d . A l k y l a t i o n enhances the s o l u b i l i t y of mesophase, but the a l k y l groups tend to be t h e r m a l l y e l i m i n a t e d (56) and thus c o n t r i b u t e l i t t l e to the f u s i b i l i t y . A l k y l a t i o n p r i o r to the hydrogénation i s thus v e r y e f f e c t i v e to i n c r e a s e the y i e l d of m o d i f i e d mesophase, as d e s c r i b e d above ( 4 7 ) . O x i d a t i v e t r e a t m e n t s can be a p p l i e d to remove hydrogen from o r t o I n t r o d u c e oxygen i n t o the mesophase ( 3 4 , 5 7 , 5 8 ) , thus l e a d i n g to further condensation of mesophase constituent molecules (oxidative condensation) sufficient for thermosetting. The r e a c t i o n s h o u l d be performed at temperatures below the s o f t e n i n g p o i n t and thus proceeds a t a slow r a t e . Mesophase r e a c t i v i t y may o r i g i n a t e from the c o n s t i t u e n t m o l e c u l e s . Naphthenic hydrogen i s known t o be much more r e a c t i v e t h a n a r o m a t i c hydrogen, as i n d i c a t e d by hydrogenated pyrene ( 3 3 ) · The b e n z y l group may be also susceptible. Thus the i n t r o d u c t i o n of a s u f f i c i e n t number of such groups f o r the i n d u c t i o n of t h e r m o s e t t i n g p r o p e r t i e s may be u s e f u l t o s h o r t e n the i n f u s i b i l i z a t i o n p r o c e d u r e . Oxygen may be i n c o r p o r a t e d i n t o the mesophase (57,58) · I n c o r p o r a t e d oxygen i s l o s t as carbon monoxide o r d i o x i d e i n the c a l c i n a t i o n s t e p so t h a t the o x y g e n a t i o n r e a c t i o n s h o u l d be minimized. The p r e c i s e c o n t r o l of such mesophase m o d i f i c a t i o n r e a c t i o n s may be a f u t u r e p r o b l e m .

Literature Cited 1. 2. 3. 4. 5. 6.

Marsh, H.; Walker, P. L . , Jr. Chem. and Phys. of Carbon 1979, 5, 229. Lewis, L. S. Fuel 1981, 60, 839. Bradshaw, W.; Mamone, V. In "Petroleum Derived Carbons"; Deviney, M. L.; O'Grady, T. M. Eds.; ACS SYMPOSIUM SERIES No. 21, American Chemical Society: Washington, DC, 1976. Patrick, J . W.; Wilkinson, H. C. In "Analytical Methods for Coal and Coal Products", Karr, C., J r . , Ed.; Academic: New York, 1978; Vol. II, p. 339. Brooks, J . D.; Taylor, G. H. Chem. and Phys. of Carbon 1968, 4, 243. Hoover, K. S.; Davis, Α.; Perrotta, A. J.; Spackman, W. Ext. Abstr. 14th Conf. Carbon, 1979, p. 393.


2. MOCHIDA AND KORAI 7. 8. 9. 10. 11. 12. 13.


14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36.


Buechler, M.; Ng, C. B.; White, J . L. Ext. Abstr. 15th Conf. Carbon, 1981, p. 182. Mochida, I.; Matsuoka, H.; Fujitsu, H.; Korai, Y.; Takeshita, K. Carbon 1981, 19, 213. Diefendorf, R. J . Ext. Abstr. 16th Conf. Carbon, 1983, p. 26. Mochida, I.; Korai, Y.; Fujitsu, H.; Takeshita, K.; Komatsubara, Y.; Koba, Y. Fuel 1981, 60, 1083. Fitzer, E.; Mueller, K.; Schaefer, W. Chem. and Phys. of Carbon 1971, 7, 237. Mochida, I.; Marsh, H. Fuel 1979, 58 626. Mochida, I.; Nakamura E.; Maeda, K.; Takeshita, K. Carbon 1975, 13, 489. Mochida, I.; Nakamura, E.; Maeda, K.; Takeshita, K. Carbon 1976, 14, 123. Mochida, I.; Maeda, K.; Takeshita, K. High Temp. High Press. 1977, 9, 123. Mochida, I.; Ando, T.; Maeda, K.; Takeshita, K. Carbon 1978, 16, 453. Mochida, I.; Ando, T.; Maeda, K.; Fujitsu, H.; Takeshita, K. Carbon 1980, 8, 319. Weintraub, Α.; Walker, P. L . , Jr. Proc. 3rd Int. Conf. on Carbon and Graphites 1971, p. 136. Korai, Y.; Fujitsu, H.; Takeshita, K.; Mochida, I. Fuel 1981, 60, 1106. Mochida, I.; Amamoto, K.; Maeda, K.; Takeshita, K. Fuel 1977, 56, 1977. Mochida, I.; Amamoto, K.; Maeda, K.; Takeshita, K. Fuel 1978, 57, 225. Mochida, I.; Marsh, H.; Grint, A. Fuel 1979, 58, 633. Ōtani, S.; Okamoto, Y.; Oshima, T.; Ōya, A. Tanso 1977, No. 8, 9. Mochida, I.; Sakata, K.; Maeda, K.; Fujitsu, H.; Takeshita, K. Fuel Process Tech. 1980, 3, 207. Marsh, H.; Akitt, J . W.; Hurley, J . M.; Melvin, J.; Warburton, A. P. J . Appl. Chem. 1971, 21, 251. Evans, S.; Marsh, H. Carbon 1971, 9, 733. Marsh, H.; Macefield, I.; Smith, J. Ext. Abstr., 13th Conf. Carbon 1977, p. 21. Mochida, I.; Korai, Y.; Fujitsu, H.; Takeshita, K.; Mukai, K.; Migitaka, W.; Suetsugu, F. Fuel 1981, 60, 405. Mochida, I.; Matsuoka, H.; Korai, Y.; Fujitsu, H. Takeshita, K. Fuel 1982, 61, 595. Mochida, I.; Matsuoka, H.; Korai Y.; Fujitsu, H.; Takeshita, K. Fuel 1982, 61, 595. Korai, Y.; Mochida, I. Fuel 1983, 62, 893. Yamada, Y.; Honda, H. Japanese patent 58-18421, 1983. Mochida, I.; Tamaru, K.; Korai, Y.; Fujitsu, H.; Takeshita, K. Carbon 1982, 20, 231. Mochida, I.; Inaba, T.; Korai, Y.; Fujitsu, H.; Takeshita, K. Carbon 1983, 21. 535. Mochida, I.; Takeshita, Y.; Korai, Y.; Fujitsu, H.; Takeshita, K. Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 505. Mochida, I.; Oishi, T.; Korai, Y.; Fujitsu, H.; Takeshita, K. Fuel Process Tech. 1983, 7, 109.


43

44

PETROLEUM-DERIVED CARBONS 37. 38. 39. 40. 41.


42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58.

Mochida, I.; Matsuoka, H.; Fujitsu, H.; Korai, Y . ; Takeshita, K. Carbon 1981, 19, 213. Mochida, I.; Iwamoto, K.; Korai, Y . ; Takeshita, K. J. Japan Petrol. Inst. 1983, 26, 201. Mochida, I.; Inaba, T.; Korai, Y . ; Takeshita, K. Carbon, in press. Ōtani, S. Mol Cryst. L i q . Cryst. 1981, 63, 249. Mochida, I.; Sone, Y . ; Matsuoka, H.; Korai, Y. Abstracts, Int. Symp. on Carbon (Toyohashi, Japan) 1982, p. 157. Chwastiak, S. U.K. Patent 2005 2984, 1979. Diefendorf, R. J.; Riggs, D. M. U.K. Patent GB 2002 024A, 1979. Ō t a n i , S. Japanese Patent 57-100186, 1982. Mochida, I.; Marsh, H. Fuel 1979, 58, 797. Mochida, I.; Maeda K.; Korai, Y . ; Fujitsu, H.; Takeshita, K. Fuel 1981, 60, 747. Mochida, I.; Maeda, K.; Korai, Y. Ext. Abstr. 16th Conf. Carbon 1983, p. 32. Mochida, I.; Moriguchi, Y . ; Shimohara, T.; Korai, Y . ; Fujitsu, H.; Takeshita, K. Fuel 1982, 61, 1015. Mochida, I.; Maeda, K.; Takeshita, K. Carbon 1977, 15, 17. Mochida, I.; Maeda, K.; Takeshita, K. Carbon 1978, 16, 459. Yamada, Y . ; Imamura, T.; Kakiyama, H.; Honda, H.; Oi, S.; Fukuda, K. Carbon 1974, 12, 307. Marsh, H.; Latham, C. S. This volume. Chwastiak, S.; Lewis, I . C. Carbon 1978, 16, 156. Brown, J. K.; Ladner, W. R. Fuel 1954, 33, 79. Mochida, I.; Maeda, K. J. Materials S c i . 1983, 18, 3736. Greinke, R. Α.; Lewis, I. C. Ext. Abstr. 16th Conf. Carbon 1983, p. 7. Ōtani, S. Carbon 1965, 3, 31. Barr, J. B.; Lewis, I. C. Carbon 1978, 16, 439.

RECEIVED November 27, 1985


Chemical Characterization and Preparation of the Carbonaceous

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