Novel Materials in Heterogeneous Catalysis - American Chemical

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Chapter 13

Cobalt Clays and Double-Layered Hydroxides as Fischer—Tropsch Catalysts

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L. Bruce, J. Takos, and T. W. Turney Division of Materials Science and Technology, Commonwealth Scientific and Industrial Research Organisation, Locked Bag 33, Clayton, Victoria, Australia, 3168

Cobalt-containing clays and double layered hydroxides have been synthesized and examined as Fischer Tropsch catalysts. Hydrothermal treatment of cobalt-aluminum hydroxysillcate gels affords 2:1 phyllosilicate clays with structures intermediate between smectites and chlorites. The clays become more chloritic on increasing Al content. Calcination results in decomposition of the hydroxide interlayer region and formation of small Co(Co,Al) O crystallites decorating the edges of the clay platelets. Hydrogen reduction converts these crystallites to ca. 100Å metallic Co particles. Catalysts containing metallic Co were also prepared from Co/Al hydroxycarbonates with a hydrotalcite structure. Comparison of the Fischer Tropsch activity of these catalysts showed the hydrotalcite-derived system to have longer lifetime and lower water gas shift activity. 2

4

T h i s work r e p r e s e n t s p a r t o f a program d e s i g n e d t o i d e n t i f y F i s c h e r - T r o p s c h (FT) c a t a l y s t s f o r the p r o c e s s i n g of H - r i c h s y n t h e s i s gas d e r i v e d from n a t u r a l gas. C u r r e n t advanced gas p r o c e s s e s a r e g e n e r a l l y a c o m b i n a t i o n o f p a r t i a l o x i d a t i o n and steam r e f o r m i n g , r e s u l t i n g i n H :C0 r a t i o s o f about 1.5-2.3. C o b a l t - r a t h e r t h a n i r o n - b a s e d FT c a t a l y s t s have been examined, i n o r d e r t o m i n i m i z e t h e competing water-gas s h i f t r e a c t i o n , which would r e s u l t i n a lowered carbon e f f i c i e n c y . Most c o b a l t FT c a t a l y s t s have been p r e p a r e d by c o p r e c i p i t a t i o n o f Co s a l t s w i t h v a r i o u s promoters onto a s l u r r i e d o x i d e support t o a f f o r d mixed phase systems ( 1 ) . R e d u c t i o n t o t h e a c t i v e c a t a l y s t was c o n t r o l l e d by a d d i t i o n of v a r i o u s promoters ( e . g . MgO, T h 0 , A 1 0 ) ( 2 ) . I n p a r t , these promoters a r e n e c e s s a r y t o m a i n t a i n good m e t a l d i s p e r s i o n i n t h e c a t a l y s t and r e s i s t a n c e t o s i n t e r i n g . Dispersion 2

2

2

2

3

0097-6156/90Λ)437-0129$06.00Α) © 1990 American Chemical Society In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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NOVEL MATERIALS IN HETEROGENEOUS CATALYSIS

i n multicomponent systems s h o u l d be more e f f e c t i v e l y c o n t r o l l e d t h r o u g h f o r m a t i o n of s t r u c t u r a l l y w e l l - d e f i n e d p r e c u r s o r c h e m i c a l compounds. Low d i m e n s i o n a l , l a m e l l a r s o l i d s , such as c l a y s or h y d r o t a l c i t e s , a r e e s p e c i a l l y a t t r a c t i v e f o r t h i s purpose, o f f e r i n g both r e a d i l y v a r i e d c o m p o s i t i o n and the p o t e n t i a l f o r h i g h component d i s p e r s i o n and s u r f a c e a r e a s . Experimental

Section

D e t a i l s of c h a r a c t e r i z a t i o n by x - r a y and e l e c t r o n d i f f r a c t i o n , t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y and XPS are g i v e n elsewhere (3)· Temperature programmed r e d u c t i o n (TPR) s t u d i e s u s i n g a 3%H /N gas m i x t u r e , were performed on 40 mg c a t a l y s t samples i n a t u b u l a r f u r ­ nace ( h e a t i n g r a t e 10°C/min), u s i n g TCD and FID d e t e c t i o n . Prior to TPR, the sample was heated i n a i r at a s p e c i f i e d temperature f o r 3h.

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2

2

H y d r o t h e r m a l s y n t h e s e s were performed i n an a u t o c l a v e ( P a r r , 2 L ) , u s i n g p o r t i o n s of a s l u r r y d e r i v e d from a d d i n g a hot (85-90°C) s o l u t i o n of ΰ ο ( Ν 0 ) · 6 Η 0 (0.5M, 1L) to a r a p i d l y s t i r r e d s o l u t i o n of hot NaOH (4M, 0.5L), i n t o which A 1 ( N 0 ) . 9 H 0 and fumed s i l i c a had p r e v i o u s l y been d i s s o l v e d . The a t o m i c r a t i o s of A l and S i employed, at c o n s t a n t r a t i o Co/Al+Si = 1.5, are shown i n T a b l e I . 3

2

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3

3

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T a b l e I . Dependence of C o b a l t C l a y Phases on Aluminum Content Al/Al+Si 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.8 1.0

Major Phases Smectite Fibres + smectite + c h r y s o t i l e Smectite C h l o r i t e + smectite Chlorite Chlorite Chlorite + CoO C h l o r i t e + CoO + C o ( C o , A l ) 0 4 CoO + C o ( C o , A l ) 0 ^ 2

2

The a u t o c l a v e was p r e s s u r i z e d w i t h hydrogen (400 kPa) and heated to 250°C f o r 16 h b e f o r e c o o l i n g and v e n t i n g . The c l a y was c o l l e c t e d by c e n t r i f u g a t i o n and washed r e p e a t e d l y w i t h water to remove i o n i c i m p u r i t i e s . C o b a l t h y d r o t a l c i t e s were o b t a i n e d by c o - p r e c i p i t a t i o n and a g e i n g of s l u r r i e s i n d i c a t e d i n T a b l e I I , u s i n g a t e c h n i q u e s i m i l a r to t h a t of R e i c h l e ( 4 ) , v i z , r e a c t i o n of Na0H/Na C0 (3/1 r a t i o ) s o l u t i o n (3.5 M i n 0H~) w i t h a mixed C o / A l n i t r a t e s o l u t i o n (1.5 M i n Co ), such t h a t the number of e q u i v a l e n t s of base added e q u a l l e d the number of e q u i v a l e n t s of Co+Al i o n s i n s o l u t i o n . F o r a g e i n g the s l u r r y a t 200°C, an a u t o c l a v e was u s e d . The degree of c r y s t a l l i n i t y was e s t i m a t e d from the w i d t h of d(Q03) P * 2

3

e a k

i n

In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

X R D

13.

Cobalt Clays & Double-Layered

BRUCE ET AL.

Table I I .

Metal Ratio Co/Al

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Hydroxides

P r o d u c t i o n o f Cobait/Aluminum Hydroxide C a t a l y s t Precursors

Temp.

Time

(°C)

(h)

£ (003)>

Hydrotalcite Hydrotalcite Hydrotalcite + Co(0H) Hydrotalcite Hydrotalcite

1.7 3.0 7.0

100 100 100

60 60 60

3.0 3.0

65 200

72 18

Phases p r e s e n t d

A

Xtallinity

]

[7.54] [7.65] [7.4]

Medium High Poor

[7.70] [7.62]

High V.high

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2

F i s c h e r T r o p s c h performance was examined at a t m o s p h e r i c p r e s s u r e , i n a down-flow m i c r o r e a c t o r w i t h a s s o c i a t e d o n - l i n e gas phase a n a l y s i s , which was automated and l i n k e d t o computerized d a t a reduction (5). Samples were p r e p a r e d f o r t e s t i n g by i n i t i a l d r y i n g o v e r n i g h t a t 90°C, f o l l o w e d by c a l c i n a t i o n a t 620°C f o r 2 h and s i z i n g (60-80#)· The r e a c t o r temperature was ramped i n f l o w i n g H at 6° m i n t o 400°C, and h e l d f o r a t l e a s t 3 h, b e f o r e l o w e r i n g the temperature t o 180°C and commencing f l o w o f ^ / C O ( 1 . 5 / 1 ) . Gas phase a n a l y s e s were made a t 0.5 h i n t e r v a l s , and the r e a c t i o n temperature r a i s e d i n s t e p s t o 2 2 0 ° C An o v e r n i g h t run a t about 10% c o n v e r s i o n l e v e l e n a b l e d c o l l e c t i o n of condensate f o r a n a l y s i s by liquid injection. 2

- 1

Results

and D i s c u s s i o n

C o b a l t C l a y S y n t h e s e s . P r e c i p i t a t i o n o f c o b a l t s a l t s by a l k a l i m e t a l s i l i c a t e / h y d r o x i d e m i x t u r e s produces c o b a l t h y d r o x y s i l i c a t e g e l s , which a f f o r d a wide range o f 1:1 and 1:2 c o b a l t c l a y s on hydrothermal treatment. I t has been shown p r e v i o u s l y t h a t the type of c l a y o b t a i n e d v a r i e s w i t h the a l k a l i m e t a l and i n i t i a l C o / S i r a t i o ( 3 ) . A d d i t i o n o f aluminium s a l t s t o these h y d r o x y s i l i c a t e g e l s e n a b l e d the s y n t h e s i s o f new c l a y phases (as w e l l as a f i b r o u s phase) as i n d i c a t e d i n T a b l e I . The new mixed Al/Co-containing c l a y s were p i n k t o l i l a c i n c o l o u r and were produced as the major phase (>95 wt%) i n the range o f a t o m i c r a t i o s , A l / S i + A l - 0.2-0.5 at a c o b a l t c o n t e n t o f C o / S i + A l = 1.5. C h l o r i t i c S t r u c t u r e o f Co/Al C l a y s . Each of the c l a y p r e p a r a t i o n s e x h i b i t e d an XRD b a s a l s p a c i n g i n the r e g i o n , 14.7-15.3 Â, as w e l l as h i g h e r o r d e r (001) and (hkO) r e f l e c t i o n s t y p i c a l o f a w e l l o r d e r e d 2:1 p h y l l o s i l i c a t e ( F i g u r e l a ) . The s w e l l i n g p r o p e r t i e s o f t h e s e c l a y s v a r i e d o v e r the A l / A l + S i c o m p o s i t i o n range. I n the absence of A l , a b r i g h t blue c l a y , w i t h s w e l l i n g and t h e r m a l p r o p e r t i e s t y p i c a l o f a s m e c t i t e , was o b t a i n e d ( 3 ) . I n the range, A l / A l + S i = 0.1-0.3, h e a t i n g of the c l a y phase r e s u l t e d i n l a y e r c o n t r a c t i o n i n s t a g e s [d(001) = 12Â a t 300°C and 9.7Â a t 6 0 0 ° C ] . T h i s i s t y p i c a l b e h a v i o u r o f a s w e l l i n g s m e c t i t e c l a y , such as montmorillonite. However, w i t h the A l - c o n t a i n i n g c l a y s , t r e a t m e n t

In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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I I I I I I I I 1 I 1 III I 1 I I I I I I I I M I II

ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι

Μ

ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι

a.

11

'

I I I I I I I

ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι

2

2-Theta (degrees)

F i g u r e 1. X - r a y powder d i f f r a c t i o n p a t t e r n o f a C o / A l c h l o r i t e p h a s e , A l / A l + S i • 0.4. a ) C a l c i n e d a t 600°C, 3 h, showing s p i n e l phase, C o ( C o , A l ) 0 4 ( * ) , b) A i r - d r i e d sample. 2

In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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13. BRUCE ET ΑΧ»

Cobalt Clays & Double-Layered

133

Hydroxides

w i t h e t h y l e n e g l y c o l o r g l y c e r o l d i d not r e s u l t i n i n c r e a s e d b a s a l s p a c i n g s , as would a l s o be e x p e c t e d f o r a s m e c t i t e . A t A l / A l + S i • 0.33-0.8, l a y e r c o n t r a c t i o n on m i l d h e a t i n g was l e s s ( f o r A l / A l + S i - 0.4, d x = 14.lA a t 4 5 0 ° C ) . These A l - c o n t a i n i n g c l a y phases a r e b e l i e v e d t o c o n s i s t o f p o s i t i v e l y c h a r g e d , mixed C o / A l h y d r o x i d e s h e e t s w i t h i n the i n t e r l a y e r r e g i o n o f a t r i o c t a h e d r a l C o / A l c l a y . As A l / A l + S i i n c r e a s e s , the i n t e r l a y e r C o / A l h y d r o x i d e sheet becomes p r o g r e s s i v e l y more complete, r e s u l t i n g i n the c l a y s t r u c t u r e and p r o p e r t i e s g r a d i n g from t h o s e e x p e c t e d f o r a s m e c t i t e t o those o f a typical chlorite. A t r a n s m i s s i o n e l e c t r o n micrograph o f a t y p i c a l c h l o r i t i c phase i s shown i n F i g u r e 2a.

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( 0 Q 1

C a l c i n a t i o n and R e d u c t i o n o f C l a y Phases. The i n t e r l a y e r r e g i o n i n c h l o r i t e s and r e l a t e d hydroxy i n t e r l a y e r e d c l a y s i s r e p o r t e d t o d e h y d r a t e between 250°C and 650°C, d e p e n d i n g upon the i n t e r l a y e r c a t i o n . T h i s causes c o l l a p s e o f the i n t e r l a y e r r e g i o n , but a p p a r e n t l y w i t h o u t d e s t r u c t i o n o f t h e 1:2 l a y e r system ( 6 - 7 ) . I n a c c o r d w i t h t h i s model, the b a s a l s p a c i n g o f the p r e s e n t C o / A l c l a y s d e c r e a s e d t o c a 9.7Â, on h e a t i n g i n a i r t o 600°C. This l a t t i c e c o n t r a c t i o n was accompanied by f o r m a t i o n o f c r y s t a l l i n e 3 ° 4 ( F i g u r e l b ) . E l e c t r o n m i c r o g r a p h s ( F i g u r e 2b) r e v e a l e d t h a t t h e l i b e r a t e d COOOA p a r t i c l e s d e c o r a t e d t h e edges o f t h e c l a y p l a t e l e t s , whose morphology had o t h e r w i s e remained unchanged. The f o r m a t i o n o f C 0 3 O 4 m i c r o c r y s t a l l i t e s was c o n f i r m e d by s e l e c t e d a r e a e l e c t r o n d i f f r a c t i o n (SAD). As e x p e c t e d , h e a t i n g caused the c l a y samples t o t u r n b l a c k , due t o o x i d a t i o n o f C o ( I I ) t o C o ( I I I ) . C o

Temperature programmed r e d u c t i o n o f c a l c i n e d c l a y samples r e v e a l e d t h a t t h e i r r e d u c i b i l i t y was s e n s i t i v e both t o the t h e r m a l p r e t r e a t m e n t c o n d i t i o n s as w e l l as t o A l / A l + S i r a t i o ( F i g u r e 3 ) . T a b l e I I I summarizes the d e g r e e o f r e d u c t i o n t o Co, as measured from the H2 uptake t o 450°C. On samples p r e v i o u s l y c a l c i n e d t o 600°C, r e d u c t i o n t o form m e t a l l i c Co was c o n f i r m e d by both XPS and SAD.

Table

I I I . Dependence o f C a t a l y s t R e d u c i b i l i t y on Aluminum Content and C a l c i n a t i o n Temperature In C o b a l t C l a y s

Al/Al+Si ratio 0.0 0.1 0.2 0.3 0.4 0.5 0.6

C a l c i n a t i o n Temperature 110°C 620°C