Novel Materials in Heterogeneous Catalysis - American Chemical

Chapter 14

Pillared Hydrotalcites Synthesis, Characterization, and Catalytic Activity

Downloaded by QUEENSLAND UNIV OF TECHNOLOGY on October 14, 2014 | http://pubs.acs.org Publication Date: September 21, 1990 | doi: 10.1021/bk-1990-0437.ch014

Mark A. Drezdzon Amoco Chemical Company, Amoco Research Center, Naperville, IL 60566

Exchange of terephthalate-pillared hydrotalcite with molybdate or vanadate salts under mildly acidic conditions affords the corresponding isopolymetalate-pillared hydrotalcite in high yield. Synthesis of mixed terephthalate/isopolymetalate-pillared hydrotalcites followed by calcination yields higher surface area products. Isopolymetalate-pillared hydrotalcites have been found to produce both propylene and acetone when screened for 2-propanol conversion activity, indicating the presence of both acidic and basic catalytic sites. Over t h e p a s t 15-20 y e a r s , t h e r e has been a renewed and growing i n t e r e s t i n t h e u s e o f c l a y m i n e r a l s as c a t a l y s t s o r c a t a l y s t supports. Most o f t h i s i n t e r e s t has f o c u s e d on t h e p i l l a r i n g o f s m e c t i t e c l a y s , such as m o n t m o r i l l o n i t e , w i t h v a r i o u s t y p e s o f c a t i o n s , such as h y d r a t e d m e t a l c a t i o n s , alkylammonium c a t i o n s and p o l y c a t i o n s , and p o l y n u c l e a r h y d r o x y m e t a l c a t i o n s ( 1 - 1 7 ) . By changing the s i z e o f the c a t i o n used to separate the a n i o n i c sheets i n t h e c l a y s t r u c t u r e , m o l e c u l a r s i e v e - l i k e m a t e r i a l s c a n be made w i t h p o r e s i z e s much l a r g e r t h a n t h o s e o f c o n v e n t i o n a l z e o l i t e s . Clays c o n t a i n i n g hydrated metal c a t i o n s or organic-based c a t i o n s c o l l a p s e upon moderate h e a t i n g due t o t h e t h e r m a l i n s t a b i l i t y o f t h e s e p i l l a r i n g agents (10-16). Using polynuclear h y d r o x y m e t a l c a t i o n s such as [ A 1 0 ( 0 H ) ( H 0 ) ] , s t a b l e p o r o u s c l a y m a t e r i a l s c a n be made ( 1 - 9 ) . However, t h e number o f m e t a l s t h a t form s u i t a b l e p o l y m e r i c s p e c i e s i s l i m i t e d . The a n i o n i c p o l y o x o m e t a l a t e s comprise a much l a r g e r c l a s s o f m e t a l o x i d e - b a s e d p i l l a r i n g agents ( 1 8 ) . I t was p r o p o s e d t h a t h y d r o t a l c i t e s ( a l s o r e f e r r e d t o as l a y e r e d d o u b l e h y d r o x i d e s , o r LDHs) would be s u i t a b l e h o s t m a t e r i a l s f o r p i l l a r i n g w i t h polyoxometalates. Subsequently, a route f o r s y n t h e s i z i n g isopolymetalate-pillared hydrotalcites v i a organic-anion-pillared p r e c u r s o r s was d e v e l o p e d (19-20). T h i s paper summarizes work done on t h e s y n t h e s i s and c h a r a c t e r i z a t i o n o f t e r e p h t h a l a t e - , i s o p o l y m e t a l a t e - , and terephthalate/isopolymetalate-pillared hydrotalcites. Several test 7 +

1 3

A

2 4

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1 2

0097-6156y90/0437-0140$06.00A) © 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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r e a c t i o n s aimed a t u n d e r s t a n d i n g the c a t a l y t i c p r o p e r t i e s of these m a t e r i a l s , i n c l u d i n g t h e i r a c i d - b a s e p r o p e r t i e s , w i l l a l s o be reported.


Experimental D e t a i l s on the m a t e r i a l s and methods u s e d t o s y n t h e s i z e and c h a r a c t e r i z e the p i l l a r e d h y d r o t a l c i t e s d i s c u s s e d i n t h i s paper been p r e v i o u s l y published (19). For the i n t e r e s t e d reader's convenience, a b r i e f d e s c r i p t i o n of the s y n t h e s i s of s e v e r a l pillared hydrotalcites follows.

have

Synthesis of T e r e p h t h a l a t e - P i l l a r e d H y d r o t a l c i t e . A 5-L, threeneck, round-DOttom f l a s k equipped w i t h a r e f l u x condenser, t h e r m o m e t e r , m e c h a n i c a l s t i r r e r , a n d e l e c t r i c h e a t i n g m a n t l e was c h a r g e d w i t h 1 6 0 0 mL d e i o n i z e d w a t e r , 1 3 3 . 1 g t e r e p h t h a l i c a c i d , a n d 575 g 50% NaOH s o l u t i o n . A s o l u t i o n c o n t a i n i n g 1 2 8 0 mL d e i o n i z e d w a t e r , 4 1 0 . 1 g M g ( N 0 ) ' 6 H 0 , a n d 3 0 0 . 0 g Α 1 ( Ν 0 ) · 9 Η 0 was t h e n a d d e d d r o p w i s e t o t h e t e r e p h t h a l a t e / N a O H s o l u t i o n a t room t e m p e r a t u r e o v e r a p e r i o d o f 90 m i n . The r e s u l t i n g g e l - l i k e m i x t u r e w a s d i g e s t e d a t 7 3 - 7 4 ° C f o r 18 h . U p o n c o o l i n g , t h e p r o d u c t was i s o l a t e d b y f i l t r a t i o n , washed w i t h d e i o n i z e d w a t e r , and s t o r e d as a 7.39 wt % w a t e r s l u r r y ( n e t w e i g h t o f p i l l a r e d h y d r o t a l c i t e - 2 0 3 g , 86% yield). A p r o d u c t d - s p a c i n g o f 1 4 . 4 Â was d e t e r m i n e d b y X R D . The f o r m u l a Mg A1 (0H) ( T A ) 4 H 0 (TA - t e r e p h t h a l a t e ) i s c o n s i s t e n t w i t h e l e m e n t a l a n a l y s e s p e r f o r m e d on the p r o d u c t . BET s u r f a c e a r e a o f t h e p r o d u c t - 35 m /g ( o u t g a s s i n g t e m p e r a t u r e - 2 0 0 ° C ) , w h i c h i n c r e a s e d t o 2 9 8 m /g a f t e r c a l c i n a t i o n a t 5 0 0 ° C f o r 12 h . 3

4

2 7

2

1 2

2

2

3

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3

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Synthesis of M o l v b d a t e - P i l l a r e d H y d r o t a l c i t e . To a 2 5 0 0 - g p o r t i o n o f t h e p r e c e d i n g t e r e p h t h a l a t e - p i l l a r e d h y d r o t a l c i t e s l u r r y was a d d e d a s o l u t i o n c o n s i s t i n g o f 2 5 4 g N a M o O * 2 H 0 i n 4 5 0 mL d e i o n i z e d water. ( T h e a m o u n t o f m o l y b d a t e a d d e d c o r r e s p o n d s t o a 50% e x c e s s o f t h a t needed f o r s t o i c h i o m e t r i c exchange o f t e r e p h t h a l a t e w i t h Mo 0 ".) A p p r o x i m a t e l y 3 5 0 mL 4N H N 0 was s l o w l y a d d e d t o t h e m i x t u r e w i t h v i g o r o u s s t i r r i n g , r e s u l t i n g i n a pH d r o p f r o m 1 2 . 0 t o 4.4. A f t e r 5 m i n a d d i t i o n a l s t i r r i n g ( p H — 3 . 7 ) , t h e p r o d u c t was f i l t e r e d , washed, and d r i e d a t 125°C o v e r n i g h t . The y i e l d o f h a r d , s l i g h t l y o f f - w h i t e c h u n k s o f p r o d u c t was 2 1 5 g ( 9 4 % ) . A p r o d u c t d - s p a c i n g o f 1 2 . 2 À was d e t e r m i n e d b y XRD. The formula Mg A1 (0H) (Mo 0 )'6H 0 i s consistent with elemental a n a l y s e s performed on the p r o d u c t . BET s u r f a c e a r e a o f t h e p r o d u c t - 27 m /g ( o u t g a s s i n g t e m p e r a t u r e « 2 0 0 ° C , w h i c h i n c r e a s e d t o 32 m /g a f t e r c a l c i n a t i o n a t 5 0 0 ° C f o r 12 h . 2

7

2 A

6

A

2

3

1 2

2 6

6

3 6

5 2

7

2 A

2

2

2

Synthesis of V a n a d a t e - P i l l a r e d H y d r o t a l c i t e . To a 1 4 0 0 - g p o r t i o n o f a t e r e p h t h a l a t e - p i l l a r e d h y d r o t a l c i t e was a d d e d a s o l u t i o n c o n t a i n i n g 7 3 . 9 1 g N a V 0 (50% e x c e s s ) i n 5 0 0 mL d e i o n i z e d w a t e r . A p p r o x i m a t e l y 3 2 0 mL 4N H N 0 was s l o w l y a d d e d t o t h e m i x t u r e w i t h v i g o r o u s s t i r r i n g , r e s u l t i n g i n a pH d r o p f r o m 9 . 0 t o 4 . 5 . After 5 m i n a d d i t i o n a l s t i r r i n g ( p H « 4 . 9 ) , t h e p r o d u c t was f i l t e r e d , washed, and d r i e d a t 125°C o v e r n i g h t . The y i e l d o f h a r d , y e l l o w i s h c h u n k s o f p r o d u c t was 1 0 5 g ( 8 5 % ) . 3

3

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

A p r o d u c t d - s p a c i n g o f 1 1 . 8 À was d e t e r m i n e d b y X R D . The formula Mg Al (OH) (V O )'6H 0 i s consistent with elemental a n a l y s e s p e r f o r m e d on the p r o d u c t . BET s u r f a c e a r e a o f t h e p r o d u c t = 30 m /g ( o u t g a s s i n g t e m p e r a t u r e = 2 0 0 ° C ) , w h i c h i n c r e a s e d t o 32 m /g a f t e r c a l c i n a t i o n a t 5 0 0 ° C f o r 12 h r . 1 2

8 3

6

3 7

6 6

1 0

2 8

2

2

2

Synthesis of Terephthalate/Vanadate-Pillared H y d r o t a l c i t e . To a 1 3 5 0 - g p o r t i o n o f a t e r e p h t h a l a t e - p i l l a r e d h y d r o t a l c i t e was a d d e d a s o l u t i o n c o n s i s t i n g o f 1 4 . 0 g N a V 0 i n 1 0 0 mL d e i o n i z e d w a t e r . (The a m o u n t o f v a n a d a t e a d d e d c o r r e s p o n d s t o 25% o f t h a t n e e d e d f o r s t o i c h i o m e t r i c exchange o f t e r e p h t h a l a t e w i t h V O ". ) A p p r o x i m a t e l y 1 0 0 mL 4N H N 0 was s l o w l y a d d e d t o t h e m i x t u r e w i t h v i g o r o u s s t i r r i n g , r e s u l t i n g i n a pH d r o p f r o m 9 . 3 t o 4 . 5 . After 5 m i n a d d i t i o n a l s t i r r i n g ( p H = 4 . 8 ) , t h e p r o d u c t was f i l t e r e d , washed, and d r i e d a t 125°C o v e r n i g h t . The y i e l d o f h a r d , p a l e y e l l o w i s h c h u n k s o f p r o d u c t was 97 g ( 8 1 % ) . A p r o d u c t d - s p a c i n g o f 1 4 . 4 3 Â was d e t e r m i n e d b y X R D . The formula Mg A1 (OH) (V 0 ) (TA) /6H 0 is consistent with e l e m e n t a l a n a l y s e s p e r f o r m e d on the p r o d u c t . BET s u r f a c e a r e a o f t h e p r o d u c t - 4 0 m /g ( o u t g a s s i n g t e m p e r a t u r e = 2 0 0 ° C ) , w h i c h i n c r e a s e d t o 166 m /g a f t e r c a l c i n a t i o n a t 5 0 0 ° C f o r 12 h r .


3

1 0

2 8

6

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6

3 7

9 6

1 0

2 8

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Results

and

Discussion

P r e v i o u s l y r e p o r t e d methods f o r s y n t h e s i z i n g polyoxometalatep i l l a r e d h y d r o t a l c i t e s i n v o l v e the exchange o f c h l o r i d e - c o n t a i n i n g hydrotalcites (21-23). In t h i s work, the n o v e l approach taken i n s y n t h e s i z i n g p i l l a r e d h y d r o t a l c i t e s has been to prepare an o r g a n i c a n i o n - p i l l a r e d p r e c u r s o r , which i s subsequently exchanged w i t h the a p p r o p r i a t e i s o p o l y m e t a l a t e under m i l d l y a c i d i c c o n d i t i o n s (Figure 1). S y n t h e s i s and C h a r a c t e r i z a t i o n of T e r e p h t h a l a t e - P i l l a r e d Hydrotalcite. Molecular modeling studies indicated that organic a n i o n s c o n t a i n i n g an a r o m a t i c r i n g , such as the t e r e p h t h a l a t e d i a n i o n , w o u l d be l a r g e enough t o p r e s e p a r a t e the m e t a l h y d r o x i d e l a y e r s of h y d r o t a l c i t e f o r polyoxometalate exchange. Therefore, a s e r i e s o f h y d r o t a l c i t e s y n t h e s e s was p e r f o r m e d b y p u b l i s h e d c o p r e c i p i t a t i o n / d i g e s t i o n procedures (24-27) except f o r the s u b s t i t u t i o n o f the d e s i r e d o r g a n i c a n i o n , s u c h as t e r e p h t h a l a t e , for carbonate. I n t h i s manner, t e r e p h t h a l a t e - p i l l a r e d h y d r o t a l c i t e was p r e p a r e d , a s c o n f i r m e d b y X - r a y d i f f r a c t i o n a n d e l e m e n t a l analysis. The 1 4 . 4 À d - s p a c i n g m e a s u r e d f o r t e r e p h t h a l a t e - p i l l a r e d h y d r o t a l c i t e agrees w e l l w i t h t h a t c a l c u l a t e d from m o l e c u l a r models assuming the plane of the aromatic r i n g i s p e r p e n d i c u l a r to the brucite layers. In samples d r i e d below 125°C, the r e m a i n i n g g a l l e r y space i s f i l l e d w i t h water. H e a t i n g to 3 0 0 - 3 5 0 ° C removes t h i s i n t e r l a y e r water (approximately 4 H 0 per terephthalate anion). C a l c i n a t i o n at h i g h e r temperatures r e s u l t s i n decomposition of the organic anion. In n e u t r a l or b a s i c media, t e r e p h t h a l a t e - p i l l a r e d h y d r o t a l c i t e is stable indefinitely. A c i d i f i c a t i o n w i t h d i l u t e HC1 o r H N 0 r e s u l t s i n the formation of the corresponding c h l o r i d e - or n i t r a t e 2

3


14.

PiUared

DREZDZON

143

Hydrotalcites

c o n t a i n i n g h y d r o t a l c i t e , as i n d i c a t e d by a c o l l a p s e o f the d - s p a c i n g to 7.8 Â . Treatment o f t e r e p h t h a l a t e - p i l l a r e d h y d r o t a l c i t e w i t h m o n o m e t a l a t e s a l t s (Na MoO o r N a V 0 ) a t room t e m p e r a t u r e w i t h o u t a c i d i f i c a t i o n r e s u l t s i n no exchange.


2

A

3

S y n t h e s i s and C h a r a c t e r i z a t i o n o f I s o p o l y m e t a l a t e - P i l l a r e d Hydrotalcites. Exchange o f t e r e p h t h a l a t e - p i l l a r e d h y d r o t a l c i t e w i t h t h e a p p r o p r i a t e m e t a l a t e u n d e r m i l d l y a c i d i c c o n d i t i o n s (pH — 4 . 5 ) proceeds smoothly to y i e l d the corresponding i s o p o l y m e t a l a t e p i l l a r e d h y d r o t a l c i t e , as i n d i c a t e d by X - r a y d i f f r a c t i o n and elemental analysis. The d - s p a c i n g s o f t h e p r o d u c t s c o n f i r m t h a t t h e shortest dimension of the isopolymetalate i s perpendicular to the metal hydroxide layers. Figure 2 i l l u s t r a t e s the thermal behavior of a heptamolybdatep i l l a r e d h y d r o t a l c i t e ; the thermal behavior of a decavanadatep i l l a r e d h y d r o t a l c i t e i s s i m i l a r , with g a l l e r y water l o s s below 300350°C a n d b r u c i t e l a y e r d e h y d r o x y l a t i o n up t o 5 0 0 ° C . Highr e s o l u t i o n TEM m i c r o g r a p h s o f m a t e r i a l s c a l c i n e d u p t o 5 0 0 ° C show t h a t the l a y e r e d h y d r o t a l c i t e s t r u c t u r e i s s t i l l i n t a c t ; however, the exact f a t e o f the polyoxometalate p i l l a r s w i t h r e s p e c t to p o s s i b l e rearrangement i s s t i l l under i n v e s t i g a t i o n . Total d e s t r u c t i o n o f t h e l a y e r e d s t r u c t u r e o c c u r s above 600°C, where t h e c o r r e s p o n d i n g magnesium m e t a l a t e i s p r o d u c e d . Variable P i l l a r Spacing i n P i l l a r e d Hydrotalcites. As i l l u s t r a t e d in Figure 3, the p i l l a r s i n a decavanadate-pillared h y d r o t a l c i t e are v e r y c l o s e l y s p a c e d , w i t h j u s t enough room f o r g a l l e r y w a t e r molecules. P i l l a r e d h y d r o t a l c i t e s w i t h l a r g e r p o r e s may b e s y n t h e s i z e d by t h e p a r t i a l exchange o f t e r e p h t h a l a t e - p i l l a r e d h y d r o t a l c i t e , f o l l o w e d b y c a l c i n a t i o n t o remove t h e r e m a i n i n g o r g a n i c m a t e r i a l ( 2 0 ) . Table I summarizes d a t a o b t a i n e d on a s e r i e s of p i l l a r e d h y d r o t a l c i t e s w i t h v a r y i n g terephthalate/decavanadate pillar ratio. As expected, the s u r f a c e area o f the c a l c i n e d p i l l a r e d h y d r o t a l c i t e s i n c r e a s e s a s t h e number o f d e c a v a n a d a t e p i l l a r s i n the g a l l e r y decreases. Table

%

Exchange 91 82 60 39 21 0

I.

Exchange R e a c t i o n s o f Hydrotalcite Clays

Terephthalate-Pillared w i t h Decavanadate

d-Spacing (k) TA Exchanged 11.76 14.39 11.89 14.39 14.40 11.76,14.27 14.32 14.38 14.43 14.40 14.40 —

S u r f a c e A r e a (m /e) Calcined Drv 38 38 45 33 64 38 123 40 166 40 298 36 2

C l a y s w h i c h h a v e a l a r g e m a j o r i t y (>80% e x c h a n g e ) o f polyoxometalate p i l l a r s exhibit a d-spacing consistent with a polyoxometalate-pillared material. When l e s s t h a n h a l f o f t h e t e r e p h t h a l a t e has been r e p l a c e d by polyoxometalate, the d-spacing does n o t change, r e f l e c t i n g t h e l a r g e r e f f e c t i v e p i l l a r i n g s i z e o f the t e r e p h t h a l a t e anion. A t i n t e r m e d i a t e exchange l e v e l s (50-75%)


144


co;

d = 14.4 AO

M0O4

VOS

co;


OH"

M07O24

H*

14.4

'

I

A

t

OH"

CO*

Y d = 12.2 A

Y 6

M07024 "

d = 11.9 Α

ν,οΟίβ*

-

F i g u r e 1. Schematic r e p r e s e n t a t i o n of t e r e p h t h a l a t e exchange w i t h heptamolybdate and decavanadate i n h y d r o t a l c i t e . Pillared h y d r o t a l c i t e d - s p a c i n g s were c a l c u l a t e d from models. ( R e p r o d u c e d f r o m R e f . 1 9 . C o p y r i g h t 1988 A m e r i c a n C h e m i c a l S o c i e t y . )

Temperature (°C) Figure 2. Thermal a n a l y s i s f o r h e p t a m o l y b d a t e - p i l l a r e d hydrotalcite: ( a ) TGA; ( b ) DTA. (Reproduced from Ref. 19. C o p y r i g h t 1988 A m e r i c a n C h e m i c a l S o c i e t y . )


14.

DREZDZON

Pillared

145

Hydrotalcites

d - s p a c i n g s c o r r e s p o n d i n g t o b o t h the t e r e p h t h a l a t e - and p o l y o x o m e t a l a t e - p i l l a r e d h y d r o t a l c i t e s are detected. Since a n a l y t i c a l e l e c t r o n m i c r o s c o p y shows a r e l a t i v e l y homogeneous e l e m e n t a l d i s t r i b u t i o n t h r o u g h o u t the sample, exchange on a p a r t i c l e - b y - p a r t i c l e b a s i s i s not o c c u r r i n g . One cause o f the d u a l d - s p a c i n g s might be s a g g i n g o f the b r u c i t e l a y e r s i n a r e a s where r a f t s o f p o l y o x o m e t a l a t e p i l l a r s may have been i n c o r p o r a t e d .


5 1

F i g u r e 4 shows the V MAS-NMR s p e c t r a o f s e v e r a l terephthalate/decavanadate-pillared h y d r o t a l c i t e s . These s p e c t r a are c o n s i s t e n t w i t h that p r e v i o u s l y p u b l i s h e d f o r ( N H ) V O (23) . The changes w h i c h o c c u r t o the decavanadate p i l l a r s upon c a l c i n a t i o n o f the p i l l a r e d h y d r o t a l c i t e a r e c u r r e n t l y b e i n g i n v e s t i g a t e d . I t i s known, however, t h a t i n the c a s e o f p i l l a r e d m a t e r i a l s c o n t a i n i n g as l i t t l e as 25-30% p o l y o x o m e t a l a t e p i l l a r s the o v e r a l l l a y e r e d s t r u c t u r e i s m a i n t a i n e d up t o 500°C ( F i g u r e 5 ) . 4

Catalytic Activity of Pillared

6

1 0

2 8

Hydrotalcites

I t has p r e v i o u s l y been r e p o r t e d t h a t h y d r o t a l c i t e c a t a l y z e s t h e a l d o l c o n d e n s a t i o n o f acetone (25). P o l y o x o m e t a l a t e s a r e known t o d e h y d r a t e a l c o h o l s due t o t h e i r a c i d i c n a t u r e ( 1 8 ) . I n order t o compare the r e l a t i v e b a s i c i t y o f p o l y o x o m e t a l a t e - p i l l a r e d h y d r o t a l c i t e s to that o f h y d r o t a l c i t e i t s e l f , a v a r i e t y o f h y d r o t a l c i t e s were s c r e e n e d f o r 2 - p r o p a n o l c o n v e r s i o n ( T a b l e I I ) . T h i s r e a c t i o n i s known t o g i v e p r o p y l e n e when the c a t a l y s t c o n t a i n s a c i d i c s i t e s (such as a l u m i n a ) and a c e t o n e when the c a t a l y s t c o n t a i n s b a s i c s i t e s (such as magnesium o x i d e ) . Table I I .

2 - P r o p a n o l C o n v e r s i o n Over V a r i o u s

C a t a l y s t s (a)

% S e l e c t i v i t v (b) Acetone Propvlene Catalvst % Conv 0 100 Alumina 100 12 87 V -Hydrotalcite 85 51 47 Mo -Hydrotalcite 35 70 25 Hydrotalcite 24 90 6 Magnesium Oxide 8 (a) Temperature = 350°C; n e a t 2 - p r o p a n o l f e e d r a t e - 1 g/min (no d i l u e n t ) ; c a t a l y s t b e d l e n g t h » 3". (b) The b a l a n c e o f the p r o d u c t s a r e h i g h e r - m o l e c u l a r - w e i g h t o l i g o m e r s formed from the a l d o l c o n d e n s a t i o n o f a c e t o n e . 1 0

7

The r e s u l t s i n T a b l e I I s u g g e s t t h a t molybdate- and v a n a d a t e p i l l a r e d h y d r o t a l c i t e s c o n t a i n b o t h a c i d i c and b a s i c s i t e s , t h e b a s i c s i t e s l o c a t e d on the m e t a l h y d r o x i d e s h e e t s , and the a c i d i c s i t e s l o c a t e d on the p o l y o x o m e t a l a t e p i l l a r s . P o l y o x o m e t a l a t e - p i l l a r e d h y d r o t a l c i t e s were a l s o s c r e e n e d f o r a c t i v i t y f o r the d e h y d r o g e n a t i o n o f t - b u t y l e t h y l b e n z e n e to t butylstyrene. T a b l e I I I shows t h a t m o l y b d a t e - p i l l a r e d h y d r o t a l c i t e s outperformed other p i l l a r e d h y d r o t a l c i t e s .



146


F i g u r e 3. Model o f d e c a v a n a d a t e - p i l l a r e d h y d r o t a l c i t e v i e w e d n o r m a l t o t h e m e t a l h y d r o x i d e l a y e r s . Atom t y p e s a r e as follows: (a) Mg o r A l ; (b) OH group i n t h e m e t a l h y d r o x i d e l a y e r ; ( c ) 0 from t h e decavanadate p i l l a r i n c o n t a c t w i t h t h e m e t a l h y d r o x i d e l a y e r ; (d) V. (Model was c o n s t r u c t e d u s i n g Chem-X, d e v e l o p e d and d i s t r i b u t e d b y C h e m i c a l D e s i g n L t d . , Oxford, England.) (Reproduced from Ref. 19. C o p y r i g h t 1988 A m e r i c a n C h e m i c a l S o c i e t y . )

-400

-600 -800 ppm

F i g u r e 4. Vanadium-51 s o l i d s t a t e MAS-NMR o f t e r e p h t h a l a t e p i l l a r e d h y d r o t a l c i t e s p a r t i a l l y exchanged w i t h d e c a v a n a d a t e : (a) 39% exch.; (b) 60% exch.; ( c ) 91% exch.



14.

Pillared

DREZDZON

Hydrotalcites

147

Figure 5. TEM m i c r o g r a p h o f a t e r e p h t h a l a t e / d e c a v a n a d a t e p i l l a r e d h y d r o t a l c i t e (39% d e c a v a n a d a t e e x c h a n g e d ) after c a l c i n a t i o n a t 500°.

Table

III.

Dehydrogenation of t-butylethylbenzene Over V a r i o u s I s o p o l y m e t a l a t e - P i l l a r e d H y d r o t a l c i t e s (a)

% Sel Temp (°C) % Conv T i m e (h) 80 54 500 8 68 57 550 12 55 38 V -Hydrotalcite 3 515 55 31 6 515 57 21 W -Hydrotalcite 6 520 83 23 9 520 ( a ) R e a c t i o n s w e r e r u n u s i n g 8% o x y g e n i n n i t r o g e n a s a d i l u e n t . R e a c t i o n c o n d i t i o n s were n o t o p t i m i z e d .

Catalyst Mo -Hydrotalcite 7

1 0

7

I t was i n i t i a l l y h o p e d t h a t r u n n i n g t h e r e a c t i o n u n d e r o x i d a t i v e dehydrogenation c o n d i t i o n s would l e a d to higher conversions. Studies w i t h m o l y b d a t e - p i l l a r e d h y d r o t a l c i t e s under more o p t i m i z e d c o n d i t i o n s showed t h a t t h e p r e s e n c e o f o x y g e n i n t h e f e e d s i m p l y k e p t t h e c a t a l y s t f r o m c o k i n g up as f a s t a s i f oxygen were n o t i n c l u d e d . U n f o r t u n a t e l y , even w i t h the presence o f oxygen i n t h e f e e d , m o l y b d a t e - p i l l a r e d h y d r o t a l c i t e s were found t o l o s e a p p r o x i m a t e l y 40% o f t h e i r a c t i v i t y a f t e r 1 0 0 - 1 5 0 h o u r s o n s t r e a m .

American Chemical Society Library 1155 16th St.,Catalysis; N.W. Baker, R., et al.; In Novel Materials in Heterogeneous ACS Symposium Series; American Chemical Washington, O.C. Society: 20036Washington, DC, 1990.

148



Acknowledgments T e c h n i c a l a s s i s t a n c e by W. J . Dangles, S. M. Oswald, and P. A. Shope has b e e n g r e a t l y a p p r e c i a t e d . The New C a t a l y t i c M a t e r i a l s Group a t Amoco i s g r a t e f u l l y acknowledged f o r u s e f u l d i s c u s s i o n s . Marc K u l l b e r g and E r i c Moore r a n t - b u t y l e t h y l b e n z e n e d e h y d r o g e n a t i o n reactions. C l a i r e Grey c o l l e c t e d and a i d e d i n t h e i n t e r p r e t a t i o n o f NMR d a t a . C o n s u l t a t i o n w i t h A. K. Cheetham and A. C l e a r f i e l d h a s a l s o been h e l p f u l d u r i n g the course o f t h i s p r o j e c t . Finally, I w i s h t o thank Amoco C h e m i c a l Company f o r p e r m i s s i o n t o p u b l i s h and p r e s e n t t h i s work.

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Novel Materials in Heterogeneous Catalysis - American Chemical

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