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Hydrogenation of Benzene Over Rhodium on Zeolites
G.M. Pajonk, S.J. Teichner and J.E. Germain (Editors), Spillouer o f Adsorbed Species 1983 Elsevier Science Publishers B.V..Amsterdam -Printed in The Netherlands
301
0
HYDROGENATION OF BENZENE OVER RHODIUM ON ZEOLITES G. d e l ANGEL, B.COQ, R. DUTARTRE, F. FAJULA, F. FIGUERAS, C . LECLERCQ'
L a b o r a t o i r e de Chimie Organique Physique e t C i n e t i q u e Chimiques Appl iquees,
ERA 954 du CNRS, ENSCM, M o n t p e l l i e r , FRANCE ' I n s t i t u t de Recherches s u r l a Catalyse du CNRS, V i l l e u r b a n n e , FRANCE
ABSTRACT Rhodium has been i n t r o d u c e d by i o n exchange o n t o Nay, NH4Y, Na-n, NH4-n, NH -mordenite, H - e r i o n i t e and H - o f f r e t i t e . Good d i s p e r s i o n s o f rhodium a r e measur e % by H a d s o r p t i o n and e l e c t i o n microscopy. The a c t i v i t i e s f o r benzene and 1-hexene h y d r o g e n a t i o n s have been determined. Rhodium s u p p o r t e d by NaY and NH4Y has t h e same i n t r i n s i c a c t i v i t y as Rh/A1203. The i n t r o d u c t i o n o f 4 % Rh i n t o m o r d e n i t e y i e l d s a c a t a l y s t which e x h i b i t s good d i s p e r s i o n s by hydrogen chemisorpt i o n i s a c t i v e f o r t h e h y d r o g e n a t i o n o f alkenes b u t n o t f o r t h e h y d r o g e n a t i o n of benzene. I t i s proposed t h a t rhodium i s l o c a t e d a t s i t e s which a r e n o t a c c e s s i b l e f o r benzene. T h e r e f o r e a d i r e c t c o o r d i n a t i o n o f t h e benzene molecule t o t h e metal i s necessary f o r t h e r e a c t i o n and s p i l l e d o v e r hydrogen cannot r e a c t w i t h t h e benzene molecules adsorbed on t h e z e o l it e . RESUME Des c a t a l y s e u r s au rhodium o n t e t @ prepares p a r @change d ' i o n s en u t i l i s a n t une s e r i e de z e o l i t h e s comme s u p p o r t . Les systemes e t u d i e s i n c l u e n t l e s z e o l i t h e s s u i v a n t e s : Nay, NHqY, Na-n, N H 4 - ~ , NH4-mordenite, H - e r i o n i t e e t H - o f f r e t i t e . Les d i s p e r s i o n s du rhodium s o n t bonnes d ' a p r e s l a c h i m i s o r p t i o n d'hydrogsne e t l a m i c r o s c o p i e G l e c t r o n i q u e . Rh/NaY e t Rh/NH Y o n t l a &me a c t i v i t e i n t r i n s e q u e que Kh/Al 0 3 . Par i n t r o d u c t i o n de 4 % Rh d h s l a m o r d e n i t e on o b t i e n t un c a t a l y s e u r p o u r ?equel 1 ' a d s o r p t i o n d'hydrogene i n d i q u e une s u r f a c e metal 1 i q u e i m p o r t a n t e , q u i hydrogene l e s a l c e n e s mais t r e s peu l e benzene. I 1 semble donc que l e metal s o i t s i t 6 dans des s i t e s i n a c c e s s i b l e s a u benzene. En consequence, l a c o o r d i n a t i o n d i r e c t e du benzene s u r l e metal e s t n e c e s s a i r e p o u r que l ' h y d r o g e n a t i o n se p r o d u i s e . L'hydrogene epandu s u r l a z e o l i t h e ne r e a g i t pas s u r l e benzene adsorbe s u r l e s s i t e s de l a z e o l i t h e . INTRODUCTION The phenomenon o f s p i l l o v e r has been demonstrated f o r many y e a r s ( r e f . 1 ) and i s p r o b a b l y i m p l i e d i n t h e r e d u c t i o n o f s u p p o r t s observed i n s t r o n g m e t a l - s u p p o r t i n t e r a c t i o n s . I t has been a l s o c l e a r l y demonstrated f o r systems s u p p o r t e d by zeolites
( r e f . 2 ) . One i m p o r t a n t p o i n t i s n o t w e l l e s t a b l i s h e d : t h e r e a c t i v i t y
towards an o r g a n i c s u b s t r a t e o f t h e d i s s o c i a t e d hydrogen s p e c i e s which has m i g r a ted
o n t o t h e s u p p o r t has n o t y e t been demonstrated. I f t h i s s p i l l e d o v e r h y d r o -
gen was c a t a l y t i c a l l y a c t i v e , many i n t e r p r e t a t i o n s o f t h e c a t a l y t i c a c t i v i t i e s of m e t a l s would become d o u b t f u l , s i n c e t h e y a r e based on t h e i d e a t h a t t h e r e a c t i o n i s r e s t r i c t e d t o t h e m e t a l . The e l u c i d a t i o n o f t h a t p o i n t i s t h e r e f o r e r a t h e r
w
0
Table 1 : Some c h a r a c t e r i s t i c p r o p e r t i e s o f R h / z e o l i t e c a t a l y s t s Catalyst ~~~
%Me ~
G A l NaY (SK-40) GA2 NaY GA3 NH4Y (SK-41) GA3A NH4Y (SK-41) GA4 NH4 Q GA4A NH4 Q GA5 NH4 mordenite G A I O NH4 mordenite GAIO-1 NH4 mordenite G A l l NH4 mordenite GA6 GA7 GA8 E05 E01
ZSM5 ( I F P ) erionite offretite
Na
Q
NH4Y
C a l c i n a t i o n ("C)
N
Reduction ("C)
%D 100 100 70 57 80 60 78 49 50 21 19 47 74
1 .o 1 .o 1 .o 1 .o 1.36 1.36 1 .o 4.0
500 500 500 500 500 500 500 500
2.0
500
300 400 300 40 0 300 40 0 300 400 400 40 0
1 .o 1.0 1 .o 1.5 1.5
500 500 500 500 500
300 40 0 40 0 40 0 400
-
-
303
important. When t h e r e a c t a n t i s an o l e f i n , t h e i n t e r p r e t a t i o n o f t h e c a t a l y t i c r e s u l t s may be obscured by t h e f a c t t h a t t h e c o r r e s p o n d i n g a l k a n e may be formed by two mechanisms : a c l a s s i c a l h y d r o g e n a t i o n p r o c e e d i n g by t h e a d d i t i o n o f H atoms t o t h e alkene and hydrogen t r a n s f e r c a t a l y z e d by a c i d i c s i t e s . The l a t t e r r e a c t i o n produces coke and i s o f t e n n o t d i s t i n g u i s h e d f r o m t h e metal c a t a l y z e d hydrogenation. W i t h aromatics, hydrogen t r a n s f e r i s i m p o s s i b l e , t h e n cyclohexane w i l l be f o r med o n l y b y h y d r o g e n a t i o n . Moreover t h i s r e a c t i o n i s 100 t o 1000 t i m e s s l o w e r t h a n t h e h y d r o g e n a t i o n o f o l e f i n s ; l i m i t a t i o n s a r i s i n g f r o m h e a t and mass t r a n s f e r a r e t h e n l e s s c r i t i c a l . However t h e p r o b l e m remains t o decide whether c y c l o hexane i s formed b y a metal c a t a l y z e d r e a c t i o n , o r by r e a c t i o n o f benzene adsorbed on a c i d i c s i t e s on m i g r a t i n g H species. I n o u r p r e v i o u s work on t h e i n f l u e n c e o f t h e s u p p o r t on t h e c a t a l y t i c p r o p e r t i e s o f p a l l a d i u m ( r e f . 3 ) we observed t h a t t h e h i g h e r a c i d i t y o f z e o l i t e s i n d u ced a h i g h e r a c t i v i t y f o r benzene h y d r o g e n a t i o n . Amorphous s i 1 ica-a1 uminas y i e l ded s i m i l a r r e s u l t s . We i n t e r p r e t a t e d o u r r e s u l t s i n terms o f some m o d i f i c a t i o n of t h e e l e c t r o n i c p r o p e r t i e s o f t h e m e t a l , i n
agreement w i t h t h e s h i f t o f t h e
v i b r a t i o n of CO adsorbed on Pd. However a d i f f e r e n t i n t e r p r e t a t i o n was used f o r P t / H Y ( r e f . 4 ) i n which t h e a u t h o r s suppose t h a t t o l u e n e may be adsorbed on t h e s u p p o r t and r e a c t w i t h s p i l l e d o v e r hydrogen. R e c e n t l y T e i c h n e r and coworkers ( r e f . 5 6 ) observed t h a t an a l u m i n a a c t i v a t e d b y s p i l l e d o v e r hydrogen has some a c t i v i t y f o r e t h y l e n e hydrog e n a t i o n . I t may be remarked t h a t t h i s a c t i v i t y i s q u i t e s m a l l compared t o t h e a c t i v i t i e s observed on m e t a l s : s e v e r a l hours a r e necessary
t o c o n v e r t C2H4 com-
p a r e d t o minutes on a metal s u r f a c e .
Ift h e r e a c t i v i t y o f t h a t hydrogen i s s i g n i f i c a n t ,
i t s h o u l d e x i s t w i t h many
metals, s i n c e most m e t a l s r e a d i l y d i s s o c i a t e d i h y d r o g e n ; i t i s t h e n e x p e c t e d t h a t a l l noble metals introduced i n t o z e o l i t e s w i l l e x h i b i t a higher a c t i v i t y t h a n alumina. S p i l l o v e r would t e n d t o decrease t h e v a r i a t i o n s o f c a t a l y t i c a c t i v i t i e s by t h e occurence o f t h e r e a c t i o n c a t a l y z e d b y t h e s u p p o r t . We have i n t e n t e d t o check t h i s h y p o t h e s i s b y s t u d y i n g rhodium c a t a l y s t s s u p p o r t e d b y s e v e r a l z e o l i t e s and we p r e s e n t h e r e some o f t h e r e s u l t s o f o u r i n v e s t i g a t i o n s . METHODS Rhodium was i n t r o d u c e d by i o n exchange u s i n g rhodium ammine
RhCl("H3)5
C1
d i s s o l v e d i n w a t e r a t pH = 7. The s o l i d was f i l t e r e d , d r i e d , c a l c i n e d i n a i r a t 500°C
t h e n reduced i n f l o w i n g hydrogen
a t 300°C o r 400OC. The z e o l i t h e s used
h e r e were:NaY and NH4Y f r o m L i n d e (SK40 and SK41) a sample
o f NH4n f u r n i s h e d b y
Union Carbide, NH4-mordenite f r o m Norton, NaK e r i o n i t e f r o m Grande P a r o i s s e and
304
o f f r e t ite. Some c h a r a c t e r i s t i c f e a t u r e s o f these c a t a l y s t s a r e summarized i n Table I . Rh/A1203 was used as a s t a n d a r d t o determine t h e a c t i v i t y o f rhodium on a c l a s s i c a l support. Rhodium d i s p e r s i o n s were measured by v o l umetry u s i n g h y d r o g e n a d s o r p t i o n a t room temperature. The r e s u l t s were checked by e l e c t r o n microscopy : carbon r e p l i ca and c u t s were examined t o i n v e s t i g a t e t h e s i z e o f p a r t i c l e s i n c l u d e d i n t h e l a t t i c e o r deposited outside the c r y s t a l . C a t a l y t i c a c t i v i t i e s were measured i n a f l o w r e a c t o r , o p e r a t e d a t d i f f e r e n t i a l conversions, a t 8OoC f o r benzene and 25°C f o r n - a l k e n e s . A n a l i q u o t o f t h e samp l e used f o r d i s p e r s i o n measurements was a c t i v a t e d under H2 a t 400"C, t h e n used f o r the c a t a l y t i c t e s t . RESULTS I n most c a s e s good d i s p e r s i o n s o f rhodium a r e o b t a i n e d . These d i s p e r s i o n s agree w i t h e l e c t r o n micrographs showing t h e presence of v e r y s m a l l p a r t i c l e s i n t h e r e d u c e d c a t a l y s t s . E r i o n i t e i s an e x c e p t i o n , poor d i s p e r s i o n a r e observed : i n t h a t case t h e p r e p a r a t i o n used i n d u c e d m i g r a t i o n o f t h e metal o u t s i d e t h e crystal o f zeolite. W i t h t h e m o r d e n i t e sample l o a d e d w i t h 4 % Rh, t h e observed d i s p e r s i o n of 50 % 0
corresponds t o a b i n o m i a l d i s t r i b u t i o n o f s i z e s : s m a l l p a r t i c u l e s , i n t h e 10 A range a r e o c c l u d e d i n t h e z e o l i t e l a t t i c e ( f i g u r e 2) and some b i g p a r t i c l e s a r e formed o u t s i d e t h e c r y s t a l s ( f i g u r e 1). The c a t a l y t i c a c t i v i t i e s r e p o r t e d i n Table 2 v a r y n o t i c e a b l y w i t h t h e s u p p o r t . The samoles s u p o o r t e d b y
Y z e o l i t e s e x h i b i t t h e same t u r n o v e r frequency as
Rh/A1203, w h i l e t h e samples s u p p o r t e d b y s m a l l e r p o r e z e o l i t e s e x h i b i t l o w a c t i v i t i e s , which may f a l l t o z e r o a t l o w l o a d i n g s .
Cole and Kouwen-
hoeven ( r e f . 7 ) r e p o r t e d a s i m i l a r b e h a v i o u r f o r t h e c o n v e r s i o n o f p a r a f f i n s on Pd/n
c a t a l y s t s : a h i g h Pd l o a d i n g was necessary t o g e t good a c t i v i t y
and i t
was h y p o t h e t i z e d t h a t P t arld Pd were l o c a t e d a t s i t e s i n a c c e s s i b l e t o r e a c t a n t molecules. F o r t h e h y d r o g e n a t i o n o f n - a l k e n e s on 4 % Rh-mordenite, t h e a c t i v i t y i s g r e a t e r ; t h i s t h e r e f o r e shows t h a t m e t a l l i c rhodium i s r e a l l y p r e s e n t i n t h e c a t a l y s t . DISCUSS ION
The comparison o f c h e m i s o r p t i o n w i t h e l e c t r o n microscopy shows t h a t these c a t a l y s t s have a n o t i c e a b l e p a r t o f rhodium i n t h e f o r m o f s m a l l m e t a l l i c p a r t i cles.
305
Fig. 1 .
E l e c t r o n micrograph o f t h e 4 % Rh-mordenite sample showing t h e b i g
p a r t i c l e s outside t h e c r y s t a l . D i r e c t examination o f t h e powdered c a t a l y s t .
Fig. 2 . E l e c t r o n micrograph o f a s l i c e o f z e o l i t e (4% Rh-mordenite), showing t h e small p a r t i c l e s o f rhodium occluded i n t h e c r y s t a l .
306
Table 2 C a t a l y t i c a c t i v i t i e s of Rhodium s u p p o r t e d c a t a l y s t s ~~
Support
%Rh
%D
Turn over frequency f o r 1 pentene Benzene 1 hexene a t 25°C a t 25°C a t 80°C
1
53
234
2.35
68
380
0.44
87
7 26
1.85
22
54 0
Na Y
1
100
200-231
NH4Y
1
70
31 2
NH4 R
1.36
80
0
1.36
60
0
1.5
74
2.2
1.0
78
3
4.0
49
60-77
A' 2'3
Na R NH4-mordenite
id
50
40
2.0
21
49
Herionite
1.0
19
1760
H offretite
1.0
47
50.5
14
33
18
41.4
389
825
307
Two p o i n t s may be p o i n t e d o u t :
1) t h e c l a s s i f i c a t i o n o f z e o l i t i c supports i n two groups i n f u n c t i o n o f p o r e d i a m e t e r s . Large p o r e Y g i v e s t u r n o v e r numbers s i m i l a r t o Rh/A1203. Small p o r e c o n t a i n i n g z e o l i t e s as m o r d e n i t e , omega and o f f r e t i t e y i e l d low a c t i v i t i e s . 11) a s p e c t a c u l a r i n c r e a s e o f t h e t u r n o v e r f r e q u e n c y o f Rh-mordenite c a t a l y s t s w i t h rhodium l o a d i n g , which i s r e m i n i s c e n t o f t h e r e s u l t s r e p o r t e d by Cole and Kouwenhoeven ( r e f . 7 ) . I n sharp c o n t r a s t w i t h t h e case o f p a l l a d i u m , t h e use o f an a c i d i c s u p p o r t does n o t induce an i n c r e a s e o f t h e a c t i v i t y f o r h y d r o g e n a t i o n . The absence o f t h e s u p p o r t e f f e c t observed on p a l l a d i u m i s a l s o evidenced by t h e s i m i l a r i t y o f t h e a c t i v i t i e s measured w i t h Rh/NaY and Rh/HY. The t h r e e z e o l i t e s which g i v e a low a c t i v i t y have some s i m i l a r i t i e s i n t h e i r 0
s t r u c t u r e : t h e y possess 6 r i n g channels, 2,3 A i n d i a m e t e r i n a d d i t i o n of t h e 12 r i n g p o r e s r e a c h i n g 6-7 located
i n d i a m e t e r . Mg2+ o c c u p i e s p r e f e r e n t i a l l y t h e s i t e s
t h e r e i n n a t u r a l o f f r e t i t e ( r e f . 9 ) and rhodium does p r o b a b l y t h e same.
The r e d u c t i o n o f t h i s m a t e r i a l t h e r e f o r e y i e l d s v e r y small m e t a l 1 i c p a r t i c l e s l o c a t e d i n s i t e s a c c e s s i b l e f o r hydrogen, as shown by c h e m i s o r p t i o n and e l e c t r o n microscopy b u t p o o r l y a c t i v e f o r h y d r o g e n a t i o n . The l o w a c t i v i t y may be e x p l a i ned e i t h e r by t h e s m a l l s i z e o f t h e m e t a l l i c p a r t i c l e s o r t h e i n a c c e s s i b i l i t y of t h e s i t e s t o t h e o r g a n i c r e a c t a n t . However, t h e f a c t t h a t some h y d r o g e n a t i o n of a l k e n e s i s observed on Rh/R samples which a r e i n a c t i v e f o r benzene s u p p o r t s t h e the
h y p o t h e s i s o f l i m i t e d a c c e s s i b i l i t y o f t h e m e t a l l i c s i t e s . By i n c r e a s i n g
rhodium c o n t e n t t h e metal o c c u p i e s s i t e s i n t h e l a r g e pores which a r e a c c e s s i b l e t o t h e r e a c t a n t s , and t h e s p e c i f i c a c t i v i t y i n c r e a s e s . I n c o n c l u s i o n , rhodium i n t r o d u c e d i n m o r d e n i t e ,
o r o f f r e t i t e has a m e t a l l i c
c h a r a c t e r a t t e s t e d by hydrogen a d s o r p t i o n and a1 kene h y d r o g e n a t i o n b u t shows a l o w r e a c t i v i t y towards benzene. The hydrogen atoms adsorbed on t h e m e t a l surface have t h e p o s s i b i l i t y
o f m i g r a t i o n o n t o t h e z e o l i t i c l a t t i c e , because t h e pheno-
menon of s p i l l o v e r has been evidenced w i t h many m e t a l s as N i ( r e f . 6 ) ,
P t (ref.2)
and many s u p p o r t s . However t h e s e hydrogen atoms do n o t possess any r e a c t i v t y towards benzene. The u s e i n c a t a l y s i s o f t h i s s p i l l e d o v e r hydrogen i s t h e n h i g h l y q u e s t i o n a b l e . The p r e s e n t r e s u l t s g e t a l s o a new i n s i g h t on a second h y p o t h e s i s , which has been proposed t o e x p l a i n e d t h e h i g h e r a c t i v i t y o f P t and Pd i n z e o l i t e s . Some a u t h o r s ( r e f . 1 0 ) suggest t h a t t h e r e a c t a n t c o u l d b e d e s t a b i l i z e d by t h e e f f e c t of t h e h i g h e l e c t o s t a t i c f i e l d e x i s t i n g i n t h e l a t t i c e , and t h u s r e a c t f a s t e r .
The
comparison o f Rh/HY, Rh/NaY and Rh/A1203 shows t h a t t h i s e f f e c t does n o t o c c u r i n t h e case of rhodium s u p p o r t e d by t h e z e o l i t e s . T h i s r e s u l t i s a l s o c o n s i s t e n t w i t h t h e p r e v i o u s o b s e r v a t i o n ( r e f , 3) t h a t amorphous s i 1ica-a1 umina and z e o l it e s g i v e comparable a c t i v i t i e s when t h e i n f l u e n c e o f such an e l e c t r o s t a t i c f i e l d would have g i v e n d i f f e r e n t r e s u l t s .
308
I n conclusion, the hydrogenation o f benzene i s thought t o proceed by a c l a s s i c a l mechanism, i m p l y i n g t h a t benzene must be adsorbed a t the metal s u r f a c e t o be hydrogenated. No i n d i c a t i o n has been obtained t h a t hydrogen a c t i v a t e d on t h e rhodium s i t e s non a c c e s s i b l e t o benzene may hydrogenate the r e a c t a n t .
REFERENCES
1 For a review : P.A. Sermon and G.C. Bond, C a t a l . Rev.; 5 (1973), 211. 2 W.C. Neikam and M.A. Vannice, Proc. 5 t h . I n t e r n . Conqr. a t a l y s i s , P a l m Beach, 1972, North H o l l a n d (Amsterdam), 1972, paper 41. M.A. Vannice and W.C. Neikam, J. C a t a l . , 20, (1971), 250. 3 F. Figueras, R. Gomez and M. Primet, Adv.Chem. Ser.,l21, (1973), 480. 4 J. Bandiera, C. Naccache and B. I m e l i k , J. Chim. Phys., 75, (1978), 406. 5 G.E.E. Gardes, G.M. Pajonk and S . J . Teichner, J . C a t a l ., 33, (1974), 145. 6 D. Bianchi, G.E.E. Gardes, G.M. Pajonk and S.J. Teichner, J. Catal., 38, (1975), 135. 7 J.F. Cole and H.W. Kouwenhoeven, Adv. Chem. Ser., 121, (1973), 583. 8 D.W. Breck, " Z e o l i t e Molecular Sieves", Wiley, New York 1974. 9 J.V. Smith, i n " Z e o l i t e Chemistry and Chemistry", ACS monograph 171, J.A. Raboed, Washington 1976. 10 C . Naccache, M. Kaufherr, M. Dufaux, J . Bandine and B. I m e l i k , Molecular Sieves 11, ACS Symposium Series no 40, (1377), 538.
301
0
HYDROGENATION OF BENZENE OVER RHODIUM ON ZEOLITES G. d e l ANGEL, B.COQ, R. DUTARTRE, F. FAJULA, F. FIGUERAS, C . LECLERCQ'
L a b o r a t o i r e de Chimie Organique Physique e t C i n e t i q u e Chimiques Appl iquees,
ERA 954 du CNRS, ENSCM, M o n t p e l l i e r , FRANCE ' I n s t i t u t de Recherches s u r l a Catalyse du CNRS, V i l l e u r b a n n e , FRANCE
ABSTRACT Rhodium has been i n t r o d u c e d by i o n exchange o n t o Nay, NH4Y, Na-n, NH4-n, NH -mordenite, H - e r i o n i t e and H - o f f r e t i t e . Good d i s p e r s i o n s o f rhodium a r e measur e % by H a d s o r p t i o n and e l e c t i o n microscopy. The a c t i v i t i e s f o r benzene and 1-hexene h y d r o g e n a t i o n s have been determined. Rhodium s u p p o r t e d by NaY and NH4Y has t h e same i n t r i n s i c a c t i v i t y as Rh/A1203. The i n t r o d u c t i o n o f 4 % Rh i n t o m o r d e n i t e y i e l d s a c a t a l y s t which e x h i b i t s good d i s p e r s i o n s by hydrogen chemisorpt i o n i s a c t i v e f o r t h e h y d r o g e n a t i o n o f alkenes b u t n o t f o r t h e h y d r o g e n a t i o n of benzene. I t i s proposed t h a t rhodium i s l o c a t e d a t s i t e s which a r e n o t a c c e s s i b l e f o r benzene. T h e r e f o r e a d i r e c t c o o r d i n a t i o n o f t h e benzene molecule t o t h e metal i s necessary f o r t h e r e a c t i o n and s p i l l e d o v e r hydrogen cannot r e a c t w i t h t h e benzene molecules adsorbed on t h e z e o l it e . RESUME Des c a t a l y s e u r s au rhodium o n t e t @ prepares p a r @change d ' i o n s en u t i l i s a n t une s e r i e de z e o l i t h e s comme s u p p o r t . Les systemes e t u d i e s i n c l u e n t l e s z e o l i t h e s s u i v a n t e s : Nay, NHqY, Na-n, N H 4 - ~ , NH4-mordenite, H - e r i o n i t e e t H - o f f r e t i t e . Les d i s p e r s i o n s du rhodium s o n t bonnes d ' a p r e s l a c h i m i s o r p t i o n d'hydrogsne e t l a m i c r o s c o p i e G l e c t r o n i q u e . Rh/NaY e t Rh/NH Y o n t l a &me a c t i v i t e i n t r i n s e q u e que Kh/Al 0 3 . Par i n t r o d u c t i o n de 4 % Rh d h s l a m o r d e n i t e on o b t i e n t un c a t a l y s e u r p o u r ?equel 1 ' a d s o r p t i o n d'hydrogene i n d i q u e une s u r f a c e metal 1 i q u e i m p o r t a n t e , q u i hydrogene l e s a l c e n e s mais t r e s peu l e benzene. I 1 semble donc que l e metal s o i t s i t 6 dans des s i t e s i n a c c e s s i b l e s a u benzene. En consequence, l a c o o r d i n a t i o n d i r e c t e du benzene s u r l e metal e s t n e c e s s a i r e p o u r que l ' h y d r o g e n a t i o n se p r o d u i s e . L'hydrogene epandu s u r l a z e o l i t h e ne r e a g i t pas s u r l e benzene adsorbe s u r l e s s i t e s de l a z e o l i t h e . INTRODUCTION The phenomenon o f s p i l l o v e r has been demonstrated f o r many y e a r s ( r e f . 1 ) and i s p r o b a b l y i m p l i e d i n t h e r e d u c t i o n o f s u p p o r t s observed i n s t r o n g m e t a l - s u p p o r t i n t e r a c t i o n s . I t has been a l s o c l e a r l y demonstrated f o r systems s u p p o r t e d by zeolites
( r e f . 2 ) . One i m p o r t a n t p o i n t i s n o t w e l l e s t a b l i s h e d : t h e r e a c t i v i t y
towards an o r g a n i c s u b s t r a t e o f t h e d i s s o c i a t e d hydrogen s p e c i e s which has m i g r a ted
o n t o t h e s u p p o r t has n o t y e t been demonstrated. I f t h i s s p i l l e d o v e r h y d r o -
gen was c a t a l y t i c a l l y a c t i v e , many i n t e r p r e t a t i o n s o f t h e c a t a l y t i c a c t i v i t i e s of m e t a l s would become d o u b t f u l , s i n c e t h e y a r e based on t h e i d e a t h a t t h e r e a c t i o n i s r e s t r i c t e d t o t h e m e t a l . The e l u c i d a t i o n o f t h a t p o i n t i s t h e r e f o r e r a t h e r
w
0
Table 1 : Some c h a r a c t e r i s t i c p r o p e r t i e s o f R h / z e o l i t e c a t a l y s t s Catalyst ~~~
%Me ~
G A l NaY (SK-40) GA2 NaY GA3 NH4Y (SK-41) GA3A NH4Y (SK-41) GA4 NH4 Q GA4A NH4 Q GA5 NH4 mordenite G A I O NH4 mordenite GAIO-1 NH4 mordenite G A l l NH4 mordenite GA6 GA7 GA8 E05 E01
ZSM5 ( I F P ) erionite offretite
Na
Q
NH4Y
C a l c i n a t i o n ("C)
N
Reduction ("C)
%D 100 100 70 57 80 60 78 49 50 21 19 47 74
1 .o 1 .o 1 .o 1 .o 1.36 1.36 1 .o 4.0
500 500 500 500 500 500 500 500
2.0
500
300 400 300 40 0 300 40 0 300 400 400 40 0
1 .o 1.0 1 .o 1.5 1.5
500 500 500 500 500
300 40 0 40 0 40 0 400
-
-
303
important. When t h e r e a c t a n t i s an o l e f i n , t h e i n t e r p r e t a t i o n o f t h e c a t a l y t i c r e s u l t s may be obscured by t h e f a c t t h a t t h e c o r r e s p o n d i n g a l k a n e may be formed by two mechanisms : a c l a s s i c a l h y d r o g e n a t i o n p r o c e e d i n g by t h e a d d i t i o n o f H atoms t o t h e alkene and hydrogen t r a n s f e r c a t a l y z e d by a c i d i c s i t e s . The l a t t e r r e a c t i o n produces coke and i s o f t e n n o t d i s t i n g u i s h e d f r o m t h e metal c a t a l y z e d hydrogenation. W i t h aromatics, hydrogen t r a n s f e r i s i m p o s s i b l e , t h e n cyclohexane w i l l be f o r med o n l y b y h y d r o g e n a t i o n . Moreover t h i s r e a c t i o n i s 100 t o 1000 t i m e s s l o w e r t h a n t h e h y d r o g e n a t i o n o f o l e f i n s ; l i m i t a t i o n s a r i s i n g f r o m h e a t and mass t r a n s f e r a r e t h e n l e s s c r i t i c a l . However t h e p r o b l e m remains t o decide whether c y c l o hexane i s formed b y a metal c a t a l y z e d r e a c t i o n , o r by r e a c t i o n o f benzene adsorbed on a c i d i c s i t e s on m i g r a t i n g H species. I n o u r p r e v i o u s work on t h e i n f l u e n c e o f t h e s u p p o r t on t h e c a t a l y t i c p r o p e r t i e s o f p a l l a d i u m ( r e f . 3 ) we observed t h a t t h e h i g h e r a c i d i t y o f z e o l i t e s i n d u ced a h i g h e r a c t i v i t y f o r benzene h y d r o g e n a t i o n . Amorphous s i 1 ica-a1 uminas y i e l ded s i m i l a r r e s u l t s . We i n t e r p r e t a t e d o u r r e s u l t s i n terms o f some m o d i f i c a t i o n of t h e e l e c t r o n i c p r o p e r t i e s o f t h e m e t a l , i n
agreement w i t h t h e s h i f t o f t h e
v i b r a t i o n of CO adsorbed on Pd. However a d i f f e r e n t i n t e r p r e t a t i o n was used f o r P t / H Y ( r e f . 4 ) i n which t h e a u t h o r s suppose t h a t t o l u e n e may be adsorbed on t h e s u p p o r t and r e a c t w i t h s p i l l e d o v e r hydrogen. R e c e n t l y T e i c h n e r and coworkers ( r e f . 5 6 ) observed t h a t an a l u m i n a a c t i v a t e d b y s p i l l e d o v e r hydrogen has some a c t i v i t y f o r e t h y l e n e hydrog e n a t i o n . I t may be remarked t h a t t h i s a c t i v i t y i s q u i t e s m a l l compared t o t h e a c t i v i t i e s observed on m e t a l s : s e v e r a l hours a r e necessary
t o c o n v e r t C2H4 com-
p a r e d t o minutes on a metal s u r f a c e .
Ift h e r e a c t i v i t y o f t h a t hydrogen i s s i g n i f i c a n t ,
i t s h o u l d e x i s t w i t h many
metals, s i n c e most m e t a l s r e a d i l y d i s s o c i a t e d i h y d r o g e n ; i t i s t h e n e x p e c t e d t h a t a l l noble metals introduced i n t o z e o l i t e s w i l l e x h i b i t a higher a c t i v i t y t h a n alumina. S p i l l o v e r would t e n d t o decrease t h e v a r i a t i o n s o f c a t a l y t i c a c t i v i t i e s by t h e occurence o f t h e r e a c t i o n c a t a l y z e d b y t h e s u p p o r t . We have i n t e n t e d t o check t h i s h y p o t h e s i s b y s t u d y i n g rhodium c a t a l y s t s s u p p o r t e d b y s e v e r a l z e o l i t e s and we p r e s e n t h e r e some o f t h e r e s u l t s o f o u r i n v e s t i g a t i o n s . METHODS Rhodium was i n t r o d u c e d by i o n exchange u s i n g rhodium ammine
RhCl("H3)5
C1
d i s s o l v e d i n w a t e r a t pH = 7. The s o l i d was f i l t e r e d , d r i e d , c a l c i n e d i n a i r a t 500°C
t h e n reduced i n f l o w i n g hydrogen
a t 300°C o r 400OC. The z e o l i t h e s used
h e r e were:NaY and NH4Y f r o m L i n d e (SK40 and SK41) a sample
o f NH4n f u r n i s h e d b y
Union Carbide, NH4-mordenite f r o m Norton, NaK e r i o n i t e f r o m Grande P a r o i s s e and
304
o f f r e t ite. Some c h a r a c t e r i s t i c f e a t u r e s o f these c a t a l y s t s a r e summarized i n Table I . Rh/A1203 was used as a s t a n d a r d t o determine t h e a c t i v i t y o f rhodium on a c l a s s i c a l support. Rhodium d i s p e r s i o n s were measured by v o l umetry u s i n g h y d r o g e n a d s o r p t i o n a t room temperature. The r e s u l t s were checked by e l e c t r o n microscopy : carbon r e p l i ca and c u t s were examined t o i n v e s t i g a t e t h e s i z e o f p a r t i c l e s i n c l u d e d i n t h e l a t t i c e o r deposited outside the c r y s t a l . C a t a l y t i c a c t i v i t i e s were measured i n a f l o w r e a c t o r , o p e r a t e d a t d i f f e r e n t i a l conversions, a t 8OoC f o r benzene and 25°C f o r n - a l k e n e s . A n a l i q u o t o f t h e samp l e used f o r d i s p e r s i o n measurements was a c t i v a t e d under H2 a t 400"C, t h e n used f o r the c a t a l y t i c t e s t . RESULTS I n most c a s e s good d i s p e r s i o n s o f rhodium a r e o b t a i n e d . These d i s p e r s i o n s agree w i t h e l e c t r o n micrographs showing t h e presence of v e r y s m a l l p a r t i c l e s i n t h e r e d u c e d c a t a l y s t s . E r i o n i t e i s an e x c e p t i o n , poor d i s p e r s i o n a r e observed : i n t h a t case t h e p r e p a r a t i o n used i n d u c e d m i g r a t i o n o f t h e metal o u t s i d e t h e crystal o f zeolite. W i t h t h e m o r d e n i t e sample l o a d e d w i t h 4 % Rh, t h e observed d i s p e r s i o n of 50 % 0
corresponds t o a b i n o m i a l d i s t r i b u t i o n o f s i z e s : s m a l l p a r t i c u l e s , i n t h e 10 A range a r e o c c l u d e d i n t h e z e o l i t e l a t t i c e ( f i g u r e 2) and some b i g p a r t i c l e s a r e formed o u t s i d e t h e c r y s t a l s ( f i g u r e 1). The c a t a l y t i c a c t i v i t i e s r e p o r t e d i n Table 2 v a r y n o t i c e a b l y w i t h t h e s u p p o r t . The samoles s u p o o r t e d b y
Y z e o l i t e s e x h i b i t t h e same t u r n o v e r frequency as
Rh/A1203, w h i l e t h e samples s u p p o r t e d b y s m a l l e r p o r e z e o l i t e s e x h i b i t l o w a c t i v i t i e s , which may f a l l t o z e r o a t l o w l o a d i n g s .
Cole and Kouwen-
hoeven ( r e f . 7 ) r e p o r t e d a s i m i l a r b e h a v i o u r f o r t h e c o n v e r s i o n o f p a r a f f i n s on Pd/n
c a t a l y s t s : a h i g h Pd l o a d i n g was necessary t o g e t good a c t i v i t y
and i t
was h y p o t h e t i z e d t h a t P t arld Pd were l o c a t e d a t s i t e s i n a c c e s s i b l e t o r e a c t a n t molecules. F o r t h e h y d r o g e n a t i o n o f n - a l k e n e s on 4 % Rh-mordenite, t h e a c t i v i t y i s g r e a t e r ; t h i s t h e r e f o r e shows t h a t m e t a l l i c rhodium i s r e a l l y p r e s e n t i n t h e c a t a l y s t . DISCUSS ION
The comparison o f c h e m i s o r p t i o n w i t h e l e c t r o n microscopy shows t h a t these c a t a l y s t s have a n o t i c e a b l e p a r t o f rhodium i n t h e f o r m o f s m a l l m e t a l l i c p a r t i cles.
305
Fig. 1 .
E l e c t r o n micrograph o f t h e 4 % Rh-mordenite sample showing t h e b i g
p a r t i c l e s outside t h e c r y s t a l . D i r e c t examination o f t h e powdered c a t a l y s t .
Fig. 2 . E l e c t r o n micrograph o f a s l i c e o f z e o l i t e (4% Rh-mordenite), showing t h e small p a r t i c l e s o f rhodium occluded i n t h e c r y s t a l .
306
Table 2 C a t a l y t i c a c t i v i t i e s of Rhodium s u p p o r t e d c a t a l y s t s ~~
Support
%Rh
%D
Turn over frequency f o r 1 pentene Benzene 1 hexene a t 25°C a t 25°C a t 80°C
1
53
234
2.35
68
380
0.44
87
7 26
1.85
22
54 0
Na Y
1
100
200-231
NH4Y
1
70
31 2
NH4 R
1.36
80
0
1.36
60
0
1.5
74
2.2
1.0
78
3
4.0
49
60-77
A' 2'3
Na R NH4-mordenite
id
50
40
2.0
21
49
Herionite
1.0
19
1760
H offretite
1.0
47
50.5
14
33
18
41.4
389
825
307
Two p o i n t s may be p o i n t e d o u t :
1) t h e c l a s s i f i c a t i o n o f z e o l i t i c supports i n two groups i n f u n c t i o n o f p o r e d i a m e t e r s . Large p o r e Y g i v e s t u r n o v e r numbers s i m i l a r t o Rh/A1203. Small p o r e c o n t a i n i n g z e o l i t e s as m o r d e n i t e , omega and o f f r e t i t e y i e l d low a c t i v i t i e s . 11) a s p e c t a c u l a r i n c r e a s e o f t h e t u r n o v e r f r e q u e n c y o f Rh-mordenite c a t a l y s t s w i t h rhodium l o a d i n g , which i s r e m i n i s c e n t o f t h e r e s u l t s r e p o r t e d by Cole and Kouwenhoeven ( r e f . 7 ) . I n sharp c o n t r a s t w i t h t h e case o f p a l l a d i u m , t h e use o f an a c i d i c s u p p o r t does n o t induce an i n c r e a s e o f t h e a c t i v i t y f o r h y d r o g e n a t i o n . The absence o f t h e s u p p o r t e f f e c t observed on p a l l a d i u m i s a l s o evidenced by t h e s i m i l a r i t y o f t h e a c t i v i t i e s measured w i t h Rh/NaY and Rh/HY. The t h r e e z e o l i t e s which g i v e a low a c t i v i t y have some s i m i l a r i t i e s i n t h e i r 0
s t r u c t u r e : t h e y possess 6 r i n g channels, 2,3 A i n d i a m e t e r i n a d d i t i o n of t h e 12 r i n g p o r e s r e a c h i n g 6-7 located
i n d i a m e t e r . Mg2+ o c c u p i e s p r e f e r e n t i a l l y t h e s i t e s
t h e r e i n n a t u r a l o f f r e t i t e ( r e f . 9 ) and rhodium does p r o b a b l y t h e same.
The r e d u c t i o n o f t h i s m a t e r i a l t h e r e f o r e y i e l d s v e r y small m e t a l 1 i c p a r t i c l e s l o c a t e d i n s i t e s a c c e s s i b l e f o r hydrogen, as shown by c h e m i s o r p t i o n and e l e c t r o n microscopy b u t p o o r l y a c t i v e f o r h y d r o g e n a t i o n . The l o w a c t i v i t y may be e x p l a i ned e i t h e r by t h e s m a l l s i z e o f t h e m e t a l l i c p a r t i c l e s o r t h e i n a c c e s s i b i l i t y of t h e s i t e s t o t h e o r g a n i c r e a c t a n t . However, t h e f a c t t h a t some h y d r o g e n a t i o n of a l k e n e s i s observed on Rh/R samples which a r e i n a c t i v e f o r benzene s u p p o r t s t h e the
h y p o t h e s i s o f l i m i t e d a c c e s s i b i l i t y o f t h e m e t a l l i c s i t e s . By i n c r e a s i n g
rhodium c o n t e n t t h e metal o c c u p i e s s i t e s i n t h e l a r g e pores which a r e a c c e s s i b l e t o t h e r e a c t a n t s , and t h e s p e c i f i c a c t i v i t y i n c r e a s e s . I n c o n c l u s i o n , rhodium i n t r o d u c e d i n m o r d e n i t e ,
o r o f f r e t i t e has a m e t a l l i c
c h a r a c t e r a t t e s t e d by hydrogen a d s o r p t i o n and a1 kene h y d r o g e n a t i o n b u t shows a l o w r e a c t i v i t y towards benzene. The hydrogen atoms adsorbed on t h e m e t a l surface have t h e p o s s i b i l i t y
o f m i g r a t i o n o n t o t h e z e o l i t i c l a t t i c e , because t h e pheno-
menon of s p i l l o v e r has been evidenced w i t h many m e t a l s as N i ( r e f . 6 ) ,
P t (ref.2)
and many s u p p o r t s . However t h e s e hydrogen atoms do n o t possess any r e a c t i v t y towards benzene. The u s e i n c a t a l y s i s o f t h i s s p i l l e d o v e r hydrogen i s t h e n h i g h l y q u e s t i o n a b l e . The p r e s e n t r e s u l t s g e t a l s o a new i n s i g h t on a second h y p o t h e s i s , which has been proposed t o e x p l a i n e d t h e h i g h e r a c t i v i t y o f P t and Pd i n z e o l i t e s . Some a u t h o r s ( r e f . 1 0 ) suggest t h a t t h e r e a c t a n t c o u l d b e d e s t a b i l i z e d by t h e e f f e c t of t h e h i g h e l e c t o s t a t i c f i e l d e x i s t i n g i n t h e l a t t i c e , and t h u s r e a c t f a s t e r .
The
comparison o f Rh/HY, Rh/NaY and Rh/A1203 shows t h a t t h i s e f f e c t does n o t o c c u r i n t h e case of rhodium s u p p o r t e d by t h e z e o l i t e s . T h i s r e s u l t i s a l s o c o n s i s t e n t w i t h t h e p r e v i o u s o b s e r v a t i o n ( r e f , 3) t h a t amorphous s i 1ica-a1 umina and z e o l it e s g i v e comparable a c t i v i t i e s when t h e i n f l u e n c e o f such an e l e c t r o s t a t i c f i e l d would have g i v e n d i f f e r e n t r e s u l t s .
308
I n conclusion, the hydrogenation o f benzene i s thought t o proceed by a c l a s s i c a l mechanism, i m p l y i n g t h a t benzene must be adsorbed a t the metal s u r f a c e t o be hydrogenated. No i n d i c a t i o n has been obtained t h a t hydrogen a c t i v a t e d on t h e rhodium s i t e s non a c c e s s i b l e t o benzene may hydrogenate the r e a c t a n t .
REFERENCES
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