- Email: [email protected]
The Activator System of Oxidic Hydrodesulfurization Catalysts: Preparational Aspects
Preparation of Catalysts, edited by B. Delmon, P.A. Jacobs and G . Poncelet o 1976, Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherland THE ACTIVATOR SYSTFM OF OXIDIC HYDRODESULFURIZATION CATALYSTS: PREPARATIONAL ASPECTS
R. M O k Akzo Chemie Nederland bv, Research C e n t r e Amsterdam, The N e t h e r l a n d s .
SUMMARY The s t r u c t u r e of t h e c o b a l t and n i c k e l i o n s i n o x i d i c h y d r o d e s u l f u r i z a t i o n c a t a l y s t s h a s been s t u d i e d . I t i s shown t h a t t h e c a l c i n a t i o n temper a t u r e s t r o n g l y d e t e r m i n e s t h e p o s i t i o n of t h e promotor i o n s . C o b a l t and n i c k e l a r e s t i l l o u t s i d e t h e alumina l a t t i c e a f t e r c a l c i n a t i o n up t o 500°C, because t h e VIS r e f l e c t a n c e s p e c t r a of CoAl204 r e s p . NiA1204 a r e n o t o b s e r ved and t h e B M n s t e d a c i d s i t e s , a s p r e s e n t i n MoO3-alumina a r e s t i l l n e u t r a l i z e d . A new Lewis a c i d s i t e i s i n t r o d u c e d by t h e i n t e r a c t i o n between t h e promotor i o n s arid MOO -alumina, a s i n d i c a t e d by t h e I R s p e c t r a of a d -
3
s o r b e d p y r i d i n e . The p r e s e n c e of t h i s band is c o r r e l a t e d w i t h t h e c a t a l y t i c a c t i v i t y . H e a t i n g t o 650-700% c a u s e s m i g r a t i o n of t h e promotor i o n s i n t o t h e u p p e r l a y e r s of t h e alumina. The new Lewis a c i d band remains p r e s e n t
for t,he c o b a l t promoted c a t a l y s t , w h i l e t h e i n t e n s i t y d e c r e a s e s f o r t h e rlick-i promoted c a t a l y s t . S u l f i d e d and r e g e n e r a t e d c a t a l y s t s have a l s o h e e n s t u d i e d i n o r d e r t o get, i n f o r m a t i o n a b o u t t h e p o s i t i o n of t h e promo-
*or' ions i n the s u l f i d e d c a t a l y s t . ? h e voLlicls which have beer] p r e s e n t e d r e c e n t l y f o r t h e a c t i v a t o r system
of h ~ ~ r o d e s u l f u r i z a t , i o rci a t a l y s t s a r e e v a l u a t e d , IRTHODUCTI ON
'The c a t a l y t i c ~ i y d r . o t 1 e s u l f u r i z a t i o np r o c e s s d a t e s from a b o u t 1940 [l] I n i t , i a l l y a " c o b a l t moiybdate" c a t a l y s t was d e v e l o p e d , c o n s i s t i n g of c o b a l t and molybdcrium oxid? on an aliimina s u p p o r t , w i t h a s t o i c h i o m e t r i c r'at,io of Co:Mo
=
1 : i . L a t e r on more o p t i m i z e d c a t a l y s t s have been i n t r o -
duced w i t h 2.owpr ~'o:Mo r a t i o s . A t r i a l and error approach p l a y s an import.ant r o l
P
i n d e v e l o p i n g i n r p r o v ~ dz a t a l y s h . But a l s o an u n d e r s t a n d i n g of
t,he c a t . n l y s t s t , r u c t u r c niight bc h e l p f u l . 'This F s p e r d e a l s w i t h r e s u l t s
o b t . a i n d by t h e l a s t a p p r o a c h . v i z t h e c h a r a c t e r i z a t i o n of t h e c a t a l y s t s t r u c t u r e , espp:ially
with w g a r d
t o t h e l o c a t i o n of t h e promotor i o n s .
.
382 MPF,lUM13NrYAL PART The c a t a l y s t s were made by a d o u b l e impregnation of a f- alumina c a r r i e r , SA = 253 m2/g.
F i r s t t h e impregnation w i t h ammonium molybdate was
c a r r i e d o u t , a f t e r w a r d s t h e promotor was d e p o s i t e d . Each i m p r e g n a t i o n was f o l l o w e d by d r y i n g o v e r n i g h t and c a l c i n a t i o n a t
650Oc a f t e r t h e impregna-
t i o n w i t h molybdate a i d t e m p e r a t u r e s v a r y i n g between 400 and 700°C a f t e r t h e impregnation w i t h c o b a l t o r n i c k e l . Composition of t h e samples: MoCo124, 12 w t
% Moo3
and
MoNi-153, 15 w t $ MOO
4wt %
3
COO; MoCo-153, 15 w t
%
Moo3 and
3 w t % COO;
and 3 w t $ N i O .
The r e f l e c t a n c e s p e c t r a were recorded w i t h an O p t i c a Milano CF
4
s p e c t r o p h o t o m e t e r , u s i n g MgO a s a r e f e r e n c e . The I R s p e c t r a were r e c o r d e d w i t h a Perkiri Elmer 621. The s e l f s u p p o r t i n g d i s c method was a p p l i e d . The d i s c s were r e h y d r o x y l a t e d f i r s t on s t a n d i n g i n humid a i r d u r i n g two
. Outgassing took place
d a y s , b e f o r e t h e y were p l a c e d i n t h e I R c e l l [ 2 ]
a t 42OoC, f i n a l p r e s s u r e b e t t e r t h a n 10-5 t o r r . P y r i d i n e was a d s o r b e d a t room t e m p e r a t u r e and d e s o r b e d a t 150 and 25OoC. The s u l f i d i n g of t h e c a t a l y s t s h a s been c a r r i e d o u t i n two d i f f e r e n t ways; i n t h e I R c e l l and i n a t e s t r e a c t o r . For t h e i n s i t u s u l f i d i n g i n t h e I R c e l l , a H2/112S m i x t u r e (20 v o l $ H2S) was l e d o v e r t h e c a t a l y s t a t 200°C d u r i n g 1
.5 h r
and a t 320°C d u r i n g 1
.5 h r
s u c c e s s i v e l y . Then t h e sam-
p l e was cooled down t o room t e m p e r a t u r e and s t r i p p e d w i t h Hg. The c e l l was a t t a c h e d t o t h e h i g h vacuum system a n d evacuated a t 35OoC d u r i n g two
hours. A f t e r t h i s p r e t r e a t m e n t t h e p y r i d i n e a d s o r p t i o n experiments were carried out. The s u l f i d i n g i n t h e t e s t r e a c t o r took p l a c e w i t h a m i x t u r e of 112 and t h e f e e d s t o c k (vacuum g a s o i l ) . 3 w t
%
b u t a n e t h i o l was added t o t h e f e e d -
s t o c k . The s u l f i d i n g was c a r r i e d o u t i n two s t e p s , f i r s t a t 204OC d u r i n g
3 h r , a f t e r w a r d s a t 3 1 6 O c d u r i n g 16 h r . The t o t a l p r e s s u r e was 21 atm. The a c t i v i t y of t h e s u l f i d e d samples was d e t e r m i n e d a t t e s t run, t h e samples were s t r i p p e d w i t h N2,
365%.
A t t h e end of t h e
c o o l e d down t o room tempera-
t u r e and t a k e n o u t of t h e r e a c t o r . Then t h e c a t a l y s t s were p l a c e d i n a miuffle f u r n a c e and h e a t e d i n t h e a i r up t o 45OoC.
'The c a t a l y s t s were brought
back i n t h e o x i d i c form by t h i s h e a t t r e a t m e n t .
MoO?-alumiria.
The f i r s t s t e p i n o u r c a t a l y s t p r e p a r a t i o n p r o c e d u r e i s
t h e impregnation of the alumina c a r r i e r w i t h molybdate. I R s p e c t r a of py-
383 and on MoO3-alumina a r e shown i n f i g 1 .
r i d i n e adsorbed on $-alumina
8-alumina is c h a r a c t e r i z e d by two Lewis bands, which correspond w i t h s t r o n g (1622 cm-’) and weak (1614 cm-’) Lewis a c i d s i t e s . The bonding between p y r i d i n e and t h e weak Lewis a c i d s i t e s i s a l r e a d y broken a t a d e s o r p t i o n t e m p e r a t u r e of 250’12.
Impregnation o f t h e alumina c a r r i e r w i t h
c o b a l t o r n i c k e l does n o t change the a c i d i t y of t h e c a r r i e r as h a s been shown b e f o r e r2.31.
F o r Mo03-alumina only one Lewis band (1622 cm-l ) is
observed. Also B m n s t e d a c i d s i t e s are p r e s e n t , c h a r a c t e r i z e d by the
1636
and 1 3 0 cm-l bands.
1700
1600
1500
1400 1700
1600
1500 1400 wwenumberkm-1)
F i g . 1 . S p e c t r a of p y r i d i n e adsorbed on 2-alumina ( a ) and on Moo3-alumina ( b )
.
Cobalt Impregnation.
The second s t e p i n t h e c a t a l y s t p r e p a r a t i o n i s
t h e impregnation w i t h t h e promotor i o n s . The c o b a l t i o n s a r e d e p o s i t e d on t h e c a t a l y s t s u r f a c e and move i n t o t h e alumina on c a l c i n a t i o n . The change i n t h e c o b a l t c o o r d i n a t i o r i can be followed w i t h r e f l e c t a n c e s p e c t r o s c o p y . F i g 2 shows t h e r e f l e c t a n c e spectrum of t h e c o b a l t impregnated c a t a l y s t a f t e r d r y i n g a t l2OoC. The bands a t 540 and 630 nm a r e c h a r a c t e r i s t i c for o c t a h e d r a l l y c o o r d i n a t e d c o b a l t i o n s . C o b a l t is s t i l l p r e s e n t on t h e c a t a l y s t s u r f a c e . A l s o t h e spectrum of
65ooc is
4wt %
COO o n x - a l u m i n a , c a l c i n e d a t
g i v e n . The t r i p l e t band is c h a r a c t e r i s t i c f o r t e t r a h e d r a l l y coor-
d i n a t e d c o b a l t (CoAl2O4) i n d i c a t i n g t h a t t h e c o b a l t i o n s have moved i n t o t h e alumina l a t t i c e . F i g 3 shows t h e r e f l e c t a n c e s p e c t r a of MoCo-124 c a t a l y s t s , c a l c i n e d a t d i f f e r e n t t e m p e r a t u r e s . I t a p p e a r s t h a t t h e i n t e n s i t y of t h e t r i p l e t
384
fig 2
fig 3
OI
F i g . ? . R e f l e c t a n c e s p e c t r a . a ) MOO -alumina. impregnated w i t h C O ( N O ~ ) ~ , 3 d r i e d a t 120°C, b ) 4 w t $ Coo-alumina c a l c i n e d 1 h r 6 5 0 O ~ . F i g . 3 . R e f l e c t a n c e s p e c t r a . MoCo-124 c a l c i n e d a t d i f f e r e n t t e m p e r a t u r e s . barid s t r o n g l y i n c r e a s e s on c a l c i n a t i o n above 60OOC. Below t h i s t e m p e r a t u r e
a broad a b s o r p t i o n band i s observed, c o v e r i n g t h e whole s p e c t r a l r a n g e with a weak s u p e r p o s i t i o n of a t r i p l e t band. T h i s i n d i c a t e s t h a t below
6OO0C t h e c o b a l t i o n s a r e s t i l l p r e s e n t o u t s i d e t h e alumina l a t t i c e , w h i l e above t h i s t e m p e r a t u r e t h e c o b a l t i o n s move i n t o t h e alumina. S o t h e p o s i t i o n of t h e c o b a l t promotor i o n s is s t r o n g l y dependent on t h e c a l c i n a t i o n temperature. S p e c t r a of adsorbed p y r i d i n e on t h e 500°C
(low c a l c i n e d ) and 650°C
( h i g h c a l c i n e d ) c a t a l y s t s a r e shown i n f i g . 4 . The Br’bnsted a c i d bands a t
1636 and 1 9 0 cm-’ ( s e e f i g . 1 ) have d i s a p p e a r e d f o r t h e g r e a t e r p a r t i n t h e spectrum of t h e low c a l c i n e d c a t a l y s t . The bands r e a p p e a r for t h e h i g h c a l c i n e d c a t a l y s t . This p o i n t s o u t t o t h e f a c t t h a t Br‘bnsted
acid sites,
p r e s e n t i n MOO -alumina a r e n e u t r a l i z e d by t h e c o b a l t i o n s f o r t h e low c a l -
3
c i n e d sample. The c o b a l t i o n s do n o t n e u t r a l i z e t h e
s i t e s any more f o r t h e
h i g h c a l c i n e d c a t a l y s t . T h i s might be expected because t h e r e f l e c t a n c e s p e c t r a i n d i c a t e t h a t t h e c o b a l t i o n s have moved i n t o t h e alumina.
F o r b o t h t h e low a s w e l l a s t h e h i g h c a l c i n e d sample a new Lewis barid i s observed a t 1612 cm-l. T h i s Lewis band d i f f e r s from t h e low f r e q u e n c y 1 band (1614 cm- ) of t h e alumina c a r r i e r , because a ) t h e p o s i t i o n is s l i g h t -
l y d i f f e r e n t and b ) t h e 1612 cm-l band i s s t i l l observed a f t e r d e s o r p t i o n
a t 25OoC, w h i l e t h e weak Lewis band of t h e alumina h a s d i s a p p e a r e d a t t h i s t e m p e r a t u r e . S o t h e i n t r o d u c t i o n of t h e 1612 em-’ band for t h e c o b a l t promoted c a t a l y s t i s due t o a n i n t e r a c t i o n of t h e c o b a l t i o n s w i t h t h e
386
1700
1600
1600
1400 1700
1500
1500
1400
wavenumber (cm-1) F i g . 4 . S p e c t r a of 'adsorbed p y r l d i n e . MoCo-124. c a l c i n e d a t 500°C ( a ) and a t 65OoC ( b ) .
3- alumina s u r f a c e l a y e r .
MOO
E f f e c t on t h e a c t i v i t y .
The i n f l u e n c e of t h e c a l c i n a t i o n a f t e r t h e i m -
p r e g n a t i o n with t h e promotor i o n s on t h e c a t a l y t i c a c t i v i t y is shown i n f i g
5. The a c t i v i t y of t h e c o b a l t promoted c a t a l y s t is n o t s t r o n g l y i n f l u e n c e d .
---
N i c k e l promoted c a t a l y s t s however a r e v e r y s e n s i t i v e f o r t o o h i g h c a l c i n a t i o n temperatures.
N i c k e l Promoted C a t a l y s t s .
R e f l e c t a n c e s p e c t r a of t h e MoNi-153 c a t a -
l y s t , c a l c i n e d a t 480 and 65OoC a r e shown i n f i g
6. A b s o r p t i o n bands a r e
fg 5
weight
xtivity
Moco-124
100
500
600
700
'C
calcination temperature
1
fig 6
15 1i waverumber (cm-1)
xld
F i g . 5 . C a t a l y t i c a c t i v i t y of MoCo-124 and MoNi-153 as a f u n c t i o n of t h e c a l c i n a t i o n t e m p e r a t u r e . T r e a c t o r = 365OC I P t o t a l = 41 atm. Fig.6. R e f l e c t a n c e s p e c t r a of MoNi-153.
386 p r e s e n t a t 14.2 and 12.8 x
Id
cm-l. These bands a r e c h a r a c t e r i s t i c f o r
o c t a h e d r a l l y c o o r d i n a t e d N i i o n s . The spectrum of t h e high c a l c i n e d catal y s t is c h a r a c t e r i s t i c f o r NiA1204. This i n d i c a t e s t h a t t h e N i i o n s move i n t o t h e alumina l a t t i c e a t the h i g h e r c a l c i n a t i o n t e m p e r a t u r e s .
1600
1700
1500
650‘c 1400 1700
1500 1400 wavenumber (cm-1)
1600
F i g . 7 . S p e c t r a of adsorbed p y r i d i n e on McCo-153 ( a ) and MoNi-153 ( b ) . S p e c t r a of p y r i d i n e adsorbed on MoNi-153 a r e shown i n f i g 7. These s p e c t r a a r e compared w i t h t h o s e f o r MoCo-153, a c o b a l t c o n t a i n i n g c a t a l y s t of t h e same composition. The s p e c t r a of t h e low c a l c i n e d c a t a l y s t s a r e s i m i l a r . The i n t e n s i t y of t h e Br’6nsted bands i s low, w h i l e t h e s p l i t t i n g of t h e Lewis band i s c l e a r l y p r e s e n t . The Br‘bnsted a c i d s i t e s r e a p p e a r i n t h e s p e c t r a of t h e high c a l c i n e d samples. The i n t e n s i t y r a t i o of t h e low f r e quency Lewis band (1612 cm-l ) r e l a t i v e t o t h e h i g h f r e q u e n c y component 1 (1622 cm- ) is lower f o r t h e n i c k e l c o n t a i n i n g c a t a l y s t t h a n for t h e c o b a l t c o n t a i n i n g one. Sulfided Catalysts.
S p e c t r a of adsorbed p y r i d i n e on i n s i t u s u l f i d e d
c a t a l y s t s have been r e g i s t r a t e d . No change h a s been observed i n t h e s p e c t r a o f &-alumina and Coo-alumina, due t o t h e s u l f i d i n g . S p e c t r a f o r s u l f i d e d MoOj-alumina
and f o r MoCo-124 ( f i n a l c a l c i n a t i o n 650°C) a r e shown i n f i g
One Lewis band is observed a t 1617 em-’
8.
i n t h e spectrum of s u l f i d e d Mo03-
alumina. A same spectrum is observed f o r MoCo-124. No s p l i t t i n g of t h e Lewis band is p r e s e n t , a s h a s been observed for t h e o x i d i c c a t a l y s t . Regenerated C a t a l y s t s .
S u l f i d e d c a t a l y s t s from t h e t e s t r e a c t o r have
been used f o r t h e i n v e s t i g a t i o n of t h e s t r u c t u r e of r e g e n e r a t e d c a t a l y s t s . McCo-124 c a t a l y s t s , c a l c i n e d a t 550 and 650Oc a f t e r t h e c o b a l t impregnation
387 w i l l be d i s c u s s e d h e r e . S p e c t r a of adsorbed p y r i d i n e a r e shown i n f i g They c l o s e l y resemble t h o s e of t h e f r e s h c a t a l y s t s as g i v e n i n f i g
9.
4 a,b.
The r e f l e c t a n c e s p e c t r a of t h e f r e s h and r e g e n e r a t e d MoCo-124 c a t a l y s t s a r e compared i n f i g 10. The r e f l e c t a n c e s p e c t r a of t h e h i g h c a l c i n e d c a t a c
fig 8
1600
1700
1400
1500
1700
1500
1600
1400
wavenumber (cm-1) F i g . 8 . S p e c t r a of adsorbed p y r i d i n e on s u l f i d e d MOO -alumina ( a ) 3 and on MoCo-124 ( b ) , Tdesorption = 150°C. Fig.9. S p e c t r a of adsorbed p y r i d i n e on r e g e n e r a t e d MoCo-124 c a t a l y s t s . Tdesorption = 15OoC.
a
OI
1
500
o
600
o
700
b
L
800
'
O
O
w
a
o
wavetength (nm) R p f l e c t a n z e s p e c t r a of f r e s h and r e g e n e r a t e d ( r e g . ) :
o
388 l y s t s are n e a r l y similar. This shows t h a t t h e c o b a l t i o n s a r e p r e s e n t i n t h e alumina l a t t i c e a l s o f o r t h e r e g e n e r a t e d c a t a l y s t s .
DISCUSS1ON Model.
Based on t h e s p e c t r o s c o p i c d a t a we propose t h e f o l l o w i n g model
f o r t h e s t r u c t u r e of t h e o x i d i c c a t a l y s t . The molybdate l a y e r , which is formed on t h e alumina c a r r i e r by impregnation and c a l c i n a t i o n , is c h a r a c t e r i z e d by t h e presence of Br'dnsted a c i d s i t e s and one t y p e of s t r o n g Lewis a c i d s i t e s (1622 cm-l band). The promotor i o n s a r e d e p o s i t e d on t h i s l a y e r . A f t e r c a l c i n a t i o n a t 500°C we d o n o t observe t h e p r e s e n c e of a cob a l t o r n i c k e l a l u m i n a t e phase and t h e Br'dnsted a c i d s i t e s a r e n e u t r a l i z e d . T h i s shows t h a t t h e promotor i o n s a r e s t i l l on t h e c a t a l y s t s u r f a c e . These c o n f i g u r a t i o n s a r e i n d i c a t e d i n f i g 1 1 by a " c o b a l t molybdate" r e s p . " n i c k e l molybdate"
.
Another i m p o r t a n t evidence f o r t h e i n t e r a c t i o n between t h e molybdate l a y e r and t h e promotor i o n s i s t h e a p p e a r a n c e of a second Lewis band a t 1612 cm-l. T h i s band is n o t p r e s e n t f o r Mo03-alumina
o r Coo-alumina.
H
?/
molybdate surface layer characterized by Bronsted and Lewis acid sites
-MD-
alumina
impregnat ion calcining 500°C
/
-Mo-
Cobalt
co2+
@ -MoL ?
-MoLCo
alumina
Nickel
@
Ni 2+
-Mo? A
- M o' - Ni
alumina
calcining 650'C surface CoAl204
some surface N1Al204 bulk NiAl204
Fig.11.
Scheme of t h e s t r u c t u r e of o x i d i c c a t a l y s t s .
For t h e h i g h c a l c i n e d c a t a l y s t s t h e f o r m a t i o n of a c o b a l t r e s p . n i c k e l a l u m i n a t e phase is i n d i c a t e d by t h e r e f l e c t a n c e s p e c t r a . Also t h e r e a p pearance of t h e Br'bnsted a c i d s i t e s p o i n t s i n t h i s d i r e c t i o n . An i m p o r t a n t d i f f e r e n c e is n o t i c e d f o r t h e i n t e n s i t y r a t i o of t h e 1612 cm-'
low f r e q u e n -
cy Lewis band r e l a t i v e t o t h e h i g h frequency component (1622 c m - l ) . This
789 r a t i o remains h i g h . w h i l e t h e i n t e n s i t y r a t i o st,rongl:, d p - r e a s p s f o r t h e n i c k e l promoted c a t a l y s t . The promoting a c t i o n of t h e n i c k e l i o n s a l s o s t r o n g l y decreases with i n c r e a s i n g c a l c i n a t i o n temperatures ( f i g
5). Hence
t h e i n t e n s i t y of t h e 1612 cm-l Lewis band i s r e l a t e d w i t h t h e c a t a l y t i c a c t i v i t y . A p p a r e n t l y t h e c o b a l t i o n s remain p r e s e n t n e a r t h e molybdate s u r f a c e . T h i s p o s i t i o n is i n d i c a t e d i n our model by a s u r f a c e CoAl2O4 l a y e r . For t h e n i c k e l c o n t a i n i n g c a t a l y s t t h e promotor i o n s g e t l o s t i n t h e alumina l a t t i c e under f o r m a t i o n of a bulk n i c k e l a l u m i n a t e phase. T h i s o x i d i c c a t a l y s t i s only t h e p r e c u r s o r of t h e a c t i v e c a t a l y s t , which is o b t a i n e d by s u l f i d i n g . So t h e q u e s t i o n rises where the promotor ions a r e located i n t h e sulfided c a t a l y s t . Sulfided Catalysts.
R e c e n t l y two models have been p r e s e n t e d for t h e
s u l f i d e d c a t a l y s t . The i n t e r c a l a t i o n model of F a r r a g h e r and Cossee[4]
is
based on o b s e r v a t i o n s f o r t h e non-supported Ni-WSg system. They assume t h a t t h i s model can be extended f o r a d e s c r i p t i o n of s u p p o r t e d s y s t e m s , l i k e t h e Co and N i promoted molybrlenum-alumina S c h u i t and Gates
[ 5J
c a t a l y s t s . The model of
is based on t h e p r e s e n c e of a molybdate monolayer on
the alumina s u r f a c e , s t a b i l i z e d by c o b a l t i o n s . These i o n s occupy t e t r a h e d r a l p o s i t i o n s i n t h e alumina l a t t i c e and r e p l a c e a n e q u i v a l e n t amount of aluminum
i o n s . These aluminiun i o n s a r e i n c o r p o r a t e d i n t o
t h e molybdate monolayer and s t a b i l i z e i n t h i s way t h e a c t i v e molybdate surface.
Our o b s e r v a t i o n s f o r t h e o x i d i c c o b a l t promoted c a t a l y s t show t h a t t h e r e is n o p r e f e r e n c e f o r a p o s i t i o n e i t h e r on t h e molybdate s u r f a c e or i n t h e upper l a y e r of t h e alumina l a t t i c e . The n i c k e l promotor i o n s however h a v e t o be on t h e c a t a l y s t s u r f a c e . The a c t i v i t y d r o p s when t h e n i c k e l i o n s move i n t o t h e alumina. This i n d i c a t e s t h a t a p o s i t i o n of t h e promotor i o n s i n t h e alumina, as s u g g e s t e d i n t h e monolayer model is n o t e s s e n t i a l . We t h e r e f o r e d i s a g r e e w i t h t h e e x p l a n a t i o n g i v e n t o t h e prornot o r i o n s i n t h a t model. D e B e e r [ 6 ] r e c e n t l y p r e s e n t e d a modified monolayer model. I n t h i s mo-
d e l t h e f u n c t i o n of t h e a l u m i n a s u p p o r t is t o b r i n g t h e rnolybdate i n a w e l l d i s p e r s e d form. Durjng t h e s u l f i d i n g s t e p , MoS2 c r y s t a l l i t e s a r e formed, which a r e promoted by i n t e r c a l a t i o n of t h e c o b a l t i o n s . The c o b a l t i o n s have t o come o u t of t h e alumina i n o r d e r t h a t i n t e r c a l a t i o n can occur. De B e e r t r i e d t o prove t h a t a t r a n s p o r t of t h e c o b a l t i o n s from t h e alumina l a t t i c e i n t o t h e s u l f i d e d molybdate l a y e r t a k e s p l a c e . We have i n v e s t i g a t e d whether s u c h a t r a n s p o r t t a k e s p l a c e i n our c a t a l y s t s . I R s p e c t r a of p y r i d i n e adsorbed on s u l f i d e d MOOT-alumina and MoCo-124
(650%)
h a v e been
390 compared ( f i g 8 a , b ) . A change i n t h e s u r f a c e l a y e r can be expected when t h e s u r f a c e l a y e r is modified by i n t e r c a l a t i o n of c o b a l t i o n s . The s p e c t r a
a r e however n e a r l y i d e n t i c a l . Also r e g e n e r a t e d c a t a l y s t s have been s t u d i e d . The c a t a l y s t s were exposed t o a t e m p e r a t u r e of 4 5 0 O C d u r i n g t h e r e g e n e r a t i o n . I n c a s e t h e c o b a l t i o n s come o u t of t h e alumina l a t t i c e d u r i n g s u l f i d i n g , one s h o u l d e x p e c t - t h a t a f t e r r e g e n e r a t i o n t h e c a t a l y s t s w i l l have t h e low c a l c i n e d form. The spectrum of p y r i d i n e a d s o r b e d on t h e r e g e n e r a t e d MoCo-124 (650%) c a t a l y s t ( f i g 9 b ) is t y p i c a l f o r t h e h i g h c a l c i n e d formi ( s e e f i g 4 ) . A l s o t h e r e f l e c t a n c e spectrum ( f i g l o b ) i n d i c a t e s t h a t t h e c o b a l t i o n s a r e s t i l l i n t h e alumina l a t t i c e . T h i s s u g g e s t s t h a t i t i s d o u b t f u l whether t h e c o b a l t i o n s l e a v e t h e alumina l a t t i c e f o r t h e Ma:, c r y s t a l l i t e s on s u l f i d a t i o n . I n t e r c a l a t i o n might t a k e p l a c e f o r t h e low c a l c i n e d MoCo-124 c a t a l y s t , where t h e c o b a l t i o n s a r e p r e s e n t on t h e s u r f a c e . However t h e c o n t r i b u t i o n
of i n t e r c a l a t i o n t o t h e a c t i v i t y w i l l n o t be t o o h i g h because no g r p a t d i f f e r e n c e i n c a t a l y t i c a c t i v i t y is observed between t h e low and h i g h c a l c i n e d MoCo-124 c a t a l y s t s . The c o n c l u s i o n w i t h r e g a r d t o t h e p r e p a r a t i o n of t h e o x i d i c c a t a l y s t
is t h a t t h e promotor i o n s s h o u l d be brought i n t h e iieighbourhood of t h e molybdate l a y e r . The m o b i l i t y of t h e prornotor i o n s a t t h e t e m p e r a t u r e of s u l f i d i n g is t o o s m a l l t h a t t h i s s i t u a t i o n w i l l be reached d u r i n g t h e sulfidation step. REFERENCES Byrns, W.E.
Lee, I n d . Eng. Chem.
3,1160
1
A.C.
2
R . Mon‘e and L. Moscou, A m e r . Chern. Soc., Div. P e t r o l . Chem. P r e p r i n t 48 (1975).
Bradley and M.W.
(1943).
n,
123P (1973). 3 F.E. K i v i a t and L. P e t r a k i s , J. Phys. Chern. 4 A.L. F a r r a g h e r and P. Cossee, Proc. I n t . Congr. C a t a l . , 5 t h , Palm Beach 1972 p. 1301.
5 G.C.A. Schuyt and B.C. G a t e s , AIChE 3 , 417 (1973). 6 V.H.F. de B e e r , T h e s i s , Eindhoven U n i v e r s i t y of Technology, 19(5.
391
DISCUSSION P.C.H.
MITCHELL :
Do you have any i d e a on t h e c h e m i c a l n a t u r e
of t h e new L e w i s a c id s i t e c r e a t e d by t h e i n t e r a c t i o n of t h e promotor i o n w i t h Mo-A1203 ?
Is i t a s s o c i a t e d w i t h t h e promotor
i o n o r w i t h MO ? R.
MONE :
The c o b a l t promotor i o n s a r e p r e s e n t i n t h e alumina
l a t t i c e b e n e a t h t h e molybdate s u r f a c e l a y e r f o r t h e h i g h c a l c i ned c a t a l y s t . served.
S t i l l t h e new L e w i s band a t 1612 c m - l
c a n b e ob-
T h i s i n d i c a t e s i n my o p i n i o n , t h a t t h e new L e w i s s i t e
i s r e l a t e d t o t h e molybdate g r o u p s .
The promotor i o n s a p p a r e n t -
l y weaken t h e L e w i s a c i d i t y of t h e molybdate s u r f a c e .
E.J. NEWSON :
My q u e s t i o n c o n c e r n s t h e i n t e r e s t i n g b a l a n c e you
c a n p r o d u c e w i t h Lewis-Brbnsted a c i d i t y by u s i n g c a l c i n a t i o n t e m p e r a t u r e s between 500°C and 65OoC.
Does t h i s mean t h a t you
would recommend one c a l c i n a t i o n t e m p e r a t u r e f o r d i s t i l l a t e hyd r o t r e a t i n g and a n o t h e r t e m p e r a t u r e f o r t h e h e a v i e r , more uns a t u r a t e d f e e d s t o c k s such a s r e s i d u a ? R. MONE :
T h e B r a n s t e d a c i d i t y h a s been o b s e r v e d o n l y f o r t h e
oxidic catalysts.
N o Brbnsted a c i d s i t e s a r e p r e s e n t f o r t h e
sulfided catalyst (fig. 8 ) .
I do not
expect t h a t t h e calcina-
t i o n t e m p e r a t u r e h a s a n e f f e c t on t h e B r b n s t e d a c i d i t y o f t h e ultimate catalyst.
R. M O k Akzo Chemie Nederland bv, Research C e n t r e Amsterdam, The N e t h e r l a n d s .
SUMMARY The s t r u c t u r e of t h e c o b a l t and n i c k e l i o n s i n o x i d i c h y d r o d e s u l f u r i z a t i o n c a t a l y s t s h a s been s t u d i e d . I t i s shown t h a t t h e c a l c i n a t i o n temper a t u r e s t r o n g l y d e t e r m i n e s t h e p o s i t i o n of t h e promotor i o n s . C o b a l t and n i c k e l a r e s t i l l o u t s i d e t h e alumina l a t t i c e a f t e r c a l c i n a t i o n up t o 500°C, because t h e VIS r e f l e c t a n c e s p e c t r a of CoAl204 r e s p . NiA1204 a r e n o t o b s e r ved and t h e B M n s t e d a c i d s i t e s , a s p r e s e n t i n MoO3-alumina a r e s t i l l n e u t r a l i z e d . A new Lewis a c i d s i t e i s i n t r o d u c e d by t h e i n t e r a c t i o n between t h e promotor i o n s arid MOO -alumina, a s i n d i c a t e d by t h e I R s p e c t r a of a d -
3
s o r b e d p y r i d i n e . The p r e s e n c e of t h i s band is c o r r e l a t e d w i t h t h e c a t a l y t i c a c t i v i t y . H e a t i n g t o 650-700% c a u s e s m i g r a t i o n of t h e promotor i o n s i n t o t h e u p p e r l a y e r s of t h e alumina. The new Lewis a c i d band remains p r e s e n t
for t,he c o b a l t promoted c a t a l y s t , w h i l e t h e i n t e n s i t y d e c r e a s e s f o r t h e rlick-i promoted c a t a l y s t . S u l f i d e d and r e g e n e r a t e d c a t a l y s t s have a l s o h e e n s t u d i e d i n o r d e r t o get, i n f o r m a t i o n a b o u t t h e p o s i t i o n of t h e promo-
*or' ions i n the s u l f i d e d c a t a l y s t . ? h e voLlicls which have beer] p r e s e n t e d r e c e n t l y f o r t h e a c t i v a t o r system
of h ~ ~ r o d e s u l f u r i z a t , i o rci a t a l y s t s a r e e v a l u a t e d , IRTHODUCTI ON
'The c a t a l y t i c ~ i y d r . o t 1 e s u l f u r i z a t i o np r o c e s s d a t e s from a b o u t 1940 [l] I n i t , i a l l y a " c o b a l t moiybdate" c a t a l y s t was d e v e l o p e d , c o n s i s t i n g of c o b a l t and molybdcrium oxid? on an aliimina s u p p o r t , w i t h a s t o i c h i o m e t r i c r'at,io of Co:Mo
=
1 : i . L a t e r on more o p t i m i z e d c a t a l y s t s have been i n t r o -
duced w i t h 2.owpr ~'o:Mo r a t i o s . A t r i a l and error approach p l a y s an import.ant r o l
P
i n d e v e l o p i n g i n r p r o v ~ dz a t a l y s h . But a l s o an u n d e r s t a n d i n g of
t,he c a t . n l y s t s t , r u c t u r c niight bc h e l p f u l . 'This F s p e r d e a l s w i t h r e s u l t s
o b t . a i n d by t h e l a s t a p p r o a c h . v i z t h e c h a r a c t e r i z a t i o n of t h e c a t a l y s t s t r u c t u r e , espp:ially
with w g a r d
t o t h e l o c a t i o n of t h e promotor i o n s .
.
382 MPF,lUM13NrYAL PART The c a t a l y s t s were made by a d o u b l e impregnation of a f- alumina c a r r i e r , SA = 253 m2/g.
F i r s t t h e impregnation w i t h ammonium molybdate was
c a r r i e d o u t , a f t e r w a r d s t h e promotor was d e p o s i t e d . Each i m p r e g n a t i o n was f o l l o w e d by d r y i n g o v e r n i g h t and c a l c i n a t i o n a t
650Oc a f t e r t h e impregna-
t i o n w i t h molybdate a i d t e m p e r a t u r e s v a r y i n g between 400 and 700°C a f t e r t h e impregnation w i t h c o b a l t o r n i c k e l . Composition of t h e samples: MoCo124, 12 w t
% Moo3
and
MoNi-153, 15 w t $ MOO
4wt %
3
COO; MoCo-153, 15 w t
%
Moo3 and
3 w t % COO;
and 3 w t $ N i O .
The r e f l e c t a n c e s p e c t r a were recorded w i t h an O p t i c a Milano CF
4
s p e c t r o p h o t o m e t e r , u s i n g MgO a s a r e f e r e n c e . The I R s p e c t r a were r e c o r d e d w i t h a Perkiri Elmer 621. The s e l f s u p p o r t i n g d i s c method was a p p l i e d . The d i s c s were r e h y d r o x y l a t e d f i r s t on s t a n d i n g i n humid a i r d u r i n g two
. Outgassing took place
d a y s , b e f o r e t h e y were p l a c e d i n t h e I R c e l l [ 2 ]
a t 42OoC, f i n a l p r e s s u r e b e t t e r t h a n 10-5 t o r r . P y r i d i n e was a d s o r b e d a t room t e m p e r a t u r e and d e s o r b e d a t 150 and 25OoC. The s u l f i d i n g of t h e c a t a l y s t s h a s been c a r r i e d o u t i n two d i f f e r e n t ways; i n t h e I R c e l l and i n a t e s t r e a c t o r . For t h e i n s i t u s u l f i d i n g i n t h e I R c e l l , a H2/112S m i x t u r e (20 v o l $ H2S) was l e d o v e r t h e c a t a l y s t a t 200°C d u r i n g 1
.5 h r
and a t 320°C d u r i n g 1
.5 h r
s u c c e s s i v e l y . Then t h e sam-
p l e was cooled down t o room t e m p e r a t u r e and s t r i p p e d w i t h Hg. The c e l l was a t t a c h e d t o t h e h i g h vacuum system a n d evacuated a t 35OoC d u r i n g two
hours. A f t e r t h i s p r e t r e a t m e n t t h e p y r i d i n e a d s o r p t i o n experiments were carried out. The s u l f i d i n g i n t h e t e s t r e a c t o r took p l a c e w i t h a m i x t u r e of 112 and t h e f e e d s t o c k (vacuum g a s o i l ) . 3 w t
%
b u t a n e t h i o l was added t o t h e f e e d -
s t o c k . The s u l f i d i n g was c a r r i e d o u t i n two s t e p s , f i r s t a t 204OC d u r i n g
3 h r , a f t e r w a r d s a t 3 1 6 O c d u r i n g 16 h r . The t o t a l p r e s s u r e was 21 atm. The a c t i v i t y of t h e s u l f i d e d samples was d e t e r m i n e d a t t e s t run, t h e samples were s t r i p p e d w i t h N2,
365%.
A t t h e end of t h e
c o o l e d down t o room tempera-
t u r e and t a k e n o u t of t h e r e a c t o r . Then t h e c a t a l y s t s were p l a c e d i n a miuffle f u r n a c e and h e a t e d i n t h e a i r up t o 45OoC.
'The c a t a l y s t s were brought
back i n t h e o x i d i c form by t h i s h e a t t r e a t m e n t .
MoO?-alumiria.
The f i r s t s t e p i n o u r c a t a l y s t p r e p a r a t i o n p r o c e d u r e i s
t h e impregnation of the alumina c a r r i e r w i t h molybdate. I R s p e c t r a of py-
383 and on MoO3-alumina a r e shown i n f i g 1 .
r i d i n e adsorbed on $-alumina
8-alumina is c h a r a c t e r i z e d by two Lewis bands, which correspond w i t h s t r o n g (1622 cm-’) and weak (1614 cm-’) Lewis a c i d s i t e s . The bonding between p y r i d i n e and t h e weak Lewis a c i d s i t e s i s a l r e a d y broken a t a d e s o r p t i o n t e m p e r a t u r e of 250’12.
Impregnation o f t h e alumina c a r r i e r w i t h
c o b a l t o r n i c k e l does n o t change the a c i d i t y of t h e c a r r i e r as h a s been shown b e f o r e r2.31.
F o r Mo03-alumina only one Lewis band (1622 cm-l ) is
observed. Also B m n s t e d a c i d s i t e s are p r e s e n t , c h a r a c t e r i z e d by the
1636
and 1 3 0 cm-l bands.
1700
1600
1500
1400 1700
1600
1500 1400 wwenumberkm-1)
F i g . 1 . S p e c t r a of p y r i d i n e adsorbed on 2-alumina ( a ) and on Moo3-alumina ( b )
.
Cobalt Impregnation.
The second s t e p i n t h e c a t a l y s t p r e p a r a t i o n i s
t h e impregnation w i t h t h e promotor i o n s . The c o b a l t i o n s a r e d e p o s i t e d on t h e c a t a l y s t s u r f a c e and move i n t o t h e alumina on c a l c i n a t i o n . The change i n t h e c o b a l t c o o r d i n a t i o r i can be followed w i t h r e f l e c t a n c e s p e c t r o s c o p y . F i g 2 shows t h e r e f l e c t a n c e spectrum of t h e c o b a l t impregnated c a t a l y s t a f t e r d r y i n g a t l2OoC. The bands a t 540 and 630 nm a r e c h a r a c t e r i s t i c for o c t a h e d r a l l y c o o r d i n a t e d c o b a l t i o n s . C o b a l t is s t i l l p r e s e n t on t h e c a t a l y s t s u r f a c e . A l s o t h e spectrum of
65ooc is
4wt %
COO o n x - a l u m i n a , c a l c i n e d a t
g i v e n . The t r i p l e t band is c h a r a c t e r i s t i c f o r t e t r a h e d r a l l y coor-
d i n a t e d c o b a l t (CoAl2O4) i n d i c a t i n g t h a t t h e c o b a l t i o n s have moved i n t o t h e alumina l a t t i c e . F i g 3 shows t h e r e f l e c t a n c e s p e c t r a of MoCo-124 c a t a l y s t s , c a l c i n e d a t d i f f e r e n t t e m p e r a t u r e s . I t a p p e a r s t h a t t h e i n t e n s i t y of t h e t r i p l e t
384
fig 2
fig 3
OI
F i g . ? . R e f l e c t a n c e s p e c t r a . a ) MOO -alumina. impregnated w i t h C O ( N O ~ ) ~ , 3 d r i e d a t 120°C, b ) 4 w t $ Coo-alumina c a l c i n e d 1 h r 6 5 0 O ~ . F i g . 3 . R e f l e c t a n c e s p e c t r a . MoCo-124 c a l c i n e d a t d i f f e r e n t t e m p e r a t u r e s . barid s t r o n g l y i n c r e a s e s on c a l c i n a t i o n above 60OOC. Below t h i s t e m p e r a t u r e
a broad a b s o r p t i o n band i s observed, c o v e r i n g t h e whole s p e c t r a l r a n g e with a weak s u p e r p o s i t i o n of a t r i p l e t band. T h i s i n d i c a t e s t h a t below
6OO0C t h e c o b a l t i o n s a r e s t i l l p r e s e n t o u t s i d e t h e alumina l a t t i c e , w h i l e above t h i s t e m p e r a t u r e t h e c o b a l t i o n s move i n t o t h e alumina. S o t h e p o s i t i o n of t h e c o b a l t promotor i o n s is s t r o n g l y dependent on t h e c a l c i n a t i o n temperature. S p e c t r a of adsorbed p y r i d i n e on t h e 500°C
(low c a l c i n e d ) and 650°C
( h i g h c a l c i n e d ) c a t a l y s t s a r e shown i n f i g . 4 . The Br’bnsted a c i d bands a t
1636 and 1 9 0 cm-’ ( s e e f i g . 1 ) have d i s a p p e a r e d f o r t h e g r e a t e r p a r t i n t h e spectrum of t h e low c a l c i n e d c a t a l y s t . The bands r e a p p e a r for t h e h i g h c a l c i n e d c a t a l y s t . This p o i n t s o u t t o t h e f a c t t h a t Br‘bnsted
acid sites,
p r e s e n t i n MOO -alumina a r e n e u t r a l i z e d by t h e c o b a l t i o n s f o r t h e low c a l -
3
c i n e d sample. The c o b a l t i o n s do n o t n e u t r a l i z e t h e
s i t e s any more f o r t h e
h i g h c a l c i n e d c a t a l y s t . T h i s might be expected because t h e r e f l e c t a n c e s p e c t r a i n d i c a t e t h a t t h e c o b a l t i o n s have moved i n t o t h e alumina.
F o r b o t h t h e low a s w e l l a s t h e h i g h c a l c i n e d sample a new Lewis barid i s observed a t 1612 cm-l. T h i s Lewis band d i f f e r s from t h e low f r e q u e n c y 1 band (1614 cm- ) of t h e alumina c a r r i e r , because a ) t h e p o s i t i o n is s l i g h t -
l y d i f f e r e n t and b ) t h e 1612 cm-l band i s s t i l l observed a f t e r d e s o r p t i o n
a t 25OoC, w h i l e t h e weak Lewis band of t h e alumina h a s d i s a p p e a r e d a t t h i s t e m p e r a t u r e . S o t h e i n t r o d u c t i o n of t h e 1612 em-’ band for t h e c o b a l t promoted c a t a l y s t i s due t o a n i n t e r a c t i o n of t h e c o b a l t i o n s w i t h t h e
386
1700
1600
1600
1400 1700
1500
1500
1400
wavenumber (cm-1) F i g . 4 . S p e c t r a of 'adsorbed p y r l d i n e . MoCo-124. c a l c i n e d a t 500°C ( a ) and a t 65OoC ( b ) .
3- alumina s u r f a c e l a y e r .
MOO
E f f e c t on t h e a c t i v i t y .
The i n f l u e n c e of t h e c a l c i n a t i o n a f t e r t h e i m -
p r e g n a t i o n with t h e promotor i o n s on t h e c a t a l y t i c a c t i v i t y is shown i n f i g
5. The a c t i v i t y of t h e c o b a l t promoted c a t a l y s t is n o t s t r o n g l y i n f l u e n c e d .
---
N i c k e l promoted c a t a l y s t s however a r e v e r y s e n s i t i v e f o r t o o h i g h c a l c i n a t i o n temperatures.
N i c k e l Promoted C a t a l y s t s .
R e f l e c t a n c e s p e c t r a of t h e MoNi-153 c a t a -
l y s t , c a l c i n e d a t 480 and 65OoC a r e shown i n f i g
6. A b s o r p t i o n bands a r e
fg 5
weight
xtivity
Moco-124
100
500
600
700
'C
calcination temperature
1
fig 6
15 1i waverumber (cm-1)
xld
F i g . 5 . C a t a l y t i c a c t i v i t y of MoCo-124 and MoNi-153 as a f u n c t i o n of t h e c a l c i n a t i o n t e m p e r a t u r e . T r e a c t o r = 365OC I P t o t a l = 41 atm. Fig.6. R e f l e c t a n c e s p e c t r a of MoNi-153.
386 p r e s e n t a t 14.2 and 12.8 x
Id
cm-l. These bands a r e c h a r a c t e r i s t i c f o r
o c t a h e d r a l l y c o o r d i n a t e d N i i o n s . The spectrum of t h e high c a l c i n e d catal y s t is c h a r a c t e r i s t i c f o r NiA1204. This i n d i c a t e s t h a t t h e N i i o n s move i n t o t h e alumina l a t t i c e a t the h i g h e r c a l c i n a t i o n t e m p e r a t u r e s .
1600
1700
1500
650‘c 1400 1700
1500 1400 wavenumber (cm-1)
1600
F i g . 7 . S p e c t r a of adsorbed p y r i d i n e on McCo-153 ( a ) and MoNi-153 ( b ) . S p e c t r a of p y r i d i n e adsorbed on MoNi-153 a r e shown i n f i g 7. These s p e c t r a a r e compared w i t h t h o s e f o r MoCo-153, a c o b a l t c o n t a i n i n g c a t a l y s t of t h e same composition. The s p e c t r a of t h e low c a l c i n e d c a t a l y s t s a r e s i m i l a r . The i n t e n s i t y of t h e Br’6nsted bands i s low, w h i l e t h e s p l i t t i n g of t h e Lewis band i s c l e a r l y p r e s e n t . The Br‘bnsted a c i d s i t e s r e a p p e a r i n t h e s p e c t r a of t h e high c a l c i n e d samples. The i n t e n s i t y r a t i o of t h e low f r e quency Lewis band (1612 cm-l ) r e l a t i v e t o t h e h i g h f r e q u e n c y component 1 (1622 cm- ) is lower f o r t h e n i c k e l c o n t a i n i n g c a t a l y s t t h a n for t h e c o b a l t c o n t a i n i n g one. Sulfided Catalysts.
S p e c t r a of adsorbed p y r i d i n e on i n s i t u s u l f i d e d
c a t a l y s t s have been r e g i s t r a t e d . No change h a s been observed i n t h e s p e c t r a o f &-alumina and Coo-alumina, due t o t h e s u l f i d i n g . S p e c t r a f o r s u l f i d e d MoOj-alumina
and f o r MoCo-124 ( f i n a l c a l c i n a t i o n 650°C) a r e shown i n f i g
One Lewis band is observed a t 1617 em-’
8.
i n t h e spectrum of s u l f i d e d Mo03-
alumina. A same spectrum is observed f o r MoCo-124. No s p l i t t i n g of t h e Lewis band is p r e s e n t , a s h a s been observed for t h e o x i d i c c a t a l y s t . Regenerated C a t a l y s t s .
S u l f i d e d c a t a l y s t s from t h e t e s t r e a c t o r have
been used f o r t h e i n v e s t i g a t i o n of t h e s t r u c t u r e of r e g e n e r a t e d c a t a l y s t s . McCo-124 c a t a l y s t s , c a l c i n e d a t 550 and 650Oc a f t e r t h e c o b a l t impregnation
387 w i l l be d i s c u s s e d h e r e . S p e c t r a of adsorbed p y r i d i n e a r e shown i n f i g They c l o s e l y resemble t h o s e of t h e f r e s h c a t a l y s t s as g i v e n i n f i g
9.
4 a,b.
The r e f l e c t a n c e s p e c t r a of t h e f r e s h and r e g e n e r a t e d MoCo-124 c a t a l y s t s a r e compared i n f i g 10. The r e f l e c t a n c e s p e c t r a of t h e h i g h c a l c i n e d c a t a c
fig 8
1600
1700
1400
1500
1700
1500
1600
1400
wavenumber (cm-1) F i g . 8 . S p e c t r a of adsorbed p y r i d i n e on s u l f i d e d MOO -alumina ( a ) 3 and on MoCo-124 ( b ) , Tdesorption = 150°C. Fig.9. S p e c t r a of adsorbed p y r i d i n e on r e g e n e r a t e d MoCo-124 c a t a l y s t s . Tdesorption = 15OoC.
a
OI
1
500
o
600
o
700
b
L
800
'
O
O
w
a
o
wavetength (nm) R p f l e c t a n z e s p e c t r a of f r e s h and r e g e n e r a t e d ( r e g . ) :
o
388 l y s t s are n e a r l y similar. This shows t h a t t h e c o b a l t i o n s a r e p r e s e n t i n t h e alumina l a t t i c e a l s o f o r t h e r e g e n e r a t e d c a t a l y s t s .
DISCUSS1ON Model.
Based on t h e s p e c t r o s c o p i c d a t a we propose t h e f o l l o w i n g model
f o r t h e s t r u c t u r e of t h e o x i d i c c a t a l y s t . The molybdate l a y e r , which is formed on t h e alumina c a r r i e r by impregnation and c a l c i n a t i o n , is c h a r a c t e r i z e d by t h e presence of Br'dnsted a c i d s i t e s and one t y p e of s t r o n g Lewis a c i d s i t e s (1622 cm-l band). The promotor i o n s a r e d e p o s i t e d on t h i s l a y e r . A f t e r c a l c i n a t i o n a t 500°C we d o n o t observe t h e p r e s e n c e of a cob a l t o r n i c k e l a l u m i n a t e phase and t h e Br'dnsted a c i d s i t e s a r e n e u t r a l i z e d . T h i s shows t h a t t h e promotor i o n s a r e s t i l l on t h e c a t a l y s t s u r f a c e . These c o n f i g u r a t i o n s a r e i n d i c a t e d i n f i g 1 1 by a " c o b a l t molybdate" r e s p . " n i c k e l molybdate"
.
Another i m p o r t a n t evidence f o r t h e i n t e r a c t i o n between t h e molybdate l a y e r and t h e promotor i o n s i s t h e a p p e a r a n c e of a second Lewis band a t 1612 cm-l. T h i s band is n o t p r e s e n t f o r Mo03-alumina
o r Coo-alumina.
H
?/
molybdate surface layer characterized by Bronsted and Lewis acid sites
-MD-
alumina
impregnat ion calcining 500°C
/
-Mo-
Cobalt
co2+
@ -MoL ?
-MoLCo
alumina
Nickel
@
Ni 2+
-Mo? A
- M o' - Ni
alumina
calcining 650'C surface CoAl204
some surface N1Al204 bulk NiAl204
Fig.11.
Scheme of t h e s t r u c t u r e of o x i d i c c a t a l y s t s .
For t h e h i g h c a l c i n e d c a t a l y s t s t h e f o r m a t i o n of a c o b a l t r e s p . n i c k e l a l u m i n a t e phase is i n d i c a t e d by t h e r e f l e c t a n c e s p e c t r a . Also t h e r e a p pearance of t h e Br'bnsted a c i d s i t e s p o i n t s i n t h i s d i r e c t i o n . An i m p o r t a n t d i f f e r e n c e is n o t i c e d f o r t h e i n t e n s i t y r a t i o of t h e 1612 cm-'
low f r e q u e n -
cy Lewis band r e l a t i v e t o t h e h i g h frequency component (1622 c m - l ) . This
789 r a t i o remains h i g h . w h i l e t h e i n t e n s i t y r a t i o st,rongl:, d p - r e a s p s f o r t h e n i c k e l promoted c a t a l y s t . The promoting a c t i o n of t h e n i c k e l i o n s a l s o s t r o n g l y decreases with i n c r e a s i n g c a l c i n a t i o n temperatures ( f i g
5). Hence
t h e i n t e n s i t y of t h e 1612 cm-l Lewis band i s r e l a t e d w i t h t h e c a t a l y t i c a c t i v i t y . A p p a r e n t l y t h e c o b a l t i o n s remain p r e s e n t n e a r t h e molybdate s u r f a c e . T h i s p o s i t i o n is i n d i c a t e d i n our model by a s u r f a c e CoAl2O4 l a y e r . For t h e n i c k e l c o n t a i n i n g c a t a l y s t t h e promotor i o n s g e t l o s t i n t h e alumina l a t t i c e under f o r m a t i o n of a bulk n i c k e l a l u m i n a t e phase. T h i s o x i d i c c a t a l y s t i s only t h e p r e c u r s o r of t h e a c t i v e c a t a l y s t , which is o b t a i n e d by s u l f i d i n g . So t h e q u e s t i o n rises where the promotor ions a r e located i n t h e sulfided c a t a l y s t . Sulfided Catalysts.
R e c e n t l y two models have been p r e s e n t e d for t h e
s u l f i d e d c a t a l y s t . The i n t e r c a l a t i o n model of F a r r a g h e r and Cossee[4]
is
based on o b s e r v a t i o n s f o r t h e non-supported Ni-WSg system. They assume t h a t t h i s model can be extended f o r a d e s c r i p t i o n of s u p p o r t e d s y s t e m s , l i k e t h e Co and N i promoted molybrlenum-alumina S c h u i t and Gates
[ 5J
c a t a l y s t s . The model of
is based on t h e p r e s e n c e of a molybdate monolayer on
the alumina s u r f a c e , s t a b i l i z e d by c o b a l t i o n s . These i o n s occupy t e t r a h e d r a l p o s i t i o n s i n t h e alumina l a t t i c e and r e p l a c e a n e q u i v a l e n t amount of aluminum
i o n s . These aluminiun i o n s a r e i n c o r p o r a t e d i n t o
t h e molybdate monolayer and s t a b i l i z e i n t h i s way t h e a c t i v e molybdate surface.
Our o b s e r v a t i o n s f o r t h e o x i d i c c o b a l t promoted c a t a l y s t show t h a t t h e r e is n o p r e f e r e n c e f o r a p o s i t i o n e i t h e r on t h e molybdate s u r f a c e or i n t h e upper l a y e r of t h e alumina l a t t i c e . The n i c k e l promotor i o n s however h a v e t o be on t h e c a t a l y s t s u r f a c e . The a c t i v i t y d r o p s when t h e n i c k e l i o n s move i n t o t h e alumina. This i n d i c a t e s t h a t a p o s i t i o n of t h e promotor i o n s i n t h e alumina, as s u g g e s t e d i n t h e monolayer model is n o t e s s e n t i a l . We t h e r e f o r e d i s a g r e e w i t h t h e e x p l a n a t i o n g i v e n t o t h e prornot o r i o n s i n t h a t model. D e B e e r [ 6 ] r e c e n t l y p r e s e n t e d a modified monolayer model. I n t h i s mo-
d e l t h e f u n c t i o n of t h e a l u m i n a s u p p o r t is t o b r i n g t h e rnolybdate i n a w e l l d i s p e r s e d form. Durjng t h e s u l f i d i n g s t e p , MoS2 c r y s t a l l i t e s a r e formed, which a r e promoted by i n t e r c a l a t i o n of t h e c o b a l t i o n s . The c o b a l t i o n s have t o come o u t of t h e alumina i n o r d e r t h a t i n t e r c a l a t i o n can occur. De B e e r t r i e d t o prove t h a t a t r a n s p o r t of t h e c o b a l t i o n s from t h e alumina l a t t i c e i n t o t h e s u l f i d e d molybdate l a y e r t a k e s p l a c e . We have i n v e s t i g a t e d whether s u c h a t r a n s p o r t t a k e s p l a c e i n our c a t a l y s t s . I R s p e c t r a of p y r i d i n e adsorbed on s u l f i d e d MOOT-alumina and MoCo-124
(650%)
h a v e been
390 compared ( f i g 8 a , b ) . A change i n t h e s u r f a c e l a y e r can be expected when t h e s u r f a c e l a y e r is modified by i n t e r c a l a t i o n of c o b a l t i o n s . The s p e c t r a
a r e however n e a r l y i d e n t i c a l . Also r e g e n e r a t e d c a t a l y s t s have been s t u d i e d . The c a t a l y s t s were exposed t o a t e m p e r a t u r e of 4 5 0 O C d u r i n g t h e r e g e n e r a t i o n . I n c a s e t h e c o b a l t i o n s come o u t of t h e alumina l a t t i c e d u r i n g s u l f i d i n g , one s h o u l d e x p e c t - t h a t a f t e r r e g e n e r a t i o n t h e c a t a l y s t s w i l l have t h e low c a l c i n e d form. The spectrum of p y r i d i n e a d s o r b e d on t h e r e g e n e r a t e d MoCo-124 (650%) c a t a l y s t ( f i g 9 b ) is t y p i c a l f o r t h e h i g h c a l c i n e d formi ( s e e f i g 4 ) . A l s o t h e r e f l e c t a n c e spectrum ( f i g l o b ) i n d i c a t e s t h a t t h e c o b a l t i o n s a r e s t i l l i n t h e alumina l a t t i c e . T h i s s u g g e s t s t h a t i t i s d o u b t f u l whether t h e c o b a l t i o n s l e a v e t h e alumina l a t t i c e f o r t h e Ma:, c r y s t a l l i t e s on s u l f i d a t i o n . I n t e r c a l a t i o n might t a k e p l a c e f o r t h e low c a l c i n e d MoCo-124 c a t a l y s t , where t h e c o b a l t i o n s a r e p r e s e n t on t h e s u r f a c e . However t h e c o n t r i b u t i o n
of i n t e r c a l a t i o n t o t h e a c t i v i t y w i l l n o t be t o o h i g h because no g r p a t d i f f e r e n c e i n c a t a l y t i c a c t i v i t y is observed between t h e low and h i g h c a l c i n e d MoCo-124 c a t a l y s t s . The c o n c l u s i o n w i t h r e g a r d t o t h e p r e p a r a t i o n of t h e o x i d i c c a t a l y s t
is t h a t t h e promotor i o n s s h o u l d be brought i n t h e iieighbourhood of t h e molybdate l a y e r . The m o b i l i t y of t h e prornotor i o n s a t t h e t e m p e r a t u r e of s u l f i d i n g is t o o s m a l l t h a t t h i s s i t u a t i o n w i l l be reached d u r i n g t h e sulfidation step. REFERENCES Byrns, W.E.
Lee, I n d . Eng. Chem.
3,1160
1
A.C.
2
R . Mon‘e and L. Moscou, A m e r . Chern. Soc., Div. P e t r o l . Chem. P r e p r i n t 48 (1975).
Bradley and M.W.
(1943).
n,
123P (1973). 3 F.E. K i v i a t and L. P e t r a k i s , J. Phys. Chern. 4 A.L. F a r r a g h e r and P. Cossee, Proc. I n t . Congr. C a t a l . , 5 t h , Palm Beach 1972 p. 1301.
5 G.C.A. Schuyt and B.C. G a t e s , AIChE 3 , 417 (1973). 6 V.H.F. de B e e r , T h e s i s , Eindhoven U n i v e r s i t y of Technology, 19(5.
391
DISCUSSION P.C.H.
MITCHELL :
Do you have any i d e a on t h e c h e m i c a l n a t u r e
of t h e new L e w i s a c id s i t e c r e a t e d by t h e i n t e r a c t i o n of t h e promotor i o n w i t h Mo-A1203 ?
Is i t a s s o c i a t e d w i t h t h e promotor
i o n o r w i t h MO ? R.
MONE :
The c o b a l t promotor i o n s a r e p r e s e n t i n t h e alumina
l a t t i c e b e n e a t h t h e molybdate s u r f a c e l a y e r f o r t h e h i g h c a l c i ned c a t a l y s t . served.
S t i l l t h e new L e w i s band a t 1612 c m - l
c a n b e ob-
T h i s i n d i c a t e s i n my o p i n i o n , t h a t t h e new L e w i s s i t e
i s r e l a t e d t o t h e molybdate g r o u p s .
The promotor i o n s a p p a r e n t -
l y weaken t h e L e w i s a c i d i t y of t h e molybdate s u r f a c e .
E.J. NEWSON :
My q u e s t i o n c o n c e r n s t h e i n t e r e s t i n g b a l a n c e you
c a n p r o d u c e w i t h Lewis-Brbnsted a c i d i t y by u s i n g c a l c i n a t i o n t e m p e r a t u r e s between 500°C and 65OoC.
Does t h i s mean t h a t you
would recommend one c a l c i n a t i o n t e m p e r a t u r e f o r d i s t i l l a t e hyd r o t r e a t i n g and a n o t h e r t e m p e r a t u r e f o r t h e h e a v i e r , more uns a t u r a t e d f e e d s t o c k s such a s r e s i d u a ? R. MONE :
T h e B r a n s t e d a c i d i t y h a s been o b s e r v e d o n l y f o r t h e
oxidic catalysts.
N o Brbnsted a c i d s i t e s a r e p r e s e n t f o r t h e
sulfided catalyst (fig. 8 ) .
I do not
expect t h a t t h e calcina-
t i o n t e m p e r a t u r e h a s a n e f f e c t on t h e B r b n s t e d a c i d i t y o f t h e ultimate catalyst.