- Email: [email protected]
Dynamic Activation, Deactivation, and Coking on PT and PTRE Catalysts for Dehydrogenation of Methylcyclohexane (MCH)
B. Imelik e t al. (Editors), MetalSupport and Metal-Additiue Effects in Catalysis
307
0 1982 Elsevier Scientific Publishing Company, Amsterdam -Printed in The Netherlands
DYNAMIC ACTIVATION, DEACTIVATION, AND C O K I N G ON PT AND PTRE CATALYSTS FOR
DEHYDROGENATION OF METHYLCYCLOHEXANE (MCH) R o b e r t W. C o u g h l i n , Koel Kawakami, Akran Hasan and Paul Buu Department o f Chemical E n g i n e e r i n g , U n i v e r s i t y o f C o n n e c t i c u t , S t o r r s , CT 06268 (USA)
RESUNE Le depBt de coke d e s a c t i v e l ’ h y d r o g e n o l y s e dans une p r e m i e r e etape, p u i s l a deshydrogenation du IICH. L ’ a d d i t i o n de Re c a t a l y s e 1 ’ 6 l i m i n a t i o n du coke p a r hydrogPnation. Le t r a i t e m e n t p a r ti2 du PtRe non s u l f u r 6 e n l @ v e p l u s de coke e t
a c e q u i e s t observe avec P t . La p r e s u l f u r a t i o n p a r H2S ou 10 ppm de t h i o p h e n e dans l e HCH a m e l i o r e n t l e rendement des
restaure l’hydrogenolyse, c o n t r a i r e n e n t
2 c a t a l y s e u r s au t o l u e n e . Mais 75 ppn de t h i o p h e n e d i m i n u e n t l e rendement de PtRe.
ABSTRACT Coke d e p o s i t i o n f i r s t d e a c t i v a t e s h y d r o g e n o l y s i s r e a c t i o n s and l a t e r dehydrog e n a t i o n o f MCH.
Added Re c a t a l y z e s coke removal by h y d r o g e n a t i o n .
Hydroqen
t r e a t m e n t o f u n s u l f i d e d PtRe c a t a l y s t s removes more coke and r e s t o r e s c o n s i d e r a b l e hydrogenolysis a c t i v i t y i n c o n t r a s t w i t h P t c a t a l y s t s .
Presulfiding with
H2S improves t o l u e n e y i e l d f o r e i t h e r c a t a l y s t , b u t e s p e c i a l l y PtRe.
Thiophene
( 1 0 ppm) i n t h e MCH f e e d improved t o l u e n e y i e l d w i t h e i t h e r c a t a l y s t , whereas 75 ppm improved t h e P t c a t a l y s t b u t l o w e r e d t o l u e n e y i e l d f o r t h e PtRe c a t a l y s t . INTRODUCTION Because dehydrogenation o f naphthenes t o a r o m a t i c s o v e r P t c a t a l y s t s i s r a p i d and o f g r e a t economic importance f o r i n c r e a s i n g t h e o c t a n e v a l u e s o f naphthas, we have i n v e s t i g a t e d t h i s r e a c t i o n f o r m e t h y l c y c l o h e x a n e as a model compound.
Commercial r e f o r m i n g c a t a l y s t s were e i t h e r m o n o m e t a l l i c (0.35% P t )
o r b i m e t a l l i c (0.32%, 0.325% Re) s u p p o r t e d on c h l o r i d e d y-alumina.
Only a f t e r
t h e c a t a l y s t has been on stream s e v e r a l hours, d u r i n g which a d e p o s i t o f hydrocarbonaceous r e s i n o r coke accumulates, does t h e e f f e c t o f t h e Re promoter become e v i d e n t ( 1 , 2 ) . accumulates
-
R e - c o n t a i n i n g c a t a l y s t s d e a c t i v a t e l e s s r a p i d l y as coke
t h e s e b i m e t a l l i c c a t a l y s t s can m a i n t a i n r e f o r m a t e p r o d u c t octane
308
number a t a s p e c i f i c desired level over the course of time-on-stream without the necessity of increasing temperature or space time as severely as required f o r monometallic P t c a t a l y s t s ( 3 ) . I t has been suggested ( 4 , 5 ) t h a t the a c i d i c alumina s u p p o r t of such catal y s t s may be modified by the Re, and shown ( 6 ) t h a t the Re i s present as an oxide such as Re02 associated with the surface of the alumina. Ludlum and Eischens ( 7 ) conducted an i n f r a r e d study of differences in the chemical natures of such d e p o s i t s ; they observed t h a t some carbon atoms of such deposits a r e bonded t o oxygen ( a s indicated by carboxylate bands a t 1570 and 1450 cm- 1 ) b u t t h a t the presence of Re decreases the f r a c t i o n of such carboxylate in t h e deposits. More recently much a t t e n t i o n has focused on t h e p o s s i b i l i t y of a l l o y format i o n between P t and Re a s well a s between P t and several other second-metal promoters. For example, alloying P t with Cu ( 8 ) o r with Sn ( 9 ) produces s e l e c t i v i t y changes dubbed "ensemble e f f e c t s " and viewed as s h i f t s from m u l t i s i t e mechanisms on c o l l e c t i o n s of contiguous P t atoms in P t - r i c h a l l o y s toward s i n g l e - s i t e mechanisms on P t d i l u t e d with the promoting metal.
In the case of
PtRe c a t a l y s t s i t has a l s o been shown (10) t h a t the combined action of Re plus S converts P t i n t o a c a t a l y s t s e l e c t i v e f o r mild hydrogenation and argued t h a t
sulfur-covered rhenium atoms divide t h e surface of the platinum metal i n t o ensembles of contiguous P t atoms.
Other evidence (11) t h a t P t and Re a r e in
intimate contact within bimetallic c l u s t e r s or ensembles has a l s o been provided by temperature programmed reduction. Many publications have reported c o n f l i c t i n g evidence concerning the degree of association of P t and Re in t h e reduced c a t a l y s t .
Alloying has been advo-
cated (5,10,12,13) as well as s e p a r a t e dispersion (4,14,15) of the two metals. More recently Bertolacini and P e l l e t (16) showed t h a t a physical mixture of P t and Re supported on separate p e l l e t s performed s i m i l a r l y t o p e l l e t s coimpregnated with P t and Re f o r reforming a naphtha t o higher-octane products. Similarly Short e t a1 (17) showed by X-ray absorption spectroscopy t h a t Re i s + not s i g n i f i c a n t l y associated with P t and l i k e l y in the 4 valence s t a t e in reduced bimetal 1 i c c a t a l y s t s .
We have a l s o performed experiments comparing MCH dehydrogenation over P t , PtRe and physical mixtures of supported P t and supported Re and observed behavior s i m i l a r t o t h a t reported by Bertolacini and P e l l e t ( 1 6 ) . Experimental. Catalyst compositions, surface a r e a s , metal dispersions and other experimental d e t a i l s a r e given elsewhere (18). All reactions were conducted a t atmospheric pressure and 50OoC. For the experiments in which
309
c o n v e r s i o n s were measured, t h e H2/MCH m o l a r r a t i o was 9.0, WHSV was 0.14 g MCH/g c a t - h r , t h e mole f r a c t i o n o f MCH was 1/19, and N2 was i n c l u d e d as an a d d i t i o n a l d i l u e n t i n amounts e a u i m o l a r w i t h H2.
The i s o t h e r m a l , heated r e a c t o r
c o n t a i n e d 3 g c a t a l y s t d i l u t e d w i t h 14 g o f 3-mm d i a m e t e r g l a s s beads.
Accel-
e r a t e d coke d e p o s i t i o n and h y d r o g e n a t i o n experiments were conducted u s i n g a DuPont Model 951 Thermogravimetric A n a l y z e r i n which f o u r preweighed c a t a l y s t p e l l e t s (90-95 mg t o t a l mass) were h e l d i n t h e c o n t i n u o u s l y weighed P t sample pan.
The s e n s i t i v i t y o f t h e g r a v i m e t r i c measurement was about 5 ug, t h e TGA
space v e l o c i t y was 3 g MCH/g c a t - h r , and t h e mole f r a c t i o n o f MCH i n t h e gas was a b o u t 0.05. Procedure. 500°C,
The c a t a l y s t charge was s l o w l y r a i s e d f r o m room temperature t o
h e l d f o r one hour a t t h a t t e m p e r a t u r e i n a f l o w ( 5 0 ml/min) o f d r y a i r ,
t h e n t r e a t e d i n f l o w i n g hydrogen ( 5 0 ml/min) f o r t h r e e hours a t 50OoC.
For t h e
f i r s t c y c l e ( c o k i n g and h y d r o g e n a t i o n ) t h e gas c o n t a i n i n g MCH, H2 and N2 i n a 1:9:9 m o l a r r a t i o was f e d t o t h e r e a c t o r ; a f t e r 20 hours on stream, t h e f e e d was s w i t c h e d t o p u r e hydrogen (80 m l / m i n ) f o r 5 hours h y d r o g e n a t i o n .
This
c y c l e was r e p e a t e d u s i n g v a r i o u s t i m e s o f exposure t o hydrocarbon and p u r e hydrogen.
For t h e TGA experiments t h e procedure was t h e same e x c e p t t h e f l o w
f o r each gas stream was 20 ml/min.
Conversions and p r o d u c t y i e l d s were measured
a t 500°C u s i n g b o t h P t and Pt-Re c a t a l y s t s .
The r e a c t i o n p r o d u c t s were sampled
and analyzed a t v a r i o u s t i m e s by gas chromatography. RESULTS AND D I S C U S S I O N F i g u r e s 1 and 2 p r o v i d e a comparison o f t h e b e h a v i o r o f t h e P t and t h e PtRe c a t a l y s t s f o r t h e c y c l i c on-stream o p e r a t i o n .
D u r i n g t h e f i r s t 20 hours o f
o p e r a t i o n w h i l e f e e d i n g MCH, t o l u e n e y i e l d r i s e s t o a maximum a f t e r s e v e r a l hours w i t h t h e P t c a t a l y s t and t h e n f a l l s .
W i t h t h e PtRe c a t a l y s t , however,
t h e r i s e i n t o l u e n e y i e l d i s e s s e n t i a l l y monotonic d u r i n g t h i s i n i t i a l 20 hour period.
A l t h o u g h benzene y i e l d s f a l l m o n o t o n i c a l l y f o r b o t h c a t a l y s t s t h e
decreases a r e less r a p i d f o r t h e PtRe c a t a l y s t .
The f a l l i n benzene y i e l d can
be a s s o c i a t e d i n p a r t w i t h r i s e i n t o l u e n e y i e l d ( p r o b a b l y by a decrease i n s i d e c h a i n c r a c k i n g ) b u t t h e t r a d e - o f f i s n o t complete.
D u r i n g t h i s f i r s t 20
hours t h e n e t y i e l d o f t o t a l a r o m a t i c s (benzene and t o l u e n e ) a l s o f a l l s o f f , b u t t h i s decrease i s s m a l l e r f o r t h e R e - c o n t a i n i n g c a t a l y s t .
This lower r a t i o
o f d e a c t i v a t i o n f o r t h e PtRe c a t a l y s t i s a l s o e v i d e n t w i t h r e s p e c t t o t o t a l c o n v e r s i o n o f MCH d u r i n g t h i s p e r i o d .
The f i r s t r e a c t i o n t o be d e a c t i v a t e d i s
s i d e - c h a i n c r a c k i n g as evidenced by t h e f a l l i n benzene and t h e r i s e i n t o l u e n e y i e l d as carbon d e p o s i t e d on t h e c a t a l y s t d u r i n g t h e f i r s t few hours.
Towards
t h e end o f t h e f i r s t 20 h r , coke d e p o s i t i o n begins t o r e t a r d t h e dehydrogenat i o n f u n c t i o n o f t h e P t b u t n o t t h e PtRe.
310
Fig. 1
Fig. 2
311 S w i t c h i n g o f f t h e hydrocarbon ( b u t m a i n t a i n i n g t h e f l o w o f H2 and t h e temp e r a t u r e ) from t = 2 0 hours t o t=25 hours a l s o has r e m a r k a b l y d i f f e r e n t e f f e c t s on t h e two c a t a l y s t s .
Most n o t a b l e i s t h e v e r y sharp decrease i n t o l u e n e
y i e l d w i t h t h e PtRe c a t a l y s t i n . c o n t r a s t t o a s l i g h t i n c r e a s e i n t o l u e n e y i e l d f o r the P t c a t a l y s t .
The c r a c k i n g a c t i v i t y ( t o C6-products) o f t h e PtRe c a t a -
l y s t i s a l s o g r e a t l y i n c r e a s e d by t h i s t r e a t m e n t i n H2 f o r 5 hours whereas t h a t for the P t c a t a l y s t i s affected f a r less.
I t appears t h a t t h e hydrogen
t r e a t m e n t r e s t o r e s c r a c k i n g s i t e s and d e a c t i v a t e s s i t e s f o r dehydroaromatizat i o n s i t e s on t h e PtRe c a t a l y s t , b u t a f f e c t s c r a c k i n g s i t e s l i t t l e and r e s t o r e s d e h y d r o a r o m a t i z a t i o n s i t e s on t h e P t c a t a l y s t . w i t h pure
A second t r e a t m e n t
H2 [from t = 3 2 hours t o t = 4 2 h o u r d produces v e r y s i m i l a r e f f e c t s .
A l t h o u g h t r e a t m e n t i n p u r e H 2 l o w e r s y i e l d and s e l e c t i v i t y f o r t o l u e n e and t o t a l a r o m a t i c s o v e r t h e PtRe c a t a l y s t , i t r a i s e s these f a v o r a b l e p r o p e r t i e s f o r the P t c a t a l y s t .
N e v e r t h e l e s s , t h i s t r e n d r e v e r s e s a f t e r t h e MCH i s
s w i t c h e d back onstream; a f t e r about o n l y f i v e hours o f exposure t o MCH t h e PtRe c a t a l y s t b e g i n s t o surpass t h e P t c a t a l y s t i n y i e l d and s e l e c t i v i t y f o r t o l u e n e and a r o m a t i c s . Less coke d e p o s i t s , and d e p o s i t s more s l o w l y , on t h e PtRe c a t a l y s t t h a n on the P t catalyst.
D u r i n g t h e f i r s t 20 hours, t h e TGA measurements showed an
i n i t i a l l a g i n w e i g h t g a i n due t o coke d e p o s i t i o n on t h e PtRe c a t a l y s t b u t n e a r l y no l a g f o r t h e P t c a t a l y s t .
D u r i n g exposure t o hydrogen a l o n e more
coke i s removed and removed more r a p i d l y , f r o m t h e PtRe c a t a l y s t .
Figure 3
shows t h e d i f f e r e n c e s between t h e two c a t a l y s t s f o r coke removal by hydrog e n a t i o n i n t h e 20-25 h r i n t e r v a l , and subsequent coke d e p o s i t i o n d u r i n g r e exposure t o MCH i n t h e 25-35 h r i n t e r v a l . y e t a n o t h e r c y c l e o f exposure t o p u r e H 2 ,
S i m i l a r r e s u l t s were o b t a i n e d f o r t h e n MCH.
I t may be t h a t t h e coke
a c t i v a t e s d e h y d r o a r o m a t i z a t i o n s i t e s and d e a c t i v a t e s c r a c k i n g s i t e s on t h e PtRe c a t a l y s t , b u t t h e e f f e c t s a r e d i f f e r e n t w i t h t h e P t c a t a l y s t .
It i s
w e l l known t h a t H2 can suppress coke f o r m a t i o n b u t t h e p r e s e n t r e s u l t s a l s o demonstrate t h a t t h e presence o f Re a l s o suppresses coking. F i g u r e 4 shows r e s u l t s f o r PtRe a f t e r t h e c a t a l y s t was p r e t r e a t e d w i t h 10 ppm o f H2S f o r 26 hours b e f o r e use.
I t i s seen t h a t t h i s p r e t r e a t m e n t
g r e a t l y i n c r e a s e s t h e t o l u e n e y i e l d and i t s p e r s i s t a n c e , l o w e r s t h e d e a c t i v a t i o n r a t e and suppresses s i d e c h a i n c r a c k i n g t o benzene.
F i g u r e 5 shows
t h a t p r e s u l p h i d i n g t h e P t c a t a l y s t does n o t improve i t s performance n e a r l y a s much as f o r PtRe shown i n F i g u r e 4.
Even so, t h e H2S p r e t r e a t m e n t does improve
t o l u e n e y i e l d s s l i g h t l y f o r t h e P t c a t a l y s t as r e v e a l e d by comparing F i g u r e s
5 and 1 .
I t appears t h a t p r e s u l f i d i n g w i t h H2S d e a c t i v a t e s s i t e s f o r b r e a k i n g
C-C bonds; s i m i l a r l y coke d e p o s i t i o n seems t o s l o w l y d e a c t i v a t e such c r a c k i n g
( F i g u r e s 1 and 2 ) b u t appears t o be f a r l e s s e f f e c t i v e t h a n p r e s u l p h i d i n g w i t h
312
4.0
I p
2.86%
I
3.5 ap
3 I
I-
z
?
5 3.0
0
w Y
0 0
2.5 20
23
TIME,HR
25
30
TIME,hR
Fig. 3
I
s
v)
8o TOLUENE
0
70-
0
\
3
0"
P4 -Re CAT S/Re = 0.75
-.
60305
PENZENE--
0
Fig. 4
I
I
4
8
I
I
12
16
--XI
IJ'
1
1
---x7-4-
I
I/
28
32
. I
I
44
TIMI . HR
313
H2S. B e r t o l a c i n i and P e l l e t ( 1 6 ) showed Re s e l e c t i v e l y removes coke p r e c u r s o r s and F i g u r e 3 shows t h a t Re i s a c a t a l y s t f o r removing coke by h y d r o g e n a t i o n . P r e s u l f i d i n g w i t h H2S appears t o d e a c t i v a t e s e l e c t i v e l y t h e h y d r o c r a c k i n g f u n c t i o n o f PtRe so t h a t s i d e c h a i n c r a c k i n g i s r e t a r d e d b u t t h e dehydroa r o m a t i z a t i o n i s u n a f f e c t e d , o r perhaps improved.
Surprisingly, treating
a l r e a d y coked PtRe w i t h H2S has d e l e t e r i o u s e f f e c t s on t o l u e n e y i e l d s ; perhaps t h e dehydrogenation s i t e s g e t d e a c t i v a t e d by S o n l y a f t e r t h e c r a c k i n g s i t e s a r e a l r e a d y b l o c k e d , e.g.,
by coke.
This behavior i s i n accord w i t h Blakely
and S o m o r j a i ’ s view ( 1 9 ) o f t h e P t atoms a t k i n k s i n s u r f a c e s t e p s as p r e f e r e n t i a l s i t e s f o r C - C bond b r e a k i n g ; such atoms o f l o w e r c o o r d i n a t i o n number would be more s u s c e p t i b l e t o bonding w i t h p o i s o n s l i k e S. Experiments w i t h 10 ppm o f t h i o p h e n e i n t h e MCH f e e d a l s o improved t h e performance of t h e P t and t h e PtRe c a t a l y s t o v e r t h e course o f about 40 hours. Compared t o H2S p r e t r e a t m e n t , however, improvements due t o t h i o p h e n e i n t h e f e e d became a p p a r e n t much l a t e r : 20 h r i n t h e case o f PtRe.
8-10 h o u r s i n t h e case o f P t and a l m o s t
U s i n g 75 ppm t h i o p h e n e i n t h e MCH f e e d gave s t i l l
g r e a t e r improvement w i t h t h e P t c a t a l y s t b u t i m p a i r e d t o l u e n e y i e l d w i t h t h e PtRe c a t a l y s t .
W i t h 75 ppm thiophene, P t gave b e t t e r performance t h a n when
p r e s u l f i d e d by H2S, b u t PtRe gave worse performance t h a n n o n - s u l f u r t r e a t e d PtRe.
I t i s n o t e w o r t h y t h a t when s u l f i d e d ( b y e i t h e r H2S o r t h i o p h e n e )
PtRe was s u b s e q u e n t l y exposed t o p u r e HZ a f t e r exposure t o MCH, t h e hydrog e n a t i o n d i d n o t cause r e g e n e r a t i o n o f h y d r o g e n o l y s i s a c t i v i t y as was c l e a r l y t h e case f o r PtRe c a t a l y s t s i n t h e absence o f S. h y d r o g e n o l y s i s s i t e s on PtRe i s s t r o n g l y bound.
I
0
0 -
Fig. 5 .
Apparently t h e S blocking
314
REFERENCES 1 H.E. Kluksdahl, U.S. P a t e n t 3,415,737 ( 1 9 6 8 ) . 2 R . L . J a c o b s o n , H . E . Kluksdahl, C.S. McCoy and R.W. D a v i s , Proc. Am. P e t r o l . I n s t . , p. 504 ( 1 9 6 9 ) . 3 F.G. C i a p e t t a and D.M. Wallace, C a t a l . Revs. 5 ( 1 9 7 1 ) 6 7 . 4 M . F . L . Johnson, C a t a l . 39(1975)487. 5 N.W. Webb, 3. C a t a l . 39(1975)485. 6 M . F . L . Johnson and V . M . LeRoy, J . C a t a l . 3 5 ( 1 9 7 4 ) . 7 K . H . Ludlurn, and R . P . E i s c h e n s , p r e p r i n t Div. o f Petroleum Chem., Arner. Chern. SOC. 21(1976)375. 8 J.C. d e J o n g s t e , V. Ponec and F.G. G a u l t , J . C a t a l . 63(1980)395. 9 F.M. Dautzenberg, J.N. H e l l e , P. Biloen and W . M . H . S a c h t l e r , J. C a t a l . 63(1980)119. 1 0 P . B i l o e n , J.M. H e l l e , H.Verbeek, F.M. D a u t z e n b e r t and W . M . H . S a c h t l e r , 63(1980)112. 11 N. Wagstaff and R. Prins, J . C a t a l . 59(1979)434. 12 J . F r e e l , R e p r i n t s Div. o f P e t r o l . Chern. p. 11 ACS D a l l a s Meeting, A p r i l 8, 1973. 1 3 C . B o l i v a r , H. C h a r c o s s e t , R. F r e t y , M. Primet, t. Tourneyon, C . B e t i z e a u , G. L e c l e r q and R. Maurel, J . C a t a l . 39(1975)249; 45( 1976)163; 45(1576)179. 1 4 B . D . McNicol, J . Catal 46(1977)438. 1 5 J.B. Peri, J . C a t a l . 52(1978)144. 1 6 R.J. B e r t o l a c i n i and R.J. P e l l e t , Proc. I n t l . Symp. on C a t a l y s t D e a c t i v a t i o n , Vol. 6 , E l s e v i e r , 1980, p. 73. 1 7 D . R . S h o r t , S.M. Khalid, J.R. Katzer and F1.J. K e l l e y , s u b m i t t e d f o r publication t o J. Catal. 1 8 P. B u u , M.S. D i s s e r t a t i o n , U n i v e r s i t y of C o n n e c t i c u t , 1980. 19 D.W. B l a k e l y and S o m o r j a i , J . C a t a l . 42(1976)181.
307
0 1982 Elsevier Scientific Publishing Company, Amsterdam -Printed in The Netherlands
DYNAMIC ACTIVATION, DEACTIVATION, AND C O K I N G ON PT AND PTRE CATALYSTS FOR
DEHYDROGENATION OF METHYLCYCLOHEXANE (MCH) R o b e r t W. C o u g h l i n , Koel Kawakami, Akran Hasan and Paul Buu Department o f Chemical E n g i n e e r i n g , U n i v e r s i t y o f C o n n e c t i c u t , S t o r r s , CT 06268 (USA)
RESUNE Le depBt de coke d e s a c t i v e l ’ h y d r o g e n o l y s e dans une p r e m i e r e etape, p u i s l a deshydrogenation du IICH. L ’ a d d i t i o n de Re c a t a l y s e 1 ’ 6 l i m i n a t i o n du coke p a r hydrogPnation. Le t r a i t e m e n t p a r ti2 du PtRe non s u l f u r 6 e n l @ v e p l u s de coke e t
a c e q u i e s t observe avec P t . La p r e s u l f u r a t i o n p a r H2S ou 10 ppm de t h i o p h e n e dans l e HCH a m e l i o r e n t l e rendement des
restaure l’hydrogenolyse, c o n t r a i r e n e n t
2 c a t a l y s e u r s au t o l u e n e . Mais 75 ppn de t h i o p h e n e d i m i n u e n t l e rendement de PtRe.
ABSTRACT Coke d e p o s i t i o n f i r s t d e a c t i v a t e s h y d r o g e n o l y s i s r e a c t i o n s and l a t e r dehydrog e n a t i o n o f MCH.
Added Re c a t a l y z e s coke removal by h y d r o g e n a t i o n .
Hydroqen
t r e a t m e n t o f u n s u l f i d e d PtRe c a t a l y s t s removes more coke and r e s t o r e s c o n s i d e r a b l e hydrogenolysis a c t i v i t y i n c o n t r a s t w i t h P t c a t a l y s t s .
Presulfiding with
H2S improves t o l u e n e y i e l d f o r e i t h e r c a t a l y s t , b u t e s p e c i a l l y PtRe.
Thiophene
( 1 0 ppm) i n t h e MCH f e e d improved t o l u e n e y i e l d w i t h e i t h e r c a t a l y s t , whereas 75 ppm improved t h e P t c a t a l y s t b u t l o w e r e d t o l u e n e y i e l d f o r t h e PtRe c a t a l y s t . INTRODUCTION Because dehydrogenation o f naphthenes t o a r o m a t i c s o v e r P t c a t a l y s t s i s r a p i d and o f g r e a t economic importance f o r i n c r e a s i n g t h e o c t a n e v a l u e s o f naphthas, we have i n v e s t i g a t e d t h i s r e a c t i o n f o r m e t h y l c y c l o h e x a n e as a model compound.
Commercial r e f o r m i n g c a t a l y s t s were e i t h e r m o n o m e t a l l i c (0.35% P t )
o r b i m e t a l l i c (0.32%, 0.325% Re) s u p p o r t e d on c h l o r i d e d y-alumina.
Only a f t e r
t h e c a t a l y s t has been on stream s e v e r a l hours, d u r i n g which a d e p o s i t o f hydrocarbonaceous r e s i n o r coke accumulates, does t h e e f f e c t o f t h e Re promoter become e v i d e n t ( 1 , 2 ) . accumulates
-
R e - c o n t a i n i n g c a t a l y s t s d e a c t i v a t e l e s s r a p i d l y as coke
t h e s e b i m e t a l l i c c a t a l y s t s can m a i n t a i n r e f o r m a t e p r o d u c t octane
308
number a t a s p e c i f i c desired level over the course of time-on-stream without the necessity of increasing temperature or space time as severely as required f o r monometallic P t c a t a l y s t s ( 3 ) . I t has been suggested ( 4 , 5 ) t h a t the a c i d i c alumina s u p p o r t of such catal y s t s may be modified by the Re, and shown ( 6 ) t h a t the Re i s present as an oxide such as Re02 associated with the surface of the alumina. Ludlum and Eischens ( 7 ) conducted an i n f r a r e d study of differences in the chemical natures of such d e p o s i t s ; they observed t h a t some carbon atoms of such deposits a r e bonded t o oxygen ( a s indicated by carboxylate bands a t 1570 and 1450 cm- 1 ) b u t t h a t the presence of Re decreases the f r a c t i o n of such carboxylate in t h e deposits. More recently much a t t e n t i o n has focused on t h e p o s s i b i l i t y of a l l o y format i o n between P t and Re a s well a s between P t and several other second-metal promoters. For example, alloying P t with Cu ( 8 ) o r with Sn ( 9 ) produces s e l e c t i v i t y changes dubbed "ensemble e f f e c t s " and viewed as s h i f t s from m u l t i s i t e mechanisms on c o l l e c t i o n s of contiguous P t atoms in P t - r i c h a l l o y s toward s i n g l e - s i t e mechanisms on P t d i l u t e d with the promoting metal.
In the case of
PtRe c a t a l y s t s i t has a l s o been shown (10) t h a t the combined action of Re plus S converts P t i n t o a c a t a l y s t s e l e c t i v e f o r mild hydrogenation and argued t h a t
sulfur-covered rhenium atoms divide t h e surface of the platinum metal i n t o ensembles of contiguous P t atoms.
Other evidence (11) t h a t P t and Re a r e in
intimate contact within bimetallic c l u s t e r s or ensembles has a l s o been provided by temperature programmed reduction. Many publications have reported c o n f l i c t i n g evidence concerning the degree of association of P t and Re in t h e reduced c a t a l y s t .
Alloying has been advo-
cated (5,10,12,13) as well as s e p a r a t e dispersion (4,14,15) of the two metals. More recently Bertolacini and P e l l e t (16) showed t h a t a physical mixture of P t and Re supported on separate p e l l e t s performed s i m i l a r l y t o p e l l e t s coimpregnated with P t and Re f o r reforming a naphtha t o higher-octane products. Similarly Short e t a1 (17) showed by X-ray absorption spectroscopy t h a t Re i s + not s i g n i f i c a n t l y associated with P t and l i k e l y in the 4 valence s t a t e in reduced bimetal 1 i c c a t a l y s t s .
We have a l s o performed experiments comparing MCH dehydrogenation over P t , PtRe and physical mixtures of supported P t and supported Re and observed behavior s i m i l a r t o t h a t reported by Bertolacini and P e l l e t ( 1 6 ) . Experimental. Catalyst compositions, surface a r e a s , metal dispersions and other experimental d e t a i l s a r e given elsewhere (18). All reactions were conducted a t atmospheric pressure and 50OoC. For the experiments in which
309
c o n v e r s i o n s were measured, t h e H2/MCH m o l a r r a t i o was 9.0, WHSV was 0.14 g MCH/g c a t - h r , t h e mole f r a c t i o n o f MCH was 1/19, and N2 was i n c l u d e d as an a d d i t i o n a l d i l u e n t i n amounts e a u i m o l a r w i t h H2.
The i s o t h e r m a l , heated r e a c t o r
c o n t a i n e d 3 g c a t a l y s t d i l u t e d w i t h 14 g o f 3-mm d i a m e t e r g l a s s beads.
Accel-
e r a t e d coke d e p o s i t i o n and h y d r o g e n a t i o n experiments were conducted u s i n g a DuPont Model 951 Thermogravimetric A n a l y z e r i n which f o u r preweighed c a t a l y s t p e l l e t s (90-95 mg t o t a l mass) were h e l d i n t h e c o n t i n u o u s l y weighed P t sample pan.
The s e n s i t i v i t y o f t h e g r a v i m e t r i c measurement was about 5 ug, t h e TGA
space v e l o c i t y was 3 g MCH/g c a t - h r , and t h e mole f r a c t i o n o f MCH i n t h e gas was a b o u t 0.05. Procedure. 500°C,
The c a t a l y s t charge was s l o w l y r a i s e d f r o m room temperature t o
h e l d f o r one hour a t t h a t t e m p e r a t u r e i n a f l o w ( 5 0 ml/min) o f d r y a i r ,
t h e n t r e a t e d i n f l o w i n g hydrogen ( 5 0 ml/min) f o r t h r e e hours a t 50OoC.
For t h e
f i r s t c y c l e ( c o k i n g and h y d r o g e n a t i o n ) t h e gas c o n t a i n i n g MCH, H2 and N2 i n a 1:9:9 m o l a r r a t i o was f e d t o t h e r e a c t o r ; a f t e r 20 hours on stream, t h e f e e d was s w i t c h e d t o p u r e hydrogen (80 m l / m i n ) f o r 5 hours h y d r o g e n a t i o n .
This
c y c l e was r e p e a t e d u s i n g v a r i o u s t i m e s o f exposure t o hydrocarbon and p u r e hydrogen.
For t h e TGA experiments t h e procedure was t h e same e x c e p t t h e f l o w
f o r each gas stream was 20 ml/min.
Conversions and p r o d u c t y i e l d s were measured
a t 500°C u s i n g b o t h P t and Pt-Re c a t a l y s t s .
The r e a c t i o n p r o d u c t s were sampled
and analyzed a t v a r i o u s t i m e s by gas chromatography. RESULTS AND D I S C U S S I O N F i g u r e s 1 and 2 p r o v i d e a comparison o f t h e b e h a v i o r o f t h e P t and t h e PtRe c a t a l y s t s f o r t h e c y c l i c on-stream o p e r a t i o n .
D u r i n g t h e f i r s t 20 hours o f
o p e r a t i o n w h i l e f e e d i n g MCH, t o l u e n e y i e l d r i s e s t o a maximum a f t e r s e v e r a l hours w i t h t h e P t c a t a l y s t and t h e n f a l l s .
W i t h t h e PtRe c a t a l y s t , however,
t h e r i s e i n t o l u e n e y i e l d i s e s s e n t i a l l y monotonic d u r i n g t h i s i n i t i a l 20 hour period.
A l t h o u g h benzene y i e l d s f a l l m o n o t o n i c a l l y f o r b o t h c a t a l y s t s t h e
decreases a r e less r a p i d f o r t h e PtRe c a t a l y s t .
The f a l l i n benzene y i e l d can
be a s s o c i a t e d i n p a r t w i t h r i s e i n t o l u e n e y i e l d ( p r o b a b l y by a decrease i n s i d e c h a i n c r a c k i n g ) b u t t h e t r a d e - o f f i s n o t complete.
D u r i n g t h i s f i r s t 20
hours t h e n e t y i e l d o f t o t a l a r o m a t i c s (benzene and t o l u e n e ) a l s o f a l l s o f f , b u t t h i s decrease i s s m a l l e r f o r t h e R e - c o n t a i n i n g c a t a l y s t .
This lower r a t i o
o f d e a c t i v a t i o n f o r t h e PtRe c a t a l y s t i s a l s o e v i d e n t w i t h r e s p e c t t o t o t a l c o n v e r s i o n o f MCH d u r i n g t h i s p e r i o d .
The f i r s t r e a c t i o n t o be d e a c t i v a t e d i s
s i d e - c h a i n c r a c k i n g as evidenced by t h e f a l l i n benzene and t h e r i s e i n t o l u e n e y i e l d as carbon d e p o s i t e d on t h e c a t a l y s t d u r i n g t h e f i r s t few hours.
Towards
t h e end o f t h e f i r s t 20 h r , coke d e p o s i t i o n begins t o r e t a r d t h e dehydrogenat i o n f u n c t i o n o f t h e P t b u t n o t t h e PtRe.
310
Fig. 1
Fig. 2
311 S w i t c h i n g o f f t h e hydrocarbon ( b u t m a i n t a i n i n g t h e f l o w o f H2 and t h e temp e r a t u r e ) from t = 2 0 hours t o t=25 hours a l s o has r e m a r k a b l y d i f f e r e n t e f f e c t s on t h e two c a t a l y s t s .
Most n o t a b l e i s t h e v e r y sharp decrease i n t o l u e n e
y i e l d w i t h t h e PtRe c a t a l y s t i n . c o n t r a s t t o a s l i g h t i n c r e a s e i n t o l u e n e y i e l d f o r the P t c a t a l y s t .
The c r a c k i n g a c t i v i t y ( t o C6-products) o f t h e PtRe c a t a -
l y s t i s a l s o g r e a t l y i n c r e a s e d by t h i s t r e a t m e n t i n H2 f o r 5 hours whereas t h a t for the P t c a t a l y s t i s affected f a r less.
I t appears t h a t t h e hydrogen
t r e a t m e n t r e s t o r e s c r a c k i n g s i t e s and d e a c t i v a t e s s i t e s f o r dehydroaromatizat i o n s i t e s on t h e PtRe c a t a l y s t , b u t a f f e c t s c r a c k i n g s i t e s l i t t l e and r e s t o r e s d e h y d r o a r o m a t i z a t i o n s i t e s on t h e P t c a t a l y s t . w i t h pure
A second t r e a t m e n t
H2 [from t = 3 2 hours t o t = 4 2 h o u r d produces v e r y s i m i l a r e f f e c t s .
A l t h o u g h t r e a t m e n t i n p u r e H 2 l o w e r s y i e l d and s e l e c t i v i t y f o r t o l u e n e and t o t a l a r o m a t i c s o v e r t h e PtRe c a t a l y s t , i t r a i s e s these f a v o r a b l e p r o p e r t i e s f o r the P t c a t a l y s t .
N e v e r t h e l e s s , t h i s t r e n d r e v e r s e s a f t e r t h e MCH i s
s w i t c h e d back onstream; a f t e r about o n l y f i v e hours o f exposure t o MCH t h e PtRe c a t a l y s t b e g i n s t o surpass t h e P t c a t a l y s t i n y i e l d and s e l e c t i v i t y f o r t o l u e n e and a r o m a t i c s . Less coke d e p o s i t s , and d e p o s i t s more s l o w l y , on t h e PtRe c a t a l y s t t h a n on the P t catalyst.
D u r i n g t h e f i r s t 20 hours, t h e TGA measurements showed an
i n i t i a l l a g i n w e i g h t g a i n due t o coke d e p o s i t i o n on t h e PtRe c a t a l y s t b u t n e a r l y no l a g f o r t h e P t c a t a l y s t .
D u r i n g exposure t o hydrogen a l o n e more
coke i s removed and removed more r a p i d l y , f r o m t h e PtRe c a t a l y s t .
Figure 3
shows t h e d i f f e r e n c e s between t h e two c a t a l y s t s f o r coke removal by hydrog e n a t i o n i n t h e 20-25 h r i n t e r v a l , and subsequent coke d e p o s i t i o n d u r i n g r e exposure t o MCH i n t h e 25-35 h r i n t e r v a l . y e t a n o t h e r c y c l e o f exposure t o p u r e H 2 ,
S i m i l a r r e s u l t s were o b t a i n e d f o r t h e n MCH.
I t may be t h a t t h e coke
a c t i v a t e s d e h y d r o a r o m a t i z a t i o n s i t e s and d e a c t i v a t e s c r a c k i n g s i t e s on t h e PtRe c a t a l y s t , b u t t h e e f f e c t s a r e d i f f e r e n t w i t h t h e P t c a t a l y s t .
It i s
w e l l known t h a t H2 can suppress coke f o r m a t i o n b u t t h e p r e s e n t r e s u l t s a l s o demonstrate t h a t t h e presence o f Re a l s o suppresses coking. F i g u r e 4 shows r e s u l t s f o r PtRe a f t e r t h e c a t a l y s t was p r e t r e a t e d w i t h 10 ppm o f H2S f o r 26 hours b e f o r e use.
I t i s seen t h a t t h i s p r e t r e a t m e n t
g r e a t l y i n c r e a s e s t h e t o l u e n e y i e l d and i t s p e r s i s t a n c e , l o w e r s t h e d e a c t i v a t i o n r a t e and suppresses s i d e c h a i n c r a c k i n g t o benzene.
F i g u r e 5 shows
t h a t p r e s u l p h i d i n g t h e P t c a t a l y s t does n o t improve i t s performance n e a r l y a s much as f o r PtRe shown i n F i g u r e 4.
Even so, t h e H2S p r e t r e a t m e n t does improve
t o l u e n e y i e l d s s l i g h t l y f o r t h e P t c a t a l y s t as r e v e a l e d by comparing F i g u r e s
5 and 1 .
I t appears t h a t p r e s u l f i d i n g w i t h H2S d e a c t i v a t e s s i t e s f o r b r e a k i n g
C-C bonds; s i m i l a r l y coke d e p o s i t i o n seems t o s l o w l y d e a c t i v a t e such c r a c k i n g
( F i g u r e s 1 and 2 ) b u t appears t o be f a r l e s s e f f e c t i v e t h a n p r e s u l p h i d i n g w i t h
312
4.0
I p
2.86%
I
3.5 ap
3 I
I-
z
?
5 3.0
0
w Y
0 0
2.5 20
23
TIME,HR
25
30
TIME,hR
Fig. 3
I
s
v)
8o TOLUENE
0
70-
0
\
3
0"
P4 -Re CAT S/Re = 0.75
-.
60305
PENZENE--
0
Fig. 4
I
I
4
8
I
I
12
16
--XI
IJ'
1
1
---x7-4-
I
I/
28
32
. I
I
44
TIMI . HR
313
H2S. B e r t o l a c i n i and P e l l e t ( 1 6 ) showed Re s e l e c t i v e l y removes coke p r e c u r s o r s and F i g u r e 3 shows t h a t Re i s a c a t a l y s t f o r removing coke by h y d r o g e n a t i o n . P r e s u l f i d i n g w i t h H2S appears t o d e a c t i v a t e s e l e c t i v e l y t h e h y d r o c r a c k i n g f u n c t i o n o f PtRe so t h a t s i d e c h a i n c r a c k i n g i s r e t a r d e d b u t t h e dehydroa r o m a t i z a t i o n i s u n a f f e c t e d , o r perhaps improved.
Surprisingly, treating
a l r e a d y coked PtRe w i t h H2S has d e l e t e r i o u s e f f e c t s on t o l u e n e y i e l d s ; perhaps t h e dehydrogenation s i t e s g e t d e a c t i v a t e d by S o n l y a f t e r t h e c r a c k i n g s i t e s a r e a l r e a d y b l o c k e d , e.g.,
by coke.
This behavior i s i n accord w i t h Blakely
and S o m o r j a i ’ s view ( 1 9 ) o f t h e P t atoms a t k i n k s i n s u r f a c e s t e p s as p r e f e r e n t i a l s i t e s f o r C - C bond b r e a k i n g ; such atoms o f l o w e r c o o r d i n a t i o n number would be more s u s c e p t i b l e t o bonding w i t h p o i s o n s l i k e S. Experiments w i t h 10 ppm o f t h i o p h e n e i n t h e MCH f e e d a l s o improved t h e performance of t h e P t and t h e PtRe c a t a l y s t o v e r t h e course o f about 40 hours. Compared t o H2S p r e t r e a t m e n t , however, improvements due t o t h i o p h e n e i n t h e f e e d became a p p a r e n t much l a t e r : 20 h r i n t h e case o f PtRe.
8-10 h o u r s i n t h e case o f P t and a l m o s t
U s i n g 75 ppm t h i o p h e n e i n t h e MCH f e e d gave s t i l l
g r e a t e r improvement w i t h t h e P t c a t a l y s t b u t i m p a i r e d t o l u e n e y i e l d w i t h t h e PtRe c a t a l y s t .
W i t h 75 ppm thiophene, P t gave b e t t e r performance t h a n when
p r e s u l f i d e d by H2S, b u t PtRe gave worse performance t h a n n o n - s u l f u r t r e a t e d PtRe.
I t i s n o t e w o r t h y t h a t when s u l f i d e d ( b y e i t h e r H2S o r t h i o p h e n e )
PtRe was s u b s e q u e n t l y exposed t o p u r e HZ a f t e r exposure t o MCH, t h e hydrog e n a t i o n d i d n o t cause r e g e n e r a t i o n o f h y d r o g e n o l y s i s a c t i v i t y as was c l e a r l y t h e case f o r PtRe c a t a l y s t s i n t h e absence o f S. h y d r o g e n o l y s i s s i t e s on PtRe i s s t r o n g l y bound.
I
0
0 -
Fig. 5 .
Apparently t h e S blocking
314
REFERENCES 1 H.E. Kluksdahl, U.S. P a t e n t 3,415,737 ( 1 9 6 8 ) . 2 R . L . J a c o b s o n , H . E . Kluksdahl, C.S. McCoy and R.W. D a v i s , Proc. Am. P e t r o l . I n s t . , p. 504 ( 1 9 6 9 ) . 3 F.G. C i a p e t t a and D.M. Wallace, C a t a l . Revs. 5 ( 1 9 7 1 ) 6 7 . 4 M . F . L . Johnson, C a t a l . 39(1975)487. 5 N.W. Webb, 3. C a t a l . 39(1975)485. 6 M . F . L . Johnson and V . M . LeRoy, J . C a t a l . 3 5 ( 1 9 7 4 ) . 7 K . H . Ludlurn, and R . P . E i s c h e n s , p r e p r i n t Div. o f Petroleum Chem., Arner. Chern. SOC. 21(1976)375. 8 J.C. d e J o n g s t e , V. Ponec and F.G. G a u l t , J . C a t a l . 63(1980)395. 9 F.M. Dautzenberg, J.N. H e l l e , P. Biloen and W . M . H . S a c h t l e r , J. C a t a l . 63(1980)119. 1 0 P . B i l o e n , J.M. H e l l e , H.Verbeek, F.M. D a u t z e n b e r t and W . M . H . S a c h t l e r , 63(1980)112. 11 N. Wagstaff and R. Prins, J . C a t a l . 59(1979)434. 12 J . F r e e l , R e p r i n t s Div. o f P e t r o l . Chern. p. 11 ACS D a l l a s Meeting, A p r i l 8, 1973. 1 3 C . B o l i v a r , H. C h a r c o s s e t , R. F r e t y , M. Primet, t. Tourneyon, C . B e t i z e a u , G. L e c l e r q and R. Maurel, J . C a t a l . 39(1975)249; 45( 1976)163; 45(1576)179. 1 4 B . D . McNicol, J . Catal 46(1977)438. 1 5 J.B. Peri, J . C a t a l . 52(1978)144. 1 6 R.J. B e r t o l a c i n i and R.J. P e l l e t , Proc. I n t l . Symp. on C a t a l y s t D e a c t i v a t i o n , Vol. 6 , E l s e v i e r , 1980, p. 73. 1 7 D . R . S h o r t , S.M. Khalid, J.R. Katzer and F1.J. K e l l e y , s u b m i t t e d f o r publication t o J. Catal. 1 8 P. B u u , M.S. D i s s e r t a t i o n , U n i v e r s i t y of C o n n e c t i c u t , 1980. 19 D.W. B l a k e l y and S o m o r j a i , J . C a t a l . 42(1976)181.