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Toluene Disproportionation and Coke Formation on Mordenites Effect of Catalyst Modifications and of Operating Conditions
B. Delmon and G.F. Froment (Editors), Catalyst Deactivation 0 1980 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
79
TOLIJENE DISPROPOR'I'IONATION AND COKE FORMATION ON MORDENITES
EFFECT OF CATALYST MODIFICATIONS AND OF OPERATING CONDITIONS
N.S.
GNEP
1
,
M.L.
MARTIN d e ARMANDOl, C. MARCILLY
2
,
B.H.
HA
3+
and M.
GUISNET
1++
1 - L a b o r a t o i r e d e Chimie 7 - Groupe d e Recherches s u r l a C a t a l y s e e n Chimie Organique (ERA CNRS 371)
- 4 0 , avenue du R e c t e u r P i n e a u - 8 6 0 2 2 POITIERS
CEDEX
FRANCE.
2 - S e r v i c e d e C a t a l y s e - I n s t i t u t FranGais du P e t r o l e - RUEIL MALMAISON. 3 - L a b o r a t o i r e d e C a t a l y s e Organique - L.A.
CNRS 231
-
VILLEURBANNE.
ABSTRACT Improving c a t a l y t i c s t a b i l i t y o f m o r d e n i t e h a s been a t t e m p t e d by u s i n g t h r e e t y p e s of t r e a t m e n t : d e a l u m i n a t i o n , w e t a i r h e a t i n g , n i c k e l - i o n - e x c h a n g e .
The
r a t e s o f d i s p r o p o r t i o n a t i o n and c o k i n g on t h e s e samples have been compared under d i f f e r e n t o p e r a t i n g c o n d i t i o n s : low and h i g h hydrogen o r n i t r o g e n p r e s s u r e s . Under n i t r o g e n , m o r d e n i t e d e a c t i v a t i o n i s v e r y f a s t . Dealumination i n c r e a s e s i t s a c t i v i t y f o r d i s p r o p o r t i o n a t i o n and c o k i n g a s w e l l a s t h e d e a c t i v a t i o n r a t e , w h i l e
w e t a i r t r e a t m e n t d e c r e a s e s i t s c o k i n g a c t i v i t y . T h i s d e c r e a s e may b e c o n n e c t e d w i t h t h e e l i m i n a t i o n o f B r d n s t e d a c i d s i t e s . The c a t a l y t i c s t a b i l i t y o f a l l samples i s c o n s i d e r a b l y improved by o p e r a t i n g under h i g h hydrogen p r e s s u r e , t h e b e s t c a t a l y s t being t h e w e t a i r t r e a t e d nickel-exchanged mordenite. T h i s s t a b i l i z i n g e f f e c t
of hydrogen i s t h e r e s u l t o f c o k i n g i n h i b i t i o n and o f a b e t t e r d i s t r i b u t i o n o f c o k e i n t h e b u l k o f t h e c a t a l y s t p e l l e t t h a n under n i t r o g e n .
I t i s suggested t h a t
c o k i n g i n h i b i t i o n comes from t h e p r e s e n c e o n m o r d e n i t e o f s i t e s c a p a b l e o f a c t i v a t i n g m o l e c u l a r hydrogen.
INTRODUCTION Numerous s t u d i e s a r e d e v o t e d t o t h e t r a n s f o r m a t i o n s o f h y d r o c a r b o n s o n morden i t e s , and i n p a r t i c u l a r t o t h e h y d r o c r a c k i n g and h y d r o i s o m e r i z a t i o n of p a r a f f i n s , t h e t r a n s a l k y l a t i o n and i s o m e r i z a t i o n o f aromatics [ 1 , 2 ] . s e n t v a r i o u s advantages o v e r t h e widely used Y-zeolites
Indeed, mordenites pre: they
a r e t h e r m a l l y more
s t a b l e ; t h e y a r e a l s o more a c i d i c , and c o n s e q u e n t l y more a c t i v e [11 ; f i n a l l y t h e i r p o r o u s s t r u c t u r e g i v e s them a s h a p e - s e l e c t i v i t y w i t h r e g a r d t o c e r t a i n reacU n i v e r s i t y , S e o u l , Corea ++Hanayang To whom c o r r e s p o n d e n c e s h o u l d b e a d d r e s s e s . +
-,
..
80 t i o n s [3-51.
On t h e o t h e r hand, t h i s p o r o u s s t r u c t u r e makes them v e r y s e n s i t i v e
t o c o k i n g and t h e i r d e a c t i v a t i o n i s g e n e r a l l y v e r y f a s t [ I ] . The aim of t h e p r e s e n t work i s t o d e t e r m i n e how d i f f e r e n t t r e a t m e n t s and o p e r a t i n g c o n d i t i o n s i n f l u e n c e t h e c a t a l y t i c s t a b i l i t y and t h e c o k i n g o f m o r d e n i t e d u r i n g t h e d i s p r o p o r t i o n a t i o n o f t o l u e n e . The f o l l o w i n g treatment.5 were e x p e r i m e n t e d : cnemical dealumination, nickel-exchange,
a c t i v a t i o n i n d r y o r wet a i r . The r a t e s
o f t h e d i s p r o p o r t i o n a t i o n and c o k i n g r e a c t i o n s on t h e c a t a l y s t s t h u s t r e a t e d were compared under d i f f e r i n g o p e r a t i n g c o n d i t i o n s : low and h i g h p r e s s u r e s of hydroqen and n i trogen.
EXPERIMENTAL The m o r d e n i t e s have been d e s c r i b e d p r e v i o u s l y [ 61. T h e i r p r o p e r t i e s a r e r e p r o duced i n t a b l e 1. These m o r d e n i t e s were used under t h e forms i n reference [6], i.e.
(I)o r
air
of w e t a i r
(1)and (2) d e f i n e d
a f t e r a t r e a t m e n t a t 500°C f o r 15 h o u r s i n a f l o w o f d r y
(2). The
c r y s t a l l i n i t y of t h e s e c a t a l y s t s r e m a i n s h i g h a f t e r
both treatments. The s i l i c a - a l u m i n a
(LA3P from K e t j e n ) w a s p r e t r e a t e d a t 500°C f o r 15 h o u r s i n
d r y a i r flow b e f o r e use. The r e a c t i o n was s t u d i e d i n t h e p r e v i o u s l y d e s c r i b e d dynamic r e a c t o r s [ 7 , 8 ] a t 450'12 w i t h a hydrogen
( o r n i t r o g e n ) t o t o l u e n e m o l a r r a t i o e q u a l t o 4 . The r e a c t o r
p r o d u c t s were a n a l y z e d by G.L.C.
The p e r c e n t c o n v e r s i o n i s g e n e r a l l y l e s s t h a n 15 % .
The coke c o n t e n t o f t h e c a t a l y s t s was d e t e r m i n e d by t h e " C e n t r e d e M i c r o a n a l y s e du CNRS, V i l l e u r b a n n e , F r a n c e " . TABLE 1
Chemical c o m p o s i t i o n a n d p r e p a r a t i o n o f t h e m o r d e n i t e s Mordenites
SiO / A 1 2 0 3
wt
wt
% N i
Preparation
M
10
0.36
0
NH4 i o n - exchange of N a 900 Zeolon ( N o r t o n )
MD
18
0.15
0
dealumination of M with H C l
MNi
10
0.36
1.45
N i ion
- exchange o f
M
MDNi
18
0.15
0.5
N i ion
- exchange o f
MD
2
% Na
RESULTS D i s p r o p o r t i o n a t i o n o f t o l u e n e under a t m o s p h e r i c p r e s s u r e Under hydrogen a l l t h e m o r d e n i t e s p r e s e n t a h i g h i n i t i a l a c t i v i t y ( a l 0 i n t a b l e 2 ) r a n g i n g from 50 t o 120,10-4 mole.h-l.g-'., t h a t o f t h e most a c t i v e f l u o r i n a t e d alumina
i.e.
from 5 t o 10 t i m e s h i g h e r t h a n
[91. The t r e a t m e n t of t h e m o r d e n i t e ,
h a s l i t t l e e f f e c t o n i t s i n i t i a l a c t i v i t y , e x c e p t f o r i t s chemscal d e a l u m i n a t i o n which m u l t i p l i e s t h e i n i t i a l a c t i v i t y by a f a c t o r o f 1 . 3 t o 2. The d e a c t i v a t i o n
81 of t i i f
m o r d e n i t e s i s v e r y f a s t ( f i g u r e 1) and a f t e r 7 hours o f r e a c t i o n , the a c t i -
v j . t y is l e s s t h a n 5
‘t2
of t h e i n i t i a l a c t i v i t y (table 2 ) .
121 I
100
3
I
75
i
I
i’. ar
ri
2 I
2
50
25
Fig. 1 . Disproportionation of toluene on mordenites a t 4 5 O o C ; pH2 = 0.8 b a r , p toluene = 0 . 2 b a r . A c t i v i t y (a,) a g a i n s t t i m e on stream ( t )
.
The expression (1)proposed by Voorhies
10 1 accounts f o r t h e e v o l u t i o n of t h e d i s -
p r o p o r t i o n a t i o n a c t i v i t y a of mordenites with t h e r e a c t i o n time t
:
The value f o r n and consequently t h e r a t e of d e a c t i v a t i o n , i n c r e a s e s with dealumin a t i o n , b u t decreases s l i g h t l y with nickel-exchange and s t r o n g l y with wet a i r treatment (table 2 ) . The amount of t o l u e n e d i s p r o p o r t i o n a t e d during each run w a s c a l c u l a t e d by i n t e g r a t i o n of t h e a c t i v i t y curves i n f i g u r e 1. This q u a n t i t y of d i s p r o p r t i o n a t e d t o l u e n e mD, i n c r e a s e s with dealumination and with wet a i r t r e a t ment. The amount of toluene transformed i n t o coke during each experiment, mk, v a r i e s by a f a c t o r of 6 according t o t h e treatment of t h e mordenite Aealumination and d e c r e a s e s with nickel-exchange. nickel-exchange and wet a i r treatment ( t a b l e 2).
:
i t i n c r e a s e s with
The r a t i o mk/mD decreases
W i t h
82 The t r a n s f o r m a t i o n o f t o l u e n e h a s been i n v e s t i g a t e d under n i t r o g e n on t h r e e d e a l u m i n a t e d m o r d e n i t e s MDII), _. MD(3) and MDNi(3), and on t h e m o r d e n i t e M(1). With t h e m o r d e n i t e s t r e a t e d under d r y a i r M(1) and MD(L), t h e r e s u l t s o b t a i n e d under n i t r o g e n a r e s i m i l a r t o t h o s e o b t a i n e d under hydrogen ( t a b l e 2 ) . On t h e o t h e r hand, t h e d e a c t i v a t i o n o f t h e m o r d e n i t e s t r e a t e d i n w e t a i r i s much f a s t e r under n i t r o i j e n t h a n under hydrogen, and t h e s e m o r d e n i t e s l o s t a l l t h e i r a c t i v i t y a f t e r t h r e e h o u r s o f work. However, t h e r a t i o o f t h e amount o f t o l u e n e t r a n s f o r m e d i n t o coke t o t h a t o f d i s p r o p o r t i o n a t e d t o l u e n e i s t h e same under n i t r o g e n a s under hydrogen f o r MD(2), w h i l e f o r M D N i ( i ) , t h i s r a t i o i s t h r e e t i m e s h i g h e r under n i t r o g e n t h a n under hydrogen ( t a b l e 2 ) . TABLE 2
T r a n s f o r m a t i o n o f t o l u e n e a t 45OoC, p t o l u e n e = 0.2 b a r ; pH o r pN2 = 0 . 8 b a r . 2 a F ( 10-4mole.h-1 . g - l . ) : f i n a l d i s p r o p o r t i o n a t i o n a c t i v i t y o f m o r d e n i t e ( t i m e between b r a c k e t s ) ; a10 ( 10-4mole.h-1 . g - l ) : d i s p r o p o r t i o n a t i o n a c t i v i t y o f m o r d e n i t e a f t e r 10 m i n u t e s ; n : d e a c t i v a t i o n c o e f f i c i e n t f o m V o o r h i e s [ l o ] ; m ( 9 . g - l . of morden i t e ) : d i s p r o p o r t i o n a t e d t o l u e n e ; mk (g.g-'. of m o r d e n i t e ) : t o f u e n e t r a n s f o r m e d i n t o coke d u r i n g t h e e x p e r i m e n t s .
n
a10
Catalyst
H M (1)
2
N2
aF'a 10
m
D
3 10 mk
3 10 mk/mD
63
1.5
O.Ol(120mn)
0.25
18
70
50
2.0
0.005(120mn)
0.20
20
100 25
MNi(L)
H2
79
1.1
0.015(400mn)
0.45
11
MNi(3)
H2
57
0.7
0.05(400mn)
0.7
7
10
H
82
2
0.00 (200mn)
0.55
45
80
MD
(1)
2
N2 MD
(3)
H2 N2
H MDNi
(2)
2
N,
130
3.3
O.OO(200mn)
0.75
45
65
117
1.1
0.05(400mn)
1.9
13
7
12
10
115
2.5
O.OO(200mn)
1.2
121
1.1
0.04(400mn)
1 .o
95
2.5
O.OO(200mn)
0.4
8.5 11
8.5 25
D i s p r o p o r t i o n a t i o n o f t o l u e n e under h i g h p r e s s u r e T h i s r e a c t i o n w a s performed under hydrogen and t o l u e n e p r e s s u r e s o f 12 b a r and 3 b a r r e s p e c t i v e l y . F i g u r e 2 shows how t h e d i s p r o p o r t i o n a t i o n a c t i v i t y o f m o r d e n i t e s e v o l v e s w i t h r e a c t i o n t i m e . T h i s a c t i v i t y i s i n i t i a l l y low, t h e n it i n c r e a s e s r a p i d l y d u r i n g t h e 2 o r 3 f i r s t h o u r s o f work and f i n a l l y d e c r e a s e s s l o w l y . The v a l u e f o r t h e a c t i v i t i e s o b t a i n e d by e x t r a p o l a t i n g t h e d e c r e a s i n g l i n e a r p a r t o f t h e c u r v e s
a t z e r o t i m e , ao , ( f i g u r e 2 ) v a r i e s from 15 t o 100 a c c o r d i n g t o t h e m o r d e n i t e t r e a t m e n t . The d e a l u m i n a t i o n c a u s e s a s t r o n g d e c r e a s e i n a o , a t l e a s t f o r t h e mord e n i t e s t r e a t e d under d r y a i r , w h i l e t h e n i c k e l - e x c h a n g e and t h e w e t a i r t r e a t m e n t have a n o p p o s i t e e f f e c t ( t a b l e 3 ) . A f t e r 9 h o u r s o f work, t h e a c t i v i t y o f t h e
83 Illordcnitfs remains general~lyhigher t h a n 70
h
of the a.
activity. The amounts of
dis~)roportioiiat~~d toluene, mu, are similar for all the mordenites treated under wet air and for MNi M(1)
(2.) ;
they arc2 markedly higher than the amounts obtained on
and M D ( I ) . The amounts of toluene transformed into coke are often less than
10 mg per gram of mordenite
; the
ratio of the amounts of toluene transformed int.0
coke to those of disproportionated toluene is generally close to 1/1000 (table 3 ) .
dl
Fig. 2. Disproportionation of toluene on mordenites and silica-alumina ( S A ) at 450°C ; pH2 - 12 h r , p toluene = 3 bar. Activity (aD) against time on stream (t).
84
TABLE 3 TransForiation of toluene a t 4SO"C, p toluene = 3 bar ; pH2 o r pN2 = 12 bar. a" ( 1 mole.h-l ~ ~ .g-l . I : disproportionation activity extrapolated d t time zero ; m o l e . k ~ - ' . g - ~. ) : disproportionation activity after 9 hours ; a9 ( rnD(g.g-l. of catalyst) : disproportionated toluene, mk (g.g-l. of catalyst) : toluene
transformed into coke during the experiments.
55
H
+
85
0
0.25
0.88
6.4
8
0
0.3
30
0.92
8.1
7
2 N-
90
M(a
H,
102
MNi(3
H,,
107
1
9
MD(U
H3
12
0.5
1.4
MD(L)
H2
73
0.9
6.7
MNi (A)
MDNi(1) H
H SA +
N
2
+
31
4 .-5 28
135 1.25 100
o .9 0.5 20
103
0.73
7.4
3.5
10
0.5
0.5
8.5
17
10
0.5
0.55
9.5
17
2
2
0.5
activity after 10 minutes.
.
T,c/ L
50
I 0
Iar
I
1
I 5
,,m,=
1 2 ba
pH2 = 1 2 bar
10
Fig. 3. Substitution of nitrogen for hydrogen during disproportionation of toluene at 400°C on wet air dealuminated mordenite MD(2).
85 O n c e r t a i n c a t a l y s t s n i t r o g e n was s u b s t i t u t e d f o r hydrogen. When t h i s was
c a r r i e d o u t d u r i n g t o l u e n e d i s p r o p o r t i o n a t i o n on MD(3) - and MDNi(?), t h e i r a c t i v i t y seemed t o i n c r e a s e , b u t t h e i r d e a c t i v a t i o n became v e r y f a s t
;
t h e r e t u r n t o hydro-
gen d i d n o t c a u s e any r e a c t i v a t i o n o f t h e c a t a l y s t ( f i g u r e 3 ) . T h i s v e r y r a p i d d e a c t i v a t i o n under n i t r o g e n p r e s s u r e w a s a l s o found w i t h t h e m o r d e n i t e s t r e a t e d under d r y a i r M(l-1
and M N i ( l ) , when f r e s h samples were u s s d . T h e i r i n i t i a l a c t i v i t y
was h i g h b u t t h e i r d e a c t i v a t i o n v e r y f a s t ( n
=
2.7
f o r t h e two c a t a l y s t s ) . A f t e r
9 h o u r s o f work u n d e r n i t r o g e n p r e s s u r e , t h e i r a c t i v i t y i s z e r o . The amount o f t o l u e n e t r a n s f o r m e d i n t o c o k e , mk, i s 3 t i m e s h i g h e r under n i t r o g e n t h a n under hydrogen and t h e r a t i o of mk t o t h e amount o f d i s p r o p o r t i o n a t e d t o l u e n e , mD, i s from 30 t o 80 t i m e s h i g h e r ( t a b l e 3 ) . These m o r d e n i t e s d e a c t i v a t e d under n i t r o g e n r e g a i n t h e i r a c t i v i t y by a n o v e r n i g h t t r e a t m e n t under hydrogen a t 450°C [ l l ] . P a r t o f coke w a s removed d u r i n g t h i s t r e a t m e n t : M ( 1 ) contained about 3 w t
%
coke
;
and M N i ( L ) u s e d under n i t r o g e n
a f t e r t h e o v e r n i g h t t r e a t m e n t and d i s p r o p o r t i o n a t i o n
o f t h e t o l u e n e f o r 8 h o u r s under hydrogen p r e s s u r e , t h e y c o n t a i n e d o n l y 1 . 2 w t
%
and 1.3 w t % c o k e . On t h e c o n t r a r y , t h e b e h a v i o u r o f s i l i c a - a l u m i n a i s s i m i l a r under n i t r o g e n and under hydrogen ( t a b l e 3 ) .
DISCUSSION Under a l l o p e r a t i n g c o n d i t i o n s and whatever i t s t r e a t m e n t , t h e m o r d e n i t e p r e s e n t s a n i n i t i a l d i s p r o p o r t i o n a t i o n a c t i v i t y h i g h e r t h a n t h a t o f f l u o r i n a t e d alumina [ 9 o r silica-alumina
1
( t a b l e 3 ) . Under n i t r o g e n o r l o w hydrogen p r e s s u r e , i t s d e a c t i v a -
t i o n i s much more r a p i d t h a n t h a t o f t h e s e two c a t a l y s t s , y e t i t becomes s l o w e r under h i g h hydrogen p r e s s u r e . Under h i g h hydrogen p r e s s u r e , t h i s s t a b i l i z a t i o n o f t h e c a t a l y t i c a c t i v i t y i s o b s e r v e d f o r a l l t h e m o r d e n i t e s , w h i l e under low hydrogen p r e s s u r e , i t o c c u r s o n l y f o r w e t a i r t r e a t e d samples ( t a b l e 2 ) . T h i s e f f e c t i s connected with a v e r y important d e c r e a s e i n t h e r a t i o of t h e coking t o d i s p r o p o r t i o n a t i o n r a t e s (m /m k
D
)
; f o r example when n i t r o g e n i s r e p l a c e d by hydro-
i s d i v i d e d by 35 on M(L) and by 80 on M N i ( L ) ( t a b l e 3 ) . gen under h i g h p r e s s u r e m /m k D The most s t a b l e and s e l e c t i v e m o r d e n i t e i s t h a t exchanged by n i c k e l and t h e n t r e a t e d
under wet a i r : M N i ( 3 ) . I t s a c t i v i t y i s 10 t i m e s h i g h e r t h a n t h a t o f s i l i c a -
i s 4 t i m e s lower t h a n on s i l i c a - a l u m i n a and t h i s makes t h i s c a t a alumina ; m /m k D l y s t v e r y s u i t a b l e f o r t h e d i s p r o p o r t i o n a t i o n o f t o l u e n e [ 1 2 1.
Influence of t h e mordenite treatments The e f f e c t of t h e d i f f e r e n t t r e a t m e n t s a p p l i e d t o t h e m o r d e n i t e on i t s p h y s i c o c h e m i c a l p r o p e r t i e s and e s p e c i a l l y on i t s a c i d i t y h a s been examined by C. M i r o d a t o s
e t al.
[61. The c a t a l y t i c m o d i f i c a t i o n s i n d u c e d by t h e s e t r e a t m e n t s w i l l be d i s -
c u s s e d i n c o n n e c t i o n w i t h t h e o b s e r v a t i o n s o f t h e s e a u t h o r s . For t h i s p u r p o s e ,
86 w e s h a l l u s e o n l y t h e r e s u l t s o b t a i n e d under n i t r o g e n , assuming t h a t c o n t r a r y t o
hydrogen,
i t p l o y s no r 6 1 e i n t h e r e a c t i o n .
Chemical d e a l u m i n a t i o n . Dealuminated m o r d e n i t e t r e a t e d under d r y a i r i s t w i c e a s a c t i v e f o r t o l u e n e d i s p r o p o r t i o n a t i o n and f o r coke f o r m a t i o n a s non-dealuminated m o r d e n i t e ; i t s d e a c t i v a t i o n i s a l i t t l e more r a p i d . T h i s c o u l d q u i t e p l a u s i b l y be e x p l a i n e d by t h e f o r m a t i o n o f new s t r o n g a c i d s i t e s f o l l o w i n g s l i g h t dealuminat i o n a s r e c e n t l y proposed by i n v e s t i g a t o r s o f t h e Norton Company [ 13 1. Unfortunat e l y , t h e a c i d i t y measurements c a r r i e d o u t o n o u r s a m p l e s by C. M i r o d a t o s e t
a l . 161 show t h a t t h e r e i s no i n c r e a s e i n t h e numiier o f a c i d s i t e s b u t on t h e c o n t r a r y a d e c r e a s e b o t h i n t h e i r number and s t r e n g t h . Dry and wet a i r t r e a t m e n t s . The comparison o f MD(1) - and MD(2) shows t h a t t h e wet a i r t r e a t e d mordenite e x h i b i t s an i n i t i a l d i s p r o p o r t i o n a t i o n a c t i v i t y s l i g h t l y lower t h a n t h a t o f d r y a i r t r e a t e d m o r d e n i t e .
Its coking a c t i v i t y i s c o n s i d e r a b l y
lower and i t s d e a c t i v a t i o n i s a l i t t l e l e s s r a p i d ( t a b l e 2 ) . The d e c r e a s e i n d i s p r o p o r t i o n a t i o n and c o k i n g a c t i v i t i e s may be c o n n e c t e d w i t h t h e e l i m i n a t i o n of t h e BrBnsted a c i d s i t e s r e s p o n s i b l e f o r t h e 3600 cm-'
I R band
[6 1. The d i s p r o p o r t i o n a -
t i o n o f t o l u e n e and t h e f o r m a t i o n o f coke would t h e r e f o r e b e c a t a l y z e d by B r d n s t e d a c i d s i t e s . The v e r y s t r o n g a c i d s i t e s c r e a t e d by wet a i r t r e a t m e n t [6] do n o t seem t o b e v e r y a c t i v e under n i t r o g e n . Nickel mordenites.
The d i s p r o p o r t i o n a t i o n a c t i v i t y o f MNi(1) - under n i t r o g e n
high p r e s s u r e i s s l i g h t l y higher than t h a t of M ( 1 ) .
The c o k i n g a c t i v i t y o f b o t h
c a t a l y s t s i s i d e n t i c a l , a s i s t h e i r d e a c t i v a t i o n r a t e . These o b s e r v a t i o n s c a n b e connected w i t h t h e s l i g h t i n c r e a s e i n t h e p r o t o n i c a c i d i t y of mordenites due t o t h e reduction of nickel ions [ 6
1.
R e l a t i o n s h i p between d e a c t i v a t i o n and c o k i n g Figure 4 r e p r e s e n t s t h e r e s i d u a l a c t i v i t y o f mordenites,
taken a s t h e r a t i o
o f t h e f i n a l a c t i v i t y t o t h e i n i t i a l o n e a g a i n s t t h e i r coke c o n t e n t . Under a l l o p e r a t i n g c o n d i t i o n s t h i s a c t i v i t y d e c r e a s e s w i t h t h e coke c o n t e n t . T h e r e f o r e , t h e d e a c t i v a t i o n o f t h e c a t a l y s t p r o b a b l y comes from c o k e p o i s o n i n g . A s i s gener a l l y a d m i t t e d f o r m o r d e n i t e s [14-161,
coke a c t s by b l o c k i n g t h e p o r e s and n o t
by s i m p l y c o v e r i n g t h e a c t i v e s i t e s [ 1 7 ] . q u a n t i t y o f coke However,
(2
Indeed, t h e d e p o s i t of a v e r y s m a l l
1 w t % f i g u r e 4 ) e l i m i n a t e s more t h a n 95 % o f t h e a c t i v i t y .
t h e a g e i n g e f f e c t o f coke s t r o n g l y depends on t h e o p e r a t i n g c o n d i t i o n s
under h i g h hydrogen p r e s s u r e t h e a g e i n g e f f e c t i s v e r y small ( c u r v e A ,
figure 4 ) ,
w h i l e it i s v e r y i m p o r t a n t under n i t r o g e n and s t i l l i m p o r t a n t under low hydrogen p r e s s u r e (curve B,
figure 4 ) .
:
87
I
0
Iw? 1'
% I 2
wt
%
3 coke
-4
5
Fig. 4. Disproportionation of toluene : Residual a c t i v i t y of mordenites a g a i n s t coke c o n t e n t i n c a t a l y s t . Molar r a t i o o f c a r r i e r g a s t o t o l u e n e = 4 ; T = 450'C.
The g r e a t e r c o k e t o x i c i t y under l o w hydrogen p r e s s u r e i s p r o b a b l y d u e t o d i f f u s i o n a l i n t r a g r a n u l a r l i m i t a t i o n s . Indeed, comparing t h e v a l u e o f t h e T h i e l e modulus 0 t o 1 [181,
t h e r e a c t i o n under h i g h hydrogen p r e s s u r e i s n o t found t o be d i f f u s i o n
l i m i t e d ( e f f e c t i v e n e s s f a c t o r q e q u a l t o 1 ) whereas i t i s under l o w hydrogen p r e s s u r e . On t h e o t h e r hand a naked eye e x a m i n a t i o n o f a p e l l e t shows t h a t , a s i n cumene
1,
d e a l k y l a t i o n [19,20
t h e coke formed d u r i n g t o l u e n e d i s p r o p o r t i o n a t i o n forms a
s h e l l around t h e m o r d e n i t e p e l l e t w i t h o u t r e a c h i n g t h e c e n t e r . I t i s w e l l known t h a t t h i s c o k e s h e l l d e p o s i t h a s a g r e a t e r p o i s o n i n g e f f e c t on t h e r e a c t i o n [ 211 when i t i s l i m i t e d by i n t r a g r a n u l a r d i f f u s i o n a s i s t h e c a s e under low hydrogen p r e s s u r e . However,
t h i s e x p l a n a t i o n d o e s n o t a c c o u n t f o r t h e g r e a t s e n s i t i v i t y of
m o r d e n i t e s t o t h e c o k e d e p o s i t e d under n i t r o g e n a s compared t o what i s o b s e r v e d under hydrogen. I n d e e d , on t h e f r e s h c a t a l y s t s , t h e e f f e c t i v e n e s s f a c t o r r? h a s s i m i l a r v a l u e s under b o t h hydrogen and n i t r o g e n . To a c c o u n t f o r t h e d i f f e r e n c e i n a g e i n g e f f e c t s o n e must assume e i t h e r t h a t t h e coke i s d i s t r i b u t e d d i f f e r e n t l y i n t h e p e l l e t o r t h a t i t s n a t u r e i s n o t t h e same under hydrogen as under n i t r o g e n . A s a matter o f f a c t ,
t h e h i g h e r r a t e o f c o k i n g under n i t r o g e n (mk i n t a b l e s 2 and 3 )
c a n l e a d t o a v e r y i m p o r t a n t coke d e p o s i t o n t h e p e l l e t ' s o u t e r s u r f a c e and conseq u e n t l y t o a v e r y r a p i d d e a c t i v a t i o n of t h e m o r d e n i t e . But. t h e mass s p e c t r u m o f t h e p r o d u c t s d e s o r b e d a t 4 0 O o C o u t o f t h e coked m o r d e n i t e shows t h a t t h e n a t u r e o f t h e coke depends o n o p e r a t i n g c o n d i t i o n s
:
t h e coke d e p o s i t e d under n i t r o g e n
c o n t a i n s compounds which have a h i g h e r m o l e c u l a r w e i g h t t h a n t h o s e o f coke d e p o s i t e d under hydrogen pressure [ 2 2
1.
88
I n f l u e n c e o f hydrogen on d i s p r o p o r t i o n a t i o n and c o k i n g r a t e s Under h i g h hydrogen p r e s s u r e , t h e d i s p r o p o r t i o n a t i o n a c t i v i t y o f m o r d e n i t e s ( a o t a b l e 3 ) i s of t h e same o r d e r of magnitude a s t h e i n i t i a l a c t i v i t y found under
low hydrogen p r e s s u r e ( a by 1 5 . Now a.
10
t a b l e 2 1 , even though t h e t o l u e n e p r e s s u r e i s m u l t i p l i e d
i s c l e a r l y higher than t h e a c t i v i t y obtained a t t h e very begining
o f t h e e x p e r i m e n t s ( f i g u r e 2 ) . These o b s e r v a t i o n s a r e c o n s i s t e n t w i t h t h e i n h i b i t i n g e f f e c t o f hydrogen on t o l u e n e d i s p r o p o r t i o n a t i o n , a s r e v e a l e d by a k i n e t i c s t u d y of the reaction[231.
Hydrogen a l s o i n h i b i t s c o k i n g t o a l a r g e e x t e n t , a s i s shown
by t h e amount o f t o l u e n e t r a n s f o r m e d i n t o c o k e , m k ,
and more p a r t i c u l a r l y by t h e
( t a b l e s 2 a n d 3 ) . To a c c o u n t f o r a n i n h i b i t i n g e f f e c t o f hydrogen d e c r e a s e o f m /m k D on c e r t a i n p a r a f f i n t r a n s f o r m a t i o n s o v e r a c i d c a t a l y s t s [24-261, t h e r e a c t i o n (2) whose e x i s t e n c e i s c l e a r l y shown i n a s u p e r a c i d medium [ 2 7 - 2 8 1 , R+
+ n, --+RH + n+
h a s been s u g g e s t e d
:
(2)
T h i s p r o p o s i t i o n o f f e r s a good e x p l a n a t i o n f o r o u r r e s u l t s s i n c e t h e i n t e r m e d i a t e s f o r c o k i n g and d i s p r o p o r t i o n a t i o n r e a c t i o n s c a n b e carbonium i o n s s u c h a s b e n z i l i c carbonium i o n s [29]. R e a c t i o n ( 2 ) r e d u c e s t h e c o n c e n t r a t i o n o f carbonium s p e c i e s and c o n s e q u e n t l y i n h i b i t s t h e c o k i n g and d i s p r o p o r t i o n a t i o n r e a c t i o n s which o c c u r t h r o u g h t h e s e i n t e r m e d i a t e s . Under t h i s h y p o t h e s i s t h e f o r m a t i o n o f coke which r e q u i r e s t h e p a r t i c i p a t i o n o f s e v e r a l m o l e c u l e s and t h e r e f o r e s e v e r a l r e a c t i o n s i n v o l v i n g carbonium i o n s , s h o u l d b e more i n h i b i t e d t h a n t h e b i m o l e c u l a r r e a c t i o n o f d i s p r o p o r t i o n a t i o n , which i s a c t u a l l y what o u r e x p e r i m e n t s show. R e a c t i o n ( 2 ) a l s o e x p l a i n s t h e coke d e s o r p t i o n by a l o n g t r e a t m e n t under hydrogen p r e s s u r e i f o n e s u p p o s e s t h a t t h e coke i s formed by b u l k y carbonium i o n s d i f f i c u l t t o d e s o r b The i n h i b i t i o n by hydrogen of t h e r e a c t i o n s o f d i s p r o p o r t i o n a t i o n and c o k i n g o c c u r s o n l y on m o r d e n i t e s ; no e f f e c t i s found on s i l i c a - a l u m i n a
( t a b l e 3 ) . This
o b s e r v a t i o n i m p l i e s t h a t , u n l i k e s i l i c a - a l u m i n a , mordenites are capable of a c t i v a t i n g hydrogen i n s u c h a way as t o e n a b l e i t t o remove t h e a d s o r b e d carbonium i o n s p e c i e s . I t i s w e l l known t h a t m o l e c u l a r hydrogen c a n b e a c t i v a t e d i n a s u p e r a c i d medium,
by s t r o n g a c i d s i t e s [ 3 0 ] . One c a n suppose t h a t m o r d e n i t e , which e x h i b i t s v e r y s t r o n g a c i d s i t e s [l],
c a n a l s o a c t i v a t e m o l e c u l a r hydrogen. However, it i s a l s o
w e l l known t h a t t h e m o l e c u l a r hydrogen a c t i v a t e d by h y d r o g e n a t i n g s i t e s c a n remove coke on a c i d s i t e s [ 3 1 ] . Hydrogenating c e n t e r s p r o b a b l y e x i s t on nickel-exchanged m o r d e n i t e , s i n c e a g r e a t p a r t o f n i c k e l i o n s are r e d u c e d d u r i n g p r e t r e a t m e n t s [6]. Moreover, s t r o n g a c i d s i t e s c r e a t e d by t r e a t m e n t o f m o r d e n i t e s under w e t a i r a r e probably a s s o c i a t e d with hydrogenating s i t e s , s i n c e t h e s e mordenites are a b l e t o decompose ammonia [ 6 ] .
Thus, t h e a c t i v a t i o n o f m o l e c u l a r hydrogen by n i c k e l f r e e
mordenites could be due t o t h e presence of i m p u r i t i e s such a s i r o n , a s r e p o r t e d i n t h e case o f X and Y z e o l i t e s [ 3 2 , 331. ACKNOWLEDGEMENT
:
The a u t h o r s t h a n k D.G.R.S.T.
f o r s u p p o r t i n g t h i s work.
89 REFERENCES P o u t x m , i n J.A. Rabo ( E d . ) , Z e o l i t e Chemi.stry and C a t a l y s i s , ACS Monograph, L71, Wa.;l,ingtori, 1 9 7 6 , cl:. E , 13.437. 2 . Braun, k’. F e t t i n g and H . S c h o e n e b e r g e r , 111 J . K . K a t z e r ( E d . ) , M o l e c u l a r
1 4.L.
2
;ieves-11, ACS Symposium S e r i e s 4 0 , Washington, 1977, pp. 504-514. B u r b i d g e , I.M. Keen and M . K . E y l e s , i n R.F. Gould ( E d . ) , M o l e c u l a r S i e v e 5 e o l i t e s - 1 1 , A d v . Chem. S e r . , 102, Washington, 1971, pp.400-409. 4 ;.M. C s i c s e r y , i n J . A . Rabo ( E d . ) , Z e o l i t e C h e m i s t r y and C a t a l y s i s , ACS Monograph, - 7 1 , Washingtoc, 1976, c h . 12, p.680. N a m b a , 0. I w a s e , N. T a k a h a s h i , T. Yashima and N . Hara, J. C a t a l . , 56 ( 1 9 7 9 ) 5 145-452. M i r o d a t o s , B . H . Ha, K . o t s u k a and D. Barthomeuf, F i f t h I n t e r n a t i o n a l C o n f e r e n c e 6 )n Z e o l i t e , N a p l e s , 1980, i n p r e s s . 7 i. Maurel and J.E. Germain, B u l l . Soc. C h i m . , ( 1 9 6 0 ) 930-934. 8 J . S . Gnep and M. G u i s n e t , B u l l . SOC. C h i m . , ( 1 9 7 7 ) 429-434. 9 I . M a r s i c o b e t r e , N.S. Gnep, M. G u i s n e t and R. Maurel, Rev. P o r t . Quim., 18 ( 1 9 7 6 ) 313-3 16. 10 A. V o o r h i e s , I n d . Eng. Chem., 37 ( 1 9 4 5 ) 318-322. 11 N.S. Gnep, M.L. M a r t i n d e Armando and M. G u i s n e t , R e a c t i o n K i n e t i c s and C a t a l y s i s Letters, i n press. 12 C . M a r c i l l y , F r e n c h P a t e n t s 75/33.601, 77/01.265. G o y e t t e and T.M. Notermann, J. C a t a l . , 52 ( 1 9 7 8 ) 25-31. 13 J . R . Kiovsky, W.J. 14 H . S . Bierenbaum, R.D. P a r t r i d g e and A . H . Weiss, i n W.M. Meier and J . B . U y t t e r hoeven ( E d s . ) , M o l e c u l a r S i e v e s , Adv. Chem. S e r . , 121, Washington, 1978, pp.605617. 1 5 S.M. C s i c s e r y , J . C a t a l . , 23 ( 1 9 7 1 ) 124-130. K a t z e r and W.R. V i e t h , I n d . Eng. Chem. Fundamentals, 16 C.N. S a t t e r f i e l d , J . R . 10 (1971) 478-486. Beeckman and G.F. Froment, I n d . Eng. Chem. Fundamentals, 18 (1979) 245-256. 17 J . N . 18 J . M . Smith, i n Chemical E n g i n e e r i n g K i n e t i c s , Chem. Eng. S e r . , New York, 1970, p.435. 19 J . B . B u t t , S . D e l g a d o - D i a z a n d W.E. Muno, J. C a t a l . , 3 7 ( 1 9 7 5 ) 158-165. 20 J . B . B u t t , J. C a t a l . , 41 ( 1 9 7 6 ) - 190-191. 2 1 J . B . B u t t , i n R.F. Gould ( E d . ) , Chemical R e a c t i o n E n g i n e e r i n g , Adv. Chem. S e r . , 109, Washington, 1972, pp.259-495. 2 2 N.S. Gnep, F. C h e v a l i e r and M. G u i s n e t , E x p e r i m e n t s u n d e r p r o g r e s s . 23 N.S. Gnep, M. G u i s n e t , i n p r e p a r a t i o n . 24 K h . Minachev, V. G a r a n i n , T. I s a k o v a , V . Kharlamov and V. Bogomolov, i n R.F. Gould ( E d . ) , M o l e c u l a r S i e v e Z e o l i t e s - 1 1 , Adv. Chern. S e r . , 102, Washington, 1971, pp.441-450. Kouwenhoven, i n W.M. Meier and J . B . U y t t e r h o e v e n ( a s . ) Adv. Chem. S e r . , 25 H.W. 121, Washington, 1973, pp.529-539. 26 F.E. Condon, i n P.H. E m m e t t ( E d . ) , C a t a l y s i s , Vol. V I , R e i n h o l d P u b l i s h i n g C o r p o r a t i o n , N e w York, 1958, c h . 2 , p . 4 3 . 27 H. Hogeveen a n d C.J. Gasbeek, R e c . T r a v . Chim. Pays-Bas, 88 ( 1 9 6 9 ) 719-724. 28 H. Hogeveen, R e C . T r a v . Chim. Pays-Bas, 89 (1970) 74-76. 29 A. S t r e i t w i e s e r a n d L. R e i f , J . Amer. Chem. Scc., 8 2 (1960) 5003-5005. 30 G.A. Olah, J. Shen and R.H. S c h t o s b e r g , J . Amer. Chem. SOC., 92 ( 1 9 6 4 ) 3831-3832. 31 P . J . L u c c h e s i , J . L . C a r t e r and J . H . S i n f e l t , J. h e r . Chem. SOC., 86 (1964) 1494-1497. 32 C.F. Heylen, P.A. J a c o b s and J . B . U y t t e r h o e v e n , J . C a t a l . , 43 ( 1 9 7 6 ) 99-110. 33 P.A. J a c o b s a n d J . B . U y t t e r h o e v e n , J . C a t a l . , 50 (1977) 109-114.
3
I3 . W .
:.
:.
79
TOLIJENE DISPROPOR'I'IONATION AND COKE FORMATION ON MORDENITES
EFFECT OF CATALYST MODIFICATIONS AND OF OPERATING CONDITIONS
N.S.
GNEP
1
,
M.L.
MARTIN d e ARMANDOl, C. MARCILLY
2
,
B.H.
HA
3+
and M.
GUISNET
1++
1 - L a b o r a t o i r e d e Chimie 7 - Groupe d e Recherches s u r l a C a t a l y s e e n Chimie Organique (ERA CNRS 371)
- 4 0 , avenue du R e c t e u r P i n e a u - 8 6 0 2 2 POITIERS
CEDEX
FRANCE.
2 - S e r v i c e d e C a t a l y s e - I n s t i t u t FranGais du P e t r o l e - RUEIL MALMAISON. 3 - L a b o r a t o i r e d e C a t a l y s e Organique - L.A.
CNRS 231
-
VILLEURBANNE.
ABSTRACT Improving c a t a l y t i c s t a b i l i t y o f m o r d e n i t e h a s been a t t e m p t e d by u s i n g t h r e e t y p e s of t r e a t m e n t : d e a l u m i n a t i o n , w e t a i r h e a t i n g , n i c k e l - i o n - e x c h a n g e .
The
r a t e s o f d i s p r o p o r t i o n a t i o n and c o k i n g on t h e s e samples have been compared under d i f f e r e n t o p e r a t i n g c o n d i t i o n s : low and h i g h hydrogen o r n i t r o g e n p r e s s u r e s . Under n i t r o g e n , m o r d e n i t e d e a c t i v a t i o n i s v e r y f a s t . Dealumination i n c r e a s e s i t s a c t i v i t y f o r d i s p r o p o r t i o n a t i o n and c o k i n g a s w e l l a s t h e d e a c t i v a t i o n r a t e , w h i l e
w e t a i r t r e a t m e n t d e c r e a s e s i t s c o k i n g a c t i v i t y . T h i s d e c r e a s e may b e c o n n e c t e d w i t h t h e e l i m i n a t i o n o f B r d n s t e d a c i d s i t e s . The c a t a l y t i c s t a b i l i t y o f a l l samples i s c o n s i d e r a b l y improved by o p e r a t i n g under h i g h hydrogen p r e s s u r e , t h e b e s t c a t a l y s t being t h e w e t a i r t r e a t e d nickel-exchanged mordenite. T h i s s t a b i l i z i n g e f f e c t
of hydrogen i s t h e r e s u l t o f c o k i n g i n h i b i t i o n and o f a b e t t e r d i s t r i b u t i o n o f c o k e i n t h e b u l k o f t h e c a t a l y s t p e l l e t t h a n under n i t r o g e n .
I t i s suggested t h a t
c o k i n g i n h i b i t i o n comes from t h e p r e s e n c e o n m o r d e n i t e o f s i t e s c a p a b l e o f a c t i v a t i n g m o l e c u l a r hydrogen.
INTRODUCTION Numerous s t u d i e s a r e d e v o t e d t o t h e t r a n s f o r m a t i o n s o f h y d r o c a r b o n s o n morden i t e s , and i n p a r t i c u l a r t o t h e h y d r o c r a c k i n g and h y d r o i s o m e r i z a t i o n of p a r a f f i n s , t h e t r a n s a l k y l a t i o n and i s o m e r i z a t i o n o f aromatics [ 1 , 2 ] . s e n t v a r i o u s advantages o v e r t h e widely used Y-zeolites
Indeed, mordenites pre: they
a r e t h e r m a l l y more
s t a b l e ; t h e y a r e a l s o more a c i d i c , and c o n s e q u e n t l y more a c t i v e [11 ; f i n a l l y t h e i r p o r o u s s t r u c t u r e g i v e s them a s h a p e - s e l e c t i v i t y w i t h r e g a r d t o c e r t a i n reacU n i v e r s i t y , S e o u l , Corea ++Hanayang To whom c o r r e s p o n d e n c e s h o u l d b e a d d r e s s e s . +
-,
..
80 t i o n s [3-51.
On t h e o t h e r hand, t h i s p o r o u s s t r u c t u r e makes them v e r y s e n s i t i v e
t o c o k i n g and t h e i r d e a c t i v a t i o n i s g e n e r a l l y v e r y f a s t [ I ] . The aim of t h e p r e s e n t work i s t o d e t e r m i n e how d i f f e r e n t t r e a t m e n t s and o p e r a t i n g c o n d i t i o n s i n f l u e n c e t h e c a t a l y t i c s t a b i l i t y and t h e c o k i n g o f m o r d e n i t e d u r i n g t h e d i s p r o p o r t i o n a t i o n o f t o l u e n e . The f o l l o w i n g treatment.5 were e x p e r i m e n t e d : cnemical dealumination, nickel-exchange,
a c t i v a t i o n i n d r y o r wet a i r . The r a t e s
o f t h e d i s p r o p o r t i o n a t i o n and c o k i n g r e a c t i o n s on t h e c a t a l y s t s t h u s t r e a t e d were compared under d i f f e r i n g o p e r a t i n g c o n d i t i o n s : low and h i g h p r e s s u r e s of hydroqen and n i trogen.
EXPERIMENTAL The m o r d e n i t e s have been d e s c r i b e d p r e v i o u s l y [ 61. T h e i r p r o p e r t i e s a r e r e p r o duced i n t a b l e 1. These m o r d e n i t e s were used under t h e forms i n reference [6], i.e.
(I)o r
air
of w e t a i r
(1)and (2) d e f i n e d
a f t e r a t r e a t m e n t a t 500°C f o r 15 h o u r s i n a f l o w o f d r y
(2). The
c r y s t a l l i n i t y of t h e s e c a t a l y s t s r e m a i n s h i g h a f t e r
both treatments. The s i l i c a - a l u m i n a
(LA3P from K e t j e n ) w a s p r e t r e a t e d a t 500°C f o r 15 h o u r s i n
d r y a i r flow b e f o r e use. The r e a c t i o n was s t u d i e d i n t h e p r e v i o u s l y d e s c r i b e d dynamic r e a c t o r s [ 7 , 8 ] a t 450'12 w i t h a hydrogen
( o r n i t r o g e n ) t o t o l u e n e m o l a r r a t i o e q u a l t o 4 . The r e a c t o r
p r o d u c t s were a n a l y z e d by G.L.C.
The p e r c e n t c o n v e r s i o n i s g e n e r a l l y l e s s t h a n 15 % .
The coke c o n t e n t o f t h e c a t a l y s t s was d e t e r m i n e d by t h e " C e n t r e d e M i c r o a n a l y s e du CNRS, V i l l e u r b a n n e , F r a n c e " . TABLE 1
Chemical c o m p o s i t i o n a n d p r e p a r a t i o n o f t h e m o r d e n i t e s Mordenites
SiO / A 1 2 0 3
wt
wt
% N i
Preparation
M
10
0.36
0
NH4 i o n - exchange of N a 900 Zeolon ( N o r t o n )
MD
18
0.15
0
dealumination of M with H C l
MNi
10
0.36
1.45
N i ion
- exchange o f
M
MDNi
18
0.15
0.5
N i ion
- exchange o f
MD
2
% Na
RESULTS D i s p r o p o r t i o n a t i o n o f t o l u e n e under a t m o s p h e r i c p r e s s u r e Under hydrogen a l l t h e m o r d e n i t e s p r e s e n t a h i g h i n i t i a l a c t i v i t y ( a l 0 i n t a b l e 2 ) r a n g i n g from 50 t o 120,10-4 mole.h-l.g-'., t h a t o f t h e most a c t i v e f l u o r i n a t e d alumina
i.e.
from 5 t o 10 t i m e s h i g h e r t h a n
[91. The t r e a t m e n t of t h e m o r d e n i t e ,
h a s l i t t l e e f f e c t o n i t s i n i t i a l a c t i v i t y , e x c e p t f o r i t s chemscal d e a l u m i n a t i o n which m u l t i p l i e s t h e i n i t i a l a c t i v i t y by a f a c t o r o f 1 . 3 t o 2. The d e a c t i v a t i o n
81 of t i i f
m o r d e n i t e s i s v e r y f a s t ( f i g u r e 1) and a f t e r 7 hours o f r e a c t i o n , the a c t i -
v j . t y is l e s s t h a n 5
‘t2
of t h e i n i t i a l a c t i v i t y (table 2 ) .
121 I
100
3
I
75
i
I
i’. ar
ri
2 I
2
50
25
Fig. 1 . Disproportionation of toluene on mordenites a t 4 5 O o C ; pH2 = 0.8 b a r , p toluene = 0 . 2 b a r . A c t i v i t y (a,) a g a i n s t t i m e on stream ( t )
.
The expression (1)proposed by Voorhies
10 1 accounts f o r t h e e v o l u t i o n of t h e d i s -
p r o p o r t i o n a t i o n a c t i v i t y a of mordenites with t h e r e a c t i o n time t
:
The value f o r n and consequently t h e r a t e of d e a c t i v a t i o n , i n c r e a s e s with dealumin a t i o n , b u t decreases s l i g h t l y with nickel-exchange and s t r o n g l y with wet a i r treatment (table 2 ) . The amount of t o l u e n e d i s p r o p o r t i o n a t e d during each run w a s c a l c u l a t e d by i n t e g r a t i o n of t h e a c t i v i t y curves i n f i g u r e 1. This q u a n t i t y of d i s p r o p r t i o n a t e d t o l u e n e mD, i n c r e a s e s with dealumination and with wet a i r t r e a t ment. The amount of toluene transformed i n t o coke during each experiment, mk, v a r i e s by a f a c t o r of 6 according t o t h e treatment of t h e mordenite Aealumination and d e c r e a s e s with nickel-exchange. nickel-exchange and wet a i r treatment ( t a b l e 2).
:
i t i n c r e a s e s with
The r a t i o mk/mD decreases
W i t h
82 The t r a n s f o r m a t i o n o f t o l u e n e h a s been i n v e s t i g a t e d under n i t r o g e n on t h r e e d e a l u m i n a t e d m o r d e n i t e s MDII), _. MD(3) and MDNi(3), and on t h e m o r d e n i t e M(1). With t h e m o r d e n i t e s t r e a t e d under d r y a i r M(1) and MD(L), t h e r e s u l t s o b t a i n e d under n i t r o g e n a r e s i m i l a r t o t h o s e o b t a i n e d under hydrogen ( t a b l e 2 ) . On t h e o t h e r hand, t h e d e a c t i v a t i o n o f t h e m o r d e n i t e s t r e a t e d i n w e t a i r i s much f a s t e r under n i t r o i j e n t h a n under hydrogen, and t h e s e m o r d e n i t e s l o s t a l l t h e i r a c t i v i t y a f t e r t h r e e h o u r s o f work. However, t h e r a t i o o f t h e amount o f t o l u e n e t r a n s f o r m e d i n t o coke t o t h a t o f d i s p r o p o r t i o n a t e d t o l u e n e i s t h e same under n i t r o g e n a s under hydrogen f o r MD(2), w h i l e f o r M D N i ( i ) , t h i s r a t i o i s t h r e e t i m e s h i g h e r under n i t r o g e n t h a n under hydrogen ( t a b l e 2 ) . TABLE 2
T r a n s f o r m a t i o n o f t o l u e n e a t 45OoC, p t o l u e n e = 0.2 b a r ; pH o r pN2 = 0 . 8 b a r . 2 a F ( 10-4mole.h-1 . g - l . ) : f i n a l d i s p r o p o r t i o n a t i o n a c t i v i t y o f m o r d e n i t e ( t i m e between b r a c k e t s ) ; a10 ( 10-4mole.h-1 . g - l ) : d i s p r o p o r t i o n a t i o n a c t i v i t y o f m o r d e n i t e a f t e r 10 m i n u t e s ; n : d e a c t i v a t i o n c o e f f i c i e n t f o m V o o r h i e s [ l o ] ; m ( 9 . g - l . of morden i t e ) : d i s p r o p o r t i o n a t e d t o l u e n e ; mk (g.g-'. of m o r d e n i t e ) : t o f u e n e t r a n s f o r m e d i n t o coke d u r i n g t h e e x p e r i m e n t s .
n
a10
Catalyst
H M (1)
2
N2
aF'a 10
m
D
3 10 mk
3 10 mk/mD
63
1.5
O.Ol(120mn)
0.25
18
70
50
2.0
0.005(120mn)
0.20
20
100 25
MNi(L)
H2
79
1.1
0.015(400mn)
0.45
11
MNi(3)
H2
57
0.7
0.05(400mn)
0.7
7
10
H
82
2
0.00 (200mn)
0.55
45
80
MD
(1)
2
N2 MD
(3)
H2 N2
H MDNi
(2)
2
N,
130
3.3
O.OO(200mn)
0.75
45
65
117
1.1
0.05(400mn)
1.9
13
7
12
10
115
2.5
O.OO(200mn)
1.2
121
1.1
0.04(400mn)
1 .o
95
2.5
O.OO(200mn)
0.4
8.5 11
8.5 25
D i s p r o p o r t i o n a t i o n o f t o l u e n e under h i g h p r e s s u r e T h i s r e a c t i o n w a s performed under hydrogen and t o l u e n e p r e s s u r e s o f 12 b a r and 3 b a r r e s p e c t i v e l y . F i g u r e 2 shows how t h e d i s p r o p o r t i o n a t i o n a c t i v i t y o f m o r d e n i t e s e v o l v e s w i t h r e a c t i o n t i m e . T h i s a c t i v i t y i s i n i t i a l l y low, t h e n it i n c r e a s e s r a p i d l y d u r i n g t h e 2 o r 3 f i r s t h o u r s o f work and f i n a l l y d e c r e a s e s s l o w l y . The v a l u e f o r t h e a c t i v i t i e s o b t a i n e d by e x t r a p o l a t i n g t h e d e c r e a s i n g l i n e a r p a r t o f t h e c u r v e s
a t z e r o t i m e , ao , ( f i g u r e 2 ) v a r i e s from 15 t o 100 a c c o r d i n g t o t h e m o r d e n i t e t r e a t m e n t . The d e a l u m i n a t i o n c a u s e s a s t r o n g d e c r e a s e i n a o , a t l e a s t f o r t h e mord e n i t e s t r e a t e d under d r y a i r , w h i l e t h e n i c k e l - e x c h a n g e and t h e w e t a i r t r e a t m e n t have a n o p p o s i t e e f f e c t ( t a b l e 3 ) . A f t e r 9 h o u r s o f work, t h e a c t i v i t y o f t h e
83 Illordcnitfs remains general~lyhigher t h a n 70
h
of the a.
activity. The amounts of
dis~)roportioiiat~~d toluene, mu, are similar for all the mordenites treated under wet air and for MNi M(1)
(2.) ;
they arc2 markedly higher than the amounts obtained on
and M D ( I ) . The amounts of toluene transformed into coke are often less than
10 mg per gram of mordenite
; the
ratio of the amounts of toluene transformed int.0
coke to those of disproportionated toluene is generally close to 1/1000 (table 3 ) .
dl
Fig. 2. Disproportionation of toluene on mordenites and silica-alumina ( S A ) at 450°C ; pH2 - 12 h r , p toluene = 3 bar. Activity (aD) against time on stream (t).
84
TABLE 3 TransForiation of toluene a t 4SO"C, p toluene = 3 bar ; pH2 o r pN2 = 12 bar. a" ( 1 mole.h-l ~ ~ .g-l . I : disproportionation activity extrapolated d t time zero ; m o l e . k ~ - ' . g - ~. ) : disproportionation activity after 9 hours ; a9 ( rnD(g.g-l. of catalyst) : disproportionated toluene, mk (g.g-l. of catalyst) : toluene
transformed into coke during the experiments.
55
H
+
85
0
0.25
0.88
6.4
8
0
0.3
30
0.92
8.1
7
2 N-
90
M(a
H,
102
MNi(3
H,,
107
1
9
MD(U
H3
12
0.5
1.4
MD(L)
H2
73
0.9
6.7
MNi (A)
MDNi(1) H
H SA +
N
2
+
31
4 .-5 28
135 1.25 100
o .9 0.5 20
103
0.73
7.4
3.5
10
0.5
0.5
8.5
17
10
0.5
0.55
9.5
17
2
2
0.5
activity after 10 minutes.
.
T,c/ L
50
I 0
Iar
I
1
I 5
,,m,=
1 2 ba
pH2 = 1 2 bar
10
Fig. 3. Substitution of nitrogen for hydrogen during disproportionation of toluene at 400°C on wet air dealuminated mordenite MD(2).
85 O n c e r t a i n c a t a l y s t s n i t r o g e n was s u b s t i t u t e d f o r hydrogen. When t h i s was
c a r r i e d o u t d u r i n g t o l u e n e d i s p r o p o r t i o n a t i o n on MD(3) - and MDNi(?), t h e i r a c t i v i t y seemed t o i n c r e a s e , b u t t h e i r d e a c t i v a t i o n became v e r y f a s t
;
t h e r e t u r n t o hydro-
gen d i d n o t c a u s e any r e a c t i v a t i o n o f t h e c a t a l y s t ( f i g u r e 3 ) . T h i s v e r y r a p i d d e a c t i v a t i o n under n i t r o g e n p r e s s u r e w a s a l s o found w i t h t h e m o r d e n i t e s t r e a t e d under d r y a i r M(l-1
and M N i ( l ) , when f r e s h samples were u s s d . T h e i r i n i t i a l a c t i v i t y
was h i g h b u t t h e i r d e a c t i v a t i o n v e r y f a s t ( n
=
2.7
f o r t h e two c a t a l y s t s ) . A f t e r
9 h o u r s o f work u n d e r n i t r o g e n p r e s s u r e , t h e i r a c t i v i t y i s z e r o . The amount o f t o l u e n e t r a n s f o r m e d i n t o c o k e , mk, i s 3 t i m e s h i g h e r under n i t r o g e n t h a n under hydrogen and t h e r a t i o of mk t o t h e amount o f d i s p r o p o r t i o n a t e d t o l u e n e , mD, i s from 30 t o 80 t i m e s h i g h e r ( t a b l e 3 ) . These m o r d e n i t e s d e a c t i v a t e d under n i t r o g e n r e g a i n t h e i r a c t i v i t y by a n o v e r n i g h t t r e a t m e n t under hydrogen a t 450°C [ l l ] . P a r t o f coke w a s removed d u r i n g t h i s t r e a t m e n t : M ( 1 ) contained about 3 w t
%
coke
;
and M N i ( L ) u s e d under n i t r o g e n
a f t e r t h e o v e r n i g h t t r e a t m e n t and d i s p r o p o r t i o n a t i o n
o f t h e t o l u e n e f o r 8 h o u r s under hydrogen p r e s s u r e , t h e y c o n t a i n e d o n l y 1 . 2 w t
%
and 1.3 w t % c o k e . On t h e c o n t r a r y , t h e b e h a v i o u r o f s i l i c a - a l u m i n a i s s i m i l a r under n i t r o g e n and under hydrogen ( t a b l e 3 ) .
DISCUSSION Under a l l o p e r a t i n g c o n d i t i o n s and whatever i t s t r e a t m e n t , t h e m o r d e n i t e p r e s e n t s a n i n i t i a l d i s p r o p o r t i o n a t i o n a c t i v i t y h i g h e r t h a n t h a t o f f l u o r i n a t e d alumina [ 9 o r silica-alumina
1
( t a b l e 3 ) . Under n i t r o g e n o r l o w hydrogen p r e s s u r e , i t s d e a c t i v a -
t i o n i s much more r a p i d t h a n t h a t o f t h e s e two c a t a l y s t s , y e t i t becomes s l o w e r under h i g h hydrogen p r e s s u r e . Under h i g h hydrogen p r e s s u r e , t h i s s t a b i l i z a t i o n o f t h e c a t a l y t i c a c t i v i t y i s o b s e r v e d f o r a l l t h e m o r d e n i t e s , w h i l e under low hydrogen p r e s s u r e , i t o c c u r s o n l y f o r w e t a i r t r e a t e d samples ( t a b l e 2 ) . T h i s e f f e c t i s connected with a v e r y important d e c r e a s e i n t h e r a t i o of t h e coking t o d i s p r o p o r t i o n a t i o n r a t e s (m /m k
D
)
; f o r example when n i t r o g e n i s r e p l a c e d by hydro-
i s d i v i d e d by 35 on M(L) and by 80 on M N i ( L ) ( t a b l e 3 ) . gen under h i g h p r e s s u r e m /m k D The most s t a b l e and s e l e c t i v e m o r d e n i t e i s t h a t exchanged by n i c k e l and t h e n t r e a t e d
under wet a i r : M N i ( 3 ) . I t s a c t i v i t y i s 10 t i m e s h i g h e r t h a n t h a t o f s i l i c a -
i s 4 t i m e s lower t h a n on s i l i c a - a l u m i n a and t h i s makes t h i s c a t a alumina ; m /m k D l y s t v e r y s u i t a b l e f o r t h e d i s p r o p o r t i o n a t i o n o f t o l u e n e [ 1 2 1.
Influence of t h e mordenite treatments The e f f e c t of t h e d i f f e r e n t t r e a t m e n t s a p p l i e d t o t h e m o r d e n i t e on i t s p h y s i c o c h e m i c a l p r o p e r t i e s and e s p e c i a l l y on i t s a c i d i t y h a s been examined by C. M i r o d a t o s
e t al.
[61. The c a t a l y t i c m o d i f i c a t i o n s i n d u c e d by t h e s e t r e a t m e n t s w i l l be d i s -
c u s s e d i n c o n n e c t i o n w i t h t h e o b s e r v a t i o n s o f t h e s e a u t h o r s . For t h i s p u r p o s e ,
86 w e s h a l l u s e o n l y t h e r e s u l t s o b t a i n e d under n i t r o g e n , assuming t h a t c o n t r a r y t o
hydrogen,
i t p l o y s no r 6 1 e i n t h e r e a c t i o n .
Chemical d e a l u m i n a t i o n . Dealuminated m o r d e n i t e t r e a t e d under d r y a i r i s t w i c e a s a c t i v e f o r t o l u e n e d i s p r o p o r t i o n a t i o n and f o r coke f o r m a t i o n a s non-dealuminated m o r d e n i t e ; i t s d e a c t i v a t i o n i s a l i t t l e more r a p i d . T h i s c o u l d q u i t e p l a u s i b l y be e x p l a i n e d by t h e f o r m a t i o n o f new s t r o n g a c i d s i t e s f o l l o w i n g s l i g h t dealuminat i o n a s r e c e n t l y proposed by i n v e s t i g a t o r s o f t h e Norton Company [ 13 1. Unfortunat e l y , t h e a c i d i t y measurements c a r r i e d o u t o n o u r s a m p l e s by C. M i r o d a t o s e t
a l . 161 show t h a t t h e r e i s no i n c r e a s e i n t h e numiier o f a c i d s i t e s b u t on t h e c o n t r a r y a d e c r e a s e b o t h i n t h e i r number and s t r e n g t h . Dry and wet a i r t r e a t m e n t s . The comparison o f MD(1) - and MD(2) shows t h a t t h e wet a i r t r e a t e d mordenite e x h i b i t s an i n i t i a l d i s p r o p o r t i o n a t i o n a c t i v i t y s l i g h t l y lower t h a n t h a t o f d r y a i r t r e a t e d m o r d e n i t e .
Its coking a c t i v i t y i s c o n s i d e r a b l y
lower and i t s d e a c t i v a t i o n i s a l i t t l e l e s s r a p i d ( t a b l e 2 ) . The d e c r e a s e i n d i s p r o p o r t i o n a t i o n and c o k i n g a c t i v i t i e s may be c o n n e c t e d w i t h t h e e l i m i n a t i o n of t h e BrBnsted a c i d s i t e s r e s p o n s i b l e f o r t h e 3600 cm-'
I R band
[6 1. The d i s p r o p o r t i o n a -
t i o n o f t o l u e n e and t h e f o r m a t i o n o f coke would t h e r e f o r e b e c a t a l y z e d by B r d n s t e d a c i d s i t e s . The v e r y s t r o n g a c i d s i t e s c r e a t e d by wet a i r t r e a t m e n t [6] do n o t seem t o b e v e r y a c t i v e under n i t r o g e n . Nickel mordenites.
The d i s p r o p o r t i o n a t i o n a c t i v i t y o f MNi(1) - under n i t r o g e n
high p r e s s u r e i s s l i g h t l y higher than t h a t of M ( 1 ) .
The c o k i n g a c t i v i t y o f b o t h
c a t a l y s t s i s i d e n t i c a l , a s i s t h e i r d e a c t i v a t i o n r a t e . These o b s e r v a t i o n s c a n b e connected w i t h t h e s l i g h t i n c r e a s e i n t h e p r o t o n i c a c i d i t y of mordenites due t o t h e reduction of nickel ions [ 6
1.
R e l a t i o n s h i p between d e a c t i v a t i o n and c o k i n g Figure 4 r e p r e s e n t s t h e r e s i d u a l a c t i v i t y o f mordenites,
taken a s t h e r a t i o
o f t h e f i n a l a c t i v i t y t o t h e i n i t i a l o n e a g a i n s t t h e i r coke c o n t e n t . Under a l l o p e r a t i n g c o n d i t i o n s t h i s a c t i v i t y d e c r e a s e s w i t h t h e coke c o n t e n t . T h e r e f o r e , t h e d e a c t i v a t i o n o f t h e c a t a l y s t p r o b a b l y comes from c o k e p o i s o n i n g . A s i s gener a l l y a d m i t t e d f o r m o r d e n i t e s [14-161,
coke a c t s by b l o c k i n g t h e p o r e s and n o t
by s i m p l y c o v e r i n g t h e a c t i v e s i t e s [ 1 7 ] . q u a n t i t y o f coke However,
(2
Indeed, t h e d e p o s i t of a v e r y s m a l l
1 w t % f i g u r e 4 ) e l i m i n a t e s more t h a n 95 % o f t h e a c t i v i t y .
t h e a g e i n g e f f e c t o f coke s t r o n g l y depends on t h e o p e r a t i n g c o n d i t i o n s
under h i g h hydrogen p r e s s u r e t h e a g e i n g e f f e c t i s v e r y small ( c u r v e A ,
figure 4 ) ,
w h i l e it i s v e r y i m p o r t a n t under n i t r o g e n and s t i l l i m p o r t a n t under low hydrogen p r e s s u r e (curve B,
figure 4 ) .
:
87
I
0
Iw? 1'
% I 2
wt
%
3 coke
-4
5
Fig. 4. Disproportionation of toluene : Residual a c t i v i t y of mordenites a g a i n s t coke c o n t e n t i n c a t a l y s t . Molar r a t i o o f c a r r i e r g a s t o t o l u e n e = 4 ; T = 450'C.
The g r e a t e r c o k e t o x i c i t y under l o w hydrogen p r e s s u r e i s p r o b a b l y d u e t o d i f f u s i o n a l i n t r a g r a n u l a r l i m i t a t i o n s . Indeed, comparing t h e v a l u e o f t h e T h i e l e modulus 0 t o 1 [181,
t h e r e a c t i o n under h i g h hydrogen p r e s s u r e i s n o t found t o be d i f f u s i o n
l i m i t e d ( e f f e c t i v e n e s s f a c t o r q e q u a l t o 1 ) whereas i t i s under l o w hydrogen p r e s s u r e . On t h e o t h e r hand a naked eye e x a m i n a t i o n o f a p e l l e t shows t h a t , a s i n cumene
1,
d e a l k y l a t i o n [19,20
t h e coke formed d u r i n g t o l u e n e d i s p r o p o r t i o n a t i o n forms a
s h e l l around t h e m o r d e n i t e p e l l e t w i t h o u t r e a c h i n g t h e c e n t e r . I t i s w e l l known t h a t t h i s c o k e s h e l l d e p o s i t h a s a g r e a t e r p o i s o n i n g e f f e c t on t h e r e a c t i o n [ 211 when i t i s l i m i t e d by i n t r a g r a n u l a r d i f f u s i o n a s i s t h e c a s e under low hydrogen p r e s s u r e . However,
t h i s e x p l a n a t i o n d o e s n o t a c c o u n t f o r t h e g r e a t s e n s i t i v i t y of
m o r d e n i t e s t o t h e c o k e d e p o s i t e d under n i t r o g e n a s compared t o what i s o b s e r v e d under hydrogen. I n d e e d , on t h e f r e s h c a t a l y s t s , t h e e f f e c t i v e n e s s f a c t o r r? h a s s i m i l a r v a l u e s under b o t h hydrogen and n i t r o g e n . To a c c o u n t f o r t h e d i f f e r e n c e i n a g e i n g e f f e c t s o n e must assume e i t h e r t h a t t h e coke i s d i s t r i b u t e d d i f f e r e n t l y i n t h e p e l l e t o r t h a t i t s n a t u r e i s n o t t h e same under hydrogen as under n i t r o g e n . A s a matter o f f a c t ,
t h e h i g h e r r a t e o f c o k i n g under n i t r o g e n (mk i n t a b l e s 2 and 3 )
c a n l e a d t o a v e r y i m p o r t a n t coke d e p o s i t o n t h e p e l l e t ' s o u t e r s u r f a c e and conseq u e n t l y t o a v e r y r a p i d d e a c t i v a t i o n of t h e m o r d e n i t e . But. t h e mass s p e c t r u m o f t h e p r o d u c t s d e s o r b e d a t 4 0 O o C o u t o f t h e coked m o r d e n i t e shows t h a t t h e n a t u r e o f t h e coke depends o n o p e r a t i n g c o n d i t i o n s
:
t h e coke d e p o s i t e d under n i t r o g e n
c o n t a i n s compounds which have a h i g h e r m o l e c u l a r w e i g h t t h a n t h o s e o f coke d e p o s i t e d under hydrogen pressure [ 2 2
1.
88
I n f l u e n c e o f hydrogen on d i s p r o p o r t i o n a t i o n and c o k i n g r a t e s Under h i g h hydrogen p r e s s u r e , t h e d i s p r o p o r t i o n a t i o n a c t i v i t y o f m o r d e n i t e s ( a o t a b l e 3 ) i s of t h e same o r d e r of magnitude a s t h e i n i t i a l a c t i v i t y found under
low hydrogen p r e s s u r e ( a by 1 5 . Now a.
10
t a b l e 2 1 , even though t h e t o l u e n e p r e s s u r e i s m u l t i p l i e d
i s c l e a r l y higher than t h e a c t i v i t y obtained a t t h e very begining
o f t h e e x p e r i m e n t s ( f i g u r e 2 ) . These o b s e r v a t i o n s a r e c o n s i s t e n t w i t h t h e i n h i b i t i n g e f f e c t o f hydrogen on t o l u e n e d i s p r o p o r t i o n a t i o n , a s r e v e a l e d by a k i n e t i c s t u d y of the reaction[231.
Hydrogen a l s o i n h i b i t s c o k i n g t o a l a r g e e x t e n t , a s i s shown
by t h e amount o f t o l u e n e t r a n s f o r m e d i n t o c o k e , m k ,
and more p a r t i c u l a r l y by t h e
( t a b l e s 2 a n d 3 ) . To a c c o u n t f o r a n i n h i b i t i n g e f f e c t o f hydrogen d e c r e a s e o f m /m k D on c e r t a i n p a r a f f i n t r a n s f o r m a t i o n s o v e r a c i d c a t a l y s t s [24-261, t h e r e a c t i o n (2) whose e x i s t e n c e i s c l e a r l y shown i n a s u p e r a c i d medium [ 2 7 - 2 8 1 , R+
+ n, --+RH + n+
h a s been s u g g e s t e d
:
(2)
T h i s p r o p o s i t i o n o f f e r s a good e x p l a n a t i o n f o r o u r r e s u l t s s i n c e t h e i n t e r m e d i a t e s f o r c o k i n g and d i s p r o p o r t i o n a t i o n r e a c t i o n s c a n b e carbonium i o n s s u c h a s b e n z i l i c carbonium i o n s [29]. R e a c t i o n ( 2 ) r e d u c e s t h e c o n c e n t r a t i o n o f carbonium s p e c i e s and c o n s e q u e n t l y i n h i b i t s t h e c o k i n g and d i s p r o p o r t i o n a t i o n r e a c t i o n s which o c c u r t h r o u g h t h e s e i n t e r m e d i a t e s . Under t h i s h y p o t h e s i s t h e f o r m a t i o n o f coke which r e q u i r e s t h e p a r t i c i p a t i o n o f s e v e r a l m o l e c u l e s and t h e r e f o r e s e v e r a l r e a c t i o n s i n v o l v i n g carbonium i o n s , s h o u l d b e more i n h i b i t e d t h a n t h e b i m o l e c u l a r r e a c t i o n o f d i s p r o p o r t i o n a t i o n , which i s a c t u a l l y what o u r e x p e r i m e n t s show. R e a c t i o n ( 2 ) a l s o e x p l a i n s t h e coke d e s o r p t i o n by a l o n g t r e a t m e n t under hydrogen p r e s s u r e i f o n e s u p p o s e s t h a t t h e coke i s formed by b u l k y carbonium i o n s d i f f i c u l t t o d e s o r b The i n h i b i t i o n by hydrogen of t h e r e a c t i o n s o f d i s p r o p o r t i o n a t i o n and c o k i n g o c c u r s o n l y on m o r d e n i t e s ; no e f f e c t i s found on s i l i c a - a l u m i n a
( t a b l e 3 ) . This
o b s e r v a t i o n i m p l i e s t h a t , u n l i k e s i l i c a - a l u m i n a , mordenites are capable of a c t i v a t i n g hydrogen i n s u c h a way as t o e n a b l e i t t o remove t h e a d s o r b e d carbonium i o n s p e c i e s . I t i s w e l l known t h a t m o l e c u l a r hydrogen c a n b e a c t i v a t e d i n a s u p e r a c i d medium,
by s t r o n g a c i d s i t e s [ 3 0 ] . One c a n suppose t h a t m o r d e n i t e , which e x h i b i t s v e r y s t r o n g a c i d s i t e s [l],
c a n a l s o a c t i v a t e m o l e c u l a r hydrogen. However, it i s a l s o
w e l l known t h a t t h e m o l e c u l a r hydrogen a c t i v a t e d by h y d r o g e n a t i n g s i t e s c a n remove coke on a c i d s i t e s [ 3 1 ] . Hydrogenating c e n t e r s p r o b a b l y e x i s t on nickel-exchanged m o r d e n i t e , s i n c e a g r e a t p a r t o f n i c k e l i o n s are r e d u c e d d u r i n g p r e t r e a t m e n t s [6]. Moreover, s t r o n g a c i d s i t e s c r e a t e d by t r e a t m e n t o f m o r d e n i t e s under w e t a i r a r e probably a s s o c i a t e d with hydrogenating s i t e s , s i n c e t h e s e mordenites are a b l e t o decompose ammonia [ 6 ] .
Thus, t h e a c t i v a t i o n o f m o l e c u l a r hydrogen by n i c k e l f r e e
mordenites could be due t o t h e presence of i m p u r i t i e s such a s i r o n , a s r e p o r t e d i n t h e case o f X and Y z e o l i t e s [ 3 2 , 331. ACKNOWLEDGEMENT
:
The a u t h o r s t h a n k D.G.R.S.T.
f o r s u p p o r t i n g t h i s work.
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2
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3
I3 . W .
:.
:.