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Al Ratio
D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak (Editors), Methane Conuersion 0 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
579
INVESTIGATION OF ACIDIC PROPERTIES OF H-ZEOLITES AS A FUNCTION OF Si/Al RATIO K. SEGAWA,+ M. SAKAGUCHI and Y.
KURUSU
Department o f Chemistry, F a c u l t y o f Science and Technology, Sophia U n i v e r s i t y , 7-1 Kioi-cho, Chi yoda-ku, Tokyo 102( Japan)
ABSTRACT A c i d i c p r o p e r t i e s o f f o u r d i f f e r e n t t y p e s o f H - z e o l i t e c a t a l y s t s were A c i d i c s i t e s o f these z e o l i t e s s t u d i e d : H-Y, La-Y, H-M(mordenite) and H-ZSM5. have been c h a r a c t e r i z e d by TPD and high-temperature c a l o r i m e t r y o f N H 3 , and b y I R spectroscopy. The comparison c o n s i s t e n t l y a f f o r d e d t h e f o l l o w i n sequence The 2 9 S i - and "Al-MASNMR f o r t h e a c i d s t r e n g t h : H-M > H-ZSM5 1. H-Y > La-Y. e x h i b i t e d d e a l u m i n a t i o n b e h a v i o r d u r i n g t h e p r o t o n a t i o n o f t h e ammonium forms o f synthetic zeolites. The r e s o n a n c e l i n e o f '7Al-MASNMR attributed t o o c t a h e d r a l aluminum ions(dea1uminated s p e c i e s from z e o l i t e framework) appeared f o r H-Y a f t e r c a l c i n a t i o n i n a i r a t 773 K, whereas t h e o c t a h e d r a l peaks i n La-Y, H-M and H-ZSM5 were a p p r e c i a b l y i n h i b i t e d . Changes o f microenvironments o f S i - and A l - t e t r a h e d r a d u r i n g t h e p r o t o n a t i o n o f H - z e o l i t e s h a v e been i n v e s t i g a t e d : t h e s e would affect t h e a c i d i c p r o p e r t i e s .
INTRODUCTION Currently,
much a t t e n t i o n
i s focused on t h e a c i d i c p r o p e r t i e s and shape
1 ) . Since t h e
s e l e c t i v i t i e s o f c a t a l y t i c reactions over acid-type z e o l i t e s ( r e f .
a c i d i t y as w e l l as t h e s t r u c t u r e s t r o n g l y a f f e c t s t h e c a t a l y t i c behavior,
it i s
o f extreme importance t o c h a r a c t e r i z e t h e z e o l i t e c a t a l y s t as a f u n c t i o n o f Si/Al
r a t i o from t h e viewpoint o f t h e a c i d i t y ( r e f .
during t h e preparation o f acid-type zeolites, acidic
some d e a l u m i n a t i o n
properties o f zeolites.
zeolites
has o c c u r r e d ( r e f . T h i s work w i l l
3):
1246.0).
H-M(Si02/A1203=9.
and a c i d p r o p e r t i e s .
On t h e o t h e r hand,
7-20.4)
o f Na-form
t h i s would
r e f l e c t the
H-Y(Si02/A1203=4.8-5.6),
and H-ZSMS(Si02/Al203
=24.6-
We s t u d i e d t h e micro-environments o f S i - and A l -
t e t r a h e d r a i n t h e z e o l i t e framework by h i g h r e s o l u t i o n s o l i d s t a t e "Si27Al-MASNMR(magic
of
e l u c i d a t e t h e dealumination
behavior d u r i n g t h e preparation o f acid-type zeolites:
La-Y(Si02/Al203=4.8-5.6),
2).
by ion-exchange
a n g l e s p i n n i n g NMR),
and
and we compared t h e a c i d p r o p e r t i e s o f
z e o l i t e s by TPD( temperature-programmed d e s o r p t i o n ) and high-temperature m i c r o c a l o r i m e t r y o f NH3
*
and by I R spectroscopy.
To whom a l l correspondence should be addressed.
580
EXPERIMENTAL Z e o l i t e samples Z e o l i t e s a m p l e s w e r e s u p p l i e d by t h e C a t a l y s i s S o c i e t y o f J a p a n ( J a p a n Reference C a t a l y s t s : zeolites
were
(Y-type,
from JRC-Z-Y),
H-ZSM5(24.6,
JRC-Z-)
studied:
80.0,
form:
H-M(9.7,
5.6)(Y-type. 14.7,
1246)(ZSM5-type,
each code o f z e o l i t e ,
exchanged);
and H-M,
and
Numbers i n parentheses a f t e r
i s t h e Si02/A1203 mole r a t i o i n Na-
by e m i s s i o n s p e c t r o c h e m i c a l a n a l y s i s . o f Na-form
L a ( N 0 3 ) 3 f o r La-Y
ion-exchanged
5.6)
La-Y(4.8, from JRC-Z-M)
20.4)(mordenite-type,
z e o l i t e s were prepared by ion-exchange NH4N03 f o r H-Y
Four d i f f e r e n t a c i d - t y p e
from JRC-Z-Y).
from JRC-Z5).
such as H-Y(4.8),
t h i s was determined
1).
i n Na-form(ref.
H-Y(4.8,
w i t h aqueous
Acid-type solution o f
a n d H C 1 f o r H-ZSM5(above 9 9 %
s a m p l e s w e r e d r i e d a t 373 K f o r 24 h and t h e n
c a l c i n e d a t 773 K f o r 5 h. MASNMR spectroscopy The MASNMR has been e m p l o y e d f o r z e o l i t e s a m p l e s a f t e r t h e y h a d been hydrated
i n a desicator
t e m p e r a t u r e f o r 24 h,
saturated
aq.
solution
o f NH4C1 a t room
l i n e w i d t h o f 27A1-MASNMR
The MASNMR s p e c t r a were o b t a i n e d a t 53.7 MHz f o r 2 9 S i and 70.4 MHz
spectra. f o r 27Al
with
i n order t o minimize t h e
on a F o u r i e r t r a n s f o r m p u l s e d NMR spectrometer(JE0L JNM-GX270)
was equipped w i t h a CP/MAS
u n i t (JEOL NM-GSH27MU).
w i t h magic a n g l e spinning(MAS) d u r i n g data acquisition.
which
A l l NMR s p e c t r a combined
s p e c t r a were measured w i t h p r o t o n d e c o u p l i n g
Cross p o l a r i z a t i o n ( C P )
p r o t o n s do n o t a t t a c h d i r e c t l y t o "Si
was n o t employed,
and 2 7 A l n u c l e i .
c o l l e c t e d w i t h 700 t o 1300 scans accumulated p e r spectrum. a r e c a l i b r a t e d by TMS and Al(H20)63+
f o r 29Si-
since the
8 K d a t a p o i n t s were The chemical s h i f t s
and 27A1-MASNMR measurements.
respectively. High-temperature m i c r o c a l o r i m e t r y M i c r o c a l o r i m e t r i c measurements o f NH3 on a c i d - t y p e w i t h a high-temperature
calorimeter(HAC-450G.
z e o l i t e s were performed
Tokyo R i k o ) a t 473 K.
c a l o r i m e t r i c experiments, t h e samples were evacuated o v e r n i g h t a t 673 a pressure o f
Before
K
down t o
Pa f o r t h e e l i m i n a t i o n o f w a t e r molecules.
A d s o r p t i o n measurements and TPD Both a d s o r p t i o n measurements and TPD o f NH3 were performed under vacuum conditions.
About 150 mg o f a sample was charged i n a q u a r t z b a s k e t a t t a c h e d
t o a s t a n d a r d vacuum s y s t e m ( l ~ l O - ~ Pa). hung down t o t h e sample basket.
A McBain-type q u a r t z s p i r a l s p r i n g was
The sample was evacuated f o r 5 h a t 773 K.
The w e i g h t change b e f o r e o r a f t e r NH3 a d s o r p t i o n a t 373 K was d e t e r m i n e d by t h e
581 change
i n t h e l e n g t h o f the spring.
meter(type-2U,
Shinko E l e c t r o n i c ) .
which was equipped w i t h a d i s p l a c e m e n t A f t e r a d s o r p t i o n o f NH3,
t h e sample was
e v a c u a t e d a t 373 K u n t i l t h e p r e s s u r e r e a c h e d I x I O - ~ Pa(3-4 determination o f a c i d i t y .
F o r TPD experiments,
for the
t h e p r e s s u r e change from t h e
e l i m i n a t i o n o f NH3 a t e l e v a t e d temperature(373-773 by an i o n i z a t i o n gauge(G1-K,
h).
K, 10 K m i n - I ) was m o n i t o r e d
ULVAC) which r e c o r d e d a u t o m a t i c a l l y .
I R spectroscopy
I R c e l l w i t h K B r windows was designed t o f i t an i n f r a r e d
A vacuum-tight spectrometer(270-30,
H i t a c h i ) and t o be a t t a c h e d t o a vacuum ~ y s t e m ( l x l O - Pa). ~
I R s p e c t r a were o b t a i n e d a t room t e m p e r a t u r e w i t h t h e s p e c t r o m e t e r o p e r a t i n g i n t h e absorbance mode.
S e l f s u p p o r t i n g z e o l i t e wafers were pressed:
t h i c k n e s s was 10.2 mg could
be lowered
examination,
t h e sample
The c e l l was arranged such t h a t t h e z e o l i t e w a f e r
into slots
between t h e o p t i c a l
windows
for
spectroscopic
and withdrawn upward by t h e a c t i o n o f a magnet i n t o a heated
p o r t i o n f o r t h e p r e t r e a t m e n t and a d s o r p t i o n o f NH3, P y r i d i n e and C o l l i d i n e . RESULTS AND DISCUSSION
MASNMR spectroscopy I n framework a l u m i n o s i l i c a t e s , formula Si(nAl),
where n = 0.
t h e r e a r e f i v e p o s s i b i l i t i e s , d e s c r i b e d by t h e
1, 2, 3 o r 4.
These f i v e b a s i c u n i t s o f Si(nA1)
express t h e f a c t t h a t each silicon atom i s l i n k e d ,
A
v i a oxygens,
B
-90
-100
Ppm from TMS
-110
150
t o n aluminum
0
100
50
0
-50
ppm from A ~ ( H ~ o ) ~ ~ +
Fig. 1. "Si-MASNMR(A) and 27A1-MASNMR(B) s p e c t r a o f Na-Y(4.8). Open c i r c l e s denote A1 atoms, c l o s e d c i r c l e s S i atoms.
582 S
NH4-Y (4.8)
Si
H-Y (4.8) ( a i r )
H-Y(4.8) ( v a c )
La-Y (4.8) I
I
I
I
I
I
I
-60 -80 -100 -120
ppm from TMS
I
I
,
,
I
and 27A1-MASNMR s p e c t r a o f Y(4.8). Fig. 2. "Sia t 773 K; (vac), evacuated a t 773 K.
(air),
( S i /A1 ) nmr*
Si(4A1) Si(3A1) Si(EA1) S i ( l A 1 ) Si(OA1) ~~~
* *i6
0 0 0 0 0
11 11 4 8 10
44 43 14 38 37
c a l c u l a t e d by eqn. 1. determined by chemical a n a l y s i s .
36 37 38 42 39
~~
9 9 44 12 14
calcined i n a i r
with Si/A1 ratios.
peak i n t e n s i t i e s i n Y(4.8)
Normalized peak i n t e n s i t i e s
Na-Y (4.8) NH -Y(4.8) H-!( 4.8) ( a i r ) H-Y (4.8) (vac) La-Y (4.8)
I
2 7 -MASNMR ~ ~
29Si-MASNMR
TABLE 1 H i g h - r e s o l u t i o n "Si-MASNMR
I
0 -100 ppm from A1(H20)6 3+ 100
~~~
( S i /A1 )ca3b* ~~
2.5 2.6 5.1 2.8 2.8
2.4 2.4 2.4 2.4 2.4
583
H-Y (5.6)
La-Y (5.6)
H-M( 20.4)
H-ZSM5(80.0)
I
-60
I
I
I
I
,
I
l
I
-80 -100 -120
"Si-MASNMR "Si-
I
1
0
I
-100
2 7 ~-MASNMR 1
and E7A1-MASNMR s p e c t r a o f a c i d - t y p e z e o l i t e s .
The "Si-MASNMR
neighbors(ref.4).
d i f f e r e n t resonance peaks a t -89 kinds o f Si-tetrahedra Si(3A1),
I
ppm from A I ( H ~ O ) ~ ~ +
ppm from TMS
F i g . 3.
I
100
Si(ZAl),
s p e c t r a o f Na-Y(4.8) t o -105
i n Fig.
ppm from TMS,
1 showed f o u r
which r e p r e s e n t f o u r
a t t a c h e d t o d i f f e r e n t numbers o f a j a c e n t A l - t e t r a h e d r a ;
Si(lA1)
and Si(OA1).
The P7A1-MASNMR spectrum i n Fig.
1
g i v e s one sharp resonance peak a t 59 ppm f r o m A l ( H ~ 0 ) 6 ~ + ,which i s assigned t o t e t r a h e d r a l framework s i t e s occupied b y t h e aluminum(ref. shows d i r e c t l y s i x - c o o r d i n a t e d
aluminum(A1-octahedra)
o f ' t h e f o u r - c o o r d i n a t e d aluminum i n t h e by t h e dashed
l i n e i n t h e spectrum.
framework
4).
27A1-MASNMR a l s o
b u i l d up a t t h e expense
a t 0 ppm, as i s r e p r e s e n t e d
In case o f Na-Y(4.8).
no d e t e c t a b l e
584
o c t a h e d r a l aluminum has been observed.
Therefore,
a l l t h e aluminum atoms a r e
t e t r a h e d r a l l y c o o r d i n a t e d t o oxygens i n t h e framework. F i g u r e 2 g i v e s t h e "Si-
and 27A1-MASNMR
s p e c t r a o f NH4-Y(4.8),
c a l c i n e d i n a i r a t 773 K o r i n vacuum a t 773 K and La-Y(4.8). exchange f r o m Na-Y observed.
t o NH4-Y(see
When t h e NH4-Y
Fig.
1 and 2),
no s e r i o u s d e a l u m i n a t i o n was
was c a l c i n e d i n a i r a t 773 K ,
occurred.
B u t t h e o c t a h e d r a l aluminum peaks i n H-Y(4.8)
La-Y (4.8)
were a p p r e c i a b l y i n h i b i t e d ( s e e Fig. 2).
The d i s t r i b u t i o n s o f Si(nA1) i n "Si-MASNMR
H-Y(4.8) A f t e r ion-
some d e a l u m i n a t i o n
c a l c i n e d i n vacuum and
s p e c t r a o f Y(4.8)
were o b t a i n e d
by computer s i m u l a t i o n o f each spectrum, based on Gaussian peak shapes. and ( S i / A l )
1 summarizes t h e peak i n t e n s i t i e s o f Y(4.8) chemical
analysis,
t h a t denote ( S i / A l ) n m r
and ( S i / A l ) c a
respectively.
(Si/Al)nmr
r a t i o s a r e c a l c u l a t e d from t h e f o l l o w i n g e q u a t i o n ( r e f .
(si'A1)nmr
=
4 n=O
Y,
The
6):
4 . :Asi(nAl) n=O
i s t h e peak area o f each d e c o n v o l u t e d c u r v e o f "Si-MASNMR
Here As~(,,A~) spectra.
/
ASi(nAl)
Table
r a t i o s f r o m NMR and
When t h e d e a l u m i n a t i o n o c c u r r e d a f t e r h e a t t r e a t m e n t o f NH4-Y
o r La-
t h e aluminum atoms o f Si(3A1) and Si(2A1) a r e p r e f e r e n t i a l l y removed f r o m
t h e framework,
which may decrease t h e a c i d i t y .
(Si/Al)nmr from ( S i / A l ) c a calcined i n air,
And t h e i n c r e a s e d v a l u e s o f
i n d i c a t e t h e dealumination. E s p e c i a l l y f o r H-Y(4.8) s e r i o u s dealumination from t h e framework has been
observed. F i g u r e 3 and Table 2 show t h e NMR r e s u l t s o f H-Y(5.6). and H-ZSM5(80.0).
F o r H-M and H-ZSM5,
La-Y(5.6).
H-M(20.4)
t h e m a j o r A l - s i t e s would be S i ( l A 1 ) .
no s e r i o u s d e a l u m i n a t i o n o c c u r r e d d u r i n g t h e p r e p a r a t i o n . TABLE 2 H i g h - r e s o l u t i o n "Si-MASNMR with S i / A l ratios.
peak i n t e n s i t i e s i n a c i d - t y p e z e o l i t e s
Normalized peak i n t e n s i t i e s Si(4A1) Si(3A1) Si(2A1) S i ( l A 1 ) Si(OA1) ~
H-Y(5.6) La-Y (5.6) H-M( 9.7) H-M( 14.7) H-M( 20.4) H-ZSM5( 24.6) H-ZSM5( 80.0) H-ZSM5( 1246) 3tx
(Si/Al)nmr*
(Si/Al)ca+*
~
7 12 0 0 0 0 0 0
33 35 10 4 4 0 0 0
c a l c u l a t e d by eqn. 1. determined by chemical a n a l y s i s .
46 40 60 45 33 26 10 0
14 13 30 51 63 74 90 100
3.0 2.7 5.0 7.5 9.8 15.0 40.0 -
2.8 2.8 4.8 7.4 10.2 12.3 40.0 623
and
585
h
c,
.r
-0 .r
u
m
/j 0. 0
0.0
0.1
,
,
0 . O
0.2
0.3
0.4
1
0 : H-Y 0 : La-Y 0 : H-M
A: H-ZSM5
0.5
Al/Si ratio F i g . 4.
The a c i d i t y p e r S i atom as a f u n c t i o n o f A l / S i r a t i o .
2 f u n c t i o n o f Si/A1 r a t i o
Acidity
A c i d i t y p e r Si-atom o f a c i d - t y p e z e o l i t e s were measured by NH3 c h e m i s o r p t i o n a t 373 K as a f u n c t i o n o f A l / S i r a t i o .
We assumed t h a t one A l -
h e d r a l p a i r c o r r e s p o n d i n g t o one S i ( l A 1 ) g i v e s one a c i d s i t e . t h e observed a c i d i t y would be on t h e s o l i d l i n e i n Fig.
and S i - t e t r a I n t h i s case,
4.
The a c i d i t y o f
H-ZSM5 and H-M a r e c o n s i s t e n t w i t h t h e s o l i d l i n e , and t h e v a l u e s a r e i n c r e a s e d w i t h i n c r e a s i n g numbers o f Al-atoms.
B u t on H-Y and La-Y l o w e r v a l u e s t h a n t h e
e s t i m a t e d v a l u e a r e found.
This is because of the larger number of sites which do not result in a proportional increase in acid sites.
Si(d1)
High-temperature m i c r o c a l o r i m e t r y r e p o r t e d m i c r o c a l o r i m e t r y measurements o f NH3 a t 423 K t o
Vedrine e t al.,
c h a r a c t e r i z e t h e a c i d c e n t e r s i n H-ZSM5(ref.
7-8).
They found t h a t t h e a c i d
d i s t r i b u t i o n o f s t r o n g e r a c i d s i t e i s more' homogeneous i n H-ZSM5. the
temperature
at
473 K f o r
c a l o r i m e t r y measurements
information o f the stronger acid s i t e s .
We p r e f e r r e d
t o collect the
The r e s u l t s a r e summarized i n Fig. 5.
A l l m i c r o c a l o r i m e t r i c curves a r e decreased w i t h i n c r e a s i n g coverage o f NH3 on zeolites.
H-M(20.4)
showed a h i g h e r i n i t i a l
heat o f adsorption(higher
s t r e n g t h ) and a l a r g e r amount o f s t r o n g e r a c i d s i t e s t h a n o t h e r s . H-M(20.4)
and H-ZSM5(80.0)
acid
I n addition,
gave a d r a s t i c a l decrease o f t h e h e a t o f a d s o r p t i o n
a t p a r t i c u l a r narrow domain o f coverage o f NH3.
The r e s u l t s suggest t h a t H-M
and H-ZSM5
h a v e more homogeneous A1 d i s t r i b u t i o n a l o n g t h e c h a n n e l s o f
zeolites.
The c a l o r i m e t r i c c u r v e s o f H-Y(5.6)
indicate the wider acid
586
-
150
c 4)
7
0
E
. 3 Y
0 1
I
I
1 .o
0.0
2.0
NH3 adsorbed / mmol g - l Fig. 5.
High-temperature m i c r o c a l o r i m e t r y o f NH3 on a c i d type z e o l i t e s .
d i s t r i b u t i o n than H-M
and H-ZSM5.
From t h e m i c r o c a l o r i m e t r y and t h e TPD o f
NH3. we conclude t h e f o l l o w i n g sequence f o r t h e a c i d s t r e n g t h :
H-M > H-ZSM5
> H-Y > La-Y.
I R spectroscopy The OH s t r e t c h i n g v i b r a t i o n s o f acid-type z e o l i t e s a f t e r evacuation a t 773 K a r e shown i n F i g .
6.
Two o r t h r e e t y p e s o f OH g r o u p s a r e o b s e r v e d a t
wavenumbers from 3800 t o 3500 cm-l.
9-10)
zeolite(ref,
Terminal Si-OH groups a t o u t e r s u r f a c e o f
g i v e s a band a t 3738 o r 3735 cm-’.
Those t e r m i n a l Si-OH
groups showed very weak chemical i n t e r a c t i o n between p y r i d i n e and NH3.
The
a c i d i c OH bands whose i n t e n s i t i e s reduced o r disappeared a f t e r a d s o r p t i o n o f base molecules g i v e bands a t 3673 and 3648 cm-I La-Y(5.6).
3610 cm-l
f o r H-M(20.4)
expense o f those a c i d i c OH bands, region.
hydroxyls(ref. 3610 cm-l
NH bands
However, t h e band a t 3557 cm-I
base molecules,
f o r H-Y(5.6).
and 3613 cm-l
3676 cm-’
f o r H-ZSM5(80.0).
for
A t the
b u i l t up a t 3500 t o 3000 cm-’
i n La-Y(5.6)
does n o t i n t e r a c t w i t h
and t h e band can be assigned t o OH s t r e t c h i n g o f Lanthanum 11).
For H-M(20.4),
about 25-30 % o f t h e a c i d i c OH bands a t
s t i l l remained even a f t e r a d s o r p t i o n a t room temperature.
was i n t r o d u c e d t o H-M.
t h e a c i d i c band almost vanished.
When NH3
We conclude t h a t about
25-30 % o f OH groups i n H-M a r e l o c a t e d i n s m a l l e r channels, such as so-called s i d e pockets, and t h e r e s t a r e i n main channels o r on o u t e r surfaces. The r i n g v i b r a t i o n r e g i o n s o f I R s p e c t r a a f t e r ’ a d s o r p t i o n a t 473 K on acidt y p e z e o l i t e s are shown i n Fig. 7.
The major a c i d s i t e s a r e Bronsted sites(BPy
587
."..\
mmm
mm m, mm
mr.u
r.IDI0
mmm
1 ',
, 3000 4000 3000 4000 3000 4000 3000
4000
wave number
H-Y (5.6)
La-Y (5.6)
/ cm-l H-ZSM5 (80.0)
H-M(20.4)
F i g . 6. I R s p e c t r a o f OH s t r e t c h i n g o f a c i d - t y p e z e o l i t e s : ( a ) a f t e r e v a c u a t i o n a t 773 K, ( b ) p y r i d i n e adsorbed and evacuated a t 473 K. ( c ) NH3 adsorbed and evacuated a t room temperature.
h
1
0.1
cn
-
d
7
- m h m a M
-
h
b
----
1700
1400 1700
1400 1700
wave number
H-Y (5.6) Fig. 7.
La-Y (5.6)
1400 1700
/ cm-l H-M( 20.4)
A d s o r p t i o n o f p y r i d i n e on a c i d - t y p e z e o l i t e s .
1400
H-ZS?l5( 80.0)
588
at
1543 cm-’)
(5.6)(BPy:
for
65 %).
83 Z),
H-ZSM5(80.0)(BPy: However, f o r H-Y,
H-M(20.4)(BPy:
80 % ) and La-Y
o n l y 26 Z o f BPy was observed.
After the
d e a l u m i n a t i o n , t h e c o n c e n t r a t i o n o f BPy tended t o decrease. When more b u l k y base m o l e c u l e s ( c o 1 l i d i n e : i n t r o d u c e d on a c i d - t y p e z e o l i t e s ,
2,4,6-trimethyl-pyridine)
were
o n l y H-ZSM5 d i d n o t i n t e r a c t w i t h c o l l i d i n e .
The r e s u l t s suggest t h a t a l l t h e a c i d i c s i t e s o f H-ZSM5 a r e l o c a t e d i n t h e channels b u t n o t on t h e o u t e r surfaces. I n conclusion,
d e a l u m i n a t i o n o f a l u m i n o s i l i c a t e s has o c c u r r e d a t t h e
c a l c i n a t i o n s t e p from ammonium-form t o H-form. change t o (A10)’
The p a r t of A l - t e t r a h e d r a may
o r n o n - a c i d i c aluminum o x i d e species.
We found d e a l u m i a t i o n
o c c u r r e d on Si(3A1) o r Si(2A1) s i t e s w i t h w a t e r vapor which would a c t as an a c i d i c r e a g e n t a t h i g h e r temperatures.
Most o f t h e S i ( l A 1 ) s i t e s o f H-M and
H-ZSM5 a r e a c i d i c and 80 % o f a c i d i c s i t e s a r e s t r o n g e r Bronsted s i t e s .
REFERENCES 1
7
8 9 10 11
M. Niwa. M. Iwamoto and K. Seqawa, B u l l . Chem. SOC. Jpn., 59(1986) 37 3 5- 3 739. W.O. Haag. R.M. Lago and P.B. Weisz, Nature, 309(1984) 589-591. P.A. Jacobs and H.K. Beyer, J. Phys. Chem., 83(1979) 1174-1177. J.M. Thomas and J. K l i n o w s k i . Adv. Catal., 33(1985) 199-374. J. K l i n o w s k i , J.M. Thomas, C.A. F y f e and G.C. Gobbi, Nature, 296(1982) 533-537. C.A. Fyfe. J.M. Thomas, J. K l i n o w s k i and G.C. Gobbi, Angew. Chem. I n t . Ed. Engl., 22(1983) 259-267. J. Vedrine, A. Auroux, P. Dejave, V. Ducarme. H. Hoser and S Zhou, J. Catal., 73(1982) 147-160. A. Auroux, V. B o l i s . P. Wierzchowki, P.C. G r a v e l l e and J.C. Vedrine, J. Chem. SOC. Faraday Trans. 11, 75(1979) 2544-2555. P.A. Jacobs and R. von Ballmoos. J. Phys. Chem.. 86(1982) 3050-3052. M. B. Sayed, R.A. Kydd and R. P. Cooney, J. Catal., 88( 1984) 137-149. J. Scherzer and J.L. Bass, J. C a t a l . , 46(1977) 100-108.
579
INVESTIGATION OF ACIDIC PROPERTIES OF H-ZEOLITES AS A FUNCTION OF Si/Al RATIO K. SEGAWA,+ M. SAKAGUCHI and Y.
KURUSU
Department o f Chemistry, F a c u l t y o f Science and Technology, Sophia U n i v e r s i t y , 7-1 Kioi-cho, Chi yoda-ku, Tokyo 102( Japan)
ABSTRACT A c i d i c p r o p e r t i e s o f f o u r d i f f e r e n t t y p e s o f H - z e o l i t e c a t a l y s t s were A c i d i c s i t e s o f these z e o l i t e s s t u d i e d : H-Y, La-Y, H-M(mordenite) and H-ZSM5. have been c h a r a c t e r i z e d by TPD and high-temperature c a l o r i m e t r y o f N H 3 , and b y I R spectroscopy. The comparison c o n s i s t e n t l y a f f o r d e d t h e f o l l o w i n sequence The 2 9 S i - and "Al-MASNMR f o r t h e a c i d s t r e n g t h : H-M > H-ZSM5 1. H-Y > La-Y. e x h i b i t e d d e a l u m i n a t i o n b e h a v i o r d u r i n g t h e p r o t o n a t i o n o f t h e ammonium forms o f synthetic zeolites. The r e s o n a n c e l i n e o f '7Al-MASNMR attributed t o o c t a h e d r a l aluminum ions(dea1uminated s p e c i e s from z e o l i t e framework) appeared f o r H-Y a f t e r c a l c i n a t i o n i n a i r a t 773 K, whereas t h e o c t a h e d r a l peaks i n La-Y, H-M and H-ZSM5 were a p p r e c i a b l y i n h i b i t e d . Changes o f microenvironments o f S i - and A l - t e t r a h e d r a d u r i n g t h e p r o t o n a t i o n o f H - z e o l i t e s h a v e been i n v e s t i g a t e d : t h e s e would affect t h e a c i d i c p r o p e r t i e s .
INTRODUCTION Currently,
much a t t e n t i o n
i s focused on t h e a c i d i c p r o p e r t i e s and shape
1 ) . Since t h e
s e l e c t i v i t i e s o f c a t a l y t i c reactions over acid-type z e o l i t e s ( r e f .
a c i d i t y as w e l l as t h e s t r u c t u r e s t r o n g l y a f f e c t s t h e c a t a l y t i c behavior,
it i s
o f extreme importance t o c h a r a c t e r i z e t h e z e o l i t e c a t a l y s t as a f u n c t i o n o f Si/Al
r a t i o from t h e viewpoint o f t h e a c i d i t y ( r e f .
during t h e preparation o f acid-type zeolites, acidic
some d e a l u m i n a t i o n
properties o f zeolites.
zeolites
has o c c u r r e d ( r e f . T h i s work w i l l
3):
1246.0).
H-M(Si02/A1203=9.
and a c i d p r o p e r t i e s .
On t h e o t h e r hand,
7-20.4)
o f Na-form
t h i s would
r e f l e c t the
H-Y(Si02/A1203=4.8-5.6),
and H-ZSMS(Si02/Al203
=24.6-
We s t u d i e d t h e micro-environments o f S i - and A l -
t e t r a h e d r a i n t h e z e o l i t e framework by h i g h r e s o l u t i o n s o l i d s t a t e "Si27Al-MASNMR(magic
of
e l u c i d a t e t h e dealumination
behavior d u r i n g t h e preparation o f acid-type zeolites:
La-Y(Si02/Al203=4.8-5.6),
2).
by ion-exchange
a n g l e s p i n n i n g NMR),
and
and we compared t h e a c i d p r o p e r t i e s o f
z e o l i t e s by TPD( temperature-programmed d e s o r p t i o n ) and high-temperature m i c r o c a l o r i m e t r y o f NH3
*
and by I R spectroscopy.
To whom a l l correspondence should be addressed.
580
EXPERIMENTAL Z e o l i t e samples Z e o l i t e s a m p l e s w e r e s u p p l i e d by t h e C a t a l y s i s S o c i e t y o f J a p a n ( J a p a n Reference C a t a l y s t s : zeolites
were
(Y-type,
from JRC-Z-Y),
H-ZSM5(24.6,
JRC-Z-)
studied:
80.0,
form:
H-M(9.7,
5.6)(Y-type. 14.7,
1246)(ZSM5-type,
each code o f z e o l i t e ,
exchanged);
and H-M,
and
Numbers i n parentheses a f t e r
i s t h e Si02/A1203 mole r a t i o i n Na-
by e m i s s i o n s p e c t r o c h e m i c a l a n a l y s i s . o f Na-form
L a ( N 0 3 ) 3 f o r La-Y
ion-exchanged
5.6)
La-Y(4.8, from JRC-Z-M)
20.4)(mordenite-type,
z e o l i t e s were prepared by ion-exchange NH4N03 f o r H-Y
Four d i f f e r e n t a c i d - t y p e
from JRC-Z-Y).
from JRC-Z5).
such as H-Y(4.8),
t h i s was determined
1).
i n Na-form(ref.
H-Y(4.8,
w i t h aqueous
Acid-type solution o f
a n d H C 1 f o r H-ZSM5(above 9 9 %
s a m p l e s w e r e d r i e d a t 373 K f o r 24 h and t h e n
c a l c i n e d a t 773 K f o r 5 h. MASNMR spectroscopy The MASNMR has been e m p l o y e d f o r z e o l i t e s a m p l e s a f t e r t h e y h a d been hydrated
i n a desicator
t e m p e r a t u r e f o r 24 h,
saturated
aq.
solution
o f NH4C1 a t room
l i n e w i d t h o f 27A1-MASNMR
The MASNMR s p e c t r a were o b t a i n e d a t 53.7 MHz f o r 2 9 S i and 70.4 MHz
spectra. f o r 27Al
with
i n order t o minimize t h e
on a F o u r i e r t r a n s f o r m p u l s e d NMR spectrometer(JE0L JNM-GX270)
was equipped w i t h a CP/MAS
u n i t (JEOL NM-GSH27MU).
w i t h magic a n g l e spinning(MAS) d u r i n g data acquisition.
which
A l l NMR s p e c t r a combined
s p e c t r a were measured w i t h p r o t o n d e c o u p l i n g
Cross p o l a r i z a t i o n ( C P )
p r o t o n s do n o t a t t a c h d i r e c t l y t o "Si
was n o t employed,
and 2 7 A l n u c l e i .
c o l l e c t e d w i t h 700 t o 1300 scans accumulated p e r spectrum. a r e c a l i b r a t e d by TMS and Al(H20)63+
f o r 29Si-
since the
8 K d a t a p o i n t s were The chemical s h i f t s
and 27A1-MASNMR measurements.
respectively. High-temperature m i c r o c a l o r i m e t r y M i c r o c a l o r i m e t r i c measurements o f NH3 on a c i d - t y p e w i t h a high-temperature
calorimeter(HAC-450G.
z e o l i t e s were performed
Tokyo R i k o ) a t 473 K.
c a l o r i m e t r i c experiments, t h e samples were evacuated o v e r n i g h t a t 673 a pressure o f
Before
K
down t o
Pa f o r t h e e l i m i n a t i o n o f w a t e r molecules.
A d s o r p t i o n measurements and TPD Both a d s o r p t i o n measurements and TPD o f NH3 were performed under vacuum conditions.
About 150 mg o f a sample was charged i n a q u a r t z b a s k e t a t t a c h e d
t o a s t a n d a r d vacuum s y s t e m ( l ~ l O - ~ Pa). hung down t o t h e sample basket.
A McBain-type q u a r t z s p i r a l s p r i n g was
The sample was evacuated f o r 5 h a t 773 K.
The w e i g h t change b e f o r e o r a f t e r NH3 a d s o r p t i o n a t 373 K was d e t e r m i n e d by t h e
581 change
i n t h e l e n g t h o f the spring.
meter(type-2U,
Shinko E l e c t r o n i c ) .
which was equipped w i t h a d i s p l a c e m e n t A f t e r a d s o r p t i o n o f NH3,
t h e sample was
e v a c u a t e d a t 373 K u n t i l t h e p r e s s u r e r e a c h e d I x I O - ~ Pa(3-4 determination o f a c i d i t y .
F o r TPD experiments,
for the
t h e p r e s s u r e change from t h e
e l i m i n a t i o n o f NH3 a t e l e v a t e d temperature(373-773 by an i o n i z a t i o n gauge(G1-K,
h).
K, 10 K m i n - I ) was m o n i t o r e d
ULVAC) which r e c o r d e d a u t o m a t i c a l l y .
I R spectroscopy
I R c e l l w i t h K B r windows was designed t o f i t an i n f r a r e d
A vacuum-tight spectrometer(270-30,
H i t a c h i ) and t o be a t t a c h e d t o a vacuum ~ y s t e m ( l x l O - Pa). ~
I R s p e c t r a were o b t a i n e d a t room t e m p e r a t u r e w i t h t h e s p e c t r o m e t e r o p e r a t i n g i n t h e absorbance mode.
S e l f s u p p o r t i n g z e o l i t e wafers were pressed:
t h i c k n e s s was 10.2 mg could
be lowered
examination,
t h e sample
The c e l l was arranged such t h a t t h e z e o l i t e w a f e r
into slots
between t h e o p t i c a l
windows
for
spectroscopic
and withdrawn upward by t h e a c t i o n o f a magnet i n t o a heated
p o r t i o n f o r t h e p r e t r e a t m e n t and a d s o r p t i o n o f NH3, P y r i d i n e and C o l l i d i n e . RESULTS AND DISCUSSION
MASNMR spectroscopy I n framework a l u m i n o s i l i c a t e s , formula Si(nAl),
where n = 0.
t h e r e a r e f i v e p o s s i b i l i t i e s , d e s c r i b e d by t h e
1, 2, 3 o r 4.
These f i v e b a s i c u n i t s o f Si(nA1)
express t h e f a c t t h a t each silicon atom i s l i n k e d ,
A
v i a oxygens,
B
-90
-100
Ppm from TMS
-110
150
t o n aluminum
0
100
50
0
-50
ppm from A ~ ( H ~ o ) ~ ~ +
Fig. 1. "Si-MASNMR(A) and 27A1-MASNMR(B) s p e c t r a o f Na-Y(4.8). Open c i r c l e s denote A1 atoms, c l o s e d c i r c l e s S i atoms.
582 S
NH4-Y (4.8)
Si
H-Y (4.8) ( a i r )
H-Y(4.8) ( v a c )
La-Y (4.8) I
I
I
I
I
I
I
-60 -80 -100 -120
ppm from TMS
I
I
,
,
I
and 27A1-MASNMR s p e c t r a o f Y(4.8). Fig. 2. "Sia t 773 K; (vac), evacuated a t 773 K.
(air),
( S i /A1 ) nmr*
Si(4A1) Si(3A1) Si(EA1) S i ( l A 1 ) Si(OA1) ~~~
* *i6
0 0 0 0 0
11 11 4 8 10
44 43 14 38 37
c a l c u l a t e d by eqn. 1. determined by chemical a n a l y s i s .
36 37 38 42 39
~~
9 9 44 12 14
calcined i n a i r
with Si/A1 ratios.
peak i n t e n s i t i e s i n Y(4.8)
Normalized peak i n t e n s i t i e s
Na-Y (4.8) NH -Y(4.8) H-!( 4.8) ( a i r ) H-Y (4.8) (vac) La-Y (4.8)
I
2 7 -MASNMR ~ ~
29Si-MASNMR
TABLE 1 H i g h - r e s o l u t i o n "Si-MASNMR
I
0 -100 ppm from A1(H20)6 3+ 100
~~~
( S i /A1 )ca3b* ~~
2.5 2.6 5.1 2.8 2.8
2.4 2.4 2.4 2.4 2.4
583
H-Y (5.6)
La-Y (5.6)
H-M( 20.4)
H-ZSM5(80.0)
I
-60
I
I
I
I
,
I
l
I
-80 -100 -120
"Si-MASNMR "Si-
I
1
0
I
-100
2 7 ~-MASNMR 1
and E7A1-MASNMR s p e c t r a o f a c i d - t y p e z e o l i t e s .
The "Si-MASNMR
neighbors(ref.4).
d i f f e r e n t resonance peaks a t -89 kinds o f Si-tetrahedra Si(3A1),
I
ppm from A I ( H ~ O ) ~ ~ +
ppm from TMS
F i g . 3.
I
100
Si(ZAl),
s p e c t r a o f Na-Y(4.8) t o -105
i n Fig.
ppm from TMS,
1 showed f o u r
which r e p r e s e n t f o u r
a t t a c h e d t o d i f f e r e n t numbers o f a j a c e n t A l - t e t r a h e d r a ;
Si(lA1)
and Si(OA1).
The P7A1-MASNMR spectrum i n Fig.
1
g i v e s one sharp resonance peak a t 59 ppm f r o m A l ( H ~ 0 ) 6 ~ + ,which i s assigned t o t e t r a h e d r a l framework s i t e s occupied b y t h e aluminum(ref. shows d i r e c t l y s i x - c o o r d i n a t e d
aluminum(A1-octahedra)
o f ' t h e f o u r - c o o r d i n a t e d aluminum i n t h e by t h e dashed
l i n e i n t h e spectrum.
framework
4).
27A1-MASNMR a l s o
b u i l d up a t t h e expense
a t 0 ppm, as i s r e p r e s e n t e d
In case o f Na-Y(4.8).
no d e t e c t a b l e
584
o c t a h e d r a l aluminum has been observed.
Therefore,
a l l t h e aluminum atoms a r e
t e t r a h e d r a l l y c o o r d i n a t e d t o oxygens i n t h e framework. F i g u r e 2 g i v e s t h e "Si-
and 27A1-MASNMR
s p e c t r a o f NH4-Y(4.8),
c a l c i n e d i n a i r a t 773 K o r i n vacuum a t 773 K and La-Y(4.8). exchange f r o m Na-Y observed.
t o NH4-Y(see
When t h e NH4-Y
Fig.
1 and 2),
no s e r i o u s d e a l u m i n a t i o n was
was c a l c i n e d i n a i r a t 773 K ,
occurred.
B u t t h e o c t a h e d r a l aluminum peaks i n H-Y(4.8)
La-Y (4.8)
were a p p r e c i a b l y i n h i b i t e d ( s e e Fig. 2).
The d i s t r i b u t i o n s o f Si(nA1) i n "Si-MASNMR
H-Y(4.8) A f t e r ion-
some d e a l u m i n a t i o n
c a l c i n e d i n vacuum and
s p e c t r a o f Y(4.8)
were o b t a i n e d
by computer s i m u l a t i o n o f each spectrum, based on Gaussian peak shapes. and ( S i / A l )
1 summarizes t h e peak i n t e n s i t i e s o f Y(4.8) chemical
analysis,
t h a t denote ( S i / A l ) n m r
and ( S i / A l ) c a
respectively.
(Si/Al)nmr
r a t i o s a r e c a l c u l a t e d from t h e f o l l o w i n g e q u a t i o n ( r e f .
(si'A1)nmr
=
4 n=O
Y,
The
6):
4 . :Asi(nAl) n=O
i s t h e peak area o f each d e c o n v o l u t e d c u r v e o f "Si-MASNMR
Here As~(,,A~) spectra.
/
ASi(nAl)
Table
r a t i o s f r o m NMR and
When t h e d e a l u m i n a t i o n o c c u r r e d a f t e r h e a t t r e a t m e n t o f NH4-Y
o r La-
t h e aluminum atoms o f Si(3A1) and Si(2A1) a r e p r e f e r e n t i a l l y removed f r o m
t h e framework,
which may decrease t h e a c i d i t y .
(Si/Al)nmr from ( S i / A l ) c a calcined i n air,
And t h e i n c r e a s e d v a l u e s o f
i n d i c a t e t h e dealumination. E s p e c i a l l y f o r H-Y(4.8) s e r i o u s dealumination from t h e framework has been
observed. F i g u r e 3 and Table 2 show t h e NMR r e s u l t s o f H-Y(5.6). and H-ZSM5(80.0).
F o r H-M and H-ZSM5,
La-Y(5.6).
H-M(20.4)
t h e m a j o r A l - s i t e s would be S i ( l A 1 ) .
no s e r i o u s d e a l u m i n a t i o n o c c u r r e d d u r i n g t h e p r e p a r a t i o n . TABLE 2 H i g h - r e s o l u t i o n "Si-MASNMR with S i / A l ratios.
peak i n t e n s i t i e s i n a c i d - t y p e z e o l i t e s
Normalized peak i n t e n s i t i e s Si(4A1) Si(3A1) Si(2A1) S i ( l A 1 ) Si(OA1) ~
H-Y(5.6) La-Y (5.6) H-M( 9.7) H-M( 14.7) H-M( 20.4) H-ZSM5( 24.6) H-ZSM5( 80.0) H-ZSM5( 1246) 3tx
(Si/Al)nmr*
(Si/Al)ca+*
~
7 12 0 0 0 0 0 0
33 35 10 4 4 0 0 0
c a l c u l a t e d by eqn. 1. determined by chemical a n a l y s i s .
46 40 60 45 33 26 10 0
14 13 30 51 63 74 90 100
3.0 2.7 5.0 7.5 9.8 15.0 40.0 -
2.8 2.8 4.8 7.4 10.2 12.3 40.0 623
and
585
h
c,
.r
-0 .r
u
m
/j 0. 0
0.0
0.1
,
,
0 . O
0.2
0.3
0.4
1
0 : H-Y 0 : La-Y 0 : H-M
A: H-ZSM5
0.5
Al/Si ratio F i g . 4.
The a c i d i t y p e r S i atom as a f u n c t i o n o f A l / S i r a t i o .
2 f u n c t i o n o f Si/A1 r a t i o
Acidity
A c i d i t y p e r Si-atom o f a c i d - t y p e z e o l i t e s were measured by NH3 c h e m i s o r p t i o n a t 373 K as a f u n c t i o n o f A l / S i r a t i o .
We assumed t h a t one A l -
h e d r a l p a i r c o r r e s p o n d i n g t o one S i ( l A 1 ) g i v e s one a c i d s i t e . t h e observed a c i d i t y would be on t h e s o l i d l i n e i n Fig.
and S i - t e t r a I n t h i s case,
4.
The a c i d i t y o f
H-ZSM5 and H-M a r e c o n s i s t e n t w i t h t h e s o l i d l i n e , and t h e v a l u e s a r e i n c r e a s e d w i t h i n c r e a s i n g numbers o f Al-atoms.
B u t on H-Y and La-Y l o w e r v a l u e s t h a n t h e
e s t i m a t e d v a l u e a r e found.
This is because of the larger number of sites which do not result in a proportional increase in acid sites.
Si(d1)
High-temperature m i c r o c a l o r i m e t r y r e p o r t e d m i c r o c a l o r i m e t r y measurements o f NH3 a t 423 K t o
Vedrine e t al.,
c h a r a c t e r i z e t h e a c i d c e n t e r s i n H-ZSM5(ref.
7-8).
They found t h a t t h e a c i d
d i s t r i b u t i o n o f s t r o n g e r a c i d s i t e i s more' homogeneous i n H-ZSM5. the
temperature
at
473 K f o r
c a l o r i m e t r y measurements
information o f the stronger acid s i t e s .
We p r e f e r r e d
t o collect the
The r e s u l t s a r e summarized i n Fig. 5.
A l l m i c r o c a l o r i m e t r i c curves a r e decreased w i t h i n c r e a s i n g coverage o f NH3 on zeolites.
H-M(20.4)
showed a h i g h e r i n i t i a l
heat o f adsorption(higher
s t r e n g t h ) and a l a r g e r amount o f s t r o n g e r a c i d s i t e s t h a n o t h e r s . H-M(20.4)
and H-ZSM5(80.0)
acid
I n addition,
gave a d r a s t i c a l decrease o f t h e h e a t o f a d s o r p t i o n
a t p a r t i c u l a r narrow domain o f coverage o f NH3.
The r e s u l t s suggest t h a t H-M
and H-ZSM5
h a v e more homogeneous A1 d i s t r i b u t i o n a l o n g t h e c h a n n e l s o f
zeolites.
The c a l o r i m e t r i c c u r v e s o f H-Y(5.6)
indicate the wider acid
586
-
150
c 4)
7
0
E
. 3 Y
0 1
I
I
1 .o
0.0
2.0
NH3 adsorbed / mmol g - l Fig. 5.
High-temperature m i c r o c a l o r i m e t r y o f NH3 on a c i d type z e o l i t e s .
d i s t r i b u t i o n than H-M
and H-ZSM5.
From t h e m i c r o c a l o r i m e t r y and t h e TPD o f
NH3. we conclude t h e f o l l o w i n g sequence f o r t h e a c i d s t r e n g t h :
H-M > H-ZSM5
> H-Y > La-Y.
I R spectroscopy The OH s t r e t c h i n g v i b r a t i o n s o f acid-type z e o l i t e s a f t e r evacuation a t 773 K a r e shown i n F i g .
6.
Two o r t h r e e t y p e s o f OH g r o u p s a r e o b s e r v e d a t
wavenumbers from 3800 t o 3500 cm-l.
9-10)
zeolite(ref,
Terminal Si-OH groups a t o u t e r s u r f a c e o f
g i v e s a band a t 3738 o r 3735 cm-’.
Those t e r m i n a l Si-OH
groups showed very weak chemical i n t e r a c t i o n between p y r i d i n e and NH3.
The
a c i d i c OH bands whose i n t e n s i t i e s reduced o r disappeared a f t e r a d s o r p t i o n o f base molecules g i v e bands a t 3673 and 3648 cm-I La-Y(5.6).
3610 cm-l
f o r H-M(20.4)
expense o f those a c i d i c OH bands, region.
hydroxyls(ref. 3610 cm-l
NH bands
However, t h e band a t 3557 cm-I
base molecules,
f o r H-Y(5.6).
and 3613 cm-l
3676 cm-’
f o r H-ZSM5(80.0).
for
A t the
b u i l t up a t 3500 t o 3000 cm-’
i n La-Y(5.6)
does n o t i n t e r a c t w i t h
and t h e band can be assigned t o OH s t r e t c h i n g o f Lanthanum 11).
For H-M(20.4),
about 25-30 % o f t h e a c i d i c OH bands a t
s t i l l remained even a f t e r a d s o r p t i o n a t room temperature.
was i n t r o d u c e d t o H-M.
t h e a c i d i c band almost vanished.
When NH3
We conclude t h a t about
25-30 % o f OH groups i n H-M a r e l o c a t e d i n s m a l l e r channels, such as so-called s i d e pockets, and t h e r e s t a r e i n main channels o r on o u t e r surfaces. The r i n g v i b r a t i o n r e g i o n s o f I R s p e c t r a a f t e r ’ a d s o r p t i o n a t 473 K on acidt y p e z e o l i t e s are shown i n Fig. 7.
The major a c i d s i t e s a r e Bronsted sites(BPy
587
."..\
mmm
mm m, mm
mr.u
r.IDI0
mmm
1 ',
, 3000 4000 3000 4000 3000 4000 3000
4000
wave number
H-Y (5.6)
La-Y (5.6)
/ cm-l H-ZSM5 (80.0)
H-M(20.4)
F i g . 6. I R s p e c t r a o f OH s t r e t c h i n g o f a c i d - t y p e z e o l i t e s : ( a ) a f t e r e v a c u a t i o n a t 773 K, ( b ) p y r i d i n e adsorbed and evacuated a t 473 K. ( c ) NH3 adsorbed and evacuated a t room temperature.
h
1
0.1
cn
-
d
7
- m h m a M
-
h
b
----
1700
1400 1700
1400 1700
wave number
H-Y (5.6) Fig. 7.
La-Y (5.6)
1400 1700
/ cm-l H-M( 20.4)
A d s o r p t i o n o f p y r i d i n e on a c i d - t y p e z e o l i t e s .
1400
H-ZS?l5( 80.0)
588
at
1543 cm-’)
(5.6)(BPy:
for
65 %).
83 Z),
H-ZSM5(80.0)(BPy: However, f o r H-Y,
H-M(20.4)(BPy:
80 % ) and La-Y
o n l y 26 Z o f BPy was observed.
After the
d e a l u m i n a t i o n , t h e c o n c e n t r a t i o n o f BPy tended t o decrease. When more b u l k y base m o l e c u l e s ( c o 1 l i d i n e : i n t r o d u c e d on a c i d - t y p e z e o l i t e s ,
2,4,6-trimethyl-pyridine)
were
o n l y H-ZSM5 d i d n o t i n t e r a c t w i t h c o l l i d i n e .
The r e s u l t s suggest t h a t a l l t h e a c i d i c s i t e s o f H-ZSM5 a r e l o c a t e d i n t h e channels b u t n o t on t h e o u t e r surfaces. I n conclusion,
d e a l u m i n a t i o n o f a l u m i n o s i l i c a t e s has o c c u r r e d a t t h e
c a l c i n a t i o n s t e p from ammonium-form t o H-form. change t o (A10)’
The p a r t of A l - t e t r a h e d r a may
o r n o n - a c i d i c aluminum o x i d e species.
We found d e a l u m i a t i o n
o c c u r r e d on Si(3A1) o r Si(2A1) s i t e s w i t h w a t e r vapor which would a c t as an a c i d i c r e a g e n t a t h i g h e r temperatures.
Most o f t h e S i ( l A 1 ) s i t e s o f H-M and
H-ZSM5 a r e a c i d i c and 80 % o f a c i d i c s i t e s a r e s t r o n g e r Bronsted s i t e s .
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7
8 9 10 11
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