68 Acidic and Oxidizing Properties of Rare Earth Exchanged Y Zeolites
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YOUNES BEN TAARIT, MICHEL-VITAL MATHIEU, and CLAUDE NACCACHE Institut de Recherches sur la Catalyse, C.N.R.S., 39, Boulevard du Onze Novembre 1918, 69-Villeurbanne, France
Infrared techniques are used to study the nature of the OH groups present on rare earth forms of Y zeolite. The infrared spectra of pyridine adsorbed on REY, after calcination at a series of temperatures, show that Bronsted and Lewis acidities are present. ESR techniques are used to measure the number of anthracene radical ions which can be formed on the REY zeolite. Electron transfer occurs at the tricoordinated aluminum atoms in the case of LaY zeolite, while for Ce Y, the centers have been identified as Ce ions. 4+
4+
C y n t h e t i c Y t y p e zeolites h a v e b e e n s t u d i e d w i d e l y i n recent years.
Many
^
deter
techniques were used—particularly infrared spectroscopy—to
m i n e t h e n a t u r e of a c t i v e sites. A t first, o n l y t h e H X a n d H Y f o r m s w e r e c o n c e r n e d , a n d t h e a c i d i c n a t u r e of t h e h y d r o g e n h e l d o n these catalysts w a s d e m o n s t r a t e d b y several i n f r a r e d investigations (10, 20).
T h e origin
of t h e s t r u c t u r a l h y d r o x y l g r o u p s w a s a subject of g e n e r a l agreement. O n the c o n t r a r y , t h e a t t r i b u t i o n of some ( O H ) b a n d s , s u c h as t h e b a n d at 3550 c m " , r e m a i n s u n d e r d i s c u s s i o n (6, 9,18, 21, 26). 1
M o r e recently, the
M ( I I ) - a n d M (III)-exchanged X a n d Y forms came into study. Similar h y d r o x y l g r o u p s w e r e o b s e r v e d a n d , u n d e r c e r t a i n c o n d i t i o n s , some O H g r o u p s w e r e r e c o g n i z e d as b o u n d to t h e c a t i o n (4, 13, 25).
T h e origin of
these h y d r o x y l groups has b e e n a t t r i b u t e d u n a n i m o u s l y t o the i o n i z a t i o n of w a t e r m o l e c u l e s b y t h e s t r o n g electrostatic fields i n t h e c a t i o n n e i g h b o r h o o d , as stated b y R a b o
(13).
H o w e v e r , t h e n a t u r e a n d r o l e of
h y d r o x y l g r o u p s is n o t c l e a r l y d e m o n s t r a t e d . R e c e n t l y , W a r d (24)
con
c l u d e d a n exhaustive s t u d y o f t h e n a t u r e o f active sites f o r c u m e n e crack i n g i n this w a y : " I n c o n c l u s i o n , a s t u d y of v a r i o u s c a t i o n - e x c h a n g e d X zeolites has s h o w n that t h e c a t a l y t i c a l l y a c t i v e f o r m s a r e B r o n s t e d a c i d s 362
In Molecular Sieve Zeolites-II; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
68.
T A A R i T ET
BEN
Rare Earth Exchanged
AL.
363
Y Zeolites
a n d c o n t a i n s t r u c t u r a l h y d r o x y l g r o u p s . . . . H o w e v e r , there is n o a p p a r ent r e l a t i o n s h i p b e t w e e n the c o n c e n t r a t i o n of B r o n s t e d acids sites a n d c a t a l y t i c a c t i v i t y . It is possible that t h e a c i d site strength is i m p o r t a n t . " It s e e m e d of some interest to test the a b i l i t y of a series of R E Y zeolites to i o n i z e p o l y n u c l e a r aromatics since the o x i d i z i n g properties of zeolites w e r e p o i n t e d o u t (8, 16), b u t t h e n a t u r e of t h e e l e c t r o n acceptor site is s t i l l u n d e r d i s c u s s i o n . H a l l et al. ( 5 ) , s t u d y i n g d e h y d r o x y l a t e d H Y zeo lites, p r e s u m e d i t t o b e m o l e c u l a r o x y g e n t r a p p e d i n a n a n i o n v a c a n c y , w h i l e H i r s c h l e r ( 7 ) asserted that t h e protons m a y b e t h e o x i d a t i v e c e n Downloaded by IOWA STATE UNIV on November 3, 2014 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0102.ch068
ters. I n a p r e v i o u s w o r k , as stated b y T u r k e v i c h et al. (16), w e c o n c l u d e d that t h e active sites are L e w i s centers, w h i l e t h e c h e m i s o r b e d increases t h e i r electron affinity (27).
oxygen
I n a recent w o r k , R i c h a r d s o n
(14)
r e l a t e d s p i n c o n c e n t r a t i o n to t h e electron affinity of t h e c a t i o n , p r e s u m i n g t h a t t h e e l e c t r o n transfer took p l a c e f r o m t h e anthracene t o t h e c u p r i c i o n , b u t h e c o u l d n o t observe a n y v a r i a t i o n of t h e C u
2 +
p e a k intensities.
T h e slightest change i n t h e p r e t r e a t m e n t c o n d i t i o n s m a y h a v e a great influence o n t h e o x i d a t i v e properties of t h e sample, m a k i n g a simultaneous i n f r a r e d s t u d y necessary. Experimental Materials.
A l l the samples are issued f r o m the same L i n d e N a Y
s t a r t i n g zeolite. T h e N H C e Y , C e N a Y , N a L a Y , a n d N a C u Y f o r m s w e r e 4
o b t a i n e d b y c o n v e n t i o n a l ion-exchange
technique.
I n the case of t h e
N H C e Y , the a m m o n i u m f o r m is o b t a i n e d first a n d t h e n e x c h a n g e d w i t h 4
C e ( N 0 ) s o l u t i o n to the d e s i r e d l e v e l . 3
3
T h e N a C e Y , N a L a Y , a n d N a C u Y are e x c h a n g e d w i t h nitrate s o l u tions; t h e degree of i o n exchange is d e t e r m i n e d b y flame s p e c t r o m e t r y analysis of t h e r e s i d u a l s o d i u m . T h e results a r e set i n T a b l e I.
X-ray
e x a m i n a t i o n s h o w e d a l l the samples to b e h i g h l y c r y s t a l l i n e . T h e p y r i d i n e is d r i e d over a 5 - A m o l e c u l a r sieve. a r o m a t i c solutions are d r i e d a n d degassed
T h e polynuclear
b y the f r e e z e - p u m p - t h a w
technique. Techniques. I n f r a r e d measurements w e r e c o n d u c t e d i n a p r e v i o u s l y d e s c r i b e d c e l l ( I I ) , a l l o w i n g t h e sample t h i n wafers t o b e h e a t e d u n d e r vacuum or equilibrated w i t h pyridine vapor. Table I. Sample NaCeY NaNH CeY NaCuY NaLaY 4
Sodium 38 23 16 14
Ion
T h e samples w e r e corn-
Degree of Ion Exchange Ce Ion 6 6 0 0
La
Ion 0 0 0 14
NH
A
Ion
Cupric
0 15 0 0
In Molecular Sieve Zeolites-II; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
0 0 20 0
Ion
364
MOLECULAR
SIEVE
ZEOLITES
II
pressed u n d e r 6000 k g / c m i n t o 1 8 - m m - d i a m e t e r disks. T h e s e disks c o n 2
tain 12-16
m g of s o l i d / c m . S p e c t r a w e r e r e c o r d e d after c o o l i n g the 2
s a m p l e at r o o m t e m p e r a t u r e o n a P e r k i n - E l m e r M o d e l 125 s p e c t r o p h o tometer u s i n g a s p e c t r a l slit w i d t h of 3 c m " b e t w e e n 4000 a n d 1200 c m " . 1
1
T h e E P R studies are c a r r i e d out i n a s p e c i a l c e l l a l l o w i n g the a d s o r p t i o n of aromatics a n d s c r e e n i n g the catalyst f r o m the a c t i o n of m o i s t u r e or o x y g e n . T h e spectra are r e c o r d e d c o n t i n u o u s l y before a n d after a n t h r a cene a d s o r p t i o n . A f t e r e q u i l i b r i u m , the s p i n c o n c e n t r a t i o n is m e a s u r e d b y c o m p a r i n g the r a d i c a l c a t i o n E P R s p e c t r u m w i t h that of c a l i b r a t e d Downloaded by IOWA STATE UNIV on November 3, 2014 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0102.ch068
D P P H solution. T h e samples either are d e h y d r a t e d d i r e c t l y u n d e r v a c u u m or c a l c i n e d in oxygen a n d evacuated. Results Samples w h i c h h a v e n o t b e e n p r e t r e a t e d w i t h 0
2
show no radical ion
f o r m a t i o n ; w i t h t h e same p r e t r e a t m e n t c o n d i t i o n s , N H Y forms h a v e the 4
same b e h a v i o r .
T h e s e results seem s o m e w h a t d i s c o n c e r t i n g , since
the
o x y g e n - p r e t r e a t e d N H Y z e o l i t e w a s o x i d a t i v e e n o u g h to i o n i z e a n t h r a 4
cene e v e n at l o w exchange levels. S u c h strange results m a y b e e x p l a i n e d b y the f o r m a t i o n of a coke deposit. T h e c a r b o n m a y either p o i s o n the sites or m e r e l y p r e v e n t the anthracene f r o m c o m i n g close e n o u g h to the a c t i v e centers. W h e n C e ( I I I ) Y zeolites are c a l c i n e d u n d e r o x y g e n at 4 0 0 ° - 5 0 0 ° C , they turn pale yellow, probably b y C e
3 +
o x i d a t i o n to C e . A d s o r p t i o n of 4 +
anthracene o n these samples gives rise to a strong E P R s i g n a l . T h e same spectrum was observed b y Rooney and P i n k on s i l i c a - a l u m i n a (J5)
and
i n t e r p r e t e d as a r i s i n g f r o m a n anthracene p o s i t i v e r a d i c a l . Results f o r N a C e ( I V ) Y , N H C e ( I V ) Y , a n d N a C u Y are g i v e n i n 2 +
4
T a b l e II. I n the same w a y , the C e ( I I I ) Y samples w h i c h h a v e b e e n
heated
u n d e r v a c u u m are i n a c t i v e . F u r t h e r o x i d a t i o n gives t h e m the same p r o p erties as i f t h e y w e r e o x y g e n - p r e t r e a t e d , i n contrast to the N H Y f o r m . 4
T h e electron-acceptor p r o p e r t y of C e Y zeolites m a y be a t t r i b u t e d to Ce
4 +
ions. T h e s e results c o u l d b e c o m p a r e d to R i c h a r d s o n ' s w o r k s o n C u Y
zeolites (14).
L a Y is m o r e stable t h a n C e Y , a n d L a 3 +
3 +
3 +
r e m a i n s i n the
t h r e e - v a l e n c y state w h e n t r e a t e d u n d e r o x y g e n . F i g u r e 1 gives the s p i n c o n c e n t r a t i o n vs. the a c t i v a t i o n t e m p e r a t u r e f o r L a Y . T h e n u m b e r of Table II.
Anthracene Positive Radicals Concentration
Sample Calcination temperature (A. )/gram +
NaCeY
NH CeY
500 2.10
500 16.10
19
NaCuY
4
19
500 6.10
In Molecular Sieve Zeolites-II; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
18
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68.
B E N TAARiT E T A L .
Figure 1.
Rare Earth
Exchanged
I
I
400
500
Y Zeolites
365
•
600
Concentration of anthracene positive ions formed on LaY as a function of activation temperature; = rehydrated
3600 Figure 2.
Spectra of the stretching hydroxyls of LaY
(a) Evacuated at 200°C (b) Evacuated at 450° C (c) Evacuated at 550°C a c t i v e sites increases w i t h i n c r e a s i n g t e m p e r a t u r e u n t i l 4 0 0 ° C a n d t h e n r e m a i n s constant.
T h e s l i g h t decrease o b s e r v e d after 6 0 0 ° C m a y b e
In Molecular Sieve Zeolites-II; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
366
M O L E C U L A R SIEVE ZEOLITES
Π
a t t r i b u t e d to a structure c o l l a p s e o r at least t o a p r o b a b l e decrease i n t h e d i a m e t e r of t h e cavities.
R e h y d r a t i o n results i n a large decrease o f t h e
spin concentration. Infrared Measurements.
O H GROUPS.
I n f r a r e d spectra i n t h e h y
d r o x y l s t r e t c h i n g r e g i o n are s h o w n i n F i g u r e 2 f o r L a Y zeolite. P h y s i c a l l y a d s o r b e d w a t e r is r e m o v e d b y d e s o r p t i o n o f t h e s a m p l e at 200 ° C , as m a y b e c o n c l u d e d f r o m t h e d i s a p p e a r a n c e of t h e 1640 c m " b a n d . I n s u c h c o n d i t i o n s , ( O H ) b a n d s at 3745, 3640, a n d 3530 c m " w i t h 1
a s h o u l d e r at 3550 c m
- 1
are o b s e r v e d . F u r t h e r d e h y d r a t i o n r e d u c e d these
b a n d s ' intensities, except f o r t h e one at 3745 c m . E v a c u a t i o n at 5 0 0 ° C Downloaded by IOWA STATE UNIV on November 3, 2014 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0102.ch068
- 1
affects the 3550 c m a n d 3640 c m b a n d s , w h i l e the 3530 c m " b a n d s t i l l 1
1
1
Figure 3. Spectra of pyridine ad sorbed on LaY activated at 450°C (a) Evacuated at 150°C (b) Evacuated at 250°C
In Molecular Sieve Zeolites-II; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
1
68.
T A A R i T ET
BEN
Rare Earth
AL.
Exchanged
Y Zeolites
367
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remains. T h e s e spectra are q u i t e s i m i l a r to those r e p o r t e d b y R a b o et ah (13) a n d W a r d (25).
ι
:
'
3700
1 ι 1450cm
L J I
3600
1550
1
Figure 4. Spectra of Ce(IV)NH J activated at 450°C under water vapor pressure, then evacuated at 450°C f
(a) Initial (b) Pyridine evacuated at 150° C (c) Further evacuation at 250° C I n s i m i l a r d e h y d r a t i o n c o n d i t i o n s , t h e 3640 c m f o r t h e N H C e Y samples. 4
- 1
b a n d is m o r e stable
T h e s e results m a y b e r e l a t e d t o W a r d ' s c o n
clusions a b o u t N H M g Y zeolites 4
(23);
i t is p r o b a b l e t h a t c e r i u m ions
s t a b i l i z e t h e H Y f o r m zeolite. C a l c i n a t i o n of C e Y samples u n d e r 20 torr o f w a t e r v a p o r at 450 ° C produces
slight m o d i f i c a t i o n s i n t h e i n f r a r e d spectra
of the hydroxyl
In Molecular Sieve Zeolites-II; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
368
M O L E C U L A R SIEVE ZEOLITES
g r o u p s : t h e 3640 a n d 3550 c m " are r e m o v e d at 4 5 0 ° C . 1
II
Simultaneously, a
b a n d at 3680 c m " is d e v e l o p e d . 1
PYRIDINE ADSORPTION.
P y r i d i n e a d s o r p t i o n has b e e n u s e d t o deter
m i n e t h e n a t u r e o f a c i d sites (2, 12).
T h e spectra a r e r e c o r d e d
after
e q u i l i b r a t i n g t h e s a m p l e w i t h a n excess o f p y r i d i n e v a p o r a n d f u r t h e r e v a c u a t i o n at v a r i o u s temperatures.
T y p i c a l spectra are s h o w n i n F i g u r e s
3, 4, a n d 5. T a b l e I I I gives t h e f r e q u e n c i e s o f t h e o b s e r v e d b a n d s b e t w e e n 1700 a n d 1400 c m " . 1
A f t e r p y r i d i n e a d s o r p t i o n , t h e b a n d a t 3550 c m " is b r o a d e n e d w h i l e 1
that at 3640 a n d 3680 c m " d i s a p p e a r . T h e 3680 a n d 3640 c m " b a n d s a r e Downloaded by IOWA STATE UNIV on November 3, 2014 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0102.ch068
1
1
restored b y e v a c u a t i o n at 2 5 0 ° a n d 3 5 0 ° C , r e s p e c t i v e l y . M o r e i n t e r e s t i n g results are o b t a i n e d f o r t h e N a C e Y samples; w h e n p y r i d i n e is d e s o r b e d
I transmittance
3700 Figure 5.
J
3500 cm"
1
OH spectra of Ce(III)Y activated at 450°C
(a) Initial (b) Pyridine evacuated at 150°C (c) At 250°C (d) At 350°C
In Molecular Sieve Zeolites-II; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
68.
BEN
TAARIT
ET
Rare Earth
AL.
Table III.
19b Bronsted (Β)
C e ( I V ) Y (450°C) C e ( I V ) Y (600°C) Ce(IV)NH Y (450°C) Ce(IV)NH Y (600°C) L a Y (200°C)
19a (B +
L)
19b Lewis ( L )
1541
1489
1443
1541
1489
1451-1443
1544 1538
1490 1483
1444 1451-1444
1630
1540
1488
1451-1444
1620-1592 1630-1610 1590 1630-1610 1590
1538 1540
1489 1489
1453-1444 1440
1540
1489
1451-1442
1635-1625 1608-1600 1625-1608 1600 1630-1593 1620-1597
C e ( I I I ) Y (600°C)
369
Y Zeolites
Infrared Bands of Adsorbed Pyridine 8a-8b
C e ( I I I ) Y (450°C)
Exchanged
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4
4
LaY
(450°C)
at 2 5 0 ° C , t h e 19b a n d 19a v i b r a t i o n s are s p l i t i n t o 2 c o m p o n e n t s
(1550
a n d 1430 f o r 19b; 1495 a n d 1488 f o r 1 9 a ) . F u r t h e r e v a c u a t i o n at 350 ° C p r o d u c e s a greater decrease f o r the l o w - f r e q u e n c y c o m p o n e n t s t h a n f o r the h i g h - f r e q u e n c y ones. T h e last r e m a i n i n g c o m p o n e n t f o r 19a v i b r a t i o n is 12 c m " s h i f t e d to the h i g h e r frequencies. 1
Discussion T h e 3745 c m
- 1
b a n d m a y b e a t t r i b u t e d either to S i - O H groups i n h e r
ent to siliceous i m p u r i t i e s o r s i l a n o l groups w h i c h t e r m i n a t e t h e z e o l i t e lattice as stated b y different authors
(1,3,17),
w h i l e the 3640 a n d 3550
c m " b a n d s w e r e assigned to a c i d i c O H g r o u p s . T h e disagreement i n t h e 1
assignment of t h e 3 6 8 0 - 9 0 c m " b a n d s (1, 3,19,
22) seems to b e o w i n g to
the
or to d i v e r s i t y of
1
different o r i g i n
m a l treatment.
of t h e s t u d i e d zeolites
ther
Nevertheless, i n the riresent w o r k , i t seems inconsistent to
assign t h e 3680 c m " b a n d t o w a t e r m o l e c u l e s associated w i t h t h e c a t i o n 1
since n o b a n d is o b s e r v e d at 3400 a n d at 1640 c m " . M o r e o v e r , the c o n 1
d i t i o n s of the f o r m a t i o n o f these O H a n d t h e i r t h e r m a l s t a b i l i t y m u s t b e
r e l a t e d r a t h e r t o t h e f o r m a t i o n of A l - O H groups b y a n
\
/ b r i d g e h y d r o l y s i s at 460 ° C . T h e b a n d at 3530 c m b y W a r d (25)
a n d R a b o (13)
to R E - O H
1
/
Al
\
Ο
\
/
Si
/
\
is assigned as stated
groups.
T h i s b a n d has n o
a c i d i c p r o p e r t y . O n t h e c o n t r a r y , t h e a c i d i c n a t u r e of t h e 3640 a n d 3680 c m " b a n d s is d e f i n i t e l y established since t h e y d i s a p p e a r w h e n p y r i d i n e is 1
a d s o r b e d a n d , s i m u l t a n e o u s l y , characteristic i n f r a r e d b a n d s of P y H
+
ions
are o b s e r v e d i n t h e s p e c t r a l r e g i o n 1700-1400 c m " . F u r t h e r m o r e , t h e 1
In Molecular Sieve Zeolites-II; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
370
M O L E C U L A R SIEVE ZEOLITES
II
e v a c u a t i o n o f p y r i d i n e points out the difference i n the a c i d strength o f the 2 h y d r o x y l g r o u p s , since t h e latter is restored b y e v a c u a t i o n a t 250 ° C w h i l e the f o r m e r is restored o n l y after e v a c u a t i o n at 3 5 0 ° C .
O n the other
h a n d , L a Y a n d C e ( I I I ) Y seem to b e rather p r o t o n i c a c i d solids after e v a c u a t i o n at 200 ° C , t h o u g h a d s o r p t i o n o f p y r i d i n e o n s u c h solids gives rise to a b a n d at 1445 c m (22)
- 1
w h i c h m a y b e assigned as suggested b y W a r d
a n d H a l l (4) t o p y r i d i n e c o o r d i n a t e d b y the c a t i o n . O n the other
h a n d , the samples c a l c i n e d at 5 0 0 ° C c o n t a i n m o r e L e w i s a c i d sites t h a n B r o n s t e d a c i d sites. T h o u g h characteristic P y H
+
b a n d s are s t i l l present i n
the spectra, the a p p e a r a n c e of a m a r k e d s h o u l d e r at 1451 c m " shows the Downloaded by IOWA STATE UNIV on November 3, 2014 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0102.ch068
1
existence of true L e w i s a c i d sites. T h e s e L e w i s sites c a n o n l y b e c r e a t e d b y d e h y d r o x y l a t i o n at t h e expense o f B r o n s t e d a c i d h y d r o x y l groups. S u c h p h e n o m e n a h a d a l r e a d y b e e n o b s e r v e d b y W a r d (22). T h e o r i g i n a l result o b t a i n e d f o r N a C e ( I V ) Y samples is m o r e d i f f i c u l t to e x p l a i n ; the 19b v i b r a t i o n s p l i t t i n g m a y b e i n t e r p r e t e d as a n e v i d e n c e of the existence of different a c i d site strengths.
T h e s p l i t t i n g o f the 19a v i b r a t i o n , v e r y
insensitive t o the b o n d i n g t y p e o f the Ν a t o m , is m u c h m o r e d i f f i c u l t t o e x p l a i n . H o w e v e r , i t seems t h a t there are 2 types of s i l a n o l a c i d g r o u p s — one near a s o d i u m i o n , the other near a c e r i u m i o n . T h e f o r m e r t y p e w o u l d give rise after p y r i d i n e e q u i l i b r a t i o n t o the l o w - f r e q u e n c y b a n d . S i n c e this b a n d is r e m o v e d at 250 ° C , the strength o f s u c h sites is rather w e a k . T h e latter t y p e w o u l d p r o v i d e a n e w species : a Π b o n d i n g is estab l i s h e d b e t w e e n the p y r i d i n e a n d the e m p t y d orbitals of the eerie i o n . B y a c t i v a t i o n a t 350 ° C , these species w o u l d g i v e rise t o a complex C H N H 5
5
2 +
charge-transfer
. T h e p e r t u r b a t i o n of the Π e l e c t r o n is strong e n o u g h
to a c c o u n t f o r s u c h a n i m p o r t a n t shift t o h i g h e r frequencies o f the 19a vibration. T h e E P R spectra o f a n t h r a c e n e a d s o r b e d o n R E Y are
characteristic
of the a n t h r a c e n e p o s i t i v e r a d i c a l . O u r results p r o v i d e strong e v i d e n c e o f the o x i d i z i n g properties o f these solids, as p o i n t e d out b y H i r s c h l e r (8). A n t h r a c e n e i o n i z a t i o n is i n h i b i t e d b y coke deposit o n the s a m p l e d u r i n g its a c t i v a t i o n u n d e r v a c u u m . N o r a d i c a l i o n g e n e r a t i o n is o b s e r v e d b y a d s o r p t i o n o f anthracene or p e r y l e n e o n N H Y e v a c u a t e d at 500 ° C e v e n 4
if the s a m p l e is oxygen-treated later. A n o p p o s i t e b e h a v i o r is o b s e r v e d for C e (III) Y f o r m : a n evacuated sample is inactive but further activation u n d e r o x y g e n p r o d u c e s a p a l e y e l l o w s o l i d , a n d a strong E P R s i g n a l is o b s e r v e d after a d s o r p t i o n o f anthracene.
T h e d i r e c t r o l e o f the c a t i o n i n
the e l e c t r o n transfer, as has b e e n s h o w n f o r C u Y (14), i s once m o r e demonstrated.
T h e e l e c t r o n a c c e p t o r sites w o u l d b e t h e C e
4 +
ions, w h i c h
h a v e p o w e r f u l o x i d i z i n g p r o p e r t i e s ; results f o r the N H C e ( I I I ) Y f o r m 4
corroborate these hypotheses since no anthracene o x i d a t i o n occurs o n s u c h solids after a c t i v a t i o n u n d e r v a c u u m at 5 0 0 ° C , t h o u g h i n f r a r e d e v i d e n c e of the existence o f L e w i s a c i d sites i s g i v e n b y the presence o f the b a n d
In Molecular Sieve Zeolites-II; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
68.
B E N
371
Rare Earth Exchanged Y Zeolites
TAARiT E T A L .
at 1451 c m " o n t h e spectra after p y r i d i n e a d s o r p t i o n . T h e l a c k o f a c t i v i t y 1
is a t t r i b u t e d once m o r e to coke p o i s o n i n g . I n a s i m i l a r m a n n e r , t h e h y d r o c a r b o n i o n i z a t i o n needs t h e s o l i d to b e a c t i v a t e d u n d e r o x y g e n a n d h e n c e Ce
t o b e f o r m e d . L a Y zeolite seems to b e h a v e i n a s i m i l a r w a y as N H Y .
4 +
4
T h e d e p e n d e n c e of t h e s p i n c o n c e n t r a t i o n o n a c t i v a t i o n t e m p e r a t u r e is s i m i l a r . M o r e o v e r , t h e n u m b e r s of r a d i c a l ions are i n t h e same m a g n i t u d e r a n g e a n d the r e h y d r a t i o n of t h e s o l i d gives rise to a l a r g e decrease o f s p i n n u m b e r s ; o n t h e other h a n d , t h e L a
3 +
has n o o x i d i z i n g properties.
It s e e m e d reasonable to postulate that the L a Downloaded by IOWA STATE UNIV on November 3, 2014 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0102.ch068
t h e e l e c t r o n transfer.
ions are n o t i n v o l v e d i n
3 +
T h e s i m i l a r b e h a v i o r of L a Y a n d N H Y makes us 4
t h i n k that t h e active sites are t h e L e w i s a c i d sites as i n t h e case o f t h e dehydroxylated N H Y . 4
T h i s seems consistent since a b a n d at 1451 c m "
appears w h e n p y r i d i n e is a d s o r b e d o n a c t i v a t e d L a Y samples,
giving
strong e v i d e n c e of the presence of s u c h L e w i s sites at the surface of this solid. I n c o n c l u s i o n , s t r u c t u r a l h y d r o x y l groups are o b s e r v e d o n the surface of R E Y . T h e i r f o r m a t i o n is a t t r i b u t e d to w a t e r i o n z a t i o n b y strong
fields
near t h e c a t i o n . T h i s h y d r o l y s i s gives rise to R E - O H a n d a p r o t o n ; t h e latter
reacts w i t h
a surface
oxygen
i n the w a y stated
b y several
investigators:
κ\/\
/ \ / \
T h e a c i d i c properties o f some h y d r o x y l groups h a v e b e e n
demonstrated
b y t h e existence of a 1540 c m " b a n d i n a l l t h e spectra of a d s o r b e d p y r i d i n e 1
o n solids a c t i v a t e d at l o w t e m p e r a t u r e .
F u r t h e r d e h y d r a t e d samples b e
have either as B r o n s t e d or L e w i s a c i d solids; the latter are c r e a t e d b y d e h y d r o x y l a t i o n of t h e zeolite, w h i c h
occurs
i n t w o different w a y s :
First Η
I [ R E (OH) ]+ + 2
\
Ο
Al
\
/
Ο
S i ->[RE(OH)] + + 2
\ _ / \ / Al
Si
or s e c o n d
In Molecular Sieve Zeolites-II; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
1
372
M O L E C U L A R SIEVE ZEOLITES
II
H
I
ο
\ 2
\ / Al
Si
->
\-/
Al
ο
\ / Si
\ T h e anion vacancy, R E ( O H )
2 +
, or R E
3 +
\ +
/ Al
/
+Si
\ /
\
m a y a l l coordinate
pyridine.
S i m u l t a n e o u s l y , N a Y zeolite e x c h a n g e d w i t h high-electron-affinity cations
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s u c h as C u
2 +
andCe
4 +
acquires i m p o r t a n t o x i d i z i n g properties.
T h e elec
t r o n is transfered f r o m t h e a r o m a t i c h y d r o c a r b o n to t h e eerie o r c u p r i c i o n , w h e r e a s L a Y zeolite behaves l i k e a d e h y d r o x y l a t e d N H Y zeolite. 4
It
seems that t h e N a C e ( I V ) Y f o r m also p r o v i d e s species s u c h as P y H . 2 +
Literature Cited (1) Angell, C. L., Schaffer, P.C.,J. Phys. Chem. 1965, 69, 3436. (2) Basila, M. R., Kantner, T. R., J. Phys. Chem. 1966, 70, 1681. (3) Carter, J. L., Lucchesi, P. J., Yates, D. J. C., J. Phys. Chem. 1964, 68, 1385. (4) Christner, L. G., Liengme, D. V., Hall, W. K., Trans. Faraday Soc. 1968, 64, 1679. (5) Dollish, F. R., Hall, W. K.,J.Phys. Chem. 1967, 71, 1005. (6) Habgood, H. W.,J.Phys. Chem. 1965, 69, 1764. (7) Hirschler, A. E., J. Catalysis 1966, 5, 196. (8) Hirschler, A. E., Neikam, W. C., Barmby, D. S., James, R. L.,J.Catalysis 1965, 4, 628. (9) Hugues, T. R., White, H. M.,J.Phys. Chem. 1967, 71, 2192. (10) Liengme, Β. V., Hall, W. K., Trans. Faraday Soc. 1966, 62, 3229. (11) Mathieu, M. V., Pichat, P., "La Catalyse au Laboratoire et dans l'In dustrie," p. 320, Editions Masson, Paris, 1967. (12) Parry, Ε. P.,J.Catalysis 1962, 2, 371. (13) Rabo, J. Α., Angell, C. L., Kasai, P. H., Schoemaker, V., Discussions Fara day Soc. 1966, 41, 328. (14) Richardson, J. T.,J.Catalysis 1967, 9, 172. (15) Rooney, A. J., Pink, R.C.,Proc. Chem. Soc. 1961, 70. (16) Stammires, D., Turkevich, J., J. Am. Chem. Soc. 1964, 86, 749. (17) Uytterhoeven, J. B., Christner, L. G., Hall, W. K., J. Phys. Chem. 1965, 69, 2117. (18) Uytterhoeven, J. B., Jacobs, P., Makay, K., Shoonheydt, R., J. Phys. Chem. 1968, 72, 1768. (19) Uytterhoeven, J. B., Shoonheydt, R., Liengme, Β. V., Hall, W. K., J. Catalysis 1969, 13, 425. (20) Ward, J. W., J. Catalysis 1967, 9, 225. (21) Ibid., 1967, 9, 396. (22) Ibid., 1968, 10, 34. (23) Ibid., 1968, 11, 251. (24) Ibid., 1969, 14, 365. (25) Ward, J. W., J. Phys. Chem. 1968, 72, 4211. (26) White, J. L., Jelli, A. N., Andre, J. Α., Frippiat, J.J.,Trans. Faraday Soc. 1967, 63, 461. (27) Ben Taarit, Y., Naccache,C.,Imelik, B., J. Chim. Phys. 1970, 67, 389.
RECEIVED March 10, 1970.
In Molecular Sieve Zeolites-II; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
68.
B E N TAARIT
E T A L .
Rare Earth
Exchanged
Y
373
Zeolites
Discussion B. D . M c N i c o l (Koninklijke/Shell Laboratorium, Amsterdam, Neth erlands ) : W i t h reference to F i g u r e 1 of y o u r p a p e r r e g a r d i n g t h e s p i n c o n c e n t r a t i o n of p o s i t i v e anthracene
ions, y o u state that " r e h y d r a t i o n
results i n a large decrease i n s p i n c o n c e n t r a t i o n . "
T h i s is c e r t a i n l y n o t
a p p a r e n t i n the figure. A l s o , w e r e n o measurements m a d e o n r e h y d r a t e d samples w h i c h h a d b e e n a c t i v a t e d a b o v e 5 5 0 ° C ?
T h i s w o u l d s h e d some
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l i g h t o n the reason f o r t h e decrease i n s p i n c o n c e n t r a t i o n o b s e r v e d a b o v e 550°C.
I n the absence of x-ray or other e v i d e n c e , this decrease m a y n o t
b e d u e to structure collapse. C . N a c c a c h e : F i g u r e 1 shows that r e h y d r a t i o n results i n a large d e crease of s p i n concentration. figure
F o r e x a m p l e , as c a n b e d e t e r m i n e d i n this
( d o t t e d l i n e corresponds to s a m p l e a c t i v a t e d at 4 5 0 °
a n d then
r e h y d r a t e d ) , L a Y z e o l i t e d e h y d r a t e d at 4 5 0 ° gives a b o u t 1.5 Χ 1 0
18
posi
t i v e r a d i c a l ions, w h i l e after r e h y d r a t i o n the n u m b e r of r a d i c a l ions is o n l y 1.1 χ
1 0 . T h e decrease i n electron-acceptor 18
sites r e s u l t i n g f r o m
r e h y d r a t i o n o f t h e z e o l i t e is d e m o n s t r a t e d . I n fact, t h e decrease of s p i n c o n c e n t r a t i o n o b s e r v e d above 550 ° C c o u l d b e better e x p l a i n e d b y m i g r a t i o n of cations i n Si p o s i t i o n w h e n t h e c a t i o n is t h e active site ( C e
4 +
a n d C u , f o r e x a m p l e ). I n the case o f L a Y , 2 +
w e h a v e n o other e x p l a n a t i o n t h a n that g i v e n i n o u r p a p e r . I w o u l d l i k e to k n o w i f y o u h a v e another hypothesis w h i c h c a n better e x p l a i n this decrease.
In Molecular Sieve Zeolites-II; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.