Acidic and Oxidizing Properties of Rare Earth Exchanged Y Zeolites

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


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


18


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


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


1

68.

T A A R i T ET

BEN

Rare Earth

AL.

Exchanged

Y Zeolites

367


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


368


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


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


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


370


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


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


1

372


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


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.


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


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.


Acidic and Oxidizing Properties of Rare Earth Exchanged Y Zeolites

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