Investigation of Oxygen Mobility in Synthetic Zeolites by Isotopic

41 Investigation of Oxygen Mobility in Synthetic Zeolites by Isotopic Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 1, 2015 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1971-0101.ch041

Exchange Method G. V. ANTOSHIN, KH. M. MINACHEV, E. N. SEVASTJANOV, D. A. KONDRATJEV, and CHAN ZUI NEWY N. D. Zelinsky Institute of Organic Chemistry, Academy of Sciences, Moscow, USSR 18

A study has been carried out on the O exchange between O and following zeolites: NaX, NaY (SiO /A O = 3.4), NaY (SiO /Al O = 4.7), Na-mordenite, and Cu and Ag forms of Y zeolite. The sodium forms have similar kinetic characteristics of oxygen mobility. Activation energies of exchange for these forms are about 45-50 kcal/mole. The replacement of Na by Cu in Y zeolite causes a large increase of oxygen mobility. The same effect, but less pronounced, has been found for silver. 2

2

2

2

2

3

3

' T ^ h e present p a p e r reports some d a t a o n t h e o x y g e n m o b i l i t y o f N a m o r d e n i t e , a n d N a X , a n d N a Y zeolites, a n d t h a t o f s i l v e r a n d c o p p e r forms of z e o l i t e Y w i t h v a r i o u s degrees of r e p l a c e m e n t of s o d i u m b y t h e cations.

These have been obtained b y studying the zeolite-molecular

o x y g e n i s o t o p i c exchange k i n e t i c s . S u c h d a t a m a y p r o v e v a l u a b l e i n gaining insight into the correlation dependence between zeolite chemical c o m p o s i t i o n a n d t h e i r p r o p e r t i e s , s u c h as c a t a l y t i c a c t i v i t y a n d t h e r m a l s t a b i l i t y (1,4,6).

N o d a t a o n i s o t o p i c exchange b e t w e e n 0

2

a n d zeolites

are a v a i l a b l e i n t h e l i t e r a t u r e , except one w o r k (3) i n w h i c h t h e exchange between 0

2

1 8

a n d z e o l i t e L i n d e 5 A w a s n o t f o u n d at 250 ° C .

Experimental Materials. T h e samples w e r e c o m m e r c i a l l y s u p p l i e d zeolites N a X ( S i 0 / A l 0 = 2 . 4 5 ) , N a Y ( S i 0 / A l 0 = 3.4 a n d 4 . 7 ) , a n d N a - m o r d e n i t e ( S i 0 / A l 0 = 10) w i t h o u t b i n d e r . T h e c r y s t a l sizes of zeolites u s e d w e r e 1-2/x,. S i l v e r a n d c o p p e r forms of z e o l i t e Y w e r e p r e p a r e d b y 2

2

3

2

2

2

2

3

3

514 In Molecular Sieve Zeolites-I; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.

41.

ANTOSHIN ET A L .

Oxygen

Mobility

in Synthetic

515

Zeolites

c a t i o n exchange b e t w e e n the s t a r t i n g s o d i u m c o n t a i n i n g z e o l i t e Y w i t h S i 0 / A l 0 = 3.4 r a t i o a n d s i l v e r n i t r a t e or c o p p e r acetate s o l u t i o n . B e f o r e experiments, the zeolites w e r e w a s h e d b y d i s t i l l e d w a t e r . P e l l e t e d forms of zeolites 4 m m i n d i a m e t e r a n d 5 m m h i g h w e r e u s e d . S u r f a c e areas ( B E T ) of X a n d Y zeolites w e r e ca. 700 m / g r a m . T h e c o r r e s p o n d i n g v a l u e for N a - m o r d e n i t e w a s 300 m / g r a m . O x y g e n e n r i c h e d i n O w a s p r e p a r e d b y electrolysis of h e a v y - o x y g e n w a t e r . O x y g e n w i t h O c o n c e n t r a t i o n a b o u t 7 % a t m ( e q u i l i b r i u m m i x t u r e of different i s o t o p i c species ) w a s u s e d i n most of experiments. Apparatus. A v a c u u m c i r c u l a r static u n i t was u s e d to s t u d y t h e i s o t o p i c exchange. T h e t e m p e r a t u r e i n the z e o l i t e b e d w a s m a i n t a i n e d w i t h a c c u r a c y of d = l ° C . O m e g a t r o n e was e m p l o y e d to a n a l y z e the gas isotopic composition. Procedure. Z e o l i t e samples w e r e t r e a t e d i n h i g h v a c u u m ( 10" m m ) at 7 0 0 ° C for 5 hours a n d t h e n k e p t i n o x y g e n at the same t e m p e r a t u r e for n o t less t h a n 5 hours at a pressure of 5 - 1 0 m m . Isotopic exchange was s t u d i e d at 6 0 0 ° - 7 0 0 ° C u n d e r 5 - 6 0 m m pressure. T h e r e a c t i o n vessel was fitted w i t h traps a n d c o o l e d w i t h l i q u i d n i t r o g e n to p r e v e n t grease f r o m g e t t i n g i n t o the zeolites. T h e c a p a c i t y of the z e o l i t e samples u s e d was d e t e r m i n e d against n i t r o g e n at the t e m p e r a t u r e of l i q u i d n i t r o g e n before a n d after the exchange experiments. 2

2

3

2

2

1 8

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 1, 2015 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1971-0101.ch041

1 8

5

Results

and

Discussion

P r e l i m i n a r y experiments w i t h zeolite N a Y ( S i 0 / A l 0 2

2

3

=

s h o w n o x y g e n m o b i l i t y ( e x c h a n g e b e t w e e n p h a s e s ) to b e

3.4 ) h a v e measurable

o n l y at t e m p e r a t u r e s e x c e e d i n g 600 ° C after v a c u u m - t r e a t i n g the zeolite s h o w e d v a r y i n g i n c r e a s e d a c t i v i t y . T h i s w a s decreased to a constant v a l u e b y m a i n t a i n i n g the z e o l i t e i n oxygen.

T h e data obtained allowed

a c h o i c e of c o n d i t i o n s for the p r e - t r e a t m e n t of the samples.

The

0

1 8

c o n c e n t r a t i o n increase i n z e o l i t e r e s u l t i n g f r o m one exchange w i t h the e n r i c h e d gas w a s n e g l i g i b l e .

( T h e r a t i o b e t w e e n the n u m b e r of o x y g e n

atoms i n the z e o l i t e a n d that i n the gas was a b o u t 200 i n most of the experiments. ) T h i s m a d e possible a series of experiments w i t h the same z e o l i t e s a m p l e to s t u d y the d e p e n d e n c e of the exchange rate o n o x y g e n t e m p e r a t u r e a n d pressure. A f t e r the series of e x p e r i m e n t s , the i n i t i a l one w a s r e p e a t e d u n d e r the s t a r t i n g c o n d i t i o n s . T h e r e p r o d u c i b i l i t y of results was

10%. E v a l u a t i o n of the e x p e r i m e n t a l d a t a has s h o w n the m o l e c u l a r o x y g e n -

z e o l i t e i s o t o p i c exchange

k i n e t i c s to o b e y

first

o r d e r equations.

e x a m p l e , F i g u r e 1 presents some k i n e t i c d a t a o n the exchange 0

2

a n d s o d i u m forms of zeolites s t u d i e d at 620° a n d 7 0 0 ° C .

For

between T h e s e are

t r e a t e d a c c o r d i n g to the e q u a t i o n

In Molecular Sieve Zeolites-I; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.


516

MOLECULAR SIEVE ZEOLITES

40

80

120 %

160

180

1

220

[min]

Figure 1. Kinetics of exchange between 0 and sodium forms of zeolites studied at temperatures 620° C (1) and 700°C (2). Oxygen pressure was 5 mm Hg. 2

w h e r e R is the i s o t o p i c exchange rate constant, a t o m s / g r a m m i n ; N i is the n u m b e r of o x y g e n atoms i n the gas; N

2

is the n u m b e r of o x y g e n atoms

i n the z e o l i t e ; F is the degree of exchange; g is the zeolite ( g r a m s ) ; τ is the exchange t i m e ( m i n ) .

specimen

T h e linear dependence

shown

i n F i g u r e 1 p r o v i d e s e v i d e n c e that a l l o x y g e n of the zeolites s t u d i e d is e q u i v a l e n t as to its exchange w i t h the gas. T h e s e results w i l l b e discussed further. T a b l e I s u m m a r i z e s the m a i n k i n e t i c characteristics of the i s o t o p i c exchange b e t w e e n m o l e c u l a r o x y g e n a n d the zeolites s t u d i e d : R is the rate constant of exchange at 6 7 0 ° C a n d 5 - m m pressure, a t o m s / g r a m m i n ; Ε is the a p p a r e n t a c t i v a t i o n energy, k c a l / m o l e ; η is the r e a c t i o n o r d e r


41.

Oxygen Mobility

ANTOSHIN E T A L .

Table I.

in Synthetic

Kinetic Characteristics of O Exchange between Zeolites and Molecular Oxygen 1 8


R X 10Atoms Zeolite

Ratio

NaX NaY NaY Na-mordenite CuNaY CuNaY CuNaY AgNaY AgNaY

2.45 3.4 4.7 10 3.4 3.4 3.4 3.4 3.4

a

517

Zeolites

Gram

1 7

Min

8.40 5.70 2.35 2.88 37.5 121 433 12.7 21.0

— — — 0.14 0.26 0.42 0.14 0.26

E, Kcal Mole

η

47 45 49 45 38 33 23 38 39

1.0 1.0 1.0 1.0 0.9 1.0 0.9 0.8

Ζ = degree of sodium replacement by the corresponding cation.

against o x y g e n pressure. A s seen f r o m the d a t a p r e s e n t e d i n T a b l e I , the o x y g e n m o b i l i t y k i n e t i c characteristics of the s o d i u m forms of the zeolites s t u d i e d are s i m i l a r . T h e r e p l a c e m e n t of s o d i u m b y c o p p e r causes a strong p r o m o t i n g effect.

F o r e x a m p l e , the exchange rate constant is 6 times as h i g h for a

N a Y sample w i t h Ζ =

0.14 as for the s t a r t i n g zeolite, the exchange a c t i

v a t i o n e n e r g y d e c r e a s i n g f r o m 45 to 38 k c a l / m o l e . W h e n the degree of c a t i o n i c exchange increases u p to 0.42, the rate constant increases f u r t h e r a n d the a c t i v a t i o n e n e r g y decreases

to 23 k c a l / m o l e .

The promoting

effect also is c a u s e d w h e n s o d i u m is r e p l a c e d b y s i l v e r , b u t i n this case the effect is less p r o n o u n c e d . O n e m a y observe

compensating

dependence

between

the

preex-

p o n e n t i a l f a c t o r i n the A r r h e n i u s e q u a t i o n a n d the a c t i v a t i o n energy. T h i s d e p e n d e n c e is s h o w n i n F i g u r e 2. A s the a c t i v a t i o n e n e r g y i n the s a m p l e w i t h the exchange degree 0.42 decreases

to 23 k c a l / m o l e , the

p r e e x p o n e n t i a l factor b e c o m e s 3 orders l o w e r . Some experiments w i t h v a r i o u s c a t i o n i c forms of z e o l i t e Y at the h i g h e s t temperatures h a v e b e e n

c a r r i e d out u s i n g the gas w i t h

O

1 8

c o n c e n t r a t i o n a b o u t 4 0 % a t m , the content of v a r i o u s i s o t o p i c m o l e c u l e s of o x y g e n ( 0

1 6

0

1 6

, 0

1 6

0

1 8

, and 0

1 8

0

1 8

) i n i t b e i n g close to e q u i l i b r i u m .

W i t h s o d i u m a n d s i l v e r f o r m s , the e q u i h b r i u m i n the gas phase

was

r e t a i n e d , whereas w i t h the c o p p e r forms a s h a r p d e v i a t i o n f r o m e q u i l i b r i u m w a s o b s e r v e d at the i n i t i a l stage of exchange. T h i s i n d i c a t e d ( 2 ) that i n the cases of silver a n d s o d i u m forms t h e exchange w i t h one i o n of zeolite o x y g e n w o u l d b e p r e d o m i n a n t , w h i l e i n the case of c o p p e r forms it w o u l d b e w i t h 2 ions.


518

MOLECULAR SIEVE ZEOLITES—I

30 Η


29 h

30

40 E [

k c a l

/mole]

Figure 2. The compensatory dependence between parameters in the Arrhenius equation for the exchange between 0 and the copper forms of zeolite Y 2

T h u s , as f o l l o w s f r o m t h e analysis of t h e exchange k i n e t i c c h a r a c teristics, t h e s u b s t i t u t i o n o f c o p p e r

f o r s o d i u m causes a c o n s i d e r a b l e

c h a n g e i n t h e exchange m e c h a n i s m . T h e temperatures o f t h e p r e l i m i n a r y t r e a t m e n t a n d exchange ex p e r i m e n t s w e r e sufficiently h i g h that one m i g h t suggest that t h e s t r u c t u r e of the samples u n d e r s t u d y s h o u l d change. H o w e v e r , t h e d a t a o n n i t r o g e n a d s o r p t i o n p r o v i d e d e v i d e n c e i n f a v o r o f r e t a i n i n g t h e structure. N o w , w e a r e to c o n s i d e r t h e p r o b l e m of zeolite o x y g e n e q u i v a l e n c e w i t h respect to exchange.

S i n c e most of t h e experiments h a v e b e e n c a r

r i e d o u t w i t h t h e r a t i o N /N 2

x

= 200, t h a t is, i n a l a r g e excess o f o x y g e n

i n t h e s o l i d phase, t h e k i n e t i c s o b s e r v e d c o u l d b e c h a r a c t e r i s t i c o f the exchange o f a s m a l l p a r t o f z e o l i t e o x y g e n .

T h e n the conclusion about

the o x y g e n e q u i v a l e n c e c o u l d b e true o n l y f o r a s m a l l p a r t of t h e o x y g e n i n v o l v e d i n t h e exchange r a t h e r t h a n f o r t h e z e o l i t e o x y g e n as a w h o l e . However, i n the experiment w i t h N a Y ( S i 0 / A l 0 2

a n d a t o x y g e n pressure o f 5 m m , w i t h Ν /Νχ 2

2

3

=

3.4) at 670 ° C

r a t i o b e i n g 20.8, t h e rate

constant v a l u e p r a c t i c a l l y c o i n c i d e s w i t h t h e c o r r e s p o n d i n g v a l u e p r e sented i n T a b l e I. F u r t h e r m o r e , a series o f as m a n y as 1 0 - 1 5 experiments w i t h t h e same s a m p l e s h o w e d

that t h e e x p e r i m e n t a l values of e q u i

l i b r i u m concentrations o b t a i n e d d u r i n g t h e exchange b e t w e e n n e w p o r -


41.

Oxygen

ANTOSHIN E T A L .

Mobility

in Synthetic

519

Zeolites

tions o f gas a n d t h e same s a m p l e d i d n o t exceed 0 . 5 - 0 . 7 %

atm.

These

d a t a a l l o w one to assume that a l l the o x y g e n o f zeolites w o u l d b e i n v o l v e d a n d , hence, the c o n c l u s i o n a b o u t its e q u i v a l e n c e w i t h respect to exchange w o u l d b e true f o r t h e z e o l i t e o x y g e n as a w h o l e . T h e k i n e t i c dependences o b s e r v e d m i g h t b e c a u s e d b y the exchange d i f f u s i o n h i n d r a n c e . I t has b e e n s h o w n e x p e r i m e n t a l l y that the exchange is n o t l i m i t e d b y outer d i f f u s i o n ; t h e results o b t a i n e d f r o m t h e e x p e r i ments w i t h various rates o f gas c i r c u l a t i o n c o i n c i d e .

Exchange probably

is n o t l i m i t e d b y diffusion i n zeolite pores. I n fact, w i t h as little as 1 4 % Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 1, 2015 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1971-0101.ch041

of s o d i u m r e p l a c e d b y c o p p e r , w h i c h does n o t p r a c t i c a l l y affect the porous s t r u c t u r e , t h e exchange rate constant o f z e o l i t e Y b e c o m e s m o r e t h a n 6 times as h i g h . T h e h i g h a c t i v a t i o n e n e r g y values f o r s o d i u m forms also p r o v i d e e v i d e n c e against diffusion h i n d r a n c e . T h u s , o n e cannot r e l y o n t h e as s u m p t i o n that exchange is l i m i t e d b y the d i f f u s i o n process to account f o r o x y g e n e q u i v a l e n c e w i t h respect to exchange.

I t is possible to e x p l a i n

i t b y m a k i n g t h e f o l l o w i n g assumptions. a ) T h e l i m i t i n g stage of exchange m a y b e one of t h e a d s o r p t i o n o r d e s o r p t i o n stages, w i t h a l l t h e a d s o r p t i o n - d e s o r p t i o n sites b e i n g e q u i v alent i n t h e i r p r o p e r t i e s . b ) A l l o x y g e n ions o f z e o l i t e m a y b e i d e n t i c a l , n o m a t t e r w h a t t h e i r close s u r r o u n d i n g is ( f o r e x a m p l e , o w i n g to t h e u n i f o r m d i s t r i b u t i o n o f the excess n e g a t i v e c h a r g e t h r o u g h o u t t h e f r a m e w o r k o f z e o l i t e ) . T h e d a t a a v a i l a b l e d o n o t a l l o w o n e to d e c i d e b e t w e e n

these 2

assumptions. S t u d y i n this d i r e c t i o n is u n d e r w a y . F i n a l l y , a f e w w o r d s s h o u l d b e s a i d a b o u t results that f e l l b e y o n d the

scope

o f this report.

T h e reduced

copper

forms

possess a h i g h c a t a l y t i c a c t i v i t y i n r e l a t i o n to the r e a c t i o n Q 2 0

1 6

0

1 8

at temperatures as l o w as 1 0 0 ° - 3 0 0 ° C .

o f zeolite Y 2

1 6

+ 0

2

1 8

=

T h u s , zeolites c o u l d b e

u s e d as o x i d i z i n g catalysts. T o c o n c l u d e , t h e zeolites s t u d i e d r a n k as a n i n t e r m e d i a t e b e t w e e n a l u m i n a a n d s i l i c a i n t h e i r o x y g e n m o b i l i t y ( 5 ) ; zeolites are less a c t i v e i n t h e exchange w i t h 0

2

t h a n a l u m i n a a n d m o r e active t h a n s i l i c a .

Literature Cited (1) Minachev, Kh. M., Garanin, V. I., Isakov, Ya. I., Usp. Khim. 1966, 35, 2151. (2) Muzykantov, V. S., Popovskii, V. V., Boreskow, G. K., Kinetics Catalysis 1964, 5, 624. (3) Saxena, S. C., Taylor, T. I., J. Inorg. Nucl. Chem. 1962, 25, 261. (4) Venuto, P. B., Landis, P. S., Advan. Catalysis 1968, 18, 259. (5) Winter, E. R. S., J. Chem. Soc. 1968, (A) 12, 2889. (6) Zhdanov, F. P., Egorova, Ε. M., "Chemistry of Zeolites," Nauka, Lenin grad, 1968. RECEIVED February 13, 1970.


Investigation of Oxygen Mobility in Synthetic Zeolites by Isotopic

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