Molecular Sieve Zeolites-II - ACS Publications

47 Heats of Immersion of Outgassed Ion-Exchanged Zeolites R. M. BARRER and P. J. CRAM

1

Downloaded by PRINCETON UNIV on September 30, 2014 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0102.ch047

Physical Chemistry Laboratories, Imperial College, L o n d o n S . W . 7, E n g l a n d

Heats of immersion in water have been determined for a number of outgassed porous crystals enriched by ion ex change in various cations (zeolites X, Y, A, chabazite, and synthetic ferrierite), and for clinoptilolite and mordenite in their Na-forms, decationated, and in various stages of dealumination. Finally, heats of immersion were determined in NaX, NaY, NaA, and (Ca,Na) chabazite in which the crys tals initially contained various known loadings of zeolitic water. From the results, the influence of the exchange cat ions upon integal heats of sorption of water, ΔΗ, and other derived heats have been evaluated and discussed. " Q e r h a p s the most i m p o r t a n t of a l l z e o l i t e - s o r b a t e complexes are those i n w h i c h w a t e r is t h e guest m o l e c u l e . W a t e r is essential f o r t h e s y n thesis of zeolites a n d is present i n a l l the n a t u r a l a n d s y n t h e t i c m e m b e r s of the g r o u p , c e r t a i n of w h i c h find a p p l i c a t i o n as i n d u s t r i a l desiccants. A c c o r d i n g l y , as f u l l a n u n d e r s t a n d i n g as possible is n e e d e d of w a t e r z e o l i t e c o m p l e x e s , e s p e c i a l l y of the b i n d i n g energy of the w a t e r w i t h i n the crystals. I n f o r m a t i o n a b o u t this e n e r g y has b e e n o b t a i n e d f r o m i s o t h e r m measurements over a t e m p e r a t u r e range (3, 7, 8, 9), b y c a l o r i m e t r y (18, 19),

a n d b y d i f f e r e n t i a l t h e r m a l analysis ( I I ) .

b y d i r e c t c a l o r i m e t r y , the isosteric heats, q , st

F r o m the isotherms a n d

m a y b e f o u n d as f u n c t i o n s

of the a m o u n t of w a t e r s o r b e d . H o w e v e r , some d i s a d v a n t a g e s m a y b e associated w i t h e a c h p r o c e d u r e . S u c h is t h e affinity b e t w e e n w a t e r a n d zeolites that to d e t e r m i n e q

st

f o r s m a l l uptakes m a y r e q u i r e i s o t h e r m

measurements at temperatures a b o v e 2 0 0 ° C . A t these temperatures, lat tice b r e a k d o w n c a n take p l a c e b y side reactions i n v o l v i n g t h e w a t e r . Present address: P e t r o l e u m Recovery Research Institute, T h e U n i v e r s i t y of C a l g a r y , Calgary, Alberta, Canada. 1

105

In Molecular Sieve Zeolites-II; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.

106

MOLECULAR

Table I. Starting

Zeolites Studied

Materials

ZEOLITES

II

0

Modified

N a X (Union Carbide) N a Y (Union Carbide) N a A (Union Carbide) (Ca, N a ) chabazite N a mordenite (Na-Zeolon, N o r t o n Co.) S r ferrierite ( S r D )

SIEVE

Forms

L i , K , R b , C s , M g , C a , Sr, B a , T l , a n d P b X L i , K , R b , C s , M g , C a , Sr, B a , T l , a n d P b Y L i , K , R b , Cs, M g , C a , B a , and T1A L i , N a , K , R b , C s , C a , a n d T l chabazites Η-forms (from 2 N , 6 N , a n d 1 2 N H C l - t r e a t e d Na-Zeolon) : H-mordenite (H-Zeolon) L i , N a , a n d C a D : Η-forms (from 0.09N H C l treated S r D , a n d H D f r o m heated N H D ) Η-forms ( f r o m 0 . 2 5 N , 0.5N, I.ON, a n d 2.0N H C l - t r e a t e d clinoptilolite) 4


N a - r i c h clinoptilolite

° I n Tables I a n d I I . the representation of a zeolite as, for example, R b X , should not be t a k e n to i m p l y 100% exchange of the original N a b y R b .

S e c o n d l y , c a l o r i m e t r i c measurements f r o m the v a p o r phase m a y refer to n o n e q u i l i b r i u m d i s t r i b u t i o n s of w a t e r w i t h i n the crystals a n d t h r o u g h the z e o l i t e b e d .

T h e v e r y energetic w a t e r - z e o l i t e b o n d , e s p e c i a l l y for

smaller w a t e r uptakes, means that w a t e r m o l e c u l e s m a y stick o n sites w h e r e t h e y first l a n d . S u b s e q u e n t r e d i s t r i b u t i o n c a n be v e r y s l o w o n the t i m e scale of the experiment, p a r t i c u l a r l y at the l o w temperatures p l o y e d (19, 21),

2 3 ° a n d 44 ° C .

em

F i n a l l y , the i n f o r m a t i o n d e r i v e d f r o m

d i f f e r e n t i a l t h e r m a l analysis is q u a l i t a t i v e or at best o n l y s e m i q u a n t i t a t i v e . A n alternative c a l o r i m e t r i c p r o c e d u r e , w h i c h i n p r i n c i p l e m a y e l i m inate the r e d i s t r i b u t i o n p r o b l e m of d i r e c t c a l o r i m e t r y , consists i n m e a s u r i n g heats of i m m e r s i o n as a f u n c t i o n of the a m o u n t of p r e s o r b e d l i q u i d . T h e b u l b c o n t a i n i n g the z e o l i t e a n d its p r e s o r b e d w a t e r c a n be to a s u i t a b l y h i g h t e m p e r a t u r e

heated

to p r o m o t e w a t e r m i g r a t i o n a n d t h e n

c o o l e d s l o w l y to the e x p e r i m e n t a l temperature.

T h e b u l b is b r o k e n u n d e r

l i q u i d w a t e r a n d the heat of w e t t i n g m e a s u r e d .

T h i s m e t h o d has

been

u s e d i n the present w o r k for a n u m b e r of zeolites of d i f f e r i n g k n o w n structures, w i t h different exchange ions, a n d also for i n d i v i d u a l structures decationated a n d progressively dealuminized. Experimental Materials. T h e zeolites s t u d i e d are s u m m a r i z e d i n T a b l e I. T h e u n i t c e l l contents of the d e h y d r a t e d forms of the starting materials w e r e : NaX NaY NaA Chabazite Mordenite Clinoptilolite S r ferrierite

8 7 N a + [87 A 1 0 " · 105SiO ] 57 (Ho.i, N a . ) [ 5 7 A 1 0 - · 1 3 5 S i 0 ] 12Na+ [ 1 2 A 1 0 - · 1 2 S i 0 ] 0.4 N a A 1 0 1.6Na+ 4 . 4 C a [10.4 A 1 0 ~ · 25.6 S i O , ] 8 (Ho.i N a . ) [8AIO2- · 4 0 S i O ] 6 N a + [6AIO2- · 30SiO ] O x i d e f o r m u l a : 0.94SrO · A 1 0 · 1 2 . 3 S i 0 2

0

9

2

+

2

2

2 +

0

9

2

2

2

2

+

2

2

2

3


2

47.

BARRER

AND

C R A M

Heats of

107

Immersion

T h e o x i d e f o r m u l a of Sr ferrierite o n l y is g i v e n because the y i e l d of this synthetic zeolite m a y h a v e b e e n less t h a n 1 0 0 %

(12).

T h e r e are

72

o x y g e n atoms i n the o r t h o r h o m b i c u n i t c e l l . T h e i o n - e x c h a n g e d forms of sieves X , Y , a n d A w e r e p r e p a r e d b y s h a k i n g a suspension of the starting materials i n a s o l u t i o n of the a p p r o priate c a t i o n , at r o o m t e m p e r a t u r e , for m o r e t h a n 6 hours i n e a c h treat ment. A n a l a R c h l o r i d e solutions w e r e u s e d w h e r e suitable. M g , T l , a n d P b f o r m s w e r e p r e p a r e d u s i n g sulfate, acetate, a n d nitrate respectively.

T h e e x c h a n g i n g solutions c o n t a i n e d a n i n i t i a l

solutions,

five-fold

ex


cess of e x c h a n g i n g i o n , except for R b , C s , a n d T l forms, w h e r e the i n i t i a l excess was a b o u t t w o - f o l d . F o l l o w i n g ion-exchange, the samples

were

w a s h e d t h o r o u g h l y w i t h n e a r l y b o i l i n g w a t e r to r e m o v e e n t r a i n e d salt. T h e entire process w a s r e p e a t e d at least 4 times. O t h e r z e o l i t e m o d i f i c a tions ( ferrierites, m o r d e n i t e s , chabazites, a n d c l i n o p t i l o l i t e s ) w e r e those p r e p a r e d b y other w o r k e r s i n these laboratories (4, 5, 6, 12, 24).

A l l the

samples s t u d i e d w e r e d r i e d o v e r n i g h t i n a 1 1 0 ° C o v e n a n d stored over saturated N H C 1 s o l u t i o n at r o o m t e m p e r a t u r e ( 2 0 ° to 2 3 ° C ) f o r at least 4

Table II.

Saturation Water Contents, Grams per G r a m of Hydrated Zeolite X I 0 0 Ion-Exchanged

Cation Form

Zeolite

Li Na Κ Rb Cs Mg Ca Sr Ba Tl Pb

X

27.8 25.8 22.5 19.4 17.5 29.3 27.6 25.2 22.2 12.3 15.9

Zeolite 27.6 25.9 21.9 20.4 17.6 28.1 26.1 25.4 23.6 16.7

a h

Chabazite

22.1 21.5 18.4 14.2 15.7 27.2 23.0

22.0 19.5 17.3 13.9 12.3 22.1,

— -

-

-

-

21.1

14.2 11.2

e

-

-

and Dealuminized

Na-Zeolon 2N H C l - t r e a t e d SN H C l - t r e a t e d 12ΛΓ H C l - t r e a t e d H-Zeolon

15.6 13.4

9.5

8.7

-

Forms Ferrierite

b

14.0 14.5 14.3 14.4 15.4

Ferrierite

-

Mordenite

1

Natural 0.257V H C l - t r e a t e d 0.5iV H C l - t r e a t e d 1.0N H C l - t r e a t e d 2.0N H C l - t r e a t e d

Zeolite A

-

Decationated Clinoptilolite

Y

Forms

13.2 -

0.09N H C l - t r e a t e d 15.7 H D from N H D 4

17.9 14.3

V a l u e for natural ( C a , N a ) chabazite. T h e fractions of A l removed i n the a c i d treatment are shown i n T a b l e I V .


108

M O L E C U L A R

Table III.


Li Na Κ Rb, N a Cs, N a Mg Ca Sr Ba Tl Pb

63.i

87.i

Zeolite Y 4.1

6Ο.3

8I.5

128.3 113.6 92.3

8

6

74., 64.o 134.x

60.3

52. 94.

8 7

103.4

143.0

94. 80.e

126.

8

7

103.6

21.2

42.

Ba, N a Tl, N a

4

2

52.4

67.i

47.2

59.4

40.

49.5

8

121.

87.4

73.3 69.5*

67.

3

27.0

Li Na Κ Rb, N a Cs, N a Mg, N a Ca

II

74.

6

99.3 93. 2

88. 32.

2 4

7

9

5

9

3

3

8

3 t

73.9 71.4

83.o 1.8 19.!

z

15.4 X 10 15.3 X 10 13.7 X 10 12.7 X 10 12.6 X 10 17.1 X 10 18.8 X 10 19.3 X 10 17.0 X 10 7.0 X 10 10.2 X 10

6 5 5 6 5 5 5 5 6

6

7

5

12.7!

6

0

3

5.1 2.8

3

8

6.I1 6.I9

0

20.9

3.9

5

6

0

4.9

6

134.5 107.7

5

Que, Cat per N Unit Cells

5.Ο4

4.3ι 4.1 4.1 5.6

6.9 3.2 5.4 6.4 6.5

29. 56.

25.3 47.7 98.0

Cal per Gram Equiv. of Cation X 10~

10.3 X 10 10.4 X 10 9 . I 9 Χ 10 9.1 Χ 10 8.5 Χ 10 15.0ι Χ 10 12.2 Χ 10

2

4.I9

Zeolite A 96.2 94.2 87.5

9

Q G E ,

z

24., 50.

84. 71.

Sr, N a

qD, Cal per Gram

Qi, Cal per Mole of Water X 10~ Zeolite X 5.9 5.8 5.7 5.5 5.4 5.8! 6.7 6.7 6.5 3.2 4.7,

92.5

Li Na Κ Rb, N a Cs, N a Mg, N a Ca, N a

ZEOLITES

Heats of Immersion of Cation Forms of Zeolites

qH, Cal per Gram

Cation Form

SIEVE

9

X 8 9

5

4

5

2

5

5

2

5

6

5

5

9

Χ

10

5

12.9 Χ 10 6.5 Χ 10

5

8

5

14.8 16.3 16.9

5

Χ Χ Χ 6.67 Χ 13.2 Χ 22. Χ 18.3 Χ

10 10 10 10 10 10 10

8.I7 X

10

2

4

4 4

4

4 4 4

17.7 17.6 15.7 14.6 14.5 19.6 21.6

22. 19.5 8.1; 11.8 2

18.2 18.3 16.1 16.0 15.0 26. 21. 3

6

22.

3

22. 11.5 7

12.3 13.6 14.1 5.5 11.0 18.5 15.3

6

!

Tl Li Na Κ Rb Cs, C a Ca, N a Ca Tl

73.2

67.

2

52.2 39H 30.4 76.7 8O.0*

22.6

93.9 83.5 63.2 45.7 34.7 97.2 102.5 25.0

Chabazite 5.9 6.2i 5.4 5.1χ 4.4 6.5 6.5 4.2 9

4

6

6

3

8

21.5 21.4 16.4 14.0o 11.7 23.5 24.9 IO.87 6

7

7

6

6

0

X X X X X X X X

4

6.81

10" 10 10 10" 10 10 10 10

20.13 20.0 15.3, 13.0 10.9 22.0i 23.2 10.1,

4 4

4

4

4

4


e

8

8

7

47.

BARRER

AND

H eat s of

C R A M

Immersion

Table III.


Cation Form

Cat per Gram

Li Na Ca Sr H (from NH +) H (0.09N HC1) 4

Continued Qi, Cal per Mole of Water X 10~

QD,

QH,

Cat per Gram

109

QGE,

Que, Cal per No Unit Cells

3

Zeolite D (Synthetic 40.2 3.9 35.4 4.1, 41.5 4.5 28.2 4.0

Cat per Gram Equiv. of Cation X ίο-*

ferrierite)

17.5 15.9 18.6 13.3

2

2

2

34.5

34. *

40,

4

3.9

5

2 weeks to ensure that w a t e r u p t a k e h a d r e a c h e d e q u i l i b r i u m . T h e satu r a t i o n w a t e r contents, d e t e r m i n e d b y T G A u s i n g a S t a n t o n t h e r m o b a l ance, are g i v e n i n T a b l e II. Outgassing. Samples w e r e w e i g h e d out i n t h i n - w a l l e d b o r o s i l i c a t e glass b u l b s of d i a m e t e r ~ 0 . 8 r e s i d u a l pressure < 1 0 "

5

c m , outgassed at 3 6 0 ° C f o r 24 hours to a

m m of H g , a n d sealed off u n d e r v a c u u m .

ples of N a X , N a Y , N a A , a n d

( C a , N a )-chabazite,

Sam

containing varying

amounts of p r e s o r b e d w a t e r , w e r e p r e p a r e d b y p a r t i a l r e m o v a l of w a t e r f r o m the saturated materials.

E q u a l samples w e r e p a r t i a l l y outgassed

i n pairs t h r o u g h a c o m m o n t a p at temperatures

controlled b y an oil

thermostat b e t w e e n 0 ° a n d 190 ° C f o r times b e t w e e n 2 a n d 12 hours.

The

heat of i m m e r s i o n of 1 sample w a s m e a s u r e d , a n d the r e s i d u a l w a t e r content of the s e c o n d w a s d e t e r m i n e d b y i g n i t i o n at 1 1 0 0 ° C to constant w e i g h t i n a P t c r u c i b l e . T h e r e s i d u a l w a t e r contents of e a c h s a m p l e w e r e assumed, i n i n t e r p r e t i n g the heat of i m m e r s i o n , to b e the same.

T o en

sure a n e q u i l i b r i u m d i s t r i b u t i o n of w a t e r p r i o r to the c a l o r i m e t r i c m e a surement, the p a r t i a l l y outgassed samples i n sealed b u l b s w e r e p l a c e d i n a n o v e n at 100 ° C f o r 36 hours a n d t h e n c o o l e d s l o w l y to r o o m t e m perature d u r i n g a p e r i o d of 6 hours. Calorimeter.

A d i f f e r e n t i a l c a l o r i m e t e r , o p e r a t i n g at 25.0 ° C u n d e r

n e a r - i s o t h e r m a l c o n d i t i o n s , w a s u s e d f o r a l l heat measurements.

Similar

calorimeters, d e s i g n e d for d e t e r m i n i n g heats of i o n exchange i n zeolites, h a v e b e e n d e s c r i b e d p r e v i o u s l y ( 5 , 6, 14, 15).

T h e calorimeter was cali

b r a t e d b y m e a s u r i n g the heat of s o l u t i o n of p o t a s s i u m c h l o r i d e i n w a t e r . T h e ratio of the area u n d e r t h e c u r v e t r a c e d b y the r e c o r d e r p e n to the heat p r o d u c e d was 1.50 ± 0.04 c m p e r calorie. N o heat c o u l d b e d e t e c t e d 2

w h e n an empty evacuated b u l b was broken under water.


110

M O L E C U L A R

Errors.

SIEVE

ZEOLITES

E r r o r s w e r e e s t i m a t e d f r o m d u p l i c a t e measurements.

c a l o r i m e t r i c measurements

are accurate

to a b o u t 4 % ,

the

II

The

saturation

w a t e r contents to 1 % , a n d the r e s i d u a l w a t e r contents are r e p r o d u c i b l e to a b o u t 3 % , except at the lowest coverages, w h e r e the error is greater. Results Heat of Immersion and Exchange Cation.

H e a t s of i m m e r s i o n i n

w a t e r w e r e d e t e r m i n e d f o r t h e outgassed c a t i o n i c f o r m s of the zeolites


(Table III).

T h e heats g i v e n i n this table are the f o l l o w i n g :

a. q

H

=

b. q

D

=

calories p e r g r a m of h y d r a t e d z e o l i t e ^

c. 0 / = —

H

calories p e r g r a m of d e h y d r a t e d zeolite

=

w

X 18.016 =

d . Que =

qD X WN =

e. QGE =

0™L =

0

calories p e r m o l e of w a t e r i m b i b e d calories p e r A v o g a d r o N o . of u n i t cells

calories p e r g r a m e q u i v a l e n t of cations

H e r e w is t h e w e i g h t of w a t e r p e r g r a m of h y d r a t e d zeolite e q u i l i b r a t e d o v e r saturated a m m o n i u m c h l o r i d e s o l u t i o n ; w

is the w e i g h t of a n A v o

No

g a d r o n u m b e r of d e h y d r a t e d u n i t cells of zeolite; a n d n

c

is t h e n u m b e r

of c a t i o n i c charges p e r u n i t c e l l . T h e values of qH w e r e d e r i v e d d i r e c t l y f r o m the c a l o r i m e t e r measurements.

T h e other q u a n t i t i e s are d e r i v a b l e ,

a s s u m i n g that a l l the w a t e r is r e m o v e d b y the outgassing at 3 6 0 ° C , a n d that w h e r e n o d i r e c t c h e m i c a l analysis w a s a v a i l a b l e , i o n exchange h a d r e a c h e d the l i m i t i n d i c a t e d b y p u b l i s h e d i o n exchange isotherms (5, 15, 31, 33).

6,14,

A l l results are t h e average of 2 or m o r e measurements except

those m a r k e d w i t h a n asterisk, w h e r e o n l y 1 m e a s u r e m e n t w a s m a d e . Dealumination and Heat of Immersion.

T r e a t m e n t s of m o r d e n i t e ,

c l i n o p t i l o l i t e , a n d f e r r i e r i t e w i t h d i l u t e a c i d i n the first instance r e m o v e m e t a l l i c cations a n d y i e l d the h y d r o g e n forms (4, 12, 13). also result b y h e a t i n g the a m m o n i u m zeolites

(24).

Such forms

Treatment

stronger a c i d solutions removes i n c r e a s i n g amounts of A l (4, ( T a b l e I V ) . T h e m a r k e d effect this has u p o n q

H

a n d d i f f e r e n t i a l (AH)

26)

is s h o w n i n the table.

^ H e a t s of Sorption as Functions of Amount Sorbed. (AH)

13,

with

T h e integral

heats of s o r p t i o n of w a t e r v a p o r w e r e o b

t a i n e d f r o m the r e l a t i o n s h i p s Qn

0

where

~

η

=

(η.

η,) (AH

-

àH ) L

a n d so


(D

47.

BARRER

AND

Heats of

C R A M

Immersion

= AH

ΔΗ + ( η . - no) Here q

n s

and q

no

111

(2)

are heats of i m m e r s i o n o f t h e z e o l i t e i n i t i a l l y c o n t a i n i n g

n a n d n moles o f p r e s o r b e d w a t e r p e r g r a m , a n d AH s

0

is t h e m o l a r heat

L

of c o n d e n s a t i o n o f w a t e r , t a k e n as 10.51 k c a l p e r m o l e . T h e degree of p r e s a t u r a t i o n , 0, is g i v e n b y _ i n i t i a l w a t e r content

_


s a t u r a t i o n w a t e r content where n

s a t

n

s

n

aat

is t h e n u m b e r o f moles of w a t e r n e e d e d to saturate t h e i n t r a -

c r y s t a l l i n e f r e e v o l u m e . S m o o t h e d curves of q

ns

vs. θ w e r e d r a w n ( F i g u r e

1 ), a n d t h e l o w e r l i m i t s o f θ c h o s e n f o r t h e c a l c u l a t i o n s of i n t e g r a l heats were: Zeolite: θ :

NaX 0.033

NaY 0

NaA 0.1

( C a , N a ) chabazite 0.133

T h e values o f — AH a n d —AH d e r i v e d f r o m t h e s m o o t h e d c u r v e s of F i g u r e 1 a r e s h o w n i n F i g u r e 2. A t t h e lowest values of Θ, —AH is v e r y large b u t d i m i n i s h e s as θ increases.

T h e curves f o r N a Y , N a A , a n d

( C a , N a ) c h a b a z i t e flatten for h i g h e r 0, w h i l e that for N a X has a m i n i m u m f o l l o w e d b y a r i s i n g section.

F o r N a X , the w a t e r - w a t e r self-potential

energy, i n c r e a s i n g w i t h 0, m o r e t h a n balances t h e d e c l i n i n g values of w a t e r - s o r b e n t interactions. A f t e r s a t u r a t i o n ( 0 ^ 1 ) , f o r a l l systems w o u l d b e e x p e c t e d to a p p r o a c h Table I V .

— AH a n d — AH

—AH . L

Heats of Immersion in Decationated and Dealuminized Zeolites

Sample

7c Al Relative to Original

Original 0.25N H C 1 0.5N H C 1 1.0N H C 1 2.0N HC1

100 58 33 7 0

N a Zeolon Η Zeolon 2.0iV H C 1 6.0N H C 1 12.0ΛΓ H C 1

100 87 49 32 25

Cal per Gram Clinoptilolite 41.o 41.3 32.4 27.3

23.

6

Mordenite 46a 34. 38. * 7

3

36.7* 34.8*

QD,

Cal per Cram 47.7 48.3

37. 31. 27.

8 9 9

qi, Kcal per Mole of Water 5.2 5.1 4.0 3.4 2.7

53.i 40.

6.2 4.3 -

42.4

3.5

5

-

que, Cal. per No Unit Cells X 10~

7 x 8 2 6

9 7

-

4

10.3, 9.5 7.3 6.0 5.2 3

3

7

9

16.2i 11.5 7

-

0


11.6i

112

M O L E C U L A R

SIEVE

ZEOLITES

II

Discussion Exchange Ions and Heats of Immersion. T h r e e c a t i o n i c f o r m s give heats of i m m e r s i o n w h i c h are out of l i n e w i t h other results. O n e o f these is B a A , f o r w h i c h q

H

is o n l y 1.8 c a l p e r g r a m . T h i s f o r m c a n lose m u c h

o f its structure o n outgassing ( 3 2 ) ,

a n d the w a t e r u p t a k e a n d heat of

i m m e r s i o n are r e d u c e d a c c o r d i n g l y . ( R b , N a ) A a n d ( M g , N a ) X also m a y lose some c r y s t a l l i n i t y d u r i n g outgassing. R a s t r e n e n k o et al. ( 2 9 ) q

H

extensive lattice b r e a k d o w n . P l a n k (28) Downloaded by PRINCETON UNIV on September 30, 2014 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0102.ch047

found

f o r ( R b , N a ) A to b e o n l y 3 c a l p e r g r a m , w h i c h p r o b a b l y i n d i c a t e s reported partial breakdown i n

( M g , N a ) X a l t h o u g h this w a s not c o n f i r m e d b y others (16, B e c a u s e of the a b n o r m a l i t i e s i n q

H

20, 27,

36).

observed w i t h B a A , ( R b , N a ) A , and

( M g , N a ) X , these 3 w i l l b e d i s r e g a r d e d i n the f o l l o w i n g c o m p a r i s o n s . O u r values of q

H

for L i , ( M g , N a ) , a n d C a forms of zeolite A are 12 to

2 0 % l o w e r t h a n those of R a s t r e n e n k o et al., b u t o u r v a l u e f o r Κ A is a p p r e c i a b l y h i g h e r . H o w e v e r , w i t h the e x c e p t i o n of K A , the trends i n

q

H

are t h e same, a n d n u m e r i c a l differences m a y arise f r o m differences i n the zeolite s a m p l e . O f the c a t i o n forms of c h a b a z i t e , C s - c h a b a z i t e contains a b o u t

20%

of r e s i d u a l C a w h i l e the other m o d i f i c a t i o n s s h o u l d c o n t a i n less t h a n 1 0 % of this i o n (6).

I n zeolite X , the R b , C s , M g , a n d B a forms c o n t a i n a b o u t

30, 30, 27, a n d 23 r e s i d u a l N a ions p e r u n i t c e l l (14) +

out of a t o t a l i n o u r

Figure 1. Curves of q vs. θ for (a) NaX, (b) (c) NaA, and (d) (Ca,Na)chabazite n s

NaY,


47.

BARRER

Heats of

A N D C R A M

113

Immersion

to;

(a) 25h

-ΔΗ -ΔΗ -ΔΗι_ ( 2 5 ° C )


AH|_ (25 C)

0-2

0-4 0 6

θ

0-8

0-2

0-4

θ

0-6

08

Figure 2. Curves of —AH and — Δ Η plotted against θ for (a) NaX, (b) NaY, (c) NaA, and (d) (Ca,Na)chabazite s a m p l e o f 87 ions. O f these n u m b e r s , 16 N a ions i n e a c h case p r o b a b l y +

o c c u p y sites w i t h i n t h e h e x a g o n a l p r i s m s a n d thus are c o m p l e t e l y s h i e l d e d f r o m w a t e r m o l e c u l e s . T h e r e m a i n d e r m u s t h a v e some c o n t a c t w i t h w a t e r a n d thus m o d i f y t h e heat of i m m e r s i o n c o m p a r e d w i t h t h e h o m o i o n i c f o r m . I n zeolite A , t h e R b , C s , a n d M g f o r m s c o n t a i n a b o u t 29, 54, a n d 33%

of N a ions, r e s p e c t i v e l y , a l l of w h i c h c a n interact w i t h w a t e r . I n +

z e o l i t e Y , a l t h o u g h the R b , C s , C a , Sr, B a , a n d T l f o r m s e a c h c o n t a i n a b o u t 16 N a

+

p e r u n i t c e l l ( 5 ) , these N a ions are p r o b a b l y w i t h i n t h e +

h e x a g o n a l p r i s m s a n d i f so w o u l d n o t interact w i t h w a t e r o r m o d i f y t h e heats o f i m m e r s i o n . T h e m a g n i t u d e of q

H

is d e t e r m i n e d l a r g e l y b y t h e w a t e r content p e r

g r a m ( T a b l e I I ), w i t h f u r t h e r i n f l u e n c e b y t h e n u m b e r , size, a n d v a l e n c e of cations a n d t h e f r a m e w o r k c o n f i g u r a t i o n .

A m o n g the alkali metal

c a t i o n s — e x c l u d i n g ( R b , N a ) A — t h e r e is a r e g u l a r decrease i n q

H

and q

i n t h e sequence. L i + > N a + > K + > R b + > Cs+ F o r t h e d i v a l e n t i o n s — e x c l u d i n g B a A a n d ( M g , N a ) X — t h e o r d e r is


D

114

M O L E C U L A R

Mg + > 2

Ca + > 2

Sr + >

Ba

2

SIEVE

ZEOLITES

II

2 +

w h i l e the heat of i m m e r s i o n f o r a g i v e n zeolite c o n t a i n i n g a d i v a l e n t exchange i o n is u n i f o r m l y larger t h a n this heat i n the same zeolite c o n t a i n i n g a m o n o v a l e n t i o n of s i m i l a r radius—e.g., C a

and Na —despite

2 +

+

the smaller n u m b e r of d i v a l e n t ions. T h e i n f l u e n c e of charge d e n s i t y i n 2 o t h e r w i s e i d e n t i c a l structures is seen c l e a r l y b y c o m p a r i n g q

H

or

q

D

f o r the same exchange i o n i n zeolites X a n d Y . I n a l l cases except that of T l , the heat of i m m e r s i o n is c o n s i d e r a b l y larger i n X . +

For T l

+

the

reverse is true, p r i m a r i l y because the w a t e r content of T 1 X is less t h a n Downloaded by PRINCETON UNIV on September 30, 2014 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0102.ch047

that of T1Y ( T a b l e

II).

T h e effect of p o l a r i t y of the f r a m e w o r k u p o n the heat of i m m e r s i o n c a n b e s h o w n i n a m o r e g e n e r a l w a y . T h e l a t t i c e - f o r m i n g units of the a n i o n i c f r a m e w o r k c a n be c o n s i d e r e d as ( A l ^ S i d . ^ )0 ~ x

2

30 k c a l p e r m o l e 30 k c a l p e r mole 22.7 k c a l per mole 17 k c a l p e r mole

F o r h i g h w a t e r l o a d i n g s , i n t e g r a l heats at t h e largest m e a s u r e d of θ c o m m o n to a l l t h e zeolites m a y i n d i c a t e j t h e


desiccants at s u c h l o a d i n g s . F o r θ =

value

r e l a t i v e usefulness as

0.8, —AH has t h e values

( C a , N a ) chabazite NaA NaX NaY

18.9 17.7 16.9 15.2

kcal kcal kcal kcal

per per per per

mole mole mole mole

T h i s o r d e r is t h e same as that of t h e i n i t i a l heats. R e t e n t i v i t y f o r w a t e r at h i g h l o a d i n g s also d e p e n d s

u p o n t h e i n t r a c r y s t a l l i n e free

volumes

w h i c h a r e ( J ) a b o u t 0.54 c m p e r c m f o r N a Y a n d N a X , a n d a b o u t 0.46 3

cm

3

3

per c m for N a A a n d chabazite. 3

A c o m p a r i s o n of w a t e r r e t e n t i v i t y is g i v e n i n F i g u r e 7 i n w h i c h 0 ( N a Y ) , 0 ( N a A ) , a n d Θ(chabazite)

are p l o t t e d against 0 ( N a X ) .

I n each

case, starting w i t h saturated crystals, outgassing w a s c o n d u c t e d f o r the 4 zeolites u n d e r i d e n t i c a l c o n d i t i o n s .

C u r v e s s h o w n i n F i g u r e 7 are n o t

O(Na-X) Figure 7. θ for NaY ( • ), NaA (Φ), and chaba zite (M) plotted against θ for NaX, following identical outgassing conditions


120

MOLECULAR

SIEVE

ZEOLITES

II

at e q u i l i b r i u m so that factors s u c h as crystallite size m a y h a v e a n i n f l u ence. N e v e r t h e l e s s , w i t h t h e e x c e p t i o n of a n a n o m a l o u s r e g i o n f o r N a A i n t h e r a n g e 0.25 < θ

0

(c) B

-.- ( r / ) w h e r e rj is the r a d i u s of the w a t e r m o l e c u l e p l u s A the r a d i u s of t h e c a t i o n i n a h y p o t h e t i c a l nonelectrostatic e n v i r o n m e n t . T h i s latter r a d i u s w a s t a k e n as the r a d i u s of the i n e r t gas a t o m h a v i n g the s a m e electronic c o n f i g u r a t i o n ( N e , 1.60A; A r , 1 . 9 2 A ) . ( d ) T h e h a r d sphere m o d e l w i t h B = oo f o r r ^ r a n d B = 0 f o r r > r . r w a s a g a i n set e q u a l to r . T h e m a x i m u m i n t e r a c t i o n energy, c o r r e s p o n d i n g w i t h t h e e q u i l i b s

=

6

4

e

e

e

4

0

r i u m distance of separation, r , w a s o b t a i n e d f r o m the plots of φ = e

+

Φ^ρ +

φρ +

φβ +

(φ^

μ

φΛ) against r. T h e values of r a c c o r d i n g to t h e c

v a l u e of Β t a k e n , a n d the c o r r e s p o n d i n g values of the c o m p o n e n t s of φ, are s h o w n i n T a b l e V I . T h e values of φ a n d r associated w i t h B e

x

seem

u n r e a l i s t i c , so a n average v a l u e f o r φ w a s c a l c u l a t e d f r o m t h e other 3 values. T h u s , the m a x i m u m energies of i n t e r a c t i o n a r e : Na+-H 0

—25 k c a l per mole

Ca

—61 k c a l per mole

2

2 +

-H 0 2

T h e r e l a t i o n s h i p b e t w e e n φ a n d AH at 0 ° K is ( φ — φ ) 0

φ

0

is t h e zero p o i n t energy.

=

AH w h e r e

I n v i e w of t h e a p p r o x i m a t i o n s m a d e , φ is


0

124

MOLECULAR

SIEVE

n e g l e c t e d , a n d it is assumed also that ΔΗ 98°κ == AH o . 2

coverages e x a m i n e d , the values of AH

0

ZEOLITES

A t the lowest

K

are

NaX NaY

θ = 0.05 θ~ 0

Δ/7 = AH =

NaA

θ = 0.1

Ai/=

—29 k c a l per m o l e

Δ#

— 32 k c a l per mole

( C a , N a ) c h a b a z i t e θ = 0.15

II

- 2 4 k c a l per m o l e - 2 0 k c a l per mole

=

T h e heats of s o r p t i o n at l o w coverages f o r the N a - f o r m s thus are c o m parable w i t h the calculated value. F o r N a X , N a A , a n d ( C a , N a )


z i t e , — A H at θ =

0 p r e s u m a b l y w o u l d be s o m e w h a t greater.

chaba

This may

e x p l a i n i n p a r t w h y the m e a s u r e d heat f o r c h a b a z i t e is s u b s t a n t i a l l y less t h a n that c a l c u l a t e d f o r a n i s o l a t e d C a

2 +

- H 0 pair. A further possibility 2

is that the N a ions i n n a t u r a l c h a b a z i t e o c c u p y m o r e exposed positions +

than the more numerous C a

2 t

ions.

T h e n u m e r i c a l l y largest e x o t h e r m a l t e r m at r is φρ e

(Table V I ) .

β

A

d i r e c t m e t h o d of e s t i m a t i n g t h e r e l a t i v e i m p o r t a n c e of the energy c o m ponents (φ

0

+

ΦΒ +

φρ) a n d (φ

Εβ

- f Φ^)

has b e e n d e v e l o p e d

(2).

T h e i n i t i a l heats of s o r p t i o n f o r a n u m b e r of n o n p o l a r , s t r u c t u r a l l y s i m p l e m o l e c u l e s w e r e p l o t t e d against t h e i r p o l a r i z a b i l i t y , a, for a g i v e n sorbent. I n this w a y , a characteristic c u r v e g i v i n g the c o n t r i b u t i o n to AH (φο +

ΦΒ +

from

Φ ρ) as a f u n c t i o n of a w a s o b t a i n e d . T h i s c o n t r i b u t i o n c a n

b e i n t e r p o l a t e d t h e n for m o l e c u l e s of i n t e r m e d i a t e p o l a r i z a b i l i t i e s w h i c h also h a v e p e r m a n e n t electric m o m e n t s . of ( ψ ^

μ

B y difference, the c o n t r i b u t i o n

- f φ^ρ) to AH c a n be f o u n d . T h i s m e t h o d demonstrates

again

t h e d o m i n a n c e of the electrostatic energy components i n w a t e r - z e o l i t e systems, as is a p p a r e n t f r o m T a b l e V I I . T h e c a l c u l a t e d values of (φ , Ρμ

Molecular Sieve Zeolites-II - ACS Publications

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