A Correlation of the Calculated Intracrystalline Void Volumes and

29

Downloaded by UNIV OF PITTSBURGH on May 4, 2015 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0121.ch029

A Correlation of the Calculated Intracrystalline Void Volumes and Limiting Adsorption Volumes in Zeolites D. W. BRECK and R. W. GROSE Union Carbide Corp., Tarrytown Technical Center, Tarrytown, Ν. Y. 10591

The limiting adsorption volumes for various adsorbates (H O, N , O , neopentane) in the zeolites A, X, L, mordenite, omega, and a synthetic offretite type have been determined from is therms. These have been compared with the void volumes c lated from the known crystal structures. For most adsorbate the measured and calculated void volumes are in good agreem However, helium and nitrogen exhibit anomalous behavior. void volume-framework density relation for zeolites is given. 2

2

2

' T ' h e adsorption of gases and vapors on dehydrated zeolites is characterized by the rectangular type I adsorption isotherm. The saturation capacity for an adsorbate, χ , corresponds to complete filling of the zeolite micropores and may be obtained from the isotherm. The adsorbate in most cases has a density, d , equal to that of the normal liquid at that tem perature. Consequently, the total micropore volume, V , available in the dehydrated zeolite may be calculated 8

&

p

V is in cm /gram, i in grams/gram, and d is in grams/cm . This has generally been referred to as the Gurvisch rule (1) and is frequently obeyed by many different adsorbates on different types of microporous adsorbents including silica gel and carbon. It also applies to dehydrated zeolites (2). p

3

X s

a

s

3

Unlike the usual amorphous, microporous adsorbents, it is possible to calculate the theoretical micropore volume of a dehydrated zeolite from the known crystal structure. We have performed these calculations here for several of the better known zeolites including zeolite A , zeolite X , zeo lite L , mordenite (Zeolon), (3) zeolite omega, (4) and the zeolite Ο (offretite 319 In Molecular Sieves; Meier, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

320

MOLECULAR SIEVES

t y p e ) (5 6). These zeolites are c h a r a c t e r i z e d b y t w o types of micropores. O n e t y p e is large enough t o a c c o m m o d a t e molecules s u c h as n-paraffins o r isoparaffin h y d r o c a r b o n s , a n d t h e o t h e r t y p e w i l l a c c o m m o d a t e o n l y s m a l l p o l a r molecules s u c h as w a t e r . C o n s e q u e n t l y , where a p p l i c a b l e , w e h a v e c a l c u l a t e d b o t h the t o t a l pore v o l u m e a n d t h e v o l u m e of t h e large pores o n l y a n d c o m p a r e d these values w i t h observed pore v o l u m e s as de t e r m i n e d f r o m e q u i l i b r i u m a d s o r p t i o n measurements a n d t h e G u r v i s c h rule. y


Experimental Zeolites A , X , L , omega, a n d Ο were s y n t h e s i z e d i n h i g h p u r i t y . The " l a r g e - p o r t " m o r d e n i t e (Zeolon 100) was o b t a i n e d i n t h e N a a n d h y d r o g e n forms f r o m t h e N o r t o n C o . S y n t h e s i z e d zeolites L , omega, a n d Ο were c o n v e r t e d t o t h e a m m o n i u m forms b y i o n exchange. N H a n d T M A ions were r e m o v e d b y c a l c i n i n g t h e zeolites i n 0 a t 4 5 0 ° C for 24 h o u r s . I n zeolites L a n d O , r e s i d u a l K ions were assumed t o o c c u p y sites i n t h e c a n c r i n i t e cages or double six r i n g s . A d s o r p t i o n isotherms were deter m i n e d b y a c o n v e n t i o n a l g r a v i m e t r i c a p p a r a t u s of t h e M c B a i n - B a k r t y p e . 4

2

+

T h e t o t a l pore v o l u m e of the v o i d s was d e t e r m i n e d f r o m t h e a m o u n t of adsorbed w a t e r at s a t u r a t i o n a s s u m i n g t h a t the w a t e r is present as a n o r m a l l i q u i d w i t h a n average d e n s i t y assumed t o be t h a t of l i q u i d w a t e r . O t h e r adsorbates u t i l i z e d i n t h i s s t u d y i n c l u d e t h e gases o x y g e n a n d n i t r o g e n a t t h e i r respective b o i l i n g p o i n t s a n d η-butane a n d neopentane a t r o o m t e m perature. Results Z e o l i t e A . T h e s t r u c t u r e of zeolite A contains t w o t y p e s of v o i d s : (1) t h e a cage, 11.4 A i n d i a m e t e r , a n d (2) t h e β cage (or sodalite u n i t ) , 6.6 A i n d i a m e t e r (7). T a b l e I compares e x p e r i m e n t a l l y d e t e r m i n e d p o r e v o l u m e s of zeolite A w i t h the v o i d v o l u m e as c a l c u l a t e d f r o m t h e s t r u c t u r e (no influence of cations considered). Since the s o d i u m zeolite A does n o t adsorb n o r m a l paraffins, d a t a are i n c l u d e d for t h e c a l c i u m - e x c h a n g e d f o r m . A l s o s h o w n i n c o l u m n 5 is the v o i d f r a c t i o n , V f , as c a l c u l a t e d b y F,

=

d /d

XB

0

a

(2)

where d is the d e n s i t y of t h e d e h y d r a t e d zeolite c r y s t a l as c a l c u l a t e d f r o m the unit cell composition a n d volume. c

A l s o g i v e n i n each t a b l e are t h e u n i t c e l l v o l u m e , V , a n d n , t h e n u m b e r of u n i t cells per g r a m of outgassed zeolite. T h e l a s t c o l u m n gives the o b served pore v o l u m e i n u n i t s of c u b i c A n g s t r o m s . T h e c a l c u l a t e d t o t a l v o i d v o l u m e is 0.32 c m / g r a m whereas t h e v o l u m e f r o m t h e large a cages 3

In Molecular Sieves; Meier, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

29.

Introcrystalline

BRECK AND GROSE

321

Voids in Zeolites

( w h i c h adsorb n o r m a l paraffins) is 0.27 c m / g r a m (8). F u r t h e r , t h e t o t a l v o i d v o l u m e is e q u i v a l e n t t o t h e measured p o r e v o l u m e s f o r t h e adsorbates w a t e r a n d n i t r o g e n . T h e v o l u m e of the a cages is e q u i v a l e n t t o t h e o x y g e n pore volume.


3

T h e a m o u n t of adsorbed w a t e r a n d n i t r o g e n i n zeolite A c a n n o t be a c c o u n t e d f o r o n the basis t h a t t h e y are n o r m a l l i q u i d s filling j u s t the large a cages. E i t h e r these adsorbates o c c u p y t h e t o t a l v o i d v o l u m e , i n c l u d i n g the β cages, o r the d e n s i t y of the adsorbed phase is c o n s i d e r a b l y greater t h a n t h e n o r m a l l i q u i d d e n s i t y a t t h e t e m p e r a t u r e concerned. This anomaly o n t h e p a r t of w a t e r a n d n i t r o g e n is observed i n several zeolites (see below). W a t e r molecules m a y o c c u p y t h e β cages, b u t t h e n i t r o g e n molecule c a n n o t enter t h e β cages a t l o w temperatures. Table I. Zeolite

Adsorbate

NaA

H 0 C0 2

2

CaA

o

2

o

2

H 0 2

N ft-C^io 2

V o i d Volume i n Zeolite A

Temp, °C

grams/gram

cm /gram

v

25 -75 -183 25 -183 -196 25

0.289 0.30 0.242 0.305 0.276 0.239 0.131

0.289 0.252 0.213 0.305 0.242 0.297 0.226

0.45 0.39 0.33 0.48 0.38 0.47 0.35

z

V ,A P

f

3

842 725 612 885 700 857 655

For CaA, d = 1.57 grams/cm , V = 1843 A , η = 3.46 X 10 /gram, 7 (calcd) = V + ν = 926 A / U C = 0.32 cmVgram, V = 7 7 5 A / U C = 0.27 c m / g r a m . 3

0

a

3

β

20

3

p

3

3

a

T h e presence of a large c a t i o n m a y s u b s t a n t i a l l y reduce t h e p o r e v o l u m e . I o n exchange b y t h a l l i u m reduced t h e p o r e v o l u m e b y a b o u t 250 A p e r u n i t cell (7). R e p l a c e m e n t of s o d i u m b y c a l c i u m increases t h e pore v o l u m e since t h e t o t a l c a t i o n d e n s i t y is reduced. T h e D u b i n i n P o l a n y i p o r e v o l u m e filling t h e o r y gave a m e a s u r e d v a l u e of t h e l i m i t i n g a d s o r p t i o n v o l u m e , W f o r l i g h t h y d r o c a r b o n s i n c a l c i u m - e x c h a n g e d zeolite A of 0.23 c m / g r a m . T h i s is i n close agreement w i t h the v a l u e x = 0.226 for n o r m a l b u t a n e ( T a b l e I) (9). 3

0}

3

s

Z e o l i t e X . T h e pore v o l u m e i n zeolite X as d e t e r m i n e d f r o m t h e a d s o r p t i o n of v a r i o u s molecules i n c l u d i n g w a t e r , gases, a n d h y d r o c a r b o n s i s s h o w n i n T a b l e I I . T h e w a t e r p o r e v o l u m e is e q u i v a l e n t t o 7908 A p e r u n i t cell. F o r most molecules, except for w a t e r a n d n i t r o g e n , i t is a p p a r e n t t h a t o n l y t h e large super-cages are occupied. T h e t o t a l pore v o l u m e of these large cages w a s d e t e r m i n e d f r o m t h e a d s o r p t i o n of o x y g e n a n d is a b o u t 6900 A p e r u n i t cell. A b o u t 1200 A corresponds t o t h e eight β cages w h i c h a r e a v a i l a b l e o n l y t o w a t e r . T h e c a l c u l a t e d i n t e r n a l v o l u m e of a single β cage is 151 A . T h e v o l u m e of each large 2 6 - h e d r o n has been c a l c u l a t e d t o be 822 A per u n i t cell (10). T h e t o t a l v o i d v o l u m e i n zeolite X is 7832 A . T h i s is i n g o o d agreement w i t h t h e observed v a l u e of 7908 3

3

3

3

3

3


322

MOLECULAR SIEVES

A . T h e t o t a l c a l c u l a t e d v o i d v o l u m e f o r t h e large v o i d s i s 0.296 c m / g r a m w h i c h is consistent w i t h t h e observed pore v o l u m e s for o x y g e n a n d h y d r o c a r b o n s as s h o w n i n T a b l e I I . These results agreed closely w i t h those of B a r r e r a n d S u t h e r l a n d (11). T h e i r results, as o r i g i n a l l y p u b l i s h e d , were r e c a l c u l a t e d o n t h e basis of outgassed zeolite a n d give a t y p i c a l v a l u e of a b o u t 0.31 c m / g r a m for n o r m a l paraffin h y d r o c a r b o n s . T h e v o i d frac t i o n i n zeolite X is n e a r l y 5 0 % of the t o t a l c r y s t a l v o l u m e . 3

3

3

Table I I . Void Volume i n Zeolite X Temp, Vt Adsorbate grams/gram cm /gram °C e

Fp,A3


3

H 0 C0 o N n-C Hi Isooctane Neopentane 2

2

2

2

5

2

25 -78 -183 -196 25 25 25

0.51 0.48 0.45 0.50 0.42 0.39 0.38

0.36 0.33 0.31 0.35 0.30 0.27 0.26

0.355 0.395 0.356 0.279 0.184 0.186 0.157

7908 7360 6923 7680 6581 6006 5860

d = 1.43, V = 15530 A / U C , η = 0.45 X 10 /gram, 7 (calcd) = 7832 A / U C , total = 0.352 cm /gram, V (large voids only) = 6576 A = 0.296 cm /gram 3

c

3

a

20

3

p

3

p

8

Sodium cation form.

D u b i n i n et al. measured t h e l i m i t i n g a d s o r p t i o n of H 0 a n d N o n X zeolites a n d c o m p a r e d these d a t a w i t h c a l c u l a t e d v o i d v o l u m e s . Their d a t a f o r H 0 a n d N compare w i t h ours a n d also show t h a t t h e measured H 0 a n d N pore v o l u m e s are the same, 0.34-0.35 c m / g r a m (19), Z e o l i t e L . S i m i l a r d a t a f o r zeolite L are s h o w n i n T a b l e I I I (12). I n zeolite L , t h e m a i n a d s o r p t i o n channels p a r a l l e l t h e c d i r e c t i o n of t h e h e x a g o n a l c r y s t a l s t r u c t u r e a n d are f o r m e d b y n e a r l y p l a n a r 12-membered rings w i t h a d i a m e t e r of 7.4 A (13). These channels are l i n k e d o n l y t h r o u g h v e r y s m a l l apertures w h i c h w i l l n o t pass even H 0 molecules. T h e linked c a n c r i n i t e u n i t s f o r m a t w o - d i m e n s i o n a l pore s y s t e m w h i c h does n o t c o n nect w i t h t h e m a i n channels. I t is possible f r o m t h e s t r u c t u r e t o estimate t h e v o i d v o l u m e a n d c o m p a r e t h i s w i t h t h e measured values. A s s h o w n i n T a b l e I I I w a t e r molecules m u s t o c c u p y v o i d space w h i c h is n o t a v a i l a b l e to oxygen. S i m i l a r results were o b t a i n e d for a r g o n a n d k r y p t o n . N i t r o g e n , however, does appear t o fill v o i d s w h i c h are n o t a v a i l a b l e t o these 2

2

2

2

2

3

2

2

other molecules. I f t h e s t r u c t u r e is correct, t h e a d d i t i o n a l space a v a i l a b l e t o w a t e r m u s t consist of t h e s m a l l v o i d s f r o m t h e c a n c r i n i t e - t y p e s t r u c t u r e u n i t s w h i c h l i n k together t o f o r m t h e c h a n n e l w a l l s . T h e c a l c u l a t e d v o i d v o l u m e of t h e m a i n channels is 614 A p e r u n i t cell. T h i s v a l u e compares w i t h t h e measured pore v o l u m e of 619-642 A as d e t e r m i n e d f r o m t h e a d s o r p t i o n of oxygen a n d η-butane. N i t r o g e n is a g a i n anomalous since i t 3


29.

323

Intracrystalline Voids in Zeolites

BRECK AND GROSE

T a b l e I I I . V o i d V o l u m e i n Z e o l i t e L« Temp, , 7p, Adsorbate cm /gram °C grams/gram Ff X s

7p,A*

z

25 -183 -196 25 25 25

H 0 o N n-C Hio Isobutane Neopentane 2

2

2

4

0.21 0.172 0.181 0.166 0.157 0.132

0.210 0.196 0.146 0.096 0.087 0.081

784 642 675 619 587 493

0.36 0.29 0.31 0.28 0.27 0.22

do = 1.70, F = 2205 A / U C , = 2.68 Χ 10 , F (calcd) = 614 A (large channel) = 0.15 cmVgram, F (calcd) tai = 686 A = 0.18 cm /gram 8


3

NH -exchanged L ; N H thermally removed. 4

8

p

8

to

p

α

20

n

3

Composition: K2.4AI9.0S127O72.

cannot o c c u p y the c a n c r i n i t e cages. W a t e r m a y enter t h e c a n c r i n i t e u n i t s w h i c h are n o t occupied b y K ions. " L a r g e - P o r t " M o r d e n i t e . T h e t o t a l v o i d v o l u m e for m o r d e n i t e has been e s t i m a t e d f r o m t h e s t r u c t u r e (14)I n a d d i t i o n t o t h e m a i n c-axis channels, s m a l l p e r m a n e n t gas molecules seem t o o c c u p y v o i d s i n n i c h e t y p e cavities w h i c h l i e o n the sides. B a r r e r a n d P e t e r s o n observed t h a t η-paraffins a n d isoparaffins were excluded f r o m t h e m a i n channels i n so d i u m m o r d e n i t e . H o w e v e r , o u r results show t h a t the m a i n channels are r e a d i l y a v a i l a b l e t o h y d r o c a r b o n s s u c h as neopentane ( T a b l e I V ) . The t o t a l e s t i m a t e d v o i d v o l u m e i n m o r d e n i t e i s , therefore, 0.21 c m / g r a m , of w h i c h 0.11 c m / g r a m consists of t h e m a i n channels. P r e v i o u s l y , D u b i n i n et al. (IS) e s t i m a t e d t h e t o t a l v o i d v o l u m e i n m o r d e n i t e as 0.16 c m / g r a m a n d observed t h a t t h e l i m i t i n g a d s o r p t i o n v o l u m e s for n i t r o g e n , a r g o n , a n d w a t e r were also 0.16 c m / g r a m . +

3

3

3

3

O m e g a . S i m i l a r results for w a t e r , o x y g e n , n i t r o g e n , a n d h y d r o c a r b o n s for t h e zeolites omega a n d zeolite Ο (offretite t y p e ) are s h o w n i n T a b l e s V a n d V I . T h e h y d r o c a r b o n pore v o l u m e o b s e r v e d i n zeolite o m e g a c o r responds to t h e c a l c u l a t e d v o i d space of the m a i n c h a n n e l . T h i s is b a s e d u p o n t h e a s s u m p t i o n t h a t t h e t e t r a m e t h y l a m m o n i u m i o n has been t h e r m a l l y decomposed. T h e g m e l i n i t e - t y p e cage i n t h e s t r u c t u r e of o m e g a c o n t r i b u t e s a b o u t h a l f of the t o t a l v o i d space of 0.19 c m / g r a m , a n d a d s o r p t i o n d a t a show t h a t t h i s space m u s t be o c c u p i e d b y w a t e r a n d t h e gases N and 0 . 3

2

2

I n t e r p r e t a t i o n of i n f r a r e d s p e c t r a of zeolite o m e g a is i n c o n s i s t e n t w i t h t h e proposed s t r u c t u r e (16) a n d is m o r e consistent w i t h a s t r u c t u r e b a s e d o n s o d a l i t e - t y p e u n i t s (17). T h e a d s o r p t i o n s a t u r a t i o n v a l u e s c a n o n l y be a c c o u n t e d for b y t h e proposed s t r u c t u r e i f i t is a s s u m e d t h a t H 0 a n d N c a n o c c u p y t h e g m e l i n i t e - t y p e cages. H o w e v e r , t h i s p r o p o s e d s t r u c t u r e does n o t p r o v i d e for access t o t h e g m e l i n i t e - t y p e u n i t s f r o m the m a i n c h a n n e l w h i c h are large enough t o pass N . Access t o the t w o d i m e n s i o n a l 2

2


2

324

MOLECULAR SIEVES

Void Volume i n Mordenite (Zeolon 100) Temp, Xs, cm /gram °C grams/gram 7,

Table IV. Cation Form Na

Adsorbate H 0 0 N Neopentane o Isobutane Neopentane

a

25 -183 -196 25 -183 25 25

2

2

2

H*

V A*

3

2

0.165 0.172 0.138 0.059 0.201 0.049 0.054

0.165 0.151 0.171 0.096 0.176 0.089 0.088

V}

831 760 861 485 844 424 422

0.30 0.27 0.31 0.17 0.30 0.15 0.15

Na6.8Al7.4Si4c.eO9e, d = 1.80, V = 2794 A , η = 1.99 Χ 10 . Nao.29Al5.8Si42.2O9e, d = 1.71, V = 2794 A , η = 2.09 Χ 10 . Total F (calcd) = 1086 A / U C = 0.21 cm /gram, F (calcd) (large channel) = 554 A = 0.11 cm /gram (lft. a

20

3

0

8


3

0

6

3

20

p

3

p

Table V . Void Volume in Zeolite Omega* Temp, , 7 , Adsorbate °C grams/gram cm /gram Ff Xb

p

V ,A*

1

H 0 o N n-C Hi Neopentane 2

2

2

4

0

25 -183 -196 25 25

0.36 0.25 0.33 0.10 0.091

0.213 0.146 0.194 0.059 0.054

0.213 0.166 0.157 0.034 0.033

P

780 533 712 215 197

N H exchanged; N H and T M A removal thermally. Composition: Na .2Al .8S127.2O72. d = 1.69, F = 2168 A , η = 2.73 Χ 10 . Total F (calcd) = 679 A / U C = 0.19 cmVgram. F (large channel) = 335 A / U C = 0.091 cm /gram. a

4

3

2

3

0

20

p

Table VI.

3

Void Volume in Zeolite Ο (Offretite Type)»

Adsorbate

Temp, °C

χ, grams/gram

F, cm /gram

H 0 o N n-C Hio Neopentane

25 -183 -196 25 25

0.269 0.250 0.219 0.104 0.048

0.269 0.219 0.271 0.180 0.078

2

2

2

4

8

3

p

3

8

F

3

F ,A' P

£

0.43 0.35 0.43 0.17 0.076

498 406 503 333 145

N H exchanged; N H and T M A removed thermally. Composition: K0.99AI4Sii 0 . d = 1.59, F = 1164, η = 5.4 X 10 . Total F (calcd) = 452 A / U C including cancrinite cage = 0.24 cm /gram. F (calcd) (main channel plus 17-hedron) = 416 A / U C = 0.21 cm /gram. F (calcd) (main channel) = 244 A / U C = 0.13 cm /gram. 0

4

4

8e

8

20

0

3

3

3

3

P

p

3

p

3

c h a n n e l s y s t e m c o m p r i s e d of these u n i t s c a n be a v a i l a b l e o n l y a t t h e c r y s t a l surface, s i m i l a r t o zeolite L . Offretite Type. T h e s y n t h e t i c offretite-type zeolite, T M A - O , consists of a f r a m e w o r k s t r u c t u r e f o r m e d b y l i n k e d c a n c r i n i t e - t y p e u n i t s i n c o l u m n s a n d e n c l o s i n g a large C - a x i s c h a n n e l (18). T h e s e c o l u m n s are f u r t h e r j o i n e d b y g m e l i n i t e - t y p e u n i t s . T h e c a l c u l a t e d t o t a l v o i d space i n c l u d i n g t h e c a n c r i n i t e u n i t s i s 0.244 c m / g r a m . T h e m e a s u r e d a d s o r p t i o n pore v o l u m e s s h o w n i n T a b l e V I s h o w t h a t e v e n a h y d r o c a r b o n s u c h as n - b u t a n e 3


29.

BRECK AND GROSE

Introcrystalline Voids in Zeolites

325

occupies t h e g m e l i n i t e - t y p e 17-hedra. I t also i n d i c a t e s t h a t t h e s m a l l e r c a n c r i n i t e cages m a y be o c c u p i e d b y w a t e r . T h e m e a s u r e d pore v o l u m e s are consistent w i t h t h e offretite s t r u c t u r e . W a t e r appears t o o c c u p y t h e t o t a l v o i d v o l u m e (0.24 c m / g r a m ) , o x y g e n a n d n-C Hio appear t o o c c u p y t h e m a i n channels a n d g m e l i n i t e - t y p e cages (0.21 c m / g r a m ) , a n d neopentane c a n o c c u p y o n l y t h e m a i n C - a x i s c h a n n e l (0.13 c m / g r a m ) . N i t r o g e n is a n o m a l o u s (discussed b e l o w ) . 3

4

3

3

Discussion T h e measured pore v o l u m e s for v a r i o u s adsorbates are r e l a t e d i n Figure 1 to the calculated v o i d volume, 7 . N i t r o g e n a n d water give c o n s i s t e n t l y higher v a l u e s w h e n c o m p a r e d w i t h o x y g e n . A l t h o u g h t h e measured pore v o l u m e o c c u p i e d b y w a t e r is i n reasonable agreement w i t h t h e t o t a l v o i d s c a l c u l a t e d f r o m t h e s t r u c t u r e s , i t is n o t reasonable t o c o n clude t h a t n i t r o g e n c a n also o c c u p y these v o i d s (10). A t low tempera tures n i t r o g e n i s n o t a d s o r b e d b y zeolite A , a n d N molecules do n o t pass t h r o u g h the e i g h t - m e m b e r e d r i n g s a l t h o u g h o x y g e n molecules are r a p i d l y o c c l u d e d (8). B y c o m p a r i s o n , t h e s i x - m e m b e r e d r i n g of t h e s o d a l i t e u n i t or β cage has a free d i a m e t e r of 2.6 A . N i t r o g e n has a k i n e t i c d i a m e t e r of 3.64 A as c o m p a r e d w i t h 3.46 A for o x y g e n . Consequently, nitrogen cannot enter the s m a l l e r β cages a t l o w t e m p e r a t u r e s . P e r h a p s n i t r o g e n i n t e r a c t s w i t h cations i n the large cages of t h e s t r u c t u r e so as t o increase t h e d e n s i t y of t h e a d s o r b e d phase b y 2 0 % over t h e n o r m a l l i q u i d d e n s i t y . A l s o , S 0 w i t h a k i n e t i c d i a m e t e r of 3.6 A is n o t a d s o r b e d b y h y d r o x y sodalite a t r o o m t e m p e r a t u r e (23).


P

2

2

I f we assume t h a t n i t r o g e n does o c c u p y t h e v o i d v o l u m e accessible t o other gases s u c h as a r g o n o r o x y g e n , t h e n , based u p o n t h e v o i d v o l u m e of c a l c i u m A , s o d i u m X , a n d other zeolites, t h e average d e n s i t y of t h e a d sorbed n i t r o g e n is a b o u t 0.95 g r a m / c m as c o m p a r e d w i t h 0.80 g r a m / c m for l i q u i d n i t r o g e n a t i t s b o i l i n g p o i n t . T h i s v a l u e corresponds t o t h e d e n s i t y of l i q u i d n i t r o g e n at a t e m p e r a t u r e of 4 6 ° K . A t 4 6 ° K l i q u i d n i t r o g e n has a v a p o r pressure of 0.14 t o r r . T h u s , t h e a d s o r b e d n i t r o g e n b e haves as i f i t s b o i l i n g p o i n t were 31° b e l o w n o r m a l . 3

3

H a n d H e behave i n a v e r y a n o m a l o u s m a n n e r a t 4 . 2 ° K . T h e d a t a of D a u n t a n d R o s e n y i e l d values of χ o n zeolite X of 290 c m N T P / g r a m a n d 309 c m N T P / g r a m , r e s p e c t i v e l y , after c o r r e c t i o n for t h e presence of 2 0 % i n e r t b i n d e r (20). T h e s e v a l u e s of χ c o r r e s p o n d t o v o i d v o l u m e s of 0.42-0.44 c m / g r a m based o n n o r m a l l i q u i d h e l i u m . I t is n o t l i k e l y t h a t h e l i u m penetrates t h e sodalite cages for t h e reasons discussed a b o v e . F r o m the c a l c u l a t e d v o l u m e of t h e super-cage, one c a n e s t i m a t e the m e a n d e n s i t y of t h e a d s o r b e d h e l i u m . F o r H e t h i s is 0.17 g r a m / c m a n d for H e , 0.18 g r a m / c m . T h e c a l c u l a t e d m o l a r v o l u m e of t h e a d s o r b e d h e l i u m is 23 cm /mole. F o r c o m p a r i s o n t h e m o l a r v o l u m e of h e l i u m at 4 . 2 ° K is 32. 3

4

3

8

3

8

3

3

3

4

3

3


326

MOLECULAR SIEVES


fi

CT L

Z

"t

Zt

L,

O

O

t

A

X

A

t

X

t

V (calc) c m ' / g P

Figure 1. Relationship between the measured adsorption volumes, V (measd) and calculated void volume V of several zeolites. The dashed line corresponds to V (measd) = V (calcd). The symbols represent the zeolites as described in Tables I-VI: A,X,L,Z (mordenite Zeolon), omega (Ω), and offretite-type 0. Vertical shaded areas containing plotted values of V (measd) correspond to calculated values of V for the main pore systems. The narrow area, 0*, corresponds to the main c-axis void of zeolite 0. The value of V for Z = V for zeolite 0. Symbols with the subscnpt t (A , Z , etc.) represent values of V for the total void volume shown by narrow shaded areas. The neopentane (NP) volumes lie consistently below the dashed line thus show ing a packing effect. In all of these zeolites of varying structure, the H 0 and N volumes correspond with complete filling of the total voids even though this is not possible in the case of 2V in zeolites A, X, and L. p

p

p

p

p

p

p

t

t

p

p

t

2

2

2

U s i n g t h e D u b i n i n e q u a t i o n (21), C o i n t o t has d e t e r m i n e d t h e l i m i t i n g a d s o r p t i o n v o l u m e s , W , for zeolite A t o be 0.26 for w a t e r , 0.267 for a m m o n i a , a n d 0.282 c m / g r a m for S O 2 as c o m p a r e d w i t h a c a l c u l a t e d v a l u e of 0.290 (22). F o r zeolite X the c a l c u l a t e d v o i d v o l u m e was 0.31. These figures are l o w w h e n c o m p a r e d w i t h those g i v e n above. T h e zeolite Ρ e x h i b i t e d a d u a l b e h a v i o r of t w o l i m i t i n g a d s o r p t i o n v o l u m e s , 0.08 c m / g r a m for l o w pressures a n d 0.23 c m / g r a m near t h e s a t u r a t i o n pressure. 0

3

3

3

I t is of interest t o relate t h e v o i d v o l u m e as c a l c u l a t e d f r o m w a t e r a d s o r p t i o n v o l u m e t o t h e f r a m e w o r k d e n s i t y of t h e zeolites—i.e., t h e d e n s i t y of t h e zeolite s t r u c t u r e w i t h n o c o n s i d e r a t i o n of t h e n o n f r a m e w o r k a t o m s (cations a n d w a t e r ) . T h e f r a m e w o r k d e n s i t y m a y be expressed i n t e r m s of g r a m s p e r c u b i c centimeter or i n t e r m s of t h e n u m b e r of t e t r a h e d r a p e r u n i t v o l u m e of 1000 A . A p l o t of t h e f r a m e w o r k d e n s i t y for zeolites of k n o w n s t r u c t u r e vs. t h e v o i d f r a c t i o n , Vf, as d e t e r m i n e d f r o m w a t e r 3


29.

327

Intracrystalline Voids in Zeolites

BRECK AND GROSE

a d s o r p t i o n , is s h o w n i n F i g u r e 2. W h e n expressed as t e t r a h e d r a p e r 1000 A , the f r a m e w o r k d e n s i t y is r e l a t e d t o t h e d e n s i t y i n u n i t s of g r a m s per c u b i c centimeter b y a f a c t o r of n e a r l y 10—i.e., a f r a m e w o r k d e n s i t y of 20 corresponds t o a d e n s i t y of 2.0 g r a m s / c m . 3


3

F i g u r e 2 shows t h a t those zeolites w h i c h c o n t a i n t h e larger p o l y h e d r a l u n i t s a n d d o u b l e rings h a v e v o i d f r a c t i o n s greater t h a n 0.3 c o r r e s p o n d i n g t o f r a m e w o r k densities of less t h a n 17. T h e p a c k i n g of t h e d o u b l e - r i n g u n i t s i n c o n j u n c t i o n w i t h t h e larger p o l y h e d r a l u n i t s results i n l o w e r d e n s i t y zeolite f r a m e w o r k s . T h e m a x i m u m o b s e r v e d v o i d f r a c t i o n appears t o be 0.5. T h e a c t u a l f r a m e w o r k structures consist of densely p a c k e d o x y g e n t e t r a h e d r a w i t h a d e n s i t y the same as t h a t of the t e t r a h e d r a i n forms of s i l i c a s u c h as q u a r t z . T h e r e l a t i o n of v o i d f r a c t i o n t o f r a m e w o r k d e n s i t y corresponds t o a s t r a i g h t l i n e d r a w n between p o i n t s c o r r e s p o n d i n g t o 1 0 0 % v o i d s a n d 0 f r a m e w o r k d e n s i t y a n d c o r r e s p o n d i n g t o t h e d e n s i t y of n e p h e line at a v o i d f r a c t i o n of z e r o ; t h e e x p e r i m e n t a l g r o u p i n g is q u i t e g o o d .

A

0

\

Ο

C h a b a z i t e

ZK-5 Τ Ot5

Offretite

L Q Philliprite

0.3

Q

&

%

Sodalite Hydrate

Mordenite Ο

Vf

O

Meqglite

Natrolite Q

Bikitaite

V Analcime

10

12

I

1

14

16

I

18

Q

I

I

20

22

I

24

Nepheline

% —

26

d, Tetrahedra/1000 A 3

Figure 2. Relation between the measured void volume, expressed as the void fraction Vf, and the framework density, d. The dashed line connects the points corresponding to Vi = 1.0 at d = 0 and V = 0 atd = 26. The line is therefore expressed by 7 = d/26 + 1. The points corresponding to natrolite and analcime deviate because the water molecules in these structures are tightly bound to cations and framework atoms indicating a "feldspathoid" character. The point representing sodalite hydrate cor responds to no occluded NaOH which is normally present. Typically, synthetic sodalite hydrate, or basic sodalite, contains occluded NaOH, and the void fraction is accordingly much less, about 0.19 cm /cm . f

f

3

3


328

MOLECULAR SIEVES

The known zeolite structures suggest that the maximum observed void fraction is about 0.5; values of 0.6 have been postulated (13). Stabilityfactors may rule out the likelihood of the formation of such a zeolite. The framework density of a zeolite with a void fraction of 0.6 is 10 tetrahedra per 1000 A which corresponds to the density of normal water. Not con sidering cations, the hydrated density of such a structure would be 1.6 grams/cm . Hypothetical structures related to that of zeolite L have been proposed by Barrier and Villiger (13). In one series these structures pro vide for a channel parelleling the hexagonal c axis formed by 18-membered rings. The free diameter of this channel is about 16 A. These structures would provide for a void fraction of about 0.6. Based upon the relation ship shown in Figure 2 and using the hypothetical structure N N F as an example, the void fraction V{ for this structure would be about 0.55 cor responding to a framework density of 11.5 tetrahedra/1000 A . 3


3

3

Summary We have compared and calculated void volumes in zeolites of diverse structure. In general, good agreement between the observed and calcu lated void volumes is found. Exceptions are shown by nitrogen, water, and helium. Adsorbed nitrogen at low temperatures is denser than the liquid. Secondly, we have shown a simple relationship between the total measured void fraction of a dehydrated zeolite and the packing density of tetrahedra in the framework structure. This relationship is linear, giving a framework density of 25-26 tetrahedra per 1000 A corresponding to 0 void fraction. At the other extreme the maximum observed void volume appears to be 0.5 cm /cm which corresponds to a framework den sity of about 13. A zeolite containing a void space of more than 50% has not been discovered. Pore volumes determined from adsorption satu ration capacities of H 0 , N , 0 , and hydrocarbons are consistent with void volumes calculated from known structures of the zeolites A, X , mor denite (Zeolon), and Ο (offretite). Pore volumes measured on the zeolites L and omega are not totally consistent with the reported structures. 3

3

2

2

3

2

A cknowledgm ent The authors wish to thank Ε . M . Flanigen for her suggestions and comments and Union Carbide Corp. for permission to publish this paper. Literature Cited 1. 2. 3. 4.

Gurvitsch, L. G., Phys. Chem. Soc. Russ. (1915) 47, 805. Barrer, R. M., Lee, J. Α., Surface Sci. (1968) 12, 341. Sand, L. B., Mol.Sieves,Pap. Conf. (1967) 71 (1968). Flanigen, Ε. M., Netherlands Patent 6,710,729 (1968).


29.

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Intracrystalline Voids in Zeolitss


5. 6. 7. 8.

329

Aiello, R., Barrer, R. M., J. Chem. Soc. (A) (1970) 1470. Rubin, M. K., German Patent 1,806,154 (1969). Reed, T. B., Breck, D. W., J. Amer. Chem. Soc. (1956) 78, 5972. Breck, D. W., Eversole, W. G., Milton, R. M., Reed, T. B., Thomas, T. L., J. Amer. Chem. Soc. (1956) 78, 5963. 9. Loughlin, K. F., Ruthven, D. M., J. Phys. Chem. Solids (1971) 32, 2451. 10. Dubinin, M. M., Zhukovskaya, E . G., Murdma, K. O., Izv. Akad. Nauk SSSR, Ser. Khim. (1962), 760. 11. Barrer, R. M., Sutherland, J. W., Proc. Roy. Soc. (1956) 237A, 439. 12. Breck, D. W., Flanigen, E. M., Mol. Sieves, Pap. Conf. (1967) 47 (1968). 13. Barrer, R. M., Villiger, H., Z. Kristallogr. (1969) 128, 352. 14. Barrer, R. M., Peterson, D. L., Proc. Roy. Soc. (1964) 280A, 466. 15. Dubinin, M. M., Zhukovskaya, E. G., Lukyanovich, V. M., Murdma, K. O., Polystyanov, E. F., Senderov, Ε. E., Izv. Akad. Nauk SSSR, Ser. Khim. (1965) 1500. 16. Barrer, R. M., Villiger, H., J. Chem. Soc. D (1969) 659. 17. Flanigen, Ε. M., Khatami, H., Szymanski, Η. Α., ADVAN. CHEM. SER. (1971) 101, 201. 18. Gard, J. Α., Tait, J. M., Acta Crystallogr. Sect. B. (1972) 28, 825. 19. Dubinin, M. M., Zhukovskaya, E . G., Murdma, K. O., Izv. Akad. Nauk. Ser. Khim. (1962), 960. 20. Daunt, J. G., Rosen, C. Z., Low Temp. Phys. (1970) 3, 89. 21. Dubinin, M. M., J. Colloid Interface Sci. (1967) 23, 487. 22. Cointot, Α., Cruchaudet, J., Simonot-Grange, M., Bull. Soc. Chim. France (1970), 497. 23.

Barrer, R. M., Denny, A. F., J. Chem. Soc. (1964) 4684.

RECEIVED December 1, 1972.


A Correlation of the Calculated Intracrystalline Void Volumes and

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