Fluid Adsorption in Zeolites at High Pressures

P.A. Jacohs and R.A. van Santen (Editors), Zeolites: Facts, Figures, Firirtre 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

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FLUID ALjSORPTION El ZEOLITBS AT €IIGH I33SSUFGS

A.A. FOhKIN The I n s t i t u t e o f P h y s i c a l Chemistry, t h e USSR Academg of S c i e n c e s , 31, Leninsky p r o s p e k t , Moscow 117, U S S R

ABSTRAC 'I? A c onc l us io n on th e f a c t t h a t a d s o r b a t e i n z e o l i t e mic ro p o re s i s a s p e c i a l s t a t e of su b stan ce was drawn as a r e s u l t of t h e a n a l y s i s of e xp erimen tal a d s o r p t i o n o f g a s e s , vapours and liq u i d s of v a r i o u s n a t u r e on N a X z e o l i t e i n a broad p r e s s u r e range (from a b o u t 0.1 Pa t o ab o u t 100 MPa) and te mp e ra tu re range (from about 100 t o ab o u t 600K). Lin ea r c h a r a c t e r of i s o s t e r e s and maxima i n c u r v e s of p a r t i a l molar i s o s t e r i c h e a t c a p a c i t y o f ads or ba t e a t h i g h molecule c o n c e n t r a t i o n s i n c a v i t i e s of z e o l i t e s a r e l i k e l y t o a t t e s t t o t h e presence of second-order phase t r a n s i t i o n s i n th e adsorbed su b stan ce . INTRODUCTION A s ubs t a n ce adsorbed i n micropores of z e o l i t e s i s under c o n d i t i o n s which q u a l i t a t i v e l y d i f f e r from th o s e under which a s ubs t a nc e adsorbed on exposed s u r f a c e s e x i s t s ( r e f . 1 ) . One o f t h e main d i f f e r e n c e s r e s i d e s i n t h e f a c t t h a t a d s o r b a t e i n micr o p o r e s i s i n an a d s o r p t i o n f i e l d which i s c r e a t e d by t h e ads o r b e n t onl y, whereas a d s o r b a t e on the exposed s u r f a c e i s i n a f i e l d c r e a t e d by b o th th e a d s o r b e n t and a n e q u i l i b r i u m t h r e e -dimensional phase ( a d s o r b t i v e ) S p e c i a l c h a r a c t e r o f p r o p e r t i e s of adsorbed s u b s t a n c e s i n micropores m a n i f e s t themselves i n t h e c h a r a c t e r of iso th erm s and i s o s t e r e s of a d s o r p t i o n , i s o t h e r i c h e a t , p a r t i a l molar i s o s t e r i c h e a t c a p a c i t i e s of a d s o r b a t e and o t h e r c h a r a c t e r i s t i c s of a d s o r p t i o n systems i n c a r r y i n g o u t the a n a l y s i s i n broad p r e s s u r e and temperature ra n g e s .

.

kESULTS AND DISCUSSION

Isotherms of a b s o l u t e a d s o r p t i o n The a n a l y s i s of a d s o r p t i o n o f v a r i o u s g a s e s (Xe , K r , A r , CH4,CF3C1,C02) on I:& z e o l i t e ( r e f . 2) i n broad te mp e ra tu re r a n g e s (from ab o u t 100 t o ab o u t 600K) and p r e s s u r e r a n g e s (from about 0.1 Pa t o ab o u t 20 nPa) and a l s o u s i n g o t h e r exper i m e n t a l d a t a ( c f . , e.g ( r e f . 3 ) ) shows t h a t a d s o r p t i o n f o r

954

such sy&ems under i s o t h e r m a l c o n d i t i o n s i s a monotonously in c r e a f i n g f u n c t i o n of p r e s s u r e . Fig.1 g i v e s as an example i s o tKermS of CH4 a d s o r p t i o n on NaX z e o l i t e . C r y s t a l l i n e N a X zeo l i t e of t h e f o l l o w i n g composion: 0.98 Na20*A1203*2.36Si02* 0.02H20 was used f o r th e t e s t s ; c r y s t a l l a t t i c e p a ra me te r was 2.496 nm. I r r e s p e c t i v e of th e s t a t e o f a d s o r b a t e ( g a s , vapour or l i q u i d ) , chemical p o t e n t i a l s o f a d s o r p t i v e and a d s o r b a t e i n micropores a r e i d e n t i c a l . This means that i s o t h e r m of a d s o r p t i o n should be one and t h e same n o t o n l y w i t h g a s a d s o r p t i o n , b u t a l s o w i t h a b s o r p t i o n o r vapour and a d s o r p t i o n of l i q u i d comp r e s s e d by a h i g h h y d r o s t a t i c p r e s s u r e . mis assumption w a s t e s t e d i n two a d s o r p t i o n systems: H20/NaX z e o l i t e and C6%/NaX z e o l i t e ( r e f .4). Fig. 2 shows i s o t h e r m s o f a d s o r p t i o n o f t h e s e s u b s t a n c e s as r e l a t i o n s h i p , o b t a i n e d by s u p e r p o s i t i o n o f d a t a on a d s o r p t i o n of vapour, l i q u i d under p r e s s u r e of i t s s a t u r a t e d vapour, and l i q u i d under h i g h h y d r o s t a t i c p r e s s u r e s (up t o 100 Milpa). m e chemical p o t e n t i a l o f l i q u i d compressed under a n was c l a c u l a t e d by t h e formula: hydrostatic pressure J Q

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wherein; I / - pressu re of s a t u r a t e d l i q u i d vapour a t a gauge h y d r o s t a t i c p r e s s u r e e q u a l t o s e r o ; u ( p ) - s p e c i f i c volume o f compressed l i q u i d . The v a l u e o f chemical potentialb(P') w a s s ta ke n as a c o n v e n t i o n a l zero datum p o i n t a t a g i v e n te mp e r a tu r e . It f o l l o w s from (1) t h a t p r e s s u r e o f s a t u r a t e d vapour of compressed l i q u i d can be c a l c u l a t e d a s f u n c t i o n o f hgdros t a t i c p r e s s u r e as fo llo ws:

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(2) As v a l u e s o f p: were n o t h i g h , formulae (I) and (2) d i d n o t call for transition to volatilities. The absence of s t e p s i n i s o t h e r m s of a d s o r p t i o n i n l i k e l y t o a t t e s t t o th e absence o f f i r s t - o r d e r phase t r a n s i t i o n s i n microporous ad so rb en ts. High d i s p e r s i t y of adsorbed m a t t e r , when maximum s e v e r a l s c o r e s of molecules c a n be p r e s e n t i n e a c h c a v i t y of z e o l i t e , i s l i k e l y t o hamper f o r m a t i o n o f germs o f a new phase.

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I s o s t e r e s of a d s o r p t i o n L i ne a r c h a r a c t e r o f i s o s t e r e s o f a d s o r p t i o n was f r e q u e n t l y r e p o r t e d i n l i t e r a t u r e ( r e f s . 5 , 6 ) . 'Phe a n a l y s i s of c h a r a c t e r of i s o s t e r e s of a d s o r p t i o n f o r the abovementioned systems showed t h a t i s o s t e r e s a r e l i n e a r , w i t h f a i r a p p r o x ima tio n , w i t h i n a broad temperature and p r e s s u r e range ( r e f . 2 ) . Fig.3 g i v e s as a n example i s o s t e r e s of a d s o r p t i o n o f Xe on NaX z e o l i t e . It w i l l be a p p a r e n t from t h i s Fig u re t h a t i s o s t e r e s remain l i n e a r o v e r t h e whole p r e s s u r e and temp eratu re ra n g e a n d , which i s e s p e c i a l l y i m p o r t a n t , i s o s t e r e s i n t e r r u p t e d a t the s a t u r a t e d vapour l i n e would r e a p p e a r w i t h th e l i n e a r c h a r a c t e r i n t h e s u p e rc r i t i c a l a r e a . Dotted l i n e s i n Pig.3 show t h e e x p e c te d c h a r a c t e r o f i s o s t e r e s i n the a r e a i n which th e e q u i l i b r i u m phase i s i n the form of compressed l i q u i d . "he p o s s i b i l i t y of i s o s t e r e s having t h i s c h a r a c t e r i s confirmed f o r water/NaX z e o l i t e and benzene/NaX z e o l i t e systems ( r e f .4). Fig.4 shows i s o s t e r e s o f a d s o r p t i o n f o r t h e same systems. I t w i l l be a p p a r e n t from t h i s Figure t h a t i s o s t e r e s of a d s o r p t i o n b e g i n n i n g i n t h e vapour a r e a c ont i nue i n t h e a r e a of compressed l i q u i d s t a t e i n th e l i n e a r form. L i n e a r i t y o f i s o s t e r e s of a d s o r p t i o n i n t h e a r e a where g a s e s l a r g e l y d e v i a t e from th e i d e a l s t a t e and t h e i r re a p p e a rance i n t h e l i n e a r form i n t h e a r e a o f l i q u i d s t a t e of adsorpt i v e and i n t h e s u p e r c r i t i c a l a r e a a l l t h i s a t t e s t s t o th e f a c t t h a t a d s o r b a t e i n a microporous a d s o r b e n t i s a s p e c i a l s t a t e o f s ubs tan ce. This assumption i s a l s o confirmed by i n v e s t i g a t i o n s i n t o a d s o r p t i o n of v a r i o u s l i q u i d s o n NaX z e o l i t e a l o n g s a t u r a t e d vapour p r e s s u r e l i n e ( r e f .7). The main r e s u l t s o f t h e s e i n v e s t i g a t i o n s a r e as fo llo ws: average d e n s i t y o f adsorbed hydrocarbons and i n e r t g a s e s i n th e a r e a of low t e m p e r a t u r e s i s lo wer, and n e a r t o c r i t i c a l temp e r a t u r e i s h i g h e r th an d e n s i t y o f e q u i l i b r i u m l i q u i d phase. Temperature a t which th ey a r e i d e n t i c a l i s a b o u t 0.8 J' ? cr ; average d e n s i t i e s of adsorbed p o l a r s u b s t a n c e s (H20 c H ' 6 69 C,E50H) over t h e whole temp eratu re rangc 273 473IC a r e h i g h e r th a n l i q u i d d e n s i t y and v a r y much slovver w i t h t e m p e r a t u r e , It i s remarkable t o n o t e t h a t d e n s i t y curve f o r adsorbed vvater d o e s n o t have a m a x i m u m a t 277.15K i n h e r e n t i n normal - w a t e r ( r e f . 8 ) (Big.5).

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C e r t a i n p r o p e r t i e s of adsorption systems a t high p r e s s u r e s of p r o p e r t i e s of substances a v a i l a b l e i n micropores can be obtained from the analysi6 of the c h a r a c t e r of thermodynamic funct i o n s of an adsorption system depending: on parameters of the adsorption equilibrium

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I s o s t e r i c h e a t of adsorption If a two-component two-phase system i s i n thermodynamic equilibrium, the following equation can be w r i t t e n ( r e f . 9 ) :

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wherein Sg=Ho/T,Sa=Qa/T- molar entropy of equilibrium phase and p a r t i a l molar entropy of adsorbate, r e s p e c t i v e l y , ( Hg, Ha a r e molar enthalpy of equilibrium phase and p a r t i a l molar enthalpy of adsorbate ) By d e f i n i t i o n , the d i f f e r e n c e between molar enthalpy o f equilibrium phase and p a r t i a l molar enthalpy of adsorbate i s r e f e r r e d t o as i s o s t e r i c h e a t of adsorption

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If the adsorbent i s i n e r t , i.e.

i t does n o t undergo deformation during adsorption,q=dV/dU= o( and n, a r e adsorbent volume and i t s amount, r e s p e c t i v e l y , = pvg/RT ). A t l o w pre ssur e s 2 = 1, and

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A t high pressures a l l these assumptions a r e n o t t r u e , and c a l c u l a t i o n should be c a r r i e d o u t using formula (5). F i l l i n g of the adsorption space of micropores i n c r e a s e s w i t h pressure increase under isothermal conditions. It i s nat u r a l t o r e p r e s e n t the system microporous adsorbent/adsorbate a s a s o l u t i o n a t i n i t i a l s t a g e s of a d s o r p t i o n ( r e f .I). Viith high concentration o f molecules i n c a v i t i e s of z e o l i t e , adsorbed substance forms a s t r o n g l y dispersed phase. C l o t s o f

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F i g . 7 . P a r t i a l molar i s o s t e r i c h e a t c a p a c i t y o f Xe adsorbed on N ~ Xz e o l i t e v. a, m o l e / g : 1-1.0; 2-2.0; 3-4.0; 4-4.5; 5-4.7;

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adsorbed molecules a r e l o c a t e d i n c a v i t i e s i n accordance w i t h z e o l i t e s t r u c t u r e . It may be assumed t h a t , i n t h e end of th e d a y , a b s o r b a t e h a s a c e r t a i n average d e n s i t y and average molar volume which, s i m i l a r l y t o the case of phase f o r m a t i o n , i s e q u a l t o t h e p a r t i a l molar volume o f a d s o r b a t e . Assessments of a d s o r p t i o n d efo rm a tio n show (ref .lo) t h a t w i t h low a d s o r p t i o n ( a d s o r p t i o n s o l u t i o n ) t h e f i r s t t i e r m i n (5) i s c l o s e t o u n i t y . On t h e o t h e r hand,pa<
- .

P a r t i a l molar i s o s t e r i c heat c a p a c i t y of a d s o r b a t e By making use o f Kirch h o ff I s fo r mu la

we have c a l c u l a t e d p a r t i a l molar i s o s t e r i c h e a t c a p a c i t y of a d s o r b a t e ~ ~ = ( a ~ ~ / Id tT sh ~ .o u ld be n o t e d t h a t Y and H, 9 were o b t a i n e d as s t a n d a r d r e f e r e n c e d a t a , Pig.7 shows r e l a t i o n s h i p s of i s o s t e r i c h e a t c a p a c i t y f o r lie adsorbed i n N a X z e o l i t e v. temperature. I t can be s e e n t h a t a d s o r b a t e h e a t c a p a c i t y w i t h low f i l l i n g r a t i o s i s c l o s e t o t h e i s o b a r i c h e a t c a p a c i t y o f e q u i l i b r i u m gas. R i t h a growth o f f i l l i n g r a t i o i n th e high-temperature a r e a , a maximum a p p e a r s which becomes more and more pronounced. I s o s t e r i c h e a t c a p a c i t y o f a d s o r b a t e i n c r e a s e s w i t h t he h i g h e s t f i l l i n g r a t i o s t o be i n t e r r u p t e d a t t h e satur a t e d vapour l i n e and t o r e a p p e a r i n t h e a r e a o f s u p e r c r i t i c a l tem pe r a t ur e s .

96 I

T h i s r e s u l t can be ex p lain ed i f we assume that mo le c u la r a s s o c i a t e s form i n ad so rb ate w i t h high enough c o n c e n t r a t i o n s o f molecules i n micropores. Th eir presence i n vapours can be det e c t e d , e.g. by a s u b s t a n t i a l i n c r e a s e i n h e a t c a p a c i t y , and i n g a s e s , a t s u p e r c r i t i c a l temp eratu res, by a t r a n s i t i o n o f h e a t c a p a c i t y through i t s maximum. Adsorbate a v a i l a b l e i n p o r e s o f z e o l i t e i s l i k e l y t o behave i n the similar manner: a s s o c i a t s f a i l t o form w i t h low € i l l i n g r a t i o s and a t low p r e s s u r e s , and h e a t c a p a c i t y of ad so rb ate i s c l o s e t o t h a t of e q u i l i b r i u m gas. D e vi a t i ons occur (Sig.7) at-3 mmole/g which c o r r e s p o n d s t o about 4-5 molecules l o c a t e d i n each c a v i t y of' N a X z e o l i t e . Mol e c u l a r a s s o c i a t e s a r e l i k e l y t o appear even under t h e s e condit i o n s . ;,!mima of i s o s t e r i c h e a t c a p a c i t y i n c r e a s i n g w i t h a n in c r e a s e i n a d s o r p t i o n and s h i f t e d i n t o th e low-temperature a~-r e a appear w i t h h i g h e r f i l l i n g r a t i o s of z e o l i t e c a v i t i e s w i t h adsorbed molecules. Yhe f o l l o w i n g co n clu sio n s may be drawn based on t h e i n v e s t i g a t i o n s d i s c u s s e d above: 1. L i n e a r i t y of i s o s t e r e s o f a d s o r p t i o n , which f a i r l y p e r s i s t s i r r e s p e c t i v e of s t a t e of e q u i l i b r i u m phase, absence o f s t e p s i n i s o s t e r e s of a d s o r p t i o n upon t r a n s i t i o n i n t o t h e l i q u i d s t a t e a r e a , s u b s t a n t i a l d i f f e r e n c e s i n t h e c h a r a c t e r of r e l a t i o n s h i p s of ad so rb ate and a d s o r p t i v e d e n s i t i e s on th e saturat i o n l i n e V . temperature a t t e s t t o th e f a c t t h a t a d s o r b a t e i n micropores of' z e o l i t e i s a s p e c i a l s t a t e of s u b s ta n c e . 2. L i n e a r i t y of i s o s t e r e s and maxima of c u r v e s o f i s o s t e r i c h e a t c a p a c i t y o f a d s o r b a t e a t h i g h c o n c e n t r a t i o n o f mol e c u l e s i n z e o l i t e c a v i t i e s a r e l i k e l y t o a t t e s t t o th e presence o f second-order phase t r a n s i t i o n s i n adsorbed s u b s t a n c e .

IIEF&FiENC YS 1 B:P. Bering, A.L. I I a j e r s , V.V. S e r p i n s k y , Dokl.Akad.Nauk S ~ S K ,1970, V O ~ . 193, pp. 119-122. 2 A . A . Fomkin, In: Ad so rb cija i ad so r b e n ty , Nauka, Moskva, 1987, ppe 10-17. 3 D. Brek, Ceo lito v y e molekuljarnye s i t a , M i r , Moskva, 1976, 778 P. 4 A.A. Pomkin, V.A. Avramenko, 1.1. S e l i v e r s t o v a , V.V. Serpi ns ky, Uokl. Akad. Nauk SSSR, 1986, vo1.288, pp.678-681. 5 P.M. B a r r e r , H. Papadopoulos, P r o s . R. SOC., London, 1972, 6

A3269 PP- 315-3309 K.11. Avgul, A.V. K i s e l e v , D.P. POskus, Ad s o rb c ija gazov i parov na odnorodnykh p o v erk h n o stja h , Khimija, Moskva, 1975, 984 p .

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7 1.1. S e l i v e r s t o v a , A.A. Fomkin, V.V.

Serpinsky, 1zv.Akad. Nauk SSSR, s e r . K h i m . , 1986, pp. 1231-1236; 1.1. S e l i v e r s t o v a , A.A. Fomkin, V.V. Serpinsky, M.M. Dubinin, Izv. Akad. Nauk SSSR, s e r Xhim., 1983, pp. 493-498. 8 B.Kh. Rakhmukov, 1.1. S e l i v e r s t o v a , V.V. Serpinsky, A.A. Fomkin, Izv. Akad. Nauk SSSR, s e r . K h i m . , 1979, pp. 2419-2422. 9 D.M. Young, A.D. Crowell, Physical Adsorption of Gases, Butterworths, London, 1962, 426 p. 10 O.K. Krasilnikova, B.P. Bering, V.V. Serpinsky, Id.K.Dubinin, Izv. Akad. Nauk SSSR, s e r . Khim., 1977, pp. 1194-1196.

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