on Molecular Sieves

33 Heats of Adsorption of CO and SO on Molecular Sieves 2

Downloaded by UNIV OF MASSACHUSETTS AMHERST on February 6, 2017 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0121.ch033

2

ANDREW A. HUANG and IMRE ZWIEBEL Worcester Polytechnic Institute, Worcester, Mass. 01609

Calorimetric (isothermal) heats of adsorption were measured tween 0°C and 100°C for CO on 5A, Na-mordenite, H-mordenite, and Na-X sieves, and for SO on Na-mordenite and H-mordenite using a Bebbe-type instrument The CO heat curves were horizontal on 5A sieve and were of the type obtained on heterogeneous surfaces on the mordenites and the Na-X sieve. Neithe the isotherm nor the heat of adsorption data could be correlated with uniform surface models. Using Halsey and Taylor's site distribution model, Hill's isotherm for mobile interacting adsorbates was extended to heterogeneous surfaces. The resultin equilibrium expression correlated the adsorption data over an e tended region, and its differentiated form for the isosteric heat predicted the measured calorimetric data. The isotherms and the calculated heats for the SO -mordenite systems were anom alous. 2

2

2

2

T n p r e s e n t i n g t h e a d s o r p t i v e properties of m o l e c u l a r sieve zeolites, m o s t a u t h o r s (1, 2) r e p o r t isosteric heats. T h e s e are o b t a i n e d f r o m t h e a p p l i c a t i o n of t h e t h e r m o d y n a m i c a l l y d e r i v e d C l a u s i u s - C l a p e y r o n t y p e equation to experimentally measured equilibrium data. A t a constant A

* - -'timl a d s o r b e n t l o a d i n g t h e e q u i l i b r i u m pressure is p l o t t e d as a f u n c t i o n of t h e inverse t e m p e r a t u r e o n s e m i l o g a r i t h m i c coordinates, a n d the slopes of the s t r a i g h t - l i n e isosteres y i e l d t h e isosteric heats. A l t e r n a t i v e l y , a n a p p r o p r i a t e i s o t h e r m expression m a y be s u b s t i t u t e d i n E q u a t i o n 1 t o o b t a i n a n e x p l i c i t r e l a t i o n s h i p b e t w e e n the isosteric heat of a d s o r p t i o n a n d t h e a d s o r b e n t l o a d i n g (8). T h e v a l i d i t y of these c a l c u l a t e d heats is based u p o n t h e a s s u m p t i o n s t h a t t h e differential a d s o r p t i o n 374 Meier and Uytterhoeven; Molecular Sieves Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

33.

HUANG AND

zwiEBEL

375

Heats of Adsorption

process is b o t h i s o t h e r m a l a n d isobaric (4), t h a t t h e gas phase obeys t h e i d e a l gas l a w , t h a t t h e adsorbent is " i n e r t , " t h a t t h e v o l u m e of t h e a d sorbed phase is negligible c o m p a r e d w i t h t h e v o l u m e of t h e gas phase, a n d t h a t t h e i s o t h e r m e q u a t i o n a d e q u a t e l y describes the a d s o r p t i o n m e c h a n i s m . U p t o n o w few i n v e s t i g a t o r s h a v e r e p o r t e d c a l o r i m e t r i c a l l y m e a s u r e d heats of a d s o r p t i o n d a t a for m o l e c u l a r sieves {6-8). T h i s paper s u m m a rizes t h e results of C 0 a n d S 0 a d s o r p t i o n measurements o n s e v e r a l m o l e c u l a r sieve zeolites u s i n g a m o d i f i e d B e e b e (9) c a l o r i m e t e r o p e r a t e d i n a n i s o t h e r m a l m o d e (10). A p p r o x i m a t e l y 4 g r a m s of p o w d e r e d c o m m e r c i a l sieve samples, p e l l e t i z e d , t o p r e v e n t clogging of a r e t a i n i n g screen, were charged t o t h e c a l o r i m e t e r a n d degassed at 360° C a n d near zero pressure ( < 1 0 t o r r ) for 20 h o u r s . P r e d e t e r m i n e d a n d m e a s u r e d doses of t h e gas were c h a r g e d t o t h e i n s t r u m e n t ; each s a m p l e was sufficiently s m a l l t h a t t h e t e m p e r a t u r e of t h e s y s t e m n e v e r increased b y m o r e t h a n 0 . 2 ° C . The e s t i m a t e d p r e c i s i o n of t h e m e a s u r e d heats of a d s o r p t i o n was =fc 6 % .


2

2

- 5

Results and Analysis T h e C 0 c a l o r i m e t r i c heats a n d t h e e x p e r i m e n t a l isosteric heats o b t a i n e d f r o m E q u a t i o n 1 c o m p a r e f a v o r a b l y for t h e 5 A sieve ( F i g u r e 1), a n d t h e y agree m o d e r a t e l y w e l l for t h e H - m o r d e n i t e ( F i g u r e 2). F o r t h e N a X s y s t e m ( F i g u r e 3), however, t h e c o m p a r i s o n is a t best q u a l i t a t i v e . S i m i l a r results were o b t a i n e d a t o t h e r t e m p e r a t u r e s i n t h e 0 ° C - 1 0 0 ° C r e g i o n . 2

20

ο

Figure 1.

0.2

0.6 0.4 0.8 Fractional Coverage θ

1.0

Heat of adsorption on δA sieve

Meier and Uytterhoeven; Molecular Sieves Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

MOLECULAR SIEVES

15

Δ

^

Δ ο Calorimetric

I

Ο


Ε Experimental

σ ο

Isoeteric

J^-x^v**. χ

,

— « Predicted (Eq-4) χ

BETLangmuir

0.4 0.6 0.8 Fractional Coverage θ

0.2

Figure 2.

1.0

Heat of adsorption on H-mordenite

20

Δ o Calori metric χ Ε xperimental Isoeteric

0.2

0.4

0.6

0.8

i.o

Fractional Coverage θ Figure 3.

Heat of adsorption on NaX sieve


33.

377

Heats of Adsorption

zwiEBEL

HUANG AND

T h e p r e d i c t e d isosteric heats, b a s e d o n t h e L a n g m u i r a n d t h e B E T equations ( w i t h the coefficients o b t a i n e d b y fitting t h e l i n e a r i z e d forms of these equations t o t h e e x p e r i m e n t a l data) i n c o n j u n c t i o n w i t h E q u a t i o n 1, d e p a r t s i g n i f i c a n t l y f r o m c a l o r i m e t r i c d a t a o v e r a w i d e range of adsorbent l o a d i n g s . T h i s is t r u e even i n t h e regions w h e r e these e q u i l i b r i u m e q u a t i o n s correlate t h e a d s o r p t i o n d a t a q u i t e w e l l , i.e., Θ ^ 0.6 ( w i t h n d e t e r m i n e d f r o m t h e L a n g m u i r p l o t s a t t h e i s o t h e r m t e m p e r a t u r e i n ques t i o n ) . T h e observed d e v i a t i o n s m a y be a t t r i b u t e d t o t h e l i m i t a t i o n s associated w i t h these a d s o r p t i o n m o d e l s . N e g l i g i b l e a d s o r b a t e - a d s o r b a t e i n t e r a c t i o n s , i m m o b i l e a d s o r p t i o n , a n d adsorbent h o m o g e n e i t y are t h e most l i k e l y a s s u m p t i o n s t h a t are n o t a p p l i c a b l e t o systems a t h a n d .


m

U s i n g t h e e q u i l i b r i u m e q u a t i o n developed b y H i l l (11) a n d a d a p t e d b y K i s e l e v (12) Wo

=

θ 1—0

h In

θ

In φ = In Κ

1 - θ

+

λ

Κφ

(2)

w h i c h a c c o u n t s for t h e i n t e r a c t i o n s a n d t h e adsorbate m o b i l i t y , a m o r e s a t i s f a c t o r y p r e d i c t i o n w o u l d be expected. H o w e v e r , t h i s was n o t r e a l i z e d . W h e n t h e e x p e r i m e n t a l d a t a are p l o t t e d o n t h e a p p r o p r i a t e co o r d i n a t e s , curves w i t h m i n i m a are o b t a i n e d i n s t e a d of s t r a i g h t lines w i t h p o s i t i v e slope (see F i g u r e 4). T h e d r a m a t i c d e v i a t i o n s f r o m t h e o r y , es p e c i a l l y a t t h e l o w adsorbent l o a d i n g regions, suggest t h a t a d s o r b e n t heterogeneity p l a y s a m o s t i m p o r t a n t r o l e i n t h e a d s o r p t i o n o n sieves. T h i s w o u l d b e expected since t h e electrostatic fields associated w i t h t h e ionic species w i t h i n t h e c r y s t a l l i n e m a t r i x n a t u r a l l y c o n t r i b u t e t o adsorbent nonuniformities. H a l s e y a n d T a y l o r (13) a d a p t e d t h e L a n g m u i r m o d e l t o a d s o r p t i o n o n heterogeneous surfaces b y a s s u m i n g a n e x p o n e n t i a l site energy d i s t r i b u t i o n a n d i n t e g r a t i n g over a n i n f i n i t e c o n t i n u u m of p o s i t i v e energies. F o l l o w i n g s i m i l a r procedures, H u a n g (10) m o d i f i e d H i l l ' s e q u a t i o n t o heterogeneous surface a d s o r p t i o n . T h e r e s u l t i n g i s o t h e r m e q u a t i o n is Wn

= In K

x

θ

where

Wu =

+

(3)

Κφ +

x

+ In —

In φ

(3a)

S u b s t i t u t i n g E q u a t i o n 3 i n t o E q u a t i o n 1 t h e c o r r e s p o n d i n g expression for t h e isosteric h e a t becomes

*-{

1 +

ϊ [ ! Μ Ι - · · [^].· +

+

T h e v a l u e of e , t h e average a d s o r p t i v e energy f r o m the adsorbent h e t e r o m


MOLECULAR SIEVES

378

5-A Sieve H-Mordenite

ο Δ

-0 C

10

e

,ΐβ-ο-ΐβ-β-τ

0 .


ΙΑ

\

70 g/ e

^ Δ Δ^.

"θ

Α

7

°

β

.1

(

I

^Δ'

)

.2

.3

.4

Fractional Coverage

.5

.6

.7

θ

Figure 4. Correlation of C0 isotherms according to Η ill's model (Equation 2) 2

geneity, m a y b e e v a l u a t e d f r o m t h e e q u i l i b r i u m d a t a p l o t t e d o n l o g a r i t h m i c coordinates a n d t h e a p p l i c a t i o n of n o n l i n e a r least-squares c u r v e - f i t t i n g techniques t o t h e slope of t h e c u r v e . T h e constants Ki a n d K are e v a l u a t e d f r o m t h e s t r a i g h t - l i n e p l o t s of Wn vs. 0, as i n d i c a t e d b y E q u a t i o n 3 a n d s h o w n i n F i g u r e 5. T h e r e s u l t i n g constants K a n d e v a r y exponen t i a l l y w i t h t h e inverse t e m p e r a t u r e w h i l e K has a l i n e a r inverse t e m p e r a t u r e dependence. U s i n g t h e a p p r o p r i a t e v a l u e s o f t h e coefficients i n E q u a t i o n 4 values of t h e heats of a d s o r p t i o n c a n b e p r e d i c t e d . T h e r e s u l t i n g agree m e n t w i t h t h e e x p e r i m e n t a l results (see F i g u r e s 1-3) shows significant i m p r o v e m e n t o v e r t h e s i m p l e homogeneous m o d e l cases. 2

x

m

2

Discussion T h e heat of a d s o r p t i o n d a t a of t h e C 0 - 5 A sieve s y s t e m e x h i b i t h o m o geneous surface properties as s h o w n b y t h e r e l a t i v e l y i n v a r i a n t heat o f a d s o r p t i o n . T h e agreement between t h e c a l o r i m e t r i c heats a n d those p r e d i c t e d b y E q u a t i o n 1 are r e a s o n a b l y good, a n d t h e a p p l i c a t i o n of E q u a t i o n 4 provides little improvement. O n the other h a n d , t h e C 0 - N a X sieve s y s t e m a n d t h e C 0 - H - m o r d e n i t e s y s t e m s h o w significant h e t e r o geneous c h a r a c t e r . I n these cases t h e m o d e l described b y E q u a t i o n s 3 a n d 4 is necessary t o o b t a i n acceptable p r e d i c t i o n s . 2

2

2


33.

379

Heats of Adsorption

HUANG AND ZWIEBEL

T h e N a X a n d m o r d e n i t e results agree q u i t e w e l l w i t h p r e v i o u s l y p u b l i s h e d C 0 a d s o r p t i o n d a t a (1). T h e 5 A sieve results, h o w e v e r , a r e q u i t e different b o t h i n f o r m a n d i n m a g n i t u d e (1); these c a n best be ex p l a i n e d b y differences i n t h e q u a l i t y a n d c h a r a c t e r of t h e adsorbents w h i c h is s u p p o r t e d b y i n d e p e n d e n t l y measured i s o t h e r m d a t a . E q u a t i o n 4 m a y be v i e w e d as a t h r e e - c o n s t a n t extension of t h e s i m p l e r equations, a n d t h e r e s u l t i n g i m p r o v e m e n t i n agreement between t h e p r e d i c t i o n s a n d t h e e x p e r i m e n t a l v a l u e s m a y be a t t r i b u t e d t o t h e i n c l u s i o n of e x t r a a r b i t r a r y constants. I f so, s i m i l a r models, s u c h as t h e K i s e l e v e q u a t i o n , w h i c h has t h e same n u m b e r of constants, w o u l d be expected t o p r o v i d e t h e desired p r e d i c t i o n s . H o w e v e r , n e i t h e r t h i s n o r other models d i d correlate t h e e x p e r i m e n t a l d a t a . T h e r e f o r e , u s i n g models t h a t i n c l u d e t e r m s t h a t describe t h e s t i p u l a t e d p r e v a i l i n g p h e n o m e n a (heterogeneity, m o b i l i t y , adsorbate i n t e r a c t i o n s ) p r o v i d e s a m o r e r e a l i s t i c m o d e l of t h e a c t u a l m e c h a n i s m a n d t h u s enables more a c c u r a t e p r e d i c t i o n s . T h e a p p l i c a t i o n of e x p e r i m e n t a l d a t a t o t h e proposed t e c h n i q u e requires o n l y s l i g h t increase i n effort, w h i c h is negligible w h e n c o m p u t e r s are e m p l o y e d , despite t h e fact t h a t t h e equations are m o r e c o m p l e x t h a n t h e expressions d e r i v e d f r o m t h e s i m p l e models.


2

N o n p h y s i c a l a d s o r p t i v e effects m a y s i g n i f i c a n t l y c o m p l i c a t e t h e s y s t e m b e h a v i o r . T h e a d s o r p t i o n of S 0 o n m o r d e n i t e ( b o t h H a n d N a forms) 2

12 ο 5-A Sieve Δ , H-Mordenîte 1

0

.1

.2

.3

4

Fractional Coverage Figure 5.

.5

.6

.7

θ

Correlation of C0 isotherms according to the heterogeneous model (Equation 3) 2


380

MOLECULAR SIEVES

35

ι

Ν α - Mordenit e Η - Mordenit •

Δ

30

I

ο Ε \ 25

σ


Ο c ο

*20

\

Q.

Ι

1 5

ο ο Χ

ν

Calorime ter

χ

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Δ*

01

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on Molecular Sieves

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