Perspectives in Molecular Sieve Science - American Chemical Society

C h a p t e r 27

Temperature-Programmed Desorption of Hydrocarbons from ZSM-5, ZSM-11, and Theta-1

Downloaded by UNIV OF NORTH CAROLINA on July 15, 2016 | http://pubs.acs.org Publication Date: May 27, 1988 | doi: 10.1021/bk-1988-0368.ch027

n-Hexane from Different Cation Forms 1

Chen Li-feng and Lovat V. C. Rees Physical Chemistry Laboratories, Imperial College of Science and Technology, London SW7 2 A Y , England

The temperature programmed desorption (t.p.d) of n-hexane from the sodium and hydrogen forms of ZSM-5, ZSM-11 and THETA-1 have been studied. The t.p.d profiles have been analysed by a newly developed method. From these analyses peak temperatures, peak widths, maximum rates of desorption and activation energies of desorption as a function of coverage have been obtained. The saturation capacities of these high silica zeolites for n-hexane have also been determined. The effect of change of cation on all of these quantities is demonstrated. In P a r t 1 ( 1 ) t h e t e m p e r a t u r e programmed d e s o r p t i o n ( t p d ) of n-hexane and η - o c t a n e from the (Na, H) forms f o r ZSM-5, ZSM-11 and T h e t a - 1 was r e p o r t e d . In P a r t 1 1 ( 2 ) t h e tpd o f p - x y l e n e and benzene from the same z e o l i t e samples was p r e s e n t e d . In t h e l a t t e r study the d e s o r p t i o n of p - x y l e n e from ZSM-5 and ZSM-11 z e o l i t e s o f v a r y i n g S i / A l r a t i o was a l s o i n c l u d e d . In t h e p r e s e n t study t h e e f f e c t o f d i f f e r e n t c a t i o n s i n t h e c h a n n e l s and i n t e r s e c t i o n s o f ZSM-5, ZSM-11 and T h e t a - 1 on t h e d e s o r p t i o n o f n-hexane i s r e p o r t e d .

Experimental The (NaH)- T h e t a - 1 used p r e v i o u s l y ( J ^ 2 ) c o m p o s i t i o n o f HQ 4 5 Ν « 2 4 β A l 2 g i S i g 3

Q

with a unit c e l l ( u . c ) g 0 1 9 2 was c o n v e r t e d by

i o n exchange i n t o i t s sodium and hydrogen f o r m s . The u . c . c o m p o s i t i o n s o f t h e s e f o r m s w e r e N a 0 n 4 A l 0 ft4 S i „ „ ftft 0 and 2.91 2.91 93.09 192 4 n n

N a

0.09

H

2.82

A l

2.91

S i

93.09

°192

r e s

P

e c t i v e l

V ·

T

n

e

P

u r e

sodium

forms

NOTE: This chapter is part 3 in a series. Current address: Nanchang Institute of Aeronautical Technology, No. 2 Shanghai Road, Nanchang Jiangxi, China 0097-6156/88/0368-0440$06.00/0 © 1988 American Chemical Society

Flank and Whyte; Perspectives in Molecular Sieve Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Desorption of Hydrocarbons

27. LI-FENG AND REES

441

o f t h e ( N a H ) - f o r m s o f Z S M - 5 a n d ZSM-11 u s e d i n P a r t s 1 ( 1 ) and 1 1 ( 2 ) w e r e a l s o p r e p a r e d by i o n e x c h a n g e . The u . c . c o m p o s i t i o n s o f t h e s t a r t i n g m a t e r i a l s w e r e H, Na4 K O A l c S i o n 4.48 1 . 3Ζ ο 9 0 0 i g 2 and H4 3 β N a 1 β 4 A l g S i g o 0 1 Q 2 r e s p e c t i v e l y . A f t e r exchange both

of these

s a m p l e s h a d t h e same u . c . c o m p o s i t i o n o f Na

Al 6


Si

g Q

0

1 Q 2

; i . e . both

Z S M - 5 a n d ZSM-11

samples have

t h e same

6

Si/Al

r a t i o of 15. To p r e p a r e t h e p u r e s o d i u m f o r m 0 . 1 M N a C l s o l u t i o n a t pH " 1 0 w a s u s e d w h i l e t h e h y d r o g e n f o r m was p r e p a r e d u s i n g 0 . 1 M N H ^ C l s o l u t i o n a t pH 5 ^ 6 . I n b o t h e x c h a n g e s t h e z e o l i t e p h a s e was c o n t a c t e d w i t h e x c e s s s o l u t i o n p h a s e w i t h c o n t i n u o u s s t i r r i n g f o r 48 h o u r s a t r o o m t e m p e r a t u r e . The h y d r o g e n f o r m was f o r m e d by c a l c i n a t i o n o f t h e ammonium e x c h a n g e d s a m p l e s a t 5 0 0 C for 1 hour. The c r y s t a l l i n i t y o f t h e s t a r t i n g m a t e r i a l s a n d t h e i o n e x c h a n g e d f o r m s was d e t e r m i n e d i n a l l c a s e s by x - r a y d i f f r a c t i o n . A l l s a m p l e s gave good d i f f r a c t i o n p a t t e r n s i n d i c a t i n g h i g h crystallinity. The t p d p r o f i l e s w e r e d e t e r m i n e d u s i n g a S t a n t o n - R e d c r o f t T6 762 t h e r m o g r a v i m e t r i c b a l a n c e w i t h o n - l i n e Commodore CBM c o m p u t e r f o r d a t a c o l l a t i o n and a n a l y s e s . ( 3 . 4 ) The p r o f i l e s w e r e a n a l y s e d u s i n g t h e s i n g l e h e a t i n g r a t e , v a r i a b l e c o v e r a g e (SHR) m e t h o d . (4.) In t h i s method f i r s t - o r d e r d e s o r p t i o n k i n e t i c s , t h e A r r h e n i u s e q u a t i o n and a l i n e a r s a m p l e h e a t i n g r a t e a r e c o m b i n e d to give

-de/θ

= (A/0) exp (-E/RT)

dT

(1)

where θ i s t h e degree o f c o v e r a g e , A i s t h e p r e - e x p o n e n t i a l f a c t o r and Ε t h e a c t i v a t i o n energy o f d e s o r p t i o n from t h e A r r h e n i u s e q u a t i o n and β i s t h e h e a t i n g r a t e . E q u a t i o n (1) was i n t e g r a t e d between t h e l i m i t s ( θ , Τ ) and ( θ . , T . ) w h i c h O

represent the starting respectively to give

] •

V

j |

O

l

c o n d i t i o n s and t h o s e

exp (-E/RT)

l

dT

l

after

some t i m e

t

(2)

ο H o w e v e r , t h e t e m p e r a t u r e i n t e g r a l i n e q u a t i o n 2 h a s no e x a c t analytical solution. I n t h e SHR m e t h o d o f a n a l y s i s ( 4 . ) a new v a r i a b l e u = E/RT i s i n t r o d u c e d a n d o n i n t e g r a t i o n t h e r i g h t h a n d s i d e (RHS) o f e q u a t i o n 2 becomes

p(x)

- p(xQ)


442

PERSPECTIVES IN MOLECULAR SIEVE SCIENCE

where

On

ρ (χ)

=

integration

-In


exp

(-χ)

equation

Θ. ι

E q u a t i o n (4) was relationship

=

exp

(2)

— 0R

(-u)

(3)

du

c a n now be w r i t t e n

as:

p(E/RT.)-p(E/RT ) ι ο

fitted

to

In

θ.

the

experimental

(4)

data

through

the 2

In

equation

(5)

variable

AE ^

{p(E/RT.)

changes

a s T^

-

ρ

changes

(E/RT )}

(5)

Q

and A and Ε

are

independent v a r i a b l e s . To o b t a i n t h e b e s t f i t two c o n d i t i o n s i n v o l v i n g the p a r t i a l d e r i v a t i v e s of the summation w i t h r e s p e c t t o t h e t w o i n d e p e n d e n t v a r i a b l e s h a v e t o be s a t i s f i e d , i . e . Ô O / Ô A = 0 and Ô Q / Ô E = o . A r e i t e r a t i v e programme i s u s e d t o o b t a i n t h e o p t i m u m v a l u e s o f A and Ε o v e r l i m i t e d a r e a s o f θ f r o m θ = 0 t o 1 . F r o m t h e a n a l y s i s p e a k t e m p e r a t u r e s , p e a k w i d t h s , maximum r a t e s o f d e s o r p t i o n and a c t i v a t i o n e n e r g i e s , E d , a s a f u n c t i o n o f c o v e r a g e w e r e o b t a i n e d i n t h e same m a n n e r a s b e f o r e . ( 1 . 2 ) A h e a t i n g r a t e o f l O K m i n " 1 was u s e d i n a l l t p d r u n s . The n h e x a n e v a p o u r , a t a r e l a t i v e p r e s s u r e o f p / p o * 0 . 5 , was f l o w e d o v e r t h e z e o l i t e s a m p l e , i n s i t u i n t h e TG b a l a n c e a t room t e m p e r a t u r e u s i n g a r g o n as t h e c a r r i e r gas u n t i l s o r p t i o n e q u i l i b r i u m was a t t a i n e d . Results

and

Discussion

The s a t u r a t i o n c a p a c i t i e s o f t h e N a - , ( N a / H ) - and H- f o r m s of Theta-1 f o r n-hexane are l i s t e d i n Table I. From t h e s a t u r a t i o n c a p a c i t y of H-Theta-1 the micropore volume, W q , i s c a l c u l a t e d to previously(j>)

3 -1 be 0 . 0 8 4 c m g , a value c l o s e to that f o r the volume of the o n e - d i m e n s i o n a l

reported channel

network of Theta-1 of 0.0B9cm3g 1 . T a b l e I c l e a r l y shows t h a t the i n t r o d u c t i o n of ^ 2.8Na i o n s per u . c . i n t o the H-form decreases the s a t u r a t i o n c a p a c i t y f o r n-hexane from 3.66 to 2.30 molecules per u . c . ( m / u . c ) . T h i s d e c r e a s e i s much g r e a t e r t h a n t h e v o l u m e o f t h e Na i o n s i n t r o d u c e d , suggesting t h a t the s t e r i c h i n d r a n c e of the Na+ i o n s i n the o n e - d i m e n s i o n a l channel system is large. S i n c e t h e r e a r e no i n t e r s e c t i o n s i n t h e + c h a n n e l s y s t e m w h i c h w o u l d a l l o w n - h e x a n e m o l e c u l e s t o b y - p a s s Na b a r r i e r s , nh e x a n e m o l e c u l e s m u s t d i f f u s e o v e r t h e s e b a r r i e r s t o be s o r b e d up


27.


LI-FENG AND REES

443

TABLE I : S a t u r a t i o n C a p a c i t i e s and E^ Values Obtained by SHR Method o f A n a l y s i s . 0 = 10 K.min"

1

n-hexane

(Na.H)

Theta-1

H Theta-1

2.30

2.72

3.66

80

74

69

amount


adsorbed , (m/u.c)1"

Na T h e t a - 1

average

activation

energy,

E_, d

(kJ.mol"1)

the ^

a c t i v a t i o n energy

molecules per

averaged over

the whole coverage

range

u.c.

to the 2.30 m/u.c l i m i t . The l a r g e d e c r e a s e i n t h e s a t u r a t i o n c a p a c i t y o f t h e N a - f o r m o v e r t h a t o f t h e H- f o r m m u s t , t h e r e f o r e , be t h e r e s u l t o f i n e f f i c i e n t p a c k i n g o f t h e n - h e x a n e molecules i n the channel system. I t w o u l d seem t h a t n - h e x a n e m o l e c u l e s a r e e x c l u d e d f r o m c h a n n e l s e g m e n t s b e t w e e n a d j a c e n t Na

+

i o n s i f t h e s e Na4 i o n s a r e c l o s e r t h a n 1.03nm a p a r t , t h e l e n g t h of a n-hexane m o l e c u l e . This i n t e r p r e t a t i o n of the packing of nh e x a n e i s s u p p o r t e d by t h e l a r g e d e c r e a s e o f 0 . 4 2 m / u . c o f n h e x a n e r e s u l t i n g f r o m t h e i n t r o d u c t i o n o f an a d d i t i o n a l 0 . 4 6 N a per u . c . i n t o the (Na/H)- form (the d i f f e r e n c e i n s a t u r a t i o n c a p a c i t i e s o f t h e ( N a / H ) - and N a - f o r m s o f T h e t a - 1 ) . The a d d i t i o n a l 0 . 4 6 Na per u . c . would s i g n i f i c a n t l y i n c r e a s e the number o f s u c h c h a n n e l s e g m e n t s . The t p d p r o f i l e s o f n - h e x a n e d e s o r b i n g f r o m t h e N a - T h e t a - 1 and H - T h e t a - 1 a r e shown i n F i g u r e s 1 a n d 2 r e s p e c t i v e l y . The v a l u e s o f t h e p e a k t e m p e r a t u r e , p e a k w i d t h and maximum r a t e o f d e s o r p t i o n o b t a i n e d from these tpd p r o f i l e s are l i s t e d i n Table 2, which a l s o l i s t s the c o r r e s p o n d i n g v a l u e s found i n Part 1(1) f o r the ( N a , H ) - form of Theta-1 f o r c o m p a r i s o n . T a b l e I I d e m o n s t r a t e s t h e i n c r e a s e i n p e a k t e m p e r a t u r e s and w i d t h s and t h e d e c r e a s e i n d e s o r p t i o n r a t e c a u s e d by t h e i n t r o d u c t i o n o f Na i o n s i n t o the channel system of T h e t a - 1 .



444


F i g u r e 1. Theta-1.

Temperature β = 10Kmin

programmed d e s o r p t i o n o f n - h e x a n e f r o m Nam' = mass o f s a m p l e a n d s o r b a t e .

650

Figure 2. Temperature Theta-1 β = 10Kmin"

programmed d e s o r p t i o n o f n - h e x a n e

f r o m H-


27.


LI-FENG AND REES

445

TABLE I I : Peak Temperatures and Widths and Maximum Rates o f D e s o r p t i o n of n- hexane from Theta- 1 .

Na-Theta-1


peak (Κ)

(NaH)-Theta-l

-1

H-Theta-1

peak

I

330

320

315

peak

II

405

395

390

temperature

larger

peak

peak

maximum r a t e (mg.s )

peak (K)

β s 10 K.min

I

peak

I

peak

I

1.323E-03

1 .662E-03

2.186E-03

peak

I

300*370

300*350

300*350

peak

II

370*450

350*410

350*410

width

From t h e r e s u l t s i n T a b l e I I ( a s p r o p o s e d p r e v i o u s l y ( U ) ) the a c t i v a t i o n energies of d e s o r p t i o n , Ed, should f o l l o w the order Na-Theta-1 > (Na/H)-Theta-1 > H-Theta-1. The a c t i v a t i o n e n e r g i e s o b t a i n e d by t h e SHR m e t h o d ( 4 . ) f o r t h e s e t h r e e s a m p l e s a s a f u n c t i o n o f c o v e r a g e a r e shown i n F i g u r e 3 a n d t h e a c t i v a t i o n e n e r g i e s averaged over t h e whole coverage range, Ë ^ , a r e g i v e n i n Table I. These a v e r a g e e n e r g i e s do f o l l o w t h e above p r e d i c t e d o r d e r b u t F i g u r e 3 shows t h a t t h e d e s o r p t i o n e n e r g i e s o f t h e l a s t 1 . 5 m / u + c . t o be d e s o r b e d a r e v e r y s i m i l a r i n a l l t h r e e s a m p l e s . The Na i o n s s h o u l d i n t r o d u c e an a d d i t i o n a l e l e c t r o s t a t i c i n t e r a c t i o n w i t h the n-hexane molecules which should i n c r e a s e the i n i t i a l h e a t o f a d s o r p t i o n and t h u s i n c r e a s e , E d , t h e a c t i v a t i o n e n e r g y f o r d e s o r p t i o n o f t h e l a t e r m o l e c u l e s t o be d e s o r b e d . Since the channel system of Theta-1 i s o n e - d i m e n s i o n a l the e l e c t r o s t a t i c i n t e r a c t i o n i n t r o d u c e d by t h e N a * i o n s w i l l t e n d t o be l i m i t e d t o t h e e n d C H 3 g r o u p o f t h e n - h e x a n e m o l e c u l e . This a d d i t i o n a l i n t e r a c t i o n e n e r g y seems t o be t o o s m a l l t o be o b s e r v e d by t h e SHR m e t h o d o f a n a l y s i s e v e n t h o u g h t h e p e a k t e m p e r a t u r e s a n d w i d t h s d o show a s m a l l e f f e c t o f t h e Na ions. A l t h o u g h t h e Na i o n s m u s t e n h a n c e t h e a c t i v a t i o n e n e r g y f o r d i f f u s i o n of n-hexane molecules i n t h e channel system of Theta-1 t h e SHR m e t h o d o f a n a l y s i s a s s u m e s t h a t d e s o r p t i o n i s n o t d i f f u s i o n c o n t r o l l e d a n d t h u s a n y i n c r e a s e i n Ed w i t h Na content c a n n o t be i n t e r p r e t e d a s d u e t o an i n c r e a s e i n t h e a c t i v a t i o n energy of d i f f u s i o n . Although the a c t i v a t i o n energy of d i f f u s i o n o f n - h e x a n e has n o t been m e a s u r e d , as y e t , as a f u n c t i o n o f c a t i o n c o n t e n t o r t y p e , i t i s c e r t a i n t h a t t h i s e n e r g y w i l l be much s m a l l e r t h a n t h e v a l u e s o f Ê H r e p o r t e d i n T a b l e I .



446


C o r r e s p o n d i n g s t u d i e s were c a r r i e d o u t on t h e p u r e N a - f o r m s o f Z S M - 5 a n d ZSM-11 b o t h s a m p l e s o f w h i c h h a d S i / A l r a t i o s o f 1 5 . The s a t u r a t i o n c a p a c i t i e s o f t h e s e s a m p l e s f o r n - h e x a n e a r e g i v e n i n T a b l e I I I , w h e r e t h e v a l u e s may be c o m p a r e d w i t h t h e c o r r e s p o n d i n g s a t u r a t i o n c a p a c i t i e s f o r t h e (Na/H)- forms o b t a i n e d p r e v i o u s l y . (J.) A l l saturation capacities l i s t e d i n T a b l e I I I a r e much s m a l l e r t h a n t h e t h e o r e t i c a l s a t u r a t i o n c a p a c i t i e s of t h e pure s i l i c a forms of these frameworks^ ^ S i l i c a l i t e - 1 has a t h e o r e t i c a l c h a n n e l volume o f 0.19cm g which corresponds to 8.42 molecules of n-hexane per u . c . at s a t u r a t i o n i n d i c a t i n g t h a t segments o f two d i f f e r e n t n - h e x a n e m o l e c u l e s a r e l o c a t e d a t each i n t e r s e c t i o n . The t h e o r e t i c a l s a t u r a t i o n c a p a c i t y o f s i l i c a l i t e - 2 i s somewhat g r e a t e r d u e t o t h e 30Z g r e a t e r volume o f t h e s t r a i g h t c h a n n e l segments w h i c h r e p l a c e t h e s i n u s o i d a l c h a n n e l segments o f s i l i c a l i t e - 1 . TABLE I I I :

Saturation by

Capacities

SHR M e t h o d .

and Ê

H

Values

0 = 10K.min ZSM-11

ZSM-5

NaZSM-5

amount

Obtained

(NaH)ZSM-5

adsorbed

NaZSM- 11

(NaH)ZSM-ll

6.18

6.80

6.42

6.99

92

84

91

87

(m/u.c)

average

activation _ * Erf

energies; (kJ.mol

the

)

activation

Table

III

energy

shows

that

averaged

over

t h e whole

the introduction

coverage

of 1.52Na+

range

ions per

u . c . i n t o ZSM-5 a n d 1 . 6 6 N a + i o n s p e r u . c . i n t o ZSM-11 r e d u c e s t h e s a t u r a t i o n c a p a c i t y o f n-hexane t o 6 . 8 0 and 6 . 9 9 m / u . c respectively. L i k e Theta-1, i n t r o d u c t i o n of Na+ i n t o the channel network decreases the packing e f f i c i e n c y of n-hexane m o l e c u l e s . H o w e v e r , on i n c r e a s i n g t h e N a + i o n c o n c e n t r a t i o n s t o 6 p e r u . c . , the s a t u r a t i o n c a p a c i t i e s only d e c r e a s e s l i g h t l y t o 6 . 1 8 and 6.42 m/u.c. r e s p e c t i v e l y . Now, u n l i k e t h e T h e t a - 1 c a s e , t h e s e q u i t e l a r g e N a + i o n c o n c e n t r a t i o n i n c r e a s e s have l i t t l e e f f e c t the n-hexane s a t u r a t i o n c a p a c i t i e s . Thus, the channel


on

27.

447


LI-FENG AND REES

i n t e r s e c t i o n s w h i c h a r e p r e s e n t i n b o t h ZSM-5 a n d ZSM-11 a l l o w more e f f i c i e n t p a c k i n g o f n - h e x a n e m o l e c u l e s i n t h e p r e s e n c e o f Na+ i o n contents. The t p d p r o f i l e s o f n - h e x a n e d e s o r b i n g f r o m N a - Z S M - 5 a n d N a ZSM-11 a r e shown i n F i g u r e s 4 a n d 5 r e s p e c t i v e l y . The v a l u e s o f t h e p e a k t e m p e r a t u r e , p e a k w i d t h a n d maximum r a t e o f d e s o r p t i o n obtained from these p r o f i l e s are l i s t e d i n Table IV. The c o r r e s p o n d i n g v a l u e s f o r t h e (Na/H)- forms o b t a i n e d p r e v i o u s l y ( 1 ) are i n c l u d e d f o r c o m p a r i s o n .


high

TABLE IV: Peak Temperatures and Widths and Maximum Rates o f D e s o r p t i o n o f n-hexane from ZSM-5 and ZSM-11.

0 = lOK.min"

ZSM-11

ZSM- •5

(NaH)ZSM-5

NaZSM-5

peak (Κ)

NaZSM-11

(NaH)ZSM-11

peak

I

350

340

350

340

peak

II

415

405

415

405

temperature

larger

peak

peak

maximum r a t e (mg.s )

peak

1

I

peak

I

peak

I

peak

I

2.312E-3

2.89E-03

2.653E-03

3.234E-03

peak

I

300*380

300*370

300*380

300*370

peak

II

380*460

370*440

380*460

370*440

Theta-1,

Table

width

As

found w i t h

IV c l e a r l y shows

that

i n c r e a s i n g N a * c o n t e n t s i n t h e c h a n n e l s o f ZSM-5 a n d ZSM-11 i n c r e a s e s t h e peak t e m p e r a t u r e s and w i d t h s and d e c r e a s e s t h e rates of d e s o r p t i o n . On a n a l y s i s o f t h e t p d p r o f i l e s i n F i g u r e s 4 a n d 5 by t h e SHR m e t h o d ( 4 . ) t h e v a r i a t i o n o f t h e a c t i v a t i o n e n e r g y o f d e s o r p t i o n , E J t w i t h c o v e r a g e shown i n F i g u r e s 6 a n d 7 f o r t h e d Z S M - 5 a n d ZSM-11 activation

s a m p l e s r e s p e c t i v e l y was o b t a i n e d .

energy,

Ê , d

averaged

over

The

t h e whole coverage range f o r

t h e s e ZSM-5 a n d ZSM-11 s a m p l e s a r e l i s t e d t h e c a s e o f T h e t a - 1 and i n a g r e e m e n t w i t h

i n Table the data

III. As i n i n Table IV,

American Chemical Society Library

1155 15th St., N.W. Flank and Whyte; Perspectives in Molecular Sieve Science Washington, D.C. 2 00361 DC, 1988. ACS Symposium Series; American Chemical Society: Washington,


448


1.0

dm/dT

10.5

Λ50 500 Τ / Κ Figure ZSM-5.

4.

Temperature 0 = 10Kmin"

programmed

desorption

of n-hexane

f r o m Na-





Τ /

Figure

Temperature

5.

ZSM-11 .

β =

449

Κ

programmed d e s o r p t i o n o f

n-hexane

from

Na-

10Kmin

120 A

ο

NaZSM-5

•

(Na.H) ZSM-5

100 Ed KJ-MOL -1 80

60 Η

AO Η

1.0

2.0

3.0

U.0

5.0

6.0

m/ u c F i g u r e 6. A c t i v a t i o n energy of d e s o r p t i o n of n-hexane from 0 Na-ZSM-5; Q ( N a . H ) - Z S M - 5 a s a f u n c t i o n o f s o r b a t e l o a d i n g molecules per u n i t c e l l . SHR m e t h o d o f a n a l y s i s .


in


450


AO

1.0

2.0

3.0 L0 m / uc

5.0

6.0

F i g u r e 7. A c t i v a t i o n e n e r g y o f d e s o r p t i o n o f n - h e x a n e 0 Na-ZSM-11; • (Na,H)-ZSM-11 as a f u n c t i o n of s o r b a t e molecules per u n i t c e l l . SHR m e t h o d o f a n a l y s i s .

from loading in




451

these energies increase with increasing Na* content per u.c. It is also interesting to note that these energies have similar values in these ZSM-5 and ZSM-11 samples which have the same Na ion concentrations. (N.B: Both of these zeolites had the same Si/Al ratios.) Unlike the case of Theta-1 the E_, values at low d loadings are much larger for the pure Na* forms of both ZSM-5 and


+

ZSM-11, indicating that the electric field around each Na* ion is introducing a significant electrostatic component to the total interaction energy. It is interesting to note on comparing the Êd values in Table I and III that the average desorption energies in the onedimensional channel network of Theta-1 are smaller than the corresponding energies for the three dimensional channel networks of ZSM-5 and ZSM-11, which have four intersections per u.c. These results are further evidence that Ed is closely related to the heat of adsorption of n-hexane rather than the activation energy for diffusion in these frameworks. The higher Ed values for ZSM-5 and ZSM-11 over those for Theta-1 are the result of the stronger electrostatic interactions which exist in the frameworks with higher Al concentrations. REFERENCES 1. 2. 3. 4. 5. 6. 7.

Chen, L.F. and Rees, L.V.C., Zeolites, in press. Chen, L.F. and Rees, L.V.C., Zeolites, in press. Richards, R.E. and Rees, L.V.C., Zeolites, 1986, 6, 17. Dima, E. and Rees, L.V.C., Zeolites, 1987, 7, 219. Barri, S.A.I., Smith, G.W., White, D. and Young, D., Nature, 1984, 312, 533. Tokoro, Υ., Misono, Μ., Uchijima, T. and Yoneda, Y., Bull. Chem. Soc. Japan, 1978, 51, 85. Czanderna, A.W., Biegen, J.R. and Kollen,W., J. Coll. Interface Science, 1970, 34, 406.

RECEIVED January 25, 1988


Perspectives in Molecular Sieve Science - American Chemical Society

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