Adsorption Phenomena in Zeolites as Studied by Nuclear Magnetic

zeolites of faujasite type per Vb unit cell. (corresponding to one supercage;. S i t e maximum number. NaX n=1.37. Β aï n=r2.6. NaGeY70x ') η = 2.6...
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4 Adsorption Phenomena in Zeolites as Studied by Nuclear Magnetic Resonance

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0034.ch004

H. PFEIFER Sektion Physik der Kark-Marx-Universität, DDR 701 Leipzig, Linnéstrasse 5

Introduction Zeolites are porous crystals with a well-defined structure and a high specific surface area, widely used i n industry as molecular sieves, as catalysts, and as ion exchangers. Their common formula is (AlO2)-M+(SiO2)n (1) +

where M denotes an exchangeable cation which can be also replaced by /2 M + or /3 M , and n is a number greater or equal 1. The adsorption phenomena to be discussed i n this report are concerned with molecules i n the pores of a special "family" of zeolites (faujasite group) to which belong the so­ -called zeolites NaX and NaY generally denoted as NaF. These synthetic zeolites, available as small c r y s t a l l i t e s with a mean diameter between 1 and 100 um, have the same structure and differ only i n the value of n. Their pore system accessible to hydrocarbons, i s a three-dimensional network of nearly spherical cavities, commonly referred to as supercages, each with a mean free diameter of 11.6 Åand connected tetrahedrally through windows of 8 - 9 Ådiameter. Because each s i l i c o n or aluminium ion of the zeolite l a t t i c e i s surrounded by four larger oxygens, the internal surfaces of ideal crystals consist of oxygen, the only other elements exposed to adsorbates being the ex­ changeable cations which are sodium ions i n the case of inNaFzeolites. Some of these cations are not accessible to Hydrocarbons since they are localized at sites SI and SI' outside of the super­ cages, some are localized at the walls of the 1

2

1

3+

36

Resing and Wade; Magnetic Resonance in Colloid and Interface Science ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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Adsorption

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37

supercages opposite t o the windows ( s i t e s SII) and some c o u l d not be l o c a l i z e d by X-ray measurements, they are assumed, to be l o o s e l y bound ( s i t e s S3) to the w a l l s of the supercages between s i t e s S I I . As was mentioned already i t i s p o s s i b l e t o r e p l a c e Na by other c a t i o n s , and since d i - and e s p e c i a l l y t r i v a l e n t c a t i o n s occupy f i r s t of a l l s i t e s S I and S I , the number of sodium i o n s per supercage can be v a r i e d not only by n, but also through an i o n exchange. Examples are given i n Table I· 1

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:

Table 1 ' dumber of exchangeable c a t i o n s i n dehydrated z e o l i t e s of f a u j a s i t e type per Vb u n i t c e l l (corresponding to one supercage; maximum number

Site

2

SI SI' SII

4 4 6

S3

NaX n=1.37

Β aï n=r2.6 0.7 2.3 3.3

1.2 1.7 3.2 4.0

0.4

NaGeY70 ' x

)

η = 2.6

I

1.5 J+1.5

0.5

Nat. 0e£ Ma +

+

(nonloc. ) WceYyO i s a NaY z e o l i t e where 70 % of iMa has been r e p l a c e d by ue^ »

+

+

The p h y s i c a l s t a t e of adsorbed molecules de­ pends on the r a t i o of i n t e r m o l e c u l a r i n t e r a c t i o n energy to the energy of i n t e r a c t i o n between a mole­ cule and the s u r f a c e . According to K i s e l e v (1) molecules and surfaces may be c l a s s i f i e d witΕ r e g a r d to t h e i r c a p a c i t y f o r n o n s p e c i f i c and s p e c i f i c i n t e r ­ a c t i o n . Nonspecific i n t e r a c t i o n i s caused mainly by d i s p e r s i o n f o r c e s and i s present t h e r e f o r e i n a l l cases. S p e c i f i c i n t e r a c t i o n may appear a d d i t i o n a l l y whenever e l e c t r o n density (e.g. I f bonds of molecules) or a p o s i t i v e charge (Ma i n the case of Natf z e o l i t e s ) i s l o c a l i z e d on the periphery between the adsorbed molecules and the surface. W have studied the p h y s i c a l state of c y c l i c hydrocarbons, adsorbed i n NaF z e o l i t e s where the degree of s p e c i f i c i n t e r a c t i o n has been s y s t e m a t i c a l ­ l y v a r i e d through the number of ΤΓ e l e c t r o n s per molecule (cyelohexane, cyclohexene, eyelohexadiene benzene and toluene) and through the l o c a t i o n and e

9

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38

M A G N E T I C

RESONANCE

number o f sodium i o n s a t t h e w a l l s o f t h e s u p e r cages. Adsorbed

cyclohexane

U s i n g v a r i o u s m i x t u r e s o f c y c l o h e x a n e and d e u t e r a t e d c y c l o h e x a n e i t c o u l d be shown ( 2 ) t h a t t h e magnetic i n t e r a c t i o n w i t h paramagnetic i m p u r i t i e s of the z e o l i t e ( c a l l e d proton e l e c t r o n interaction) i s c o n t r o l l i n g the proton r e l a x a t i o n t i m e s (1L and T~) of C E L a d s o r b e d i n a c o m m e r c i a l NaY z e o l i t e (VEB Chemlekumbinat i i i t t e r f e l d , η =2.6) These, p a r a m a g n e t i c i m p u r i t i e s a r e F e ^ i o n s l o c a t e d at A l ^ * s i t e s o f t h e z e o l i t e s k e l e t o n , a l t h o u g h an even g r e a t e r number i s l o c a l i z e d a t s i t e s S I o r a s an i r o n o x i d e - l i k e phase on t h e c r y s t a l s ( 3 ) · The t o t a l c o n c e n t r a t i o n , d e t e r m i n e d o p t i c a l l y c o r r e ­ sponds t o about ^QO ppm Fe^O^ f o r t h i s z e o l i t e . The t e m p e r a t u r e dependence ox I L and T h a s been measured a t 16 MEz between - 140°C and +90°C f o r p o r e f i l l i n g f a c t o r s θ = 0.16, 0.4, 0 . 7 , and 0.88. T h i s q u a n t i t y θ i s t h e r a t i o o f t h e a d s o r b e d amount of c y c l o h e x a n e t o i t s maximum v a l u e w h i c h c o r r e ­ sponds t o a o o u t f o u r m o l e c u l e s p e r s u p e r c a g e . I n a l l c a s e s t h e minimum o f Τ.·, o c c u r e d a t t h e same t e m p e r a t u r e (-100°U) w h i l e t h e r a t i o o f l V | t o 1*2 a t "uhis p o i n t i n c r e a s e d f r o m a b o u t 2 t o 4, and T^ v e r s u s r e c i p r o c a l a b s o l u t e t e m p e r a t u r e becomes f l a t t e r w i t h i n c r e a s i n g coverage. Using T o r r e y s model f o r t r a n s l a t i o n a l m o t i o n ( 4 ) t h e f o l l o w i n g f o r m u l a e c a n be d e r i v e d f o r t h i s c a s e (£) where £ a n d CJ^ a r e t h e Larmor f r e q u e n c i e s o f t h e p r o t o n and p a r a m a g n e t i c i o n r e s p e c t i v e l y , Κ i s a constant proportional to the concentration o f p a r a m a g n e t i c i o n s , d d e n o t e s t h e minimum d i s t a n c e between p r o t o n and i o n ^ a n d C i s t h e c o r r e l a t i o n time f o r t h e t r a n s l a t i o n a l motion o f t h e adsorbed molecule: c

Resing and Wade; Magnetic Resonance in Colloid and Interface Science ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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where OC



-

(5)

» 2 The mean q u a d r a t i c jump l e n g t h < r > , t h e a v e r a g e t i m e V between two jumps a n d t h e d i f f u s i o n c o n s t a n t D a r e r e l a t e d by E i n s t e i n ' s e q u a t i o n Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0034.ch004

The f u n c t i o n f

t

= ( oc ,

6 3>Γ.

(6)

V ) i s defined as c

(7)

where 01

A f i t t i n g p r o c e d u r e ( 6 ) y i e l d s OC ^1/5 f o r θ = 0 . 8 8 , and s i n c e f o r t h i s v a l u e o f at j a t t h e minimum of T i t i s UJ^X^s 1.58, we h a v e 1

I n c o n t r a s t , f o r t h e l o w e s t c o v e r a g e (θ = 0·16) i t f o l l o w s from the f i t t i n g procedure B e c a u s e i n t h i s c a s e t h e minimum o f occurs i f Od^V = 1§ so t h a t we have c

T h e r e f o r e "C i s a b o u t 1 6 .·· 2 0 n s a t ~100°C a n d does n o t depend on c o v e r a g e w i t h i n t h e s e l i m i t s . The mean q u a d r a t i c jump l e n g t h o f t r a n s l a t i o n a l m o t i o n however, d e c r e a s e s w i t h i n c r e a s i n g c o v e r a g e which i s a d i r e c t h i n t f o r n o n l o c a l i z e d adsorption o f c y e l o h e x a n e . I n a c c o r d a n c e w i t h t h i s , measure­ ments o f p r o t o n r e l a x a t i o n i n z e o l i t e s w i t h différait

Resing and Wade; Magnetic Resonance in Colloid and Interface Science ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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number o f s o d i u m i o n s ( n = 1.37.(2) a n d n = 1 * 2 , ( 8 ) ) r e v e a l e d a b o u t t h e same c o r r e l a t i o n t i m e s as i n t h e above s y s t e m ( n = 2.6)· S i n c e t h e s e z e o l i t e s a r e o f h i g h p u r i t y ( o n l y 2 . . . 3 PPm JtepCU i n t h e c a s e o f η = 1.37) a n d t h e c r y s t a l l i t e s o f H s x g d i a m e t e r ( 1 5 ... 2 5 / u m i n t h e c a s e o f η = 1 . 2 ) i t was p o s s i b l e t o d e t e r m i n e t h e i n t r a c r y s t a l l i n e s e l f - d i f f u s i o n c o e f f i c i e n t D by t h e p u l s e d f i e x d g r a d i e n t t e c h n i q u e , The r e s u l t s a r e shown i n Table 2 . Table 2 S e l f - d i f f u s i o n c o e f f i c i e n t o f C^-H^p i n E a X z e o l i t e s a t room t e m p e r a t u r e ( D ^ ) and a c t i v a t i o n e n e r g y ( Ε ) 2

η = 1.37 θ = 0.3

20 = 7

β

10~

fo

2

cm /s

E

D

= ( 3 . 0

±

0 . 2 ) g g "

(2) η = θ

1*2 0.25 2

"20

[cm /s]

5.5*10'

0.2?

0.45

0.66

-6 6.10' -6 3.8.10-6 9*10'r 7

υ. 69 ^ 7 5 9.10

(â) From equs. ( 1 0 ) and ( 1 1 ) and t h e o b s e r v e d a c t i v a t i o n energy ή = ( 3 · 0 + 0 . 3 ) k c a l / m o l e , t h e v a l u e o f V a t room t e m p e r a t u r e c o m e s o u t ijo be ~ 0 . 5 n s s o t h a t t h e rms jump l e n g t h | ( c f . equ.(6)) and T a b l e 2)isabout 1 3 S , a n d l e s s t h a n 2 X t o r θ = υ · 2 and 0 · 8 r e s p e c t i v e l y . Summarized, c y c l o h e x a n e a d s o r b e d i n z e o l i t e s o f f a u j a s i t e t y p e behaves l i k e an i n t r a c r y s t a l l i n e l i q u i d : t h e m o l e c u l e s do n o t jump b e t w e e n s i t e s w h i c h a r e f i x e d i n s p a c e . The mean jump l e n g t h d u r i n g t r a n s l a t i o n a l m o t i o n d e c r e a s e s w i t h i n c r e a s i n g coverage as a consequence of mutual h i n d r a n c e , w h i l e t h e average t i m e between jumps does n o t change s i g n i f i c a n t l y . 9

Adsorbed n o n s a t u r a t e d c y c l i c hydrocarbons In Table 3 the r a t i o s o f proton r e l a x a t i o n times a n d Tp measured a t t h e minima o f for v a r i o u s c y c l i c h y d r o c a r b o n s a d s o r b e d i n t h e same c o m m e r c i a l NaY z e o l i t e (n=2.6) a s above, a r e l i s t e d

(2) Resing and Wade; Magnetic Resonance in Colloid and Interface Science ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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Adsorption Phenomena in Zeolites

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Table 3 T^/Tp a t t h e minimum o f TA$ a c t i v a t i o n e n e r g y 1 and c o r r e l a t i o n t i m e T 'for various c y c l i c h y d r o c a r b o n s a d s o r b e d i n a c o m m e r c i a l NaT z e o l i t e ( n s 2.6) c

Η

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θ =0.2 θ =0.8 c

LiôIëJ

at

P

20 X5

°6 6

°6 8

Η

°6 10

°6 12

1.8

1.8

1.8

2.1

1.8

1.9

1.9

Θ

=0.2

Θ

=0.8

e-

=0.2 13

Θ

=0.8

|

50

3.4

±

Η

0.3

3

7.7 '

Η

2.8

8

9

+ 0.3

r =^2~0.25 t

V=T

~ 0 . 5

As c a n b e s e e n , t h e s e r a t i o s a r e c l o s e t o t h e t h e o r e t i c a l v a l u e (1·83) f o r d o m i n a t i n g p r o t o n e l e c t r o n i n t e r a c t i o n and t h e r m a l m o t i o n c h a r a c t e r i z e d by a s i n g l e c o r r e l a t i o n time V (10). T h e r e f o r e we h a v e c

and C c a n be r e a d i l y d e t e r m i n e d a s a f u n c t i o n o f T, s i n c e T,. h a s b e e n measured between about -Ί00 U a n d + 2 0 0 ° G ( ^ ) . The l o g a r i t h m o f T p l o t t e d v e r s u s Ύ Τ g i v e s s t r a i g h t l i n e s , so t h a t an a c t i v a t i o n e n e r g y Ε c a n be d e f i n e d w h i c h i s l i s t e d i n T a b l e 2 together with absolute values f o r a t room t e m p e r a t u r e , u'rom t h e s e r e s u l t s t h e f o l l o w i n g c o n ­ c l u s i o n s concerning non^jsaturated c y c l i c hydrocarbons a d s o r b e d i n NaX c a n be drawn: ( i ) E v e n a t h i g h p o r e f i l l i n g f a c t o r s (Θ=0.8) t h e r e i s no d i s t r i b u t i o n o f c o r r e l a t i o n t i m e s : The m o l e c u l e s a r e a d s o r b e d a t s i t e s f i x e d i n s p a c e ( l o c a l i z e d a d s o r p t i o n ) . These a d s o r p t i o n c e n t r e s w i l l be t h e sodium i o n s a t s i t e s S I I as we s h a l l s e e l a t e r . ( i i ) As a c o n s e q u e n c e o f l o c a l i z e d a d s o r p t i o n u j y r i n g ^ e q u a t i o n c a n be a p p l i e d : c

1

c

Resing and Wade; Magnetic Resonance in Colloid and Interface Science ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

42

MAGNETIC

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v

c =

ép

4-

z*?t t/nT)

A

RESONANCE

( 1 3 )

B

where t h e i n i t i a l s t a t e ( p a r t i t i o n functioç ) c o r r e s p o n d s t o t h e m o l e c u l e a d s o r b e d o n l a and t h e a c t i v a t e d s t a t e ( p a r t i t i o n f u n c t i o n ρ^φ) c o r r e s p o n d s t o t h e m o l e c u l e d u r i n g i t s jump f r o m one Ν a t o a n o t h e r . I n a c c o r d a n c e w i t h t h i s m o d e l the energy o f a c t i v a t i o n Ε i s l e s s t h a n t h e heat o f a d s o r p t i o n ( & = 1 8 Jfccal/mole f o r b e n z e n e ) . I t does n o t depend o n t h e number o f ΤΓ e l e c t r o n s . The o b ­ served increase o f V from G H t o 0 Η must be e x p l a i n e d by a change o f t h e p r e e x p o n e n t i a l f a c t o r and e s p e c i a l l y b y a d e c r e a s e o f since a stronger i n t e r a c t i o n always l e a d s t o a decrease i n t h e p a r t i t i o n f u n c t i o n . C o r r e s p o n d i n g l y , we have a n increasing r e s t r i c t i o n o f molecular mobility i n t h e a c t i v a t e d s t a t e w i t h i n c r e a s i n g number o f 1Γ e l e c t r o n s . ( i i i ) The i n c r e a s e o f V w i t h i n c r e a s i n g c o v e r a g e i s a l s o i n accordance w i t h e q u . ( 1 3 ) because t h e p r e e x p o n e n t i a l f a c t o r i s p r o p o r t i o n a l t o /u, where u d e n o t e s t h e number o f u n o c c u p i e d a d s o r p t i o n s i t e s w h i c h c a n be r e a c h e d f r o m t h e a c t i v a t e d s t a t e : We c a n assume u ^ 3 and u £ 1 f o r Θ» = 0.2 a n d 0.8 r e s p e c t i v e l y , s i n c e t h e r e a r e about 4 a d s o r p t i o n c e n t r e s p e r s u p e r c a g e , and θ = 1 c o r r e s p o n d s r o u g h l y t o t h e same number o f m o l e c u l e s . F o r a f u r t h e r c h e c k o f t h i s model we have measured the p r o t o n r e l a x a t i o n times o f t o l u e n e adsorbed on t h e same z e o l i t e . A l t h o u g h benzene a n d t o l u e n e d i f f e r i n t h e i r i n t e r m o l e c u l a r i n t e r a c t i o n s , as c a n be s e e n f r o m t h e i r q u i t e d i f f e r e n t m e l t i n g p o i n t s (+ 5 · 5 ° 0 a n d -95°C r e s p e c t i v e l y ) , m o l e c u l a r m o t i o n i n t h e adsorbed s t a t e s h o u l d be s i m u l a r , s i n c e i t i s d e t e r m i n e d b y t h e i n t e r a c t i o n between t h e r i n g o f t h e m o l e c u l e s and t h e sodium i o n s a t s i t e s S I I . T h i s i n d e e d h a s b e e n o b s e r v e d : There i s no d i f f e r e n c e i n p r o t o n r e l a x a t i o n t i m e s between benzene a n d t o l u e n e f r o m a b o u t -20°0 t o +180°C ( 1 1 ) . I n a d d i t i o n , t h i s r e s u l t states that the r o t a t i o n of t h e benzene molecule around i t s s i x f o l d a x i s o f symmetry does n o t i n f l u e n c e p r o t o n r e l a x a t i o n i n t h i s temperature i n t e r v a l . 0

6

1 0

&

6

c

1

Resing and Wade; Magnetic Resonance in Colloid and Interface Science ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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I f benzene i s a d s o r b e d i n a c o m m e r c i a l ΙΗΘΧ z e o l i t e (VEB Ohemiekombinat B i t t e r f e l d , η = Ί . 8 , A^500 ppm F e 0 o ) t h e minimum o f s h i f t s t o much l o w e r t e m p e r a t u r e s ( f i g . 1) a n d t h e r a t i o Ty./Tp i n c r e a s e s s l i g h t l y f r o m 1 . 8 t o about 2 . 4 · T h e r e ­ f o r e t h e m o b i l i t y o f benzene m o l e c u l e s a d s o r b e d i n NaX i s much h i g h e r ( a t room t e m p e r a t u r e V * Λ 0 n s i n c o m p a r i s o n t o 50 n s i n NaX) a n d a s l i g h t d i s ­ t r i b u t i o n o f c o r r e l a t i o n t i m e s seems p r o b a b l e . The much h i g h e r m o b i l i t y o f benzene m o l e c u l e s a d s o r b e d i n HaZ z e o l i t e s h o u l d b e d i s c u s s e d i n c o n n e c t i o n w i t h r e s u l t s f o r w a t e r where t h e m o b i l i t y d o e s n o t depend o n s i l i c o n t o a l u m i n i u m r a t i o η o f z e o l i t e s (12). F o r a pore f i l l i n g f a c t o r θ s 0 . 8 t h e r e a r e about 24 w a t e r m o l e c u l e s and o n l y about 4 b e n z e n e m o l e c u l e s p e r s u p e r c a g e . T h e r e f o r e i n NaX ( n = 2 . 6 ) a l l a r o m a t i c m o l e c u l e s w i l l b e s t r o n g l y bound b y s p e c i f i c i n t e r a c t i o n to t h e l o c a l i z e d sodium i o n s which a r e f i x e d a t the w a l l o f t h e supercage ( s i t e s S I I , c f . T b l e 1 ) · This e x p l a i n s t h e low m o b i l i t y o£ t h e s e m o l e c u l e s . I n NaX z e o l i t e s ( n = 1 . 8 ) however, a r o m a t i c m o l e ­ c u l e s w i l l be bound t o t h e n o n l o c a l i z a b l e s o d i u m i o n s a v a i l a b l e h e r e , s i n c e t h e s e a r e more e x p o s e d than N a a t s i t e s S I I and a c t as primary a d s o r p t i o n c e n t r e s ( 1 3 ) » So t h e h i g h m o b i l i t y i n NaX c a n be e x p l a i n e d T5y t r a n s l a t i o n a l a n d r o t a t i o n a l m o t i o n o f a r o m a t i c m o l e c u l e - s o d i u m ion-complexes· I n agreement w i t h t h i s , i t was f o u n d r e c e n t l y (14) t h a t w i t h i n ­ c r e a s i n g S i / A l - r a t i o (n) t h e m o b i l i t y decreases a t f i r s t , b u t r e m a i n s c o n s t a n t above η = 2 . 4 3 where a l l s o d i u m i o n s a r e l o c a l i z e d , a n o t h e r model h a s been proposed b e f o r e (15), a c c o r d i n g t o w h i c h t h e d i f f e r e n c e i n m o b i l i t y i s caused by d i f f e r e n t e l e c t r i c f i e l d s i n NaX a n d NaX z e o l i t e s . B u t t h i s m o d e l w o u l d p r e d i c t a monotonous d e c r e a s e o f mobility with increasing values of η oppositet o the e x p e r i m e n t a l r e s u l t s . I n c o n t r a s t t o benzene, f o r w a t e r a t h i g h e r p o r e f i l l i n g f a c t o r s (Θ 0 . 5 ) the mobility o f a d s o r b e d m o l e c u l e s does n o t depend on n , s o that the reduced m o b i l i t y w i t h respect t o l i q u i d w a t e r c a n n o t be c a u s e d b y t h e sodium i o n s . T h i s i s c o n f i r m e d by two e x p e r i m e n t a l f i n d i n g s : F i r s t l y , e v e n i n c o n c e n t r a t e d aqueous s o l u t i o n s o f N a t h e m o b i l i t y o f water m o l e c u l e s i s only s l i g h t l y r e ­ duced a n d c o r r e s p o n d s a t room t e m p e r a t u r e t o a n increase o f c o r r e l a t i o n time from the f r e e water v a l u e 2 . 5 p s (16) t o o n l y about 5 p s (17) f o r a 2

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c

a

+

+

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water molecule i n the f i r s t c o o r d i n a t i o n sphere o f a sodium i o n . S e c o n d l y t h e s m a l l w i d t h o f t h e d i s ­ t r i b u t i o n f u n c t i o n f o r V (11), (12) i s e q u i v a l e n t to the statement t h a t the c o r r e l a t i o n time of a w a t e r m o l e c u l e a d j a c e n t t o a sodium i o n c a n n o t be v e r y d i f f e r e n t from the c o r r e l a t i o n time o f water m o l e c u l e s s u r r o u n d e d by oxygen atoms b e l o n g i n g t o the w a l l o f t h e s u p e r c a g e o r t o o t h e r w a t e r m o l e ­ c u l e s , because the e x p e r i m e n t a l l y determined h a l f width of the d i s t r i b u t i o n function ( f o r the distri­ b u t i o n p a r a m e t e r β a s d e f i n e d i n (18) i t has been found B é 1 ) corresponds t o a r a t i o o f c o r r e l a t i o n t i m e s o f l e s s t h a n about 5 · f h i s o n l y s m a l l change i n m o b i l i t y o f w a t e r m o l e c u l e s c a u s e d by N a i s due t o t h e s t r o n g w a t e r - w a t e r i n t e r a c t i o n (19) and t h e r e f o r e t h e r e d u c e d m o b i l i t y i n t h e a l s o r b e d s t a t e ( Τ \ =200 ps at room t e m p e r a t u r e ) must be a c o l l e c t i v e e f f e c t o f the r i g i d oxygen l a t t i c e o f t h e z e o l i t e w h i c h s t a b i l i z e s t h e ensemble o f w a t e r m o l e c u l e s i n front of i t (,"stabilization e f f e c t " (11)). To show d i r e c t l y t h e i n f l u e n c e o f s o d i u m i o n s on t h e m o b i l i t y o f Denzene, measurements o f p r o t o n r e l a x a t i o n t i m e s i n NaCeY 70 z e o l i t e have b e e n . p e r f o r m e d , where t h r o u g h an exchange o f 70 % Na by C e ^ t h e number o f sodium i o n s i n t h e s u p e r c a g e s has been d r a s t i c a l l y r e d u c e d ( c f . T a b l e 1 ) . A l t h o u g h t h e minimum o f T^ i s s h i f t e d t o extreme low t e m p e r a t u r e s ( t o l e s s t h a n -1^0°ϋ (19)) t h i s c a n n o t be t a k e n a s a p r o o f f o r a h i g h e r m o b i l i t y o f t h e a d s o r b e d benzene m o l e c u l e s , s i n c e i t i s an a r t i f a c t o f n u c l e a r m a g n e t i c r e l a x a t i o n method. The v a l u e s o f t h e r e l a x a t i o n t i m e s a r e significantly g r e a t e r t h a n i n NaY i n c o n t r a s t t o the u s u a l e f f e c t i f paramagnetic i o n s (Qe^*) a r e i n t r o d u c e d i n t o a sample. The r e a s o n i s t h a t Ce^ r e d u c e s by m a g n e t i c d i p o l e i n t e r a c t i o n the e l e c t r o n r e l a x a t i o n t i m e o f t h e p a r a m a g n e t i c i m p u r i t i e s which c o n t r o l (Pe3 ions located at A l ^ s i t e s of the z e o l i t e s k e l e t o n ) the proton r e l a x a t i o n o f a d s o r b e d benzene t o s u c h a v a l u e t h a t i s becomes shorter than the thermal c o r r e l a t i o n time (19). T h e r e f o r e a z e o l i t e was p r e p a r e d where t h e very s i m i l a r but d i a magnetic L a ^ , i n s t e a d of the p a r a m a g n e t i c (Je3 was i n t r o d u c e d . The r e s u l t , shown i n P i g . 1, i s i n f u i l agreement w i t h t h e above model, s i n c e one o b s e r v e s t h a t t ? a t room t e m p e r a t u r e d e c r e a s e s f r o m 50 n s f o r NaY t o 10 ns or t o ΝαΥ, η = 2.6; ·, ΝαΧ, η = 1.8; Χ, NaLaY74,n = 2.6

Resing and Wade; Magnetic Resonance in Colloid and Interface Science ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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introduced (NaX) or i f these and the l o c a l i z e d sodium ions are removed from the supercageâ (NaLaY 74). The broad d i s t r i b u t i o n of c o r r e l a t i o n times i n the l a t t e r case which can be deduced from the great value of T^/Tp at the minimum of T* may be due t o a few t i g h t l y bound benzene molecules (e.g. at the few ha+ s t i l l occupying s i t e s S I I ) and p a r t l y due t o n o n l o c a l i z e d adsorption o f the major part of adsorbate (small jump lengths during the course o f t r a n s l a t i o n a l motion). F i n a l l y i t shoula be mentioned that the benzene-sodium ion-complex suggested by these measurements was studied also t h e o r e t i c a l l y using the semiemyirical quantum chemical Ci\D0/2 method (20) . From energy minima of f u l l p o t e n t i a l curves tïïe optimum arrangement and s t a b i l i z a t i o n energy coula oe obtained. The π est favourable c o n r i g u r a t i o n has been ιουηα when the sodium i o n i s l y i n g on the s i x f o l d axis o f symmetry of the aromatic r i n g (3·2 8 j , because i n t h i s case there i s a good p o s s i b i l i t y f o r overlapping of the tree e l e c t r o n o r b i t a l s of N a with the if o r b i t a l s of the adsorbed molecules, ana an e l e c t r o n t r a n s f e r w.s found from the If system towara the c a t i o n . This mouel of benzene adsorption nas "been confirmed experimentally (21)fcnrar broad l i n e technique, i n s o f a r , as the independence o f second moments of proton resonance at 77 Κ on pore f i l l i n g f a c t o r s could be explained only by a f l a t arrangement o f benzene molecules i n f r o n t o f sodium ions occupying s i t e s S I I . +

c

References (1) (2) (3) (4) (5) (6) (7) (8) (9)

Kiselev, A . V . , Disc.Farad.Soc. (1966) 40, 205-218 Michel, D . , Thöring, J., Z.phys.Chemie (Leipzig) (1971) 247, 85-90 Derouane, E . G . , Mestdagh, M.M., Vielvoye, L., J.Catalysis (1974), 33, 169-175 Torrey, H . C . , Phys.Rev. (1953) 92, 962-969 Krüger, G.J., Z.Naturforsch. (1969), 24a, 560-565 Michel, D . , Thesis (Prom.B) Karl-Marx-Uni­ versität Leipzig (1973) Lorenz, P . , Diplomarbeit, Karl-Marx-Uni­ versität Leipzig (1974) Walter, Α . , Thesis i n prep. Karl-Marx-Uni­ versität Leipzig Nagel, Μ., Pfeifer, Η . , W i n k l e r , Η . , Ζ.phys.Chemie (Leipzig) (1974), 255, 283-292

Resing and Wade; Magnetic Resonance in Colloid and Interface Science ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

4. PFEIFER

Adsorption Phenomena in Zeolites

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(10)

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Pfeifer, Η . , NMR - Basic Principles and Progress Vol.7, p.53-153, Springer, New York 1972 (11) Pfeifer, Η . , Surface Phenomena Investigated by Nuclear Magnetic Resonance, Phys.Reports (in press) (12) Pfeifer, Η . , Gutsze, Α . , Shdanov, S . P . , Z.phys.Chemie (Leipzig) (in press) (13) Dzhigit, O.M., Kiselev, A.V. et al., Trans.Farad.Soc.(1971), 67, 458-467 (14) Lechert, H . , Haupt, W., Kacirek, H., Ζ . N a t u r f o r s c h . (1975), 30a, 1207-1210 (15) L e c h e r t , Η . , Hennig, H.J., Mirtsch, Sch., Surface S c i . (1974), 43, 88-100 (16) Hindman, J.C., Z i e l e n , A.J. et al., J.Chem.Phys. (1971), 54, 621-634 (17) Hertz, H . G . , Angew.Chemie (1970), 82, 91-106 (18) Nowick, A . S . , Berry, B . S . , JBM J.Res. Development (1961), 5, 297-320 (19) Geschke, D., Pfeifer, Η . , Z.phys.Chemie (Leipzig) (in press) (20) Geschke,D., Hoffmann, W.-D., Deininger, D . , Surface S c i . (in press) (21) Hoffmann, W.-D., Z.phys.Chemie (Leipzig) (in press)

Resing and Wade; Magnetic Resonance in Colloid and Interface Science ACS Symposium Series; American Chemical Society: Washington, DC, 1976.