Spectroscopic Studies of Zeolite Synthesis: Evidence for a Solid-State

12 Spectroscopic Studies of Zeolite Synthesis: Evidence for a Solid-State Mechanism B. D. McNICOL, G. T. POTT, K. R. LOOS, and N. MULDER

Downloaded by MONASH UNIV on April 13, 2016 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0121.ch012

Koninklijke/Shell-Laboratorium, Amsterdam, Badhuisweg 3, Amsterdam-N., The Netherlands

The crystallization of zeolites from alkaline aluminosilicate gels was studied by luminescence and Raman spectroscopy. Trace amounts of Fe ions substituted for Al in the tetrahedral aluminosilicate gel framework exhibit characteristic phosphorescence spectra, which have been used to follow the buildup of the zeolite framework. Phosphorescence spectra of exchanged Eu cations and Raman spectra of (CH)N cations present in the solid phase of the gel indicate that no zeolitic cages exist in this phase during the induction period. Raman spectra of the liquid phase of the gel system show only the presence of SiO2(OH)2- and Al(OH)4- anions. Our results demonstrate that crystallization of zeolites occurs within the solid phase of the gel, which is believed to consist of amorphous tetrahedral aluminosilicate species. 3+

3+

3+

+

34

'he m e c h a n i s m of zeolite synthesis f r o m a l k a l i n e a l u m i n o s i l i c a t e gels has c o n t i n u a l l y p u z z l e d zeolite chemists (1). O n e reason t h i s process has n o t been u n d e r s t o o d is t h e c o m p l i c a t e d r e a c t i o n m e d i u m f r o m w h i c h t h e zeolite c r y s t a l l i z e s — a n a m o r p h o u s s o d i u m a l u m i n o s i l i c a t e gel " s k e l e t o n " w i t h i n a l i q u i d phase c o n t a i n i n g s i m i l a r ions (2). U n t i l n o w , few p h y s i c a l t e c h n i q u e s h a v e b e e n s u i t a b l y used t o s t u d y s u c h a s y s t e m . T h e c r y s t a l l i z a t i o n c a n be f o l l o w e d b y x - r a y d i f f r a c t i o n (1,3), b u t i t is difficult t o find a t e c h n i q u e for t r a c i n g t h e processes w h i c h o c c u r d u r i n g t h e i n d u c t i o n p e r i o d before c r y s t a l l i z a t i o n . T h e b e h a v i o r of t h e s y s t e m d u r i n g i n d u c t i o n r e m a i n s a n e n i g m a a n d is t h e k e y t o u n d e r s t a n d i n g t h e m e c h a n i s m of zeolite synthesis. A

S e v e r a l proposals h a v e b e e n a d v a n c e d for t h i s m e c h a n i s m . O n t h e basis of e l e c t r o n m i c r o s c o p y studies a n d c h e m i c a l a n a l y s i s of a l u m i n o s i l i c a t e gels B r e c k a n d F l a n i g e n c o n c l u d e d t h a t c r y s t a l l i z a t i o n occurs f r o m t h e s o l i d gel phase (3, 4). T h e i n d u c t i o n p e r i o d was p o s t u l a t e d t o be a t i m e 152 Meier and Uytterhoeven; Molecular Sieves Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

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d u r i n g w h i c h t h e n u c l e i w h i c h f o r m e d i n t h e s o l i d phase g r e w i n size. T h i s v i e w was f u r t h e r s u b s t a n t i a t e d b y t h e fact t h a t t h e e l e m e n t a l c o m p o s i t i o n of t h e c r y s t a l l i n e zeolite was a l m o s t i d e n t i c a l t o t h a t of t h e i n i t i a l s o l i d phase e x t r a c t e d f r o m t h e gel (1).


A n a l t e r n a t i v e h y p o t h e s i s , d e v e l o p e d f r o m studies of t h e s y n t h e s i s of L i n d e A zeolite c a r r i e d o u t b y K e r r (6) a n d C i r i c (6), p o i n t e d t o g r o w t h o c c u r r i n g f r o m s o l u t i o n . T h e gel was b e l i e v e d t o be a t least p a r t i a l l y d i s s o l v e d i n s o l u t i o n , f o r m i n g a c t i v e a l u m i n o s i l i c a t e species as w e l l as s i l i c a t e a n d a l u m i n a t e ions. T h e s e species l i n k e d t o f o r m t h e basic b u i l d i n g b l o c k s of t h e zeolite s t r u c t u r e a n d r e t u r n e d to t h e s o l i d phase. A i e l l o et al. (7) f o l l o w e d t h e synthesis f r o m a h i g h l y a l k a l i n e clear a l u m i n o s i l i c a t e s o l u t i o n b y electron microscopy, electron diffraction, a n d x-ray diffraction. These a u t h o r s o b s e r v e d t h e f o r m a t i o n of t h i n plates (lamellae) of a m o r p h o u s a l u minosilicates prior to actual crystal formation. R e c e n t l y we s h o w e d t h a t s m a l l q u a n t i t i e s of F e + ions c a n s u b s t i t u t e for A l i n t h e t e t r a h e d r a l zeolite f r a m e w o r k a n d e x h i b i t c h a r a c t e r i s t i c phosphorescence a n d e x c i t a t i o n s p e c t r a (8) (see F i g u r e 1). W e used t h i s finding t o o b t a i n i n f o r m a t i o n r e g a r d i n g b u i l d u p of t h e a l u m i n o s i l i c a t e f r a m e w o r k d u r i n g synthesis. T o m o n i t o r changes i n t h e e n v i r o n m e n t of t h e exchangeable cations, N a + , we f o l l o w e d t h e changes i n t h e E u phosphorescence s p e c t r u m of a p a r t i a l l y E u + - e x c h a n g e d gel. R a m a n spectroscopy was used t o s t u d y t h e l i q u i d a n d s o l i d phases of zeolitic gels. T h i s t e c h n i q u e is g o o d for s t u d y i n g aqueous systems a n d for d e t e c t i n g t h e presence of c o m p l e x ions s u c h as C 1 0 ~ a n d ( C H ) N w h i c h m i g h t be o c c l u d e d i n zeolitic cages a n d t h e r e b y serve as R a m a n probes (9). 3

3

+

3

+

3

4

3

4

+

Experimental M a t e r i a l s a n d P h o s p h o r e s c e n c e . T h e gel w a s g e n e r a l l y p r e p a r e d b y m i x i n g aqueous solutions of B a k e r A n a l y z e d A l ( O H ) a n d N a O H w i t h solutions of M a l l i n c k r o d t S i 0 - x H 0 a n d N a O H . T r a c e a m o u n t s of F e ( ~ 0 . 0 1 w t % o n t o t a l solids) were a d d e d as F e - d o p e d S i 0 . F o r t h e L i n d e A synthesis c o m p r i s i n g m o s t of o u r e x p e r i m e n t s , a gel of c o m p o s i t i o n 6 N a s O - A l s O i - 1 1 . 7 S i O - 3 7 0 H 0 was used. G e l formation occurred almost immediately at r o o m temperature. T h e gel was t h e n h e a t e d t o b o i l i n g (ca. 105°C) a n d refluxed. G e l samples of a b o u t 0.5 m l w e r e ext r a c t e d a t v a r i o u s t i m e s , i n s e r t e d i n t o 2 - m m i d q u a r t z tubes, a n d q u e n c h e d i n l i q u i d n i t r o g e n . Phosphorescence a n d / o r e x c i t a t i o n s p e c t r a were m e a s u r e d o n a sensitive B e c q u e r e l - t y p e phosphorescence spectrometer described p r e v i o u s l y (10). I n t h i s w a y gels t r a n s f o r m i n g m o s t l y t o L i n d e A , b u t also t o sodalite, zeolite X , zeolite P , a n d a s y n t h e t i c g m e l i n i t e , w e r e s t u d i e d . T h e L i n d e A s y s t e m was also s t u d i e d i n D 0 solutions t o m i n i m i z e radiationless t r a n s i t i o n s of h y d r o x y l a t e d F e ions. For the E u phosphorescence studies special p r e c a u t i o n s h a d t o be t a k e n since E u salts p r e c i p i t a t e as E u ( O H ) w h e n a d d e d t o basic s o l u tions. I n addition, the E u phosphorescence i n H 0 is w e a k because of a h i g h p r o b a b i l i t y for radiationless t r a n s i t i o n s (11). T h i s c a n be o v e r c o m e b y u s i n g D 0 as s o l v e n t i n s t e a d of H 0 . A n u n d o p e d zeolite gel was p r e 3

2

3 +

2

3 +

2

2

2

2

3 +

3 +

3 +

3

3 +

2

2

2

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

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p a r e d i n D 0 , a n d samples were e x t r a c t e d , c e n t r i f u g e d , a n d w a s h e d w i t h D 0 t o p H 8. T h e w a s h e d gel was s t i r r e d w i t h a k n o w n v o l u m e of 0 . 0 5 M E u ( N 0 ) s / D 0 s o l u t i o n f o r five m i n u t e s , c e n t r i f u g e d a g a i n , w a s h e d t h o r o u g h l y a g a i n , a n d p l a c e d i n t o 2 - m m i d q u a r t z tubes. Q u e n c h i n g i n l i q u i d n i t r o g e n was n o t necessary because t h e E u luminescence c o u l d b e m e a s u r e d at r o o m t e m p e r a t u r e . L a s e r R a m a n S t u d i e s . G e l samples were e x t r a c t e d a t v a r i o u s t i m e i n t e r v a l s a n d c e n t r i f u g e d . T h e l i q u i d f r a c t i o n w a s t h e n separated a n d s t o r e d a t 0 ° C u n t i l m e a s u r e m e n t . W h e n t h e s p e c t r u m of t h e s o l i d phase was r e q u i r e d , t h e s a m p l e was first w a s h e d a n d t h e n d r i e d a t 1 2 0 ° C . W e s t u d i e d t h e synthesis of L i n d e A u s i n g t h e L i n d e A recipe b u t r e p l a c i n g 6 0 % of t h e N a + ions b y ( C H ) N + as ( C H ) N O H , together w i t h a s l i g h t excess of S i 0 . A recipe f o r C I O 4 " - c o n t a i n i n g sodalite b y C o l e a n d B a r r e r (12) w a s used i n studies of sodalite c r y s t a l l i z a t i o n . R a m a n s p e c t r a were m e a s u r e d w i t h a S p e x R a m a l o g spectrometer e q u i p p e d w i t h a coherent r a d i a t i o n m o d e l 52 a r g o n i o n laser. S i n c e m o s t b a c k g r o u n d fluorescence o c c u r r i n g i n t h i s t y p e of s a m p l e was caused b y t r a n s i t i o n m e t a l i m p u r i t i e s , especially F e , b u t also C r + , M n + , a n d M n , we u s e d s t a r t i n g m a t e r i a l s designed t o give zeolites c o n t a i n i n g < 1 0 p p m of t r a n s i t i o n m e t a l i m p u r i t i e s . T h e r e p r o d u c i b i l i t y of t h e R a m a n t e c h n i q u e was w i t h i n 1 0 % , a n d t h e d e t e c t i o n l i m i t for a l u m i n a t e a n d s i l i c a t e anions ~ 0 . 1 w t % . 2

2

3

2

3 +

3

4

3

4


2

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3

2

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Results Phosphorescence.

H 0. 2

PHOSPHORESCENCE OF F E

3 +

-DOPED

In

GELS.

I n a l l systems s t u d i e d t h e phosphorescence of t h e i n i t i a l

Fe

3 +

-

d o p e d gel showed a w e a k e m i s s i o n at 690 n m w i t h a shoulder at ca. 720 n m . E m i s s i o n a n d e x c i t a t i o n s p e c t r a were s i m i l a r t o those s h o w n i n F i g u r e 1. T h e phosphorescence comes f r o m t h e 141 other transitions i n the F e

3 +

4

7 i transition while the various

i o n i n t e t r a h e d r a l c o o r d i n a t i o n are a p p a r e n t

i n t h e e x c i t a t i o n s p e c t r u m (8, 10).

Throughout the induction period

e m i s s i o n i n t e n s i t y increased o n l y s l i g h t l y , t h e peak a t 690 n m i n c r e a s i n g s o m e w h a t m o r e t h a n t h e shoulder a t 720 n m .

A f t e r induction the signal

rose r a p i d l y u n t i l a c o n s t a n t i n t e n s i t y was a t t a i n e d .

T h e r a p i d increase

i n i n t e n s i t y of t h e phosphorescence s i g n a l after i n d u c t i o n ended p a r a l l e l e d t h e i n t e n s i t y increase of t h e x - r a y d i f f r a c t i o n lines. I n L i n d e A a n d sodalite syntheses t h e s i g n a l g r e w t o a b o u t 20 t i m e s its i n i t i a l i n t e n s i t y .

I n other systems, s u c h as f a u j a s i t e , t h e increase

was s o m e w h a t s m a l l e r .

T h e increase seemed t o d e p e n d u p o n t h e S i / A l

r a t i o of t h e r e s u l t a n t zeolite c r y s t a l s — i . e . , t h e smallest increase o c c u r r e d for m o r d e n i t e c r y s t a l l i z a t i o n s h a v i n g a n S i / A l r a t i o of 5 (for L i n d e A a n d sodalite S i / A l = 1). phase of t h e gel. ditions F e

3 +

No F e

3 +

phosphorescence was o b s e r v e d i n t h e l i q u i d

I n three e x p e r i m e n t s c a r r i e d o u t u n d e r i d e n t i c a l c o n -

phosphorescence studies of t h e g r o w t h k i n e t i c s gave i d e n t i c a l

r e s u l t s ( i n d u c t i o n periods e q u a l w i t h i n 5 % , F e

3 +

i n t e n s i t y increase o n

crystallization equal within 10%). In D 0. 2

O n l y t h e L i n d e A s y s t e m was s t u d i e d w i t h D 0 as a s o l v e n t . 2

I n i t i a l l y , a n emission b a n d p e a k i n g a t 7 2 0 n m w i t h a shoulder a t 690



12.

MCNicoL

E T AL.

Figure 1.

155


Phosphorescence (a) and excitation (b) spectra of Fe zeolites

i+

Figure 2.

Fe

z+

phosphorescence of Linde A/D 0 system 2

(a) Î minute after gel formation at room temperature (b) after 90 minutes boiling under reflux (c) after 120 minutes boiling under reflux


in

156

MOLECULAR

SIEVES

n m ( F i g u r e 2a) was m e a s u r e d i n t h e gel. T h e e x c i t a t i o n s p e c t r u m d i f fered s u b s t a n t i a l l y f r o m t h e s p e c t r u m measured i n H 0 . D u r i n g t h e p e r i o d r e q u i r e d t o heat t h e gel t o b o i l i n g the 6 9 0 - n m shoulder became more p r o n o u n c e d . A s i m i l a r effect o c c u r r e d w h e n the gel was a l l o w e d t o " a g e " o v e r n i g h t a t r o o m t e m p e r a t u r e . T h e r e l a t i v e increase of t h e shoulder c o n t i n u e d t h r o u g h o u t t h e i n d u c t i o n . A t t h e onset of c r y s t a l l i z a t i o n the 2

6 9 0 - n m a n d 7 2 0 - n m b a n d s were a l m o s t e q u a l l y intense ( F i g u r e 2b) whereas i n t h e f u l l y c r y s t a l l i z e d m a t e r i a l the 6 9 0 - n m b a n d was stronger ( F i g u r e 2c), a n d t h e phosphorescence a n d e x c i t a t i o n s p e c t r a were essentially i d e n t i c a l t o those measured i n H 0 systems. 2

E u + PHOSPHORESCENCE. T h e s p e c t r u m of t h e s t a r t i n g gel s h o w i n g the c h a r a c t e r i s t i c E u phosphorescence is i l l u s t r a t e d i n F i g u r e 3. D o m i n a t i n g i n the s p e c t r u m are the D Fi, Do F and D F bands at 595, 618, a n d 700 n m , r e s p e c t i v e l y . T h e 5 9 5 - n m b a n d is a m a g n e t i c d i pole t r a n s i t i o n , t h e other t w o b e i n g electric dipole t r a n s i t i o n s (11). A change i n s y m m e t r y of t h e E u site w o u l d be reflected i n a n a l t e r e d i n t e n s i t y d i s t r i b u t i o n between t h e t w o electric dipole a n d the m a g n e t i c d i pole t r a n s i t i o n s . 3

3 +


5

0

7

5

7

2)

5

7

0

4

3 +

E x t e n s i v e washings of t h e gel w i t h D 0 h a d no influence o n t h e s i g n a l shape or i n t e n s i t y , i n d i c a t i n g t h a t the E u was b o u n d t o t h e gel. T h r o u g h o u t i n d u c t i o n no changes i n the s p e c t r u m were detectable. Only when cryst a l l i z a t i o n o c c u r r e d , r e s u l t i n g i n the f o r m a t i o n of L i n d e A , d i d a significant change i n t h e i n t e n s i t y d i s t r i b u t i o n o v e r t h e b a n d envelope appear (see F i g u r e 3). T h u s , t h e site s y m m e t r y of t h e E u changes a t t h e e n d of t h e induction period. 2

3 +

3 +

LASER

RAMAN

SPECTROSCOPY.

Crystal

growth

in

the

Linde

A,

sodalite, a n d faujasite systems was s t u d i e d . S p e c t r a of the silicate s t a r t i n g solutions f e a t u r e d b a n d s a t 772 a n d 925 c m associated w i t h m o n o m e r i c - 1

S i 0 ( O H ) ~ species (13), a n d t h a t of the a l u m i n a t e s o l u t i o n showed one b a n d a t 618 c m f r o m A l ( O H ) ~ (14)· T h e s e results d e m o n s t r a t e t h a t u n d e r t h e c o n d i t i o n s we used to prepare the gels o n l y m o n o m e r i c silicate a n d a l u m i n a t e ions were present i n i t i a l l y . [This is b y no means t h e case i n other zeolite syntheses. F r e q u e n t l y a source of silicate anions is used w h i c h contains significant a m o u n t s of p o l y m e r i c silicate anions i n a d d i t i o n to t h e m o n o m e r i c f o r m . I n p a r t i c u l a r , systems s t u d i e d b y K e r r (15), w h o used s o d i u m m e t a s i l i c a t e as the S i 0 source, h a v e been s h o w n b y us t o consist of m i x t u r e s of m o n o m e r i c a n d p o l y m e r i c s i l i c a t e anions. T h e effect of v a r y i n g the r e l a t i v e concentrations of these components a n d t h e i r r e l a t i o n s h i p to t h e m o n o m e r i c systems s t u d i e d here, w h i l e of possible i n terest for f u r t h e r s t u d y , has n o t been i n v e s t i g a t e d i n d e t a i l here. ] 2

2

2

- 1

4

2

U p o n gel f o r m a t i o n after m i x i n g the silicate a n d a l u m i n a t e solutions the l i q u i d phase s t i l l p r o d u c e d t h e b a n d s a t t r i b u t e d t o the i n i t i a l l y present silicate a n d a l u m i n a t e anions, b u t these were m u c h l o w e r i n i n t e n s i t y . T h r o u g h o u t the i n d u c t i o n t h e i n t e n s i t y of these b a n d s r e m a i n e d c o n s t a n t


12.

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MCNICOL E T A L .

Eu * IN GEL 5

Eu * IN CRYSTALS 3

tNTENSTTY (ARBITRARY UNITS)


ft

I

ι 600

Figure 3. Eu

3+

ι 700

ι 800 WAVELENGTH,ηm

phosphorescence in gel and Linde A crystals, measured at room temperature

i n t h e l i q u i d - p h a s e spectra. F u r t h e r , n o evidence for the existence of a n a l u m i n o s i l i c a t e a n i o n i c species was f o u n d . T h e s p e c t r a d i d n o t change even after c r y s t a l l i z a t i o n h a d o c c u r r e d . S p e c t r a of t h e s o l i d phase began t o r e v e a l discrete b a n d s associated w i t h the zeolite f r a m e w o r k after c r y s t a l lization h a d commenced. I n the synthesis of L i n d e A c o n t a i n i n g ( C H ) N O H n o changes were detected i n the l i q u i d - p h a s e s p e c t r u m t h r o u g h o u t i n d u c t i o n a n d c r y s t a l l i z a t i o n . D u r i n g i n d u c t i o n t h e s o l i d phase gave rise t o a w e a k b a n d a t 754 c m " " , w h i c h d i d n o t d i s a p p e a r even after w a s h i n g t o p H 8. T h i s b a n d r e m a i n e d c o n s t a n t t h r o u g h o u t i n d u c t i o n a n d was r e p l a c e d b y a n e w b a n d a t 768 c m a t t h e onset of c r y s t a l l i z a t i o n (evidenced b y the rise i n Fe phosphorescence signal). T h i s b a n d t h e n increased i n i n t e n s i t y as the 7 5 4 - c m " b a n d d i m i n i s h e d , u n t i l u l t i m a t e l y u p o n complete c r y s t a l l i z a t i o n o n l y t h e 7 6 8 - c m b a n d r e m a i n e d ( F i g u r e 4). I n contrast, s p e c t r a o b t a i n e d of the C 1 0 " - c o n t a i n i n g sodalite s y s t e m d i d n o t s h o w a n y b a n d for t h e w a s h e d s o l i d phases of t h e gel d u r i n g i n d u c t i o n . A t the onset of c r y s t a l l i z a t i o n a C10 ~~ b a n d was o b s e r v e d ; i t grew 3

4

1

- 1

3 +

1

- 1

4

4


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i n intensity, again paralleling the F e

3 +

phosphorescence s p e c t r u m t h r o u g h -

o u t t h e c r y s t a l l i z a t i o n u n t i l a m a x i m u m was reached a t t h e e n d o f c r y s t a l lization.

T h i s C I O 4 - s i g n a l d i d n o t disappear u p o n repeated washings.

Discussion T h e L i q u i d P h a s e . T h e l i q u i d phase consisted of a s o l u t i o n of A l ( O H ) ~ , S i 0 ( O H ) ~ , N a , a n d 0 H ~ ions, whose concentrations (determ i n e d b y R a m a n spectroscopy) d i d n o t change s i g n i f i c a n t l y d u r i n g i n d u c t i o n a n d c r y s t a l l i z a t i o n . N o evidence w a s f o u n d for t h e existence of a n y soluble a l u m i n o s i l i c a t e anions. 4

2

2

2

+


INTENSITY (ARBITRARY UNITS)

750

500

250 WAVE NUMBER, cm" 1

Figure 4.

Raman spectra of crystallizing gels: (CHs)JÏ[+/Linde A system

T h e S o l i d P h a s e . T h e R a m a n spectra of the L i n d e A systems c o n t a i n i n g ( C H ) N showed a s i g n a l at 754 c m w h i c h d i d n o t disappear after repeated gel washings. T h i s b a n d was caused b y t h e s y m m e t r i c s t r e t c h i n g of a l l f o u r N - C H b o n d s a n d o c c u r r e d v i r t u a l l y a t t h e same p o s i t i o n as f o u n d for aqueous solutions o f ( C H ) N (752 c m ) . However, the fact t h a t i t r e m a i n e d e v e n after w a s h i n g t o p H 8 i n d i c a t e s t h a t t h i s ( C H ) N + species was b o u n d t o t h e gel n e t w o r k . I n t h e synthesis of C 1 0 ~ - d o p e d sodalite n o R a m a n s p e c t r u m of C 1 0 ~ was observed for t h e w a s h e d gel. T h i s i n d i c a t e s t h a t C 1 0 ~ was n o t associated w i t h t h e gel n e t w o r k as was the case for ( C H ) N ; t h i s i s n o t unexpected since r e t e n t i o n of C 1 0 ~ w o u l d be e l e c t r o s t a t i c a l l y u n f a v o r a b l e . 3

4

+

- 1

3

3

4

+

- 1

3

4

4

4

4

3

4

+


4

12.


MCNICOL E T A L .

159

E x c h a n g e of E u for N a was d e m o n s t r a t e d b y t h e phosphorescence s p e c t r a o b t a i n e d for these systems, w h i c h also showed t h a t t h e s p e c t r u m d i d n o t change d u r i n g i n d u c t i o n ; t h i s suggests t h a t n o change i n s y m m e t r y o c c u r r e d a t the c a t i o n sites d u r i n g t h i s t i m e . 3 +

+

F e + phosphorescence studies r e v e a l e d t h a t i m m e d i a t e l y u p o n t h e f o r m a t i o n of the F e - d o p e d a l u m i n o s i l i c a t e gel a c h a r a c t e r i s t i c p h o s p h o r e s cence s i g n a l was generated. W e h a v e s h o w n t h a t s m a l l q u a n t i t i e s of F e s u b s t i t u t e for A l i n t h e f r a m e w o r k of zeolites (8) a n d give s i m i l a r s p e c t r a (i.e., a p e a k a t 690 n m a n d a s h o u l d e r a t 720 n m ) . T h e s h o u l d e r a t 720 n m decreased i n i n t e n s i t y , g o i n g f r o m t h e f r e s h l y p r e p a r e d gel t o t h e e n d of i n d u c t i o n a n d decreased a g a i n w h e n the gel was t r a n s f o r m e d i n t o c r y s t a l l i n e zeolite. T w o sites are possible for t h e F e present i n t h e d o p e d g e l : i t can be c o m p l e t e l y c o o r d i n a t e d t o - O - S i groups i n t h e b u l k of t h e gel, o r i t can be o n t h e surface of t h e gel p a r t i a l l y c o o r d i n a t e d t o O H groups. These partially hydroxylated F e ions are i n a c r y s t a l field different f r o m t h e b u l k Fe ions, a n d we expect a different phosphorescence s p e c t r u m . I t is t h u s possible t h a t t h e p e a k a t 690 n m is caused b y t e t r a h e d r a l l y c o o r d i nated F e i n t h e b u l k gel, F e ( O S i ) ~ whereas t h e 720 n m s h o u l d e r comes f r o m t e t r a h e d r a l l y c o o r d i n a t e d F e + o n t h e surface—e.g., F e ( O S i ) ( O H ) " ~ or F e ( O S i ) O H ~ . T h i s h y p o t h e s i s was c o n f i r m e d b y phosphorescence measurements i n D 0 systems. R e p l a c e m e n t of O H b y O D is expected t o reduce t h e r a d i a t i o n l e s s t r a n s i t i o n s i n the h y d r o x y l a t e d F e ions a n d t h e r e fore w i l l increase t h e luminescence i n t e n s i t y . A c c o r d i n g t o t h e R o b i n s o n F o r s c h t h e o r y (16) r a d i a t i o n l e s s t r a n s i t i o n s are p r o v i d e d b y a s m a l l b u t finite o v e r l a p of v i b r a t i o n a l eigenfunctions of g r o u n d a n d e x c i t e d states, a n d s u c h o v e r l a p is f a v o r e d b y h i g h a m p l i t u d e v i b r a t i o n a l m a n i f o l d s i n t h e d i r e c t e n v i r o n m e n t of t h e center. H i g h energy i n t r a m o l e c u l a r v i b r a t i o n s i n v o l v i n g h y d r o g e n a t o m s , s u c h as t h e O H s t r e t c h i n g v i b r a t i o n , are p r e d i c t e d t o be v e r y a c t i v e i n t h i s respect a n d w o u l d be expected t o r e s u l t i n increased n o n - r a d i a t i v e t r a n s i t i o n s for t h e h y d r o x y l a t e d F e ions. T h e effect of r e p l a c i n g O H b y O D is t o l o w e r t h e f r e q u e n c y of t h e i n t r a m o l e c u l a r s t r e t c h i n g v i b r a t i o n s a n d p r e s u m a b l y t o d i m i n i s h t h e effectiveness of t h i s p a t h w a y t o r a d i a t i o n l e s s t r a n s i t i o n s . T h u s , one finds t h a t E u i n D 0 gives a n intense phosphorescence whereas t h e e m i s s i o n of E u in H 0 is h a r d l y detectable. F i g u r e 2 shows t h a t for s p e c t r a o b t a i n e d i n D 0 t h e 7 2 0 - n m b a n d increases i n i n t e n s i t y , t h u s i n d i c a t i n g t h a t i t c a n be assigned t o a p a r t i a l l y h y d r o x y l a t e d f o u r - c o o r d i n a t e d F e + center. 3

3 +

3 +

3

+


3 +

3 +

3 +

3 +

4

3

2

2

3

2

3 +

3 +

3 +

3

2

+

2

2

3

F r o m o u r e x p e r i m e n t a l results we a r r i v e at t h e f o l l o w i n g p i c t u r e of t h e gel d u r i n g i n d u c t i o n . T h e i n i t i a l l y f o r m e d gel c o n t a i n s s m a l l a m o r phous a l u m i n o s i l i c a t e species w h i c h g r o w b y c o n d e n s a t i o n of t e r m i n a l h y d r o x y l a t e d centers u p o n a g i n g a n d h e a t i n g . T h i s is e v i d e n t f r o m t h e r e l a t i v e increase of b u l k F e centers (690 n m ) w i t h r e g a r d t o t h e n u m b e r of surface ( h y d r o x y l a t e d ) F e centers (720 n m ) ( F i g u r e 2). D u r i n g i n d u c t i o n t h e particles c o n t i n u e t o g r o w w i t h o u t a n y cage f o r m a t i o n ; t h i s is 3 +

3 +


160

MOLECULAR SIEVES +

indicated by the lack of evidence for occluded CIO4- or (CH )4N ions and the absence of Raman bands from zeolite framework vibrations. Chemical analysis shows that the number of Na+ ions equal the number of Al + ions, and therefore, the gel framework consists exclusively of tetrahedral Si0 , A10 ~, and hydroxylated A l units balanced by N a ions which are exchangeable, as in zeolites. Crystallization. At the onset of crystallization we observed an ac celerated increase of the 690-nm F e phosphorescence band at the ex pense of the 720-nm band (Figure 2). This is a consequence of the con densation of the hydroxylated tetrahedra to non-hydroxylated tetrahedra in the zeolite crystals, which have a much smaller fraction of external hydroxylated tetrahedra. Replacement of the (CH ) N+ band at 754 c m by a new band at 768 c m at the onset of crystallization indicates that the ( C H ) N is now being occluded into the sodalite cages of Linde A. Previous studies of the Raman spectra of (CH ) N+ ions occluded in cages of different zeolites showed that this band was always shifted to higher fre quency as a result of the restrictions imposed by the zeolite cages (9). A similar occlusion occurs for the C10 ~ ion in the sodalite system; the only difference here is that no C10 ~" was associated with the gel, as was found for (CH ) N+. These results parallel those obtained from the E u phos phorescence studies, which also indicate a change in symmetry of the E u site upon crystallization, consistent with the hypothesis of cage formation at this point. Our results support a crystallization mechanism which occurs in the solid phase of the gel. We do not find any evidence for changes either in concentration or composition of the liquid phase throughout the induction and crystallization periods. Further, no evidence was found for cage-like building blocks in solution or in the solid before crystallization began. The solid phase is believed to consist of amorphous aluminosilicate species which grow during induction until critical nuclei, or seed crystals, are formed, whereupon rapid crystallization of the remaining mass occurs. 3

3

3 +

4

+

4

3 +

3


- 1

4

- 1

+

3

4

3

4

4

4

3 +

3

4

3 +

Literature Cited 1.

Zhdanov, S. P., ADVAN. CHEM. SER. (1971) 101, 20.

2. Fahlke, B., Wieker, W., Thilo, Ε., Z. Anorg. Allgem. Chem. (1966) 347, 82. 3. Breck, D. W., Flanigen, Ε. M., "Molecular Sieves," p. 47, Society of Chemical Industry, London, 1968. 4. Flanigen, Ε. M., Breck, D. W., "Abstracts of Papers," 137th Meeting, ACS, Cleveland, 1960. 5. Kerr, G. T., J. Phys. Chem. (1966) 70, 1947. 6. Ciric, J., J. Colloid Interface Sci. (1968) 28, 315. 7. Aiello, R., Barrer, R. M., Kerr, I. S., ADVAN. CHEM. SER. (1971) 101, 44. 8. Pott, G. T., McNicol, B. D., Chem. Phys. Lett. (1971) 12, 62. McNicol, B. D., Pott, G. T., J. Catalyst (1972) 25, 223. McNicol, B. D., Pott, G. T., Loos, K. R., J. Phys. Chem. (1972) 76, 3388. 9. Loos, K. R., Cole, J. F., unpublished results.


12. MCNICOL ET AL.


161

10. Pott, G. T., McNicol, B. D., J. Chem. Phys. (1972) 56, 5246. 11. Haas, Y., Stein, G., J. Phys. Chem. (1971) 75, 3668. 12. Barrer, R. M., Cole, J. F., J. Chem. Soc. A (1970) 1516. 13. Fortnum, D., Edwards, S.O.,J.Inorg. Nucl. Chem. (1955) 2, 264. 14. Moolenaar, R. J., Evans, J. C., McKeever, L. D., J. Phys. Chem. (1970) 74, 3629. 15. Kerr, G. T., J. Phys. Chem. (1968) 72, 1385. 16. Robinson, G. W., Frosch, R. P., J. Chem. Phys. (1962) 37, 1962.


RECEIVED November 24, 1972.


Spectroscopic Studies of Zeolite Synthesis: Evidence for a Solid-State

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