MÃ¶ssbauer and Magnetic Studies of Bifunctional Medium-Pore Zeolite

27 Mössbauer and Magnetic Studies of Bifunctional Medium-Pore Zeolite-Iron

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Catalysts Used i n Synthesis Gas Conversion C A R Y LO—Physics Department, Pennsylvania State University, University Park, P A 16802 K. R. P. M . R A O and L . N . MULAY —Department of Materials Science and Engineering, 136 Materials Research Laboratory, Pennsylvania State University, University Park, P A 16802 1

2

V. U . S. R A O , R. T . OBERMYER , and R. J. GORMLEY—Pittsburgh Energy Technology Center, U.S. Department of Energy, P.O. Box 10940, Pittsburgh, P A 15236 3

Medium-pore (diameter ~ 6 Å) zeolites such as ZSM-5 and

Silicalite impregnated with Group VIII metals provide selective catalytic pathways for the conversion of coal derived synthesis gas to gasoline or olefins. Mössbauer and magnetic studies on these catalysts containing iron or iron plus cobalt are reported. The zeolites were impregnated with metal nitrate solutions, reduced, and carbided to yield the active catalyst. The freshly impregnated samples showed Fe type spectra. The ZSM-5 (14.7% Fe) and Silicalite (13.6% Fe) samples exposed to H (450°C) showed an approximate 85% reduction to the metallic state. The carbided ZSM-5 (14.7% Fe) revealed a spectrum of Hagg carbide (Fe C ), an active component of the catalyst. The used catalysts showed mixtures of Hagg carbide (Fe C ) and cementite (Fe C). It is suggested that the selectivity of ZSM-5 (5.6% Fe, 4.5% Co) resulted from iron-cobalt alloy formation. 3+

2

5

2

5

2

3

Current address: Bhabha Atomic Research Center, Trombay, Bombay, India. * Address inquiries to this author. Current address: Physics Department, Pennsylvania State University, McKeesport, PA 15132. 1

3

0065-2393/81/0194-0573$05.00/0 © 1981 American Chemical Society Stevens and Shenoy; Mössbauer Spectroscopy and Its Chemical Applications Advances in Chemistry; American Chemical Society: Washington, DC, 1981.

574

MOSSBAUER SPECTROSCOPY A N D ITS C H E M I C A L A P P L I C A T I O N S

l

he interest of M u l a y ' s g r o u p ( P e n n State U n i v e r s i t y ) i n t h i s c h a l -

-•- l e n g i n g a r e a of catalysis r e s e a r c h s t e m m e d f r o m h i s e a r l i e r b a s i c r e s e a r c h (1-4)

o n s u p e r p a r a m a g n e t i c dispersions of a - F e 0 . 2

This dis

3

p e r s i o n w a s o b t a i n e d b y a n o v e l t e c h n i q u e , w h i c h c o n s i s t e d of d u c i n g v a p o r s of F e ( C O )

5

intro

[ m a x i m u m dimension ~ 9 A ] into the cage

s t r u c t u r e of L i n d e - 1 3 X z e o l i t e , v i a its a p e r t u r e of a b o u t 10 A . T h e s u p e r paramagnetism

of

the

dispersions

was

elucidated by

m e a s u r e m e n t s as a f u n c t i o n of t h e field (H) as

by

5 7

Fe

Mossbauer

spectroscopy.

magnetization

a n d t e m p e r a t u r e (T)

The

technological

as w e l l

interest

V . U . S. Rao's g r o u p ( P i t t s b u r g h E n e r g y T e c h n o l o g y C e n t e r )

of

centered

a r o u n d m a k i n g r e l a t i v e l y m o r e efficient z e o l i t e - S i l i c a l i t e - b a s e d catalysts Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: July 1, 1981 | doi: 10.1021/ba-1981-0194.ch027

f o r synthesis gas c o n v e r s i o n .

T h e o v e r l a p p i n g of t h e interests of these

t w o r e s e a r c h g r o u p s has l e d to a f r u i t f u l c o l l a b o r a t i o n , w h i c h exemplifies i n t e r d i s c i p l i n a r y as w e l l as i n t e r o r g a n i z a t i o n a l efforts b e t w e e n a f e d e r a l and a university laboratory. T h e c a t a l y t i c c o n v e r s i o n of c o a l - d e r i v e d synthesis gas ( C O + to gasoline-range catalysts (5-7)

hydrocarbons

2

is of m u c h interest t o d a y . T h e m e d i u m - p o r e ( d i a m e t e r

6 A ) z e o l i t e Z S M - 5 i n c o m b i n a t i o n w i t h i r o n (5,6) (7)

H )

a n d olefins u s i n g b i f u n c t i o n a l z e o l i t e ^

or w i t h i r o n - c o b a l t

w a s s h o w n t o y i e l d a h i g h f r a c t i o n of a r o m a t i c s i n t h e p r o d u c t ,

resulting i n a favorable octane n u m b e r ( >

It was indicated

80).

(7)

t h a t t h e b i m e t a l l i c i r o n - c o b a l t o n Z S M - 5 c o u l d a l t e r t h e p r o d u c t selec t i v i t y , m a i n l y i n r e d u c i n g t h e shift c o n v e r s i o n of C O + H 0 to C 0 2

2

from

t h e h i g h shift y i e l d s of i r o n o n Z S M - 5 . T h e a r o m a t i c s i n t h e p r o d u c t d e c r e a s e d b y the a d d i t i o n of c o b a l t to t h e z e o l i t e

(7).

I n contrast t o Z S M - 5 , S i l i c a l i t e l a c k s a l u m i n u m , a l t h o u g h t h e t w o a p p e a r to possess s i m i l a r c r y s t a l structures ( 8 , 9 ) . T h e y c r y s t a l l i z e w i t h t h e o r t h o r h o m b i c space g r o u p P n m a or Pn2ia w i t h a = A; c =

13.4 A . T h e f r a m e w o r k s t r u c t u r e consists of

20.1 A ; b =

five-membered

19.9 rings

of S i ( A l ) - 0 t e t r a h e d r a . T h e p o r e s t r u c t u r e ( F i g u r e 1) consists of i n t e r -

Elliptical 10-ring straight channel (5.7 % x 5.1 1)

Near-circular 10-ring zig-zag channel (dia 5.4,

Figure 1.

Possible model of the pore structure of ZSM-5

and

Silicalite

Stevens and Shenoy; Mössbauer Spectroscopy and Its Chemical Applications Advances in Chemistry; American Chemical Society: Washington, DC, 1981.

27.

Medium-Pore

LO ET AL.

Table I .

Zeolite-Iron

Catalysts

575

Comparison of Z S M - 5 and Silicalite ZSM-5

Composition

Na,(A10 ).(Si0 )ioo-. x = 1 t o 25

Absent

Orthorhombic a = 20.1 A b = 19.9 A c = 13.3 A

Orthorhombic a = 20.06 A 6 = 19.80 A c = 13.36 A


Structure

Structure

Sorption

of

Thermal

4

+

+

S t r a i g h t channels a l o n g b - a x i s . Zig-zag along a-axis. Pore diameter = 6 A .

H0 2

Stability

2

x = 0

Present EXCH Calcine Na >NH > H _ +

Pore

Si0

2

2

Ion Exchange properties Crystal

Silicalite

Same as Z S M - 5

L o w for h i g h s i l i c o n content

Low

Increases w i t h s i l i c o n content

High

s e c t i n g c h a n n e l s d e f i n e d b y t e n rings of o x y g e n atoms.

T h e elliptical

s t r a i g h t c h a n n e l s of cross section 5.7 A X 5.1 A a l o n g t h e &-axis a n d t h e c i r c u l a r z i g z a g channels

of d i a m e t e r

5.4 A i n t e r c o n n e c t

the straight

channels. W h i l e the s i l i c o n / a l u m i n u m ratio i n Z S M - 5 can be varied from 3 to o v e r 100, S i l i c a l i t e has essentially n o a l u m i n u m . H e n c e i t appears t h a t S i l i c a l i t e is t h e l i m i t i n g f o r m of Z S M - 5 w h e n t h e a l u m i n u m c o n c e n t r a t i o n is v a n i s h i n g l y s m a l l .

A comparison

of t h e p r o p e r t i e s

of Z S M - 5 a n d

S i l i c a l i t e is s h o w n i n T a b l e I . O w i n g to t h e absence of cations t h a t c a n b e e x c h a n g e d w i t h p r o t o n s , S i l i c a l i t e has n o a c i d i t y , w h i l e H Z S M - 5 is a h i g h l y a c i d i c zeolite.

Recent

i n v e s t i g a t i o n s ( 7 ) h a v e s h o w n t h a t t h e difference i n s e l e c t i v i t y f o r s y n thesis gas c o n v e r s i o n b y Z S M - 5 ( i r o n )

a n d Silicalite (iron)

results f r o m t h e p r e v i o u s l y m e n t i o n e d difference

i n acidity.

catalysts The main

influence w a s o n t h e p r o d u c t i o n of a r o m a t i c s a n d olefins; the f o r m e r w e r e d o m i n a n t w i t h t h e a c i d i c Z S M - 5 ( i r o n ) catalyst a n d t h e latter w i t h t h e n o n a c i d i c S i l i c a l i t e ( i r o n ) catalyst. M o s s b a u e r a n d m a g n e t i c i n v e s t i g a t i o n s , d e s c r i b e d i n this c h a p t e r , were conducted

to d e t e r m i n e t h e state of i r o n a n d i r o n - c o b a l t i n t h e

z e o l i t e c a t a l y s t at different stages of catalyst p r e p a r a t i o n a n d use. A m o n g t h e several a i m s of t h e i n v e s t i g a t i o n w e r e t h e d e t e r m i n a t i o n of: 1. t h e v a l e n c e state of t h e t r a n s i t i o n m e t a l i n t h e f r e s h l y i m p r e g n a t e d state; 2. t h e extent of r e d u c t i o n o n exposure t o H ; 2

3.

t h e a c t i v e c a t a l y t i c species after c a r b i d i n g w i t h synthesis gas;


576


4.

the species present i n t h e u s e d c a t a l y s t ; a n d

5.

i n t h e case of i r o n - c o b a l t , t h e p o s s i b l e f o r m a t i o n of b i m e t a l l i c o r a l l o y clusters a n d t h e i r influence o n t h e selec t i v i t y of t h e catalyst.


Experimental Preparation of Samples. Z S M - 5 and Silicalite were prepared using methods described i n the literature (10,11). X - r a y powder diffraction pat terns revealed no phases other than Z S M - 5 or Silicalite. T h e metal component was introduced b y gradually adding the metal nitrate solution to the zeolite until incipient wetness was reached. T h e impregnation w i t h the metal ( F e or F e + C o ) nitrate solution was carried out for 1 h under vacuum to enable the nitrate solution to enter the pore of the zeolite. T h e material is dried initially w i t h constant stirring over a boiling water bath, and further dried i n air at 110°C for 12 h . T h e amount of iron and cobalt i n the samples was determined b y standard wet chemical techniques and atomic absorption. T h e zeolite impregnated w i t h iron or iron plus cobalt was reduced i n flow i n g H at 450°C for 24 h . It was then carbided i n flowing synthesis gas at 250°C for 24 h to yield the active catalyst. T h e catalysts were tested (7) for synthesis gas conversion i n both a fixed-bed microreactor and a Berty (continuous-flow stirred-tank) reactor. F o r catalytic testing, these steps on the metal-impreg nated zeolite were all carried out i n the reactor. F o r Mossbauer studies, a l l the samples except the used catalysts were prepared separately under the condi tions just described. In the ensuing discussion such samples w i l l be described variously as (a) freshly impregnated, (b) reduced, (c) carbided, and (d) used catalysts. T h e last mentioned was taken from the reactor after its use for periods lasting from one to three weeks under synthesis gas under temperatures ranging from 280° to 320°C. X - r a y diffraction studies were carried out on the samples after steps (a) and ( d ) . Characterization Techniques. T h e apparatus used for Mossbauer spec troscopy and magnetic measurements is described separately under the corre sponding sections. 2

Results and Discussion Mossbauer Studies. Table II were recorded spectrometer

T h e M o s s b a u e r s p e c t r a of catalysts l i s t e d i n utilizing a conventional

made b y N u c l e a r Science

constant-acceleration

and Engineering Corporation

and Nuclear D a t a N D - 1 0 0 multichannel analyzer i n M C S mode. spectra were calibrated w i t h a standard N B S i r o n foil. background observed

i n t h e s p e c t r a arose f r o m t h e g e o m e t r y

M o s s b a u e r setup. A least-squares p r o g r a m

fitting,

The

The parabolic of

the

i n progress, has j u s t i

fied o u r i n t e r p r e t a t i o n s . T h e s p e c t r a w e r e r e c o r d e d at r o o m t e m p e r a t u r e using an 8 0 - m C i C o i n R h matrix. T h e spectra were analyzed, i n gen 5 7

e r a l , o n the basis of the studies o n c a r b i d e s b y R a u p p a n d D e l g a s s

(12).

T h e M o s s b a u e r s p e c t r a of t h e different catalysts w e r e r e c o r d e d v a r i o u s stages; n a m e l y , ( a ) after i m p r e g n a t i o n w i t h F e ( N 0 ) ; 3

reduction i n H

2

at 4 5 0 ° C f o r 2 4 h ; ( c )

3

(b)

at on

o n c a r b i d i n g i n 1:1 H / C O s y n 2


27.

Medium-Pore

LO ET AL.

Zeolite-Iron

thesis gas at 2 5 0 ° C f o r 2 4 h ; a n d ( d )

577

Catalysts finally

after u t i l i z a t i o n of

the

c a t a l y s t i n t h e c o n v e r s i o n of t h e synthesis gas t o g a s o l i n e - r a n g e h y d r o carbons.

T h e s p e c t r a h a v e r e v e a l e d t h e existence of v a r i o u s

phases,

f o r m e d at different stages, a n d h a v e g i v e n clues to t h e n a t u r e of

the

a c t i v e c o m p o n e n t ( s ) r e s p o n s i b l e f o r the efficient c o n v e r s i o n of synthesis gas i n t o gasoline. T h e s p e c t r u m of a f r e s h c a t a l y s t , i n g e n e r a l , c o n s i s t e d of a d o u b l e t w i t h a n i s o m e r shift of a b o u t + 0 . 3 5 m m / s a n d a q u a d r u p o l e s p l i t t i n g of a b o u t 0.75 m m / s , w h i c h i n d i c a t e t h a t t h e v a l e n c e state of i r o n i n t h e s t a r t i n g m a t e r i a l is F e .

A t y p i c a l s p e c t r u m f o r a f r e s h c a t a l y s t of

3 +

Silicalite impregnated w i t h 13.6%

iron using F e ( N 0 ) 3

3

is s h o w n i n


F i g u r e 2. A s d i s c u s s e d i n a l a t e r section, t h e m a g n e t i z a t i o n ( M ) vs. m a g n e t i c field

m e a s u r e m e n t s o n t h e f r e s h c a t a l y s t gave a m a g n e t i c m o m e n t

(H)

of a b o u t 5.96 /x , w h i c h f u r t h e r c o n f i r m e d t h a t t h e i r o n i o n is i n a h i g h B

spin F e

3 +

state. I t s h o u l d b e n o t e d t h a t t h e m a g n i t u d e of t h e q u a d r u p o l e

s p l i t t i n g a n d i s o m e r shift s i g n i f i c a n t l y d e p e n d u p o n t h e n a t u r e of t h e s u p p o r t u s e d a n d the size of t h e i r o n p a r t i c l e s ( 1 3 ) . T h e s p e c t r u m of a r e d u c e d c a t a l y s t c o n s i s t e d of a s i x - l i n e p a t t e r n c o r r e s p o n d i n g m o s t l y to i r o n m e t a l , i n a d d i t i o n t o i n d i c a t i n g t h e p r e s e n c e of a s m a l l q u a n t i t y of a n o x i d e , i f the c a t a l y s t c o n t a i n e d o n l y i r o n , o n

« *4l

3 3

1

1

1

H5

DOPPLER VELOCITY

Figure

2.

Mossbauer

1

J

1

1

3

'

^

(am/sec.)

spectrum of Silicalite impregnated iron using Fe(NO )

with

s s


13.6%

578


Table II.


Sample

No.

S u m m a r y of

Sample

Mossbauer State

M l

Z S M - 5 (14.7% Fe)

Reduced

M2

Z S M - 5 (14.7% Fe)

Carbided

M3

Z S M - 5 (14.7% Fe)

Used

M4

Z S M - 5 (5.4% F e +

1.3 % C o )

Reduced

M5

Z S M - 5 (5.4% F e +

1.3 % C o )

Carbided

M6

Z S M - 5 (5.4% F e + 4.5% Co)

Used

M7

Silicalite (13.6% Fe)

Fresh

M8

Silicalite (13.6% F e )

Used

M9

Silicalite (4.4% F e +

3% C o )

Reduced

M10

Silicalite (4.4% F e +

3%

Used

Co)

t h e one h a n d ; w h e r e a s t h e s p e c t r u m of a c a t a l y s t c o n s i s t i n g of b o t h i r o n a n d c o b a l t c l e a r l y i n d i c a t e d t h e f o r m a t i o n of a n i r o n - c o b a l t a l l o y o n r e d u c t i o n , o n t h e o t h e r h a n d . T h e s p e c t r u m of r e d u c e d Z S M - 5 c o n t a i n i n g 1 4 . 7 % i r o n , s h o w n i n F i g u r e 3 A , essentially c o r r e s p o n d s to t h a t of m e t a l l i c i r o n . H o w e v e r , t h e r e is a s m a l l a m o u n t of u n r e d u c e d i r o n i n t h e f o r m of a n o x i d e , p r o b a b l y a - F e 0 . T h e r e d u c t i o n i n t h i s case is a b o u t 8 5 % . 2

3

T h i s is also c o n f i r m e d b y t h e m a g n e t i z a t i o n m e a s u r e m e n t s , w h i c h i n d i cated an approximate 1 5 %

l o w e r i n g i n the observed saturation m a g

n e t i z a t i o n of i r o n . T h e s p e c t r u m of r e d u c e d Z S M - 5 c o n t a i n i n g 5 . 4 %

iron and

1.3%

cobalt, shown i n F i g u r e 3 B , revealed a six-line pattern corresponding to a n average i n t e r n a l m a g n e t i c field of a b o u t 340 k O e , a n d i n d i c a t e s t h e f o r m a t i o n of a n i r o n - c o b a l t a l l o y . A t y p i c a l s p e c t r u m of r e d u c e d S i l i c a l i t e c o n t a i n i n g 4 . 4 % 3.0%

c o b a l t is s h o w n i n F i g u r e 4.

iron and

T h e s p e c t r u m consists of a w e l l -

defined six-line pattern corresponding to a n internal magnetic

field

of

345 db 3 k O e , w h i c h is m u c h l a r g e r t h a n t h e 3 3 0 - k O e field e x p e c t e d f o r m e t a l l i c i r o n . S e c o n d , t h e i s o m e r shift o b s e r v e d is 0.18 m m / s w i t h respect t o i r o n m e t a l , w h i c h i n d i c a t e s t h a t the e l e c t r o n d e n s i t y at t h e i r o n n u c l e u s is s m a l l e r i n t h e i r o n - c o b a l t a l l o y t h a n i n i r o n m e t a l . T h i s decrease i n t h e e l e c t r o n d e n s i t y i n t h e i r o n - c o b a l t a l l o y is consistent w i t h t h e e x p e r i m e n t a l results r e p o r t e d b y V a n d e r W o u d e a n d S a w a t s k y (14).

T h e for

m a t i o n of a n i r o n - c o b a l t a l l o y is s u p p o r t e d f u r t h e r b y t h e m a g n e t i z a t i o n


27.

LO ET A L .

Medium-Pore

Zeolite-Iron

579

Catalysts

R e s u l t s of V a r i o u s C a t a l y s t s Remarks I r o n m e t a l a n d s m a l l a m o u n t of a n oxide ( a - F e 0 ) 2

Fe C 5

2

and F e C 3

F e C , F e C , and F e 0 5

2

Fe C 5

2

3

3

3

4

has been r e l a t i v e l y reduced as c o m p a r e d to t h a t i n S a m p l e N o . 2

Iron-cobalt alloy Fe C , Fe C 5

2

3

a n d a strong d o u b l e t

7


Iron-cobalt alloy C o b a l t seems to have i n h i b i t e d the f o r m a t i o n of carbides. S t r o n g d o u b l e t c o r r e s p o n d i n g to F e Fe C 5

2

3 +

and F e C 3

Iron-cobalt alloy F e C , F e C , a n d a s t r o n g doublet. F e C a n d F e C are c o n s i d e r a b l y s m a l l c o m p a r e d to those observed i n S a m p l e 8. Presence of c o b a l t seems to h a v e i n h i b i t e d the f o r m a t i o n of carbides. 5

2

3

5

2

3

measurements o n this catalyst, w h i c h i n d i c a t e d a m a g n e t i c m o m e n t i n t e r m e d i a t e b e t w e e n t h e m o m e n t s c o r r e s p o n d i n g to i r o n a n d c o b a l t . T h e s p e c t r a of c a r b i d e d catalysts consist of s u p e r p o s i t i o n of at least t w o a p p a r e n t s i x - l i n e patterns c o r r e s p o n d i n g to at least t w o

different

i r o n - c a r b o n phases. A t y p i c a l s p e c t r u m of c a r b i d e d Z S M - 5 w i t h 1 4 . 7 % is s h o w n i n F i g u r e 5 A . T h i s s p e c t r u m represents the p r e s e n c e of H a g g c a r b i d e ( F e C ) a n d c e m e n t i t e ( F e C ) . T h e f o r m e r has three i n e q u i v a 5

2

3

l e n t i r o n sites, w h e r e a s the l a t t e r has o n l y one.

T h e p o s s i b i l i t y of t h e

p r e s e n c e of s m a l l q u a n t i t i e s of less stable e' a n d c c a r b i d e s ( F e F e C ) c a n n o t b e r u l e d out. A d o u b l e t d u e to F e 2

3 +

2 2

C and

was not apparent i n

F i g u r e 5 A ; f u r t h e r l o w - t e m p e r a t u r e studies are i n progress to d i s c e r n a n y superparamagnetic behavior. T h e s p e c t r u m of c a r b i d e d Z S M - 5 c o n t a i n i n g 5 . 4 %

iron and

c o b a l t , s h o w n i n F i g u r e 5 B , i n d i c a t e s t h e presence of b o t h F e C 5

F e C i n a d d i t i o n to a s t r o n g d o u b l e t . 3

1.3% 2

and

A p p a r e n t l y fewer carbides have

b e e n f o r m e d i n this catalyst t h a n i n those c o n t a i n i n g n o c o b a l t ; i t a p p e a r s t h a t the presence of c o b a l t has s o m e w h a t i n h i b i t e d t h e f o r m a t i o n of c a r bides.

T h e d o u b l e t m a y b e a t t r i b u t a b l e p a r t l y to

superparamagnetic

b e h a v i o r . S i m i l a r s p e c t r a also h a v e b e e n o b s e r v e d i n the case of S i l i c a l i t e b a s e d catalysts as w e l l . F u r t h e r l o w - t e m p e r a t u r e studies are u n d e r w a y to i d e n t i f y t h e n a t u r e of the d o u b l e t . T h e s p e c t r a of u s e d catalysts are i n g e n e r a l v e r y c o m p l i c a t e d a n d s e e m to consist of three or m o r e m a g n e t i c a l l y s p l i t h y p e r f i n e spectra.


A

580

MOSSBAUER

SPECTROSCOPY

A N D ITS C H E M I C A L

APPLICATIONS

A

Fe-

metal


°< - F e

-1 0

-8

-6

-4

-2

0

DOPPLLR VLLOCITY

„„ g> ee

_ se.ee M

tee.ee

ise.ro

aro.ee

2

0

3

2

4

b

8

10

(mm/sec.)

CHANNEL NUMBER 2se.ee 3ee.ee 3se.ee

4ea.ee

4se.ee

sea.ee

sse.oe

s

ect? i

-2 0 2 DOPPLER VELOCITY (ran/sec.)

Figure

3.

Mossbauer

spectrum 5.4%

of

reduced

Fe,1.3%

Z S M - 5 (A)

14.7

Fe

and

Co


(B)

27.

Medium-Pore

LO ET AL.

Zeolite-Iron

581

Catalysts


eog.ee

i Figure 4.

i

i

J i DOPPLER VELOCITY (••/••c.)

i

i

Mossbauer spectrum of reduced Silicalite (4.4%

i

?r

Fe, 3%

Co)

t y p i c a l s p e c t r u m of u s e d Z S M - 5 w i t h 1 4 . 7 % i r o n is s h o w n i n F i g u r e 6. T h i s s p e c t r u m c a n b e e x p l a i n e d i n terms of the presence of H a g g c a r b i d e , c e m e n t i t e , a n d F e 0 . I t is n o t e w o r t h y t h a t t h e c e m e n t i t e c o n t e n t has 3

4

r e l a t i v e l y i n c r e a s e d at the expense of t h e H a g g c a r b i d e i n t h e c a t a l y s t as c o m p a r e d to t h a t f o u n d i n t h e c a r b i d e d catalyst.

(See

used the

lines m a r k e d b y a r r o w s i n F i g u r e 5 A a n d F i g u r e 6.) T h e s p e c t r u m of u s e d Z S M - 5 w i t h 5 . 6 % i r o n a n d 4 . 5 % c o b a l t s h o w n i n F i g u r e 7 c o n s i s t e d of a s i x - l i n e p a t t e r n c o r r e s p o n d i n g to a n i n t e r n a l m a g n e t i c field of 344 zb 3 k O e a n d a n i s o m e r shift of + 0 . 1 5 m m / s w i t h respect to i r o n m e t a l , a n d a p p e a r s to i n d i c a t e t h e f o r m a t i o n of a n i r o n c o b a l t a l l o y . I t is t o b e n o t e d t h a t t h e c a r b i d e s , w h i c h w e r e present i n t h e case of u s e d Z S M - 5 c o n t a i n i n g o n l y i r o n , are s u r p r i s i n g l y absent i n this case. O n c e a g a i n , the presence of a l a r g e a m o u n t of c o b a l t a p p e a r s t o i n h i b i t the f o r m a t i o n of c a r b i d e s i n these samples. T h e x - r a y p o w d e r p a t t e r n s s h o w e d t h e presence of a b c c i r o n - c o b a l t a l l o y p h a s e i n a d d i t i o n to the Z S M - 5 phase. T h e s p e c t r u m of u s e d S i l i c a l i t e c o n t a i n i n g 1 3 . 6 % i r o n is s h o w n i n F i g u r e 8. I t consists of b o t h H a g g c a r b i d e a n d c e m e n t i t e a n d is s i m i l a r t o t h a t o b s e r v e d f o r Z S M - 5 w i t h 1 4 . 7 % i r o n , except t h a t t h i s s p e c t r u m does n o t i n d i c a t e t h e presence Silicalite containing both 5 %

of a n y o x i d e .

iron and 5 %

T h e s p e c t r u m of

used

c o b a l t is s h o w n i n F i g u r e 9.


582

MOSSBAUER SPECTROSCOPY A N D ITS C H E M I C A L A P P L I C A T I O N S a

a

°


150.00

200.08

i

2S0.B0

380.00

358.60

408.08

4S0.06

660.00

656.60

r

5

OPPLER VELOCITY ( a m / s e c . )

1

1

1

n D O m i l VELOCITY

Figure

5.

Mossbauer

T

1

i

w

r

(H/MC.)

spectrum of carbided ZSM-5 (B)5.4% Fe,1.3% Co

(A) 14.7%

Fe and


27.

Medium-Pore

LO ET AL.

Zeolite-Iron

Catalysts

583


s

DOPPLER VELOCITY (on/sec.)

Figure 6.

Mossbauer spectrum of used ZSM-5 (14.7% Fe)

DOPPLER VELOCITY

Figure 7.

(mm/sec.)

Mossbauer spectrum of used ZSM-5 (5.4% Fe, 4.5% Co)


584



CHANNEL NUMBER 3sa.ee

ee$.o

4se.ee see.ee

DOPPLER VELOCITY («•»/••

MÃ¶ssbauer and Magnetic Studies of Bifunctional Medium-Pore Zeolite

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