The Characterization of Modified ZSM-5 Catalysts Prepared Via a Solid State Reaction for Propane Aromatization

H.G. Karge, J. Weitkamp (Editors), Zeolites as Catalysts, Sorbents and Detergent Builders 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

ThE CYARACTERIZATION OF MODIFIED ZSM-5 CATALYSTS PREPARED V I A A SOLID STATE REACTION FOR PROPANE AROMATIZFITION

YANG Yashu, GUO Xiexiari, DENG Maicun, WANG Limin and FU Zaihui Dalian Institute of Chemical Physics, Chinese FIcademy o f Sciences P.O. SOX 100, Dalian, P.3.CHINA

I.iESTRAZT This paper dsscribes a simple method o f solid state reaction for preparation o f Zn-, V o - , a;>d Cr-ZSM-5 catalysts instead o f ion exchange. NH,-TPD, I R , TPR, ESR and UPS techniques x e r e used to characterize the interaction o f HZSM-5 with Znn, %C1:.I and CrC.. ahich leads to introduction o f cations into the channels o f zeo1i:cs. Zn-ZSM-5 is more active f o r propane conversion and gives the better BTX selectivity. Over Mo-ZSM-5, propane mainly undergoes cracking into methane a:id ethane, J X ~ the loading o f CrT’ o f ZSM-5 enhances the propar@ dehydrogenation t o propsne.

iXTPODUCT1ON The

aromatiiation o f liqht hydrocarbons h a s been investigated since the

eal-ly ;;eventies ’. 1 - i ’ )

.

Interest in studies of modified ZSM-5 catalysts

propane aromatizatian was greatly stimulated developed by UC)P 3nd prepared via

the

RP.

by

the Cylar

for

jointly

Process

in yeneral, the modified high-silica zeolites a r e

introduction o f metal cations by meaiis o f an ion exchange.

This DrrJccss requires a lor,g period of refluwing in the ion-exchange solution, is iI!convenient in industry. Thus the introduction

:ghiih

higti-iilica . : e o I i t e s .via

a

of

metal ions into

s o l i d state reaction deserves attmtion from both

the.7retiral a n d zractical p o i r t s of view.

Thjs paper dESCi-ihS the characteri-

z a t i o n of Zn, ?‘u and C r ior% introduced by a solid state reaction C~si t:a;is in 2 3 - 5 z ~ o iltes. TPR,

!a,

ESR and XPS

Ohto

EdtiiJniC

Chemical and physical nethods such as T P D - W ~ ,

;,Ere used.

4s the

test reaction, propane aromatization was

c a r r i e d o u t in a fired--Sedpulse reactor.

EXPERIYFNT M a T a 1a t 5 Catdly3ts of Z11(2.0wt%l-, Ma(3.5wtXI- and Cr(l.3vct%;-ZSM-5 wore prepared by mixing

and grinding the powder o f HZSM-5 (obtained f r o m Nan Gai University, SiOc/

A1.Ua=34)

wjth

ZnO, MoCl- and CrO-. respectively.

crus’led t o 40-60 mesh aiid calcined

3%

CIfter

shown in Tab:e 1 .

pelleting, they were

850 TABLE 1 Ka t e r i a l s

CATOLYSTS

Preparation Conditions

ZnO

H e a t i n g a f Zn a c e t a t e a t 540C i n a i r f o r 6 h r

CrO::

H e a t i n g o f Cr(NOsr:., a t 550°C i n a i r f o r 4 h r

roc1:.

Y9.99%

Zn/ZS#-5

Calcination of

Zn/ZS?l-S(i)

I o n e x c h a n g e o f 29 o f NH,,ZS?I-5

d

m i x t u r e o f ZnOtHZSM-5 i n He a t 980nC f o r 1 h r w i t h 2 0 0 ml o f 0.1M O F Zn(NO:,):..

s o l u t i o n f o r 16 h r a t 8O'.,C, t h o r o u g h w a s h i n g w i t h w a t e r t o f r e e d r i e d o v e r n i g h t a t l 2 O r ' C , c a l c i n e d i n a i r a t 540DC f o r 3 h r

NO:,,

Mo/ZSM-5

C a l c i n a t i o n o f a m i x t u r e o f MoCI:;+HZSM-J

Cr /ZSM-5

Calcination o f

d

ii? Ar a t 450'X f o r 4 h r

m i x t u r e o f CI-C:,*ICSM-~i n s i r a t 54cS'T f o r 4 h r

F r o p a n e h a s 99.0% p u r e a n d f r e e d f r s m t r a c e s o f 0:: arid w a t e r p r i o r t o use. CATFLYST C;H~RAC:TERIZATION NH-?-TPD s p e c t r a were measured w i t h a c c n v e n b i o n a l TP3 a p p a r a t u s .

NH-.:-TPD

About 0.29 o f s d s p l e m a t e r i a l w i s p l a c e d i n a q u a i t z r e s c t o i - a n d a d s o r p t i o n s a t u r a t e d by ptjlses o f dry ammonia a t lEO<'C.

TPD *as c a r r i e d o u t f r o m 120°C

t o 5OO.C w i t h

3

h e a t i n g r a t e o f @ - 3 2 T / m i n arid w i t h H e ( 3 0 nrl/min) as t h e

c a r r i e r gas.

. W e t o t a l NH., u p t a b e was d e t e r m i n e d b y a t h e r m a l c o n d u c t i v i t y

d e t e c t o r , c o l n p 3 r i n g tbe i r ? t e g r a t e d a r e a b e l o w t h e c u r v s w i t h t h a t o f a known v o l m e o f NH3. T h e s a m p l e s were p r e s s e d i n t o s e l f - s u p p o r t i n g wafers and

19 SPECTROSCCPY

placed i n a quartz i n - s i t u I R c e l l .

The s a m p l e s here p r e t r e a t e d a t 580'C

i n He

(30 m l / a i n ) f a r 1 h r , evacu.ited a t 50rJr'C t o 10"" T o r r f o r 4 lir, c o o l e d t o 230r'C and e x p o s e d t o

d

s a t u r a t e d v a p o r p r e s s u r e o f p y r i d i w o f 2rJ,C, a f t e r 1 h o f dd-

s o r o t i o n . t h e e x c e 5 s a c d w e a k l y a d s o r b e d p y r i d i n e was 1-evoked by e v a c u a t i o n a t :he sime t e m p e r a t u r e f o r 30 min t~ S e l o w 10-" T o r r t a pyi-idioe on

the s u r f a c e .

Ie3've

only chmisorbed

1R s p e c t r a luere r e c o r d e d o n a Per t i ri-E 1rner 580

s;>ecCrometer .at r o o m t e m p e r a t u r e . X-RAY

XPS s p e c t r a * e r e r e c a r d e d o n a PHI 550

PHOTOELECTRON SPECTROSCOPY

;pectrometcr

u s i n g Al-K6

X-rays,

r e f e r e n c i n g t o t h e BE o f C,,,

b i r d i n g e n e r g i e s (EE) wer

(?86.4

EV).

E!

i o r r x t e d by

Pie s u r f a c e a t o m i c r a t i o s o f S i , A I ,

a n d 21-1 were c a l c u l a t e d f r o m t h e i n t e G r a t e d XPS s i g n a l i n t e n s i t y c o r r e c t e d f o r atomic s e n s i t i v i t y f a c t o r s " + ) .

F o r t h e XPS s t u d i e s ? s e l f - s ~ ; p p o r t e dwafers were

l o a d e d i n t o a s t a i n l e s s steel h o l d e r and e v a c u a t e d t o lo-" Torr a t 25-500'-'C f o r 1 hr.

TPR

The TPR e x p x i m n t a l p r o c e d u r e u s e d :*as e s s e n t i a l l y s i m i l a r t o t h a t d e s -

c r i b e d by McNicol'r.".

An i n - s i t u a d e o r b e n t t r a p h o u s e d a h e a d o f ttie r e a c t o r

was u s e d t o remove the l a s t traces o f H,JJ and O:.. From che c a r r i e r q a s .

861

a b o u t 0.29 o f c a t a l y s t was p l a c e d i n a q u a r t z r e a c t o r , p r e t r e a t e d a t 590°C i n d r y He (30 m l l m i n ) f o r 1 h r and c o o l e d i n He t o room t e r p e r a t u r e . The gas s t r e a m was t h e n s w i t c h c d t o a mixture of 6% Ii,: i n Or b e f o r e r u n n i n g t h e TFR. The f l o w r a t e of t h e r e d t i c i n 5 gas was 30 ml/cnin and t h e h e a t i n g r a t e was i6"C/

min.The t o t a l !ir:u p t a k e bas !neiisured b y i n t e g r a t i n g t h e a r e a below t h e c u r v e and c a l i b r a t i n g w i t h a known volume o f He. ESR SPECTROSCOPY

T h e c a t a l y s t s a m p l e s (40-hO mesh) were l o a d e d i n t o a n ESR

t u b e , h e a t e d i n Ye o r a i r (30ml/min) a t 5O0-55OFC f o r 1 h r and t h e n e v a c u a t e d a t the stlme t e m p e r a t u r e t o lo-.'' Tori- f o r 4h p r i o r t o ESR measurements. s p e c t r a Hiere r u n on a JES-FEZSG s p e c t r o m e t e r a t 20-C.

The ESR

The g f a c t o r s f o r t h e ESR

s i g n a l were d e t e r m i n e d I - e l a t i v e t o DPPH w i t h g-2.0036. CFITNYTIC KTIVITY

y l s c m i c r o r e a c t o r was used w i t h 0.29 o f c a t a l y s t .

Thi?

p u l s e volume o f 0.4Bml o f p r o p a n e was c a r r i e d through t h e c a t a l y s t bed by cle a t a f l o w r a t e o f 35 ml/min. and 5OOrsC f o r 1 h r .

The c a t a l y s t was p r e t r e a t e d in He a t 580 C f o r 30 min

R e a c t i o n p r o d u c t s u e r e a n a l y z e d o n l i n e u5iiv.l a gas chromato-

g r a p h w i t h a ttrermal c o n d u c t i v i t y d e t e c t o r .

A porapak

I)

column programmed from

20 t o 200'>C N J S used , The e l u t i o : ) t i m e was a b o u t 60 min f o r Cv a r o m a t i c s .

RES!JL'TS FIN3

D I SCtJSS I Chi

I ) In/ZStI-J

NHv-TPD and IR s p e c t r o s c o p y ai-e t h e nost p o t e n t methods f a r i n v e s t i g a t i n g t h e 3 c i d p r o p e r t i e s o f ZSM-5 t y p e z e o l i t e s . E M - 5 a r e shown i n F i q . 1 .

T y p i c a l TPD p a t t e r n s o n HZSM-5 and Zn/

T h e r e 31-8 two p e a k s o f t h e 1 ( o r @aboLt 220°C) and h

( o r l a b o u t 430eC) s t a t e s , i n acjreement w i t h Topsfie e t a l . ( * ' .

The amounts o f NH,:

d e s o r b e d froin t h e s e c a t a l y s t s arid t h e rlraxima t e m p e r a t u r e s a r e summarized i n T a b l e 2. TASLE 2

NH.!-TPD RESULTS

H2SM-5

Zn/Z5M-5

Max imum load ii-q (mmo 1/g 1

0.92

0.90

FIctual loadingOnmol/g)

1.37

1.10

Cat a 1 y s t

Peak maxima (T,."C)

1 h %

a r e a under t h e peak 1 h

215

229

439

lt30

53

63

47

37

852

100

200

300

400

--

600

500

TEMPERATURE ["C]

Figure 1

TPD spectra of NH3 desorbing from HZSM-5 and ZnnSM-5;

B = 8'Umin.

1

1

1

1

1

I

7

w

x

4

k 3

n

I

--

I \ I I I,/

1900

3500

1600

IR spectra, obtained at 2O"C, of HZSM-5 and ZnRSM-5 (a) evacuated at 500°C, followed by pyridine adsorption at 2OO"C, then evacuated at (b) 200"C, (c) 450°C.

1

1

600

1

--

1

1

800

TEMPERATURE ["C]

WAVENUMBER [crn"] Figure 2

,

400

Figure 3

TPR spectra of ZnO and ZnRSM-5 0 = 16"Umin; sample weight: 0.2 g; flow rate of H Ar gas mixture: &rnl/rnin.

853

The NHS TPD r e s u 1 t s . m t h e a b o v e c a t a l y s t s show t h a t ( 1 ) t h e t o t a l NH3 u p t a k e &a5

lowered by 20% on Zn/ZSM-5 w i t h t h e h p e a k b e i n g p r e f e r e n t i a l l y d e c r e a s e d

rathec- t h a n t h e 1 peak.

( 2 ) t h e Tm o f t h e 1 peak i n c r e a s e d and t h a t o f t h e h

I R s p e c t r a on Zn/ZSM-5 and HZSM-5 are shown

peak d e c r e a s e d by 5 c C o n Zn/ZSM-5.

The i n t e n s i t i e s o f t h e b a n d s a t 3600 cm-'

i n Fig.2a.. Zn/ZSM-5.

and 3740cm'-l a r e l o w e r on

When p y r i d i n e was a d s o r b e d and e v a c u a t e d a t 20OC8C, t h e 3600cm-1 band

,

d i s a p p e a r e d more q u i c k l y t h a n t h e band a t 3740cm-' ( B r k n s t e d a c i d s i t e s ) , a t 1455cm-'(Lewis

s i t z s ) appeared

acid sites

and new b a n d s a t 1 5 4 3 ~ m - - ~ )

rnd a t 1 4 8 8 c r n - ' ( t o t a l

acid

a s shown i n F i g 2b. The r e s u l t s o f p y r i d i n e a d s o r p t i o n a r e sum-

T h i s shows t h a t t h e i n t e n s i t y c f I , at: 1543 cm-'' was

m a r i z e d i n T a b l e 3.

lowered f r o m 6.21 on HZSM-5 t o 3.30 on Zn/ZSM-5, avd I l . a t 1455cm.' was i n c r e a s e d c o r r e s p o n d i n g l y f r o m 1 . 3 6 t o 3.85 w i t h l i t t l e l o s s o f t o t a l a c i d s i t e s a t 1WBcm-l. IR INTENSITIES OF PVRIDINE ADSOSPTION

TABLE 3

Cata l y s t

HZSM-5

Evacaat ion t e m p e r a t u r e

230

1.36

1,

4.57

1l./:* NH.,-TPD

7n:ZSM-5

?00

450

450

3.39

3.05

1.21

6.46

0.86

1.CG

and I R s t u d i e s o f t h e a c i d i c p r o p e r t i e s o f Z5M-5 z e o l i t e s h a d e b e e n

d e s c r i b e d i n v a r i o u s ~ a p e r s ( ' ~ - ~ 'I .t is w ? l l known t h a t the a c t i v e a c i d s i t e s a r e c h a r a c t e r i z e d by ~n IR bar:d a t 3600 c m ' ' arid s t a t o a r o u n d 670-770k.

d

d e s o r p t i o n peak o f t t : e h o r

Weaker a c i d s i t e s a r e c h a r a c t e r i z e d by a 3724-3740rm.'

band w h i c h i s c o r r e l a t e 9 w i t h d e s o r p t i o n o f YlW, a t a h o u t X O I : f o r t h e 1 state.

The Si-OH-Rl

r

g r o u p s a t 3hDOcm-* arid thF? hvdro.cy1 group:;

shown t o b e B r t n s t e d a c i d s i t e s .

T ~ EI R

01'

0

a t 3 7 2 0 c 1 n - ~i r e

and NH..-TPD e:!perini;+ntal results des-

c r i b e d a b o v e a g r e e i n i n d i c a t i n g t h a t the Br.#risted a c i d s i t e s on HZSII-5 r e a c t w i t h Zinc ions, i.e,

t h e Zne'

ions s e l e c t i v e l y r e d u c e t h e number o f s t r o n g a c l d

s i t e s , and c h a n g e t h e a c i d d i s t r i b u t i o n , l , - / I , .

, and

T,,

as w e l l .

TPR and XPS d e s c r i b e t h e 280 d i s t r i b b t i o n i n t h e channels o f ZSM-5 z e o l i t e s . The a f f e c t o f Z n z e o l i t e i n t e r a c t i o n on t h e TPD p r o f i l e o f n o n - s u p p o r t e d

ZnO

and s u p o r t e d 2-10 i r e shown in F i g . 3 , and the t o t a l Ii!>u p t a k e i s s u n r n a r i z e d i n T a b l e 4.

The t o t a l HI* u p t a k e d u r i n g TPR is much h i g h e r o n Zn/iSM-5

f o r which t h e r e l a t i v e r e d u c i b i l i t y is 5.4%.

t h a n c n ZnO

T h s H:; t i p t a k ~ so n t h e 2nIZSM-5

c a t a l v s t s a r e n e a r l y c o n s t a n t f o r d i f f e r e n t Zn l o a d i n g s f;-on 0.4 t o 2.0% a r d f o r

catalv5t5 prepared hv ion exchange. This implies that ZnO is widely dispersed in the channels o f the zeolites. Measurements of H, consumption indicate that the TABLE 4

TPR" DATA F3R ZnO AND Zn/ZSM-J peak temp.

He uptake

re1 at i ve

mmol/q-Zn

reducibility X

Sample

Zn(wt%)

pretreatment

1.7I

T"

ihO

80.3

S40r,C,Air,6 hr

386

675

0.14

Zii/ZSM-S

2.3

58OC'C!He, 1 hr

347

623

2.55

ZnIZSM-5

0.4

580"C,He, 1 hr

515 700

2.26

88.6

L'n/XM-SD

3.L

540°C. a i r

420 680

2.37

09.R

,I

5.4

100

( a ) sample weight 0.29 ( t ) prepared b y ion exchange method

extant o f reductioii o f Zn". to Zn is abaut I & % , i.e.

it

is lot4 charge density

of Zn+-' ions and n o t Zn metdl that are formed during 7PR MI exhibits trio peaks:

. The TPR profile of

low temperature peak T m j o f 386% attributes to

easily reduced senall particles, and h i g h temperature peak T,,+: 60 large particles mors difficult to reduce.

of 675 C rslates

The low teniperature peak. rapidly

decrca?;es f o r ZilO supoorted on ZSM-5 zeolites, when the 211 loading is reduced fr.om 2.0 to O.G%, t L r e pc;k

the tiiqh temperature peak splits into two and the low terrpera-

disappears.

The change in the TPR profile is due ta an interaction

between Zn iors and the zeolites on ZnIZSM-5 which gives rise to the high reducing temperature around !r20-70O0C.

%PS of

XPS data are listed in Table 5. These show that the surface

Si/Al ratio

H X M - 5 is higher than the n m i n a l hulk composition o f S i / A l =17, and that

this surface Si/P1 ratio increases from 25.0 to 30.3 Khen the evacuation temp e r a t u r e is increased from 25°C t a 500'.C. nf

This suggests that thare is a loss

5urface A i atoms from tiZSM-5 under v d c w m even at 25lSC, end this 105s is

increased at elevated temperatures.

En Zn/ZSM-S, the surface Si/Al r a t i o shows

only a modest increase t o 19.2 after eracuation at 2S"C and to 2G.4 dfter c t l J t i o n at 500°C.

L'VI-

The higher eracuation temperature causes increases in t h e

Zn/51 and Zn/Al ratios from 4.BxlO-e to 6 . 7 ~ 1 0 -.md ~ 0.92 to 1.37 respectively. The Zn/Si and Zn/A1 data from Zn/ZSM-5 is further evidence that the solid state reaction of HZSM-5 w i t h ZnO causes the migration of Zn ions from the outer stirface into the chanriels of the zeolites even at room temperature, with more extensive migration at the hipher temperature.

Zn ions located near R 1 atoms

protect the A1 atom; f r a m escaping from the surface. CATALYTIC K T I V I T Y

The catalytic activity for propane conversion is Shown in

Tabje 6. This shows a higher activity and selectivity for propane aromatilation on Zn/ZSM-5 prepared either by sol id calcination or liouid ion-exchange, than

855

200

400

600

TEMPERATURE ["C] Figure 4

TPD spectra of NH3 desorbing from HZSM-5 and MoESM-5; fl = 32'Umin.

gl'= 1.99

100G H H-

Figure 5

ESR spectra of M o (V)at 20°C; mixture of H-ZSM-5and MoC15 calcined in He at 450°C.

Figure 6

ESR spectra of Cr (V); CrESM-5 calcined in He at (a) 500°C. (b) 550°C

856

on HZSM-5.

Zn ions enhance the rate of dehydrogenatioi of propane and t.he transformation of intermediates into aromatics. With regard t o Zn state and Zn*'-ZSM-5 bifunctional action on Zn/ZSM-5 catalysts will be discussed elsswhere (:C).

TC\BCE 5

XPS CATA ~

Ca tal yst E v a c . Temp.( C )

El. ( O v : , e v )

~~

~~

~~

25

500

532.8

~

~~

;7n/ZSM-S

H-2SM-5

25

532.1

532.0

500 532.0

En (4lry. , e v )

74.c

74.4

74.3

74.4

Es (Sir:,. ,av)

103.6

103.2

103.0

iC3.1

1021.5

1021.3

503.3

502.9

Et. ftnTw , e v )

E,,,, :Zn,ev)

Surface atomic ratio

O/Si

1.91

Si/FI1

1.84

2.13

30.3

I .91

13.2

20.4

Z d S i ( x lo+)

4.8

6.7

Zn//S:

0.92

1.37

Catalyst

25.0

Zn/ZSM-5

H-2SM-5

?I (at % )

Mo/ZSM-S

C r IZSM-J

2.0

1.5

Coiiversion(C%)

17.4

96.4

79.9

81.4

70.4

16.5

BTX

27.1

50.7

42.0

37.1

a.6

3.6

i7.0

33.7

I 9

se1ec.X

1 * 5 4 < i , 3.5

Producu t d i 5 tr ihu t ion(C% )

.

methane

16.8

e thy1 ene

24.5

1.1

1.7

1 .D

3.b

13.9

ethane

11.3

27.5

36.0

42.0

54.3

1.9

propyl ene

a.

1.3

.-..

bu tene

2.6

butane

15.8

14.4

17.0

1.5

2.0

--.. 1.0

_....I

0.4

----

-..1.2 --.- -

- _ I

?O.? 0.b

0.9

1.2

4.2

0.4

3.1

- ---

h.7

1.8

C!,..

1 .b

3.3

benzene

8.7

39. I

30.6

28.3

toluene

13.1

10.E

11.0

8.2

1.3

1 .r7

xylmes

5.3

0.8

0.4

0.6

0.6

0.6

0.29 catalyst calcined at 580C for 0.5 hr in He prior- to r e a c t i o n :

Reaction Temp. 500C, flow rate o f He 35ml/rnin. (except f o r Znc,, )

867

2 ) Mo/ZSM-5 and Cr/ZSM-5 ESR s p e c t r a h a v e b e e n a p p l i e d t o s t u d y t h e s o l i d s t a t e r e a c t i o n o f h i g h -

si1:ca

Calcination of a

z e o l i t e s w i t h t h e compounds o f MoCl?. a n d CrOo"l'.

m i x t u r e o f MoCI,; and H%SM-J(in He), CrOn and HZSM-5 ( i n a i r , f o l l o w e d b y e v a c u a t i o n ) r e s u l t s i n t h e a p p e a r a n c e o f i n t s n s e e.5.r.

s i g n a l s of M o ( v )

( g =1.93, g =1.89) and C r ( v ) fg=1.967) shown i n F i g . 4 a n d F i g . 5 r e s p e c t i v e l y . s p e c t r a shown i n F i g . 6 i n d i c a t s s t h a t t h e h i g h t e m p e r a t u r e p e a k is

NH,,-TPD

very

I t is b e l i e v e d t h a t t h e r e a c t i o n b e t w e e n HZSM-5

weak o n M o C S M - 5 .

and M C C I , ~ , Cr03 i n t h e s o l i d s t a t e l e a d s t o t h e i n t r o d u c t i o n o f M o ( v ) , C r ( v ) ions onto c a t i o n i c p o s i t i o n s o f the high-silica

zeolites.

The h i g h t e m p e r a t u r e

treatment of Ci-/ZSM-S shows t h a t t h e b i n d i n y o f C r ( v ) t o c a t i o n p o s i t i c n s o n the z e o i i t e framework is s t r o n g e r t h a n t h a t o n t h e s u r f a c e o f s i l i c a - a l u m i n a

a t 550-C i n a i r l e a d s t o a c o n s i d e r a b l e

c a l c i n a t i o n o f :he a i x t u r e (HZSM-S+CrO.,) d e c r e a s e i n t h e i n t e n s i t y o f t h e e.5.r. of hyperfine s p ! i t t i n q

s i g n a l o f c r ( v ) i o n s and t h e a p p e a r a n c e

a s shown i n F i q . 5 .

(h.f.s.)

A t atoms a r e h i p h l y d i l u t e d j n t h e z e o l i : e

A 1 atonis is l a r p e .

In h i g h - s i l i c a

zeolites the

framework, and t h e d i s t a n c e b e t w e e n

+

+

T h e r e f o r e , i t is s u a g e s t e d t h a t i t is CrOn, M o C l , a n d n o t

j s o l a t e c ! i o r . 5 ( C I - ~ + , Mo5:,')

t h a t are coordirated t o t h e z e o l i t e c a t i o n i c posi-

t is t h e c o o r d i n a t e d C r ( v ) , V o ( v ) large molecc;lcs t h a t s h i e l d s

tions.

because

acid

s i t e s r:eecled f o r o l i g o m e r i z j t i o n and c : ~ c l l z a t i o n , a n d l e a d s t o a low s e l e c t i v i t y f o r DrGpane a r o m a t i z a t i o n and a 9 i g h a c t i v i t y f o r p r o p a n e c r a c k i n g o n Mo/ZSt?-lj

and p r o p a n e d e h ~ ~ d r o g e n a t i oonn Cr/XM-5, as shown i n TPblc- 6. Evidently, t h e Dossibilty o f c a t i o n introduction i n t o high-silica

zeolites

by a s o l i d s t a t e r e a c t i o n is d e t e r m i n e d by p h v s i c a l p r o p e r t i e s o f metal o x i d e s , s u c h as m e l t i n g p o i n t , s u b l i m a t i o n p o i n t , e t c . and t h e p r u s e n c e of a c i d c i t e s z e o l i t e s kchirh may b e c o n r i d e r e d 9 s p o w e r f u l t r a p s f o r t h e m i g r a t i n g S i n c e PioCI,, : 2 t B * ' C ) and C r O n ( 1 6 7 ° C ) a r e low-me1 tii-q-point-compounds, z r c h a n p takes p l a c e s m m t t i l y 1-1;5;1-5

+ mctal-ccmpounds

iii

ion-

the c o n d i t i o n s of c a l c i i m t i o n of t h e ini*tur€

s t a b o v e 500 C.

The m e l t i n g poin!

P ~ o t h e rf a c t o r i s r e p o r t e d

(1978"'C).

On

CatiJii5.

o f 3 0 is h i g h

t o e x p l a i n t h e ion-exchange

o c c u r e d i n the m i x t u r e HZSM-5 + ZnO t h r o u g h s o l i d s t a t e r e a c t i o n a b o v e 500

c.

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