Structural and Catalytic Properties of Nickel Modified Molecular Sieves

75 Structural and Catalytic Properties of Nickel

Downloaded by CALIFORNIA INST OF TECHNOLOGY on February 7, 2017 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0102.ch075

Modified Molecular Sieves V. PENCHEV, H. MINCHEV, V. KANAZIREV, and I. TSOLOVSKI

1

Institute of Organic Chemistry, Bulgarian Academy of Sciences, Sofia

The limit of stability of the crystal framework at different extents of Ni ion exchange of type A molecular sieve is shown by means of electron microscopy, differential thermal analysis, and x-ray diffraction. The data obtained from catalytic studies are in accord with the results of physical methods, showing preservation of the molecular sieve properties after reduction of the Ni ions. Metallic Ni aggregates on the external surface of the zeolite. In the dehydrogenation of cyclohexane and the hydrogenolysis of n-hexane, type A molecular sieve shows the properties of metallic Ni on an inert support. When NiNaA is mixed mechanically with CaY, a typical bifunctional catalyst is obtained.

" X T i c k e l catalysts o n a s u p p o r t h a v e b e e n a n d c o n t i n u e to b e a subject ^

of m u l t i l a t e r a l studies, the results of w h i c h h a v e h e l p e d the d e v e l

o p m e n t of the t h e o r y of the p o l y f u n c t i o n a l catalyst. erties of N i m o l e c u l a r sieves i n the presence of H studied completely.

2

T h e catalytic prop

(2,7,

8) h a v e n o t b e e n

S t u d y of zeolite s t r u c t u r a l changes a n d the c o n d i

t i o n of t h e m e t a l after t h e r m a l a n d h y d r o g e n treatment comparatively poor.

have

been

T h e same is true f o r the influence of zeolite t y p e

o n t h e c a t a l y t i c a c t i v i t y of the m e t a l . T h i s study aims to give a d d i t i o n a l i n f o r m a t i o n o n the subject. A s t y p e A m o l e c u l a r sieve m o d i f i e d w i t h N i proves to b e a suitable subject f o r c l a r i f y i n g some of t h e s t r u c t u r a l a n d c a t a l y t i c p e c u l i a r i t i e s of zeolites, i t is g i v e n m a i n c o n s i d e r a t i o n . Present address : Institute of G e n e r a l a n d Inorganic C h e m i s t r y , B u l g a r i a n A c a d e m y of Sciences, Sofia. 1

434

Flanigen and Sand; Molecular Sieve Zeolites-II Advances in Chemistry; American Chemical Society: Washington, DC, 1971.

75.

Nickel

PENCHEV E T A L .

Modified

Molecular

435

Sieves

Experimental F o r this s t u d y , L i n d e N a A z e o l i t e w a s u s e d . T h e i o n e x c h a n g e w a s c a r r i e d out w i t h 0 . 1 N s o l u t i o n of N i ( N 0 ) , a c c o r d i n g to o u r m e t h o d (6). I n T a b l e I , some of t h e d a t a o b t a i n e d are s h o w n . T h e a c i d i t y of t h e s a m ples w a s d e t e r m i n e d b y p o t e n t i o m e t r i c t i t r a t i o n w i t h C H O K ( 9 ) . T h e t h e r m o g r a p h i c studies w e r e c a r r i e d o u t u n d e r c o n d i t i o n s d e s c r i b e d i n Ref. 11. T h e r e d u c t i o n was c h e c k e d o n a s a m p l e p r e v i o u s l y r e d u c e d w i t h H u n d e r c o n d i t i o n s s i m i l a r to those i n c a t a l y t i c a c t i v a t i o n . T h e same p r o c e d u r e was c a r r i e d out i n N f l o w f o r c o m p a r i s o n . F i g u r e 1 illustrates the results. X - r a y studies of zeolite w i t h N i i n i o n f o r m w e r e c a r r i e d o u t m a k i n g use of C r r a d i a t i o n (10). T h e x-ray patterns o f the r e d u c e d samples w e r e m a d e w i t h C u r a d i a t i o n , t h e samples b e i n g p l a c e d i n 0 . 5 - m m d i a m e t e r L i n d e m a n c a p i l l a r y tubes. O n platinum—carbon r e p l i c a , e l e c t r o n m i c r o s c o p e studies w e r e c a r r i e d o u t (12). I n o r d e r to c l a r i f y the changes i n the zeolite w h e n heated, a d d i t i o n a l x-ray a n d electron m i c r o s c o p e studies w e r e c a r r i e d o u t o n Samples 3 a n d 4, h e a t e d at D T A s p e e d u p to 3 6 0 ° , 4 4 0 ° , 6 4 0 ° , 7 4 0 ° , 8 8 0 ° , a n d 1 0 7 0 ° C . T h e c a t a l y t i c properties w e r e e x a m i n e d i n the process of d e h y d r o g e n a t i o n of c y c l o hexane a n d h y d r o g e n o l y s i s a n d h y d r o i s o m e r i z a t i o n of n-hexane. T h e tests u n d e r a t m o s p h e r i c pressure w e r e c a r r i e d o u t i n a fixed-bed flow m i c r o r e a c t o r . T h e catalyst was r e d u c e d w i t h H i n the c a t a l y t i c r e a c t o r at 400 ° C d u r i n g a 4 - h o u r p e r i o d . D e h y d r o g e n a t i o n w a s c a r r i e d o u t at 3 0 0 ° C , space v e l o c i t y (F/W) \Ah~\ m o l e r a t i o H / C H — 5 : 1 , a n d d i l u t i o n o f t h e catalyst b y a n i n e r t m a t e r i a l . T h e c o n d i t i o n s of h y d r o genolysis a n d i s o m e r i z a t i o n are as f o l l o w s : t e m p e r a t u r e r a n g e : 3 0 0 ° 4 0 0 ° C , F/W 2h~\ H / C H 5 : 1 m o l e , a n d d i l u t i o n w i t h i n e r t m a t e r i a l . T h e p r o d u c t s w e r e a n a l y z e d b y t h e gas c h r o m a t o g r a p h i c m e t h o d . 3

2

3


2

2

2

2

2

Table I.

Sample D a t a DTA

No. of Sample 1 2 3 4 5 4a 4b Results

Ni, Wt %

Degree of Exchange, %Eqv

0 3.6 4.0 5.9 10.6 5.9 5.9

0 18 23 34 55 34 34

and

Acidity Mg Eqv

X 10\ H /Gram +

in Ν2

in H

0.50 1.56

0.45 3.45

0.60 0.71 0.60

1.16 1.12

2

1.16

Data

Loss of Wt,

Initial Temp, of Destruc tion, °C

22.4

840

24.0 25.0 26.3 23.0 22.0

780 750 640 740 755

%

Discussion

T h e x-ray studies (6) s h o w that after i o n exchange, N i zeolites r e t a i n t h e i r p r i m a r y structure.

M o r e d e t a i l e d i n f o r m a t i o n o n the s t a b i l i t y

of t h e N i zeolite w a s o b t a i n e d b y a t h e r m o g r a p h i c m e t h o d ( F i g u r e 1 ) .



436

MOLECULAR

SIEVE ZEOLITES

II

260° 1 10

Figure

1.

I

I 30

I

I 50

1

' 70

1

Differential thermograms and NiNaA zeolites

A p r o n o u n c e d e n d o t h e r m a l effect ( 1 6 0 ° - 2 4 0 ° C )

90 Trnin

of NaA a n d 2 high-temperature

e x o t h e r m a l effects ( 7 0 0 ° - 9 8 0 ° C ) w e r e o b s e r v e d i n a l l samples. N i f o r m s are c h a r a c t e r i z e d b y another e x o t h e r m a l effect ( 3 7 0 ° C ) , w h i c h is w e a k e r t h a n the other t w o . C o m p a r i s o n of D T A a n d D T G curves shows that the e n d o t h e r m a l effect corresponds to the l i b e r a t i o n o f w a t e r f r o m t h e m o l e c u l a r sieves.

T h e w a t e r l i b e r a t i o n is i n stages, a n d c o m p l e t e d e h y

d r a t i o n occurs o n l y after d e s t r u c t i o n of t h e m o l e c u l a r sieve f r a m e w o r k . E v e n at temperatures h i g h e r t h a n 6 0 0 ° C , a b o u t 1% w e l l - b o n d e d w a t e r r e m a i n s i n t h e zeolite. A l t h o u g h this q u a n t i t y of w a t e r is s m a l l , i t p l a y s a definite p a r t i n t h e s t r u c t u r a l s t a b i l i t y a n d the c a t i o n c o m p a r i s o n

(4).

T h e o b s e r v e d character of d e h y d r a t i o n is s h o w n e v e n better i n t h e N i sieves. T h e p e a k of the e n d o t h e r m a l r e a c t i o n is s p l i t i n t w o , a n d its shift to l o w e r temperatures has b e e n o b s e r v e d .

T h e total quantity of water

a d s o r b e d b y t h e zeolite increases f r o m 22 to 2 6 % w i t h i n c r e a s i n g degree of i o n exchange ( T a b l e I ) , w h i c h is i n a c c o r d w i t h t h e greater h y d r a t i o n a b i l i t y of t h e N i ions ( 3 ) . W h e n N i is i n t r o d u c e d , i t decreases t h e r e l a t i v e d y n a m i c t h e r m a l s t a b i l i t y ; t h e b e g i n n i n g of t h e first h i g h - t e m p e r a t u r e exoeffect n o t e d o n t h e D T A p l o t s , w h i c h m a r k s t h e d e s t r u c t i o n o f t h e f r a m e w o r k ( I ) , decreases f r o m 8 4 0 ° to 6 4 0 ° C ( T a b l e I ) . T h e a d d i t i o n a l e l e c t r o n m i c r o s c o p e ( F i g u r e 2c,d,e) a n d x-ray studies ( F i g u r e 3 ) s h o w



75.

PENCHEV ET

AL.

Figure 2.

Nickel

Modified

Molecular

Sieves

437

Electron microscope photos of zeolite samples a. h. c. d. e.

No. 1 No. 4 No. 4 heated to 640° No. 4 heated to 870° No. 4 heated to 1070°

that the zeolite structure r e m a i n s u n c h a n g e d u n t i l t h e b e g i n n i n g of t h e first h i g h - t e m p e r a t u r e exoeffect a n d later transforms into a state a m o r p h o u s to x-rays.

A t the e n d , a n e w c r y s t a l phase is f o r m e d w i t h a

n e p h e l i n e structure.

T h e r m o g r a p h i c studies f o r S a m p l e 4 treated u n d e r

c o n d i t i o n s analogous to t h e c a t a l y t i c a c t i v a t i o n s h o w c o n s i d e r a b l e changes i n the endoeffect o n l y c h a r a c t e r i s t i c of the r e d u c e d s a m p l e ( F i g u r e 1 ) . Y e t , after h e a t i n g at 400 ° C , some n o n r e v e r s i b l e changes o c c u r , m a r k e d


438

MOLECULAR

SIEVE

ZEOLITES

II

b y a decrease o f t h e t o t a l a d s o r p t i o n c a p a c i t y b y 2 - 3 % ( T a b l e I ) a n d d i s a p p e a r a n c e o f some o f t h e w e a k e r x-ray lines ( F i g u r e 3 ) .

ί 1 11 11


H i

I

, . Ι ι I, i . l

i Lill

i . 1—i—1_1

ili.il 11 h il

\ I.,!.. i i i l l j l l 20

α

I

I

I, Ιΐ,ΙΙ,ιιιιΙ 60

40

/A4inH

}

1 I 1

I . , . . , .

III

.

.

Il

M4inN

t

1 1

Mi I 11,111 u ι i- I ' • • • 100 120 80 29—Cu~Kei 1070'C

ι ι

.,,111

11,1,

ί

. ιI

1 1 i l 1 1 11 1Il

eeo'c

IL

, ι il

?40'c

I ι II

L_L

JLL

I • Il

il lui

111

111 I ..

440'C 360'C

ι

Lull I. 40

640'C

il i 120

80

M*

140 2G~Cr-K«

Figure 3. X-ray diffraction data on Sample 4; the arrows note lines characteristic of Ni and NiO a. Heating in air b. Heating to 400° in H or N flow 2

2

A l t h o u g h t h e c y c l o h e x a n e m o l e c u l e has a b i g g e r c r i t i c a l d i a m e t e r size ( 6 . 1 A ) t h a n t h e e n t r y openings o f t y p e A m o l e c u l a r sieve, d e h y d r o g e n a t i o n occurs to a c o n s i d e r a b l e degree.

T h i s r e s u l t confirms i n a cata

l y t i c w a y f o r t y p e A zeolites t h e fact, a c c e p t e d f o r m o l e c u l a r sieves X a n d Y ( 7 , 1 3 ) , that after r e d u c t i o n w i t h H , N i aggregates o n t h e e x t e r n a l 2

z e o l i t e surface i n crystals o f c o n s i d e r a b l e size. T a b l e I I shows that the specific a c t i v i t y o f t h e m e t a l decreases as its c o n c e n t r a t i o n increases o n the catalyst. T h i s effect m i g h t b e c o n s i d e r e d a n i n d i c a t i o n o f a n increase of N i c r y s t a l size. X - r a y studies ( F i g u r e 3 ) c o n f i r m t h e presence o f m e t a l l i c N i . D e t e r m i n a t i o n o f N i c r y s t a l sizes f r o m t h e l i n e breadths is d i f f i c u l t because o f t h e c o i n c i d e n c e o f a l l d i f f r a c t i o n lines of N i a n d N i O w i t h t h e lines o f t h e zeolite. T h e c o n s i d e r a b l y l o w e r specific a c t i v i t y o f


75.

Table I I .

Mol Sieve


Nickel

PENCHEV E T AL.

Quantity Wt% Ni

Modified

Molecular

439

Sieves

Specific A c t i v i t y and Concentration of N i Hydrogenolysis of n-Hexane, Grams C

Structural and Catalytic Properties of Nickel Modified Molecular Sieves

Recommend Documents