Molecular Sieves

41 Catalytic Functions of Metal—Zeolite Systems

Downloaded by JOHNS HOPKINS UNIV on October 21, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0121.ch041

V. PENCHEV, N. DAVIDOVA, V. KANAZIREV, H. MINCHEV, and Y. NEINSKA Institute of Organic Chemistry, Bulgarian Academy of Sciences, Sofia, Bulgaria

The paper deals with some new data concerning the state of the metal after reduction and the catalytic functions of zeolite cata lysts containing nickel and platinum. By using the molecular sieve selectivity in the hydrogenation of mesitylene it has be proved that metal (platinum) is contained in the volume of the zeolite crystal. The temperature dependence of the formation of nickel crystals was investigated. The aluminosilicate structure and the zeolite composition influence mainly the for mation of the metal surface which determines the catalytic activity. In the hydrocracking of cumene and disproportiona tion of toluene a bifunctional action of catalysts has been es tablished. Hydrogen retarded the reaction. ^ e o l i t e catalysts m o d i f i e d b y t r a n s i t i o n metals are i n t e r e s t i n g a n d difficult subjects t o s t u d y . I n one of t h e first studies of zeolites as c a t a l y s t s , R a b o a n d co-workers (1) used a zeolite c a t a l y s t c o n t a i n i n g 0 . 5 % p l a t i n u m for i s o m e r i z a t i o n of η-paraffins. I n t h i s r e a c t i o n t h e m e t a l zeolite s y s t e m a c t e d as a t y p i c a l representative of the b i f u n c t i o n a l c a t a l y s t s . Studies of zeolites m o d i f i e d b y t r a n s i t i o n metals (2, 3, 4) s h o w e d t h a t t h e i r p o l y f u n c t i o n a l properties are d e t e r m i n e d b y the s t r u c t u r a l a n d c h e m i c a l properties of t h e zeolite a n d b y the state of the m e t a l i n i t . I n t h i s paper we discuss new d a t a o n the m e t a l state after r e d u c t i o n as w e l l as t h e c a t a l y t i c functions of zeolite catalysts c o n t a i n i n g n i c k e l a n d p l a t i num. Experimental I n these experiments, s y n t h e t i c zeolites of the f a u j a s i t e - t y p e w i t h o u t b i n d i n g substance were used. C a l c i u m a n d n i c k e l - c a l c i u m samples i n i o n i c f o r m were o b t a i n e d b y i o n exchange u n d e r c o n d i t i o n s e n s u r i n g s t a b i l i t y of the c r y s t a l s t r u c t u r e (5). P l a t i n u m a d d i t i o n was c a r r i e d out b y i o n exchange w i t h P t ( N H ) C l (6). 3

6

4

461 In Molecular Sieves; Meier, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

462

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T h e surface a n d size of t h e m e t a l p a r t i c l e s after r e d u c t i o n were d e t e r m i n e d b y gas c h r o m a t o g r a p h i c i m p u l s e t i t r a t i o n i n a flow s y s t e m — f o r t h e p l a t i n u m samples h y d r o g e n - o x y g e n t i t r a t i o n i n n i t r o g e n (7) was u s e d , whereas c h e m i s o r p t i o n of o x y g e n i n h e l i u m flow (8) was u s e d f o r t h e n i c k e l samples. S o m e samples were a n a l y z e d b y e l e c t r o n m i c r o s c o p i c e x a m i n a t i o n s d e s c r i b e d i n R e f . 9. C a t a l y t i c i n v e s t i g a t i o n s were p e r f o r m e d i n a glass flow a p p a r a t u s a t a t m o s p h e r i c pressure. A p p r o p r i a t e p r e t r e a t m e n t a n d c a t a l y s t r e d u c t i o n for each c a t a l y t i c r e a c t i o n was c a r r i e d o u t .


Results and

Discussion

F o r correct i n t e r p r e t a t i o n of t h e b e h a v i o r of metal-zeolite c a t a l y s t s i t is i m p o r t a n t t o elucidate the m e t a l state i n t h e zeolite s t r u c t u r e . U s i n g gas c h r o m a t o g r a p h i c i m p u l s e t i t r a t i o n i n a flow s y s t e m we f o u n d t h a t the average d i a m e t e r of t h e p l a t i n u m particles was i n the range 30 t o 80 A . M e t a l p a r t i c l e s h a v e different diameters d e p e n d i n g o n t h e r e d u c t i o n c o n d i t i o n s a n d zeolite c o m p o s i t i o n . These d a t a were c o n f i r m e d b y electron m i c r o s c o p i c e x a m i n a t i o n . T h e m a x i m u m d i a m e t e r of p l a t i n u m c r y s t a l s is ca. 100-120 A . T h e average d i a m e t e r does n o t exclude t h e p o s s i b i l i t y t h a t a significant p a r t of t h e m e t a l is a fine dispersion w i t h p a r t i c l e sizes e q u a l t o or smaller t h a n t h e zeolite pores, as r e p o r t e d b y some a u t h o r s (1, 10, 11). T h e p r o b l e m of w h e t h e r t h e m e t a l is p l a c e d o n l y o n the surface of t h e zeolite c r y s t a l o r is present i n t h e v o l u m e of t h e c r y s t a l is of great i m p o r tance i n catalysis. I n f o r m a t i o n o n t h e presence of m e t a l particles i n t h e v o l u m e of t h e zeolite c r y s t a l s was o b t a i n e d b y c a t a l y t i c studies u s i n g selective m o l e c u l a r sieves. F o r m e r i n v e s t i g a t i o n s (3) s h o w e d t h a t after r e d u c t i o n of t h e m e t a l , c a t i o n m o l e c u l a r sieves of f a u j a s i t e - t y p e preserve t h e i r c r y s t a l s t r u c t u r e . H e n c e , molecules w i t h a d i a m e t e r l a r g e r t h a n the pores of t h e zeolites w o u l d n o t react i f t h e r e a c t i o n is d e t e r m i n e d b y t h e surface of m e t a l particles i n t h e v o l u m e of t h e zeolite c r y s t a l . F o r t h i s purpose we s t u d i e d mesitylene h y d r o g e n a t i o n o n t y p e X zeolites c o n t a i n i n g different a m o u n t s of p l a t i n u m . T h e a c t i v i t y of the same c a t a l y s t s i n t h e deh y d r o g e n a t i o n of cyclohexane was tested as w e l l . P l a t i n u m c a t a l y s t o n a l u m i n i u m oxide was used for c o m p a r i s o n . T h e results are g i v e n i n T a b l e I . T h e t w o zeolite samples show s i m i l a r a c t i v i t y o n cyclohexane d e h y d r o g e n a t i o n despite the f o u r f o l d greater a m o u n t of p l a t i n u m i n sample 2. Table I.

Ν 1 2 3

D e h y d r o g e n a t i o n of C y c l o h e x a n e a n d H y d r o g e n a t i o n of M e s i t y l e n e Dehydrogenation Hydrogenation at Cyclohexane of Mesitylene at 200°C, at 300°C, Platinum, wt% wt% Supporter wt % CaX CaX A1 0 2

3

0.53 1.83 0.52

37 46 50

In Molecular Sieves; Meier, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

0 10 21

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A t the same t i m e t h e zeolite w i t h lower p l a t i n u m c o n t e n t is i n a c t i v e t o w a r d s mesitylene hydrogénation w h i l e sample 2 shows a considerable a c t i v i t y . T h e p l a t i n u m - a l u m i n i u m oxide c a t a l y s t w i t h a n average p o r e size of 40 A appears a c t i v e i n b o t h t y p e s of reactions. E v i d e n t l y t h e r e sults c o u l d be e x p l a i n e d b y t h e i n a b i l i t y of the mesitylene molecules t o reach the m e t a l i n the v o l u m e of the zeolite c r y s t a l . B y i n c r e a s i n g t h e p l a t i n u m c o n t e n t i n t h e zeolite (sample 2), a c o n s i d e r a b l y greater p a r t of t h e m e t a l aggregates o n t h e surface of t h e zeolite c r y s t a l s ( w h i c h is c o n f i r m e d b y electron microscopic studies), r e s u l t i n g i n h i g h e r a c t i v i t y o n mesitylene hydrogénation. T h e influence of the r e d u c t i o n t e m p e r a t u r e was s t u d i e d for n i c k e l zeolite c a t a l y s t s . M o l e c u l a r sieves of t y p e A , X , a n d Y w h i c h c o n t a i n e d a b o u t 7-8 w t % n i c k e l were used. F i g u r e 1 shows t h e results of t h e s t u d y o n t h e f o r m a t i o n of m e t a l surface i n r e d u c t i o n temperatures f r o m 250 t o 600° C . R e d u c t i o n of n i c k e l w i t h h y d r o g e n begins a t 2 5 0 - 3 0 0 ° C for a l l

2ft

510

490

5S0

Figure 1. Dependence of nickel surface in the zeolite on the reduction temperature: 1, sample NiA (Ni content 7.1 wt %, Si0 /Al 0 = 2.0); 2, sample NiX (Ni content 8.1 wt %, Si0 /Al O = 2.6); 3, NiY (Ni content 8.0 wt%,Si0 /Al 0 = 3.8) 2

2

3

2

2

2

z

2

3


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t h r e e types of zeolite c a t a l y s t s . M a x i m u m surface of n i c k e l was o b t a i n e d a t r e d u c t i o n temperatures 4 5 0 - 5 0 0 ° C . A t higher r e d u c t i o n temperatures s i n t e r i n g of m e t a l particles takes place w i t h a corresponding decrease of t h e specific surface. I t c o u l d be expected t h a t samples w i t h greater specific (metal) surface s h o u l d h a v e higher c a t a l y t i c a c t i v i t y . T h i s a s s u m p t i o n w a s confirmed i n the s t u d y of benzene hydrogénation p e r f o r m e d a t 1 1 0 ° C . F i g u r e 2 presents the results of benzene hydrogénation o n t w o t y p e s of zeolites, N a A w i t h 7.1 w t % n i c k e l a n d N a X w i t h 8.1 w t % n i c k e l . B y v a r y i n g the t h e r m a l c o n d i t i o n s five samples w i t h different size n i c k e l c r y s t a l s were o b t a i n e d . T h e c a t a l y t i c a c t i v i t y per w e i g h t u n i t of r e d u c e d m e t a l decreases w i t h the increase of t h e d i a m e t e r of t h e n i c k e l c r y s t a l s . T h e a l u m i n o s i l i c a t e s t r u c t u r e influences m a i n l y the f o r m a t i o n of m e t a l surface w h i c h determines t h e c a t a l y t i c a c t i v i t y .

ZOO

300

i/DO

SBO

600

W

&00

if

Figure 2. Dependence of catalytic activity of zeolites type A and X on size of nickel crystals in benzene hydrogénation: · = type AO = type X T h e a c i d i c n a t u r e of N i C a Y after r e d u c t i o n of the m e t a l c a n be i l l u s t r a t e d b y u s i n g t h e m o d e l r e a c t i o n of c r a c k i n g of cumene. F i g u r e 3 shows the c a t a l y t i c a c t i v i t y a t v a r i o u s temperatures a n d t h e y i e l d s of the p r o d u c t s . T h e c a t a l y s t possesses h i g h a c t i v i t y even a t 2 0 0 ° C , where t h e c o n v e r s i o n is 20.5 mole % . A t 4 0 0 ° C t h e a c t i v i t y increases a n d the conversion reaches 97.1 mole % . A t 2 0 0 ° C d e a l k y l a t i o n is a c c o m p a n i e d b y d i s p r o p o r t i o n a t i o n w i t h f o r m a t i o n of d i i s o p r o p y l b e n z e n e . W i t h increasing temperature t h e d i s p r o p o r t i o n a t i o n decreases, w h i l e h y d r o g e n o l y s i s of the a l k y l c h a i n is s t r o n g l y increased. T o l u e n e d i s p r o p o r t i o n a t i o n depends on the a c i d i c properties of the c a t a l y s t s used. T h i s r e a c t i o n a l l o w s u s t o f o l l o w t h e b e h a v i o r of t h e l a t t e r a t higher temperatures (12). F i g u r e 4 shows t h e t o t a l c o n v e r s i o n of toluene o n zeolite C a Y m o d i f i e d b y n i c k e l a n d p l a t i n u m . N i C a Y shows h i g h e r c a t a l y t i c a c t i v i t y a n d has a d i s t i n c t l y expressed i n i t i a l a c t i v a t i o n


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moil

Figure 3. Extent of conversion and composition of liquid products in cumene cracking over NiCaY catalyst (24 wt % nickel), depending on temperature (volume rate 1.10h~ , ratio of hydrogen: cumene 10): 1, total conversion extent; 2, benzene; 3, toluene; 4, ethylbenzene; 5, diisopropyïbenzene l

p e r i o d . T h e changes i n t h e c a t a l y t i c a c t i v i t y of N i N a Y a n d N i C a Y zeolites d e p e n d o n the extent of i o n exchange w i t h n i c k e l , as s h o w n i n F i g u r e 5. Zeolite N a Y becomes a c t i v e o n l y after a d d i t i o n of a c e r t a i n a m o u n t of n i c k e l , after w h i c h i t s a c t i v i t y increases w i t h i n c r e a s i n g n i c k e l concentration. C a Y zeolite is c h a r a c t e r i z e d b y s t r o n g l y increased a c t i v i t y after a d d i t i o n of a b o u t 2 % n i c k e l . T h e difference i n the a c t i v i t y of these t w o c a t a l y s t s is p r o b a b l y due t o t h e different p e n e t r a t i o n of N i ( I I ) i n t h e c r y s t a l l a t t i c e i n t h e course of the ion-exchange process, w h i c h depends o n the c o m p e n s a t i n g c a t i o n . T h e p o s i t i o n of N i ( I I ) i n the ionic f o r m of t h e zeolite determines the different l o c a t i o n of the m e t a l particles a n d sites free f r o m cations i n the zeolite s t r u c t u r e after r e d u c t i o n . T h e studies o n t h e effect of c a r r i e r gas ( h y d r o g e n a n d nitrogen) a n d the r a t i o of h y d r o g e n : toluene p r o v i d e i n f o r m a t i o n for e l u c i d a t i o n of t h e role of n i c k e l i n toluene d i s p r o p o r t i o n a t i o n . A r a p i d d e a c t i v a t i o n of t h e


MOLECULAR SIEVES


Aw

0

60

420

HO

tfO

JOO

360

r

Figure 4- Change with time of activity toward toluene disproportionation (temperature 470° C volume rate 1.16 h' , ratio of hydrogen : toluene 10): 1, catalyst NiCaY (Ni content 2.4 wt %; 2, catalyst PtCaY (Pt content 0.5 wt %) }

1

Figure 5. Dependence of catalytic activity on nickel content of catalyst vs. toluene disproportionation (temperature 450° C, volume rate 1.16 h' , ratio of hydrogen'.toluene 10): 1, catalyst NaY; 2, catalyst CaY 1


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50

10

60

110

180

110

300

360

T f m i n )

Figure 6. Dependence of catalytic activity of NiCaY catalyst (Ni content 24 wt %) on molar ratio of hydrogen:toluene in toluene disproportionate (temperature 450° C, volume rate 1.16 h~ ): 1, hydrogen: toluene = 5; 2, hydrogen-.toluene = 10; 3, hydrogen-.toluene = 15; 4, hydrogen: toluene = 20; 5, nitrogen: toluene = 10 l

catalyst was observed i n a nitrogen flow (Figure 6), whereas hydrogen shows a stabilizing effect. When the molar ratio of hydrogen : toluene was increased, we noted a decrease i n the activity and an increase i n the activation period of the catalyst. This effect is similar to that studied b y Minachev and Isakov (13), namely, the retardation effect of hydrogen on isomerization of w-parafnns. I n toluene disproportionation, hydrogen promotes the hydrogenation of some reaction products whose deposition on the catalyst would cause its deactivation. Another possibility for blocking the reaction is the hydrogenation of some intermediate products at high ratios of hydrogen : toluene. Literature Cited 1. Rabo, J. Α., Pickert, P. E., Stamires, D. N., Boyle, J. E., Proc. 2nd Int. Congr. Catalysis (Paris) 1960, 2, 2055. 2. Richardson, J. T., J. Catalysis (1971) 21, 122. 3. Minachev, K. M., Garanin, V. I., Kharlamov, V. V., Isakova, Τ. Α., Kinetics Catalysis (1972) 13, 1101. 4. Penchev, V., Minchev, H., Kanazirev, V., Tsolovski, I., ADVAN. CHEM. SER. (1970) 102, 434. 5. Penchev, V., Minchev, H., Bakurdjiev, I., Tsolovski, I., Compt. Rend. Acad. Bulgare Sci. (1968) 21, 143.


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6. Penchev, V., Kanazirev, V., Commun. Dept. Chem. Bulg. Acad. Sci. (1972) 5, 4. 7. Grubner, Ο. Z., Phys. Chem. (1961) 216, 287. 8. Buyanova, N. E., Karnauhov, A. P., Kefely, L. M., Ratner, I. D., Chernyav skaya, O., Kinetics Catalysis (1967) 8, 868. 9. Penchev, V., Commun. Dept. Chem. Bulg. Acad. Sci. (1971) 4, 573. 10. Kubo, T., Arai, H., Tominaga, H., Kunugi, T., Bull. Chem. Soc. Japan (1972) 45, 607. 11. Dalla Betta, R. Α., Boudart, M., Int. Congr. Catalysis, 5th, Preprint. 12. Penchev, V., Davidova, N., Commun. Dept. Chem. Bulg. Acad. Sci. (1971) 4, 409. 13.

Minachev, Κ. M., Isakov, Y. I., Neftekhimiya (1970) 10, 805.


RECEIVED DECEMBER 5, 1972.


Molecular Sieves

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