Catalytic Active Centers in Cerium-Exchanged Faujasite Zeolites

47 Catalytic Active Centers in Cerium-Exchanged

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Faujasite Zeolites HANNA HOSER, ANDRZEJ DABROWSKI, and STANISLAW KRZYZANOWSKI Institute of Organic Chemistry, Polish Academy of Sciences, 01-224 Warsaw, Poland

ABSTRACT The catalytic activity of cerium-exchanged zeolites for n-butene isomerization depends upon the Si/Al ratio, the extent of cation exchange, the temperature and method of activation. A l l these factors have the influence on the character and "concentration" of the catalytic active centres in the zeolites. Introduction Our e a r l i e r study (1) on the isomerization o f ol e f i n s catalyzed by cobalt-exchanged X z e o l i t e s a l l owed one t o conclude that the r e a c t i o n i n v o l v e s a c a rbonium c a t i o n intermediate, and that a c i d i c z e o l i t e hydroxyl groups are the a c t i v e centres responsible f o r the course o f the r e a c t i o n . IR s p e c t r a l studies (2-4) d i s c l o s e d that i f t e r v a l e n t cation-exchanged z e o l i t e s are subjected to thermal dehydration the number o f the r e s u l t i n g a c i d hydroxyl groups i s higher than that produced i n b i v a l e n t cation-exchanged zeol i t e s . This f a c t allows us to propose that cerium-exchanged z e o l i t e s should be h i g h l y a c t i v e c a t a l y s t s f o r o l e f i n s i s o m e r i z a t i o n . On the other hand, numerous i n v e s t i g a t o r s (5.-^8) have long reported that c e r t a i n z e o l i t e s , i n c l u d i n g rare earth-exchanged z e o l i t e s , when brought i n t o contact with unsaturated hydrocarbons, are capable of generating organic r a d i c a l s at

room temperature. Studies on CuNaX z e o l i t e - c a t a l y z e d i s o m e r i z a t i o n of butènes have l e d Dimitrov and Leach(8) to p o s t u l a te a r a d i c a l mechanism f o r the reaction~on the ground of an i n d u c t i o n p e r i o d observed. However, K e i i ^ t a l . (9), who also have demonstrated formation o f organic 572 Katzer; Molecular Sieves—II ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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r a d i c a l s during i s o m e r i z a t i o n of butènes on CeNaX and e s p e c i a l l y CeNaY, have r e j e c t e d the r a d i c a l mechanism because they found the r e a c t i o n to proceed at a f a s t e r rate than d i d the r a d i c a l formation. One of the present authors(10,11) who studied the mechanism of organic r a d i c a l s formation during adsorption of o l e f i n s on cerium-exchanged z e o l i t e s , showed that both the r a t e of r a d i c a l formation and concentration are g r e a t l y a f f e c t e d by the a c t i v a t i n g pretreatment and that number of r a d i c a l s can be cons i d e r a b l y enhanced i f appropriate conditions of a c t i v a t i o n are used. Therefore, i t was deemed advisable to study whether a c t i v a t i o n of cerium-exchanged zeol i t e s under s t r i c t l y s i m i l a r conditions would also r e s u l t i n m o d i f i c a t i o n of t h e i r c a t a l y t i c a c t i v i t y . Isomerization of but-1-ene was chosen as the t e s t r e a c t i o n because i t s mechanism has been w e l l recognized f o r various types of a c t i v e centres operative i n the c a t a l y s t s employed. Experimental Materials. S e r i e s of cerium-exchanged z e o l i t e s were prepared by i o n exchange. The s t a r t i n g mater i a l was a 13 NaX z e o l i t e (the Inowroclaw Soda Works, M^twy, Poland), S i / A l = 1 . 2 ? + . 0 2 , NaY ( I n s t i t u t e of I n d u s t r i a l Chemistry, Warsaw!, S i / A l = 2 . 3 7 + - 0 2 and d e a l uminated NaY ( I n s t i t u t e of C a t a l y s i s , Lyon, Prance), Si/Al=4.14+.02, a l l used as a powder containing no clay-binders. The exchange procedure was done at pH=6.2 to prevent p r e c i p i t a t i o n of cerium hydroxides. This procedure i n the case of X-type z e o l i t e s caused not only the exchange but also p a r t l y replacement of sodium by hydronium ions. The h y d r o l y s i s degree was not the same f o r a l l samples. I t was ca 15% f o r the samples of the exchange r a t i o lower than 56% of cerium. In the case of the highest extent of the exchange, the hydr o l y s i s degree was much lower, e.g. f o r CeNaX-68 i t was ca 5%· In comparison, f o r NaX alone(the blank t e s t ) , the h y d r o l y s i s degree was 13·6%· In the case of Y type z e o l i t e s the h y d r o l y s i s degree was r a t h e r low (1-2.5%) and i n the case of dealuminated z e o l i t e s i t was n e g l i g i b l e . The ion-exchange procedure and the a n a l y s i s have been described i n d e t a i l s elsewhere(1). Pure-grade but-1-ene (Pluka) was degassed and p u r i f i e d by repeated vacuum bulb-to-bulb d i s t i l l a t i o n . Apparatus and procedure. A 235-nuL glass r e a c t o r was used,operated i n conjunction with a conventional

Katzer; Molecular Sieves—II ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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vacuum apparatus which enabled c a t a l y s t s to be p r e l i m i n a r i l y outgassed and a c t i v a t e d i n a dry oxygen or argon atmosphere and r e a c t a n t s to be added i n pre determined amounts. For each experiment a f r e s h por t i o n of a hydrous-form c a t a l y s t r w a s weighed, 0.100g. The c a t a l y s t was outgassed ( l O ^ T o r r ) i n i t i a l l y at room temperature, and l a t e r the r e a c t o r with the c a t a l y s t was placed i n a thermostated e l e c t r i c a l oven and heated to a required temperature. Afterwards, the ze o l i t e c a t a l y s t , a c t i v a t e d i n vacuum f o r 3 h, was coo l e d to 50 0 and used d i r e c t l y f o r the r e a c t i o n . The second s e r i e s of c a t a l y s t s a f t e r a c t i v a t i o n f o r 2 h i n vacuum was r e a c t i v a t e d at the same temperature f o r 30 min i n a d r i e d and p u r i f i e d oxygen atmosphere (ca p=250 T o r r ) . The oxygen excess was then removed and the c a t a l y s t was outgassed f o r 30,-min at an i d e n t i c a l temperature u n t i l a vacuum of l O ^ T o r r was achieved. In a few cases the c a t a l y s t s were a c t i v a t e d i n dry argon. A f t e r having been a c t i v a t e d , the c a t a l y s t was cooled to 50 0 (the thermostated oven was changed), and used f o r the r e a c t i o n immediately a f t e r the con d i t i o n s become e s t a b l i s h e d . The isomerization was c a r r i e d out i n s t a t i c sys tem at 50 0· Argon-born but-1-ene (14 mole %) was ad mitted to the r e a c t o r by opening a greaseless valve which l e d to a part of the apparatus (equipped with a manometer) c a l i b r a t e d so as to produce a pressure of 200 T o r r i n the r e a c t o r . Reaction time was recorded from the moment of valve opening. The r e a c t o r was then closed and samples f o r a n a l y s i s were withdrawn through a rubber septum at 15 or 30-sec i n t e r v a l s . The samples were stored i n gas-tight syringes and then i n j e c t e d into a dimethylsulpholane (20% by wt./ chromosorb P) column, 3m long, operated at room temp. The IR spectra of tested z e o l i t e s have been r e c orded with UR-20 (Jena, GDR) and Perkin-Elmer 325 spectrometers applying the method descriebed by P i c h at(12). The pure z e o l i t e disks f o r IR measurements were a c t i v a t e d i n vacuum quartz-glass c e l l s at the same temperatures and atmosphere as the samples f o r catalytic tests. The EPR measurements have been described i n de t a i l s elsewhere(11). Results But-1-ene was isomerized s e l e c t i v e l y to n-butenes over each of the i n v e s t i g a t e d c a t a l y s t . A deposit


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ca.10%, was found to occur on the c a t a l y s t . The expe rimental r a t e constants for the disappearance of but1-ene. which was assumed to "be a f i r s t - o r d e r r e a c t i o n ( 1 ) , were used as a measure of c a t a l y s t a c t i v i t y . The k i n e t i c equation, log(X - X) = - k t + l o g X , pro ved to be a good f i t up to about 75% of the e q u i l i b r ium conversion. In t h i s equation, X and X stand f o r the conversions recorded a f t e r a p e r i o d of time t and at the e q u i l i b r i u m set up at a given r e a c t i o n temper ature, r e s p e c t i v e l y . The c a t a l y t i c a c t i v i t y of the z e o l i t e s i s r e l a t ed to the S i / A l r a t i o , the extent of c a t i o n exchange, and the temperature and mode of a c t i v a t i o n . As the r a t i o S i / A l i s increased, the a c t i v i t y r i s e s as may be seen from Table I . TABLE I . -

E f f e c t of S i / A l r a t i o

s Zeolite

8

Ce-^/unit c e l l

But-1-ene disappear ance r a t e constant k χ 10 (molecule^ l7

NaX NaY NaY dealum. CeNaX-68 CeNaY-6? CeNaY dealum.-70 § -each c a t a l y s t 400°C.

0 0 0 19.25 12.51 5.54

inactive inactive 0,00987 0.986 3.21 4.30

examined was

a c t i v a t e d i n vacuum at

Effect of exchange extent. The p l o t s i n Fig.1 show the z e o l i t e a c t i v i t y to be c l o s e l y r e l a t e d toCe3+ ion content. The NaX, NaHX-13.6, and CeNaX (below 30% of exchange) proved to be p r a c t i c a l l y i n a c t i v e . As the cerium ion content i s r a i s e d above 30%, the z e o l i t e a c t i v i t y r i s e s only s l i g h t l y , but, at contents excee ding 4-5%, i t r i s e s quite r a p i d l y . Whether below or above 45%, the r i s e i s almost r e c t i l i n e a r . Figure 2 shows IR spectra of the z e o l i t e s . As cerium i o n content i s increased, the band at ca. 3640 cm~* r i s e s i n i n t e n s i t y and the absorption maximum s h i f t s i t s e l f to lower frequencies. With the z e o l i t e s more than 40% Ce-exchanged, a new band i s produced at ca. 3520 cm"'', which becomes more intense as the Ce3+ i s f u r t h e r increased (curves d - f ). The degree of hy d r o l y s i s was p r a c t i c a l l y constant at ca. 15% (curves b -g).


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577

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Table I I . K i n e t i c data on i s o m e r i z a t i o n o f but-1-ene over cerium-exchanged z e o l i t e s

Catalyst

Activa tion temp.ι o„

Rate constants of but-1-ene disappearance IflO^molecule dg^s"^ Activation i n : vacuo oxygen

200 300 400 500

2.43 4.79 0.986 0.599 3.28 5.28 3.21 1.30

2.78 3.89 4.09 3.91 3.44 4.44 3.89 2.38

300 400 500

6.83 4.58 2.05

5.05 5.50 2.45

CeNaX-68

200 300 400 500

CeNaY-67

CeNaY-de alum-70


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E f f e c t of a c t i v a t i o n temperature. With the zeol i t e s a c t i v a t e d i n vacuum at 200 - 500°C (Table I I ) , the c a t a l y t i c a c t i v i t y r i s e s as the outgassing temperature i s increased, a t t a i n s maximum at 300°C and, at s t i l l higher temperatures, f a l l s r a p i d l y . With the CeNaY and CeNaY dealuminated, t h i s f a l l i s much small e r than that observed with CeNaX. Again, at a c t i v a t i o n temperatures higher than 300°C, the i n t e n s i t y o f the the 3500 - 3700 cm" bands a t t r i b u t a b l e to hydrox y l groups v i b r a t i o n s becomes considerably reduced. The maximum changes occur with the z e o l i t e X ( c f . curves a i n P i g . 3A, 3B and 3C). E f f e c t of a c t i v a t i o n mode. R e a c t i v a t i o n o f Ceexchanged z e o l i t e s i n oxygen has r e s u l t e d i n considerable m o d i f i c a t i o n of the c a t a l y t i c a c t i v i t y ofthe vacuum-activated specimens (Table I I ) . Except f o r the specimens vacuum-activated at 300°C, the oxygen-react i v a t e d z e o l i t e s are the more a c t i v e . Those r e a c t i v a ted at s t i l l higher temperatures, and e s p e c i a l l y the z e o l i t e X, exhibited the most pronounced changes. As the vacuum a c t i v a t i o n temperature i s r a i s e d to 400°C, the a c t i v i t y of each z e o l i t e r i s e s and past t h i s temperature f a l l s . This a c t i v i t y f a l l i s , however, l e s s pronounced than that observed with the specimens a c t i vated only i n vacuum. R e a c t i v a t i o n i n oxygen ( e s p e c i a l l y of the z e o l i t e X at the higher temperature) has also a d i s t i n c t e f f ect on the product r a t i o s observed at the i n i t i a l r e a c t i o n stages. The c i s / t r a n s mole r a t i o evaluated from the data obtained at low conversions was p l o t t e d as a f u n c t i o n of but-1-ene conversion. The r a t i o s evaluated at zero conversion are l i s t e d i n Table I I I . 1

TABLE I I I . -

Catalyst

E f f e c t of temperature and mode of a c t i v a t i o n on c i s / t r a n s but-2-ene r a t i o Product r a t i o Temperature of a c t i v a t i o n A c t i v a t i o n c a r r i e d out xn Oxygen Vacuum °C

CeNaX-56

200 300 400 500

1.23 1.39 1.44 1.50

CeNaX-68

200 300 400 500

1.11 1.38 1.43 1.52

1.19 1.00 0.94 0.89 1.08 1.06 0.92 0.88


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Again, r e a c t i v a t i o n has considerably a f f e c t e d the IR spectra o f z e o l i t e s ( F i g . 3 ) · With the z e o l i t es r e a c t i v a t e d at 4O0°C and 500°C, the ca. 3640 c n T band r i s e s i n i n t e n s i t y whereas the ca. 3520 cmr band diminishes to n i l . 1

1

Discussion A c t i v e centres i n vacuum-activated cerium-exchanged z e o l i t e s . Comparison o f the a c t i v i t y data (Fig.1) with the IR s p e c t r a l evidence (Pig.2) i n d i c a t e s the f o l l o w i n g conclusions: The i n i t i a l increase i n the Ce5+ i o n content (up to 40% exchange) while r e s u l t i n g i n a much higher i n t e n s i t y o f ca. 3640 cm"" band ( r i s e of hydroxyl group concentration i n the z e o l i t e ) , only s l i g h t l y enhances the c a t a l y t i c a c t i v i t y o f the z e o l i t e s . On the other hand, a f u r t h e r increase i n the Ce* i o n content while l e a d i n g to p r o g r e s s i v e l y high ca. 3640 cm"" band i n t e n s i t i e s and r e s u l t i n g i n a new and i n c r e a s i n g l y intense band at ca. 3520 cm"", does at the same time r a p i d l y enhances the c a t a l y t i c a c t i v i ty o f the z e o l i t e s . T h i s f a c t may be regarded as t y p i c a l , because analogous r e l a t i o n s h i p s were found by the authors (13) i n the study o£ i s o m e r i z a t i o n of butènes over C o N a Y z e o l i t e s and by Ward (14-) i n the isomerizat i o n o f o-xylene over v a r i o u s l y Na*-exchanged NaHY zeo l i t e s . To i n t e r p r e t Ward's data , Dempsey (15) has pos t u l a t e d the occurence o f hydroxyl groups d i f f e r i n g i n a c i d strength but occupying i d e n t i c a l l a t t i c e s i t e s . According t o Dempsey, the a c i d i c p r o p e r t i e s o f the r e s u l t i n g hydroxyl groups are a f f e c t e d by the number o f A l ions present i n the adjacent l a t t i c e element. Dempsey b e l i e v e s , that a t low N a f o r H+ exchange, there are formed weakly a c i d i c hydroxyl groups responsible f o r the c a 3640 cm"" band, whereas a t higher exchange degrees, strongly a c i d i c hydroxyl groups are formed responsible f o r the ca. 3640 cm- and 3540 cm"" bands. The order o f the i n c r e a s i n g a c i d strength assumed f o r the hydroxyl group formation to f o l l o w the progress i n the exchange o f N a ions f o r protons i n a z e o l i t e Y i s consistent with the present data concerning CeNaX z e o l i t e s . Therefore, the z e o l i t e a c t i v e centres r e s ponsible f o r i s o m e r i z a t i o n appear to involve a c i d i c hydroxyl groups which are formed i n the CeNaX a f t e r a s u i t a b l e degree o f cerium i o n exchange has been a c h i e ved* Dempsey's i n t e r p r e t a t i o n (14·) provides an easy expianation f o r the commonly observed r i s e i n the c a t a l y t i c a c t i v i t y o f the z e o l i t e s with the increase i n the S i / A l r a t i o . The f a l l i n the A l i o n proportion i n 1

+

1

1

+

1

f

1

+


1

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582

the z e o l i t e l a t t i c e gives r i s e to a reduced number of weakly a c i d i c hydroxyl groups per u n i t c e l l . The a c t i v i t y f a l l observed i n cerium-exchanged z e o l i t e s a c t i v a t e d at temperatures exceeding 300°C i s associated with progressive dehydroxylation which i s evident from the diminishing i n t e n s i t y of the ca.3520 and 3640 cm"' absorption bands i n the IR spectrum (c£ curves a i n F i g . 3A 3B and 3C). In the case of the CeNaX p r e a c t i v a t e d at 500°0 the bands are b a r e l y v i sible. A c t i v e centres i n Ce-exchanged z e o l i t e s r e a c t i vated i n oxygenT The a c t i v i t y or the r e a c t i v a t e d zeo l i t e s i s seen (Table I I ) to vary a great deal with p a r t i c u l a r l y high d i f f e r e n c e s o c c u r r i n g f o r CeNaX act i v a t e d at the higher temperatures. Reference to the IR s p e c t r a l evidence (Pig. 3) hardly a s s i s t s i n exp l a i n i n g these v a r i a t i o n s . Reaction at 400 C and esp e c i a l l y at 500° C gives r i s e to a considerably more intense ca. 3640 cm" band and at the same time to a considerably l e s s intense ca.3520 cm"" band as compared with those recorded upon vacuum a c t i v a t i o n o f the specimens ( F i g . 3 ) . Our e a r l i e r EPR studies on Ce-exchanged oxygen-reactivated z e o l i t e s (10,11) have shown that under these conditions r a d i c a l oxygen centres are formed, representing both paramagnetic (Oj) and diamagnetic (Oé~) adsorbed oxygen species (x = 1, 2 ) . The nature of the a r i s i n g oxygen centres i s r e l a t e d p r i m a r i l y to thetemperature of z e o l i t e a c t i v a t i o n . Confronting the e a r l i e r (10,11) with the present r e s u l t s r e v e a l s a c o r r e l a t i o n to e x i s t between the number of r a d i c a l oxygen species i n z e o l i t e s and t h e i r c a t a l y t i c a c t i v i t y i n i s o m e r i z a t i o n of but-1-ene. Changes observed i n the i n i t i a l c i s / t r a n s but-2-ene product r a t i o s (Table I I I ) suggest that r a d i c a l centr e s p a r t i c i p a t e i n the isomerization, which would thus account f o r the enhanced c a t a l y t i c a c t i v i t y of the oxygen-reactivated z e o l i t e s . M o d i f i e d r a d i c a l - g e n e r a t i n g c a p a c i t i e s of v a r i o u s f a u j a s i t e - t y p e z e o l i tes (CeNaX > CeNaY dealum. > CeNaY) may also be r e s ponsible f o r the changes observed i n the c a t a l y t i c p r o p e r t i e s of the z e o l i t e s r e a c t i v a t e d i n oxygen at higher temperatures. Summing up, we b e l i e v e t h a t , on being a c t i v a t e d , cerium z e o l i t e s produce at l e a s t two types of a c t i v e centres which are responsible f o r the course of isomer i z a t i o n , v i z . , i o n i c centres a s s o c i a t e d with the p r e sence o f strongly a c i d i c hydroxyl groups and r a d i c a l centres a s s o c i a t e d with the presence of adsorbed oxygen species. The a p p l i c a t i o n of s u i t a b l e conditions f

1

1


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f o r the a c t i v a t i o n o f z e o l i t e s r e s u l t s i n a higher o r lower concentration" o f a given type o f centres. I n vacuum a c t i v a t i o n as w e l l as i n oxygen r e a c t i v a t i o n , though only at lower temperatures, the i o n i c centres predominate i n Ce-exchanged z e o l i t e s , whereby the i s o merization proceeds mainly through a carbonium c a t i o n as an intermediate stage. On the other hand, r e a c t i v a t i o n i n oxygen, e s p e c i a l l y o f CeNaX a t higher tem peratures, increases the concentration o f the a c i d i c s i t e s but also gives r i s e to a considerably higher nu mber o f r a d i c a l centres, which i n our opinion r e s u l t s i n at l e a s t p a r t i a l m o d i f i c a t i o n o f the r e a c t i o n me chanism toward the r a d i c a l mode. ft

Literature Cited 1. Hoser, H., Krzyzanowski, S., J . Catal. (1975), 38, 366. 2. Jacobs, P.Α., Uytterhoeven, J.Β., J . Chem. Soc., Faraday Trans. I. (1973), 69, 359, 373. 3. Smith, J . V., Advances in "Chemistry Series No 101, Molecular Sieve Zeolites-I, Chapter 15, p 171, ACS (1971). 4. Ward, J.W., J . Catal. (1969), 14, 365. 5. Hirschler, A.E., Neikam, W.C., Barmby, D.S., James R.L., J . Catal. (1965), 4, 628. 6. Raseev, G., J. Catal. (1971), 20, 120. 7. Krzyżanowski, S., J . Chem. Soc., Chem. Commun. (1974), 1036. 8. Dimitrov, Chr., Leach, H.F., J. Catal. (1969), 14, 337. 9. Suzuki, J., Honda, Y., Ono, Y., Keii, T., Proceedings of Vth Intern. Congress on Catalysis, Palm Beach, (1973), Vol 2, p 1377. 10. Krzyżanowski, S., Bull. Acad. Polon. Sci., Ser. Sci Chim. (1976), 24, 165, 231. 11. Krzyżanowski, S., J. Chem. Soc., Faraday Trans.I. (1976), 72, 1573. 12. Pichat, P., Chapter in "Spectroscopies Infrarouge et Raman"., Monographies du Centre d'Actualisation Scientifique et Technique de l'INSA, No 9, Masson et C , Éditeurs, Paris (1974), p 240. 13. Hoser, H., Dąbrowski, Α., Krżyzanowski, S., to be published. 14. Ward, J.W., Hansford, R.C., J. Catal. (1969), 13, 364. 15. Dempsey, E., J . Catal. (1975), 39, 155. ie


Catalytic Active Centers in Cerium-Exchanged Faujasite Zeolites

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