Properties of the Metallic Nickel in Reduced NaNiY Zeolite Catalysts

43 Properties of the Metallic Nickel in Reduced NaNiY Zeolite Catalysts K. H. BAGER, F. VOGT, and H . BREMER

Downloaded by UNIV LAVAL on May 7, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0040.ch043

Department of Chemistry, Technical University "Carl Schorlemmer" Leuna-Merseburg, Merseburg, West Germany

ABSTRACT The reducibility of the Ni2+ cations, the dispersion and location of the metallic phase and the catalytic dehydrogenation a c t i v i t y were investigated on zeolites NaNiY (- 4 wt. % Ni) modified by mono, two and threevalent cations (NH4+, Ca2+, Ce3+). The metallic nickel i s localized on the external surface as well as i n the zeolite cages depending on the temperature of reduction and the nature of the second-cation.

Introduction Zeolite catalysts which contain cations of group VIII b exhibit bifunctional properties after reduction. Therefore most zeolite catalysts used for hydrocracking, hydroiscmerization and selectofontiing contain noble metals l i k e Pt, Pd or the cheaper nickel as hydrogénation /dehydrogenation carponents. For optimal hydrogénation /dehydrogenation properties the content, the dispersion and the location of the metal are of essential importance. I t should be possible to modify these properties to a certain extent by choice of the pretreatment conditions, the degree of cation exchange and a controlled directional effect on the N i ion location, e. g., by exchange of a second-cation with different s i t e selectivity. The reducibility of N i ions i n faujasite type zeolites and characterization of the metal by X-ray, electron microscopy, magnetic, chemisorption and catalytic methods have been the subject of many investigations (1-14). Most authors agree that after reduction the nickel migrates t o the external surface where i t deposits yielding large agglcmerates. Nevertheless there i s l i t t l e information on which conditions of pretreatment and of reduction lead to the reduced nickel remaining within the zeolite cages and on what i t s properties are. The site-directing influence of a 528 Katzer; Molecular Sieves—II ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

BAGER ET

43.

AL.

Metallic

Nickel

in Reduced

NaNiY

Catalysts

529

second-cation on the properties of the reduced nickel zeolites has not been investigated. The present work deals with these problems.

Experimental Materials. The zeolites studied are summarized i n Table I. The samples were prepared by a consecutive ion exchange 12 h) at 70 ^C with 0.1 Ν nitrate solutions of the Ni , NH , Ca and Ce ions, respectively. The degree of exchange was determined by analyzing the s o l i d for the amount of remaining sodium and exchanged cations. Downloaded by UNIV LAVAL on May 7, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0040.ch043

3

4

Table I.-

Zeolites Studied Si0 /Al 0 2

S a r n p l e

2

Exchange Degree

3

Mole Ratio

NaY Na 0.3QNÎY 0.30CaNa 0.31NÎY 0.47CeNa 0.32NÎY 0.49NH Na 0.32NÎY 4

equ. % of .2+ _

5.2 5.2 5.2 5.2 5.2

0 30 31 32 32

equ. % of Second cation 0 0 30 47 49

Pretreatment of Samples. A l l samples were reduced with hydrogen (4 1/h) for 2 h at temperatures between 300 and 500 °C after drying at 110 C. For comparison parts of the dryed samples were reduced after pretreatment (15 min.) at 500 °C i n a stream of argon (4 1/h) and a i r (4 1/h), respectively. Experimental Technique. The degree of reduction of nickel was determined by an iodimetric t i t r a t i o n . The metallic nickel was solved by an acid 1.0 M K C r 0 solution (15). XPS investigations were carried out on an A.E.I, spectrometer, ES-100 with Algo^-radiation (E= 1486-6 eV). The samples were pretreated at 400 C i n vacuum (5·10~ torr) and f o r 2 h i n a hydrogen atmosphere, followed by a vacuum treatment for 2 h (16) . The electron ferromagnetic resonance (EFR) spectra were recorded by ESR spectrometer ER 9 (VEB Carl Zeiss Jena) i n the regigg of 600-5000 Oe. After reduction samples were evacuated (10~ torr) 2

a

2

7

Ttiese investigations were carried out at the Academy of Sciences of USSR, Institute of Organic Chemistry, Mosccw, i n the laboratory of Prof. Dr. Kh. M. Minachev.

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

MOLECULAR SIEVES—Π

530

for 2 h at 250 C and sealed under vacuum. Chemisorption of oxygen was carried out at 0 °C by a pulse chromatographic method. For electron microscopy we used a microscope of type SEJA 3/2 (VEB Werk fur Fernsehelektronik Berlin). The reduced samples were investigated by means of coal replica techniques. Catalytic a c t i v i t y i n cyclohexane dehydrogenation was measured at 280-320 C by a pulse technique.


Results In Figure 1 the degree of reduction i s given as function of reduction tenperature for a l l unpretreated samples and for the pretreated NaNiY. For a l l reduction temperatures the same sequence of degree of reduction (Ni /Ni 4Ni , %) was observed: CaNaNiY > NaNiY > NH NaNiY > CeNaNiY. Three different regions of température can be clearly distinguished f o r the reduction of Ni ions e s p e c i a l l y i n the case of the zeolites CaNaNiY and NaNiY. U n t i l about 350 C the degree of reduction strongly increased with temperature, followed by only slight changes i n the reduction degree. Above 420 C a further increase of the amount of reduced nickel was observed. For NELNaNiY and CeNaNiY a significant reduction of the nickel occurs only at temperatures above 420 C. In Figure 1 the influence of pretreatment on the degree of reduction for NaNiY i s also shown. The typical reduction behavior remains unchanged, but the dehydration before reduction diminished the extent of reduction. The d i s t i n c t differences existing at lcwer temperatures are no longer presenc at reduction temperatures of 500 °C. The degree of reduction determined by XPS after 2 h of reduction at 400 C i s summarized i n Table I I . For catparison the values for the reduction degrees as measured by the chemical method are given i n parenthesis. 4

f

Q

Table I I . -

Degree of Reduction of Nickel

Sample

Degree of Reduction (Ni°/Hi°-«îi

2+

after Vacuum TreatCaNaNiY NaNiY CeNaNiY NH NaNiY 4

(XPS)

58 49 32 27

(42) (28) ( 6) (18)

%)

after A i r Treat38 35 18-20 10-11

(28) (20) ( 6) (12)



43.

BAGER ET A L .

Metallic

Nickel

in Reduced

NaNiY

Catalysts

531

The values obtained by XPS are higher than those measured by the chemical method. While XPS i s only sensitive to the nickel i n the external surface layers the chemical method determines the integral value for surface and bulk. In spite of these differences the sequence of reduction degrees i n the samples examined i s the same with both methods (exception: CeNaNiY). Fran the EFR spectra we got the l i n e intensity (Ipgr), the l i n e with ( Δ Η) and the g-values (17). The change of tnese parameters with tenperature of reduction i s shewn i n Figure 2 for the unpretreated NaNiY (CaNaNiY shows an andjogous behavior) and NH^NaNiY. These differences i n the reduction behavior i n the case of CaNaNiY and NaNiY on the one hand and i n the case of NH.NaNiY and CeNaNiY on the other hand are reflected i n the -values. While the Δ Η- and g-values for the NaNiY, CaNaNiY and CeNaNiY are constant with increasing reduction tenperature, bothe parameters for the NH-NaNiY sample are characterized by a maximum value a t reduction tmperatures of 450 C. The L _ , values (I related to the amount of nickel) f o r a l l un- ^ pretreated samples and a sample of NaNiY which were thermally treated (vacuum, 580 C, 50 h) after the reduction are shewn i n Figure 3. The curve course for a l l samples was similar with the exception of NH^NaNiY. For reduction temperatures up to 400 C the Irçrç .-values are essentially unchanged. Above 400 C they decreaseί At 450 C f o r the NH-NaNiY a significant maximum i s evident. U p t o 400 °C the sequence of X j _ «.-values i s : CeNâNiY> NàNiY (580 C, 50 h) > CaNaNiY>NaNiY ^ ^ J ^ NH^NàNiY. Figure 4 shews f o r a i l unpretreated samples the results obtained i n the oxygen chemisorption as function of reduction tenperature. The F-value characterizes the proportion of the nickel surface atems (aval ai M e for oxygen) t o the t o t a l number of nickel atcms. For the calculation of the average p a r t i c l e size frcm the chemisorption data the egde distance of regular octahedrons was taken as p a r t i c l e diameter (ÇL.). An adgorption stoichicmetry of Ni:01:1 and an effective Area of 7,2 A N i per oxygen atcm were assumed. NH-NaNiY shows the highest F-values and corresponding the lowest N i p a r t i c l e sizes (1-30 A) changing strongly with increasing reduction temperature. For the other samples the N i p a r t i c l e sizes (60-300 A) nearly independent of reduction tenperature. Results of the electron microcopy of the airpretreated samples and the specific nickel surface area of the unpretreated samples are summarized i n Table III. This table shews that with increasing reduction tenperature the c r y s t a l l i t e s a t the external surface of the NaNiY and CaNaNiY increase. The nickel i n CeNaNiY at high tenperatures tends to migrate and c r y s t a l l i z e a t the external zeolite surface upon reduction. For NH^NaNiY at a l l reduction temperatures no nickel c r y s t a l l i t e s were detectable a t the surface. Although the NH^NaNiY and CeNaNiY exhibit comparable degrees of reduction of nickel, the specific nickel surface area E

N

1


L

?


532

300


TEMPERATURE

400

500

OF REDUCTION,°C

Figure 1. Temperature dependence of the degree of nickel reduction for unpretreated samples and pretreated NaNiY

ο

Να Ni Y

•

NH. Να Ni Y 4

20, o—ro

/

s "

10

15001

u

»1000

500

2.4

Figure 2. Temperature dependence of the EFR parameters for the NaNiY and ΝΗ,,ΝαΝίΥ

2.2 300

400

500

TEMPERATURE OF REDUCTION,


X

Metallic

BAGER ET AL.

30f

·

No Ni Y

X

CtNoNi Y

•

Co Να Ni Y

o

Nickel in Reduced

NaNiY

Catalysts

533

C50h.580 °C )

No Ni Y

•

NH^NoNi Y

20\

7o

ν

Ο V

V

— ν

o

X /


1.0

W0 500 TEMPERATURE 0F REDUCTION . °c

Figure 3. EFR line intensity ÇLRBLJSH) as a function of reduction temperature

10| 12

α 18

\ d

o *

>0.5h

NH 4 No Ni Y

23

No Ni Y Co No Ni Y

x C t Να Ni Y

32

*~ 58 72

^

100 250

300

400

500

TEMPERATURE 0F REDUCTION, °C

Figure 4. Dependence of crystallite size (m) and F-value on the tempera ture of reduction



534

d i f f e r s by threefold. For a l l samples there i s a maximum of the specific nickel surface area at a reduction tenperature of about 450 ° C Table I I I . - Sizes of the Nickel C r y s t a l l i t e s , 0^. (Electron Microscope) and Specific Nickel Surface Area, Sj,. (Oxygen Chemisorption) * 2

S^, m /gNi

Sample after Reduction at Downloaded by UNIV LAVAL on May 7, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0040.ch043

386 °C NaNiY CaNaNiY CeNaNiY NH NaNiY

50 160

4

486 °C 135 260 260

after Reduction at 400 °C 450 °C 500 °C 2.2 2.4 0.8 4.2

2.6 2.6 1.8 4.8

2,4 2«4 1.6 4.6

Based on c r i t i c a l molecular diameter the dehydrogenation reaction of cyclohexane to benzene should occur on the metallic nickel on the external zeolite surface as well as on the f i n e l y dispersed nickel i n the supercages. The apparent (per g catalyst) and specific (per niNi) constants of reaction rate (calculated according to the equation of Basset and Habgood (18) frem the degrees of conversion determined at 290°C) are shown i n Figures 5 and 6 for the unpretreated sanples as a function of the reduction tenperature. NH^NaNiY reduced between 350-40ofchas the highest dehydrogenation a c t i v i t y despite the extremely lew degree of reduction. With the exception of CeNaNiY a l l the other sanples have their maximum catalytic a c t i v i t y after reduction at 350-400 C. The NaNiY has a second maximum at the reduction tenperature at which also the maximum catalytic a c t i v i t y of the CeNaNiY occurs (460 °C). For NaNiY the specific catalytic N i a c t i v i t y decreases only s l i g h t l y with increasing reduction tenperature; i t decreases more significantly for the other samples (Figure 6). Reduced NaNiY have the highest specific dehydrogenation a c t i v i t y of a l l sanples investigated.

Discussion Nickel zeolite carrier catalysts exhibit good hydrogénation/ dehydrogenation properties when they have a large and accessible metallic surface. The c a t a l y t i c a l l y effective metallic surface i s determined by the amount, the dispersion and the location of the metal i n zeolite cages.


Metallic

BAGER ET A L . α o

ΜΗ. No Ni No Ni ν Co No Ni X Co No Ni

Nickel

in Reduced

NaNiY

Catalysts

Y Y Y Y

4.0r

/ 0 0

Λ α -

2D|


UJ é

Σ O * 101

300

400

500

TEMPERATURE OF REDUCTION, °c

T£ 20Γ

ο

No

Ni

Figure 5. Apparent reaction rate constants as a function of reduction temperature

Υ

ν Co Να Ni Υ Χ C t No Ni Y

15

0.5^ 300 TEMPERATURE Figure 6.

400

500

OF REDUCTION,

Influence of reduction temperature on specific reac tion rate constants



536

Figure 1 clearly i l l u s t r a t e s that the amount of nickel reduced between 300500 C increases considerably. While at 500 C the differences i n the extent of reduction of the sanples are not large; for lcwer temperatures significant differences are observed, Fran t h i s i t f o l l e t s that a reduction températures belcw 400 C (Figure 1) the Ni ions i n the supercages are reduced and that only at higher temperatures does reduction of Ni i n t h e small cages take place. While cations which support the Ni location i n the small cages (S , 5 ,, S ,) decrease the nickel reducibility (NH , Ce ), Ca cations (high S selectivity) cause an increased reducibility. By the choice of the cation present not only nickel reducibility but also the dispersion of the metal formed can be influenced. In the case of CaNaNiY and NaNiY the reduction leads tQ r e l a t i v e l y large metal c r y s t a l l i t e s (Figure 2 and 3) which have superparamagnetic and ferranagnetic properties, respectively. Metal c r y s t a l l i t e s with these properties may only be formed at the external suface. The oxygen chemisorption measurements and the electron microscopic investigations (Figure 4 and Table III) confirm the migration and the location of the nickel at the external surface, A remarkable reduction of CeNaNiY begins only at températures above 400 C and leads to the formation of only a few but r e l a t i v e l y large nickel c r y s t a l l i t e s at the external surface (Figure 3 and 4, Table I I I ) , On thg other hand the reduction of the nickel i n NH^NaNiY belcw 400 C results i n the formation of very small nickel particles expressed by r e l a t i v e l y s n a i l values of T , Λ Η and g (Figure 2 and 3), The high s t a b i l i t y of the metal dispersion i s probably due to a strong interaction of the nickel with defects of the zeolite l a t t i c e . For reduction temperatures above 400 C the nickel increas singly agglomerates i n the supercage forms clusters and the size of the active metal surface decreases (Figure 5), But on the other hand the reduction of the nickel i n the small cages begins. Both procedures are reflected by a maKiraum of I ™ . -. at a reduction tenperature of 450 C (Figure 3). ^ The decrease of I _ . at reduction temperatures above 400 C indicates i n a l l ' sanples an increase i n the amount of finely dispersed nickel as a consequence of further reduction. Even a vacuum treatment (50 h) at 580 C of reduced NaNiY d i d not a l t e r this curvature which i s characteristic f o r a l l sanples (Figure 3) and confirms the high s t a b i l i t y of the reduced nickel remaining i n the small cages. Amount, dispersion and location of the metallic nickel correspond well with the catalytic properties of the samples examined (Figure 5 and 6). The dehydrogenation a c t i v i t y increases with increasing reduction temperature and i s caused by the i n creasing amount of accessible nickel. At temperatures above 400 C the process of metal agglomeration predaninates. This process as well as the formation of metallic nickel i n the small cages decrease the accessible nickel surface and decrease the catalytic +

T

t

TT

J

z


4

R R r

f

x

N

l

N

i


43.

BAGER ET A L .

Metallic

Nickel

in Reduced

NaNïY

Catalysts

537

a c t i v i t y . The optiinum conditions with respect t o the accessibility and catatytic a c t i v i t y of the metal surface are realized i n the NaNiY sample.


Literature Cited 1. Yates, D.J.C., J . Phys. Chem. (1965) 69, 1676 2. Rabo, J.A., Angell, C.L., Kasai, P.H., Schcmaker, V., Disc. Faraday Soc. (1966) 41, 328 3. Rubinshtein, A.M., Minachev, Kh.M., Slinkin, A.A., Garanin, V.I., Ashavskaya, G.A., Izv. Akad. Nauk SSSR, Ser. Khim. (1968) 786 4. Riekert, L., Ber. Bunsenges. Phys. Chem. (1969) 73, 331 5. Lawson, J.D., Rase, H.F., Ind. Eng. Chem., Prod. Pes. Develop, (1970) 9, 317 6. Richardson, J.T., J . Catalysis (1971) 21, 122 7. Romanovski, W., Roczniki Chem. [Ann, Soc. Chim. Polon.] (1971) 45, 427 8. Bredichina, T.N., Evdokimov, V.B., Zh. F i z . Khim. (1967) 41, 2975 9. Brooks, C.S., Christopher, G.L.M., J . Catalysis (1968) 10, 211 10. Schmidt, F., Gunsser, W,, Knappwost, A., Ber. Bunsenges. Phys. Chem., (1973) 77, 1022 11. Herd, A.C., Pope, C.G., J. Chem. Soc., Faraday Trans. I (1973) 5, 833 12. Selenina, M., Z. anorg. a l l g . Chem. (1972) 387, 179 13. Vogt, F., Forner, Ch., Bremer, H., Becker, Κ., Weber, Μ., Chem. Techn. (1975) 27, 460 14. Minchev, H., Steinbach, F., Penchev, V., Z.Phys. Chem. (Frankfurt am Main) (1976) 99, 223 15. Bremer, H., Bager, Κ.Η., Vogt. F., Z. Chem. (1974) 14, 199 16. Minachev, Kh.M., Antoshin, G.V., Shpiro, E.S., Izv. Akad. Nauk SSSR, Ser. Khim. (1974) 1012 17. Slinkin, A.A., Usp. Khim. (1968) 8, 1531 18. Bassett, D.W., Habgood, H.W., J . Phys. Chem. (1960) 64, 769


Properties of the Metallic Nickel in Reduced NaNiY Zeolite Catalysts

Recommend Documents