Polymerization of Ethylene over Cr-Y Zeolite

52 Polymerization of Ethylene over Cr-Y Zeolite

Downloaded by CALIFORNIA INST OF TECHNOLOGY on September 19, 2017 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0040.ch052

TATSUAKI YASHIMA, JUN-ICHI NAGATA, and YUJI SHIMAZAKI Department of Chemistry, Faculty of Science, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan NOBUYOSHI HARA Department of Engineering Chemistry, Faculty of Engineering, Tokyo University of Agriculture and Technology, Naka-machi, Koganei-shi, Tokyo 184, Japan

ABSTRACT

The polymerization of ethylene over a Cr-Y catalyst without solvent was studied at relatively low reaction temperature. It was found that the polyethylene thus produced showed a high melting point and linear chain molecules without branches. It was then concluded that the active sites on Cr-Y were composed of divalent chromium ions. Introduction I t has been found that t r a n s i t i o n - m e t a l c a t i o n s are s t a b l y supported on z e o l i t e s by the ion-exchange procedure and promote some r e a c t i o n s j u s t l i k e complex c a t a l y s t s or metal oxide c a t a l y s t s . T r a n s i t i o n - m e t a l c a t i o n exchanged z e o l i t e c a t a l y s t s have r e c e n t l y been used f o r the o x i d a t i o n of hydrocarbons at a r e l a t i v e l y low r e a c t i o n temperature(1-3), and i n the amination of chlorobenzene with ammonia(4). We have found that over Ni-Y, Rh-Y, and Ru-Y the d i m e r i z a t i o n o f ethylene to n-butenes proceeds s e l e c t i v e l y at r e l a t i v e l y low r e a c t i o n temperature, 0-100°C(5)· We have concluded by i n f r a r e d and ESR studies that the a c t i v e s i t e s on Ni-Y and Rh-Y f o r the ethylene d i m e r i z a t i o n are zero v a l e n t N i and Rh, r e s p e c t i v e l y , which are h i g h l y dispersed i n the z e o l i t e framework(5). On the other hand, we have found that over the Cr-Y c a t a l y s t the polymerization of ethylene proceeds, and the polyethylene produced has a high melting p o i n t , high molecular weight, high d e n s i t y , and l i n e a r chain s t r u c t u r e without branches. In t h i s paper, the polymerization of ethylene over the Cr-Y c a t a l y s t and some p r o p e r t i e s of the polyethylene are s t u d i e d , and the c h a r a c t e r i s t i c s of the a c t i v e s i t e s on the c a t a l y s t are discussed.

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

52.

YASHiMA E T A L .

Polymerization of Ethylene over Cr-Y Zeolite

627

Experimental Methods


M a t e r i a l . Ethylene, having a p u r i t y of over 99.8%, was obtained from a commercial source. C a t a l y s t . Cr-Y was prepared by a conventional c a t i o n exchange procedure u s i n g Linde SK-40 and 0.2N aqueous s o l u t i o n of CrCl3(pH:3.7) at 70°C. The exchanged Cr-Y was washed by pure water, d r i e d at 100°C, p e l l e t e d without a b i n d e r , crushed and s i z e d i n 14-20 mesh. The Cr-Y c a t a l y s t used had an i o n exchange of 76% determined by atomic a b s o r p t i o n spectrophotometry, that i s , the atomic r a t i o obtained was Na : 3Cr : Al=24 : 76 : 100. The s p e c i f i c s u r f a c e area of t h i s Cr-Y was 870 m^/g, which was measured by BET method. Apparatus and procedure. The experiments f o r polyethylene synthesis were c a r r i e d out i n a s t a i n l e s s s t e e l autoclave(200ml). The c a t a l y s t ( 0 . 5 g ) was placed i n an e l e c t r i c a l l y heated quartz tube, a c t i v a t e d at v a r i o u s temperatures under evacuation, and was sealed i n a g l a s s tube to make an ampoule. The ampoule was put i n t o the autoclave and heated at the v a r i o u s r e a c t i o n temperatures a f t e r evacuation. The r e a c t i o n s t a r t e d when the ampoule was broken by the pressure of ethylene and i t proceeded under constant ethylene pressure without a s o l v e n t . The polyethylene formed was put i n t o water to separate i t from the broken pieces of g l a s s , and an HF aqueous s o l u t i o n was added to d i s s o l v e the c a t a l y s t . The polyethylene was then washed by a mixture of methanol and HC1 aqueous s o l u t i o n , followed by washing with acetone, and was d r i e d at room temperature. The p o l y ethylene was then used as a sample f o r measurements of m e l t i n g p o i n t , d e n s i t y , and i n f r a r e d s p e c t r a . The experiments f o r the observation of the e f f e c t of the c a l c i n a t i o n atmosphere and the e f f e c t of a d d i t i v e s fed i n t o the r e a c t i o n system were c a r r i e d out i n a f i x e d bed type apparatus. The c a t a l y s t ( 0 . 8 g ) was placed i n an e l e c t r i c a l l y heated quartz r e a c t o r , and c a l c i n e d at v a r i o u s temperatures i n v a r i o u s atmospheres, such as oxygen, hydrogen, CO as w e l l as i n a vacuo. Then the r e a c t o r was kept at a given r e a c t i o n temperature using a water bath. Ethylene was fed at 200 T o r r a f t e r the c a t a l y s t had adsorbed v a r i o u s amounts of water, NO, CO, or oxygen i n a gaseous s t a t e and ethylene was c i r c u l a t e d through the c a t a l y s t bed. The r a t e of pressure drop was measured with a mercury manometer. I n f r a r e d measurement. The sample f i l m s of polyethylene were made by p r e s s i n g with hot p l a t e s . The sample wafers of the c a t a l y s t were prepared i n a s e l f - s u p p o r t i n g form by p r e s s i n g the f i n e powder at 200 kg/cm . The wafers were e l e c t r i c a l l y heated at v a r i o u s temperatures and v a r i o u s atmospheres i n an i n f r a r e d cell. A f t e r the evacuation, the wafers were exposed to NO at 2

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

628

MOLECULAR

SIEVES—Π


40 T o r r f o r 30 min followed by evacuation at room temperature. A l l the i n f r a r e d s p e c t r a were recorded at room temperature. ESR measurement. The Cr-Y c a t a l y s t s were e l e c t r i c a l l y heated at v a r i o u s temperatures and v a r i o u s atmospheres i n a quartz tube. A f t e r evacuation, the samples were exposed to NO at 200 T o r r f o r 30 min followed by evacuation at room temperature. A l l the ESR measurements were c a r r i e d out at -196°C with an X-band spectrometer. R e f l e c t i v e v i s i b l e measurement. A l l v i s i b l e measurements were made with Cr-Y i n the same form as the sample of i n f r a r e d measurement. A f t e r evacuation, the wafers were exposed to water, NO, ethylene, and ammonia at 40 T o r r i n a quartz c e l l . A l l the s p e c t r a of the wafers were recorded at room temperature. Results and D i s c u s s i o n Synthesis of polyethylene. The c a t a l y s t w i t h the higher exchange degree of Cr i o n showed the higher a c t i v i t y f o r the p o l y m e r i z a t i o n of ethylene. However, the maximum degree of i o n exchange was 76% when our i o n exchange procedure was used. Table l(Run No. 1-5) shows the e f f e c t of evacuation c o n d i t i o n s on the c a t a l y s t a c t i v a t i o n . I t was found that more than 200°C was needed f o r the evacuation temperature, and the optimum temperature was 350°C. These r e s u l t s show that the water i n the z e o l i t i c cage depresses the p o l y m e r i z a t i o n , and on the other hand, at higher evacuation temperatures the c r y s t a l l i n i t y of Cr-Y i s decreased by the d e s t r u c t i o n of z e o l i t i c s t r u c t u r e ; the BET surface area has decreased from 870 to 820 m /g at 550°C of evacuation temperature. One hour evacuation at 350°C was s u f f i c i e n t to a c t i v a t e the Cr-Y c a t a l y s t f o r p o l y m e r i z a t i o n . Table l(Run No. 2,6-11) shows the e f f e c t of r e a c t i o n c o n d i t i o n s on the polyethylene y i e l d . I t was found that the p o l y m e r i z a t i o n proceeded at r e l a t i v e l y low r e a c t i o n temperatures, and the optimum temperature was 50-70°C. Even at 0°C, the polyethylene could be formed a f t e r 5 hr of r e a c t i o n time. The y i e l d of polyethylene increased l i n e a r l y with the pressure of ethylene i n the range from 5 to 50 atm, and increased l i n e a r l y with the r e a c t i o n time up to 10 h r . However, the i n c r e a s e of polyethylene y i e l d g r a d u a l l y reached a c e i l i n g a f t e r more than 10 hr of r e a c t i o n time. When 34.5g/g-cat. of the y i e l d ( a f t e r 10 hr of r e a c t i o n time) was reached, the c a t a l y s t s u r f a c e was s u f f i c i e n t l y covered by the polyethylene produced. The step i n which ethylene d i f f u s e d through the polyethylene l a y e r to the a c t i v e s i t e s on the c a t a l y s t seems to have some p a r t i n determining the r a t e of p o l y m e r i z a t i o n . In the case of hydrogen a d d i t i o n at 10 atm to the r e a c t i o n system, the y i e l d of polyethylene d i d not change. However, the a d d i t i o n of benzene as a solvent decreased the y i e l d of p o l y e t h y l e n e . The benzene 2


Katzer; Molecular Sieves—II ACS Symposium Series; American Chemical Society: Washington, DC, 1977. 3.0

350

350

350

350

350

350

5

6

7

8

350

11(c) 3.0

3.0

3.0

3.0

1.0

0.5

3.0

50

50

50

50

110

0

50

50

50

50

50

Temperature (°C)

40

40

50

10

40

40

40

40

40

40

40

Ethylene pressure(atm)

Reaction c o n d i t i o n s

3.8

13.1

16.4

3.0

4.7

2.5

13.6

8.7

7.2

13.8

4.6

Polyethylene yield(a) (g/g-cat.)

on

140

135

140

140

134

142

140

140

140

140

138

Melting point (°C)

( a ) : The values a f t e r 5 hr of r e a c t i o n time, ( b ) : The a d d i t i o n of hydrogen at 10 atm i n t o the r e a c t i o n system, ( c ) : The a d d i t i o n of 30 ml of benzene as solvent i n t o the r e a c t i o n system.

350

10(b)

9

3.0

500

3

4

3.0

350

2

3.0

Time (hr)

200

Evacuation temperature(°C)

conditions

1

Run No.

Activation

Table 1. E f f e c t of a c t i v a t i o n and r e a c t i o n c o n d i t i o n s y i e l d and melting p o i n t of p o l y e t h y l e n e


630

MOLECULAR

SIEVES—Π


used was p u r i f i e d by the a d d i t i o n of Na metal followed by d i s t i l l a t i o n , but might p o s s i b l y s t i l l c o n t a i n some i m p u r i t i e s . Some p r o p e r t i e s of polyethylene. The melting p o i n t s of a l l polyethylene samples are shown i n Table 1. The polyethylene produced at lower r e a c t i o n temperature showed the higher melting p o i n t s ; 142, 140, and 134°C at 0, 50, and 110°C, r e s p e c t i v e l y . In the case of a d d i t i o n of hydrogen i n t o the r e a c t i o n system, the melting point of the polyethylene produced was 135°C. These low melting p o i n t s would be r e l a t e d to the degree of branching of the polyethylene. The i n f r a r e d s p e c t r a of the polyethylene samples are shown i n F i g u r e .1. Absorption bands were observed at 1303, 1353, and 1369 cm"" , and a small shoulder band was observed at 1375 cm" i n the range between 1300 and 1400 cm" . The absorption bands at 1303, 1353, and 1369 cm" were a t t r i b u t e d to the CH2 group i n the l i n e a r chain of p o l y ethylene and the band at 1375 cm" was a t t r i b u t e d to the CH3 group of the branch(6). The very small absorption band at 1375 cm" shows very l i t t l e branching i n the l i n e a r chain polyethylene. However, the s p e c t r a of polyethylenes produced at 110°C and produced under the presence of hydrogen showed a rather l a r g e r a b s o r p t i o n at 1375 cm" . These r e s u l t s suggest that these polyethylenes had a small amount of branching i n the l i n e a r chain polymer. 1

1

1

1

1

1

1

The d e n s i t y of polyethylene produced was 0.962 g/cm**, and i t was not changed by the change of r e a c t i o n c o n d i t i o n s . T h i s value shows that the polyethylene produced over Cr-Y c a t a l y s t has high c r y s t a l l i n i t y . The molecular weight of the p o l y ethylene samples could not be measured, because the g e l a t i o n of the polyethylene occurred i n p-xylene as a s o l v e n t . We estimate that the molecular weight of a l l polyethylene samples i s more than 2 m i l l i o n . Nature of a c t i v e s i t e s . I t i s suggested that the p r o p e r t i e s of the polyethylene produced by t h i s work are s i m i l a r to those of the polyethylene produced by the P h i l l i p s Process u s i n g chromium oxides as a c a t a l y s t . In the study on the P h i l l i p s c a t a l y s t , many papers have reported on the a c t i v e s i t e s , e s p e c i a l l y on the valence s t a t e of chromium i n the a c t i v e center. Recently, K a z a n s k i i et a l . ( 7 ) have concluded that the exposed Cr3+ ions i n low c o o r d i n a t i o n s t a t e are the a c t i v e c e n t e r s . On the other hand, Krauss et al.(8,9) and Ermakov et al.(10) proposed that C r ions were contained i n the a c t i v e c e n t e r s . We present some i n f o r m a t i o n on the nature of a c t i v e s i t e s of Cr-Y c a t a l y s t f o r the p o l y m e r i z a t i o n of ethylene. A f t e r a c t i v a t i o n under evacuation at 350°C f o r 3 hr, the c a t a l y s t was c a l c i n e d i n oxygen at 350°C f o r 10-60 min followed by evacuation f o r 1 h r . The c a l c i n e d c a t a l y s t showed lower a c t i v i t y than the uncalcined c a t a l y s t , and the longer the c a t a l y s t was o x i d i z e d , the lower the a c t i v i t y . On the other 2 +



52.

YASHiMA E T A L .


631

hand, the c a t a l y s t s which were c a l c i n e d i n hydrogen or CO at 350°C f o r 10-180 min, showed lower a c t i v i t y than the uncalcined c a t a l y s t . The Cr-Y a c t i v a t e d under evacuation was the most a c t i v e c a t a l y s t . These r e s u l t s suggest that the valence s t a t e of chromium ions which compose the a c t i v e s i t e s i s n e i t h e r higher than t r i v a l e n t nor completely reduced metal. Figure 2 shows that the a d d i t i o n of water and NO decreases the c a t a l y t i c a c t i v i t y of Cr-Y. The poisoning e f f e c t of water was r e l a t i v e l y small, though NO remarkably poisoned the a c t i v e s i t e s . The a d d i t i o n of oxygen and CO remarkably decreased the c a t a l y t i c a c t i v i t y , but a f t e r some p e r i o d the a c t i v i t y recovered. The length of t h i s i n a c t i v e p e r i o d r e l a t e d to the amount of a d d i t i v e s ( F i g u r e 3). I t i s shown that NO s t r o n g l y i n t e r a c t s with the a c t i v e s i t e s . The i n f r a r e d s p e c t r a o f NO adsorbed on Cr-Y a c t i v a t e d under evacuation at 350°C f o r 3 hr were measured. The absorption bands were observed at 1780 and 1900 cm" . These absorption bands could not be found i n the case of Cr-Y evacuated up to 200°C which had no c a t a l y t i c a c t i v i t y f o r ethylene p o l y m e r i z a t i o n . These r e s u l t s show that these absorption bands can be a t t r i b u t e d to the i n t e r a c t i o n between the a c t i v e s i t e s on the c a t a l y s t and NO. These bands gradually decreased with the evacuation temperature a f t e r adsorption, but held t h e i r absorption up to 100°C. Naccache et al.(11) a t t r i b u t e d these bands to the n i t r o s y l complex formed by NO with C r on the z e o l i t e . An ESR s i g n a l was observed on the untreated Cr-Y. T h i s signal(g-value=l.97, width=500G) can be a t t r i b u t e d to C r . When the Cr-Y was a c t i v a t e d under evacuation at 350°C f o r 3 h r , only a broad s i g n a l was observed. T h i s s i g n a l would be a background of the z e o l i t e , because the same s i g n a l was observed on Na-Y. When NO was added i n t o the system i n which Cr-Y has been a c t i v a t e d under evacuation, a new ESR s i g n a l was observed; gi=1.999, g,/=1.918. T h i s s i g n a l i s caused by (3d )Cr+ i n the l i n e a r n i t r o s y l complex of C r + , and was reported by Naccache et al.(11) and Shelef et a l . ( 1 2 ) . On the other hand, when Cr-Y was c a l c i n e d i n oxygen at 350°C, a c l e a r ESR s i g n a l was observed, and was a t t r i b u t e d to Cr5+ by the data of g-value=1.98 and width=50G. On the Cr-Y r e d u c t i v e l y t r e a t e d by hydrogen, a weak and broad s i g n a l was observed, but i t could not be a t t r i b u t e d to any valence s t a t e of chromium, because t h i s s i g n a l overlapped with the background. F i g u r e 4 shows the r e f l e c t i v e v i s i b l e spectra of Cr-Y t r e a t e d under v a r i o u s c o n d i t i o n s . The absorption bands on untreated Cr-Y were observed at 410 and 580 nm. These bands show that the s t a t e of chromium ions i n the z e o l i t e i s t r i v a l e n t and coordinates with s i x water molecules(13). When the Cr-Y was a c t i v a t e d under evacuation at 350°C, one absorption band was observed at 750 nm, and d i d not s h i f t by the a d d i t i o n of water, NO, or ethylene. However, the a d d i t i o n of ammonia i n t h i s system s h i f t e d the absorption band to 630 nm. These phenomena were 1

2 +

3 +

5

2



MOLECULAR

1300

1400

1300

Wave number

1400

(cm"l)

Figure 1. Infrared spectra of polyethylene. The numbers in the figure correspond to the run numbers in Table 1. Amount of NO 0

2

4

6

Amount of water

(/jmol/g-cat. ) 8

10

12

(mmol/g-cat.)

Figure 2. Effect of addition of NO and water. Reaction conditions: catalyst, Cr-Y activated under evacuation at 350°C for 3 hr; reaction temperature, 60°C; ethylene initial pressure, 200 torr.


SIEVES-


52.

YASHIMA


E TA L .

Figure 3. Effect of addition of CO and oxygen. (0),no additives. (A), CO addition; (1), 1.6 μηιοί/ g-cat; (2), 3.3 ^mol/g-cat.; (3), 6.5 μτηοΐ/g-cat. (B), oxygen addi tion; (1), 0.5 μτηοΐ/g-cat.; (2), 1.0 μηιοί/g-cat.; (3), 1.5 μηιοί/g-cat. Reaction conditions: catalyst, CrY activated under evacuation at 350°C for 3 hr; reaction tempera ture, 60°C; ethylene initial pres sure, 200 torr.

Reaction time (min)

I

1

t

ι

\

•

c ο

ι

\

-»-> ίΟ

~

•

.

\

(c)

JQ «C

(B) 40 —

^

\ (D)

30 -

(A)

1 400

633

1

1

1

600 Wave length

1 800

(nm)

Figure 4. Visible spectra of Cr-Y catalyst. (A), uncalcined; (B), activated under evacuation at 350°C for 3 hr; (C), ammonia adsorbed on (B); (D), calcined (B) in oxygen at 350°C for 1 hr.


MOLECULAR

634

SIEVES—Π

c h a r a c t e r i s t i c s of d i v a l e n t Cr ion(_7) . Thus, i t i s suggested that the d i v a l e n t Cr ions are formed by the a c t i v a t i o n . In consequence, i t i s concluded that the a c t i v e s i t e s of Cr-Y c a t a l y s t are composed by d i v a l e n t Cr ions which are s t a b l y supported on the z e o l i t e .


Literature Cited

(1) Van Sickle, D. Ε., and Prest, M. L., J. Catal., (1970), 19, 209. (2) Mochida, I., Hayata, S., Kato, Α., and Seiyama, T., J. Catal., (1969), 15, 314; (1970), 19, 405; (1971), 23, 31. (3) Kubo, T., Kumada, F., Tominaga, H., and Kunugi, T., Nippon Kagaku Kaishi, (1972), 1972, 1621. (4) Hatada, Κ., Ono, Y., and Keii, T., Advan. Chem. Ser., (1973), 121, 501. (5) Yashima, T., Ushida, Y., Ebisawa, Μ., and Hara, N., J. Catal., (1975), 36, 320. (6) Krimm, S., Liang, C. Y., and Sutherland, G. Β. Β. M., J. Chem. Phys., (1956), 25, 549. (7) Przhevalskaya, L. K., Shvets, V. Α., and Kazanskii, V. B., Kinet. Katal., (1970), 11, 1310; J. Catal., (1975), 39, 363. (8) Krauss, H. L., and Stach, H., Inorg. Nucl. Chem. Letters, (1968), 4, 393. (9) Krauss, H. L., and Schmidt, Η., Z. Anorg. Allg. Chem., (1972), 392, 258. (10) Druzhkov, V. N., Zakharov, V. Α., and Ermakov, Yu. I., Kinet. Katal., (1973), 14, 998. (11) Naccache, C., and Taarit, Y. B., Faraday Trans. I, (1973), 69, 1475. (12) Shelef, M., J. Catal., (1969), 15, 289. (13) Anderson, J. H. Jr., J. Catal., (1973), 28, 76.


Polymerization of Ethylene over Cr-Y Zeolite

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