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SiO2 Catalyst
Science and Technology in Catalysis 1998 Copyright © 1999 by Kodansha Ltd.
85 Kinetic Study of Ethylene Polymerization over Cr/Si02 Catalyst
Toshiya SAITO, Manabu MOTEGI, Hiroyuki FURUHASHI and Satoshi UEKI Technical Development Center, Tonen Chemical Corporation, Kawasaki-ku, Kawasaki 210-0865, Japan Abstract Ethylene homopolymerizations have been performed at different temperatures in the range of 368378K over industrial Cr catalyst. Kinetic parameters such as rate constant of elementary reaction of polymerization or concentration of active centers ([C*]) were calculated by kinetic approach. Two types of transfer reactions of polymer chains, i.e. chain transfer with monomer and spontaneous chain transfer, occur over Cr catalyst. Chain transfer with monomer proceeded dominantly compared to spontaneous chain transfer. However, at elevated temperature the proportion of spontaneous chain transfer increased because of higher activation energy of this reaction. From the values of activation energy of transfer and propagation reaction, it is strongly suggested that the rate determining step of propagation reaction and chain transfer with monomer is ethylene coordination. l.INTRODUCTION More than forty years have passed since the discovery by Hogan and Banks [1] at Phillips Petroleum Co. that ethylene could be converted under relatively low pressures to solid polymer by using chromia supported 30000 on silica or silica-alumina substrates. Because the molecular weight of this polymer is one of the most important 25000 product property, this control is an area of major industri^ interest. In contrast to Mn Ziegler-Natta catalyst, the molecular weight 20000 of a polymer is mainly controlled by polymerization temperature in Cr catalyst. Moreover, each Cr catalyst has a different 15000 dependency of the average molecular weight 370 375 380 365 (Mn) on polymerization temperature as Polymerization Temperature (K) shown in Fig. 1. Therefore, this study was performed to investigate this polymerization Fig.1 Temperature Dependency of Mn behavior by kinetic study. 2.EXPERIMENTAL 2.1.Catalysts Three types of industrial Cr catalyst (AB,C) were used in this work. All of the supported chromium precursor samples were supplied by GRACE Corp. In order to activate these samples, they were calcined in dry air at temperatures higher than 923K in advance.
477
478 T.Sa'iio etal.
2.2.Ethylene Polymerization Polymerization was carried out in a 2L autoclave with different temperatures in the range of 368378K and at different concentration of ethylene in diluent in the range of 0.5-1.5 mol/1. Prescribed amount of catalyst and i-butane as a diluent were introduced into the reactor under nitrogen atmosphere. Then ethylene was charged at the polymerization temperature. The apparatus allowed a constant pressure to be maintained in the reactor during the polymerization by continuously adding ethylene through a pressure control valve. After polymerization, the produced polymer was recovered and its weight was measured. The average molecular weight was determined by GPC. 3.RESULTS AND DISSCUSIONS Fig.2 and 3 show the time dependency of polymer yield(Yt) and number-average degree of
2 106,
800
/ 1.5 106
600
Yt /mol-C2=\ \mol-Cr /
/
P n t 400
5 105| 0
/ ^
/
200
/ 50
100
150
200
250
50
100
150
200
250
t (min)
t (min)
Fig.2 Time Dependency of Yt
Fig.3 Time Dependency of Pnt
polymerization (Pnt) over Catalyst A at 373K of polymerization temperature. This catalyst system exhibits characteristically slow initiation reaction which is considered to be due to slow activation of precursors by ethylene. Yt and Pnt are generally expressed as follows. Yt= j ^ Rpdt= j ^ kp[M][C*]dt= j ^ kp[M][C*]oo {l-exp(-ki t)} dt
(A)
Pnt= j j Rpdt/([C*]+jJ Rtrdt)
(B)
Rpirate of propagation reaction kp:rate constant of propagation reaction [M]:concentration of ethylene in diluent [C*]:concentration of active centers [C*] oo xoncentration of active centers at stationary rate ki:rate constant of initiation reaction Rtr:rate of transfer reaction Moreover, because two types of transfer reaction, namely chain transfer with monomer and spontaneous chain transfer[2], occur over Cr catalyst ,Rtr is expressed as follows. Rtr = ktr[C*] = (ktr,s+ktr,m[M])[C*]
(C)
ktrirate constant of transfer reaction ktr,s:rate constant of spontaneous chain transfer ktr,m:rate constant of chain transfer with monomer In equations (A) and (B), since the value of [M], Yt and Pnt were measured experimentally, the values of [C*] oo , ki, kp and ktr were calculated, respectively. Furthermore, ktr,m and ktr,s can be evaluated from the dependency of Rtr on [M] and the value of kp. Results were shown in Table 1.
479
Tablel Kinetic Parameters of Catalyst A,B and C (at 373K) ki(1/sec.)
kp (I / mol sec.)
[C*]cx) ktr,m (I / mol sec.) ktr,s(1/sec.)
catalyst A
0.000135
3200
0.035
0.14
0.017
catalyst B
0.000137
3400
0.030
0.12
0.012
catalyst C
0.000138
3600
0.032
0.13
0.009
This is the first case of calculation of [C*] oo and kp using kinetic data over Cr catalyst. The determination of these parameters by quenching technique were given in several papers [3-5]. Tait determined the value of kp and [C*] oo to be about 4000 and 0.05 , respectively, by CO quenching technique[5]. Comparison with these values support the validity of our method. Besides, this method has advantage of determination of ki. Because the values of ktr,m [M\ are 5-15 times larger than the values of ktr,s in every catalyst, chain transfer with monomer is dominant. This result agrees with a previous report[2]. Moreover, the dependency of temperature on kp, ktr,m and ktr,s was measured and each activation energy was calculated. Results are summarized in Table2. Table 2 Activation Energy of Catalyst A, B and C Ep (kcal/mol)
Etr,m (kcal/mol)
Etr,s (kcal/mol)
catalyst A
8
12
29
catalyst B
8
7
30
catalyst C
6
6
8
Ep; activation energy of propagation reaction Etr,m; activation energy of chain transfer with monomer Etr,s; activation energy of spontaneous chain transfer
Etr,s was larger compared to Ep or Etr,m in catalyst A and B. This means that the increase of the rate of spontaneous chain transfer at elevated temperature is greater than that of other reactions. Therefore molecular weight decHnes with increasing polymerization temperature. Etr,s is only slightly larger than Etr,m in catalyst C. It is considered that these differences between Etr,m and Etr,s reflect the dependency of Mn on polymerization temperature as shown in Fig. 1 Propagation reaction and transfer reaction are believed to occur as shown in Fig. 4. The active species with a hving polymer chain is coordinated by ethylene(l) or transferred spontaneously(4). After coordination of ethylene, propagation reaction(2) or transfer reaction (referred as chain transfer with monomer)(3) proceeds. The results that Ep and Etr,m are ahnost equal and less than Etr,s are consistent with this mechanism. Moreover, it is strongly suggested thatfiierate determining step of propagation reaction and chain transfer with monomer is ethylene coordination(l).
480 T.Saiio etal.
^ CH, I ^CHg Cr--CH2— CH2
living polymer chain CH3 I CH2 4" \ Cr
propagataion reaction
,<© CH II Q^
chain transfer with monomer
.© H ^Cr
A
+
CH II CH2
spontaneous chain transfer
: rate determining step
Fig.4 Plausible mechanism of propagation and transfer reaction
4.SUMMARY -Kinetic parameters such as ki, kp and [C*] 00 were determined by the data of time dependency of Yt and Pnt. -Chain transfer with monomer is proceeds dominantly compared to spontaneous chain transfer. -The proportion of spontaneous chain transfer increases at elevated temperature because of higher activation energy. -It is strongly suggested that the rate determining step of propagation reaction and chain transfer with monomer is eSiylene coordination. References [1] Belg.Pat. 530617 (1995) [2] R.Blom, A.Follestad and O.Noel, J. Mol. Catal. 91(1994) 237 [3] C.Eden, H.Feilchenfeld and Y.Hass, J. Catal. 11(1968) 263 [4] J.Hogan, J. Polym. Sci., Part A-1 8(1970) 2637 [5] S.Wang and P.J.T.Tait, J. Mol. Catal. 65(1991) 237
85 Kinetic Study of Ethylene Polymerization over Cr/Si02 Catalyst
Toshiya SAITO, Manabu MOTEGI, Hiroyuki FURUHASHI and Satoshi UEKI Technical Development Center, Tonen Chemical Corporation, Kawasaki-ku, Kawasaki 210-0865, Japan Abstract Ethylene homopolymerizations have been performed at different temperatures in the range of 368378K over industrial Cr catalyst. Kinetic parameters such as rate constant of elementary reaction of polymerization or concentration of active centers ([C*]) were calculated by kinetic approach. Two types of transfer reactions of polymer chains, i.e. chain transfer with monomer and spontaneous chain transfer, occur over Cr catalyst. Chain transfer with monomer proceeded dominantly compared to spontaneous chain transfer. However, at elevated temperature the proportion of spontaneous chain transfer increased because of higher activation energy of this reaction. From the values of activation energy of transfer and propagation reaction, it is strongly suggested that the rate determining step of propagation reaction and chain transfer with monomer is ethylene coordination. l.INTRODUCTION More than forty years have passed since the discovery by Hogan and Banks [1] at Phillips Petroleum Co. that ethylene could be converted under relatively low pressures to solid polymer by using chromia supported 30000 on silica or silica-alumina substrates. Because the molecular weight of this polymer is one of the most important 25000 product property, this control is an area of major industri^ interest. In contrast to Mn Ziegler-Natta catalyst, the molecular weight 20000 of a polymer is mainly controlled by polymerization temperature in Cr catalyst. Moreover, each Cr catalyst has a different 15000 dependency of the average molecular weight 370 375 380 365 (Mn) on polymerization temperature as Polymerization Temperature (K) shown in Fig. 1. Therefore, this study was performed to investigate this polymerization Fig.1 Temperature Dependency of Mn behavior by kinetic study. 2.EXPERIMENTAL 2.1.Catalysts Three types of industrial Cr catalyst (AB,C) were used in this work. All of the supported chromium precursor samples were supplied by GRACE Corp. In order to activate these samples, they were calcined in dry air at temperatures higher than 923K in advance.
477
478 T.Sa'iio etal.
2.2.Ethylene Polymerization Polymerization was carried out in a 2L autoclave with different temperatures in the range of 368378K and at different concentration of ethylene in diluent in the range of 0.5-1.5 mol/1. Prescribed amount of catalyst and i-butane as a diluent were introduced into the reactor under nitrogen atmosphere. Then ethylene was charged at the polymerization temperature. The apparatus allowed a constant pressure to be maintained in the reactor during the polymerization by continuously adding ethylene through a pressure control valve. After polymerization, the produced polymer was recovered and its weight was measured. The average molecular weight was determined by GPC. 3.RESULTS AND DISSCUSIONS Fig.2 and 3 show the time dependency of polymer yield(Yt) and number-average degree of
2 106,
800
/ 1.5 106
600
Yt /mol-C2=\ \mol-Cr /
/
P n t 400
5 105| 0
/ ^
/
200
/ 50
100
150
200
250
50
100
150
200
250
t (min)
t (min)
Fig.2 Time Dependency of Yt
Fig.3 Time Dependency of Pnt
polymerization (Pnt) over Catalyst A at 373K of polymerization temperature. This catalyst system exhibits characteristically slow initiation reaction which is considered to be due to slow activation of precursors by ethylene. Yt and Pnt are generally expressed as follows. Yt= j ^ Rpdt= j ^ kp[M][C*]dt= j ^ kp[M][C*]oo {l-exp(-ki t)} dt
(A)
Pnt= j j Rpdt/([C*]+jJ Rtrdt)
(B)
Rpirate of propagation reaction kp:rate constant of propagation reaction [M]:concentration of ethylene in diluent [C*]:concentration of active centers [C*] oo xoncentration of active centers at stationary rate ki:rate constant of initiation reaction Rtr:rate of transfer reaction Moreover, because two types of transfer reaction, namely chain transfer with monomer and spontaneous chain transfer[2], occur over Cr catalyst ,Rtr is expressed as follows. Rtr = ktr[C*] = (ktr,s+ktr,m[M])[C*]
(C)
ktrirate constant of transfer reaction ktr,s:rate constant of spontaneous chain transfer ktr,m:rate constant of chain transfer with monomer In equations (A) and (B), since the value of [M], Yt and Pnt were measured experimentally, the values of [C*] oo , ki, kp and ktr were calculated, respectively. Furthermore, ktr,m and ktr,s can be evaluated from the dependency of Rtr on [M] and the value of kp. Results were shown in Table 1.
479
Tablel Kinetic Parameters of Catalyst A,B and C (at 373K) ki(1/sec.)
kp (I / mol sec.)
[C*]cx) ktr,m (I / mol sec.) ktr,s(1/sec.)
catalyst A
0.000135
3200
0.035
0.14
0.017
catalyst B
0.000137
3400
0.030
0.12
0.012
catalyst C
0.000138
3600
0.032
0.13
0.009
This is the first case of calculation of [C*] oo and kp using kinetic data over Cr catalyst. The determination of these parameters by quenching technique were given in several papers [3-5]. Tait determined the value of kp and [C*] oo to be about 4000 and 0.05 , respectively, by CO quenching technique[5]. Comparison with these values support the validity of our method. Besides, this method has advantage of determination of ki. Because the values of ktr,m [M\ are 5-15 times larger than the values of ktr,s in every catalyst, chain transfer with monomer is dominant. This result agrees with a previous report[2]. Moreover, the dependency of temperature on kp, ktr,m and ktr,s was measured and each activation energy was calculated. Results are summarized in Table2. Table 2 Activation Energy of Catalyst A, B and C Ep (kcal/mol)
Etr,m (kcal/mol)
Etr,s (kcal/mol)
catalyst A
8
12
29
catalyst B
8
7
30
catalyst C
6
6
8
Ep; activation energy of propagation reaction Etr,m; activation energy of chain transfer with monomer Etr,s; activation energy of spontaneous chain transfer
Etr,s was larger compared to Ep or Etr,m in catalyst A and B. This means that the increase of the rate of spontaneous chain transfer at elevated temperature is greater than that of other reactions. Therefore molecular weight decHnes with increasing polymerization temperature. Etr,s is only slightly larger than Etr,m in catalyst C. It is considered that these differences between Etr,m and Etr,s reflect the dependency of Mn on polymerization temperature as shown in Fig. 1 Propagation reaction and transfer reaction are believed to occur as shown in Fig. 4. The active species with a hving polymer chain is coordinated by ethylene(l) or transferred spontaneously(4). After coordination of ethylene, propagation reaction(2) or transfer reaction (referred as chain transfer with monomer)(3) proceeds. The results that Ep and Etr,m are ahnost equal and less than Etr,s are consistent with this mechanism. Moreover, it is strongly suggested thatfiierate determining step of propagation reaction and chain transfer with monomer is ethylene coordination(l).
480 T.Saiio etal.
^ CH, I ^CHg Cr--CH2— CH2
living polymer chain CH3 I CH2 4" \ Cr
propagataion reaction
,<© CH II Q^
chain transfer with monomer
.© H ^Cr
A
+
CH II CH2
spontaneous chain transfer
: rate determining step
Fig.4 Plausible mechanism of propagation and transfer reaction
4.SUMMARY -Kinetic parameters such as ki, kp and [C*] 00 were determined by the data of time dependency of Yt and Pnt. -Chain transfer with monomer is proceeds dominantly compared to spontaneous chain transfer. -The proportion of spontaneous chain transfer increases at elevated temperature because of higher activation energy. -It is strongly suggested that the rate determining step of propagation reaction and chain transfer with monomer is eSiylene coordination. References [1] Belg.Pat. 530617 (1995) [2] R.Blom, A.Follestad and O.Noel, J. Mol. Catal. 91(1994) 237 [3] C.Eden, H.Feilchenfeld and Y.Hass, J. Catal. 11(1968) 263 [4] J.Hogan, J. Polym. Sci., Part A-1 8(1970) 2637 [5] S.Wang and P.J.T.Tait, J. Mol. Catal. 65(1991) 237