Ethylene was polymerized at 5 bar in a stirred powder bed reactor with silica supported rac-Me2Si[Ind]2ZrCl2/methylaluminoxane (MAO) at temperatures between 40°C and 80°C using NaCl as support bed and triethylaluminium (TEA) as a scavenger for impurities. For this fixed recipe and a given charge of catalyst. the average catalyst activity is reproducible within 10% for low temperatures. The polymerization rate and the rate of deactivation increase with increasing temperature. The deactivation could be modeled using a first order dependence with respect to the polymerization rate

Meier, G.B.

Roos, P.

Samson, J.J.C.

Samson, Job Jan C.

Weickert, G.

Westerterp, K.R.

English

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Gas phase polymerization of ethylene with a silica-supported metallocene catalyst: influence of temperature on deactivation.

University of Twente Research Information

Macromol. Rapid Commun. 18,319-324 (1997) 319 Gas phase polymerization of ethylene with a silica- supported metallocene catalyst: influence of temperature on deactivation Peter Roos, Gerben B. Meier; Job Jan C. Samson, Giinter Weickert, K. Roe1 Westerterp* University of Twente, Faculty of Chemical Engineering, P.O. Box 217, NL-7500 AE, Enschede, The Netherlands (Received: August 27, 1996; revised manuscript of October 1, 1996) SUMMARY Ethylene was polymerized at 5 bar in a stirred powder bed reactor with silica supported r~c-Me~Si[Ind]~ZrC1~/rnethylaluminoxane (MAO) at temperatures between 40 "C and 80 "C using NaCl as support bed and triethylaluminium (TEA) as a scavenger for impuri- ties. For this fixed recipe and a given charge of catalyst, the average catalyst activity is reproducible within 10% for low temperatures. The polymerization rate and the rate of deactivation increase with increasing temperature. The deactivation could be modeled using a first order dependence with respect to the polymerization rate. Introduction In recent years, versatile catalysts based upon methylaluminoxane (MAO) acti- vated metallocenes have been developed'.2). These organometallic complexes are able to polymerize a-olefins stereoselectively with high activity and narrow molecu- lar weight  distribution^^^). Almost all public research is directed to the development of new metallocenes and conducted in the slurry phase'-''). Tsutsui and Kashiwa have investigated the gas phase polymerization of propylene with a non-supported solid complex catalyst obtained from Et[Ind],ZICl, and methylaluminoxane using NaCl as a disperse medium"). Up to now no data is available in the open literature describing the reproducibility of polymerization rate measurements for gas phase olefin polymerization with supported metallocenes. In this paper we report on the influence of temperature on the gas phase polymer- ization of ethylene with silica-supported Me,Si[Ind],ZrCI,. Experimental part Materials Triethylaluminium (TEA) and (dimethylsilylene)bis(~5-inden-l-ylidene)zirconium dichloride or Me2Si[IndI2ZrCl2 (Witco) were used without further purification. The Me2Si[IndI2ZrCl2 catalyst has been heterogenized on silica gel (1 wt%). The silica (SD3216-30, 10-110 pm, average 51 pm, provided by Witco) has been pretreated with methylaluminoxane (MAO) to a concentration of 25 wt% Al, giving an [Al]/[Zr] mole ratio of 386. Ethylene 2.8 and argon 5.0 were further purified through BTS (reduced state) and molecular sieves columns. NaCl was purified by drying it under vacuum at 90°C. 0 1997, Hiithig & Wepf Verlag, Zug CCC 1022- 1336/97/$02.50 320 P. Roos, G. B. Meier, J. Jan C. Samson, G. Weickert, K. R. Westerterp Polymerization procedures In a 1 L glass autoclave equipped with a helical stirrer, 50 g of NaCl is placed as a support bed. The system is vacuum dried at the polymerization temperature and flushed with argon several times. 0.5 mL of the scavenger (TEA) is injected with argon. After that, the system is flushed with ethylene several times. Subsequently, approximately 0.2 g of catalyst is injected with argon. Immediately after the injection the reactor is flushed with ethylene to remove the argon. Polymerization is carried out at 5 bar for 200 min. The product is washed with water to remove the salt followed by filtration. The obtained product is vacuum dried for 24 h at 70 "C. Gas purities of ethylene and argon were investigated by depressurising the reactor to 1.5 bar followed by refilling with new ethylene to 5 bar during the experiment and by flushing with argon to stop the experiment for 18 h followed by flushing with ethylene to restart the experiment. None of these procedures influenced the polymerization activity significantly. Therefore deactivation by impurities in the monomer can be exc1udedl2). Results Tab. 1 shows the results of the polymerization experiments. Fig. 1 shows the poly- merization rate profiles of the experiments listed in Tab. 1. Flushing the reactor with ethylene was carried out within 2 min after the catalyst injection, with the exception of the experiments 120396, 130396 and 140396 where the first 25 min couldn't be measured due to the long flushing period. Tab. 1. Results of ethylene polymerization experiments with MezSi[Ind]zZrC1z as cata- lyst . ~ ~~ Exp. no. Temp. Yield Initial Average in "C in g activitya* b, activitya) in kg/(g/h) in kg/(g/h) 120396 220 396 250396 180 396 130 396 210396 I40 396 200 396 40 40 50 60 60 70 80 80 9.6 10.9 19.6 27.1 25.1 40.5 53.5 38.9 1.7 1.8 3.5 4.7 4.8 6.7 9.6 7.9 1.3 1.4 2.7 3.9 3.6 4.9 7.8 5.2 a) Average and initial activity in kg polyethylene per gram metallocene per hour. b, Initial activity has been extrapolated from the experimental data. The irregular behavior of the rate profiles in the first 30 min (especially for the higher temperatures) were caused by the 'temperature control. Temperature control by cooling a particle/gas system through the relatively thick glass wall is slow. Visual inspection of the product indicated that particle agglomeration and break-up could also contribute to the fluctuations in temperature and activity at the higher temperatures. Gas phase polymerization of ethylene ... 321 - .; v 5 . -  a, CCI -6 0 -  100 200 Time in min Fig. 1. Ethylene polymerization rate profiles at different temperatures with Me2Si[IndI2ZrCl2 as catalyst; (a) 40°C + duplo, (b) 50°C (c) 60°C + duplo, (d) 70°C (e) 80 "C + duplo It can be concluded that the reproducibility of the average activity is rather good (within 10%) with the exception of the experiments at the higher temperatures. The spread in deactivation rate at 80°C is much larger, which is most likely due to uncontrollable agglomeration behaviour and a non-isothermal single parti~le '~).  Application of prepolymerization techniques will probably improve the reproduci- bility at higher temperatures. All curves show a typical high initial activity, followed by a decline in activity due to deactivation. This decline seems to be higher when the activity is higher. With a Ti-based Ziegler-Natta catalyst, Samson et al. have found similar experimen- tal results for the activity-time  curve^'^). Model It is generally supposed that the reaction rate is first order in the monomer con- centration and in the concentration of active centers. This can be described with the following equation: R, = k, C, C* In this equation R, is the polymerization rate (expressed in kg polymer per g metallocene per hour), k, is the propagation constant, C, the monomer mass concen- tration at the active center and C* the number of active centers per g of metallocene. The propagation constant is assumed to be dependent on the temperature according to Arrhenius: Fiom Fig. 1 it can be seen that the deactivation of the catalyst increases with increasing polymerization rate. The same effect has been found under isothermal 322 P. Roos, G. B. Meier, J. Jan C. Samson, G. Weickert, K. R. Westerterp -6.2 -6.7 -7.2.- -7.7 -8.2 conditions when the polymerization rate increases with increasing pre~sure'~). We conclude that the rate of monomer insertion itself is the critical step, producing deac- tivated sites in a statistical way. This leads to Eq. (3) for the rate of change of the number of active sites: -- --  -- -- -- - -kd R, dC* dt (3 )  The deactivation constant kd is a lumped value for all the deactivation reactions. The deactivation constant is assumed to be dependent on the temperature according to Arrhenius, with Ed the deactivation energy: Under the conditions described above there is no need to implement an activation step. The catalyst is preactivated and shows full activity at the beginning. When the temperature and monomer pressure are constant, the following equation can be found by combining Eq. (1) and Eq. (3): = -kp k d  C,  R, dt ( 5 )  or R, = RPo e(-kp kd Crn I )  RN = kP,- e(kE,IRT) C* o m  C (6) with (7) For each temperature, Rpo and k,kd have been determined from the experimental data. In Fig. 2a, ln(RdC,,,) against 1000/T is given to determine the activation energy and k,,Ci according to Eq. (7). 0 -0.L -0.8 -1.2 (b) 0 -8.7 2.8 2.9 3.0 3.1 3.2 2.8 2.9 3.0 3.1 3.2 T-'/K-' lo3. T'/K-' a) ln(R&,) against 1000/T to determine Ep and kF Cg; b) ln(k, kd) against Fig. 2. 1 000/T to determine Ed and kp.. kdo, Gas phase polymerization of ethylene ... 323 C,,, has been taken equal to the mass concentration of the bulk and is calculated with the ideal gas law. In Fig. 2b, In(kpkd) against 1 000/T is given to determine the deactivation energy and k$kd0. The determined kinetic constants are: Ep = 39.2 W/mol Ed = -4.0 kJ/mol kpC:, = 1.26 * lo6 m3/(h * scat) kp kda  = 201 m3/(kg - min) The negative deactivation energy is physically not possible of course, but can be interpreted as being zero. With our model this indicates that a certain percentage of the monomer insertions (represented by k d )  leads to deactivated sites independent of temperature. In Fig. 3 the experimental profiles are compared with the modelled pro- files. As can be seen from Fig. 3, the fit between model and experimental data is good at temperatures of 70°C and lower. 0, Y C .- C 0 c( N .- c .- L h s 01 I t 0 100 200 Time in min Fig. 3. Experimental and model profiles at different temperatures with Me2Si[IndI2ZrC1, as catalyst; (a) 40"C, (b) 50"C, (c) 60"C, (d) 70"C, (e) 80°C a Conclusions We consider the active center of a supported metallocene catalyst as a surface- fixed microreactor which produces polymer independent of other microreactors. Therefore the deactivation of such an active center is not a function of the distance between those centers, the active center concentration. Our kinetic measurements of gas phase polymerization of supported metallocenes lead us to the hypothesis that the deactivation is proportional to the polymerization rate, which can be influenced by a number of physical and chemical factors. From our present data it can be deduced that it is most likely that a certain percentage of the insertions lead to deac- tivated sites. How this comes about is not clear. It could be the result of a blocking insertion of a monomer molecule or of a complex activation and reactivation mechanism'). On the other hand, mechanisms of dilution of reactants by polymer 324 P. Roos, G. B. Meier, J. Jan C. Samson, G. Weickert, K. R. Westerterp formation or local overheating, which are also coupled to polymerization activity, could also play a role. An important indication for the blocked site hypothesis could be the generally noticed increase in activity in the presence of H2 as a chain transfer agent. The small H2 molecule could easily break the bond between blocking poly- mer and site by chain transfer, reactivating the site in this way. However, this effect has been found only for propylene polymerizations. List of symbols C* C ,  monomer mass concentration Ep activation energy Ed deactivation energy kp propagation constant kd overall deactivation constant Rp polymerization rate t time 7' temperature number of active centers per gram of metallocene V. K. Gupta, S. Satish, I. S. Bhardwaj, J.  M .  S. - Rev. Macromol. Chem. Phys., C34(3), 439 (1994) 2, H. H. Brintzinger, D. Fischer, R. Miilhaupt, B. Rieger, R. Waymouth, Angew. Chem., Int. Ed. Engl. 34, 1143 (1995) 3, W. Tsai, J. Chien, J.  Polym. Sci., Part A: Polym. Chem. 32, 149 (1994) 4, W. Kaminsky, R. Engehausen, K. Zoumis, W. Spaleck, J. Rohrmann, Makromol. Chem. 193,1643 (1992) B. Rieger, X. Mu, T. Mallin, M. D. Rausch, J. W. Chien, Macromolecules 23, 3559 ( 1990) 6, S. Jungling, R. Miilhaupt, U. Stehling, H. Brintzinger, D. Fischer, F. Langhauser, J. Polym. Sci., Part A: Polym. Chem. 33, 1305 (1995) 7, M. Meneghetti, M. Forte, J. Dupont, Polym. Bull. 35,431 (1995) *) B. Rieger, C. Janiak, Angew. Makromol. Chem. 215,35 (1994) 9, B. Liang, Y. Li, G. Xie, Macromol. Rapid Commun. 17, 193 (1996) lo) R. Quijada, J. Dupont, D. CorrEa Silveira, M. Lacerda Miranda, R. Scipioni, Macro- 11) T. Tsutsui, N. Kashiwa, Polymer32,2671 (1991) I*) G. Weickert, P. Roos, G. B. Meier, J. J. C. Samson, K. R. Westerterp, Pmc. 4th Meet- ing Working Party Polymer Reaction Engineering, Thessaloniki (1 996) 13) J. J. C. Samson, A. P. H Westenbroek, J. A. M. Kuipers, G. Weickert, K. R. Wester- terp, Proc. XI11 Int. Con$ Chem. Reac. 11,79 (1996) 14) J. J. C. Samson, G. Weickert, K. R. Westerterp, in preparation 15) P. Roos, G. Weickert, K. R. Westerterp, in preparation rnol. Rapid Commun. 16,357 (1995) 

https://doi.org/10.1002/marc.1997.030180408

Gas phase polymerization of ethylene with a silica-supported metallocene catalyst: influence of temperature on deactivation

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