Influence of Reaction Conditions on Catalyst Behavior during the Early

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Influence of Reaction Conditions on Catalyst Behavior during the Early Stages of Gas Phase Ethylene Homo- and Copolymerization Estevan Tioni,†,‡ Jean Pierre Broyer,† Vincent Monteil,*,† and Timothy McKenna*,† †

Université de Lyon, Univ. Lyon 1, CPE Lyon, CNRS, UMR 5265 Laboratoire de Chimie Catalyse Polymères et Procédés (C2P2), LCPP team, Bat 308F, 43 Bd du 11 novembre 1918, F-69616 Villeurbanne, France ‡ Dutch Polymer Institute DPI, P.O. Box 902, 5600 AX Eindhoven, The Netherlands ABSTRACT: A packed bed stopped flow minireactor (3 mL) suitable for performing gas phase polymerizations of olefins has been used to study the initial phases of ethylene homo- and copolymerization with two supported metallocene catalysts. The reactor can be used to perform gas phase polymerizations at times as short as 100 ms under industrially relevant conditions. It has been used to follow the evolution of the rate of polymerization, the gas phase temperature (and indirectly the particle temperature), and the polymer properties (molecular weight distribution, melting temperature, and crystallinity) for the two catalysts. It is shown that polymerization activity during the first 2−5 s of reaction can be up to 20 times higher than what is measured at longer polymerization times. The main consequence is the release of a significant amount of heat due to the rapid reaction that has to be efficiently evacuated in order to avoid particle overheating and melting. It has been seen that insufficient heat removal can strongly influence the behavior of the active sites, eventually leading to uncontrolled transfer reactions and polymers with unusually broad molecular weight distributions (MWD). It is also observed that the kinetic behavior of the two types of catalyst is similar at short times. Finally, some influence of particle size on reaction rate and molecular weight is observed between the largest and smallest catalyst particle cuts.

1. INTRODUCTION The annual production of polyethylene (PE) and polypropylene (PP) in processes using supported catalysts is likely to be close to 90 million at the current time.1 This process, which has been used commercially since the 1950s is clearly commercially significant and has been the object of many industrial and academic studies far too numerous to cite here. However, despite the intense research efforts that have been made in the past 5 decades, there remains much to understand about these processes. In particular, the events surrounding the transformation of the catalyst particle into a growing polymer particle still need to be better understood.2 The most common types of catalyst supports used in olefin polymerization are magnesium dichloride for Ziegler−Natta (ZN) catalysts, and silica which is used to support chromium oxide and metallocene active sites (although hybrid supports of MgCl2 on silica are used for certain types of ZN catalysts). Both types of support have a high surface area and porosity that allow for the deposition of a large number of active sites throughout the structure, and both types of support undergo physical transformations as soon as they are injected into the reactor. It can be said that the nature of the steps by which the particles of supported catalyst are transformed into growing polymer particles is certainly understood from a qualitative point of view: once the virgin catalyst particles are injected into the reactor, monomer diffuses from the bulk phase and begins to react at the active sites on the surface of the catalyst support. Polymer then quickly accumulates, generating pressure throughout the particle and provoking a local fragmentation of the support. Once the fragmentation step is complete, the resulting particle (now referred to as a polymer particle) will continue to grow as long as monomer arrives at the active sites. © 2012 American Chemical Society

For a more in-depth discussion of the process of catalyst fragmentation and growth, the reader is referred to earlier reviews from our group, as well as the references therein.2,3 The fragmentation step is quite short with respect to the average residence time of the reactor; fractions of a second up to several tens of seconds, depending on the type of support and reaction conditions, versus 1−3 h, respectively. Despite the almost negligible amount of time the fragmentation and initial growth phases take with respect to the length of an industrial reaction, it is at this point that the most extreme changes in particle morphology occur, that the production of fines due to an abrupt particle fragmentation is likely, and when severe overheating of the particles can be a significant problem. Given that the rates of mass transfer into the particles and heat transfer out of the particles are strongly dependent on the morphology (pore network, pore size, pore volume, and especially particle size), this can be also the time range in which the temperature and concentration values inside the catalyst particle can vary most abruptly. For instance, strong concentration gradients can lead to different parts of the particle core expanding more rapidly than others, with consequences as extreme as the production of hollow particles.4,5 Excessive temperature gradients in the boundary layer (temperature profile inside the particle is known to be less important6,7) can lead to polymer melting and reaction extinction. Received: Revised: Accepted: Published: 14673

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different purification columns before use: a first one filled with reduced BASF R3-16 catalyst (CuO on alumina), a second one filled with molecular sieves (13X, 3A, Sigma-Aldrich), and a last one filled with Selexsorb COS (Alcoa). Butene (purity >99%), helium (purity >99.999%), and carbon dioxide (purity >99.995%) are purchased from Air Liquide and used without further purification. The catalyst support was Grace 948 silica from Grace Davison, with an average particle diameter of 58 μm. It was treated at 200 °C for 4 h under vacuum (10−5 mbar) before use to fix a concentration of hydroxyl groups on the surface of around 5 OH/nm2.25 Before reaching the 200 °C plateau, the solid was heated under vacuum at 130 °C for 30 min to remove the water adsorbed on the surface. The zirconocene complexes ((EtInd 2 ZrCl 2 ) and (nBuCp)2ZrCl2) were used as received from Sigma Aldrich and supported on the aforementioned silica treated with methylaluminoxane (MAO). Both a 10 wt % MAO solution in toluene (Sigma-Aldrich, Saint-Quentin Fallavier, France) and a 30 wt % MAO solution in toluene (Albermarle) were used for this study. The impregnation method has been reported earlier.22 As the molar masses of the two metallocenes are close each other and as we used the same mass of catalyst precursor during the supporting procedure, the Al and Zr contents of the final catalysts are similar for both complexes: the average metal loading of the reference (nBuCp)2ZrCl2 supported catalyst is of 0.31 wt % Zr and 7.16 wt % Al (unless otherwise reported) and the EtInd2ZrCl2 catalyst showed average values of 0.32 and 7.63 wt % for Al and Zr, respectively. The inert seedbed used in this work is composed of 30−50 μm agglomerates of fine NaCl particles (80

271 267 270 270 269

24.5 24.4 24.6 24.6 24.6

1.66 1.63 1.66 1.66 1.65

5.4 5.2 5.7 5.8 5.6

0.31 0.27 0.27 N.A. 0.23

7.16 7.44 7.34 N.A. 7.12

Figure 13. Relative weight of each fraction after sieving (replicates).

absorption technique. It can be seen from Table 3 that there appears to be no difference in pore size, pore volume, or specific surface area between the different cuts of silica. On the other hand, the catalysts made from the different cuts of silica appear to have slightly lower levels of Zr than the reference catalyst. Nevertheless, the difference is very slight, and it seems reasonable that we can at least compare the activities of the different cuts to each other. For this study the cuts 36−45, 45−63, and >80 have been used as supports for catalyst heterogeneisation. Each catalyst has been tested in a 2 L gas phase reactor for 1 h before performing short reaction times. Results are summarized in Table 4.

Figure 14. Influence of support size on (a) observed activity; (b) estimated average solid temperature; and (c) outlet gas phase temperature for polymerization on a silica supported (nBuCp)2ZrCl2 catalyst at 80 °C (inlet) and 6 bar of ethylene, 3 bar He.

Table 4. Influence of Support Size on Activity and Polymer Properties in Long Term Gas Phase Reactions particle size (μm) all 36−45 45−63 >80

activity (gPE/(mol Zr h))

Mn (g/mol)

Mw (g/mol)

PDI

× × × ×

59000 67000 39200 NA

165200 192400 128900 NA

2.8 2.7 3.2 NA

2.4 3.3 1.5 0.8

106 106 106 106

the results seen in the longer reactions. On the basis of these results alone, it is not reasonable to affirm that the differences in the activities seen after 30 s are due to mass transfer resistances. However, the evolution of the molecular weights can help clarify this point. There is also very little difference between the evolution of the estimated solid temperatures for the different cuts, or between the evolution of the outlet gas temperatures. The evolution of the weight average molecular weight of polymer made on three different cuts is shown in Figure 15. It can be seen from this figure that there is a clear trend, with the molecular weights of the larger cuts being consistently lower than those of the small cuts. This, combined with the evidence from the rate data presented above, leads us to propose that there is some mass transfer resistance, since the largest particles have the lowest molecular weights. However, it is also possible that if the larger particles are slightly hotter, one could also see the same results. Both sets of data in Figure 15 points to lower monomer concentrations (on average) inside the larger particles than the smaller ones.

As already shown previously,20 in an investigation of the polymerization of ethylene on silica-supported EtInd2ZrCl2 catalyst precursors in slurry, the largest particle cut produced catalysts with a lower activity than the smaller cuts. If we compare the cuts to each other, the results in Table 4 suggest that there is some mass transfer resistance as the particles get larger, since the rates drop as the size of the cut increases, and, at least for the 2 smaller cuts, the molecular weight also diminishes. The activity profiles and outlet gas temperatures are shown in Figure 14. It can be seen that the differences between the activity profiles and temperature profiles are small at shorter times. For times longer than 30 s the activity is lowest in the large particles and highest in the smallest ones. This agrees with 14682

dx.doi.org/10.1021/ie301682u | Ind. Eng. Chem. Res. 2012, 51, 14673−14684

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rates are rather low compared to those found in commercial catalysts. Finally, the results in this paper show that the fixed bed gas phase stopped flow reactor is a useful tool for studying many aspects of the important initial phase of gas phase polymerization reactions. It has been shown that the online measurement of the inlet and outlet temperatures can be used to get a good approximation of the average surface temperature of the polymerizing particles as a function of time. This is a tremendous advantage with respect to existing reactors34 that have been used to study short time polymerizations such IR microscopy (see above) since this reactor allows us to approximate many different flow conditions under a range of conditions, including realistic ones, rather than being limited to static conditions.

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Figure 15. Evolution of the weight average molecular weight of polymer produced on three different cuts of silica.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.M.K.); monteil@ lcpp.cpe.fr (V.M.).

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CONCLUSIONS In this work we have studied the behavior of supported metallocene catalysts during the first seconds of the gas phase ethylene polymerization. It was shown that rapid reaction rates are measured for the first 2−5 s, with values that can be 20 times higher than those measured at longer times. This behavior was observed for both homopolymerization and in copolymerization and cannot be explained only with mass transfer limitations arising because of the polymer layer formed around the active site, nor by an increase in the local particle temperature. The deactivation seems to coincide with the beginning of the temperature excursions measured during the first seconds, but given the fact that the increase in solid temperature and outlet gas temperature begins af ter the rate begins to decrease, it is unlikely that drop in the rate is caused by the increase in temperature. In fact, the magnitude of the decay in the initial rate suggests that there exists some active sites that work for only a short period of time. The MWD of the produced polymers is constant throughout the studied reaction time if optimum conditions for heat transfer are used. On the contrary in the case of insufficient heat removal (too low gas velocity or too much catalyst), broad MWDs have been measured for reaction times between 5 and 75 s even if the same decay type reaction rate profile is maintained. These reaction conditions are responsible in fact for high temperature gradients in the reactor thus proving that an insufficient heat removal at the reaction start-up can lead to uncontrolled transfer reactions and poor polymer properties even if polymer melting is avoided. That the ratio of Al/Zr influences the kinetics is clear; however, it is not easy to draw strong conclusions. While the reference conditions (Al/Zr = 80) gives the highest activity in the long run and leads to the hottest solid phase temperature, the other two ratios (65 and 40) lead to a much more rapid “light off” of the reaction and a very rapid rise in temperature that occur at times just after the onset of the decay in the reaction rate. Subsequently, there appears to be very little impact of the initial particle size on the reaction rates, although molecular weight measurements suggest that there is perhaps some mass transfer resistance in the larger particles, or that the larger particles are slightly hotter and thus the masses are slightly lower. However, it should be underscored here that the reaction

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is part of the Research Programme of the Dutch Polymer Institute (DPI, Eindhoven,The Netherlands), project no. 636.

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REFERENCES

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