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Title

Role of Dispersion State of Ti Species in
Deactivation of MgCl̲2‑Supported Ziegler‑Natta
Catalysts

Author(s)Taniike, Toshiaki; Wada, Toru; Kouzai, Iku;
Takahashi, Shougo; Terano, Minoru

Citation

Macromolecular Research, 18(9): 839‑844

Issue Date

2010

Type

Journal Article

Text version

author

URL

http://hdl.handle.net/10119/9878

RightsThis is the author‑created version of Springer,
Toshiaki Taniike, Toru Wada, Iku Kouzai, Shougo
Takahashi, Minoru Terano, Macromolecular
Research, 18(9), 2010, 839‑844. The original
publication is available at www.springerlink.com,
http://dx.doi.org/10.1007/s13233‑010‑0914‑1

Description

Japan Advanced Institute of Science and Technology

Role of Dispersion State of Ti Species in Deactivation of MgCl2-Supported
Ziegler-Natta Catalysts

Toshiaki Taniike, Toru Wada, Iku Kouzai, Shogo Takahashi, Minoru Terano*

School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1
Asahidai, Nomi, Ishikawa, 923-1292, Japan

Corresponding author
Prof. Dr. Minoru Terano
TEL: +81-761-51-1620, FAX: +81-761-51-1625, E-mail: terano@jaist.ac.jp

Running Head: Role of Ti Dispersion State in Catalyst Deactivation

Abstract
Deactivation behaviors of TiCl3/MgCl2 model catalysts with molecularly dispersed TiCl3
were investigated to clarify the role of the dispersion state of the Ti species in thedeactivation of MgCl2-supported Ziegler-Natta (ZN) catalysts for propylene and ethylene
polymerization. The propylene and ethylene polymerization activities of clustered Ti
species supported on MgCl2 were about one tenth of those of isolated Ti species, indicativeof a significance of the aggregation of Ti species in the deactivation of ZN catalysts forolefin polymerization.

Moreover, the time-course depression of the monomer

consumption for the clustered Ti species was slower than that for the isolated Ti species inpropylene polymerization, while the dispersion state hardly affected the depression rate in
ethylene polymerization. The reduction rate of the Ti species was concluded as important
for the time-course depression of propylene consumption, while ethylene polymerization
was insensitive to the oxidation state of the Ti species.

Introduction
MgCl2-supported Ziegler-Natta (ZN) catalysts have played a dominant role in the industrial
polyolefin production.1 Enormous scientific researches have been accumulated on the ZN
catalysts with the aim of getting better understanding for all the aspects of olefin
polymerization catalysis such as the polymerization mechanism, active site structure, andetc. However, only a limited attention has been paid to the deactivation behavior of theZN catalysts and to its mechanism, even though it is one of the most important catalytic
features. Deactivation of the ZN catalysts has been generally defined as a time-coarse
depression of olefin monomer consumption during polymerization, and is normally aphenomenon in the time scale of hours.2

Several mechanisms have been proposed so far,which are briefly reviewed in the introduction.
Keii et al. estimated an active site concentration [C*] and a propagation rate constant kp
as a function of polymerization time for a TiCl4/MgCl2 catalyst by means of a CO
quenching technique. They found that a decrease of [C*], i.e. deactivation of the active
sites, was the main reason of the decay, while kp was rather kept unchanged in the absenceof donors.3 For the deactivation of each active site, the role of the oxidation state of Ti
species has been believed as crucial. Generally, in the production of polyolefin with theZN catalysts, an alkylaluminum activator is required to convert TiCl4 precursor that is
supported on MgCl2 into active sites through alkylation and reduction of TiCl4.1
oxidation state of Ti species varies from IV to II during polymerization.

The

Soga et al.

concluded that both of Ti2+ and Ti3+ species were active for ethylene polymerization but
α-olefins including propylene could polymerize only with Ti3+ species.4

Kashiwa et al.

found that a completely deactivated TiCl4/MgCl2 catalyst for propylene polymerization
through aging with AlEt3 for 1 h at 60°C could be reactivated with an introduction of

various Cl-based oxidants (for instance, Cl2).5 They indicated that the formation of Ti2+,
called as an over-reduction, was a major reason of the deactivation of the active sites in ZN
olefin polymerization. On the contrary, Albizzati et al. reported that a MgCl2-supported
η6-BenzeneTi(II)Al2Cl8 complex was quite active for propylene polymerization.6

Thoughthe supported complex including benzene and Al2Cl8 might not be a good active site model
for real Ti2+ species produced by the over-reduction of TiCl4, the over-reduction of the Ti
species must not be the sole reason of the deactivation. Another source of the active sites
deactivation is aggregation of Ti species which proceeds in parallel with the reduction
during polymerization, leading to a decrease in the active site concentration. Several
electron spin resonance (ESR) studies revealed that a contact of TiCl4/MgCl2 withalkylaluminum produced a large amount of ESR-silent Ti3+ species, assigned as clusteredTi3+ species.7-10

Recently, Tregubov et al. reported that the dispersion state of the Ti

species was vital for the ethylene polymerization activity.11
Aside from the said chemical deactivation of the active sites, physical origins were also
proposed for the decay of the ZN catalysts.

One famous proposal is related to theinhibition of monomer diffusion from reactor medium to active sites by encapsulation of
the active sites with growing polymer layers, so called a filter effect.2

However, Keii et al.

found that a contact of a TiCl4/MgCl2 catalyst with AlEt3 in the absence or in the presenceof propylene similarly promoted the decay of subsequent monomer consumption, claiming
a dominant role of the chemical deactivation of the active sites rather than the inhibitedmonomer diffusion at least for propylene polymerization.3 On the other hand, it is certainthat the time-course change of the monomer consumption, namely polymerization kinetics,
is deeply related to the morphology development of polymer/catalyst particles. Generally,polymer chains growing from active sites fill catalyst pores or channels to produce a stress,

resulting in disintegration of catalyst particles. Ti species located deep inside catalyst
particles and unable to be in contact with alkylaluminum at an early stage ofpolymerization get exposed in the course of polymerization, and lead to new active sites.2
The decay curve of monomer consumption thus results from the balance between theactive sites deactivation and morphology-driven formation of new active sites.

Tait et al.

found that the kinetic profile of the ZN catalysts was strongly influenced by the catalyst
synthesis route.12

We have also proposed that the spatial distribution of Ti species inside

catalyst particles was strongly dependent on the synthetic procedure to diverse kinetic
profiles and morphology developments among differently prepared ZN catalysts.13
In summary, three factors (the over-reduction and aggregation of Ti species, and thedisintegration of catalyst particles) are believed to collaborate in polymerization with
MgCl2-supported ZN catalysts in order to give a specific decay curve of monomer
consumption.

It is essential to study these three factors individually for investigating thedeactivation behavior of the ZN catalysts.

Recently we have prepared a TiCl3/MgCl2

model catalyst from a TiCl3·3C5H5N molecular precursor,14-16 where TiCl3 is molecularly
dispersed on the MgCl2 surface. In this model catalyst, the dispersion state of TiCl3 isprecisely controllable by varying the Ti content in the catalyst, where a lower content leads
to more isolated Ti species, while a higher content gives more aggregated Ti species.

An

initially-determined dispersion state of the Ti species is believed to be maintained during
polymerization since TiCl3 much more strongly adsorbs on MgCl2 and hardly migrates on
the surface during polymerization, whereas TiCl4 is rather mobile on the surface and boundto reduced Ti species to cause the aggregation of Ti species. Moreover, TiCl3 is supported
on the exposed surface of MgCl2 without breaking or reconstructing the original
morphology of MgCl2, plausibly making the contribution from the disintegration of thecatalyst particles to the decay curves of the catalysts rather negligible. Thus, the novel
TiCl3/MgCl2 model catalyst allowed us to investigate the influences of the dispersion state
and reduction of the Ti species separately on the deactivation behavior of MgCl2-supported
Ziegler-Natta catalysts.
The deactivation behaviors of the TiCl3/MgCl2 model catalysts having an “isolated” or“clustered” Ti species were acquired for propylene and ethylene polymerization in thisarticle.

Both the aggregation and reduction of the Ti species were found to be important

for the depression of the propylene polymerization activity, while the ethylene
polymerization activity was only sensitive for the aggregation.

Experimental partMaterials
Dehydration of heptane was performed by passing through a column filled withmolecular sieve 13X.

Pyridine (C5H5N) was dried with molecular sieve 13X.

Propylene,

ethylene and nitrogen of research grade, MgCl2 (65.1 m2·g–1) and aluminum-free T-TiCl3(both donated by Toho Titanium Co.), and AlEt2Cl and AlEt3 (both donated by Tosoh
Finechem Co.) were used without further purification.
Synthesis of TiCl3/MgCl2 model catalysts
Preparation of the TiCl3/MgCl2 model catalysts was conducted according to thepreviously reported procedure.14-16

T-TiCl3 (4.0 g) and C5H5N (20 ml) were mixed

vigorously in nitrogen atmosphere for 8 h at 140°C. The mixture was washed adequately
with heptane and dried in vacuo to obtain TiCl3·3C5H5N as greenish fine powder.

Themeasured amounts of MgCl2 and the TiCl3·3C5H5N complex were placed in a flask undernitrogen. Heptane solution of AlEt2Cl (Al/Ti = 78 mol·mol–1) was introduced into theflask and stirred at room temperature for 3 h, then washed adequately with heptane toobtain the catalyst.

The interaction between TiCl3·3C5H5N and AlEt2Cl is believed totransform TiCl3·3C5H5N into some partly soluble or mobile species through a removal of
the Py ligands by AlEt2Cl so as to disperse TiCl3 units on MgCl2 surface at a molecularlevel.

The Ti contents ranged from 0.004-1.3 wt.-% were determined by titration.

Semi-batch slurry polymerization
A 500 ml glass flask with a magnetic stirrer was filled with a measured volume ofheptane kept at 30°C.

Monomer gas was saturated in heptane at 0.015-0.020 MPa above

the atmospheric pressure, and then AlEt3 was introduced, observing the monomer flow
with a mass flow meter (Kofloc Model 3100). After the stabilization of the baseline in
the flow meter, a measured amount of catalyst slurry (the total liquid volume was 300 ml)
was injected to start the polymerization.

During polymerization, the propylene or

ethylene pressure was kept at constant within the above range and the monomer
consumption was recorded on line using a mass flow meter throughout the polymerizationfor 2 h.

The polymerization was terminated by adding 20 ml of ethanol containing 20

vol.-% of HCl. The amount of the injected catalysts was adjusted in order to produce
around 10 g of polymers for all the conditions. The Al concentration was varied from6.25 to 125 mol·L–1.

Results and discussion
In our previous paper,16 the dispersion state of Ti species in the TiCl3/MgCl2 model catalyst
was controlled by varying the Ti content to investigate its influences on the propylenepolymerization performance.

The results for the catalyst activity and stereoregularity ofthe made polypropylenes are summarized in Figure 1.

The stereoregularity was increased

with increasing the Ti content over 0.1-1.3 wt.-%, while it became constant below 0.1
wt.-%.

The increase of the catalyst stereospecificity at a higher Ti content came from theformation of clustered Ti species, which generally have a higher stereospecificity than anisolated Ti species.

The convergence of the stereoregularity below 0.1 wt.-%, on theother hand, indicated the loss of the Ti-Ti interaction, i.e. each Ti species existed on the
MgCl2 surface in an isolated manner.

The propylene polymerization activity of the model

catalyst with 0.08 wt.-% of the Ti content was ca. 10 times higher than that of the catalyst
with 0.9 wt.-%, clearly indicating that the aggregation of the Ti species significantly
promoted the depression of the monomer consumption plausibly due to a much lower
active site concentration.

It is also notable that both the catalysts gave poorly

crystallizable polypropylenes, and therefore the fliter effect was considered to beminimized.
Hereafter, two catalysts with the Ti content of 0.08 and 0.90 wt.-% (Cat-0.08 andCat-0.90) were employed as representatives of the catalysts including “isolated” and“clustered” Ti species in order to investigate the deactivation behaviors for propylene andethylene polymerizations.

The shapes of the kinetic profiles were quantified andcompared by four parameters (Tables 1 and 2); Rp,ave and Rp,max respectively as average andmaximum instantaneous consumptions of monomers, t(Rp,max) as the time when the activity
reaches the maximum value, and Rp,max/Rp(2 h) as a decay index on average.

Figure 2shows the kinetic profiles of propylene polymerization for Cat-0.08 and Cat-0.90 with the
Al concentration of 12.5 mmol/L–1. Rp of Cat-0.08 was around 10 times higher than thatof Cat-0.90, indicating that the clustering of the Ti species gave a significant effect to thetotal activity due to decrease in the number of surface Ti species being able to react withalkylaluminum.

Cat-0.08 exhibited faster increase and depression of the monomer

consumption, respectively quantified by t(Rp,max) and Rp,max/Rp(2 h), than Cat-0.90.
t(Rp,max) and Rp,max/Rp(2 h) for propylene polymerization were plotted as a function of the
Al concentration in Figures 3 and 4.

The increase in the Al concentration made t(Rp,max)smaller and Rp,max/Rp(2 h) larger, similarly for the two catalysts.

As was alreadydescribed, the dispersion state of the Ti species for the TiCl3/MgCl2 model catalysts was
believed to hardly change during the polymerization, and therefore the time-course
depression of the monomer consumption was considered to purely reflect the progress ofthe over-reduction of the Ti species by AlEt3.

The difference in the activation anddeactivation rates between the two catalysts suggested that the dispersion state of the Ti
species affected not only the absolute activity but also the reaction rate of the Ti species
with AlEt3.

The reason of the measured slower activation and deactivation for theclustered Ti species was safely explainable, as follows.

In the clustered Ti species, one Ti

ion can exchange its electron with neighboring Ti ion through bridging Cl ions. For
instance, a deactivated active site (Ti2+) as a result of the over-reduction can be reactivatedby transferring its excess electron to neighboring Ti3+ species, where the neighboring Ti
species works as an oxidant.

In other words, Ti species belonging to a cluster is able topool the surplus electron to slow down the rate of the average reduction rate in the cluster.
This mechanism resembles the reactivation of a TiCl4/MgCl2 catalyst by the addition of
Cl-based oxidants reported by Kashiwa et al.5 It is also considered that Ti ions in acluster have a larger coordination number in terms of Cl ions than isolated Ti ions, andtherefore a larger amount of Cl should be removed from the clustered Ti species than fromthe isolated Ti species in order to prepare a vacant site for monomer coordination that isessential to make an active site.

Thus, the activation of the clustered Ti species should be

slower.
The kinetic profiles of ethylene polymerization for the two catalysts with the Alconcentration of 12.5 mmol/L–1 are shown in Figure 5. The dependences of t(Rp,max) andRp,max/Rp(2 h) on the Al concentration are plotted in Figures 6 and 7. Rp of Cat-0.08
exceeded that of Cat-0.90 by an order of magnitude in the same fashion as for propylene
polymerization (Tables 1 and 2), again indicating a significance of the aggregation for thecatalyst activity.

As can be seen from t(Rp,max) and Rp,max/Rp(2 h) in Tables 1 and 2, theactivation and deactivation for the ethylene polymerization were much slower than thosefor propylene polymerization. It was previously reported that the most active oxidation
state of Ti species for ethylene polymerization was II,17 implying a slower activation.
t(Rp,max) were about 30-50 min for ethylene polymerization, while the propylene
consumption rate already became 10-20% of the maximum value at the timing.

Moreover,there was no drastic decay of the activity even after 2 h of polymerization on the contrary
to the case of propylene polymerization, indicative of the insensitivity of ethylene
polymerization to the oxidation state of Ti species as Kashiwa et al. and Tregubov et al.
pointed out.

It is also taken into consideration that the diffusion of ethylene must play animportant role based on the following two reasons: i) the crystallinity of the produced
polyethylenes was not low for the two catalysts, and ii) the solubility of ethylene is much
lower than that of propylene, while the reactivity of ethylene is much larger than that ofpropylene.18 Thus, it was not easy to give a clear explanation to the deactivation behaviorof the catalysts for ethylene polymerization. However, the importance of the aggregationand the immateriality of the over-reduction of Ti species were safely concluded for the
depression of the ethylene polymerization activity.

Conclusion
The catalyst deactivation behaviors were investigated for propylene and ethylene
polymerization using TiCl3/MgCl2 model catalysts. The advantage of the TiCl3/MgCl2
model catalyst, namely, the controlled dispersion state of Ti species, enabled us to study indetail the origin of the deactivation of active sites in MgCl2-supported Ziegler-Nattacatalysts. The aggregation of the Ti species significantly reduced both the propylene andethylene polymerization activities, and was identified as one of the major deactivation
mechanisms.

The time-course depression of the monomer consumption which arose fromthe over-reduction of Ti species was prominent for propylene polymerization, while it was
subtle for ethylene polymerization. The role of the aggregation and over-reduction of Ti
species were successfully understood in relation to the deactivation of MgCl2-supported
Ziegler-Natta catalysts, which is expected to be valuable for the design of highly active
industrial olefin polymerization catalysts.

Acknowledgement
This work was supported by Toho Titanium Co., Ltd. and Tosoh Finechem Co., Ltd.

Reference(1) E. Albizzati, U. Giannini, G. Collina, L. Noristi, and L. Resconi, in Propylene
Handbook, E. P. Moore, Eds., Hanser, New York, 1996, Chapter 2.
(2) J. Boor, in Ziegler-Natta Catalysts and Polymerization. Academic Press, New York,
1979, Chapter 8 and 18.
(3) T. Keii, Makromol. Chem., 183, 2285 (1982).
(4) K. Soga, S. Chen, and R. Ohnishi, Polym. Bull., 8, 473 (1982).
(5) N. Kashiwa, and J. Yoshitake, Makromol. Chem., 185, 1133 (1984).
(6) E. Albizzati, U. Giannini, I. Camurati, J. C. Chadwick, T. Dall'occo, Y. Dubitsky. M.
Galimberti, G. Morini, and A. Maldotti, J. Polym. Sci. A. Polym. Chem., 35. 2645 (1997).
(7) J. C. Chien, and J.-C. Wu, J. Polym. Sci., 20, 2461 (1982).
(8) V. A. Zakharov, S. I. Makhtarulin, V. A. Poluboyarov, and V. F. Anufrienko, Makromol.
Chem., 185, 1781 (1984).
(9) S. A. Sergei, V. A. Poluboyyarov, V. A. Zakharov, V. F. Anufrienko, and G. D. Bukatov,
Makromol. Chem., 186, 243 (1985),(10) P. Brant, and A. N. Speca, Macromolecules, 20, 2740 (1987).
(11) A. A. Tregubov, V. A. Zakharov, and T. B. Mikenas, J. Polym. Sci. A. Polym. Chem.,
47. 6362 (2009).
(12) P. J. T. Tait, G. H. Zohuri, A. M. Kells, and I. D. Mckenzie, in Ziegler Catalysts, G.
fink, R. Mülhaupt, and H. H. Bruntzunger, Eds., Springer-Verlag, Berlin, P 343.
(13) A. Dashti, A. Ramazani SA, Y. Hiraoka, S. Y. Kim, T. Taniike, and M. Terano, Polym.
Int., 58, 40 (2009).
(14) K. Soga, and M. Terano, Makromol. Chem., 182, 2439 (1981).
(15 I. Kouzai, B. Liu, T. Wada, and M. Terano, Macromol. React. Eng. 1, 160 (2007).

(16) T. Wada, T. Taniike, I. Kouzai, S. Takahashi, and M. Terano, Macromol. Rapid
Commun. 30, 887 (2009).
(17) D. B. Ludlu, A. W. Anderson, and C. E. Ashby, J. Am. Chem. Soc., 80, 1380 (1958).
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(2008).

Figure/Scheme Captions
Figure 1. Stereoregularity (●) of the polypropylenes and activity (○) with respect to the
Ti content. Propylene polymerization was conducted under normal pressure at 30ºC in
heptane with 12.5 mmol·L–1 of AlEt3 for 30 min.

Figure 2.

(a) Kinetic profiles of propylene polymerization with Cat-0.08 (●) andCat-0.90 (●), and (b) the magnification of the initial stage of the polymerization.

The

Al concentration was 12.5 mmol·L–1.

Figure 3.

t(Rp,max) vs. the Al concentration for propylene polymerization with Cat-0.08(■) and Cat-0.90 (□)

Figure 4. Rp,max/Rp(2 h) vs. the Al concentration for propylene polymerization with
Cat-0.08 (■) and Cat-0.90 (□)

Figure 5.

(a) Kinetic profiles of ethylene polymerization with Cat-0.08 (●) andCat-0.90 (●), and (b) the magnification of the initial stage of the polymerization.

The

Al concentration was 12.5 mmol·L–1.

Figure 6. t(Rp,max) vs. the Al concentration for ethylene polymerization with Cat-0.08(■) and Cat-0.90 (□).

Figure 7. Rp,max/Rp(2 h) vs. the Al concentration for ethylene polymerization with
Cat-0.08 (■) and Cat-0.90 (□)

Table 1.

Kinetic parameters of propylene polymerization with the TiCl3/MgCl2 model

catalysts.

Ti contents / wt.-%
Al concentration /mmol·L–1
Rp,avea x 10–3/
kg-C3H6·mol-Ti–1·h–

0.08

0.90

6.25

12.5

62.5

125

6.25

12.5

62.5

125

1.4

1.2

0.94

0.94

0.16

0.14

0.11

0.10

Rp,maxa x 10–3 /kg-C3H6·mol-Ti–1·h–

5.4

5.1

4.3

4.5

0.44

0.43

0.38

0.34

t(Rp,max) / minb

5.9

5.3

4.2

3.3

9.2

8.4

7.3

6.6

Rp,max/Rp(2 h)b

11

9.9

13

15

6.1

6.9

7.5

9.1

1

1

a

Averaged and maximum instantaneous consumptions of propylene.

bTime when the activity reaches the maximum value, and decay index on average.

Table 2. Kinetic parameters of ethylene polymerization with the TiCl3/MgCl2 model
catalysts.

Ti contents / wt.-%
Al concentration /mmol·L–1
Rp,ave x 10–3 /kg-C2H4·mol-Ti–1·h–1
Rp,max x 10–3 /kg-C2H4·mol-Ti–1·h–1t(Rp,max) / min
Rp,max/Rp(2 h)

0.08

0.90

6.25

12.5

62.5

125

6.25

12.5

62.5

125

4.9

5.7

6.6

6.2

0.52

0.64

0.70

0.64

6.0

7.1

8.3

8.1

0.64

0.78

0.88

0.83

55

47

37

24

64

41

32

21

1.2

1.3

1.5

1.5

1.3

1.2

1.3

1.6

Figure 1. Taniike et al.

Figure 2. Taniike et al.

Figure 3. Taniike et al.

Figure 4. Taniike et al.

Figure 5. Taniike et al.

Figure 6. Taniike et al.

Figure 7. Tanike et al.

Table of content
Deactivation behaviors of TiCl3/MgCl2 model catalysts with molecularly dispersed TiCl3
were investigated to clarify the role of the dispersion state of the Ti species in thedeactivation of MgCl2-supported Ziegler-Natta catalysts for propylene and ethylene
polymerization.