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Title

Structure‑Performance Relationship in Ziegler‑
Natta Olefin Polymerization with Novel Core‑Shell
MgO/MgCl̲2/TiCl̲4 Catalysts

Author(s)Taniike, Toshiaki; Chammingkwan, Patchanee;
Terano, Minoru

Citation

Catalysis Communications, 27: 13‑16

Issue Date

2012

Type

Journal Article

Text version

author

URL

http://hdl.handle.net/10119/11450

RightsNOTICE: This is the author's version of a work
accepted for publication by Elsevier. Toshiaki
Taniike, Patchanee Chammingkwan, Minoru Terano,
Catalysis Communications, 27, 2012, 13‑16,
http://dx.doi.org/10.1016/j.catcom.2012.06.015

Description

Japan Advanced Institute of Science and Technology

Structure-Performance Relationship in Ziegler-Natta Olefin
Polymerization with Novel Core-Shell MgO/MgCl2/TiCl4 Catalysts

Toshiaki Taniike, Patchanee Chammingkwan, and Minoru Terano*

School of Materials Science, Japan Advance Institute of Science and Technology, 1-1 Asahidai,
Nomi, Ishikawa 923-1292, Japan*CORRESPONDING AUTHOR
Tel: +81-761-51-1620; Fax: +81-761-51-1625; E-mail: terano@jaist.ac.jp1

ABSTRACT: The impact of support architectures on the olefin polymerization performance ofheterogeneous Ziegler-Natta catalysts has been scarcely understood due to the complexity ofpore system and the fragmentation-induced structural changes during polymerization. In thiscommunication, a series of novel core-shell MgO/MgCl2/TiCl4 catalysts were synthesized byutilizing poreless single-crystal MgO nanoparticles as a non-fragmentable core material.
Employing these catalysts, we have successfully established a structure-performance
relationship (SPR) that the propylene polymerization activity is perfectly proportional to thecatalyst surface area for the first time. On the other hand, polymer properties were found to be
decided by active site natures independently of the surface area.

KEYWORDS: Ziegler-Natta polymerization; polyolefins; structure-performance relationship;core-shell; nanoparticle2

1. Introduction
Elucidation of the structure-performance relationship (SPR) is one of the ultimate goals for
systematic and tailored design of advanced materials. In heterogeneous catalysis, both active
site structures and support architectures either competitively or synergistically contribute tocatalytic performances. For example, a linear relationship between catalyst surface area andactivity has been found in many catalytic systems, like in photocatalytic decomposition ofaldehyde on Bi2WO6 [1]. The pore dimension of support materials can affect both activity andselectivity through restricted diffusion and formation of bulkier molecules, e.g. the pore size ofmesoporous silicate regulates the alkylation degree of benzene [2]. As was demonstrated in CO
oxidation on Pt surfaces [3], catalytic performance are often very sensitive to surface structures(plane and defect), which is especially important in shaping metal nanoparticles, and vice versa.
In heterogeneous Ziegler-Natta (ZN) catalysts (TiCl4 supported on MgCl2) for olefin
polymerization, SPR for active sites has been gradually uncovered as a result of tremendous
efforts. For instance, ethylene can be polymerized irrespective of the oxidation state of Ti
species, while Ti2+ species is inactive for polymerization of -olefin [4]. The polymer molecularweight tends to be larger for a lower oxidation state [4]. Stereoselectivity in propylene
polymerization is influenced by the presence or absence of bulky ligands at metal sites adjacent
to a Ti center, as was proposed in the three-site model [5,6]. On the contrary, SPR for support
architectures has been scarcely established in the long history, even for the most basic SPR
between the catalyst surface area and activity. Gerbasi et al. [7] plotted the ethylene
polymerization activity of a TiCl4/MgCl2 catalyst along the grinding time, where the activity
was correlated with the structural disorder of MgCl2 support rather than the catalyst surface area.

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Marigo et al. [8] evaluated the surface areas of several ZN catalysts prepared from
MgCl2-alcohol adduct by means of the BET analysis in N2 adsorption and small-angle X-ray
scattering (SAXS). They found that the propylene polymerization activity was never correlatedwith the BET surface area, while some level of correlation was found for the SAXS surface area.
However, the obtained correlation was rather rough and could not be extrapolated to the origin(i.e. zero activity at zero surface area) plausibly due to the contribution of closed pores to the
SAXS surface area. This result implies an intrinsic problem to apply the BET analysis to thesurface area of typical ZN catalysts. Bozik et al. [9] examined the ethylene polymerization
activities of various metallophosphate-supported TiCl4 catalysts. They suggested the presence ofsome but very rough correlation (R2~0.8) between the activity and the BET surface area. Besidethe surface area, the importance of pore architectures of MgCl2 support has been empiricallyrecognized. Nevertheless, their influences on polymerization have been hardly studied in aquantitative manner owing to the shape irregularity and size distribution of pore structures.
Furthermore, the fragmentation of catalyst particles (which consist of hierarchically
agglomerated smaller structural units) causes time-course variations of support architecturesduring polymerization, thus messing up the initially characterized architectures. Since theperformance of a ZN catalyst results from interplay among the polymerization reaction on
catalyst surfaces, the monomer transportation within pores and the particle fragmentation during
polymerization, it is almost impossible to explore clear SPR using a typical industrial ZN
catalyst.
In this communication, we report the first SPR establishment for support architectures ofheterogeneous ZN catalysts on the basis of novel core-shell MgO/MgCl2/TiCl4 catalysts.

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Starting from single-crystal MgO nanoparticles with various dimensions and surface areas, aseries of core-shell MgO/MgCl2/TiCl4 catalysts were fabricated, where poreless andnon-fragmentable MgO core was thinly covered by the catalytic components. While thesecatalysts exhibited active site natures typical for MgCl2-supported ZN catalysts in propylene
polymerization, the poreless and non-fragmentable features gave a perfectly proportional
relation between the catalyst surface area and activity.

2. Experimental
2.1. Materials
Analytical grades of magnesium nitrate hexahydrate (Mg(NO)2·6H2O) and oxalic aciddihydrate ((COOH)2·2H2O) purchased from Wako Pure Chemical Industries, Ltd. were used as
the precursors for the synthesis of MgO. Anhydrous ethanol and titanium tetrachloride (TiCl4)
purchased from Wako Pure Chemical Industries, Ltd. and triisobutyl aluminum (TiBA) donatedby Tosoh FineChem Co., Ltd. were used as received. Heptane was used after dehydration bypassing through a column of molecular sieve 4A. Propylene of research grade was used without
further purification.

2.2. Preparation of MgO nanoparticles
Single-crystal MgO nanoparticles were synthesized according to the sol-gel method describedby Kumar and Kumar [10]. Mg(NO)2·6H2O and (COOH)2·2H2O were dissolved separately inethanol. The two precursors were mixed at the molar ratio of 1:1 to form white gel. The gel wasdigested for 12 h under vigorous mixing and dried at 100ºC for 24 h to form MgC2O4·2H2O.

5

The dried gel was then calcined in air at 800, 750, 700, 650 and 500ºC to obtain MgO
nanoparticles having different sizes and surface areas, denoted as MgO1, MgO2, MgO3, MgO4
and MgO5, respectively.

2.3. Preparation of MgO/MgCl2/TiCl4 catalyst
2.0 g of MgO powder was thermally pre-treated at 130°C under nitrogen flow for 6 h toremove physisorbed water, and subsequently treated with 30 ml of refluxing TiCl4 at 140°C for
2 h. The solid was washed with heptane 10 times and kept as slurry under nitrogen.

2.4. Polymerization
Propylene polymerization was performed in a 1 L stainless steel autoclave equipped with amechanical stirrer. 200 ml of heptane as solvent was introduced to the reactor under nitrogenand then saturated with 0.5 MPa of propylene at 50ºC for 30 min. Following the addition of 2.0
mmol of TiBA as an activator, 30 mg of a catalyst was charged to the reactor to startpolymerization. The polymerization was continued for 30 min at the constant pressure andtemperature, and finally terminated by the addition of acidic ethanol.

2.5. Characterizations
Powder X-ray diffraction (XRD, RINT 2100, Rigaku) was used to identify and determine thecrystallite structures and dimensions of MgO and MgO/MgCl2/TiCl4 particles. Transmission
electron microscope (TEM) was recorded on Hitachi H-7100 for the observation of particle
morphology. N2 adsorption/desorption experiments were conducted on Belsorp-max at 77 K to

6

determine the surface area based on the BET method.
The molecular weight (Mn) of produced polypropylene (PP) was measured by gel permeation
chromatography (GPC, Viscotek, HT-GPC 350) with Shodex UT-806M columns at 140ºC usingo-dichlorobenzene as solvent. The stereostructure of PP was analyzed by 13C NMR (Bruker 400
MHz NMR spectrometer) at 120ºC using 1,1,2,2-tetrachloroethane-d2 as an internal lock andreference.

3. Results and discussion
Fig. 1 shows representative TEM images of MgO1-5 synthesized by the sol-gel method andsubsequent calcination at different temperatures. The particles were dense with cubic andpartially polygonal shapes for all the samples. They were separated from each other withrelatively uniform sizes ranging approximately 8-35 nm according to the calcination
temperature. At a higher calcination temperature, particles were larger due to thermally-driven
particle growth. The formation of MgO pure phase with a cubic crystal structure was confirmed
by XRD for all the samples (not shown). Table 1 lists the crystallite sizes estimated from the(200) diffraction peak of MgO using the Scherrer equation along with the BET surface areas.
The crystallite sizes increased from 8 to 32 nm as the calcination temperature increased. The
quantitative coincidence between the crystallite sizes and the particle sizes in TEM indicatedthat the MgO nanoparticles were single-crystal. The change in the specific surface area was also
correlated with the crystallite sizes, where a larger crystallite size led to a lower surface area.
The particle sizes estimated from the BET surface areas under the assumption of a cubic shape(Table 1) again well coincided with the sizes obtained from XRD and TEM except for MgO1.

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This fact suggested that the single-crystal MgO nanoparticles were uniform in size and freefrom internal pore structures. The substantially greater size estimated from the BET surface areafor MgO1 was due to the deviation from the cubic shape. The reiterated calculation with theassumption of a spherical shape resulted in a smaller size than the XRD and TEM sizes,
indicating that MgO1 could have an intermediate shape between cubic and spherical,
consistently with the polygonal shape observed in TEM. In this way, a series of porelesssingle-crystal MgO nanoparticles were obtained with the surface area systematically altered by
the calcination temperature.
The MgO nanoparticles with well-defined architectures were chlorinated and titanated with
refluxing TiCl4 to synthesize core-shell MgO/MgCl2/TiCl4 catalysts. The Ti content increased
with the increase of the MgO surface area (Table 1). The XRD patterns of the obtained catalysts
are shown in Fig. 2. Beside five prominent peaks of MgO pure phase at 37º, 43º, 62º, 74º and78º (which correspond to (111), (200), (220), (331) and (222) reflections [10]),
MgO1-4/MgCl2/TiCl4 catalysts exhibited minor and broad peaks, characteristic of anhydrous
δ-MgCl2, at around 15º, 30-35º and 50º [11]. The crystallite sizes of MgO cores calculated fromthe XRD patterns as well as the BET surface areas were almost kept unchanged before and after
the TiCl4 treatment. These facts suggested that thin MgCl2 overlayer was formed on the MgO
core without altering the particle structures of MgO. The uniform sizes of MgO nanoparticles
observed in the TEM images let us to presume that the formation of the MgCl2 overlayer andimmobilization of Ti species on the surfaces occurred homogenously, following the reaction
schemes proposed by Tatizawa et al. [12],MgO + TiCl4 → MgCl2 + TiOCl2

(1),

8

MgO + 2TiCl4 → MgCl2TiCl4 + TiOCl2(2).

On the contrary to MgO1-4/MgCl2/TiCl4, MgO5/MgCl2/TiCl4 exhibited sharp diffraction peaks
at 2θ ≈ 15º, 30º, 35º and 50º, corresponding to the Miller indices of (003), (012), (104) and (110)of -MgCl2, respectively [8], while the characteristic peaks of MgO almost disappeared.
Correspondingly, the surface area of MgO5/MgCl2/TiCl4 significantly decreased from 221.5 to
47.0 m2/g. Hence, the smallest MgO5 particles underwent deep chlorination into the bulk andwere mostly converted into -MgCl2, which accompanied sintering and agglomeration of theparticles. Due to the loss of the original particle structures, MgO5/MgCl2/TiCl4 was excluded in
the following polymerization experiments.
Fig. 3 shows the propylene polymerization activity of the core-shell MgO/MgCl2/TiCl4
catalysts (except sintered MgO5). The activity (per g of catalyst) was found to be completely
proportional to the catalyst surface area, crossing the origin (Fig. 3). At our best knowledge, thisis the first experimental results on a linear relationship between the surface area and activity,
while typical ZN catalysts suffer from the fragmentation phenomena, the complexity of porearchitectures, and so on. On the contrary, the Ti content (which did not linearly increase with thecatalyst surface area (Table 1)) was not clearly correlated with the activity, even though a higher
Ti content was expected to give a higher activity. It was presumed that active site concentration
per surface area reaches a plateau in the presence of excess alkylaluminum, resulting in aconstant yield per surface area. The polymer properties were quite insensitive to the support
variables (Table 2), suggesting that active sites are situated in similar environments on thesurfaces of the catalysts: the stereoregularities were similarly about 50 mol% in mmmm and Mn
was in a range of 2.7-3.1  104 g/mol with the Mw/Mn values of 6.1-6.8, both of which were9

typical values for donor-free TiCl4/MgCl2 catalysts. It was concluded that the core-shell
MgO/MgCl2/TiCl4 catalysts reproduce the active site environments of typical ZN catalysts, but
the novelty of their poreless and non-fragmentable structures has enabled the establishment of
the first clear SPR for support architectures in heterogeneous ZN olefin polymerization.

4. Conclusions
A series of poreless single-crystal MgO nanoparticles with different sizes were prepared by
the sol-gel method. The MgO nanoparticles were utilized as a non-fragmentable core material toprepare novel core-shell MgO/MgCl2/TiCl4 catalysts for olefin polymerization. Characterizationof the MgO nanoparticles and MgO/MgCl2/TiCl4 catalysts demonstrated the formation of
TiCl4/δ-MgCl2 overlayer without the shrinkage of MgO core except MgO5, which underwent
deep chlorination into bulk and severely agglomerated. With these core-shell MgO/MgCl2/TiCl4
catalysts, SPR between the catalyst surface area and propylene polymerization activity was
successfully acquired. We concluded that the catalyst surface area is primarily important forolefin polymerization activity, while polymer properties are dependent mainly on active site
structures.

Acknowledgement
The Authors appreciate Japan Polychem Co., Mitsui Chemicals, Inc., Sumitomo ChemicalCo., Ltd., Toho Titanium Co., Ltd., and Tosoh Finechem Co. for the donation of reagents.

References10[1] F. Amano, K. Nogami, B. Ohtani, Langmuir 26 (2010) 7174.
[2] X. Hu, M.L. Foo, G.Kh. Chuah, S. Jeanicke, J. Catal. 412 (2000) 195.
[3] K. Nakao, S. Ito, K. Tomishige, K. Kunimori, J. Phys. Chem. B 109 (2005) 17579.
[4] D. Fregonese, S. Mortara, S. Bresadola, J. Mol. Catal. A: Chem. 172 (2001) 89.
[5] V. Busico, R. Cipullo, G. Monaco, G. Talarico, M. Vacatello, J.C. Chadwick, A.L. Segre, O.
Sudmeijer, Macromolecules 32 (1999) 4173.
[6] B. Liu, T. Nitta, H. Nakatani, M. Terano, Macromol. Chem. Phys. 204 (2003) 395.
[7] R. Gerbasi, A. Marigo, A. Martorana, R. Zannetti, G.P. Guidetti, G. Baruzzi, Eur. Polym. J.
20 (1984) 967.
[8] A. Marigo, C. Marega, R. Zannetti, G. Morini, G. Ferrara, Eur. Polym. J. 36 (2000) 1921.
[9] J.E. Bozik, R.F. Vogel, Y.V. Kissin, D.L. Beach, J. Appl. Polym. Sci. 29 (1984) 3491.
[10] A. Kumar, J. Kumar, J. Phys. Chem. Solids 69 (2008) 2764.
[11] J.C.J. Bart, J. Mater. Sci. 28 (1993) 278.
[12] N. Tatizawa, R. Quijada, Eur. Polym. J. 28 (1992) 139.

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Figure captions
Fig. 1. TEM images of MgO particles: (a) MgO1, (b) MgO2, (c) MgO3, (d) MgO4 and (e)MgO5Fig. 2. XRD patterns of MgO/MgCl2/TiCl4 catalysts: (a) MgO1/MgCl2/TiCl4, (b)MgO2/MgCl2/TiCl4,(c)

MgO3/MgCl2/TiCl4,(d)

MgO4/MgCl2/TiCl4

and

(e)

MgO5/MgCl2/TiCl4

Fig. 3. Relationship between the catalyst surface area and propylene polymerization activity.
The color code is the same as for Fig. 2.

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Table 1
Characterization results for MgO nanoparticles and MgO/MgCl2/TiCl4 catalysts.
Sample MgO nanoparticles
BET surface
Crystal size inaarea [m2/g]
XRD [nm]

Particle size in
BETb [nm]MgO/MgCl2/TiCl4 catalyst
BET surface
Ti contentdarea [m2/g][wt.-%]

MgO1

32.0 (30.3c)

32.6

51.4 (20.7c)

32.7

0.88

MgO2

19.5

72.9

23.0

99.8

2.39

MgO3

16.3

103.4

16.2

106.8

6.16

MgO4

12.8

128.7

13.0

148.5

7.72

MgO5

8.0

221.5

7.6

47.0

9.16

a

Based on the Scherrer equation for the (200) peak of MgO (K = 0.94 for cubic crystal).
Based on dp = 6×103/SBETρ for cubic particles, where SBET is the BET surface area and ρ is thedensity of MgO (3.58 g/cm3).
c
Assuming spherical particles.
dMeasured by titration.
b

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Table 2
Results of polymer analyses.
Catalyst
mmmma [mol%]
MgO1/MgO/TiCl4 52.7
MgO2/MgO/TiCl4 49.4
MgO3/MgO/TiCl4 50.8
MgO4/MgO/TiCl4 50.0aDetermined with 13C-NMR.
b
Determined with GPC.

Mnb [g/mol]
3.0104
2.7104
3.1104
2.7104

14

Mw/Mnb
6.8
6.8
6.1
6.7

Fig. 1.

Fig. 2.

15

Fig. 3.

16