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Ethylene polymerization over mgo-supported zirconocene catalysts.

INTRODUCTION

Metallocene catalysts have received much attention in industry because they can produce polymers with a narrow molecular weight distribution, uniform chemical composition or a stereo-specificity. Several types of metallocene catalysts have been developed, with [Cp.sub.2][ZrCl.sub.2] being one of the most widely studied (1-5). However, in order to use the metallocene catalysts in slurxy or gas phase processes, it is necessary to support the catalyst properly. For this reason, much effort has been made to prepare efficient supported catalysts. Catalysts supported on [SiO.sub.2], [Al.sub.2][O.sub.3]. [MgCl.sub.2] and zeolite have been found to have high activities and to produce polymers with a morphology in olefin polymerization (6-15).

It is known that if metallocene is direcily impregnated onto a support, both the degree of impregnation and the catalytic activity are usually low (16). But pretreatment of [SiO.sub.2] with MAO was found to yield metallocene catalysts with higher metal loading.

MgO has similar physical properties to [SiO.sub.2] but no reports have been made on the metallocene catalyst supported on MgO. In this work, MgO-supported metallocene catalysts were prepared by various supporting methods, including support treatments, and the catalytic properties were examined in ethylene polymerization to find their applicability. Especially, the influence of the supporting method on the polymerization behaviors of the catalysts was investigated in detail.

EXPERIMENTAL

Materials

All experiments were carried out under nitrogen atmosphere using standard Schlenk techniques. The MgO, supplied by Shinyo Pure Chemicals Co. (Osaka, Japan), dried for 6 h at 400[degrees] under a nitrogen atmosphere prior to use. [Cp.sub.2][ZrCl.sub.2] was purchased from

Acros Co. and was used without further purification. The modified methylaluminoxane (MMAO, Type 4) was obtained from Akzo Nobel Chemicals Inc. as a 7.8 wt% toluene solution (density 0.878 g/[cm.sup.3]). Ethylene (polymerization grade, 99.5% purity), supplied by Air Products Co., was dried by passing through two columns filled with RIDOX and a molecular sieve respectively. Commercial toluene was obtained from DUKSAN Pure Chemical Co. (Korea) and further purified by refluxing over a sodium/benzophenone complex and distilled under inert gas.

Preparation of MgO Supported Catalysts

The types of MgO-supported metallocene catalysts were prepared according to the supporting method. Figure 1 shows the schematic diagram for catalyst preparation.

CAT. 1 (Direct Impregnation): 2 g of MgO was suspended in 50 ml of toluene followed by the addition of a solution containing [Cp.sub.2]Zr[Cl.sub.2]. The mixture was gradually heated to 50[degrees]C and stirred at this temperature for 10 h. After stirring, the precipitate was washed 8 times with 50 ml distilled toluene and dried under vacuum at 100[degrees]C for 12 h.

CAT. 2 (Supporting on MAO-treated MgO): In a 150 ml flask equipped with a magnetic stirrer, 2 g of MgO and 50 ml of toluene were added and then by the addition of a toluene solution of MAO (2.8 mmol Al). The mixture was stirred at 30[degrees]C for 1 h and washed with 50 ml distilled toluene. Subsequently, 20 ml solution of the [Cp.sub.2]Zr[Cl.sub.2] in toluene was added to the MAO/MgO precursor and stirred for 10 h at 50[degrees]C. The resulting precipitate was washed and dried as mentioned above.

CAT. 3 (Supporting of MAO-treated Metallocene): 50 ml of toluene and the required amount of MAO were placed in 150 ml flask equipped with a magnetic stirrer, followed by the addition of 0.2 g of [Cp.sub.2]Zr[Cl.sub.2]. The mixture was stirred at 30[degrees]C for 1 h. The MAO-pretreated [Cp.sub.2]Zr[Cl.sub.2] solution was carefully transferred to a flask containing 2 g MgO and stirred for 10 h at 50[degrees]C. The resulting precipitate was washed and dried as described above.

Polymerization

Ethylene polymerization was carried out in a 250 ml glass reactor equipped with a magnetic stirrer. The reactor was degassed under reduced pressure and 100 ml toluene was added under a continuous nitrogen stream at room temperature. Ethylene was subsequently introduced into the reactor and stirred at 50[degrees]C. After the solvent was saturated with ethylene, polymerization was initiated by injecting the appropriate amount of catalyst and co-catalyst. Polymerization was terminated by addition of methanol to the reaction mixture. The product was treated in a methanol/ HCl mixture, filtered and dried in vacuo.

Characterization

The surface areas of the supports were determined by the BET method (adsorption of [N.sub.2] at the liquid [N.sub.2] temperature), and their particle sizes were measured by Scanning Electron Microscopy (SEM) (JSM-35, JEOL). The water content of MgO was determined by thermogravimetric analysis (TGA7, Perkin-Elmer). The zirconium concentration of the catalysts was measured using Inductively Coupled Plasma/Mass Spectroscopy (ICP/MS) (Ultramass700, Varian). The melting temperature ([T.sub.m]) of polymer product was determined by DSC (DSC 821e, Mettler Toledo) at a heating rate of 10[degrees]C/min. Gel Permeation Chromatography (GPC) (PL-2 10, Polymer Laboratory) was used to measure the molecular weight (MW) and molecular weight distribution (MWD) of polyethylene at 160[degrees]C with 1, 2, 4-trichlorobenzene as a solvent.

RESULTS AND DISCUSSION

Characterization of [Mg.sub.O] Support

The water content in the MgO was analyzed by TGA since it affected the physical property of the support significantly. The measured weight reduction during the slow heating up to 600[degrees] for MgO and MgO dried at 400[degrees]C were 18.7% and 5.1% respectively, which correspond to MgO - 0.5 [H.sub.2]O and MgO . 0.12 [H.sub.2]O according to Bart and Roover's study (17).

The average particle size and surface area of the supports are shown in Table 1. The surface area of MgO was found to be larger than other conventional supports such as Mg[Cl.sub.2] or Si[O.sub.2] and the particle sizes of MgO were comparable to that of silica, indicating that MgO has suitable physical features for supporting metallocene catalyst.

Effect of of Support Pretreatment on Catalyst Loading

The amounts of [Cp.sub.2][ZrCl.sub.2] impregnated at different temperatures for each method are shown in Table 2 and Fig. 2. Table 2 shows that the metal loadings of the CAT. 2 are higher than those of the CAT. 1, which shows that the modified MgO surface is better suited for impregnation of [Cp.sub.2][ZrCl.sub.2] than original MgO. This result may be attributed to the easier combination of the zirconocene cation with the MAO anchored on the MgO support than with the base MgO surface. Chien et al suggested a mechanism in which Si-O-Al formed on the Si[O.sub.2] surface by a reaction of Silica and MAO, which a zirconocene methyl cation is produced by elimination of C1 from zirconocene (18). Soga suggested that MAO is anchored to the hydroxyl groups (-OH) present on the surface of the Si[O.sub.2] support and that this anchored MAO reacts with another MAO, which is combined to the zirconocene to form the active species (19). The above results and previous reports suggest that the aluminoxane compound (MAO) combin es with the hydroxyl groups present in MgO to form Mg-O-(AI) followed by a reaction with zirconocene to form the active cationic species. Catalysts prepared by supporting MAO-treated metallocene (CAT. 3) show the least amount of impregnation. In this case, the metallocene catalyst reacted with MAO to form a complex before supporting. During the supporting procedure, the bulky metallocene-MAO complex had to combine with the bare MgO. Therefore this procedure became the least efficient in supporting to lead the lowest metal loading.

Polymerization of Ethylene

The characteristics of polyethylene produced on the catalytic properties of the MgO supported catalysts, which were prepared by the three methods, were investigated in ethylene polymerization. Figure 3 shows the change in the activity profiles of the three catalysts with reaction temperature. The CAT. 1 catalyst showed low activities at all temperature in the range, which is probably due to low Zr loading of the catalyst supported on MgO directly. The activities of CAT. 2 catalyst were much higher than that of CAT. 1 catalyst. It reached a maximum value at 70[degrees]C and decreased afterwards. The activities of the CAT. 3 catalyst were lower than those of CAT. 2 catalyst, but higher than those of the CAT. 1 catalyst, even though the Zr loading of CAT. 2 catalyst was lower than CAT. 1 catalyst. It seems that metallocene catalyst pretreated with MAO and then supported on Mg is more efficient in provoking polymerization than the metallocene supported directly on [SiO.sub.2]. Table 3 also shows the properties of the polymer obtained from the three catalysts, the [T.sub.m], the MW, and MWD. Polymers obtained from the CAT. 2 and CAT. 3 catalysts had a higher [T.sub.m] and a higher MW than the CAT. 1 catalyst. It seems that impregnation of metallocene on MAO treated support in the most desirable supporting method for MgO. Figure 4 shows the activity profile with respect to the reaction time. The reaction rate of the CAT. 2 catalyst began with an initial low value, but soon rose to a relatively high level, maintaining that level throughout the reaction. Stable activities were also observed in the CAT. 1 and CAT. 3 catalysts than in CAT. 1 catalyst. These reaction profiles for metallocene supported on MgO are in contrast to those for homogeneous metallocene catalysts where the activities decrease rapidly with time and show typical reaction characteristics of supported metallocene catalysts (20). These results indicate that [Cp.sub.2]Zr[Cl.sub.2] catalyst was successfully impregnated on MgO irrespective of the supporting method.

In Fig. 5a, the GPC results from the three catalysts are shown. In the case of polyethylene obtained from CAT. 1 catalyst which was prepared by directly impregnation of [Cp.sub.2]Zr[Cl.sub.2] on MAO, a narrow peak appeared around log MW = 4.5. It is a similar tendency to the case of the [Cp.sub.2]Zr[Cl.sub.2] supported on Mg[Cl.sub.2] pretreated with MAO (21). However, the CAT. 2 catalyst ([Cp.sub.2]Zr[Cl.sub.2] supported on MAO treated MgO), exhibited bimodal peaks formed around log MW = 4.5 and 5.8, yielding a broad MWD. In the case of CAT. 3 (prepared by impregnation of MAO-treated [Cp.sub.2]Zr[Cl.sub.2] on MgO), bimodal peaks of polyethylene were observed in the neighborhood of log MW = 4.5 and 5.8 more distinctively with a similar intensity. Figure 5b shows that this bimodal molecular weight distribution is more apparent for the polymer obtained in the reaction at a lower temperature. And the high melting temperature peak and the low melting temperature peak are partly overlapped. This suggests that this polymer is composed of two lamella structures, which are polymerized by two different active sites. Comparing the peaks corresponding to the polymers from CAT. 1, CAT. 2, and CAT. 3 catalyst in Fig. 5a, it can be seen that the low MW peak corresponding to the polyethylene produced on the CAT. 1 catalyst is produced by the metallocene directly supported on MgO, while the high MW peak that appears only for CAT. 2 and CAT. 3 is obtained from the MAO-metallocene complex on MgO. This indicates that two different activities are present on CAT. 2 and CAT. 3. The low MW peak was formed by [Cp.sub.2]Zr[Cl.sub.2] directly impregnated on MgO, while the high MW peak was formed by [Cp.sub.2]Zr[Cl.sub.2] impregnated on MgO modified by MAO. The [AI-NMR.sup.27] spectra of the CAT. 2, CAT. 3 catalysts, and MgO/MAO were analyzed to find the effect of MAO on metallocene. The A1 peaks for the CAT. 2 and CAT. 3 catalysts appeared at 3.6216 ppm and at 8.3012 ppm respectively, while that of MAO-pretreated MgO appeared at -5.7 376 ppm. The electron density of [Cp.sub.2]Zr[Cl.sub.2] seemed to be de-deshielded Al peak in the MAO-treated support, so that the Al peak appeared downfield shifted from -5.7376 to 3.6216 and 8.3012 ppm. This suggests that strong interaction between the MAO and the metallocene catalyst exists, and that MAO plays a role in impregnating the catalysts.

CONCLUSIONS

In this study, the polymerization behaviors of [Cp.sub.2]Zr[Cl.sub.2] catalysts supported on MgO were investigated. The direct impregnation extent of [Cp.sub.2]Zr[Cl.sub.2] on MgO and its catalytic activity for ethylene polymerization were relatively low. However, both the impregnation amount and polymerization activity were relatively high in the case of the [Cp.sub.2]Zr[Cl.sub.2] supported on MgO pretreated with MAO, indicating that impregnation of metallocene on MAO-treated support is the most desirable supporting method for MgO. Pretreatment with MAO during the supporting step invoked two types of catalytic sites, which was evidenced by the bimodal molecular weight distribution of the polymer products. These results indicate that MgO seems to have potential as a support for metallocene catalysts.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5a OMITTED]

[FIGURE 5b OMITTED]
Table 1

The average Particle Size and Surface Area of the supports.

Supports Vendor Avg. particle size ([micro]m)

Mg[CI.sub.2] Junsel 322.56
Si[O.sub.2] strem 18.21
MgO shinyo 26.00
MgO-dried at 400[degrees]C shinyo 33.29

Supports Surface area ([m.sub.2]/g)

Mg[CI.sub.2] 1.18
Si[O.sub.2] 0.18
MgO 42.7
MgO-dried at 400[degrees]C 21.2

Pretreatment condition: evacuation up to [10.sub.-5] torr at
130[degrees]C, 2 h.

Table 2

The Effect of impregnation Method and Temperature on the Impregnated
Amount of Zr (wt%). (a)

 Impregnation Temperature

Catalyst 30[degrees]C 50[degrees]C 70[degrees]C 100[degrees]C

CAT. 1 0.50 0.65 1.06 1.01
CAT. 2 0.71 0.92 1.64 1.50
CAT. 3 0.31 0.53 0.82 0.71

(a)The amount of impregnated catalyst was measured by ICP.

Table 3

Polymerization Results of the MgO- Supported [Cp.sub.2]Zr[Cl.sub.2]
Catalysts and the Physical Properties of Their PE.

 Polymerization temp

 30[degrees]C

Catalyst Activity (a) [M.sub.w.sup.b] MWD [T.sub.m.sup.c]

CAT. 1 57.4 -- -- --
CAT. 2 250 347,100 16,220 133.19
CAT. 3 82.1 459,200 24,168 135.52

 Polymerization temp

 50[degrees]C

Catalyst Activity (a) [M.sub.w.sup.b] MWD [T.sub.m.sup.c]

CAT. 1 57.6 69,400 5.179 129.48
CAT. 2 307 177,200 11.145 131.76
CAT. 3 352 229,200 15.179 131.35

 Polymerization temp

 70[degrees]C

Catalyst Activity (a) [M.sub.w.sup.b] MWD [T.sub.m.sup.c]

CAT. 1 55.0 33,400 2,088 130.76
CAT. 2 558 57,500 6.741 131.13
CAT. 3 339 98,900 7.667 131.06

Polymerization condition: [P.sub.C2H4] = 1.3 bar, [P.sub.H2] = 0.2 bar,
polym'n time = 30 min, diluent = toluene.

(a)Activity: kg*PE/g-cal. * atm * hr.

(b)[M.sub.w]: g/mol.

(c)[T.sub.m]: [degrees]C.


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SOO JIN KIM (1a), WHA YOUNG LEE (1) *, YEUNGHO PARK (2), WANSOO HUH (3), and YOUNG GWAN KO (1)

(1.) School of Chemical Engineering Seoul National University Shilim-Dong, Kwanak-ku, Seoul, 151-742, South Korea

(2.) Department of Chemical Engineering Hanyang University 1271-I, Sa-dong, Ansan-si, Kyungki-do, 425-791, South Korea

(3.) Department of Chemical Engineering Soongsil University 1-1, Sangdo-dong, Dongiak-ku. Seoul, 156-743, South Korea

(1a.) Clean Technology Team Korea Institute of Industrial Technology 35-3, HongchonRi, IpjangMyum chonAn, 330-825, South-Korea

(a.) Corresponding author. E-mail: wyl@snu.ac.kr
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Author:Kim, Soo Jin; Lee, Wha Young; Park, Yeungho; Huh, Wansoo; Ko, Young Gwan
Publication:Polymer Engineering and Science
Geographic Code:9SOUT
Date:May 1, 2003
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