1 Introduction
Metallocene catalysts are single-site catalysts that form one active site per catalyst particle; these active sites have identical properties. However, due to instability of the catalyst during polymerization, de- crease in catalytic activity and fouling of the reactor may occur. To address this, metallocene catalysts were heterogenized through immobilization on supports such as silica, alumina, magnesium compounds, cyclodextrin, and polymers[1,2]. For olefin polymerization, MgCl2 and SiO2 are mainly used.
In the case of supported catalyst, the activity and mechanical properties of the supported catalyst are affected by the chemical composition and particle shape of the support[3,4]. Recrystallization of MgCl2 by dissolving it in alcoholic hydrocarbon improves the polymerization ac-tivity of the catalyst through changes in its chemical composition and crystal structure. On the other hand, SiO2 is advantageous in that it has a spherical shape with a large surface area[5]. To combine the advantages of these two supports, binary support was synthesized by recrystallizing MgCl2 into a hydrocarbon solution and then supporting it on SiO2. Sensarma et al. reported that when a Zr- or Ti-based metallocene catalyst supported on a binary support synthesized with MgCl2 and SiO2 dissolved in THF then subjected to ethylene polymerization, a higher activity and a narrower molecular weight distribution than the catalysts supported on MgCl2 or SiO2 was observed[6]. Patthamasang et al. synthesized a SiO2/MgCl2/TiCl4 catalyst by dissolving MgCl2 in various molar ratios of [EtOH]/[MgCl2]. The catalyst with [EtOH]/[MgCl2] molar ratio 7 has high activity, large surface area and spherical particles[5]. Cho et al. synthesized a hybrid catalyst with metallocene catalyst and a Ziegler-Natta catalyst supported on either MgCl2 or SiO2. This hybrid catalyst has two active sites and can produce polymer blends, and it is reported that MW and MWD can be controlled by changing the ratio of metallocene catalyst to Ziegler-Natta catalyst[7].
In this study, metallocene catalyst was supported on SiO2/MgCl2 binary support. SiO2/MgCl2 binary support was synthesized by dissolving in the molar ratio of [EtOH]/[MgCl2] then supporting on SiO2. After surface treatment with various alkyl aluminum compounds, metallocene catalyst was loaded on the synthesized binary support. The synthesized bi-supported metallocene catalyst were compared and analyzed according to the kind of alkyl aluminum compound used in the surface treatment and was used on ethylene polymerization and ethylene-1-hexene copolymerization.
2 Experimental Method
2.1 Reagents
All experiments were carried out in a nitrogen atmosphere using schlenk technique. Magnesium chloride (MgCl2, Aldrich) was used without any additional treatment. Silica (SP948) was purchased from Grace Davision Company and calcined at 500 ℃ for 10 hours. Heptane (Aldrich, 99%) and ethanol (Aldrich, 95%) used in MgCl2 recrystallization were used without further treatment. The metallocene catalyst bis-(n-butylcyclopentadienyl) zirconium dichloride ((n-BuCp)2ZrCl2, Aldrich) was used without further treatment. The structure of the metallocene catalyst is shown in Figure 1. Trimethylaluminum (TMA, Aldrich), triethylaluminum (TEAL, Aldrich), methylaluminoxane (MAO, Albemarle), ethylaluminum sesquichloride (EASC, Acros organics) were used without further processing. The structure and molecular weight of the alkyl aluminum compounds are shown in Table 1. Ethylene (SK energy, Korea, 99.999%) and nitrogen (Air Products, Korea, 99.999%) were passed through Fisher REDOX Oxygen Removal Column and 5A/13X molecular sieve to remove moisture and oxygen. Toluene (J.T Baker) and hexane (J.T Baker), which were used as a solvent, were prepared by refluxing over sodium metal and benzophenone in a nitrogen atmosphere.
2.2 Catalyst synthesis
2.2.1 SiO2/MgCl2 binary support synthesis
1 g of MgCl2 was suspended in 10 mL of heptane then 4.3 mL of ethanol was added. After stirring at 89 ℃ for 1 hour, 2 g of SiO2 suspended in 30 mL heptane at 89 ℃ was added. The reaction proceeded at 89 ℃ for 1 hour with vigorous stirring. After completion of the reaction, the reaction mixture was washed four times with heptane at 70 ℃ then vacuum-dried at 40 ℃ for 1 hour to synthesize a SiO2/MgCl2 binary support.
2.2.2 Surface treatment of binary support using alkyl aluminum compound
1 g of SiO2/MgCl2 binary support was suspended in 30 mL of toluene, maintained at 0 ℃ with water and ice, and then an appropriate amount of cocatalyst alkyl aluminum compound was injected. The reaction proceeded with vigorous stirring at 0 ℃ for 2 hours. After completion of the reaction, the catalyst was washed with toluene five times then vacuum dried at 40 ℃ for 1 hour to synthesize a SiO2/MgCl2/ alkyl aluminum support.
2.2.3 Supporting the catalyst
1 g of SiO2/MgCl2/alkyl aluminum support was suspended in 30 mL of toluene then a solution of 0.35 mmol of (n-BuCp)2ZrCl2 catalyst and 8 mmol of MAO was injected. The reaction proceeded at 70 ℃ for 3 hours with vigorous stirring. After completion of the reaction, the catalyst was washed five times with toluene and vacuum-dried at 40 ℃ for 1 hour to synthesize SiO2/MgCl2/alkyl aluminum/(n-BuCp)2ZrCl2 catalyst.
2.3 Polymerization
The ethylene polymerization was carried out in a 500 mL high pressure steel reactor with a magnetic stirrer for stirring. 280 mL hexane and 2 mmol cocatalyst TEAL were injected into the reactor then temperature was increased to 70 ℃. A certain amount of catalyst was added in a slurry state with 5 mL of hexane. The polymerization was initiated with agitation after the reactor was saturated with monomers and a pressure of 7 bar was achieved. This pressure was maintained during the reaction. After polymerization, the resulting polymer was vacuum filtered, washed with ethanol, and dried. The concentration of ethylene and 1-hexene in the liquid phase of hexane was calculated by PRO/II.
2.4 Characterization
The surface area of the supports and catalysts used in the experiments was measured by Micromeritics ASAP 2010 instrument. The sample was dried before measurement and then outgassed under vacuum at 150 ℃ for 12 hours. The surface area of the support was calculated by the BET (Brunauer-Emmett-Teller) equation. The binding energies of Mg and Zr supported on the support were measured by X-ray photoelectron spectroscopy (XPS). The Mg, Al, and Zr contents in the supported catalysts were measured by inductively coupled plasma atomic emission spectroscopy (Perkin-Elmer, Optima 200DV) respectively. The melting point of the resulting polymer was measured by differential scanning calorimetry (DSC, TA 2010) at a rate of 10 ℃/min at 25 ℃ to 180 ℃ to obtain DSC curves. The comonomer content of the copolymer was measured by 13C-NMR (Bruker AMX-300 FT NMR spectrometer). The molecular weight and molecular weight distribution of the resulting polymer were determined by GPC (Waters Associates Chromatograph, Model ALC-GPC-150C) analysis. The particle shape of the support, catalyst, and resulting polymer was observed through a Field Emission Scanning Electron Microscope (FE-SEM, TESCAN, MIRA LMH).
3 Results and Discussion
3.1 Characterization of support and supported catalyst
In this study, MgCl2 suspended in heptane was dissolved in ethanol and supported on SiO2 to synthesize SiO2/MgCl2 binary support. Surface treatment of the support with various alkyl aluminum compounds was carried out to investigate the influence of the type of alkyl aluminum compound to catalytic performance and copolymer properties. The molar ratio of [Al]/[EtOH] was fixed to 2. The TGA results of the synthesized binary supports and catalyst are shown in Figure 2. SiO2 showed mass reduction before 125 ℃ due to the removal of CO2 and H2O. On the other hand, 35.7% mass reduction for SiO2/MgCl2 binary support was observed. This is due to the removal of residual ethanol present in the binary support. When MgCl2 is recrystallized with alcohol, the alcohol remaining in the support poisons the metallocene catalyst. Therefore, to prevent deactivation of the catalyst, it is necessary to remove the remaining alcohol in the support by introducing an appropriate alkyl aluminum compound[8,9].
Figure 3 shows the results of the XPS Mg 2p analysis of the SiO2/MgCl2 binary support and the supported catalyst. The binding energy of Mg 2p of MgCl2 is known to be around 52.3 eV[10,11]. Mg 2p binding energies of SiO2/MgCl2 binary support and supported catalysts ranged from 51.3 eV to 52.5 eV. The Zr 3d spectrum of (n-BuCp)2ZrCl2, a metallocene catalyst, is shown in Figure 4. The Zr binding energy peaks of (n-BuCp)2ZrCl2 were found to be 184.7 eV (Zr 3d5/2) and 182.3 eV (Zr 3d3/2). Zr binding energy of the metallocene catalyst supported on SiO2/MgCl2/alkyl aluminum was measured in the range of 183.0~185.9 eV (Zr 3d5/2) and 185.3~185.9 eV (Zr 3d3/2). The Zr binding energy of the metallocene catalyst supported on SiO2/MgCl2/alkyl aluminum is slightly lower than the Zr binding energy of (n-BuCp)2ZrCl2, which is the metallocene catalyst. This is due to the change in electronic environment.
Table 2 shows the results of BET and ICP analysis of SiO2/MgCl2 binary support and supported catalyst. The SiO2/MgCl2 binary support has less surface area, and pore volume and size compared to the conventional SiO2 due to the presence of recrystallized MgCl2 on it. It could also be seen that the surface area of the SiO2/MgCl2 binary support is reduced upon surface treatment with the alkyl aluminum com- pound and metallocene catalyst loading. Furthermore, ICP analysis shows that the Zr content of catalysts supported on SiO2/MgCl2/alkyl aluminum is relatively lower compared to the catalyst supported on SiO2. This could be attributed to the reduction in surface area of SiO2 due to the presence of recrystallized MgCl2 and alkyl aluminum.
SiO2/MgCl2/TMA/(n-BuCp)2ZrCl2, which has larger surface area compared to most catalyst on SiO2/MgCl2/alkyl aluminum support, has more Al content and Zr content. Likewise, SiO2/MgCl2/TEAL/ (n-BuCp)2 catalyst, which has the smallest surface area, has least Zr content. As shown in Table 2, SiO2/MgCl2/EASC/(n-BuCp)2 catalyst has the highest Al content. This is because Al of EASC is strongly fixed to Cl and Mg-O-(Et).
Figure 5 shows the particle formation of SiO2/MgCl2 binary support and catalysts. It could be seen that the catalyst has rough surface but retained the spherical morphology of SiO2. In the case of SiO2/MgCl2 binary support, recrystallized MgCl2 accumulates on the surface of SiO2. Through surface treatment with alkyl aluminum, agglomerated MgCl2 on SiO2 surface was removed. Figure 6 shows the distribution of Si, Mg, Al, and Zr atoms by SEM-EDX analysis of the supported catalysts. Al and Zr atoms are uniformly distributed on the Mg and Si surfaces. The process of supporting the metallocene catalyst on the SiO2/MgCl2 binary support is shown in Figure 7.
3.2 Copolymerization Activity
The activity of the catalyst supported on the SiO2/MgCl2 binary support can be determined by the following equation: SiO2/MgCl2/TMA/ (n-BuCp)2ZrCl2 > SiO2/MgCl2/TEAL/(n-BuCp)2ZrCl2 > SiO2/MgCl2/EASC/ (n-BuCp)2ZrCl2. Alkyl aluminum compound used in the surface treatment of SiO2/MgCl2 binary support causes a steric hindrance to the diffusion of ethylene monomer as the size of the alkyl ligand increases [12]. The catalysts supported on SiO2/MgCl2/EASC has the lowest Zr content while having the highest activity of 1.9 kg-PE/(mmol-Zr*h) as seen in Table 3. Additionally, its copolymerization activity slightly increases with increasing C6/C2 ratio while the rest shows a decrease in activity with increasing C6/C2 ratio as shown in Figure 8. It is believed that, unlike other alkyl aluminum compounds, EASC acts as a strong Lewis acid which improves the polymerization activity.
3.3 Copolymer Characterization
Among the SiO2/MgCl2/alkyl aluminum supported catalysts, catalyst supported on SiO2/MgCl2/EASC produced the copolymer with the highest 1-hexene content as seen in Table 3. On the other hand, the melting point of the copolymers produced using the SiO2/MgCl2/alkyl aluminum supported catalysts showed a decrease as C6/C2 molar ratio increases as seen in Table 3. This means that concentration of 1-hexene inserted into the polyethylene chain changes as the C6/C2 molar ratio changes. The molecular weight and distribution curve, and polydispersity index (PDI) of the resulting polymer are shown in Figure 9 and Table 3. The molecular weight of the resulting polymer increased as the alkyl ligand size of the alkyl aluminum compound used in the surface treatment of the SiO2/MgCl2 binary support increased. This is because the larger the volume of the alkyl ligand in the aluminum compound, steric hindrance decreases the termination reaction of the ethylene chain[12]. The PDI values measured by GPC analysis show that (n-BuCp)2ZrCl2 catalyst adsorbed on different SiO2/MgCl2/alkyl aluminum surface produced various types of active sites. The PDI values of the resulting SiO2/MgCl2/alkyl aluminum/(n-BuCp)2ZrCl2 decreased as the alkyl group size of the alkyl aluminum compound increased. As there were various types of active sites and agglomeration of MgCl2 on SiO2 surface, due to replication phenomena, the copolymers produced using the different synthesized surface-treated supported catalysts have different surface morphologies as seen in Figure 10. It could be observed that the larger the ligand of alkyl aluminum used, the rougher the particle surface of the resulting polymer.
4 Conclusion
SiO2/MgCl2 binary support was synthesized to combine the advantages of SiO2 and MgCl2. This binary support was surface treated with various alkyl aluminum then was loaded with metallocene catalyst, (n-BuCp)2ZrCl2. The supported catalysts were used in ethylene/1-octene copolymerization. Through this study, it was found that the alkyl aluminum compound used for surface treatment affects the catalytic performance and copolymer properties. In comparison with SiO2 supported catalyst, better catalytic activity was observed depending on the alkyl aluminum compound used for the surface treated SiO2/MgCl2 binary support.
