Immobilizing metallocene by solid polymethylaluminoxane “sMAO” for slurry-phase ethylene-1-hexene copolymerization

. In the field of olefin catalytic polymerization, metallocene catalysts show higher activity than traditional Ziegler-Natta catalysts. In this study, benzoic acid catalyzed the controlled hydrolysis of trimethylaluminum (TMA) to methylaluminoxane (MAO). After pyrolysis, an insoluble form of solid polymethylaluminoxane (sMAO) was synthesized. The influence of synthesis conditions on the particle size and morphy was systematically explored. The synthetic sMAO is used as a carrier to support the metallocene catalyst, and the catalyst is applied to the ethylene/1-hexene copolymerization. The experimental results show that the activity can be as high as 12149.8 gPEꞏ(gcat)-1 when the slurry reaction is catalyzed in a 5L kettle.


Introduction
Polyolefins account for nearly half of the world plastic production. [1] Thanks to their low cost and versatility, they are used in packaging, automotive, construction, pharmaceutical/medical, and electronics/electrical applications. Low pressure olefin polymerization processes that use supported coordination catalysts (Ziegler-Natta, Phillips, and metallocene) are the main route for making polyolefins. The discovery of metallocene-based catalysts for the polymerization of a-olefins has opened the possibility of synthesizing new materials. The single site metallocene catalysts offer several advantages with respect to the traditional Ziegler-Natta catalysts, as narrow molar mass distribution, control of comonomer incorporation, high activity, and low residual metal content. [2] Methylaluminoxane (MAO) is the most commonly used activator and co-catalyst for transition metal containing, single-site complexes in olefin polymerisation. [3] The combination of MAO, an inert inorganic carrier material (most commonly silica) and a precatalyst complex have been described by Severn as the ''Holy Trinity'' of supported single-site catalysts. [4] However, there is also growing interest in single-site catalysts in particle forming polymerisation processes that are free from an inert inorganic carrier. These require an activating support material, which can be combined with the precatalyst complex in a single synthetic step. This process has several advantages from an industrial viewpoint, as it reduces the time and energy-intensive drying steps and hence can lower manufacturing costs. Furthermore, these 'self-supported' catalyst systems have an advantage over silica-supported systems as the complex loading can be increased significantly, which leads to correspondingly higher polymerisation activities. [5][6] In 2013, Tosoh Finechem Corporation reported, in the patent literature, an insoluble form of solid methylaluminoxane (sMAO) formed via controlled hydrolysis of trimethylaluminium with benzoic acid. [7] We have recently reported the laboratory scale synthesis and detailed characterisation of sMAO, and demonstrated its function as a solid-phase support, scavenger and activator in slurry-phase ethylene polymerisation. [8][9][10]

Materials
All manipulations were carried out using standard Schlenk techniques under N 2 , orin an glovebox under N 2 . All glassware was dried at 160 °C overnight prior to use. Hexane and toluene were dried and degassed using an MBraun SPS-800 solvent purification system. Dried solvents were collected, degassed and stored over N 2 in K mirrored ampoules.Trimethylaluminum, Benzoic acid were purchased from Sigma-Aldrich.

Preparation of sMAO:
In a typical procedure an ampoule containing a 30 mL solution of TMA (2mol/L) in toluene was cooled to 15 °C with rapid stirring, and benzoic acid (3.019 g, 2.478 mmol) was added under a flush of N2 over a period of 30 minutes. Effervescence (presumably methane gas) was observed and the reaction mixture appeared as a white suspension, which was allowed to warm to room temperature. After 30 minutes the mixture appeared as a colorless solution and was heated at 70 °C for 32 h (a stir rate of 500 rpm was used throughout these experiments). The mixture obtained was a colorless solution free of gelatinous material. Above mixtrue was concentrated to 16mL by vocumm and subsequently heated at 100 °C for 14 h. The reaction mixture was cooled to room temperature and hexane (150 mL) added, resulting in the precipitation of a white solid which was isolated by filtration, washed with hexane (2 x 100 mL) and dried in vacuo for 3 h. Total yield = 3.478 g (86% based on 40.3 wt% Al).

Immobilized catalysts:
To a Schlenk flask charged with sMAO (300 mg, 4.58 mmolAl) and rac-(EBI)ZrCl2 (9.6 mg, 0.0229 mmol) was added toluene (40 mL), and the resulting orange dispersion was heated at 80 °C for 2 h with regular swirling. The mixture was allowed to cool to room temperature and a pink-orange solid settled below a colorless supernatant solution. The supernatant was removed by decantation and the remaining slurry was dried in vacuo overnight, to afford a free-flowing orange solid. Total yield = 249 mg (86% based on 41.3 wt% Al).

Polymerization
The immobilized catalyst (nearly 40.0 mg), triisobutylaluminum scavenger (150 mg), and hexane (2 L) were added to a high-pressure reactor. Ethylene gas was continuously fed into the ampoule at 1.0MPa overpressure at 70 °C. After 30 minutes, the reaction was stopped by removing the ampoule from the oil bath, and degassing in vacuum. The polymer was isolated on a frit and vacuum dried at room temperature for 3 hours. Each polymerization experiment was conducted at least twice to ensure the reproducibility of the corresponding outcome, and mean activities are quoted in units of gPE/gcat.

Characterization
Diameter of particles were measured by Mastersizer 2000 Laser Diffraction Particle Size Analyzer (Malvern Instruments, Britain). The microspheres size distribution was characterized as SPAN value, defined as follows (Eq. (1)): where D90, D50 and D10 are the particle sizes at which 90%, 50% and 10% of the distribution fall below.
Scanning electron microscopy (SEM) measurements were conducted using Philips XL30 ESEM and Hitachi S-4800 instrument at an operation voltage of 20.0 keV and 0.7 keV. Elemental analysis was performed by energy dispersive spectroscopy (EDS) attached to the SEM. Thermograms were obtained using a PerkinElmer Diamond differential scanning calorimeter (DSC) system. PE productions were heated from 25 to 250 °C at a heating rate of 10 °C /min under constant purging of N 2 at 20 mL/min. Changing the volume ratio of toluene and hexane may influence the morphology and diameter of the sMAO supports (Figure 2). At lower toluene/hexane molar ratios, the sMAO particles exhibited nonspherical morphology (Figure 1a), while individually adding hexane led to an increase in the dispersity of the catalyst particles ( Figure  1b)，because the solubility of MAO in hexane is lower than it in toluene. Although the mean particle diameter was obtained in the 0.5-1 μm range with a narrow distribution as well as spherical shape, the aggregation was obvious. To eliminate the aggregation, the toluene /hexane volume ratio was increased to 1:4. The diameter of sMAO ranged from 3-5 μm, and distributed homogenously (Figure 1c). Upon increasing the volume ratio to 1:6, a spherical morphology with an approximately 1-2 μm particle diameter was observed (Figure 1d).  Figure 2 shows the effect of the hexane-addting temperature on the diameter and morphology of the sMAO. The particles diameter exhibits a relation with the hexane-addting temperature and the D50 diameter of the catalyst increases from 27.8 μm to 6.75 μm as the temperature was changed from 50 °C to 20 °C. The surface morphology of the sMAO particles was also related to the temperature, with the particles obtained at higher temperatures exhibiting a broader diameter distribution. The small particles display regular spherical morphology while the larger particles collapsed ( Figure  2b) or even aggregated (Figure 2a). The lower temperature (20 °C) is the optimum temperature that helps to maintain the smooth surface and uniform diameter of the particles (Figure 2c).

Figure 3
Energy dispersive X-ray spectrometry results for the catalyst Figure 4 Particle size distribution of the catalyst EDS measurements of the catalyst show that the C, O， Al，Cl and Zr elements were distributed homogeneously in the catalyst system ( Figure 3). Figure9 shows the size distribution of the catalyst particles. The results show that the catalyst particle distribution is concentrated at approximately 7µm with a narrow particle size distribution and with less fine content ( Figure 4). Since the polymer particle morphology is mainly determined by the morphology of the catalysts through the replication phenomenon, the low fine content of the catalysts results in a low content of polyethylene fine powder, avoiding equipment blockage. Conditions:, ethylene(1MPa), 2h, cocatalyst((i-Bu) 3 Al),Al/Zr (3500), solvent(2L), 80℃.
Obviously enhancement in activity was observed for ethylene/1-hexene copolymerisation compared to ethylene homopolymerisation (6120.9 gPE/gcat). When the ratio of 1-hexene increased from 0%-2.0%, A large increase in ethylene copolymerization was presented (from 6120.9 to 12149.8 gPE/gcat).Many theories have been proposed for the positive comonomer effect, including fracturing of catalyst particles exposing new sites, the formation of new active species by coordination of a-oleffins, and activation of dormant active sites as well as improved diffusion of ethylene close to the catalytic site, which improves polymerisation activity. However, further increase the amount of 1-hexene (from 2.0%-4.0%) lead to a decrease of activity, indicating that the negative comonomer effects outweigh the positive effects. The negative effects of comonomer addition are proposed to be due to competitive binding between ethylene and aoleffins and, if the rate of migratory insertion of the aoleffin is slower than that of ethylene, the rate of chain propagation will decrease leading to a decrease in polymerization activity. The negative effects of comonomers on ethylene polymerisation activity may also be due to slower rates of insertion; the increased steric bulk of a-oleffin comonomers in the polymer chain can lead to reduced rates of ethylene insertion.

Conclusion
In this report we have developed an optimized laboratoryscale synthesis for solid polymethylaluminoxane (sMAO). This amorphous solid has been extensively characterized using SEM imaging and BET physisorption provide insight into the surface structure and porosity of sMAO. sMAO is shown to be a very promising bifunctional support and activator for single site metallocene-based pre-catalysts in slurry-phase ethylene polymerization reactions.