Study On Silica And Silica-supported Metallocene Catalyst For Ethylene Polymerization

Process for preparing silica and titanium-containing silica was studied and the support silicas suitable for olefin polymerization catalyst were obtained. The metallocene catalyst was supported on as-prepared silicas and the influencing factor was investigated. Both properties of resultant supported catalyst and polymers could meet the need of industrial technologies.Silicas with desired physico-chemical properties for ethylene polymerization catalysts were prepared via sol-gel process, all the operating conditions in the process were selected and optimized. The results showed that temperature for nucleation of mother liquor was 70～80℃,pH value was 9.0～9.5, time was 40～50min, SiO₂ mass concentration in mother liquor was 3%～5%;In the process of sol particlegrowth, the mole ratio of SiO₂ added subsequently to SiO₂ remained in mother liquor was controlled at 7～10, pH at 9.0～9.5, feed flow of water glass at 150～250ml/h(d water glass was 1.11g/cm³), temperature at 80～90℃; hydrosol gelfication was controlled at pH 6.0～8.0,temperature 60～70℃; aging time for hydrogel stabilization was 5～6h, pH value was 9.0; the silica was dried by spray drying, the homogenization conditions for aqueous silica filter cake before drying were: the stirring speed 600r/min, time 40～50 min, solid mass fraction in slurry 14%～16%; the rotating speed of spray dryer was kept at 12000r/min, the temperature at inlet and outlet at 300～320℃and 90～130℃, respectively.Titanium-containing silica with Ti mass fraction of 0.1%～0.3% was prepared via sol-gel process and the hydrolysis rates of Ti and Si sources were controlled .The results indicated that either inorganic Ti(SO₄)₂ or organic Ti(OC₄H₉)₄ was suitable for producing qualified titanium-containing silica. In the reaction system, predetermined amount of H₂O₂ was added to complex quickly with a great deal of Ti(OH)₄ formed during Ti source hydrolysis to establish a dynamic equilibrium with H₂O₂ ,thus ensuring the match between hydrolysis rates of Ti and Si sources and perfect incorporation of Ti from raw material into framework. A mole ratio of H₂O₂/Ti above 5 was sufficient to realize this object. Low Ti contents in raw material (should below mass ratio of 3%) and reaction temperature between 30～40℃were better for incorporation of Ti into the framework.The metallocene catalyst was supported on as-prepared silicas. The results indicated that in the supporting process, the addition of methylaluminoxane (MAO) in two-step was better than in one-step. The catalyst should be filtered and washed before drying. Optimized parameters were MAO/silica(ml/g) of 10:1, Al loading mass fraction of 15%～17%, Zr loading mass fraction of 0.13%～0.45%. For ethylene polymerization with the as-supported metallocene catalyst, the polymerization activities were greater than 6200gPE/g.cat, polyethylene (PE) particle size distribution , molecular weight and its distribution ,and melting temperature all met the requirements from existing industrial technology. The as-prepared silica was successfully applied to Unipol gas PE process as well. In addition, titanium-containing silica used as support for metallocene catalysts led to partial breaking and exhibited lower ethylene polymerization activities and more inferior PE morphology compared with that supported by self-made titanium-free silica. The problems remaining need to be solved.