Synthesis and Characterization of Solid Polymethylaluminoxane: A Bifunctional Activator and Support for Slurry-Phase Ethylene Polymerization

摘要

An insoluble form of methylaluminoxane, also known as solid polymethylaluminoxane (sMAO), has been synthesized by the controlled hydrolysis of trimethylaluminum (TMA) with benzoic acid, followed by thermolysis. Characterization of sMAO by multinuclear NMR spectroscopy in solution and the solid state reveals an aluminoxane structure that features "free" and bound TMA and incorporation of a benzoate residue. Total X-ray scattering (or pair distribution function, PDF) measurements on sMAO allow comparisons to be made with simulated data for density functional theory (DFT) modeled structures of methylaluminoxane (MAO). Several TMA-bound (AlOMe) cage and nanotubular structures with n > 10 are consistent with the experimental data. The measured Brunauer-Emmett-Teller (BET) surface area of sMAO ranges between 312 and 606 m(2) g(-1) and shows an N-2, adsorption/desorption isotherm consistent with a nonporous material. sMAO can be utilized to support metallocene precatalysts in slurry phase ethylene polymerization reactions. Metallocene precatalyst rac-ethylenebis(1-indenyl)-dichlorozirconium, rac-(EBI)ZrCl2, was immobilized on sMAO samples, to afford solids which showed very high polymerization activities in hexane, comparable to those of the respective homogeneous catalysts formed by treatment of the precatalysts with MAO. rac-(EBI)ZrCl2 immobilized on an sMAO containing an Al:O ratio of 1.2 gave the highest ethylene polymerization activity.