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Synthesis, spectroscopy and catalysis of [Cr(acac)3] complexes grafted onto MCM-41 materials: formation of polyethylene nanofibres within mesoporous crystalline aluminosilicates

By B.M. Weckhuysen, R. Ramachandra Rao, P. Bodart, G. Debras, O. Collart, P. van der Voort, R.A. Schoonheydt and E.F. Vansant


Chromium acetyl acetonate [Cr(acac)3] complexes have been grafted onto the surface of two mesoporous crystalline materials; pure silica MCM-41 (SiMCM-41) and Al-containing silica MCM-41 with an Si:Al ratio of 27 (AlMCM-41). The materials were characterized with X-ray diffraction, N2 adsorption, thermogravimetrical analysis, diffuse reflectance spectroscopy in the UV-Vis-NIR region (DRS), electron spin resonance (ESR) and Fourier transform infrared spectroscopy. Hydrogen bonding between surface hydroxyls and the acetylacetonate (acac) ligands is the only type of interaction between [Cr(acac)3] complexes and SiMCM-41, while the deposition of [Cr(acac)3] onto the surface of AlMCM-41 takes place through either a ligand exchange reaction or a hydrogen-bonding mechanism. In the as-synthesized materials, Cr3+ is present as a surface species in pseudo-octahedral coordination. This species is characterized by high zero-field ESR parameters D and E, indicating a strong distortion from Oh symmetry. After calcination, Cr3+ is almost completely oxidized to Cr6+, which is anchored onto the surface as dichromate, some chromate and traces of small amorphous Cr2O3 clusters and square pyramidal Cr5+ ions. These materials are active in the gas-phase and slurry-phase polymerization of ethylene at 100 °C. The polymerization activity is dependent on the Cr loading, precalcination temperature and the support characteristics; a 1 wt % [Cr(acac)3]-AlMCM-41 catalyst pretreated at high temperatures was found to be the most active material with a polymerization rate of 14 000 g polyethylene per gram of Cr per hour. Combined DRS-ESR spectroscopies were used to monitor the reduction process of Cr6+/5+ and the oxidation and coordination environment of Crn+ species during catalytic action. It will be shown that the polymer chains initially produced within the mesopores of the Cr-MCM-41 material form nanofibres of polyethylene with a length of several microns and a diameter of 50 to 100 nanometers. These nanofibres (partially) cover the outer surface of the MCM-41 material. The catalyst particles also gradually break up during ethylene polymerization resulting in the formation of crystalline and amorphous polyethylene with a low bulk density and a melt flow index between 0.56 and 1.38 g per 10 min; this indicates the very high molecular weight of the polymer

Topics: Scheikunde, aluminosilicates, catalysis, chromium, mesoporosity, polyethylene
Year: 2000
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