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

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 pseudooctahedral coordination. This species is characterized by high zero-field ESR parameters D and E, indicating a strong distortion from O_h 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 14000 g polyethylene per gram of Cr per hour. Combined DRS-ESR spectroscopies were used to monitor the reduction process of Cr^6+/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.