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 th e surface of two mesoporous crystalline materials; pure silica MCM-41 (SiMC M-41) and Al-containing silica MCM-41 with an Si:AI ratio of 27 (AlMCM-41). The materials were characterized with X-ray diffraction, N-2 adsorption, t hermogravimetrical analysis. diffuse reflectance spectroscopy in the UV-Vis -NIR region (DRS), electron spin resonance (ESR) and Fourier transform infr ared spectroscopy. Hydrogen bonding between surface hydroxyls and the acety lacetonate (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 p resent as a surface species in pseudo-octahedral coordination. This species is characterized by high zero-field ESR parameters D and E, indicating a s trong distortion from O-h symmetry. After calcination, Cr3+ is almost compl etely oxidized to Cr6+, which is anchored onto the surface as dichromate, s ome chromate and traces of small amorphous Cr2O3 clusters and square pyrami dal Cr5+ ions. These materials are active in the gas-phase and slurry-phase polymerization of ethylene at 100 degrees C. The polymerization activity i s dependent on the Cr loading, precalcination temperature and the support c haracteristics: a 1 wt% [Cr(acac)(3)]-AlMCM-41 catalyst pretreated at high temperatures was found to be the most active material with a of 14000 g pol yethylene per gram of Cr per hour. Combined DRS-ESR spectroscopies the redu ction process or Cr6+/5+ and the oxidation and coordination environment of Crn+ species during catalytic action. Tt will be shown that the polymer cha ins 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 surfa ce of the MCM-41 material. The catalyst particles also gradually break up d uring ethylene polymerization resulting in the formation of crystalline and amorphous polyethylene with a low bulk density and a melt flow index betwe en 0.56 and 1.38 g per 10 min; this indicates the very high molecular weigh t of the polymer.