ABSTRACT
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 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 degrees 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 Cr(n+) 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 1.38g per 10 min; this indicates the very high molecular weight of the polymer.
ABSTRACT
Aqueoussuspensions of different Na-smectite type clay minerals were exchanged with methylene blue (Mb) and analyzed by visible spectroscopy. The spectra show bands of two types of monomers, protonated Mb, Mb-dimers, and higher aggregates. Their relative importance and the bandwidth was found to depend on parameters such as the particle morphology, the degree of dispersion, and the extent and location of the layer charge of the smectite. This can be qualitatively explained by the relative importance of three types of interactions, Mb-surface, H(2)O-surface, and Mb-Mb interactions. For hectorite and laponite at small loadings, H(2)O-surface interactions are dominant. Mb-islands are formed with a characteristic monomer absorption at 653 nm. Monomers at the surface absorb at 670 nm as found in barasym and saponite. Wyoming bentonite takes an intermediate position. As the loading increases Mb-Mb and Mb-surface interactions become dominant, giving rise to monomers absorbing at 670 nm, dimers, and higher aggregates. The bandwidths of the absorption bands reflect the structure of the clay particle associates. Copyright 1999 Academic Press.
