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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, N 2 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, Cr 3‡ 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, Cr 3‡ is almost completely oxidized to Cr 6‡ , which is anchored onto the surface as dichromate, some chro-mate and traces of small amorphous Cr 2 O 3 clusters and square pyramidal Cr 5‡ ions. These materials are active in the gas-phase and slurry-phase polymer-ization of ethylene at 100 8C. The poly-merization activity is dependent on the Cr loading, precalcination temperature and the support characteristics; a 1 wt % [Cr(acac) 3 ]-AlMCM-41 catalyst pre-treated at high temperatures was found to be the most active material with a polymerization rate of 14 000 g poly-ethylene 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 nano-fibres 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 .

A series of chemically dealuminated mordenites were investigated by 29Si and 27Al NMR spectroscopy. High-resolution magic-angle-spinning (HRMAS) solid state 29Si NMR lines with varying intensity were observed at δ=−95, −105 and −110 ppm. When it is assumed that the Al atoms are located exclusively in the 4-membered rings of the structure, these variations can be explained by assigning the resonance lines to silicon atoms surrounded by 2, 1 and 0 Al atoms respectively. The 27Al NMR spectrum yields quantitative information on the amount of octahedrally coordinated aluminium formed during various chemical treatments of Na-mordenite.

High resolution solid state MAS 11B-NMR was used to detect quantitatively variously coordinated BOx structural units in zeolites. Boron stemming from Pyrex autoclaves is progressively incorporated into the tetrahedral sites of ZSM-5 and mordenite frameworks during synthesis. The final crystalline zeolite phases may contain up to 0.8 B atoms in their unit cells.