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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.

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On the fine scale revealed by TEM, the material is characterized by predominant dendritic 3C crystals. All these dendritic crystals have equivalent orientations and are not polycrystals like those found in dendritic growth of metals. The branching-off characteristic of 3C dendritic growth is thus due to symmetry of its having three equivalent close-packed directions. Since both trunks and branches have exactly the same orientation and since the direction of the spines is parallel to the < 1 1 1 > directions, S i c crystals seem to grow in one of the < 1 1 1 > directions under the present conditions. Some X crystals were also found to grow in a columnar fashion in one of the < I 11> directions, much as in S i c whiskers. These columnar crystals exhibit twin bands perpendicular to the growth direction.Both the 2H and the one-dimensionally-disordered crystals are uniaxial and grow in acolumnarfashion in thec direction. SinceSiC does not usually form regular crystalline boundaries between crystals of different orientation, as observed in common metals, presumably because of the high energy involved in dislocation formation, these crystals do not form dendritic structures with branching-off as do the 3C crystals. However, several onedimensionally-disordered crystals were found to grow together with a columnar habit, making small angles with one another. Precise details of the structure of such small-angle grain boundaries are not known .The one-dimensionally-disordered crystals are characterized by the fact that the stacking order of the crystal is almost completely random, whereas the structure is perfect in the other two dimensions. The existence of the one-dimensionally-disordered structure reflects the general characteristic of polytypism of S i c , i.e. that the energy of the structure does not depend on its stacking order and that any polytype is equally pr~bable.~.'" The situation is also reflected by the extremely low stacking-fault energy.6 The stacking order of S i c crystals thus seems to be affected by local growth conditions,'" as demonstrated by the distribution of disordered crystals in the present case. One-dimensionally-disordered crystals, however, should not be considered simply as heavily faulted 3C crystals. ,A study of such one-dimensionally-disordered crystals by highresolution electron microscopyz1 revealed that the structure con-sisted of random layers of thin a-and P-phase lamellas of various thicknesses.2t Acknowledgments: The writers are indebted for valuable discussions to J. E.

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