Chromium/silica catalyst support properties

When controlled nitridation of surface layers is required, as for example in the modification of the chemical properties of the surface of a support, the atomic layer deposition (ALD) technique can be applied." This technique is based upon repeated separate saturating reactions of at least two different reactants with the surface, which leads to the controlled build-up of thin films via reaction of the second component with the chemisorbed residues of the first reactant. Aluminium nitride surfaces have been prepared on both alumina and silica supports by this method wherein reaction cycles of trimethylaluminium and ammonia have been performed with the respective supports, retaining their high surface areas." This method has been applied to the modification of the support composition for chromium catalysts supported on alumina." ... 

An investigation of chromia supported on alumina or silica as catalyst for partial oxidation of paraxylene [144] revealed influence of the oxidation states and degree of oligomerization of ehannium on the catalytic activity. Superior catalytic properties were obtained when chromium was supported on alumina. 

Several green and sustainable chemistry concepts are supported by the use of this reaction. The first is the use a chromium complex immobilised on a silica support to give a heterogeneous catalyst for easier recyclability and separation from the product mixture. Moreover, the use of ionic liquids is favourable for catalyst recyclability. The chemical nature of the catalyst possessing ionic or polar properties can fix the catalyst in the ionic liquids... 

Phillips Chromox Catalyst. Impregnation of chromium oxide into porous, amorphous silica-alumina followed by calcination in dry air at 400-800°C produces a precatalyst that presumably is reduced by ethylene during an induction period to form an active polymerization catalyst (47). Other supports such as silica, alumina, and titanium-modified silicas can be used and together with physical factors such as calcination temperature will control polymer properties such as molecular weight. The precatalyst can be reduced by CO to an active state. The percent of metal sites active for polymerization, their oxidation state, and their structure are the subject of debate. These so-called chromox catalysts are highly active and have been licensed extensively by Phillips for use in a slurry loop process (Fig. 14). While most commonly used to make HDPE, they can incorporate a-olefins to make LLDPE. The molecular weight distributions of the polymers are very broad with PDI > 10. The catalysts are very sensitive to air, moisture, and polar impurities. 

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