Chromium/silica catalyst composition

Common catalyst compositions include oxides of chromium or molybdenum, or cobalt and nickel metals, supported on silica, alumina, titania, zirconia, or activated carbon. 

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." ... 

A refractory oxide (usually silica) is essential to the performance of Phillips catalysts. Indeed, chromium compounds used in the production of Phillips catalysts are not stable at elevated temperatures and would decompose rapidly under the conditions typically employed for activation of Phillips catalysts (>500 °C). For example, CrOj decomposes to Cr Oj and above 200 °C. However, when affixed ("chemisorbed") onto silica surfaces, supported Cr compositions are stable to temperatures at least up to 1000 °C (7). 

The data of Table 55 show how the polymer composition varied with activation temperature. Such observations have been reported from this and other laboratories for catalysts made with several different organo-chromium compounds [301,640,644,654], and most recently by Bade et al. [311], who used chromium allyl to make their catalyst. Presumably, the calcination temperature of the silica resulted in the formation of two very different species. Cr(DMPD)2 reacted with silica treated at 250 and at 400 °C to yield di-attached or coordinated species, whereas it reacted differently with silica treated at 600 °C, because on that support only a single oxide attachment can form. Clearly, the higher OH group population has a major effect on the behavior of the site. 

The widely investigated Phillips catalyst, which is alkyl free, can be prepared by impregnating a silica-alumina (87 13 composition [101-103] or a silica support with an aqueous solution of Cr03). High surface supports with about 400 to 600 g/m are used [104]. After the water is removed, the powdery catalyst is fluidized and activated by a stream of dry air at temperatures of 400 to 800 °C to remove the bound water. The impregnated catalysts contain 1 to 5wt% chromium oxides. When this catalyst is heated in the presence of carbon monoxide, a more active catalyst is obtained [105]. The Phillips catalyst specifically catalyzes the polymerization of ethene to high-density polyethene. To obtain poly ethene of lower crystallinity, copolymers with known amounts of an a-olefin, usually several percent of 1-butene ean be synthesized. The polymerization can be carried out by a solution, slurry, or gas-phase (vapor phase) process. 

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