Basic Chemistry of Phillips Catalysts

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 chemistry of Phillips catalysts is intimately linked to interactions between inorganic chromium compounds and silica. Basic steps are summarized below  

Chromium is present chiefly as the monochromate however, some is chemisorbed as the dichromate. Though it is known that monochromate species are precursors to active centers for polymerization, it is possible that dichromate may also form active sites (7). However, monochromate is widely believed to be the dominant precursor and will be used in this discussion. 

Phillips catalysts are produced by reaction of a chromium compound (usually CrO ) with dehydrated silica  

Productivity of the Phillips catalysts is typically 3 kg/g of catalyst (8). At the typical chromium loading of 1 %, residual chromium in the polymer is less than 5 ppm, and post-reactor treatment to remove catalyst residues is not necessary. 

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