The activation of Phillips Cr/silica catalysts: V. Stability of Cr(VI)

This report attempts to define the underlying principles governing activation of the Phillips Cr/silica catalyst during fluid bed calcining. Cr(VI) is destabilized at high temperatures by water vapor, and therefore the key to a successful activation is in balancing two conflicting goals: (1) achieving extreme dehydroxylation of the silica surface, while (2) simultaneously minimizing Cr(VI) exposure to the H2O by-product. Moisture can lead to the growth of large crystallites of α-chromia which are difficult to reoxidize. Three main variables can be used to control moisture concentration in the gas stream during commercial activation—catalyst charge weight, air flow, and thermal ramp profile. In this study the sensitivity of Cr(VI)/silica to moisture, or to the absence of O2, has been defined as a function of temperature. Such laboratory data then allows one to model and predict the performance of Cr/silica catalyst activations when carried out on a commercial scale using a wide range of recipe variables.

Although critical to activity and polymer properties, activation of the Phillips Cr/silica polymerization catalyst is never accomplished as efficiently on a commercial scale as in the laboratory. This is because Cr(VI) thermal stability depends on minimizing moisture released from the necessary dehydroxylation. This principle is used to successfully model commercial results from laboratory data. Figure optionsDownload as PowerPoint slide