A Novel Laboratory Slurry Reactor for Ethylene Polymerization Studies

Abstract

A novel laboratory slurry reactor useful for studies of polymerization of olefins with solid catalysts such as Cr/silica is described. The operation of this reactor, in which gaseous impurities are minimized by purification of incoming gases and by in situ activation of the catalyst, has enabled the early stages of the homopolymerization of ethylene to be studied without interference from catalyst deactivation effects. By standardizing the conditions for the experimental runs, the reactor demonstrated its usefulness in isolating and assessing the relative physical roles of different silica supports. Four catalysts supported on different silica supports were compared, and their differences in promoting the polymerization were interpreted in terms of porosity, average particle size, and changes in specific surface area. During early polymerization, the fracturing of catalysts prepared by impregnation of the porous silica support ensures access to these original Cr sites, in the absence of additional concealed sites. At low yields, the polymerization process is clearly affected by the physical character of the silica support; however, at higher yields, the evidence suggests that other causes such as chemical effects are responsible for the continuing acceleration in rate. The porous Cr/silica catalyst tested did not fracture instantaneously; hence, current models of nascent polymer do not describe this process. The "hardcore" model of nascent polymer seems applicable to polymerization on the nonporous Cr/Cab-O-Sil catalyst tested.

The Structure of Active Centers and the Ethylene Polymerization Mechanism on the Cr/SiO2 Catalyst: A Frontier for the Characterization Methods

“…Rebenstorf 319 obtained, at a temperature of 353 K and at an ethylene pressure of 500 Torr, a TOF of about 0.44 C 2 H 4 molecules/s. Szymura et al 320 reported a polymer yield of 25.5 g (PE)/g (catalyst) for a 300 m 2 /g silica loaded with 5 wt % Cr, during polymerization at 300 K and atmospheric pressure over a COprereduced catalyst. This value corresponds to a TOF of about 0.26 C 2 H 4 molecules/s at atmospheric pressure.…”

“…Of course, it is very difficult to compare these three values, because they have been obtained in different conditions of temperature and pressure on catalysts characterized by different chromium concentrations and different activation procedures. Nevertheless, we can try to convert reported TOF values 133,319,320 into the expected TOF values for a polymerization reaction conducted in conditions typical of spectroscopic characterization experiments, i.e., RT and low ethylene pressure (20 Torr). As the spread in the temperature adopted in the three experiments is quite small (300-353 K), in this simple conversion, the temperature effects can be neglected.…”

Realistic Surface Science Models of Supported Catalysts: The Chromium Oxide Ethylene Polymerization Catalyst

“…The polymerization experiments reported by Szymura et al 24 give us a feeling of how our model catalyst performs in comparison to the industrial chromium on silica. These authors tested a series of chromium on high surface area silica for ethylene polymerization in a slurry reactor at 25 "C. Although the catalyst pre-treatment was more involved than ours, the calcination step, 500 "C in O,, is close to the activation temperature we used for the model catalyst.…”

Kinetic modeling studies of ethylene polymerization reactions using supported chromium catalysts

“…1) support this observation. This fragmentation effect, which is particularly important when the rate of polymerization is high and when enough polymer to push the particles apart has been formed, has already been observed both in Cr/SiO 2 12 and in silica‐supported metallocene catalysts 8, 13…”