Phillips Cr/silica catalyst

The Phillips Cr/silica catalyst is prepared by impregnating a chromium compound (commonly chromic acid) onto a support material, most commonly a wide-pore silica, and then calcining in oxygen at 923 K. In the industrial process, the formation of the propagation centers takes place by reductive interaction of Cr(VI) with the monomer (ethylene) at about 423 K [4]. This feature makes the Phillips catalyst unique among all the olefin polymerization catalysts, but also the most controversial one [17]. 

Keywords Ethylene polymerization mechanisms Heterogeneous model catalysts Homogeneous model catalysts Molecular modeling Phillips Cr/silica catalyst Polyethylene Polymerization kinetics... 

Scheme 1 Plausible structures of surface-stabilized hexavalent chromate species Cr(VI)0 c,surf the silica surface of the Phillips Cr/silica catalyst (n > 1)...

Fig. 1 EPMA map (a) and line curves (b) of the chromium distribution on the Phillips Cr/silica catalyst. The line curves of A-A and B-B in (b) correspond to the chromium distribution at the straight-line positions of A-A and B-B in (a), respectively. The red patches marked in circles in (a) correspond to small aggregates of surface Cr species...

Activation of the Phillips catalyst directly by ethylene monomer was further investigated by XPS and TPD-MS methods in order to shed some light on the reaction mechanisms during the induction period. Deconvolution of the XPS spectra for industrial Phillips Cr/silica catalysts treated in ethylene atmosphere at RT for 2 h revealed that surface chromium species presented in three oxidatimi states surface chromate Cr(Vl)0 c,surf species surface-stabilized trivalent Cr(III) species and surface-stabilized Cr(II) species. Compared to the original catalyst before ethylene treatment, about one-third of chromate Cr(VIX) c,surf species (i.e., ca. 22.6% of the whole surface Cr) was reduced to Cr species in lower oxidation states during the ethylene treatment, even under ambient conditions [67]. 

Scheme 6 Plausible mechanistic routes for the formation of the first hydrocarbon species, propylene, during the induction period over the non-pre-reduced Phillips Cr/silica catalyst through interaction with ethylene under various conditions...

Scheme 9 Heterogeneous models reported in the literature for the Phillips Cr/silica catalyst...

Scheme 15 Plausible monomer reaction mechanism between ethylene and monochromate site on the Phillips Cr/silica catalyst during the induction period of ethylene polymerization...

Erratum to Phillips Cr/Silica Catalyst for Ethylene Polymerization... 

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