Chromium catalyst, Phillips supported reduction

Chromium-containing Phillips catalysts are prepared by adsorption of a chromium compound, mostly chromium trioxide, onto an amorphous silica support and a subsequent reduction by exposure to ethylene. The resulting catalysts are... 

Phillips supported chromium (II) catalyst, the most commonly used for high density polyethylene (HDPE) manufacture, possibly behaves in a similar manner, but the olefin insertion reaction is faster by several orders of magnitude. In the original Zeigler catalyst systems for HDPE, an aluminium alkyl is used to reductively alkylate the primary component, most frequently a titanium compound, to give the true catalytic species. 

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]. 

The typical Phillips catalyst comprises chemically anchored chromium species on a silica support. The formation of a surface silyl chromate, and eventually silyl dichromate [scheme (29)], is significant during the catalyst preparation, because at the calcination temperature chromium trioxide would decompose to lower-valent oxides. Chromium trioxide probably binds to the silica as the chromate initially, at least for the ordinary 1% loading. However, some rearrangement to the dichromate at high temperature may occur. It is incorrect to regard only one particular valence state of chromium as the only one capable of catalysing ethylene polymerisation. On the commercial CrOs/silica catalyst the predominant active species after reduction by ethylene or carbon monoxide [scheme (59)] is probably Cr(II), but other species, particularly Cr(III), may also polymerise ethylene under certain conditions ... 

It has been calculated that between 0.1 and 0.4 wt% of the total chromium forms active centers [105]. A difficult question relates to the valences of chromium in the active sites. Valences of II, III, IV, V, and VI have been established [106]. Because of the small number of total chromium atoms that are active centers, it has not been possible to unequivocally assign the active valence [107,108]. Krauss and Hums [109] concluded that the reduction of hexavalent chromium centers linked to support produced coordinately unsaturated Cr(II) surface compounds. A speciality of the Phillips catalyst is that there is no influence of hydrogen to control the molecular weight of the polyethylene. Only by higher activation temperatures can the molecular weight be lowered. 

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