Silyl-chromate

Numerous modifications of chromium-based catalysts have been made through the introduction of various additives, the most effective of which are titanium alkoxides (38,39). These additives apparentiy reduce surface silyl chromate moieties to chromium titanates, which are then oxidized to titanyl chromates. These catalysts offer a better control of the resin molecular weight (39). 

The following stage of the propagation center formation occurs through the reduction of Cr(VI) to the lower oxidation state. The compounds of Cr(II) seem to be active in polymerization in the solution of bis-triphenyl-silyl-chromate (109). For the formation of these compounds the following scheme taking into account the results (110) concerning the study of the reaction of bis-triphenylsilyl-chromate with olefins was considered (109) ... 

Polyethylene (chromium catalyst). The chromium on silica catalyst is quickly reduced from Cr(VI) to Cr(II). The active site consists of a single chromium ion present as silyl chromate before reduction with ethylene. Ethylene adds to the chromium as indicated. 

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 ... 

BIS(TRIPHENYL SILYL)CHROMATE see BLS750 BISCTRIPHENYLTIN)ACETYI-ENEDICARBOXYLATE see BLS900... 

Cann K (2010) Silica-supported silyl chromate-based ethylene polymerizatirai catalysts. In Hoff R, Mathers RT (eds) Handbook of transition metal polymerization catalysts. Wiley, Blackwell... 

Silyl Chromate Pressure (Psig) Silica/ Alumina (g) Temp (°C) Time (hrs) Yield mg) Ml (12) FI/ MP Activity... 

Silyl Chromate Al Alkyl Temp CQ Time (hrs) Yield PE(g) Flow Index (FI) Activity... 

Li L, Wu Y, Dong Q, et al Effect of Al-alkyl cocatalyst on ethylene polymerization behavior using silyl chromate (SC)/Si02 catalyst, Asia PacJ Chem Eng 8(4) 539—546, 2013. 

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