Silica supported chromium

Thermal reduction at 623 K by means of CO is a common method of producing reduced and catalytically active chromium centers. In this case the induction period in the successive ethylene polymerization is replaced by a very short delay consistent with initial adsorption of ethylene on reduce chromium centers and formation of active precursors. In the CO-reduced catalyst, CO2 in the gas phase is the only product and chromium is found to have an average oxidation number just above 2 [4,7,44,65,66], comprised of mainly Cr(II) and very small amount of Cr(III) species (presumably as Q -Cr203 [66]). Fubini et al. [47] reported that reduction in CO at 623 K of a diluted Cr(VI)/Si02 sample (1 wt. % Cr) yields 98% of the silica-supported chromium in the +2 oxidation state, as determined from oxygen uptake measurements. The remaining 2 wt. % of the metal was proposed to be clustered in a-chromia-like particles. As the oxidation product (CO2) is not adsorbed on the surface and CO is fully desorbed from Cr(II) at 623 K (reduction temperature), the resulting catalyst acquires a model character in fact, the siliceous part of the surface is the same of pure silica treated at the same temperature and the anchored chromium is all in the divalent state. 

Future challenges for polymerization model catalysts are to study the structure of polymers below their melting point in what is called the nascent morphology. Such work can be undertaken on silica-supported chromium catalysts as discussed above, or on so-called single-site catalysts, such as metallocenes, applied on flat silica supports. 

The chemical anchoring of complexes to a solid, such as silica-supported chromium catalyst, has been successfully used by Union Carbide for ethylene polymerization (see Section 6.2). 

Hogan and Banks (Phillips Petroleum chemists) produce high MW linear HOPE with a silica-supported chromium catalyst... 

Phillips catalyst silica-supported chromium catalyst for HOPE developed by Phillips Petro. in the 1950s... 

In the literature, most of the early discussion of the "active" valence is in reference to silica-supported chromium oxide catalysts. However, many organochromium compounds of widely differing valence are also known to be active upon contact with a support and subsequent exposure to ethylene. For example, as early as 1961, Walker et al. showed that diare-nechromium(O) compounds polymerize ethylene when deposited onto silica or another support [280,281]. The Cr(0) is probably oxidized by silanol groups to Cr(I), consistent with the inference that it too can be an active precursor. 

Silica supported chromium catalysts that polymerize ethylene to polyethylene with as many as 12 methyl branches/1000 carbon atoms have been reported. The small amount of branching observed in the ethylene homopolymers prepared by these supported chromocene catalysts was attributed to a chain isomerization process (a) Karol, F. J. Karapinka, G. L. Wu,... 

Catalysts based on other metals, such as gallium and vanadium oxides, can be also employed in DH processes [8, 9]. For example, silica-supported gallium oxide catalyst has been found to be moderately active, but quite selective in propane dehydrogenation (up to 80%) and results in much less coking, 1/10 of that using a silica-supported chromium oxide [8], There is an extensive research aimed to find new DH catalysts that will perform well at moderate temperatures, suffer less from coke deposition and maintain catalytic activity for long periods of time without regeneration. 

Supported metal complexes and clusters with well-defined structures offer the advantages of catalysts that are selective and structures that can be understood in depth. Such catalysts can be synthesized precisely with organometallic precursors, as illustrated in this review. Synthetic methods are illustrated with examples, including silica-supported chromium and titanium complexes for alkene polymerization rhodium carbonyls bonded predominantly at crystallographically specific sites in a zeolite and metal clusters, including Ir4, Rhg, OsjC, and bimetallics. 

Zecchina A, Garrone E, Ghiotti G, Morterra C, Boiello E. (1975) Chemistry of silica supported chromium ions. I. Characterization of the samples. J Phys Chem 79 (10) 966-972... 

In the 1960s, scientists at the Union Carbide Corporation developed two additional silica-supported, chromium-based catalysts that are used in a gas-phase process for the manufacture of HDPE. One catalyst is based on... 

Karol and coworkers [34] reported silica-supported chromium-based catalysts based on Bis(indenyl) chromium (II), Bis(fluorenyl) chromium (II) and Bis(9-methyIfluorenyI) chromium (II) in place of chromocene. These catalysts were prepared at room temperature by reacting the dehydrated silica with a hexane solution containing one of the Cr(II) compounds. The Bis(indenyl)-based catalyst exhibited good activity under some polymerization conditions, while the Bis(fluorenyl)-based catalyst was significantly less active than the chromocene-based catalyst. Table 3.8 summarizes some polymerization data for these catalyst systems. 

Liu L, Li H, Zhang Y (2007) Mesoporous silica-supported chromium catalyst characterization and excellent performance in dehydrogenation of propane to propylene with carbon dioxide. Catal Commun 8 565-570... 

Autoradiography was used to show non-uniform distributions of radiolabelled additives in PE, PP and PS [732]. XRM and high-resolution (to 5 jxm) tiCl have allowed 2D and 3D imaging of the non-uniform void and silica-supported chromium catalyst fragment distribution within PE particles [732a]. 

Ghiotti G, Garrone E, Zecchina A An infrared study of carbon monoxide/ethylene coadsorption and reaction on silica-supported chromium(II) ions, J Mol Catal 65(1-2) 73-83, 1991. 

Tonosaki K, Taniike T, Terano M Origin of broad molecular weight distribution of polyethylene produced by Phihips-type silica-supported chromium catalyst, J Mol Catal A Ghent 340(l-2) 33-38, 2011b. 

S.W. Webb, W.C. Cornier, and R.L. Laurence, Monomer Transport Influences in the Nascoit Polymerization of Ethylene by Silica-Supported Chromium Oxide Catalyst, Macromolecules 22(7) (1989) 2885-2895. 

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