Chromium-silica catalyst

Formulation example selection of chromium/silica catalysts, Ziegler-Natta catalysts, or metallocene catalysts makes it possible to achieve low degree of branching required to obtain high density polyethylene (density >0.94), paraffin or cycioparaffin are used as diiuents ... 

Catalyst - chromium/silica catalysts, Ziegler-Natta catalysts, or metallocene ... 

The active component of the chromium oxide catalyst is a surface compound of Cr(VI). In the case of silica as a support this stage may be presented by the schemes ... 

However, when using supports with weak linkage between the primary particles of the catalyst, its splitting occurs quickly and it is unlikely to influence the shape of the kinetic curve. For example, in the case of chromium oxide catalyst reduced by CO supported on aerosil-type silica, steady-state polymerization with a very short period of increasing rate is possible (see curve 1, Fig. 1). 

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]. 

More than three decades ago, skeletal rearrangement processes using alkane or cycloalkane reactants were observed on platinum/charcoal catalysts (105) inasmuch as the charcoal support is inert, this can be taken as probably the first demonstration of the activity of metallic platinum as a catalyst for this type of reaction. At about the same time, similar types of catalytic conversions over chromium oxide catalysts were discovered (106, 107). Distinct from these reactions was the use of various types of acidic catalysts (including the well-known silica-alumina) for effecting skeletal reactions via carbonium ion mechanisms, and these led... 

Figure 9.26 Cr 2p spectra and negative SIMS spectra of two model catalysts and a blank reference. The blank (bottom) shows only Si,0, fragments on a chromium-loaded catalyst CriCf fragments appear after thermal activation (Ar/02). If desorption of chromium is made impossible (in oxygen-free argon), Cr2Or clusters can also be detected. In combination, this is strong evidence that chromate anchors to the silica surface as a monomer (courtesy of P.C. Thiine and R. Linke, Eindhoven).

It is incorrect to regard only one particular valence state of chromium as the only one capable of catalyzing ethylene polymerization. Active catalysts have been made from organochromium compounds with every valence from Cr(I) to Cr(IV). On the commercial Cr(VI)/silica catalyst the predominant active valence after reduction by ethylene is probably Cr(II), but other states, particularly Cr(III), may also polymerize ethylene under certain conditions. 

Although titania is not itself a good carrier for Cr(VI), its presence in small amounts on Cr/silica catalysts does have a promotional effect on both activity and termination rate (74, 75). The beneficial effect probably results from a change in the electronic environment on the chromium, which possibly becomes linked to the titania during calcining. 

Figure 13 demonstrates the promotional effect of titania on the activity of Cr/silica catalysts. These samples were made by coprecipitation. The chromium was then added and each sample was calcined at 760°C to form surface attached Cr(VI). For comparison, titania concentrations are expressed as Ti atoms per square nanometer of surface, even though a good part of the titania may actually be in the bulk. 

The active site concentration on the organochromium catalysts may be higher than that of the oxide catalysts. The activity usually assumes a more linear increase with chromium loading than on the oxide catalysts, at least up to 2% Cr. Yermakov and Zakharov, studying allyl-Cr(III)/silica catalysts, stopped the polymerization with radioactive methanol, and found that the kill mechanism is different from that on the oxide catalysts (59). The proton of the methanol, and not the alkoxide, became attached to the polymer. This suggests a polarity opposite to that of the oxide catalysts, with the site being more positive than the chain. 

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 use of supports in heterogeneous catalysis was well understood by 1950 and flourished with the discovery in 1954 by Hogan and Banks [220,221] of highly active chromium trioxide catalysts supported on silica, which could... 

Polymerization with Complex Catalysts. High density polyethylene reached a domestic production of 1.25 billion pounds in 1968. It is made either with a stereospecific Ziegler-Natta catalyst or on a supported chromium oxide catalyst. The latter forms a complex with the silica-alumina and is activated by treatment with air and steam at elevated temperature. The mechanism is such that electrons are donated to the catalyst in order to be returned under polymerizational-promoting conditions, consequently lowering the energy of the system ... 

The details of the mechanism are not well understood yet. Reasonable speculations based on evidence from ESR measurements have been published by van Reijen and Cossee (17) and by Pecherskaya and Kazan-skii (15). Van Reijen and Cossee speculate that the active site in a chromium oxide-silica catalyst is a tetrahedrally coordinated chromium ion. They picture the Cr04 tetrahedron linked to the SiC>2 network by... 

Table V gives pertinent experiments for a chromium oxide-silica catalyst.

Chromocene (CrCp2) supported on silica is used to generate certain chromium-based catalysts for the polymerization of ethylene (e.g., Phillips and Union Carbide catalysts). The nature of the organometallic species responsible for the catalysis is not known with certainty, though it is noteworthy that some Crm alkyls such as [Cp Cr(CH2Ph)(THF)2]+BPh catalyze the polymerization of ethylene.19... 

Jehng, J.M. et al., Surface chemistry of silica-titania-supported chromium oxide catalysts,, 7 Chem. Soc. Faraday Trans., 91, 953, 1995,... 

The supported chromium oxide catalysts can be prepared by impregnating a silica-alumina support with a solution of chromium ions or by coprecipitating the oxides. The preferred impregnating solutions contain dissolved Cr(N03)s.9H20 or CrOs in nitric acid because catalysts made from chromium chlorides or sulfates retain some of the anions after calcination. The solid mixture of chromium-silicon-aluminum compounds is calcined in dry air at 400-700° C or higher to obtain the desired oxide. This probably results in the reaction of surface hydroxy groups in the support material with CrOs to form chromate (IV) and dichromate (V) species ... 

In 1968, new copolymers were introduced containing 1-hexene instead of 1-butene. This change provided improved physical properties of polymers made with Cr/silica catalysts. A new process to produce LLDPE was announced by Phillips in 1969 [22,23]. Polymers with densities as low as 0.925 g mL-1 were produced in a modified PF process with chromium-containing catalysts. Nevertheless, polymers with densities <0.93 g mL 1 were not common among Phillips licensees because of the tendency of low-density polymer to swell, and this swelling limited reactor output. 

FIGURE 5 Silanol group measurements of Cr/silica catalysts. The AOH/Cr ratio was determined from the OH group content measured with and without Cr03 applied. It is proportional to the original OH population, so that the fraction of OH groups displaced by any chromium loading is constant with calcination temperature. 

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