Chromium/silica catalyst reduced

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). 

Cr(VI) /silica catalysts, reduced in CO at 350 °C, are used commercially to make polymers very similar to those obtained from Cr(VI). The predominant (sometimes exclusive) valence of the chromium in these commercial catalysts (before contact with ethylene) is Cr(II) oxide. Thus, Cr(II) is an active precursor. However, it is probable that a further change in formal oxidation state occurs upon exposure of the catalyst to ethylene, either to Cr(III) or Cr(IV), as indicated by XPS experiments [141]. Oxidation to Cr(IV) by olefin during the initiation of polymerization, which has been suggested by many researchers [52,251,315-325], is a reasonable hypothesis from our present state of knowledge. 

Again (as mentioned in Section V,C) sulfur compounds perform better than CO, as can be seen in Fig. 20, because they are better dehydrating agents. When Cr/silica is reduced by COS or CS2 a black chromium sulfide forms. Reoxidation then converts it back to the hexavalent oxide. The catalyst retains no sulfur, but it often takes on a new reddish hue and the activity is greatly improved. This is probably an extension of the trend already observed in Fig. 10, which shows both activity and termination to increase as the catalyst is dehydrated. Perhaps the color change from yellow to orange, and finally to red for sulfided catalysts, indicates a transition from chromate to dichromate, or maybe just less coordination to hydroxyls. Adding water vapor to a sulfided catalyst completely reverses the benefit. 

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. 

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]. 

Other Early Developments. In addition to the breakthrough by Ziegler, two other discoveries of ethylene polymerization catalysts were made in the early 1950s. A patent by Standard Oil of Indiana, filed in 1951, disclosed reduced molybdenum oxide or cobalt molybdate on alumina (13). At the same time, Phillips discovered supported chromium oxide catalysts, prepared by impregnation of a silica-alumina support with Cr03 (14 16). Both the Phillips catalyst and titanium chloride based Ziegler catalysts are widely used in the production of high density polyethylene (HDPE). 

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. 

Licensors offer a variety of catalysts to promote the isomerization— silica alumina by itself or enhanced with a noble metal like platinum or a non-noble metal like chromium. Another uses hydrofluoric acid with boron trifluoride In the case of the noble metal catalytic process, the feed enters a vessel with a fixed catalyst bed at 850°F and 14.5 psi. As is often the case, a small amount of hydrogen is present to reduce the amount of coke laying down on the catalyst. The effluent is processed in a standard fashion to separate the hydrogen, the para- and ortho-xylene, and any unreacted or miscellaneous compounds. Yields of para-xylene are in the 70% range. 

A supported catalyst for ethylene polymerization which requires no alkyl aluminum for activation was first claimed by the Phillips Petroleum Company (32). It consists of chromium oxide on silica, reduced with hydrogen. Krauss and Stach (93) showed that the active sites are Cr(II) centers. The presence of solvent, or even aluminum alkyls, diminishes... 

Sheldon and co-workers have circumvented this problem to some extent by three approaches 46 the use of sulfuric acid to reduce the pH, by addition of ammonium fluoride and by addition of ammonia. Ammonia stabilizes monomeric chromium(III) species via the formation of amine complexes, and the fluoride effects dissolution of the silica at near-neutral pH. The three catalysts that were synthesized were evaluated in the oxidative cleavage of styrene with 35% m/m hydrogen peroxide in 1,2-dichloroethane at 70 °C (Table 4.4). The... 

Catalysts. Two types of silica support were used in these experiments. Davison grade 952 silica had a pore volume of 1.6 cc/g and a surface area of about 280 nr/g. The other support was a coprecipitated silica-titania (3.3 wt% Ti02) having a pore volume of 2.5 cc/g and a surface area of about 450 m /g. Ordinarily both supports were first treated with chromium (III) acetate to yield 1 wt Cr. Activation was accomplished in a shallow bed fluidized by air or another gas predried through alumina columns. Gases other than air were also deoxygenated through columns of specially reduced Cr/silica-alumina catalyst. 

Organic sources of Cr(VI) have also been investigated as the chromium source. Baker and Carrick [148] first investigated bis(triphenylsilyl) chromate as a homogeneous model for the surface chromate structures postulated to exist on the Phillips catalyst. This chromate ester is quite stable, but like Cr(VI) /silica, it can also be reduced by olefins under polymerization conditions to give the corresponding aldehyde and Cr(II) or Cr(III). Thus, it mimics the behavior of Cr(VI)/silica in many respects [149]. Bis(triphenylsilyl) chromate does catalyze ethylene polymerization,... 

Supported bis(triphenylsilyl) chromate is widely used as a low-activity substitute for chromium oxide in fluidized-bed reactors with gas-phase reactants. To generate sufficient activity, it is necessary to add an organoa-luminum compound (e.g., AlEt3 or AlEt2OEt) to reduce and alkylate the catalyst. The aluminum alkyl is usually impregnated onto the silica-supported bis(triphenylsilyl) chromate. These catalysts usually provide a broader MW distribution than simple catalysts made from chromium oxide on silica, and the two types are often contrasted with each other [150]. Elowever, catalysts made from chromium oxide on silica can be similarly impregnated with such cocatalysts (Section 17) and they then produce the same broad MW distribution [155-159]. 

The induction period can also be shortened or even eliminated by the addition of reducing agents either to the catalyst or to the reactor. Particularly effective are the alkyls or hydrides of aluminum, boron, zinc, lithium, magnesium, etc. When added in ppm quantities, they can eliminate the induction time of Cr(VI)/silica and also raise the steady-state polymerization rate. Some metal alkyls can remove poisons and redox byproducts. All metal alkyls no doubt help reduce the Cr(VI), perhaps to Cr(IV). And some may even help alkylate the chromium, similar to the chemistry of Ziegler catalysts. Figure 16 shows how triethylaluminum cocatalyst can be used to shorten the induction time [52],... 

Most commercial manufacture of polyethylene with the Phillips catalyst is carried out with Cr(VI)/silica as the catalyst. Cr(VI) is reduced by ethylene in the reactor to form the active precursor, probably Cr(II). However, in some cases, it is desirable to reduce the Cr(VI) to Cr(II) before the catalyst goes into the reactor. Treatment in CO at about 350 °C reduces the hexavalent chromium almost quantitatively to a divalent species. In practice, one must be careful to fully flush out the CO with an inert gas like N2 before the catalyst is allowed to cool. Otherwise, CO is chemisorbed and poisons the catalyst. Actually, CO begins to react with Cr(VI) / silica even at room temperature, although that reaction is slow. Over a period of hours, the Cr(VI) can become highly reduced at 25 °C. However, purging with N2 at higher temperatures is still needed to remove the adsorbed CO. Otherwise, the catalyst will be inactive. 

This conclusion is consistent with other data. Cr(VI) /alumina catalyst can be treated with CO at 350 °C to yield a catalyst with a substantial portion of the chromium coordinatively unsaturated and in the divalent state. The reduction is not as clean on alumina as it is on silica, and considerable Cr(III) is also obtained [104]. Nevertheless, the reduced Cr/alumina catalyst exhibits chemiluminescence upon exposure to air, and the wavelength of light emitted, characteristic of oxygen, is similar to that of light emitted by Cr(II)/silica [274]. An example is shown in Figures 13 and 14. 

Sometimes the chromium species generated by reaction of the catalyst with the cocatalyst become highly sensitive to H2 as a MW regulator, much like the organochromium catalysts. For example, an attempt was made to minimize the MI (raise the MW) by choosing a combination of catalyst and reaction variables that are all known to raise the polymer MW. A low pore volume Cr/silica was activated at only 600 °C, and the catalyst was treated with fluoride to increase the activity. It was then reduced in CO at 340 °C, again to improve activity and to lower the MI potential of the catalyst. To further lower the MI (actually the HLMI in... 

Studies (90, 91) with Cr02/Si02 catalyst have shown that formation of a surface chromate takes place by reaction of Cr02 and surface silanol groups on silica (Reaction 17). Reaction of this chemisorbed chromate with ethylene results in an oxidation-reduction reaction (90-95) with formation of a low-valent chromium center (Reaction 18). Proposals for Cr(II) as the active site are based on studies of the catalyst after reduction by ethylene, carbon monoxide, or hydrogen. One study (93. 94) showed that the polymerization rate increased with the fraction of Cr(II) in the catalyst. Another study (92) showed by polarography that the chromium is reduced to a divalent state by ethylene. 

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