Activity chromium oxides

In contradiction thereto, Rabes and Schenk185 assume that metallic copper and activating chromium oxide are the effective constituents of the reduced catalyst. 

Mixed catalysts have been prepared by reaction of an activated chromium oxide silica catalyst with an organo-chromium compound. Mixed catalysts are... 

C and 19,600 kPa (2800 psi). The catalyst is a complex aluminum—ca dmium —chromium oxide that has high activity and exceptionally long life. The process is claimed to give a conversion of ester to alcohol of about 99% retaining essentially all of the original double bonds. 

Processes for HDPE with Broad MWD. Synthesis of HDPE with a relatively high molecular weight and a very broad MWD (broader than that of HDPE prepared with chromium oxide catalysts) can be achieved by two separate approaches. The first is to use mixed catalysts containing two types of active centers with widely different properties (50—55) the second is to employ two or more polymerization reactors in a series. In the second approach, polymerization conditions in each reactor are set drastically differendy in order to produce, within each polymer particle, an essential mixture of macromolecules with vasdy different molecular weights. Special plants, both slurry and gas-phase, can produce such resins (74,91—94). 

The preferred catalyst is one which contains 5% of chromium oxides, mainly Cr03, on a finely divided silica-alumina catalyst (75-90% silica) which has been activated by heating to about 250°C. After reaction the mixture is passed to a gas-liquid separator where the ethylene is flashed off, catalyst is then removed from the liquid product of the separator and the polymer separated from the solvent by either flashing off the solvent or precipitating the polymer by cooling. 

Titanium, vanadium or chromium oxides activated with chlorine-free organo-aluminum compounds, triethyl- or triisobutyl aluminum, have also been used as catalysts [285],... 

So far the problem of active center formation in chromium oxide catalysts amounted mainly to a discussion of the oxidation number of chromium that is necessary for catalytic activity. As an active species chromium ions having practically every possible oxidation number—... 

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 ... 

Fig. 1. Examples of the kinetic curves during ethylene polymerization by chromium oxide catalysts. Support—SiOs temperature—80°C polymerization at constant ethylene pressure in perfect mixing reactor. Curve 1—catalyst reduced by CO at 300°C. Curve 2— catalyst activated in vacuum (400°C) polymerization in the case of (1) and (2) in solvent (heptane) ethylene pressure 10 kg/cm2 02 content in ethylene 1 ppm, HsO 3 ppm. Curves 3, 4, 5, 6—catalyst activated in vacuum (400°C) polymerization without solvent ethylene pressure 19 (curve 3), 13 (curve 4), 4 (curve 5), and 2 (curve 6) kg/cm2 02 content in ethylene 1 ppm, HsO = 12 ppm.

If the catalyst active centers are nonuniform, a time variation of the average value of Kp may be caused by the change of the proportion between the centers with various reactivity during polymerization. However, in the case of chromium oxide catalysts the experimental data show that the... 

It is evident [see Eq. (5), Section II[] that for catalysts of the same or similar composition the number of active centers determined must be consistent with the catalytic activity it can be expected that only in the case of highly active supported catalysts a considerable part of the surface transition metal ions will act as propagation centers. However, the results published by different authors for chromium oxide catalysts are hardly comparable, as the polymerization parameters as a rule were very different, and the absolute polymerization rate was not reported. 

Two ions of the transition metal take part in this reaction. However, in the case of supported one-component catalysts the formation of the active bond seems to occur on the interaction of the monomer with isolated ions of the transition metal. That may be illustrated by the data showing that the activity of chromium oxide catalysts decreases linearly with decreasing chromium content (or even increases per chromium ion) to the rather low (0.01%) chromium concentrations on the catalyst surface (62, 69). In... 

In polymerization by one-component catalysts [chromium oxide catalyst (75), titanium dichloride 159) at ethylene concentrations higher than 1 mole/liter and temperatures below 90°C the transfer with the monomer is a prevailing process. The spontaneous transfer, having a higher activation energy, plays an essential role at higher temperatures and lower concentrations of the monomer. 

Table IV presents the results of the determination of polyethylene radioactivity after the decomposition of the active bonds in one-component catalysts by methanol, labeled in different positions. In the case of TiCU (169) and the catalyst Cr -CjHsU/SiCU (8, 140) in the initial state the insertion of tritium of the alcohol hydroxyl group into the polymer corresponds to the expected polarization of the metal-carbon bond determined by the difference in electronegativity of these elements. The decomposition of active bonds in this case seems to follow the scheme (25) (see Section V). But in the case of the chromium oxide catalyst and the catalyst obtained by hydrogen reduction of the supported chromium ir-allyl complexes (ir-allyl ligands being removed from the active center) (140) C14 of the...

Chromium compounds as catalysts, 188 Chromium oxide in catalytic converter, 62 Chromium oxide catalysts, 175-184 formation of active component, 176,177 of Cr-C bonds, 177, 178 propagation centers formation of, 175-178 number of, 197, 198 change in, 183, 184 reduction of active component, 177 Clear Air Act of 1970, 59, 62 Cobalt oxide in catalytic converter, 62 Cocatalysts, 138-141, 152-154 Competitive reactions, 37-43 Copper chromite, oxidation of CO over, 86-88... 

Kinetic studies of the oxidation of sulphoxides to sulphones by chromium(VI) species have been carried out131-133. The reaction has been found to be first order with respect to the chromium(VI) species and the sulphoxide and second order with respect to acid. At high sulphoxide concentrations the order with respect to sulphoxide is two. The proposed mechanism involves an electron transfer from the sulphoxide to the active chromium(VI) species (HCr03+ in strong acidic media) in the rate-determining step producing a sulphoxide radical cation which further reacts to give the sulphone. 

The fluorination of CF3CH2CI into CF3CH2F over chromium oxides is accompanied by a dehydrofluorination reaction (formation mainly of CF2=CHC1). This dehydrofluorination is responsible for the deactivation of the catalyst. A study of the dehydrofluorination reaction of CF3CH2CI proves that the reaction is favoured when the degree of fluorination of chromium oxide increases. Consequently it would be favoured on strong acid sites. Adding nickel to chromium oxide decreases the formation of alkenes and increases the selectivity for fluorination while the total activity decreases. Two kinds of active sites would be present at the catalyst surface. The one would be active for both the reactions of dehydrofluorination and of fluorination, the other only for the fluorination reaction. 

In table 1, we show the influence of the degree of fluorination of the chromium oxide (measured by F/Cr atomic ratio.) on the transformation of CF3CH2CI. We remarked that the longer the fluorination of the catalyst lasts, the greater the degree of fluorination Moreover this fluorination decreases slightly the specific surface of the catalyst.. Fluorination and dehydrofluorination activities per unit of surface show the same evolution whatever the degree of fluorination... 

Catalytic activity for the fluorination of CF3CH2CI and for the alkene formation after 5 hours of CF3CH2CI pulses over chromium oxide with different degree of fluorination... 

The nickel addition in chromium oxide decreased the formation of alkenes which was smaller than the one observed in the presence of just chromium oxide. It is to be remarked that the decrease of alkene formation was independent of the quantity of nickel in the catalyst. However, the catalytic activity for the fluorination reaction decreased when the nickel content increased. Thus the addition of nickel in small quantities allowed to increase the selectivity for the fluorination reaction. We could suggest that nickel substitute... 

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. 

All mechanisms proposed in Scheme 7 start from the common hypotheses that the coordinatively unsaturated Cr(II) site initially adsorbs one, two, or three ethylene molecules via a coordinative d-7r bond (left column in Scheme 7). Supporting considerations about the possibility of coordinating up to three ethylene molecules come from Zecchina et al. [118], who recently showed that Cr(II) is able to adsorb and trimerize acetylene, giving benzene. Concerning the oxidation state of the active chromium sites, it is important to notice that, although the Cr(II) form of the catalyst can be considered as active , in all the proposed reactions the metal formally becomes Cr(IV) as it is converted into the active site. These hypotheses are supported by studies of the interaction of molecular transition metal complexes with ethylene [119,120]. Groppo et al. [66] have recently reported that the XANES feature at 5996 eV typical of Cr(II) species is progressively eroded upon in situ ethylene polymerization. 

To my knowledge, the first transition metal-catalyzed reaction utilizing S-S bond activation was reported by Holmquist el al. in 1960 [14]. The reaction of (PhS)2 with CO (950 atm) in the presence of chromium oxide on AI2O3 at 275°C furnished thioester 57 in 31% yield (Eq. 7.42). 

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