Chromium Oxide Catalysts Research Articles

Kinetic study of ethylene polymerization with chromium oxide catalysts by a radiotracer technique

AbstractThe quenching of polymerization with a chromium oxide catalyst by radioactive methanol 14CH3OH enables one to determine the concentration of propagation centers and then to calculate the rate constant of the propagation. The dependence of the concentration of propagation centers and the polymerization rate on reaction time, ethylene concentration, and temperature was investigated. The change of the concentration of propagation centers with the duration of polymerization was found to be responsible for the time dependence of the overall polymerization rate. The propagation reaction is of first order on ethylene concentration in the pressure range 2–25 kg/cm2. For catalysts of different composition, the temperature dependence of the overall polymerization rate and the propagation rate constant were determined, and the overall activation energy Eov and activation energy of the propagation state Ep were calculated. The difference between Eov and Ep is due to the change of the number of propagation centers with temperature. The variation of catalyst composition and preliminary reduction of the catalyst influence the shape of the temperature dependence of the propagation center concentration and change Eov.

Nature of active sites of supported chromic oxide polymerization catalysts

The active sites of chromic oxide polymerization catalysts form by interaction of the catalyst surface with monomer. Part of the Cr 6+ atoms are reduced to the trivalent state and alkylated. The active sites are alkylated trivalent chromium atoms in combination with a Lewis acid. By lowering the electron density on the Cr 3+ atom, the Lewis acid strengthens the Cr 3+C bond in the polymer chain and increases the ability of the Cr 3+ atoms to coordinate with the monomer molecules. On the other hand, coordination weakens the Cr 3+C bond, which grows stronger again after the monomer molecule enters the chain. Polymerization occurs as a result of alternative strengthening and weakening of this bond. An effective carrier increases the oxidative power of the chromic anhydride and enhances the donor-acceptor properties of the chromium atoms. There is a direct dependence between the oxidative power of chromic oxide catalysts and the polyethylene yield on them, which persists up to a certain limit. The subsequent drop in activity is due to decreasing stability of the catalyst.

Some surface and catalytic studies on chromic-oxide gel

Three different samples of precipitated chromic oxide gel were prepared, and their surface and catalytic properties were studied. The techniques of DTA, TGA, X-ray diffraction, and electron microscopy, together with adsorption measurements, have been used to provide information regarding the water content, porosity, and surface area of the samples. In particular, the manner in which these properties change as the pretreatment temperature is varied has been studied. The isomerization of n-but-1-ene, to cis-but-2-ene and trans-but-2-ene at 25 °C, has been used as a test reaction to examine the manner in which the catalytic activity is dependent upon pretreatment temperature. The initial cis-but-2-ene/ trans-but-2-ene product ratio can give indications as to the type of reaction mechanism operative. The present work suggests that there are two different types of active sites on the chromic-oxide catalysts. At relatively low pretreatment temperatures, the reaction appears to proceed via a carbonium ion type of mechanism; and the sites responsible for such activity are believed to be associated with surface hydroxyl groups. At higher out-gassing temperatures the initial product ratio is greatly increased, and the reaction mechanism is best explained in terms of an allylic type of intermediate. It is suggested that the active sites responsible for this latter type of behavior are surface sites that are strained, such as would be produced by the removal of water from two adjacent hydroxyl groups.

Supported Chromium Oxide Catalyst for Olefin Polymerization. V. Measurement of Catalytic Activity in an Integral Dynamic Reactor

Abstract An integral dynamic reactor has been used for the continuous measurement of the instantaneous polymerization rate. With ethylene as monomer, the rate goes through a maximum after a few minutes, then levels off, and finally decreases owing to the blocking of the surface by the polymer. The catalytic activity may be calculated safely from the maximum instantaneous rate, after correction for the monomer consumption along the column of catalyst. As examples of application, it is shown that the catalytic activity goes through a maximum with either the chromium content at a fixed temperature of polymerization or with the temperature at a fixed chromium content.

Supported chromium oxide catalyst for olefin polymerization: IV. Physico-chemical study of the activation process

Both magnetic susceptibility and oxygen chemisorption measurements on chromium oxide supported on silica-alumina catalyst show a limited evolution of the dispersion state of the chromium oxide during the time of the activation treatment in air at 550 °C. The Cr(III) domains tend to be more isotropic, and the reverse is true for the Cr(VI) domains. However, the activity for ethylene polymerization is only slightly stabilized by a longer activation time.

Dependence of reactivity of propagation centers of oxide catalysts for ethylene polymerization on catalyst composition and activation conditions

AbstractThe propagation rate constant Kp was used as a measure of reactivity of propagation centers in ethylene polymerization with oxide catalysts. This constant was determined by a radiotracer quenching technique for oxide catalysts of different compositions and activation conditions. For catalysts based on various transition metal oxides, an increase of Kp was observed in the series W < Mo < Cr and V < Cr. In the case of chromium oxide catalyst it was shown that Kp value does not depend on the content of the transition metal in a catalyst. A change of propagation center reactivity was found when oxides of different composition (SiO2, Al2O3, ZrO2, TiO2) were used as supports. An increase of the vacuum activation temperature of a catalyst results in increasing Kp. Pretreatment of catalyst with different reducing agents (SO2, CO2, NH3, HCN) results in the change of Kp value in comparison in comparison with the catalyst activated by the vacuum treatment only. The data obtained on the variation of the reactivity of the propagation centers permit one to draw a conclusion about the composition of surface compounds as acitve centers of the oxide polymerization catalysts.

Olefin Polymerization on Supported Chromium Oxide Catalysts

Abstract There are at least three heterogeneous catalyst systems of interest for low-pressure polymerization of olefins to high polymers:

Mechanism of dehydrocyclization of paraffinic hydrocarbons on chromium oxide catalysts

Dehydrocyclization of normal paraffins to aromatic hydrocarbons on chromium oxide catalysts does not include the stage of the formation of intermediate cyclohexanes or their chemisorbed forms (complexes of the cyclohexanes with the catalyst).

Catalytic Vapor-Phase Ammoxidation of Substituted Toluenes over Chromium Oxide

Abstract Ammoxidation of substituted toluenes over a chromium oxide catalyst has been studied in a flow system at atmospheric pressure. The reactions give satisfactory yields (ca.70%) of corresponding nitriles at high conversion of toluenes. The rate constant for the reaction of substrate adsorbed on the catalyst surface, kT0, is independent of the substituent of substrate, while the adsorption equilibrium constant of gaseous substrate, KT depends on the substituent, affording Hammett’s ρ of −0.9 for σ+. These results support our mechanism reported previously for the analogous oxidation of toluene and ethylbenzene over Cr2O3, which involves an electron transfer from adsorbed hydrocarbons to dissociated oxygen.

The oxidation of CO by O 2 and by NO on supported chromium oxide and other metal oxide catalysts

The oxidation of CO by NO and by O 2 has been studied on a number of supported transition metal oxide catalysts. On supported chromia the oxidation of CO by NO is faster than by O 2; however, with mixtures of NO and O 2 the reaction is selective towards O 2. The application of continuous mass-spectrometer monitoring to the study of surface state changes in chromia catalysts is outlined. The method was extended to determine the oxidation state of the surface in situ during reaction. In the oxidation-reduction cycle involving alternate passage of HeCO and HeO 2 mixtures at 500 °, the majority of the surface atoms undergo a change of oxidation state from Cr 6+ to Cr 2+. The extent of this change diminishes with decreasing temperature. Mixtures of He and NO oxidize the surface of supported chromia to a lesser extent than HeO 2. During the CONO reaction the average surface oxidation state is lower than in the presence of oxygen. A tentative explanation is offered for the selectivity for oxygen over nitric oxide in the oxidation-reduction reactions on commonly employed catalysts.

Reduction studies on supported metal oxide catalysts

A procedure is described for determining the reduction characteristics of metal oxidepromoted catalysts. The method involves circulating a measured volume of hydrogen in a closed system that includes a suitable tube for the catalyst, an absorption tube for removing the water formed, and a manometer for measuring the consumption of hydrogen. Reductions may be conducted either at a definite temperature or the temperature may be increased on a fixed time schedule. In the latter case, regular temperature and pressure readings provide information necessary to plot “reduction profiles” in which the reduction rate is plotted versus temperature (or time). Characteristic curves are obtained for various promoter-support combinations and the areas below the “reduction profile” curves are proportional to the amount of reduction. However, the latter is obtained more precisely by accurate measurement of the hydrogen consumption. The procedure was used for studying the reduction characteristics of supported nickel oxide and chromium oxide catalysts as affected by methods of preparation, kinds of support, and the effects of heat treatment, temperature, and promoter concentrations. The results showed that promoter oxide-support oxide interaction increased the difficulty of reducing the promoter metal oxide. This effect was most pronounced for nickel oxide when supported on alumina and for chromium oxide supported on silica. Coprecipitated catalysts were more difficult to reduce than those prepared by impregnation, apparently because of better distribution and therefore better opportunity for interaction. For the same reason, higher temperatures of catalyst heat treatment resulted in increased difficulty of reduction. The interaction between the metal oxide promoter and the support oxide appears to have an important effect on the catalytic properties of metal oxidepromoted catalysts. This is illustrated by results obtained with chromium oxide—silicaalumina catalysts.

Kinetics of the catalytic vapor-phase ammoxidation of ethylbenzene over chromium oxide

The kinetics of the catalytic vapor-phase ammoxidation of ethylbenzene over a chromium oxide catalyst have been studied in a flow system at atmospheric pressure. Styrene and benzonitrile were the main products in the ammoxidation of ethylbenzene, but neither acetophenone nor benzyl cyanide was detectable. The rate of ammoxidation of ethylbenzene is expressed as: k[PhCH 2CH 3] 0·73[O 2] 0·39, while the rate of formation of styrene as: k[PhCH 2CH 3] 0·78[O 2] 0·25 and that of the formation of benzonitrile as: k[PhCH 2CH 3] 0·55[O 2] 0·90. All these rates are almost independent of the concentrations of ammonia. The apparent activation energy for the ammoxidation of ethylbenzene is ca. 23 kcal/mole. The rate of ammoxidation of styrene is expressed as: k[PhCHCH 2] −0.10[O 2] 0·50, and independnet of the concentrations of ammonia. These results suggest that styrene is formed by the reaction between the adsorbed ethylbenzene and the adsorbed oxygen in a dissociated form (i.e. oxidative dehydrogenation), and that the main pathway for the ammoxidation of ethylbenzene is a consecutive reaction: PhCH 2CH 3 → PhCHCH 2 → PhCN. The rate equations of the formation of styrene and benzonitrile and the effectof the addition of styrene were interpreted in terms of the Langmuir-Hinshelwood mechanism. The rate constants ( k°) for the reaction of substrates on the catalyst surface and the adsorption equilibrium constants of reactant gases ( K) were compared with those in the ammoxidation of toluene.

Kinetics of the catalytic vapor-phase ammoxidation of toluene over chromium oxide

The kinetics of the catlytic vapor-phase ammoxidation of toluene over a chromium oxide catalyst have been studied in a flow system at atmospheric pressure. The reaction consists of the oxidation of toluene and the ammoxidation of formed benzaldehyde. The rate of ammoxidation of toluene is expressed as k[PhCH 3] 0·45 [O 2] 0·45 and independent of concentrations of ammonia and products. The rate of ammoxidation of benzaldehyde is equal to that of ammonolysis of benzaldehyde, and is expressed as k[PhCHO] 0·47–0·53 [NH 3] 0·42–0·44 which is independent of oxygen and products, but the formation of benzonitrile is dependent on oxygen. The apparent activation energies for ammoxidations of toluene and benzaldehyde are ca. 23 and ca. 11 kcal/mole, respectively. These results suggest a main pathway involving benzaldehyde and benzylidenimine for the ammoxidation. The rate equations are interpreted in terms of the Langmuir-Hinshelwood mechanism.

Texture of Chromium Oxide Catalysts

Texture of Chromium Oxide Catalysts

Supported chromium oxide catalysts for propene polymerization: III. Magnetic properties and dispersion state of the chromium oxide

Measurements of the magnetic susceptibility of chromium oxide on silica-alumina polymerization catalysts, carried out in the range 4–300 °K, show the character of superantiferromagnetism of the chromium oxide domains. These measurements allow us to determine more precisely the dispersion state of the oxide upon its support and give some indications about the distribution of the size of the single domains of the oxide.

Supported chromium oxide catalysts for polymerization of ethylene. The reason for stability of hexavalent chromium

Supported chromium oxide catalysts for polymerization of ethylene. The reason for stability of hexavalent chromium

Supported chromium oxide catalysts for propene polymerization: I. Relationship between the catalytic activity and the dispersion state of the chromium supported on a silica gel with 13.5% alumina

The impregnation of a silica-alumina catalyst by chromic acid and the activation of the resulting catalyst causes an important change in the texture which may be explained by a chemical interaction between the carrier and the chromium oxide. The area covered by the chromium oxide, measured by oxygen chemisorption, is always a low fraction of the total available area. It goes through a maximum with chromium content, and decreases with the average of oxidation number. Linear relations between catalytic activity and area are observed to hold only in limited ranges, and the quality of the coverage seems to be as important as the value of the area covered.

Supported chromium oxide catalysts for propene polymerization: II. Catalytic activity and dispersion state of the chromium on various carriers—General relationship

Measurements of the activity in the propene polymerization of the area covered by chromium oxide, and of the specific area of several activated catalysts prepared by impregnation of various materials such as silica, alumina, or aluminum silicate with chromic acid were carried out. They confirm the existence of chemical interactions between the supporting material and the deposited oxide, which govern the dispersion of the chromium and the activity of the catalysts. The results suggest a distribution of sites which have different activities and are characterized by the number of hexavalent chromium oxide layers.

Chain transfer reactions in the polymerization of ethylene with a chromium oxide catalyst

Chain transfer reactions in the polymerization of ethylene with a chromium oxide catalyst

Mechanism of Polymerization of α‐Olefins on Oxide Catalysts

AbstractThe polymerization of ethylene on a chromic oxide catalyst with and without a solvent has been studied. It was found that the active catalyst surface is formed exclusively as a result of its interaction with ethylene. This interaction is accompanied by the formation of products which poison the surface of the catalyst when they are sorbed on it in the absence of a solvent. A catalyst which contains no Cr+6 atoms as a result of reduction by alcohol is inactive. On the other hand, a catalyst which contains only Cr+6 atoms becomes active only after it has been partially reduced. The most probable product of this reduction is trivalent chromium atoms. The results obtained have given grounds for the assumption that the active complex contains Cr+6 and Cr+3 atoms. A possible mechanism of the reaction is discussed. Owing to the oxidative action of CrO3 on the ethylene molecules, part of the Cr+6 is reduced to Cr+3, and the trivalent chromium becomes alkylated. The monomer molecule is added at the Cr+3—C bond thus formed. A strong Lewis acid, CrO3, lowers the electron density on the Cr+3 atom. This increases the strength of the Cr+3—C bond and the ability of the Cr+3 atom to coordinate with the monomer molecule. The monomer molecule enters the chain at the moment when the strength of the Cr−3—C bond is weakened due to coordination of this molecule with the Cr+3 atom.