On the mechanism of propylene oxidation over gallium-molybdenum oxide catalysts

The products of selective oxidation are shown to be formed via a stepwise mechanism with participation of surface allylic complexes. A successive transformation of -allyl to -allyl and further to acrolein is possible. The products of complete oxidation may be formed through surface carbonate-carboxylate complexes via either a stepwise mechanism involving the catalyst's oxygen, or a concerted mechanism with the participation of molecular oxygen.

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Active state of molybdenum in molybdenum-alumina catalysts for propylene oxidation

Catalytic properties of molybdenum-alumina catalysts for propylene oxidation are shown to be determined by water-soluble molybdenum compounds, being apparently aluminomolybdenum heteropoly acids bonded to the catalyst surface.

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Role of acid centers of Sn/Mo oxide catalyst in propylene conversion to acetone

Infrared spectroscopic studies of the nature of catalyst centers indicate the presence of Brönsted and Lewis acid centers on the surface. Propylene conversion to acetone proceeds via proton transfer from the surface to propylene with the formation of a (CH₃)₂–C(H)–O–Me type compound, and its subsequent dehydration.

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Catalytic properties of silica-molybdenum catalysts in propylene oxidation

The activity of silica-molybdenum catalyts in propylene oxidation is shown to be determined by the partially dehydrated silica-molybdenum heteropolyacid stabilized by the SiO₂ surface.

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Kinetic evidence for the identical nature of active centers for partial and deep oxidation of ethylene on silver

Kinetics of interaction of ethylene with pre-adsorbed oxygen on a silver film was studied at 473 K and different initial values of surface coverage with oxygen. Dependencies of the initial rates of partial and deep ethylene oxidation on the oxygen surface coverage manifest themselves as peaked curves with coinciding maxima. The results are considered as evidence that the processes of partial and deep ethylene oxidation on silver, when they occur at optimal conditions, proceed viaidentical active centers.     

Bond energy of surface oxygen in multicomponent oxide catalysts for propylene oxidation to acrolein

Bond energy of surface oxygen for the multicomponent oxide catalyst Mo₁₂Bi₁Ni_2.5Fe₃Co_4.5K_0.07P₁/SiO₂ has been measured in conditions of propylene oxidation and found to be 272 kJ/mol.

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Mo₁₂Bi₁Ni_2,5Fe₃Co_4,5K_0.07P₁/SiO₂, , 272 /.

     

Nonsteady-state oxidation of propylene on supported silver catalysts

Nonsteady-state epoxidation of propylene on silver has been studied to establish the reason for the low catalyst activity and selectivity that appear to be due to side reactions of the formation of hydrogen-deficient deposits.

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Selective oxidation of propylene to acrolein by silica-supported bismuth molybdate catalysts

Silica-supported bismuth molybdate catalysts have been prepared by impregnation, structurally characterized and examined as improved catalysts for the selective oxidation of propylene to acrolein. Catalysts with a wide range of loadings (from 10 to 90 wt%) of beta bismuth molybdate (??-Bi₂Mo₂O₉) were studied to provide a better understanding about the distribution of active sites, and to elucidate the role of lattice oxygen in the reaction. The catalyst containing 50 wt% of beta bismuth molybdate on SiO₂ was found to possess good distribution of active sites and sufficiently high lattice oxygen, which resulted in an extraordinary increase of the catalytic activity.     

Study of the mobility of lattice oxygen in complex oxide catalysts for partial oxidation of propylene to acrolein

The amount of oxygen in the lattice of solids that participates in the elementary stages of partial propylene oxidation is determined for two types of Co-Mo-Bi-Fe-Sb-K-O catalysts (I, II) differing in the method of introduction of antimony and potassium. Two independent methods are used: (1) on the basis of the yield of the oxygen-containing products of propylene oxidation by oxygen of the catalyst in a pulse regime and (2) with the use of Möessbauer spectroscopy. Coincidence of the results obtained by both methods indicates that the active oxygen of the catalyst lattice is formed during redox transformations of iron(III) molybdate entering the composition of the catalysts. Data on the reduction of the catalysts in a pulse regime at various temperatures, which were processed in the framework of the diffusion model, allowed the estimation of the rate constants for diffusion of the lattice oxygen. An increase in the mobility of the lattice oxygen in catalyst I, which is modified with a small amount of antimony as compared to catalyst II, results in an increase in the overall productivity of the sample and in a decrease in the selectivity of propylene oxidation to acrolein. This correlates with the increase in the total amount of the lattice oxygen participating in the process.     

Cyclopentadienyl titanium hydroxylaminato complexes as highly active catalysts for the polymerization of propylene

Half sandwich complexes of titanium bearing eta1 or eta2 bound nitroxide ligands are highly active catalysts for the polymerisation of propylene to high molecular weight atactic poly(propylene).     

Structure and oxidation state of silica-supported manganese oxide catalysts and reactivity for acetone oxidation with ozone

Silica-supported manganese oxide catalysts with loadings of 3, 10, 15, and 20 wt % (as MnO2) were characterized with use of X-ray absorption spectroscopy and X-ray diffraction (XRD). The edge positions in the X-ray absorption spectra indicated that the oxidation state for the manganese decreased with increasing metal oxide loading from a value close to that of Mn2O3 (+3) to a value close to that of Mn3O4 (+2(2)/3). The XRD was consistent with these results as the diffractograms for the supported catalysts of higher manganese oxide loading matched those of a Mn3O4 reference. The reactivity of the silica-supported manganese oxide catalysts in acetone oxidation with ozone as an oxidant was studied over the temperature range of 300 to 600 K. Both oxygen and ozone produced mainly CO2 as the product of oxidation, but in the case of ozone the reaction temperature and activation energy were significantly reduced. The effect of metal oxide loading was investigated, and the activity for acetone oxidation was greater for a 10 wt % MnOx/SiO2 catalyst sample compared to a 3 wt % MnOx/SiO2 sample.     

Temperature dependence of stereospecificity of active sites in heterogeneous catalysts for propylene polymerization

Temperature dependences of the stereoregularity parameters of the most stereospecific active centres of α-TiCl₃-AlEt₃ catalytic system from ?25 to 120 and of VCl₃-AlEt₃ catalytic system from ?15° to 90° have been measured. It was found that this temperature dependence could be represented by a curve with a minimum at 20–50. The results could be explained by a two-step mechanism of isotactic chain growth (propagation) with preliminary co-ordination of monomer on the active centres.     

Bi-Mo-Nb复合氧化物的结构与对丙烯氧化的催化活性

本文用XRD, IR, Raman, SEM和ESR等方法研究了系列氧化物Bi~2Mo~3-3XNb~2xO~12-4X(X=0.00, 0.02, 0.05, 0.10, 0.15, 0.20, 0.25) 的结构和对丙烯氧化的催化活性。结果表明, 在X<0.25范围内, 催化剂基本保持典型的α-Bi~2Mo~3O~12结构, 少量Nb^5+的掺杂, 可取代晶格中的Mo^6+, 产生氧空位,无序分布的氧空位的浓度先随X值的增加而增加, 当X=0.15时达到最大值, 催化剂对丙烯氧化的催化活性与这种氧空位的浓度成正比, 反应遵循Redox机理。     

Nature of the adsorption centers of oxide catalysts and the direction of propylene conversion

According to the analysis of spectral data on adsorbed propylene forms, an attempt has been made to establish a relationship between the nature of olefin adsorption center, the adsorbed form during precatalysis and the direction of conversion.

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Catalytic properties of Ti?Mo oxide catalysts in propylene oxidation

Catalytic properties of Ti–Mo oxide catalysts are shown to be determined to a great extent by the Ti–Mo heteropoly acid anchored to the TiO₂ surface which is formed during the catalyst preparation.

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Calorimetric measurements of adsorption power for active centers of aluminosilicate catalysts

Direct calorimetric measurements for the interaction energy of cumene and benzene with active centers of an aluminosilicate catalyst have been carried out to determine the potential energies of the centers catalyzing selective cracking with minimum coke formation. Kinetics of cumene cracking has been examined.

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