N. I. Kuznetsova

Hydrocarbon oxidation with an oxygen-hydrogen mixture: Catalytic systems based on the interaction of platinum or palladium with a heteropoly compound

A series of studies of hydrocarbon oxidation by the O2 + H2 mixture in the presence of catalytic systems based on Pt or Pd and a heteropoly compound (HPC) is reviewed. The catalytic systems were prepared from Pd(II) complexes with the heteropoly tungstate anions PW11O297− and PW9O349−, the complex salt [Pt(NH3)4][H2Mo12O40]2 · 7H2O, mixtures of H2PtCl4 or H2PtCl6 with H3 + nPMo12 − nVnO40 (n = 0–3) heteropoly acids, or supported platinum dispersed in HPC solutions. The interaction of metal ions and particles with HPCs in the initial state and after thermal and redox treatments was investigated by NMR, IR spectroscopy, XPS, EXAFS, HREM, and TPR. The catalytic systems were tested in the liquid-phase oxidation of alkanes, cyclohexane, cycloalkenes, benzene, toluene, and phenol with the O2 + H2 mixture at low temperatures. Effective supported catalysts based on platinum nanoparticles associated with the redox-active HPCs H3PMo12O40 and H4PMo11VO40 were prepared for gas-phase benzene oxidation into phenol. The oxidation mechanism includes the interaction between dioxygen and platinum (or palladium) and the participation of the HPC in the formation of active oxygen species of radical nature.

Reductive Activation of Dioxygen in Catalytic Systems Including Platinum and Heteropoly Compounds: Oxidation of Cyclohexane

Catalytic action of the system based on platinum and heteropoly compound (HPC) was studied in the oxidation of cyclohexane with O2H2 gases to produce cyclohexanol and cyclohexanone. The active composition was represented by a solid bi-component catalyst prepared from the [Pt(NH3)4][H2PMo12O40]2·7H2O complex salt through calcination and redox treatments. The bi-component catalysts were characterized by HREM, XPS, and IR spectroscopy. The active samples consisted of undestroyed crystalline HPC with finely dispersed Pt species, which contained both metallic and ionic states. Reversible Mo6+/Mo5+ electron transfer in HPC was easily realized under conditions of catalytic reaction. Based on the state of the active catalysts, a scheme of O2/H2 activation and cyclohexane oxidation was suggested. According to the scheme, oxidation proceeded via radical hydroxyl intermediate.

Catalytic properties of heteropoly compounds in 1,3-butadiene oxidation with hydrogen peroxide

The homogeneous oxidation of 1,3-butadiene (BD) in H2O2-HPC-CH3CN (HPC = heteropoly compound) solutions has been investigated. The route of the reaction depends on the nature of the metal capable of coordinating with active oxygen in the HPC. The products of radical BD oxidation (acrolein, 3-butene-1,2-diol, 2-butene-1,4-diol, furan) form in the presence of H3+nPMo12 − nVnO40 (n = 1, 2) acids. 3,4-Epoxy-1-butene (EB) and acrolein + furan, which form in equal amounts in the presence of the (n-Bu4N)5PW11O39Fe(OH) salt, result, respectively, from the electrophilic addition of hydrogen peroxide to BD and from radical BD oxidation on iron-oxygen complexes in the HPC composition. The reaction carried out in the presence of (n-Bu4N)3{PO4[WO(O2)2]4}, (n-Bu4N)5Na0.6H1.4PW11O39, or (EMIm)5NaHPW11O39 yields EB with high selectivity on the reacted BD basis (up to 97%) and H2O2 (about 100%). The formation and conversion of the phosphotungstate peroxo complexes PWnOmα−(n = 2, 3, 4) that are active in BD epoxidation have been investigated by 31PNMR spectroscopy. The role of the tetrabutylammonium and ethylmethylimidazolium cations in the formation of these complexes has been demonstrated.

Copper catalysts based on fiberglass supports for hydrocarbon oxidation reactions with the participation of hydrogen peroxide

Copper-containing catalysts were prepared by the adsorption of the ammonia complexes of Cu(II) on the surface of a silicate fiberglass material followed by the thermal and oxidative treatment of the samples. The states of copper after the adsorption of ammonia complexes and in the prepared samples were characterized using electronic diffuse reflectance spectroscopy. The catalytic activity of the samples in hydrogen peroxide decomposition and cyclohexane oxidation reactions was studied. It was found that molecular oxygen can be involved in the radical process of hydrogen peroxide oxidation. Based on spectroscopic data, it was hypothesized that partially reduced Cu(I)–Cu(O) compounds are active species in the catalysts of this type.

Catalytic properties of platinum-promoted acid cesium salts of molybdophosphoric and molybdovanadophosphoric heteropoly acids in the gas-phase oxidation of benzene to phenol with an O2 + H2 mixture

Acid salts CsxH3 + n − xPMo12 − nVnO40 (n = 0, 1, 2, or 3; x = 2.5 or 3.5) with coprecipitated or supported platinum were studied using thermogravimetry, IR spectroscopy, and temperature-programmed reduction. The thermal region of the full stability of these salts is limited by the decomposition temperature of the corresponding acid H3PMo12O40 (∼400°C) or H3 + nPMo12 − nVnO40 (∼300–350°C). The degree of reduction of heteropoly anions with hydrogen is regulated by temperature. Deeply reduced heteropoly anions (at 300°C) are slowly oxidized with oxygen with structure and composition regeneration. The states of molybdenum and vanadium on the surface of samples with coprecipitated platinum Pt0.1-Cs2.5H0.5PMo12O40 (1) and Pt0.1-Cs2.5H2.5PMo10V2O40 (2), which were studied using XPS, correspond to reduced or reoxidized heteropoly anions in the bulk. Platinum metal particles of ∼5 nm in size were observed in high-resolution TEM images obtained after the reduction and storage of sample 1 in air. A heteropoly compound forms two texture levels: spherical nanoparticles of 10–20 nm in size are collected in closely packed globules of 100–300 nm in size. Detailed texture studies, which were performed using nitrogen adsorption isotherms, demonstrated texture mobility under the ambient conditions. The cesium salts of the heteropoly acids were tested in the gas-phase oxidation of benzene to phenol with an O2 + H2 mixture at 180°. The effect of platinum concentration on the specific catalytic activity in the presence of deeply reduced heteropoly anions was monitored. The samples containing the salt Cs2.5H0.5PMo12O40 exhibited the highest activity in the formation of phenol. The introduction of vanadium into the heteropoly anion impaired the catalytic performance of both deeply and slightly reduced samples.

Oxidation of 1,3-butadiene over Pd/C and Pd-Te/C catalysts in polar media

The oxidation of 1,3-butadiene over the Pd/C and Pd-Te/C heterogeneous catalysts occurs in organic solvents containing water at a temperature of 100°C and an oxygen partial pressure of

Cyclohexane oxidation with an O 2 –H 2 mixture in the presence of a two-component Pt/C–heteropoly acid catalyst and ionic liquids

P_{\left[ {O_2 } \right]} = 4

The mechanism of formation of ethylene glycol monoacetate from ethylene in the system MeCO2H + LiNO3+ Pd(OAc)2

atm. Crotonaldehyde dominates among the three major products of oxidation over the Pd catalyst. The introduction of Te into the catalyst increases the methyl vinyl ketone yield, the furan yield being the lowest in all cases. X-ray photoelectron spectroscopy (XPS) showed that the active catalyst components can be in a partially oxidized state, particularly after storing the catalysts in air. Additional hydrogen treatment results in almost complete reduction of the active components to metals and enhances the catalytic activity. It is supposed that the oxidation of 1,3-butadiene over the Pd-Te catalysts proceeds via the activation of dioxygen over the Pd0 sites, with oxidized Pd and Te participating in subsequent chemical transformations.