L. I. Kuznetsova

Complexes of palladium(II) and platinum(II) with the PW11O7−39 heteropolyanion as catalytically active species in benzene oxidation

Aqueous solutions prepared from Pd(H2O)2+4 and PtCl2−4 with heteropolytungstate PW11O7−39 at the Pd(II) or Pt(II) to PW11 ratio from 1 to 8 and pH 2 and 4 have been characterized by means of UV-VIS and IR spectroscopy and 31P and 195Pt NMR. Three types of complexes have been detected in Pd(II) containing solutions. Two of them, [PW11O39Pd] (1) and [PW11O39PdOPdO39W11P] (2) had sharp peaks at δ1 −13.2 and δ2 −12.8 ppm in 31P NMR spectra. Possessing a broad signal at about − 13 ppm, complex (3) represented a water-soluble oligomeric Pd(II) hydroxide stabilized by the heteropolytungstate. The complex of type (1) (PW11O39Pt) was predominant in solutions of Pt(II). The Pd(II) complexes proved capable to catalyze the oxidation of benzene into phenol with a mixture of O2 and H2 gases in a two-phase water-benzene system at the temperature of 10 to 40°C. The yield of phenol for 1 h depended on the Pd(II) to PW11 ratio and on pH and reached a maximum at an O2H2 ratio equal to 2. The catalytic action was attributed mainly to complexes (1) and (2). Under the reaction conditions, these complexes have been found to be reduced into Pd(0) species which readily activate dioxygen and are stabilized in solution by heteropolytungstate. Pt(II) containing systems were one order of magnitude less active than Pd(II) containing ones.

Catalytic properties of heteropoly complexes containing Fe( III) ions in benzene oxidation by hydrogen peroxide

Abstract Elemental analysis, magnetic measurements, IR, 31P NMR, and UV-VIS spectroscopy were used to study heteropoly complexes (HPC), containing Fe(III) ions and heteropoly anion [PW11O39]7−, isolated from aqueous solutions as tetrabutylammonium (TBA) salts and dissolved in acetonitrile. The complexes identified are: Fe(III)-substituted complexes [PW11O39Fe(H2O)]4− (1) and [PW11O39Fe(SO4)]6− (1′); Fe(III)-substituted hydroxo complex [PW11O39Fe(OH)]5− (2′) formed during the precipitation of binuclear μ-oxo complex [(PW11O39Fe)2O]10− (2) from aqueous solutions (pH = 3 ÷ 5) by TBA cations; polynuclear Fe(III)- hydroxo complexes [PW11O39FenOxHy]m− (3) (n ∼ 8). The catalytic activity of complexes both in the hydrogen peroxide decomposition and benzene oxidation in a one-phase system HPC + CH3CN + H2O2(aq) + C6H6, with [HPC] = 6 · 10−3, [H2O2] = 0.175 ÷ 1.6, and [C6H6] = 1.4 ÷ 5.6 M at 70°C have been studied. In the presence of 1 or 1′, the molar ratio between phenol formed and H2O2 decomposed equals 10–20%. HPC containing SO2−4 ions shows a far lower activity in both reactions. The kinetics of PhOH accumulation conforms to the chain mechanism of H2O2 decomposition. Benzene is oxidized by OH radicals coordinated to HPC. In the presence of complex 2′ the rate of H2O2 decomposition is considerably lower. Thus the molar ratio of phenol formed and H2O2 decomposed is almost 60%. The mechanism of H2O2 activation by 2′ includes most likely the initial formation of a peroxo complex which was observed in absorption spectra. The state of HPC affected by the reaction mixture components was studied by UV-VIS spectra. The products of one-electron phenol (or pyrocatechol) oxidation inhibit the catalytic activity due to the complex formation with Fe(III) ions in HPC.

Amination of benzene and toluene with hydroxylamine in the presence of transition metal redox catalysts

Abstract The amination of benzene and toluene to aniline and toluidines with hydroxylamine sulfate has been investigated in water–acetic acid and water–acetic acid–sulfuric acid media in the presence of transition metal compounds as catalysts. The process yields are strongly dependent on the temperature, added sulfuric acid and the composition of catalyst. For the amination of benzene, the soluble catalysts, NaVO3 and Fe(III) salts produce high yields of aniline without addition of H2SO4, whereas Na2MoO4 and FeSO4 exhibit substantial activity only in 5 M H2SO4. Amination is accompanied by a disproportionation of hydroxylamine catalyzed by the redox active transition metal ions. The favorable effect of H2SO4 on the amination is due mostly to the greater stability of hydroxylamine in the strongly acidic medium. Mixed oxides containing V(V) and Mo(VI) are active amination catalysts when suspended in 5 M solution of H2SO4 in acetic acid. Introduction of metallic Pd into these oxide catalysts improves performance increasing the yield and selectivity of amination with respect to the aromatic substrate. Toluene exhibited a close to benzene reactivity in amination giving approximately equal yields of o-, m-, p-toluidines. Mechanistic considerations based on literature data and results of ab initio quantum mechanics calculations suggest that the aminating species is the protonated amino radical ⋅NH3+, which in the rate-determining step reacts with benzene and toluene to yield the corresponding aminocyclohexadienyl and aminomethylcyclohexadienyl radical intermediates. These are then oxidatively aromatized to give, respectively, aniline and a non-regiospecific mixture of toluidines.

Catalytic properties of Cr-containing heteropolytungstates in H2O2 participated reactions: H2O2 decomposition and oxidation of unsaturated hydrocarbons with H2O2

Abstract Cr-containing derivatives of PW11O7−39 heteropolyanion (PW11Cr) were synthesized under variation of precursors and procedure (increasing temperature, prolonged keeping of solution, varying molar ratio of starting reagents and acidity) and characterized by elemental analysis, UV-Vis and IR spectroscopy. Tetrabutylammonium salts of PW11Cr were examined in catalysis of H2O2 decomposition and oxygen transfer from H2O2 to the hydrocarbon substrates, cyclohexene and benzene. With all the other conditions of synthesis identical, increasing the pH gave samples more active in H2O2 decomposition. In the H 2 O 2 PW 11 Cr system, benzene formed phenol and cyclohexene formed a mixture of cyclohexenone, cyclohexenol and cyclohexene oxide. The activity of PW11Cr in hydrocarbon oxidation was sensitive to the procedure used for sample synthesis. Variable spectral characteristics and catalytic properties of PW11Cr samples were suggested to depend on the nature of bond between Cr(III) ion and PW11O7−39 anion. As distinguished from typical M-substituted polyoxometalates, the samples obtained contain Cr(III) ions which are probably connected with surface oxygen atoms of PW11O7−39 heteropolyanion near the vacancy of the Keggin structure. Surface location of Cr(III) ions is supposed to favor oxygen transfer from H2O2 to hydrocarbon. In the presence of H2O2, PW11Cr solutions showed the increase of visible absorption and a shift of its maximum from 15200–16300 to 15500–18000 cm−1, that arose from the reaction of PW11Cr with intermediate products of the H2O2 decomposition. The resulting oxidized species PW11Cr[O] are active oxidants in the reaction with unsaturated hydrocarbons.

Relation between structure and catalytic properties of transition metal complexes with heteropolyanion PW11O7−39 in oxidative reactions

The content and structural features of transition metals Fe(III), Cr(III), Ru(IV), Ti(IV) and V(IV) complexes with the heteropolyanion PW11O7−39 depending on the starting M-compound, molar ratio of reagents, temperature and pH of the solution were studied. Heteropolycomplexes (HPC) of three types were identified using NMR 31P, 17O, 183W, IR, UV-VIS spectroscopy, magnetic measurements and elemental analysis: I — M-substituted HPC, II — M-supported HPC, III — polynuclear hydroxocomplexes of M, stabilized by the heteropolyanion. The catalytic properties of HPC obtained have been studied in oxidation of cyclohexene, benzene, alcohols, aldehydes with hydrogen peroxide and other oxygen containing oxidants. It was demonstrated, that the nature of bonding of metal ions with the heteropolyanion, additional ligands of M ions in HPC content, as well as the composition of reaction mixture are the governing factors of HPC catalytic performance. A mechanistic study of hydrogen peroxide participated oxidations in the presence of Cr(III)- and Fe(III)-containing HPC was performed.

Liquid-Phase Oxidation of Benzothiophene and Dibenzothiophene by Cumyl Hydroperoxide in the Presence of Catalysts Based on Supported Metal Oxides

The liquid-phase oxidation of benzothiophene and dibenzothiophene by cumyl hydroperoxide in the presence of supported metal oxide catalysts was carried out in octane in an N2 atmosphere at 50–80°C. The cumyl hydroperoxide, benzothiophene, and dibenzothiophene conversions and the yield of sulfones were determined for catalysts of various natures. In the presence of MoO3/SiO2, the most efficient and most readily regenerable catalyst, the benzothiophene conversion was ∼60% and the dibenzothiophene conversion was as high as 100% upon almost complete consumption of cumyl hydroperoxide. The influence of unsaturated and aromatic compounds (oct-1-ene, toluene) on the catalytic effect was studied. The kinetics of substrate oxidation and cumyl hydroperoxide decomposition and an analysis of the cumyl hydroperoxide conversion products suggested a benzothiophene and dibenzothiophene oxidation mechanism including the formation of an intermediate complex of the hydroperoxide with the catalyst and the substrate and its transformation via heterolytic and homolytic routes.

Oxidation of hydrocarbons by dioxygen reductively activated on platinum and heteropoly compounds

Abstract Based on Pt and heteropoly compounds (HPC), catalysts are applied to liquid-phase oxidation of cyclohexane and cyclohexene with a mixture of O 2 and H 2 gases. Platinum catalyst in the presence of PW 11 and PW 11 Fe HPC represent the most active catalytic systems for alkene oxidation, whereas highest reactivity of cyclohexane was exhibited in the presence of PMo 12 HPC. Activity of the catalytic systems and composition of the oxygenated products are controlled by the nature of active intermediates generated under the action of the different HPC.

Reactivity of transition-metal-modified, Keggin-type heteropolycomplexes in the homogeneous oxidation of cyclohexane and adamantane

Abstract The oxidation of cyclohexane and adamantane catalyzed by Keggin-type transition-metal-containing heteropoly-complexes (TMC-HPCs) is reported. The reactions were carried out at 20°C in CCl 4 or in CH 2 Cl 2 as reaction solvent in the presence of t -butylhydroperoxide ( t -BuOOH) as oxidizing agent. A radical chain mechanism of oxidation by t -BuOOH in the presence of molecular oxygen is proposed for all complexes tested, involving a RH hydrogen atom abstraction by an intermediate active form of the TMC-HPC. Under the examined conditions molecular oxygen was necessary to initiate the reaction. The preparation of Co(II), Fe(III) and Cr(III)-HPC samples and their composition under reaction conditions are also discussed, based on 31 P-NMR and UV-VIS absorption spectra.

Interaction of platinum and molybdophosphoric heteropoly acid under conditions of catalyst preparation for benzene oxidation to phenol with an O2-H2 gas mixture

The transformations of platinum and a heteropoly acid (HPA) in binary systems prepared from H2PtCl6 or H2PtCl4 and H3PMo12O40 were studied using IR and UV-VIS spectroscopy, elemental analysis, XPS, EXAFS, TPR, and HREM. The calcination of platinum chloride with the HPA to 450°C resulted in the formation of a platinum salt of the HPA along with decomposition products (mixture I). The reduction of calcined samples containing Pt: HPA = 1: 1 with hydrogen at 300°C (mixture II) followed by exposure to air resulted in the regeneration of the HPA structure. The resulting solid samples of Pt1−n0PtnIIClmOxHy) (H3+pPMo12−pVIMopVO40) (III) contained platinum and molybdenum in both oxidized and reduced states. The following association species were isolated from mixtures I and II by dissolving in water: [PtnIIPMo12O40] (Is) (n = 0.3−0.8) and [Ptn0PMo12red O40] (IIs) (n ≈ 1). Under exposure to air, the solutions of Is were stable (pH ∼2), whereas Ptmet was released from IIs. After the drying of Is, the solid association species (PtnIIClmOxHy). (H3PMo12O40), where n = 0.3−0.8, m = 0.2−1, and x = 3−0, (Isolid) were obtained. The Isolid/SiO2 supported samples were prepared by impregnating SiO2 with a solution of Is and drying at 100°C. Platinum metal particles of size ∼20 Å and a mixed-valence association species of platinum with the HPA were observed after the reduction of Isolid/SiO2 with hydrogen at 100–250°C. These samples were active in the gas-phase oxidation of benzene to phenol at 180°C with the use of an O2-H2-N2 mixture.

Palladium(II), Copper(II), Iron(III), and Vanadium(V) Complexes with Heteropolyanion PW9O9–34: 31P, 183W, 51V NMR and IR Spectroscopy Studies

The formation of Pd(II)-containing and mixed Pd(II),Cu(II), Pd(II),Fe(III), and Pd(II),V(V) complexes with heteropolyanion PW9O9–34was studied using 31P, 183W, 51V NMR, visible UV and IR spectroscopy, and the differentiating dissolution methods. In an aqueous solution and at optimal pH (3.7), the monometallic complexes [Pd3(PW9O34)2]12–and [Pd3(PW9O34)2PdnOxHy]q–(nav= 3), the bimetallic complexes [Pd2Cu(PW9O34)2]12–, [Pd2Fe(PW9O34)2]11–, and [PdFe2(PW9O34)2]10–, and a mixture of the [Pd3(PW9O34)2PdnOxHy]q–(nav≈ 10) + [(VO)3(PW9O34)2]9–complexes are formed. The title complexes were isolated from solution as Cs+solid salts belonging to the same [M3(PW9O34)2] structural type.