G. M. Maksimov

Titanium-Substituted Heteropolytungstates as Model Catalysts for Studying the Mechanisms of Selective Oxidation by Hydrogen Peroxide

The 31P NMR method shows that four forms of titanium(IV)-monosubstituted Keggin-type heteropolytungstate (Ti–HPA) exist in MeCN: the dimer (Bu4N)7[{PTiW11O39}2OH] (in the abbreviated form, (PW11Ti)2OH or H1), its conjugate base (PW11Ti)2O (1), and two monomers, PW11TiO (2) and PW11TiOH (H2). The ratio between the forms depends on the concentrations of H+and H2O. Dimer H1is produced from 2in MeCN when H+(1.5 mol) is added, and monomer H2is the key intermediate in this process. The catalytic activity of Ti–HPA in the oxidation of thioethers by H2O2correlates with their activity in peroxo complex formation and decreases in the order H2> H1> 2. The reaction of 2with H2O2in MeCN occurs slowly to form the inactive peroxo complex PW11TiO2(A). The addition of H2O2to H1and H2most likely results in the formation of the active hydroperoxo complex PW11TiOOH (B). Complexes Aand Btransform into each other when H+or OH–(1 mol) is added per 1 mol of Aor B, respectively. The activity of Btoward thioethers in the stoichiometric reaction is proven by 31PNMR and optical spectroscopy.

WO3/MO2 (M = Zr, Sn, Ti) heterogeneous acid catalysts: Synthesis, study, and use in cumene hydroperoxide decomposition

Thirty (5–40)% WO3/MO2 (M = Zr, Ti, Sn), heterogeneous acidic catalysts have been synthesized by two methods, specifically, via homogeneous acid solutions and from solutions brought to pH 9 with ammonia, both followed by calcination at 600–900°C. The catalysts have been characterized by IR spectroscopy and scanning electron microscopy, and their aqueous washings have been analyzed. Their acidity has been determined by the thermal analysis of samples containing adsorbed pyridine, and in terms of the proton affinity scale. Catalytic activities have been compared for cumene hydroperoxide (CHP) decomposition at 40°C in cumene and acetone. For all M, the catalysts are one type and contain W in strongly and weakly bound states, the latter being a polyoxometalate that can be washed off. Both tungstate phases are active in acid catalysis. Brønsted acid sites with a broad strength distribution have been found. The strongest of them are heteropolyacid protons. The catalysts 30% WO3/SnO2 and 20% WO3/ZrO2 (in acetone) and 10–20% WO3/TiO2 (in cumene) are the most active in CHP decomposition, and their activity is not related to their total acidity. Phases containing W6+ that form during the high-temperature synthesis are responsible for the high acidity, and additional protons that may appear owing to W6+ reduction can play only a minor role.

Preparation of polyoxometalate-stabilized colloidal solutions of palladium metal and catalysts supported on them

Sols containing Pd(0) clusters with polyoxo anions are prepared by the reduction of colloidal solutions of polyhydroxo complexes of Pd(II) in the presence of Mo(VI), W(VI), V(V), and Nb(V) polyoxo anions. The cluster sizes varied within the limits of 1–10 nm depending on the nature of the polyoxo anion. The stability of sols toward coagulation depends on the ratio between the palladium and polyoxo anion amounts in solution and on the composition of the solvent. Supported Pd catalysts are obtained by the adsorption of particles from sols; Pd can exist in these catalysts as individual particles or associates or form filamentary structures.

Thermal transformations of α-H6P2Mo18O62·nH2O heteropolyacid

According to the 31P NMR spectroscopy, heteropolyacid (HPA) H6P2Mo18O62·nH2O (P2Mo18), α-isomer of the Dawson structure, transforms upon heating above 80 °C partially (up to 30%) to γ-isomer, in which both polar groups Mo3O13 of the heteropolyanion are turned by 60° around the N3 axis, and partially to β-isomer in which only one group is turned. The β- and γ-isomers of P2Mo18 have been found for the first time. Their transformation into the α-isomer occurs upon rehydration in one week in air and in 1 h in an aqueous solution. HPA P2Mo18 decomposes on heating up to 350 °C to HPA H3PMo12O40 (PMo12) and a previously unknown phase of the HPMo6I21 composition, which in its turn decomposes at ∼375 °C to molybdenyl phosphates and IiI3. The PMo12 decomposition occurs via two routes to form the same products at temperatures of ∼400 and 450 °C with corresponding exotherms of IiI3 crystallization.

Acidity of heteropoly acids with various structures and compositions studied by IR spectroscopy of the pyridinium salts

The acidity on the “proton affinity” scale was determined by IR spectroscopy of the pyridinium salts for nineteen heteropoly acids of nine structural types (including two with the previously unknown structure) and one isopoly acid. All heteropoly acids exhibited a high acidity at the level of CF3SO3H and HClO4. H3PW12O40 was the strongest acid.

Acidity of Solutions of Heteropoly Acids with Various Structures and Compositions

Hammett acidity functions H0 of solutions of heteropoly acids H5PW11XO40 (X(IV) = Ti, Zr), H3PW12O40, H4SiW12O40, H6P2W21O71, and H21B3W39O132, as well as HClO4 and CF3SO3H, in water and 90% aqueous acetone and acetonitrile, are measured at 20°C by the indicator method. In aqueous solutions all acids under study have the same strength, and in organic solvents their acidities differ. A correlation between the catalytic activity and acidity of the solution is found for the condensation of acetone to mesityl oxide.

A study of complexation of chloral hydrate with heteropoly anions having various structures

Abstract1H NMR was applied to study the interaction of chloral hydrate in deuterionitrobenzene solution with tetrabutylammonium salts of the heteropoly acids (HPA) belonging to five structural types: Keggin (H3PW12O40, H3PMo12O40, H4SiW12O40), Dawson (α-H6P2W18O62, α-H6P2Mo18O62, α-H4S2Mo18O62), H6P2W21O71(H2O)3, H6As2W21O69(H2O), and H21B3W39O132. The surface of the HPA anions is nonuniform in acid-base properties. A general rule for all HPA was found, namely, that the HPA acidity increases with a decrease in the specific anion charge (per W or Mo atom).

A study of the acid properties of structurally and compositionally different heteropoly acids in acetic acid

The acid properties of heteropoly acids of the following three structure types were studied by conductometry in acetic acid: Keggin (H3PW12O40, H3PMo12O40, H4SiW12O40, H3PW11ThO39; and H5PW11XO40, where X(IV) = Ti or Zr), Dawson (α-H6P2W18O62and α-H6P2Mo18O62), and H6P2W21O71(H2O)3. These compounds are electrolytes that dissociate in only the first step of this solvent. The thermodynamic dissociation constants of the heteropoly acids were calculated by the Fuoss–Kraus method. The Hammett acidity functions H0of the solutions of H5PW11XO40, H3PW12O40, H4SiW12O40, and H6P2W21O71(H2O)3in 85% acetic acid at 25°C were determined by the indicator method. All of the test heteropoly acids were found to be strong acids.

Coupling of phenol with ketones in the presence of heteropoly acids with different structures and compositions

The reactions of phenol coupling with ketones MeCOR (R = CH3, C2H5, C3H7, and C4H9) are studied in the presence of heteropoly acids with different structures and compositions in toluene solutions ([PhOH]/[MeCOR] = (2–8)/1 mol/mol; 50–70°C) with thioglycolic acid added as a promoter. The reaction rate depends on ketone and heteropoly acid, and the yield of bisphenols is as high as 24–72%. The reaction orders are 0.68, 0.77, and 0.97 with respect to H6P2W21O71, H3PW12O40, and H4SiW12O40, respectively, and the activation energies are 25.1, 21.0, and 20.6 kcal/mol, respectively. Heteropoly acids of the Dawson structure exhibited the highest activity.

Propane chlorination over ruthenium oxychloride catalysts

The gas-phase chlorination of propane over different catalysts, including those containing ruthenium oxychlorides as the active component, has been investigated. The propylene and chlorine-containing product formation selectivities in propane chlorination at 150–450°C in a fixed-bed flow reactor have been determined.