Ammoxidation of methylpyrazine over phosphorus-modified vanadium-titanium catalysts

Vanadium-titanium catalysts modified with phosphorus additives (1-15 wt.% of P₂O₅) have been studied in methylpyrazine ammoxidation. Introduction of high amounts of the additive (≥ 10 wt.% of P₂O₅) results in a decrease in activity and selectivity of the catalysts due to formation of low active phase - a ternary compound with the component ratio V : P : Ti ≈ 1 : 1 : 1 with only one type of slightly distorted tetrahedral vanadium (5+) bound via oxygen to phosphorus (5+). The nature of the active sites of the samples modified with 1-5 wt.% of P₂O₅, similarly to that in the V-Ti-O catalyst, was found to include the V⁵⁺ cations strongly bound with TiO₂ and located in the significantly distorted octahedral oxygen environment. This revised version was published online in June 2006 with corrections to the Cover Date.     

Methylpyrazine Ammoxidation over Binary Oxide Systems: V. Effect of Phosphorus Additives on the Physicochemical and Catalytic Properties of a Vanadium–Titanium Catalyst in Methylpyrazine Ammoxidation

Oxide vanadium–titanium catalysts modified by phosphorus additives (20V₂O₅–(80 –n)TiO₂–nP₂O₅, n = 1, 3, 5, 10, and 15 wt %) are studied in methylpyrazine ammoxidation. Two regions of compositions are found corresponding to radically different catalytic properties, namely, catalysts with a low (5 wt % P₂O₅) and high (10 wt % P₂O₅) concentration of the additive. In the first case, the introduction of phosphorus is accompanied by a gradual increase in the activity. In the second case, an increase in the additive concentration results in a decrease in the activity and selectivity to the target product, pyrazineamide, and a simultaneous increase in the selectivities to by-products, pyrazine and carbon oxides. The catalysts are characterized by X-ray diffraction analysis, differential dissolution, IR, and NMR spectroscopic data. As in the binary system, the active sites of the samples with a low concentration of phosphorus contain V⁵⁺ cations in a strongly distorted octahedral oxygen environment, which are strongly bound to a support due to the formation of V–O–Ti bonds. The catalytic properties of the samples containing 10 wt % P₂O₅ are due to the presence of the phase of a triple V–P–Ti compound with an atomic ratio V : P : Ti approximately equal to 1 : 1 : 1. The V⁵⁺ cations in this compound occur in a weakly distorted tetrahedral oxygen environment and are bound to the tetrahedral P⁵⁺ cations.     

Analytical Chemistry of Phases of Variable Composition and the Principles of Stoichiographic Determination of Such Phases in Multielement Multiphase Samples

The general problems of determining the stoichiometry and content of solid phases of variable composition in multielement multiphase substances are discussed. The possibilities and conditions of application of a reference-free stoichiographic method based on the differential dissolution (DD) of components for the analysis of samples containing spatially homogeneous and inhomogeneous phases are considered. The results of mathematical modeling of the DD process for a mixture of phases of constant and variable composition are presented. These data substantiate the use of the stoichiogram affinity criterion for the identification of spatially inhomogeneous phases of variable composition. Examples of applying the DD method to the phase analysis of various substances containing phases of variable composition are given.     

Determination of the chemical composition of fiberglass silicate materials by the differential dissolution method

The standardless method of differential dissolution (DD) was used for determining the chemical composition of aluminosilicate and zirconium silicate glass-fiber clothes at different stages of their preparation and modification. Conditions for the detection, identification, and quantitative determination of various forms of heterogeneity in the elemental, phase, and surface compositions of these materials are considered. The distribution of the elements that constitute the glass-fiber clothes between various forms—surface ion-exchange (Na), hydrated (Al, Si), and framework (Al, Si) species—was quantitatively determined for the first time.     

Formation of vanadia-titania oxide catalysts

The formation of vanadia-titania catalysts was studied with a complex of physicochemical methods. The use of highly dispersed anatase with a defect structure results in the formation of coherent boundaries of coalescence of the V₂O₅ and TiO₂ crystallites with the ratio V : Ti =1 : 1 in a wide range of vanadium and titanium concentrations. The catalysts containing coherent boundaries are active and selective in Β-picoline oxidation to nicotinic acid.     

In situ XRD study of nanocrystalline cobalt oxide reduction

The reduction of nanocrystalline cobalt oxide samples (single-phase and supported on γ-Al₂O₃) was studied using in situ X-ray diffraction (XRD) analysis. The atomic structures of single-phase and supported Co₃O₄ samples were refined, and the occurrence of cationic vacancies was demonstrated. A set of methods (XRD, temperature-programmed reduction, and differential dissolution) was used to find that the reduction of supported and unsupported model cobalt oxide was considerably different. The single-phase sample was reduced in undiluted hydrogen to cobalt metal with a hexagonal closely packed structure. The reduction of the supported sample (unlike the single-phase sample) occurred through the formation of a crystalline CoO phase to the formation of cobalt metal with a face-centered cubic structure. Interaction of cobalt oxide with the γ-Al₂O₃ support, which hinders the reduction to cobalt metal, was detected.     

Oxidative ammonolysis of methylpyrazine over binary catalytic systems: III. Phosphorus-molybdenum system: Catalytic properties and the active component

The phase composition of the binary phosphorus-molybdenum system with a Mo/P ratio of 3-24.5 and its catalytic properties in the reaction of oxidative ammonolysis of methylpyrazine are studied. X-ray amorphous phases of molybdenyl phosphate and phosphorus modified molybdenum trioxide are active in the formation of pyrazinonitrilee.     

Ammoxidation of methylpyrazine over vanadium-titanium catalysts modified by alkali additives

Vanadium-titanium catalysts modified with sodium or potassium additives (1-15 wt.% of Me₂O) have been studied in methylpyrazine ammoxidation. Introduction of the additives results in a decrease in the activity and selectivity of the catalysts due to formation of low-active phase - bronzes (MeV₆O₁₅) and vanadates (α-NaVO₃, KVO₃ and K₃V₅O₁₄). The active sites of the modified samples, similar to those in the V-Ti-O catalyst, are found to be V⁵⁺ cations strongly bound to TiO₂ and located in a significantly distorted octahedral oxygen environment. This revised version was published online in August 2006 with corrections to the Cover Date.     

1A1⇔5T2 Spin Transition in Heterometallic Solid Phases FexNi1-x(Htrz)3(NO3)2 · H2O (Htrz = 1,2,4‐Triazole)

Solid phases Fe_xNi_1-x(Htrz)₃(NO₃)₂ · H₂O (0.4 x 0.8$) and Ni(Htrz)₃(NO₃)₂ · H₂O were synthesized and studied. The phases were studied by means of magnetochemistry, powder Xray difraction analysis, and electronic and IR spectroscopy. The heterometallic phases are described by the stoichiographic method of differentiating dissolution (DD). The values of x were determined by two methods — atomic absorption and DD. Magnetochemical data showed that the solid phases exhibit a hightemperature ¹A₁ ⁵T₂ 0.5 x 0.8 and disappears at x = 0.4. The spin transition is accompanied by thermochromism (color changed from pink to white at 0.6 x 1 and from pink to light lilac at x = 0.5). A decrease in x leads to a decrease in the temperature of the forward (under heating T_c ) and reverse (under cooling T_c ) transitions, a decrease in hysteresis value ( T_c), and a smearing of the spin transition.     

Monolithic FeO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts for ammonia oxidation and nitrous oxide decomposition

(1.2–8.3)%FeO_х/Al₂O₃ monolith catalysts have been prepared by impregnating alumina with aqueous solutions of iron(III) nitrate and oxalate and have been tested in NH₃ oxidation and in the selective decomposition of N₂O in mixtures resulting from ammonia oxidation over a Pt–Rh gauze pack under conditions of nitric acid synthesis (800–900°C). In the case of the support calcined at 1200°C, the catalyst is dominated by bulk Fe₂O₃ particles localized on the Al₂O₃ surface. The activity of these samples in both reactions decreases with a decreasing active component content, thus limiting the potential of Fe₂(C₂O₄)₃ · 5H₂O, an environmentally friendlier but poorly soluble compound, as a substitute for Fe(NO₃)₃ · 9H₂O. Decreasing the support calcination temperature to 1000°C or below leads to the formation of a highly defective Fe–Al–O solid solution in the (1.2–2.7)%FeO_х/Al₂O₃ catalysts. The surface layers of the solid solution are enriched with iron ions or stabilize ultrafine FeO_х particles. The catalytic activity of these samples in both reactions is close to the activities measured for ~8%FeO_х/Al₂O₃ samples prepared using iron nitrate.