Effect of the nature of the active-component precursor on the properties of Pt/MgAlO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> catalysts in propane and <Emphasis Type="Italic">n</Emphasis>-decane dehydrogenation

The Pt/MgAlO _x catalysts, in which a mixed aluminum–magnesium oxide obtained by the heat treatment of corresponding layered double hydroxides was used as a support, were studied. The effects of the nature of active-component precursors ([PtCl₆]^2–, [PtCl₄]^2–, and [Pt₃(CO)₆] ₆ ^2- ), and the Mg/Al ratio in the support on the anchoring of complexes, the disperse state of platinum, and the properties of the obtained catalysts in the reactions of propane and n-decane dehydrogenation were found.     

Chemical composition optimization and characterization of the NiO/B<Subscript>2</Subscript>O<Subscript>3</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> system as a catalyst for ethylene oligomerization

The samples of the NiO/B₂O₃-Al₂O₃ system with NiO contents from 0.48 to 38.30 wt % were synthesized by the impregnation of borate-containing alumina (20 wt % B₂O₃). It was found that nickel oxide occurred in an X-ray amorphous state in the samples containing to 23.20 wt % NiO. At a NiO content of 4.86 wt % or higher, the support was blocked by the modifier to cause a decrease in the specific surface area from 234 to 176 m²/g and in the amount of acid sites from 409–424 to 333 μmol/g. An extremal character of the dependence of catalyst activity in ethylene oligomerization on NiO content was found with a maximum in the range of 4.86–9.31 wt %. Based on spectroscopic data, it was found that ethylene activation on the NiO/B₂O₃-Al₂O₃ catalyst can be associated with the presence of Ni²⁺ cations, which chemically interact with the support. The catalyst containing 4.86 wt % NiO at 200°C, a pressure of 4 MPa, and an ethylene supply rate of 1.1 h⁻¹ provided almost complete ethylene conversion at the yield of liquid oligomerization products to 90.0 wt %; the total concentration of C⁸⁺ alkenes in these products was 89.0 wt %.     

Effect of the concentration of Pt(IV) and Pd(II) chloro complexes on the proportions of their ion-exchanged and coordinatively bound species on the γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> surface

The interaction of platinum(IV) and palladium(II) chloro complexes with the γ-Al₂O₃ surface in a wide range of surface metal concentrations is reported. Varying the concentration of the adsorbed metal complex on the alumina surface causes changes both in the proportions of weakly and strongly bound desorbable platinum species and in the proportions of desorbable (ion-exchanged) and nondesorbable (coordinatively bound) complexes. The adsorbed palladium complexes are more uniform in chemical composition and binding strength and consist largely of desorbable species removable from the surface by competitive sorption of anions. The absolute amount of coordinatively bound platinum and palladium species increases as the total metal content of the sample is raised to 1.0% and remains almost invariable at higher metal contents.     

In situ study of the interaction between <Emphasis Type="Italic">tert</Emphasis>-butyl chloride and aluminum activated with liquid In-Ga eutectic

The interaction between tert-butyl chloride and activated aluminum was studied by attenuated total reflectance Fourier transform infrared spectroscopy near room temperature (18–25°C). A long induction period of ∼240–260 min was observed. The ionic aluminum chloride complexes [Al _n Cl_3n+1]⁻ (n = 1, 2) and the molecular species AlCl₃ were identified at the activated aluminum/tert-butyl chloride interface during the reaction. The formation of the ion in the AlCl₄⁻ ion in the liquid medium and the presence of the same ion and a molecular AlCl₃-tert-butyl chloride complex in the resinous products of the reaction were confirmed by ²⁷Al NMR spectroscopy. The reaction products were analyzed qualitatively by GC/MS. The reactivities of activated aluminum and anhydrous aluminum chloride toward tert-butyl chloride under the same conditions were compared. A distinctive feature of the interaction activated aluminum and tert-butyl chloride is the dominant formation of the AlCl₄⁻ ion. By contrast, the interaction between aluminum chloride and tert-butyl chloride yields the polynuclear ion Al₂Cl₇⁻ and, likely, Al₃Cl₁₀⁻.     

Formation of platinum sites on layered double hydroxides type basic supports: I. Effect of the nature of the interlayer anion on the structure characteristics of the layered aluminum-magnesium hydroxide and the formation of an oxide phase

A layered aluminum-magnesium hydroxide of the hydrotalcite type containing interlayer carbonate counterions (HT-CO₃) and activated hydrotalcite containing interlayer OH⁻ ions (HT-OH) were studied for the subsequent use as the precursors of supports for platinum catalysts. It was found that the nature of an interlayer anion in the composition of an aluminum-magnesium layered hydroxide is an important factor affecting both the formation of the oxide support and its texture characteristics. The replacement of the interlayer CO₃²⁻ anion by OH⁻ resulted in changes in the structural parameters of the initial double hydroxide: a decrease in the interlayer distance with the retention of the Mg/Al ratio and an increase in the imperfection of the layered material. X-ray diffraction studies in the temperature range of 30–900°C showed that HT-OH is characterized by the ability to form low-temperature spinel at 375°C. As a result, two types of aluminum-magnesium oxide supports, which were characterized by different pore space organizations at the same Mg: Al ratio, were obtained from the given layered hydroxides.     

Mechanochemical synthesis of nanocrystalline nickel-molybdenum compounds,their morphology and application in catalysis: I. Effect of the Ni: Mo atomic ratio on the structure and properties of nickel-molybdenum compounds prepared under mechanochemical synthesis conditions

Mechanochemical activation in high-energy planetary activators was used for the preparation of highly dispersed nickel-molybdenum compounds. Nickel hydroxocarbonate [NiCO₃ · 2Ni(OH)₂ · nH₂O] and ammonium paramolybdate [(NH₄)₆Mo₇O₂₄ · 4H₂O] were chosen as starting compounds. The effect of the Ni: Mo atomic ratio on the composition and structure of products formed in the process of mechanochemical activation followed by calcination was studied. It was found that, at the Ni: Mo atomic ratios of 1.0 and 1.4, the mechanically activated product after calcination at 520°C contained 70–100% β-NiMoO₄, which is a stable phase at temperatures lower than 180°C.     

Liquid-phase oxidation of α-pinene with oxygen catalyzed by carbon-supported platinum metals

The liquid-phase oxidation of α-pinene with oxygen at 70–90°C is studied in the presence of Pd, Pt, Ru, Rh, and Ir supported on carbon. The conversion of α-pinene and the selectivity of formation of the main reaction products, namely, verbenol (1), verbenone (2), and α-pinene oxide (3), depends on the nature of the metal, on its oxidation state and extent of dispersion, and on the admixtures introduced into the system. In the presence of the Pt catalysts and promoting admixtures of tetrahexylammonium chloride (Hex₄NCl), the selectivity of formation of the most valuable oxidation products (1 + 2) reaches 50% at an α-pinene conversion of 20–30%. The fraction of resinlike oxidation products decreases in the presence of the catalysts. The results obtained are discussed in the framework of the radical mechanism of α-pinene oxidation.     

Preparation of the Pd/C catalysts: A molecular-level study of active site formation

This review summarizes the results of molecular-level studies on the mechanism of Pd/C catalyst formation from the PdCl₂ precursor. Two processes occur in acidic media during the contact of H₂PdCl₄ with carbon: (a) adsorption of palladium chloride to form surface complexes and (b) redox interaction between PdCl₂ and carbon with the formation of palladium metal particles. The ratio between these adsorbed palladium species depends on the conditions of adsorption and especially on the size of carbon support grains and the oxidative atmosphere. The observations are explained by the fact that carbon support exhibits electrochemical and ligand properties. X-ray diffraction, X-ray scattering, XPS, and high-resolution electron microscopy revealed that the nanostructure of carbon materials, in particular the extent of their three-dimensional ordering, is crucial for the ligand properties. The presence of two forms, metallic and ionic, of sorbed palladium determines the bimodal size distribution of the metal. After the reduction of ionic species, metal particles are “blocked” with support. The nature of the ionic forms of palladium (mostly (PdCl₂)_n) clusters chemically and epitaxially bound to the carbon surface suggests the mechanisms of the bimodal distribution of the supported metal particles on the surface and the methods for the control of the ratio between “blocked,” low-dispersed, and highly-dispersed particles in the catalyst. One of these methods is the use of palladium polynuclear hydroxo complexes (PHCs) with low oxidation potentials as starting compounds for catalysts preparation. The data on the PHC structure in a solution and its change upon the adsorption of PHC on the surface of the carbon material obtained by the¹⁷O,²³Na,¹³³Cs, and³⁵Cl NMR techniques are discussed. PHCs are shown to be a clew of the [Pd(OH)₂]_n polymeric filament, whose fractions are bound with alkali metal ions. When PHC is adsorbed on the surface of the carbon support and then dried, palladium oxide is formed from which highly dispersed metal particles are formed during reduction. The nature of alkali metal ions in PHC affects the activity of the Pd/C catalyst. An important role of the ligand, electrochemical, and lyophilic properties of carbon material during the formation of the species of the active catalyst component is discussed.     

Kinetic study of liquid-phase hydrodechlorination of hexachlorobenzene on Ni/C and 2%PdNi/C

Liquid-phase hydrodechlorination of hexachlorobenzene was kinetically studied in the presence of both nickel (Ni/C) and palladium-promoted nickel (2%PdNi/C) catalysts under different reaction conditions. Molecular hydrogen (at 1 and 20 atm) and sodium borohydride (NaBH₄) were used as reducing agents. In the presence of the nickel catalyst, the hydrodechlorination of C₆Cl₆ occurs via a consecutive mechanism (removal of one chlorine atom from the substrate at each stage), whereas with the 2%PdNi/C catalyst, the transformation of C₆Cl₆ occurs via both consecutive and multiplet mechanism (with the elimination of several chlorine atoms without desorption of the chloroaromatic substrate from the catalyst surface). The promotion of the nickel catalysts with palladium substantially changes the selectivity of formation of intermediate products of C₆C1₆ dechlorination. The mechanism of hydrodechlorination of hexachlorobenzene was suggested that explained the presence of only certain products of partial dechlorination of hexachlorobenzene in the reaction medium.     

Effect of the reduction conditions of the supported palladium precursor on the activity of Pd/C catalysts in hydrogenation of sodium 2,4,6-trinitrobenzoate

The study addresses the effect of the reduction conditions of palladium polynuclear hydroxo complexes (PHC) supported on the Sibunit carbon material on the dispersion of the metal particles and the activity of 0.5%Pd/Sibunit catalysts in the selective hydrogenation of sodium 2,4,6-trinitrobenzoate to 1,3,5-triaminobenzene in an aqueous solution (temperature of 323 or 343 K, pressure of 0.5 MPa). The palladium PHC were reduced using the most common methods pertaining to catalyst preparation: liquid-phase reduction with sodium formate and reduction in a hydrogen flow at elevated temperature. It was found that high-temperature reduction in the gas phase gives rise to Pd particles with a markedly lower dispersion compared with the sample obtained under mild liquid-phase reduction conditions. The catalytic activity of the sample containing large Pd particles proved to be higher than the activity of the catalyst obtained by reduction with sodium formate.