Home – Home – Jornals – Chemistry for Sustainable Development 2013 number 1

We discuss the tasks and methodological problems of molecular absorption spectrometry as a method for the analysis of unseparated mixtures of organic compounds (petroleum products, pharmaceuticals, and others), which method in some cases can successfully compete with chromatography. Using the chemometric algorithms, in particular the method of projection to latent structures, allows successfully solving three major problems: to eliminate the influence of inter–reference spectral overlapping, to take into account a non-additive character of absorption, to level the detection sensitivity with respect to similar analytes. A brief review of relevant studies carried out within recent years in the Omsk State University is reported.

Despite the abundance of available experimental techniques, the X-ray absorption spectroscopy often remains the only acceptable means for solving a number of problems: studying X-ray amorphous substances, determining the phase composition at a lower concentration limit for impurity samples, determining the valence state of a chemical element in the case of a low content thereof in the sample (mass percent quota). The present review reports the theory and examples of successful applying the methods of EXAFS and XANES to solve complicated problems.

Results are reported on studying the interaction of Pt (IV) anionic chloride complexes with layered magnesium-aluminum hydroxides in the course of synthesizing Pt/MgAlO_x catalysts. As the oxide precursors there were layered hydroxides used with different nature of interlayer anions (and ОН^–) and with Mg/Al ratio ranging within 2–4. The nature of an interlayer of anion is an important parameter affecting the amount of fixed platinum, the composition of surface species and the sites of location thereof in the structure of layered material. In the case of using a hydroxide carrier with the carbonate counterion the fixation of platinum occurs with the participation of the OH groups of hydroxide layers, via adsorbing the hydrolyzed forms of the complexes on the surface. The reduction thereof leads to the formation of isometric platinum particles of 2–4 nm in size. When attaching the platinum complexes to aluminum-magnesium hydroxide with interlayer anions ОН^–, the precursor of the active component is mainly located in the interlayer space, with further forming the particles of flat morphology 50 nm long and the thickness close to the interlayer distance in the layered carrier (0.75 nm). In the case of varying the Mg/Al ratio, an increase of triply charged aluminum cation causes changing the structural characteristics and adsorption properties of aluminum-magnesium hydroxide to result in the formation of an oxide phase with a more developed surface area and higher pore volume. Increasing the positive charge density and enhancing the electrostatic interaction promote the adsorption of a greater amount of platinum (IV) anionic complexes and a stronger interaction thereof with the hydroxide layers. In the case of changing the Mg/Al ratio from 4 to 2, one can observe increasing the dispersity level of the supported platinum particles and increasing the activity of Pt/MgAlO_x catalysts in the propane dehydrogenation reaction.

Zirconium sulphate catalysts applied onto SiO₂ and Al₂O₃ were prepared and investigated. Phase composition inherent in the catalysts, acidic properties according to the adsorption of CO and textural characteristics thereof were investigated. Catalytic activity level in the isomerization reaction of n–hexane was determined. It has been demonstrated that the most active catalytic system is presented by a catalyst based on Al₂O₃ carrier. The yield of hexane isomers wit the use of this catalyst amounts up to 75 mass % within the temperature range of 200–220 °C. It has been found that in the case of using alumina as a carrier the catalysts exhibit forming strong acid centers with the absorption bands of adsorbed CO being at 2210 and 2224 cm^–1.

Equilibrium was investigated concerning the sorption of palladium from the H₂PdCl₄ and Pd(NH₃)₄Cl₂ solutions onto B₂O₃–Al₂O₃ carrier. It is demonstrated that the sorption equilibrium is described by Langmuir equation. According the results of testing the Pd/B₂O₃–Al₂O₃ samples in the course of oligomerization it has been found that the optimum content of palladium in the catalyst should be equal to 0.5 mass %, whereas the optimum precursor of palladium should be presented by palladium chloride complex. The catalytic properties of Pd–Re₂O₇/B₂O₃–Al₂O₃ system in the one-stage propylene synthesis from ethylene were studied. It has been demonstrated that it is possible to obtain propylene with the yield equal to 77–83 % with respect to the theoretically determined one.

The effect of chemical composition on the formation of the crystal structure and texture characteristics of the oxide system B₂O₃–Al₂O₃ obtained through impregnation of pseudoboehmite with the solutions of ortho-boric acid was studied. It was shown that the modification with boron oxide slows down the crystallization of aluminium oxide. According to the high-resolution TEM data, borate-containing aluminium oxides are composed of amorphous and crystalline primary particles. It was established by means of ²⁷Al MAS NMR and ¹¹В MAS NMR that aluminium atoms in the samples occur in octahedral, tetrahedral and penta-coordination, while boron atoms occur in trigonal and tetrahedral coordinations; their relative content depends on chemical composition. The dependence of the specific surface of B₂O₃–Al₂O₃ system on its composition has an extremal character; the maximum is achieved with B₂O₃ content 5 mass %.

TThe structure of oxides obtained by the calcination of layered Mg–Al and Ni–Al (Mg/Al = Ni/Al = 2 : 1) double hydroxides with the structure of hydrotalcite. It is demonstrated that the formation mechanisms for the oxides are different: the Mg–Al oxide retains the original cation ratio, whereas the structure of the oxide formed represents a kind of layered defective spinel. Calcining the Ni–Al hydroxide results in the fact that the majority of Al³⁺ ions reaches the surface of the oxide particles, thereby forming aluminum hydroxide with retaining the nickel oxide phase in the course of hydration. It is established that, depending on the number of aluminum atoms in the bulk of the particles there are different structures of oxides formed, those determine the oxide ability either of restoring the layered oxide structure in the case of Mg–Al hydrotalcite, or saving the structure Ni–Al oxide in the course of hydration.

Studies concerning supported aluminum-palladium catalysts obtained via surface self-propagating thermal synthesis (SSTS) for the selective liquid-phase hydrogenation of acetylene into ethylene in the presence of CO were performed. According to the results of catalytic testing, in the case of running the reaction at 60 °C the SSTS catalysts are inferior to the samples prepared in a conventional manner. At a temperature of 90 °C, the catalyst prepared via SSTS with using citric acid as a fuel additive is almost similar to the sample prepared via traditional method, in the catalytic properties thereof. According to the results obtained by EXAFS and XPS, the original SSTS catalysts contain palladium in oxidized and reduced forms, whose ratio depends on the nature and content of the fuel additive. It has been found that palladium oxide in SSTS catalysts can be quickly reduced under the reaction conditions to produce zero-valent Pd, which is indicated by the results of studying the SSTS catalysts before and after the reaction by means of EXAFS.

The effect of reaction conditions (methane concentration, flow rate, catalyst position, gas dynamic mode) on methane pyrolysis within temperature range 700–1200 °C on preliminarily annealed Fechral wire heated with electric current, at a ratio СН₄/О₂ = 15 : 1 was investigated. It was shown that independently of conditions the process runs in two temperature regions differing in the selectivity of the formation of major products – С₂–hydrocarbons. Reaction conditions have a substantial effect on methane conversion and selectivity with respect to reaction products. The maximal selectivity with respect to С₂ products can be achieved with the transversal coil position to the gas flow and with the use of flow-circulation set-up instead of flow one. An increase in methane concentration in the initial mixture has ambiguous effect on the selectivity with respect to С₂ hydrocarbons within different temperature ranges: for temperature not hgiher than 1000 °C, this causes an increase in selectivity with respect to ethylene and acetylene, while at 1000 °С and above this causes a decrease in selectivity with respect to these components.

Catalysts of liquid-phase hydrogenation of acetylene deposited on the oxides of aluminium, gallium, and indium were studied. According to the results of catalytic tests and examination using the physical methods, decoration of the active component by partially reduced support is observed in Pd/Ga₂O₃ and Pd/In₂O₃ catalysts. An increase in the atomic catalytic activity for the samples deposited on gallium and indium oxides can be connected with the change of the electron density of the active component or with the formation of bimetallic phases.

The structural features and morphology of the particles (globules) of technical carbon (TC) samples, both initial and those treated with pulsed laser radiation, were studied by means of high-resolution transmission electron microscopy. Measurements were carried out; the average distances between graphene layers (<i>d</i>₀₀₂), length and curvature of graphene layers in TC globules were calculated. Results of the investigation of structural transformations in TC samples irradiated with nanosecond laser pulses are presented. Changes of the average diameter of globules in TC samples determine the essential difference in the morphology of irradiated nanoparticles and in the lattice parameter <i>d</i>₀₀₂.

The possibility to use available and easily synthesized borate-containing aluminium oxide as an acidic support for the catalyst of hydroisomerization of benzene-containing gasoline fractions was demonstrated for the first time. The phase composition, texture characteristics and acid properties of B₂O₃–Al₂O₃ systems with boron oxide content 0.9–39.0 mass %, obtained by mixing pseudo-boehmite with solid ortho–boric acid followed by annealing at a temperature of 600 °C were studied. For hydroisomerization of the model mixture of n-heptane with benzene it was shown that the most efficient sample is Pt/B₂O₃–Al₂O₃ with the support containing 27.5 mass % of boron oxide. This catalyst provides almost complete benzene hydrogenation Данный and the maximal yield of methylcyclopentane (12.1 mass %) and isoheptanes (43.5 mass %) at a temperature of 350 °C. Higher selectivity of isomerization reactions in this case is connected with the smaller amount of strong acid centres on the catalyst surface.