R. A. Buyanov

Catalysts and Processes for Paraffin and Olefin Dehydrogenation1

General trends in the development of industrial processes and catalysts for the dehydrogenation of lower C3–C5 paraffins and olefins are considered. A brief review of studies on the improvement of commercial nickel–calcium phosphate and iron oxide catalysts for the dehydrogenation of olefins and ethylbenzene to dienes and styrene, respectively, performed in the Laboratory of Dehydrogenation at the Boreskov Institute of Catalysis (Siberian Division, Russian Academy of Sciences) in collaboration with OAO NPO Yarsintez is given. The results of studies on the development of new spinel-supported bimetallic Pt–Sn catalysts for the steam dehydrogenation of lower paraffins are presented.

Decomposition of Chlorinated Hydrocarbons on Iron-Group Metals

The decomposition of 1,2-dichloroethane and chlorobenzene on nickel–alumina, cobalt–alumina, and iron–alumina catalysts at 400–600°C was studied. Thermodynamic calculations demonstrated that the susceptibility of metals to chlorination under exposure to HCl increases in the order Ni < Co < Fe. The addition of hydrogen to the reaction mixture was found to dramatically decrease the rate of carbon deposition in the decomposition of 1,2-dichloroethane because of the intense hydrogenation of intermediates that are graphite precursors. Two fundamentally different reaction paths were found in the degradation of 1,2-dichloroethane: decomposition via a carbide-cycle mechanism with the formation of carbon as the main product (under conditions of a deficiency of hydrogen) and 1,2-dichloroethane hydrodechlorination accompanied by methanation of the formed carbon (under conditions of an excess of hydrogen). The degradation of chlorobenzene diluted with hydrogen in a molar ratio of 1 : 50 was not accompanied by carbon formation on the catalyst. A comparison between the selectivity for reaction products on nickel–alumina and cobalt–alumina catalysts indicated that the former catalyst is more active in the rupture of C–C bonds and in the methanation reaction of deposited carbon, whereas the latter is more favorable for hydrodechlorination. The optimum conditions and thermal regime for efficient and stable operation of the catalysts were found.

The Relationship between the State of Active Species in a Ni/Al2O3Catalyst and the Mechanism of Growth of Filamentous Carbon

The formation of active particles and their changes in the course of 1,3-butadiene decomposition on a Ni/Al2O3catalyst at temperatures from 400 to 800°C were studied by high-resolution electron microscopy. It was found that carbon filaments of different types were formed at 400–800°C. The growth of thin filaments (20–30 nm in diameter) takes place at 400–600°C on a conical Ni particle located at the growing end of the filament, whereas di-symmetrical filaments 50–100 nm in diameter grow on biconical metal particles. As the carbonization temperature was increased to 700–800°C, graphite nanotubes 5–20 nm in diameter were formed. It was found that the mechanism of formation and the structure of filaments are related to the state of catalytically active species, which consist of a solid solution of carbon in the metal. It is suggested that the metastable surface nickel carbide Ni3C1 – xis an intermediate compound in the catalytic formation of graphite filaments from 1,3-butadiene. Upon termination of the reaction, the metastable Ni3C1 – xmicrophase is decomposed with the formation of hexagonal nickel microinclusions. The role of epitaxy in the nucleation and growth of a graphite phase on the metal is discussed. Models are presented for the growth of structurally different carbon filaments depending on the formation of active metal species at various temperatures. Considerable changes in the structure of carbon and the formation of nanotubes at 700–800°C are related to the appearance of a viscous-flow state of metal–carbon particles.

Morphology of carbon from methane on nickel-containing catalysts

Abstract High-resolution electron microscopy (HREM) and X-ray techniques were used to study the reasons for the various morphology and structure formations of filamentous carbon from methane on Ni-containing catalysts.

Catalytic properties of massive iron-subgroup metals in dichloroethane decomposition into carbon products

The formation of nanocarbon materials on massive nickel, nichrome, and some other alloys via the carbide cycle mechanism is reported using 1,2-dichloroethane decomposition as an example. The role of the physical stage of the carbide cycle is elucidated, and massive metal surface activation methods ensuring the realization of this stage are considered. The surface layer of massive nickel or some nickel alloys is most effectively activated by the action of chlorine resulting from the catalytic decomposition of 1,2-dichloroethane. It has been demonstrated by ferromagnetic resonance (FMR) spectroscopy that the activation of the massive metal surface in 1,2-dichloroethane decomposition to nanocarbon is due to the surface undergoing crystal chemical restructuring. The microstructuring of the surface yields fine Ni particles similar in size (0.2–0.3 μm) and shape, whose FMR spectra are anisotropic and have similar magnetic resonance parameters. Both chlorine-free and chlorinated hydrocarbons decompose over these particles via the carbide cycle mechanism. It is demonstrated that it is possible to design catalytic reactors packed with massive nickel or its alloy. The nanocarbon material obtained in such a reactor will not be contaminated by components of conventional catalyst supports (Al, Mg, etc.). The stable performance temperature of the catalyst will be increased, and this will allow the equilibrium outlet methane concentration to be reduced.

Catalytic Synthesis of Nanosized Feathery Carbon Structures via the Carbide Cycle Mechanism

The morphology of carbon nanostructures obtained by 1,2-dichloroethane decomposition on the 90% Ni/Al2O3 catalyst under different reaction conditions was studied by high-resolution transmission electron microscopy. A new carbon product was discovered, which received the name of feathery carbon. The product has an extremely loose disordered structure consisting of separate fragments of a graphite-like phase. The structural disordering is assumed to be caused by the variation of chlorohydrocarbon decomposition conditions on the frontal face of the metal particle. This changes the character of carbon atom diffusion from the frontal face to the backside face of the nickel particles and finally results in a feathery morphology of the carbon phase. The specific surface area of feathery carbon is 300–400 m2/g.

Current Trends in the Improvement and Development of Catalyst Preparation Methods

The state of the art in the scientific foundations of catalyst preparation is analyzed. New lines and trends in the development of conventional catalyst preparation methods that have appeared in the last 10–15 years are discussed. The theoretical and experimental foundations of the syntheses of porous materials by the sol-gel processing of alkoxides are considered. The synthesis of fine MgO aerogel, which is a unique destructive sorbent and catalyst, is described as one of the numerous examples of the use of this method in combination with supercritical drying. The synthesis of complex oxide and supported metallic catalysts by the sol-gel method is analyzed. Some new approaches to active catalyst deposition are considered, including the deposition-precipitation method. Unconventional methods of catalyst preparation are classified. Tasks are formulated for the development of the scientific basis of these methods.

Scientific grounds for the application of mechanochemistry to catalyst preparation

It is shown that mechanochemical activation is efficient in creating waste-free energy-saving methods for the preparation of hydride catalysts, heteropoly acid catalysts, and catalysts for hydrocarbon decomposition into hydrogen and carbon materials, as well for the syntheses of earlier unknown catalytic systems. The capabilities of the mechanochemical methods are demonstrated on modifying the catalytic properties of catalysts and supports: an increase in the strength and catalytic activity, sorption properties, etc. Adhesion theory applied to melts helps to describe the mechanism of mechanochemical synthesis of catalytic systems.

Processing of organochlorine waste components on bulk metal catalysts

A method for destroying chloroorganic waste components on catalysts, particularly bulk metal nickel (99.99%), nichrome (80% Ni and 20% Cr), and chromel (90% Ni and 10% Cr) is proposed. The process is accompanied by the formation of carbon nanofibers (CNFs) with feathery morphology. Catalytic destruction of 1,2-dichloroethane on bulk nickel catalysts is characterized by a long induction period (∼3 h) with spontaneous activation of the alloy’s surface. Preactivation of the catalyst with acids or by alternative treatment in oxidizing and reducing environments shortens the induction period by one order of magnitude. The state of the surface before and after activation is studied by SEM, TEM, and EDX. The activity of catalysts is determined for the decomposition of 1,2-dichloroethane at temperatures of 500 to 700°C. Nichrome exhibited the greatest activity (yield of CNFs, 400 g/g of catalyst); the yield of CNFs on catalysts prepared by coprecipitation and mechanical activation was considerably lower. The proposed approach combines organochlorine waste disposal with the production of a useful product (CNFs). The use of bulk metal catalysts is promising since it simplifies the technology for their preparation, and the absence of carriers makes it easy to cleanse CNFs of impurities of catalyst fragments.

Carbon erosion of hardware made of iron subgroup metals and their alloys

A paper published in 1952 described a previously unknown phenomenon of the formation of carbon tubes ~0.03–0.50 µm in diameter in carbon monoxide decomposition on dispersed iron particles at 873 K [1]. At that time, this paper almost completely eluded the attention of scientists. The interest in this phe� nomenon abruptly increased only in 1970–1980s. These studies became particularly important in the last decade in the context of the development of nanotech� nology and synthesis of nanomaterials. The nanosized carbon products obtained by the present day have a variety of morphological features and are a class of