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Among the known synthesis procedures and reagents for unsupported Ni–Mo–W catalysts, there is no consensus about optimal preparation conditions of their precursors. In the present work, Ni–Mo–W precursors were prepared via three preparation techniques—hydrothermal synthesis, precipitation method and spray drying—after the synthesis of complex compounds in solution. Ni–Mo–W precursors were studied by the XRD analysis, SEM methods, Raman and UV-vis spectroscopies and XPS measurements and used for the hydrotreatment of straight-run gasoil. Precursors prepared by hydrothermal synthesis contain particles with stacked plate shapes, while other methods provide spherical particles. The formation of different amounts of individual molybdates, tungstates or mixed phases such as W1−xMoxO3 possibly doped by Ni was detected. The precipitation technique results in the formation of spheres, with W located at the center and is unavailable for catalysis. The catalytic activity increased when all active metals are available for the feedstock, and a more mixed phase containing Ni, Mo and W is formed. This mixed phase is realized when the synthesis of the Ni–Mo–W precursors is carried out in solution followed by spray drying. The resulting catalyst has 1.2–4 times higher activity than catalysts prepared by other methods. [ABSTRACT FROM AUTHOR]

Valence and coordination states of molybdenum ions formed upon thermal treatment of Mo/H-ZSM-5 catalyst for methane dehydroaromatization in Ar and Ar/CH4 media at 573-973 K have been studied by ESR and UV-VIS spectroscopy. For comparison, the characteristic ESR spectra of thermolyzed bulk ammonium heptamolybdate have been studied and analyzed in detail. The nature of earlier observed Mo5+ ions has been verified, and new paramagnetic states of molybdenum in Mo/H-ZSM-5 catalysts have been detected: Mo3+ ions, and Mo5+ ions in tetrahedral coordination with delocalization of unpaired electron to Al and H or Al and N atoms. [ABSTRACT FROM AUTHOR]

The effect of alumina structure (γ‐, η‐, χ‐Al2O3) on the long‐term stability of industrial‐like Cr2O3/Al2O3 dehydrogenation catalysts under industrial dehydrogenation conditions is studied. It is shown that the type of alumina support determines physicochemical and catalytic stability of the catalyst: η‐Al2O3 is the most stable against irreversible deactivation, whereas χ‐Al2O3 is the least stable. One of the possible reasons of predominant stability of η‐Al2O3‐based catalyst is its relatively high sintering stability under real plant conditions. High‐temperature (>800 °C) calcination, sometimes used to compare stabilities of chromia/alumina catalysts, appears to be unable to simulate industrial aging because of the inconsistency of the phase composition of industrially and artificially aged catalysts. [ABSTRACT FROM AUTHOR]

Laser vaporized xCr/Al 2 O 3 nanopowders with the particle size of ca. 15 nm were studied by XRF, EDX, HRTEM, XRD, DD, BET, TGA, UV–Vis DRS, Raman and PL spectroscopy methods as a catalytic system in dehydrogenation of isobutane. The nanopowders were found to contain not only the Cr6+ ions located on the surface of nanoparticles, but also two different types of Cr3+ sites as components of the γ-Al 2 O 3 solid solution. One type consists of the bulk Cr3+ b sites in a strong crystal field. Another type is represented by the Cr3+ s sites residing near the surface of xCr/Al 2 O 3 nanoparticles. Such sites are in a weak crystal field. It was shown that catalytic activity of the nanostructured xCr/Al 2 O 3 system is provided by the high content of surface Cr3+ s sites. The results obtained make it possible to propose the nanopowders obtained by laser synthesis as a promising catalytic system that can be used in the development of highly active catalysts for dehydrogenation of alkanes with the decreased chromium content. Unlabelled Image • Model CrO x /Al 2 O 3 nanoparticle catalysts were obtained by laser vaporization • Structure, morphology and optical properties of CrO x /Al 2 O 3 nanopowders were studied • Cr3+ s sites are located near the surface of CrO x /Al 2 O 3 nanoparticles • Cr3+ s sites play the key role in the isobutane dehydrogenation reaction • CrO x /Al 2 O 3 nanoparticles are highly active catalysts with the decreased Cr content [ABSTRACT FROM AUTHOR]