Influence of the composition and morphology of nanosized transition metal sulfides prepared using the Anderson-type heteropoly compounds [X(OH)6Mo6O18]n? (X = Co, Ni, Mn, Zn) and [Co2Mo10O38H4]6? on their catalytic properties

Using the Anderson-type heteropoly compounds (HPCs) [X(OH)₆Mo₆O₁₈] _n− (X = Co, Ni, Mn, Zn) and [Co₂Mo₁₀O₃₈H₄]₆₋ and cobalt (or nickel) nitrate, XMo/Al₂O₃ and Co(Ni)-XMo/Al₂O₃ catalysts were prepared. The catalysts were studied by low-temperature nitrogen adsorption, X-ray diffraction, and high-resolution transmission electron microscopy. The average length of the active-phase particles of the catalysts was 3.5 to 3.9 nm, and the average number of MoS₂ layers in a packet was 1.4 to 2.1. The catalytic properties of the samples, which were estimated in dibenzothiophene (DBT) hydrodesulfurization and in the hydrotreating of the diesel fraction, are considerably dependent upon both the type and composition of the HPC, and the nature of the applied promoter (Ni or Co). As compared to the Ni-promoted catalysts, the Co-promoted samples exhibit a higher desulfurization activity, whereas the hydrogenation ability of the Ni-XMo/Al₂O₃ catalysts surpasses that of the Co-XMo/Al₂O₃ ones. The catalytic properties depend on the morphology of the nanostructured active phase. With a growing number of MoS₂ layers in the packet of the catalysts’ active phase, the DBT hydrodesulfurization rate constants for both the direct desulfurization route and the preliminary hydrogenation rote rise linearly and the selectivity falls linearly for the hydrogenation route. The selectivity of Ni-XMo/Al₂O₃ decreases to a greater extent than that of Co-XMo/Al₂O₃. The dependences of the catalytic properties on the morphology of the catalysts’ active phase are consistent with the “dynamic” model of the functioning of the active sites of transition metal sulfides.     

Conversion of synthesis gas into alcohols on supported cobalt-molybdenum sulfide catalysts promoted with potassium

The use of transition metal sulfides as catalysts for the synthesis of alcohols can solve the problem of catalyst resistance to sulfur. Catalysts based on molybdenum sulfide of different compositions (promoted with Co and K) were synthesized with the use of various supports (aluminum oxide, aluminum oxide modified with silicon oxide, Sibunit, and titanium silicate) and tested in the reactions of alcohol synthesis and the hydrofining of a mixture of thiophene with n-1-hexene. The dependence of catalyst activity in the synthesis of alcohols on support pore size was demonstrated. It was found that an increase in the potassium content of the active phase of a catalyst increased its activity in the synthesis of alcohols and decreased it in hydrodesulfurization and hydrogenation reactions. Transmission electron microscopy data made it possible to quantitatively evaluate the effect of a potassium additive on the morphology of the active phase; the hypothesis that potassium was intercalated between the layers of molybdenum sulfide was proposed.     

Influence of the Pore Structure of a Catalyst for Demetallization of Petroleum Feedstock on the Process Results

Demetallization of a mixture of vacuum gas oil with heavy coker gas oil on CoMo/γ-Al₂O₃ catalysts with different pore structures was studied, and the influence of the catalyst pore structure on the process results was demonstrated. For the demetallization catalysts to be effective, their pore size should be restricted not only from below but also from above. When using samples with broad pores, effects of the hydrodesulfurization and hydrodemetallization inhibition can arise owing to adsorption of supramolecular structures of heavy residues, in particular, of asphaltenes.

     

Modern concepts on catalysis of hydroprocessing and synthesis of alcohols from syngas by transition metal sulfides

A concept on the dynamic nature of active centers (AC) of the catalysts based on transition metal sulfides is described. The concept formed the basis of a “dynamic” model, according to which AC formed and functioning under the reaction conditions can oscillate between layers of promoted molybdenum sulfide. The model assumes the existence of “rapid” and “slow” AC and the possibility of their intertransformation due to the reversible migration of sulfur and promoter between the crystallite layers in a hydrogen atmosphere. The frequency of these migrations (oscillations) determines the catalyst activity. An assumption is substantiated that the hydrogenation sites are localized at the rims of Co(Ni)MoS₂ crystallites and desulfurization (hydrodesulfurization) sites are localized on the edges. The proposed model makes it possible to develop criteria for the evaluation of the efficiency of catalytic performance for hydrodesulfurization of hydrocarbon raw materials of various types and for synthesis of higher alcohols from syngas.