Cobalt-molybdenum catalysts activity

Minaev, V. Z. Zaidman, N. M. Spirina, G. A., et al., Effect of Pore Structure of Alumina-Cobalt-Molybdenum Catalyst on Activity and Stability in Hydrodesulfurization of Heavy Feedstocks. Chemistry and Technology of Fuels and Oils, 1975. 11(6) pp. 436-39. 

We begin with the structure of a noble metal catalyst. The emphasis is on the preparation of rhodium on aluminum oxide and the nature of the metal-support interaction. Next we focus on a promoted surface in a review of potassium on noble metals. This section illustrates how single crystal techniques have been applied to investigate to what extent promoters perturb the surface of a catalyst. The third study deals with the sulfidic cobalt-molybdenum catalysts used in hydrotreating reactions. Here we are concerned with the composition and structure of the catalytically active... 

Cobalt-molybdenum catalysts are in general much more active for HDS than single molybdenum catalysts. Thus, it is essential to investigate the state of cobalt in the sulfided Co-Mo/Al203 catalyst. 

Activity of carbon-supported cobalt-molybdenum catalysts. 

Relative Volume HDS Activity At Compared to tha Fresh Cobalt/Molybdenum Catalyst. 

Figure 5 Is a histogram showing the distribution of pore volume vs. pore diameter for alumina carrier, fresh cobalt molybdenum catalyst and used cobalt molybdenum catalyst. There was a slight change In mode diameter when the carrier was loaded with about 20% active metal oxides. The pore volume was reduced from 0,60 to 0.53 ml/g. However, accumulation of about 17% coke during the processing of West Coast resld greatly shifted the mode downward and reduced the total pore volume from 0.53 to 0.30 ml/g. (All of these pore volumes have been normalized to 1.0 gram of alumina).

T-310 About 10-12% nickel as the oxide on an activated alumina T-606 Specially compounded refractory oxide support G-39 A cobalt-molybdenum catalyst, used for simultaneous hydrodesulfurization of sulfur compounds and hydrogenation of olefins... 

M. Breysse, M. Activation of off site presulfided cobalt-molybdenum catalysts. Catal. Today 1996,... 

The first description of a synergetic effect due to a mixed cobalt-molybdenum catalyst (oxides and sulfides) was in 1933 by Pease and Keithon (11) at the Princeton University. Their catalytic system was active for the HDS of a mixture of benzene and thiophene. However, difficulty in reproducing their results already pointed out the complexity of this promotion effect, highly dependent on the conditions of preparation and pretreatment and the experimental conditions. 

Systematic assessment of alumina-supported cobalt-molybdenum nitride catalyst Relationship between nitriding conditions, innate properties and CO hydrogenation activity... 

Another SIMS study on model systems concerns molybdenum sulfide catalysts. The removal of sulfur from heavy oil fractions is carried out over molybdenum catalysts promoted with cobalt or nickel, in processes called hydrodesulfurization (HDS) [17]. Catalysts are prepared in the oxidic state but have to be sulfided in a mixture of H2S and H2 in order to be active. SIMS sensitively reveals the conversion of Mo03 into MoSi, in model systems of MoCf supported on a thin layer of Si02 [21]. 

The sulfidation mechanisms of cobalt- or nickel-promoted molybdenum catalysts are not yet known in the same detail as that of M0O3, but are not expected to be much different, as TPS patterns of Co-Mo/A1203 and Mo/Al203 are rather similar [56J. However, interactions of the promoter elements with the alumina support play an important role in the ease with which Ni and Co convert to the sulfidic state. We come back to this after we have discussed the active phase for the hydrodesulfurization reaction in more detail. 

A favorable combination of valence forces of both components seems to be the basic principle of the nickel-molybdenum ammonia catalyst. It has been found (50) that an effective catalyst of this type requires the presence of two solid phases consisting of molybdenum and nickel on the one hand and an excess of metallic molybdenum on the other. Similar conditions prevail for molybdenum-cobalt and for molybdenum-iron catalysts their effectiveness depends on an excess of free metal, molybdenum for the molybdenum-cobalt combination and iron for the molybdenum-iron combination, beyond the amounts of the two components which combine with each other. A simple explanation for the working mechanism of such catalysts is that at the boundary lines between the two phases, an activation takes place. In the case of the nickel-molybdenum catalyst, the nickel-molybdenum phase will probably act preferentially on the hydrogen and the molybdenum phase on the nitrogen. 

Reduction. Benzene can be reduced to cyclohexane [110-82-7], C5H12, or cycloolefins. At room temperature and ordinary pressure, benzene, either alone or in hydrocarbon solvents, is quantitatively reduced to cyclohexane with hydrogen and nickel or cobalt (14) catalysts. Catalytic vapor-phase hydrogenation of benzene is readily accomplished at about 200°C with nickel catalysts. Nickel or platinum catalysts are deactivated by the presence of sulfur-containing impurities in the benzene and these metals should only be used with thiophene-free benzene. Catalysts less active and less sensitive to sulfur, such as molybdenum oxide or sulfide, can be used when benzene is contaminated with sulfur-containing impurities. Benzene is reduced to 1,4-cydohexadiene [628-41-1], C6HS, with alkali metals in liquid ammonia solution in the presence of alcohols (15). 

The object of this review is threefold (1) to discuss the various characterization techniques which have been applied to this catalyst system, (2) to relate what each technique reveals about the nature of the catalyst, and (3) to present an overall picture of the state of the catalyst as it now appears. We will not discuss the vast literature on catalyst activity testing, kinetics, or mechanisms here. These are subjects for review themselves. However, we will mention some selective catalyst activity tests which were designed to give some fundamental insight into the catalyst state or active sites present. Also, we will not discuss in detail the considerable work reported on pure compounds (unsupported) of molybdenum, cobalt, and/or aluminum but we will have occasion to compare some of their properties to our catalyst systems to assess to what degree they may be present in the catalyst. 

Molybdenum oxide - alumina systems have been studied in detail (4-8). Several authors have pointed out that a molybdate surface layer is formed, due to an interaction between molybdenum oxide and the alumina support (9-11). Richardson (12) studied the structural form of cobalt in several oxidic cobalt-molybdenum-alumina catalysts. The presence of an active cobalt-molybdate complex was concluded from magnetic susceptibility measurements. Moreover cobalt aluminate and cobalt oxide were found. Only the active cobalt molybdate complex would contribute to the activity and be characterized by octahedrally coordinated cobalt. Lipsch and Schuit (10) studied a commercial oxidic hydrodesulfurization catalyst, containing 12 wt% M0O3 and 4 wt% CoO. They concluded that a cobalt aluminate phase was present and could not find indications for an active cobalt molybdate complex. Recent magnetic susceptibility studies of the same type of catalyst (13) confirmed the conclusion of Lipsch and Schuit. 

The promoting action of cobalt on the activity for hydrodesulfurization has been shown already in the pioneering work of Byrns, Bradley and Lee (14). This promoting action might be linked with the sulfiding step, since the actual catalyst is the sulfided form of cobalt- or nickel-molybdenum-alumina. Voorhoeve and Stuiver (15) and Farragher and Cossee (16) demonstrated the promoting action for the unsupported Ni-WS2 system. Their intercalation model was based on these experiments. 

The optimal activity for a cobalt-molybdenum-alumina catalyst is obtained by calcination at the higher temperatures. This means that the cobalt ions, present as a cobalt aluminate phase according to the reflectance spectra and the magnetic susceptibility measurements, still have a pronounced promoting action after this calcination. The assumption of cobalt present in the surface layer of the alumina lattice explains both the high activity due to the cobalt promotion as well as the presence of the second Lewis band. This configuration is shown schematically in Figure lib. 

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