Cobalt-molybdenum catalysts unsupported

X-Ray Absorption Fine Structure, Mossbauer, and Reactivity Studies of Unsupported Cobalt-Molybdenum Hydrotreating Catalysts... 

Kushiyama et al (129) reported that phosphorus also promotes the activity of highly divided unsupported molybdenum and cobalt-molybdenum sulfide catalysts for HDS (and HDM) of crude oil, with a maximum at a P/(Co + Mo) atomic ratio of 2. This result warrants attention because it shows that phosphorus may directly modify the active sulfide phase(s) even in the absence of the alumina support. 

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. 

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. 

Catalytic reductions have been carried out under an extremely wide range of reaction conditions. Temperatures of 20 C to over 300 C have been described. Pressures from atmospheric to several thousand pounds have been used. Catal3rsts have included nickel, copper, cobalt, chromium, iron, tin, silver, platinum, palladium, rhodium, molybdenum, tungsten, titanium and many others. They have been used as free metals, in finely divided form for enhanced activity, or as compounds (such as oxides or sulfides). Catalysts have been used singly and in combination, also on carriers, such as alumina, magnesia, carbon, silica, pumice, clays, earths, barium sulfate, etc., or in unsupported form. Reactions have been carried out with organic solvents, without solvents, and in water dispersion. Finally, various additives, such as sodium acetate, sodium hydroxide, sulfuric acid, ammonia, carbon monoxide, and others, have been used for special purposes. It is obvious that conditions must be varied from case to case to obtain optimum economics, yield, and quality. 

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