Transition-metal sulfide catalysts importance

The catalysts applied in hydroprocessing operations are typically sulfided CoO—Mo03—A1203 or NiO—M0O3—A1203. The results of relevant studies281 and the application in refinery processes of these and other transition-metal sulfide catalysts were reviewed.282 Selection of catalysts and reactors for particular feeds and products is also an important issue.257,280,283,284... 

Many minerals or their synthetic equivalents are of industrial importance, and the possibilities of greater understanding of their behavior and properties through application of the methods described in this book is a major stimulus to research. Two examples are chosen for discussion here The first is the zeolites, a group of framework alumino-silicates (with interstitial Na+, Ca +, and HjO), characterized by very open frameworks with large interconnecting spaces or channels the second is the transition-metal sulfide catalysts, materials already generally discussed in Chapter 6, but considered here specifically in relation to their catalytic properties. 

Catalytic hydrodesulfurization (HDS), the removal of sulfide in the form of H2S from petroleum, is a critical step in the industrial refinement process and one of increasing importance as the cleaner world supplies of petroleum feedstocks dwindle and the poorer quality feedstocks have to be used. The removal of sulfur (and certain other impurities such as nitrogen in hydrodenitrogenation) is undertaken using transition-metal sulfide catalysts (Weisser and Landa, 1973). The most widely used materials... 

The importance of edge planes also arises in the industrially important promoted transition metal sulfide catalyst systems. It has been known for many years that the presence of a second metal such as Co or Ni to a M0S2 or WS2 catalyst leads to promotion (an increase in activity for HDS or hydrogenation in excess of the activity of the individual components) ( ). Promotion effects can easily be observed in supported or unsupported catalysts. The supported catalysts are currently the most important industrial catalysts, but the unsupported catalysts are easier to characterize and study. Unsupported, promoted catalysts have been prepared by many different methods, but one convenient way of preparing these catalysts is by applying the nonaqueous precipitation method described above. For example, for Co/Mo, appropriate mixtures of C0CI2 MoCl are reacted with Li2S in ethyl acetate ... 

In this section, we will emphasize the importance of the electronic structure, the crystallographic structure and the sulfur vacancies in understanding catalytic activity and selectivity of transition-metal sulfide catalysts. 

This work is a contribution to the understanding of the effect of spillover hydrogen in a type of catalyst of considerable industrial importance, namely that composed of transition metal sulfides and amorphous acidic solids. This is typically the case of sulfided CoMo supported on silica-alumina used for mild hydrocracking. 

The chapter Fundamental Studies of Transition-Metal Sulfide Catalytic Materials by Chianelli, Daage, and Ledoux reviews current understanding of the relationship between structural and other properties of these catalysts and their catalytic activity and selectivity in hydrodesulfurization. In view of increasing environmental demands, this field has been heavily researched. The authors show how systematic studies and applications of novel methods can provide considerable understanding of these important catalysts. 

The Transition Metal Sulfides are a group of solids which form the basis for an extremely useful class of industrial hydrotreating and hydroprocessing catalysts. Solid state chemistry plays an important role in understanding and controlling the catalytic properties of these sulfide catalysts. This report discusses the preparation of sulfide catalysts, the role of disorder and anisotropy in governing catalytic properties, and the role of structure in the promotion of molybdenum disulfide by cobalt. 

In the preceding part of this paper the predominance of the "periodic effect on HDS by sulfide catalysts was described. Because periodicity dominates, crystal structure is of secondary importance. However, in this section we briefly examine the effect of crystal structure on the catalytic properties of the transition metal sulfides. In the case of catalysts such as M0S2 and WS2, the most industrially important catalysts, the effect of crystal structure is quite pronounced. An understanding of the effect of crystal structure in these catalysts is essential to optimizing their catalytic properties for a given application. 

Solid state chemistry plays an important role in the catalysis by Transition Metal Sulfides however, it is a role that is somewhat different than the role usually assigned to solid state chemistry in catalysis. In catalysis, by sulfides, the chemistry of ternary phases is not now important and thus, the usual role of solid state chemistry in preparing ternary phases and systematically studying the effect on catalytic properties through variation of the composition of these ternary phases is absent. Nevertheless, preparation of the Transition Metal Sulfides is crucial in controlling the properties of the catalysts. Low temperature solid state preparations are the key to obtaining good catalysts in reasonable surface area for catalytic measurements. 

Particularly important here is the role of transition metal sulfides. In 1988 Wachtershauser proposed that pyrite, abundant in hydrothermal vent systems, provided an energy source for the first life. He suggested that pyrite provided the catalyst necessary to drive a number of essential chemical reactions which are important precursors to life. More recent studies have confirmed this view and have shown that the sulfides of Fe, Ni, Co, and Zn can play an important role in the fixation of carbon in a prebiotic world (Cody et al., 2004). Transition metal sulfides also play a role in more advance organic synthesis, and Huber and Wachtershauser (1998) showed how amino acids were converted into their peptides using a (NiFe)S catalyst. 

Transition-metal sulfide (TMS) catalysts play an important role in the petroleum industry. TMS are unique catalysts for the removal of heteroatoms (N, S, 0) in the presence of large amovmts of hydrogen (3). Hydrodesulfiirization (HDS) of petroleum feedstocks are commercially achieved with M0S2 or WS2 supported on alumina and promoted by Co or Ni, (3,4). Co-promoted catalysts are mainly used for HDS, whereas Ni-promoted catalysts are superior for HDN and hydrogenation reactions (5). Catal3rsts currently employed need to be improved to satisfy the imminent restrictions that require the removal of the most refractory species, mainly alkyl-substituted polyaromatic thiophenes. 

Transition Metal Salts and Oxides on Alumina. Transition metal salts, particularly chlorides and nitrates, are frequently used as starting materials for the preparation of supported transition metal oxides or supported precursors for supported metal catalysts. Also, many catalytic materials, particularly supported molybdenum and tungsten oxide and sulfide catalysts, contain transition metal ions, namely Co, Ni , and Fe " as promoters. Thus, it is interesting to study the spreading and wetting behavior of salts of these transition metals and of their oxides. This is of particular importance for promoted catalyst materials, since in practice the incorporation of the active phase and the promoter should be possible in one step for economic reasons. 

Oxides and sulfides of the transition metals are the most active, most selective, and industrially most important catalysts. According to electronic theory, numerous other semiconductors should have good catalytic properties, which could be... 

It is possible to oxidise and reduce atoms in the framework and also those within the pores of microporous (and mesoporous) solids of appropriate chemical compositions. Although pure aluminosilicate, silicate and aluminophosphate frameworks cannot be oxidised or reduced, transition metal and some lanthanide cations within the framework can exist in different oxidation states. Also, although typical alkali, alkali metal and most lanthanide cations in extraframework positions possess no redox chemistry, transition metal cations such as nickel, copper and platinum do. In the case of the transition metals, this enables them to become important catalysts. The included sulfide species in ultramarine-related pigments described above are also prepared through the reduction of sulfate species. 

Heavy metals and arsenic Heavy metals are routinely determined, often with one or other form of sulfide precipitation. These compendial tests are performed from the viewpoint of safety and the general limits (l-30pgml g ) are now more often related to the dose. For metals such as mercury, lead, cadmium, or nickel, atomic absorption spectrometry or other instrumental methods are often prescribed. Copper and other transition metals can act as catalysts in certain degradation reactions and thus require special attention. Arsenic tests that were important at the beginning of the previous century are now being phased out. 

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