Transition metal sulfides catalysts

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... 

Eijsbouts, S., Mayo, S.W. and Fujita, K. (2007) Unsupported transition metal sulfide catalysts from fundamentals to industrial application. Appl. Catal. A Gen., 322, 58. 

Figure 1 Plot of the rates of dibenzothiophene HDS as a function of the transition metal sulfide catalyst. (Reprinted from Ref 19. 1981, with permission from Elsevier)...

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 ... 

The Role of Disorder in Layered Transition Metal Sulfide Catalysts... 

This section is concerned with transition metal clusters that, in addition to metal atoms, contain sulfur atoms in the cluster core (rather than just in the peripheral ligands). Metal-metal bonds often supplement sulfur-atom bridges in stabilizing the structures encountered. Generally, such clusters are likely to resemble the HDS active phases of heterogeneous metal-sulfide catalysts to some extent, e.g. in terms of coordination sphere and metallic oxidation states. Because of the large number of molecular metal-sulfide clusters now known, we shall focus on homometallic clusters of Mo(W) and heterobimetallic clusters of Mo(W)-Co(Ni) (next two sections), i.e. molecular clusters containing the elements that are relevant for industrial... 

The group of transition metal. sulfide clusters can be viewed as constituting discrete molecular fragments of the metal sulfide phases present in heterogeneous catalysts. However, with a few very notable exceptions, it has so far been difficult to identify sulfide cluster chemistry that directly reflects the mechanisms of HDS. The main exception is the desulfurization of e.g. thiophene by the sulfide vacancy cluster [(Cp )2Mo2Co2(CO)4(//3-S)2( 4-S)], a reaction which closely mimics the mechanism envisaged for the heterogeneous metal sulfide catalysts. 

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. 

---