Heterogeneous Sulfide Catalysts

Finally, it is important to describe the effects of H2S on the reactivity of different surface sulfide phases. The influence of HgS can be four-fold. It can react with an oxide or transition metal thus converting it to the sulfide  

On non-reducible oxides, H2S can exchange with surface hydroxyl groups. 

The S-H bond is weaker than the 0-H bond, which should increase the acidity of the sulfide. H2S can also create acidic sulfhydryl groups via dissociative adsorption  

We will encounter these three different influences on the reactivity that result from the presence of H2S. 

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The SRC-II process, shown in Figure 2, was developed in order to minimise the production of soHds from the SRC-I coal processing scheme. The principal variation of the SRC-II process relative to SRC-I was incorporation of a recycle loop for the heavy ends of the primary Hquefaction process. It was quickly realized that minerals which were concentrated in this recycle stream served as heterogeneous hydrogenation catalysts which aided in the distillate production reactions. In particular, pyrrhotites, non stoichiometric iron sulfides, produced by reduction of iron pyrite were identified as being... 

Figure 2. These high-resolution micrographs show how a so-called x-ray amorphous, nonstoichiometric molybdenum sulfide catalyst exhibits structural (as well as compositional) heterogeneity. Amorphous, quasi-crystalline, and crystalline regions coexist at the ultramicro level (18,).

Catalysts are heterogeneous sulfided nickel (or cobalt) molybdenum compounds on a y-alumina. The reaction has been extensively studied with substrates such as thiophene (Figure 2.40) as the model compound mainly with the aims of improving the catalyst performance. The mechanism on the molecular level has not been established. In recent years the reaction has also attracted the interest of organometallic chemists who have tried to contribute to the mechanism by studying the reactions of organometallic complexes with thiophene [41], Many possible co-ordination modes for thiophene have been described. 

In 2001, Hill and co-workers (151) used a heterogeneous Ag5PV2Moio04o catalyst to mediate the oxidation of 2-chloroethyl ethyl sulfide to the corresponding sulfoxide at room temperature under 1.0 atm of air (Fig. 37). 

Commercial heterogeneous HDS catalysts for refinery use consist, almost without exception, of nickel- and/or cobalt-promoted molybdenum oxide located on a high surface area (approx. 300 m g ) alumina or silica-alumina support. Cobalt and nickel promoters increase the catalytic activity, particularly towards thiophenes whether Co or Ni is used as a promoter depends on the specific function for which the catalyst should be optimal. The catalyst material is shaped into porous pellets, a few millimeters in size, and these pellets are loaded into the reactor, forming a catalyst bed of 30-200 m volume. During start-up of a freshly loaded reactor, the catalyst bed, which is in the oxidic form, is sulfided, typically by treatment with an oil feed which has been spiked with a reactive sulfur compound that readily generates H2S in situ. The oxidic precursor phases (non-stoichiometric CoMo or NiMo surface oxides) are thereby converted into sulfidic phases termed Co-Mo-S and Ni-Mo-S. The conversion from the oxidic phase to the sulfidic is accompanied by a reduction in Mo oxidation state from +6 to +4. 

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. 

The oxidation of sulfides to sulfoxides is a key routine organic transformation. Oakes et al. have developed conditions to enable this reaction to be carried out in supercritical carbon dioxide, with clean sulfoxide formation and no overoxidation to the sulfone (1999). Using TBHP as the oxidant and Amberlyst 15 as a heterogeneous acid catalyst, a range of alkyl, aryl, and benzyl sulfides have been selectively oxidized to the sulfoxide in quantitative yield. 

The chiral vanadyl salen complex was anchored on mesoporous materials by a covalent grafting method. These heterogenized complex catalysts were evaluated as asymmetric catalysts for the asymmetric oxidation of sulfides to sulfoxides [92]. 

Sulfur is widely distributed as sulfide ores, which include galena, PbS cinnabar, HgS iron pyrite, FeS, and sphalerite, ZnS (Fig. 15.11). Because these ores are so common, sulfur is a by-product of the extraction of a number of metals, especially copper. Sulfur is also found as deposits of the native element (called brimstone), which are formed by bacterial action on H,S. The low melting point of sulfur (115°C) is utilized in the Frasch process, in which superheated water is used to melt solid sulfur underground and compressed air pushes the resulting slurry to the surface. Sulfur is also commonly found in petroleum, and extracting it chemically has been made inexpensive and safe by the use of heterogeneous catalysts, particularly zeolites (see Section 13.14). One method used to remove sulfur in the form of H2S from petroleum and natural gas is the Claus process, in which some of the H2S is first oxidized to sulfur dioxide ... 

In Chapter 1 we emphasized that the properties of a heterogeneous catalyst surface are determined by its composition and structure on the atomic scale. Hence, from a fundamental point of view, the ultimate goal of catalyst characterization should be to examine the surface atom by atom under the reaction conditions under which the catalyst operates, i.e. in situ. However, a catalyst often consists of small particles of metal, oxide, or sulfide on a support material. Chemical promoters may have been added to the catalyst to optimize its activity and/or selectivity, and structural promoters may have been incorporated to improve the mechanical properties and stabilize the particles against sintering. As a result, a heterogeneous catalyst can be quite complex. Moreover, the state of the catalytic surface generally depends on the conditions under which it is used. 

Traditionally, monoqrclic arene hydrogenation is carried out in drastic conditions with heterogeneous catalysts [9-18] such as Rh/Al203 and Raney Nickel or metal sulfides. Nevertheless, some pure homogeneous systems have been reported [19-23]. 

The catalyst effectiveness decreased upon increasing the concentration of the intercalated complexes via aggregation of the closely associated complexes.164 Sulfides were also oxidized by heterogeneous Co(II) complexes.163 Catalytic oxidation of thiols was mediated by Mo complex intercalated in a layered double hydroxide.166... 

The deuteration pattern of the tetrahydroquinoline (Scheme 53)258 is rather similar to that reported by Laine for the hydrogenation of (110) with the clusters H2Os3(CO)10 and Os3(CO)12,260 as well as with sulfide/Co-Mo/7-Al203 heterogeneous catalysts.248 249 The... 

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