Transition metal sulfides mechanisms

This article is focused on HDN, the removal of nitrogen from compounds in oil fractions. Hydrodemetallization, the removal of nickel and vanadium, is not discussed, and HDS is discussed only as it is relevant to HDN. Section II is a discussion of HDN on sulfidic catalysts the emphasis is on the mechanisms of HDN and how nitrogen can be removed from specific molecules with the aid of sulfidic catalysts. Before the discussion of these mechanisms, Section II.A provides a brief description of the synthesis of the catalyst from the oxidic to the sulfidic form, followed by current ideas about the structure of the final, sulfidic catalyst and the catalytic sites. All this information is presented with the aim of improving our understanding of the catalytic mechanisms. Section II.B includes a discussion of HDN mechanisms on sulfidic catalysts to explain the reactions that take place in today s industrial HDN processes. Section II.C is a review of the role of phosphate and fluorine additives and current thinking about how they improve catalytic activity. Section II.D presents other possibilities for increasing the activity of the catalyst, such as by means of other transition-metal sulfides and the use of supports other than alumina. 

Decompositions of transition-metal sulfides, notably those of Fe, Ni, Cu and Co, have been of technological importance in ore refining. Some of the published work is concerned with naturally-occurring minerals, while other studies used synthetic preparations. Reactions often proceed by a contracting interface mechanism and the rates are decreased when gaseous product is present, or its escape is opposed by an inert gas. On heating in air, several metal sulfides form sulfates or oxysulfates as intermediates in a sequence of reactions which finally yield metal oxides [43]. 

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. 

Aromatic organosulfur compounds such as thiophenes, benzothiophenes and dibenzothiophenes are frequently contained in fossil oil and their sulfur atoms are generally difficult to remove in HDS process [106], In the industrial HDS process, Mo/Co/S or Ni/Mo/S heterogeneous catalysts supported on alumina are widely employed. In order to obtain ideas to develop more efficient catalysts as well as to shed some light on their mechanisms at a molecular level, transition metal complex-mediated cleavages of C-S bond are extensively studied. On the other hand, thiiranes and thietanes are frequently employed for preparation of transition metal sulfides, in which their C-S bonds are smoothly cleaved. In this section, the C-S bond cleavages of thiophene derivatives, thiiranes, thietanes, vinylic sulfides, allylic sulfides, thiols and dithioacetals are overviewed. 

The presence of V3S4 crystals can only be attributed either to an autocatalytic mechanism of this type or the migration of the deposited metals. It is known that deposited Ni and V sulfides possess some catalytic activity (see Section IV). Slurry processes have been proposed which utilize Ni and V deposited from the oil onto a slurry material (Bearden and Aldridge, 1981). Studies have appeared in the literature demonstrating that nearly all of the transition metals are catalytically active for HDS reactions and presumably for HDM (Harris and Chianelli, 1984). Rankel and Rollmann (1983) impregnated an alumina catalyst base with Ni and V and concluded that these sulfides display an order of magnitude lower activity than the standard Co-Mo sulfide catalyst for HDS reactions, but exhibited similar activity for HDM reactions. 

With respect to the overall mechanism, it is worthwhile to comment briefly on the APCI results of high elemental sulfur yields, even in the absence of air. The results suggest that the reaction responsible for the elemental sulfur formation under these conditions may be reaction (12). The main reason for this contention is the fact that transition metals and their sulfides are known to be very good catalysts for H2S decomposition (11,... 

An organic disulfide sorbed into a transition-metal form of a zeolite may dissociate to give two thiol radicals. These may coordinate to cations by a mechanism which involves electron sharing or electron transfer, for example to oxidize the cation fiorther and to generate coordinated organic sulfide ions. This was observed with dimethyldisulfide sorbed into Co -exchanged zeolite... 

Thermo-oxidation of polyphthalamides (PPA-1 and PPA-2) display the same regularities (kinetics of mass losses, oxygen absorption and release of the main volatile products, composition of volatile and heavy products relative to the polymer structure, oxidation inhibiting by adding PCA and transition metal compounds, etc.), which are described in detail for PAI, PPQ, polybenzoxazole (PBO), PEI, PI, PAIM, polyphenyl sulfide (PSP), PES and LCP. The degradation mechanism will now be discussed in more detail using PPA-1 and PPA-2 as examples (see Figure 7.5 and 7.6). 

Eq. (10.12), may oxidize the dissolved SO2 to sulfate in the outer layers. The species within rectangular boxes represent the solution constituents and those in ovals represent the corrosion products. Dotted ovals represent reactions or chemical compounds for which there is no evidence by laboratory or field studies. The chemical reactions that have been described and confirmed by laboratory studies are presented as sohd arrows. The dotted arrows represent the mechanisms that are uncertain. TMI in the upper part of Fig. 10.3 stands for transition metal ions and soot catalyzes the S(IV) to S(VI). The ferrous cations react with reduced sulfur to produce several insoluble sulfides. Multistep processes including Fe(II), Fe(III), and OH produce hydroxysulfate mixed salts [5]. 

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