Catalyst sulphide phase

In this paper, the phenomena occuring in catalysts used to hydrocrack petroleum residua are discussed. Reaction sites are provided by the catalyst sulphide phase (Mo is the majority cation) and by the catalyst oxide phase (A1 is the majority cation). The influence of the promoter cations (typically Co or Ni) is also described. The catalyst is deactivated by coke and by metals. Furthermore, the reaction rate is often controlled by the rate of diffusion of the large carbonaceous molecules in the residua. All of these factors have been considered in mathematical simulations of the phenomena occuring in the catalyst. 

Species in sulphided M0O3 (H2/thiophen) have been identified (XPS) as M0S2, M0O2, and another Mo sulphide. Comparison of Mo/S ratios from chemical analysis (5.7) and XPS (1.5-3,4) showed that the sulphided phase was concentrated at the catalyst surface. The catalyst also contained Mo (e.s.r.) and its concentration correlated with activity for thiophen hds and with the S content of the catalyst. 

Physical techniques applied in the characterization of C0-M0/AI2O3 catalysts are summarized in Table 2. The presence in the sulphided catalysts of M0S2 C09S8 is fairly well established but occasionally disputed (see below). Other species which have been reported are Mo, M0O2, reduced Co (Co or Co ), and mixed Co-Mo sulphide phases (possibly involving Co V). 

As described above both the oxide phase (alumina) and the sulphide phase (MoS containing excess non-stoichiometric sulphur) of the catalyst contain electron acceptor states (rp. The data in Figure 1 are for catalysts having different amounts of sulphide and oxide phases. As the proportion of molybdenum increases, the number of molybdenum electron acceptor states will increase and the number of alumina electron acceptor states will decrease. 

The lower part of Figure 1 shows the amount of coke on the catalyst as a function of the catalyst molybdenum content. Equation 13 indicates that undesirable reactions are only dependent upon the number of electron acceptor states (D in the catalyst. In Figure 1, as the number of sulphide phase electron states increase, the number of oxide phase (alumina) electron acceptor states decrease. It has been shown [1] that the Fermi level of the oxide phase is much lower than the Fermi level of the sulphide phase. This means that coke will be bonded much more strongly to the oxide phase. Therefore, as the number of oxide phase electron acceptor states decrease, the quantity of coke on the catalyst also decreases, as shown in Figure 1. 

The application of ly transition metal carbides as effective substitutes for the more expensive noble metals in a variety of reactions has hem demonstrated in several studies [ 1 -2]. Conventional pr aration route via high temperature (>1200K) oxide carburization using methane is, however, poorly understood. This study deals with the synthesis of supported tungsten carbide nanoparticles via the relatively low-tempoatine propane carburization of the precursor metal sulphide, hi order to optimize the carbide catalyst propertira at the molecular level, we have undertaken a detailed examination of hotii solid-state carburization conditions and gas phase kinetics so as to understand the connectivity between plmse kinetic parametera and catalytically-important intrinsic attributes of the nanoparticle catalyst system. 

The conversion of S-alkyl isothiouronium salts into thioethers is aided by the addition of a phase-transfer catalyst (4.1.4.E) [30]. Similarly, a,co-dihaloalkanes are converted in a one-pot reaction into bis-sulphides (> 90%) via the isothiouronium salts (Scheme 4.3) [31]. 

As an alternative to the oxidation of sulphides and sulphoxides (see Chapter 10), sulphones can be prepared by the nucleophilic substitution reaction of the sulphinite anion on haloalkanes. In the absence of a phase-transfer catalyst, the reaction times are generally long and the yields are low, and undesirable O-alkylation of the sulphinite anion competes with S-alkylation. The stoichiometric reaction of the preformed tetra-n-butylammonium salt of 4-toluenesulphinic acid with haloalkanes produces 4-tolyl sulphones in high yield [1], but it has been demonstrated that equally good... 

A variety of solids is used as catalysts metals, alloys, clays, metal oxides, sulphides, nitrides, carbides and so on. Catalysts may be single-phase substances or multiphasic mixtures they may be crystalline, microcrystalline or even amorphous. Catalysts can be electrically insulating, semiconducting or metallic. Some examples of heterogeneously catalysed reactions are given in Table 8.1. Two aspects of catalysts are important activity and selectivity. Activity refers to the ability of catalysts to accelerate chemical reactions so that equilibrium is achieved rapidly. The degree of acceleration... 

Before World War II, the use of molybdenum based catalysts was popular, with molybdenum oxide (and sometimes sulphide) used for a number of isomerization processes.4 These types of catalyst were less sensitive to sulphur in the feed but were phased out owing to inferior selectivity.5... 

Catalysts are mixed metal sulphides on a carrier. The major examples are supported mixed phases of CoS and M0S2, of NiS and M0S2 and NiS and WS2. There is as yet no full agreement on the structure. For the first catalyst the best description is small MoS2 clusters with CoS units at the edges. Typical dimensions of these clusters are 1 nm and a stacking between 1 and 3. 

Plasticised PVC was surface modified by nucleophilic substitution of the chlorine atoms of PVC by sulphide ions in aqueous media in the presence of a phase-transfer catalyst. The modified PVC was sterilised by steam autoclaving and gamma irradiation and subjected to plasticiser extraction. Surfaces were evaluated by contact angle measurements, SEM, cell culture studies, haemolysis assay and whole-blood clotting time measurements. Mechanical properties were examined. 31 refs. 

Figure 11.17 Under the same sulphidation conditions a W/alumina catalyst shows a mixture of oxide and sulphur phases whilst a nicket-pronDOted catalyst is entirely sulphided (taken from the Journal of Physics IV J. Lynch, p. C9-253, 1994, with the permission of EDP Sciences).

The vast majority of catalysts used in modern chemical industry are oxides. Because of their ability to take part in the exchange of electrons, as well as in the exchange of protons or oxide ions, oxides are used as catalysts in both redox and acid-base reactions. They constitute the active phase not only in oxide catalysts but also in the case of many metal catalysts, which in the conditions of catalytic reaction are covered by a surface layer of a reactive oxide. Properties of oxides are also important in the case of preparation of many metal and sulphide catalysts, which are... 

Further extensions of the use of pyrylium salts as synthons have appeared for example, their reaction with hydrazines. The xanthyUum salt (27 A = 0) reacted with amines under very mild conditions and in good yield and it has been used to synthesize sulphones, sulphides, and ethers in the presence of phase-transfer catalysts for example, 2-hexyloxynaphthalene was obtained in 70% yield by heating the acridinium salt (27 A = NC Hia) with sodium 2-naphthoxide and Bu4N A phase-transfer catalyst has... 

Micro- and mesoporous materials containing niobium in the framework or extra framework positions were studied in the oxidation of dibutyl sulphide with H2O2. Leaching of Nb from the solid to the liquid phase was considered. Some of the catalysts prepared via the impregnation with Nb-salts show some leaching of Nb to the liquid phase and the oxidation partially occurs homogeneously in the liquid phase. The reaction proceeds mainly on the catalyst surface when the mesoporous molecular sieves containing Nb in the framework are used. 

More complicated problems are presented by the mixed-metal Co-Mo hydrodesulphurisation catalysts, which have been the subject of some interest. The molybdenum site of the sulphided catalyst seems to be part of a MoS2 like phase, with EXAFS results indicating 6 sulphur nearest neighbours ( 2.48) and - 4 Mo atoms at 3.28 distance. These parameters are essentially... 

A steady loss of catalytic activity with the reaction time was observed over two catalysts and the rate of deactivation of in situ activated catalyst is much quicker than ex situ activated catalyst. In ammonium sulphide solution the total amount of sulphur is in excess in comparison with their stoichiometric value for M0S2 plus CoS. After impregnation in ammonium sulphide solution, both the CogSg and thiomolybdate complexes will be formed which is conformed from X-ray diffraction measurements. However, after pre-reduction in the presence of H2 these phases were disappeared. It can be understood that the... 

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