Transition-metal sulfide catalysts activity

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. 

We have already referred to the Mo/Ru/S Chevrel phases and related catalysts which have long been under investigation for their oxygen reduction properties. Reeve et al. [19] evaluated the methanol tolerance, along with oxygen reduction activity, of a range of transition metal sulfide electrocatalysts, in a liquid-feed solid-polymer-electrolyte DMFC. The catalysts were prepared in high surface area by direct synthesis onto various surface-functionalized carbon blacks. The intrinsic... 

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 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. 

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. 

Earlier work has been reviewed by Weisser and Landa (1973) and certain more recent studies reviewed by Chianelli (1983), who points to the primary catalytic effects being electronic in nature, related to the position occupied by the metal concerned in the periodic table, with first-row transition-metal sulfides being relatively inactive and second- and third-row transition metals (Ru and Os) showing maximum activity (by three orders of magnitude). A secondary effect appears to be geometric in origin for example, MoSj catalysts seem to exhibit activity associated particularly with edges rather than basal planes. 

Researchers at Haldor Topsoe and their collaborators in academic institutions have contributed significantly to both the advances in research on fundamental aspects of catalytically active sites of transition metal sulfides and the development of new and more active commercial hydrotreating catalysts and processes.15 79 80... 

The effect of crystal structure may be investigated by preparing catalysts, as described above, at various temperatures which assures a set of catalysts having variable surface areas, pore size distributions, and crystallinity. Measuring the catalytic activity as a function of these physical properties will help to define the role of crystal structure for the particular transition metal sulfide. In general, the HDS is poorly correlated to N2 BET surface area. This non-correlation can be most easily seen by preparing a... 

The necessity to develop hydrotreating catalysts with enhanced activity stimulates the search for alternative catalyst supports. It was shown that clay-supported transition metal sulfides can efficiently catalyze hydrodesulfurization (HDS) of thiophene [1-3]. However, the large scale application of the catalysts based on natural clays is still hampered, mainly due to the difficulties in controlling the chemical composition and textural properties. Synthetic clays do not suffer from these drawbacks. Recently, a novel non-hydrothermal approach was proposed for the synthesis of some trioctahedral smectites, namely saponite... 

Transition metal sulfides of the 3" row show high activity in hydrotreating reactions and some of them are studied as potential promoters of conventional catalysts in order to improve their performance. Carbon supported Pt sulfide was highly active in hydrodesulfurization (HDS) of thiophene and hydrodenitrogenation (HDN) of quinoline and pyridine [1,2]. The Pt/silica-alumina sulfide catalyst has been investigated as the promising candidate for deep HDS [3]. 

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... 

Inorganic. - Aray et correlated the topology of p of pyrite-type transition metal sulfides with their catalytic activity in hydrodesulfurization. The most active catalysts are characterized by intermediate values at the M-S BCP. This result supports the consistency of transition-metal-sulfide-catalysed hydrodesulfurization with the Sabatier principle. 

Researchers at Haldor Topsoe and their collaborators in academic institutions have contributed significantly to both the advances in research on fundamental aspects of catalytically active sites of transition metal sulfides and the development of new and more active commercial hydrotreating catalysts and processes . Haldor Topsoe has commercialized more active catalysts for HDS. Its TK-554 catalyst is analogous to Akzo Nobel s KF 756 catalyst, while its newer, more active catalyst is termed TK-574. For example, in pilot plant studies, under conditions where TK-554 produces 400 ppmw sulfur in SRGO, I K 574 will produce 280 ppmw. Under more severe conditions, TK-554 will produce 60 ppmw, while TK 574 will produce 30 ppmw, and similar benefits are found with a mixture of straight run and... 

Metal oxides, sulfides, and hydrides form a transition between acid/base and metal catalysts. They catalyze hydrogenation/dehydro-genation as well as many of the reactions catalyzed by acids, such as cracking and isomerization. Their oxidation activity is related to the possibility of two valence states which allow oxygen to be released and reabsorbed alternately. Common examples are oxides of cobalt, iron, zinc, and chromium and hydrides of precious metals that can release hydrogen readily. Sulfide catalysts are more resistant than metals to the formation of coke deposits and to poisoning by sulfur compounds their main application is in hydrodesulfurization. 

Solid catalysts for the metathesis reaction are mainly transition metal oxides, carbonyls, or sulfides deposited on high surface area supports (oxides and phosphates). After activation, a wide variety of solid catalysts is effective, for the metathesis of alkenes. Table I (1, 34 38) gives a survey of the more efficient catalysts which have been reported to convert propene into ethene and linear butenes. The most active ones contain rhenium, molybdenum, or tungsten. An outstanding catalyst is rhenium oxide on alumina, which is active under very mild conditions, viz. room temperature and atmospheric pressure, yielding exclusively the primary metathesis products. 

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