Sulfided metal catalysts

Although supported Pd catalysts have been the most extensively studied for butadiene hydrogenation, a number of other catalysts have also been the object of research studies. Some examples are Pd film catalysts, molybdenum sulfide, metal catalysts containing Fe, Co, Ni, Ru, Rh, Os, Ir, Pt, Cu, MgO, HCo(CN) on supports, and LaCoC Perovskite. There are many others (79—85). Studies on the weU-characteri2ed Mo(II) monomer and Mo(II) dimer on siUca carrier catalysts have shown wide variations not only in catalyst performance, but also of activation energies (86). 

Catalysis by Metal Sulfides. Metal sulfides such as M0S2, WS2, and many others catalyze numerous reactions that are catalyzed by metals (98). The metal sulfides are typically several orders of magnitude less active than the metals, but they have the unique advantage of not being poisoned by sulfur compounds. They are thus good catalysts for appHcations with sulfur-containing feeds, including many fossil fuels. 

On sulfided metallic phases the hydrotreatment reactions also takes place. Noble metal catalysts usually include a zeolitic support. They are particularly used for fulfilling two different objectives, in the case of a gasoline oriented HCK their cracking and isomerization activity is the most important (increasing high octane and conversion yield). In a diesel HCK unit, the noble metal catalyst is mainly oriented to aromatic saturation and cetane improvement. However, in this latter case, also sulfided metal catalysts are used, especially NiW. 

However, residuum hydrotreating catalysts themselves are susceptible to irreversible deactivation caused by the accumulation of sulfided metal impurities. The gradual buildup of these impurities in the pores of a hydroprocessing catalyst causes plugging and deactivation. 

These cracking and H-addition processes also require catalysts, and a major engineering achievement of the 1970s was the development of hydroprocessing catalysts, in particular cobalt molybdate on alumina catalysts. The active catalysts are metal sulfides, which are resistant to sulfur poisoning. One of the major tasks was the design of porous pellet catalysts with wide pore structures that are not rapidly poisoned by heavy metals. 

The main classes of materials employed as catalysts are metals (generally transition and noble metals), oxides (including transition-metal oxides), transition-metal sulfides and zeolites. In the following sections, we discuss some of the more common structures and chemistry exhibited by catalytic systems. 

Which is the best catalyst for accelerating the reaction depends on the nature of the working materials. For the reaction of hydrogen or oxygen in potassium hydroxide solution, nickel or silver is suitable for carbonaceous fuels as well as for the reaction of oxygen in acid electrolytes platinum metals were up to the middle 60s, the only known catalysts. Precious metals are ruled out by price for wide application in fuel cells, and the search for cheaper catalysts has been actively pursued in many research laboratories. Many classes of inorganic substances (carbides, nitrides, oxides, sulfides, phosphides, etc.) have been investigated and, in particular, several chelates. 

Metals accumulate more slowly on the catalyst surfaces because the inlet concentrations of metals are lower than for coke precursors. The accumulation of metals can be even greater than coke, for example the vanadium concentration can reach 30-50 wt% of the catalyst on a fresh catalyst basis (Thakur and Thomas, 1985). Demetallization reactions can be considered autocatalytic in the sense that once the surface of the catalyst is covered with metal sulfides the catalyst remains quite active and continues to accumulate metal sulfides. The final loss of catalyst activity is usually associated with the filling of pore mouths in the catalyst by metal sulfide deposits. 

Although metals or even promoted metals have very low sulfur tolerances in synthesis reactions, other materials, such as metal oxides, nitrides, borides, and sulfides, may have greater tolerance to sulfur poisoning because of their potential ability to resist sulfidation (18). The extremely low steady-state activities of Co, Ni, and Ru metals in a sulfur-contaminated stream actually correspond to the activities of the sulfided metal surfaces. However, if more active sulfides could be found, their activity/selectivity properties would be presumably quite stable in a reducing, H2S-containing environment. This is, in fact, the basis for the recent development of sulfur active synthesis catalysts (211-215), which are reported to maintain stable activity/ selectivity properties in methanation and Fischer-Tropsch synthesis at H2S levels of 1% or greater. Happel and Hnatow (214), for example, reported in a recent patent that rare-earth and actinide-metal-promoted molybdenum oxide catalysts are reasonably active for methanation in the presence of 1-3% H2S. None of these patents, however, have reported intrinsic activities... 

Although the hydrogenation activity of metal sulfides is lower by several orders of magnitude than that of metal catalysts, sulfides allow operations under conditions that are impractical for metals. They are generally used as highly dispersed materials on a high surface area support, such as y-alumina, in fixed bed operation. Most important is catalyst design to minimize deactivation due to the deposition of metals (V, Ni) in the feed and of coke at the mouths of the pores. Metal sulfides can also be used as finely dispersed phases in continuous slurry reactors to reduce the mass transport limitations of heavy oils. 

The hydroformylation of alkenes is commonly run using soluble metal carbonyl complexes as catalysts but there are some reports of heterogeneously catalyzed reactions of olefins with hydrogen and carbon monoxide. Almost all of these are vapor phase reactions of ethylene or propylene with hydrogen and carbon monoxide catalyzed by rhodium, " 20 ruthenium,nickel, 22,123 cobalt, 23,124 and cobalt-molybdenum 23 catalysts as well as various sulfided metal catalysts. 23,125,126... 

Mo " ". Cobalt became also essentially sulfided and S is characteristic of a sulfide S -. We may conclude that the presulfided catalyst after the activation results in giving similar active metal forms to those in the catalyst sulfided by a mixture of H2S and H2. 

One of the primary requisites for a good redox catalyst is metallic character. Any material which loses this property upon exposure to automobile exhaust will not be an effective NO catalyst. From the above considerations, one would predict that all noble metals actiye for NO reduction will be immune from chemical poisoning by sulfur and oxygen. On the other hand, any base metal active for NO reduction will be chemically poisoned by sulfide or sulfate formation as long as any sulfur is present in the system. These predictions are generally confirmed by experience. Any base metal other than copper will probably be chemically poisoned by oxygen in the molecular form or derived from water. 

He examined various sulfided metal catalysts for the hydrogenolysls of (II). Our results in Table 3 indicate that PtSx is far superior to other metal sulfides or Pd for the hydrogenation of 6-chloro-2(IH)-hydroxyquinoxaline-4- oxides to... 

SYNONYMS (nickel) elemental nickel, nickel catalyst, nickel metal, raney alloy, raney nickel (nickel carbonate) basic nickel carbonate, carbon acid nickel salt (1 1), nickelous carbonate (nickel sulfide) mononickel monosulfide, nickel (II) sulfide, nickel monosulfide, nickelous sulfide. 

The infrared spectra did have new bands below 700 wavenumbers that possibly could be from inorganic sulfides. Metal analysis showed an extremely large amount of zinc, 4.5%, and less than 100 ppm aluminum from the catalyst support. This amount of zinc is one atom for every 15 sulfur atoms. 

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