Molybdenum-based catalyst systems supported

Abstract Metathesis-based polymerizations of 1-alkynes and cyclopolymerizations of 1,6-heptadiynes using late transition metal catalysts are reviewed. Results obtained with both binary, ternary, and quaternary catalytic systems and well-defined molybdenum- and ruthenium-based catalysts are presented. Special consideration is given to advancements in catalyst design and mechanistic understanding that have been made in this area over the last few years advancements that have facilitated tailor-made syntheses of poly(ene)s. In addition, the first supported ruthenium-based cyclopolymerization-active systems are summarized. Finally, selected structure-dependent properties will be outlined where applicable. 

Up to now, only a few catalyst systems based on organic polymers such as molybdenum compounds supported on benzimidazole, polystyrene, or poly(gly-cidyl methacrylate) [9] as well as micelle-incorporated manganese-porphyrin catalysts [66] have been tested in the epoxidation of propene. Molybdenum-doped epoxy resins were also employed in the epoxidation of propene with TBHP and propene oxide yields of up to 88% were obtained [65]. The catalysts were employed repeatedly in up to 10 reactions without significant loss of activity and metal leaching proved to be very low. 

The catalyst systems employed are based on molybdenum and phosphorus. They also contain Various additives (oxides of bismuth, antimony, thorium, chromium, copper, zirconium, etc.) and occur in the form of complex phosphomolybdates, or preferably heteropolyacids deposited on an inert support (silicon carbide, a-alumina, diatomaceous earths, titanium dioxide, etc.). This makes them quite different from the catalysts used to produce acrylic acid, which do not offer sufficient activity in this case. With residence times of 2 to 5 s, once-through conversion is better than 90 to 95 per cent, and the molar yield of methacrylic acid is up to 85 to 90 per cent The main by-products formed are acetic add, acetone, acrylic add, CO, C02, etc. The major developments in this area were conducted by Asahi Glass, Daicel, Japan Catalytic Chemical, Japanese Gem, Mitsubishi Rayon, Nippon Kayaku, Standard Oil, Sumitomo Chemical, Toyo Soda, Ube, etc. A number of liquid phase processes, operating at about 30°C, in die presence of a catalyst based on silver or cobalt in alkaline medium, have been developed by ARCO (Atlantic Richfield Co,), Asahi, Sumitomo, Union Carbide, etc. 

In recent years, a lot of research effort has been directed towards dehydroaromatisation of methane in which methane is converted to aromatic products such as benzene and naphthalene in addition to hydrogen. Perhaps the most well studied system has been that employing Mo/ZSM-5 based catalysts, where the bifunctional interaction between the zeolite Bronsted acidity and molybdenum species is well recognised. Under reaction conditions, the active molybdenum species are known to be in the form of carbides or oxycarbides, and recently it has been proposed that the a-MoCi-x phase is the most active form. Deactivation, primarily due to coke formation, is well precedented in this reaction and represents a major obstacle to be overcome in the successful application of these catalysts. In this respect, it is interesting to note that Ichikawa and co-workers have published studies indicating that the inclusion of low levels of CO or CO2 in the feed can promote the reaction via the suppression of coke formation in the case of both Mo/HZSM-5 and Re/HZSM-5 catalysts. Other approaches adopted towards this aim have been the inclusion of second metal components and a reduction of the acid strength of the HZSM-5 support. ... 

Molybdenum- and tungsten-based hydrotreating catalysts supported on AI2O3 are widely used in the industry for HDS and HDN processes. Cobalt and nickel are the promoters of choice for HDS and HDN catalyst system based on MS2 (M = Mo or W), and their presence greatly increases both HDS and HDN activity. For HDN, Ni is the promoter of choice, whereas Co is preferred for HDS. Other additives, such as fluorine, chlorine, boron, and phosphorus, have been incorporated into both promoted and unpromoted MoS2-type catalysts with beneficial effect on HDN and HDS processes [116],... 

It may be advantageous to carry out the WGS reaction on the raw S5mgas from gasification of coal and heavy hydrocarbons by a so-called sour shift catalyst. This allows removal of CO2 and H2S in the same wash system (see Section 1.5.3). It requires a catalyst that is sulphur-tolerant and capable of working at low H2O/C ratios. The conventional iron-based HTS catalyst can operate in the presence of sulphur, but it requires addition of significant amounts of steam to eliminate the problem of the carbide formation reaction. This problem is solved by using a molybdenum sulphide-based catalyst [168] [232]. The catalyst is promoted and is based on alumina support. It requires the presence of... 

Catalysts based on molybdenum disulfide, M0S2, and cobalt or nickel as promoters are used for the hydrodesulfurization (HDS) and hydrodenitrogenadon (HDN) of heavy oil fractions [48,49]. The catalyst, containing at least five elements (Mo, S, Co or Ni, as well as O and A1 or Si of the support), is rather complex and represents a real challenge for the spectroscopist. Nevertheless, owing largely to research in the last twenty years, the sulfided C0-M0/AI2O3 system is one of the few industrial catalysts for which we know the structure in almost atomic detail [49, 50],... 

The results in Table V show that active disproportionation catalysts are obtained when molybdenum and tungsten hydrocarbyls are supported on silica or alumina. These catalytic systems have not been optimized, and it is highly probable that more active catalysts based on these systems can be obtained. No obvious pattern emerges from the results both silica and alumina supports confer activity on the organometallic compounds which are themselves inactive in homogeneous solution under similar conditions. 

Solid catalysts can be used at elevated temperatures, though their acidities are much weaker than those of liquid ones. From this point of view, solid superacids based on Lewis acids and liquid superacids discussed in Sections II—1V are not sufficiently stable Nafion-H is also unsatisfactory, its maximum operating temperature being below 200°C. A new type of the sulfate-supported metal oxides is more stable because of preparatory heat treatment at high temperatures, but elimination of the sulfate is sometimes observed during reaction, thus it is hoped to synthesize superacids with the system of metal oxides. Another type of superacid, tungsten or molybdenum oxide supported on zirconia, has been prepared by a new preparation method, and its stability is satisfactory so far. It is hoped that the preparation method will be extensively applied to other metal oxides for new solid superacids. 

These Mo-based cyanide and insulin systems are suggested to operate similarly to the molybdothiol systems, via the formation of a coordinatively unsaturated oxo-molybdenum species as catalyst, which binds N2 side-on and reduces it by two electrons to N2H2 (see Equations 23, 24, 25). The intermediacy of N2H2 in the formation of NH3 is supported by the reduction of fumarate to succinate by these systems when operating under N2. Succinate is not produced when other substrates, for example, acetylene, are being reduced. 

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