Cobalt-molybdenum-alumina system

Molybdenum oxide - alumina systems have been studied in detail (4-8). Several authors have pointed out that a molybdate surface layer is formed, due to an interaction between molybdenum oxide and the alumina support (9-11). Richardson (12) studied the structural form of cobalt in several oxidic cobalt-molybdenum-alumina catalysts. The presence of an active cobalt-molybdate complex was concluded from magnetic susceptibility measurements. Moreover cobalt aluminate and cobalt oxide were found. Only the active cobalt molybdate complex would contribute to the activity and be characterized by octahedrally coordinated cobalt. Lipsch and Schuit (10) studied a commercial oxidic hydrodesulfurization catalyst, containing 12 wt% M0O3 and 4 wt% CoO. They concluded that a cobalt aluminate phase was present and could not find indications for an active cobalt molybdate complex. Recent magnetic susceptibility studies of the same type of catalyst (13) confirmed the conclusion of Lipsch and Schuit. 

Catalysts used in hydrotreatment (hydrodesulfurization, HDS) processes are the same as those developed in Germany for coal hydrogenation during World War II. The catalysts should be sulfur-resistant. The cobalt-molybdenum system supported on alumina was found to be an effective catalyst. 

The promoting action of cobalt on the activity for hydrodesulfurization has been shown already in the pioneering work of Byrns, Bradley and Lee (14). This promoting action might be linked with the sulfiding step, since the actual catalyst is the sulfided form of cobalt- or nickel-molybdenum-alumina. Voorhoeve and Stuiver (15) and Farragher and Cossee (16) demonstrated the promoting action for the unsupported Ni-WS2 system. Their intercalation model was based on these experiments. 

In order to achieve the goal of reducing sulfur levels in fuels, there is a clear need for understanding the mechanism of the reaction (Chapter 4) in conjunction with the nature of the catalyst and support. Most of the work has been carried out with the traditional cobalt-molybdenum catalyst supported on alumina. This system is a time-tested and effective. 

Natural gas contains both organic and inorganic sulfur compounds that must be removed to protect both the reforming and downstream methanol synthesis catalysts. Hydrodesulfurization across a cobalt or nickel molybdenum—zinc oxide fixed-bed sequence is the basis for an effective purification system. For high levels of sulfur, bulk removal in a Hquid absorption—stripping system followed by fixed-bed residual clean-up is more practical (see Sulfur REMOVAL AND RECOVERY). Chlorides and mercury may also be found in natural gas, particularly from offshore reservoirs. These poisons can be removed by activated alumina or carbon beds. 

For the most highly developed processes, maf coal conversion can be as high as 90 to 95 % with a C4+ distillate yield of 60 to 75 wt % and a hydrogen consumption of 5 to 7 wt %. When an external catalyst is used, it is typically some combination of cobalt, nickel, and molybdenum on a solid acid support, such as silica alumina. In slurry hydrogenation processes, catalyst life is typically fairly short because of the large number of potential catalyst poisons present in the system. 

A picture has been formed of the way in which the promotor ions are built in the M0O3-AI2O3 system. The neutralization of the Brdnsted acid sites, as originally present in M0O3-AI2O3 systems by the cobalt ions for the catalysts calcined at low temperatures ( 500°C) indicates that the cobalt ions are present on the catalyst surface. The liberation of these sites in catalysts calcined at high temperatures ( 650°C) and the observation of the characteristic reflectance spectrum of C0AI2O4 show that the cobalt ions enter the alumina lattice. However the interaction between cobalt and molybdenum, as indicated by the second Lewis band remains present. This leads to the conclusion that the cobalt ions are present in the surface layers of the alumina lattice. 

The usual catalysts are based on cobalt, nickel, molybdenum and tungsten sulfides, generally combined and deposited on alumina. The most widely used formula is a composite sulfide of molybdenum and colbalt oo alumina. Run length and catalyst life are longer than those of tbe catalytic systems employed in brst step hydrogenation, Le. 6 to 12 months and 3 to 5 years, and the regeneration method is identical... 

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. 

The need to develop new materials for artificial hip joints is driven, in part, by the local and systemic biological consequences of wear debris arising from the currently used materials. As a result, most studies of artificial joint materials, such as alumina, cobalt-chromium-molybdenum alloys (CoCrMo), and ultrahigh molecular weight polyethylene (UHMWPE), concentrate on wear analyses, most reliably carried out with a hip-joint simulator and involving wear-... 

Standard of Indiana Catalyst. The first low pressure polyethylene catalyst invented (46), the Standard of Indiana catalyst system, saw relatively little commercial practice. Their 1951 patent discloses reduced molybdenum oxide or cobalt molybdate on alumina for ethylene polymerization, preferably in aromatic solvents. Later, work concerning the use of promoters was also disclosed. 

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