Co Ni -Mo W -S Phase

Several research groups have been involved in determining the structure of hydroprocessing catalysts. The contributions of Topsoe et u/. to the 

Ni-W-S phase in the NiW/AC catalysts. In this case, the WS2 particle growth in the c direction was observed on the addition of Ni. The NiW/AC catalyst was more active than the CoW/AC catalyst. Although Co-Mo-S phase was detected, this catalyst was prone to the formation of CogSg. For the same amounts of active metals, presence of the Ni-W-S phase in the NiW/AC catalyst was more evident than the Co-W-S phase in the CoW/AC catalyst. A similar observation was also made for the CoMo/AC catalyst.  

Craje et used Mossbauer emission spectroscopy to confirm the pres- 

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The information on carbon-supported catalysts has been dominated by cata-lytically active metals that are part of the conventional hydroprocessing catalysts, i.e. Co(Ni)Mo(W). In a sulfided form, the structure of the Co(Ni)-Mo(W)-S active phase in these catalysts should approach that of Type-II phase observed on the y-Al203-supported catalysts after a high-temperature sulfidation. " Apparently, there is a sufficient driving force for a direct interaction of carbon with either Mo or sulfur leading to the formation of the Mo-C(S) bonds. Then, in carbon-supported catalysts, the presence of another active phase, i.e. Co-Mo C(S), appears to be plausible. The formation of metal carbides may take place if the supply of sulfur to maintain the catalyst surface in a sulfided form becomes limited, particularly if such a state persists for an extended period. ... 

In association with infrared data, Raman spectra are invaluable in assigning CO-stretching frequencies. Few Raman data for carbonyl compounds are available, however, because of the experimental difficulties involved. Solutions of the compounds at high concentrations are required to obtain acceptable Raman data unfortunately, many carbonyl compounds are insufficiently soluble in the appropriate solvents. There are a few examples, however, where this problem of solubility has been circumvented by employing another phase. Thus, Raman data have been reported for the compounds, M(CO)5 (M = Mo, Cr, or W) (8), M2(CO)io [M = Re (104, 122, 179, 220) or Mn (122)] -, Mn(CO)sBr (122), Re(CO)6l (179), and Re3(CO)i2H3 (277) in the solid state and Ni(CO)4 in the gas phase (39). Another limitation is that decomposition or isomerism of the compound may occur on irradiation. The fact that all colored compounds absorb the mercury excitation line at 4358 A poses an additional problem. However, it is possible to use the helium lines at 5876, 6678, and 7065 A to obtain Raman spectra, as has been done for the compounds Fe(CO)s (283), HFe(CO)4-, Fe(CO) -, Co(CO)4- (282), M[Co(CO)4]2 (M = Cd or Hg) (281), and Ni(CO)4 (280). Further, the use of laser sources in the measurement of Raman spectra overcomes many of these difficulties this technique is now being applied extensively (122, 179, 198). 

A very important class of organometallic compounds is complexes of transition metals with carbonyl (CO) ligands (see examples in Tables S3.13) [228-231]. The coordination number in these complexes is determined by the FAN mle, hence the stable complexes are tetrahedral Ni(CO)4 and Pd(CO)4, trigonal-bipyramidal [Mn(CO)5] andFe(CO)s, octahedral Cr(CO)e, Mo(CO)6 and W(CO)6. Formally CO is an inorganic ligand, the bond in it is shorter (1.128 A in the gas phase) than in CO2 (1.160 A) and is nearly triple (C O) in character. Usually CO coordinates with metal in a linear fashion which can be formally described by the valence-bond scheme M=C=0. However, metal carbonyls are typical k complexes in their... 

The catalyst used for this process is again based on complex mixed oxides, for example, Nippon Shokubai s Mo-W-Te-Sn-Co-O or Nb-W-Co- Ni-Bi-Fe-Mn-Si-Zr-0 catalysts giving 65 and 73% yield, respectively [10,11]. The temperature of the reaction lies between 325 and 350°C in order to afford economic propene conversion, but under such conditions, significant total oxidation takes place. Gas-phase homogeneous reaction also occurs, which implies that the size of the catalyst bed, the void volume, and the shape of the reactor are important. These are the reasons why the one-step process yields much less acrylic acid than the two-step process, making the latter the commercially preferred route. 

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