Activation of Mo

Nishizawa, M., Okazaki, K., Furuno, N., Watanabe, N., and Sagata, N. (1992). The second-codon rule and autosphophorylation govern the stability and activity of Mos during the meiotic cell cycle in Xenopus oocytes. EMBO J. 11 2433-2446. 

Figure 5. Specific activity of "Mo in fallout and U7Fm in cloud vs. particle size...

Twenty-six moles of NH3 and 19 moles of H2 are produced per mole of cluster in 4 hr. The catalytic activity of [Mo-Fe]m is superior to that of [4-Fe]n with respect to NH3 formation, and in both cases, higher pH favors the fonnation of NH3. The maximum current efficiency using [Mo-Fe]m catalyst in MeOH/thf is 97%. 

The addition of Co promoter favours the HDS activity of Mo catalysts was established a long time ago, but the explanation of the promoting effect is still a subject of controversy the latest studies of Tops e s group65 however propose a convincing new model, the CoMoS phase. This active HDS phase... 

Metal Effect. Schrauzer and Schlesinger surveyed the relative activity of various transition-metal ions as catalysts in the reduction of acetylene to ethylene in an aqueous solution containing 1-thioglycerol and excess Na2S204 (8). Except for the remarkably specific activity of Mo, only iridium showed appreciable activity, converting acetylene to ethylene at 15% of the rate of the Mo system. In the catalytic system of cysteine and NaBH4, tungsten, rhodium, rhenium, and ruthenium demonstrated the catalytic activity of approximate 7.0, 2.7, 2.0, and 1.5%, respectively, relative to the Mo system (9). 

In the case of V nitrogenase the efficiency with which N2 competes with H+ as reducible substrate increases as the temperature is decreased from 30°C (47). Within the temperature range 30-5°C, assays with purified enzymes showed that the activity of Mo nitrogenase toward N2 as a substrate decreased 10-fold more than for V nitrogenase. This difference may account for the persistence in nature of what, under some conditions, appears to be an inefficient system. 

Mo(VI)/HZSM-5 catalysts are active and selective catalysts for the conversion of CH4 to aromatics and the best activity can be obtained when Mo loading is about 2-3%. The activity and the stability of the catalyst can be improved by W addition. W(VI)/HZSM-5 catalyst exhibits excellent selectivity for ethane aromatization and thus it may be potential to be used in ethane aromatization process. The higher activity of Mo(VI)/HZSM-5 for the aromatization of CH4 and for the C-C bond cleavage in C2H6 can be attributed to the formation of CH2 that is stabilized by the formation of Mo=CH2 species. Carbon deposit on the catalyst may take part in the conversion of CH4 to aromaties. 

Promotional effects of Co or Ni are also construed in terms of a "remote control" theory proposed by Delmon and co-workers [3]. On the basis of the findings that catalytic synergies are generated even for physical mixtures of supported component sulfides, it is claimed in this theory that the catalytic, activity of Mo sulfides is enhanced by spillover hydrogen originally generated on highly dispersed Co or Ni promoter sulfides in the proximity. 

The authors have tried the method to apply photoreaction of metal carbonyl on solid surface for preparation of catalysts, and have found that metal carbonyls, M(CO)g(M=Mo, W), adsorbed on porous Vycor glass are activated as catalysts by their irradiation with UV light in the presence of reactants (20). Fig.6 shows a time course of the propene reaction carried out on the Mo(CO)g-, W(CO)g- or Cr(CO)g-adsorbed porous Vycor glass(denoted by Mo(CO)g/PVG, etc., hereafter). No reaction occurs under darlc. Propene is converted to equal molar amounts of ethene and butene under the irradiation of Mo(CO)g/PVG and W(CO)g/PVG, but no activation of Cr(CO)g/PVG has been observed. This result indicates the photoinduction of the catalytic activity of Mo(CO)g/PVG and W(CO)g/PVG for the metathesis of propene. The reaction continues for about 30 min even after the stop of the... 

Microsomal MO take part in the reactions of oxidation and reduction of exogenous substances. Some of these reactions are included in processes of activation of OPC (i.e. oxidation), while others represent a part of detoxication mechanisms (i.e. reduction). Activity of MO largely depends on the presence of NADPH and molecular oxygen. 

An advantage of fission-product Mo is the high specific activity of Mo (> 10 Ci/g Mo). Disadvantages are the (1) elaborate and expensive post-irradiation processing facilities, (2) chemical separation of the highly-toxic a-emitters (transuranic radionuclides) and pure yff-emitters, (3) special problems of quality control, and (4) large quantities of long-lived radioactive waste. 

Table 3 Mo loading, pore volume, and specific activity of MoS /zeolite catalysts...

On the basis of the results obtained one may draw a conclusion that there exists a certain relationship between the concentration of acidic sites of different types of zeolite and Mo content. In this coimection, to produce a catalyst e diibiting a high activity in the process of methane dehydroaromatization, it is necessary to optimise the relationship between the acidic sites number of a zeolite and the number of active sites connected with different Mo forms. The highest methane conversion per one run and maximal yield of aromatic hydrocarbons are reached for the sample containing 4.0 mass% of Mo nanopowder. The development of the mesoporous zeolite structure is an important factor promoting the activity of Mo-ZSM-5 in the reactions of the formation of high-molecular aromatic compounds. 

Several other methods have been employed for the preparation of carbon-supported catalysts, although to a lesser extent that impregnation methods. Nakamura et al. [38] prepared molybdenum catalysts for ethene homologation by physical deposition of gaseous [Mo(CO)6]. Their supports were commercial activated carbons that were subjected to different treatments to modify then-surface. The authors compared these supports with oxidic supports and concluded that the interaction between the metal carbonyl and the carbon supports were weaker. Furthermore, they observed that oxidation of the carbon surface was effective in enhancing the catalytic activity of Mo/C, and they ascribed this effect to the contribution of the surface oxygen groups to the partial oxidation of decomposed [Mo(CO)6]. 

Some physical correlations with the catalytic activity of Mo(VI)-grafted carboxylated resins used as epoxidation catalysts... 

Mo(CO)g and W(CO)6 adsorbed on porous Vycor glass or NaY zeolite become active when irradiated with a mercury lamp (> 290 nm) at room temperature in the presence of propene (Wada 1988). They remain active after irradiation ceases, indicating that the catalytically active species is formed by photoreduction of the metal carbonyl on the surface. The activity of Mo(CO)6/Al203 is enhanced by irradiation. 

Sulfur coverage resulted in a decrease in HDS activity of Mo and the catalyst lost its HDS activity after prolonged ( 12 hr) thiophene treatment. In the author s opinion, this is a consequence of M0S2 overlayer formation. 

Wachowski et al. [13] studied Co-Mo sulphides supported on alumina-modified with La203 for the WGS reaction. Results reveal that only higher concentrations of La203 increase the WGS activity. After that they studied the effect of support (alumina, titania, zirconia) on the WGS activity of Mo and Mo-Ni catalysts. Mo-impregnated Ti02 catalyst shows much better activity compared to alumina and zirconia supported catalysts. The addition of Ni to... 

In 1982, Slegeir et al. [24] also reported tungsten-based catalysts for homogeneous WGS reaction. In 1992 Ng and Tsakiri [25] reported Mo(CO)6 catalysts for homogeneous WGS reaction. They investigated methanol-water system and toluene-water systems by using KOH as a base. Table 7.13 shows the effect of Mo(CO)6 and KOH concentrations on the WGS activity of Mo(CO)6 for methanol-water system. 

Apart from UV-cleavable DMNB linkers, a bromohydroxyquinoline (BHQ) linker was also introduced into hairpin cMOs, which enables two-photon activation of MO function at 750 nm [6]. This BHQ linker can provide improved spatial resolution and is less likely to cause any phototoxicity because of UV exposure. 

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