Molybdenum removal

Helz GR, Miller CV, Chamock JM, Mosselmans JEW, Pattrick RAD, Gamer CD, Vaughn DJ (1996) Mechanism of molybdenum removal from the sea and its concentration in black shales EXAFS evidence. Geochim Cosmochim Acta 60 3631-3642 Hille R (1996) The mononuclear molybdenum enzymes. Chem Rev 96 2757-2816... 

Molybdenum Removal by Adsorption on Activated Carbon [5.34]. The principle of the method is the selective adsorption of thiomolybdate by activated carbon while tungstate will not be retained. 

In a series column operation, thiomolybdate is sorbed in the first column (resin 930) and tungstate in the second (resin 937). The molybdenum removal is better than 99%. The impiffity to WO3 ratios are ... 

Similarly, neither iron nor copper can substitute for molybdenum once removed from metal-free xanthine or aldehyde oxidase, and up to 5000 times the number of molecules of molybdenum removed are necessary for reactivation of one molecule of apoenzyme (Mahler and Green, 1954). 

Mechanism of molybdenum removal from the sea and its concentration in black shales. Geochim. Cosmochim. Acta., 60, 3631-3642. 

Molybdenum removal. Mo is really a special member of the NFPN group. Its solubility at 375°C, probably the coldest fuel temperature, is estimated to be less than 1 ppm. Moreover, it is produced at a rate equivalent to 0.38 ppm/day. Thus, probably the most feasible way to remove Mo would be to precipitate it out of solution onto cold fingers immersed in the circulating fuel. The precipitation rate for a 500-Mw reactor would be 54 g/day, most of which would be stable Mo. Even with cold traps, some Mo will... 

Another type of demasking involves formation of new complexes or other compounds that are more stable than the masked species. For example, boric acid is used to demask fluoride complexes of tin(IV) and molybdenum(VI). Formaldehyde is often used to remove the masking action of cyanide ions by converting the masking agent to a nonreacting species through the reaction ... 

Thiophene [110-02-17, C H S, and dibenzothiophene [132-65-OJ C22HgS, are models for the organic sulfur compounds found in coal, as well as in petroleum and oil shale. Cobalt—molybdenum and nickel—molybdenum catalysts ate used to promote the removal of organic sulfur (see Coal CONVERSION... 

The conversion of CO to CO2 can be conducted in two different ways. In the first, gases leaving the gas scmbber are heated to 260°C and passed over a cobalt—molybdenum catalyst. These catalysts typically contain 3—4% cobalt(II) oxide [1307-96-6] CoO 13—15% molybdenum oxide [1313-27-5] MoO and 76—80% alumina, JSifDy and are offered as 3-mm extmsions, SV about 1000 h . On these catalysts any COS and CS2 are converted to H2S. Operating temperatures are 260—450°C. The gases leaving this shift converter are then scmbbed with a solvent as in the desulfurization step. After the first removal of the acid gases, a second shift step reduces the CO content in the gas to 0.25—0.4%, on a dry gas basis. The catalyst for this step is usually Cu—Zn, which may be protected by a layer of ZnO. 

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. 

Most of the Moco enzymes catalyze oxygen atom addition or removal from their substrates. Molybdenum usually alternates between oxidation states VI and IV. The Mo(V) state forms as an intermediate as the active site is reconstituted by coupled proton—electron transfer processes (62). The working of the Moco enzymes depends on the 0x0 chemistry of Mo (VI), Mo(V), and Mo (TV). 

Phthalocyanines are excellent lubricants at temperatures of 149—343°C (191). Combinations with other lubricants, like grease, molybdenum, or tungsten sulfides, have found appHcations in the automotive industry or professional drilling equipment (192—195). Further uses include indicators for iron(Il), molybdenum(V), and uranium(IV) (196) or redox reactions (197), medical appHcations like hemoglobin replacements (198) or sterilisation indicators (199), or uses like in gas filters for the removal of nitrogen oxides from cigarette smoke (200). 

The tert-huty hydroperoxide is then mixed with a catalyst solution to react with propylene. Some TBHP decomposes to TBA during this process step. The catalyst is typically an organometaHic that is soluble in the reaction mixture. The metal can be tungsten, vanadium, or molybdenum. Molybdenum complexes with naphthenates or carboxylates provide the best combination of selectivity and reactivity. Catalyst concentrations of 200—500 ppm in a solution of 55% TBHP and 45% TBA are typically used when water content is less than 0.5 wt %. The homogeneous metal catalyst must be removed from solution for disposal or recycle (137,157). Although heterogeneous catalysts can be employed, elution of some of the metal, particularly molybdenum, from the support surface occurs (158). References 159 and 160 discuss possible mechanisms for the catalytic epoxidation of olefins by hydroperoxides. 

Heating and Cooling. Heat must be appHed to form the molten zones, and this heat much be removed from the adjacent sohd material (4,70). In principle, any heat source can be used, including direct flames. However, the most common method is to place electrical resistance heaters around the container. In air, nichrome wine is useflil to ca 1000°C, Kanthal to ca 1300°C, and platinum-rhodium alloys to ca 1700°C. In an inert atmosphere or vacuum, molybdenum, tungsten, and graphite can be used to well over 2000°C. 

Berylha ceramic parts ate frequendy used in electronic and microelectronic apphcations requiting thermal dissipation (see Ceramics as ELECTRICAL materials). Berylha substrates are commonly metallized using refractory metallizations such as molybdenum—manganese or using evaporated films of chromium, titanium, and nickel—chromium alloys. Semiconductor devices and integrated circuits (qv) can be bonded by such metallization for removal of heat. 

Reclamation, Disposal, and Toxicity. Removal of poisons and inorganic deposits from used catalysts is typically difficult and usually uneconomical. Thus some catalysts are used without regeneration, although they may be processed to reclaim expensive metal components. Used precious metal catalysts, including automobile exhaust conversion catalysts, are treated (often by the suppHers) to extract the metals, and recovery efficiencies are high. Some spent hydroprocessing catalysts may be used as sources of molybdenum and other valuable metals. 

Ammonia production from natural gas includes the following processes desulfurization of the feedstock primary and secondary reforming carbon monoxide shift conversion and removal of carbon dioxide, which can be used for urea manufacture methanation and ammonia synthesis. Catalysts used in the process may include cobalt, molybdenum, nickel, iron oxide/chromium oxide, copper oxide/zinc oxide, and iron. 

On molybdenum sulfate roasting, downstream of a scrubber, to remove ammonium sulfite and sulfate aerosols which form in the ammonia scrubbing stage, and SO2,... 

The virgin naphtha feed after having been hydrofined over a cobalt molybdenum catalyst to remove sulftir compounds passes through the Powerformer. After stabilization, the aromatics are recovered by extracdon with... 

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