Activity methanation

Nickel. As a methanation catalyst, nickel is presently preeminent. It is relatively cheap, it is very active, and it is the most selective to methane of all the metals. Its main drawback is that it is easily poisoned by sulfur, a fault common to all the known active methanation catalysts. The nickel content of commercial nickel catalysts is 25-77 wt %. Nickel is dispersed on a high-surface-area, refractory support such as alumina or kieselguhr. Some supports inhibit the formation of carbon by Reaction 4. Chromia-supported nickel has been studied by Czechoslovakian and Russian investigators. 

Kanazawa, S., Chang, J.S., Round, G. et al. (1997) Removal of NOx from flue gas by corona discharge activated methane radical showers, J. Electrostatics 40 41, 651-6. 

Other soft, electrophilic metals, including mercury(ll) analogs, are also known to activate methane.16... 

The resulting radicals are not usually observed, but thermal desorption products indicate the nature of the surface intermediates. Molybdenum(V) dispersed on silica also gives rise to 0 and O2 ions when exposed to N2O and O29 respectively. The 0 ion on this surface may be used to activate methane and ethane in a catalytic cycle which leads to their partial oxidation. 

Copper enzymes are involved in reactions with a large number of other, mostly inorganic substrates. In addition to its role in oxygen and superoxide activation described above, copper is also involved in enzymes that activate methane, nitrite and nitrous oxide. The structure of particulate methane mono-oxygenase from the methanotrophic bacteria Methylococcus capsulatus has been determined at a resolution of 2.8 A. It is a trimer with an a3P33 polypeptide arrangement. Two metal centres, modelled as mononuclear and dinuclear copper, are located in the soluble part of each P-subunit, which resembles CcOx subunit II. A third metal centre, occupied by Zn in the crystal, is located within the membrane. 

Cp Th(CH3)2] MgCl2.3oo (73) than for surface species 67. This proves the importance of cahonic surface species in polymerizahon reachons, since the number of active sites is >35% for the former. The reactivity of [Cp Th(CH3)2] MgCl2.3oo (73) was further examined toward propylene and/or 3,3 -dimethylbutene. This study rather suggests an aUyhc C-H bond activation/methane ehmination (Equation 12.1) followed by olefin inserhon than direct propylene insertion into the Th-R bond (Equahon 12.2). This observed reactivity is in agreement with that one described previously for organolanthanide complexes [CpJLnR] [142, 180, 181]. 

Bimetallic Catalysts and Promoters. Shah and co-workers compared the methane decomposition activities and stabilities for monometallic (Pd, Mo or Ni) and bimetallic M-Fe (M = Pd, Mo or Ni) catalyst above 673 Their studies showed that the bimetallic M-Fe catalysts produced hydrogen at significantly higher rates than the monometallic (M) catalysts. The Pd-Fe catalyst was found to be the most active methane decomposition catalyst at 973 K. 

Rhodium is a unique metal since it can catalyze several transformations.222,223 It is an active methanation catalyst and yields saturated hydrocarbons on an inert support. Methanol is the main product in the presence of rhodium on Mg(OH)2. Transition-metal oxides as supports or promoters shift the selectivity toward the formation of C2 and higher oxygenates. 

Doubly activated methane compounds with aryl substituents can also be converted to formazans 31 by elimination of both of the activating groups... 

CuCl2 supported on silica (in presence of KC1, LaCl3 or A1C13 as cocatalysts) is also an active catalyst in oxychlorination of ethylene and other hydrocarbons104. Silica is also the support of choice for a Rh(III) complex which has been discovered to activate methane for chlorination via an electrophilic mechanism105. 

M. J. Wax, J. M. Stryker, J. M. Buchanan, C. A. Kovac, and R. G. Bergman, Reversible C—H Insertion/Reductive Elimination in ( 5-Pentamethylcyclopentadienyl)(trimethyl-phosphine)iridium Complexes. Use in Determining Relative Metal-Carbon Bond Energies and Thermally Activating Methane, J. Am. Chem. Soc. 106, 1121-1122 (1984). 

Watson was the first to show that methane could be attacked by a Group IB metal reagent, in a reaction termed cr-bond metathesis . This reaction probably proceeds via electrophilic attack on the C— bond by the reagent (equation 8). Marks describ a nondegenerate example (equation 9) and Wolczan-ski has shown that a zirconium imidate (Zr=NR) can also activate methane, the basic amine group receiving the proton released from methane Equation 10). 

Fox, B. G., and Lipscomb, J. D., 1988, Purification of a high specific activity methane monooxygenase hydroxylase component from a type II methanotroph, Biochem. Biophys. Res. Commun. 154 165nl70. 

Rataj, M. J., Kauth, J. E., and Donnelly, M. L, 1991, Oxidation of deuterated compounds by high specific activity methane monooxygenase from Methylosinus trichosporium. Mechanistic implications, J. Biol. Chem. 266 18684918690. 

The necessity of a larger ensemble for the dissolution of carbon into nickel than for activating methane corresponds to observations in surface physics. Adsorbed carbon atoms result in a distortion of the metal atom geometry whereas the bonding of adsorbed methane may require only 3-4 free nickel atoms. In simple terms, the two-dimensional surface sulfide prevents a distortion of the surface being necessary for the diffusion of surface carbon atoms into the bulk nickel phase. 

In addition to the aluminosilicates, some obvious synthetic analogues, such as borosilicates, gallosilicates and ferrisilicates, have also been explored. The boro-silicates are more difficult to prepare, probably because the much smaller boron, with its tendency towards planar, three-fold coordination, is not an obvious substitute for the role of aluminum. Gallium and iron(III) materials, though, are well known and usually have properties similar to those of the aluminosilicates. There is evidence, however, for some unique behavior with gallosilicates, as in their ability to catalyze cyclization reactions and activate methane [30]. Other silicate-based systems include the titanosilicates, mentioned earlier, and some recently discovered vanadium silicates that show excellent thermal stability and can potentially be activated for catalysis [31]. 

Copper enzymes are involved in reactions with a large number of other, mostly inorganic, substrates. In addition to its role in oxygen and superoxide activation described above, copper is also involved in enzymes which activate methane, nitrite, and nitrous oxide. 

The first well-characterized example of oxidative addition of the unreactive C—H bond of an alkane was based on Ir complexes containing a Cp ligand (eq (72) and (73) [82,83,51]. Photoirradiation is needed to create active lrCp L species. However, a cationic complex [IrCp (PMe3)(Me)(ClCH2Cl)] [B(Arf)4]" can thermally activate methane and alkane at 10 °C (eq (74)) [84]. 

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