Methanation turnover numbers

Catalyst Methanation turnover numbers (sec Activation energy (kJ/mol)... 

The iridium complex [Ir(cod)(//2-,PrPCH2CH2OMe)]+BF4 (22) in dichloro-methane at 25 °C at 1 bar H2 is a particularly active catalyst for the hydrogenation of phenyl acetylene to styrene [29]. In a typical experiment, an average TOF of 50 mol mol-1 h-1 was obtained (calculated from a turnover number, TON, of 125) with a selectivity close to 100%. The mechanism of this reaction has been elucidated by a combination of kinetic, chemical and spectroscopic data (Scheme 14.10). 

During the 1970s Shilov published extensively on the reactions of alkanes in aqueous solutions of platinum(II) complexes [3]. The reactions are typically carried out in aqueous hydrochloric acid as solvent at <100°C with chloride salts of Pt(II) as catalyst and the chloride salts of Pt(IV) as the stoichiometric oxidant. Typical reaction yields, based on added methane, are less than 3% with >75% selectivity to methanol and methyl chloride. It was proposed the reaction proceeded via C-H activation to generate alkyl platinum intermediates in reactions with alkanes and later results are consistent with this proposal [4]. This system is one of the first systems proposed to operate via the C-H activation reaction and to generate potentially useful functionalized products. The key disadvantages of the Shilov system were the low rates (catalyst tum-over-frequency, TOF, <10 s ), short catalyst life (turnover-number, TON, <20), and the use of Pt (IV) as a stoichiometric oxidant. 

In fact, with the expected trend toward upgrading coal and the depletion of United States natural gas reserves, methanation should further increase in importance. The methanation reaction is thermodynamically favorable even at high temperatures and pressures and there are a variety of metals which catalyze this reaction. Vannice (231) compared the turnover numbers for a variety of Al203-supported group VIII metals and showed that the rate of... 

Vannice (232) measured turnover numbers for methanation on a variety of well-characterized palladium catalysts. The supported catalysts were all more active than unsupported palladium (Pd black) and PdHY was intermediate between Pd/Si02 and Pd/Al203 in specific activity (see Table VII). As is evident from the data there is no obvious correlation betwen particle size and turnover number. It was therefore suggested that the enhanced activity of various supported catalysts was due to a metal-support interaction. Figureas et al. (105) also found good evidence for a support effect during benzene hydrogenation studies. In this case the palladium zeolite... 

The same workers (234) also studied the methanation behavior of bimetallic clusters of Ru/Ni and Ru/Cu in zeolite Y. Such clusters can be formed by metals, such as ruthenium and copper which are immiscible as bulk metals (235, 236). The turnover numbers versus bimetallic cluster composition are shown in Fig. 22. Dilution of ruthenium with copper clearly causes a marked decrease in specific activity. This decrease in activity is also accompanied by a decrease in methanation selectivity. This was attributed to an inhibiting effect of copper on the ruthenium hydrogenolysis activity. 

The turnover numbers and activation energies for methanation over all four metals freshly reduced and under sulfur-free conditions (Table XVI) were in good agreement with values reported for supported metal catalysts (220). At 673 K, Ni and Ru exhibited only very slow losses in activity apparently due to slow carbon deposition, whereas Co and Fe underwent rapid, severe carbon deactivation after maintaining their fresh catalyst activity for a few hours. After rapid deactivation the final steady-state activity, which was about 100-fold lower than the activity of the fresh catalyst, was approached slowly this activity region was referred to as the lower pseudosteady state. Likewise, the fresh catalyst behavior was referred to as the upper pseudo-... 

Treatment of catalyst Dispersion (%) Turnover number x lO sec"i) Nn Ni to isobutane and to isopentane methane Selectivity JVi/ATh... 

Thauer et al., 2008). MCR converts methyl-CoM (methyl-SCoM) and N -mercaptoheptanoylthreonine phosphate (CoBSH) to methane and the CoB-SS-CoM heterodisulfide (Table 15.1, Reaction 6), with a turnover number of 100 s and a kcat/K (methyl-SCoM) of 1 x 10 s . The structure of the enzyme from M. 

Quantitative data on the turnover numbers for methane production, and their variation with CO and H2 partial pressure, as summarized in Table... 

Methane conversion and turnover number (TON) calculated at 325°C of fresh and aged catalysts calcined at 500 and 800°C. 

These fluorinated catalysts were also employed in the functionalization of the first members of the series of alkanes, from methane to butane. To that end, the reaction was carried out in supercritical carbon dioxide, mixed with the alkane, in a fluid mixture that also dissolved the catalyst and EDA (Scheme 14). In this manner, a significant amount of methane (and the other alkanes) was available to react with the in situ generated metallocarbene intermediate, with no other C—H bond in the reaction mixture. Methane has been converted into ethyl propionate with turnover numbers up to 750 at 40 °C, and a total pressure of 250 atm. 

Turnover number for production of methane (molecules/Rh atom/sec). 

Table II shows the results with C2, C3, and cycloCfi and manganese clusters 1-4 (Figure 1) with t-butyl hydroperoxide at room temperature in acetonitrile (methane did not react under the reaction conditions). The important observation of no catalyst decomposition upon continual addition of t-butyl hydroperoxide to again provide the initial turnover number is an extremely important characteristic of any biomimetic catalyst. It is interesting to note that this increase in catalyst lifetimes occurred in acetonitrile and not methylene chloride and shows the dramatic effect of a coordinating solvent We did not see any indication of acetonitrile activation.

Fig. 15. Methanation rates for 100-torr CO and 400-torr H2 on a Pt foil. The turnover number (NCh4) is in units of molecules/site/second and is based on I x 1015 sites/cm2 for the...

Other works on alkane oxidations in trifluoroacetic acid have been also described [56]. Finally, it has been found that heating an aqueous solution of sodium vanadate in the presence of methane, carbon monoxide and air gives rise to the formation of acetic acid, as well as methanol and formaldehyde in smaller amounts [57]. The yield of CH3COOH attains 3700% based on vanadium after 50 h at 100 °C, the total turnover number being 49. The reaction is sensitive to the pH of the solution. For example, the yields of the products decrease noticeably if pH > 5. The reaction apparently involves hydrogen atom abstraction from methane by a radical or radical-hke species which could be generated via the reduction of V(V) with carbon monoxide. 

Yamanaka et al. have demonstrated that aerobic hydroxylation of methane in trifluoroacetic acid at 40 °C is catalyzed by NH4VO3 or VO(acac)2 if zinc powder is present [56m]. Methyl trifluoroacetate is formed with a turnover number ca. 10 after 1 h. The authors suggested that the active oxygen species in... 

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