Methane hydrogen transfer process

Alternatively, the BDE values may be reported relative to the C-H bond dissociation energy in methane (3) as the reference. This is quantitatively described in Equation 5.3 as a formal hydrogen transfer process between methane (3) and a substituted carbon-centered radical 2. The reaction enthalpy for this process is often interpreted as the stabilizing influence of substituents Rj, R2, and R3 on the radical center and thus referred to as the radical stabihzation energy (RSE). When defined as in Equation 5.3, positive values imply a stabilizing influence of the substituents on the radical center. The RSE energies are connected to the BDE values in Equations 5.land 5.2 as described in Equation 5.4. 

It was found, that also Ru and Os colloids can act as catalysts for the photoreduction of carbon dioxide to methane [94, 95]. [Ru(bpy)3]2+ plays a role of a photosensitizer, triethanolamine (TEOA) works as an electron donor, while bipyridinium electron relays (R2+) mediate the electron transfer process. The production of hydrogen, methane, and small amounts of ethylene may be observed in such a system (Figure 21.1). Excited [Ru(bpy)3]2+ is oxidized by bipyridinium salts, whereas formed [Ru(bpy)3]3+ is reduced back to [Ru(bpy)3]2+ by TEOA. The reduced bipyridinium salt R + reduces hydrogen and C02 in the presence of metal colloids. Recombination of surface-bound H atoms competes with a multi-electron C02 reduction. More selective reduction of C02 to CH4, ethylene, and ethane was obtained using ruthenium(II)-trisbipyrazine, [Ru(bpz)3]2+/TEOA/Ru colloid system. The elimination of hydrogen evolution is thought to be caused by a kinetic barrier towards H2 evolution in the presence of [Ru(bpz)3]2+ and noble metal catalysts [96]. 

An important deactivation process for MAO-activated catalytic systems is a-hydrogen transfer which leads to the production of methane.The condensation reaction of the metallocenium alkyl + MAO forms Zr—CH2—A1 or Zr—CH2—Zr structures (eq 47), and these species are considered to be... 

In Equation 8.68, the plus sign concerns the case when volatile substance is transferred from aqueous phase into the bubble volume and the minus sign concerns the case when the diffusion of volatile substance occurs in opposite direction. And the concentration gradient has the plus sign when the process of water degassing is analyzed (i.e., the evolution of chlorine, chloroform, ammonia, methane, hydrogen sulfide, and other gases analyzed, if their concentration in air is considerably lower than that in aqueous phase). 

P(OMe)3] with Ati=4.2x 10 s" (30 °C). As with the osmium complexes just discussed this rate is several orders of magnitude lower than that of methane elimination from the corresponding methyl hydride while reductive elimination of methane from [Co(Me)2 P(OMe)s 4]+ exhibits an intermediate reactivity. The latter reaction is consequently thought to proceed via a multistage mechanism in which carbene complexes (resulting from a-hydrogen transfer to the metal) play an important role. Elimination of CH4 from the methyl hydride complex is probably a concerted process according to these workers. 

In a later publication, Kolbel et al. (K16) have proposed a less empirical model based on the assumption that the rate-determining steps for a slurry process are the catalytic reaction and the mass transfer across the gas-liquid interface. When used for the hydrogenation of carbon monoxide to methane, the process rate is expressed as moles carbon monoxide consumed per hour and per cubic meter of slurry ... 

Kolbel et al. (K16) examined the conversion of carbon monoxide and hydrogen to methane catalyzed by a nickel-magnesium oxide catalyst suspended in a paraffinic hydrocarbon, as well as the oxidation of carbon monoxide catalyzed by a manganese-cupric oxide catalyst suspended in a silicone oil. The results are interpreted in terms of the theoretical model referred to in Section IV,B, in which gas-liquid mass transfer and chemical reaction are assumed to be rate-determining process steps. Conversion data for technical and pilot-scale reactors are also presented. 

Steinfeld et al. [133] demonstrated the technical feasibility of solar decomposition of methane using a reactor with a fluidized bed of catalyst particulates. Experimentation was conducted at the Paul Scherrer Institute (PSI, Switzerland) solar furnace delivering up to 15 kW with a peak concentration ratio of 3500 sun. A quartz reactor (diameter 2 cm) with a fluidized bed of Ni (90%)/Al2O3 catalyst and alumina grains was positioned in the focus of the solar furnace. The direct irradiation of the catalyst provided effective heat transfer to the reaction zone. The temperature was maintained below 577°C to prevent rapid deactivation of the catalyst. The outlet gas composition corresponded to 40% conversion of methane to H2 in a single pass. Concentrated solar radiation was used as a source of high-temperature process heat for the production of hydrogen and filamentous... 

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