Active sites ethane

Complexes with thiolate/thioether ligands such as dttd [dttd = l,2-bis(2-merca-ptophenylthio)ethane] have been extensively investigated [7] as they provide a coordination sphere similar to that found at the active site of various metalloen-zymes [1-4]. [Mo(NO)Cl(dttd)] 6 showed a rich electrode-induced reactivity [27]. 6 undergoes two diffusion-controlled reduction steps, the first one =... 

The nature of the 0(s) species for ethane activation probably varies depending on the catalyst. On reduced Mo/Si02, the surface O species generated by the decomposition of N20 has been shown to react readily with ethane (15), and O has also been suggested as the active species for the Li-Mg oxide catalyst (17). Oxygen vacancies or surface vanadyl groups may also be active sites. 

The main minor product is ethane. (The distribution of the minor products on 0.48% Pt/Si02 catalyst CH4 = 2.6%, CO = 10.6%, C = 32.4%, C = 54.4%.) The minor products are produced by the decarbonylation of methyloxirane, but only the hydrocarbons desorb, the CO remaining adsorbed on the surface. During the decarbonylation process, C-D and C-C bond ruptures occur. It is well known that kink sites are the active sites of C-C hydrogenolysis, so it is understandable that decarbonylation will poison the kink sites. 

Hydrogenation of alkenes on ZnO was studied by means of IR spectroscopy17. The interaction of -adsorbed ethylene and ZnH species was concluded to yield adsorbed ethyl, which reacts with ZnOH to form the product ethane. Higher alkenes adsorb as allylic species. Active sites for hydrogenation and those for exchange and isomerization are independent of ZnO. As a result, the main product in the deuteration of alkenes is the d2 isotopomer. 

However, attempts to develop similar selective catalysts failed in the case of reactions that require one oxygen atom, like the oxidation of methane, ethane and other alkanes to alcohols, aromatic compounds to phenols, alkenes to epoxides, and many others. These mechanistically simple reactions assume one difficult condition the presence of active sites that upon obtaining two atoms from gas-phase 02 can transfer only one of them to the molecule to be oxidized, reserving the second atom for the next catalytic cycle with another molecule. This problem remains a hard challenge for chemical catalysis. 

Fig. 8.8 Product yields evaluated by PAS detector on catalytic ethane oxidation over N02-treated catalysts. N02 gas was flowed onto each catalyst before reaction to produce active site (reproduced by permission of Elsevier from [20]).

Broad empirical experience shows that organic reactivity in hydrocarbon solvents is no less versatile than in water. Indeed, many terran enzymes are believed to catalyze reactions by having an active site that is not waterlike. Further, with ethane as a solvent, a hypothetical form of life would be able to use hydrogen bonding more effectively these bonds would have the strength appropriate for the low temperature. Further, hydrocarbons with polar groups can be hydrocarbon-phobic acetonitrile and hexane, for example, form two phases. It is possible to conceive of liquid/liquid phase separation in bulk hydrocarbons that could achieve the isolation necessary for Darwinian evolution. 

These observations are consistent with those of other UV-vis experiments (Puurunen and Weckhuysen, 2002 Puurunen et al., 2001). Raman spectroscopy of a working alumina-supported vanadia catalyst, showed that the surface population ratio of polymeric-to-isolated vanadia species decreased during reduction (as indicated by a relative loss in intensity of the 1009-cm 1 band relative to that of the 1017-cm 1 band), whereas the total activity and selectivity in propane ODF1 essentially remained unaffected (Garcia-Cortez and Banares, 2002). This result suggested that the active sites for ODH of propane and of ethane on alumina-supported vanadia should be isolated surface vanadia sites, whereas other arrangements of vanadium sites such as polymeric species did not seem to be crucial. 

These results can be easily rationalized if ethane hydrogenolysis requires an ensemble of multiple Ni surface atoms to form an active site. The concentration of active ensembles will decline sharply with composition as the active Ni atoms are diluted with inactive Cu atoms in the surface.. Additional information on how bimetallic catalysts affect chemical reactions can be found in the excellent monograph by Sinfelt (J. H. Sinfelt, Bimetallic Catalysts Discoveries, Concepts, and Applications, Wiley, New York, 9st, . 

The results obtained show that the active sites responsible for N2O catalytic decomposition apparently differ from those leading ethane oxidation by O2. 

The catalytic activity for the conversion of ethane on V and/or Mg containing aiuminophosphates are shown in Table 1. VAPO-5 and MgVAPO-5 samples show an reaction rate higher than the corresponding ALPO4-5 and MgAPO-5. For this reason, it can be concluded that vanadium species are the active sites for the alkane conversion. This has also been proposed in supported vanadium catalysts [1,2,14]. 

LTO ] species cannot be detected under oxidative coupling conditions however, it is probable that the active site participating in the activaticm of methane is in the 0 form. This view is consistent with the report by Peil et al. who determined that the role of the Li/MgO catalyst is more extensive than the generation of methyl radicals. These authors concluded that the catalyst provides two separate pathways for the conversion of methane. One of them is selective, which allows for the fcxmation of ethane, while the second one is active for the fcxmaticxi of carbon oxides. In addition, it was determined that the presrace of lithium increased the mobility... 

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