Activity of ethane

Acetic acid can be synthesized from methane using an aqueous-phase homogeneous system comprising RhCI as catalyst, CO and 02.17 Side-products included methanol and formic acid, although yields of acetic acid increased upon addition of either Pd/C or iodide ions. The active species is thought to be a CH3-Rh(l) derivative, formed from the C-H activation of methane. The activation of ethane was also achieved, although selectivities were lower, with products including acetic and propionic acids and ethanol (Equation (9)). 

Electron-rich iridium(l) complexes can perform C—H activation reactions under mild conditions [13]. In this line, acetone-dis solutions of the [(ri -l,3,5-C 5H3Me3)) lr(Ti -C2H4)(P Pr3)]BF4 complex, at room temperature, show deuterium incorporation to the ethane ligand, most likely due to the participation of hydrido vinyl iridium(lll) species, formed by the C—H activation of ethane, according to Scheme 2.25 [21]. 

Unlike higher alkanes, ethane contains only primary C—H bonds, and the dehydrogenation product ethene contains only vinylic C—H bonds. As shown in Table I, these are strong bonds. Thus one would expect that, compared to other alkanes, the activation of ethane would require the highest temperature, but the reaction might be the most selective in terms of the formation of alkene. Indeed, this appears to be the case. 

The hydrodynamic behavior of liquid films can be characterized by the dimensionless Weber number We = (82 p g sina)/c. In Figure 4 the Weber number of a falling film (since = 1) is plotted against the activity of ethane for the system oleic acid/ethane. At low temperatures the temperature dependence of the Weber number is small. The Weber number changes little at low activities. For activities greater than 0.8 the Weber number increases sharply. The transition region from the first appearance of instability to disintegration into droplets or trickles corresponds with the sharp increase of the Weber number. 

For instance the activation of ethane proceeds after initial cleavage of the CH bonds, whereas the C-C bond is weaker than the CH bond. This is because the C-C bond can only be activated once removal of some hydrogen atoms has made both C atoms accessible for activation. When, for instance, due to alloying the ensemble of atoms is not large enough to accommodate the fragments of dissociated ethane only CH activation occurs and the preferred product will be ethylene, a molecule in which the number of C-C bonds is maintained. 

In recent studies, 3c-2e transition states have been found along the reaction coordinate at various levels of theory for the activation of methane by Sc , Zr , and group 11 metal ions (Cu , Ag , Au ), as well as for the activation of ethane by Co and Nb . Schwarz and coworkers have recently made... 

The importance of transient intermediates derived from both of the competitors is further suggested by the relative molar activity of ethane produced in the reaction (cf. Table VII). With a sixfold excess of N(4S) this is derived equally from ethylene and from propane. With a twofold excess it derives to a greater extent from ethylene. Extensive data of this sort could be instructive as to the nature and reactions of such transients. 

Activation of ethane with these tethered dinuclear species was observed to give exclusive C—H activation, despite the fact that C—C bond cleavage would be thermodynamically more favorable (170). For steric reasons, C—H activation of methane is kinetically favored over C—H activation of ethane or methanol substrates in these systems (second-order rate constants in the order H2 > CH4 > MeOH > Et > MePh). Activation parameters and kinetic isotope effects derived from kinetic studies of C—H activation processes with these tethered complexes are consistent with the previous conclusions derived from reactions with [Rh°(TMP)]. 

This greatly simplifies analysis of the system since the two rates differ only in the fundamental rate constant and the concentration of the reactants. Computer simulation of the consecutive reaction mechanism allows one to determine the 2/ 1 needed to achieve the desired yield of C2H4. Figure 6 shows how the selectivity to B varies as a function of conversion of A for various 2/ 1 ratios. (Since oxidative activation of ethane to give an ethyl radical results in an ethylene product, it only serves to consume oxidant and can be ignored. If direct ethane conversion to CO2 occurs this will reduce C2 yield.) The results in Figure 4 are fit well by a 2/ 1 about 6. 

Frash M, van Santen RA (2000) Activation of ethane on Zn-exchanged zeolites a theoretiad study. Phys Chem Chem Phys 2 1085... 

RebeUleau-Dassomieville M, Rosini S, van Veen A C, Farrusseng D and Mirodato C (2005), Oxidative activation of ethane on catalytic modified dense ionic oxygen conducting membranes , Catal Today, 104,131-137. 

Wang H,Tablet C, SchiestelT and Caro J (2006b), Hollow fiber membrane reactors for the oxidative activation of ethane , Catal Today, 118,98-103. 

The activation of C-H bonds for different hydrocarbons can occur both at Zn + and ZnOZn + sites. We will first discuss hydrocarbon activation by Zn +. The results presented here are based on quantum-chemical cluster calculations. The reaction energies involved in the overall catalytic cycle for the activation of ethane over a Zn + cation and a ZnOZn " " oxycation adsorbed on a representative cluster chosen to model the ZSM-5 adsorption site are compared in Fig. 4.22. 

Figure 4.22a. The structures and energies involved in the catalytic activation of ethane by the Zn + exchanged at a ZSM-5 adsorption site .

Figure 4.22b. Activation of ethane by an ion-exchanged ZnOZn + cluster .

Wang, H., Tablet, C., Schiestel, T., et al (2006). Hollow Fiber Membrane Reactors for the Oxidative Activation of Ethane, Catal Today, 118, pp. 98-103. 

RebeiUeau-DassonneviUe, M., Rosini, S., Van Veen, A., et al. (2005). Oxidative Activation of Ethane on Catalytic Modified Dense Ionic Oxygen Conducting Membranes, Catal. Today, 194, pp. 131-137. 

Despite much effort exerted to identify new 16-electron fragments capable of reacting with C—H bonds in methane and other saturated hydrocarbons, only a few such systems are known. The benchmark remains [Cp ML] (M = Rh, Ir L = PMea, CO). These complexes continue to appear in the literature in mechanistic and application studies. The activation of ethane by irradiation of [Cp Ir(CO)2] in supercritical C02(SC—CO2) is greatly accelerated in solutions saturated with The extremely photolabile... 

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