Reaction mechanisms for hydrogenation

Although the basic chemical reactions have been thought, for many years, to be similar with either sulfuric or hydro -fluoric acid catalyst, extensive work with sulfuric acid described by Albright et al (24) has demonstrated that this is probably not the case. The need for more detailed work with hydrofluoric acid is cited. The following summarizes their report "The reaction mechanism for hydrogen fluoride alkylation also seems to be radically different than that for sulfuric acid alkylation. The alkylation mechanism which has been widely accepted in the past seems to be somewhat more satisfactory for hydrogen fluoride alkylation even though it is not for sulfuric acid... 

Full reaction mechanism for hydrogen oxidation including rate data and sources... 

H2/O2/N2 flames have been investigated in a first step for the simplicity of their chemical schemes compared to hydrocarbons. Furtfiermore the reaction mechanisms for hydrogen flames are quite well established to date and reasonable agreement between simulations and experiments (e.g. for ignition or planar, laminar flames) is reported. Moreover, a thorough understanding of the H2/O2/N2 mecha-... 

Strohle, J., Myhrvold, T. Reduction of a detailed reaction mechanism for hydrogen combustion under gas turbine conditions. Combust. Flame 144, 545-557 (2006)... 

J. Strohle, T. Myhrvoid, An evaluation of detailed reaction mechanisms for hydrogen combustion under gas turbine conditions, hit. J. Hydrogen Energy 32(1), 125-135 (2007)... 

The reaction takes place extremely rapidly, and if D2O is present in excess, all the alcohol is converted to ROD. This hydrogen-deuterium exchange can be catalyzed by either acids or bases. If D30 is the catalyst in acid solution and DO the catalyst in base, write reasonable reaction mechanisms for the conversion of ROH to ROD under conditions of (a) acid catalysis and (b) base catalysis. 

Carefully study the reaction mechanism for the stearoyl-CoA desaturase in Figure 25.14, and account for all of the electrons flowing through the reactions shown. Also account for all of the hydrogen and oxygen atoms involved in this reaction, and convince yourself that the stoichiometry is correct as shown. 

In this section we apply the adaptive boundary value solution procedure and the pseudo-arclength continuation method to a set of strained premixed hydrogen-air flames. Our goal is to predict accurately and efficiently the extinction behavior of these flames as a function of the strain rate and the equivalence ratio. Detailed transport and complex chemical kinetics are included in all of the calculations. The reaction mechanism for the hydrogen-air system is listed in Table... 

Figure 9.8. Global reaction mechanism for the hydrodesulfurization of thiophene, in which the first step involves hydrogenation of the unsaturated ring, followed by cleavage ofthe C-S bond in two steps. Butadiene is assumed to be the first sulfur-free product,...

Reaction step 5 in Scheme 3.1 can be rnled ont becanse the flnoranil ketyl radical (FAH ) reaches a maximum concentration within 100 ns as the triplet state ( FA) decays by reaction step 2 while the fluoranil radical anion (FA ) takes more than 500 ns to reach a maximum concentration. This difference snggests that the flnoranil radical anion (FA ) is being produced from the fluoranil ketyl radical (FAH ). Reaction steps 1 and 2 are the most likely pathway for prodncing the flnoranil ketyl radical (FAH ) from the triplet state ( FA) and is consistent with the TR resnlts above and other experiments in the literatnre. The kinetic analysis of the TR experiments indicates the fluoranil radical anion (FA ) is being prodnced with a hrst order rate constant and not a second order rate constant. This can be nsed to rnle ont reaction step 4 and indicates that the flnoranil radical anion (FA ) is being prodnced by reaction step 3. Therefore, the reaction mechanism for the intermolecular hydrogen abstraction reaction of fluoranil with 2-propanol is likely to predominantly occur through reaction steps 1 to 3. 

Based on the data listed in Table 20.1, a value of 0.42% P was calculated for an anchored catalyst having three triphenylphosphine ligands, 0.28% P with two phosphine groups and 0.14% with one triphenylphosphene. An analytical value of 0.37% P was found which indicates that all three triphenyl-phosphines (TPP) are present in the catalyst as depicted by 4 in Scheme 20.2. However, only 0.11% P was found in the catalyst sample taken after catalyst pre-hydrogenation indicating that only one TPP is present on the active entity. Because of steric constraints between the bulky TPP and the HP A, it would appear that the TPP should be in the axial position as in 5. A proposed reaction mechanism for the anchored Wilkinson based on that shown in Scheme 20.1 is shown in Scheme 20.2. 

The primary radical yields are often 3. A much higher value (>10) indicates chain reaction. In fact, the chain reaction mechanism for the formation of HC1 from a gaseous mixture of hydrogen and chlorine exposed to radium irradiation is one of the earliest example of this kind, although the detailed chemistry was later shown to involve dissociated atoms rather than electrons and ions, as was originally proposed (see Bansal and Freeman, 1971). 

Further mechanistic insights into hydrogenations catalyzed by HRuCl(PPh3)3 (7, p. 83) have been obtained indirectly, from studies on hydrogenation of some ruthenium(III) phosphine complexes (83). A frequently considered mechanism for hydrogen reduction of metal salts involves slow formation of an intermediate monohydride, followed by a faster reaction between the hydride and starting complex (/, p. 72), Eqs. (2) and (3) ... 

Because of the complexity of the rhodium-catalyzed reduction of benzaldehyde to benzyl alcohol with CO and H20, it is not possible to fully elucidate the mechanism of catalytic reduction given the extent of the kinetic studies performed to date. However, the results do allow us to draw several important conclusions about the reaction mechanism for benzaldehyde hydrogenation and several related reactions. 

The reaction mechanism for the solid state reduction is the same as that described above for the hydrogen reduction of haematite, namely the formation of a porous iron product which results from the penetration of pores in the reacting pellets by reducing gases, and the migration of the reaction products, C02 and H20 through these pores back into the gaseous phase. 

The two main reaction mechanisms are analogous to the mechanisms for hydrogen evolution. The equivalent scheme to the Volmer-Tafel mechanism is ... 

The isotope effect is observed with the hydrogen atom of the formate and not with the hydrogen atom of the water molecule. The result is similar to that observed on ZnO, where the ratedetermining step of the formate decomposition is suggested to be dissociation of the CH bond of the bidentate formate. In summary, the reaction mechanism for the catalytic WGS reaction on Rh/Ce02 is essentially the same as that on ZnO. 

Figure 1. Reaction mechanism for the hydrogenation process of CO2 proposed by Noyori and co-workers [12].

Nevertheless, the mechanism of the Shvo s catalyst has been one of the most controversial regarding the nature of the hydrogen-transfer process (84). The analysis of this reaction mechanism served as an example of comparison of both the inner- and outer-sphere reaction pathways for hydrogenation of polar, C=0 (85-87) and C=N (88—95) and unpolar bonds (95). In the next subsections are presented the mechanistic studies we carried out for the hydrogenation of ketones, imines, alkenes, and alkynes (29,87,95). 

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