THE ACTIVATION OF HYDROGEN

Hydrogen is one of the key components in HDS as well as of all the other reactions implicated in hydrotreating, and therefore the way in which hydrogen reacts with the catalysts and the nature of the sites at which such activation takes place are of prime concern. Two main types of hydrogen activation processes have been considered [14, 15, 18] one involves a simple homolytic splitting on surface (S-S) units to produce two -SH groups (Eq. 1.3), which some authors think is the major pathway for hydrogen activation  

A more elaborate and perhaps more appealing proposal advanced by several authors [54] involves the heterolytic activation of H2 on e.g. a Mo=S or Mo-S-Mo bond to form Mo-H plus Mo-SH species. The metal hydride is sometimes though to take part in HDS catalysis, but also its oxidation by molybdenum to form a second -SH group can be envisaged as a facile process for a highly mobile H atom in such an sulfur rich surface (Eq. 1.4)  

or a heterolytic activation leading to a metal monohydride and usually requiring an external base to remove the proton, as depicted in Eq. 1.6 for the reaction of RuCl2(PPh3)3 with Hj in the presence of triethylamine the latter mechanism is analogous to the activation proposed on M=S or M-S-M units in which the sulfur would act as the base for proton abstraction. 

Another well-documented way of activating hydrogen on metal complexes is the homolytic cleavage of dihydrogen on two adjacent metal centers which yields also monohydrides, as exemplified for CojCCOjs in Eq. 1.7. 

Additionally, extensive theoretical calculations on such complexes have provided a very deep understanding of the bonding between thiophenes and metal centers and this knowledge has been very helpful in distinguishing the most reasonable proposals to be envisaged in surface chemistry [21-34], This subject is discussed in detail in Chapter 2. 

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Griess has observed crevice corrosion of titanium in hot concentrated solutions of Cl , SOj I ions, and considers that the formation of acid within the crevice is the major factor in the mechanism. He points out that at room temperature Ti(OH)3 precipitates at pH 3, and Ti(OH)4 at pH 0-7, and that at elevated temperatures and at the high concentrations of Cl ions that prevail within a crevice the activity of hydrogen ions could be even greater than that indicated by the equilibrium pH values at ambient temperatures. Alloys that remain passive in acid solutions of the same pH as that developed within a crevice should be more immune to crevice attack than pure titanium, and this appears to be the case with alloys containing 0-2% Pd, 2% Mo or 2[Pg.169]

Nevertheless, Ta5+ and Nb5+ interact with aqueous media containing fluorine ions, such as solutions of hydrofluoric acid. On the other hand, as was clearly shown by Majima et al. [448 - 450], the increased hydrogen ion activity can also significantly enhance the dissolution rate of oxides. The activity of hydrogen ions can be increased by the addition of mineral salts or mineral acids to the solution. 

The use of molybdenum catalysts in combination with hydrogen peroxide is not so common. Nevertheless, there are a number of systems in which molybdates have been employed for the activation of hydrogen peroxide. A catalytic amount of sodium molybdate in combination with monodentate ligands (e.g., hexaalkyl phosphorus triamides or pyridine-N-oxides), and sulfuric acid allowed the epoxidation of simple linear or cyclic olefins [46]. The selectivity obtained by this method was quite low, and significant amounts of diol were formed, even though highly concentrated hydrogen peroxide (>70%) was employed. 

Begue and coworkers recently achieved an improvement in this method by performing the epoxidation reaction in hexafluoro-2-propanol [120]. They found that the activity of hydrogen peroxide was significantly increased in this fluorous alcohol, in relation to trifluoroethanol, which allowed for the use of 30% aqueous H202. Interestingly, the nature of the substrate and the choice of additive turned out to have important consequences for the lifetime of the catalyst. Cyclic dis-ubstituted olefins were efficiently epoxidized with 0.1 mol% of MTO and 10 mol%... 

The dependence of the equilibrium potential on the activities of hydrogen and sulfuric acid is given by the corresponding Nernst equation ... 

Recently, other authors when studying the activation of hydrogen by nickel and nickel-copper catalysts in the hydrogen-deuterium exchange reaction concentrated for example only on the role of nickel in these alloys (56) or on a correlation between the true nickel concentration in the surface layer of an alloy, as stated by the Auger electron spectroscopy, and the catalytic activity (57). 

The activity of hydrogen is obtained from the Nemst equation (equation (3.8)) remember that two protons are involved in the balanced redox reaction, so the numerator within the bracket is written as (H ). Pressure and activity are the same for gaseous mixtures of this type, so the pressure of hydrogen gas is 6.5 Pa. 

Point (3) has been interpreted by Kieboom and van Bekkum (59) as evidence of the similarity in the electronic character of the initial and transition states. However, an alternative explanation would be that the ratedetermining step does not involve the unsaturated compound but only the activation of hydrogen the overall rate then will be determined by the equilibrium adsorption of the unsaturated compound, the extent of which is sensitive to steric effects. 

Let us now express the activities of the main ionic species in solution as a function of the activity of hydrogen ions and f co2(gy Combining equations 8.82 to 8.86, we obtain... 

Even more effective in hydrogen activation are frustrated Lewis pairs [220] containing NHCs as Lewis bases. In 2008 two research groups reported the activation of hydrogen by frustrated Lewis pairs made up from a suitably Af,A -substituted NHC and B(CgF5)3 (Fig. 30) [221, 222]. The corresponding reaction of the frustrated Lewis pair with primary and secondary alkyl amines resulted in the formation of aminoboranes [222]. 

There is an eqnilibrinm constant for the dissolution of hydrogen in each solid phase, Ka and Kb, respectively. Similarly, the activity of hydrogen at the interface, is related to the concentration at the interface and equilibrium constants, Cj = Kaci and c = Kbu, so that Eq. (4.88) becomes... 

Combine your answers to calculate the activity of hydrogen at the interface. 

Cm++. Information about the activation of hydrogen by Cu++ is derived largely from kinetic measurements on the cupric perchlorate catalyzed hydrogenation of dichromate [Equation (4)]. The rate-law for this reaction is of the form... 

Cupric and Cuprous Salts in Inert Solvents. The reduction of cupric heptanoate by hydrogen to the cuprous salt proceeds homogeneously in a variety of nonpolar solvents. In heptanoic acid solution, both the cupric and cuprous salt contribute to the activation of hydrogen, the latter being more active (Chalk and Halpern, 28). The reaction is thus autocatalytic (Fig. 4), the rate-law being of the form... 

The rate-law for the activation of hydrogen (Table I) is second order in the cuprous salt. This contrasts with the rate law for cuprous heptanoate in heptanoic acid but resembles that for the low-temperature path of activation of hydrogen by Ag+ in aqueous solution. As in the latter case, it seems likely that hydrogen is split homolytically in this system to give CuH+ as an intermediate. 

There is some doubt about the kinetics of the activation of hydrogen by cuprous acetate in the closely related solvent, pyridine. Wright, Weller, and Mills (34) have reported that the rate-law in this solvent (and in dodecyl-amine) is first-order in cuprous acetate, suggesting heterolytic splitting of hydrogen. On the other hand, Wilmarth (33) has observed a second-order dependence similar to that in quinoline. The reasons for this discrepancy and for the difference between pyridine and quinoline, if real, are not clear. 

However, selectivity continued to be a problem in the process due to the hard conditions required by the method. Thus, in parallel research, Haruta et al. [253] and Ishihara and coworkers [254] achieved the reaction at only 10 °C. These studies compared the activity of Au/Si02 and Au-Pd/Si02 catalysts and the authors concluded that the enhancement observed when Pd was added to Au was directly related to the activation of hydrogen. However, excess Pd also induced rapid decomposition of H202. 

The definition of pH represents the measure of the activity of hydrogen ions in a solution at a given temperature. It is derived from a combination of p for the word power and H for the symbol for the element hydrogen. Mathematically, pH is the negative log of the activity of hydrogen ions. This relationship is illustrated in the formula... 

Except for some vitamin B12-dependent reactions, the cleavage or formation of carbon-carbon bonds usually depends upon the participation of carbonyl groups. For this reason, carbonyl groups have a central mechanistic role in biosynthesis. The activation of hydrogen atoms (3 to carbonyl groups permits (3 condensations to occur during biosynthesis. Aldol or Claisen condensations require the participation of two carbonyl compounds. Carbonyl compounds are also essential to thiamin diphosphate-dependent condensations and the aldehyde pyridoxal phosphate is needed for most C-C bond cleavage or formation within amino acids. 

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