Metals, activated hydrogenation

Figure 5.10 Oxidation of thiols to sulfonic acid salts using metal activated hydrogen peroxide.

Other interesting polymerizations include the use of metal-activated hydrogen peroxide to deliver low molecular weight pol5uners (83,84), continuous polymerization of water-soluble monomers in extruders (85), dry polymerization of acrylic acid in super critical carbon dioxide (86,87) and on a powder bed (88), and the use of sodium nitrate mediated aqueous pol5unerization to allow high solids (89). 

However, because of the high temperature nature of this class of peroxides (10-h half-life temperatures of 133—172°C) and their extreme sensitivities to radical-induced decompositions and transition-metal activation, hydroperoxides have very limited utiUty as thermal initiators. The oxygen—hydrogen bond in hydroperoxides is weak (368-377 kJ/mol (88.0-90.1 kcal/mol) BDE) andis susceptible to attack by higher energy radicals ... 

A number of indices relate metal activity to hydrogen and coke production. (These indices predate the use of metal passivation in the FCC process but are still reliable). The most commonly used index is 4 X Nickel + Vanadium. This indicates that nickel is four times as actiw as vanadium in producing hydrogen. Other indices [9] used are ... 

Among the three commonly used metal catalysts mentioned above which activate hydrogen, nickel and palladium form hydride phases of essentially the same type. The existence of a platinum hydride has not so far been proved. 

The catalytic activity of the pure /3-palladium hydride has been studied under the appropriate temperature and pressure conditions. The palladium sample was converted into the hydride in a manner which bypassed the area of coexistence of the phases. This was achieved by suitably saturating the metal with hydrogen at 35 atm above the critical temperature and then subsequently cooling the sample to the required temperature and reducing the hydrogen pressure. This method of sample prepare tion allowed one to avoid cracking the palladium crystallites, which would... 

Moreover, a specially active hydrogen species present in a reaction mixture (e.g. atomic hydrogen, protons) (83) or forming during the surface reaction (37) can penetrate into a metal catalyst lattice and become... 

Activation methods can be divided into two groups. Activation by addition of selected metals (a few wt%), mainly transition metals, e.g., fine powders of Fe, Ni, Co, Cr, Pt, Pd, etc. ", or chlorides of these metals when these are reducible to the metal by hydrogen during presintering. The mechanism of activation is not understood (surface tension, surface diffusion, etc.) but is related to the electronic structure of the metal additive. Activation by carbon is also effective. Alternatively, activation utilizes powders in a specially activated state, e.g., very fine (submicronic) powders. ... 

The role of catalytic metal oxide is not only to etrhance the step from S to S, but also, mote importantly, to enhance the step from S to S, which evidently is relatively slow. To achieve this requires a massive supply of active hydrogen ftom hydrocarbons by virtue of interactions with the catalyst. This can be represented by combining the first three equations above as follows ... 

As an additional probe of metal activity, we monitored benzene hydrogenation activity. As seen in Figure 9, Pt-containing rare earth catalysts have lower hydrogenation activity than chlorided alumina catalysts this result reflects inhibition of metal activity on these supports relative to conventional transitional alumina supports. Whereas the acid strength can be adjusted close to that of chlorided and flourided aluminas, metal activity is somewhat inhibited on these catalysts relative to halided aluminas. This inhibition is not due to dispersion, and perhaps indicates a SMSI interaction between Pt and the dispersed Nd203 phase. 

It is noteworthy that the relative proportion of amine 44 and bicumyl (43) which reflects the ratio of the rate of electronation to the rate of reaction with M(H) (the competition between electronation and reaction with M(H)), varies with the Raney metal (compare entries 1 and 3 of Table 1, and entries 2 and 4) and with the electrode potential (compare entries 1 and 2). The more negative is the potential, the faster is the rate of electronation and the higher should be the proportion of bicumyl (43) as observed (entries 1 and 2). The less active the Raney metal as hydrogenation catalyst, the slower is the rate of reaction with M(H) (the lower is the amount of M(H) at the surface of the electrode) and the lower is the amount of aminocumene (44). RCu is the least active catalyst and the proportion of aminocumene (44) is indeed the lowest at the RCu cathode (entry 4). 

The metal-oxo molecular models outlined above have a quite remarkable potential for studying the metal activity in a quite unusual environment. Some of the possibilities could be (1) the generation and the chemistry of M—C, M=C, M=C functionalities (2) the interaction with alkenes, alkynes, hydrocarbons, and hydrogen (3) the activation of small molecules like N26 and CO (4) the support of metal-metal bonded functionalities and (5) the generation of highly reactive low-valent metals. 

In agreement with this lining-up of alkene molecules on the catalyst surface, and the probable approach of activated hydrogen molecules from the body of the metal, it might be expected that hydrogenation would proceed stereoselectively SYN. This is broadly true, and has often been of synthetic/structural importance, e.g. ... 

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