Catalytic properties, viii

The successive demethylation scheme of hydrogenolysis just discussed for iron, cobalt, and nickel clearly does not apply to the noble metals of group VIII. This can be seen by examining the product distribution data in Table IV. The amounts of methane observed are much lower than would be expected if the hydrogenolysis occurred by successive demethylation steps. Thus, we have another indication that the noble and nonnoble metals of group VIII behave as two separate classes with regard to their catalytic properties in the hydrogenolysis of hydrocarbons. 

Catalysts for this codimerization reaction can be derived from prac-tially all the Group VIII transition metal compounds. Their catalytic properties, such as rate, efficiency, yield, selectivity, and stereoselectivity, vary from poor to amazingly good. Some better-known catalyst systems and their product distributions are listed in Table I. As one can see, the major codimerization product under the given condition is the linear 1 1 addition product, 1,4-hexadiene. The formation of this diene and its related C6 products will become the center of our discussions. The catalyst systems that have been investigated rather extensively are derived from Rh, Ni, Co, and Fe. We shall cover these systems in some detail. A lesser-known catalyst system based on Pd will also be briefly discussed. 

Metals and alloys. The principal industrial metallic catalysts ate found in periodic group VIII which are transition elements with almost completed 3d, 4d, and 5d electron orbits. According to one theory, electrons from adsorbed molecules can fill the vacancies in the incomplete shells and thus make a chemical bond. What happens subsequently will depend on the operating conditions. Platinum, palladium, and nickel, for example, form both hydrides and oxides they are effective in hydrogenation (vegetable oils, for instance) and oxidation (ammonia or sulfur dioxide, for instance). Alloys do not always have catalytic properties intermediate between those of the pure metals since the surface condition may be different from the bulk and the activity is a property of the surface. Addition of small amounts of rhenium to Pt/A12Q3 results in a smaller decline of activity with higher temperature and slower deactivation rate. The mechanism of catalysis by alloys is in many instances still controversial. 

Krocher, O., Koppel, R.A., Froba, M. and Baiker, A. (1998) Silica hybrid gel catalysts containing group(VIII) transition metal complexes preparation, structural, and catalytic properties in the synthesis of N, N-dimethylformamide and methyl formate from supercritical carbon dioxide. Journal of Catalysis, 178, 284-298. 

Present knowledge of the electronic, surface, and catalytic properties of non-metals is substantially less advanced than that of metals. This is partly due to the fact that various physicochemical techniques are less rapidly applicable to non-metal catalysts. Relatively few metal oxide catalysts have been investigated by XAES techniques (Table VIII and Ref. 18). 

A process for the preparation of fluorobcnzencs comprises the heating of fluorobenzaldehydes in the presence of a catalyst. Suitable catalysts are transition metals from the B groups 1, 11. VI. VII and VIII. The best catalytical properties seem to be held by rhodium and the metals of the platinum group, e.g. formation of 1.3-difluorobenzene (5). The reaction maybe carried out in homogeneous solution with soluble rhodium catalysts (Wilkinson s catalyst) or in heterogeneous phase with the catalyst fixed on a carrier. ... 

The effect of FSO3H on HF alkylation was very much like that of CF3SO3H. The 9-to-l blend of Isobutane with refinery olefins, alkylated earlier with HF alone (Table IV), was used to study the catalytic properties of HF-FSO3H blends. Table VIII gives the results of runs made at 4 C and 1.0 minute... 

Many reactions of organic chemistry are catalyzed by complexes of transition-metal ions, most notably those of Group VIII. Here, die catalyst is a system of different complexes that are linked by ligand-exchange and ligand-dissociation equilibria and differ in their catalytic properties, occasionally with rather counterintuitive results such as a decrease in rate with increase in pressure in a reaction in which gas is consumed. 

When bulky alkenes are used, addition of a second alkene molecule to produce an oxo-bis(l,2-diolato)osmate(VI) 7 is preferred. Subsequent osmium oxidation and hydrolysis releases the 1,2-diol and regenerates the trioxo(l,2-diolato)osmate(VIII) 4 that can reenter the catalytic cycle. In this latter case amine addition has no major effect. Proof for the existence of species of type 7 (X-ray) as well as for the catalytic properties of 4 have recently been provided38. 

To understand the catalytic properties of the various group VIII metal surfaces, the interactions among neighboring adsorbed species and their... 

The catalytic properties of H-, Li-, Na-, K-, Mg-, Ca-, Zn-, Cd-, and Al-forms of synthetic mordenite in the reactions of cyclohexane and n-pentane isomerization and benzene hydrogenation have been studied. The cation forms of mordenite that do not involve the metals of column VIII of the Mendeleyev Table show high activity in these reactions. To elucidate the mechanism of n-pentane isomerization, the kinetics of the reaction on H-mordenite have been studied. Carbonium ion is supposed to result from splitting off hydride ion from hydrocarbon molecule. Na-mordenite catalytic activity in benzene hydrogenation reaction decreases linearly with the increase of decationization. This indicates that cations are responsible for the catalytic activity of zeolite. The high activity of cations of nontransition metals in oxidation-reduction reactions seems to be quite unexpected and may provide evidence for some uncommon mechanism of benzene hydrogenation. 

Heterogeneous Catalytic Properties of the Group VIII Metals... 

Homogeneous Catalytic Properties of Group VIII Salts and Complexes... 

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