Catalytic activity of transition metal

We have summarized below recent results concerning spectroscopic / flow reactor investigations of hydrocarbons partial and total oxidation on different transition metal oxide catalysts. The aim of this study is to have more information on the mechanisms of the catalytic activity of transition metal oxides, to better establish selective and total oxidation ways at the catalyst surface, and to search for partial oxidation products from light alkane conversion. 

The second example demonstrated immobilization via ship in a bottle , ionic, metal center, and covalent bonding approaches of the metal-salen complexes. Zeolites X and Y were highly dealuminated by a succession of different dealumi-nation methods, generating mesopores completely surrounded by micropores. This method made it possible to form cavities suitable to accommodate bulky metal complexes. The catalytic activity of transition metal complexes entrapped in these new materials (e.g, Mn-S, V-S, Co-S, Co-Sl) was investigated in stereoselective epoxidation of (-)-a-pinene using 02/pivalic aldehyde as the oxidant. The results obtained with the entrapped organometallic complex were comparable with those of the homogeneous complex. 

Figure 8.18 Correlation between catalytic activity of transition metals towards ethane hydrogenolysis and percentage d-character. The closed points represent activities and open points percentage d-character. (Following Sinfelt, 1977.)...

Correlations between catalytic activity and a variety of bulk properties of semiconductors have been reported (i) the average band gap of III-V and II-VI semiconductors and activity towards hydrogenation of isopropanol (ii) enthalpy of oxides and their activity towards oxidation of propylene and (iii) number of d-electrons (and crystal field stabilization energy) or 3rf-metal oxides and their activity towards N2O decomposition. The last correlation, due to Dowden (1972), is important since it provides a connection between heterogeneous catalysis and coordination chemistry of transition-metal compounds. A correlation between the catalytic activity of transition-metal sulphides towards hydrodesulphurization of aromatic compounds and the position of the transition metal in the periodic table has been made by Whittingham ... 

Figure 8.19 Catalytic activity of transition-metal sulphides for hydrodesulphurization of dibenzothiophene. (After Whittingham Chianelli,...

This is the same order of the catalytic activity of transition metal in exchanged zeolites for aniline formation. Irving and Williams (15, 16) pointed out also that there is a clear correlation between complex stability and the second ionization potential. As a matter of fact, a good correlation was found between the catalytic activity and the second ionization potential of divalent ions. (We thank the reviewer for pointing out this correlation.)... 

The catalytic activities of transition metal sulphides were classified in the DHN of THT, the HDS of thiophene and the HN of BP. The results clearly evidence strong similarities between the activities of the most active catalysts in the three reactions, which implies that the catalytic sites involved in each of these reactions are comparable. 

The elevated catalytic activities of transition metals have been attributed to their partially unoccupied d-electron states. Then, information regarding the band structure of these metals is obviously of interest for understanding the catalytic action of these metals [30-34],... 

Whether or not the model systems described here bear any relevance to the B12-coenzyme catalyzed rearrangements must await the test of time. Nevertheless, it is intriguing to speculate that the o 7r = catalytic activity of transition-metals complexes understood by chemists for the past three decades, may have been appreciated and used by nature in the Bi2-coenzyme catalyzed rearrangements for the past three billion years ... 

In considering the effect of the electronic structure of catalysts on activity, Dowden (33) suggested that carbides, and similarly nitrides and carbonitrides, should be less active for synthesis than the corresponding metal since the interstitial atoms may contribute electrons to the unfilled d-shells of the metal, which are believed to be essential for the catalytic activity of transition metals in hydrogenation reactions. This hypothesis is supported by the low activity of cobalt carbide compared with that of reduced cobalt (28,29). For iron catalysts the hypothesis... 

Several parameters influence the catalytic activity of transition metal containing zeolites in thiophene HDS reactions. These are the loading and sulfidation state of the metal, the dispersion, distribution and location of the metal-sulfide phase, and the interaction of this phase with the zeolite support itself and with its acid sites. 

TABLE 4.8. Catalytic Activity of Transition Metal Oxides and Sulfides... 

The catalytic activity of transition metal oxides decreases C03O4 > Cu s NiO > MnOz > Cr203 > Fe203 > ZnO > VjOj > Ti02- The rate-controlling step in these oxidations involves cleavage of an oxygen-catalyst bond. 

The above reaction is also catalysed by several transition metal ions and metal complexes provided the coordination sphere is accessible for H Oj and/or HOj. In our laboratory catalytic activity of transition metal ions and metal complexes supported on various zirconium based inorganic ion exchangers has been studied in detail for the disproportionation of hydrogen peroxide ". 

Catalytic activity of transition metal mixed amorphous aluminium phosphate towards alcohol conversion reactions. 

Figure 7.3. Catalytic activities of transition metals across the periodic table [1881 for the hydrogenolysis of the C-C bond in ethane, the C-N bond in methylamine, and the C-Cl bond in methyl chloride.

The study of gas-phase activation of H-H, C-H and C-C bonds of the hydrogen molecule and saturated hydrocarbons, respectively, by bare transition metal atoms and cations is very attractive for getting insight to the mechanisms and factors (such as nature of metal atoms and their lower-lying electronic states) controlHng catalytic activities of transition metal complexes. Such studies, which are free from the ligand and solvent effects, have been subject of many experimental [2] and theoretical [3] papers in the past 10-15 years. Experimental studies indicate that reaction of some transition metal cations (such as Fe+, Co+, and Rh ) with methane exclusively leads to the ion-molecule complex M+(CH4), while others (such as Sc+ and Ir ) pro-... 

Above we have presented four different factors that control the catalytic activity of transition metals toward o-bonds. In the mono-nuclear transition metal systems (1) the availability of the lower lying s d° and s d" states of the transition metal atoms, and (2) the nature of the ligands facihtating the reduction of the energy gap between the different oxidative states of the transition metal centers are very crucial. Meanwhile, as was demonstrated, in the transition metal clusters the cooperative (or cluster ) effects play important roles in the catalytic activities of these clusters. Another factor, which could be very important for catalytic activity of the transition metal systems is shown to be their redox activity. 

However, those four factors are definitely not the only ones that play crucial roles in the catalytic activity of transition metal systems with o-bonds. The transition metal catalyzed o-bond activation and formation are very complex processes and need more detailed investigations. 

In 1964 a short investigation was made of EPR signals of platinum on alumina by F. Nozaki, D. Stamires and Turkevich.(70) The relation of catalytic activity of transition metal oxides to their EPR properties was studied. Thus in 1967 Kazanski investigated the chromium oxide on silica and its ability to carry out low temperature polymerization of ethylene. 

Metals normally have a narrow d band. The catalytic properties are strongly influenced by the occupational density of the electrons in this band. In many cases a direct relationship has been found between the catalytic activity of transition metals... 

Fig. 5-26 Dependence of the catalytic activity of transition metals in the hydrogenation of ethylene on the percentual d character of the metallic bonding...

Estiu, G., A. Jubert, J. Molina, J. Costamagna, J. Canales, and J. Vargas (1995). Semiempirical SCF/CI interpretation of the origin of the catalytic activity of transition metal-macrocycle complexes. J. Mol. Struct Teochem. 330(1-3), 201-210. 

A. M. Tokmachev, A. L. Tchougreeff and I. A. Misurkin Effective Hamiltonian approach to catalytic activity of transition metal complexes, Int. J. Quantum Chem. 84, 99-109 (2001). 

Seiyama T, Arakawa T, Matsuda T et al (1977) Catalytic activity of transition metal ion exchanged Y zeolites in the reduction of nitric oxide with ammonia. Journal of Catalysis 48 1-7... 

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