Activation 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 activity of transition metal catalysts depends on both the metal and the ligands. In addition, solvent effects, etc. can play a role. Table 3.10 shows examples of transition- metal catalysts with the reactions for which they are active (Farkas, 1986). 

Schatzschneider, U. Photoactivated Biological Activity of Transition-Metal Complexes. Eur. J. Inorg. Chem. 2010,1451-1467 (2010). 

Suslick KS, Johnson RE (1984) Sonochemical activation of transition metals. J Am Chem Soc 106 6856-6858... 

The activity of transition metal allyl compounds for the polymerization of vinyl monomers has been studied by Ballard, Janes, and Medinger (10) and their results are summarized in Table II. Monomers that polymerize readily with anionic initiators, such as sodium or lithium alkyls, polymerize vigorously with allyl compounds typical of these are acrylonitrile, methyl methacrylate, and the diene isoprene. Vinyl acetate, vinyl chloride, ethyl acrylate, and allylic monomers do not respond to these initiators under the conditions given in Table II. 

The above substitution effects appear to be independent of the nature of the ligand (16) since the benzyl compounds behave similarly, Table XI. It would appear from these observations that the introduction of anionic ligand would be sufficient to increase activity of transition metal alkyl compounds for polymerization. This, however, is probably an oversimplifica-... 

Another recent application to the activation of transition metals was reported (247) by Bonnemann, Bogdavovic, and co-workers, in which an extremely reactive Mg species was used to reduce metal salts in the presence of cyclopentadiene, 1,5-cyclo-octadiene, and other ligands to form their metal complexes. The reactive Mg species, characterized as Mg(THF)3 (anthracene), was produced from Mg powder in THF solutions containing a catalytic amount of anthracene by use of an ultrasonic cleaning bath. A plausible scheme for this reaction has been suggested ... 

Activated metals, generation via the reduction of metal salts, 223-225 Activation of transition metal slurries, mechanisms, 202 Addition reactions... 

Radicals are versatile synthetic intermediates. One of the efficient procedures for radical generation is based on one-electron oxidation or reduction with transition metal compounds. An important feature is that the redox activity of transition metal compounds can be controlled by appropriate ligands, in order to attain chemoselectivity in the generation of radicals. The application to small ring compounds provides useful methods for organic syntheses. Reductive transformation are first reviewed here. 

In general the activity of transition metal complexes for alkene isomerisation is low in the presence of carbon monoxide, but HCo(CO)4 is an exception to this rule. Depending on conditions, full equilibration of the alkene isomers is obtained. 

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,...

Moderately good correlations are being established for the relative activities of transition metals for the total oxidation of hydrocarbons to C02. Moro-Oka, Morikawa, and Ozaki (13) show a fair relationship between oxidation rate of 2-C4H8, C2H2, or C2H4 and the heat of formation of the oxide per oxygen atom contained therein. Maximum activity is observed at the lowest values of heat of formation. 

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.)... 

Crystalline phases, surface areas (Sggip), specific and intrinsic activities of transition metal sulphides... 

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. 

Photochemical activation of transition metal carbonyls has been used as a preparative tool for substitution of carbonyl ligands by donor molecules or unsaturated hydrocarbons for many years (7-6). The advantage of photochemical activation in comparison with thermal activation is the possibility of conducting reactions at fairly low temperatures. Hence even thermolabile products can be prepared and isolated by appropriate treatment of the reaction mixtures. However, due to the various activation modes of transition metal carbonyls by UV light, often more than one product is obtained, and chromatographic separation is necessary. Limitations are set primarily by the amount of substance which can be irradiated in solution at one time. 

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... 

Fig. 10. Activities of transition metal ions loaded on zeolite X (space velocity 23,400 cm3 (gcatalyst) 1 hr, NO 300 ppm, 02 6.4 vol %). (Reproduced from Ref. 51 with permission from the authors and the American Chemical Society.)...

While commercial HDS catalysts are based on Mo, other transition metals are also active, some more active than Mo. In Figme 1 is a plot of the HDS activities of transition metal... 

---