Transition Metal Oxide Initiators

Attempts to achieve selective oxidations of hydrocarbons or other compounds when the desired site of attack is remote from an activating functional group are faced with several difficulties. With powerful transition-metal oxidants, the initial oxidation products are almost always more susceptible to oxidation than the starting material. When a hydrocarbon is oxidized, it is likely to be oxidized to a carboxylic acid, with chain cleavage by successive oxidation of alcohol and carbonyl intermediates. There are a few circumstances under which oxidations of hydrocarbons can be synthetically useful processes. One group involves catalytic industrial processes. Much effort has been expended on the development of selective catalytic oxidation processes and several have economic importance. We focus on several reactions that are used on a laboratory scale. 

Alkali leach methods axe exemplified by the Bayer process for the preparation of pure a-A C for electrolysis (Section 17.5) from the mineral bauxite. Bauxite consists mainly of a-AlO(OH) (diaspore) and/or 7-A10(0H) (boehmite), the difference between these being essentially that the oxygen atoms form hep and ccp arrays, respectively. The chief contaminants are silica, some clay minerals, and iron(III) oxides/hydroxides, which impart a red-brown color to the mineral. Aluminum (III) is much more soluble than iron(III) or aluminosilicates in alkali, so that it can be leached out with aqueous NaOH (initially 10-15 mol L 1) at 165 °C under approximately 0.6 MPa pressure, leaving a red mud of iron (and other transition metal) oxides/hydroxides and aluminosilicates ... 

The sulfate promoted transition metal oxides focussed considerable attention in recent years due to attractive catalytic properties. Most of the research carried out to date centered on sulfated zirconias,1 5 not surprisingly perhaps, as they exhibit the highest surface acidity (Ho <-16.04) among the members of this family of materials and appear to be able to initiate isomerization reactions in temperatures as low as 298 K. Far less interest attracted sulfated porous titanias, mainly owing to a lower surface acidity,6 although it may be a useful property in many catalytic situations. Thus closer inspection of the preparation procedures for sulfated titanias may be of interest, in particular as the reports on preparation and properties of these materials are scarce and we are not familiar with any work dealing with titania-sulfate aerogels. 

The most remarkable point worthy of mention in relation to the physical aspect of the CTs of transition metal oxides and graphite is the linear relationship between the initial current level fin, and the potential step A . In these studies, the term initial current level indicates the value of current at a time of 2-10 s during the CT experiments, rather than at a moment of application of potential step [96]. 

As with NVC-Zeolite (13X and SK-500) systems, the initiation in the NVC-MMT system was suggested [56-58] to be cationic involving Br( )nsted acid sites in MMT arising from the dissociation of interlayer water molecules coordinated to the exchangeable cations [59]. Yet another possibility, especially with NVC, was that the transition metal oxides Fe203/Ti02 present in MMT could also lead to cationic initiation of NVC (Scheme 1) [32,59] ... 

The mechanisms by which transition-metal oxidizing agents convert alcohols to aldehydes and ketones are rather complicated and will not be dealt with in detail here. In broad ontline, chromic acid oxidation involves initial formation of an alkyl chromate ... 

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