Silicates divalent state

Thermal reduction at 623 K by means of CO is a common method of producing reduced and catalytically active chromium centers. In this case the induction period in the successive ethylene polymerization is replaced by a very short delay consistent with initial adsorption of ethylene on reduce chromium centers and formation of active precursors. In the CO-reduced catalyst, CO2 in the gas phase is the only product and chromium is found to have an average oxidation number just above 2 [4,7,44,65,66], comprised of mainly Cr(II) and very small amount of Cr(III) species (presumably as Q -Cr203 [66]). Fubini et al. [47] reported that reduction in CO at 623 K of a diluted Cr(VI)/Si02 sample (1 wt. % Cr) yields 98% of the silica-supported chromium in the +2 oxidation state, as determined from oxygen uptake measurements. The remaining 2 wt. % of the metal was proposed to be clustered in a-chromia-like particles. As the oxidation product (CO2) is not adsorbed on the surface and CO is fully desorbed from Cr(II) at 623 K (reduction temperature), the resulting catalyst acquires a model character in fact, the siliceous part of the surface is the same of pure silica treated at the same temperature and the anchored chromium is all in the divalent state. 

The study of coordination compounds of the lanthanides dates in any practical sense from around 1950, the period when ion-exchange methods were successfully applied to the problem of the separation of the individual lanthanides,131-133 a problem which had existed since 1794 when J. Gadolin prepared mixed rare earths from gadolinite, a lanthanide iron beryllium silicate. Until 1950, separation of the pure lanthanides had depended on tedious and inefficient multiple crystallizations or precipitations, which effectively prevented research on the chemical properties of the individual elements through lack of availability. However, well before 1950, many principal features of lanthanide chemistry were clearly recognized, such as the predominant trivalent state with some examples of divalency and tetravalency, ready formation of hydrated ions and their oxy salts, formation of complex halides,134 and the line-like nature of lanthanide spectra.135... 

Iron occurs in two oxidation states, the divalent (Fe ) ion or trivalent (Fe ) ion, and sedimentary rocks contain iron in these various forms with ferric oxides being the most common. When iron is weathered out of the rocks, it is not retained in solution but, depending upon conditions, it is redeposited as oxides or hydroxides. In addition, Fe can replace aluminium in some silicate minerals. An important chemical feature of iron (in solution) is its tendency to form complexes with organic materials. Such complexes are considerably more stable and consequently survive in solution or in the soil for longer periods of time. Specific examples of Fe-organic complexes will be discussed in later sections. 

Figure 10.14 shows the calculated ionization states of soluble silica based on Eqs. 10.20 to 10.23 as a function of solution pH at 25°C. In neutral solutions, only silicic acid exists. For pH values from 10 to almost 13, the monovalent species dominates, whereas for pH values greater than 13, the divalent species prevails. Not until very high alkalinities do the tri- and tetravalent ions appear in solution. 

Siloxide ligands are able to coordinate to rare earth metals in various oxidation states and coordination numbers to primarily form mono- and dinuclear complexes. In particular, the synthetic and stmctural chemistry of trivalent rare earth siloxides are well documented in the literature and show analogies with rare earth alkoxides. It is fair to state, however, that the field of divalent and tetravalent rare earth siloxides is poorly developed and that applications pertaining to the design of siloxide-based homogeneous and heterogeneous rare earth metal catalysts as well as the development of novel silicate-based materials are scarce. Although the few results of the catalytic activity of some of the rare earth siloxides in olefin... 

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