Neutral inorganic precursors

In 1995, Tanev and Pinnavaia [1] have reported the synthesis of a new type of mesoporous molecular sieve designated as the hexagonal mesoporous silica (HMS). Instead of using the ionic inorganic precursor and surfactant as in the case of MCM-41 [2], HMS is manufactured by hydrolysis reaction between a neutral inorganic precursor, tetraethyl-orthosilicate (TEOS) and a neutral primary amine surfactant (8-18 carbons). HMS possesses numerous favourable characteristics, but, like MCM-41, its synthesis process can only be concluded by the removal of the surfactant. This was reportedly done either by calcination at 630°C or by warm ethanol extraction [1]. 

PCH materials offer new opportunities for the rational design of heterogeneous catalyst systems, because the pore size distributions are in the supermicropore to small mesopore range (14-25A) and chemical functionality (e.g., acidity) can be introduced by adjusting the composition of the layered silicate host. The approach to designing PCH materials is based on the use of intercalated quaternary ammonium cations and neutral amines as co-surfactants to direct the interlamellar hydrolysis and condensation polymerization of neutral inorganic precursor (for example, tetraethylorthosilicate, TEOS) within the galleries of an ionic lamellar solid. 

Pinnavaia etal. used neutral alkylamines as templates to form disordered mesoporous silica, named hexagonal molecular sieves (HMS). The S°I° formation mechanism was proposed between neutral amine micelles (S°) and neutral inorganic precursors (1°). The interactions between S° and 1° were assumed to be hydrogen bonding. The resulting HMS has a worm-like pore structure, with thicker framework walls and smaller X-ray scattering domain sizes compared to M41S. 

Apart from cetyltrimethylammonium bromide (CTAB), the classical surfactant in M41S syntheses, a variety of differently sized cationic, anionic and neutral surfactants exists (figure 1), which can be used in order to mesostructure various inorganic precursors. 

Tanev et al. have reported the synthesis of mesoporous materials via a route which involves self-assembly between neutral primary amines and neutral inorganic framework precursors.12 The regularity of the pore structure in these materials has been illustrated by lattice images which show a honeycomb like structure. The system of channels of these molecular sieves produces solids with very high internal surface area and pore volume. This fact combined with the possibility of generating active sites within the channels produces a very unique type of acid catalyst. In the case of transition metal substituted M41S, the principal interest lies in their potential as oxidation catalysts, especially Ti and V substituted MCM and HMS type materials, and more recently synthesised large pore materials.13... 

Ferrihydrite has indeed been found in association with magnetite in Magnetospirillum magnetotacticum (Frankel et ah, 1983). It seems essential that the cell solution is sufficiently buffered to maintain a neutral pH and thus ensure that the solubility product of magnetite is always exceeded. This is a reaction which easily takes place in a purely inorganic system at ambient temperature (see chap. 14). Lepidocrocite has also been suggested as a magnetite precursor (Abe et al. 1983). 

Since the discovery of hexacarbonylvanadium(O) and hexacarbonylvanadate( 1-) by Calderazzo and co-workers in 1959 and 1960, these substances have been key precursors to a variety of vanadium compounds, including inorganic noncarbonyl species, organovanadium complexes," and other vanadium carbonyls. Neutral V(CO)6 is of special interest in that it is the only isolable 17-electron homoleptic metal carbonyl and exhibits fascinating chemical properties that are often reminiscent of iodine and classic pseudohalogens. ... 

Neutral and Anionic Lithiumsilylamides as Precursors of Inorganic Ring Systems... 

The most frequently used electron-reservoir complex for stoichiometric singleelectron transfer reactions is (Fe Cp( / -C6Me6)], because of its stability and ease of preparation, and since it has one of the most negative redox potentials in the series. It can reduce most inorganic and organometallic cations [2]. For instance, it is very useful to synthesize neutral 19-electron complexes (C in the equation below) such as other (Fe Cp( / -arene)] complexes and (Fe ( / -C6Me6H)( / -C6Me6)] from the 18-electron cationic precursors C[PF6 ... 

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