Precursor synthesis techniques

The use of precursor synthesis techniques as described above is driven by the fact that decomposition of the precursor material into the catalyst often results in catalysts that have activity or selectivity superior to that of preferred products. The amine thiomolybdate decomposition described above results in MoS2 catalysts with surface areas exceeding several hundred square meters per gram (27). The HDS activity increases correspondingly and unpromoted catalysts have activities approaching those of promoted systems. 

Because of the sequential nature of atomic layer deposition, it is a slow method for preparing thin films. The sequential nature, however, also produces a film of uniform thickness, referred to as a conformal film. This is important when the surface being coated is not atomically flat, but rather, has troughs and islands to be coated. Some of the most important technological materials, such as silicon and germanium, have not shown themselves to be amenable to the atomic layer deposition technique. This points to the need for continued research in the field of precursor synthesis. 

Ferrite ceramics have found widespread applications as materials for permanent magnets and recording media due to their low cost and attractive magnetic properties. The conventional method for manufacturing ferrite ceramics involves solid state reaction of oxide or carbonate precursors at high temperature. Although technically simple, this method does not readily allow control of the product s microstructure and purity, which is necessary for the attainment of optimal magnetic properties. As a result, there has been considerable research into the development of alternative synthesis techniques. 

The foregoing powder synthesis techniques, along with others such as polymerization and decomposition of complex double alkoxides and the melt and spray decomposition of mixed nitrates, produce precursor powders with a high degree of chemical homogeneity. However upon subsequent calcination of the products of these reactions, the crystallization path invariably passes through the following sequence of reactions ... 

This study is aimed at using and identifying Al- and Co-containing Anderson-type heteropolyanions in the synthesis of (Co)Mo/alumina and (Co)Mo/zeolite HDS catalysts oxidic precursors. Various techniques such as Raman spectroscopy, NMR and EXAFS are used to show the apparition of the AlMo6024H6 entity upon impregnation of an alumina or a zeolite with an ammonium heptamolybdate solution. The question of the Mo-Co interaction in the promoted precursors is also tackled through the use of the CoMo6024H6 entity. 

From carbohydrates, several diphosphines have been derived and used as ligands in hydroformy-lation reactions (Section D.l. 5.8.). a,a -TREDFP (a,a -40) is prepared from commercially available a, a -trehalose (39) by the reaction sequence outlined42,43 similarly, the / ,/f-anomer is obtained from / ,/ -trehalose44. L-Iditol is commercially available but expensive, but may be prepared by catalytic reduction of L-sorbose and separation from the isomeric L-sorbitol by fractionated crystallization of the hexaacetate45. Conversion to the diphosphine 42 involves a further fractional crystallization for the separation of isomeric ditosylates46. The reactions for the synthesis of the precursors employ techniques described in Section 4.4. for the derivatization of carbohydrates. 

Syntheses reported for the pentaammine(trifluoromethanesulfonato-O) complexes can be readily adapted for other amine or multidentate amine analogs. Syntheses of coIbalt(III) complexes with 1,2-ethanediamine or A -ethyl-l,2-eth-anediamine ligands have been reported earlier in this series. To exemplify the procedures further, trifluoromethanesulfonato-O complexes of cobalt(III), chro-mium(III), and rhodium(III) with unidentate methylamine ligands based on the readily prepared [M(NH3)5Cl]Cl2 precursors are reported here. The following sections report syntheses of 1,2-ethanediamine complexes of Rh(III) and Irflll) and of Ru(II) and Os(II) diimines with trifluoromethanesulfonato ligands. Such syntheses indicate the diversity of the synthesis technique, and the complexes described are excellent precursors for other compounds. 

Metal-containing polymers may be produced by various methods, such as chemical reactions of precursors— in particular, reactions of metal salts in polymer solutions, the treatment of polymers with metal vapors, or the polymerization of various metal-monomer systems [1-4], Depending on the metal nature and the polymer structure, these processes lead to organometallic units incorporated into polymer chains, metal-polymer complexes, or metal clusters and nanoparticles physically connected with polymer matrix. Of special interest are syntheses with the use of metal vapors. In this case, metal atoms or clusters are not protected by complexones or solvate envelopes and consequently have specific high reactivity. It should be noted that the apparatus and principles of metal vapor synthesis techniques are closely related to many industrial processes with participation of atomic and molecular species [5]—for example, manufacturing devices for microelectronic from different metals and metal containing precursors [6]. Vapor synthesis methods employ varying metals and... 

Recently Cabrera et al. [47] developed a new synthesis of ordered mesoporous solids involving the use of atrane complexes as precursors. This technique was tested to obtain silica, Al and Ti mesoporous oxides and simply and doubly doped oxides with a large list of metals [see Table 3 in reference 47]. Other dopants or chemical groups were added to MCM type materials. For... 

Regardless of the precise structure of the chosen half southern synthon, the two main problems to be solved are the establishment of the carbon skeleton and the introduction of the necessary chirahty into the molecule. The published approaches have introduced chirality either by resolution, by starting with a chiral precursor, or via use of asymmetric synthesis techniques. The carbon skeleton has been established by use of a wide variety of techniques including the Diels-Alder and other cycloaddition reactions, heteroatom induced cyclizations, intramolecular Michael or Aldol cyclizations, intramolecular ether formation, and radical cyclization. 

The chemistry of the alumina precursor used for fiber spinning varies with synthesis technique. This chemistry is maintained throughout the pyrolysis process and affects the nature of the crystals formed at higher temperatures. For instance, the presence of the AI13 complex leads to highly ordered spinel structures, whereas the presence of 5-coordinated Al species in solution produces poorly ordered crystals (Wood et al., 1990). 

Chemically the interlayers in the modular phases described previously are relatively reactive. This has been exploited to fabricate large numbers of phases that are not available via the normal solid-state chemistry preparative method of high-temperature synthesis from mixed oxides or oxide precursors. The technique generally employs low temperatures, typically room temperature to 300°C, and long reactions times, of the order of 1 week. This procedure, termed chimie douce , is often translated as soft chemistry but is better called mild chemistry or gentle chemistry . Under such conditions, much of the stmcture remains intact, and the product phases have a strong topotactic relationship to the starting stmctures. For the modular perovskite phases, in... 

The potential of advanced synthesis techniques to develop CaO-based sorbents stabilized by a support material has been demonstrated recently by Broda et al. [74] who synthesized AhOs-stabilized CO2 sorbents by employing a sol-gel technique. Here, aluminium iso-propoxide was added to reverse osmosis water (15 Mf2 cm) and allowed to hydrolyse for 30 min at 75 °C. Subsequently, a calcium precursor, i.e. calcium acetate hydrate (Ca(CH300)2-H20), calcium nitrate tetrahydrate (Ca(N03)2-4H20) or calcium acetylacetonate (Ca(CsH702)), was dissolved in water and, together with an acid (either acetic or nitric acid), added... 

The most conmum synthesis technique for carbon cryogels from polymer precursors is based on their thermal processing in a non-oxidizing atmosphere, usually argOTi or nitrogen, at 800-1,000 °C. The polymer precursor is mainly synthesized by... 

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