Structure-directed synthesis

Girreser U., Giuffrida D., Kohnke F. H., Mathias J. P., Philp D., Stoddart J. F. The Structure-Directed Synthesis of Cyclacene and Polyacene Derivatives Pure... 

Eschenmoser [15] has recently alluded to the concepts of structure-directed synthesis and autochthonous reactivity — the propensity of certain systems to undergo reactions as a consequence of their intrinsic molecular structural factors — within the context of certain biomolecules, particularly the organic cofactor, vitamin B12. The formation of the AD ring junction in the corrin ligand had to be regarded as a major hurdle in any proposed chemical synthesis of cobyric acid (5) (Fig. 2), and hence of vitamin B12. However, driven initially by the desire to identify a dark, and therefore potentially biomimetic, variant of the photochemical... 

This approach mimics familiar biological self-assembly phenomena such as protein folding [ 192], protein aggregation [ 192] and nucleotide pairing [ 188]. It incorporates features described in each of the above strategies (i.e., I—III), to give specialized nanoscopic structures, that can be precisely designed, usually with excellent control over CMDPs. Recent examples include so called structure directed synthesis by Stoddart [3a] (see Chapter 1 of this book) to produce toroidal bis-bipyridinium cyclophanes that are reminiscent of a molecular abacus , melamine-cyanuric acid lattices by Whitesides [193] and unique helical structures based on coordination of bipyridyl units to copper (II) ions by Lehn [194],... 

Keywords Materials / Microporous Solids / Organometallic complexes / Porosils / Structure-directed Synthesis... 

The use of organic molecules in the synthesis of zeotype solids is an especially interesting preparative method for extended inorganic solid materials. Organic molecules cannot survive the harsh conditions of the classical high-temperature route involving reaction of the components in the solid state. Structure-directed synthesis thus belongs to the "soft chemistry" routes for the preparation of solid-state compounds [12, 13]. 

Keywords Structure-Directed Synthesis / Mesoporous Materials / Mesostructured Materials / Trialkoxysilanes / M41S Materials... 

The size and shape of the intracrystalline pore/cavity system are critical for the utility of microporous crystals. Thus, the ability to direct the synthesis towards crystals of desirable pore structure, particularly of large-pore 3-dimensional networks, is of great importance. Therefore, new studies should be directed to the characterization of structural precursors at the molecular level. In particular, emphasis should be placed on synthesis media incorporating cations, molecular species, and/or templates that are structure-directing. These factors may ultimately determine the nature of the pores as well as the crystal lattice. Chemical studies coupled with NMR and other in-situ methods of characterization may provide information leading to predictable structure-directing synthesis methods. 

Quaternary ammonium compounds as structure directing agents in zeolite synthesis... 

The importance of quats as structure-directing agents in zeolite synthesis was recently underlined (ref. 5) by the synthesis of the new zeolites SSZ-26 and -33, which combine 10- and 12-ring pores. The templates applied are shown in Figure 3. 

Since then, organic amines, quaternary-ammonium bases, metal complexes, and other compounds have been extensively used in zeolite synthesis, acting as space fillers with low specificity, structure-directing agent, or true templates ... 

For the sake of developing commercial reactors with high performance for direct synthesis of DME process, a novel circulating slurry bed reactor was developed. The reactor consists of a riser, down-comer, gas-liquid separator, gas distributor and specially designed internals for mass transfer and heat removal intensification [3], Due to density difference between the riser and down-comer, the slurry phase is eirculated in the reactor. A fairly good flow structure can be obtained and the heat and mass transfer can be intensified even at a relatively low superficial gas velocity. 

The same periodic structures can also be formed from alternating AIO4 and PO4 tetrahedra the resulting aluminophosphates are not called zeolites but AlPOs. Zeolites are made by hydrothermal synthesis under pressure in autoclaves, in the presence of template molecules such as tetramethylammonium, which act as structure directing agents. 

V-Sb-oxide based catalysts show interesting catal)dic properties in the direct synthesis of acrylonitrile from propane [1,2], a new alternative option to the commercial process starting from propylene. However, further improvement of the selectivity to acrylonitrile would strengthen interest in the process. Optimization of the behavior of Sb-V-oxide catalysts requires a thorough analysis of the relationship between structural/surface characteristics and catalytic properties. Various studies have been reported on the analysis of this relationship [3-8] and on the reaction kinetics [9,10], but little attention has been given to the study of the surface reactivity of V-Sb-oxide in the transformation of possible intermediates and on the identification of the sxirface mechanism of reaction. 

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