Silica phase transition

In addition to the external forces, the catalyst must also resist internal forces imposed on the pellet as phase transitions in the catalyst material progress. These transitions, including e.g. transformation of the amorohous silica carrier into crystalline a-cristobalite, precipitation of V4+ and compounds, and destruction of the carrier by the melt, may eventually cause the catalyst to break up in smaller particles or even to catalyst powder. 

Pyrene has been used to investigate the extent of water penetration into micelles and to accurately determine critical micellar concentrations (Kalyanasundaram, 1987). Polarity studies of silica or alumina surfaces have also been reported. In lipid vesicles, measurement of the ratio Ii/Iui provides a simple tool for determination of phase transition temperatures and also the effect of cholesterol addition. 

Demiralp E, Cagin T, Goddard WA(1999) Morse stretch potential charge equilibrium force field for ceramics Application to the quartz-stishovite phase transition and to silica glass. Phys Rev l tt 82 1708-1711 Dewar MJS (1977) Ground states of molecules. The MNDO method. Approximations and parameters. J Am Chem Soc 99 4899-4907... 

Figure 7.9A shows the NaAlSi04-Si02 (nepheline-silica) system, after Schairer and Bowen (1956). Let us first examine the Si02-rich side of the join. At P = 1 bar, the pure component Si02 crystallizes in the cristobalite form (Cr) at r = 1713 °C (cf figure 2.6). At P = 1470 °C, there is a phase transition to tridymite (Tr), which does not appreciably affect the form of the liquidus curve, which reaches the eutectic point at P = 1062 °C. 

NMR spectra heteronuclear gold cluster compounds, 39 345-348 Phalaris canariensis esophageal cancer, 36 144-145 scanning proton microprobe, 36 149 structural motifs of silicas, 36 146 Pharmaceuticals, 18 177 Phase transitions, in chalcogenide halide compounds, 23 332, 408, 412 [PhCHjMejNAlHjlj, 41 225-226 [(PhCH2)jNLi]3 molecular structure, 37 94, 96 in solution, 37 107-108... 

LB films of 1,4,8,11,15,18-liexaoctyl-22,25-bis-(carboxypropyl)-phthalocyanine (2), an asymmetrically substituted phthalocyanine, were stable monolayers formed at the water—air interface that could be transferred onto hydrophilic silica substrates (32—34). When a monolayer film of the phthalocyanine derivative was heated, there was a remarkable change in the optical spectrum. This, by comparison to the spectrum of the bulk material, indicated a phase transition from the low temperature herringbone packing, to a high temperature hexagonal packing. 

A new catalyst formulation containing alkali metals and W on a silica support gives more promising results.549 Alkali metals are able to lower the phase transition temperature from amorphous silica to a-crystoballite, shown to be critically important for an effective catalyst, while incorporation of W enhances catalytic activity to ensure high methane conversion and excellent ethylene selectivity. An alkali-stabilized tungsten oxo species is thought to be the active site. 

Ternary transition metal oxides within mesoporous MCM-48 silica phases Synthesis and characterization... 

Since pure mesoporous silica phases does not show any catalytic activity many successful attempts have been made to vary the inorganic composition towards transition metal oxides or metal chalcogenides [5-12], In particular the semiconducting properties of the latter offer a great range of possible applications in materials chemistry. 

Ternary Transition Metal Oxides within Mesoporous MCM-48 Silica Phases 341... 

The phase transformation behavior of Py-D3C was far simpler than that reported for tetrahydrofuran/N2- and tetrahydrofuran/Xe-D3C in reference 10. We attribute this difference in part to impurities in the samples employed, samples that contained methanol and ethylenediamine according to 13 C CPMAS NMR spectroscopy. We have observed that use of Si(OCH3)4 as a silica source or ethylenediamine as a catalyst in clathrasil synthesis introduces defects that can alter phase transition temperatures by as much as 30 °C and/or introduce new phase transformations. 

In ZSM-5, which is nominally isostructural with silicalite, the temperature,Tt, at which the transition occurs depends on the level of "impurities (i.e. species other than silica, such as the residual aluminium in the framework, cations, adsorbed water and organics). ZSM-5 with relatively high Al contents is orthorhombic at room temperature because the transition occurs at Tt<20°C. The sample of silicalite treated with 2M (but not with 0.5M) base solution was orthorhombic, and it is clear that treatment with a strong base introduces sufficient amount of "impurities" for the phase transition to take place below the room temperature. 

Thermal stability. Thermal stability of several common ceramic and metallic membrane materials has been briefly reviewed in Chapter 4. The materials include alumina, glass, silica, zirconia, titania and palladium. As the reactor temperature increases, phase transition of the membrane material may occur. Even if the temperature has not reached but is approaching the phase transition temperature, the membrane may still undergo some structural change which could result in corresponding permeability and permselectivity changes. These issues for the more common ceramic membranes will be further discussed here. 

Fig. 1 Plot showing the relationship between the phase transition temperature from the amorphous to the tetragonal state and the composition of silica in the composite material. (From Ref [12].)...

In the specific case of zirconia-silica composites, the stationary phase could be prepared through relatively simple processes using either coprecipitation or coating methods. The addition of silica to the zirconia matrix increases the phase transition temperature from the amorphous phase to the tetragonal phase, which in turn stabilizes the tetragonal phase. The pore structure can be controlled through processes similar to those employed in the preparation of zirconia. However, the type of pore structure obtained appears to be dependent on the method of preparation. Calcination in the presence of salts improves the pore shapes. Zirconia-silica phases can also be surface-modified and Cg and ion-exchange media have been prepared. Composite zirconia-silica stationary... 

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