Alumina-silica system

Figure 4.29. Effect of grinding on the Si spectra of the alumina-silica system. A. Mixture of gibbsite, Al(OH)3 and silica gel, and B. Diphasic gel of mullite composition. Note in both systems the evidence of Si-O-Al bond formation (peak at about — 84 ppm). Adapted from Temuujin et al.

Figure 7. Calcia-alumina-silica system. This illustration compares the melt viscosity of the inviscid binary aluminate melts from 60% CaO to 75% alumina. Addition of MgO and CaO produces quaternary aluminate melts, raises the viscosity significantly and produces a fragile melt with high viscosities below the liquidus temperature and low viscosities above the liquidus temperature. Redrawn from F. T. Wallenberger and S. D. Brown, High modulus glass fibers for new transportation and infrastructure composites and for new infrared uses, Composites Science and Technology, 51.243-263 (1994).

From the widely diverse observations it is obvious that the alumina-silica system is very complex. Under the right conditions alumina can remove silica from solution down to a very low level, but this is difficult to accomplish under practical conditions. 

In Section II,B,8 we discussed the question of determining site densities using high-conversion data. We developed a method applicable in the inter-conversion of three isomers when there is a common surface complex for the three possible reactions. We have tested this method using the conversion of 1-butene to cis- and rrans-2-butene over silica-alumina, a system that, according to Hightower and Hall, proceeds through a common surface complex (111). Their conclusion has been confirmed experimentally (112) and by semiempirical quantum-chemical calculations (113). 

Residual fuel oils and heavy marine fuels are composed of high-boiling-petroleum fractions, gas oils and cracked components. Residual and clarified oil streams from the FCC process can contain degraded alumina/silica catalyst fines. These 20- to 70-micron-diameter fines are known to contribute to a variety of problems in fuel injection and combustion systems. In marine engines, excessive injector pump wear, piston ring wear, and cylinder wall wear can all be due to the abrasive action of catalyst fines on these fuel system parts. 

However, a larger portion of materials which may be of concern as trace components will not be extractable or generally soluble in aqueous systems. This presents the necessity to concentrate components contained in organic solvents. When the organic solution is too complex, or the components sought are too volatile to allow concentration simply by evaporation, then more complex means must be pursued. Under these circumstances, liquid column processes are again commonly used. Alumina, silica gel,... 

Microwave irradiation also shows a beneficial effect in the preparation of solid-supported palladium catalysts for hydrogenation reactions. Thus, alumina- and silica-supported palladium catalysts were synthesised by conventional and microwave heating, and their physical properties and catalytic activity in the hydrogenation of benzene were compared. The alumina-based system prepared under microwave conditions showed turnover numbers an order of magnitude higher than the conventionally prepared catalysts28. 

Microwave heating has proven to be of benefit particularly for reactions under dry media (e.g., solvent-free conditions) in open vessel systems (i.e., in the absence of a solvent, on solid support with or without catalysts) [4]. Reactions under dry conditions were originally developed in the late 1980 s [51], but solventless systems under microwave conditions offer several additional advantages. The absence of solvent reduces the risk of explosions when the reaction takes place in a closed vessel. Moreover, aprotic dipolar solvents with high boiling points are expensive and difficult to remove from the reaction mixtures. During microwave induction of reactions under dry conditions, the reactants adsorbed on the surface of alumina, silica gel, clay, and other mineral supports absorb microwaves whereas the support does not, and transmission of microwaves is not restricted. Moreover, microwaves can interact directly with reagents and, therefore, can more efficiently drive chemical reactions. The possible accelerations of such reactions are expected... 

It is interesting that the presence of the alumina-silica carrier in the catalyst is sufficient enough for the activation of the titanocene for the polymerisation, similarly like the presence of methylaluminoxane in the titanocene homogeneous polymerisation system CpTiCl3 alone does not exhibit catalytic activity in conjugated diene polymerisation. 

Although the decomposition of ozone to dioxygen is a thermodynamically favoured process,126 it is thermally stable up to 523 K and catalysts are needed to decompose it at ambient temperature in ventilation systems, in the presence of water vapour and at high space velocity. A limited number of catalysts have been evaluated and active components are mainly metals such as platinum, palladium and rhodium, and metal oxides including those of manganese, cobalt, copper, iron, nickel and silver. Supports that have been used include 7-alumina, silica, zirconia, titania and activated carbon.125,170... 

Oblad, Milliken, and Mills (28) were among the first chemists to be fully aware of the chemical significance of surface stresses in catalysis. They explained the extremely high value for the heat of hydration of a thoroughly degassed alumina-silica catalyst on the basis of stresses. The author prefers to describe a system under tensile stress as acidic because of the incompletely screened cations. Using intemuclear distances and the screening demands of cations to describe the difference in surface films carries the macroscopic description of a solid under tension into the atomic dimensions (36). 

Work has been reported on the conversion of aldehyde 81, the Diels-Alder cycloadduct of acrolein and cyclopentadiene, to its fully saturated primary alcohol and thereafter to bicyclo[3.3.0]octyl hydrocarbons.133 A silica/alumina catalyst system provides cu-bicyclo[3.3.0]octane, while phosphoric acid and Kieselguhr gives the A2-olefin. Since this cyclic alkene reacts under Koch-Haaf conditions to produce the... 

Alumina is much more adapted to the particulate sol route while silica and titania are more flexible in that they can lead to both particulate and polymeric gels as the membrane precursors. This is particularly true with the silica system. Hydrolysis and condensation reactions in silica systems are known to be slower than most other alkoxides. Their reactions, therefore, are easier to control. The fact that it is much easier to prepare alumina sol-gel membranes than their titania equivalents may suggest that the plate-shaped particles like alumina can be more readily processed to form defect-free membranes than spherical particles such as titania [van Praag et al., 1989]. 

Berman R. G. (1983) A thermodynamic model for multi-component melts, with application to the system calcium oxide-magnesium oxide-alumina-silica. PhD Thesis, University of British Columbia. 

As previously mentioned, GC is a two-phase system that consists primarily of a stationary (solid and/or liquid) and mobile (gas) phase. When a liquid stationary phase is used (GLC), the liquid is immobilized as a thin film supported on a finely divided, inert solid support usually consisting of siliceous earth, crushed firebrick, glass beads, or in some cases, the inner wall of a glass tube. In GSC, the stationary phase is an active adsorbent, such as alumina, silica gel, or carbon, which is tightly packed into a tube. 

The phase diagram for the Na20-Al203-Si02 system illustrates the thermochemical relationships between these three components [2-4], Knowledge of the equilibrium relations in this system is pertinent to the attack of alumina-silica ceramics by alkali vapors. 

---