Silica keatite

Kea.tlte, Keatite has been prepared (65) by the crystallisation of amorphous precipitated silica ia a hydrothermal bomb from dilute alkah hydroxide or carbonate solutions at 380—585°C and 35—120 MPa (345—1180 atm). The stmcture (66) is tetragonal. There are 12 Si02 units ia the unit cell ttg = 745 pm and Cg = 8604 pm the space group is P42. Keatite has a negative volumetric expansion coefficient from 20—550°C. It is unchanged by beating at 1100°C, but is transformed completely to cristobahte ia three hours at 1620°C. 

Only with silica was the nature of the surface groups studied as extensively as with carbon. Silica, like carbon, has several polymorphs. Apart from the amorphous state, it is known to exist in numerous crystalline modifications. The most important forms are quartz, tridymite, and cristobalite. Each of these can occur in a low-temperature form and in a high-temperature form of somewhat higher symmetry. Tridymite is only stable if small amounts of alkali ions are present in the lattice 159). Ar. Weiss and Al. Weiss 160) discovered an unstable fibrous modification with the SiSj structure. Recently, a few high-pressure modifications have been synthesized keatite 161), coesite 162), and stishovite 16S). The high-pressure forms have been found in nature in impact craters of meteorites, e.g., in the Arizona crater or in the Ries near Nbrdlingen (Bavaria). 

Crystalline Silica Three principal polymorphic forms exist at atmospheric pressure. These are quartz, tridymite, and cristobalite. Quartz is stable below 870°C. It transforms to tridymite form at about 870°C. Tridymite is stable up to 1,470°C and transforms to cristobahte at 1,470°C. High cristobalite melts around 1,723°C. Other than these three polymorphs, there are also three high pressure phases of crystalline sihca keatite, coesite, and stishovite. 

In addition to the three principal polymorphs of silica, three high pressure phases have been prepared keatite [17679-64-0], coesite, and stishovite. The pressure—temperature diagram in Figure 5 shows the approximate stability relationships of coesite, quartz, tridymite, and cristobalite. A number of other phases, eg, silica O, silica X, silicalite, and a cubic form derived from the mineral melanophlogite, have been identified (9), along with a structurally unique fibrous form, silica W. 

If the catalyst is placed inside the membrane tubes and also the separative layer is coated at the inside of the tube, any compounds such as potassiumoxide from the catalyst might react with the silica separative layer to form keatite. This will destroy the molecular sieving properties of the silica toplayer, see paragraph 3.1. Additionally there is a risk that catalyst loading will damage the membrane layer. 

Much attention should be paid to the choice of the catalyst for the reaction. Various catalysts of which M0S2 is the most common, are possible. In a separate subtask, the impact of the used catalyst on the stability of the membrane should be studied. E.g., alkali doping might have a negative effect on the stability of the silica layer due to keatite fomation as described in this thesis. 

Influence of potassium from the catalyst on the stability of the silica layer (possible keatite formation). 

The main points of interest of the structures of these polymorphs are (i) the analogies with silica and silicate structures, (ii) the presence of two interpenetrating frameworks in the most dense forms vi and vii (viii), and (iii) the ordering of the protons. Analogies with silica and silicate structures are noted in Table 15.1, namely, ice-iii with a keatite-like structure, ice-vi with two interpenetrating frameworks of the edingtonite type (p. 828), and ice-vii (and viii) with two interpenetrating cristobalite-like frameworks. In these structures, related to those of... 

Cristobalite, the highest-temperature polymorph of silica, was named after the place where it was discovered, the San Cristobal mountain in Mexico. Interestingly, silicate phases including cristobalite have also been found in cosmic dust collected by space vehicles [9], The high cosmic and terrestrial abundance of silicas makes knowledge of their physical and chemical properties especially important in fields such as geology, chemistry, and physics. Cristobalite, like tridymite and keatite, is isostructural with ice polymorphs (i.e. cubic ice Ic). 

Vitreous Si02 occurs as tectites, obsidian and the rare mineral lechatelierite. Synthetic forms include keatite and W-silica. Opals are an exceedingly complex crystalline aggregate of partly hydrated silica. 

In crystalline silica the silicon coordination is four for quartz (53221), tridymite (51 and Cc), cristobalite (P4i2i2), coesite (C2/c) and keatite (54i2i2), six in stishovite (rutile type structure P42/mnm) and in the recently high pressure phase with a CaCl2 structure (Pnnm) and eight in a fluorite type (5 /w3/w)model structure which is one of the hypothetical post-stishovite modifications investigated by modeling techniques. Of course, the... 

Two insoluble polymorphs of beryllium polyphosphate, [Be(P03)2l , have crystalline structures which are probably covalent in type. They contain long chains of corner-shared PO4 tetrahedra which are cross-linked by Be04 tetrahedra to give three-dimensional networks containing Be04 and PO4 units in a 2 1 ratio [49] (Figure 5.22a). One form is isostructural with keatite, a polymorph of silica (see above). 

The three phases metastable at ordinary pressure were recognized only recently. Keatite was discovered by Paul Keat (56) in 1954. and its formation via cristobalite and transformation to quartz were studied by Carr and Fyfe (57). Hoover (58), in a patent filed In 1954, described the preparation of a very similar if not identical material from silicic acid , that is, hydrated amorphous silica powder, by heating it in water at about 3000 atm pressure and 500-625 C in the presence of about 1% alkali based on silica. 

Coesite was discovered by Coes, in 1953 (59). It is made from amorphous silica in the same temperature range as for keatite, but at 10 times the pressure and with weakly acidic catalysts such as boric acid or ammonium chloride (59). It was found in nature in I960 at Meteor Crater, Arizona, apparently formed under the high temperature and pressure conditions of the impact. 

Carr and Fyfe (57) observed that amorphous silica in water crystallizes via cristo-balite and keatite to quartz at 335 C in 840 hr under 15.000 psi pressure, but in 18 hr at 45,000 psi. 

Kaye Disk Centrifuge. A device for particle-size analysis (1-50 pm) by means of a beam of light passed through a liquid suspension of the particles while the suspension is rotated in a transparent disk-shaped centrifuge. The prototype was designed by B. H. Kaye (Brit. Pat, 895,222,2/5/62) now made by Coulter Electronics Ltd., St. Albans, England. Keatite. A form of silica resulting from the crystallization of amorphous precipitated silica at 380-585°C and water pressures of 350-1250 bars sp. gr. 2.50. Named from its discoverer, P. P. Keat (Science, 120, 328,1954). 

Keatite, sometimes referred to as silica K, is a high-pressure form of Si02, which has neither been recognized in nature nor appears to have any field of thermodynamic stability without the additions of alkalis or water. The phase may be synthesized at 0.1 GPa and 800 K from silica gel. 

The lack of ambient-pressure polymorphism in B2O3 is in stark contrast to the situation observed in most simple oxide systems. By ambient-pressure polymorphs, we mean crystals built upon the same structural unit and thus in which the networkforming cations have the same coordination as the ambient glass. In silica for instance, more than 20 polymorphs (quartz, coesite, cristobalite, keatite, moganite, tridymite... 

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