Polysilicic acid

Microscopic sheets of amorphous silica have been prepared in the laboratory by either (/) hydrolysis of gaseous SiCl or SiF to form monosilicic acid [10193-36-9] (orthosihcic acid), Si(OH)4, with simultaneous polymerisation in water of the monosilicic acid that is formed (7) (2) freesing of colloidal silica or polysilicic acid (8—10) (J) hydrolysis of HSiCl in ether, followed by solvent evaporation (11) or (4) coagulation of silica in the presence of cationic surfactants (12). Amorphous silica fibers are prepared by drying thin films of sols or oxidising silicon monoxide (13). Hydrated amorphous silica differs in solubility from anhydrous or surface-hydrated amorphous sdica forms (1) in that the former is generally stable up to 60°C, and water is not lost by evaporation at room temperature. Hydrated sdica gel can be prepared by reaction of hydrated sodium siUcate crystals and anhydrous acid, followed by polymerisation of the monosilicic acid that is formed into a dense state (14). This process can result in a water content of approximately one molecule of H2O for each sdanol group present. 

Poly-morphie, /. polymorphism, polymorphy, -nom, n. polynomial, -saure, f. poly add. -schwefelwasserstoff, m. hydrogen persulfide (either H2S2 or HaSa). -sUiciumsaure,/. polysilicic acid, -styrol, n. polystyrene, -thion s ure, /. polythionic acid, -zimtsaure, /. polycinnamic acid, polyzyklisch, a. polycyclic. 

Hofmann and collaborators 168) were probably the first to postulate that the free valences of silicon atoms in the surface of silicates must be saturated with silanol groups. Carman 169) visualized the structure of a particle of colloidal silica as a network of interlinked Si04 tetrahedra with hydroxy groups attached to the surface, due to the tendency of silicon to complete tetrahedral coordination. Each particle of silica can be considered as a macromolecule of polysilicic acid. 

Ionization constants of polysilicic acids. The ionization constant Ki for equilibrium a) of a species of ionization degree i is the product of i ionization constants k corresponding to successive dissociation reactions. He aesume that all tetrahedra within any species dissociate once before the second possible dissociation of any tetrahedron occurs, and so on for successive dissociations. 

Fig. 2.21. Spray dried particles prepared from 50% colloidal silica (-20 nm diameter) and 10% silica derived from polysilicic acid [13].

Monosilicic acid is stable in aqueous solution only at low pH and very low concentration (Her, 1979, p. 209). The rate of condensation-polymerization of silicic acid is dependent on pH, concentration and temperature. At a certain stage, the polysilicic acid sol is converted into either a precipitate (i.e. a flocculated system) or a hydrogel. 

In the case of the VPO catalyst for the butane oxidation process and the MCM catalyst for the acrylonitrile process, the preferred precursor of the peripheral hard phase is polysilicic acid (PSA). The term "polysilicic acid" is generally reserved for those "silicic acids that have been formed and partially polymerized in the pH range 1-4 and consist of ultimate silica particles generally smaller than 3-4 nm diameter" (4). Small, discrete particles of colloidal silica also migrate to the periphery of the droplet, but they do not coalesce as extensively as PSA in drying. The larger the particle size, the lower the mechanical strength of the coalesced dry product. 

In our case a 6% Si02 solution of polysilicic acid was prepared by deionizing to pH 3 a dilute solution of filtered Du Pont JM grade sodium silicate with Dowex HCR-W2-H resin, a strongly acidic nuclear sulfonic acid cation exchanger supplied by Dow Chemical Company. 

Micrographs obtained by EPMA are shown in Figure 4. The sample in this case was VPO with 10% amorphous silica as the hard phase. This composition was prepared by spray drying an aqueous slurry with about 40% solids made of l-2 im particles of the VPO catalyst precursor and polysilicic acid. The back-scattered image shows all elements present in the porous microsphere. The X-ray image of silicon clearly shows this element concentrated exclusively on the periphery of the microsphere. Independent X-ray diffraction and electron diffraction analysis of the peripheral layer of the microspheres showed that the silicon is present as amorphous silica. 

Figure 2. Attrition resistance of catalyst powders with and without polysilicic acid (PSA).

The trend to improved attrition resistance as the CP particle size decreases is illustrated in Figure 6. The bar graph plots the AI against median particle diameter of the aluminas. We suspect that CP-2 offers the most binding sites per unit volume of particles. Also, there are more nuclei of CP-2 than CP-25 in equivalent slurry volumes. Consequently, more surface functionality is available for condensation of the polysilicic acid moieties and the CP-2 material is the least attritable. 

Alcoa s rehydratable CP alumina powders can be used effectively to improve a viable FCC catalyst formulation. Well-formed microspheres which have superior attrition resistance can be fabricated by controlling the pH and viscosity of the FCC slurry. At this time, the preferred formulation uses CP-2 as the free alumina source and a silica sol which has aged at conditions conducive to the formation of chains of polysilicic acid aggregates. The addition of the rehydratable alumina can also have a beneficial effect on the cracking activity of the catalyst. The conversion and selectivity of a CP-2 formulated sample were comparable to a commercial grade catalyst and an experimental reference, which was alumina-free. After heavy metals poisoning, the CP-2 material had activity which was superior to the reference formulation. 

The rate of silica dissolution is proportional to its surface area. Silicic acid being passed into solution undergoes polymerization giving water and the molecules of polysilicic acids of variable complexity. At pH... 

Association between polysilicic acid and polar organic compounds. 

Dietzel M. (2000) Dissolution of silicates and the stability of polysilicic acid. Geochim. Cosmochim. Acta 64, 3275-3281. 

Asbestos, the first inorganic fiber material used, is currently still exclusively produced from natural mineral deposits. It is formed by the hydrothermal conversion of basic and ultrabasic volcanic rock (olivine and pyroxene) to serpentine upon which the actual asbestos formation takes place leading to two asbestos sorts with different structures serpentine asbestos and amphibole asbestos. Asbestos can be produced synthetically by several hours heating of a polysilicic acid/metal oxide mixture (e.g. Mg, Fe, Co, Ni) in water at 300 to 350°C and 90 to 160 bar. The properties of four important asbestos types are summarized in Table 5.2-2. 

The study of alkaline silicate and aluminosilicate solutions is of fundamental importance for a better understanding of the mechanism of zeolite synthesis (1,2). Real progress has been realized durinq the last fifteen years thanks to the use of new techniques such as the trimethylsilylation (3,4) of polysilicic acids (followed by a chromatographic separation of the derivatives) and, even more, 29Si and 27Ai NMR spectroscopy (5-11). The latter techniques have enabled the identification of about twenty types of oligomers in silicate solutions and in some cases estimates of their respective concentrations. The investigation of alkaline aluminosilicate solutions is... 

In alkaline solutions, silica exists in the form of silicate ions e.g., Si03 ). In dilute solutions (up to 0.1 mg of Si per ml) between pH 1 and 8, water-soluble monomeric silicic acid is the stable form. In more concentrated solutions of the same acidity, monosilicic acid condenses to disilicic acid and polysilicic acids which can be transformed into colloidal species. 

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