Chromium silicates

The oxidation of both linear and cyclic ethers to the corresponding acids and lactones by aqueous H202 as catalyzed by TS-1 and TS-2 was reported by Sasidharan et al. (241) (Scheme 17 and Table XXXV). The titanosilicates exhibited significantly better activity (about 55% conversion) and selectivity (98%) than chromium silicates, although vanadium silicates totally failed to catalyze the reaction. Such conversions are usually accomplished using either stoichiometric amounts of chromium trioxide, lead tetraacetate, or ruthenium tetroxide as oxidants (242) or catalytic amounts of Ru04 in the presence of... 

Scheme 51. Synthesis of microporous chromium silicate materials from 153.

Fixed Residue.—2 grams of the substance are calcined in a porcelain crucible, which is heated at first gently and afterwards gradually more and more intensely to bright redness, the residue being weighed. If this is in appreciable quantity, it is analysed qualitatively, especially for lead, iron, chromium, silicates and barium and calcium sulphates. 

For synthesis of biomimetic catalysts four versions of granulated carriers were used neutral and activated A1203, NaX zeolite and synthetic amorphous aluminum-magnesium silicate and aluminum-chromium silicate. 

Garnet Ferrous, magnesium, manganese, aluminum, calcium, chromium silicate H2SO4 pH3 4 Excess acid Acid medium R-801, 825 F-77 Fuel oil Acid... 

Thus the Cr + ions show a peculiar inertness relative to monomeric silica, in marked contrast to the behavior of AF. When amorphous silica was heated under pressure with a mixture of Cr(OH), and Al(OH)3 for 2 days at 300 C, only the alumina combined with silica (63). This behavior of chromium probably explains the rarity of chromium silicate minerals. 

Iron silicate Iron-silicon alloys Iron-silicon-chromium Iron sponge... 

Molten salts promote rapid corrosion of many constmctional materials at relatively low temperatures. Low-melting-point salts include sodium salts from saline atmospheres, fireside ash, silicate insulation, contaminants in the feed, etc. Corrosion rates of several mm/year can be observed at temperatures as low as 520°C. High chromium- and nickel-containing alloys up to 50% Cr/50% Ni are employed. 

Determination of titanium with tannic acid and phenazone Discussion. This method affords a separation from iron, aluminium, chromium, manganese, nickel, cobalt, and zinc, and is applicable in the presence of phosphates and silicates. Small quantities of titanium (2-50 mg) may be readily determined. 

Thermal reduction at 623 K by means of CO is a common method of producing reduced and catalytically active chromium centers. In this case the induction period in the successive ethylene polymerization is replaced by a very short delay consistent with initial adsorption of ethylene on reduce chromium centers and formation of active precursors. In the CO-reduced catalyst, CO2 in the gas phase is the only product and chromium is found to have an average oxidation number just above 2 [4,7,44,65,66], comprised of mainly Cr(II) and very small amount of Cr(III) species (presumably as Q -Cr203 [66]). Fubini et al. [47] reported that reduction in CO at 623 K of a diluted Cr(VI)/Si02 sample (1 wt. % Cr) yields 98% of the silica-supported chromium in the +2 oxidation state, as determined from oxygen uptake measurements. The remaining 2 wt. % of the metal was proposed to be clustered in a-chromia-like particles. As the oxidation product (CO2) is not adsorbed on the surface and CO is fully desorbed from Cr(II) at 623 K (reduction temperature), the resulting catalyst acquires a model character in fact, the siliceous part of the surface is the same of pure silica treated at the same temperature and the anchored chromium is all in the divalent state. 

A colorless mineral known as corundum (composed of aluminum oxide) is colorless. A red variety of corundum known as ruby, a precious stone, owes its color to impurities of chromium within the crystal structure of corundum. Blue and violet varieties of corundum are classified as sapphires, the blue being the result of iron and titanium impurities, and the violet of vanadium impurities within the corundum crystal structure. Another colorless mineral is beryl (composed of beryllium aluminum silicate) but blue aquamarine, green emerald, and pink morganite, are precious varieties of beryl including different impurities aquamarine includes iron, emerald chromium and vanadium, and morganite manganese. 

Chromium has a similar electron configuration to Cu, because both have an outer electronic orbit of 4s. Since Cr3+, the most stable form, has a similar ionic radius (0.64 A0) to Mg (0.65 A0), it is possible that Cr3+ could readily substitute for Mg in silicates. Chromium has a lower electronegativity (1.6) than Cu2+ (2.0) and Ni (1.8). It is assumed that when substitution in an ionic crystal is possible, the element having a lower electronegativity will be preferred because of its ability to form a more ionic bond (McBride, 1981). Since chromium has an ionic radius similar to trivalent Fe (0.65°A), it can also substitute for Fe3+ in iron oxides. This may explain the observations (Han and Banin, 1997, 1999 Han et al., 2001a, c) that the native Cr in arid soils is mostly and strongly bound in the clay mineral structure and iron oxides compared to other heavy metals studied. On the other hand, humic acids have a high affinity with Cr (III) similar to Cu (Adriano, 1986). The chromium in most soils probably occurs as Cr (III) (Adriano, 1986). The chromium (III) in soils, especially when bound to... 

UG2 Morensky Reef Plant B recovery = 80-84% Ore PGM-dominated ores - with very little sulphides and the main gangue minerals include silicate, mica, aluminosilicate and some chromium... 

Huebner I S., Lipin B. R., and Wiggins L. B. (1976). Partitioning of chromium between silicate crystal and melts. Proc. Seventh Lunar Scl Conf, 1195-1220. 

Schreiber H. D. (1976). The experimental determination of redox states, properties, and distribution of Chromium in synthetic silicate phases and application to basalt petrogenesis. Ph.D. diss., University of Wisconsin. 

Baleizao, C. Gigante, B. Sabater, M. J. Garcia, H. Corma, A. (2002) On the activity of chiral chromium salen complexes covalently bound to solid silicates for the enantioselective epoxide ring o emag Applied Catalysis A General 228 279-288. 

The Oxide Ores (e.g., ores of iron, chromium, manganese, tin).— The study of these ores involves high-temperature investigations similar to those on the silicates, and also studies of the hydrated and colloidal oxides. 

Similarly, it combines with silica and chromium(VI) oxide at elevated temperatures, forming lead silicate and lead chromate, respectively ... 

Sevenson Environmental Services, Inc. (Sevenson), is the owner of the MAECTITE chemical treatment process for the precipitation and stabilization of toxic heavy metals. Chemical treatment by the MAECTITE process converts teachable lead, hexavalent chromium, or other heavy metals into insoluble minerals and mixed mineral forms within the material or waste matrix. The technology can be used as an in situ or an ex situ method and does not use pozzolanic or siliceous binders to stabilize the treated material. 

Omotoso, O. E., Ivey, D. G. Mikula, R. 1998a. Hexavalent chromium in tricalcium silicate Part 1 Quantitative X-ray diffraction analysis of crystalline hydration products. Journal of Materials Science, 33, 507-513. 

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