Sepiolite formation

Figure 3D. Continued. Vanadium remains mostly on the sepiolite because of strong metal-surface interactions and vanadate formation.

Sepiolite passivates most of the nickel via formation of non interactive silicate-like materials. Heating at high temperatures induces migration of nickel to the interior and of vanadium to the exterior of the catalyst surface. Metal-surface interactions are observed also in Ni-loaded kaolin microspheres however, V on kaolin behaves like bulk V20g with respect to reduction, thus explaining this xlay s inability to passivate V-contaminants. 

Ovarlez S, Chaze A-M, Giulieii F, Delamare F (2006) Indigo chemisorption in sepiolite. Application to Maya blue formation. Compt Ren Chim 9 1243-1248. 

Sepiolite and palygorskite have a rather special composition and seem to be related to specific mineral parageneses. They appear to be stably associated with montmorillonite, corrensite, serpentine, chert, sulfates, carbonates and various salts. They are found in deposits typified by processes of chemical precipitation or solution-solid equilibria (Millot, 1964) and are therefore rarely associated in sediments with large quantities of detrital minerals. Their chemical environment of formation is in all evidence impoverished in alumina and divalent iron. Their frequent association with evaporites, carbonates and cherts indicate that they came from solutions with high chlorinity. 

Sepiolite has been made exchangable by chemical treatments and Mg2+ at the border of the channels has been substituted by Al3+. In this way sepiolite with mild acidity, controlled mesopore, and improved stability has been obtained. This material is active for gasoil cracking, giving a good bottom conversion, and light cycle oil products without excessive gas and coke formation. Meanwhile, it is active for vanadium passivation. 

Table I.— A Partial List Of The Thermodynamic Data Employed In Computing The Revised Values For The Free Energy Of Formation Of Kaolinite And Sepiolite...

The experimental values for the free energies of formation of kaolinite and sepiolite are given in Tbble II. The value of -907.7 +1.33 kcal/mol recommended for kaolinite, is the mean of three recomputed free energies of formation weighed equally in the computation, and was obtained from calorimetry, dissolution, and precipitation data. Several values in the -905 to -906.0 kcal/mol range probably reflect the more soluble nature of small particles typically present in bulk samples. 

A free energy of formation for sepiolite of -1105.4 kcal/mol is based on extrapolation to 25°C of results from measurements made at 51, 70, and 90 C by Christ and others (25). In support of... 

Clay minerals are present in almost all surface-water and ground-water systems, and in many instances may be controlling the concentration of aluminum, silica, iron, magnesium, or other cations in solution. The thermodynamic data necessary to evaluate the state of reaction (saturation) are not available for some clay minerals, and for those minerals with published values, the data are in disagreement by as much as 10 kilocalories per mole for the same clay mineral. A critical review of the available data for kaolinite and sepiolite, incorporating both the most recent thermodynamic data for the components in the reaction schemes and a more complete computation for the solubility data, yields the values of -907.7 +1.3 and 1105.6 +0.4 kilocalories per mole for the free energy of formation of kaolinite and sepiolite, respectively. 

Irradiation at 300 nm in CCl, CHCl, or CH2CI2 promotes am irreversible reaction and formation of what is probably [Os(IV)(TTP)Cl2l. Optimum conditions have been reported for determination of osmium by measurement of the luminescence of its 1 3-complex with 1,10-phenanthroline.A catalyst, prepared by reducing the product of grafting OsO on to the C-C bond of sepiolite, has been found to mediate the photooxidation of water but to do so less efficiently than RuOjj. This is the first exaunple of a dispersed water oxidation catalyst grafted on to a solid support. 

The conditions necessary for artificial synthesis of smectites suggest the kind of natural environment in which these minerals will be found. This environment is alkaline as a result of restricted drainage and/or evaporative salt accumulation (see Chapter 8), so that the normally mobile alkaline and alkaline earth ions (Na, K, Ca, Mg) accumulate, along with silica. Neoformation of smectites from Na-rich saline water, and possibly illites from more K-rich water, is favored under these conditions. Other very silica-rich silicate minerals, such as attapulgites, sepiolites, and zeolites, are also known to fomi under these conditions. Less alkaline conditions may be necessary for Fe and Al-rich (dioctahedral) smectite formation, and a reducing environment may assist crystallization. 

---