Transition metal phosphate vanadium structures

Mixed-valence transition metal phosphates have been found for Ti, V, and Nb. Their crystal chemistry and properties are more closely related to those of the Mo analogs in that they have mixed frameworks of MOe octahedra that are partially or wholly isolated from one another by intervening phosphate groups and the d electrons do not appear to be delocalized. A particularly rich chemistry exists for the vanadium analogs. Details of their structural properties have been reviewed by Borel and coworkers. ... 

This article is focused on HDN, the removal of nitrogen from compounds in oil fractions. Hydrodemetallization, the removal of nickel and vanadium, is not discussed, and HDS is discussed only as it is relevant to HDN. Section II is a discussion of HDN on sulfidic catalysts the emphasis is on the mechanisms of HDN and how nitrogen can be removed from specific molecules with the aid of sulfidic catalysts. Before the discussion of these mechanisms, Section II.A provides a brief description of the synthesis of the catalyst from the oxidic to the sulfidic form, followed by current ideas about the structure of the final, sulfidic catalyst and the catalytic sites. All this information is presented with the aim of improving our understanding of the catalytic mechanisms. Section II.B includes a discussion of HDN mechanisms on sulfidic catalysts to explain the reactions that take place in today s industrial HDN processes. Section II.C is a review of the role of phosphate and fluorine additives and current thinking about how they improve catalytic activity. Section II.D presents other possibilities for increasing the activity of the catalyst, such as by means of other transition-metal sulfides and the use of supports other than alumina. 

When considering the roles of promoters in vanadium phosphate catalysts, care must be taken to decouple the effects of structural promotion which increases the surface area, and hence the activity of catalysts. A vast array of transition metals, alkali metals, alkali earth metals, and lanthanides have all been studied as promoters but only a handful have been found to have promotional effects that are independent of the surface area of the catalyst. 

Although the structural chemistry of molybdenum and vanadium phosphates is very diverse these compounds seem to be prototypes for open framework transition and main group metal phosphates. Recent results have shown that zeolite-like compounds like zinc/cobalt phosphates [731, indium phosphates [74], nickel/aluminum phosphates [75], cobalt/boron phosphates [76], manganese and cobalt phosphonates with giant 44-membered rings forming ellipsoidal cavities [77] can be synthesized. All these examples show that the diverse chemistry of octahedral-tetrahedral framework structures is a vast field where... 

Fluorides represent another alternative to oxides, and may have higher redox potentials compared with similar oxides [54]. In comparison with the alkali fluoride salts used in the preparation of vanadium phosphates [62,63], Kohl et al. [54] prepared lithiated transition metal fluorides, LigMFg, using acetyl acet-onate salts as metal precursors and organic solutions of HF as a fluoride source. Extensive structural characterizations were presented for M = V, Mn, Cr, and Fe and the crystalline domain size was found to vary between 30 and 200 nm. However, no electrochemical characterization was reported. 

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