Pyrovanadates

Sodium triperoxychromate Sodium peroxymolybdate Sodium peroxynickelate Sodium diperoxytungstate Potassium peroxyferrate Potassium peroxynickelate Potassium hyperoxytungstate Potassium peroxy pyrovanadate Calcium diperoxysulphate Calcium peroxychromate Zinc tetraaminoperoxydisulphate Ammonium peroxyborate Ammonium peroxymanganate Ammonium peroxychromate... 

Barium (II) (Continued) pyrovanadate Ba2V207 s White Insol ... 

Mg pyrovanadate (Mg2V207) was also found to be a selective catalyst for this reaction (30). A later study further showed that the catalytic performance of Mg2V207 was insensitive to the method of preparation, such that there were only minor differences whether or not the oxide contained small amounts of potassium (25). In contrast, the presence of K in Mg orthovanadate degraded its selectivity noticeably. Temperature-programmed reduction with H2 and electrical conductivity characterization in the presence of propane showed that Mg pyrovanadate could be reduced by both H2 and propane faster than Mg orthovanadate containing K (31). 

As to the method of preparation, it was found that V-Mg oxide catalysts prepared with a Mg(OH)2 precursor that was precipitated with KOH was less selective than one prepared with a MgC03 purecursor precipitated with (NH4)2C03 (25). Interestingly, unlike the butane reaction, there was no effect of preparation on the oxidative dehydrogenation of propane using the same catalysts, as mentioned earlier (25, 30). Unlike the oxidation of propane, Mg pyrovanadate was nonselective for butane (25, 26). Mg metavanadate was nonselective as well (26). 

General.—It seems certain that the free acids corresponding to these salts do not exist in the solid state, and that, with the possible exception of hexavanadic acid, mentioned below, they are also incapable of existing in solution, although salts of all the acids are known. The most stable class of salts is the metavanadates, the next in order of stability being the pyrovanadates, while the orthovanadates are few in number and undergo rapid hydrolysis even in the cold, to give the pyro-salts ... 

Sodium Orthovanadate, Na3V04.12H20, is the orthovanadate most frequently met with. It is readily obtained by adding excess of caustic soda to a solution of sodium pyrovanadate ... 

Pyrovanadates, R 4V207 or 2R a0.Va05.—The alkali pyrovana-dates are prepared by dissolving the equivalent quantity of vanadium pentoxide in solutions of alkalis, or by the spontaneous decomposition in solution of the alkali orthovanadates. Pyrovanadates of other metals are obtained by fusing vanadium pentoxide with the salts or hydroxides of the metals in molecular proportions, or, when they are sufficiently insoluble, by double decomposition between an alkali pyro-vanadate and a salt of the metal required. 

Pyrovanadates are more stable than orthovanadates, but, in consequence of the weakly add character of pyrovanadic add, they undergo easy conversion into the metavanadates. Sodium pyrovanadate in solution is thus converted by carbon dioxide into the metavanadate ... 

Ammonium pyrovanadate does not appear to exist addition of... 

These reactions suggest that sodium pyrovanadate undergoes partial hydrolysis in solution ... 

The introduction of carbon dioxide into the solution removes the caustic soda (or, from the point of view of the ionic theory, the OH ions), so that the equilibrium is disturbed and the reaction then proceeds completely from left to right. Similarly, addition of ionised ammonium chloride suppresses the concentration of the OH ions already present in solution more of the pyrovanadate therefore undergoes hydrolysis, in order that the equilibrium concentrations of ions shall be maintained, until all the pyrovanadate is converted into metavanadate. 

The weakly acid nature of pyrovanadic acid is also shown by the fact that solutions of pyrovanadates react alkaline to phenolphthalein. The solutions have been shown to contain the colourless (Va07)"" ion.1... 

Silver pyrovanadate gives esters of pyrovanadic acid only with the higher alkyl halides, e.g. amyl pyrovanadate, (C5H11)4Va07.a... 

Barium Pyrovanadate, BaaVaO is precipitated on addition of barium chloride to a solution of sodium pyrovanadate or of other vanadates in the presence of ammonia.3 It has more recently been prepared by the action of barium peroxide on vanadium pentoxide.4 It is a white, amorphous powder which melts above 868° C.5... 

Calcium Pyrovanadate, 2Ca2Va07.5Ha0, is formed by adding calcium chloride to a solution of sodium pyrovanadate and drying the precipitate at 100° C.6 The dihydrate, CaaVa07.2Ha0, has been obtained in transparent needles by adding calcium chloride to ammonium metavanadate solution and then excess of ammonium hydroxide.7... 

Lead Pyrovanadate, PbaVa07.—This salt is of interest in that it is the artificial form of descloizite, one of the important natural ores of vanadium. It is obtained by boiling mixed solutions of lead nitrate and ammonium metavanadate in the presence of acetic acid.11 A pale yellow basic pyrovanadate of lead, 2PbaVa07.PbO, is obtained by... 

Manganese Pyrovanadate, MnaV207, is obtained in large, brilliant brown needles by fusing together a mixture of vanadium pentoxide, sodium bromide and manganese bromide.4... 

Potassium Pyrovanadate, K4V207.4lI20, separates out in colourless, transparent crystals when one molecular proportion of vanadium pentoxide is dissolved in two molecular proportions of potassium hydroxide. These lose water when heated, melt, and leave a crystalline mass of the anhydrous salt K4Va07,5 which is deliquescent and melts at 910° C.6 A white, crystalline, xeadily soluble trihydrate, K4V207. SH20, has also been prepared.7... 

Silver Pyrovanadate, Ag4V207, is obtained as a dense yellow precipitate by the addition of neutral silver nitrate to sodium pyrovanadate solution. It has also been obtained as brilliant yellow, transparent plates.9 M.pt. 383° C.1 ... 

Thallium Pyrovanadate, T14V20 is precipitated as a light yellow powder by the addition of thallium sulphate to a cold, saturated solution of sodium orthovanadate. It also results on fusing a mixture of vanadium pentoxide and thallium carbonate.14 It melts at 454°15 or 416° C.,16 and is soluble in about 5000 parts of water at 14° C. 

Thorium Pyrovanadate, Th2V207.6H20, is greenish-yellow, and is obtained by the action of thorium chloride in dilute solution on ammonium metavanadate.1... 

Zinc Pyrovanadate, Zn2V207, forms orange-red prisms when vanadium pentoxide is fused with a mixture of sodium bromide and zinc bromide.2 It is appreciably soluble in water. 

Several vanadates are known the compositions of whieh are intermediate between that of the pyrovanadates, 2R 2O.V2Os, and that of the metavanadates, R 2O.V205. They can be looked upon as basic metavanadates. The following have been described —... 

Basic Potassium Metavanadate, 5K20.4V20S.7H20 or K20. KV03. 7H20, separates in white crystals when a solution of potassium pyrovanadate, K4V207, is acidified with acetic acid and concentrated.4... 

T120.5V20s or Tl2O.10TlVO3 is prepared by the action of thallium sulphate on sodium pyrovanadate, Na4V207, in the presence of excess of vanadium pentoxide.7 It dissolves in 9372 parts of water at 11° C. and in 83C6 parts of water at 100 0 C. 

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