Vanadyl hydrogen phosphate

The catalyst used in the production of maleic anhydride from butane is vanadium—phosphoms—oxide (VPO). Several routes may be used to prepare the catalyst (123), but the route favored by industry involves the reaction of vanadium(V) oxide [1314-62-1] and phosphoric acid [7664-38-2] to form vanadyl hydrogen phosphate, VOHPO O.5H2O. This material is then heated to eliminate water from the stmcture and irreversibly form vanadyl pyrophosphate, (V(123,124). Vanadyl pyrophosphate is befleved to be the catalyticaHy active phase required for the conversion of butane to maleic anhydride (125,126). 

Torardi, C. and Calabrese, J. (1984). Ambient- and Low-temperature Crystal Structure of Vanadyl Hydrogen Phosphate, Inorg. Chem., 23, pp. 1308-1310. 

The VPO catalysts are prepared by thermal dehydration of its precursor, vanadyl(IV) hydrogen phosphate hemihydrate, VOHPO4 0.5H2O. The catalytic performance of the VPO catalysts depends on (i) the method of V0HP04 ... 

B. VANADYL(IV) HYDROGEN PHOSPHATE HEMIHYDRATE, (VO)(HP04) 0.5H20 (V0)P04-2H20/s-Bu0H METHOD... 

The fact that divalent metal ions, the vanadyl ion, and their chelates, do not catalyze the hydrolysis forms [Ll and [IV] to an appreciable extent is in accord with the proposed mechanism, since coordination of the phosphate group by the metal ion would be prevented or greatly reduced by the presence of two protons. Accordingly metal ion catalysis by Cu and VO ions increases in effectiveness as the number of protons on the substrate is successively reduced. Such behavior would not be expected if transfer of a hydrogen-bonded proton from the carboxyl group to the phenolic ester oxygen were the only pathway for the reaction. Metal ion catalysis of the hydrolysis of [V], [VI],. and [VII] was not measured because of the formation of a solid phase in the presence of Cu and VO ions. 

The ion-exchange technique allows to prepare VO modified titanium phosphates. Different vanadium loadings can be obtained by properly controlling the operating exchange conditions and precursor phase. Vanadyl modified titanium phosphates catalysts were found active and selective towards SCR reaction, either as hydrogen or pyrophosphate phase. The results obtained in this paper indicate that the activity of the materi s can be relate to vanadyl species whose redox properties affect the catalytic behaviour. 

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