No abstract

Acetic acid and acetic anhydride are dissociatively adsorbed to give acetate anions on the surface of vanadyl pyrophosphate. Undissociated acetic acid molecules ligated to Lewis-acidic surface sites are partially displaced as the surface concentration of acetate anions increased. The products of adsorption of carbon dioxide and carbon monoxide are probably carbonate or bicarbonate species resulting from reaction with surface oxide ions. Adsorbed acetate and carbonate ions are predominantly in the bidentate configuration.An infrared study1 of the adsorption of water and pyridine has established that Brarnsted-acidic hydroxy groups and Lewis-acidic sites exist on the surface of anydrous vanadyl pyrophosphate (VO),P,O,, which is an important component of vanadiumphosphorus mixed-oxide catalysts for the oxidation of hydrocarbons to maleic anhydride.,? The mechanism of the oxidative reaction involves surface oxide ions as reactive intermediate specie^.^-^ The reactivity of surface oxide ions is here monitored by infrared studies of the adsorption on (VO),P,O, of the probe molecules carbon monoxide, carbon dioxide and acetic acid. Carbon monoxide and acetic acid were chosen also because of their ability to ligate to coordinatively unsaturated exposed cations in the surface of solids. It was hoped that comparison of adsorption data for water, pyridine,l carbon monoxide and acetic acid might facilitate the further characterisation of Lewis-acidic surface sites. Acetic anhydride was chosen as a model adsorbate to probe the interactions between a simple acid anhydride and the surface of (VO),P,O, because it was expected that at least some of the resulting adsorbed species would resemble species produced by the adsorption of acetic acid.

Brrnsted-acidic hydroxy groups on the surface of vanadyl pyrophosphate act as catalytically active sites for the isomerisation of but-l-ene to but-2-enes. Progressive poisoning of the reaction is attributed to the formation of alkyl species, possibly alkoxy groups, on the (VO),P,O, surface. Adsorbed alkyl groups also result from buta-1,3-diene adsorption. Furan adsorbs on (VO),P,O, through coordinative interactions with Lewis-acidic surface sites. The presence of oxygen promotes the oxidation of furan to maleic anhydride.The importance has been established of vanadyl pyrophosphate as a component of vanadium-phosphorus mixed-oxide catalysts for the oxidation of butane or but-1 -ene to maleic anh~dride.l-~ The surface character of (VO),P207 has been partly established by infrared studies of the adsorption of water, pyridine, CO, CO,, acetic acid and acetic a n h ~d r i d e . ~? ~The present paper reports an infrared study of the adsorption on (VO),P207 of the species (butane, but-1 -ene, buta-1,3-diene, furan and maleic anhydride) involved as reactants, intermediates or product in the oxidation reactions. ExperimentalVanadyl pyrophosphate (surface area 11 m2 g-l) was prepared by reacting orthophosphoric acid and vanadium pentoxide in strongly acidic solution and decomposing the reaction product by thermal activation in vacuum. The purity of the final product was confirmed by infrared spectroscopy and X-ray powder diffracti0n.l Compressed (3.3 ton) self-supporting discs (25 mm diameter) of (VO),P207 (ca. 55 mg) were mounted in an evacuable infrared cell with fluorite windows and an external furnace and were heated at 673 K (17 h) in vacuum before the admission of adsorbate gases. Spectra of discs at ca. 300 K were recorded using a Perkin-Elmer 580A spectrophotometer. High-purity commercial samples of butane, but-1 -ene, buta-1,3-diene and furan were freed from permanent gases by a series of freeze-thaw cycles. Maleic anhydride was purified by sublimation under reduced pressure to remove maleic acid. ResultsContact between butane vapour (1.3 kN m-2) and (VO),P,O, at temperatures in the range 293-723 K gave no detectable change in the infrared spectrum of the gas phase and no infrared bands due to adsorbed species. However, at elevated temperatures the disc turned black, implying that carbon deposition had occurred.Exposure of (VO),P,O, at the ambient temperature in the sample beam of the spectrophotometer (ca. 300 K) to but-1-ene (2.5 kN m-,) gave spectra [fig. 1 (b)] in which the observed bands could primarily be attributed to but-1-ene in the gas phase. The spectra did not vary with time but removal of the gas phase by evacuation left residual 100 3033 FAR 1