Promotion of vanadium phosphate catalysts using gallium compounds: effect of low Ga/V molar ratios

The doping of vanadium phosphate catalysts by low levels of gallium is described an discussed. VOHPO4·0.5H2O precursors doped with Ga were prepared using a two stage method in which V2O5 is initially reacted with isobutanol before reaction with H3PO4 and Ga(acac)3. These were transformed to (VO)2P2O7 by reaction with 1.7% n-butane in air at 400°C for 72h. The materials were characterised using a combination of powder XRD, BET surface area measurement, laser Raman spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy. The addition of 0.1mol% Ga significantly enhances the activity of the catalyst. The effect is due in part to a structural effect as the surface area of the catalyst is increased by ca. 50–100% when up to 1mol% Ga is added. However, the intrinsic activity (mol maleic anhydride produced/m2/h) is also significantly enhanced at these low doping levels. Hence, the promotional effect is considered to be due to both structural and electronic effects. The source of Ga was found to be and experiments were carried out with Ga2O3 and GaPO4 in place of Ga(acac)3. GaPO4 was found to give some enhancement in activity, but neither Ga2O3 nor GaPO4 gave an enhancement in surface area.