Control of the crystal morphology of VOHPO₄·0.5H₂O precursors prepared via light alcohols-assisted solvothermal synthesis and influence on the selective oxidation of n-butane

The catalytic performance of vanadyl pyrophosphate (VPP) catalysts in the oxidation of n-butane to maleic anhydride (MA) depends strongly on the display of the active and selective {0 0 1} faces, that may be controlled during the preparation of VOHPO₄·0.5H₂O precursor by solvothermal technique using C₂ to C₄ alkanoic alcohols. Intercalated metastable vanadyl-alkyl-phosphates were formed when linear alcohols (ethanol, n-butanol) were used, at variance with iso-alcohols. {0 0 1} platelets of VOHPO₄·0.5H₂O yielded {2 0 0} platelets to (VO)₂P₂O₇ in nitrogen or in situ in the reactor, as revealed by structural analyses. The catalysts prepared in ethanol or n-butanol in situ activated at 440 °C were more active and selective to MA than in iso-alcohols, but if the equilibration was performed at 380 °C the influence of alcohol was mitigated. The highest MA yield was obtained with the ethanol-derived catalyst, which exhibited the smallest platy crystallites of (VO)₂P₂O₇, as well as surface V⁵⁺ species as seen by XPS.