Vanadium Phosphate Oxide Seeds and Their Influence on the Formation of Vanadium Phosphate Catalyst Precursors

摘要

Vanadium phosphate materials, based on a VOHPO4-0.5 H2O precursor and with (V0)2P2O7 as an active high-temperature phase, are used as catalysts for the oxidation of alkanes. VOHPO4-0.5H2O is prepared from VOPO4·2H2O using 1-octanol, 3-octanol, 2-butanol, or 2-methyl-1-propanol as both solvent and reducing agent. With 1-octanol, the reaction temperature was found to be crucial in obtaining a high yield of the precursor phase. At temperatures of 160 C or greater, a solution containing V~(4+) ions formed in preference to VOHPO4·0.5 H2O. However, VOHPO4·0.5H2O formation can be achieved above 160 C by carrying out the reduction process in the presence of a small amount of vanadium phosphate material, which ef- fectively acts as a templating seed. The use of this seeding concept is shown to have a dramatic effect on the morphology of the final activated catalyst. In contrast, when 3-octanol is used, solely V0(H2PO4)_2 is generated, except in the presence of a vanadium phosphate seed where significant amounts of VOHPO4·0.5H2O can also be formed. Furthermore, VO(H2PO4)_2 can be transformed to VOHPO4·0.5 H2O by heating at reflux with an alcohol in the presence of VOHPO4·0.5H2O precursor seeds. The findings reported herein show that both the phase composition and morphology of vanadium phosphates can be influenced by the use of seeds during the preparation process.