Effect of water vapor on the transformation of VOHPO4·0.5H2O into (VO)2P2O7 Original Research Article

The transformation of VOHPO4·0.5H2O to (VO)2P2O7 has been studied in the absence and presence of water vapor (0−40 vol%) using small (340 nm × 40 nm) and large (10,000 nm × 410 nm) VOHPO4·0.5H2O crystallites. The small VOHPO4·0.5H2O crystallites were transformed into a single-phase of well-crystallized (VO)2P2O7 within 5 h under a reactant gas (0.9% n-butane, 10% O2 and He(balance)) containing 40% water vapor, whereas the transformation took more than 100 h in reactant gas without water vapor. In the large-sized VOHPO4·0.5H2O crystallites, (VO)2P2O7 was the main phase formed in the presence of reactant gas containing 40% water vapor, whereas αII-VOPO4 was the main product in the absence of water vapor. It was found that, under the reaction conditions, water vapor accelerated two processes in the transformation of VOHPO4·0.5H2O to (VO)2P2O7; the crystallization of the amorphous VP phase containing V4+ and V5+ to (VO)2P2O7 and δ-VOPO4, and the transformation of δ-VOPO4 to (VO)2P2O7. On the other hand, water vapor inhibited the topotactic transformation of VOHPO4·0.5H2O to (VO)2P2O7. The two processes accelerated by water vapor resulted in the rapid transformation of VOHPO4·0.5H2O to (VO)2P2O7 under the reactant gas. The catalyst obtained in the presence of water vapor gave stationary conversion and selectivity for MA from immediately after the beginning of the reaction. This catalyst showed a high selectivity for MA in comparison to the catalyst transformed in the absence of water vapor. Furthermore, over the catalyst formed in the absence of water vapor, it took more than 100 h to reach stationary conversion and selectivity.