Several biphasic VOP04/(VO)2P2O7
redox partners seem to be active in the catalytic cycles involved in the
mild oxidation of simple hydrocarbons to maleic anhydride. The present X-ray
time resolved thermal-diffractometric study reveals the existence of a new
anhydrous variety of the oxidized form, ω-VOPO4, which may appear
as an intermediate in the formation of the well known β-VOPO4.
This new phase only remains well crystallized at relatively high
temperatures, and when it is allowed to cool down in wet air
VOPO4·2H2O results. In turn, another well defined
phase, VOPO4·1.58H2O, can be recognized in the course
of the hydration process.