Studies of molybdena-alumina catalysts

Abstract

MoO 3Al 2O 3 catalysts over the loading range 3.9 to 14.9 wt% MoO 3 have been studied. Although the BET surface areas in the higher valent (MoO 3) form are compatible with a monolayer model, the areas after H 2 reduction at 500 °C are not; the appropriate model would be porous or finely crystalline MoO 2 produced by reduction. For reduced catalysts, the curve for O 2 chemisorption (at −78 °C) vs molybdena loading is concave upwards. The origin of this behavior seems to be incomplete reduction (of MoO 3 to MoO 2, after 6 hr at 500 °C in flowing H 2) for catalysts containing less than about 11 wt% MoO 3. If correction is made for the actual extent of reduction at 500 °C, then O 2 chemisorption vs loading is essentially a straight line through the origin. Activity for cyclohexane dehydrogenation at 500 °C vs molybdena loading is also a curve concave upward; again, correction for the actual amount of MoO 3 reduced during prereduction changes the curve to one that is slightly convex upward. The sites responsible for both O 2 chemisorption and cyclohexane dehydrogenation may correspond to surface Mo 4+ ions, but this is not proven.