Electron microscopy of some molybdenum oxide phases after use as catalysts in oxidative ammonolysis and ammoxidation of toluene

Abstract

Five molybdenum oxide samples, subjected to conditions of oxidative ammonolysis and ammoxidation of toluene at 450 and 460°C, respectively, have been characterized using X-ray diffraction, scanning and transmission electron microscopy, and specific surface area measurements. In oxidative ammonolysis, the relatively large, freshly prepared MoO 3 crystals are reduced to smaller MoO 2 crystals with a crystallite size of 5–30 nm. This process gives a perfectly pseudomorphous product with pores less than 5 nm. The specific surface area increases from <0.1 m 2/g to almost 40 m 2/g. In subsequent ammoxidation, MoO 2 transforms first into orthorhombic Mo 4O 11 and finally into MoO 3. The crystals of Mo 4O 11 are about 1 μm in diameter, and their formation leads to a decrease of specific surface area. The original MoO 3 morphology is retained even after the sequence of transformation as follows: MoO 3 → MoO 2 → Mo 4O 11 (→MoO 3). In some cases, the new generation of MoO 3 crystals grows parallel to the original MoO 3 crystals.