Highly dispersed transition-metal-containing polyoxomolybdates [XMo6O24n−; X = Fe, Co, and Ni] on alumina modified with a silane agent and their catalytic features for partial-methanol-oxidation Original Research Article

The molecular units of polyoxomolybdates containing transition metal ions [XMo6O24n−; XMo6, X = Fe, Co, Ni] were quite homogeneous and thus could be densely dispersed on alumina chemically modified with N-(2-aminoethyl)-3-aminopropyltrimethoxysilane by an equilibrium adsorption method [XMo6/DAPS-Al2O3]. The catalytic features of XMo6/DAPS-Al2O3 thus prepared for methanol-oxidation were extensively examined. On XMo6/DAPS-Al2O3, the methanol-conversion increased up to 100% with an increase in the reaction temperature (200 → 300 °C) while maintaining the formaldehyde-selectivity at a high level such as 80–90% or more; e.g., the high selectivity of 91% was found in the 83%-conversion on the most efficient catalyst, FeMo6/DAPS-Al2O3, at 300 °C. Such catalytic features were found to be induced from the disk-like XMo6-molecular units that were projected perpendicular and densely over the DAPS-Al2O3 surfaces in the saturated states. The mechanistic aspects were also easy to explain based on the XMo6-dispersed model. Finally, we proposed that the premodification method of the carrier surfaces with silane agents should be widely applied as a very useful technique for the preparation of fixed-polyoxometalate catalysts.