Dehydration, dehydrogenation, and condensation of alcohols on supported oxide catalysts based on cyclic (WO₃)₃ and (MoO₃)₃ clusters

Supported early transition metal oxides have important applications in numerous catalytic reactions. In this article, we review the synthesis and activity of well-defined model WO₃ and MoO₃ catalysts that are prepared via deposition of cyclic gas-phase (WO₃)₃ and (MoO₃)₃ clusters generated by sublimation of WO₃ and MoO₃ powders. Conversion of small aliphatic alcohols to alkenes, aldehydes/ketones, and ethers is employed to probe the structure-activity relationships on model catalysts ranging from unsupported (WO₃)₃ and (MoO₃)₃ clusters embedded in alcohol matrices, to (WO₃)₃ clusters supported on surfaces of other oxides, and epitaxial and nanoporous WO₃ films. Detailed theoretical calculations reveal the underlying reaction mechanisms and provide insight into the origin of the differences in the WO₃ and MoO₃ reactivity. The catalytic activity for a range of interrogated (WO₃)₃ motifs (from unsupported clusters to nanoporous films) further sheds light onto the role structure and binding of (WO₃)₃ clusters with the support play in determining their catalytic activity.