Fischer Indole Synthesis over Hydrous Zirconia-Supported Niobium Oxide

Supported niobium oxides are investigated as green catalysts for Fischer indole reaction. By means of wet impregnation, 10-40 wt-% Nb2O5 were loaded onto hydrous zirconia as a support. Pore size distribution curves showed that the niobium oxide overlayer was uniformly dispersed onto the mesoporous support. Samples with close to a monolayer coverage of niobium oxide had the highest activity in the Fischer indole reaction of phenylhydrazine with both 3-heptanone and cyclohexanone. A coverage higher than a monolayer led to lower activity. In comparison, the supported catalysts were more active than bulk niobium oxide or a sample prepared by co-precipitation of hydrous zirconia and niobium oxide.