Fischer indole synthesis over hydrous-zirconia supported niobium oxide and boria-zirconia catalysts

Abstract

Supported niobium oxides and boria zirconia catalysts are investigated as green catalysts for Fischer indole reaction. By means of wet impregnation, Nb2O5 and B2O3 were loaded onto hydrous zirconia and zirconia as a support. Pore size distribution curves showed that the overlayer was uniformly dispersed onto the mesoporous support. Samples with close to a monolayer coverage had the highest activity in the Fischer indole reaction of phenylhydrazine with 3-heptanone and with cyclohexanone. A coverage higher than a monolayer led to lower activity. From base titration results, both of supported niobium oxides and boria zirconia catalysts are highly acidic. Uncalcined supported niobium oxide exhibited better activity than calcined samples whereas the best calcination temperature for boria zirconia catalaysts was 500B0C. The catalytic activity of used supported niobium oxides can be fully recovered by immersion the solid in H2O2 solution at 40B0C while boria zirconia catalysts were easily regenerated by calcinations. Both acidity and activity of the catalysts were fully recovered.